DKIM Access Control and Differential Changes

Abstract

This document specifies a DKIM (RFC 6376) extension that allows cryptographic verification of SMTP (RFC 5321) envelope data, and of DKIM signatures prior to IMF (RFC 5322) message content changes along the message path, addressing thus security glitches, and offering a new world of email solutions that move complexity away from lower network layers, where problems cannot be solved, complying with David Wheeler's fundamental theorem of software engineering, that "We can solve any problem by introducing an extra level of indirection". It updates DKIM to obsolete certain aspects that reality has proven to be superfluous, incomplete, or obsoleted. It is the future of email for email of the future.¶

1. Preface

Due to an IETF tooling bug iteration -7 of this draft could not become submitted on July 21st of 2025, before the IETF meeting in Madrid. A week later [rt5.ietf.org #44510] is still open, so this gives me time to add this preface with the presumed outcome of the session, introduction of the completely new DKIM2 that tracks in public SMTP envelope data beyond RFC 3461 "final delivery for the [original] message" that is, in mind. This proposal here would thus become "DKIM3" in the event that it is revived if the internet community (in large) objects using DKIM2.¶

So, shall this draft be revived at some later time, and if compatibility to the original DKIMv1 is not an issue, i would vote to simply go for DKIX- prefix all the time, and use the shorter DKIX-Sig:; i would furtherly vote for using header abbreviations for certain standard headers, as in, for example, D:F:T:L=IU[:.:xy] instead of Date:From:To:List-ID:List-Unsubscribe, where custom names come after standardized abbreviations after a separating field, "." in the above (but care for syntax), otherwise simply an empty field.¶

If compatibility to DKIMv1 is an issue, then maybe the below is of some value "as-is". Greetings, and Ciao! from Germany.¶

2. Introduction

DKIM[RFC6376] was not designed to cover SMTP[RFC5321] envelope data, allowing replay of valid, verifiable messages to an infinite set of recipients by malicious third parties, undetectable by sender and recipients. (To aid SMTP delivery to recipients in various conditions the optional "x=" expiration tag timestamp must be chosen so far in the future that malicious players have plenty of time to misuse messages.)¶

Whereas DKIM[RFC6376] standardized rudimentary, incomplete approaches to undo modifications of IMF[RFC5322] message content that happen along the message path, the overall design was agreed in not to survive them (compare, for example, [RFC6377]). The resulting paradigm of DKIM is "as long as one signature can be verified cryptographically, DKIM verification will succeed". This is problematic as message content changes may be falsely attributed to (the) address(es) in the IMF originator field(s). (Later policy-enforcing standards effectively complicated the situation, in that false attribution may now technically be avoidable, but mitigations of practice like "user A via B" will still be attributed to "A" by a human for one, and, in short, anything is valid if one DKIM signature is.)¶

Potentially many DKIM signatures may exist in a message. DKIM[RFC6376] gives hints on how verification can be performed, but, in practice, mitigations are applied in order to reduce excessive and useless verifications on hops down the message path: elder signatures are removed, or renamed, as changes are performed on message content, for example, by mailing-lists. An approach to avoid excessive network traffic and CPU work during message verification mitigates careless configurations.¶

The presented ACDC extension addresses these and more issues, backward and forward compatible, easy adoptable, and easy integratable into the current, existing infrastucture.¶

3. DKIM Vivid Adjustments

The adult, mature, and rock solid foundation of DKIM[RFC6376] is built upon. But this document obsoletes certain unused / incomplete aspects, and adjusts certain vivid parts of the DKIM core, as follows. The full context of the changes will become clear as the remains of this document unfold.¶

• The signature expiration tag "x=" is no longer optional. It MUST be used within DKIM-Signature: header fields to place a lifetime constraint when creating signatures.¶

  The maximum "t=" to "x=" delta MUST NOT be greater than 864000 seconds (ten days: to reach into the next working week). Example delta values for tag auto-generation may be the bounce defaults 432000 seconds (five days: used for example by the Mailman2 and mlmmj mailing-list managers and the postfix MTA), 345600 seconds (four days: OpenSMTPD MTA), 172800 seconds (two days: Exim MTA).¶

• ACDC, as below, aware software is urged to "oversign" aka "seal" aka sign fields that are not present at the time of signing, how DKIM (section 3.5, "h=" tag description) calls it. Modifications and/or additions of "regular" intermediates will then be covered by their "regular" signature, and the differences will be discoverable via ACDC.¶

• The advice of DKIM section 5.4.1, on recommended signature content, still applies, and IANA is asked to introduce a registry of further header fields to be included in cryptographic signature protection. This document hereby includes the Author Header Field[RFC9057] in both lists.¶

  DKIM section 3.7 defines how "Computing the Message Hashes" has to be performed. Different to the RSA algorithm (RFC 8017) solely defined for DKIM at the time section 3.7 was written, modern algorithms include checksumming themselves. Section 3.7 is hereby modified in that the input to "sig-alg", the "data-hash", can adapt to standardized algorithms as appropriate. If an algorithm chooses adaption, "hash-alg" is only used to produce the "body-hash", whereas the input formerly used to create the "data-hash" is fed in full into "sig-alg", instead of to "hash-alg". More formally, the new pseudo-code for the signature algorithm is:¶

  body-hash = hash-alg (canon-body, l-param)
  data-hash = hash-alg (h-headers, D-SIG, body-hash)
  signature = sig-alg (d-domain, selector,
                       data-hash / (h-headers, D-SIG, body-hash))
  

  ¶

4. DKIM ACDC

The DKIM[RFC6376] extension Access Control and Differential Changes:¶

• Places DKIM signatures in an ordered, numbered, random-accessible sequence which' state correlate. Identical DKIM signatures generated at the same hop, but which differ in only the used algorithm, share, however, a sequence number. With ACDC it can be, and usually is, sufficient to verify only the cheaply detectable highest numbered signature.¶
• Adds reversible data difference tracking, and as such supports cryptographical content verification of any (ACDC ware) intermediate message state, up to the initial variant as sent by the originator.¶
• Cryptographically protects the SMTP[RFC5321] envelope, that is, RCPT TO addresses as well as the MAIL FROM address. Replay of valid messages to initially not addressed recipients, as well as backscatter bounces to random addresses instead of the originator, become detectable.¶
• Allows cryptographically verifiable collection of statistics of organizational trust ([RFC5863], section 2.5) along the entire message path.¶
• Allows recognition of certain flagged conditions (along the message path) only by looking at the highest numbered signature.¶

The DKIM[RFC6376] extension Access Control and Differential Changes is announced by adding an "acdc=" tag to the DKIM-Signature: header field. (For efficiency reasons it SHOULD be placed early, before tags like "h=", "bh=" and "b=", for example.)¶

The tag starts with "sequence", a decimal number starting at 1, or incremented by 1 from the highest ACDC "sequence" encountered in the message; the maximum value is 999: if incrementing would result in overflow, the message MUST be rejected; detected sequence holes MUST also cause rejection (but see below); in both cases SMTP[RFC5321] reply code 550 is to be used; with enhanced SMTP status codes[RFC3463] 5.5.4 MUST be used.¶

Multiple DKIM-Signature: header fields with the same "sequence" MAY be generated by a domain, in which case each field MUST use a different "s=" selector and algorithm.¶

Flag description is normative. (Note the missing FWS separators around =.) ABNF[RFC5234]:¶

acdc = %x61 %x63 %x64 %x63 = sequence ":" 1*flag [ ":" id ]
sequence = 1*3DIGIT; DIGIT from RFC 5234
flag = "A" / "D" / "E" / "I" / "O" / "P" / "R" / "S" / "s" /
       "V" / "v" / "X" / "x" / "Y" / "y" / "Z" / "z"
id = *42(ALPHA / DIGIT / "+" / "-"); optional (bounce) identifier

A

Alongside the "V" flag (see there) only: all existing signatures were verified.¶

D

The message was modified at this hop, differential changes were generated, and are stored in a DKIM-DC: header field.¶

The "Y" flag has to be set.¶

E

The SMTP[RFC5321] envelope (MAIL FROM and/or RCPT TO) was modified. A new "Access Control" (see there) evaluation has been performed.¶

The "O" flag has to be set if the MAIL FROM changed. The "y" flag has to be set.¶

I

This DKIM-Signature: header field was generated at ingress. The purpose of such a field is that its flags can be used to query the verification state of the message, verifiably. (Also see the "R", "S", "s", "V" and "v" flags.)¶

All signature instances with this flag set MUST be removed when messages enter and leave the email system. (First.)¶

O

This hop claims the message origin.¶

This either means that the message originated at this hop, in which case the signature (usually, DKIM-typical) refers to the first address of the From: header field, and the "sequence" is 1. TODO: As in the "Mitigations for Future" section below, i (and not alone) would love to revive the original email concept of From:/Sender:, which is counteracted by DMARC, unfortunately.¶

It can also mean that the current hop was the, quoting [RFC3461], "final delivery for the [original] message", that the message got a "new envelope return address", that is, the MAIL FROM of the SMTP envelope was changed. In this case the "E" flag has to be set.¶

An "Access Control" (see there) evaluation has been performed.¶

P

Postmaster mode. With this flag set the behavior of ACDC borders test mode in that rejections must not occur (due to ACDC). This is to allow for a communication possibility window in a situation where messages would always be rejected, due to misconfigurations et cetera, and as such reflects SMTP[RFC5321] section 4.5.1 Minimum Implementation.¶

If the "sequence" is 1, message recipients have to be inspected. If the IMF[RFC5322] header fields To: and Cc: only contain a single addressee with the local part postmaster[RFC1123], and if the same "postmaster" is addressed as a SMTP[RFC5321] RCPT TO recipient, and if no more than two RCPT TO recipients exist in total, then the "P" flag has to be set.¶

Once set, all future ACDC signatures must copy it. It MUST, however, be removed when in conjunction with the "D" and/or "E" flags the according SMTP envelope conditions are no longer satisfied.¶

R

Reputation check to collect organizational trust ([RFC5863], section 2.5) along the signature chain was performed.¶

On top of the "V" (and possibly "A") flag(s) this means that all differential changes have been applied, and all signatures along the chain have been verified, and the entire chain validated correctly.¶

Only in signatures with a "sequence" greater than 1, and without the "Z" or "z" flag.¶

Informative remark: The presence of "R" reveals local state publically; however, in a chain of trust this seems desirable even. The use of organizational trust may for example mean to perform full reputation checks more and more sparingly, the higher the trust, falling back to only random checks.¶

S

Only in conjunction with the "I" flag: upon ingress the SPF[RFC7208] state was successfully verified.¶

s

Only in conjunction with the "I" flag: upon ingress the SPF[RFC7208] verification failed.¶

V

ACDC signature verified successfully.¶

The signature with the highest "sequence" has been verified correctly, the sequence of ACDC signatures is complete, and their flags make sense (in the sequence). In conjunction with the flag "R" even deeper inspection was performed.¶

If multiple signatures with the same highest "sequence" exist, the verifier behavior is unspecified in that "V" signals success: at least one signature was checked, and all tested signatures verified successfully. If all signatures were verified, the "A" flag MUST be set; in single-signature cases the "A" flag MAY be omitted.¶

Only in signatures with a "sequence" greater than 1.¶

v

DKIM signature verified successfully.¶

In signatures with "sequence" 1, then missing the "O" flag, it means the message originated at a non ACDC aware host, and normal DKIM processing was performed and succeeded. Unless DKIM processing succeeded for the DKIM signature which covered the messages' From: header field address, the "Z" flag must be set, otherwise the "z" flag.¶

In messages with a higher "sequence" it comes alongside the "X" flag: necessarily the ACDC chain was broken, and the message changed, by an intermediate non ACDC aware hop. The "z" flag must be set.¶

X

ACDC verification failed ("V" flag conditions not met); however, the normal DKIM signature verification was performed, and succeeded.¶

The "z" flag must be set.¶

x

DKIM verification failed.¶

In signatures with "sequence" 1, then missing the "O" flag, it means the message originated at a non ACDC aware host, and normal DKIM processing was performed and failed. The "z" flag must be set.¶

In messages with a higher "sequence" it comes alongside the "X" flag: necessarily the ACDC chain was broken, and the message changed, by an intermediate non ACDC aware hop. The "z" flag must be set.¶

Y

The message has seen IMF[RFC5322] modifications: somewhere along the chain the message data was modified. Once set, all future ACDC signatures must copy it.¶

y

The message has seen SMTP[RFC5321] envelope modifications: somewhere along the chain the envelope was modified. Once set, all future ACDC signatures must copy it.¶

Z

Announces the ACDC chain is incomplete. The message was processed by ACDC unaware hops. However, the message verifies correctly and seems to have never been modified non-reversibly. Once set, all future ACDC signatures must copy it, unless later downgraded to the "z" flag.¶

z

The message has seen non-reversible modifications, and cannot be cryptographically verified back to its origin. Once set, all future ACDC signatures must copy it.¶

If this flag is set ACDC looses its decisive meaning and "degrades" to normal DKIM (except for access control): no more differential data is generated, and messages are distributed further / accepted if just any signature verifies. (Software configuration MAY allow otherwise.)¶

"id"

The optional "bounce identifier" offers enough room to store Universally Unique IDentifiers[RFC9562].¶

It may be generated to help sending domains to uniquely identify messages within the DKIM "t=" and "x=" time delta, as well as to ensure that successively sent identical messages are not detected as being the same.¶

Receiving domains SHOULD NOT use this identifier due to the denial of service attack surface, regardless of collected organizational trust (see "R" flag).¶

Unknown flags MUST be ignored. Invalid flag combinations and flag misuse MUST result in rejection with SMTP reply code 550; if enhanced status codes[RFC3463] are used, 5.5.4 MUST be used. (This includes the "P" flag upon incorrect use.)¶

5. The DKIM-Store header field

The DKIM-Store header field has no meaning in the email system. The sole purpose of mentioning it is to announce that it MUST be removed when messages enter and leave the email system.¶

It could for example be temporarily created and used by non-integrated mail filter (milter) software to pass informational data in between the "ingress" and the "egress" processing side. To aid in software bugs and possible configuration errors this specification enforces removal of all occurrences.¶

Informative remark: In order to achieve locality it is suggested to encrypt data passed around in this temporary header field with a key internal to the "local" email processing system.¶

6. Access Control

SMTP[RFC5321] delivers messages to individual domains. With ACDC, when a SMTP envelope was created or changed, all distinct domain-names found within the list of intended SMTP RCPT TO addressees are collected, as the message needs to be forged on this individual domain base: ACDC will create DKIM-AC: header fields covering SMTP envelopes, and include them as messages are sent to individual domains.¶

The domains _dkimacdc DNS entries, as below, are queried. Any domain that announces ACDC support can be served by a single message for all recipients (possible limits, as below, aside).¶

For other domains, to guarantee anonymity, it is necessary to differentiate in between public recipients in the To: and Cc: header fields, and recipients not covered by these header fields. However, another anonymous approach is presented below.¶

Remarks: quality-of-service: for simplicity messages may always be forged on a single recipient base, individually.¶

In any case the completely prepared message, including the readily prepared DKIM-Signature:(s), is forged, (a) DKIM-AC: header field(s) is/are generated which cover(s) the logical recipient subset, and the resulting message is then sent.¶

ACDC aware recipient domains are expected to manage a message DKIM-AC: identity cache to mitigate replay attacks. (Hint: the DKIM-AC: signature seems like a natural cache key source, see below.) The now mandatory and constrained "x=" tag allows for finite identity cache sizes.¶

To keep the identity cache a write-once data structure, ACDC senders MUST NOT generate DKIM-AC: header fields with more than half of the 100 recipients that SMTP[RFC5321] section 4.5.3.1.8 guarantees as a minimum, unless a DNSSEC[RFC4033][RFC4034][RFC4035] protected, or otherwise known trustworthy, _dkimacdc DNS entry announced a different limit.¶

If more recipients need to be addressed on a single domain, multiple message forges with recipient subsets must be generated: like this each message forge is "atomic", and the DKIM-AC: header field covers all the SMTP envelope.¶

Informative remark: Implementations MAY offer configuration options to specify other (higher, lower) recipient limits. Like this the much higher limits in actual use (for example, the Exim MTA default is 50000) can be utilized.¶

An ACDC aware recipient domain that receives an "acdc=" tagged message without a DKIM-AC: header field MUST reject the message with SMTP reply code 550; if enhanced status codes[RFC3463] are used, 5.5.4 MUST be used.¶

It MUST likewise fail if the DKIM-AC: header field does not cover the SMTP envelope data. It MUST test for a superset of recipients, and only fail if an envelope recipient is not included in the DKIM-AC: header field.¶

It MUST reject messages which fail the signature check of a DKIM-AC: or DKIM-Signature: header field, or the condition and flag check verification, with SMTP reply code 550; the enhanced status code MUST be 5.7.7.¶

(Senders MAY use Delivery Status Notifications[RFC3461] to fine-tune the resulting behavior.)¶

7. Differential Changes

Whenever an ACDC enabled domain detects during DKIM-Signature: creation that the relaxed representation of a message was modified along its flight from ingress to egress, for example, when it was processed by a mailing-list which tagged the subject and added a message footer, a DKIM-DC: header field has to be created.¶

Informative remark: In an unbroken chain of ACDC signatures the DKIM-DC: covered changes can be applied in reverse order of creation in order to cryptographically verify all intermediate message states and signatures, back to the original version as sent by the sender.¶

8. Mitigations for Future

As of the time of this writing the email infrastructure is deeply divided due to standards like DMARC and SPF, which require mitigations to be applied in order to keep existing infrastructures in a usable state.¶

For example, SPF will not survive a single hop, which means that alias expansion, a widely used core feature of the email infrastructure, does no longer work. The IETF has no solution for this problem, but the FOSS scene has created a "Sender Rewriting Scheme" so that aliases can be used regardless.¶

As another example, DMARC causes a lot of mailing-lists to apply mitigations of various form and style: old DKIM signatures are removed, or renamed, often the From: header field is rewritten in a "User A via List B" style.¶

9. Example

An example that shows the flow of a single message with multiple different recipients, including mailing-lists and aliases.¶

Originator (yet forged for recipient domain f.g):

  MAIL FROM: <a@b.c>
  RCPT TO: <d@f.g>
  RCPT TO: <e@f.g>
  ...
  DKIM-AC: sn=1; s=K1; f=a@b.c; d=f.g; t=d; t=e; b=...
  DKIM-Signature: acdc=1:O; s=K1; ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a@b.c
  To: d@f.g, e@f.g, x@y.z, u@v.w, r@s.t, o@p.q
  ...

f.g, local delivery (to d@ and e@):

  ...
  DKIM-Signature: acdc=2:AIV; ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a@b.c
  ...

x@y.z -- a mailing-list!
It redistributes after RFC 2369 and RFC 2919 additions,
in-message unsubscribe footer, and From: mitigated
(in best RFC 3461 manner):

  MAIL FROM: <x@y.z>
  RCPT TO: <l@m.n>
  ...
  DKIM-AC: sn=2; s=K2; f=x@y.z; d=m.n; t=l; b=...
  DKIM-DC: sn=2; c=xz; h=BASE64; b=BASE64
  DKIM-Signature: acdc=2:ADEOVYy; s=K2 ...
  DKIX-Signature: acdc=2:ADEOVYy; s=K2 ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a(AT)b(DOT)c via x@y.z <x@y.z>
  ...
  List-Unsubscribe: bla

u@v.w -- an expanded alias!
The host honours RFC 3461, and changes MAIL FROM:

  MAIL FROM: <u@v.w>
  RCPT TO: <realu@realv.realw>
  ...
  DKIM-AC: sn=2; s=K2; f=u@v.w; d=realv.realw; t=realu; b=...
  DKIM-Signature: acdc=2:EOVy; s=K2 ...
  DKIX-Signature: acdc=2:EOVy; s=K2 ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a@b.c
  ...

r@s.t -- an expanded alias!
Note: will later fail SPF of a@b.c without Mitigations!

  MAIL FROM: <a@b.c>
  RCPT TO: <realr@reals.realt>
  ...
  DKIM-AC: sn=2; s=K2; f=a@b.c; d=reals.realt; t=realr; b=...
  DKIM-Signature: acdc=2:EVy; s=K2 ...
  DKIX-Signature: acdc=2:EVy; s=K2 ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a@b.c
  ...

o@p.q -- a mailing-list!
It redistributes after RFC 2369 and RFC 2919 additions,
in-message unsubscribe footer, without From: mitigation.
Note: will later fail DMARC of a@b.c without Mitigations!
(This is why DKIM-Signature "1" is not removed.)

  MAIL FROM: <o@p.q>
  RCPT TO: <X@X.X>
  ...
  DKIM-AC: sn=2; s=K2; f=o@p.q; d=X.X; t=X; b=...
  DKIM-DC: sn=2; c=xz; h=BASE64; b=BASE64
  DKIM-Signature: acdc=2:DEOVYy; s=K2 ...
  DKIX-Signature: acdc=2:DEOVYy; s=K2 ...
  DKIM-Signature: acdc=1:O; s=K1; ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a@b.c
  ...
  List-Unsubscribe: bla

l@m.n (recipient of x.y.z mailing-list), local delivery:

  ...
  DKIM-Signature: acdc=3:IVYy;
  DKIM-DC: sn=2; c=xz; h=BASE64; b=BASE64
  DKIX-Signature: acdc=2:ADEOVYy; s=K2 ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a(AT)b(DOT)c via x@y.z <x@y.z>
  ...

realr@reals.realt -- expanded alias target, local delivery:

  ...
  DKIM-Signature: acdc=3:IVy;
  DKIX-Signature: acdc=2:EVy; s=K2 ...
  DKIX-Signature: acdc=1:O; s=K1; ...
  From: a@b.c
  ...

12. References