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- Committer: Bazaar Package Importer
- Author(s): Scott Kitterman
- Date: 2008-07-03 01:56:39 UTC
- mto: (3.3.1 sid) (1.1.15)
- mto: This revision was merged to the branch mainline in revision 11.
- Revision ID: james.westby@ubuntu.com-20080703015639-ybv18yuz405il9i7
Tags: upstream-2.6.0.dfsg
Import upstream version 2.6.0.dfsg
- files removed:
- README_FILES/rfc4871.txt
- files modified:
- MANIFEST
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README_FILES/rfc4871.txt
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Network Working Group E. Allman
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Request for Comments: 4871 Sendmail, Inc.
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Obsoletes: 4870 J. Callas
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Category: Standards Track PGP Corporation
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M. Delany
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M. Libbey
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Yahoo! Inc
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J. Fenton
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M. Thomas
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Cisco Systems, Inc.
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May 2007
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DomainKeys Identified Mail (DKIM) Signatures
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Status of This Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Copyright (C) The IETF Trust (2007).
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Abstract
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DomainKeys Identified Mail (DKIM) defines a domain-level
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authentication framework for email using public-key cryptography and
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key server technology to permit verification of the source and
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contents of messages by either Mail Transfer Agents (MTAs) or Mail
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User Agents (MUAs). The ultimate goal of this framework is to permit
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a signing domain to assert responsibility for a message, thus
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protecting message signer identity and the integrity of the messages
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they convey while retaining the functionality of Internet email as it
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is known today. Protection of email identity may assist in the
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global control of "spam" and "phishing".
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Allman, et al. Standards Track [Page 1]
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RFC 4871 DKIM Signatures May 2007
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
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1.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 5
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1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 5
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1.3. Simple Key Management . . . . . . . . . . . . . . . . . . 5
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2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5
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2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.3. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 6
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2.4. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 6
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2.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7
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2.6. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 7
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3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
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3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8
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3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10
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3.3. Signing and Verification Algorithms . . . . . . . . . . . 11
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3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 13
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3.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 17
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3.6. Key Management and Representation . . . . . . . . . . . . 25
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3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 29
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3.8. Signing by Parent Domains . . . . . . . . . . . . . . . . 31
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4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 32
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4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 32
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4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 33
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5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 34
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5.1. Determine Whether the Email Should Be Signed and by
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Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
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5.2. Select a Private Key and Corresponding Selector
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Information . . . . . . . . . . . . . . . . . . . . . . . 35
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5.3. Normalize the Message to Prevent Transport Conversions . . 35
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5.4. Determine the Header Fields to Sign . . . . . . . . . . . 36
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5.5. Recommended Signature Content . . . . . . . . . . . . . . 38
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5.6. Compute the Message Hash and Signature . . . . . . . . . . 39
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5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 40
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6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 40
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6.1. Extract Signatures from the Message . . . . . . . . . . . 41
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6.2. Communicate Verification Results . . . . . . . . . . . . . 46
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6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 47
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7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
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7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 48
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7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 49
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7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 49
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7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 50
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7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 50
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7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 51
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7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 51
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7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 52
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Allman, et al. Standards Track [Page 2]
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7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 52
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8. Security Considerations . . . . . . . . . . . . . . . . . . . 52
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8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 52
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8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 53
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8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 54
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8.4. Attacks Against the DNS . . . . . . . . . . . . . . . . . 54
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8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 55
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8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 55
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8.7. Intentionally Malformed Key Records . . . . . . . . . . . 56
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8.8. Intentionally Malformed DKIM-Signature Header Fields . . . 56
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8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 56
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8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 56
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8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 56
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8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 56
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8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 57
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
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9.1. Normative References . . . . . . . . . . . . . . . . . . . 57
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9.2. Informative References . . . . . . . . . . . . . . . . . . 58
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Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 60
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A.1. The user composes an email . . . . . . . . . . . . . . . . 60
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A.2. The email is signed . . . . . . . . . . . . . . . . . . . 61
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A.3. The email signature is verified . . . . . . . . . . . . . 61
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Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 62
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B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 63
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B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 65
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Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 67
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Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 68
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Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 69
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Allman, et al. Standards Track [Page 3]
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1. Introduction
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DomainKeys Identified Mail (DKIM) defines a mechanism by which email
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messages can be cryptographically signed, permitting a signing domain
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to claim responsibility for the introduction of a message into the
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mail stream. Message recipients can verify the signature by querying
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the signer's domain directly to retrieve the appropriate public key,
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and thereby confirm that the message was attested to by a party in
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possession of the private key for the signing domain.
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The approach taken by DKIM differs from previous approaches to
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message signing (e.g., Secure/Multipurpose Internet Mail Extensions
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(S/MIME) [RFC1847], OpenPGP [RFC2440]) in that:
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o the message signature is written as a message header field so that
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neither human recipients nor existing MUA (Mail User Agent)
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software is confused by signature-related content appearing in the
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message body;
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o there is no dependency on public and private key pairs being
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issued by well-known, trusted certificate authorities;
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o there is no dependency on the deployment of any new Internet
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protocols or services for public key distribution or revocation;
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o signature verification failure does not force rejection of the
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message;
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o no attempt is made to include encryption as part of the mechanism;
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o message archiving is not a design goal.
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DKIM:
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o is compatible with the existing email infrastructure and
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transparent to the fullest extent possible;
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o requires minimal new infrastructure;
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o can be implemented independently of clients in order to reduce
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deployment time;
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o can be deployed incrementally;
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o allows delegation of signing to third parties.
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1.1. Signing Identity
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DKIM separates the question of the identity of the signer of the
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message from the purported author of the message. In particular, a
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signature includes the identity of the signer. Verifiers can use the
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signing information to decide how they want to process the message.
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The signing identity is included as part of the signature header
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field.
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INFORMATIVE RATIONALE: The signing identity specified by a DKIM
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signature is not required to match an address in any particular
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header field because of the broad methods of interpretation by
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recipient mail systems, including MUAs.
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1.2. Scalability
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DKIM is designed to support the extreme scalability requirements that
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characterize the email identification problem. There are currently
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over 70 million domains and a much larger number of individual
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addresses. DKIM seeks to preserve the positive aspects of the
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current email infrastructure, such as the ability for anyone to
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communicate with anyone else without introduction.
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1.3. Simple Key Management
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DKIM differs from traditional hierarchical public-key systems in that
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no Certificate Authority infrastructure is required; the verifier
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requests the public key from a repository in the domain of the
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claimed signer directly rather than from a third party.
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The DNS is proposed as the initial mechanism for the public keys.
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Thus, DKIM currently depends on DNS administration and the security
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of the DNS system. DKIM is designed to be extensible to other key
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fetching services as they become available.
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2. Terminology and Definitions
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This section defines terms used in the rest of the document. Syntax
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descriptions use the form described in Augmented BNF for Syntax
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Specifications [RFC4234].
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
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2.1. Signers
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Elements in the mail system that sign messages on behalf of a domain
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are referred to as signers. These may be MUAs (Mail User Agents),
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MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other
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agents such as mailing list exploders. In general, any signer will
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be involved in the injection of a message into the message system in
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some way. The key issue is that a message must be signed before it
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leaves the administrative domain of the signer.
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2.2. Verifiers
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Elements in the mail system that verify signatures are referred to as
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verifiers. These may be MTAs, Mail Delivery Agents (MDAs), or MUAs.
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In most cases it is expected that verifiers will be close to an end
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user (reader) of the message or some consuming agent such as a
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mailing list exploder.
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2.3. Whitespace
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There are three forms of whitespace:
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o WSP represents simple whitespace, i.e., a space or a tab character
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(formal definition in [RFC4234]).
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o LWSP is linear whitespace, defined as WSP plus CRLF (formal
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definition in [RFC4234]).
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o FWS is folding whitespace. It allows multiple lines separated by
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CRLF followed by at least one whitespace, to be joined.
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The formal ABNF for these are (WSP and LWSP are given for information
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only):
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WSP = SP / HTAB
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LWSP = *(WSP / CRLF WSP)
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FWS = [*WSP CRLF] 1*WSP
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The definition of FWS is identical to that in [RFC2822] except for
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the exclusion of obs-FWS.
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2.4. Common ABNF Tokens
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The following ABNF tokens are used elsewhere in this document:
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hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
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base64string = 1*(ALPHA / DIGIT / "+" / "/" / [FWS])
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[ "=" [FWS] [ "=" [FWS] ] ]
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2.5. Imported ABNF Tokens
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The following tokens are imported from other RFCs as noted. Those
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RFCs should be considered definitive.
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The following tokens are imported from [RFC2821]:
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o "Local-part" (implementation warning: this permits quoted strings)
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o "sub-domain"
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The following tokens are imported from [RFC2822]:
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o "field-name" (name of a header field)
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o "dot-atom-text" (in the Local-part of an email address)
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The following tokens are imported from [RFC2045]:
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o "qp-section" (a single line of quoted-printable-encoded text)
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o "hex-octet" (a quoted-printable encoded octet)
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INFORMATIVE NOTE: Be aware that the ABNF in RFC 2045 does not obey
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the rules of RFC 4234 and must be interpreted accordingly,
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particularly as regards case folding.
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Other tokens not defined herein are imported from [RFC4234]. These
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are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
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etc.
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2.6. DKIM-Quoted-Printable
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The DKIM-Quoted-Printable encoding syntax resembles that described in
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Quoted-Printable [RFC2045], Section 6.7: any character MAY be encoded
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as an "=" followed by two hexadecimal digits from the alphabet
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"0123456789ABCDEF" (no lowercase characters permitted) representing
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the hexadecimal-encoded integer value of that character. All control
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characters (those with values < %x20), 8-bit characters (values >
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%x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon
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(";", %x3B) MUST be encoded. Note that all whitespace, including
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SPACE, CR, and LF characters, MUST be encoded. After encoding, FWS
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MAY be added at arbitrary locations in order to avoid excessively
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long lines; such whitespace is NOT part of the value, and MUST be
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removed before decoding.
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ABNF:
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dkim-quoted-printable =
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*(FWS / hex-octet / dkim-safe-char)
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; hex-octet is from RFC 2045
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dkim-safe-char = %x21-3A / %x3C / %x3E-7E
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; '!' - ':', '<', '>' - '~'
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; Characters not listed as "mail-safe" in
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; RFC 2049 are also not recommended.
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INFORMATIVE NOTE: DKIM-Quoted-Printable differs from Quoted-
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Printable as defined in RFC 2045 in several important ways:
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1. Whitespace in the input text, including CR and LF, must be
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encoded. RFC 2045 does not require such encoding, and does
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not permit encoding of CR or LF characters that are part of a
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CRLF line break.
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2. Whitespace in the encoded text is ignored. This is to allow
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tags encoded using DKIM-Quoted-Printable to be wrapped as
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needed. In particular, RFC 2045 requires that line breaks in
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the input be represented as physical line breaks; that is not
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the case here.
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3. The "soft line break" syntax ("=" as the last non-whitespace
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character on the line) does not apply.
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4. DKIM-Quoted-Printable does not require that encoded lines be
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no more than 76 characters long (although there may be other
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requirements depending on the context in which the encoded
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text is being used).
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3. Protocol Elements
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Protocol Elements are conceptual parts of the protocol that are not
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specific to either signers or verifiers. The protocol descriptions
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for signers and verifiers are described in later sections (Signer
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Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This
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section must be read in the context of those sections.
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3.1. Selectors
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To support multiple concurrent public keys per signing domain, the
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key namespace is subdivided using "selectors". For example,
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selectors might indicate the names of office locations (e.g.,
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"sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
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(e.g., "january2005", "february2005", etc.), or even the individual
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user.
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Selectors are needed to support some important use cases. For
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example:
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o Domains that want to delegate signing capability for a specific
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address for a given duration to a partner, such as an advertising
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provider or other outsourced function.
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o Domains that want to allow frequent travelers to send messages
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locally without the need to connect with a particular MSA.
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o "Affinity" domains (e.g., college alumni associations) that
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provide forwarding of incoming mail, but that do not operate a
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mail submission agent for outgoing mail.
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Periods are allowed in selectors and are component separators. When
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keys are retrieved from the DNS, periods in selectors define DNS
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label boundaries in a manner similar to the conventional use in
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domain names. Selector components might be used to combine dates
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with locations, for example, "march2005.reykjavik". In a DNS
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implementation, this can be used to allow delegation of a portion of
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the selector namespace.
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ABNF:
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selector = sub-domain *( "." sub-domain )
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The number of public keys and corresponding selectors for each domain
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is determined by the domain owner. Many domain owners will be
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satisfied with just one selector, whereas administratively
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distributed organizations may choose to manage disparate selectors
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and key pairs in different regions or on different email servers.
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Beyond administrative convenience, selectors make it possible to
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seamlessly replace public keys on a routine basis. If a domain
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wishes to change from using a public key associated with selector
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"january2005" to a public key associated with selector
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"february2005", it merely makes sure that both public keys are
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advertised in the public-key repository concurrently for the
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transition period during which email may be in transit prior to
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verification. At the start of the transition period, the outbound
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email servers are configured to sign with the "february2005" private
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key. At the end of the transition period, the "january2005" public
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key is removed from the public-key repository.
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INFORMATIVE NOTE: A key may also be revoked as described below.
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The distinction between revoking and removing a key selector
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record is subtle. When phasing out keys as described above, a
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signing domain would probably simply remove the key record after
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the transition period. However, a signing domain could elect to
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revoke the key (but maintain the key record) for a further period.
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There is no defined semantic difference between a revoked key and
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a removed key.
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While some domains may wish to make selector values well known,
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others will want to take care not to allocate selector names in a way
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that allows harvesting of data by outside parties. For example, if
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per-user keys are issued, the domain owner will need to make the
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decision as to whether to associate this selector directly with the
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user name, or make it some unassociated random value, such as a
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fingerprint of the public key.
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INFORMATIVE OPERATIONS NOTE: Reusing a selector with a new key
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(for example, changing the key associated with a user's name)
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makes it impossible to tell the difference between a message that
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didn't verify because the key is no longer valid versus a message
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that is actually forged. For this reason, signers are ill-advised
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to reuse selectors for new keys. A better strategy is to assign
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new keys to new selectors.
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3.2. Tag=Value Lists
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DKIM uses a simple "tag=value" syntax in several contexts, including
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in messages and domain signature records.
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Values are a series of strings containing either plain text, "base64"
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text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid,
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Section 6.7), or "dkim-quoted-printable" (as defined in Section 2.6).
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The name of the tag will determine the encoding of each value.
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Unencoded semicolon (";") characters MUST NOT occur in the tag value,
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since that separates tag-specs.
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INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined
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below (as "tag-value") only includes 7-bit characters, an
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implementation that wished to anticipate future standards would be
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advised not to preclude the use of UTF8-encoded text in tag=value
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lists.
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Formally, the syntax rules are as follows:
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tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
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tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
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tag-name = ALPHA 0*ALNUMPUNC
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tag-value = [ tval 0*( 1*(WSP / FWS) tval ) ]
573
; WSP and FWS prohibited at beginning and end
574
tval = 1*VALCHAR
575
VALCHAR = %x21-3A / %x3C-7E
576
; EXCLAMATION to TILDE except SEMICOLON
577
ALNUMPUNC = ALPHA / DIGIT / "_"
578
579
Note that WSP is allowed anywhere around tags. In particular, any
580
WSP after the "=" and any WSP before the terminating ";" is not part
581
of the value; however, WSP inside the value is significant.
582
583
Tags MUST be interpreted in a case-sensitive manner. Values MUST be
584
processed as case sensitive unless the specific tag description of
585
semantics specifies case insensitivity.
586
587
Tags with duplicate names MUST NOT occur within a single tag-list; if
588
a tag name does occur more than once, the entire tag-list is invalid.
589
590
Whitespace within a value MUST be retained unless explicitly excluded
591
by the specific tag description.
592
593
Tag=value pairs that represent the default value MAY be included to
594
aid legibility.
595
596
Unrecognized tags MUST be ignored.
597
598
Tags that have an empty value are not the same as omitted tags. An
599
omitted tag is treated as having the default value; a tag with an
600
empty value explicitly designates the empty string as the value. For
601
example, "g=" does not mean "g=*", even though "g=*" is the default
602
for that tag.
603
604
3.3. Signing and Verification Algorithms
605
606
DKIM supports multiple digital signature algorithms. Two algorithms
607
are defined by this specification at this time: rsa-sha1 and rsa-
608
sha256. The rsa-sha256 algorithm is the default if no algorithm is
609
specified. Verifiers MUST implement both rsa-sha1 and rsa-sha256.
610
Signers MUST implement and SHOULD sign using rsa-sha256.
611
612
613
614
615
616
617
618
Allman, et al. Standards Track [Page 11]
619
620
RFC 4871 DKIM Signatures May 2007
621
622
623
INFORMATIVE NOTE: Although sha256 is strongly encouraged, some
624
senders of low-security messages (such as routine newsletters) may
625
prefer to use sha1 because of reduced CPU requirements to compute
626
a sha1 hash. In general, sha256 should always be used whenever
627
possible.
628
629
3.3.1. The rsa-sha1 Signing Algorithm
630
631
The rsa-sha1 Signing Algorithm computes a message hash as described
632
in Section 3.7 below using SHA-1 [FIPS.180-2.2002] as the hash-alg.
633
That hash is then signed by the signer using the RSA algorithm
634
(defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the
635
signer's private key. The hash MUST NOT be truncated or converted
636
into any form other than the native binary form before being signed.
637
The signing algorithm SHOULD use a public exponent of 65537.
638
639
3.3.2. The rsa-sha256 Signing Algorithm
640
641
The rsa-sha256 Signing Algorithm computes a message hash as described
642
in Section 3.7 below using SHA-256 [FIPS.180-2.2002] as the hash-alg.
643
That hash is then signed by the signer using the RSA algorithm
644
(defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the
645
signer's private key. The hash MUST NOT be truncated or converted
646
into any form other than the native binary form before being signed.
647
648
3.3.3. Key Sizes
649
650
Selecting appropriate key sizes is a trade-off between cost,
651
performance, and risk. Since short RSA keys more easily succumb to
652
off-line attacks, signers MUST use RSA keys of at least 1024 bits for
653
long-lived keys. Verifiers MUST be able to validate signatures with
654
keys ranging from 512 bits to 2048 bits, and they MAY be able to
655
validate signatures with larger keys. Verifier policies may use the
656
length of the signing key as one metric for determining whether a
657
signature is acceptable.
658
659
Factors that should influence the key size choice include the
660
following:
661
662
o The practical constraint that large (e.g., 4096 bit) keys may not
663
fit within a 512-byte DNS UDP response packet
664
665
o The security constraint that keys smaller than 1024 bits are
666
subject to off-line attacks
667
668
o Larger keys impose higher CPU costs to verify and sign email
669
670
671
672
673
674
Allman, et al. Standards Track [Page 12]
675
676
RFC 4871 DKIM Signatures May 2007
677
678
679
o Keys can be replaced on a regular basis, thus their lifetime can
680
be relatively short
681
682
o The security goals of this specification are modest compared to
683
typical goals of other systems that employ digital signatures
684
685
See [RFC3766] for further discussion on selecting key sizes.
686
687
3.3.4. Other Algorithms
688
689
Other algorithms MAY be defined in the future. Verifiers MUST ignore
690
any signatures using algorithms that they do not implement.
691
692
3.4. Canonicalization
693
694
Empirical evidence demonstrates that some mail servers and relay
695
systems modify email in transit, potentially invalidating a
696
signature. There are two competing perspectives on such
697
modifications. For most signers, mild modification of email is
698
immaterial to the authentication status of the email. For such
699
signers, a canonicalization algorithm that survives modest in-transit
700
modification is preferred.
701
702
Other signers demand that any modification of the email, however
703
minor, result in a signature verification failure. These signers
704
prefer a canonicalization algorithm that does not tolerate in-transit
705
modification of the signed email.
706
707
Some signers may be willing to accept modifications to header fields
708
that are within the bounds of email standards such as [RFC2822], but
709
are unwilling to accept any modification to the body of messages.
710
711
To satisfy all requirements, two canonicalization algorithms are
712
defined for each of the header and the body: a "simple" algorithm
713
that tolerates almost no modification and a "relaxed" algorithm that
714
tolerates common modifications such as whitespace replacement and
715
header field line rewrapping. A signer MAY specify either algorithm
716
for header or body when signing an email. If no canonicalization
717
algorithm is specified by the signer, the "simple" algorithm defaults
718
for both header and body. Verifiers MUST implement both
719
canonicalization algorithms. Note that the header and body may use
720
different canonicalization algorithms. Further canonicalization
721
algorithms MAY be defined in the future; verifiers MUST ignore any
722
signatures that use unrecognized canonicalization algorithms.
723
724
Canonicalization simply prepares the email for presentation to the
725
signing or verification algorithm. It MUST NOT change the
726
727
728
729
730
Allman, et al. Standards Track [Page 13]
731
732
RFC 4871 DKIM Signatures May 2007
733
734
735
transmitted data in any way. Canonicalization of header fields and
736
body are described below.
737
738
NOTE: This section assumes that the message is already in "network
739
normal" format (text is ASCII encoded, lines are separated with CRLF
740
characters, etc.). See also Section 5.3 for information about
741
normalizing the message.
742
743
3.4.1. The "simple" Header Canonicalization Algorithm
744
745
The "simple" header canonicalization algorithm does not change header
746
fields in any way. Header fields MUST be presented to the signing or
747
verification algorithm exactly as they are in the message being
748
signed or verified. In particular, header field names MUST NOT be
749
case folded and whitespace MUST NOT be changed.
750
751
3.4.2. The "relaxed" Header Canonicalization Algorithm
752
753
The "relaxed" header canonicalization algorithm MUST apply the
754
following steps in order:
755
756
o Convert all header field names (not the header field values) to
757
lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
758
759
o Unfold all header field continuation lines as described in
760
[RFC2822]; in particular, lines with terminators embedded in
761
continued header field values (that is, CRLF sequences followed by
762
WSP) MUST be interpreted without the CRLF. Implementations MUST
763
NOT remove the CRLF at the end of the header field value.
764
765
o Convert all sequences of one or more WSP characters to a single SP
766
character. WSP characters here include those before and after a
767
line folding boundary.
768
769
o Delete all WSP characters at the end of each unfolded header field
770
value.
771
772
o Delete any WSP characters remaining before and after the colon
773
separating the header field name from the header field value. The
774
colon separator MUST be retained.
775
776
3.4.3. The "simple" Body Canonicalization Algorithm
777
778
The "simple" body canonicalization algorithm ignores all empty lines
779
at the end of the message body. An empty line is a line of zero
780
length after removal of the line terminator. If there is no body or
781
no trailing CRLF on the message body, a CRLF is added. It makes no
782
783
784
785
786
Allman, et al. Standards Track [Page 14]
787
788
RFC 4871 DKIM Signatures May 2007
789
790
791
other changes to the message body. In more formal terms, the
792
"simple" body canonicalization algorithm converts "0*CRLF" at the end
793
of the body to a single "CRLF".
794
795
Note that a completely empty or missing body is canonicalized as a
796
single "CRLF"; that is, the canonicalized length will be 2 octets.
797
798
3.4.4. The "relaxed" Body Canonicalization Algorithm
799
800
The "relaxed" body canonicalization algorithm:
801
802
o Ignores all whitespace at the end of lines. Implementations MUST
803
NOT remove the CRLF at the end of the line.
804
805
o Reduces all sequences of WSP within a line to a single SP
806
character.
807
808
o Ignores all empty lines at the end of the message body. "Empty
809
line" is defined in Section 3.4.3.
810
811
INFORMATIVE NOTE: It should be noted that the relaxed body
812
canonicalization algorithm may enable certain types of extremely
813
crude "ASCII Art" attacks where a message may be conveyed by
814
adjusting the spacing between words. If this is a concern, the
815
"simple" body canonicalization algorithm should be used instead.
816
817
3.4.5. Body Length Limits
818
819
A body length count MAY be specified to limit the signature
820
calculation to an initial prefix of the body text, measured in
821
octets. If the body length count is not specified, the entire
822
message body is signed.
823
824
INFORMATIVE RATIONALE: This capability is provided because it is
825
very common for mailing lists to add trailers to messages (e.g.,
826
instructions how to get off the list). Until those messages are
827
also signed, the body length count is a useful tool for the
828
verifier since it may as a matter of policy accept messages having
829
valid signatures with extraneous data.
830
831
INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables
832
an attack in which an attacker modifies a message to include
833
content that solely benefits the attacker. It is possible for the
834
appended content to completely replace the original content in the
835
end recipient's eyes and to defeat duplicate message detection
836
algorithms. To avoid this attack, signers should be wary of using
837
838
839
840
841
842
Allman, et al. Standards Track [Page 15]
843
844
RFC 4871 DKIM Signatures May 2007
845
846
847
this tag, and verifiers might wish to ignore the tag or remove
848
text that appears after the specified content length, perhaps
849
based on other criteria.
850
851
The body length count allows the signer of a message to permit data
852
to be appended to the end of the body of a signed message. The body
853
length count MUST be calculated following the canonicalization
854
algorithm; for example, any whitespace ignored by a canonicalization
855
algorithm is not included as part of the body length count. Signers
856
of MIME messages that include a body length count SHOULD be sure that
857
the length extends to the closing MIME boundary string.
858
859
INFORMATIVE IMPLEMENTATION NOTE: A signer wishing to ensure that
860
the only acceptable modifications are to add to the MIME postlude
861
would use a body length count encompassing the entire final MIME
862
boundary string, including the final "--CRLF". A signer wishing
863
to allow additional MIME parts but not modification of existing
864
parts would use a body length count extending through the final
865
MIME boundary string, omitting the final "--CRLF". Note that this
866
only works for some MIME types, e.g., multipart/mixed but not
867
multipart/signed.
868
869
A body length count of zero means that the body is completely
870
unsigned.
871
872
Signers wishing to ensure that no modification of any sort can occur
873
should specify the "simple" canonicalization algorithm for both
874
header and body and omit the body length count.
875
876
3.4.6. Canonicalization Examples (INFORMATIVE)
877
878
In the following examples, actual whitespace is used only for
879
clarity. The actual input and output text is designated using
880
bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a
881
tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
882
For example, "X <SP> Y" and "X<SP>Y" represent the same three
883
characters.
884
885
Example 1: A message reading:
886
887
A: <SP> X <CRLF>
888
B <SP> : <SP> Y <HTAB><CRLF>
889
<HTAB> Z <SP><SP><CRLF>
890
<CRLF>
891
<SP> C <SP><CRLF>
892
D <SP><HTAB><SP> E <CRLF>
893
<CRLF>
894
<CRLF>
895
896
897
898
Allman, et al. Standards Track [Page 16]
899
900
RFC 4871 DKIM Signatures May 2007
901
902
903
when canonicalized using relaxed canonicalization for both header and
904
body results in a header reading:
905
906
a:X <CRLF>
907
b:Y <SP> Z <CRLF>
908
909
and a body reading:
910
911
<SP> C <CRLF>
912
D <SP> E <CRLF>
913
914
Example 2: The same message canonicalized using simple
915
canonicalization for both header and body results in a header
916
reading:
917
918
A: <SP> X <CRLF>
919
B <SP> : <SP> Y <HTAB><CRLF>
920
<HTAB> Z <SP><SP><CRLF>
921
922
and a body reading:
923
924
<SP> C <SP><CRLF>
925
D <SP><HTAB><SP> E <CRLF>
926
927
Example 3: When processed using relaxed header canonicalization and
928
simple body canonicalization, the canonicalized version has a header
929
of:
930
931
a:X <CRLF>
932
b:Y <SP> Z <CRLF>
933
934
and a body reading:
935
936
<SP> C <SP><CRLF>
937
D <SP><HTAB><SP> E <CRLF>
938
939
3.5. The DKIM-Signature Header Field
940
941
The signature of the email is stored in the DKIM-Signature header
942
field. This header field contains all of the signature and key-
943
fetching data. The DKIM-Signature value is a tag-list as described
944
in Section 3.2.
945
946
The DKIM-Signature header field SHOULD be treated as though it were a
947
trace header field as defined in Section 3.6 of [RFC2822], and hence
948
SHOULD NOT be reordered and SHOULD be prepended to the message.
949
950
951
952
953
954
Allman, et al. Standards Track [Page 17]
955
956
RFC 4871 DKIM Signatures May 2007
957
958
959
The DKIM-Signature header field being created or verified is always
960
included in the signature calculation, after the rest of the header
961
fields being signed; however, when calculating or verifying the
962
signature, the value of the "b=" tag (signature value) of that DKIM-
963
Signature header field MUST be treated as though it were an empty
964
string. Unknown tags in the DKIM-Signature header field MUST be
965
included in the signature calculation but MUST be otherwise ignored
966
by verifiers. Other DKIM-Signature header fields that are included
967
in the signature should be treated as normal header fields; in
968
particular, the "b=" tag is not treated specially.
969
970
The encodings for each field type are listed below. Tags described
971
as qp-section are encoded as described in Section 6.7 of MIME Part
972
One [RFC2045], with the additional conversion of semicolon characters
973
to "=3B"; intuitively, this is one line of quoted-printable encoded
974
text. The dkim-quoted-printable syntax is defined in Section 2.6.
975
976
Tags on the DKIM-Signature header field along with their type and
977
requirement status are shown below. Unrecognized tags MUST be
978
ignored.
979
980
v= Version (MUST be included). This tag defines the version of this
981
specification that applies to the signature record. It MUST have
982
the value "1". Note that verifiers must do a string comparison
983
on this value; for example, "1" is not the same as "1.0".
984
985
ABNF:
986
987
sig-v-tag = %x76 [FWS] "=" [FWS] "1"
988
989
INFORMATIVE NOTE: DKIM-Signature version numbers are expected
990
to increase arithmetically as new versions of this
991
specification are released.
992
993
a= The algorithm used to generate the signature (plain-text;
994
REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256";
995
signers SHOULD sign using "rsa-sha256". See Section 3.3 for a
996
description of algorithms.
997
998
ABNF:
999
1000
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
1001
sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
1002
sig-a-tag-k = "rsa" / x-sig-a-tag-k
1003
sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h
1004
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT) ; for later extension
1005
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT) ; for later extension
1006
1007
1008
1009
1010
Allman, et al. Standards Track [Page 18]
1011
1012
RFC 4871 DKIM Signatures May 2007
1013
1014
1015
b= The signature data (base64; REQUIRED). Whitespace is ignored in
1016
this value and MUST be ignored when reassembling the original
1017
signature. In particular, the signing process can safely insert
1018
FWS in this value in arbitrary places to conform to line-length
1019
limits. See Signer Actions (Section 5) for how the signature is
1020
computed.
1021
1022
ABNF:
1023
1024
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data
1025
sig-b-tag-data = base64string
1026
1027
bh= The hash of the canonicalized body part of the message as limited
1028
by the "l=" tag (base64; REQUIRED). Whitespace is ignored in
1029
this value and MUST be ignored when reassembling the original
1030
signature. In particular, the signing process can safely insert
1031
FWS in this value in arbitrary places to conform to line-length
1032
limits. See Section 3.7 for how the body hash is computed.
1033
1034
ABNF:
1035
1036
sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
1037
sig-bh-tag-data = base64string
1038
1039
c= Message canonicalization (plain-text; OPTIONAL, default is
1040
"simple/simple"). This tag informs the verifier of the type of
1041
canonicalization used to prepare the message for signing. It
1042
consists of two names separated by a "slash" (%d47) character,
1043
corresponding to the header and body canonicalization algorithms
1044
respectively. These algorithms are described in Section 3.4. If
1045
only one algorithm is named, that algorithm is used for the
1046
header and "simple" is used for the body. For example,
1047
"c=relaxed" is treated the same as "c=relaxed/simple".
1048
1049
ABNF:
1050
1051
sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg
1052
["/" sig-c-tag-alg]
1053
sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg
1054
x-sig-c-tag-alg = hyphenated-word ; for later extension
1055
1056
d= The domain of the signing entity (plain-text; REQUIRED). This is
1057
the domain that will be queried for the public key. This domain
1058
MUST be the same as or a parent domain of the "i=" tag (the
1059
signing identity, as described below), or it MUST meet the
1060
requirements for parent domain signing described in Section 3.8.
1061
When presented with a signature that does not meet these
1062
requirement, verifiers MUST consider the signature invalid.
1063
1064
1065
1066
Allman, et al. Standards Track [Page 19]
1067
1068
RFC 4871 DKIM Signatures May 2007
1069
1070
1071
Internationalized domain names MUST be encoded as described in
1072
[RFC3490].
1073
1074
ABNF:
1075
1076
sig-d-tag = %x64 [FWS] "=" [FWS] domain-name
1077
domain-name = sub-domain 1*("." sub-domain)
1078
; from RFC 2821 Domain, but excluding address-literal
1079
1080
h= Signed header fields (plain-text, but see description; REQUIRED).
1081
A colon-separated list of header field names that identify the
1082
header fields presented to the signing algorithm. The field MUST
1083
contain the complete list of header fields in the order presented
1084
to the signing algorithm. The field MAY contain names of header
1085
fields that do not exist when signed; nonexistent header fields
1086
do not contribute to the signature computation (that is, they are
1087
treated as the null input, including the header field name, the
1088
separating colon, the header field value, and any CRLF
1089
terminator). The field MUST NOT include the DKIM-Signature
1090
header field that is being created or verified, but may include
1091
others. Folding whitespace (FWS) MAY be included on either side
1092
of the colon separator. Header field names MUST be compared
1093
against actual header field names in a case-insensitive manner.
1094
This list MUST NOT be empty. See Section 5.4 for a discussion of
1095
choosing header fields to sign.
1096
1097
ABNF:
1098
1099
sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name
1100
0*( *FWS ":" *FWS hdr-name )
1101
hdr-name = field-name
1102
1103
INFORMATIVE EXPLANATION: By "signing" header fields that do not
1104
actually exist, a signer can prevent insertion of those
1105
header fields before verification. However, since a signer
1106
cannot possibly know what header fields might be created in
1107
the future, and that some MUAs might present header fields
1108
that are embedded inside a message (e.g., as a message/rfc822
1109
content type), the security of this solution is not total.
1110
1111
INFORMATIVE EXPLANATION: The exclusion of the header field name
1112
and colon as well as the header field value for non-existent
1113
header fields prevents an attacker from inserting an actual
1114
header field with a null value.
1115
1116
1117
1118
1119
1120
1121
1122
Allman, et al. Standards Track [Page 20]
1123
1124
RFC 4871 DKIM Signatures May 2007
1125
1126
1127
i= Identity of the user or agent (e.g., a mailing list manager) on
1128
behalf of which this message is signed (dkim-quoted-printable;
1129
OPTIONAL, default is an empty Local-part followed by an "@"
1130
followed by the domain from the "d=" tag). The syntax is a
1131
standard email address where the Local-part MAY be omitted. The
1132
domain part of the address MUST be the same as or a subdomain of
1133
the value of the "d=" tag.
1134
1135
Internationalized domain names MUST be converted using the steps
1136
listed in Section 4 of [RFC3490] using the "ToASCII" function.
1137
1138
ABNF:
1139
1140
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name
1141
1142
INFORMATIVE NOTE: The Local-part of the "i=" tag is optional
1143
because in some cases a signer may not be able to establish a
1144
verified individual identity. In such cases, the signer may
1145
wish to assert that although it is willing to go as far as
1146
signing for the domain, it is unable or unwilling to commit
1147
to an individual user name within their domain. It can do so
1148
by including the domain part but not the Local-part of the
1149
identity.
1150
1151
INFORMATIVE DISCUSSION: This document does not require the value
1152
of the "i=" tag to match the identity in any message header
1153
fields. This is considered to be a verifier policy issue.
1154
Constraints between the value of the "i=" tag and other
1155
identities in other header fields seek to apply basic
1156
authentication into the semantics of trust associated with a
1157
role such as content author. Trust is a broad and complex
1158
topic and trust mechanisms are subject to highly creative
1159
attacks. The real-world efficacy of any but the most basic
1160
bindings between the "i=" value and other identities is not
1161
well established, nor is its vulnerability to subversion by
1162
an attacker. Hence reliance on the use of these options
1163
should be strictly limited. In particular, it is not at all
1164
clear to what extent a typical end-user recipient can rely on
1165
any assurances that might be made by successful use of the
1166
"i=" options.
1167
1168
l= Body length count (plain-text unsigned decimal integer; OPTIONAL,
1169
default is entire body). This tag informs the verifier of the
1170
number of octets in the body of the email after canonicalization
1171
included in the cryptographic hash, starting from 0 immediately
1172
following the CRLF preceding the body. This value MUST NOT be
1173
larger than the actual number of octets in the canonicalized
1174
message body.
1175
1176
1177
1178
Allman, et al. Standards Track [Page 21]
1179
1180
RFC 4871 DKIM Signatures May 2007
1181
1182
1183
INFORMATIVE IMPLEMENTATION WARNING: Use of the "l=" tag might
1184
allow display of fraudulent content without appropriate
1185
warning to end users. The "l=" tag is intended for
1186
increasing signature robustness when sending to mailing lists
1187
that both modify their content and do not sign their
1188
messages. However, using the "l=" tag enables attacks in
1189
which an intermediary with malicious intent modifies a
1190
message to include content that solely benefits the attacker.
1191
It is possible for the appended content to completely replace
1192
the original content in the end recipient's eyes and to
1193
defeat duplicate message detection algorithms. Examples are
1194
described in Security Considerations (Section 8). To avoid
1195
this attack, signers should be extremely wary of using this
1196
tag, and verifiers might wish to ignore the tag or remove
1197
text that appears after the specified content length.
1198
1199
INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76
1200
decimal digits. This constraint is not intended to predict
1201
the size of future messages or to require implementations to
1202
use an integer representation large enough to represent the
1203
maximum possible value, but is intended to remind the
1204
implementer to check the length of this and all other tags
1205
during verification and to test for integer overflow when
1206
decoding the value. Implementers may need to limit the
1207
actual value expressed to a value smaller than 10^76, e.g.,
1208
to allow a message to fit within the available storage space.
1209
1210
ABNF:
1211
1212
sig-l-tag = %x6c [FWS] "=" [FWS] 1*76DIGIT
1213
1214
q= A colon-separated list of query methods used to retrieve the
1215
public key (plain-text; OPTIONAL, default is "dns/txt"). Each
1216
query method is of the form "type[/options]", where the syntax
1217
and semantics of the options depend on the type and specified
1218
options. If there are multiple query mechanisms listed, the
1219
choice of query mechanism MUST NOT change the interpretation of
1220
the signature. Implementations MUST use the recognized query
1221
mechanisms in the order presented.
1222
1223
Currently, the only valid value is "dns/txt", which defines the DNS
1224
TXT record lookup algorithm described elsewhere in this document.
1225
The only option defined for the "dns" query type is "txt", which
1226
MUST be included. Verifiers and signers MUST support "dns/txt".
1227
1228
1229
1230
1231
1232
1233
1234
Allman, et al. Standards Track [Page 22]
1235
1236
RFC 4871 DKIM Signatures May 2007
1237
1238
1239
ABNF:
1240
1241
sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
1242
*([FWS] ":" [FWS] sig-q-tag-method)
1243
sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
1244
["/" x-sig-q-tag-args]
1245
x-sig-q-tag-type = hyphenated-word ; for future extension
1246
x-sig-q-tag-args = qp-hdr-value
1247
1248
s= The selector subdividing the namespace for the "d=" (domain) tag
1249
(plain-text; REQUIRED).
1250
1251
ABNF:
1252
1253
sig-s-tag = %x73 [FWS] "=" [FWS] selector
1254
1255
t= Signature Timestamp (plain-text unsigned decimal integer;
1256
RECOMMENDED, default is an unknown creation time). The time that
1257
this signature was created. The format is the number of seconds
1258
since 00:00:00 on January 1, 1970 in the UTC time zone. The
1259
value is expressed as an unsigned integer in decimal ASCII. This
1260
value is not constrained to fit into a 31- or 32-bit integer.
1261
Implementations SHOULD be prepared to handle values up to at
1262
least 10^12 (until approximately AD 200,000; this fits into 40
1263
bits). To avoid denial-of-service attacks, implementations MAY
1264
consider any value longer than 12 digits to be infinite. Leap
1265
seconds are not counted. Implementations MAY ignore signatures
1266
that have a timestamp in the future.
1267
1268
ABNF:
1269
1270
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
1271
1272
x= Signature Expiration (plain-text unsigned decimal integer;
1273
RECOMMENDED, default is no expiration). The format is the same
1274
as in the "t=" tag, represented as an absolute date, not as a
1275
time delta from the signing timestamp. The value is expressed as
1276
an unsigned integer in decimal ASCII, with the same constraints
1277
on the value in the "t=" tag. Signatures MAY be considered
1278
invalid if the verification time at the verifier is past the
1279
expiration date. The verification time should be the time that
1280
the message was first received at the administrative domain of
1281
the verifier if that time is reliably available; otherwise the
1282
current time should be used. The value of the "x=" tag MUST be
1283
greater than the value of the "t=" tag if both are present.
1284
1285
1286
1287
1288
1289
1290
Allman, et al. Standards Track [Page 23]
1291
1292
RFC 4871 DKIM Signatures May 2007
1293
1294
1295
INFORMATIVE NOTE: The "x=" tag is not intended as an anti-replay
1296
defense.
1297
1298
ABNF:
1299
1300
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT
1301
1302
z= Copied header fields (dkim-quoted-printable, but see description;
1303
OPTIONAL, default is null). A vertical-bar-separated list of
1304
selected header fields present when the message was signed,
1305
including both the field name and value. It is not required to
1306
include all header fields present at the time of signing. This
1307
field need not contain the same header fields listed in the "h="
1308
tag. The header field text itself must encode the vertical bar
1309
("|", %x7C) character (i.e., vertical bars in the "z=" text are
1310
metacharacters, and any actual vertical bar characters in a
1311
copied header field must be encoded). Note that all whitespace
1312
must be encoded, including whitespace between the colon and the
1313
header field value. After encoding, FWS MAY be added at
1314
arbitrary locations in order to avoid excessively long lines;
1315
such whitespace is NOT part of the value of the header field, and
1316
MUST be removed before decoding.
1317
1318
The header fields referenced by the "h=" tag refer to the fields in
1319
the RFC 2822 header of the message, not to any copied fields in
1320
the "z=" tag. Copied header field values are for diagnostic use.
1321
1322
Header fields with characters requiring conversion (perhaps from
1323
legacy MTAs that are not [RFC2822] compliant) SHOULD be converted
1324
as described in MIME Part Three [RFC2047].
1325
1326
ABNF:
1327
sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy
1328
*( [FWS] "|" sig-z-tag-copy )
1329
sig-z-tag-copy = hdr-name ":" qp-hdr-value
1330
qp-hdr-value = dkim-quoted-printable ; with "|" encoded
1331
1332
INFORMATIVE EXAMPLE of a signature header field spread across
1333
multiple continuation lines:
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
Allman, et al. Standards Track [Page 24]
1347
1348
RFC 4871 DKIM Signatures May 2007
1349
1350
1351
DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane;
1352
c=simple; q=dns/txt; i=@eng.example.net;
1353
t=1117574938; x=1118006938;
1354
h=from:to:subject:date;
1355
z=From:foo@eng.example.net|To:joe@example.com|
1356
Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
1357
bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
1358
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
1359
VoG4ZHRNiYzR
1360
1361
3.6. Key Management and Representation
1362
1363
Signature applications require some level of assurance that the
1364
verification public key is associated with the claimed signer. Many
1365
applications achieve this by using public key certificates issued by
1366
a trusted third party. However, DKIM can achieve a sufficient level
1367
of security, with significantly enhanced scalability, by simply
1368
having the verifier query the purported signer's DNS entry (or some
1369
security-equivalent) in order to retrieve the public key.
1370
1371
DKIM keys can potentially be stored in multiple types of key servers
1372
and in multiple formats. The storage and format of keys are
1373
irrelevant to the remainder of the DKIM algorithm.
1374
1375
Parameters to the key lookup algorithm are the type of the lookup
1376
(the "q=" tag), the domain of the signer (the "d=" tag of the DKIM-
1377
Signature header field), and the selector (the "s=" tag).
1378
1379
public_key = dkim_find_key(q_val, d_val, s_val)
1380
1381
This document defines a single binding, using DNS TXT records to
1382
distribute the keys. Other bindings may be defined in the future.
1383
1384
3.6.1. Textual Representation
1385
1386
It is expected that many key servers will choose to present the keys
1387
in an otherwise unstructured text format (for example, an XML form
1388
would not be considered to be unstructured text for this purpose).
1389
The following definition MUST be used for any DKIM key represented in
1390
an otherwise unstructured textual form.
1391
1392
The overall syntax is a tag-list as described in Section 3.2. The
1393
current valid tags are described below. Other tags MAY be present
1394
and MUST be ignored by any implementation that does not understand
1395
them.
1396
1397
1398
1399
1400
1401
1402
Allman, et al. Standards Track [Page 25]
1403
1404
RFC 4871 DKIM Signatures May 2007
1405
1406
1407
v= Version of the DKIM key record (plain-text; RECOMMENDED, default
1408
is "DKIM1"). If specified, this tag MUST be set to "DKIM1"
1409
(without the quotes). This tag MUST be the first tag in the
1410
record. Records beginning with a "v=" tag with any other value
1411
MUST be discarded. Note that verifiers must do a string
1412
comparison on this value; for example, "DKIM1" is not the same as
1413
"DKIM1.0".
1414
1415
ABNF:
1416
1417
key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1"
1418
1419
g= Granularity of the key (plain-text; OPTIONAL, default is "*").
1420
This value MUST match the Local-part of the "i=" tag of the DKIM-
1421
Signature header field (or its default value of the empty string
1422
if "i=" is not specified), with a single, optional "*" character
1423
matching a sequence of zero or more arbitrary characters
1424
("wildcarding"). An email with a signing address that does not
1425
match the value of this tag constitutes a failed verification.
1426
The intent of this tag is to constrain which signing address can
1427
legitimately use this selector, for example, when delegating a
1428
key to a third party that should only be used for special
1429
purposes. Wildcarding allows matching for addresses such as
1430
"user+*" or "*-offer". An empty "g=" value never matches any
1431
addresses.
1432
1433
ABNF:
1434
1435
key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart
1436
key-g-tag-lpart = [dot-atom-text] ["*" [dot-atom-text] ]
1437
1438
h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
1439
allowing all algorithms). A colon-separated list of hash
1440
algorithms that might be used. Signers and Verifiers MUST
1441
support the "sha256" hash algorithm. Verifiers MUST also support
1442
the "sha1" hash algorithm.
1443
1444
ABNF:
1445
1446
key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg
1447
0*( [FWS] ":" [FWS] key-h-tag-alg )
1448
key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg
1449
x-key-h-tag-alg = hyphenated-word ; for future extension
1450
1451
1452
1453
1454
1455
1456
1457
1458
Allman, et al. Standards Track [Page 26]
1459
1460
RFC 4871 DKIM Signatures May 2007
1461
1462
1463
k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and
1464
verifiers MUST support the "rsa" key type. The "rsa" key type
1465
indicates that an ASN.1 DER-encoded [ITU.X660.1997] RSAPublicKey
1466
[RFC3447] (see Sections 3.1 and A.1.1) is being used in the "p="
1467
tag. (Note: the "p=" tag further encodes the value using the
1468
base64 algorithm.)
1469
1470
ABNF:
1471
1472
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type
1473
key-k-tag-type = "rsa" / x-key-k-tag-type
1474
x-key-k-tag-type = hyphenated-word ; for future extension
1475
1476
n= Notes that might be of interest to a human (qp-section; OPTIONAL,
1477
default is empty). No interpretation is made by any program.
1478
This tag should be used sparingly in any key server mechanism
1479
that has space limitations (notably DNS). This is intended for
1480
use by administrators, not end users.
1481
1482
ABNF:
1483
1484
key-n-tag = %x6e [FWS] "=" [FWS] qp-section
1485
1486
p= Public-key data (base64; REQUIRED). An empty value means that
1487
this public key has been revoked. The syntax and semantics of
1488
this tag value before being encoded in base64 are defined by the
1489
"k=" tag.
1490
1491
INFORMATIVE RATIONALE: If a private key has been compromised
1492
or otherwise disabled (e.g., an outsourcing contract has been
1493
terminated), a signer might want to explicitly state that it
1494
knows about the selector, but all messages using that
1495
selector should fail verification. Verifiers should ignore
1496
any DKIM-Signature header fields with a selector referencing
1497
a revoked key.
1498
1499
ABNF:
1500
1501
key-p-tag = %x70 [FWS] "=" [ [FWS] base64string ]
1502
1503
INFORMATIVE NOTE: A base64string is permitted to include white
1504
space (FWS) at arbitrary places; however, any CRLFs must be
1505
followed by at least one WSP character. Implementors and
1506
administrators are cautioned to ensure that selector TXT
1507
records conform to this specification.
1508
1509
1510
1511
1512
1513
1514
Allman, et al. Standards Track [Page 27]
1515
1516
RFC 4871 DKIM Signatures May 2007
1517
1518
1519
s= Service Type (plain-text; OPTIONAL; default is "*"). A colon-
1520
separated list of service types to which this record applies.
1521
Verifiers for a given service type MUST ignore this record if the
1522
appropriate type is not listed. Currently defined service types
1523
are as follows:
1524
1525
* matches all service types
1526
1527
email electronic mail (not necessarily limited to SMTP)
1528
1529
This tag is intended to constrain the use of keys for other
1530
purposes, should use of DKIM be defined by other services in the
1531
future.
1532
1533
ABNF:
1534
1535
key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type
1536
0*( [FWS] ":" [FWS] key-s-tag-type )
1537
key-s-tag-type = "email" / "*" / x-key-s-tag-type
1538
x-key-s-tag-type = hyphenated-word ; for future extension
1539
1540
t= Flags, represented as a colon-separated list of names (plain-
1541
text; OPTIONAL, default is no flags set). The defined flags are
1542
as follows:
1543
1544
y This domain is testing DKIM. Verifiers MUST NOT treat
1545
messages from signers in testing mode differently from
1546
unsigned email, even should the signature fail to verify.
1547
Verifiers MAY wish to track testing mode results to assist
1548
the signer.
1549
1550
s Any DKIM-Signature header fields using the "i=" tag MUST have
1551
the same domain value on the right-hand side of the "@" in
1552
the "i=" tag and the value of the "d=" tag. That is, the
1553
"i=" domain MUST NOT be a subdomain of "d=". Use of this
1554
flag is RECOMMENDED unless subdomaining is required.
1555
1556
ABNF:
1557
1558
key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag
1559
0*( [FWS] ":" [FWS] key-t-tag-flag )
1560
key-t-tag-flag = "y" / "s" / x-key-t-tag-flag
1561
x-key-t-tag-flag = hyphenated-word ; for future extension
1562
1563
Unrecognized flags MUST be ignored.
1564
1565
1566
1567
1568
1569
1570
Allman, et al. Standards Track [Page 28]
1571
1572
RFC 4871 DKIM Signatures May 2007
1573
1574
1575
3.6.2. DNS Binding
1576
1577
A binding using DNS TXT records as a key service is hereby defined.
1578
All implementations MUST support this binding.
1579
1580
3.6.2.1. Namespace
1581
1582
All DKIM keys are stored in a subdomain named "_domainkey". Given a
1583
DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag
1584
of "foo.bar", the DNS query will be for
1585
"foo.bar._domainkey.example.com".
1586
1587
INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records (e.g.,
1588
*.bar._domainkey.example.com) do not make sense in this context
1589
and should not be used. Note also that wildcards within domains
1590
(e.g., s._domainkey.*.example.com) are not supported by the DNS.
1591
1592
3.6.2.2. Resource Record Types for Key Storage
1593
1594
The DNS Resource Record type used is specified by an option to the
1595
query-type ("q=") tag. The only option defined in this base
1596
specification is "txt", indicating the use of a TXT Resource Record
1597
(RR). A later extension of this standard may define another RR type.
1598
1599
Strings in a TXT RR MUST be concatenated together before use with no
1600
intervening whitespace. TXT RRs MUST be unique for a particular
1601
selector name; that is, if there are multiple records in an RRset,
1602
the results are undefined.
1603
1604
TXT RRs are encoded as described in Section 3.6.1.
1605
1606
3.7. Computing the Message Hashes
1607
1608
Both signing and verifying message signatures start with a step of
1609
computing two cryptographic hashes over the message. Signers will
1610
choose the parameters of the signature as described in Signer Actions
1611
(Section 5); verifiers will use the parameters specified in the DKIM-
1612
Signature header field being verified. In the following discussion,
1613
the names of the tags in the DKIM-Signature header field that either
1614
exists (when verifying) or will be created (when signing) are used.
1615
Note that canonicalization (Section 3.4) is only used to prepare the
1616
email for signing or verifying; it does not affect the transmitted
1617
email in any way.
1618
1619
The signer/verifier MUST compute two hashes, one over the body of the
1620
message and one over the selected header fields of the message.
1621
1622
1623
1624
1625
1626
Allman, et al. Standards Track [Page 29]
1627
1628
RFC 4871 DKIM Signatures May 2007
1629
1630
1631
Signers MUST compute them in the order shown. Verifiers MAY compute
1632
them in any order convenient to the verifier, provided that the
1633
result is semantically identical to the semantics that would be the
1634
case had they been computed in this order.
1635
1636
In hash step 1, the signer/verifier MUST hash the message body,
1637
canonicalized using the body canonicalization algorithm specified in
1638
the "c=" tag and then truncated to the length specified in the "l="
1639
tag. That hash value is then converted to base64 form and inserted
1640
into (signers) or compared to (verifiers) the "bh=" tag of the DKIM-
1641
Signature header field.
1642
1643
In hash step 2, the signer/verifier MUST pass the following to the
1644
hash algorithm in the indicated order.
1645
1646
1. The header fields specified by the "h=" tag, in the order
1647
specified in that tag, and canonicalized using the header
1648
canonicalization algorithm specified in the "c=" tag. Each
1649
header field MUST be terminated with a single CRLF.
1650
1651
2. The DKIM-Signature header field that exists (verifying) or will
1652
be inserted (signing) in the message, with the value of the "b="
1653
tag deleted (i.e., treated as the empty string), canonicalized
1654
using the header canonicalization algorithm specified in the "c="
1655
tag, and without a trailing CRLF.
1656
1657
All tags and their values in the DKIM-Signature header field are
1658
included in the cryptographic hash with the sole exception of the
1659
value portion of the "b=" (signature) tag, which MUST be treated as
1660
the null string. All tags MUST be included even if they might not be
1661
understood by the verifier. The header field MUST be presented to
1662
the hash algorithm after the body of the message rather than with the
1663
rest of the header fields and MUST be canonicalized as specified in
1664
the "c=" (canonicalization) tag. The DKIM-Signature header field
1665
MUST NOT be included in its own h= tag, although other DKIM-Signature
1666
header fields MAY be signed (see Section 4).
1667
1668
When calculating the hash on messages that will be transmitted using
1669
base64 or quoted-printable encoding, signers MUST compute the hash
1670
after the encoding. Likewise, the verifier MUST incorporate the
1671
values into the hash before decoding the base64 or quoted-printable
1672
text. However, the hash MUST be computed before transport level
1673
encodings such as SMTP "dot-stuffing" (the modification of lines
1674
beginning with a "." to avoid confusion with the SMTP end-of-message
1675
marker, as specified in [RFC2821]).
1676
1677
With the exception of the canonicalization procedure described in
1678
Section 3.4, the DKIM signing process treats the body of messages as
1679
1680
1681
1682
Allman, et al. Standards Track [Page 30]
1683
1684
RFC 4871 DKIM Signatures May 2007
1685
1686
1687
simply a string of octets. DKIM messages MAY be either in plain-text
1688
or in MIME format; no special treatment is afforded to MIME content.
1689
Message attachments in MIME format MUST be included in the content
1690
that is signed.
1691
1692
More formally, the algorithm for the signature is as follows:
1693
body-hash = hash-alg(canon_body)
1694
header-hash = hash-alg(canon_header || DKIM-SIG)
1695
signature = sig-alg(header-hash, key)
1696
1697
where "sig-alg" is the signature algorithm specified by the "a=" tag,
1698
"hash-alg" is the hash algorithm specified by the "a=" tag,
1699
"canon_header" and "canon_body" are the canonicalized message header
1700
and body (respectively) as defined in Section 3.4 (excluding the
1701
DKIM-Signature header field), and "DKIM-SIG" is the canonicalized
1702
DKIM-Signature header field sans the signature value itself, but with
1703
"body-hash" included as the "bh=" tag.
1704
1705
INFORMATIVE IMPLEMENTERS' NOTE: Many digital signature APIs
1706
provide both hashing and application of the RSA private key using
1707
a single "sign()" primitive. When using such an API, the last two
1708
steps in the algorithm would probably be combined into a single
1709
call that would perform both the "hash-alg" and the "sig-alg".
1710
1711
3.8. Signing by Parent Domains
1712
1713
In some circumstances, it is desirable for a domain to apply a
1714
signature on behalf of any of its subdomains without the need to
1715
maintain separate selectors (key records) in each subdomain. By
1716
default, private keys corresponding to key records can be used to
1717
sign messages for any subdomain of the domain in which they reside;
1718
e.g., a key record for the domain example.com can be used to verify
1719
messages where the signing identity ("i=" tag of the signature) is
1720
sub.example.com, or even sub1.sub2.example.com. In order to limit
1721
the capability of such keys when this is not intended, the "s" flag
1722
may be set in the "t=" tag of the key record to constrain the
1723
validity of the record to exactly the domain of the signing identity.
1724
If the referenced key record contains the "s" flag as part of the
1725
"t=" tag, the domain of the signing identity ("i=" flag) MUST be the
1726
same as that of the d= domain. If this flag is absent, the domain of
1727
the signing identity MUST be the same as, or a subdomain of, the d=
1728
domain. Key records that are not intended for use with subdomains
1729
SHOULD specify the "s" flag in the "t=" tag.
1730
1731
1732
1733
1734
1735
1736
1737
1738
Allman, et al. Standards Track [Page 31]
1739
1740
RFC 4871 DKIM Signatures May 2007
1741
1742
1743
4. Semantics of Multiple Signatures
1744
1745
4.1. Example Scenarios
1746
1747
There are many reasons why a message might have multiple signatures.
1748
For example, a given signer might sign multiple times, perhaps with
1749
different hashing or signing algorithms during a transition phase.
1750
1751
INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be
1752
insufficiently strong, and DKIM usage transitions to SHA-1024. A
1753
signer might immediately sign using the newer algorithm, but
1754
continue to sign using the older algorithm for interoperability
1755
with verifiers that had not yet upgraded. The signer would do
1756
this by adding two DKIM-Signature header fields, one using each
1757
algorithm. Older verifiers that did not recognize SHA-1024 as an
1758
acceptable algorithm would skip that signature and use the older
1759
algorithm; newer verifiers could use either signature at their
1760
option, and all other things being equal might not even attempt to
1761
verify the other signature.
1762
1763
Similarly, a signer might sign a message including all headers and no
1764
"l=" tag (to satisfy strict verifiers) and a second time with a
1765
limited set of headers and an "l=" tag (in anticipation of possible
1766
message modifications in route to other verifiers). Verifiers could
1767
then choose which signature they preferred.
1768
1769
INFORMATIVE EXAMPLE: A verifier might receive a message with two
1770
signatures, one covering more of the message than the other. If
1771
the signature covering more of the message verified, then the
1772
verifier could make one set of policy decisions; if that signature
1773
failed but the signature covering less of the message verified,
1774
the verifier might make a different set of policy decisions.
1775
1776
Of course, a message might also have multiple signatures because it
1777
passed through multiple signers. A common case is expected to be
1778
that of a signed message that passes through a mailing list that also
1779
signs all messages. Assuming both of those signatures verify, a
1780
recipient might choose to accept the message if either of those
1781
signatures were known to come from trusted sources.
1782
1783
INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing
1784
lists to which they have subscribed and that have acceptable anti-
1785
abuse policies so as to accept messages sent to that list even
1786
from unknown authors. They might also subscribe to less trusted
1787
mailing lists (e.g., those without anti-abuse protection) and be
1788
willing to accept all messages from specific authors, but insist
1789
on doing additional abuse scanning for other messages.
1790
1791
1792
1793
1794
Allman, et al. Standards Track [Page 32]
1795
1796
RFC 4871 DKIM Signatures May 2007
1797
1798
1799
Another related example of multiple signers might be forwarding
1800
services, such as those commonly associated with academic alumni
1801
sites.
1802
1803
INFORMATIVE EXAMPLE: A recipient might have an address at
1804
members.example.org, a site that has anti-abuse protection that is
1805
somewhat less effective than the recipient would prefer. Such a
1806
recipient might have specific authors whose messages would be
1807
trusted absolutely, but messages from unknown authors that had
1808
passed the forwarder's scrutiny would have only medium trust.
1809
1810
4.2. Interpretation
1811
1812
A signer that is adding a signature to a message merely creates a new
1813
DKIM-Signature header, using the usual semantics of the h= option. A
1814
signer MAY sign previously existing DKIM-Signature header fields
1815
using the method described in Section 5.4 to sign trace header
1816
fields.
1817
1818
INFORMATIVE NOTE: Signers should be cognizant that signing DKIM-
1819
Signature header fields may result in signature failures with
1820
intermediaries that do not recognize that DKIM-Signature header
1821
fields are trace header fields and unwittingly reorder them, thus
1822
breaking such signatures. For this reason, signing existing DKIM-
1823
Signature header fields is unadvised, albeit legal.
1824
1825
INFORMATIVE NOTE: If a header field with multiple instances is
1826
signed, those header fields are always signed from the bottom up.
1827
Thus, it is not possible to sign only specific DKIM-Signature
1828
header fields. For example, if the message being signed already
1829
contains three DKIM-Signature header fields A, B, and C, it is
1830
possible to sign all of them, B and C only, or C only, but not A
1831
only, B only, A and B only, or A and C only.
1832
1833
A signer MAY add more than one DKIM-Signature header field using
1834
different parameters. For example, during a transition period a
1835
signer might want to produce signatures using two different hash
1836
algorithms.
1837
1838
Signers SHOULD NOT remove any DKIM-Signature header fields from
1839
messages they are signing, even if they know that the signatures
1840
cannot be verified.
1841
1842
When evaluating a message with multiple signatures, a verifier SHOULD
1843
evaluate signatures independently and on their own merits. For
1844
example, a verifier that by policy chooses not to accept signatures
1845
with deprecated cryptographic algorithms would consider such
1846
signatures invalid. Verifiers MAY process signatures in any order of
1847
1848
1849
1850
Allman, et al. Standards Track [Page 33]
1851
1852
RFC 4871 DKIM Signatures May 2007
1853
1854
1855
their choice; for example, some verifiers might choose to process
1856
signatures corresponding to the From field in the message header
1857
before other signatures. See Section 6.1 for more information about
1858
signature choices.
1859
1860
INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate
1861
valid signatures with invalid signatures in an attempt to guess
1862
why a signature failed are ill-advised. In particular, there is
1863
no general way that a verifier can determine that an invalid
1864
signature was ever valid.
1865
1866
Verifiers SHOULD ignore failed signatures as though they were not
1867
present in the message. Verifiers SHOULD continue to check
1868
signatures until a signature successfully verifies to the
1869
satisfaction of the verifier. To limit potential denial-of-service
1870
attacks, verifiers MAY limit the total number of signatures they will
1871
attempt to verify.
1872
1873
5. Signer Actions
1874
1875
The following steps are performed in order by signers.
1876
1877
5.1. Determine Whether the Email Should Be Signed and by Whom
1878
1879
A signer can obviously only sign email for domains for which it has a
1880
private key and the necessary knowledge of the corresponding public
1881
key and selector information. However, there are a number of other
1882
reasons beyond the lack of a private key why a signer could choose
1883
not to sign an email.
1884
1885
INFORMATIVE NOTE: Signing modules may be incorporated into any
1886
portion of the mail system as deemed appropriate, including an
1887
MUA, a SUBMISSION server, or an MTA. Wherever implemented,
1888
signers should beware of signing (and thereby asserting
1889
responsibility for) messages that may be problematic. In
1890
particular, within a trusted enclave the signing address might be
1891
derived from the header according to local policy; SUBMISSION
1892
servers might only sign messages from users that are properly
1893
authenticated and authorized.
1894
1895
INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign
1896
Received header fields if the outgoing gateway MTA obfuscates
1897
Received header fields, for example, to hide the details of
1898
internal topology.
1899
1900
If an email cannot be signed for some reason, it is a local policy
1901
decision as to what to do with that email.
1902
1903
1904
1905
1906
Allman, et al. Standards Track [Page 34]
1907
1908
RFC 4871 DKIM Signatures May 2007
1909
1910
1911
5.2. Select a Private Key and Corresponding Selector Information
1912
1913
This specification does not define the basis by which a signer should
1914
choose which private key and selector information to use. Currently,
1915
all selectors are equal as far as this specification is concerned, so
1916
the decision should largely be a matter of administrative
1917
convenience. Distribution and management of private keys is also
1918
outside the scope of this document.
1919
1920
INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
1921
private key when the selector containing the corresponding public
1922
key is expected to be revoked or removed before the verifier has
1923
an opportunity to validate the signature. The signer should
1924
anticipate that verifiers may choose to defer validation, perhaps
1925
until the message is actually read by the final recipient. In
1926
particular, when rotating to a new key pair, signing should
1927
immediately commence with the new private key and the old public
1928
key should be retained for a reasonable validation interval before
1929
being removed from the key server.
1930
1931
5.3. Normalize the Message to Prevent Transport Conversions
1932
1933
Some messages, particularly those using 8-bit characters, are subject
1934
to modification during transit, notably conversion to 7-bit form.
1935
Such conversions will break DKIM signatures. In order to minimize
1936
the chances of such breakage, signers SHOULD convert the message to a
1937
suitable MIME content transfer encoding such as quoted-printable or
1938
base64 as described in MIME Part One [RFC2045] before signing. Such
1939
conversion is outside the scope of DKIM; the actual message SHOULD be
1940
converted to 7-bit MIME by an MUA or MSA prior to presentation to the
1941
DKIM algorithm.
1942
1943
If the message is submitted to the signer with any local encoding
1944
that will be modified before transmission, that modification to
1945
canonical [RFC2822] form MUST be done before signing. In particular,
1946
bare CR or LF characters (used by some systems as a local line
1947
separator convention) MUST be converted to the SMTP-standard CRLF
1948
sequence before the message is signed. Any conversion of this sort
1949
SHOULD be applied to the message actually sent to the recipient(s),
1950
not just to the version presented to the signing algorithm.
1951
1952
More generally, the signer MUST sign the message as it is expected to
1953
be received by the verifier rather than in some local or internal
1954
form.
1955
1956
1957
1958
1959
1960
1961
1962
Allman, et al. Standards Track [Page 35]
1963
1964
RFC 4871 DKIM Signatures May 2007
1965
1966
1967
5.4. Determine the Header Fields to Sign
1968
1969
The From header field MUST be signed (that is, included in the "h="
1970
tag of the resulting DKIM-Signature header field). Signers SHOULD
1971
NOT sign an existing header field likely to be legitimately modified
1972
or removed in transit. In particular, [RFC2821] explicitly permits
1973
modification or removal of the Return-Path header field in transit.
1974
Signers MAY include any other header fields present at the time of
1975
signing at the discretion of the signer.
1976
1977
INFORMATIVE OPERATIONS NOTE: The choice of which header fields to
1978
sign is non-obvious. One strategy is to sign all existing, non-
1979
repeatable header fields. An alternative strategy is to sign only
1980
header fields that are likely to be displayed to or otherwise be
1981
likely to affect the processing of the message at the receiver. A
1982
third strategy is to sign only "well known" headers. Note that
1983
verifiers may treat unsigned header fields with extreme
1984
skepticism, including refusing to display them to the end user or
1985
even ignoring the signature if it does not cover certain header
1986
fields. For this reason, signing fields present in the message
1987
such as Date, Subject, Reply-To, Sender, and all MIME header
1988
fields are highly advised.
1989
1990
The DKIM-Signature header field is always implicitly signed and MUST
1991
NOT be included in the "h=" tag except to indicate that other
1992
preexisting signatures are also signed.
1993
1994
Signers MAY claim to have signed header fields that do not exist
1995
(that is, signers MAY include the header field name in the "h=" tag
1996
even if that header field does not exist in the message). When
1997
computing the signature, the non-existing header field MUST be
1998
treated as the null string (including the header field name, header
1999
field value, all punctuation, and the trailing CRLF).
2000
2001
INFORMATIVE RATIONALE: This allows signers to explicitly assert
2002
the absence of a header field; if that header field is added later
2003
the signature will fail.
2004
2005
INFORMATIVE NOTE: A header field name need only be listed once
2006
more than the actual number of that header field in a message at
2007
the time of signing in order to prevent any further additions.
2008
For example, if there is a single Comments header field at the
2009
time of signing, listing Comments twice in the "h=" tag is
2010
sufficient to prevent any number of Comments header fields from
2011
being appended; it is not necessary (but is legal) to list
2012
Comments three or more times in the "h=" tag.
2013
2014
2015
2016
2017
2018
Allman, et al. Standards Track [Page 36]
2019
2020
RFC 4871 DKIM Signatures May 2007
2021
2022
2023
Signers choosing to sign an existing header field that occurs more
2024
than once in the message (such as Received) MUST sign the physically
2025
last instance of that header field in the header block. Signers
2026
wishing to sign multiple instances of such a header field MUST
2027
include the header field name multiple times in the h= tag of the
2028
DKIM-Signature header field, and MUST sign such header fields in
2029
order from the bottom of the header field block to the top. The
2030
signer MAY include more instances of a header field name in h= than
2031
there are actual corresponding header fields to indicate that
2032
additional header fields of that name SHOULD NOT be added.
2033
2034
INFORMATIVE EXAMPLE:
2035
2036
If the signer wishes to sign two existing Received header fields,
2037
and the existing header contains:
2038
2039
Received: <A>
2040
Received: <B>
2041
Received: <C>
2042
2043
then the resulting DKIM-Signature header field should read:
2044
2045
DKIM-Signature: ... h=Received : Received : ...
2046
2047
and Received header fields <C> and <B> will be signed in that
2048
order.
2049
2050
Signers should be careful of signing header fields that might have
2051
additional instances added later in the delivery process, since such
2052
header fields might be inserted after the signed instance or
2053
otherwise reordered. Trace header fields (such as Received) and
2054
Resent-* blocks are the only fields prohibited by [RFC2822] from
2055
being reordered. In particular, since DKIM-Signature header fields
2056
may be reordered by some intermediate MTAs, signing existing DKIM-
2057
Signature header fields is error-prone.
2058
2059
INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
2060
header fields to be reordered (with the exception of Received
2061
header fields), reordering of signed header fields with multiple
2062
instances by intermediate MTAs will cause DKIM signatures to be
2063
broken; such anti-social behavior should be avoided.
2064
2065
INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this
2066
specification, all end-user visible header fields should be signed
2067
to avoid possible "indirect spamming". For example, if the
2068
Subject header field is not signed, a spammer can resend a
2069
previously signed mail, replacing the legitimate subject with a
2070
one-line spam.
2071
2072
2073
2074
Allman, et al. Standards Track [Page 37]
2075
2076
RFC 4871 DKIM Signatures May 2007
2077
2078
2079
5.5. Recommended Signature Content
2080
2081
In order to maximize compatibility with a variety of verifiers, it is
2082
recommended that signers follow the practices outlined in this
2083
section when signing a message. However, these are generic
2084
recommendations applying to the general case; specific senders may
2085
wish to modify these guidelines as required by their unique
2086
situations. Verifiers MUST be capable of verifying signatures even
2087
if one or more of the recommended header fields is not signed (with
2088
the exception of From, which must always be signed) or if one or more
2089
of the disrecommended header fields is signed. Note that verifiers
2090
do have the option of ignoring signatures that do not cover a
2091
sufficient portion of the header or body, just as they may ignore
2092
signatures from an identity they do not trust.
2093
2094
The following header fields SHOULD be included in the signature, if
2095
they are present in the message being signed:
2096
2097
o From (REQUIRED in all signatures)
2098
2099
o Sender, Reply-To
2100
2101
o Subject
2102
2103
o Date, Message-ID
2104
2105
o To, Cc
2106
2107
o MIME-Version
2108
2109
o Content-Type, Content-Transfer-Encoding, Content-ID, Content-
2110
Description
2111
2112
o Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-Cc,
2113
Resent-Message-ID
2114
2115
o In-Reply-To, References
2116
2117
o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post,
2118
List-Owner, List-Archive
2119
2120
The following header fields SHOULD NOT be included in the signature:
2121
2122
o Return-Path
2123
2124
o Received
2125
2126
o Comments, Keywords
2127
2128
2129
2130
Allman, et al. Standards Track [Page 38]
2131
2132
RFC 4871 DKIM Signatures May 2007
2133
2134
2135
o Bcc, Resent-Bcc
2136
2137
o DKIM-Signature
2138
2139
Optional header fields (those not mentioned above) normally SHOULD
2140
NOT be included in the signature, because of the potential for
2141
additional header fields of the same name to be legitimately added or
2142
reordered prior to verification. There are likely to be legitimate
2143
exceptions to this rule, because of the wide variety of application-
2144
specific header fields that may be applied to a message, some of
2145
which are unlikely to be duplicated, modified, or reordered.
2146
2147
Signers SHOULD choose canonicalization algorithms based on the types
2148
of messages they process and their aversion to risk. For example,
2149
e-commerce sites sending primarily purchase receipts, which are not
2150
expected to be processed by mailing lists or other software likely to
2151
modify messages, will generally prefer "simple" canonicalization.
2152
Sites sending primarily person-to-person email will likely prefer to
2153
be more resilient to modification during transport by using "relaxed"
2154
canonicalization.
2155
2156
Signers SHOULD NOT use "l=" unless they intend to accommodate
2157
intermediate mail processors that append text to a message. For
2158
example, many mailing list processors append "unsubscribe"
2159
information to message bodies. If signers use "l=", they SHOULD
2160
include the entire message body existing at the time of signing in
2161
computing the count. In particular, signers SHOULD NOT specify a
2162
body length of 0 since this may be interpreted as a meaningless
2163
signature by some verifiers.
2164
2165
5.6. Compute the Message Hash and Signature
2166
2167
The signer MUST compute the message hash as described in Section 3.7
2168
and then sign it using the selected public-key algorithm. This will
2169
result in a DKIM-Signature header field that will include the body
2170
hash and a signature of the header hash, where that header includes
2171
the DKIM-Signature header field itself.
2172
2173
Entities such as mailing list managers that implement DKIM and that
2174
modify the message or a header field (for example, inserting
2175
unsubscribe information) before retransmitting the message SHOULD
2176
check any existing signature on input and MUST make such
2177
modifications before re-signing the message.
2178
2179
The signer MAY elect to limit the number of bytes of the body that
2180
will be included in the hash and hence signed. The length actually
2181
hashed should be inserted in the "l=" tag of the DKIM-Signature
2182
header field.
2183
2184
2185
2186
Allman, et al. Standards Track [Page 39]
2187
2188
RFC 4871 DKIM Signatures May 2007
2189
2190
2191
5.7. Insert the DKIM-Signature Header Field
2192
2193
Finally, the signer MUST insert the DKIM-Signature header field
2194
created in the previous step prior to transmitting the email. The
2195
DKIM-Signature header field MUST be the same as used to compute the
2196
hash as described above, except that the value of the "b=" tag MUST
2197
be the appropriately signed hash computed in the previous step,
2198
signed using the algorithm specified in the "a=" tag of the DKIM-
2199
Signature header field and using the private key corresponding to the
2200
selector given in the "s=" tag of the DKIM-Signature header field, as
2201
chosen above in Section 5.2
2202
2203
The DKIM-Signature header field MUST be inserted before any other
2204
DKIM-Signature fields in the header block.
2205
2206
INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this
2207
is to insert the DKIM-Signature header field at the beginning of
2208
the header block. In particular, it may be placed before any
2209
existing Received header fields. This is consistent with treating
2210
DKIM-Signature as a trace header field.
2211
2212
6. Verifier Actions
2213
2214
Since a signer MAY remove or revoke a public key at any time, it is
2215
recommended that verification occur in a timely manner. In many
2216
configurations, the most timely place is during acceptance by the
2217
border MTA or shortly thereafter. In particular, deferring
2218
verification until the message is accessed by the end user is
2219
discouraged.
2220
2221
A border or intermediate MTA MAY verify the message signature(s). An
2222
MTA who has performed verification MAY communicate the result of that
2223
verification by adding a verification header field to incoming
2224
messages. This considerably simplifies things for the user, who can
2225
now use an existing mail user agent. Most MUAs have the ability to
2226
filter messages based on message header fields or content; these
2227
filters would be used to implement whatever policy the user wishes
2228
with respect to unsigned mail.
2229
2230
A verifying MTA MAY implement a policy with respect to unverifiable
2231
mail, regardless of whether or not it applies the verification header
2232
field to signed messages.
2233
2234
Verifiers MUST produce a result that is semantically equivalent to
2235
applying the following steps in the order listed. In practice,
2236
several of these steps can be performed in parallel in order to
2237
improve performance.
2238
2239
2240
2241
2242
Allman, et al. Standards Track [Page 40]
2243
2244
RFC 4871 DKIM Signatures May 2007
2245
2246
2247
6.1. Extract Signatures from the Message
2248
2249
The order in which verifiers try DKIM-Signature header fields is not
2250
defined; verifiers MAY try signatures in any order they like. For
2251
example, one implementation might try the signatures in textual
2252
order, whereas another might try signatures by identities that match
2253
the contents of the From header field before trying other signatures.
2254
Verifiers MUST NOT attribute ultimate meaning to the order of
2255
multiple DKIM-Signature header fields. In particular, there is
2256
reason to believe that some relays will reorder the header fields in
2257
potentially arbitrary ways.
2258
2259
INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as
2260
a clue to signing order in the absence of any other information.
2261
However, other clues as to the semantics of multiple signatures
2262
(such as correlating the signing host with Received header fields)
2263
may also be considered.
2264
2265
A verifier SHOULD NOT treat a message that has one or more bad
2266
signatures and no good signatures differently from a message with no
2267
signature at all; such treatment is a matter of local policy and is
2268
beyond the scope of this document.
2269
2270
When a signature successfully verifies, a verifier will either stop
2271
processing or attempt to verify any other signatures, at the
2272
discretion of the implementation. A verifier MAY limit the number of
2273
signatures it tries to avoid denial-of-service attacks.
2274
2275
INFORMATIVE NOTE: An attacker could send messages with large
2276
numbers of faulty signatures, each of which would require a DNS
2277
lookup and corresponding CPU time to verify the message. This
2278
could be an attack on the domain that receives the message, by
2279
slowing down the verifier by requiring it to do a large number of
2280
DNS lookups and/or signature verifications. It could also be an
2281
attack against the domains listed in the signatures, essentially
2282
by enlisting innocent verifiers in launching an attack against the
2283
DNS servers of the actual victim.
2284
2285
In the following description, text reading "return status
2286
(explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
2287
means that the verifier MUST immediately cease processing that
2288
signature. The verifier SHOULD proceed to the next signature, if any
2289
is present, and completely ignore the bad signature. If the status
2290
is "PERMFAIL", the signature failed and should not be reconsidered.
2291
If the status is "TEMPFAIL", the signature could not be verified at
2292
this time but may be tried again later. A verifier MAY either defer
2293
the message for later processing, perhaps by queueing it locally or
2294
issuing a 451/4.7.5 SMTP reply, or try another signature; if no good
2295
2296
2297
2298
Allman, et al. Standards Track [Page 41]
2299
2300
RFC 4871 DKIM Signatures May 2007
2301
2302
2303
signature is found and any of the signatures resulted in a TEMPFAIL
2304
status, the verifier MAY save the message for later processing. The
2305
"(explanation)" is not normative text; it is provided solely for
2306
clarification.
2307
2308
Verifiers SHOULD ignore any DKIM-Signature header fields where the
2309
signature does not validate. Verifiers that are prepared to validate
2310
multiple signature header fields SHOULD proceed to the next signature
2311
header field, should it exist. However, verifiers MAY make note of
2312
the fact that an invalid signature was present for consideration at a
2313
later step.
2314
2315
INFORMATIVE NOTE: The rationale of this requirement is to permit
2316
messages that have invalid signatures but also a valid signature
2317
to work. For example, a mailing list exploder might opt to leave
2318
the original submitter signature in place even though the exploder
2319
knows that it is modifying the message in some way that will break
2320
that signature, and the exploder inserts its own signature. In
2321
this case, the message should succeed even in the presence of the
2322
known-broken signature.
2323
2324
For each signature to be validated, the following steps should be
2325
performed in such a manner as to produce a result that is
2326
semantically equivalent to performing them in the indicated order.
2327
2328
6.1.1. Validate the Signature Header Field
2329
2330
Implementers MUST meticulously validate the format and values in the
2331
DKIM-Signature header field; any inconsistency or unexpected values
2332
MUST cause the header field to be completely ignored and the verifier
2333
to return PERMFAIL (signature syntax error). Being "liberal in what
2334
you accept" is definitely a bad strategy in this security context.
2335
Note however that this does not include the existence of unknown tags
2336
in a DKIM-Signature header field, which are explicitly permitted.
2337
2338
Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag
2339
that is inconsistent with this specification and return PERMFAIL
2340
(incompatible version).
2341
2342
INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course,
2343
choose to also verify signatures generated by older versions of
2344
this specification.
2345
2346
If any tag listed as "required" in Section 3.5 is omitted from the
2347
DKIM-Signature header field, the verifier MUST ignore the DKIM-
2348
Signature header field and return PERMFAIL (signature missing
2349
required tag).
2350
2351
2352
2353
2354
Allman, et al. Standards Track [Page 42]
2355
2356
RFC 4871 DKIM Signatures May 2007
2357
2358
2359
INFORMATIONAL NOTE: The tags listed as required in Section 3.5 are
2360
"v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there be a
2361
conflict between this note and Section 3.5, Section 3.5 is
2362
normative.
2363
2364
If the DKIM-Signature header field does not contain the "i=" tag, the
2365
verifier MUST behave as though the value of that tag were "@d", where
2366
"d" is the value from the "d=" tag.
2367
2368
Verifiers MUST confirm that the domain specified in the "d=" tag is
2369
the same as or a parent domain of the domain part of the "i=" tag.
2370
If not, the DKIM-Signature header field MUST be ignored and the
2371
verifier should return PERMFAIL (domain mismatch).
2372
2373
If the "h=" tag does not include the From header field, the verifier
2374
MUST ignore the DKIM-Signature header field and return PERMFAIL (From
2375
field not signed).
2376
2377
Verifiers MAY ignore the DKIM-Signature header field and return
2378
PERMFAIL (signature expired) if it contains an "x=" tag and the
2379
signature has expired.
2380
2381
Verifiers MAY ignore the DKIM-Signature header field if the domain
2382
used by the signer in the "d=" tag is not associated with a valid
2383
signing entity. For example, signatures with "d=" values such as
2384
"com" and "co.uk" may be ignored. The list of unacceptable domains
2385
SHOULD be configurable.
2386
2387
Verifiers MAY ignore the DKIM-Signature header field and return
2388
PERMFAIL (unacceptable signature header) for any other reason, for
2389
example, if the signature does not sign header fields that the
2390
verifier views to be essential. As a case in point, if MIME header
2391
fields are not signed, certain attacks may be possible that the
2392
verifier would prefer to avoid.
2393
2394
6.1.2. Get the Public Key
2395
2396
The public key for a signature is needed to complete the verification
2397
process. The process of retrieving the public key depends on the
2398
query type as defined by the "q=" tag in the DKIM-Signature header
2399
field. Obviously, a public key need only be retrieved if the process
2400
of extracting the signature information is completely successful.
2401
Details of key management and representation are described in
2402
Section 3.6. The verifier MUST validate the key record and MUST
2403
ignore any public key records that are malformed.
2404
2405
When validating a message, a verifier MUST perform the following
2406
steps in a manner that is semantically the same as performing them in
2407
2408
2409
2410
Allman, et al. Standards Track [Page 43]
2411
2412
RFC 4871 DKIM Signatures May 2007
2413
2414
2415
the order indicated (in some cases, the implementation may
2416
parallelize or reorder these steps, as long as the semantics remain
2417
unchanged):
2418
2419
1. Retrieve the public key as described in Section 3.6 using the
2420
algorithm in the "q=" tag, the domain from the "d=" tag, and the
2421
selector from the "s=" tag.
2422
2423
2. If the query for the public key fails to respond, the verifier
2424
MAY defer acceptance of this email and return TEMPFAIL (key
2425
unavailable). If verification is occurring during the incoming
2426
SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
2427
code. Alternatively, the verifier MAY store the message in the
2428
local queue for later trial or ignore the signature. Note that
2429
storing a message in the local queue is subject to denial-of-
2430
service attacks.
2431
2432
3. If the query for the public key fails because the corresponding
2433
key record does not exist, the verifier MUST immediately return
2434
PERMFAIL (no key for signature).
2435
2436
4. If the query for the public key returns multiple key records, the
2437
verifier may choose one of the key records or may cycle through
2438
the key records performing the remainder of these steps on each
2439
record at the discretion of the implementer. The order of the
2440
key records is unspecified. If the verifier chooses to cycle
2441
through the key records, then the "return ..." wording in the
2442
remainder of this section means "try the next key record, if any;
2443
if none, return to try another signature in the usual way".
2444
2445
5. If the result returned from the query does not adhere to the
2446
format defined in this specification, the verifier MUST ignore
2447
the key record and return PERMFAIL (key syntax error). Verifiers
2448
are urged to validate the syntax of key records carefully to
2449
avoid attempted attacks. In particular, the verifier MUST ignore
2450
keys with a version code ("v=" tag) that they do not implement.
2451
2452
6. If the "g=" tag in the public key does not match the Local-part
2453
of the "i=" tag in the message signature header field, the
2454
verifier MUST ignore the key record and return PERMFAIL
2455
(inapplicable key). If the Local-part of the "i=" tag on the
2456
message signature is not present, the "g=" tag must be "*" (valid
2457
for all addresses in the domain) or the entire g= tag must be
2458
omitted (which defaults to "g=*"), otherwise the verifier MUST
2459
ignore the key record and return PERMFAIL (inapplicable key).
2460
Other than this test, verifiers SHOULD NOT treat a message signed
2461
with a key record having a "g=" tag any differently than one
2462
without; in particular, verifiers SHOULD NOT prefer messages that
2463
2464
2465
2466
Allman, et al. Standards Track [Page 44]
2467
2468
RFC 4871 DKIM Signatures May 2007
2469
2470
2471
seem to have an individual signature by virtue of a "g=" tag
2472
versus a domain signature.
2473
2474
7. If the "h=" tag exists in the public key record and the hash
2475
algorithm implied by the a= tag in the DKIM-Signature header
2476
field is not included in the contents of the "h=" tag, the
2477
verifier MUST ignore the key record and return PERMFAIL
2478
(inappropriate hash algorithm).
2479
2480
8. If the public key data (the "p=" tag) is empty, then this key has
2481
been revoked and the verifier MUST treat this as a failed
2482
signature check and return PERMFAIL (key revoked). There is no
2483
defined semantic difference between a key that has been revoked
2484
and a key record that has been removed.
2485
2486
9. If the public key data is not suitable for use with the algorithm
2487
and key types defined by the "a=" and "k=" tags in the DKIM-
2488
Signature header field, the verifier MUST immediately return
2489
PERMFAIL (inappropriate key algorithm).
2490
2491
6.1.3. Compute the Verification
2492
2493
Given a signer and a public key, verifying a signature consists of
2494
actions semantically equivalent to the following steps.
2495
2496
1. Based on the algorithm defined in the "c=" tag, the body length
2497
specified in the "l=" tag, and the header field names in the "h="
2498
tag, prepare a canonicalized version of the message as is
2499
described in Section 3.7 (note that this version does not
2500
actually need to be instantiated). When matching header field
2501
names in the "h=" tag against the actual message header field,
2502
comparisons MUST be case-insensitive.
2503
2504
2. Based on the algorithm indicated in the "a=" tag, compute the
2505
message hashes from the canonical copy as described in
2506
Section 3.7.
2507
2508
3. Verify that the hash of the canonicalized message body computed
2509
in the previous step matches the hash value conveyed in the "bh="
2510
tag. If the hash does not match, the verifier SHOULD ignore the
2511
signature and return PERMFAIL (body hash did not verify).
2512
2513
4. Using the signature conveyed in the "b=" tag, verify the
2514
signature against the header hash using the mechanism appropriate
2515
for the public key algorithm described in the "a=" tag. If the
2516
signature does not validate, the verifier SHOULD ignore the
2517
signature and return PERMFAIL (signature did not verify).
2518
2519
2520
2521
2522
Allman, et al. Standards Track [Page 45]
2523
2524
RFC 4871 DKIM Signatures May 2007
2525
2526
2527
5. Otherwise, the signature has correctly verified.
2528
2529
INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to
2530
initiate the public-key query in parallel with calculating the
2531
hash as the public key is not needed until the final decryption is
2532
calculated. Implementations may also verify the signature on the
2533
message header before validating that the message hash listed in
2534
the "bh=" tag in the DKIM-Signature header field matches that of
2535
the actual message body; however, if the body hash does not match,
2536
the entire signature must be considered to have failed.
2537
2538
A body length specified in the "l=" tag of the signature limits the
2539
number of bytes of the body passed to the verification algorithm.
2540
All data beyond that limit is not validated by DKIM. Hence,
2541
verifiers might treat a message that contains bytes beyond the
2542
indicated body length with suspicion, such as by truncating the
2543
message at the indicated body length, declaring the signature invalid
2544
(e.g., by returning PERMFAIL (unsigned content)), or conveying the
2545
partial verification to the policy module.
2546
2547
INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
2548
at the indicated body length might pass on a malformed MIME
2549
message if the signer used the "N-4" trick (omitting the final
2550
"--CRLF") described in the informative note in Section 3.4.5.
2551
Such verifiers may wish to check for this case and include a
2552
trailing "--CRLF" to avoid breaking the MIME structure. A simple
2553
way to achieve this might be to append "--CRLF" to any "multipart"
2554
message with a body length; if the MIME structure is already
2555
correctly formed, this will appear in the postlude and will not be
2556
displayed to the end user.
2557
2558
6.2. Communicate Verification Results
2559
2560
Verifiers wishing to communicate the results of verification to other
2561
parts of the mail system may do so in whatever manner they see fit.
2562
For example, implementations might choose to add an email header
2563
field to the message before passing it on. Any such header field
2564
SHOULD be inserted before any existing DKIM-Signature or preexisting
2565
authentication status header fields in the header field block.
2566
2567
INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
2568
search for results header fields to visibly mark authenticated
2569
mail for end users should verify that such header field was added
2570
by the appropriate verifying domain and that the verified identity
2571
matches the author identity that will be displayed by the MUA. In
2572
particular, MUA filters should not be influenced by bogus results
2573
2574
2575
2576
2577
2578
Allman, et al. Standards Track [Page 46]
2579
2580
RFC 4871 DKIM Signatures May 2007
2581
2582
2583
header fields added by attackers. To circumvent this attack,
2584
verifiers may wish to delete existing results header fields after
2585
verification and before adding a new header field.
2586
2587
6.3. Interpret Results/Apply Local Policy
2588
2589
It is beyond the scope of this specification to describe what actions
2590
a verifier system should make, but an authenticated email presents an
2591
opportunity to a receiving system that unauthenticated email cannot.
2592
Specifically, an authenticated email creates a predictable identifier
2593
by which other decisions can reliably be managed, such as trust and
2594
reputation. Conversely, unauthenticated email lacks a reliable
2595
identifier that can be used to assign trust and reputation. It is
2596
reasonable to treat unauthenticated email as lacking any trust and
2597
having no positive reputation.
2598
2599
In general, verifiers SHOULD NOT reject messages solely on the basis
2600
of a lack of signature or an unverifiable signature; such rejection
2601
would cause severe interoperability problems. However, if the
2602
verifier does opt to reject such messages (for example, when
2603
communicating with a peer who, by prior agreement, agrees to only
2604
send signed messages), and the verifier runs synchronously with the
2605
SMTP session and a signature is missing or does not verify, the MTA
2606
SHOULD use a 550/5.7.x reply code.
2607
2608
If it is not possible to fetch the public key, perhaps because the
2609
key server is not available, a temporary failure message MAY be
2610
generated using a 451/4.7.5 reply code, such as:
2611
2612
451 4.7.5 Unable to verify signature - key server unavailable
2613
2614
Temporary failures such as inability to access the key server or
2615
other external service are the only conditions that SHOULD use a 4xx
2616
SMTP reply code. In particular, cryptographic signature verification
2617
failures MUST NOT return 4xx SMTP replies.
2618
2619
Once the signature has been verified, that information MUST be
2620
conveyed to higher-level systems (such as explicit allow/whitelists
2621
and reputation systems) and/or to the end user. If the message is
2622
signed on behalf of any address other than that in the From: header
2623
field, the mail system SHOULD take pains to ensure that the actual
2624
signing identity is clear to the reader.
2625
2626
The verifier MAY treat unsigned header fields with extreme
2627
skepticism, including marking them as untrusted or even deleting them
2628
before display to the end user.
2629
2630
2631
2632
2633
2634
Allman, et al. Standards Track [Page 47]
2635
2636
RFC 4871 DKIM Signatures May 2007
2637
2638
2639
While the symptoms of a failed verification are obvious -- the
2640
signature doesn't verify -- establishing the exact cause can be more
2641
difficult. If a selector cannot be found, is that because the
2642
selector has been removed, or was the value changed somehow in
2643
transit? If the signature line is missing, is that because it was
2644
never there, or was it removed by an overzealous filter? For
2645
diagnostic purposes, the exact reason why the verification fails
2646
SHOULD be made available to the policy module and possibly recorded
2647
in the system logs. If the email cannot be verified, then it SHOULD
2648
be rendered the same as all unverified email regardless of whether or
2649
not it looks like it was signed.
2650
2651
7. IANA Considerations
2652
2653
DKIM introduces some new namespaces that have been registered with
2654
IANA. In all cases, new values are assigned only for values that
2655
have been documented in a published RFC that has IETF Consensus
2656
[RFC2434].
2657
2658
7.1. DKIM-Signature Tag Specifications
2659
2660
A DKIM-Signature provides for a list of tag specifications. IANA has
2661
established the DKIM-Signature Tag Specification Registry for tag
2662
specifications that can be used in DKIM-Signature fields.
2663
2664
The initial entries in the registry comprise:
2665
2666
+------+-----------------+
2667
| TYPE | REFERENCE |
2668
+------+-----------------+
2669
| v | (this document) |
2670
| a | (this document) |
2671
| b | (this document) |
2672
| bh | (this document) |
2673
| c | (this document) |
2674
| d | (this document) |
2675
| h | (this document) |
2676
| i | (this document) |
2677
| l | (this document) |
2678
| q | (this document) |
2679
| s | (this document) |
2680
| t | (this document) |
2681
| x | (this document) |
2682
| z | (this document) |
2683
+------+-----------------+
2684
2685
DKIM-Signature Tag Specification Registry Initial Values
2686
2687
2688
2689
2690
Allman, et al. Standards Track [Page 48]
2691
2692
RFC 4871 DKIM Signatures May 2007
2693
2694
2695
7.2. DKIM-Signature Query Method Registry
2696
2697
The "q=" tag-spec (specified in Section 3.5) provides for a list of
2698
query methods.
2699
2700
IANA has established the DKIM-Signature Query Method Registry for
2701
mechanisms that can be used to retrieve the key that will permit
2702
validation processing of a message signed using DKIM.
2703
2704
The initial entry in the registry comprises:
2705
2706
+------+--------+-----------------+
2707
| TYPE | OPTION | REFERENCE |
2708
+------+--------+-----------------+
2709
| dns | txt | (this document) |
2710
+------+--------+-----------------+
2711
2712
DKIM-Signature Query Method Registry Initial Values
2713
2714
7.3. DKIM-Signature Canonicalization Registry
2715
2716
The "c=" tag-spec (specified in Section 3.5) provides for a specifier
2717
for canonicalization algorithms for the header and body of the
2718
message.
2719
2720
IANA has established the DKIM-Signature Canonicalization Algorithm
2721
Registry for algorithms for converting a message into a canonical
2722
form before signing or verifying using DKIM.
2723
2724
The initial entries in the header registry comprise:
2725
2726
+---------+-----------------+
2727
| TYPE | REFERENCE |
2728
+---------+-----------------+
2729
| simple | (this document) |
2730
| relaxed | (this document) |
2731
+---------+-----------------+
2732
2733
DKIM-Signature Header Canonicalization Algorithm Registry
2734
Initial Values
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
Allman, et al. Standards Track [Page 49]
2747
2748
RFC 4871 DKIM Signatures May 2007
2749
2750
2751
The initial entries in the body registry comprise:
2752
2753
+---------+-----------------+
2754
| TYPE | REFERENCE |
2755
+---------+-----------------+
2756
| simple | (this document) |
2757
| relaxed | (this document) |
2758
+---------+-----------------+
2759
2760
DKIM-Signature Body Canonicalization Algorithm Registry
2761
Initial Values
2762
2763
7.4. _domainkey DNS TXT Record Tag Specifications
2764
2765
A _domainkey DNS TXT record provides for a list of tag
2766
specifications. IANA has established the DKIM _domainkey DNS TXT Tag
2767
Specification Registry for tag specifications that can be used in DNS
2768
TXT Records.
2769
2770
The initial entries in the registry comprise:
2771
2772
+------+-----------------+
2773
| TYPE | REFERENCE |
2774
+------+-----------------+
2775
| v | (this document) |
2776
| g | (this document) |
2777
| h | (this document) |
2778
| k | (this document) |
2779
| n | (this document) |
2780
| p | (this document) |
2781
| s | (this document) |
2782
| t | (this document) |
2783
+------+-----------------+
2784
2785
DKIM _domainkey DNS TXT Record Tag Specification Registry
2786
Initial Values
2787
2788
7.5. DKIM Key Type Registry
2789
2790
The "k=" <key-k-tag> (specified in Section 3.6.1) and the "a=" <sig-
2791
a-tag-k> (specified in Section 3.5) tags provide for a list of
2792
mechanisms that can be used to decode a DKIM signature.
2793
2794
IANA has established the DKIM Key Type Registry for such mechanisms.
2795
2796
2797
2798
2799
2800
2801
2802
Allman, et al. Standards Track [Page 50]
2803
2804
RFC 4871 DKIM Signatures May 2007
2805
2806
2807
The initial entry in the registry comprises:
2808
2809
+------+-----------+
2810
| TYPE | REFERENCE |
2811
+------+-----------+
2812
| rsa | [RFC3447] |
2813
+------+-----------+
2814
2815
DKIM Key Type Initial Values
2816
2817
7.6. DKIM Hash Algorithms Registry
2818
2819
The "h=" <key-h-tag> (specified in Section 3.6.1) and the "a=" <sig-
2820
a-tag-h> (specified in Section 3.5) tags provide for a list of
2821
mechanisms that can be used to produce a digest of message data.
2822
2823
IANA has established the DKIM Hash Algorithms Registry for such
2824
mechanisms.
2825
2826
The initial entries in the registry comprise:
2827
2828
+--------+-------------------+
2829
| TYPE | REFERENCE |
2830
+--------+-------------------+
2831
| sha1 | [FIPS.180-2.2002] |
2832
| sha256 | [FIPS.180-2.2002] |
2833
+--------+-------------------+
2834
2835
DKIM Hash Algorithms Initial Values
2836
2837
7.7. DKIM Service Types Registry
2838
2839
The "s=" <key-s-tag> tag (specified in Section 3.6.1) provides for a
2840
list of service types to which this selector may apply.
2841
2842
IANA has established the DKIM Service Types Registry for service
2843
types.
2844
2845
The initial entries in the registry comprise:
2846
2847
+-------+-----------------+
2848
| TYPE | REFERENCE |
2849
+-------+-----------------+
2850
| email | (this document) |
2851
| * | (this document) |
2852
+-------+-----------------+
2853
2854
DKIM Service Types Registry Initial Values
2855
2856
2857
2858
Allman, et al. Standards Track [Page 51]
2859
2860
RFC 4871 DKIM Signatures May 2007
2861
2862
2863
7.8. DKIM Selector Flags Registry
2864
2865
The "t=" <key-t-tag> tag (specified in Section 3.6.1) provides for a
2866
list of flags to modify interpretation of the selector.
2867
2868
IANA has established the DKIM Selector Flags Registry for additional
2869
flags.
2870
2871
The initial entries in the registry comprise:
2872
2873
+------+-----------------+
2874
| TYPE | REFERENCE |
2875
+------+-----------------+
2876
| y | (this document) |
2877
| s | (this document) |
2878
+------+-----------------+
2879
2880
DKIM Selector Flags Registry Initial Values
2881
2882
7.9. DKIM-Signature Header Field
2883
2884
IANA has added DKIM-Signature to the "Permanent Message Header
2885
Fields" registry (see [RFC3864]) for the "mail" protocol, using this
2886
document as the reference.
2887
2888
8. Security Considerations
2889
2890
It has been observed that any mechanism that is introduced that
2891
attempts to stem the flow of spam is subject to intensive attack.
2892
DKIM needs to be carefully scrutinized to identify potential attack
2893
vectors and the vulnerability to each. See also [RFC4686].
2894
2895
8.1. Misuse of Body Length Limits ("l=" Tag)
2896
2897
Body length limits (in the form of the "l=" tag) are subject to
2898
several potential attacks.
2899
2900
8.1.1. Addition of New MIME Parts to Multipart/*
2901
2902
If the body length limit does not cover a closing MIME multipart
2903
section (including the trailing "--CRLF" portion), then it is
2904
possible for an attacker to intercept a properly signed multipart
2905
message and add a new body part. Depending on the details of the
2906
MIME type and the implementation of the verifying MTA and the
2907
receiving MUA, this could allow an attacker to change the information
2908
displayed to an end user from an apparently trusted source.
2909
2910
2911
2912
2913
2914
Allman, et al. Standards Track [Page 52]
2915
2916
RFC 4871 DKIM Signatures May 2007
2917
2918
2919
For example, if attackers can append information to a "text/html"
2920
body part, they may be able to exploit a bug in some MUAs that
2921
continue to read after a "</html>" marker, and thus display HTML text
2922
on top of already displayed text. If a message has a
2923
"multipart/alternative" body part, they might be able to add a new
2924
body part that is preferred by the displaying MUA.
2925
2926
8.1.2. Addition of new HTML content to existing content
2927
2928
Several receiving MUA implementations do not cease display after a
2929
""</html>"" tag. In particular, this allows attacks involving
2930
overlaying images on top of existing text.
2931
2932
INFORMATIVE EXAMPLE: Appending the following text to an existing,
2933
properly closed message will in many MUAs result in inappropriate
2934
data being rendered on top of existing, correct data:
2935
<div style="position: relative; bottom: 350px; z-index: 2;">
2936
<img src="http://www.ietf.org/images/ietflogo2e.gif"
2937
width=578 height=370>
2938
</div>
2939
2940
8.2. Misappropriated Private Key
2941
2942
If the private key for a user is resident on their computer and is
2943
not protected by an appropriately secure mechanism, it is possible
2944
for malware to send mail as that user and any other user sharing the
2945
same private key. The malware would not, however, be able to
2946
generate signed spoofs of other signers' addresses, which would aid
2947
in identification of the infected user and would limit the
2948
possibilities for certain types of attacks involving socially
2949
engineered messages. This threat applies mainly to MUA-based
2950
implementations; protection of private keys on servers can be easily
2951
achieved through the use of specialized cryptographic hardware.
2952
2953
A larger problem occurs if malware on many users' computers obtains
2954
the private keys for those users and transmits them via a covert
2955
channel to a site where they can be shared. The compromised users
2956
would likely not know of the misappropriation until they receive
2957
"bounce" messages from messages they are purported to have sent.
2958
Many users might not understand the significance of these bounce
2959
messages and would not take action.
2960
2961
One countermeasure is to use a user-entered passphrase to encrypt the
2962
private key, although users tend to choose weak passphrases and often
2963
reuse them for different purposes, possibly allowing an attack
2964
against DKIM to be extended into other domains. Nevertheless, the
2965
decoded private key might be briefly available to compromise by
2966
malware when it is entered, or might be discovered via keystroke
2967
2968
2969
2970
Allman, et al. Standards Track [Page 53]
2971
2972
RFC 4871 DKIM Signatures May 2007
2973
2974
2975
logging. The added complexity of entering a passphrase each time one
2976
sends a message would also tend to discourage the use of a secure
2977
passphrase.
2978
2979
A somewhat more effective countermeasure is to send messages through
2980
an outgoing MTA that can authenticate the submitter using existing
2981
techniques (e.g., SMTP Authentication), possibly validate the message
2982
itself (e.g., verify that the header is legitimate and that the
2983
content passes a spam content check), and sign the message using a
2984
key appropriate for the submitter address. Such an MTA can also
2985
apply controls on the volume of outgoing mail each user is permitted
2986
to originate in order to further limit the ability of malware to
2987
generate bulk email.
2988
2989
8.3. Key Server Denial-of-Service Attacks
2990
2991
Since the key servers are distributed (potentially separate for each
2992
domain), the number of servers that would need to be attacked to
2993
defeat this mechanism on an Internet-wide basis is very large.
2994
Nevertheless, key servers for individual domains could be attacked,
2995
impeding the verification of messages from that domain. This is not
2996
significantly different from the ability of an attacker to deny
2997
service to the mail exchangers for a given domain, although it
2998
affects outgoing, not incoming, mail.
2999
3000
A variation on this attack is that if a very large amount of mail
3001
were to be sent using spoofed addresses from a given domain, the key
3002
servers for that domain could be overwhelmed with requests. However,
3003
given the low overhead of verification compared with handling of the
3004
email message itself, such an attack would be difficult to mount.
3005
3006
8.4. Attacks Against the DNS
3007
3008
Since the DNS is a required binding for key services, specific
3009
attacks against the DNS must be considered.
3010
3011
While the DNS is currently insecure [RFC3833], these security
3012
problems are the motivation behind DNS Security (DNSSEC) [RFC4033],
3013
and all users of the DNS will reap the benefit of that work.
3014
3015
DKIM is only intended as a "sufficient" method of proving
3016
authenticity. It is not intended to provide strong cryptographic
3017
proof about authorship or contents. Other technologies such as
3018
OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements.
3019
3020
A second security issue related to the DNS revolves around the
3021
increased DNS traffic as a consequence of fetching selector-based
3022
data as well as fetching signing domain policy. Widespread
3023
3024
3025
3026
Allman, et al. Standards Track [Page 54]
3027
3028
RFC 4871 DKIM Signatures May 2007
3029
3030
3031
deployment of DKIM will result in a significant increase in DNS
3032
queries to the claimed signing domain. In the case of forgeries on a
3033
large scale, DNS servers could see a substantial increase in queries.
3034
3035
A specific DNS security issue that should be considered by DKIM
3036
verifiers is the name chaining attack described in Section 2.3 of the
3037
DNS Threat Analysis [RFC3833]. A DKIM verifier, while verifying a
3038
DKIM-Signature header field, could be prompted to retrieve a key
3039
record of an attacker's choosing. This threat can be minimized by
3040
ensuring that name servers, including recursive name servers, used by
3041
the verifier enforce strict checking of "glue" and other additional
3042
information in DNS responses and are therefore not vulnerable to this
3043
attack.
3044
3045
8.5. Replay Attacks
3046
3047
In this attack, a spammer sends a message to be spammed to an
3048
accomplice, which results in the message being signed by the
3049
originating MTA. The accomplice resends the message, including the
3050
original signature, to a large number of recipients, possibly by
3051
sending the message to many compromised machines that act as MTAs.
3052
The messages, not having been modified by the accomplice, have valid
3053
signatures.
3054
3055
Partial solutions to this problem involve the use of reputation
3056
services to convey the fact that the specific email address is being
3057
used for spam and that messages from that signer are likely to be
3058
spam. This requires a real-time detection mechanism in order to
3059
react quickly enough. However, such measures might be prone to
3060
abuse, if for example an attacker resent a large number of messages
3061
received from a victim in order to make them appear to be a spammer.
3062
3063
Large verifiers might be able to detect unusually large volumes of
3064
mails with the same signature in a short time period. Smaller
3065
verifiers can get substantially the same volume of information via
3066
existing collaborative systems.
3067
3068
8.6. Limits on Revoking Keys
3069
3070
When a large domain detects undesirable behavior on the part of one
3071
of its users, it might wish to revoke the key used to sign that
3072
user's messages in order to disavow responsibility for messages that
3073
have not yet been verified or that are the subject of a replay
3074
attack. However, the ability of the domain to do so can be limited
3075
if the same key, for scalability reasons, is used to sign messages
3076
for many other users. Mechanisms for explicitly revoking keys on a
3077
per-address basis have been proposed but require further study as to
3078
their utility and the DNS load they represent.
3079
3080
3081
3082
Allman, et al. Standards Track [Page 55]
3083
3084
RFC 4871 DKIM Signatures May 2007
3085
3086
3087
8.7. Intentionally Malformed Key Records
3088
3089
It is possible for an attacker to publish key records in DNS that are
3090
intentionally malformed, with the intent of causing a denial-of-
3091
service attack on a non-robust verifier implementation. The attacker
3092
could then cause a verifier to read the malformed key record by
3093
sending a message to one of its users referencing the malformed
3094
record in a (not necessarily valid) signature. Verifiers MUST
3095
thoroughly verify all key records retrieved from the DNS and be
3096
robust against intentionally as well as unintentionally malformed key
3097
records.
3098
3099
8.8. Intentionally Malformed DKIM-Signature Header Fields
3100
3101
Verifiers MUST be prepared to receive messages with malformed DKIM-
3102
Signature header fields, and thoroughly verify the header field
3103
before depending on any of its contents.
3104
3105
8.9. Information Leakage
3106
3107
An attacker could determine when a particular signature was verified
3108
by using a per-message selector and then monitoring their DNS traffic
3109
for the key lookup. This would act as the equivalent of a "web bug"
3110
for verification time rather than when the message was read.
3111
3112
8.10. Remote Timing Attacks
3113
3114
In some cases, it may be possible to extract private keys using a
3115
remote timing attack [BONEH03]. Implementations should consider
3116
obfuscating the timing to prevent such attacks.
3117
3118
8.11. Reordered Header Fields
3119
3120
Existing standards allow intermediate MTAs to reorder header fields.
3121
If a signer signs two or more header fields of the same name, this
3122
can cause spurious verification errors on otherwise legitimate
3123
messages. In particular, signers that sign any existing DKIM-
3124
Signature fields run the risk of having messages incorrectly fail to
3125
verify.
3126
3127
8.12. RSA Attacks
3128
3129
An attacker could create a large RSA signing key with a small
3130
exponent, thus requiring that the verification key have a large
3131
exponent. This will force verifiers to use considerable computing
3132
resources to verify the signature. Verifiers might avoid this attack
3133
by refusing to verify signatures that reference selectors with public
3134
keys having unreasonable exponents.
3135
3136
3137
3138
Allman, et al. Standards Track [Page 56]
3139
3140
RFC 4871 DKIM Signatures May 2007
3141
3142
3143
In general, an attacker might try to overwhelm a verifier by flooding
3144
it with messages requiring verification. This is similar to other
3145
MTA denial-of-service attacks and should be dealt with in a similar
3146
fashion.
3147
3148
8.13. Inappropriate Signing by Parent Domains
3149
3150
The trust relationship described in Section 3.8 could conceivably be
3151
used by a parent domain to sign messages with identities in a
3152
subdomain not administratively related to the parent. For example,
3153
the ".com" registry could create messages with signatures using an
3154
"i=" value in the example.com domain. There is no general solution
3155
to this problem, since the administrative cut could occur anywhere in
3156
the domain name. For example, in the domain "example.podunk.ca.us"
3157
there are three administrative cuts (podunk.ca.us, ca.us, and us),
3158
any of which could create messages with an identity in the full
3159
domain.
3160
3161
INFORMATIVE NOTE: This is considered an acceptable risk for the
3162
same reason that it is acceptable for domain delegation. For
3163
example, in the example above any of the domains could potentially
3164
simply delegate "example.podunk.ca.us" to a server of their choice
3165
and completely replace all DNS-served information. Note that a
3166
verifier MAY ignore signatures that come from an unlikely domain
3167
such as ".com", as discussed in Section 6.1.1.
3168
3169
9. References
3170
3171
9.1. Normative References
3172
3173
[FIPS.180-2.2002] U.S. Department of Commerce, "Secure Hash
3174
Standard", FIPS PUB 180-2, August 2002.
3175
3176
[ITU.X660.1997] "Information Technology - ASN.1 encoding rules:
3177
Specification of Basic Encoding Rules (BER),
3178
Canonical Encoding Rules (CER) and Distinguished
3179
Encoding Rules (DER)", ITU-T Recommendation X.660,
3180
1997.
3181
3182
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose
3183
Internet Mail Extensions (MIME) Part One: Format
3184
of Internet Message Bodies", RFC 2045,
3185
November 1996.
3186
3187
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
3188
Extensions) Part Three: Message header field
3189
Extensions for Non-ASCII Text", RFC 2047,
3190
November 1996.
3191
3192
3193
3194
Allman, et al. Standards Track [Page 57]
3195
3196
RFC 4871 DKIM Signatures May 2007
3197
3198
3199
[RFC2119] Bradner, S., "Key words for use in RFCs to
3200
Indicate Requirement Levels", BCP 14, RFC 2119,
3201
March 1997.
3202
3203
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol",
3204
RFC 2821, April 2001.
3205
3206
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
3207
April 2001.
3208
3209
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key
3210
Cryptography Standards (PKCS) #1: RSA Cryptography
3211
Specifications Version 2.1", RFC 3447,
3212
February 2003.
3213
3214
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
3215
"Internationalizing Domain Names in Applications
3216
(IDNA)", RFC 3490, March 2003.
3217
3218
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF
3219
for Syntax Specifications: ABNF", RFC 4234,
3220
October 2005.
3221
3222
9.2. Informative References
3223
3224
[BONEH03] Proc. 12th USENIX Security Symposium, "Remote
3225
Timing Attacks are Practical", 2003.
3226
3227
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
3228
"Security Multiparts for MIME: Multipart/Signed
3229
and Multipart/Encrypted", RFC 1847, October 1995.
3230
3231
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for
3232
Writing an IANA Considerations Section in RFCs",
3233
BCP 26, RFC 2434, October 1998.
3234
3235
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R.
3236
Thayer, "OpenPGP Message Format", RFC 2440,
3237
November 1998.
3238
3239
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths
3240
for Public Keys Used For Exchanging Symmetric
3241
Keys", RFC 3766, April 2004.
3242
3243
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the
3244
Domain Name System (DNS)", RFC 3833, August 2004.
3245
3246
3247
3248
3249
3250
Allman, et al. Standards Track [Page 58]
3251
3252
RFC 4871 DKIM Signatures May 2007
3253
3254
3255
[RFC3851] Ramsdell, B., "S/MIME Version 3 Message
3256
Specification", RFC 3851, June 1999.
3257
3258
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul,
3259
"Registration Procedures for Message Header
3260
Fields", BCP 90, September 2004.
3261
3262
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D.,
3263
and S. Rose, "DNS Security Introduction and
3264
Requirements", RFC 4033, March 2005.
3265
3266
[RFC4686] Fenton, J., "Analysis of Threats Motivating
3267
DomainKeys Identified Mail (DKIM)", RFC 4686,
3268
September 2006.
3269
3270
[RFC4870] Delany, M., "Domain-Based Email Authentication
3271
Using Public Keys Advertised in the DNS
3272
(DomainKeys)", RFC 4870, May 2007.
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
Allman, et al. Standards Track [Page 59]
3307
3308
RFC 4871 DKIM Signatures May 2007
3309
3310
3311
Appendix A. Example of Use (INFORMATIVE)
3312
3313
This section shows the complete flow of an email from submission to
3314
final delivery, demonstrating how the various components fit
3315
together. The key used in this example is shown in Appendix C.
3316
3317
A.1. The User Composes an Email
3318
3319
3320
From: Joe SixPack <joe@football.example.com>
3321
To: Suzie Q <suzie@shopping.example.net>
3322
Subject: Is dinner ready?
3323
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
3324
Message-ID: <20030712040037.46341.5F8J@football.example.com>
3325
3326
Hi.
3327
3328
We lost the game. Are you hungry yet?
3329
3330
Joe.
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
Allman, et al. Standards Track [Page 60]
3363
3364
RFC 4871 DKIM Signatures May 2007
3365
3366
3367
A.2. The Email Is Signed
3368
3369
This email is signed by the example.com outbound email server and now
3370
looks like this:
3371
3372
DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
3373
c=simple/simple; q=dns/txt; i=joe@football.example.com;
3374
h=Received : From : To : Subject : Date : Message-ID;
3375
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
3376
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
3377
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
3378
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
3379
4bmp/YzhwvcubU4=;
3380
Received: from client1.football.example.com [192.0.2.1]
3381
by submitserver.example.com with SUBMISSION;
3382
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
3383
From: Joe SixPack <joe@football.example.com>
3384
To: Suzie Q <suzie@shopping.example.net>
3385
Subject: Is dinner ready?
3386
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
3387
Message-ID: <20030712040037.46341.5F8J@football.example.com>
3388
3389
Hi.
3390
3391
We lost the game. Are you hungry yet?
3392
3393
Joe.
3394
3395
The signing email server requires access to the private key
3396
associated with the "brisbane" selector to generate this signature.
3397
3398
A.3. The Email Signature Is Verified
3399
3400
The signature is normally verified by an inbound SMTP server or
3401
possibly the final delivery agent. However, intervening MTAs can
3402
also perform this verification if they choose to do so. The
3403
verification process uses the domain "example.com" extracted from the
3404
"d=" tag and the selector "brisbane" from the "s=" tag in the DKIM-
3405
Signature header field to form the DNS DKIM query for:
3406
3407
brisbane._domainkey.example.com
3408
3409
Signature verification starts with the physically last Received
3410
header field, the From header field, and so forth, in the order
3411
listed in the "h=" tag. Verification follows with a single CRLF
3412
followed by the body (starting with "Hi."). The email is canonically
3413
prepared for verifying with the "simple" method. The result of the
3414
query and subsequent verification of the signature is stored (in this
3415
3416
3417
3418
Allman, et al. Standards Track [Page 61]
3419
3420
RFC 4871 DKIM Signatures May 2007
3421
3422
3423
example) in the X-Authentication-Results header field line. After
3424
successful verification, the email looks like this:
3425
3426
X-Authentication-Results: shopping.example.net
3427
header.from=joe@football.example.com; dkim=pass
3428
Received: from mout23.football.example.com (192.168.1.1)
3429
by shopping.example.net with SMTP;
3430
Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
3431
DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
3432
c=simple/simple; q=dns/txt; i=joe@football.example.com;
3433
h=Received : From : To : Subject : Date : Message-ID;
3434
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
3435
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
3436
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
3437
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
3438
4bmp/YzhwvcubU4=;
3439
Received: from client1.football.example.com [192.0.2.1]
3440
by submitserver.example.com with SUBMISSION;
3441
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
3442
From: Joe SixPack <joe@football.example.com>
3443
To: Suzie Q <suzie@shopping.example.net>
3444
Subject: Is dinner ready?
3445
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
3446
Message-ID: <20030712040037.46341.5F8J@football.example.com>
3447
3448
Hi.
3449
3450
We lost the game. Are you hungry yet?
3451
3452
Joe.
3453
3454
Appendix B. Usage Examples (INFORMATIVE)
3455
3456
DKIM signing and validating can be used in different ways, for
3457
different operational scenarios. This Appendix discusses some common
3458
examples.
3459
3460
NOTE: Descriptions in this Appendix are for informational purposes
3461
only. They describe various ways that DKIM can be used, given
3462
particular constraints and needs. In no case are these examples
3463
intended to be taken as providing explanation or guidance
3464
concerning DKIM specification details, when creating an
3465
implementation.
3466
3467
3468
3469
3470
3471
3472
3473
3474
Allman, et al. Standards Track [Page 62]
3475
3476
RFC 4871 DKIM Signatures May 2007
3477
3478
3479
B.1. Alternate Submission Scenarios
3480
3481
In the most simple scenario, a user's MUA, MSA, and Internet
3482
(boundary) MTA are all within the same administrative environment,
3483
using the same domain name. Therefore, all of the components
3484
involved in submission and initial transfer are related. However, it
3485
is common for two or more of the components to be under independent
3486
administrative control. This creates challenges for choosing and
3487
administering the domain name to use for signing, and for its
3488
relationship to common email identity header fields.
3489
3490
B.1.1. Delegated Business Functions
3491
3492
Some organizations assign specific business functions to discrete
3493
groups, inside or outside the organization. The goal, then, is to
3494
authorize that group to sign some mail, but to constrain what
3495
signatures they can generate. DKIM selectors (the "s=" signature
3496
tag) and granularity (the "g=" key tag) facilitate this kind of
3497
restricted authorization. Examples of these outsourced business
3498
functions are legitimate email marketing providers and corporate
3499
benefits providers.
3500
3501
Here, the delegated group needs to be able to send messages that are
3502
signed, using the email domain of the client company. At the same
3503
time, the client often is reluctant to register a key for the
3504
provider that grants the ability to send messages for arbitrary
3505
addresses in the domain.
3506
3507
There are multiple ways to administer these usage scenarios. In one
3508
case, the client organization provides all of the public query
3509
service (for example, DNS) administration, and in another it uses DNS
3510
delegation to enable all ongoing administration of the DKIM key
3511
record by the delegated group.
3512
3513
If the client organization retains responsibility for all of the DNS
3514
administration, the outsourcing company can generate a key pair,
3515
supplying the public key to the client company, which then registers
3516
it in the query service, using a unique selector that authorizes a
3517
specific From header field Local-part. For example, a client with
3518
the domain "example.com" could have the selector record specify
3519
"g=winter-promotions" so that this signature is only valid for mail
3520
with a From address of "winter-promotions@example.com". This would
3521
enable the provider to send messages using that specific address and
3522
have them verify properly. The client company retains control over
3523
the email address because it retains the ability to revoke the key at
3524
any time.
3525
3526
3527
3528
3529
3530
Allman, et al. Standards Track [Page 63]
3531
3532
RFC 4871 DKIM Signatures May 2007
3533
3534
3535
If the client wants the delegated group to do the DNS administration,
3536
it can have the domain name that is specified with the selector point
3537
to the provider's DNS server. The provider then creates and
3538
maintains all of the DKIM signature information for that selector.
3539
Hence, the client cannot provide constraints on the Local-part of
3540
addresses that get signed, but it can revoke the provider's signing
3541
rights by removing the DNS delegation record.
3542
3543
B.1.2. PDAs and Similar Devices
3544
3545
PDAs demonstrate the need for using multiple keys per domain.
3546
Suppose that John Doe wanted to be able to send messages using his
3547
corporate email address, jdoe@example.com, and his email device did
3548
not have the ability to make a Virtual Private Network (VPN)
3549
connection to the corporate network, either because the device is
3550
limited or because there are restrictions enforced by his Internet
3551
access provider. If the device was equipped with a private key
3552
registered for jdoe@example.com by the administrator of the
3553
example.com domain, and appropriate software to sign messages, John
3554
could sign the message on the device itself before transmission
3555
through the outgoing network of the access service provider.
3556
3557
B.1.3. Roaming Users
3558
3559
Roaming users often find themselves in circumstances where it is
3560
convenient or necessary to use an SMTP server other than their home
3561
server; examples are conferences and many hotels. In such
3562
circumstances, a signature that is added by the submission service
3563
will use an identity that is different from the user's home system.
3564
3565
Ideally, roaming users would connect back to their home server using
3566
either a VPN or a SUBMISSION server running with SMTP AUTHentication
3567
on port 587. If the signing can be performed on the roaming user's
3568
laptop, then they can sign before submission, although the risk of
3569
further modification is high. If neither of these are possible,
3570
these roaming users will not be able to send mail signed using their
3571
own domain key.
3572
3573
B.1.4. Independent (Kiosk) Message Submission
3574
3575
Stand-alone services, such as walk-up kiosks and web-based
3576
information services, have no enduring email service relationship
3577
with the user, but users occasionally request that mail be sent on
3578
their behalf. For example, a website providing news often allows the
3579
reader to forward a copy of the article to a friend. This is
3580
typically done using the reader's own email address, to indicate who
3581
the author is. This is sometimes referred to as the "Evite problem",
3582
3583
3584
3585
3586
Allman, et al. Standards Track [Page 64]
3587
3588
RFC 4871 DKIM Signatures May 2007
3589
3590
3591
named after the website of the same name that allows a user to send
3592
invitations to friends.
3593
3594
A common way this is handled is to continue to put the reader's email
3595
address in the From header field of the message, but put an address
3596
owned by the email posting site into the Sender header field. The
3597
posting site can then sign the message, using the domain that is in
3598
the Sender field. This provides useful information to the receiving
3599
email site, which is able to correlate the signing domain with the
3600
initial submission email role.
3601
3602
Receiving sites often wish to provide their end users with
3603
information about mail that is mediated in this fashion. Although
3604
the real efficacy of different approaches is a subject for human
3605
factors usability research, one technique that is used is for the
3606
verifying system to rewrite the From header field, to indicate the
3607
address that was verified. For example: From: John Doe via
3608
news@news-site.com <jdoe@example.com>. (Note that such rewriting
3609
will break a signature, unless it is done after the verification pass
3610
is complete.)
3611
3612
B.2. Alternate Delivery Scenarios
3613
3614
Email is often received at a mailbox that has an address different
3615
from the one used during initial submission. In these cases, an
3616
intermediary mechanism operates at the address originally used and it
3617
then passes the message on to the final destination. This mediation
3618
process presents some challenges for DKIM signatures.
3619
3620
B.2.1. Affinity Addresses
3621
3622
"Affinity addresses" allow a user to have an email address that
3623
remains stable, even as the user moves among different email
3624
providers. They are typically associated with college alumni
3625
associations, professional organizations, and recreational
3626
organizations with which they expect to have a long-term
3627
relationship. These domains usually provide forwarding of incoming
3628
email, and they often have an associated Web application that
3629
authenticates the user and allows the forwarding address to be
3630
changed. However, these services usually depend on users sending
3631
outgoing messages through their own service providers' MTAs. Hence,
3632
mail that is signed with the domain of the affinity address is not
3633
signed by an entity that is administered by the organization owning
3634
that domain.
3635
3636
With DKIM, affinity domains could use the Web application to allow
3637
users to register per-user keys to be used to sign messages on behalf
3638
of their affinity address. The user would take away the secret half
3639
3640
3641
3642
Allman, et al. Standards Track [Page 65]
3643
3644
RFC 4871 DKIM Signatures May 2007
3645
3646
3647
of the key pair for signing, and the affinity domain would publish
3648
the public half in DNS for access by verifiers.
3649
3650
This is another application that takes advantage of user-level
3651
keying, and domains used for affinity addresses would typically have
3652
a very large number of user-level keys. Alternatively, the affinity
3653
domain could handle outgoing mail, operating a mail submission agent
3654
that authenticates users before accepting and signing messages for
3655
them. This is of course dependent on the user's service provider not
3656
blocking the relevant TCP ports used for mail submission.
3657
3658
B.2.2. Simple Address Aliasing (.forward)
3659
3660
In some cases, a recipient is allowed to configure an email address
3661
to cause automatic redirection of email messages from the original
3662
address to another, such as through the use of a Unix .forward file.
3663
In this case, messages are typically redirected by the mail handling
3664
service of the recipient's domain, without modification, except for
3665
the addition of a Received header field to the message and a change
3666
in the envelope recipient address. In this case, the recipient at
3667
the final address' mailbox is likely to be able to verify the
3668
original signature since the signed content has not changed, and DKIM
3669
is able to validate the message signature.
3670
3671
B.2.3. Mailing Lists and Re-Posters
3672
3673
There is a wide range of behaviors in services that take delivery of
3674
a message and then resubmit it. A primary example is with mailing
3675
lists (collectively called "forwarders" below), ranging from those
3676
that make no modification to the message itself, other than to add a
3677
Received header field and change the envelope information, to those
3678
that add header fields, change the Subject header field, add content
3679
to the body (typically at the end), or reformat the body in some
3680
manner. The simple ones produce messages that are quite similar to
3681
the automated alias services. More elaborate systems essentially
3682
create a new message.
3683
3684
A Forwarder that does not modify the body or signed header fields of
3685
a message is likely to maintain the validity of the existing
3686
signature. It also could choose to add its own signature to the
3687
message.
3688
3689
Forwarders which modify a message in a way that could make an
3690
existing signature invalid are particularly good candidates for
3691
adding their own signatures (e.g., mailing-list-name@example.net).
3692
Since (re-)signing is taking responsibility for the content of the
3693
message, these signing forwarders are likely to be selective, and
3694
forward or re-sign a message only if it is received with a valid
3695
3696
3697
3698
Allman, et al. Standards Track [Page 66]
3699
3700
RFC 4871 DKIM Signatures May 2007
3701
3702
3703
signature or if they have some other basis for knowing that the
3704
message is not spoofed.
3705
3706
A common practice among systems that are primarily redistributors of
3707
mail is to add a Sender header field to the message, to identify the
3708
address being used to sign the message. This practice will remove
3709
any preexisting Sender header field as required by [RFC2822]. The
3710
forwarder applies a new DKIM-Signature header field with the
3711
signature, public key, and related information of the forwarder.
3712
3713
Appendix C. Creating a Public Key (INFORMATIVE)
3714
3715
The default signature is an RSA signed SHA256 digest of the complete
3716
email. For ease of explanation, the openssl command is used to
3717
describe the mechanism by which keys and signatures are managed. One
3718
way to generate a 1024-bit, unencrypted private key suitable for DKIM
3719
is to use openssl like this:
3720
3721
$ openssl genrsa -out rsa.private 1024
3722
3723
For increased security, the "-passin" parameter can also be added to
3724
encrypt the private key. Use of this parameter will require entering
3725
a password for several of the following steps. Servers may prefer to
3726
use hardware cryptographic support.
3727
3728
The "genrsa" step results in the file rsa.private containing the key
3729
information similar to this:
3730
3731
-----BEGIN RSA PRIVATE KEY-----
3732
MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC
3733
jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb
3734
to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB
3735
AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX
3736
/1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ
3737
gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO
3738
n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m
3739
3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/
3740
eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj
3741
7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA
3742
qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf
3743
eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX
3744
GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc=
3745
-----END RSA PRIVATE KEY-----
3746
3747
To extract the public-key component from the private key, use openssl
3748
like this:
3749
3750
$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
3751
3752
3753
3754
Allman, et al. Standards Track [Page 67]
3755
3756
RFC 4871 DKIM Signatures May 2007
3757
3758
3759
This results in the file rsa.public containing the key information
3760
similar to this:
3761
3762
-----BEGIN PUBLIC KEY-----
3763
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM
3764
oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R
3765
tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI
3766
MmPSPDdQPNUYckcQ2QIDAQAB
3767
-----END PUBLIC KEY-----
3768
3769
This public-key data (without the BEGIN and END tags) is placed in
3770
the DNS:
3771
3772
brisbane IN TXT ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ"
3773
"KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt"
3774
"IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v"
3775
"/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi"
3776
"tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB")
3777
3778
Appendix D. MUA Considerations
3779
3780
When a DKIM signature is verified, the processing system sometimes
3781
makes the result available to the recipient user's MUA. How to
3782
present this information to the user in a way that helps them is a
3783
matter of continuing human factors usability research. The tendency
3784
is to have the MUA highlight the address associated with this signing
3785
identity in some way, in an attempt to show the user the address from
3786
which the mail was sent. An MUA might do this with visual cues such
3787
as graphics, or it might include the address in an alternate view, or
3788
it might even rewrite the original From address using the verified
3789
information. Some MUAs might indicate which header fields were
3790
protected by the validated DKIM signature. This could be done with a
3791
positive indication on the signed header fields, with a negative
3792
indication on the unsigned header fields, by visually hiding the
3793
unsigned header fields, or some combination of these. If an MUA uses
3794
visual indications for signed header fields, the MUA probably needs
3795
to be careful not to display unsigned header fields in a way that
3796
might be construed by the end user as having been signed. If the
3797
message has an l= tag whose value does not extend to the end of the
3798
message, the MUA might also hide or mark the portion of the message
3799
body that was not signed.
3800
3801
The aforementioned information is not intended to be exhaustive. The
3802
MUA may choose to highlight, accentuate, hide, or otherwise display
3803
any other information that may, in the opinion of the MUA author, be
3804
deemed important to the end user.
3805
3806
3807
3808
3809
3810
Allman, et al. Standards Track [Page 68]
3811
3812
RFC 4871 DKIM Signatures May 2007
3813
3814
3815
Appendix E. Acknowledgements
3816
3817
The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
3818
Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin,
3819
Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman,
3820
Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto,
3821
Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur
3822
Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman,
3823
Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig
3824
Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S.
3825
Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon
3826
Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau,
3827
Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada,
3828
Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud,
3829
Scott Renfro, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector
3830
Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert
3831
Sanders, Rand Wacker, Sam Weiler, and Dan Wing for their valuable
3832
suggestions and constructive criticism.
3833
3834
The DomainKeys specification was a primary source from which this
3835
specification has been derived. Further information about DomainKeys
3836
is at [RFC4870].
3837
3838
Authors' Addresses
3839
3840
Eric Allman
3841
Sendmail, Inc.
3842
6425 Christie Ave, Suite 400
3843
Emeryville, CA 94608
3844
USA
3845
3846
Phone: +1 510 594 5501
3847
EMail: eric+dkim@sendmail.org
3848
URI:
3849
3850
3851
Jon Callas
3852
PGP Corporation
3853
3460 West Bayshore
3854
Palo Alto, CA 94303
3855
USA
3856
3857
Phone: +1 650 319 9016
3858
EMail: jon@pgp.com
3859
3860
3861
3862
3863
3864
3865
3866
Allman, et al. Standards Track [Page 69]
3867
3868
RFC 4871 DKIM Signatures May 2007
3869
3870
3871
Mark Delany
3872
Yahoo! Inc
3873
701 First Avenue
3874
Sunnyvale, CA 95087
3875
USA
3876
3877
Phone: +1 408 349 6831
3878
EMail: markd+dkim@yahoo-inc.com
3879
URI:
3880
3881
3882
Miles Libbey
3883
Yahoo! Inc
3884
701 First Avenue
3885
Sunnyvale, CA 95087
3886
USA
3887
3888
EMail: mlibbeymail-mailsig@yahoo.com
3889
URI:
3890
3891
3892
Jim Fenton
3893
Cisco Systems, Inc.
3894
MS SJ-9/2
3895
170 W. Tasman Drive
3896
San Jose, CA 95134-1706
3897
USA
3898
3899
Phone: +1 408 526 5914
3900
EMail: fenton@cisco.com
3901
URI:
3902
3903
3904
Michael Thomas
3905
Cisco Systems, Inc.
3906
MS SJ-9/2
3907
170 W. Tasman Drive
3908
San Jose, CA 95134-1706
3909
3910
Phone: +1 408 525 5386
3911
EMail: mat@cisco.com
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
Allman, et al. Standards Track [Page 70]
3923
3924
RFC 4871 DKIM Signatures May 2007
3925
3926
3927
Full Copyright Statement
3928
3929
Copyright (C) The IETF Trust (2007).
3930
3931
This document is subject to the rights, licenses and restrictions
3932
contained in BCP 78, and except as set forth therein, the authors
3933
retain all their rights.
3934
3935
This document and the information contained herein are provided on an
3936
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
3937
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
3938
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
3939
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
3940
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
3941
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
3942
3943
Intellectual Property
3944
3945
The IETF takes no position regarding the validity or scope of any
3946
Intellectual Property Rights or other rights that might be claimed to
3947
pertain to the implementation or use of the technology described in
3948
this document or the extent to which any license under such rights
3949
might or might not be available; nor does it represent that it has
3950
made any independent effort to identify any such rights. Information
3951
on the procedures with respect to rights in RFC documents can be
3952
found in BCP 78 and BCP 79.
3953
3954
Copies of IPR disclosures made to the IETF Secretariat and any
3955
assurances of licenses to be made available, or the result of an
3956
attempt made to obtain a general license or permission for the use of
3957
such proprietary rights by implementers or users of this
3958
specification can be obtained from the IETF on-line IPR repository at
3959
http://www.ietf.org/ipr.
3960
3961
The IETF invites any interested party to bring to its attention any
3962
copyrights, patents or patent applications, or other proprietary
3963
rights that may cover technology that may be required to implement
3964
this standard. Please address the information to the IETF at
3965
ietf-ipr@ietf.org.
3966
3967
Acknowledgement
3968
3969
Funding for the RFC Editor function is currently provided by the
3970
Internet Society.
3971
3972
3973
3974
3975
3976
3977
3978
Allman, et al. Standards Track [Page 71]
3979
Loggerhead is a web-based interface for Breezy
Version: 2.0.1