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DKIM                                                     D. Crocker, Ed.
Internet-Draft                               Brandenburg InternetWorking
Intended status: Standards Track                        January 25, 2009
Expires: July 29, 2009


    RFC 4871 DomainKeys Identified Mail (DKIM) Signatures -- Errata
                   draft-ietf-dkim-rfc4871-errata-00

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Abstract

   This documents and resolves errata for RFC 4871, DomainKeys
   Identified Mail (DKIM) Signatures.  Specifically the document



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   clarifies the nature, roles and relationship of the two DKIM
   identifier tag values that are candidates for payload delivery to a
   receiving processing module.

Errata Contact Fields

   Name:   Dave Crocker

   Email:   dcrocker@bbiw.net

Type

   Technical


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Section 2.7 Identity Assessor . . . . . . . . . . . . . . . . . 3
   3.  Section 2.8 Signing Domain Identifier (SDID)  . . . . . . . . . 3
   4.  Section 2.9 User Agent Identifier (UAID)  . . . . . . . . . . . 4
   5.  Section 3.9 Relationship Between SDID and UAID  . . . . . . . . 4
   6.  Section 3.5 The DKIM-Signature Header Field . . . . . . . . . . 5
   7.  Section 3.5 The DKIM-Signature Header Field . . . . . . . . . . 5
   8.  Section 3.8.  Signing by Parent Domains . . . . . . . . . . . . 5
   9.  Section 6.3.  Interpret Results/Apply Local Policy  . . . . . . 6
   10. Section 6.3.  Interpret Results/Apply Local Policy  . . . . . . 6
   11. Appendix D.  MUA Considerations . . . . . . . . . . . . . . . . 7
   12. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   13. Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7



















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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, a validator and a consumer of the validated
   signing domain.  In effect, the purpose of DKIM is to communicate
   that "responsible" domain name to a receive-side consuming module.
   For DKIM to be interoperable and useful, signer and consumer must
   share the same understanding of the details about the name.

   However the specification defines two, potentially different
   identifiers that are carried in the DKIM-Signature: header field and
   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 consumers with the potential for
   having differing -- and therefore non-interoperable --
   interpretations of how DKIM operates.

   This erratum resolves this confusion.  It defines new labels for the
   two values, clarifies their nature, and specifies and their
   relationship.


2.  Section 2.7 Identity Assessor

   Original Text:   (None.  Additional text.)

   Corrected Text:   The name of the module which consumes DKIM's
      primary payload, the responsible Signing Domain Identifier (SDID).
      It can optionally consume the User Agent Identifier (UAID) value,
      if provided to the module.  Details of this module are outside the
      scope of the DKIM specification.


3.  Section 2.8 Signing Domain Identifier (SDID)

   Original Text:   (None.  Additional text.)







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   Corrected Text:   A single, opaque value that is the mandatory
      payload output of DKIM and which refers to the identity claiming
      responsibility for the introduction of a message into the mail
      stream.


4.  Section 2.9 User Agent Identifier (UAID)

   Original Text:   (None.  Additional text.)

   Corrected Text:   A single, opaque value that is the optional,
      additional payload output of DKIM and which identifies the user or
      agent on behalf of whom the SDID has taken responsibility.


5.  Section 3.9 Relationship Between SDID and UAID

   Original Text:   (None.  This is an addition.)

   Corrected Text:   DKIM's primary task is to communicate from the
      Signer to a recipient-side Identity Assessor a single, responsible
      Signing Domain Identifier (SDID).  The SDID MUST be the value of
      the d= tag.  DKIM MAY optionally provide a single responsible User
      Agent Identifier (UAID); if the UAID is present, it MUST be the
      value of the i= tag.

      The SDID MUST correspond to a valid DNS name under which the DKIM
      key record is published.

      The UAID 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 -- and known only to -- the Signer.  The Signer MAY
      choose to use the same namespace for its UAIDs as its users' email
      addresses, or MAY choose other means of representing its users.
      However, the signer SHOULD use the same UAID 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 UAID as a finer grained, stable identifier than the
      SDID.

      Hence, DKIM delivers to receive-side Identity Assessors
      responsible Identifiers that are individual, opaque values.  Their
      sub-structures and particular semantics are not publicly defined
      and, therefore, cannot be assumed by an Identity Assessor.



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      A receive-side DKIM validator 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 chooses to attempt to interpret 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.


6.  Section 3.5 The DKIM-Signature Header Field

   Original Text:   d= The domain of the signing entity.

   Corrected Text:   d= Specifies the SDID claiming responsibility for
      the introduction of a message into the mail stream.


7.  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

   Corrected Text:   i= The responsible User Agent Identifier (UAID) on
      behalf of which the SDID is taking responsibility.


8.  Section 3.8.  Signing by Parent Domains

   the section where the error appears

   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=







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   Corrected Text:   For example, a key record for the domain
      example.com can be used to verify messages where the UAID ("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 UAID.  If the referenced key record contains the "s"
      flag as part of the "t=" tag, the domain of the UAID ("i=" flag)
      MUST be the same as that of the SDID (d=) domain.  If this flag is
      absent, the domain of the UAID MUST be the same as, or a subdomain
      of, the SDID.


9.  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.

   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.


10.  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 UAID is not the same as the address in the From:
      header field, the mail system SHOULD take pains to ensure that the
      actual UAID is clear to the reader.




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11.  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 UAID,
      in an attempt to show the user the identity that is claiming
      responsibility for the message.


12.  Security Considerations

   This Errata document 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.


13.  Normative References

   [RFC4871]  Sendmail, Inc., PGP Corporation, Yahoo! Inc, Yahoo! Inc,
              Cisco Systems, Inc., and Cisco Systems, Inc., "DomainKeys
              Identified Mail (DKIM) Signatures", RFC 4871, May 2007.


Appendix A.  Acknowledgements

   This document was initially formulated by an ad hoc design team,
   comprising: Jon Callas, J D Falk, Jim Fenton, Tony Hansen, Murray
   Kucherawy, John Levine, Michael Hammer, Jeff Macdonald, Ellen Siegel,
   Wietse Venema.  It represents a rough consensus of that effort,
   although the final wording of the submitted draft is the sole
   responsibility (d=) of the listed author.


Author's Address

   D. Crocker (editor)
   Brandenburg InternetWorking

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net






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