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Header Protection for Cryptographically Protected E-mail
draft-ietf-lamps-header-protection-24

Document Type Active Internet-Draft (lamps WG)
Authors Daniel Kahn Gillmor , Bernie Hoeneisen , Alexey Melnikov
Last updated 2024-10-21 (Latest revision 2024-09-04)
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Header protection conventions sent to IESG for standards track publication
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draft-ietf-lamps-header-protection-24
LAMPS Working Group                                        D. K. Gillmor
Internet-Draft                            American Civil Liberties Union
Updates: 8551 (if approved)                                 B. Hoeneisen
Intended status: Standards Track                             pEp Project
Expires: 8 March 2025                                        A. Melnikov
                                                               Isode Ltd
                                                        4 September 2024

        Header Protection for Cryptographically Protected E-mail
                 draft-ietf-lamps-header-protection-24

Abstract

   S/MIME version 3.1 introduced a mechanism to provide end-to-end
   cryptographic protection of e-mail message headers.  However, few
   implementations generate messages using this mechanism, and several
   legacy implementations have revealed rendering or security issues
   when handling such a message.

   This document updates the S/MIME specification (RFC8551) to offer a
   different mechanism that provides the same cryptographic protections
   but with fewer downsides when handled by legacy clients.
   Furthermore, it offers more explicit usability, privacy, and security
   guidance for clients when generating or handling e-mail messages with
   cryptographic protection of message headers.

   The Header Protection scheme defined here is also applicable to
   messages with PGP/MIME cryptographic protections.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://dkg.gitlab.io/lamps-header-protection/.  Status information
   for this document may be found at https://datatracker.ietf.org/doc/
   draft-ietf-lamps-header-protection/.

   Discussion of this document takes place on the LAMPS Working Group
   mailing list (mailto:spasm@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/spasm/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/spasm/.

   Source for this draft and an issue tracker can be found at
   https://gitlab.com/dkg/lamps-header-protection.

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Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 March 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.1.  Update to RFC 8551  . . . . . . . . . . . . . . . . . . .   7
       1.1.1.  Problems with RFC 8551 Header Protection  . . . . . .   8
     1.2.  Risks of Header Protection for Legacy MUA Recipients  . .   9
     1.3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  10
       1.3.1.  Backward Compatibility  . . . . . . . . . . . . . . .  10
       1.3.2.  Deliverability  . . . . . . . . . . . . . . . . . . .  11
     1.4.  Other Protocols to Protect E-Mail Header Fields . . . . .  11
     1.5.  Applicability to PGP/MIME . . . . . . . . . . . . . . . .  12
     1.6.  Requirements Language . . . . . . . . . . . . . . . . . .  12
     1.7.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     1.8.  Document Scope  . . . . . . . . . . . . . . . . . . . . .  14
       1.8.1.  In Scope  . . . . . . . . . . . . . . . . . . . . . .  14
       1.8.2.  Out of Scope  . . . . . . . . . . . . . . . . . . . .  15
     1.9.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  15

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   2.  Internet Message Format Extensions  . . . . . . . . . . . . .  18
     2.1.  Content-Type parameters . . . . . . . . . . . . . . . . .  18
       2.1.1.  Content-Type parameter: hp  . . . . . . . . . . . . .  18
       2.1.2.  Content-Type parameter: hp-legacy-display . . . . . .  19
     2.2.  The HP-Outer Header Field . . . . . . . . . . . . . . . .  19
       2.2.1.  HP-Outer Header Field Definition  . . . . . . . . . .  21
   3.  Header Confidentiality Policy . . . . . . . . . . . . . . . .  21
     3.1.  HCP Definition  . . . . . . . . . . . . . . . . . . . . .  22
       3.1.1.  HCP Avoids Changing From addr-spec  . . . . . . . . .  23
     3.2.  Initial Registered HCPs . . . . . . . . . . . . . . . . .  23
       3.2.1.  Baseline Header Confidentiality Policy  . . . . . . .  23
       3.2.2.  Shy Header Confidentiality Policy . . . . . . . . . .  24
       3.2.3.  No Header Confidentiality Policy  . . . . . . . . . .  24
     3.3.  Default Header Confidentiality Policy . . . . . . . . . .  25
     3.4.  HCP Evolution . . . . . . . . . . . . . . . . . . . . . .  25
       3.4.1.  Offering More Ambitious Header Confidentiality  . . .  25
       3.4.2.  Expert Guidance for Registering Header Confidentiality
               Policies  . . . . . . . . . . . . . . . . . . . . . .  26
   4.  Receiving Guidance  . . . . . . . . . . . . . . . . . . . . .  26
     4.1.  Identifying that a Message has Header Protection  . . . .  27
     4.2.  Extracting Protected and Unprotected ("Outer") Header
            Fields . . . . . . . . . . . . . . . . . . . . . . . . .  28
       4.2.1.  HeaderSetsFromMessage . . . . . . . . . . . . . . . .  28
     4.3.  Updating the Cryptographic Summary  . . . . . . . . . . .  29
       4.3.1.  HeaderFieldProtection . . . . . . . . . . . . . . . .  29
     4.4.  Handling Mismatch of From Header Fields . . . . . . . . .  31
       4.4.1.  Definitions . . . . . . . . . . . . . . . . . . . . .  31
       4.4.2.  Warning for From Header Field Mismatch  . . . . . . .  32
       4.4.3.  From Header Field Rendering . . . . . . . . . . . . .  32
       4.4.4.  Handling Protected From Header Field when
               Responding  . . . . . . . . . . . . . . . . . . . . .  33
       4.4.5.  Matching addr-specs . . . . . . . . . . . . . . . . .  33
     4.5.  Rendering a Message with Header Protection  . . . . . . .  34
       4.5.1.  Example Signed-only Message . . . . . . . . . . . . .  34
       4.5.2.  Example Signed-and-Encrypted Message  . . . . . . . .  34
       4.5.3.  Do Not Render Legacy Display Elements . . . . . . . .  35
     4.6.  Implicitly rendered Header Fields . . . . . . . . . . . .  37
     4.7.  Handling Undecryptable Messages . . . . . . . . . . . . .  37
     4.8.  Guidance for Automated Message Handling . . . . . . . . .  38
       4.8.1.  Interpret Only Protected Header Fields  . . . . . . .  39
       4.8.2.  Ignore Legacy Display Elements  . . . . . . . . . . .  39
     4.9.  Affordances for Debugging and Troubleshooting . . . . . .  40
     4.10. Handling RFC8551HP Messages (Backward Compatibility)  . .  40
       4.10.1.  Identifying an RFC8551HP Message . . . . . . . . . .  40
       4.10.2.  Rendering or Responding to an RFC8551HP message  . .  41
     4.11. Rendering Other Schemes . . . . . . . . . . . . . . . . .  42
   5.  Sending Guidance  . . . . . . . . . . . . . . . . . . . . . .  42

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     5.1.  Composing a Cryptographically Protected Message Without
           Header Protection . . . . . . . . . . . . . . . . . . . .  43
       5.1.1.  ComposeNoHeaderProtection . . . . . . . . . . . . . .  43
     5.2.  Composing a Message with Header Protection  . . . . . . .  44
       5.2.1.  Compose . . . . . . . . . . . . . . . . . . . . . . .  45
       5.2.2.  Adding a Legacy Display Element to a text/plain
               Part  . . . . . . . . . . . . . . . . . . . . . . . .  47
       5.2.3.  Adding a Legacy Display Element to a text/html
               Part  . . . . . . . . . . . . . . . . . . . . . . . .  48
       5.2.4.  Only Add a Legacy Display Element to Main Body
               Parts . . . . . . . . . . . . . . . . . . . . . . . .  49
       5.2.5.  Do Not Add a Legacy Display Element to Other
               Content-Types . . . . . . . . . . . . . . . . . . . .  50
   6.  Replying and Forwarding Guidance  . . . . . . . . . . . . . .  50
     6.1.  Avoid Leaking Encrypted Header Fields in Replies and
           Forwards  . . . . . . . . . . . . . . . . . . . . . . . .  50
       6.1.1.  ReferenceHCP  . . . . . . . . . . . . . . . . . . . .  51
     6.2.  Avoid Misdirected Replies . . . . . . . . . . . . . . . .  53
   7.  Unprotected Header Fields Added in Transit  . . . . . . . . .  54
     7.1.  Mailing list Header Fields: List-* and Archived-At  . . .  54
   8.  E-mail Ecosystem Evolution  . . . . . . . . . . . . . . . . .  55
     8.1.  Dropping Legacy Display Elements  . . . . . . . . . . . .  55
     8.2.  More Ambitious Default Header Confidentiality Policy  . .  56
     8.3.  Deprecation of Messages Without Header Protection . . . .  57
   9.  Usability Considerations  . . . . . . . . . . . . . . . . . .  57
     9.1.  Mixed Protections Within a Message Are Hard To
           Understand  . . . . . . . . . . . . . . . . . . . . . . .  57
     9.2.  Users Should Not Have To Choose a Header Confidentiality
           Policy  . . . . . . . . . . . . . . . . . . . . . . . . .  58
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  59
     10.1.  From Address Spoofing  . . . . . . . . . . . . . . . . .  59
       10.1.1.  From Rendering Reasoning . . . . . . . . . . . . . .  60
     10.2.  Avoid Cryptographic Summary Confusion from hp
            Parameter  . . . . . . . . . . . . . . . . . . . . . . .  63
     10.3.  Caution about Composing with Legacy Display Elements . .  64
     10.4.  Plaintext Attacks  . . . . . . . . . . . . . . . . . . .  65
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  65
     11.1.  Leaks When Replying  . . . . . . . . . . . . . . . . . .  65
     11.2.  Encrypted Header Fields Are Not Always Private . . . . .  65
       11.2.1.  Encrypted Header Fields Can Leak Unwanted Information
               to the Recipient  . . . . . . . . . . . . . . . . . .  66
       11.2.2.  Encrypted Header Fields Can Be Inferred From External
               or Internal Metadata  . . . . . . . . . . . . . . . .  67
       11.2.3.  Encrypted Header Fields May Not Be Fully Masked by
               HCP . . . . . . . . . . . . . . . . . . . . . . . . .  67
     11.3.  A Naive Recipient May Overestimate the Cryptographic
            Status of a Header Field in an Encrypted Message . . . .  67

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     11.4.  Privacy and Deliverability Risks with Bcc and Encrypted
            Messages . . . . . . . . . . . . . . . . . . . . . . . .  68
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  69
     12.1.  Register the HP-Outer Header Field . . . . . . . . . . .  69
     12.2.  Update Reference for Content-Type Header Field due to hp
            and hp-legacy-display Parameters . . . . . . . . . . . .  69
     12.3.  New Registry: Mail Header Confidentiality Policies . . .  70
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  72
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  72
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  72
     14.2.  Informative References . . . . . . . . . . . . . . . . .  73
   Appendix A.  Table of Pseudocode Listings . . . . . . . . . . . .  76
   Appendix B.  Possible Problems with Legacy MUAs . . . . . . . . .  77
     B.1.  Problems Viewing Messages in a List View  . . . . . . . .  78
     B.2.  Problems when Rendering a Message . . . . . . . . . . . .  78
     B.3.  Problems when Replying to a Message . . . . . . . . . . .  79
   Appendix C.  Test Vectors . . . . . . . . . . . . . . . . . . . .  80
     C.1.  Baseline Messages . . . . . . . . . . . . . . . . . . . .  80
       C.1.1.  No Cryptographic Protections Over a Simple Message  .  80
       C.1.2.  S/MIME Signed-only signedData Over a Simple Message, No
               Header Protection . . . . . . . . . . . . . . . . . .  81
       C.1.3.  S/MIME Signed-only multipart/signed Over a Simple
               Message, No Header Protection . . . . . . . . . . . .  83
       C.1.4.  S/MIME Signed and Encrypted Over a Simple Message, No
               Header Protection . . . . . . . . . . . . . . . . . .  85
       C.1.5.  No Cryptographic Protections Over a Complex
               Message . . . . . . . . . . . . . . . . . . . . . . .  90
       C.1.6.  S/MIME Signed-only signedData Over a Complex Message,
               No Header Protection  . . . . . . . . . . . . . . . .  92
       C.1.7.  S/MIME Signed-only multipart/signed Over a Complex
               Message, No Header Protection . . . . . . . . . . . .  95
       C.1.8.  S/MIME Signed and Encrypted Over a Complex Message, No
               Header Protection . . . . . . . . . . . . . . . . . .  98
     C.2.  Signed-only Messages  . . . . . . . . . . . . . . . . . . 105
       C.2.1.  S/MIME Signed-only signedData Over a Simple Message,
               Header Protection . . . . . . . . . . . . . . . . . . 105
       C.2.2.  S/MIME Signed-only multipart/signed Over a Simple
               Message, Header Protection  . . . . . . . . . . . . . 107
       C.2.3.  S/MIME Signed-only signedData Over a Complex Message,
               Header Protection . . . . . . . . . . . . . . . . . . 110
       C.2.4.  S/MIME Signed-only multipart/signed Over a Complex
               Message, Header Protection  . . . . . . . . . . . . . 113
       C.2.5.  S/MIME Signed-only signedData Over a Complex Message,
               Legacy RFC 8551 Header Protection . . . . . . . . . . 117
       C.2.6.  S/MIME Signed-only multipart/signed Over a Complex
               Message, Legacy RFC 8551 Header Protection  . . . . . 121
     C.3.  Signed-and-Encrypted Messages . . . . . . . . . . . . . . 124

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       C.3.1.  S/MIME Signed and Encrypted Over a Simple Message,
               Header Protection With hcp_baseline . . . . . . . . . 124
       C.3.2.  S/MIME Signed and Encrypted Over a Simple Message,
               Header Protection With hcp_baseline (+ Legacy
               Display)  . . . . . . . . . . . . . . . . . . . . . . 130
       C.3.3.  S/MIME Signed and Encrypted Over a Simple Message,
               Header Protection With hcp_shy  . . . . . . . . . . . 135
       C.3.4.  S/MIME Signed and Encrypted Over a Simple Message,
               Header Protection With hcp_shy (+ Legacy Display) . . 141
       C.3.5.  S/MIME Signed and Encrypted Reply Over a Simple
               Message, Header Protection With hcp_baseline  . . . . 147
       C.3.6.  S/MIME Signed and Encrypted Reply Over a Simple
               Message, Header Protection With hcp_baseline (+ Legacy
               Display)  . . . . . . . . . . . . . . . . . . . . . . 153
       C.3.7.  S/MIME Signed and Encrypted Reply Over a Simple
               Message, Header Protection With hcp_shy . . . . . . . 160
       C.3.8.  S/MIME Signed and Encrypted Reply Over a Simple
               Message, Header Protection With hcp_shy (+ Legacy
               Display)  . . . . . . . . . . . . . . . . . . . . . . 166
       C.3.9.  S/MIME Signed and Encrypted Over a Complex Message,
               Header Protection With hcp_baseline . . . . . . . . . 174
       C.3.10. S/MIME Signed and Encrypted Over a Complex Message,
               Header Protection With hcp_baseline (+ Legacy
               Display)  . . . . . . . . . . . . . . . . . . . . . . 181
       C.3.11. S/MIME Signed and Encrypted Over a Complex Message,
               Header Protection With hcp_shy  . . . . . . . . . . . 190
       C.3.12. S/MIME Signed and Encrypted Over a Complex Message,
               Header Protection With hcp_shy (+ Legacy Display) . . 197
       C.3.13. S/MIME Signed and Encrypted Reply Over a Complex
               Message, Header Protection With hcp_baseline  . . . . 206
       C.3.14. S/MIME Signed and Encrypted Reply Over a Complex
               Message, Header Protection With hcp_baseline (+ Legacy
               Display)  . . . . . . . . . . . . . . . . . . . . . . 214
       C.3.15. S/MIME Signed and Encrypted Reply Over a Complex
               Message, Header Protection With hcp_shy . . . . . . . 223
       C.3.16. S/MIME Signed and Encrypted Reply Over a Complex
               Message, Header Protection With hcp_shy (+ Legacy
               Display)  . . . . . . . . . . . . . . . . . . . . . . 231
       C.3.17. S/MIME Signed and Encrypted Over a Complex Message,
               Legacy RFC 8551 Header Protection With hcp_baseline . 240
   Appendix D.  Composition Examples . . . . . . . . . . . . . . . . 248
     D.1.  New message composition . . . . . . . . . . . . . . . . . 248
       D.1.1.  Unprotected message . . . . . . . . . . . . . . . . . 249
       D.1.2.  Encrypted with hcp_baseline and Legacy Display  . . . 249
     D.2.  Composing a Reply . . . . . . . . . . . . . . . . . . . . 251
       D.2.1.  Unprotected message . . . . . . . . . . . . . . . . . 252
       D.2.2.  Encrypted with hcp_no_confidentiality and Legacy
               Display . . . . . . . . . . . . . . . . . . . . . . . 253

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   Appendix E.  Rendering Examples . . . . . . . . . . . . . . . . . 257
     E.1.  Example text/plain Cryptographic Payload with Legacy
           Display Elements  . . . . . . . . . . . . . . . . . . . . 257
     E.2.  Example text/html Cryptographic Payload with Legacy Display
           Elements  . . . . . . . . . . . . . . . . . . . . . . . . 258
   Appendix F.  Other Header Protection Schemes  . . . . . . . . . . 260
     F.1.  Original RFC 8551 Header Protection . . . . . . . . . . . 260
     F.2.  Pretty Easy Privacy (pEp) . . . . . . . . . . . . . . . . 260
     F.3.  "draft-autocrypt" Protected Headers . . . . . . . . . . . 261
   Appendix G.  Document Changelog . . . . . . . . . . . . . . . . . 261
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 269

1.  Introduction

   Privacy and security issues regarding e-mail Header Protection in S/
   MIME and PGP/MIME have been identified for some time.  Most current
   implementations of cryptographically protected electronic mail
   protect only the body of the message, which leaves significant room
   for attacks against otherwise-protected messages.  For example, lack
   of Header Protection allows an attacker to substitute the message
   subject and/or author.

   This document describes how to cryptographically protect message
   headers, and provides guidance for the implementer of a Mail User
   Agent (MUA) that generates, interprets, and replies to such a
   message.  It uses the term "Legacy MUA" to refer to an MUA that does
   not implement this specification.  This document takes particular
   care to ensure that messages interact reasonably well with Legacy
   MUAs.

1.1.  Update to RFC 8551

   An older scheme for Header Protection was specified in S/MIME 3.1
   ([RFC8551]), which involves wrapping a message/rfc822 MIME object
   with a Cryptographic Envelope around the message to protect.  This
   document refers to that scheme as RFC 8551 Header Protection, or
   "RFC8551HP".  Substantial testing has shown that RFC8551HP does not
   interact well with some Legacy MUAs (see Section 1.1.1).

   This specification supersedes RFC8551HP, effectively replacing the
   final two paragraphs of Section 3.1 of [RFC8551].

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   In this specification, all Header Fields gain end-to-end
   cryptographic integrity and authenticity by being copied directly
   into the Cryptographic Payload without using an intervening message/
   rfc822 MIME object.  In an encrypted message, some Header Fields can
   also be made confidential by removing or obscuring them from the
   outer Header Section.

   This specification also offers substantial security, privacy, and
   usability guidance for sending and receiving MUAs that was not
   considered in RFC 8551.

1.1.1.  Problems with RFC 8551 Header Protection

   Several Legacy MUAs have difficulty rendering a message that uses
   RFC8551HP.  These problems can appear on signed-only messages, as
   well as signed-and-encrypted messages.

   In some cases, some mail user agents cannot render message/rfc822
   message subparts at all, in violation of baseline MIME requirements
   as defined on page 5 of [RFC2049].  A message using RFC8551HP is
   unreadable by any recipient using such an MUA.

   In other cases, the user sees an attachment suggesting a forwarded
   e-mail message, which -- in fact -- contains the protected e-mail
   message that should be rendered directly.  In most of these cases,
   the user can click on the attachment to view the protected message.

   However, viewing the protected message as an attachment in isolation
   may strip it of any security indications, leaving the user unable to
   assess the cryptographic properties of the message.  Worse, for
   encrypted messages, interacting with the protected message in
   isolation may leak contents of the cleartext, for example, if the
   reply is not also encrypted.

   Furthermore, RFC8551HP lacks any discussion of the following points,
   all of which are provided in this specification:

   *  Which Header Fields should be given end-to-end cryptographic
      integrity and authenticity protections (this specification
      mandates protection of all Header Fields that the sending MUA
      knows about).

   *  How to securely indicate the sender's intent to offer Header
      Protection and encryption, which lets a receiving MUA detect
      messages whose cryptographic properties may have been modified in
      transit (see Section 2.1.1).

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   *  Which Header Fields should be given end-to-end cryptographic
      confidentiality protections in an encrypted message, and how (see
      Section 3).

   *  How to securely indicate the sender's choices about which Header
      Fields were made confidential, which lets a receiving MUA reply or
      forward an encrypted message safely without accidentally leaking
      confidential material (see Section 2.2).

   These stumbling blocks with Legacy MUAs, missing mechanisms, and
   missing guidance create a strong disincentive for existing MUAs
   generate messages using RFC8551HP.  Because few messages have been
   produced, there has been little incentive for those MUAs capable of
   upgrading to bother interpreting them better.

   In contrast, the mechanisms defined here are safe to adopt and
   produce messages with very few problems for Legacy MUAs.  And,
   Section 4.10 provides useful guidance for rendering and replying to
   RFC8551HP messages.

1.2.  Risks of Header Protection for Legacy MUA Recipients

   Producing a signed-only message using this specification is risk-
   free.  Such a message will render in the same way on any Legacy MUA
   as a Legacy Signed Message (that is, a signed message without Header
   Protection).  An MUA conformant to this specification that encounters
   such a message will be able to gain the benefits of end-to-end
   cryptographic integrity and authenticity for all Header Fields.

   An encrypted message produced according to this specification that
   has some user-facing Header Fields removed or obscured may not render
   as desired in a Legacy MUA.  In particular, those Header Fields that
   were made confidential will not be visible to the user of a Legacy
   MUA.  For example, if the Subject Header Field outside the
   Cryptographic Envelope is replaced with [...], a Legacy MUA will
   render the [...] anywhere the Subject is normally seen.  This is the
   only risk of producing an encrypted message according to this
   specification.

   A workaround "Legacy Display" mechanism is provided in this
   specification (see Section 2.1.2).  Legacy MUAs will render "Legacy
   Display Elements" to the user, albeit not in the same location that
   the Header Fields would normally be rendered.

   Alternately, if the sender of an encrypted message is particularly
   concerned about the experience of a recipient using a Legacy MUA, and
   they are willing to accept leaking the user-facing Header Fields,
   they can simply adopt the No Header Confidentiality Policy (see

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   Section 3.2.3).  A signed and encrypted message composed using the No
   Header Confidentiality Policy offers no usability risk for a reader
   using a Legacy MUA, and retains end-to-end cryptographic integrity
   and authenticity properties for all Header Fields for any reader
   using a conformant MUA.  Of course, such a message has the same (non-
   existent) confidentiality properties for all Header Fields as a
   Legacy Encrypted Message (that is, an encrypted message made without
   Header Protection).

1.3.  Motivation

   Users generally do not understand the distinction between message
   body and message header.  When an e-mail message has cryptographic
   protections that cover the message body, but not the Header Fields,
   several attacks become possible.

   For example, a Legacy Signed Message has a signature that covers the
   body but not the Header Fields.  An attacker can therefore modify the
   Header Fields (including the Subject header) without invalidating the
   signature.  Since most readers consider a message body in the context
   of the message's Subject header, the meaning of the message itself
   could change drastically (under the attacker's control) while still
   retaining the same cryptographic indicators of integrity and
   authenticity.

   In another example, a Legacy Encrypted Message has its body
   effectively hidden from an adversary that snoops on the message.  But
   if the Header Fields are not also encrypted, significant information
   about the message (such as the message Subject) will leak to the
   inspecting adversary.

   However, if the sending and receiving MUAs ensure that cryptographic
   protections cover the message Header Section as well as the message
   body, these attacks are defeated.

1.3.1.  Backward Compatibility

   If the sending MUA is unwilling to generate such a fully protected
   message due to the potential for rendering, usability,
   deliverability, or security issues, these defenses cannot be
   realized.

   The sender cannot know what MUA (or MUAs) the recipient will use to
   handle the message.  Thus, an outbound message format that is
   backward compatible with as many legacy implementations as possible
   is a more effective vehicle for providing the whole-message
   cryptographic protections described above.

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   This document aims for backward compatibility with Legacy MUAs to the
   extent possible.  In some cases, like when a user-visible header like
   the Subject is cryptographically hidden, a Legacy MUA will not be
   able to render or reply to the message exactly the same way as a
   conformant MUA would.  But accommodations are described here that
   ensure a rough semantic equivalence for Legacy MUA even in these
   cases.

1.3.2.  Deliverability

   A message with perfect cryptographic protections that cannot be
   delivered is less useful than a message with imperfect cryptographic
   protections that can be delivered.  Senders want their messages to
   reach the intended recipients.

   Given the current state of the Internet mail ecosystem, encrypted
   messages in particular cannot shield all of their Header Fields from
   visibility and still be guaranteed delivery to their intended
   recipient.

   This document accounts for this concern by providing a mechanism
   (Section 3) that prioritizes initial deliverability (at the cost of
   some header leakage) while facilitating future message variants that
   shield more header metadata from casual inspection.

1.4.  Other Protocols to Protect E-Mail Header Fields

   A separate pair of protocols also provides some cryptographic
   protection for the e-mail message header integrity: DomainKeys
   Identified Mail (DKIM) [RFC6376], as used in combination with Domain-
   based Message Authentication, Reporting, and Conformance (DMARC)
   [RFC7489].  This pair of protocols provides a domain-based reputation
   mechanism that can be used to mitigate some forms of unsolicited
   e-mail (spam).

   However, the DKIM+DMARC suite provides cryptographic protection at a
   different scope.  DKIM+DMARC typically provide MTA-to-MTA protection,
   whereas this specification provides MUA-to-MUA protection.  This is
   because DKIM+DMARC are typically applied to messages by (and
   interpreted by) MTAs, whereas the mechanisms in this document are
   typically applied and interpreted by MUAs.

   A receiving MUA that relies on DKIM+DMARC for sender authenticity
   should note Section 10.1.

   Furthermore, the DKIM+DMARC suite only provides cryptographic
   integrity and authentication, not encryption.  So cryptographic
   confidentiality is not available from that suite.

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   The DKIM+DMARC suite can be used on any message, including messages
   formed as defined in this document.  There should be no conflict
   between DKIM+DMARC and the specification here.

   Though not strictly e-mail, similar protections have been in use on
   Usenet for signing and verification of message headers for years.
   See [PGPCONTROL] and [PGPVERIFY-FORMAT] for more details.  Like DKIM,
   these Usenet control protections offer only integrity and
   authentication, not confidentiality.

1.5.  Applicability to PGP/MIME

   This document describes end-to-end cryptographic protections for
   e-mail messages in reference to S/MIME ([RFC8551]).

   Comparable end-to-end cryptographic protections can also be provided
   by PGP/MIME ([RFC3156]).

   The mechanisms in this document should be applicable in the PGP/MIME
   protections as well as S/MIME protections, but analysis and
   implementation in this document focuses on S/MIME.

   To the extent that any divergence from the mechanism defined here is
   necessary for PGP/MIME, that divergence is out of scope for this
   document.

1.6.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The key words "SPECIFICATION REQUIRED" and "IETF REVIEW" that appear
   in this document when used to describe namespace allocation are to be
   interpreted as described in [RFC8126].

1.7.  Terms

   The following terms are defined for the scope of this document:

   *  S/MIME: Secure/Multipurpose Internet Mail Extensions (see
      [RFC8551])

   *  PGP/MIME: MIME Security with OpenPGP (see [RFC3156])

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   *  Message: An E-Mail Message consisting of Header Fields
      (collectively called "the Header Section of the message")
      followed, optionally, by a Body; see [RFC5322].

      Note: To avoid ambiguity, this document avoids using the terms
      "Header" or "Headers" in isolation, but instead always uses
      "Header Field" to refer to the individual field and "Header
      Section" to refer to the entire collection.

   *  Header Field: A Header Field includes a field name, followed by a
      colon (":"), followed by a field body (value), and terminated by
      CRLF; see Section 2.2 of [RFC5322] for more details.

   *  Header Section: The Header Section is a sequence of lines of
      characters with special syntax as defined in [RFC5322].  The
      Header Section of a Message contains the Header Fields associated
      with the Message itself.  The Header Section of a MIME part (that
      is, a subpart of a message) typically contains Header Fields
      associated with that particular MIME part.

   *  Body: The Body is the part of a Message that follows the Header
      Section and is separated from the Header Section by an empty line
      (that is, a line with nothing preceding the CRLF); see [RFC5322].
      It is the (bottom) section of a Message containing the payload of
      a Message.  Typically, the Body consists of a (possibly multipart)
      MIME [RFC2045] construct.

   *  Header Protection (HP): cryptographic protection of e-mail Header
      Sections (or parts of it) by means of signatures and/or
      encryption.

   *  Cryptographic Layer, Cryptographic Payload, Cryptographic
      Envelope, Cryptographic Summary, Structural Header Fields, Main
      Body Part, User-Facing Header Fields, and MUA are all used as
      defined in [I-D.ietf-lamps-e2e-mail-guidance]

   *  Legacy MUA: an MUA that does not understand Header Protection as
      defined in this document.  A Legacy Non-Crypto MUA is incapable of
      doing any end-to-end cryptographic operations.  A Legacy Crypto
      MUA is capable of doing cryptographic operations, but does not
      understand or generate messages with Header Protection.

   *  Legacy Signed Message: an e-mail message that was signed by a
      Legacy MUA, and therefore has no cryptographic authenticity or
      integrity protections on its Header Fields.

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   *  Legacy Encrypted Message: an e-mail message that was signed and
      encrypted by a Legacy MUA, and therefore has no cryptographic
      authenticity, integrity, or confidentiality protections on any of
      its Header Fields.

   *  Header Confidentiality Policy (HCP): a functional specification of
      which Header Fields should be removed or obscured when composing
      an encrypted message with Header Protection.  An HCP is considered
      more "conservative" when it removes or obscures fewer Header
      Fields.  When it removes or obscures more Header fields, it is
      more "ambitious".  See Section 3.

   *  Ordinary User: a user of an MUA who follows a simple and minimal
      experience, focused on sending and receiving e-mails.  A user who
      opts into advanced configuration, expert mode, or the like is not
      an "Ordinary User".

1.8.  Document Scope

   This document describes sensible, simple behavior for a program that
   generates an e-mail message with standard end-to-end cryptographic
   protections, following the guidance in
   [I-D.ietf-lamps-e2e-mail-guidance].  An implementation conformant to
   this document will produce messages that have cryptographic
   protection that covers the message's Header Fields as well as its
   body.

1.8.1.  In Scope

   This document also describes sensible, simple behavior for a program
   that interprets such a message, in a way that can take advantage of
   these protections covering the Header Fields as well as the body.

   The message generation guidance aims to minimize negative
   interactions with any Legacy receiving MUA while providing actionable
   cryptographic properties for modern receiving clients.

   In particular, this document focuses on two standard types of
   cryptographic protection that cover the entire message:

   *  A cleartext message with a single signature, and

   *  An encrypted message that contains a single cryptographic
      signature.

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1.8.2.  Out of Scope

   The message composition guidance in this document (in Section 5.2)
   aims to provide minimal disruption for any Legacy MUA that receives
   such a message.  However, a Legacy MUA by definition does not
   implement any of the guidance here.  Therefore, the document does not
   attempt to provide guidance for Legacy MUAs directly.

   Furthermore, this document does not explicitly contemplate other
   variants of cryptographic message protections, including any of
   these:

   *  Encrypted-only message (Without a cryptographic signature.  See
      Section 5.3 of [I-D.ietf-lamps-e2e-mail-guidance].)

   *  Triple-wrapped message

   *  Signed message with multiple signatures

   *  Encrypted message with a cryptographic signature outside the
      encryption.

   All such messages are out of scope of this document.

1.9.  Example

   This section gives an overview by providing an example of how MIME
   messages with Header Protection look like.

   Consider the following MIME message:

   A └─╴application/pkcs7-mime; smime-type="enveloped-data"
      ↧ (decrypts to)
   B  └─╴application/pkcs7-mime; smime-type="signed-data"
       ⇩ (unwraps to)
   C   └┬╴multipart/alternative; hp="cipher"
   D    ├─╴text/plain; hp-legacy-display="1"
   E    └─╴text/html; hp-legacy-display="1"

   Observe that:

   *  Node A and B are collectively called the Cryptographic Envelope.
      Node C (including its sub-nodes D and E) is called the
      Cryptographic Payload ([I-D.ietf-lamps-e2e-mail-guidance]).

   *  Node A contains the traditional unprotected ("outer") Header
      Fields.  Node C contains the protected ("inner") Header Fields.

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   *  The presence of the hp attribute (see Section 2.1.1) on the
      Content-Type of node C allows the receiver to know that the sender
      applied Header Protection.  Its value allows the receiver to
      distinguish whether the sender intended for the message to be
      confidential (hp="cipher") or not (hp="clear"), since encryption
      may have been added in transit (see Section 10.2).

   The "outer" Header Section on node A looks as follows:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: [...]
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: application/pkcs7-mime; smime-type="enveloped-data"
   MIME-Version: 1.0

   The "inner" Header Section on node C looks as follows:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: Handling the Jones contract
   Keywords: Contract, Urgent
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: multipart/alternative; hp="cipher"
   MIME-Version: 1.0
   HP-Outer: Date: Wed, 11 Jan 2023 16:08:43 -0500
   HP-Outer: From: Bob <bob@example.net>
   HP-Outer: To: Alice <alice@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <20230111T210843Z.1234@lhp.example>

   Observe that:

   *  Between node C and node A, some Header Fields are copied as-is
      (Date, From, To, Message-ID), some are obscured (Subject), and
      some are removed (Keywords).

   *  The HP-Outer Header Fields (see Section 2.2) of node C contain a
      protected copy of the Header Fields in node A.  The copy allows
      the receiver to recompute for which Header Fields the sender
      provided confidentiality by removing or obscuring them.

   *  The copying/removing/obscuring and the HP-Outer only apply to Non-
      Structural Header Fields, not to Structural Header Fields like
      Content-Type or MIME-Version (see Section 1.1 of
      [I-D.ietf-lamps-e2e-mail-guidance]).

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   *  If the sender intends no confidentiality and doesn't encrypt the
      message, it doesn't remove or obscure Header Fields.  All Non-
      Structural Header Fields are copied as-is.  No HP-Outer Header
      Fields are present.

   Node D looks as follows:

   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";

   Subject: Handling the Jones contract
   Keywords: Contract, Urgent

   Please review and approve or decline by Thursday, it's critical!

   Thanks,
   Bob

   --
   Bob Gonzalez
   ACME, Inc.

   Observe that:

   *  The sender adds the removed and obscured User-Facing Header Fields
      (see Section 1.1.2 of [I-D.ietf-lamps-e2e-mail-guidance]) to the
      main body (note the empty line after the Content-Type).  This is
      called the Legacy Display Element.  It allows a user with a Legacy
      MUA which doesn't implement this document to understand the
      message, since the Header Fields will be shown as part of the main
      body.

   *  The hp-legacy-display="1" attribute (see Section 2.1.2) indicates
      that the sender added a Legacy Display Element.  This allows
      receivers that implement this document to recognise the Legacy
      Display Element and distinguish it from user-added content.  The
      receiver then hides the Legacy Display Element and doesn't display
      it to the user.

   *  The hp-legacy-display is added to the node to which it applies,
      not on any outer nodes (e.g., not to node C).

   For more examples, see Appendix D and Appendix E.

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2.  Internet Message Format Extensions

   This section describes relevant, backward-compatible extensions to
   the Internet Message Format ([RFC5322]).  Subsequent sections offer
   concrete guidance for an MUA to make use of these mechanisms,
   including policy decisions and recommended pseudocode.

2.1.  Content-Type parameters

   This document introduces two parameters for the Content-Type Header
   Field, which have distinct semantics and use cases.

2.1.1.  Content-Type parameter: hp

   This specification defines a parameter for the Content-Type Header
   Field named hp (for Header Protection).  This parameter is only
   relevant on the Content-Type Header Field at the root of the
   Cryptographic Payload.  The presence of this parameter at the root of
   the Cryptographic Payload indicates that the sender intends for this
   message to have end-to-end cryptographic protections for the Header
   Fields.

   The parameter's defined values describe the sender's cryptographic
   intent when producing the message:

   +========+==============+=========+=================+==============+
   |hp Value| Authenticity |Integrity| Confidentiality | Description  |
   +========+==============+=========+=================+==============+
   |"clear" | yes          |yes      | no              | This message |
   |        |              |         |                 | has been     |
   |        |              |         |                 | signed by    |
   |        |              |         |                 | the sender   |
   |        |              |         |                 | with Header  |
   |        |              |         |                 | Protection   |
   +--------+--------------+---------+-----------------+--------------+
   |"cipher"| yes          |yes      | yes             | This message |
   |        |              |         |                 | has been     |
   |        |              |         |                 | signed by    |
   |        |              |         |                 | the sender,  |
   |        |              |         |                 | with Header  |
   |        |              |         |                 | Protection,  |
   |        |              |         |                 | and is       |
   |        |              |         |                 | encrypted to |
   |        |              |         |                 | the          |
   |        |              |         |                 | recipients   |
   +--------+--------------+---------+-----------------+--------------+

           Table 1: hp parameter for Content-Type Header Field

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   A sending implementation MUST NOT produce a Cryptographic Payload
   with parameter hp="cipher" for an non-encrypted message (that is,
   where none of the Cryptographic Layers in the Cryptographic Envelope
   of the message provide encryption).  Likewise, if a sending
   implementation is sending an encrypted message with Header
   Protection, it MUST emit an hp="cipher" parameter, regardless of
   which Header Fields were made confidential.

   Note that hp="cipher" indicates that the message itself has been
   encrypted by the sender to the recipients, but makes no assertions
   about which Header Fields have been removed or obscured.  This can be
   derived from the Cryptographic Payload itself (see Section 4.2).

   A receiving implementation MUST NOT mistake the presence of an
   hp="cipher" parameter in the Cryptographic Payload for the actual
   presence of a Cryptographic Layer that provides encryption.

2.1.2.  Content-Type parameter: hp-legacy-display

   This specification also defines an hp-legacy-display parameter for
   the Content-Type Header Field.  The only defined value for this
   parameter is 1.

   This parameter is only relevant on a leaf MIME node of Content-Type
   text/html or text/plain within a well-formed message with end-to-end
   cryptographic protections.  Its presence indicates that the MIME node
   it is attached to contains a decorative "Legacy Display Element".
   The Legacy Display Element itself is used for backward-compatible
   visibility of any removed or obscured User-Facing Header Field in a
   Legacy MUA.

   Such a Legacy Display Element need not be rendered to the user of an
   MUA that implements this specification, because the MUA already knows
   the correct Header Field information, and can render it to the user
   in the appropriate part of the MUA's user interface rather than in
   the body of the message.

   See Section 5.2.2 for how to insert a Legacy Display Element into a
   text/plain Main Body Part.  See Section 5.2.3 for how to insert a
   Legacy Display Element into a text/html Main Body Part.  See
   Section 4.5.3 for how to avoid rendering a Legacy Display Element.

2.2.  The HP-Outer Header Field

   This document also specifies a new Header Field: HP-Outer.

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   This Header Field is used only in the Header Section of the
   Cryptographic Payload of an encrypted message.  It is not relevant
   for signed-only messages.  It documents, with the same cryptographic
   guarantees shared by the rest of the message, the sender's choices
   about Header Field confidentiality.  It does so by embedding a copy
   within the Cryptographic Envelope of every non-structural Header
   Field that the sender put outside the Cryptographic Envelope.  This
   Header Field enables the MUA receiving the encrypted message to
   reliably identify whether the sending MUA intended to make a Header
   Field confidential (see Section 11.3).

   The HP-Outer Header Fields in a message's Cryptographic Payload are
   useful for ensuring that any confidential Header Field will not be
   automatically leaked in the clear if the user replies to or forwards
   the message.  They may also be useful for an MUA that indicates the
   confidentiality status of any given Header Field to the user.

   An implementation that composes encrypted e-mail MUST include a copy
   of all non-structural Header Fields deliberately exposed to the
   outside of the Cryptographic Envelope using a series of HP-Outer
   Header Fields within the Cryptographic Payload.  These HP-Outer MIME
   Header Fields should only ever appear directly within the Header
   Section of the Cryptographic Payload of a Cryptographic Envelope
   offering confidentiality.  They MUST be ignored for the purposes of
   evaluating the message's Header Protection if they appear in other
   places.

   Each instance of HP-Outer contains a non-structural Header Field name
   and the value that this Header Field was set in the outer
   (unprotected) Header Section.  The HP-Outer Header Field can appear
   multiple times in the Header Section of a Cryptographic Payload.

   If a non-structural Header Field name Z is present in Header
   Section of the Cryptographic Payload, but doesn't appear in an HP-
   Outer Header Field value at all, then the sender is effectively
   asserting that every instance of Z was made confidential by removal
   from the Outer Header Section.  Specifically, it means that no Header
   Field Z was included on the outside of the message's Cryptographic
   Envelope by the sender at the time the message was injected into the
   mail system.

   See Section 5.2 for how to insert HP-Outer Header Fields into an
   encrypted message.  See Section 4.3 for how to determine the end-to-
   end confidentiality of a given Header Field from an encrypted message
   with Header Protection using HP-Outer.  See Section 6.1 for how an
   MUA can safely reply to (or forward) an encrypted message without
   leaking confidential Header Fields by default.

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2.2.1.  HP-Outer Header Field Definition

   The syntax of this Header Field is defined using the following ABNF
   [RFC5234], where field-name, WSP, VCHAR, and FWS are defined in
   [RFC5322]:

   hp-outer     =   "HP-Outer:" [FWS] field-name ": "
                       hp-outer-value CRLF

   hp-outer-value  =   (*([FWS] VCHAR) *WSP)

   Note that hp-outer-value is the same as unstructured from
   Section 3.2.5 of [RFC5322], but without the obsolete obs-unstruct
   option.

3.  Header Confidentiality Policy

   An MUA composing an encrypted message according to this specification
   may make any given Header Field confidential by removing it from
   Header Section outside the Cryptographic Envelope, or by obscuring it
   by rewriting it to a different value in that outer Header Section.
   The composing MUA faces a choice for any new message: which Header
   Fields should be made confidential, and how?

   This section defines the "Header Confidentiality Policy" (or HCP) as
   a well-defined abstraction to encourage MUA developers to consider,
   document, and share reasonable policies across the community.  It
   establishes a registry of known HCPs, defines a small number of
   simple HCPs in that registry, and makes a recommendation for a
   reasonable default.

   Note that such a policy is only needed when the end-to-end
   protections include encryption (confidentiality).  No comparable
   policy is needed for other end-to-end cryptographic protections
   (integrity and authenticity), as they are simply uniformly applied so
   that all Header Fields known by the sender have these protections.

   This asymmetry is a consequence of complexities in existing message
   delivery systems, some of which may reject, drop, or delay messages
   where all Header Fields are removed from the top-level MIME object.

   Note that no representation of the HCP itself ever appears "on the
   wire".  However, the consumer of the encrypted message can see the
   decisions that were made by the sender's HCP via the HP-Outer Header
   Fields (see Section 2.2).

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3.1.  HCP Definition

   In this document, we represent that Header Confidentiality Policy as
   a function hcp:

   *  hcp(name, val_in) → val_out: this function takes a non-structural
      Header Field identified by name with initial value val_in as
      arguments, and returns a replacement header value val_out.  If
      val_out is the special value null, it means that the Header Field
      in question should be removed from the set of Header Fields
      visible outside the Cryptographic Envelope.

   In the pseudocode descriptions of various choices of HCP in this
   document, any comparison with the name input is done case-
   insensitively.  This is appropriate for Header Field names, as
   described in [RFC5322].

   Note that hcp is only applied to non-structural Header Fields.  When
   composing a message, Structural Header Fields are dealt with
   separately, as described in Section 5.2.

   As an example, an MUA that obscures the Subject Header Field by
   replacing it with the literal string "[...]", hides all Cc'ed
   recipients, and does not offer confidentiality to any other Header
   Fields would be represented as (in pseudocode):

   hcp_example_hide_cc(name, val_in) → val_out:
       if lower(name) is 'subject':
           return '[...]'
       else if lower(name) is 'cc':
           return null
       else:
           return val_in

   For alignment with common practice as well as the ABNF in
   Section 2.2.1 for HP-Outer, val_out MUST be one of the following:

   *  identical to val_in, or

   *  the special value null (meaning that the Header Field will be
      removed from the outside of the message), or

   *  a sequence of printable and whitespace (that is, space or tab)
      7-bit clean ASCII characters (of course, non-ASCII text can be
      encoded as ASCII using the encoded-word construct from [RFC2047])

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   The HCP can compute val_out using any technique describable in
   pseudocode, such as copying a fixed string or invocations of other
   pseudocode functions.  If it alters the value, it MUST NOT include
   control or NUL characters in val_out. val_out SHOULD match the
   expected ABNF for the Header Field identified by name.

3.1.1.  HCP Avoids Changing From addr-spec

   The From Header Field should also be treated specially by the HCP, to
   enable defense against possible e-mail address spoofing (see
   Section 10.1).  In particular, for hcp("From", val_in), the addr-spec
   of val_in and the addr-spec of val_out SHOULD match according to
   Section 4.4.5, unless the sending MUA has additional knowledge
   coordinated with the receiving MUA about more subtle addr-spec
   equivalence or certificate validity.

3.2.  Initial Registered HCPs

   This document formally defines three Header Confidentiality Policies
   with known and reasonably well-understood characteristics as a way to
   compare and contrast different possible behavioral choices for a
   composing MUA.  These definitions are not meant to preclude the
   creation of other HCPs.

   (The example hypothetical HCP described in Section 3.1 above,
   hcp_example_hide_cc, is deliberately not formally registered, as it
   has not been evaluated in practice.)

3.2.1.  Baseline Header Confidentiality Policy

   The most conservative recommended Header Confidentiality Policy only
   provides confidentiality for Informational Fields, as defined in
   Section 3.6.5 of [RFC5322].  These fields are "only human-readable
   content" and thus their content should not be relevant to transport
   agents.  Since most Internet messages today do have a Subject Header
   Field, and some filtering engines might object to a message without a
   Subject, this policy is conservative and merely obscures that Header
   Field by replacing it with a fixed string [...].  By contrast,
   Comments and Keywords are comparatively rare, so these fields are
   removed entirely from the Outer Header Section.

   hcp_baseline(name, val_in) → val_out:
       if lower(name) is 'subject':
           return '[...]'
       else if lower(name) is in ['comments', 'keywords']:
           return null
       else:
           return val_in

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   hcp_baseline is the recommended default HCP for a new implementation,
   as it provides meaningful confidentiality protections and is unlikely
   to cause deliverability or usability problems.

3.2.2.  Shy Header Confidentiality Policy

   Alternately, a slightly more ambitious (and therefore more privacy-
   preserving) Header Confidentiality Policy might avoid leaking human-
   interpretable data that MTAs generally don't care about.  The
   additional protected data isn't related to message routing or
   transport, but but might reveal sensitive information about the
   sender or their relationship to the recipients.  This "shy" HCP
   builds on hcp_baseline, but also:

   *  avoids revealing the display-name of each identified e-mail
      address, and

   *  avoids leaking the sender's locally-configured time zone in the
      Date Header Field.

   hcp_shy(name, val_in) → val_out:
      if lower(name) is 'from':
         if val_in is an RFC 5322 mailbox:
            return the RFC 5322 addr-spec part of val_in
      if lower(name) in ['to', 'cc']:
         if val_in is an RFC 5322 mailbox-list:
            let val_out be an empty mailbox-list
            for each mailbox in val_in:
               append the RFC 5322 addr-spec part of mailbox to val_out
            return val_out
      if lower(name) is 'date':
         if val_in is an RFC 5322 date-time:
             return the UTC form of val_in
      else if lower(name) is 'subject':
         return '[...]'
      else if lower(name) is in ['comments', 'keywords']:
         return null
      return val_in

   hcp_shy requires more sophisticated parsing and Header Field
   manipulation, and is not recommended as a default HCP for new
   implementations.

3.2.3.  No Header Confidentiality Policy

   Legacy MUAs can be conceptualized as offering a "No Header
   Confidentiality" Policy, which offers no confidentiality protection
   to any Header Field:

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   hcp_no_confidentiality(name, val_in) → val_out:
       return val_in

   A conformant MUA that is not modified by local policy or
   configuration MUST NOT use hcp_no_confidentiality by default.

3.3.  Default Header Confidentiality Policy

   An MUA MUST have a default Header Confidentiality Policy that offers
   confidentiality for the Subject Header Field at least.  Local policy
   and configuration may alter this default, but the MUA SHOULD NOT
   require the user to select an HCP.

   hcp_baseline provides confidentiality for the Subject Header Field by
   replacing it with the literal string "[...]".  It also provides
   confidentiality for the other less common Informational Header Fields
   (Comments and Keywords) by removing them entirely from the outer
   Header Section.  This is a sensible default because most users treat
   the Informational Fields of a message (particularly the Subject) the
   same way that they treat the body, and they are surprised to find
   that the Subject of an encrypted message is visible.

3.4.  HCP Evolution

   This document does not mandate any particular Header Confidentiality
   Policy, though it offers guidance for MUA implementers in selecting
   one in Section 3.3.  Future documents may recommend or mandate such a
   policy for an MUA with specific needs.  Such a recommendation might
   be motivated by descriptions of metadata-derived attacks, or stem
   from research about message deliverability, or describe new
   signalling mechanisms, but these topics are out of scope for this
   document.

3.4.1.  Offering More Ambitious Header Confidentiality

   An MUA MAY offer even more ambitious confidentiality for Header
   Fields of an encrypted message than defined in Section 3.2.2.  For
   example, it might implement an HCP that removes the To and Cc Header
   Fields entirely, relying on the SMTP envelope to ensure proper
   routing.  Or it might remove References and In-Reply-To so that
   message threading is not visible to any MTA.  Any more ambitious
   choice might result in deliverability, rendering, or usability issues
   for the relevant messages, so testing and documentation will be
   valuable to get this right.

   The authors of this document hope that implementers with deployment
   experience will document their chosen Header Confidentiality Policy
   and the rationale behind their choice.

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3.4.2.  Expert Guidance for Registering Header Confidentiality Policies

   There is no formal syntax specified for the Header Confidentiality
   Policy, but any attempt to specify an HCP for inclusion in the
   registry needs to provide:

   *  a stable reference document clearly indicating the distinct name
      for the proposed HCP

   *  pseudocode that other implementers can clearly and unambiguously
      interpret

   *  a clear explanation of why this HCP is different from all other
      registered HCPs

   *  any relevant considerations related to deployment of the HCP (for
      example, known or expected deliverability, rendering, or privacy
      challenges and possible mitigations)

   When the proposed HCP produces any non-null output for a given Header
   Field name, val_out SHOULD match the expected ABNF for that Header
   Field.  If the proposed HCP does not match the expected ABNF for that
   Header Field, the documentation should explicitly identify the
   relevant circumstances and provide a justification for the deviation.

   An entry should not be marked as "Recommended" unless it has been
   shown to offer confidentiality or privacy improvements over the
   status quo and have minimal or mitigatable negative impact on
   messages to which it is applied, considering factors such as message
   deliverability and security.  Only one entry in the table
   (hcp_baseline) is initially marked as "Recommended".  In the future,
   more than one entry may be marked as "Recommended".

4.  Receiving Guidance

   An MUA that receives a cryptographically protected e-mail will render
   it for the user.

   The receiving MUA will render the message body, a selected subset of
   Header Fields, and (as described in Section 3 of
   [I-D.ietf-lamps-e2e-mail-guidance]) provide a summary of the
   cryptographic properties of the message.

   Most MUAs only render a subset of Header Fields by default.  For
   example, most MUAs render From, To, Cc, Date, and Subject Header
   Fields to the user, but few render Message-Id or Received.

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   An MUA that knows how to handle a message with Header Protection
   makes the following four changes to its behavior when rendering a
   message:

   *  If the MUA detects that an incoming message has protected Header
      Fields:

      -  For a Header Field that is present in the protected Header
         Section, the MUA SHOULD render the protected value, and ignore
         any unprotected counterparts that may be present (with a
         special exception for the From Header Field (see Section 4.4).

      -  For a Header Field that is present only in the unprotected
         Header Section, the MUA SHOULD NOT render that value.  If it
         does render the value, the MUA SHOULD indicate that the
         rendered value is unprotected.  For an exception to this, see
         Section 7 for a discussion of some specific Header Fields that
         are known to be added in transit, and therefore are not
         expected to have end-to-end cryptographic protections.

   *  The MUA SHOULD include information in the message's Cryptographic
      Summary to indicate the types of protection that applied to each
      rendered Header Field (if any).

   *  If any Legacy Display Elements are present in the body of the
      message, it does not render them.

   *  When replying to a message with confidential Header Fields, the
      replying MUA avoids leaking into the cleartext of the reply any
      Header Fields which were confidential in the original.  It does
      this even if its own Header Confidentiality Policy would not have
      treated those Header Fields as confidential.  See Section 6 for
      more details.

   Note that an MUA that handles a message with Header Protection does
   _not_ need to render any new Header Fields that it did not render
   before.

4.1.  Identifying that a Message has Header Protection

   An incoming message can be identified as having Header Protection
   using the following test:

   *  The Cryptographic Payload has parameter hp set to "clear" or
      "cipher".  See Section 4.5 for rendering guidance.

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   When consuming a message, an MUA MUST ignore the hp parameter to
   Content-Type when it encounters it anywhere other than the root of
   the message's Cryptographic Payload.

4.2.  Extracting Protected and Unprotected ("Outer") Header Fields

   When a message is encrypted and it uses Header Protection, an MUA
   extracts a list of protected Header Fields (names and values), as
   well as a list of Header Fields that were added by the original
   message sender in unprotected form to the outside of the message's
   Cryptographic Envelope.

   The following algorithm takes a reference message refmsg as input,
   which is encrypted with Header Protection as described in this
   document (that is, the Cryptographic Envelope includes a
   Cryptographic Layer that provides encryption, and the hp parameter
   for the Content-Type Header Field of the Cryptographic Payload is
   cipher).  It produces as output a pair of lists of (h,v) Header
   Fields.

4.2.1.  HeaderSetsFromMessage

   Method Signature:

   HeaderSetsFromMessage(refmsg) → (refouter, refprotected)

   Procedure:

   1.  Let refheaders be the list of (h,v) protected Header Fields found
       in the root of the Cryptographic Payload

   2.  Let refouter be an empty list of Header Field names and values

   3.  Let refprotected be an empty list of Header Field names and
       values

   4.  For each (h,v) in refheaders:

       i.   If h is HP-Outer:

            a.  Split v into (h1,v1) on the first colon (:) followed by
                any amount of whitespace.

            b.  Append (h1,v1) to refouter

       ii.  Else:

            a.  Append (h,v) to refprotected

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   5.  Return refouter, refprotected

   Note that this algorithm is independent of the unprotected Header
   Fields.  It derives its output only from the normal Header Fields and
   the HP-Outer Header Fields, both contained inside the Cryptographic
   Payload.

4.3.  Updating the Cryptographic Summary

   Regardless of whether a cryptographically protected message has
   protected Header Fields, the Cryptographic Summary of the message
   should be modified to indicate what protections the Header Fields
   have.  This field-by-field status is complex and isn't necessarily
   intended to be presented in full to the user.  Rather, it represents
   the state of the message internally within the MUA, and may be used
   to influence behavior like replying to the message (see Section 6.1).

   Each Header Field individually has exactly one of the following
   protection states:

   *  unprotected (has no Header Protection)

   *  signed-only (bound into the same validated signature as the
      enclosing message, but also visible in transit)

   *  encrypted-only (only appears within the Cryptographic Payload; the
      corresponding external Header Field was either removed or
      obscured)

   *  signed-and-encrypted (same as encrypted-only, but additionally is
      under a validated signature)

   If the message does not have Header Protection (as determined by
   Section 4.1), then all of the Header Fields are by definition
   unprotected.

   If the message has Header Protection, an MUA SHOULD use the following
   algorithm to compute the protection state of a protected Header Field
   (h,v) (that is, an element of refprotected from Section 4.2):

4.3.1.  HeaderFieldProtection

   Method signature:

   HeaderFieldProtection(msg, h, v) → protection_state

   Procedure:

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   1.  Let ct be the Content-Type of the root of the Cryptographic
       Payload of msg.

   2.  Compute (refouter, refprotected) from HeaderSetsFromMessage(msg).

   3.  If (h, v) is not in refprotected):

       i.  Abort, v is not a valid value for header h

   4.  Let is_sig_valid be false

   5.  If the message is signed:

       i.  Let is_sig_valid be the result of validating the signature

   6.  If the message is encrypted, and if ct has a parameter
       hp="cipher", and if (h,v) is not in refouter:

       i.  Return signed-and-encrypted if is_sig_valid otherwise
           encrypted-only

   7.  Return signed-only if is_sig_valid otherwise unprotected

   Note that:

   *  This algorithm is independent of the unprotected Header Fields.
      It derives the protection state only from (h,v) and the set of HP-
      Outer Header Fields, both of which are inside the Cryptographic
      Envelope.

   *  If the signature fails validation, the MUA lowers the affected
      state to unprotected or encrypted-only without warning the user,
      as specified by Section 3.1 of [I-D.ietf-lamps-e2e-mail-guidance].

   *  Data from signed-and-encrypted and encrypted-only Header Fields
      may still not be fully private (see Section 11.2).

   *  Encryption may have been added in transit to an originally signed-
      only message.  Thus only consider Header Fields to be confidential
      if the sender indicates it with the hp="cipher" parameter.

   *  The protection state of a Header Field may be weaker than that of
      the message body.  For example, a message body can be signed-and-
      encrypted, but a Header Field that is copied unmodified to the
      unprotected Header Section is signed-only.

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   If the message has Header Protection, Header Fields that are not in
   refprotected (e.g., because they were added in transit), are
   unprotected.

   Rendering the cryptographic status of each Header Field is likely to
   be complex and messy --- users may not understand it.  It is beyond
   the scope of this document to suggest any specific graphical
   affordances or user experience.  Future work should include examples
   of successful rendering of this information.

4.4.  Handling Mismatch of From Header Fields

   End-to-end (MUA-to-MUA) Header Protection is good for authenticity,
   integrity, and confidentiality, but it potentially introduces new
   issues when an MUA depends on its MTA to authenticate parts of the
   Header Section.  The latter is typically the case in modern e-mail
   systems.

   In particular, when an MUA depends on its MTA to ensure that the
   e-mail address in the (unprotected) From Header Field is authentic,
   but the MUA renders the e-mail address of the protected From Header
   Field that differs from the address visible to the MTA, this could
   create a risk of sender address spoofing (see Section 10.1).  This
   potential risk applies to signed-only messages as well as signed-and-
   encrypted messages.

4.4.1.  Definitions

4.4.1.1.  From Header Field Mismatch

   "From Header Field Mismatch" is defined as follows:

   The addr-spec of the inner From Header Field doesn't match the addr-
   spec of the outer From Header Field (see Section 4.4.5).

   Note: The unprotected From Header Field used in this comparison is
   the actual outer Header Field (as seen by the MTA), not the value
   indicated by any potential inner HP-Outer.

4.4.1.2.  No Valid and Correctly Bound Signature

   "No Valid and Correctly Bound Signature" is defined as follows:

   There is no valid signature made by a certificate for which the MUA
   has a valid binding to the protected From address.  This includes:

   *  the message has no signature, or

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   *  the message has a broken signature, or

   *  the message has a valid signature, but the receiving MUA does not
      see any valid binding between the signing certificate and the
      addr-spec of the inner From Header Field.

   Note: There are many possible ways that an MUA could choose to
   validate a certificate-to-address binding.  For example, the MUA
   could ensure the certificate is issued by one of a set of trusted
   certification authorities, it could rely on the user to do a manual
   out-of-band comparison, it could rely on a DNSSEC signal ([RFC7929]
   or [RFC8162]), and so on.  It is beyond the scope of this document to
   describe all possible ways an MUA might validate the certificate-to-
   address binding, or to choose among them.

4.4.2.  Warning for From Header Field Mismatch

   To mitigate the above described risk of sender address spoofing, an
   MUA SHOULD warn the user whenever both of the following conditions
   are met:

   *  From Header Field Mismatch (as defined in Section 4.4.1.1), and

   *  No Valid and Correctly Bound Signature (as defined in
      Section 4.4.1.2)

   This warning should be comparable to the MUA's warning about messages
   that are likely spam or phishing, and it SHOULD show both of the non-
   matching From Header Fields.

4.4.3.  From Header Field Rendering

   Furthermore, a receiving MUA that depends on its MTA to authenticate
   the unprotected (outer) From Header Field SHOULD render the outer
   From Header Field (as an exception to the guidance in the beginning
   of Section 4), if both of the following conditions are met:

   *  From Header Field Mismatch (as defined in Section 4.4.1.1), and

   *  No Valid and Correctly Bound Signature (as defined in
      Section 4.4.1.2)

   An MUA MAY apply a local preference to render a different display
   name (e.g., from an address book).

   See Section 10.1.1 for an detailed explanation of this rendering
   guidance.

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4.4.4.  Handling Protected From Header Field when Responding

   When responding to a message, an MUA has different ways to populate
   the recipients of the new message.  Depending on whether it is a
   Reply, a Reply-All, or a Forward, an MUA may populate the composer
   view using a combination of the referenced message's From, To, Cc,
   Reply-To, Mail-Followup-To Header Fields, or any other signals.

   When responding to a message with Header Protection, an MUA MUST only
   use the protected Header Fields when populating the recipients of the
   new message.

   This avoids compromise of message confidentiality when a MITM
   attacker modifies the unprotected From address of an encrypted
   message, attempting to learn the contents through a misdirected
   reply.  Note that with the rendering guidance above, a MITM attacker
   can cause the unprotected From Header Field to be displayed.  Thus
   when responding, the populated To address may differ from the
   rendered From address.  However, this change in addresses should not
   cause more user confusion than the address change caused by a Reply-
   To in a Legacy Message does.

4.4.5.  Matching addr-specs

   When generating (Section 3.1.1) or consuming (Section 4.4) a
   protected From Header Field, the MUA considers the equivalence of two
   different addr-spec values.

   First, the MUA MUST check whether the domain part of an addr-spec
   being compared contains any U-label [RFC5890].  If it does, it MUST
   be converted to the A-label form is described in [RFC5891].  We call
   such converted version (or the original domain, if it didn't contain
   any U-label) "the ASCII version of the domain part".  Second, the MUA
   MUST compare the ASCII version of the domain part of the two addr-
   specs by standard DNS comparison: assume ASCII text, and compare
   alphabetic characters case-insensitively, as described in Section 3.1
   of [RFC1035].  If the domain parts match, then the two local-parts
   are matched against each other.  The simplest and most common
   comparison for the local-part is also an ASCII-based, case-
   insensitive match.  If the MUA has special knowledge about the domain
   and, when composing, it can reasonably expect the receiving MUAs to
   have the same information, it MAY match the local-part using a more
   sophisticated and inclusive matching algorithm.

   It is beyond the scope of this document to recommend a more
   sophisticated and inclusive matching algorithm.

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4.5.  Rendering a Message with Header Protection

   When the Cryptographic Payload's Content-Type has the parameter hp
   set to "clear" or "cipher", the values of the protected Header Fields
   are drawn from the Header Fields of the Cryptographic Payload, and
   the body that is rendered is the Cryptographic Payload itself.

4.5.1.  Example Signed-only Message

   Consider a message with this structure, where the MUA is able to
   validate the cryptographic signature:

   A └─╴application/pkcs7-mime; smime-type="signed-data"
      ⇩ (unwraps to)
   B  └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
   C   ├─╴text/plain
   D   └─╴text/html

   The message body should be rendered the same way as this message:

   B └┬╴multipart/alternative
   C  ├─╴text/plain
   D  └─╴text/html

   The MUA should render Header Fields taken from part B.

   Its Cryptographic Summary should indicate that the message was signed
   and all rendered Header Fields were included in the signature.

   Because this message is signed-only, none of its parts will have a
   Legacy Display Element.

   The MUA should ignore Header Fields from part A for the purposes of
   rendering.

4.5.2.  Example Signed-and-Encrypted Message

   Consider a message with this structure, where the MUA is able to
   validate the cryptographic signature:

   E └─╴application/pkcs7-mime; smime-type="enveloped-data"
      ↧ (decrypts to)
   F  └─╴application/pkcs7-mime; smime-type="signed-data"
       ⇩ (unwraps to)
   G   └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
   H    ├─╴text/plain
   I    └─╴text/html

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   The message body should be rendered the same way as this message:

   G └┬╴multipart/alternative
   H  ├─╴text/plain
   I  └─╴text/html

   It should render Header Fields taken from part G.

   Its Cryptographic Summary should indicate that the message is signed-
   and-encrypted.

   When rendering the Cryptographic Status of a Header Field and when
   composing a reply, each Header Field found in G should be considered
   against all HP-Outer Header Fields found in G.  If an HP-Outer Header
   Field is found that matches both the name and value, the Header
   Field's Cryptographic Status is just signed-only, even though the
   message itself is signed-and-encrypted.  If no matching HP-Outer
   Header Field is found, the Header Field's Cryptographic Status is
   signed-and-encrypted, like the rest of the message.

   If any of the User-Facing Header Fields are removed or obscured, the
   composer of this message may have placed Legacy Display Elements in
   parts H and I.

   The MUA should ignore Header Fields from part E for the purposes of
   rendering.

4.5.3.  Do Not Render Legacy Display Elements

   As described in Section 2.1.2, a message with cryptographic
   confidentiality protection MAY include Legacy Display Elements for
   backward-compatibility with Legacy MUAs.  These Legacy Display
   Elements are strictly decorative, unambiguously identifiable, and
   will be discarded by compliant implementations.

   The receiving MUA MUST avoid rendering the identified Legacy Display
   Elements to the user at all, since it is aware of Header Protection
   and can render the actual protected Header Fields.

   If a text/html or text/plain part within the Cryptographic Envelope
   is identified as containing Legacy Display Elements, those elements
   MUST be hidden when rendering and MUST be dropped when generating a
   draft reply or inline forwarded message.  Whenever a Message or MIME
   subtree is exported, downloaded, or otherwise further processed, if
   there is no need to retain a valid cryptographic signature, the
   implementer MAY drop the Legacy Display Elements.

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4.5.3.1.  Identifying a Part with Legacy Display Elements

   A receiving MUA acting on a message that contains an encrypting
   Cryptographic Layer identifies a MIME subpart within the
   Cryptographic Payload as containing Legacy Display Elements based on
   the Content-Type of the subpart.  The subpart's Content-Type:

   *  contains a parameter hp-legacy-display with value set to 1, and

   *  is either text/html (see Section 4.5.3.3) or text/plain (see
      Section 4.5.3.2).

   Note that the term "subpart" above is used in the general sense: if
   the Cryptographic Payload is a single part, that part itself may
   contain a Legacy Display Element if it is marked with the hp-legacy-
   display=1 parameter.

4.5.3.2.  Omitting Legacy Display Elements from text/plain

   If a text/plain part within the Cryptographic Payload has the
   Content-Type parameter hp-legacy-display="1", it should be processed
   before rendering in the following fashion:

   *  Discard the leading lines of the body of the part up to and
      including the first entirely blank line.

   Note that implementing this strategy is dependent on the charset used
   by the MIME part.

   See Appendix E.1 for an example.

4.5.3.3.  Omitting Legacy Display Elements from text/html

   If a text/html part within the Cryptographic Payload has the Content-
   Type parameter hp-legacy-display="1", it should be processed before
   rendering in the following fashion:

   *  If any element of the HTML <body> is a <div> with class attribute
      header-protection-legacy-display, that entire element should be
      omitted.

   This cleanup could be done, for example, as a custom rule in the
   MUA's HTML sanitizer, if one exists.  Another implementation strategy
   for an HTML-capable MUA would be to add an entry to the [CSS]
   stylesheet for such a part:

   body div.header-protection-legacy-display { display: none; }

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4.6.  Implicitly rendered Header Fields

   While From, To, Cc, Subject, and Date Header Fields are often
   explicitly rendered to the user, some Header Fields do affect message
   display, without being explicitly rendered.

   For example, Message-Id, References, and In-Reply-To Header Fields
   may collectively be used to place a message in a "thread" or series
   of messages.

   In another example, Section 6.2 observes that the value of the Reply-
   To field can influence the draft reply message.  So while the user
   may never see the Reply-To Header Field directly, it is implicitly
   "rendered" when the user interacts with the message by replying to
   it.

   An MUA that depends on any implicitly rendered Header Field in a
   message with Header Protection MUST use the value from the protected
   Header Field, and SHOULD NOT use any value found outside the
   cryptographic protection unless it is known to be a Header Field
   added in transit, as specified in Section 7.

4.7.  Handling Undecryptable Messages

   An MUA might receive an apparently encrypted message that it cannot
   currently decrypt.  For example, when an MUA does not have regular
   access to the secret key material needed for decryption, it cannot
   know the cryptographically protected Header Fields or even whether
   the message has any cryptographically protected Header Fields.

   Such an undecrypted message will be rendered by the MUA as a message
   without any Header Protection.  This means that the message summary
   may well change how it is rendered when the user is finally able to
   supply the secret key.

   For example, the rendering of the Subject Header Field in a mailbox
   summary might change from [...] to the real message subject when the
   message is decrypted.  Or the message's placement in a message thread
   might change if, say, References or In-Reply-To have been removed or
   obscured (see Section 4.6).

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   Additionally, if the MUA does not retain access to the decrypting
   secret key, and it drops the decrypted form of a message, the
   message's rendering may revert to the encrypted form.  For example,
   if an MUA follows this behavior, the Subject Header Field in a
   mailbox summary might change from the real message subject back to
   [...].  Or the message might be displayed outside of its current
   thread if the MUA loses access to a removed References or In-Reply-To
   header.

   These behaviors are likely to surprise the user.  However, an MUA has
   several possible ways of reducing or avoiding all of these surprises,
   including:

   *  Ensuring that the MUA always has access to decryption-capable
      secret key material.

   *  Rendering undecrypted messages in a special quarantine view until
      the decryption-capable secret key material is available.

   To reduce or avoid the surprises associated with a decrypted message
   with removed or obscured Header Fields becoming undecryptable, the
   MUA could also:

   *  Securely cache metadata from a decrypted message's protected
      Header Fields so that its rendering doesn't change after the first
      decryption.

   *  Securely store the session key associated with a decrypted
      message, so that attempts to read the message when the long-term
      secret key are unavailable can proceed using only the session key
      itself.  See, for example, the discussion about stashing session
      keys in Section 9.1 of [I-D.ietf-lamps-e2e-mail-guidance].

4.8.  Guidance for Automated Message Handling

   Some automated systems have a control channel that is operated by
   e-mail.  For example, an incoming e-mail message could subscribe
   someone to a mailing list, initiate the purchase of a specific
   product, approve another message for redistribution, or adjust the
   state of some shared object.

   To the extent that such a system depends on end-to-end cryptographic
   guarantees about the e-mail control message, Header Protection as
   defined in this document should improve the system's security.  This
   section provides some specific guidance for systems that use e-mail
   messages as a control channel that want to benefit from these
   security improvements.

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4.8.1.  Interpret Only Protected Header Fields

   Consider the situation where an e-mail-based control channel depends
   on the message's cryptographic signature and the action taken depends
   on some Header Field of the message.

   In this case, the automated system MUST rely on information from the
   Header Field that is protected by the mechanism defined in this
   document.  It MUST NOT rely on any Header Field found outside the
   Cryptographic Payload.

   For example, consider an administrative interface for a mailing list
   manager that only accepts control messages that are signed by one of
   its administrators.  When an inbound message for the list arrives, it
   is queued (waiting for administrative approval) and the system
   generates and listens for two distinct e-mail addresses related to
   the queued message -- one that approves the message, and one that
   rejects it.  If an administrator sends a signed control message to
   the approval address, the mailing list verifies that the protected To
   Header Field of the signed control message contains the approval
   address before approving the queued message for redistribution.  If
   the protected To Header Field does not contain that address, or there
   is no protected To Header Field, then the mailing list logs or
   reports the error and does not act on that control message.

4.8.2.  Ignore Legacy Display Elements

   Consider the situation where an e-mail-based control channel expects
   to receive an end-to-end encrypted message -- for example, where the
   control messages need confidentiality guarantees -- and where the
   action taken depends on the contents of some MIME part within the
   message body.

   In this case, the automated system that decrypts the incoming
   messages and scans the relevant MIME part MUST identify when the MIME
   part contains a Legacy Display Element (see Section 4.5.3.1), and it
   MUST parse the relevant MIME part with the Legacy Display Element
   removed.

   For example, consider an administrative interface of a confidential
   issue tracking software.  An authorized user can confidentially
   adjust the status of a tracked issue by a specially formatted first
   line of the message body (for example, severity #183 serious).  When
   the user's MUA encrypts a plain text control message to this issue
   tracker, depending on the MUA's HCP and its choice of legacy value,
   it may add a Legacy Display Element.  If it does so, then the first
   line of the message body will contain a decorative copy of the
   confidential Subject Header Field.  The issue tracking software

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   decrypts the incoming control message, identifies that there is a
   Legacy Display Element in the part (see Section 4.5.3.1), strips the
   lines comprising the Legacy Display Element (including the first
   blank line), and only then parses the remaining top line to look for
   the expected special formatting.

4.9.  Affordances for Debugging and Troubleshooting

   Note that advanced users of an MUA may need access to the original
   message, for example to troubleshoot problems with the rendering MUA
   itself, or problems with the SMTP transport path taken by the
   message.

   An MUA that applies these rendering guidelines SHOULD ensure that the
   full original source of the message as it was received remains
   available to such a user for debugging and troubleshooting.

   If a troubleshooting scenario demands information about the
   cryptographically protected values of Header Fields, and the message
   is encrypted, the debugging interface SHOULD also provide a "source"
   view of the Cryptographic Payload itself, alongside the full original
   source of the message as received.

4.10.  Handling RFC8551HP Messages (Backward Compatibility)

   Section 1.1.1 describes some drawbacks to the Header Protection
   scheme defined in [RFC8551], referred to here as RFC8551HP.  An MUA
   MUST NOT generate an RFC8551HP message.  However, for backward
   compatibility an MUA MAY try to render or respond to such a message
   as though the message has standard Header Protection.

   The following two sections contain guidance for identifying,
   rendering and replying to RFC8551HP messages.  Corresponding test
   vectors are provided in Appendix C.2.5, Appendix C.2.6, and
   Appendix C.3.17.

4.10.1.  Identifying an RFC8551HP Message

   An RFC8551HP Message can be identified by its MIME structure, given
   that all of the following conditions are met:

   *  It has a well-formed Cryptographic Envelope consisting of at least
      one Cryptographic Layer as the outermost MIME object.

   *  The Cryptographic Payload is a single message/rfc822 object

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   *  The message that constitutes the Cryptographic Payload does not
      itself have a well-formed Cryptographic Envelope; that is, its
      outermost MIME object is not a Cryptographic Layer.

   *  No Content-Type parameter of hp= is set on either the
      Cryptographic Payload, or its immediate MIME child.

   Here is the MIME structure of an example signed-and-encrypted
   RFC8551HP message:

   A └─╴application/pkcs7-mime; smime-type="enveloped-data"
      ↧ (decrypts to)
   B  └─╴application/pkcs7-mime; smime-type="signed-data"
       ⇩ (unwraps to)
   C   └┬╴message/rfc822 [Cryptographic Payload]
   D    └┬╴multipart/alternative [Rendered Body]
   E     ├─╴text/plain
   F     └─╴text/html

   This meets the definition of an RFC8551HP message because:

   *  Cryptographic Layers A and B form the Cryptographic Envelope.

   *  The Cryptographic Payload, rooted in part C has Content-Type:
      message/rfc822.

   *  Part D (the MIME root of the message at C) is itself not a
      Cryptographic Layer.

   *  Neither part C nor part D have any hp parameter set on their
      Content-Type.

4.10.2.  Rendering or Responding to an RFC8551HP message

   When it has precisely identified a message as an RFC8551HP message,
   an MUA MAY render or respond to that message as though it were a
   message with Header Protection as defined in this document by making
   the following adjustments:

   *  Rather than rendering the message body as the Cryptographic
      Payload itself (part C in the example above), render the RFC8551HP
      message's body as the MIME subtree that is the Cryptographic
      Payload's immediate child (part D).

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   *  Make a comparable modification to HeaderSetsFromMessage
      (Section 4.2.1) and HeaderFieldProtection (Section 4.3.1): both
      algorithms currently look for the protected Header Fields on the
      Cryptographic Payload (part C), but they should instead look at
      the Cryptographic Payload's immediate child (part D).

   *  If the Cryptographic Envelope is signed-only, behave as though
      there is an hp="clear" parameter for the Cryptographic Payload; if
      the Envelope contains encryption, behave as though there is an
      hp="cipher" parameter.  That is, infer the sender's cryptographic
      intent from the structure of the message.

   *  If the Cryptographic Envelope contains encryption, further modify
      HeaderSetsFromMessage to derive refouter from the actual outer
      message Header Fields (those found in part A in the example
      above), rather than looking for HP-Outer Header Fields with the
      other protected Header Fields.  That is, infer Header Field
      confidentiality based on the unprotected headers.

   The inferences in the above modifications are not based on any strong
   end-to-end guarantees.  An intervening MTA may tamper with the
   message's outer Header Section or wrap the message in an encryption
   layer to undetectably change the recipient's understanding of the
   confidentiality of the message's Header Fields or the message body
   itself.

4.11.  Rendering Other Schemes

   Other MUAs may have generated different structures of messages that
   aim to offer end-to-end cryptographic protections that include Header
   Protection.  This document is not normative for those schemes, and it
   is NOT RECOMMENDED to generate these other schemes, as they can
   either have structural flaws or simply render poorly on Legacy MUAs.
   A conformant MUA MAY attempt to infer Header Protection when
   rendering an existing message that appears to use some other scheme
   not documented here.  Pointers to some known other schemes can be
   found in Appendix F.

5.  Sending Guidance

   This section describes the process an MUA should use to apply
   cryptographic protection to an e-mail message with Header Protection.

   When composing a message with end-to-end cryptographic protections,
   an MUA SHOULD apply Header Protection.

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   When generating such a message, an MUA MUST add the hp parameter (see
   Section 2.1.1) only to the Content-Type Header Field at the root of
   the message's Cryptographic Payload.  The value of the parameter MUST
   indicate whether the Cryptographic Envelope contains a layer that
   provides encryption.

5.1.  Composing a Cryptographically Protected Message Without Header
      Protection

   For contrast, we first consider the typical message composition
   process of a Legacy Crypto MUA which does not provide any Header
   Protection.

   This process is described in Section 5.1 of
   [I-D.ietf-lamps-e2e-mail-guidance].  We replicate it here for
   reference.  The inputs to the algorithm are:

   *  origbody: the traditional unprotected message body as a well-
      formed MIME tree (possibly just a single MIME leaf part).  As a
      well-formed MIME tree, origbody already has structural Header
      Fields (Content-*) present.

   *  origheaders: the intended non-structural Header Fields for the
      message, represented here as a list of (h,v) pairs, where h is a
      Header Field name and v is the associated value.  Note that these
      are Header Fields that the MUA intends to be visible to the
      recipient of the message.  In particular, if the MUA uses the Bcc
      Header Field during composition, but plans to omit it from the
      message (see Section 3.6.3 of [RFC5322]), it will not be in
      origheaders.

   *  crypto: The series of cryptographic protections to apply (for
      example, "sign with the secret key corresponding to X.509
      certificate X, then encrypt to X.509 certificates X and Y").  This
      is a routine that accepts a MIME tree as input (the Cryptographic
      Payload), wraps the input in the appropriate Cryptographic
      Envelope, and returns the resultant MIME tree as output.

   The algorithm returns a MIME object that is ready to be injected into
   the mail system.

5.1.1.  ComposeNoHeaderProtection

   Method Signature:

   ComposeNoHeaderProtection(origbody, origheaders, crypto) →
   mime_message

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   Procedure:

   1.  Apply crypto to MIME part origbody, producing MIME tree output

   2.  For each Header Field name and value (h,v) in origheaders:

       i.  Add Header Field h to output with value v

   3.  Return output

5.2.  Composing a Message with Header Protection

   To compose a message using Header Protection, the composing MUA uses
   the following inputs:

   *  All the inputs described in Section 5.1

   *  hcp: a Header Confidentiality Policy, as defined in Section 3

   *  respond: if the new message is a response to another message
      (e.g., "Reply", "Reply All", "Forward", etc), the MUA function
      corresponding to the user's action (see Section 6.1), otherwise
      null

   *  refmsg: if the new message is a response to another message, the
      message being responded to, otherwise null

   *  legacy: a boolean value, indicating whether any recipient of the
      message is believed to have a Legacy MUA.  If all recipients are
      known to implement this document, legacy should be set to false.
      (How an MUA determines the value of legacy is out of scope for
      this document; an initial implementation can simply set it to
      true)

   To enable visibility of User-Facing but now removed/obscured Header
   Fields for decryption-capable Legacy MUAs, the Header Fields are
   included as a decorative Legacy Display Element in specially marked
   parts of the message (see Section 2.1.2).  This document recommends
   two mechanisms for such a decorative adjustment: one for a text/html
   Main Body Part of the e-mail message, and one for a text/plain Main
   Body Part.  This document does not recommend adding a Legacy Display
   Element to any other part.

   Please see Section 7.1 of [I-D.ietf-lamps-e2e-mail-guidance] for
   guidance on identifying the parts of a message that are a Main Body
   Part.

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5.2.1.  Compose

   Method Signature:

   Compose(origbody, origheaders, crypto, hcp, respond, refmsg, legacy)
   → mime_message

   Procedure:

   1.  Let newbody be a copy of origbody

   2.  If crypto contains encryption, and legacy is true:

       i.    Create ldlist, an empty list of (header, value) pairs

       ii.   For each Header Field name and value (h,v) in origheaders:

             a.  If h is User-Facing (see Section 1.1.2 of
                 [I-D.ietf-lamps-e2e-mail-guidance]):

                 I.  If hcp(h,v) is not v:

                     A.  Add (h,v) to ldlist

       iii.  If ldlist is not empty:

             a.  Identify each leaf MIME part of newbody that represents
                 the "main body" of the message.

             b.  For each "Main Body Part" bodypart of type text/plain
                 or text/html:

                 I.  Adjust bodypart by inserting a Legacy Display
                     Element header list ldlist into its content, and
                     adding a Content-Type parameter hp-legacy-display
                     with value 1 (see Section 5.2.2 for text/plain and
                     Section 5.2.3 for text/html)

   3.  For each Header Field name and value (h,v) in origheaders:

       i.  Add Header Field h to MIME part newbody with value v

   4.  If crypto does not contain encryption:

       i.   Set the hp parameter on the Content-Type of MIME part
            newbody to clear

       ii.  Let newheaders be a copy of origheaders

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   5.  Else (if crypto contains encryption):

       i.    Set the hp parameter on the Content-Type of MIME part
             newbody to cipher

       ii.   If refmsg is not null, respond is not null, and refmsg
             itself is encrypted with header protection:

             a.  Let response_hcp be a single-use HCP derived from
                 respond and refmsg (see Section 6.1)

       iii.  Else (if this is not a response to an encrypted, header-
             protected message):

             a.  Set response_hcp to hcp_no_confidentiality

       iv.   Create new empty list of Header Field names and values
             newheaders

       v.    For each Header Field name and value (h,v) in origheaders:

             a.  Let newval be hcp(h,v)

             b.  If newval is v:

                 I.  Let newval be response_hcp(h,v)

             c.  If newval is not null):

                 I.  Add (h,newval) to newheaders

       vi.   For each Header Field name and value (h,v) in newheaders:

             a.  Let string record be the concatenation of h, a literal
                 ": " (ASCII colon (0x3A) followed by ASCII space
                 (0x20)), and v

             b.  Add Header Field "HP-Outer" to MIME part newbody with
                 value record

   6.  Apply crypto to MIME part newbody, producing MIME tree output

   7.  For each Header Field name and value (h,v) in newheaders:

       i.  Add Header Field h to output with value v

   8.  Return output

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   Note that both new parameters (hcp and legacy) are effectively
   ignored if crypto does not contain encryption.  This is by design,
   because they are irrelevant for signed-only cryptographic
   protections.

5.2.2.  Adding a Legacy Display Element to a text/plain Part

   For a list of obscured and removed User-Facing Header Fields
   represented as (header, value) pairs, concatenate them as a set of
   lines, with one newline at the end of each pair.  Add an additional
   trailing newline after the resultant text, and prepend the entire
   list to the body of the text/plain part.

   The MUA MUST also add a Content-Type parameter of hp-legacy-display
   with value 1 to the MIME part to indicate that a Legacy Display
   Element was added.

   For example, if the list of obscured Header Fields was [("Cc",
   "alice@example.net"), ("Subject", "Thursday's meeting")], then a
   text/plain Main Body Part that originally looked like this:

   Content-Type: text/plain; charset=UTF-8

   I think we should skip the meeting.

   Would become:

   Content-Type: text/plain; charset=UTF-8; hp-legacy-display=1

   Subject: Thursday's meeting
   Cc: alice@example.net

   I think we should skip the meeting.

   Note that the Legacy Display Element (the lines beginning with
   Subject: and Cc:) are part of the body of the MIME part in question.

   This example assumes that the Main Body Part in question is not the
   root of the Cryptographic Payload.  For instance, it could be a leaf
   of a multipart/alternative Cryptographic Payload.  This is why no
   additional Header Fields have been injected into the MIME part in
   this example.

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5.2.3.  Adding a Legacy Display Element to a text/html Part

   Adding a Legacy Display Element to a text/html part is similar to how
   it is added to a text/plain part (see Section 5.2.2).  Instead of
   adding the obscured or removed User-Facing Header Fields to a block
   of text delimited by a blank line, the composing MUA injects them in
   an HTML <div> element annotated with a class attribute of header-
   protection-legacy-display.

   The content and formatting of this decorative <div> have no strict
   requirements, but they MUST represent all the obscured and removed
   User-Facing Header Fields in a readable fashion.  A simple approach
   is to assemble the text in the same way as Section 5.2.2, wrap it in
   a verbatim <pre> element, and put that element in the annotated
   <div>.

   The annotated <div> should be placed as close to the start of the
   <body> as possible, where it will be visible when viewed with a
   standard HTML renderer.

   The MUA MUST also add a Content-Type parameter of hp-legacy-display
   with value 1 to the MIME part to indicate that a Legacy Display
   Element was added.

   For example, if the list of obscured Header Fields was [("Cc",
   "alice@example.net"), ("Subject", "Thursday's meeting")], then a
   text/html Main Body Part that originally looked like this:

   Content-Type: text/html; charset=UTF-8

   <html><head><title></title></head><body>
   <p>I think we should skip the meeting.</p>
   </body></html>

   Would become:

   Content-Type: text/html; charset=UTF-8; hp-legacy-display=1

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>Subject: Thursday's meeting
   Cc: alice@example.net</pre></div>
   <p>I think we should skip the meeting.</p>
   </body></html>

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   This example assumes that the Main Body Part in question is not the
   root of the Cryptographic Payload.  For instance, it could be a leaf
   of a multipart/alternative Cryptographic Payload.  This is why no
   additional Header Fields have been injected into the MIME part in
   this example.

5.2.3.1.  Step-by-step Example for Inserting Legacy Display Element to
          text/html

   A composing MUA MAY insert the Legacy Display Element anywhere
   reasonable within the message as long as it prioritizes visibility
   for the reader using a Legacy decryption-capable MUA.  This decision
   may take into account special message-specific HTML formatting
   expectations if the MUA is aware of them.  However, some MUAs may not
   have any special insight into the user's preferred HTML formatting,
   and still want to insert a Legacy Display Element.  This section
   offers a non-normative, simple, and minimal step-by-step approach for
   a composing MUA that has no other information or preferences to fall
   back on.

   The process below assumes that the MUA already has the full HTML
   object that it intends to send, including all of the text supplied by
   the user.

   1.  Assemble the text exactly as specified for text/plain (see
       Section 5.2.2).

   2.  Wrap that text in a verbatim <pre> element.

   3.  Wrap that <pre> element in a <div> element annotated with the
       class header-protection-legacy-display.

   4.  Find the <body> element of the full HTML object.

   5.  Insert the <div> element as the first child of the <body>
       element.

5.2.4.  Only Add a Legacy Display Element to Main Body Parts

   Some messages may contain a text/plain or text/html subpart that is
   _not_ a Main Body Part.  For example, an e-mail message might contain
   an attached text file or a downloaded webpage.  Attached documents
   need to be preserved as intended in the transmission, without
   modification.

   The composing MUA MUST NOT add a Legacy Display Element to any part
   of the message that is not a Main Body Part.  In particular, if a
   part is annotated with Content-Disposition: attachment, or if it does

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   not descend via the first child of any of its multipart/mixed or
   multipart/related ancestors, it is not a Main Body Part, and MUST NOT
   be modified.

   See Section 7.1 of [I-D.ietf-lamps-e2e-mail-guidance] for more
   guidance about common ways to distinguish Main Body Parts from other
   MIME parts in a message.

5.2.5.  Do Not Add a Legacy Display Element to Other Content-Types

   The purpose of injecting a Legacy Display Element into each Main Body
   MIME part is to enable rendering of otherwise obscured Header Fields
   in Legacy MUAs that are capable of message decryption, but don't know
   how to follow the rest of the guidance in this document.

   The authors are unaware of any Legacy MUA that would render any MIME
   part type other than text/plain and text/html as the Main Body.  A
   generating MUA SHOULD NOT add a Legacy Display Element to any MIME
   part with any other Content-Type.

6.  Replying and Forwarding Guidance

   An MUA might create a new message in response to another message,
   thus acting both as a receiving MUA and as a sending MUA.  For
   example, the user of an MUA viewing any given message might take an
   action like "Reply", "Reply All", "Forward", or some comparable
   action to start the composition of a new message.  The new message
   created this way effectively references the original message that was
   viewed at the time.

   For encrypted messages, special guidance applies, because information
   can leak in at least two ways: leaking previously confidential Header
   Fields, and leaking the entire message by sending the reply or
   forward to the wrong party.

6.1.  Avoid Leaking Encrypted Header Fields in Replies and Forwards

   As noted in Section 5.4 of [I-D.ietf-lamps-e2e-mail-guidance], an MUA
   in this position MUST NOT leak previously encrypted content in the
   clear in a follow-up message.  The same is true for protected Header
   Fields.

   Values from any Header Field that was identified as either encrypted-
   only or signed-and-encrypted based on the steps outlined above MUST
   NOT be placed in cleartext output when generating a message.

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   In particular, if Subject was encrypted, and it is copied into the
   draft encrypted reply, the replying MUA MUST obscure the unprotected
   (cleartext) Subject Header Field.

   When crafting the Header Fields for a reply or forwarded message, the
   composing MUA SHOULD make use of the HP-Outer Header Fields from
   within the Cryptographic Envelope of the reference message to ensure
   that Header Fields derived from the reference message do not leak in
   the reply.

   On a high-level, this can be achieved as follows: Consider a Header
   Field in a reply message that is generated by derivation from a
   Header Field in the reference message.  For example, the To Header
   Field is typically derived from the reference message's Reply-To or
   From Header Fields.  When generating the outer copy of the Header
   Field, the composing MUA first applies its own Header Confidentiality
   Policy.  If the Header Field's value is changed by the HCP, then it
   is applied to the outside header.  If the Header Field's value is
   unchanged, the composing MUA re-generates the Header Field using the
   Header Fields that had been on the outside of the original message at
   sending time.  These can be inferred from the HP-Outer Header Fields
   located within the Cryptographic Payload of the referenced message.
   If that value is itself different than the protected value, then it
   is applied to the outside header.  If the value is the same as the
   protected value, then it is simply copied to the outside header
   directly.  Whether it was changed or not, it is noted in the
   protected Header Section using HP-Outer, as described in
   Section 2.2.1.

   See Appendix D.2 for a simple worked example of this process.

   Below we describe a supporting algorithm to handles this.  It
   produces a list of Header Fields that should be obscured or removed
   in the new message even if the sender's choice of Header
   Confidentiality Policy wouldn't normally remove or obscure the Header
   Field in question.  This is effectively a single-use HCP.  The normal
   sending guidance in Section 5.2 applies this single-use HCP to
   implement the high-level guidance above.

6.1.1.  ReferenceHCP

   The algorithm takes two inputs:

   *  A single referenced message refmsg, and

   *  A built-in MUA function respond associated with the user's action.
      respond takes as input a list of headers from a referenced message
      and generates a list of initial candidate message Header Field

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      names and values that are used to populate the message composition
      interface.  Something like this function already exists in most
      MUAs, though it may differ across responsive actions.  For
      example, the respond function that implements "Reply All" is
      likely to be a different from the respond that implements "Reply".

   As an output, it produces an ephemeral single-use Header
   Confidentiality Policy, specific to this kind of response to this
   specific message.

   Method signature:

   ReferenceHCP(refmsg, respond) → ephemeral_hcp

   Procedure:

   1.  If refmsg is not encrypted with Header Protection:

       i.  Return hcp_no_confidentiality (there is no header
           confidentiality in the reference message that needs
           protection)

   2.  Extract refouter, refprotected from refmsg as described in
       Section 4.2

   3.  Let genprotected be a list of (h,v) pairs generated by
       respond(refprotected)

   4.  Let genouter be a list of (h,v) pairs generated by
       respond(refouter)

   5.  For each (h,v) in genprotected:

       i.  If (h,v) is in genouter:

           a.  Remove (h,v) from both genprotected and genouter (this
               Header Field does not need additional confidentiality)

   6.  Let confmap be a mapping from a Header Field name and value (h,v)
       to either a string or the special value null (this mapping is
       initially empty)

   7.  For each (h,v) remaining in genprotected:

       i.    Set result to the special value null

       ii.   For each (h1,v1) in genouter:

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             a.  If h1 is h:

                 I.  Set result to v1

       iii.  Insert (h,v) -> result into confmap

   8.  Return a new HCP from confmap that tests whether (name,val_in)
       are in confmap; if so, return confmap[(name,val_in)]; otherwise,
       return val_in

   Note that the key idea here is to reuse the MUA's existing respond
   function.  The algorithm simulates how the MUA would pre-populate a
   reply to two traditional messages whose Header Fields have the values
   refouter and refprotected respectively (independent of any
   cryptographic protections).  Then it uses the difference to derive a
   one-time HCP.  This HCP takes into account both the referenced
   message's sender's preferences and the derivations that can happen to
   Header Field values when responding.  Note that while some of these
   derivations are straight forward (e.g., In-Reply-To is usually
   derived from Message-ID), others are non-trivial.  For example, the
   From address may be derived from To, Cc, or from the MUA's local
   address preference (especially when the MUA received the referenced
   message via Bcc).  Similarly, To may be derived from To, From, and/or
   Cc Header Fields depending on the MUA implementation and depending on
   whether the user clicked "Reply", "Reply All", "Forward", or any
   other action that generates a response to a message.  Reusing the
   MUA's existing respond function incorporates these nuances without
   requiring any extra configuration choices or additional maintenance
   burden.

6.2.  Avoid Misdirected Replies

   When replying to a message, the Composing MUA typically decides who
   to send the reply to based on:

   *  the Reply-To, Mail-Followup-To, or From Header Fields

   *  optionally, the other To or Cc Header Fields (if the user chose to
      "reply all")

   When a message has Header Protection, the replying MUA MUST populate
   the destination fields of the draft message using the protected
   Header Fields, and ignore any unprotected Header Fields.

   This mitigates against an attack where Mallory gets a copy of an
   encrypted message from Alice to Bob, and then replays the message to
   Bob with an additional Cc to Mallory's own e-mail address in the
   message's outer (unprotected) Header Section.

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   If Bob knows Mallory's certificate already, and he replies to such a
   message without following the guidance in this section, it's likely
   that his MUA will encrypt the cleartext of the message directly to
   Mallory.

7.  Unprotected Header Fields Added in Transit

   Some Header Fields are legitimately added in transit and could not
   have been known to the sender at message composition time.

   The most common of these Header Fields are Received and DKIM-
   Signature, neither of which are typically rendered, either explicitly
   or implicitly.

   If a receiving MUA has specific knowledge about a given Header Field,
   including that:

   *  the Header Field would not have been known to the original sender,
      and

   *  the Header Field might be rendered explicitly or implicitly,

   then the MUA MAY decide to operate on the value of that Header Field
   from the unprotected Header Section, even though the message has
   Header Protection.

   The MUA MAY prefer to verify that the Header Fields in question have
   additional transit-derived cryptographic protections before rendering
   or acting on them.  For example, the MUA could verify whether these
   Header Fields are covered by an appropriate and valid ARC-
   Authentication-Results (see [RFC8617]) or DKIM-Signature (see
   [RFC6376]) Header Field.

   Specific examples of user-meaningful Header Fields commonly added by
   transport agents appear below.

7.1.  Mailing list Header Fields: List-* and Archived-At

   If the message arrives through a mailing list, the list manager
   itself may inject Header Fields (most have a List- prefix) in the
   message:

   *  List-Archive

   *  List-Subscribe

   *  List-Unsubscribe

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   *  List-Id

   *  List-Help

   *  List-Post

   *  Archived-At

   For some MUAs, these Header Fields are implicitly rendered, by
   providing buttons for actions like "Subscribe", "View Archived
   Version", "Reply List", "List Info", etc.

   An MUA that receives a message with Header Protection that contains
   these Header Fields in the unprotected section, and that has reason
   to believe the message is coming through a mailing list MAY decide to
   render them to the user (explicitly or implicitly) even though they
   are not protected.

8.  E-mail Ecosystem Evolution

   This document is intended to offer tooling needed to improve the
   state of the e-mail ecosystem in a way that can be deployed without
   significant disruption.  Some elements of this specification are
   present for transitional purposes, but would not exist if the system
   were designed from scratch.

   This section describes these transitional mechanisms, as well as some
   suggestions for how they might eventually be phased out.

8.1.  Dropping Legacy Display Elements

   Any decorative Legacy Display Element added to an encrypted message
   that uses Header Protection is present strictly for enabling Header
   Field visibility (most importantly, the Subject Header Field) when
   the message is viewed with a decryption-capable Legacy MUA.

   Eventually, the hope is that most decryption-capable MUAs will
   conform to this specification, and there will be no need for
   injection of Legacy Display Elements in the message body.  A survey
   of widely used decryption-capable MUAs might be able to establish
   when most of them do support this specification.

   At that point, a composing MUA could set the legacy parameter defined
   in Section 5.2 to false by default or could even hard-code it to
   false, yielding a much simpler message construction set.

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   Until that point, an end user might want to signal that their
   receiving MUAs are conformant to this document so that a peer
   composing a message to them can set legacy to false.  A signal
   indicating capability of handling messages with Header Protection
   might be placed in the user's cryptographic certificate, or in
   outbound messages.

   This document does not attempt to define the syntax or semantics of
   such a signal.

8.2.  More Ambitious Default Header Confidentiality Policy

   This document defines a few different forms of Header Confidentiality
   Policy.  An MUA implementing an HCP for the first time SHOULD deploy
   hcp_baseline as recommended in Section 3.3.  This HCP offers the most
   commonly expected protection (obscuring the Subject Header Field)
   without risking deliverability or rendering issues.

   The HCPs proposed in this document are relatively conservative and
   still leak a significant amount of metadata for encrypted messages.
   This is largely done to ensure deliverability (see Section 1.3.2) and
   usability, as messages without some critical Header Fields are more
   likely to not reach their intended recipient.

   In the future, some mail transport systems may accept and deliver
   messages with even less publicly visible metadata.  Many MTA
   operators today would ask for additional guarantees about such a
   message to limit the risks associated with abusive or spammy mail.

   This specification offers the HCP formalism itself as a way for MUA
   developers and MTA operators to describe their expectations around
   message deliverability.  MUA developers can propose a more ambitious
   default HCP, and ask MTA operators (or simply test) whether their
   MTAs would be likely to deliver or reject encrypted mail with that
   HCP applied.  Proponents of a more ambitious HCP should explicitly
   document the HCP and name it clearly and unambiguously to facilitate
   this kind of interoperability discussion.

   Reaching widespread consensus around a more ambitious global default
   HCP is a challenging problem of coordinating many different actors.
   A piecemeal approach might be more feasible, where some signalling
   mechanism allows a message recipient, MTA operator, or third-party
   clearinghouse to announce what kinds of HCPs are likely to be
   deliverable for a given recipient.  In such a situation, the default
   HCP for an MUA might involve consulting the signalled acceptable HCPs
   for all recipients, and combining them (along with a default for when
   no signal is present) in some way.

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   If such a signal were to reach widespread use, it could also be used
   to guide reasonable statistical default HCP choices for recipients
   with no signal.

   This document does not attempt to define the syntax or semantics of
   such a signal.

8.3.  Deprecation of Messages Without Header Protection

   At some point, when the majority of MUA clients that can generate
   cryptographically protected messages with Header Protection, it
   should be possible to deprecate any cryptographically protected
   message that does not have Header Protection.

   For example, as noted in Section 9.1, it's possible for an MUA to
   render a signed-only message that has no Header Protection the same
   as an unprotected message.  And a signed-and-encrypted message
   without Header Protection could likewise be marked as not fully
   protected.

   These stricter rules could be adopted immediately for all messages.
   Or an MUA developer could roll them out immediately for any new
   message, but still treat an old message (based on the Date Header
   Field and cryptographic signature timestamp) more leniently.

   A decision like this by any popular receiving MUA could drive
   adoption of this standard for sending MUAs.

9.  Usability Considerations

   This section describes concerns for MUAs that are interested in easy
   adoption of Header Protection by normal users.

   While they are not protocol-level artifacts, these concerns motivate
   the protocol features described in this document.

   See also the Usability commentary in Section 2 of
   [I-D.ietf-lamps-e2e-mail-guidance].

9.1.  Mixed Protections Within a Message Are Hard To Understand

   When rendering a message to the user, the ideal circumstance is to
   present a single cryptographic status for any given message.
   However, when message Header Fields are present, some message Header
   Fields do not have the same cryptographic protections as the main
   message.

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   Representing such a mixed set of protection statuses is very
   difficult to do in a way that a Ordinary User can understand.  There
   are at least three scenarios that are likely to be common, and poorly
   understood:

   *  A signed message with no Header Protection.

   *  A signed-and-encrypted message with no Header Protection.

   *  A signed-and-encrypted message with Header Protection as defined
      in this document, where some User-Facing Header Fields have
      confidentiality but some do not.

   An MUA should have a reasonable strategy for clearly communicating
   each of these scenarios to the user.  For example, an MUA operating
   in an environment where it expects most cryptographically protected
   messages to have Header Protection could use the following rendering
   strategy:

   *  When rendering a message with signed-only cryptographic status but
      no Header Protection, an MUA may decline to indicate a positive
      security status overall, and only indicate the cryptographic
      status to a user in a message properties or diagnostic view.  That
      is, the message may appear identical to an unsigned message except
      if a user verifies the properties through a menu option.

   *  When rendering a message with signed-and-encrypted or encrypted-
      only cryptographic status but no Header Protection, overlay a
      warning flag on the typical cryptographic status indicator.  That
      is, if a typical signed-and-encrypted message displays a lock
      icon, display a lock icon with a warning sign (e.g., an
      exclamation point in a triangle) overlaid.  See, for example, the
      graphics in [chrome-indicators].

   *  When rendering a message with signed-and-encrypted or encrypted-
      only cryptographic status, with Header Protection, but where the
      Subject Header Field has not been removed or obscured, place a
      warning sign on the Subject line.

   Other simple rendering strategies could also be reasonable.

9.2.  Users Should Not Have To Choose a Header Confidentiality Policy

   This document defines the abstraction of a Header Confidentiality
   Policy object for the sake of communication between implementers and
   deployments.

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   Most e-mail users are unlikely to understand the tradeoffs between
   different policies.  In particular, the potential negative side
   effects (e.g., poor deliverability) may not be easily attributable by
   a normal user to a particular HCP.

   Therefore, MUA implementers should be conservative in their choice of
   default HCP, and should not require the Ordinary User to make an
   incomprehensible choice that could cause unfixable, undiagnosable
   problems.  The safest option is for the MUA developer to select a
   known, stable HCP (this document recommends hcp_baseline in
   Section 3.3) on the user's behalf.  An MUA should not expose the
   Ordinary User to a configuration option where they are expected to
   manually select (let alone define) an HCP.

10.  Security Considerations

   Header Protection improves the security of cryptographically
   protected e-mail messages.  Following the guidance in this document
   improves security for users by more directly aligning the underlying
   messages with user expectations about confidentiality, authenticity,
   and integrity.

   Nevertheless, helping the user distinguish between cryptographic
   protections of various messages remains a security challenge for
   MUAs.  This is exarcebated by the fact that many existing messages
   with cryptographic protections do not employ Header Protection.  MUAs
   encountering these messages (e.g., in an archive) will need to handle
   older forms (without Header Protection) for quite some time, possibly
   forever.

   The security considerations from Section 6 of [RFC8551] continue to
   apply for any MUA that offers S/MIME cryptographic protections, as
   well as Section 3 of [RFC5083] (Authenticated-Enveloped-Data in CMS)
   and Section 14 of [RFC5652] (CMS more broadly).  Likewise, the
   security considerations from Section 8 of [RFC3156] continue to apply
   for any MUA that offers PGP/MIME cryptographic protections, as well
   as Section 13 of [I-D.ietf-openpgp-crypto-refresh-13] (OpenPGP
   itself).  In addition, these underlying security considerations are
   now also applicable to the contents of the message header, not just
   the message body.

10.1.  From Address Spoofing

   If the From Header Field were treated by the receiving MUA like any
   other protected Header Field, this scheme would enable sender address
   spoofing.

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   To prevent sender spoofing, many receiving MUAs implicitly rely on
   their receiving MTA to inspect the unprotected Header Section and
   verify that the From Header Field is authentic.  If a receiving MUA
   displays a From address that doesn't match the From address that the
   receiving and/or sending MTAs filtered on, the MUA may be vulnerable
   to spoofing.

   Consider a malicious MUA that sets the following Header Fields on an
   encrypted message with Header Protection:

   *  Outer: From: <alice@example.com>

   *  Inner: HP-Outer: From: <alice@example.com>

   *  Inner: From: <bob@example.org>

   During sending, the MTA of example.com validates that the sending MUA
   is authorized to send from alice@example.com.  Since the message is
   encrypted, the sending and receiving MTAs cannot see the protected
   Header Fields.  A naive receiving MUA might follow the algorithms in
   this document without special consideration for the From Header
   Field.  Such an MUA might display the email as coming from
   bob@example.org to the user, resulting in a spoofed address.

   This problem applies both between domains and within a domain.

   This problem always applies to signed-and-encrypted messages.  This
   problem also applies to signed-only messages because MTAs typically
   do not look at the protected Header Fields when confirming From
   address authenticity.

   Sender address spoofing is relevant for two distinct security
   properties:

   *  Sender authenticity: relevant for rendering the message (which
      address to show the user?).

   *  Message confidentiality: relevant when replying to a message (a
      reply to the wrong address can leak the message contents).

10.1.1.  From Rendering Reasoning

   Section 4.4.3 provides guidance for rendering the From Header Field.
   It recommends a receiving MUA that depends on its MTA to authenticate
   the unprotected (outer) From Header Field to render the outer From
   Header Field, if both of the following conditions are met:

   *  From Header Field Mismatch (as defined in Section 4.4.1.1)

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   *  No Valid and Correctly Bound Signature (as defined in
      Section 4.4.1.2)

   Note: The second condition effectively means that the inner (expected
   to be protected) From Header Field appears to have insufficient
   protection.

   This may seem surprising since it causes the MUA to render a mix of
   both protected and unprotected values.  This section provides an
   argument as to why this guidance makes sense.

   We proceed by case distinction:

   *  Case 1: Malicious sending MUA.

      -  Attack situation: the sending MUA puts a different inner From
         Header Field to spoof the sender address.

      -  In this case, it is "better" to fall back and render the outer
         From Header Field because this is what the receiving MTA can
         validate.  Otherwise this document would introduce a new way
         for senders to spoof the From address of the message.

      -  This does not preclude a future document from updating this
         document to specify a protocol for legitimate sender address
         hiding.

   *  Case 2: Malicious sending/transiting/receiving MTA (or anyone
      meddling between MTAs).

      -  Attack situation: an on-path attacker changes the outer From
         Header Field (possibly with other meddling to break the
         signature, see below).  Their goal is to get the receiving MUA
         to show a different From address than the sending MUA intended
         (breaking MUA-to-MUA sender authenticity).

      -  Case 2.a: The sending MUA submitted an unsigned or encrypted-
         only message to the email system.  In this case, there can be
         no sender authenticity anyway.

      -  Case 2.b: The sending MUA submitted a signed-only message to
         the email system.

         o  Case 2.b.i: The attacker removes or breaks the signature.
            In this case, the attacker can also modify the inner From
            Header Field to their liking.

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         o  Case 2.b.ii: The signature is valid, but the receiving MUA
            does not see any valid binding between the signing
            certificate and the addr-spec of the inner From Header
            Field.  In this case, there can be no sender authenticity
            anyways (the certificate could have been generated by the
            on-path attacker).  This case is indistinguishable from a
            malicious sending MUA, hence it is "better" to fall back to
            the outer From that the MTA can validate.  Note that once
            the binding is validated (e.g., after an out-of-band
            comparison), the rendering may change from showing the outer
            From address (and a warning) to showing the inner, now
            validated From address.  In some cases, the binding may be
            instantly validated even for previously unseen certificates
            (e.g., if the certificate is issued by a trusted
            certification authority).

      -  Case 2.c: The sending MUA submitted a signed-and-encrypted
         message to the email system.

         o  Case 2.c.i: The attacker removes or breaks the signature.
            Note that the signature is inside the ciphertext (see
            Section 5.2 of [I-D.ietf-lamps-e2e-mail-guidance]).  Thus,
            assuming the encryption is non-malleable, any on-path
            attacker cannot break the signature while ensuring that the
            message still decrypts successfully.

         o  Case 2.c.ii: The signature is valid, but the receiving MUA
            does not see any valid binding between the signing
            certificate and the addr-spec of the inner From Header
            Field.  See case 2.b.ii.

   As the case distinction shows, the outer From Header Field is either
   the preferred fallback (in particular, to avoid introducing a new
   spoofing channel), or it is just as good (because just as modifiable)
   as the inner From Header Field.

   Rendering the outer From Header Field does carry the risk of a
   "temporary downgrade attack" in cases 2.b.ii and 2.c.ii, where a
   malicious MTA keeps the signature intact but modifies the outer From
   Header Field.  The MUA can resolve this temporary downgrade by
   validating the certificate-to-addr-spec binding.  If the MUA never
   does this validation, the entire message could be fake.

   If there were a signalling channel where the MTA can tell the MUA
   whether it authenticated the From Header Field, an MUA could use this
   in its rendering decision.  In the absence of such a signal, and when
   end-to-end authenticity is unavailable, this document prefers to fall
   back to the outer From Header Field.  This default is based on the

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   assumption that most MTAs apply some filtering based on the outer
   From Header Field (whether the MTA can authenticate it or not).
   Rendering the unprotected outer From Header Field (instead of the
   protected inner one) in case of a mismatch retains this ability for
   MTAs.

   If the MUA decides not to rely on the MTA to authenticate the outer
   From Header Field, it may prefer the inner From Header Field.

10.2.  Avoid Cryptographic Summary Confusion from hp Parameter

   When parsing a message, the recipient MUA infers the message's
   Cryptographic Status from the Cryptographic Layers, as described in
   Section 4.6 of [I-D.ietf-lamps-e2e-mail-guidance].

   The Cryptographic Layers that make up the Cryptographic Envelope
   describe an ordered list of cryptographic properties as present in
   the message after it has been delivered.  By contrast, the hp
   parameter to the Content-Type Header Field contains a simpler
   indication: whether the sender originally tried to encrypt the
   message or not.  In particular, for a message with Header Protection,
   the Cryptographic Payload should have a hp parameter of cipher if the
   message is encrypted (in addition to signed), and clear if no
   encryption is present (that is, the message is signed-only).

   As noted in Section 2.1.1, the receiving implementation should not
   inflate its estimation of the confidentiality of the message or its
   Header Fields based on the sender's intent, if it can see that the
   message was not actually encrypted.  A signed-only message that
   happens to have an hp parameter of cipher is still signed-only.

   Conversely, since the encrypting Cryptographic Layer is typically
   outside the signature layer (see Section 5.2 of
   [I-D.ietf-lamps-e2e-mail-guidance]), an originally signed-only
   message could have been wrapped in an encryption layer by an
   intervening party before receipt, to appear encrypted.

   If a message appears to be wrapped in an encryption layer, and the hp
   parameter is present but is not set to cipher, then it is likely that
   the encryption layer was not added by the original sender.  For such
   a message, the lack of any HP-Outer Header Field in the Header
   Section of the Cryptographic Payload MUST NOT be used to infer that
   all Header Fields were removed from the message by the original
   sender.  In such a case, the receiving MUA SHOULD treat every Header
   Field as though it was not confidential.

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10.3.  Caution about Composing with Legacy Display Elements

   When composing a message, it's possible for a Legacy Display Element
   to contain risky data that could trigger errors in a rendering
   client.

   For example, if the value for a Header Field to be included in a
   Legacy Display Element within a given body part contains folding
   whitespace, it should be "unfolded" before generating the Legacy
   Display Element: all contiguous folding whitespace should be replaced
   with a single space character.  Likewise, if the header value was
   originally encoded with [RFC2047], it should be decoded first to a
   standard string and re-encoded using the charset appropriate to the
   target part.

   When including a Legacy Display Element in a text/plain part (see
   Section 5.2.2), if the decoded Subject Header Field contains a pair
   of newlines (e.g., if it is broken across multiple lines by encoded
   newlines), any newline MUST be stripped from the Legacy Display
   Element.  If the pair of newlines is not stripped, a receiving MUA
   that follows the guidance in Section 4.5.3.2 might leave the later
   part of the Legacy Display Element in the rendered message.

   When including a Legacy Display Element in a text/html part (see
   Section 5.2.3), any material in the header values should be
   explicitly HTML escaped to avoid being rendered as part of the HTML.
   At a minimum, the characters <, >, and & should be escaped to &lt;,
   &gt;, and &amp;, respectively (see for example [HTML-ESCAPES]).  If
   unescaped characters from removed or obscured header values end up in
   the Legacy Display Element, a receiving MUA that follows the guidance
   in Section 4.5.3.3 might fail to identify the boundaries of the
   Legacy Display Element, cutting out more than it should, or leaving
   remnants visible.  And a Legacy MUA parsing such a message might
   misrender the entire HTML stream, depending on the content of the
   removed or obscured header values.

   The Legacy Display Element is a decorative addition solely to enable
   visibility of obscured or removed Header Fields in decryption-capable
   Legacy MUAs.  When it is produced, it should be generated minimally
   and strictly, as described above, to avoid damaging the rest of the
   message.

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10.4.  Plaintext Attacks

   An encrypted e-mail message using S/MIME or PGP/MIME tends to have
   some amount of predictable plaintext.  For example, the standard MIME
   headers of the Cryptographic Payload of a message are often a
   predictable sequence of bytes, even without Header Protection, when
   they only include the Structural Header Fields MIME-Version and
   Content-Type.  This is a potential risk for known-plaintext attacks.

   Including protected Header Fields as defined in this document
   increases the amount of known plaintext.  Since some of those headers
   in a reply will be derived from the message being replied to, this
   also creates a potential risk for chosen-plaintext attacks, in
   addition to known-plaintext attacks.

   Modern message encryption mechanisms are expected to be secure
   against both known-plaintext attacks and chosen-plaintext attacks.
   An MUA composing an encrypted message should ensure that it is using
   such a mechanism, regardless of whether it does Header Protection.

11.  Privacy Considerations

11.1.  Leaks When Replying

   The encrypted Header Fields of a message may accidentally leak when
   replying to the message.  See the guidance in Section 6.

11.2.  Encrypted Header Fields Are Not Always Private

   For encrypted messages, depending on the sender's HCP, some Header
   Fields may appear both within the Cryptographic Envelope and on the
   outside of the message (e.g., Date might exist identically in both
   places).  Section 4.3 identifies such a Header Field as signed-only.
   These Header Fields are clearly _not_ private at all, despite a copy
   being inside the Cryptographic Envelope.

   A Header Field whose name and value are not matched verbatim by any
   HP-Outer Header Field from the same part will have encrypted-only or
   signed-and-encrypted status.  But even Header Fields with these
   stronger levels of cryptographic confidentiality protection might not
   be as private as the user would like.

   See the examples below.

   This concern is true for any encrypted data, including the body of
   the message, not just the Header Fields: if the sender isn't careful,
   the message contents or session keys can leak in many ways that are
   beyond the scope of this document.  The message recipient has no way

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   in principle to tell whether the apparent confidentiality of any
   given piece of encrypted content has been broken via channels that
   they cannot perceive.  Additionally, an active intermediary aware of
   the recipient's public key can always encrypt a cleartext message in
   transit to give the recipient a false sense of security.

11.2.1.  Encrypted Header Fields Can Leak Unwanted Information to the
         Recipient

   For encrypted messages, even with an ambitious HCP that successfully
   obscures most Header Fields from all transport agents, Header Fields
   will be ultimately visible to all intended recipients.  This can be
   especially problematic for Header Fields that are not user-facing,
   which the sender may not expect to be injected by their MUA.
   Consider the three following examples:

   *  The MUA may inject a User-Agent Header Field that describes itself
      to every recipient, even though the sender may not want the
      recipient to know the exact version of their OS, hardware
      platform, or MUA.

   *  The MUA may have an idiosyncratic way of generating a Message-ID
      header, which could embed the choice of MUA, a time zone, a
      hostname, or other subtle information to a knowledgeable
      recipient.

   *  The MUA may erroneously include a Bcc Header Field in the
      origheaders of a copy of a message sent to the named recipient,
      defeating the purpose of using Bcc instead of Cc (see Section 11.4
      for more details about risks related to Bcc).

   Clearly, no end-to-end cryptographic protection of any Header Field
   as defined in this document will hide such a sensitive field from the
   intended recipient.  Instead, the composing MUA MUST populate the
   origheaders list for any outbound message with only information the
   recipient should have access to.  This is true for messages without
   any cryptographic protection as well, of course, and it is even worse
   there: such a leak is exposed to the transport agents as well as the
   recipient.  An encrypted message with Header Protection and a more
   ambitious Header Confidentiality Policy avoid these leaks exposing
   information to the transport agents but cannot defend against such a
   leak to the recipient.

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11.2.2.  Encrypted Header Fields Can Be Inferred From External or
         Internal Metadata

   For example, if the To and Cc Header Fields are removed from the
   unprotected Header Section, the values in those fields might still be
   inferred with high probability by an adversary who looks at the
   message either in transit or at rest.  If the message is found in, or
   being delivered to a mailbox for bob@example.org, it's likely that
   Bob was in either To or Cc.  Furthermore, encrypted message
   ciphertext may hint at the recipients: for S/MIME messages, the
   RecipientInfo, and for PGP/MIME messages the key ID in the Public Key
   Encrypted Session Key (PKESK) packets will all hint at a specific set
   of recipients.  Additionally, an MTA that handles the message may add
   a Received Header Field (or some other custom Header Field) that
   leaks some information about the nature of the delivery.

11.2.3.  Encrypted Header Fields May Not Be Fully Masked by HCP

   In another example, if the HCP modifies the Date header to mask out
   high-resolution time stamps (e.g., rounding to the most recent hour),
   some information about the date of delivery will still be attached to
   the e-mail.  At the very least, the low resolution, global version of
   the date will be present on the message.  Additionally, Header Fields
   like Received that are added during message delivery might include
   higher-resolution timestamps.  And if the message lands in a mailbox
   that is ordered by time of receipt, even its placement in the mailbox
   and the non-obscured Date Header Fields of the surrounding messages
   could leak this information.

   Some Header Fields like From may be impossible to fully obscure, as
   many modern message delivery systems depend on at least domain
   information in the From Header Field for determining whether a
   message is coming from a domain with "good reputation" (that is, from
   a domain that is not known for leaking spam).  So even if an
   ambitious HCP opts to remove the human-readable part from any From
   Header Field, and to standardize/genericize the local part of the
   From address, the domain will still leak.

11.3.  A Naive Recipient May Overestimate the Cryptographic Status of a
       Header Field in an Encrypted Message

   When an encrypted (or signed-and-encrypted) message is in transit, an
   active intermediary can strip or tamper with any Header Field that
   appears outside the Cryptographic Envelope.  A receiving MUA that
   naively infers cryptographic status from differences between the
   external Header Fields and those found in the Cryptographic Envelope
   could be tricked into overestimating the protections afforded to some
   Header Fields.

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   For example, if the original sender's HCP passes through the Cc
   Header Field unchanged, a cleanly delivered message would indicate
   that the Cc Header Field has a cryptographic status of signed.  But
   if an intermediary attacker simply removes the Header Field from the
   unprotected Header Section before forwarding the message, then the
   naive recipient might believe that the field has a cryptographic
   status of signed-and-encrypted.

   This document offers protection against such an attack by way of the
   HP-Outer Header Fields that can be found on the Cryptographic
   Payload.  If a Header Field appears to have been obscured by
   inspection of the outer message, but an HP-Outer Header Field matches
   it exactly, the receiving MUA can indicate to the user that the
   Header Field in question may not have been confidential.

   In such a case, a cautious MUA may render the Header Field in
   question as signed (because the sender did not hide it), but still
   treat it as signed-and-encrypted during reply, to avoid accidental
   leakage of the cleartext value in the reply message, as described in
   Section 6.1.

11.4.  Privacy and Deliverability Risks with Bcc and Encrypted Messages

   As noted in Section 9.3 of [I-D.ietf-lamps-e2e-mail-guidance],
   handling Bcc when generating an encrypted e-mail message can be
   particularly tricky.  With Header Protection, there is an additional
   wrinkle.  When an encrypted e-mail message with Header Protection has
   a Bcc'ed recipient, and the composing MUA explicitly includes the
   Bcc'ed recipient's address in their copy of the message (see the
   "second method" in Section 3.6.3 of [RFC5322]), that Bcc Header Field
   will always be visible to the Bcc'ed recipient.

   In this scenario, though, the composing MUA has one additional
   choice: whether to hide the Bcc Header Field from intervening message
   transport agents, by returning null when the HCP is invoked for Bcc.
   If the composing MUA's rationale for including an explicit Bcc in the
   copy of the message sent to the Bcc recipient is to ensure
   deliverability via a message transport agent that inspects message
   Header Fields, then stripping the Bcc field during encryption may
   cause the intervening transport agent to drop the message entirely.
   This is why Bcc is not explicitly stripped in hcp_baseline.

   If, on the other hand, deliverability to a Bcc'ed recipient is not a
   concern, the most privacy-preserving option is to simply omit the Bcc
   Header Field from the protected Header Section in the first place.
   An MUA that is capable of receiving and processing such a message can
   infer that since their user's address was not mentioned in any To or
   Cc Header Field, they were likely a Bcc recipient.

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   Please also see Section 9.3 of [I-D.ietf-lamps-e2e-mail-guidance] for
   more discussion about Bcc and encrypted messages.

12.  IANA Considerations

   This document registers an e-mail Header Field, describes parameters
   for the Content-Type Header Field, and establishes a registry for
   Header Confidentiality Policies to facilitate HCP evolution.

12.1.  Register the HP-Outer Header Field

   This document requests IANA to register the following Header Field in
   the "Permanent Message Header Field Names" registry within "Message
   Headers" in accordance with [RFC3864].

      +============+==========+==========+==========+===============+
      | Header     | Template | Protocol | Status   | Reference     |
      | Field Name |          |          |          |               |
      +============+==========+==========+==========+===============+
      | HP-Outer   |          | mail     | standard | Section 2.2.1 |
      |            |          |          |          | of RFCXXXX    |
      +------------+----------+----------+----------+---------------+

           Table 2: Additions to 'Permanent Message Header Field
                              Names' registry

   The Author/Change Controller of these two entries (Section 4.5 of
   [RFC3864]) should be the IETF itself.

12.2.  Update Reference for Content-Type Header Field due to hp and hp-
       legacy-display Parameters

   This document also defines the Content-Type parameters known as hp
   (in Section 2.1.1) and hp-legacy-display (in Section 2.1.2).
   Consequently, the Content-Type row in the "Permanent Message Header
   Field Names" registry should add a reference to this RFC to its
   "References" column.

   That is, the current row:

     +===================+==========+==========+========+===========+
     | Header Field Name | Template | Protocol | Status | Reference |
     +===================+==========+==========+========+===========+
     | Content-Type      |          | MIME     |        | [RFC4021] |
     +-------------------+----------+----------+--------+-----------+

         Table 3: Existing row in 'Permanent Message Header Field
                             Names' registry

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   Should be updated to have the following values:

     +===================+==========+==========+========+===========+
     | Header Field Name | Template | Protocol | Status | Reference |
     +===================+==========+==========+========+===========+
     | Content-Type      |          | MIME     |        | [RFC4021] |
     |                   |          |          |        | [RFCXXXX] |
     +-------------------+----------+----------+--------+-----------+

       Table 4: Replacement row in 'Permanent Message Header Field
                             Names' registry

12.3.  New Registry: Mail Header Confidentiality Policies

   This document also requests IANA to create a new registry in the
   "Mail Parameters" protocol group (https://www.iana.org/assignments/
   mail-parameters/) titled Mail Header Confidentiality Policies with
   the following content:

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   +========================+=================+=========+=============+
   | Header Confidentiality | Description     |Reference| Recommended |
   | Policy Name            |                 |         |             |
   +========================+=================+=========+=============+
   | hcp_no_confidentiality | No header       |Section  | N           |
   |                        | confidentiality |3.2.3 of |             |
   |                        |                 |RFCXXX   |             |
   |                        |                 |(this    |             |
   |                        |                 |document)|             |
   +------------------------+-----------------+---------+-------------+
   | hcp_baseline           | Confidentiality |Section  | Y           |
   |                        | for             |3.2.1 of |             |
   |                        | Informational   |RFCXXX   |             |
   |                        | Header Fields:  |(this    |             |
   |                        | Subject Header  |document)|             |
   |                        | Field is        |         |             |
   |                        | obscured,       |         |             |
   |                        | Keywords and    |         |             |
   |                        | Comments are    |         |             |
   |                        | removed         |         |             |
   +------------------------+-----------------+---------+-------------+
   | hcp_shy                | Obscure         |Section  | N           |
   |                        | Subject, remove |3.2.2 of |             |
   |                        | Keywords and    |RFCXXX   |             |
   |                        | Comments,       |(this    |             |
   |                        | remove the time |document)|             |
   |                        | zone from Date, |         |             |
   |                        | and obscure     |         |             |
   |                        | display-names   |         |             |
   +------------------------+-----------------+---------+-------------+

          Table 5: Mail Header Confidentiality Policies registry

   hcp_example_hide_cc is offered as an example in Section 3 but is not
   formally registered by this document.

   Please add the following textual note to this registry:

      The Header Confidentiality Policy Name never appears on the wire.
      This registry merely tracks stable references to implementable
      descriptions of distinct policies.  Any addition to this registry
      should be governed by guidance in Section 3.4.2 of RFC XXX (this
      document).

   Adding an entry to this registry with an N in the "Recommended"
   column follows the registration policy of SPECIFICATION REQUIRED.
   Adding an entry to this registry with a Y in the "Recommended" column
   or changing the "Recommended" column in an existing entry (from N to

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   Y or vice versa) requires IETF REVIEW.  During IETF REVIEW, the
   designated expert must also be consulted.  Guidance for the
   designated expert can be found in Section 3.4.2.

13.  Acknowledgments

   Alexander Krotov identified the risk of From address spoofing (see
   Section 10.1) and helped provide guidance to MUAs.

   Thore Göbel identified significant gaps in earlier versions of this
   document, and proposed concrete and substantial improvements.  Thanks
   to his contributions, the document is clearer, and the protocols
   described herein are more useful.

   Additionally, the authors would like to thank the following people
   who have provided helpful comments and suggestions for this document:
   Berna Alp, Bernhard E. Reiter, Carl Wallace, Claudio Luck, Daniel
   Huigens, David Wilson, Hernani Marques, juga, Krista Bennett, Kelly
   Bristol, Lars Rohwedder, Michael StJohns, Nicolas Lidzborski, Orie
   Steele, Peter Yee, Phillip Tao, Robert Williams, Rohan Mahy, Roman
   Danyliw, Russ Housley, Sofia Balicka, Steve Kille, Volker Birk, and
   Wei Chuang.

14.  References

14.1.  Normative References

   [I-D.ietf-lamps-e2e-mail-guidance]
              Gillmor, D. K., Hoeneisen, B., and A. Melnikov, "Guidance
              on End-to-End E-mail Security", Work in Progress,
              Internet-Draft, draft-ietf-lamps-e2e-mail-guidance-16, 16
              March 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lamps-e2e-mail-guidance-16>.

   [I-D.ietf-openpgp-crypto-refresh-13]
              Wouters, P., Huigens, D., Winter, J., and N. Yutaka,
              "OpenPGP", Work in Progress, Internet-Draft, draft-ietf-
              openpgp-crypto-refresh-13, 4 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-
              crypto-refresh-13>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2045>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/rfc/rfc3864>.

   [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              DOI 10.17487/RFC5083, November 2007,
              <https://www.rfc-editor.org/rfc/rfc5083>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/rfc/rfc5322>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5652>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/rfc/rfc8551>.

14.2.  Informative References

   [chrome-indicators]
              Schechter, E., "Evolving Chrome's security indicators",
              May 2018, <https://blog.chromium.org/2018/05/evolving-
              chromes-security-indicators.html>.

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   [CSS]      World Wide Web Consortium, "Cascading Style Sheets Level 2
              Revision 2 (CSS 2.2) Specification", 12 April 2016,
              <https://www.w3.org/TR/2016/WD-CSS22-20160412/>.

   [HTML-ESCAPES]
              W3C, "Using character escapes in markup and CSS", n.d.,
              <https://www.w3.org/International/questions/qa-
              escapes#use>.

   [I-D.autocrypt-lamps-protected-headers]
              Einarsson, B. R., "juga", and D. K. Gillmor, "Protected
              Headers for Cryptographic E-mail", Work in Progress,
              Internet-Draft, draft-autocrypt-lamps-protected-headers-
              02, 20 December 2019,
              <https://datatracker.ietf.org/doc/html/draft-autocrypt-
              lamps-protected-headers-02>.

   [I-D.pep-email]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Email Formats and Protocols", Work in Progress, Internet-
              Draft, draft-pep-email-02, 16 December 2022,
              <https://datatracker.ietf.org/doc/html/draft-pep-email-
              02>.

   [I-D.pep-general]
              Birk, V., Marques, H., and B. Hoeneisen, "pretty Easy
              privacy (pEp): Privacy by Default", Work in Progress,
              Internet-Draft, draft-pep-general-02, 16 December 2022,
              <https://datatracker.ietf.org/doc/html/draft-pep-general-
              02>.

   [PGPCONTROL]
              UUNET Technologies, Inc., "Authentication of Usenet Group
              Changes", 27 October 2016,
              <https://ftp.isc.org/pub/pgpcontrol/>.

   [PGPVERIFY-FORMAT]
              Lawrence, D. C., "Signing Control Messages, Verifying
              Control Messages", n.d.,
              <https://www.eyrie.org/~eagle/usefor/other/pgpverify>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.

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   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2047>.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2049>.

   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
              "MIME Security with OpenPGP", RFC 3156,
              DOI 10.17487/RFC3156, August 2001,
              <https://www.rfc-editor.org/rfc/rfc3156>.

   [RFC3851]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, DOI 10.17487/RFC3851, July 2004,
              <https://www.rfc-editor.org/rfc/rfc3851>.

   [RFC4021]  Klyne, G. and J. Palme, "Registration of Mail and MIME
              Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
              2005, <https://www.rfc-editor.org/rfc/rfc4021>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <https://www.rfc-editor.org/rfc/rfc5751>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5890>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5891>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/rfc/rfc6376>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/rfc/rfc7489>.

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   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,
              <https://www.rfc-editor.org/rfc/rfc7929>.

   [RFC8162]  Hoffman, P. and J. Schlyter, "Using Secure DNS to
              Associate Certificates with Domain Names for S/MIME",
              RFC 8162, DOI 10.17487/RFC8162, May 2017,
              <https://www.rfc-editor.org/rfc/rfc8162>.

   [RFC8617]  Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
              Kucherawy, Ed., "The Authenticated Received Chain (ARC)
              Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
              <https://www.rfc-editor.org/rfc/rfc8617>.

   [RFC9216]  Gillmor, D. K., Ed., "S/MIME Example Keys and
              Certificates", RFC 9216, DOI 10.17487/RFC9216, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9216>.

Appendix A.  Table of Pseudocode Listings

   This document contains guidance with pseudocode descriptions.  Each
   algorithm is listed here for easy reference.

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    +===========================+=========================+===========+
    | Method Name               | Description             | Reference |
    +===========================+=========================+===========+
    | HeaderSetsFromMessage     | Derive "outer" and      | Section   |
    |                           | "protected" sets of     | 4.2.1     |
    |                           | Header Fields from a    |           |
    |                           | given message           |           |
    +---------------------------+-------------------------+-----------+
    | HeaderFieldProtection     | Calculate cryptographic | Section   |
    |                           | protections for a       | 4.3.1     |
    |                           | Header Field in a given |           |
    |                           | message                 |           |
    +---------------------------+-------------------------+-----------+
    | ReferenceHCP              | Produce an ephemeral    | Section   |
    |                           | HCP to use when         | 6.1.1     |
    |                           | responding to a given   |           |
    |                           | message                 |           |
    +---------------------------+-------------------------+-----------+
    | ComposeNoHeaderProtection | Legacy message          | Section   |
    |                           | composition with end-   | 5.1.1     |
    |                           | to-end cryptographic    |           |
    |                           | protections (but no     |           |
    |                           | header protection)      |           |
    +---------------------------+-------------------------+-----------+
    | Compose                   | Compose a message with  | Section   |
    |                           | end-to-end              | 5.2.1     |
    |                           | cryptographic           |           |
    |                           | protections including   |           |
    |                           | header protection       |           |
    +---------------------------+-------------------------+-----------+

                   Table 6: Table of Pseudocode Listings

Appendix B.  Possible Problems with Legacy MUAs

   When an e-mail message with end-to-end cryptographic protection is
   received by a mail user agent, the user might experience many
   different possible problematic interactions.  A message with Header
   Protection may introduce new forms of user experience failure.

   In this section, the authors enumerate different kinds of failures we
   have observed when reviewing, rendering, and replying to messages
   with different forms of Header Protection in different Legacy MUAs.
   Different Legacy MUAs demonstrate different subsets of these
   problems.

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   A conformant MUA would not exhibit any of these problems.  An
   implementer updating their Legacy MUA to be compliant with this
   specification should consider these concerns and try to avoid them.

   Recall that "protected" refers to the "inner" values, e.g., the real
   Subject, and "unprotected" refers to the "outer" values, e.g., the
   dummy Subject.

B.1.  Problems Viewing Messages in a List View

   *  Unprotected Subject, Date, From, To Header Fields are visible
      (instead of being replaced by protected values)

   *  Threading is not visible

B.2.  Problems when Rendering a Message

   *  Unprotected Subject is visible

   *  Protected Subject (on its own) is visible in the body

   *  Protected Subject, Date, From, and To Header Fields visible in the
      body

   *  User interaction needed to view whole message

   *  User interaction needed to view message body

   *  User interaction needed to view protected subject

   *  Impossible to view protected Subject

   *  Nuisance alarms during user interaction

   *  Impossible to view message body

   *  Appears as a forwarded message

   *  Appears as an attachment

   *  Security indicators not visible

   *  Security indicators do not identify protection status of Header
      Fields

   *  User has multiple different methods to reply (e.g., reply to
      outer, reply to inner)

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   *  User sees English "Subject:" in body despite message itself being
      in non-English

   *  Security indicators do not identify protection status of Header
      Fields

   *  Header Fields in body render with local Header Field names (e.g.,
      showing "Betreff" instead of "Subject") and dates (TZ, locale)

B.3.  Problems when Replying to a Message

   Note that the use case here is:

   *  User views message, to the point where they can read it

   *  User then replies to message, and they are shown a message
      composition window, which has some UI elements

   *  If the MUA has multiple different methods to reply to a message,
      each way may need to be evaluated separately

   This section also uses the shorthand UI:x to mean "the UI element
   that the user can edit that they think of as x."

   *  Unprotected Subject is in UI:subject (instead of the protected
      Subject)

   *  Protected Subject is quoted in UI:body (from Legacy Display
      Element)

   *  Protected Subject leaks when the reply is serialised into MIME

   *  Protected Subject is not anywhere in UI

   *  Message body is _not_ visible/quoted in UI:body

   *  User cannot reply while viewing protected message

   *  Reply is not encrypted by default (but is for legacy signed-and-
      encrypted messages without Header Protection)

   *  Unprotected From or Reply-To Header Field is in UI:To (instead of
      the protected From or Reply-To Header Field)

   *  User's locale (lang, TZ) leaks in quoted body

   *  Header Fields not protected (and in particular, Subject is not
      obscured) by default

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Appendix C.  Test Vectors

   This section contains sample messages using the specification defined
   above.  Each sample contains a MIME object, a textual and
   diagrammatic view of its structure, and examples of how an MUA might
   render it.

   The cryptographic protections used in this document use the S/MIME
   standard, and keying material and certificates come from [RFC9216].

   These messages should be accessible to any IMAP client at
   imap://bob@header-protection.cmrg.net/ (any password should
   authenticate to this read-only IMAP mailbox).

   You can also download copies of these test vectors separately at
   https://header-protection.cmrg.net.

   If any of the messages downloaded differ from those offered here,
   this document is the canonical source.

C.1.  Baseline Messages

   These messages offer no header protection at all, and can be used as
   a baseline.  They are provided in this document as a counterexample.
   An MUA implementer can use these messages to verify that the reported
   cryptographic summary of the message indicates no header protection.

C.1.1.  No Cryptographic Protections Over a Simple Message

   This message uses no cryptographic protection at all.  Its body is a
   text/plain message.

   It has the following structure:

   └─╴text/plain 152 bytes

   Its contents are:

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   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit
   Subject: no-crypto
   Message-ID: <no-crypto@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:00:02 -0500
   User-Agent: Sample MUA Version 1.0

   This is the
   no-crypto
   message.

   This message uses no cryptographic protection at all.  Its body
   is a text/plain message.

   --
   Alice
   alice@smime.example

C.1.2.  S/MIME Signed-only signedData Over a Simple Message, No Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message.  It uses no header protection.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 3856 bytes
    ⇩ (unwraps to)
    └─╴text/plain 206 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part
   Message-ID: <smime-one-part@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:01:02 -0500
   User-Agent: Sample MUA Version 1.0

   MIILGQYJKoZIhvcNAQcCoIILCjCCCwYCAQExDTALBglghkgBZQMEAgEwggFCBgkq
   hkiG9w0BBwGgggEzBIIBL01JTUUtVmVyc2lvbjogMS4wDQpDb250ZW50LVR5cGU6
   IHRleHQvcGxhaW47IGNoYXJzZXQ9InV0Zi04Ig0KQ29udGVudC1UcmFuc2Zlci1F

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   bmNvZGluZzogN2JpdA0KDQpUaGlzIGlzIHRoZQ0Kc21pbWUtb25lLXBhcnQNCm1l
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   AQ0FAAOCAQEAc4miNqfOqaBpI3f+CpJDhxtuZ2P9HjQEQ+v6BdP7GKJ19naIs3Bj
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   UyBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3JpdHkCEzdBBXntdX9CqaJcOvT4as6a

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   qdcwCwYJYIZIAWUDBAIBoGkwGAYJKoZIhvcNAQkDMQsGCSqGSIb3DQEHATAcBgkq
   hkiG9w0BCQUxDxcNMjEwMjIwMTUwMTAyWjAvBgkqhkiG9w0BCQQxIgQgrhyFjywc
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   bqYT1i46156CSOqyxA==

C.1.2.1.  S/MIME Signed-only signedData Over a Simple Message, No Header
          Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message. It uses no header protection.

   --
   Alice
   alice@smime.example

C.1.3.  S/MIME Signed-only multipart/signed Over a Simple Message, No
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a text/plain message.  It uses no
   header protection.

   It has the following structure:

   └┬╴multipart/signed 4187 bytes
    ├─╴text/plain 224 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

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   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="253";
    micalg="sha-256"
   Subject: smime-multipart
   Message-ID: <smime-multipart@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:02:02 -0500
   User-Agent: Sample MUA Version 1.0

   --253
   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a text/plain
   message. It uses no header protection.

   --
   Alice
   alice@smime.example

   --253
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

   MIIJ4AYJKoZIhvcNAQcCoIIJ0TCCCc0CAQExDTALBglghkgBZQMEAgEwCwYJKoZI
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   LomKSVRTUG11eEhmRmxIY4wKhwc0U9PKjCQFrhu3t1ZkGSfPn9jvdNTJkg85WUpk
   WqmOyrup6DH4Gb84By+0IMk3vflrOyAw3kbsj6Ij+zymAlH61YypnAvddFBIuZPL
   2LYdIHPLmq8KGrzcgjkjP+Y58hf9U+6gp0KPuS8DAGOvxYs0

   --253--

C.1.4.  S/MIME Signed and Encrypted Over a Simple Message, No Header
        Protection

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses no header protection.

   It has the following structure:

Gillmor, et al.           Expires 8 March 2025                 [Page 85]
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   └─╴application/pkcs7-mime [smime.p7m] 6720 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 3960 bytes
     ⇩ (unwraps to)
     └─╴text/plain 241 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: smime-signed-enc
   Message-ID: <smime-signed-enc@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:03:02 -0500
   User-Agent: Sample MUA Version 1.0

   MIITXAYJKoZIhvcNAQcDoIITTTCCE0kCAQAxggMQMIIBhAIBADBsMFUxDTALBgNV
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   VPABjb/fqf6lzO3gszY2RE6wI7zHydlz2DgkpFdjyVk1Jub2+QkrQA7Brn9gES/I
   gshjTIF+OL3me4UBxww0Bxtt46yz8FpVVOK4MunYel4U4p1SR1WEZGRLPDL+bydN
   vXdstX39Eg8YChAdt5o5pPQ7bUo3Qkk0X9glJdyVNsTpWREj+F+/6do/JPStJSQt
   TYgnXdjkHP4/w6+xqOcogfEVp6in7KkwfZ0v+SdZK++IPm/rMOsZlP9MbM9LkOA1
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   RUFYTkhF5/g+pJ/Ysw9lVIvAOXHtnrbsTOxbrsIzL5wbkvCDW6ZTQIQ4kP9D0NTl
   1JcxNVj10GprUztmYgqOy+wIJj3DlXHSSdugy3S/qEjiCCZwN8zAVl+c8AiifgfP
   zpI4QU1552EC8HyoIUZSQP5O/dIy6ABLEDcwZKJ8nGJdSLurpD0V68p/hWk+Q6mu

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   I7DqidlNT0yehBKvZRE8jr7wclUm73xX/PhOqY158N6wNsekUHOHYERKU0BzRScQ
   +YJ9tcsmPldE6jAzJB/vjjgoiLxIMci0PAXVdGjisxY3QhLh4DJTwKwhIr4kMkv0
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   0t+Rr4Ul0oXs/0ERZLxQQbJLIRIxwsfekwvFBZ8QXp30mfQ+4M4lCO/f6cNO0TpF
   LlNM6YyjWYQ38UDpirxgrp+ySOmCCFF+OjVC5AHsS+Orozv8IOWG8A8KKgryfNMs
   tLrLctIOXLL900J4DOP3noqEQnYOI9Qq9X7f2Zv+f1G2sp0qrA8+frrxyB9H1VKu
   Nqo+S2qq/c3d1EvDtVG8YYU4gCFeZzUq2nAsZcIoD157z7M512cQrCabLcZAYG4T
   /PwRQpb9EqPwzuEBPZq997VbzzWKzqOuJPx4TeT8ksJawZzvs0/Gi5YL8inCV0Hx
   vz2vmsWlL2sDDCus6vcl7X5pOqckNW5A7J/uGOXylkb2ZxTR0xP1wd4P3Ncw0S8m
   3TVIiSKsNDHd3/ZEBkTeVIcmkprNeApZ6toTc3/izJO2OgLDtdjfu85nEVTIsalg
   Syq8uGagBIQPpNb/EmICF1s78/b7MPu/NtF47Z0j8LIljS5xac1s/mT9XOEPw28z
   ZmL6/5I+UKMKsJuaoSAJ5TcK13TONCdOteBt0dxMZHbw4Ix/YKESkCFu9B3IyoLq
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   qCwlzzZMs/P+PSkZtwwsQ9Zkn1b/wq1AFDqxjs3cysQeBLt0wAGBIRtnetvsWht9
   yxAMLanLX01Wh8PtNewJY2LZZkhkOWCxP30VSqrzmwhGyX6lwMH2AAv+mu6hD3ci
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   RTit8UQJBGqReS9YJGh14U2ra1dvKLoZHIZdyxob12fu4QkTDAjGIvDzYuxuVaZL
   W0NaHpBNIlOQUitx5e6JvyjIKtwM6Y/3/0o9pInhXDezk3t78NYctFR08xFQY3LJ
   DN3S2EgXj1jWmd5E0/z+Tccg7d8hEn+0vVCRRQksqiPIEcZ1f/xgfm01FOfnI1Pb
   OJfUSuZpTvnWtvCTOn62XmWj+4jzxBmopauAqf9XzDj6NsHGkrPVrdotEhFoYYRu
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   jCz1WNqQXPFny6j60BJzZ8wd6nFshHcYbvCP+BKxx7WB3j5Pqxr3/s9S9daCgMQ4
   gWiPMOzuSgoTz2ggjqv31QMAXvkBSE+DIauh9BPw5pwoMsdMYT9eV+DrbN4dhy6t
   P/4zCB4NQcyU2vP8P9piBLhcjunadSdITTna3D/fA6VdhidmuF5ieCzo1sTAGH6H
   /VRPjxvA9gBeDtko120xoIaLpBF7I75UuFziIzuGuSE1lAf1S+I4NOD9tw0Gw+xU
   /lvzqk4NHZ/j91GvRxTRj0eFWRuTKXDvVj6Z07vW1l8tJs+IpslaZgo5/sE7Ntx/
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   qIcxIldwnbKDufmd6kKgu66wjtfxKcGK+JQ09r2G+E0vDHLO3CUHjVafLEN1Rwt9
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   8AsdOxVIbgYYalaB01pWcQE/jRv4D7cO0D2OM1DQzED9Ydzvl51jHE+71LVUbSkA

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   LQoYXJzLNj16DRYbSynXXFiRPmgAq9sfPEf+CoR47zpQUVXACRPLieRSDajlnj/U
   XaoLV6JVFLY7+FQeW/W0YElIz4R2NJXdBXtaNNBjLnrS+8sW99cVY/yzMUjsohys
   5Vjun8GPVRYVyAx003J5bdzefPLxoUhy7Of46lJxL0kBELzWAtCMm+MwBbrJCphS
   0PlziAmYr5EGUEhA2pmv5O5Ok83Z7C4lmdbrRDraw++N0fq7mSm9ZgJRwbslrP+D
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   1OtoJc+0XZ617xSFoKWnyA==

C.1.4.1.  S/MIME Signed and Encrypted Over a Simple Message, No Header
          Protection, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

   MIILPAYJKoZIhvcNAQcCoIILLTCCCykCAQExDTALBglghkgBZQMEAgEwggFlBgkq
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   dGlvbiBBdXRob3JpdHkwIBcNMTkxMTIwMDY1NDE4WhgPMjA1MjA5MjcwNjU0MTha

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   MDsxDTALBgNVBAoTBElFVEYxETAPBgNVBAsTCExBTVBTIFdHMRcwFQYDVQQDEw5B
   bGljZSBMb3ZlbGFjZTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAJqV
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   eBAuDLsatbtKtl4FzkgRyOg31/+Cw7H8e30iLrPIFlWN1qjHrjgOyIs5AQ/hgxLv
   Lir3hEUV2Z3MRsMtjH2x9SG91PEM046gfPnc9gMGHjMTg1qvaKcLQP5UzpEYPLro
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C.1.4.2.  S/MIME Signed and Encrypted Over a Simple Message, No Header
          Protection, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses no header protection.

   --
   Alice
   alice@smime.example

C.1.5.  No Cryptographic Protections Over a Complex Message

   This message uses no cryptographic protection at all.  Its body is a
   multipart/alternative message with an inline image/png attachment.

   It has the following structure:

   └┬╴multipart/mixed 1402 bytes
    ├┬╴multipart/alternative 794 bytes
    │├─╴text/plain 206 bytes
    │└─╴text/html 304 bytes
    └─╴image/png inline 232 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="e68"
   Subject: no-crypto-complex
   Message-ID: <no-crypto-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:00:02 -0500
   User-Agent: Sample MUA Version 1.0

   --e68
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="f70"

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   --f70
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   no-crypto-complex
   message.

   This message uses no cryptographic protection at all.  Its body
   is a multipart/alternative message with an inline image/png
   attachment.

   --
   Alice
   alice@smime.example
   --f70
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>no-crypto-complex</b>
   message.</p>
   <p>This message uses no cryptographic protection at all.  Its body
   is a multipart/alternative message with an inline image/png
   attachment.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --f70--

   --e68
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --e68--

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C.1.6.  S/MIME Signed-only signedData Over a Complex Message, No Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline image/png
   attachment.  It uses no header protection.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 5253 bytes
    ⇩ (unwraps to)
    └┬╴multipart/mixed 1288 bytes
     ├┬╴multipart/alternative 882 bytes
     │├─╴text/plain 260 bytes
     │└─╴text/html 355 bytes
     └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-complex
   Message-ID: <smime-one-part-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:01:02 -0500
   User-Agent: Sample MUA Version 1.0

   MIIPIwYJKoZIhvcNAQcCoIIPFDCCDxACAQExDTALBglghkgBZQMEAgEwggVMBgkq
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   b2FkIGlzIGEgbXVsdGlwYXJ0L2FsdGVybmF0aXZlIG1lc3NhZ2Ugd2l0aCBhbiBp

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   bmxpbmUNCmltYWdlL3BuZyBhdHRhY2htZW50LiBJdCB1c2VzIG5vIGhlYWRlciBw
   cm90ZWN0aW9uLjwvcD4NCjxwPjx0dD4tLSA8YnIvPkFsaWNlPGJyLz5hbGljZUBz
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   yrZlRAzEf7GHqgB/NyjOad3pdpVYeDh4ciNKjbs+aEoTWgAkoqENt1sRxlcvb7HV

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   X524bKZa1oPTUNlm6QpivtqDIdqGJdGf8L1zLfXBuo2zL3HR+M9CDr4Opq2JCkzP
   0Qhp7poIccGE6I9Tsg+RrOA9iCQsPn1+Tg8YedjGzUWF07rNmT0TzPCVzUAuBlr+
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   qRbwZbtOjbrhAszzkRG5P8jR+35FIkG6abSF8hwYix0fJokUn3YnU7G6pRM7DSGg
   S9MtDUy34GTkdUQ7OXFlLa5kpQfUFBbQ5qflKUvIrBsYX6qjWAVs

C.1.6.1.  S/MIME Signed-only signedData Over a Complex Message, No
          Header Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="533"

   --533
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="931"

   --931
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part-complex
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses no header protection.

   --
   Alice
   alice@smime.example
   --931
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

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   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-one-part-complex</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses no header protection.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --931--

   --533
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --533--

C.1.7.  S/MIME Signed-only multipart/signed Over a Complex Message, No
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a multipart/alternative message
   with an inline image/png attachment.  It uses no header protection.

   It has the following structure:

   └┬╴multipart/signed 5230 bytes
    ├┬╴multipart/mixed 1344 bytes
    │├┬╴multipart/alternative 938 bytes
    ││├─╴text/plain 278 bytes
    ││└─╴text/html 376 bytes
    │└─╴image/png inline 232 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="4e5";
    micalg="sha-256"
   Subject: smime-multipart-complex
   Message-ID: <smime-multipart-complex@example>
   From: Alice <alice@smime.example>

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   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:02:02 -0500
   User-Agent: Sample MUA Version 1.0

   --4e5
   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="0be"

   --0be
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="cb6"

   --cb6
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart-complex
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no header protection.

   --
   Alice
   alice@smime.example
   --cb6
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-multipart-complex</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no header protection.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --cb6--

   --0be
   Content-Type: image/png
   Content-Transfer-Encoding: base64

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   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --0be--

   --4e5
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   --4e5--

C.1.8.  S/MIME Signed and Encrypted Over a Complex Message, No Header
        Protection

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses no
   header protection.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 8710 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 5434 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 1356 bytes
      ├┬╴multipart/alternative 950 bytes
      │├─╴text/plain 295 bytes
      │└─╴text/html 390 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

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   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: smime-signed-enc-complex
   Message-ID: <smime-signed-enc-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:03:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.1.8.1.  S/MIME Signed and Encrypted Over a Complex Message, No Header
          Protection, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   165ernT9O5NLFflHUjURca3ynqEBBV4DmhnZp8eDhv3t6dXyCjNHT82S6DgCReZu

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   rxSLzwv5QtKJCaDfYw==

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C.1.8.2.  S/MIME Signed and Encrypted Over a Complex Message, No Header
          Protection, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="508"

   --508
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="804"

   --804
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no header protection.

   --
   Alice
   alice@smime.example
   --804
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no header protection.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --804--

   --508
   Content-Type: image/png
   Content-Transfer-Encoding: base64

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   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --508--

C.2.  Signed-only Messages

   These messages are signed-only, using different schemes of header
   protection and different S/MIME structure.  The use no Header
   Confidentiality Policy because the hcp is only relevant when a
   message is encrypted.

C.2.1.  S/MIME Signed-only signedData Over a Simple Message, Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message.  It uses the Header Protection
   scheme from the draft.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 4189 bytes
    ⇩ (unwraps to)
    └─╴text/plain 233 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-hp
   Message-ID: <smime-one-part-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:06:02 -0500
   User-Agent: Sample MUA Version 1.0

   MIIMEAYJKoZIhvcNAQcCoIIMATCCC/0CAQExDTALBglghkgBZQMEAgEwggI5Bgkq
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   VXNlci1BZ2VudDogU2FtcGxlIE1VQSBWZXJzaW9uIDEuMA0KQ29udGVudC1UeXBl

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

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C.2.1.1.  S/MIME Signed-only signedData Over a Simple Message, Header
          Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-one-part-hp
   Message-ID: <smime-one-part-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:06:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="clear"

   This is the
   smime-one-part-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message. It uses the Header Protection
   scheme from the draft.

   --
   Alice
   alice@smime.example

C.2.2.  S/MIME Signed-only multipart/signed Over a Simple Message,
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a text/plain message.  It uses
   the Header Protection scheme from the draft.

   It has the following structure:

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   └┬╴multipart/signed 4435 bytes
    ├─╴text/plain 250 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="78f";
    micalg="sha-256"
   Subject: smime-multipart-hp
   Message-ID: <smime-multipart-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:07:02 -0500
   User-Agent: Sample MUA Version 1.0

   --78f
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-multipart-hp
   Message-ID: <smime-multipart-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:07:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="clear"

   This is the
   smime-multipart-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft.

   --
   Alice
   alice@smime.example

   --78f
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   iT9IwijZa6psfTSFjzUWTIc0jGx3GcLZr+BIm+MEBCSRzDum

   --78f--

C.2.3.  S/MIME Signed-only signedData Over a Complex Message, Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline image/png
   attachment.  It uses the Header Protection scheme from the draft.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 5647 bytes
    ⇩ (unwraps to)
    └┬╴multipart/mixed 1570 bytes
     ├┬╴multipart/alternative 934 bytes
     │├─╴text/plain 287 bytes
     │└─╴text/html 382 bytes
     └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-complex-hp
   Message-ID: <smime-one-part-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:06:02 -0500
   User-Agent: Sample MUA Version 1.0

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   KRV8ASPw546vzfN3DzAfBgNVHSMEGDAWgBSRMI58BxcMp/EJKGU2GmccaHb0WTAN
   BgkqhkiG9w0BAQ0FAAOCAQEAgUl4oJyxMpwWpAylOvK6NEbMl1gD5H14EC4Muxq1
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   I2juwdRrjFBSXkk7pWpjXwsA3A5GOtz0FpfgyC7OxsVcF7q4WHWZWleYXFKlQHJD

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   73nQwXP968+A/3rBX7PhO0DBbZnfitOLPgPEwjTtdg0VQQ6Wz+CRQ/YbHPKaw7aR
   phZO63dKvIKp4cQVtkWQHi6syTjGsgkLcLNau5LZDQUdsGV+SAo3nBdWCRYV+I65
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   TF/KtmVEDMR/sYeqAH83KM5p3el2lVh4OHhyI0qNuz5oShNaACSioQ23WxHGVy9v
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   0Or/+EuyWAzp0r/GCsSGXt2owaAkGPuZf6kPc0mLhb/VFdeY16wy9J0=

C.2.3.1.  S/MIME Signed-only signedData Over a Complex Message, Header
          Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-one-part-complex-hp
   Message-ID: <smime-one-part-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:06:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="e2e"; hp="clear"

   --e2e
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="200"

   --200
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

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   This is the
   smime-one-part-complex-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the Header Protection scheme from
   the draft.

   --
   Alice
   alice@smime.example
   --200
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-one-part-complex-hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the Header Protection scheme from
   the draft.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --200--

   --e2e
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --e2e--

C.2.4.  S/MIME Signed-only multipart/signed Over a Complex Message,
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a multipart/alternative message
   with an inline image/png attachment.  It uses the Header Protection
   scheme from the draft.

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   It has the following structure:

   └┬╴multipart/signed 5520 bytes
    ├┬╴multipart/mixed 1628 bytes
    │├┬╴multipart/alternative 990 bytes
    ││├─╴text/plain 304 bytes
    ││└─╴text/html 402 bytes
    │└─╴image/png inline 232 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="ba4";
    micalg="sha-256"
   Subject: smime-multipart-complex-hp
   Message-ID: <smime-multipart-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:07:02 -0500
   User-Agent: Sample MUA Version 1.0

   --ba4
   MIME-Version: 1.0
   Subject: smime-multipart-complex-hp
   Message-ID: <smime-multipart-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:07:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="b14"; hp="clear"

   --b14
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="f1a"

   --f1a
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart-complex-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a

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   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft.

   --
   Alice
   alice@smime.example
   --f1a
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-multipart-complex-hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --f1a--

   --b14
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --b14--

   --ba4
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

   MIIJ4AYJKoZIhvcNAQcCoIIJ0TCCCc0CAQExDTALBglghkgBZQMEAgEwCwYJKoZI
   hvcNAQcBoIIHpjCCA88wggK3oAMCAQICEw8tJb0ROZdKzkJUh6HuPTQGirQwDQYJ
   KoZIhvcNAQENBQAwVTENMAsGA1UEChMESUVURjERMA8GA1UECxMITEFNUFMgV0cx
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   acKTg8cc2OtJ9ZSed6U3jUoiZVpMLcP3MUKtLeLg9r1mAfIDlB/wlbdmadXPmrsz
   yidmbuZmOpB5voVQfiLYYy3iOx7YOqzXrl6udP07k0sV+UdSNRFxrfKeoQEFXgOa
   Gdmnx4OG/e3p1fIKM0dPzZLoOAJF5m5O0xzXPL74zFCWp2f1ZkuE4A6l41koaZXC

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   7XV/QqmiXDr0+GrOmqnXMA0GCSqGSIb3DQEBDQUAMFUxDTALBgNVBAoTBElFVEYx
   ETAPBgNVBAsTCExBTVBTIFdHMTEwLwYDVQQDEyhTYW1wbGUgTEFNUFMgUlNBIENl
   cnRpZmljYXRpb24gQXV0aG9yaXR5MCAXDTE5MTEyMDA2NTQxOFoYDzIwNTIwOTI3
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   --ba4--

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C.2.5.  S/MIME Signed-only signedData Over a Complex Message, Legacy RFC
        8551 Header Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline image/png
   attachment.  It uses the legacy RFC 8551 header protection
   (RFC8551HP) scheme.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 5696 bytes
    ⇩ (unwraps to)
    └┬╴message/rfc822 1660 bytes
     └┬╴multipart/mixed 1612 bytes
      ├┬╴multipart/alternative 974 bytes
      │├─╴text/plain 296 bytes
      │└─╴text/html 394 bytes
      └─╴image/png inline 232 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-complex-rfc8551hp
   Message-ID: <smime-one-part-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:26:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.2.5.1.  S/MIME Signed-only signedData Over a Complex Message, Legacy
          RFC 8551 Header Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: message/rfc822

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="e68"
   Subject: smime-one-part-complex-rfc8551hp
   Message-ID: <smime-one-part-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:26:02 -0500
   User-Agent: Sample MUA Version 1.0

   --e68
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="bba"

   --bba
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0

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   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part-complex-rfc8551hp
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the legacy RFC 8551 header
   protection (RFC8551HP) scheme.

   --
   Alice
   alice@smime.example
   --bba
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-one-part-complex-rfc8551hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the legacy RFC 8551 header
   protection (RFC8551HP) scheme.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --bba--

   --e68
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --e68--

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C.2.6.  S/MIME Signed-only multipart/signed Over a Complex Message,
        Legacy RFC 8551 Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a multipart/alternative message
   with an inline image/png attachment.  It uses the legacy RFC 8551
   header protection (RFC8551HP) scheme.

   It has the following structure:

   └┬╴multipart/signed 5624 bytes
    ├┬╴message/rfc822 1718 bytes
    │└┬╴multipart/mixed 1670 bytes
    │ ├┬╴multipart/alternative 1030 bytes
    │ │├─╴text/plain 324 bytes
    │ │└─╴text/html 422 bytes
    │ └─╴image/png inline 232 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="a61";
    micalg="sha-256"
   Subject: smime-multipart-complex-rfc8551hp
   Message-ID: <smime-multipart-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:27:02 -0500
   User-Agent: Sample MUA Version 1.0

   --a61
   MIME-Version: 1.0
   Content-Type: message/rfc822

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="91c"
   Subject: smime-multipart-complex-rfc8551hp
   Message-ID: <smime-multipart-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:27:02 -0500
   User-Agent: Sample MUA Version 1.0

   --91c
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="b87"

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   --b87
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart-complex-rfc8551hp
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 header protection
   (RFC8551HP) scheme.

   --
   Alice
   alice@smime.example
   --b87
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-multipart-complex-rfc8551hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 header protection
   (RFC8551HP) scheme.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --b87--

   --91c
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --91c--

   --a61

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   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   9w0BCQMxCwYJKoZIhvcNAQcBMBwGCSqGSIb3DQEJBTEPFw0yMTAyMjAxNzI3MDJa
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   --a61--

C.3.  Signed-and-Encrypted Messages

   These messages are signed and encrypted.  They use PKCS#7 signedData
   inside envelopedData, with different header protection schemes and
   different Header Confidentiality Policies.

C.3.1.  S/MIME Signed and Encrypted Over a Simple Message, Header
        Protection With hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_baseline Header Confidentiality Policy.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 7825 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 4786 bytes
     ⇩ (unwraps to)
     └─╴text/plain 329 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:09:02 -0500
   User-Agent: Sample MUA Version 1.0

   MIIWjAYJKoZIhvcNAQcDoIIWfTCCFnkCAQAxggMQMIIBhAIBADBsMFUxDTALBgNV
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   UFMgUlNBIENlcnRpZmljYXRpb24gQXV0aG9yaXR5AhMPLSW9ETmXSs5CVIeh7j00

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C.3.1.1.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   G5uhNtW6SURCQjN+d6PNa182OqCW3w==

C.3.1.2.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline
   Message-ID: <smime-signed-enc-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:09:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-baseline@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:09:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-baseline
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

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C.3.2.  S/MIME Signed and Encrypted Over a Simple Message, Header
        Protection With hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_baseline Header Confidentiality Policy with a "Legacy
   Display" part.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 8085 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 4968 bytes
     ⇩ (unwraps to)
     └─╴text/plain 414 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:10:02 -0500
   User-Agent: Sample MUA Version 1.0

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   WhVik6d6oJaGviNjcZaw4C5kuZ5bKHUCiMLv05uAtQOOyPiddgfZXymBoKCjndge
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   lRXXfx7senJDd2tXmXvYkA==

C.3.2.1.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_baseline (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   WxxyPU2C+RwECSI=

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C.3.2.2.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_baseline (+ Legacy Display), Decrypted and
          Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline-legacy
   Message-ID: <smime-signed-enc-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:10:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-baseline-legacy@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:10:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-baseline-legacy

   This is the
   smime-signed-enc-hp-baseline-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy with a
   "Legacy Display" part.

   --
   Alice
   alice@smime.example

C.3.3.  S/MIME Signed and Encrypted Over a Simple Message, Header
        Protection With hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_shy Header Confidentiality Policy.

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   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 7760 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 4732 bytes
     ⇩ (unwraps to)
     └─╴text/plain 319 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:12:02 +0000
   User-Agent: Sample MUA Version 1.0

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

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Gillmor, et al.           Expires 8 March 2025                [Page 138]
Internet-Draft    Cryptographic MIME Header Protection    September 2024

C.3.3.1.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

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C.3.3.2.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

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   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy
   Message-ID: <smime-signed-enc-hp-shy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:12:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-shy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:12:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-shy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.4.  S/MIME Signed and Encrypted Over a Simple Message, Header
        Protection With hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_shy Header Confidentiality Policy with a "Legacy Display"
   part.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 8170 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 5046 bytes
     ⇩ (unwraps to)
     └─╴text/plain 502 bytes

   Its contents are:

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   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy-legacy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:13:02 +0000
   User-Agent: Sample MUA Version 1.0

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C.3.4.1.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

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   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.4.2.  S/MIME Signed and Encrypted Over a Simple Message, Header
          Protection With hcp_shy (+ Legacy Display), Decrypted and
          Unwrapped

   The inner signed-data layer unwraps to:

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   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy-legacy
   Message-ID: <smime-signed-enc-hp-shy-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:13:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-shy-legacy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:13:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-shy-legacy
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:13:02 -0500

   This is the
   smime-signed-enc-hp-shy-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy with a "Legacy
   Display" part.

   --
   Alice
   alice@smime.example

C.3.5.  S/MIME Signed and Encrypted Reply Over a Simple Message, Header
        Protection With hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_baseline Header Confidentiality Policy.

   It has the following structure:

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   └─╴application/pkcs7-mime [smime.p7m] 8300 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 5136 bytes
     ⇩ (unwraps to)
     └─╴text/plain 335 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline@example>
   References: <smime-signed-enc-hp-baseline@example>

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   fqdfr1EdJoe+GEdrT8J3NVl1AYzS3t3zTQdQ5yNzrP0kVyOUIbiyd5MpNBxLquLS
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   j4nVpvjAa34DWgZ+RgQCXF95kSztyoSAL3Jnq1fQOZ8=

C.3.5.1.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.5.2.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline-reply
   Message-ID: <smime-signed-enc-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline@example>
   References: <smime-signed-enc-hp-baseline@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-baseline-reply@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:15:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-hp-baseline@example>
   HP-Outer: References: <smime-signed-enc-hp-baseline@example>
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-baseline-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.6.  S/MIME Signed and Encrypted Reply Over a Simple Message, Header
        Protection With hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_baseline Header Confidentiality Policy with a "Legacy
   Display" part.

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   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 8625 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 5368 bytes
     ⇩ (unwraps to)
     └─╴text/plain 426 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline-legacy@example>
   References: <smime-signed-enc-hp-baseline-legacy@example>

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   LHK2foRU983nLGdDrp2q4TWCoMGVSmOwBasUjVHiUA8=

C.3.6.1.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_baseline (+ Legacy Display),
          Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.6.2.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_baseline (+ Legacy Display),
          Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

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   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline-legacy-reply
   Message-ID: <smime-signed-enc-hp-baseline-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline-legacy@example>
   References: <smime-signed-enc-hp-baseline-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-baseline-legacy-reply@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:16:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer:
    In-Reply-To: <smime-signed-enc-hp-baseline-legacy@example>
   HP-Outer:
    References: <smime-signed-enc-hp-baseline-legacy@example>
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-baseline-legacy-reply

   This is the
   smime-signed-enc-hp-baseline-legacy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy with a
   "Legacy Display" part.

   --
   Alice
   alice@smime.example

C.3.7.  S/MIME Signed and Encrypted Reply Over a Simple Message, Header
        Protection With hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_shy Header Confidentiality Policy.

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   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 8190 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 5054 bytes
     ⇩ (unwraps to)
     └─╴text/plain 325 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:18:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy@example>
   References: <smime-signed-enc-hp-shy@example>

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   J9Hv77iPBZMmURHGiRkK0hBxYlRGUFZm/6/Y/aX4vG/1K+A8l2ksWdLpqXRQpcuD
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C.3.7.1.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   K7iaKJzRBDxl4Qt/m6VHxtvkTXjkOOD+7g==

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C.3.7.2.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy-reply
   Message-ID: <smime-signed-enc-hp-shy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:18:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy@example>
   References: <smime-signed-enc-hp-shy@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-shy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:18:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-hp-shy@example>
   HP-Outer: References: <smime-signed-enc-hp-shy@example>
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-shy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.8.  S/MIME Signed and Encrypted Reply Over a Simple Message, Header
        Protection With hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from the draft with
   the hcp_shy Header Confidentiality Policy with a "Legacy Display"
   part.

   It has the following structure:

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   └─╴application/pkcs7-mime [smime.p7m] 8690 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 5418 bytes
     ⇩ (unwraps to)
     └─╴text/plain 514 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy-legacy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:19:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy-legacy@example>
   References: <smime-signed-enc-hp-shy-legacy@example>

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Gillmor, et al.           Expires 8 March 2025                [Page 167]
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Gillmor, et al.           Expires 8 March 2025                [Page 168]
Internet-Draft    Cryptographic MIME Header Protection    September 2024

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Gillmor, et al.           Expires 8 March 2025                [Page 169]
Internet-Draft    Cryptographic MIME Header Protection    September 2024

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   HNeqqUVBSzDveFdaylcw2HkJpm8D9BoC3Y0n/WMW5VE=

C.3.8.1.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   lIw=

C.3.8.2.  S/MIME Signed and Encrypted Reply Over a Simple Message,
          Header Protection With hcp_shy (+ Legacy Display), Decrypted
          and Unwrapped

   The inner signed-data layer unwraps to:

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   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy-legacy-reply
   Message-ID: <smime-signed-enc-hp-shy-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:19:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy-legacy@example>
   References: <smime-signed-enc-hp-shy-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-shy-legacy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:19:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-hp-shy-legacy@example>
   HP-Outer: References: <smime-signed-enc-hp-shy-legacy@example>
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-shy-legacy-reply
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:19:02 -0500

   This is the
   smime-signed-enc-hp-shy-legacy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy with a "Legacy
   Display" part.

   --
   Alice
   alice@smime.example

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C.3.9.  S/MIME Signed and Encrypted Over a Complex Message, Header
        Protection With hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_baseline Header
   Confidentiality Policy.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 10035 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 6412 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2054 bytes
      ├┬╴multipart/alternative 1124 bytes
      │├─╴text/plain 383 bytes
      │└─╴text/html 478 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:09:02 -0500
   User-Agent: Sample MUA Version 1.0

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

C.3.9.1.  S/MIME Signed and Encrypted Over a Complex Message, Header
          Protection With hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   22OH4ConlB8f7R7s1LM2cSYxggIAMIIB/AIBATBsMFUxDTALBgNVBAoTBElFVEYx

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   ETAPBgNVBAsTCExBTVBTIFdHMTEwLwYDVQQDEyhTYW1wbGUgTEFNUFMgUlNBIENl
   cnRpZmljYXRpb24gQXV0aG9yaXR5AhM3QQV57XV/QqmiXDr0+GrOmqnXMAsGCWCG
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   9Dgce5pbZYQgeB27mLBCpsgRgYKbQ/+NBPBexT6Kxixd4sND++AZ6kUie+AvUpXo
   +kGun/Q=

C.3.9.2.  S/MIME Signed and Encrypted Over a Complex Message, Header
          Protection With hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline
   Message-ID: <smime-signed-enc-complex-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:09:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-complex-hp-baseline@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:09:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="e03"; hp="cipher"

   --e03
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="799"

   --799
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-baseline
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a

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   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --799
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-baseline</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --799--

   --e03
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --e03--

C.3.10.  S/MIME Signed and Encrypted Over a Complex Message, Header
         Protection With hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_baseline Header
   Confidentiality Policy with a "Legacy Display" part.

   It has the following structure:

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   └─╴application/pkcs7-mime [smime.p7m] 10640 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 6856 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2367 bytes
      ├┬╴multipart/alternative 1415 bytes
      │├─╴text/plain 476 bytes
      │└─╴text/html 636 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:10:02 -0500
   User-Agent: Sample MUA Version 1.0

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

C.3.10.1.  S/MIME Signed and Encrypted Over a Complex Message, Header
           Protection With hcp_baseline (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

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C.3.10.2.  S/MIME Signed and Encrypted Over a Complex Message, Header
           Protection With hcp_baseline (+ Legacy Display), Decrypted
           and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline-legacy
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:10:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:10:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="308"; hp="cipher"

   --308
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="fff"

   --fff
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

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   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-baseline-legacy

   This is the
   smime-signed-enc-complex-hp-baseline-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy with a
   "Legacy Display" part.

   --
   Alice
   alice@smime.example
   --fff
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-baseline-legacy
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-baseline-legacy</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy with a
   "Legacy Display" part.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --fff--

   --308
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA

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   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --308--

C.3.11.  S/MIME Signed and Encrypted Over a Complex Message, Header
         Protection With hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_shy Header
   Confidentiality Policy.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 9925 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 6342 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2003 bytes
      ├┬╴multipart/alternative 1104 bytes
      │├─╴text/plain 373 bytes
      │└─╴text/html 468 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-shy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:12:02 +0000
   User-Agent: Sample MUA Version 1.0

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C.3.11.1.  S/MIME Signed and Encrypted Over a Complex Message, Header
           Protection With hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

   MIIR/gYJKoZIhvcNAQcCoIIR7zCCEesCAQExDTALBglghkgBZQMEAgEwgggnBgkq
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   bjogaW5saW5lDQoNCmlWQk9SdzBLR2dvQUFBQU5TVWhFVWdBQUFCUUFBQUFVQ0FZ
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   ZmljYXRpb24gQXV0aG9yaXR5AhM3QQV57XV/QqmiXDr0+GrOmqnXMAsGCWCGSAFl
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   nzc=

C.3.11.2.  S/MIME Signed and Encrypted Over a Complex Message, Header
           Protection With hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy
   Message-ID: <smime-signed-enc-complex-hp-shy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:12:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-complex-hp-shy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:12:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="1fa"; hp="cipher"

   --1fa
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="601"

   --601
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-shy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft

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   with the hcp_shy Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --601
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-shy</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --601--

   --1fa
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --1fa--

C.3.12.  S/MIME Signed and Encrypted Over a Complex Message, Header
         Protection With hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_shy Header
   Confidentiality Policy with a "Legacy Display" part.

   It has the following structure:

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   └─╴application/pkcs7-mime [smime.p7m] 10920 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 7072 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2519 bytes
      ├┬╴multipart/alternative 1597 bytes
      │├─╴text/plain 564 bytes
      │└─╴text/html 736 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-shy-legacy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:13:02 +0000
   User-Agent: Sample MUA Version 1.0

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C.3.12.1.  S/MIME Signed and Encrypted Over a Complex Message, Header
           Protection With hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   LTA1MDANCjwvcHJlPg0KPC9kaXY+PHA+VGhpcyBpcyB0aGUNCjxiPnNtaW1lLXNp

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   AQ/hgxLvLir3hEUV2Z3MRsMtjH2x9SG91PEM046gfPnc9gMGHjMTg1qvaKcLQP5U
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   ElnAtJ7BtZcsl7dUy9u9COgEykRiVokFQgqQ7XNDU+r3SeOWwks7AgMBAAGjga8w

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   gawwDAYDVR0TAQH/BAIwADAXBgNVHSAEEDAOMAwGCmCGSAFlAwIBMAEwHgYDVR0R
   BBcwFYETYWxpY2VAc21pbWUuZXhhbXBsZTATBgNVHSUEDDAKBggrBgEFBQcDBDAO
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   U3Zqnl/G

C.3.12.2.  S/MIME Signed and Encrypted Over a Complex Message, Header
           Protection With hcp_shy (+ Legacy Display), Decrypted and
           Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy-legacy
   Message-ID: <smime-signed-enc-complex-hp-shy-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:13:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-complex-hp-shy-legacy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:13:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="cd5"; hp="cipher"

   --cd5
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="582"

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   --582
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-shy-legacy
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:13:02 -0500

   This is the
   smime-signed-enc-complex-hp-shy-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy with a "Legacy
   Display" part.

   --
   Alice
   alice@smime.example
   --582
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-shy-legacy
   From: Alice &lt;alice@smime.example&gt;
   To: Bob &lt;bob@smime.example&gt;
   Date: Sat, 20 Feb 2021 12:13:02 -0500
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-shy-legacy</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy with a "Legacy
   Display" part.</p>

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   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --582--

   --cd5
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --cd5--

C.3.13.  S/MIME Signed and Encrypted Reply Over a Complex Message,
         Header Protection With hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_baseline Header
   Confidentiality Policy.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 10575 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 6820 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2345 bytes
      ├┬╴multipart/alternative 1136 bytes
      │├─╴text/plain 389 bytes
      │└─╴text/html 484 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-baseline@example>

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   References: <smime-signed-enc-complex-hp-baseline@example>

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C.3.13.1.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.13.2.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

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   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline-reply
   Message-ID: <smime-signed-enc-complex-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-baseline@example>
   References: <smime-signed-enc-complex-hp-baseline@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-baseline-reply@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:15:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer:
    In-Reply-To: <smime-signed-enc-complex-hp-baseline@example>
   HP-Outer:
    References: <smime-signed-enc-complex-hp-baseline@example>
   Content-Type: multipart/mixed; boundary="b2f"; hp="cipher"

   --b2f
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="6e8"

   --6e8
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-baseline-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --6e8
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

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   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-baseline-reply</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --6e8--

   --b2f
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   --b2f--

C.3.14.  S/MIME Signed and Encrypted Reply Over a Complex Message,
         Header Protection With hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_baseline Header
   Confidentiality Policy with a "Legacy Display" part.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 11205 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 7278 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2666 bytes
      ├┬╴multipart/alternative 1419 bytes
      │├─╴text/plain 478 bytes
      │└─╴text/html 638 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

Gillmor, et al.           Expires 8 March 2025                [Page 214]
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   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-lgc-rpl@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   References:
    <smime-signed-enc-complex-hp-baseline-legacy@example>

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Gillmor, et al.           Expires 8 March 2025                [Page 218]
Internet-Draft    Cryptographic MIME Header Protection    September 2024

C.3.14.1.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_baseline (+ Legacy Display),
           Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.14.2.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_baseline (+ Legacy Display),
           Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline-lgc-rpl
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-lgc-rpl@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   References:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-baseline-lgc-rpl@example>
   HP-Outer: From: Alice <alice@smime.example>

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   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:16:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   HP-Outer: References:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   Content-Type: multipart/mixed; boundary="63c"; hp="cipher"

   --63c
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="802"

   --802
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-baseline-lgc-rpl

   This is the
   smime-signed-enc-complex-hp-baseline-lgc-rpl
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy with a
   "Legacy Display" part.

   --
   Alice
   alice@smime.example
   --802
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-baseline-lgc-rpl
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-baseline-lgc-rpl</b>

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   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_baseline Header Confidentiality Policy with a
   "Legacy Display" part.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --802--

   --63c
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --63c--

C.3.15.  S/MIME Signed and Encrypted Reply Over a Complex Message,
         Header Protection With hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_shy Header
   Confidentiality Policy.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 10445 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 6716 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2273 bytes
      ├┬╴multipart/alternative 1116 bytes
      │├─╴text/plain 379 bytes
      │└─╴text/html 474 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

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   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-shy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:18:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy@example>
   References: <smime-signed-enc-complex-hp-shy@example>

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   +Hvwh/mIF5Uc0hssRDYNn9YxYA0jCLsjpxjMcDJCMUA=

C.3.15.1.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   Pg0KUmVmZXJlbmNlczogPHNtaW1lLXNpZ25lZC1lbmMtY29tcGxleC1ocC1zaHlA

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   ZXhhbXBsZT4NCkhQLU91dGVyOiBTdWJqZWN0OiBbLi4uXQ0KSFAtT3V0ZXI6DQog
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C.3.15.2.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy-reply
   Message-ID: <smime-signed-enc-complex-hp-shy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:18:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy@example>
   References: <smime-signed-enc-complex-hp-shy@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-complex-hp-shy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:18:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-complex-hp-shy@example>
   HP-Outer: References: <smime-signed-enc-complex-hp-shy@example>
   Content-Type: multipart/mixed; boundary="46f"; hp="cipher"

   --46f
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="fa5"

   --fa5
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-shy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --fa5

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   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-shy-reply</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --fa5--

   --46f
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --46f--

C.3.16.  S/MIME Signed and Encrypted Reply Over a Complex Message,
         Header Protection With hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from the draft with the hcp_shy Header
   Confidentiality Policy with a "Legacy Display" part.

   It has the following structure:

   └─╴application/pkcs7-mime [smime.p7m] 11505 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 7508 bytes
     ⇩ (unwraps to)
     └┬╴multipart/mixed 2832 bytes
      ├┬╴multipart/alternative 1621 bytes
      │├─╴text/plain 576 bytes
      │└─╴text/html 748 bytes
      └─╴image/png inline 236 bytes

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   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-signed-enc-complex-hp-shy-legacy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:19:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy-legacy@example>
   References: <smime-signed-enc-complex-hp-shy-legacy@example>

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Internet-Draft    Cryptographic MIME Header Protection    September 2024

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Gillmor, et al.           Expires 8 March 2025                [Page 235]
Internet-Draft    Cryptographic MIME Header Protection    September 2024

C.3.16.1.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.16.2.  S/MIME Signed and Encrypted Reply Over a Complex Message,
           Header Protection With hcp_shy (+ Legacy Display), Decrypted
           and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy-legacy-reply
   Message-ID:
    <smime-signed-enc-complex-hp-shy-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:19:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy-legacy@example>
   References: <smime-signed-enc-complex-hp-shy-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-shy-legacy-reply@example>
   HP-Outer: From: alice@smime.example

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   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:19:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer:
    In-Reply-To: <smime-signed-enc-complex-hp-shy-legacy@example>
   HP-Outer:
    References: <smime-signed-enc-complex-hp-shy-legacy@example>
   Content-Type: multipart/mixed; boundary="d37"; hp="cipher"

   --d37
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="d3e"

   --d3e
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-shy-legacy-reply
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:19:02 -0500

   This is the
   smime-signed-enc-complex-hp-shy-legacy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy with a "Legacy
   Display" part.

   --
   Alice
   alice@smime.example
   --d3e
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-shy-legacy-reply

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   From: Alice &lt;alice@smime.example&gt;
   To: Bob &lt;bob@smime.example&gt;
   Date: Sat, 20 Feb 2021 12:19:02 -0500
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-shy-legacy-reply</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from the draft
   with the hcp_shy Header Confidentiality Policy with a "Legacy
   Display" part.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --d3e--

   --d37
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --d37--

C.3.17.  S/MIME Signed and Encrypted Over a Complex Message, Legacy RFC
         8551 Header Protection With hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   legacy RFC 8551 header protection (RFC8551HP) scheme with the
   hcp_baseline Header Confidentiality Policy.

   It has the following structure:

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   └─╴application/pkcs7-mime [smime.p7m] 9580 bytes
    ↧ (decrypts to)
    └─╴application/pkcs7-mime [smime.p7m] 6082 bytes
     ⇩ (unwraps to)
     └┬╴message/rfc822 1876 bytes
      └┬╴multipart/mixed 1828 bytes
       ├┬╴multipart/alternative 1166 bytes
       │├─╴text/plain 392 bytes
       │└─╴text/html 490 bytes
       └─╴image/png inline 232 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-enc-signed-complex-rfc8551hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:28:02 -0500
   User-Agent: Sample MUA Version 1.0

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   sG1/1VBptLyt3UY3jh1tUw==

C.3.17.1.  S/MIME Signed and Encrypted Over a Complex Message, Legacy
           RFC 8551 Header Protection With hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   4YMS7y4q94RFFdmdzEbDLYx9sfUhvdTxDNOOoHz53PYDBh4zE4Nar2inC0D+VM6R

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   GDy66K9l+D+bl8Wj9CyGUc1ppMNURexTg+z3web/eDOdu+F2MVtluLihne0Bp1GU
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   mX58BvQt6GHQZ4TR2NVDvq3z+/CAlzsR5xmNH1C+uDH99ORoy3w6CHmv4aTTmRM9
   S+uZXg==

C.3.17.2.  S/MIME Signed and Encrypted Over a Complex Message, Legacy
           RFC 8551 Header Protection With hcp_baseline, Decrypted and
           Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: message/rfc822

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="266"
   Subject: smime-enc-signed-complex-rfc8551hp-baseline

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   Message-ID:
    <smime-enc-signed-complex-rfc8551hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:28:02 -0500
   User-Agent: Sample MUA Version 1.0

   --266
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="db6"

   --db6
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-enc-signed-complex-rfc8551hp-baseline
   message.

   This is an encrypted and signed S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 header protection
   (RFC8551HP) scheme with the hcp_baseline Header Confidentiality
   Policy.

   --
   Alice
   alice@smime.example
   --db6
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-enc-signed-complex-rfc8551hp-baseline</b>
   message.</p>
   <p>This is an encrypted and signed S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 header protection
   (RFC8551HP) scheme with the hcp_baseline Header Confidentiality
   Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --db6--

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   --266
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --266--

Appendix D.  Composition Examples

   This section offers step-by-step examples of message composition.

D.1.  New message composition

   A typical MUA composition interface offers the user a place to
   indicate the message recipients, the subject, and the body.  Consider
   a composition window filled out by the user like so:

    .------------------------------------------------------.
   |                 Composing New Message          .----.  |
   |          +---------------------------------+  | Send | |
   |      To: | Alice <alice@example.net>       |   '----'  |
   |          +---------------------------------+---------+ |
   | Subject: | Handling the Jones contract               | |
   |          +-------------------------------------------+ |
   +--------------------------------------------------------+
   | Please review and approve or decline by Thursday, it's |
   | critical!                                              |
   |                                                        |
   | Thanks,                                                |
   | Bob                                                    |
   |                                                        |
   | --                                                     |
   | Bob Gonzalez                                           |
   | ACME, Inc.                                             |
   |                                                        |
   +--------------------------------------------------------+

              Figure 1: Example Message Composition Interface

   When Bob clicks "Send", his MUA generates values for Message-ID,
   From, and Date Header Fields, and converts the message body into the
   appropriate format.

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D.1.1.  Unprotected message

   The resulting message would look something like this if it was sent
   without cryptographic protections:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: Handling the Jones contract
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0

   Please review and approve or decline by Thursday, it's critical!

   Thanks,
   Bob

   --
   Bob Gonzalez
   ACME, Inc.

D.1.2.  Encrypted with hcp_baseline and Legacy Display

   Now consider the message to be generated if it is to be
   cryptographically signed and encrypted, using HCP hcp_baseline, and
   the legacy variable is set.

   For each Header Field, Bob's MUA passes its name and value through
   hcp_baseline.  This returns the same value for every Header Field,
   except that:

   hcp_baseline("Subject", "Handling the Jones contract") yields
   "[...]".

D.1.2.1.  Cryptographic Payload

   The Cryptographic Payload that will be signed and then encrypted is
   very similar to the unprotected message in Appendix D.1.1.  Note the
   addition of:

   *  The hp="cipher" parameter for the Content-Type

   *  The appropriate HP-Outer Header Field for Subject

   *  The hp-legacy-display="1" parameter for the Content-Type

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   *  The Legacy Display Element (the simple pseudo-header and its
      trailing newline) in the Main Body Part.

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: Handling the Jones contract
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   MIME-Version: 1.0
   HP-Outer: Date: Wed, 11 Jan 2023 16:08:43 -0500
   HP-Outer: From: Bob <bob@example.net>
   HP-Outer: To: Alice <alice@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <20230111T210843Z.1234@lhp.example>

   Subject: Handling the Jones contract

   Please review and approve or decline by Thursday, it's critical!

   Thanks,
   Bob

   --
   Bob Gonzalez
   ACME, Inc.

D.1.2.2.  External Header Section

   The Cryptographic Payload from Appendix D.1.2.1 is then wrapped in
   the appropriate Cryptographic Layers.  For this example, using S/
   MIME, it is wrapped in an application/pkcs7-mime; smime-type="signed-
   data" layer, which is in turn wrapped in an application/pkcs7-mime;
   smime-type="enveloped-data" layer.

   Then an external Header Section is applied to the outer MIME object,
   which looks like this:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: [...]
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   MIME-Version: 1.0

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   Note that the Subject Header Field has been obscured appropriately by
   hcp_baseline.  The output of the CMS enveloping operation is
   base64-encoded and forms the body of the message.

D.2.  Composing a Reply

   Next we consider a typical MUA reply interface, where we see Alice
   replying to Bob's message from Appendix D.1.

   When Alice clicks "Reply" to Bob's signed-and-encrypted message with
   Header Protection, she might see something like this:

    .--------------------------------------------------------.
   |  Replying to Bob ("Handling the Jones Contract") .----.  |
   |          +-----------------------------------+  | Send | |
   |      To: | Bob <bob@example.net>             |   '----'  |
   |          +-----------------------------------+---------+ |
   | Subject: | Re: Handling the Jones contract             | |
   |          +---------------------------------------------+ |
   +----------------------------------------------------------+
   | On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:           |
   |                                                          |
   | > Please review and approve or decline by Thursday,      |
   | > it's critical!                                         |
   | >                                                        |
   | > Thanks,                                                |
   | > Bob                                                    |
   | >                                                        |
   | > --                                                     |
   | > Bob Gonzalez                                           |
   | > ACME, Inc.                                             |
   |                                                          |
   | --                                                       |
   | Alice Jenkins                                            |
   | ACME, Inc.                                               |
   |                                                          |
   +----------------------------------------------------------+

            Figure 2: Example Message Reply Interface (unedited)

   Note that because Alice's MUA is aware of Header Protection, it knows
   what the correct Subject header is, even though it was obscured.  It
   also knows to avoid including the Legacy Display Element in the
   quoted/attributed text that it includes in the draft reply.

   Once Alice has edited the reply message, it might look something like
   this:

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    .--------------------------------------------------------.
   |  Replying to Bob ("Handling the Jones Contract") .----.  |
   |          +-----------------------------------+  | Send | |
   |      To: | Bob <bob@example.net>             |   '----'  |
   |          +-----------------------------------+---------+ |
   | Subject: | Re: Handling the Jones contract             | |
   |          +---------------------------------------------+ |
   +----------------------------------------------------------+
   | On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:           |
   |                                                          |
   | > Please review and approve or decline by Thursday,      |
   | > it's critical!                                         |
   |                                                          |
   | I'll get right on it, Bob!                               |
   |                                                          |
   | Regards,                                                 |
   | Alice                                                    |
   |                                                          |
   | --                                                       |
   | Alice Jenkins                                            |
   | ACME, Inc.                                               |
   |                                                          |
   +----------------------------------------------------------+

             Figure 3: Example Message Reply Interface (edited)

   When Alice clicks "Send", the MUA generates values for Message-ID,
   From, and Date Header Fields, populates the In-Reply-To, and
   References Header Fields, and also converts the reply body into the
   appropriate format.

D.2.1.  Unprotected message

   The resulting message would look something like this if it were to be
   sent without any cryptographic protections:

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   Date: Wed, 11 Jan 2023 16:48:22 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Re: Handling the Jones contract
   Message-ID: <20230111T214822Z.5678@lhp.example>
   In-Reply-To: <20230111T210843Z.1234@lhp.example>
   References: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0

   On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:

   > Please review and approve or decline by Thursday,
   > it's critical!

   I'll get right on it, Bob!

   Regards,
   Alice

   --
   Alice Jenkins
   ACME, Inc.

   Of course, this would leak not only the contents of Alice's message,
   but also the contents of Bob's initial message, as well as the
   Subject Header Field!  So Alice's MUA won't do that; it is going to
   create a signed-and-encrypted message to submit to the network.

D.2.2.  Encrypted with hcp_no_confidentiality and Legacy Display

   This example assumes that Alice's MUA uses hcp_no_confidentiality,
   not hcp_baseline.  That is, by default, it does not obscure or remove
   any Header Fields, even when encrypting.

   However, it follows the guidance in Section 6.1, and will make use of
   the HP-Outer field in the Cryptographic Payload of Bob's original
   message (Appendix D.1.2.1) to determine what to obscure.

   When crafting the Cryptographic Payload, its baseline HCP
   (hcp_no_confidentiality) leaves each field untouched.  To uphold the
   confidentiality of the sender's values when replying, the MUA
   executes the following steps (for brevity only Subject and Message-
   ID/In-Reply-To are shown):

   *  Extract the referenced header fields (see Section 4.2):

      -  refouter contains:

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         o  Date: Wed, 11 Jan 2023 16:08:43 -0500

         o  From: Bob <bob@example.net>

         o  To: Alice <alice@example.net>

         o  Subject: [...]

         o  Message-ID: <20230111T210843Z.1234@lhp.example>

      -  refprotected contains:

         o  Date: Wed, 11 Jan 2023 16:08:43 -0500

         o  From: Bob <bob@example.net>

         o  To: Alice <alice@example.net>

         o  Subject: Handling the Jones contract

         o  Message-ID: <20230111T210843Z.1234@lhp.example>

   *  Apply the response function:

      -  respond(refouter) contains:

         o  From: Alice <alice@example.net>

         o  To: Bob <bob@example.net>

         o  Subject: Re: [...]

         o  In-Reply-To: <20230111T210843Z.1234@lhp.example>

         o  References: <20230111T210843Z.1234@lhp.example>

      -  respond(refprotected) contains:

         o  From: Alice <alice@example.net>

         o  To: Bob <bob@example.net>

         o  Subject: Re: Handling the Jones contract

         o  In-Reply-To: <20230111T210843Z.1234@lhp.example>

         o  References: <20230111T210843Z.1234@lhp.example>

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   *  Compute the ephemeral response_hcp (see Section 6.1):

      -  Note that all headers except Subject are the same.

      -  confmap contains only ("Subject", "Re: Handling the Jones
         contract") -> "Re: [...]"

   Thus all Header Fields that were signed are passed through untouched.
   The reply's Subject is obscured as Subject: Re: [...] if and only if
   the user does not edit the subject line from that initially proposed
   by the MUA's reply interface.  If the user edits the subject line,
   e.g., to Subject: Re: Handling the Jones contract ASAP, the
   response_hcp will _not_ obscure it, and instead pass it through in
   the clear.

   For stronger header confidentiality, the replying MUA should use a
   reasonable HCP (not hcp_no_confidentiality).  Also recall that the
   local HCP is applied first, and that response_hcp is only applied to
   what is left unchanged by the local HCP.

D.2.2.1.  Cryptographic Payload

   Consequently, the Cryptographic Payload for Alice's reply looks like
   this:

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   Date: Wed, 11 Jan 2023 16:48:22 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Re: Handling the Jones contract
   Message-ID: <20230111T214822Z.5678@lhp.example>
   In-Reply-To: <20230111T210843Z.1234@lhp.example>
   References: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   MIME-Version: 1.0
   HP-Outer: Date: Wed, 11 Jan 2023 16:48:22 -0500
   HP-Outer: From: Alice <alice@example.net>
   HP-Outer: To: Bob <bob@example.net>
   HP-Outer: Subject: Re: [...]
   HP-Outer: Message-ID: <20230111T214822Z.5678@lhp.example>
   HP-Outer: In-Reply-To: <20230111T210843Z.1234@lhp.example>
   HP-Outer: References: <20230111T210843Z.1234@lhp.example>

   Subject: Re: Handling the Jones contract

   On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:

   > Please review and approve or decline by Thursday,
   > it's critical!

   I'll get right on it, Bob!

   Regards,
   Alice

   --
   Alice Jenkins
   ACME, Inc.

   Note the following features:

   *  the hp="cipher" parameter to Content-Type

   *  the appropriate HP-Outer Header Field for Subject,

   *  the hp-legacy-display="1" parameter for the Content-Type

   *  the Legacy Display Element (the simple pseudo-header and its
      trailing newline) in the Main Body Part.

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D.2.2.2.  External Header Section

   The Cryptographic Payload from Appendix D.2.2.1 is then wrapped in
   the appropriate Cryptographic Layers.  For this example, using S/
   MIME, it is wrapped in an application/pkcs7-mime; smime-type="signed-
   data" layer, which is in turn wrapped in an application/pkcs7-mime;
   smime-type="enveloped-data" layer.

   Then an external Header Section is applied to the outer MIME object,
   which looks like this:

   Date: Wed, 11 Jan 2023 16:48:22 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Re: [...]
   Message-ID: <20230111T214822Z.5678@lhp.example>
   In-Reply-To: <20230111T210843Z.1234@lhp.example>
   References: <20230111T210843Z.1234@lhp.example>
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   MIME-Version: 1.0

   Note that the Subject Header Field has been obscured appropriately
   even though hcp_no_confidentiality would not have touched it by
   default.  The output of the CMS enveloping operation is
   base64-encoded and forms the body of the message.

Appendix E.  Rendering Examples

   This section offers example Cryptographic Payloads (the content
   within the Cryptographic Envelope) that contain Legacy Display
   Elements.

E.1.  Example text/plain Cryptographic Payload with Legacy Display
      Elements

   Here is a simple one-part Cryptographic Payload (Header Section and
   body) of a message that includes Legacy Display Elements:

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   Date: Fri, 21 Jan 2022 20:40:48 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Dinner plans
   Message-ID: <text-plain-legacy-display@lhp.example>
   MIME-Version: 1.0
   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   HP-Outer: Date: Fri, 21 Jan 2022 20:40:48 -0500
   HP-Outer: From: Alice <alice@example.net>
   HP-Outer: To: Bob <bob@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <text-plain-legacy-display@lhp.example>

   Subject: Dinner plans

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

   A compatible MUA will recognize the hp-legacy-display="1" parameter
   and render the body of the message as:

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

   A legacy decryption-capable MUA that is unaware of this mechanism
   will ignore the hp-legacy-display="1" parameter and instead render
   the body including the Legacy Display Elements:

   Subject: Dinner plans

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

E.2.  Example text/html Cryptographic Payload with Legacy Display
      Elements

   Here is a modern one-part Cryptographic Payload (Header Section and
   body) of a message that includes Legacy Display Elements:

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   Date: Fri, 21 Jan 2022 20:40:48 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Dinner plans
   Message-ID: <text-html-legacy-display@lhp.example>
   MIME-Version: 1.0
   Content-Type: text/html; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   HP-Outer: Date: Fri, 21 Jan 2022 20:40:48 -0500
   HP-Outer: From: Alice <alice@example.net>
   HP-Outer: To: Bob <bob@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <text-html-legacy-display@lhp.example>

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>Subject: Dinner plans</pre>
   </div>
   <p>
   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.
   </p>
   </body>
   </html>

   A compatible MUA will recognize the hp-legacy-display="1" parameter
   and mask out the Legacy Display div, rendering the body of the
   message as a simple paragraph:

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

   A legacy decryption-capable MUA that is unaware of this mechanism
   will ignore the hp-legacy-display="1" parameter and instead render
   the body including the Legacy Display Elements:

   Subject: Dinner plans

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

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Appendix F.  Other Header Protection Schemes

   Other Header Protection schemes have been proposed in the past.
   However, those typically have drawbacks such as sparse
   implementation, known problems with legacy interoperability (in
   particular with rendering), lack of clear signalling of sender
   intent, and/or incomplete cryptographic protections.  This section
   lists such schemes known at the time of the publication of this
   document out of historical interest.

F.1.  Original RFC 8551 Header Protection

   S/MIME [RFC8551] (as well as its predecessors [RFC5751] and
   [RFC3851]) defined a form of cryptographic Header Protection that has
   never reached wide adoption, and has significant drawbacks compared
   to the mechanism in this draft.  See Section 1.1.1 for more
   discussion of the differences and Section 4.10 for guidance on how to
   handle such a message.

F.2.  Pretty Easy Privacy (pEp)

   The pEp (pretty Easy privacy) [I-D.pep-general] project specifies two
   different MIME schemes that include Header Protection for Signed-and-
   Encrypted e-mail messages in [I-D.pep-email]: One scheme -- referred
   as pEp Email Format 1 (PEF-1) -- is generated towards MUAs not known
   to be pEp-capable, while the other scheme -- referred as PEF-2 -- is
   used between MUAs discovered to be compatible with pEp.  Signed-only
   messages are not recommended in pEp.

   Although the PEF-2 scheme is only meant to be used between PEF-2
   compatible MUAs, PEF-2 messages may end up at MUAs unaware of PEF-2
   (in which case they typically render badly).  This is due to
   signalling mechanism limitations.

   As the PEF-2 scheme is an enhanced variant of the RFC8551HP scheme
   (with an additional MIME Layer), it is similar to the RFC8551HP
   scheme (see Section 4.10).  The basic PEF-2 MIME structure looks as
   follows:

   A └┬╴multipart/encrypted [Outer Message]
   B  ├─╴application/pgp-encrypted
   C  └─╴application/octet-stream inline [Cryptographic Payload]
   D   ↧ (decrypts to)
   E   └┬╴multipart/mixed
   F    ├─╴text/plain
   G    ├┬╴message/rfc822
   H    │└─╴[Inner Message]
   I    └─╴application/pgp-keys

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   The MIME structure at part H contains the Inner Message to be
   rendered to the user.

   It is possible for a normal MUA to accidentally produce a message
   that happens to have the same MIME structure as used for PEF-2
   messages.  Therefore, a PEF-2 message cannot be identified by MIME
   structure alone.

   The lack of a mechanism comparable to HP-Outer (see Section 2.2)
   makes it impossible for the recipient of a PEF-2 message to safely
   determine which Header Fields are confidential or not, while
   forwarding or replying to a message (see Section 6).

   Note: As this document is not normative for PEF-2 messages, it does
   not provide any guidance for handling them.  Please see
   [I-D.pep-email] for more guidance.

F.3.  "draft-autocrypt" Protected Headers

   [I-D.autocrypt-lamps-protected-headers] describes a scheme similar to
   the Header Protection scheme specified in this document.  However,
   instead of adding Legacy Display Elements to existing MIME parts (see
   Section 5.2.2), "draft-autocrypt" injects a new MIME element "Legacy
   Display Part", thus modifying the MIME structure of the Cryptographic
   Payload.  These modified Cryptographic Payloads cause significant
   rendering problems on some common Legacy MUAs.

   The lack of a mechanism comparable to hp="cipher" and hp="clear" (see
   Section 2.1.1) means the recipient of an encrypted "draft-autocrypt"
   message cannot be cryptographically certain whether the sender
   intended for the message to be confidential or not.  The lack of a
   mechanism comparable to HP-Outer (see Section 2.2) makes it
   impossible for the recipient of an encrypted "draft-autocrypt" to
   safely determine which Header Fields are confidential or not, while
   forwarding or replying to a message (see Section 6).

Appendix G.  Document Changelog

   [[ RFC Editor: This section is to be removed before publication ]]

   *  draft-ietf-lamps-header-protection-24

      -  Deal with From spoofing risk: when inner and outer From differ
         with no valid signature, render outer From and warn

      -  Add test vectors to show historical 8551HP variants

      -  clarify PEF-2 and draft-autocrypt commentary

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   *  draft-ietf-lamps-header-protection-23

      -  normalize on "signed-and-encrypted" across the document

      -  replace hcp_strong with hcp_shy

      -  Remove "Wrapped Message" scheme

      -  Rename "Injected Headers" to "Header Protection"

      -  Add guidance about From Header Field spoofing risk

      -  offer guidance on handling RFC8551HP messages when received

   *  draft-ietf-lamps-header-protection-22

      -  Reorganize document for better readability.

      -  Add more details about problems with draft-autocrypt.

      -  Rename hcp_minimal to hcp_baseline: in addition to obscuring
         Subject, it now removes other Informational Header Fields
         Comments and Keywords.

      -  Add an example message up front for easier explainability.

      -  Unwrap sample message test vectors.

      -  Name pseudocode algorithms, number steps.

      -  Reply guidance also applies to forwarded messages.

      -  hcp_strong: stop rewriting Message-Id.

   *  draft-ietf-lamps-header-protection-21

      -  HP-Outer mechanism replaces HP-Removed and HP-Obscured.  This
         enables the recipient to easily calculate the sender's actions
         around header confidentiality.

      -  Replace Content-Type parameter protected-headers= with hp= and
         hp-scheme=. The presence of hp= indicates that the sender used
         Header Protection according to this document, and the value
         indicates whether the sender tried to encrypt and sign the
         message or just sign it. hp-scheme="wrapped" advises the
         recipient that they should look for the protected Header Fields
         in subtly different place.

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      -  Provide a clear algorithm for reasonably safe handling of
         confidential headers during Reply and Forward operations.

      -  Do not register the example HCP hcp_hide_cc, rename to
         hcp_example_hide_cc

      -  Rename hcp_null to hcp_no_confidentiality

      -  Provide a clear algorithm for the recipient to compute the
         protection state of each Header Field.

   *  draft-ietf-lamps-header-protection-20

      -  clarify IANA guidance about registration policy and designated
         expert review

      -  emphasize that Content-Type parameter hp-legacy-display=1
         belongs on all main body parts with a legacy display element

      -  clean up/normalize pseudocode variable names and text (no
         algorithm changes)

   *  draft-ietf-lamps-header-protection-19

      -  improve text, capitalize defined terms, fix typos

      -  Clean up from AD review:

      -  updates RFC 8551 explicitly

      -  add "Legacy Signed Message" and "Ordinary User" explicitly to
         terms

      -  tighten up SHOULDs/MUSTs for conformant MUAs

      -  expand references to other relevant Security Considerations

      -  drop nudge about non-existent Content-Type Parameters registry

      -  clarify IANA notes to align with table columns

      -  explicitly request HCP registry

      -  add references to other header protections schemes, but move
         all of them to appendix

   *  draft-ietf-lamps-header-protection-18

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      -  only allow US-ASCII as modified output of HCP, adjusted ABNF to
         match

   *  draft-ietf-lamps-header-protection-17

      -  More edits from WGLC:

      -  clean up definition of "Header Field"

      -  note leakage of encrypted recipient hints

      -  clarify explanation of LDE generation

      -  clarify how some obscured headers might not actually be private

   *  draft-ietf-lamps-header-protection-16

      -  correct variable names in message composition algorithms

      -  make text more readable

   *  draft-ietf-lamps-header-protection-15

      -  include clarifications, typos, etc from comments received
         during WGLC

   *  draft-ietf-lamps-header-protection-14

      -  provide section references for draft-ietf-lamps-e2e-mail-
         guidance

      -  encouarge a future IANA named HCP registry if HCP development
         takes off

   *  draft-ietf-lamps-header-protection-13

      -  Retitle from "Header Protection for S/MIME" to "Header
         Protection for Cryptographically Protected E-mail"

   *  draft-ietf-lamps-header-protection-12

      -  MUST produce HP-Obscured and HP-Removed when generating
         encrypted messages with non-null HCP

      -  Wrapped Message: move from forwarded=no to protected-
         headers=wrapped

      -  Wrapped Message: recommend Content-Disposition: inline

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   *  draft-ietf-lamps-header-protection-11

      -  Remove most of the Bcc text (transferred general discussion to
         e2e-mail-guidance)

      -  Fix bug in algorithm for generating HP-Obscured and HP-Removed

      -  More detail about handling Reply messages

      -  Considerations around handling risky Legacy Display Elements

      -  Narrative descriptions of some worked examples

      -  Describe potential leaks to recipients

      -  Clarify debugging/troubleshooting UX affordances

   *  draft-ietf-lamps-header-protection-10

      -  Clarify that HCP doesn't apply to Structural Header Fields

      -  Drop out-of-date "Open Issues" section

      -  Brief commentary on UI of messages with intermediate/mixed
         protections

      -  Deprecation prospects for messages without protected headers

      -  Describe generating replies to encrypted messages with stronger
         HCP

   *  draft-ietf-lamps-header-protection-09

      -  clarify terminology

      -  add privacy and security considerations

      -  clarify HCP examples and baselines

      -  recommend hcp_minimal as default HCP

      -  add HP-Obscured and HP-Removed (avoids reasoning about
         differences between outside and inside the Cryptographic
         Envelope)

      -  regenerated test vectors

   *  draft-ietf-lamps-header-protection-08

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      -  MUST compose injected headers, MAY compose wrapped messages

      -  MUST parse both schemes

      -  cleanup and restructure document

   *  draft-ietf-lamps-header-protection-07

      -  move from legacy display MIME part to legacy display elements
         within main body part

   *  draft-ietf-lamps-header-protection-06

      -  document observed problems with legacy MUAs

      -  avoid duplicated outer Message-IDs in hcp_strong test vectors

   *  draft-ietf-lamps-header-protection-05

      -  fix multipart/signed wrapped test vectors

   *  draft-ietf-lamps-header-protection-04

      -  add test vectors

      -  add "problems with Injected Messages" subsection

   *  draft-ietf-lamps-header-protection-03

      -  dkg takes over from Bernie as primary author

      -  Add Usability section

      -  describe two distinct formats "Wrapped Message" and "Injected
         Headers"

      -  Introduce Header Confidentiality Policy model

      -  Overhaul message composition guidance

      -  Simplify document creation workflow, move public face to gitlab

   *  draft-ietf-lamps-header-protection-02

      -  editorial changes / improve language

   *  draft-ietf-lamps-header-protection-01

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      -  Add DKG as co-author

      -  Partial Rewrite of Abstract and Introduction [HB/AM/DKG]

      -  Adding definitions for Cryptographic Layer, Cryptographic
         Payload, and Cryptographic Envelope (reference to
         [I-D.ietf-lamps-e2e-mail-guidance]) [DKG]

      -  Enhanced MITM Definition to include Machine- / Meddler-in-the-
         middle [HB]

      -  Relaxed definition of Original message, which may not be of
         type "message/rfc822" [HB]

      -  Move "memory hole" option to the Appendix (on request by Chair
         to only maintain one option in the specification) [HB]

      -  Updated Scope of Protection Levels according to WG discussion
         during IETF-108 [HB]

      -  Obfuscation recommendation only for Subject and Message-Id and
         distinguish between Encrypted and Unencrypted Messages [HB]

      -  Removed (commented out) Header Field Flow Figure (it appeared
         to be confusing as is was) [HB]

   *  draft-ietf-lamps-header-protection-00

      -  Initial version (text partially taken over from draft-ietf-
         lamps-header-protection-requirements

Index

   C H R

      C

         Compose  Table 6
         ComposeNoHeaderProtection  Table 6

      H

         HCP  Section 1.7, Paragraph 2.12.1; Section 3, Paragraph 2;
            Section 3, Paragraph 5; Section 3.1, Paragraph 3;
            Section 3.1, Paragraph 9; Section 3.1.1, Paragraph 1;
            Section 3.2, Paragraph 2; Section 3.2.1, Paragraph 3;
            Section 3.2.2, Paragraph 1; Section 3.2.2, Paragraph 4;
            Section 3.3, Paragraph 1; Section 3.4.1, Paragraph 1;

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            Section 3.4.2, Paragraph 1; Section 3.4.2, Paragraph 2.1.1;
            Section 3.4.2, Paragraph 2.3.1; Section 3.4.2, Paragraph
            2.4.1; Section 3.4.2, Paragraph 3; Section 4.8.2, Paragraph
            3; Section 5.2.1, Paragraph 4.5.2.2.2.1.1; Section 6.1,
            Paragraph 5; Section 6.1, Paragraph 7; Section 6.1.1,
            Paragraph 7.8.1; Section 6.1.1, Paragraph 8; Section 8.2,
            Paragraph 1; Section 8.2, Paragraph 4; Section 8.2,
            Paragraph 5; Section 8.2, Paragraph 6; Section 9.2,
            Paragraph 2; Section 9.2, Paragraph 3; Section 11.2,
            Paragraph 1; Section 11.2.1, Paragraph 1; Section 11.2.3,
            Paragraph 1; Section 11.2.3, Paragraph 2; Section 11.3,
            Paragraph 2; Section 11.4, Paragraph 2; Section 12,
            Paragraph 1; Table 6; Appendix D.1.2, Paragraph 1;
            Appendix D.2.2, Paragraph 3; Appendix D.2.2, Paragraph 6;
            Appendix G, Paragraph 2.4.2.4.1; Appendix G, Paragraph
            2.6.2.9.1; Appendix G, Paragraph 2.7.2.1.1; Appendix G,
            Paragraph 2.11.2.2.1; Appendix G, Paragraph 2.13.2.1.1;
            Appendix G, Paragraph 2.15.2.1.1; Appendix G, Paragraph
            2.15.2.5.1; Appendix G, Paragraph 2.16.2.3.1; Appendix G,
            Paragraph 2.16.2.4.1
         Header Confidentiality Policy  Section 1.2, Paragraph 4;
            Section 1.7, Paragraph 2.12.1; Section 3, Paragraph 2;
            Section 3.1, Paragraph 1; Section 3.2.1, Paragraph 1;
            Section 3.2.2, Paragraph 1; Section 3.3, Paragraph 1;
            Section 3.4, Paragraph 1; Section 3.4.1, Paragraph 2;
            Section 3.4.2, Paragraph 1; Section 4, Paragraph 5.4.1;
            Section 5.2, Paragraph 2.2.1; Section 6.1, Paragraph 5;
            Section 6.1, Paragraph 7; Section 6.1.1, Paragraph 3;
            Section 8.2, Paragraph 1; Section 9.2, Paragraph 1;
            Section 11.2.1, Paragraph 3; Section 12.3, Paragraph 5.1.1;
            Appendix C.2, Paragraph 1; Appendix C.3.1, Paragraph 1;
            Appendix C.3.2, Paragraph 1; Appendix C.3.3, Paragraph 1;
            Appendix C.3.4, Paragraph 1; Appendix C.3.5, Paragraph 1;
            Appendix C.3.6, Paragraph 1; Appendix C.3.7, Paragraph 1;
            Appendix C.3.8, Paragraph 1; Appendix C.3.9, Paragraph 1;
            Appendix C.3.10, Paragraph 1; Appendix C.3.11, Paragraph 1;
            Appendix C.3.12, Paragraph 1; Appendix C.3.13, Paragraph 1;
            Appendix C.3.14, Paragraph 1; Appendix C.3.15, Paragraph 1;
            Appendix C.3.16, Paragraph 1; Appendix C.3.17, Paragraph 1;
            Appendix G, Paragraph 2.22.2.4.1
         HeaderFieldProtection  Section 4.10.2, Paragraph 2.2.1; Table 6
         HeaderSetsFromMessage  Section 4.3.1, Paragraph 4.2.1;
            Section 4.10.2, Paragraph 2.2.1; Section 4.10.2, Paragraph
            2.4.1; Table 6

      R

         ReferenceHCP  Table 6

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         RFC8551HP  Section 1.1, Paragraph 1; Section 1.1, Paragraph 2;
            Section 1.1.1, Paragraph 1; Section 1.1.1, Paragraph 2;
            Section 1.1.1, Paragraph 5; Section 1.1.1, Paragraph 7;
            Section 1.1.1, Paragraph 8; Section 4.10, Paragraph 1;
            Section 4.10, Paragraph 2; Section 4.10.1, Paragraph 1;
            Section 4.10.1, Paragraph 3; Section 4.10.1, Paragraph 5;
            Section 4.10.2, Paragraph 1; Section 4.10.2, Paragraph
            2.1.1; Appendix C.2.5, Paragraph 1; Appendix C.2.6,
            Paragraph 1; Appendix C.3.17, Paragraph 1; Appendix F.2,
            Paragraph 3; Appendix G, Paragraph 2.2.2.6.1

Authors' Addresses

   Daniel Kahn Gillmor
   American Civil Liberties Union
   125 Broad St.
   New York, NY,  10004
   United States of America
   Email: dkg@fifthhorseman.net

   Bernie Hoeneisen
   pEp Project
   Oberer Graben 4
   CH- 8400 Winterthur
   Switzerland
   Email: bernie@ietf.hoeneisen.ch
   URI:   https://pep-project.org/

   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex
   TW12 2NP
   United Kingdom
   Email: alexey.melnikov@isode.com

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