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Network Working Group B. Hoeneisen
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Internet-Draft pEp Foundation
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Intended status: Informational A. Melnikov
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Expires: January 10, 2021 Isode Ltd
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July 09, 2020
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[krb: the terms mail user agent, Mail User Agent, user agent, and MUA are all used interchangeably throughout the draft. I'd suggest using Mail User Agent (MUA) at the beginning of the doc and then using MUA for the rest, unless there's a grammatical need to use the full term, which I'll do my best to note. This will maintain consistency and reduce confusion.]
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Header Protection for S/MIME
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draft-ietf-lamps-header-protection-00
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Abstract
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Privacy and security issues with email header protection in S/MIME
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have been identified for some time. However, the desire to fix these
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issues has only recently been expressed in the IETF LAMPS Working
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Group. The existing S/MIME specification is to be updated regarding
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header protection.
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This document describes the problem statement, generic use cases, and
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the S/MIME specification for header protection.
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Status of This Memo
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This Internet-Draft is submitted in full conformance with the
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provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF). Note that other groups may also distribute
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working documents as Internet-Drafts. The list of current Internet-
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Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on January 10, 2021.
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Copyright Notice
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Copyright (c) 2020 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
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(https://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 1]
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Internet-Draft Header Protection S/MIME July 2020
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to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
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1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
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1.2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 4
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2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
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2.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.2. Security . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.3. Usability . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.4. Interoperability . . . . . . . . . . . . . . . . . . . . 7
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3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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3.1. Interactions . . . . . . . . . . . . . . . . . . . . . . 7
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3.1.1. Main Use Case . . . . . . . . . . . . . . . . . . . . 7
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3.1.2. Backward Compatibility Use Cases . . . . . . . . . . 7
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3.2. Protection Levels . . . . . . . . . . . . . . . . . . . . 9
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4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 9
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4.1. Main Use Case . . . . . . . . . . . . . . . . . . . . . . 10
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4.1.1. MIME Format . . . . . . . . . . . . . . . . . . . . . 10
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4.1.2. Inner Message Header Fields . . . . . . . . . . . . . 15
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4.1.3. Wrapper . . . . . . . . . . . . . . . . . . . . . . . 16
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4.1.4. Outer Message Header Fields . . . . . . . . . . . . . 16
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4.1.5. Receiving User Facing Message Header Fields . . . . . 18
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4.1.6. Header Field Flow . . . . . . . . . . . . . . . . . . 18
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4.1.7. Sending Side Message Processing . . . . . . . . . . . 20
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4.1.8. Receiving Side Message Processing . . . . . . . . . . 21
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4.2. Backward Compatibility Use Cases . . . . . . . . . . . . 21
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4.2.1. Receiving Side MIME-Conformant . . . . . . . . . . . 21
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4.2.2. Receiving Side Not MIME-Conformant . . . . . . . . . 22
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5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
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6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23
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7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
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8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
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9.1. Normative References . . . . . . . . . . . . . . . . . . 23
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9.2. Informative References . . . . . . . . . . . . . . . . . 24
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9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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Appendix A. Additional information . . . . . . . . . . . . . . . 25
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A.1. Stored Variants of Messages with Bcc . . . . . . . . . . 25
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Appendix B. Document Changelog . . . . . . . . . . . . . . . . . 26
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Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 26
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 2]
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Internet-Draft Header Protection S/MIME July 2020
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1. Introduction
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A range of protocols for the protection of electronic mail (email)
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exists, which allows to assess the authenticity and integrity of the
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email headers section or selected header fields (HF) from the domain-
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level perspective, specifically DomainKeys Identified Mail (DKIM)
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[RFC6376] and Sender Policy Framework (SPF) [RFC7208], and Domain-
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based Message Authentication, Reporting, and Conformance (DMARC)
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[RFC7489]. These protocols, while essential to responding to a range
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of attacks on email, do not offer (full) end-to-end protection to the
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header section and are not capable of providing privacy for the
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information contained therein.
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The need for means of Data Minimization, which includes data
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sparseness and hiding all technically concealable information
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whenever possible, has grown in importance over the past several
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years.
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A standard for end-to-end protection of the email header section
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exists for S/MIME version 3.1 and later. (cf. [RFC8551]):
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In order to protect outer, non-content-related message header
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fields (for instance, the "Subject", "To", "From", and "Cc"
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fields), the sending client MAY wrap a full MIME message in a
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message/RFC822 wrapper in order to apply S/MIME security services
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to these header fields.
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No mechanism for header protection (HP) has been standardized for
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PGP/MIME (Pretty Good Privacy) [RFC3156] yet.
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Several varying implementations of end-to-end protections for email
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header sections exist, though the total number of such
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implementations appears to be rather low.
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Some LAMPS WG participants expressed the opinion that regardless of
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the mechanism chosen, it should not be limited to S/MIME, but also
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applicable to PGP/MIME.
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This document describes the problem statement (Section 2), generic
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use cases (Section 3) and the specification for Header Protection
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(Section 4).
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[I-D.ietf-lamps-header-protection-requirements] defines the
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requirements that this specification is based on.
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This document is in early draft state and contains a proposal on
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which to base future discussions of this topic. In any case, the
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final mechanism is to be determined by the IETF LAMPS WG.
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1.1. Requirements Language
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
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1.2. Terms
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The following terms are defined for the scope of this document:
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o Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A
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form of active wiretapping attack in which the attacker intercepts
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and selectively modifies communicated data to masquerade as one or
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more of the entities involved in a communication association."
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o S/MIME: Secure/Multipurpose Internet Mail Extensions (cf.
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[RFC8551])
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o PGP/MIME: MIME Security with OpenPGP (cf. [RFC3156])
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o Message: An Email Message consisting of Header Fields
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(collectively called "the Header Section of the message")
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followed, optionally, by a Body; cf. [RFC5322].
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Note: To avoid ambiguity, this document does not use the terms
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"Header" or "Headers" in isolation, but instead always uses
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"Header Field" to refer to the individual field and "Header
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Section" to refer to the entire collection; cf. [RFC5322].
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o Header Field (HF): cf. [RFC5322] Header Fields are lines beginning
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with a field name, followed by a colon (":"), followed by a field
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body (value), and terminated by CRLF; cf. [RFC5322].
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o Header Section (HS): The Header Section is a sequence of lines of
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characters with special syntax as defined in [RFC5322]. It is the
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(top) section of a Message containing the Header Fields.
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o Body: The Body is simply a sequence of characters that follows the
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Header Section and is separated from the Header Section by an
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empty line (i.e., a line with nothing preceding the CRLF); cf
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[RFC5322]. It is the (bottom) section of Message containing the
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payload of a Message. Typically, the Body consists of a
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(multipart) MIME [RFC2045] construct.
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o MIME Header Fields: Header Fields describing content of a MIME
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entity [RFC2045], in particular the MIME structure. Each MIME
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Header Field name starts with "Content-" prefix.
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 4]
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o MIME Header Section (part): The collection of MIME Header Fields.
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"MIME Header Section" refers to a Header Sections that contains
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only MIME Header Fields, whereas "MIME Header Section part" refers
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to the MIME Header Fields of a Header Section that - in addition
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to MIME Header Fields - also contains non-MIME Header Fields.
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o Essential Header Fields (EHF): The minimum set of Header Fields an
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Outer Message Header Section SHOULD contain; cf. Section 4.1.4.
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o Header Protection (HP): cryptographic protection of email Header
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Sections (or parts of it) for signatures and/or encryption
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o Protection Levels (PL): One of 'signature and encryption',
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'signature only' or 'encryption only' (cf. Section 3.2)
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o Protected: Protected refers to the parts of a Message where
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protection measures of any Protection Level have been applied to.
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o Protected Message: A Message that protection measures of any
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Protection Levels have been applied to.
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o Unprotected: Unprotected refers to the parts of a Message where no
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protection measures of any Protection Levels have been applied to.
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o Unprotected Message: A Message that no protection measures of any
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Protection Levels have been applied to.
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o Original Message (OrigM): The message to be protected before any
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protection related processing has been applied on the sending
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side.
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o Inner Message (InnerM): The message to be protected, i.e. which
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wrapping and protection measures are applied to on the sending
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side or the result of decryption and unwrapping on the receiving
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side respectively. Typically, the Inner Message is in clear text.
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The Inner Message is a subset of (or the same as) the Original
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Message (cf. Section 4.1.2). The Inner Message must be the same
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on the sending and the receiving side.
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o Outer Message (OuterM): The Message as handed over to the
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Submission Entity or received from the last hop respectively. The
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Outer Message normally differs on the sending and the receiving
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side (e.g. new Header Fields are added by intermediary nodes).
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o Receiving User Facing Message (RUFM): The message used for
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rendering at the receiving side. Typically this is the same as
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the Inner Message.
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 5]
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o Submission Entity: The entity taking care of further processing of
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the Message (incl. transport towards the receiver), after
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protection measures have been applied to. It typically determines
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the destination recipients by reading the To, Cc and Bcc Header
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Fields (of the Outer Message).
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Note: The Submission Entity varies among implementations, mainly
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depending on the stage, where protection measures are applied to:
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It could be e.g. a Message Submission Agent (MSA) [RFC6409] or
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another (proprietary) solution. The latter is particularly
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relevant, if protection is implemented as a plugin solution or for
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mixnet networks, i.e. "onion routing" for email (e.g.
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[pEp.mixnet]).
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o Data Minimization: Data sparseness and hiding of all technically
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concealable information whenever possible.
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2. Problem Statement
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The LAMPS charter contains the following Work Item:
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Update the specification for the cryptographic protection of email
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headers - both for signatures and encryption - to improve the
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implementation situation with respect to privacy, security,
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usability and interoperability in cryptographically-protected
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electronic mail. Most current implementations of
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cryptographically-protected electronic mail protect only the body
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of the message, which leaves significant room for attacks against
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otherwise-protected messages.
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In the following a set of challenges to be addressed:
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[[ TODO: enhance this section, add more items to the following ]]
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2.1. Privacy
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o Data Minimization, which includes data sparseness and hiding all
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technically concealable information whenever possible
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2.2. Security
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o MITM attacks (cf. [RFC4949])
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2.3. Usability
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o User interaction / User experience
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2.4. Interoperability
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o Interoperability with [RFC8551] implementations
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3. Use Cases
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In the following, the reader can find a list of the generic use cases
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that need to be addressed for Messages with Header Protection (HP).
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These use cases apply regardless of technology (S/MIME, PGP/MIME,
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etc.) used to achieve HP.
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3.1. Interactions
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The following use cases assume that at least the sending side
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supports Header Protection as specified in this document. Receiving
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sides that support this specification are expected to be able to
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distinguish between Messages that Header Protection - as specified in
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this document - has been applied to and (legacy) Mail user Agents [KRB: capitalize "user"]
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(MUAs) not implementing this specification.
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[[TODO: Verify once solution is stable and update last sentence ]]
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3.1.1. Main Use Case
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Both peers (sending and receiving side) (fully) [krb: why is "fully" in parentheses? I'd reword as follows: "Both the sending and receiving peers fully support..."] support Header
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Protection as specified in this document.
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The main use case is specified in Section 4.1.
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3.1.2. Backward Compatibility Use Cases
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Regarding backwards compatibility [krb: add comma after compatibility] the main distinction is based on
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whether or not the receiving side conforms to MIME according to
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[RFC2046], ff., which in particular [krb: delete "particular"] also includes Section 2 of
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[RFC2049] on "MIME Conformance". In the following an excerpt of
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paragraphs relevant in this context:
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A mail user agent that is MIME-conformant MUST:
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[krb: suggest total reword. Easier to type it out rather than individual marks. This is intended to replace the two paragraphs above. "According to [RFC2046], the main concern with regard to backwards compatibility is based upon whether or not the receiving side conforms to MIME. This document also references Section 2 of [RFC2049] on "Mime Conformance", which states that a Mail User Agent (MUA) that is MIME-conformant MUST handle messages as follows:"]
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[...]
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-- Recognize and display at least the RFC822 message
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encapsulation (message/rfc822) in such a way as to
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preserve any recursive structure, that is, displaying
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or offering to display the encapsulated data in
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accordance with its media type.
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-- Treat any unrecognized subtypes as if they were
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"application/octet-stream".
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[...]
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[krb: maybe a display issue on my end, but this is indented further than other non-quote paragraphs] A user agent that meets the above conditions is said to be MIME-
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conformant. The meaning of this phrase is that it is assumed to be
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"safe" to send virtually any kind of properly-marked data to users
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of such mail systems, because such systems will at least be able to
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treat the data as undifferentiated binary, and will not simply
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splash it onto the screen of unsuspecting users. [krb: again, suggesting total reword. "A Mail User Agent that meets the above criteria is considered MIME-conformant, and is assumed to be "safe" to send properly-marked data to users of this MUA. MIME-conformant MUAs are able to treat data as undifferentiated binary, and will not default to displaying it to unsuspecting users." or something like this. :) ]
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Note: The compatibility of legacy HP systems with this new solution,
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and how to handle issues surrounding future maintenance for these
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legacy systems, will be decided by the LAMPS WG.
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3.1.2.1. Receiving Side MIME-Conformant
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The sending side (fully) [krb: again, why is this in parentheses] supports Header Protection as specified in
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this document, while the receiving side does not support this
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specification. The receiving side is MIME-conformant according to
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[RFC2045], ff. (cf. Section 3.1.2),
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This use case is specified in Section 4.2.1.
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Note: This case is expected to just work fine [krb: suggest "This case should perform as expected if the sending side applies this specification as outlined in Section 4.1."], if the sending side
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applies specification for the main use case Section 4.1.
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[[TODO: Verify once solution is stable and update last sentence ]]
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3.1.2.2. Receiving Side Not MIME-Conformant
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The sending side (fully) [krb: again, why is this in parentheses] supports Header Protection as specified in
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this document, while the receiving side does not support this
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specification. The receiving side is *not* MIME-conformant according
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to [RFC2045], ff. (cf. Section 3.1.2) either [krb: delete "either"].
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This use case is specified in Section 4.2.2. [krb: this should be with the rest of the above paragraph.]
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3.2. Protection Levels
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The following protection levels need to be considered:
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a) Signature and encryption
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Messages containing a cryptographic signature, which are also
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encrypted. [krb: "...signature and are encrypted."]
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b) Signature only
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Messages containing a cryptographic signature, but which are not
|
|
encrypted. [krb: "...signature but are not encrypted."]
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c) Encryption only
|
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Messages that are encrypted, but do not contain a cryptographic
|
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signature.
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4. Specification
|
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|
|
This section contains the specification for Header Protection in
|
|
S/MIME to update and it clarifies Section 3.1 of [RFC8551] (S/MIME
|
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4.0). [krb: This is a little unclear. Do you mean that this spec both updates and clarifies the linked section of 8551? If so, change "...to update and clarify Section..."]
|
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Furthermore, it is likely that PGP/MIME [RFC3156] will also
|
|
incorporate this specification or parts of it.
|
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|
This specification applies to the protection levels "signature &
|
|
encryption" and "signature only" (cf. Section 3.2):
|
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|
Sending and receiving sides MUST implement "signature and
|
|
encryption" [krb: "...implement the "signature and encryption" protection level, which is the default level for the sending side."], which is the default to use on the sending side. [krb: question -- does this mean the "signature and encryption" level must be implemented in a way that it's the default? If that's the case, you may want to make this clearer, because as written, it almost sounds optional.]
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|
Certain implementations MAY decide to send "signature only" messages,
|
|
depending on the circumstances and customer requirements. Sending
|
|
side MAY and receiving sides MUST implement "signature only". [krb: "signature only" what, messages? Protection level? Please clarify.]
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|
It generally is NOT RECOMMENDED to send a message with protection
|
|
level "encryption only". On the other hand, messages with protection
|
|
level "encryption only" might arrive at the receiving side. [krb: Why or how would this happen? Consider including an example.] While
|
|
not targeted to protection level "encryption only", this
|
|
specification is assumed to also function for "encryption only".
|
|
Receiving sides SHOULD implement "encryption only". [krb: Is this implementation at a minimum? If so, please clarify.]
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 9]
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Internet-Draft Header Protection S/MIME July 2020
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Note: It is for further study whether or not more guidance for
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handling messages with protection level "encryption only" at the
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receiving side is needed. [krb: "Further study is necessary to determine whether additional guidance for handling messages with "encryption only" protection is needed."]
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|
4.1. Main Use Case
|
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|
|
This section applies to the main use case, where both peers (sending
|
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and receiving side) (fully) [krb: again, why is this in parentheses, and I suggest rewording: "both the sending and receiving peers fully support..."] support Header Protection as specified
|
|
herein (cf. Section 3.1.1).
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|
Note: The sending side specification of the main use case is also
|
|
applicable to the cases, where [krb: "...to cases where the..."] the sending side (fully) [krb: again, why is this in parentheses] supports
|
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Header Protection as specified herein, while the receiving side does
|
|
not support this specification, but [krb: "...side does not, but is MIME-conformant..."] is MIME-conformant according to
|
|
[RFC2045], ff. (cf. Section 3.1.2) and Section 3.1.2.1)
|
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|
|
Further backward compatibility cases are defined in Section 4.2. [krb: I'd add this to the first paragraph of this section.]
|
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|
4.1.1. MIME Format
|
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|
|
Currently there are two options in discussion:
|
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|
|
1. The option according to the current S/MIME specification (cf.
|
|
[RFC8551])
|
|
|
|
2. An alternative option that is based on the former "memory hole"
|
|
approach (cf. [I-D.autocrypt-lamps-protected-headers])
|
|
|
|
4.1.1.1. S/MIME Specification
|
|
|
|
As per S/MIME version 3.1 and later (cf. [RFC8551]), the sending
|
|
client MAY wrap a full MIME message in a message/RFC822 wrapper in
|
|
order to apply S/MIME security services to these header fields.
|
|
|
|
To help the receiving side to distinguish between forwarded and
|
|
wrapped message [krb: "messages"], a [krb: "a" -> "the"] Content-Type header field parameter "forwarded" is
|
|
added as defined in [I-D.melnikov-iana-reg-forwarded]. Certain
|
|
mailing applications might display the Inner Message as [krb: add "an"] attachment
|
|
otherwise.
|
|
|
|
The MIME structure of an Email message looks as follows:
|
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 10]
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|
|
Internet-Draft Header Protection S/MIME July 2020
|
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|
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|
|
<Outer Message Header Section (unprotected)>
|
|
|
|
<Outer Message Body (protected)>
|
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|
|
<MIME Header Section (wrapper)>
|
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|
|
<Inner Message Header Section>
|
|
|
|
<Inner Message Body>
|
|
|
|
|
|
The following example demonstrates how [krb: add "the"] header section and payload of
|
|
a protected body part might look like [krb: "look like" -> "appear"]. For example, this will be the
|
|
first body part [krb: "The following example illustrates the first body portion of a..."] of a multipart/signed message or the signed and/or
|
|
encrypted payload of the application/pkcs7-mime body part. Lines
|
|
prepended by "O: " are the Outer Message Header Section. Lines
|
|
prepended by "I: " are the Inner Message Header Section. Lines
|
|
prepended by "W: " are the wrapper (MIME Header Section):
|
|
[krb: this is structured oddly. Suggest a new paragraph.
|
|
"Lines are prepended as follows:
|
|
O: Outer Message Header Section
|
|
I: Inner Message Header Section
|
|
W: Wrapper Section]
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 11]
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|
Internet-Draft Header Protection S/MIME July 2020
|
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|
|
O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
|
|
O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
|
|
O: Subject: Meeting at my place
|
|
O: From: "Alexey Melnikov" <alexey.melnikov@example.net>
|
|
O: To: somebody@example.net
|
|
O: MIME-Version: 1.0
|
|
O: Content-Type: multipart/signed; charset=us-ascii; micalg=sha1;
|
|
O: protocol="application/pkcs7-signature";
|
|
O: boundary=.cbe16d2a-e1a3-4220-b821-38348fc97237
|
|
|
|
This is a multipart message in MIME format.
|
|
--.cbe16d2a-e1a3-4220-b821-38348fc97237
|
|
W: Content-Type: message/RFC822; forwarded=no
|
|
W:
|
|
I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
|
|
I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
|
|
I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
|
|
I: MIME-Version: 1.0
|
|
I: MMHS-Primary-Precedence: 3
|
|
I: Subject: Meeting at my place
|
|
I: To: somebody@example.net
|
|
I: X-Mailer: Isode Harrier Web Server
|
|
I: Content-Type: text/plain; charset=us-ascii
|
|
|
|
This is an important message that I don't want to be modified.
|
|
|
|
--.cbe16d2a-e1a3-4220-b821-38348fc97237
|
|
Content-Transfer-Encoding: base64
|
|
Content-Type: application/pkcs7-signature
|
|
|
|
[[base-64 encoded signature]]
|
|
|
|
--.cbe16d2a-e1a3-4220-b821-38348fc97237--
|
|
|
|
|
|
The Outer Message Header Section is unprotected, while the remainder
|
|
(Outer Message Body) is protected. The Outer Message Body consists
|
|
of the wrapper (MIME Header Section) and the Inner Message (Header
|
|
Section and Body).
|
|
|
|
The wrapper is a simple MIME Header Section with media type "message/
|
|
RFC822" containing a Content-Type header field parameter
|
|
"forwarded=no" followed by an empty line.
|
|
|
|
The Inner Message Header Section is the same as (or a subset of) the
|
|
Original Message Header Section (cf. Section 4.1.2).
|
|
|
|
The Inner Message Body is the same as the Original Message Body.
|
|
|
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|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 12]
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|
|
Internet-Draft Header Protection S/MIME July 2020
|
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|
|
|
The Original Message itself may contain any MIME structure.
|
|
|
|
4.1.1.2. Alternative Option Autocrypt "Protected Headers" (Ex-"Memory
|
|
Hole")
|
|
|
|
An alternative option (based on the former autocrypt "Memory Hole"
|
|
approach) to be considered, is described in
|
|
[I-D.autocrypt-lamps-protected-headers].
|
|
|
|
Unlike the option described in Section 4.1.1.1, this option does not
|
|
use a "message/RFC822" wrapper to unambiguously delimit the Inner
|
|
Message.
|
|
|
|
Before choosing this option, [krb: add "the following"] two issues must be assessed to ensure, [krb: remove comma]
|
|
no interoperability issues result from it:
|
|
|
|
1. How current MIME parser implementations treat non-MIME Header
|
|
Fields, which are not part of the outermost MIME entity and not
|
|
part of a message wrapped into a MIME entity of media type
|
|
"message/rfc822", and how such messages are rendered to the user.
|
|
|
|
[I-D.autocrypt-lamps-protected-headers] provides some examples
|
|
for testing this.
|
|
|
|
2. MIME-conformance, i.e. whether or not this option is (fully) [krb: again, parentheses]
|
|
MIME-conformant [RFC2045] ff., in particular also Section 5.1. of
|
|
[RFC2046] on "Multipart Media Type). In the following an excerpt
|
|
of paragraphs that may be relevant in this context: [krb: suggest total reword. Easier to type it out rather than individual marks. "Whether or not this option is fully MIME-conformant per [RFC2045] ff., and Section 5.1 of [RFC2046] on "Multipart Media Type". The following excerpt from Section 5.1 are of particular relevance here:" Also, not sure if the quotes themselves are formatted properly--double check.]
|
|
|
|
The only header fields that have defined meaning for body parts
|
|
are those the names of which begin with "Content-". All other
|
|
header fields may be ignored in body parts. Although they
|
|
should generally be retained if at all possible, they may be
|
|
discarded by gateways if necessary. Such other fields are
|
|
permitted to appear in body parts but must not be depended on.
|
|
"X-" fields may be created for experimental or private
|
|
purposes, with the recognition that the information they
|
|
contain may be lost at some gateways.
|
|
|
|
|
|
|
|
|
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|
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|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 13]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
NOTE: The distinction between an RFC 822 message and a body
|
|
part is subtle, but important. A gateway between Internet and
|
|
X.400 mail, for example, must be able to tell the difference
|
|
between a body part that contains an image and a body part
|
|
that contains an encapsulated message, the body of which is a
|
|
JPEG image. In order to represent the latter, the body part
|
|
must have "Content-Type: message/rfc822", and its body (after
|
|
the blank line) must be the encapsulated message, with its own
|
|
"Content-Type: image/jpeg" header field. The use of similar
|
|
syntax facilitates the conversion of messages to body parts,
|
|
and vice versa, but the distinction between the two must be
|
|
understood by implementors. (For the special case in which
|
|
parts actually are messages, a "digest" subtype is also
|
|
defined.)
|
|
|
|
The MIME structure of an Email message looks as follows:
|
|
|
|
<Outer Message Header Section (unprotected)>
|
|
|
|
<Outer Message Body (protected)>
|
|
|
|
<Inner Message Header Section>
|
|
|
|
<Inner Message Body>
|
|
|
|
|
|
The following example demonstrates how the header section and payload
|
|
of a protected body part might appear. For example, this will [krb: "This example could..."] be the
|
|
first body part of a multipart/signed message [krb: add a comma] or the signed and/or
|
|
encrypted payload of the application/pkcs7-mime body part. Lines
|
|
prepended by "O: " are the outer header section. Lines prepended by
|
|
"I: " are the inner header section. [krb: As above, this is structured oddly. Suggest a new paragraph.
|
|
"Lines are prepended as follows:
|
|
O: Outer Message Header Section
|
|
I: Inner Message Header Section]
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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|
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|
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|
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|
|
|
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|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 14]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
O: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
|
|
O: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
|
|
O: Subject: Meeting at my place
|
|
O: From: "Alexey Melnikov" <alexey.melnikov@example.net>
|
|
O: MIME-Version: 1.0
|
|
O: Content-Type: multipart/signed; charset=us-ascii; micalg=sha1;
|
|
O: protocol="application/pkcs7-signature";
|
|
O: boundary=.cbe16d2a-e1a3-4220-b821-38348fc97237
|
|
|
|
This is a multipart message in MIME format.
|
|
--.cbe16d2a-e1a3-4220-b821-38348fc97237
|
|
I: Date: Mon, 25 Sep 2017 17:31:42 +0100 (GMT Daylight Time)
|
|
I: From: "Alexey Melnikov" <alexey.melnikov@example.net>
|
|
I: Message-ID: <e4a483cb-1dfb-481d-903b-298c92c21f5e@m.example.net>
|
|
I: MIME-Version: 1.0
|
|
I: MMHS-Primary-Precedence: 3
|
|
I: Subject: Meeting at my place
|
|
I: To: somebody@example.net
|
|
I: X-Mailer: Isode Harrier Web Server
|
|
I: Content-Type: text/plain; charset=us-ascii
|
|
|
|
This is an important message that I don't want to be modified.
|
|
|
|
--.cbe16d2a-e1a3-4220-b821-38348fc97237
|
|
Content-Transfer-Encoding: base64
|
|
Content-Type: application/pkcs7-signature
|
|
|
|
[[base-64 encoded signature]]
|
|
|
|
--.cbe16d2a-e1a3-4220-b821-38348fc97237--
|
|
|
|
|
|
The Outer Message Header Section is unprotected, while the remainder
|
|
(Outer Message Body) is protected. The Outer Message Body consists
|
|
of the Inner Message (Header Section and Body).
|
|
|
|
The Inner Message Header Section is the same as (or a subset of) the
|
|
Original Message Header Section (cf. Section 4.1.2).
|
|
|
|
The Inner Message Body is the same as the Original Message Body.
|
|
|
|
The Original Message itself may contain any MIME structure.
|
|
|
|
4.1.2. Inner Message Header Fields
|
|
|
|
It is RECOMMENDED that the Inner Messages [krb: "Message"] contains all the [krb: remove "the"] Header
|
|
Fields of the Original Message with the exception of the following
|
|
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 15]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
Header Field, which MUST NOT be included within the Inner Message nor
|
|
within any other protected part of the message:
|
|
|
|
o Bcc
|
|
|
|
[[ TODO: Bcc handling needs to be further specified (see also
|
|
Appendix A.1). Certain MUAs cannot properly decrypt messages with
|
|
Bcc recipients. ]]
|
|
|
|
4.1.3. Wrapper
|
|
|
|
The wrapper is a simple MIME Header Section followed by an empty line
|
|
preceding the Inner Message (inside the Outer Message Body). The
|
|
media type of the wrapper MUST be "message/RFC822" and MUST contain
|
|
the Content-Type header field parameter "forwarded=no" as defined in
|
|
[I-D.melnikov-iana-reg-forwarded]. The wrapper delimits
|
|
unambiguously [krb: "unambiguously delimits"] the Inner Message from the rest of the message.
|
|
|
|
4.1.4. Outer Message Header Fields
|
|
|
|
To maximize Privacy, it is strongly RECOMMENDED to follow the
|
|
principle of Data Minimization (cf. Section 2.1).
|
|
|
|
However, the Outer Message Header Section SHOULD contain the
|
|
Essential Header Fields and, in addition, MUST contain the Header
|
|
Fields of the MIME Header Section part to describe the encryption or
|
|
signature as per [RFC8551].
|
|
|
|
The following Header Fields are defined as the Essential Header
|
|
Fields:
|
|
|
|
o From
|
|
|
|
o To (if present in the Original Message)
|
|
|
|
o Cc (if present in the Original Message)
|
|
|
|
o Bcc (if present in the Original Message, see also Section 4.1.2
|
|
and Appendix A.1)
|
|
|
|
o Date
|
|
|
|
o Message-ID
|
|
|
|
o Subject
|
|
|
|
Further processing by the Submission Entity normally depends on part
|
|
of these Header Fields, e.g. From and Date HFs are required by
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 16]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
[RFC5322]. Furthermore, not including certain Header Fields may
|
|
trigger spam detection to flag the message and/or lead to user
|
|
experience (UX) issues.
|
|
|
|
For further Data Minimization, the value of the Subject Header Field
|
|
SHOULD be obfuscated. In addition, the value of other Essential
|
|
Header Fields MAY be obfuscated. Further Header Fields MAY be
|
|
obfuscated, though simply not adding those to the Outer Message
|
|
Header Section SHOULD be preferred over obfuscation. Header Field
|
|
obfuscation is further specified in Section 4.1.4.1. Header Fields
|
|
not obfuscated should contain the same values as in the Original
|
|
Message.
|
|
|
|
The MIME Header Section part is the collection of MIME Header Fields
|
|
describing the following MIME structure as defined in [RFC2045]. A
|
|
MIME Header Section part typically includes the following Header
|
|
Fields:
|
|
|
|
o Content-Type
|
|
|
|
o Content-Transfer-Encoding
|
|
|
|
o Content-Disposition
|
|
|
|
The following example shows the MIME Header Section part of an S/MIME
|
|
signed message (using application/pkcs7-mime with SignedData):
|
|
|
|
MIME-Version: 1.0
|
|
Content-Type: application/pkcs7-mime; smime-type=signed-data;
|
|
name=smime.p7m
|
|
Content-Transfer-Encoding: base64
|
|
Content-Disposition: attachment; filename=smime.p7m
|
|
|
|
|
|
Depending on the scenario, further Header Fields MAY be exposed in
|
|
the Outer Message Header Section, which is NOT RECOMMENDED unless
|
|
justified. Such Header Fields may include e.g.:
|
|
|
|
o References
|
|
|
|
o Reply-To
|
|
|
|
o In-Reply-To
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 17]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
4.1.4.1. Obfuscation of Outer Message Header Fields
|
|
|
|
If the values of the following Outer Message Header Fields are
|
|
obfuscated, those SHOULD assume the following values:
|
|
|
|
* Subject: ...
|
|
* Message-ID: <new randomly generated Message-ID>
|
|
* Date: Thu, 01 Jan 1970 00:00:00 +0000 (UTC)
|
|
|
|
[[ TODO: Consider alternatives for Date e.g. set to Monday 9am of the
|
|
same week. ]]
|
|
|
|
In certain implementations also the From, To, and/or Cc Header Field
|
|
MAY be obfuscated. Those may be replaced by e.g.
|
|
|
|
o To: Obfuscated anonymous@anonymous.invalid [1]
|
|
|
|
Such implementations may need to ensure that the Submission Entity
|
|
has access to the content of these Header Fields in clear text and is
|
|
capable of processing those. This is particularly relevant, if
|
|
proprietary Submission Entities are used.
|
|
|
|
A use case for obfuscation of all Outer Message Header Fields is
|
|
mixnet networks, i.e. "onion routing" for email (e.g. [pEp.mixnet]).
|
|
|
|
Note: It is for further study to what extent Header Field obfuscation
|
|
adversely impacts spam filtering.
|
|
|
|
4.1.5. Receiving User Facing Message Header Fields
|
|
|
|
The Receiving User Facing Message SHOULD be a verbatim copy of the
|
|
Inner Message.
|
|
|
|
4.1.6. Header Field Flow
|
|
|
|
The Following figure depicts the different message representations
|
|
(OrigM, InnerM, OuterM, RUFM) and which parts those are constructed
|
|
from:
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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|
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|
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|
|
OrigM InnerM Outer(S) OuterM(R) RUFM
|
|
|
|
<Trace-HF>
|
|
From (OrigM) = From
|
|
To (OrigM) = To
|
|
Cc (OrigM) = Cc
|
|
Bcc (OrigM) = Bcc*
|
|
Date (OrigM) = Date
|
|
Message-ID (OrigM)= Message-ID
|
|
Subject (new) = Subject
|
|
<MIME-HSp> (new) = <MIME-HSp>
|
|
|
|
PROTECTED: PROTECTED:
|
|
<Wrapper> (new) = <Wrapper>
|
|
From > From > From = From > From
|
|
To > To > To = To > To
|
|
Cc* > Cc > Cc = Cc > Cc
|
|
Bcc*
|
|
Date > Date > Date = Date > Date
|
|
Message-ID > Message-ID > Message-ID = Message-ID > Message-ID
|
|
Subject > Subject > Subject = Subject > Subject
|
|
<More HF> > <More HF> > <More HF> = <More HF> > <More-HF>
|
|
<MIME-HSp> > <MIME-HSp> > <MIME-HSp> = <MIME-HSp> > <MIME-HSp>
|
|
<Body> > <Body> > <Body> = <Body> > <Body>
|
|
<Signature>* (new)= <Signature>
|
|
|
|
|
|
Legend:
|
|
|
|
o OuterM(S): Outer Message (OuterM) at sending side (before handing
|
|
it over to the Submission Entity)
|
|
|
|
o OuterM(R): Outer Message at receiving side (as received by the
|
|
last hop, before decryption and/or signature verification is
|
|
applied to)
|
|
|
|
o InnerM: Inner Message (that protection is applied to)
|
|
|
|
o RUFM: Receiving User Facing Message
|
|
|
|
o More-HF: Additional Header Fields (HF) in the Original Message
|
|
(OrigM)
|
|
|
|
o Wrapper: MIME Header Section; with media type (message/RFC822) to
|
|
unambiguously delimit the inner message from the rest of the
|
|
message.
|
|
|
|
|
|
|
|
|
|
|
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 19]
|
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|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
o MIME-HSp: MIME Header Section part to describe the encryption or
|
|
signature as per [RFC8551]
|
|
|
|
o Trace-HF: Header Fields added in Transit (between sending and
|
|
receiving side) as per [RFC5322]
|
|
|
|
o >: taken over / copied from last column
|
|
|
|
o =: propagates unchanged, unless something unusual (e.g. attack)
|
|
happens
|
|
|
|
o *: HF that is often not present (also further HFs, e.g. To, may
|
|
not be present). If a HF is not present, naturally it can neither
|
|
be taken over nor propagated.
|
|
|
|
o (new) / (OrigM): HF or MIME-HSp is generated depending on the
|
|
decision in Section 4.1.7.1, while '(new)' / '(OrigM)' designate
|
|
the default.
|
|
|
|
4.1.7. Sending Side Message Processing
|
|
|
|
For a protected message the following steps are applied before a
|
|
message is handed over to the Submission Entity:
|
|
|
|
4.1.7.1. Step 1: Decide on Protection Level and Information Disclosure
|
|
|
|
The entity applying protection to a message must decide:
|
|
|
|
o Which Protection Level (signature and/or encryption) is applied to
|
|
the message? This depends on user request and/or local policy as
|
|
well as availability of cryptographic keys.
|
|
|
|
o Which Header Fields of the Original Message shall be part of the
|
|
Outer Message Header Section? This typically depends on local
|
|
policy. By default the Essential Header Fields are part of the
|
|
Outer Message Header Section; cf. Section 4.1.4.
|
|
|
|
o Which of these Header Fields are to be obfuscated? This depends
|
|
on local policy and/or specific Privacy requirements of the user.
|
|
By default only the Subject Header Field is obfuscated; cf.
|
|
Section 4.1.4.1.
|
|
|
|
4.1.7.2. Step 2: Compose the Outer Message Header Section
|
|
|
|
Depending on the decision in Section 4.1.7.1, compose the Outer
|
|
Message Header Section. (Note that this also includes the necessary
|
|
MIME Header Section part for the following protection layer.)
|
|
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 20]
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|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
Outer Header Fields that are not obfuscated should contain the same
|
|
values as in the Original Message (except for MIME Header
|
|
Section part, which depends on the protection level selected in
|
|
Section 4.1.7.1).
|
|
|
|
4.1.7.3. Step 3: Apply Protection to the Original Message
|
|
|
|
Depending on the Protection Level selected in Section 4.1.7.1, apply
|
|
signature and/or encryption to the Original Message, including the
|
|
wrapper (as per [RFC8551]), and set the result to the message as
|
|
Outer Message Body.
|
|
|
|
The resulting (Outer) Message is then typically handed over to the
|
|
Submission Entity.
|
|
|
|
[[ TODO: Example ]]
|
|
|
|
4.1.8. Receiving Side Message Processing
|
|
|
|
When a protected message is received, the following steps are
|
|
applied:
|
|
|
|
4.1.8.1. Step 1: Decrypt message and/or check signature
|
|
|
|
Depending on the protection level, the received message is decrypted
|
|
and/or its signature is checked as per [RFC8551].
|
|
|
|
4.1.8.2. Step 2: Construct the Receiving User Facing Message
|
|
|
|
The Receiving User Facing Message is constructed according to
|
|
Section 4.1.5.
|
|
|
|
The resulting message is handed over for further processing, which
|
|
typically involves rendering it for the user.
|
|
|
|
Note: Further study is needed to determine whether or not the Outer
|
|
Message Header Section, as received from the last hop, is preserved
|
|
for the user, and if so, how this is to be achieved.
|
|
|
|
4.2. Backward Compatibility Use Cases
|
|
|
|
4.2.1. Receiving Side MIME-Conformant
|
|
|
|
This section applies to the case where the sending side (fully)
|
|
supports Header Protection as specified in this document, while the
|
|
receiving side does not support this specification, but is MIME-
|
|
conformant according to [RFC2045], ff. (cf. Section 3.1.2) and
|
|
Section 3.1.2.1)
|
|
|
|
|
|
|
|
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|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
The sending side specification of the main use case (cf. {#main-use-
|
|
case}}) MUST ensure that receiving sides, that do not support this
|
|
specification, but are MIME-conformant according to [RFC2045], ff.
|
|
can still recognize and display the Inner Message (or rather the
|
|
RUFM) in such a way as to preserve any recursive structure, that is,
|
|
displaying or offering to display the encapsulated data in accordance
|
|
with its media type (cf. [RFC2049], Section 2).
|
|
|
|
[[TODO: Verify once solution is stable and update last sentence ]]
|
|
|
|
4.2.2. Receiving Side Not MIME-Conformant
|
|
|
|
This section applies to the case where the sending side (fully)
|
|
supports Header Protection as specified in this document, while the
|
|
receiving side neither supports this specification and *nor* is MIME-
|
|
conformant according to [RFC2045], ff. (cf. {{uc-ia-backward-
|
|
compatibility-use-cases}) and Section 3.1.2.2).
|
|
|
|
[I-D.autocrypt-lamps-protected-headers] describes a possible way to
|
|
achieve backward compatibility with existing S/MIME (and PGP/MIME)
|
|
implementations that predate this specification and are not MIME-
|
|
conformant (Legacy Display) either. It mainly focuses on email
|
|
clients that do not render emails using header protection (in a user
|
|
friendly manner) and may confuse the user. While this has been
|
|
observed occasionally in PGP/MIME (cf. [RFC3156]), the extent of
|
|
this problem with S/MIME implementations is still unclear. (Note: At
|
|
this time, none of the samples in
|
|
[I-D.autocrypt-lamps-protected-headers] apply header protection as
|
|
specified in Section 3.1 of [RFC8551], which is wrapping as Media
|
|
Type "message/RFC822".)
|
|
|
|
Should serious backward compatibility issues with rendering at the
|
|
receiver be discovered, the Legacy Display format described in
|
|
[I-D.autocrypt-lamps-protected-headers] may serve as a basis to
|
|
mitigate those issues (cf. Section 4.2).
|
|
|
|
Another variant of backward compatibility has been implemented by pEp
|
|
[I-D.pep-email], i.e. pEp Email Format 1.0. At this time pEp has
|
|
implemented this for PGP/MIME, but not yet S/MIME.
|
|
|
|
5. Security Considerations
|
|
|
|
[[ TODO ]]
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
6. Privacy Considerations
|
|
|
|
[[ TODO ]]
|
|
|
|
7. IANA Considerations
|
|
|
|
This document requests no action from IANA.
|
|
|
|
[[ RFC Editor: This section may be removed before publication. ]]
|
|
|
|
8. Acknowledgments
|
|
|
|
The authors would like to thank the following people who have
|
|
provided helpful comments and suggestions for this document: Berna
|
|
Alp, Claudio Luck, Daniel Kahn Gillmor, David Wilson, Hernani
|
|
Marques, Krista Bennett, Kelly Bristol, Robert Williams, Sofia
|
|
Balicka, Steve Kille, Volker Birk, and Wei Chuang.
|
|
|
|
9. References
|
|
|
|
9.1. Normative References
|
|
|
|
[I-D.ietf-lamps-header-protection-requirements]
|
|
Melnikov, A. and B. Hoeneisen, "Problem Statement and
|
|
Requirements for Header Protection", draft-ietf-lamps-
|
|
header-protection-requirements-01 (work in progress),
|
|
October 2019.
|
|
|
|
[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/info/rfc2045>.
|
|
|
|
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
|
|
Extensions (MIME) Part Two: Media Types", RFC 2046,
|
|
DOI 10.17487/RFC2046, November 1996,
|
|
<https://www.rfc-editor.org/info/rfc2046>.
|
|
|
|
[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/info/rfc2049>.
|
|
|
|
[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/info/rfc2119>.
|
|
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 23]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
|
|
DOI 10.17487/RFC5322, October 2008,
|
|
<https://www.rfc-editor.org/info/rfc5322>.
|
|
|
|
[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/info/rfc8551>.
|
|
|
|
9.2. Informative References
|
|
|
|
[I-D.autocrypt-lamps-protected-headers]
|
|
Einarsson, B., juga, j., and D. Gillmor, "Protected
|
|
Headers for Cryptographic E-mail", draft-autocrypt-lamps-
|
|
protected-headers-02 (work in progress), December 2019.
|
|
|
|
[I-D.melnikov-iana-reg-forwarded]
|
|
Melnikov, A. and B. Hoeneisen, "IANA Registration of
|
|
Content-Type Header Field Parameter 'forwarded'", draft-
|
|
melnikov-iana-reg-forwarded-00 (work in progress),
|
|
November 2019.
|
|
|
|
[I-D.pep-email]
|
|
Marques, H., "pretty Easy privacy (pEp): Email Formats and
|
|
Protocols", draft-marques-pep-email-02 (work in progress),
|
|
October 2018.
|
|
|
|
[pEp.mixnet]
|
|
pEp Foundation, "Mixnet", June 2020,
|
|
<https://dev.pep.foundation/Mixnet>.
|
|
|
|
[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/info/rfc3156>.
|
|
|
|
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
|
|
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
|
|
<https://www.rfc-editor.org/info/rfc4949>.
|
|
|
|
[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/info/rfc6376>.
|
|
|
|
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
|
|
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
|
|
<https://www.rfc-editor.org/info/rfc6409>.
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 24]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
|
|
Authorizing Use of Domains in Email, Version 1", RFC 7208,
|
|
DOI 10.17487/RFC7208, April 2014,
|
|
<https://www.rfc-editor.org/info/rfc7208>.
|
|
|
|
[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/info/rfc7489>.
|
|
|
|
9.3. URIs
|
|
|
|
[1] mailto:anonymous@anonymous.invalid
|
|
|
|
Appendix A. Additional information
|
|
|
|
A.1. Stored Variants of Messages with Bcc
|
|
|
|
Messages containing at least one recipient address in the Bcc header
|
|
field may appear in up to three different variants:
|
|
|
|
1. The message for the recipient addresses listed in To or Cc header
|
|
fields, which must not include the Bcc header field neither for
|
|
signature calculation nor for encryption.
|
|
|
|
2. The message(s) sent to the recipient addresses in the Bcc header
|
|
field, which depends on the implementation:
|
|
|
|
a) One message for each recipient in the Bcc header field
|
|
separately, with a Bcc header field containing only the address
|
|
of the recipient it is sent to.
|
|
|
|
b) The same message for each recipient in the Bcc header field
|
|
with a Bcc header field containing an indication such as
|
|
"Undisclosed recipients", but no addresses.
|
|
|
|
c) The same message for each recipient in the Bcc header field
|
|
which does not include a Bcc header field (this message is
|
|
identical to 1. / cf. above).
|
|
|
|
3. The message stored in the 'Sent'-Folder of the sender, which
|
|
usually contains the Bcc unchanged from the original message,
|
|
i.e., with all recipient addresses.
|
|
|
|
The most privacy preserving method of the alternatives (2a, 2b, and
|
|
2c) is to standardize 2a, as in the other cases (2b and 2c),
|
|
information about hidden recipients is revealed via keys. In any
|
|
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 25]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
case, the message has to be cloned and adjusted depending on the
|
|
recipient.
|
|
|
|
Appendix B. Document Changelog
|
|
|
|
[[ RFC Editor: This section is to be removed before publication ]]
|
|
|
|
o draft-ietf-lamps-header-protection-00
|
|
|
|
* Initial version (text partially taken over from
|
|
[I-D.ietf-lamps-header-protection-requirements]
|
|
|
|
Appendix C. Open Issues
|
|
|
|
[[ RFC Editor: This section should be empty and is to be removed
|
|
before publication. ]]
|
|
|
|
o Ensure "protected header" (Ex-Memory-Hole) option is (fully)
|
|
compliant with the MIME standard, in particular also [RFC2046],
|
|
Section 5.1. (Multipart Media Type) Section 4.1.1.2.
|
|
|
|
o Decide on format of obfuscated HFs, in particular Date HF
|
|
(Section 4.1.4.1)
|
|
|
|
o Impact on spam filtering, if HFs are obfuscated (Section 4.1.4.1)
|
|
|
|
o More examples (e.g. in Section 4.1.7)
|
|
|
|
o Should Outer Message Header Section (as received) be preserved for
|
|
the user? (Section 4.1.8.2)
|
|
|
|
o Change adding Content-Type header field parameter "forwarded" from
|
|
SHOULD to MUST (Section 4.1.3)?
|
|
|
|
o Decide on whether or not merge requirements from
|
|
[I-D.ietf-lamps-header-protection-requirements] into this
|
|
document.
|
|
|
|
o Decide what parts of [I-D.autocrypt-lamps-protected-headers] to
|
|
merge into this document.
|
|
|
|
o Enhance Introduction and Problem Statement sections
|
|
|
|
o Decide on whether or not specification for more legacy HP
|
|
requirements should be added to this document
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Hoeneisen & Melnikov Expires January 10, 2021 [Page 26]
|
|
|
|
Internet-Draft Header Protection S/MIME July 2020
|
|
|
|
|
|
o Verify ability to distinguish between Messages with Header
|
|
Protection as specified in this document and legacy clients and
|
|
update Section 3.1 accordingly.
|
|
|
|
o Verify simple backward compatibility case (Receiving Side MIME-
|
|
Conformant) is working; once solution is stable and update
|
|
paragraphs in {#uc-ia-bc-receiving-side-mime-conformant}} and
|
|
Section 4.2.1 accordingly.
|
|
|
|
o Improve definitions in Section 3.2
|
|
|
|
o Privacy Considerations Section 6
|
|
|
|
o Security Considerations Section 5
|
|
|
|
Authors' Addresses
|
|
|
|
Bernie Hoeneisen
|
|
pEp Foundation
|
|
Oberer Graben 4
|
|
CH-8400 Winterthur
|
|
Switzerland
|
|
|
|
Email: bernie.hoeneisen@pep.foundation
|
|
URI: https://pep.foundation/
|
|
|
|
|
|
Alexey Melnikov
|
|
Isode Ltd
|
|
14 Castle Mews
|
|
Hampton, Middlesex TW12 2NP
|
|
UK
|
|
|
|
Email: alexey.melnikov@isode.com
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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Hoeneisen & Melnikov Expires January 10, 2021 [Page 27]
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