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
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.
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
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
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
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
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
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
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
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
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
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 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,
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
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 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,
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"
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
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.
If the Signer wishes to sign two existing Received header fields,
and the existing header contains:
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
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
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
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
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
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
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
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
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
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
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
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
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
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 Values7.3. DKIM-Signature Query Method Registry
The "q=" tag-spec (specified in Section 3.5) provides for a list of
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 Values7.4. 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
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.