--- 1/draft-ietf-dkim-rfc4871bis-08.txt 2011-05-02 04:16:27.000000000 +0200 +++ 2/draft-ietf-dkim-rfc4871bis-09.txt 2011-05-02 04:16:27.000000000 +0200 @@ -1,167 +1,163 @@ Network Working Group D. Crocker, Ed. Internet-Draft Brandenburg InternetWorking Obsoletes: 4871, 5672 T. Hansen, Ed. (if approved) AT&T Laboratories Intended status: Standards Track M. Kucherawy, Ed. -Expires: October 29, 2011 Cloudmark - April 27, 2011 +Expires: November 2, 2011 Cloudmark + May 1, 2011 DomainKeys Identified Mail (DKIM) Signatures - draft-ietf-dkim-rfc4871bis-08 + draft-ietf-dkim-rfc4871bis-09 Abstract DomainKeys Identified Mail (DKIM) permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message. This can be an author's organization, an operational relay or one of their agents. DKIM separates the question of the identity of the signer of the message from the purported author of the message. Assertion of responsibility is validated through a cryptographic signature and querying the signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. - This memo obsoletes RFC4871 and RFC5672 {DKIM 14}. + This memo obsoletes RFC4871 and RFC5672. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on October 29, 2011. + This Internet-Draft will expire on November 2, 2011. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents - 1. Notes to Editor and Reviewers . . . . . . . . . . . . . . . . 5 - 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 2.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 - 2.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 - 2.3. Simple Key Management . . . . . . . . . . . . . . . . . . 6 - 2.4. Data Integrity . . . . . . . . . . . . . . . . . . . . . . 6 - 3. Terminology and Definitions . . . . . . . . . . . . . . . . . 7 - 3.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7 - 3.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 - 3.3. Identity . . . . . . . . . . . . . . . . . . . . . . . . . 7 - 3.4. Identifier . . . . . . . . . . . . . . . . . . . . . . . . 8 - 3.5. Signing Domain Identifier (SDID) . . . . . . . . . . . . . 8 - 3.6. Agent or User Identifier (AUID) . . . . . . . . . . . . . 8 - 3.7. Identity Assessor . . . . . . . . . . . . . . . . . . . . 8 - 3.8. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 8 - 3.9. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 9 - 3.10. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 9 - 3.11. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 10 - 4. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 11 - 4.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 11 - 4.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 13 - 4.3. Signing and Verification Algorithms . . . . . . . . . . . 14 - 4.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 15 - 4.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 20 - 4.6. Key Management and Representation . . . . . . . . . . . . 29 - 4.7. Computing the Message Hashes . . . . . . . . . . . . . . . 33 - 4.8. Input Requirements . . . . . . . . . . . . . . . . . . . . 35 - 4.9. Signing by Parent Domains . . . . . . . . . . . . . . . . 36 - 4.10. Relationship between SDID and AUID . . . . . . . . . . . . 36 - 5. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 37 - 5.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 37 - 5.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 38 - 6. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 39 - 6.1. Determine Whether the Email Should Be Signed and by + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 1.1. DKIM Architecture Documents . . . . . . . . . . . . . . . 6 + 1.2. Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 + 1.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 + 1.4. Simple Key Management . . . . . . . . . . . . . . . . . . 6 + 1.5. Data Integrity . . . . . . . . . . . . . . . . . . . . . . 7 + 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 7 + 2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 + 2.3. Identity . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 2.4. Identifier . . . . . . . . . . . . . . . . . . . . . . . . 8 + 2.5. Signing Domain Identifier (SDID) . . . . . . . . . . . . . 8 + 2.6. Agent or User Identifier (AUID) . . . . . . . . . . . . . 8 + 2.7. Identity Assessor . . . . . . . . . . . . . . . . . . . . 8 + 2.8. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 8 + 2.9. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 9 + 2.10. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 9 + 2.11. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 10 + 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 11 + 3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 11 + 3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 13 + 3.3. Signing and Verification Algorithms . . . . . . . . . . . 14 + 3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 15 + 3.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 19 + 3.6. Key Management and Representation . . . . . . . . . . . . 28 + 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 32 + 3.8. Input Requirements . . . . . . . . . . . . . . . . . . . . 35 + 3.9. Output Requirements . . . . . . . . . . . . . . . . . . . 35 + 3.10. Signing by Parent Domains . . . . . . . . . . . . . . . . 35 + 3.11. Relationship between SDID and AUID . . . . . . . . . . . . 36 + 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 37 + 4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 37 + 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 38 + + 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 39 + 5.1. Determine Whether the Email Should Be Signed and by Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 - 6.2. Select a Private Key and Corresponding Selector + 5.2. Select a Private Key and Corresponding Selector Information . . . . . . . . . . . . . . . . . . . . . . . 40 - 6.3. Normalize the Message to Prevent Transport Conversions . . 40 - 6.4. Determine the Header Fields to Sign . . . . . . . . . . . 41 - 6.5. Recommended Signature Content . . . . . . . . . . . . . . 43 - 6.6. Compute the Message Hash and Signature . . . . . . . . . . 45 - 6.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 45 - 7. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 46 - 7.1. Extract Signatures from the Message . . . . . . . . . . . 46 - 7.2. Communicate Verification Results . . . . . . . . . . . . . 51 - 7.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 52 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53 - 8.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 53 - 8.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 54 - 8.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 54 - 8.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 55 - 8.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 56 - 8.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 56 - 8.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 56 - 8.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 57 - 8.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 57 - 9. Security Considerations . . . . . . . . . . . . . . . . . . . 57 - 9.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 57 - 9.2. Misappropriated Private Key . . . . . . . . . . . . . . . 58 - 9.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 59 - 9.4. Attacks Against the DNS . . . . . . . . . . . . . . . . . 59 - 9.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 60 - 9.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 61 - 9.7. Intentionally Malformed Key Records . . . . . . . . . . . 61 - 9.8. Intentionally Malformed DKIM-Signature Header Fields . . . 61 - 9.9. Information Leakage . . . . . . . . . . . . . . . . . . . 61 - 9.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 61 - 9.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 61 - 9.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 62 - 9.13. Inappropriate Signing by Parent Domains . . . . . . . . . 62 - 9.14. Attacks Involving Addition of Header Fields . . . . . . . 62 - 9.15. Malformed Inputs . . . . . . . . . . . . . . . . . . . . . 63 - 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 64 - 10.1. Normative References . . . . . . . . . . . . . . . . . . . 64 - 10.2. Informative References . . . . . . . . . . . . . . . . . . 65 - Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 66 - A.1. The User Composes an Email . . . . . . . . . . . . . . . . 66 - A.2. The Email is Signed . . . . . . . . . . . . . . . . . . . 67 - A.3. The Email Signature is Verified . . . . . . . . . . . . . 68 - Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 69 - B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 69 - B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 71 - Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 73 - C.1. Compatibility with DomainKeys Key Records . . . . . . . . 74 - Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 74 - Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 75 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 76 - -1. Notes to Editor and Reviewers - - This version of the memo contains notations such as "{DKIM 2}". - These correspond to DKIM working group issue tracker items. They - should be deleted prior to publication. + 5.3. Normalize the Message to Prevent Transport Conversions . . 40 + 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 41 + 5.5. Recommended Signature Content . . . . . . . . . . . . . . 43 + 5.6. Compute the Message Hash and Signature . . . . . . . . . . 45 + 5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 45 + 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 45 + 6.1. Extract Signatures from the Message . . . . . . . . . . . 46 + 6.2. Communicate Verification Results . . . . . . . . . . . . . 51 + 6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 52 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53 + 7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 53 + 7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 54 + 7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 54 + 7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 55 + 7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 55 + 7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 56 + 7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 56 + 7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 56 + 7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 57 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 57 + 8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 57 + 8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 57 + 8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 58 + 8.4. Attacks Against the DNS . . . . . . . . . . . . . . . . . 58 + 8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 59 + 8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 59 + 8.7. Intentionally Malformed Key Records . . . . . . . . . . . 60 + 8.8. Intentionally Malformed DKIM-Signature Header Fields . . . 60 + 8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 60 + 8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 60 + 8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 60 + 8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 61 + 8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 61 + 8.14. Attacks Involving Addition of Header Fields . . . . . . . 61 + 8.15. Malformed Inputs . . . . . . . . . . . . . . . . . . . . . 62 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 63 + 9.1. Normative References . . . . . . . . . . . . . . . . . . . 63 + 9.2. Informative References . . . . . . . . . . . . . . . . . . 64 + Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 65 + A.1. The User Composes an Email . . . . . . . . . . . . . . . . 65 + A.2. The Email is Signed . . . . . . . . . . . . . . . . . . . 66 + A.3. The Email Signature is Verified . . . . . . . . . . . . . 67 + Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 68 + B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 68 + B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 70 + Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 72 + C.1. Compatibility with DomainKeys Key Records . . . . . . . . 73 + Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 73 + Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 74 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 75 -2. Introduction +1. Introduction DomainKeys Identified Mail (DKIM) permits a person, role, or organization to claim some responsibility for a message by associating a domain name [RFC1034] with the message [RFC5322], which they are authorized to use. This can be an author's organization, an operational relay or one of their agents. Assertion of responsibility is validated through a cryptographic signature and querying the signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message @@ -198,135 +194,142 @@ o requires minimal new infrastructure; o can be implemented independently of clients in order to reduce deployment time; o can be deployed incrementally; o allows delegation of signing to third parties. -2.1. Signing Identity +1.1. DKIM Architecture Documents + + Readers are advised to be familiar with the material in [RFC4686], + [RFC5585] and [RFC5863], which respectively provide the background + for the development of DKIM, an overview of the service, and + deployment and operations guidance and advice. + +1.2. Signing Identity DKIM separates the question of the identity of the signer of the message from the purported author of the message. In particular, a signature includes the identity of the signer. Verifiers can use the signing information to decide how they want to process the message. The signing identity is included as part of the signature header field. INFORMATIVE RATIONALE: The signing identity specified by a DKIM signature is not required to match an address in any particular header field because of the broad methods of interpretation by recipient mail systems, including MUAs. -2.2. Scalability +1.3. Scalability DKIM is designed to support the extreme scalability requirements that characterize the email identification problem. There are currently over 70 million domains and a much larger number of individual addresses. DKIM seeks to preserve the positive aspects of the current email infrastructure, such as the ability for anyone to communicate with anyone else without introduction. -2.3. Simple Key Management +1.4. Simple Key Management DKIM differs from traditional hierarchical public-key systems in that no Certificate Authority infrastructure is required; the verifier requests the public key from a repository in the domain of the claimed signer directly rather than from a third party. The DNS is proposed as the initial mechanism for the public keys. Thus, DKIM currently depends on DNS administration and the security of the DNS system. DKIM is designed to be extensible to other key fetching services as they become available. -2.4. Data Integrity +1.5. Data Integrity A DKIM signature associates the d= name with the computed hash of some or all of the message (see Section 3.7) in order to prevent the re-use of the signature with different messages. Verifying the signature asserts that the hashed content has not changed since it was signed, and asserts nothing else about "protecting" the end-to- end integrity of the message. -3. Terminology and Definitions +2. Terminology and Definitions This section defines terms used in the rest of the document. DKIM is designed to operate within the Internet Mail service, as defined in [RFC5598]. Basic email terminology is taken from that specification. Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. -3.1. Signers +2.1. Signers Elements in the mail system that sign messages on behalf of a domain are referred to as signers. These may be MUAs (Mail User Agents), MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other agents such as mailing list exploders. In general, any signer will be involved in the injection of a message into the message system in some way. The key issue is that a message must be signed before it leaves the administrative domain of the signer. -3.2. Verifiers +2.2. Verifiers Elements in the mail system that verify signatures are referred to as verifiers. These may be MTAs, Mail Delivery Agents (MDAs), or MUAs. In most cases it is expected that verifiers will be close to an end user (reader) of the message or some consuming agent such as a mailing list exploder. -3.3. Identity +2.3. Identity A person, role, or organization. In the context of DKIM, examples include the author, the author's organization, an ISP along the handling path, an independent trust assessment service, and a mailing list operator. -3.4. Identifier +2.4. Identifier A label that refers to an identity. -3.5. Signing Domain Identifier (SDID) +2.5. Signing Domain Identifier (SDID) A single domain name that is the mandatory payload output of DKIM and that refers to the identity claiming some responsibility for the - message by signing it. It is specified in Section 4.5. + message by signing it. It is specified in Section 3.5. -3.6. Agent or User Identifier (AUID) +2.6. Agent or User Identifier (AUID) A single identifier that refers to the agent or user on behalf of whom the Signing Domain Identifier (SDID) has taken responsibility. The AUID comprises a domain name and an optional . The domain name is the same as that used for the SDID or is a sub-domain of it. For DKIM processing, the domain name portion of the AUID has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. It is specified in - Section 4.5 . + Section 3.5 . -3.7. Identity Assessor +2.7. Identity Assessor A module that consumes DKIM's mandatory payload, which is the responsible Signing Domain Identifier (SDID). The module is dedicated to the assessment of the delivered identifier. Other DKIM (and non-DKIM) values can also be delivered to this module as well as to a more general message evaluation filtering engine. However, this additional activity is outside the scope of the DKIM signature specification. -3.8. Whitespace +2.8. Whitespace There are three forms of whitespace: o WSP represents simple whitespace, i.e., a space or a tab character (formal definition in [RFC5234]). o LWSP is linear whitespace, defined as WSP plus CRLF (formal definition in [RFC5234]). o FWS is folding whitespace. It allows multiple lines separated by @@ -334,21 +337,21 @@ The formal ABNF for these are (WSP and LWSP are given for information only): WSP = SP / HTAB LWSP = *(WSP / CRLF WSP) FWS = [*WSP CRLF] 1*WSP The definition of FWS is identical to that in [RFC5322] except for the exclusion of obs-FWS. -3.9. Imported ABNF Tokens +2.9. Imported ABNF Tokens The following tokens are imported from other RFCs as noted. Those RFCs should be considered definitive. The following tokens are imported from [RFC5321]: o "Local-part" (implementation warning: this permits quoted strings) o "sub-domain" @@ -365,30 +368,30 @@ o "hex-octet" (a quoted-printable encoded octet) INFORMATIVE NOTE: Be aware that the ABNF in [RFC2045] does not obey the rules of [RFC5234] and must be interpreted accordingly, particularly as regards case folding. Other tokens not defined herein are imported from [RFC5234]. These are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF, etc. -3.10. Common ABNF Tokens +2.10. Common ABNF Tokens The following ABNF tokens are used elsewhere in this document: hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ] ALPHADIGITPS = (ALPHA / DIGIT / "+" / "/") base64string = ALPHADIGITPS *([FWS] ALPHADIGITPS) [ [FWS] "=" [ [FWS] "=" ] ] hdr-name = field-name qp-hdr-value = dkim-quoted-printable ; with "|" encoded -3.11. DKIM-Quoted-Printable +2.11. DKIM-Quoted-Printable The DKIM-Quoted-Printable encoding syntax resembles that described in Quoted-Printable [RFC2045], Section 6.7: any character MAY be encoded as an "=" followed by two hexadecimal digits from the alphabet "0123456789ABCDEF" (no lowercase characters permitted) representing the hexadecimal-encoded integer value of that character. All control characters (those with values < %x20), 8-bit characters (values > %x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon (";", %x3B) MUST be encoded. Note that all whitespace, including SPACE, CR, and LF characters, MUST be encoded. After encoding, FWS @@ -420,29 +423,29 @@ the case here. 3. The "soft line break" syntax ("=" as the last non-whitespace character on the line) does not apply. 4. DKIM-Quoted-Printable does not require that encoded lines be no more than 76 characters long (although there may be other requirements depending on the context in which the encoded text is being used). -4. Protocol Elements +3. Protocol Elements Protocol Elements are conceptual parts of the protocol that are not specific to either signers or verifiers. The protocol descriptions for signers and verifiers are described in later sections (Signer - Actions (Section 6) and Verifier Actions (Section 7)). NOTE: This + Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This section must be read in the context of those sections. -4.1. Selectors +3.1. Selectors To support multiple concurrent public keys per signing domain, the key namespace is subdivided using "selectors". For example, selectors might indicate the names of office locations (e.g., "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date (e.g., "january2005", "february2005", etc.), or even an individual user. Selectors are needed to support some important use cases. For example: @@ -505,29 +508,29 @@ value, such as a fingerprint of the public key. INFORMATIVE OPERATIONS NOTE: Reusing a selector with a new key (for example, changing the key associated with a user's name) makes it impossible to tell the difference between a message that didn't verify because the key is no longer valid versus a message that is actually forged. For this reason, signers are ill-advised to reuse selectors for new keys. A better strategy is to assign new keys to new selectors. -4.2. Tag=Value Lists +3.2. Tag=Value Lists DKIM uses a simple "tag=value" syntax in several contexts, including in messages and domain signature records. Values are a series of strings containing either plain text, "base64" text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid, Section 6.7), or "dkim-quoted-printable" (as defined in - Section 3.11). The name of the tag will determine the encoding of + Section 2.11). The name of the tag will determine the encoding of each value. Unencoded semicolon (";") characters MUST NOT occur in the tag value, since that separates tag-specs. INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined below (as "tag-value") only includes 7-bit characters, an implementation that wished to anticipate future standards would be advised not to preclude the use of UTF8-encoded text in tag=value lists. Formally, the ABNF syntax rules are as follows: @@ -557,53 +560,53 @@ Tag=value pairs that represent the default value MAY be included to aid legibility. Unrecognized tags MUST be ignored. Tags that have an empty value are not the same as omitted tags. An omitted tag is treated as having the default value; a tag with an empty value explicitly designates the empty string as the value. -4.3. Signing and Verification Algorithms +3.3. Signing and Verification Algorithms DKIM supports multiple digital signature algorithms. Two algorithms are defined by this specification at this time: rsa-sha1 and rsa- sha256. Signers MUST implement and SHOULD sign using rsa-sha256. Verifiers MUST implement rsa-sha256. INFORMATIVE NOTE: Although rsa-sha256 is strongly encouraged, some senders might prefer to use rsa-sha1 when balancing security strength against performance, complexity, or other needs. However, compliant verifiers might not implement rsa-sha1; they - will treat such messages as unsigned. {DKIM 13} + will treat such messages as unsigned. -4.3.1. The rsa-sha1 Signing Algorithm +3.3.1. The rsa-sha1 Signing Algorithm The rsa-sha1 Signing Algorithm computes a message hash as described - in Section 4.7 below using SHA-1 [FIPS-180-2-2002] as the hash-alg. + in Section 3.7 below using SHA-1 [FIPS-180-2-2002] as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. The signing algorithm SHOULD use a public exponent of 65537. -4.3.2. The rsa-sha256 Signing Algorithm +3.3.2. The rsa-sha256 Signing Algorithm The rsa-sha256 Signing Algorithm computes a message hash as described - in Section 4.7 below using SHA-256 [FIPS-180-2-2002] as the hash-alg. + in Section 3.7 below using SHA-256 [FIPS-180-2-2002] as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. -4.3.3. Key Sizes +3.3.3. Key Sizes Selecting appropriate key sizes is a trade-off between cost, performance, and risk. Since short RSA keys more easily succumb to off-line attacks, signers MUST use RSA keys of at least 1024 bits for long-lived keys. Verifiers MUST be able to validate signatures with keys ranging from 512 bits to 2048 bits, and they MAY be able to validate signatures with larger keys. Verifier policies may use the length of the signing key as one metric for determining whether a signature is acceptable. @@ -619,26 +622,26 @@ o Larger keys impose higher CPU costs to verify and sign email o Keys can be replaced on a regular basis, thus their lifetime can be relatively short o The security goals of this specification are modest compared to typical goals of other systems that employ digital signatures See [RFC3766] for further discussion on selecting key sizes. -4.3.4. Other Algorithms +3.3.4. Other Algorithms Other algorithms MAY be defined in the future. Verifiers MUST ignore any signatures using algorithms that they do not implement. -4.4. Canonicalization +3.4. Canonicalization Some mail systems modify email in transit, potentially invalidating a signature. For most signers, mild modification of email is immaterial to validation of the DKIM domain name's use. For such signers, a canonicalization algorithm that survives modest in-transit modification is preferred. Other signers demand that any modification of the email, however minor, result in a signature verification failure. These signers prefer a canonicalization algorithm that does not tolerate in-transit @@ -661,32 +664,32 @@ algorithms MAY be defined in the future; verifiers MUST ignore any signatures that use unrecognized canonicalization algorithms. Canonicalization simply prepares the email for presentation to the signing or verification algorithm. It MUST NOT change the transmitted data in any way. Canonicalization of header fields and body are described below. NOTE: This section assumes that the message is already in "network normal" format (text is ASCII encoded, lines are separated with CRLF - characters, etc.). See also Section 6.3 for information about + characters, etc.). See also Section 5.3 for information about normalizing the message. -4.4.1. The "simple" Header Canonicalization Algorithm +3.4.1. The "simple" Header Canonicalization Algorithm The "simple" header canonicalization algorithm does not change header fields in any way. Header fields MUST be presented to the signing or verification algorithm exactly as they are in the message being signed or verified. In particular, header field names MUST NOT be case folded and whitespace MUST NOT be changed. -4.4.2. The "relaxed" Header Canonicalization Algorithm +3.4.2. The "relaxed" Header Canonicalization Algorithm The "relaxed" header canonicalization algorithm MUST apply the following steps in order: o Convert all header field names (not the header field values) to lowercase. For example, convert "SUBJect: AbC" to "subject: AbC". o Unfold all header field continuation lines as described in [RFC5322]; in particular, lines with terminators embedded in continued header field values (that is, CRLF sequences followed by @@ -697,40 +700,40 @@ character. WSP characters here include those before and after a line folding boundary. o Delete all WSP characters at the end of each unfolded header field value. o Delete any WSP characters remaining before and after the colon separating the header field name from the header field value. The colon separator MUST be retained. -4.4.3. The "simple" Body Canonicalization Algorithm +3.4.3. The "simple" Body Canonicalization Algorithm The "simple" body canonicalization algorithm ignores all empty lines at the end of the message body. An empty line is a line of zero length after removal of the line terminator. If there is no body or no trailing CRLF on the message body, a CRLF is added. It makes no other changes to the message body. In more formal terms, the "simple" body canonicalization algorithm converts "0*CRLF" at the end of the body to a single "CRLF". Note that a completely empty or missing body is canonicalized as a single "CRLF"; that is, the canonicalized length will be 2 octets. The sha1 value (in base64) for an empty body (canonicalized to a "CRLF") is: uoq1oCgLlTqpdDX/iUbLy7J1Wic= The sha256 value is: frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY= -4.4.4. The "relaxed" Body Canonicalization Algorithm +3.4.4. The "relaxed" Body Canonicalization Algorithm The "relaxed" body canonicalization algorithm MUST apply the following steps (a) and (b) in order: a. Reduce whitespace: * Ignore all whitespace at the end of lines. Implementations MUST NOT remove the CRLF at the end of the line. * Reduce all sequences of WSP within a line to a single SP @@ -747,69 +750,58 @@ 2jmj7l5rSw0yVb/vlWAYkK/YBwk= The sha256 value is: 47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU= INFORMATIVE NOTE: It should be noted that the relaxed body canonicalization algorithm may enable certain types of extremely crude "ASCII Art" attacks where a message may be conveyed by adjusting the spacing between words. If this is a concern, the "simple" body canonicalization algorithm should be used instead. -4.4.5. Body Length Limits +3.4.5. Body Length Limits A body length count MAY be specified to limit the signature calculation to an initial prefix of the body text, measured in octets. If the body length count is not specified, the entire message body is signed. INFORMATIVE RATIONALE: This capability is provided because it is very common for mailing lists to add trailers to messages (e.g., instructions how to get off the list). Until those messages are also signed, the body length count is a useful tool for the verifier since it may as a matter of policy accept messages having valid signatures with extraneous data. INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables an attack in which an attacker modifies a message to include content that solely benefits the attacker. It is possible for the appended content to completely replace the original content in the - end recipient's eyes and to defeat duplicate message detection - algorithms. To avoid this attack, signers should be wary of using - this tag, and verifiers might wish to ignore the tag, {DKIM 2} - perhaps based on other criteria. + end recipient's eyes, such as via alterations to the MIME + structure or exploiting lax HTML parsing in the MUA, and to defeat + duplicate message detection algorithms. To avoid this attack, + signers should be wary of using this tag, and verifiers might wish + to ignore the tag, perhaps based on other criteria. The body length count allows the signer of a message to permit data to be appended to the end of the body of a signed message. The body length count MUST be calculated following the canonicalization algorithm; for example, any whitespace ignored by a canonicalization - algorithm is not included as part of the body length count. Signers - of MIME messages that include a body length count SHOULD be sure that - the length extends to the closing MIME boundary string. - - INFORMATIVE IMPLEMENTATION NOTE: A signer wishing to ensure that - the only acceptable modifications are to add to the MIME postlude - would use a body length count encompassing the entire final MIME - boundary string, including the final "--CRLF". A signer wishing - to allow additional MIME parts but not modification of existing - parts would use a body length count extending through the final - MIME boundary string, omitting the final "--CRLF". Note that this - only works for some MIME types, e.g., multipart/mixed but not - multipart/signed. + algorithm is not included as part of the body length count. A body length count of zero means that the body is completely unsigned. Signers wishing to ensure that no modification of any sort can occur should specify the "simple" canonicalization algorithm for both header and body and omit the body length count. -4.4.6. Canonicalization Examples (INFORMATIVE) +3.4.6. Canonicalization Examples (INFORMATIVE) In the following examples, actual whitespace is used only for clarity. The actual input and output text is designated using bracketed descriptors: "" for a space character, "" for a tab character, and "" for a carriage-return/line-feed sequence. For example, "X Y" and "XY" represent the same three characters. Example 1: A message reading: A: X @@ -839,51 +831,52 @@ and a body reading: C D E Example 3: When processed using relaxed header canonicalization and simple body canonicalization, the canonicalized version has a header of: a:X b:Y Z + and a body reading: C D E -4.5. The DKIM-Signature Header Field +3.5. The DKIM-Signature Header Field The signature of the email is stored in the DKIM-Signature header field. This header field contains all of the signature and key- fetching data. The DKIM-Signature value is a tag-list as described - in Section 4.2. + in Section 3.2. The DKIM-Signature header field SHOULD be treated as though it were a trace header field as defined in Section 3.6 of [RFC5322], and hence SHOULD NOT be reordered and SHOULD be prepended to the message. The DKIM-Signature header field being created or verified is always included in the signature calculation, after the rest of the header fields being signed; however, when calculating or verifying the signature, the value of the "b=" tag (signature value) of that DKIM- Signature header field MUST be treated as though it were an empty string. Unknown tags in the DKIM-Signature header field MUST be included in the signature calculation but MUST be otherwise ignored by verifiers. Other DKIM-Signature header fields that are included in the signature should be treated as normal header fields; in particular, the "b=" tag is not treated specially. The encodings for each field type are listed below. Tags described as qp-section are encoded as described in Section 6.7 of MIME Part One [RFC2045], with the additional conversion of semicolon characters to "=3B"; intuitively, this is one line of quoted-printable encoded - text. The dkim-quoted-printable syntax is defined in Section 3.11. + text. The dkim-quoted-printable syntax is defined in Section 2.11. Tags on the DKIM-Signature header field along with their type and requirement status are shown below. Unrecognized tags MUST be ignored. v= Version (MUST be included). This tag defines the version of this specification that applies to the signature record. It MUST have the value "1". Note that verifiers must do a string comparison on this value; for example, "1" is not the same as "1.0". @@ -905,44 +899,44 @@ sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT) ; for later extension x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT) ; for later extension b= The signature data (base64; REQUIRED). Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length - limits. See Signer Actions (Section 6) for how the signature is + limits. See Signer Actions (Section 5) for how the signature is computed. ABNF: sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag-data = base64string bh= The hash of the canonicalized body part of the message as limited by the "l=" tag (base64; REQUIRED). Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length - limits. See Section 4.7 for how the body hash is computed. + limits. See Section 3.7 for how the body hash is computed. ABNF: sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data sig-bh-tag-data = base64string c= Message canonicalization (plain-text; OPTIONAL, default is "simple/simple"). This tag informs the verifier of the type of canonicalization used to prepare the message for signing. It consists of two names separated by a "slash" (%d47) character, corresponding to the header and body canonicalization algorithms - respectively. These algorithms are described in Section 4.4. If + respectively. These algorithms are described in Section 3.4. If only one algorithm is named, that algorithm is used for the header and "simple" is used for the body. For example, "c=relaxed" is treated the same as "c=relaxed/simple". ABNF: sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg ["/" sig-c-tag-alg] sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg x-sig-c-tag-alg = hyphenated-word ; for later extension @@ -950,21 +944,21 @@ into the mail stream (plain-text; REQUIRED). Hence, the SDID value is used to form the query for the public key. The SDID MUST correspond to a valid DNS name under which the DKIM key record is published. The conventions and semantics used by a signer to create and use a specific SDID are outside the scope of the DKIM Signing specification, as is any use of those conventions and semantics. When presented with a signature that does not meet these requirements, verifiers MUST consider the signature invalid. Internationalized domain names MUST be encoded as A-Labels, as - described in Section 2.3 of [RFC5890]. {DKIM 4}. + described in Section 2.3 of [RFC5890]. ABNF: sig-d-tag = %x64 [FWS] "=" [FWS] domain-name domain-name = sub-domain 1*("." sub-domain) ; from RFC5321 Domain, excluding address-literal h= Signed header fields (plain-text, but see description; REQUIRED). A colon-separated list of header field names that identify the header fields presented to the signing algorithm. The field MUST @@ -972,21 +966,21 @@ to the signing algorithm. The field MAY contain names of header fields that do not exist when signed; nonexistent header fields do not contribute to the signature computation (that is, they are treated as the null input, including the header field name, the separating colon, the header field value, and any CRLF terminator). The field MUST NOT include the DKIM-Signature header field that is being created or verified, but may include others. Folding whitespace (FWS) MAY be included on either side of the colon separator. Header field names MUST be compared against actual header field names in a case-insensitive manner. This list - MUST NOT be empty. See Section 6.4 for a discussion of choosing + MUST NOT be empty. See Section 5.4 for a discussion of choosing header fields to sign. ABNF: sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name 0*( [FWS] ":" [FWS] hdr-name ) INFORMATIVE EXPLANATION: By "signing" header fields that do not actually exist, a signer can prevent insertion of those header fields before verification. However, since a signer cannot possibly know what header fields might be created in the @@ -1002,21 +996,21 @@ i= The Agent or User Identifier (AUID) on behalf of which the SDID is taking responsibility (dkim-quoted-printable; OPTIONAL, default is an empty Local-part followed by an "@" followed by the domain from the "d=" tag). The syntax is a standard email address where the Local-part MAY be omitted. The domain part of the address MUST be the same as, or a subdomain of, the value of the "d=" tag. Internationalized domain names MUST be encoded as A-Labels, as - described in Section 2.3 of [RFC5890]. {DKIM 4}. + described in Section 2.3 of [RFC5890]. ABNF: sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name The AUID is specified as having the same syntax as an email address, but is not required to have the same semantics. Notably, the domain name is not required to be registered in the DNS -- so it might not resolve in a query -- and the Local-part MAY be drawn @@ -1070,23 +1064,23 @@ allow display of fraudulent content without appropriate warning to end users. The "l=" tag is intended for increasing signature robustness when sending to mailing lists that both modify their content and do not sign their messages. However, using the "l=" tag enables attacks in which an intermediary with malicious intent modifies a message to include content that solely benefits the attacker. It is possible for the appended content to completely replace the original content in the end recipient's eyes and to defeat duplicate message detection algorithms. Examples are described in Security - Considerations Section 9. To avoid this attack, signers should + Considerations Section 8. To avoid this attack, signers should be extremely wary of using this tag, and verifiers might wish - to ignore the tag. {DKIM 2} + to ignore the tag. INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76 decimal digits. This constraint is not intended to predict the size of future messages or to require implementations to use an integer representation large enough to represent the maximum possible value, but is intended to remind the implementer to check the length of this and all other tags during verification and to test for integer overflow when decoding the value. Implementers may need to limit the actual value expressed to a value smaller than 10^76, e.g., to allow a @@ -1116,21 +1110,21 @@ *([FWS] ":" [FWS] sig-q-tag-method) sig-q-tag-method = "dns/txt" / x-sig-q-tag-type ["/" x-sig-q-tag-args] x-sig-q-tag-type = hyphenated-word ; for future extension x-sig-q-tag-args = qp-hdr-value s= The selector subdividing the namespace for the "d=" (domain) tag (plain-text; REQUIRED). Internationalized selector names MUST be encoded as A-Labels, as - described in Section 2.3 of [RFC5890]. {DKIM 4}. + described in Section 2.3 of [RFC5890]. ABNF: sig-s-tag = %x73 [FWS] "=" [FWS] selector t= Signature Timestamp (plain-text unsigned decimal integer; RECOMMENDED, default is an unknown creation time). The time that this signature was created. The format is the number of seconds since 00:00:00 on January 1, 1970 in the UTC time zone. The value is expressed as an unsigned integer in decimal ASCII. This value is not constrained to fit into a 31- or 32-bit integer. @@ -1199,21 +1193,21 @@ multiple continuation lines: DKIM-Signature: v=1; a=rsa-sha256; d=example.net; s=brisbane; c=simple; q=dns/txt; i=@eng.example.net; t=1117574938; x=1118006938; h=from:to:subject:date; z=From:foo@eng.example.net|To:joe@example.com| Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700; bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZVoG4ZHRNiYzR -4.6. Key Management and Representation +3.6. Key Management and Representation Signature applications require some level of assurance that the verification public key is associated with the claimed signer. Many applications achieve this by using public key certificates issued by a trusted third party. However, DKIM can achieve a sufficient level of security, with significantly enhanced scalability, by simply having the verifier query the purported signer's DNS entry (or some security-equivalent) in order to retrieve the public key. DKIM keys can potentially be stored in multiple types of key servers @@ -1217,33 +1211,32 @@ security-equivalent) in order to retrieve the public key. DKIM keys can potentially be stored in multiple types of key servers and in multiple formats. The storage and format of keys are irrelevant to the remainder of the DKIM algorithm. Parameters to the key lookup algorithm are the type of the lookup (the "q=" tag), the domain of the signer (the "d=" tag of the DKIM- Signature header field), and the selector (the "s=" tag). public_key = dkim_find_key(q_val, d_val, s_val) - This document defines a single binding, using DNS TXT records to distribute the keys. Other bindings may be defined in the future. -4.6.1. Textual Representation +3.6.1. Textual Representation It is expected that many key servers will choose to present the keys in an otherwise unstructured text format (for example, an XML form would not be considered to be unstructured text for this purpose). The following definition MUST be used for any DKIM key represented in an otherwise unstructured textual form. - The overall syntax is a tag-list as described in Section 4.2. The + The overall syntax is a tag-list as described in Section 3.2. The current valid tags are described below. Other tags MAY be present and MUST be ignored by any implementation that does not understand them. v= Version of the DKIM key record (plain-text; RECOMMENDED, default is "DKIM1"). If specified, this tag MUST be set to "DKIM1" (without the quotes). This tag MUST be the first tag in the record. Records beginning with a "v=" tag with any other value MUST be discarded. Note that verifiers must do a string comparison on this value; for example, "DKIM1" is not the same as @@ -1244,38 +1237,38 @@ v= Version of the DKIM key record (plain-text; RECOMMENDED, default is "DKIM1"). If specified, this tag MUST be set to "DKIM1" (without the quotes). This tag MUST be the first tag in the record. Records beginning with a "v=" tag with any other value MUST be discarded. Note that verifiers must do a string comparison on this value; for example, "DKIM1" is not the same as "DKIM1.0". ABNF: key-v-tag = %x76 [FWS] "=" [FWS] %x44 %x4B %x49 %x4D %x31 + h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to allowing all algorithms). A colon-separated list of hash algorithms that might be used. Unrecognized algorithms MUST be - ignored. Refer to Section 4.3 for a discussion of the hash + ignored. Refer to Section 3.3 for a discussion of the hash algorithms implemented by Signers and Verifiers. The set of algorithms listed in this tag in each record is an operational choice made by the Signer. ABNF: key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg 0*( [FWS] ":" [FWS] key-h-tag-alg ) key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg x-key-h-tag-alg = hyphenated-word ; for future extension - k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and verifiers MUST support the "rsa" key type. The "rsa" key type indicates that an ASN.1 DER-encoded [ITU-X660-1997] RSAPublicKey - [RFC3447] (see Sections Section 4.1 and A.1.1) is being used in + [RFC3447] (see Sections Section 3.1 and A.1.1) is being used in the "p=" tag. (Note: the "p=" tag further encodes the value using the base64 algorithm.) Unrecognized key types MUST be ignored. ABNF: key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type key-k-tag-type = "rsa" / x-key-k-tag-type x-key-k-tag-type = hyphenated-word ; for future extension n= Notes that might be of interest to a human (qp-section; OPTIONAL, default is empty). No interpretation is made by any program. @@ -1343,56 +1337,56 @@ unless subdomaining is required. ABNF: key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag 0*( [FWS] ":" [FWS] key-t-tag-flag ) key-t-tag-flag = "y" / "s" / x-key-t-tag-flag x-key-t-tag-flag = hyphenated-word ; for future extension Unrecognized flags MUST be ignored. -4.6.2. DNS Binding +3.6.2. DNS Binding A binding using DNS TXT records as a key service is hereby defined. All implementations MUST support this binding. -4.6.2.1. Namespace +3.6.2.1. Namespace All DKIM keys are stored in a subdomain named "_domainkey". Given a DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag of "foo.bar", the DNS query will be for "foo.bar._domainkey.example.com". -4.6.2.2. Resource Record Types for Key Storage +3.6.2.2. Resource Record Types for Key Storage The DNS Resource Record type used is specified by an option to the query-type ("q=") tag. The only option defined in this base specification is "txt", indicating the use of a TXT Resource Record (RR). A later extension of this standard may define another RR type. Strings in a TXT RR MUST be concatenated together before use with no intervening whitespace. TXT RRs MUST be unique for a particular selector name; that is, if there are multiple records in an RRset, the results are undefined. - TXT RRs are encoded as described in Section 4.6.1 + TXT RRs are encoded as described in Section 3.6.1 -4.7. Computing the Message Hashes +3.7. Computing the Message Hashes Both signing and verifying message signatures start with a step of computing two cryptographic hashes over the message. Signers will choose the parameters of the signature as described in Signer Actions - Section 6; verifiers will use the parameters specified in the DKIM- + Section 5; verifiers will use the parameters specified in the DKIM- Signature header field being verified. In the following discussion, the names of the tags in the DKIM-Signature header field that either exists (when verifying) or will be created (when signing) are used. - Note that canonicalization (Section 4.4) is only used to prepare the + Note that canonicalization (Section 3.4) is only used to prepare the email for signing or verifying; it does not affect the transmitted email in any way. The signer/verifier MUST compute two hashes, one over the body of the message and one over the selected header fields of the message. Signers MUST compute them in the order shown. Verifiers MAY compute them in any order convenient to the verifier, provided that the result is semantically identical to the semantics that would be the case had they been computed in this order. @@ -1421,53 +1415,53 @@ All tags and their values in the DKIM-Signature header field are included in the cryptographic hash with the sole exception of the value portion of the "b=" (signature) tag, which MUST be treated as the null string. All tags MUST be included even if they might not be understood by the verifier. The header field MUST be presented to the hash algorithm after the body of the message rather than with the rest of the header fields and MUST be canonicalized as specified in the "c=" (canonicalization) tag. The DKIM-Signature header field MUST NOT be included in its own h= tag, although other DKIM-Signature - header fields MAY be signed (see Section 5). + header fields MAY be signed (see Section 4). When calculating the hash on messages that will be transmitted using base64 or quoted-printable encoding, signers MUST compute the hash after the encoding. Likewise, the verifier MUST incorporate the values into the hash before decoding the base64 or quoted-printable text. However, the hash MUST be computed before transport level encodings such as SMTP "dot-stuffing" (the modification of lines beginning with a "." to avoid confusion with the SMTP end-of-message marker, as specified in [RFC5321]). With the exception of the canonicalization procedure described in - Section 4.4, the DKIM signing process treats the body of messages as + Section 3.4, the DKIM signing process treats the body of messages as simply a string of octets. DKIM messages MAY be either in plain-text or in MIME format; no special treatment is afforded to MIME content. Message attachments in MIME format MUST be included in the content that is signed. More formally, pseudo-code for the signature algorithm is: body-hash = hash-alg (canon-body, l-param) data-hash = hash-alg (h-headers, D-SIG, content-hash) signature = sig-alg (d-domain, selector, data-hash) where: body-hash: is the output from hashing the body, using hash-alg. hash-alg: is the hashing algorithm specified in the "a" parameter. canon-body: is a canonicalized representation of the body, produced by using the body algorithm specified in the "c" - parameter, as defined in Section 4.4 and excluding the + parameter, as defined in Section 3.4 and excluding the DKIM-Signature field. l-param: is the length-of-body value of the "l" parameter. data-hash: is the output from using the hash-alg algorithm, to hash the header including the DKIM-Signature header, and the body hash. h-headers: is the list of headers to be signed, as specified in the "h" parameter. @@ -1485,47 +1479,62 @@ d-domain: is the domain name specified in the "d" parameter. selector: is the selector value specified in the "s" parameter. NOTE: Many digital signature APIs provide both hashing and application of the RSA private key using a single "sign()" primitive. When using such an API, the last two steps in the algorithm would probably be combined into a single call that would perform both the "a-hash-alg" and the "sig-alg". -4.8. Input Requirements +3.8. Input Requirements DKIM's design is predicated on valid input. Therefore, signers and verifiers SHOULD take reasonable steps to ensure that the messages they are processing are valid according to [RFC5322], [RFC2045], and - any other relevant message format standards. See Section 9.15 for + any other relevant message format standards. See Section 8.15 for additional discussion and references. -4.9. Signing by Parent Domains +3.9. Output Requirements + + For each signature that verifies successfully or produces a TEMPFAIL + result, the output of a DKIM verifier module MUST include the set of: + + o The domain name, taken from the "d=" signature tag; and + + o The result of the verification attempt for that signature. + + The output MAY include other signature properties or result meta- + data, including PERMFAILed or otherwise ignored signatures, for use + by modules that consume those results. + + See Section 6.1 for discussion of signature validation result codes. + +3.10. Signing by Parent Domains In some circumstances, it is desirable for a domain to apply a signature on behalf of any of its subdomains without the need to maintain separate selectors (key records) in each subdomain. By default, private keys corresponding to key records can be used to sign messages for any subdomain of the domain in which they reside; for example, a key record for the domain example.com can be used to verify messages where the AUID ("i=" tag of the signature) is sub.example.com, or even sub1.sub2.example.com. In order to limit the capability of such keys when this is not intended, the "s" flag MAY be set in the "t=" tag of the key record, to constrain the validity of the domain of the AUID. If the referenced key record contains the "s" flag as part of the "t=" tag, the domain of the AUID ("i=" flag) MUST be the same as that of the SDID (d=) domain. If this flag is absent, the domain of the AUID MUST be the same as, or a subdomain of, the SDID. -4.10. Relationship between SDID and AUID +3.11. Relationship between SDID and AUID DKIM's primary task is to communicate from the Signer to a recipient- side Identity Assessor a single Signing Domain Identifier (SDID) that refers to a responsible identity. DKIM MAY optionally provide a single responsible Agent or User Identifier (AUID). Hence, DKIM's mandatory output to a receive-side Identity Assessor is a single domain name. Within the scope of its use as DKIM output, the name has only basic domain name semantics; any possible owner- specific semantics are outside the scope of DKIM. That is, within @@ -1553,23 +1562,23 @@ is a broad and complex topic and trust mechanisms are subject to highly creative attacks. The real-world efficacy of any but the most basic bindings between the SDID or AUID and other identities is not well established, nor is its vulnerability to subversion by an attacker. Hence, reliance on the use of such bindings should be strictly limited. In particular, it is not at all clear to what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the SDID or AUID. -5. Semantics of Multiple Signatures +4. Semantics of Multiple Signatures -5.1. Example Scenarios +4.1. Example Scenarios There are many reasons why a message might have multiple signatures. For example, a given signer might sign multiple times, perhaps with different hashing or signing algorithms during a transition phase. INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be insufficiently strong, and DKIM usage transitions to SHA-1024. A signer might immediately sign using the newer algorithm, but continue to sign using the older algorithm for interoperability with verifiers that had not yet upgraded. The signer would do @@ -1612,26 +1621,26 @@ services, such as those commonly associated with academic alumni sites. INFORMATIVE EXAMPLE: A recipient might have an address at members.example.org, a site that has anti-abuse protection that is somewhat less effective than the recipient would prefer. Such a recipient might have specific authors whose messages would be trusted absolutely, but messages from unknown authors that had passed the forwarder's scrutiny would have only medium trust. -5.2. Interpretation +4.2. Interpretation A signer that is adding a signature to a message merely creates a new DKIM-Signature header, using the usual semantics of the h= option. A signer MAY sign previously existing DKIM-Signature header fields - using the method described in Section 6.4 to sign trace header + using the method described in Section 5.4 to sign trace header fields. INFORMATIVE NOTE: Signers should be cognizant that signing DKIM- Signature header fields may result in signature failures with intermediaries that do not recognize that DKIM-Signature header fields are trace header fields and unwittingly reorder them, thus breaking such signatures. For this reason, signing existing DKIM- Signature header fields is unadvised, albeit legal. INFORMATIVE NOTE: If a header field with multiple instances is @@ -1651,41 +1660,41 @@ messages they are signing, even if they know that the signatures cannot be verified. When evaluating a message with multiple signatures, a verifier SHOULD evaluate signatures independently and on their own merits. For example, a verifier that by policy chooses not to accept signatures with deprecated cryptographic algorithms would consider such signatures invalid. Verifiers MAY process signatures in any order of their choice; for example, some verifiers might choose to process signatures corresponding to the From field in the message header - before other signatures. See Section 7.1 for more information about + before other signatures. See Section 6.1 for more information about signature choices. INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate valid signatures with invalid signatures in an attempt to guess why a signature failed are ill-advised. In particular, there is no general way that a verifier can determine that an invalid signature was ever valid. - Verifiers SHOULD ignore failed signatures as though they were not - present in the message. Verifiers SHOULD continue to check - signatures until a signature successfully verifies to the - satisfaction of the verifier. To limit potential denial-of-service - attacks, verifiers MAY limit the total number of signatures they will - attempt to verify. + Verifiers SHOULD ignore those signatures that produce a PERMFAIL + result (see Section 6.1), acting as though they were not present in + the message. Verifiers SHOULD continue to check signatures until a + signature successfully verifies to the satisfaction of the verifier. + To limit potential denial-of-service attacks, verifiers MAY limit the + total number of signatures they will attempt to verify. -6. Signer Actions +5. Signer Actions The following steps are performed in order by signers. -6.1. Determine Whether the Email Should Be Signed and by Whom +5.1. Determine Whether the Email Should Be Signed and by Whom A signer can obviously only sign email for domains for which it has a private key and the necessary knowledge of the corresponding public key and selector information. However, there are a number of other reasons beyond the lack of a private key why a signer could choose not to sign an email. INFORMATIVE NOTE: Signing modules may be incorporated into any portion of the mail system as deemed appropriate, including an MUA, a SUBMISSION server, or an MTA. Wherever implemented, @@ -1697,41 +1706,41 @@ authenticated and authorized. INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign Received header fields if the outgoing gateway MTA obfuscates Received header fields, for example, to hide the details of internal topology. If an email cannot be signed for some reason, it is a local policy decision as to what to do with that email. -6.2. Select a Private Key and Corresponding Selector Information +5.2. Select a Private Key and Corresponding Selector Information This specification does not define the basis by which a signer should choose which private key and selector information to use. Currently, all selectors are equal as far as this specification is concerned, so the decision should largely be a matter of administrative convenience. Distribution and management of private keys is also outside the scope of this document. INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a private key when the selector containing the corresponding public key is expected to be revoked or removed before the verifier has an opportunity to validate the signature. The signer should anticipate that verifiers may choose to defer validation, perhaps until the message is actually read by the final recipient. In particular, when rotating to a new key pair, signing should immediately commence with the new private key and the old public key should be retained for a reasonable validation interval before being removed from the key server. -6.3. Normalize the Message to Prevent Transport Conversions +5.3. Normalize the Message to Prevent Transport Conversions Some messages, particularly those using 8-bit characters, are subject to modification during transit, notably conversion to 7-bit form. Such conversions will break DKIM signatures. In order to minimize the chances of such breakage, signers SHOULD convert the message to a suitable MIME content transfer encoding such as quoted-printable or base64 as described in [RFC2045] before signing. Such conversion is outside the scope of DKIM; the actual message SHOULD be converted to 7-bit MIME by an MUA or MSA prior to presentation to the DKIM algorithm. @@ -1750,21 +1759,21 @@ bare CR or LF characters (used by some systems as a local line separator convention) MUST be converted to the SMTP-standard CRLF sequence before the message is signed. Any conversion of this sort SHOULD be applied to the message actually sent to the recipient(s), not just to the version presented to the signing algorithm. More generally, the signer MUST sign the message as it is expected to be received by the verifier rather than in some local or internal form. -6.4. Determine the Header Fields to Sign +5.4. Determine the Header Fields to Sign The From header field MUST be signed (that is, included in the "h=" tag of the resulting DKIM-Signature header field). Signers SHOULD NOT sign an existing header field likely to be legitimately modified or removed in transit. In particular, [RFC5321] explicitly permits modification or removal of the Return-Path header field in transit. Signers MAY include any other header fields present at the time of signing at the discretion of the signer. INFORMATIVE OPERATIONS NOTE: The choice of which header fields to @@ -1844,40 +1853,38 @@ instances by intermediate MTAs will cause DKIM signatures to be broken; such anti-social behavior should be avoided. INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this specification, all end-user visible header fields should be signed to avoid possible "indirect spamming". For example, if the Subject header field is not signed, a spammer can resend a previously signed mail, replacing the legitimate subject with a one-line spam. -6.5. Recommended Signature Content - - {DKIM 20} +5.5. Recommended Signature Content The purpose of the DKIM cryptographic algorithm is to affix an identifier to the message in a way that is both robust against normal transit-related changes and resistant to kinds of replay attacks. An essential aspect of satisfying these requirements is choosing what header fields to include in the hash and what fields to exclude. The basic rule for choosing fields to include is to select those fields that constitute the "core" of the message content. Hence, any replay attack will have to include these in order to have the signature succeed; but with these included, the core of the message is valid, even if sent on to new recipients. Common examples of fields with addresses and fields with textual content related to the body are: - o From (REQUIRED; see Section 6.4) + o From (REQUIRED; see Section 5.4) o Reply-To o Subject o Date o To, Cc o Resent-Date, Resent-From, Resent-To, Resent-Cc @@ -1874,30 +1881,40 @@ o Reply-To o Subject o Date o To, Cc o Resent-Date, Resent-From, Resent-To, Resent-Cc + o In-Reply-To, References o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-Owner, List-Archive - If the "l=" signature tag is in use (see Section 4.5), the Content- + If the "l=" signature tag is in use (see Section 3.5), the Content- Type field is also a candidate for being included as it could be replaced in a way that causes completely different content to be rendered to the receiving user. + There are tradeoffs in the decision of what constitutes the "core" of + the message, which for some fields is a subjective concept. + + Including fields such as "Message-ID" for example is useful if one + considers a mechanism for being able to distinguish separate + instances of the same message to be core content. Similarly, "In- + Reply-To" and "References" might be desirable to include if one + considers message threading to be a core part of the message. + Another class of fields that may be of interest are those that convey security-related information about the message, such as Authentication-Results [RFC5451]. The basic rule for choosing field to exclude is to select those fields for which there are multiple fields with the same name, and fields that are modified in transit. Examples of these are: o Return-Path @@ -1927,61 +1944,61 @@ Signers SHOULD NOT use "l=" unless they intend to accommodate intermediate mail processors that append text to a message. For example, many mailing list processors append "unsubscribe" information to message bodies. If signers use "l=", they SHOULD include the entire message body existing at the time of signing in computing the count. In particular, signers SHOULD NOT specify a body length of 0 since this may be interpreted as a meaningless signature by some verifiers. -6.6. Compute the Message Hash and Signature +5.6. Compute the Message Hash and Signature - The signer MUST compute the message hash as described in Section 4.7 + The signer MUST compute the message hash as described in Section 3.7 and then sign it using the selected public-key algorithm. This will result in a DKIM-Signature header field that will include the body hash and a signature of the header hash, where that header includes the DKIM-Signature header field itself. Entities such as mailing list managers that implement DKIM and that modify the message or a header field (for example, inserting unsubscribe information) before retransmitting the message SHOULD check any existing signature on input and MUST make such modifications before re-signing the message. The signer MAY elect to limit the number of bytes of the body that will be included in the hash and hence signed. The length actually hashed should be inserted in the "l=" tag of the DKIM-Signature header field. -6.7. Insert the DKIM-Signature Header Field +5.7. Insert the DKIM-Signature Header Field Finally, the signer MUST insert the DKIM-Signature header field created in the previous step prior to transmitting the email. The DKIM-Signature header field MUST be the same as used to compute the hash as described above, except that the value of the "b=" tag MUST be the appropriately signed hash computed in the previous step, signed using the algorithm specified in the "a=" tag of the DKIM- Signature header field and using the private key corresponding to the selector given in the "s=" tag of the DKIM-Signature header field, as - chosen above in Section 6.2 + chosen above in Section 5.2 The DKIM-Signature header field MUST be inserted before any other DKIM-Signature fields in the header block. INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this is to insert the DKIM-Signature header field at the beginning of the header block. In particular, it may be placed before any existing Received header fields. This is consistent with treating DKIM-Signature as a trace header field. -7. Verifier Actions +6. Verifier Actions Since a signer MAY remove or revoke a public key at any time, it is recommended that verification occur in a timely manner. In many configurations, the most timely place is during acceptance by the border MTA or shortly thereafter. In particular, deferring verification until the message is accessed by the end user is discouraged. A border or intermediate MTA MAY verify the message signature(s). An MTA who has performed verification MAY communicate the result of that @@ -1994,37 +2011,37 @@ A verifying MTA MAY implement a policy with respect to unverifiable mail, regardless of whether or not it applies the verification header field to signed messages. Verifiers MUST produce a result that is semantically equivalent to applying the following steps in the order listed. In practice, several of these steps can be performed in parallel in order to improve performance. -7.1. Extract Signatures from the Message +6.1. Extract Signatures from the Message The order in which verifiers try DKIM-Signature header fields is not defined; verifiers MAY try signatures in any order they like. For example, one implementation might try the signatures in textual order, whereas another might try signatures by identities that match the contents of the From header field before trying other signatures. Verifiers MUST NOT attribute ultimate meaning to the order of multiple DKIM-Signature header fields. In particular, there is reason to believe that some relays will reorder the header fields in potentially arbitrary ways. INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as a clue to signing order in the absence of any other information. However, other clues as to the semantics of multiple signatures (such as correlating the signing host with Received header fields) - may also be considered. + might also be considered. A verifier SHOULD NOT treat a message that has one or more bad signatures and no good signatures differently from a message with no signature at all; such treatment is a matter of local policy and is beyond the scope of this document. When a signature successfully verifies, a verifier will either stop processing or attempt to verify any other signatures, at the discretion of the implementation. A verifier MAY limit the number of signatures it tries to avoid denial-of-service attacks. @@ -2039,73 +2056,72 @@ by enlisting innocent verifiers in launching an attack against the DNS servers of the actual victim. In the following description, text reading "return status (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL") means that the verifier MUST immediately cease processing that signature. The verifier SHOULD proceed to the next signature, if any is present, and completely ignore the bad signature. If the status is "PERMFAIL", the signature failed and should not be reconsidered. If the status is "TEMPFAIL", the signature could not be verified at - this time but may be tried again later. A verifier MAY either defer - the message for later processing, perhaps by queueing it locally or - issuing a 451/4.7.5 SMTP reply, or try another signature; if no good - signature is found and any of the signatures resulted in a TEMPFAIL - status, the verifier MAY save the message for later processing. The - "(explanation)" is not normative text; it is provided solely for - clarification. + this time but may be tried again later. A verifier MAY either + arrange to defer the message for later processing, or try another + signature; if no good signature is found and any of the signatures + resulted in a TEMPFAIL status, the verifier MAY arrange to defer the + message for later processing. The "(explanation)" is not normative + text; it is provided solely for clarification. Verifiers SHOULD ignore any DKIM-Signature header fields where the signature does not validate. Verifiers that are prepared to validate multiple signature header fields SHOULD proceed to the next signature - header field, should it exist. However, verifiers MAY make note of - the fact that an invalid signature was present for consideration at a + header field, if one exists. However, verifiers MAY make note of the + fact that an invalid signature was present for consideration at a later step. INFORMATIVE NOTE: The rationale of this requirement is to permit messages that have invalid signatures but also a valid signature to work. For example, a mailing list exploder might opt to leave the original submitter signature in place even though the exploder knows that it is modifying the message in some way that will break that signature, and the exploder inserts its own signature. In this case, the message should succeed even in the presence of the known-broken signature. For each signature to be validated, the following steps should be performed in such a manner as to produce a result that is semantically equivalent to performing them in the indicated order. -7.1.1. Validate the Signature Header Field +6.1.1. Validate the Signature Header Field Implementers MUST meticulously validate the format and values in the DKIM-Signature header field; any inconsistency or unexpected values MUST cause the header field to be completely ignored and the verifier to return PERMFAIL (signature syntax error). Being "liberal in what you accept" is definitely a bad strategy in this security context. Note however that this does not include the existence of unknown tags in a DKIM-Signature header field, which are explicitly permitted. Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag that is inconsistent with this specification and return PERMFAIL (incompatible version). INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course, choose to also verify signatures generated by older versions of this specification. - If any tag listed as "required" in Section 4.5 is omitted from the + If any tag listed as "required" in Section 3.5 is omitted from the DKIM-Signature header field, the verifier MUST ignore the DKIM- Signature header field and return PERMFAIL (signature missing required tag). - INFORMATIONAL NOTE: The tags listed as required in Section 4.5 are + INFORMATIONAL NOTE: The tags listed as required in Section 3.5 are "v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there be a - conflict between this note and Section 4.5, Section 4.5 is + conflict between this note and Section 3.5, Section 3.5 is normative. If the DKIM-Signature header field does not contain the "i=" tag, the verifier MUST behave as though the value of that tag were "@d", where "d" is the value from the "d=" tag. Verifiers MUST confirm that the domain specified in the "d=" tag is the same as or a parent domain of the domain part of the "i=" tag. If not, the DKIM-Signature header field MUST be ignored and the verifier should return PERMFAIL (domain mismatch). @@ -2124,56 +2140,51 @@ "com" and "co.uk" may be ignored. The list of unacceptable domains SHOULD be configurable. Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (unacceptable signature header) for any other reason, for example, if the signature does not sign header fields that the verifier views to be essential. As a case in point, if MIME header fields are not signed, certain attacks may be possible that the verifier would prefer to avoid. -7.1.2. Get the Public Key +6.1.2. Get the Public Key The public key for a signature is needed to complete the verification process. The process of retrieving the public key depends on the query type as defined by the "q=" tag in the DKIM-Signature header field. Obviously, a public key need only be retrieved if the process of extracting the signature information is completely successful. Details of key management and representation are described in - Section 4.6. The verifier MUST validate the key record and MUST + Section 3.6. The verifier MUST validate the key record and MUST ignore any public key records that are malformed. NOTE: The use of a wildcard TXT record that covers a queried DKIM domain name will produce a response to a DKIM query that is unlikely to be valid DKIM key record. This problem is not specific to DKIM and applies to many other types of queries. Client software that processes DNS responses needs to take this problem into account. When validating a message, a verifier MUST perform the following steps in a manner that is semantically the same as performing them in the order indicated -- in some cases the implementation may parallelize or reorder these steps, as long as the semantics remain unchanged: - 1. Retrieve the public key as described in Section 4.6 using the + 1. Retrieve the public key as described in Section 3.6 using the algorithm in the "q=" tag, the domain from the "d=" tag, and the selector from the "s=" tag. 2. If the query for the public key fails to respond, the verifier - MAY defer acceptance of this email and return TEMPFAIL (key - unavailable). If verification is occurring during the incoming - SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply - code. Alternatively, the verifier MAY store the message in the - local queue for later trial or ignore the signature. Note that - storing a message in the local queue is subject to denial-of- - service attacks. + MAY seek a later verification attempt by returning TEMPFAIL (key + unavailable). 3. If the query for the public key fails because the corresponding key record does not exist, the verifier MUST immediately return PERMFAIL (no key for signature). 4. If the query for the public key returns multiple key records, the verifier may choose one of the key records or may cycle through the key records performing the remainder of these steps on each record at the discretion of the implementer. The order of the key records is unspecified. If the verifier chooses to cycle @@ -2198,36 +2209,36 @@ been revoked and the verifier MUST treat this as a failed signature check and return PERMFAIL (key revoked). There is no defined semantic difference between a key that has been revoked and a key record that has been removed. 8. If the public key data is not suitable for use with the algorithm and key types defined by the "a=" and "k=" tags in the DKIM- Signature header field, the verifier MUST immediately return PERMFAIL (inappropriate key algorithm). -7.1.3. Compute the Verification +6.1.3. Compute the Verification Given a signer and a public key, verifying a signature consists of actions semantically equivalent to the following steps. 1. Based on the algorithm defined in the "c=" tag, the body length specified in the "l=" tag, and the header field names in the "h=" tag, prepare a canonicalized version of the message as is - described in Section 4.7 (note that this version does not + described in Section 3.7 (note that this version does not actually need to be instantiated). When matching header field names in the "h=" tag against the actual message header field, comparisons MUST be case-insensitive. 2. Based on the algorithm indicated in the "a=" tag, compute the message hashes from the canonical copy as described in - Section 4.7. + Section 3.7. 3. Verify that the hash of the canonicalized message body computed in the previous step matches the hash value conveyed in the "bh=" tag. If the hash does not match, the verifier SHOULD ignore the signature and return PERMFAIL (body hash did not verify). 4. Using the signature conveyed in the "b=" tag, verify the signature against the header hash using the mechanism appropriate for the public key algorithm described in the "a=" tag. If the signature does not validate, the verifier SHOULD ignore the @@ -2243,107 +2254,109 @@ the "bh=" tag in the DKIM-Signature header field matches that of the actual message body; however, if the body hash does not match, the entire signature must be considered to have failed. A body length specified in the "l=" tag of the signature limits the number of bytes of the body passed to the verification algorithm. All data beyond that limit is not validated by DKIM. Hence, verifiers might treat a message that contains bytes beyond the indicated body length with suspicion, such as by declaring the signature invalid (e.g., by returning PERMFAIL (unsigned content)), - or conveying the partial verification to the policy module. {DKIM 2} + or conveying the partial verification to the policy module. -7.2. Communicate Verification Results +6.2. Communicate Verification Results Verifiers wishing to communicate the results of verification to other parts of the mail system may do so in whatever manner they see fit. For example, implementations might choose to add an email header field to the message before passing it on. Any such header field SHOULD be inserted before any existing DKIM-Signature or preexisting authentication status header fields in the header field block. The Authentication-Results: header field ([RFC5451]) MAY be used for this purpose. INFORMATIVE ADVICE to MUA filter writers: Patterns intended to search for results header fields to visibly mark authenticated mail for end users should verify that such header field was added by the appropriate verifying domain and that the verified identity matches the author identity that will be displayed by the MUA. In particular, MUA filters should not be influenced by bogus results header fields added by attackers. To circumvent this attack, - verifiers may wish to delete existing results header fields after - verification and before adding a new header field. + verifiers MAY wish to request deletion of existing results header + fields after verification and before arranging to add a new header + field. -7.3. Interpret Results/Apply Local Policy +6.3. Interpret Results/Apply Local Policy It is beyond the scope of this specification to describe what actions an Identity Assessor can make, but mail carrying a validated SDID presents an opportunity to an Identity Assessor that unauthenticated email does not. Specifically, an authenticated email creates a predictable identifier by which other decisions can reliably be managed, such as trust and reputation. Conversely, unauthenticated email lacks a reliable identifier that can be used to assign trust and reputation. It is reasonable to treat unauthenticated email as lacking any trust and having no positive reputation. - In general, verifiers SHOULD NOT reject messages solely on the basis - of a lack of signature or an unverifiable signature; such rejection - would cause severe interoperability problems. However, if the - verifier does opt to reject such messages (for example, when - communicating with a peer who, by prior agreement, agrees to only - send signed messages), and the verifier runs synchronously with the - SMTP session and a signature is missing or does not verify, the MTA + In general, modules that consume DKIM verification output SHOULD NOT + determine message acceptability based solely on a lack of any + signature or on an unverifiable signature; such rejection would cause + severe interoperability problems. If an MTA does wish to reject such + messages during an SMTP session (for example, when communicating with + a peer who, by prior agreement, agrees to only send signed messages), + and a signature is missing or does not verify, the handling MTA SHOULD use a 550/5.7.x reply code. - If it is not possible to fetch the public key, perhaps because the - key server is not available, a temporary failure message MAY be - generated using a 451/4.7.5 reply code, such as: + Where the verifier is integrated within the MTA and it is not + possible to fetch the public key, perhaps because the key server is + not available, a temporary failure message MAY be generated using a + 451/4.7.5 reply code, such as: 451 4.7.5 Unable to verify signature - key server unavailable Temporary failures such as inability to access the key server or other external service are the only conditions that SHOULD use a 4xx SMTP reply code. In particular, cryptographic signature verification - failures MUST NOT return 4xx SMTP replies. + failures MUST NOT provoke 4xx SMTP replies. Once the signature has been verified, that information MUST be conveyed to the Identity Assessor (such as an explicit allow/ whitelist and reputation system) and/or to the end user. If the SDID is not the same as the address in the From: header field, the mail system SHOULD take pains to ensure that the actual SDID is clear to the reader. While the symptoms of a failed verification are obvious -- the signature doesn't verify -- establishing the exact cause can be more difficult. If a selector cannot be found, is that because the selector has been removed, or was the value changed somehow in transit? If the signature line is missing, is that because it was never there, or was it removed by an overzealous filter? For diagnostic purposes, the exact reason why the verification fails - SHOULD be made available to the policy module and possibly recorded - in the system logs. If the email cannot be verified, then it SHOULD - be treated {DKIM 2} the same as all unverified email regardless of - whether or not it looks like it was signed. + SHOULD be made available and possibly recorded in the system logs. + If the email cannot be verified, then it SHOULD be treated the same + as all unverified email regardless of whether or not it looks like it + was signed. -8. IANA Considerations +7. IANA Considerations DKIM has registered namespaces with IANA. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus [RFC5226]. This memo updates these registries as described below. Of note is the addition of a new "status" column. All registrations into these namespaces MUST include the name being registered, the document in which it was registered or updated, and an indication of its current status which MUST be one of "active" (in current use) or "historic" (no longer in current use). -8.1. DKIM-Signature Tag Specifications +7.1. DKIM-Signature Tag Specifications A DKIM-Signature provides for a list of tag specifications. IANA has established the DKIM-Signature Tag Specification Registry for tag specifications that can be used in DKIM-Signature fields. The updated entries in the registry comprise: +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ @@ -2358,42 +2371,42 @@ | l | (this document) | active | | q | (this document) | active | | s | (this document) | active | | t | (this document) | active | | x | (this document) | active | | z | (this document) | active | +------+-----------------+--------+ Table 1: DKIM-Signature Tag Specification Registry Updated Values -8.2. DKIM-Signature Query Method Registry +7.2. DKIM-Signature Query Method Registry - The "q=" tag-spec (specified in Section 4.5) provides for a list of + The "q=" tag-spec (specified in Section 3.5) provides for a list of query methods. IANA has established the DKIM-Signature Query Method Registry for mechanisms that can be used to retrieve the key that will permit validation processing of a message signed using DKIM. The updated entry in the registry comprises: +------+--------+-----------------+--------+ | TYPE | OPTION | REFERENCE | STATUS | +------+--------+-----------------+--------+ | dns | txt | (this document) | active | +------+--------+-----------------+--------+ DKIM-Signature Query Method Registry Updated Values -8.3. DKIM-Signature Canonicalization Registry +7.3. DKIM-Signature Canonicalization Registry - The "c=" tag-spec (specified in Section 4.5) provides for a specifier + The "c=" tag-spec (specified in Section 3.5) provides for a specifier for canonicalization algorithms for the header and body of the message. IANA has established the DKIM-Signature Canonicalization Algorithm Registry for algorithms for converting a message into a canonical form before signing or verifying using DKIM. The updated entries in the header registry comprise: +---------+-----------------+--------+ @@ -2411,21 +2424,21 @@ +---------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------------+--------+ | simple | (this document) | active | | relaxed | (this document) | active | +---------+-----------------+--------+ DKIM-Signature Body Canonicalization Algorithm Registry Updated Values -8.4. _domainkey DNS TXT Record Tag Specifications +7.4. _domainkey DNS TXT Record Tag Specifications A _domainkey DNS TXT record provides for a list of tag specifications. IANA has established the DKIM _domainkey DNS TXT Tag Specification Registry for tag specifications that can be used in DNS TXT Records. The updated entries in the registry comprise: +------+-----------------+----------+ | TYPE | REFERENCE | STATUS | @@ -2436,146 +2449,116 @@ | k | (this document) | active | | n | (this document) | active | | p | (this document) | active | | s | (this document) | active | | t | (this document) | active | +------+-----------------+----------+ DKIM _domainkey DNS TXT Record Tag Specification Registry Updated Values -8.5. DKIM Key Type Registry +7.5. DKIM Key Type Registry - The "k=" (specified in Section 4.6.1) and the "a=" (specified in Section 4.5) tags provide for a list of + The "k=" (specified in Section 3.6.1) and the "a=" (specified in Section 3.5) tags provide for a list of mechanisms that can be used to decode a DKIM signature. IANA has established the DKIM Key Type Registry for such mechanisms. The updated entry in the registry comprises: +------+-----------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------+--------+ | rsa | [RFC3447] | active | +------+-----------+--------+ DKIM Key Type Updated Values -8.6. DKIM Hash Algorithms Registry +7.6. DKIM Hash Algorithms Registry - The "h=" (specified in Section 4.6.1) and the "a=" (specified in Section 4.5) tags provide for a list of + The "h=" (specified in Section 3.6.1) and the "a=" (specified in Section 3.5) tags provide for a list of mechanisms that can be used to produce a digest of message data. IANA has established the DKIM Hash Algorithms Registry for such mechanisms. The updated entries in the registry comprise: +--------+-------------------+--------+ | TYPE | REFERENCE | STATUS | +--------+-------------------+--------+ | sha1 | [FIPS-180-2-2002] | active | | sha256 | [FIPS-180-2-2002] | active | +--------+-------------------+--------+ DKIM Hash Algorithms Updated Values -8.7. DKIM Service Types Registry +7.7. DKIM Service Types Registry - The "s=" tag (specified in Section 4.6.1) provides for a + The "s=" tag (specified in Section 3.6.1) provides for a list of service types to which this selector may apply. IANA has established the DKIM Service Types Registry for service types. The updated entries in the registry comprise: +-------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +-------+-----------------+--------+ | email | (this document) | active | | * | (this document) | active | +-------+-----------------+--------+ DKIM Service Types Registry Updated Values -8.8. DKIM Selector Flags Registry +7.8. DKIM Selector Flags Registry - The "t=" tag (specified in Section 4.6.1) provides for a + The "t=" tag (specified in Section 3.6.1) provides for a list of flags to modify interpretation of the selector. IANA has established the DKIM Selector Flags Registry for additional flags. The updated entries in the registry comprise: +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | y | (this document) | active | | s | (this document) | active | +------+-----------------+--------+ DKIM Selector Flags Registry Updated Values -8.9. DKIM-Signature Header Field +7.9. DKIM-Signature Header Field IANA has added DKIM-Signature to the "Permanent Message Header Fields" registry (see [RFC3864]) for the "mail" protocol, using this document as the reference. -9. Security Considerations +8. Security Considerations It has been observed that any mechanism that is introduced that attempts to stem the flow of spam is subject to intensive attack. DKIM needs to be carefully scrutinized to identify potential attack vectors and the vulnerability to each. See also [RFC4686]. -9.1. Misuse of Body Length Limits ("l=" Tag) - - Body length limits (in the form of the "l=" tag) are subject to - several potential attacks. - -9.1.1. Addition of New MIME Parts to Multipart/* - - If the body length limit does not cover a closing MIME multipart - section (including the trailing "--CRLF" portion), then it is - possible for an attacker to intercept a properly signed multipart - message and add a new body part. Depending on the details of the - MIME type and the implementation of the verifying MTA and the - receiving MUA, this could allow an attacker to change the information - displayed to an end user from an apparently trusted source. - - For example, if attackers can append information to a "text/html" - body part, they may be able to exploit a bug in some MUAs that - continue to read after a "" marker, and thus display HTML text - on top of already displayed text. If a message has a "multipart/ - alternative" body part, they might be able to add a new body part - that is preferred by the displaying MUA. - -9.1.2. Addition of new HTML content to existing content - - Several receiving MUA implementations do not cease display after a - """" tag. In particular, this allows attacks involving - overlaying images on top of existing text. - - INFORMATIVE EXAMPLE: Appending the following text to an existing, - properly closed message will in many MUAs result in inappropriate - data being rendered on top of existing, correct data: +8.1. Misuse of Body Length Limits ("l=" Tag) -
-
+ As noted in Section 3.4.5, use of the "l=" signature tag enables a + variety of attacks in which added content can partially or completely + change the recipient's view of the message. -9.2. Misappropriated Private Key +8.2. Misappropriated Private Key If the private key for a user is resident on their computer and is not protected by an appropriately secure mechanism, it is possible for malware to send mail as that user and any other user sharing the same private key. The malware would not, however, be able to generate signed spoofs of other signers' addresses, which would aid in identification of the infected user and would limit the possibilities for certain types of attacks involving socially engineered messages. This threat applies mainly to MUA-based implementations; protection of private keys on servers can be easily @@ -2602,38 +2585,38 @@ A somewhat more effective countermeasure is to send messages through an outgoing MTA that can authenticate the submitter using existing techniques (e.g., SMTP Authentication), possibly validate the message itself (e.g., verify that the header is legitimate and that the content passes a spam content check), and sign the message using a key appropriate for the submitter address. Such an MTA can also apply controls on the volume of outgoing mail each user is permitted to originate in order to further limit the ability of malware to generate bulk email. -9.3. Key Server Denial-of-Service Attacks +8.3. Key Server Denial-of-Service Attacks Since the key servers are distributed (potentially separate for each domain), the number of servers that would need to be attacked to defeat this mechanism on an Internet-wide basis is very large. Nevertheless, key servers for individual domains could be attacked, impeding the verification of messages from that domain. This is not significantly different from the ability of an attacker to deny service to the mail exchangers for a given domain, although it affects outgoing, not incoming, mail. A variation on this attack is that if a very large amount of mail were to be sent using spoofed addresses from a given domain, the key servers for that domain could be overwhelmed with requests. However, given the low overhead of verification compared with handling of the email message itself, such an attack would be difficult to mount. -9.4. Attacks Against the DNS +8.4. Attacks Against the DNS Since the DNS is a required binding for key services, specific attacks against the DNS must be considered. While the DNS is currently insecure [RFC3833], these security problems are the motivation behind DNS Security (DNSSEC) [RFC4033], and all users of the DNS will reap the benefit of that work. DKIM is only intended as a "sufficient" method of proving authenticity. It is not intended to provide strong cryptographic @@ -2649,21 +2632,21 @@ A specific DNS security issue that should be considered by DKIM verifiers is the name chaining attack described in Section 2.3 of [RFC3833]. A DKIM verifier, while verifying a DKIM-Signature header field, could be prompted to retrieve a key record of an attacker's choosing. This threat can be minimized by ensuring that name servers, including recursive name servers, used by the verifier enforce strict checking of "glue" and other additional information in DNS responses and are therefore not vulnerable to this attack. -9.5. Replay Attacks +8.5. Replay Attacks In this attack, a spammer sends a message to be spammed to an accomplice, which results in the message being signed by the originating MTA. The accomplice resends the message, including the original signature, to a large number of recipients, possibly by sending the message to many compromised machines that act as MTAs. The messages, not having been modified by the accomplice, have valid signatures. Partial solutions to this problem involve the use of reputation @@ -2672,191 +2655,191 @@ spam. This requires a real-time detection mechanism in order to react quickly enough. However, such measures might be prone to abuse, if for example an attacker resent a large number of messages received from a victim in order to make them appear to be a spammer. Large verifiers might be able to detect unusually large volumes of mails with the same signature in a short time period. Smaller verifiers can get substantially the same volume of information via existing collaborative systems. -9.6. Limits on Revoking Keys +8.6. Limits on Revoking Keys When a large domain detects undesirable behavior on the part of one of its users, it might wish to revoke the key used to sign that user's messages in order to disavow responsibility for messages that have not yet been verified or that are the subject of a replay attack. However, the ability of the domain to do so can be limited if the same key, for scalability reasons, is used to sign messages for many other users. Mechanisms for explicitly revoking keys on a per-address basis have been proposed but require further study as to their utility and the DNS load they represent. -9.7. Intentionally Malformed Key Records +8.7. Intentionally Malformed Key Records It is possible for an attacker to publish key records in DNS that are intentionally malformed, with the intent of causing a denial-of- service attack on a non-robust verifier implementation. The attacker could then cause a verifier to read the malformed key record by sending a message to one of its users referencing the malformed record in a (not necessarily valid) signature. Verifiers MUST thoroughly verify all key records retrieved from the DNS and be robust against intentionally as well as unintentionally malformed key records. -9.8. Intentionally Malformed DKIM-Signature Header Fields +8.8. Intentionally Malformed DKIM-Signature Header Fields Verifiers MUST be prepared to receive messages with malformed DKIM- Signature header fields, and thoroughly verify the header field before depending on any of its contents. -9.9. Information Leakage +8.9. Information Leakage An attacker could determine when a particular signature was verified by using a per-message selector and then monitoring their DNS traffic for the key lookup. This would act as the equivalent of a "web bug" for verification time rather than when the message was read. -9.10. Remote Timing Attacks +8.10. Remote Timing Attacks In some cases it may be possible to extract private keys using a remote timing attack [BONEH03]. Implementations should consider obfuscating the timing to prevent such attacks. -9.11. Reordered Header Fields +8.11. Reordered Header Fields Existing standards allow intermediate MTAs to reorder header fields. If a signer signs two or more header fields of the same name, this can cause spurious verification errors on otherwise legitimate messages. In particular, signers that sign any existing DKIM- Signature fields run the risk of having messages incorrectly fail to verify. -9.12. RSA Attacks +8.12. RSA Attacks An attacker could create a large RSA signing key with a small exponent, thus requiring that the verification key have a large exponent. This will force verifiers to use considerable computing resources to verify the signature. Verifiers might avoid this attack by refusing to verify signatures that reference selectors with public keys having unreasonable exponents. In general, an attacker might try to overwhelm a verifier by flooding it with messages requiring verification. This is similar to other MTA denial-of-service attacks and should be dealt with in a similar fashion. -9.13. Inappropriate Signing by Parent Domains +8.13. Inappropriate Signing by Parent Domains - The trust relationship described in Section 4.9 could conceivably be + The trust relationship described in Section 3.10 could conceivably be used by a parent domain to sign messages with identities in a subdomain not administratively related to the parent. For example, the ".com" registry could create messages with signatures using an "i=" value in the example.com domain. There is no general solution to this problem, since the administrative cut could occur anywhere in the domain name. For example, in the domain "example.podunk.ca.us" there are three administrative cuts (podunk.ca.us, ca.us, and us), any of which could create messages with an identity in the full domain. INFORMATIVE NOTE: This is considered an acceptable risk for the same reason that it is acceptable for domain delegation. For example, in the example above any of the domains could potentially simply delegate "example.podunk.ca.us" to a server of their choice and completely replace all DNS-served information. Note that a verifier MAY ignore signatures that come from an unlikely domain - such as ".com", as discussed in Section 7.1.1. + such as ".com", as discussed in Section 6.1.1. -9.14. Attacks Involving Addition of Header Fields +8.14. Attacks Involving Addition of Header Fields Many email implementations do not enforce [RFC5322] with strictness. - As discussed in Section 6.3 DKIM processing is predicated on a valid + As discussed in Section 5.3 DKIM processing is predicated on a valid mail message as its input. However, DKIM implementers should be aware of the potential effect of having loose enforcement by email components interacting with DKIM modules. For example, a message with multiple From: header fields violates Section 3.6 of [RFC5322]. With the intent of providing a better user experience, many agents tolerate these violations and deliver the message anyway. An MUA then might elect to render to the user the - value of the first {DKIM 16}, or "top", From: field. This may also - be done simply out of the expectation that there is only one, where a - "find first" algorithm would have the same result. Such code in an - MUA can be exploited to fool the user if it is also known that the - other From: field is the one checked by arriving message filters. - Such is the case with DKIM; although the From: field must be signed, - a malformed message bearing more than one From: field might only have + value of the first, or "top", From: field. This may also be done + simply out of the expectation that there is only one, where a "find + first" algorithm would have the same result. Such code in an MUA can + be exploited to fool the user if it is also known that the other + From: field is the one checked by arriving message filters. Such is + the case with DKIM; although the From: field must be signed, a + malformed message bearing more than one From: field might only have the first ("bottom") one signed, in an attempt to show the message with some "DKIM passed" annotation while also rendering the From: field that was not authenticated. (This can also be taken as a demonstration that DKIM is not designed to support author validation.) Note that the technique for using "h=...:from:from:...", described in - Section 9.15 below, is of no effect in the case of an attacker that - is also the signer. {DKIM 16} + Section 8.15 below, is of no effect in the case of an attacker that + is also the signer. The From: field is used above to illustrate this issue, but it is only one of several fields that Section 3.6 of [RFC5322] constrains - in this way. In reality any agent that forgives such malformations - {DKIM 16}, or is careless about identifying which parts of a message - were authenticated, is open to exploitation. + in this way. In reality any agent that forgives such malformations, + or is careless about identifying which parts of a message were + authenticated, is open to exploitation. -9.15. Malformed Inputs +8.15. Malformed Inputs DKIM allows additional header fields to be added to a signed message without breaking the signature. This tolerance can be abused, for - example in a replay attack or a man-in-the-middle attack {DKIM 16}. - The attack is accomplished by creating additional instances of header - fields to an already signed message, without breaking the signature. - These then might be displayed to the end user or are used as - filtering input. Salient fields could include From: and Subject:, + example in a replay attack or a man-in-the-middle attack. The attack + is accomplished by creating additional instances of header fields to + an already signed message, without breaking the signature. These + then might be displayed to the end user or are used as filtering + input. Salient fields could include From: and Subject:, The resulting message violates section 3.6 of [RFC5322]. The way such input will be handled and displayed by an MUA is unpredictable, - but it will commonly {DKIM 16} display the newly added header fields - rather than those that are part of the originally signed message - alongside some "valid DKIM signature" annotation. This might allow - an attacker to replay a previously sent, signed message with a - different Subject:, From: or To: field. + but it will commonly display the newly added header fields rather + than those that are part of the originally signed message alongside + some "valid DKIM signature" annotation. This might allow an attacker + to replay a previously sent, signed message with a different + Subject:, From: or To: field. However, [RFC5322] also tolerates obsolete message syntax, which does allow things like multiple From: fields on messages. The implementation of DKIM thus potentially creates a more stringent layer of expectation regarding the meaning of an identity, while that additional meaning is either obscured from or incorrectly presented to an end user in this context. Implementers need to consider this possibility when designing their input handling functions. Outright rejection of messages that violate the relevant standards such as [RFC5322], [RFC2045], etc. - will interfere with delivery of {DKIM 16} legacy formats. Instead, - given such input, a signing module could return an error rather than - generate a signature; a verifying module might return a syntax error - code or arrange not to return a positive result even if the signature + will interfere with delivery of legacy formats. Instead, given such + input, a signing module could return an error rather than generate a + signature; a verifying module might return a syntax error code or + arrange not to return a positive result even if the signature technically validates. Senders concerned that their messages might be particularly vulnerable to this sort of attack and who do not wish to rely on receiver filtering of invalid messages can ensure that adding additional header fields will break the DKIM signature by including two copies of the header fields about which they are concerned in the signature (e.g. "h= ... from:from:to:to:subject:subject ..."). See Sections 3.5 and 5.4 for further discussion of this mechanism. Specific validity rules for all known header fields can be gleaned from the IANA "Permanent Header Field Registry" and the reference documents it identifies. -10. References +9. References -10.1. Normative References +9.1. Normative References [FIPS-180-2-2002] U.S. Department of Commerce, "Secure Hash Standard", FIPS PUB 180-2, August 2002. [ITU-X660-1997] "Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", 1997. @@ -2888,21 +2871,21 @@ [RFC5322] Resnick, P., "Internet Message Format", RFC 5322, October 2008. [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. [RFC5890] Klensin, J., "Internationalizing Domain Names in Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. -10.2. Informative References +9.2. Informative References [BONEH03] "Remote Timing Attacks are Practical", Proceedings 12th USENIX Security Symposium, 2003. [RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted", RFC 1847, October 1995. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, @@ -2936,24 +2919,32 @@ [RFC4880] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP Message Format", RFC 4880, November 2007. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5451] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, April 2009. + [RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys + Identified Mail (DKIM) Service Overview", RFC 5585, + July 2009. + [RFC5751] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 5751, January 2010. + [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, + "DomainKeys Identified Mail (DKIM) Development, + Deployment, and Operations", RFC 5863, May 2010. + Appendix A. Example of Use (INFORMATIVE) This section shows the complete flow of an email from submission to final delivery, demonstrating how the various components fit together. The key used in this example is shown in Appendix C. A.1. The User Composes an Email From: Joe SixPack To: Suzie Q Subject: Is dinner ready? @@ -3293,21 +3284,21 @@ similar to this: -----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI MmPSPDdQPNUYckcQ2QIDAQAB -----END PUBLIC KEY----- This public-key data (without the BEGIN and END tags) is placed in the DNS: - $ORIGIN _domainkey.example.org. {DKIM 10} + $ORIGIN _domainkey.example.org. brisbane IN TXT ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ" "KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt" "IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v" "/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi" "tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB") C.1. Compatibility with DomainKeys Key Records DKIM key records were designed to be backwards-compatible in many cases with key records used by DomainKeys [RFC4870] (sometimes @@ -3372,20 +3363,26 @@ Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau, Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker, Sam Weiler, and Dan Wing. The earlier DomainKeys was a primary source from which DKIM was derived. Further information about DomainKeys is at [RFC4870]. + This revision received contributions from: Steve Atkins, Mark Delany, + J.D. Falk, Jim Fenton, Michael Hammer, Barry Leiba, John Levine, + Charles Lindsey, Jeff Macdonald, Franck Martin, Brett McDowell, Doug + Otis, Bill Oxley, Hector Santos, Rolf Sonneveld, Michael Thomas, and + Alessandro Vesely. + Authors' Addresses D. Crocker (editor) Brandenburg InternetWorking 675 Spruce Dr. Sunnyvale USA Phone: +1.408.246.8253 Email: dcrocker@bbiw.net