



Network Working Group                                         R. Clayton
Internet-Draft                                                     Yahoo
Intended status: Standards Track                               W. Chuang
Expires: 22 October 2026                                          Google
                                                             B. Gondwana
                                                        Fastmail Pty Ltd
                                                           20 April 2026


            DomainKeys Identified Mail Signatures v2 (DKIM2)
                     draft-ietf-dkim-dkim2-spec-01

Abstract

   DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or
   organization that owns a signing domain to document that it has
   handled an email message by associating their domain with the
   message.  This is achieved by providing a hash value that has been
   calculated on the current contents of the message and then applying a
   cryptographic signature that covers the hash values and other details
   about the transmission of the message.  Verification is performed by
   querying an entry within the signing domain's DNS space to retrieve
   an appropriate public key.  As a message is transferred from author
   to recipient systems that alter the body or header fields will
   provide details of their changes and calculate new hash values.
   Further signatures will be added to provide a validatable "chain".
   This permits validators to identify the nature of changes made by
   intermediaries and apply a reputation to the systems that made
   changed.  DKIM2 also allows recipients to detect when messages have
   been unexpectedly "replayed" and will ensure that Delivery Status
   Notifications are only sent to entities that were involved in the
   transmission of a message.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on 22 October 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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  DKIM2 Architecture Documents  . . . . . . . . . . . . . .   4
   2.  Terminology and Definitions . . . . . . . . . . . . . . . . .   4
     2.1.  Signer  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Forwarder . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Reviser . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Verifier  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.5.  Signing Domain  . . . . . . . . . . . . . . . . . . . . .   6
     2.6.  Originator  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.7.  Header Field  . . . . . . . . . . . . . . . . . . . . . .   6
     2.8.  Tag . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.9.  Message Body  . . . . . . . . . . . . . . . . . . . . . .   7
     2.10. Hash  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.11. Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.12. Whitespace  . . . . . . . . . . . . . . . . . . . . . . .   8
     2.13. Imported ABNF Tokens  . . . . . . . . . . . . . . . . . .   8
     2.14. Common ABNF Tokens  . . . . . . . . . . . . . . . . . . .   8
   3.  Signing and Verification Cryptographic Algorithms . . . . . .   9
     3.1.  The SHA256 Hashing Algorithm  . . . . . . . . . . . . . .   9
     3.2.  The RSA-SHA256 Signing Algorithm  . . . . . . . . . . . .   9
     3.3.  The Ed25519-SHA256 Signing Algorithm  . . . . . . . . . .  10
     3.4.  Other Algorithms  . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Selectors . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.6.  Key Management  . . . . . . . . . . . . . . . . . . . . .  11
   4.  Recipes . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Header Recipes  . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Body Recipes  . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Message Hash Values . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Computing the Body Hash . . . . . . . . . . . . . . . . .  15
     5.2.  Computing the Header Fields Hash  . . . . . . . . . . . .  16



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   6.  The Message-Instance Header Field . . . . . . . . . . . . . .  17
     6.1.  m= the revision number of the Message-Instance header
           field . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.2.  r= recipes to recreate the previous instance of the
           message . . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.3.  h= the hash values for the message  . . . . . . . . . . .  18
   7.  The DKIM2-Signature Header Field  . . . . . . . . . . . . . .  19
     7.1.  i= the sequence number of the DKIM2-Signature header
           field . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  m= the highest numbered Message-Instance header field . .  20
     7.3.  n= nonce value  . . . . . . . . . . . . . . . . . . . . .  20
     7.4.  t= signature timestamp  . . . . . . . . . . . . . . . . .  20
     7.5.  mf= the MAIL FROM used when the message was sent  . . . .  21
     7.6.  rt= the RCPT TO value(s) used when the message was
           sent  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     7.7.  d= the domain associated with this signature. . . . . . .  22
     7.8.  s= the signature value(s) for the message . . . . . . . .  22
     7.9.  f= flags  . . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  Signer Actions  . . . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Add any Necessary Message-Instance Header Fields  . . . .  24
     8.2.  Provide a "Chain of Custody" for the Message  . . . . . .  24
     8.3.  The Relaxed Domain Match Algorithm  . . . . . . . . . . .  25
     8.4.  Select a Private Key and Corresponding Selector Value . .  26
     8.5.  Calculate a Signature Value . . . . . . . . . . . . . . .  26
   9.  Verification Requirements . . . . . . . . . . . . . . . . . .  27
     9.1.  Check Most Recent Signature and Hashes for the Message  .  27
     9.2.  Checking the Message-Instance Header Fields . . . . . . .  28
     9.3.  Checking the DKIM2-Signature Header Fields  . . . . . . .  28
     9.4.  Interpret Results/Apply Local Policy  . . . . . . . . . .  28
   10. Verifier Actions  . . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Output States  . . . . . . . . . . . . . . . . . . . . .  29
     10.2.  Ensure that the DKIM2 Header Fields are Valid  . . . . .  30
     10.3.  Check the timestamps . . . . . . . . . . . . . . . . . .  31
     10.4.  Check the Chain-of-Custody . . . . . . . . . . . . . . .  31
     10.5.  Fetch the Public Key . . . . . . . . . . . . . . . . . .  32
     10.6.  Perform the Signature Verification Calculation . . . . .  32
     10.7.  Validating Body and Header hashes  . . . . . . . . . . .  33
   11. Delivery Status Notifications in the DKIM2 ecosystem  . . . .  33
     11.1.  DSN contents . . . . . . . . . . . . . . . . . . . . . .  34
       11.1.1.  Bounce Propagation . . . . . . . . . . . . . . . . .  34
       11.1.2.  Authentication of Inbound Bounce Notifications . . .  34
   12. Preventing Transport Conversions  . . . . . . . . . . . . . .  35
   13. EAI (RFC6530) Considerations for DKIM2  . . . . . . . . . . .  36
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  36
   16. Changes from Earlier Versions . . . . . . . . . . . . . . . .  36
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  37



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     17.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or
   organization to document that they have handled an email message by
   associating a domain name [RFC1034] with the message [RFC5322].  A
   public key signature is used to record that they have been able to
   read the contents of the message and write to it.

   Verification of claims is achieved by fetching a public key stored in
   the DNS under the relevant domain and then checking the signature.

   Message transit from author to recipient is through Forwarders that
   typically make no substantive change to the message content and thus
   preserve the DKIM2 signature.  Where they do make a change the
   changes they have made are documented so that these can be "undone"
   and the original signature validated.

   When a message is forwarded from one system to another an additional
   DKIM2 signature is added on each occasion.  This chain of custody
   assists validators in distinguishing between messages that were
   intended to be sent to a particular email address and those that are
   being "replayed" to that address.

   The chain of custody can also be used to ensure that delivery status
   notifications are only sent to entities that were involved in the
   transmission of a message.

   Organizations that process a message can add to their signature a
   request for feedback as to any opinion (for example, that the email
   was considered to be spam) that the eventual recipient of the message
   wishes to share.

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.






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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].  These words take their normative meanings only when they
   are presented in ALL UPPERCASE.

   DKIM2 is designed to operate within the Internet Mail service, as
   defined in [RFC5598].  Basic email terminology is taken from that
   specification.

   DKIM2 inherits many ideas from DKIM ([RFC6376]) which, for clarity we
   refer to in this specification as DKIM1.  In addition, some features
   were influenced by experience with (see [CONCLUDEARC]) the
   experimental ARC protocol ([RFC8617]).

   Syntax descriptions use Augmented BNF (ABNF) [RFC5234].

   This document uses JSON [RFC8259] to encode the "recipes" which
   record changes made to a message header fields or body.  The JSON
   objects are then base64 encoded.  This means that a standard JSON
   parser can be used to create what may be quite complex data
   structures.  Unrecognised fields within JSON objects MUST be ignored.

2.1.  Signer

   Elements in the mail system that sign messages on behalf of a domain
   are referred to as Signers.  These may be MUAs (Mail User Agents),
   MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other
   agents such as mailing list "exploders".  In general, any Signer will
   be involved in the injection of a message into the message system in
   some way.  The key point is that a message must be signed before it
   leaves the administrative domain of the Signer.

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.








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2.3.  Reviser

   As will be seen, a Forwarder may alter the message content or header
   fields, in such a way that existing signatures on the message will no
   longer validate.  If so, then a record will be made of these changes.
   We call a Forwarder that makes such changes a Reviser.

2.4.  Verifier

   Elements in the mail system that verify signatures are referred to as
   Verifiers.  These may be Forwarders, Revisers, MTAs, Mail Delivery
   Agents (MDAs), or MUAs.  It is an expectation of DKIM2 that a
   recipient of a message will wish to verify some or all signatures
   before determining whether or not to accept the message or pass it on
   to another entity.

2.5.  Signing Domain

   A domain name associated with a signature.  This domain may be
   associated with the author of an email, their organization, a company
   hired to deliver the email, a mailing list operator, or some other
   entity that handles email.  What they have in common is that at some
   point they had access to the entire contents of the email and were in
   a position to add their signature to the email.

2.6.  Originator

   The entity that creates and sends the initial form of a message.  The
   Originator adds the first Message-Instance header field (m=1) and the
   first DKIM2-Signature header field (i=1) to the message.

2.7.  Header Field

   As defined in [RFC5322], a header field is a single logical line in
   the message header consisting of a field name, a colon, and a field
   body (value).  In this document "header field" always refers to a
   single field; "header fields" (plural) refers to multiple fields.
   The unqualified term "header" is avoided to prevent ambiguity.

2.8.  Tag

   A named element within a header field (see Section 6 and Section 7).
   A tag consists of a tag-name and a tag-value separated by an equals
   sign.  Tags are separated by semicolons within the header field.







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2.9.  Message Body

   The content of an email message that follows the blank line after the
   header fields, treated as a sequence of octets.  In this document,
   the terms "body" and "message body" are used interchangeably.

2.10.  Hash

   A fixed-length value produced by applying a cryptographic hash
   function (such as SHA-256) to an input.  DKIM2 uses hashes to create
   a compact, verifiable representation of message header fields and the
   message body.

2.11.  Glossary

   The following terms are used throughout this document:

   DKIM1  The original DomainKeys Identified Mail protocol as specified
      in [RFC6376].

   DKIM2-Signature  A header field containing a cryptographic signature
      over the Message-Instance and DKIM2-Signature header fields of a
      message, along with metadata about the signing domain, SMTP
      envelope, and timestamp.

   Message-Instance  A header field containing cryptographic hashes of
      the message header fields and body, along with optional recipes
      that allow undoing changes made at that hop.

   Recipe  A set of instructions encoded as a JSON object within the r=
      tag of a Message-Instance header field.  Recipes allow a Verifier
      to reconstruct the previous state of a message from its current
      state, by specifying which parts of the header fields or body to
      copy and which literal values to substitute.

   Chain of Custody  The sequence of DKIM2-Signature header fields on a
      message, each recording the SMTP envelope addresses (MAIL FROM and
      RCPT TO) used at each hop.  A valid chain of custody demonstrates
      that the message followed a plausible path from Originator to the
      current recipient.

   Selector  A subdivision of the key namespace for a signing domain,
      used to look up the public key in DNS.  The selector value is
      combined with the signing domain to form the DNS query name:
      selector._domainkey.domain.






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2.12.  Whitespace

   There are two forms of whitespace used in this specification:

   *  WSP represents simple whitespace, i.e., a space or a tab character
      (formal definition in [RFC5234]).

   *  FWS is folding whitespace.  It allows multiple lines separated by
      CRLF followed by at least one whitespace, to be joined.

   The formal ABNF for these are (WSP given for information only):

   WSP =   SP / HTAB
   FWS =   [*WSP CRLF] 1*WSP

   The definition of FWS is identical to that in [RFC5322] except for
   the exclusion of obs-FWS.

2.13.  Imported ABNF Tokens

   The following tokens are imported from other RFCs as noted.  Those
   RFCs should be considered definitive.

   The following tokens are imported from [RFC5321]:

   *  "Domain"

   *  "Forward-path"

   *  "reverse-path"

   The following tokens are imported from [RFC5322]:

   *  "field-name" (name of a header field)

   Other tokens not defined herein are imported from [RFC5234].  These
   are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
   etc.

2.14.  Common ABNF Tokens

   The following ABNF tokens are used elsewhere in this document:









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   ALPHADIGITD  = (ALPHA / DIGIT / "-" / "_")

   textstring   =  [FWS] ALPHADIGITD *(ALPHADIGITD) [FWS]

   ALPHADIGITPS =  (FWS / ALPHA / DIGIT / "+" / "/")

   base64string =  ALPHADIGITPS *(ALPHADIGITPS) [[FWS] "=" [[FWS] "="]]

   Note that base64strings are defined in [RFC4648], but that document
   does not contain any ABNF.  Note that a base64string MUST be padded
   with trailing = characters if needed.

   Note that the definition of base64string allows for the presence of
   FWS, which simplifies folding header fields to an allowable line
   length.  FWS within base64strings will be ignored when their value is
   being used.

3.  Signing and Verification Cryptographic Algorithms

   DKIM2 supports multiple hashing and digital signature algorithms.
   One hash function (SHA256) is 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.

3.1.  The SHA256 Hashing Algorithm

   The SHA256 hashing algorithm is used to compute body and header
   hashes as defined in Section 5.1 and Section 5.2.

   The resultant values are identified by the text string "sha256" and
   placed into Message-Instance header fields.

3.2.  The RSA-SHA256 Signing Algorithm

   The RSA-SHA256 Signing Algorithm computes a hash over all the
   Message-Instance and DKIM2-Signature header fields as described in
   Section 8.5 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.



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   The signature value (expressed in base64) is placed (with the
   identifying text string "rsa-sha256") into DKIM2-Signature header
   fields.

3.3.  The Ed25519-SHA256 Signing Algorithm

   The Ed25519-SHA256 Signing Algorithm computes a hash over all the
   Message-Instance and DKIM2-Signature fields as described in
   Section 8.5 using SHA-256 (FIPS-180-4-2015) as the hash-alg.  It
   signs the hash with the PureEdDSA variant Ed25519, as defined in
   Section 5.1 of [RFC8032].

   The signature value (expressed in base64) is placed (with the
   identifying text string "ed25519-sha256") into DKIM2-Signature header
   fields.

3.4.  Other Algorithms

   Other algorithms MAY be defined in the future.  Verifiers MUST ignore
   any hashes or signatures using algorithms that they do not implement.

3.5.  Selectors

   To support multiple concurrent public keys per signing domain, the
   key namespace is subdivided using "selectors".

   The number of public keys and corresponding selectors for each domain
   is determined by the domain owner.  Many domain owners will use just
   one selector, whereas administratively distributed organizations can
   choose to manage disparate selectors and key pairs in different
   regions or on different email servers.  Selectors can also be used to
   delegate a signing authority, which can be withdrawn at any time.
   Selectors also make it possible to seamlessly replace keys on a
   routine basis by signing with a new selector, while keeping the key
   associated with the old selector available.

   Periods are allowed in selectors and are component separators.
   Periods in selectors define DNS label boundaries in a manner similar
   to the conventional use in domain names.  This will allow portions of
   the selector namespace to be delegated.

   ABNF:

   selector = Domain







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3.6.  Key Management

   Some level of assurance is required that a public key is associated
   with the claimed Signer.  DKIM2 does this by fetching the key from
   the DNS for the domain specified in the d= field of the
   DKIM2-Signature header field.

   DKIM2 keys are stored in a subdomain named "_domainkey".  Given a
   DKIM2-Signature field with a "d=" tag of "example.com" and a selector
   of "foo.bar", the DNS query will be for
   "foo.bar._domainkey.example.com".

   NOTE: these keys are no different, and are stored in the same
   locations as those for DKIM1 ([RFC6376]).

   Further details can be found in [DKIMKEYS].

4.  Recipes

   A set of "recipes" is used to recreate the previous version of the
   body and/or header fields of a message.  The recipes are provided
   within a JSON object with the schema:

 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://dkim2.org/schemas/recipe-v1",
   "title": "DKIM2 recipes",
   "description": "see draft-dkim-dkim2-spec",
   "type": "object",
   "properties": {
     "h": {
       "description": [ "recipes to recreate header fields",
         "keys are header field names matched case-insensitively",
         "and there MUST NOT be keys that differ only in case"],
       "oneOf": [
         {
           "description": "per-field-name recipe arrays",
           "type": "object",
           "minProperties": 1,
           "additionalProperties": { "$ref": "#/$defs/recipe-steps" }
         },
         {
           "description": "previous header state cannot be recreated",
           "type": "null"
         }
       ]
     },
     "b": {



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       "description": "recipes to recreate the body",
       "oneOf": [
         {
           "description": "body recipes",
           "$ref": "#/$defs/recipe-steps"
         },
         {
           "description": "previous body state cannot be recreated",
           "type": "null"
         },
         {
           "description": "body was truncated (DSN)",
           "type": "object",
           "properties": {
             "z": { "type": "boolean", "const": true }
           },
           "required": ["z"], "additionalProperties": false
         }
       ]
     }
   },
   "anyOf": [
     { "required": ["h"] },
     { "required": ["b"] }
   ],
   "$defs": {
     "recipe-steps": {
       "type": "array",
       "items": {
         "oneOf": [
           {
             "description": "copy lines/fields, start to end inclusive",
             "type": "object",
             "properties": {
               "c": { "type": "array",
                 "items": { "type": "integer", "minimum": 1 },
                 "minItems": 2, "maxItems": 2
               }
             },
             "required": ["c"], "additionalProperties": false
           },
           {
             "description": "data lines/values to emit",
             "type": "object",
             "properties": {
               "d": { "type": "array",
                 "items": { "type": "string" },
                 "minItems": 1



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               }
             },
             "required": ["d"], "additionalProperties": false
           }
         ]
       }
     }
   }
 }

   Note that the specification of JSON schemas is maintained by the JSON
   Schema organisation, and the relevant specification document is
   linked to by the $schema field in each JSON schema.

4.1.  Header Recipes

   A Header Recipe is an array of instructions applied to the specified
   header fields with the given header field name.  These instructions
   are applied in order to the message which has been received so as to
   recreate the message as it was before modifications were made.

   If there is no "h" field in the JSON object then there was no
   modification to the header fields.

   If the "h" field value is null (there are no recipes for any header
   field) then the previous state of the header fields cannot be
   recreated.  Verifiers of the message may be able to determine, by
   seeing which entity makes this declaration, that this is acceptable
   to them because, for example, that entity is providing a
   contractually arranged service.

   Matching of header field names is always done without regard to case.

   If a header field name is not present in the JSON object then all
   header fields with that header field name are to be retained.

   If the recipe array for a header field name that is present in the
   JSON object is empty then all instances of that header field are to
   be removed to reinstate the previous state of the message.

   Header fields are numbered "bottom up" (the opposite direction to the
   body lines).  That is to say, when walking the header fields from the
   top of the message to end of the header fields then the last header
   field instance encountered with any particular header field name is
   numbered 1, the header field (with the same header field name) above
   that is numbered 2, and so on.





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   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.

   The recipes are processed in order and the resulting header fields
   are emitted so that later header field will appear above earlier
   header fields in the recreated message.

   Each recipe step is a JSON object with exactly one key:

   A "c" step has the form {"c": [start, end]}. The relevant header
   field instances numbered from start to end inclusive, are to be
   emitted.  The start value of each "c" step MUST be in ascending order
   and MUST be greater than the end value of all preceding "c" steps for
   this header field name.

   A "d" step has the form {"d": ["value1", "value2", ...]}. Each string
   in the array is treated as a value to which the relevant header field
   name and a colon is prepended and a CRLF is appended and the
   resultant string is then emitted.  Note that the way in which hashes
   are calculated (see Section 5.2) means that no heed needs to be taken
   of wrapping or the case of the header field name.  The text strings
   MUST NOT contain CR or LF characters.  If a string is empty then the
   CRLF will immediately follow the header field name and colon.

4.2.  Body Recipes

   A Body Recipe is an array of instructions applied to the message body
   which can recreate the message as it was before modifications were
   made.

   If there is no "b" field in the JSON object then there was no
   modification to the message body.  Note that the JSON schema requires
   either "h" or "b" to be present.

   If the "b" field is null (there are no recipes) then the previous
   state of the message body cannot be recreated.  Verifiers of the
   message may be able to determine, by seeing which entity makes this
   declaration, that this is acceptable to them because, for example,
   that entity is providing a contractually arranged service.

   Body lines are numbered "top down" (the opposite direction to the
   header fields).  The first line of the body (immediately after the
   blank line that indicates that there are no more header fields) is
   numbered 1.






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   The recipes are processed in order and the resulting body lines
   fields are emitted so that later lines will appear below earlier
   lines in the recreated message.

   Each recipe step is a JSON object with exactly one key:

   A "c" step has the form {"c": [start, end]}. The message body lines
   from start to end, inclusive, are to be emitted.  The start value of
   each "c" step MUST be in ascending order and MUST be greater than the
   end value of all preceding "c" steps.

   A "d" step has the form {"d": ["line1", "line2", ...]}. Each string
   in the array has a CRLF appended and the resultant string is emitted.
   The text strings MUST NOT contain CR or LF characters.  If a string
   is empty then just a CRLF is emitted.

   A "z" step has the form {"z": true} and indicates that the body was
   truncated (see the DSN handling in Section 11).

5.  Message Hash Values

   A set of cryptographic "hashes" are used to record the current
   message body and header fields.  The hashes are placed into the h=
   tag of a Message-Instance header field.

   To provide for algorithmic dexterity more that one hash value, using
   a different algorithm MAY be supplied in the same Message-Instance
   header field.

   Since Message-Instance header fields are ignored when calculating the
   header hash value, the body hash and header hash may be calculated in
   any convenient order.

5.1.  Computing the Body Hash

   The body of messages is treated as merely a string of octets.  DKIM2
   messages MAY be either in plain-text or in MIME format; no special
   treatment is afforded to MIME content.  Message attachments in MIME
   format MUST be included in the content that is signed.

   The DKIM2 body hash is calculated in the same manner as DKIM1's
   "simple" scheme:

   All empty lines at the end of the message body are ignored.  An empty
   line is a line of zero length after removal of the line terminator.
   If there is no body or no trailing CRLF on the message body, a CRLF
   is added.  That is "*CRLF" at the end of the body is converted to
   "CRLF".



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   No other changes are made to the body, which is then processed by the
   relevant hash algorithm(s).  The name of the hash and the hash value
   (converted to base64 form) is then inserted into (Signers) or
   compared to (Verifiers) the value of the "h=" tag of the Message-
   Instance header field that is being created/verified.  If multiple
   hashes are calculated then multiple entries within the "h=" value
   will be inserted/compared.

5.2.  Computing the Header Fields Hash

   The header fields hash calculation done by a Signer MUST apply the
   following steps in the order given.  A Verifier will need to do the
   equivalent steps in order to check that the hash they have received
   is correct.

   *  Ignore some header fields

      When calculating the header field hash "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 field name starts with "ARC-"
      MUST be ignored.  Not including DKIM1 and ARC signatures means
      that systems that wish to add other types of signature as well as
      a DKIM2 signature are free to do this in any convenient order.

   *  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 were
      likely to have been placed into the message header, that is from
      the last within the header upwards (the same ordering as is used
      in the header recipes (see Section 4.1).

      It is sometimes suggested that some MTAs re-order header fields
      after they receive an email.  If an MTA does change the order of
      header fields with the same header field name (and those header
      fields will be included in the hash calculation) then it is their
      responsibility to recover the original order before verifying an
      existing signature or passing a previously signed message to
      another MTA that may wish to do such verification.

   *  The hash(es) of the concatenated header fields are calculated.

   The name of the hash and the hash value (converted to base64 form) is
   then inserted into (Signers) or compared to (Verifiers) the value of
   the "h=" tag of the Message-Instance header field that is being
   created/verified.  If multiple hashes are calculated then multiple
   entries within the "h=" value will be inserted/compared.

6.  The Message-Instance Header Field

   A Message-Instance header field documents the current contents of the
   message and, in the case of a Reviser, records any relevant changes
   that have been made to the incoming message.




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   The Message-Instance header field is a list of tag values as
   described below.  The m= and h= tags MUST be present.  The r= tag is
   optional.

   The tag identifiers (before the = sign) MUST be treated as case
   insignificant, the tag value (after the = sign) is case significant.
   The tags may appear in any order, but MUST be only one of each kind.
   Unknown tags, for extensions, MUST be ignored.

   ABNF:

   mi-field    = "Message-Instance:" mi-tag-list
   mi-tag-list = *([FWS] mi-tag [FWS] ";" [FWS])
   mi-tag      = mi-m-tag / mi-h-tag / mi-r-tag / x-tag
   x-tag       = ALPHA *(ALPHA / DIGIT / "_") "=" %x21-3A / %x3C-7E
                 ; for extension

6.1.  m= the revision number of the Message-Instance header field

   The Originator of a message uses the value 1.  Further Message-
   Instance header fields are added with a value one more than the
   current highest numbered Message-Instance header field.  Gaps in the
   numbering MUST be treated as making the whole message impossible to
   verify.

   ABNF:

   mi-m-tag    = %x6d [FWS] "=" [FWS] 1*DIGIT

6.2.  r= recipes to recreate the previous instance of the message

   The r= tag value is the base64 encoded version of the JSON object
   that contains the recipes that allow the previous instance of the
   message to be recreated (see Section 4}.

   ABNF:

   mi-r-tag    = %x72 [FWS] "=" base64string

6.3.  h= the hash values for the message

   The h= tag value contains the hash name, header hash value and body
   hash value.  Calculating the hash values is explained in Section 5.

   ABNF:






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   mi-h-tag    = %x68 [FWS] "=" hash-set *("," hash-set )
   hash-set    = [FWS] hash-name [FWS] ":" header-hash ":" body-hash
   hash-name   = "sha256" / x-hash-name
   header-hash = base64string
   body-hash   = base64string
   x-hash-name = textstring ; for later expansion

7.  The DKIM2-Signature Header Field

   The signature of the email is stored in a DKIM2-Signature header
   field.  This header field contains tag values that provide the
   signature and key-fetching data.  The i=, m=, t=, mf=, rt=, d= and s=
   tags MUST be present.  The other tags are optional.

   The tag identifiers (before the = sign) MUST be treated as case
   insignificant, the tag value (after the = sign) is case significant.
   The tags may appear in any order, but MUST be only one of each kind.
   Unknown tags, for extensions, MUST be ignored.

   ABNF:

   sig-field    = "DKIM2-Signature:" sig-tag-list
   sig-tag-list = *([FWS] sig-tag [FWS] ";" [FWS])
   sig-tag      = sig-i-tag / sig-m-tag / sig-t-tag / sig-mf-tag /
                  sig-rt-tag / sig-d-tag / sig-s-tag / sig-n-tag /
                  sig-f-tag / x-tag

   It will be noted that we have not included a version number.
   Experience from IMF onwards shows that it is essentially impossible
   to change version numbers.  If it becomes necessary to change DKIM2
   in the sort of incompatible way that a v=2 / v=3 version number would
   support, it is expected that header fields will be labelled as DKIM3
   instead.

7.1.  i= the sequence number of the DKIM2-Signature header field

   The Originator of a message uses the value 1.  Further
   DKIM2-Signature header fields are added with a value one more than
   the current highest numbered DKIM2-Signature header field.  Gaps in
   the numbering MUST be treated as making the whole message unsigned.

   ABNF:

   sig-i-tag = %x69 [FWS] "=" [FWS] 1*DIGIT







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7.2.  m= the highest numbered Message-Instance header field

   This value allows verifiers to determine which entity made a
   particular revision to the message header fields or body.

   ABNF:

   sig-m-tag = %x6d [FWS] "=" [FWS] 1*DIGIT

7.3.  n= nonce value

   This text value, if present, has a meaning to the creator of the
   signature but MUST NOT be assumed to have any meaning to any other
   entity.  It MAY be used as an index into a database to assist in
   handling Delivery Status Notifications or for any other purpose.

   To discourage use of this tag field as an alternative to the use of
   more appropriate header fields, the length of the string MUST NOT
   exceed 64 characters and implementations SHOULD reject messages where
   this limit has been ignored.

   Note the value MUST be simple ASCII and MUST NOT contain semicolon.

   ABNF:

   sig-n-tag   = %x6e [FWS] "=" [FWS] nonce-value
   nonce-value = *64(%x21-3A / %x3C-7E)
                     ; printable ASCII except semicolon, max 64 chars

7.4.  t= signature timestamp

   The time that this header field was created.  The format is the
   number of seconds since 00:00:00 on January 1, 1970 in the UTC time
   zone.  The value is expressed as an unsigned integer in decimal
   ASCII.  This value is not constrained to fit into a 31- or 32-bit
   integer.

   Implementations SHOULD be prepared to handle values up to at least
   10^12 (until approximately AD 200,000; this fits into 40 bits).

   Implementations MAY ignore signatures that have a timestamp in the
   future.  Implementations MAY ignore signatures that are more than 14
   days old.

   ABNF:

   sig-t-tag    = %x74 [FWS] "=" [FWS] 1*DIGIT




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7.5.  mf= the MAIL FROM used when the message was sent

   DKIM2 records the [RFC5321] MAIL FROM value that was used when the
   message was transmitted over an SMTP link from the signing MTA.  Note
   that MAIL FROM may be just "<>", for example for a Delivery Status
   Notification.

   The value is recorded as the base64 encoding of the [RFC5321]
   reverse-path because of the complex syntax of reverse-path values
   (which can include characters which would confuse naive parsers of
   DKIM2-Signature header fields).  The angle brackets MUST be included,
   but any "Mail-parameters" that were present after the reverse-path
   MUST NOT be included.

   ABNF:

   sig-mf-tag  = %x6d %x66 [FWS] "=" base64string

7.6.  rt= the RCPT TO value(s) used when the message was sent

   DKIM2 records the [RFC5321] RCPT TO value(s) that were used when the
   message was transmitted over an SMTP link from the signing MTA.

   The value is recorded as the base64 encoding of the [RFC5321]
   Forward-path because of the complex syntax of Forward-path values
   (which can include characters which would confuse naive parsers of
   DKIM2-Signature header fields).  The angle brackets MUST be included,
   but any "Rcpt-parameters" that were present after the Forward-path
   MUST NOT be included.

   When a message is intended for more than one recipient then the RCPT
   TO values provided MAY include all of the recipients so that a single
   copy of the email MAY be sent to all of the recipients in a single
   SMTP transaction.  Alternatively, multiple copies of the email may be
   generated so as to not immediately reveal who else received the
   email.

   However, if "bcc:" recipients are involved then in order to meet the
   requirements of [RFC5322] Section 3.6.3 each and every bcc recipients
   MUST NOT revealed to any other message recipient.

   ABNF:

   sig-rt-tag = %x72 %x74 [FWS] "=" base64string *("," base64string)







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7.7.  d= the domain associated with this signature.

   This domain is used to form the query for the public key.  The domain
   MUST be a valid DNS name under which the DKIM2 key record is
   published.

   The domain name in the d= tag MUST exactly match the rightmost labels
   of the domain name of the mf= tag.  That is to say, the domain name
   of the mf= tag MUST either match the d= domain exactly or be a sub-
   domain of the d= domain name.

   When the mf= domain is empty ("<>"), as will be the case for Delivery
   Status Notifications (DSNs), then no match is required.

   ABNF:

   sig-d-tag   = %x64 [FWS] "=" [FWS] Domain

7.8.  s= the signature value(s) for the message

   The s= tag value contains the selector, signature algorithm name and
   signature value.  Calculating the value is explained in Section 8.5.

   The selector values subdivides the namespace for the domain being
   used for signing.

   The algorithm value is the one used to generate the signature.
   Verifiers MUST support "RSA-SHA256" for which the string "rsa-sha256"
   is used and "Ed25519-SHA256" for which the string "ed25519-sha256" is
   used.  See Section 3 for a description of these algorithms.

   To provide for algorithmic dexterity more than one signature, using
   different algorithms, MAY be supplied.  Since the DNS lookup for the
   public key will check that the k= algorithm value matches, a
   different selector MUST necessarily be used for each signature.

   ABNF:

   sig-s-tag   = %x73 [FWS] "=" [FWS] sig-set *( "," sig-set )
   sig-set     = selector [FWS] ":" [FWS] sig-name [FWS] ":" message-sig
   sig-name    = "rsa-sha256" / "ed25519-sha256" / x-sig-name
   x-sig-name  = textstring     ; for later extension
   message-sig = base64string








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7.9.  f= flags

   Flags serve two purposes; they either report what has been done to
   the message by the system creating the DKIM2-Signature or they make a
   request to systems that handle the mail thereafter.  Flags are
   separated by commas, and optional white-space allows systems to add
   several flags without creating long lines.

   If a flag value is not recognised it MUST be ignored.

   The flag values that report things are:

   "exploded": this message (identified by its unique header hash value
   (recorded in the h= JSON object of the relevant Message-Instance) is
   being sent to more than one email address.  An MTA which receives a
   message MAY use this information to help it distinguish between
   malicious "DKIM replay" and legitimate activity performed by mailing
   list.  If this flag is not present in at least one DKIM2-Signature
   header field then an MTA MAY assume that only one copy of a
   particular message (identified by relevant cryptographic hash values)
   is intended to exist;

   The flags values that make requests are:

   "donotexplode": this Signer requests that the message not be sent to
   more than one recipient.  A system that, by local policy, ignores
   this request MUST NOT allow any of the copies it creates to be
   forwarded on to any MTA outside its control.

   "donotmodify": this Signer requests that the message not be modified
   from the form in which it is sent.  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] "," [FWS] sig-f-tag-data)
   sig-f-tag-data   = "donotmodify" | "donotexplode" | "feedback" |
                      "exploded" | x-sig-f-tag-data
   x-sig-f-tag-data = textstring ; for later extension





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8.  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.

8.1.  Add any Necessary Message-Instance Header Fields

   If a system is generating the initial form of a message or if it is a
   Reviser that has made changes to the message body and/or header
   fields then it MUST compute the body hash as described in Section 5.1
   and the hash of the header fields as described in Section 5.2.

   If the message does not contain a Message-Instance header field then
   one MUST be added.

   If hashing the message body or relevant header fields does not give
   the same hash values as those recorded in the highest version (m=)
   Message-Instance header field then a new Message-Instance header
   field MUST be added and if they are the same a new Message-Instance
   header field SHOULD NOT be added.

   A 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 null
   recipe 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 first (m=1) Message-Instance header field MAY contain
   "recipes" if it is wished to record any changes made to a message as
   it enters the DKIM2 ecosystem.  All other Message-Instance header
   fields SHOULD contain at least one "recipe".

8.2.  Provide a "Chain of Custody" for the Message

   The DKIM2-Signature header field contains the MAIL FROM and RCPT TO
   values that will be used when the message is transmitted, so these
   [RFC5321] "envelope" values MUST be available to (or deducible by) a
   Signer.

   The receiver of a message will check for an exact match (including
   the local parts of the email addresses) between the MAIL FROM / RCPT
   TO [RFC5321] protocol values and the mf= and rt= values in the
   highest numbered (most recent) DKIM2-Signature header field.  It is



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   acceptable for there to be more RCPT TO email addresses recorded in
   rt= than are actually used in the SMTP conversation, but any RCPT TO
   value which is used MUST be present.

   Verifiers will check for a relaxed domain match (see Section 8.3)
   between the signing domain (d=) and the domain in the MAIL FROM
   value.

   When the message being signed already has a DKIM2-Signature header
   field (i.e. it has already been transmitted at least once) then a
   valid "chain of custody" MUST be apparent when all of the
   DKIM2-Signature header fields are considered.  This "chain of
   custody" contributes to the way in which DKIM2 tackles "DKIM replay"
   attacks.

   In any situation where a message will be forwarded in such a way that
   the mf= on the outgoing message is such that the "chain of custody"
   would be broken then the Signer MUST 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 RCPT TO entry.  This is often achieved by the
   Signer creating the private key and never sharing it and then taking
   one of two approaches to publishing the public key.  The first is to
   provide the public key (and selector value) to the domain owner who
   creates an appropriate DNS entry.  The alternative is for the Signer
   to create a public key DNS entry within a part of the DNS that they
   control and the domain owner publishes a CNAME pointing at this.

   If an MTA does not change anything in the message which would require
   a new Message-Instance header field and it is going to send it
   onwards to a system that be able to verify the existing message (that
   is no changes are made to the MAIL FROM and RCPT TO values) and there
   is no other reason to add a DKIM2-Signature header field then the MTA
   MAY choose not to add one.  This means that an essentially
   transparent SMTP forwarding system need not be made "DKIM2 aware".

8.3.  The Relaxed Domain Match Algorithm

   To assist in addressing the "DKIM replay" problem DKIM2 provides a
   "chain of custody" for every message.  This is established by
   checking that the MAIL FROM value recorded in every DKIM2-Signature
   header field (except of course the i=1 instance) can be matched with
   a RCPT TO value of the next lower numbered DKIM2-Signature header
   field.



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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.

   To allow systems to use existing "bounce-handling" schemes with
   special subdomains in their MAIL FROM values a "relaxed" approach is
   taken to the matches between these values.

   *  Only the domain part of the MAIL FROM and RCPT TO values is used
      for these matches The local part (and the @) are ignored.

   *  If there is not an exact match between the domain names then
      labels are removed, one by one from the left hand side of the MAIL
      FROM domain name and the comparison is repeated.

   *  If no labels remain then there is no match.

8.4.  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.

8.5.  Calculate a Signature Value

   A Signer calculates a signature solely over the Message-Instance and
   DKIM2-Signature header fields of the message.  The hashes of the body
   and other header fields are covered by the hashes in the highest
   version (m=) Message-Instance header field and hence the signature
   will in practice be signing the message as a whole.

   Most cryptographic schemes proceed by first calculating a hash value
   and then signing the hash value, but the DKIM2-Signature header field
   only provides the final signature value.  This means that there is no
   difficulty if the hash value is inordinately long, or is not emitted
   by the cryptographic routine being used.

   The signature algorithm MUST apply the following steps in the order
   given (which are not quite the same as the steps undertaken in
   calculating header hashes).

   *  Convert all relevant header field names (not the header field
      values) to lowercase.  For example, convert "DKIM2-signature" to
      "dkim2-signature".






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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.

   *  Delete all WSP characters.  This means all WSP characters before
      and after the colon separating the header field name from the
      header field value, all WSP characters within the unfolded header
      field value and all trailing WPS characters before the CRLF.  The
      colon separator and the CRLF MUST be retained.

   *  Place the header fields in order.  First come the Message-Instance
      header fields in ascending instance (m=) order.  Second are the
      DKIM2-Signature header fields in ascending sequence (i=) order.
      Last of all is an incomplete DKIM2-Signature header field (the one
      that this system is creating) with all tags present except that
      the signature value(s) within the (s=) value are set to the null
      string ("").  The incomplete header field MUST be unfolded, MUST
      have a trailing CRLF and MUST have spaces removed in just the same
      way as the complete header fields being processed.

   *  The concatenated header fields are then fed to the signature
      algorithm(s).  Once all the values are available the null
      signature value strings are replaced by the base64 values of the
      signatures.

9.  Verification Requirements

   The details of verification appear in Section 10 below.  This section
   considers when verification should be performed and how thorough it
   needs to be.

9.1.  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 (m=) Message-Instance before accepting an email.

   If these checks do not pass then a Delivery Status Notification (DSN)
   for the email MUST NOT be generated thereafter -- hence the best
   strategy, if the email is not wanted, is to reject it (with a 5xx
   error code) whilst the relevant SMTP conversation is still ongoing.
   If the check gives a TEMPFAIL result then a 4xx error code SHOULD be
   used to allow the sending MTA to understand the situation.






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   If the checks do pass and it is later determined that the email is
   unacceptable for any reason then a DSN MAY be created and passed to
   the system that delivered the email.  The details of this procedure
   appear in Section 11.

9.2.  Checking the Message-Instance Header Fields

   If the message has been modified since its original creation then the
   Message-Instance header fields will enable a Verifier to determine
   whether or not all the changes made are correctly recorded by using
   the "recipes" to construct each preceding version of the message.

   Note that if it is only the first form of the message is of interest
   then all the "recipes" can be applied in turn and only one hash value
   checked -- the correctness of the intermediate hash values are not
   relevant to this assessment.

9.3.  Checking the DKIM2-Signature Header Fields

   However, in order to check the chain of custody, to assess whether
   the message has been exploded, to pick out "feedback" requests to be
   honoured or to assign reputation to Revisers then all of the
   DKIM2-Signature header fields will have to checked for validity.  The
   TBA document explores these issues in more detail.

9.4.  Interpret Results/Apply Local Policy

   It is beyond the scope of this specification to describe what actions
   the recipient of an email performs, but mail carrying valid DKIM2
   signatures gives the recipient opportunities that unauthenticated
   email would not.  Specifically, an authenticated email provides
   predictable information by which other decisions can reliably be
   managed, such as trust and reputation.  Conversely, it is hard to
   assign trust or reputation to unauthenticated email.

   If an MTA wishes to reject messages where signatures are missing or
   do not verify, the handling MTA SHOULD use a 550/5.7.x reply code.

   Where the Verifier is integrated within the MTA and it is not
   possible to fetch the public key, perhaps because the key server is
   not available, a temporary failure message MAY be generated using a
   451/4.7.5 reply code.

   Temporary failures such as inability to access the key server or
   other external service are the only conditions that SHOULD use a 4xx
   SMTP reply code.  In particular, cryptographic signature verification
   failures MUST NOT provoke 4xx SMTP replies.




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10.  Verifier Actions

   This section discusses the detail of the actions taken by a Verifier.
   In essence this will involve repeating all the actions taken by a
   Signer to produce a Message-Instance or DKIM2-Signature header field.
   To avoid a lot of repetition these actions will not be spelled out in
   detail.  Once a hash value has been calculated it is then compared
   with the value reported by the Signer, or the Signer's public key is
   used to determine whether a signature that has been provided is
   correct.

   When a Verifier is determining whether a particular DKIM2-Signature
   header field it MUST consider the state of the message when that
   header field was added to the message.  That means it MUST first
   apply all relevant recipes to reconstruct the body and header fields
   and it MUST ignore any Message-Instance and DKIM2-Signature fields
   that were added after that point.

10.1.  Output States

   For compatibility with the Authentication-Results header field
   defined in [RFC8601] a verification will result in one of four
   states:

   PASS: The message was successfully verified.

   FAIL: The message could be verified but a hash or signature was not
   correct.

   PERMERROR: The message could not be verified due to some error that
   is unrecoverable, such as a required header field being absent or
   malformed.

   TEMPERROR: The message could not be verified due a temporary
   inability to retrieve a public key.  A later attempt may produce a
   different.

   A Verifier MAY cease verifying once a single failure is detected.













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   Verifiers wishing to communicate the results of verification to other
   parts of the mail system may do so in whatever manner they see fit.
   If they wish to provide a human-readable string to describe a failure
   to verify (any state except PASS) then in order to provide the
   maximum possible assistance to senders they SHOULD use the text
   strings specified in this document.  These human-readable messages
   are described with m=<x> or tag=<y> placeholders, the <x> and <y>
   MUST be replaced with the relevant ordinal or tag name (without the <
   and > characters).  Similarly <value> MUST be replaced by a relevant
   string for the particular message.

   If the verification is being performed during an SMTP protocol
   conversation the human-readable string SHOULD be part of the 5xx or
   4xx response string.

   If the results of the verification are being communicated in a
   Delivery Status Notification message ([RFC3461]) the human-readable
   string should be included.

   If, by local policy, a system wishes to accept a message which has
   failed authentication it might choose to add an email header field to
   the message before passing it on.  Any such header field SHOULD
   include the human-readable string and SHOULD be inserted before any
   existing DKIM2-Signature or pre-existing authentication status header
   fields in the header field block.  The Authentication-Results: header
   field ([RFC8601]) MAY be used for this purpose.  It should be noted
   that any "Authentication-Results" header field will count as a
   modification to the email if any further DKIM2-Signature header
   fields are to be generated.

10.2.  Ensure that the DKIM2 Header Fields are Valid

   Verifiers MUST meticulously validate the format and values of all
   relevant Message-Instance and DKIM2-Signature header fields.  It MUST
   also ensure that all required instances of these header fields are
   present and that all required tags are present.  Recall however that
   unknown tags MUST be ignored.

   As a special case, there MUST NOT be a Message-Instance field with a
   higher m= value than occurs in any DKIM2-Signature field.

   Possible errors:









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   PERMERROR Message-Instance m=<x> missing
   PERMERROR Message-Instance m=<x> syntax error
   PERMERROR Message-Instance m=<x> tag=<y> missing
   PERMERROR Message-Instance m=<x> is not signed
   PERMERROR DKIM2-Signature i=<x> missing
   PERMERROR DKIM2-Signature i=<x> syntax error
   PERMERROR DKIM2-Signature i=<x> tag=<y> missing

10.3.  Check the timestamps

   Verifiers SHOULD return a failure it is more than 14 days since the
   timestamp recorded in the "t=" tag of any DKIM2-Signature header
   field.

   Possible errors:

   PERMERROR DKIM2-Signature i=<x> signature expired

10.4.  Check the Chain-of-Custody

   As explained in Section 8.2 a Verifier MUST check an exact match
   between the MAIL FROM and RCPT TO parameters used when delivering a
   message and the values found in the mf= and rt= tags of the highest
   numbered DKIM2-Signature header field.  There may be extra values in
   the rt= value, but all RCPT TO values actually used for delivery MUST
   be present.

   The values of domains MUST BE put into lower-case before doing these
   checks.  As is usual in email protocols the case of the local part of
   an email address is assumed to matter.  Note that these checks MUST
   NOT use the relaxed domain match algorithm.

   A Verifier SHOULD check that there is a relaxed domain match (see
   {relaxed-domain-match}) between the signing domain of the most
   recently applied DKIM2-Signature header field and the mf= value in
   that header field.

   Possible errors:

   PERMERROR: MAIL FROM <value> did not match
   PERMERROR: RCPT TO <value> did not match
   PERMERROR: MAIL FROM and d= do not match









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10.5.  Fetch the Public Key

   The public keys of all the signatures in DKIM2-Signature fields are
   needed to complete the verification process.  Details of key
   management and representation are described in Section 3.6 and
   [DKIMKEYS].  The Verifier MUST validate the key record and MUST NOT
   use any public key records that are malformed.

   Note that DNS timeouts MUST be reported as TEMPERROR but a DNS result
   that indicates the key is absent MUST be reported as a PERMERROR.
   Additionally, as [DKIMKEYS] makes clear, if more than one record is
   returned this is an error.  The human-readable error message SHOULD
   provide the selector value so that it is clear which key has caused a
   problem.

   Note that [DKIMKEYS] has retired the h= field and DKIM2
   implementations MUST ignore this tag if it is present.

   Possible errors:

TEMPERROR: DKIM2-Signature i=<x> public key <value> could not be fetched
PERMERROR: DKIM2-Signature i=<x> public key <value> does not exist
PERMERROR: DKIM2-Signature i=<x> public key <value> has multiple records
PERMERROR: DKIM2-Signature i=<x> public key <value> has a syntax error
PERMERROR: DKIM2-Signature i=<x> public key <value> algorithm mismatch
PERMERROR: DKIM2-Signature i=<x> public key <value> has been revoked

10.6.  Perform the Signature Verification Calculation

   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 8.5.  The
       signature value(s) themselves will need to be removed to
       correspond with what was actually signed.  Note that this
       canonicalized version does not actually replace the original
       content.

   2.  Use the relevant public key value(s) to check the signature(s).

   3.  If there is more than one signature provided then they MUST all
       be checked if the Verifier is able to do so.  If any signature
       fails then an error SHOULD be reported.  If all signatures that
       can be checked fail then PERMFAIL MUST be reported.






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   4.  If some signatures fail and other pass then any error that is
       reported should provide that information (e.g. PERMFAIL "rsa-
       sha256 signature passed, ed25519-sha256 signature failed").

   The reasoning for requiring that all signatures pass is that if a
   signature scheme has recently become deprecated because it is known
   to be cryptographically flawed then Signers will use a second
   (unbroken) signature scheme.  However, such a Signer may still
   provide the other signature for the benefit of Verifiers that have
   yet to upgrade -- reasoning perhaps that attacks are too expensive to
   be a very significant security issue.  A Verifier that determines
   that one signature passes whilst the other fails may well be in a
   position to prevent an attack.

   Possible errors:

   FAIL: DKIM2-Signature i=<x> public key <value> incorrect signature

10.7.  Validating Body and Header hashes

   Verifying a hash value requires a Verifier to repeat the hash
   calculation performed by the Signer as set out in Section 5.1 and
   Section 5.1.  The values can then be directly compared.

   Since there may be more than one hash algorithm given the human-
   readable error message SHOULD indicate which algorithm's result
   failed to match.

   Possible errors:

   FAIL: Message Instance m=<x> header hash <value> mismatch
   FAIL: Message Instance m=<x> body hash <value> mismatch

11.  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 DSN
   MUST NOT be sent.





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11.1.  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 containing a {"z": true} step.

11.1.1.  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.

11.1.2.  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:







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   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.

   2.  The last (highest i= tag) DKIM2-Signature header field of the
       returned message will be one that was generated by the system
       receiving the bounce notification, determined by examining the d=
       and mf= tags of that DKIM2-Signature header field.

   3.  The header fields of the embedded message (in the message/rfc822
       MIME part) can be verified.  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.

12.  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.

13.  EAI ([RFC6530]) Considerations for DKIM2

   TBA

14.  IANA Considerations

   TBA

15.  Security Considerations

   TBA

16.  Changes from Earlier Versions

   draft-ietf-dkim-dkim2-spec-01

   Additions to terninology.  Improved ABNF.  Removed definition of tag-
   list and placed relevant text in the two header field definitions.
   Untangled he description of what needs to be verified from the
   description of how to verify and provided a list of human-readable
   strings to generate for errors.

   draft-ietf-dkim-dkim2-spec-00

   Removed JSON for hashes, signatures and SMTP parameters.  Provided
   valid JSON for recipes and added "z" for truncated body.  Changed
   algorithm names for signing.  Simplified the canonicalisation
   performed for the header fields signed by DKIM2-Signature.  Changed
   v= to m= for message instance numbering.

   General tidying up of specifying tag=value specifications and
   associated ABNF.  Various other fixes for issues flagged in WG.

   [[This section to be removed by RFC Editor]]




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17.  References

17.1.  Normative References

   [DKIMKEYS] Chuang, W., "Domain Name Specification for DKIM2", Work in
              Progress, Internet-Draft, draft-chuang-dkim2-dns-04, 18
              March 2026, <https://datatracker.ietf.org/doc/html/draft-
              chuang-dkim2-dns-04>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/rfc/rfc1034>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2045>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/rfc/rfc2047>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [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>.

   [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>.






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   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [RFC8601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 8601,
              DOI 10.17487/RFC8601, May 2019,
              <https://www.rfc-editor.org/rfc/rfc8601>.

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>.

   [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>.







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   [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
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