DomainKeys Identified Mail (DKIM)
SignaturesSendmail, Inc.6425 Christie Ave, Suite 400EmeryvilleCA94608USA+1 510 594 5501eric+dkim@sendmail.orgPGP Corporation3460 West BayshorePalo AltoCA94303USA+1 650 319 9016jon@pgp.comYahoo! Inc701 First AvenueSunnyvaleCA95087USA+1 408 349 6831markd+dkim@yahoo-inc.comYahoo! Inc701 First AvenueSunnyvaleCA95087USAmlibbeymail-mailsig@yahoo.comCisco Systems, Inc.MS SJ-9/2170 W. Tasman DriveSan JoseCA95134-1706USA+1 408 526 5914fenton@cisco.comCisco Systems, Inc.MS SJ-9/2170 W. Tasman DriveSan JoseCA95134-1706+1 408 525 5386mat@cisco.comDKIMinternet mailauthenticationspamphishingspoofingdigital signatureDomainKeys 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".DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain to
claim responsibility for the introduction of a message into the mail
stream. Message recipients can verify the signature by querying the
signer's domain directly to retrieve the appropriate public key, and
thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain.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. Syntax
descriptions use the form described in Augmented
BNF for Syntax Specifications .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.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 formal ABNF for these are (WSP and LWSP are given for
information only):The definition of FWS is identical to that in except for the exclusion of obs-FWS.The following ABNF tokens are used elsewhere in this document: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-domainThe 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 RFC 2045 does not
obey the rules of RFC 4234 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.ABNF:' - '~'
; Characters not listed as "mail-safe" in
; RFC 2049 are also not recommended.]]>INFORMATIVE NOTE: DKIM-Quoted-Printable differs from
Quoted-Printable as defined in RFC 2045 in several important
ways:Whitespace in the input text, including CR and LF, must be
encoded. RFC 2045 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, RFC 2045 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 the 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.ABNF:The number of public keys and corresponding selectors for each
domain is determined by the domain owner. Many domain owners will be
satisfied with just one selector, whereas administratively distributed
organizations 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
user name, 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 ). 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.Formally, the syntax rules are as follows:Note that WSP is allowed anywhere around tags. In particular, any
WSP after the "=" and any WSP before the terminating ";" is not part
of the value; however, WSP inside the value is significant.Tags MUST be interpreted in a case-sensitive manner. Values MUST be
processed as case sensitive unless the specific tag description of
semantics specifies case insensitivity.Tags with duplicate names MUST NOT occur within a single tag-list;
if a tag name does occur more than once, the entire tag-list is
invalid.Whitespace within a value MUST be retained unless explicitly
excluded by the specific tag description.Tag=value pairs that represent the default value MAY be included to
aid legibility.Unrecognized tags MUST be ignored.Tags that have an empty value are not the same as omitted tags. An
omitted tag is treated as having the default value; a tag with an
empty value explicitly designates the empty string as the value. 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. The rsa-sha256 algorithm is the default if no algorithm is
specified. Verifiers MUST implement both 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.Empirical evidence demonstrates that some mail servers and relay
systems modify email in transit, potentially invalidating a signature.
There are two competing perspectives on such modifications. For most
signers, mild modification of email is immaterial to the
authentication status of the email. 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.The "relaxed" body canonicalization algorithm:Ignores all whitespace at the end of lines. Implementations
MUST NOT remove the CRLF at the end of the line.Reduces all sequences of WSP within a line to a single SP
character.Ignores all empty lines at the end of the message body.
"Empty line" is defined in .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.Example 1: A message reading: X
B : Y Z C
D E ]]>when canonicalized using relaxed canonicalization for
both header and body results in a header reading:
b:Y Z ]]>and a body reading: C
D E ]]>Example 2: The same message canonicalized using simple
canonicalization for both header and body results in a header
reading: X
B : Y Z ]]>and a body reading: C
D E ]]>Example 3: When processed using relaxed header
canonicalization and simple body canonicalization, the
canonicalized version has a header of:
b:Y Z ]]>and a body reading: C
D E ]]>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".ABNF: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.ABNF: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.ABNF: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.ABNF: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".ABNF:The domain of the signing entity (plain-text;
REQUIRED). This is the domain that will be queried for the public
key. This domain MUST be the same as or a parent domain of the
"i=" tag (the signing identity, as described below), or it MUST
meet the requirements for parent domain signing described in . When presented with a
signature that does not meet these requirement, verifiers MUST
consider the signature invalid.Internationalized domain names MUST be encoded as described in
.ABNF: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.ABNF:INFORMATIVE 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.Identity of the user or agent (e.g., a mailing
list manager) on behalf of which this message is signed
(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.ABNF: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 may 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 document 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.ABNF: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.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".ABNF:The selector subdividing the namespace for the
"d=" (domain) tag (plain-text; REQUIRED).ABNF: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.ABNF: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.ABNF: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 metacharacters, 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 RFC 2822 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 .ABNF:INFORMATIVE EXAMPLE of a signature header field spread across
multiple continuation lines: Signature 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. Version 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+*" or "*-offer". An
empty "g=" value never matches any addresses.ABNF: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.ABNF: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 3.1 and A.1.1) is being
used in the "p=" tag. (Note: the "p=" tag further encodes the
value using the base64 algorithm.)ABNF: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.ABNF: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.ABNF: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. 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.ABNF: Flags, represented as a colon-separated list of
names (plain-text; OPTIONAL, default is no flags set). 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.ABNF: Unrecognized flags MUST be ignored.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 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.More formally, the algorithm for the signature is as
follows: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; e.g., a
key record for the domain example.com can be used to verify messages
where the signing identity ("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 record to exactly the domain
of the signing identity. If the referenced key record contains the
s flag as part of the "t=" tag, the domain
of the signing identity ("i=" flag) MUST be the same as that of the d=
domain. If this flag is absent, the domain of the signing identity
MUST be the same as, or a subdomain of, the d= domain. Key records
that are not intended for use with subdomains SHOULD specify the
s flag in the "t=" tag.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 MIME
Part One before signing. Such conversion is outside the scope
of DKIM; the actual message SHOULD be converted to 7-bit MIME by an
MUA or MSA prior to presentation to the DKIM algorithm.If the message is submitted to the signer with any local encoding
that will be modified before transmission, that modification to
canonical 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:
Received:
Received: ]]> then the resulting DKIM-Signature header field should
read: 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 disrecommended header fields is signed. Note that verifiers do
have the option of ignoring signatures that do not cover a sufficient
portion of the header or body, just as they may ignore signatures from
an identity they do not trust.The following header fields SHOULD be included in the signature, if
they are present in the message being signed: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. 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 a verifier system should make, but an authenticated email
presents an opportunity to a receiving system that unauthenticated
email cannot. Specifically, an authenticated email creates a
predictable identifier by which other decisions can reliably be
managed, such as trust and reputation. Conversely, unauthenticated
email lacks a reliable identifier that can be used to assign trust and
reputation. It is reasonable to treat unauthenticated email as lacking
any trust and having no positive reputation.In general, verifiers SHOULD NOT reject messages solely on the
basis of a lack of signature or an unverifiable signature; such
rejection would cause severe interoperability problems. However, if
the verifier does opt to reject such messages (for example, when
communicating with a peer who, by prior agreement, agrees to only send
signed messages), and the verifier runs synchronously with the SMTP
session and a signature is missing or does not verify, the MTA SHOULD
use a 550/5.7.x reply code.If it is not possible to fetch the public key, perhaps because the
key server is not available, a temporary failure message MAY be
generated using a 451/4.7.5 reply code, such as: 451 4.7.5 Unable to verify signature - key server
unavailableTemporary failures such as inability to access the key server
or other external service are the only conditions that SHOULD use a
4xx SMTP reply code. In particular, cryptographic signature
verification failures MUST NOT return 4xx SMTP replies.Once the signature has been verified, that information MUST be
conveyed to higher-level systems (such as explicit allow/whitelists
and reputation systems) and/or to the end user. If the message is
signed on behalf of any address other than that in the From: header
field, the mail system SHOULD take pains to ensure that the actual
signing identity is clear to the reader.The verifier MAY treat unsigned header fields with extreme
skepticism, including marking them as untrusted or even deleting them
before display to the end user.While the symptoms of a failed verification are obvious — the
signature doesn't verify — establishing the exact cause can be
more difficult. If a selector cannot be found, is that because the
selector has been removed, or was the value changed somehow in
transit? If the signature line is missing, is that because it was
never there, or was it removed by an overzealous filter? For
diagnostic purposes, the exact reason why the verification fails
SHOULD be made available to the policy module and possibly recorded in
the system logs. If the email cannot be verified, then it SHOULD be
rendered the same as all unverified email regardless of whether or not
it looks like it was signed.DKIM introduces some new namespaces that have been registered with
IANA. In all cases, new values are assigned only for values that have
been documented in a published RFC that has IETF Consensus .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:TYPEREFERENCEv(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:TYPEOPTIONREFERENCEdnstxt(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 header registry
comprise:TYPEREFERENCEsimple(this document)relaxed(this document)The initial entries in the body registry
comprise:TYPEREFERENCEsimple(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:TYPEREFERENCEv(this document)g(this document)h(this document)k(this document)n(this document)p(this document)s(this document)t(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:TYPEREFERENCErsaThe "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:TYPEREFERENCEsha1sha256The "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:TYPEREFERENCEemail(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:TYPEREFERENCEy(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 the
DNS Threat Analysis. 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 .Secure Hash StandardU.S. Department of CommerceInformation 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 header
fields, 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 field 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, RFC
2049, 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 field
Extensions for Non-ASCII TextUniversity of Tennessee107 Ayres HallKnoxville TN 37996-1301moore@cs.utk.edu
Applications
American Standard Code for Information
Interchangemailmultipurpose internet mail extensionsSTD 11, RFC 822, defines a message representation protocol
specifying considerable detail about US-ASCII message header
fields, 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 field 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:which specifies the various header
fields used to describe the structure of MIME messages.which defines the general structure of
the MIME media typing system and defines an initial set of
media types,which specifies various IANA
registration procedures for MIME-related facilities, andwhich 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.Key words for use in RFCs to Indicate
Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138+1 617 495 3864sob@harvard.edu
General
keywordIn 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 ProtocolKInternet Message FormatPublic-Key Cryptography Standards (PKCS) #1: RSA Cryptography
Specifications Version 2.1Internationalizing 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:
ABNFInternet Mail Consortium675 Spruce Dr.SunnyvaleCA94086US+1 408 246 8253+1 408 249 6205dcrocker@imc.orgDemon Internet Ltd.Dorking Business ParkDorkingSurreyEnglandRH4 1HNUKpaulo@turnpike.comABNFAugmentedBackus-NaurFormelectronicmailRemote Timing Attacks are PracticalProc. 12th USENIX Security SymposiumSecurity 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
RFCsTOpenPGP 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 privacyPGPsecurityThis 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 Keys2HS/MIME Version 3 Message SpecificationWorldtalk17720 NE 65th StreetSuite 201RedmondWA98052US+1 425 376 0225blaker@deming.comThreat Analysis of the Domain Name System (DNS)Registration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orghttp://www.ninebynine.netBEA Systems235 Montgomery St., Level 15San FranciscoCA94104USAmnot@pobox.comHP Labs1501 Page Mill RoadPalo AltoCA94304USJeffMogul@acm.orgDNS Security Introduction and RequirementsAnalysis 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)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 .
To: Suzie Q
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.]]>This email is signed by the example.com outbound email server and
now looks like this:
To: Suzie Q
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.]]>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: Signature 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:
To: Suzie Q
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.]]>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.The genrsa step results in the file
rsa.private containing the key information similar to this: To extract the public-key component from the private key, use openssl
like this: This results in the file rsa.public containing the key
information similar to this: This public-key data (without the BEGIN and END tags) is placed
in the DNS: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 address associated with this signing identity in some
way, in an attempt to show the user the address from which the mail was
sent. 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 authors wish to thank 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 Fältström, Mark Fanto,
Stephen Farrell, Duncan Findlay, Elliot Gillum, Ólafur
Guðmundsson, 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 for their valuable suggestions and constructive criticism.The DomainKeys specification was a primary source from which this
specification has been derived. Further information about DomainKeys is
at .
]]>