]> Yahoo

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Google

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Fastmail Pty Ltd

Level 2, 114 William Street 3000 Australia +61 457 416 436 brong@fastmailteam.com

Internet-Draft 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 applying a cryptographic signature to 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 further signatures will be added to provide a validatable "chain". This permits validators to identify when messages have been unexpectedly "replayed" and can ensure that delivery status notifications are only sent to entities that were involved in the transmission of a message.

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 with the message . 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 it was considered to be spam) that the eventual recipient of the message wishes to share.

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.

Terminology and Definitions This section defines terms used in the rest of the document. DKIM2 is designed to operate within the Internet Mail service, as defined in . Basic email terminology is taken from that specification. DKIM2 inherits many ideas from DKIM () which, for clarity we refer to in this specification as DKIM1. Syntax descriptions use Augmented BNF (ABNF) . 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 . These words take their normative meanings only when they are presented in ALL UPPERCASE.

Forwarder 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. As will be seen, Forwarders may alter message content or envelopes but will create a signed record of what they have done.

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.

Verifiers Elements in the mail system that verify signatures are referred to as Verifiers. These may be Forwarders, 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.

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.

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 ). 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):

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

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 : "local-part" (implementation warning: this permits quoted strings) "sub-domain" The following tokens are imported from : "field-name" (name of a header field) Other tokens not defined herein are imported from . These are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF, etc.

Common ABNF Tokens The following ABNF tokens are used elsewhere in this document:

Protocol Elements Protocol Elements are conceptual parts of the protocol that are not specific to either Signers or Verifiers. The protocol descriptions for Signers and Verifiers are described in later sections ("Signer Actions" () and "Verifier Actions" (). NOTE: This section must be read in the context of those sections.

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:

The number of public keys and corresponding selectors for each domain is determined by the domain owner. Many domain owners will be satisfied with just one selector, whereas administratively distributed organizations can choose to manage disparate selectors and key pairs in different regions or on different email servers. Beyond administrative convenience, selectors make it possible to seamlessly replace public keys on a routine basis. If a domain wishes to change from using a public key associated with selector "january2026" to a public key associated with selector "february2026", it merely makes sure that both public keys are advertised in the public key repository concurrently for the transition period during which email may be in transit prior to verification. At the start of the transition period, the outbound email servers are configured to sign with the "february2026" private key. At the end of the transition period, the "january2026" public key is removed from the public key repository. INFORMATIVE NOTE: A key may also be revoked as described below. The distinction between revoking and removing a key selector record is subtle. When phasing out keys as described above, a signing domain would probably simply remove the key record after the transition period. However, a signing domain could elect to revoke the key (but maintain the key record) for a further period. There is no defined semantic difference between a revoked key and a removed key.

Tag=Value Lists DKIM2 uses a simple "tag=value" syntax in several contexts, including in messages and domain signature records (see ). Values are a series of strings containing either plain text or "base64" text (as defined in , 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. INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined below (as "tag-value") only includes 7-bit characters, an implementation that wished to anticipate future standards would be advised not to preclude the use of UTF-8-encoded () text in tag=value lists. Formally, the ABNF syntax rules are as follows:

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.

Signing and Verification Algorithms DKIM2 supports multiple digital signature algorithms. Two algorithms are defined by this specification: RSA-SHA256 and Ed25519-SHA256. Signers SHOULD implement both RSA-SHA256 and Ed25519-SHA256. Verifiers MUST implement both RSA-SHA256 and Ed25519-SHA256.

The RSA-SHA256 Signing Algorithm The RSA-SHA256 Signing Algorithm computes a message hash as described in 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 ) 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 SHOULD use a public exponent of 65537. Signers MUST use RSA keys of at least 1024 bits. Verifiers MUST be able to validate signatures with keys ranging from 1024 bits to 2048 bits, and they MAY be able to validate signatures with larger keys.

The Ed25519-SHA256 Signing Algorithm The Ed25519-SHA256 signing algorithm computes a message hash as defined in Section 3 of 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 .

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

Canonicalization Some mail systems modify email in transit, potentially invalidating a signature. DKIM2 provides a method of documenting such changes as they occur. However, in order to reduce the necessity to document minor changes to header fields DKIM2 uses an algorithm for computing header hashes which permits minor changes. This is identical to the "relaxed" algorithm of DKIM1 (). For the body no modifications are permitted (equivalent to the DKIM1 "relaxed" algorithm. Canonicalization simply prepares the email for presentation to the signing or verification algorithm. It MUST NOT change the transmitted data in any way. Canonicalization of header fields and body are described below. NOTE: This section assumes that the message is already in "network normal" format (text is ASCII encoded, lines are separated with CRLF characters, etc.). See also for information about normalizing the message.

The Header Canonicalization Algorithm All header fields present in a message are signed, with the exception of Trace Header Fields. "Trace headers" are described in and and documented in an IANA registry established by . At present the only header fields designated as "Trace" are Received and Return-path. The header canonicalization algorithm MUST apply the following steps in order: Ignore any trace header fields that are present. Convert all header field names (not the header field values) to lowercase. For example, convert "SUBJect: AbC" to "subject: AbC". Unfold all header field continuation lines as described in ; 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, except for any header fields that start "DKIM2" which are placed last. If there is more than one header with the same header field name then the header fields are placed in the order in which they occur in the email header. It is sometimes suggested that some MTAs re-order header fields after they receive an email, if they do then it is their responsibility to recover the original order of any header fields with identical header field names (that are part of a signature calculation) before verifying an existing signature or passing a previously signed message to another MTA that is going to wish to verify a signature. The DKIM2 header fields are placed at the end of the list of header fields to be signed, ordered first by their "i=" value (in ascending numerical order) and then by their header field name. Note that the special rules for ordering DKIM2 header fields are intended to assist systems that wish to pre-calculate a hash value for all the other header fields and then finish off with the actual DKIM2 headers that they will be adding.

The Body Canonicalization Algorithm The body canonicalization algorithm MUST apply the following steps in order: Ignore all whitespace at the end of lines. Implementations MUST NOT remove the CRLF at the end of the line. Reduce all sequences of WSP within a line to a single SP character. Ignore all empty lines at the end of the message body. 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. Note that a completely empty or missing body is canonicalized as a single "CRLF"; that is, the canonicalized length will be 2 octets.

The DKIM2-Signature Header Field The signature of the email is stored in the DKIM2-Signature header field. This header field contains all of the signature and key- fetching data. The DKIM2-Signature value is a tag-list as described in . The DKIM2-Signature header field SHOULD be treated as though it were a trace header field as defined in Section 3.6 of and hence SHOULD NOT be reordered and SHOULD be prepended to the message. The DKIM2-Signature header field being created or verified is always included in the signature calculation, after the rest of the header fields being signed; however, when calculating or verifying the signature, the value of the "b=" tag (signature value) of that DKIM- Signature header field MUST be treated as though it were an empty string. Unknown tags in the DKIM2-Signature header field MUST be included in the signature calculation. Tags on the DKIM2-Signature header field along with their type, encoding and requirement status are shown below. It will be noted that we have not included a version number. Experience from IMF onwards shows that it is essentially impossible to change version numbers. If it becomes necessary to change DKIM2 in the sort of incompatible way that a v=2 / v=3 version number would support, it is expected that header fields will be labelled as DKIM3 instead. i= The sequence number of the DKIM2-Signature header field.

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:

n= Nonce value

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

t= Signature Timestamp

The time that this header field was created. The format is the number of seconds since 1970-01-01T00:00:00 UTC. 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:

mf= The MAIL FROM value to be used when this message is transmitted over an SMTP link from the signing MTA.

Note that MAIL FROM may be just "<>", for example for a Delivery Status Notification. ABNF:

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rt= The RCPT TO value(s) to be used when this message is transmitted over an SMTP link from the signing MTA.

When a message is intended for more than one recipient then this field MAY include all of the recipients so that a single copy of the email MAY be sent to all of the recipients in a single SMTP transaction. Note that if "bcc:" recipients are involved then in order to meet the requirements of Section 3.6.3 each bcc recipient MUST be sent a message with just their email address appearing in this tag. ABNF:

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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. Internationalized domain names MUST be encoded as A-labels, as described in Section 2.3 of . The domain in the d= tag MUST exactly match the rightmost labels of the domain-name part of the mf= tag. That is to say, the domain-name of the mf= tag MUST either match the d= domain exactly or be a sub-domain of d= domain. ABNF:

s1= The selector subdividing the namespace for the "d=" (domain) tag.

Internationalized selector names MUST be encoded as A-labels, as described in Section 2.3 of . ABNF:

a1= The algorithm used to generate the signature.

Verifiers MUST support "rsa-sha256" and "ed25519"; See for a description of the algorithms. ABNF:

b1= The signature data.

Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length limits. See "Signer Actions" () for how the signature is computed. ABNF:

bh1= The hash of the canonicalized body part of the message.

Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length limits. See for how the body hash is computed. ABNF:

s2=, a2= b2= bh2= Further signatures (equivalent to s1, a1, b2 and bh1)

To provide for algorithmic dexterity a second signature, using a different algorithm MAY be supplied. A verifier MUST check all signatures that it understands and SHOULD treat any failure as invalidating all signatures. 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 b1= and perhaps b2=)) is being sent to more than one email address. An MTA which receives a message MAY use this information to help it distinguish between malicious "DKIM replay" and legitimate activity performed by mailing lists. "modifiedbody": the body of the message has been altered in some way. A corresponding DKIM2-Delta-Body MUST be present with the same i= value as defined in . This flag MUST NOT be present when i=1; "modifiedheader": the header of the message has been altered in some way. A corresponding DKIM2-Delta-Header must be present with the same i= value as defined in . This flag MUST NOT be present when i=1; 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:

Key Management Signature applications require some level of assurance that the 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. These keys are no different, and are stored in the same locations as those for DKIM1 (). All DKIM 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".

Computing the Message Hashes Both signing and verifying message signatures start with a step of computing two cryptographic hashes over the message. Signers will choose the parameters of the signature as described in "Signer Actions" (); Verifiers will use the parameters specified in the DKIM2-Signature header field being verified. In the following discussion, the names of the tags in the DKIM2-Signature header field that either exists (when verifying) or will be created (when signing) are used. Note that canonicalization () is only used to prepare the email for signing or verifying; it does not affect the transmitted email in any way. For clarity this section will discuss the first pair of signatures (s1=, a1=, h1=, bh1). If a second pair (s2=, a2=, h2=, bh2=) is present then that is calculated or verified in an identical manner except using the "2" values rather than the "1" values. The Signer/Verifier MUST compute two hashes: one over the body of the message and one over relevant header fields of the message. Signers MUST compute them in the order shown. Verifiers MAY compute them in any order convenient to the Verifier, provided that the result is semantically identical to the semantics that would be the case had they been computed in this order. In hash step 1, the Signer/Verifier MUST hash the message body, canonicalized using the body canonicalization algorithm. That hash value is then converted to base64 form and inserted into (Signers) or compared to (Verifiers) the "bh1=" tag of the DKIM- Signature header field. In hash step 2, the Signer/Verifier MUST pass header fields to the hash algorithm as specified in . After these header fields add any DKIM2-Delta-Header header field (as specified in for the current i= value. This header field has its CRLF included in the hash in the normal way. Finally add a canonicalized copy of the DKIM2-Signature header field that exists (verifying) or will be inserted (signing) in the message, with the value of the "b=" tag (including all surrounding whitespace) deleted (i.e., treated as the empty string) without a trailing CRLF. 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 ). With the exception of the canonicalization procedure described in , the DKIM2 signing process treats the body of messages as simply 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.

Input Requirements A message that is not compliant with , , and can be subject to attempts by intermediaries to correct or interpret such content. See Section 8 of 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. Accordingly, DKIM2's design is predicated on valid input. Therefore, Signers and Verifiers SHOULD take reasonable steps to ensure that the messages they are processing are valid according to , , and any other relevant message format standards.

Output Requirements The evaluation of each signature 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 TEMPFAIL: a temporary, recoverable error such as a DNS query timeout For each signature that verifies successfully or produces a TEMPFAIL result, output of the DKIM2 algorithm MUST include the set of: The domain name, taken from the "d=" signature tag; and The result of the verification attempt for that signature. The output MAY include other signature properties or result meta- data, including PERMFAILed or otherwise ignored signatures, for use by modules that consume those results. See for discussion of signature validation result codes.

Semantics of Multiple Signatures In DKIM2 a new signature (with an incremented i= value) is added by every MTA that handles a message. However, if these MTAs are handling email within an organization then it is only necessary for the MTA that sends the email outside of that organization to apply a DKIM2 signature. Verifiers may check the entire chain of signatures to establish that there is a valid chain from start to finish; however where the chain shows that the email has not been modified during its travels it will generally be sufficient to check only the first and last signature. When there is evidence of "DKIM replay" inspection of the entire chain will enable the location of the replay to be determined and appropriate action can then be taken. Further information about multiple signatures can be found in TBA.

Signer Actions The following steps are performed in order by Signers.

Document modifications made to a message MTAs that accept a DKIM2 signed message and send it onward to another MTA MUST record the changes that they have made to the message. A scheme for generating appropriate DKIM2-Delta-Header header fields to describe such modifications can be found in . Note in particular that adding header fields must be documented, with the exception of trace header fields (such as Received:) that are not signed. Failure to record modifications will mean that an MTA that subsequently handles the message SHOULD detect this and this MAY lead to the message being rejected.

Determine what "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 or Verifier. 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. If this will not be the case then a Signer MUST generate an extra DKIM2-Signature field that causes values to match, noting that the creation of this extra header field will require the Signer to have access to the DKIM2 key value associated with a domain in the rt= entry.

Select a Private Key and Corresponding Selector Information This specification does not define the basis by which a Signer should choose which private key and selector information to use. Currently, all selectors are equal as far as this specification is concerned, so the decision should largely be a matter of administrative convenience. Distribution and management of private keys is also outside the scope of this document.

Normalize the Message to Prevent Transport Conversions Some messages, particularly those using 8-bit characters, are subject to modification during transit, notably conversion to 7-bit form. Such conversions will break DKIM2 signatures. Hence the message SHOULD be converted to only contain 7-bit characters, for example by employing a suitable MIME content-transfer encoding such as quoted-printable or base64 as described in . The way in which this is achieved is outside the scope of this specification. If the message contains local encoding that will be modified before transmission, that modification to canonical 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.

Compute the Message Hash and Signature{#signer-compute} The Signer MUST compute the message hash as described in and then sign it using the selected public key algorithm. This will result in a DKIM2-Signature header field that will include the body hash and a signature of the header hash, where that header includes the DKIM2-Signature header field itself. Entities such as mailing list managers that implement DKIM2 and that modify the message or a header field (for example, inserting unsubscribe information) before retransmitting the message SHOULD check any existing signature on input and MUST make such modifications before re-signing the message.

Insert the DKIM2-Signature Header Field Finally, the Signer MUST insert the DKIM2-Signature header field created in the previous step prior to transmitting the email. The DKIM2-Signature header field MUST be the same as used to compute the hash as described above, except that the value of the "b=" tag MUST be the appropriately signed hash computed in the previous step, signed using the algorithm specified in the "a=" tag of the DKIM2- Signature header field and using the private key corresponding to the selector given in the "s=" tag of the DKIM2-Signature header field, as chosen above in }. The DKIM2-Signature header field MUST be inserted before any other DKIM2-Signature fields in the header block. INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this is to insert the DKIM2-Signature header field at the beginning of the header block before any existing Received header fields. This is consistent with treating DKIM2-Signature as a trace header field.

Verifier Actions A border or intermediate MTA MAY verify the message signature(s). An MTA that has performed verification MAY communicate the result of that verification by adding a verification header field to incoming messages. This simplifies things considerably for the user, who can now use an existing mail user agent. Most MUAs have the ability to filter messages based on message header fields or content; these filters would be used to implement whatever policy the user wishes with respect to unsigned mail. A verifying MTA MAY implement a policy with respect to unverifiable mail, regardless of whether or not it applies the verification header field to signed messages. Verifiers MUST produce a result that is semantically equivalent to applying the steps listed in , , and in order. In practice, several of these steps can be performed in parallel in order to improve performance.

Extract Signatures from the Message A Verifier SHOULD check the most recently applied (highest numbered i= value) DKIM2-Signature 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 till ongoing. A Verifier that wishes to ascertain whether or not a message has been modified since it was first created need only check the first (i=1) DKIM2-Signature. If the message has been modified since it's original creation then the DKIM2-Modification header fields (see ) will enable a Verifier to determine whether or not all the changes made are correctly recorded. For each signature to be validated, the following steps should be performed in such a manner as to produce a result that is semantically equivalent to performing them in the indicated order. Where the status is TEMPFAIL then it may be possible for the Verifier to determine the validity of the signature at a later time. If the SMTP conversation is still ongoing then a 4xx error code SHOULD be used to allow the sending MTA to understand the situation.

Validate the Signature Header Field Implementers MUST meticulously validate the format and values in the DKIM2-Signature header field; any inconsistency or unexpected values MUST cause the header field to be completely ignored and the Verifier to return PERMFAIL (signature syntax error). Being "liberal in what you accept" is definitely a bad strategy in this security context. Note, however, that this does not include the existence of unknown tags in a DKIM2-Signature header field, which are explicitly permitted. Verifiers MAY return PERMFAIL (signature expired) if it is more than 14 days since the timestamp recorded in the "t=" tag.

Get the Public Key The public key for a signature is needed to complete the verification process. Details of key management and representation are described in . The Verifier MUST validate the key record and MUST ignore any public key records that are malformed. NOTE: The use of a wildcard TXT RR that covers a queried DKIM domain name will produce a response to a DKIM query that is unlikely to be a valid DKIM key record. This problem is not specific to DKIM and applies to many other types of queries. Client software that processes DNS responses needs to take this problem into account. When validating a message, a Verifier MUST perform the following steps in a manner that is semantically the same as performing them in the order indicated; in some cases, the implementation may parallelize or reorder these steps, as long as the semantics remain unchanged: The Verifier retrieves the public key as described in using the algorithm in the "a=" tag, the domain from the "d=" tag, and the selector from the "s=" tag. If the query for the public key fails to respond, the Verifier MAY seek a later verification attempt by returning TEMPFAIL (key unavailable). If the query for the public key fails because the corresponding key record does not exist, the Verifier MUST return PERMFAIL (no key for signature). If the query for the public key returns multiple key records, the Verifier can choose one of the key records or may cycle through the key records, performing the remainder of these steps on each record at the discretion of the implementer. The order of the key records is unspecified. If the Verifier chooses to cycle through the key records, then the "return ..." wording in the remainder of this section means "try the next key record, if any; if none, return to try another signature in the usual way". If the result returned from the query does not adhere to the format defined in the DKIM key specification , the Verifier MUST ignore the key record and return PERMFAIL (key syntax error). Verifiers are urged to validate the syntax of key records carefully to avoid attempted attacks. In particular, the Verifier MUST ignore keys with a version code ("v=" tag) that they do not implement. If the public key data (the "p=" tag) is empty, then this key has been revoked and the Verifier MUST treat this as a failed signature check and return PERMFAIL (key revoked). There is no defined semantic difference between a key that has been revoked and a key record that has been removed. If the public key data is not suitable for use with the algorithm and key types defined by the "a=" and "k=" tags in the DKIM- Signature header field, the Verifier MAY immediately return PERMFAIL (inappropriate key algorithm). However, these fields are now deprecated so DKIM2 implementations MAY ignore them altogether. If the "h=" tag exists in the public key record and the hash algorithm implied by the "a=" tag in the DKIM2-Signature header field is not included in the contents of the "h=" tag, the Verifier MUST ignore the key record and return PERMFAIL (inappropriate hash algorithm).

Compute the Verification Given a Signer and a public key, verifying a signature consists of actions semantically equivalent to the following steps. Prepare a canonicalized version of the message as is described in (note that this canonicalized version does not actually replace the original content). Based on the algorithm indicated in the "a=" tag, compute the message hashes from the canonical copy as described in . Verify that the hash of the canonicalized message body computed in the previous step matches the hash value conveyed in the "bh=" tag. If the hash does not match, the Verifier SHOULD ignore the signature and return PERMFAIL (body hash did not verify). Using the signature conveyed in the "b=" tag, verify the signature against the header hash using the mechanism appropriate for the public key algorithm described in the "a=" tag. If the signature does not validate, the Verifier SHOULD ignore the signature and return PERMFAIL (signature did not verify). Otherwise, the signature has correctly verified.

Communicate Verification Results 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 () MAY be used for this purpose.

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. In general, modules that consume DKIM2 verification results SHOULD NOT determine message acceptability based solely on a lack of any signature or on an unverifiable signature; such rejection would cause severe interoperability problems. If an MTA does wish to reject such messages during an SMTP session (for example, when communicating with a peer who, by prior agreement, agrees to only send signed messages), and a signature is missing or does 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. If the email cannot be verified, then it SHOULD be treated the same as all unverified email, regardless of whether or not it looks like it was signed.

IANA Considerations TBA

Security Considerations TBA

This RFC is the revised basic definition of The Domain Name System. It obsoletes RFC-882. This memo describes the domain style names and their used for host address look up and electronic mail forwarding. It discusses the clients and servers in the domain name system and the protocol used between them. This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] This particular document is the third document in the series. It describes extensions to RFC 822 to allow non-US-ASCII text data in Internet mail header fields. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. Although SMTP was designed as a mail transport and delivery protocol, this specification also contains information that is important to its use as a "mail submission" protocol for "split-UA" (User Agent) mail reading systems and mobile environments. [STANDARDS-TRACK] This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK] This document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS-TRACK] DomainKeys Identified Mail (DKIM) permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message. This can be an author's organization, an operational relay, or one of their agents. DKIM separates the question of the identity of the Signer of the message from the purported author of the message. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. This memo obsoletes RFC 4871 and RFC 5672. [STANDARDS-TRACK] This memo splits message submission from message relay, allowing each service to operate according to its own rules (for security, policy, etc.), and specifies what actions are to be taken by a submission server. Message relay is unaffected, and continues to use SMTP over port 25. When conforming to this document, message submission uses the protocol specified here, normally over port 587. This separation of function offers a number of benefits, including the ability to apply specific security or policy requirements. [STANDARDS-TRACK] This document specifies a message header field called "Authentication-Results" for use with electronic mail messages to indicate the results of message authentication efforts. Any receiver-side software, such as mail filters or Mail User Agents (MUAs), can use this header field to relay that information in a convenient and meaningful way to users or to make sorting and filtering decisions. This document obsoletes RFC 7601. Fastmail Pty Ltd This memo describes a method for describing the changes made to an email during common email modifications, for example those caused by mailing lists and forwarders. While this is general enough to be used for any changes, it is anticipated that this method will normally be used for removing added data rather than large complex changes. Google The updated DomainKeys Identified Mail (DKIM2) permits an organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message through a digital signature. This is done by publishing to Domain Name Service (DNS) of the domain a public key that is then associated to the domain and where messages can be signed by the corresponding private key. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer’s domain directly to retrieve the appropriate public key. This document describes DKIM2 DNS record format and how to find the record. Over its thirty-five-year history, Internet Mail has changed significantly in scale and complexity, as it has become a global infrastructure service. These changes have been evolutionary, rather than revolutionary, reflecting a strong desire to preserve both its installed base and its usefulness. To collaborate productively on this large and complex system, all participants need to work from a common view of it and use a common language to describe its components and the interactions among them. But the many differences in perspective currently make it difficult to know exactly what another participant means. To serve as the necessary common frame of reference, this document describes the enhanced Internet Mail architecture, reflecting the current service. This memo provides information for the Internet community. This document provides recommendations for the implementation of public-key cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes. This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 3447. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided.

Changes from Earlier Versions [[This section to be removed by RFC Editor]]