DomainKeys Identified Mail Signatures v2 (DKIM2)

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.¶

2.1. 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.¶

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):¶

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.¶

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:¶

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.¶

4.1. 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.¶

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].¶

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.¶

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.¶

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.¶

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.¶

• 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.¶

10.1. 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.¶

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.¶

10.3. 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.¶

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.¶

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>.

[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>.

[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>.