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Versions: 00 01 02 03 04 05 RFC 5016

DKIM Working Group                                             M. Thomas
Internet-Draft                                             Cisco Systems
Intended status: Informational                        September 15, 2006
Expires: March 19, 2007


           Requirements for a DKIM Signing Practices Protocol
                  draft-ietf-dkim-ssp-requirements-01

Status of this Memo

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   This Internet-Draft will expire on March 19, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).














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Abstract

   DomainKeys Identified Mail [DKIM] [I-D.ietf-dkim-base] provides a
   cryptographic mechanism for domains to assert responsibility for the
   messages they sign.  A related mechanism would allow an administrator
   to publish various statements about their email accountability
   practices.  This draft defines the requirement for this additional
   mechanism.











































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Requirements Language

   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 RFC 2119 [RFC2119].


Table of Contents

   1.  Preface  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5

   3.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6

   4.  Use Scenarios  . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Scenario 1: Bigbank.example.com  . . . . . . . . . . . . .  7
     4.2.  Scenario 2: DKIM Signing Complete State  . . . . . . . . .  8
     4.3.  Scenario 3: Outsourced First Party Signing . . . . . . . .  8
     4.4.  Scenario 4: Resent Original Mail . . . . . . . . . . . . .  9
     4.5.  Scenario 5: Incremental Deployment of Signing  . . . . . .  9

   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Discovery Requirements . . . . . . . . . . . . . . . . . . 11
     5.2.  Transport requirements . . . . . . . . . . . . . . . . . . 11
     5.3.  Practice and Expectation Requirements  . . . . . . . . . . 12
       5.3.1.  Negative Commentary  . . . . . . . . . . . . . . . . . 12
     5.4.  Extensibility and Forward Compatibilty Requirements  . . . 15

   6.  Security Requirements  . . . . . . . . . . . . . . . . . . . . 16

   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17

   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18

   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19

   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative References . . . . . . . . . . . . . . . . . . 20

   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22








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1.  Preface

   The purpose of this draft is get out into the open a range of issues
   related to the perceived need for a signing practices information
   service primarily focused on DKIM.  This document is intended to
   document well-agreed upon problems and requirements, in addition to
   less well-agreed upon requirements in an attempt to capture the issue
   as well as generalize the requirement as much as possible.  These
   latter requirements will be noted as "[PROVISIONAL]" to indicate that
   there is not yet solid consensus, or that the problem is not well
   understood.  A winnowing process is envisioned where the more
   difficult and/or speculative problems/requirement will be eliminated
   unless concrete problems with proven constituencies can be
   demonstrated, along with reasonable plausibility that they do not
   contradict more well agreed upon requirements.




































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2.  Definitions

   o  Domain Holder: the entity that ultimately controls the contents of
      the DNS subtree starting at the domain, either directly or by
      delegation via NS records it controls.

   o  First Party Address: For DKIM, a first party address is defined to
      be the [RFC2822].From address in the message header; a first party
      address is also known as an Author address

   o  First Party Signature: a first party signature is a valid
      signature where the domain tag (d= or the more specific identity
      i= tag) matches the first party address.  "Matches" in this
      context is defined in [I-D.ietf-dkim-base]

   o  Third Party Signature: a third party signature is a valid
      signature that does not qualify as a First Party Signature.  Note
      that a DKIM third party signature does is not required to
      correspond to a third party address such as Sender or Listid, etc.

   o  DKIM Signer Complete: the state where the domain holder believes
      that all legitimate mail purportedly from the domain was sent with
      a valid DKIM signature.

   o  The Protocol: in this document, The Protocol is used as
      placeholder for a protocol that will meet the requirements set in
      this draft.
























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3.  Introduction

   The DomainKeys Identified Mail working group is chartered to create a
   base signing mechanism for email.  This work is contained in
   [I-D.ietf-dkim-base].  In addition there are two other documents
   [I-D.ietf-dkim-overview] and [I-D.ietf-dkim-threats] which give an
   overview and a threat analysis of the chartered work.  This draft
   reflects the requirements for the last part of the chartered work to
   define a protocol to publish DKIM signing practices.

   While the base signing document defines a mechanism for signing and
   verifying DKIM signatures, there has been a great deal of interest in
   a signing practices protocol.  The most pressing case seems to be the
   bid down attack inherent with almost all systems that allow optional
   authentication: how does a receiver know whether or not it should
   expect a message to contain authentication information?  For email
   this is an especially difficult problem since there is generally no a
   priori knowledge of a sending domain's practices.  If a protocol
   could be developed for a domain to publish its DKIM signing
   practices, a message verifier could take that into account when it
   receives a unsigned piece of email.

   This draft is organized into two main sections: a Usage Scenario
   section which attempts to describe some common usage scenarios that
   DKIM is likely to be deployed in and the problems that are not solved
   by DKIM alone.  The second is the Requirements that arise because of
   those usage scenarios, in addition to more basic protocol
   requirements.























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4.  Use Scenarios

   The email world is a diverse world with many deployment scenarios.
   This section tries to outline some usage scenarios that it is
   expected that DKIM signing/verifying will take place in, and how a
   new protocol might be helpful to clarify the relevance of DKIM signed
   mail.

4.1.  Scenario 1: Bigbank.example.com

   There seems to be a class of mail -- mostly transactional mail from
   high value domains -- that are the target of phishing attacks.  In
   particular, the phishing scams forge the [RFC2822].From address in
   addition to spoofing much of the content to trick unsuspecting users
   into revealing sensitive information.  Domain holders sending this
   kind of mail would like the ability to give better guarantees that
   mail sent in their name is with the consent of the domain holder.
   The first step is, of course, to use DKIM-base to sign all of the
   outgoing mail so that a receiver can make a determination that the
   mail is from the domain holder in question.

   The problem with this scenario is that a receiver in the general case
   doesn't know what the practices are for a given domain, or what their
   expectations are for unsigned mail.  An information service which
   allowed a receiver to query for those practices and expectations
   could be useful to close the gap where an attacker merely sends
   unsigned mail to exploit a bid down attack.  It is assumed that
   receivers would use this information to treat such questionable mail
   with prejudice.

   Note that for the foreseeable future, DKIM signature breakage for
   unrestricted use patterns (ie with users and especially where users
   are members of mailing lists) will likely suffer occasional damage in
   transit.  While probably not a large percentage of total traffic, the
   kind (quality) of breakage may be significant for certain usage
   patterns.  As such, this scenario defines a more limited situation
   where the risk of a legitimate piece of mail being mislabeled as
   unsigned outweights the risk of illegitimate mail being delivered in
   the eyes of the sender.

   1.  Mail with a [RFC2822].From A purportedly sends to B with a
       missing or broken DKIM signature from A

   2.  B would like to know whether that is an acceptable state of
       affairs.






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4.2.  Scenario 2: DKIM Signing Complete State

   After auditing their outgoing mail and deploying DKIM signing for all
   of their legitimate outgoing mail, a domain could be said to be DKIM
   signing complete.  That is, the domain has to the best of its ability
   insured that all mail legitimately purporting to have come from that
   domain contained a valid DKIM signature.  Given the likelihood of
   signature damage in the current mail infrastructure as noted above, a
   domain can fit the DKIM signing complete scenario without wanting to
   take the risks associated with the more narrow scope of use in the
   previous scenario.  A receiver, on the other hand, may be able to
   take advantage of the knowledge the domain's practice of signing all
   mail in order to use it to bias filters against the unexpected
   arrival of a piece of unsigned or damaged in transit mail.

4.3.  Scenario 3: Outsourced First Party Signing

   Many domains do not run their own mail infrastructure, or may
   outsource parts of it to third parties.  It is desirable for a domain
   holder to have the ability delegate to other entities the ability to
   sign for the domain holder with either a first party signature or the
   equivalent.  One obvious use scenario is a domain holder for a small
   domain that needs to have the ability for their outgoing ISP to sign
   all of their mail on behalf of the domain holder.  Other use
   scenarios include outsourced bulk mail for marketing campaigns, as
   well as outsourcing various business functions such as insurance
   benefits, etc.

   That said, DKIM uses DNS to store selectors.  Thus there is always
   the ability for a domain holder to delegate all or parts of the
   _domainkey subdomain to a third party of the domain holder's
   choosing.  That is, the domain holder may be able to set a NS record
   for _domainkey.example.com to, say, an email provider who manages
   that namespace.  There is also the ability for the domain holder to
   partition its namespace into subdomains to further constrain how
   third parties.  For example, a domain holder could delegate only
   _domainkey.benefits.example.com to a third party to constrain the
   third party to only be able to produce valid signatures in the
   benefits.example.com subdomain.

   There have been concerns expressed about how well this would scale
   when the third party is, say, a large ISP that signs for thousands of
   domains.  There has been concern about how well this would work for
   multiple delegations.  Another concern is that not all DNS providers
   give the ability to specify delegations.  Lastly, using NS
   delegations requires that the signer actively cooperate with the
   domain for whom it is signing.  That is, it requires that the signer
   actively manage the _domainkey delegation for the domain holder.  A



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   domain holder would not, for example, be able to make a statement
   that ISP.com signing on its behalf was acceptable without ISP.com's
   cooperation.  This by extension also applies to other third parties
   that a domain might like to effectively "whitelist" such as mailing
   lists that re-sign mail that the domain holder holds in esteem.

4.4.  Scenario 4: Resent Original Mail

   Resent mail is a common occurrence in many scenarios in the email
   world of today.  For example, Alice sends a DKIM signed message with
   a published practice of signing all messages sent from Alice's domain
   to Bob's mailing list in another domain.  Bob, being a good net
   citizen, wants to be able to take his part of the responsibility of
   the message in question, so he DKIM signs the message.

   Note that this scenario is completely orthogonal to whether Alice's
   signature survived Bob's mailing list: Bob merely wants to assert his
   part in the chain of accountability for the benefit of the ultimate
   receivers.  It would be useful for this practice to be encouraged as
   it gives a more accurate view of who handled the message, and the
   trustworthiness of the handlers.  It also has the side benefit that
   remailers that are not friendly to DKIM first party signatures (ie,
   break them) can still be potentially assessed by the receiver based
   on the receiver's opinion of the signing domains that actually
   survived.

4.5.  Scenario 5: Incremental Deployment of Signing

   As a practical matter, it may be difficult for a domain to roll out
   DKIM signing such that they can publish the DKIM Signing Complete
   practice for its given the complexities of the user population,
   outsourced vendors sending on its behalf, etc.  This leaves open an
   exploit that high-value mail must be classified to the least common
   denominator of the published practices.  It would be desirable to
   allow a domain holder to publish a list of exceptions which would
   have a stronger practices statement.

   In this situation, bigbank.example.com might be ready to say that
   statements@bigbank.example.com is always signed, but the rest of the
   domain, say, is not.  Another situation is that the practices of some
   address local parts in a given domain are not the same as practices
   of other local parts.  Using the same example of
   statements@bigbank.example.com being a transactional kind of email
   which would like to publish very strong practices, mixed in with the
   rest of the user population local parts which may go through mailing
   lists, etc, for which a less strong statement is appropriate.  It
   should be said that DKIM, through the use of subdomains, can already
   support this kind of differentiation.  That is, in order to publish a



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   strong practice, one only has to segregate those cases into different
   subdomains.  For example: *@accounts.bigbank.example.com would
   publish a strong practice while *@bigbank.example.com could publish a
   more permissive practice.















































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5.  Requirements

   This section defines the requirements for The Protocol.  As with most
   requirements drafts, these requirements define the MINIMUM
   requirements that a candidate protocol must provide.  It should also
   be noted that The Protocol must fulfill all of the requirements.

      [Informative Note: it's not clear to the author that all of the
      provisional requirements can fulfill the harder requirements.  If
      this is determined to be true, the provisional requirement should
      either be dropped or the harder requirements revised]

5.1.  Discovery Requirements

   1.  Discovery mechanism MUST be rooted in DNS.

   2.  Discovery mechanism MUST converge in a deterministic number of
       exchanges.

          [Informative Note: this, for all intents and purposes is a
          prohibition on anything that might produce loops; also though
          "deterministic" doesn't specify how many exchanges, the
          expectation is "few".]

   3.  Discovery mechanism MUST NOT overload semantics of existing DNS
       resource records where name space collisions are plausible.  For
       the purposes of this requirement, neither an underscore prefixed
       namespace nor a new resource record type are considered
       plausible.

5.2.  Transport requirements

   1.  Widespread deployment of the transport layer would be highly
       desirable, especially if riding on top of a true transport layer
       (eg, TCP, UDP).

   2.  A low-cost query/response in terms of latency time and the number
       of packets involved is highly desirable.

   3.  If the infrastructure doesn't provide caching (ala DNS), the
       records retrieved will need time-to-live values to allow querying
       verifiers to maintain their own caches.  Existing caching
       infrastructure is, however, highly desirable.

   4.  Multiple geographically and topologically diverse servers must be
       supported for high availability





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5.3.  Practice and Expectation Requirements

   In this section, a Practice is defined as a true statement according
   to the [RFC2822].From domain holder of its intended externally
   viewable behavior.  An Expectation combines with a Practice to convey
   what the domain holder considers the likely outcome of the
   survivability of the Practice at a receiver.  For example, a Practice
   that X is true when it leaves the domain, and an Expectation that it
   will|will-not|may|may-not remain true for some/all receivers.

   1.   The Protocol MUST be able to make Practices and Expectation
        assertions about the [RFC2822].From address in the context of
        DKIM.  The Protocol will not make assertions about other
        addresses for DKIM at this time.

   2.   The Protocol MUST be able to publish a Practice that the domain
        doesn't send mail.

   3.   The Protocol MUST be able to publish a Practice that the
        domain's signing behavior is "DKIM Signing Complete"

   4.   The Protocol MUST be able to publish an Expectation that a
        verifiable First Party DKIM Signature should be expected on
        receipt of a message.

           [Informative Note: the DKIM Signing Complete Practice seems
           to be a pre-requisite for this Expectation]

   5.   [PROVISIONAL] A domain MUST be able to delegate responsibility
        for signing its messages to a non-related domain in such a way
        that it does not require active participation by the non-related
        domain.  That is, the published information MUST have a way to
        specify the domains that are allowed to sign on its behalf.

     5.3.1.  Negative Commentary

        1.  Scaling Concerns for DNS: the addition of third party
            signers begs the question of how that information is stored
            in the DNS if that is the repository (which seems pretty
            likely).  The naive approach of just encoding them into one
            resource record could quite plausiblely exceed the maximum
            DNS UDP MTU which would be highly undesirable.  Other
            approaches have been suggested, but they seemingly trade off
            complexity, number of lookups, non-deterministic completion,
            etc.

        2.  Alternate mechanism to NS delegation, and threats not well
            understood: the mechanism for delegating zones within DNS is



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            a very well understood problem which well understood
            threats.  The threats involved with another form of
            indirection are far less clear, though undoubtably present.
            There is a great deal of concern that (re)discovering those
            threats, deciding whether they are valid, whether they need
            to mitigated, etc, etc are a worthwhile exercise given that
            this provides minimal functionality over what currently
            exists with DKIM base.

        3.  The seems to be a strong consensus for most of requirements,
            and quite a bit less for this, is there harm if we don't do
            this now?  That is, can it extended later if needed based on
            much more experience?

        4.  Concerns about conflation of responsibility and/or
            reputation: there are concerns of adding to the confusion
            about who is taking responsibility for what.  Is it the d=
            domain in the DKIM-signature?  Is it the domain in the From:
            address?  Is it both?  Is it neither?  Keeping this simple
            by using the NS delegation mechanism would not raise these
            interesting, if not thorny questions.

        5.  It's not clear that this requirement actually delivers on
            the less complexity aspect of its billing: one of the
            attractions of this requirement is that there would be
            little or no interaction required between the first party
            and the designated third party signer.  Given the security
            issues with this approach, it is not clear that the
            interaction required between the first and third parties are
            any less onerous.

        6.  Security Threat with DKIM base, a first party signer can
            always clarify which address it is signing on behalf of by
            using the i= tag.  That is, when there's ambiguity between,
            say, From: and Sender: the signer has the ability to clarify
            which address the signature was on behalf of if it so
            desires.  For a third party signature, there is no clarity
            since the signature by definition has no relationship to the
            origination addresses.

               A legitimate flow follows:

               -  An ISP signs for both a.com and mailinglist.com

               -  mailinglist.com sends a piece of mail From:
                  president@a.com, Sender: list@mailinglist.com





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               -  ISP signs the message with d=ISP.com

               -  The receiver at this point has no idea who ISP.com was
                  signing on behalf of because they are both
                  legitimately signed by ISP.com

            The attack is essentially identical: it only requires that
            mailinglist.com spoof the From: address to whatever other
            customer of ISP.com.  That is, mailinglist.com can be any
            example.com with bad intentions.  Worse, is that the ISP
            will ordinarily have no clue as to whether its customers are
            running mailing lists or not, so it would not have the
            ability "legitimate" From: spoofing (ie, a real mailing
            list) from illegitimate spoofing.

   6.   [PROVISIONAL] The protocol MUST have the ability to provide
        practices and expecations keyed off of the local part of the
        [RFC2822].From address.  As with all provisional requirements,
        this requirement must not be in conflict with other
        requirements, including DNS considerations, etc.

           [INFORMATIVE NOTE: this requirement seems to have rather weak
           support.  It's mainly been added so that it can be issue-
           tracked. /mat]

   7.   Practices and Expectations MUST be presented as an information
        service from the sender to be consumed as an added factor to the
        receiver's local policy.  In particular, a Practice or
        Expectation MUST NOT mandate any disposition stance on the
        receiver.

   8.   If the Discovery process would be shortened by publication of a
        "null" practice, the protocol SHOULD provide a mechanism to
        publish such a practice.

           [INFORMATIVE NOTE: there seems to be widespread consensus
           that a "neutral" or "I sign some mail" practice is useless to
           receivers.  However, a null practice may help to cut short
           the policy lookup mechanism if it's published, and if that
           the case it seems worthwhile.  Also, a null policy may have
           some forensic utility, such as gaging the number of domains
           considering/using DKIM for example.]

   9.   There is no requirement that The Protocol publish a Practices of
        any/all third parties that MUST NOT sign on the domain holder's
        behalf.  This should be considered out of scope.





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           [INFORMATIVE NOTE: this is essentially saying that the
           protocol doesn't have to concern itself with being a
           blacklist repository.]

   10.  The Protocol MUST NOT be required to be invoked if a valid first
        party signature is found.

   11.  [PROVISIONAL] A domain holder MUST be able to publish a Practice
        which enumerates the acceptable cryptographic algorithms for
        signatures purportedly from that domain.

           [INFORMATIVE NOTE: this is to counter a bid down attack; some
           comments indicated that this need only be done if the
           algorithm was considered suspect by the receiver; I'm not
           sure that I've captured that nuance correctly]

   12.  Given the considerations in scenario 4, the protocol MUST NOT
        provide a mechanism which impugns the mere existence of third
        party signatures in a message.  A corrolary of this requirement
        is that the protocol MUST NOT in any way tie practices of first
        party signers with third party signers.

           [INFORMATIVE NOTE: the main thrust of this requirement is
           that practices should only be published for that which the
           publisher has control, and should not restate what is
           ultimately the local policy of the receiver.]

5.4.  Extensibility and Forward Compatibilty Requirements

   1.  The Protocol MUST NOT extend to any other than DKIM for email at
       this time.

   2.  The Protocol MUST be able to add new Practices and Expectations
       within the existing discovery/transport/practices in a backward
       compatible fashion.

   3.  [PROVISIONAL] The Protocol MUST be able to extend for new
       protocols signed by DKIM

   4.  [PROVISIONAL] The Protocol MUST be able to extend for protocols
       other than DKIM










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6.  Security Requirements

   1.  Minimize DoS potential: The Protocol for a high-value domain is
       potentially a high-value DoS target, especially since the
       unavailability of The Protocol's record could make unsigned
       messages less suspicious.

   2.  Amplification Attacks: The Protocol MUST NOT make highly
       leveraged amplification or make-work attacks possible.  In
       particular any amplification must be O(1).

   3.  Authenticity: The Protocol MUST have the ability for a domain
       holder to provide The Protocol's data such that a receiver can
       determine that it is authentically from the domain holder with a
       large degree of certainty.  The Protocol may provide means which
       provide less certainty in trade off for ease of deployment.



































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7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.













































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8.  Security Considerations

   This draft defines requirements for a new protocol and the security
   related requirements are defined above.  There is an expectation that
   The Protocol will not always be required to have source
   authentication of the practices information which is noteworthy.













































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9.  Acknowledgements

   still free to good home
















































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10.  References

10.1.  Normative References

   [I-D.ietf-dkim-base]
              Allman, E., "DomainKeys Identified Mail (DKIM)
              Signatures", draft-ietf-dkim-base-04 (work in progress),
              July 2006.

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

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

10.2.  Informative References

   [I-D.ietf-dkim-overview]
              Hansen, T., "DomainKeys Identified Mail (DKIM) Service
              Overview", draft-ietf-dkim-overview-01 (work in progress),
              June 2006.

   [I-D.ietf-dkim-threats]
              Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", draft-ietf-dkim-threats-03 (work
              in progress), May 2006.

























Thomas                   Expires March 19, 2007                [Page 20]


Internet-Draft                DKIM-SSP-REQ                September 2006


Author's Address

   Michael Thomas
   Cisco Systems
   606 Sanchez St
   San Francisco, California  94114
   USA

   Phone: +1-408-525-5386
   Fax:   +1-408-525-5386
   Email: mat@cisco.com








































Thomas                   Expires March 19, 2007                [Page 21]


Internet-Draft                DKIM-SSP-REQ                September 2006


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Thomas                   Expires March 19, 2007                [Page 22]


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