DomainKeys Identified Mail Signatures v2 (DKIM2)
draft-ietf-dkim-dkim2-spec-05
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Authors | Richard Clayton , Wei Chuang , Bron Gondwana | ||
| Last updated | 2026-08-25 (Latest revision 2026-07-05) | ||
| Replaces | draft-clayton-dkim2-spec | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-dkim-dkim2-spec-05
Network Working Group R. Clayton
Internet-Draft Yahoo
Intended status: Standards Track W. Chuang
Expires: 26 February 2027 Google
B. Gondwana
Fastmail Pty Ltd
25 August 2026
DomainKeys Identified Mail Signatures v2 (DKIM2)
draft-ietf-dkim-dkim2-spec-05
Abstract
DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or
organization that owns a signing domain to document that it has
handled an email message by associating their domain with the
message. This is achieved by providing a hash value that has been
calculated on the current contents of the message and then applying a
cryptographic signature that covers the hash values and other details
about the transmission of the message. Verification is performed by
querying an entry within the signing domain's DNS space to retrieve
an appropriate public key. As a message is transferred from author
to recipient systems that alter the body or header fields will
provide details of their changes and calculate new hash values.
Further signatures will be added to provide a validatable "chain".
This permits validators to identify the nature of changes made by
intermediaries and apply a reputation to the systems that made
changed. DKIM2 also allows recipients to detect when messages have
been unexpectedly "replayed" and will ensure that Delivery Status
Notifications are only sent to entities that were involved in the
transmission of a message.
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."
Clayton, et al. Expires 26 February 2027 [Page 1]
Internet-Draft DKIM2 Signatures August 2026
This Internet-Draft will expire on 26 February 2027.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. DKIM2 architecture documents . . . . . . . . . . . . . . 5
2. Terminology and definitions . . . . . . . . . . . . . . . . . 5
2.1. Signer . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Forwarder . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Reviser . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Signing domain . . . . . . . . . . . . . . . . . . . . . 6
2.6. Originator . . . . . . . . . . . . . . . . . . . . . . . 6
2.7. Header field . . . . . . . . . . . . . . . . . . . . . . 7
2.8. Tag . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.9. Message body . . . . . . . . . . . . . . . . . . . . . . 7
2.10. Hash . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.11. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 7
2.12. Whitespace . . . . . . . . . . . . . . . . . . . . . . . 8
2.13. Imported ABNF tokens . . . . . . . . . . . . . . . . . . 8
2.14. Common ABNF tokens . . . . . . . . . . . . . . . . . . . 9
3. Signing and verification cryptographic algorithms . . . . . . 9
3.1. The SHA256 and SHA512 hashing algorithms . . . . . . . . 9
3.2. The RSA-SHA256 signing algorithm . . . . . . . . . . . . 10
3.3. The Ed25519-SHA256 signing algorithm . . . . . . . . . . 10
3.4. Other algorithms . . . . . . . . . . . . . . . . . . . . 10
3.5. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. Key management . . . . . . . . . . . . . . . . . . . . . 11
4. Unsigned header fields . . . . . . . . . . . . . . . . . . . 11
4.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Recipes . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Header Recipes . . . . . . . . . . . . . . . . . . . . . 15
5.2. Body Recipes . . . . . . . . . . . . . . . . . . . . . . 16
6. Message hash values . . . . . . . . . . . . . . . . . . . . . 17
Clayton, et al. Expires 26 February 2027 [Page 2]
Internet-Draft DKIM2 Signatures August 2026
6.1. Computing the body hash . . . . . . . . . . . . . . . . . 17
6.2. Computing the header fields hash . . . . . . . . . . . . 18
7. The Message-Instance header field . . . . . . . . . . . . . . 19
7.1. m= the revision number of the Message-Instance header
field . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. r= recipes to recreate the previous instance of the
message . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.3. h= the hash values for the message . . . . . . . . . . . 20
8. The DKIM2-Signature header field . . . . . . . . . . . . . . 21
8.1. i= the sequence number of the DKIM2-Signature header
field . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.2. m= the highest numbered Message-Instance header field . . 21
8.3. n= nonce value . . . . . . . . . . . . . . . . . . . . . 22
8.4. t= signature timestamp . . . . . . . . . . . . . . . . . 22
8.5. mf= the MAIL FROM used when the message was sent . . . . 22
8.6. rt= the RCPT TO value(s) used when the message was
sent . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.7. nd= the domain that will appear in the next DKIM2-Signature
header field . . . . . . . . . . . . . . . . . . . . . . 23
8.8. d= the domain associated with this signature. . . . . . . 24
8.9. s= the signature value(s) for the message . . . . . . . . 24
8.10. f= flags . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Signer actions . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Add any necessary Message-Instance header fields . . . . 26
9.2. Provide a Chain of Custody for the message . . . . . . . 27
9.3. Chain of Custody for imaginary hops . . . . . . . . . . . 28
9.4. The relaxed domain match algorithm . . . . . . . . . . . 29
9.5. Select a private key and corresponding Selector value . . 29
9.6. Calculate a signature value . . . . . . . . . . . . . . . 29
10. Verification requirements . . . . . . . . . . . . . . . . . . 30
10.1. Check the most recent signature and hashes for the
message . . . . . . . . . . . . . . . . . . . . . . . . 30
10.2. Checking the Message-Instance header fields . . . . . . 31
10.3. Checking the DKIM2-Signature header fields . . . . . . . 31
10.4. Interpret results/Apply local policy . . . . . . . . . . 31
11. Verifier actions . . . . . . . . . . . . . . . . . . . . . . 32
11.1. Output states . . . . . . . . . . . . . . . . . . . . . 32
11.2. Ensure that the DKIM2 header fields are valid . . . . . 33
11.3. Check the timestamps . . . . . . . . . . . . . . . . . . 34
11.4. Check the Chain of Custody . . . . . . . . . . . . . . . 34
11.5. Fetch the public key . . . . . . . . . . . . . . . . . . 35
11.6. Perform the signature verification calculation . . . . . 35
11.7. Validating body and header hashes . . . . . . . . . . . 36
11.8. Check if donotmodify and donotexplode requests were
honored . . . . . . . . . . . . . . . . . . . . . . . . 37
12. Delivery Status Notifications in the DKIM2 ecosystem . . . . 37
12.1. DSN contents . . . . . . . . . . . . . . . . . . . . . . 37
12.1.1. DSN propagation . . . . . . . . . . . . . . . . . . 38
Clayton, et al. Expires 26 February 2027 [Page 3]
Internet-Draft DKIM2 Signatures August 2026
12.1.2. Authentication of inbound DSNs . . . . . . . . . . . 38
13. Preventing transport conversions . . . . . . . . . . . . . . 39
14. EAI (RFC6530) considerations for DKIM2 . . . . . . . . . . . 40
15. IANA considerations . . . . . . . . . . . . . . . . . . . . . 40
16. Security considerations . . . . . . . . . . . . . . . . . . . 40
17. Changes from Earlier Versions . . . . . . . . . . . . . . . . 40
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
18.1. Normative References . . . . . . . . . . . . . . . . . . 42
18.2. Informative References . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or
organization to document that they have handled an email message by
associating a domain name [RFC1034] with the message [RFC5322]. A
public key signature is used to record that they have been able to
read the contents of the message and write to it.
Verification of claims is achieved by fetching a public key stored in
the DNS under the relevant domain and then checking the signature.
Message transit from author to recipient is through Forwarders that
typically make no substantive change to the message content and thus
preserve the DKIM2 signature. Where they do make a change the
changes they have made are documented so that these can be "undone"
and the original signature validated.
When a message is forwarded from one system to another an additional
DKIM2 signature is added on each occasion. This Chain of Custody
assists validators in distinguishing between messages that were
intended to be sent to a particular email address and those that are
being "replayed" to that address.
The Chain of Custody can also be used to ensure that delivery status
notifications are only sent to entities that were involved in the
transmission of a message.
Organizations that process a message can add to their signature a
request for feedback as to any opinion (for example, that the email
was considered to be spam) that the eventual recipient of the message
wishes to share.
Clayton, et al. Expires 26 February 2027 [Page 4]
Internet-Draft DKIM2 Signatures August 2026
1.1. DKIM2 architecture documents
Readers are advised to be familiar with the material in TBA, TBA and
TBA which provide the background for the development of DKIM2, an
overview of the service, and deployment and operations guidance and
advice.
2. Terminology and definitions
This section defines terms used in the rest of the document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. These words take their normative meanings only when they
are presented in ALL UPPERCASE.
DKIM2 is designed to operate within the Internet Mail service, as
defined in [RFC5598]. Basic email terminology is taken from that
specification.
DKIM2 inherits many ideas from DKIM ([RFC6376]) which, for clarity we
refer to in this specification as DKIM1. In addition, some features
were influenced by experience with (see [CONCLUDEARC]) the
experimental ARC protocol ([RFC8617]).
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
This document uses JSON [RFC8259] to encode the Recipes which record
changes made to a message header fields or body. The JSON objects
are then base64 encoded. This means that a standard JSON parser can
be used to create what may be quite complex data structures.
Unrecognised fields within JSON objects MUST be ignored.
2.1. Signer
Elements in the mail system that sign messages on behalf of a domain
are referred to as Signers. These may be MUAs (Mail User Agents),
MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other
agents such as mailing list "exploders". In general, any Signer will
be involved in the injection of a message into the message system in
some way. The key point is that a message must be signed before it
leaves the administrative domain of the Signer.
Clayton, et al. Expires 26 February 2027 [Page 5]
Internet-Draft DKIM2 Signatures August 2026
2.2. Forwarder
[RFC5598] defines a Relay as transmitting or retransmitting a message
but states that it will not modify the envelope information or the
message content semantics. It also defines a Gateway as a hybrid of
User and Relay that connects heterogeneous mail services. In this
document we use the concept of a Forwarder which is an MTA that
receives a message and then, as an alternative to delivering it into
a destination mailbox, can forward it on to another system in an
automated, pre-determined, manner.
2.3. Reviser
As will be seen, a Forwarder may alter the message content or header
fields, in such a way that existing signatures on the message will no
longer validate. If so, then a record will be made of these changes.
We call a Forwarder that makes such changes a Reviser.
2.4. Verifier
Elements in the mail system that verify signatures are referred to as
Verifiers. These may be Forwarders, Revisers, MTAs, Mail Delivery
Agents (MDAs), or MUAs. It is an expectation of DKIM2 that a
recipient of a message will wish to verify some or all signatures
before determining whether or not to accept the message or pass it on
to another entity.
2.5. Signing domain
A domain name associated with a signature. This domain may be
associated with the author of an email, their organization, a company
hired to deliver the email, a mailing list operator, or some other
entity that handles email. What they have in common is that at some
point they had access to the entire contents of the email and were in
a position to add their signature to the email.
2.6. Originator
The entity that creates and sends the initial form of a message. The
Originator adds the first Message-Instance header field (m=1) and the
first DKIM2-Signature header field (i=1) to the message.
Clayton, et al. Expires 26 February 2027 [Page 6]
Internet-Draft DKIM2 Signatures August 2026
2.7. Header field
As defined in [RFC5322], a header field is a single logical line in
the message header consisting of a field name, a colon, and a field
body (value). In this document "header field" always refers to a
single field; "header fields" (plural) refers to multiple fields.
The unqualified term "header" is avoided to prevent ambiguity.
2.8. Tag
A named element within a header field (see Section 7 and Section 8).
A tag consists of a tag-name and a tag-value separated by an equals
sign. Tags are separated by semicolons within the header field.
2.9. Message body
The content of an email message that follows the blank line after the
header fields, treated as a sequence of octets. In this document,
the terms "body" and "message body" are used interchangeably.
2.10. Hash
A fixed-length value produced by applying a cryptographic hash
function (such as SHA-256) to an input. DKIM2 uses hashes to create
a compact, verifiable representation of message header fields and the
message body.
2.11. Glossary
The following terms are used throughout this document:
DKIM1 The original DomainKeys Identified Mail protocol as specified
in [RFC6376].
DKIM2-Signature A header field containing a cryptographic signature
over the Message-Instance and DKIM2-Signature header fields of a
message, along with metadata about the signing domain, SMTP
envelope, and timestamp.
Message-Instance A header field containing cryptographic hashes of
the message header fields and body, along with optional Recipes
that allow undoing changes made at that hop.
Recipe A set of instructions encoded as a JSON object within the r=
tag of a Message-Instance header field. Recipes allow a Verifier
to reconstruct the previous state of a message from its current
state, by specifying which parts of the header fields or body to
copy and which literal values to substitute.
Clayton, et al. Expires 26 February 2027 [Page 7]
Internet-Draft DKIM2 Signatures August 2026
Chain of Custody The sequence of DKIM2-Signature header fields on a
message, each recording the SMTP envelope addresses (MAIL FROM and
RCPT TO) used at each hop. A valid Chain of Custody demonstrates
that the message followed a plausible path from Originator to the
current recipient.
Selector A subdivision of the key namespace for a signing domain,
used to look up the public key in DNS. The Selector value is
combined with the signing domain to form the DNS query name:
selector._domainkey.domain.
2.12. Whitespace
There are two forms of whitespace used in this specification:
* WSP represents simple whitespace, i.e., a space or a tab character
(formal definition in [RFC5234]).
* FWS is folding whitespace. It allows multiple lines separated by
CRLF followed by at least one whitespace, to be joined.
The formal ABNF for these are (WSP given for information only):
WSP = SP / HTAB
FWS = [*WSP CRLF] 1*WSP
The definition of FWS is identical to that in [RFC5322] except for
the exclusion of obs-FWS.
2.13. Imported ABNF tokens
The following tokens are imported from other RFCs as noted. Those
RFCs should be considered definitive.
The following tokens are imported from [RFC5321]:
* "Domain"
* "Forward-path"
* "reverse-path"
The following tokens are imported from [RFC5322]:
* "field-name" (name of a header field)
Clayton, et al. Expires 26 February 2027 [Page 8]
Internet-Draft DKIM2 Signatures August 2026
Other tokens not defined herein are imported from [RFC5234]. These
are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
etc.
2.14. Common ABNF tokens
The following ABNF tokens are used elsewhere in this document:
ALPHADIGITD = (ALPHA / DIGIT / "-" / "_")
textstring = [FWS] ALPHADIGITD *(ALPHADIGITD) [FWS]
ALPHADIGITPS = (FWS / ALPHA / DIGIT / "+" / "/")
base64string = ALPHADIGITPS *(ALPHADIGITPS) [[FWS] "=" [[FWS] "="]]
Note that base64strings are defined in [RFC4648], but that document
does not contain any ABNF. A base64string MUST be padded with zero
bits and trailing = characters provided if needed. This allows
implementations to compare base64string values without decoding them.
Note that the definition of base64string allows for the presence of
FWS, which simplifies folding header fields to an allowable line
length. FWS within base64strings will be ignored when their value is
being used.
3. Signing and verification cryptographic algorithms
DKIM2 supports multiple hashing and digital signature algorithms.
Two hashing algorithms (SHA256 and SHA512) and two signing algorithms
(RSA-SHA256 and Ed25519-SHA256) are defined in this specification.
Signers MAY implement either or both of the two hashing algorithms
and either or both of the two signing algorithms. Verifiers MUST
implement all four algorithms.
3.1. The SHA256 and SHA512 hashing algorithms
The SHA256 hashing algorithm may be used to compute body and header
hashes as defined in Section 6.1 and Section 6.2. As an alternative
SHA512 may be used. Both hash functions are specified in (FIPS-
180-4-2015).
The resultant values are identified by the text string "sha256" or
"sha512" as appropriate and are placed into Message-Instance header
fields.
Clayton, et al. Expires 26 February 2027 [Page 9]
Internet-Draft DKIM2 Signatures August 2026
3.2. The RSA-SHA256 signing algorithm
The RSA-SHA256 signing algorithm computes a hash over all the
Message-Instance and DKIM2-Signature header fields as described in
Section 9.6 using SHA-256 (FIPS-180-4-2015) as the hash-alg. That
hash is then signed by the Signer using the RSA algorithm (defined in
PKCS#1 version 1.5 [RFC8017]) as the crypt-alg and the Signer's
private key. The hash MUST NOT be truncated or converted into any
form other than the native binary form before being signed. The
signing algorithm MUST use a public exponent of 65537.
Signers MUST use RSA keys of at least 1024 bits. Verifiers MUST be
able to validate signatures with keys ranging from 1024 bits to 2048
bits, and they MAY be able to validate signatures with larger keys.
The signature value (expressed in base64) is placed (with the
identifying text string "rsa-sha256") into DKIM2-Signature header
fields.
3.3. The Ed25519-SHA256 signing algorithm
The Ed25519-SHA256 signing algorithm computes a hash over all the
Message-Instance and DKIM2-Signature fields as described in
Section 9.6 using SHA-256 (FIPS-180-4-2015) as the hash-alg. It
signs the hash with the PureEdDSA variant Ed25519, as defined in
Section 5.1 of [RFC8032].
The signature value (expressed in base64) is placed (with the
identifying text string "ed25519-sha256") into DKIM2-Signature header
fields.
3.4. Other algorithms
Other algorithms MAY be defined in the future. Verifiers MUST ignore
any hashes or signatures using algorithms that they do not implement.
3.5. Selectors
To support multiple concurrent public keys per signing domain, the
key namespace is subdivided using Selectors.
Clayton, et al. Expires 26 February 2027 [Page 10]
Internet-Draft DKIM2 Signatures August 2026
The number of public keys and corresponding Selectors for each domain
is determined by the domain owner. Many domain owners will use just
one Selector, whereas administratively distributed organizations can
choose to manage disparate Selectors and key pairs in different
regions or on different email servers. Selectors can also be used to
delegate a signing authority, which can be withdrawn at any time.
Selectors also make it possible to seamlessly replace keys on a
routine basis by signing with a new Selector, while keeping the key
associated with the old Selector available.
Periods are allowed in Selectors and are component separators.
Periods in Selectors define DNS label boundaries in a manner similar
to the conventional use in domain names. This will allow portions of
the Selector namespace to be delegated.
ABNF:
selector = Domain
3.6. Key management
Some level of assurance is required that a public key is associated
with the claimed Signer. DKIM2 does this by fetching the key from
the DNS for the domain specified in the d= field of the
DKIM2-Signature header field.
DKIM2 keys are stored in a subdomain named "_domainkey". Given a
DKIM2-Signature field with a "d=" tag of "example.com" and a Selector
of "foo.bar", the DNS query will be for
"foo.bar._domainkey.example.com".
NOTE: these keys are no different, and are stored in the same
locations as those for DKIM1 ([RFC6376]).
Further details can be found in [DKIMKEYS].
4. Unsigned header fields
As explained in detail below, DKIM2 will provide a cryptographic
signature for all header fields that are present within a message,
whether they have a standardised meaning or are just placed there at
the whim of a particular sender.
However, for reasons of simplicity some header fields are not signed
and processing of the message MUST be performed as if they were not
present.
The exceptions fall into three classes:
Clayton, et al. Expires 26 February 2027 [Page 11]
Internet-Draft DKIM2 Signatures August 2026
1. Trace Headers
"Received" or "Return-Path" header fields MUST be ignored. These
are Trace Headers as described in [RFC5321] and serve only to
document details of the SMTP transmission process.
The experimental "Delivered-To:" header field ([RFC9228]) MUST
also be ignored.
Other specifications designate further header fields as trace
fields. A survey of such specifications, using the methodology
of [HDRMAINT], identifies "Auto-Submitted", "DL-Expansion-
History", "Original-Recipient", "SIO-Label-History", "VBR-Info"
and "X400-Trace", which MUST also be ignored.
The same survey records header fields which are added by handling
agents in transit even though no specification designates them as
trace fields: "Apparently-To" and "X400-Received". For DKIM2 it
is that behaviour, rather than any formal designation, which
matters, so these MUST also be ignored.
Any header field whose field name starts with "Received-", such
as "Received-SPF" ([RFC7208]), MUST be ignored. This allows
future specifications to create new trace header fields of this
form without affecting DKIM2 processing.
2. Other email authentication mechanisms
"DKIM-Signature" header fields and the "ARC-Authentication-
Results", "ARC-Message-Signature" and "ARC-Seal" header fields
([RFC8617]) MUST be ignored.
Not including DKIM1 and ARC header fields means that systems that
wish to add other types of signature as well as a DKIM2 signature
are free to do this in any convenient order.
3. Headers with purely local meaning
"Authentication-Results header fields MUST be ignored. These
header fields are defined in [RFC8601] where it is made clear
that their contents are only intended for use within the the
""trust boundary" of an Administrative Management Domain". As
they can only used reliably within that trust boundary there has
been no need to secure them cryptographically and given how they
may be added or removed as messages cross trust boundaries it is
inconvenient and unnecessary for DKIM2 to sign them.
Clayton, et al. Expires 26 February 2027 [Page 12]
Internet-Draft DKIM2 Signatures August 2026
Header fields with a header field name starting with "X-" MUST be
ignored. Currently deployed email systems use these fields as
proprietary Trace headers or for other proprietary reasons. They
have no defined meaning for other systems and it considerably
simplifies reporting on changes to header fields to ignore them.
Note that the recommendations in [RFC6648] mean that it is most
unlikely that any header field name starting "X-" will be
developed within the IETF.
4.1. Summary
These header fields are not signed:
Apparently-To:
ARC-Authentication-Results:
ARC-Message-Signature:
ARC-Seal:
Authentication-Results:
Auto-Submitted:
Delivered-To:
DKIM-Signature:
DL-Expansion-History:
Original-Recipient:
Received:
Received-*:
Return-Path:
SIO-Label-History:
VBR-Info:
X-*:
X400-Received:
X400-Trace:
5. Recipes
A set of Recipes is used to recreate the previous version of the body
and/or header fields of a message. The Recipes are provided within a
JSON object with the schema:
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"$id": "https://dkim2.org/schemas/recipe-v1",
"title": "DKIM2 Recipes",
"description": "See draft-dkim-dkim2-spec",
"type": "object",
"properties": {
"h": {
"description": "Recipes to recreate specific header fields",
"type": "object",
Clayton, et al. Expires 26 February 2027 [Page 13]
Internet-Draft DKIM2 Signatures August 2026
"minProperties": 1,
"additionalProperties": { "$ref": "#/$defs/recipe-steps" }
},
"b": {
"description": "Recipes to recreate the body",
"oneOf": [
{
"description": "Body Recipes",
"$ref": "#/$defs/recipe-steps"
},
{
"description": "Previous body state cannot be recreated",
"type": "null"
}
]
}
},
"anyOf": [
{ "required": ["h"] },
{ "required": ["b"] }
],
"$defs": {
"recipe-steps": {
"type": "array",
"items": {
"oneOf": [
{
"description": "Copy lines/fields, start to end inclusive",
"type": "object",
"properties": {
"c": { "type": "array",
"items": { "type": "integer", "minimum": 1 },
"minItems": 2, "maxItems": 2
}
},
"required": ["c"], "additionalProperties": false
},
{
"description": "Data lines/values to emit",
"type": "object",
"properties": {
"d": { "type": "array",
"items": { "type": "string" },
"minItems": 1
}
},
"required": ["d"], "additionalProperties": false
}
Clayton, et al. Expires 26 February 2027 [Page 14]
Internet-Draft DKIM2 Signatures August 2026
]
}
}
}
}
Note that the specification of JSON schemas is maintained by the JSON
Schema organisation, and the relevant specification document is
linked to by the $schema field in each JSON schema.
5.1. Header Recipes
A Header Recipe is an array of instructions applied to the specified
header fields with the given header field name. These Recipes are
applied in order to the message which has been received so as to
recreate the message as it was before modifications were made.
Recipes MUST be provided for any relevant header field (see Section 4
for header fields that are not relevant) that is changed; i.e. it is
not possible to indicate that the previous state of the header fields
cannot be recreated.
If there is no "h" field in the JSON object then there was no
modification to the header fields.
Header field names in the JSON keys MUST be in lower case, but they
will be matched with the message without regard to case. Recipes
SHOULD NOT be specified for header fields that will not be signed
(see Section 4
If a header field name is not present in the JSON object then all
header fields with that header field name are to be retained.
If the Recipe array for a header field name that is present in the
JSON object is empty then all instances of that header field are to
be removed to reinstate the previous state of the message.
Header fields are numbered "bottom up" (the opposite direction to the
body lines). That is to say, when walking the header fields from the
top of the message to end of the header fields then the last header
field instance encountered with any particular header field name is
numbered 1, the header field (with the same header field name) above
that is numbered 2, and so on.
The header fields should be treated as being unwrapped (in the normal
[RFC5321] manner). That is, all of the physical lines that form a
single header field are processed under the same logical number.
Clayton, et al. Expires 26 February 2027 [Page 15]
Internet-Draft DKIM2 Signatures August 2026
The Recipes are processed in order and the resulting header fields
are emitted so that later header field will appear above earlier
header fields in the recreated message.
Each Recipe step is a JSON object with exactly one key:
A "c" step has the form {"c": [start, end]}. The relevant header
field instances numbered from start to end inclusive, are to be
emitted. The start value of each "c" step MUST be in ascending order
and MUST be greater than the end value of all preceding "c" steps for
this header field name.
A "d" step has the form {"d": ["value1", "value2", ...]}. Each string
in the array is treated as a value to which the relevant header field
name and a colon is prepended and a CRLF is appended and the
resultant string is then emitted. Note that the way in which hashes
are calculated (see Section 6.2) means that no heed needs to be taken
of wrapping or the case of the header field name. The text strings
MUST NOT contain CR or LF characters. If a string is empty then the
CRLF will immediately follow the header field name and colon.
5.2. Body Recipes
A Body Recipe is an array of instructions applied to the message body
which can recreate the message as it was before modifications were
made.
If there is no "b" field in the JSON object then there was no
modification to the message body. Note that the JSON schema requires
either "h" or "b" to be present.
If the "b" field is null (there are no Recipes) then the previous
state of the message body cannot be recreated. Verifiers of the
message may be able to determine, by seeing which entity makes this
declaration, that this is acceptable to them because, for example,
that entity is providing a contractually arranged service.
Body lines are numbered "top down" (the opposite direction to the
header fields). The first line of the body (immediately after the
blank line that indicates that there are no more header fields) is
numbered 1.
The Recipes are processed in order and the resulting body lines
fields are emitted so that later lines will appear below earlier
lines in the recreated message.
Each Recipe step is a JSON object with exactly one key:
Clayton, et al. Expires 26 February 2027 [Page 16]
Internet-Draft DKIM2 Signatures August 2026
A "c" step has the form {"c": [start, end]}. The message body lines
from start to end, inclusive, are to be emitted. The start value of
each "c" step MUST be in ascending order and MUST be greater than the
end value of all preceding "c" steps.
A "d" step has the form {"d": ["line1", "line2", ...]}. Each string
in the array has a CRLF appended and the resultant string is emitted.
The text strings MUST NOT contain CR or LF characters. If a string
is empty then just a CRLF is emitted.
6. Message hash values
A set of cryptographic "hashes" are used to record the current
message body and header fields. The hashes are placed into the h=
tag of a Message-Instance header field.
Since Message-Instance header fields are ignored when calculating the
header hash value, the body hash and header hash may be calculated in
any convenient order.
6.1. Computing the body hash
The body of messages is treated as merely a string of octets. DKIM2
messages MAY be either in plain-text or in MIME format; no special
treatment is afforded to MIME content. Message attachments in MIME
format MUST be included in the content that is signed.
The DKIM2 body hash is calculated in the same manner as DKIM1's
"simple" scheme:
All empty lines at the end of the message body are ignored. An empty
line is a line of zero length after removal of the line terminator.
If there is no body or no trailing CRLF on the message body, a CRLF
is added. That is "*CRLF" at the end of the body is converted to
"CRLF".
No other changes are made to the body, which is then processed by the
relevant hash algorithm(s). The name of the hash and the hash value
(converted to base64 form) is then inserted into (Signers) or
compared to (Verifiers) the value of the "h=" tag of the Message-
Instance header field that is being created/verified. If multiple
hashes are calculated then multiple entries within the "h=" value
will be inserted/compared.
Clayton, et al. Expires 26 February 2027 [Page 17]
Internet-Draft DKIM2 Signatures August 2026
6.2. Computing the header fields hash
The header fields hash calculation done by a Signer MUST apply the
following steps in the order given. A Verifier will need to do the
equivalent steps in order to check that the hash they have received
is correct.
* Ignore some header fields
When calculating the header field hash some header fields are
entirely ignored. See Section 4 for the list of header field
names that MUST be ignored.
When calculating the header field hash any "Message-Instance" or
"DKIM2-Signature" header fields MUST be ignored. These header
fields will be included in the hash value that will be signed by a
DKIM2-Signature header field and it simplifies implementations if
they are not included twice, especially when determining whether
all modifications to a message have been correctly declared.
* Convert all header field names (not the header field values) to
lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
* Unfold all header field continuation lines as described in
[RFC5322]; in particular, lines with terminators embedded in
continued header field values (that is, CRLF sequences followed by
WSP) MUST be interpreted without the CRLF. Implementations MUST
NOT remove the CRLF at the end of the header field value.
* Convert all sequences of one or more WSP characters to a single SP
character. WSP characters here include those before and after a
line folding boundary.
* Delete all WSP characters at the end of each unfolded header field
value.
* Delete any WSP characters remaining before and after the colon
separating the header field name from the header field value. The
colon separator MUST be retained.
* Place the header fields in alphabetical order by the header field
name.
* If there is more than one header with the same header field name
then the header fields are placed in the order in which they were
likely to have been placed into the message header, that is from
the last within the header upwards (the same ordering as is used
in the header Recipes (see Section 5.1).
Clayton, et al. Expires 26 February 2027 [Page 18]
Internet-Draft DKIM2 Signatures August 2026
It is sometimes suggested that some MTAs re-order header fields
after they receive an email. If an MTA does change the order of
header fields with the same header field name (and those header
fields will be included in the hash calculation) then it is their
responsibility to recover the original order before verifying an
existing signature or passing a previously signed message to
another MTA that may wish to do such verification.
* The hash(es) of the concatenated header fields are calculated.
The name of the hash and the hash value (converted to base64 form) is
then inserted into (Signers) or compared to (Verifiers) the value of
the "h=" tag of the Message-Instance header field that is being
created/verified. If multiple hashes are calculated then multiple
entries within the "h=" value will be inserted/compared.
7. The Message-Instance header field
A Message-Instance header field documents the current contents of the
message and, in the case of a Reviser, records any relevant changes
that have been made to the incoming message.
The Message-Instance header field is a list of tag values as
described below. The m= and h= tags MUST be present. The r= tag is
optional. Note that the syntax is such that semi-colons will never
occur within tags or values, they only act as a separators.
The tag identifiers (before the = sign) MUST be treated as case
insignificant, the tag value (after the = sign) is case significant.
The tags may appear in any order, but MUST be only one of each kind.
Unknown tags, for extensions, MUST be ignored.
ABNF:
mi-field = "Message-Instance:" mi-tag-list
mi-tag-list = *([FWS] mi-tag [FWS] ";" [FWS])
mi-tag = mi-m-tag / mi-h-tag / mi-r-tag / x-tag
; x-tag is for extension
x-tag = x-tag-name [FWS] "=" [FWS] [x-tag-value]
x-tag-name = ALPHA *(ALPHA / DIGIT / "_")
x-tag-char = %x21-3A / %x3C-7E
x-tag-value = x-tag-char *([FWS] x-tag-char)
Clayton, et al. Expires 26 February 2027 [Page 19]
Internet-Draft DKIM2 Signatures August 2026
7.1. m= the revision number of the Message-Instance header field
The Originator of a message uses the value 1. Further Message-
Instance header fields are added with a value one more than the
current highest numbered Message-Instance header field. Gaps in the
numbering MUST be treated as making the whole message impossible to
verify.
ABNF:
mi-m-tag = %x6d [FWS] "=" [FWS] 1*DIGIT
7.2. r= recipes to recreate the previous instance of the message
The r= tag value is the base64 encoded version of the JSON object
that contains the Recipes that allow the previous instance of the
message to be recreated (see Section 5}.
ABNF:
mi-r-tag = %x72 [FWS] "=" base64string
7.3. h= the hash values for the message
The h= tag value contains the hash name, header hash value and body
hash value. Calculating the hash values is explained in Section 6.
To provide for algorithmic dexterity more that one pair of hash
values, using a different algorithm MAY be supplied in the same
Message-Instance header field. An algorithm MUST NOT be present more
than once in a single Message-Instance header field; if the hash
values were the same this would be pointless and if they differ then
the Sender is presumably attempting to confuse a Verifier.
ABNF:
mi-h-tag = %x68 [FWS] "=" hash-set *("," hash-set)
hash-set = [FWS] hash-name [FWS] ":" header-hash ":" body-hash
hash-name = "sha256" / "sha512" / x-hash-name
header-hash = base64string
body-hash = base64string
x-hash-name = textstring ; for later expansion
Clayton, et al. Expires 26 February 2027 [Page 20]
Internet-Draft DKIM2 Signatures August 2026
8. The DKIM2-Signature header field
The signature of the email is stored in a DKIM2-Signature header
field. This header field contains tag values that provide the
signature and key-fetching data. Note that the syntax is such that
semi-colons will never occur within tags or values, they only act as
a separators.
The i=, m=, t=, d= and s= tags MUST be present. There MUST be either
an nd= tag or both mf= and rt= tags. The other tags are optional.
The tag identifiers (before the = sign) MUST be treated as case
insignificant, the tag value (after the = sign) is case significant.
The tags may appear in any order, but there MUST be only one of each
kind. Unknown tags, for extensions, MUST be ignored.
ABNF:
sig-field = "DKIM2-Signature:" sig-tag-list
sig-tag-list = *([FWS] sig-tag [FWS] ";" [FWS])
sig-tag = sig-i-tag / sig-m-tag / sig-t-tag / sig-mf-tag /
sig-rt-tag / sig-nd-tag / sig-d-tag / sig-s-tag /
sig-n-tag / sig-f-tag / x-tag
It will be noted that we have not included a version number.
Experience from IMF onwards shows that it is essentially impossible
to change version numbers. If it becomes necessary to change DKIM2
in the sort of incompatible way that a v=2 / v=3 version number would
support, it is expected that header fields will be labelled as DKIM3
instead.
8.1. i= the sequence number of the DKIM2-Signature header field
The Originator of a message uses the value 1. Further
DKIM2-Signature header fields are added with a value one more than
the current highest numbered DKIM2-Signature header field. Gaps in
the numbering MUST be treated as making the whole message unsigned.
ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] 1*DIGIT
8.2. m= the highest numbered Message-Instance header field
This value allows Verifiers to determine which entity made a
particular revision to the message header fields or body.
ABNF:
Clayton, et al. Expires 26 February 2027 [Page 21]
Internet-Draft DKIM2 Signatures August 2026
sig-m-tag = %x6d [FWS] "=" [FWS] 1*DIGIT
8.3. n= nonce value
This text value, if present, has a meaning to the creator of the
signature but MUST NOT be assumed to have any meaning to any other
entity. It MAY be used as an index into a database to assist in
handling Delivery Status Notifications or for any other purpose.
To discourage use of this tag field as an alternative to the use of
more appropriate header fields, the length of the string MUST NOT
exceed 64 characters and implementations SHOULD reject messages where
this limit has been ignored.
Note the value MUST be simple ASCII and MUST NOT contain semicolon.
ABNF:
sig-n-tag = %x6e [FWS] "=" [FWS] nonce-value
nonce-value = *64(%x21-3A / %x3C-7E)
; printable ASCII except semicolon, max 64 chars
8.4. t= signature timestamp
The time that this header field was created. The format is the
number of seconds since 00:00:00 on January 1, 1970 in the UTC time
zone. The value is expressed as an unsigned integer in decimal
ASCII. This value is not constrained to fit into a 31- or 32-bit
integer.
Implementations SHOULD be prepared to handle values up to at least
10^12 (until approximately AD 200,000; this fits into 40 bits).
Implementations MAY ignore signatures that have a timestamp in the
future. Implementations MAY ignore signatures that are more than 14
days old.
ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*DIGIT
8.5. mf= the MAIL FROM used when the message was sent
DKIM2 records the [RFC5321] MAIL FROM value that was used when the
message was transmitted over an SMTP link from the signing MTA. Note
that MAIL FROM may be just "<>", for example for a Delivery Status
Notification.
Clayton, et al. Expires 26 February 2027 [Page 22]
Internet-Draft DKIM2 Signatures August 2026
The value is recorded as the base64 encoding of the [RFC5321]
reverse-path because of the complex syntax of reverse-path values
(which can include characters which would confuse naive parsers of
DKIM2-Signature header fields). The angle brackets MUST be included,
but any "Mail-parameters" that were present after the reverse-path
MUST NOT be included.
ABNF:
sig-mf-tag = %x6d %x66 [FWS] "=" base64string
8.6. rt= the RCPT TO value(s) used when the message was sent
DKIM2 records the [RFC5321] RCPT TO value(s) that were used when the
message was transmitted over an SMTP link from the signing MTA.
The value is recorded as the base64 encoding of the [RFC5321]
Forward-path because of the complex syntax of Forward-path values
(which can include characters which would confuse naive parsers of
DKIM2-Signature header fields). The angle brackets MUST be included,
but any "Rcpt-parameters" that were present after the Forward-path
MUST NOT be included.
When a message is intended for more than one recipient then the RCPT
TO values provided MAY include all of the recipients so that a single
copy of the email MAY be sent to all of the recipients in a single
SMTP transaction. Alternatively, multiple copies of the email may be
generated so as to not immediately reveal who else received the
email.
However, if "bcc:" recipients are involved then in order to meet the
requirements of [RFC5322] Section 3.6.3 each and every bcc recipients
MUST NOT be revealed to any other message recipient.
ABNF:
sig-rt-tag = %x72 %x74 [FWS] "=" base64string *("," base64string)
8.7. nd= the domain that will appear in the next DKIM2-Signature header
field
This tag is used in order to provide a valid Chain of Custody when a
message is being forwarded from a different domain than the one to
which it arrived. The domain MUST be a valid DNS name under which a
DKIM2 key record is published.
The domain name in the nd= tag MUST exactly match the domain name of
the d= tag of the next DKIM2-Signature in sequence (i=) order.
Clayton, et al. Expires 26 February 2027 [Page 23]
Internet-Draft DKIM2 Signatures August 2026
Note that when an nd= tag is present both mf= and rt= MUST be
omitted. It is permissible to have a sequence of more than one
DKIM2-Signature header fields with an nd= tag, but this sequence (in
unbroken ascending numeric order of i= tags) MUST end with a
DKIM2-Signature that contains mf= and rt= tags.
ABNF:
sig-nd-tag = %x6e %x64 [FWS] "=" [FWS] Domain
8.8. d= the domain associated with this signature.
This domain is used to form the query for the public key. The domain
MUST be a valid DNS name under which the DKIM2 key record is
published.
The domain name in the d= tag MUST exactly match the rightmost labels
of the domain name of the mf= tag. That is to say, the domain name
of the mf= tag MUST either match the d= domain exactly or be a sub-
domain of the d= domain name.
When the mf= domain is empty ("<>"), as will be the case for Delivery
Status Notifications (DSNs), then no match is required.
ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] Domain
8.9. s= the signature value(s) for the message
The s= tag value contains the Selector, signature algorithm name and
signature value. Calculating the value is explained in Section 9.6
and the algorithms are described in Section 3.
The Selector values subdivide the namespace for the domain being used
for signing. Note that the DNS lookup for the public key will check
that the k= algorithm value is a match for the algorithm supplied in
the tag.
To provide for algorithmic dexterity more than one signature, using
different algorithms, MAY be supplied. One additional signature
using the same algorithm MAY be present provided that a different
Selector is used.
A Selector MUST NOT be present more than once in a single
DKIM2-Signature header field; if the signature values were the same
this would be pointless and if they differ then the Sender is
presumably attempting to confuse a Verifier.
Clayton, et al. Expires 26 February 2027 [Page 24]
Internet-Draft DKIM2 Signatures August 2026
ABNF:
sig-s-tag = %x73 [FWS] "=" [FWS] sig-set *( "," sig-set )
sig-set = selector [FWS] ":" [FWS] sig-name [FWS] ":" message-sig
sig-name = "rsa-sha256" / "ed25519-sha256" / x-sig-name
x-sig-name = textstring ; for later extension
message-sig = base64string
8.10. f= flags
Flags serve two purposes; they either report what has been done to
the message by the system creating the DKIM2-Signature or they make a
request to systems that handle the mail thereafter. Flags are
separated by commas, and optional white-space allows systems to add
several flags without creating long lines.
If a flag value is not recognised it MUST be ignored.
The flag values that report things are:
"exploded": this message (identified by its unique header hash value
(recorded in the h= JSON object of the relevant Message-Instance) is
being sent to more than one email address. An MTA which receives a
message MAY use this information to help it distinguish between
malicious "DKIM replay" and legitimate activity performed by mailing
list. If this flag is not present in at least one DKIM2-Signature
header field then an MTA MAY assume that only one copy of a
particular message (identified by relevant cryptographic hash values)
is intended to exist;
The flags values that make requests are:
"donotexplode": this Signer requests that the message not be sent to
more than one recipient. A system that, by local policy, ignores
this request MUST NOT allow any of the copies it creates to be
forwarded on to any MTA outside its control.
"donotmodify": this Signer requests that the message not be modified
from the form in which it is sent. If this request is honored then
the body MUST NOT be changed in any way that alters the body hash and
header fields MUST NOT be removed or changed. It is permissible to
add header fields but a subsequent receiver SHOULD consider whether
such header fields have an impact on how the message will be viewed
or responded to and MAY reject the message if it has concerns. A
system that, by local policy, ignores this request and makes other
changes MUST NOT allow the message to be forwarded on to any MTA
outside its control.
Clayton, et al. Expires 26 February 2027 [Page 25]
Internet-Draft DKIM2 Signatures August 2026
"feedback": this Signer requests feedback about how this message is
handled during delivery and thereafter. This document does not
describe what such feedback might be or where it might be delivered.
If this flag is absent then feedback is explicitly not required.
"feedhere": this Signer requests that any feedback about how this
message is handled during delivery and thereafter is relayed via this
hop. This flag will be set by privacy-conscious Forwarders when a
message has a "feedback" flag set and the Forwarder does not wish the
identity of systems to which the message is forwarded to be reveal to
the requestor of feedback.
ABNF:
sig-f-tag = %x66 [FWS] "=" [FWS] sig-f-tag-data
*( [FWS] "," [FWS] sig-f-tag-data)
sig-f-tag-data = "donotmodify" | "donotexplode" | "feedback" |
"feedhere" | "exploded" | x-sig-f-tag-data
x-sig-f-tag-data = textstring ; for later extension
9. Signer actions
This section gives the actions that need to be undertaken by the
Signer of a message. They may be done in any appropriate order.
9.1. Add any necessary Message-Instance header fields
If a system is generating the initial form of a message or if it is a
Reviser that has made changes to the message body and/or header
fields then it MUST compute the body hash as described in Section 6.1
and the hash of the header fields as described in Section 6.2.
If the message does not contain a Message-Instance header field then
one MUST be added.
If hashing the message body or relevant header fields does not give
the same hash values as those recorded in the highest version (m=)
Message-Instance header field then a new Message-Instance header
field MUST be added.
A Message-Instance header field MUST contain Recipes to be able to
recreate the message corresponding to the hash values in the
previously highest numbered Message-Instance header field, or a null
Recipe to indicate that recreating the previous version of the
message will not be possible.
Clayton, et al. Expires 26 February 2027 [Page 26]
Internet-Draft DKIM2 Signatures August 2026
A system may add more than one Message-Instance header field if it
wishes to do so, but the DKIM2 design allows all modifications made
by any single system to be documented in a single Message-Instance
header field.
Note that the first (m=1) Message-Instance header field MAY contain
Recipes if it is wished to record any changes made to a message as it
enters the DKIM2 ecosystem. All other Message-Instance header
fields, will contain at least one Recipe.
Adding a Message-Instance field with no Recipes (and thus the same
hash values as the previously highest numbered Message-Instance
field) is not forbidden; however, it is most likely to be pointless
and a waste of time and energy for all concerned.
9.2. Provide a Chain of Custody for the message
The DKIM2-Signature header field contains the MAIL FROM and RCPT TO
values that will be used when the message is transmitted, so these
[RFC5321] "envelope" values MUST be available to (or deducible by) a
Signer.
The receiver of a message will check for an exact match (including
the local parts of the email addresses) between the MAIL FROM / RCPT
TO [RFC5321] protocol values and the mf= and rt= values in the
highest numbered (most recent) DKIM2-Signature header field. It is
acceptable for there to be more RCPT TO email addresses recorded in
rt= than are actually used in the SMTP conversation, but any RCPT TO
value which is used MUST be present.
Verifiers will check for a relaxed domain match (see Section 9.4)
between the signing domain (d=) and the domain in the MAIL FROM
value.
When the message being signed already has a DKIM2-Signature header
field (i.e. it has already been transmitted at least once) then a
valid Chain of Custody MUST be apparent when all of the
DKIM2-Signature header fields are considered. This Chain of Custody
contributes to the way in which DKIM2 tackles "DKIM replay" attacks.
If an MTA does not change anything in the message which would require
a new Message-Instance header field and it is going to send it
onwards to a system that be able to verify the existing message (that
is no changes are made to the MAIL FROM and RCPT TO values) and there
is no other reason to add a DKIM2-Signature header field then the MTA
MAY choose not to add one. This means that an essentially
transparent SMTP forwarding system need not be made "DKIM2 aware".
Clayton, et al. Expires 26 February 2027 [Page 27]
Internet-Draft DKIM2 Signatures August 2026
9.3. Chain of Custody for imaginary hops
In any situation where a message will be forwarded in such a way that
the mf= on the outgoing message is such that the Chain of Custody
would be broken then the Signer MUST ensure this is not the case.
One way of doing this would be to generate an extra DKIM2-Signature
header field that causes values to match, i.e. a record is fabricated
that documents the mail being passed from one domain to another with
appropriate "mf=" and "rt=" tag values which document the imaginary
transfer of the email from one domain to the next.
Alternatively the "nd=" tag may be used. When present the next
higher sequence numbered DKIM2-Signature header field MUST be signed
by the specified domain. However, since there are no mf= or rt= tag
values there is no need to provide imaginary values for the imaginary
"hop" between the domain used for receiving the message and the
domain used to forward it.
It will be noted that the creation of an extra DKIM2-Signature header
field (whether the nd= scheme is used or not) will require the Signer
to have access to a DKIM2 private key associated with a domain in the
RCPT TO entry of the message as it arrived.
Access to this private key is often achieved by the Signer creating
the private key and never sharing it. One of two approaches is then
taken to publishing the public key. The first is to provide the
public key (and Selector value) to the domain owner who creates an
appropriate DNS entry. The alternative is for the Signer to create a
public key DNS entry within a part of the DNS that they control and
the domain owner publishes a CNAME pointing at this.
The general expectation is that signatures with an nd= tag have been
applied by the same system as the next DKIM2-Signature field without
an nd= tag. However, as a special case (where arrangements have been
put in place to cover the situation) a message with an nd= tag in the
highest numbered DKIM2-Signature header field may be sent over an
SMTP connection. The receiving system will not be able to validate
the MAIL FROM and RCPT TO parameters and MUST rely on out-of-band
information to deem the message acceptable. The system MUST then
provide a DKIM2-Signature header field with a d= tag that exactly
matches the nd= tag. Adding such a header field will signal that the
break in the Chain of Custody is intentional.
Clayton, et al. Expires 26 February 2027 [Page 28]
Internet-Draft DKIM2 Signatures August 2026
9.4. The relaxed domain match algorithm
To assist in addressing the "DKIM replay" problem DKIM2 provides a
Chain of Custody for every message. This is established by checking
that the MAIL FROM value recorded in every DKIM2-Signature header
field (except of course the i=1 instance) can be matched with a RCPT
TO value of the next lower numbered DKIM2-Signature header field.
It is also necessary to check DKIM2-Signature header fields for a
match between the signing domain (specified in the d= tag) and the
MAIL FROM domain.
To allow systems to use existing "bounce-handling" schemes with
special subdomains in their MAIL FROM values a "relaxed" approach is
taken to the matches between these values.
* Only the domain part of the MAIL FROM and RCPT TO values is used
for these matches The local part (and the @) are ignored.
* If there is not an exact match between the domain names then
labels are removed, one by one from the left hand side of the MAIL
FROM domain name and the comparison is repeated.
* If no labels remain then there is no match.
9.5. Select a private key and corresponding Selector value
This specification does not define the basis by which a Signer should
choose which private key and Selector value to use -- this will be a
matter of administrative convenience. Distribution and management of
private keys is also outside the scope of this document.
9.6. Calculate a signature value
A Signer calculates a signature solely over the Message-Instance and
DKIM2-Signature header fields of the message. The hashes of the body
and other header fields are covered by the hashes in the highest
version (m=) Message-Instance header field and hence the signature
will in practice be signing the message as a whole.
Most cryptographic schemes proceed by first calculating a hash value
and then signing the hash value, but the DKIM2-Signature header field
only provides the final signature value. This means that there is no
difficulty if the hash value is inordinately long, or is not emitted
by the cryptographic routine being used.
Clayton, et al. Expires 26 February 2027 [Page 29]
Internet-Draft DKIM2 Signatures August 2026
The signature algorithm MUST apply the following steps in the order
given (which are not quite the same as the steps undertaken in
calculating header hashes).
* Convert all relevant header field names (not the header field
values) to lowercase. For example, convert "DKIM2-signature" to
"dkim2-signature".
* Unfold all header field continuation lines as described in
[RFC5322]; in particular, lines with terminators embedded in
continued header field values (that is, CRLF sequences followed by
WSP) MUST be interpreted without the CRLF. Implementations MUST
NOT remove the CRLF at the end of the header field value.
* Delete all WSP characters. This means all WSP characters before
and after the colon separating the header field name from the
header field value, all WSP characters within the unfolded header
field value and all trailing WSP characters before the CRLF. The
colon separator and the CRLF MUST be retained.
* Place the header fields in order. First come the Message-Instance
header fields in ascending instance (m=) order. Second are the
DKIM2-Signature header fields in ascending sequence (i=) order.
Last of all is an incomplete DKIM2-Signature header field (the one
that this system is creating) with all tags present except that
the signature value(s) within the (s=) value are set to the null
string (""). The incomplete header field MUST be unfolded, MUST
have a trailing CRLF and MUST have spaces removed in just the same
way as the complete header fields being processed.
* The concatenated header fields are then fed to the signature
algorithm(s). Once all the values are available the null
signature value strings are replaced by the base64 values of the
signatures.
10. Verification requirements
The details of verification appear in Section 11 below. This section
considers when verification should be performed and how thorough it
needs to be.
10.1. Check the most recent signature and hashes for the message
A Verifier SHOULD check the validity of the most recently applied
(highest numbered i= value) DKIM2-Signature header field and the
associated (m=) Message-Instance before accepting an email.
Clayton, et al. Expires 26 February 2027 [Page 30]
Internet-Draft DKIM2 Signatures August 2026
If these checks do not pass then a Delivery Status Notification (DSN)
for the email MUST NOT be generated thereafter -- hence the best
strategy, if the email is not wanted, is to reject it (with a 5xx
error code) whilst the relevant SMTP conversation is still ongoing.
If the check gives a TEMPFAIL result then a 4xx error code SHOULD be
used to allow the sending MTA to understand the situation.
If the checks do pass and it is later determined that the email is
unacceptable for any reason then a DSN MAY be created and passed to
the system that delivered the email. The details of this procedure
appear in Section 12.
10.2. Checking the Message-Instance header fields
If the message has been modified since its original creation then the
Message-Instance header fields will enable a Verifier to determine
whether or not all the changes made are correctly recorded by using
the Recipes to construct each preceding version of the message.
Note that if it is only the first form of the message is of interest
then all the Recipes can be applied in turn and only one hash value
checked -- the correctness of the intermediate hash values are not
relevant to this assessment.
10.3. Checking the DKIM2-Signature header fields
However, in order to check the Chain of Custody, to assess whether
the message has been exploded, to pick out "feedback" requests to be
honoured or to assign reputation to Revisers then all of the
DKIM2-Signature header fields will have to checked for validity. The
TBA document explores these issues in more detail.
10.4. Interpret results/Apply local policy
It is beyond the scope of this specification to describe what actions
the recipient of an email performs, but mail carrying valid DKIM2
signatures gives the recipient opportunities that unauthenticated
email would not. Specifically, an authenticated email provides
predictable information by which other decisions can reliably be
managed, such as trust and reputation. Conversely, it is hard to
assign trust or reputation to unauthenticated email.
If an MTA wishes to reject messages where signatures are missing or
do not verify, the handling MTA SHOULD use a 550/5.7.x reply code.
Clayton, et al. Expires 26 February 2027 [Page 31]
Internet-Draft DKIM2 Signatures August 2026
Where the Verifier is integrated within the MTA and it is not
possible to fetch the public key, perhaps because the key server is
not available, a temporary failure message MAY be generated using a
451/4.7.5 reply code.
Temporary failures such as inability to access the key server or
other external service are the only conditions that SHOULD use a 4xx
SMTP reply code. In particular, cryptographic signature verification
failures MUST NOT provoke 4xx SMTP replies.
11. Verifier actions
This section discusses the detail of the actions taken by a Verifier.
In essence this will involve repeating all the actions taken by a
Signer to produce a Message-Instance or DKIM2-Signature header field.
To avoid a lot of repetition these actions will not be spelled out in
detail. Once a hash value has been calculated it is then compared
with the value reported by the Signer, or the Signer's public key is
used to determine whether a signature that has been provided is
correct.
When a Verifier is determining whether a particular DKIM2-Signature
header field it MUST consider the state of the message when that
header field was added to the message. That means it MUST first
apply all relevant Recipes to reconstruct the body and header fields
and it MUST ignore any Message-Instance and DKIM2-Signature fields
that were added after that point.
11.1. Output states
For compatibility with the Authentication-Results header field
defined in [RFC8601] a verification will result in one of four
states:
PASS: The message was successfully verified.
FAIL: The message could be verified but a hash or signature was not
correct.
PERMERROR: The message could not be verified due to some error that
is unrecoverable, such as a required header field being absent or
malformed.
TEMPERROR: The message could not be verified due a temporary
inability to retrieve a public key. A later attempt may produce a
different.
A Verifier MAY cease verifying once a single failure is detected.
Clayton, et al. Expires 26 February 2027 [Page 32]
Internet-Draft DKIM2 Signatures August 2026
Verifiers wishing to communicate the results of verification to other
parts of the mail system may do so in whatever manner they see fit.
If they wish to provide a human-readable string to describe a failure
to verify (any state except PASS) then in order to provide the
maximum possible assistance to senders they SHOULD use the text
strings specified in this document. These human-readable messages
are described with m=<x> or tag=<y> placeholders, the <x> and <y>
MUST be replaced with the relevant ordinal or tag name (without the <
and > characters). Similarly <value> MUST be replaced by a relevant
string for the particular message.
If the verification is being performed during an SMTP protocol
conversation the human-readable string SHOULD be part of the 5xx or
4xx response string.
If the results of the verification are being communicated in a
Delivery Status Notification message ([RFC3462]) the human-readable
string should be included.
If, by local policy, a system wishes to accept a message which has
failed authentication it might choose to add an email header field to
the message before passing it on. Any such header field SHOULD
include the human-readable string and SHOULD be inserted before any
existing DKIM2-Signature or pre-existing authentication status header
fields in the header field block. The Authentication-Results: header
field ([RFC8601]) MAY be used for this purpose. It should be noted
that any "Authentication-Results" header field does not count as a
modification to the email if any further DKIM2-Signature header
fields are to be generated.
11.2. Ensure that the DKIM2 header fields are valid
Verifiers MUST meticulously validate the format and values of all
relevant Message-Instance and DKIM2-Signature header fields. It MUST
also ensure that all required instances of these header fields are
present and that all required tags are present and tags that cannot
appear together do not do so. Recall however that unknown tags MUST
be ignored.
As a special case, there MUST NOT be a Message-Instance field with a
higher m= value than occurs in any DKIM2-Signature field.
There are also requirements on not repeating hash algorithms or
Selectors or providing the same signing algorithm more than twice.
To assist debugging, errors in a JSON object specifying Recipes
should be called out specifically.
Clayton, et al. Expires 26 February 2027 [Page 33]
Internet-Draft DKIM2 Signatures August 2026
Possible errors:
PERMERROR Message-Instance m=<x> missing
PERMERROR Message-Instance m=<x> syntax error
PERMERROR Message-Instance m=<x> tag=<y> missing
PERMERROR Message-Instance m=<x> is not signed
PERMERROR Message-Instance m=<x> has a duplicate hash algorithm
PERMERROR Message-Instance m=<x> contains invalid JSON
PERMERROR DKIM2-Signature i=<x> missing
PERMERROR DKIM2-Signature i=<x> syntax error
PERMERROR DKIM2-Signature i=<x> tag=<y> missing
PERMERROR DKIM2-Signature i=<x> tag=<y> was unexpected
PERMERROR DKIM2-Signature i=<x> has a duplicate selector
PERMERROR DKIM2-Signature i=<x> has too many signatures
11.3. Check the timestamps
Verifiers SHOULD return a failure if it is more than 14 days since
the timestamp recorded in the "t=" tag of any DKIM2-Signature header
field.
Possible errors:
PERMERROR DKIM2-Signature i=<x> signature expired
11.4. Check the Chain of Custody
As explained in Section 9.2 a Verifier MUST check for an exact match
between the MAIL FROM and RCPT TO parameters used when delivering a
message and the values found in the mf= and rt= tags of the highest
numbered DKIM2-Signature header field. There may be extra values in
the rt= value, but all RCPT TO values actually used for delivery MUST
be present.
The values of domains MUST BE put into lower-case before doing these
checks. As is usual in email protocols the case of the local part of
an email address is assumed to matter. Note that these checks MUST
NOT use the relaxed domain match algorithm.
A Verifier SHOULD check that there is a relaxed domain match (see
{relaxed-domain-match}) between the signing domain of the most
recently applied DKIM2-Signature header field and the mf= value in
that header field.
If an nd= tag has been used instead of mf= and rt= tags then a
Verifier MUST ensure that there is an exact match with the d= value
in the next DKIM2-Signature field in sequence number order.
Clayton, et al. Expires 26 February 2027 [Page 34]
Internet-Draft DKIM2 Signatures August 2026
Verifiers SHOULD ensure that there are out-of-band arrangements in
place to assess message validity when the highest numbered
DKIM2-Signature header field contains an nd= tag.
Possible errors:
PERMERROR: DKIM2-Signature i=<x> MAIL FROM <value> did not match
PERMERROR: DKIM2-Signature i=<x> RCPT TO <value> did not match
PERMERROR: DKIM2-Signature i=<x> MAIL FROM and d= do not match
PERMERROR: DKIM2-Signature i=<x> MAIL nd= does not match
PERMERROR: DKIM2-Signature i=<x> unexpected nd= tag
11.5. Fetch the public key
The public keys of all the signatures in DKIM2-Signature fields are
needed to complete the verification process. Details of key
management and representation are described in Section 3.6 and
[DKIMKEYS]. The Verifier MUST validate the key record and MUST NOT
use any public key records that are malformed.
Note that DNS timeouts MUST be reported as TEMPERROR but a DNS result
that indicates the key is absent MUST be reported as a PERMERROR.
Additionally, as [DKIMKEYS] makes clear, if more than one record is
returned this is an error. The human-readable error message SHOULD
provide the Selector value so that it is clear which key has caused a
problem.
Note that [DKIMKEYS] has retired the h= field and DKIM2
implementations MUST ignore this tag if it is present.
Possible errors:
TEMPERROR: DKIM2-Signature i=<x> public key <value> could not be fetched
PERMERROR: DKIM2-Signature i=<x> public key <value> does not exist
PERMERROR: DKIM2-Signature i=<x> public key <value> has multiple records
PERMERROR: DKIM2-Signature i=<x> public key <value> has a syntax error
PERMERROR: DKIM2-Signature i=<x> public key <value> algorithm mismatch
PERMERROR: DKIM2-Signature i=<x> public key <value> has been revoked
11.6. Perform the signature verification calculation
Verifying a signature consists of actions semantically equivalent to
the following steps:
Clayton, et al. Expires 26 February 2027 [Page 35]
Internet-Draft DKIM2 Signatures August 2026
1. Prepare a canonicalized version of the Message-Instance and
DKIM2-Signature header fields as described in Section 9.6. The
signature value(s) themselves will need to be removed to
correspond with what was actually signed. Note that this
canonicalized version does not actually replace the original
content.
2. Use the relevant public key value(s) to check the signature(s).
3. If there is more than one signature provided then they MUST all
be checked if the Verifier is able to do so. If any signature
fails then an error SHOULD be reported. If all signatures that
can be checked fail then PERMFAIL MUST be reported.
4. If some signatures fail and other pass then any error that is
reported should provide that information (e.g. PERMFAIL "rsa-
sha256 signature passed, ed25519-sha256 signature failed").
The reasoning for requiring that all signatures pass is that if a
signature scheme has recently become deprecated because it is known
to be cryptographically flawed then Signers will use a second
(unbroken) signature scheme. However, such a Signer may still
provide the other signature for the benefit of Verifiers that have
yet to upgrade -- reasoning perhaps that attacks are too expensive to
be a very significant security issue. A Verifier that determines
that one signature passes whilst the other fails may well be in a
position to prevent an attack.
Possible errors:
FAIL: DKIM2-Signature i=<x> public key <value> incorrect signature
11.7. Validating body and header hashes
Verifying a hash value requires a Verifier to repeat the hash
calculation performed by the Signer as set out in Section 6.1 and
Section 6.1. The values can then be directly compared.
Since there may be more than one hash algorithm given the human-
readable error message SHOULD indicate which algorithm's result
failed to match.
Possible errors:
FAIL: Message Instance m=<x> header hash <value> mismatch
FAIL: Message Instance m=<x> body hash <value> mismatch
Clayton, et al. Expires 26 February 2027 [Page 36]
Internet-Draft DKIM2 Signatures August 2026
11.8. Check if donotmodify and donotexplode requests were honored
If a Verifier receives a message with a nonotmodify request and a
later hop has altered the message body or removed or altered header
fields then the message SHOULD be rejected.
If a Verifier receives a message with a donotexplode request and a
later hop has indicated that it has exploded the message then the
message SHOULD be rejected.
Possible errors:
FAIL: Message has been modified despite a donotmodify request
FAIL: Message has been exploded despite a donotexplode request
12. Delivery Status Notifications in the DKIM2 ecosystem
In the DKIM2 ecosystem, when a message cannot be delivered then this
is reported to the sending machine by means of an [RFC5321] return
code or, if the SMTP session has completed, by generating a Delivery
Status Notification (DSN), as defined in [RFC3462].
A DSN MUST be addressed to the MTA that sent the message. This
prevents "backscatter" by passing failures back along the chain of
MTAs that were in involved in passing the message forwards. This is
achieved by using the mf= tag from the highest numbered
DKIM2-Signature field. If this field is null ("mf=<>") then a DSN
MUST NOT be sent.
12.1. DSN contents
As set out in [RFC3462], a DSN has a top-level MIME part of type
multipart/report.
A valid DSN generated by a DKIM2 system MUST always contain three
sections, the human-readable text, the machine readable status (in a
message/delivery-status MIME part and either a MIME part of type
message/rfc822 that holds the original message exactly as it was
submitted by the sending system or `text/rfc822-headers' which has
just the header fields of that message.
All relevant DKIM2-Signature header fields (and Message-Instance
header fields if the message body is supplied) MUST verify.
The DSN itself MUST have appropriate Message-Instance and
DKIM2-Signature fields, noting that the MAIL FROM to be used will be
null ("<>").
Clayton, et al. Expires 26 February 2027 [Page 37]
Internet-Draft DKIM2 Signatures August 2026
12.1.1. DSN propagation
A Forwarder which receives a DSN MAY propagate this DSN to the MAIL
FROM address used to deliver the message to it (which can be found in
the relevant DKIM2-Signature header field).
The DSN MUST be rebuilt so that the message (or just the message
header fields) are placed into the DSN to reflect the state the
message was in when it was forwarded. This means that the
DKIM2-Signature header field that was added MUST be removed.
Additionally, if the message was modified by the Forwarder on the
outward journey those modifications must be undone and the Message-
Instance header field documenting those changes MUST be removed.
If it is impossible to regenerate the body of the message, because it
was changed on the outgoing path and a "null Recipe" means that the
previous version cannot be reconstructed then the body MUST be
removed from the third part of the DSN and a text/rfc822-headers MIME
part must be supplied.
Where appropriate the reason for the delivery failure (as recorded in
the first part of the DSN) should be rewritten to remove destination
specific information.
The resultant DSN is sent to the MAIL FROM address from the now
highest numbered DKIM2-Signature header field. The DSN itself MUST
have only one DKIM2-Signature field and only one Message-Instance
header : i.e. it is a new message.
Following these procedures will ensure that limited details of where
the message was forwarded to will be revealed to the previous hop.
12.1.2. Authentication of inbound DSNs
When a system receives a DKIM2 signed DSN, and the included original
message is also DKIM2 signed, it SHOULD verify that this message (or
just the header fields if the body is not present) has not been
altered.
This means:
1. The DSN's DKIM2-Signature will have a signing domain that is
aligned with the recipient of the message that is being returned.
The recipient's address is located in the rt= tag of the last
(highest i= tag) DKIM2-Signature in the returned message.
Clayton, et al. Expires 26 February 2027 [Page 38]
Internet-Draft DKIM2 Signatures August 2026
2. The last (highest i= tag) DKIM2-Signature header field of the
returned message will be one that was generated by the system
receiving the DSN, determined by examining the d= and mf= tags of
that DKIM2-Signature header field.
3. The header fields of the embedded message (in the message/rfc822
MIME part) can be verified by checking the header hash values in
the highest numbered Message-Instance header field. If the
message body is present then that can be verified by inspecting
the body hash value in the highest numbered Message-Instance
header field.
If the verification fails then the DSN MUST NOT be propagated any
further. If verification has been performed prior to accepting the
DSN from the sender the DSN SHOULD be rejected with a 550/5.7.x
return code. If the verification cannot be completed because of a
temporary issue (with DNS lookups) then a 4xx return code should be
used.
13. Preventing transport conversions
DKIM2's design is predicated on valid input.
In order to be signed a message will need to be in "network normal"
format (text is ASCII encoded, lines are separated with CRLF
characters, etc.).
A message that is not compliant with [RFC5322], [RFC2045], [RFC2047]
and other relevant message format standards can be subject to
attempts by intermediaries to correct or interpret such content. See
Section 8 of [RFC6409] for examples of changes that are commonly
made. Such "corrections" may invalidate DKIM2 signatures or have
other undesirable effects, including some that involve changes to the
way a message is presented to an end user.
When calculating the hash on messages that will be transmitted using
base64 or quoted-printable encoding, Signers MUST compute the hash
after the encoding. Likewise, the Verifier MUST incorporate the
values into the hash before decoding the base64 or quoted-printable
text. However, the hash MUST be computed before transport-level
encodings such as SMTP "dot-stuffing" (the modification of lines
beginning with a "." to avoid confusion with the SMTP end-of-message
marker, as specified in [RFC5321]).
Further, if the message contains local encoding that will be modified
before transmission, that modification to canonical [RFC5322] form
MUST be done before signing. In particular, bare CR or LF characters
(used by some systems as a local line separator convention) MUST be
Clayton, et al. Expires 26 February 2027 [Page 39]
Internet-Draft DKIM2 Signatures August 2026
converted to the SMTP-standard CRLF sequence before the message is
signed. Any conversion of this sort SHOULD be applied to the message
actually sent to the recipient(s), not just to the version presented
to the signing algorithm.
More generally, the Signer MUST sign the message as it is expected to
be received by the Verifier rather than in some local or internal
form.
14. EAI ([RFC6530]) considerations for DKIM2
TBA
15. IANA considerations
TBA
16. Security considerations
TBA
17. Changes from Earlier Versions
draft-ietf-dkim-dkim2-spec-05
Updated list of unsigned header fields to align with [HDRMAINT].
Added sha512 to allow testing of hash function dexterity.
Removed SHOULD NOT for speciously added Message-Instance fields, we
now just point out they are pointless.
Added text to forbid repetition of the same hash algorithm or the
same Selector in any given header field. Provided messages for
reporting infractions.
Now specifies that header field names in the JSON Recipes MUST be
lower case; and Verifiers should now report JSON issues specifically.
Made Capitalisation Consistent
draft-ietf-dkim-dkim2-spec-04
Provided summary of unsigned header fields.
Added text for situation when the highest DKIM2-Signature field of a
message has an nd= tag.
Clayton, et al. Expires 26 February 2027 [Page 40]
Internet-Draft DKIM2 Signatures August 2026
Added text to clarify checking of Message-Instance in a DSN.
draft-ietf-dkim-dkim2-spec-03
Move list of ignored header fields into its own section. Added the
experimental Delivered-To: header field to the list.
Remove possibility of a null Recipe for header field changes.
Add nd= tag as an alternative to mf= rt= for imaginary hops.
Change DSN propagation rules so that a DSN always contains the
message headers up to the point at which the DSN creator saw the
message on the outward journey.
"feedhere" flag added.
draft-ietf-dkim-dkim2-spec-02
Made explicit that base64strings must be padded with zero bits.
Fixed header field description in JSON to be single string. and moved
the removed info into the textual discussion. Removed the z body
Recipe since DSNs do not actually need this.
Added text to emphasise that semi-colons are (easy to parse)
separators in Message-Instance and DKIM2-Signature. Also fixed the
syntax for extension tags.
Added Authentication-Results to the list of headers that are excluded
from being signed.
Added text about what donotmodify means and added text to the
verification section about this and the donotexplode flag.
draft-ietf-dkim-dkim2-spec-01
Additions to terminology. Improved ABNF. Removed definition of tag-
list and placed relevant text in the two header field definitions.
Untangled the description of what needs to be verified from the
description of how to verify and provided a list of human-readable
strings to generate for errors.
draft-ietf-dkim-dkim2-spec-00
Clayton, et al. Expires 26 February 2027 [Page 41]
Internet-Draft DKIM2 Signatures August 2026
Removed JSON for hashes, signatures and SMTP parameters. Provided
valid JSON for Recipes and added "z" for truncated body. Changed
algorithm names for signing. Simplified the canonicalisation
performed for the header fields signed by DKIM2-Signature. Changed
v= to m= for message instance numbering.
General tidying up of specifying tag=value specifications and
associated ABNF. Various other fixes for issues flagged in WG.
[[This section to be removed by RFC Editor]]
18. References
18.1. Normative References
[DKIMKEYS] Chuang, W., "Domain Name Specification for DKIM2", Work in
Progress, Internet-Draft, draft-chuang-dkim2-dns-04, 18
March 2026, <https://datatracker.ietf.org/doc/html/draft-
chuang-dkim2-dns-04>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/rfc/rfc1034>.
[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/rfc/rfc2045>.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC2047, November 1996,
<https://www.rfc-editor.org/rfc/rfc2047>.
[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/rfc/rfc2119>.
[RFC3462] Vaudreuil, G., "The Multipart/Report Content Type for the
Reporting of Mail System Administrative Messages",
RFC 3462, DOI 10.17487/RFC3462, January 2003,
<https://www.rfc-editor.org/rfc/rfc3462>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
Clayton, et al. Expires 26 February 2027 [Page 42]
Internet-Draft DKIM2 Signatures August 2026
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/rfc/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/rfc/rfc5322>.
[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/rfc/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/rfc/rfc6409>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/rfc/rfc6530>.
[RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648,
DOI 10.17487/RFC6648, June 2012,
<https://www.rfc-editor.org/rfc/rfc6648>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[RFC8601] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 8601,
DOI 10.17487/RFC8601, May 2019,
<https://www.rfc-editor.org/rfc/rfc8601>.
[RFC9228] Crocker, D., Ed., "Delivered-To Email Header Field",
RFC 9228, DOI 10.17487/RFC9228, April 2022,
<https://www.rfc-editor.org/rfc/rfc9228>.
18.2. Informative References
Clayton, et al. Expires 26 February 2027 [Page 43]
Internet-Draft DKIM2 Signatures August 2026
[CONCLUDEARC]
Adams, J. T. and J. R. Levine, "Concluding the ARC
Experiment", Work in Progress, Internet-Draft, draft-
adams-arc-experiment-conclusion-01, 4 December 2025,
<https://datatracker.ietf.org/doc/html/draft-adams-arc-
experiment-conclusion-01>.
[HDRMAINT] Gondwana, B., "Maintenance of the IANA Message Header
Field Registries", Work in Progress, Internet-Draft,
draft-gondwana-email-header-maintenance-02, 29 July 2026,
<https://datatracker.ietf.org/doc/html/draft-gondwana-
email-header-maintenance-02>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/rfc/rfc5598>.
[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/rfc/rfc7208>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/rfc/rfc8017>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>.
[RFC8617] Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
Kucherawy, Ed., "The Authenticated Received Chain (ARC)
Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
<https://www.rfc-editor.org/rfc/rfc8617>.
Authors' Addresses
Richard Clayton
Yahoo
Email: rclayton@yahooinc.com
Wei Chuang
Google
Email: weihaw@google.com
Clayton, et al. Expires 26 February 2027 [Page 44]
Internet-Draft DKIM2 Signatures August 2026
Bron Gondwana
Fastmail Pty Ltd
Level 2, 114 William Street
3000
Australia
Phone: +61 457 416 436
Email: brong@fastmailteam.com
Clayton, et al. Expires 26 February 2027 [Page 45]