Network Working Group B. Gondwana Internet-Draft Fastmail Pty Ltd Intended status: Standards Track 3 November 2025 Expires: 7 May 2026 A method for describing changes to emails draft-gondwana-dkim2-mailversion-00 Abstract This memo describes a method for describing the changes made to an email during common email modifications, for example those caused by mailing lists and forwarders. While this is general enough to be used for any changes, it is anticipated that this method will normally be used for removing added data rather than large complex changes. This method also captures hashes of important features of the message, allowing validation that the changes were described correctly, and allowing a signature which covers the Mail-Version header to, by extention, ensure that the important content of the message is unchanged. 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 2026. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. Gondwana Expires 7 May 2026 [Page 1] Internet-Draft Email Modification Versioning November 2025 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. Background and motivations . . . . . . . . . . . . . . . . . 2 2. The Mail-Version Header Field . . . . . . . . . . . . . . . . 3 2.1. Mail-Version Header . . . . . . . . . . . . . . . . . . . 3 2.2. body-recipe . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. header-recipe . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Iterative application . . . . . . . . . . . . . . . . . . . . 7 4. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Normative References . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Changes from Earlier Versions . . . . . . . . . . . 8 A.1. draft-gondwana-dkim2-mailversion-00 . . . . . . . . . . . 8 A.2. draft-gondwana-dkim2-modification-algebra-03 . . . . . . 9 A.3. draft-gondwana-dkim2-modification-algebra-02 . . . . . . 9 A.4. draft-gondwana-dkim2-modification-algebra-02 . . . . . . 9 A.5. draft-gondwana-dkim2-modification-algebra-01 . . . . . . 9 A.6. draft-gondwana-dkim2-modification-algebra-00 . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 1. Background and motivations Currently, when an email is sent with a DKIM signature, the message can go through multiple forwarders and still be authenticated, however if a single change is made to a header which is covered by the signature, or to the body, then the signature no longer validates - and it's impossible for the receiver to know what was changed, or even if the entire message was replaced. By producing a way to describe changes, the recipient can examine the sections which were changed and determine whether the change was malicious. By undoing the changes, it is possible to recreate a message which matches the original signature, and hence provide accountability for the content which was present in the copy of the message to which the signature was applied. Gondwana Expires 7 May 2026 [Page 2] Internet-Draft Email Modification Versioning November 2025 2. The Mail-Version Header Field This document describes an ordered set of header fields, each of which describes the message at a specific version, along with instructions on how to convert that version back to the previous version. To well formed, a message must have a monotonically increasing set of Mail-Version header fields, with the first having mv=1 and each having an increasing integer number, with no gaps. 2.1. Mail-Version Header The format of the value is a tag-list. Tag-lists contain key=value; key2=value2; - they can be wrapped, and all whitespace is normalised to a single space. TODO: we need to specify tag-list. Gondwana Expires 7 May 2026 [Page 3] Internet-Draft Email Modification Versioning November 2025 +==========+=============+==========================================+ | Tag | Type | Value | +==========+=============+==========================================+ | mv | position | Revision number (range: 1 to 100) | +----------+-------------+------------------------------------------+ | h | headerlist | List of headers signed by the hh | | | | field. | +----------+-------------+------------------------------------------+ | a | hash-alg | Hash Algorithm used (at least | | | | sha256 must be supported); used | | | | for hh, bh and ph | +----------+-------------+------------------------------------------+ | hh | base64 | Header Hash for the named headers, | | | | using the 'relaxed' header | | | | algorith from [DKIM] | +----------+-------------+------------------------------------------+ | bh | base64 | Body Hash, using the 'relaxed' | | | | body algorithm from [DKIM] | +----------+-------------+------------------------------------------+ | ph.n.m | base64 | Hash for the binary representation | | | | of the numbered mime part, after | | | | removal of any content-transfer- | | | | encoding (this is the data | | | | returned by FETCH BINARY[n.m] as | | | | described in [IMAP] section 7.5.2) | +----------+-------------+------------------------------------------+ | b | body-recipe | Recipe to replicate the previous | | | | version of the message body | +----------+-------------+------------------------------------------+ | h.header | head-recipe | Recipe to replicate the previous | | | | version of the named header field | +----------+-------------+------------------------------------------+ Table 1 TODO: do we want to add 'bh.n.m' to hash the un-decoded body of individual MIME parts, and hh.n.m to hash the headers of the MIME part as well? 2.2. body-recipe The Body Recipe is a comma separated list of instructions. Each instruction starts with a prefix. Commas can be followed by optional whitespace. The presence of a recipe replaces the previous value, so the tag- value b= creates an empty body, while the tag-value b=b: creates a message with a single blank line for the body. Gondwana Expires 7 May 2026 [Page 4] Internet-Draft Email Modification Versioning November 2025 +========+===========+===========================================+ | Prefix | Value | Action | +========+===========+===========================================+ | c: | start-end | Copy the lines (inclusive) numbered from | | | | 1. E.g. "c:1-3" copies the first three | | | | lines. | +--------+-----------+-------------------------------------------+ | b: | base64 | Decode the base64 to get the value of a | | | | line to insert. The base64 value does | | | | NOT include the trailing CRLF, which must | | | | be added at the end of the line, hence | | | | "b:" means insert an empty line, and | | | | "b:SGVsbG8=" inserts the line "Hello". | +--------+-----------+-------------------------------------------+ | z | none | If present, says that changes have been | | | | made to the body which can not be | | | | described to get back to the earlier | | | | version, meaning the signing system takes | | | | accountability for the full content. | +--------+-----------+-------------------------------------------+ Table 2 2.3. header-recipe The Header Recipe is a comma separated list of instructions. Each instruction starts with a prefix. Commas can be followed by optional whitespace. While key names are case insensitive, implementations SHOULD create the header with the same case as the key. The presence of a recipe removes every instance of the named header field before applying the recipe, so the tag-value h.foo= removes all instances of Foo from the message. Gondwana Expires 7 May 2026 [Page 5] Internet-Draft Email Modification Versioning November 2025 +========+===========+========================================+ | Prefix | Value | Action | +========+===========+========================================+ | c: | start-end | Copy the values of the header-fields | | | | with the indexes, numbered from 1 and | | | | starting at the bottom; | +--------+-----------+----------------------------------------+ | b: | base64 | Decode the base64 to get the value of | | | | a header field to insert. The base64 | | | | value does NOT include the trailing | | | | CRLF, which must be added at the end | | | | of the line, hence "b:" means insert a | | | | header-field with an empty value. | +--------+-----------+----------------------------------------+ | z | none | If present, says that changes have | | | | been made to the named header field | | | | which can not be described to get back | | | | to the earlier version, meaning the | | | | signing system takes accountability | | | | for the full content of this message. | +--------+-----------+----------------------------------------+ Table 3 NOTE: the headers are inserted from the bottom, i.e. prepended to the message in the order that they are named. e.g. given a message with headers: Foo: three Foo: two Foo: one And the recipe: h.Foo=c:1-1,b:Zm91cg==,c:2-3 The output will be: ~~~ Foo: three Foo: two Foo: four Foo: one ~~~ 2.4. Examples Example for a message which has had Subject and From replaced, and Reply-To added. Gondwana Expires 7 May 2026 [Page 6] Internet-Draft Email Modification Versioning November 2025 From: brong@fastmailteam.com.dmarc.fail To: dkim2@lists.ietf.org Reply-To: dkim2@lists.ietf.org Mail-Version: mv=2; h.Subject=b:QSByZXBsYWNlbWVudCBmb3IgREtJTQ==; h.From=b:YnJvbmdAZmFzdG1haWx0ZWFtLmNvbQo=; h.Reply-To= Example: Mail-Version: mv=3; b=c:1-500,c:520-520 Example - a URL was substituted in the content of the body (complex, but still easily doable!) Mail-Version: mv=4; b=c:1-500, b:PGEgaHJlZj0iaHR0cHM6Ly93d3cuZXhhbXBsZS5jb20iPkV4YW1wbGU8L2E+Cg==, c:501-702 It is expected that 'c' will normally be used to copy lines directly from the new message, however in cases where a message needs to transit 7 bit systems cleanly, the email modifier may need to re- encode the octets of the original message, and this allows for doing so, albeit at some expense in header bloat! 3. Iterative application To get back to the original message and confirm that it was unchanged, it is necessary to apply this algorith iteratively. For example if you receive a message for which there is a modification to the headers at mv=3 and a modification to both headers and body at mv=2, to recreate the original message you would first apply the header changes from mv=3, then apply the header and body changes for mv=2. If this doesn't create a message which validates with the initial mv=1 hash, then some hop has corrupted the message. 4. Security Since a Mail-Version header can be used to recreate any email content, implementations need to be aware that this could be used to bypass security checks, and passing the generated message to a parser could expose it to content that would otherwise be blocked by earlier security checks, e.g. the base64 output could generate 8 bit content or NULL bytes that would otherwise be blocked by a simple filter. Gondwana Expires 7 May 2026 [Page 7] Internet-Draft Email Modification Versioning November 2025 5. IANA Considerations IANA is requested to add to the Permanent Message Header Field Names registry the following record. * Header Field Name: Mail-Version * Template: * Protocol: mail * Status: standard * Trace: no * Reference: this document 6. Normative References [DKIM] 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, . [IMAP] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message Access Protocol (IMAP) - Version 4rev2", RFC 9051, DOI 10.17487/RFC9051, August 2021, . Appendix A. Changes from Earlier Versions A.1. draft-gondwana-dkim2-mailversion-00 * Rename this draft to "mailversion". * Rename the header field to 'Mail-Version' * Change 'v=' to 'mv=' so it matches DKIM2-Signature and doesn't confuse protocol versions * Removed 't:' optional fields entirely. * Added ha= to select the hash algorithm and documentation about how the hashes are calculated * Renamed bin.partspec to ph.partspec for "part hash" and described it with reference to IMAP. Gondwana Expires 7 May 2026 [Page 8] Internet-Draft Email Modification Versioning November 2025 * Added security considerations * Added IANA considerations A.2. draft-gondwana-dkim2-modification-algebra-03 * Rename header to 'MailVersion' * Remove all requirements that it integrates with DKIM2. * Add body hash and per-mime-part hashes (NOTE: this is a bunch of extra calculation, so definitely to discuss) A.3. draft-gondwana-dkim2-modification-algebra-02 * change the header format to have unique keys, making it fit the ABNF for these types of headers. * allow easier editing of multi-value headers by always popping the first header and always prepending newly added headers. * change body to use d.0, d.1, etc with the program in the value, so that the program ordering is reliable regardless of the parser used to read the header. A.4. draft-gondwana-dkim2-modification-algebra-02 * change to using line numbers rather than octet offsets * remove d= base64 decoding capability * for multiple lines; require a separate b= or t= for each line A.5. draft-gondwana-dkim2-modification-algebra-01 * rename 'DKIM2-Diff' headers to 'DKIM2-Delta' * add 'z=y' option to DKIM2-Delta-Body for "complete replacement" * add d= base64 decoding option to DKIM2-Delta-Body A.6. draft-gondwana-dkim2-modification-algebra-00 * original version [[This section to be removed by RFC Editor]] Gondwana Expires 7 May 2026 [Page 9] Internet-Draft Email Modification Versioning November 2025 Author's Address Bron Gondwana Fastmail Pty Ltd Level 2, 114 William Street 3000 Australia Phone: +61 457 416 436 Email: brong@fastmailteam.com Gondwana Expires 7 May 2026 [Page 10]