DKIM Working Group                                             M. Thomas
Internet-Draft                                             Cisco Systems
Intended status: Informational                             March 6, 2007
Expires: September 7, 2007

           Requirements for a DKIM Signing Practices Protocol

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   DomainKeys Identified Mail (DKIM) provides a cryptographic mechanism
   for domains to assert responsibility for the messages they handle.  A
   related mechanism will allow an administrator to publish various
   statements about their DKIM signing practices.  This document defines
   requirements for this mechanism.

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5

   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6

   3.  SSP Problem Scenarios  . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Problem Scenario 1: All Mail Signed with DKIM  . . . . . .  7
     3.2.  Problem Scenario 2: Illegitimate Domain Name Use . . . . .  8

   4.  SSP Deployment Scenarios . . . . . . . . . . . . . . . . . . . 10
     4.1.  Deployment Scenario 1: Outsourced Signing  . . . . . . . . 10
     4.2.  Deployment Scenario 2: Determinism in Lookup Mechanism
           and Subdomain Coverage . . . . . . . . . . . . . . . . . . 10
     4.3.  Deployment Scenario 3: Resent Original Mail  . . . . . . . 10
     4.4.  Deployment Scenario 4: Incremental Deployment of
           Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.5.  Deployment Scenario 5: Transport Scenarios . . . . . . . . 11
     4.6.  Deployment Scenario 6: Human Legibility of Practices . . . 12
     4.7.  Deployment Scenario 7: Cryptographic Downgrade Attacks . . 12
     4.8.  Deployment Scenario 8: Extensibility . . . . . . . . . . . 12
     4.9.  Deployment Scenario 9: Security  . . . . . . . . . . . . . 12

   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Discovery Requirements . . . . . . . . . . . . . . . . . . 13
     5.2.  SSP Transport Requirements . . . . . . . . . . . . . . . . 14
     5.3.  Practice and Expectation Requirements  . . . . . . . . . . 14
     5.4.  Extensibility and Forward Compatibility Requirements . . . 17

   6.  Security Requirements  . . . . . . . . . . . . . . . . . . . . 18

   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19

   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20

   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21

   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     10.2. Informative References . . . . . . . . . . . . . . . . . . 22

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   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24

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1.  Introduction

   DomainKeys Identified Mail [I-D.ietf-dkim-base]defines a message
   level signing and verification mechanism for email.  While a DKIM
   signed message speaks for itself, there is ambiguity if a message
   doesn't have a valid first party signature: is this to be expected or
   not?.  For email this is an especially difficult problem since there
   is no expectation of a priori knowledge of a sending domain's
   practices.  This ambiguity can be used to mount a bid down attack
   which is inherent with systems that allow optional authentication
   like email: if a receiver doesn't know otherwise, it should not
   assume that the lack of a valid signature is a priori invalid.  Thus,
   an attacker can take advantage of the ambiguity and simply not sign
   messages.  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 an unsigned piece of email.

   This document defines the requirements for a mechanism that permits
   the publication of Sender Signing Practices (SSP).  The document is
   organized into two main sections: a Problem and Deployment Scenario
   section which describes the problems that SSP is intended to address
   as well as the deployment issues surrounding the base problems.  The
   second section is the Requirements that arise because of those

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2.  Definitions

   o  Domain Holder: the entity that 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 is not required to correspond to
      a third party address such as Sender or Listid, etc.

   o  Practice: a statement according to the [RFC2822].From domain
      holder of externally verifiable behavior in the email messages it
      sends.  A practice should always be true when received by a
      topologically adjacent SMTP.

   o  Expectation: an Expectation combines with a Practice to convey
      what the domain holder considers the likely survivability of the
      Practice for a non-topologically adjacent receiver.

   o  DKIM Signing Complete: a Practice where the domain holder asserts
      that all legitimate mail will be sent with a valid First Party

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3.  SSP Problem Scenarios

   The email world is a diverse place 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

3.1.  Problem Scenario 1: All Mail Signed with DKIM

   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 contains a valid DKIM signature.

   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.
   Thus the receiver is at a disadvantage in that it does not know if it
   should expect mail to be signed from a given domain or not.  This
   knowledge gap leads to a trivially exploitable bid-down attack where
   the attacker merely sends unsigned mail; since the receiver doesn't
   know the practices of the signing domain, it cannot treat the message
   any more harshly for lack of a valid signature.

   An information service which allowed a receiver to query for the
   practices and expectations of the first party domain when no valid
   first party signature is found could be useful in closing this gap.
   A receiver could use this information to treat such questionable mail
   with varying degrees of prejudice.

   Note that for the foreseeable future, unrestricted use patterns of
   mail (eg where users may be members of mailing lists, etc) will
   likely suffer occasional non-malicious signature failure in transit.
   While probably not a large percentage of total traffic, the kind of
   breakage may be a significant concern for those usage patterns.  This
   scenario defines where the sender cannot set any expectation as to
   whether an individual message will arrive intact.

   Even without that expectation, a receiver may be able to take
   advantage of the knowledge that the domain's practice is to sign all
   mail and bias filters against unsigned or damaged in transit mail.
   This information should not be expected to be used in a binary yes/no
   fashion, but instead as a data point among others in a filtering

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

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

   3.  A provides information that it signs all outgoing mail, but
       places no expectation on whether it will arrive with an intact
       first party signature.

   4.  B could use this information to bias its filters such that it
       looks somewhat more suspicious.

3.2.  Problem Scenario 2: Illegitimate Domain Name Use

   A class of mail typified by transactional mail from high value
   domains is the target of phishing attacks.  In particular, many
   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 an enhanced guarantee that mail
   sent in their name should always arrive with the proof that the
   domain holder consented to its transmission.  That is, the message
   should contain a valid first party signature as defined above.

   From a receiver's standpoint, knowing that a domain not only signs
   all of its mail, but places a very high value on the receipt of a
   valid first party signature from that domain is helpful.  Hence a
   receiver can know that the domain not only signs all of its mail, but
   also feels it essential that legitimate mail must have its first
   party signatures survive transit.  A receiver with the knowledge of
   the sender's expectations in hand might choose to process messages
   not conforming to the published practices in a special manner.

      [Informative Note: in terms of a receiving filter, one may choose
      to treat scenario 2 much more harshly than scenario 1; where
      scenario 1 looks odd, scenario 2 looks like something is very

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

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

   3.  A provides information that it signs all outgoing mail, but
       places an expectation that it should arrive with an intact first
       party signature, and that the receiver should be suspicious if it

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       does not.

   4.  B could use this information to bias its filters such that it
       looks much more suspicious.

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4.  SSP Deployment Scenarios

   Given the problems enumerated above for which we'd like SSP to
   provide information to recipients, there are a number of scenarios
   that are not related to the problems that are to be solved, per se,
   but the actual mechanics of implementing/deploying the information
   service that SSP would provide.

4.1.  Deployment Scenario 1: Outsourced 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.  One obvious use scenario is a domain
   holder from 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.

4.2.  Deployment Scenario 2: Determinism in Lookup Mechanism and
      Subdomain Coverage

   SSP's client will generally be deployed on incoming mail streams to
   provide the information as proposed in the problem scenarios.  The
   RFC2822.From address will be used as a basis for the lookup.  More
   precisely the domain part of the first address of the RFC2822.From
   will form the trust basis to fetch the published information.  A
   trivial attack to circumvent finding the published information could
   be mounted by simply using a subdomain of the parent which doesn't
   have published information.  This attack is called the subdomain
   attack: that is, a domain needs to not only publish a policy for a
   given DNS label it controls, but it also may need to protect all
   subdomains of that label as well.  If this characteristic is not met,
   an attacker would need only create a possibly fictitious subdomain
   which was not covered by SSP's information service.  Thus, it would
   be advantageous for The Protocol to not only cover a given domain,
   but all subdomains of that domain as well.

4.3.  Deployment Scenario 3: 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 to Bob's mailing list.
   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, perhaps corresponding to the Sender address.

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   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.  It also
   has the side benefit that remailers that are not friendly to DKIM
   first party signatures (ie, break them) can be potentially assessed
   by the receiver based on the receiver's opinion of the signing
   domains that actually survived.

4.4.  Deployment Scenario 4: 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 given the complexities of the user population, outsourced
   vendors sending on its behalf, etc.  This leaves open an exploit that
   high-value mail such as in Problem Scenario 2 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.

   For example, might be ready to say that 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 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 strong practice, one only has to segregate those cases into
   different subdomains.  For example:
   would publish constrained practices while might publish more permissive

4.5.  Deployment Scenario 5: Transport Scenarios

   Email service provides an any-any mesh of potential connections: all
   that is required is the publication of an MX record and a SMTP
   listener to receive mail.  Thus the use of SSP is likely to fall into
   two main scenarios, the first of which are large, well known domains
   who are in constant contact with one another.  In this case caching
   of records is essential for performance, including the caching of the
   non-existence of records (ie, negative caching).

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   The second main scenario is when a domain exchanges mail with a much
   smaller volume domain.  This scenario can be both perfectly normal as
   with the case of vanity domains, and sadly a vector for those sending
   mail for anti-social reasons.  In this case we'd like the burden to
   SSP querier to be low, since many of the lookups will not provide a
   useful answer.  Likewise, it would be advantageous to have upstream
   caching here as well so that, say, a mailing list exploder on a small
   domain does not result in an explosion of queries back at the root
   server for the small domain.

4.6.  Deployment Scenario 6: Human Legibility of Practices

   While SSP records are likely to be primarily consumed by an
   automaton, for the foreseeable future they are also likely to be
   inspected by hand.  It would be nice to have the practices stated in
   a fashion which is also intuitive to the human inspectors.

4.7.  Deployment Scenario 7: Cryptographic Downgrade Attacks

   There is a downgrade attack possible when a DKIM signature is hashed
   with a previously acceptable but now insecure hash algorithm.  This
   could allow attackers to send their chosen text which is apparently
   signed by the targeted domain.  It would be advantageous for a domain
   to publish what the allowable signing/hashing algorithms are to
   prevent this downgrade attack.

4.8.  Deployment Scenario 8: Extensibility

   While this document pertains only to requirements surrounding DKIM
   signing practices, it would be beneficial for the protocol to be able
   to extend to other protocols.

4.9.  Deployment Scenario 9: Security

   SSP must be able to withstand life in a hostile open internet
   environment.  These include DoS attacks, and especially DoS attacks
   that leverage themselves through amplification inherent in the
   protocol.  In addition, while a useful protocol may be built without
   strong source authentication provided by the information service, a
   path to strong source authentication should be provided by the
   protocol, or underlying protocols.

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5.  Requirements

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

5.1.  Discovery Requirements

   Receivers need a means of obtaining information about a sender's DKIM
   practices.  This requires a means of discovering where the
   information is and what it contains.

   1.  The author is the first-party sender of a message, as specified
       in the [rfc2822].From field.  SSP's information is associated
       with the author's domain name and is published subordinate to
       that domain name.

   2.  In order to limit the cost of its use, any query service
       supplying SSP's information MUST provide a definitive responsive
       within a small, deterministic number of query exchanges.

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

          [Refs: Deployment Scenario 2]

   3.  SSP's publishing mechanism MUST be defined such that it does not
       lead to multiple records of different protocols residing at the
       same location.

          [Informative note: An example is multiple resource record of
          the same type within a common DNS leaf.  Hence, uniquely
          defined leaf names or uniquely defined resource record types
          will ensure unambiguous reference.]

          [Refs: Deployment Scenario 2]

   4.  SSP must be capable of providing coverage for not only the domain
       but all of its subdomains as well.  If all subdomains cannot be
       directly associated with the parent's information, the protocol
       MUST be able to communicate that the domain name is suspicious.
       The process of obtaining the parent domain's practices MUST
       complete in a deterministic number of steps, preferably few.  In
       widening the scope to have the possibility of all subdomains
       inherit the parent practice, a number of algorithms could be

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       employed -- all seemingly with their own set of engineering
       tradeoffs.  A common theme in the production of this document was
       that the number of lookups, on average should be small, and that
       the discovery process should always be bound to some small finite
       number of queries.

          [Refs: Deployment Scenario 2

5.2.  SSP Transport Requirements

   The publication and query mechanism will operate over the Internet
   message exchange.  This lower layer service must exhibit basic

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

          [Refs: Deployment Scenario 5, 8]

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

          [Refs: Deployment Scenario 5]

   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.

          [Refs: Deployment Scenario 5]

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

          [Refs: Deployment Scenario 5, 8]

5.3.  Practice and Expectation Requirements

   As stated in the definitions a Practice is a statement according to
   the [RFC2822].From domain holder of externally verifiable behavior in
   the email messages it sends.  As a silly example, a Practice might be
   defined that all email messages will contain an X-Silly header.
   Since there is a possibility of alteration between what a sender
   sends and a receiver examines, an Expectation combines with a
   Practice to convey what the domain holder considers the likely
   outcome of the survivability of the Practice for at a receiver.  For
   example, a Practice that X-Silly is present when it is sent from the

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   domain, and an Expectation that it will remain present for receivers
   whether topologically adjacent or not.

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

           [Refs: Problem Scenario 1,2]

   2.   SSP MUST provide a concise linkage between the [RFC2822].From
        and the identity in the DKIM i= tag, or its default if it is
        missing in the signature.  That is, SSP MUST precisely define
        the semantics of what qualifies as a First Party Signature.

           [Refs: Problem Scenario 1,2]

   3.   SSP MUST be able to publish a Practice that the domain's signing
        behavior is "DKIM Signing Complete".  That is, all messages were
        transmitted with a valid first party signature.

           [Refs: Problem Scenario 1]

   4.   SSP MUST be able to publish an Expectation that a verifiable
        first party DKIM Signature should be expected on receipt of a

           [Refs: Problem Scenario 2]

   5.   Practices and Expectations MUST be presented in SSP syntax using
        as intuitive a descriptor as possible.  For example, p=? would
        be better represented as p=unknown.

           [Refs: Deployment Scenario 6]

   6.   Because DKIM uses DNS to store selectors, there is always the
        ability for a domain holder to delegate all or parts of the
        _domainkey subdomain to an affiliated party of the domain
        holder's choosing.  That is, the domain holder may be able to
        set a NS record for 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 third parties.  For example, a domain
        holder could delegate only to a
        third party to constrain the third party to only be able to
        produce valid signatures in the subdomain.
        Last, a domain holder can even use CNAME's to delegate
        individual leaf nodes.  Thus SSP need not invent a different

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        means of allowing affiliated parties to sign on a domain's
        behalf at this time.

   7.   Practices and Expectations MUST be presented as an information
        service from the signing domain 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.

           [Refs: Problem Scenario 1, 2, 3]

   8.   There is no requirement that SSP 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.

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

           [Refs: Problem Scenario 1-2]

   9.   SSP MUST NOT be required to be invoked if a valid first party
        signature is found.

           [Refs: Deployment Scenario 2]

   10.  [PROVISIONAL] The signing policy statement MUST be capable of
        fully describing a signing practice in which multiple signatures
        are always provided such that the policy is of utility to any
        verifier is capable of verifying any of the signatures that are
        always provided.  Such a mechanism MUST NOT:

        *  Require the verifier to perform any additional DNS lookups

        *  Require duplication of configuration data

        *  In particular not require the policy record to provide for
           the description of any cryptographic or cannonicalization

           INFORMATIVE NOTE: The ability to specify multiple signatures
           is necessary in order to permit orderly transitions to new
           cryptographic and canonicalization algorithms.  Unless the
           policy language is not sufficiently expressive to allow the
           signer to describe the actual signature practice in this case
           there is an opportunity for an attacker to exploit the fact
           that there are verifiers that do not yet support the new

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           [Refs: Deployment Scenario 7]

   11.  SSP MUST NOT provide a mechanism which impugns the existence of
        non-first party signatures in a message.  A corollary of this
        requirement is that the protocol MUST NOT link practices of
        first party signers with the practices of 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 meddle in what is
           ultimately the local policy of the receiver.]

           [Refs: Deployment Scenario 3]

5.4.  Extensibility and Forward Compatibility Requirements

   1.  SSP MUST NOT extend to any other protocol than DKIM for email at
       this time.  SSP SHOULD be able to extend for protocols other than

          [Refs: Deployment Scenario 8]

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

          [Refs: Deployment Scenario 8]

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6.  Security Requirements

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

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

          [Author's question: is it really O(1)? or O(n)?]

          [Refs: Deployment Scenario 9]

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

          [Refs: Deployment Scenario 9]

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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

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8.  Security Considerations

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

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9.  Acknowledgments

   Due to flipping in the market and raising interest rates, this home
   is no longer free.  Dave Crocker and Jim Fenton helped me raise the
   walls on this document and are accorded a room at the inn.  The inn
   is not yet full, however.

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10.  References

10.1.  Normative References

              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

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

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

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Author's Address

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

   Phone: +1-408-525-5386
   Fax:   +1-408-525-5386

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