DKIM E. Allman
Internet-Draft Sendmail, Inc.
Expires: November 23, 2006 J. Callas
PGP Corporation
M. Delany
M. Libbey
Yahoo! Inc
J. Fenton
M. Thomas
Cisco Systems, Inc.
May 22, 2006
DomainKeys Identified Mail Signatures (DKIM)
draft-ietf-dkim-base-02
Status of this Memo
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This Internet-Draft will expire on November 23, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level
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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 proving
and protecting message sender identity and the integrity of the
messages they convey while retaining the functionality of Internet
email as it is known today. Proof and protection of email identity,
including repudiation and non-repudiation, may assist in the global
control of "spam" and "phishing".
Requirements Language
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6
1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions . . . . . . . . . . . . . . . . 6
2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7
2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 8
3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10
3.3 Signing and Verification Algorithms . . . . . . . . . . . 11
3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 12
3.5 The DKIM-Signature header field . . . . . . . . . . . . . 17
3.6 Key Management and Representation . . . . . . . . . . . . 24
3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 28
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 29
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 30
5.1 Determine if the Email Should be Signed and by Whom . . . 30
5.2 Select a private-key and corresponding selector
information . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 Normalize the Message to Prevent Transport Conversions . . 31
5.4 Determine the header fields to Sign . . . . . . . . . . . 31
5.5 Compute the Message Hash and Signature . . . . . . . . . . 33
5.6 Insert the DKIM-Signature header field . . . . . . . . . . 34
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 35
6.1 Extract the Signature from the Message . . . . . . . . . . 36
6.2 Get the Public Key . . . . . . . . . . . . . . . . . . . . 37
6.3 Compute the Verification . . . . . . . . . . . . . . . . . 38
6.4 Communicate Verification Results . . . . . . . . . . . . . 40
6.5 Interpret Results/Apply Local Policy . . . . . . . . . . . 40
6.6 MUA Considerations . . . . . . . . . . . . . . . . . . . . 41
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 42
8. Security Considerations . . . . . . . . . . . . . . . . . . 42
8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 43
8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 43
8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 44
8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 44
8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 45
8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 46
8.7 Intentionally malformed Key Records . . . . . . . . . . . 46
8.8 Intentionally Malformed DKIM-Signature header fields . . . 46
8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 46
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8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 46
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
9.1 Normative References . . . . . . . . . . . . . . . . . . . 47
9.2 Informative References . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 49
A.1 The user composes an email . . . . . . . . . . . . . . . . 50
A.2 The email is signed . . . . . . . . . . . . . . . . . . . 50
A.3 The email signature is verified . . . . . . . . . . . . . 51
B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 52
B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 52
B.2 Outsourced Business Functions . . . . . . . . . . . . . . 52
B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 52
B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 53
B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 53
B.6 Third-party Message Transmission . . . . . . . . . . . . . 54
C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 54
D. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56
E. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 56
E.1 Changes since -ietf-01 version . . . . . . . . . . . . . . 56
E.2 Changes since -ietf-00 version . . . . . . . . . . . . . . 57
E.3 Changes since -allman-01 version . . . . . . . . . . . . . 57
E.4 Changes since -allman-00 version . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . 59
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1. Introduction
[[Note: text in double square brackets (such as this text) will be
deleted before publication.]]
1.1 Overview
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. 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 are 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 it makes no attempt to include encryption as part of the
mechanism.
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 does not require the use of a trusted third party (such as a
certificate authority or other entity) which might impose
significant costs or introduce delays to deployment
o can be deployed incrementally
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o allows delegation of signing to third parties
o is not intended be used for archival purposes
A "selector" mechanism allows multiple keys per domain, including
delegation of the right to authenticate a portion of the namespace to
a trusted third party.
1.2 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.
INFORMATIVE RATIONALE: The signing address associated with a DKIM
signature is not required to match a particular header field
because of the broad methods of interpretation by recipient mail
systems, including MUAs.
1.3 Scalability
DKIM is designed to support the extreme scalability requirements
which 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.
1.4 Simple Key Management
DKIM differs from traditional hierarchical public-key systems in that
no key signing infrastructure is required; the verifier requests the
public key from the claimed signer directly.
The DNS is proposed as the initial mechanism for publishing public
keys. 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].
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2.1 Signers
Elements in the mail system that sign messages 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 White Space
There are three forms of white space:
o WSP represents simple white space, i.e., a space or a tab
character, and is inherited from[RFC2822].
o SWSP is streaming white space; it adds the CR and LF characters.
o FWS, also from [RFC2822], is folding white space. It allows
multiple lines separated by CRLF followed by at least one white
space, to be joined.
The formal ABNF for SWSP is:
SWSP = CR / LF / WSP ; streaming white space
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 / "+" / "/" / "=" / SWSP)
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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. However, all tokens having
names beginning with "obs-" should be excluded from this import, as
they have been obsoleted and are expected to go away in future
editions of those RFCs.
The following tokens are imported from [RFC2821]:
o Local-part (implementation warning: this permits quoted strings)
o Domain (implementation warning: this permits address-literals)
o sub-domain
The following definitions are imported from [RFC2822]:
o WSP (space or tab)
o FWS (folding white space)
o field-name (name of a header field)
o dot-atom (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)
Other tokens not defined herein are imported from [RFC4234]. These
are intuitive primitives such as SP, ALPHA, CRLF, etc.
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
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user.
Selectors are needed to support some important use cases. For
example:
o Domains which 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 which 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) which
provide forwarding of incoming mail but which do not operate a
mail submission agent for outgoing mail.
Periods are allowed in selectors and are component separators. If
keys are stored in DNS, the period defines sub-domain boundaries.
Sub-selectors might be used to combine dates with locations; for
example, "march2005.reykjavik". This can be used to allow delegation
of a portion of the selector name-space.
ABNF:
selector = sub-domain *( "." sub-domain )
The number of public keys and corresponding selectors for each domain
are 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.
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. E.g., if per-user
keys are issued, the domain owner will need to make the decision as
to whether to make this selector associated directly with the user
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name, or make it some unassociated random value, such as a
fingerprint of the public key.
INFORMATIVE IMPLEMENTERS' 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. Signers should not change the key
associated with a selector. When creating a new key, signers
should associate it with a new selector.
3.2 Tag=Value Lists
DKIM uses a simple "tag=value" syntax in several contexts, including
in messages, domain signature records, and policy records.
Values are a series of strings containing either base64 text, plain
text, or quoted printable text, as defined in [RFC2045], section 6.7.
The name of the tag will determine the encoding of each value;
however, no encoding may include the semicolon (";") character, since
that separates tag-specs.
Formally, the syntax rules are:
tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tag-name = ALPHA 0*ALNUMPUNC
tag-value = 0*VALCHAR ; SWSP prohibited at beginning and end
VALCHAR = %9 / %d32 - %d58 / %d60 - %d126
; HTAB and SP to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / "_"
Note that WSP is allowed anywhere around tags; in particular, WSP
between the tag-name and the "=", and any WSP before the terminating
";" is not part of the value.
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 be specified within a single tag-
list.
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.
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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 key signing/verification 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.
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 as the hash-alg. That hash is then
signed by the signer using the RSA algorithm (defined in PKCS#1
version 1.5 [RFC3447]; in particular see section 5.2) with an
exponent of 65537 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.
3.3.2 The rsa-sha256 Signing Algorithm
The rsa-sha256 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-256 as the hash-alg. That hash is
then signed by the signer using the RSA algorithm (actually PKCS#1
version 1.5 [RFC3447]; in particular see section 5.2) with an
exponent of 65537 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 Other algorithms
Other algorithms MAY be defined in the future. Verifiers MUST ignore
any signatures using algorithms that they do not understand.
3.3.4 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
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validate signatures with larger keys. Security 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:
o The practical constraint that large 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
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 public-key systems
See RFC3766 [RFC3766] for further discussion of selecting key sizes.
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 an authentication 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 white-space replacement and
header field line re-wrapping. 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
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for both header and body. Verifiers MUST implement both
canonicalization algorithms. Further canonicalization algorithms MAY
be defined in the future; verifiers MUST ignore any signatures that
use unrecognized canonicalization algorithms.
In all cases, the header fields of the message are presented to the
signing algorithm first in the order indicated by the signature
header field and canonicalized using the indicated algorithm. Only
header fields listed as signed in the signature header field are
included. Note: the signature header field itself is presented at
the end of the hash, not with the other headers. The CRLF separating
the header field from the body is then presented, followed by the
canonicalized body. Note that the header and body may use different
canonicalization algorithms.
Canonicalization simply prepares the email for presentation to the
signing or verification algorithm. It MUST NOT change the
transmitted data in any way. Canonicalization of header fields and
body are described below.
NOTE: This section assumes that the message is already in "network
normal" format (e.g., 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 Field 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 white space MUST NOT be changed.
3.4.2 The "relaxed" Header Field 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
lower case. 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.
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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. It makes no other
changes to the message body. In more formal terms, the "simple" body
canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the
body to a single "CRLF".
3.4.4 The "relaxed" Body Canonicalization Algorithm
[[This section may be deleted; see discussion below.]] The "relaxed"
body canonicalization algorithm:
o Ignores all white space 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.
[[NON-NORMATIVE DISCUSSION: The authors are undecided whether to
leave the "relaxed" body canonicalization algorithm in to the
specification or delete it entirely. We believe that for the vast
majority of cases, the "simple" body canonicalization algorithm
should be sufficient. We simply do not have enough data to know
whether to retain the "relaxed" body canonicalization algorithm or
not.]]
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. If the body
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length count is not specified then the entire message body is signed
and verified.
INFORMATIVE IMPLEMENTATION NOTE: Body length limits could be
useful in increasing signature robustness when sending to a
mailing list that both appends to content sent to it and does not
sign its messages. However, using such limits enables an attack
in which a sender 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. To avoid this attack, signers
should be wary of using this tag, and verifiers might wish to
ignore the tag or remove text that appears after the specified
content length, perhaps based on other criteria.
The body length count allows the signer of a message to permit data
to be appended to the end of the body of a signed message. The body
length count is made following the canonicalization algorithm; for
example, any white space ignored by a canonicalization algorithm is
not included as part of the body length count.
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.
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".
A body length count of zero means that the body is completely
unsigned.
Note that verifiers MAY choose to truncate messages that have body
content beyond that specified by the body length count.
Alternatively, verifiers MAY ignore signatures that do not cover the
entire message body.
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Signers wishing to ensure that no modification of any sort can occur
should specify the "simple" algorithm and omit the body length count.
3.4.6 Example
(In the following examples, actual white space is used only for
clarity. The actual input and output text is designated using
bracketed descriptors: "<SP>" for a space character, "<TAB>" for a
tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
For example, "X <SP> Y" and "X<SP>Y" represent the same three
characters.)
Example 1: A message reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <TAB><CRLF>
<TAB> Z <SP><SP><CRLF>
<CRLF>
<SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF>
<CRLF>
<CRLF>
when canonicalized using relaxed canonicalization for both header and
body results in a header reading:
a:X <CRLF>
b:Y <SP> Z <CRLF>
and a body reading:
<SP> C <CRLF>
D <SP> E <CRLF>
(postamble)
Example 2: The same message canonicalized using simple
canonicalization for both header and body results in a header
reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <TAB><CRLF>
<TAB> Z <SP><SP><CRLF>
and a body reading:
<SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF>
(postamble)
Example 3: When processed using relaxed header canonicalization and
simple body canoniccalization, the canonicalized version has a header
of:
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a:X <CRLF>
b:Y <SP> Z <CRLF>
and a body reading:
<SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF>
(postamble)
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.
In particular, the "DKIM-Signature" header field SHOULD precede the
original email header fields presented to the canonicalization and
signature algorithms.
The "DKIM-Signature:" header field is always included in the
signature calculation, after the body of the message; however, when
calculating or verifying the signature, the value of the b= tag
(signature value) MUST be treated as though it were the null string.
Unknown tags MUST be signed and verified but MUST be otherwise
ignored by verifiers.
The encodings for each field type are listed below. Tags described
as quoted-printable are as described in section 6.7 of MIME Part One
[RFC2045], with the additional conversion of semicolon characters to
"=3B".
Tags on the DKIM-Signature header field along with their type and
requirement status are shown below. Defined tags are described
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 0.2.
ABNF:
sig-v-tag = %x76 [FWS] "=" [FWS] "0.2"
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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 = "rsa-sha1" / "rsa-sha256" / x-sig-a-tag-alg
x-sig-a-tag-alg = hyphenated-word ; for later extension
b= The signature data (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when re-assembling tthe 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 body part of the message (base64; REQUIRED).
Whitespace is ignored in this value and MUST be ignored when re-
assembling 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.
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:
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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). When presented with
a signature that does not meet this requirement, verifiers MUST
consider the signature invalid.
ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] Domain
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, includiing 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.
Folding white space (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 sender
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.
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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.
i= Identity of the user or agent (e.g., a mailing list manager) on
behalf of which this message is signed (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.
ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" Domain
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 field 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.
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l= Body count (plain-text decimal integer; OPTIONAL, default is
entire body). This tag informs the verifier of the number of
bytes in the body of the email after canonicalization included in
the cryptographic hash, starting from 0 immediately following the
CRLF preceding the body.
INFORMATIVE IMPLEMENTATION WARNING: Use of the l= tag might
allow display of fraudulent content without appropriate
warning to end users. The l= tag is intended for increasing
signature robustness when sending to mailing lists that both
modify their content and do not sign their messages.
However, using the l= tag enables man-in-the-middle 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.
ABNF:
sig-l-tag = %x6c [FWS] "=" [FWS] 1*DIGIT
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 depends on the type and specified
options. If there are multiple query mechanisms listed, the
choice of query mechanism MUST NOT change the interpretation of
the signature. Implementations MUST use the recognized query
mechanisms in the order presented.
Currently the only valid value is "dns/txt" which defines the DNS
TXT record lookup algorithm described elsewhere in this document.
The only option defined for the "dns" query type is "txt", which
MUST be included. Verifiers and signers MUST support "dns/txt".
ABNF:
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sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([FWS] ":" [FWS] sig-q-tag-method)
sig-q-tag-method = sig-q-tag-type ["/" sig-q-tag-args]
sig-q-tag-type = "dns" / x-sig-q-tag-type
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] subdomain *( "." sub-domain )
t= Signature Timestamp (pplain-text; 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.
ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
x= Signature Expiration (plain-text; 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 contraints on the value in the
"t=" tag. Signatures MAY be considered invalid if the
verification time at the verifier is past the expiration date.
The verification time should be the time that the message was
first received at the administrative domain of the verifier if
that time is reliably available; otherwise the current time
should be used. The value of the "x=" tag MUST be greater than
the value of the "t=" tag if both are present.
INFORMATIVE NOTE: The x= tag is not intended as an anti-
replay defense.
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ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT
z= Copied header fields (plain-text, but see description; OPTIONAL,
default is null). A vertical-bar-separated list of selected
header field names and copies of header field values present when
the message was signed. 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. Copied
header field values MUST immediately follow the header field name
with a colon separator (no white space permitted). Header field
values MUST be represented as Quoted-Printable [RFC2045] with
vertical bars, colons, semicolons, and white space encoded in
addition to the usual requirements.
Verifiers MUST NOT use the header field names or copied values
for checking the signature in any way. Copied header field
values are for diagnostic use onnly.
Header fields with characters requiring conversion (perhaps from
legacy MTAs which 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 ":" [FWS] qp-hdr-value
qp-hdr-value = <quoted-printable text with WS, "|", ":",
and ";" encoded>
; needs to be updated with real definition
; (could be messy)
INFORMATIVE EXAMPLE of a signature header field spread across
multiple continuation lines:
DKIM-Signature: a=rsa-sha1; d=example.net; s=brisbane
c=simple; q=dns; 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
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR
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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 responsible signer (the "d=" tag of
the DKIM-Signature header field), the signing identity (the "i="
tag), and the selector (the "s=" tag). The "i=" tag value could be
ignored by some key services.
public_key = dkim_find_key(q_val, d_val, i_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 MMUST be used for any DKIM key represented in
an otherwise unstructured textual form.
The overall syntax is a key-value-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.
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
response. Responses beginning with a "v=" tag with any other
value MUST be discarded.
ABNF:
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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 signing address, with
a "*" character acting as a wildcard. The intent of this tag is
to constrain which signing address can legitimately use this
selector. An email with a signing address that does not match
the value of this tag constitutes a failed verification.
Wildcarding allows matching for addresses such as "user+*". 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] ["*"] [dot-atom]
[[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a dot-
atom. This should probably use a different character for
wildcarding. Unfortunately, the options are non-mnemonic
(e.g., "@", "(", ":"). Alternatively we could insist on
escaping a "*" intended as a literal "*" in the address.]]
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 "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
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 RSA public key, as defined in [RFC3447],
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:
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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
[[NON-NORMATIVE DISCUSSION NOTE: In some cases it can be
hard to separate h= and k=; for example DSA implies that
SHA-1 will be used. This might be an actual change to the
spec depending on how we decide to fix this.]]
n= Notes that might be of interest to a human (quoted-printable;
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).
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 is defined by the
k= tag.
ABNF:
key-p-tag = %x70 [FWS] "=" [FWS] base64string
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:
* matches all service types
email electronic mail (not necessarily limited to SMTP)
This tag is intended to permit senders to constrain the use of
delegated keys, e.g., where a company is willing to delegate the
right to send mail in their name to an outsourcer, but not to
send IM or make VoIP calls. (This of course presumes that these
keys are used in other services in the future.)
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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:
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 modee results to assist
the signer.
ABNF:
key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag
0*( [FWS] ":" [FWS] key-t-tag-flag )
key-t-tag-flag = "y" / x-key-t-tag-flag
x-key-t-tag-flag = hyphenated-word ; for future extension
Unrecognized flags MUST be ignored.
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 Name Space
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 ""sample"", the DNS query will be for
""sample._domainkey.example.com"".
The value of the "i=" tag is not used by the DNS binding.
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 RR record. A
later extension of this standard may define another Resource Record
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type, tentatively dubbed "DKK".
TXT records are encoded as described in Section 3.6.1.
3.7 Computing the Message Hashes
Both signing and verifying message signatures starts 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 which 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 or verifier must compute two hashes, one over the body of
the message and one over the header of the message. Signers MUST
compute them in the order shown. Verifiers MAY compute them in any
order convenient to the verifier, provided that the result is
semantically identical to the semantics that would be the case had
they been computed in this order.
In hash step 1, the signer or verifier MUST hash the message body,
canonicalized using the body canonicalization algorithm specified in
the "c=" tag and truncated to the length specified in the "l=" tag.
That hash value is then converted to base64 form and inserted into
the "bh=" tag of the DKIMM-Signature: header field.
In hash step 2, the signer or 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
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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.
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."
With the exception of the canonicalization procedure described in
Section 3.4, the DKIM signing process treats the body of messages as
simply a string of characters. 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 which is signed.
More formally, the algorithm for the signature is:
body-hash = hash-alg(canon_body)
header-hash = hash-alg(canon_header || DKIM-SIG)
signature = crypt-alg(header-hash, key)
where crypt-alg is the encryption 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.
4. Semantics of Multiple Signatures
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 headers using the
method described in section Section 5.4 to sign trace headers.
Signers should be cognizant that signing DKIM-Signature headers may
result in signature failures with intermediaries that do not
recognize that DKIM-Signature's are trace headers and unwittingly
reorder them.
When evaluating a message with multiple signatures, a receiver should
evaluate signatures independently and on their own merits. For
example, a receiver that by policy chooses not to accept signatures
with deprecated crypto algorithms should consider such signatures
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invalid. As with messages with a single signature, receievers are at
liberty to use the presence of valid signatures as an input to local
policy; likewise, the interpretation of multiple valid signatures in
combination is a local policy decision of the receiver.
Signers SHOULD NOT remove any DKIM-Signature header fields from
messages they are signing, even if they know that the signatures
cannot be verified.
5. Signer Actions
The following steps are performed in order by signers.
5.1 Determine if 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.
A SUBMISSION server MAY sign if the sender is authenticated by some
secure means, e.g., SMTP AUTH. Within a trusted enclave the signing
address MAY be derived from the header field according to local
signer policy. Within a trusted enclave an MTA MAY do the signing.
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.
A signer MUST NOT sign an email if it is unwilling to be held
responsible for the message; in particular, the signer SHOULD ensure
that the submitter has a bona fide relationship with the signer and
that the submitter has tthe right to use the address being claimed.
If an email cannot be signed for some reason, it is a local policy
decision as to what to do with that email.
5.2 Select a private-key and corresponding selector information
This specification does not define the basis by which a signer should
choose which private-key and selector information to use. Currently,
all selectors are equal as far as this specification is concerned, so
the decision should largely be a matter of administrative
convenience. Distribution and management of private-keys is also
outside the scope of this document.
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A signer SHOULD NOT sign with a key that is expected to expire within
seven days; that is, when rotating to a new key, signing should
immediately commence with the new key and the old key SHOULD be
retained for at least seven days 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.
Should the message be submitted to the signer with any local encoding
that will be modified before transmission, such conversion to
canonical form MUST be done before signing. In particular, some
systems use local line separator conventions (such as the Unix
newline character) internally rather than the SMTP-standard CRLF
sequence. All such local conventions MUST be converted to canonical
format before signing.
More generally, the signer MUST sign the message as it will be
received by the verifier rather than in some local or internal form.
5.4 Determine the header fields to Sign
The From header field MUST be signed (that is, included in the h= tag
of the resulting DKIM-Signature header field); any header field that
describes the role of the signer (for example, the Sender or Resent-
From header field if the signature is on behalf of the corresponding
address and that address is different from the From address) MUST
also be included. The signed header fields SHOULD also include the
Subjectt and Date header fields as well as all MIME header fields.
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. It is RECOMMENDED that all other existing, non-repeatable
header fields be signed.
The DKIM-Signature header field is always implicitly signed and MUST
NOT be included in the h= tag except to indicate that other
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preexisting signatures are also signed.
Signers MUST sign any header fields that the signers wish to assert
were present at the time of signing. Put another way, verifiers MAY
treat unsigned header fields with extreme skepticism, up to and
including refusing to display them to the end user.
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.
Signers choosing to sign an existing replicated header field (such as
Received) MUST sign the physically last instance of that header field
in the header field block. Signers wishing to sign multiple
instances of an existing replicated 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 header field names than there are actual corresponding header
fields to indicate that additional header fields of that name SHOULD
NOT be added.
INFORMATIVE EXAMPLE:
If the signer wishes to sign two existing Received header fields,
and the existing header contains:
Received: <A>
Received: <B>
Received: <C>
then the resulting DKIM-Signature header field should read:
DKIM-Signature: ... h=Received : Received : ...
and Received header fields <C> and <B> will be signed in that
order.
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Signers SHOULD NOT sign header fields that might be replicated
(either at the time of signing or potentially in the future), with
the exception of trace header fields such as Received. Comment and
non standard header fields (including X-* header fields) are
permitted by [RFC2822] to be replicated; however, many such header
fields are, by convention, not replicated. Signers need to
understand the implications of signing header fields that might later
be replicated, especially in the face of header field reordering. In
particular, [RFC2822] only requires that trace header fields retain
the original order.
INFORMATIVE RATIONALE: Received: is allowed because these header
fields, as well as Resent-* header fields, are already order-
sensitive.
INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
header field blocks to be reordered (with the exception of
Received header fields), reordering of signed replicated header
fields 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.
INFORMATIVE NOTE: There has been some discussion that a Sender
Signing Policy include the list of header fields that the signer
always signs. N.B. In theory this is unnecessary, since as long
as the signer really always signs the indicated header fields
there is no possibility of an attacker replaying an existing
message that has such an unsigned header field.
5.5 Compute the Message Hash and Signature
The signer MUST compute the message hash as described in Section 3.7
and then sign it using the selected public-key algorithm. This will
result in a DKIM-Signature header field which will include the body
hash and a signature of the header hash, where that header includes
the DKIM-Signature header field itself.
To avoid possible ambiguity, a signer SHOULD either sign or remove
any preexisting header fields which convey the results of previous
verifications of the message signature prior to preparation for
signing and transmission. Such header fields MUST NOT be signed if
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the signer is uncertain of the authenticity of the preexisting header
field, for example, if it is not locally generated or signed by a
previous DKIM-Signature line that the current signer has verified.
Entities such as mailing list managers that implement DKIM and which
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; such signing SHOULD
include any prior verification status, if any, that was inserted upon
message receipt.
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.
INFORMATIVE NOTE: A possible value to include in the "l=" tag
would include the entire length of the message being signed,
thereby allowing intermediate agents to append further information
to the message without breaking the signature (e.g., a mailing
list manager might add unsubscribe information to the body). A
signer wishing to permit such intermediate agents to add another
MIME body part to a "multipart/mixed" message should use a length
that covers the entire presented message except for the trailing
"--CRLF" characters; this is known as the "N-4" approach. Note
that more than four characters may need to be stripped, since
there could be postlude information that needs to be ignored.
5.6 Insert the DKIM-Signature header field
Finally, the signer MUST insert the "DKIM-Signature:" header field
created in the previous step prior to transmitting the email. The
"DKIM-Signature" header field MUST be the same as used to compute the
hash as described above, except that the value of the "b=" tag MUST
be the appropriately signed hash computed in the previous step,
signed using the algorithm specified in the "a=" tag of the "DKIM-
Signature" header field and using the private key corresponding to
the selector given in the "s=" tag of the "DKIM-Signature" header
field, as chosen above in Section 5.2
The "DKIM-Signature" SHOULD be inserted before any header fields that
it signs 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
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existing Received header fields. This is consistent with treating
"DKIM-Signature" as a trace header.
6. Verifier Actions
Since a signer MAY expire a public key at any time, it is recommended
that verification occur in a timely manner with the most timely place
being during acceptance by the border MTA.
A border or intermediate MTA MAY verify the message signatures and
add 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.
In the following description, text reading "return with
DKIM_STAT_something" 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. There are two special cases: DKIM_STAT_PARTIALSIG
indicates that only a portion of the message was actually signed, and
DKIM_STAT_TEMPFAIL means that the signature could not be verified at
this time but should be tried again later. In the former case, the
action a verifier takes is a matter of local policy. In the latter
case, a verifier MAY either defer the message for later processing,
perhaps by queueing it local or issuing a 451/4.7.5 SMTP reply, or
try another signature; if no good signature is found and any of the
signatures resulted in a DKIM_STAT_TEMPFAIL status, the verifier
SHOULD save the message for later processing. Note that an
implementation is not constrained to use these status codes; these
are for explanatory purposes only, and an implementation may define
fewer or more status codes.
The order in which signatures are tried is a matter of local policy
for the verifier and is not defined here. 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;
again, this is 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
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discretion of the implementation.
Verifiers MUST apply the following steps in the order listed.
6.1 Extract the Signature from the Message
The signature and associated signing identity is included in the
value of the DKIM-Signature header field. The order in which
verifiers try DKIM-Signature header fields is not defined; verifiers
MAY try signatures in any order they would like. For example, one
implementation might prefer to try the signatures in textual order,
whereas another might want to prefer signatures by identities that
match the contents of the "From" header field over other identities.
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 with DKIM_STAT_SYNTAX. 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 with
DKIM_STAT_INCOMPAT.
INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of
course, choose to also verify signatures generated by older
versions of this specification.
If the DKIM-Signature header field does not contain any of the tags
listed as required in Section 3.5 the verifier MUST ignore the DKIM-
Signature header field and return with DKIM_STAT_SYNTAX.
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 superdomain of the domain part of the "i=" tag. If
not, the DKIM-Signature header field MUST be ignored and the verifier
should return with DKIM_STAT_SYNTAX.
Verifiers MAY ignore the DKIM-Signature header field and return with
DKIM_STAT_EXPIRED if it contains an "x=" tag and the signature has
expired.
Verifiers MUST NOT attribute ultimate meaning to the order of
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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 informaation.
However, other clues as to the semantics of multiple signatures
must be considered before using ordering.
If there are no valid signatures remaining after this step, a
verifier MUST NOT proceed to the next step.
6.2 Get the Public Key
The public key 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 should only be retrieved if the process of
extracting the signature information is completely successful.
Details of key management and representation are described in
Section 3.6. The verifier MUST validate the key record and MUST
ignore any public key records that are malformed.
When validating a message, a verifier MUST perform the following
steps in a manner that is semantically the same as performing them in
the order indicated (in some cases the implementation may parallelize
or reorder these steps, as long as the semantics remain unchanged):
1. Retrieve the public key as described in (Section 3.6) using the
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
SHOULD defer acceptance of this email and return with
DKIM_STAT_TEMPFAIL. If verification is occuring 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
with DKIM_STAT_NOKEY.
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
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key records is unspecified. If the verifier chooses to cycle
through the key records, then the "return with ..." wording in
the remainder of this section means "try the next key record, if
any; if none, try the next DKIM-Signature header field."
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 aand return with DKIM_STAT_NOKEY. Verifiers are
urged to validate the syntax of key records carefully to avoid
attempted attacks.
6. If the "g=" tag in the public key does not match the local part
of the "i=" tag on the message signature, the verifier MUST
ignore the key record and return with DKIM_STAT_INAPPLICABLE. 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 not present (which defaults to *), otherwise the
verifier MUST ignore the key record and return with
DKIM_STAT_INAPPLICABLE. Other than this test, verifiers SHOULD
NOT treat a message signed with a key record having a g= tag any
differently than one without; in particular, verifiers SHOULD NOT
prefer messages that seem to have an individual signature by
virtue of a g= tag vs. 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 is
not included in the contents of the "h=" tag, the verifier MUST
ignore the key record and return with DKIM_STAT_INAPPLICABLE.
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 with DKIM_STAT_REVOKED.
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
with DKIM_STAT_INAPPLICABLE.
6.3 Compute the Verification
Given a signer and a public key, verifying a signature consists of
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, create a canonicalized copy of the email as is described in
Section 3.7. When matching header field names in the "h=" tag
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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. 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 validatee, the verifier SHOULD ignore the
signature and return with DKIM_STAT_INVALIDSIG.
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.
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.
If a body length is specified in the "l=" tag of the signature,
verifiers MUST only verify the number of bytes indicated in the body
length. Verifiers MAY decide to treat a message containing bytes
beyond the indicated body length with suspicion. Verifiers MAY
truncate the message at the indicated body length, reject the
signature outright, or convey the partial verification to the policy
module using DKIM_STAT_PARTIALSIG.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
at the indicated body length might pass on a malformed MIME
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message if the signer used the "N-4" trick described in the
informative note in Section 5.5. 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 displayeed to the end user.
6.4 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. An example proposal for a
header field is the Authentication-Results header field [ID-AUTH-
RES]. 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 sender identity that will be displayed by the MUA. In
particular, MUA filters should not be influenced by bogus results
header fields added by attackers.
6.5 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. However, if the
verifier does opt to reject such messages, and the verifier runs
synchronously with the SMTP session and a signature is missing or
does not verify, the MTA SHOULD reject the message with an error such
as:
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550 5.7.1 Unsigned messages not accepted
550 5.7.5 Message signature incorrect
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, such as:
451 4.7.5 Unable to verify signature - key server unavailable
A temporary failure of the key server or other external service is
the only condition that should use a 4xx SMTP reply code. In
particular, signature verification failures MUST NOT return 4xx SMTP
replies.
Oncee the signature has been verified, that information MUST be
conveyed to higher level systems (such as explicit allow/white lists
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.
INFORMATIVE NOTE: If the authentication status is to be stored in
the message header field, the Authentication-Results header field
[ID-AUTH-RES] may be used to convey this information.
The verifier MAY treat unsigned header fields with extreme
skepticism, including marking them as untrusted or even deleting them
before display to the end user.
While the symptoms of a failed verification are obvious -- the
signature doesn't verify -- establishing the exact cause can be more
difficult. If a selector cannot be found, is that because the
selector has been removed or was the value changed somehow in
transit? If the signature line is missing is that because it was
never there, or was it removed by an over-zealous filter? For
diagnostic purposes, the exact reason why the verification fails
SHOULD be recorded in the "Authentication-Results" header field and
possibly the system logs. However in terms of presentation to the
end user, the result SHOULD be presented as a simple binary result:
either the email is verified or it is not. If the email cannot be
verified, then it SHOULD be rendered the same as all unverified email
regardless of whether it looks like it was signed or not.
6.6 MUA Considerations
In order to retain the current semantics and visibility of the From
header field, verifying mail agents SHOULD take steps to ensure that
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the signing address is prominently visible to the user if it is
different from the From address. MUAs MAY visually mark the
unverified part of the body in a distinctive font or color to the end
user.
If MUA implementations that highlight the signed address are not
available, this MAY be done by the validating MTA or MDA by rewriting
the From address in a manner which remains compliant with [RFC2822].
Such modifications MUST be performed after the final verification
step since they will break the signature. If performed, the
rewriting SHOULD include the name of the signer in the address. For
example:
From: John Q. User <user@example.com>
might be converted to
From: "John Q. User via <asrg-admin@ietf.org>" <user@example.com>
This sort of addrress inconsistency is expected for mailing lists, but
might be otherwise used to mislead the verifier, for example if a
message supposedly from administration@your-bank.com had a Sender
address of fraud@badguy.com.
Under no circumstances should an unsigned header field be displayed
in any context that might be construed by the end user as having been
signed. Notably, unsigned header fields SHOULD be hidden from the
end user to the extent possible.
The MUA MAY hide or mark portions of the message body that are not
signed when using the "l=" tag.
7. IANA Considerations
To avoid conflicts, tag names for the DKIM-Signature header and key
records should be registered with IANA.
Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature
header field, and the "h=", "k=", "s=", and "t" tags in key records
should be registered with IANA for the same reason.
The DKK RR type must be registered by IANA.
8. Security Considerations
It has been observed that any mechanism that is introduced which
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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 [ID-DKIM-THREATS].
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.
*** Example appropriate here ***
8.1.2 Addition of new HTML content to existing content
Several receiving MUA implementations do not cease display after a
""</html>"" tag. In particular, this allows attacks involving
overlaying images on top of existing text.
INFORMATIVE EXAMPLE: Appending the following text to an existing,
properly closed message will in many MUAs result in inappropriate
data being rendered on top of existing, correct data:
<div style="position: relative; bottom: 350px; z-index: 2;">
<img src="http://www.ietf.org/images/ietflogo2e.gif"
width=578 height=370>
</div>
8.2 Misappropriateed Private Key
If the private key for a user is resident on their computer and is
not protected by an appropriately secure mechanism, it is possible
for malware to send mail as that user and any other user sharing the
same private key. The malware would, however, not be able to
generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user and would limit the
possibilities for certain types of attacks involving socially-
engineered messages.
A larger problem occurs if malware on many users' computers obtains
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the private keys for those users and transmits them via a covert
channel to a site where they can be shared. The compromised users
would likely not know of the misappropriation until they receive
"bounce" messages from messages they are purported to have sent.
Many users might not understand the significance of these bounce
messages and would not take action.
One countermeasure is to use a user-entered passphrase to encrypt the
private key, although users tend to choose weak passphrases and often
reuse them for different purposes, possibly allowing an attack
against DKIM to be extended into other domains. Nevertheless, the
decoded private key might be briefly available to compromise by
malware when it is entered, or might be discovered via keystroke
logging. The added complexity of entering a passphrase each time one
sends a message would also tend to discourage the use of a secure
passphrase.
A somewhat more effective countermeasure is to send messages through
an outgoing MTA that can authenticate the submitter using existing
techniques (e.g., SMTP Authentication), possibly validate the message
itself (e.g., verify that the header is legitimate and that the
content passes a spam content check), and sign the message using a
key appropriate for the submitter address. Such an MTA can also
apply controls on the volume of outgoing mail each user is permitted
to originate in order to further limit the ability of malware to
generate bulk email.
8.3 Key Server Denial-of-Service Attacks
Since the key servers are distributed (potentially separate for each
domain), the number of servers that would need to be attacked to
defeat this mechanism on an Internet-wide basis is very large.
Nevertheless, key servers for individual domains could be attacked,
impeding the verification of messages from that domain. This is not
significantly different from the ability of an attacker to deny
service to the mail exchangers for a given domain, aalthough it
affects outgoing, not incoming, mail.
A variation on this attack is that if a very large amount of mail
were to be sent using spoofed addresses from a given domain, the key
servers for that domain could be overwhelmed with requests. However,
given the low overhead of verification compared with handling of the
email message itself, such an attack would be difficult to mount.
8.4 Attacks Against DNS
Since DNS is a required binding for key services, specific attacks
against DNS must be considered.
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While the DNS is currently insecure [RFC3833], it is expected that
the security problems should and will be solved by DNSSEC [RFC4033],
and all users of the DNS will reap the benefit of that work.
Secondly, the types of DNS attacks relevant to DKIM are very costly
and are far less rewarding than DNS attacks on other Internet
applications.
To systematically thwart the intent of DKIM, an attacker must conduct
a very costly and very extensive attack on many parts of the DNS over
an extended period. No one knows for sure how attackers will
respond, however the cost/benefit of conducting prolonged DNS attacks
of this nature is expected to be uneconomical.
Finally, DKIM is only intended as a "sufficient" method of proving
authenticity. It is not intended to provide strong cryptographic
proof about authorship or contents. Other technologies such as
OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements.
A second security issue related to the DNS revolves around the
increased DNS traffic as a consequence of fetching Selector-based
data as well as fetching signing domain policy. Widespread
deployment of DKIM will result in a significant increase in DNS
queries to the claimed signing domain. In the case of forgeries on a
large scale, DNS servers could see a substantial increase in queries.
8.5 Replay Attacks
In this attack, a spammer sends a message to be spammed to an
accomplice, which results in the message being signed by the
originating MTA. The accomplice resends the message, including the
original signature, to a large number of recipients, possibly by
sending the message to many compromised machines that act as MTAs.
The messages, not having been modified by the accomplice, have valid
signatures.
Partial solutions to this problem involve the use of reputation
services to convey the fact that the specific email address is being
used for spam, and that messages from that signer are likely to be
spam. This requires a real-time detection mechanissm in order to
react quickly enough. However, such measures might be prone to
abuse, if for example an attacker resent a large number of messages
received from a victim in order to make them appear to be a spammer.
Large verifiers might be able to detect unusually large volumes of
mails with the same signature in a short time period. Smaller
verifiers can get substantially the same volume information via
existing collaborative systems.
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8.6 Limits on Revoking Keys
When a large domain detects undesirable behavior on the part of one
of its users, it might wish to revoke the key used to sign that
user's messages in order to disavow responsibility for messages which
have not yet been verified or which are the subject of a replay
attack. However, the ability of the domain to do so can be limited
if the same key, for scalability reasons, is used to sign messages
for many other users. Mechanisms for explicitly revoking keys on a
per-address basis have been proposed but require further study as to
their utility and the DNS load they represent.
8.7 Intentionally malformed Key Records
It is possible for an attacker to publish key records in DNS which
are intentionally malformed, with the intent of causing a denial-of-
service attack on a non-robust verifier implementation. The attacker
could then cause a verifier to read the malformed key record by
sending a message to one of its users referencing the malformed
record in a (not necessarily valid) signature. Verifiers MUST
thoroughly verify all key records retrieved from DNS and be robust
against intentionally as well as unintentionally malformed key
records.
8.8 Intentionally Malformed DKIM-Signature header fields
Verifiers MUST be prepared to receive messages with malformed DKIM-
Signature header fields, and thoroughly verify the header field
before depending on any of its contents.
8.9 Information Leakage
An attacker could determine when a particular signature was verified
by using a per-message selector and then monitoring their DNS traffic
for the key lookup. This would act as the equivalent of a "web bug"
for verification time rather than when the message was read.
8.10 Remote Timing Attacks
In some cases it may be possible to extract private keys using a
remote timing attack [BONEH03]. Implementations should consider
obfuscating the timing to prevent such attacks.
9. References
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9.1 Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message header field Extensions for Non-ASCII
Text", RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
9.2 Informative References
[BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing
Attacks are Practical", 2003, <http://www.usenix.org/
publications/library/proceedings/sec03/tech/brumley.html>.
[ID-AUTH-RES]
Kucherawy, M., "Message header field for Indicating Sender
Authentication Status",
draft-kucherawy-sender-auth-header-02 (work in progress),
February 2006.
[ID-DKIM-THREATS]
Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", draft-fenton-dkim-threats-02
(work in progress), April 2006.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
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"OpenPGP Message Format", RFC 2440, November 1998.
[RFC3766] Orman, H. and P. Hoffman, "Determing Strengths for Public
Keys Used For Exchanging Symmetric Keys", RFC 3766,
April 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004.
[RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 3851, June 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
Authors' Addresses
Eric Allman
Sendmail, Inc.
6425 Christie Ave, Suite 400
Emeryville, CA 94608
USA
Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org
URI:
Jon Callas
PGP Corporation
3460 West Bayshore
Palo Alto, CA 94303
USA
Phone: +1 650 319 9016
Email: jon@pgp.com
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Mark Delany
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
URI:
Miles Libbey
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Email: mlibbeymail-mailsig@yahoo.com
URI:
Jim Fenton
Cisco Systems, Inc.
MS SJ-24/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
Michael Thomas
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
Phone: +1 408 525 5386
Email: mat@cisco.com
Appendix A. Example of Use (INFORMATIVE)
This section shows the complete flow of an email from submission to
final delivery, demonstrating how the various components fit
together.
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A.1 The user composes an email
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
A.2 The email is signed
This email is signed by the example.com outbound email server and now
looks like this:
DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com;
c=simple; q=dns; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR;
Received: from dsl-10.2.3.4.football.example.com [10.2.3.4]
by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
The signing email server requires access to thhe private-key
associated with the "brisbane" selector to generate this signature.
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A.3 The email signature is verified
The signature is normally verified by an inbound SMTP server or
possibly the final delivery agent. However, intervening MTAs can
also perform this verification if they choose to do so. The
verification process uses the domain "example.com" extracted from the
"d=" tag and the selector "brisbane" from the "s=" tag in the "DKIM-
Signature" header field to form the DNS DKIM query for:
brisbane._domainkey.example.com
Signature verification starts with the physically last "Received"
header field, the "From" header field, and so forth, in the order
listed in the "h=" tag. Verification follows with a single CRLF
followed by the body (starting with "Hi."). The email is canonically
prepared for verifying with the "simple" method. The result of the
query and subsequent verification of the signature is stored in the
"Authentication-Results" header field line. After successful
verification, the email looks like this:
Authentication-Results: shopping.example.net
header.from=joe@football.example.com; dkim=pass
Received: from mout23.football.example.com (192.168.1.1)
by shopping.example.net with SMTP;
Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com;
c=simple; q=dns; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR
Received: from dsl-10.2.3.4.network.example.com [10.2.3.4]
by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
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Appendix B. Usage Examples (INFORMATIVE)
Studies in this appendix are for informational purposes only. In no
case should these examples be used as guidance when creating an
implementation.
B.1 Simple Message Forwarding
In some cases the recipient may request forwarding of email messages
from the original address tto another, through the use of a Unix
.forward file or equivalent. In this case messages are typically
forwarded without modification, except for the addition of a Received
header field to the message and a change in the Envelope-to address.
In this case, the eventual recipient should be able to verify the
original signature since the signed content has not changed, and
attribute the message correctly.
B.2 Outsourced Business Functions
Outsourced business functions represent a use case that motivates the
need for selectors (the "s=" signature tag) and granularity (the "g="
key tag). Examples of outsourced business functions are legitimate
email marketing providers and corporate benefits providers. In
either case, the outsourced function would like to be able to send
messages using the email domain of the client company. At the same
time, the client may be reluctant to register a key for the provider
that grants the ability to send messages for any address in the
domain.
The outsourcing company can generate a keypair and the client company
can register the public key using a unique selector for a specific
address such as winter-promotions@example.com by specifying a
granularity of "g=winter-promotions" or "g=*-promotions" (to allow a
range of addresses). This would enable the provider to send messages
using that specific address and have them verify properly. The
client company retains control over the email address because it
retains the ability to revoke the key at any time.
B.3 PDAs and Similar Devices
PDAs are one example of the use of multiple keys per user. Suppose
that John Doe wanted to be able to send messages using his corporate
email address, jdoe@example.com, and the device did not have the
ability to make a VPN connection to the corporate network. If the
device was equipped with a private key registered for
jdoe@example.com by the administrator of that domain, and appropriate
software to sign messages, John could send signed messages through
the outgoing network of the PDA service provider.
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B.4 Mailing Lists
There is a wide range of behavior in forwarders and mailing lists
(collectively called "forwarders" below), ranging from those which
make no modification to the message itself (other than to add a
Received header field and change the envelope information) to those
which may add header fields, change the Subject header field, add
content to the body (typically at the end), or reformat the body in
some manner.
Forwarders which do noot modify the body or signed header fields of a
message with a valid signature may re-sign the message as described
below.
Forwarders which make any modification to a message that could result
in its signature becoming invalid should sign or re-sign using an
appropriate identification (e.g., mailing-list-name@example.net).
Since in so doing the (re-)signer is taking responsibility for the
content of the message, modifying forwarders may elect to forward or
re-sign only for messages which were received with valid signatures
or other indications that the messages being signed are not spoofed.
Forwarders which wish to re-sign a message must apply a Sender header
field to the message to identify the address being used to sign the
message and must remove any preexisting Sender header field as
required by [RFC2822]. The forwarder applies a new DKIM-Signature
header field with the signature, public key, and related information
of the forwarder.
B.5 Affinity Addresses
"Affinity addresses" are email addresses that users employ to have an
email address that is independent of any changes in email service
provider they may choose to make. They are typically associated with
college alumni associations, professional organizations, and
recreational organizations with which they expect to have a long-term
relationship. These domains usually provide forwarding of incoming
email, but (currently) usually depend on the user to send outgoing
messages through their own service provider's MTA. They usually have
an associated Web application which authenticates the user and allows
the forwarding address to be changed.
With DKIM, affinity domains could use the Web application to allow
users to register their own public keys to be used to sign messages
on behalf of their affinity address. This is another application
that takes advantage of user-level keying, and domains used for
affinity addresses would typically have a very large number of user-
level keys. Alternatively, the affinity domain could handle outgoing
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mail, operating a mail submission agent that authenticates users
before accepting and signing messages for them. This is of course
dependent on the user's service provider not blocking the relevant
TCP ports used for mail submission.
B.6 Third-party Message Transmission
Third-party message transmission refers to the authorized sending of
mail by an Internet application on behalf of a user. For example, a
website providing news may allow the reader to forward a copy of the
message to a friend; this is typically done using the reader's email
address. This is sometimes referred to as the "Evite problem", named
after the website of the same name that allows a user to send
invitations to friends.
One way this can be handled is to continue to put the reader's email
address in the From field of the message, but put an address owned by
the site into the Sender field, and sign the message on behalf of the
Sender. A verifying MTA should accept this and rewrite the From
field to indicate the address that was verified, i.e., From: John
Doe via news@news-site.com <jdoe@example.com>.
Appendix C. Creating a public key (INFORMATIVE)
XXX Update to 1024 bit key and SHA-256 and adjust examples
accordingly. XXX
The default signature is an RSA signed SHA1 digest of the complete
email. For ease of explanation, the openssl command is used to
describe the mechanism by which keys and signatures are managed. One
way to generate a 768 bit private-key suitable for DKIM, is to use
openssl like this:
$ openssl genrsa -out rsa.private 768
This results in the file rsa.private containing the key information
similar to this:
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-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
To extract the public-key component from the private-key, use openssl
like this:
$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
This results in the file rsa.public containing the key information
similar to this:
-----BEGIN PUBLIC KEY-----
MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l
MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E
XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB
-----END PUBLIC KEY-----
This public-key data (without the BEGIN and END tags) is placed in
the DNS. With the signature, canonical email contents and puublic
key, a verifying system can test the validity of the signature. The
openssl invocation to verify a signature looks like this:
openssl dgst -verify rsa.public -sha1 -signature signature.file \
<input.file
Once a private-key has been generated, the openssl command can be
used to sign an appropriately prepared email, like this:
$ openssl dgst -sign rsa.private -sha1 <input.file
This results in signature data similar to this when represented in
Base64 [MIME] format:
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aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz
msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl
How this signature is added to the email is discussed elsewhere in
this document.
Appendix D. Acknowledgements
The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
Steve Atkins, Fred Baker, Mark Baugher, Nathaniel Borenstein, Dave
Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Patrik
Faltstrom, Duncan Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony
Hansen, Arvel Hathcock, Amir Herzberg, Craig Hughes, Don Johnsen,
Harry Katz, Murray S. Kucherawy, Barry Leiba, John Levine, Simon
Longsdale, David Margrave, Justin Mason, David Mayne, Steve Murphy,
Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer
Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro,
Eric Rescorla, Dave Rossetti, Hector Santos, the Spamhaus.org team,
Malte S. Stretz, Robert Sanders, Rand Wacker, and Dan Wing for their
valuable suggestions and constructive criticism.
The DomainKeys specification was a primary source from which this
specification has been derived. Further information about DomainKeys
is at
<http://domainkeys.sourceforge.net/license/patentlicense1-1.html>.
Appendix E. Edit History
[[This section to be removed before publication.]]
E.1 Changes since -ietf-01 version
The following changes were made between draft-ietf-dkim-base-01 and
draft-ietf-dkim-base-02:
o Change wording on "x=" tag in DKIM-Signature header field
regarding verifier handling of expired signatures from MUST to MAY
(per 20 April Jabber session). Also, make it clear that received
time is to be preferred over current time if reliably available.
o Several changes to limit wording that would intrude into verifier
policy. This is largely changing statements such as "... MUST
reject the message" to "... MUST consider the signature invalid."
o Drop normative references to ID-DKIM-RR, OpenSSL, PEM, and
Stringprep.
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o Change "v=" tag in DKIM-Signature from "MUST NOT" to "MUST"; the
version number is 0.2 for this draft, with the expectation that
the first official version will be "v=1". (Per 18 May Jabber
session.)
o Change "q=dns" query access method to "q=dnstxt" to emphasize the
use of the TXT record. The expectation is that a later extension
will define "q=dnsdkk" to indicate use of a DKK record. (Per 18
May Jabber session.)
o Several typos fixed, including removing a paragraph that implied
that the DKIM-Signature header field should be hashed with the
body (it should not).
E.2 Changes since -ietf-00 version
The following changes were made between draft-ietf-dkim-base-00 and
draft-ietf-dkim-base-01:
o Added section 8.9 (Information Leakage).
o Replace section 4 (Multiple Signatures) with much less vague text.
o Fixed ABNF for base64string.
o Added rsa-sha256 signing algorithm.
o Expanded several examples.
o Changed signing algorithm to use separate hash of the body of the
message; this is represented as the "bh=" tag in the DKIM-
Signature header field.
o Changed "z=" tag so that it need not have the same header field
names as the "h=" tag.
o Significant wordsmithing.
E.3 Changes since -allman-01 version
The following changes were made between draft-allman-dkim-base-01 and
draft-ietf-dkim-base-00:
o Remove references to Sender Signing Policy document. Such
consideration is implicitly included in Section 6.5.
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o Added ABNF for all tags.
o Updated references (still includes some references to expired
drafts, notably [ID-AUTH-RES].
o Significant wordsmithing.
E.4 Changes since -allman-00 version
The following changes were made between draft-allman-dkim-base-00 and
draft-allman-dkim-base-01:
o Changed "c=" tag to separate out header from body
canonicalization.
o Eliminated "nowsp" canonicalization in favor of "relaxed", which
is somewhat less relaxed (but more secure) than "nowsp".
o Moved the (empty) Compliance section to the Sender Signing Policy
document.
o Added several IANA Considerations.
o Fixed a number of grammar and formatting errors.
Allman, et al. Expires November 23, 2006 [Page 588]
Internet-Draft DKIM Signing May 2006
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Allman, et al. Expires November 23, 2006 [Page 59]
Internet-Draft DKIM Signing May 2006
Acknowledgment
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Allman, et al. Expires November 23, 2006 [Page 60]
