Protected Headers for Cryptographic E-mail
draft-autocrypt-lamps-protected-headers-02
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2019-12-20
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openpgp B.R. Einarsson
Internet-Draft Mailpile ehf
Intended status: Informational . juga
Expires: 22 June 2020 Independent
D.K. Gillmor
ACLU
20 December 2019
Protected Headers for Cryptographic E-mail
draft-autocrypt-lamps-protected-headers-02
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 22 June 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 22 June 2020 [Page 1]
Internet-Draft Protected Headers for Cryptographic E-mail December 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 . . . . . . . . . . . . . . . . . . . . . . . . 4
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 Cryptographic Layers . . . . . . . . . . . . 7
3.1.2. S/MIME Cryptographic Layers . . . . . . . . . . . . . 8
3.2. Cryptographic Envelope . . . . . . . . . . . . . . . . . 9
3.3. Cryptographic Payload . . . . . . . . . . . . . . . . . . 9
3.3.1. Simple Cryptographic Payloads . . . . . . . . . . . . 9
3.3.2. Multilayer Cryptographic Envelopes . . . . . . . . . 10
3.3.3. A Baroque Example . . . . . . . . . . . . . . . . . . 10
3.4. Exposed Headers are Outside . . . . . . . . . . . . . . . 11
4. Message Composition . . . . . . . . . . . . . . . . . . . . . 11
4.1. Copying All Headers . . . . . . . . . . . . . . . . . . . 11
4.2. Confidential Subject . . . . . . . . . . . . . . . . . . 11
4.3. Obscured Headers . . . . . . . . . . . . . . . . . . . . 11
4.4. Message Composition without Protected Headers . . . . . . 12
4.5. Message Composition with Protected Headers . . . . . . . 12
5. Legacy Display . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Message Generation: Including a Legacy Display Part . . . 14
5.1.1. Legacy Display Transformation . . . . . . . . . . . . 15
5.1.2. When to Generate Legacy Display . . . . . . . . . . . 15
5.2. Message Rendering: Omitting a Legacy Display Part . . . . 16
5.2.1. Legacy Display Detection Algorithm . . . . . . . . . 16
5.3. Legacy Display is Decorative and Transitional . . . . . . 16
6. Message Interpretation . . . . . . . . . . . . . . . . . . . 17
6.1. Reverse-Copying . . . . . . . . . . . . . . . . . . . . . 17
6.2. Signature Invalidation . . . . . . . . . . . . . . . . . 17
6.3. The Legacy Display Part . . . . . . . . . . . . . . . . . 18
6.4. Replying to a Message with Obscured Headers . . . . . . . 18
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