draft-ietf-dkim-base-08.txt   draft-ietf-dkim-base-09.txt 
DKIM E. Allman DKIM E. Allman
Internet-Draft Sendmail, Inc. Internet-Draft Sendmail, Inc.
Expires: July 22, 2007 J. Callas Intended status: Standards Track J. Callas
PGP Corporation Expires: August 15, 2007 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.
January 18, 2007 February 11, 2007
DomainKeys Identified Mail (DKIM) Signatures DomainKeys Identified Mail (DKIM) Signatures
draft-ietf-dkim-base-08 draft-ietf-dkim-base-09
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 41 skipping to change at page 1, line 41
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 22, 2007. This Internet-Draft will expire on August 15, 2007.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (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
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global control of "spam" and "phishing". global control of "spam" and "phishing".
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 1.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 6
1.2 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Simple Key Management . . . . . . . . . . . . . . . . . . 6 1.3. Simple Key Management . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6
2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. White Space . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7 2.4. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7
2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8 2.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8
2.6 DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8 2.6. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 9 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11 3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11
3.3 Signing and Verification Algorithms . . . . . . . . . . . 12 3.3. Signing and Verification Algorithms . . . . . . . . . . . 12
3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 14 3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 14
3.5 The DKIM-Signature header field . . . . . . . . . . . . . 18 3.5. The DKIM-Signature header field . . . . . . . . . . . . . 18
3.6 Key Management and Representation . . . . . . . . . . . . 26 3.6. Key Management and Representation . . . . . . . . . . . . 26
3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 30 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 30
3.8 Signing by Parent Domains . . . . . . . . . . . . . . . . 32 3.8. Signing by Parent Domains . . . . . . . . . . . . . . . . 32
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 33 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 33
4.1 Example Scenarios . . . . . . . . . . . . . . . . . . . . 33 4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 33
4.2 Interpretation . . . . . . . . . . . . . . . . . . . . . . 34 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 34
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 35 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 35
5.1 Determine if the Email Should be Signed and by Whom . . . 35 5.1. Determine if the Email Should be Signed and by Whom . . . 35
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 . . 36
5.4 Determine the Header Fields to Sign . . . . . . . . . . . 37 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 36
5.5 Compute the Message Hash and Signature . . . . . . . . . . 40 5.5. Recommended Signature Content . . . . . . . . . . . . . . 39
5.6 Insert the DKIM-Signature Header Field . . . . . . . . . . 40 5.6. Compute the Message Hash and Signature . . . . . . . . . . 40
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 41 5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 41
6.1 Extract Signatures from the Message . . . . . . . . . . . 41 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 41
6.2 Communicate Verification Results . . . . . . . . . . . . . 47 6.1. Extract Signatures from the Message . . . . . . . . . . . 42
6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 47 6.2. Communicate Verification Results . . . . . . . . . . . . . 47
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 49 6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 48
7.1 DKIM-Signature Tag Specifications . . . . . . . . . . . . 49 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
7.2 DKIM-Signature Query Method Registry . . . . . . . . . . . 49 7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 49
7.3 DKIM-Signature Canonicalization Registry . . . . . . . . . 50 7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 50
7.4 _domainkey DNS TXT Record Tag Specifications . . . . . . . 50 7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 50
7.5 DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 51 7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 51
7.6 DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 51 7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 51
7.7 DKIM Service Types Registry . . . . . . . . . . . . . . . 52 7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 52
7.8 DKIM Selector Flags Registry . . . . . . . . . . . . . . . 52 7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 52
7.9 DKIM-Signature Header Field . . . . . . . . . . . . . . . 53 7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 53
8. Security Considerations . . . . . . . . . . . . . . . . . . 53 7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 53
8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 53 8. Security Considerations . . . . . . . . . . . . . . . . . . . 53
8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 54 8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 53
8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 54 8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 54
8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 55 8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 55
8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 55 8.4. Attacks Against DNS . . . . . . . . . . . . . . . . . . . 55
8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 56 8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 56
8.7 Intentionally malformed Key Records . . . . . . . . . . . 56 8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 56
8.8 Intentionally Malformed DKIM-Signature header fields . . . 56 8.7. Intentionally malformed Key Records . . . . . . . . . . . 57
8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 57 8.8. Intentionally Malformed DKIM-Signature header fields . . . 57
8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 57 8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 57
8.11 Reordered Header Fields . . . . . . . . . . . . . . . . 57 8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 57
8.12 RSA Attacks . . . . . . . . . . . . . . . . . . . . . . 57 8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 57
8.13 Inappropriate Signing by Parent Domains . . . . . . . . 57 8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 57
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 58 8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 58
9.1 Normative References . . . . . . . . . . . . . . . . . . . 58 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 58
9.2 Informative References . . . . . . . . . . . . . . . . . . 59 9.1. Normative References . . . . . . . . . . . . . . . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 59 9.2. Informative References . . . . . . . . . . . . . . . . . . 59
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 . . . . . . . . . . . . . . . . . . . 60
A.3 The email signature is verified . . . . . . . . . . . . . 62 A.3. The email signature is verified . . . . . . . . . . . . . 61
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
C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 68 Appendix C. Creating a public key (INFORMATIVE) . . . . . . . . . 67
D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 70 Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 69
E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 70 Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 69
F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 71 Appendix F. Edit History . . . . . . . . . . . . . . . . . . . . 70
F.1 Changes since -ietf-07 version . . . . . . . . . . . . . . 71 F.1. Changes since -ietf-08 version . . . . . . . . . . . . . . 70
F.2 Changes since -ietf-06 version . . . . . . . . . . . . . . 72 F.2. Changes since -ietf-07 version . . . . . . . . . . . . . . 70
F.3 Changes since -ietf-05 version . . . . . . . . . . . . . . 73 F.3. Changes since -ietf-06 version . . . . . . . . . . . . . . 72
F.4 Changes since -ietf-04 version . . . . . . . . . . . . . . 73 F.4. Changes since -ietf-05 version . . . . . . . . . . . . . . 72
F.5 Changes since -ietf-03 version . . . . . . . . . . . . . . 74 F.5. Changes since -ietf-04 version . . . . . . . . . . . . . . 73
F.6 Changes since -ietf-02 version . . . . . . . . . . . . . . 75 F.6. Changes since -ietf-03 version . . . . . . . . . . . . . . 73
F.7 Changes since -ietf-01 version . . . . . . . . . . . . . . 76 F.7. Changes since -ietf-02 version . . . . . . . . . . . . . . 74
F.8 Changes since -ietf-00 version . . . . . . . . . . . . . . 76 F.8. Changes since -ietf-01 version . . . . . . . . . . . . . . 75
F.9 Changes since -allman-01 version . . . . . . . . . . . . . 77 F.9. Changes since -ietf-00 version . . . . . . . . . . . . . . 76
F.10 Changes since -allman-00 version . . . . . . . . . . . . 77 F.10. Changes since -allman-01 version . . . . . . . . . . . . . 77
Intellectual Property and Copyright Statements . . . . . . . 79 F.11. Changes since -allman-00 version . . . . . . . . . . . . . 77
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 77
Intellectual Property and Copyright Statements . . . . . . . . . . 80
1. Introduction 1. Introduction
[[Note: text in double square brackets (such as this text) will be [[Note: text in double square brackets (such as this text) will be
deleted before publication.]] 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
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o requires minimal new infrastructure; o requires minimal new infrastructure;
o can be implemented independently of clients in order to reduce o can be implemented independently of clients in order to reduce
deployment time; deployment time;
o can be deployed incrementally; o can be deployed incrementally;
o allows delegation of signing to third parties. o allows delegation of signing to third parties.
1.1 Signing Identity 1.1. Signing Identity
DKIM separates the question of the identity of the signer of the DKIM separates the question of the identity of the signer of the
message from the purported author of the message. In particular, a message from the purported author of the message. In particular, a
signature includes the identity of the signer. Verifiers can use the signature includes the identity of the signer. Verifiers can use the
signing information to decide how they want to process the message. signing information to decide how they want to process the message.
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
which characterize the email identification problem. There are which characterize the email identification problem. There are
currently over 70 million domains and a much larger number of currently over 70 million domains and a much larger number of
individual addresses. DKIM seeks to preserve the positive aspects of individual addresses. DKIM seeks to preserve the positive aspects of
the current email infrastructure, such as the ability for anyone to the 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].
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 be
involved in the injection of a message into the message system in 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. White Space
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 white space, i.e., a space or a tab
character (formal definition in [RFC4234]). character (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
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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 / "+" / "/" / LWSP)
[ "=" LWSP [ "=" LWSP ] ] [ "=" LWSP [ "=" LWSP ] ]
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"
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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 the rules of RFC 4234 and must be interpreted accordingly, obey 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), eight-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
skipping to change at page 9, line 44 skipping to change at page 9, line 44
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:
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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.
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aid legibility. aid legibility.
Unrecognized tags MUST be ignored. Unrecognized tags MUST be ignored.
Tags that have an empty value are not the same as omitted tags. An Tags that have an empty value are not the same as omitted tags. An
omitted tag is treated as having the default value; a tag with an omitted tag is treated as having the default value; a tag with an
empty value explicitly designates the empty string as the value. For empty value explicitly designates the empty string as the value. For
example, "g=" does not mean "g=*", even though "g=*" is the default example, "g=" does not mean "g=*", even though "g=*" is the default
for that tag. for that tag.
3.3 Signing and Verification Algorithms 3.3. Signing and Verification Algorithms
DKIM supports multiple 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 [SHA] as the hash-alg. That hash is in Section 3.7 below using SHA-1 [SHA] as the hash-alg. That hash is
then signed by the signer using the RSA algorithm (defined in PKCS#1 then signed by the signer using the RSA algorithm (defined in PKCS#1
version 1.5 [RFC3447]) as the crypt-alg and the signer's private key. version 1.5 [RFC3447]) as the crypt-alg and the signer's private key.
The hash MUST NOT be truncated or converted into any form other than The hash MUST NOT be truncated or converted into any form other than
the native binary form before being signed. the native binary form before being signed. The signing algorithm
SHOULD use an exponent of 65537.
INFORMATIVE IMPLEMENTATION WARNING: Low-valued exponents should
be avoided, as they are believed to be subject to attack.
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 [SHA] as the hash-alg. That hash in Section 3.7 below using SHA-256 [SHA] as the hash-alg. That hash
is then signed by the signer using the RSA algorithm (defined in is then signed by the signer using the RSA algorithm (defined in
PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's
private key. The hash MUST NOT be truncated or converted into any private key. The hash MUST NOT be truncated or converted into any
form other than the native binary form before being signed. 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.
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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
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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 (e.g., text is ASCII encoded, lines are separated with
CRLF characters, etc.). See also Section 5.3 for information about CRLF characters, etc.). See also Section 5.3 for information about
normalizing the message. normalizing the message.
3.4.1 The "simple" Header 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: lower case. 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
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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.
o Delete all WSP characters at the end of each unfolded header field o Delete all WSP characters at the end of each unfolded header field
value. value.
o Delete any WSP characters remaining before and after the colon o Delete any WSP characters remaining before and after the colon
separating the header field name from the header field value. The separating the header field name from the header field value. The
colon separator MUST be retained. colon separator MUST be retained.
3.4.3 The "simple" Body Canonicalization Algorithm 3.4.3. The "simple" Body Canonicalization Algorithm
The "simple" body canonicalization algorithm ignores all empty lines The "simple" body canonicalization algorithm ignores all empty lines
at the end of the message body. An empty line is a line of zero at the end of the message body. An empty line is a line of zero
length after removal of the line terminator. If there is no body or length after removal of the line terminator. If there is no body or
no trailing CRLF on the message body, a CRLF is added. It makes no no trailing CRLF on the message body, a CRLF is added. It makes no
other changes to the message body. In more formal terms, the other changes to the message body. In more formal terms, the
"simple" body canonicalization algorithm converts "0*CRLF" at the end "simple" body canonicalization algorithm converts "0*CRLF" at the end
of the body to a single "CRLF". of the body to a single "CRLF".
Note that a completely empty or missing body is canonicalized as a Note that a completely empty or missing body is canonicalized as a
single "CRLF"; that is, the canonicalized length will be 2 octets. single "CRLF"; that is, the canonicalized length will be 2 octets.
3.4.4 The "relaxed" Body Canonicalization Algorithm 3.4.4. The "relaxed" Body Canonicalization Algorithm
The "relaxed" body canonicalization algorithm: The "relaxed" body canonicalization algorithm:
o Ignores all white space at the end of lines. Implementations MUST o Ignores all white space at the end of lines. Implementations MUST
NOT remove the CRLF at the end of the line. NOT remove the CRLF at the end of the line.
o Reduces all sequences of WSP within a line to a single SP o Reduces all sequences of WSP within a line to a single SP
character. character.
o Ignores all empty lines at the end of the message body. "Empty o Ignores all empty lines at the end of the message body. "Empty
line" is defined in Section 3.4.3. line" is defined in Section 3.4.3.
3.4.5 Body Length Limits INFORMATIVE NOTE: It should be noted that the relaxed body
canonicalization algorithm may enable certain types of extremely
crude "ASCII Art" attacks where a message may be conveyed by
adjusting the spacing between words. If this is a concern, the
"simple" body canonicalization algorithm should be used instead.
3.4.5. Body Length Limits
A body length count MAY be specified to limit the signature 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 then 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
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only works for some MIME types, e.g., multipart/mixed but not only works for some MIME types, e.g., multipart/mixed but not
multipart/signed. multipart/signed.
A body length count of zero means that the body is completely A body length count of zero means that the body is completely
unsigned. unsigned.
Signers wishing to ensure that no modification of any sort can occur Signers wishing to ensure that no modification of any sort can occur
should specify the "simple" canonicalization algorithm for both should specify the "simple" canonicalization algorithm for both
header and body and omit the body length count. header and body and omit the body length count.
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>
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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 trace header field as defined in section 3.6 of [RFC2822], and were a trace header field as defined in section 3.6 of [RFC2822], and
hence SHOULD NOT be reordered and SHOULD be prepended to the message. hence SHOULD NOT be reordered and SHOULD be prepended to the message.
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DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane; DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane;
c=simple; q=dns/txt; i=@eng.example.net; c=simple; q=dns/txt; i=@eng.example.net;
t=1117574938; x=1118006938; t=1117574938; x=1118006938;
h=from:to:subject:date; h=from:to:subject:date;
z=From:foo@eng.example.net|To:joe@example.com| z=From:foo@eng.example.net|To:joe@example.com|
Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700; Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=; bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR VoG4ZHRNiYzR
3.6 Key Management and Representation 3.6. Key Management and Representation
Signature applications require some level of assurance that the Signature applications require some level of assurance that the
verification public key is associated with the claimed signer. Many verification public key is associated with the claimed signer. Many
applications achieve this by using public key certificates issued by applications achieve this by using public key certificates issued by
a trusted third party. However, DKIM can achieve a sufficient level a trusted third party. However, DKIM can achieve a sufficient level
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
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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
would not be considered to be unstructured text for this purpose). would not be considered to be unstructured text for this purpose).
The following definition MUST be used for any DKIM key represented in The following definition MUST be used for any DKIM key represented in
an otherwise unstructured textual form. an otherwise unstructured textual form.
The overall syntax is a tag-list as described in Section 3.2. The The overall syntax is a tag-list as described in Section 3.2. The
current valid tags are described below. Other tags MAY be present current valid tags are described below. Other tags MAY be present
and MUST be ignored by any implementation that does not understand and MUST be ignored by any implementation that does not understand
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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- [[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a
atom-text". This should probably use a different character "dot-atom-text". This should probably use a different
for wildcarding. Unfortunately, the options are non-mnemonic character for wildcarding. Unfortunately, the options are
(e.g., "@", "(", ":"). Alternatively we could insist on non-mnemonic (e.g., "@", "(", ":"). Alternatively we could
escaping a "*" intended as a literal "*" in the address.]] 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
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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. Name Space
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.
3.6.2.2 Resource Record Types for Key Storage 3.6.2.2. Resource Record Types for Key Storage
The DNS Resource Record type used is specified by an option to the The DNS Resource Record type used is specified by an option to the
query-type ("q=") tag. The only option defined in this base query-type ("q=") tag. The only option defined in this base
specification is "txt", indicating the use of a TXT Resource Record specification is "txt", indicating the use of a TXT Resource Record
(RR). A later extension of this standard may define another RR type. (RR). A later extension of this standard may define another RR type.
Strings in a TXT RR MUST be concatenated together before use with no 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 starts 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-Signature" header field being verified. In the following "DKIM-Signature" header field being verified. In the following
discussion, the names of the tags in the "DKIM-Signature" header discussion, the names of the tags in the "DKIM-Signature" header
field which either exists (when verifying) or will be created (when field which either exists (when verifying) or will be created (when
signing) are used. Note that canonicalization (Section 3.4) is only signing) are used. Note that canonicalization (Section 3.4) is only
used to prepare the email for signing or verifying; it does not used to prepare the email for signing or verifying; it does not
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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
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of the record to exactly the domain of the signing identity. If the of the record to exactly the domain of the signing identity. If the
referenced key record contains the "s" flag as part of the t= tag, referenced key record contains the "s" flag as part of the t= tag,
the domain of the signing identity (i= flag) MUST be the same as that the domain of the signing identity (i= flag) MUST be the same as that
of the d= domain. If this flag is absent, the domain of the signing of the d= domain. If this flag is absent, the domain of the signing
identity MUST be the same as, or a subdomain of, the d= domain. Key identity MUST be the same as, or a subdomain of, the d= domain. Key
records which are not intended for use with subdomains SHOULD specify records which are not intended for use with subdomains SHOULD specify
the "s" flag in the t= tag. 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 that 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
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even from unknown authors. They might also subscribe to less even from unknown authors. They might also subscribe to less
trusted mailing lists (e.g., those without anti-abuse protection) trusted mailing lists (e.g., those without anti-abuse protection)
and be willing to accept all messages from specific authors, but and be willing to accept all messages from specific authors, but
insist on doing additional abuse scanning for other messages. insist 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
alumni.example.edu, 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 which 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 Section 5.4 to sign trace
header fields. header 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
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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 if the Email Should be Signed and by Whom
A signer can obviously only sign email for domains for which it has a A signer can obviously only sign email for domains for which it has a
private key and the necessary knowledge of the corresponding public private key and the necessary knowledge of the corresponding public
key and Selector information. However there are a number of other key and Selector information. However there are a number of other
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,
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authenticated and authorized. authenticated and authorized.
INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not
sign Received header fields if the outgoing gateway MTA obfuscates sign 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
Some messages, particularly those using 8-bit characters, are subject Some messages, particularly those using 8-bit characters, are subject
to modification during transit, notably conversion to 7-bit form. to modification during transit, notably conversion to 7-bit form.
Such conversions will break DKIM signatures. In order to minimize Such conversions will break DKIM signatures. In order to minimize
the chances of such breakage, signers SHOULD convert the message to a the chances of such breakage, signers SHOULD convert the message to a
suitable MIME content transfer encoding such as quoted-printable or suitable MIME content transfer encoding such as quoted-printable or
base64 as described in MIME Part One [RFC2045] before signing. Such base64 as described in MIME Part One [RFC2045] before signing. Such
conversion is outside the scope of DKIM; the actual message SHOULD be conversion is outside the scope of DKIM; the actual message SHOULD be
converted to 7-bit MIME by an MUA or MSA prior to presentation to the converted to 7-bit MIME by an MUA or MSA prior to presentation to the
DKIM algorithm. DKIM algorithm.
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bare CR or LF characters (used by some systems as a local line bare CR or LF characters (used by some systems as a local line
separator convention) MUST be converted to the SMTP-standard CRLF separator convention) MUST be converted to the SMTP-standard CRLF
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= tag
of the resulting DKIM-Signature header field). Signers SHOULD NOT of the resulting DKIM-Signature header field). Signers SHOULD NOT
sign an existing header field likely to be legitimately modified or sign an existing header field likely to be legitimately modified or
removed in transit. In particular, [RFC2821] explicitly permits 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
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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.
5.4.1 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
situations. Verifiers MUST be capable of verifying signatures even situations. Verifiers MUST be capable of verifying signatures even
if one or more of the recommended header fields is not signed (with if one or more of the recommended header fields is not signed (with
the exception of From, which must always be signed) or if one or more the exception of From, which must always be signed) or if one or more
of the disrecommended header fields is signed. Note that verifiers of the disrecommended header fields is signed. Note that verifiers
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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 which 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 include all or nearly all of the body content when Signers SHOULD choose canonicalization algorithms based on the types
specifying the body length count (l= tag) in the signature. In of messages they process and their aversion to risk. For example,
particular, signers SHOULD NOT specify a body length of 0 since this e-commerce sites sending primarily purchase receipts, which are not
may be interpreted as a meaningless signature by some verifiers. expected to be processed by mailing lists or other software likely to
modify messages, will generally prefer "simple" canonicalization.
Sites sending primarily person-to-person email will likely prefer to
be more more resilient to modification during transport by using
"relaxed" canonicalization.
5.5 Compute the Message Hash and Signature Signers SHOULD NOT use l= unless they intend to accomodate
intermediate mail processors that append text to a message. For
example, many mailing list processors append "unsubscribe"
information to message bodies. If signers use l=, they SHOULD
include the entire message body existing at the time of signing in
computing the count. In particular, signers SHOULD NOT specify a
body length of 0 since this may be interpreted as a meaningless
signature by some verifiers.
5.6. Compute the Message Hash and Signature
The signer MUST compute the message hash as described in Section 3.7 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 which 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 which
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.
5.6 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 Selector given in the "s=" tag of the "DKIM-Signature" header the Selector given in the "s=" tag of the "DKIM-Signature" header
field, as chosen above in Section 5.2 field, as chosen above in Section 5.2
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A verifying MTA MAY implement a policy with respect to unverifiable A verifying MTA MAY implement a policy with respect to unverifiable
mail, regardless of whether or not it applies the verification header mail, regardless of whether or not it applies the verification header
field to signed messages. field to signed messages.
Verifiers 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 would like.
For example, one implementation might prefer to try the signatures in For example, one implementation might prefer to try the signatures in
textual order, whereas another might want to prefer signatures by textual order, whereas another might want to prefer signatures by
identities that match the contents of the "From" header field over identities that match the contents of the "From" header field over
other identities. Verifiers MUST NOT attribute ultimate meaning to other identities. Verifiers MUST NOT attribute ultimate meaning to
the order of multiple DKIM-Signature header fields. In particular, the order of multiple DKIM-Signature header fields. In particular,
there is reason to believe that some relays will reorder the header there is reason to believe that some relays will reorder the header
fields in potentially arbitrary ways. fields in potentially arbitrary ways.
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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
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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
used by the signer in the d= tag is not associated with a valid
signing entity. For example, signatures with d= values such as "com"
and "co.uk" may be ignored.
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.
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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
described in Section 3.7 (note that this version does not described in Section 3.7 (note that this version does not
actually need to be instantiated). When matching header field actually need to be instantiated). When matching header field
names in the "h=" tag against the actual message header field, names in the "h=" tag against the actual message header field,
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at the indicated body length might pass on a malformed MIME at the indicated body length might pass on a malformed MIME
message if the signer used the "N-4" trick (omitting the final message if the signer used the "N-4" trick (omitting the final
"--CRLF") described in the informative note in Section 3.4.5. "--CRLF") described in the informative note in Section 3.4.5.
Such verifiers may wish to check for this case and include a Such verifiers may wish to check for this case and include a
trailing "--CRLF" to avoid breaking the MIME structure. A simple trailing "--CRLF" to avoid breaking the MIME structure. A simple
way to achieve this might be to append "--CRLF" to any "multipart" way to achieve this might be to append "--CRLF" to any "multipart"
message with a body length; if the MIME structure is already message with a body length; if the MIME structure is already
correctly formed, this will appear in the postlude and will not be correctly formed, this will appear in the postlude and will not be
displayed to the end user. displayed to the end user.
6.2 Communicate Verification Results 6.2. Communicate Verification Results
Verifiers wishing to communicate the results of verification to other Verifiers wishing to communicate the results of verification to other
parts of the mail system may do so in whatever manner they see fit. parts of the mail system may do so in whatever manner they see fit.
For example, implementations might choose to add an email header For example, implementations might choose to add an email header
field to the message before passing it on. Any such header field field to the message before passing it on. Any such header field
SHOULD be inserted before any existing DKIM-Signature or preexisting SHOULD be inserted before any existing DKIM-Signature or preexisting
authentication status header fields in the header field block. authentication status header fields in the header field block.
INFORMATIVE ADVICE to MUA filter writers: Patterns intended to INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
search for results header fields to visibly mark authenticated search for results header fields to visibly mark authenticated
mail for end users should verify that such header field was added mail for end users should verify that such header field was added
by the appropriate verifying domain and that the verified identity by the appropriate verifying domain and that the verified identity
matches the author identity that will be displayed by the MUA. In matches the author identity that will be displayed by the MUA. In
particular, MUA filters should not be influenced by bogus results particular, MUA filters should not be influenced by bogus results
header fields added by attackers. To circumvent this attack, header fields added by attackers. To circumvent this attack,
verifiers may wish to delete existing results header fields after verifiers may wish to delete existing results header fields after
verification and before adding a new header field. verification and before adding a new header field.
6.3 Interpret Results/Apply Local Policy 6.3. Interpret Results/Apply Local Policy
It is beyond the scope of this specification to describe what actions It is beyond the scope of this specification to describe what actions
a verifier system should make, but an authenticated email presents an a verifier system should make, but an authenticated email presents an
opportunity to a receiving system that unauthenticated email cannot. opportunity to a receiving system that unauthenticated email cannot.
Specifically, an authenticated email creates a predictable identifier Specifically, an authenticated email creates a predictable identifier
by which other decisions can reliably be managed, such as trust and by which other decisions can reliably be managed, such as trust and
reputation. Conversely, unauthenticated email lacks a reliable reputation. Conversely, unauthenticated email lacks a reliable
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, for example: SHOULD use a 550/5.7.x reply code.
550 5.7.1 Unsigned messages not accepted
550 5.7.5 Message signature incorrect
If it is not possible to fetch the public key, perhaps because the 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
skipping to change at page 49, line 11 skipping to change at page 49, line 16
never there, or was it removed by an over-zealous filter? For never there, or was it removed by an over-zealous 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 it
looks like it was signed or not. looks like it was signed or not.
7. IANA Considerations 7. IANA Considerations
DKIM introduces some new namespaces that require IANA registry. In DKIM introduces some new namespaces that require IANA registry. In
all cases, new values are assigned only for Standards Track RFCs all cases, new values are assigned only for values that have
approved by the IESG. documented in a published RFC having 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 is
requested to establish the DKIM Signature Tag Specification Registry, requested to establish the DKIM Signature Tag Specification Registry,
for tag specifications that can be used in DKIM-Signature fields and for tag specifications that can be used in DKIM-Signature fields and
that have been specified in any published RFC. 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 |
skipping to change at page 49, line 42 skipping to change at page 49, line 47
| 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) |
+------+-----------------+ +------+-----------------+
7.2 DKIM-Signature Query Method Registry DKIM Signature Tag Specification Registry Initial Values
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, as 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 is requested to establish the DKIM 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 and have been
specified in any published RFC. 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) |
+------+--------+-----------------+ +------+--------+-----------------+
7.3 DKIM-Signature Canonicalization Registry DKIM-Signature Query Method Registry Initial Values
7.3. DKIM-Signature Canonicalization Registry
The "c=" tag-spec, as specified in Section 3.5 provides for a The "c=" tag-spec, as specified in Section 3.5 provides for a
specifier for canonicalization algorithms for the header and body of specifier for canonicalization algorithms for the header and body of
the message. the message.
IANA is requested to establish the DKIM Canonicalization Algorithm IANA is requested to establish the DKIM 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 and have been specified
in any published RFC. 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
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) |
+---------+-----------------+ +---------+-----------------+
7.4 _domainkey DNS TXT Record Tag Specifications DKIM-Signature Body Canonicalization Algorithm Registry Initial
Values
7.4. _domainkey DNS TXT Record Tag Specifications
A _domainkey DNS TXT record provides for a list of tag A _domainkey DNS TXT record provides for a list of tag
specifications. IANA is requested to establish the DKIM _domainkey specifications. IANA is requested to establish the DKIM _domainkey
DNS TXT Tag Specification Registry, for tag specifications that can DNS TXT Tag Specification Registry, for tag specifications that can
be used in DNS TXT Records and that have been specified in any be used in DNS TXT Records and that have been specified in any
published RFC. published RFC.
The initial entries in the registry comprise: The initial entries in the registry comprise:
+------+-----------------+ +------+-----------------+
skipping to change at page 51, line 20 skipping to change at page 51, line 39
| 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) |
+------+-----------------+ +------+-----------------+
7.5 DKIM Key Type Registry DKIM _domainkey DNS TXT Record Tag Specification Registry Initial
Values
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> (as specified in Section 3.6.1) and the "a="
<sig-a-tag-k> (Section 3.5) tags provide for a list of mechanisms <sig-a-tag-k> (Section 3.5) tags provide for a list of mechanisms
that can be used to decode a DKIM signature. that can be used to decode a DKIM signature.
IANA is requested to establish the DKIM Key Type Registry, for such IANA is requested to establish the DKIM Key Type Registry, for such
mechanisms that have been specified in any published RFC. 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] |
+------+-----------+ +------+-----------+
7.6 DKIM Hash Algorithms Registry DKIM Key Type Initial Values
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> list (specified in Section 3.6.1) and the "a="
<sig-a-tag-h> (Section 3.5) provide for a list of mechanisms that can <sig-a-tag-h> (Section 3.5) provide for a list of mechanisms that can
be used to produce a digest of message data. be used to produce a digest of message data.
IANA is requested to establish the DKIM Hash Algorithms Registry, for IANA is requested to establish the DKIM Hash Algorithms Registry, for
such mechanisms that have been specified in any published RFC. such mechanisms 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 |
+--------+-----------+ +--------+-----------+
| sha1 | [SHA] | | sha1 | [SHA] |
| sha256 | [SHA] | | sha256 | [SHA] |
+--------+-----------+ +--------+-----------+
7.7 DKIM Service Types Registry DKIM Hash Algorithms Initial Values
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> list (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 is requested to establish the DKIM Service Types Registry, for
service types that have been specified in any published RFC. service types 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 |
+-------+-----------------+ +-------+-----------------+
| email | (this document) | | email | (this document) |
| * | (this document) | | * | (this document) |
+-------+-----------------+ +-------+-----------------+
7.8 DKIM Selector Flags Registry DKIM Hash Algorithms Initial Values
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> list (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 is requested to establish the DKIM Selector Flags Registry, for
additional flags that have been specified in any published RFC. additional flags 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 |
+------+-----------------+ +------+-----------------+
| y | (this document) | | y | (this document) |
| s | (this document) | | s | (this document) |
+------+-----------------+ +------+-----------------+
7.9 DKIM-Signature Header Field DKIM Hash Algorithms Initial Values
IANA is requested to add DKIM-Signature to the "Permanent Header 7.9. DKIM-Signature Header Field
Messages" registry for the "mail" protocol, using this document as
the Reference. IANA is requested to add DKIM-Signature to the "Permanent Message
Header Fields" registry (see [RFC3864]) for the "mail" protocol,
using this 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 which
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 an attacker 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 "multipart/ on top of already displayed text. If a message has a
alternative" body part, they might be able to add a new body part "multipart/alternative" body part, they might be able to add a new
that is preferred by the displaying MTA. 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
overlaying images on top of existing text. overlaying images on top of existing text.
INFORMATIVE EXAMPLE: Appending the following text to an existing, INFORMATIVE EXAMPLE: Appending the following text to an existing,
properly closed message will in many MUAs result in inappropriate properly closed message will in many MUAs result in inappropriate
data being rendered on top of existing, correct data: data being rendered on top of existing, correct data:
<div style="position: relative; bottom: 350px; z-index: 2;"> <div style="position: relative; bottom: 350px; z-index: 2;">
<img src="http://www.ietf.org/images/ietflogo2e.gif" <img src="http://www.ietf.org/images/ietflogo2e.gif"
width=578 height=370> width=578 height=370>
</div> </div>
8.2 Misappropriated Private Key 8.2. Misappropriated Private Key
If the private key for a user is resident on their computer and is If the private key for a user is resident on their computer and is
not protected by an appropriately secure 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, however, not be able to
generate signed spoofs of other signers' addresses, which would aid generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user and would limit the in identification of the infected user and would limit the
possibilities for certain types of attacks involving socially- possibilities for certain types of attacks involving socially-
engineered messages. 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
skipping to change at page 54, line 46 skipping to change at page 55, line 18
A somewhat more effective countermeasure is to send messages through A somewhat more effective countermeasure is to send messages through
an outgoing MTA that can authenticate the submitter using existing an outgoing MTA that can authenticate the submitter using existing
techniques (e.g., SMTP Authentication), possibly validate the message techniques (e.g., SMTP Authentication), possibly validate the message
itself (e.g., verify that the header is legitimate and that the itself (e.g., verify that the header is legitimate and that the
content passes a spam content check), and sign the message using a content passes a spam content check), and sign the message using a
key appropriate for the submitter address. Such an MTA can also key appropriate for the submitter address. Such an MTA can also
apply controls on the volume of outgoing mail each user is permitted apply controls on the volume of outgoing mail each user is permitted
to originate in order to further limit the ability of malware to to originate in order to further limit the ability of malware to
generate bulk email. generate bulk email.
8.3 Key Server Denial-of-Service Attacks 8.3. Key Server Denial-of-Service Attacks
Since the key servers are distributed (potentially separate for each Since the key servers are distributed (potentially separate for each
domain), the number of servers that would need to be attacked to domain), the number of servers that would need to be attacked to
defeat this mechanism on an Internet-wide basis is very large. defeat this mechanism on an Internet-wide basis is very large.
Nevertheless, key servers for individual domains could be attacked, Nevertheless, key servers for individual domains could be attacked,
impeding the verification of messages from that domain. This is not impeding the verification of messages from that domain. This is not
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 DNS
Since DNS is a required binding for key services, specific attacks Since DNS is a required binding for key services, specific attacks
against DNS must be considered. against 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 DNSSEC [RFC4033], and all users of
the DNS will reap the benefit of that work. 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
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A specific DNS security issue which should be considered by DKIM A specific DNS security issue which 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.
skipping to change at page 56, line 21 skipping to change at page 56, line 41
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 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 which
have not yet been verified or which are the subject of a replay have not yet been verified or which are the subject of a replay
attack. However, the ability of the domain to do so can be limited 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 which
are intentionally malformed, with the intent of causing a denial-of- are intentionally malformed, with the intent of causing a denial-of-
service attack on a non-robust verifier implementation. The attacker service attack on a non-robust verifier implementation. The attacker
could then cause a verifier to read the malformed key record by could then cause a verifier to read the malformed key record by
sending a message to one of its users referencing the malformed sending a message to one of its users referencing the malformed
record in a (not necessarily valid) signature. Verifiers MUST record in a (not necessarily valid) signature. Verifiers MUST
thoroughly verify all key records retrieved from DNS and be robust thoroughly verify all key records retrieved from DNS and be robust
against intentionally as well as unintentionally malformed key against intentionally as well as 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
verify. verify.
8.12 RSA Attacks 8.12. RSA Attacks
An attacker could create a large RSA signing key with a small An attacker could create a large RSA signing key with a small
exponent, thus requiring that the verification key have a large exponent, thus requiring that the verification key have a large
exponent. This will force verifiers to use considerable computing exponent. This will force verifiers to use considerable computing
resources to verify the signature. Verifiers might avoid this attack resources to verify the signature. Verifiers might avoid this attack
by refusing to verify signatures that reference selectors with public by refusing to verify signatures that reference selectors with public
keys having unreasonable exponents. keys having unreasonable exponents.
In general, an attacker might try to overwhelm a verifier by flooding In general, an attacker might try to overwhelm a verifier by flooding
it with messages requiring verification. This is similar to other it with messages requiring verification. This is similar to other
MTA denial-of-service attacks and should be dealt with in a similar MTA denial-of-service attacks and should be dealt with in a similar
fashion. fashion.
8.13 Inappropriate Signing by Parent Domains 8.13. Inappropriate Signing by Parent Domains
The trust relationship described in Section 3.8 could conceivably be The trust relationship described in Section 3.8 could conceivably be
used by a parent domain to sign messages with identities in a used by a parent domain to sign messages with identities in a
subdomain not administratively related to the parent. For example, subdomain not administratively related to the parent. For example,
the ".com" registry could create messages with signatures using an the ".com" registry could create messages with signatures using an
"i=" value in the example.com domain. There is no general solution "i=" value in the example.com domain. There is no general solution
to this problem, since the administrative cut could occur anywhere in to this problem, since the administrative cut could occur anywhere in
the domain name. For example, in the domain "example.podunk.ca.us" the domain name. For example, in the domain "example.podunk.ca.us"
there are three administrative cuts (podunk.ca.us, ca.us, and us), there are three administrative cuts (podunk.ca.us, ca.us, and us),
any of which could create messages with an identity in the full any of which could create messages with an identity in the full
domain. domain.
INFORMATIVE NOTE: This is considered an acceptable risk for the INFORMATIVE NOTE: This is considered an acceptable risk for the
same reason that it is acceptable for domain delegation. For same reason that it is acceptable for domain delegation. For
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. and completely replace all DNS-served information. Note that a
verifier MAY ignore signatures that come from an unlikely domain
such as ".com", as discussed in Section 6.1.1.
9. References 9. References
9.1 Normative References 9.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996. Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message header field Extensions for Non-ASCII Part Three: Message header field Extensions for Non-ASCII
Text", RFC 2047, November 1996. 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 Indicate
skipping to change at page 59, line 5 skipping to change at page 59, line 24
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
[SHA] U.S. Department of Commerce, "Secure Hash Standard", FIPS [SHA] U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 180-2, August 2002. PUB 180-2, August 2002.
[X.660] "ITU-T Recommendation X.660 Information Technology - ASN.1 [X.660] "ITU-T Recommendation X.660 Information Technology - ASN.1
encoding rules: Specification of Basic Encoding Rules encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished (BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER)", 1997. Encoding Rules (DER)", 1997.
9.2 Informative References 9.2. Informative References
[BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing [BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing
Attacks are Practical", 2003, <http://www.usenix.org/ Attacks are Practical", 2003, <http://www.usenix.org/
publications/library/proceedings/sec03/tech/brumley.html>. publications/library/proceedings/sec03/tech/brumley.html>.
[RFC-DK] "DomainKeys specification (to be published with this [RFC-DK] "DomainKeys specification (to be published with this
RFC)", 2005. 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 and
Multipart/Encrypted", RFC 1847, October 1995. Multipart/Encrypted", RFC 1847, October 1995.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considers Section in RFCs", BCP 26, October 1998.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998. "OpenPGP Message Format", RFC 2440, November 1998.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths for [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths for
Public Keys Used For Exchanging Symmetric Keys", RFC 3766, Public Keys Used For Exchanging Symmetric Keys", RFC 3766,
April 2004. April 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004. Name System (DNS)", RFC 3833, August 2004.
[RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification", [RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 3851, June 1999. RFC 3851, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header 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., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005. RFC 4033, March 2005.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006. Identified Mail (DKIM)", RFC 4686, September 2006.
Authors' Addresses
Eric Allman
Sendmail, Inc.
6425 Christie Ave, Suite 400
Emeryville, CA 94608
USA
Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org
URI:
Jon Callas
PGP Corporation
3460 West Bayshore
Palo Alto, CA 94303
USA
Phone: +1 650 319 9016
Email: jon@pgp.com
Mark Delany
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
URI:
Miles Libbey
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Email: mlibbeymail-mailsig@yahoo.com
URI:
Jim Fenton
Cisco Systems, Inc.
MS SJ-24/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
Michael Thomas
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
Phone: +1 408 525 5386
Email: mat@cisco.com
Appendix A. Example of Use (INFORMATIVE) Appendix A. Example of Use (INFORMATIVE)
This section shows the complete flow of an email from submission to This section shows the complete flow of an email from submission to
final delivery, demonstrating how the various components fit final delivery, demonstrating how the various components fit
together. together.
A.1 The user composes an email A.1. The user composes an email
From: Joe SixPack <joe@football.example.com> From: Joe SixPack <joe@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: a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com; c=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=jpltwNFTq83Bkjt/Y2ekyqr/+i296daNkFZSdaz8VCY=;
b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B
7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt 7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt
skipping to change at page 62, line 31 skipping to change at page 61, line 31
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
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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 only. They describe various ways that DKIM can be used, purposes only. They describe various ways that DKIM can be used,
given particular constraints and needs. In no case are these given particular constraints and needs. In no case are these
examples intended to be taken as providing explanation or guidance examples 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.
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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 VPN connection to the corporate
network, either because the device is limited or because there are network, either because the device is limited or because there are
restrictions enforced by his Internet access provider. If the device restrictions enforced by his Internet access provider. If the device
was equipped with a private key registered for jdoe@example.com by was equipped with a private key registered for jdoe@example.com by
the administrator of the example.com domain, and appropriate software the administrator of the example.com domain, and appropriate software
to sign messages, John could sign the message on the device itself to sign messages, John could sign the message on the device itself
before transmission through the outgoing network of the access before transmission through the outgoing network of the access
service provider. 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 the user occasionally requests that mail be sent
on their behalf. For example, a website providing news often allows on their behalf. For example, a website providing news often allows
the reader to forward a copy of the article to a friend. This is the reader to forward a copy of the article to a friend. This is
typically done using the reader's own email address, to indicate who 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.
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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 which
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 the user's sending
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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 to
cause automatic redirection of email messages from the original 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 which 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 which 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 produces messages that are quite similar to
the automated alias services. More elaborate systems essentially the automated alias services. More elaborate systems essentially
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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
any other information that may, in the opinion of the MUA author, be any other information that may, in the opinion of the MUA author, be
deemed important to the end user. deemed important to the end user.
Appendix E. Acknowledgements Appendix E. Acknowledgements
The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann, The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
Steve Atkins, 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 Faltstrom, Mark Fanto, Stephen Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto,
Farrell, Duncan Findlay, Elliot Gillum, Olafur Gu[eth]mundsson, Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur
Phillip Hallam-Baker, Tony Hansen, Sam Hartman, Arvel Hathcock, Amir Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman,
Herzberg, Paul Hoffman, Russ Housley, Craig Hughes, Cullen Jennings, Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig
Don Johnsen, Harry Katz, Murray S. Kucherawy, Barry Leiba, John Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S.
Levine, Charles Lindsey, Simon Longsdale, David Margrave, Justin Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon
Mason, David Mayne, Steve Murphy, Russell Nelson, Dave Oran, Doug Longsdale, David Margrave, Justin Mason, David Mayne, Steve Murphy,
Otis, Shamim Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer
Ramsdell, Christian Renaud, Scott Renfro, Neil Rerup, Eric Rescorla, Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro,
Dave Rossetti, Hector Santos, Jim Schaad, the Spamhaus.org team, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector Santos, Jim Schaad,
Malte S. Stretz, Robert Sanders, Rand Wacker, Sam Weiler, and Dan the Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker,
Wing for their valuable suggestions and constructive criticism. Sam Weiler, and Dan Wing for their valuable suggestions and
constructive criticism.
The DomainKeys specification was a primary source from which this The DomainKeys specification was a primary source from which this
specification has been derived. Further information about DomainKeys specification has been derived. Further information about DomainKeys
is at [RFC-DK]. is at [RFC-DK].
Appendix F. Edit History Appendix F. Edit History
[[This section to be removed before publication.]] [[This section to be removed before publication.]]
F.1 Changes since -ietf-07 version F.1. 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.2. Changes since -ietf-07 version
The following changes were made between draft-ietf-dkim-base-07 and The following changes were made between draft-ietf-dkim-base-07 and
draft-ietf-dkim-base-08: draft-ietf-dkim-base-08:
o Drop reference to "trusted third party" in section 1; it was o Drop reference to "trusted third party" in section 1; it was
redundant with existing bullet points and created confusion. redundant with existing bullet points and created confusion.
o Drop the wording on re-using keys from normative to an operational o Drop the wording on re-using keys from normative to an operational
note. note.
skipping to change at page 72, line 25 skipping to change at page 72, line 10
o Add sentence in section 8.11 to emphasize that signing existing o Add sentence in section 8.11 to emphasize that signing existing
DKIM-Signature header fields may result in incorrect validation DKIM-Signature header fields may result in incorrect validation
failures, as requested by Security Area review. failures, as requested by Security Area review.
o Added section 8.14 (RSA Attacks) based on DNS-dir review from o Added section 8.14 (RSA Attacks) based on DNS-dir review from
Olafur Gu[eth]mundsson. Olafur Gu[eth]mundsson.
o Added section 8.15 (Inappropriate Signing by Parent Domains). o Added section 8.15 (Inappropriate Signing by Parent Domains).
F.2 Changes since -ietf-06 version F.3. Changes since -ietf-06 version
The following changes were made between draft-ietf-dkim-base-06 and The following changes were made between draft-ietf-dkim-base-06 and
draft-ietf-dkim-base-07: draft-ietf-dkim-base-07:
o Added section 8.11 regarding header reordering. o Added section 8.11 regarding header reordering.
o Added informative note to section 3.3 regarding use of sha256. o Added informative note to section 3.3 regarding use of sha256.
o Added informative rationale to section 3.6.1, "p=", regarding key o Added informative rationale to section 3.6.1, "p=", regarding key
revocation. revocation.
skipping to change at page 73, line 5 skipping to change at page 72, line 34
o Minor modification of the second informative note in section 6.1 o Minor modification of the second informative note in section 6.1
regarding DoS attacks. regarding DoS attacks.
o Added explicit mention of v= to section 6.1.2, step 5. 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 Updated paragraph 3 of section 8.4 regarding DNS attacks.
o Added section 7.9 (DKIM-Signature IANA Registry) per IANA request. o Added section 7.9 (DKIM-Signature IANA Registry) per IANA request.
F.3 Changes since -ietf-05 version F.4. Changes since -ietf-05 version
The following changes were made between draft-ietf-dkim-base-05 and The following changes were made between draft-ietf-dkim-base-05 and
draft-ietf-dkim-base-06: draft-ietf-dkim-base-06:
o Fix an error in an example in Appendix C. o Fix an error in an example in Appendix C.
o Substantial updates to Appendixes B and D. o Substantial updates to Appendixes B and D.
o Clarify ABNF for tag-value. o Clarify ABNF for tag-value.
skipping to change at page 73, line 29 skipping to change at page 73, line 10
o Add normative reference to SHA1/SHA256 FIPS publication 180-2. o Add normative reference to SHA1/SHA256 FIPS publication 180-2.
o Several minor edits based on AD Review. o Several minor edits based on AD Review.
o Move discussion of not re-using a selector (i.e., changing the o Move discussion of not re-using a selector (i.e., changing the
public key for a single selector) from informational to normative. public key for a single selector) from informational to normative.
o Assorted wordsmithing based on external review. o Assorted wordsmithing based on external review.
F.4 Changes since -ietf-04 version F.5. Changes since -ietf-04 version
The following changes were made between draft-ietf-dkim-base-04 and The following changes were made between draft-ietf-dkim-base-04 and
draft-ietf-dkim-base-05: draft-ietf-dkim-base-05:
o Clarified definition of "plain text" in section 3.2 (issue 1316). o Clarified definition of "plain text" in section 3.2 (issue 1316).
o Added some clarification about multiple listings of non-existent o Added some clarification about multiple listings of non-existent
header field names in h= in section 5.4 (issue 1316). header field names in h= in section 5.4 (issue 1316).
o Finished filling out IANA registries in section 7 (issue 1320). o Finished filling out IANA registries in section 7 (issue 1320).
skipping to change at page 74, line 5 skipping to change at page 73, line 32
o Clarified handling of bare CR and LF in section 5.3 (issue 1326). 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 o Listed the required tags in section 6.1.1 as an informational note
(issue 1330). (issue 1330).
o Changed IDNA reference from 3492 to 3490 (issue 1331). o Changed IDNA reference from 3492 to 3490 (issue 1331).
o Changed the reference for WSP to 4234; changed the definition of o Changed the reference for WSP to 4234; changed the definition of
SWSP to exclude bare CR and LF (issue 1332). SWSP to exclude bare CR and LF (issue 1332).
F.5 Changes since -ietf-03 version F.6. Changes since -ietf-03 version
The following changes were made between draft-ietf-dkim-base-03 and The following changes were made between draft-ietf-dkim-base-03 and
draft-ietf-dkim-base-04: draft-ietf-dkim-base-04:
o Re-worded Abstract to avoid use of "prove" and "non-repudiation". 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 Use dot-atom-text instead of dot-atom to avoid inclusion of CFWS.
o Capitalize Selector throughout. o Capitalize Selector throughout.
skipping to change at page 75, line 4 skipping to change at page 74, line 32
o Add several examples; update some others. o Add several examples; update some others.
o Considerable minor editorial updating to clarify language, delete o Considerable minor editorial updating to clarify language, delete
redundant text, fix spelling errors, etc. redundant text, fix spelling errors, etc.
Still to be resolved: Still to be resolved:
o How does "simple" body canonicalization interact with BINARYMIME o How does "simple" body canonicalization interact with BINARYMIME
data? data?
o Deal with "relaxed" body canonicalizations, especially in regard o Deal with "relaxed" body canonicalizations, especially in regard
to bare CRs and NLs. to bare CRs and NLs.
o How to handle "*" in g= dot-atom-text (which allows "*" as a o How to handle "*" in g= dot-atom-text (which allows "*" as a
literal character). literal character).
o The IANA Considerations need to be completed and cleaned up. o The IANA Considerations need to be completed and cleaned up.
F.6 Changes since -ietf-02 version F.7. Changes since -ietf-02 version
The following changes were made between draft-ietf-dkim-base-02 and The following changes were made between draft-ietf-dkim-base-02 and
draft-ietf-dkim-base-03: draft-ietf-dkim-base-03:
o Section 5.2: changed key expiration text to be informational; o Section 5.2: changed key expiration text to be informational;
drop "seven day" wording in favor of something vaguer. drop "seven day" wording in favor of something vaguer.
o Don't indicate that the "i=" tag value should be passed to the key 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. lookup service; this can be added as an extension if required.
skipping to change at page 76, line 13 skipping to change at page 75, line 42
may contain the content. may contain the content.
o Use dkim-quoted-printable as the encoding used in z= rather than o Use dkim-quoted-printable as the encoding used in z= rather than
referring to RFC2045, since they are different. referring to RFC2045, since they are different.
o Rewrite description of g= tag in the key record. o Rewrite description of g= tag in the key record.
o Deleted use of Domain in ABNF, which permits address-literals; o Deleted use of Domain in ABNF, which permits address-literals;
define domain-name to act in stead. define domain-name to act in stead.
F.7 Changes since -ietf-01 version F.8. Changes since -ietf-01 version
The following changes were made between draft-ietf-dkim-base-01 and The following changes were made between draft-ietf-dkim-base-01 and
draft-ietf-dkim-base-02: draft-ietf-dkim-base-02:
o Change wording on "x=" tag in DKIM-Signature header field o Change wording on "x=" tag in DKIM-Signature header field
regarding verifier handling of expired signatures from MUST to MAY regarding verifier handling of expired signatures from MUST to MAY
(per 20 April Jabber session). Also, make it clear that received (per 20 April Jabber session). Also, make it clear that received
time is to be preferred over current time if reliably available. time is to be preferred over current time if reliably available.
o Several changes to limit wording that would intrude into verifier o Several changes to limit wording that would intrude into verifier
skipping to change at page 76, line 44 skipping to change at page 76, line 26
o Change "q=dns" query access method to "q=dnstxt" to emphasize the 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 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 will define "q=dnsdkk" to indicate use of a DKK record. (Per 18
May Jabber session.) May Jabber session.)
o Several typos fixed, including removing a paragraph that implied o Several typos fixed, including removing a paragraph that implied
that the DKIM-Signature header field should be hashed with the that the DKIM-Signature header field should be hashed with the
body (it should not). body (it should not).
F.8 Changes since -ietf-00 version F.9. Changes since -ietf-00 version
The following changes were made between draft-ietf-dkim-base-00 and The following changes were made between draft-ietf-dkim-base-00 and
draft-ietf-dkim-base-01: draft-ietf-dkim-base-01:
o Added section 8.9 (Information Leakage). o Added section 8.9 (Information Leakage).
o Replace section 4 (Multiple Signatures) with much less vague text. o Replace section 4 (Multiple Signatures) with much less vague text.
o Fixed ABNF for base64string. o Fixed ABNF for base64string.
skipping to change at page 77, line 22 skipping to change at page 77, line 5
o Changed signing algorithm to use separate hash of the body of the o Changed signing algorithm to use separate hash of the body of the
message; this is represented as the "bh=" tag in the DKIM- message; this is represented as the "bh=" tag in the DKIM-
Signature header field. Signature header field.
o Changed "z=" tag so that it need not have the same header field o Changed "z=" tag so that it need not have the same header field
names as the "h=" tag. names as the "h=" tag.
o Significant wordsmithing. o Significant wordsmithing.
F.9 Changes since -allman-01 version F.10. Changes since -allman-01 version
The following changes were made between draft-allman-dkim-base-01 and The following changes were made between draft-allman-dkim-base-01 and
draft-ietf-dkim-base-00: draft-ietf-dkim-base-00:
o Remove references to Sender Signing Policy document. Such o Remove references to Sender Signing Policy document. Such
consideration is implicitly included in Section 6.3. consideration is implicitly included in Section 6.3.
o Added ABNF for all tags. o Added ABNF for all tags.
o Updated references (still includes some references to expired o Updated references (still includes some references to expired
drafts, notably ID-AUTH-RES. drafts, notably ID-AUTH-RES.
o Significant wordsmithing. o Significant wordsmithing.
F.10 Changes since -allman-00 version F.11. Changes since -allman-00 version
The following changes were made between draft-allman-dkim-base-00 and The following changes were made between draft-allman-dkim-base-00 and
draft-allman-dkim-base-01: draft-allman-dkim-base-01:
o Changed "c=" tag to separate out header from body o Changed "c=" tag to separate out header from body
canonicalization. canonicalization.
o Eliminated "nowsp" canonicalization in favor of "relaxed", which o Eliminated "nowsp" canonicalization in favor of "relaxed", which
is somewhat less relaxed (but more secure) than "nowsp". is somewhat less relaxed (but more secure) than "nowsp".
o Moved the (empty) Compliance section to the Sender Signing Policy o Moved the (empty) Compliance section to the Sender Signing Policy
document. document.
o Added several IANA Considerations. o Added several IANA Considerations.
o Fixed a number of grammar and formatting errors. o Fixed a number of grammar and formatting errors.
Intellectual Property Statement Authors' Addresses
Eric Allman
Sendmail, Inc.
6425 Christie Ave, Suite 400
Emeryville, CA 94608
USA
Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org
URI:
Jon Callas
PGP Corporation
3460 West Bayshore
Palo Alto, CA 94303
USA
Phone: +1 650 319 9016
Email: jon@pgp.com
Mark Delany
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
URI:
Miles Libbey
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Email: mlibbeymail-mailsig@yahoo.com
URI:
Jim Fenton
Cisco Systems, Inc.
MS SJ-24/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
Michael Thomas
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
Phone: +1 408 525 5386
Email: mat@cisco.com
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
skipping to change at page 79, line 29 skipping to change at page 80, 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.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2007). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is provided by the IETF
Internet Society. Administrative Support Activity (IASA).
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