



Network Working Group                                         R. Clayton
Internet-Draft                                                     Yahoo
Intended status: Standards Track                               W. Chuang
Expires: 15 June 2026                                             Google
                                                             B. Gondwana
                                                        Fastmail Pty Ltd
                                                        12 December 2025


            DomainKeys Identified Mail Signatures v2 (DKIM2)
                      draft-clayton-dkim2-spec-04

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 can also 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."




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   This Internet-Draft will expire on 15 June 2026.

Copyright Notice

   Copyright (c) 2025 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  . . . . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . .  14
   7.  Computing the Body Hash . . . . . . . . . . . . . . . . . . .  19
     7.1.  Preventing Transport Conversions  . . . . . . . . . . . .  20
   8.  Computing the Header Fields Hash  . . . . . . . . . . . . . .  20
   9.  Signer Actions  . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Add any Necessary Message-Instance Header Fields  . . . .  22
     9.2.  Determine what RFC5321 "envelope" will be used for the
           message . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.3.  Select a Private Key and Corresponding Selector Value . .  23



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     9.4.  Calculate a Signature Value . . . . . . . . . . . . . . .  24
   10. Verifier Actions  . . . . . . . . . . . . . . . . . . . . . .  25
     10.1.  Output States  . . . . . . . . . . . . . . . . . . . . .  25
     10.2.  Check Most Recent Signature and Hashes for the
            Message  . . . . . . . . . . . . . . . . . . . . . . . .  26
       10.2.1.  Validation of Tag Fields . . . . . . . . . . . . . .  26
       10.2.2.  Get the Public Key . . . . . . . . . . . . . . . . .  27
       10.2.3.  Perform the Signature Verification Calculation . . .  28
       10.2.4.  Validation of a Message-Instance Header Field  . . .  29
     10.3.  Interpret Results/Apply Local Policy . . . . . . . . . .  29
   11. Delivery Status Notifications in the DKIM2 ecosystem  . . . .  30
   12. EAI (RFC6530) considerations for DKIM2  . . . . . . . . . . .  30
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     15.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Appendix A.  Changes from Earlier Versions  . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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.





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   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 either delivers it into a destination
   mailbox or forwards 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 9) and "Verifier Actions" (Section 10) 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 9.4 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 9.4 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*2DIGIT

   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.

   r= A "recipe" to recreate the previous version of the message body.
























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   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).

   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. If the endnumber is omitted then all lines
   (from the startnumber line onwards) are to be copied.

   b: base64string

   Decode the base64string to get the value of a line to be inserted.
   If the base64string contains an encoded CRLF then more than one
   line is being added but note that a CRLF will automatically be
   added after the decoded text -- i.e. if only one line is being
   added there MUST NOT be an encoded CRLF present. 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-r-tag       = %x72 [FWS] "=" [FWS] mi-r-tag-data
   mi-r-tag-data  = mi-r-recipes / %x7a
   mi-r-recipes   = mi-r-recipe * ("," [FWS] mi-r-recipe)
   mi-r-recipe    = %x63 ":" 1*DIGIT "-" [ 1*DIGIT ] /
                    %x62 ":" [ base64string ]



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   r.header= A "recipe" to replicate the previous version of a header
   field.

   plain text; OPTIONAL

   A Header Recipe is a comma separated list of instructions. Commas can
   be followed by optional whitespace. Each
   set of instructions refers to a particular header field name. There
   MUST be only one set of instructions for any given header field name

   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 directtion to
   the body lines). That is to say, when walking the header from top
   to bottom 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 of header fields with that
   header field name are to be retained.

   c: startnumber-endnumber

   Keep the header field (with the relevant header field name)
   from position startnumber to endnumber.

   b: base64string

   Decode the base64string value to get the header field to be inserted.
   The header field will start with the field name and a colon. These
   MUST NOT be specified again. A CRLF will be added after the text.
   When the base64string has been decoded it MUST NOT contain a CRLF.
   If the base64string is absent then there will be no text after the
   colon.



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   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       = %x68 "." field-name [FWS] "=" [FWS] mi-rh-tag-data
   mi-rh-tag-data  = mi-rh-recipes / %07A
   mi-rh-recipes   = mi-rh-recipe * ("," [FWS] mi-rh-recipe)
   mi-rh-recipe    = %x63 ":" 1*DIGIT /
                     %x62 ":" base64string

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.




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   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.

   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).






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   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] dat

   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

   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.





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   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"; See Section 4 for
   a description of the algorithms.

   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 9) for
   how the signature is computed.




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   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.

   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.




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   "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-data
                      *("," [FWS] sig-f-tag-data)
   sig-f-tag-data   = "modifiedbody" | "modifiedheader" | "exploded" |
                      "donotmodify" | "donotexplode" | "feedback"
   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.

   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.





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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]).

   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






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      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".

   *  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.




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   *  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.  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 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 contain a Message-Instance header field then one
   MUST be added.  This MUST NOT contain any "recipes" (b=, h.field=).

   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 field then a new Message-Instance MUST be added.
   This Message-Instance field MUST contain "recipes" to be able to
   recreate the message corresponding to the hash values in the highest
   numbered Message-Instance field, or a "recipe" of "z" to indicate
   that recreating the previous version of the message will not be
   possible.








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   A system may add more than one Message-Instance 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
   contains any "recipes" and any other Message-Instance field MUST
   contain at least one "recipe".

9.2.  Determine what [RFC5321] "envelope" will be used 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 MUST be available to (or deducible by) a Signer.

   When the message being signed already has a DKIM2-Signature header
   field (i.e. it has already been transmitted at least once) then the
   mf= of the message to be signed MUST match with an entry in the rt=
   of currently highest numbered (i=) DKIM2-Signature header field.

   The match algorithm only considers the domain part of the rt= and mf=
   values, that is the local part is ignored.  If there is not an exact
   match then labels are removed, one by one from the left hand side of
   the mf= domain and compared with the rt= domain until there is an
   exact match or no labels remain.  This approach allows systems to use
   existing "bounce-handling" schemes with special subdomains in their
   MAIL FROM values.

   In any situation where a messages will be forwarded in such a way
   that the mf= on the outgoing message would not match with the rt=
   value then the Signer MUST generate an extra DKIM2-Signature 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.

9.3.  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.




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9.4.  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
      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.







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   *  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 9.3}.

   *  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.

10.  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.

10.1.  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
   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.




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10.2.  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 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 rt= and mf= values in the most
   recent DKIM2-Signature header field are consistent with the MAIL FROM
   and RCPT TO values from the SMTP protocol interaction that delivered
   the email to the Verifier.

   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.  If the
   only concern is whether or not a particular stage of modification is
   correct (for example the very first form of the message) it is not
   necessary to check very hash and signature in order to do this, if
   applying "recipes" produces a message with the correct hashes for
   Message-Instance field of interest then this is sufficient.

   Verifiers who wish to honour "feedback" requests or who wish to
   assess assign reputation to Revisers SHOULD check all relevant
   signatures and hashes.  The TBA document explores these issues in
   more detail.

   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.

10.2.1.  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.






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   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.

10.2.2.  Get 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 9.3
       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, the
       Verifier can choose one of the key records or may cycle through
       the key records, performing the remainder of these steps on each
       record at the discretion of the implementer.  The order of the
       key records is unspecified.  If the Verifier chooses to cycle
       through the key records, then the "return ..." wording in the
       remainder of this section means "try the next key record, if any;
       if none, return to try another signature in the usual way".









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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").

10.2.3.  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 9.4.  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.

10.2.4.  Validation of a Message-Instance Header Field

   Implementers MUST meticulously validate the format and values in any
   Message-Instance header field they rely on; any inconsistency or
   unexpected values MUST cause the header field to be completely
   ignored and the Verifier to return PERMFAIL (signature syntax error).
   Being "liberal in what you accept" is definitely a bad strategy in
   this security context.

   Note, however, that the presence of unknown tags in a Message-
   Instance header field, MUST NOT cause a verification to fail.

   In order to check the hash values in a Message-Instance header field
   are correct it will be necessary to repeat the steps taken by the
   Signer as set out in Section 7 and Section 8.

10.3.  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.

11.  Delivery Status Notifications in the DKIM2 ecosystem

   [BOUNCES] should be incorporated here.

12.  EAI ([RFC6530]) considerations for DKIM2

   TBA

13.  IANA Considerations

   TBA

14.  Security Considerations

   TBA

15.  References

15.1.  Normative References

   [BOUNCES]  Robinson, A., "DKIM2 Procedures for bounce processing",
              Work in Progress, Internet-Draft, draft-robinson-dkim2-
              bounce-processing-01, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-robinson-
              dkim2-bounce-processing-01>.

   [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>.





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   [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>.

   [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>.







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   [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>.

15.2.  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>.

   [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>.

Appendix A.  Changes from Earlier Versions

   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.




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   draft-clayton-dkim2-spec-02

   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]]

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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