Brandenburg InternetWorking

+1.408.246.8253 dcrocker@bbiw.net

Security DKIM This updates RFC 4871, DomainKeys Identified Mail (DKIM) Signatures. Specifically the document clarifies the nature, roles and relationship of the two DKIM identifier tag values that are candidates for payload delivery to a receiving processing module. The Update is in the style of an Errata entry, albeit a rather long one.

About the purpose for DKIM, states: The ultimate goal of this framework is to permit a signing domain to assert responsibility for a message, thus protecting message signer identity... Hence, DKIM has a signer that produces a signed message, a verifier that confirms the signature and an assessor that consumes the validated signing domain. So the simple purpose of DKIM is to communicate an identifier to a receive-side assessor module. The identifier is in the form of a domain name that refers to a responsible identity. For DKIM to be interoperable and useful, signer and assessor must share the same understanding of the details about the identifier. However the RFC4871 specification defines two, potentially different identifiers that are carried in the DKIM-Signature: header field, d= and i=. Either might be delivered to a receiving processing module that consumes validated payload. The DKIM specification fails to clearly define what is "payload" to be delivered to a consuming module, versus what is internal and merely in support of achieving payload delivery. This currently leaves signers and assessors with the potential for having differing -- and therefore non-interoperable -- interpretations of how DKIM operates. This update resolves this confusion. It defines new labels for the two values, clarifies their nature, and specifies and their relationship. The text provided here updates . All references and all appearances of RFC-2119 keywords are replacing text in RFC 4871. Hence those references are in that document and are not needed here.

The ultimate goal of this framework is to permit a signing domain to assert responsibility for a message, The ultimate goal of this framework is to permit a person, role or organization that owns the signing domain to assert responsibility for a message,

...permitting a signing domain to claim responsibility permitting a person, role or organization that owns the signing domain to claim responsibility

(None. Additional text.) A person, role or organization. In the context of DKIM, examples include author, author's organization, an ISP along the handling path, an independent trust assessment service, and a mailing list operator.

(None. Additional text.) A label that refers to an identity.

(None. Additional text.) A single domain name that is the mandatory payload output of DKIM and that refers to the identity claiming responsibility for introduction of a message into the mail stream. For DKIM processing, the name has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. It is specified in section 3.5.

(None. Additional text.) A single identifier that refers to the agent or user on behalf of whom the 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 sub-domain of it. For DKIM processing, the domain name portion of the AUID has only basic domain name semantics; any possible owner-specific semantics are outside the scope of DKIM. It is specified in section 3.5.

(None. Additional text.) The name of the module that consumes DKIM's mandatory payload, the responsible Signing Domain Identifier (SDID). The module is dedicated to the assessment of the delivered identifier. Other DKIM (and non-DKIM) values can also be delivered to this module as well as to a more general message evaluation filtering engine. However this additional activity is outside the scope of the DKIM signature specification.

Original Text:

Corrected Text: d= Specifies the SDID claiming responsibility for an introduction of a message into the mail stream (plain-text; REQUIRED). This is the domain that will be queried for the public key. The SDID MUST correspond to a valid DNS name under which the DKIM key record is published. The conventions and semantics used by a signer to create and use a specific SDID are outside the scope of the DKIM Signing specification, as is any use of those conventions and semantics. When presented with a signature that does not meet these requirements, verifiers MUST consider the signature invalid. Internationalized domain names MUST be encoded as described in [RFC3490]. ABNF:

Original Text:

Corrected Text: i= The Agent or User Identifier (AUID) on behalf of which the SDID is taking responsibility (dkim-quoted-printable; OPTIONAL, default is an empty Local-part followed by an "@" followed by the domain from the "d=" tag). The syntax is a standard email address where the Local-part MAY be omitted. The domain part of the address MUST be the same as, or a subdomain of the value of, the "d=" tag. Internationalized domain names MUST be converted using the steps listed in Section 4 of [RFC3490] using the "ToASCII" function. ABNF:

The AUID is specified as having the same syntax as an email address, but is not required to have the same semantics. Notably, the domain name is not required to be registered in the DNS -- so it might not resolve in a query -- and the Local-part MAY be drawn from a namespace that does not contain the user's 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 the DKIM Signing 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 finer grained, stable identifier 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 their domain. It can do so by including the domain part but not the Local-part of the identity.

Original Text:

Corrected Text: ...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.

(None. This is an addition.) 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. A receive-side DKIM verifier MUST communicate the Signing Domain Identifier (d=) to a consuming Identity Assessor module and MAY communicate the User Agent 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. 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 the identity in any message header fields. This is considered to be an assessor policy issue. Constraints between the value of the SDID or AUID 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 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 these options should be strictly limited. In particular, it is not at all clear to what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the SDID or AUID.

Original Text:

Corrected Text: 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.

Original Text:

Corrected Text: 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.

The tendency is to have the MUA highlight the address associated with this signing identity in some way, in an attempt to show the user the address from which the mail was sent. 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.

This Update clarifies core details about DKIM's payload. As such it affects interoperability, semantic characterization, and the expectations for the identifiers carried with a DKIM signature. Clarification of these details is likely to limit misinterpretation of DKIM's semantics. Since DKIM is fundamentally a security protocol, this should improve its security characteristics.

This document has no actions for IANA.

Sendmail, Inc. PGP Corporation Yahoo! Inc Yahoo! Inc Cisco Systems, Inc. Cisco Systems, Inc.

This document was initially formulated by an ad hoc design team, comprising: Jon Callas, D. Crocker, J. D. Falk, Michael Hammer, Tony Hansen, Murray Kucherawy, John Levine, Jeff Macdonald, Ellen Siegel and Wietse Venema. The final version of the document was developed through vigorous discussion in the IETF DKIM working group.