--- 1/draft-ietf-dkim-base-03.txt 2006-07-17 22:12:32.000000000 +0200 +++ 2/draft-ietf-dkim-base-04.txt 2006-07-17 22:12:32.000000000 +0200 @@ -1,25 +1,25 @@ DKIM E. Allman Internet-Draft Sendmail, Inc. -Expires: December 27, 2006 J. Callas +Expires: January 16, 2007 J. Callas PGP Corporation M. Delany M. Libbey Yahoo! Inc J. Fenton M. Thomas Cisco Systems, Inc. - June 25, 2006 + July 15, 2006 DomainKeys Identified Mail (DKIM) Signatures - draft-ietf-dkim-base-03 + draft-ietf-dkim-base-04 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 @@ -30,126 +30,131 @@ 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 December 27, 2006. + This Internet-Draft will expire on January 16, 2007. Copyright Notice Copyright (C) The Internet Society (2006). 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 proving - and protecting message signer identity and the integrity of the - messages they convey while retaining the functionality of Internet - email as it is known today. Proof and protection of email identity, - including repudiation and non-repudiation, may assist in the global - control of "spam" and "phishing". + a signing domain to assert responsibility for a message, thus + protecting message signer identity and the integrity of the messages + they convey while retaining the functionality of Internet email as it + is known today. Protection of email identity may assist in the + 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 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 1.2 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 - 1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 - 1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6 + 1.1 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 + 1.2 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 + 1.3 Simple Key Management . . . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . 6 - 2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 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 + 2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7 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 . . . . . . . . . . . . . . . . . . . . . 13 + 3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 14 3.5 The DKIM-Signature header field . . . . . . . . . . . . . 18 - 3.6 Key Management and Representation . . . . . . . . . . . . 25 + 3.6 Key Management and Representation . . . . . . . . . . . . 26 3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 30 - 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 31 + 3.8 Signing by Parent Domains . . . . . . . . . . . . . . . . 32 + 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 32 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 32 - 5.1 Determine if the Email Should be Signed and by Whom . . . 32 + 5.1 Determine if the Email Should be Signed and by Whom . . . 33 5.2 Select a private-key and corresponding selector - information . . . . . . . . . . . . . . . . . . . . . . . 32 - 5.3 Normalize the Message to Prevent Transport Conversions . . 33 - 5.4 Determine the header fields to Sign . . . . . . . . . . . 33 - 5.5 Compute the Message Hash and Signature . . . . . . . . . . 35 + information . . . . . . . . . . . . . . . . . . . . . . . 33 + 5.3 Normalize the Message to Prevent Transport Conversions . . 34 + 5.4 Determine the header fields to Sign . . . . . . . . . . . 34 + 5.5 Compute the Message Hash and Signature . . . . . . . . . . 36 5.6 Insert the DKIM-Signature header field . . . . . . . . . . 36 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 37 6.1 Extract Signatures from the Message . . . . . . . . . . . 37 - 6.2 Communicate Verification Results . . . . . . . . . . . . . 42 - 6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 42 + 6.2 Communicate Verification Results . . . . . . . . . . . . . 43 + 6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 43 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 44 - 8. Security Considerations . . . . . . . . . . . . . . . . . . 44 - 8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 44 - 8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 45 - 8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 45 - 8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 46 - 8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 46 - 8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 47 - 8.7 Intentionally malformed Key Records . . . . . . . . . . . 47 - 8.8 Intentionally Malformed DKIM-Signature header fields . . . 47 - 8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 48 - 8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 48 - 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 - 9.1 Normative References . . . . . . . . . . . . . . . . . . . 48 - 9.2 Informative References . . . . . . . . . . . . . . . . . . 49 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 50 - A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 51 - A.1 The user composes an email . . . . . . . . . . . . . . . . 51 - A.2 The email is signed . . . . . . . . . . . . . . . . . . . 51 - A.3 The email signature is verified . . . . . . . . . . . . . 52 - B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 53 - B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 53 - B.2 Outsourced Business Functions . . . . . . . . . . . . . . 53 - B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 54 - B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 54 - B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 55 - B.6 Third-party Message Transmission . . . . . . . . . . . . . 55 - C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 56 - D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 57 - E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 58 - F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 58 - F.1 Changes since -ietf-02 version . . . . . . . . . . . . . . 58 - F.2 Changes since -ietf-01 version . . . . . . . . . . . . . . 59 - F.3 Changes since -ietf-00 version . . . . . . . . . . . . . . 60 - F.4 Changes since -allman-01 version . . . . . . . . . . . . . 60 - F.5 Changes since -allman-00 version . . . . . . . . . . . . . 61 - Intellectual Property and Copyright Statements . . . . . . . 62 + 7.1 DKIM-Signature Tag Specifications . . . . . . . . . . . . 44 + 7.2 DKIM-Signature Query Method Registry . . . . . . . . . . . 45 + 7.3 DKIM-Signature Canonicalization Registry . . . . . . . . . 45 + 7.4 _domainkey DNS TXT Record Tag Specifications . . . . . . . 46 + 7.5 DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 47 + 7.6 DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 47 + 8. Security Considerations . . . . . . . . . . . . . . . . . . 47 + 8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 47 + 8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 48 + 8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 49 + 8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 49 + 8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 50 + 8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 51 + 8.7 Intentionally malformed Key Records . . . . . . . . . . . 51 + 8.8 Intentionally Malformed DKIM-Signature header fields . . . 51 + 8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 51 + 8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 51 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 52 + 9.1 Normative References . . . . . . . . . . . . . . . . . . . 52 + 9.2 Informative References . . . . . . . . . . . . . . . . . . 52 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 53 + A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 54 + A.1 The user composes an email . . . . . . . . . . . . . . . . 55 + A.2 The email is signed . . . . . . . . . . . . . . . . . . . 55 + A.3 The email signature is verified . . . . . . . . . . . . . 56 + B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 57 + B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 57 + B.2 Outsourced Business Functions . . . . . . . . . . . . . . 57 + B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 57 + B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 58 + B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 58 + B.6 Third-party Message Transmission . . . . . . . . . . . . . 59 + B.7 SMTP Servers for Roaming Users . . . . . . . . . . . . . . 59 + C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 59 + D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 61 + E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 62 + F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 62 + F.1 Changes since -ietf-03 version . . . . . . . . . . . . . . 62 + F.2 Changes since -ietf-02 version . . . . . . . . . . . . . . 63 + F.3 Changes since -ietf-01 version . . . . . . . . . . . . . . 64 + F.4 Changes since -ietf-00 version . . . . . . . . . . . . . . 65 + F.5 Changes since -allman-01 version . . . . . . . . . . . . . 66 + F.6 Changes since -allman-00 version . . . . . . . . . . . . . 66 + Intellectual Property and Copyright Statements . . . . . . . 67 1. Introduction [[Note: text in double square brackets (such as this text) will be deleted before publication.]] -1.1 Overview - 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 the signer's domain directly to retrieve the appropriate public key, and thereby confirm that the message was attested to by a party in possession of the private key for the signing domain. The approach taken by DKIM differs from previous approaches to message signing (e.g. S/MIME [RFC1847], OpenPGP [RFC2440]) in that: @@ -158,70 +163,68 @@ neither human recipients nor existing MUA (Mail User Agent) software are confused by signature-related content appearing in the message body, o there is no dependency on public and private key pairs being issued by well-known, trusted certificate authorities, o there is no dependency on the deployment of any new Internet protocols or services for public key distribution or revocation, - o it makes no attempt to include encryption as part of the - mechanism. + o signature verification failure does not result in rejection of the + message, + + o no attempt is made to include encryption as part of the mechanism, + + o archival is not a design goal. DKIM: o is compatible with the existing email infrastructure and transparent to the fullest extent possible o requires minimal new infrastructure o can be implemented independently of clients in order to reduce deployment time - o does not require the use of a trusted third party (such as a - certificate authority or other entity) which might impose - significant costs or introduce delays to deployment - + o does not require the use of an additional trusted third party + (such as a certificate authority or other entity) which might + impose significant costs or introduce delays to deployment o can be deployed incrementally - o allows delegation of signing to third parties - - o is not intended be used for archival purposes - A "selector" mechanism allows multiple keys per domain, including - delegation of the right to authenticate a portion of the namespace to - a trusted third party. + o allows delegation of signing to third parties -1.2 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 associated with 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. + 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. -1.3 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. -1.4 Simple Key Management +1.3 Simple Key Management DKIM differs from traditional hierarchical public-key systems in that no key signing infrastructure is required; the verifier requests the public key from the claimed signer directly. The DNS is proposed as the initial mechanism for publishing public keys. DKIM is designed to be extensible to other key fetching services as they become available. 2. Terminology and Definitions @@ -289,21 +292,21 @@ o "sub-domain" The following definitions are imported from [RFC2822]: o "WSP" (space or tab) o "FWS" (folding white space) o "field-name" (name of a header field) - o "dot-atom" (in the local-part of an email address) + o "dot-atom-text" (in the local-part of an email address) The following tokens are imported from [RFC2045]: o "qp-section" (a single line of quoted-printable-encoded text) 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. @@ -361,95 +364,112 @@ 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 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., + 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: o Domains which want to delegate signing capability for a specific address for a given duration to a partner, such as an advertising - provider or other outsourced function. + provider or other out-sourced function. o Domains which want to allow frequent travelers to send messages locally without the need to connect with a particular MSA. o "Affinity" domains (e.g., college alumni associations) which provide forwarding of incoming mail but which do not operate a mail submission agent for outgoing mail. - Periods are allowed in selectors and are component separators. If - keys are stored in DNS, the period defines sub-domain boundaries. - Sub-selectors might be used to combine dates with locations; for - example, "march2005.reykjavik". This can be used to allow delegation - of a portion of the selector name-space. + Periods are allowed in Selectors and are component separators. When + keys are retrieved from the DNS, periods in Selectors define DNS + label boundaries in a manner similar to the conventional use in + domain names. Selector components might be used to combine dates + with locations; for example, "march2005.reykjavik". In a DNS + implementation, this can be used to allow delegation of a portion of + the Selector name-space. ABNF: selector = sub-domain *( "." sub-domain ) - The number of public keys and corresponding selectors for each domain + The number of public keys and corresponding Selectors for each domain are determined by the domain owner. Many domain owners will be - satisfied with just one selector whereas administratively distributed - organizations may choose to manage disparate selectors and key pairs + satisfied with just one Selector whereas administratively distributed + organizations may choose to manage disparate Selectors and key pairs in different regions or on different email servers. - Beyond administrative convenience, selectors make it possible to + Beyond administrative convenience, Selectors make it possible to seamlessly replace public keys on a routine basis. If a domain - wishes to change from using a public key associated with selector - "january2005" to a public key associated with selector + wishes to change from using a public key associated with Selector + "january2005" to a public key associated with Selector "february2005", it merely makes sure that both public keys are advertised in the public-key repository concurrently for the transition period during which email may be in transit prior to verification. At the start of the transition period, the outbound email servers are configured to sign with the "february2005" private- key. At the end of the transition period, the "january2005" public key is removed from the public-key repository. - While some domains may wish to make selector values well known, - others will want to take care not to allocate selector names in a way + INFORMATIVE NOTE: A key may also be revoked as described below. + The distinction between revoking and removing a key selector + record is subtle. When phasing out keys as described above, a + signing domain would probably simply remove the key record after + the transition period. However, a signing domain could elect to + revoke the key (but maintain the key record) for a further period. + There is no defined semantic difference between a revoked key and + a removed key. + + While some domains may wish to make Selector values well known, + others will want to take care not to allocate Selector names in a way that allows harvesting of data by outside parties. E.g., if per-user keys are issued, the domain owner will need to make the decision as - to whether to associate this selector directly with the user name, or + to whether to associate this Selector directly with the user name, or make it some unassociated random value, such as a fingerprint of the public key. - INFORMATIVE IMPLEMENTERS' NOTE: reusing a selector with a new key + INFORMATIVE IMPLEMENTERS' 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. Signers should not change the key - associated with a selector. When creating a new key, signers - should associate it with a new selector. + associated with a Selector. When creating a new key, signers + should associate it with a new Selector. 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 above). The name - of the tag will determine the encoding of each value; however, no - encoding may include the semicolon (";") character, since that + 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 above). The + name of the tag will determine the encoding of each value; however, + no encoding may include the semicolon (";") character, since that separates tag-specs. + INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" + defined below (as "tag-value") only includes 7-bit characters, an + implementation that wished to anticipate future standards would be + advised to not preclude the use of UTF8-encoded text in tag=value + lists. + Formally, the syntax rules are: tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ] tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] tag-name = ALPHA 0*ALNUMPUNC tag-value = [ 1*VALCHAR 0*( 1*(WSP / FWS) 1*VALCHAR ) ] ; WSP and FWS prohibited at beginning and end VALCHAR = %x21-3A / %x3C-7E ; EXCLAMATION to TILDE except SEMICOLON ALNUMPUNC = ALPHA / DIGIT / "_" @@ -473,120 +493,117 @@ 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 - DKIM supports multiple key signing/verification 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. + 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. 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 as the hash-alg. That hash is then signed by the signer using the RSA algorithm (defined in PKCS#1 - version 1.5 [RFC3447]; in particular see section 5.2) with an - exponent of 65537 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. + 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. 3.3.2 The rsa-sha256 Signing Algorithm The rsa-sha256 Signing Algorithm computes a message hash as described in Section 3.7 below using SHA-256 as the hash-alg. That hash is then signed by the signer using the RSA algorithm (actually PKCS#1 - version 1.5 [RFC3447]; in particular see section 5.2) with an - exponent of 65537 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. + 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 Other algorithms Other algorithms MAY be defined in the future. Verifiers MUST ignore - any signatures using algorithms that they do not understand. + any signatures using algorithms that they do not implement. 3.3.4 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. Security policies may use the + validate signatures with larger keys. Verifier policies may use the length of the signing key as one metric for determining whether a signature is acceptable. Factors that should influence the key size choice include: o The practical constraint that large 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 public-key systems - See RFC3766 [RFC3766] for further discussion of selecting key sizes. + See [RFC3766] for further discussion of selecting key sizes. 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 - minor, result in an authentication failure. These signers prefer a - canonicalization algorithm that does not tolerate in-transit + minor, result in a signature verification failure. These signers + prefer a canonicalization algorithm that does not tolerate in-transit modification of the signed email. Some signers may be willing to accept modifications to header fields that are within the bounds of email standards such as [RFC2822], but are unwilling to accept any modification to the body of messages. To satisfy all requirements, two canonicalization algorithms are defined for each of the header and the body: a "simple" algorithm that tolerates almost no modification and a "relaxed" algorithm that tolerates common modifications such as white-space replacement and header field line re-wrapping. A signer MAY specify either algorithm for header or body when signing an email. If no canonicalization algorithm is specified by the signer, the "simple" algorithm defaults for both header and body. Verifiers MUST implement both - canonicalization algorithms. Further canonicalization algorithms MAY - be defined in the future; verifiers MUST ignore any signatures that - use unrecognized canonicalization algorithms. + canonicalization algorithms. Note that the header and body may use + different canonicalization algorithms. Further canonicalization + algorithms MAY be defined in the future; verifiers MUST ignore any + signatures that use unrecognized canonicalization algorithms. - In all cases, the header fields of the message are presented to the - signing algorithm first in the order indicated by the signature - header field and canonicalized using the indicated algorithm. Only - header fields listed as signed in the signature header field are - included. Note: the signature header field itself is presented at - the end of the hash, not with the other headers. The CRLF separating - the header field from the body is then presented, followed by the - canonicalized body. Note that the header and body may use different - canonicalization algorithms. + [[WORKING GROUP DISCUSSION POINT: If a message is transmitted + using CHUNKING (that is, BDAT instead of the DATA command) and + BODY=BINARYMIME [RFC3030] then the body should be treated as a + binary stream, and no canonicalization whatsoever should be done. + Do we want to leave this for the future, say that canonicalization + is ignored in this circumstance, or add a third "binary" body + canonicalization algorithm? Or something else, of course.]] 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. @@ -622,102 +639,94 @@ 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 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. It makes no other - changes to the message body. In more formal terms, the "simple" body - canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the - body to a single "CRLF". + length after removal of the line terminator. If there is no trailing + CRLF on the message, 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". 3.4.4 The "relaxed" Body Canonicalization Algorithm [[This section may be deleted; see discussion below.]] 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. - [[NON-NORMATIVE DISCUSSION: The authors are undecided whether to - leave the "relaxed" body canonicalization algorithm in to the - specification or delete it entirely. We believe that for the vast - majority of cases, the "simple" body canonicalization algorithm - should be sufficient. We simply do not have enough data to know - whether to retain the "relaxed" body canonicalization algorithm or - not.]] + [[WORKING GROUP DISCUSSION POINT (ISSUE 1326): Mike Thomas has + found bare CRs in the wild that are getting converted to CRLF by + some MTAs and thus breaking signatures. Shall we (a) drop + "relaxed" until we can figure out how to do it right and then put + it in as an extension, (b) change "relaxed" to handle this case, + probably by having it convert bare CR and LF to CRLF, or (c) + something else?]] 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 and verified. - - INFORMATIVE IMPLEMENTATION NOTE: Body length limits could be - useful in increasing signature robustness when sending to a - mailing list that both appends to content sent to it and does not - sign its messages. However, using such limits enables an attack - in which an attacker modifies a message to include content that - solely benefits the attacker. It is possible for the appended - content to completely replace the original content in the end - recipient's eyes and to defeat duplicate message detection - algorithms. To avoid this attack, signers should be wary of using - this tag, and verifiers might wish to ignore the tag or remove - text that appears after the specified content length, perhaps - based on other criteria. - - The body length count allows the signer of a message to permit data - to be appended to the end of the body of a signed message. The body - length count is made following the canonicalization algorithm; for - example, any white space ignored by a canonicalization algorithm is - not included as part of the body length count. + message body is signed. INFORMATIVE RATIONALE: This capability is provided because it is very common for mailing lists to add trailers to messages (e.g., instructions how to get off the list). Until those messages are also signed, the body length count is a useful tool for the verifier since it may as a matter of policy accept messages having valid signatures with extraneous data. - Signers of MIME messages that include a body length count SHOULD be - sure that the length extends to the closing MIME boundary string. + INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables + an attack in which an attacker modifies a message to include + content that solely benefits the attacker. It is possible for the + appended content to completely replace the original content in the + end recipient's eyes and to defeat duplicate message detection + algorithms. To avoid this attack, signers should be wary of using + this tag, and verifiers might wish to ignore the tag or remove + text that appears after the specified content length, perhaps + based on other criteria. + + The body length count allows the signer of a message to permit data + to be appended to the end of the body of a signed message. The body + length count MUST be calculated following the canonicalization + algorithm; for example, any white space ignored by a canonicalization + algorithm is not included as part of the body length count. Signers + of MIME messages that include a body length count SHOULD be sure that + the length extends to the closing MIME boundary string. INFORMATIVE IMPLEMENTATION NOTE: A signer wishing to ensure that the only acceptable modifications are to add to the MIME postlude would use a body length count encompassing the entire final MIME boundary string, including the final "--CRLF". A signer wishing to allow additional MIME parts but not modification of existing parts would use a body length count extending through the final MIME boundary string, omitting the final "--CRLF". A body length count of zero means that the body is completely unsigned. - INFORMATIVE IMPLEMENTATION NOTE: Note that verifiers may choose - to modify their interpretation of messages with unsigned content, - including truncating the unsigned part, refusing to display the - unsigned part to the user, or simply treating the signature as - invalid. - Signers wishing to ensure that no modification of any sort can occur - should specify the "simple" algorithm and omit the body length count. + should specify the "simple" canonicalization algorithm for both + header and body and omit the body length count. 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.) @@ -728,90 +737,85 @@ C D E when canonicalized using relaxed canonicalization for both header and body results in a header reading: a:X b:Y Z - and a body reading: C D E - (postamble) Example 2: The same message canonicalized using simple canonicalization for both header and body results in a header reading: A: X B : Y Z + and a body reading: C D E - (postamble) 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 - (postamble) 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. - In particular, the "DKIM-Signature" header field SHOULD precede the - original email header fields presented to the canonicalization and - signature algorithms. The "DKIM-Signature:" header field being created or verified is always included in the signature calculation, after the body of the message; however, when calculating or verifying the signature, the value of the b= tag (signature value) of that DKIM-Signature header - field MUST be treated as though it were the null string. Unknown + field MUST be treated as though it were an empty string. Unknown tags in the "DKIM-Signature:" header field MUST be included in the signature calculation but MUST be otherwise ignored by verifiers. Other "DKIM-Signature:" header fields that are included in the signature should be treated as normal header fields; in particular, the b= tag is not treated specially. The encodings for each field type are listed below. Tags described as qp-section are as described in section 6.7 of MIME Part One [RFC2045], with the additional conversion of semicolon characters to "=3B"; intuitively, this is one line of quoted-printable encoded - text. Tags described as dkim-quoted-printable are as defined above. + text. Tags described as dkim-quoted-printable are as defined in + Section 2.6. Tags on the DKIM-Signature header field along with their type and - requirement status are shown below. Defined tags are described - below. Unrecognized tags MUST be ignored. + requirement status are shown below. Unrecognized tags MUST be + ignored. - v= Version (MUST be included). This tag defines the version of - this specification that applies to the signature record. It MUST - have the value 0.3. + v= Version (MUST be included). This tag defines the version of this + specification that applies to the signature record. It MUST have + the value 0.4. ABNF: - sig-v-tag = %x76 [FWS] "=" [FWS] "0.3" + sig-v-tag = %x76 [FWS] "=" [FWS] "0.4" INFORMATIVE NOTE: DKIM-Signature version numbers are expected to increase arithmetically as new versions of this specification are released. [[INFORMATIVE NOTE: Upon publication, this version number should be changed to "1", and this note should be deleted.]] a= The algorithm used to generate the signature (plain-text; REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256"; @@ -831,26 +835,27 @@ this value and MUST be ignored when re-assembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length limits. See Signer Actions (Section 5) for how the signature is computed. ABNF: sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag-data = base64string - bh= The hash of the body part of the message (base64; REQUIRED). - Whitespace is ignored in this value and MUST be ignored when re- - assembling the original signature. In particular, the signing - process can safely insert FWS in this value in arbitrary places - to conform to line-length limits. See Section 3.7 for how the - body hash is computed. + + bh= The hash of the canonicalized body part of the message as limited + by the "l=" tag (base64; REQUIRED). Whitespace is ignored in + this value and MUST be ignored when re-assembling the original + signature. In particular, the signing process can safely insert + FWS in this value in arbitrary places to conform to line-length + limits. See Section 3.7 for how the body hash is computed. ABNF: sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data sig-bh-tag-data = base64string c= Message canonicalization (plain-text; OPTIONAL, default is "simple/simple"). This tag informs the verifier of the type of canonicalization used to prepare the message for signing. It consists of two names separated by a "slash" (%d47) character, @@ -860,55 +865,53 @@ header and "simple" is used for the body. For example, "c=relaxed" is treated the same as "c=relaxed/simple". ABNF: sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg ["/" sig-c-tag-alg] sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg x-sig-c-tag-alg = hyphenated-word ; for later extension - d= The domain of the signing entity (plain-text; REQUIRED). This - is the domain that will be queried for the public key. This - domain MUST be the same as or a parent domain of the "i=" tag - (the signing identity, as described below). If the "t=s" tag is - specified in the key record referenced by the selector in the - "s=" tag, then the domain in the "d=" tag must be identical to - the domain specified in the "i=" tag. When presented with a - signature that does not meet these requirement, verifiers MUST - consider the signature invalid. + d= The domain of the signing entity (plain-text; REQUIRED). This is + the domain that will be queried for the public key. This domain + MUST be the same as or a parent domain of the "i=" tag (the + signing identity, as described below), or it MUST meet the + requirements for parent domain signing described in Section 3.8. + When presented with a signature that does not meet these + requirement, verifiers MUST consider the signature invalid. Internationalized domain names MUST be punycode-encoded [RFC3492]. ABNF: sig-d-tag = %x64 [FWS] "=" [FWS] domain-name domain-name = sub-domain 1*("." sub-domain) ; from RFC 2821 Domain, but excluding address-literal - h= Signed header fields (plain-text, but see description; - REQUIRED). A colon-separated list of header field names that - identify the header fields presented to the signing algorithm. - The field MUST contain the complete list of header fields in the - order presented to the signing algorithm. The field MAY contain - names of header fields that do not exist when signed; nonexistent - header fields do not contribute to the signature computation - (that is, they are treated as the null input, including the - header field name, the separating colon, the header field value, - and any CRLF terminator). The field MUST NOT include the DKIM- - Signature header field that is being created or verified, but may - include others. Folding white space (FWS) MAY be included on - either side of the colon separator. Header field names MUST be - compared against actual header field names in a case insensitive - manner. This list MUST NOT be empty. See Section 5.4 for a - discussion of choosing header fields to sign. + h= Signed header fields (plain-text, but see description; REQUIRED). + A colon-separated list of header field names that identify the + header fields presented to the signing algorithm. The field MUST + contain the complete list of header fields in the order presented + to the signing algorithm. The field MAY contain names of header + fields that do not exist when signed; nonexistent header fields + do not contribute to the signature computation (that is, they are + treated as the null input, including the header field name, the + separating colon, the header field value, and any CRLF + terminator). The field MUST NOT include the DKIM-Signature + header field that is being created or verified, but may include + others. Folding white space (FWS) MAY be included on either side + of the colon separator. Header field names MUST be compared + against actual header field names in a case insensitive manner. + This list MUST NOT be empty. See Section 5.4 for a discussion of + choosing header fields to sign. ABNF: sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name 0*( *FWS ":" *FWS hdr-name ) hdr-name = field-name INFORMATIVE EXPLANATION: By "signing" header fields that do not actually exist, a signer can prevent insertion of those header fields before verification. However, since a signer @@ -923,20 +926,23 @@ actual header field with a null value. i= Identity of the user or agent (e.g., a mailing list manager) on behalf of which this message is signed (dkim-quoted-printable; OPTIONAL, default is an empty local-part followed by an "@" followed by the domain from the "d=" tag). The syntax is a standard email address where the local-part MAY be omitted. The domain part of the address MUST be the same as or a subdomain of the value of the "d=" tag. + Internationalized domain names MUST be punycode-encoded + [RFC3492]. + ABNF: sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name INFORMATIVE NOTE: The local-part of the "i=" tag is optional because in some cases a signer may not be able to establish a verified individual identity. In such cases, the signer may wish to assert that although it is willing to go as far as signing for the domain, it is unable or unwilling to commit to an individual user name within their domain. It can do so @@ -953,27 +959,27 @@ complex topic and trust mechanisms are subject to highly creative attacks. The real-world efficacy of any but the most basic bindings between the "i=" value and other identities is not well established, nor is its vulnerability to subversion by an attacker. Hence reliance on the use of these options should be strictly limited. In particular it is not at all clear to what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the "i=" options. - l= Body length count (plain-text unsigned decimal integer; - OPTIONAL, default is entire body). This tag informs the verifier - of the number of octets in the body of the email after - canonicalization included in the cryptographic hash, starting - from 0 immediately following the CRLF preceding the body. This - value MUST NOT be larger than the actual number of octets in the - canonicalized message body. + l= Body length count (plain-text unsigned decimal integer; OPTIONAL, + default is entire body). This tag informs the verifier of the + number of octets in the body of the email after canonicalization + included in the cryptographic hash, starting from 0 immediately + following the CRLF preceding the body. This value MUST NOT be + larger than the actual number of octets in the canonicalized + message body. INFORMATIVE IMPLEMENTATION WARNING: Use of the l= tag might allow display of fraudulent content without appropriate warning to end users. The l= tag is intended for increasing signature robustness when sending to mailing lists that both modify their content and do not sign their messages. However, using the l= tag enables attacks in which an intermediary with malicious intent modifies a message to include content that solely benefits the attacker. It is possible for the appended content to completely replace the @@ -1008,29 +1014,30 @@ Currently the only valid value is "dns/txt" which defines the DNS TXT record lookup algorithm described elsewhere in this document. The only option defined for the "dns" query type is "txt", which MUST be included. Verifiers and signers MUST support "dns/txt". ABNF: sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method *([FWS] ":" [FWS] sig-q-tag-method) - sig-q-tag-method = "txt/dns" / x-sig-q-tag-type ["/" x-sig-q-tag-args] + sig-q-tag-method = "dns/txt" / x-sig-q-tag-type ["/" x-sig-q-tag-args] x-sig-q-tag-type = hyphenated-word ; for future extension x-sig-q-tag-args = qp-hdr-value - s= The selector subdividing the namespace for the "d=" (domain) tag + + s= The Selector subdividing the namespace for the "d=" (domain) tag (plain-text; REQUIRED). ABNF: - sig-s-tag = %x73 [FWS] "=" [FWS] subdomain *( "." sub-domain ) + sig-s-tag = %x73 [FWS] "=" [FWS] selector t= Signature Timestamp (plain-text unsigned decimal integer; RECOMMENDED, default is an unknown creation time). The time that this signature was created. The format is the number of seconds since 00:00:00 on January 1, 1970 in the UTC time zone. The value is expressed as an unsigned integer in decimal ASCII. This value is not constrained to fit into a 31- or 32-bit integer. Implementations SHOULD be prepared to handle values up to at least 10^12 (until approximately AD 200,000; this fits into 40 bits). To avoid denial of service attacks, implementations MAY @@ -1052,35 +1059,36 @@ if that time is reliably available; otherwise the current time should be used. The value of the "x=" tag MUST be greater than the value of the "t=" tag if both are present. INFORMATIVE NOTE: The x= tag is not intended as an anti- replay defense. ABNF: sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT - z= Copied header fields (dkim-quoted-printable, but see - description; OPTIONAL, default is null). A vertical-bar- - separated list of selected header fields present when the message - was signed, including both the field name and value. It is not - required to include all header fields present at the time of - signing. This field need not contain the same header fields - listed in the "h=" tag. The header field text itself must encode - the vertical bar ("|", %x7C) character (i.e., vertical bars in - the z= text are metacharacters, and any actual vertical bar - characters in a copied header field must be encoded). Note that - all white space must be encoded, including white space between - the colon and the header field value. After encoding, SWSP MAY - be added at arbitrary locations in order to avoid excessively - long lines; such white space is NOT part of the value of the - header field, and MUST be removed before decoding. + + z= Copied header fields (dkim-quoted-printable, but see description; + OPTIONAL, default is null). A vertical-bar-separated list of + selected header fields present when the message was signed, + including both the field name and value. It is not required to + include all header fields present at the time of signing. This + field need not contain the same header fields listed in the "h=" + tag. The header field text itself must encode the vertical bar + ("|", %x7C) character (i.e., vertical bars in the z= text are + metacharacters, and any actual vertical bar characters in a + copied header field must be encoded). Note that all white space + must be encoded, including white space between the colon and the + header field value. After encoding, SWSP MAY be added at + arbitrary locations in order to avoid excessively long lines; + such white space is NOT part of the value of the header field, + and MUST be removed before decoding. Verifiers MUST NOT use the header field names or copied values for checking the signature in any way. Copied header field values are for diagnostic use only. Header fields with characters requiring conversion (perhaps from legacy MTAs which are not [RFC2822] compliant) SHOULD be converted as described in MIME Part Three [RFC2047]. ABNF: @@ -1089,137 +1097,141 @@ sig-z-tag-copy = hdr-name ":" qp-hdr-value qp-hdr-value = dkim-quoted-printable ; with "|" encoded INFORMATIVE EXAMPLE of a signature header field spread across multiple continuation lines: 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 + 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 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 and in multiple formats. The storage and format of keys are irrelevant to the remainder of the DKIM algorithm. Parameters to the key lookup algorithm are the type of the lookup (the "q=" tag), the domain of the responsible signer (the "d=" tag of - the DKIM-Signature header field), and the selector (the "s=" tag). + 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 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 key-value-list as described in Section 3.2. - The current valid tags are described below. Other tags MAY be - present and MUST be ignored by any implementation that does not - understand them. + The overall syntax is a tag-list as described in Section 3.2. The + current valid tags are described below. Other tags MAY be present + and MUST be ignored by any implementation that does not understand + them. v= Version of the DKIM key record (plain-text; RECOMMENDED, default is "DKIM1"). If specified, this tag MUST be set to "DKIM1" (without the quotes). This tag MUST be the first tag in the - response. Responses beginning with a "v=" tag with any other - value MUST be discarded. + record. Records beginning with a "v=" tag with any other value + MUST be discarded. ABNF: key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1" g= granularity of the key (plain-text; OPTIONAL, default is "*"). This value MUST match the Local-part of the "i=" tag of the DKIM- Signature header field (or its default value of the empty string if "i=" is not specified), with a "*" character matching a sequence of zero or more arbitrary characters ("wildcarding"). The intent of this tag is to constrain which signing address can - legitimately use this selector. An email with a signing address + legitimately use this Selector. An email with a signing address that does not match the value of this tag constitutes a failed verification. Wildcarding allows matching for addresses such as "user+*". 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] ["*"] [dot-atom] + key-g-tag-lpart = [dot-atom-text] ["*"] [dot-atom-text] - [[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a dot- - atom. This should probably use a different character for - wildcarding. Unfortunately, the options are non-mnemonic + [[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 "sha1" hash algorithm. + support the "sha256" hash algorithm. Verifiers MUST also support + the "sha1" hash algorithm. ABNF: key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg 0*( [FWS] ":" [FWS] key-h-tag-alg ) key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg x-key-h-tag-alg = hyphenated-word ; for future extension k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and verifiers MUST support the "rsa" key type. The "rsa" key type - indicates that an RSA public key, as defined in [RFC3447], - sections 3.1 and A.1.1, is being used in the p= tag. (Note: the - p= tag further encodes the value using the base64 algorithm.) + indicates that an ASN.1 DER-encoded [X.660] RSAPublicKey + [RFC3447] (see sections 3.1 and A.1.1) is being used in the p= + tag. (Note: the p= tag further encodes the value using the + base64 algorithm.) ABNF: key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type key-k-tag-type = "rsa" / x-key-k-tag-type x-key-k-tag-type = hyphenated-word ; for future extension [[NON-NORMATIVE DISCUSSION NOTE: In some cases it can be hard to separate h= and k=; for example DSA implies that SHA-1 will be used. This might be an actual change to the spec depending on how we decide to fix this.]] - n= Notes that might be of interest to a human (qp-section; - OPTIONAL, default is empty). No interpretation is made by any - program. This tag should be used sparingly in any key server - mechanism that has space limitations (notably DNS). + + n= Notes that might be of interest to a human (qp-section; OPTIONAL, + default is empty). No interpretation is made by any program. + This tag should be used sparingly in any key server mechanism + that has space limitations (notably DNS). ABNF: key-n-tag = %x6e [FWS] "=" [FWS] qp-section p= Public-key data (base64; REQUIRED). An empty value means that this public key has been revoked. The syntax and semantics of this tag value before being encoded in base64 is defined by the k= tag. ABNF: - key-p-tag = %x70 [FWS] "=" [FWS] base64string + key-p-tag = %x70 [FWS] "=" [ [FWS] base64string ] s= Service Type (plain-text; OPTIONAL; default is "*"). A colon- separated list of service types to which this record applies. Verifiers for a given service type MUST ignore this record if the appropriate type is not listed. Currently defined service types are: * matches all service types email electronic mail (not necessarily limited to SMTP) @@ -1241,52 +1253,55 @@ y This domain is testing DKIM. Verifiers MUST NOT treat messages from signers in testing mode differently from unsigned email, even should the signature fail to verify. Verifiers MAY wish to track testing mode results to assist the signer. s Any DKIM-Signature header fields using the "i=" tag MUST have the same domain value on the right hand side of the "@" in the "i=" tag and the value of the "d=" tag. That is, the - "i=" domain MUST NOT be a subdomain of "d=". + "i=" domain MUST NOT be a subdomain of "d=". Use of this + flag is RECOMMENDED unless subdomaining is required. ABNF: key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag 0*( [FWS] ":" [FWS] key-t-tag-flag ) key-t-tag-flag = "y" / "s" / x-key-t-tag-flag x-key-t-tag-flag = hyphenated-word ; for future extension Unrecognized flags MUST be ignored. 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 - 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 ""sample"", the DNS query will be for - ""sample._domainkey.example.com"". + 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". - The value of the "i=" tag is not used by the DNS binding. + Wildcard DNS records (e.g., *.bar._domainkey.example.com) MUST 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 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 RR record. A later extension of this standard may define another Resource Record - type, tentatively dubbed "DKK". + type. TXT records are encoded as described in Section 3.6.1. 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 @@ -1335,47 +1350,73 @@ When calculating the hash on messages that will be transmitted using base64 or quoted-printable encoding, signers MUST compute the hash after the encoding. Likewise, the verifier MUST incorporate the values into the hash before decoding the base64 or quoted-printable text. However, the hash MUST be computed before transport level encodings such as SMTP "dot-stuffing." With the exception of the canonicalization procedure described in Section 3.4, the DKIM signing process treats the body of messages as - simply a string of characters. DKIM messages MAY be either in plain- - text or in MIME format; no special treatment is afforded to MIME - content. Message attachments in MIME format MUST be included in the - content which is signed. + simply a string of octets. DKIM messages MAY be either in plain-text + or in MIME format; no special treatment is afforded to MIME content. + Message attachments in MIME format MUST be included in the content + which is signed. More formally, the algorithm for the signature is: body-hash = hash-alg(canon_body) header-hash = hash-alg(canon_header || DKIM-SIG) signature = sig-alg(header-hash, key) - where sig-alg is the signature algorithm specified by the "a=" tag, - hash-alg is the hash algorithm specified by the "a=" tag, - canon_header and canon_body are the canonicalized message header and - body (respectively) as defined in Section 3.4 (excluding the DKIM- - Signature header field), and DKIM-SIG is the canonicalized DKIM- - Signature header field sans the signature value itself, but with - body-hash included as the "bh=" tag. + where "sig-alg" is the signature algorithm specified by the "a=" tag, + "hash-alg" is the hash algorithm specified by the "a=" tag, + "canon_header" and "canon_body" are the canonicalized message header + and body (respectively) as defined in Section 3.4 (excluding the + DKIM-Signature header field), and "DKIM-SIG" is the canonicalized + DKIM-Signature header field sans the signature value itself, but with + "body-hash" included as the "bh=" tag. + + INFORMATIVE 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 + + 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 + may be set in the t= tag of the key record to constrain the validity + 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 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 headers using the method described in section Section 5.4 to sign trace headers. Signers should be cognizant that signing DKIM-Signature headers may result in signature failures with intermediaries that do not - recognize that DKIM-Signature's are trace headers and unwittingly + recognize that DKIM-Signatures are trace headers and unwittingly reorder them. When evaluating a message with multiple signatures, a verifier should evaluate signatures independently and on their own merits. For example, a verifier that by policy chooses not to accept signatures with deprecated cryptographic algorithms should consider such signatures invalid. As with messages with a single signature, verifiers are at liberty to use the presence of valid signatures as an input to local policy; likewise, the interpretation of multiple valid signatures in combination is a local policy decision of the @@ -1386,56 +1427,55 @@ cannot be verified. 5. Signer Actions The following steps are performed in order by signers. 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 + 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. - A SUBMISSION server MAY sign if the submitter is authenticated by - some secure means, e.g., SMTP AUTH. Within a trusted enclave the - signing address MAY be derived from the header field according to - local signer policy. Within a trusted enclave an MTA MAY do the - signing. + 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, + signers should beware of signing (and thereby asserting + responsibility for) messages that may be problematic. In + particular, within a trusted enclave the signing address might be + derived from the header according to local policy; SUBMISSION + servers might only sign messages from users that are properly + 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. - A signer MUST NOT sign an email if it is unwilling to be held - responsible for the message; in particular, the signer SHOULD ensure - that the submitter has a bona fide relationship with the signer and - that the submitter has the right to use the address being claimed. - 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 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 + 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 removed before the verifier has an - opportunity to validate the signature. The signer should + 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 Some messages, particularly those using 8-bit characters, are subject @@ -1449,197 +1489,162 @@ DKIM algorithm. Should the message be submitted to the signer with any local encoding that will be modified before transmission, such conversion to canonical form MUST be done before signing. In particular, some systems use local line separator conventions (such as the Unix newline character) internally rather than the SMTP-standard CRLF sequence. All such local conventions MUST be converted to canonical format before signing. - More generally, the signer MUST sign the message as it will be - received by the verifier rather than in some local or internal form. + 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 The From header field MUST be signed (that is, included in the h= tag - of the resulting DKIM-Signature header field); any header field that - describes the role of the signer (for example, the Sender or Resent- - From header field if the signature is on behalf of the corresponding - address and that address is different from the From address) MUST - also be included. The signed header fields SHOULD also include the - Subject and Date header fields as well as all MIME header fields. - 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. It is RECOMMENDED that all other existing, non-repeatable - header fields be signed. + 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 + sign is non-obvious. One strategy is to sign all existing, non- + repeatable header fields. An alternative strategy is to sign only + header fields that are likely to be displayed to or otherwise be + likely to affect the processing of the message at the receiver. A + third strategy is to sign only "well known" headers. Note that + verifiers may treat unsigned header fields with extreme + skepticism, including refusing to display them to the end user or + even ignore the signature if it does not cover certain header + fields. For this reason signing fields present in the message + such as Date, Subject, Reply-To, Sender, and all MIME headers is + highly advised. The DKIM-Signature header field is always implicitly signed and MUST NOT be included in the h= tag except to indicate that other preexisting signatures are also signed. - Signers MUST sign any header fields that the signers wish to assert - were present at the time of signing. Put another way, verifiers MAY - treat unsigned header fields with extreme skepticism, up to and - including refusing to display them to the end user. - Signers MAY claim to have signed header fields that do not exist (that is, signers MAY include the header field name in the h= tag even if that header field does not exist in the message). When computing the signature, the non-existing header field MUST be treated as the null string (including the header field name, header field value, all punctuation, and the trailing CRLF). INFORMATIVE RATIONALE: This allows signers to explicitly assert the absence of a header field; if that header field is added later the signature will fail. - Signers choosing to sign an existing replicated header field (such as - Received) MUST sign the physically last instance of that header field - in the header field block. Signers wishing to sign multiple - instances of an existing replicated header field MUST include the - header field name multiple times in the h= tag of the DKIM-Signature - header field, and MUST sign such header fields in order from the - bottom of the header field block to the top. The signer MAY include - more header field names than there are actual corresponding header - fields to indicate that additional header fields of that name SHOULD - NOT be added. + Signers choosing to sign an existing header field that occurs more + than once in the message (such as Received) MUST sign the physically + last instance of that header field in the header block. Signers + wishing to sign multiple instances of such a header field MUST + include the header field name multiple times in the h= tag of the + DKIM-Signature header field, and MUST sign such header fields in + order from the bottom of the header field block to the top. The + signer MAY include more instances of a header field name in h= than + there are actual corresponding header fields to indicate that + additional header fields of that name SHOULD NOT be added. INFORMATIVE EXAMPLE: If the signer wishes to sign two existing Received header fields, and the existing header contains: Received: Received: Received: then the resulting DKIM-Signature header field should read: DKIM-Signature: ... h=Received : Received : ... and Received header fields and will be signed in that order. - Signers SHOULD NOT sign header fields that might be replicated - (either at the time of signing or potentially in the future), with - the exception of trace header fields such as Received. Comment and - non standard header fields (including X-* header fields) are - permitted by [RFC2822] to be replicated; however, many such header - fields are, by convention, not replicated. Signers need to - understand the implications of signing header fields that might later - be replicated, especially in the face of header field reordering. In - particular, [RFC2822] only requires that trace header fields retain - the original order. - - INFORMATIVE RATIONALE: Received: is allowed because these header - fields, as well as Resent-* header fields, are already order- - sensitive. + Signers should be careful of signing header fields that might have + additional instances added later in the delivery process, since such + header fields might be inserted after the signed instance or + otherwise reordered. Trace header fields (such as Received and DKIM- + Signature) and Resent-* blocks are the only fields prohibited by + [RFC2822] from being reordered. INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits - header field blocks to be reordered (with the exception of - Received header fields), reordering of signed replicated header - fields by intermediate MTAs will cause DKIM signatures to be + header fields to be reordered (with the exception of Received + header fields), reordering of signed header fields with multiple + 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. - INFORMATIVE NOTE: There has been some discussion that a Sender - Signing Policy include the list of header fields that the signer - always signs. N.B. In theory this is unnecessary, since as long - as the signer really always signs the indicated header fields - there is no possibility of an attacker replaying an existing - message that has such an unsigned header field. - 5.5 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. - To avoid possible ambiguity, a signer SHOULD either sign or remove - any preexisting header fields which convey the results of previous - verifications of the message signature prior to preparation for - signing and transmission. Such header fields MUST NOT be signed if - the signer is uncertain of the authenticity of the preexisting header - field, for example, if it is not locally generated or signed by a - previous DKIM-Signature line that the current signer has verified. - 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; such signing SHOULD - include any prior verification status, if any, that was inserted upon - message receipt. + 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. - INFORMATIVE NOTE: A possible value to include in the "l=" tag - would include the entire length of the message being signed, - thereby allowing intermediate agents to append further information - to the message without breaking the signature (e.g., a mailing - list manager might add unsubscribe information to the body). A - signer wishing to permit such intermediate agents to add another - MIME body part to a "multipart/mixed" message should use a length - that covers the entire presented message except for the trailing - "--CRLF" characters; this is known as the "N-4" approach. Note - that more than four characters may need to be stripped, since - there could be postlude information that needs to be ignored. - 5.6 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 + the Selector given in the "s=" tag of the "DKIM-Signature" header field, as chosen above in Section 5.2 - - The "DKIM-Signature" SHOULD be inserted before any header fields that - it signs in the header block. + The "DKIM-Signature" MUST be inserted before any other DKIM-Signature + fields in the header block. INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this is to insert the "DKIM-Signature" header field at the beginning of the header block. In particular, it may be placed before any existing Received header fields. This is consistent with treating "DKIM-Signature" as a trace header. 6. Verifier Actions - Since a signer MAY expire a public key at any time, it is recommended - that verification occur in a timely manner with the most timely place - being during acceptance by the border MTA. + Since a signer MAY remove or revoke a public key at any time, it is + recommended that verification occur in a timely manner with the most + timely place being during acceptance by the border MTA. - A border or intermediate MTA MAY verify the message signatures and - add a verification header field to incoming messages. This - considerably simplifies things for the user, who can now use an - existing mail user agent. Most MUAs have the ability to filter - messages based on message header fields or content; these filters - would be used to implement whatever policy the user wishes with - respect to unsigned mail. + A border or intermediate MTA MAY verify the message signature(s). An + MTA who has performed verification MAY communicate the result of that + verification by adding a verification header field to incoming + messages. This considerably simplifies things for the user, who can + now use an existing mail user agent. Most MUAs have the ability to + filter messages based on message header fields or content; these + filters would be used to implement whatever policy the user wishes + with respect to unsigned mail. 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. @@ -1650,31 +1655,43 @@ 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. INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as a clue to signing order in the absence of any other information. + However, other clues as to the semantics of multiple signatures - must be considered before using ordering. + (such as correlating the signing host with Received headers) may + also be considered. A verifier SHOULD NOT treat a message that has one or more bad signatures and no good signatures differently from a message with no - signature at all; this is local policy and is beyond the scope of - this document. + signature at all; such treatment is a matter of local policy and is + beyond the scope of this document. When a signature successfully verifies, a verifier will either stop processing or attempt to verify any other signatures, at the - discretion of the implementation. + discretion of the implementation. A verifier MAY limit the number of + signatures it tries to avoid denial-of-service attacks. + + INFORMATIVE NOTE: An attacker could send messages with large + numbers of faulty signatures, each of which would require a DNS + lookup. This could be an attack on the domain that receives the + message, by slowing down the verifier by requiring it to do large + number of DNS lookups and signature verifications. It could also + be an attack against the domains listed in the signatures, + essentially by enlisting innocent verifiers in launching an attack + against the DNS servers of the actual victim. In the following description, text reading "return status (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL") means that the verifier MUST immediately cease processing that signature. The verifier SHOULD proceed to the next signature, if any is present, and completely ignore the bad signature. If the status is "PERMFAIL", the signature failed and should not be reconsidered. If the status is "TEMPFAIL", the signature could not be verified at this time but may be tried again later. A verifier MAY either defer the message for later processing, perhaps by queueing it locally or @@ -1725,50 +1742,61 @@ If the DKIM-Signature header field does not contain any of the tags listed as required in Section 3.5 the verifier MUST ignore the DKIM- Signature header field and return PERMFAIL (signature missing required tag). If the "DKIM-Signature" header field does not contain the "i=" tag, the verifier MUST behave as though the value of that tag were "@d", where "d" is the value from the "d=" tag. Verifiers MUST confirm that the domain specified in the "d=" tag is - the same as or a superdomain of the domain part of the "i=" tag. If - not, the DKIM-Signature header field MUST be ignored and the verifier - should return PERMFAIL (domain mismatch). + the same as or a parent domain of the domain part of the "i=" tag. + If not, the DKIM-Signature header field MUST be ignored and the + verifier should return PERMFAIL (domain mismatch). + + 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 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 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. When validating a message, a verifier MUST perform the following steps in a manner that is semantically the same as performing them in the order indicated (in some cases the implementation may parallelize or reorder these steps, as long as the semantics remain unchanged): 1. Retrieve the public key as described in (Section 3.6) using the - domain from the "d=" tag and the selector from the "s=" tag. + domain from the "d=" tag and the Selector from the "s=" tag. 2. If the query for the public key fails to respond, the verifier MAY defer acceptance of this email and return TEMPFAIL (key - unavailable). If verification is occuring during the incoming + unavailable). If verification is occurring during the incoming SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply code. Alternatively, the verifier MAY store the message in the local queue for later trial or ignore the signature. Note that storing a message in the local queue is subject to denial-of- service attacks. 3. If the query for the public key fails because the corresponding key record does not exist, the verifier MUST immediately return PERMFAIL (no key for signature). @@ -1802,38 +1830,41 @@ domain signature. 7. If the "h=" tag exists in the public key record and the hash algorithm implied by the a= tag in the DKIM-Signature header is not included in the contents of the "h=" tag, the verifier MUST ignore the key record and return PERMFAIL (inappropriate hash algorithm). 8. If the public key data (the "p=" tag) is empty then this key has been revoked and the verifier MUST treat this as a failed - signature check and return PERMFAIL (key revoked). + 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 Given a signer and a public key, verifying a signature consists of - the following steps. + 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, create a canonicalized copy of the email as is described in - Section 3.7. When matching header field names in the "h=" tag - against the actual message header field, comparisons MUST be - case-insensitive. + 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, + comparisons MUST be case-insensitive. 2. Based on the algorithm indicated in the "a=" tag, compute the message hashes from the canonical copy as described in Section 3.7. 3. Verify that the hash of the canonicalized message body computed in the previous step matches the hash value conveyed in the "bh=" tag. 4. Using the signature conveyed in the "b=" tag, verify the @@ -1870,33 +1901,33 @@ 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 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. An example proposal for a - header field is the Authentication-Results header field [ID-AUTH- - RES]. Any such header field SHOULD be inserted before any existing - DKIM-Signature or preexisting authentication status header fields in - the header field block. + 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. + header fields added by attackers. Verifiers may wish to delete + existing results header fields after verification and before + adding a new header field to circumvent this attack. 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 @@ -1931,40 +1962,171 @@ signed on behalf of any address other than that in the From: header field, the mail system SHOULD take pains to ensure that the actual signing identity is clear to the reader. The verifier MAY treat unsigned header fields with extreme skepticism, including marking them as untrusted or even deleting them before display to the end user. While the symptoms of a failed verification are obvious -- the signature doesn't verify -- establishing the exact cause can be more - difficult. If a selector cannot be found, is that because the - selector has been removed or was the value changed somehow in + difficult. If a Selector cannot be found, is that because the + Selector has been removed or was the value changed somehow in transit? If the signature line is missing is that because it was never there, or was it removed by an 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. However in terms of presentation to the end user, the result SHOULD be presented as a simple binary result: either the email is verified or it is not. 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 - To avoid conflicts, tag names for the DKIM-Signature header and key - records should be registered with IANA. + DKIM introduces some new namespaces that require IANA registry. - Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature - header field, and the "h=", "k=", "s=", and "t" tags in key records - should be registered with IANA for the same reason. + [[Missing registries for signature t= (flags) tags, as well as key + record s= (service type) and t= (flags).]] + +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 | RFC | + +------+-----------------+ + | v | (this document) | + | a | (this document) | + | b | (this document) | + | bh | (this document) | + | c | (this document) | + | d | (this document) | + | 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 + + 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 | RFC | + +------+--------+-----------------+ + | dns | txt | (this document) | + +------+--------+-----------------+ + +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 | RFC | + +---------+-----------------+ + | simple | (this document) | + | relaxed | (this document) | + +---------+-----------------+ + + The initial entries in the body registry comprise: + + +---------+-----------------+ + | TYPE | RFC | + +---------+-----------------+ + | simple | (this document) | + | relaxed | (this document) | + +---------+-----------------+ + +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: + + +------+-----------------+ + | TYPE | RFC | + +------+-----------------+ + | 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 + + 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 | RFC | + +------+---------+ + | rsa | RFC3447 | + +------+---------+ + +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 | RFC | + +--------+-----+ + | sha1 | ? | + | sha256 | ? | + +--------+-----+ 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 [ID-DKIM-THREATS]. 8.1 Misuse of Body Length Limits ("l=" Tag) @@ -1974,21 +2136,26 @@ 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. - *** Example appropriate here *** + 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. 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: @@ -2000,21 +2166,23 @@ 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. + engineered messages. This threat applies mainly to MUA-based + implementations; protection of private keys on servers can be easily + achieved through the use of specialized cryptographic hardware. A larger problem occurs if malware on many users' computers obtains the private keys for those users and transmits them via a covert channel to a site where they can be shared. The compromised users would likely not know of the misappropriation until they receive "bounce" messages from messages they are purported to have sent. Many users might not understand the significance of these bounce messages and would not take action. One countermeasure is to use a user-entered passphrase to encrypt the @@ -2134,21 +2302,21 @@ 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 An attacker could determine when a particular signature was verified - by using a per-message selector and then monitoring their DNS traffic + by using a per-message Selector and then monitoring their DNS traffic for the key lookup. This would act as the equivalent of a "web bug" for verification time rather than when the message was read. 8.10 Remote Timing Attacks In some cases it may be possible to extract private keys using a remote timing attack [BONEH03]. Implementations should consider obfuscating the timing to prevent such attacks. 9. References @@ -2176,46 +2344,45 @@ Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Application(IDNA)", March 2003. [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. + [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 [BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing Attacks are Practical", 2003, . - [ID-AUTH-RES] - Kucherawy, M., "Message header field for Indicating Sender - Authentication Status", - draft-kucherawy-sender-auth-header-02 (work in progress), - February 2006. - [ID-DKIM-THREATS] Fenton, J., "Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)", draft-fenton-dkim-threats-02 (work in progress), April 2006. [RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted", RFC 1847, October 1995. [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, "OpenPGP Message Format", RFC 2440, November 1998. - [RFC3766] Orman, H. and P. Hoffman, "Determing Strengths for Public - Keys Used For Exchanging Symmetric Keys", RFC 3766, + [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. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", @@ -2299,71 +2465,73 @@ We lost the game. Are you hungry yet? Joe. 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-sha1; s=brisbane; d=example.com; + DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com; c=simple; q=dns/txt; i=joe@football.example.com; h=Received : From : To : Subject : Date : Message-ID; + bh=ZSVEYuq4ri3LR9S+qjlzCP+LxvJrIfrOI2g5hxp5+MI=; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR; Received: from dsl-10.2.3.4.football.example.com [10.2.3.4] by submitserver.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) 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. The signing email server requires access to the private-key - associated with the "brisbane" selector to generate this signature. + associated with the "brisbane" Selector to generate this signature. A.3 The email signature is verified 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- + "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 Signature verification starts with the physically last "Received" header field, the "From" header field, and so forth, in the order listed in the "h=" tag. Verification follows with a single CRLF followed by the body (starting with "Hi."). The email is canonically prepared for verifying with the "simple" method. The result of the query and subsequent verification of the signature is stored in the "Authentication-Results" header field line. After successful verification, the email looks like this: - Authentication-Results: shopping.example.net + X-Authentication-Results: shopping.example.net header.from=joe@football.example.com; dkim=pass Received: from mout23.football.example.com (192.168.1.1) by shopping.example.net with SMTP; Fri, 11 Jul 2003 21:01:59 -0700 (PDT) - DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com; + DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com; c=simple; q=dns/txt; i=joe@football.example.com; h=Received : From : To : Subject : Date : Message-ID; + bh=ZSVEYuq4ri3LR9S+qjlzCP+LxvJrIfrOI2g5hxp5+MI=; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR Received: from dsl-10.2.3.4.network.example.com [10.2.3.4] by submitserver.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) 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> @@ -2387,31 +2555,31 @@ .forward file or equivalent. In this case messages are typically forwarded without modification, except for the addition of a Received header field to the message and a change in the Envelope-to address. In this case, the eventual recipient should be able to verify the original signature since the signed content has not changed, and attribute the message correctly. B.2 Outsourced Business Functions Outsourced business functions represent a use case that motivates the - need for selectors (the "s=" signature tag) and granularity (the "g=" + need for Selectors (the "s=" signature tag) and granularity (the "g=" key tag). Examples of outsourced business functions are legitimate email marketing providers and corporate benefits providers. In either case, the outsourced function would like to be able to send messages using the email domain of the client company. At the same time, the client may be reluctant to register a key for the provider that grants the ability to send messages for any address in the domain. The outsourcing company can generate a keypair and the client company - can register the public key using a unique selector for a specific + can register the public key using a unique Selector for a specific address such as winter-promotions@example.com by specifying a granularity of "g=winter-promotions" or "g=*-promotions" (to allow a range of addresses). This would enable the provider to send messages using that specific address and have them verify properly. The client company retains control over the email address because it retains the ability to revoke the key at any time. B.3 PDAs and Similar Devices PDAs are one example of the use of multiple keys per user. Suppose @@ -2489,32 +2657,53 @@ One way this can be handled is to continue to put the reader's email address in the From header field of the message, but put an address owned by the site into the Sender header field, and sign the message on behalf of that address. A verifying MTA could accept this and rewrite the From header field to indicate the address that was verified, i.e., From: John Doe via news@news-site.com . (However, such rewriting must be done after the verification pass is complete, and will break any later attempts to re-verify.) +B.7 SMTP Servers for Roaming Users + + Roaming users may find themselves in circumstances where it is + convenient or necessary to use an SMTP server other than their home + server; examples are IETF conferences and many hotels. In such + circumstances the signature, if any, will be added by a party other + than 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 may be high. If neither of these are possible, + these roaming users will not be able to send mail signed using their + own domain key. + Appendix C. Creating a public key (INFORMATIVE) The default signature is an RSA signed SHA256 digest of the complete email. For ease of explanation, the openssl command is used to describe the mechanism by which keys and signatures are managed. One - way to generate a 768 bit private-key suitable for DKIM, is to use - openssl like this: + way to generate a 1024 bit, unencrypted private-key suitable for + DKIM, is to use openssl like this: $ openssl genrsa -out rsa.private 1024 - This results in the file rsa.private containing the key information - similar to this: + For increased security, the "-passin" parameter can also be added to + encrypt the private key. Use of this parameter will require entering + a password for several of the following steps. Servers may prefer to + use hardware cryptographic support. + + The "genrsa" step results in the file rsa.private containing the key + information similar to this: -----BEGIN RSA PRIVATE KEY----- MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX /1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m 3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/ @@ -2555,20 +2744,26 @@ This results in signature data similar to this when represented in Base64 [MIME] format: aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl How this signature is added to the email is discussed elsewhere in this document. + The final record entered into a DNS zone file would be: + + brisbane IN TXT ("v=DKIM1; p=aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvct" + "qc4rSEFYby9+omKD3pJ/TVxATeTzmsybuW3WZiamb+mvn7f" + "3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl" ) + Appendix D. MUA Considerations When a DKIM signature is verified, one of the results is a validated signing identity. MUAs might highlight the address associated with this identity in some way to show the user the address from which the mail is sent. An MUA might do this with visual cues such as graphics, or it might include the address in an alternate views, or it might even rewrite the original "From:" address using the verified information. Some MUAs might want to indicate which headers were covered in a validated DKIM signature. This might be done with a @@ -2603,21 +2798,77 @@ The DomainKeys specification was a primary source from which this specification has been derived. Further information about DomainKeys is at . Appendix F. Edit History [[This section to be removed before publication.]] -F.1 Changes since -ietf-02 version +F.1 Changes since -ietf-03 version + + The following changes were made between draft-ietf-dkim-base-03 and + draft-ietf-dkim-base-04: + + o Re-worded Abstract to avoid use of "prove" and "non-repudiation". + + o Use dot-atom-text instead of dot-atom to avoid inclusion of CFWS. + + o Capitalize Selector throughout. + + o Add discussion of plain text, mentioning informatively that + implementors should plan for eventual 8-bit requirements. + + o Drop RSA requirement of exponent of 65537 (not required, since it + is already in the key) and clarify the key format. + + o Drop SHOULD that DKIM-Signature should precede header fields that + it signs. + + o Mention that wildcard DNS records MUST NOT be used for selector + records. + + o Add section 3.8 to clarify the t=s flag. + + o Change the list of header fields that MUST be signed to include + only From. + + o Require that verifier check that From is in the list of signed + header fields. + + o Drop all reference to draft-kucherawy-sender-auth-header draft. + + o Substantially expand Section 7 (IANA Considerations) to include + initial registries. + + o Add section B.7 (use case: SMTP Servers for Roaming Users). + + o Add several examples; update some others. + + o Considerable minor editorial updating to clarify language, delete + redundant text, fix spelling errors, etc. + + Still to be resolved: + + o How does "simple" body canonicalization interact with BINARYMIME + data? + + o Deal with "relaxed" body canonicalizations, especially in regard + to bare CRs and NLs. + + o How to handle "*" in g= dot-atom-text (which allows "*" as a + literal character). + + o The IANA Considerations need to be completed and cleaned up. + +F.2 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. @@ -2651,21 +2902,21 @@ 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.2 Changes since -ietf-01 version +F.3 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 @@ -2681,21 +2933,21 @@ 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.3 Changes since -ietf-00 version +F.4 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. @@ -2705,36 +2957,36 @@ 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.4 Changes since -allman-01 version +F.5 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]. + drafts, notably ID-AUTH-RES. o Significant wordsmithing. -F.5 Changes since -allman-00 version +F.6 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".