draft-ietf-dkim-base-10.txt   rfc4871.txt 
DKIM E. Allman Network Working Group E. Allman
Internet-Draft Sendmail, Inc. Request for Comments: 4871 Sendmail, Inc.
Intended status: Standards Track J. Callas Obsoletes: 4870 J. Callas
Expires: August 19, 2007 PGP Corporation Category: Standards Track 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 15, 2007
DomainKeys Identified Mail (DKIM) Signatures DomainKeys Identified Mail (DKIM) Signatures
draft-ietf-dkim-base-10
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
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
contents of messages by either Mail Transfer Agents (MTAs) or Mail contents of messages by either Mail Transfer Agents (MTAs) or Mail
User Agents (MUAs). The ultimate goal of this framework is to permit User Agents (MUAs). The ultimate goal of this framework is to permit
a signing domain to assert responsibility for a message, thus a signing domain to assert responsibility for a message, thus
protecting message signer identity and the integrity of the messages protecting message signer identity and the integrity of the messages
they convey while retaining the functionality of Internet email as it they convey while retaining the functionality of Internet email as it
is known today. Protection of email identity may assist in the is known today. Protection of email identity may assist in the
global control of "spam" and "phishing". global control of "spam" and "phishing".
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 1.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 5
1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Simple Key Management . . . . . . . . . . . . . . . . . . 6 1.3. Simple Key Management . . . . . . . . . . . . . . . . . . 5
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5
2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. White Space . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7 2.4. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 6
2.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7 2.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7
2.6. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8 2.6. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 7
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 9 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11 3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10
3.3. Signing and Verification Algorithms . . . . . . . . . . . 12 3.3. Signing and Verification Algorithms . . . . . . . . . . . 11
3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 14 3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 13
3.5. The DKIM-Signature header field . . . . . . . . . . . . . 18 3.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 17
3.6. Key Management and Representation . . . . . . . . . . . . 26 3.6. Key Management and Representation . . . . . . . . . . . . 25
3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 30 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 29
3.8. Signing by Parent Domains . . . . . . . . . . . . . . . . 32 3.8. Signing by Parent Domains . . . . . . . . . . . . . . . . 31
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 33 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 32
4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 33 4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 32
4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 34 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 33
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 35 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 34
5.1. Determine if the Email Should be Signed and by Whom . . . 35 5.1. Determine Whether the Email Should Be Signed and by
Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2. Select a Private Key and Corresponding Selector 5.2. Select a Private Key and Corresponding Selector
Information . . . . . . . . . . . . . . . . . . . . . . . 36 Information . . . . . . . . . . . . . . . . . . . . . . . 35
5.3. Normalize the Message to Prevent Transport Conversions . . 36 5.3. Normalize the Message to Prevent Transport Conversions . . 35
5.4. Determine the Header Fields to Sign . . . . . . . . . . . 37 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 36
5.5. Recommended Signature Content . . . . . . . . . . . . . . 39 5.5. Recommended Signature Content . . . . . . . . . . . . . . 38
5.6. Compute the Message Hash and Signature . . . . . . . . . . 40 5.6. Compute the Message Hash and Signature . . . . . . . . . . 39
5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 41 5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 40
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 41 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 40
6.1. Extract Signatures from the Message . . . . . . . . . . . 42 6.1. Extract Signatures from the Message . . . . . . . . . . . 41
6.2. Communicate Verification Results . . . . . . . . . . . . . 47 6.2. Communicate Verification Results . . . . . . . . . . . . . 46
6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 48 6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 47
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 49 7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 48
7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 50 7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 49
7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 50 7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 49
7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 51 7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 50
7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 52 7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 50
7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 52 7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 51
7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 53 7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 51
7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 53 7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 52
7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 54 7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 52
8. Security Considerations . . . . . . . . . . . . . . . . . . . 54 8. Security Considerations . . . . . . . . . . . . . . . . . . . 52
8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 54 8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 52
8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 55 8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 53
8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 56 8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 54
8.4. Attacks Against DNS . . . . . . . . . . . . . . . . . . . 56 8.4. Attacks Against the DNS . . . . . . . . . . . . . . . . . 54
8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 57 8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 55
8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 57 8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 55
8.7. Intentionally malformed Key Records . . . . . . . . . . . 57 8.7. Intentionally Malformed Key Records . . . . . . . . . . . 56
8.8. Intentionally Malformed DKIM-Signature header fields . . . 58 8.8. Intentionally Malformed DKIM-Signature Header Fields . . . 56
8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 58 8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 56
8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 58 8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 56
8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 58 8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 56
8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 58 8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 56
8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 58 8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 57
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 59 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
9.1. Normative References . . . . . . . . . . . . . . . . . . . 59 9.1. Normative References . . . . . . . . . . . . . . . . . . . 57
9.2. Informative References . . . . . . . . . . . . . . . . . . 60 9.2. Informative References . . . . . . . . . . . . . . . . . . 58
Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 61 Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 60
A.1. The user composes an email . . . . . . . . . . . . . . . . 61 A.1. The user composes an email . . . . . . . . . . . . . . . . 60
A.2. The email is signed . . . . . . . . . . . . . . . . . . . 61 A.2. The email is signed . . . . . . . . . . . . . . . . . . . 61
A.3. The email signature is verified . . . . . . . . . . . . . 62 A.3. The email signature is verified . . . . . . . . . . . . . 61
Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 63 Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 62
B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 64 B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 63
B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 66 B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 65
Appendix C. Creating a public key (INFORMATIVE) . . . . . . . . . 68 Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 67
Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 70 Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 68
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 70 Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 69
Appendix F. Edit History . . . . . . . . . . . . . . . . . . . . 71
F.1. Changes since -ietf-09 version . . . . . . . . . . . . . . 71
F.2. Changes since -ietf-08 version . . . . . . . . . . . . . . 71
F.3. Changes since -ietf-07 version . . . . . . . . . . . . . . 72
F.4. Changes since -ietf-06 version . . . . . . . . . . . . . . 73
F.5. Changes since -ietf-05 version . . . . . . . . . . . . . . 74
F.6. Changes since -ietf-04 version . . . . . . . . . . . . . . 74
F.7. Changes since -ietf-03 version . . . . . . . . . . . . . . 75
F.8. Changes since -ietf-02 version . . . . . . . . . . . . . . 76
F.9. Changes since -ietf-01 version . . . . . . . . . . . . . . 77
F.10. Changes since -ietf-00 version . . . . . . . . . . . . . . 77
F.11. Changes since -allman-01 version . . . . . . . . . . . . . 78
F.12. Changes since -allman-00 version . . . . . . . . . . . . . 78
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 79
Intellectual Property and Copyright Statements . . . . . . . . . . 81
1. Introduction 1. Introduction
[[Note: text in double square brackets (such as this text) will be
deleted before publication.]]
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.
The approach taken by DKIM differs from previous approaches to The approach taken by DKIM differs from previous approaches to
message signing (e.g. S/MIME [RFC1847], OpenPGP [RFC2440]) in that: message signing (e.g., Secure/Multipurpose Internet Mail Extensions
(S/MIME) [RFC1847], OpenPGP [RFC2440]) in that:
o the message signature is written as a message header field so that o the message signature is written as a message header field so that
neither human recipients nor existing MUA (Mail User Agent) neither human recipients nor existing MUA (Mail User Agent)
software are confused by signature-related content appearing in software is confused by signature-related content appearing in the
the message body; message body;
o there is no dependency on public and private key pairs being o there is no dependency on public and private key pairs being
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 signature verification failure does not force rejection of the o signature verification failure does not force rejection of the
message; message;
skipping to change at page 6, line 21 skipping to change at page 5, line 21
The signing identity is included as part of the signature header The signing identity is included as part of the signature header
field. field.
INFORMATIVE RATIONALE: The signing identity specified by a DKIM INFORMATIVE RATIONALE: The signing identity specified by a DKIM
signature is not required to match an address in any particular signature is not required to match an address in any particular
header field because of the broad methods of interpretation by header field because of the broad methods of interpretation by
recipient mail systems, including MUAs. recipient mail systems, including MUAs.
1.2. Scalability 1.2. Scalability
DKIM is designed to support the extreme scalability requirements DKIM is designed to support the extreme scalability requirements that
which characterize the email identification problem. There are characterize the email identification problem. There are currently
currently over 70 million domains and a much larger number of over 70 million domains and a much larger number of individual
individual addresses. DKIM seeks to preserve the positive aspects of addresses. DKIM seeks to preserve the positive aspects of the
the current email infrastructure, such as the ability for anyone to current email infrastructure, such as the ability for anyone to
communicate with anyone else without introduction. communicate with anyone else without introduction.
1.3. Simple Key Management 1.3. Simple Key Management
DKIM differs from traditional hierarchical public-key systems in that DKIM differs from traditional hierarchical public-key systems in that
no Certificate Authority infrastructure is required; the verifier no Certificate Authority infrastructure is required; the verifier
requests the public key from a repository in the domain of the requests the public key from a repository in the domain of the
claimed signer directly rather than from a third party. claimed signer directly rather than from a third party.
The DNS is proposed as the initial mechanism for the public keys. The DNS is proposed as the initial mechanism for the public keys.
Thus, DKIM currently depends on DNS administration and the security Thus, DKIM currently depends on DNS administration and the security
of the DNS system. DKIM is designed to be extensible to other key of the DNS system. DKIM is designed to be extensible to other key
fetching services as they become available. fetching services as they become available.
2. Terminology and Definitions 2. Terminology and Definitions
This section defines terms used in the rest of the document. Syntax This section defines terms used in the rest of the document. Syntax
descriptions use the form described in Augmented BNF for Syntax descriptions use the form described in Augmented BNF for Syntax
Specifications [RFC4234]. Specifications [RFC4234].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2.1. Signers 2.1. Signers
Elements in the mail system that sign messages on behalf of a domain Elements in the mail system that sign messages on behalf of a domain
are referred to as signers. These may be MUAs (Mail User Agents), are referred to as signers. These may be MUAs (Mail User Agents),
MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other
agents such as mailing list exploders. In general any signer will be agents such as mailing list exploders. In general, any signer will
involved in the injection of a message into the message system in be involved in the injection of a message into the message system in
some way. The key issue is that a message must be signed before it some way. The key issue is that a message must be signed before it
leaves the administrative domain of the signer. leaves the administrative domain of the signer.
2.2. Verifiers 2.2. Verifiers
Elements in the mail system that verify signatures are referred to as Elements in the mail system that verify signatures are referred to as
verifiers. These may be MTAs, Mail Delivery Agents (MDAs), or MUAs. verifiers. These may be MTAs, Mail Delivery Agents (MDAs), or MUAs.
In most cases it is expected that verifiers will be close to an end In most cases it is expected that verifiers will be close to an end
user (reader) of the message or some consuming agent such as a user (reader) of the message or some consuming agent such as a
mailing list exploder. mailing list exploder.
2.3. White Space 2.3. Whitespace
There are three forms of white space: There are three forms of white space:
o WSP represents simple white space, i.e., a space or a tab o WSP represents simple whitespace, i.e., a space or a tab character
character (formal definition in [RFC4234]). (formal definition in [RFC4234]).
o LWSP is linear white space, defined as WSP plus CRLF (formal o LWSP is linear white space, defined as WSP plus CRLF (formal
definition in [RFC4234]). definition in [RFC4234]).
o FWS is folding white space. It allows multiple lines separated by o FWS is folding white space. It allows multiple lines separated by
CRLF followed by at least one white space, to be joined. CRLF followed by at least one white space, to be joined.
The formal ABNF for these are (WSP and LWSP are given for information The formal ABNF for these are (WSP and LWSP are given for information
only): only):
WSP = SP / HTAB WSP = SP / HTAB
LWSP = *(WSP / CRLF WSP) LWSP = *(WSP / CRLF WSP)
FWS = [*WSP CRLF] 1*WSP FWS = [*WSP CRLF] 1*WSP
The definition of FWS is identical to that in [RFC2822] except for The definition of FWS is identical to that in [RFC2822] except for
the exclusion of obs-FWS. the exclusion of obs-FWS.
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 / "-") (ALPHA / DIGIT) ] hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
base64string = 1*(ALPHA / DIGIT / "+" / "/" / LWSP) base64string = 1*(ALPHA / DIGIT / "+" / "/" / [FWS])
[ "=" LWSP [ "=" LWSP ] ] [ "=" [FWS] [ "=" [FWS] ] ]
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. RFCs should be considered definitive.
The following tokens are imported from [RFC2821]: The following tokens are imported from [RFC2821]:
o "Local-part" (implementation warning: this permits quoted o "Local-part" (implementation warning: this permits quoted strings)
strings)
o "sub-domain" o "sub-domain"
The following tokens are imported from [RFC2822]: The following tokens are imported from [RFC2822]:
o "field-name" (name of a header field) o "field-name" (name of a header field)
o "dot-atom-text" (in the local-part of an email address) o "dot-atom-text" (in the Local-part of an email address)
The following tokens are imported from [RFC2045]: The following tokens are imported from [RFC2045]:
o "qp-section" (a single line of quoted-printable-encoded text) o "qp-section" (a single line of quoted-printable-encoded text)
o "hex-octet" (a quoted-printable encoded octet) o "hex-octet" (a quoted-printable encoded octet)
INFORMATIVE NOTE: Be aware that the ABNF in RFC 2045 does not INFORMATIVE NOTE: Be aware that the ABNF in RFC 2045 does not obey
obey the rules of RFC 4234 and must be interpreted accordingly, the rules of RFC 4234 and must be interpreted accordingly,
particularly as regards case folding. particularly as regards case folding.
Other tokens not defined herein are imported from [RFC4234]. These Other tokens not defined herein are imported from [RFC4234]. These
are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF, are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
etc. etc.
2.6. DKIM-Quoted-Printable 2.6. DKIM-Quoted-Printable
The DKIM-Quoted-Printable encoding syntax resembles that described in The DKIM-Quoted-Printable encoding syntax resembles that described in
Quoted-Printable [RFC2045] section 6.7: any character MAY be encoded Quoted-Printable [RFC2045], Section 6.7: any character MAY be encoded
as an "=" followed by two hexadecimal digits from the alphabet as an "=" followed by two hexadecimal digits from the alphabet
"0123456789ABCDEF" (no lower case characters permitted) representing "0123456789ABCDEF" (no lower case characters permitted) representing
the hexadecimal-encoded integer value of that character. All control the hexadecimal-encoded integer value of that character. All control
characters (those with values < %x20), eight-bit characters (values > characters (those with values < %x20), 8-bit characters (values >
%x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon %x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon
(";", %x3B) MUST be encoded. Note that all white space, including (";", %x3B) MUST be encoded. Note that all white space, including
SPACE, CR and LF characters, MUST be encoded. After encoding, FWS SPACE, CR, and LF characters, MUST be encoded. After encoding, FWS
MAY be added at arbitrary locations in order to avoid excessively MAY be added at arbitrary locations in order to avoid excessively
long lines; such white space is NOT part of the value, and MUST be long lines; such white space is NOT part of the value, and MUST be
removed before decoding. removed before decoding.
ABNF: ABNF:
dkim-quoted-printable = dkim-quoted-printable =
*(FWS / hex-octet / dkim-safe-char) *(FWS / hex-octet / dkim-safe-char)
; hex-octet is from RFC 2045 ; hex-octet is from RFC 2045
dkim-safe-char = %x21-3A / %x3C / %x3E-7E dkim-safe-char = %x21-3A / %x3C / %x3E-7E
; '!' - ':', '<', '>' - '~' ; '!' - ':', '<', '>' - '~'
; Characters not listed as "mail-safe" in ; Characters not listed as "mail-safe" in
; RFC 2049 are also not recommended. ; RFC 2049 are also not recommended.
INFORMATIVE NOTE: DKIM-Quoted-Printable differs from Quoted- INFORMATIVE NOTE: DKIM-Quoted-Printable differs from Quoted-
Printable as defined in RFC 2045 in several important ways: Printable as defined in RFC 2045 in several important ways:
skipping to change at page 9, line 28 skipping to change at page 8, line 29
encoded. RFC 2045 does not require such encoding, and does encoded. RFC 2045 does not require such encoding, and does
not permit encoding of CR or LF characters that are part of a not permit encoding of CR or LF characters that are part of a
CRLF line break. CRLF line break.
2. White space in the encoded text is ignored. This is to allow 2. White space in the encoded text is ignored. This is to allow
tags encoded using DKIM-Quoted-Printable to be wrapped as tags encoded using DKIM-Quoted-Printable to be wrapped as
needed. In particular, RFC 2045 requires that line breaks in needed. In particular, RFC 2045 requires that line breaks in
the input be represented as physical line breaks; that is not the input be represented as physical line breaks; that is not
the case here. the case here.
3. The "soft line break" syntax ("=" as the last non-white-space 3. The "soft line break" syntax ("=" as the last non-whitespace
character on the line) does not apply. character on the line) does not apply.
4. DKIM-Quoted-Printable does not require that encoded lines be 4. DKIM-Quoted-Printable does not require that encoded lines be
no more than 76 characters long (although there may be other no more than 76 characters long (although there may be other
requirements depending on the context in which the encoded requirements depending on the context in which the encoded
text is being used). text is being used).
3. Protocol Elements 3. Protocol Elements
Protocol Elements are conceptual parts of the protocol that are not Protocol Elements are conceptual parts of the protocol that are not
specific to either signers or verifiers. The protocol descriptions specific to either signers or verifiers. The protocol descriptions
for signers and verifiers are described in later sections (Signer for signers and verifiers are described in later sections (Signer
Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This
section must be read in the context of those sections. section must be read in the context of those sections.
3.1. Selectors 3.1. Selectors
To support multiple concurrent public keys per signing domain, the To support multiple concurrent public keys per signing domain, the
key namespace is subdivided using "Selectors". For example, key namespace is subdivided using "selectors". For example,
Selectors might indicate the names of office locations (e.g., selectors might indicate the names of office locations (e.g.,
"sanfrancisco", "coolumbeach", and "reykjavik"), the signing date "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
(e.g., "january2005", "february2005", etc.), or even the individual (e.g., "january2005", "february2005", etc.), or even the individual
user. user.
Selectors are needed to support some important use cases. For Selectors are needed to support some important use cases. For
example: example:
o Domains which want to delegate signing capability for a specific o Domains that want to delegate signing capability for a specific
address for a given duration to a partner, such as an advertising address for a given duration to a partner, such as an advertising
provider or other out-sourced function. provider or other outsourced function.
o Domains which want to allow frequent travelers to send messages o Domains that want to allow frequent travelers to send messages
locally without the need to connect with a particular MSA. locally without the need to connect with a particular MSA.
o "Affinity" domains (e.g., college alumni associations) which o "Affinity" domains (e.g., college alumni associations) that
provide forwarding of incoming mail but which do not operate a provide forwarding of incoming mail, but that do not operate a
mail submission agent for outgoing mail. mail submission agent for outgoing mail.
Periods are allowed in Selectors and are component separators. When Periods are allowed in selectors and are component separators. When
keys are retrieved from the DNS, periods in Selectors define DNS keys are retrieved from the DNS, periods in selectors define DNS
label boundaries in a manner similar to the conventional use in label boundaries in a manner similar to the conventional use in
domain names. Selector components might be used to combine dates domain names. Selector components might be used to combine dates
with locations; for example, "march2005.reykjavik". In a DNS with locations, for example, "march2005.reykjavik". In a DNS
implementation, this can be used to allow delegation of a portion of implementation, this can be used to allow delegation of a portion of
the Selector name-space. the selector namespace.
ABNF: ABNF:
selector = sub-domain *( "." sub-domain ) selector = sub-domain *( "." sub-domain )
The number of public keys and corresponding Selectors for each domain The number of public keys and corresponding selectors for each domain
are determined by the domain owner. Many domain owners will be is determined by the domain owner. Many domain owners will be
satisfied with just one Selector whereas administratively distributed satisfied with just one selector, whereas administratively
organizations may choose to manage disparate Selectors and key pairs distributed organizations may choose to manage disparate selectors
in different regions or on different email servers. and key pairs in different regions or on different email servers.
Beyond administrative convenience, Selectors make it possible to Beyond administrative convenience, selectors make it possible to
seamlessly replace public keys on a routine basis. If a domain seamlessly replace public keys on a routine basis. If a domain
wishes to change from using a public key associated with Selector wishes to change from using a public key associated with selector
"january2005" to a public key associated with Selector "january2005" to a public key associated with selector
"february2005", it merely makes sure that both public keys are "february2005", it merely makes sure that both public keys are
advertised in the public-key repository concurrently for the advertised in the public-key repository concurrently for the
transition period during which email may be in transit prior to transition period during which email may be in transit prior to
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.
INFORMATIVE NOTE: A key may also be revoked as described below. INFORMATIVE NOTE: A key may also be revoked as described below.
The distinction between revoking and removing a key selector The distinction between revoking and removing a key selector
record is subtle. When phasing out keys as described above, a record is subtle. When phasing out keys as described above, a
signing domain would probably simply remove the key record after signing domain would probably simply remove the key record after
the transition period. However, a signing domain could elect to the transition period. However, a signing domain could elect to
revoke the key (but maintain the key record) for a further period. revoke the key (but maintain the key record) for a further period.
There is no defined semantic difference between a revoked key and There is no defined semantic difference between a revoked key and
a removed key. a removed key.
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. For example, if that allows harvesting of data by outside parties. For example, if
per-user keys are issued, the domain owner will need to make the per-user keys are issued, the domain owner will need to make the
decision as to whether to associate this Selector directly with the decision as to whether to associate this selector directly with the
user name, or make it some unassociated random value, such as a user name, or make it some unassociated random value, such as a
fingerprint of the public key. fingerprint of the public key.
INFORMATIVE OPERATIONS NOTE: Reusing a Selector with a new key INFORMATIVE OPERATIONS NOTE: Reusing a selector with a new key
(for example, changing the key associated with a user's name) (for example, changing the key associated with a user's name)
makes it impossible to tell the difference between a message that makes it impossible to tell the difference between a message that
didn't verify because the key is no longer valid versus a message didn't verify because the key is no longer valid versus a message
that is actually forged. For this reason, signers are ill-advised that is actually forged. For this reason, signers are ill-advised
to reuse selectors for new keys. A better strategy is to assign to reuse selectors for new keys. A better strategy is to assign
new keys to new selectors. new keys to new selectors.
3.2. Tag=Value Lists 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 and domain signature records. in messages and domain signature records.
Values are a series of strings containing either plain text, "base64" Values are a series of strings containing either plain text, "base64"
text (as defined in [RFC2045], section 6.8), "qp-section" (ibid, text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid,
section 6.7), or "dkim-quoted-printable" (as defined in Section 2.6). Section 6.7), or "dkim-quoted-printable" (as defined in Section 2.6).
The name of the tag will determine the encoding of each value. The name of the tag will determine the encoding of each value.
Unencoded semicolon (";") characters MUST NOT occur in the tag value, Unencoded semicolon (";") characters MUST NOT occur in the tag value,
since that separates tag-specs. since that separates tag-specs.
INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined
defined below (as "tag-value") only includes 7-bit characters, an below (as "tag-value") only includes 7-bit characters, an
implementation that wished to anticipate future standards would be implementation that wished to anticipate future standards would be
advised to not preclude the use of UTF8-encoded text in tag=value advised not to preclude the use of UTF8-encoded text in tag=value
lists. lists.
Formally, the syntax rules are: Formally, the syntax rules are as follows:
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 = [ tval 0*( 1*(WSP / FWS) tval ) ] tag-value = [ tval 0*( 1*(WSP / FWS) tval ) ]
; WSP and FWS prohibited at beginning and end ; WSP and FWS prohibited at beginning and end
tval = 1*VALCHAR tval = 1*VALCHAR
VALCHAR = %x21-3A / %x3C-7E VALCHAR = %x21-3A / %x3C-7E
; EXCLAMATION to TILDE except SEMICOLON ; EXCLAMATION to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / "_" ALNUMPUNC = ALPHA / DIGIT / "_"
Note that WSP is allowed anywhere around tags; in particular, any WSP Note that WSP is allowed anywhere around tags. In particular, any
after the "=" and any WSP before the terminating ";" is not part of WSP after the "=" and any WSP before the terminating ";" is not part
the value; however, WSP inside the value is significant. of the value; however, WSP inside the value is significant.
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
semantics specifies case insensitivity. semantics specifies case insensitivity.
Tags with duplicate names MUST NOT occur within a single tag-list; if Tags with duplicate names MUST NOT occur within a single tag-list; if
a tag name does occur more than once, the entire tag-list is invalid. a tag name does occur more than once, the entire tag-list is invalid.
Whitespace within a value MUST be retained unless explicitly excluded Whitespace within a value MUST be retained unless explicitly excluded
by the specific tag description. by the specific tag description.
skipping to change at page 12, line 44 skipping to change at page 11, line 45
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 digital signature algorithms. Two algorithms DKIM supports multiple digital signature algorithms. Two algorithms
are defined by this specification at this time: rsa-sha1, and rsa- are defined by this specification at this time: rsa-sha1 and rsa-
sha256. The rsa-sha256 algorithm is the default if no algorithm is sha256. The rsa-sha256 algorithm is the default if no algorithm is
specified. Verifiers MUST implement both rsa-sha1 and rsa-sha256. specified. Verifiers MUST implement both rsa-sha1 and rsa-sha256.
Signers MUST implement and SHOULD sign using rsa-sha256. Signers MUST implement and SHOULD sign using rsa-sha256.
INFORMATIVE NOTE: Although sha256 is strongly encouraged, some INFORMATIVE NOTE: Although sha256 is strongly encouraged, some
senders of low-security messages (such as routine newsletters) may senders of low-security messages (such as routine newsletters) may
prefer to use sha1 because of reduced CPU requirements to compute prefer to use sha1 because of reduced CPU requirements to compute
a sha1 hash. In general, sha256 should always be used whenever a sha1 hash. In general, sha256 should always be used whenever
possible. possible.
3.3.1. The rsa-sha1 Signing Algorithm 3.3.1. The rsa-sha1 Signing Algorithm
The rsa-sha1 Signing Algorithm computes a message hash as described The rsa-sha1 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-1 [FIPS.180-2.2002] as the hash-alg. in Section 3.7 below using SHA-1 [FIPS.180-2.2002] as the hash-alg.
That hash is then signed by the signer using the RSA algorithm That hash is then signed by the signer using the RSA algorithm
(defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the
signer's private key. The hash MUST NOT be truncated or converted signer's private key. The hash MUST NOT be truncated or converted
into any form other than the native binary form before being signed. into any form other than the native binary form before being signed.
The signing algorithm SHOULD use an exponent of 65537. The signing algorithm SHOULD use a public exponent of 65537.
3.3.2. The rsa-sha256 Signing Algorithm 3.3.2. The rsa-sha256 Signing Algorithm
The rsa-sha256 Signing Algorithm computes a message hash as described The rsa-sha256 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-256 [FIPS.180-2.2002] as the hash-alg. in Section 3.7 below using SHA-256 [FIPS.180-2.2002] as the hash-alg.
That hash is then signed by the signer using the RSA algorithm That hash is then signed by the signer using the RSA algorithm
(defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the
signer's private key. The hash MUST NOT be truncated or converted signer's private key. The hash MUST NOT be truncated or converted
into any form other than the native binary form before being signed. into any form other than the native binary form before being signed.
3.3.3. Key sizes 3.3.3. 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. Since short RSA keys more easily succumb to performance, and risk. Since short RSA keys more easily succumb to
off-line attacks, signers MUST use RSA keys of at least 1024 bits for 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 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 keys ranging from 512 bits to 2048 bits, and they MAY be able to
validate signatures with larger keys. Verifier policies may use the validate signatures with larger keys. Verifier policies may use the
length of the signing key as one metric for determining whether a length of the signing key as one metric for determining whether a
signature is acceptable. signature is acceptable.
Factors that should influence the key size choice include: Factors that should influence the key size choice include the
following:
o The practical constraint that large (e.g., 4096 bit) keys may not o The practical constraint that large (e.g., 4096 bit) keys may not
fit within a 512 byte DNS UDP response packet fit within a 512-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 off-line 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 other systems that employ digital signatures typical goals of other systems that employ digital signatures
See [RFC3766] for further discussion on selecting key sizes. See [RFC3766] for further discussion on selecting key sizes.
3.3.4. Other algorithms 3.3.4. 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 implement. any signatures using algorithms that they do not implement.
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.
Other signers demand that any modification of the email, however Other signers demand that any modification of the email, however
minor, result in a signature verification failure. These signers minor, result in a signature verification failure. These signers
prefer a canonicalization algorithm that does not tolerate in-transit prefer a canonicalization algorithm that does not tolerate in-transit
modification of the signed email. modification of the signed email.
Some signers may be willing to accept modifications to header fields Some signers may be willing to accept modifications to header fields
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 whitespace replacement and
header field line re-wrapping. A signer MAY specify either algorithm header field line rewrapping. 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. Verifiers MUST implement both for both header and body. Verifiers MUST implement both
canonicalization algorithms. Note that the header and body may use canonicalization algorithms. Note that the header and body may use
different canonicalization algorithms. Further canonicalization different canonicalization algorithms. Further canonicalization
algorithms MAY be defined in the future; verifiers MUST ignore any algorithms MAY be defined in the future; verifiers MUST ignore any
signatures that use unrecognized canonicalization algorithms. signatures that use unrecognized canonicalization algorithms.
Canonicalization simply prepares the email for presentation to the Canonicalization simply prepares the email for presentation to the
signing or verification algorithm. It MUST NOT change the signing or verification algorithm. It MUST NOT change the
transmitted data in any way. Canonicalization of header fields and transmitted data in any way. Canonicalization of header fields and
body are described below. body are described below.
NOTE: This section assumes that the message is already in "network NOTE: This section assumes that the message is already in "network
normal" format (e.g., text is ASCII encoded, lines are separated with normal" format (text is ASCII encoded, lines are separated with CRLF
CRLF characters, etc.). See also Section 5.3 for information about characters, etc.). See also Section 5.3 for information about
normalizing the message. normalizing the message.
3.4.1. The "simple" Header Canonicalization Algorithm 3.4.1. The "simple" Header 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 and white space MUST NOT be changed. case folded and white space MUST NOT be changed.
3.4.2. The "relaxed" Header Canonicalization Algorithm 3.4.2. The "relaxed" Header 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: lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
AbC".
o Unfold all header field continuation lines as described in o Unfold all header field continuation lines as described in
[RFC2822]; in particular, lines with terminators embedded in [RFC2822]; in particular, lines with terminators embedded in
continued header field values (that is, CRLF sequences followed by continued header field values (that is, CRLF sequences followed by
WSP) MUST be interpreted without the CRLF. Implementations MUST WSP) MUST be interpreted without the CRLF. Implementations MUST
NOT remove the CRLF at the end of the header field value. NOT remove the CRLF at the end of the header field value.
o Convert all sequences of one or more WSP characters to a single SP o Convert all sequences of one or more WSP characters to a single SP
character. WSP characters here include those before and after a character. WSP characters here include those before and after a
line folding boundary. line folding boundary.
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INFORMATIVE NOTE: It should be noted that the relaxed body INFORMATIVE NOTE: It should be noted that the relaxed body
canonicalization algorithm may enable certain types of extremely canonicalization algorithm may enable certain types of extremely
crude "ASCII Art" attacks where a message may be conveyed by crude "ASCII Art" attacks where a message may be conveyed by
adjusting the spacing between words. If this is a concern, the adjusting the spacing between words. If this is a concern, the
"simple" body canonicalization algorithm should be used instead. "simple" body canonicalization algorithm should be used instead.
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, measured in calculation to an initial prefix of the body text, measured in
octets. If the body length count is not specified then the entire octets. If the body length count is not specified, the entire
message body is signed. message body is signed.
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
also signed, the body length count is a useful tool for the also signed, the body length count is a useful tool for the
verifier since it may as a matter of policy accept messages having verifier since it may as a matter of policy accept messages having
valid signatures with extraneous data. valid signatures with extraneous data.
INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables
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3.4.6. Canonicalization Examples (INFORMATIVE) 3.4.6. Canonicalization Examples (INFORMATIVE)
In the following examples, actual white space is used only for In the following examples, actual white space is used only for
clarity. The actual input and output text is designated using clarity. The actual input and output text is designated using
bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a
tab character, and "<CRLF>" for a carriage-return/line-feed sequence. tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
For example, "X <SP> Y" and "X<SP>Y" represent the same three For example, "X <SP> Y" and "X<SP>Y" represent the same three
characters. characters.
Example 1: A message reading: Example 1: A message reading:
A: <SP> X <CRLF> A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF> B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF> <HTAB> Z <SP><SP><CRLF>
<CRLF> <CRLF>
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF> D <SP><HTAB><SP> E <CRLF>
<CRLF> <CRLF>
<CRLF> <CRLF>
when canonicalized using relaxed canonicalization for both header and when canonicalized using relaxed canonicalization for both header and
body results in a header reading: body results in a header reading:
a:X <CRLF> a:X <CRLF>
b:Y <SP> Z <CRLF> b:Y <SP> Z <CRLF>
and a body reading: and a body reading:
<SP> C <CRLF> <SP> C <CRLF>
D <SP> E <CRLF> D <SP> E <CRLF>
Example 2: The same message canonicalized using simple Example 2: The same message canonicalized using simple
canonicalization for both header and body results in a header canonicalization for both header and body results in a header
reading: reading:
A: <SP> X <CRLF> A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF> B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF> <HTAB> Z <SP><SP><CRLF>
and a body reading: and a body reading:
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF> D <SP><HTAB><SP> E <CRLF>
Example 3: When processed using relaxed header canonicalization and Example 3: When processed using relaxed header canonicalization and
simple body canonicalization, the canonicalized version has a header simple body canonicalization, the canonicalized version has a header
of: of:
a:X <CRLF> a:X <CRLF>
b:Y <SP> Z <CRLF> b:Y <SP> Z <CRLF>
and a body reading: and a body reading:
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF> D <SP><HTAB><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 signature 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
were a trace header field as defined in section 3.6 of [RFC2822], and trace header field as defined in Section 3.6 of [RFC2822], and hence
hence SHOULD NOT be reordered and SHOULD be prepended to the message. SHOULD NOT be reordered and SHOULD be prepended to the message.
The "DKIM-Signature:" header field being created or verified is The DKIM-Signature header field being created or verified is always
always included in the signature calculation, after the rest of the included in the signature calculation, after the rest of the header
header fields being signed; however, when calculating or verifying fields being signed; however, when calculating or verifying the
the signature, the value of the b= tag (signature value) of that signature, the value of the "b=" tag (signature value) of that DKIM-
DKIM-Signature header field MUST be treated as though it were an Signature header field MUST be treated as though it were an empty
empty string. Unknown tags in the "DKIM-Signature:" header field string. Unknown tags in the DKIM-Signature header field MUST be
MUST be included in the signature calculation but MUST be otherwise included in the signature calculation but MUST be otherwise ignored
ignored by verifiers. Other "DKIM-Signature:" header fields that are by verifiers. Other DKIM-Signature header fields that are included
included in the signature should be treated as normal header fields; in the signature should be treated as normal header fields; in
in particular, the b= tag is not treated specially. particular, the "b=" tag is not treated specially.
The encodings for each field type are listed below. Tags described The encodings for each field type are listed below. Tags described
as qp-section are encoded as described in section 6.7 of MIME Part as qp-section are encoded as described in Section 6.7 of MIME Part
One [RFC2045], with the additional conversion of semicolon characters One [RFC2045], with the additional conversion of semicolon characters
to "=3B"; intuitively, this is one line of quoted-printable encoded to "=3B"; intuitively, this is one line of quoted-printable encoded
text. The dkim-quoted-printable syntax is defined in Section 2.6. text. The dkim-quoted-printable syntax is defined in Section 2.6.
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. Unrecognized tags MUST be requirement status are shown below. Unrecognized tags MUST be
ignored. ignored.
v= Version (MUST be included). This tag defines the version of this v= Version (MUST be included). This tag defines the version of this
specification that applies to the signature record. It MUST have specification that applies to the signature record. It MUST have
the value 0.5. Note that verifiers must do a string comparison the value "1". Note that verifiers must do a string comparison
on this value; for example, "1" is not the same as "1.0". on this value; for example, "1" is not the same as "1.0".
ABNF: ABNF:
sig-v-tag = %x76 [FWS] "=" [FWS] "0.5" sig-v-tag = %x76 [FWS] "=" [FWS] "1"
INFORMATIVE NOTE: DKIM-Signature version numbers are INFORMATIVE NOTE: DKIM-Signature version numbers are expected
expected to increase arithmetically as new versions of this to increase arithmetically as new versions of this
specification are released. specification are released.
[[INFORMATIVE NOTE: Upon publication, this version number
should be changed to "1" (two places), and this note should
be deleted.]]
a= The algorithm used to generate the signature (plain-text; a= The algorithm used to generate the signature (plain-text;
REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256"; REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256";
signers SHOULD sign using "rsa-sha256". See Section 3.3 for a signers SHOULD sign using "rsa-sha256". See Section 3.3 for a
description of algorithms. description of algorithms.
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 = sig-a-tag-k "-" sig-a-tag-h sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / x-sig-a-tag-k sig-a-tag-k = "rsa" / x-sig-a-tag-k
skipping to change at page 19, line 39 skipping to change at page 19, line 4
description of algorithms. description of algorithms.
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 = sig-a-tag-k "-" sig-a-tag-h sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / x-sig-a-tag-k sig-a-tag-k = "rsa" / x-sig-a-tag-k
sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT) ; for later extension x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT) ; for later extension
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT) ; for later extension x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT) ; 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 reassembling 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-tag-data = base64string
bh= The hash of the canonicalized body part of the message as limited bh= The hash of the canonicalized body part of the message as limited
by the "l=" tag (base64; REQUIRED). Whitespace is ignored in by the "l=" tag (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when re-assembling the original this value and MUST be ignored when reassembling 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 Section 3.7 for how the body hash is computed. limits. See Section 3.7 for how the body hash is computed.
ABNF: ABNF:
sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
sig-bh-tag-data = base64string sig-bh-tag-data = base64string
c= Message canonicalization (plain-text; OPTIONAL, default is c= Message canonicalization (plain-text; OPTIONAL, default is
skipping to change at page 21, line 24 skipping to change at page 20, line 27
contain the complete list of header fields in the order presented contain the complete list of header fields in the order presented
to the signing algorithm. The field MAY contain names of header to the signing algorithm. The field MAY contain names of header
fields that do not exist when signed; nonexistent header fields fields that do not exist when signed; nonexistent header fields
do not contribute to the signature computation (that is, they are do not contribute to the signature computation (that is, they are
treated as the null input, including the header field name, the treated as the null input, including the header field name, the
separating colon, the header field value, and any CRLF separating colon, the header field value, and any CRLF
terminator). The field MUST NOT include the DKIM-Signature terminator). The field MUST NOT include the DKIM-Signature
header field that is being created or verified, but may include header field that is being created or verified, but may include
others. Folding white space (FWS) MAY be included on either side others. Folding white space (FWS) MAY be included on either side
of the colon separator. Header field names MUST be compared of the colon separator. Header field names MUST be compared
against actual header field names in a case insensitive manner. against actual header field names in a case-insensitive manner.
This list MUST NOT be empty. See Section 5.4 for a discussion of This list MUST NOT be empty. See Section 5.4 for a discussion of
choosing header fields to sign. 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
not actually exist, a signer can prevent insertion of those actually exist, a signer can prevent insertion of those
header fields before verification. However, since a signer header fields before verification. However, since a signer
cannot possibly know what header fields might be created in cannot possibly know what header fields might be created in
the future, and that some MUAs might present header fields the future, and that some MUAs might present header fields
that are embedded inside a message (e.g., as a message/rfc822 that are embedded inside a message (e.g., as a message/rfc822
content type), the security of this solution is not total. content type), the security of this solution is not total.
INFORMATIVE EXPLANATION: The exclusion of the header field INFORMATIVE EXPLANATION: The exclusion of the header field name
name and colon as well as the header field value for non- and colon as well as the header field value for non-existent
existent header fields prevents an attacker from inserting an header fields prevents an attacker from inserting an actual
actual header field with a null value. 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 (dkim-quoted-printable; behalf of which this message is signed (dkim-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 MAY be omitted. The standard email address where the Local-part MAY be omitted. The
domain part of the address MUST be the same as or a subdomain of domain part of the address MUST be the same as or a subdomain of
the value of the "d=" tag. the value of the "d=" tag.
Internationalized domain names MUST be converted using the steps Internationalized domain names MUST be converted using the steps
listed in section 4 of [RFC3490] using the "ToASCII" function. listed in Section 4 of [RFC3490] using the "ToASCII" function.
ABNF: ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name
INFORMATIVE NOTE: The local-part of the "i=" tag is optional INFORMATIVE NOTE: The Local-part of the "i=" tag is optional
because in some cases a signer may not be able to establish a because in some cases a signer may not be able to establish a
verified individual identity. In such cases, the signer may verified individual identity. In such cases, the signer may
wish to assert that although it is willing to go as far as wish to assert that although it is willing to go as far as
signing for the domain, it is unable or unwilling to commit signing for the domain, it is unable or unwilling to commit
to an individual user name within their domain. It can do so to an individual user name within their domain. It can do so
by including the domain part but not the local-part of the by including the domain part but not the Local-part of the
identity. identity.
INFORMATIVE DISCUSSION: This document does not require the INFORMATIVE DISCUSSION: This document does not require the value
value of the "i=" tag to match the identity in any message of the "i=" tag to match the identity in any message header
header field fields. This is considered to be a verifier fields. This is considered to be a verifier policy issue.
policy issue. Constraints between the value of the "i=" tag Constraints between the value of the "i=" tag and other
and other identities in other header fields seek to apply identities in other header fields seek to apply basic
basic authentication into the semantics of trust associated authentication into the semantics of trust associated with a
with a role such as content author. Trust is a broad and role such as content author. Trust is a broad and complex
complex topic and trust mechanisms are subject to highly topic and trust mechanisms are subject to highly creative
creative attacks. The real-world efficacy of any but the attacks. The real-world efficacy of any but the most basic
most basic bindings between the "i=" value and other bindings between the "i=" value and other identities is not
identities is not well established, nor is its vulnerability well established, nor is its vulnerability to subversion by
to subversion by an attacker. Hence reliance on the use of an attacker. Hence reliance on the use of these options
these options should be strictly limited. In particular it should be strictly limited. In particular, it is not at all
is not at all clear to what extent a typical end-user clear to what extent a typical end-user recipient can rely on
recipient can rely on any assurances that might be made by any assurances that might be made by successful use of the
successful use of the "i=" options. "i=" options.
l= Body length count (plain-text unsigned decimal integer; OPTIONAL, l= Body length count (plain-text unsigned decimal integer; OPTIONAL,
default is entire body). This tag informs the verifier of the default is entire body). This tag informs the verifier of the
number of octets in the body of the email after canonicalization number of octets in the body of the email after canonicalization
included in the cryptographic hash, starting from 0 immediately included in the cryptographic hash, starting from 0 immediately
following the CRLF preceding the body. This value MUST NOT be following the CRLF preceding the body. This value MUST NOT be
larger than the actual number of octets in the canonicalized larger than the actual number of octets in the canonicalized
message body. message body.
INFORMATIVE IMPLEMENTATION WARNING: Use of the l= tag might INFORMATIVE IMPLEMENTATION WARNING: Use of the "l=" tag might
allow display of fraudulent content without appropriate allow display of fraudulent content without appropriate
warning to end users. The l= tag is intended for increasing warning to end users. The "l=" tag is intended for
signature robustness when sending to mailing lists that both increasing signature robustness when sending to mailing lists
modify their content and do not sign their messages. that both modify their content and do not sign their
However, using the l= tag enables attacks in which an messages. However, using the "l=" tag enables attacks in
intermediary with malicious intent modifies a message to which an intermediary with malicious intent modifies a
include content that solely benefits the attacker. It is message to include content that solely benefits the attacker.
possible for the appended content to completely replace the It is possible for the appended content to completely replace
original content in the end recipient's eyes and to defeat the original content in the end recipient's eyes and to
duplicate message detection algorithms. Examples are defeat duplicate message detection algorithms. Examples are
described in Security Considerations (Section 8). To avoid described in Security Considerations (Section 8). To avoid
this attack, signers should be extremely wary of using this this attack, signers should be extremely wary of using this
tag, and verifiers might wish to ignore the tag or remove tag, and verifiers might wish to ignore the tag or remove
text that appears after the specified content length. text that appears after the specified content length.
INFORMATIVE NOTE: The value of the l= tag is constrained to INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76
76 decimal digits. This constraint is not intended to decimal digits. This constraint is not intended to predict
predict the size of future messages or to require the size of future messages or to require implementations to
implementations to use an integer representation large enough use an integer representation large enough to represent the
to represent the maximum possible value, but is intended to maximum possible value, but is intended to remind the
remind the implementer to check the length of this and all implementer to check the length of this and all other tags
other tags during verification and to test for integer during verification and to test for integer overflow when
overflow when decoding the value. Implementers may need to decoding the value. Implementers may need to limit the
limit the actual value expressed to a value smaller than actual value expressed to a value smaller than 10^76, e.g.,
10^76, e.g., to allow a message to fit within the available to allow a message to fit within the available storage space.
storage space.
ABNF: ABNF:
sig-l-tag = %x6c [FWS] "=" [FWS] 1*76DIGIT sig-l-tag = %x6c [FWS] "=" [FWS] 1*76DIGIT
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/txt"). Each public key (plain-text; OPTIONAL, default is "dns/txt"). Each
query method is of the form "type[/options]", where the syntax query method is of the form "type[/options]", where the syntax
and semantics of the options depends on the type and specified and semantics of the options depend on the type and specified
options. If there are multiple query mechanisms listed, the options. If there are multiple query mechanisms listed, the
choice of query mechanism MUST NOT change the interpretation of choice of query mechanism MUST NOT change the interpretation of
the signature. Implementations MUST use the recognized query the signature. Implementations MUST use the recognized query
mechanisms in the order presented. mechanisms in the order presented.
Currently the only valid value is "dns/txt" which defines the DNS Currently, the only valid value is "dns/txt", which defines the DNS
TXT record lookup algorithm described elsewhere in this document. TXT record lookup algorithm described elsewhere in this document.
The only option defined for the "dns" query type is "txt", which The only option defined for the "dns" query type is "txt", which
MUST be included. Verifiers and signers MUST support "dns/txt". MUST be included. Verifiers and signers MUST support "dns/txt".
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 = "dns/txt" / x-sig-q-tag-type sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
["/" x-sig-q-tag-args] ["/" x-sig-q-tag-args]
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] selector sig-s-tag = %x73 [FWS] "=" [FWS] selector
t= Signature Timestamp (plain-text unsigned decimal integer; t= Signature Timestamp (plain-text unsigned decimal integer;
RECOMMENDED, default is an unknown creation time). The time that RECOMMENDED, default is an unknown creation time). The time that
this signature was created. The format is the number of seconds this signature was created. The format is the number of seconds
since 00:00:00 on January 1, 1970 in the UTC time zone. The since 00:00:00 on January 1, 1970 in the UTC time zone. The
value is expressed as an unsigned integer in decimal ASCII. This value is expressed as an unsigned integer in decimal ASCII. This
value is not constrained to fit into a 31- or 32-bit integer. value is not constrained to fit into a 31- or 32-bit integer.
Implementations SHOULD be prepared to handle values up to at Implementations SHOULD be prepared to handle values up to at
least 10^12 (until approximately AD 200,000; this fits into 40 least 10^12 (until approximately AD 200,000; this fits into 40
bits). To avoid denial of service attacks, implementations MAY bits). To avoid denial-of-service attacks, implementations MAY
consider any value longer than 12 digits to be infinite. Leap consider any value longer than 12 digits to be infinite. Leap
seconds are not counted. Implementations MAY ignore signatures seconds are not counted. Implementations MAY ignore signatures
that have a timestamp in the future. that have a timestamp in the future.
ABNF: ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
x= Signature Expiration (plain-text unsigned decimal integer; x= Signature Expiration (plain-text unsigned decimal integer;
RECOMMENDED, default is no expiration). The format is the same RECOMMENDED, default is no expiration). The format is the same
skipping to change at page 25, line 5 skipping to change at page 24, line 5
time delta from the signing timestamp. The value is expressed as time delta from the signing timestamp. The value is expressed as
an unsigned integer in decimal ASCII, with the same constraints an unsigned integer in decimal ASCII, with the same constraints
on the value in the "t=" tag. Signatures MAY be considered on the value in the "t=" tag. Signatures MAY be considered
invalid if the verification time at the verifier is past the invalid if the verification time at the verifier is past the
expiration date. The verification time should be the time that expiration date. The verification time should be the time that
the message was first received at the administrative domain of the message was first received at the administrative domain of
the verifier if that time is reliably available; otherwise the the verifier if that time is reliably available; otherwise the
current time should be used. The value of the "x=" tag MUST be current time should be used. The value of the "x=" tag MUST be
greater than the value of the "t=" tag if both are present. 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
replay defense. defense.
ABNF: ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT
z= Copied header fields (dkim-quoted-printable, but see description; z= Copied header fields (dkim-quoted-printable, but see description;
OPTIONAL, default is null). A vertical-bar-separated list of OPTIONAL, default is null). A vertical-bar-separated list of
selected header fields present when the message was signed, selected header fields present when the message was signed,
including both the field name and value. It is not required to including both the field name and value. It is not required to
include all header fields present at the time of signing. This include all header fields present at the time of signing. This
field need not contain the same header fields listed in the "h=" field need not contain the same header fields listed in the "h="
tag. The header field text itself must encode the vertical bar tag. The header field text itself must encode the vertical bar
("|", %x7C) character (i.e., vertical bars in the z= text are ("|", %x7C) character (i.e., vertical bars in the "z=" text are
metacharacters, and any actual vertical bar characters in a metacharacters, and any actual vertical bar characters in a
copied header field must be encoded). Note that all white space copied header field must be encoded). Note that all white space
must be encoded, including white space between the colon and the must be encoded, including white space between the colon and the
header field value. After encoding, LWSP MAY be added at header field value. After encoding, FWS MAY be added at
arbitrary locations in order to avoid excessively long lines; arbitrary locations in order to avoid excessively long lines;
such white space is NOT part of the value of the header field, such whitespace is NOT part of the value of the header field, and
and MUST be removed before decoding. MUST be removed before decoding.
The header fields referenced by the h= tag refer to the fields in The header fields referenced by the "h=" tag refer to the fields in
the 2822 header of the message, not to any copied fields in the the RFC 2822 header of the message, not to any copied fields in
z= tag. Copied header field values are for diagnostic use. the "z=" tag. Copied header field values are for diagnostic use.
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 that are not [RFC2822] compliant) SHOULD be converted
converted as described in MIME Part Three [RFC2047]. 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 ":" qp-hdr-value sig-z-tag-copy = hdr-name ":" qp-hdr-value
qp-hdr-value = dkim-quoted-printable ; with "|" encoded qp-hdr-value = dkim-quoted-printable ; with "|" encoded
INFORMATIVE EXAMPLE of a signature header field spread across INFORMATIVE EXAMPLE of a signature header field spread across
multiple continuation lines: multiple continuation lines:
skipping to change at page 26, line 31 skipping to change at page 25, line 31
of security, with significantly enhanced scalability, by simply of security, with significantly enhanced scalability, by simply
having the verifier query the purported signer's DNS entry (or some having the verifier query the purported signer's DNS entry (or some
security-equivalent) in order to retrieve the public key. 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 signer (the "d=" tag of the DKIM- (the "q=" tag), the domain of the signer (the "d=" tag of the DKIM-
Signature header field), and the Selector (the "s=" tag). Signature header field), and the selector (the "s=" tag).
public_key = dkim_find_key(q_val, d_val, s_val) public_key = dkim_find_key(q_val, d_val, s_val)
This document defines a single binding, using DNS TXT records to This document defines a single binding, using DNS TXT records to
distribute the keys. Other bindings may be defined in the future. distribute the keys. Other bindings may be defined in the future.
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 an otherwise unstructured text format (for example, an XML form in an otherwise unstructured text format (for example, an XML form
skipping to change at page 27, line 17 skipping to change at page 26, line 17
(without the quotes). This tag MUST be the first tag in the (without the quotes). This tag MUST be the first tag in the
record. Records beginning with a "v=" tag with any other value record. Records beginning with a "v=" tag with any other value
MUST be discarded. Note that verifiers must do a string MUST be discarded. Note that verifiers must do a string
comparison on this value; for example, "DKIM1" is not the same as comparison on this value; for example, "DKIM1" is not the same as
"DKIM1.0". "DKIM1.0".
ABNF: ABNF:
key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1" key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1"
g= granularity of the key (plain-text; OPTIONAL, default is "*"). g= Granularity of the key (plain-text; OPTIONAL, default is "*").
This value MUST match the Local-part of the "i=" tag of the DKIM- This value MUST match the Local-part of the "i=" tag of the DKIM-
Signature header field (or its default value of the empty string Signature header field (or its default value of the empty string
if "i=" is not specified), with a single, optional "*" character if "i=" is not specified), with a single, optional "*" character
matching a sequence of zero or more arbitrary characters matching a sequence of zero or more arbitrary characters
("wildcarding"). An email with a signing address that does not ("wildcarding"). An email with a signing address that does not
match the value of this tag constitutes a failed verification. match the value of this tag constitutes a failed verification.
The intent of this tag is to constrain which signing address can The intent of this tag is to constrain which signing address can
legitimately use this Selector, for example, when delegating a legitimately use this selector, for example, when delegating a
key to a third party that should only be used for special key to a third party that should only be used for special
purposes. Wildcarding allows matching for addresses such as purposes. Wildcarding allows matching for addresses such as
"user+*" or "*-offer". An empty "g=" value never matches any "user+*" or "*-offer". An empty "g=" value never matches any
addresses. addresses.
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-text] ["*" [dot-atom-text] ] key-g-tag-lpart = [dot-atom-text] ["*" [dot-atom-text] ]
[[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a
"dot-atom-text". This should probably use a different
character for wildcarding. Unfortunately, the options are
non-mnemonic (e.g., "@", "(", ":"). Alternatively we could
insist on escaping a "*" intended as a literal "*" in the
address.]]
h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to h= Acceptable 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 "sha256" hash algorithm. Verifiers MUST also support support the "sha256" hash algorithm. Verifiers MUST also support
the "sha1" hash algorithm. 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 )
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algorithms that might be used. Signers and Verifiers MUST algorithms that might be used. Signers and Verifiers MUST
support the "sha256" hash algorithm. Verifiers MUST also support support the "sha256" hash algorithm. Verifiers MUST also support
the "sha1" hash algorithm. 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" / "sha256" / 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 ASN.1 DER-encoded [ITU.X660.1997] RSAPublicKey indicates that an ASN.1 DER-encoded [ITU.X660.1997] RSAPublicKey
[RFC3447] (see sections 3.1 and A.1.1) is being used in the p= [RFC3447] (see Sections 3.1 and A.1.1) is being used in the "p="
tag. (Note: the p= tag further encodes the value using the tag. (Note: the "p=" tag further encodes the value using the
base64 algorithm.) base64 algorithm.)
ABNF: ABNF:
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type
key-k-tag-type = "rsa" / x-key-k-tag-type key-k-tag-type = "rsa" / x-key-k-tag-type
x-key-k-tag-type = hyphenated-word ; for future extension x-key-k-tag-type = hyphenated-word ; for future extension
n= Notes that might be of interest to a human (qp-section; OPTIONAL, n= Notes that might be of interest to a human (qp-section; OPTIONAL,
default is empty). No interpretation is made by any program. default is empty). No interpretation is made by any program.
This tag should be used sparingly in any key server mechanism This tag should be used sparingly in any key server mechanism
that has space limitations (notably DNS). This is intended for that has space limitations (notably DNS). This is intended for
use by administrators, not end users. use by administrators, not end users.
ABNF: ABNF:
key-n-tag = %x6e [FWS] "=" [FWS] qp-section key-n-tag = %x6e [FWS] "=" [FWS] qp-section
p= Public-key data (base64; REQUIRED). An empty value means that p= Public-key data (base64; REQUIRED). An empty value means that
this public key has been revoked. The syntax and semantics of this public key has been revoked. The syntax and semantics of
this tag value before being encoded in base64 is defined by the this tag value before being encoded in base64 are defined by the
k= tag. "k=" tag.
INFORMATIVE RATIONALE: If a private key has been compromised INFORMATIVE RATIONALE: If a private key has been compromised
or otherwise disabled (e.g., an outsourcing contract has been or otherwise disabled (e.g., an outsourcing contract has been
terminated), a signer might want to explicitly state that it terminated), a signer might want to explicitly state that it
knows about the selector, but all messages using that knows about the selector, but all messages using that
selector should fail verification. Verifiers should ignore selector should fail verification. Verifiers should ignore
any DKIM-Signature header fields with a selector referencing any DKIM-Signature header fields with a selector referencing
a revoked key. a revoked key.
ABNF: ABNF:
key-p-tag = %x70 [FWS] "=" [ [FWS] base64string ] key-p-tag = %x70 [FWS] "=" [ [FWS] base64string ]
INFORMATIVE NOTE: A base64string is permitted to include white
space (FWS) at arbitrary places; however, any CRLFs must be
followed by at least one WSP character. Implementors and
administrators are cautioned to ensure that selector TXT
records conform to this specification.
s= Service Type (plain-text; OPTIONAL; default is "*"). A colon- s= Service Type (plain-text; OPTIONAL; default is "*"). A colon-
separated list of service types to which this record applies. separated list of service types to which this record applies.
Verifiers for a given service type MUST ignore this record if the Verifiers for a given service type MUST ignore this record if the
appropriate type is not listed. Currently defined service types appropriate type is not listed. Currently defined service types
are: are as follows:
* 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 constrain the use of keys for other This tag is intended to constrain the use of keys for other
purposes, should use of DKIM be defined by other services in the purposes, should use of DKIM be defined by other services in the
future. future.
ABNF: ABNF:
key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type
0*( [FWS] ":" [FWS] key-s-tag-type ) 0*( [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
as follows:
y This domain is testing DKIM. Verifiers MUST NOT treat y This domain is testing DKIM. Verifiers MUST NOT treat
messages from signers in testing mode differently from messages from signers in testing mode differently from
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.
s Any DKIM-Signature header fields using the "i=" tag MUST have s Any DKIM-Signature header fields using the "i=" tag MUST have
the same domain value on the right hand side of the "@" in the same domain value on the right-hand side of the "@" in
the "i=" tag and the value of the "d=" tag. That is, the the "i=" tag and the value of the "d=" tag. That is, the
"i=" domain MUST NOT be a subdomain of "d=". Use of this "i=" domain MUST NOT be a subdomain of "d=". Use of this
flag is RECOMMENDED unless subdomaining is required. flag is RECOMMENDED unless subdomaining is required.
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" / "s" / x-key-t-tag-flag key-t-tag-flag = "y" / "s" / x-key-t-tag-flag
x-key-t-tag-flag = hyphenated-word ; for future extension 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 TXT records as a key service is hereby defined. A binding using DNS TXT records as a key service is hereby defined.
All implementations MUST support this binding. All implementations MUST support this binding.
3.6.2.1. Name Space 3.6.2.1. Namespace
All DKIM keys are stored in a subdomain named "_domainkey". Given a All DKIM keys are stored in a subdomain named "_domainkey". Given a
DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag
of "foo.bar", the DNS query will be for of "foo.bar", the DNS query will be for
"foo.bar._domainkey.example.com". "foo.bar._domainkey.example.com".
INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records (e.g., INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records (e.g.,
*.bar._domainkey.example.com) do not make sense in this context *.bar._domainkey.example.com) do not make sense in this context
and should not be used. Note also that wildcards within domains and should not be used. Note also that wildcards within domains
(e.g., s._domainkey.*.example.com) are not supported by the DNS. (e.g., s._domainkey.*.example.com) are not supported by the DNS.
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Strings in a TXT RR MUST be concatenated together before use with no Strings in a TXT RR MUST be concatenated together before use with no
intervening white space. TXT RRs MUST be unique for a particular intervening white space. TXT RRs MUST be unique for a particular
selector name; that is, if there are multiple records in an RRset, selector name; that is, if there are multiple records in an RRset,
the results are undefined. the results are undefined.
TXT RRs are encoded as described in Section 3.6.1. TXT RRs are encoded as described in Section 3.6.1.
3.7. Computing the Message Hashes 3.7. Computing the Message Hashes
Both signing and verifying message signatures starts with a step of Both signing and verifying message signatures start with a step of
computing two cryptographic hashes over the message. Signers will computing two cryptographic hashes over the message. Signers will
choose 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-
"DKIM-Signature" header field being verified. In the following Signature header field being verified. In the following discussion,
discussion, the names of the tags in the "DKIM-Signature" header the names of the tags in the DKIM-Signature header field that either
field which either exists (when verifying) or will be created (when exists (when verifying) or will be created (when signing) are used.
signing) are used. Note that canonicalization (Section 3.4) is only Note that canonicalization (Section 3.4) is only used to prepare the
used to prepare the email for signing or verifying; it does not email for signing or verifying; it does not affect the transmitted
affect the transmitted email in any way. email in any way.
The signer/verifier MUST compute two hashes, one over the body of the The signer/verifier MUST compute two hashes, one over the body of the
message and one over the selected header fields of the message. message and one over the selected header fields of the message.
Signers MUST compute them in the order shown. Verifiers MAY compute Signers MUST compute them in the order shown. Verifiers MAY compute
them in any order convenient to the verifier, provided that the them in any order convenient to the verifier, provided that the
result is semantically identical to the semantics that would be the result is semantically identical to the semantics that would be the
case had they been computed in this order. case had they been computed in this order.
In hash step 1, the signer/verifier MUST hash the message body, In hash step 1, the signer/verifier MUST hash the message body,
canonicalized using the body canonicalization algorithm specified in canonicalized using the body canonicalization algorithm specified in
the "c=" tag and then truncated to the length specified in the "l=" the "c=" tag and then truncated to the length specified in the "l="
tag. That hash value is then converted to base64 form and inserted tag. That hash value is then converted to base64 form and inserted
into (signers) or compared to (verifiers) the "bh=" tag of the DKIM- into (signers) or compared to (verifiers) the "bh=" tag of the DKIM-
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Signers MUST compute them in the order shown. Verifiers MAY compute Signers MUST compute them in the order shown. Verifiers MAY compute
them in any order convenient to the verifier, provided that the them in any order convenient to the verifier, provided that the
result is semantically identical to the semantics that would be the result is semantically identical to the semantics that would be the
case had they been computed in this order. case had they been computed in this order.
In hash step 1, the signer/verifier MUST hash the message body, In hash step 1, the signer/verifier MUST hash the message body,
canonicalized using the body canonicalization algorithm specified in canonicalized using the body canonicalization algorithm specified in
the "c=" tag and then truncated to the length specified in the "l=" the "c=" tag and then truncated to the length specified in the "l="
tag. That hash value is then converted to base64 form and inserted tag. That hash value is then converted to base64 form and inserted
into (signers) or compared to (verifiers) the "bh=" tag of the DKIM- into (signers) or compared to (verifiers) the "bh=" tag of the DKIM-
Signature: header field. Signature header field.
In hash step 2, the signer/verifier MUST pass the following to the In hash step 2, the signer/verifier MUST pass the following to the
hash algorithm in the indicated order. hash algorithm in the indicated order.
1. The header fields specified by the "h=" tag, in the order 1. The header fields specified by the "h=" tag, in the order
specified in that tag, and canonicalized using the header specified in that tag, and canonicalized using the header
canonicalization algorithm specified in the "c=" tag. Each canonicalization algorithm specified in the "c=" tag. Each
header field MUST be terminated with a single CRLF. header field MUST be terminated with a single CRLF.
2. 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 value of the "b=" be inserted (signing) in the message, with the value of the "b="
tag deleted (i.e., treated as the empty string), canonicalized tag deleted (i.e., treated as the empty string), canonicalized
using the header canonicalization algorithm specified in the "c=" using the header canonicalization algorithm specified in the "c="
tag, and without a trailing CRLF. tag, and without a trailing CRLF.
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
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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" (the modification of lines encodings such as SMTP "dot-stuffing" (the modification of lines
beginning with a "." to avoid confusion with the SMTP end-of-message beginning with a "." to avoid confusion with the SMTP end-of-message
marker, as specified in [RFC2821]). marker, as specified in [RFC2821]).
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 octets. DKIM messages MAY be either in plain-text simply a string of octets. DKIM messages MAY be either in plain-text
or in MIME format; no special treatment is afforded to MIME content. 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. that is signed.
More formally, the algorithm for the signature is: More formally, the algorithm for the signature is as follows:
body-hash = hash-alg(canon_body) body-hash = hash-alg(canon_body)
header-hash = hash-alg(canon_header || DKIM-SIG) header-hash = hash-alg(canon_header || DKIM-SIG)
signature = sig-alg(header-hash, key) signature = sig-alg(header-hash, key)
where "sig-alg" is the signature algorithm specified by the "a=" tag, where "sig-alg" is the signature algorithm specified by the "a=" tag,
"hash-alg" is the hash algorithm specified by the "a=" tag, "hash-alg" is the hash algorithm specified by the "a=" tag,
"canon_header" and "canon_body" are the canonicalized message header "canon_header" and "canon_body" are the canonicalized message header
and body (respectively) as defined in Section 3.4 (excluding the and body (respectively) as defined in Section 3.4 (excluding the
DKIM-Signature header field), and "DKIM-SIG" is the canonicalized DKIM-Signature header field), and "DKIM-SIG" is the canonicalized
DKIM-Signature header field sans the signature value itself, but with DKIM-Signature header field sans the signature value itself, but with
"body-hash" included as the "bh=" tag. "body-hash" included as the "bh=" tag.
INFORMATIVE IMPLEMENTERS' NOTE: Many digital signature APIs INFORMATIVE IMPLEMENTERS' NOTE: Many digital signature APIs
provide both hashing and application of the RSA private key using provide both hashing and application of the RSA private key using
a single "sign()" primitive. When using such an API the last two a single "sign()" primitive. When using such an API, the last two
steps in the algorithm would probably be combined into a single steps in the algorithm would probably be combined into a single
call that would perform both the "hash-alg" and the "sig-alg". call that would perform both the "hash-alg" and the "sig-alg".
3.8. Signing by Parent Domains 3.8. Signing by Parent Domains
In some circumstances, it is desirable for a domain to apply a In some circumstances, it is desirable for a domain to apply a
signature on behalf of any of its subdomains without the need to signature on behalf of any of its subdomains without the need to
maintain separate selectors (key records) in each subdomain. By maintain separate selectors (key records) in each subdomain. By
default, private keys corresponding to key records can be used to default, private keys corresponding to key records can be used to
sign messages for any subdomain of the domain in which they reside, sign messages for any subdomain of the domain in which they reside;
e.g., a key record for the domain example.com can be used to verify e.g., a key record for the domain example.com can be used to verify
messages where the signing identity (i= tag of the signature) is messages where the signing identity ("i=" tag of the signature) is
sub.example.com, or even sub1.sub2.example.com. In order to limit sub.example.com, or even sub1.sub2.example.com. In order to limit
the capability of such keys when this is not intended, the "s" flag the capability of such keys when this is not intended, the "s" flag
may be set in the t= tag of the key record to constrain the validity may be set in the "t=" tag of the key record to constrain the
of the record to exactly the domain of the signing identity. If the validity of the record to exactly the domain of the signing identity.
referenced key record contains the "s" flag as part of the t= tag, If the referenced key record contains the "s" flag as part of the
the domain of the signing identity (i= flag) MUST be the same as that "t=" tag, the domain of the signing identity ("i=" flag) MUST be the
of the d= domain. If this flag is absent, the domain of the signing same as that of the d= domain. If this flag is absent, the domain of
identity MUST be the same as, or a subdomain of, the d= domain. Key the signing identity MUST be the same as, or a subdomain of, the d=
records which are not intended for use with subdomains SHOULD specify domain. Key records that are not intended for use with subdomains
the "s" flag in the t= tag. SHOULD specify the "s" flag in the "t=" tag.
4. Semantics of Multiple Signatures 4. Semantics of Multiple Signatures
4.1. Example Scenarios 4.1. Example Scenarios
There are many reasons that a message might have multiple signatures. There are many reasons why a message might have multiple signatures.
For example, a given signer might sign multiple times, perhaps with For example, a given signer might sign multiple times, perhaps with
different hashing or signing algorithms during a transition phase. different hashing or signing algorithms during a transition phase.
INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be
insufficiently strong, and DKIM usage transitions to SHA-1024. A insufficiently strong, and DKIM usage transitions to SHA-1024. A
signer might immediately sign using the newer algorithm, but signer might immediately sign using the newer algorithm, but
continue to sign using the older algorithm for interoperability continue to sign using the older algorithm for interoperability
with verifiers that had not yet upgraded. The signer would do with verifiers that had not yet upgraded. The signer would do
this by adding two DKIM-Signature header fields, one using each this by adding two DKIM-Signature header fields, one using each
algorithm. Older verifiers that did not recognize SHA-1024 as an algorithm. Older verifiers that did not recognize SHA-1024 as an
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the verifier might make a different set of policy decisions. the verifier might make a different set of policy decisions.
Of course, a message might also have multiple signatures because it Of course, a message might also have multiple signatures because it
passed through multiple signers. A common case is expected to be passed through multiple signers. A common case is expected to be
that of a signed message that passes through a mailing list that also that of a signed message that passes through a mailing list that also
signs all messages. Assuming both of those signatures verify, a signs all messages. Assuming both of those signatures verify, a
recipient might choose to accept the message if either of those recipient might choose to accept the message if either of those
signatures were known to come from trusted sources. signatures were known to come from trusted sources.
INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing
lists to which they have subscribed and which have acceptable lists to which they have subscribed and that have acceptable anti-
anti-abuse policies so as to accept messages sent to that list abuse policies so as to accept messages sent to that list even
even from unknown authors. They might also subscribe to less from unknown authors. They might also subscribe to less trusted
trusted mailing lists (e.g., those without anti-abuse protection) mailing lists (e.g., those without anti-abuse protection) and be
and be willing to accept all messages from specific authors, but willing to accept all messages from specific authors, but insist
insist on doing additional abuse scanning for other messages. on doing additional abuse scanning for other messages.
Another related example of multiple signers might be forwarding Another related example of multiple signers might be forwarding
services, such as those commonly associated with academic alumni services, such as those commonly associated with academic alumni
sites. sites.
INFORMATIVE EXAMPLE: A recipient might have an address at INFORMATIVE EXAMPLE: A recipient might have an address at
members.example.org, a site that has anti-abuse protection that is members.example.org, a site that has anti-abuse protection that is
somewhat less effective than the recipient would prefer. Such a somewhat less effective than the recipient would prefer. Such a
recipient might have specific authors whose messages would be recipient might have specific authors whose messages would be
trusted absolutely, but messages from unknown authors which had trusted absolutely, but messages from unknown authors that had
passed the forwarder's scrutiny would have only medium trust. passed the forwarder's scrutiny would have only medium trust.
4.2. Interpretation 4.2. Interpretation
A signer that is adding a signature to a message merely creates a new A signer that is adding a signature to a message merely creates a new
DKIM-Signature header, using the usual semantics of the h= option. A DKIM-Signature header, using the usual semantics of the h= option. A
signer MAY sign previously existing DKIM-Signature header fields signer MAY sign previously existing DKIM-Signature header fields
using the method described in section Section 5.4 to sign trace using the method described in Section 5.4 to sign trace header
header fields. fields.
INFORMATIVE NOTE: Signers should be cognizant that signing DKIM- INFORMATIVE NOTE: Signers should be cognizant that signing DKIM-
Signature header fields may result in signature failures with Signature header fields may result in signature failures with
intermediaries that do not recognize that DKIM-Signature header intermediaries that do not recognize that DKIM-Signature header
fields are trace header fields and unwittingly reorder them, thus fields are trace header fields and unwittingly reorder them, thus
breaking such signatures. For this reason, signing existing DKIM- breaking such signatures. For this reason, signing existing DKIM-
Signature header fields is unadvised, albeit legal. Signature header fields is unadvised, albeit legal.
INFORMATIVE NOTE: If a header field with multiple instances is INFORMATIVE NOTE: If a header field with multiple instances is
signed, those header fields are always signed from the bottom up. signed, those header fields are always signed from the bottom up.
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present in the message. Verifiers SHOULD continue to check present in the message. Verifiers SHOULD continue to check
signatures until a signature successfully verifies to the signatures until a signature successfully verifies to the
satisfaction of the verifier. To limit potential denial-of-service satisfaction of the verifier. To limit potential denial-of-service
attacks, verifiers MAY limit the total number of signatures they will attacks, verifiers MAY limit the total number of signatures they will
attempt to verify. attempt to verify.
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 Whether 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
reasons beyond the lack of a private key why a signer could choose reasons beyond the lack of a private key why a signer could choose
not to sign an email. not to sign an email.
INFORMATIVE NOTE: Signing modules may be incorporated into any INFORMATIVE NOTE: Signing modules may be incorporated into any
portion of the mail system as deemed appropriate, including an portion of the mail system as deemed appropriate, including an
MUA, a SUBMISSION server, or an MTA. Wherever implemented, MUA, a SUBMISSION server, or an MTA. Wherever implemented,
signers should beware of signing (and thereby asserting signers should beware of signing (and thereby asserting
responsibility for) messages that may be problematic. In responsibility for) messages that may be problematic. In
particular, within a trusted enclave the signing address might be particular, within a trusted enclave the signing address might be
derived from the header according to local policy; SUBMISSION derived from the header according to local policy; SUBMISSION
servers might only sign messages from users that are properly servers might only sign messages from users that are properly
authenticated and authorized. authenticated and authorized.
INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign
sign Received header fields if the outgoing gateway MTA obfuscates Received header fields if the outgoing gateway MTA obfuscates
Received header fields, for example to hide the details of Received header fields, for example, to hide the details of
internal topology. internal topology.
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. Distribution and management of private keys is also convenience. Distribution and management of private keys is also
outside the scope of this document. outside the scope of this document.
INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
private key when the Selector containing the corresponding public private key when the selector containing the corresponding public
key is expected to be revoked or removed before the verifier has key is expected to be revoked or removed before the verifier has
an opportunity to validate the signature. The signer should an opportunity to validate the signature. The signer should
anticipate that verifiers may choose to defer validation, perhaps anticipate that verifiers may choose to defer validation, perhaps
until the message is actually read by the final recipient. In until the message is actually read by the final recipient. In
particular, when rotating to a new key pair, signing should particular, when rotating to a new key pair, signing should
immediately commence with the new private key and the old public immediately commence with the new private key and the old public
key should be retained for a reasonable validation interval before key should be retained for a reasonable validation interval before
being removed from the key server. being removed from the key server.
5.3. Normalize the Message to Prevent Transport Conversions 5.3. Normalize the Message to Prevent Transport Conversions
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sequence before the message is signed. Any conversion of this sort sequence before the message is signed. Any conversion of this sort
SHOULD be applied to the message actually sent to the recipient(s), SHOULD be applied to the message actually sent to the recipient(s),
not just to the version presented to the signing algorithm. not just to the version presented to the signing algorithm.
More generally, the signer MUST sign the message as it is expected to More generally, the signer MUST sign the message as it is expected to
be received by the verifier rather than in some local or internal be received by the verifier rather than in some local or internal
form. form.
5.4. Determine the Header Fields to Sign 5.4. Determine the Header Fields to Sign
The From header field MUST be signed (that is, included in the h= tag The From header field MUST be signed (that is, included in the "h="
of the resulting DKIM-Signature header field). Signers SHOULD NOT tag of the resulting DKIM-Signature header field). Signers SHOULD
sign an existing header field likely to be legitimately modified or NOT sign an existing header field likely to be legitimately modified
removed in transit. In particular, [RFC2821] explicitly permits or removed in transit. In particular, [RFC2821] explicitly permits
modification or removal of the "Return-Path" header field in transit. modification or removal of the Return-Path header field in transit.
Signers MAY include any other header fields present at the time of Signers MAY include any other header fields present at the time of
signing at the discretion of the signer. signing at the discretion of the signer.
INFORMATIVE OPERATIONS NOTE: The choice of which header fields to INFORMATIVE OPERATIONS NOTE: The choice of which header fields to
sign is non-obvious. One strategy is to sign all existing, non- sign is non-obvious. One strategy is to sign all existing, non-
repeatable header fields. An alternative strategy is to sign only repeatable header fields. An alternative strategy is to sign only
header fields that are likely to be displayed to or otherwise be header fields that are likely to be displayed to or otherwise be
likely to affect the processing of the message at the receiver. A likely to affect the processing of the message at the receiver. A
third strategy is to sign only "well known" headers. Note that third strategy is to sign only "well known" headers. Note that
verifiers may treat unsigned header fields with extreme verifiers may treat unsigned header fields with extreme
skepticism, including refusing to display them to the end user or skepticism, including refusing to display them to the end user or
even ignore the signature if it does not cover certain header even ignoring the signature if it does not cover certain header
fields. For this reason signing fields present in the message fields. For this reason, signing fields present in the message
such as Date, Subject, Reply-To, Sender, and all MIME header such as Date, Subject, Reply-To, Sender, and all MIME header
fields is highly advised. fields are highly advised.
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 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=" 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.
INFORMATIVE NOTE: A header field name need only be listed once INFORMATIVE NOTE: A header field name need only be listed once
more than the actual number of that header field in a message at more than the actual number of that header field in a message at
the time of signing in order to prevent any further additions. the time of signing in order to prevent any further additions.
For example, if there is a single "Comments" header field at the For example, if there is a single Comments header field at the
time of signing, listing "Comments" twice in the h= tag is time of signing, listing Comments twice in the "h=" tag is
sufficient to prevent any number of Comments header fields from sufficient to prevent any number of Comments header fields from
being appended; it is not necessary (but is legal) to list being appended; it is not necessary (but is legal) to list
"Comments" three or more times in the h= tag. Comments three or more times in the "h=" tag.
Signers choosing to sign an existing header field that occurs more Signers choosing to sign an existing header field that occurs more
than once in the message (such as Received) MUST sign the physically than once in the message (such as Received) MUST sign the physically
last instance of that header field in the header block. Signers last instance of that header field in the header block. Signers
wishing to sign multiple instances of such a header field MUST wishing to sign multiple instances of such a header field MUST
include the header field name multiple times in the h= tag of the include the header field name multiple times in the h= tag of the
DKIM-Signature header field, and MUST sign such header fields in DKIM-Signature header field, and MUST sign such header fields in
order from the bottom of the header field block to the top. The order from the bottom of the header field block to the top. The
signer MAY include more instances of a header field name in h= than signer MAY include more instances of a header field name in h= than
there are actual corresponding header fields to indicate that there are actual corresponding header fields to indicate that
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Signature header fields is error-prone. Signature header fields is error-prone.
INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
header fields to be reordered (with the exception of Received header fields to be reordered (with the exception of Received
header fields), reordering of signed header fields with multiple header fields), reordering of signed header fields with multiple
instances by intermediate MTAs will cause DKIM signatures to be instances by intermediate MTAs will cause DKIM signatures to be
broken; such anti-social behavior should be avoided. broken; such anti-social behavior should 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.
5.5. Recommended Signature Content 5.5. Recommended Signature Content
In order to maximize compatibility with a variety of verifiers, it is In order to maximize compatibility with a variety of verifiers, it is
recommended that signers follow the practices outlined in this recommended that signers follow the practices outlined in this
section when signing a message. However, these are generic section when signing a message. However, these are generic
recommendations applying to the general case; specific senders may recommendations applying to the general case; specific senders may
wish to modify these guidelines as required by their unique wish to modify these guidelines as required by their unique
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o Date, Message-ID o Date, Message-ID
o To, Cc o To, Cc
o MIME-Version o MIME-Version
o Content-Type, Content-Transfer-Encoding, Content-ID, Content- o Content-Type, Content-Transfer-Encoding, Content-ID, Content-
Description Description
o Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-cc, o Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-Cc,
Resent-Message-ID Resent-Message-ID
o In-Reply-To, References o In-Reply-To, References
o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post,
List-Owner, List-Archive List-Owner, List-Archive
The following header fields SHOULD NOT be included in the signature: The following header fields SHOULD NOT be included in the signature:
o Return-Path o Return-Path
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o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post,
List-Owner, List-Archive List-Owner, List-Archive
The following header fields SHOULD NOT be included in the signature: The following header fields SHOULD NOT be included in the signature:
o Return-Path o Return-Path
o Received o Received
o Comments, Keywords o Comments, Keywords
o Bcc, Resent-Bcc o Bcc, Resent-Bcc
o DKIM-Signature o DKIM-Signature
Optional header fields (those not mentioned above) normally SHOULD Optional header fields (those not mentioned above) normally SHOULD
NOT be included in the signature, because of the potential for NOT be included in the signature, because of the potential for
additional header fields of the same name to be legitimately added or additional header fields of the same name to be legitimately added or
reordered prior to verification. There are likely to be legitimate reordered prior to verification. There are likely to be legitimate
exceptions to this rule, because of the wide variety of application- exceptions to this rule, because of the wide variety of application-
specific header fields which may be applied to a message, some of specific header fields that may be applied to a message, some of
which are unlikely to be duplicated, modified, or reordered. which are unlikely to be duplicated, modified, or reordered.
Signers SHOULD choose canonicalization algorithms based on the types Signers SHOULD choose canonicalization algorithms based on the types
of messages they process and their aversion to risk. For example, of messages they process and their aversion to risk. For example,
e-commerce sites sending primarily purchase receipts, which are not e-commerce sites sending primarily purchase receipts, which are not
expected to be processed by mailing lists or other software likely to expected to be processed by mailing lists or other software likely to
modify messages, will generally prefer "simple" canonicalization. modify messages, will generally prefer "simple" canonicalization.
Sites sending primarily person-to-person email will likely prefer to Sites sending primarily person-to-person email will likely prefer to
be more more resilient to modification during transport by using be more resilient to modification during transport by using "relaxed"
"relaxed" canonicalization. canonicalization.
Signers SHOULD NOT use l= unless they intend to accomodate Signers SHOULD NOT use "l=" unless they intend to accommodate
intermediate mail processors that append text to a message. For intermediate mail processors that append text to a message. For
example, many mailing list processors append "unsubscribe" example, many mailing list processors append "unsubscribe"
information to message bodies. If signers use l=, they SHOULD information to message bodies. If signers use "l=", they SHOULD
include the entire message body existing at the time of signing in include the entire message body existing at the time of signing in
computing the count. In particular, signers SHOULD NOT specify a computing the count. In particular, signers SHOULD NOT specify a
body length of 0 since this may be interpreted as a meaningless body length of 0 since this may be interpreted as a meaningless
signature by some verifiers. signature by some verifiers.
5.6. Compute the Message Hash and Signature 5.6. 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. This will and then sign it using the selected public-key algorithm. This will
result in a DKIM-Signature header field which will include the body result in a DKIM-Signature header field that will include the body
hash and a signature of the header hash, where that header includes hash and a signature of the header hash, where that header includes
the DKIM-Signature header field itself. the DKIM-Signature header field itself.
Entities such as mailing list managers that implement DKIM and which Entities such as mailing list managers that implement DKIM and that
modify the message or a 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. modifications before re-signing the message.
The signer MAY elect to limit the number of bytes of the body that 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 will be included in the hash and hence signed. The length actually
hashed should be inserted in the "l=" tag of the "DKIM-Signature" hashed should be inserted in the "l=" tag of the DKIM-Signature
header field. header field.
5.7. Insert the DKIM-Signature Header Field 5.7. 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
created in the previous step prior to transmitting the email. The created in the previous step prior to transmitting the email. The
"DKIM-Signature" header field MUST be the same as used to compute the DKIM-Signature header field MUST be the same as used to compute the
hash as described above, except that the value of the "b=" tag MUST hash as described above, except that the value of the "b=" tag MUST
be the appropriately signed hash computed in the previous step, be the appropriately signed hash computed in the previous step,
signed using the algorithm specified in the "a=" tag of the "DKIM- signed using the algorithm specified in the "a=" tag of the DKIM-
Signature" header field and using the private key corresponding to Signature header field and using the private key corresponding to the
the Selector given in the "s=" tag of the "DKIM-Signature" header selector given in the "s=" tag of the DKIM-Signature header field, as
field, as chosen above in Section 5.2 chosen above in Section 5.2
The "DKIM-Signature" MUST be inserted before any other DKIM-Signature The DKIM-Signature header field MUST be inserted before any other
fields in the header block. DKIM-Signature fields 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. In particular, it may be placed before any the header block. In particular, it may be placed before any
existing Received header fields. This is consistent with treating existing Received header fields. This is consistent with treating
"DKIM-Signature" as a trace header field. DKIM-Signature as a trace header field.
6. Verifier Actions 6. Verifier Actions
Since a signer MAY remove or revoke a public key at any time, it is Since a signer MAY remove or revoke a public key at any time, it is
recommended that verification occur in a timely manner. In many recommended that verification occur in a timely manner. In many
configurations, the most timely place is during acceptance by the configurations, the most timely place is during acceptance by the
border MTA or shortly thereafter. In particular, deferring border MTA or shortly thereafter. In particular, deferring
verification until the message is accessed by the end user is verification until the message is accessed by the end user is
discouraged. discouraged.
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field to signed messages. field to signed messages.
Verifiers MUST produce a result that is semantically equivalent to Verifiers MUST produce a result that is semantically equivalent to
applying the following steps in the order listed. In practice, applying the following steps in the order listed. In practice,
several of these steps can be performed in parallel in order to several of these steps can be performed in parallel in order to
improve performance. improve performance.
6.1. Extract Signatures from the Message 6.1. Extract Signatures from the Message
The order in which verifiers try DKIM-Signature header fields is not The order in which verifiers try DKIM-Signature header fields is not
defined; verifiers MAY try signatures in any order they would like. defined; verifiers MAY try signatures in any order they like. For
For example, one implementation might prefer to try the signatures in example, one implementation might try the signatures in textual
textual order, whereas another might want to prefer signatures by order, whereas another might try signatures by identities that match
identities that match the contents of the "From" header field over the contents of the From header field before trying other signatures.
other identities. Verifiers MUST NOT attribute ultimate meaning to Verifiers MUST NOT attribute ultimate meaning to the order of
the order of multiple DKIM-Signature header fields. In particular, multiple DKIM-Signature header fields. In particular, there is
there is reason to believe that some relays will reorder the header reason to believe that some relays will reorder the header fields in
fields 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
(such as correlating the signing host with Received header fields) (such as correlating the signing host with Received header fields)
may also be considered. may also be considered.
A verifier SHOULD NOT treat a message that has one or more bad A verifier SHOULD NOT treat a message that has one or more bad
signatures and no good signatures differently from a message with no signatures and no good signatures differently from a message with no
signature at all; such treatment is a matter of local policy and is signature at all; such treatment is a matter of local policy and is
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When a signature successfully verifies, a verifier will either stop When a signature successfully verifies, a verifier will either stop
processing or attempt to verify any other signatures, at the processing or attempt to verify any other signatures, at the
discretion of the implementation. A verifier MAY limit the number of discretion of the implementation. A verifier MAY limit the number of
signatures it tries to avoid denial-of-service attacks. signatures it tries to avoid denial-of-service attacks.
INFORMATIVE NOTE: An attacker could send messages with large INFORMATIVE NOTE: An attacker could send messages with large
numbers of faulty signatures, each of which would require a DNS numbers of faulty signatures, each of which would require a DNS
lookup and corresponding CPU time to verify the message. This lookup and corresponding CPU time to verify the message. This
could be an attack on the domain that receives the message, by could be an attack on the domain that receives the message, by
slowing down the verifier by requiring it to do large number of slowing down the verifier by requiring it to do a large number of
DNS lookups and/or signature verifications. It could also be an DNS lookups and/or signature verifications. It could also be an
attack against the domains listed in the signatures, essentially attack against the domains listed in the signatures, essentially
by enlisting innocent verifiers in launching an attack against the by enlisting innocent verifiers in launching an attack against the
DNS servers of the actual victim. DNS servers of the actual victim.
In the following description, text reading "return status In the following description, text reading "return status
(explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL") (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
means that the verifier MUST immediately cease processing that means that the verifier MUST immediately cease processing that
signature. The verifier SHOULD proceed to the next signature, if any signature. The verifier SHOULD proceed to the next signature, if any
is present, and completely ignore the bad signature. If the status is present, and completely ignore the bad signature. If the status
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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 even in the presence of the
known-broken signature. known-broken signature.
For each signature to be validated, the following steps should be For each signature to be validated, the following steps should be
performed in such a manner as to produce a result that is performed in such a manner as to produce a result that is
semantically equivalent to performing them in the indicated order. semantically equivalent to performing them in the indicated order.
6.1.1. Validate the Signature Header Field 6.1.1. Validate the Signature Header Field
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 values DKIM-Signature header field; any inconsistency or unexpected values
MUST cause the header field to be completely ignored and the verifier MUST cause the header field to be completely ignored and the verifier
to return PERMFAIL (signature syntax error). Being "liberal in what to return PERMFAIL (signature syntax error). Being "liberal in what
you accept" is definitely a bad strategy in this security context. you accept" is definitely a bad strategy in this security context.
Note however that this does not include the existence of unknown tags Note however that this does not include the existence of unknown tags
in a DKIM-Signature header field, which are explicitly permitted. in a DKIM-Signature header field, which are explicitly permitted.
Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag
that is inconsistent with this specification and return PERMFAIL that is inconsistent with this specification and return PERMFAIL
(incompatible version). (incompatible version).
INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course,
course, choose to also verify signatures generated by older choose to also verify signatures generated by older versions of
versions of this specification. this specification.
If any tag listed as "required" in Section 3.5 is omitted from the If any tag listed as "required" in Section 3.5 is omitted from the
DKIM-Signature header field, the verifier MUST ignore the DKIM- DKIM-Signature header field, the verifier MUST ignore the DKIM-
Signature header field and return PERMFAIL (signature missing Signature header field and return PERMFAIL (signature missing
required tag). required tag).
INFORMATIONAL NOTE: The tags listed as required in Section 3.5 INFORMATIONAL NOTE: The tags listed as required in Section 3.5 are
are "v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there "v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there be a
be a conflict between this note and Section 3.5, Section 3.5 is conflict between this note and Section 3.5, Section 3.5 is
normative. normative.
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
the verifier MUST behave as though the value of that tag were "@d", verifier MUST behave as though the value of that tag were "@d", where
where "d" is the value from the "d=" tag. "d" is the value from the "d=" tag.
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 parent domain of the domain part of the "i=" tag. the same as or a parent domain of the domain part of the "i=" tag.
If not, the DKIM-Signature header field MUST be ignored and the If not, the DKIM-Signature header field MUST be ignored and the
verifier should return PERMFAIL (domain mismatch). verifier should return PERMFAIL (domain mismatch).
If the "h=" tag does not include the "From" header field the verifier If the "h=" tag does not include the From header field, the verifier
MUST ignore the DKIM-Signature header field and return PERMFAIL (From MUST ignore the DKIM-Signature header field and return PERMFAIL (From
field not signed). field not signed).
Verifiers MAY ignore the DKIM-Signature header field and return Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (signature expired) if it contains an "x=" tag and the PERMFAIL (signature expired) if it contains an "x=" tag and the
signature has expired. signature has expired.
Verifiers MAY ignore the DKIM-Signature header field if the domain Verifiers MAY ignore the DKIM-Signature header field if the domain
used by the signer in the d= tag is not associated with a valid used by the signer in the "d=" tag is not associated with a valid
signing entity. For example, signatures with d= values such as "com" signing entity. For example, signatures with "d=" values such as
and "co.uk" may be ignored. The list of unacceptable domains SHOULD "com" and "co.uk" may be ignored. The list of unacceptable domains
be configurable. SHOULD be configurable.
Verifiers MAY ignore the DKIM-Signature header field and return Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (unacceptable signature header) for any other reason, for PERMFAIL (unacceptable signature header) for any other reason, for
example, if the signature does not sign header fields that the example, if the signature does not sign header fields that the
verifier views to be essential. As a case in point, if MIME header verifier views to be essential. As a case in point, if MIME header
fields are not signed, certain attacks may be possible that the fields are not signed, certain attacks may be possible that the
verifier would prefer to avoid. verifier would prefer to avoid.
6.1.2. Get the Public Key 6.1.2. Get the Public Key
The public key for a signature is needed to complete the verification The public key for a signature is needed to complete the verification
process. The process of retrieving the public key depends on the process. The process of retrieving the public key depends on the
query type as defined by the "q=" tag in the "DKIM-Signature:" header query type as defined by the "q=" tag in the DKIM-Signature header
field. Obviously, a public key need only be retrieved if the process field. Obviously, a public key need only be retrieved if the process
of extracting the signature information is completely successful. of 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 perform the following When validating a message, a verifier MUST perform the following
steps in a manner that is semantically the same as performing them in steps in a manner that is semantically the same as performing them in
the order indicated (in some cases the implementation may parallelize the order indicated (in some cases, the implementation may
or reorder these steps, as long as the semantics remain unchanged): 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
algorithm in the "q=" tag, the domain from the "d=" tag, and the algorithm in the "q=" tag, the domain from the "d=" tag, and the
Selector from the "s=" tag. 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
MAY defer acceptance of this email and return TEMPFAIL (key MAY defer acceptance of this email and return TEMPFAIL (key
unavailable). If verification is occurring during the incoming unavailable). If verification is occurring during the incoming
SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
code. Alternatively, the verifier MAY store the message in the code. Alternatively, the verifier MAY store the message in the
local queue for later trial or ignore the signature. Note that local queue for later trial or ignore the signature. Note that
storing a message in the local queue is subject to denial-of- storing a message in the local queue is subject to denial-of-
service attacks. service attacks.
skipping to change at page 45, line 40 skipping to change at page 44, line 32
key record does not exist, the verifier MUST immediately return key record does not exist, the verifier MUST immediately return
PERMFAIL (no key for signature). PERMFAIL (no key for signature).
4. If the query for the public key returns multiple key records, the 4. If the query for the public key returns multiple key records, the
verifier may choose one of the key records or may cycle through verifier may choose one of the key records or may cycle through
the key records performing the remainder of these steps on each the key records performing the remainder of these steps on each
record at the discretion of the implementer. The order of the record at the discretion of the implementer. The order of the
key records is unspecified. If the verifier chooses to cycle key records is unspecified. If the verifier chooses to cycle
through the key records, then the "return ..." wording in the through the key records, then the "return ..." wording in the
remainder of this section means "try the next key record, if any; remainder of this section means "try the next key record, if any;
if none, return to try another signature in the usual way." if none, return to try another signature in the usual way".
5. If the result returned from the query does not adhere to the 5. If the result returned from the query does not adhere to the
format defined in this specification, the verifier MUST ignore format defined in this specification, the verifier MUST ignore
the key record and return PERMFAIL (key syntax error). Verifiers the key record and return PERMFAIL (key syntax error). Verifiers
are urged to validate the syntax of key records carefully to are urged to validate the syntax of key records carefully to
avoid attempted attacks. In particular, the verifier MUST ignore avoid attempted attacks. In particular, the verifier MUST ignore
keys with a version code ("v=" tag) that they do not implement. keys with a version code ("v=" tag) that they do not implement.
6. If the "g=" tag in the public key does not match the Local-part 6. If the "g=" tag in the public key does not match the Local-part
of the "i=" tag in the message signature header field, the of the "i=" tag in the message signature header field, the
verifier MUST ignore the key record and return PERMFAIL verifier MUST ignore the key record and return PERMFAIL
(inapplicable key). If the Local-part of the "i=" tag on the (inapplicable key). If the Local-part of the "i=" tag on the
message signature is not present, the g= tag must be * (valid for message signature is not present, the "g=" tag must be "*" (valid
all addresses in the domain) or the entire g= tag must be omitted for all addresses in the domain) or the entire g= tag must be
(which defaults to "g=*"), otherwise the verifier MUST ignore the omitted (which defaults to "g=*"), otherwise the verifier MUST
key record and return PERMFAIL (inapplicable key). Other than ignore the key record and return PERMFAIL (inapplicable key).
this test, verifiers SHOULD NOT treat a message signed with a key Other than this test, verifiers SHOULD NOT treat a message signed
record having a g= tag any differently than one without; in with a key record having a "g=" tag any differently than one
particular, verifiers SHOULD NOT prefer messages that seem to without; in particular, verifiers SHOULD NOT prefer messages that
have an individual signature by virtue of a g= tag versus a seem to have an individual signature by virtue of a "g=" tag
domain signature. versus a domain signature.
7. If the "h=" tag exists in the public key record and the hash 7. If the "h=" tag exists in the public key record and the hash
algorithm implied by the a= tag in the DKIM-Signature header algorithm implied by the a= tag in the DKIM-Signature header
field is not included in the contents of the "h=" tag, the field is not included in the contents of the "h=" tag, the
verifier MUST ignore the key record and return PERMFAIL verifier MUST ignore the key record and return PERMFAIL
(inappropriate hash algorithm). (inappropriate hash algorithm).
8. If the public key data (the "p=" tag) is empty then this key has 8. If the public key data (the "p=" tag) is empty, then this key has
been revoked and the verifier MUST treat this as a failed been revoked and the verifier MUST treat this as a failed
signature check and return PERMFAIL (key revoked). There is no signature check and return PERMFAIL (key revoked). There is no
defined semantic difference between a key that has been revoked defined semantic difference between a key that has been revoked
and a key record that has been removed. and a key record that has been removed.
9. If the public key data is not suitable for use with the algorithm 9. If the public key data is not suitable for use with the algorithm
and key types defined by the "a=" and "k=" tags in the "DKIM- and key types defined by the "a=" and "k=" tags in the DKIM-
Signature" header field, the verifier MUST immediately return Signature header field, the verifier MUST immediately return
PERMFAIL (inappropriate key algorithm). PERMFAIL (inappropriate key algorithm).
6.1.3. Compute the Verification 6.1.3. 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
actions semantically equivalent to the following steps. actions semantically equivalent to the following steps.
1. 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, prepare a canonicalized version of the message as is tag, prepare a canonicalized version of the message as is
skipping to change at page 48, line 27 skipping to change at page 47, line 20
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
identifier that can be used to assign trust and reputation. It is identifier that can be used to assign trust and reputation. It is
reasonable to treat unauthenticated email as lacking any trust and reasonable to treat unauthenticated email as lacking any trust and
having no positive reputation. having no positive reputation.
In general verifiers SHOULD NOT reject messages solely on the basis In general, verifiers SHOULD NOT reject messages solely on the basis
of a lack of signature or an unverifiable signature; such rejection of a lack of signature or an unverifiable signature; such rejection
would cause severe interoperability problems. However, if the would cause severe interoperability problems. However, if the
verifier does opt to reject such messages (for example, when verifier does opt to reject such messages (for example, when
communicating with a peer who, by prior agreement, agrees to only communicating with a peer who, by prior agreement, agrees to only
send signed messages), and the verifier runs synchronously with the send signed messages), and the verifier runs synchronously with the
SMTP session and a signature is missing or does not verify, the MTA SMTP session and a signature is missing or does not verify, the MTA
SHOULD use a 550/5.7.x reply code. SHOULD use a 550/5.7.x reply code.
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 using a 451/4.7.5 reply code, such as: generated using a 451/4.7.5 reply code, such as:
451 4.7.5 Unable to verify signature - key server unavailable 451 4.7.5 Unable to verify signature - key server unavailable
Temporary failures such as inability to access the key server or Temporary failures such as inability to access the key server or
other external service are the only conditions that SHOULD use a 4xx other external service are the only conditions that SHOULD use a 4xx
SMTP reply code. In particular, cryptographic signature verification SMTP reply code. In particular, cryptographic signature verification
failures MUST NOT return 4xx SMTP replies. failures MUST NOT return 4xx SMTP replies.
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/whitelists
and reputation systems) and/or to the end user. If the message is and reputation systems) and/or to the end user. If the message is
signed on behalf of any address other than that in the From: header signed on behalf of any address other than that in the From: header
field, the mail system SHOULD take pains to ensure that the actual field, the mail system SHOULD take pains to ensure that the actual
signing identity is clear to the reader. signing identity is clear to the reader.
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
never there, or was it removed by an over-zealous filter? For never there, or was it removed by an overzealous filter? For
diagnostic purposes, the exact reason why the verification fails diagnostic purposes, the exact reason why the verification fails
SHOULD be made available to the policy module and possibly recorded SHOULD be made available to the policy module and possibly recorded
in the system logs. If the email cannot be verified, then it SHOULD in the system logs. If the email cannot be verified, then it SHOULD
be rendered the same as all unverified email regardless of whether it be rendered the same as all unverified email regardless of whether or
looks like it was signed or not. not it looks like it was signed.
7. IANA Considerations 7. IANA Considerations
DKIM introduces some new namespaces that require IANA registry. In DKIM introduces some new namespaces that have been registered with
all cases, new values are assigned only for values that have IANA. In all cases, new values are assigned only for values that
documented in a published RFC having IETF Consensus [RFC2434]. have been documented in a published RFC that has IETF Consensus
[RFC2434].
7.1. DKIM-Signature Tag Specifications 7.1. DKIM-Signature Tag Specifications
A DKIM-Signature provides for a list of tag specifications. IANA is A DKIM-Signature provides for a list of tag specifications. IANA has
requested to establish the DKIM Signature Tag Specification Registry, established the DKIM-Signature Tag Specification Registry for tag
for tag specifications that can be used in DKIM-Signature fields and specifications that can be used in DKIM-Signature fields.
that have been specified in any published RFC.
The initial entries in the registry comprise: The initial entries in the registry comprise:
+------+-----------------+ +------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+------+-----------------+ +------+-----------------+
| v | (this document) | | v | (this document) |
| a | (this document) | | a | (this document) |
| b | (this document) | | b | (this document) |
| bh | (this document) | | bh | (this document) |
skipping to change at page 50, line 26 skipping to change at page 48, line 51
| h | (this document) | | h | (this document) |
| i | (this document) | | i | (this document) |
| l | (this document) | | l | (this document) |
| q | (this document) | | q | (this document) |
| s | (this document) | | s | (this document) |
| t | (this document) | | t | (this document) |
| x | (this document) | | x | (this document) |
| z | (this document) | | z | (this document) |
+------+-----------------+ +------+-----------------+
DKIM Signature Tag Specification Registry Initial Values DKIM-Signature Tag Specification Registry Initial Values
7.2. DKIM-Signature Query Method Registry 7.2. DKIM-Signature Query Method Registry
The "q=" tag-spec, as specified in Section 3.5 provides for a list of The "q=" tag-spec (specified in Section 3.5) provides for a list of
query methods. query methods.
IANA is requested to establish the DKIM Query Method Registry, for IANA has established the DKIM-Signature Query Method Registry for
mechanisms that can be used to retrieve the key that will permit mechanisms that can be used to retrieve the key that will permit
validation processing of a message signed using DKIM and have been validation processing of a message signed using DKIM.
specified in any published RFC.
The initial entry in the registry comprises: The initial entry in the registry comprises:
+------+--------+-----------------+ +------+--------+-----------------+
| TYPE | OPTION | REFERENCE | | TYPE | OPTION | REFERENCE |
+------+--------+-----------------+ +------+--------+-----------------+
| dns | txt | (this document) | | dns | txt | (this document) |
+------+--------+-----------------+ +------+--------+-----------------+
DKIM-Signature Query Method Registry Initial Values DKIM-Signature Query Method Registry Initial Values
7.3. DKIM-Signature Canonicalization Registry 7.3. DKIM-Signature Canonicalization Registry
The "c=" tag-spec, as specified in Section 3.5 provides for a The "c=" tag-spec (specified in Section 3.5) provides for a specifier
specifier for canonicalization algorithms for the header and body of for canonicalization algorithms for the header and body of the
the message. message.
IANA is requested to establish the DKIM Canonicalization Algorithm IANA has established the DKIM-Signature Canonicalization Algorithm
Registry, for algorithms for converting a message into a canonical Registry for algorithms for converting a message into a canonical
form before signing or verifying using DKIM and have been specified form before signing or verifying using DKIM.
in any published RFC.
The initial entries in the header registry comprise: The initial entries in the header registry comprise:
+---------+-----------------+ +---------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+---------+-----------------+ +---------+-----------------+
| simple | (this document) | | simple | (this document) |
| relaxed | (this document) | | relaxed | (this document) |
+---------+-----------------+ +---------+-----------------+
DKIM-Signature Header Canonicalization Algorithm Registry Initial DKIM-Signature Header Canonicalization Algorithm Registry
Values Initial Values
The initial entries in the body registry comprise: The initial entries in the body registry comprise:
+---------+-----------------+ +---------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+---------+-----------------+ +---------+-----------------+
| simple | (this document) | | simple | (this document) |
| relaxed | (this document) | | relaxed | (this document) |
+---------+-----------------+ +---------+-----------------+
DKIM-Signature Body Canonicalization Algorithm Registry Initial DKIM-Signature Body Canonicalization Algorithm Registry
Values Initial Values
7.4. _domainkey DNS TXT Record Tag Specifications 7.4. _domainkey DNS TXT Record Tag Specifications
A _domainkey DNS TXT record provides for a list of tag A _domainkey DNS TXT record provides for a list of tag
specifications. IANA is requested to establish the DKIM _domainkey specifications. IANA has established the DKIM _domainkey DNS TXT Tag
DNS TXT Tag Specification Registry, for tag specifications that can Specification Registry for tag specifications that can be used in DNS
be used in DNS TXT Records and that have been specified in any TXT Records.
published RFC.
The initial entries in the registry comprise: The initial entries in the registry comprise:
+------+-----------------+ +------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+------+-----------------+ +------+-----------------+
| v | (this document) | | v | (this document) |
| g | (this document) | | g | (this document) |
| h | (this document) | | h | (this document) |
| k | (this document) | | k | (this document) |
| n | (this document) | | n | (this document) |
| p | (this document) | | p | (this document) |
| s | (this document) | | s | (this document) |
| t | (this document) | | t | (this document) |
+------+-----------------+ +------+-----------------+
DKIM _domainkey DNS TXT Record Tag Specification Registry Initial DKIM _domainkey DNS TXT Record Tag Specification Registry
Values Initial Values
7.5. DKIM Key Type Registry 7.5. DKIM Key Type Registry
The "k=" <key-k-tag> (as specified in Section 3.6.1) and the "a=" The "k=" <key-k-tag> (specified in Section 3.6.1) and the "a=" <sig-
<sig-a-tag-k> (Section 3.5) tags provide for a list of mechanisms a-tag-k> (specified in Section 3.5) tags provide for a list of
that can be used to decode a DKIM signature. mechanisms that can be used to decode a DKIM signature.
IANA is requested to establish the DKIM Key Type Registry, for such IANA has established the DKIM Key Type Registry for such mechanisms.
mechanisms that have been specified in any published RFC.
The initial entry in the registry comprises: The initial entry in the registry comprises:
+------+-----------+ +------+-----------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+------+-----------+ +------+-----------+
| rsa | [RFC3447] | | rsa | [RFC3447] |
+------+-----------+ +------+-----------+
DKIM Key Type Initial Values DKIM Key Type Initial Values
7.6. DKIM Hash Algorithms Registry 7.6. DKIM Hash Algorithms Registry
The "h=" <key-h-tag> list (specified in Section 3.6.1) and the "a=" The "h=" <key-h-tag> (specified in Section 3.6.1) and the "a=" <sig-
<sig-a-tag-h> (Section 3.5) provide for a list of mechanisms that can a-tag-h> (specified in Section 3.5) tags provide for a list of
be used to produce a digest of message data. mechanisms that can be used to produce a digest of message data.
IANA is requested to establish the DKIM Hash Algorithms Registry, for IANA has established the DKIM Hash Algorithms Registry for such
such mechanisms that have been specified in any published RFC. mechanisms.
The initial entries in the registry comprise: The initial entries in the registry comprise:
+--------+-------------------+ +--------+-------------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+--------+-------------------+ +--------+-------------------+
| sha1 | [FIPS.180-2.2002] | | sha1 | [FIPS.180-2.2002] |
| sha256 | [FIPS.180-2.2002] | | sha256 | [FIPS.180-2.2002] |
+--------+-------------------+ +--------+-------------------+
DKIM Hash Algorithms Initial Values DKIM Hash Algorithms Initial Values
7.7. DKIM Service Types Registry 7.7. DKIM Service Types Registry
The "s=" <key-s-tag> list (specified in Section 3.6.1) provides for a The "s=" <key-s-tag> tag (specified in Section 3.6.1) provides for a
list of service types to which this selector may apply. list of service types to which this selector may apply.
IANA is requested to establish the DKIM Service Types Registry, for IANA has established the DKIM Service Types Registry for service
service types that have been specified in any published RFC. types.
The initial entries in the registry comprise: The initial entries in the registry comprise:
+-------+-----------------+ +-------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+-------+-----------------+ +-------+-----------------+
| email | (this document) | | email | (this document) |
| * | (this document) | | * | (this document) |
+-------+-----------------+ +-------+-----------------+
DKIM Hash Algorithms Initial Values DKIM Service Types Registry Initial Values
7.8. DKIM Selector Flags Registry 7.8. DKIM Selector Flags Registry
The "t=" <key-t-tag> list (specified in Section 3.6.1) provides for a The "t=" <key-t-tag> tag (specified in Section 3.6.1) provides for a
list of flags to modify interpretation of the selector. list of flags to modify interpretation of the selector.
IANA is requested to establish the DKIM Selector Flags Registry, for IANA has established the DKIM Selector Flags Registry for additional
additional flags that have been specified in any published RFC. flags.
The initial entries in the registry comprise: The initial entries in the registry comprise:
+------+-----------------+ +------+-----------------+
| TYPE | REFERENCE | | TYPE | REFERENCE |
+------+-----------------+ +------+-----------------+
| y | (this document) | | y | (this document) |
| s | (this document) | | s | (this document) |
+------+-----------------+ +------+-----------------+
DKIM Hash Algorithms Initial Values DKIM Selector Flags Registry Initial Values
7.9. DKIM-Signature Header Field 7.9. DKIM-Signature Header Field
IANA is requested to add DKIM-Signature to the "Permanent Message IANA has added DKIM-Signature to the "Permanent Message Header
Header Fields" registry (see [RFC3864]) for the "mail" protocol, Fields" registry (see [RFC3864]) for the "mail" protocol, using this
using this document as the Reference. document as the reference.
8. Security Considerations 8. Security Considerations
It has been observed that any mechanism that is introduced which It has been observed that any mechanism that is introduced that
attempts to stem the flow of spam is subject to intensive attack. attempts to stem the flow of spam is subject to intensive attack.
DKIM needs to be carefully scrutinized to identify potential attack DKIM needs to be carefully scrutinized to identify potential attack
vectors and the vulnerability to each. See also [RFC4686]. vectors and the vulnerability to each. See also [RFC4686].
8.1. Misuse of Body Length Limits ("l=" Tag) 8.1. Misuse of Body Length Limits ("l=" Tag)
Body length limits (in the form of the "l=" tag) are subject to Body length limits (in the form of the "l=" tag) are subject to
several potential attacks. several potential attacks.
8.1.1. Addition of new MIME parts to multipart/* 8.1.1. Addition of New MIME Parts to Multipart/*
If the body length limit does not cover a closing MIME multipart If the body length limit does not cover a closing MIME multipart
section (including the trailing ""--CRLF"" portion), then it is section (including the trailing "--CRLF" portion), then it is
possible for an attacker to intercept a properly signed multipart possible for an attacker to intercept a properly signed multipart
message and add a new body part. Depending on the details of the message and add a new body part. Depending on the details of the
MIME type and the implementation of the verifying MTA and the MIME type and the implementation of the verifying MTA and the
receiving MUA, this could allow an attacker to change the information receiving MUA, this could allow an attacker to change the information
displayed to an end user from an apparently trusted source. displayed to an end user from an apparently trusted source.
For example, if an attacker can append information to a "text/html" For example, if attackers can append information to a "text/html"
body part, they may be able to exploit a bug in some MUAs that body part, they may be able to exploit a bug in some MUAs that
continue to read after a "</html>" marker, and thus display HTML text continue to read after a "</html>" marker, and thus display HTML text
on top of already displayed text. If a message has a on top of already displayed text. If a message has a
"multipart/alternative" body part, they might be able to add a new "multipart/alternative" body part, they might be able to add a new
body part that is preferred by the displaying MUA. body part that is preferred by the displaying MUA.
8.1.2. Addition of new HTML content to existing content 8.1.2. Addition of new HTML content to existing content
Several receiving MUA implementations do not cease display after a Several receiving MUA implementations do not cease display after a
""</html>"" tag. In particular, this allows attacks involving ""</html>"" tag. In particular, this allows attacks involving
skipping to change at page 55, line 15 skipping to change at page 53, line 31
<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 mechanism, 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 not, however, 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. This threat applies mainly to MUA-based engineered messages. This threat applies mainly to MUA-based
implementations; protection of private keys on servers can be easily implementations; protection of private keys on servers can be easily
achieved through the use of specialized cryptographic hardware. achieved through the use of specialized cryptographic hardware.
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
would likely not know of the misappropriation until they receive would likely not know of the misappropriation until they receive
"bounce" messages from messages they are purported to have sent. "bounce" messages from messages they are purported to have sent.
Many users might not understand the significance of these bounce Many users might not understand the significance of these bounce
skipping to change at page 56, line 22 skipping to change at page 54, line 35
significantly different from the ability of an attacker to deny significantly different from the ability of an attacker to deny
service to the mail exchangers for a given domain, although it service to the mail exchangers for a given domain, although it
affects outgoing, not incoming, mail. affects outgoing, not incoming, mail.
A variation on this attack is that if a very large amount of mail A variation on this attack is that if a very large amount of mail
were to be sent using spoofed addresses from a given domain, the key were to be sent using spoofed addresses from a given domain, the key
servers for that domain could be overwhelmed with requests. However, servers for that domain could be overwhelmed with requests. However,
given the low overhead of verification compared with handling of the given the low overhead of verification compared with handling of the
email message itself, such an attack would be difficult to mount. email message itself, such an attack would be difficult to mount.
8.4. Attacks Against DNS 8.4. Attacks Against the DNS
Since DNS is a required binding for key services, specific attacks Since the DNS is a required binding for key services, specific
against DNS must be considered. attacks against the DNS must be considered.
While the DNS is currently insecure [RFC3833], these security While the DNS is currently insecure [RFC3833], these security
problems are the motivation behind DNSSEC [RFC4033], and all users of problems are the motivation behind DNS Security (DNSSEC) [RFC4033],
the DNS will reap the benefit of that work. and all users of the DNS will reap the benefit of that work.
DKIM is only intended as a "sufficient" method of proving 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 signing 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.
A specific DNS security issue which should be considered by DKIM A specific DNS security issue that should be considered by DKIM
verifiers is the name chaining attack described in section 2.3 of the verifiers is the name chaining attack described in Section 2.3 of the
DNS Threat Analysis [RFC3833]. A DKIM verifier, while verifying a DNS Threat Analysis [RFC3833]. A DKIM verifier, while verifying a
DKIM-Signature header field, could be prompted to retrieve a key DKIM-Signature header field, could be prompted to retrieve a key
record of an attacker's choosing. This threat can be minimized by record of an attacker's choosing. This threat can be minimized by
ensuring that name servers, including recursive name servers, used by ensuring that name servers, including recursive name servers, used by
the verifier enforce strict checking of "glue" and other additional the verifier enforce strict checking of "glue" and other additional
information in DNS responses and are therefore not vulnerable to this information in DNS responses and are therefore not vulnerable to this
attack. attack.
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
sending the message to many compromised machines that act as MTAs. sending the message to many compromised machines that act as MTAs.
The messages, not having been modified by the accomplice, have valid The messages, not having been modified by the accomplice, have valid
signatures. signatures.
Partial solutions to this problem involve the use of reputation Partial solutions to this problem involve the use of reputation
services to convey the fact that the specific email address is being services to convey the fact that the specific email address is being
used for spam, and that messages from that signer are likely to be used for spam and that messages from that signer are likely to be
spam. This requires a real-time detection mechanism in order to spam. This requires a real-time detection mechanism in order to
react quickly enough. However, such measures might be prone to react quickly enough. However, such measures might be prone to
abuse, if for example an attacker resent a large number of messages abuse, if for example an attacker resent a large number of messages
received from a victim in order to make them appear to be a spammer. received from a victim in order to make them appear to be a spammer.
Large verifiers might be able to detect unusually large volumes of Large verifiers might be able to detect unusually large volumes of
mails with the same signature in a short time period. Smaller mails with the same signature in a short time period. Smaller
verifiers can get substantially the same volume information via verifiers can get substantially the same volume of information via
existing collaborative systems. existing collaborative systems.
8.6. Limits on Revoking Keys 8.6. Limits on Revoking Keys
When a large domain detects undesirable behavior on the part of one When a large domain detects undesirable behavior on the part of one
of its users, it might wish to revoke the key used to sign that of its users, it might wish to revoke the key used to sign that
user's messages in order to disavow responsibility for messages which user's messages in order to disavow responsibility for messages that
have not yet been verified or which are the subject of a replay have not yet been verified or that are the subject of a replay
attack. However, the ability of the domain to do so can be limited attack. However, the ability of the domain to do so can be limited
if the same key, for scalability reasons, is used to sign messages if the same key, for scalability reasons, is used to sign messages
for many other users. Mechanisms for explicitly revoking keys on a for many other users. Mechanisms for explicitly revoking keys on a
per-address basis have been proposed but require further study as to per-address basis have been proposed but require further study as to
their utility and the DNS load they represent. their utility and the DNS load they represent.
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 that are
are intentionally malformed, with the intent of causing a denial-of- 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 the DNS and be
against intentionally as well as unintentionally malformed key robust against intentionally as well as unintentionally 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 8.9. Information Leakage
An attacker could determine when a particular signature was verified An attacker could determine when a particular signature was verified
by using a per-message Selector and then monitoring their DNS traffic 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 the key lookup. This would act as the equivalent of a "web bug"
for verification time rather than when the message was read. for verification time rather than when the message was read.
8.10. Remote Timing Attacks 8.10. Remote Timing Attacks
In some cases it may be possible to extract private keys using a In some cases, it may be possible to extract private keys using a
remote timing attack [BONEH03]. Implementations should consider remote timing attack [BONEH03]. Implementations should consider
obfuscating the timing to prevent such attacks. obfuscating the timing to prevent such attacks.
8.11. Reordered Header Fields 8.11. Reordered Header Fields
Existing standards allow intermediate MTAs to reorder header fields. Existing standards allow intermediate MTAs to reorder header fields.
If a signer signs two or more header fields of the same name, this If a signer signs two or more header fields of the same name, this
can cause spurious verification errors on otherwise legitimate can cause spurious verification errors on otherwise legitimate
messages. In particular, signers that sign any existing DKIM- messages. In particular, signers that sign any existing DKIM-
Signature fields run the risk of having messages incorrectly fail to Signature fields run the risk of having messages incorrectly fail to
skipping to change at page 59, line 23 skipping to change at page 57, line 35
example, in the example above any of the domains could potentially example, in the example above any of the domains could potentially
simply delegate "example.podunk.ca.us" to a server of their choice simply delegate "example.podunk.ca.us" to a server of their choice
and completely replace all DNS-served information. Note that a and completely replace all DNS-served information. Note that a
verifier MAY ignore signatures that come from an unlikely domain verifier MAY ignore signatures that come from an unlikely domain
such as ".com", as discussed in Section 6.1.1. such as ".com", as discussed in Section 6.1.1.
9. References 9. References
9.1. Normative References 9.1. Normative References
[FIPS.180-2.2002] [FIPS.180-2.2002] U.S. Department of Commerce, "Secure Hash
U.S. Department of Commerce, "Secure Hash Standard", FIPS Standard", FIPS PUB 180-2, August 2002.
PUB 180-2, August 2002.
[ITU.X660.1997] [ITU.X660.1997] "Information Technology - ASN.1 encoding rules:
"Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER),
Specification of Basic Encoding Rules (BER), Canonical Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (CER) and Distinguished Encoding Rules Encoding Rules (DER)", ITU-T Recommendation X.660,
(DER)", ITU-T Recommendation X.660, 1997. 1997.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2045] Freed, N. and N. Borenstein, "Multipurpose
Extensions (MIME) Part One: Format of Internet Message Internet Mail Extensions (MIME) Part One: Format
Bodies", RFC 2045, November 1996. of Internet Message Bodies", RFC 2045,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
Part Three: Message header field Extensions for Non-ASCII Extensions) Part Three: Message header field
Text", RFC 2047, November 1996. Extensions for Non-ASCII Text", RFC 2047,
November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to
Requirement Levels", BCP 14, RFC 2119, March 1997. Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, [RFC2821] Klensin, J., "Simple Mail Transfer Protocol",
April 2001. RFC 2821, April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, [RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001. April 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key
Standards (PKCS) #1: RSA Cryptography Specifications Cryptography Standards (PKCS) #1: RSA Cryptography
Version 2.1", RFC 3447, February 2003. Specifications Version 2.1", RFC 3447,
February 2003.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", "Internationalizing Domain Names in Applications
RFC 3490, March 2003. (IDNA)", RFC 3490, March 2003.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF
Specifications: ABNF", RFC 4234, October 2005. for Syntax Specifications: ABNF", RFC 4234,
October 2005.
9.2. Informative References 9.2. Informative References
[BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing [BONEH03] Proc. 12th USENIX Security Symposium, "Remote
Attacks are Practical", 2003. Timing Attacks are Practical", 2003.
[RFC-DK] "DomainKeys specification (to be published with this
RFC)", 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
Multipart/Encrypted", RFC 1847, October 1995. and Multipart/Encrypted", RFC 1847, October 1995.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for
IANA Considers Section in RFCs", BCP 26, October 1998. Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 2434, October 1998.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R.
"OpenPGP Message Format", RFC 2440, November 1998. Thayer, "OpenPGP Message Format", RFC 2440,
November 1998.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths for [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths
Public Keys Used For Exchanging Symmetric Keys", RFC 3766, for Public Keys Used For Exchanging Symmetric
April 2004. Keys", RFC 3766, April 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the
Name System (DNS)", RFC 3833, August 2004. Domain Name System (DNS)", RFC 3833, August 2004.
[RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification", [RFC3851] Ramsdell, B., "S/MIME Version 3 Message
RFC 3851, June 1999. Specification", RFC 3851, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul,
Procedures for Message Header Fields", BCP 90, "Registration Procedures for Message Header
September 2004. Fields", BCP 90, September 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D.,
Rose, "DNS Security Introduction and Requirements", and S. Rose, "DNS Security Introduction and
RFC 4033, March 2005. Requirements", RFC 4033, March 2005.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys [RFC4686] Fenton, J., "Analysis of Threats Motivating
Identified Mail (DKIM)", RFC 4686, September 2006. DomainKeys Identified Mail (DKIM)", RFC 4686,
September 2006.
[RFC4870] Delany, M., "Domain-Based Email Authentication
Using Public Keys Advertised in the DNS
(DomainKeys)", RFC 4870, May 2007.
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. The key used in this example is shown in Appendix C.
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@football.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.
A.2. The email is signed A.2. The Email Is Signed
This email is signed by the example.com outbound email server and now This email is signed by the example.com outbound email server and now
looks like this: looks like this:
DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com; DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com; c=simple/simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID; h=Received : From : To : Subject : Date : Message-ID;
bh=jpltwNFTq83Bkjt/Y2ekyqr/+i296daNkFZSdaz8VCY=; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt 4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
zEn8/OHpTCcdLOJaTp8/mKz69/RpatVBas2OqWas7jrlaLGfHdBkt KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
Hs6fxOzzAB7Wro=; 4bmp/YzhwvcubU4=;
Received: from client1.football.example.com [192.0.2.1] Received: from client1.football.example.com [192.0.2.1]
by submitserver.example.com with SUBMISSION; by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT) Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com> From: Joe SixPack <joe@football.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.
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.
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:
brisbane._domainkey.example.com brisbane._domainkey.example.com
Signature verification starts with the physically last "Received" Signature verification starts with the physically last Received
header field, the "From" header field, and so forth, in the order header field, the From header field, and so forth, in the order
listed in the "h=" tag. Verification follows with a single CRLF listed in the "h=" tag. Verification follows with a single CRLF
followed by the body (starting with "Hi."). The email is canonically followed by the body (starting with "Hi."). The email is canonically
prepared for verifying with the "simple" method. The result of the prepared for verifying with the "simple" method. The result of the
query and subsequent verification of the signature is stored (in this query and subsequent verification of the signature is stored (in this
example) in the "X-Authentication-Results" header field line. After example) in the X-Authentication-Results header field line. After
successful verification, the email looks like this: successful verification, the email looks like this:
X-Authentication-Results: shopping.example.net X-Authentication-Results: shopping.example.net
header.from=joe@football.example.com; dkim=pass header.from=joe@football.example.com; dkim=pass
Received: from mout23.football.example.com (192.168.1.1) Received: from mout23.football.example.com (192.168.1.1)
by shopping.example.net with SMTP; by shopping.example.net with SMTP;
Fri, 11 Jul 2003 21:01:59 -0700 (PDT) Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com; DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com; c=simple/simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID; h=Received : From : To : Subject : Date : Message-ID;
bh=jpltwNFTq83Bkjt/Y2ekyqr/+i296daNkFZSdaz8VCY=; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt 4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
zEn8/OHpTCcdLOJaTp8/mKz69/RpatVBas2OqWas7jrlaLGfHdBkt KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
Hs6fxOzzAB7Wro=; 4bmp/YzhwvcubU4=;
Received: from client1.football.example.com [192.0.2.1] Received: from client1.football.example.com [192.0.2.1]
by submitserver.example.com with SUBMISSION; by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT) Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com> From: Joe SixPack <joe@football.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.
INFORMATIVE NOTE: The key used to compute this signature is shown
in Appendix C.
Appendix B. Usage Examples (INFORMATIVE) Appendix B. Usage Examples (INFORMATIVE)
DKIM signing and validating can be used in different ways, for DKIM signing and validating can be used in different ways, for
different operational scenarios. This Appendix discusses some common different operational scenarios. This Appendix discusses some common
examples. examples.
NOTE: Descriptions in this Appendix are for informational NOTE: Descriptions in this Appendix are for informational purposes
purposes only. They describe various ways that DKIM can be used, only. They describe various ways that DKIM can be used, given
given particular constraints and needs. In no case are these particular constraints and needs. In no case are these examples
examples intended to be taken as providing explanation or guidance intended to be taken as providing explanation or guidance
concerning DKIM specification details, when creating an concerning DKIM specification details, when creating an
implementation. implementation.
B.1. Alternate Submission Scenarios B.1. Alternate Submission Scenarios
In the most simple scenario, a user's MUA, MSA, and Internet In the most simple scenario, a user's MUA, MSA, and Internet
(boundary) MTA are all within the same administrative environment, (boundary) MTA are all within the same administrative environment,
using the same domain name. Therefore, all of the components using the same domain name. Therefore, all of the components
involved in submission and initial transfer are related. However it involved in submission and initial transfer are related. However, it
is common for two or more of the components to be under independent is common for two or more of the components to be under independent
administrative control. This creates challenges for choosing and administrative control. This creates challenges for choosing and
administering the domain name to use for signing, and for its administering the domain name to use for signing, and for its
relationship to common email identity header fields. relationship to common email identity header fields.
B.1.1. Delegated Business Functions B.1.1. Delegated Business Functions
Some organizations assign specific business functions to discrete Some organizations assign specific business functions to discrete
groups, inside or outside the organization. The goal, then, is to groups, inside or outside the organization. The goal, then, is to
authorize that group to sign some mail, but to constrain what authorize that group to sign some mail, but to constrain what
signatures they can generate. DKIM Selectors (the "s=" signature signatures they can generate. DKIM selectors (the "s=" signature
tag) and granularity (the "g=" key tag) facilitate this kind of tag) and granularity (the "g=" key tag) facilitate this kind of
restricted authorization. Examples of these outsourced business restricted authorization. Examples of these outsourced business
functions are legitimate email marketing providers and corporate functions are legitimate email marketing providers and corporate
benefits providers. benefits providers.
Here, the delegated group needs to be able to send messages that are Here, the delegated group needs to be able to send messages that are
signed, using the email domain of the client company. At the same signed, using the email domain of the client company. At the same
time, the client often is reluctant to register a key for the time, the client often is reluctant to register a key for the
provider that grants the ability to send messages for arbitrary provider that grants the ability to send messages for arbitrary
addresses in the domain. addresses in the domain.
There are multiple ways to administer these usage scenarios. In one There are multiple ways to administer these usage scenarios. In one
case, the client organization provides all of the public query case, the client organization provides all of the public query
service (for example, DNS) administration, and in another it uses DNS service (for example, DNS) administration, and in another it uses DNS
delegation to enable all on-going administration of the DKIM key delegation to enable all ongoing administration of the DKIM key
record by the delegated group. record by the delegated group.
If the client organization retains responsibility for all of the DNS If the client organization retains responsibility for all of the DNS
administration, the outsourcing company can generate a key pair, administration, the outsourcing company can generate a key pair,
supplying the public key to the client company, which then registers supplying the public key to the client company, which then registers
it in the query service, using a unique Selector that authorizes a it in the query service, using a unique selector that authorizes a
specific From header field local-part. For example, a client with specific From header field Local-part. For example, a client with
the domain "example.com" could have the Selector record specify the domain "example.com" could have the selector record specify
"g=winter-promotions" so that this signature is only valid for mail "g=winter-promotions" so that this signature is only valid for mail
with a From address of "winter-promotions@example.com". This would with a From address of "winter-promotions@example.com". This would
enable the provider to send messages using that specific address and enable the provider to send messages using that specific address and
have them verify properly. The client company retains control over have them verify properly. The client company retains control over
the email address because it retains the ability to revoke the key at the email address because it retains the ability to revoke the key at
any time. any time.
If the client wants the delegated group to do the DNS administration, If the client wants the delegated group to do the DNS administration,
it can have the domain name that is specified with the selector point it can have the domain name that is specified with the selector point
to the provider's DNS server. The provider then creates and to the provider's DNS server. The provider then creates and
maintains all of the DKIM signature information for that Selector. maintains all of the DKIM signature information for that selector.
Hence, the client cannot provide constraints on the local-part of Hence, the client cannot provide constraints on the Local-part of
addresses that get signed, but it can revoke the provider's signing addresses that get signed, but it can revoke the provider's signing
rights by removing the DNS delegation record. rights by removing the DNS delegation record.
B.1.2. PDAs and Similar Devices B.1.2. PDAs and Similar Devices
PDAs demonstrate the need for using multiple keys per domain. PDAs demonstrate the need for using multiple keys per domain.
Suppose that John Doe wanted to be able to send messages using his Suppose that John Doe wanted to be able to send messages using his
corporate email address, jdoe@example.com, and his email device did corporate email address, jdoe@example.com, and his email device did
not have the ability to make a VPN connection to the corporate not have the ability to make a Virtual Private Network (VPN)
network, either because the device is limited or because there are connection to the corporate network, either because the device is
restrictions enforced by his Internet access provider. If the device limited or because there are restrictions enforced by his Internet
was equipped with a private key registered for jdoe@example.com by access provider. If the device was equipped with a private key
the administrator of the example.com domain, and appropriate software registered for jdoe@example.com by the administrator of the
to sign messages, John could sign the message on the device itself example.com domain, and appropriate software to sign messages, John
before transmission through the outgoing network of the access could sign the message on the device itself before transmission
service provider. through the outgoing network of the access service provider.
B.1.3. Roaming Users B.1.3. Roaming Users
Roaming users often find themselves in circumstances where it is Roaming users often find themselves in circumstances where it is
convenient or necessary to use an SMTP server other than their home convenient or necessary to use an SMTP server other than their home
server; examples are conferences and many hotels. In such server; examples are conferences and many hotels. In such
circumstances a signature that is added by the submission service circumstances, a signature that is added by the submission service
will use an identity that is different from the user's home system. will use an identity that is different from the user's home system.
Ideally roaming users would connect back to their home server using Ideally, roaming users would connect back to their home server using
either a VPN or a SUBMISSION server running with SMTP AUTHentication either a VPN or a SUBMISSION server running with SMTP AUTHentication
on port 587. If the signing can be performed on the roaming user's on port 587. If the signing can be performed on the roaming user's
laptop then they can sign before submission, although the risk of laptop, then they can sign before submission, although the risk of
further modification is high. If neither of these are possible, further modification is high. If neither of these are possible,
these roaming users will not be able to send mail signed using their these roaming users will not be able to send mail signed using their
own domain key. own domain key.
B.1.4. Independent (Kiosk) Message Submission B.1.4. Independent (Kiosk) Message Submission
Stand-alone services, such as walk-up kiosks and web-based Stand-alone services, such as walk-up kiosks and web-based
information services, have no enduring email service relationship information services, have no enduring email service relationship
with the user, but the user occasionally requests that mail be sent with the user, but users occasionally request that mail be sent on
on their behalf. For example, a website providing news often allows their behalf. For example, a website providing news often allows the
the reader to forward a copy of the article to a friend. This is reader to forward a copy of the article to a friend. This is
typically done using the reader's own email address, to indicate who typically done using the reader's own email address, to indicate who
the author is. This is sometimes referred to as the "Evite problem", the author is. This is sometimes referred to as the "Evite problem",
named after the website of the same name that allows a user to send named after the website of the same name that allows a user to send
invitations to friends. invitations to friends.
A common way this is handled is to continue to put the reader's email A common way this is handled is to continue to put the reader's email
address in the From header field of the message, but put an address address in the From header field of the message, but put an address
owned by the email posting site into the Sender header field. The owned by the email posting site into the Sender header field. The
posting site can then sign the message, using the domain that is in posting site can then sign the message, using the domain that is in
the Sender field. This provides useful information to the receiving the Sender field. This provides useful information to the receiving
email site, which is able to correlate the signing domain with the email site, which is able to correlate the signing domain with the
initial submission email role. initial submission email role.
Receiving sites often wish to provide their end users with Receiving sites often wish to provide their end users with
information about mail that is mediated in this fashion. Although information about mail that is mediated in this fashion. Although
the real efficacy of different approaches is a subject for human the real efficacy of different approaches is a subject for human
factors usability research, one technique that is used is for the factors usability research, one technique that is used is for the
verifying system to rewrite the From header field, to indicate the verifying system to rewrite the From header field, to indicate the
address that was verified. For example: From: John Doe via address that was verified. For example: From: John Doe via
news@news-site.com <jdoe@example.com>. (Note that, such rewriting news@news-site.com <jdoe@example.com>. (Note that such rewriting
will break a signature, unless it is done after the verification pass will break a signature, unless it is done after the verification pass
is complete.) is complete.)
B.2. Alternate Delivery Scenarios B.2. Alternate Delivery Scenarios
Email is often received at a mailbox that has an address different Email is often received at a mailbox that has an address different
from the one used during initial submission. In these cases, an from the one used during initial submission. In these cases, an
intermediary mechanism operates at the address originally used and it intermediary mechanism operates at the address originally used and it
then passes the message on to the final destination. This mediation then passes the message on to the final destination. This mediation
process presents some challenges for DKIM signatures. process presents some challenges for DKIM signatures.
B.2.1. Affinity Addresses B.2.1. Affinity Addresses
"Affinity addresses" allow a user to have an email address that "Affinity addresses" allow a user to have an email address that
remains stable, even as the user moves among different email remains stable, even as the user moves among different email
providers. They are typically associated with college alumni providers. They are typically associated with college alumni
associations, professional organizations, and recreational associations, professional organizations, and recreational
organizations with which they expect to have a long-term 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, and they often have an associated Web application which email, and they often have an associated Web application that
authenticates the user and allows the forwarding address to be authenticates the user and allows the forwarding address to be
changed. However these services usually depend on the user's sending changed. However, these services usually depend on users sending
outgoing messages through their own service provider's MTA. Hence, outgoing messages through their own service providers' MTAs. Hence,
mail that is signed with the domain of the affinity address is not mail that is signed with the domain of the affinity address is not
signed by an entity that is administered by the organization owning signed by an entity that is administered by the organization owning
that domain. that domain.
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 per-user keys to be used to sign messages on behalf users to register per-user keys to be used to sign messages on behalf
of their affinity address. The user would take away the secret half of their affinity address. The user would take away the secret half
of the key pair for signing, and the affinity domain would publish of the key pair for signing, and the affinity domain would publish
the public half in DNS for access by verifiers. the public half in DNS for access by verifiers.
This is another application that takes advantage of user-level This is another application that takes advantage of user-level
keying, and domains used for affinity addresses would typically have keying, and domains used for affinity addresses would typically have
a very large number of user-level keys. Alternatively, the affinity a very large number of user-level keys. Alternatively, the affinity
domain could handle outgoing mail, operating a mail submission agent domain could handle outgoing mail, operating a mail submission agent
that authenticates users before accepting and signing messages for that authenticates users before accepting and signing messages for
them. This is of course dependent on the user's service provider not them. This is of course dependent on the user's service provider not
blocking the relevant TCP ports used for mail submission. blocking the relevant TCP ports used for mail submission.
B.2.2. Simple Address Aliasing (.forward) B.2.2. Simple Address Aliasing (.forward)
In some cases a recipient is allowed to configure an email address to In some cases, a recipient is allowed to configure an email address
cause automatic redirection of email messages from the original to cause automatic redirection of email messages from the original
address to another, such as through the use of a Unix .forward file. address to another, such as through the use of a Unix .forward file.
In this case messages are typically redirected by the mail handling In this case, messages are typically redirected by the mail handling
service of the recipient's domain, without modification, except for service of the recipient's domain, without modification, except for
the addition of a Received header field to the message and a change the addition of a Received header field to the message and a change
in the envelope recipient address. In this case, the recipient at in the envelope recipient address. In this case, the recipient at
the final address' mailbox is likely to be able to verify the the final address' mailbox is likely to be able to verify the
original signature since the signed content has not changed, and DKIM original signature since the signed content has not changed, and DKIM
is able to validate the message signature. is able to validate the message signature.
B.2.3. Mailing Lists and Re-Posters B.2.3. Mailing Lists and Re-Posters
There is a wide range of behaviors in services that take delivery of There is a wide range of behaviors in services that take delivery of
a message and then resubmit it. A primary example is with mailing a message and then resubmit it. A primary example is with mailing
lists (collectively called "forwarders" below), ranging from those lists (collectively called "forwarders" below), ranging from those
which make no modification to the message itself, other than to add a that make no modification to the message itself, other than to add a
Received header field and change the envelope information, to those Received header field and change the envelope information, to those
which add header fields, change the Subject header field, add content that add header fields, change the Subject header field, add content
to the body (typically at the end), or reformat the body in some to the body (typically at the end), or reformat the body in some
manner. The simple ones produces messages that are quite similar to manner. The simple ones produce messages that are quite similar to
the automated alias services. More elaborate systems essentially the automated alias services. More elaborate systems essentially
create a new message. create a new message.
A Forwarder which does not modify the body or signed header fields of A Forwarder that does not modify the body or signed header fields of
a message is likely to maintain the validity of the existing a message is likely to maintain the validity of the existing
signature. It also could choose to add its own signature to the signature. It also could choose to add its own signature to the
message. message.
Forwarders which modify a message in a way that could make an Forwarders which modify a message in a way that could make an
existing signature invalid are particularly good candidates for existing signature invalid are particularly good candidates for
adding their own signatures (e.g., mailing-list-name@example.net). adding their own signatures (e.g., mailing-list-name@example.net).
Since (re-)signing is taking responsibility for the content of the Since (re-)signing is taking responsibility for the content of the
message, these signing forwarders are likely to be selective, and message, these signing forwarders are likely to be selective, and
forward or re-sign only those messages which are received with a forward or re-sign a message only if it is received with a valid
valid signature or some other basis for knowing that the messages signature or if they have some other basis for knowing that the
being signed is not spoofed. message is not spoofed.
A common practice among systems that are primarily re-distributors of A common practice among systems that are primarily redistributors of
mail is to add a Sender header field to the message, to identify the mail is to add a Sender header field to the message, to identify the
address being used to sign the message. This practice will remove address being used to sign the message. This practice will remove
any preexisting Sender header field as required by [RFC2822]. The any preexisting Sender header field as required by [RFC2822]. The
forwarder applies a new DKIM-Signature header field with the forwarder applies a new DKIM-Signature header field with the
signature, public key, and related information of the forwarder. signature, public key, and related information of the forwarder.
Appendix C. Creating a public key (INFORMATIVE) Appendix C. Creating a Public Key (INFORMATIVE)
The default signature is an RSA signed SHA256 digest of the complete The default signature is an RSA signed SHA256 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 1024 bit, unencrypted private key suitable for way to generate a 1024-bit, unencrypted private key suitable for DKIM
DKIM, is to use openssl like this: is to use openssl like this:
$ openssl genrsa -out rsa.private 1024 $ openssl genrsa -out rsa.private 1024
For increased security, the "-passin" parameter can also be added to For increased security, the "-passin" parameter can also be added to
encrypt the private key. Use of this parameter will require entering encrypt the private key. Use of this parameter will require entering
a password for several of the following steps. Servers may prefer to a password for several of the following steps. Servers may prefer to
use hardware cryptographic support. use hardware cryptographic support.
The "genrsa" step results in the file rsa.private containing the key The "genrsa" step results in the file rsa.private containing the key
information similar to this: information similar to this:
skipping to change at page 69, line 18 skipping to change at page 68, line 15
similar to this: similar to this:
-----BEGIN PUBLIC KEY----- -----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM
oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R
tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI
MmPSPDdQPNUYckcQ2QIDAQAB MmPSPDdQPNUYckcQ2QIDAQAB
-----END PUBLIC KEY----- -----END PUBLIC KEY-----
This public-key data (without the BEGIN and END tags) is placed in This public-key data (without the BEGIN and END tags) is placed in
the DNS. With the signature, canonical email contents, and public the DNS:
key, a verifying system can test the validity of the signature. The
openssl invocation to verify a signature looks like this:
openssl dgst -verify rsa.public -sha256 -signature signature.file \
<input.file
Once a private key has been generated, the openssl command can be
used to sign an appropriately prepared email, like this:
$ openssl dgst -sign rsa.private -sha256 <input.file
This results in signature data similar to this when represented as a
base64string:
aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz
msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl
How this signature is added to the email is discussed elsewhere in
this document.
The final record entered into a DNS zone file would be:
brisbane IN TXT ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ" brisbane IN TXT ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ"
"KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt" "KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt"
"IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v" "IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v"
"/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi" "/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi"
"tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB") "tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB")
Appendix D. MUA Considerations Appendix D. MUA Considerations
When a DKIM signature is verified, the processing system sometimes When a DKIM signature is verified, the processing system sometimes
makes the result available to the recipient user's MUA. How to makes the result available to the recipient user's MUA. How to
present this information to the user in a way that helps them is a present this information to the user in a way that helps them is a
matter of continuing human factors usability research. The tendency matter of continuing human factors usability research. The tendency
is to have the MUA highlight the address associated with this signing is to have the MUA highlight the address associated with this signing
identity in some way, in an attempt to show the user the address from identity in some way, in an attempt to show the user the address from
which the mail was sent. An MUA might do this with visual cues such which the mail was sent. An MUA might do this with visual cues such
as graphics, or it might include the address in an alternate view, or as graphics, or it might include the address in an alternate view, or
it might even rewrite the original "From:" address using the verified it might even rewrite the original From address using the verified
information. Some MUAs might indicate which header fields were information. Some MUAs might indicate which header fields were
protected by the validated DKIM signature. This could be done with a protected by the validated DKIM signature. This could be done with a
positive indication on the signed header fields, or with a negative positive indication on the signed header fields, with a negative
indication on the unsigned header fields or by visually hiding the indication on the unsigned header fields, by visually hiding the
unsigned header fields, or some combination of these. If an MUA uses unsigned header fields, or some combination of these. If an MUA uses
visual indications for signed header fields, the MUA probably needs visual indications for signed header fields, the MUA probably needs
to be careful not to display unsigned header fields in a way that to be careful not to display unsigned header fields in a way that
might be construed by the end user as having been signed. If the might be construed by the end user as having been signed. If the
message has an l= tag whose value does not extend to the end of the message has an l= tag whose value does not extend to the end of the
message, the MUA might also hide or mark the portion of the message message, the MUA might also hide or mark the portion of the message
body that was not signed. body that was not signed.
The aforementioned information is not intended to be exhaustive. The The aforementioned information is not intended to be exhaustive. The
MUA may choose to highlight, accentuate, hide, or otherwise display MUA may choose to highlight, accentuate, hide, or otherwise display
skipping to change at page 70, line 50 skipping to change at page 69, line 16
The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann, The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin, Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin,
Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman, Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman,
Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto, Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto,
Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur
Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman, Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman,
Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig
Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S. Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S.
Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon
Longsdale, David Margrave, Justin Mason, David Mayne, Steve Murphy, Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau,
Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada,
Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro, Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud,
Neil Rerup, Eric Rescorla, Dave Rossetti, Hector Santos, Jim Schaad, Scott Renfro, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector
the Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker, Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert
Sam Weiler, and Dan Wing for their valuable suggestions and Sanders, Rand Wacker, Sam Weiler, and Dan Wing for their valuable
constructive criticism. 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 [RFC-DK]. is at [RFC4870].
Appendix F. Edit History
[[This section to be removed before publication.]]
F.1. Changes since -ietf-09 version
The following changes were made between draft-ietf-dkim-base-09 and
draft-ietf-dkim-base-10:
o Section 6.1.1, clarify that the list of unlikely domains should be
configurable in some fashion.
o Section 4.2, add informative note about the dangers of trying to
correlate valid and invalid signatures.
o Reference details added in XML (may not show up in .txt versions).
o Some XML adjustments for formatting.
F.2. Changes since -ietf-08 version
The following changes were made between draft-ietf-dkim-base-08 and
draft-ietf-dkim-base-09:
o Section 3.3.1, recommend use of an RSA exponent of 65537.
o Section 3.4.4, mention theoretical "ASCII Art" attack for relaxed
body canonicalization.
o Section 5.4.1 moved to 5.5 (with old 5.5 et seq. pushed down) to
talk more generally about use of l= and canonicalization
algorithms.
o Section 6.1.1, make an explicit mention that verifiers may reject
signatures from unlikely domains such as "com" and "co.uk".
o Section 6.3, try to clarify the wording about SMTP rejections.
o Section 7, change IANA registration requirement to be any RFC
having "IETF Consensus" (as defined in RFC2434), not necessarily
standards-track, as a result of overwhelming WG consensus.
o Informative References, add RFC 2434.
F.3. Changes since -ietf-07 version
The following changes were made between draft-ietf-dkim-base-07 and
draft-ietf-dkim-base-08:
o Drop reference to "trusted third party" in section 1; it was
redundant with existing bullet points and created confusion.
o Drop the wording on re-using keys from normative to an operational
note.
o Add a sentence to 3.4.3 to clarify how empty or missing bodies are
canonicalized.
o Section 3.5, t= tag: note that a verifier MAY ignore signatures
with a timestamp in the future.
o Section 3.5, z= tag: dropped the MUST NOT wording.
o Clarify the description of g= in section 3.6.1.
o Mention that n= is not intended for end-user use in section 3.6.1.
o Modify wording of s= in section 3.6.1 so as to not imply possible
future uses.
o Add a reference to RFC 2821 in section 3.7 to describe dot-
stuffing.
o Fairly extensive update of section 4 as requested during IESG
review.
o DKIM-Signature is not a "trace header field" as defined by RFC
2822 (section 5.4).
o Add sentence in section 5.4 to discourage signing of existing
DKIM-Signature header fields.
o Added section 5.4.1 describing recommended headers for signing per
IESG review.
o Dropped comment about presenting binary result to end user in
section 6.3 (out of scope, and by IESG request).
o Clarify that SMTP-level rejects are discouraged, but that if they
are used they should use the indicated reply codes (section 6.3).
o Add text to section 7 (IANA Considerations) to make it clear that
registry updates require a standards-track document.
o Rewrote section 8.4 per request by Security Area review.
o Add sentence in section 8.11 to emphasize that signing existing
DKIM-Signature header fields may result in incorrect validation
failures, as requested by Security Area review.
o Added section 8.14 (RSA Attacks) based on DNS-dir review from
Olafur Gu[eth]mundsson.
o Added section 8.15 (Inappropriate Signing by Parent Domains).
F.4. Changes since -ietf-06 version
The following changes were made between draft-ietf-dkim-base-06 and
draft-ietf-dkim-base-07:
o Added section 8.11 regarding header reordering.
o Added informative note to section 3.3 regarding use of sha256.
o Added informative rationale to section 3.6.1, "p=", regarding key
revocation.
o Added second informative note in section 4 regarding signing
multiple DKIM-Signature header fields.
o Minor modification of the second informative note in section 6.1
regarding DoS attacks.
o Added explicit mention of v= to section 6.1.2, step 5.
o Updated paragraph 3 of section 8.4 regarding DNS attacks.
o Added section 7.9 (DKIM-Signature IANA Registry) per IANA request.
F.5. Changes since -ietf-05 version
The following changes were made between draft-ietf-dkim-base-05 and
draft-ietf-dkim-base-06:
o Fix an error in an example in Appendix C.
o Substantial updates to Appendixes B and D.
o Clarify ABNF for tag-value.
o Changed SWSP (streaming white space) to LWSP (linear white space
from RFC 4234); LWSP makes it clear that white space is required
after a CRLF.
o Add normative reference to SHA1/SHA256 FIPS publication 180-2.
o Several minor edits based on AD Review.
o Move discussion of not re-using a selector (i.e., changing the
public key for a single selector) from informational to normative.
o Assorted wordsmithing based on external review.
F.6. Changes since -ietf-04 version
The following changes were made between draft-ietf-dkim-base-04 and
draft-ietf-dkim-base-05:
o Clarified definition of "plain text" in section 3.2 (issue 1316).
o Added some clarification about multiple listings of non-existent
header field names in h= in section 5.4 (issue 1316).
o Finished filling out IANA registries in section 7 (issue 1320).
o Clarified handling of bare CR and LF in section 5.3 (issue 1326).
o Listed the required tags in section 6.1.1 as an informational note
(issue 1330).
o Changed IDNA reference from 3492 to 3490 (issue 1331).
o Changed the reference for WSP to 4234; changed the definition of
SWSP to exclude bare CR and LF (issue 1332).
F.7. Changes since -ietf-03 version
The following changes were made between draft-ietf-dkim-base-03 and
draft-ietf-dkim-base-04:
o Re-worded Abstract to avoid use of "prove" and "non-repudiation".
o Use dot-atom-text instead of dot-atom to avoid inclusion of CFWS.
o Capitalize Selector throughout.
o Add discussion of plain text, mentioning informatively that
implementors should plan for eventual 8-bit requirements.
o Drop RSA requirement of exponent of 65537 (not required, since it
is already in the key) and clarify the key format.
o Drop SHOULD that DKIM-Signature should precede header fields that
it signs.
o Mention that wildcard DNS records MUST NOT be used for selector
records.
o Add section 3.8 to clarify the t=s flag.
o Change the list of header fields that MUST be signed to include
only From.
o Require that verifier check that From is in the list of signed
header fields.
o Drop all reference to draft-kucherawy-sender-auth-header draft.
o Substantially expand Section 7 (IANA Considerations) to include
initial registries.
o Add section B.7 (use case: SMTP Servers for Roaming Users).
o Add several examples; update some others.
o Considerable minor editorial updating to clarify language, delete
redundant text, fix spelling errors, etc.
Still to be resolved:
o How does "simple" body canonicalization interact with BINARYMIME
data?
o Deal with "relaxed" body canonicalizations, especially in regard
to bare CRs and NLs.
o How to handle "*" in g= dot-atom-text (which allows "*" as a
literal character).
o The IANA Considerations need to be completed and cleaned up.
F.8. Changes since -ietf-02 version
The following changes were made between draft-ietf-dkim-base-02 and
draft-ietf-dkim-base-03:
o Section 5.2: changed key expiration text to be informational;
drop "seven day" wording in favor of something vaguer.
o Don't indicate that the "i=" tag value should be passed to the key
lookup service; this can be added as an extension if required.
o Move Section 6.6 (MUA Considerations) to be Appendix D and modify
it to avoid any hint of normative language.
o Soften the DKIM_STAT_ language in section 6 so that it doesn't
appear normative. This involved using only PERMFAIL and TEMPFAIL
as status, with parenthetical explanations.
o Restructured section 6 to make it clearer which steps apply on a
per-signature basis versus a per-message basis.
o Clarification of "signing identity" in several places.
o Clarification that DKIM-Signature header fields being signed by
another DKIM-Signature header field should be treated as a normal
header field (i.e., their "b=" field is unchanged).
o Change ABNF on a= tag to separate the public key algorithm from
the hash algorithm.
o Add t=s flag in key record to disallow subdomains in the i= tag
relative to the d= tag of the DKIM-Signature header field.
o Add a new definition for "dkim-quoted-printable", which is a
simple case of quoted-printable from RFC2045. dkim-quoted-
printable requires that all white space in the original text be
escaped, and all unescaped white space in the encoded field should
be ignored to allow arbitrary wrapping of the header fields which
may contain the content.
o Use dkim-quoted-printable as the encoding used in z= rather than
referring to RFC2045, since they are different.
o Rewrite description of g= tag in the key record.
o Deleted use of Domain in ABNF, which permits address-literals;
define domain-name to act in stead.
F.9. Changes since -ietf-01 version
The following changes were made between draft-ietf-dkim-base-01 and
draft-ietf-dkim-base-02:
o Change wording on "x=" tag in DKIM-Signature header field
regarding verifier handling of expired signatures from MUST to MAY
(per 20 April Jabber session). Also, make it clear that received
time is to be preferred over current time if reliably available.
o Several changes to limit wording that would intrude into verifier
policy. This is largely changing statements such as "... MUST
reject the message" to "... MUST consider the signature invalid."
o Drop normative references to ID-DKIM-RR, OpenSSL, PEM, and
Stringprep.
o Change "v=" tag in DKIM-Signature from "MUST NOT" to "MUST"; the
version number is 0.2 for this draft, with the expectation that
the first official version will be "v=1". (Per 18 May Jabber
session.)
o Change "q=dns" query access method to "q=dnstxt" to emphasize the
use of the TXT record. The expectation is that a later extension
will define "q=dnsdkk" to indicate use of a DKK record. (Per 18
May Jabber session.)
o Several typos fixed, including removing a paragraph that implied
that the DKIM-Signature header field should be hashed with the
body (it should not).
F.10. 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.
F.11. Changes since -allman-01 version
The following changes were made between draft-allman-dkim-base-01 and
draft-ietf-dkim-base-00:
o Remove references to Sender Signing Policy document. Such
consideration is implicitly included in Section 6.3.
o Added ABNF for all tags.
o Updated references (still includes some references to expired
drafts, notably ID-AUTH-RES.
o Significant wordsmithing.
F.12. Changes since -allman-00 version
The following changes were made between draft-allman-dkim-base-00 and
draft-allman-dkim-base-01:
o Changed "c=" tag to separate out header from body
canonicalization.
o Eliminated "nowsp" canonicalization in favor of "relaxed", which
is somewhat less relaxed (but more secure) than "nowsp".
o Moved the (empty) Compliance section to the Sender Signing Policy
document.
o Added several IANA Considerations.
o Fixed a number of grammar and formatting errors.
Authors' Addresses Authors' Addresses
Eric Allman Eric Allman
Sendmail, Inc. Sendmail, Inc.
6425 Christie Ave, Suite 400 6425 Christie Ave, Suite 400
Emeryville, CA 94608 Emeryville, CA 94608
USA USA
Phone: +1 510 594 5501 Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org EMail: eric+dkim@sendmail.org
URI: URI:
Jon Callas Jon Callas
PGP Corporation PGP Corporation
3460 West Bayshore 3460 West Bayshore
Palo Alto, CA 94303 Palo Alto, CA 94303
USA USA
Phone: +1 650 319 9016 Phone: +1 650 319 9016
Email: jon@pgp.com EMail: jon@pgp.com
Mark Delany Mark Delany
Yahoo! Inc Yahoo! Inc
701 First Avenue 701 First Avenue
Sunnyvale, CA 95087 Sunnyvale, CA 95087
USA USA
Phone: +1 408 349 6831 Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com EMail: markd+dkim@yahoo-inc.com
URI: URI:
Miles Libbey Miles Libbey
Yahoo! Inc Yahoo! Inc
701 First Avenue 701 First Avenue
Sunnyvale, CA 95087 Sunnyvale, CA 95087
USA USA
Email: mlibbeymail-mailsig@yahoo.com EMail: mlibbeymail-mailsig@yahoo.com
URI: URI:
Jim Fenton Jim Fenton
Cisco Systems, Inc. Cisco Systems, Inc.
MS SJ-24/2 MS SJ-9/2
170 W. Tasman Drive 170 W. Tasman Drive
San Jose, CA 95134-1706 San Jose, CA 95134-1706
USA USA
Phone: +1 408 526 5914 Phone: +1 408 526 5914
Email: fenton@cisco.com EMail: fenton@cisco.com
URI: URI:
Michael Thomas Michael Thomas
Cisco Systems, Inc. Cisco Systems, Inc.
MS SJ-9/2 MS SJ-9/2
170 W. Tasman Drive 170 W. Tasman Drive
San Jose, CA 95134-1706 San Jose, CA 95134-1706
Phone: +1 408 525 5386 Phone: +1 408 525 5386
Email: mat@cisco.com EMail: mat@cisco.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
skipping to change at page 81, line 45 skipping to change at page 71, line 45
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.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgment Acknowledgement
Funding for the RFC Editor function is provided by the IETF Funding for the RFC Editor function is currently provided by the
Administrative Support Activity (IASA). Internet Society.
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