--- 1/draft-ietf-dkim-base-08.txt 2007-02-13 22:12:11.000000000 +0100 +++ 2/draft-ietf-dkim-base-09.txt 2007-02-13 22:12:12.000000000 +0100 @@ -1,25 +1,25 @@ DKIM E. Allman Internet-Draft Sendmail, Inc. -Expires: July 22, 2007 J. Callas - PGP Corporation +Intended status: Standards Track J. Callas +Expires: August 15, 2007 PGP Corporation M. Delany M. Libbey Yahoo! Inc J. Fenton M. Thomas Cisco Systems, Inc. - January 18, 2007 + February 11, 2007 DomainKeys Identified Mail (DKIM) Signatures - draft-ietf-dkim-base-08 + draft-ietf-dkim-base-09 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that @@ -30,25 +30,25 @@ 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on July 22, 2007. + This Internet-Draft will expire on August 15, 2007. Copyright Notice - Copyright (C) The Internet Society (2007). + Copyright (C) The IETF Trust (2007). Abstract DomainKeys Identified Mail (DKIM) defines a domain-level authentication framework for email using public-key cryptography and key server technology to permit verification of the source and contents of messages by either Mail Transfer Agents (MTAs) or Mail User Agents (MUAs). The ultimate goal of this framework is to permit a signing domain to assert responsibility for a message, thus protecting message signer identity and the integrity of the messages @@ -57,105 +57,107 @@ global control of "spam" and "phishing". Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 - 1.1 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 - 1.2 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 - 1.3 Simple Key Management . . . . . . . . . . . . . . . . . . 6 - 2. Terminology and Definitions . . . . . . . . . . . . . . . . 6 - 2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 - 2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7 - 2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7 - 2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8 - 2.6 DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8 - 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 9 - 3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9 - 3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11 - 3.3 Signing and Verification Algorithms . . . . . . . . . . . 12 - 3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 14 - 3.5 The DKIM-Signature header field . . . . . . . . . . . . . 18 - 3.6 Key Management and Representation . . . . . . . . . . . . 26 - 3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 30 - 3.8 Signing by Parent Domains . . . . . . . . . . . . . . . . 32 - 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 33 - 4.1 Example Scenarios . . . . . . . . . . . . . . . . . . . . 33 - 4.2 Interpretation . . . . . . . . . . . . . . . . . . . . . . 34 - 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 35 - 5.1 Determine if the Email Should be Signed and by Whom . . . 35 - 5.2 Select a Private Key and Corresponding Selector - Information . . . . . . . . . . . . . . . . . . . . . . . 36 - 5.3 Normalize the Message to Prevent Transport Conversions . . 36 - 5.4 Determine the Header Fields to Sign . . . . . . . . . . . 37 - 5.5 Compute the Message Hash and Signature . . . . . . . . . . 40 - 5.6 Insert the DKIM-Signature Header Field . . . . . . . . . . 40 - 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 41 - 6.1 Extract Signatures from the Message . . . . . . . . . . . 41 - 6.2 Communicate Verification Results . . . . . . . . . . . . . 47 - 6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 47 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 49 - 7.1 DKIM-Signature Tag Specifications . . . . . . . . . . . . 49 - 7.2 DKIM-Signature Query Method Registry . . . . . . . . . . . 49 - 7.3 DKIM-Signature Canonicalization Registry . . . . . . . . . 50 - 7.4 _domainkey DNS TXT Record Tag Specifications . . . . . . . 50 - 7.5 DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 51 - 7.6 DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 51 - 7.7 DKIM Service Types Registry . . . . . . . . . . . . . . . 52 - 7.8 DKIM Selector Flags Registry . . . . . . . . . . . . . . . 52 - 7.9 DKIM-Signature Header Field . . . . . . . . . . . . . . . 53 - 8. Security Considerations . . . . . . . . . . . . . . . . . . 53 - 8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 53 - 8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 54 - 8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 54 - 8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 55 - 8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 55 - 8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 56 - 8.7 Intentionally malformed Key Records . . . . . . . . . . . 56 - 8.8 Intentionally Malformed DKIM-Signature header fields . . . 56 - 8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 57 - 8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 57 - 8.11 Reordered Header Fields . . . . . . . . . . . . . . . . 57 - 8.12 RSA Attacks . . . . . . . . . . . . . . . . . . . . . . 57 - 8.13 Inappropriate Signing by Parent Domains . . . . . . . . 57 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 58 - 9.1 Normative References . . . . . . . . . . . . . . . . . . . 58 - 9.2 Informative References . . . . . . . . . . . . . . . . . . 59 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 59 - A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 61 - A.1 The user composes an email . . . . . . . . . . . . . . . . 61 - A.2 The email is signed . . . . . . . . . . . . . . . . . . . 61 - A.3 The email signature is verified . . . . . . . . . . . . . 62 - B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 63 - B.1 Alternate Submission Scenarios . . . . . . . . . . . . . . 64 - B.2 Alternate Delivery Scenarios . . . . . . . . . . . . . . . 66 - C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 68 - D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 70 - E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 70 - F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 71 - F.1 Changes since -ietf-07 version . . . . . . . . . . . . . . 71 - F.2 Changes since -ietf-06 version . . . . . . . . . . . . . . 72 - F.3 Changes since -ietf-05 version . . . . . . . . . . . . . . 73 - F.4 Changes since -ietf-04 version . . . . . . . . . . . . . . 73 - F.5 Changes since -ietf-03 version . . . . . . . . . . . . . . 74 - F.6 Changes since -ietf-02 version . . . . . . . . . . . . . . 75 - F.7 Changes since -ietf-01 version . . . . . . . . . . . . . . 76 - F.8 Changes since -ietf-00 version . . . . . . . . . . . . . . 76 - F.9 Changes since -allman-01 version . . . . . . . . . . . . . 77 - F.10 Changes since -allman-00 version . . . . . . . . . . . . 77 - Intellectual Property and Copyright Statements . . . . . . . 79 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 1.1. Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 + 1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 + 1.3. Simple Key Management . . . . . . . . . . . . . . . . . . 6 + 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6 + 2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 + 2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 + 2.3. White Space . . . . . . . . . . . . . . . . . . . . . . . 7 + 2.4. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7 + 2.5. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8 + 2.6. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8 + 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 9 + 3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9 + 3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11 + 3.3. Signing and Verification Algorithms . . . . . . . . . . . 12 + 3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 14 + 3.5. The DKIM-Signature header field . . . . . . . . . . . . . 18 + 3.6. Key Management and Representation . . . . . . . . . . . . 26 + 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 30 + 3.8. Signing by Parent Domains . . . . . . . . . . . . . . . . 32 + 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 33 + 4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 33 + 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 34 + 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 35 + 5.1. Determine if the Email Should be Signed and by Whom . . . 35 + 5.2. Select a Private Key and Corresponding Selector + Information . . . . . . . . . . . . . . . . . . . . . . . 35 + 5.3. Normalize the Message to Prevent Transport Conversions . . 36 + 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 36 + 5.5. Recommended Signature Content . . . . . . . . . . . . . . 39 + 5.6. Compute the Message Hash and Signature . . . . . . . . . . 40 + 5.7. Insert the DKIM-Signature Header Field . . . . . . . . . . 41 + 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 41 + 6.1. Extract Signatures from the Message . . . . . . . . . . . 42 + 6.2. Communicate Verification Results . . . . . . . . . . . . . 47 + 6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 48 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49 + 7.1. DKIM-Signature Tag Specifications . . . . . . . . . . . . 49 + 7.2. DKIM-Signature Query Method Registry . . . . . . . . . . . 50 + 7.3. DKIM-Signature Canonicalization Registry . . . . . . . . . 50 + 7.4. _domainkey DNS TXT Record Tag Specifications . . . . . . . 51 + 7.5. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 51 + 7.6. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 52 + 7.7. DKIM Service Types Registry . . . . . . . . . . . . . . . 52 + 7.8. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 53 + 7.9. DKIM-Signature Header Field . . . . . . . . . . . . . . . 53 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 53 + 8.1. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 53 + 8.2. Misappropriated Private Key . . . . . . . . . . . . . . . 54 + 8.3. Key Server Denial-of-Service Attacks . . . . . . . . . . . 55 + 8.4. Attacks Against DNS . . . . . . . . . . . . . . . . . . . 55 + 8.5. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 56 + 8.6. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 56 + 8.7. Intentionally malformed Key Records . . . . . . . . . . . 57 + 8.8. Intentionally Malformed DKIM-Signature header fields . . . 57 + 8.9. Information Leakage . . . . . . . . . . . . . . . . . . . 57 + 8.10. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 57 + 8.11. Reordered Header Fields . . . . . . . . . . . . . . . . . 57 + 8.12. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 57 + 8.13. Inappropriate Signing by Parent Domains . . . . . . . . . 58 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 58 + 9.1. Normative References . . . . . . . . . . . . . . . . . . . 58 + 9.2. Informative References . . . . . . . . . . . . . . . . . . 59 + Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 60 + A.1. The user composes an email . . . . . . . . . . . . . . . . 60 + A.2. The email is signed . . . . . . . . . . . . . . . . . . . 60 + A.3. The email signature is verified . . . . . . . . . . . . . 61 + Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 62 + B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 63 + B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 65 + Appendix C. Creating a public key (INFORMATIVE) . . . . . . . . . 67 + Appendix D. MUA Considerations . . . . . . . . . . . . . . . . . 69 + Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 69 + Appendix F. Edit History . . . . . . . . . . . . . . . . . . . . 70 + F.1. Changes since -ietf-08 version . . . . . . . . . . . . . . 70 + F.2. Changes since -ietf-07 version . . . . . . . . . . . . . . 70 + F.3. Changes since -ietf-06 version . . . . . . . . . . . . . . 72 + F.4. Changes since -ietf-05 version . . . . . . . . . . . . . . 72 + F.5. Changes since -ietf-04 version . . . . . . . . . . . . . . 73 + F.6. Changes since -ietf-03 version . . . . . . . . . . . . . . 73 + F.7. Changes since -ietf-02 version . . . . . . . . . . . . . . 74 + F.8. Changes since -ietf-01 version . . . . . . . . . . . . . . 75 + F.9. Changes since -ietf-00 version . . . . . . . . . . . . . . 76 + F.10. Changes since -allman-01 version . . . . . . . . . . . . . 77 + F.11. Changes since -allman-00 version . . . . . . . . . . . . . 77 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 77 + Intellectual Property and Copyright Statements . . . . . . . . . . 80 1. Introduction [[Note: text in double square brackets (such as this text) will be deleted before publication.]] DomainKeys Identified Mail (DKIM) defines a mechanism by which email messages can be cryptographically signed, permitting a signing domain to claim responsibility for the introduction of a message into the mail stream. Message recipients can verify the signature by querying @@ -191,80 +193,80 @@ 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. -1.1 Signing Identity +1.1. 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. -1.2 Scalability +1.2. Scalability DKIM is designed to support the extreme scalability requirements which 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. -1.3 Simple Key Management +1.3. 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. Terminology and Definitions This section defines terms used in the rest of the document. Syntax descriptions use the form described in Augmented BNF for Syntax Specifications [RFC4234]. -2.1 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. -2.2 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. -2.3 White Space +2.3. White Space There are three forms of white space: o WSP represents simple white space, i.e., a space or a tab character (formal definition in [RFC4234]). o LWSP is linear white space, defined as WSP plus CRLF (formal definition in [RFC4234]). o FWS is folding white space. It allows multiple lines separated by @@ -273,29 +275,29 @@ 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 [RFC2822] except for the exclusion of obs-FWS. -2.4 Common ABNF Tokens +2.4. Common ABNF Tokens The following ABNF tokens are used elsewhere in this document. hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ] base64string = 1*(ALPHA / DIGIT / "+" / "/" / LWSP) [ "=" LWSP [ "=" LWSP ] ] -2.5 Imported ABNF Tokens +2.5. 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 [RFC2821]: o "Local-part" (implementation warning: this permits quoted strings) o "sub-domain" @@ -313,21 +315,21 @@ o "hex-octet" (a quoted-printable encoded octet) INFORMATIVE NOTE: Be aware that the ABNF in RFC 2045 does not obey the rules of RFC 4234 and must be interpreted accordingly, particularly as regards case folding. Other tokens not defined herein are imported from [RFC4234]. These are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF, etc. -2.6 DKIM-Quoted-Printable +2.6. 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 lower case characters permitted) representing the hexadecimal-encoded integer value of that character. All control characters (those with values < %x20), eight-bit characters (values > %x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon (";", %x3B) MUST be encoded. Note that all white space, including SPACE, CR and LF characters, MUST be encoded. After encoding, FWS @@ -367,21 +369,21 @@ text is being used). 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 5) and Verifier Actions (Section 6)). NOTE: This section must be read in the context of those sections. -3.1 Selectors +3.1. Selectors To support multiple concurrent public keys per signing domain, the 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 the individual user. Selectors are needed to support some important use cases. For example: @@ -444,21 +446,21 @@ 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. -3.2 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 2.6). 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. @@ -498,56 +500,54 @@ 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. For example, "g=" does not mean "g=*", even though "g=*" is the default for that tag. -3.3 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. The rsa-sha256 algorithm is the default if no algorithm is specified. Verifiers MUST implement both rsa-sha1 and rsa-sha256. Signers MUST implement and SHOULD sign using rsa-sha256. INFORMATIVE NOTE: Although sha256 is strongly encouraged, some senders of low-security messages (such as routine newsletters) may prefer to use sha1 because of reduced CPU requirements to compute a sha1 hash. In general, sha256 should always be used whenever possible. -3.3.1 The rsa-sha1 Signing Algorithm +3.3.1. The rsa-sha1 Signing Algorithm The rsa-sha1 Signing Algorithm computes a message hash as described in Section 3.7 below using SHA-1 [SHA] as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 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. - - INFORMATIVE IMPLEMENTATION WARNING: Low-valued exponents should - be avoided, as they are believed to be subject to attack. + the native binary form before being signed. The signing algorithm + SHOULD use an exponent of 65537. -3.3.2 The rsa-sha256 Signing Algorithm +3.3.2. The rsa-sha256 Signing Algorithm The rsa-sha256 Signing Algorithm computes a message hash as described in Section 3.7 below using SHA-256 [SHA] as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. -3.3.3 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. @@ -556,31 +556,31 @@ o The practical constraint that large (e.g., 4096 bit) keys may not fit within a 512 byte DNS UDP response packet o The security constraint that keys smaller than 1024 bits are subject to off-line attacks 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. -3.3.4 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. -3.4 Canonicalization +3.4. Canonicalization Empirical evidence demonstrates that some mail servers and relay systems modify email in transit, potentially invalidating a signature. There are two competing perspectives on such modifications. For most signers, mild modification of email is immaterial to the authentication status of the email. For such signers a canonicalization algorithm that survives modest in-transit modification is preferred. Other signers demand that any modification of the email, however @@ -608,29 +608,29 @@ 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 (e.g., text is ASCII encoded, lines are separated with CRLF characters, etc.). See also Section 5.3 for information about normalizing the message. -3.4.1 The "simple" Header Canonicalization Algorithm +3.4.1. The "simple" Header Canonicalization Algorithm The "simple" header canonicalization algorithm does not change header 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 white space MUST NOT be changed. -3.4.2 The "relaxed" Header Canonicalization Algorithm +3.4.2. The "relaxed" Header Canonicalization Algorithm The "relaxed" header canonicalization algorithm MUST apply the following steps in order: o Convert all header field names (not the header field values) to lower case. For example, convert "SUBJect: AbC" to "subject: AbC". o Unfold all header field continuation lines as described in [RFC2822]; in particular, lines with terminators embedded in @@ -642,47 +642,53 @@ 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. -3.4.3 The "simple" Body Canonicalization Algorithm +3.4.3. The "simple" Body Canonicalization Algorithm The "simple" body canonicalization algorithm ignores all empty lines 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. -3.4.4 The "relaxed" Body Canonicalization Algorithm +3.4.4. The "relaxed" Body Canonicalization Algorithm The "relaxed" body canonicalization algorithm: o Ignores all white space at the end of lines. Implementations MUST NOT remove the CRLF at the end of the line. o Reduces all sequences of WSP within a line to a single SP character. o Ignores all empty lines at the end of the message body. "Empty line" is defined in Section 3.4.3. -3.4.5 Body Length Limits + INFORMATIVE NOTE: It should be noted that the relaxed body + canonicalization algorithm may enable certain types of extremely + crude "ASCII Art" attacks where a message may be conveyed by + adjusting the spacing between words. If this is a concern, the + "simple" body canonicalization algorithm should be used instead. + +3.4.5. Body Length Limits A body length count MAY be specified to limit the signature calculation to an initial prefix of the body text, measured in octets. If the body length count is not specified then 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 @@ -717,21 +723,21 @@ only works for some MIME types, e.g., multipart/mixed but not multipart/signed. 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. -3.4.6 Canonicalization Examples (INFORMATIVE) +3.4.6. Canonicalization Examples (INFORMATIVE) In the following examples, actual white space 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 @@ -765,21 +771,21 @@ 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 -3.5 The DKIM-Signature header field +3.5. The DKIM-Signature header field The signature of the email is stored in the "DKIM-Signature:" header 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 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 [RFC2822], and hence SHOULD NOT be reordered and SHOULD be prepended to the message. @@ -1113,21 +1119,21 @@ DKIM-Signature: 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+yuU4zGeeruD00lszZ VoG4ZHRNiYzR -3.6 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 @@ -1136,21 +1142,21 @@ 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. -3.6.1 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 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 @@ -1180,25 +1186,26 @@ key to a third party that should only be used for special purposes. Wildcarding allows matching for addresses such as "user+*" or "*-offer". An empty "g=" value never matches any addresses. ABNF: key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart key-g-tag-lpart = [dot-atom-text] ["*" [dot-atom-text] ] - [[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a "dot- - atom-text". This should probably use a different character - for wildcarding. Unfortunately, the options are non-mnemonic - (e.g., "@", "(", ":"). Alternatively we could insist on - escaping a "*" intended as a literal "*" in the address.]] + [[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a + "dot-atom-text". This should probably use a different + character for wildcarding. Unfortunately, the options are + non-mnemonic (e.g., "@", "(", ":"). Alternatively we could + insist on escaping a "*" intended as a literal "*" in the + address.]] h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to allowing all algorithms). A colon-separated list of hash algorithms that might be used. Signers and Verifiers MUST support the "sha256" hash algorithm. Verifiers MUST also support the "sha1" hash algorithm. ABNF: key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg @@ -1284,52 +1291,52 @@ 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. -3.6.2 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. -3.6.2.1 Name Space +3.6.2.1. Name Space 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". INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records (e.g., *.bar._domainkey.example.com) do not make sense in this context and should not be used. Note also that wildcards within domains (e.g., s._domainkey.*.example.com) are not supported by the DNS. -3.6.2.2 Resource Record Types for Key Storage +3.6.2.2. Resource Record Types for Key Storage The DNS Resource Record type used is specified by an option to the 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 white space. 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 3.6.1. -3.7 Computing the Message Hashes +3.7. Computing the Message Hashes Both signing and verifying message signatures starts with a step of computing two cryptographic hashes over the message. Signers will choose the parameters of the signature as described in Signer Actions (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 which either exists (when verifying) or will be created (when signing) are used. Note that canonicalization (Section 3.4) is only used to prepare the email for signing or verifying; it does not @@ -1402,21 +1409,21 @@ DKIM-Signature header field), and "DKIM-SIG" is the canonicalized DKIM-Signature header field sans the signature value itself, but with "body-hash" included as the "bh=" tag. INFORMATIVE IMPLEMENTERS' 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 "hash-alg" and the "sig-alg". -3.8 Signing by Parent Domains +3.8. Signing by Parent Domains In some circumstances, it is desirable for a domain to apply a 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, e.g., a key record for the domain example.com can be used to verify messages where the signing identity (i= tag of the signature) is 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 @@ -1424,21 +1431,21 @@ of the record to exactly the domain of the signing identity. If the referenced key record contains the "s" flag as part of the t= tag, the domain of the signing identity (i= flag) MUST be the same as that of the d= domain. If this flag is absent, the domain of the signing identity MUST be the same as, or a subdomain of, the d= domain. Key records which are not intended for use with subdomains SHOULD specify the "s" flag in the t= tag. 4. Semantics of Multiple Signatures -4.1 Example Scenarios +4.1. Example Scenarios There are many reasons that 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 @@ -1475,27 +1482,27 @@ even from unknown authors. They might also subscribe to less trusted mailing lists (e.g., those without anti-abuse protection) and be willing to accept all messages from specific authors, but insist on doing additional abuse scanning for other messages. Another related example of multiple signers might be forwarding services, such as those commonly associated with academic alumni sites. INFORMATIVE EXAMPLE: A recipient might have an address at - alumni.example.edu, a site that has anti-abuse protection that is + members.example.org, a site that has anti-abuse protection that is somewhat less effective than the recipient would prefer. Such a recipient might have specific authors whose messages would be trusted absolutely, but messages from unknown authors which had passed the forwarder's scrutiny would have only medium trust. -4.2 Interpretation +4.2. Interpretation A signer that is adding a signature to a message merely creates a new 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 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 @@ -1534,21 +1541,21 @@ 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. 5. Signer Actions The following steps are performed in order by signers. -5.1 Determine if the Email Should be Signed and by Whom +5.1. Determine if the Email Should be Signed and by Whom A signer can obviously only sign email for domains for which it has a 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, @@ -1560,41 +1567,41 @@ 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. -5.2 Select a Private Key and Corresponding Selector Information +5.2. Select a Private Key and Corresponding Selector Information This specification does not define the basis by which a signer should 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. -5.3 Normalize the Message to Prevent Transport Conversions +5.3. Normalize the Message to Prevent Transport Conversions Some messages, particularly those using 8-bit characters, are subject 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 MIME Part One [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. @@ -1605,21 +1612,21 @@ 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. -5.4 Determine the Header Fields to Sign +5.4. Determine the Header Fields to Sign The From header field MUST be signed (that is, included in the h= tag 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, [RFC2821] 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 @@ -1701,21 +1708,21 @@ 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. -5.4.1 Recommended Signature Content +5.5. Recommended Signature Content In order to maximize compatibility with a variety of verifiers, it is recommended that signers follow the practices outlined in this section when signing a message. However, these are generic recommendations applying to the general case; specific senders may wish to modify these guidelines as required by their unique situations. Verifiers MUST be capable of verifying signatures even if one or more of the recommended header fields is not signed (with the exception of From, which must always be signed) or if one or more of the disrecommended header fields is signed. Note that verifiers @@ -1762,45 +1768,58 @@ o DKIM-Signature Optional header fields (those not mentioned above) normally SHOULD NOT be included in the signature, because of the potential for additional header fields of the same name to be legitimately added or reordered prior to verification. There are likely to be legitimate exceptions to this rule, because of the wide variety of application- specific header fields which may be applied to a message, some of which are unlikely to be duplicated, modified, or reordered. - Signers SHOULD include all or nearly all of the body content when - specifying the body length count (l= tag) in the signature. In - particular, signers SHOULD NOT specify a body length of 0 since this - may be interpreted as a meaningless signature by some verifiers. + Signers SHOULD choose canonicalization algorithms based on the types + of messages they process and their aversion to risk. For example, + e-commerce sites sending primarily purchase receipts, which are not + expected to be processed by mailing lists or other software likely to + modify messages, will generally prefer "simple" canonicalization. + Sites sending primarily person-to-person email will likely prefer to + be more more resilient to modification during transport by using + "relaxed" canonicalization. -5.5 Compute the Message Hash and Signature + Signers SHOULD NOT use l= unless they intend to accomodate + intermediate mail processors that append text to a message. For + example, many mailing list processors append "unsubscribe" + information to message bodies. If signers use l=, they SHOULD + include the entire message body existing at the time of signing in + computing the count. In particular, signers SHOULD NOT specify a + body length of 0 since this may be interpreted as a meaningless + signature by some verifiers. + +5.6. Compute the Message Hash and Signature The signer MUST compute the message hash as described in Section 3.7 and then sign it using the selected public-key algorithm. This will result in a DKIM-Signature header field which will include the body hash and a signature of the header hash, where that header includes the DKIM-Signature header field itself. Entities such as mailing list managers that implement DKIM and which 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. -5.6 Insert the DKIM-Signature Header Field +5.7. Insert the DKIM-Signature Header Field Finally, the signer MUST insert the "DKIM-Signature:" header field 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 5.2 @@ -1834,21 +1853,21 @@ 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. -6.1 Extract Signatures from the Message +6.1. Extract Signatures from the Message The order in which verifiers try DKIM-Signature header fields is not defined; verifiers MAY try signatures in any order they would like. For example, one implementation might prefer to try the signatures in textual order, whereas another might want to prefer signatures by identities that match the contents of the "From" header field over other identities. 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. @@ -1907,21 +1927,21 @@ 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. -6.1.1 Validate the Signature Header Field +6.1.1. Validate the Signature Header Field Implementers MUST meticulously validate the format and values in the 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 @@ -1952,28 +1972,33 @@ verifier should return PERMFAIL (domain mismatch). If the "h=" tag does not include the "From" header field the verifier MUST ignore the DKIM-Signature header field and return PERMFAIL (From field not signed). Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (signature expired) if it contains an "x=" tag and the signature has expired. + Verifiers MAY ignore the DKIM-Signature header field if the domain + used by the signer in the d= tag is not associated with a valid + signing entity. For example, signatures with d= values such as "com" + and "co.uk" may be ignored. + Verifiers MAY ignore the DKIM-Signature header field and return 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. -6.1.2 Get the Public Key +6.1.2. Get the Public Key The public key for a signature is needed to complete the verification 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 3.6. The verifier MUST validate the key record and MUST ignore any public key records that are malformed. @@ -2039,21 +2064,21 @@ 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. 9. If the public key data is not suitable for use with the algorithm and key types defined by the "a=" and "k=" tags in the "DKIM- Signature" header field, the verifier MUST immediately return PERMFAIL (inappropriate key algorithm). -6.1.3 Compute the Verification +6.1.3. Compute the Verification Given a signer and a public key, verifying a signature consists of 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 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, @@ -2098,63 +2123,59 @@ at the indicated body length might pass on a malformed MIME message if the signer used the "N-4" trick (omitting the final "--CRLF") described in the informative note in Section 3.4.5. Such verifiers may wish to check for this case and include a trailing "--CRLF" to avoid breaking the MIME structure. A simple way to achieve this might be to append "--CRLF" to any "multipart" message with a body length; if the MIME structure is already correctly formed, this will appear in the postlude and will not be displayed to the end user. -6.2 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. 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. -6.3 Interpret Results/Apply Local Policy +6.3. Interpret Results/Apply Local Policy It is beyond the scope of this specification to describe what actions a verifier system should make, but an authenticated email presents an opportunity to a receiving system that unauthenticated email cannot. 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 - SHOULD use a 550/5.7.x reply code, for example: - - 550 5.7.1 Unsigned messages not accepted - - 550 5.7.5 Message signature incorrect + 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: 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 @@ -2179,24 +2200,24 @@ never there, or was it removed by an over-zealous 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 rendered the same as all unverified email regardless of whether it looks like it was signed or not. 7. IANA Considerations DKIM introduces some new namespaces that require IANA registry. In - all cases, new values are assigned only for Standards Track RFCs - approved by the IESG. + all cases, new values are assigned only for values that have + documented in a published RFC having IETF Consensus [RFC2434]. -7.1 DKIM-Signature Tag Specifications +7.1. DKIM-Signature Tag Specifications A DKIM-Signature provides for a list of tag specifications. IANA is requested to establish the DKIM Signature Tag Specification Registry, for tag specifications that can be used in DKIM-Signature fields and that have been specified in any published RFC. The initial entries in the registry comprise: +------+-----------------+ | TYPE | REFERENCE | @@ -2210,68 +2231,77 @@ | h | (this document) | | i | (this document) | | l | (this document) | | q | (this document) | | s | (this document) | | t | (this document) | | x | (this document) | | z | (this document) | +------+-----------------+ -7.2 DKIM-Signature Query Method Registry + DKIM Signature Tag Specification Registry Initial Values + +7.2. DKIM-Signature Query Method Registry The "q=" tag-spec, as specified in Section 3.5 provides for a list of query methods. IANA is requested to establish the DKIM Query Method Registry, for mechanisms that can be used to retrieve the key that will permit validation processing of a message signed using DKIM and have been specified in any published RFC. The initial entry in the registry comprises: +------+--------+-----------------+ | TYPE | OPTION | REFERENCE | +------+--------+-----------------+ | dns | txt | (this document) | +------+--------+-----------------+ -7.3 DKIM-Signature Canonicalization Registry + DKIM-Signature Query Method Registry Initial Values + +7.3. DKIM-Signature Canonicalization Registry The "c=" tag-spec, as specified in Section 3.5 provides for a specifier for canonicalization algorithms for the header and body of the message. IANA is requested to establish the DKIM Canonicalization Algorithm Registry, for algorithms for converting a message into a canonical form before signing or verifying using DKIM and have been specified in any published RFC. The initial entries in the header registry comprise: +---------+-----------------+ | TYPE | REFERENCE | +---------+-----------------+ | simple | (this document) | | relaxed | (this document) | +---------+-----------------+ + DKIM-Signature Header Canonicalization Algorithm Registry Initial + Values The initial entries in the body registry comprise: +---------+-----------------+ | TYPE | REFERENCE | +---------+-----------------+ | simple | (this document) | | relaxed | (this document) | +---------+-----------------+ -7.4 _domainkey DNS TXT Record Tag Specifications + DKIM-Signature Body Canonicalization Algorithm Registry Initial + Values + +7.4. _domainkey DNS TXT Record Tag Specifications A _domainkey DNS TXT record provides for a list of tag specifications. IANA is requested to establish the DKIM _domainkey DNS TXT Tag Specification Registry, for tag specifications that can be used in DNS TXT Records and that have been specified in any published RFC. The initial entries in the registry comprise: +------+-----------------+ @@ -2280,139 +2310,150 @@ | v | (this document) | | g | (this document) | | h | (this document) | | k | (this document) | | n | (this document) | | p | (this document) | | s | (this document) | | t | (this document) | +------+-----------------+ -7.5 DKIM Key Type Registry + DKIM _domainkey DNS TXT Record Tag Specification Registry Initial + Values + +7.5. DKIM Key Type Registry The "k=" (as specified in Section 3.6.1) and the "a=" (Section 3.5) tags provide for a list of mechanisms that can be used to decode a DKIM signature. IANA is requested to establish the DKIM Key Type Registry, for such mechanisms that have been specified in any published RFC. The initial entry in the registry comprises: +------+-----------+ | TYPE | REFERENCE | +------+-----------+ | rsa | [RFC3447] | +------+-----------+ -7.6 DKIM Hash Algorithms Registry + DKIM Key Type Initial Values + +7.6. DKIM Hash Algorithms Registry The "h=" list (specified in Section 3.6.1) and the "a=" (Section 3.5) provide for a list of mechanisms that can be used to produce a digest of message data. IANA is requested to establish the DKIM Hash Algorithms Registry, for such mechanisms that have been specified in any published RFC. The initial entries in the registry comprise: +--------+-----------+ | TYPE | REFERENCE | +--------+-----------+ | sha1 | [SHA] | | sha256 | [SHA] | +--------+-----------+ -7.7 DKIM Service Types Registry + DKIM Hash Algorithms Initial Values + +7.7. DKIM Service Types Registry The "s=" list (specified in Section 3.6.1) provides for a list of service types to which this selector may apply. IANA is requested to establish the DKIM Service Types Registry, for service types that have been specified in any published RFC. The initial entries in the registry comprise: +-------+-----------------+ | TYPE | REFERENCE | +-------+-----------------+ | email | (this document) | | * | (this document) | +-------+-----------------+ -7.8 DKIM Selector Flags Registry + DKIM Hash Algorithms Initial Values + +7.8. DKIM Selector Flags Registry The "t=" list (specified in Section 3.6.1) provides for a list of flags to modify interpretation of the selector. IANA is requested to establish the DKIM Selector Flags Registry, for additional flags that have been specified in any published RFC. The initial entries in the registry comprise: +------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | y | (this document) | | s | (this document) | +------+-----------------+ -7.9 DKIM-Signature Header Field + DKIM Hash Algorithms Initial Values - IANA is requested to add DKIM-Signature to the "Permanent Header - Messages" registry for the "mail" protocol, using this document as - the Reference. +7.9. DKIM-Signature Header Field + + IANA is requested to add DKIM-Signature to the "Permanent Message + Header Fields" registry (see [RFC3864]) for the "mail" protocol, + using this document as the Reference. 8. Security Considerations It has been observed that any mechanism that is introduced which 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]. -8.1 Misuse of Body Length Limits ("l=" Tag) +8.1. Misuse of Body Length Limits ("l=" Tag) Body length limits (in the form of the "l=" tag) are subject to several potential attacks. -8.1.1 Addition of new MIME parts to multipart/* +8.1.1. Addition of new MIME parts to multipart/* If the body length limit does not cover a closing MIME multipart 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 an attacker 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 MTA. + 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. -8.1.2 Addition of new HTML content to existing content +8.1.2. Addition of new HTML content to existing content Several receiving MUA implementations do not cease display after a """" 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:
-8.2 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, however, not 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 @@ -2439,38 +2480,38 @@ 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. -8.3 Key Server Denial-of-Service Attacks +8.3. Key Server Denial-of-Service Attacks Since the key servers are distributed (potentially separate for each 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. -8.4 Attacks Against DNS +8.4. Attacks Against DNS Since DNS is a required binding for key services, specific attacks against DNS must be considered. While the DNS is currently insecure [RFC3833], these security problems are the motivation behind 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 @@ -2487,28 +2528,27 @@ A specific DNS security issue which should be considered by DKIM verifiers is the name chaining attack described in section 2.3 of the DNS Threat Analysis [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. -8.5 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 services to convey the fact that the specific email address is being used for spam, and that messages from that signer are likely to be 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. @@ -2511,108 +2551,110 @@ 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 information via existing collaborative systems. -8.6 Limits on Revoking Keys +8.6. Limits on Revoking Keys When a large domain detects undesirable behavior on the part of one of its users, it might wish to revoke the key used to sign that user's messages in order to disavow responsibility for messages which have not yet been verified or which are the subject of a replay attack. However, the ability of the domain to do so can be limited 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. -8.7 Intentionally malformed Key Records +8.7. Intentionally malformed Key Records It is possible for an attacker to publish key records in DNS which 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 DNS and be robust against intentionally as well as unintentionally malformed key records. -8.8 Intentionally Malformed DKIM-Signature header fields +8.8. Intentionally Malformed DKIM-Signature header fields Verifiers MUST be prepared to receive messages with malformed DKIM- Signature header fields, and thoroughly verify the header field before depending on any of its contents. -8.9 Information Leakage +8.9. Information Leakage An attacker could determine when a particular signature was verified 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. -8.10 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. -8.11 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. -8.12 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. -8.13 Inappropriate Signing by Parent Domains +8.13. Inappropriate Signing by Parent Domains The trust relationship described in Section 3.8 could conceivably be 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. + and completely replace all DNS-served information. Note that a + verifier MAY ignore signatures that come from an unlikely domain + such as ".com", as discussed in Section 6.1.1. 9. References -9.1 Normative References +9.1. Normative References [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message header field Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate @@ -2636,134 +2678,81 @@ Specifications: ABNF", RFC 4234, October 2005. [SHA] U.S. Department of Commerce, "Secure Hash Standard", FIPS PUB 180-2, August 2002. [X.660] "ITU-T Recommendation X.660 Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", 1997. -9.2 Informative References +9.2. Informative References [BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing Attacks are Practical", 2003, . [RFC-DK] "DomainKeys specification (to be published with this RFC)", 2005. [RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted", RFC 1847, October 1995. + [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an + IANA Considers Section in RFCs", BCP 26, October 1998. + [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP Message Format", RFC 2440, November 1998. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths for Public Keys Used For Exchanging Symmetric Keys", RFC 3766, April 2004. [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name System (DNS)", RFC 3833, August 2004. [RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 3851, June 1999. + [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration + Procedures for Message Header Fields", BCP 90, + September 2004. + [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)", RFC 4686, September 2006. -Authors' Addresses - - Eric Allman - Sendmail, Inc. - 6425 Christie Ave, Suite 400 - Emeryville, CA 94608 - USA - - Phone: +1 510 594 5501 - Email: eric+dkim@sendmail.org - URI: - - Jon Callas - PGP Corporation - 3460 West Bayshore - Palo Alto, CA 94303 - USA - - Phone: +1 650 319 9016 - Email: jon@pgp.com - - Mark Delany - Yahoo! Inc - 701 First Avenue - Sunnyvale, CA 95087 - USA - - Phone: +1 408 349 6831 - Email: markd+dkim@yahoo-inc.com - URI: - - Miles Libbey - Yahoo! Inc - 701 First Avenue - Sunnyvale, CA 95087 - USA - - Email: mlibbeymail-mailsig@yahoo.com - URI: - - Jim Fenton - Cisco Systems, Inc. - MS SJ-24/2 - 170 W. Tasman Drive - San Jose, CA 95134-1706 - USA - - Phone: +1 408 526 5914 - Email: fenton@cisco.com - URI: - - Michael Thomas - Cisco Systems, Inc. - MS SJ-9/2 - 170 W. Tasman Drive - San Jose, CA 95134-1706 - - Phone: +1 408 525 5386 - Email: mat@cisco.com - Appendix A. Example of Use (INFORMATIVE) This section shows the complete flow of an email from submission to final delivery, demonstrating how the various components fit together. -A.1 The user composes an email +A.1. The user composes an email From: Joe SixPack To: Suzie Q Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com> Hi. We lost the game. Are you hungry yet? Joe. -A.2 The email is signed +A.2. The email is signed This email is signed by the example.com outbound email server and now looks like this: DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com; c=simple; q=dns/txt; i=joe@football.example.com; h=Received : From : To : Subject : Date : Message-ID; bh=jpltwNFTq83Bkjt/Y2ekyqr/+i296daNkFZSdaz8VCY=; b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B 7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt @@ -2780,21 +2769,21 @@ Hi. We lost the game. Are you hungry yet? Joe. The signing email server requires access to the private key associated with the "brisbane" Selector to generate this signature. -A.3 The email signature is verified +A.3. The email signature is verified The signature is normally verified by an inbound SMTP server or possibly the final delivery agent. However, intervening MTAs can also perform this verification if they choose to do so. The verification process uses the domain "example.com" extracted from the "d=" tag and the Selector "brisbane" from the "s=" tag in the "DKIM- Signature" header field to form the DNS DKIM query for: brisbane._domainkey.example.com @@ -2844,32 +2833,32 @@ different operational scenarios. This Appendix discusses some common examples. NOTE: Descriptions in this Appendix are for informational purposes only. They describe various ways that DKIM can be used, given particular constraints and needs. In no case are these examples intended to be taken as providing explanation or guidance concerning DKIM specification details, when creating an implementation. -B.1 Alternate Submission Scenarios +B.1. Alternate Submission Scenarios In the most simple scenario, a user's MUA, MSA, and Internet (boundary) MTA are all within the same administrative environment, using the same domain name. Therefore, all of the components involved in submission and initial transfer are related. However it is common for two or more of the components to be under independent administrative control. This creates challenges for choosing and administering the domain name to use for signing, and for its relationship to common email identity header fields. -B.1.1 Delegated Business Functions +B.1.1. Delegated Business Functions Some organizations assign specific business functions to discrete groups, inside or outside the organization. The goal, then, is to authorize that group to sign some mail, but to constrain what signatures they can generate. DKIM Selectors (the "s=" signature tag) and granularity (the "g=" key tag) facilitate this kind of restricted authorization. Examples of these outsourced business functions are legitimate email marketing providers and corporate benefits providers. @@ -2899,51 +2888,51 @@ any time. If the client wants the delegated group to do the DNS administration, it can have the domain name that is specified with the selector point to the provider's DNS server. The provider then creates and maintains all of the DKIM signature information for that Selector. Hence, the client cannot provide constraints on the local-part of addresses that get signed, but it can revoke the provider's signing rights by removing the DNS delegation record. -B.1.2 PDAs and Similar Devices +B.1.2. PDAs and Similar Devices PDAs demonstrate the need for using multiple keys per domain. Suppose that John Doe wanted to be able to send messages using his corporate email address, jdoe@example.com, and his email device did not have the ability to make a VPN connection to the corporate network, either because the device is limited or because there are restrictions enforced by his Internet access provider. If the device was equipped with a private key registered for jdoe@example.com by the administrator of the example.com domain, and appropriate software to sign messages, John could sign the message on the device itself before transmission through the outgoing network of the access service provider. -B.1.3 Roaming Users +B.1.3. Roaming Users Roaming users often find themselves in circumstances where it is convenient or necessary to use an SMTP server other than their home server; examples are conferences and many hotels. In such circumstances a signature that is added by the submission service will use an identity that is different from the user's home system. Ideally roaming users would connect back to their home server using either a VPN or a SUBMISSION server running with SMTP AUTHentication on port 587. If the signing can be performed on the roaming user's laptop then they can sign before submission, although the risk of further modification is high. If neither of these are possible, these roaming users will not be able to send mail signed using their own domain key. -B.1.4 Independent (Kiosk) Message Submission +B.1.4. Independent (Kiosk) Message Submission Stand-alone services, such as walk-up kiosks and web-based information services, have no enduring email service relationship with the user, but the user occasionally requests that mail be sent on their behalf. For example, a website providing news often allows the reader to forward a copy of the article to a friend. This is typically done using the reader's own email address, to indicate who the author is. This is sometimes referred to as the "Evite problem", named after the website of the same name that allows a user to send invitations to friends. @@ -2959,29 +2948,29 @@ Receiving sites often wish to provide their end users with information about mail that is mediated in this fashion. Although the real efficacy of different approaches is a subject for human factors usability research, one technique that is used is for the verifying system to rewrite the From header field, to indicate the address that was verified. For example: From: John Doe via news@news-site.com . (Note that, such rewriting will break a signature, unless it is done after the verification pass is complete.) -B.2 Alternate Delivery Scenarios +B.2. Alternate Delivery Scenarios Email is often received at a mailbox that has an address different from the one used during initial submission. In these cases, an intermediary mechanism operates at the address originally used and it then passes the message on to the final destination. This mediation process presents some challenges for DKIM signatures. -B.2.1 Affinity Addresses +B.2.1. Affinity Addresses "Affinity addresses" allow a user to have an email address that remains stable, even as the user moves among different email providers. They are typically associated with college alumni associations, professional organizations, and recreational organizations with which they expect to have a long-term relationship. These domains usually provide forwarding of incoming email, and they often have an associated Web application which authenticates the user and allows the forwarding address to be changed. However these services usually depend on the user's sending @@ -2997,34 +2986,34 @@ the public half in DNS for access by verifiers. This is another application that takes advantage of user-level keying, and domains used for affinity addresses would typically have a very large number of user-level keys. Alternatively, the affinity domain could handle outgoing mail, operating a mail submission agent that authenticates users before accepting and signing messages for them. This is of course dependent on the user's service provider not blocking the relevant TCP ports used for mail submission. -B.2.2 Simple Address Aliasing (.forward) +B.2.2. Simple Address Aliasing (.forward) In some cases a recipient is allowed to configure an email address to cause automatic redirection of email messages from the original address to another, such as through the use of a Unix .forward file. In this case messages are typically redirected by the mail handling service of the recipient's domain, without modification, except for the addition of a Received header field to the message and a change in the envelope recipient address. In this case, the recipient at the final address' mailbox is likely to be able to verify the original signature since the signed content has not changed, and DKIM is able to validate the message signature. -B.2.3 Mailing Lists and Re-Posters +B.2.3. Mailing Lists and Re-Posters There is a wide range of behaviors in services that take delivery of a message and then resubmit it. A primary example is with mailing lists (collectively called "forwarders" below), ranging from those which make no modification to the message itself, other than to add a Received header field and change the envelope information, to those which add header fields, change the Subject header field, add content to the body (typically at the end), or reformat the body in some manner. The simple ones produces messages that are quite similar to the automated alias services. More elaborate systems essentially @@ -3155,42 +3145,68 @@ The aforementioned information is not intended to be exhaustive. The MUA may choose to highlight, accentuate, hide, or otherwise display any other information that may, in the opinion of the MUA author, be deemed important to the end user. Appendix E. Acknowledgements The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann, Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin, Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman, - Jutta Degener, Frank Ellermann, Patrik Faltstrom, Mark Fanto, Stephen - Farrell, Duncan Findlay, Elliot Gillum, Olafur Gu[eth]mundsson, - Phillip Hallam-Baker, Tony Hansen, Sam Hartman, Arvel Hathcock, Amir - Herzberg, Paul Hoffman, Russ Housley, Craig Hughes, Cullen Jennings, - Don Johnsen, Harry Katz, Murray S. Kucherawy, Barry Leiba, John - Levine, Charles Lindsey, Simon Longsdale, David Margrave, Justin - Mason, David Mayne, 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 for their valuable suggestions and constructive criticism. + Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto, + Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur + Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman, + Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig + Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S. + Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon + Longsdale, David Margrave, Justin Mason, David Mayne, 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 for their valuable suggestions and + constructive criticism. The DomainKeys specification was a primary source from which this specification has been derived. Further information about DomainKeys is at [RFC-DK]. Appendix F. Edit History [[This section to be removed before publication.]] -F.1 Changes since -ietf-07 version +F.1. Changes since -ietf-08 version + + The following changes were made between draft-ietf-dkim-base-08 and + draft-ietf-dkim-base-09: + + o Section 3.3.1, recommend use of an RSA exponent of 65537. + + o Section 3.4.4, mention theoretical "ASCII Art" attack for relaxed + body canonicalization. + + o Section 5.4.1 moved to 5.5 (with old 5.5 et seq. pushed down) to + talk more generally about use of l= and canonicalization + algorithms. + + o Section 6.1.1, make an explicit mention that verifiers may reject + signatures from unlikely domains such as "com" and "co.uk". + + o Section 6.3, try to clarify the wording about SMTP rejections. + + o Section 7, change IANA registration requirement to be any RFC + having "IETF Consensus" (as defined in RFC2434), not necessarily + standards-track, as a result of overwhelming WG consensus. + + o Informative References, add RFC 2434. + +F.2. Changes since -ietf-07 version The following changes were made between draft-ietf-dkim-base-07 and draft-ietf-dkim-base-08: o Drop reference to "trusted third party" in section 1; it was redundant with existing bullet points and created confusion. o Drop the wording on re-using keys from normative to an operational note. @@ -3237,21 +3253,21 @@ o Add sentence in section 8.11 to emphasize that signing existing DKIM-Signature header fields may result in incorrect validation failures, as requested by Security Area review. o Added section 8.14 (RSA Attacks) based on DNS-dir review from Olafur Gu[eth]mundsson. o Added section 8.15 (Inappropriate Signing by Parent Domains). -F.2 Changes since -ietf-06 version +F.3. Changes since -ietf-06 version The following changes were made between draft-ietf-dkim-base-06 and draft-ietf-dkim-base-07: o Added section 8.11 regarding header reordering. o Added informative note to section 3.3 regarding use of sha256. o Added informative rationale to section 3.6.1, "p=", regarding key revocation. @@ -3261,21 +3277,21 @@ o Minor modification of the second informative note in section 6.1 regarding DoS attacks. o Added explicit mention of v= to section 6.1.2, step 5. o Updated paragraph 3 of section 8.4 regarding DNS attacks. o Added section 7.9 (DKIM-Signature IANA Registry) per IANA request. -F.3 Changes since -ietf-05 version +F.4. Changes since -ietf-05 version The following changes were made between draft-ietf-dkim-base-05 and draft-ietf-dkim-base-06: o Fix an error in an example in Appendix C. o Substantial updates to Appendixes B and D. o Clarify ABNF for tag-value. @@ -3285,21 +3301,21 @@ o Add normative reference to SHA1/SHA256 FIPS publication 180-2. o Several minor edits based on AD Review. o Move discussion of not re-using a selector (i.e., changing the public key for a single selector) from informational to normative. o Assorted wordsmithing based on external review. -F.4 Changes since -ietf-04 version +F.5. Changes since -ietf-04 version The following changes were made between draft-ietf-dkim-base-04 and draft-ietf-dkim-base-05: o Clarified definition of "plain text" in section 3.2 (issue 1316). o Added some clarification about multiple listings of non-existent header field names in h= in section 5.4 (issue 1316). o Finished filling out IANA registries in section 7 (issue 1320). @@ -3307,21 +3323,21 @@ o Clarified handling of bare CR and LF in section 5.3 (issue 1326). o Listed the required tags in section 6.1.1 as an informational note (issue 1330). o Changed IDNA reference from 3492 to 3490 (issue 1331). o Changed the reference for WSP to 4234; changed the definition of SWSP to exclude bare CR and LF (issue 1332). -F.5 Changes since -ietf-03 version +F.6. Changes since -ietf-03 version The following changes were made between draft-ietf-dkim-base-03 and draft-ietf-dkim-base-04: o Re-worded Abstract to avoid use of "prove" and "non-repudiation". o Use dot-atom-text instead of dot-atom to avoid inclusion of CFWS. o Capitalize Selector throughout. @@ -3354,29 +3370,30 @@ o Add several examples; update some others. o Considerable minor editorial updating to clarify language, delete redundant text, fix spelling errors, etc. Still to be resolved: o How does "simple" body canonicalization interact with BINARYMIME data? + o Deal with "relaxed" body canonicalizations, especially in regard to bare CRs and NLs. o How to handle "*" in g= dot-atom-text (which allows "*" as a literal character). o The IANA Considerations need to be completed and cleaned up. -F.6 Changes since -ietf-02 version +F.7. Changes since -ietf-02 version The following changes were made between draft-ietf-dkim-base-02 and draft-ietf-dkim-base-03: o Section 5.2: changed key expiration text to be informational; drop "seven day" wording in favor of something vaguer. o Don't indicate that the "i=" tag value should be passed to the key lookup service; this can be added as an extension if required. @@ -3410,21 +3427,21 @@ may contain the content. o Use dkim-quoted-printable as the encoding used in z= rather than referring to RFC2045, since they are different. o Rewrite description of g= tag in the key record. o Deleted use of Domain in ABNF, which permits address-literals; define domain-name to act in stead. -F.7 Changes since -ietf-01 version +F.8. Changes since -ietf-01 version The following changes were made between draft-ietf-dkim-base-01 and draft-ietf-dkim-base-02: o Change wording on "x=" tag in DKIM-Signature header field regarding verifier handling of expired signatures from MUST to MAY (per 20 April Jabber session). Also, make it clear that received time is to be preferred over current time if reliably available. o Several changes to limit wording that would intrude into verifier @@ -3441,21 +3458,21 @@ o Change "q=dns" query access method to "q=dnstxt" to emphasize the use of the TXT record. The expectation is that a later extension will define "q=dnsdkk" to indicate use of a DKK record. (Per 18 May Jabber session.) o Several typos fixed, including removing a paragraph that implied that the DKIM-Signature header field should be hashed with the body (it should not). -F.8 Changes since -ietf-00 version +F.9. Changes since -ietf-00 version The following changes were made between draft-ietf-dkim-base-00 and draft-ietf-dkim-base-01: o Added section 8.9 (Information Leakage). o Replace section 4 (Multiple Signatures) with much less vague text. o Fixed ABNF for base64string. @@ -3465,54 +3482,130 @@ o Changed signing algorithm to use separate hash of the body of the message; this is represented as the "bh=" tag in the DKIM- Signature header field. o Changed "z=" tag so that it need not have the same header field names as the "h=" tag. o Significant wordsmithing. -F.9 Changes since -allman-01 version +F.10. Changes since -allman-01 version The following changes were made between draft-allman-dkim-base-01 and draft-ietf-dkim-base-00: o Remove references to Sender Signing Policy document. Such consideration is implicitly included in Section 6.3. o Added ABNF for all tags. o Updated references (still includes some references to expired drafts, notably ID-AUTH-RES. o Significant wordsmithing. -F.10 Changes since -allman-00 version +F.11. Changes since -allman-00 version The following changes were made between draft-allman-dkim-base-00 and draft-allman-dkim-base-01: o Changed "c=" tag to separate out header from body canonicalization. o Eliminated "nowsp" canonicalization in favor of "relaxed", which is somewhat less relaxed (but more secure) than "nowsp". o Moved the (empty) Compliance section to the Sender Signing Policy document. o Added several IANA Considerations. o Fixed a number of grammar and formatting errors. -Intellectual Property Statement +Authors' Addresses + + Eric Allman + Sendmail, Inc. + 6425 Christie Ave, Suite 400 + Emeryville, CA 94608 + USA + + Phone: +1 510 594 5501 + Email: eric+dkim@sendmail.org + URI: + + Jon Callas + PGP Corporation + 3460 West Bayshore + Palo Alto, CA 94303 + USA + + Phone: +1 650 319 9016 + Email: jon@pgp.com + + Mark Delany + Yahoo! Inc + 701 First Avenue + Sunnyvale, CA 95087 + USA + + Phone: +1 408 349 6831 + Email: markd+dkim@yahoo-inc.com + URI: + + Miles Libbey + Yahoo! Inc + 701 First Avenue + Sunnyvale, CA 95087 + USA + + Email: mlibbeymail-mailsig@yahoo.com + URI: + + Jim Fenton + Cisco Systems, Inc. + MS SJ-24/2 + 170 W. Tasman Drive + San Jose, CA 95134-1706 + USA + + Phone: +1 408 526 5914 + Email: fenton@cisco.com + URI: + + Michael Thomas + Cisco Systems, Inc. + MS SJ-9/2 + 170 W. Tasman Drive + San Jose, CA 95134-1706 + + Phone: +1 408 525 5386 + Email: mat@cisco.com + +Full Copyright Statement + + Copyright (C) The IETF Trust (2007). + + This document is subject to the rights, licenses and restrictions + contained in BCP 78, and except as set forth therein, the authors + retain all their rights. + + This document and the information contained herein are provided on an + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND + THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS + OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF + THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED + WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. @@ -3522,35 +3615,14 @@ such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. - The IETF has been notified of intellectual property rights claimed in - regard to some or all of the specification contained in this - document. For more information consult the online list of claimed - rights. - -Disclaimer of Validity - - This document and the information contained herein are provided on an - "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS - OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET - ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, - INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE - INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED - WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. - -Copyright Statement - - Copyright (C) The Internet Society (2007). This document is subject - to the rights, licenses and restrictions contained in BCP 78, and - except as set forth therein, the authors retain all their rights. - Acknowledgment - Funding for the RFC Editor function is currently provided by the - Internet Society. + Funding for the RFC Editor function is provided by the IETF + Administrative Support Activity (IASA).