RFC 4871 DomainKeys Identified Mail (DKIM) Signatures -- Update

Abstract

This document 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.

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  RFC 4871, Abstract . . . . . . . . . . . . . . . . . . . . . .  4
   3.  RFC 4871, Section 1, Introduction  . . . . . . . . . . . . . .  4
   4.  RFC 4871, Section 2.7, Identity  . . . . . . . . . . . . . . .  4
   5.  RFC 4871, Section 2.8, Identifier  . . . . . . . . . . . . . .  5
   6.  RFC 4871, Section 2.9, Signing Domain Identifier (SDID)  . . .  5
   7.  RFC 4871, Section 2.10, Agent or User Identifier (AUID)  . . .  5
   8.  RFC 4871, Section 2.11, Identity Assessor  . . . . . . . . . .  6
   9.  RFC 4871, Section 3.5, The DKIM-Signature Header Field . . . .  6
   10. RFC 4871, Section 3.5, The DKIM-Signature Header Field . . . .  7
   11. RFC 4871, Section 3.8, Signing by Parent Domains  . . . . . . . 9
   12. RFC 4871, Section 3.9, Relationship between SDID and AUID  . . 10
   13. RFC 4871, Section 6.3, Interpret Results/Apply Local Policy  . 11
   14. RFC 4871, Section 6.3, Interpret Results/Apply Local Policy  . 11
   15. RFC 4871, Appendix D, MUA Considerations . . . . . . . . . . . 12
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   17. Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Appendix A.  ABNF Fragments  . . . . . . . . . . . . . . . . . . . 13
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 14

1. Introduction

About the purpose for DKIM, [RFC4871] 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, the signer and assessor must share the same understanding of the details about the identifier.

However, the RFC 4871 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 which is the "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 making different interpretations between the two identifiers and may lead to interoperability problems. A signer could intend one to be used for assessment, and have a different intent in setting the value in the other. However the verifier might choose the wrong value to deliver to the assessor, thereby producing an unintended (and inaccurate) assessment.

This Update resolves that confusion. It defines additional, semantic labels for the two values, clarifies their nature, and specifies their relationship. More specifically, it clarifies that the identifier intended for delivery to the assessor -- such as one that consults a whitelist -- is the value of the "d=" tag. However, this does not prohibit message filtering engines from using the "i=" tag, or any other information in the message's header, for filtering decisions.

For signers and verifiers that have been using the i= tag as the primary value that is delivered to the assessor, a software change to using the d= tag is intended.

So, this Update clarifies the formal interface to DKIM, after signature verification has been performed. It distinguishes DKIM's internal signing and verification activity, from its standardized delivery of data to that interface.

The focus of the Update is on the portion of DKIM that is much like an API definition. If DKIM is implemented as a software library for use by others, it needs to define what outputs are provided, that is, what data that an application developer who uses the library can expect to obtain as a result of invoking DKIM on a message.

This Update defines the output of that library to include the yes/no result of the verification and the "d=" value. In other words, it says what (one) identifier was formally specified for use by the signer and whether the use of that identifier has been validated. For a particular library, other information can be provided at the discretion of the library developer, since developers of assessors -- these are the consumers of the DKIM library -- well might want more information than the standardized two pieces of information. However, that standardized set is the minimum that is required to be provided to a consuming module, in order to be able to claim that the library is DKIM compliant.

This does not state what the implicit value of "i=" is, relative to "d=". In this context, that fact is irrelevant.

Another example is the difference between the socket interface to TCP versus the TCP protocol itself. There is the activity within the protocol stack, and then there is the activity within in the software libraries that are actually used.

NOTE: The text provided here updates [RFC4871]. Text appearing in

the "Corrected Text:" replaces text in RFC 4871. Hence, references that appear in the "Original Text:" can be found in RFC 4871, and are not duplicated in this document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. RFC 4871, Abstract

Original Text:

The ultimate goal of this framework is to permit a signing domain to assert responsibility for a message,

Corrected Text:

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,

3. RFC 4871, Section 1, Introduction

Original Text:

      ...permitting a signing domain to claim responsibility

Corrected Text:

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

4. RFC 4871, Section 2.7, Identity

Original Text:

      (None.  New section.  Additional text.)

Corrected 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.

5. RFC 4871, Section 2.8, Identifier

Original Text:

      (None.  New section.  Additional text.)

Corrected Text:

A label that refers to an identity.

6. RFC 4871, Section 2.9, Signing Domain Identifier (SDID)

Original Text:

      (None.  New section.  Additional text.)

Corrected 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.

7. RFC 4871, Section 2.10, Agent or User Identifier (AUID)

Original Text:

      (None.  New section.  Additional text.)

Corrected Text:

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 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.

8. RFC 4871, Section 2.11, Identity Assessor

Original Text:

      (None.  New section.  Additional text.)

Corrected Text:

A module that consumes DKIM's mandatory payload, which is the responsible Signing Domain Identifier (SDID). The module is dedicated to the assessment of the delivered identifier. Other DKIM (and non-DKIM) values can also be delivered to this module as well as to a more general message evaluation filtering engine. However, this additional activity is outside the scope of the DKIM signature specification.

9. RFC 4871, Section 3.5, The DKIM-Signature Header Field

Original Text:

   d=  The domain of the signing entity (plain-text; REQUIRED).  This is
      the domain that will be queried for the public key.  This domain
      MUST be the same as or a parent domain of the "i=" tag (the
      signing identity, as described below), or it MUST meet the
      requirements for parent domain signing described in Section 3.8.
      When presented with a signature that does not meet these
      requirement, verifiers MUST consider the signature invalid.

Internationalized domain names MUST be encoded as described in [RFC3490].

ABNF:

         sig-d-tag       = %x64 [FWS] "=" [FWS] domain-name
         domain-name     = sub-domain 1*("." sub-domain)
                  ; from RFC 2821 Domain,
                    but excluding address-literal

Corrected Text:

      d=

Specifies 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 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:

            sig-d-tag   = %x64 [FWS] "=" [FWS] domain-name
           domain-name = sub-domain 1*("." sub-domain)
                         ; from RFC 5321 Domain,
                           but excluding address-literal

10. RFC 4871, Section 3.5, The DKIM-Signature Header Field

Original Text:

   i=  Identity of the user or agent (e.g., a mailing list manager) on
   
      behalf of which this message is signed (dkim-quoted-printable;
      OPTIONAL, default is an empty Local-part followed by an "@"
      followed by the domain from the "d=" tag).  The syntax is a
      standard email address where the Local-part MAY be omitted.  The
      domain part of the address MUST be the same as or a subdomain of
      the value of the "d=" tag.

Internationalized domain names MUST be converted using the steps listed in Section 4 of [RFC3490] using the "ToASCII" function.

ABNF:

         sig-i-tag =  %x69 [FWS] "=" [FWS]
                      [ Local-part ] "@" domain-name

INFORMATIVE NOTE: The Local-part of the "i=" tag is optional because in some cases a signer may not be able to establish a verified individual identity. In such cases, the signer may wish to assert that although it is willing to go as far as signing for the domain, it is unable or unwilling to commit to an individual user name within their domain. It can do so by including the domain part but not the Local-part of the identity.

INFORMATIVE DISCUSSION: This document 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
bindings between the "i=" value and other identities is not well established, nor is its vulnerability to subversion by an attacker. Hence reliance on the use of these options should be strictly limited. In particular, it is not at all clear to what extent a typical end-user recipient can rely on any assurances that might be made by successful use of the "i=" options.

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:

            sig-i-tag =  %x69 [FWS] "=" [FWS]
                        [ Local-part ] "@" domain-name

The AUID is specified as having the same syntax as an email address, but is not required to have the same semantics. Notably, the domain name is not required to be registered in the DNS -- so it might not resolve in a query -- and the Local- part MAY be drawn 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 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 their domain. It can do so by including the domain part but not the Local-part of the identity.

11. RFC 4871, Section 3.8, Signing by Parent Domains

Original Text:

e.g., a key record for the domain example.com can be used to verify messages where the signing identity ("i=" tag of the signature) is sub.example.com, or even sub1.sub2.example.com. In order to limit the capability of such keys when this is not intended, the "s" flag may be set in the "t=" tag of the key record to constrain the validity of the record to exactly the domain of the signing identity. If the referenced key record contains the "s" flag as part of the "t=" tag, the domain of the signing identity ("i=" flag) MUST be the same as that of the d= domain. If this flag is absent, the domain of the signing identity MUST be the same as, or a subdomain of, the d= domain.

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.

12. RFC 4871, Section 3.9, Relationship between SDID and AUID

   Original Text:   (None.  New section.  Additional text.)

Corrected Text:

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 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. 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 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.

13. RFC 4871, Section 6.3, Interpret Results/Apply Local Policy

Original Text:

It is beyond the scope of this specification to describe what actions a verifier system should make, but an authenticated email presents an opportunity to a receiving system that unauthenticated email cannot. Specifically, an authenticated email creates a predictable identifier by which other decisions can reliably be managed, such as trust and reputation. Conversely, unauthenticated email lacks a reliable identifier that can be used to assign trust and reputation.

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.

14. RFC 4871, Section 6.3, Interpret Results/Apply Local Policy

Original Text:

Once the signature has been verified, that information MUST be conveyed to higher-level systems (such as explicit allow/ whitelists and reputation systems) and/or to the end user. If the message is signed on behalf of any address other than that in the From: header field, the mail system SHOULD take pains to ensure that the actual signing identity is clear to the reader.

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.

15. RFC 4871, Appendix D, MUA Considerations

Original Text:

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.

Corrected Text:

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.

16. Security Considerations

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.

17. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
   
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.
   
   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

Appendix A. ABNF Fragments

This appendix contains the full set of corrected ABNF fragments defined in this document.

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This version of this MIB module is part of RFC 5672; see the RFC itself for full legal notices.

            sig-d-tag   = %x64 [FWS] "=" [FWS] domain-name
           domain-name = sub-domain 1*("." sub-domain)
                         ; from RFC 5321 Domain,
                           but excluding address-literal
           
            sig-i-tag =  %x69 [FWS] "=" [FWS]
                        [ Local-part ] "@" domain-name

Appendix B. Acknowledgements

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.

Author's Address

   D. Crocker (editor)
   Brandenburg InternetWorking
   
   Phone: +1.408.246.8253
   EMail: dcrocker@bbiw.net