RFC 4871
DomainKeys Identified Mail (DKIM) Signatures
4. Semantics of Multiple Signatures
4.1. Example Scenarios
There are many reasons why a message might have multiple signatures. For example, a given signer might sign multiple times, perhaps with different hashing or signing algorithms during a transition phase. INFORMATIVE 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 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 headers and no "l=" tag (to satisfy strict verifiers) and a second time with a limited set of headers and an "l=" tag (in anticipation of possible message modifications in route to other verifiers). Verifiers could then choose which signature they preferred. INFORMATIVE EXAMPLE: A verifier might receive a message with two signatures, one covering more of the message than the other. If the signature covering more of the message verified, then the verifier could make one set of policy decisions; if that signature failed but the signature covering less of the message verified, the verifier might make a different set of policy decisions. 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. INFORMATIVE EXAMPLE: 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.
INFORMATIVE 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.
INFORMATIVE NOTE: 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 ignore failed signatures as though they were not
present in the message. 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.
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: Signing modules may be incorporated into 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 address 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 may 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 MIME Part One [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 [RFC2822] 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.
5.4. 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, [RFC2821] 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 non-existing 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.
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 to indicate that
additional header fields of that name SHOULD NOT be 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.
Signers should 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 [RFC2822] 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 [RFC2822] permits
header fields to be reordered (with the exception of Received
header fields), reordering of signed header fields with multiple
instances by intermediate MTAs will cause DKIM signatures to be
broken; such anti-social 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.
5.5. Recommended Signature Content
In order to maximize compatibility with a variety of verifiers, it is recommended that signers follow the practices outlined in this section when signing a message. However, these are generic recommendations applying to the general case; specific senders may wish to modify these guidelines as required by their unique situations. Verifiers MUST be capable of verifying signatures even if one or more of the recommended header fields is not signed (with the exception of From, which must always be signed) or if one or more of the disrecommended header fields is signed. Note that verifiers do have the option of ignoring signatures that do not cover a sufficient portion of the header or body, just as they may ignore signatures from an identity they do not trust. The following header fields SHOULD be included in the signature, if they are present in the message being signed: o From (REQUIRED in all signatures) o Sender, Reply-To o Subject o Date, Message-ID o To, Cc o MIME-Version o Content-Type, Content-Transfer-Encoding, Content-ID, Content- Description o Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-Cc, Resent-Message-ID o In-Reply-To, References o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-Owner, List-Archive The following header fields SHOULD NOT be included in the signature: o Return-Path o Received o Comments, Keywords
o Bcc, Resent-Bcc o DKIM-Signature Optional header fields (those not mentioned above) normally SHOULD NOT be included in the signature, because of the potential for additional header fields of the same name to 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 may 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. Signers SHOULD NOT use "l=" unless they intend to accommodate intermediate mail processors that append text to a message. For example, many mailing list processors append "unsubscribe" information to message bodies. If signers use "l=", they SHOULD include the entire message body existing at the time of signing in computing the count. In particular, signers SHOULD NOT specify a body length of 0 since this may be interpreted as a meaningless signature by some verifiers.5.6. 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. The signer MAY elect to limit the number of bytes of the body that will be included in the hash and hence signed. The length actually hashed should be inserted in the "l=" tag of the DKIM-Signature header field.
5.7. 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 recommended 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 considerably simplifies things 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 following steps in the order listed. 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) may also be considered. 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; such treatment is a matter of local policy and is beyond the scope of this document. 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 to avoid denial-of-service attacks. INFORMATIVE NOTE: An attacker could send messages with large numbers of faulty signatures, each of which would require a DNS lookup and corresponding CPU time to verify the message. This could be an attack on the domain that receives the message, by slowing down the verifier by requiring it to do a large number of DNS lookups and/or signature verifications. It could also be an attack against the domains listed in the signatures, essentially by enlisting innocent verifiers in launching an attack against the DNS servers of the actual victim. 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 any 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 defer the message for later processing, perhaps by queueing it locally or issuing a 451/4.7.5 SMTP reply, or try another signature; if no good
signature is found and any of the signatures resulted in a TEMPFAIL
status, the verifier MAY save the message for later processing. The
"(explanation)" is not normative text; it is provided solely for
clarification.
Verifiers SHOULD ignore any DKIM-Signature header fields where the
signature does not validate. Verifiers that are prepared to validate
multiple signature header fields SHOULD proceed to the next signature
header field, should it exist. 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 ignore DKIM-Signature header fields with a "v=" tag
that is inconsistent with this specification and return PERMFAIL
(incompatible version).
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).
INFORMATIONAL 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" may 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.
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. Retrieve 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 defer acceptance of this email and return TEMPFAIL (key
unavailable). If verification is occurring during the incoming
SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
code. Alternatively, the verifier MAY store the message in the
local queue for later trial or ignore the signature. Note that
storing a message in the local queue is subject to denial-of-
service attacks.
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 may 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 "g=" tag in the public key does not match the Local-part
of the "i=" tag in the message signature header field, the
verifier MUST ignore the key record and return PERMFAIL
(inapplicable key). If the Local-part of the "i=" tag on the
message signature is not present, the "g=" tag must be "*" (valid
for all addresses in the domain) or the entire g= tag must be
omitted (which defaults to "g=*"), otherwise the verifier MUST
ignore the key record and return PERMFAIL (inapplicable key).
Other than this test, verifiers SHOULD NOT treat a message signed
with a key record having a "g=" tag any differently than one
without; in particular, verifiers SHOULD NOT prefer messages that
seem to have an individual signature by virtue of a "g=" tag
versus a domain signature.
7. 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).
8. 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.
9. 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 version does not
actually need to be instantiated). 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, such as by truncating the
message at the indicated body length, declaring the signature invalid
(e.g., by returning PERMFAIL (unsigned content)), or conveying the
partial verification to the policy module.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
at the indicated body length might pass on a malformed MIME
message if the signer used the "N-4" trick (omitting the final
"--CRLF") described in the informative note in Section 3.4.5.
Such verifiers may wish to check for this case and include a
trailing "--CRLF" to avoid breaking the MIME structure. A simple
way to achieve this might be to append "--CRLF" to any "multipart"
message with a body length; if the MIME structure is already
correctly formed, this will appear in the postlude and will not be
displayed to the end user.
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.
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 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 delete existing results header fields after
verification and before adding a new header field.
6.3. Interpret Results/Apply Local Policy
It is beyond the scope of this specification to describe what actions
a verifier system should make, but an authenticated email presents an
opportunity to a receiving system that unauthenticated email cannot.
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, verifiers SHOULD NOT reject messages solely on the basis
of a lack of signature or an unverifiable signature; such rejection
would cause severe interoperability problems. However, if the
verifier does opt to reject such messages (for example, when
communicating with a peer who, by prior agreement, agrees to only
send signed messages), and the verifier runs synchronously with the
SMTP session and a signature is missing or does not verify, the MTA
SHOULD use a 550/5.7.x reply code.
If 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 return 4xx SMTP replies.
Once the signature has been verified, that information MUST be
conveyed to higher-level systems (such as explicit allow/whitelists
and reputation systems) and/or to the end user. If the message is
signed on behalf of any address other than that in the From: header
field, the mail system SHOULD take pains to ensure that the actual
signing identity is clear to the reader.
The verifier MAY treat unsigned header fields with extreme
skepticism, including marking them as untrusted or even deleting them
before display to the end user.
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 to the policy module and possibly recorded in the system logs. If the email cannot be verified, then it SHOULD be rendered the same as all unverified email regardless of whether or not it looks like it was signed.7. IANA Considerations
DKIM introduces some new namespaces that have been registered with IANA. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus [RFC2434].7.1. DKIM-Signature Tag Specifications
A DKIM-Signature provides for a list of tag specifications. IANA has established the DKIM-Signature Tag Specification Registry for tag specifications that can be used in DKIM-Signature fields. The initial entries in the registry comprise: +------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | v | (this document) | | a | (this document) | | b | (this document) | | bh | (this document) | | c | (this document) | | d | (this document) | | h | (this document) | | i | (this document) | | l | (this document) | | q | (this document) | | s | (this document) | | t | (this document) | | x | (this document) | | z | (this document) | +------+-----------------+ DKIM-Signature Tag Specification Registry Initial Values
7.2. 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. The initial entry in the registry comprises: +------+--------+-----------------+ | TYPE | OPTION | REFERENCE | +------+--------+-----------------+ | dns | txt | (this document) | +------+--------+-----------------+ DKIM-Signature Query Method Registry Initial Values7.3. 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 Algorithm Registry for algorithms for converting a message into a canonical form before signing or verifying using DKIM. The initial entries in the header registry comprise: +---------+-----------------+ | TYPE | REFERENCE | +---------+-----------------+ | simple | (this document) | | relaxed | (this document) | +---------+-----------------+ DKIM-Signature Header Canonicalization Algorithm Registry Initial Values
The initial entries in the body registry comprise:
+---------+-----------------+
| TYPE | REFERENCE |
+---------+-----------------+
| simple | (this document) |
| relaxed | (this document) |
+---------+-----------------+
DKIM-Signature Body Canonicalization Algorithm Registry
Initial Values
7.4. _domainkey DNS TXT Record Tag Specifications
A _domainkey DNS TXT record provides for a list of tag
specifications. IANA has established the DKIM _domainkey DNS TXT Tag
Specification Registry for tag specifications that can be used in DNS
TXT Records.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| v | (this document) |
| g | (this document) |
| h | (this document) |
| k | (this document) |
| n | (this document) |
| p | (this document) |
| s | (this document) |
| t | (this document) |
+------+-----------------+
DKIM _domainkey DNS TXT Record Tag Specification Registry
Initial Values
7.5. 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.
The initial entry in the registry comprises:
+------+-----------+
| TYPE | REFERENCE |
+------+-----------+
| rsa | [RFC3447] |
+------+-----------+
DKIM Key Type Initial Values
7.6. 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.
The initial entries in the registry comprise:
+--------+-------------------+
| TYPE | REFERENCE |
+--------+-------------------+
| sha1 | [FIPS.180-2.2002] |
| sha256 | [FIPS.180-2.2002] |
+--------+-------------------+
DKIM Hash Algorithms Initial Values
7.7. 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.
The initial entries in the registry comprise:
+-------+-----------------+
| TYPE | REFERENCE |
+-------+-----------------+
| email | (this document) |
| * | (this document) |
+-------+-----------------+
DKIM Service Types Registry Initial Values
7.8. 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. The initial entries in the registry comprise: +------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | y | (this document) | | s | (this document) | +------+-----------------+ DKIM Selector Flags Registry Initial Values7.9. DKIM-Signature Header Field
IANA has added DKIM-Signature to the "Permanent Message Header Fields" registry (see [RFC3864]) for the "mail" protocol, using this document as the reference.
(page 52 continued on part 4)
