RFC5672

Network Working Group D. Crocker, Ed. Request for Comments: 5672 Brandenburg InternetWorking Updates: 4871 August 2009 Category: Standards Track

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 (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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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.

6. RFC 4871, Section 2.9, Signing Domain Identifier (SDID) . . . 5 7. RFC 4871, Section 2.10, Agent or User Identifier (AUID) . . . 5

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

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.

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,

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

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.

RFC 4871, Section 2.8, Identifier

Original Text:

  (None.  New section.  Additional text.)

Corrected Text:

  A label that refers to an identity.

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.

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.

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.

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: [email protected]