4. Semantics of Multiple Signatures 4.1. Example Scenarios There are many reasons why a message might have multiple signatures. For example, suppose SHA-256 is in the future found to be insufficiently strong, and DKIM usage transitions to SHA-1024. A Signer might immediately sign using the newer algorithm but also continue to sign using the older algorithm for interoperability with Verifiers that had not yet upgraded. The Signer would do this by adding two DKIM-Signature header fields, one using each algorithm. Older Verifiers that did not recognize SHA-1024 as an acceptable algorithm would skip that signature and use the older algorithm; newer Verifiers could use either signature at their option and, all other things being equal, might not even attempt to verify the other signature. Similarly, a Signer might sign a message including all header fields and no "l=" tag (to satisfy strict Verifiers) and a second time with a limited set of header fields and an "l=" tag (in anticipation of possible message modifications en route to other Verifiers). Verifiers could then choose which signature they prefer. Of course, a message might also have multiple signatures because it passed through multiple Signers. A common case is expected to be that of a signed message that passes through a mailing list that also
signs all messages. Assuming both of those signatures verify, a recipient might choose to accept the message if either of those signatures were known to come from trusted sources. In particular, recipients might choose to whitelist mailing lists to which they have subscribed and that have acceptable anti-abuse policies so as to accept messages sent to that list even from unknown authors. They might also subscribe to less trusted mailing lists (e.g., those without anti-abuse protection) and be willing to accept all messages from specific authors but insist on doing additional abuse scanning for other messages. Another related example of multiple Signers might be forwarding services, such as those commonly associated with academic alumni sites. For example, a recipient might have an address at members.example.org, a site that has anti-abuse protection that is somewhat less effective than the recipient would prefer. Such a recipient might have specific authors whose messages would be trusted absolutely, but messages from unknown authors that had passed the forwarder's scrutiny would have only medium trust. 4.2. Interpretation A Signer that is adding a signature to a message merely creates a new DKIM-Signature header, using the usual semantics of the "h=" option. A Signer MAY sign previously existing DKIM-Signature header fields using the method described in Section 5.4 to sign trace header fields. Note that Signers should be cognizant that signing DKIM-Signature header fields may result in signature failures with intermediaries that do not recognize that DKIM-Signature header fields are trace header fields and unwittingly reorder them, thus breaking such signatures. For this reason, signing existing DKIM-Signature header fields is unadvised, albeit legal. INFORMATIVE NOTE: If a header field with multiple instances is signed, those header fields are always signed from the bottom up. Thus, it is not possible to sign only specific DKIM-Signature header fields. For example, if the message being signed already contains three DKIM-Signature header fields A, B, and C, it is possible to sign all of them, B and C only, or C only, but not A only, B only, A and B only, or A and C only. A Signer MAY add more than one DKIM-Signature header field using different parameters. For example, during a transition period, a Signer might want to produce signatures using two different hash algorithms.
Signers SHOULD NOT remove any DKIM-Signature header fields from messages they are signing, even if they know that the signatures cannot be verified. When evaluating a message with multiple signatures, a Verifier SHOULD evaluate signatures independently and on their own merits. For example, a Verifier that by policy chooses not to accept signatures with deprecated cryptographic algorithms would consider such signatures invalid. Verifiers MAY process signatures in any order of their choice; for example, some Verifiers might choose to process signatures corresponding to the From field in the message header before other signatures. See Section 6.1 for more information about signature choices. INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate valid signatures with invalid signatures in an attempt to guess why a signature failed are ill-advised. In particular, there is no general way that a Verifier can determine that an invalid signature was ever valid. Verifiers SHOULD continue to check signatures until a signature successfully verifies to the satisfaction of the Verifier. To limit potential denial-of-service attacks, Verifiers MAY limit the total number of signatures they will attempt to verify. If a Verifier module reports signatures whose evaluations produced PERMFAIL results, Identity Assessors SHOULD ignore those signatures (see Section 6.1), acting as though they were not present in the message. 5. Signer Actions The following steps are performed in order by Signers. 5.1. Determine Whether the Email Should Be Signed and by Whom A Signer can obviously only sign email for domains for which it has a private key and the necessary knowledge of the corresponding public key and selector information. However, there are a number of other reasons beyond the lack of a private key why a Signer could choose not to sign an email. INFORMATIVE NOTE: A Signer can be implemented as part of any portion of the mail system as deemed appropriate, including an MUA, a SUBMISSION server, or an MTA. Wherever implemented, Signers should beware of signing (and thereby asserting responsibility for) messages that may be problematic. In particular, within a trusted enclave, the signing domain might be
derived from the header according to local policy; SUBMISSION servers might only sign messages from users that are properly authenticated and authorized. INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign Received header fields if the outgoing gateway MTA obfuscates Received header fields, for example, to hide the details of internal topology. If an email cannot be signed for some reason, it is a local policy decision as to what to do with that email. 5.2. 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. INFORMATIVE OPERATIONS ADVICE: A Signer should not sign with a private key when the selector containing the corresponding public key is expected to be revoked or removed before the Verifier has an opportunity to validate the signature. The Signer should anticipate that Verifiers can choose to defer validation, perhaps until the message is actually read by the final recipient. In particular, when rotating to a new key pair, signing should immediately commence with the new private key, and the old public key should be retained for a reasonable validation interval before being removed from the key server. 5.3. 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 DKIM signatures. In order to minimize the chances of such breakage, Signers SHOULD convert the message to a suitable MIME content-transfer encoding such as quoted-printable or base64 as described in [RFC2045] before signing. Such conversion is outside the scope of DKIM; the actual message SHOULD be converted to 7-bit MIME by an MUA or MSA prior to presentation to the DKIM algorithm. If the message is submitted to the Signer with any 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. 5.3.1. Body Length Limits A body length count MAY be specified to limit the signature calculation to an initial prefix of the body text, measured in octets. If the body length count is not specified, the entire message body is signed. INFORMATIVE RATIONALE: This capability is provided because it is very common for mailing lists to add trailers to messages (e.g., instructions on how to get off the list). Until those messages are also signed, the body length count is a useful tool for the Verifier since it can, as a matter of policy, accept messages having valid signatures with extraneous data. The length actually hashed should be inserted in the "l=" tag of the DKIM-Signature header field. (See Section 3.5.) The body length count allows the Signer of a message to permit data to be appended to the end of the body of a signed message. The body length count MUST be calculated following the canonicalization algorithm; for example, any whitespace ignored by a canonicalization algorithm is not included as part of the body length count. A body length count of zero means that the body is completely unsigned. Signers wishing to ensure that no modification of any sort can occur should specify the "simple" canonicalization algorithm for both header and body and omit the body length count. See Section 8.2 for further discussion. 5.4. Determine the Header Fields to Sign The From header field MUST be signed (that is, included in the "h=" tag of the resulting DKIM-Signature header field). Signers SHOULD NOT sign an existing header field likely to be legitimately modified or removed in transit. In particular, [RFC5321] explicitly permits
modification or removal of the Return-Path header field in transit. Signers MAY include any other header fields present at the time of signing at the discretion of the Signer. INFORMATIVE OPERATIONS NOTE: The choice of which header fields to sign is non-obvious. One strategy is to sign all existing, non- repeatable header fields. An alternative strategy is to sign only header fields that are likely to be displayed to or otherwise be likely to affect the processing of the message at the receiver. A third strategy is to sign only "well-known" headers. Note that Verifiers may treat unsigned header fields with extreme skepticism, including refusing to display them to the end user or even ignoring the signature if it does not cover certain header fields. For this reason, signing fields present in the message such as Date, Subject, Reply-To, Sender, and all MIME header fields are highly advised. The DKIM-Signature header field is always implicitly signed and MUST NOT be included in the "h=" tag except to indicate that other preexisting signatures are also signed. Signers MAY claim to have signed header fields that do not exist (that is, Signers MAY include the header field name in the "h=" tag even if that header field does not exist in the message). When computing the signature, the nonexisting header field MUST be treated as the null string (including the header field name, header field value, all punctuation, and the trailing CRLF). INFORMATIVE RATIONALE: This allows Signers to explicitly assert the absence of a header field; if that header field is added later, the signature will fail. INFORMATIVE NOTE: A header field name need only be listed once more than the actual number of that header field in a message at the time of signing in order to prevent any further additions. For example, if there is a single Comments header field at the time of signing, listing Comments twice in the "h=" tag is sufficient to prevent any number of Comments header fields from being appended; it is not necessary (but is legal) to list Comments three or more times in the "h=" tag. Refer to Section 5.4.2 for a discussion of the procedure to be followed when canonicalizing a header with more than one instance of a particular header field name. Signers need to be careful of signing header fields that might have additional instances added later in the delivery process, since such header fields might be inserted after the signed instance or
otherwise reordered. Trace header fields (such as Received) and Resent-* blocks are the only fields prohibited by [RFC5322] from being reordered. In particular, since DKIM-Signature header fields may be reordered by some intermediate MTAs, signing existing DKIM- Signature header fields is error-prone. INFORMATIVE ADMONITION: Despite the fact that [RFC5322] does not prohibit the reordering of header fields, reordering of signed header fields with multiple instances by intermediate MTAs will cause DKIM signatures to be broken; such antisocial behavior should be avoided. INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this specification, all end-user visible header fields should be signed to avoid possible "indirect spamming". For example, if the Subject header field is not signed, a spammer can resend a previously signed mail, replacing the legitimate subject with a one-line spam. 5.4.1. Recommended Signature Content The purpose of the DKIM cryptographic algorithm is to affix an identifier to the message in a way that is both robust against normal transit-related changes and resistant to kinds of replay attacks. An essential aspect of satisfying these requirements is choosing what header fields to include in the hash and what fields to exclude. The basic rule for choosing fields to include is to select those fields that constitute the "core" of the message content. Hence, any replay attack will have to include these in order to have the signature succeed; however, with these included, the core of the message is valid, even if sent on to new recipients. Common examples of fields with addresses and fields with textual content related to the body are: o From (REQUIRED; see Section 5.4) o Reply-To o Subject o Date o To, Cc o Resent-Date, Resent-From, Resent-To, Resent-Cc
o In-Reply-To, References o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-Owner, List-Archive If the "l=" signature tag is in use (see Section 3.5), the Content- Type field is also a candidate for being included as it could be replaced in a way that causes completely different content to be rendered to the receiving user. There are trade-offs in the decision of what constitutes the "core" of the message, which for some fields is a subjective concept. Including fields such as "Message-ID", for example, is useful if one considers a mechanism for being able to distinguish separate instances of the same message to be core content. Similarly, "In- Reply-To" and "References" might be desirable to include if one considers message threading to be a core part of the message. Another class of fields that may be of interest are those that convey security-related information about the message, such as Authentication-Results [RFC5451]. The basic rule for choosing fields to exclude is to select those fields for which there are multiple fields with the same name and fields that are modified in transit. Examples of these are: o Return-Path o Received o Comments, Keywords Note that the DKIM-Signature field is also excluded from the header hash because its handling is specified separately. Typically, it is better to exclude other optional fields because of the potential that additional fields of the same name will be legitimately added or reordered prior to verification. There are likely to be legitimate exceptions to this rule because of the wide variety of application-specific header fields that might be applied to a message, some of which are unlikely to be duplicated, modified, or reordered. Signers SHOULD choose canonicalization algorithms based on the types of messages they process and their aversion to risk. For example, e-commerce sites sending primarily purchase receipts, which are not expected to be processed by mailing lists or other software likely to modify messages, will generally prefer "simple" canonicalization.
Sites sending primarily person-to-person email will likely prefer to be more resilient to modification during transport by using "relaxed" canonicalization. Unless mail is processed through intermediaries, such as mailing lists that might add "unsubscribe" instructions to the bottom of the message body, the "l=" tag is likely to convey no additional benefit while providing an avenue for unauthorized addition of text to a message. The use of "l=0" takes this to the extreme, allowing complete alteration of the text of the message without invalidating the signature. Moreover, a Verifier would be within its rights to consider a partly signed message body as unacceptable. Judicious use is advised. 5.4.2. Signatures Involving Multiple Instances of a Field Signers choosing to sign an existing header field that occurs more than once in the message (such as Received) MUST sign the physically last instance of that header field in the header block. Signers wishing to sign multiple instances of such a header field MUST include the header field name multiple times in the "h=" tag of the DKIM-Signature header field and MUST sign such header fields in order from the bottom of the header field block to the top. The Signer MAY include more instances of a header field name in "h=" than there are actual corresponding header fields so that the signature will not verify if additional header fields of that name are added. INFORMATIVE EXAMPLE: If the Signer wishes to sign two existing Received header fields, and the existing header contains: Received: <A> Received: <B> Received: <C> then the resulting DKIM-Signature header field should read: DKIM-Signature: ... h=Received : Received :... and Received header fields <C> and <B> will be signed in that order.
5.5. Compute the Message Hash and Signature The Signer MUST compute the message hash as described in Section 3.7 and then sign it using the selected public-key algorithm. This will result in a DKIM-Signature header field that will include the body hash and a signature of the header hash, where that header includes the DKIM-Signature header field itself. Entities such as mailing list managers that implement DKIM 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. 5.6. Insert the DKIM-Signature Header Field Finally, the Signer MUST insert the DKIM-Signature header field created in the previous step prior to transmitting the email. The DKIM-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 DKIM- Signature header field and using the private key corresponding to the selector given in the "s=" tag of the DKIM-Signature header field, as chosen above in Section 5.2. The DKIM-Signature header field MUST be inserted before any other DKIM-Signature fields in the header block. INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this is to insert the DKIM-Signature header field at the beginning of the header block. In particular, it may be placed before any existing Received header fields. This is consistent with treating DKIM-Signature as a trace header field. 6. Verifier Actions Since a Signer MAY remove or revoke a public key at any time, it is advised that verification occur in a timely manner. In many configurations, the most timely place is during acceptance by the border MTA or shortly thereafter. In particular, deferring verification until the message is accessed by the end user is discouraged. A border or intermediate MTA MAY verify the message signature(s). An MTA who 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 Sections 6.1, 6.1.1, and 6.1.2 in order. In practice, several of these steps can be performed in parallel in order to improve performance. 6.1. Extract Signatures from the Message The order in which Verifiers try DKIM-Signature header fields is not defined; Verifiers MAY try signatures in any order they like. For example, one implementation might try the signatures in textual order, whereas another might try signatures by identities that match the contents of the From header field before trying other signatures. Verifiers MUST NOT attribute ultimate meaning to the order of multiple DKIM-Signature header fields. In particular, there is reason to believe that some relays will reorder the header fields in potentially arbitrary ways. INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as a clue to signing order in the absence of any other information. However, other clues as to the semantics of multiple signatures (such as correlating the signing host with Received header fields) might also be considered. Survivability of signatures after transit is not guaranteed, and signatures can fail to verify through no fault of the Signer. Therefore, a Verifier SHOULD NOT treat a message that has one or more bad signatures and no good signatures differently from a message with no signature at all. When a signature successfully verifies, a Verifier will either stop processing or attempt to verify any other signatures, at the discretion of the implementation. A Verifier MAY limit the number of signatures it tries, in order to avoid denial-of-service attacks (see Section 8.4 for further discussion). In the following description, text reading "return status (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL") means that the Verifier MUST immediately cease processing that signature. The Verifier SHOULD proceed to the next signature, if one
is present, and completely ignore the bad signature. If the status is "PERMFAIL", the signature failed and should not be reconsidered. If the status is "TEMPFAIL", the signature could not be verified at this time but may be tried again later. A Verifier MAY either arrange to defer the message for later processing or try another signature; if no good signature is found and any of the signatures resulted in a TEMPFAIL status, the Verifier MAY arrange to defer the message for later processing. The "(explanation)" is not normative text; it is provided solely for clarification. Verifiers that are prepared to validate multiple signature header fields SHOULD proceed to the next signature header field, if one exists. However, Verifiers MAY make note of the fact that an invalid signature was present for consideration at a later step. INFORMATIVE NOTE: The rationale of this requirement is to permit messages that have invalid signatures but also a valid signature to work. For example, a mailing list exploder might opt to leave the original submitter signature in place even though the exploder knows that it is modifying the message in some way that will break that signature, and the exploder inserts its own signature. In this case, the message should succeed even in the presence of the known-broken signature. 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. 6.1.1. Validate the Signature Header Field Implementers MUST meticulously validate the format and values in the DKIM-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 DKIM-Signature header field, which are explicitly permitted. Verifiers MUST return PERMFAIL (incompatible version) when presented a DKIM-Signature header field with a "v=" tag that is inconsistent with this specification. INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course, choose to also verify signatures generated by older versions of this specification.
If any tag listed as "required" in Section 3.5 is omitted from the DKIM-Signature header field, the Verifier MUST ignore the DKIM- Signature header field and return PERMFAIL (signature missing required tag). INFORMATIVE NOTE: The tags listed as required in Section 3.5 are "v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there be a conflict between this note and Section 3.5, Section 3.5 is normative. If the DKIM-Signature header field does not contain the "i=" tag, the Verifier MUST behave as though the value of that tag were "@d", where "d" is the value from the "d=" tag. Verifiers MUST confirm that the domain specified in the "d=" tag is the same as or a parent domain of the domain part of the "i=" tag. If not, the DKIM-Signature header field MUST be ignored, and the Verifier should return PERMFAIL (domain mismatch). If the "h=" tag does not include the From header field, the Verifier MUST ignore the DKIM-Signature header field and return PERMFAIL (From field not signed). Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (signature expired) if it contains an "x=" tag and the signature has expired. Verifiers MAY ignore the DKIM-Signature header field if the domain used by the Signer in the "d=" tag is not associated with a valid signing entity. For example, signatures with "d=" values such as "com" and "co.uk" could be ignored. The list of unacceptable domains SHOULD be configurable. Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (unacceptable signature header) for any other reason, for example, if the signature does not sign header fields that the Verifier views to be essential. As a case in point, if MIME header fields are not signed, certain attacks may be possible that the Verifier would prefer to avoid. 6.1.2. Get the Public Key The public key for a signature is needed to complete the verification process. The process of retrieving the public key depends on the query type as defined by the "q=" tag in the DKIM-Signature header field. Obviously, a public key need only be retrieved if the process of extracting the signature information is completely successful.
Details of key management and representation are described in Section 3.6. 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: 1. The Verifier retrieves the public key as described in Section 3.6 using the algorithm in the "q=" tag, the domain from the "d=" tag, and the selector from the "s=" tag. 2. If the query for the public key fails to respond, the Verifier MAY seek a later verification attempt by returning TEMPFAIL (key unavailable). 3. If the query for the public key fails because the corresponding key record does not exist, the Verifier MUST immediately return PERMFAIL (no key for signature). 4. If the query for the public key returns multiple key records, the Verifier can choose one of the key records or may cycle through the key records, performing the remainder of these steps on each record at the discretion of the implementer. The order of the key records is unspecified. If the Verifier chooses to cycle through the key records, then the "return ..." wording in the remainder of this section means "try the next key record, if any; if none, return to try another signature in the usual way". 5. If the result returned from the query does not adhere to the format defined in this 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.
6. If the "h=" tag exists in the public-key record and the hash algorithm implied by the "a=" tag in the DKIM-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). 7. 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. 8. 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 MUST immediately return PERMFAIL (inappropriate key algorithm). 6.1.3. Compute the Verification Given a Signer and a public key, verifying a signature consists of actions semantically equivalent to the following steps. 1. Based on the algorithm defined in the "c=" tag, the body length specified in the "l=" tag, and the header field names in the "h=" tag, prepare a canonicalized version of the message as is described in Section 3.7 (note that this canonicalized version does not actually replace the original content). When matching header field names in the "h=" tag against the actual message header field, comparisons MUST be case-insensitive. 2. Based on the algorithm indicated in the "a=" tag, compute the message hashes from the canonical copy as described in Section 3.7. 3. 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). 4. 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).
5. Otherwise, the signature has correctly verified. INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to initiate the public-key query in parallel with calculating the hash as the public key is not needed until the final decryption is calculated. Implementations may also verify the signature on the message header before validating that the message hash listed in the "bh=" tag in the DKIM-Signature header field matches that of the actual message body; however, if the body hash does not match, the entire signature must be considered to have failed. A body length specified in the "l=" tag of the signature limits the number of bytes of the body passed to the verification algorithm. All data beyond that limit is not validated by DKIM. Hence, Verifiers might treat a message that contains bytes beyond the indicated body length with suspicion and can choose to treat the signature as if it were invalid (e.g., by returning PERMFAIL (unsigned content)). Should the algorithm reach this point, the verification has succeeded, and DKIM reports SUCCESS for this signature. 6.2. 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 DKIM-Signature or preexisting authentication status header fields in the header field block. The Authentication-Results: header field ([RFC5451]) MAY be used for this purpose. INFORMATIVE ADVICE to MUA filter writers: Patterns intended to search for results header fields to visibly mark authenticated mail for end users should verify that such a header field was added by the appropriate verifying domain and that the verified identity matches the author identity that will be displayed by the MUA. In particular, MUA filters should not be influenced by bogus results header fields added by attackers. To circumvent this attack, Verifiers MAY wish to request deletion of existing results header fields after verification and before arranging to add a new header field.
6.3. Interpret Results/Apply Local Policy It is beyond the scope of this specification to describe what actions an Identity Assessor can make, but mail carrying a validated SDID presents an opportunity to an Identity Assessor that unauthenticated email does not. Specifically, an authenticated email creates a predictable identifier by which other decisions can reliably be managed, such as trust and reputation. Conversely, unauthenticated email lacks a reliable identifier that can be used to assign trust and reputation. It is reasonable to treat unauthenticated email as lacking any trust and having no positive reputation. In general, modules that consume DKIM verification output 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. Once the signature has been verified, that information MUST be conveyed to the Identity Assessor (such as an explicit allow/ whitelist and reputation system) and/or to the end user. If the SDID is not the same as the address in the From: header field, the mail system SHOULD take pains to ensure that the actual SDID is clear to the reader. While the symptoms of a failed verification are obvious -- the signature doesn't verify -- establishing the exact cause can be more difficult. If a selector cannot be found, is that because the selector has been removed, or was the value changed somehow in transit? If the signature line is missing, is that because it was never there, or was it removed by an overzealous filter? For diagnostic purposes, the exact reason why the verification fails SHOULD be made available and possibly recorded in the system logs.
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. See Section 8.15 for additional discussion. 7. IANA Considerations DKIM has registered namespaces with IANA. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus [RFC5226]. This memo updates these registries as described below. Of note is the addition of a new "status" column. All registrations into these namespaces MUST include the name being registered, the document in which it was registered or updated, and an indication of its current status, which MUST be one of "active" (in current use) or "historic" (no longer in current use). No new tags are defined in this specification compared to [RFC4871], but one has been designated as "historic". Also, the "Email Authentication Methods" registry is revised to refer to this update. 7.1. Email Authentication Methods Registry The "Email Authentication Methods" registry is updated to indicate that "dkim" is defined in this memo. 7.2. DKIM-Signature Tag Specifications A DKIM-Signature provides for a list of tag specifications. IANA has established the "DKIM-Signature Tag Specifications" registry for tag specifications that can be used in DKIM-Signature fields.
+------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | v | (this document) | active | | a | (this document) | active | | b | (this document) | active | | bh | (this document) | active | | c | (this document) | active | | d | (this document) | active | | h | (this document) | active | | i | (this document) | active | | l | (this document) | active | | q | (this document) | active | | s | (this document) | active | | t | (this document) | active | | x | (this document) | active | | z | (this document) | active | +------+-----------------+--------+ Table 1: DKIM-Signature Tag Specifications Registry Updated Values 7.3. DKIM-Signature Query Method Registry The "q=" tag-spec (specified in Section 3.5) provides for a list of query methods. IANA has established the "DKIM-Signature Query Method" registry for mechanisms that can be used to retrieve the key that will permit validation processing of a message signed using DKIM. +------+--------+-----------------+--------+ | TYPE | OPTION | REFERENCE | STATUS | +------+--------+-----------------+--------+ | dns | txt | (this document) | active | +------+--------+-----------------+--------+ Table 2: DKIM-Signature Query Method Registry Updated Values 7.4. DKIM-Signature Canonicalization Registry The "c=" tag-spec (specified in Section 3.5) provides for a specifier for canonicalization algorithms for the header and body of the message. IANA has established the "DKIM-Signature Canonicalization Header" Registry for algorithms for converting a message into a canonical form before signing or verifying using DKIM.
+---------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------------+--------+ | simple | (this document) | active | | relaxed | (this document) | active | +---------+-----------------+--------+ Table 3: DKIM-Signature Canonicalization Header Registry Updated Values +---------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------------+--------+ | simple | (this document) | active | | relaxed | (this document) | active | +---------+-----------------+--------+ Table 4: DKIM-Signature Canonicalization Body Registry Updated Values 7.5. _domainkey DNS TXT Resource Record Tag Specifications A _domainkey DNS TXT RR provides for a list of tag specifications. IANA has established the DKIM "_domainkey DNS TXT Record Tag Specifications" registry for tag specifications that can be used in DNS TXT resource records. +------+-----------------+----------+ | TYPE | REFERENCE | STATUS | +------+-----------------+----------+ | v | (this document) | active | | g | [RFC4871] | historic | | h | (this document) | active | | k | (this document) | active | | n | (this document) | active | | p | (this document) | active | | s | (this document) | active | | t | (this document) | active | +------+-----------------+----------+ Table 5: _domainkey DNS TXT Record Tag Specifications Registry Updated Values 7.6. DKIM Key Type Registry The "k=" <key-k-tag> (specified in Section 3.6.1) and the "a=" <sig- a-tag-k> (specified in Section 3.5) tags provide for a list of mechanisms that can be used to decode a DKIM signature.
IANA has established the "DKIM Key Type" registry for such mechanisms. +------+-----------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------+--------+ | rsa | [RFC3447] | active | +------+-----------+--------+ Table 6: DKIM Key Type Registry Updated Values 7.7. DKIM Hash Algorithms Registry The "h=" <key-h-tag> (specified in Section 3.6.1) and the "a=" <sig- a-tag-h> (specified in Section 3.5) tags provide for a list of mechanisms that can be used to produce a digest of message data. IANA has established the "DKIM Hash Algorithms" registry for such mechanisms. +--------+-------------------+--------+ | TYPE | REFERENCE | STATUS | +--------+-------------------+--------+ | sha1 | [FIPS-180-3-2008] | active | | sha256 | [FIPS-180-3-2008] | active | +--------+-------------------+--------+ Table 7: DKIM Hash Algorithms Registry Updated Values 7.8. DKIM Service Types Registry The "s=" <key-s-tag> tag (specified in Section 3.6.1) provides for a list of service types to which this selector may apply. IANA has established the "DKIM Service Types" registry for service types. +-------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +-------+-----------------+--------+ | email | (this document) | active | | * | (this document) | active | +-------+-----------------+--------+ Table 8: DKIM Service Types Registry Updated Values
7.9. DKIM Selector Flags Registry The "t=" <key-t-tag> tag (specified in Section 3.6.1) provides for a list of flags to modify interpretation of the selector. IANA has established the "DKIM Selector Flags" registry for additional flags. +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | y | (this document) | active | | s | (this document) | active | +------+-----------------+--------+ Table 9: DKIM Selector Flags Registry Updated Values 7.10. DKIM-Signature Header Field IANA has added DKIM-Signature to the "Permanent Message Header Field Names" registry (see [RFC3864]) for the "mail" protocol, using this document as the reference.