Guidance on End-to-End E-mail Security
draft-dkg-lamps-e2e-mail-guidance-00

Document Type Active Internet-Draft (individual)
Author Daniel Gillmor 
Last updated 2020-10-31
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lamps                                                       D.K. Gillmor
Internet-Draft                                                      ACLU
Intended status: Informational                           31 October 2020
Expires: 4 May 2021

                 Guidance on End-to-End E-mail Security
                  draft-dkg-lamps-e2e-mail-guidance-00

Abstract

   End-to-end cryptographic protections for e-mail messages can provide
   useful security.  However, the standards for providing cryptographic
   protection are extremely flexible.  That flexibility can trap users
   and cause surprising failures.  This document offers guidance for
   mail user agent implementers that need to compose or interpret e-mail
   messages with end-to-end cryptographic protection.  It provides a
   useful set of vocabulary as well as suggestions to avoid common
   failures.

Status of This Memo

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

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   This Internet-Draft will expire on 4 May 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Gillmor                    Expires 4 May 2021                   [Page 1]
Internet-Draft   Guidance on End-to-End E-mail Security     October 2020

   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
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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
       1.2.1.  Structural Headers  . . . . . . . . . . . . . . . . .   4
   2.  Usability . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Types of Protection . . . . . . . . . . . . . . . . . . . . .   4
   4.  Cryptographic MIME Message Structure  . . . . . . . . . . . .   5
     4.1.  Cryptographic Layers  . . . . . . . . . . . . . . . . . .   5
       4.1.1.  S/MIME Cryptographic Layers . . . . . . . . . . . . .   5
       4.1.2.  PGP/MIME Cryptographic Layers . . . . . . . . . . . .   6
     4.2.  Cryptographic Envelope  . . . . . . . . . . . . . . . . .   7
     4.3.  Cryptographic Payload . . . . . . . . . . . . . . . . . .   7
     4.4.  Types of Cryptographic Envelope . . . . . . . . . . . . .   7
       4.4.1.  Simple Cryptographic Envelopes  . . . . . . . . . . .   8
       4.4.2.  Multilayer Cryptographic Envelopes  . . . . . . . . .   8
     4.5.  Errant Crytptographic Layers  . . . . . . . . . . . . . .   8
       4.5.1.  Mailing List Wrapping . . . . . . . . . . . . . . . .   8
       4.5.2.  A Baroque Example . . . . . . . . . . . . . . . . . .   9
   5.  Message Composition . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Message Composition Algorithm . . . . . . . . . . . . . .  10
     5.2.  Encryption Outside, Signature Inside  . . . . . . . . . .  11
     5.3.  Avoid Offering Encrypted-only Messages  . . . . . . . . .  11
     5.4.  Composing a Reply Message . . . . . . . . . . . . . . . .  12
   6.  Message Interpretation  . . . . . . . . . . . . . . . . . . .  12
     6.1.  Rendering Well-formed Messages  . . . . . . . . . . . . .  12
     6.2.  Errant Cryptographic Layers . . . . . . . . . . . . . . .  13
       6.2.1.  Errant Signing Layer  . . . . . . . . . . . . . . . .  13
       6.2.2.  Errant Encryption Layer . . . . . . . . . . . . . . .  14
     6.3.  Forwarded Messages with Cryptographic Protection  . . . .  15
     6.4.  Signature failures  . . . . . . . . . . . . . . . . . . .  16
   7.  Common Pitfalls and Guidelines  . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. Document Considerations . . . . . . . . . . . . . . . . . . .  17
     10.1.  Document History . . . . . . . . . . . . . . . . . . . .  17
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