DomainKeys Identified Mail (DKIM) Authorized Third-Party Signatures
draft-kucherawy-dkim-atps-16

Individual submission                                       M. Kucherawy
Internet-Draft                                           Cloudmark, Inc.
Intended status: Experimental                            January 5, 2012
Expires: July 8, 2012

                  DKIM Authorized Third-Party Signers
                      draft-kucherawy-dkim-atps-16

Abstract

   This experimental specification proposes a modification to Domain
   Keys Identified Mail (DKIM) allowing advertisement of third-party
   signature authorizations that are to be interpreted as equivalent to
   a signature added by the administrative domain of the message's
   author.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 8, 2012.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Keywords . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  E-Mail Architecture Terminology  . . . . . . . . . . . . .  3
   3.  Roles and Scope  . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Queries and Replies  . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Hash Selection . . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  Extension to DKIM  . . . . . . . . . . . . . . . . . . . .  5
     4.3.  ATPS Query Details . . . . . . . . . . . . . . . . . . . .  5
     4.4.  ATPS Reply Details . . . . . . . . . . . . . . . . . . . .  7
   5.  Interpretation . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Relationship to ADSP . . . . . . . . . . . . . . . . . . . . .  8
   7.  Experiment Process . . . . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  ATPS Tag Registry  . . . . . . . . . . . . . . . . . . . .  9
     8.2.  Email Authentication Method Name Registry Update . . . . . 10
     8.3.  Email Authentication Result Name Registry Update . . . . . 10
     8.4.  DKIM-Signature Tag Specification Registry  . . . . . . . . 11
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     9.1.  Hash Selection . . . . . . . . . . . . . . . . . . . . . . 12
     9.2.  False Privacy  . . . . . . . . . . . . . . . . . . . . . . 12
     9.3.  Transient Security Failures  . . . . . . . . . . . . . . . 12
     9.4.  Load on the DNS  . . . . . . . . . . . . . . . . . . . . . 13
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Example Query and Reply . . . . . . . . . . . . . . . 14
   Appendix B.  Choice of DNS RR Type . . . . . . . . . . . . . . . . 15
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   [DKIM] defines a mechanism for transparent domain-level signing of
   messages for the purpose of declaring that a particular
   Administrative Mail Domain (ADMD) takes some responsibility for a
   message.

   DKIM, however, deliberately makes no binding between the DNS domain
   of the signer and any other identity found in the message.  Despite
   this, there is an automatic human perception that an author domain
   signature (one for which the RFC5322.From domain matches the DNS
   domain of the signer) is more valuable or trustworthy than any other.

   To enable a third party to apply DKIM signatures to messages, the
   DKIM specification suggests delegation to a third party of either
   subdomains or private keys, so that the third party can add DKIM
   signatures that appear to have been added by the Author ADMD.  Absent
   is a protocol by which an Author ADMD can announce that messages
   bearing specific valid DKIM signatures on its mail, which are added
   by other ADMDs, are to be treated as if they were signed by the
   Author ADMD itself.  This memo presents an experimental mechanism for
   doing so, called Authorized Third-Party Signers (ATPS).

   ATPS augments the semantics of DKIM by providing to the verifier
   multiple identifiers rather than one.  Specifically, it validates the
   identifier found in the DKIM signature, and then provides the
   RFC5322.From domain for evaluation.

   This memo also registers, per [AUTHRES], the means to indicate to
   agents downstream of the Verifier that a third-party signature
   verification occurred.

2.  Definitions

2.1.  Keywords

   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 [KEYWORDS].

2.2.  E-Mail Architecture Terminology

   Readers are advised to be familiar with the material and terminology
   discussed in [MAIL] and [EMAIL-ARCH].

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3.  Roles and Scope

   The context of this protocol involves the following roles:

   o  ADministrative Mail Domain (ADMD)s, whose DNS domain name(s)
      appear in the RFC5322.From field of a [MAIL] message;

   o  ATPS Signers, which apply [DKIM] signatures using their own
      domains, but on behalf of the message Author's ADMD; and

   o  the Verifier, who implements the signature validation procedures
      described in [DKIM].

   An ADMD implements this protocol if it wishes to announce that a
   signature from any in a set of specified DNS domains is to be
   considered equivalent to one from the ADMD itself.  One might, for
   example, wish to delegate signing authority for its DNS domain to an
   approved messaging service provider without doing the work of key
   transfer described in Appendix B.1.1 of [DKIM].  An authorized ATPS
   Signer makes a claim of this relationship via new tags in the DKIM
   signature, and the ADMD confirms this claim by publishing a specific
   TXT record in its DNS.

   A Verifier implements this protocol if it wishes to ensure that a
   message bears one or more signatures from sources authorized to sign
   mail on behalf of the ADMD, and identify for special treatment mail
   that meets (or does not meet) that criterion.  It will do so by
   treating the signer's authorization on behalf of the author's ADMD to
   mean that the signer's signature is equivalent to one affixed by the
   author's ADMD.

4.  Queries and Replies

   This section describes in detail the queries issued, the replies
   received, and how they should be interpreted and applied.

4.1.  Hash Selection

   The author's ADMD will indicate authorization of a third party to
   sign its mail via the presence of a DNS TXT record that contains an
   encoding of the third party's DNS domain name.  There are two
   supported methods for doing so, one that involves a plain copy of the
   third party's DNS domain name, and one that involves an encoded
   version of the name.  The encoding mechanism is provided so that any
   domain name can be added to the DNS in a fixed length, so that longer
   third-party domain names are not excluded from participation because
   of the overall length limit on a DNS query.

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   If selected, the encoding mechanism requires constructing a digest of
   the third party's DNS domain name.  The author ADMD MUST select a
   digest ("hash") method currently supported by DKIM (see Section 7.7
   of [DKIM]), and this selection needs to be communicated to the ATPS
   Signer as it is used in generation of its the third party signatures.

   Where the encoding mechanism is not used, the ATPS Signer MUST use a
   hash name of "none".

   The full DNS mechanism is specified in Section 4.3.

4.2.  Extension to DKIM

   [DKIM] signatures contain a "tag=value" sequence.  This protocol will
   add additional tags called "atps" and "atpsh".

   When the ATPS Signer generates a DKIM signature for another ADMD, it
   MUST put its own domain in the signature's "d" tag, and include an
   "atps" tag that has as its value the domain name of the ADMD on whose
   behalf it is signing.

   The tag name that carries the name of the selected hash algorithm is
   "atpsh".  This tag MUST also be included, as it is required as part
   of the algorithm that will be enacted by the Verifier.

   The formal syntax definition, per [ABNF]:

      dkim-atps-tag = %x61.74.70.73 *WSP "=" *WSP domain-name

      dkim-atpsh-tag = %x61.74.70.73.68 *WSP "=" *WSP
                       ( "none" / key-h-tag-alg )

   "domain-name" and "key-h-tag-alg" are defined in [DKIM].  Note that
   according to [DKIM], internationalized domain names are to be encoded
   as A-labels, as described in Section 2.3 of [IDNA].

   The registration for these tags can be found in Section 8.

4.3.  ATPS Query Details

   When a [DKIM] signature including an "atps" tag is successfully
   verified, and is considered acceptable to the Verifier according to
   any local policy requirements (which are not discussed here or in
   [DKIM]), the Verifier compares the domain name in the value of that
   tag with the one found in the RFC5322.From field of the message.  The
   match MUST be done in a case-insensitive manner.

   If they do not match, the "atps" tag MUST be ignored.

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   If they do match, the Verifier issues a DNS TXT query, as specified
   below, looking for confirmation by the Author ADMD that the ATPS
   Signer is authorized by that ADMD to sign mail on its behalf.  Where
   multiple DKIM signatures are present including valid "atps" tags,
   these queries MAY be done in any order or MAY be done in parallel.

   Where the RFC5322.From field contains multiple addresses, this
   process SHOULD be applied if the "atps" tag's value matches any of
   the domains found in that field.  These MAY be done in any order.

   Note that the algorithm uses hashing, but this is not a security
   mechanism.  See Section 9.2 for discussion.

   The name for the query is constructed as follows:

   1.  Select the hash algorithm from the "atpsh" tag in the signature.
       If the hash algorithm specified does not appear in the list
       registered with IANA as one valid for use with DKIM (see Section
       7.7 of [DKIM]), and is not the reserved name "none" as described
       above, abort the query.

   2.  Extract the value of the "d=" tag from the signature.

   3.  Convert any upper-case characters in that string to their lower-
       case equivalents.

   4.  If the selected hash algorithm is not "none", apply the following
       additional steps:

       A.  Feed the resulting string to the selected hash algorithm.

       B.  Convert the output of the hash to a string of printable ASCII
           characters by applying base32 encoding as defined in Section
           6 of [BASE32].  The base32 encoding is used because its
           output is restricted to characters that are legal for use in
           labels in the DNS, and evaluates the same way in the DNS
           whether encoded using uppercase or lowercase characters.

   5.  Append the string "._atps."

   6.  Append the domain name found in the "atps" tag of the validated
       signature.

   The query's formal syntax definition, per [ABNF]:

     atps-query = ( 1*63BASE32 / domain-name )
                  %x2e.5f.61.74.70.73.2e domain-name

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     BASE32 = ( ALPHA / %x32-37 )

   The width limit of 63 on the base32 encoding is based on the maximum
   label limit found in Section 2.3.4 of [DNS].

   See Appendix A for an example of a query construction.

4.4.  ATPS Reply Details

   In the descriptions below, the label NOERROR symbolizes DNS response
   code ("rcode") 0, and NXDOMAIN represents rcode 3.  See Section 4.1.1
   of [DNS] for further details.

   At this time, only three possibilities need to be identified in this
   specification:

   o  An answer is returned (i.e.  [DNS] reply code NOERROR with at
      least one answer) containing a valid ATPS reply.  In this case,
      the protocol has been satisfied and the Verifier can conclude that
      the signing domain is authorized by the ADMD to sign its mail.
      Further queries SHOULD NOT be initiated.

   o  No answer is returned (i.e.  [DNS] reply code NXDOMAIN, or NOERROR
      with no answers), or one or more answers have been returned as
      described above but none contain a valid ATPS reply.  In this
      case, the Signer has not been authorized to act as a third-party
      signer for this ADMD, and thus the Verifier MUST continue to the
      next query, if any.

   o  An error is returned (i.e. any other [DNS] reply code).  It is no
      longer possible to determine whether or not this message satisfies
      the ADMD's list of authorized third-party signers.  The Verifier
      SHOULD stop processing and defer the message for later processing,
      such as requesting temporary failure code from the MTA.

   If all queries are completed and return either NXDOMAIN or NOERROR
   with no answers, then the Signer was not authorized by the ADMD.

   A valid ATPS reply consists of a sequence of tag-value pairs as
   described in Section 3.2 of [DKIM].  The following tags and values
   are currently supported in ATPS records:

   d: Domain (plain-text; RECOMMENDED) This tag includes a plain-text
      copy of the DNS domain being authorized as an ATPS Signer.  This
      is included to assist with collision detections; for example, if
      the base32 encoding of this name is not the same as the base32
      portion of the query, or more simply if this name is not the same
      as that found in the "atps" tag, a hash collision could have

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      occurred.  Its use where no name hashing has occurred is
      redundant.  ABNF:

      atps-d-tag = %x64 [FWS] "=" [FWS] domain-name
                 ; FWS is defined in [DKIM]

   v: Version (plain-text; REQUIRED) This tag defines the version of
      this specification that applies to the ATPS record.  The record
      MUST be ignored if the value is not "ATPS1".  ABNF:

      atps-v-tag = %x76 [FWS] "=" [FWS] %x41.54.50.53.31
                 ; FWS is defined in [DKIM]

5.  Interpretation

   For each DKIM signature that verifies (see Section 6 of [DKIM]), if a
   Verifier succeeds in confirming that the Author's ADMD authorized the
   ATPS Signer using this protocol, then the Verifier SHOULD evaluate
   the message as though it contained a valid signature from the
   Author's ADMD.  It MAY also independently evaluate the ATPS Signer
   when determining message disposition.

   This assertion is based on the fact that the ADMD explicitly endorsed
   the ATPS Signer.  Therefore, a module assessing reputation that is
   based on DKIM signature verification SHOULD apply the reputation of
   the Author's ADMD domain instead of, or in addition to, that of the
   ATPS Signer domain.

6.  Relationship to ADSP

   [ADSP] defined a protocol by which the owner of an Author Domain can
   advertise a request to message receivers that messages bearing no
   valid author signature be treated with suspicion or even discarded.

   A Verifier implementing both ADSP and ATPS MUST test ATPS first.  If
   ATPS indicates a valid delegation, the verifier MUST act, with
   respect to ADSP, as though the message has a valid Author Domain
   Signature (because that's what the delegation means), and no ADSP
   test is required.

7.  Experiment Process

   The working group that developed DKIM considered a third-party
   mechanism such as this one to be controversial, in terms of need and
   practicality, and decided that an alternative mechanism was suffient.
   However, this was not based on actual experience as there is no
   specific history on this question.  Thus, this experiment was
   devised.

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   The experimental protocol described here has been implemented as an
   extension to DKIM in two software products, one of which is open
   source and seeing increasingly wide use.  It is included there to
   allow customers of those systems to make use of it if they believe
   such third party assertions are useful to the overall DKIM mechanism.
   Further adoption as part of the experiment is welcome and encouraged.

   Use of the protocol and anecdotes of how it affects the overall DKIM
   experience will be collected by those implementers and the author of
   this memo.  Those participating in the experiment are also advised to
   observe and report the impact of what is discussed in Section 9.4,
   especially with respect to MTA latency that may be introduced.

   If the response is substantial and positive, advancement along the
   Standards Track might be warranted.

8.  IANA Considerations

   This section enumerates requested IANA actions.

8.1.  ATPS Tag Registry

   IANA is directed to create an Authorized Third Party Signature (ATPS)
   Tag Registry, under the DomainKeys Identified Mail (DKIM) Parameters
   group, to enumerate the tags that are valid for use in ATPS records.

   New registrations or updates MUST be published in accordance with the
   "Specification Required" guidelines described in [IANA].  Such
   registry changes MUST contain the following information:

   1.  Name of the tag being registered or updated

   2.  The document where the specification is created or updated

   3.  The status of the tag, one of "current" (tag is in current use),
       "deprecated" (tag is in current use but its use is discouraged),
       or "historic" (tag is no longer in use)

   The registry's sole initial entry is:

       +-----+--------------+---------+
       | Tag | Specified In | Status  |
       +-----+--------------+---------+
       |  v  | [this memo]  | current |
       +-----+--------------+---------+

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8.2.  Email Authentication Method Name Registry Update

   The following is to be added to the Email Authentication Methods
   Registry (in the Email Authentication Parameters group) established
   by [AUTHRES] as per [IANA]:

   Method:  dkim-atps

   Defined In:  [THIS MEMO]

   ptype:  header

   property:  from

   value:  contents of the [MAIL] From: header field, with comments
      removed

8.3.  Email Authentication Result Name Registry Update

   The following are to be added to the Email Authentication Result
   Names Registry (in the Email Authentication Parameters group)
   established by [AUTHRES] as per [IANA]:

   Code:  none

   Existing/New Code:  existing

   Defined In:  [AUTHRES]

   Auth Method:  dkim-atps

   Meaning:  No valid DKIM signatures were found on the message bearing
      "atps" tags.

   Code:  pass

   Existing/New Code:  existing

   Defined In:  [AUTHRES]

   Auth Method:  dkim-atps

   Meaning:  An ATPS evaluation was performed and a valid signature from
      an authorized third-party was found on the message.

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   Code:  fail

   Existing/New Code:  existing

   Defined In:  [AUTHRES]

   Auth Method:  dkim-atps

   Meaning:  All valid DKIM signatures bearing an "atps" tag either did
      not reference a domain name found in the RFC5322.From field, or
      the ATPS test(s) performed failed to confirm a third-party
      authorization.

   Code:  temperror

   Existing/New Code:  existing

   Defined In:  [AUTHRES]

   Auth Method:  dkim-atps

   Meaning:  An ATPS evaluation could not be completed due to some error
      that is likely transient in nature, such as a temporary DNS error.
      A later attempt might produce a final result.

   Code:  permerror

   Existing/New Code:  existing

   Defined In:  [AUTHRES]

   Auth Method:  dkim-atps

   Meaning:  An ATPS evaluation could not be completed due to some error
      that is not likely transient in nature, such as a permanent DNS
      error.  A later attempt is unlikely to produce a final result.

8.4.  DKIM-Signature Tag Specification Registry

   The following are to be added to the DKIM-Signature Tag Speficication
   Registry (in the DomainKeys Identifie Mail (DKIM) Parameters group)
   established by [DKIM] as per [IANA]:

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      +-------+-------------+---------+
      | TYPE  | REFERENCE   | STATUS  |
      +-------+-------------+---------+
      | atps  | [THIS MEMO] | current |
      +-------+-------------+---------+
      | atpsh | [THIS MEMO] | current |
      +-------+-------------+---------+

9.  Security Considerations

   This section discusses potential security issues related to this
   experimental protocol.

9.1.  Hash Selection

   The selection of the hash algorithm to be used (see Section 4.1) has
   security implications, as weaker algorithms have more risk of
   collision, meaning a second DNS domain name could in theory be
   constructed to appear to have been authorized by the Author ADMD.

   At the time of publication of [DKIM], use of SHA256 was preferred
   over SHA1 for this reason, though support for both has been
   maintained.  See Section 3.3 of [DKIM] for additional discussion.

9.2.  False Privacy

   The fact that the authorized third-party domain name is hashed and
   then encoded with base32 might give some the false sense that the
   relationship between the two parties is somehow protected.  This is
   not the case.  Indeed, the very purpose of this protocol is to make
   it possible for such relationships to be discovered, so such an
   obscuration would make that process more difficult without a shared
   secret delivered out-of-band to message verifiers (which also adds
   further complexity.  Rather, the hash and encode steps are done
   merely to convert any third-party domain name to a fixed width in the
   construction of the DNS query.

9.3.  Transient Security Failures

   Approving a third party signer exposes the ADMD to the risk that the
   third party signer becomes compromised and then begins to sign
   malicious or nuisance messages on behalf of the ADMD.  This can
   obviously reflect negatively on the ADMD, and the impact of this can
   become more severe as automated domain reputation systems are
   developed and deployed.  Thorough vetting and monitoring practices by
   ADMDs of third party signers will likely need to become the norm.

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9.4.  Load on the DNS

   A Verifier participating in DKIM, ADSP and ATPS will now issue a
   number of TXT queries to the DNS equal to as many as one (for the
   ADSP query) plus the number of signatures on the message (one for
   each key that is to be verified) plus the number of signatures that
   validated which also bear an "atps" tag.  This is in addition to any
   PTR and A queries the MTA might issue at the time the actual message
   relaying or delivery is initiated.  Obviously this can be burdensome
   on the DNS at both ends, and waiting for that number of queries to
   return when they are issued in parallel could trigger timeouts in the
   MTA.

   An alternative to this that has not yet been explored is the storage
   of the ATPS data at a specific URL tied to the Author's domain name.
   This would alleviate pressure on the DNS at the expense of requiring
   the ADMD to operate a web server (which has its own security
   implications) and the addition of the establishment of a TCP
   connection.  Moreover, the Verifier would be well advised to
   implement caching of this data to prevent ATPS from being used as a
   denial-of-service vector.

   See Section 8.5 of [DKIM] for further discussion of DNS-related
   issues.

10.  References

10.1.  Normative References

   [ABNF]        Crocker, D. and P. Overell, "Augmented BNF for Syntax
                 Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [AUTHRES]     Kucherawy, M., "Message Header Field for Indicating
                 Message Authentication Status", RFC 5451, April 2009.

   [BASE32]      Josefsson, S., "The Base16, Base32, and Base64 Data
                 Encodings", RFC 4648, October 2006.

   [DKIM]        Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy,
                 Ed., "DomainKeys Identified Mail (DKIM) Signatures",
                 RFC 6376, September 2011.

   [DNS]         Mockapetris, P., "Domain names - implementation and
                 specification", STD 13, RFC 1035, November 1987.

   [EMAIL-ARCH]  Crocker, D., "Internet Mail Architecture", RFC 5598,
                 October 2008.

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

   [MAIL]        Resnick, P., Ed., "Internet Message Format", RFC 5322,
                 October 2008.

10.2.  Informative References

   [ADSP]        Allman, E., Fenton, J., Delany, M., and J. Levine,
                 "DomainKeys Identified Mail (DKIM) Author Domain
                 Signing Practices (ADSP)", RFC 5617, August 2009.

   [IANA]        Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26,
                 RFC 5226, May 2008.

   [IDNA]        Klensin, J., "Internationalized Domain Names for
                 Applications (IDNA): Definitions and Document
                 Framework", RFC 5890, August 2010.

Appendix A.  Example Query and Reply

   This section presents an example of the use of this protocol to query
   for a third-party authorization and discusses the interpretation of
   the result.

   Presume a message for which the RFC5322.From domain is "example.com",
   and it bears two valid signatures, from "one.example.net" and from
   "two.example.net", each with an "atps" tag whose value is
   "example.com", and no "atpsh" tag is present in either.  The
   following actions are taken:

   1.  A SHA1 hash of "one.example.net" is computed and then converted
       to printable form using base32 encoding, resulting in the string
       "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6".

   2.  A TXT query is issued to
       "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6._atps.example.com".

   3.  If a valid reply arrives, the algorithm stops with [AUTHRES]
       result "pass".  If a DNS error code other than NXDOMAIN is
       returned, the algorithm stops with a result of "temperror" or
       "permerror" as appropriate.

   4.  A SHA1 hash of "two.example.net" is computed and then converted
       to printable form using base32 encoding, resulting in the string
       "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX".

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   5.  A TXT query is issued to
       "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX._atps.example.com".

   6.  If a valid reply arrives, the algorithm stops with [AUTHRES]
       result "pass".  If a DNS error code other than NXDOMAIN is
       returned, the algorithm stops with a result of "temperror" or
       "permerror" as appropriate.

   7.  As there are no valid signatures left to test, the algorithm
       stops with an "unknown" result.

Appendix B.  Choice of DNS RR Type

   It was proposed that this work appear within the DNS under a new
   Resource Record (RR) Type.  Although this is possibly an appropriate
   thing to do, consideration was also given to the fact that major
   portions of DKIM already use an ASCII-based "tag=value" syntax, and
   store key and ADSP data in the DNS using TXT resource records.  To
   enable re-use of existing DKIM code, it was decided to re-use the TXT
   message scheme.

Appendix C.  Acknowledgements

   The author wishes to acknowledge Dave Crocker, Frank Ellermann, Mark
   Martinec and Phil Pennock for their review and constructive criticism
   of this proposal.

   The author also wishes to acknowledge Doug Otis and Daniel Black for
   their original draft upon which this work was based.

Author's Address

   Murray S. Kucherawy
   Cloudmark, Inc.
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Phone: +1 415 946 3800
   EMail: msk@cloudmark.com

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