Internet Engineering Task Force (IETF) D. Crocker, Ed. Request for Comments: 6376 Brandenburg InternetWorking Obsoletes: 4871, 5672 T. Hansen, Ed. Category: Standards Track AT&T Laboratories ISSN: 2070-1721 M. Kucherawy, Ed. Cloudmark September 2011 DomainKeys Identified Mail (DKIM) Signatures Abstract DomainKeys Identified Mail (DKIM) permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message. This can be an author's organization, an operational relay, or one of their agents. DKIM separates the question of the identity of the Signer of the message from the purported author of the message. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. This memo obsoletes RFC 4871 and RFC 5672. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6376. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. Crocker, et al. Standards Track [Page 1]
RFC 6376 DKIM Signatures September 2011 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. DKIM Architecture Documents . . . . . . . . . . . . . . . 5 1.2. Signing Identity . . . . . . . . . . . . . . . . . . . . . 5 1.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 5 1.4. Simple Key Management . . . . . . . . . . . . . . . . . . 6 1.5. Data Integrity . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6 2.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. Identity . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4. Identifier . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5. Signing Domain Identifier (SDID) . . . . . . . . . . . . . 7 2.6. Agent or User Identifier (AUID) . . . . . . . . . . . . . 7 2.7. Identity Assessor . . . . . . . . . . . . . . . . . . . . 7 2.8. Whitespace . . . . . . . . . . . . . . . . . . . . . . . . 8 2.9. Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8 2.10. Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 9 2.11. DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 9 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 10 3.1. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 12 3.3. Signing and Verification Algorithms . . . . . . . . . . . 13 3.4. Canonicalization . . . . . . . . . . . . . . . . . . . . . 14 3.5. The DKIM-Signature Header Field . . . . . . . . . . . . . 18 Crocker, et al. Standards Track [Page 2]
RFC 6376 DKIM Signatures September 2011 3.6. Key Management and Representation . . . . . . . . . . . . 26 3.7. Computing the Message Hashes . . . . . . . . . . . . . . . 29 3.8. Input Requirements . . . . . . . . . . . . . . . . . . . . 32 3.9. Output Requirements . . . . . . . . . . . . . . . . . . . 32 3.10. Signing by Parent Domains . . . . . . . . . . . . . . . . 33 3.11. Relationship between SDID and AUID . . . . . . . . . . . . 33 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 34 4.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 34 4.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 35 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 36 5.1. Determine Whether the Email Should Be Signed and by Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.2. Select a Private Key and Corresponding Selector Information . . . . . . . . . . . . . . . . . . . . . . . 37 5.3. Normalize the Message to Prevent Transport Conversions . . 37 5.4. Determine the Header Fields to Sign . . . . . . . . . . . 38 5.5. Compute the Message Hash and Signature . . . . . . . . . . 43 5.6. Insert the DKIM-Signature Header Field . . . . . . . . . . 43 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 43 6.1. Extract Signatures from the Message . . . . . . . . . . . 44 6.2. Communicate Verification Results . . . . . . . . . . . . . 49 6.3. Interpret Results/Apply Local Policy . . . . . . . . . . . 50 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51 7.1. Email Authentication Methods Registry . . . . . . . . . . 51 7.2. DKIM-Signature Tag Specifications . . . . . . . . . . . . 51 7.3. DKIM-Signature Query Method Registry . . . . . . . . . . . 52 7.4. DKIM-Signature Canonicalization Registry . . . . . . . . . 52 7.5. _domainkey DNS TXT Resource Record Tag Specifications . . 53 7.6. DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 53 7.7. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 54 7.8. DKIM Service Types Registry . . . . . . . . . . . . . . . 54 7.9. DKIM Selector Flags Registry . . . . . . . . . . . . . . . 55 7.10. DKIM-Signature Header Field . . . . . . . . . . . . . . . 55 8. Security Considerations . . . . . . . . . . . . . . . . . . . 55 8.1. ASCII Art Attacks . . . . . . . . . . . . . . . . . . . . 55 8.2. Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 55 8.3. Misappropriated Private Key . . . . . . . . . . . . . . . 56 8.4. Key Server Denial-of-Service Attacks . . . . . . . . . . . 56 8.5. Attacks against the DNS . . . . . . . . . . . . . . . . . 57 8.6. Replay/Spam Attacks . . . . . . . . . . . . . . . . . . . 57 8.7. Limits on Revoking Keys . . . . . . . . . . . . . . . . . 58 8.8. Intentionally Malformed Key Records . . . . . . . . . . . 58 8.9. Intentionally Malformed DKIM-Signature Header Fields . . . 58 8.10. Information Leakage . . . . . . . . . . . . . . . . . . . 58 8.11. Remote Timing Attacks . . . . . . . . . . . . . . . . . . 59 8.12. Reordered Header Fields . . . . . . . . . . . . . . . . . 59 8.13. RSA Attacks . . . . . . . . . . . . . . . . . . . . . . . 59 8.14. Inappropriate Signing by Parent Domains . . . . . . . . . 59 Crocker, et al. Standards Track [Page 3]
RFC 6376 DKIM Signatures September 2011 8.15. Attacks Involving Extra Header Fields . . . . . . . . . . 60 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 61 9.1. Normative References . . . . . . . . . . . . . . . . . . . 61 9.2. Informative References . . . . . . . . . . . . . . . . . . 62 Appendix A. Example of Use (INFORMATIVE) . . . . . . . . . . . . 64 A.1. The User Composes an Email . . . . . . . . . . . . . . . . 64 A.2. The Email is Signed . . . . . . . . . . . . . . . . . . . 65 A.3. The Email Signature is Verified . . . . . . . . . . . . . 66 Appendix B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . 67 B.1. Alternate Submission Scenarios . . . . . . . . . . . . . . 67 B.2. Alternate Delivery Scenarios . . . . . . . . . . . . . . . 69 Appendix C. Creating a Public Key (INFORMATIVE) . . . . . . . . . 71 C.1. Compatibility with DomainKeys Key Records . . . . . . . . 72 C.2. RFC 4871 Compatibility . . . . . . . . . . . . . . . . . . 73 Appendix D. MUA Considerations (INFORMATIVE) . . . . . . . . . . 73 Appendix E. Changes since RFC 4871 . . . . . . . . . . . . . . . 73 Appendix F. Acknowledgments . . . . . . . . . . . . . . . . . . . 75 1. Introduction DomainKeys Identified Mail (DKIM) permits a person, role, or organization to claim some responsibility for a message by associating a domain name [RFC1034] with the message [RFC5322], which they are authorized to use. This can be an author's organization, an operational relay, or one of their agents. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. A message can contain multiple signatures, from the same or different organizations involved with the message. The approach taken by DKIM differs from previous approaches to message signing (e.g., Secure/Multipurpose Internet Mail Extensions (S/MIME) [RFC5751], OpenPGP [RFC4880]) in that: o the message signature is written as a message header field so that neither human recipients nor existing MUA (Mail User Agent) software is 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; Crocker, et al. Standards Track [Page 4]
RFC 6376 DKIM Signatures September 2011 o signature verification failure does not force rejection of the message; o no attempt is made to include encryption as part of the mechanism; and o message archiving 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 can be deployed incrementally; and o allows delegation of signing to third parties. 1.1. DKIM Architecture Documents Readers are advised to be familiar with the material in [RFC4686], [RFC5585], and [RFC5863], which provide the background for the development of DKIM, an overview of the service, and deployment and operations guidance and advice, respectively. 1.2. Signing Identity DKIM separates the question of the identity of the Signer of the message from the purported author of the message. In particular, a signature includes the identity of the Signer. Verifiers can use the signing information to decide how they want to process the message. The signing identity is included as part of the signature header field. INFORMATIVE RATIONALE: The signing identity specified by a DKIM signature is not required to match an address in any particular header field because of the broad methods of interpretation by recipient mail systems, including MUAs. 1.3. Scalability DKIM is designed to support the extreme scalability requirements that characterize the email identification problem. There are many millions of domains and a much larger number of individual addresses. Crocker, et al. Standards Track [Page 5]
RFC 6376 DKIM Signatures September 2011 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 DKIM differs from traditional hierarchical public-key systems in that no certificate authority infrastructure is required; the Verifier requests the public key from a repository in the domain of the claimed Signer directly rather than from a third party. The DNS is proposed as the initial mechanism for the public keys. Thus, DKIM currently depends on DNS administration and the security of the DNS system. DKIM is designed to be extensible to other key fetching services as they become available. 1.5. Data Integrity A DKIM signature associates the "d=" name with the computed hash of some or all of the message (see Section 3.7) in order to prevent the reuse of the signature with different messages. Verifying the signature asserts that the hashed content has not changed since it was signed and asserts nothing else about "protecting" the end-to-end integrity of the message. 2. Terminology and Definitions This section defines terms used in the rest of the document. DKIM is designed to operate within the Internet Mail service, as defined in [RFC5598]. Basic email terminology is taken from that specification. Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. These words take their normative meanings only when they are presented in ALL UPPERCASE. 2.1. Signers Elements in the mail system that sign messages on behalf of a domain are referred to as Signers. These may be MUAs (Mail User Agents), MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other agents such as mailing list exploders. In general, any Signer will Crocker, et al. Standards Track [Page 6]
RFC 6376 DKIM Signatures September 2011 be involved in the injection of a message into the message system in some way. The key issue is that a message must be signed before it leaves the administrative domain of the Signer. 2.2. Verifiers Elements in the mail system that verify signatures are referred to as Verifiers. These may be MTAs, Mail Delivery Agents (MDAs), or MUAs. In most cases, it is expected that Verifiers will be close to an end user (reader) of the message or some consuming agent such as a mailing list exploder. 2.3. Identity A person, role, or organization. In the context of DKIM, examples include the author, the author's organization, an ISP along the handling path, an independent trust assessment service, and a mailing list operator. 2.4. Identifier A label that refers to an identity. 2.5. Signing Domain Identifier (SDID) A single domain name that is the mandatory payload output of DKIM and that refers to the identity claiming some responsibility for the message by signing it. It is specified in Section 3.5. 2.6. Agent or User Identifier (AUID) A single identifier that refers to the agent or user on behalf of whom the Signing Domain Identifier (SDID) has taken responsibility. The AUID comprises a domain name and an optional <local-part>. The domain name is the same as that used for the SDID or is a subdomain of it. For DKIM processing, the domain name portion of the AUID has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. It is specified in Section 3.5. Note that acceptable values for the AUID may be constrained via a flag in the public-key record. (See Section 3.6.1.) 2.7. Identity Assessor An element in the mail system that consumes DKIM's payload, which is the responsible Signing Domain Identifier (SDID). The Identity Assessor is dedicated to the assessment of the delivered identifier. Crocker, et al. Standards Track [Page 7]
RFC 6376 DKIM Signatures September 2011 Other DKIM (and non-DKIM) values can also be used by the Identity Assessor (if they are available) to provide a more general message evaluation filtering engine. However, this additional activity is outside the scope of this specification. 2.8. Whitespace There are three forms of whitespace: o WSP represents simple whitespace, i.e., a space or a tab character (formal definition in [RFC5234]). o LWSP is linear whitespace, defined as WSP plus CRLF (formal definition in [RFC5234]). o FWS is folding whitespace. It allows multiple lines separated by CRLF followed by at least one whitespace, to be joined. The formal ABNF for these are (WSP and LWSP are given for information only): WSP = SP / HTAB LWSP = *(WSP / CRLF WSP) FWS = [*WSP CRLF] 1*WSP The definition of FWS is identical to that in [RFC5322] except for the exclusion of obs-FWS. 2.9. Imported ABNF Tokens The following tokens are imported from other RFCs as noted. Those RFCs should be considered definitive. The following tokens are imported from [RFC5321]: o "local-part" (implementation warning: this permits quoted strings) o "sub-domain" The following tokens are imported from [RFC5322]: o "field-name" (name of a header field) 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) Crocker, et al. Standards Track [Page 8]
RFC 6376 DKIM Signatures September 2011 o "hex-octet" (a quoted-printable encoded octet) INFORMATIVE NOTE: Be aware that the ABNF in [RFC2045] does not obey the rules of [RFC5234] and must be interpreted accordingly, particularly as regards case folding. Other tokens not defined herein are imported from [RFC5234]. These are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF, etc. 2.10. Common ABNF Tokens The following ABNF tokens are used elsewhere in this document: hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ] ALPHADIGITPS = (ALPHA / DIGIT / "+" / "/") base64string = ALPHADIGITPS *([FWS] ALPHADIGITPS) [ [FWS] "=" [ [FWS] "=" ] ] hdr-name = field-name qp-hdr-value = dkim-quoted-printable ; with "|" encoded 2.11. DKIM-Quoted-Printable The DKIM-Quoted-Printable encoding syntax resembles that described in Quoted-Printable [RFC2045], Section 6.7: any character MAY be encoded as an "=" followed by two hexadecimal digits from the alphabet "0123456789ABCDEF" (no lowercase characters permitted) representing the hexadecimal-encoded integer value of that character. All control characters (those with values < %x20), 8-bit characters (values > %x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon (";", %x3B) MUST be encoded. Note that all whitespace, including SPACE, CR, and LF characters, MUST be encoded. After encoding, FWS MAY be added at arbitrary locations in order to avoid excessively long lines; such whitespace is NOT part of the value, and MUST be removed before decoding. Use of characters not listed as "mail-safe" in [RFC2049] is NOT RECOMMENDED. ABNF: dkim-quoted-printable = *(FWS / hex-octet / dkim-safe-char) ; hex-octet is from RFC2045 dkim-safe-char = %x21-3A / %x3C / %x3E-7E ; '!' - ':', '<', '>' - '~' Crocker, et al. Standards Track [Page 9]
RFC 6376 DKIM Signatures September 2011 INFORMATIVE NOTE: DKIM-Quoted-Printable differs from Quoted- Printable as defined in [RFC2045] in several important ways: 1. Whitespace in the input text, including CR and LF, must be encoded. [RFC2045] does not require such encoding, and does not permit encoding of CR or LF characters that are part of a CRLF line break. 2. Whitespace in the encoded text is ignored. This is to allow tags encoded using DKIM-Quoted-Printable to be wrapped as needed. In particular, [RFC2045] requires that line breaks in the input be represented as physical line breaks; that is not the case here. 3. The "soft line break" syntax ("=" as the last non-whitespace character on the line) does not apply. 4. DKIM-Quoted-Printable does not require that encoded lines be no more than 76 characters long (although there may be other requirements depending on the context in which the encoded text is being used). 3. Protocol Elements Protocol Elements are conceptual parts of the protocol that are not specific to either Signers or Verifiers. The protocol descriptions for Signers and Verifiers are described in later sections ("Signer Actions" (Section 5) and "Verifier Actions" (Section 6)). NOTE: This section must be read in the context of those sections. 3.1. Selectors To support multiple concurrent public keys per signing domain, the key namespace is subdivided using "selectors". For example, selectors might indicate the names of office locations (e.g., "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date (e.g., "january2005", "february2005", etc.), or even an individual user. Selectors are needed to support some important use cases. For example: o Domains that 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. o Domains that want to allow frequent travelers to send messages locally without the need to connect with a particular MSA. Crocker, et al. Standards Track [Page 10]
RFC 6376 DKIM Signatures September 2011 o "Affinity" domains (e.g., college alumni associations) that provide forwarding of incoming mail, but that do not operate a mail submission agent for outgoing mail. 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 namespace. ABNF: selector = sub-domain *( "." sub-domain ) The number of public keys and corresponding selectors for each domain is determined by the domain owner. Many domain owners will be satisfied with just one selector, whereas administratively distributed organizations can choose to manage disparate selectors and key pairs in different regions or on different email servers. 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 "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. 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. For example, if per-user keys are issued, the domain owner will need to decide Crocker, et al. Standards Track [Page 11]
RFC 6376 DKIM Signatures September 2011 whether to associate this selector directly with the name of a registered end user or make it some unassociated random value, such as a fingerprint of the public key. INFORMATIVE OPERATIONS NOTE: Reusing a selector with a new key (for example, changing the key associated with a user's name) makes it impossible to tell the difference between a message that didn't verify because the key is no longer valid and a message that is actually forged. For this reason, Signers are ill-advised to reuse selectors for new keys. A better strategy is to assign new keys to new selectors. 3.2. Tag=Value Lists DKIM uses a simple "tag=value" syntax in several contexts, including in messages and domain signature records. Values are a series of strings containing either plain text, "base64" text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid, Section 6.7), or "dkim-quoted-printable" (as defined in Section 2.11). The name of the tag will determine the encoding of each value. Unencoded semicolon (";") characters MUST NOT occur in the tag value, since that separates tag-specs. INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined below (as "tag-value") only includes 7-bit characters, an implementation that wished to anticipate future standards would be advised not to preclude the use of UTF-8-encoded ([RFC3629]) text in tag=value lists. Formally, the ABNF syntax rules are as follows: tag-list = tag-spec *( ";" tag-spec ) [ ";" ] tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] tag-name = ALPHA *ALNUMPUNC tag-value = [ tval *( 1*(WSP / FWS) tval ) ] ; Prohibits WSP and FWS at beginning and end tval = 1*VALCHAR VALCHAR = %x21-3A / %x3C-7E ; EXCLAMATION to TILDE except SEMICOLON ALNUMPUNC = ALPHA / DIGIT / "_" Note that WSP is allowed anywhere around tags. In particular, any WSP after the "=" and any WSP before the terminating ";" is not part of the value; however, WSP inside the value is significant. Crocker, et al. Standards Track [Page 12]
RFC 6376 DKIM Signatures September 2011 Tags MUST be interpreted in a case-sensitive manner. Values MUST be processed as case sensitive unless the specific tag description of semantics specifies case insensitivity. Tags with duplicate names MUST NOT occur within a single tag-list; if a tag name does occur more than once, the entire tag-list is invalid. Whitespace within a value MUST be retained unless explicitly excluded by the specific tag description. Tag=value pairs that represent the default value MAY be included to aid legibility. Unrecognized tags MUST be ignored. Tags that have an empty value are not the same as omitted tags. An omitted tag is treated as having the default value; a tag with an empty value explicitly designates the empty string as the value. 3.3. Signing and Verification Algorithms DKIM supports multiple digital signature algorithms. Two algorithms are defined by this specification at this time: rsa-sha1 and rsa- sha256. Signers MUST implement and SHOULD sign using rsa-sha256. Verifiers MUST implement both rsa-sha1 and rsa-sha256. INFORMATIVE NOTE: Although rsa-sha256 is strongly encouraged, some senders might prefer to use rsa-sha1 when balancing security strength against performance, complexity, or other needs. In general, however, rsa-sha256 should always be used whenever possible. 3.3.1. The rsa-sha1 Signing Algorithm The rsa-sha1 Signing Algorithm computes a message hash as described in Section 3.7 using SHA-1 [FIPS-180-3-2008] as the hash-alg. That hash is then signed by the Signer using the RSA algorithm (defined in Public-Key Cryptography Standards (PKCS) #1 version 1.5 [RFC3447]) as the crypt-alg and the Signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. The signing algorithm SHOULD use a public exponent of 65537. 3.3.2. The rsa-sha256 Signing Algorithm The rsa-sha256 Signing Algorithm computes a message hash as described in Section 3.7 using SHA-256 [FIPS-180-3-2008] as the hash-alg. That hash is then signed by the Signer using the RSA algorithm (defined in Crocker, et al. Standards Track [Page 13]
RFC 6376 DKIM Signatures September 2011 PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the Signer's private key. The hash MUST NOT be truncated or converted into any form other than the native binary form before being signed. The signing algorithm SHOULD use a public exponent of 65537. 3.3.3. Key Sizes Selecting appropriate key sizes is a trade-off between cost, performance, and risk. Since short RSA keys more easily succumb to off-line attacks, Signers MUST use RSA keys of at least 1024 bits for long-lived keys. Verifiers MUST be able to validate signatures with keys ranging from 512 bits to 2048 bits, and they MAY be able to validate signatures with larger keys. Verifier policies may use the length of the signing key as one metric for determining whether a signature is acceptable. Factors that should influence the key size choice include the following: o The practical constraint that large (e.g., 4096-bit) keys might not fit within a 512-byte DNS UDP response packet o The security constraint that keys smaller than 1024 bits are subject to off-line attacks o Larger keys impose higher CPU costs to verify and sign email o Keys can be replaced on a regular basis; thus, their lifetime can be relatively short o The security goals of this specification are modest compared to typical goals of other systems that employ digital signatures See [RFC3766] for further discussion on selecting key sizes. 3.3.4. Other Algorithms Other algorithms MAY be defined in the future. Verifiers MUST ignore any signatures using algorithms that they do not implement. 3.4. Canonicalization Some mail systems modify email in transit, potentially invalidating a signature. For most Signers, mild modification of email is immaterial to validation of the DKIM domain name's use. For such Signers, a canonicalization algorithm that survives modest in-transit modification is preferred. Crocker, et al. Standards Track [Page 14]
RFC 6376 DKIM Signatures September 2011 Other Signers demand that any modification of the email, however minor, result in a signature verification failure. These Signers prefer a canonicalization algorithm that does not tolerate in-transit 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 [RFC5322], 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 whitespace replacement and header field line rewrapping. 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. 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. Canonicalization simply prepares the email for presentation to the signing or verification algorithm. It MUST NOT change the transmitted data in any way. Canonicalization of header fields and body are described below. NOTE: This section assumes that the message is already in "network normal" format (text is ASCII encoded, lines are separated with CRLF characters, etc.). See also Section 5.3 for information about normalizing the message. 3.4.1. The "simple" Header Canonicalization Algorithm The "simple" header canonicalization algorithm does not change header fields in any way. Header fields MUST be presented to the signing or verification algorithm exactly as they are in the message being signed or verified. In particular, header field names MUST NOT be case folded and whitespace MUST NOT be changed. 3.4.2. The "relaxed" Header Canonicalization Algorithm The "relaxed" header canonicalization algorithm MUST apply the following steps in order: o Convert all header field names (not the header field values) to lowercase. For example, convert "SUBJect: AbC" to "subject: AbC". Crocker, et al. Standards Track [Page 15]
RFC 6376 DKIM Signatures September 2011 o Unfold all header field continuation lines as described in [RFC5322]; in particular, lines with terminators embedded in continued header field values (that is, CRLF sequences followed by WSP) MUST be interpreted without the CRLF. Implementations MUST NOT remove the CRLF at the end of the header field value. o Convert all sequences of one or more WSP characters to a single SP character. WSP characters here include those before and after a line folding boundary. o Delete all WSP characters at the end of each unfolded header field value. o Delete any WSP characters remaining before and after the colon separating the header field name from the header field value. The colon separator MUST be retained. 3.4.3. The "simple" Body Canonicalization Algorithm The "simple" body canonicalization algorithm ignores all empty lines at the end of the message body. An empty line is a line of zero length after removal of the line terminator. If there is no body or no trailing CRLF on the message body, a CRLF is added. It makes no other changes to the message body. In more formal terms, the "simple" body canonicalization algorithm converts "*CRLF" at the end of the body to a single "CRLF". Note that a completely empty or missing body is canonicalized as a single "CRLF"; that is, the canonicalized length will be 2 octets. The SHA-1 value (in base64) for an empty body (canonicalized to a "CRLF") is: uoq1oCgLlTqpdDX/iUbLy7J1Wic= The SHA-256 value is: frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY= 3.4.4. The "relaxed" Body Canonicalization Algorithm The "relaxed" body canonicalization algorithm MUST apply the following steps (a) and (b) in order: a. Reduce whitespace: * Ignore all whitespace at the end of lines. Implementations MUST NOT remove the CRLF at the end of the line. Crocker, et al. Standards Track [Page 16]
RFC 6376 DKIM Signatures September 2011 * Reduce all sequences of WSP within a line to a single SP character. b. Ignore all empty lines at the end of the message body. "Empty line" is defined in Section 3.4.3. If the body is non-empty but does not end with a CRLF, a CRLF is added. (For email, this is only possible when using extensions to SMTP or non-SMTP transport mechanisms.) The SHA-1 value (in base64) for an empty body (canonicalized to a null input) is: 2jmj7l5rSw0yVb/vlWAYkK/YBwk= The SHA-256 value is: 47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU= 3.4.5. Canonicalization Examples (INFORMATIVE) In the following examples, actual whitespace is used only for clarity. The actual input and output text is designated using bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a tab character, and "<CRLF>" for a carriage-return/line-feed sequence. For example, "X <SP> Y" and "X<SP>Y" represent the same three characters. Example 1: A message reading: A: <SP> X <CRLF> B <SP> : <SP> Y <HTAB><CRLF> <HTAB> Z <SP><SP><CRLF> <CRLF> <SP> C <SP><CRLF> D <SP><HTAB><SP> E <CRLF> <CRLF> <CRLF> when canonicalized using relaxed canonicalization for both header and body results in a header reading: a:X <CRLF> b:Y <SP> Z <CRLF> and a body reading: <SP> C <CRLF> D <SP> E <CRLF> Crocker, et al. Standards Track [Page 17]
RFC 6376 DKIM Signatures September 2011 Example 2: The same message canonicalized using simple canonicalization for both header and body results in a header reading: A: <SP> X <CRLF> B <SP> : <SP> Y <HTAB><CRLF> <HTAB> Z <SP><SP><CRLF> and a body reading: <SP> C <SP><CRLF> D <SP><HTAB><SP> E <CRLF> Example 3: When processed using relaxed header canonicalization and simple body canonicalization, the canonicalized version has a header of: a:X <CRLF> b:Y <SP> Z <CRLF> and a body reading: <SP> C <SP><CRLF> D <SP><HTAB><SP> E <CRLF> 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 [RFC5322] and hence SHOULD NOT be reordered and SHOULD be prepended to the message. The DKIM-Signature header field being created or verified is always included in the signature calculation, after the rest of the header fields being signed; however, when calculating or verifying the signature, the value of the "b=" tag (signature value) of that DKIM- Signature header field MUST be treated as though it were an empty string. Unknown tags in the DKIM-Signature header field MUST be included in the signature calculation but MUST be otherwise ignored by Verifiers. Other DKIM-Signature header fields that are included in the signature should be treated as normal header fields; in particular, the "b=" tag is not treated specially. Crocker, et al. Standards Track [Page 18]
RFC 6376 DKIM Signatures September 2011 The encodings for each field type are listed below. Tags described as qp-section are encoded as described in Section 6.7 of MIME Part One [RFC2045], with the additional conversion of semicolon characters to "=3B"; intuitively, this is one line of quoted-printable encoded text. The dkim-quoted-printable syntax is defined in Section 2.11. Tags on the DKIM-Signature header field along with their type and requirement status are shown below. Unrecognized tags MUST be ignored. v= Version (plain-text; REQUIRED). This tag defines the version of this specification that applies to the signature record. It MUST have the value "1" for implementations compliant with this version of DKIM. ABNF: sig-v-tag = %x76 [FWS] "=" [FWS] 1*DIGIT INFORMATIVE NOTE: DKIM-Signature version numbers may increase arithmetically as new versions of this specification are released. a= The algorithm used to generate the signature (plain-text; REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256"; Signers SHOULD sign using "rsa-sha256". See Section 3.3 for a description of the algorithms. ABNF: sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h sig-a-tag-k = "rsa" / x-sig-a-tag-k sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT) ; for later extension x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT) ; for later extension b= The signature data (base64; REQUIRED). Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length limits. See "Signer Actions" (Section 5) for how the signature is computed. Crocker, et al. Standards Track [Page 19]
RFC 6376 DKIM Signatures September 2011 ABNF: sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag-data = base64string bh= The hash of the canonicalized body part of the message as limited by the "l=" tag (base64; REQUIRED). Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert FWS in this value in arbitrary places to conform to line-length limits. See Section 3.7 for how the body hash is computed. ABNF: sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data sig-bh-tag-data = base64string c= Message canonicalization (plain-text; OPTIONAL, default is "simple/simple"). This tag informs the Verifier of the type of canonicalization used to prepare the message for signing. It consists of two names separated by a "slash" (%d47) character, corresponding to the header and body canonicalization algorithms, respectively. These algorithms are described in Section 3.4. If only one algorithm is named, that algorithm is used for the header and "simple" is used for the body. For example, "c=relaxed" is treated the same as "c=relaxed/simple". ABNF: sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg ["/" sig-c-tag-alg] sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg x-sig-c-tag-alg = hyphenated-word ; for later extension d= The SDID claiming responsibility for an introduction of a message into the mail stream (plain-text; REQUIRED). Hence, the SDID value is used to form the query for the public key. The SDID MUST correspond to a valid DNS name under which the DKIM key record is published. The conventions and semantics used by a Signer to create and use a specific SDID are outside the scope of this specification, as is any use of those conventions and semantics. When presented with a signature that does not meet these requirements, Verifiers MUST consider the signature invalid. Internationalized domain names MUST be encoded as A-labels, as described in Section 2.3 of [RFC5890]. Crocker, et al. Standards Track [Page 20]
RFC 6376 DKIM Signatures September 2011 ABNF: sig-d-tag = %x64 [FWS] "=" [FWS] domain-name domain-name = sub-domain 1*("." sub-domain) ; from [RFC5321] Domain, ; excluding address-literal h= Signed header fields (plain-text, but see description; REQUIRED). A colon-separated list of header field names that identify the header fields presented to the signing algorithm. The field MUST 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 MAY contain multiple instances of a header field name, meaning multiple occurrences of the corresponding header field are included in the header hash. The field MUST NOT include the DKIM-Signature header field that is being created or verified but may include others. Folding whitespace (FWS) MAY be included on either side of the colon separator. Header field names MUST be compared against actual header field names in a case-insensitive manner. This list MUST NOT be empty. See Section 5.4 for a discussion of choosing header fields to sign and Section 5.4.2 for requirements when signing multiple instances of a single field. ABNF: sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name *( [FWS] ":" [FWS] hdr-name ) INFORMATIVE EXPLANATION: By "signing" header fields that do not actually exist, a Signer can allow a Verifier to detect insertion of those header fields after signing. However, since a Signer cannot possibly know what header fields might be defined in the future, this mechanism cannot be used to prevent the addition of any possible unknown header fields. INFORMATIVE NOTE: "Signing" fields that are not present at the time of signing not only prevents fields and values from being added but also prevents adding fields with no values. i= The Agent or User Identifier (AUID) on behalf of which the SDID is taking responsibility (dkim-quoted-printable; OPTIONAL, default is an empty local-part followed by an "@" followed by the domain from the "d=" tag). Crocker, et al. Standards Track [Page 21]
RFC 6376 DKIM Signatures September 2011 The syntax is a standard email address where the local-part MAY be omitted. The domain part of the address MUST be the same as, or a subdomain of, the value of the "d=" tag. Internationalized domain names MUST be encoded as A-labels, as described in Section 2.3 of [RFC5890]. ABNF: sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name The AUID is specified as having the same syntax as an email address but it need not have the same semantics. Notably, the domain name need not be registered in the DNS -- so it might not resolve in a query -- and the local-part MAY be drawn from a namespace unrelated to any mailbox. The details of the structure and semantics for the namespace are determined by the Signer. Any knowledge or use of those details by Verifiers or Assessors is outside the scope of this specification. The Signer MAY choose to use the same namespace for its AUIDs as its users' email addresses or MAY choose other means of representing its users. However, the Signer SHOULD use the same AUID for each message intended to be evaluated as being within the same sphere of responsibility, if it wishes to offer receivers the option of using the AUID as a stable identifier that is finer grained than the SDID. 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 might 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 the domain. It can do so by including the domain part but not the local-part of the identity. INFORMATIVE DISCUSSION: This specification does not require the value of the "i=" tag to match the identity in any message header fields. This is considered to be a Verifier policy issue. Constraints between the value of the "i=" tag and other identities in other header fields seek to apply basic authentication into the semantics of trust associated with a role such as content author. Trust is a broad and complex topic, and trust mechanisms are subject to highly creative attacks. The real-world efficacy of any but the most basic bindings between the "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 Crocker, et al. Standards Track [Page 22]
RFC 6376 DKIM Signatures September 2011 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. See further discussion in Section 8.2. INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76 decimal digits. This constraint is not intended to predict the size of future messages or to require implementations to use an integer representation large enough to represent the maximum possible value but is intended to remind the implementer to check the length of this and all other tags during verification and to test for integer overflow when decoding the value. Implementers may need to limit the actual value expressed to a value smaller than 10^76, e.g., to allow a message to fit within the available storage space. ABNF: sig-l-tag = %x6c [FWS] "=" [FWS] 1*76DIGIT q= A colon-separated list of query methods used to retrieve the public key (plain-text; OPTIONAL, default is "dns/txt"). Each query method is of the form "type[/options]", where the syntax and semantics of the options depend on the type and specified options. If there are multiple query mechanisms listed, the choice of query mechanism MUST NOT change the interpretation of the signature. Implementations MUST use the recognized query mechanisms in the order presented. Unrecognized query mechanisms MUST be ignored. Currently, the only valid value is "dns/txt", which defines the DNS TXT resource record (RR) 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) Crocker, et al. Standards Track [Page 23]
RFC 6376 DKIM Signatures September 2011 sig-q-tag-method = "dns/txt" / x-sig-q-tag-type ["/" x-sig-q-tag-args] x-sig-q-tag-type = hyphenated-word ; for future extension x-sig-q-tag-args = qp-hdr-value s= The selector subdividing the namespace for the "d=" (domain) tag (plain-text; REQUIRED). Internationalized selector names MUST be encoded as A-labels, as described in Section 2.3 of [RFC5890]. ABNF: sig-s-tag = %x73 [FWS] "=" [FWS] selector t= Signature Timestamp (plain-text unsigned decimal integer; RECOMMENDED, default is an unknown creation time). The time that this signature was created. The format is the number of seconds since 00:00:00 on January 1, 1970 in the UTC time zone. The value is expressed as an unsigned integer in decimal ASCII. This value is not constrained to fit into a 31- or 32-bit integer. 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 consider any value longer than 12 digits to be infinite. Leap seconds are not counted. Implementations MAY ignore signatures that have a timestamp in the future. ABNF: sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT x= Signature Expiration (plain-text unsigned decimal integer; RECOMMENDED, default is no expiration). The format is the same as in the "t=" tag, represented as an absolute date, not as a time delta from the signing timestamp. The value is expressed as an unsigned integer in decimal ASCII, with the same constraints on the value in the "t=" tag. Signatures MAY be considered invalid if the verification time at the Verifier is past the expiration date. The verification time should be the time that the message was first received at the administrative domain of the Verifier 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. Crocker, et al. Standards Track [Page 24]
RFC 6376 DKIM Signatures September 2011 INFORMATIVE NOTE: Due to clock drift, the receiver's notion of when to consider the signature expired may not exactly match what the sender is expecting. Receivers MAY add a 'fudge factor' to allow for such possible drift. 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 meta-characters, and any actual vertical bar characters in a copied header field must be encoded). Note that all whitespace must be encoded, including whitespace between the colon and the header field value. After encoding, FWS MAY be added at arbitrary locations in order to avoid excessively long lines; such whitespace is NOT part of the value of the header field and MUST be removed before decoding. The header fields referenced by the "h=" tag refer to the fields in the [RFC5322] header of the message, not to any copied fields in the "z=" tag. Copied header field values are for diagnostic use. ABNF: sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy *( "|" [FWS] sig-z-tag-copy ) sig-z-tag-copy = hdr-name [FWS] ":" qp-hdr-value INFORMATIVE EXAMPLE of a signature header field spread across multiple continuation lines: DKIM-Signature: v=1; a=rsa-sha256; d=example.net; s=brisbane; c=simple; q=dns/txt; i=@eng.example.net; t=1117574938; x=1118006938; h=from:to:subject:date; z=From:foo@eng.example.net|To:joe@example.com| Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700; bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZVoG4ZHRNiYzR Crocker, et al. Standards Track [Page 25]
RFC 6376 DKIM Signatures September 2011 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 Signer (the "d=" tag of the DKIM- Signature header field), and the selector (the "s=" tag). public_key = dkim_find_key(q_val, d_val, s_val) This document defines a single binding, using DNS TXT RRs 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 tag-list as described in Section 3.2. The current valid tags are described below. Other tags MAY be present and MUST be ignored by any implementation that does not understand them. v= Version of the DKIM key record (plain-text; RECOMMENDED, default is "DKIM1"). If specified, this tag MUST be set to "DKIM1" (without the quotes). This tag MUST be the first tag in the record. Records beginning with a "v=" tag with any other value MUST be discarded. Note that Verifiers must do a string comparison on this value; for example, "DKIM1" is not the same as "DKIM1.0". ABNF: key-v-tag = %x76 [FWS] "=" [FWS] %x44.4B.49.4D.31 Crocker, et al. Standards Track [Page 26]
RFC 6376 DKIM Signatures September 2011 h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to allowing all algorithms). A colon-separated list of hash algorithms that might be used. Unrecognized algorithms MUST be ignored. Refer to Section 3.3 for a discussion of the hash algorithms implemented by Signers and Verifiers. The set of algorithms listed in this tag in each record is an operational choice made by the Signer. ABNF: key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg *( [FWS] ":" [FWS] key-h-tag-alg ) key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg x-key-h-tag-alg = hyphenated-word ; for future extension k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and Verifiers MUST support the "rsa" key type. The "rsa" key type indicates that an ASN.1 DER-encoded [ITU-X660-1997] RSAPublicKey (see [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.) Unrecognized key types MUST be ignored. ABNF: key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type key-k-tag-type = "rsa" / x-key-k-tag-type x-key-k-tag-type = hyphenated-word ; for future extension n= Notes that might be of interest to a human (qp-section; OPTIONAL, default is empty). No interpretation is made by any program. This tag should be used sparingly in any key server mechanism that has space limitations (notably DNS). This is intended for use by administrators, not end users. 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 are defined by the "k=" tag. INFORMATIVE RATIONALE: If a private key has been compromised or otherwise disabled (e.g., an outsourcing contract has been terminated), a Signer might want to explicitly state that it knows about the selector, but all messages using that selector Crocker, et al. Standards Track [Page 27]
RFC 6376 DKIM Signatures September 2011 should fail verification. Verifiers SHOULD return an error code for any DKIM-Signature header field with a selector referencing a revoked key. (See Section 6.1.2 for details.) ABNF: key-p-tag = %x70 [FWS] "=" [ [FWS] base64string] INFORMATIVE NOTE: A base64string is permitted to include whitespace (FWS) at arbitrary places; however, any CRLFs must be followed by at least one WSP character. Implementers and administrators are cautioned to ensure that selector TXT RRs conform to this specification. 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. Unrecognized service types MUST be ignored. Currently defined service types are as follows: * matches all service types email electronic mail (not necessarily limited to SMTP) This tag is intended to constrain the use of keys for other purposes, should use of DKIM be defined by other services in the future. ABNF: key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type *( [FWS] ":" [FWS] key-s-tag-type ) key-s-tag-type = "email" / "*" / x-key-s-tag-type x-key-s-tag-type = hyphenated-word ; for future extension t= Flags, represented as a colon-separated list of names (plain- text; OPTIONAL, default is no flags set). Unrecognized flags MUST be ignored. The defined flags are as follows: 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. Crocker, et al. Standards Track [Page 28]
RFC 6376 DKIM Signatures September 2011 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=". Use of this flag is RECOMMENDED unless subdomaining is required. ABNF: key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag *( [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 3.6.2. DNS Binding A binding using DNS TXT RRs as a key service is hereby defined. All implementations MUST support this binding. 3.6.2.1. Namespace All DKIM keys are stored in a subdomain named "_domainkey". Given a DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag of "foo.bar", the DNS query will be for "foo.bar._domainkey.example.com". 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. A later extension of this standard may define another RR type. Strings in a TXT RR MUST be concatenated together before use with no intervening whitespace. TXT RRs MUST be unique for a particular selector name; that is, if there are multiple records in an RRset, the results are undefined. TXT RRs are encoded as described in Section 3.6.1. 3.7. Computing the Message Hashes Both signing and verifying message signatures start with a step of computing two cryptographic hashes over the message. Signers will choose the parameters of the signature as described in "Signer Actions" (Section 5); Verifiers will use the parameters specified in the DKIM-Signature header field being verified. In the following discussion, the names of the tags in the DKIM-Signature header field that either exists (when verifying) or will be created (when signing) Crocker, et al. Standards Track [Page 29]
RFC 6376 DKIM Signatures September 2011 are used. Note that canonicalization (Section 3.4) is only used to prepare the email for signing or verifying; it does not affect the transmitted email in any way. The Signer/Verifier MUST compute two hashes: one over the body of the message and one over the selected header fields of the message. Signers MUST compute them in the order shown. Verifiers MAY compute them in any order convenient to the Verifier, provided that the result is semantically identical to the semantics that would be the case had they been computed in this order. In hash step 1, the Signer/Verifier MUST hash the message body, canonicalized using the body canonicalization algorithm specified in the "c=" tag and then truncated to the length specified in the "l=" tag. That hash value is then converted to base64 form and inserted into (Signers) or compared to (Verifiers) the "bh=" tag of the DKIM- Signature header field. In hash step 2, the Signer/Verifier MUST pass the following to the hash algorithm in the indicated order. 1. The header fields specified by the "h=" tag, in the order specified in that tag, and canonicalized using the header canonicalization algorithm specified in the "c=" tag. Each header field MUST be terminated with a single CRLF. 2. The DKIM-Signature header field that exists (verifying) or will be inserted (signing) in the message, with the value of the "b=" tag (including all surrounding whitespace) deleted (i.e., treated as the empty string), canonicalized using the header canonicalization algorithm specified in the "c=" tag, and without a trailing CRLF. All tags and their values in the DKIM-Signature header field are included in the cryptographic hash with the sole exception of the value portion of the "b=" (signature) tag, which MUST be treated as the null string. All tags MUST be included even if they might not be understood by the Verifier. The header field MUST be presented to the hash algorithm after the body of the message rather than with the rest of the header fields and MUST be canonicalized as specified in the "c=" (canonicalization) tag. The DKIM-Signature header field MUST NOT be included in its own "h=" tag, although other DKIM- Signature header fields MAY be signed (see Section 4). When calculating the hash on messages that will be transmitted using base64 or quoted-printable encoding, Signers MUST compute the hash after the encoding. Likewise, the Verifier MUST incorporate the Crocker, et al. Standards Track [Page 30]
RFC 6376 DKIM Signatures September 2011 values into the hash before decoding the base64 or quoted-printable text. However, the hash MUST be computed before transport-level encodings such as SMTP "dot-stuffing" (the modification of lines beginning with a "." to avoid confusion with the SMTP end-of-message marker, as specified in [RFC5321]). With the exception of the canonicalization procedure described in Section 3.4, the DKIM signing process treats the body of messages as simply a string of octets. DKIM messages MAY be either in plain-text or in MIME format; no special treatment is afforded to MIME content. Message attachments in MIME format MUST be included in the content that is signed. More formally, pseudo-code for the signature algorithm is: body-hash = hash-alg (canon-body, l-param) data-hash = hash-alg (h-headers, D-SIG, body-hash) signature = sig-alg (d-domain, selector, data-hash) where: body-hash: is the output from hashing the body, using hash-alg. hash-alg: is the hashing algorithm specified in the "a" parameter. canon-body: is a canonicalized representation of the body, produced using the body algorithm specified in the "c" parameter, as defined in Section 3.4 and excluding the DKIM-Signature field. l-param: is the length-of-body value of the "l" parameter. data-hash: is the output from using the hash-alg algorithm, to hash the header including the DKIM-Signature header, and the body hash. h-headers: is the list of headers to be signed, as specified in the "h" parameter. D-SIG: is the canonicalized DKIM-Signature field itself without the signature value portion of the parameter, that is, an empty parameter value. signature: is the signature value produced by the signing algorithm. sig-alg: is the signature algorithm specified by the "a" parameter. Crocker, et al. Standards Track [Page 31]
RFC 6376 DKIM Signatures September 2011 d-domain: is the domain name specified in the "d" parameter. selector: is the selector value specified in the "s" parameter. NOTE: Many digital signature APIs provide both hashing and application of the RSA private key using a single "sign()" primitive. When using such an API, the last two steps in the algorithm would probably be combined into a single call that would perform both the "a-hash-alg" and the "sig-alg". 3.8. Input Requirements A message that is not compliant with [RFC5322], [RFC2045], and [RFC2047] can be subject to attempts by intermediaries to correct or interpret such content. See Section 8 of [RFC4409] for examples of changes that are commonly made. Such "corrections" may invalidate DKIM signatures or have other undesirable effects, including some that involve changes to the way a message is presented to an end user. Accordingly, DKIM's design is predicated on valid input. Therefore, Signers and Verifiers SHOULD take reasonable steps to ensure that the messages they are processing are valid according to [RFC5322], [RFC2045], and any other relevant message format standards. See Section 8.15 for additional discussion. 3.9. Output Requirements The evaluation of each signature ends in one of three states, which this document refers to as follows: SUCCESS: a successful verification PERMFAIL: a permanent, non-recoverable error such as a signature verification failure TEMPFAIL: a temporary, recoverable error such as a DNS query timeout For each signature that verifies successfully or produces a TEMPFAIL result, output of the DKIM algorithm MUST include the set of: o The domain name, taken from the "d=" signature tag; and o The result of the verification attempt for that signature. Crocker, et al. Standards Track [Page 32]
RFC 6376 DKIM Signatures September 2011 The output MAY include other signature properties or result meta- data, including PERMFAILed or otherwise ignored signatures, for use by modules that consume those results. See Section 6.1 for discussion of signature validation result codes. 3.10. Signing by Parent Domains In some circumstances, it is desirable for a domain to apply a signature on behalf of any of its subdomains without the need to maintain separate selectors (key records) in each subdomain. By default, private keys corresponding to key records can be used to sign messages for any subdomain of the domain in which they reside; for example, a key record for the domain example.com can be used to verify messages where the AUID ("i=" tag of the signature) is sub.example.com, or even sub1.sub2.example.com. In order to limit the capability of such keys when this is not intended, the "s" flag MAY be set in the "t=" tag of the key record, to constrain the validity of the domain of the AUID. If the referenced key record contains the "s" flag as part of the "t=" tag, the domain of the AUID ("i=" flag) MUST be the same as that of the SDID (d=) domain. If this flag is absent, the domain of the AUID MUST be the same as, or a subdomain of, the SDID. 3.11. Relationship between SDID and AUID DKIM's primary task is to communicate from the Signer to a recipient- side Identity Assessor a single Signing Domain Identifier (SDID) that refers to a responsible identity. DKIM MAY optionally provide a single responsible Agent or User Identifier (AUID). Hence, DKIM's mandatory output to a receive-side Identity Assessor is a single domain name. Within the scope of its use as DKIM output, the name has only basic domain name semantics; any possible owner- specific semantics are outside the scope of DKIM. That is, within its role as a DKIM identifier, additional semantics cannot be assumed by an Identity Assessor. Upon successfully verifying the signature, a receive-side DKIM Verifier MUST communicate the Signing Domain Identifier (d=) to a consuming Identity Assessor module and MAY communicate the Agent or User Identifier (i=) if present. To the extent that a receiver attempts to intuit any structured semantics for either of the identifiers, this is a heuristic function that is outside the scope of DKIM's specification and semantics. Crocker, et al. Standards Track [Page 33]
RFC 6376 DKIM Signatures September 2011 Hence, it is relegated to a higher-level service, such as a delivery- handling filter that integrates a variety of inputs and performs heuristic analysis of them. INFORMATIVE DISCUSSION: This document does not require the value of the SDID or AUID to match an identifier in any other message header field. This requirement is, instead, an Assessor policy issue. The purpose of such a linkage would be to authenticate the value in that other header field. This, in turn, is the basis for applying a trust assessment based on the identifier value. Trust is a broad and complex topic, and trust mechanisms are subject to highly creative attacks. The real-world efficacy of any but the most basic bindings between the SDID or AUID and other identities is not well established, nor is its vulnerability to subversion by an attacker. Hence, reliance on the use of such bindings should be strictly limited. In particular, it is not at all clear to what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the SDID or AUID. 4. Semantics of Multiple Signatures 4.1. Example Scenarios There are many reasons why a message might have multiple signatures. For example, suppose SHA-256 is in the future found to be insufficiently strong, and DKIM usage transitions to SHA-1024. A Signer might immediately sign using the newer algorithm but also continue to sign using the older algorithm for interoperability with Verifiers that had not yet upgraded. The Signer would do this by adding two DKIM-Signature header fields, one using each algorithm. Older Verifiers that did not recognize SHA-1024 as an acceptable algorithm would skip that signature and use the older algorithm; newer Verifiers could use either signature at their option and, all other things being equal, might not even attempt to verify the other signature. Similarly, a Signer might sign a message including all header fields and no "l=" tag (to satisfy strict Verifiers) and a second time with a limited set of header fields and an "l=" tag (in anticipation of possible message modifications en route to other Verifiers). Verifiers could then choose which signature they prefer. Of course, a message might also have multiple signatures because it passed through multiple Signers. A common case is expected to be that of a signed message that passes through a mailing list that also Crocker, et al. Standards Track [Page 34]
RFC 6376 DKIM Signatures September 2011 signs all messages. Assuming both of those signatures verify, a recipient might choose to accept the message if either of those signatures were known to come from trusted sources. In particular, recipients might choose to whitelist mailing lists to which they have subscribed and that have acceptable anti-abuse policies so as to accept messages sent to that list even from unknown authors. They might also subscribe to less trusted mailing lists (e.g., those without anti-abuse protection) and be willing to accept all messages from specific authors but insist on doing additional abuse scanning for other messages. Another related example of multiple Signers might be forwarding services, such as those commonly associated with academic alumni sites. For example, a recipient might have an address at members.example.org, a site that has anti-abuse protection that is somewhat less effective than the recipient would prefer. Such a recipient might have specific authors whose messages would be trusted absolutely, but messages from unknown authors that had passed the forwarder's scrutiny would have only medium trust. 4.2. Interpretation A Signer that is adding a signature to a message merely creates a new DKIM-Signature header, using the usual semantics of the "h=" option. A Signer MAY sign previously existing DKIM-Signature header fields using the method described in Section 5.4 to sign trace header fields. Note that Signers should be cognizant that signing DKIM-Signature header fields may result in signature failures with intermediaries that do not recognize that DKIM-Signature header fields are trace header fields and unwittingly reorder them, thus breaking such signatures. For this reason, signing existing DKIM-Signature header fields is unadvised, albeit legal. INFORMATIVE NOTE: If a header field with multiple instances is signed, those header fields are always signed from the bottom up. Thus, it is not possible to sign only specific DKIM-Signature header fields. For example, if the message being signed already contains three DKIM-Signature header fields A, B, and C, it is possible to sign all of them, B and C only, or C only, but not A only, B only, A and B only, or A and C only. A Signer MAY add more than one DKIM-Signature header field using different parameters. For example, during a transition period, a Signer might want to produce signatures using two different hash algorithms. Crocker, et al. Standards Track [Page 35]
RFC 6376 DKIM Signatures September 2011 Signers SHOULD NOT remove any DKIM-Signature header fields from messages they are signing, even if they know that the signatures cannot be verified. When evaluating a message with multiple signatures, a Verifier SHOULD evaluate signatures independently and on their own merits. For example, a Verifier that by policy chooses not to accept signatures with deprecated cryptographic algorithms would consider such signatures invalid. Verifiers MAY process signatures in any order of their choice; for example, some Verifiers might choose to process signatures corresponding to the From field in the message header before other signatures. See Section 6.1 for more information about signature choices. INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate valid signatures with invalid signatures in an attempt to guess why a signature failed are ill-advised. In particular, there is no general way that a Verifier can determine that an invalid signature was ever valid. Verifiers SHOULD continue to check signatures until a signature successfully verifies to the satisfaction of the Verifier. To limit potential denial-of-service attacks, Verifiers MAY limit the total number of signatures they will attempt to verify. If a Verifier module reports signatures whose evaluations produced PERMFAIL results, Identity Assessors SHOULD ignore those signatures (see Section 6.1), acting as though they were not present in the message. 5. Signer Actions The following steps are performed in order by Signers. 5.1. Determine Whether the Email Should Be Signed and by Whom A Signer can obviously only sign email for domains for which it has a private key and the necessary knowledge of the corresponding public key and selector information. However, there are a number of other reasons beyond the lack of a private key why a Signer could choose not to sign an email. INFORMATIVE NOTE: A Signer can be implemented as part of 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 domain might be Crocker, et al. Standards Track [Page 36]
RFC 6376 DKIM Signatures September 2011 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. 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 the decision should largely be a matter of administrative convenience. Distribution and management of private keys is also outside the scope of this document. INFORMATIVE OPERATIONS ADVICE: A Signer should not sign with a private key when the selector containing the corresponding public key is expected to be revoked or removed before the Verifier has an opportunity to validate the signature. The Signer should anticipate that Verifiers can 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 to modification during transit, notably conversion to 7-bit form. Such conversions will break DKIM signatures. In order to minimize the chances of such breakage, Signers SHOULD convert the message to a suitable MIME content-transfer encoding such as quoted-printable or base64 as described in [RFC2045] before signing. Such conversion is outside the scope of DKIM; the actual message SHOULD be converted to 7-bit MIME by an MUA or MSA prior to presentation to the DKIM algorithm. If the message is submitted to the Signer with any local encoding that will be modified before transmission, that modification to canonical [RFC5322] form MUST be done before signing. In particular, bare CR or LF characters (used by some systems as a local line Crocker, et al. Standards Track [Page 37]
RFC 6376 DKIM Signatures September 2011 separator convention) MUST be converted to the SMTP-standard CRLF sequence before the message is signed. Any conversion of this sort SHOULD be applied to the message actually sent to the recipient(s), not just to the version presented to the signing algorithm. More generally, the Signer MUST sign the message as it is expected to be received by the Verifier rather than in some local or internal form. 5.3.1. Body Length Limits A body length count MAY be specified to limit the signature calculation to an initial prefix of the body text, measured in octets. If the body length count is not specified, the entire message body is signed. INFORMATIVE RATIONALE: This capability is provided because it is very common for mailing lists to add trailers to messages (e.g., instructions on 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 can, as a matter of policy, accept messages having valid signatures with extraneous data. The length actually hashed should be inserted in the "l=" tag of the DKIM-Signature header field. (See Section 3.5.) The body length count allows the Signer of a message to permit data to be appended to the end of the body of a signed message. The body length count MUST be calculated following the canonicalization algorithm; for example, any whitespace ignored by a canonicalization algorithm is not included as part of the body length count. A body length count of zero means that the body is completely unsigned. Signers wishing to ensure that no modification of any sort can occur should specify the "simple" canonicalization algorithm for both header and body and omit the body length count. See Section 8.2 for further discussion. 5.4. Determine the Header Fields to Sign The From header field MUST be signed (that is, included in the "h=" tag of the resulting DKIM-Signature header field). Signers SHOULD NOT sign an existing header field likely to be legitimately modified or removed in transit. In particular, [RFC5321] explicitly permits Crocker, et al. Standards Track [Page 38]
RFC 6376 DKIM Signatures September 2011 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 ignoring 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 header fields are 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 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 nonexisting 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. INFORMATIVE NOTE: A header field name need only be listed once more than the actual number of that header field in a message at the time of signing in order to prevent any further additions. For example, if there is a single Comments header field at the time of signing, listing Comments twice in the "h=" tag is sufficient to prevent any number of Comments header fields from being appended; it is not necessary (but is legal) to list Comments three or more times in the "h=" tag. Refer to Section 5.4.2 for a discussion of the procedure to be followed when canonicalizing a header with more than one instance of a particular header field name. Signers need to 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 Crocker, et al. Standards Track [Page 39]
RFC 6376 DKIM Signatures September 2011 otherwise reordered. Trace header fields (such as Received) and Resent-* blocks are the only fields prohibited by [RFC5322] from being reordered. In particular, since DKIM-Signature header fields may be reordered by some intermediate MTAs, signing existing DKIM- Signature header fields is error-prone. INFORMATIVE ADMONITION: Despite the fact that [RFC5322] does not prohibit the reordering of header fields, reordering of signed header fields with multiple instances by intermediate MTAs will cause DKIM signatures to be broken; such antisocial behavior should be avoided. INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this specification, all end-user visible header fields should be signed to avoid possible "indirect spamming". For example, if the Subject header field is not signed, a spammer can resend a previously signed mail, replacing the legitimate subject with a one-line spam. 5.4.1. Recommended Signature Content The purpose of the DKIM cryptographic algorithm is to affix an identifier to the message in a way that is both robust against normal transit-related changes and resistant to kinds of replay attacks. An essential aspect of satisfying these requirements is choosing what header fields to include in the hash and what fields to exclude. The basic rule for choosing fields to include is to select those fields that constitute the "core" of the message content. Hence, any replay attack will have to include these in order to have the signature succeed; however, with these included, the core of the message is valid, even if sent on to new recipients. Common examples of fields with addresses and fields with textual content related to the body are: o From (REQUIRED; see Section 5.4) o Reply-To o Subject o Date o To, Cc o Resent-Date, Resent-From, Resent-To, Resent-Cc Crocker, et al. Standards Track [Page 40]
RFC 6376 DKIM Signatures September 2011 o In-Reply-To, References o List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-Owner, List-Archive If the "l=" signature tag is in use (see Section 3.5), the Content- Type field is also a candidate for being included as it could be replaced in a way that causes completely different content to be rendered to the receiving user. There are trade-offs in the decision of what constitutes the "core" of the message, which for some fields is a subjective concept. Including fields such as "Message-ID", for example, is useful if one considers a mechanism for being able to distinguish separate instances of the same message to be core content. Similarly, "In- Reply-To" and "References" might be desirable to include if one considers message threading to be a core part of the message. Another class of fields that may be of interest are those that convey security-related information about the message, such as Authentication-Results [RFC5451]. The basic rule for choosing fields to exclude is to select those fields for which there are multiple fields with the same name and fields that are modified in transit. Examples of these are: o Return-Path o Received o Comments, Keywords Note that the DKIM-Signature field is also excluded from the header hash because its handling is specified separately. Typically, it is better to exclude other optional fields because of the potential that additional fields of the same name will be legitimately added or reordered prior to verification. There are likely to be legitimate exceptions to this rule because of the wide variety of application-specific header fields that might be applied to a message, some of which are unlikely to be duplicated, modified, or reordered. Signers SHOULD choose canonicalization algorithms based on the types of messages they process and their aversion to risk. For example, e-commerce sites sending primarily purchase receipts, which are not expected to be processed by mailing lists or other software likely to modify messages, will generally prefer "simple" canonicalization. Crocker, et al. Standards Track [Page 41]
RFC 6376 DKIM Signatures September 2011 Sites sending primarily person-to-person email will likely prefer to be more resilient to modification during transport by using "relaxed" canonicalization. Unless mail is processed through intermediaries, such as mailing lists that might add "unsubscribe" instructions to the bottom of the message body, the "l=" tag is likely to convey no additional benefit while providing an avenue for unauthorized addition of text to a message. The use of "l=0" takes this to the extreme, allowing complete alteration of the text of the message without invalidating the signature. Moreover, a Verifier would be within its rights to consider a partly signed message body as unacceptable. Judicious use is advised. 5.4.2. Signatures Involving Multiple Instances of a Field 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 so that the signature will not verify if additional header fields of that name are added. INFORMATIVE EXAMPLE: If the Signer wishes to sign two existing Received header fields, and the existing header contains: Received: <A> Received: <B> Received: <C> then the resulting DKIM-Signature header field should read: DKIM-Signature: ... h=Received : Received :... and Received header fields <C> and <B> will be signed in that order. Crocker, et al. Standards Track [Page 42]
RFC 6376 DKIM Signatures September 2011 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 that will include the body hash and a signature of the header hash, where that header includes the DKIM-Signature header field itself. Entities such as mailing list managers that implement DKIM and that modify the message or a header field (for example, inserting unsubscribe information) before retransmitting the message SHOULD check any existing signature on input and MUST make such modifications before re-signing the message. 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 field, as chosen above in Section 5.2. The DKIM-Signature header field MUST be inserted before any other DKIM-Signature fields in the header block. INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this is to insert the DKIM-Signature header field at the beginning of the header block. In particular, it may be placed before any existing Received header fields. This is consistent with treating DKIM-Signature as a trace header field. 6. Verifier Actions Since a Signer MAY remove or revoke a public key at any time, it is advised that verification occur in a timely manner. In many configurations, the most timely place is during acceptance by the border MTA or shortly thereafter. In particular, deferring verification until the message is accessed by the end user is discouraged. A border or intermediate MTA MAY verify the message signature(s). An MTA who has performed verification MAY communicate the result of that verification by adding a verification header field to incoming messages. This simplifies things considerably for the user, who can Crocker, et al. Standards Track [Page 43]
RFC 6376 DKIM Signatures September 2011 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 steps listed in Sections 6.1, 6.1.1, and 6.1.2 in order. In practice, several of these steps can be performed in parallel in order to improve performance. 6.1. Extract Signatures from the Message The order in which Verifiers try DKIM-Signature header fields is not defined; Verifiers MAY try signatures in any order they like. For example, one implementation might try the signatures in textual order, whereas another might try signatures by identities that match the contents of the From header field before trying other signatures. Verifiers MUST NOT attribute ultimate meaning to the order of multiple DKIM-Signature header fields. In particular, there is reason to believe that some relays will reorder the header fields in potentially arbitrary ways. INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as a clue to signing order in the absence of any other information. However, other clues as to the semantics of multiple signatures (such as correlating the signing host with Received header fields) might also be considered. Survivability of signatures after transit is not guaranteed, and signatures can fail to verify through no fault of the Signer. Therefore, 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. When a signature successfully verifies, a Verifier will either stop processing or attempt to verify any other signatures, at the discretion of the implementation. A Verifier MAY limit the number of signatures it tries, in order to avoid denial-of-service attacks (see Section 8.4 for further discussion). 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 one Crocker, et al. Standards Track [Page 44]
RFC 6376 DKIM Signatures September 2011 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 arrange to defer the message for later processing or try another signature; if no good signature is found and any of the signatures resulted in a TEMPFAIL status, the Verifier MAY arrange to defer the message for later processing. The "(explanation)" is not normative text; it is provided solely for clarification. Verifiers that are prepared to validate multiple signature header fields SHOULD proceed to the next signature header field, if one exists. However, Verifiers MAY make note of the fact that an invalid signature was present for consideration at a later step. INFORMATIVE NOTE: The rationale of this requirement is to permit messages that have invalid signatures but also a valid signature to work. For example, a mailing list exploder might opt to leave the original submitter signature in place even though the exploder knows that it is modifying the message in some way that will break that signature, and the exploder inserts its own signature. In this case, the message should succeed even in the presence of the known-broken signature. For each signature to be validated, the following steps should be performed in such a manner as to produce a result that is semantically equivalent to performing them in the indicated order. 6.1.1. Validate the Signature Header Field Implementers MUST meticulously validate the format and values in the DKIM-Signature header field; any inconsistency or unexpected values MUST cause the header field to be completely ignored and the Verifier to return PERMFAIL (signature syntax error). Being "liberal in what you accept" is definitely a bad strategy in this security context. Note, however, that this does not include the existence of unknown tags in a DKIM-Signature header field, which are explicitly permitted. Verifiers MUST return PERMFAIL (incompatible version) when presented a DKIM-Signature header field with a "v=" tag that is inconsistent with this specification. INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course, choose to also verify signatures generated by older versions of this specification. Crocker, et al. Standards Track [Page 45]
RFC 6376 DKIM Signatures September 2011 If any tag listed as "required" in Section 3.5 is omitted from the DKIM-Signature header field, the Verifier MUST ignore the DKIM- Signature header field and return PERMFAIL (signature missing required tag). INFORMATIVE NOTE: The tags listed as required in Section 3.5 are "v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there be a conflict between this note and Section 3.5, Section 3.5 is normative. If the DKIM-Signature header field does not contain the "i=" tag, the Verifier MUST behave as though the value of that tag were "@d", where "d" is the value from the "d=" tag. Verifiers MUST confirm that the domain specified in the "d=" tag is the same as or a parent domain of the domain part of the "i=" tag. If not, the DKIM-Signature header field MUST be ignored, and the Verifier should return PERMFAIL (domain mismatch). If the "h=" tag does not include the From header field, the Verifier MUST ignore the DKIM-Signature header field and return PERMFAIL (From field not signed). Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (signature expired) if it contains an "x=" tag and the signature has expired. Verifiers MAY ignore the DKIM-Signature header field if the domain used by the Signer in the "d=" tag is not associated with a valid signing entity. For example, signatures with "d=" values such as "com" and "co.uk" could be ignored. The list of unacceptable domains SHOULD be configurable. Verifiers MAY ignore the DKIM-Signature header field and return PERMFAIL (unacceptable signature header) for any other reason, for example, if the signature does not sign header fields that the Verifier views to be essential. As a case in point, if MIME header fields are not signed, certain attacks may be possible that the Verifier would prefer to avoid. 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. Crocker, et al. Standards Track [Page 46]
RFC 6376 DKIM Signatures September 2011 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. NOTE: The use of a wildcard TXT RR that covers a queried DKIM domain name will produce a response to a DKIM query that is unlikely to be a valid DKIM key record. This problem is not specific to DKIM and applies to many other types of queries. Client software that processes DNS responses needs to take this problem into account. When validating a message, a Verifier MUST perform the following steps in a manner that is semantically the same as performing them in the order indicated; in some cases, the implementation may parallelize or reorder these steps, as long as the semantics remain unchanged: 1. The Verifier retrieves the public key as described in Section 3.6 using the algorithm in the "q=" tag, the domain from the "d=" tag, and the selector from the "s=" tag. 2. If the query for the public key fails to respond, the Verifier MAY seek a later verification attempt by returning TEMPFAIL (key unavailable). 3. If the query for the public key fails because the corresponding key record does not exist, the Verifier MUST immediately return PERMFAIL (no key for signature). 4. If the query for the public key returns multiple key records, the Verifier can choose one of the key records or may cycle through the key records, performing the remainder of these steps on each record at the discretion of the implementer. The order of the key records is unspecified. If the Verifier chooses to cycle through the key records, then the "return ..." wording in the remainder of this section means "try the next key record, if any; if none, return to try another signature in the usual way". 5. If the result returned from the query does not adhere to the format defined in this specification, the Verifier MUST ignore the key record and return PERMFAIL (key syntax error). Verifiers are urged to validate the syntax of key records carefully to avoid attempted attacks. In particular, the Verifier MUST ignore keys with a version code ("v=" tag) that they do not implement. Crocker, et al. Standards Track [Page 47]
RFC 6376 DKIM Signatures September 2011 6. If the "h=" tag exists in the public-key record and the hash algorithm implied by the "a=" tag in the DKIM-Signature header field is not included in the contents of the "h=" tag, the Verifier MUST ignore the key record and return PERMFAIL (inappropriate hash algorithm). 7. 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). There is no defined semantic difference between a key that has been revoked and a key record that has been removed. 8. If the public-key data is not suitable for use with the algorithm and key types defined by the "a=" and "k=" tags in the DKIM- Signature header field, the Verifier MUST immediately return PERMFAIL (inappropriate key algorithm). 6.1.3. Compute the Verification Given a Signer and a public key, verifying a signature consists of actions semantically equivalent to the following steps. 1. Based on the algorithm defined in the "c=" tag, the body length specified in the "l=" tag, and the header field names in the "h=" tag, prepare a canonicalized version of the message as is described in Section 3.7 (note that this canonicalized version does not actually replace the original content). 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. If the hash does not match, the Verifier SHOULD ignore the signature and return PERMFAIL (body hash did not verify). 4. Using the signature conveyed in the "b=" tag, verify the signature against the header hash using the mechanism appropriate for the public-key algorithm described in the "a=" tag. If the signature does not validate, the Verifier SHOULD ignore the signature and return PERMFAIL (signature did not verify). Crocker, et al. Standards Track [Page 48]
RFC 6376 DKIM Signatures September 2011 5. Otherwise, the signature has correctly verified. INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to initiate the public-key query in parallel with calculating the hash as the public key is not needed until the final decryption is calculated. Implementations may also verify the signature on the message header before validating that the message hash listed in the "bh=" tag in the DKIM-Signature header field matches that of the actual message body; however, if the body hash does not match, the entire signature must be considered to have failed. A body length specified in the "l=" tag of the signature limits the number of bytes of the body passed to the verification algorithm. All data beyond that limit is not validated by DKIM. Hence, Verifiers might treat a message that contains bytes beyond the indicated body length with suspicion and can choose to treat the signature as if it were invalid (e.g., by returning PERMFAIL (unsigned content)). Should the algorithm reach this point, the verification has succeeded, and DKIM reports SUCCESS for this signature. 6.2. Communicate Verification Results Verifiers wishing to communicate the results of verification to other parts of the mail system may do so in whatever manner they see fit. For example, implementations might choose to add an email header field to the message before passing it on. Any such header field SHOULD be inserted before any existing DKIM-Signature or preexisting authentication status header fields in the header field block. The Authentication-Results: header field ([RFC5451]) MAY be used for this purpose. INFORMATIVE ADVICE to MUA filter writers: Patterns intended to search for results header fields to visibly mark authenticated mail for end users should verify that such a header field was added by the appropriate verifying domain and that the verified identity matches the author identity that will be displayed by the MUA. In particular, MUA filters should not be influenced by bogus results header fields added by attackers. To circumvent this attack, Verifiers MAY wish to request deletion of existing results header fields after verification and before arranging to add a new header field. Crocker, et al. Standards Track [Page 49]
RFC 6376 DKIM Signatures September 2011 6.3. Interpret Results/Apply Local Policy It is beyond the scope of this specification to describe what actions an Identity Assessor can make, but mail carrying a validated SDID presents an opportunity to an Identity Assessor that unauthenticated email does not. Specifically, an authenticated email creates a predictable identifier by which other decisions can reliably be managed, such as trust and reputation. Conversely, unauthenticated email lacks a reliable identifier that can be used to assign trust and reputation. It is reasonable to treat unauthenticated email as lacking any trust and having no positive reputation. In general, modules that consume DKIM verification output SHOULD NOT determine message acceptability based solely on a lack of any signature or on an unverifiable signature; such rejection would cause severe interoperability problems. If an MTA does wish to reject such messages during an SMTP session (for example, when communicating with a peer who, by prior agreement, agrees to only send signed messages), and a signature is missing or does not verify, the handling MTA SHOULD use a 550/5.7.x reply code. Where the Verifier is integrated within the MTA and it is not possible to fetch the public key, perhaps because the key server is not available, a temporary failure message MAY be generated using a 451/4.7.5 reply code, such as: 451 4.7.5 Unable to verify signature - key server unavailable Temporary failures such as inability to access the key server or other external service are the only conditions that SHOULD use a 4xx SMTP reply code. In particular, cryptographic signature verification failures MUST NOT provoke 4xx SMTP replies. Once the signature has been verified, that information MUST be conveyed to the Identity Assessor (such as an explicit allow/ whitelist and reputation system) and/or to the end user. If the SDID is not the same as the address in the From: header field, the mail system SHOULD take pains to ensure that the actual SDID is clear to the reader. While the symptoms of a failed verification are obvious -- the signature doesn't verify -- establishing the exact cause can be more difficult. If a selector cannot be found, is that because the selector has been removed, or was the value changed somehow in transit? If the signature line is missing, is that because it was never there, or was it removed by an overzealous filter? For diagnostic purposes, the exact reason why the verification fails SHOULD be made available and possibly recorded in the system logs. Crocker, et al. Standards Track [Page 50]
RFC 6376 DKIM Signatures September 2011 If the email cannot be verified, then it SHOULD be treated the same as all unverified email, regardless of whether or not it looks like it was signed. See Section 8.15 for additional discussion. 7. IANA Considerations DKIM has registered namespaces with IANA. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus [RFC5226]. This memo updates these registries as described below. Of note is the addition of a new "status" column. All registrations into these namespaces MUST include the name being registered, the document in which it was registered or updated, and an indication of its current status, which MUST be one of "active" (in current use) or "historic" (no longer in current use). No new tags are defined in this specification compared to [RFC4871], but one has been designated as "historic". Also, the "Email Authentication Methods" registry is revised to refer to this update. 7.1. Email Authentication Methods Registry The "Email Authentication Methods" registry is updated to indicate that "dkim" is defined in this memo. 7.2. DKIM-Signature Tag Specifications A DKIM-Signature provides for a list of tag specifications. IANA has established the "DKIM-Signature Tag Specifications" registry for tag specifications that can be used in DKIM-Signature fields. Crocker, et al. Standards Track [Page 51]
RFC 6376 DKIM Signatures September 2011 +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | v | (this document) | active | | a | (this document) | active | | b | (this document) | active | | bh | (this document) | active | | c | (this document) | active | | d | (this document) | active | | h | (this document) | active | | i | (this document) | active | | l | (this document) | active | | q | (this document) | active | | s | (this document) | active | | t | (this document) | active | | x | (this document) | active | | z | (this document) | active | +------+-----------------+--------+ Table 1: DKIM-Signature Tag Specifications Registry Updated Values 7.3. DKIM-Signature Query Method Registry The "q=" tag-spec (specified in Section 3.5) provides for a list of query methods. IANA has established the "DKIM-Signature Query Method" registry for mechanisms that can be used to retrieve the key that will permit validation processing of a message signed using DKIM. +------+--------+-----------------+--------+ | TYPE | OPTION | REFERENCE | STATUS | +------+--------+-----------------+--------+ | dns | txt | (this document) | active | +------+--------+-----------------+--------+ Table 2: DKIM-Signature Query Method Registry Updated Values 7.4. DKIM-Signature Canonicalization Registry The "c=" tag-spec (specified in Section 3.5) provides for a specifier for canonicalization algorithms for the header and body of the message. IANA has established the "DKIM-Signature Canonicalization Header" Registry for algorithms for converting a message into a canonical form before signing or verifying using DKIM. Crocker, et al. Standards Track [Page 52]
RFC 6376 DKIM Signatures September 2011 +---------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------------+--------+ | simple | (this document) | active | | relaxed | (this document) | active | +---------+-----------------+--------+ Table 3: DKIM-Signature Canonicalization Header Registry Updated Values +---------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------------+--------+ | simple | (this document) | active | | relaxed | (this document) | active | +---------+-----------------+--------+ Table 4: DKIM-Signature Canonicalization Body Registry Updated Values 7.5. _domainkey DNS TXT Resource Record Tag Specifications A _domainkey DNS TXT RR provides for a list of tag specifications. IANA has established the DKIM "_domainkey DNS TXT Record Tag Specifications" registry for tag specifications that can be used in DNS TXT resource records. +------+-----------------+----------+ | TYPE | REFERENCE | STATUS | +------+-----------------+----------+ | v | (this document) | active | | g | [RFC4871] | historic | | h | (this document) | active | | k | (this document) | active | | n | (this document) | active | | p | (this document) | active | | s | (this document) | active | | t | (this document) | active | +------+-----------------+----------+ Table 5: _domainkey DNS TXT Record Tag Specifications Registry Updated Values 7.6. DKIM Key Type Registry The "k=" <key-k-tag> (specified in Section 3.6.1) and the "a=" <sig- a-tag-k> (specified in Section 3.5) tags provide for a list of mechanisms that can be used to decode a DKIM signature. Crocker, et al. Standards Track [Page 53]
RFC 6376 DKIM Signatures September 2011 IANA has established the "DKIM Key Type" registry for such mechanisms. +------+-----------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------+--------+ | rsa | [RFC3447] | active | +------+-----------+--------+ Table 6: DKIM Key Type Registry Updated Values 7.7. DKIM Hash Algorithms Registry The "h=" <key-h-tag> (specified in Section 3.6.1) and the "a=" <sig- a-tag-h> (specified in Section 3.5) tags provide for a list of mechanisms that can be used to produce a digest of message data. IANA has established the "DKIM Hash Algorithms" registry for such mechanisms. +--------+-------------------+--------+ | TYPE | REFERENCE | STATUS | +--------+-------------------+--------+ | sha1 | [FIPS-180-3-2008] | active | | sha256 | [FIPS-180-3-2008] | active | +--------+-------------------+--------+ Table 7: DKIM Hash Algorithms Registry Updated Values 7.8. DKIM Service Types Registry The "s=" <key-s-tag> tag (specified in Section 3.6.1) provides for a list of service types to which this selector may apply. IANA has established the "DKIM Service Types" registry for service types. +-------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +-------+-----------------+--------+ | email | (this document) | active | | * | (this document) | active | +-------+-----------------+--------+ Table 8: DKIM Service Types Registry Updated Values Crocker, et al. Standards Track [Page 54]
RFC 6376 DKIM Signatures September 2011 7.9. DKIM Selector Flags Registry The "t=" <key-t-tag> tag (specified in Section 3.6.1) provides for a list of flags to modify interpretation of the selector. IANA has established the "DKIM Selector Flags" registry for additional flags. +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | y | (this document) | active | | s | (this document) | active | +------+-----------------+--------+ Table 9: DKIM Selector Flags Registry Updated Values 7.10. DKIM-Signature Header Field IANA has added DKIM-Signature to the "Permanent Message Header Field Names" registry (see [RFC3864]) for the "mail" protocol, using this document as the reference. 8. Security Considerations It has been observed that any introduced mechanism that attempts to stem the flow of spam is subject to intensive attack. DKIM needs to be carefully scrutinized to identify potential attack vectors and the vulnerability to each. See also [RFC4686]. 8.1. ASCII Art Attacks The relaxed body canonicalization algorithm may enable certain types of extremely crude "ASCII Art" attacks where a message may be conveyed by adjusting the spacing between words. If this is a concern, the "simple" body canonicalization algorithm should be used instead. 8.2. Misuse of Body Length Limits ("l=" Tag) 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 modified messages. However, using the "l=" tag enables attacks in which an intermediary with malicious intent can modify a message to include content that solely benefits the attacker. It is possible for the appended Crocker, et al. Standards Track [Page 55]
RFC 6376 DKIM Signatures September 2011 content to completely replace the original content in the end recipient's eyes and to defeat duplicate message detection algorithms. An example of such an attack includes altering the MIME structure, exploiting lax HTML parsing in the MUA, and defeating duplicate message detection algorithms. To avoid this attack, Signers should be extremely wary of using this tag, and Assessors might wish to ignore signatures that use the tag. 8.3. Misappropriated Private Key As with any other security application that uses private- or public- key pairs, DKIM requires caution around the handling and protection of keys. A compromised private key or access to one means an intruder or malware can send mail signed by the domain that advertises the matching public key. Thus, private keys issued to users, rather than one used by an ADministrative Management Domain (ADMD) itself, create the usual problem of securing data stored on personal resources that can affect the ADMD. A more secure architecture involves sending messages through an outgoing MTA that can authenticate the submitter using existing techniques (e.g., SMTP Authentication), possibly validate the message itself (e.g., verify that the header is legitimate and that the content passes a spam content check), and sign the message using a key appropriate for the submitter address. Such an MTA can also apply controls on the volume of outgoing mail each user is permitted to originate in order to further limit the ability of malware to generate bulk email. 8.4. Key Server Denial-of-Service Attacks Since the key servers are distributed (potentially separate for each domain), the number of servers that would need to be attacked to defeat this mechanism on an Internet-wide basis is very large. Nevertheless, key servers for individual domains could be attacked, impeding the verification of messages from that domain. This is not significantly different from the ability of an attacker to deny service to the mail exchangers for a given domain, although it affects outgoing, not incoming, mail. A variation on this attack involves a very large amount of mail being sent using spoofed signatures from a given domain: the key servers for that domain could be overwhelmed with requests in a denial-of- Crocker, et al. Standards Track [Page 56]
RFC 6376 DKIM Signatures September 2011 service attack (see [RFC4732]). However, given the low overhead of verification compared with handling of the email message itself, such an attack would be difficult to mount. 8.5. Attacks against the DNS Since the DNS is a required binding for key services, specific attacks against the DNS must be considered. While the DNS is currently insecure [RFC3833], these security problems are the motivation behind DNS Security (DNSSEC) [RFC4033], and all users of the DNS will reap the benefit of that work. DKIM is only intended as a "sufficient" method of proving authenticity. It is not intended to provide strong cryptographic proof about authorship or contents. Other technologies such as OpenPGP [RFC4880] and S/MIME [RFC5751] address those requirements. A second security issue related to the DNS revolves around the increased DNS traffic as a consequence of fetching selector-based data as well as fetching signing domain policy. Widespread deployment of DKIM will result in a significant increase in DNS queries to the claimed signing domain. In the case of forgeries on a large scale, DNS servers could see a substantial increase in queries. A specific DNS security issue that should be considered by DKIM Verifiers is the name chaining attack described in Section 2.3 of [RFC3833]. A DKIM Verifier, while verifying a DKIM-Signature header field, could be prompted to retrieve a key record of an attacker's choosing. This threat can be minimized by ensuring that name servers, including recursive name servers, used by the Verifier enforce strict checking of "glue" and other additional information in DNS responses and are therefore not vulnerable to this attack. 8.6. Replay/Spam Attacks In this attack, a spammer sends a piece of spam through an MTA that signs it, banking on the reputation of the signing domain (e.g., a large popular mailbox provider) rather than its own, and then re- sends that message to a large number of intended recipients. The recipients observe the valid signature from the well-known domain, elevating their trust in the message and increasing the likelihood of delivery and presentation to the user. Partial solutions to this problem involve the use of reputation services to convey the fact that the specific email address is being used for spam and that messages from that Signer are likely to be spam. This requires a real-time detection mechanism in order to Crocker, et al. Standards Track [Page 57]
RFC 6376 DKIM Signatures September 2011 react quickly enough. However, such measures might be prone to abuse, if, for example, an attacker re-sent a large number of messages received from a victim in order to make the victim appear to be a spammer. Large Verifiers might be able to detect unusually large volumes of mails with the same signature in a short time period. Smaller Verifiers can get substantially the same volume of information via existing collaborative systems. 8.7. Limits on Revoking Keys When a large domain detects undesirable behavior on the part of one of its users, it might wish to revoke the key used to sign that user's messages in order to disavow responsibility for messages that have not yet been verified or that are the subject of a replay attack. However, the ability of the domain to do so can be limited if the same key, for scalability reasons, is used to sign messages for many other users. Mechanisms for explicitly revoking keys on a per-address basis have been proposed but require further study as to their utility and the DNS load they represent. 8.8. Intentionally Malformed Key Records It is possible for an attacker to publish key records in DNS that are intentionally malformed, with the intent of causing a denial-of- service attack on a non-robust Verifier implementation. The attacker could then cause a Verifier to read the malformed key record by sending a message to one of its users referencing the malformed record in a (not necessarily valid) signature. Verifiers MUST thoroughly verify all key records retrieved from the DNS and be robust against intentionally as well as unintentionally malformed key records. 8.9. 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. 8.10. Information Leakage An attacker could determine when a particular signature was verified by using a per-message selector and then monitoring their DNS traffic for the key lookup. This would act as the equivalent of a "web bug" for verification time rather than the time the message was read. Crocker, et al. Standards Track [Page 58]
RFC 6376 DKIM Signatures September 2011 8.11. Remote Timing Attacks In some cases, it may be possible to extract private keys using a remote timing attack [BONEH03]. Implementations should consider obfuscating the timing to prevent such attacks. 8.12. Reordered Header Fields Existing standards allow intermediate MTAs to reorder header fields. If a Signer signs two or more header fields of the same name, this can cause spurious verification errors on otherwise legitimate messages. In particular, Signers that sign any existing DKIM- Signature fields run the risk of having messages incorrectly fail to verify. 8.13. RSA Attacks An attacker could create a large RSA signing key with a small exponent, thus requiring that the verification key have a large exponent. This will force Verifiers to use considerable computing resources to verify the signature. Verifiers might avoid this attack by refusing to verify signatures that reference selectors with public keys having unreasonable exponents. In general, an attacker might try to overwhelm a Verifier by flooding it with messages requiring verification. This is similar to other MTA denial-of-service attacks and should be dealt with in a similar fashion. 8.14. Inappropriate Signing by Parent Domains The trust relationship described in Section 3.10 could conceivably be used by a parent domain to sign messages with identities in a subdomain not administratively related to the parent. For example, the ".com" registry could create messages with signatures using an "i=" value in the example.com domain. There is no general solution to this problem, since the administrative cut could occur anywhere in the domain name. For example, in the domain "example.podunk.ca.us", there are three administrative cuts (podunk.ca.us, ca.us, and us), any of which could create messages with an identity in the full domain. INFORMATIVE NOTE: This is considered an acceptable risk for the same reason that it is acceptable for domain delegation. For example, in the case above, any of the domains could potentially simply delegate "example.podunk.ca.us" to a server of their choice Crocker, et al. Standards Track [Page 59]
RFC 6376 DKIM Signatures September 2011 and completely replace all DNS-served information. Note that a Verifier MAY ignore signatures that come from an unlikely domain such as ".com", as discussed in Section 6.1.1. 8.15. Attacks Involving Extra Header Fields Many email components, including MTAs, MSAs, MUAs, and filtering modules, implement message format checks only loosely. This is done out of years of industry pressure to be liberal in what is accepted into the mail stream for the sake of reducing support costs; improperly formed messages are often silently fixed in transit, delivered unrepaired, or displayed inappropriately (e.g., by showing only the first of multiple From: fields). Agents that evaluate or apply DKIM output need to be aware that a DKIM Signer can sign messages that are malformed (e.g., violate [RFC5322], such as by having multiple instances of a field that is only permitted once), that become malformed in transit, or that contain header or body content that is not true or valid. Use of DKIM on such messages might constitute an attack against a receiver, especially where additional credence is given to a signed message without adequate evaluation of the Signer. These can represent serious attacks, but they have nothing to do with DKIM; they are attacks on the recipient or on the wrongly identified author. Moreover, an agent would be incorrect to infer that all instances of a header field are signed just because one is. A genuine signature from the domain under attack can be obtained by legitimate means, but extra header fields can then be added, either by interception or by replay. In this scenario, DKIM can aid in detecting addition of specific fields in transit. This is done by having the Signer list the field name(s) in the "h=" tag an extra time (e.g., "h=from:from:..." for a message with one From field), so that addition of an instance of that field downstream will render the signature unable to be verified. (See Section 3.5 for details.) This, in essence, is an explicit indication that the Signer repudiates responsibility for such a malformed message. DKIM signs and validates the data it is told to and works correctly. So in this case, DKIM has done its job of delivering a validated domain (the "d=" value) and, given the semantics of a DKIM signature, essentially the Signer has taken some responsibility for a problematic message. It is up to the Identity Assessor or some other Crocker, et al. Standards Track [Page 60]
RFC 6376 DKIM Signatures September 2011 subsequent agent to act on such messages as needed, such as degrading the trust of the message (or, indeed, of the Signer), warning the recipient, or even refusing delivery. All components of the mail system that perform loose enforcement of other mail standards will need to revisit that posture when incorporating DKIM, especially when considering matters of potential attacks such as those described. 9. References 9.1. Normative References [FIPS-180-3-2008] U.S. Department of Commerce, "Secure Hash Standard", FIPS PUB 180-3, October 2008. [ITU-X660-1997] "Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", 1997. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, November 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. Crocker, et al. Standards Track [Page 61]
RFC 6376 DKIM Signatures September 2011 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. 9.2. Informative References [BONEH03] "Remote Timing Attacks are Practical", Proceedings 12th USENIX Security Symposium, 2003. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name System (DNS)", RFC 3833, August 2004. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006. [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)", RFC 4686, September 2006. [RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of- Service Considerations", RFC 4732, December 2006. Crocker, et al. Standards Track [Page 62]
RFC 6376 DKIM Signatures September 2011 [RFC4870] Delany, M., "Domain-Based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, May 2007. [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, November 2007. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5451] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, April 2009. [RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys Identified Mail (DKIM) Service Overview", RFC 5585, July 2009. [RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM) Signatures -- Update", RFC 5672, August 2009. [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification", RFC 5751, January 2010. [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, "DomainKeys Identified Mail (DKIM) Development, Deployment, and Operations", RFC 5863, May 2010. [RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and Mailing Lists", RFC 6377, September 2011. Crocker, et al. Standards Track [Page 63]
RFC 6376 DKIM Signatures September 2011 Appendix A. Example of Use (INFORMATIVE) This section shows the complete flow of an email from submission to final delivery, demonstrating how the various components fit together. The key used in this example is shown in Appendix C. A.1. The User Composes an Email From: Joe SixPack <joe@football.example.com> To: Suzie Q <suzie@shopping.example.net> 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. Figure 1: The User Composes an Email Crocker, et al. Standards Track [Page 64]
RFC 6376 DKIM Signatures September 2011 A.2. The Email is Signed This email is signed by the example.com outbound email server and now looks like this: DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com; c=simple/simple; q=dns/txt; i=joe@football.example.com; h=Received : From : To : Subject : Date : Message-ID; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=; b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB 4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV 4bmp/YzhwvcubU4=; Received: from client1.football.example.com [192.0.2.1] by submitserver.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) From: Joe SixPack <joe@football.example.com> To: Suzie Q <suzie@shopping.example.net> 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. Figure 2: The Email is Signed The signing email server requires access to the private key associated with the "brisbane" selector to generate this signature. Crocker, et al. Standards Track [Page 65]
RFC 6376 DKIM Signatures September 2011 A.3. The Email Signature is Verified The signature is normally verified by an inbound SMTP server or possibly the final delivery agent. However, intervening MTAs can also perform this verification if they choose to do so. The verification process uses the domain "example.com" extracted from the "d=" tag and the selector "brisbane" from the "s=" tag in the DKIM- Signature header field to form the DNS DKIM query for: brisbane._domainkey.example.com 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 this example) in the X-Authentication-Results header field line. After successful verification, the email looks like this: 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: v=1; a=rsa-sha256; s=brisbane; d=example.com; c=simple/simple; q=dns/txt; i=joe@football.example.com; h=Received : From : To : Subject : Date : Message-ID; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=; b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB 4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV 4bmp/YzhwvcubU4=; Received: from client1.football.example.com [192.0.2.1] by submitserver.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) From: Joe SixPack <joe@football.example.com> To: Suzie Q <suzie@shopping.example.net> 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. Figure 3: Successful Verification Crocker, et al. Standards Track [Page 66]
RFC 6376 DKIM Signatures September 2011 Appendix B. Usage Examples (INFORMATIVE) DKIM signing and validating can be used in different ways, for different operational scenarios. This Appendix discusses some common examples. NOTE: Descriptions in this Appendix are for informational purposes only. They describe various ways that DKIM can be used, given particular constraints and needs. In no case are these examples intended to be taken as providing explanation or guidance concerning DKIM specification details when creating an implementation. B.1. Alternate Submission Scenarios In the most simple scenario, a user's MUA, MSA, and Internet (boundary) MTA are all within the same administrative environment, using the same domain name. Therefore, all of the components involved in submission and initial transfer are related. However, it is common for two or more of the components to be under independent administrative control. This creates challenges for choosing and administering the domain name to use for signing and for its relationship to common email identity header fields. B.1.1. Delegated Business Functions Some organizations assign specific business functions to discrete groups, inside or outside the organization. The goal, then, is to authorize that group to sign some mail but to constrain what signatures they can generate. DKIM selectors (the "s=" signature tag) facilitate this kind of restricted authorization. Examples of these outsourced business functions are legitimate email marketing providers and corporate benefits providers. Here, the delegated group needs to be able to send messages that are signed, using the email domain of the client company. At the same time, the client often is reluctant to register a key for the provider that grants the ability to send messages for arbitrary addresses in the domain. There are multiple ways to administer these usage scenarios. In one case, the client organization provides all of the public query service (for example, DNS) administration, and in another, it uses DNS delegation to enable all ongoing administration of the DKIM key record by the delegated group. Crocker, et al. Standards Track [Page 67]
RFC 6376 DKIM Signatures September 2011 If the client organization retains responsibility for all of the DNS administration, the outsourcing company can generate a key pair, supplying the public key to the client company, which then registers it in the query service using a unique selector. The client company retains control over the use of the delegated key because it retains the ability to revoke the key at any time. If the client wants the delegated group to do the DNS administration, it can have the domain name that is specified with the selector point to the provider's DNS server. The provider then creates and maintains all of the DKIM signature information for that selector. Hence, the client cannot provide constraints on the local-part of addresses that get signed, but it can revoke the provider's signing rights by removing the DNS delegation record. B.1.2. PDAs and Similar Devices PDAs demonstrate the need for using multiple keys per domain. Suppose that John Doe wants to be able to send messages using his corporate email address, jdoe@example.com, and his email device does not have the ability to make a Virtual Private Network (VPN) connection to the corporate network, either because the device is limited or because there are restrictions enforced by his Internet access provider. If the device is equipped with a private key registered for jdoe@example.com by the administrator of the example.com domain and appropriate software to sign messages, John could sign the message on the device itself before transmission through the outgoing network of the access service provider. B.1.3. Roaming Users Roaming users often find themselves in circumstances where it is convenient or necessary to use an SMTP server other than their home server; examples are conferences and many hotels. In such circumstances, a signature that is added by the submission service will use an identity that is different from the user's home system. Ideally, roaming users would connect back to their home server using either a VPN or a SUBMISSION server running with SMTP AUTHentication on port 587. If the signing can be performed on the roaming user's laptop, then they can sign before submission, although the risk of further modification is high. If neither of these are possible, these roaming users will not be able to send mail signed using their own domain key. Crocker, et al. Standards Track [Page 68]
RFC 6376 DKIM Signatures September 2011 B.1.4. Independent (Kiosk) Message Submission Stand-alone services, such as walk-up kiosks and web-based information services, have no enduring email service relationship with the user, but users occasionally request that mail be sent on their behalf. For example, a website providing news often allows the reader to forward a copy of the article to a friend. This is typically done using the reader's own email address, to indicate who the author is. This is sometimes referred to as the "Evite" problem, named after the website of the same name that allows a user to send invitations to friends. A common way this is 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 email posting site into the Sender header field. The posting site can then sign the message, using the domain that is in the Sender field. This provides useful information to the receiving email site, which is able to correlate the signing domain with the initial submission email role. Receiving sites often wish to provide their end users with information about mail that is mediated in this fashion. Although the real efficacy of different approaches is a subject for human factors usability research, one technique that is used is for the verifying system to rewrite the From header field to indicate the address that was verified, for example: From: John Doe via news@news-site.example <jdoe@example.com>. (Note that such rewriting will break a signature, unless it is done after the verification pass is complete.) B.2. Alternate Delivery Scenarios Email is often received at a mailbox that has an address different from the one used during initial submission. In these cases, an intermediary mechanism operates at the address originally used, and it then passes the message on to the final destination. This mediation process presents some challenges for DKIM signatures. B.2.1. Affinity Addresses "Affinity addresses" allow a user to have an email address that remains stable, even as the user moves among different email providers. They are typically associated with college alumni associations, professional organizations, and recreational organizations with which they expect to have a long-term relationship. These domains usually provide forwarding of incoming email, and they often have an associated Web application that authenticates the user and allows the forwarding address to be Crocker, et al. Standards Track [Page 69]
RFC 6376 DKIM Signatures September 2011 changed. However, these services usually depend on users sending outgoing messages through their own service provider's MTAs. Hence, mail that is signed with the domain of the affinity address is not signed by an entity that is administered by the organization owning that domain. With DKIM, affinity domains could use the Web application to allow users to register per-user keys to be used to sign messages on behalf of their affinity address. The user would take away the secret half of the key pair for signing, and the affinity domain would publish the public half in DNS for access by Verifiers. This is another application that takes advantage of user-level keying, and domains used for affinity addresses would typically have a very large number of user-level keys. Alternatively, the affinity domain could handle outgoing mail, operating a mail submission agent that authenticates users before accepting and signing messages for them. This is, of course, dependent on the user's service provider not blocking the relevant TCP ports used for mail submission. B.2.2. Simple Address Aliasing (.forward) In some cases, a recipient is allowed to configure an email address to cause automatic redirection of email messages from the original address to another, such as through the use of a Unix .forward file. In this case, messages are typically redirected by the mail handling service of the recipient's domain, without modification, except for the addition of a Received header field to the message and a change in the envelope recipient address. In this case, the recipient at the final address' mailbox is likely to be able to verify the original signature since the signed content has not changed, and DKIM is able to validate the message signature. B.2.3. Mailing Lists and Re-Posters There is a wide range of behaviors in services that take delivery of a message and then resubmit it. A primary example is with mailing lists (collectively called "forwarders" below), ranging from those that make no modification to the message itself, other than to add a Received header field and change the envelope information, to those that add header fields, change the Subject header field, add content to the body (typically at the end), or reformat the body in some manner. The simple ones produce messages that are quite similar to the automated alias services. More elaborate systems essentially create a new message. Crocker, et al. Standards Track [Page 70]
RFC 6376 DKIM Signatures September 2011 A Forwarder that does not modify the body or signed header fields of a message is likely to maintain the validity of the existing signature. It also could choose to add its own signature to the message. Forwarders that modify a message in a way that could make an existing signature invalid are particularly good candidates for adding their own signatures (e.g., mailing-list-name@example.net). Since (re-)signing is taking responsibility for the content of the message, these signing forwarders are likely to be selective and forward or re-sign a message only if it is received with a valid signature or if they have some other basis for knowing that the message is not spoofed. A common practice among systems that are primarily redistributors of mail is to add a Sender header field to the message to identify the address being used to sign the message. This practice will remove any preexisting Sender header field as required by [RFC5322]. The forwarder applies a new DKIM-Signature header field with the signature, public key, and related information of the forwarder. See [RFC6377] for additional related topics and discussion. Appendix C. Creating a Public Key (INFORMATIVE) The default signature is an RSA-signed SHA-256 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 1024-bit, unencrypted private key suitable for DKIM is to use openssl like this: $ openssl genrsa -out rsa.private 1024 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: Crocker, et al. Standards Track [Page 71]
RFC 6376 DKIM Signatures September 2011 -----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/ eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj 7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc= -----END RSA PRIVATE KEY----- To extract the public-key component from the private key, use openssl like this: $ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM This results in the file rsa.public containing the key information similar to this: -----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI MmPSPDdQPNUYckcQ2QIDAQAB -----END PUBLIC KEY----- This public-key data (without the BEGIN and END tags) is placed in the DNS: $ORIGIN _domainkey.example.org. brisbane IN TXT ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ" "KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt" "IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v" "/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi" "tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB") C.1. Compatibility with DomainKeys Key Records DKIM key records were designed to be backward compatible in many cases with key records used by DomainKeys [RFC4870] (sometimes referred to as "selector records" in the DomainKeys context). One area of incompatibility warrants particular attention. The "g=" tag value may be used in DomainKeys and [RFC4871] key records to provide Crocker, et al. Standards Track [Page 72]
RFC 6376 DKIM Signatures September 2011 finer granularity of the validity of the key record to a specific local-part. A null "g=" value in DomainKeys is valid for all addresses in the domain. This differs from the usage in the original DKIM specification ([RFC4871]), where a null "g=" value is not valid for any address. In particular, see the example public-key record in Section 3.2.3 of [RFC4870]. C.2. RFC 4871 Compatibility Although the "g=" tag has been deprecated in this version of the DKIM specification (and thus MUST now be ignored), Signers are advised not to include the "g=" tag in key records because some [RFC4871]- compliant Verifiers will be in use for a considerable period to come. Appendix D. MUA Considerations (INFORMATIVE) When a DKIM signature is verified, the processing system sometimes makes the result available to the recipient user's MUA. How to present this information to users in a way that helps them is a matter of continuing human factors usability research. The tendency is to have the MUA highlight the SDID, in an attempt to show the user the identity that is claiming responsibility for the message. An MUA might do this with visual cues such as graphics, might include the address in an alternate view, or might even rewrite the original From address using the verified information. Some MUAs might indicate which header fields were protected by the validated DKIM signature. This could be done with a positive indication on the signed header fields, with a negative indication on the unsigned header fields, by visually hiding the unsigned header fields, or some combination of these. If an MUA uses visual indications for signed header fields, the MUA probably needs to be careful not to display unsigned header fields in a way that might be construed by the end user as having been signed. If the message has an "l=" tag whose value does not extend to the end of the message, the MUA might also hide or mark the portion of the message body that was not signed. The aforementioned information is not intended to be exhaustive. The MUA can choose to highlight, accentuate, hide, or otherwise display any other information that may, in the opinion of the MUA author, be deemed important to the end user. Appendix E. Changes since RFC 4871 o Abstract and introduction refined based on accumulated experience. o Various references updated. Crocker, et al. Standards Track [Page 73]
RFC 6376 DKIM Signatures September 2011 o Several errata resolved (see http://www.rfc-editor.org/): * 1376 applied * 1377 applied * 1378 applied * 1379 applied * 1380 applied * 1381 applied * 1382 applied * 1383 discarded (no longer applies) * 1384 applied * 1386 applied * 1461 applied * 1487 applied * 1532 applied * 1596 applied o Introductory section enumerating relevant architectural documents added. o Introductory section briefly discussing the matter of data integrity added. o Allowed tolerance of some clock drift. o Dropped "g=" tag from key records. The implementation report indicates that it is not in use. o Removed errant note about wildcards in the DNS. o Removed SMTP-specific advice in most places. o Reduced (non-normative) recommended signature content list, and reworked the text in that section. Crocker, et al. Standards Track [Page 74]
RFC 6376 DKIM Signatures September 2011 o Clarified signature generation algorithm by rewriting its pseudo- code. o Numerous terminology subsections added, imported from [RFC5672]. Also, began using these terms throughout the document (e.g., SDID, AUID). o Sections added that specify input and output requirements. Input requirements address a security concern raised by the working group (see also new sections in Security Considerations). Output requirements are imported from [RFC5672]. o Appendix subsection added discussing compatibility with DomainKeys ([RFC4870]) records. o Referred to [RFC5451] as an example method of communicating the results of DKIM verification. o Removed advice about possible uses of the "l=" signature tag. o IANA registry updated. o Added two new Security Considerations sections talking about malformed message attacks. o Various copy editing. Appendix F. Acknowledgments The previous IETF version of DKIM [RFC4871] was edited by Eric Allman, Jon Callas, Mark Delany, Miles Libbey, Jim Fenton, and Michael Thomas. That specification was the result of an extended collaborative effort, including participation by Russ Allbery, Edwin Aoki, Claus Assmann, Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin, Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto, Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur Gudmundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman, Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S. Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau, Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro, Neil Crocker, et al. Standards Track [Page 75]
RFC 6376 DKIM Signatures September 2011 Rerup, Eric Rescorla, Dave Rossetti, Hector Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker, Sam Weiler, and Dan Wing. The earlier DomainKeys was a primary source from which DKIM was derived. Further information about DomainKeys is at [RFC4870]. This revision received contributions from Steve Atkins, Mark Delany, J.D. Falk, Jim Fenton, Michael Hammer, Barry Leiba, John Levine, Charles Lindsey, Jeff Macdonald, Franck Martin, Brett McDowell, Doug Otis, Bill Oxley, Hector Santos, Rolf Sonneveld, Michael Thomas, and Alessandro Vesely. Authors' Addresses Dave Crocker (editor) Brandenburg InternetWorking 675 Spruce Dr. Sunnyvale, CA 94086 USA Phone: +1.408.246.8253 EMail: dcrocker@bbiw.net URI: http://bbiw.net Tony Hansen (editor) AT&T Laboratories 200 Laurel Ave. South Middletown, NJ 07748 USA EMail: tony+dkimsig@maillennium.att.com Murray S. Kucherawy (editor) Cloudmark 128 King St., 2nd Floor San Francisco, CA 94107 USA EMail: msk@cloudmark.com Crocker, et al. Standards Track [Page 76]
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