DomainKeys Identified Mail (DKIM) Signatures :: RFC4871
Network Working Group E. Allman
Request for Comments: 4871 Sendmail, Inc.
Obsoletes: 4870 J. Callas
Category: Standards Track PGP Corporation
M. Delany
M. Libbey
Yahoo! Inc
J. Fenton
M. Thomas
Cisco Systems, Inc.
May 2007
DomainKeys Identified Mail (DKIM) Signatures
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
DomainKeys 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".
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 5
1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Simple Key Management . . . . . . . . . . . . . . . . . . 5
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5
2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 6
2.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7
2.6. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 7
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10
3.3. Signing and Verification Algorithms . . . . . . . . . . . 11
3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 13
3.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 17
3.6. Key Management and Representation . . . . . . . . . . . . 25
3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 29
3.8. Signing by Parent Domains . . . . . . . . . . . . . . . . 31
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 32
4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 32
4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 33
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 34
5.1. Determine Whether the Email Should Be Signed and by
Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2. Select a Private Key and Corresponding Selector
Information . . . . . . . . . . . . . . . . . . . . . . . 35
5.3. Normalize the Message to Prevent Transport Conversions . . 35
5.4. Determine the Header Fields to Sign . . . . . . . . . . . 36
5.5. Recommended Signature Content . . . . . . . . . . . . . . 38
5.6. Compute the Message Hash and Signature . . . . . . . . . . 39
5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 40
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 40
6.1. Extract Signatures from the Message . . . . . . . . . . . 41
6.2. Communicate Verification Results . . . . . . . . . . . . . 46
6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 47
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 48
7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 49
7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 49
7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 50
7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 50
7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 51
7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 51
7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 52
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7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 52
8. Security Considerations . . . . . . . . . . . . . . . . . . . 52
8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 52
8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 53
8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 54
8.4. Attacks Against the DNS . . . . . . . . . . . . . . . . . 54
8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 55
8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 55
8.7. Intentionally Malformed Key Records . . . . . . . . . . . 56
8.8. Intentionally Malformed DKIM-Signature Header Fields . . . 56
8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 56
8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 56
8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 56
8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 56
8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 57
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
9.1. Normative References . . . . . . . . . . . . . . . . . . . 57
9.2. Informative References . . . . . . . . . . . . . . . . . . 58
Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 60
A.1. The user composes an email . . . . . . . . . . . . . . . . 60
A.2. The email is signed . . . . . . . . . . . . . . . . . . . 61
A.3. The email signature is verified . . . . . . . . . . . . . 61
Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 62
B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 63
B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 65
Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 67
Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 68
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 69
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1. Introduction
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) [RFC1847], OpenPGP [RFC2440]) in that:
o 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;
o there is no dependency on public and private key pairs being
issued by well-known, trusted certificate authorities;
o there is no dependency on the deployment of any new Internet
protocols or services for public key distribution or revocation;
o signature verification failure does not force rejection of the
message;
o no attempt is made to include encryption as part of the mechanism;
o message archiving is not a design goal.
DKIM:
o is compatible with the existing email infrastructure and
transparent to the fullest extent possible;
o requires minimal new infrastructure;
o can be implemented independently of clients in order to reduce
deployment time;
o can be deployed incrementally;
o allows delegation of signing to third parties.
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1.1. Signing Identity
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.
1.2. Scalability
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.
1.3. Simple Key Management
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.
2. Terminology and Definitions
This section defines terms used in the rest of the document. Syntax
descriptions use the form described in Augmented BNF for Syntax
Specifications [RFC4234].
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 [RFC2119].
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2.1. Signers
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.
2.2. Verifiers
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.
2.3. Whitespace
There are three forms of whitespace:
o WSP represents simple whitespace, i.e., a space or a tab character
(formal definition in [RFC4234]).
o LWSP is linear whitespace, defined as WSP plus CRLF (formal
definition in [RFC4234]).
o 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):
WSP = SP / HTAB
LWSP = *(WSP / CRLF WSP)
FWS = [*WSP CRLF] 1*WSP
The definition of FWS is identical to that in [RFC2822] except for
the exclusion of obs-FWS.
2.4. Common ABNF Tokens
The following ABNF tokens are used elsewhere in this document:
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
base64string = 1*(ALPHA / DIGIT / "+" / "/" / [FWS])
[ "=" [FWS] [ "=" [FWS] ] ]
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2.5. Imported ABNF Tokens
The following tokens are imported from other RFCs as noted. Those
RFCs should be considered definitive.
The following tokens are imported from [RFC2821]:
o "Local-part" (implementation warning: this permits quoted strings)
o "sub-domain"
The following tokens are imported from [RFC2822]:
o "field-name" (name of a header field)
o "dot-atom-text" (in the Local-part of an email address)
The following tokens are imported from [RFC2045]:
o "qp-section" (a single line of quoted-printable-encoded text)
o "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 [RFC4234]. These
are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
etc.
2.6. DKIM-Quoted-Printable
The DKIM-Quoted-Printable encoding syntax resembles that described in
Quoted-Printable [RFC2045], 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.
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ABNF:
dkim-quoted-printable =
*(FWS / hex-octet / dkim-safe-char)
; hex-octet is from RFC 2045
dkim-safe-char = %x21-3A / %x3C / %x3E-7E
; '!' - ':', '<', '>' - '~'
; 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:
1. 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.
2. 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.
3. The "soft line break" syntax ("=" as the last non-whitespace
character on the line) does not apply.
4. 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).
3. Protocol Elements
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 (Section 5) and Verifier Actions (Section 6)). NOTE: This
section must be read in the context of those sections.
3.1. Selectors
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.
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Selectors are needed to support some important use cases. For
example:
o 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.
o Domains that want to allow frequent travelers to send messages
locally without the need to connect with a particular MSA.
o "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:
selector = sub-domain *( "." sub-domain )
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
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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.
3.2. Tag=Value Lists
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 [RFC2045], Section 6.8), "qp-section" (ibid,
Section 6.7), or "dkim-quoted-printable" (as defined in Section 2.6).
The name of the tag will determine the encoding of each value.
Unencoded semicolon (";") characters MUST NOT occur in the tag value,
since that separates tag-specs.
INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined
below (as "tag-value") only includes 7-bit characters, an
implementation that wished to anticipate future standards would be
advised not to preclude the use of UTF8-encoded text in tag=value
lists.
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Formally, the syntax rules are as follows:
tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tag-name = ALPHA 0*ALNUMPUNC
tag-value = [ tval 0*( 1*(WSP / FWS) tval ) ]
; WSP and FWS prohibited at beginning and end
tval = 1*VALCHAR
VALCHAR = %x21-3A / %x3C-7E
; EXCLAMATION to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / "_"
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.
3.3. Signing and Verification Algorithms
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.
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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.
3.3.1. The rsa-sha1 Signing Algorithm
The rsa-sha1 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-1 [FIPS.180-2.2002] as the hash-alg.
That hash is then signed by the signer using the RSA algorithm
(defined in PKCS#1 version 1.5 [RFC3447]) 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.
3.3.2. The rsa-sha256 Signing Algorithm
The rsa-sha256 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-256 [FIPS.180-2.2002] as the hash-alg.
That hash is then signed by the signer using the RSA algorithm
(defined in PKCS#1 version 1.5 [RFC3447]) 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.
3.3.3. Key Sizes
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:
o The practical constraint that large (e.g., 4096 bit) keys may not
fit within a 512-byte DNS UDP response packet
o The security constraint that keys smaller than 1024 bits are
subject to off-line attacks
o Larger keys impose higher CPU costs to verify and sign email
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o Keys can be replaced on a regular basis, thus their lifetime can
be relatively short
o The security goals of this specification are modest compared to
typical goals of other systems that employ digital signatures
See [RFC3766] for further discussion on selecting key sizes.
3.3.4. Other Algorithms
Other algorithms MAY be defined in the future. Verifiers MUST ignore
any signatures using algorithms that they do not implement.
3.4. Canonicalization
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 [RFC2822], 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
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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 Section 5.3 for information about
normalizing the message.
3.4.1. The "simple" Header Canonicalization Algorithm
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.
3.4.2. The "relaxed" Header Canonicalization Algorithm
The "relaxed" header canonicalization algorithm MUST apply the
following steps in order:
o Convert all header field names (not the header field values) to
lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
o Unfold all header field continuation lines as described in
[RFC2822]; 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.
o 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.
o Delete all WSP characters at the end of each unfolded header field
value.
o 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.
3.4.3. The "simple" Body Canonicalization Algorithm
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
Allman, et al. Standards Track [Page 14]
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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.
3.4.4. The "relaxed" Body Canonicalization Algorithm
The "relaxed" body canonicalization algorithm:
o Ignores all whitespace at the end of lines. Implementations MUST
NOT remove the CRLF at the end of the line.
o Reduces all sequences of WSP within a line to a single SP
character.
o Ignores all empty lines at the end of the message body. "Empty
line" is defined in Section 3.4.3.
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.
3.4.5. Body Length Limits
A body length count MAY be specified to limit the signature
calculation to an initial prefix of the body text, measured in
octets. If the body length count is not specified, the entire
message body is signed.
INFORMATIVE RATIONALE: This capability is provided because it is
very common for mailing lists to add trailers to messages (e.g.,
instructions 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
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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.
3.4.6. Canonicalization Examples (INFORMATIVE)
In the following examples, actual whitespace is used only for
clarity. The actual input and output text is designated using
bracketed descriptors: "" for a space character, "" for a
tab character, and "" for a carriage-return/line-feed sequence.
For example, "X Y" and "XY" represent the same three
characters.
Example 1: A message reading:
A: X
B : Y
Z
C
D E
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when canonicalized using relaxed canonicalization for both header and
body results in a header reading:
a:X
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:
A: 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:
a:X
b:Y Z
and a body reading:
C
D E
3.5. The DKIM-Signature Header Field
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 Section 3.2.
The DKIM-Signature header field SHOULD be treated as though it were a
trace header field as defined in Section 3.6 of [RFC2822], and hence
SHOULD NOT be reordered and SHOULD be prepended to the message.
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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 [RFC2045], 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 Section 2.6.
Tags on the DKIM-Signature header field along with their type and
requirement status are shown below. Unrecognized tags MUST be
ignored.
v= 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:
sig-v-tag = %x76 [FWS] "=" [FWS] "1"
INFORMATIVE NOTE: DKIM-Signature version numbers are expected
to increase arithmetically as new versions of this
specification are released.
a= 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 Section 3.3 for a
description of algorithms.
ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / x-sig-a-tag-k
sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT) ; for later extension
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT) ; for later extension
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b= 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 (Section 5) for how the signature is
computed.
ABNF:
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data
sig-b-tag-data = base64string
bh= 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 Section 3.7 for how the body hash is computed.
ABNF:
sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
sig-bh-tag-data = base64string
c= 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 Section 3.4. 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:
sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg
["/" sig-c-tag-alg]
sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg
x-sig-c-tag-alg = hyphenated-word ; for later extension
d= 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 Section 3.8.
When presented with a signature that does not meet these
requirement, verifiers MUST consider the signature invalid.
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Internationalized domain names MUST be encoded as described in
[RFC3490].
ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] domain-name
domain-name = sub-domain 1*("." sub-domain)
; from RFC 2821 Domain, but excluding address-literal
h= 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 Section 5.4 for a discussion of
choosing header fields to sign.
ABNF:
sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name
0*( *FWS ":" *FWS hdr-name )
hdr-name = field-name
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.
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i= 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 [RFC3490] using the "ToASCII" function.
ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name
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.
l= 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.
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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 (Section 8). 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:
sig-l-tag = %x6c [FWS] "=" [FWS] 1*76DIGIT
q= 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".
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ABNF:
sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([FWS] ":" [FWS] sig-q-tag-method)
sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
["/" x-sig-q-tag-args]
x-sig-q-tag-type = hyphenated-word ; for future extension
x-sig-q-tag-args = qp-hdr-value
s= The selector subdividing the namespace for the "d=" (domain) tag
(plain-text; REQUIRED).
ABNF:
sig-s-tag = %x73 [FWS] "=" [FWS] selector
t= 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:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
x= 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.
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INFORMATIVE NOTE: The "x=" tag is not intended as an anti-replay
defense.
ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT
z= 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 [RFC2822] compliant) SHOULD be converted
as described in MIME Part Three [RFC2047].
ABNF:
sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy
*( [FWS] "|" sig-z-tag-copy )
sig-z-tag-copy = hdr-name ":" qp-hdr-value
qp-hdr-value = dkim-quoted-printable ; with "|" encoded
INFORMATIVE EXAMPLE of a signature header field spread across
multiple continuation lines:
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DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane;
c=simple; q=dns/txt; i=@eng.example.net;
t=1117574938; x=1118006938;
h=from:to:subject:date;
z=From:foo@eng.example.net|To:joe@example.com|
Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR
3.6. Key Management and Representation
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).
public_key = dkim_find_key(q_val, d_val, s_val)
This document defines a single binding, using DNS TXT records to
distribute the keys. Other bindings may be defined in the future.
3.6.1. Textual Representation
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 Section 3.2. The
current valid tags are described below. Other tags MAY be present
and MUST be ignored by any implementation that does not understand
them.
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v= 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:
key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1"
g= 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:
key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart
key-g-tag-lpart = [dot-atom-text] ["*" [dot-atom-text] ]
h= 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-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg
0*( [FWS] ":" [FWS] key-h-tag-alg )
key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg
x-key-h-tag-alg = hyphenated-word ; for future extension
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k= 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 [ITU.X660.1997] RSAPublicKey
[RFC3447] (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:
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type
key-k-tag-type = "rsa" / x-key-k-tag-type
x-key-k-tag-type = hyphenated-word ; for future extension
n= 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:
key-n-tag = %x6e [FWS] "=" [FWS] qp-section
p= 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:
key-p-tag = %x70 [FWS] "=" [ [FWS] base64string ]
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.
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s= 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 types
email electronic 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:
key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type
0*( [FWS] ":" [FWS] key-s-tag-type )
key-s-tag-type = "email" / "*" / x-key-s-tag-type
x-key-s-tag-type = hyphenated-word ; for future extension
t= Flags, represented as a colon-separated list of names (plain-
text; OPTIONAL, default is no flags set). The defined flags are
as follows:
y 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.
s 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:
key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag
0*( [FWS] ":" [FWS] key-t-tag-flag )
key-t-tag-flag = "y" / "s" / x-key-t-tag-flag
x-key-t-tag-flag = hyphenated-word ; for future extension
Unrecognized flags MUST be ignored.
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3.6.2. DNS Binding
A binding using DNS TXT records as a key service is hereby defined.
All implementations MUST support this binding.
3.6.2.1. Namespace
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.
3.6.2.2. Resource Record Types for Key Storage
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 Section 3.6.1.
3.7. Computing the Message Hashes
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
(Section 5); 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 (Section 3.4) 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.
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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.
1. 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.
2. 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 Section 4).
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 [RFC2821]).
With the exception of the canonicalization procedure described in
Section 3.4, the DKIM signing process treats the body of messages as
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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:
body-hash = hash-alg(canon_body)
header-hash = hash-alg(canon_header || DKIM-SIG)
signature = sig-alg(header-hash, key)
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 Section 3.4 (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".
3.8. Signing by Parent Domains
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.
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4. Semantics of Multiple Signatures
4.1. Example Scenarios
There are many reasons why a message might have multiple signatures.
For example, a given signer might sign multiple times, perhaps with
different hashing or signing algorithms during a transition phase.
INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be
insufficiently strong, and DKIM usage transitions to SHA-1024. A
signer might immediately sign using the newer algorithm, but
continue to sign using the older algorithm for interoperability
with verifiers that had not yet upgraded. The signer would do
this by adding two DKIM-Signature header fields, one using each
algorithm. Older verifiers that did not recognize SHA-1024 as an
acceptable algorithm would skip that signature and use the older
algorithm; newer verifiers could use either signature at their
option, and all other things being equal might not even attempt to
verify the other signature.
Similarly, a signer might sign a message including all headers and no
"l=" tag (to satisfy strict verifiers) and a second time with a
limited set of headers and an "l=" tag (in anticipation of possible
message modifications in route to other verifiers). Verifiers could
then choose which signature they preferred.
INFORMATIVE EXAMPLE: A verifier might receive a message with two
signatures, one covering more of the message than the other. If
the signature covering more of the message verified, then the
verifier could make one set of policy decisions; if that signature
failed but the signature covering less of the message verified,
the verifier might make a different set of policy decisions.
Of course, a message might also have multiple signatures because it
passed through multiple signers. A common case is expected to be
that of a signed message that passes through a mailing list that also
signs all messages. Assuming both of those signatures verify, a
recipient might choose to accept the message if either of those
signatures were known to come from trusted sources.
INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing
lists to which they have subscribed and that have acceptable anti-
abuse policies so as to accept messages sent to that list even
from unknown authors. They might also subscribe to less trusted
mailing lists (e.g., those without anti-abuse protection) and be
willing to accept all messages from specific authors, but insist
on doing additional abuse scanning for other messages.
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Another related example of multiple signers might be forwarding
services, such as those commonly associated with academic alumni
sites.
INFORMATIVE EXAMPLE: A recipient might have an address at
members.example.org, a site that has anti-abuse protection that is
somewhat less effective than the recipient would prefer. Such a
recipient might have specific authors whose messages would be
trusted absolutely, but messages from unknown authors that had
passed the forwarder's scrutiny would have only medium trust.
4.2. Interpretation
A signer that is adding a signature to a message merely creates a new
DKIM-Signature header, using the usual semantics of the h= option. A
signer MAY sign previously existing DKIM-Signature header fields
using the method described in Section 5.4 to sign trace header
fields.
INFORMATIVE NOTE: Signers should be cognizant that signing DKIM-
Signature header fields may result in signature failures with
intermediaries that do not recognize that DKIM-Signature header
fields are trace header fields and unwittingly reorder them, thus
breaking such signatures. For this reason, signing existing DKIM-
Signature header fields is unadvised, albeit legal.
INFORMATIVE NOTE: If a header field with multiple instances is
signed, those header fields are always signed from the bottom up.
Thus, it is not possible to sign only specific DKIM-Signature
header fields. For example, if the message being signed already
contains three DKIM-Signature header fields A, B, and C, it is
possible to sign all of them, B and C only, or C only, but not A
only, B only, A and B only, or A and C only.
A signer MAY add more than one DKIM-Signature header field using
different parameters. For example, during a transition period a
signer might want to produce signatures using two different hash
algorithms.
Signers SHOULD NOT remove any DKIM-Signature header fields from
messages they are signing, even if they know that the signatures
cannot be verified.
When evaluating a message with multiple signatures, a verifier SHOULD
evaluate signatures independently and on their own merits. For
example, a verifier that by policy chooses not to accept signatures
with deprecated cryptographic algorithms would consider such
signatures invalid. Verifiers MAY process signatures in any order of
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their choice; for example, some verifiers might choose to process
signatures corresponding to the From field in the message header
before other signatures. See Section 6.1 for more information about
signature choices.
INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate
valid signatures with invalid signatures in an attempt to guess
why a signature failed are ill-advised. In particular, there is
no general way that a verifier can determine that an invalid
signature was ever valid.
Verifiers SHOULD ignore failed signatures as though they were not
present in the message. Verifiers SHOULD continue to check
signatures until a signature successfully verifies to the
satisfaction of the verifier. To limit potential denial-of-service
attacks, verifiers MAY limit the total number of signatures they will
attempt to verify.
5. Signer Actions
The following steps are performed in order by signers.
5.1. Determine Whether the Email Should Be Signed and by Whom
A signer can obviously only sign email for domains for which it has a
private key and the necessary knowledge of the corresponding public
key and selector information. However, there are a number of other
reasons beyond the lack of a private key why a signer could choose
not to sign an email.
INFORMATIVE NOTE: Signing modules may be incorporated into any
portion of the mail system as deemed appropriate, including an
MUA, a SUBMISSION server, or an MTA. Wherever implemented,
signers should beware of signing (and thereby asserting
responsibility for) messages that may be problematic. In
particular, within a trusted enclave the signing address might be
derived from the header according to local policy; SUBMISSION
servers might only sign messages from users that are properly
authenticated and authorized.
INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign
Received header fields if the outgoing gateway MTA obfuscates
Received header fields, for example, to hide the details of
internal topology.
If an email cannot be signed for some reason, it is a local policy
decision as to what to do with that email.
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5.2. Select a Private Key and Corresponding Selector Information
This specification does not define the basis by which a signer should
choose which private key and selector information to use. Currently,
all selectors are equal as far as this specification is concerned, so
the decision should largely be a matter of administrative
convenience. Distribution and management of private keys is also
outside the scope of this document.
INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
private key when the selector containing the corresponding public
key is expected to be revoked or removed before the verifier has
an opportunity to validate the signature. The signer should
anticipate that verifiers may choose to defer validation, perhaps
until the message is actually read by the final recipient. In
particular, when rotating to a new key pair, signing should
immediately commence with the new private key and the old public
key should be retained for a reasonable validation interval before
being removed from the key server.
5.3. Normalize the Message to Prevent Transport Conversions
Some messages, particularly those using 8-bit characters, are subject
to modification during transit, notably conversion to 7-bit form.
Such conversions will break DKIM signatures. In order to minimize
the chances of such breakage, signers SHOULD convert the message to a
suitable MIME content transfer encoding such as quoted-printable or
base64 as described in MIME Part One [RFC2045] before signing. Such
conversion is outside the scope of DKIM; the actual message SHOULD be
converted to 7-bit MIME by an MUA or MSA prior to presentation to the
DKIM algorithm.
If the message is submitted to the signer with any local encoding
that will be modified before transmission, that modification to
canonical [RFC2822] form MUST be done before signing. In particular,
bare CR or LF characters (used by some systems as a local line
separator convention) MUST be converted to the SMTP-standard CRLF
sequence before the message is signed. Any conversion of this sort
SHOULD be applied to the message actually sent to the recipient(s),
not just to the version presented to the signing algorithm.
More generally, the signer MUST sign the message as it is expected to
be received by the verifier rather than in some local or internal
form.
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5.4. Determine the Header Fields to Sign
The From header field MUST be signed (that is, included in the "h="
tag of the resulting DKIM-Signature header field). Signers SHOULD
NOT sign an existing header field likely to be legitimately modified
or removed in transit. In particular, [RFC2821] explicitly permits
modification or removal of the Return-Path header field in transit.
Signers MAY include any other header fields present at the time of
signing at the discretion of the signer.
INFORMATIVE OPERATIONS NOTE: The choice of which header fields to
sign is non-obvious. One strategy is to sign all existing, non-
repeatable header fields. An alternative strategy is to sign only
header fields that are likely to be displayed to or otherwise be
likely to affect the processing of the message at the receiver. A
third strategy is to sign only "well known" headers. Note that
verifiers may treat unsigned header fields with extreme
skepticism, including refusing to display them to the end user or
even ignoring the signature if it does not cover certain header
fields. For this reason, signing fields present in the message
such as Date, Subject, Reply-To, Sender, and all MIME header
fields are highly advised.
The DKIM-Signature header field is always implicitly signed and MUST
NOT be included in the "h=" tag except to indicate that other
preexisting signatures are also signed.
Signers MAY claim to have signed header fields that do not exist
(that is, signers MAY include the header field name in the "h=" tag
even if that header field does not exist in the message). When
computing the signature, the non-existing header field MUST be
treated as the null string (including the header field name, header
field value, all punctuation, and the trailing CRLF).
INFORMATIVE RATIONALE: This allows signers to explicitly assert
the absence of a header field; if that header field is added later
the signature will fail.
INFORMATIVE NOTE: A header field name need only be listed once
more than the actual number of that header field in a message at
the time of signing in order to prevent any further additions.
For example, if there is a single Comments header field at the
time of signing, listing Comments twice in the "h=" tag is
sufficient to prevent any number of Comments header fields from
being appended; it is not necessary (but is legal) to list
Comments three or more times in the "h=" tag.
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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:
Received:
then the resulting DKIM-Signature header field should read:
DKIM-Signature: ... h=Received : Received : ...
and Received header fields and will be signed in that
order.
Signers should be careful of signing header fields that might have
additional instances added later in the delivery process, since such
header fields might be inserted after the signed instance or
otherwise reordered. Trace header fields (such as Received) and
Resent-* blocks are the only fields prohibited by [RFC2822] from
being reordered. In particular, since DKIM-Signature header fields
may be reordered by some intermediate MTAs, signing existing DKIM-
Signature header fields is error-prone.
INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
header fields to be reordered (with the exception of Received
header fields), reordering of signed header fields with multiple
instances by intermediate MTAs will cause DKIM signatures to be
broken; such anti-social behavior should be avoided.
INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this
specification, all end-user visible header fields should be signed
to avoid possible "indirect spamming". For example, if the
Subject header field is not signed, a spammer can resend a
previously signed mail, replacing the legitimate subject with a
one-line spam.
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5.5. Recommended Signature Content
In order to maximize compatibility with a variety of verifiers, it is
recommended that signers follow the practices outlined in this
section when signing a message. However, these are generic
recommendations applying to the general case; specific senders may
wish to modify these guidelines as required by their unique
situations. Verifiers MUST be capable of verifying signatures even
if one or more of the recommended header fields is not signed (with
the exception of From, which must always be signed) or if one or more
of the disrecommended header fields is signed. Note that verifiers
do have the option of ignoring signatures that do not cover a
sufficient portion of the header or body, just as they may ignore
signatures from an identity they do not trust.
The following header fields SHOULD be included in the signature, if
they are present in the message being signed:
o From (REQUIRED in all signatures)
o Sender, Reply-To
o Subject
o Date, Message-ID
o To, Cc
o MIME-Version
o Content-Type, Content-Transfer-Encoding, Content-ID, Content-
Description
o Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-Cc,
Resent-Message-ID
o In-Reply-To, References
o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post,
List-Owner, List-Archive
The following header fields SHOULD NOT be included in the signature:
o Return-Path
o Received
o Comments, Keywords
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o Bcc, Resent-Bcc
o DKIM-Signature
Optional header fields (those not mentioned above) normally SHOULD
NOT be included in the signature, because of the potential for
additional header fields of the same name to be legitimately added or
reordered prior to verification. There are likely to be legitimate
exceptions to this rule, because of the wide variety of application-
specific header fields that may be applied to a message, some of
which are unlikely to be duplicated, modified, or reordered.
Signers SHOULD choose canonicalization algorithms based on the types
of messages they process and their aversion to risk. For example,
e-commerce sites sending primarily purchase receipts, which are not
expected to be processed by mailing lists or other software likely to
modify messages, will generally prefer "simple" canonicalization.
Sites sending primarily person-to-person email will likely prefer to
be more resilient to modification during transport by using "relaxed"
canonicalization.
Signers SHOULD NOT use "l=" unless they intend to accommodate
intermediate mail processors that append text to a message. For
example, many mailing list processors append "unsubscribe"
information to message bodies. If signers use "l=", they SHOULD
include the entire message body existing at the time of signing in
computing the count. In particular, signers SHOULD NOT specify a
body length of 0 since this may be interpreted as a meaningless
signature by some verifiers.
5.6. Compute the Message Hash and Signature
The signer MUST compute the message hash as described in Section 3.7
and then sign it using the selected public-key algorithm. This will
result in a DKIM-Signature header field that will include the body
hash and a signature of the header hash, where that header includes
the DKIM-Signature header field itself.
Entities such as mailing list managers that implement DKIM and that
modify the message or a header field (for example, inserting
unsubscribe information) before retransmitting the message SHOULD
check any existing signature on input and MUST make such
modifications before re-signing the message.
The signer MAY elect to limit the number of bytes of the body that
will be included in the hash and hence signed. The length actually
hashed should be inserted in the "l=" tag of the DKIM-Signature
header field.
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5.7. Insert the DKIM-Signature Header Field
Finally, the signer MUST insert the DKIM-Signature header field
created in the previous step prior to transmitting the email. The
DKIM-Signature header field MUST be the same as used to compute the
hash as described above, except that the value of the "b=" tag MUST
be the appropriately signed hash computed in the previous step,
signed using the algorithm specified in the "a=" tag of the DKIM-
Signature header field and using the private key corresponding to the
selector given in the "s=" tag of the DKIM-Signature header field, as
chosen above in Section 5.2
The DKIM-Signature header field MUST be inserted before any other
DKIM-Signature fields in the header block.
INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this
is to insert the DKIM-Signature header field at the beginning of
the header block. In particular, it may be placed before any
existing Received header fields. This is consistent with treating
DKIM-Signature as a trace header field.
6. Verifier Actions
Since a signer MAY remove or revoke a public key at any time, it is
recommended that verification occur in a timely manner. In many
configurations, the most timely place is during acceptance by the
border MTA or shortly thereafter. In particular, deferring
verification until the message is accessed by the end user is
discouraged.
A border or intermediate MTA MAY verify the message signature(s). An
MTA who has performed verification MAY communicate the result of that
verification by adding a verification header field to incoming
messages. This considerably simplifies things for the user, who can
now use an existing mail user agent. Most MUAs have the ability to
filter messages based on message header fields or content; these
filters would be used to implement whatever policy the user wishes
with respect to unsigned mail.
A verifying MTA MAY implement a policy with respect to unverifiable
mail, regardless of whether or not it applies the verification header
field to signed messages.
Verifiers MUST produce a result that is semantically equivalent to
applying the following steps in the order listed. In practice,
several of these steps can be performed in parallel in order to
improve performance.
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6.1. Extract Signatures from the Message
The order in which verifiers try DKIM-Signature header fields is not
defined; verifiers MAY try signatures in any order they like. For
example, one implementation might try the signatures in textual
order, whereas another might try signatures by identities that match
the contents of the From header field before trying other signatures.
Verifiers MUST NOT attribute ultimate meaning to the order of
multiple DKIM-Signature header fields. In particular, there is
reason to believe that some relays will reorder the header fields in
potentially arbitrary ways.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as
a clue to signing order in the absence of any other information.
However, other clues as to the semantics of multiple signatures
(such as correlating the signing host with Received header fields)
may also be considered.
A verifier SHOULD NOT treat a message that has one or more bad
signatures and no good signatures differently from a message with no
signature at all; such treatment is a matter of local policy and is
beyond the scope of this document.
When a signature successfully verifies, a verifier will either stop
processing or attempt to verify any other signatures, at the
discretion of the implementation. A verifier MAY limit the number of
signatures it tries to avoid denial-of-service attacks.
INFORMATIVE NOTE: An attacker could send messages with large
numbers of faulty signatures, each of which would require a DNS
lookup and corresponding CPU time to verify the message. This
could be an attack on the domain that receives the message, by
slowing down the verifier by requiring it to do a large number of
DNS lookups and/or signature verifications. It could also be an
attack against the domains listed in the signatures, essentially
by enlisting innocent verifiers in launching an attack against the
DNS servers of the actual victim.
In the following description, text reading "return status
(explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
means that the verifier MUST immediately cease processing that
signature. The verifier SHOULD proceed to the next signature, if any
is present, and completely ignore the bad signature. If the status
is "PERMFAIL", the signature failed and should not be reconsidered.
If the status is "TEMPFAIL", the signature could not be verified at
this time but may be tried again later. A verifier MAY either defer
the message for later processing, perhaps by queueing it locally or
issuing a 451/4.7.5 SMTP reply, or try another signature; if no good
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signature is found and any of the signatures resulted in a TEMPFAIL
status, the verifier MAY save the message for later processing. The
"(explanation)" is not normative text; it is provided solely for
clarification.
Verifiers SHOULD ignore any DKIM-Signature header fields where the
signature does not validate. Verifiers that are prepared to validate
multiple signature header fields SHOULD proceed to the next signature
header field, should it exist. However, verifiers MAY make note of
the fact that an invalid signature was present for consideration at a
later step.
INFORMATIVE NOTE: The rationale of this requirement is to permit
messages that have invalid signatures but also a valid signature
to work. For example, a mailing list exploder might opt to leave
the original submitter signature in place even though the exploder
knows that it is modifying the message in some way that will break
that signature, and the exploder inserts its own signature. In
this case, the message should succeed even in the presence of the
known-broken signature.
For each signature to be validated, the following steps should be
performed in such a manner as to produce a result that is
semantically equivalent to performing them in the indicated order.
6.1.1. Validate the Signature Header Field
Implementers MUST meticulously validate the format and values in the
DKIM-Signature header field; any inconsistency or unexpected values
MUST cause the header field to be completely ignored and the verifier
to return PERMFAIL (signature syntax error). Being "liberal in what
you accept" is definitely a bad strategy in this security context.
Note however that this does not include the existence of unknown tags
in a DKIM-Signature header field, which are explicitly permitted.
Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag
that is inconsistent with this specification and return PERMFAIL
(incompatible version).
INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course,
choose to also verify signatures generated by older versions of
this specification.
If any tag listed as "required" in Section 3.5 is omitted from the
DKIM-Signature header field, the verifier MUST ignore the DKIM-
Signature header field and return PERMFAIL (signature missing
required tag).
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INFORMATIONAL NOTE: The tags listed as required in Section 3.5 are
"v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there be a
conflict between this note and Section 3.5, Section 3.5 is
normative.
If the DKIM-Signature header field does not contain the "i=" tag, the
verifier MUST behave as though the value of that tag were "@d", where
"d" is the value from the "d=" tag.
Verifiers MUST confirm that the domain specified in the "d=" tag is
the same as or a parent domain of the domain part of the "i=" tag.
If not, the DKIM-Signature header field MUST be ignored and the
verifier should return PERMFAIL (domain mismatch).
If the "h=" tag does not include the From header field, the verifier
MUST ignore the DKIM-Signature header field and return PERMFAIL (From
field not signed).
Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (signature expired) if it contains an "x=" tag and the
signature has expired.
Verifiers MAY ignore the DKIM-Signature header field if the domain
used by the signer in the "d=" tag is not associated with a valid
signing entity. For example, signatures with "d=" values such as
"com" and "co.uk" may be ignored. The list of unacceptable domains
SHOULD be configurable.
Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (unacceptable signature header) for any other reason, for
example, if the signature does not sign header fields that the
verifier views to be essential. As a case in point, if MIME header
fields are not signed, certain attacks may be possible that the
verifier would prefer to avoid.
6.1.2. Get the Public Key
The public key for a signature is needed to complete the verification
process. The process of retrieving the public key depends on the
query type as defined by the "q=" tag in the DKIM-Signature header
field. Obviously, a public key need only be retrieved if the process
of extracting the signature information is completely successful.
Details of key management and representation are described in
Section 3.6. The verifier MUST validate the key record and MUST
ignore any public key records that are malformed.
When validating a message, a verifier MUST perform the following
steps in a manner that is semantically the same as performing them in
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the order indicated (in some cases, the implementation may
parallelize or reorder these steps, as long as the semantics remain
unchanged):
1. Retrieve the public key as described in Section 3.6 using the
algorithm in the "q=" tag, the domain from the "d=" tag, and the
selector from the "s=" tag.
2. If the query for the public key fails to respond, the verifier
MAY defer acceptance of this email and return TEMPFAIL (key
unavailable). If verification is occurring during the incoming
SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
code. Alternatively, the verifier MAY store the message in the
local queue for later trial or ignore the signature. Note that
storing a message in the local queue is subject to denial-of-
service attacks.
3. If the query for the public key fails because the corresponding
key record does not exist, the verifier MUST immediately return
PERMFAIL (no key for signature).
4. If the query for the public key returns multiple key records, the
verifier may choose one of the key records or may cycle through
the key records performing the remainder of these steps on each
record at the discretion of the implementer. The order of the
key records is unspecified. If the verifier chooses to cycle
through the key records, then the "return ..." wording in the
remainder of this section means "try the next key record, if any;
if none, return to try another signature in the usual way".
5. If the result returned from the query does not adhere to the
format defined in this specification, the verifier MUST ignore
the key record and return PERMFAIL (key syntax error). Verifiers
are urged to validate the syntax of key records carefully to
avoid attempted attacks. In particular, the verifier MUST ignore
keys with a version code ("v=" tag) that they do not implement.
6. If the "g=" tag in the public key does not match the Local-part
of the "i=" tag in the message signature header field, the
verifier MUST ignore the key record and return PERMFAIL
(inapplicable key). If the Local-part of the "i=" tag on the
message signature is not present, the "g=" tag must be "*" (valid
for all addresses in the domain) or the entire g= tag must be
omitted (which defaults to "g=*"), otherwise the verifier MUST
ignore the key record and return PERMFAIL (inapplicable key).
Other than this test, verifiers SHOULD NOT treat a message signed
with a key record having a "g=" tag any differently than one
without; in particular, verifiers SHOULD NOT prefer messages that
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seem to have an individual signature by virtue of a "g=" tag
versus a domain signature.
7. If the "h=" tag exists in the public key record and the hash
algorithm implied by the a= tag in the DKIM-Signature header
field is not included in the contents of the "h=" tag, the
verifier MUST ignore the key record and return PERMFAIL
(inappropriate hash algorithm).
8. If the public key data (the "p=" tag) is empty, then this key has
been revoked and the verifier MUST treat this as a failed
signature check and return PERMFAIL (key revoked). There is no
defined semantic difference between a key that has been revoked
and a key record that has been removed.
9. If the public key data is not suitable for use with the algorithm
and key types defined by the "a=" and "k=" tags in the DKIM-
Signature header field, the verifier MUST immediately return
PERMFAIL (inappropriate key algorithm).
6.1.3. Compute the Verification
Given a signer and a public key, verifying a signature consists of
actions semantically equivalent to the following steps.
1. Based on the algorithm defined in the "c=" tag, the body length
specified in the "l=" tag, and the header field names in the "h="
tag, prepare a canonicalized version of the message as is
described in Section 3.7 (note that this version does not
actually need to be instantiated). When matching header field
names in the "h=" tag against the actual message header field,
comparisons MUST be case-insensitive.
2. Based on the algorithm indicated in the "a=" tag, compute the
message hashes from the canonical copy as described in
Section 3.7.
3. Verify that the hash of the canonicalized message body computed
in the previous step matches the hash value conveyed in the "bh="
tag. If the hash does not match, the verifier SHOULD ignore the
signature and return PERMFAIL (body hash did not verify).
4. Using the signature conveyed in the "b=" tag, verify the
signature against the header hash using the mechanism appropriate
for the public key algorithm described in the "a=" tag. If the
signature does not validate, the verifier SHOULD ignore the
signature and return PERMFAIL (signature did not verify).
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5. Otherwise, the signature has correctly verified.
INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to
initiate the public-key query in parallel with calculating the
hash as the public key is not needed until the final decryption is
calculated. Implementations may also verify the signature on the
message header before validating that the message hash listed in
the "bh=" tag in the DKIM-Signature header field matches that of
the actual message body; however, if the body hash does not match,
the entire signature must be considered to have failed.
A body length specified in the "l=" tag of the signature limits the
number of bytes of the body passed to the verification algorithm.
All data beyond that limit is not validated by DKIM. Hence,
verifiers might treat a message that contains bytes beyond the
indicated body length with suspicion, such as by truncating the
message at the indicated body length, declaring the signature invalid
(e.g., by returning PERMFAIL (unsigned content)), or conveying the
partial verification to the policy module.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
at the indicated body length might pass on a malformed MIME
message if the signer used the "N-4" trick (omitting the final
"--CRLF") described in the informative note in Section 3.4.5.
Such verifiers may wish to check for this case and include a
trailing "--CRLF" to avoid breaking the MIME structure. A simple
way to achieve this might be to append "--CRLF" to any "multipart"
message with a body length; if the MIME structure is already
correctly formed, this will appear in the postlude and will not be
displayed to the end user.
6.2. Communicate Verification Results
Verifiers wishing to communicate the results of verification to other
parts of the mail system may do so in whatever manner they see fit.
For example, implementations might choose to add an email header
field to the message before passing it on. Any such header field
SHOULD be inserted before any existing DKIM-Signature or preexisting
authentication status header fields in the header field block.
INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
search for results header fields to visibly mark authenticated
mail for end users should verify that such header field was added
by the appropriate verifying domain and that the verified identity
matches the author identity that will be displayed by the MUA. In
particular, MUA filters should not be influenced by bogus results
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header fields added by attackers. To circumvent this attack,
verifiers may wish to delete existing results header fields after
verification and before adding a new header field.
6.3. Interpret Results/Apply Local Policy
It is beyond the scope of this specification to describe what actions
a verifier system should make, but an authenticated email presents an
opportunity to a receiving system that unauthenticated email cannot.
Specifically, an authenticated email creates a predictable identifier
by which other decisions can reliably be managed, such as trust and
reputation. Conversely, unauthenticated email lacks a reliable
identifier that can be used to assign trust and reputation. It is
reasonable to treat unauthenticated email as lacking any trust and
having no positive reputation.
In general, verifiers SHOULD NOT reject messages solely on the basis
of a lack of signature or an unverifiable signature; such rejection
would cause severe interoperability problems. However, if the
verifier does opt to reject such messages (for example, when
communicating with a peer who, by prior agreement, agrees to only
send signed messages), and the verifier runs synchronously with the
SMTP session and a signature is missing or does not verify, the MTA
SHOULD use a 550/5.7.x reply code.
If it is not possible to fetch the public key, perhaps because the
key server is not available, a temporary failure message MAY be
generated using a 451/4.7.5 reply code, such as:
451 4.7.5 Unable to verify signature - key server unavailable
Temporary failures such as inability to access the key server or
other external service are the only conditions that SHOULD use a 4xx
SMTP reply code. In particular, cryptographic signature verification
failures MUST NOT return 4xx SMTP replies.
Once the signature has been verified, that information MUST be
conveyed to higher-level systems (such as explicit allow/whitelists
and reputation systems) and/or to the end user. If the message is
signed on behalf of any address other than that in the From: header
field, the mail system SHOULD take pains to ensure that the actual
signing identity is clear to the reader.
The verifier MAY treat unsigned header fields with extreme
skepticism, including marking them as untrusted or even deleting them
before display to the end user.
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While the symptoms of a failed verification are obvious -- the
signature doesn't verify -- establishing the exact cause can be more
difficult. If a selector cannot be found, is that because the
selector has been removed, or was the value changed somehow in
transit? If the signature line is missing, is that because it was
never there, or was it removed by an overzealous filter? For
diagnostic purposes, the exact reason why the verification fails
SHOULD be made available to the policy module and possibly recorded
in the system logs. If the email cannot be verified, then it SHOULD
be rendered the same as all unverified email regardless of whether or
not it looks like it was signed.
7. IANA Considerations
DKIM introduces some new namespaces that have been registered with
IANA. In all cases, new values are assigned only for values that
have been documented in a published RFC that has IETF Consensus
[RFC2434].
7.1. DKIM-Signature Tag Specifications
A DKIM-Signature provides for a list of tag specifications. IANA has
established the DKIM-Signature Tag Specification Registry for tag
specifications that can be used in DKIM-Signature fields.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| v | (this document) |
| a | (this document) |
| b | (this document) |
| bh | (this document) |
| c | (this document) |
| d | (this document) |
| h | (this document) |
| i | (this document) |
| l | (this document) |
| q | (this document) |
| s | (this document) |
| t | (this document) |
| x | (this document) |
| z | (this document) |
+------+-----------------+
DKIM-Signature Tag Specification Registry Initial Values
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7.2. DKIM-Signature Query Method Registry
The "q=" tag-spec (specified in Section 3.5) provides for a list of
query methods.
IANA has established the DKIM-Signature Query Method Registry for
mechanisms that can be used to retrieve the key that will permit
validation processing of a message signed using DKIM.
The initial entry in the registry comprises:
+------+--------+-----------------+
| TYPE | OPTION | REFERENCE |
+------+--------+-----------------+
| dns | txt | (this document) |
+------+--------+-----------------+
DKIM-Signature Query Method Registry Initial Values
7.3. DKIM-Signature Canonicalization Registry
The "c=" tag-spec (specified in Section 3.5) provides for a specifier
for canonicalization algorithms for the header and body of the
message.
IANA has established the DKIM-Signature Canonicalization Algorithm
Registry for algorithms for converting a message into a canonical
form before signing or verifying using DKIM.
The initial entries in the header registry comprise:
+---------+-----------------+
| TYPE | REFERENCE |
+---------+-----------------+
| simple | (this document) |
| relaxed | (this document) |
+---------+-----------------+
DKIM-Signature Header Canonicalization Algorithm Registry
Initial Values
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The initial entries in the body registry comprise:
+---------+-----------------+
| TYPE | REFERENCE |
+---------+-----------------+
| simple | (this document) |
| relaxed | (this document) |
+---------+-----------------+
DKIM-Signature Body Canonicalization Algorithm Registry
Initial Values
7.4. _domainkey DNS TXT Record Tag Specifications
A _domainkey DNS TXT record provides for a list of tag
specifications. IANA has established the DKIM _domainkey DNS TXT Tag
Specification Registry for tag specifications that can be used in DNS
TXT Records.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| v | (this document) |
| g | (this document) |
| h | (this document) |
| k | (this document) |
| n | (this document) |
| p | (this document) |
| s | (this document) |
| t | (this document) |
+------+-----------------+
DKIM _domainkey DNS TXT Record Tag Specification Registry
Initial Values
7.5. DKIM Key Type Registry
The "k=" (specified in Section 3.6.1) and the "a=" (specified in Section 3.5) tags provide for a list of
mechanisms that can be used to decode a DKIM signature.
IANA has established the DKIM Key Type Registry for such mechanisms.
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The initial entry in the registry comprises:
+------+-----------+
| TYPE | REFERENCE |
+------+-----------+
| rsa | [RFC3447] |
+------+-----------+
DKIM Key Type Initial Values
7.6. DKIM Hash Algorithms Registry
The "h=" (specified in Section 3.6.1) and the "a=" (specified in Section 3.5) tags provide for a list of
mechanisms that can be used to produce a digest of message data.
IANA has established the DKIM Hash Algorithms Registry for such
mechanisms.
The initial entries in the registry comprise:
+--------+-------------------+
| TYPE | REFERENCE |
+--------+-------------------+
| sha1 | [FIPS.180-2.2002] |
| sha256 | [FIPS.180-2.2002] |
+--------+-------------------+
DKIM Hash Algorithms Initial Values
7.7. DKIM Service Types Registry
The "s=" tag (specified in Section 3.6.1) provides for a
list of service types to which this selector may apply.
IANA has established the DKIM Service Types Registry for service
types.
The initial entries in the registry comprise:
+-------+-----------------+
| TYPE | REFERENCE |
+-------+-----------------+
| email | (this document) |
| * | (this document) |
+-------+-----------------+
DKIM Service Types Registry Initial Values
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7.8. DKIM Selector Flags Registry
The "t=" tag (specified in Section 3.6.1) provides for a
list of flags to modify interpretation of the selector.
IANA has established the DKIM Selector Flags Registry for additional
flags.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| y | (this document) |
| s | (this document) |
+------+-----------------+
DKIM Selector Flags Registry Initial Values
7.9. DKIM-Signature Header Field
IANA has added DKIM-Signature to the "Permanent Message Header
Fields" registry (see [RFC3864]) for the "mail" protocol, using this
document as the reference.
8. Security Considerations
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 [RFC4686].
8.1. Misuse of Body Length Limits ("l=" Tag)
Body length limits (in the form of the "l=" tag) are subject to
several potential attacks.
8.1.1. Addition of New MIME Parts to Multipart/*
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.
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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 "