draft-ietf-dkim-base-00.txt   draft-ietf-dkim-base-01.txt 
DKIM E. Allman DKIM E. Allman
Internet-Draft Sendmail, Inc. Internet-Draft Sendmail, Inc.
Expires: August 6, 2006 J. Callas Expires: October 15, 2006 J. Callas
PGP Corporation PGP Corporation
M. Delany M. Delany
M. Libbey M. Libbey
Yahoo! Inc Yahoo! Inc
J. Fenton J. Fenton
M. Thomas M. Thomas
Cisco Systems, Inc. Cisco Systems, Inc.
February 2, 2006 April 13, 2006
DomainKeys Identified Mail Signatures (DKIM) DomainKeys Identified Mail Signatures (DKIM)
draft-ietf-dkim-base-00 draft-ietf-dkim-base-01
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 6, 2006. This Internet-Draft will expire on October 15, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key cryptography and authentication framework for email using public-key cryptography and
key server technology to permit verification of the source and key server technology to permit verification of the source and
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1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6 1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6
2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7 2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7
2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7 2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Tag=Value Format for DKIM header fields . . . . . . . . . 10 3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10
3.3 Signing and Verification Algorithms . . . . . . . . . . . 10 3.3 Signing and Verification Algorithms . . . . . . . . . . . 11
3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 12 3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 12
3.5 The DKIM-Signature header field . . . . . . . . . . . . . 16 3.5 The DKIM-Signature header field . . . . . . . . . . . . . 16
3.6 Key Management and Representation . . . . . . . . . . . . 22 3.6 Key Management and Representation . . . . . . . . . . . . 23
3.7 Computing the Message Hash . . . . . . . . . . . . . . . . 26 3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 27
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 28 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 29
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 28 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1 Determine if the Email Should be Signed and by Whom . . . 29 5.1 Determine if the Email Should be Signed and by Whom . . . 30
5.2 Select a private-key and corresponding selector 5.2 Select a private-key and corresponding selector
information . . . . . . . . . . . . . . . . . . . . . . . 29 information . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 Normalize the Message to Prevent Transport Conversions . . 29 5.3 Normalize the Message to Prevent Transport Conversions . . 31
5.4 Determine the header fields to Sign . . . . . . . . . . . 30 5.4 Determine the header fields to Sign . . . . . . . . . . . 31
5.5 Compute the Message Hash . . . . . . . . . . . . . . . . . 32 5.5 Compute the Message Hash and Signature . . . . . . . . . . 33
5.6 Insert the DKIM-Signature header field . . . . . . . . . . 32 5.6 Insert the DKIM-Signature header field . . . . . . . . . . 34
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 33 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 35
6.1 Extract the Signature from the Message . . . . . . . . . . 33 6.1 Extract the Signature from the Message . . . . . . . . . . 35
6.2 Get the Public Key . . . . . . . . . . . . . . . . . . . . 34 6.2 Get the Public Key . . . . . . . . . . . . . . . . . . . . 36
6.3 Compute the Verification . . . . . . . . . . . . . . . . . 35 6.3 Compute the Verification . . . . . . . . . . . . . . . . . 37
6.4 Insert the Authentication-Results Header Field . . . . . . 36 6.4 Communicate Verification Results . . . . . . . . . . . . . 39
6.5 Interpret Results/Apply Local Policy . . . . . . . . . . . 37 6.5 Interpret Results/Apply Local Policy . . . . . . . . . . . 39
6.6 MUA Considerations . . . . . . . . . . . . . . . . . . . . 38 6.6 MUA Considerations . . . . . . . . . . . . . . . . . . . . 40
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
8. Security Considerations . . . . . . . . . . . . . . . . . . . 39 8. Security Considerations . . . . . . . . . . . . . . . . . . . 41
8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 39 8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 42
8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 40 8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 42
8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 41 8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 43
8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 41 8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 43
8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 42 8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 44
8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 42 8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 44
8.7 Intentionally malformed Key Records . . . . . . . . . . . 42 8.7 Intentionally malformed Key Records . . . . . . . . . . . 45
8.8 Intentionally Malformed DKIM-Signature header fields . . . 43 8.8 Intentionally Malformed DKIM-Signature header fields . . . 45
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43 8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 45
9.1 Normative References . . . . . . . . . . . . . . . . . . . 43 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.2 Informative References . . . . . . . . . . . . . . . . . . 44 9.1 Normative References . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45 9.2 Informative References . . . . . . . . . . . . . . . . . . 46
A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . . 46 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47
A.1 The user composes an email . . . . . . . . . . . . . . . . 46 A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . . 48
A.2 The email is signed . . . . . . . . . . . . . . . . . . . 46 A.1 The user composes an email . . . . . . . . . . . . . . . . 49
A.3 The email signature is verified . . . . . . . . . . . . . 47 A.2 The email is signed . . . . . . . . . . . . . . . . . . . 49
B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . . 48 A.3 The email signature is verified . . . . . . . . . . . . . 50
B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 48 B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . . 51
B.2 Outsourced Business Functions . . . . . . . . . . . . . . 48 B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 51
B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 49 B.2 Outsourced Business Functions . . . . . . . . . . . . . . 51
B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 49 B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 51
B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 50 B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 52
B.6 Third-party Message Transmission . . . . . . . . . . . . . 50 B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 52
C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . . 51 B.6 Third-party Message Transmission . . . . . . . . . . . . . 53
D. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 52 C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . . 53
E. Edit History . . . . . . . . . . . . . . . . . . . . . . . . . 52 D. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 55
E.1 Changes since -allman-01 version . . . . . . . . . . . . . 52 E. Edit History . . . . . . . . . . . . . . . . . . . . . . . . . 55
E.2 Changes since -allman-00 version . . . . . . . . . . . . . 53 E.1 Changes since -ietf-00 version . . . . . . . . . . . . . . 55
Intellectual Property and Copyright Statements . . . . . . . . 54 E.2 Changes since -allman-01 version . . . . . . . . . . . . . 56
E.3 Changes since -allman-00 version . . . . . . . . . . . . . 56
Intellectual Property and Copyright Statements . . . . . . . . 57
1. Introduction 1. Introduction
[[Note: text in double square brackets (such as this text) will be
deleted before publication.]]
1.1 Overview 1.1 Overview
DomainKeys Identified Mail (DKIM) defines a mechanism by which email DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key, the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain. possession of the private key for the signing domain.
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issued by well-known, trusted certificate authorities, issued by well-known, trusted certificate authorities,
o there is no dependency on the deployment of any new Internet o there is no dependency on the deployment of any new Internet
protocols or services for public key distribution or revocation, protocols or services for public key distribution or revocation,
o it makes no attempt to include encryption as part of the o it makes no attempt to include encryption as part of the
mechanism. mechanism.
DKIM: DKIM:
o is transparent to and compatible with the existing email o is compatible with the existing email infrastructure and
infrastructure transparent to the fullest extent possible
o requires minimal new infrastructure o requires minimal new infrastructure
o can be implemented independently of clients in order to reduce o can be implemented independently of clients in order to reduce
deployment time deployment time
o does not require the use of a trusted third party (such as a o does not require the use of a trusted third party (such as a
certificate authority or other entity) which might impose certificate authority or other entity) which might impose
significant costs or introduce delays to deployment significant costs or introduce delays to deployment
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o requires minimal new infrastructure o requires minimal new infrastructure
o can be implemented independently of clients in order to reduce o can be implemented independently of clients in order to reduce
deployment time deployment time
o does not require the use of a trusted third party (such as a o does not require the use of a trusted third party (such as a
certificate authority or other entity) which might impose certificate authority or other entity) which might impose
significant costs or introduce delays to deployment significant costs or introduce delays to deployment
o can be deployed incrementally o can be deployed incrementally
o allows delegation of signing to third parties. o allows delegation of signing to third parties.
A "selector" mechanism allows multiple keys per domain, including A "selector" mechanism allows multiple keys per domain, including
delegation of the right to authenticate a portion of the namespace to delegation of the right to authenticate a portion of the namespace to
a trusted third party. a trusted third party.
1.2 Signing Identity 1.2 Signing Identity
DKIM separates the question of the signer of the message from the DKIM separates the question of the identity of the signer of the
purported author of the message. In particular, a signature includes message from the purported author of the message. In particular, a
the identity of the signer. Recipients can use the signing signature includes the identity of the signer. Verifiers can use the
information to decide how they want to process the message. signing information to decide how they want to process the message.
INFORMATIVE RATIONALE: The signing address associated with a DKIM INFORMATIVE RATIONALE: The signing address associated with a DKIM
signature is not required to match a particular header field signature is not required to match a particular header field
because of the broad methods of interpretation by recipient mail because of the broad methods of interpretation by recipient mail
systems, including MUAs. systems, including MUAs.
1.3 Scalability 1.3 Scalability
The email identification problem is characterized by extreme The email identification problem is characterized by extreme
scalability requirements. There are currently over 70 million scalability requirements. There are currently over 70 million
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space, to be joined. space, to be joined.
The formal ABNF for SWSP is: The formal ABNF for SWSP is:
SWSP = CR / LF / WSP ; streaming white space SWSP = CR / LF / WSP ; streaming white space
2.4 Common ABNF Tokens 2.4 Common ABNF Tokens
The following ABNF tokens are used elsewhere in this document. The following ABNF tokens are used elsewhere in this document.
hyphenated-word = ALPHA *(ALPHA / DIGIT / "-") hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
base64string = 1*(ALPHA / DIGIT / "+" / "/" / SWSP) base64string = 1*(ALPHA / DIGIT / "+" / "/" / "=" / SWSP)
2.5 Imported ABNF Tokens 2.5 Imported ABNF Tokens
The following tokens are imported from other RFCs as noted. Those The following tokens are imported from other RFCs as noted. Those
RFCs should be considered definitive. However, all tokens having RFCs should be considered definitive. However, all tokens having
names beginning with "obs-" should be excluded from this import. 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]: The following tokens are imported from [RFC2821]:
o Local-part (implementation warning: this permits quoted strings) o Local-part (implementation warning: this permits quoted strings)
o Domain (implementation warning: this permits address-literals) o Domain (implementation warning: this permits address-literals)
o sub-domain o sub-domain
The following definitions are imported from [RFC2822]: The following definitions are imported from [RFC2822]:
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verification. At the start of the transition period, the outbound verification. At the start of the transition period, the outbound
email servers are configured to sign with the "february2005" private- email servers are configured to sign with the "february2005" private-
key. At the end of the transition period, the "january2005" public key. At the end of the transition period, the "january2005" public
key is removed from the public-key repository. key is removed from the public-key repository.
While some domains may wish to make selector values well known, 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 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 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 keys are issued, the domain owner will need to make the decision as
to whether to make this selector associated directly with the user to whether to make this selector associated directly with the user
name, or make it some unassociated random value, such as the name, or make it some unassociated random value, such as a
fingerprint of the public key. fingerprint of the public key.
3.2 Tag=Value Format for DKIM header fields 3.2 Tag=Value Lists
DKIM uses a simple "tag=value" syntax in several contexts, including DKIM uses a simple "tag=value" syntax in several contexts, including
in messages, domain signature records, and policy records. in messages, domain signature records, and policy records.
Values are a series of strings containing either base64 text, plain Values are a series of strings containing either base64 text, plain
text, or quoted printable text, as defined in [RFC2045], section 6.7. text, or quoted printable text, as defined in [RFC2045], section 6.7.
The name of the tag will determine the encoding of each value; The name of the tag will determine the encoding of each value;
however, no encoding may include the semicolon (";") character, since however, no encoding may include the semicolon (";") character, since
that separates tag-specs. that separates tag-specs.
Formally, the syntax rules are: Formally, the syntax rules are:
tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ] tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tag-name = ALPHA 0*ALNUMPUNC tag-name = ALPHA 0*ALNUMPUNC
tag-value = *VALCHAR ; SWSP prohibited at beginning and end tag-value = 0*VALCHAR ; SWSP prohibited at beginning and end
VALCHAR = %9 / %d32 - %d58 / %d60 - %d126 VALCHAR = %9 / %d32 - %d58 / %d60 - %d126
; HTAB and SP to TILDE except SEMICOLON ; HTAB and SP to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / "_" ALNUMPUNC = ALPHA / DIGIT / "_"
Note that WSP is allowed anywhere around tags; in particular, WSP Note that WSP is allowed anywhere around tags; in particular, WSP
between the tag-name and the "=", and any WSP before the terminating between the tag-name and the "=", and any WSP before the terminating
";" is not part of the value. ";" is not part of the value.
Tags MUST be interpreted in a case-sensitive manner. Values MUST be Tags MUST be interpreted in a case-sensitive manner. Values MUST be
processed as case sensitive unless the specific tag description of processed as case sensitive unless the specific tag description of
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Unrecognized tags MUST be ignored. Unrecognized tags MUST be ignored.
Tags that have an empty value are not the same as omitted tags. An 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 omitted tag is treated as having the default value; a tag with an
empty value explicitly designates the empty string as the value. For empty value explicitly designates the empty string as the value. For
example, "g=" does not mean "g=*", even though "g=*" is the default example, "g=" does not mean "g=*", even though "g=*" is the default
for that tag. for that tag.
3.3 Signing and Verification Algorithms 3.3 Signing and Verification Algorithms
DKIM supports multiple key signing/verification algorithms. The only DKIM supports multiple key signing/verification algorithms. Two
algorithm defined by this specification at this time is rsa-sha1, algorithms are defined by this specification at this time: rsa-sha1,
which is the default if no algorithm is specified and which MUST be and rsa-sha256. The rsa-sha256 algorithm is the default if no
supported by all implementations. 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 3.3.1 The rsa-sha1 Signing Algorithm
The rsa-sha1 Signing Algorithm computes a SHA-1 hash of the message The rsa-sha1 Signing Algorithm computes a message hash as described
header field and body as described in Section 3.7 below. That hash in Section 3.7 below using SHA-1 as the hash-alg. That hash is then
is then encrypted by the signer using the RSA algorithm (actually encrypted by the signer using the RSA algorithm (defined in PKCS#1
PKCS#1 version 1.5 [RFC3447]) and the signer's private key. The hash version 1.5 [RFC3447]) as the crypt-alg and the signer's private key.
MUST NOT be truncated or converted into any form other than the The hash MUST NOT be truncated or converted into any form other than
native binary form before being signed. the native binary form before being signed.
More formally, the algorithm for the signature using rsa-sha1 is: 3.3.2 The rsa-sha256 Signing Algorithm
RSA(SHA1(canon_message || DKIM-SIG), key)
where canon_message is the canonicalized message header and body as The rsa-sha256 Signing Algorithm computes a message hash as described
defined in Section 3.4 and DKIM-SIG is the canonicalized DKIM- in Section 3.7 below using SHA-256 as the hash-alg. That hash is
Signature header field sans the signature value itself. then encrypted by the signer using the RSA algorithm (actually PKCS#1
version 1.5 [RFC3447]) as the crypt-alg and the signer's private key.
The hash MUST NOT be truncated or converted into any form other than
the native binary form before being signed.
3.3.2 Other algorithms 3.3.3 Other algorithms
Other algorithms MAY be defined in the future. Verifiers MUST ignore Other algorithms MAY be defined in the future. Verifiers MUST ignore
any signatures using algorithms that they do not understand. any signatures using algorithms that they do not understand.
3.3.3 Key sizes 3.3.4 Key sizes
Selecting appropriate key sizes is a trade-off between cost, Selecting appropriate key sizes is a trade-off between cost,
performance and risk. All implementations MUST support keys of sizes performance and risk. Since short RSA keys more easily succumb to
512, 768, 1024, 1536 and 2048 bits and MAY support larger keys. off-line attacks, signers MUST use RSA keys of at least 1024 bits for
long-lived keys. Verifiers MUST be able to validate signatures with
keys ranging from 512 bits to 2048 bits, and they MAY be able to
validate signatures with larger keys. 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: Factors that should influence the key size choice include:
o The practical constraint that a 2048 bit key is the largest key o The practical constraint that large keys may not fit within a 512
that fits within a 512 byte DNS UDP response packet byte DNS UDP response packet
o The security constraint that keys smaller than 1024 bits are o The security constraint that keys smaller than 1024 bits are
subject to brute force attacks. subject to off-line attacks
o Larger keys impose higher CPU costs to verify and sign email 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 o Keys can be replaced on a regular basis, thus their lifetime can
be relatively short be relatively short
o The security goals of this specification are modest compared to o The security goals of this specification are modest compared to
typical goals of public-key systems typical goals of public-key systems
See RFC3766 [RFC3766] for further discussion of selecting key sizes.
3.4 Canonicalization 3.4 Canonicalization
Empirical evidence demonstrates that some mail servers and relay Empirical evidence demonstrates that some mail servers and relay
systems modify email in transit, potentially invalidating a systems modify email in transit, potentially invalidating a
signature. There are two competing perspectives on such signature. There are two competing perspectives on such
modifications. For most signers, mild modification of email is modifications. For most signers, mild modification of email is
immaterial to the authentication status of the email. For such immaterial to the authentication status of the email. For such
signers a canonicalization algorithm that survives modest in-transit signers a canonicalization algorithm that survives modest in-transit
modification is preferred. modification is preferred.
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that are within the bounds of email standards such as [RFC2822], but that are within the bounds of email standards such as [RFC2822], but
are unwilling to accept any modification to the body of messages. are unwilling to accept any modification to the body of messages.
To satisfy all requirements, two canonicalization algorithms are To satisfy all requirements, two canonicalization algorithms are
defined for each of the header and the body: a "simple" algorithm defined for each of the header and the body: a "simple" algorithm
that tolerates almost no modification and a "relaxed" algorithm that that tolerates almost no modification and a "relaxed" algorithm that
tolerates common modifications such as white-space replacement and tolerates common modifications such as white-space replacement and
header field line re-wrapping. A signer MAY specify either algorithm header field line re-wrapping. A signer MAY specify either algorithm
for header or body when signing an email. If no canonicalization for header or body when signing an email. If no canonicalization
algorithm is specified by the signer, the "simple" algorithm defaults algorithm is specified by the signer, the "simple" algorithm defaults
for both header and body. A verifier MUST be able to process email for both header and body. Verifiers MUST implement both
using either canonicalization algorithm. Further canonicalization canonicalization algorithms. Further canonicalization algorithms MAY
algorithms MAY be defined in the future; verifiers MUST ignore any be defined in the future; verifiers MUST ignore any signatures that
signatures that use unrecognized canonicalization algorithms. use unrecognized canonicalization algorithms.
In all cases, the header fields of the message are presented to the In all cases, the header fields of the message are presented to the
signing algorithm first in the order indicated by the signature signing algorithm first in the order indicated by the signature
header field and canonicalized using the indicated algorithm. Only header field and canonicalized using the indicated algorithm. Only
header fields listed as signed in the signature header field are header fields listed as signed in the signature header field are
included. The CRLF separating the header field from the body is then included. The CRLF separating the header field from the body is then
presented, followed by the canonicalized body. Note that the header presented, followed by the canonicalized body. Note that the header
and body may use different canonicalization algorithms. and body may use different canonicalization algorithms.
Canonicalization simply prepares the email for presentation to the Canonicalization simply prepares the email for presentation to the
skipping to change at page 13, line 11 skipping to change at page 13, line 24
normal" format (e.g., text is ASCII encoded, lines are separated with normal" format (e.g., text is ASCII encoded, lines are separated with
CRLF characters, etc.). See also Section 5.3 for information about CRLF characters, etc.). See also Section 5.3 for information about
normalizing the message. normalizing the message.
3.4.1 The "simple" Header Field Canonicalization Algorithm 3.4.1 The "simple" Header Field Canonicalization Algorithm
The "simple" header canonicalization algorithm does not change header The "simple" header canonicalization algorithm does not change header
fields in any way. Header fields MUST be presented to the signing or fields in any way. Header fields MUST be presented to the signing or
verification algorithm exactly as they are in the message being verification algorithm exactly as they are in the message being
signed or verified. In particular, header field names MUST NOT be signed or verified. In particular, header field names MUST NOT be
case folded. case folded and white space MUST NOT be changed.
3.4.2 The "relaxed" Header Field Canonicalization Algorithm 3.4.2 The "relaxed" Header Field Canonicalization Algorithm
The "relaxed" header canonicalization algorithm MUST apply the The "relaxed" header canonicalization algorithm MUST apply the
following steps in order: following steps in order:
o Convert all header field names (not the header field values) to o Convert all header field names (not the header field values) to
lower case. For example, convert "SUBJect: AbC" to "subject: lower case. For example, convert "SUBJect: AbC" to "subject:
AbC". AbC".
skipping to change at page 13, line 51 skipping to change at page 14, line 17
previous version of this draft is that nowsp reduces all strings previous version of this draft is that nowsp reduces all strings
of streaming white space to zero characters while "relaxed" of streaming white space to zero characters while "relaxed"
reduces strings of white space to one space.] reduces strings of white space to one space.]
3.4.3 The "simple" Body Canonicalization Algorithm 3.4.3 The "simple" Body Canonicalization Algorithm
The "simple" body canonicalization algorithm ignores all empty lines The "simple" body canonicalization algorithm ignores all empty lines
at the end of the message body. An empty line is a line of zero 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 length after removal of the line terminator. It makes no other
changes to the message body. In more formal terms, the "simple" body changes to the message body. In more formal terms, the "simple" body
canonicalization algorithm reduces "CRLF 0*CRLF" to a single "CRLF" canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the
at the end of the body. body to a single "CRLF".
3.4.4 The "relaxed" Body Canonicalization Algorithm 3.4.4 The "relaxed" Body Canonicalization Algorithm
[[This section may be deleted; see discussion below.]] The "relaxed" [[This section may be deleted; see discussion below.]] The "relaxed"
body canonicalization algorithm: body canonicalization algorithm:
o Ignores all white space at the end of lines. Implementations MUST o Ignores all white space at the end of lines. Implementations MUST
NOT remove the CRLF at the end of the line. NOT remove the CRLF at the end of the line.
o Reduces all sequences of WSP within a line to a single SP o Reduces all sequences of WSP within a line to a single SP
character. character.
o Ignores all empty lines at the end of the message body. "Empty o Ignores all empty lines at the end of the message body. "Empty
line" is defined in Section 3.4.3. line" is defined in Section 3.4.3.
[[NON-NORMATIVE DISCUSSION: The authors are undecided whether to [[NON-NORMATIVE DISCUSSION: The authors are undecided whether to
leave the "relaxed" body canonicalization algorithm in to the leave the "relaxed" body canonicalization algorithm in to the
specification or delete it entirely. We believe that for the vast specification or delete it entirely. We believe that for the vast
majority of cases, the "simple" body canonicalization algorithm majority of cases, the "simple" body canonicalization algorithm
should be sufficient. We simply do not have enough data to know should be sufficient. We simply do not have enough data to know
whether retain the "relaxed" body canonicalization algorithm or whether to retain the "relaxed" body canonicalization algorithm or
not.]] not.]]
3.4.5 Body Length Limits 3.4.5 Body Length Limits
A body length count MAY be specified to limit the signature A body length count MAY be specified to limit the signature
calculation to an initial prefix of the body text. If the body calculation to an initial prefix of the body text. If the body
length count is not specified then the entire message body is signed length count is not specified then the entire message body is signed
and verified. and verified.
INFORMATIVE IMPLEMENTATION NOTE: Body length limits could be INFORMATIVE IMPLEMENTATION NOTE: Body length limits could be
useful in increasing signature robustness when sending to a useful in increasing signature robustness when sending to a
mailing list that both appends to content sent to it and does not mailing list that both appends to content sent to it and does not
sign its messages. However, using such limits enables an attack sign its messages. However, using such limits enables an attack
in which a sender with malicious intent modifies a message to in which a sender with malicious intent modifies a message to
include content that solely benefits the attacker. It is possible include content that solely benefits the attacker. It is possible
for the appended content to completely replace the original for the appended content to completely replace the original
content in the end recipient's eyes and to defeat duplicate content in the end recipient's eyes and to defeat duplicate
message detection algorithms. To avoid this attack, signers message detection algorithms. To avoid this attack, signers
should be wary of using this tag, and verifiers might wish to should be wary of using this tag, and verifiers might wish to
ignore the tag or remove text that appears after the specified ignore the tag or remove text that appears after the specified
content length. content length, perhaps based on other criteria.
The body length count allows the signer of a message to permit data 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 to be appended to the end of the body of a signed message. The body
length count is made following the canonicalization algorithm; for length count is made following the canonicalization algorithm; for
example, any white space ignored by a canonicalization algorithm is example, any white space ignored by a canonicalization algorithm is
not included as part of the body length count. not included as part of the body length count.
INFORMATIVE RATIONALE: This capability is provided because it is INFORMATIVE RATIONALE: This capability is provided because it is
very common for mailing lists to add trailers to messages (e.g., very common for mailing lists to add trailers to messages (e.g.,
instructions how to get off the list). Until those messages are instructions how to get off the list). Until those messages are
skipping to change at page 15, line 36 skipping to change at page 15, line 50
unsigned. unsigned.
Note that verifiers MAY choose to reject or truncate messages that Note that verifiers MAY choose to reject or truncate messages that
have body content beyond that specified by the body length count. have body content beyond that specified by the body length count.
Signers wishing to ensure that no modification of any sort can occur Signers wishing to ensure that no modification of any sort can occur
should specify the "simple" algorithm and omit the body length count. should specify the "simple" algorithm and omit the body length count.
3.4.6 Example 3.4.6 Example
Assuming a "c=relaxes/relaxed" canonicalization algorithm, a message (In the following examples, actual white space is used only for
reading: 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.)
A message reading:
A: <SP> X <CRLF> A: <SP> X <CRLF>
B <SP> : <SP> Y <TAB><CRLF> B <SP> : <SP> Y <TAB><CRLF>
<TAB> Z <SP><SP><CRLF> <TAB> Z <SP><SP><CRLF>
<CRLF> <CRLF>
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF> D <SP><TAB><SP> E <CRLF>
<CRLF> <CRLF>
<CRLF> <CRLF>
when canonicalized using "relaxed" for both header and body results
in:
when canonicalized using relaxed canonicalization for both header and
body results in:
a:X<CRLF> a:X<CRLF>
b:Y<SP>Z<CRLF> b:Y<SP>Z<CRLF>
<CRLF> <CRLF>
<SP>C<CRLF> <SP>C<CRLF>
D<SP>E<CRLF> D<SP>E<CRLF>
The same message canonicalized using simple canonicalization for both
header and body results in:
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>
When processed using relaxed header canonicalization and simple body
canonicalization, the canonicalized version reads:
a:X <CRLF>
b:Y <SP> Z <CRLF>
<CRLF>
<SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF>
3.5 The DKIM-Signature header field 3.5 The DKIM-Signature header field
The signature of the email is stored in the "DKIM-Signature:" header The signature of the email is stored in the "DKIM-Signature:" header
field. This header field contains all of the signature and key- field. This header field contains all of the signaturee and key-
fetching data. The DKIM-Signature value is a tag-list as described fetching data. The DKIM-Signature value is a tag-list as described
in Section 3.2. in Section 3.2.
The "DKIM-Signature:" header field SHOULD be treated as though it 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 were a trace header field as defined in section 3.6 of [RFC2822], and
hence SHOULD NOT be reordered and SHOULD be kept in blocks prepended hence SHOULD NOT be reordered and SHOULD be prepended to the message.
to the message. In particular, the "DKIM-Signature" header field In particular, the "DKIM-Signature" header field SHOULD precede the
SHOULD precede the original email header fields presented to the original email header fields presented to the canonicalization and
canonicalization and signature algorithms. signature algorithms.
The "DKIM-Signature:" header field is always included in the The "DKIM-Signature:" header field is always included in the
signature calculation, after the body of the message; however, when signature calculation, after the body of the message; however, when
calculating or verifying the signature, the b= (signature value) calculating or verifying the signature, the value of the b= tag
value MUST be treated as though it were the null string. Unknown (signature value) MUST be treated as though it were the null string.
tags MUST be signed but MUST be otherwise ignored by verifiers. 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 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 as quoted-printable are as described in section 6.7 of MIME Part One
[RFC2045], with the additional conversion of semicolon characters to [RFC2045], with the additional conversion of semicolon characters to
"=3B". "=3B".
Tags on the DKIM-Signature header field along with their type and Tags on the DKIM-Signature header field along with their type and
requirement status are shown below. Valid tags are: requirement status are shown below. Defined tags are described
below. Unrecognized tags MUST be ignored.
v= Version (MUST NOT be included). This tag is reserved for future v= Version (MUST NOT be included). This tag is reserved for future
use to indicate a possible new, incompatible version of the use to indicate a possible new, incompatible version of the
specification. It MUST NOT be included in the DKIM-Signature specification. It MUST NOT be included in the DKIM-Signature
header field. header field.
ABNF: ABNF:
sig-v-tag = sig-v-tag =
a= The algorithm used to generate the signature (plain-text;
REQUIRED). Signers and verifiers MUST support "rsa-sha1", an
RSA-signed SHA-1 digest. See Section 3.3 for a description of
algorithms.
INFORMATIVE RATIONALE: The authors understand that SHA-1 has a= The algorithm used to generate the signature (plain-text;
been theoretically compromised. However, viable attacks REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256";
require the attacker to choose both sets of input text; given signers SHOULD sign using "rsa-sha256". See Section 3.3 for a
a preexisting input (a "preimaging" attack), it is still hard description of algorithms.
to determine another input that produces an SHA-1 collision,
and the chance that such input would be of value to an
attacker is minimal. Also, there is broad library support
for SHA-1, whereas alternatives such as SHA-256 are just
emerging. Finally, DKIM is not intended to have legal- or
military-grade requirements. There is nothing inherent in
using SHA-1 here other than implementer convenience. See
<http://www3.ietf.org/proceedings/05mar/slides/saag-3.pdf>
for a discussion of the security issues.
ABNF: ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = "rsa-sha1" / x-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 x-sig-a-tag-alg = hyphenated-word ; for later extension
b= The signature data (base64; REQUIRED). Whitespace is ignored in b= The signature data (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when re-assembling the original this value and MUST be ignored when re-assembling the original
signature. In particular, the signing process can safely insert signature. In particular, the signing process can safely insert
FWS in this value in arbitrary places to conform to line-length FWS in this value in arbitrary places to conform to line-length
limits. See Signer Actions (Section 5) for how the signature is limits. See Signer Actions (Section 5) for how the signature is
computed. computed.
ABNF: ABNF:
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data
skipping to change at page 17, line 39 skipping to change at page 18, line 14
b= The signature data (base64; REQUIRED). Whitespace is ignored in b= The signature data (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when re-assembling the original this value and MUST be ignored when re-assembling the original
signature. In particular, the signing process can safely insert signature. In particular, the signing process can safely insert
FWS in this value in arbitrary places to conform to line-length FWS in this value in arbitrary places to conform to line-length
limits. See Signer Actions (Section 5) for how the signature is limits. See Signer Actions (Section 5) for how the signature is
computed. computed.
ABNF: ABNF:
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data
sig-b-tag-data = base64string sig-b-tagg-data = base64string
c= Body canonicalization (plain-text; OPTIONAL, default is "simple/ bh= The hash of the body part of the message (base64; REQUIRED).
simple"). This tag informs the verifier of the type of 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 canonicalization used to prepare the message for signing. It
consists of two names separated by a "slash" (%d47) character, consists of two names separated by a "slash" (%d47) character,
corresponding to the header and body canonicalization algorithms corresponding to the header and body canonicalization algorithms
respectively. These algorithms are described in Section 3.4. If respectively. These algorithms are described in Section 3.4. If
only one algorithm is named, that algorithm is used for the only one algorithm is named, that algorithm is used for the
header and "simple" is used for the body. For example, "relaxed" header and "simple" is used for the body. For example,
is treated the same as "relaxed/simple". "c=relaxed" is treated the same as "c=relaxed/simple".
ABNF: ABNF:
sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg
["/" sig-c-tag-alg] ["/" sig-c-tag-alg]
sig-c-tag-alg = "simple" / "relaxed" / x-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 x-sig-c-tag-alg = hyphenated-word ; for later extension
d= The domain of the signing entity (plain-text; REQUIRED). This d= The domain of the signing entity (plain-text; REQUIRED). This
is the domain that will be queried for the public key. 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. domain MUST be the same as or a parent domain of the "i=" tag
When presented with a signature that does not meet this (the signing identity, as described below). When presented with
requirement, verifiers MUST either ignore the signature or reject a signature that does not meet this requirement, verifiers MUST
the message. either ignore the signature or reject the message.
ABNF: ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] Domain sig-d-tag = %x64 [FWS] "=" [FWS] Domain
h= Signed header fields (plain-text, but see description; h= Signed header fields (plain-text, but see description;
REQUIRED). A colon-separated list of header field names that REQUIRED). A colon-separated list of header field names that
identify the header fields presented to the signing algorithm. identify the header fields presented to the signing algorithm.
The field MUST contain the complete list of header fields in the The field MUST contain the complete list of header fields in the
order presented to the signing algorithm. The field MAY contain order presented to the signing algorithm. The field MAY contain
names of header fields that do not exist when signed; nonexistent names of header fields that do not exist when signed; nonexistent
header fields do not contribute to the signature computation header fields do not contribute to the signature computation
(that is, they are treated as the null input, including the (that is, they are treated as the null input, including the
header field name, the separating colon, the header field value, header field name, the separating colon, the header field value,
and any CRLF terminator), and when verified non-existent header and any CRLF terminator). The field MUST NOT include the DKIM-
fields MUST be treated in the same way. The field MUST NOT Signature header field that is being created or verified.
include the DKIM-Signature header field that is being created or Folding white space (FWS) MAY be included on either side of the
verified. Folding white space (FWS) MAY be included on either colon separator. Header ffield names MUST be compared against
side of the colon separator. Header field names MUST be compared actual header field names in a case insensitive manner. This
against actual header field names in a case insensitive manner. list MUST NOT be empty. See Section 5.4 for a discussion of
choosing header fields to sign.
ABNF: ABNF:
sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name
0*( *FWS ":" *FWS hdr-name ) 0*( *FWS ":" *FWS hdr-name )
hdr-name = field-name hdr-name = field-name
INFORMATIVE EXPLANATION: By "signing" header fields that do INFORMATIVE EXPLANATION: By "signing" header fields that do
not actually exist, a signer can prevent insertion of those not actually exist, a signer can prevent insertion of those
header fields before verification. However, since a sender header fields before verification. However, since a sender
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INFORMATIVE EXPLANATION: The exclusion of the header field INFORMATIVE EXPLANATION: The exclusion of the header field
name and colon as well as the header field value for non- name and colon as well as the header field value for non-
existent header fields prevents an attacker from inserting an existent header fields prevents an attacker from inserting an
actual header field with a null value. actual header field with a null value.
i= Identity of the user or agent (e.g., a mailing list manager) on i= Identity of the user or agent (e.g., a mailing list manager) on
behalf of which this message is signed (quoted-printable; behalf of which this message is signed (quoted-printable;
OPTIONAL, default is an empty local-part followed by an "@" OPTIONAL, default is an empty local-part followed by an "@"
followed by the domain from the "d=" tag). The syntax is a followed by the domain from the "d=" tag). The syntax is a
standard email address where the local-part is optional. If the standard email address where the local-part MAY be omitted. The
signing domain is unable or unwilling to commit to an individual domain part of the address MUST be the same as or a subdomain of
user name within their domain, the local-part of the address MUST the value of the "d=" tag.
be omitted. If the local-part of the address is omitted or the
"i=" tag is not present, the key used to sign MUST be valid for
any address in the domain. The domain part of the address MUST
be the same as or a subdomain of the value of the "d=" tag.
ABNF: ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" Domain 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 INFORMATIVE DISCUSSION: This document does not require the
value of the "i=" tag to match the identity in any message value of the "i=" tag to match the identity in any message
header field fields. This is considered to be a verifier header field fields. This is considered to be a verifier
policy issue, described in another document [XREF-TBD]. policy issue, described in another document [XREF-TBD].
Constraints between the value of the "i=" tag and other Constraints between the value of the "i=" tag and other
identities in other header fields seek to apply basic identities in other header fields seek to apply basic
authentication into the semantics of trust associated with a aauthentication into the semantics of trust associated with a
role such as content author. Trust is a broad and complex role such as content author. Trust is a broad and complex
topic and trust mechanisms are subject to highly creative topic and trust mechanisms are subject to highly creative
attacks. The real-world efficacy of any but the most basic attacks. The real-world efficacy of any but the most basic
bindings between the "i=" value and other identities is not bindings between the "i=" value and other identities is not
well established, nor is its vulnerability to subversion by well established, nor is its vulnerability to subversion by
an attacker. Hence reliance on the use of these options an attacker. Hence reliance on the use of these options
should be strictly limited. In particular it is not at all should be strictly limited. In particular it is not at all
clear to what extent a typical end-user recipient can rely on clear to what extent a typical end-user recipient can rely on
any assurances that might be made by successful use of the any assurances that might be made by successful use of the
"i=" options. "i=" options.
skipping to change at page 20, line 32 skipping to change at page 21, line 24
ABNF: ABNF:
sig-l-tag = %x6c [FWS] "=" [FWS] 1*DIGIT sig-l-tag = %x6c [FWS] "=" [FWS] 1*DIGIT
q= A colon-separated list of query methods used to retrieve the q= A colon-separated list of query methods used to retrieve the
public key (plain-text; OPTIONAL, default is "dns"). Each query public key (plain-text; OPTIONAL, default is "dns"). Each query
method is of the form "type[/options]", where the syntax and method is of the form "type[/options]", where the syntax and
semantics of the options depends on the type. If there are semantics of the options depends on the type. If there are
multiple query mechanisms listed, the choice of query mechanism multiple query mechanisms listed, the choice of query mechanism
MUST NOT change the interpretation of the signature. Currently MUST NOT change the interpretation of the signature. Currently
the only valid value is "dns" which defines the DNS lookup the only valid value iis "dns" which defines the DNS lookup
algorithm described elsewhere in this document. No options are algorithm described elsewhere in this document. No options are
defined for the "dns" query type, but the string "dns" MAY have a defined for the "dns" query type, but the string "dns" MAY have a
trailing "/" character. Verifiers and signers MUST support trailing "/" character. Verifiers and signers MUST support
"dns". "dns".
INFORMATIVE RATIONALE: Explicitly allowing a trailing "/" on INFORMATIVE RATIONALE: Explicitly allowing a trailing "/" on
"dns" allows for the possibility of adding options later and "dns" allows for the possibility of adding options later and
makes it clear that matching of the query type must terminate makes it clear that matching of the query type must terminate
on either "/" or end of string. on either "/" or end of string.
skipping to change at page 21, line 4 skipping to change at page 21, line 43
on either "/" or end of string. on either "/" or end of string.
ABNF: ABNF:
sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([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-method = sig-q-tag-type ["/" sig-q-tag-args]
sig-q-tag-type = "dns" / x-sig-q-tag-type sig-q-tag-type = "dns" / x-sig-q-tag-type
x-sig-q-tag-type = hyphenated-word ; for future extension x-sig-q-tag-type = hyphenated-word ; for future extension
x-sig-q-tag-args = qp-hdr-value x-sig-q-tag-args = qp-hdr-value
s= The selector subdividing the namespace for the "d=" (domain) tag s= The selector subdividing the namespace for the "d=" (domain) tag
(plain-text; REQUIRED). (plain-text; REQUIRED).
ABNF: ABNF:
sig-s-tag = %x73 [FWS] "=" [FWS] Domain sig-s-tag = %x73 [FWS] "=" [FWS] Domain
t= Signature Timestamp (plain-text; RECOMMENDED, default is an t= Signature Timestamp (plain-text; RECOMMENDED, default is an
unknown creation time). The time that this signature was unknown creation time). The time that this signature was
created. The format is the standard Unix seconds-since-1970. created. The format is the number of seconds since 00:00:00 on
The value is expressed as an unsigned integer in decimal ASCII. January 1, 1970 in the UTC time zone. The value is expressed as
This value is not constrained to fit into a 31- or 32-bit an unsigned integer in decimal ASCII. This value is not
integer. Implementations SHOULD be prepared to handle values up constrained to fit into a 31- or 32-bit integer. Implementations
to at least 10^12 (until approximately AD 200,000; this fits into SHOULD be prepared to handle values up to at least 10^12 (until
40 bits). To avoid denial of service attacks, implementations approximately AD 200,000; this fits into 40 bits). To avoid
MAY consider any value longer than 12 digits to be infinite. denial of service attacks, implementations MAY consider any value
longer than 12 digits to be infinite.
ABNF: ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
x= Signature Expiration (plain-text; RECOMMENDED, default is no x= Signature Expiration (plain-text; RECOMMENDED, default is no
expiration). Signature expiration in seconds-since-1970 format expiration). The format is the same as in the "t=" tag,
as an absolute date, not as a time delta from the signing represented as an absolute date, not as a time delta from the
timestamp. Signatures MUST NOT be considered valid if the signing timestamp. Signatures MUST NOT be considered valid if
current time at the verifier is past the expiration date. The the current time at the verifier is past the expiration date.
value is expressed as an unsigned integer in decimal ASCII, with The value is expressed as an unsigned integer in decimal ASCII,
the same contraints on the value in the "t=" tag. with the same contraints on the value in the "t=" tag. 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- INFORMATIVE NOTE: The x= tag is not intended as an anti-
replay defense. replay defense.
ABNF: ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT
z= Copied header fields (plain-text, but see description; OPTIONAL, z= Copied header fields (plain-text, but see description; OPTIONAL,
default is null). A vertical-bar-separated list of header field default is null). A vertical-bar-separated list of header field
names and copies of header field values that identify the header names and copies of header field values that identify the header
fields presented to the signing algorithm. The field MUST fields present when the message was signed. This field need not
contain the complete list of header fields in the order presented contain the same header fields listed in the "h=" tag. Copied
to the signing algorithm. Copied header field values MUST header field values MUST immediately follow the header field name
immediately follow the header field name with a colon separator with a colon separator (no white space permitted). Header field
(no white space permitted). Header field values MUST be values MUST be represented as Quoted-Printable [RFC2045] with
represented as Quoted-Printable [RFC2045] with vertical bars, vertical bars, colons, semicolons, and white space encoded in
colons, semicolons, and white space encoded in addition to the addition to the usual requirements.
usual requirements.
Verifiers MUST NOT use the copied header field values for Verifiers MUST NOT use the header field names or copied values
verification should they be present in the h= field. Copied for checking the signature in any way. Copied header field
header field values are for forensic use only. values are for diagnostic use only.
Header fields with characters requiring conversion (perhaps from Header fields with characters requiring conversion (perhaps from
legacy MTAs which are not [RFC2822] compliant) SHOULD be legacy MTAs which are not [RFC2822] compliant) SHOULD be
converted as described in MIME Part Three [RFC2047]. converted as described in MIME Part Three [RFC2047].
ABNF: ABNF:
sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy
*( [FWS] "|" sig-z-tag-copy ) *( [FWS] "|" sig-z-tag-copy )
sig-z-tag-copy = hdr-name ":" [FWS] qp-hdr-value sig-z-tag-copy = hdr-name ":" [FWS] qp-hdr-value
qp-hdr-value = <quoted-printable text with WS, "|", ":", qp-hdr-value = <quoted-printable text with WS, "|", ":",
skipping to change at page 22, line 37 skipping to change at page 23, line 35
DKIM-Signature: a=rsa-sha1; d=example.net; s=brisbane DKIM-Signature: a=rsa-sha1; d=example.net; s=brisbane
c=simple; q=dns; i=@eng.example.net; t=1117574938; x=1118006938; c=simple; q=dns; i=@eng.example.net; t=1117574938; x=1118006938;
h=from:to:subject:date; h=from:to:subject:date;
z=From:foo@eng.example.net|To:joe@example.com| z=From:foo@eng.example.net|To:joe@example.com|
Subject:demo%20run|Date:July%205,%202005%203:44:08%20PM%20-0700 Subject:demo%20run|Date:July%205,%202005%203:44:08%20PM%20-0700
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR VoG4ZHRNiYzR
3.6 Key Management and Representation 3.6 Key Management and Representation
DKIM keys do not require third party signatures by Certificate Signature applications require some level of assurance that the
Authorities in order to be trusted, since the public key is retrieved verification public key is associated with the claimed signer. Many
directly from the signer. 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 DKIM keys can potentially be stored in multiple types of key servers
and in multiple formats. The storage and format of keys are and in multiple formats. The storage and format of keys are
irrelevant to the remainder of the DKIM algorithm. irrelevant to the remainder of the DKIM algorithm.
Parameters to the key lookup algorithm are the type of the lookup 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 "q=" tag), the domain of the responsible signer (the "d=" tag of
the DKIM-Signature header field), the signing identity (the "i=" the DKIM-Signature header field), the signing identity (the "i="
tag), and the selector (the "s=" tag). The "i=" tag value could be tag), and the selector (the "s=" tag). The "i=" tag value could be
ignored by some key services. ignored by some key services.
public_key = dkim_find_key(q_val, d_val, i_val, s_val) public_key = dkim_find_key(q_val, d_val, i_val, s_val)
This document defines a single binding, using DNS to distribute the This document defines a single binding, using DNS to distribute the
keys. keys.
3.6.1 Textual Representation 3.6.1 Textual Representation
It is expected that many key servers will choose to present the keys It is expected that many key servers will choose to present the keys
in a text format. The following definition MUST be used for any DKIM in an otherwise unstructured text format (for example, an XML form
key represented in textual form. would not be considered to be unstructured text for this purpose).
The following definition MUST be used for any DKIM key represented in
an otherwise unstructured textual form.
The overall syntax is a key-value-list as described above. The The overall syntax is a key-value-list as described in Section 3.2.
current valid tags are: 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 v= Version of the DKIM key record (plain-text; RECOMMENDED, default
is "DKIM1"). If specified, this tag MUST be set to "DKIM1" is "DKIM1"). If specified, this tag MUST be set to "DKIM1"
(without the quotes). This tag MUST be the first tag in the (without the quotes). This tag MUST be the first tag in the
response. Responses beginning with a "v=" tag with any other response. Responses beginning with a "v=" tag with any other
value MUST be discarded. value MUST be discarded.
ABNF: ABNF:
key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1" key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1"
skipping to change at page 24, line 4 skipping to change at page 25, line 6
ABNF: ABNF:
key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart
key-g-tag-lpart = [dot-atom] ["*"] [dot-atom] key-g-tag-lpart = [dot-atom] ["*"] [dot-atom]
[[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a dot- [[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a dot-
atom. This should probably use a different character for atom. This should probably use a different character for
wildcarding. Unfortunately, the options are non-mnemonic wildcarding. Unfortunately, the options are non-mnemonic
(e.g., "@", "(", ":"). Alternatively we could insist on (e.g., "@", "(", ":"). Alternatively we could insist on
escaping a "*" intended as a literal "*" in the address.]] escaping a "*" intended as a literal "*" in the address.]]
h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
h= Accceptable hash algorithms (plain-text; OPTIONAL, defaults to
allowing all algorithms). A colon-separated list of hash allowing all algorithms). A colon-separated list of hash
algorithms that might be used. Signers and Verifiers MUST algorithms that might be used. Signers and Verifiers MUST
support the "sha1" hash algorithm. support the "sha1" hash algorithm.
ABNF: ABNF:
key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg
0*( [FWS] ":" [FWS] key-h-tag-alg ) 0*( [FWS] ":" [FWS] key-h-tag-alg )
key-h-tag-alg = "sha1" / x-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 x-key-h-tag-alg = hyphenated-word ; for future extension
k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and
verifiers MUST support the "rsa" key type. The "rsa" key type verifiers MUST support the "rsa" key type. The "rsa" key type
indicates that an RSA public key, as defined in [RFC3447], 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 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.) p= tag further encodes the value using the base64 algorithm.)
ABNF: ABNF:
skipping to change at page 25, line 22 skipping to change at page 26, line 26
appropriate type is not listed. Currently defined service types appropriate type is not listed. Currently defined service types
are: are:
* matches all service types * matches all service types
email electronic mail (not necessarily limited to SMTP) email electronic mail (not necessarily limited to SMTP)
This tag is intended to permit senders to constrain the use of This tag is intended to permit senders to constrain the use of
delegated keys, e.g., where a company is willing to delegate the 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 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 send IM or make VoIP calls. (This of courrse presumes that these
keys are used in other services in the future.) keys are used in other services in the future.)
ABNF: ABNF:
key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type
key-s-tag-type = "email" / "*" / x-key-s-tag-type key-s-tag-type = "email" / "*" / x-key-s-tag-type
x-key-s-tag-type = hyphenated-word ; for future extension x-key-s-tag-type = hyphenated-word ; for future extension
t= Flags, represented as a colon-separated list of names (plain- t= Flags, represented as a colon-separated list of names (plain-
text; OPTIONAL, default is no flags set). The defined flags are: text; OPTIONAL, default is no flags set). The defined flags are:
skipping to change at page 26, line 4 skipping to change at page 27, line 9
unsigned email, even should the signature fail to verify. unsigned email, even should the signature fail to verify.
Verifiers MAY wish to track testing mode results to assist Verifiers MAY wish to track testing mode results to assist
the signer. the signer.
ABNF: ABNF:
key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag
0*( [FWS] ":" [FWS] key-t-tag-flag ) 0*( [FWS] ":" [FWS] key-t-tag-flag )
key-t-tag-flag = "y" / x-key-t-tag-flag key-t-tag-flag = "y" / x-key-t-tag-flag
x-key-t-tag-flag = hyphenated-word ; for future extension x-key-t-tag-flag = hyphenated-word ; for future extension
Unrecognized flags MUST be ignored. Unrecognized flags MUST be ignored.
3.6.2 DNS binding 3.6.2 DNS binding
A binding using DNS as a key service is hereby defined. All A binding using DNS as a key service is hereby defined. All
implementations MUST support this binding. implementations MUST support this binding.
3.6.2.1 Name Space 3.6.2.1 Name Space
All DKIM keys are stored in a "_domainkey" subdomain. Given a DKIM- All DKIM keys are stored in a subdomain named ""_domainkey"". Given
Signature field with a "d=" tag of "domain" and an "s=" tag of a DKIM-Signature field with a "d=" tag of ""example.com"" and an "s="
"selector", the DNS query will be for "selector."_domainkey".domain". 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. The value of the "i=" tag is not used by the DNS binding.
3.6.2.2 Resource Record Types for Key Storage 3.6.2.2 Resource Record Types for Key Storage
[[This section needs to be fleshed out. ACTUALLY: will be addressed [[This section needs to be fleshed out. ACTUALLY: will be addressed
in another document.]] in another document.]]
Two RR types are used: DKK and TXT. Two RR types are used: DKK and TXT.
skipping to change at page 26, line 37 skipping to change at page 27, line 44
intended to allow the largest possible keys to be represented and intended to allow the largest possible keys to be represented and
transmitted in a UDP DNS packet. Details of this RR are described in transmitted in a UDP DNS packet. Details of this RR are described in
[ID-DKIM-RR]. [ID-DKIM-RR].
TXT records are encoded as described in Section 3.6.1. TXT records are encoded as described in Section 3.6.1.
Verifiers SHOULD search for a DKK RR first, if possible, followed by Verifiers SHOULD search for a DKK RR first, if possible, followed by
a TXT RR. If the verifier is unable to search for a DKK RR or a DKK a TXT RR. If the verifier is unable to search for a DKK RR or a DKK
RR is not found, the verifier MUST search for a TXT RR. RR is not found, the verifier MUST search for a TXT RR.
3.7 Computing the Message Hash 3.7 Computing the Message Hashes
Both signing and verifying message signatures starts with a step of Both signing and verifying message signatures starts with a step of
computing a cryptographic hash of the message. Signers will choose computing two cryptographic hash over the message. Signers will
the parameters of the signature as described in Signer Actions choose the parameters of the signature as described in Signer Actions
(Section 5); verifiers will use the parameters specified in the (Section 5); verifiers will use the parameters specified in the
"DKIM-Signature" header field being verified. In the following "DKIM-Signature" header field being verified. In the following
discussion, the names of the tags in the "DKIM-Signature" header discussion, the names of the tags in the "DKIM-Signature" header
field which either exists (when verifying) or will be created (when field which either exists (when verifying) or will be created (when
signing) are used. signing) are used. Note that canonicalization (Section 3.4) is only
used to prepare the email for signing or verifying; it does not
The signer or verifier MUST pass the following to the hash algorithm
in the indicated order. 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. affect the transmitted email in any way.
1. The header fields specified by the "h=" tag, in the order The signer or verifier must compute two hashes, one over the body of
specified in that tag, and canonicalized using the header the message and one over the header of the message. Signers MUST
canonicalization algorithm specified in the "c=" tag. 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.
2. A single CRLF. In hash step 1, the signer or verifier MUST hash the message body,
canonicalized using the header 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 "XXX=" tag of the DKIM-Signature: header field.
3. The message body, canonicalized using the body canonicalization In hash step 2, the signer or verifier MUST pass the following to the
algorithm specified in the "c=" tag, and truncated to the length hash algorithm in the indicated order.
specified in the "l=" tag.
4. A single CRLF. 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.
5. The "DKIM-Signature" header field that exists (verifying) or will 2. The "DKIM-Signature" header field that exists (verifying) or will
be inserted (signing) in the message, with the content of the be inserted (signing) in the message, with the value of the "b="
"b=" tag deleted (i.e., treated as the empty string), tag deleted (i.e., treated as the empty string), canonicalized
canonicalized using the header canonicalization algorithm using the header canonicalization algorithm specified in the "c="
specified in the "c=" tag, and without a trailing CRLF. tag, and without a trailing CRLF.
After the body is processed, a single CRLF followed by the "DKIM- After the body is processed, a single CRLF followed by the "DKIM-
Signature" header field being created or verified is presented to the Signature" header field being created or verified is presented to the
algorithm. The value portion of the "b=" tag (that is, the portion algorithm. The value portion of the "b=" tag (that is, the portion
after the "=" sign) must be treated as though it were empty, and the after the "=" sign) must be treated as though it were empty, and the
header field must be canonicalized according to the algorithm that is header field must be canonicalized according to the algorithm that is
specified in the "c=" tag. Any final CRLF on the "DKIM-Signature" specified in the "c=" tag. Any final CRLF on the "DKIM-Signature"
header field MUST NOT be included in the signature computation. header field MUST NOT be included in the signature computation.
All tags and their values in the DKIM-Signature header field are All tags and their values in the DKIM-Signature header field are
included in the cryptographic hash with the sole exception of the included in the cryptographic hash with the sole exception of the
value portion of the "b=" (signature) tag, which MUST be treated as 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 the null string. All tags MUST be included even if they might not be
understood by the verifier. The header field MUST be presented to understood by the verifier. The header field MUST be presented to
the hash algorithm after the body of the message rather than with the 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 rest of the headder fields and MUST be canonicalized as specified in
the "c=" (canonicalization) tag. The DKIM-Signature header field the "c=" (canonicalization) tag. The DKIM-Signature header field
MUST NOT be included in its own h= tag. MUST NOT be included in its own h= tag.
When calculating the hash on messages that will be transmitted using When calculating the hash on messages that will be transmitted using
base64 or quoted-printable encoding, signers MUST compute the hash base64 or quoted-printable encoding, signers MUST compute the hash
after the encoding. Likewise, the verifier MUST incorporate the after the encoding. Likewise, the verifier MUST incorporate the
values into the hash before decoding the base64 or quoted-printable values into the hash before decoding the base64 or quoted-printable
text. However, the hash MUST be computed before transport level text. However, the hash MUST be computed before transport level
encodings such as SMTP "dot-stuffing." encodings such as SMTP "dot-stuffing."
With the exception of the canonicalization procedure described in With the exception of the canonicalization procedure described in
Section 3.4, the DKIM signing process treats the body of messages as 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- simply a string of characters. DKIM messages MAY be either in plain-
text or in MIME format; no special treatment is afforded to MIME text or in MIME format; no special treatment is afforded to MIME
content. Message attachments in MIME format MUST be included in the content. Message attachments in MIME format MUST be included in the
content which is signed. content which is signed.
4. Semantics of Multiple Signatures More formally, the algorithm for the signature is:
body-hash = hash-alg(canon_body)
Considerable energy has been spent discussing the desirability and header-hash = crypt-alg(hash-alg(canon_header || DKIM-SIG), key)
semantics of multiple DKIM signatures in a single message,
particularly in a "re-sending" scenario such as a mailing list. On
the one hand, discarding existing signature header fields loses
information which could prove to be valuable in the future. On the
other hand, since header fields are known to be re-ordered in transit
by at least some MTAs, determining the most interesting signature
header field is non-trivial.
Further confusion could occur with multiple signatures added at the where crypt-alg is the encryption algorithm specified by the "a="
same logical "depth". For example, a signer could choose to sign tag, hash-alg is the hash algorithm specified by the "a=" tag,
using multiple signing or canonicalization algorithms. There is no a canon_header and canon_body are the canonicalized message header and
priori way to determine that two signatures are alternatives versus body (respectively) as defined in Section 3.4 (excluding the DKIM-
nested in a re-sending scenario. 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.
[[DOCUMENTATION NOTE: ISSUE: downgrade attacks by removal of sig 4. Semantics of Multiple Signatures
headers that use stronger algorithms.]]
Also, many agents are expected to break existing signatures (e.g., a A signer that is adding a signature to a message merely creates a new
mailing list that modifies Subject header fields or adds unsubscribe DKIM-Signature header, using the usual semantics of the h= option. A
information to the end of the message). Retaining signature signer MAY sign previously existing DKIM-Signature headers using the
information that is known to be bad could create more problems than method described in section NN to sign trace headers. Signers should
it solves. 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.
For these reasons, multiple signatures are not prohibited but are When evaluating a message with multiple signatures, a receiver should
left undefined. 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
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.
INFORMATIVE IMPLEMENTATION GUIDANCE: Agents that forward mail Signers SHOULD NOT remove any DKIM-Signature headers from messages
without modification could decide whether to add another signature they are signing, even if they know that the headers cannot be
or simply retain an existing signatures. Agents that are known to verified.
break existing signatures may leave the existing signature or
delete it. Agents that re-sign messages that are already signed
should verify the previous signature and should probably refuse to
sign any critical information that failed a signature
verification.
5. Signer Actions 5. Signer Actions
The following steps are performed in order by signers. The following steps are performed in order by signers.
5.1 Determine if the Email Should be Signed and by Whom 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 A signer can obviously only sign email for domains for which it has a
private-key and the necessary knowledge of the corresponding public private-key and the necessary knowledge of the corresponding public
key and selector information. However there are a number of other key and selector information. However there are a number of other
skipping to change at page 29, line 37 skipping to change at page 30, line 43
If an email cannot be signed for some reason, it is a local policy If an email cannot be signed for some reason, it is a local policy
decision as to what to do with that email. decision as to what to do with that email.
5.2 Select a private-key and corresponding selector information 5.2 Select a private-key and corresponding selector information
This specification does not define the basis by which a signer should This specification does not define the basis by which a signer should
choose which private-key and selector information to use. Currently, choose which private-key and selector information to use. Currently,
all selectors are equal as far as this specification is concerned, so all selectors are equal as far as this specification is concerned, so
the decision should largely be a matter of administrative the decision should largely be a matter of administrative
convenience. convenience. Distribution and management of private-keys is also
outside the scope of this document.
A signer SHOULD NOT sign with a key that is expected to expire within 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 seven days; that is, when rotating to a new key, signing should
immediately commence with the new key and the old key SHOULD be immediately commence with the new key and the old key SHOULD be
retained for at least seven days before being removed from the key retained for at least seven days before being removed from the key
server. server.
5.3 Normalize the Message to Prevent Transport Conversions 5.3 Normalize the Message to Prevent Transport Conversions
Some messages, particularly those using 8-bit characters, are subject Some messages, particularly those using 8-bit characters, are subject
to modification during transit, notably conversion to 7-bit form. to modification during transitt, notably conversion to 7-bit form.
Such conversions will break DKIM signatures. In order to minimize Such conversions will break DKIM signatures. In order to minimize
the chances of such breakage, signers SHOULD convert the message to a the chances of such breakage, signers SHOULD convert the message to a
suitable MIME content transfer encoding such as quoted-printable or suitable MIME content transfer encoding such as quoted-printable or
base64 as described in MIME Part One [RFC2045] before signing. Such base64 as described in MIME Part One [RFC2045] before signing. Such
conversion is outside the scope of DKIM; the actual message SHOULD be 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 converted to 7-bit MIME by an MUA or MSA prior to presentation to the
DKIM algorithm. DKIM algorithm.
Should the message be submitted to the signer with any local encoding Should the message be submitted to the signer with any local encoding
that will be modified before transmission, such conversion to that will be modified before transmission, such conversion to
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[RFC2821] explicitly permits modification or removal of the "Return- [RFC2821] explicitly permits modification or removal of the "Return-
Path" header field in transit. Signers MAY include any other header Path" header field in transit. Signers MAY include any other header
fields present at the time of signing at the discretion of the fields present at the time of signing at the discretion of the
signer. It is RECOMMENDED that all other existing, non-repeatable signer. It is RECOMMENDED that all other existing, non-repeatable
header fields be signed. header fields be signed.
The DKIM-Signature header field is always implicitly signed and MUST The DKIM-Signature header field is always implicitly signed and MUST
NOT be included in the h= tag except to indicate that other NOT be included in the h= tag except to indicate that other
preexisting signatures are also signed. preexisting signatures are also signed.
Signers MUST sign any header fields that the signers wish to have the Signers MUST sign any header fields that the signers wish to assert
verifiers treat as trusted. Put another way, verifiers MAY treat were present at the time of signing. Put another way, verifiers MAY
unsigned header fields with extreme skepticism, up to and including treat unsigned header fields with extreme skepticism, up to and
refusing to display them to the end user. including refusing to display them to the end user.
Signers MAY claim to have signed header fields that do not exist 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 (that is, signers MAY include the header field name in the h=D tag
even if that header field does not exist in the message). When even if that header field does not exist in the message). When
computing the signature, the non-existing header field MUST be computing the signature, the non-existing header field MUST be
treated as the null string (including the header field name, header treated as the null string (including the header field name, header
field value, all punctuation, and the trailing CRLF). field value, all punctuation, and the trailing CRLF).
INFORMATIVE RATIONALE: This allows signers to explicitly assert INFORMATIVE RATIONALE: This allows signers to explicitly assert
the absence of a header field; if that header field is added later the absence of a header field; if that header field is added later
the signature will fail. the signature will fail.
Signers choosing to sign an existing replicated header field (such as Signers choosing to sign an existing replicated header field (such as
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the original order. the original order.
INFORMATIVE RATIONALE: Received: is allowed because these header INFORMATIVE RATIONALE: Received: is allowed because these header
fields, as well as Resent-* header fields, are already order- fields, as well as Resent-* header fields, are already order-
sensitive. sensitive.
INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
header field blocks to be reordered (with the exception of header field blocks to be reordered (with the exception of
Received header fields), reordering of signed replicated header Received header fields), reordering of signed replicated header
fields by intermediate MTAs will cause DKIM signatures to be fields by intermediate MTAs will cause DKIM signatures to be
broken; such anti-social behavior should be avoided. broken; such anti-social behavior shoulld be avoided.
INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this
specification, all end-user visible header fields should be signed specification, all end-user visible header fields should be signed
to avoid possible "indirect spamming." For example, if the to avoid possible "indirect spamming." For example, if the
"Subject" header field is not signed, a spammer can resend a "Subject" header field is not signed, a spammer can resend a
previously signed mail, replacing the legitimate subject with a previously signed mail, replacing the legitimate subject with a
one-line spam. one-line spam.
INFORMATIVE NOTE: There has been some discussion that a Sender INFORMATIVE NOTE: There has been some discussion that a Sender
Signing Policy include the list of header fields that the signer Signing Policy include the list of header fields that the signer
always signs. N.B. In theory this is unnecessary, since as long always signs. N.B. In theory this is unnecessary, since as long
as the signer really always signs the indicated header fields as the signer really always signs the indicated header fields
there is no possibility of an attacker replaying an existing there is no possibility of an attacker replaying an existing
message that has such an unsigned header field. message that has such an unsigned header field.
5.5 Compute the Message Hash 5.5 Compute the Message Hash and Signature
The signer MUST compute the message hash as described in Section 3.7 The signer MUST compute the message hash as described in Section 3.7
and then sign it using the selected public-key algorithm. 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 To avoid possible ambiguity, a signer SHOULD either sign or remove
any preexisting header fields which convey the results of previous any preexisting header fields which convey the results of previous
verifications of the message signature prior to preparation for verifications of the message signature prior to preparation for
signing and transmission. Such header fields MUST NOT be signed if signing and transmission. Such header fields MUST NOT be signed if
the signer is uncertain of the authenticity of the preexisting header the signer is uncertain of the authenticity of the preexisting header
field, for example, if it is not locally generated or signed by a field, for example, if it is not locally generated or signed by a
previous DKIM-Signature line that the current signer has verified. previous DKIM-Signature line that the current signer has verified.
Entities such as mailing list managers that implement DKIM and which Entities such as mailing list managers that implement DKIM and which
modify the message or the header field (for example, inserting modify the message or a header field (for example, inserting
unsubscribe information) before retransmitting the message SHOULD unsubscribe information) before retransmitting the message SHOULD
check any existing signature on input and MUST make such check any existing signature on input and MUST make such
modifications before re-signing the message; such signing SHOULD modifications before re-signing the message; such signing SHOULD
include any prior verification status, if any, that was inserted upon include any prior verification status, if any, that was inserted upon
message receipt. 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 innformation 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 5.6 Insert the DKIM-Signature header field
Finally, the signer MUST insert the "DKIM-Signature:" header field Finally, the signer MUST insert the "DKIM-Signature:" header field
prior to transmitting the email. The "DKIM-Signature" header field created in the previous step prior to transmitting the email. The
MUST be the same as used to compute the hash as described above, "DKIM-Signature" header field MUST be the same as used to compute the
except that the value of the "b=" tag MUST be the appropriately hash as described above, except that the value of the "b=" tag MUST
signed hash computed in the previous step, signed using the algorithm be the appropriately signed hash computed in the previous step,
specified in the "a=" tag of the "DKIM-Signature" header field and signed using the algorithm specified in the "a=" tag of the "DKIM-
using the private key corresponding to the selector given in the "s=" Signature" header field and using the private key corresponding to
tag of the "DKIM-Signature" header field, as chosen above in the selector given in the "s=" tag of the "DKIM-Signature" header
Section 5.2 field, as chosen above in Section 5.2
The "DKIM-Signature" SHOULD be inserted before any header fields that The "DKIM-Signature" SHOULD be inserted before any header fields that
it signs in the header block. it signs in the header block.
INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this
is to insert the "DKIM-Signature" header field at the beginning of is to insert the "DKIM-Signature" header field at the beginning of
the header block. This is consistent with treating "DKIM- the header block. In particular, it may be placed before any
Signature" as a trace header. existing Received header fields. This is consistent with treating
"DKIM-Signature" as a trace header.
6. Verifier Actions 6. Verifier Actions
Since a signer MAY expire a public key at any time, it is recommended 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 that verification occur in a timely manner with the most timely place
being during acceptance by the border MTA. being during acceptance by the border MTA.
A border or intermediate MTA MAY verify the message signatures and A border or intermediate MTA MAY verify the message signatures and
add a verification header field to incoming messages. This add a verification header field to incoming messages. This
considerably simplifies things for the user, who can now use an considerably simplifies things for the user, who can now use an
existing mail user agent. Most MUAs have the ability to filter existing mail user agent. Most MUAs have the ability to filter
messages based on message header fields or content; these filters messages based on message header fields or content; these filters
would be used to implement whatever policy the user wishes with would be used to implement whatever policy the user wishes with
respect to unsigned mail. respect to unsigned mail.
A verifying MTA MAY implement a policy with respect to unverifiable A verifying MTA MAY implement a policy with respect to unverifiable
mail, regardless of whether or not it applies the verification header mail, regardless of whether or not it applies the verification header
field to signed messages. field to signed messages.
Verifiers apply the following steps in the order listed. Verifiers MUST apply the following steps in the order listed. In
many cases these steps say that a "DKIM-Signature" header field must
be ignored, e.g., because it is malformed or because the signature
verification failed. In such cases verifiers SSHOULD proceed to the
next signature, and treat the message as verified if any signature
succeeded, ignoring the bad signatures. The order in which
signatures are tried is a matter of local policy for the verifier and
is not defined here. A verifier MAY 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.
6.1 Extract the Signature from the Message 6.1 Extract the Signature from the Message
The signature and associated signing identity is included in the The signature and associated signing identity is included in the
value of the DKIM-Signature header field. value of the DKIM-Signature header field.
Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag. Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag.
Existence of such a tag indicates a new, incompatible version of the Existence of such a tag indicates a new, incompatible version of the
DKIM-Signature header field. DKIM-Signature header field.
If the "DKIM-Signature" header field does not contain the "i=" tag, 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", the verifier MUST behave as though the value of that tag were "@d",
where "d" is the value from the "d=" tag (which MUST exist). where "d" is the value from the "d=" tag (which MUST exist).
Verifiers MUST confirm that the domain specified in the "d=" tag is 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 the same as or a superdomain of the domain part of the "i=" tag. If
not, the DKIM-Signature header field MUST be ignored. not, the DKIM-Signature header field MUST be ignored.
Implementers MUST meticulously validate the format and values in the Implementers MUST meticulously validate the format and values in the
"DKIM-Signature:" header field; any inconsistency or unexpected "DKIM-Signature:" header field; any inconsistency or unexpected
values MUST result in an unverified email. Being "liberal in what values MUST cause the header field to be completely ignored. Being
you accept" is definitely a bad strategy in this security context. "liberal in what you accept" is definitely a bad strategy in this
Note however that this does not include the existence of unknown tags security context. Note however that this does not include the
in a "DKIM-Signature" header field, which are explicitly permitted. existence of unknown tags in a "DKIM-Signature" header field, which
are explicitly permitted.
Verifiers MUST NOT attribute ultimate meaning to the order of Verifiers MUST NOT attribute ultimate meaning to the order of
multiple DKIM-Signature header fields. In particular, there is multiple DKIM-Signature header fields. In particular, there is
reason to believe that some relays will reorder the header field in reason to believe that some relays will reorder the header field in
potentially arbitrary ways. potentially arbitrary ways.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as
a clue to signing order in the absence of any other information. a clue to signing order in the absence of any other information.
However, other clues as to the semantics of multiple signatures However, other clues as to the semantics of multiple signatures
must be considered before using ordering. must be considered before using ordering.
Since there can be multiple signatures in a message, a verifier Since there can be multiple signatures in a message, a verifier
SHOULD ignore an invalid signature (regardless if caused by a SHOULD ignore an invalid signature (regardless if caused by a
syntactic or semantic problem) and try other signatures. A verifier syntactic or semantic problem) and try other signatures. A verifier
MAY choose to treat a message with one or more invalid signatures and MAY choose to treat a message with one or more invalid signatures and
no valid signatures with more suspicion than a message with no no valid signatures with more suspicion than a message with no
signature at all. signature at all.
6.2 Get the Public Key 6.2 Get the Public Key
The public key is needed to complete the verification process. The The public key is needed to complete the verificatiion process. The
process of retrieving the public key depends on the query type as process of retrieving the public key depends on the query type as
defined by the "q=" tag in the "DKIM-Signature:" header field line. defined by the "q=" tag in the "DKIM-Signature:" header field line.
Obviously, a public key should only be retrieved if the process of Obviously, a public key should only be retrieved if the process of
extracting the signature information is completely successful. extracting the signature information is completely successful.
Details of key management and representation are described in Details of key management and representation are described in
Section 3.6. The verifier MUST validate the key record and MUST Section 3.6. The verifier MUST validate the key record and MUST
ignore any public key records that are malformed. ignore any public key records that are malformed.
When validating a message, a verifier MUST: 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 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. 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 2. If the query for the public key fails to respond, the verifier
SHOULD defer acceptance of this email (normally this will be SHOULD defer acceptance of this email (normally this will be
achieved with a 451/4.7.5 SMTP reply code). achieved with a 451/4.7.5 SMTP reply code).
3. If the query for the public key fails because the corresponding 3. If the query for the public key fails because the corresponding
RR does not exist, the verifier MUST ignore the signature. RR does not exist, the verifier MUST ignore the signature.
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field, the verifier MUST ignore the signature. field, the verifier MUST ignore the signature.
If the signature is to be ignored, verifiers SHOULD search for If the signature is to be ignored, verifiers SHOULD search for
another signature in the message. another signature in the message.
6.3 Compute the Verification 6.3 Compute the Verification
Given a signer and a public key, verifying a signature consists of Given a signer and a public key, verifying a signature consists of
the following steps. the following steps.
o Based on the algorithm defined in the "c=" tag, the body length 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=" 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 tag, create a canonicalized copy of the email as is described in
Section 3.7. When matching header field names in the "h=" tag Section 3.7. When matching header field names in the "h=" tag
against the actual message header field, comparisons MUST be case- against the actual message header field, comparisons MUST be
insensitive. case-insensitive.
o Based on the algorithm indicated in the "a=" tag, 2. Based on the algorithm indicated in the "a=" tag,
* Compute the message hash from the canonical copy as described * Compute the message hashes from the canonical copy as
in Section 3.7. described in Section 3.7.
* Decrypt the signature using the signer's public key. * Decrypt the signature using the signer's public key.
o Compare the decrypted signature to the message hash. If they are 3. Compare the decrypted signature to the message hash. If they are
identical, the hash computed by the signer must be the same as the identical, the hash computed by the signer must be the same as
hash computed by the verifier, and hence the signature verifies; the hash computed by the verifier, and hence the signature
otherwise, the signature fails. verifies; otherwise, the signature fails.
INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to
initiate the public-key query in parallel with calculating the initiate the public-key query in parallel with calculating the
hash as the public key is not needed until the final decryption is hash as the public key is not needed until the final decryption is
calculated. 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 match, the
entire signature must be considered to have failed.
Verifiers MUST ignore any DKIM-Signature header fields where the Verifiers SHOULD ignore any DKIM-Signature header fields where the
signature does not validate. Verifiers that are prepared to validate signature does not validate. Verifiers that are prepared to validate
multiple signature header fields SHOULD proceed to the next signature multiple signature header fields SHOULD proceed to the next signature
header field, should it exist. However, verifiers MAY make note of header field, should it exist. However, verifiers MAY make note of
the fact that an invalid signature was present for consideration at a the fact that an invalid signature was present for consideration at a
later step. later step.
INFORMATIVE NOTE: The rationale of this requirement is to permit INFORMATIVE NOTE: The rationale of this requirement is to permit
messages that have invalid signatures but also a valid signature messages that have invalid signatures but also a valid signature
to work. For example, a mailing list exploder might opt to leave to work. For example, a mailing list exploder might opt to leave
the original submitter signature in place even though the exploder the original submitter signature in place even though the exploder
knows that it is modifying the message in some way that will break knows that it is modifying the message in some way that will break
that signature, and the exploder inserts its own signature. In that signature, and the exploder inserts its own signature. In
this case the message should succeed even in the presence of the this case the message should succeed eveen in the presence of the
known-broken signature. known-broken signature.
If a body length is specified in the "l=" tag of the 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 verifiers MUST only verify the number of bytes indicated in the body
length. Verifiers MAY decide to treat a message containing bytes length. Verifiers MAY decide to treat a message containing bytes
beyond the indicated body length with suspicion. Verifiers MAY beyond the indicated body length with suspicion. Verifiers MAY
truncate the message at the indicated body length or reject the truncate the message at the indicated body length or reject the
signature outright. signature outright.
6.4 Insert the Authentication-Results Header Field INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
at the indicated body length might pass on a malformed MIME
message if the signer used the "N-4" trick described in the
informative note inSection 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 displayed to the end user.
Verifiers wishing to communicate the results of verification via an 6.4 Communicate Verification Results
email header field SHOULD use the Authentication-Results header field
[ID-AUTH-RES]. The Authentication-Results header field SHOULD be Verifiers wishing to communicate the results of verification to other
inserted before any existing DKIM-Signature or Authentication-Results parts of the mail system may do so in whatever manner they see fit.
header fields in the header field block. 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 Authentication-Results header fields in the header
field block.
INFORMATIVE ADVICE to MUA filter writers: Patterns intended to INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
search for Authentication-Results header fields to visibly mark search for results header fields to visibly mark authenticated
authenticated mail for end users should verify that the mail for end users should verify that such header field was added
Authentication-Results header field was added by the appropriate by the appropriate verifying domain and that the verified identity
verifying domain and that the verified identity matches the sender matches the sender identity that will be displayed by the MUA. In
identity that will be displayed by the MUA. In particular, MUA particular, MUA patterns should not be influenced by bogus results
patterns should not be influenced by bogus Authentication-Results
header fields added by attackers. header fields added by attackers.
6.5 Interpret Results/Apply Local Policy 6.5 Interpret Results/Apply Local Policy
It is beyond the scope of this specification to describe what actions It is beyond the scope of this specification to describe what actions
a verifier system should make, but an authenticated email presents an a verifier system should make, but an authenticated email presents an
opportunity to a receiving system that unauthenticated email cannot. opportunity to a receiving system that unauthenticated email cannot.
Specifically, an authenticated email creates a predictable identifier Specifically, an authenticated email creates a predictable identifier
by which other decisions can reliably be managed, such as trust and by which other decisions can reliably be managed, such as trust and
reputation. Conversely, unauthenticated email lacks a reliable reputation. Conversely, unauthenticated email lacks a reliable
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having no positive reputation. having no positive reputation.
If the verifying MTA is capable of verifying the public key of the If the verifying MTA is capable of verifying the public key of the
signer and check the signature on the message synchronously with the signer and check the signature on the message synchronously with the
SMTP session and such signature is missing or does not verify the MTA SMTP session and such signature is missing or does not verify the MTA
MAY reject the message with an error such as: MAY reject the message with an error such as:
550 5.7.1 Unsigned messages not accepted 550 5.7.1 Unsigned messages not accepted
550 5.7.5 Message signature incorrect 550 5.7.5 Message signature incorrect
If it is not possible to fetch the public key, perhaps because the If it is not possible to fetch the public key, perhaps because the
key server is not available, a temporary failure message MAY be key server is not available, a temporary failure message MAY be
generated, such as: generated, such as:
451 4.7.5 Unable to verify signature - key server unavailable 451 4.7.5 Unable to verify signature - key server unavailable
Once the signature has been verified, that information MUST be Once the signature has been verified, that information MUST be
conveyed to higher level systems (such as explicit allow/white lists conveyed to higher level systems (such as explicit allow/white lists
and reputation systems) and/or to the end user. If the and reputation systems) and/or to the end user. If the message is
authentication status is to be stored in the message header field, signed on behalf of any address other than that in the From: header
the Authentication-Results header field [ID-AUTH-RES] SHOULD be used field, the mail system SHOULD take pains to ensure that the actual
to convey this information. If the message is signed on behalf of signing identity is clear to the reader.
any address other than that in the From: header field, the mail
system SHOULD take pains to ensure that the actual signing identity INFORMATIVE NOTE: If the authentication status is to be stored in
is clear to the reader. 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 The verifier MAY treat unsigned header fields with extreme
skepticism, including marking them as untrusted or even deleting them skepticism, including marking them as untrusted or even deleting them
before display to the end user. before display to the end user.
While the symptoms of a failed verification are obvious -- the While the symptoms of a failed verification are obvious -- the
signature doesn't verify -- establishing the exact cause can be more signature doesn't verify -- establishing the exact cause can be more
difficult. If a selector cannot be found, is that because the difficult. If a selector cannot be found, is that because the
selector has been removed or was the value changed somehow in selector has been removed or was the value changed somehow in
transit? If the signature line is missing is that because it was transit? If the signature line is missing is that because it was
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might be otherwise used to mislead the verifier, for example if a might be otherwise used to mislead the verifier, for example if a
message supposedly from administration@your-bank.com had a Sender message supposedly from administration@your-bank.com had a Sender
address of fraud@badguy.com. address of fraud@badguy.com.
Under no circumstances should an unsigned header field be displayed Under no circumstances should an unsigned header field be displayed
in any context that might be construed by the end user as having been in any context that might be construed by the end user as having been
signed. Notably, unsigned header fields SHOULD be hidden from the signed. Notably, unsigned header fields SHOULD be hidden from the
end user to the extent possible. end user to the extent possible.
The MUA MAY hide or mark portions of the message body that are not The MUA MAY hide or mark portions of the message body that are not
signed when using the "l=" tag. signed wh€en using the "l=" tag.
7. IANA Considerations 7. IANA Considerations
Use of the _domainkey prefix in DNS records will require registration Use of the _domainkey prefix in DNS records will require registration
by IANA. by IANA.
To avoid conflicts, tag names for the DKIM-Signature header and key To avoid conflicts, tag names for the DKIM-Signature header and key
records should be registered with IANA. records should be registered with IANA.
Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature
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properly closed message will in many MUAs result in inappropriate properly closed message will in many MUAs result in inappropriate
data being rendered on top of existing, correct data: data being rendered on top of existing, correct data:
<div style="position: relative; bottom: 350px; z-index: 2;"> <div style="position: relative; bottom: 350px; z-index: 2;">
<img src="http://www.ietf.org/images/ietflogo2e.gif" <img src="http://www.ietf.org/images/ietflogo2e.gif"
width=578 height=370> width=578 height=370>
</div> </div>
8.2 Misappropriated Private Key 8.2 Misappropriated Private Key
If the private key for a user is resident on their computer and is If the private key for a user is resident on their computer and is
not protected by an appropriately secure passphrase, it is possible not protected by an appropriately secure mechanism, it is possible
for malware to send mail as that user and any other user sharing the 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 same private key. The malware would, however, not be able to
generate signed spoofs of other signers' addresses, which would aid generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user and would limit the in identification of the infected user and would limit the
possibilities for certain types of attacks involving socially- possibilities for certain types of attacks involving socially-
engineered messages. engineered messages.
A larger problem occurs if malware on many users' computers obtains A larger problem occurs if malware on many users' computers obtains
the private keys for those users and transmits them via a covert the private keys for those users and transmits them via a covert
channel to a site where they can be shared. The compromised users channel to a site where they can be shared. The compromised users
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respond, however the cost/benefit of conducting prolonged DNS attacks respond, however the cost/benefit of conducting prolonged DNS attacks
of this nature is expected to be uneconomical. of this nature is expected to be uneconomical.
Finally, DKIM is only intended as a "sufficient" method of proving Finally, DKIM is only intended as a "sufficient" method of proving
authenticity. It is not intended to provide strong cryptographic authenticity. It is not intended to provide strong cryptographic
proof about authorship or contents. Other technologies such as proof about authorship or contents. Other technologies such as
OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements. OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements.
A second security issue related to the DNS revolves around the A second security issue related to the DNS revolves around the
increased DNS traffic as a consequence of fetching Selector-based increased DNS traffic as a consequence of fetching Selector-based
data as well as fetching signing domain policy. Widespread data as well as fetching siggning domain policy. Widespread
deployment of DKIM will result in a significant increase in DNS deployment of DKIM will result in a significant increase in DNS
queries to the claimed signing domain. In the case of forgeries on a queries to the claimed signing domain. In the case of forgeries on a
large scale, DNS servers could see a substantial increase in queries. large scale, DNS servers could see a substantial increase in queries.
8.5 Replay Attacks 8.5 Replay Attacks
In this attack, a spammer sends a message to be spammed to an In this attack, a spammer sends a message to be spammed to an
accomplice, which results in the message being signed by the accomplice, which results in the message being signed by the
originating MTA. The accomplice resends the message, including the originating MTA. The accomplice resends the message, including the
original signature, to a large number of recipients, possibly by original signature, to a large number of recipients, possibly by
skipping to change at page 42, line 51 skipping to change at page 45, line 22
8.7 Intentionally malformed Key Records 8.7 Intentionally malformed Key Records
It is possible for an attacker to publish key records in DNS which It is possible for an attacker to publish key records in DNS which
are intentionally malformed, with the intent of causing a denial-of- are intentionally malformed, with the intent of causing a denial-of-
service attack on a non-robust verifier implementation. The attacker service attack on a non-robust verifier implementation. The attacker
could then cause a verifier to read the malformed key record by could then cause a verifier to read the malformed key record by
sending a message to one of its users referencing the malformed sending a message to one of its users referencing the malformed
record in a (not necessarily valid) signature. Verifiers MUST record in a (not necessarily valid) signature. Verifiers MUST
thoroughly verify all key records retrieved from DNS and be robust thoroughly verify all key records retrieved from DNS and be robust
against intentionally as well as unintentionally malformed key against intentionally as well as unintentiionally malformed key
records. records.
8.8 Intentionally Malformed DKIM-Signature header fields 8.8 Intentionally Malformed DKIM-Signature header fields
Verifiers MUST be prepared to receive messages with malformed DKIM- Verifiers MUST be prepared to receive messages with malformed DKIM-
Signature header fields, and thoroughly verify the header field Signature header fields, and thoroughly verify the header field
before depending on any of its contents. 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.
9. References 9. References
9.1 Normative References 9.1 Normative References
[ID-AUTH-RES]
Kucherawy, M., "Message header field for Indicating Sender
Authentication Status",
draft-kucherawy-sender-auth-header-02 (work in progress),
May 2005.
[ID-DKIM-RR] [ID-DKIM-RR]
"DKIM Key Resource Records (To be written)", "DKIM Key Resource Records (To be written)",
draft-dkim-dkk-rr-xx (work in progress), 2005. draft-dkim-dkk-rr-xx (work in progress), 2005.
[ID-SHA] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms [ID-SHA] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMAC-SHA)", draft-eastlake-sha2-02 (work in (SHA and HMAC-SHA)", draft-eastlake-sha2-02 (work in
progress), January 2006. progress), January 2006.
[OPENSSL] Team, C&D., "OpenSSL Documents", [OPENSSL] Team, C&D., "OpenSSL Documents",
http://www.openssl.org/docs/, http://www.openssl.org/docs/,
skipping to change at page 44, line 21 skipping to change at page 46, line 40
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003. RFC 3491, March 2003.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
9.2 Informative References 9.2 Informative References
[ID-AUTH-RES]
Kucherawy, M., "Message header field for Indicating Sender
Authentication Status",
draft-kucherawy-sender-auth-header-02 (work in progress),
May 2005.
[ID-DKIM-THREATS] [ID-DKIM-THREATS]
Fenton, J., "Analysis of Threats Motivating DomainKeys Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", draft-fenton-dkim-threats-02 Identified Mail (DKIM)", draft-fenton-dkim-threats-02
(work in progress), December 2005. (work in progress), December 2005.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed, [RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and "Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995. Multipart/Encrypted", RFC 1847, October 1995.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998. "OpenPGP Message Format", RFC 2440, November 1998.
[RFC3766] "", 2005.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004. Name System (DNS)", RFC 3833, August 2004.
[RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification", [RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 3851, June 1999. RFC 3851, June 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005. RFC 4033, March 2005.
skipping to change at page 46, line 33 skipping to change at page 49, line 7
Email: mat@cisco.com Email: mat@cisco.com
Appendix A. Example of Use (INFORMATIVE) Appendix A. Example of Use (INFORMATIVE)
This section shows the complete flow of an email from submission to This section shows the complete flow of an email from submission to
final delivery, demonstrating how the various components fit final delivery, demonstrating how the various components fit
together. together.
A.1 The user composes an email A.1 The user composes an email
From: Joe SixPack <joe@football.example.com> From: Joe SixPack <joe@foootball.example.com>
To: Suzie Q <suzie@shopping.example.net> To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready? Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com> Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi. Hi.
We lost the game. Are you hungry yet? We lost the game. Are you hungry yet?
Joe. Joe.
skipping to change at page 47, line 27 skipping to change at page 50, line 4
Message-ID: <20030712040037.46341.5F8J@football.example.com> Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi. Hi.
We lost the game. Are you hungry yet? We lost the game. Are you hungry yet?
Joe. Joe.
The signing email server requires access to the private-key The signing email server requires access to the private-key
associated with the "brisbane" selector to generate this signature. associated with the "brisbane" selector to generate this signature.
Distribution and management of private-keys is outside the scope of
this document.
A.3 The email signature is verified A.3 The email signature is verified
The signature is normally verified by an inbound SMTP server or The signature is normally verified by an inbound SMTP server or
possibly the final delivery agent. However, intervening MTAs can possibly the final delivery agent. However, intervening MTAs can
also perform this verification if they choose to do so. The also perform this verification if they choose to do so. The
verification process uses the domain "example.com" extracted from the verification process uses the domain "example.com" extracted from the
"d=" tag and the selector "brisbane" from the "s=" tag in the "DKIM- "d=" tag and the selector "brisbane" from the "s=" tag in the "DKIM-
Signature" header field to form the DNS DKIM query for: Signature" header field to form the DNS DKIM query for:
skipping to change at page 50, line 23 skipping to change at page 52, line 44
B.5 Affinity Addresses B.5 Affinity Addresses
"Affinity addresses" are email addresses that users employ to have an "Affinity addresses" are email addresses that users employ to have an
email address that is independent of any changes in email service email address that is independent of any changes in email service
provider they may choose to make. They are typically associated with provider they may choose to make. They are typically associated with
college alumni associations, professional organizations, and college alumni associations, professional organizations, and
recreational organizations with which they expect to have a long-term recreational organizations with which they expect to have a long-term
relationship. These domains usually provide forwarding of incoming relationship. These domains usually provide forwarding of incoming
email, but (currently) usually depend on the user to send outgoing email, but (currently) usually depend on the user to send outgoing
messages through their own service provider's MTA. They usually have messages through their own service provider's MTA. They usually have
an associated Web application which authenticates the user and allows an aassociated Web application which authenticates the user and allows
the forwarding address to be changed. the forwarding address to be changed.
With DKIM, affinity domains could use the Web application to allow With DKIM, affinity domains could use the Web application to allow
users to register their own public keys to be used to sign messages users to register their own public keys to be used to sign messages
on behalf of their affinity address. This is another application on behalf of their affinity address. This is another application
that takes advantage of user-level keying, and domains used for that takes advantage of user-level keying, and domains used for
affinity addresses would typically have a very large number of user- affinity addresses would typically have a very large number of user-
level keys. Alternatively, the affinity domain could handle outgoing level keys. Alternatively, the affinity domain could handle outgoing
mail, operating a mail submission agent that authenticates users mail, operating a mail submission agent that authenticates users
before accepting and signing messages for them. This is of course before accepting and signing messages for them. This is of course
skipping to change at page 51, line 8 skipping to change at page 53, line 28
One way this can be handled is to continue to put the reader's email 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 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 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 Sender. A verifying MTA should accept this and rewrite the From
field to indicate the address that was verified, i.e., From: John field to indicate the address that was verified, i.e., From: John
Doe via news@news-site.com <jdoe@example.com>. Doe via news@news-site.com <jdoe@example.com>.
Appendix C. Creating a public key (INFORMATIVE) 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 The default signature is an RSA signed SHA1 digest of the complete
email. For ease of explanation, the openssl command is used to email. For ease of explanation, the openssl command is used to
describe the mechanism by which keys and signatures are managed. One 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 way to generate a 768 bit private-key suitable for DKIM, is to use
openssl like this: openssl like this:
$ openssl genrsa -out rsa.private 768 $ openssl genrsa -out rsa.private 768
This results in the file rsa.private containing the key information This results in the file rsa.private containing the key information
similar to this: similar to this:
-----BEGIN RSA PRIVATE KEY----- -----BEGIN RSA PRIVATE KEY-----
MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5 MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5
ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo
AUsFUq+J6+OprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH AUsFUq+J6+OpprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH
+Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn +Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn
Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/ Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/
3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew 3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew
ti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs ti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs
FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb
weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG
6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U= 6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U=
-----END RSA PRIVATE KEY----- -----END RSA PRIVATE KEY-----
To extract the public-key component from the private-key, use openssl To extract the public-key component from the private-key, use openssl
skipping to change at page 52, line 30 skipping to change at page 55, line 17
How this signature is added to the email is discussed elsewhere in How this signature is added to the email is discussed elsewhere in
this document. this document.
Appendix D. Acknowledgements Appendix D. Acknowledgements
The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann, The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
Steve Atkins, Fred Baker, Mark Baugher, Nathaniel Borenstein, Dave Steve Atkins, Fred Baker, Mark Baugher, Nathaniel Borenstein, Dave
Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Patrik Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Patrik
Faltstrom, Duncan Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony Faltstrom, Duncan Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony
Hansen, Arvel Hathcock, Amir Herzberg, Don Johnsen, Harry Katz, Hansen, Arvel Hathcock, Amir Herzberg, Craig Hughes, Don Johnsen,
Murray S. Kucherawy, Barry Leiba, John Levine, Simon Longsdale, David Harry Katz, Murray S. Kucherawy, Barry Leiba, John Levine, Simon
Margrave, Justin Mason, David Mayne, Steve Murphy, Russell Nelson, Longsdale, David Margrave, Justin Mason, David Mayne, Steve Murphy,
Dave Oran, Shamim Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer
Ramsdell, Christian Renaud, Scott Renfro, Dave Rossetti, the Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro,
Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker, and Dave Rossetti, Hector Santos, the Spamhaus..org team, Malte S. Stretz,
Dan Wing for their valuable suggestions and constructive criticism. Robert Sanders, Rand Wacker, and Dan Wing for their valuable
suggestions and constructive criticism.
The DomainKeys specification was a primary source from which this The DomainKeys specification was a primary source from which this
specification has been derived. Further information about DomainKeys specification has been derived. Further information about DomainKeys
is at is at
<http://domainkeys.sourceforge.net/license/patentlicense1-1.html>. <http://domainkeys.sourceforge.net/license/patentlicense1-1.html>.
Appendix E. Edit History Appendix E. Edit History
E.1 Changes since -allman-01 version [[This section to be removed before publication.]]
E.1 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.2 Changes since -allman-01 version
The following changes were made between draft-allman-dkim-base-01 and The following changes were made between draft-allman-dkim-base-01 and
draft-ietf-dkim-base-00: draft-ietf-dkim-base-00:
o Remove references to Sender Signing Policy document. Such o Remove references to Sender Signing Policy document. Such
consideration is implicitly included in Section 6.5. consideration is implicitly included in Section 6.5.
o Added ABNF for all tags. o Added ABNF for all tags.
o Updated references (still includes some references to expired o Updated references (still includes some references to expired
drafts, notably [ID-AUTH-RES]. drafts, notably [ID-AUTH-RES].
o Significant wordsmithing. o Significant wordsmithing.
E.2 Changes since -allman-00 version E.3 Changes since -allman-00 version
The following changes were made between draft-allman-dkim-base-00 and The following changes were made between draft-allman-dkim-base-00 and
draft-allman-dkim-base-01: draft-allman-dkim-base-01:
o Changed "c=" tag to separate out header from body o Changed "c=" tag to separate out header from body
canonicalization. canonicalization.
o Eliminated "nowsp" canonicalization in favor of "relaxed", which o Eliminated "nowsp" canonicalization in favor of "relaxed", which
is somewhat less relaxed (but more secure) than "nowsp". is somewhat less relaxed (but more secure) than "nowsp".
skipping to change at page 54, line 11 skipping to change at page 57, line 11
o Added several IANA Considerations. o Added several IANA Considerations.
o Fixed a number of grammar and formatting errors. o Fixed a number of grammar and formatting errors.
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might oor might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
 End of changes. 126 change blocks. 
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