DomainKeys Identified Mail Signatures v2 (DKIM2)
draft-clayton-dkim2-spec-06
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Richard Clayton , Wei Chuang , Bron Gondwana | ||
| Last updated | 2026-01-20 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
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| Send notices to | (None) |
draft-clayton-dkim2-spec-06
Network Working Group R. Clayton
Internet-Draft Yahoo
Intended status: Standards Track W. Chuang
Expires: 24 July 2026 Google
B. Gondwana
Fastmail Pty Ltd
20 January 2026
DomainKeys Identified Mail Signatures v2 (DKIM2)
draft-clayton-dkim2-spec-06
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
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 24 July 2026.
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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. DKIM2 Architecture Documents . . . . . . . . . . . . . . 4
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 4
2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Forwarder . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Reviser . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Signing Domain . . . . . . . . . . . . . . . . . . . . . 5
2.6. Whitespace . . . . . . . . . . . . . . . . . . . . . . . 5
2.7. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . 6
2.8. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . 6
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 7
4. Signing and Verification Cryptographic Algorithms . . . . . . 8
4.1. The SHA256 Hashing Algorithm . . . . . . . . . . . . . . 8
4.2. The RSA-SHA256 Signing Algorithm . . . . . . . . . . . . 9
4.3. The Ed25519-SHA256 Signing Algorithm . . . . . . . . . . 9
4.4. Other Algorithms . . . . . . . . . . . . . . . . . . . . 9
4.5. Key Management . . . . . . . . . . . . . . . . . . . . . 9
5. The Message-Instance Header Field . . . . . . . . . . . . . . 10
6. The DKIM2-Signature Header Field . . . . . . . . . . . . . . 15
7. Computing the Body Hash . . . . . . . . . . . . . . . . . . . 19
7.1. Preventing Transport Conversions . . . . . . . . . . . . 20
8. Computing the Header Fields Hash . . . . . . . . . . . . . . 21
9. The Relaxed Domain Match Algorithm . . . . . . . . . . . . . 22
10. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Add any Necessary Message-Instance Header Fields . . . . 23
10.2. Provide a "chain of custody" for the message . . . . . . 24
10.3. Select a Private Key and Corresponding Selector Value . 25
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10.4. Calculate a Signature Value . . . . . . . . . . . . . . 25
11. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 26
11.1. Output States . . . . . . . . . . . . . . . . . . . . . 26
11.2. Validation of Tag Fields . . . . . . . . . . . . . . . . 27
11.3. Fetching the Public Key . . . . . . . . . . . . . . . . 27
11.4. Perform the Signature Verification Calculation . . . . . 28
11.5. Check Most Recent Signature and Hashes for the
Message . . . . . . . . . . . . . . . . . . . . . . . . 29
11.6. Checking the Message-Instance Header Fields . . . . . . 30
11.7. Checking the DKIM2-Signature Header Fields . . . . . . . 30
11.8. Interpret Results/Apply Local Policy . . . . . . . . . . 30
12. Delivery Status Notifications in the DKIM2 ecosystem . . . . 31
12.1. DSN contents . . . . . . . . . . . . . . . . . . . . . . 31
12.1.1. Bounce propagation . . . . . . . . . . . . . . . . . 32
12.1.2. Authentication of inbound bounce notifications . . . 32
13. EAI (RFC6530) considerations for DKIM2 . . . . . . . . . . . 33
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
15. Security Considerations . . . . . . . . . . . . . . . . . . . 33
16. Changes from Earlier Versions . . . . . . . . . . . . . . . . 33
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
17.1. Normative References . . . . . . . . . . . . . . . . . . 34
17.2. Informative References . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
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.
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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.
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.
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 from (see [CONCLUDEARC]) the
experimental ARC protocol ([RFC8617]).
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
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.
2.1. Signers
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.
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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. Verifiers
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. 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):
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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.
2.7. 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]:
* "local-part" (implementation warning: this permits quoted strings)
* "sub-domain"
The following tokens are imported from [RFC5322]:
* "field-name" (name of a header field)
Other tokens not defined herein are imported from [RFC5234]. These
are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
etc.
2.8. Common ABNF Tokens
The following ABNF tokens are used elsewhere in this document:
VALCHAR = %x21-3A / %x3C-7E
; EXCLAMATION to TILDE except SEMICOLON
ALPHADIGITPS = (ALPHA / DIGIT / "+" / "/")
ALNUMPUNC = ALPHA / DIGIT / "_"
base64string = ALPHADIGITPS *([FWS] ALPHADIGITPS)
[ [FWS] "=" [ [FWS] "=" ] ]
textstring = VALCHAR * (FWS / VALCHAR)
Note that base64strings are defined in [RFC4648], but that document
does not contain any ABNF. Note that a base64string MUST be padded
with trailing = characters if needed.
Note that the definitions of both textstring and base64string allow
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. FWS within a textstring MUST be treated
as a single space when this value is used.
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3. Protocol Elements
Protocol Elements are conceptual parts of the protocol that are not
specific to either Signers or Verifiers. The protocol descriptions
for Signers and Verifiers are described in later sections ("Signer
Actions" (Section 10) and "Verifier Actions" (Section 11) and this
section must be read in the context of those sections.
3.1. Selectors
To support multiple concurrent public keys per signing domain, the
key namespace is subdivided using "selectors".
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 = sub-domain *( "." sub-domain )
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 withdraw 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.
3.2. Tag=Value Lists
DKIM2 uses a simple "tag=value" syntax in the Message-Instance and
DKIM2-Signature header fields, as well as in domain signature records
(see [DKIMKEYS]).
Values are a series of strings containing either plain text or
"base64" text (as defined in [RFC2045], Section 6.8). The name of
the tag will determine the encoding of each value. Unencoded
semicolon (";") characters MUST NOT occur in the tag value, since
that separates tag-specs.
Formally, the ABNF syntax rules are as follows:
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tag-list = tag-spec *( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tval = 1*VALCHAR
tag-name = ALPHA *ALNUMPUNC
tag-value = [ tval *( 1*(WSP / FWS) tval ) ]
; Prohibits WSP and FWS at beginning and end
Note that WSP is allowed anywhere around tags. In particular, any
WSP after the "=" and any WSP before the terminating ";" is not part
of the value; however, WSP inside the value is significant.
Tags MUST be interpreted in a case-sensitive manner. Values MUST be
processed as case sensitive unless the specific tag description of
semantics specifies case insensitivity.
Tags with duplicate names MUST NOT occur within a single tag-list; if
a tag name does occur more than once, the entire tag-list is invalid.
Whitespace within a value MUST be retained unless explicitly excluded
by the specific tag description.
Tag=value pairs that represent the default value MAY be included to
aid legibility.
Unrecognized tags MUST be ignored.
Tags that have an empty value are not the same as omitted tags. An
omitted tag is treated as having the default value; a tag with an
empty value explicitly designates the empty string as the value.
4. Signing and Verification Cryptographic Algorithms
DKIM2 supports multiple hashing and digital signature algorithms.
One hash function (SHA256) if specified here and two signing
algorithms are defined by this specification: RSA-SHA256 and
Ed25519-SHA256. Signers and Verifiers MUST implement SHA256.
Signers SHOULD implement both RSA-SHA256 and Ed25519-SHA256.
Verifiers MUST implement both RSA-SHA256 and Ed25519-SHA256.
4.1. The SHA256 Hashing Algorithm
The SHA256 hashing algorithm is used to compute body and header
hashes as defined in Section 7 and Section 8. The resultant values
are stored within Message-Instance header fields.
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4.2. 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 10.4 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.
4.3. 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 10.4 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].
4.4. Other Algorithms
Other algorithms MAY be defined in the future. Verifiers MUST ignore
any hashes or signatures using algorithms that they do not implement.
4.5. 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.
DKIM2 keys are stored in a subdomain named "_domainkey". Given a
DKIM2-Signature field with a "d=" tag of "example.com" and an "s1="
tag 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].
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5. The Message-Instance Header Field
The hash values for a message are stored in a Message-Instance header
field. This 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 tag-list as described in
Section 3.2.
Tags on the Message-Instance header field along with their type,
encoding and requirement status are shown below.
v= The revision number of the Message-Instance header field.
plain-text unsigned decimal integer; REQUIRED
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-v-tag = %x76 [FWS] "=" [FWS] 1*DIGIT
a1= The algorithm used to generate the body hash.
plain-text; REQUIRED
ABNF:
mi-a-tag = %x61 %x31 [FWS] "=" [FWS] mi-a-tag-alg
mi-a-tag-alg = "sha256" / x-mi-a-tag-h
x-mi-a-tag-h = ALPHA *(ALPHA / DIGIT) ; for later extension
b1= The hash of the canonicalized body part of the message.
base64; REQUIRED
Whitespace is ignored in this value and MUST be ignored when
reassembling the original hash. In particular, FWS can be safely
inserted this value in arbitrary places to conform to line-length
limits. See Section 7 for how the body hash is computed.
ABNF:
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mi-b-tag = %x62 %x31 [FWS] "=" [FWS] mi-b-tag-data
mi-b-tag-data = base64string
h1= The hash of the canonicalized headers of the message.
base64; REQUIRED
Whitespace is ignored in this value and MUST be ignored when
reassembling the original hash. In particular, FWS can be safely
inserted this value in arbitrary places to conform to line-length
limits. See Section 8 for how the header hash is computed.
ABNF:
mi-h-tag = %x68 %x31 [FWS] "=" [FWS] mi-h-tag-data
mi-h-tag-data = base64string
a2=, b2=, h2= Further hashes (equivalent to a1, b1 and h1)
plain text / base64; OPTIONAL
To provide for algorithmic dexterity a second pair of hashes, using a
different algorithm MAY be supplied. A verifier MUST check all
signatures that it understands and SHOULD treat any failure as
invalidating all hashes.
rb= A "recipe" to recreate the previous version of the message body.
plain text; OPTIONAL
A Body Recipe is a comma separated list of instructions. Each
instruction starts with a prefix. Commas can be followed by optional
whitespace.
The idea is that you take the message which has been received and
apply the Body Recipes so as to recreate the message as it was before
modifications were made. Hence it is necessary to cope with body
lines being added (c. is used to indicate which lines were original)
or removed/altered (b. is used to indicate what the body line was
before the removal/alteration).
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c. startnumber-endnumber
Copy the lines numbered startnumber up to and including the line
numbered endnumber. The first line of the body (immediately after
the blank line that indicates that there are no more header fields)
is numbered 1.
The startnumber of every "c" instruction MUST be in ascending order
and MUST be greater than the endnumber of all preceding "c"
instructions. In other words a recipe is not allowed to
use "c" instructions to duplicate or re-order lines.
b. base64string
Decode the base64string to get the value of a line to be inserted.
The inserted line will have a CRLF appended. The decoded value
MUST NOT contain CR or LF characters. If the base64string is
absent then a blank line is being added.
z
If a z is present then it MUST be the only "recipe". It indicates
that the changes that have been made to the body cannot be undone.
For example, a malware attachment may have been removed and it is
inappropriate to enable the malicious content to be recreated.
Verifiers of the message may be able to inspect the first signer
of this Message-Instance header field and determine that the
presence of z is acceptable to them because, for example, that
signer is providing a contractually arranged service.
ABNF:
mi-rb-tag = %x72 %x62 [FWS] "=" [FWS] mi-rb-tag-data
mi-rb-tag-data = mi-rb-recipes / %x7a
mi-rb-recipes = mi-rb-recipe * ([FWS] "," [FWS] mi-rb-recipe)
mi-rb-recipe = %x63 "." 1*DIGIT "-" 1*DIGIT /
%x62 "." [ base64string ]
rh= A "recipe" to replicate the previous version of a header field.
plain text; OPTIONAL
A Header Recipe starts with the name of the header field name, then a
colon and then a comma separated list of instructions. Commas can be
followed by optional whitespace. There MUST be only one set of
instructions for any given header field name. If there are further
header field names then a "|" is used to introduce them.
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The idea is that you take the message which has been received and
apply the Header Recipes so as to recreate the relevant header fields
as they were before modifications were made. Hence it is necessary
to cope with header fields being added (c. is used to indicate how
many header fields, if any, should be kept so that the addition is
undone) or removed/altered (b. is used to provide the contents of the
the header field was before the removal/alteration).
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) before
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.
If there are no "recipes" for a specified header field name that
means that all instances of that header field should be removed to
reinstate the previous state of the message. If a header field name
is not present at all then all the header fields with that header
field name are to be retained.
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c. startnumber-endnumber
Keep the instances of the header fields (with the relevant header
field name) which are at position startnumber to endnumber.
The startnumber of every "c" instruction MUST be in ascending order
and MUST be greater than the endnumber of all
preceding "c" instructions.
b. base64string
Decode the base64string value to get the header field to be inserted.
The inserted header field will start with the field name and a colon
followed by the decoded value and then a CRLF. When the base64string
is decoded it MUST NOT contain CR or LF characters.
If the base64string is absent then the CRLF will immediately
follow the header field name and colon.
Note that the hashing algorithm for processing the header fields will
work on unwrapped lines -- so there is no need to wrap the header
field created by this recipe because it will never appear "on the
wire".
z
If a z is present then it MUST be the only "recipe". It indicates
that the changes that have been made to the header field
cannot be undone and/or that it is inappropriate to provide
the original value.
Verifiers of the message may be able to inspect the first signer
of this Message-Instance header field and determine that the
presence of z is acceptable to them because, for example, that
signer is providing a contractually arranged service.
ABNF:
mi-rh-tag = %x72 %x68 [FWS] "=" [FWS] mi-rh-headers
mi-rh-headers = mi-rh-header * ([FWS] %x7c [FWS] mi-rh-header)
mi-rh-header = header-field-name [FWS] ":" [FWS] mi-rh-header-data
mi-rh-item-data = / ; the no recipes case
mi-rh-recipes / %7a
mi-rh-recipes = mi-rh-recipe * ([FWS] "," [FWS] mi-rh-recipe)
mi-rh-recipe = %x63 "." 1*DIGIT "-" 1*DIGIT /
%x62 "." base64string
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6. The DKIM2-Signature Header Field
The signature of the email is stored in a DKIM2-Signature header
field. This header field contains all of the signature and key-
fetching data. The DKIM2-Signature value is a tag-list as described
in Section 3.2.
Tags on the DKIM2-Signature header field along with their type,
encoding and requirement status are shown below. 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.
i= The sequence number of the DKIM2-Signature header field.
plain-text unsigned decimal integer; REQUIRED
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
v= The highest numbered Message-Instance header field
plain-text unsigned decimal integer; REQUIRED
This value allows verifiers to determine which entity made a
particular revision to the message body or header fields.
ABNF:
sig-v-tag = %x76 [FWS] "=" [FWS] 1*DIGIT
n= Nonce value
textstring; OPTIONAL
This 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.
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To discourage use of the tag field as an alternative to the use of
more appropriate header fields, the length of the textstring MUST NOT
exceed 64 characters and implementations SHOULD reject messages where
this limit has been ignored.
ABNF:
sig-n-tag = %x6e [FWS] "=" [FWS] textstring
t= Signature Timestamp
plain-text date-time; REQUIRED
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
mf= The [RFC5321] MAIL FROM value to be used when this message is
transmitted over an SMTP link from the signing MTA.
plain-text; REQUIRED
Note that MAIL FROM may be just "<>", for example for a Delivery
Status Notification.
ABNF:
sig-mf-tag = %x6d %65 [FWS] "="
[FWS] "<" [ local-part "@" domain-name ] ">"
rt= The [RFC5321] RCPT TO value(s) to be used when this message is
transmitted over an SMTP link from the signing MTA.
plain-text; REQUIRED
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When a message is intended for more than one recipient then this
field 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. Note that if "bcc:" recipients are involved then in
order to meet the requirements of [RFC5322] Section 3.6.3 each bcc
recipient MUST be sent a message with just their email address
appearing in this tag.
ABNF:
sig-rt-tag = %72 %x74 [FWS] "="
1*( [FWS] "<" local-part "@" domain-name ">" )
d= The domain associated with this signature.
plain-text; REQUIRED
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 in the d= tag MUST exactly match the rightmost labels of
the domain-name part 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 d= domain.
ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] domain-name
domain-name = sub-domain 1*("." sub-domain)
; from [RFC5321] Domain,
; excluding address-literal
s1= The selector subdividing the namespace for the "d=" (domain) tag.
plain-text; REQUIRED
ABNF:
sig-s-tag = %x73 %x31 [FWS] "=" [FWS] selector
a1= The algorithm used to generate the signature.
plain-text; REQUIRED
Verifiers MUST support "RSA-SHA256" and "Ed25519-SHA256"; See
Section 4 for a description of these algorithms.
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ABNF:
sig-a-tag = %x61 %x31 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / "ed25519" / x-sig-a-tag-k
sig-a-tag-h = "sha256" / x-sig-a-tag-h
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT)
; for later extension
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT)
; for later extension
b1= The signature data.
base64; REQUIRED
Whitespace is ignored in this value and MUST be ignored when
reassembling the original signature. In particular, the signing
process can safely insert FWS in this value in arbitrary places to
conform to line-length limits. See "Signer Actions" (Section 10) for
how the signature is computed.
ABNF:
sig-b-tag = %x62 %x31 [FWS] "=" [FWS] sig-b-tag-data
sig-b-tag-data = base64string
s2=, a2= b2= Second signature (equivalent to s1, a1 and b1)
plain text / base64; OPTIONAL
To provide for algorithmic dexterity a second signature, using a
different algorithm MAY be supplied. A verifier MUST check all
signatures that it understands and SHOULD treat any failure as
invalidating all signatures.
Since the DNS lookup for the public key will check that the algorithm
is correct a different MUST necessarily be used for the second
signature.
f= Flags
plain text; OPTIONAL
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.
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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 b1= and perhaps b2=)) 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. A system that, by local policy,
ignores this request MUST NOT allow the message to be forwarded on to
any MTA outside its control.
"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.
ABNF:
sig-f-tag = %x66 [FWS] "=" [FWS] sig-f-tag-data
*("," [FWS] sig-f-tag-data)
sig-f-tag-data = "donotmodify" | "donotexplode" | "feedback" |
"exploded" | x-sig-f-tag-data
x-sig-f-tag-data = ALPHA *(ALPHA / DIGIT)
; for later extension
7. Computing the Body Hash
Although the body hash value will be incorporated into a Message-
Instance header field, these header fields are ignored when
calculating the header hash value and so the body hash and header
hash may be calculated in any convenient order.
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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 hash value is converted to base64
form and inserted into (Signers) or compared to (Verifiers) the
"bh1=" tag of the Message-Instance header field that is being
created. If a second hash is calculated then its base64
representation will be included in the "bh2=" tag.
7.1. 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]).
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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
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.
8. Computing the Header Fields Hash
The header fields hash calculation 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 "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.
When calculating the header field hash any header field with a
header field name starting with "X-" MUST be ignored. Currently
deployed email systems use these fields as proprietary Trace
headers which have no defined meaning for other systems and it
considerably simplifies reporting on changes to header fields to
ignore them.
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.
When calculating the header field hash any "DKIM-Signature" header
fields and any header fields whose name starts with "ARC-" MUST be
ignored. Not including DKIM1 and ARC signatures means that
systems that wish to add other types of signature are free to do
this in any convenient order.
* Convert all header field names (not the header field values) to
lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
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* 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 occur
in the email header, from the top downwards.
It is sometimes suggested that some MTAs re-order header fields
after they receive an email, if they do then it is their
responsibility to recover the original order of any header fields
with identical header field names (that are part of a signature
calculation) before verifying an existing signature or passing a
previously signed message to another MTA that is going to wish to
verify a signature.
* The hash(es) of the concatenated header fields are calculated.
The hash value is converted to base64 form and inserted into
(Signers) or compared to (Verifiers) the "h1=" tag of the Message-
Instance header field that is being created. If a second hash is
calculated then its base64 representation will be included in the
"h2=" tag.
9. 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 "mf= value of every DKIM2-Signature header field
(except of course the i=1 instance) can be matched with the rt= value
of the next lower numbered DKIM2-Signature header field.
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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 (specified in the mf= tag).
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 mf= and rt= and mf= and d=.
* Only the domain part of the rt= and mf= 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 mf=
domain name and the comparison is repeated.
* If no labels remain then there is no match.
10. 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.
10.1. Add any Necessary Message-Instance Header Fields
If a system is generating the initial form of a message or if it a
Reviser that has made to changes to the message body and/or header
fields then it MUST compute the body hash as described in Section 7
and the hash of the header fields as described in Section 8.
If the message does not contain a Message-Instance header field then
one MUST be added. This MUST NOT contain any "recipes" (r=,
r.header=).
If hashing the message body or relevant header fields does not give
the same hash values as those recorded in the highest version (v=)
Message-Instance header field then a new Message-Instance header
field MUST be added. This Message-Instance header field MUST contain
"recipes" to be able to recreate the message corresponding to the
hash values in the currently highest numbered Message-Instance header
field, or a "recipe" of "z" to indicate that recreating the previous
version of the message will not be possible.
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 v=1 Message-Instance header field MUST
NOT contain any "recipes" and any other Message-Instance field MUST
contain at least one "recipe".
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10.2. Provide a "chain of custody" for the message
The DKIM2-Signature field contains mf= and rt= values, so the MAIL
FROM and RCPT TO values that will be used when the message is
transmitted (the [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=, rt= values in the highest
numbered (most recent) DKIM2-Signature header field. It is
acceptable for there to be more email addresses in the rt= value than
occur in the RCPT TO values, but any address used in the SMTP
conversation MUST be present in the rt= value.
Verifiers will check for a relaxed domain match (see {relaxed-domain-
match}) between the signing domain (d=) and the domain in the MAIL
FROM value (mf=).
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. The "chain of custody" uses a relaxed domain match (see
{relaxed-domain-match}).
In any situation where a messages 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 generate an extra
DKIM2-Signature header field that causes values to match, i.e. a
record must be fabricated that documents the mail being passed from
one domain to another.
It will be noted that the creation of this extra header field will
require the Signer to have access to a DKIM2 private key associated
with a domain in the rt= entry. This is often achieved by the Signer
creating the private key and never sharing it. The Signer gives the
public key (and selector value) to the domain owner who creates an
appropriate DNS entry. Alternatively, the Signer creates a public
key DNS entry within a part of the DNS that they control and the
domain owner merely needs to publish a CNAME pointing at this.
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10.3. 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.
10.4. Calculate a Signature Value
A signer calculates a hash solely over the Message-Instance and
DKIM2-Signature header fields of the message and then signs this.
The hashes of the body and other header fields are covered by the
hashes in the highest version (v=) Message-Instance header field.
Note that the DKIM2-Signature header field contains a signature but
does not give the hash value that was signed. This permits
flexibility for any future signature schemes where a relevant hash
value may not be readily available (or may be inordinately long).
The signature algorithm MUST apply the following steps in the order
given.
* 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.
* 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 order. First come the Message-Instance
header fields in ascending version (v=) 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
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the signature value (b1=) is null (that is the base64 value is
absent. If there will be a second signature then the b2= tag must
be present, again with a null base64 value.
* The hash of the concatenated header fields is calculated and this
value is then signed using the algorithm specified in the "a1="
tag of the DKIM2- Signature header field and using the private key
corresponding to the selector given in the "s1=" tag of the
DKIM2-Signature header field, as chosen above in Section 10.3}.
* If a second signature is to be generated then the process if
repeated with the a2= and s2= settings.
The signature value(s) are converted to base64 form and inserted into
the "b1=" tag (and "b2=") tags of the DKIM2-Signature header field
which MUST then be placed into the message.
11. Verifier Actions
This section discusses 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 the signature that has been provided is correct.
11.1. Output States
A verification ends in one of three states, which this document
refers to as follows:
SUCCESS: a successful verification
PERMFAIL: a permanent, non-recoverable error such as a signature
verification failure or mismatched hash value
TEMPFAIL: a temporary, recoverable error such as a DNS query timeout
A verifier MAY cease verifying once a single failure is detected.
Verifiers wishing to communicate the results of verification to other
parts of the mail system may do so in whatever manner they see fit.
For example, implementations might choose to add an email header
field to the message before passing it on. Any such header field
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
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used for this purpose. It should be noted that any "Authentication-
Results" header field will count as a modification to the email if
any further DKIM2-Signature header fields are to be generated.
11.2. Validation of Tag Fields
Implementers MUST meticulously validate the format and values of
Message-Instance and DKIM2-Signature header fields. Errors SHOULD be
treated as a PERMFAIL (signature syntax error). Being "liberal in
what you accept" is an inappropriate strategy.
Note, however, that the presence of unknown tags in a DKIM2-Signature
header field (or a Message-Instance header field), MUST NOT cause a
verification to fail.
Verifiers MAY return PERMFAIL ("signature expired") if it is more
than 14 days since the timestamp recorded in the "t=" tag of a
DKIM2-Signature header field.
11.3. Fetching the Public Key
The public key of a signature is needed to complete the verification
process. Details of key management and representation are described
in Section 4.5 and [DKIMKEYS]. The Verifier MUST validate the key
record and MUST ignore any public key records that are malformed.
When validating a message, a Verifier MUST perform the following
steps in a manner that is semantically the same as performing them in
the order indicated; in some cases, the implementation may
parallelize or reorder these steps, as long as the semantics remain
unchanged:
1. The Verifier retrieves the public key as described in Section 4.5
using the algorithm in the "a1=" tag, the domain from the "d="
tag, and the selector from the "s1=" tag. If a2= and s2 tags are
present, subsequent steps are repeated for the second signature.
2. If the query for the public key fails to complete, the Verifier
MAY seek a later verification attempt by returning TEMPFAIL ("key
unavailable").
3. If the query for the public key fails because the corresponding
key record does not exist, the Verifier MUST return PERMFAIL ("no
key for signature").
4. If the query for the public key returns multiple key records,
then the return PERMFAIL ("more than one key returned" since this
is not permitted by [DKIMKEYS]).
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5. If the result returned from the query does not adhere to the
format defined in the DKIM key specification [DKIMKEYS], the
Verifier MUST ignore the key record and return PERMFAIL ("key
syntax error"). Verifiers are urged to validate the syntax of
key records carefully to avoid attacks. In particular, the
Verifier MUST ignore keys with a version code ("v=" tag) that
they do not implement.
6. If the public key data (the "p=" tag) is empty, then this key has
been revoked and the Verifier MUST treat this as a failed
signature check and return PERMFAIL ("key revoked"). There is no
defined semantic difference between a key that has been revoked
and a key record that has been removed.
7. If the public key data is not suitable for use with the algorithm
specified in the DKIM-Signature header field, the Verifier MAY
immediately return PERMFAIL ("inappropriate key algorithm").
However, the tag fields in the public key record that would cause
this to occur are now deprecated so DKIM2 implementations MAY
ignore these tag fields altogether.
8. If the "h=" tag exists in the public key record and the hash
algorithm implied by the "a1=" (or "a2=") tag in the
DKIM2-Signature header field is not included in the contents of
the "h=" tag, the Verifier MUST ignore the key record and return
PERMFAIL ("inappropriate hash algorithm").
11.4. Perform the Signature Verification Calculation
Given a Signer and a public key, verifying a signature consists of
actions semantically equivalent to the following steps.
1. Prepare a canonicalized version of the Message-Instance and
DKIM2-Signature header fields as described in Section 10.4. Note
that this canonicalized version does not actually replace the
original content.
2. Based on the algorithm indicated in the "a1=" tag, compute the
hashes of the canonical copy. Then verify that the the signature
conveyed in the "b1=" tag is correct for this hash value using
the mechanism appropriate for the public key algorithm described
in the "a1=" tag. If the signature does not validate, the
Verifier SHOULD ignore the signature and return PERMFAIL
("signature did not verify").
3. Otherwise, the signature has correctly verified.
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4. If there is more than one signature provided then the second
signature MUST be checked if the verifier is able to do so, using
a2= and b2= as appropriate.
5. If either signature fails then an error SHOULD be reported.
6. If one signature fails and the other passes then the error that
is reported should provide that information (e.g. PERMFAIL "RSA
signature passed, elliptic curve signature failed")
The reasoning for requiring that both 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.
11.5. Check 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 (v=) Message-Instance before accepting an email. If this
check does not pass then a Delivery Status Notification 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.
A Verifier SHOULD check that the mf= value in the most recent
DKIM2-Signature header field is identical to the [RFC5321] MAIL FROM
values of the SMTP protocol interaction that delivered the email to
the Verifier. A Verifier SHOULD also check that all of the [RFC5321]
RCPT TO values from the SMTP protocol occur in the most recent
DKIM2-Signature header field. The values MUST BE put into lower-case
before doing these checks. Note that these check 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.
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A Verifier SHOULD also check the chain of custody for the message
(see {chain-of-custody}) is valid, using a relaxed domain match (see
{#relaxed-domain-match}).
Should checking these signatures (all but the most recently applied)
give the status TEMPFAIL then it may be possible for the Verifier to
determine the validity of the signature at a later time. It the
TEMPFAIL status continues to occur, or if a PERMFAIL is encountered
then this MAY be reported to the sending MTA by means of a Delivery
Status Notification. However the non-validating email MUST NOT be
forwarded to any MTA outside of the current organisation.
11.6. Checking the Message-Instance Header Fields
The highest numbered (v=) Message-Instance header field SHOULD be
checked to determine that the message body has not been altered since
the body hash was calculated.
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.
11.7. 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.
11.8. 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.
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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, such as:
451 4.7.5 Unable to verify signature - key server unavailable
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.
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 [RFC3461].
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 DNS
MUST NOT be sent.
12.1. DSN contents
As set out in [RFC3461], the DSN has a top-level MIME part of type
multipart/report. Among other things, that MIME part must contain a
MIME part of type message/rfc822 that holds either the original
message exactly as it was submitted by the sending system or 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 ("<>").
If the message body has been truncated (rather than omitted
altogether) then in order to allow verification of the DNS contents a
Message-Instance header field MUST be added to the message with a
body recipe of "z".
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12.1.1. Bounce propagation
A Forwarder which receives a DSN MAY decide to 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 SHOULD
be handled in the usual way, with Message-Instance header fields
documenting any changes and a DKIM2-Signature field with an
incremented hop count value added.
The Forwarder MAY alternatively decide to reconstruct the message (or
just the message header fields) as they were when the message was
delivered to the Forwarder and construct a DSN using that
information. The information in Message-Instance header fields can
be used to achieve this. The resultant DSN is sent to the MAIL FROM
address from the now highest numbered DKIM2-Signature header field.
Doing this will ensure that details of where the message was
forwarded to will not be revealed to the previous hop.
12.1.2. Authentication of inbound bounce notifications
When a system receives a DKIM2 signed bounce notification, 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.
1) The last (highest i= tag) DKIM2-Signature header field of the
returned message will be one that was generated by the system
receiving the bounce notification, determined by examining the d= and
mf= tags of that DKIM2-Signature header field.
1) The header fields of the embedded message (in the message/rfc822
MIME part) can be verified. If the message body is present then that
can also be verified by inspecting the Message-Instance header
field(s).
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.
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13. EAI ([RFC6530]) considerations for DKIM2
TBA
14. IANA Considerations
TBA
15. Security Considerations
TBA
16. Changes from Earlier Versions
draft-clayton-dkim2-spec-06
Changed r= to rb= and r.fieldname to rh=fieldname because the tags in
a tag value field may not use "-" characters. Also fixed the ABNF.
Incorporated Allen Robinson's BOUNCE draft.
Assorted other very minor fixes.
draft-clayton-dkim2-spec-05
Assorted fixes, with particular thanks to Hannah Stern. Clarified
that there are two types of rt/mt matches performed. Specified that
recipes must not contain overlaps. Set out the need for matching mf=
and d= and put the relaxed domain match algorithm into its own
section. Set out that in practice all DKIM2-Signature header fields
will need to be checked.
draft-clayton-dkim2-spec-04
Added a definition of a Reviser. Incorporate the Message-Instance
scheme previously found in ALGEBRA. Recast the text relating to
hashes and signatures accordingly. Changed t= back to just digits.
draft-clayton-dkim2-spec-03
Removed the pp= proposal, and briefly explained how signers often
handle private keys on behalf of domain owners. Changed t= to be
human-readable. Fixed description of body canonicalisation to match
DKIM1/relaxed.
draft-clayton-dkim2-spec-02
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Further clarifications and tidying up; alignment of ALGEBRA
description with the new MailVersion header field-name; addition of
h= tag field. Also added the pp= mechanism to address forwarders who
do not have private keys to hand to make the rt/mf/rt chains
validate.
draft-clayton-dkim2-spec-01
Significant re-ordering of sections and removal of repetitious
material.
Relax the matching algorithm between rt= and mf=
[[This section to be removed by RFC Editor]]
17. References
17.1. Normative References
[DKIMKEYS] Chuang, W., "Domain Name Specification for DKIM2", Work in
Progress, Internet-Draft, draft-chuang-dkim2-dns-03, 20
October 2025, <https://datatracker.ietf.org/doc/html/
draft-chuang-dkim2-dns-03>.
[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>.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, DOI 10.17487/RFC3461, January 2003,
<https://www.rfc-editor.org/rfc/rfc3461>.
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[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>.
[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>.
[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>.
17.2. Informative References
[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>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/rfc/rfc5598>.
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[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
Bron Gondwana
Fastmail Pty Ltd
Level 2, 114 William Street
3000
Australia
Phone: +61 457 416 436
Email: brong@fastmailteam.com
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