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pre-workgroup                                                    D. Otis
Internet-Draft                                         Trend Micro, NSSG
Expires: June 24, 2006                                 December 21, 2005

         Extended Options for DomainKeys Identified Mail (DKIM)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes options that extend protections offered by
   DKIM.  These options include Binding-Advice & Role-Assertion, Opaque-
   Identifier, and an In-Channel check.  The Binding-Advice & Role-
   Assertion offers guidance in isolating the source of a message, in
   addition to establishing message signature expectations.  The Opaque-
   Identifier (O-ID) offers protection from replay abuse and intra-
   domain spoofing, even when email-addresses are not associated with
   the signing-domain.  The In-Channel check provides a means to
   mitigate DNS lookups for avoiding possible message replay abuse.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Binding-Advice & Role-Assertion  . . . . . . . . . . . . . . .  5
   4.  Opaque-Identifier  . . . . . . . . . . . . . . . . . . . . . .  8
   5.  In-Channel Check . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2   Informative References . . . . . . . . . . . . . . . . . . 12
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

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1.  Introduction

   Message signing, as exemplified by DomainKeys Identified Mail (DKIM)
   [I-D.allman-dkim-base], is a mechanism to allow an assertion of an
   accountable domain for an email message in transit.  The assertion is
   made by means of a digital signature included within a header, which
   also validates the integrity of selected headers and message body
   content subsequent to the signing of the message.

   Combining DKIM with an authorization mechanism, referenced from an
   email-address contained within the message, may result in unintended
   consequences.  The email-address domain owner may be unfairly held
   accountable for abuse found subsequent to their authorization.  As
   the email-address domain owner often has no administrative oversight
   or ability to rectify abuse issues, such accountability may place
   them in peril of having their email refused.

   Even when authorization is restricted to a single provider, the
   email-address domain owner would still be relying upon this
   provider's diligence, and may be unable to ascertain the cause for
   refusals or remedy their situation.  Such restriction on allowable
   sources for email also interferes with existing email practices, such
   as the use of list-servers or sending email using the email-address
   of one's alma mater.  To ensure fair treatment for email-address
   domain owners, and to minimize the impact upon email practices, the
   ability to refute messages should not be contingent upon the use of
   an authorization scheme.

   Indicating the results of an authorization that compares an email-
   address domain to a signing-domain would be unsafe.  Domain matching
   only indicates the email-address is within the same Administrative
   Unit as the signing-domain.  Ambiguity in the display of the email-
   address and one's limited ability to detect variations from prior
   messages means such indications may mislead the recipient into
   erroneously trusting the source of the message.

   In addition, the entity directly involved with sending email should
   be held accountable for abuse.  Such an assignment of accountability
   permits effective and timely remedies, and ensures innocent parties
   are not inadvertently harmed.  For email, such an entity could be
   discerned by the remote IP address, a verified host name, or the
   domain used to verify the DKIM-Signature.  The DKIM-Signature should
   be considered an aspect of the message transport, and not necessarily
   directly associated with message content or any contained email-

   Restoring trust and establishing the expectation of a signature being
   present within email messages can be accomplished by way of a

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   recognition strategy, instead of using an email-address authorization
   mechanism.  Perhaps one of the greatest assets of DKIM would be the
   enhanced ability to recognize previous email sources.  Simple email-
   address and signing-domain comparisons permit all too common social
   engineering techniques that are often involved in the spoofing of
   email-addresses.  A recognition strategy can safely highlight those
   messages emanating from a source specifically recognized by the
   recipient through a prior message.

   The ability to recognize a unique email source is enhanced with the
   use of the Binding-Advice & Role-Assertion, and the O-ID.  The O-ID
   and In-Channel check can further enhance protections that curtail
   abusive message replays.  The In-Channel check allows a reduction in
   the overhead associated with abusive message replay protections.

   Binding identifiers from a prior correspondent at the behest of the
   recipient allows indications of recognition without the use of
   complex and problematic email-address domain authorizations, which
   may create significant support issues.  To support the recognition of
   a prior correspondent, the MUA could simply highlight those messages
   from prior correspondents.  This approach would offer a higher level
   of assurance and trust without using any DNS lookups.  Following the
   verification of the DKIM-Signature, the identification of the message
   source would be contained completely within the message itself.

   When assuming legacy MUAs, scant protections are possible by the MTA
   even using many DNS lookups and registering thousands of look-alike
   domains.  Due to limitations of ensuring the visibility of checked
   domains, the MTA approach provides an alarmingly low level of email-
   address protections.  There is also a potential for an undesired
   exposure of email-addresses in the 'i=' parameter.

   The O-ID approach could be used to detect when a previous
   correspondent appears to be from a different source.  Without the
   O-ID, detecting intra-domain spoofing would depend upon the signing-
   domain verifying the validity of the email-address.  The signed
   message may even advise what other information should be compared
   against the email-address.  While in most cases the collected
   relationships (bindings) would be made at the behest of the recipient
   and require their approval, some relationships could be established

   When the email-address domain is within the signing-domain, and when
   the message advises that these messages should always be signed, then
   it should be safe to capture this assertion automatically.  When
   signature assurances are captured (cached), the MTA or MUA would be
   able to detect when a message violated these expected relationships.
   Before rejecting a message for not having the proper signature, a

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   check may be made to verify that the signature assurance remains

   To recognize the source of a message when there are no assurances
   being made regarding the email-address, an O-ID that tracks accounts
   could be added by the signing-domain.  This O-ID would become a part
   of the captured relationship once approved by the recipient.  A
   provision has been added to indicate when the signing role has been
   delegated.  The message from a delegated signer is not allowed to
   make "broad" recommendations with respect to the scope of a binding.
   This delegated role will also require the O-ID to equal the left-most
   label of the DKIM-Signature selector.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Terminology: Terminology conforms to [I-D.crocker-email-arch].

3.  Binding-Advice & Role-Assertion

   The displayed character-repertoire may be defined by the sender as
   result of [RFC2047] or [RFC3492].  Even displaying raw puny-code
   would represent a difficult basis for recognition, especially for
   recipients who's native language is not based upon ASCII characters.
   In addition, a large percentage of recipients only see the "display-
   name" as defined by [RFC2822] (also called the "pretty-name") where
   the email-address is not normally seen by the recipient.

   A safe indication shown to the recipient would be that a message
   source has been recognized as belonging to a prior correspondent.  To
   help achieve this goal, the signer of the message assists by
   indicating which aspects of the message's information may be used to
   isolate the message's source.

   Three roles are defined in the following tables.  For example,
   although an MSA is indicated as providing the signature, this role
   could be delegated to an MUA or another less trusted MSA.  Detecting
   the delegation of a role involves examining the DKIM-Key optional
   parameters.  Whenever the 'g=<email-address>' has an email-address
   assigned, or the 'w=<sa-validation>' first letter is 'D' then the
   role should be considered delegated.  In the case of a delegated
   role, the O-ID is derived from the DKIM-Signature 's=<selector>'
   parameter.  When the role is delegated and the 'u=<Opaque-
   Identifier>' parameter is present, it MUST match that of the left-

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   most selector label.  A "broad" assertion by a Delegated signer is
   not valid.

   | Code | Scope of Binding, and Role                                 |
   | w=b  | Always signed by MSA, broad ass. across email-domain       |
   | w=n  | Always signed by MSA, narrow ass. with email-address       |
   | w=d  | Signed by MSA, broad association across email-domain       |
   | w=a  | Signed by MSA, narrow association with email-address       |
   | w=o  | Signed by MSA, association with Opaque-Identifier          |
   | none | Signed by MSA, no association assured                      |
   | w=B  | Always signed by Mediator, broad ass. across email-domain  |
   | w=N  | Always signed by Mediator, narrow ass. with email-address  |
   | w=D  | Signed by Mediator, broad association across email-domain  |
   | w=A  | Signed by Mediator, narrow association with email-address  |
   | w=O  | Signed by Mediator, association with Opaque-Identifier     |
   | w=M  | Signed by Mediator, no association assured                 |
   | w=X  | Signed by MDA, no association assured                      |

   When the DKIM-Signature header field has the option 'w=' with a value
   of 'b','B','d', or 'D', then the email-address domain associated with
   the signing-role together with the signing-domain can be used to
   recognize the source of a message.  With a value of 'n','N','a', or
   'A', then the entire email-address associated with the signing-role
   together with the signing-domain should be used to recognize the
   source of a message.  With a value of 'o', or 'O', the O-ID together
   with the signing-domain can be used to recognize the source of a
   message.  With a value of 'M', 'X' or no 'w=' option (default), just
   the signing-domain can be used to recognize the source of a message.

   When the DKIM-Signature header field has the option 'w=' with a value
   of 'X', this is used to verify that the message has been accepted by
   the MDA of the signing-domain and the 'u=' parameter, if present,
   represents an assessment made by the MDA.  To ensure signatures are
   not misused to perpetrate abusive message replays, the MDA may
   overlay the 'b=<signature>' of other roles with "!MDA-verified" or
   "!MDA-invalid".  DKIM-Signature header fields containing the 'w=X'
   option will include other DKIM-Signature header fields containing an
   "!MDA-verified" signature overlay, in sequential order from the
   beginning of the message.  These additional DKIM-Signature header
   fields are processed immediately following the processing of the
   DKIM-Signature header field with the 'w=X' option, and before the
   remainder of the message.  A message with a DKIM-Signature header
   field signed by a domain of a different Administrative Unit with the
   'w=X' option is invalid and SHOULD be rejected.

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   When the DKIM-Signature header field has the option 'w=' with a value
   of 'B','N','D', or 'A', then the email-address associated with the
   DKIM-Signature should be found within a "Resent-*" header field.
   Each DKIM-Signature should be uniquely associated with a MSA,
   Mediator, or MDA role.  The DKIM-Signature header field added by the
   MDA or Mediator MUST be removed by the MSA prior to processing the
   message.  When a signature added by an MSA is known by the Mediator
   to be currently invalid, the DKIM-Signature header field SHOULD be
   removed or the Mediator may otherwise overlay the 'b=<signature>'
   with "!Mediator-verified" or "!Mediator-invalid".

  | Code |      Binding      |   Assurances  | Validation |   Role   |
  | w=b  |    email-domain   | always signed |  DKIM key  |    MSA   |
  | w=n  |   email-address   | always signed |  DKIM key  |    MSA   |
  | w=d  |    email-domain   |      none     |    none    |    MSA   |
  | w=a  |   email-address   |      none     |    none    |    MSA   |
  | w=o  | Opaque-Identifier |      none     |    none    |    MSA   |
  | none |        none       |      none     |    none    |    MSA   |
  | w=B  |    email-domain   | always signed |  DKIM key  | Mediator |
  | w=N  |   email-address   | always signed |  DKIM key  | Mediator |
  | w=D  |    email-domain   |      none     |    none    | Mediator |
  | w=A  |   email-address   |      none     |    none    | Mediator |
  | w=O  | Opaque-Identifier |      none     |    none    | Mediator |
  | w=M  |        none       |      none     |    none    | Mediator |
  | w=X  |        none       |      none     |    none    |    MDA   |

   When the DKIM-Signature header field has the option 'w=' with a value
   of 'n', or 'b', and the email-address domain is within the signing-
   domain as denoted by the 'd=<domain>' parameter, then the assurance
   of a signature for this domain can be automatically cached.  The
   cached information should include both the domain where an assurance
   has been made, and the label for a record used to confirm the
   continued status of the assurance.  A parameter within the DKIM-Key
   can be used to consolidate where assurances are confirmed when
   multiple DKIM-Keys are being used.  When there are no parameters
   added to the DKIM-Key, the default signature-assurance validation
   location would be determined by the 'd=<domain>' and parameters
   's=<selector>'.  The left-most label within the selector would be
   used as follows:


   When the 'w=' option is present within the DKIM-Key, the value of
   this parameter modifies the signature-assurance validation location

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   to be:


   The 'w=<sa-validation>' option would be composed of 1 to 63
   characters within the DKIM-Key and used to consolidate signature
   assurances.  The operation of this signature-assurance validation
   record mechanism would take the form of a single A record lookup
   where the existence of the record would validate a cached assurance.

     <sa-validation> ::= <role> [ [ <ldh-str> ] <let-dig> ]
     <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
     <let-dig-hyp> ::= <let-dig> | "-"
     <let-dig> ::= <letter> | <digit>
     <letter> ::= %x41-5A / %x61-7A
     <role> ::= "T" | "D"
       ; Trusted or Delegated role
     <digit> ::= %x30-39
     <sa-validation>._dkim-sa.<signing-domain> IN A

4.  Opaque-Identifier

   The Opaque-Identifier (O-ID) is an option that supports two different
   mechanisms.  One mechanism isolates the source of a message to a
   specific account as denoted by the Binding-Advice & Role-Assertion.
   The other mechanism provides a means to revoke messages being
   abusively replayed.  An O-ID added to the signature header MUST also
   be a valid domain name label.  The term 'opaque' means only the
   domain creating the identifier understands the associations indicated
   in Binding-Advice & Role-Assertion.  There are two modes for creating
   the O-ID.  One mode would make the O-ID persistent with the account
   used to access the signing-domain, and the other could be sequential
   for cases where an account is not involved.  A prefix added to a
   sequential O-ID prevents collisions with identifiers used for

   If an identifier were added to an unsigned message, this would invite
   forgery and therefore offer little value.  A standardized O-ID,
   included within the validated content of a signed message, would
   offer significant value.  A persistent O-ID would be most useful and
   could be derived from the access server that authenticates the
   account being used.

   A sequential O-ID may be appropriate when distributing bulk mailings.
   To identify abusers that may attempt to stage replay attacks, having
   a unique identifier for each recipient could prove helpful.  These

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   replay attacks could be done using the unchanged content of the
   message, but sent to recipients that would consider the information
   to be unsolicited.  The reason for such a replay attack may be to
   damage the reputation of the signing-domain.

   The persistent O-ID would greatly aid the correlation of abuse and
   the locating of compromised systems.  This identifier could be
   effective against systems compromised by Trojan programs, stolen
   passwords, and cracked wireless access points, among many other
   nefarious methods.  Abuse reports that catalog signed messages and
   that are correlated with a persistent O-ID would provide
   incontrovertible evidence of where the source of a problem exists.
   The publishing of the revocation record for the O-ID would also
   provide feedback that actions were taken to rectify a policy breach.

   In odd cases where an In-Channel check fails, a single lookup of a
   revocation record for the O-ID returning no record would be an
   indication that this particular O-ID is still authorized by the
   signing-domain.  This mechanism would be most valuable in those cases
   where the message may have been forwarded, such as at the typical
   alma mater, or where a mailing list opts to also forward signed
   messages unaltered.

   If there is a problem, the signature would offer the name of the most
   capable domain able to remedy abuse.  People can still safely use
   their forwarding email accounts given to them by their school or
   society.  Mailing lists would be given a strong identifier upon which
   to grant the replication of messages.  Complaints would also likely
   be directed to those most able to curtail future episodes of bad
   behavior, i.e. the provider of the abusive account!

   Within the signature header, a 'u=<Opaque-Identifier>' parameter or
   within the DKIM-Key, a 'w=<sa-assurance>' parameter where the first
   letter is 'D' a would indicate the use of an O-ID.  The operation of
   this revocation record mechanism takes the form of a single A record
   lookup where the return of a record indicates the O-ID has been
   revoked.  The O-ID would be composed of 1 to 63 characters and select
   a record in this fashion:

     <Opaque-Identifier> ::= <mode> [ [ <ldh-str> ] <let-dig> ]
     <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
     <let-dig-hyp> ::= <let-dig> | "-"
     <let-dig> ::= <letter> | <digit>
     <letter> ::= %x41-5A / %x61-7A
     <mode> ::= "P" |  "S"
       ; Persistent and Sequential O-ID assignment
     <digit> ::= %x30-39
     <Opaque-Identifier>._dkim-or.<signing-domain> IN A

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   When the first letter in the O-ID is 'P,' this represents an
   identifier where the portion of the identifier to the right of the
   leftmost '-' character is persistent with the account used to obtain
   access.  When the first letter is 'S,' then no portion of the
   identifier can be used to isolate which account was used to obtain

   When the signing-domain has not revoked authorization for the O-ID,
   no record would be returned and the remote DNS cache would retain the
   absence of this record for a brief period of time, see [RFC2308].
   For the majority of cases, where messages are obtained directly from
   the signing-domain, confirmation of an In-Channel check allows the
   O-ID revocation check to be skipped.

   The O-ID revocation check would be performing nearly an identical
   lookup now ubiquitously done to investigate the status of the SMTP
   client's IP address against a DNS black-hole list.  Those addresses
   or identifiers that warrant refusal are granted a long lived address
   record to ensure their immediate refusal and limit DNS traffic
   resulting from abusive sources.  Otherwise, not offering a record
   allows for the prompt cessation of an O-ID's authorization when the
   situation regarding a particular identifier changes.  The Time-To-
   Live for negative DNS caching may be determined by the recipient, or
   represent the lesser of the SOA TTL or the SOA MINIMUM field,
   depending upon the recipient's implementation.

5.  In-Channel Check

   There are two methods that can be used to ascertain whether a message
   is In-Channel.  In-Channel would be when the RCPT TO list has been
   specified by or sourced by the originating Administrative Unit.  One
   method uses a hash of the initial [RFC2821] RCPT TO: email-address
   list.  The other method verifies the EHLO using a DNS lookup for an
   address record or CSV-CSA record as defined in [I-D.crocker-csv-csa].
   When the signing-domain as noted in the DKIM-Signature 'd=<domain>'
   parameter are within the verified EHLO domain name, the message could
   be said to be In-Channel.  Another method may use a [RFC2821] RCPT
   TO: email-address hash parameter stored within the DKIM-Signature to
   confirm that the RCPT TO list has not been altered.

   When the message is determined to be In-Channel, and an O-ID option
   is being used, checking for O-ID revocation may be skipped.  When
   O-ID revocation should be checked, the receiving SMTP server may
   issue an SMTP 450 temporary error and delay acceptance for a few
   minutes.  Once the receiving SMTP server decides enough time has
   elapsed from the initial delivery attempt for the specific message, a
   O-ID revocation check would be made instead.  If the O-ID
   authorization has not been revoked, the message may be accepted.

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   When the 'm=' parameter is included, an SHA-1 hash algorithm defined
   in [RFC3174] is used to hash all [RFC2821] RCPT TO: email-addresses
   in sequence from left to right and first to last.  The hash will
   include only the [RFC2821] RCPT TO: email-addresses, and to obfuscate
   the use of a BCC header, the hash may be initialized by a special
   SMTP extension MF-SALT.  The result of the hash is stored in Base 64
   within the DKIM-Signature 'm=<mailfrom-hash>' parameter.  When the
   MF-SALT extension has been allowed, a RCPT TO parameter may return an
   SMTP extension MF-SALT-?????????????? where the fourteen '?' are
   replaced by "URL and Filename safe" Base 64 Alphabet characters as
   defined in [RFC3548] representing an 84 bit random number.  When the
   MF-SALT parameter is found within the initial RCPT TO parameter,
   without a binary conversion, the fourteen Base 64 Alphabet characters
   are hashed first, followed by the RCPT TO: email-addresses.  When the
   MF-SALT parameter is not present, just the RCPT TO: list may have
   been hashed.

6.  IANA Considerations

   The SMTP extension MF-SALT will require registration by IANA.

   Use of the _dkim-sa and _dkim-or prefix in DNS records will require
   registration by IANA.

   To avoid conflicts, tag names for the DKIM-Signature header and key
   records the following should be added to those registered with IANA.

   Tag values for the "w=", "u=", and "m=" tags in the DKIM-Signature
   header, and the "w=", tags in key records should be registered with
   IANA for the same reason.

7.  Security Considerations

   This document describes options that can be used with DomainKeys
   Identified Mail (DKIM) to improve upon the secure use of this

8.  References

8.1  Normative References

              Crocker, D., "Client SMTP Authorization (CSA)",
              draft-crocker-csv-csa-00 (work in progress), October 2005.

              Crocker, D., "Internet Mail Architecture",
              draft-crocker-email-arch-04 (work in progress),

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              March 2005.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, March 1998.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RFC3548]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 3548, July 2003.

8.2  Informative References

              Allman, E., "DomainKeys Identified Mail (DKIM)",
              draft-allman-dkim-base-01 (work in progress),
              October 2005.

Author's Address

   Douglas Otis
   Trend Micro, NSSG
   1737 North First Street, Suite 680
   San Jose, CA  95112

   Phone: +1.408.453.6277
   Email: doug_otis@trendmicro.com

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Intellectual Property Statement

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   this document or the extent to which any license under such rights
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Otis                      Expires June 24, 2006                [Page 13]

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