Protected Headers for Cryptographic E-mail
draft-autocrypt-lamps-protected-headers-01

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openpgp                                                   B.R. Einarsson
Internet-Draft                                              Mailpile ehf
Intended status: Informational                                    . juga
Expires: 7 May 2020                                          Independent
                                                            D.K. Gillmor
                                                                    ACLU
                                                         4 November 2019

               Protected Headers for Cryptographic E-mail
               draft-autocrypt-lamps-protected-headers-01

Abstract

   This document describes a common strategy to extend the end-to-end
   cryptographic protections provided by PGP/MIME, etc. to protect
   message headers in addition to message bodies.  In addition to
   protecting the authenticity and integrity of headers via signatures,
   it also describes how to preserve the confidentiality of the Subject
   header.

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 7 May 2020.

Copyright Notice

   Copyright (c) 2019 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

Einarsson, et al.          Expires 7 May 2020                   [Page 1]
Internet-Draft Protected Headers for Cryptographic E-mail  November 2019

   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.1.  User-Facing Headers . . . . . . . . . . . . . . . . .   5
       1.2.2.  Structural Headers  . . . . . . . . . . . . . . . . .   6
   2.  Protected Headers Summary . . . . . . . . . . . . . . . . . .   6
   3.  Cryptographic MIME Message Structure  . . . . . . . . . . . .   7
     3.1.  Cryptographic Layers  . . . . . . . . . . . . . . . . . .   7
       3.1.1.  PGP/MIME Signing Cryptographic Layer (multipart/
               signed) . . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  PGP/MIME Encryption Cryptographic Layer
               (multipart/encrypted) . . . . . . . . . . . . . . . .   7
     3.2.  Cryptographic Envelope  . . . . . . . . . . . . . . . . .   8
     3.3.  Cryptographic Payload . . . . . . . . . . . . . . . . . .   8
       3.3.1.  Simple Cryptographic Payloads . . . . . . . . . . . .   8
       3.3.2.  Multilayer Cryptographic Envelopes  . . . . . . . . .   8
       3.3.3.  A Baroque Example . . . . . . . . . . . . . . . . . .   9
     3.4.  Exposed Headers are Outside . . . . . . . . . . . . . . .   9
   4.  Message Composition . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Copying All Headers . . . . . . . . . . . . . . . . . . .  10
     4.2.  Confidential Subject  . . . . . . . . . . . . . . . . . .  10
     4.3.  Obscured Headers  . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Message Composition without Protected Headers . . . . . .  10
     4.5.  Message Composition with Protected Headers  . . . . . . .  11
   5.  Legacy Display  . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Message Generation: Including a Legacy Display
           Part  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.1.1.  Legacy Display Transformation . . . . . . . . . . . .  14
       5.1.2.  When to Generate Legacy Display . . . . . . . . . . .  14
     5.2.  Message Rendering: Omitting a Legacy Display Part . . . .  14
       5.2.1.  Legacy Display Detection Algorithm  . . . . . . . . .  15
     5.3.  Legacy Display is Decorative and Transitional . . . . . .  15
   6.  Message Interpretation  . . . . . . . . . . . . . . . . . . .  16
     6.1.  Reverse-Copying . . . . . . . . . . . . . . . . . . . . .  16