DomainKeys Identified Mail (DKIM)
SignaturesBrandenburg InternetWorking675 Spruce Dr.SunnyvaleUSA+1.408.246.8253dcrocker@bbiw.nethttp://bbiw.netAT&T Laboratories200 Laurel Ave. SouthMiddletownNJ07748USAtony+dkimov@maillennium.att.comCloudmarkmsk@cloudmark.comDKIMDomainKeys Identified Mail (DKIM) permits a person, role, or
organization that owns the signing domain to claim some
responsibility for a message by associating the domain with the
message. This can be an author's organization, an operational relay
or one of their agents. DKIM separates the question of the identity
of the signer of the message from the purported author of the
message. Assertion of responsibility is validated through a
cryptographic signature and querying the signer's domain directly to
retrieve the appropriate public key. Message transit from author to
recipient is through relays that typically make no substantive
change to the message content and thus preserve the DKIM
signature.DomainKeys Identified Mail (DKIM) permits a person, role, or
organization that owns the signing domain to claim some
responsibility for a message by associating the domain with the
message. This can be an author's organization, an operational relay
or one of their agents. Assertion of responsibility is validated
through a cryptographic signature and querying the signer's domain
directly to retrieve the appropriate public key. Message transit
from author to recipient is through relays that typically make no
substantive change to the message content and thus preserve the DKIM
signature. A message can contain multiple signatures, from the same
or different organizations involved with the message. The approach taken by DKIM differs from previous approaches to
message signing (e.g., Secure/Multipurpose Internet Mail Extensions
(S/MIME) , OpenPGP ) in that: the message signature is written as a message header field so
that neither human recipients nor existing MUA (Mail User
Agent) software is confused by signature-related content
appearing in the message body;there is no dependency on public and private key pairs being
issued by well-known, trusted certificate authorities; there is no dependency on the deployment of any new Internet
protocols or services for public key distribution or
revocation;signature verification failure does not force rejection of the
message;no attempt is made to include encryption as part of the
mechanism;message archiving is not a design goal.DKIM: is compatible with the existing email infrastructure and
transparent to the fullest extent possible;requires minimal new infrastructure;can be implemented independently of clients in order to reduce
deployment time;can be deployed incrementally;allows delegation of signing to third parties.DKIM separates the question of the identity of the signer of the
message from the purported author of the message. In particular,
a signature includes the identity of the signer. Verifiers can
use the signing information to decide how they want to process
the message. The signing identity is included as part of the
signature header field. INFORMATIVE RATIONALE: The signing identity specified by a
DKIM signature is not required to match an address in any
particular header field because of the broad methods of
interpretation by recipient mail systems, including
MUAs.DKIM is designed to support the extreme scalability requirements
that characterize the email identification problem. There are
currently over 70 million domains and a much larger number of
individual addresses. DKIM seeks to preserve the positive aspects
of the current email infrastructure, such as the ability for
anyone to communicate with anyone else without introduction.DKIM differs from traditional hierarchical public-key systems in
that no Certificate Authority infrastructure is required; the
verifier requests the public key from a repository in the domain
of the claimed signer directly rather than from a third
party.The DNS is proposed as the initial mechanism for the public keys.
Thus, DKIM currently depends on DNS administration and the
security of the DNS system. DKIM is designed to be extensible to
other key fetching services as they become available.This section defines terms used in the rest of the document. DKIM is designed to operate within the Internet Mail service, as
defined in . Basic email terminology is taken from that
specification.Syntax descriptions use Augmented BNF (ABNF) . The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .Elements in the mail system that sign messages on behalf of a
domain are referred to as signers. These may be MUAs (Mail User
Agents), MSAs (Mail Submission Agents), MTAs (Mail Transfer
Agents), or other agents such as mailing list exploders. In
general, any signer will be involved in the injection of a
message into the message system in some way. The key issue is
that a message must be signed before it leaves the administrative
domain of the signer. Elements in the mail system that verify signatures are referred
to as verifiers. These may be MTAs, Mail Delivery Agents (MDAs),
or MUAs. In most cases it is expected that verifiers will be
close to an end user (reader) of the message or some consuming
agent such as a mailing list exploder.A person, role, or organization. In the context of DKIM, examples
include author, author's organization, an ISP along the handling
path, an independent trust assessment service, and a mailing list
operator. A label that refers to an identity.A single domain name that is the mandatory payload output of DKIM
and that refers to the identity claiming responsibility for
introduction of a message into the mail stream. For DKIM
processing, the name has only basic domain name semantics; any
possible owner-specific semantics are outside the scope of DKIM.
It is specified in .A single identifier that refers to the agent or user on behalf of
whom the Signing Domain Identifier (SDID) has taken
responsibility. The AUID comprises a domain name and an optional
<Local-part>. The domain name is the same as that used for
the SDID or is a sub-domain of it. For DKIM processing, the
domain name portion of the AUID has only basic domain name
semantics; any possible owner-specific semantics are outside the
scope of DKIM. It is specified in .A module that consumes DKIM's mandatory payload, which is the
responsible Signing Domain Identifier (SDID). The module is
dedicated to the assessment of the delivered identifier. Other
DKIM (and non-DKIM) values can also be delivered to this module
as well as to a more general message evaluation filtering engine.
However, this additional activity is outside the scope of the
DKIM signature specification.There are three forms of whitespace: WSP represents simple whitespace, i.e., a space or a tab
character (formal definition in ).LWSP is linear whitespace, defined as WSP plus CRLF (formal
definition in ).FWS is folding whitespace. It allows multiple lines
separated by CRLF followed by at least one whitespace, to
be joined.The definition of FWS is identical to that in except for the exclusion of obs-FWS.errata 1596 The following tokens are imported from other RFCs as noted.
Those RFCs should be considered definitive.The following tokens are imported from : "Local-part" (implementation warning: this permits quoted
strings)"sub-domain"The following tokens are imported from : "field-name" (name of a header field)"dot-atom-text" (in the Local-part of an email address)The following tokens are imported from : "qp-section" (a single line of quoted-printable-encoded
text)"hex-octet" (a quoted-printable encoded octet)INFORMATIVE NOTE: Be aware that the ABNF in does not obey the rules of and must be interpreted accordingly,
particularly as regards case folding. Other tokens not defined herein are imported from . These are intuitive primitives such as
SP, HTAB, WSP, ALPHA, DIGIT, CRLF, etc.The DKIM-Quoted-Printable encoding syntax resembles that
described in Quoted-Printable , Section 6.7: any character MAY be encoded
as an "=" followed by two hexadecimal digits from the alphabet
"0123456789ABCDEF" (no lowercase characters permitted)
representing the hexadecimal-encoded integer value of that
character. All control characters (those with values < %x20),
8-bit characters (values > %x7F), and the characters DEL
(%x7F), SPACE (%x20), and semicolon (";", %x3B) MUST be encoded.
Note that all whitespace, including SPACE, CR, and LF characters,
MUST be encoded. After encoding, FWS MAY be added at arbitrary
locations in order to avoid excessively long lines; such
whitespace is NOT part of the value, and MUST be removed before
decoding.INFORMATIVE NOTE: DKIM-Quoted-Printable differs from
Quoted- Printable as defined in in several important ways: Whitespace in the input text, including CR and LF,
must be encoded. does not require such
encoding, and does not permit encoding of CR or LF
characters that are part of a CRLF line break.Whitespace in the encoded text is ignored. This is to
allow tags encoded using DKIM-Quoted-Printable to be
wrapped as needed. In particular, requires that line breaks in
the input be represented as physical line breaks;
that is not the case here.The "soft line break" syntax ("=" as the last
non-whitespace character on the line) does not
apply.DKIM-Quoted-Printable does not require that encoded
lines be no more than 76 characters long (although
there may be other requirements depending on the
context in which the encoded text is being used). Protocol Elements are conceptual parts of the protocol that are not
specific to either signers or verifiers. The protocol descriptions
for signers and verifiers are described in later sections (Signer
Actions () and Verifier Actions ()). NOTE: This section must be read in the
context of those sections.To support multiple concurrent public keys per signing domain,
the key namespace is subdivided using "selectors". For example,
selectors might indicate the names of office locations (e.g.,
"sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
(e.g., "january2005", "february2005", etc.), or even an
individual user.Selectors are needed to support some important use cases. For
example: Domains that want to delegate signing capability for a
specific address for a given duration to a partner, such as
an advertising provider or other outsourced function. Domains that want to allow frequent travelers to send
messages locally without the need to connect with a
particular MSA."Affinity" domains (e.g., college alumni associations) that
provide forwarding of incoming mail, but that do not
operate a mail submission agent for outgoing mail. Periods are allowed in selectors and are component separators.
When keys are retrieved from the DNS, periods in selectors define
DNS label boundaries in a manner similar to the conventional use
in domain names. Selector components might be used to combine
dates with locations, for example, "march2005.reykjavik". In a
DNS implementation, this can be used to allow delegation of a
portion of the selector namespace.The number of public keys and corresponding selectors for each
domain is determined by the domain owner. Many domain owners will
be satisfied with just one selector, whereas administratively
distributed organizations may choose to manage disparate
selectors and key pairs in different regions or on different
email servers.Beyond administrative convenience, selectors make it possible to
seamlessly replace public keys on a routine basis. If a domain
wishes to change from using a public key associated with selector
"january2005" to a public key associated with selector
"february2005", it merely makes sure that both public keys are
advertised in the public-key repository concurrently for the
transition period during which email may be in transit prior to
verification. At the start of the transition period, the outbound
email servers are configured to sign with the "february2005"
private key. At the end of the transition period, the
"january2005" public key is removed from the public-key
repository. INFORMATIVE NOTE: A key may also be revoked as described
below. The distinction between revoking and removing a key
selector record is subtle. When phasing out keys as
described above, a signing domain would probably simply
remove the key record after the transition period. However,
a signing domain could elect to revoke the key (but
maintain the key record) for a further period. There is no
defined semantic difference between a revoked key and a
removed key.While some domains may wish to make selector values well known,
others will want to take care not to allocate selector names in a
way that allows harvesting of data by outside parties. For
example, if per-user keys are issued, the domain owner will need
to make the decision as to whether to associate this selector
directly with the name of a registered end-user, or make it some
unassociated random value, such as a fingerprint of the public
key. INFORMATIVE OPERATIONS NOTE: Reusing a selector with a new
key (for example, changing the key associated with a user's
name) makes it impossible to tell the difference between a
message that didn't verify because the key is no longer
valid versus a message that is actually forged. For this
reason, signers are ill-advised to reuse selectors for new
keys. A better strategy is to assign new keys to new
selectors.DKIM uses a simple "tag=value" syntax in several contexts,
including in messages and domain signature records.Values are a series of strings containing either plain text,
"base64" text (as defined in , Section 6.8), "qp-section" (ibid,
Section 6.7), or "dkim-quoted-printable" (as defined in
Section 2.6). The name of the tag will determine the
encoding of each value. Unencoded semicolon (";") characters MUST
NOT occur in the tag value, since that separates tag-specs. INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text"
defined below (as "tag-value") only includes 7-bit
characters, an implementation that wished to anticipate
future standards would be advised not to preclude the use
of UTF8-encoded text in tag=value lists. Note that WSP is allowed anywhere around tags. In particular, any
WSP after the "=" and any WSP before the terminating ";" is not
part of the value; however, WSP inside the value is
significant.Tags MUST be interpreted in a case-sensitive manner. Values MUST
be processed as case sensitive unless the specific tag
description of semantics specifies case insensitivity.Tags with duplicate names MUST NOT occur within a single
tag-list; if a tag name does occur more than once, the entire
tag-list is invalid.Whitespace within a value MUST be retained unless explicitly
excluded by the specific tag description.Tag=value pairs that represent the default value MAY be included
to aid legibility. Unrecognized tags MUST be ignored.Tags that have an empty value are not the same as omitted tags.
An omitted tag is treated as having the default value; a tag with
an empty value explicitly designates the empty string as the
value. For example, "g=" does not mean "g=*", even though "g=*"
is the default for that tag.DKIM supports multiple digital signature algorithms. Two
algorithms are defined by this specification at this time:
rsa-sha1 and rsa-sha256.
Signers MUST implement and SHOULD sign using rsa-sha256. INFORMATIVE NOTE: Although sha256 is strongly encouraged,
some senders of low-security messages (such as routine
newsletters) may prefer to use sha1 because of reduced CPU
requirements to compute a sha1 hash. In general, sha256
should always be used whenever possible.The rsa-sha1 Signing Algorithm computes a message hash as
described in below using SHA-1 as the hash-alg. That hash is
then signed by the signer using the RSA algorithm (defined in
PKCS#1 version 1.5 ) as the crypt-alg and the signer's
private key. The hash MUST NOT be truncated or converted into
any form other than the native binary form before being
signed. The signing algorithm SHOULD use a public exponent of
65537.The rsa-sha256 Signing Algorithm computes a message hash as
described in below using SHA-256 as the hash-alg. That hash is
then signed by the signer using the RSA algorithm (defined in
PKCS#1 version 1.5 ) as the crypt-alg and the signer's
private key. The hash MUST NOT be truncated or converted into
any form other than the native binary form before being
signed.Selecting appropriate key sizes is a trade-off between cost,
performance, and risk. Since short RSA keys more easily
succumb to off-line attacks, signers MUST use RSA keys of at
least 1024 bits for long-lived keys. Verifiers MUST be able to
validate signatures with keys ranging from 512 bits to 2048
bits, and they MAY be able to validate signatures with larger
keys. Verifier policies may use the length of the signing key
as one metric for determining whether a signature is
acceptable.Factors that should influence the key size choice include the
following: The practical constraint that large (e.g., 4096 bit)
keys may not fit within a 512-byte DNS UDP response
packetThe security constraint that keys smaller than 1024 bits
are subject to off-line attacksLarger keys impose higher CPU costs to verify and sign
emailKeys can be replaced on a regular basis, thus their
lifetime can be relatively shortThe security goals of this specification are modest
compared to typical goals of other systems that employ
digital signaturesSee for further discussion on selecting key
sizes.Other algorithms MAY be defined in the future. Verifiers MUST
ignore any signatures using algorithms that they do not
implement.Some mail systems modify email in transit, potentially
invalidating a signature. For most signers, mild modification of
email is immaterial to validation of the DKIM domain name's use.
For such signers, a canonicalization algorithm that survives
modest in-transit modification is preferred.Other signers demand that any modification of the email, however
minor, result in a signature verification failure. These signers
prefer a canonicalization algorithm that does not tolerate
in-transit modification of the signed email.Some signers may be willing to accept modifications to header
fields that are within the bounds of email standards such as , but are unwilling to accept any
modification to the body of messages.To satisfy all requirements, two canonicalization algorithms are
defined for each of the header and the body: a "simple" algorithm
that tolerates almost no modification and a "relaxed" algorithm
that tolerates common modifications such as whitespace
replacement and header field line rewrapping. A signer MAY
specify either algorithm for header or body when signing an
email. If no canonicalization algorithm is specified by the
signer, the "simple" algorithm defaults for both header and body.
Verifiers MUST implement both canonicalization algorithms. Note
that the header and body may use different canonicalization
algorithms. Further canonicalization algorithms MAY be defined in
the future; verifiers MUST ignore any signatures that use
unrecognized canonicalization algorithms.Canonicalization simply prepares the email for presentation to
the signing or verification algorithm. It MUST NOT change the
transmitted data in any way. Canonicalization of header fields
and body are described below.NOTE: This section assumes that the message is already in
"network normal" format (text is ASCII encoded, lines are
separated with CRLF characters, etc.). See also for information about normalizing the
message.The "simple" header canonicalization algorithm does not change
header fields in any way. Header fields MUST be presented to
the signing or verification algorithm exactly as they are in
the message being signed or verified. In particular, header
field names MUST NOT be case folded and whitespace MUST NOT be
changed.The "relaxed" header canonicalization algorithm MUST apply the
following steps in order: Convert all header field names (not the header field
values) to lowercase. For example, convert "SUBJect:
AbC" to "subject: AbC".Unfold all header field continuation lines as described
in ; in particular, lines with
terminators embedded in continued header field values
(that is, CRLF sequences followed by WSP) MUST be
interpreted without the CRLF. Implementations MUST NOT
remove the CRLF at the end of the header field
value.Convert all sequences of one or more WSP characters to a
single SP character. WSP characters here include those
before and after a line folding boundary.Delete all WSP characters at the end of each unfolded
header field value.Delete any WSP characters remaining before and after the
colon separating the header field name from the header
field value. The colon separator MUST be retained.The "simple" body canonicalization algorithm ignores all empty
lines at the end of the message body. An empty line is a line
of zero length after removal of the line terminator. If there
is no body or no trailing CRLF on the message body, a CRLF is
added. It makes no other changes to the message body. In more
formal terms, the "simple" body canonicalization algorithm
converts "0*CRLF" at the end of the body to a single
"CRLF".Note that a completely empty or missing body is canonicalized
as a single "CRLF"; that is, the canonicalized length will be
2 octets.errata 1376The sha1 value (in base64) for an
empty body (canonicalized to a "CRLF") is: The sha256 value is: errata 1384 The "relaxed" body canonicalization
algorithm MUST apply the following steps (a) and (b) in order:Reduce whitespace: Ignore all whitespace at the end of lines.
Implementations MUST NOT remove the CRLF at the
end of the line.Reduce all sequences of WSP within a line to a
single SP character.Ignore all empty lines at the end of the message body.
"Empty line" is defined in Section 3.4.3. errata
1377 If the body is non-empty, but does not
end with a CRLF, a CRLF is added. (For email, this is
only possible when using extensions to SMTP or non-SMTP
transport mechanisms.)errata 1376The sha1 value (in base64) for an
empty body (canonicalized to a null input) is: The sha256 value is: INFORMATIVE NOTE: It should be noted that the relaxed
body canonicalization algorithm may enable certain types
of extremely crude "ASCII Art" attacks where a message
may be conveyed by adjusting the spacing between words.
If this is a concern, the "simple" body canonicalization
algorithm should be used instead.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 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 may as a matter of policy accept messages
having valid signatures with extraneous data.INFORMATIVE IMPLEMENTATION NOTE: Using body length
limits enables an attack in which an attacker modifies a
message to include content that solely benefits the
attacker. It is possible for the appended content to
completely replace the original content in the end
recipient's eyes and to defeat duplicate message
detection algorithms. To avoid this attack, signers
should be wary of using this tag, and verifiers might
wish to ignore the tag or remove text that appears after
the specified content length, perhaps based on other
criteria.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. Signers of MIME messages that
include a body length count SHOULD be sure that the length
extends to the closing MIME boundary string. INFORMATIVE IMPLEMENTATION NOTE: A signer wishing to
ensure that the only acceptable modifications are to add
to the MIME postlude would use a body length count
encompassing the entire final MIME boundary string,
including the final "--CRLF". A signer wishing to allow
additional MIME parts but not modification of existing
parts would use a body length count extending through
the final MIME boundary string, omitting the final
"--CRLF". Note that this only works for some MIME types,
e.g., multipart/mixed but not
multipart/signed.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.In the following examples, actual whitespace is used only for
clarity. The actual input and output text is designated using
bracketed descriptors: "<SP>" for a space character,
"<HTAB>" for a tab character, and "<CRLF>" for a
carriage-return/line-feed sequence. For example, "X <SP>
Y" and "X<SP>Y" represent the same three characters.The signature of the email is stored in the DKIM-Signature header
field. This header field contains all of the signature and key-
fetching data. The DKIM-Signature value is a tag-list as
described in .The DKIM-Signature header field SHOULD be treated as though it
were a trace header field as defined in Section 3.6 of , and hence SHOULD NOT be reordered and
SHOULD be prepended to the message.The DKIM-Signature header field being created or verified is
always included in the signature calculation, after the rest of
the header fields being signed; however, when calculating or
verifying the signature, the value of the "b=" tag (signature
value) of that DKIM- Signature header field MUST be treated as
though it were an empty string. Unknown tags in the
DKIM-Signature header field MUST be included in the signature
calculation but MUST be otherwise ignored by verifiers. Other
DKIM-Signature header fields that are included in the signature
should be treated as normal header fields; in particular, the
"b=" tag is not treated specially.The encodings for each field type are listed below. Tags
described as qp-section are encoded as described in
Section 6.7 of MIME Part One , with the additional conversion of
semicolon characters to "=3B"; intuitively, this is one line of
quoted-printable encoded text. The dkim-quoted-printable syntax
is defined in .Tags on the DKIM-Signature header field along with their type and
requirement status are shown below. Unrecognized tags MUST be
ignored. Version (MUST be included). This tag defines
the version of this specification that applies to the
signature record. It MUST have the value "1". Note that
verifiers must do a string comparison on this value; for
example, "1" is not the same as "1.0".INFORMATIVE NOTE: DKIM-Signature version numbers are
expected to increase arithmetically as new versions
of this specification are released.The algorithm used to generate the
signature (plain-text; REQUIRED). Verifiers MUST support
"rsa-sha1" and "rsa-sha256"; signers SHOULD sign using
"rsa-sha256". See for a description of
algorithms.The signature data (base64; REQUIRED).
Whitespace is ignored in this value and MUST be ignored
when reassembling the original signature. In particular,
the signing process can safely insert FWS in this value in
arbitrary places to conform to line-length limits. See
Signer Actions for how the signature is
computed.The hash of the canonicalized body part of
the message as limited by the "l=" tag (base64; REQUIRED).
Whitespace is ignored in this value and MUST be ignored
when reassembling the original signature. In particular,
the signing process can safely insert FWS in this value in
arbitrary places to conform to line-length limits. See for how the body hash is
computed.Message canonicalization (plain-text;
OPTIONAL, default is "simple/simple"). This tag informs the
verifier of the type of canonicalization used to prepare
the message for signing. It consists of two names separated
by a "slash" (%d47) character, corresponding to the header
and body canonicalization algorithms respectively. These
algorithms are described in . If only one algorithm is named, that
algorithm is used for the header and "simple" is used for
the body. For example, "c=relaxed" is treated the same as
"c=relaxed/simple".Specifies the SDID claiming responsibility
for an introduction of a message into the mail stream
(plain-text; REQUIRED). Hence, the SDID value is used to
form the query for the public key. The SDID MUST correspond
to a valid DNS name under which the DKIM key record is
published. The conventions and semantics used by a signer
to create and use a specific SDID are outside the scope of
the DKIM Signing specification, as is any use of those
conventions and semantics. When presented with a signature
that does not meet these requirements, verifiers MUST
consider the signature invalid.Internationalized domain names MUST be encoded as described
in .Signed header fields (plain-text, but see
description; REQUIRED). A colon-separated list of header
field names that identify the header fields presented to
the signing algorithm. The field MUST contain the complete
list of header fields in the order presented to the signing
algorithm. The field MAY contain names of header fields
that do not exist when signed; nonexistent header fields do
not contribute to the signature computation (that is, they
are treated as the null input, including the header field
name, the separating colon, the header field value, and any
CRLF terminator). The field MUST NOT include the
DKIM-Signature header field that is being created or
verified, but may include others. Folding whitespace (FWS)
MAY be included on either side of the colon separator.
Header field names MUST be compared against actual header
field names in a case-insensitive manner. This list MUST
NOT be empty. See for a discussion of choosing
header fields to sign.errata 1461INFORMATIVE EXPLANATION: By "signing" header fields
that do not actually exist, a signer can prevent
insertion of those header fields before verification.
However, since a signer cannot possibly know what
header fields might be created in the future, and
that some MUAs might present header fields that are
embedded inside a message (e.g., as a message/rfc822
content type), the security of this solution is not
total.INFORMATIVE EXPLANATION: The exclusion of the header
field name and colon as well as the header field
value for non-existent header fields prevents an
attacker from inserting an actual header field with a
null value.The Agent or User Identifier (AUID) on behalf
of which the SDID is taking responsibility
(dkim-quoted-printable; OPTIONAL, default is an empty
Local-part followed by an "@" followed by the domain from
the "d=" tag).The syntax is a standard email address where the Local-part
MAY be omitted. The domain part of the address MUST be the
same as, or a subdomain of the value of, the "d=" tag.Internationalized domain names MUST be converted using the
steps listed in Section 4 of using the "ToASCII" function.The AUID is specified as having the same syntax as an email
address, but is not required to have the same semantics.
Notably, the domain name is not required to be registered
in the DNS -- so it might not resolve in a query -- and the
Local-part MAY be drawn from a namespace unrelated to any
mailbox. The details of the structure and semantics for the
namespace are determined by the Signer. Any knowledge or
use of those details by verifiers or assessors is outside
the scope of the DKIM Signing specification. The Signer MAY
choose to use the same namespace for its AUIDs as its
users' email addresses or MAY choose other means of
representing its users. However, the signer SHOULD use the
same AUID for each message intended to be evaluated as
being within the same sphere of responsibility, if it
wishes to offer receivers the option of using the AUID as a
stable identifier that is finer grained than the SDID. INFORMATIVE NOTE: The Local-part of the "i=" tag is
optional because, in some cases, a signer may not be
able to establish a verified individual identity. In
such cases, the signer might wish to assert that
although it is willing to go as far as signing for
the domain, it is unable or unwilling to commit to an
individual user name within their domain. It can do
so by including the domain part but not the
Local-part of the identity.INFORMATIVE DISCUSSION: This specification does not
require the value of the "i=" tag to match the
identity in any message header fields. This is
considered to be a verifier policy issue. Constraints
between the value of the "i=" tag and other
identities in other header fields seek to apply basic
authentication into the semantics of trust associated
with a role such as content author. Trust is a broad
and complex topic and trust mechanisms are subject to
highly creative attacks. The real-world efficacy of
any but the most basic bindings between the "i="
value and other identities is not well established,
nor is its vulnerability to subversion by an
attacker. Hence reliance on the use of these options
should be strictly limited. In particular, it is not
at all clear to what extent a typical end-user
recipient can rely on any assurances that might be
made by successful use of the "i=" options.Body length count (plain-text unsigned
decimal integer; OPTIONAL, default is entire body). This
tag informs the verifier of the number of octets in the
body of the email after canonicalization included in the
cryptographic hash, starting from 0 immediately following
the CRLF preceding the body. This value MUST NOT be larger
than the actual number of octets in the canonicalized
message body.INFORMATIVE IMPLEMENTATION WARNING: Use of the "l="
tag might allow display of fraudulent content without
appropriate warning to end users. The "l=" tag is
intended for increasing signature robustness when
sending to mailing lists that both modify their
content and do not sign their messages. However,
using the "l=" tag enables attacks in which an
intermediary with malicious intent modifies a message
to include content that solely benefits the attacker.
It is possible for the appended content to completely
replace the original content in the end recipient's
eyes and to defeat duplicate message detection
algorithms. Examples are described in Security
Considerations . To avoid this attack,
signers should be extremely wary of using this tag,
and verifiers might wish to ignore the tag or remove
text that appears after the specified content
length.INFORMATIVE NOTE: The value of the "l=" tag is
constrained to 76 decimal digits. This constraint is
not intended to predict the size of future messages
or to require implementations to use an integer
representation large enough to represent the maximum
possible value, but is intended to remind the
implementer to check the length of this and all other
tags during verification and to test for integer
overflow when decoding the value. Implementers may
need to limit the actual value expressed to a value
smaller than 10^76, e.g., to allow a message to fit
within the available storage space.A colon-separated list of query methods used
to retrieve the public key (plain-text; OPTIONAL, default
is "dns/txt"). Each query method is of the form
"type[/options]", where the syntax and semantics of the
options depend on the type and specified options. If there
are multiple query mechanisms listed, the choice of query
mechanism MUST NOT change the interpretation of the
signature. Implementations MUST use the recognized query
mechanisms in the order presented. errata 1381
Unrecognized query mechanisms MUST be ignored. Currently, the only valid value is "dns/txt", which defines
the DNS TXT record lookup algorithm described elsewhere in
this document. The only option defined for the "dns" query
type is "txt", which MUST be included. Verifiers and
signers MUST support "dns/txt".The selector subdividing the namespace for
the "d=" (domain) tag (plain-text; REQUIRED).Signature Timestamp (plain-text unsigned
decimal integer; RECOMMENDED, default is an unknown
creation time). The time that this signature was created.
The format is the number of seconds since 00:00:00 on
January 1, 1970 in the UTC time zone. The value is
expressed as an unsigned integer in decimal ASCII. This
value is not constrained to fit into a 31- or 32-bit
integer. Implementations SHOULD be prepared to handle
values up to at least 10^12 (until approximately AD
200,000; this fits into 40 bits). To avoid
denial-of-service attacks, implementations MAY consider any
value longer than 12 digits to be infinite. Leap seconds
are not counted. Implementations MAY ignore signatures that
have a timestamp in the future.Signature Expiration (plain-text unsigned
decimal integer; RECOMMENDED, default is no expiration).
The format is the same as in the "t=" tag, represented as
an absolute date, not as a time delta from the signing
timestamp. The value is expressed as an unsigned integer in
decimal ASCII, with the same constraints on the value in
the "t=" tag. Signatures MAY be considered invalid if the
verification time at the verifier is past the expiration
date. The verification time should be the time that the
message was first received at the administrative domain of
the verifier if that time is reliably available; otherwise
the current time should be used. The value of the "x=" tag
MUST be greater than the value of the "t=" tag if both are
present.INFORMATIVE NOTE: The "x=" tag is not intended as an
anti-replay defense.errata 1380INFORMATIVE NOTE: Due to
clock drift, the receiver's notion of when to
consider the signature expired may not match exactly
when the sender is expecting. Receivers MAY add a
'fudge factor' to allow for such possible drift. Copied header fields (dkim-quoted-printable,
but see description; OPTIONAL, default is null). A
vertical-bar-separated list of selected header fields
present when the message was signed, including both the
field name and value. It is not required to include all
header fields present at the time of signing. This field
need not contain the same header fields listed in the "h="
tag. The header field text itself must encode the vertical
bar ("|", %x7C) character (i.e., vertical bars in the "z="
text are meta-characters, and any actual vertical bar
characters in a copied header field must be encoded). Note
that all whitespace must be encoded, including whitespace
between the colon and the header field value. After
encoding, FWS MAY be added at arbitrary locations in order
to avoid excessively long lines; such whitespace is NOT
part of the value of the header field, and MUST be removed
before decoding.The header fields referenced by the "h=" tag refer to the
fields in the header of the message, not to any
copied fields in the "z=" tag. Copied header field values
are for diagnostic use.Header fields with characters requiring conversion (perhaps
from legacy MTAs that are not compliant) SHOULD be converted as
described in MIME Part Three .errata 1379errata 1386Signature applications require some level of assurance that the
verification public key is associated with the claimed signer.
Many applications achieve this by using public key certificates
issued by a trusted third party. However, DKIM can achieve a
sufficient level of security, with significantly enhanced
scalability, by simply having the verifier query the purported
signer's DNS entry (or some security-equivalent) in order to
retrieve the public key.DKIM keys can potentially be stored in multiple types of key
servers and in multiple formats. The storage and format of keys
are irrelevant to the remainder of the DKIM algorithm.Parameters to the key lookup algorithm are the type of the lookup
(the "q=" tag), the domain of the signer (the "d=" tag of the
DKIM- Signature header field), and the selector (the "s=" tag). This document defines a single binding, using DNS TXT records to
distribute the keys. Other bindings may be defined in the
future.It is expected that many key servers will choose to present
the keys in an otherwise unstructured text format (for
example, an XML form would not be considered to be
unstructured text for this purpose). The following definition
MUST be used for any DKIM key represented in an otherwise
unstructured textual form.The overall syntax is a tag-list as described in . The current valid tags are described
below. Other tags MAY be present and MUST be ignored by any
implementation that does not understand them.errata 1487Version of the DKIM key record
(plain-text; RECOMMENDED, default is "DKIM1"). If
specified, this tag MUST be set to "DKIM1" (without the
quotes). This tag MUST be the first tag in the record.
Records beginning with a "v=" tag with any other value
MUST be discarded. Note that verifiers must do a string
comparison on this value; for example, "DKIM1" is not
the same as "DKIM1.0". ABNF: Granularity of the key (plain-text;
OPTIONAL, default is "*"). This value MUST match the
Local-part of the "i=" tag of the DKIM- Signature header
field (or its default value of the empty string if "i="
is not specified), with a single, optional "*" character
matching a sequence of zero or more arbitrary characters
("wildcarding"). An email with a signing address that
does not match the value of this tag constitutes a
failed verification. The intent of this tag is to
constrain which signing address can legitimately use
this selector, for example, when delegating a key to a
third party that should only be used for special
purposes. Wildcarding allows matching for addresses such
as "user+*", "*-offer" or "foo*bar".errata
1383 An empty "g=" value never matches any
addresses.Acceptable hash algorithms (plain-text;
OPTIONAL, defaults to allowing all algorithms). A
colon-separated list of hash algorithms that might be
used. Signers and Verifiers MUST support the "sha256"
hash algorithm. Verifiers MUST also support the "sha1"
hash algorithm. errata 1381 Unrecognized
hash algorithms MUST be ignored. Key type (plain-text; OPTIONAL, default is
"rsa"). Signers and verifiers MUST support the "rsa" key
type. The "rsa" key type indicates that an ASN.1
DER-encoded RSAPublicKey (see Sections and A.1.1) is being used in the
"p=" tag. (Note: the "p=" tag further encodes the value
using the base64 algorithm.) errata 1381
Unrecognized key types MUST be ignored. Notes that might be of interest to a human
(qp-section; OPTIONAL, default is empty). No
interpretation is made by any program. This tag should
be used sparingly in any key server mechanism that has
space limitations (notably DNS). This is intended for
use by administrators, not end users.Public-key data (base64; REQUIRED). An
empty value means that this public key has been revoked.
The syntax and semantics of this tag value before being
encoded in base64 are defined by the "k=" tag. INFORMATIVE RATIONALE: If a private key has been
compromised or otherwise disabled (e.g., an
outsourcing contract has been terminated), a
signer might want to explicitly state that it
knows about the selector, but all messages using
that selector should fail verification. Verifiers
should ignore any DKIM-Signature header fields
with a selector referencing a revoked key.INFORMATIVE NOTE: A base64string is permitted to
include white space (FWS) at arbitrary places;
however, any CRLFs must be followed by at least
one WSP character. Implementors and administrators
are cautioned to ensure that selector TXT records
conform to this specification.Service Type (plain-text; OPTIONAL;
default is "*"). A colon- separated list of service
types to which this record applies. Verifiers for a
given service type MUST ignore this record if the
appropriate type is not listed. errata 1381,
1382 Unrecognized service types MUST be
ignored. Currently defined service types are as follows: matches all service typeselectronic mail (not necessarily
limited to SMTP) This tag is intended to constrain the use of
keys for other purposes, should use of DKIM be defined
by other services in the future.Flags, represented as a colon-separated
list of names (plain- text; OPTIONAL, default is no
flags set). errata 1381 Unrecognized flags
MUST be ignored. The defined flags are as follows: This domain is testing DKIM. Verifiers
MUST NOT treat messages from signers in testing mode
differently from unsigned email, even should the
signature fail to verify. Verifiers MAY wish to track
testing mode results to assist the signer.Any DKIM-Signature header fields using the
"i=" tag MUST have the same domain value on the
right-hand side of the "@" in the "i=" tag and the value
of the "d=" tag. That is, the "i=" domain MUST NOT be a
subdomain of "d=". Use of this flag is RECOMMENDED
unless subdomaining is required.Unrecognized flags MUST be ignored.errata 1532 If a v= value is not found at the
beginning of the DKIM key record, the key record MAY be
interpreted as for DomainKeys . The definition given here is
upwardly compatible with what is used for DomainKeys, with
the exception of the "g=" value. In a DomainKeys key
record, an empty "g=" value would be interpreted as being
equivalent to DKIM's "g=*". A binding using DNS TXT records as a key service is hereby
defined. All implementations MUST support this binding.All DKIM keys are stored in a subdomain named "_domainkey".
Given a DKIM-Signature field with a "d=" tag of
"example.com" and an "s=" tag of "foo.bar", the DNS query
will be for "foo.bar._domainkey.example.com". INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records
(e.g., *.bar._domainkey.example.com) do not make
sense in this context and should not be used. Note
also that wildcards within domains (e.g.,
s._domainkey.*.example.com) are not supported by the
DNS. The DNS Resource Record type used is specified by an
option to the query-type ("q=") tag. The only option
defined in this base specification is "txt", indicating the
use of a TXT Resource Record (RR). A later extension of
this standard may define another RR type.Strings in a TXT RR MUST be concatenated together before
use with no intervening whitespace. TXT RRs MUST be unique
for a particular selector name; that is, if there are
multiple records in an RRset, the results are
undefined.TXT RRs are encoded as described in Both signing and verifying message signatures start with a step
of computing two cryptographic hashes over the message. Signers
will choose the parameters of the signature as described in
Signer Actions ; verifiers will use the parameters
specified in the DKIM- Signature header field being verified. In
the following discussion, the names of the tags in the
DKIM-Signature header field that either exists (when verifying)
or will be created (when signing) are used. Note that
canonicalization () is only used to prepare the email for
signing or verifying; it does not affect the transmitted email in
any way.The signer/verifier MUST compute two hashes, one over the body of
the message and one over the selected header fields of the
message.Signers MUST compute them in the order shown. Verifiers MAY
compute them in any order convenient to the verifier, provided
that the result is semantically identical to the semantics that
would be the case had they been computed in this order.In hash step 1, the signer/verifier MUST hash the message body,
canonicalized using the body canonicalization algorithm specified
in the "c=" tag and then truncated to the length specified in the
"l=" tag. That hash value is then converted to base64 form and
inserted into (signers) or compared to (verifiers) the "bh=" tag
of the DKIM- Signature header field.In hash step 2, the signer/verifier MUST pass the following to
the hash algorithm in the indicated order. The header fields specified by the "h=" tag, in the order
specified in that tag, and canonicalized using the header
canonicalization algorithm specified in the "c=" tag. Each
header field MUST be terminated with a single CRLF. The DKIM-Signature header field that exists (verifying) or
will be inserted (signing) in the message, with the value
of the "b=" tag (including all surrounding whitespace)
deleted (i.e., treated as the empty string), canonicalized
using the header canonicalization algorithm specified in
the "c=" tag, and without a trailing CRLF. All tags and their values in the DKIM-Signature header field are
included in the cryptographic hash with the sole exception of the
value portion of the "b=" (signature) tag, which MUST be treated
as the null string. All tags MUST be included even if they might
not be understood by the verifier. The header field MUST be
presented to the hash algorithm after the body of the message
rather than with the rest of the header fields and MUST be
canonicalized as specified in the "c=" (canonicalization) tag.
The DKIM-Signature header field MUST NOT be included in its own
h= tag, although other DKIM-Signature header fields MAY be signed
(see ).When calculating the hash on messages that will be transmitted
using base64 or quoted-printable encoding, signers MUST compute
the hash after the encoding. Likewise, the verifier MUST
incorporate the values into the hash before decoding the base64
or quoted-printable text. However, the hash MUST be computed
before transport level encodings such as SMTP "dot-stuffing" (the
modification of lines beginning with a "." to avoid confusion
with the SMTP end-of-message marker, as specified in ).With the exception of the canonicalization procedure described in , the DKIM signing process treats the body of
messages as simply a string of octets. DKIM messages MAY be
either in plain-text or in MIME format; no special treatment is
afforded to MIME content. Message attachments in MIME format MUST
be included in the content that is signed.where "sig-alg" is the signature algorithm specified by the "a="
tag, "hash-alg" is the hash algorithm specified by the "a=" tag,
"canon_header" and "canon_body" are the canonicalized message
header and body (respectively) as defined in (excluding the DKIM-Signature header field),
and "DKIM-SIG" is the canonicalized DKIM-Signature header field
sans the signature value itself, but with "body-hash" included as
the "bh=" tag. INFORMATIVE IMPLEMENTERS' NOTE: Many digital signature APIs
provide both hashing and application of the RSA private key
using a single "sign()" primitive. When using such an API,
the last two steps in the algorithm would probably be
combined into a single call that would perform both the
"hash-alg" and the "sig-alg".In some circumstances, it is desirable for a domain to apply a
signature on behalf of any of its subdomains without the need to
maintain separate selectors (key records) in each subdomain. By
default, private keys corresponding to key records can be used to
sign messages for any subdomain of the domain in which they
reside; for example, a key record for the domain example.com can
be used to verify messages where the AUID ("i=" tag of the
signature) is sub.example.com, or even sub1.sub2.example.com. In
order to limit the capability of such keys when this is not
intended, the "s" flag MAY be set in the "t=" tag of the key
record, to constrain the validity of the domain of the AUID. If
the referenced key record contains the "s" flag as part of the
"t=" tag, the domain of the AUID ("i=" flag) MUST be the same as
that of the SDID (d=) domain. If this flag is absent, the domain
of the AUID MUST be the same as, or a subdomain of, the SDID.DKIM's primary task is to communicate from the Signer to a
recipient-side Identity Assessor a single Signing Domain
Identifier (SDID) that refers to a responsible identity. DKIM MAY
optionally provide a single responsible Agent or User Identifier
(AUID).Hence, DKIM's mandatory output to a receive-side Identity
Assessor is a single domain name. Within the scope of its use as
DKIM output, the name hnamas only basic domain name semantics;
any possible owner-specific semantics are outside the scope of
DKIM. That is, within its role as a DKIM identifier, additional
semantics cannot be assumed by an Identity Assessor.A receive-side DKIM verifier MUST communicate the Signing Domain
Identifier (d=) to a consuming Identity Assessor module and MAY
communicate the Agent or User Identifier (i=) if present.To the extent that a receiver attempts to intuit any structured
semantics for either of the identifiers, this is a heuristic
function that is outside the scope of DKIM's specification and
semantics. Hence, it is relegated to a higher-level service, such
as a delivery handling filter that integrates a variety of inputs
and performs heuristic analysis of them. INFORMATIVE DISCUSSION: This document does not require the
value of the SDID or AUID to match an identifier in any
other message header field. This requirement is, instead,
an assessor policy issue. The purpose of such a linkage
would be to authenticate the value in that other header
field. This, in turn, is the basis for applying a trust
assessment based on the identifier value. Trust is a broad
and complex topic and trust mechanisms are subject to
highly creative attacks. The real-world efficacy of any but
the most basic bindings between the SDID or AUID and other
identities is not well established, nor is its
vulnerability to subversion by an attacker. Hence, reliance
on the use of such bindings should be strictly limited. In
particular, it is not at all clear to what extent a typical
end-user recipient can rely on any assurances that might be
made by successful use of the SDID or AUID.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.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 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 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.The following steps are performed in order by signers.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.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.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 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.Similarly, a message that is not compliant with RFC5322, RFC2045
correct or interpret such content. See Section 8 of for examples of changes that are commonly
made. Such "corrections" may break DKIM signatures or have other
undesirable effects. Therefore, a verifier SHOULD NOT validate a
message that is not conformant. If the message is submitted to the signer with any local encoding
that will be modified before transmission, that modification to
canonical form MUST be done before signing. In
particular, bare CR or LF characters (used by some systems as a
local line separator convention) MUST be converted to the
SMTP-standard CRLF sequence before the message is signed. Any
conversion of this sort SHOULD be applied to the message actually
sent to the recipient(s), not just to the version presented to
the signing algorithm.More generally, the signer MUST sign the message as it is
expected to be received by the verifier rather than in some local
or internal form.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, 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:
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 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 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.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 dis-recommended 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: From (REQUIRED in all signatures)Sender, Reply-ToSubjectDate, Message-IDTo, CcMIME-VersionContent-Type, Content-Transfer-Encoding, Content-ID,
Content- DescriptionResent-Date, Resent-From, Resent-Sender, Resent-To,
Resent-Cc, Resent-Message-IDIn-Reply-To, ReferencesList-Id, List-Help, List-Unsubscribe, List-Subscribe,
List-Post, List-Owner, List-ArchiveThe following header fields SHOULD NOT be included in the
signature: Return-PathReceivedComments, KeywordsBcc, Resent-BccDKIM-SignatureOptional 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.The signer MUST compute the message hash as described in and then sign it using the selected
public-key algorithm. This will result in a 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.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 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.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. 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.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 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 are "v=", "a=", "b=", "bh=",
"d=", "h=", and "s=". Should there be a conflict between
this note and , 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.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 . 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): Retrieve the public key as described in using the algorithm in the "q="
tag, the domain from the "d=" tag, and the selector from
the "s=" tag.If the query for the public key fails to respond, the
verifier MAY 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. 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).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".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.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.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).If the public key data (the "p=" tag) is empty, then
this key has been revoked and the verifier MUST treat
this as a failed signature check and return PERMFAIL
(key revoked). There is no defined semantic difference
between a key that has been revoked and a key record
that has been removed.If the public key data is not suitable for use with the
algorithm and key types defined by the "a=" and "k="
tags in the DKIM- Signature header field, the verifier
MUST immediately return PERMFAIL (inappropriate key
algorithm).Given a signer and a public key, verifying a signature
consists of actions semantically equivalent to the following
steps. 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 (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.Based on the algorithm indicated in the "a=" tag,
compute the message hashes from the canonical copy as
described in .Verify that the hash of the canonicalized message body
computed in the previous step matches the hash value
conveyed in the "bh=" tag. If the hash does not match,
the verifier SHOULD ignore the signature and return
PERMFAIL (body hash did not verify). Using the signature conveyed in the "b=" tag, verify
the signature against the header hash using the
mechanism appropriate for the public key algorithm
described in the "a=" tag. If the signature does not
validate, the verifier SHOULD ignore the signature and
return PERMFAIL (signature did not verify).Otherwise, the signature has correctly verified.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 . 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.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 () 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
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.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, 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.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 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.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.DKIM has registered new 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 .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:TYPEREFERENCE 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) The "q=" tag-spec (specified in ) 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:TYPEOPTIONREFERENCE dns txt (this document) The "c=" tag-spec (specified in ) 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 body registry comprise:TYPEREFERENCE simple (this document) relaxed (this document) 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:TYPEREFERENCE v (this document) g (this document) h (this document) k (this document) n (this document) p (this document) s (this document) t (this document) The initial entries in the body registry comprise:TYPEREFERENCE simple (this document) relaxed (this document) The "k=" <key-k-tag> (specified in ) and the "a=" <sig- a-tag-k>
(specified in ) 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:TYPEREFERENCE rsa The "h=" <key-h-tag> (specified in ) and the "a=" <sig- a-tag-h>
(specified in ) 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:TYPEREFERENCE sha1 sha256 The "s=" <key-s-tag> tag (specified in ) 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:TYPEREFERENCE email (this document) * (this document) The "t=" <key-t-tag> tag (specified in ) 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:TYPEREFERENCE y (this document) s (this document) IANA has added DKIM-Signature to the "Permanent Message Header
Fields" registry (see ) for the "mail" protocol, using this
document as the reference. It has been observed that any mechanism that is introduced that
attempts to stem the flow of spam is subject to intensive attack.
DKIM needs to be carefully scrutinized to identify potential attack
vectors and the vulnerability to each. See also . Body length limits (in the form of the "l=" tag) are subject to
several potential attacks.If the body length limit does not cover a closing MIME
multipart section (including the trailing "--CRLF" portion),
then it is possible for an attacker to intercept a properly
signed multipart message and add a new body part. Depending on
the details of the MIME type and the implementation of the
verifying MTA and the receiving MUA, this could allow an
attacker to change the information displayed to an end user
from an apparently trusted source.For example, if attackers can append information to a
"text/html" body part, they may be able to exploit a bug in
some MUAs that continue to read after a "</html>"
marker, and thus display HTML text on top of already displayed
text. If a message has a "multipart/alternative" body
part, they might be able to add a new body part that is
preferred by the displaying MUA.Several receiving MUA implementations do not cease display
after a ""</html>"" tag. In particular, this allows
attacks involving overlaying images on top of existing text. INFORMATIVE EXAMPLE: Appending the following text to an
existing, properly closed message will in many MUAs
result in inappropriate data being rendered on top of
existing, correct data: If the private key for a user is resident on their computer and
is not protected by an appropriately secure mechanism, it is
possible for malware to send mail as that user and any other user
sharing the same private key. The malware would not, however, be
able to generate signed spoofs of other signers' addresses, which
would aid in identification of the infected user and would limit
the possibilities for certain types of attacks involving socially
engineered messages. This threat applies mainly to MUA-based
implementations; protection of private keys on servers can be
easily achieved through the use of specialized cryptographic
hardware.A larger problem occurs if malware on many users' computers
obtains the private keys for those users and transmits them via a
covert channel to a site where they can be shared. The
compromised users would likely not know of the misappropriation
until they receive "bounce" messages from messages they are
purported to have sent. Many users might not understand the
significance of these bounce messages and would not take
action.One countermeasure is to use a user-entered passphrase to encrypt
the private key, although users tend to choose weak passphrases
and often reuse them for different purposes, possibly allowing an
attack against DKIM to be extended into other domains.
Nevertheless, the decoded private key might be briefly available
to compromise by malware when it is entered, or might be
discovered via keystroke logging. The added complexity of
entering a passphrase each time one sends a message would also
tend to discourage the use of a secure passphrase.A somewhat more effective countermeasure is to send messages
through an outgoing MTA that can authenticate the submitter using
existing techniques (e.g., SMTP Authentication), possibly
validate the message itself (e.g., verify that the header is
legitimate and that the content passes a spam content check), and
sign the message using a key appropriate for the submitter
address. Such an MTA can also apply controls on the volume of
outgoing mail each user is permitted to originate in order to
further limit the ability of malware to generate bulk email.Since the key servers are distributed (potentially separate for
each domain), the number of servers that would need to be
attacked to defeat this mechanism on an Internet-wide basis is
very large. Nevertheless, key servers for individual domains
could be attacked, impeding the verification of messages from
that domain. This is not significantly different from the ability
of an attacker to deny service to the mail exchangers for a given
domain, although it affects outgoing, not incoming, mail.A variation on this attack is that if a very large amount of mail
were to be sent using spoofed addresses from a given domain, the
key servers for that domain could be overwhelmed with requests.
However, given the low overhead of verification compared with
handling of the email message itself, such an attack would be
difficult to mount.Since the DNS is a required binding for key services, specific
attacks against the DNS must be considered.While the DNS is currently insecure , these security problems are the
motivation behind DNS Security (DNSSEC) , and all users of the DNS will reap the
benefit of that work.DKIM is only intended as a "sufficient" method of proving
authenticity. It is not intended to provide strong cryptographic
proof about authorship or contents. Other technologies such as
OpenPGP and S/MIME address those requirements.A second security issue related to the DNS revolves around the
increased DNS traffic as a consequence of fetching selector-based
data as well as fetching signing domain policy. Widespread
deployment of DKIM will result in a significant increase in DNS
queries to the claimed signing domain. In the case of forgeries
on a large scale, DNS servers could see a substantial increase in
queries.A specific DNS security issue that should be considered by DKIM
verifiers is the name chaining attack described in Section 2.3 of . A DKIM verifier, while verifying a
DKIM-Signature header field, could be prompted to retrieve a key
record of an attacker's choosing. This threat can be minimized by
ensuring that name servers, including recursive name servers,
used by the verifier enforce strict checking of "glue" and other
additional information in DNS responses and are therefore not
vulnerable to this attack.In this attack, a spammer sends a message to be spammed to an
accomplice, which results in the message being signed by the
originating MTA. The accomplice resends the message, including
the original signature, to a large number of recipients, possibly
by sending the message to many compromised machines that act as
MTAs. The messages, not having been modified by the accomplice,
have valid signatures.Partial solutions to this problem involve the use of reputation
services to convey the fact that the specific email address is
being used for spam and that messages from that signer are likely
to be spam. This requires a real-time detection mechanism in
order to react quickly enough. However, such measures might be
prone to abuse, if for example an attacker resent a large number
of messages received from a victim in order to make them appear
to be a spammer.Large verifiers might be able to detect unusually large volumes
of mails with the same signature in a short time period. Smaller
verifiers can get substantially the same volume of information
via existing collaborative systems.When a large domain detects undesirable behavior on the part of
one of its users, it might wish to revoke the key used to sign
that user's messages in order to disavow responsibility for
messages that have not yet been verified or that are the subject
of a replay attack. However, the ability of the domain to do so
can be limited if the same key, for scalability reasons, is used
to sign messages for many other users. Mechanisms for explicitly
revoking keys on a per-address basis have been proposed but
require further study as to their utility and the DNS load they
represent.It is possible for an attacker to publish key records in DNS that
are intentionally malformed, with the intent of causing a
denial-of- service attack on a non-robust verifier
implementation. The attacker could then cause a verifier to read
the malformed key record by sending a message to one of its users
referencing the malformed record in a (not necessarily valid)
signature. Verifiers MUST thoroughly verify all key records
retrieved from the DNS and be robust against intentionally as
well as unintentionally malformed key records.Verifiers MUST be prepared to receive messages with malformed
DKIM- Signature header fields, and thoroughly verify the header
field before depending on any of its contents.An attacker could determine when a particular signature was
verified by using a per-message selector and then monitoring
their DNS traffic for the key lookup. This would act as the
equivalent of a "web bug" for verification time rather than when
the message was read.In some cases, it may be possible to extract private keys using a
remote timing attack . Implementations should consider
obfuscating the timing to prevent such attacks.Existing standards allow intermediate MTAs to reorder header
fields. If a signer signs two or more header fields of the same
name, this can cause spurious verification errors on otherwise
legitimate messages. In particular, signers that sign any
existing DKIM- Signature fields run the risk of having messages
incorrectly fail to verify.An attacker could create a large RSA signing key with a small
exponent, thus requiring that the verification key have a large
exponent. This will force verifiers to use considerable computing
resources to verify the signature. Verifiers might avoid this
attack by refusing to verify signatures that reference selectors
with public keys having unreasonable exponents.In general, an attacker might try to overwhelm a verifier by
flooding it with messages requiring verification. This is similar
to other MTA denial-of-service attacks and should be dealt with
in a similar fashion.The trust relationship described in could conceivably be used by a parent
domain to sign messages with identities in a subdomain not
administratively related to the parent. For example, the ".com"
registry could create messages with signatures using an "i="
value in the example.com domain. There is no general solution to
this problem, since the administrative cut could occur anywhere
in the domain name. For example, in the domain
"example.podunk.ca.us" there are three administrative cuts
(podunk.ca.us, ca.us, and us), any of which could create messages
with an identity in the full domain. INFORMATIVE NOTE: This is considered an acceptable risk for
the same reason that it is acceptable for domain
delegation. For example, in the example above any of the
domains could potentially simply delegate
"example.podunk.ca.us" to a server of their choice and
completely replace all DNS-served information. Note that a
verifier MAY ignore signatures that come from an unlikely
domain such as ".com", as discussed in .Many email implementations do not enforce with strictness. As discussed in DKIM processing is predicated on a valid
mail message as its input. However, DKIM implementers should be
aware of the potential effect of having loose enforcement by
email components interacting with DKIM modules.For example, a message with multiple From: header fields violates
Section 3.6 of . With the intent of providing a better
user experience, many agents tolerate these violations and
deliver the message anyway. An MUA then might elect to render to
the user the value of the last, or "top", From: field. This may
also be done simply out of the expectation that there is only
one, where a "find first" algorithm would have the same result.
Such code in an MUA can be exploited to fool the user if it is
also known that the other From: field is the one checked by
arriving message filters. Such is the case with DKIM; although
the From: field must be signed, a malformed message bearing more
than one From: field might only have the first ("bottom") one
signed, in an attempt to show the message with some "DKIM passed"
annotation while also rendering the From: field that was not
authenticated. (This can also be taken as a demonstration that
DKIM is not designed to support author validation.) A signer wishing to be resistant to this specific attack can
include in the signed header field list an additional instance of
each field that was present at signing. For example, a proper
message with one From: field could be signed using
"h=From:From:..." Because of the way header fields are
canonicalized for input to the hash function, this would prevent
additional fields from being added, after signing, as this would
render the signature invalid. Secure Hash StandardInformation Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
BodiesInnosoft International, Inc.1050 East Garvey Avenue SouthWest CovinaCA91790US+1 818 919 3600+1 818 919 3614ned@innosoft.comFirst Virtual Holdings25 Washington AvenueMorristownNJ07960US+1 201 540 8967+1 201 993 3032nsb@nsb.fv.comSTD 11, RFC 822, defines a message representation protocol
specifying considerable detail about US-ASCII message
headers, and leaves the message content, or message body,
as flat US-ASCII text. This set of documents, collectively
called the Multipurpose Internet Mail Extensions, or MIME,
redefines the format of messages to allow for(1) textual message bodies in character sets other than
US-ASCII,(2) an extensible set of different formats for non-textual
message bodies,(3) multi-part message bodies, and(4) textual header information in character sets other than
US-ASCII.These documents are based on earlier work documented in RFC
934, STD 11, and RFC 1049, but extends and revises them.
Because RFC 822 said so little about message bodies, these
documents are largely orthogonal to (rather than a revision
of) RFC 822.This initial document specifies the various header fields
used to describe the structure of MIME messages. The second
document, RFC 2046, defines the general structure of the
MIME media typing system and defines an initial set of
media types. The third document, RFC 2047, describes
extensions to RFC 822 to allow non-US-ASCII text data in
Internet mail header fields. The fourth document, RFC 2048,
specifies various IANA registration procedures for
MIME-related facilities. The fifth and final document, , describes MIME conformance criteria
as well as providing some illustrative examples of MIME
message formats, acknowledgements, and the
bibliography.These documents are revisions of RFCs 1521, 1522, and 1590,
which themselves were revisions of RFCs 1341 and 1342. An
appendix in RFC 2049 describes differences and changes from
previous versions.MIME (Multipurpose Internet
Mail Extensions) Part Three: Message Header Extensions for
Non-ASCII TextUniversity of Tennessee107 Ayres HallKnoxville TN 37996-1301moore@cs.utk.edu
Applications
Amercian Standard Code for Information
Interchangemailmultipurpose internet mail extensions STD 11, RFC 822, defines a message representation protocol
specifying considerable detail about US-ASCII message
headers, and leaves the message content, or message body,
as flat US-ASCII text. This set of documents, collectively
called the Multipurpose Internet Mail Extensions, or MIME,
redefines the format of messages to allow for (1) textual message bodies in character sets other
than US-ASCII, (2) an extensible set of different formats for
non-textual message bodies, (3) multi-part message bodies, and (4) textual header information in character sets
other than US-ASCII. These documents are based on earlier work documented in
RFC 934, STD 11, and RFC 1049, but extends and revises
them. Because RFC 822 said so little about message bodies,
these documents are largely orthogonal to (rather than a
revision of) RFC 822. This particular document is the third document in the
series. It describes extensions to RFC 822 to allow
non-US-ASCII text data in Internet mail header fields.
Other documents in this series include: RFC 2045, which specifies the various headers used to
describe the structure of MIME messages. RFC 2046, which defines the general structure of the
MIME media typing system and defines an initial set
of media types, RFC 2048, which specifies various IANA registration
procedures for MIME-related facilities, and RFC 2049, which describes MIME conformance criteria
and provides some illustrative examples of MIME
message formats, acknowledgements, and the
bibliography. These documents are revisions of RFCs 1521, 1522, and
1590, which themselves were revisions of RFCs 1341 and
1342. An appendix in RFC 2049 describes differences and
changes from previous versions. Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
ExamplesInnosoft International, Inc.1050 East Garvey Avenue SouthWest CovinaCA 91790USA+1 818 919 3600+1 818 919 3614ned@innosoft.comFirst Virtual Holdings25 Washington AvenueMorristownNJ 07960USA+1 201 540 8967+1 201 993 3032nsb@nsb.fv.com
Applications
mailmultipurpose internet mail extensions STD 11, RFC 822, defines a message representation protocol
specifying considerable detail about US-ASCII message
headers, and leaves the message content, or message body,
as flat US-ASCII text. This set of documents, collectively
called the Multipurpose Internet Mail Extensions, or MIME,
redefines the format of messages to allow for (1) textual message bodies in character sets other
than US-ASCII, (2) an extensible set of different formats for
non-textual message bodies, (3) multi-part message bodies, and (4) textual header information in character sets
other than US-ASCII. These documents are based on earlier work documented in
RFC 934, STD 11, and RFC 1049, but extends and revises
them. Because RFC 822 said so little about message bodies,
these documents are largely orthogonal to (rather than a
revision of) RFC 822. The initial document in this set, RFC 2045, specifies the
various headers used to describe the structure of MIME
messages. The second document defines the general structure
of the MIME media typing system and defines an initial set
of media types. The third document, RFC 2047, describes
extensions to RFC 822 to allow non-US- ASCII text data in
Internet mail header fields. The fourth document, RFC 2048,
specifies various IANA registration procedures for MIME-
related facilities. This fifth and final document describes
MIME conformance criteria as well as providing some
illustrative examples of MIME message formats,
acknowledgements, and the bibliography. These documents are revisions of RFCs 1521, 1522, and
1590, which themselves were revisions of RFCs 1341 and
1342. Appendix B of this document describes differences and
changes from previous versions. Key words for use in RFCs to Indicate
Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keyword In many standards track documents several words are used
to signify the requirements in the specification. These
words are often capitalized. This document defines these
words as they should be interpreted in IETF documents.
Authors who follow these guidelines should incorporate this
phrase near the beginning of their document: The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document
are to be interpreted as described in RFC 2119. Note that the force of these words is modified by the
requirement level of the document in which they are used.
Simple Mail Transfer ProtocolThis document is a self-contained specification of the
basic protocol for the Internet electronic mail transport.
[STANDARDS TRACK]Internet Message FormatThis document specifies a syntax for text messages that are
sent between computer users, within the framework of
"electronic mail" messages. [STANDARDS TRACK]Public-Key Cryptography Standards (PKCS) #1: RSA
Cryptography Specifications Version 2.1This memo represents a republication of PKCS #1 v2.1 from
RSA Laboratories' Public-Key Cryptography Standards (PKCS)
series, and change control is retained within the PKCS
process. The body of this document is taken directly from
the PKCS #1 v2.1 document, with certain corrections made
during the publication process. This memo provides
information for the Internet community.Internationalizing Domain Names in Applications
(IDNA)Until now, there has been no standard method for domain
names to use characters outside the ASCII repertoire. This
document defines internationalized domain names (IDNs) and
a mechanism called Internationalizing Domain Names in
Applications (IDNA) for handling them in a standard
fashion. IDNs use characters drawn from a large repertoire
(Unicode), but IDNA allows the non-ASCII characters to be
represented using only the ASCII characters already allowed
in so-called host names today. This backward-compatible
representation is required in existing protocols like DNS,
so that IDNs can be introduced with no changes to the
existing infrastructure. IDNA is only meant for processing
domain names, not free text. [STANDARDS TRACK]Augmented BNF for Syntax Specifications:
ABNFBrandenburg InternetWorking675 Spruce Dr.SunnyvaleCA94086US+1.408.246.8253dcrocker@bbiw.netTHUS plc.1/2 Berkeley Square, 99 Berkeley StreetGlasgowG3 7HRUKpaul.overell@thus.netABNFAugmentedBackus-NaurFormelectronicmailInternet technical specifications often need to define a
formal syntax. Over the years, a modified version of
Backus-Naur Form (BNF), called Augmented BNF (ABNF), has
been popular among many Internet specifications. The
current specification documents ABNF. It balances
compactness and simplicity, with reasonable
representational power. The differences between standard
BNF and ABNF involve naming rules, repetition,
alternatives, order- independence, and value ranges. This
specification also supplies additional rule definitions and
encoding for a core lexical analyzer of the type common to
several Internet specifications. Internet Mail ArchitectureRemote Timing Attacks are PracticalSecurity Multiparts for MIME:
Multipart/Signed and Multipart/EncryptedTrusted Information Systems3060 Washington RoadGlenwoodMD21738US+1 301 854 6889+1 301 854 5363galvin@tis.comTrusted Information Systems3060 Washington RoadGlenwoodMD21738US+1 301 854 6889+1 301 854 5363sandy@tis.comCyberCash, Inc.2086 Hunters Crest WayViennaVA22181US+1 703 620 1222+1 703 391 2651crocker@cybercash.comInnosoft International, Inc.1050 East Garvey Avenue SouthWest CovinaCA91790US+1 818 919 3600+1 818 919 3614ned@innosoft.comThis document defines a framework within which security
services may be applied to MIME body parts. MIME, an
acronym for "Multipurpose Internet Mail Extensions",
defines the format of the contents of Internet mail
messages and provides for multi-part textual and
non-textual message bodies. The new content types are
subtypes of multipart: signed and encrypted. Each will
contain two body parts: one for the protected data and one
for the control information necessary to remove the
protection. The type and contents of the control
information body parts are determined by the value of the
protocol parameter of the enclosing multipart/signed or
multipart/encrypted content type, which is required to
be present.Guidelines for
Writing an IANA Considerations Section in RFCsIBM Corporation3039 Cornwallis Ave.PO Box 12195 - BRQA/502Research Triangle ParkNC 27709-2195919-254-7798narten@raleigh.ibm.comMaxwarePirsenteretN-7005 TrondheimNorway+47 73 54 57 97Harald@Alvestrand.no
General
Internet Assigned Numbers AuthorityIANA Many protocols make use of identifiers consisting of
constants and other well-known values. Even after a
protocol has been defined and deployment has begun, new
values may need to be assigned (e.g., for a new option type
in DHCP, or a new encryption or authentication algorithm
for IPSec). To insure that such quantities have consistent
values and interpretations in different implementations,
their assignment must be administered by a central
authority. For IETF protocols, that role is provided by the
Internet Assigned Numbers Authority (IANA). In order for the IANA to manage a given name space
prudently, it needs guidelines describing the conditions
under which new values can be assigned. If the IANA is
expected to play a role in the management of a name space,
the IANA must be given clear and concise instructions
describing that role. This document discusses issues that
should be considered in formulating a policy for assigning
values to a name space and provides guidelines to document
authors on the specific text that must be included in
documents that place demands on the IANA. OpenPGP Message FormatNetwork Associates, Inc.3965 Freedom CircleSanta ClaraCA 95054USA+1 408-346-5860jon@pgp.comIKS GmbHWildenbruchstr. 1507745 JenaGermany+49-3641-675642lutz@iks-jena.deNetwork Associates, Inc.3965 Freedom CircleSanta ClaraCA 95054USAhal@pgp.comEIS CorporationClearwaterFL 33767USArodney@unitran.com
Security
pretty good privacyPGPsecurity This document defines many tag values, yet it doesn't
describe a mechanism for adding new tags (for new
features). Traditionally the Internet Assigned Numbers
Authority (IANA) handles the allocation of new values for
future expansion and RFCs usually define the procedure to
be used by the IANA. However, there are subtle (and not so
subtle) interactions that may occur in this protocol
between new features and existing features which result in
a significant reduction in over all security. Therefore,
this document does not define an extension procedure.
Instead requests to define new tag values (say for new
encryption algorithms for example) should be forwarded to
the IESG Security Area Directors for consideration or
forwarding to the appropriate IETF Working Group for
consideration. This document is maintained in order to publish all
necessary information needed to develop interoperable
applications based on the OpenPGP format. It is not a
step-by-step cookbook for writing an application. It
describes only the format and methods needed to read,
check, generate, and write conforming packets crossing any
network. It does not deal with storage and implementation
questions. It does, however, discuss implementation issues
necessary to avoid security flaws. Open-PGP software uses a combination of strong public-key
and symmetric cryptography to provide security services for
electronic communications and data storage. These services
include confidentiality, key management, authentication,
and digital signatures. This document specifies the message
formats used in OpenPGP. This document defines many tag values, yet it doesn't
describe a mechanism for adding new tags (for new
features). Traditionally the Internet Assigned Numbers
Authority (IANA) handles the allocation of new values for
future expansion and RFCs usually define the procedure to
be used by the IANA. However, there are subtle (and not so
subtle) interactions that may occur in this protocol
between new features and existing features which result in
a significant reduction in over all security. Therefore,
this document does not define an extension procedure.
Instead requests to define new tag values (say for new
encryption algorithms for example) should be forwarded to
the IESG Security Area Directors for consideration or
forwarding to the appropriate IETF Working Group for
consideration. Determining Strengths For Public Keys Used For Exchanging
Symmetric KeysImplementors of systems that use public key cryptography to
exchange symmetric keys need to make the public keys
resistant to some predetermined level of attack. That level
of attack resistance is the strength of the system, and the
symmetric keys that are exchanged must be at least as
strong as the system strength requirements. The three
quantities, system strength, symmetric key strength, and
public key strength, must be consistently matched for any
network protocol usage. While it is fairly easy to express
the system strength requirements in terms of a symmetric
key length and to choose a cipher that has a key length
equal to or exceeding that requirement, it is harder to
choose a public key that has a cryptographic strength
meeting a symmetric key strength requirement. This document
explains how to determine the length of an asymmetric key
as a function of a symmetric key strength requirement. Some
rules of thumb for estimating equivalent resistance to
large-scale attacks on various algorithms are given. The
document also addresses how changing the sizes of the
underlying large integers (moduli, group sizes, exponents,
and so on) changes the time to use the algorithms for key
exchange. This document specifies an Internet Best Current
Practices for the Internet Community, and requests
discussion and suggestions for improvements.Threat Analysis of the Domain Name System (DNS)Although the DNS Security Extensions (DNSSEC) have been
under development for most of the last decade, the IETF has
never written down the specific set of threats against
which DNSSEC is designed to protect. Among other drawbacks,
this cart-before-the-horse situation has made it difficult
to determine whether DNSSEC meets its design goals, since
its design goals are not well specified. This note attempts
to document some of the known threats to the DNS, and, in
doing so, attempts to measure to what extent (if any)
DNSSEC is a useful tool in defending against these threats.
This memo provides information for the Internet
community.Secure/Multipurpose Internet Mail Extensions (S/MIME)
Version 3.1 Message SpecificationThis document defines Secure/Multipurpose Internet Mail
Extensions (S/MIME) version 3.1. S/MIME provides a
consistent way to send and receive secure MIME data.
Digital signatures provide authentication, message
integrity, and non-repudiation with proof of origin.
Encryption provides data confidentiality. Compression can
be used to reduce data size. This document obsoletes RFC
2633. [STANDARDS TRACK]Registration Procedures for Message Header FieldsThis specification defines registration procedures for the
message header fields used by Internet mail, HTTP, Netnews
and other applications. This document specifies an Internet
Best Current Practices for the Internet Community, and
requests discussion and suggestions for improvements.DNS Security Introduction and RequirementsThe Domain Name System Security Extensions (DNSSEC) add
data origin authentication and data integrity to the Domain
Name System. This document introduces these extensions and
describes their capabilities and limitations. This document
also discusses the services that the DNS security
extensions do and do not provide. Last, this document
describes the interrelationships between the documents that
collectively describe DNSSEC. [STANDARDS TRACK]Message Submission for MailQUALCOMMAnalysis of Threats Motivating DomainKeys Identified Mail
(DKIM)This document provides an analysis of some threats against
Internet mail that are intended to be addressed by
signature-based mail authentication, in particular
DomainKeys Identified Mail. It discusses the nature and
location of the bad actors, what their capabilities are,
and what they intend to accomplish via their attacks. This
memo provides information for the Internet community.Domain-Based Email Authentication Using Public Keys
Advertised in the DNS (DomainKeys)"DomainKeys" creates a domain-level authentication
framework for email by using public key technology and the
DNS to prove the provenance and contents of an
email.</t><t> This document defines a framework for
digitally signing email on a per-domain basis. The ultimate
goal of this framework is to unequivocally prove and
protect identity while retaining the semantics of Internet
email as it is known today.</t><t> Proof and
protection of email identity may assist in the global
control of "spam" and "phishing". This memo defines a
Historic Document for the Internet community.DomainKeys Identified Mail (DKIM) SignaturesDomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key
cryptography and key server technology to permit
verification of the source and contents of messages by
either Mail Transfer Agents (MTAs) or Mail User Agents
(MUAs). The ultimate goal of this framework is to permit a
signing domain to assert responsibility for a message, thus
protecting message signer identity and the integrity of the
messages they convey while retaining the functionality of
Internet email as it is known today. Protection of email
identity may assist in the global control of "spam" and
"phishing". [STANDARDS TRACK]Message Header Field for Indicating Message Authentication
StatusThis memo defines a new header field for use with
electronic mail messages to indicate the results of message
authentication efforts. Any receiver-side software, such as
mail filters or Mail User Agents (MUAs), may use this
message header field to relay that information in a
convenient way to users or to make sorting and filtering
decisions. [STANDARDS TRACK]This section shows the complete flow of an email from submission to
final delivery, demonstrating how the various components fit
together. The key used in this example is shown in .The signing email server requires access to the private key
associated with the "brisbane" selector to generate this
signature.The signature is normally verified by an inbound SMTP server or
possibly the final delivery agent. However, intervening MTAs can
also perform this verification if they choose to do so. The
verification process uses the domain "example.com" extracted from
the "d=" tag and the selector "brisbane" from the "s=" tag in the
DKIM- Signature header field to form the DNS DKIM query for:
brisbane._domainkey.example.comSignature verification starts with the physically last Received
header field, the From header field, and so forth, in the order
listed in the "h=" tag. Verification follows with a single CRLF
followed by the body (starting with "Hi."). The email is
canonically prepared for verifying with the "simple" method. The
result of the query and subsequent verification of the signature
is stored (in this example) in the X-Authentication-Results
header field line. After successful verification, the email looks
like this: DKIM signing and validating can be used in different ways, for
different operational scenarios. This Appendix discusses some common
examples. NOTE: Descriptions in this Appendix are for informational
purposes only. They describe various ways that DKIM can be
used, given particular constraints and needs. In no case are
these examples intended to be taken as providing explanation
or guidance concerning DKIM specification details, when
creating an implementation.In the most simple scenario, a user's MUA, MSA, and Internet
(boundary) MTA are all within the same administrative
environment, using the same domain name. Therefore, all of the
components involved in submission and initial transfer are
related. However, it is common for two or more of the components
to be under independent administrative control. This creates
challenges for choosing and administering the domain name to use
for signing, and for its relationship to common email identity
header fields.Some organizations assign specific business functions to
discrete groups, inside or outside the organization. The goal,
then, is to authorize that group to sign some mail, but to
constrain what signatures they can generate. DKIM selectors
(the "s=" signature tag) and granularity (the "g=" key tag)
facilitate this kind of restricted authorization. Examples of
these outsourced business functions are legitimate email
marketing providers and corporate benefits providers.Here, the delegated group needs to be able to send messages
that are signed, using the email domain of the client company.
At the same time, the client often is reluctant to register a
key for the provider that grants the ability to send messages
for arbitrary addresses in the domain.There are multiple ways to administer these usage scenarios.
In one case, the client organization provides all of the
public query service (for example, DNS) administration, and in
another it uses DNS delegation to enable all ongoing
administration of the DKIM key record by the delegated
group.If the client organization retains responsibility for all of
the DNS administration, the outsourcing company can generate a
key pair, supplying the public key to the client company,
which then registers it in the query service, using a unique
selector that authorizes a specific From header field
Local-part. For example, a client with the domain
"example.com" could have the selector record specify
"g=winter-promotions" so that this signature is only valid for
mail with a From address of "winter-promotions@example.com".
This would enable the provider to send messages using that
specific address and have them verify properly. The client
company retains control over the email address because it
retains the ability to revoke the key at any time.If the client wants the delegated group to do the DNS
administration, it can have the domain name that is specified
with the selector point to the provider's DNS server. The
provider then creates and maintains all of the DKIM signature
information for that selector. Hence, the client cannot
provide constraints on the Local-part of addresses that get
signed, but it can revoke the provider's signing rights by
removing the DNS delegation record.PDAs demonstrate the need for using multiple keys per domain.
Suppose that John Doe wanted to be able to send messages using
his corporate email address, jdoe@example.com, and his email
device did not have the ability to make a Virtual Private
Network (VPN) connection to the corporate network, either
because the device is limited or because there are
restrictions enforced by his Internet access provider. If the
device was equipped with a private key registered for
jdoe@example.com by the administrator of the example.com
domain, and appropriate software to sign messages, John could
sign the message on the device itself before transmission
through the outgoing network of the access service
provider.Roaming users often find themselves in circumstances where it
is convenient or necessary to use an SMTP server other than
their home server; examples are conferences and many hotels.
In such circumstances, a signature that is added by the
submission service will use an identity that is different from
the user's home system.Ideally, roaming users would connect back to their home server
using either a VPN or a SUBMISSION server running with SMTP
AUTHentication on port 587. If the signing can be performed on
the roaming user's laptop, then they can sign before
submission, although the risk of further modification is high.
If neither of these are possible, these roaming users will not
be able to send mail signed using their own domain key.Stand-alone services, such as walk-up kiosks and web-based
information services, have no enduring email service
relationship with the user, but users occasionally request
that mail be sent on their behalf. For example, a website
providing news often allows the reader to forward a copy of
the article to a friend. This is typically done using the
reader's own email address, to indicate who the author is.
This is sometimes referred to as the "Evite problem", named
after the website of the same name that allows a user to send
invitations to friends.A common way this is handled is to continue to put the
reader's email address in the From header field of the
message, but put an address owned by the email posting site
into the Sender header field. The posting site can then sign
the message, using the domain that is in the Sender field.
This provides useful information to the receiving email site,
which is able to correlate the signing domain with the initial
submission email role.Receiving sites often wish to provide their end users with
information about mail that is mediated in this fashion.
Although the real efficacy of different approaches is a
subject for human factors usability research, one technique
that is used is for the verifying system to rewrite the From
header field, to indicate the address that was verified. For
example: From: John Doe via news@news-site.com
<jdoe@example.com>. (Note that such rewriting will break
a signature, unless it is done after the verification pass is
complete.)Email is often received at a mailbox that has an address
different from the one used during initial submission. In these
cases, an intermediary mechanism operates at the address
originally used and it then passes the message on to the final
destination. This mediation process presents some challenges for
DKIM signatures."Affinity addresses" allow a user to have an email address
that remains stable, even as the user moves among different
email providers. They are typically associated with college
alumni associations, professional organizations, and
recreational organizations with which they expect to have a
long-term relationship. These domains usually provide
forwarding of incoming email, and they often have an
associated Web application that authenticates the user and
allows the forwarding address to be changed. However, these
services usually depend on users sending outgoing messages
through their own service providers' MTAs. Hence, mail that is
signed with the domain of the affinity address is not signed
by an entity that is administered by the organization owning
that domain.With DKIM, affinity domains could use the Web application to
allow users to register per-user keys to be used to sign
messages on behalf of their affinity address. The user would
take away the secret half of the key pair for signing, and the
affinity domain would publish the public half in DNS for
access by verifiers.This is another application that takes advantage of user-level
keying, and domains used for affinity addresses would
typically have a very large number of user-level keys.
Alternatively, the affinity domain could handle outgoing mail,
operating a mail submission agent that authenticates users
before accepting and signing messages for them. This is of
course dependent on the user's service provider not blocking
the relevant TCP ports used for mail submission.In some cases, a recipient is allowed to configure an email
address to cause automatic redirection of email messages from
the original address to another, such as through the use of a
Unix .forward file. In this case, messages are typically
redirected by the mail handling service of the recipient's
domain, without modification, except for the addition of a
Received header field to the message and a change in the
envelope recipient address. In this case, the recipient at the
final address' mailbox is likely to be able to verify the
original signature since the signed content has not changed,
and DKIM is able to validate the message signature.There is a wide range of behaviors in services that take
delivery of a message and then resubmit it. A primary example
is with mailing lists (collectively called "forwarders"
below), ranging from those that make no modification to the
message itself, other than to add a Received header field and
change the envelope information, to those that add header
fields, change the Subject header field, add content to the
body (typically at the end), or reformat the body in some
manner. The simple ones produce messages that are quite
similar to the automated alias services. More elaborate
systems essentially create a new message.A Forwarder that does not modify the body or signed header
fields of a message is likely to maintain the validity of the
existing signature. It also could choose to add its own
signature to the message.Forwarders which modify a message in a way that could make an
existing signature invalid are particularly good candidates
for adding their own signatures (e.g.,
mailing-list-name@example.net). Since (re-)signing is taking
responsibility for the content of the message, these signing
forwarders are likely to be selective, and forward or re-sign
a message only if it is received with a valid signature or if
they have some other basis for knowing that the message is not
spoofed.A common practice among systems that are primarily
redistributors of mail is to add a Sender header field to the
message, to identify the address being used to sign the
message. This practice will remove any preexisting Sender
header field as required by . The forwarder applies a new
DKIM-Signature header field with the signature, public key,
and related information of the forwarder. The default signature is an RSA signed SHA256 digest of the
complete email. For ease of explanation, the openssl command is used
to describe the mechanism by which keys and signatures are managed.
One way to generate a 1024-bit, unencrypted private key suitable for
DKIM is to use openssl like this: For increased security, the "-passin" parameter can also
be added to encrypt the private key. Use of this parameter will
require entering a password for several of the following steps.
Servers may prefer to use hardware cryptographic support.When a DKIM signature is verified, the processing system sometimes
makes the result available to the recipient user's MUA. How to
present this information to the user in a way that helps them is a
matter of continuing human factors usability research. The tendency
is to have the MUA highlight the SDID, in an attempt to show the
user the identity that is claiming responsibility for the message.
An MUA might do this with visual cues such as graphics, or it might
include the address in an alternate view, or it might even rewrite
the original From address using the verified information. Some MUAs
might indicate which header fields were protected by the validated
DKIM signature. This could be done with a positive indication on the
signed header fields, with a negative indication on the unsigned
header fields, by visually hiding the unsigned header fields, or
some combination of these. If an MUA uses visual indications for
signed header fields, the MUA probably needs to be careful not to
display unsigned header fields in a way that might be construed by
the end user as having been signed. If the message has an l= tag
whose value does not extend to the end of the message, the MUA might
also hide or mark the portion of the message body that was not
signed.The aforementioned information is not intended to be exhaustive. The
MUA may choose to highlight, accentuate, hide, or otherwise display
any other information that may, in the opinion of the MUA author, be
deemed important to the end user.The previous IETF version of DKIM was edited by: Eric Allman, Jon Callas, Mark
Delany, Miles Libbey, Jim Fenton and Michael Thomas.That specification was the result of an extended, collaborative
effort, including participation by: Russ Allbery, Edwin Aoki, Claus
Assmann, Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve
Bellovin, Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis
Dayman, Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark
Fanto, Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur
Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman,
Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig
Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S.
Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon
Longsdale, David Margrave, Justin Mason, David Mayne, Thierry
Moreau, Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim
Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell,
Christian Renaud, Scott Renfro, Neil Rerup, Eric Rescorla, Dave
Rossetti, Hector Santos, Jim Schaad, the Spamhaus.org team, Malte S.
Stretz, Robert Sanders, Rand Wacker, Sam Weiler, and Dan Wing. The earlier DomainKeys was a primary source from which DKIM was
derived. Further information about DomainKeys is at .Revise summary Introduction to reflect "authentic labeling"
rather than "message authentication".Review interoperability items to consider dropping unused
features.Review retention of other parameters, such as l="signatures inside parts shouldn't be processed"? Incorporate Updates RFCAdded RFC 5598 referenceErrata ID: 1376 - Verified - sha value for empty bodiesErrata ID: 1377 - Verified - no trailing CR-LFErrata ID: 1378 - Verified - no default algorithmErrata ID: 1379 - Verified - z= ABNFErrata ID: 1384 - Verified - clarify relaxed step orderErrata ID: 1461 - Verified - h= ABNF Errata ID: 1487 - Verified - v= ABNFErrata ID: 1380 - Held for Document Update - x= fudge
factorErrata ID: 1381 - Held for Document Update - unknown q=
value, h=/k=/s=/t= valueErrata ID: 1382 - Held for Document Update - unknown s=
values (dup of 1381)Errata ID: 1383 - Held for Document Update - add g=
exampleErrata ID: 1386 - Held for Document Update - fix
DKIM-Signature exampleErrata ID: 1532 - Held for Document Update - "v=DKIM1"Errata ID: 1596 - Held for Document Update - *value* of the
b= tagAdd Authentication Results RFC 5451 to 6.2