<pre>Network Working Group                                          E. Allman
Request for Comments: 4871                                Sendmail, Inc.
Obsoletes: <a href="./rfc4870">4870</a>                                                J. Callas
Category: Standards Track                                PGP Corporation
                                                               M. Delany
                                                               M. Libbey
                                                              Yahoo! Inc
                                                               J. Fenton
                                                               M. Thomas
                                                     Cisco Systems, Inc.
                                                                May 2007


              <span class="h1">DomainKeys Identified Mail (DKIM) Signatures</span>

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   DomainKeys Identified Mail (DKIM) defines a domain-level
   authentication framework for email using public-key cryptography and
   key server technology to permit verification of the source and
   contents of messages by either Mail Transfer Agents (MTAs) or Mail
   User Agents (MUAs).  The ultimate goal of this framework is to permit
   a signing domain to assert responsibility for a message, thus
   protecting message signer identity and the integrity of the messages
   they convey while retaining the functionality of Internet email as it
   is known today.  Protection of email identity may assist in the
   global control of "spam" and "phishing".












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Table of Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-4">4</a>
     <a href="#section-1.1">1.1</a>.  Signing Identity . . . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
     <a href="#section-1.2">1.2</a>.  Scalability  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
     <a href="#section-1.3">1.3</a>.  Simple Key Management  . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
   <a href="#section-2">2</a>.  Terminology and Definitions  . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
     <a href="#section-2.1">2.1</a>.  Signers  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-2.2">2.2</a>.  Verifiers  . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-2.3">2.3</a>.  Whitespace . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-2.4">2.4</a>.  Common ABNF Tokens . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-2.5">2.5</a>.  Imported ABNF Tokens . . . . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
     <a href="#section-2.6">2.6</a>.  DKIM-Quoted-Printable  . . . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
   <a href="#section-3">3</a>.  Protocol Elements  . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
     <a href="#section-3.1">3.1</a>.  Selectors  . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-8">8</a>
     <a href="#section-3.2">3.2</a>.  Tag=Value Lists  . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
     <a href="#section-3.3">3.3</a>.  Signing and Verification Algorithms  . . . . . . . . . . . <a href="#page-11">11</a>
     <a href="#section-3.4">3.4</a>.  Canonicalization . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
     <a href="#section-3.5">3.5</a>.  The DKIM-Signature Header Field  . . . . . . . . . . . . . <a href="#page-17">17</a>
     <a href="#section-3.6">3.6</a>.  Key Management and Representation  . . . . . . . . . . . . <a href="#page-25">25</a>
     <a href="#section-3.7">3.7</a>.  Computing the Message Hashes . . . . . . . . . . . . . . . <a href="#page-29">29</a>
     <a href="#section-3.8">3.8</a>.  Signing by Parent Domains  . . . . . . . . . . . . . . . . <a href="#page-31">31</a>
   <a href="#section-4">4</a>.  Semantics of Multiple Signatures . . . . . . . . . . . . . . . <a href="#page-32">32</a>
     <a href="#section-4.1">4.1</a>.  Example Scenarios  . . . . . . . . . . . . . . . . . . . . <a href="#page-32">32</a>
     <a href="#section-4.2">4.2</a>.  Interpretation . . . . . . . . . . . . . . . . . . . . . . <a href="#page-33">33</a>
   <a href="#section-5">5</a>.  Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     5.1.  Determine Whether the Email Should Be Signed and by
           Whom . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     5.2.  Select a Private Key and Corresponding Selector
           Information  . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
     <a href="#section-5.3">5.3</a>.  Normalize the Message to Prevent Transport Conversions . . <a href="#page-35">35</a>
     <a href="#section-5.4">5.4</a>.  Determine the Header Fields to Sign  . . . . . . . . . . . <a href="#page-36">36</a>
     <a href="#section-5.5">5.5</a>.  Recommended Signature Content  . . . . . . . . . . . . . . <a href="#page-38">38</a>
     <a href="#section-5.6">5.6</a>.  Compute the Message Hash and Signature . . . . . . . . . . <a href="#page-39">39</a>
     <a href="#section-5.7">5.7</a>.  Insert the DKIM-Signature Header Field . . . . . . . . . . <a href="#page-40">40</a>
   <a href="#section-6">6</a>.  Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-40">40</a>
     <a href="#section-6.1">6.1</a>.  Extract Signatures from the Message  . . . . . . . . . . . <a href="#page-41">41</a>
     <a href="#section-6.2">6.2</a>.  Communicate Verification Results . . . . . . . . . . . . . <a href="#page-46">46</a>
     <a href="#section-6.3">6.3</a>.  Interpret Results/Apply Local Policy . . . . . . . . . . . <a href="#page-47">47</a>
   <a href="#section-7">7</a>.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . <a href="#page-48">48</a>
     <a href="#section-7.1">7.1</a>.  DKIM-Signature Tag Specifications  . . . . . . . . . . . . <a href="#page-48">48</a>
     <a href="#section-7.2">7.2</a>.  DKIM-Signature Query Method Registry . . . . . . . . . . . <a href="#page-49">49</a>
     <a href="#section-7.3">7.3</a>.  DKIM-Signature Canonicalization Registry . . . . . . . . . <a href="#page-49">49</a>
     <a href="#section-7.4">7.4</a>.  _domainkey DNS TXT Record Tag Specifications . . . . . . . <a href="#page-50">50</a>
     <a href="#section-7.5">7.5</a>.  DKIM Key Type Registry . . . . . . . . . . . . . . . . . . <a href="#page-50">50</a>
     <a href="#section-7.6">7.6</a>.  DKIM Hash Algorithms Registry  . . . . . . . . . . . . . . <a href="#page-51">51</a>
     <a href="#section-7.7">7.7</a>.  DKIM Service Types Registry  . . . . . . . . . . . . . . . <a href="#page-51">51</a>
     <a href="#section-7.8">7.8</a>.  DKIM Selector Flags Registry . . . . . . . . . . . . . . . <a href="#page-52">52</a>



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     <a href="#section-7.9">7.9</a>.  DKIM-Signature Header Field  . . . . . . . . . . . . . . . <a href="#page-52">52</a>
   <a href="#section-8">8</a>.  Security Considerations  . . . . . . . . . . . . . . . . . . . <a href="#page-52">52</a>
     <a href="#section-8.1">8.1</a>.  Misuse of Body Length Limits ("l=" Tag)  . . . . . . . . . <a href="#page-52">52</a>
     <a href="#section-8.2">8.2</a>.  Misappropriated Private Key  . . . . . . . . . . . . . . . <a href="#page-53">53</a>
     <a href="#section-8.3">8.3</a>.  Key Server Denial-of-Service Attacks . . . . . . . . . . . <a href="#page-54">54</a>
     <a href="#section-8.4">8.4</a>.  Attacks Against the DNS  . . . . . . . . . . . . . . . . . <a href="#page-54">54</a>
     <a href="#section-8.5">8.5</a>.  Replay Attacks . . . . . . . . . . . . . . . . . . . . . . <a href="#page-55">55</a>
     <a href="#section-8.6">8.6</a>.  Limits on Revoking Keys  . . . . . . . . . . . . . . . . . <a href="#page-55">55</a>
     <a href="#section-8.7">8.7</a>.  Intentionally Malformed Key Records  . . . . . . . . . . . <a href="#page-56">56</a>
     <a href="#section-8.8">8.8</a>.  Intentionally Malformed DKIM-Signature Header Fields . . . <a href="#page-56">56</a>
     <a href="#section-8.9">8.9</a>.  Information Leakage  . . . . . . . . . . . . . . . . . . . <a href="#page-56">56</a>
     <a href="#section-8.10">8.10</a>. Remote Timing Attacks  . . . . . . . . . . . . . . . . . . <a href="#page-56">56</a>
     <a href="#section-8.11">8.11</a>. Reordered Header Fields  . . . . . . . . . . . . . . . . . <a href="#page-56">56</a>
     <a href="#section-8.12">8.12</a>. RSA Attacks  . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-56">56</a>
     <a href="#section-8.13">8.13</a>. Inappropriate Signing by Parent Domains  . . . . . . . . . <a href="#page-57">57</a>
   <a href="#section-9">9</a>.  References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-57">57</a>
     <a href="#section-9.1">9.1</a>.  Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-57">57</a>
     <a href="#section-9.2">9.2</a>.  Informative References . . . . . . . . . . . . . . . . . . <a href="#page-58">58</a>
   <a href="#appendix-A">Appendix A</a>.  Example of Use (INFORMATIVE)  . . . . . . . . . . . . <a href="#page-60">60</a>
     <a href="#appendix-A.1">A.1</a>.  The user composes an email . . . . . . . . . . . . . . . . <a href="#page-60">60</a>
     <a href="#appendix-A.2">A.2</a>.  The email is signed  . . . . . . . . . . . . . . . . . . . <a href="#page-61">61</a>
     <a href="#appendix-A.3">A.3</a>.  The email signature is verified  . . . . . . . . . . . . . <a href="#page-61">61</a>
   <a href="#appendix-B">Appendix B</a>.  Usage Examples (INFORMATIVE)  . . . . . . . . . . . . <a href="#page-62">62</a>
     <a href="#appendix-B.1">B.1</a>.  Alternate Submission Scenarios . . . . . . . . . . . . . . <a href="#page-63">63</a>
     <a href="#appendix-B.2">B.2</a>.  Alternate Delivery Scenarios . . . . . . . . . . . . . . . <a href="#page-65">65</a>
   <a href="#appendix-C">Appendix C</a>.  Creating a Public Key (INFORMATIVE) . . . . . . . . . <a href="#page-67">67</a>
   <a href="#appendix-D">Appendix D</a>.  MUA Considerations  . . . . . . . . . . . . . . . . . <a href="#page-68">68</a>
   <a href="#appendix-E">Appendix E</a>.  Acknowledgements  . . . . . . . . . . . . . . . . . . <a href="#page-69">69</a>























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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   DomainKeys Identified Mail (DKIM) defines a mechanism by which email
   messages can be cryptographically signed, permitting a signing domain
   to claim responsibility for the introduction of a message into the
   mail stream.  Message recipients can verify the signature by querying
   the signer's domain directly to retrieve the appropriate public key,
   and thereby confirm that the message was attested to by a party in
   possession of the private key for the signing domain.

   The approach taken by DKIM differs from previous approaches to
   message signing (e.g., Secure/Multipurpose Internet Mail Extensions
   (S/MIME) [<a href="./rfc1847" title="&quot;Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted&quot;">RFC1847</a>], OpenPGP [<a href="./rfc2440" title="&quot;OpenPGP Message Format&quot;">RFC2440</a>]) in that:

   o  the message signature is written as a message header field so that
      neither human recipients nor existing MUA (Mail User Agent)
      software is confused by signature-related content appearing in the
      message body;

   o  there is no dependency on public and private key pairs being
      issued by well-known, trusted certificate authorities;

   o  there is no dependency on the deployment of any new Internet
      protocols or services for public key distribution or revocation;

   o  signature verification failure does not force rejection of the
      message;

   o  no attempt is made to include encryption as part of the mechanism;

   o  message archiving is not a design goal.

   DKIM:

   o  is compatible with the existing email infrastructure and
      transparent to the fullest extent possible;

   o  requires minimal new infrastructure;

   o  can be implemented independently of clients in order to reduce
      deployment time;

   o  can be deployed incrementally;

   o  allows delegation of signing to third parties.






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<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Signing Identity</span>

   DKIM separates the question of the identity of the signer of the
   message from the purported author of the message.  In particular, a
   signature includes the identity of the signer.  Verifiers can use the
   signing information to decide how they want to process the message.
   The signing identity is included as part of the signature header
   field.

      INFORMATIVE RATIONALE: The signing identity specified by a DKIM
      signature is not required to match an address in any particular
      header field because of the broad methods of interpretation by
      recipient mail systems, including MUAs.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Scalability</span>

   DKIM is designed to support the extreme scalability requirements that
   characterize the email identification problem.  There are currently
   over 70 million domains and a much larger number of individual
   addresses.  DKIM seeks to preserve the positive aspects of the
   current email infrastructure, such as the ability for anyone to
   communicate with anyone else without introduction.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  Simple Key Management</span>

   DKIM differs from traditional hierarchical public-key systems in that
   no Certificate Authority infrastructure is required; the verifier
   requests the public key from a repository in the domain of the
   claimed signer directly rather than from a third party.

   The DNS is proposed as the initial mechanism for the public keys.
   Thus, DKIM currently depends on DNS administration and the security
   of the DNS system.  DKIM is designed to be extensible to other key
   fetching services as they become available.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Terminology and Definitions</span>

   This section defines terms used in the rest of the document.  Syntax
   descriptions use the form described in Augmented BNF for Syntax
   Specifications [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>].

   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 [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].







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<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  Signers</span>

   Elements in the mail system that sign messages on behalf of a domain
   are referred to as signers.  These may be MUAs (Mail User Agents),
   MSAs (Mail Submission Agents), MTAs (Mail Transfer Agents), or other
   agents such as mailing list exploders.  In general, any signer will
   be involved in the injection of a message into the message system in
   some way.  The key issue is that a message must be signed before it
   leaves the administrative domain of the signer.

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Verifiers</span>

   Elements in the mail system that verify signatures are referred to as
   verifiers.  These may be MTAs, Mail Delivery Agents (MDAs), or MUAs.
   In most cases it is expected that verifiers will be close to an end
   user (reader) of the message or some consuming agent such as a
   mailing list exploder.

<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Whitespace</span>

   There are three forms of whitespace:

   o  WSP represents simple whitespace, i.e., a space or a tab character
      (formal definition in [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>]).

   o  LWSP is linear whitespace, defined as WSP plus CRLF (formal
      definition in [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>]).

   o  FWS is folding whitespace.  It allows multiple lines separated by
      CRLF followed by at least one whitespace, to be joined.

   The formal ABNF for these are (WSP and LWSP are given for information
   only):

       WSP =   SP / HTAB
       LWSP =  *(WSP / CRLF WSP)
       FWS =   [*WSP CRLF] 1*WSP

   The definition of FWS is identical to that in [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>] except for
   the exclusion of obs-FWS.

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  Common ABNF Tokens</span>

   The following ABNF tokens are used elsewhere in this document:
     hyphenated-word =  ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
     base64string =     1*(ALPHA / DIGIT / "+" / "/" / [FWS])
                        [ "=" [FWS] [ "=" [FWS] ] ]




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<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>.  Imported ABNF Tokens</span>

   The following tokens are imported from other RFCs as noted.  Those
   RFCs should be considered definitive.

   The following tokens are imported from [<a href="./rfc2821" title="&quot;Simple Mail Transfer Protocol&quot;">RFC2821</a>]:

   o  "Local-part" (implementation warning: this permits quoted strings)

   o  "sub-domain"

   The following tokens are imported from [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>]:

   o  "field-name" (name of a header field)

   o  "dot-atom-text" (in the Local-part of an email address)

   The following tokens are imported from [<a href="./rfc2045" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies&quot;">RFC2045</a>]:

   o  "qp-section" (a single line of quoted-printable-encoded text)

   o  "hex-octet" (a quoted-printable encoded octet)

      INFORMATIVE NOTE: Be aware that the ABNF in <a href="./rfc2045">RFC 2045</a> does not obey
      the rules of <a href="./rfc4234">RFC 4234</a> and must be interpreted accordingly,
      particularly as regards case folding.

   Other tokens not defined herein are imported from [<a href="./rfc4234" title="&quot;Augmented BNF for Syntax Specifications: ABNF&quot;">RFC4234</a>].  These
   are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
   etc.

<span class="h3"><a class="selflink" id="section-2.6" href="#section-2.6">2.6</a>.  DKIM-Quoted-Printable</span>

   The DKIM-Quoted-Printable encoding syntax resembles that described in
   Quoted-Printable <a href="./rfc2045#section-6.7">[RFC2045], Section&nbsp;6.7</a>: any character MAY be encoded
   as an "=" followed by two hexadecimal digits from the alphabet
   "0123456789ABCDEF" (no lowercase characters permitted) representing
   the hexadecimal-encoded integer value of that character.  All control
   characters (those with values &lt; %x20), 8-bit characters (values &gt;
   %x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon
   (";", %x3B) MUST be encoded.  Note that all whitespace, including
   SPACE, CR, and LF characters, MUST be encoded.  After encoding, FWS
   MAY be added at arbitrary locations in order to avoid excessively
   long lines; such whitespace is NOT part of the value, and MUST be
   removed before decoding.






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   ABNF:

       dkim-quoted-printable =
                          *(FWS / hex-octet / dkim-safe-char)
                     ; hex-octet is from <a href="./rfc2045">RFC 2045</a>
       dkim-safe-char =   %x21-3A / %x3C / %x3E-7E
                     ; '!' - ':', '&lt;', '&gt;' - '~'
                     ; Characters not listed as "mail-safe" in
                     ; <a href="./rfc2049">RFC 2049</a> are also not recommended.

      INFORMATIVE NOTE: DKIM-Quoted-Printable differs from Quoted-
      Printable as defined in <a href="./rfc2045">RFC 2045</a> in several important ways:

      1.  Whitespace in the input text, including CR and LF, must be
          encoded.  <a href="./rfc2045">RFC 2045</a> does not require such encoding, and does
          not permit encoding of CR or LF characters that are part of a
          CRLF line break.

      2.  Whitespace in the encoded text is ignored.  This is to allow
          tags encoded using DKIM-Quoted-Printable to be wrapped as
          needed.  In particular, <a href="./rfc2045">RFC 2045</a> requires that line breaks in
          the input be represented as physical line breaks; that is not
          the case here.

      3.  The "soft line break" syntax ("=" as the last non-whitespace
          character on the line) does not apply.

      4.  DKIM-Quoted-Printable does not require that encoded lines be
          no more than 76 characters long (although there may be other
          requirements depending on the context in which the encoded
          text is being used).

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Protocol Elements</span>

   Protocol Elements are conceptual parts of the protocol that are not
   specific to either signers or verifiers.  The protocol descriptions
   for signers and verifiers are described in later sections (Signer
   Actions (<a href="#section-5">Section 5</a>) and Verifier Actions (<a href="#section-6">Section 6</a>)).  NOTE: This
   section must be read in the context of those sections.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Selectors</span>

   To support multiple concurrent public keys per signing domain, the
   key namespace is subdivided using "selectors".  For example,
   selectors might indicate the names of office locations (e.g.,
   "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
   (e.g., "january2005", "february2005", etc.), or even the individual
   user.



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   Selectors are needed to support some important use cases.  For
   example:

   o  Domains that want to delegate signing capability for a specific
      address for a given duration to a partner, such as an advertising
      provider or other outsourced function.

   o  Domains that want to allow frequent travelers to send messages
      locally without the need to connect with a particular MSA.

   o  "Affinity" domains (e.g., college alumni associations) that
      provide forwarding of incoming mail, but that do not operate a
      mail submission agent for outgoing mail.

   Periods are allowed in selectors and are component separators.  When
   keys are retrieved from the DNS, periods in selectors define DNS
   label boundaries in a manner similar to the conventional use in
   domain names.  Selector components might be used to combine dates
   with locations, for example, "march2005.reykjavik".  In a DNS
   implementation, this can be used to allow delegation of a portion of
   the selector namespace.

   ABNF:

      selector =   sub-domain *( "." sub-domain )

   The number of public keys and corresponding selectors for each domain
   is determined by the domain owner.  Many domain owners will be
   satisfied with just one selector, whereas administratively
   distributed organizations may choose to manage disparate selectors
   and key pairs in different regions or on different email servers.

   Beyond administrative convenience, selectors make it possible to
   seamlessly replace public keys on a routine basis.  If a domain
   wishes to change from using a public key associated with selector
   "january2005" to a public key associated with selector
   "february2005", it merely makes sure that both public keys are
   advertised in the public-key repository concurrently for the
   transition period during which email may be in transit prior to
   verification.  At the start of the transition period, the outbound
   email servers are configured to sign with the "february2005" private
   key.  At the end of the transition period, the "january2005" public
   key is removed from the public-key repository.

      INFORMATIVE NOTE: A key may also be revoked as described below.
      The distinction between revoking and removing a key selector
      record is subtle.  When phasing out keys as described above, a
      signing domain would probably simply remove the key record after



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      the transition period.  However, a signing domain could elect to
      revoke the key (but maintain the key record) for a further period.
      There is no defined semantic difference between a revoked key and
      a removed key.

   While some domains may wish to make selector values well known,
   others will want to take care not to allocate selector names in a way
   that allows harvesting of data by outside parties.  For example, if
   per-user keys are issued, the domain owner will need to make the
   decision as to whether to associate this selector directly with the
   user name, or make it some unassociated random value, such as a
   fingerprint of the public key.

      INFORMATIVE OPERATIONS NOTE: Reusing a selector with a new key
      (for example, changing the key associated with a user's name)
      makes it impossible to tell the difference between a message that
      didn't verify because the key is no longer valid versus a message
      that is actually forged.  For this reason, signers are ill-advised
      to reuse selectors for new keys.  A better strategy is to assign
      new keys to new selectors.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Tag=Value Lists</span>

   DKIM uses a simple "tag=value" syntax in several contexts, including
   in messages and domain signature records.

   Values are a series of strings containing either plain text, "base64"
   text (as defined in <a href="./rfc2045#section-6.8">[RFC2045], Section&nbsp;6.8</a>), "qp-section" (ibid,
   <a href="#section-6.7">Section 6.7</a>), or "dkim-quoted-printable" (as defined in <a href="#section-2.6">Section 2.6</a>).
   The name of the tag will determine the encoding of each value.
   Unencoded semicolon (";") characters MUST NOT occur in the tag value,
   since that separates tag-specs.

      INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined
      below (as "tag-value") only includes 7-bit characters, an
      implementation that wished to anticipate future standards would be
      advised not to preclude the use of UTF8-encoded text in tag=value
      lists.













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   Formally, the syntax rules are as follows:

        tag-list  =  tag-spec 0*( ";" tag-spec ) [ ";" ]
        tag-spec  =  [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
        tag-name  =  ALPHA 0*ALNUMPUNC
        tag-value =  [ tval 0*( 1*(WSP / FWS) tval ) ]
                          ; WSP and FWS prohibited at beginning and end
        tval      =  1*VALCHAR
        VALCHAR   =  %x21-3A / %x3C-7E
                          ; EXCLAMATION to TILDE except SEMICOLON
        ALNUMPUNC =  ALPHA / DIGIT / "_"

   Note that WSP is allowed anywhere around tags.  In particular, any
   WSP after the "=" and any WSP before the terminating ";" is not part
   of the value; however, WSP inside the value is significant.

   Tags MUST be interpreted in a case-sensitive manner.  Values MUST be
   processed as case sensitive unless the specific tag description of
   semantics specifies case insensitivity.

   Tags with duplicate names MUST NOT occur within a single tag-list; if
   a tag name does occur more than once, the entire tag-list is invalid.

   Whitespace within a value MUST be retained unless explicitly excluded
   by the specific tag description.

   Tag=value pairs that represent the default value MAY be included to
   aid legibility.

   Unrecognized tags MUST be ignored.

   Tags that have an empty value are not the same as omitted tags.  An
   omitted tag is treated as having the default value; a tag with an
   empty value explicitly designates the empty string as the value.  For
   example, "g=" does not mean "g=*", even though "g=*" is the default
   for that tag.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Signing and Verification Algorithms</span>

   DKIM supports multiple digital signature algorithms.  Two algorithms
   are defined by this specification at this time: rsa-sha1 and rsa-
   sha256.  The rsa-sha256 algorithm is the default if no algorithm is
   specified.  Verifiers MUST implement both rsa-sha1 and rsa-sha256.
   Signers MUST implement and SHOULD sign using rsa-sha256.







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      INFORMATIVE NOTE: Although sha256 is strongly encouraged, some
      senders of low-security messages (such as routine newsletters) may
      prefer to use sha1 because of reduced CPU requirements to compute
      a sha1 hash.  In general, sha256 should always be used whenever
      possible.

<span class="h4"><a class="selflink" id="section-3.3.1" href="#section-3.3.1">3.3.1</a>.  The rsa-sha1 Signing Algorithm</span>

   The rsa-sha1 Signing Algorithm computes a message hash as described
   in <a href="#section-3.7">Section 3.7</a> below using SHA-1 [<a href="#ref-FIPS.180-2.2002" title="&quot;Secure Hash Standard&quot;">FIPS.180-2.2002</a>] as the hash-alg.
   That hash is then signed by the signer using the RSA algorithm
   (defined in PKCS#1 version 1.5 [<a href="./rfc3447" title="&quot;Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1&quot;">RFC3447</a>]) as the crypt-alg and the
   signer's private key.  The hash MUST NOT be truncated or converted
   into any form other than the native binary form before being signed.
   The signing algorithm SHOULD use a public exponent of 65537.

<span class="h4"><a class="selflink" id="section-3.3.2" href="#section-3.3.2">3.3.2</a>.  The rsa-sha256 Signing Algorithm</span>

   The rsa-sha256 Signing Algorithm computes a message hash as described
   in <a href="#section-3.7">Section 3.7</a> below using SHA-256 [<a href="#ref-FIPS.180-2.2002" title="&quot;Secure Hash Standard&quot;">FIPS.180-2.2002</a>] as the hash-alg.
   That hash is then signed by the signer using the RSA algorithm
   (defined in PKCS#1 version 1.5 [<a href="./rfc3447" title="&quot;Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1&quot;">RFC3447</a>]) as the crypt-alg and the
   signer's private key.  The hash MUST NOT be truncated or converted
   into any form other than the native binary form before being signed.

<span class="h4"><a class="selflink" id="section-3.3.3" href="#section-3.3.3">3.3.3</a>.  Key Sizes</span>

   Selecting appropriate key sizes is a trade-off between cost,
   performance, and risk.  Since short RSA keys more easily succumb to
   off-line attacks, signers MUST use RSA keys of at least 1024 bits for
   long-lived keys.  Verifiers MUST be able to validate signatures with
   keys ranging from 512 bits to 2048 bits, and they MAY be able to
   validate signatures with larger keys.  Verifier policies may use the
   length of the signing key as one metric for determining whether a
   signature is acceptable.

   Factors that should influence the key size choice include the
   following:

   o  The practical constraint that large (e.g., 4096 bit) keys may not
      fit within a 512-byte DNS UDP response packet

   o  The security constraint that keys smaller than 1024 bits are
      subject to off-line attacks

   o  Larger keys impose higher CPU costs to verify and sign email





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   o  Keys can be replaced on a regular basis, thus their lifetime can
      be relatively short

   o  The security goals of this specification are modest compared to
      typical goals of other systems that employ digital signatures

   See [<a href="./rfc3766" title="&quot;Determining Strengths for Public Keys Used For Exchanging Symmetric Keys&quot;">RFC3766</a>] for further discussion on selecting key sizes.

<span class="h4"><a class="selflink" id="section-3.3.4" href="#section-3.3.4">3.3.4</a>.  Other Algorithms</span>

   Other algorithms MAY be defined in the future.  Verifiers MUST ignore
   any signatures using algorithms that they do not implement.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Canonicalization</span>

   Empirical evidence demonstrates that some mail servers and relay
   systems modify email in transit, potentially invalidating a
   signature.  There are two competing perspectives on such
   modifications.  For most signers, mild modification of email is
   immaterial to the authentication status of the email.  For such
   signers, a canonicalization algorithm that survives modest in-transit
   modification is preferred.

   Other signers demand that any modification of the email, however
   minor, result in a signature verification failure.  These signers
   prefer a canonicalization algorithm that does not tolerate in-transit
   modification of the signed email.

   Some signers may be willing to accept modifications to header fields
   that are within the bounds of email standards such as [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>], but
   are unwilling to accept any modification to the body of messages.

   To satisfy all requirements, two canonicalization algorithms are
   defined for each of the header and the body: a "simple" algorithm
   that tolerates almost no modification and a "relaxed" algorithm that
   tolerates common modifications such as whitespace replacement and
   header field line rewrapping.  A signer MAY specify either algorithm
   for header or body when signing an email.  If no canonicalization
   algorithm is specified by the signer, the "simple" algorithm defaults
   for both header and body.  Verifiers MUST implement both
   canonicalization algorithms.  Note that the header and body may use
   different canonicalization algorithms.  Further canonicalization
   algorithms MAY be defined in the future; verifiers MUST ignore any
   signatures that use unrecognized canonicalization algorithms.

   Canonicalization simply prepares the email for presentation to the
   signing or verification algorithm.  It MUST NOT change the




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   transmitted data in any way.  Canonicalization of header fields and
   body are described below.

   NOTE: This section assumes that the message is already in "network
   normal" format (text is ASCII encoded, lines are separated with CRLF
   characters, etc.).  See also <a href="#section-5.3">Section 5.3</a> for information about
   normalizing the message.

<span class="h4"><a class="selflink" id="section-3.4.1" href="#section-3.4.1">3.4.1</a>.  The "simple" Header Canonicalization Algorithm</span>

   The "simple" header canonicalization algorithm does not change header
   fields in any way.  Header fields MUST be presented to the signing or
   verification algorithm exactly as they are in the message being
   signed or verified.  In particular, header field names MUST NOT be
   case folded and whitespace MUST NOT be changed.

<span class="h4"><a class="selflink" id="section-3.4.2" href="#section-3.4.2">3.4.2</a>.  The "relaxed" Header Canonicalization Algorithm</span>

   The "relaxed" header canonicalization algorithm MUST apply the
   following steps in order:

   o  Convert all header field names (not the header field values) to
      lowercase.  For example, convert "SUBJect: AbC" to "subject: AbC".

   o  Unfold all header field continuation lines as described in
      [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>]; in particular, lines with terminators embedded in
      continued header field values (that is, CRLF sequences followed by
      WSP) MUST be interpreted without the CRLF.  Implementations MUST
      NOT remove the CRLF at the end of the header field value.

   o  Convert all sequences of one or more WSP characters to a single SP
      character.  WSP characters here include those before and after a
      line folding boundary.

   o  Delete all WSP characters at the end of each unfolded header field
      value.

   o  Delete any WSP characters remaining before and after the colon
      separating the header field name from the header field value.  The
      colon separator MUST be retained.

<span class="h4"><a class="selflink" id="section-3.4.3" href="#section-3.4.3">3.4.3</a>.  The "simple" Body Canonicalization Algorithm</span>

   The "simple" body canonicalization algorithm ignores all empty lines
   at the end of the message body.  An empty line is a line of zero
   length after removal of the line terminator.  If there is no body or
   no trailing CRLF on the message body, a CRLF is added.  It makes no




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   other changes to the message body.  In more formal terms, the
   "simple" body canonicalization algorithm converts "0*CRLF" at the end
   of the body to a single "CRLF".

   Note that a completely empty or missing body is canonicalized as a
   single "CRLF"; that is, the canonicalized length will be 2 octets.

<span class="h4"><a class="selflink" id="section-3.4.4" href="#section-3.4.4">3.4.4</a>.  The "relaxed" Body Canonicalization Algorithm</span>

   The "relaxed" body canonicalization algorithm:

   o  Ignores all whitespace at the end of lines.  Implementations MUST
      NOT remove the CRLF at the end of the line.

   o  Reduces all sequences of WSP within a line to a single SP
      character.

   o  Ignores all empty lines at the end of the message body.  "Empty
      line" is defined in <a href="#section-3.4.3">Section 3.4.3</a>.

      INFORMATIVE NOTE: It should be noted that the relaxed body
      canonicalization algorithm may enable certain types of extremely
      crude "ASCII Art" attacks where a message may be conveyed by
      adjusting the spacing between words.  If this is a concern, the
      "simple" body canonicalization algorithm should be used instead.

<span class="h4"><a class="selflink" id="section-3.4.5" href="#section-3.4.5">3.4.5</a>.  Body Length Limits</span>

   A body length count MAY be specified to limit the signature
   calculation to an initial prefix of the body text, measured in
   octets.  If the body length count is not specified, the entire
   message body is signed.

      INFORMATIVE RATIONALE: This capability is provided because it is
      very common for mailing lists to add trailers to messages (e.g.,
      instructions how to get off the list).  Until those messages are
      also signed, the body length count is a useful tool for the
      verifier since it may as a matter of policy accept messages having
      valid signatures with extraneous data.

      INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables
      an attack in which an attacker modifies a message to include
      content that solely benefits the attacker.  It is possible for the
      appended content to completely replace the original content in the
      end recipient's eyes and to defeat duplicate message detection
      algorithms.  To avoid this attack, signers should be wary of using





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      this tag, and verifiers might wish to ignore the tag or remove
      text that appears after the specified content length, perhaps
      based on other criteria.

   The body length count allows the signer of a message to permit data
   to be appended to the end of the body of a signed message.  The body
   length count MUST be calculated following the canonicalization
   algorithm; for example, any whitespace ignored by a canonicalization
   algorithm is not included as part of the body length count.  Signers
   of MIME messages that include a body length count SHOULD be sure that
   the length extends to the closing MIME boundary string.

      INFORMATIVE IMPLEMENTATION NOTE: A signer wishing to ensure that
      the only acceptable modifications are to add to the MIME postlude
      would use a body length count encompassing the entire final MIME
      boundary string, including the final "--CRLF".  A signer wishing
      to allow additional MIME parts but not modification of existing
      parts would use a body length count extending through the final
      MIME boundary string, omitting the final "--CRLF".  Note that this
      only works for some MIME types, e.g., multipart/mixed but not
      multipart/signed.

   A body length count of zero means that the body is completely
   unsigned.

   Signers wishing to ensure that no modification of any sort can occur
   should specify the "simple" canonicalization algorithm for both
   header and body and omit the body length count.

<span class="h4"><a class="selflink" id="section-3.4.6" href="#section-3.4.6">3.4.6</a>.  Canonicalization Examples (INFORMATIVE)</span>

   In the following examples, actual whitespace is used only for
   clarity.  The actual input and output text is designated using
   bracketed descriptors: "&lt;SP&gt;" for a space character, "&lt;HTAB&gt;" for a
   tab character, and "&lt;CRLF&gt;" for a carriage-return/line-feed sequence.
   For example, "X &lt;SP&gt; Y" and "X&lt;SP&gt;Y" represent the same three
   characters.

   Example 1: A message reading:

       A: &lt;SP&gt; X &lt;CRLF&gt;
       B &lt;SP&gt; : &lt;SP&gt; Y &lt;HTAB&gt;&lt;CRLF&gt;
       &lt;HTAB&gt; Z &lt;SP&gt;&lt;SP&gt;&lt;CRLF&gt;
       &lt;CRLF&gt;
       &lt;SP&gt; C &lt;SP&gt;&lt;CRLF&gt;
       D &lt;SP&gt;&lt;HTAB&gt;&lt;SP&gt; E &lt;CRLF&gt;
       &lt;CRLF&gt;
       &lt;CRLF&gt;



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   when canonicalized using relaxed canonicalization for both header and
   body results in a header reading:

       a:X &lt;CRLF&gt;
       b:Y &lt;SP&gt; Z &lt;CRLF&gt;

   and a body reading:

       &lt;SP&gt; C &lt;CRLF&gt;
       D &lt;SP&gt; E &lt;CRLF&gt;

   Example 2: The same message canonicalized using simple
   canonicalization for both header and body results in a header
   reading:

       A: &lt;SP&gt; X &lt;CRLF&gt;
       B &lt;SP&gt; : &lt;SP&gt; Y &lt;HTAB&gt;&lt;CRLF&gt;
       &lt;HTAB&gt; Z &lt;SP&gt;&lt;SP&gt;&lt;CRLF&gt;

   and a body reading:

       &lt;SP&gt; C &lt;SP&gt;&lt;CRLF&gt;
       D &lt;SP&gt;&lt;HTAB&gt;&lt;SP&gt; E &lt;CRLF&gt;

   Example 3: When processed using relaxed header canonicalization and
   simple body canonicalization, the canonicalized version has a header
   of:

       a:X &lt;CRLF&gt;
       b:Y &lt;SP&gt; Z &lt;CRLF&gt;

   and a body reading:

       &lt;SP&gt; C &lt;SP&gt;&lt;CRLF&gt;
       D &lt;SP&gt;&lt;HTAB&gt;&lt;SP&gt; E &lt;CRLF&gt;

<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  The DKIM-Signature Header Field</span>

   The signature of the email is stored in the DKIM-Signature header
   field.  This header field contains all of the signature and key-
   fetching data.  The DKIM-Signature value is a tag-list as described
   in <a href="#section-3.2">Section 3.2</a>.

   The DKIM-Signature header field SHOULD be treated as though it were a
   trace header field as defined in <a href="./rfc2822#section-3.6">Section&nbsp;3.6 of [RFC2822]</a>, and hence
   SHOULD NOT be reordered and SHOULD be prepended to the message.





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   The DKIM-Signature header field being created or verified is always
   included in the signature calculation, after the rest of the header
   fields being signed; however, when calculating or verifying the
   signature, the value of the "b=" tag (signature value) of that DKIM-
   Signature header field MUST be treated as though it were an empty
   string.  Unknown tags in the DKIM-Signature header field MUST be
   included in the signature calculation but MUST be otherwise ignored
   by verifiers.  Other DKIM-Signature header fields that are included
   in the signature should be treated as normal header fields; in
   particular, the "b=" tag is not treated specially.

   The encodings for each field type are listed below.  Tags described
   as qp-section are encoded as described in <a href="#section-6.7">Section 6.7</a> of MIME Part
   One [<a href="./rfc2045" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies&quot;">RFC2045</a>], with the additional conversion of semicolon characters
   to "=3B"; intuitively, this is one line of quoted-printable encoded
   text.  The dkim-quoted-printable syntax is defined in <a href="#section-2.6">Section 2.6</a>.

   Tags on the DKIM-Signature header field along with their type and
   requirement status are shown below.  Unrecognized tags MUST be
   ignored.

   v=  Version (MUST be included).  This tag defines the version of this
       specification that applies to the signature record.  It MUST have
       the value "1".  Note that verifiers must do a string comparison
       on this value; for example, "1" is not the same as "1.0".

   ABNF:

       sig-v-tag   = %x76 [FWS] "=" [FWS] "1"

           INFORMATIVE NOTE: DKIM-Signature version numbers are expected
           to increase arithmetically as new versions of this
           specification are released.

   a=  The algorithm used to generate the signature (plain-text;
       REQUIRED).  Verifiers MUST support "rsa-sha1" and "rsa-sha256";
       signers SHOULD sign using "rsa-sha256".  See <a href="#section-3.3">Section 3.3</a> for a
       description of algorithms.

   ABNF:

       sig-a-tag       = %x61 [FWS] "=" [FWS] sig-a-tag-alg
       sig-a-tag-alg   = sig-a-tag-k "-" sig-a-tag-h
       sig-a-tag-k     = "rsa" / x-sig-a-tag-k
       sig-a-tag-h     = "sha1" / "sha256" / x-sig-a-tag-h
       x-sig-a-tag-k   = ALPHA *(ALPHA / DIGIT)   ; for later extension
       x-sig-a-tag-h   = ALPHA *(ALPHA / DIGIT)   ; for later extension




<span class="grey">Allman, et al.              Standards Track                    [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   b=  The signature data (base64; REQUIRED).  Whitespace is ignored in
       this value and MUST be ignored when reassembling the original
       signature.  In particular, the signing process can safely insert
       FWS in this value in arbitrary places to conform to line-length
       limits.  See Signer Actions (<a href="#section-5">Section 5</a>) for how the signature is
       computed.

   ABNF:

       sig-b-tag       = %x62 [FWS] "=" [FWS] sig-b-tag-data
       sig-b-tag-data  = base64string

   bh= The hash of the canonicalized body part of the message as limited
       by the "l=" tag (base64; REQUIRED).  Whitespace is ignored in
       this value and MUST be ignored when reassembling the original
       signature.  In particular, the signing process can safely insert
       FWS in this value in arbitrary places to conform to line-length
       limits.  See <a href="#section-3.7">Section 3.7</a> for how the body hash is computed.

   ABNF:

       sig-bh-tag      = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
       sig-bh-tag-data = base64string

   c=  Message canonicalization (plain-text; OPTIONAL, default is
       "simple/simple").  This tag informs the verifier of the type of
       canonicalization used to prepare the message for signing.  It
       consists of two names separated by a "slash" (%d47) character,
       corresponding to the header and body canonicalization algorithms
       respectively.  These algorithms are described in <a href="#section-3.4">Section 3.4</a>.  If
       only one algorithm is named, that algorithm is used for the
       header and "simple" is used for the body.  For example,
       "c=relaxed" is treated the same as "c=relaxed/simple".

   ABNF:

       sig-c-tag       = %x63 [FWS] "=" [FWS] sig-c-tag-alg
                     ["/" sig-c-tag-alg]
       sig-c-tag-alg   = "simple" / "relaxed" / x-sig-c-tag-alg
       x-sig-c-tag-alg = hyphenated-word    ; for later extension

   d=  The domain of the signing entity (plain-text; REQUIRED).  This is
       the domain that will be queried for the public key.  This domain
       MUST be the same as or a parent domain of the "i=" tag (the
       signing identity, as described below), or it MUST meet the
       requirements for parent domain signing described in <a href="#section-3.8">Section 3.8</a>.
       When presented with a signature that does not meet these
       requirement, verifiers MUST consider the signature invalid.



<span class="grey">Allman, et al.              Standards Track                    [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   Internationalized domain names MUST be encoded as described in
       [<a href="./rfc3490" title="&quot;Internationalizing Domain Names in Applications (IDNA)&quot;">RFC3490</a>].

   ABNF:

       sig-d-tag       = %x64 [FWS] "=" [FWS] domain-name
       domain-name     = sub-domain 1*("." sub-domain)
                ; from <a href="./rfc2821">RFC 2821</a> Domain, but excluding address-literal

   h=  Signed header fields (plain-text, but see description; REQUIRED).
       A colon-separated list of header field names that identify the
       header fields presented to the signing algorithm.  The field MUST
       contain the complete list of header fields in the order presented
       to the signing algorithm.  The field MAY contain names of header
       fields that do not exist when signed; nonexistent header fields
       do not contribute to the signature computation (that is, they are
       treated as the null input, including the header field name, the
       separating colon, the header field value, and any CRLF
       terminator).  The field MUST NOT include the DKIM-Signature
       header field that is being created or verified, but may include
       others.  Folding whitespace (FWS) MAY be included on either side
       of the colon separator.  Header field names MUST be compared
       against actual header field names in a case-insensitive manner.
       This list MUST NOT be empty.  See <a href="#section-5.4">Section 5.4</a> for a discussion of
       choosing header fields to sign.

   ABNF:

       sig-h-tag       = %x68 [FWS] "=" [FWS] hdr-name
                     0*( *FWS ":" *FWS hdr-name )
       hdr-name        = field-name

       INFORMATIVE EXPLANATION: By "signing" header fields that do not
           actually exist, a signer can prevent insertion of those
           header fields before verification.  However, since a signer
           cannot possibly know what header fields might be created in
           the future, and that some MUAs might present header fields
           that are embedded inside a message (e.g., as a message/rfc822
           content type), the security of this solution is not total.

       INFORMATIVE EXPLANATION: The exclusion of the header field name
           and colon as well as the header field value for non-existent
           header fields prevents an attacker from inserting an actual
           header field with a null value.







<span class="grey">Allman, et al.              Standards Track                    [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   i=  Identity of the user or agent (e.g., a mailing list manager) on
       behalf of which this message is signed (dkim-quoted-printable;
       OPTIONAL, default is an empty Local-part followed by an "@"
       followed by the domain from the "d=" tag).  The syntax is a
       standard email address where the Local-part MAY be omitted.  The
       domain part of the address MUST be the same as or a subdomain of
       the value of the "d=" tag.

   Internationalized domain names MUST be converted using the steps
       listed in <a href="./rfc3490#section-4">Section&nbsp;4 of [RFC3490]</a> using the "ToASCII" function.

   ABNF:

       sig-i-tag =   %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name

       INFORMATIVE NOTE: The Local-part of the "i=" tag is optional
           because in some cases a signer may not be able to establish a
           verified individual identity.  In such cases, the signer may
           wish to assert that although it is willing to go as far as
           signing for the domain, it is unable or unwilling to commit
           to an individual user name within their domain.  It can do so
           by including the domain part but not the Local-part of the
           identity.

       INFORMATIVE DISCUSSION: This document does not require the value
           of the "i=" tag to match the identity in any message header
           fields.  This is considered to be a verifier policy issue.
           Constraints between the value of the "i=" tag and other
           identities in other header fields seek to apply basic
           authentication into the semantics of trust associated with a
           role such as content author.  Trust is a broad and complex
           topic and trust mechanisms are subject to highly creative
           attacks.  The real-world efficacy of any but the most basic
           bindings between the "i=" value and other identities is not
           well established, nor is its vulnerability to subversion by
           an attacker.  Hence reliance on the use of these options
           should be strictly limited.  In particular, it is not at all
           clear to what extent a typical end-user recipient can rely on
           any assurances that might be made by successful use of the
           "i=" options.

   l=  Body length count (plain-text unsigned decimal integer; OPTIONAL,
       default is entire body).  This tag informs the verifier of the
       number of octets in the body of the email after canonicalization
       included in the cryptographic hash, starting from 0 immediately
       following the CRLF preceding the body.  This value MUST NOT be
       larger than the actual number of octets in the canonicalized
       message body.



<span class="grey">Allman, et al.              Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


       INFORMATIVE IMPLEMENTATION WARNING: Use of the "l=" tag might
           allow display of fraudulent content without appropriate
           warning to end users.  The "l=" tag is intended for
           increasing signature robustness when sending to mailing lists
           that both modify their content and do not sign their
           messages.  However, using the "l=" tag enables attacks in
           which an intermediary with malicious intent modifies a
           message to include content that solely benefits the attacker.
           It is possible for the appended content to completely replace
           the original content in the end recipient's eyes and to
           defeat duplicate message detection algorithms.  Examples are
           described in Security Considerations (<a href="#section-8">Section 8</a>).  To avoid
           this attack, signers should be extremely wary of using this
           tag, and verifiers might wish to ignore the tag or remove
           text that appears after the specified content length.

       INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76
           decimal digits.  This constraint is not intended to predict
           the size of future messages or to require implementations to
           use an integer representation large enough to represent the
           maximum possible value, but is intended to remind the
           implementer to check the length of this and all other tags
           during verification and to test for integer overflow when
           decoding the value.  Implementers may need to limit the
           actual value expressed to a value smaller than 10^76, e.g.,
           to allow a message to fit within the available storage space.

   ABNF:

   sig-l-tag    = %x6c [FWS] "=" [FWS] 1*76DIGIT

   q=  A colon-separated list of query methods used to retrieve the
       public key (plain-text; OPTIONAL, default is "dns/txt").  Each
       query method is of the form "type[/options]", where the syntax
       and semantics of the options depend on the type and specified
       options.  If there are multiple query mechanisms listed, the
       choice of query mechanism MUST NOT change the interpretation of
       the signature.  Implementations MUST use the recognized query
       mechanisms in the order presented.

   Currently, the only valid value is "dns/txt", which defines the DNS
       TXT record lookup algorithm described elsewhere in this document.
       The only option defined for the "dns" query type is "txt", which
       MUST be included.  Verifiers and signers MUST support "dns/txt".







<span class="grey">Allman, et al.              Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   ABNF:

       sig-q-tag        = %x71 [FWS] "=" [FWS] sig-q-tag-method
                      *([FWS] ":" [FWS] sig-q-tag-method)
       sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
                      ["/" x-sig-q-tag-args]
       x-sig-q-tag-type = hyphenated-word  ; for future extension
       x-sig-q-tag-args = qp-hdr-value

   s=  The selector subdividing the namespace for the "d=" (domain) tag
       (plain-text; REQUIRED).

   ABNF:

       sig-s-tag    = %x73 [FWS] "=" [FWS] selector

   t=  Signature Timestamp (plain-text unsigned decimal integer;
       RECOMMENDED, default is an unknown creation time).  The time that
       this signature was created.  The format is the number of seconds
       since 00:00:00 on January 1, 1970 in the UTC time zone.  The
       value is expressed as an unsigned integer in decimal ASCII.  This
       value is not constrained to fit into a 31- or 32-bit integer.
       Implementations SHOULD be prepared to handle values up to at
       least 10^12 (until approximately AD 200,000; this fits into 40
       bits).  To avoid denial-of-service attacks, implementations MAY
       consider any value longer than 12 digits to be infinite.  Leap
       seconds are not counted.  Implementations MAY ignore signatures
       that have a timestamp in the future.

   ABNF:

       sig-t-tag    = %x74 [FWS] "=" [FWS] 1*12DIGIT

   x=  Signature Expiration (plain-text unsigned decimal integer;
       RECOMMENDED, default is no expiration).  The format is the same
       as in the "t=" tag, represented as an absolute date, not as a
       time delta from the signing timestamp.  The value is expressed as
       an unsigned integer in decimal ASCII, with the same constraints
       on the value in the "t=" tag.  Signatures MAY be considered
       invalid if the verification time at the verifier is past the
       expiration date.  The verification time should be the time that
       the message was first received at the administrative domain of
       the verifier if that time is reliably available; otherwise the
       current time should be used.  The value of the "x=" tag MUST be
       greater than the value of the "t=" tag if both are present.






<span class="grey">Allman, et al.              Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


       INFORMATIVE NOTE: The "x=" tag is not intended as an anti-replay
           defense.

   ABNF:

       sig-x-tag    = %x78 [FWS] "=" [FWS] 1*12DIGIT

   z=  Copied header fields (dkim-quoted-printable, but see description;
       OPTIONAL, default is null).  A vertical-bar-separated list of
       selected header fields present when the message was signed,
       including both the field name and value.  It is not required to
       include all header fields present at the time of signing.  This
       field need not contain the same header fields listed in the "h="
       tag.  The header field text itself must encode the vertical bar
       ("|", %x7C) character (i.e., vertical bars in the "z=" text are
       metacharacters, and any actual vertical bar characters in a
       copied header field must be encoded).  Note that all whitespace
       must be encoded, including whitespace between the colon and the
       header field value.  After encoding, FWS MAY be added at
       arbitrary locations in order to avoid excessively long lines;
       such whitespace is NOT part of the value of the header field, and
       MUST be removed before decoding.

   The header fields referenced by the "h=" tag refer to the fields in
       the <a href="./rfc2822">RFC 2822</a> header of the message, not to any copied fields in
       the "z=" tag.  Copied header field values are for diagnostic use.

   Header fields with characters requiring conversion (perhaps from
       legacy MTAs that are not [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>] compliant) SHOULD be converted
       as described in MIME Part Three [<a href="./rfc2047" title="&quot;MIME (Multipurpose Internet Mail Extensions) Part Three: Message header field Extensions for Non-ASCII Text&quot;">RFC2047</a>].

   ABNF:
       sig-z-tag      = %x7A [FWS] "=" [FWS] sig-z-tag-copy
                    *( [FWS] "|" sig-z-tag-copy )
   sig-z-tag-copy = hdr-name ":" qp-hdr-value
   qp-hdr-value   = dkim-quoted-printable    ; with "|" encoded

      INFORMATIVE EXAMPLE of a signature header field spread across
      multiple continuation lines:












<span class="grey">Allman, et al.              Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane;
      c=simple; q=dns/txt; i=@eng.example.net;
      t=1117574938; x=1118006938;
      h=from:to:subject:date;
      z=From:foo@eng.example.net|To:joe@example.com|
        Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
      bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
      b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
               VoG4ZHRNiYzR

<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>.  Key Management and Representation</span>

   Signature applications require some level of assurance that the
   verification public key is associated with the claimed signer.  Many
   applications achieve this by using public key certificates issued by
   a trusted third party.  However, DKIM can achieve a sufficient level
   of security, with significantly enhanced scalability, by simply
   having the verifier query the purported signer's DNS entry (or some
   security-equivalent) in order to retrieve the public key.

   DKIM keys can potentially be stored in multiple types of key servers
   and in multiple formats.  The storage and format of keys are
   irrelevant to the remainder of the DKIM algorithm.

   Parameters to the key lookup algorithm are the type of the lookup
   (the "q=" tag), the domain of the signer (the "d=" tag of the DKIM-
   Signature header field), and the selector (the "s=" tag).

       public_key = dkim_find_key(q_val, d_val, s_val)

   This document defines a single binding, using DNS TXT records to
   distribute the keys.  Other bindings may be defined in the future.

<span class="h4"><a class="selflink" id="section-3.6.1" href="#section-3.6.1">3.6.1</a>.  Textual Representation</span>

   It is expected that many key servers will choose to present the keys
   in an otherwise unstructured text format (for example, an XML form
   would not be considered to be unstructured text for this purpose).
   The following definition MUST be used for any DKIM key represented in
   an otherwise unstructured textual form.

   The overall syntax is a tag-list as described in <a href="#section-3.2">Section 3.2</a>.  The
   current valid tags are described below.  Other tags MAY be present
   and MUST be ignored by any implementation that does not understand
   them.






<span class="grey">Allman, et al.              Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   v=  Version of the DKIM key record (plain-text; RECOMMENDED, default
       is "DKIM1").  If specified, this tag MUST be set to "DKIM1"
       (without the quotes).  This tag MUST be the first tag in the
       record.  Records beginning with a "v=" tag with any other value
       MUST be discarded.  Note that verifiers must do a string
       comparison on this value; for example, "DKIM1" is not the same as
       "DKIM1.0".

       ABNF:

       key-v-tag    = %x76 [FWS] "=" [FWS] "DKIM1"

   g=  Granularity of the key (plain-text; OPTIONAL, default is "*").
       This value MUST match the Local-part of the "i=" tag of the DKIM-
       Signature header field (or its default value of the empty string
       if "i=" is not specified), with a single, optional "*" character
       matching a sequence of zero or more arbitrary characters
       ("wildcarding").  An email with a signing address that does not
       match the value of this tag constitutes a failed verification.
       The intent of this tag is to constrain which signing address can
       legitimately use this selector, for example, when delegating a
       key to a third party that should only be used for special
       purposes.  Wildcarding allows matching for addresses such as
       "user+*" or "*-offer".  An empty "g=" value never matches any
       addresses.

   ABNF:

       key-g-tag       = %x67 [FWS] "=" [FWS] key-g-tag-lpart
       key-g-tag-lpart = [dot-atom-text] ["*" [dot-atom-text] ]

   h=  Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
       allowing all algorithms).  A colon-separated list of hash
       algorithms that might be used.  Signers and Verifiers MUST
       support the "sha256" hash algorithm.  Verifiers MUST also support
       the "sha1" hash algorithm.

       ABNF:

       key-h-tag       = %x68 [FWS] "=" [FWS] key-h-tag-alg
                     0*( [FWS] ":" [FWS] key-h-tag-alg )
       key-h-tag-alg   = "sha1" / "sha256" / x-key-h-tag-alg
       x-key-h-tag-alg = hyphenated-word   ; for future extension








<span class="grey">Allman, et al.              Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   k=  Key type (plain-text; OPTIONAL, default is "rsa").  Signers and
       verifiers MUST support the "rsa" key type.  The "rsa" key type
       indicates that an ASN.1 DER-encoded [<a href="#ref-ITU.X660.1997" title="&quot;Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)&quot;">ITU.X660.1997</a>] RSAPublicKey
       [<a href="./rfc3447" title="&quot;Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1&quot;">RFC3447</a>] (see Sections <a href="#section-3.1">3.1</a> and A.1.1) is being used in the "p="
       tag.  (Note: the "p=" tag further encodes the value using the
       base64 algorithm.)

       ABNF:

       key-k-tag        = %x76 [FWS] "=" [FWS] key-k-tag-type
       key-k-tag-type   = "rsa" / x-key-k-tag-type
       x-key-k-tag-type = hyphenated-word   ; for future extension

   n=  Notes that might be of interest to a human (qp-section; OPTIONAL,
       default is empty).  No interpretation is made by any program.
       This tag should be used sparingly in any key server mechanism
       that has space limitations (notably DNS).  This is intended for
       use by administrators, not end users.

   ABNF:

       key-n-tag    = %x6e [FWS] "=" [FWS] qp-section

   p=  Public-key data (base64; REQUIRED).  An empty value means that
       this public key has been revoked.  The syntax and semantics of
       this tag value before being encoded in base64 are defined by the
       "k=" tag.

           INFORMATIVE RATIONALE: If a private key has been compromised
           or otherwise disabled (e.g., an outsourcing contract has been
           terminated), a signer might want to explicitly state that it
           knows about the selector, but all messages using that
           selector should fail verification.  Verifiers should ignore
           any DKIM-Signature header fields with a selector referencing
           a revoked key.

   ABNF:

       key-p-tag    = %x70 [FWS] "=" [ [FWS] base64string ]

       INFORMATIVE NOTE: A base64string is permitted to include white
           space (FWS) at arbitrary places; however, any CRLFs must be
           followed by at least one WSP character.  Implementors and
           administrators are cautioned to ensure that selector TXT
           records conform to this specification.






<span class="grey">Allman, et al.              Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   s=  Service Type (plain-text; OPTIONAL; default is "*").  A colon-
       separated list of service types to which this record applies.
       Verifiers for a given service type MUST ignore this record if the
       appropriate type is not listed.  Currently defined service types
       are as follows:

       *   matches all service types

       email   electronic mail (not necessarily limited to SMTP)

       This tag is intended to constrain the use of keys for other
       purposes, should use of DKIM be defined by other services in the
       future.

   ABNF:

       key-s-tag        = %x73 [FWS] "=" [FWS] key-s-tag-type
                       0*( [FWS] ":" [FWS] key-s-tag-type )
       key-s-tag-type   = "email" / "*" / x-key-s-tag-type
       x-key-s-tag-type = hyphenated-word   ; for future extension

   t=  Flags, represented as a colon-separated list of names (plain-
       text; OPTIONAL, default is no flags set).  The defined flags are
       as follows:

       y   This domain is testing DKIM.  Verifiers MUST NOT treat
           messages from signers in testing mode differently from
           unsigned email, even should the signature fail to verify.
           Verifiers MAY wish to track testing mode results to assist
           the signer.

       s   Any DKIM-Signature header fields using the "i=" tag MUST have
           the same domain value on the right-hand side of the "@" in
           the "i=" tag and the value of the "d=" tag.  That is, the
           "i=" domain MUST NOT be a subdomain of "d=".  Use of this
           flag is RECOMMENDED unless subdomaining is required.

   ABNF:

       key-t-tag        = %x74 [FWS] "=" [FWS] key-t-tag-flag
                      0*( [FWS] ":" [FWS] key-t-tag-flag )
       key-t-tag-flag   = "y" / "s" / x-key-t-tag-flag
       x-key-t-tag-flag = hyphenated-word   ; for future extension

   Unrecognized flags MUST be ignored.






<span class="grey">Allman, et al.              Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


<span class="h4"><a class="selflink" id="section-3.6.2" href="#section-3.6.2">3.6.2</a>.  DNS Binding</span>

   A binding using DNS TXT records as a key service is hereby defined.
   All implementations MUST support this binding.

<span class="h5"><a class="selflink" id="section-3.6.2.1" href="#section-3.6.2.1">3.6.2.1</a>.  Namespace</span>

   All DKIM keys are stored in a subdomain named "_domainkey".  Given a
   DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag
   of "foo.bar", the DNS query will be for
   "foo.bar._domainkey.example.com".

      INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records (e.g.,
      *.bar._domainkey.example.com) do not make sense in this context
      and should not be used.  Note also that wildcards within domains
      (e.g., s._domainkey.*.example.com) are not supported by the DNS.

<span class="h5"><a class="selflink" id="section-3.6.2.2" href="#section-3.6.2.2">3.6.2.2</a>.  Resource Record Types for Key Storage</span>

   The DNS Resource Record type used is specified by an option to the
   query-type ("q=") tag.  The only option defined in this base
   specification is "txt", indicating the use of a TXT Resource Record
   (RR).  A later extension of this standard may define another RR type.

   Strings in a TXT RR MUST be concatenated together before use with no
   intervening whitespace.  TXT RRs MUST be unique for a particular
   selector name; that is, if there are multiple records in an RRset,
   the results are undefined.

   TXT RRs are encoded as described in <a href="#section-3.6.1">Section 3.6.1</a>.

<span class="h3"><a class="selflink" id="section-3.7" href="#section-3.7">3.7</a>.  Computing the Message Hashes</span>

   Both signing and verifying message signatures start with a step of
   computing two cryptographic hashes over the message.  Signers will
   choose the parameters of the signature as described in Signer Actions
   (<a href="#section-5">Section 5</a>); verifiers will use the parameters specified in the DKIM-
   Signature header field being verified.  In the following discussion,
   the names of the tags in the DKIM-Signature header field that either
   exists (when verifying) or will be created (when signing) are used.
   Note that canonicalization (<a href="#section-3.4">Section 3.4</a>) is only used to prepare the
   email for signing or verifying; it does not affect the transmitted
   email in any way.

   The signer/verifier MUST compute two hashes, one over the body of the
   message and one over the selected header fields of the message.





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   Signers MUST compute them in the order shown.  Verifiers MAY compute
   them in any order convenient to the verifier, provided that the
   result is semantically identical to the semantics that would be the
   case had they been computed in this order.

   In hash step 1, the signer/verifier MUST hash the message body,
   canonicalized using the body canonicalization algorithm specified in
   the "c=" tag and then truncated to the length specified in the "l="
   tag.  That hash value is then converted to base64 form and inserted
   into (signers) or compared to (verifiers) the "bh=" tag of the DKIM-
   Signature header field.

   In hash step 2, the signer/verifier MUST pass the following to the
   hash algorithm in the indicated order.

   1.  The header fields specified by the "h=" tag, in the order
       specified in that tag, and canonicalized using the header
       canonicalization algorithm specified in the "c=" tag.  Each
       header field MUST be terminated with a single CRLF.

   2.  The DKIM-Signature header field that exists (verifying) or will
       be inserted (signing) in the message, with the value of the "b="
       tag deleted (i.e., treated as the empty string), canonicalized
       using the header canonicalization algorithm specified in the "c="
       tag, and without a trailing CRLF.

   All tags and their values in the DKIM-Signature header field are
   included in the cryptographic hash with the sole exception of the
   value portion of the "b=" (signature) tag, which MUST be treated as
   the null string.  All tags MUST be included even if they might not be
   understood by the verifier.  The header field MUST be presented to
   the hash algorithm after the body of the message rather than with the
   rest of the header fields and MUST be canonicalized as specified in
   the "c=" (canonicalization) tag.  The DKIM-Signature header field
   MUST NOT be included in its own h= tag, although other DKIM-Signature
   header fields MAY be signed (see <a href="#section-4">Section 4</a>).

   When calculating the hash on messages that will be transmitted using
   base64 or quoted-printable encoding, signers MUST compute the hash
   after the encoding.  Likewise, the verifier MUST incorporate the
   values into the hash before decoding the base64 or quoted-printable
   text.  However, the hash MUST be computed before transport level
   encodings such as SMTP "dot-stuffing" (the modification of lines
   beginning with a "." to avoid confusion with the SMTP end-of-message
   marker, as specified in [<a href="./rfc2821" title="&quot;Simple Mail Transfer Protocol&quot;">RFC2821</a>]).

   With the exception of the canonicalization procedure described in
   <a href="#section-3.4">Section 3.4</a>, the DKIM signing process treats the body of messages as



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   simply a string of octets.  DKIM messages MAY be either in plain-text
   or in MIME format; no special treatment is afforded to MIME content.
   Message attachments in MIME format MUST be included in the content
   that is signed.

   More formally, the algorithm for the signature is as follows:
       body-hash = hash-alg(canon_body)
       header-hash = hash-alg(canon_header || DKIM-SIG)
       signature = sig-alg(header-hash, key)

   where "sig-alg" is the signature algorithm specified by the "a=" tag,
   "hash-alg" is the hash algorithm specified by the "a=" tag,
   "canon_header" and "canon_body" are the canonicalized message header
   and body (respectively) as defined in <a href="#section-3.4">Section 3.4</a> (excluding the
   DKIM-Signature header field), and "DKIM-SIG" is the canonicalized
   DKIM-Signature header field sans the signature value itself, but with
   "body-hash" included as the "bh=" tag.

      INFORMATIVE IMPLEMENTERS' NOTE: Many digital signature APIs
      provide both hashing and application of the RSA private key using
      a single "sign()" primitive.  When using such an API, the last two
      steps in the algorithm would probably be combined into a single
      call that would perform both the "hash-alg" and the "sig-alg".

<span class="h3"><a class="selflink" id="section-3.8" href="#section-3.8">3.8</a>.  Signing by Parent Domains</span>

   In some circumstances, it is desirable for a domain to apply a
   signature on behalf of any of its subdomains without the need to
   maintain separate selectors (key records) in each subdomain.  By
   default, private keys corresponding to key records can be used to
   sign messages for any subdomain of the domain in which they reside;
   e.g., a key record for the domain example.com can be used to verify
   messages where the signing identity ("i=" tag of the signature) is
   sub.example.com, or even sub1.sub2.example.com.  In order to limit
   the capability of such keys when this is not intended, the "s" flag
   may be set in the "t=" tag of the key record to constrain the
   validity of the record to exactly the domain of the signing identity.
   If the referenced key record contains the "s" flag as part of the
   "t=" tag, the domain of the signing identity ("i=" flag) MUST be the
   same as that of the d= domain.  If this flag is absent, the domain of
   the signing identity MUST be the same as, or a subdomain of, the d=
   domain.  Key records that are not intended for use with subdomains
   SHOULD specify the "s" flag in the "t=" tag.








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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Semantics of Multiple Signatures</span>

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Example Scenarios</span>

   There are many reasons why a message might have multiple signatures.
   For example, a given signer might sign multiple times, perhaps with
   different hashing or signing algorithms during a transition phase.

      INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be
      insufficiently strong, and DKIM usage transitions to SHA-1024.  A
      signer might immediately sign using the newer algorithm, but
      continue to sign using the older algorithm for interoperability
      with verifiers that had not yet upgraded.  The signer would do
      this by adding two DKIM-Signature header fields, one using each
      algorithm.  Older verifiers that did not recognize SHA-1024 as an
      acceptable algorithm would skip that signature and use the older
      algorithm; newer verifiers could use either signature at their
      option, and all other things being equal might not even attempt to
      verify the other signature.

   Similarly, a signer might sign a message including all headers and no
   "l=" tag (to satisfy strict verifiers) and a second time with a
   limited set of headers and an "l=" tag (in anticipation of possible
   message modifications in route to other verifiers).  Verifiers could
   then choose which signature they preferred.

      INFORMATIVE EXAMPLE: A verifier might receive a message with two
      signatures, one covering more of the message than the other.  If
      the signature covering more of the message verified, then the
      verifier could make one set of policy decisions; if that signature
      failed but the signature covering less of the message verified,
      the verifier might make a different set of policy decisions.

   Of course, a message might also have multiple signatures because it
   passed through multiple signers.  A common case is expected to be
   that of a signed message that passes through a mailing list that also
   signs all messages.  Assuming both of those signatures verify, a
   recipient might choose to accept the message if either of those
   signatures were known to come from trusted sources.

      INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing
      lists to which they have subscribed and that have acceptable anti-
      abuse policies so as to accept messages sent to that list even
      from unknown authors.  They might also subscribe to less trusted
      mailing lists (e.g., those without anti-abuse protection) and be
      willing to accept all messages from specific authors, but insist
      on doing additional abuse scanning for other messages.




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   Another related example of multiple signers might be forwarding
   services, such as those commonly associated with academic alumni
   sites.

      INFORMATIVE EXAMPLE: A recipient might have an address at
      members.example.org, a site that has anti-abuse protection that is
      somewhat less effective than the recipient would prefer.  Such a
      recipient might have specific authors whose messages would be
      trusted absolutely, but messages from unknown authors that had
      passed the forwarder's scrutiny would have only medium trust.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Interpretation</span>

   A signer that is adding a signature to a message merely creates a new
   DKIM-Signature header, using the usual semantics of the h= option.  A
   signer MAY sign previously existing DKIM-Signature header fields
   using the method described in <a href="#section-5.4">Section 5.4</a> to sign trace header
   fields.

      INFORMATIVE NOTE: Signers should be cognizant that signing DKIM-
      Signature header fields may result in signature failures with
      intermediaries that do not recognize that DKIM-Signature header
      fields are trace header fields and unwittingly reorder them, thus
      breaking such signatures.  For this reason, signing existing DKIM-
      Signature header fields is unadvised, albeit legal.

      INFORMATIVE NOTE: If a header field with multiple instances is
      signed, those header fields are always signed from the bottom up.
      Thus, it is not possible to sign only specific DKIM-Signature
      header fields.  For example, if the message being signed already
      contains three DKIM-Signature header fields A, B, and C, it is
      possible to sign all of them, B and C only, or C only, but not A
      only, B only, A and B only, or A and C only.

   A signer MAY add more than one DKIM-Signature header field using
   different parameters.  For example, during a transition period a
   signer might want to produce signatures using two different hash
   algorithms.

   Signers SHOULD NOT remove any DKIM-Signature header fields from
   messages they are signing, even if they know that the signatures
   cannot be verified.

   When evaluating a message with multiple signatures, a verifier SHOULD
   evaluate signatures independently and on their own merits.  For
   example, a verifier that by policy chooses not to accept signatures
   with deprecated cryptographic algorithms would consider such
   signatures invalid.  Verifiers MAY process signatures in any order of



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   their choice; for example, some verifiers might choose to process
   signatures corresponding to the From field in the message header
   before other signatures.  See <a href="#section-6.1">Section 6.1</a> for more information about
   signature choices.

      INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate
      valid signatures with invalid signatures in an attempt to guess
      why a signature failed are ill-advised.  In particular, there is
      no general way that a verifier can determine that an invalid
      signature was ever valid.

   Verifiers SHOULD ignore failed signatures as though they were not
   present in the message.  Verifiers SHOULD continue to check
   signatures until a signature successfully verifies to the
   satisfaction of the verifier.  To limit potential denial-of-service
   attacks, verifiers MAY limit the total number of signatures they will
   attempt to verify.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Signer Actions</span>

   The following steps are performed in order by signers.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Determine Whether the Email Should Be Signed and by Whom</span>

   A signer can obviously only sign email for domains for which it has a
   private key and the necessary knowledge of the corresponding public
   key and selector information.  However, there are a number of other
   reasons beyond the lack of a private key why a signer could choose
   not to sign an email.

      INFORMATIVE NOTE: Signing modules may be incorporated into any
      portion of the mail system as deemed appropriate, including an
      MUA, a SUBMISSION server, or an MTA.  Wherever implemented,
      signers should beware of signing (and thereby asserting
      responsibility for) messages that may be problematic.  In
      particular, within a trusted enclave the signing address might be
      derived from the header according to local policy; SUBMISSION
      servers might only sign messages from users that are properly
      authenticated and authorized.

      INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign
      Received header fields if the outgoing gateway MTA obfuscates
      Received header fields, for example, to hide the details of
      internal topology.

   If an email cannot be signed for some reason, it is a local policy
   decision as to what to do with that email.




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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Select a Private Key and Corresponding Selector Information</span>

   This specification does not define the basis by which a signer should
   choose which private key and selector information to use.  Currently,
   all selectors are equal as far as this specification is concerned, so
   the decision should largely be a matter of administrative
   convenience.  Distribution and management of private keys is also
   outside the scope of this document.

      INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
      private key when the selector containing the corresponding public
      key is expected to be revoked or removed before the verifier has
      an opportunity to validate the signature.  The signer should
      anticipate that verifiers may choose to defer validation, perhaps
      until the message is actually read by the final recipient.  In
      particular, when rotating to a new key pair, signing should
      immediately commence with the new private key and the old public
      key should be retained for a reasonable validation interval before
      being removed from the key server.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Normalize the Message to Prevent Transport Conversions</span>

   Some messages, particularly those using 8-bit characters, are subject
   to modification during transit, notably conversion to 7-bit form.
   Such conversions will break DKIM signatures.  In order to minimize
   the chances of such breakage, signers SHOULD convert the message to a
   suitable MIME content transfer encoding such as quoted-printable or
   base64 as described in MIME Part One [<a href="./rfc2045" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies&quot;">RFC2045</a>] before signing.  Such
   conversion is outside the scope of DKIM; the actual message SHOULD be
   converted to 7-bit MIME by an MUA or MSA prior to presentation to the
   DKIM algorithm.

   If the message is submitted to the signer with any local encoding
   that will be modified before transmission, that modification to
   canonical [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>] form MUST be done before signing.  In particular,
   bare CR or LF characters (used by some systems as a local line
   separator convention) MUST be converted to the SMTP-standard CRLF
   sequence before the message is signed.  Any conversion of this sort
   SHOULD be applied to the message actually sent to the recipient(s),
   not just to the version presented to the signing algorithm.

   More generally, the signer MUST sign the message as it is expected to
   be received by the verifier rather than in some local or internal
   form.







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<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Determine the Header Fields to Sign</span>

   The From header field MUST be signed (that is, included in the "h="
   tag of the resulting DKIM-Signature header field).  Signers SHOULD
   NOT sign an existing header field likely to be legitimately modified
   or removed in transit.  In particular, [<a href="./rfc2821" title="&quot;Simple Mail Transfer Protocol&quot;">RFC2821</a>] explicitly permits
   modification or removal of the Return-Path header field in transit.
   Signers MAY include any other header fields present at the time of
   signing at the discretion of the signer.

      INFORMATIVE OPERATIONS NOTE: The choice of which header fields to
      sign is non-obvious.  One strategy is to sign all existing, non-
      repeatable header fields.  An alternative strategy is to sign only
      header fields that are likely to be displayed to or otherwise be
      likely to affect the processing of the message at the receiver.  A
      third strategy is to sign only "well known" headers.  Note that
      verifiers may treat unsigned header fields with extreme
      skepticism, including refusing to display them to the end user or
      even ignoring the signature if it does not cover certain header
      fields.  For this reason, signing fields present in the message
      such as Date, Subject, Reply-To, Sender, and all MIME header
      fields are highly advised.

   The DKIM-Signature header field is always implicitly signed and MUST
   NOT be included in the "h=" tag except to indicate that other
   preexisting signatures are also signed.

   Signers MAY claim to have signed header fields that do not exist
   (that is, signers MAY include the header field name in the "h=" tag
   even if that header field does not exist in the message).  When
   computing the signature, the non-existing header field MUST be
   treated as the null string (including the header field name, header
   field value, all punctuation, and the trailing CRLF).

      INFORMATIVE RATIONALE: This allows signers to explicitly assert
      the absence of a header field; if that header field is added later
      the signature will fail.

      INFORMATIVE NOTE: A header field name need only be listed once
      more than the actual number of that header field in a message at
      the time of signing in order to prevent any further additions.
      For example, if there is a single Comments header field at the
      time of signing, listing Comments twice in the "h=" tag is
      sufficient to prevent any number of Comments header fields from
      being appended; it is not necessary (but is legal) to list
      Comments three or more times in the "h=" tag.





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<span class="grey"><a href="./rfc4871">RFC 4871</a>                    DKIM Signatures                     May 2007</span>


   Signers choosing to sign an existing header field that occurs more
   than once in the message (such as Received) MUST sign the physically
   last instance of that header field in the header block.  Signers
   wishing to sign multiple instances of such a header field MUST
   include the header field name multiple times in the h= tag of the
   DKIM-Signature header field, and MUST sign such header fields in
   order from the bottom of the header field block to the top.  The
   signer MAY include more instances of a header field name in h= than
   there are actual corresponding header fields to indicate that
   additional header fields of that name SHOULD NOT be added.

      INFORMATIVE EXAMPLE:

      If the signer wishes to sign two existing Received header fields,
      and the existing header contains:

       Received: &lt;A&gt;
       Received: &lt;B&gt;
       Received: &lt;C&gt;

      then the resulting DKIM-Signature header field should read:

       DKIM-Signature: ... h=Received : Received : ...

      and Received header fields &lt;C&gt; and &lt;B&gt; will be signed in that
      order.

   Signers should be careful of signing header fields that might have
   additional instances added later in the delivery process, since such
   header fields might be inserted after the signed instance or
   otherwise reordered.  Trace header fields (such as Received) and
   Resent-* blocks are the only fields prohibited by [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>] from
   being reordered.  In particular, since DKIM-Signature header fields
   may be reordered by some intermediate MTAs, signing existing DKIM-
   Signature header fields is error-prone.

      INFORMATIVE ADMONITION: Despite the fact that [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>] permits
      header fields to be reordered (with the exception of Received
      header fields), reordering of signed header fields with multiple
      instances by intermediate MTAs will cause DKIM signatures to be
      broken; such anti-social behavior should be avoided.

      INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this
      specification, all end-user visible header fields should be signed
      to avoid possible "indirect spamming".  For example, if the
      Subject header field is not signed, a spammer can resend a
      previously signed mail, replacing the legitimate subject with a
      one-line spam.



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<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>.  Recommended Signature Content</span>

   In order to maximize compatibility with a variety of verifiers, it is
   recommended that signers follow the practices outlined in this
   section when signing a message.  However, these are generic
   recommendations applying to the general case; specific senders may
   wish to modify these guidelines as required by their unique
   situations.  Verifiers MUST be capable of verifying signatures even
   if one or more of the recommended header fields is not signed (with
   the exception of From, which must always be signed) or if one or more
   of the disrecommended header fields is signed.  Note that verifiers
   do have the option of ignoring signatures that do not cover a
   sufficient portion of the header or body, just as they may ignore
   signatures from an identity they do not trust.

   The following header fields SHOULD be included in the signature, if
   they are present in the message being signed:

   o  From (REQUIRED in all signatures)

   o  Sender, Reply-To

   o  Subject

   o  Date, Message-ID

   o  To, Cc

   o  MIME-Version

   o  Content-Type, Content-Transfer-Encoding, Content-ID, Content-
      Description

   o  Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-Cc,
      Resent-Message-ID

   o  In-Reply-To, References

   o  List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post,
      List-Owner, List-Archive

   The following header fields SHOULD NOT be included in the signature:

   o  Return-Path

   o  Received

   o  Comments, Keywords



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   o  Bcc, Resent-Bcc

   o  DKIM-Signature

   Optional header fields (those not mentioned above) normally SHOULD
   NOT be included in the signature, because of the potential for
   additional header fields of the same name to be legitimately added or
   reordered prior to verification.  There are likely to be legitimate
   exceptions to this rule, because of the wide variety of application-
   specific header fields that may be applied to a message, some of
   which are unlikely to be duplicated, modified, or reordered.

   Signers SHOULD choose canonicalization algorithms based on the types
   of messages they process and their aversion to risk.  For example,
   e-commerce sites sending primarily purchase receipts, which are not
   expected to be processed by mailing lists or other software likely to
   modify messages, will generally prefer "simple" canonicalization.
   Sites sending primarily person-to-person email will likely prefer to
   be more resilient to modification during transport by using "relaxed"
   canonicalization.

   Signers SHOULD NOT use "l=" unless they intend to accommodate
   intermediate mail processors that append text to a message.  For
   example, many mailing list processors append "unsubscribe"
   information to message bodies.  If signers use "l=", they SHOULD
   include the entire message body existing at the time of signing in
   computing the count.  In particular, signers SHOULD NOT specify a
   body length of 0 since this may be interpreted as a meaningless
   signature by some verifiers.

<span class="h3"><a class="selflink" id="section-5.6" href="#section-5.6">5.6</a>.  Compute the Message Hash and Signature</span>

   The signer MUST compute the message hash as described in <a href="#section-3.7">Section 3.7</a>
   and then sign it using the selected public-key algorithm.  This will
   result in a DKIM-Signature header field that will include the body
   hash and a signature of the header hash, where that header includes
   the DKIM-Signature header field itself.

   Entities such as mailing list managers that implement DKIM and that
   modify the message or a header field (for example, inserting
   unsubscribe information) before retransmitting the message SHOULD
   check any existing signature on input and MUST make such
   modifications before re-signing the message.

   The signer MAY elect to limit the number of bytes of the body that
   will be included in the hash and hence signed.  The length actually
   hashed should be inserted in the "l=" tag of the DKIM-Signature
   header field.



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<span class="h3"><a class="selflink" id="section-5.7" href="#section-5.7">5.7</a>.  Insert the DKIM-Signature Header Field</span>

   Finally, the signer MUST insert the DKIM-Signature header field
   created in the previous step prior to transmitting the email.  The
   DKIM-Signature header field MUST be the same as used to compute the
   hash as described above, except that the value of the "b=" tag MUST
   be the appropriately signed hash computed in the previous step,
   signed using the algorithm specified in the "a=" tag of the DKIM-
   Signature header field and using the private key corresponding to the
   selector given in the "s=" tag of the DKIM-Signature header field, as
   chosen above in <a href="#section-5.2">Section 5.2</a>

   The DKIM-Signature header field MUST be inserted before any other
   DKIM-Signature fields in the header block.

      INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this
      is to insert the DKIM-Signature header field at the beginning of
      the header block.  In particular, it may be placed before any
      existing Received header fields.  This is consistent with treating
      DKIM-Signature as a trace header field.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Verifier Actions</span>

   Since a signer MAY remove or revoke a public key at any time, it is
   recommended that verification occur in a timely manner.  In many
   configurations, the most timely place is during acceptance by the
   border MTA or shortly thereafter.  In particular, deferring
   verification until the message is accessed by the end user is
   discouraged.

   A border or intermediate MTA MAY verify the message signature(s).  An
   MTA who has performed verification MAY communicate the result of that
   verification by adding a verification header field to incoming
   messages.  This considerably simplifies things for the user, who can
   now use an existing mail user agent.  Most MUAs have the ability to
   filter messages based on message header fields or content; these
   filters would be used to implement whatever policy the user wishes
   with respect to unsigned mail.

   A verifying MTA MAY implement a policy with respect to unverifiable
   mail, regardless of whether or not it applies the verification header
   field to signed messages.

   Verifiers MUST produce a result that is semantically equivalent to
   applying the following steps in the order listed.  In practice,
   several of these steps can be performed in parallel in order to
   improve performance.




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<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Extract Signatures from the Message</span>

   The order in which verifiers try DKIM-Signature header fields is not
   defined; verifiers MAY try signatures in any order they like.  For
   example, one implementation might try the signatures in textual
   order, whereas another might try signatures by identities that match
   the contents of the From header field before trying other signatures.
   Verifiers MUST NOT attribute ultimate meaning to the order of
   multiple DKIM-Signature header fields.  In particular, there is
   reason to believe that some relays will reorder the header fields in
   potentially arbitrary ways.

      INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as
      a clue to signing order in the absence of any other information.
      However, other clues as to the semantics of multiple signatures
      (such as correlating the signing host with Received header fields)
      may also be considered.

   A verifier SHOULD NOT treat a message that has one or more bad
   signatures and no good signatures differently from a message with no
   signature at all; such treatment is a matter of local policy and is
   beyond the scope of this document.

   When a signature successfully verifies, a verifier will either stop
   processing or attempt to verify any other signatures, at the
   discretion of the implementation.  A verifier MAY limit the number of
   signatures it tries to avoid denial-of-service attacks.

      INFORMATIVE NOTE: An attacker could send messages with large
      numbers of faulty signatures, each of which would require a DNS
      lookup and corresponding CPU time to verify the message.  This
      could be an attack on the domain that receives the message, by
      slowing down the verifier by requiring it to do a large number of
      DNS lookups and/or signature verifications.  It could also be an
      attack against the domains listed in the signatures, essentially
      by enlisting innocent verifiers in launching an attack against the
      DNS servers of the actual victim.

   In the following description, text reading "return status
   (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
   means that the verifier MUST immediately cease processing that
   signature.  The verifier SHOULD proceed to the next signature, if any
   is present, and completely ignore the bad signature.  If the status
   is "PERMFAIL", the signature failed and should not be reconsidered.
   If the status is "TEMPFAIL", the signature could not be verified at
   this time but may be tried again later.  A verifier MAY either defer
   the message for later processing, perhaps by queueing it locally or
   issuing a 451/4.7.5 SMTP reply, or try another signature; if no good



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   signature is found and any of the signatures resulted in a TEMPFAIL
   status, the verifier MAY save the message for later processing.  The
   "(explanation)" is not normative text; it is provided solely for
   clarification.

   Verifiers SHOULD ignore any DKIM-Signature header fields where the
   signature does not validate.  Verifiers that are prepared to validate
   multiple signature header fields SHOULD proceed to the next signature
   header field, should it exist.  However, verifiers MAY make note of
   the fact that an invalid signature was present for consideration at a
   later step.

      INFORMATIVE NOTE: The rationale of this requirement is to permit
      messages that have invalid signatures but also a valid signature
      to work.  For example, a mailing list exploder might opt to leave
      the original submitter signature in place even though the exploder
      knows that it is modifying the message in some way that will break
      that signature, and the exploder inserts its own signature.  In
      this case, the message should succeed even in the presence of the
      known-broken signature.

   For each signature to be validated, the following steps should be
   performed in such a manner as to produce a result that is
   semantically equivalent to performing them in the indicated order.

<span class="h4"><a class="selflink" id="section-6.1.1" href="#section-6.1.1">6.1.1</a>.  Validate the Signature Header Field</span>

   Implementers MUST meticulously validate the format and values in the
   DKIM-Signature header field; any inconsistency or unexpected values
   MUST cause the header field to be completely ignored and the verifier
   to return PERMFAIL (signature syntax error).  Being "liberal in what
   you accept" is definitely a bad strategy in this security context.
   Note however that this does not include the existence of unknown tags
   in a DKIM-Signature header field, which are explicitly permitted.

   Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag
   that is inconsistent with this specification and return PERMFAIL
   (incompatible version).

      INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course,
      choose to also verify signatures generated by older versions of
      this specification.

   If any tag listed as "required" in <a href="#section-3.5">Section 3.5</a> is omitted from the
   DKIM-Signature header field, the verifier MUST ignore the DKIM-
   Signature header field and return PERMFAIL (signature missing
   required tag).




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      INFORMATIONAL NOTE: The tags listed as required in <a href="#section-3.5">Section 3.5</a> are
      "v=", "a=", "b=", "bh=", "d=", "h=", and "s=".  Should there be a
      conflict between this note and <a href="#section-3.5">Section 3.5</a>, <a href="#section-3.5">Section 3.5</a> is
      normative.

   If the DKIM-Signature header field does not contain the "i=" tag, the
   verifier MUST behave as though the value of that tag were "@d", where
   "d" is the value from the "d=" tag.

   Verifiers MUST confirm that the domain specified in the "d=" tag is
   the same as or a parent domain of the domain part of the "i=" tag.
   If not, the DKIM-Signature header field MUST be ignored and the
   verifier should return PERMFAIL (domain mismatch).

   If the "h=" tag does not include the From header field, the verifier
   MUST ignore the DKIM-Signature header field and return PERMFAIL (From
   field not signed).

   Verifiers MAY ignore the DKIM-Signature header field and return
   PERMFAIL (signature expired) if it contains an "x=" tag and the
   signature has expired.

   Verifiers MAY ignore the DKIM-Signature header field if the domain
   used by the signer in the "d=" tag is not associated with a valid
   signing entity.  For example, signatures with "d=" values such as
   "com" and "co.uk" may be ignored.  The list of unacceptable domains
   SHOULD be configurable.

   Verifiers MAY ignore the DKIM-Signature header field and return
   PERMFAIL (unacceptable signature header) for any other reason, for
   example, if the signature does not sign header fields that the
   verifier views to be essential.  As a case in point, if MIME header
   fields are not signed, certain attacks may be possible that the
   verifier would prefer to avoid.

<span class="h4"><a class="selflink" id="section-6.1.2" href="#section-6.1.2">6.1.2</a>.  Get the Public Key</span>

   The public key for a signature is needed to complete the verification
   process.  The process of retrieving the public key depends on the
   query type as defined by the "q=" tag in the DKIM-Signature header
   field.  Obviously, a public key need only be retrieved if the process
   of extracting the signature information is completely successful.
   Details of key management and representation are described in
   <a href="#section-3.6">Section 3.6</a>.  The verifier MUST validate the key record and MUST
   ignore any public key records that are malformed.

   When validating a message, a verifier MUST perform the following
   steps in a manner that is semantically the same as performing them in



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   the order indicated (in some cases, the implementation may
   parallelize or reorder these steps, as long as the semantics remain
   unchanged):

   1.  Retrieve the public key as described in <a href="#section-3.6">Section 3.6</a> using the
       algorithm in the "q=" tag, the domain from the "d=" tag, and the
       selector from the "s=" tag.

   2.  If the query for the public key fails to respond, the verifier
       MAY defer acceptance of this email and return TEMPFAIL (key
       unavailable).  If verification is occurring during the incoming
       SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
       code.  Alternatively, the verifier MAY store the message in the
       local queue for later trial or ignore the signature.  Note that
       storing a message in the local queue is subject to denial-of-
       service attacks.

   3.  If the query for the public key fails because the corresponding
       key record does not exist, the verifier MUST immediately return
       PERMFAIL (no key for signature).

   4.  If the query for the public key returns multiple key records, the
       verifier may choose one of the key records or may cycle through
       the key records performing the remainder of these steps on each
       record at the discretion of the implementer.  The order of the
       key records is unspecified.  If the verifier chooses to cycle
       through the key records, then the "return ..." wording in the
       remainder of this section means "try the next key record, if any;
       if none, return to try another signature in the usual way".

   5.  If the result returned from the query does not adhere to the
       format defined in this specification, the verifier MUST ignore
       the key record and return PERMFAIL (key syntax error).  Verifiers
       are urged to validate the syntax of key records carefully to
       avoid attempted attacks.  In particular, the verifier MUST ignore
       keys with a version code ("v=" tag) that they do not implement.

   6.  If the "g=" tag in the public key does not match the Local-part
       of the "i=" tag in the message signature header field, the
       verifier MUST ignore the key record and return PERMFAIL
       (inapplicable key).  If the Local-part of the "i=" tag on the
       message signature is not present, the "g=" tag must be "*" (valid
       for all addresses in the domain) or the entire g= tag must be
       omitted (which defaults to "g=*"), otherwise the verifier MUST
       ignore the key record and return PERMFAIL (inapplicable key).
       Other than this test, verifiers SHOULD NOT treat a message signed
       with a key record having a "g=" tag any differently than one
       without; in particular, verifiers SHOULD NOT prefer messages that



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       seem to have an individual signature by virtue of a "g=" tag
       versus a domain signature.

   7.  If the "h=" tag exists in the public key record and the hash
       algorithm implied by the a= tag in the DKIM-Signature header
       field is not included in the contents of the "h=" tag, the
       verifier MUST ignore the key record and return PERMFAIL
       (inappropriate hash algorithm).

   8.  If the public key data (the "p=" tag) is empty, then this key has
       been revoked and the verifier MUST treat this as a failed
       signature check and return PERMFAIL (key revoked).  There is no
       defined semantic difference between a key that has been revoked
       and a key record that has been removed.

   9.  If the public key data is not suitable for use with the algorithm
       and key types defined by the "a=" and "k=" tags in the DKIM-
       Signature header field, the verifier MUST immediately return
       PERMFAIL (inappropriate key algorithm).

<span class="h4"><a class="selflink" id="section-6.1.3" href="#section-6.1.3">6.1.3</a>.  Compute the Verification</span>

   Given a signer and a public key, verifying a signature consists of
   actions semantically equivalent to the following steps.

   1.  Based on the algorithm defined in the "c=" tag, the body length
       specified in the "l=" tag, and the header field names in the "h="
       tag, prepare a canonicalized version of the message as is
       described in <a href="#section-3.7">Section 3.7</a> (note that this version does not
       actually need to be instantiated).  When matching header field
       names in the "h=" tag against the actual message header field,
       comparisons MUST be case-insensitive.

   2.  Based on the algorithm indicated in the "a=" tag, compute the
       message hashes from the canonical copy as described in
       <a href="#section-3.7">Section 3.7</a>.

   3.  Verify that the hash of the canonicalized message body computed
       in the previous step matches the hash value conveyed in the "bh="
       tag.  If the hash does not match, the verifier SHOULD ignore the
       signature and return PERMFAIL (body hash did not verify).

   4.  Using the signature conveyed in the "b=" tag, verify the
       signature against the header hash using the mechanism appropriate
       for the public key algorithm described in the "a=" tag.  If the
       signature does not validate, the verifier SHOULD ignore the
       signature and return PERMFAIL (signature did not verify).




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   5.  Otherwise, the signature has correctly verified.

      INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to
      initiate the public-key query in parallel with calculating the
      hash as the public key is not needed until the final decryption is
      calculated.  Implementations may also verify the signature on the
      message header before validating that the message hash listed in
      the "bh=" tag in the DKIM-Signature header field matches that of
      the actual message body; however, if the body hash does not match,
      the entire signature must be considered to have failed.

   A body length specified in the "l=" tag of the signature limits the
   number of bytes of the body passed to the verification algorithm.
   All data beyond that limit is not validated by DKIM.  Hence,
   verifiers might treat a message that contains bytes beyond the
   indicated body length with suspicion, such as by truncating the
   message at the indicated body length, declaring the signature invalid
   (e.g., by returning PERMFAIL (unsigned content)), or conveying the
   partial verification to the policy module.

      INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
      at the indicated body length might pass on a malformed MIME
      message if the signer used the "N-4" trick (omitting the final
      "--CRLF") described in the informative note in <a href="#section-3.4.5">Section 3.4.5</a>.
      Such verifiers may wish to check for this case and include a
      trailing "--CRLF" to avoid breaking the MIME structure.  A simple
      way to achieve this might be to append "--CRLF" to any "multipart"
      message with a body length; if the MIME structure is already
      correctly formed, this will appear in the postlude and will not be
      displayed to the end user.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Communicate Verification Results</span>

   Verifiers wishing to communicate the results of verification to other
   parts of the mail system may do so in whatever manner they see fit.
   For example, implementations might choose to add an email header
   field to the message before passing it on.  Any such header field
   SHOULD be inserted before any existing DKIM-Signature or preexisting
   authentication status header fields in the header field block.

      INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
      search for results header fields to visibly mark authenticated
      mail for end users should verify that such header field was added
      by the appropriate verifying domain and that the verified identity
      matches the author identity that will be displayed by the MUA.  In
      particular, MUA filters should not be influenced by bogus results





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      header fields added by attackers.  To circumvent this attack,
      verifiers may wish to delete existing results header fields after
      verification and before adding a new header field.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Interpret Results/Apply Local Policy</span>

   It is beyond the scope of this specification to describe what actions
   a verifier system should make, but an authenticated email presents an
   opportunity to a receiving system that unauthenticated email cannot.
   Specifically, an authenticated email creates a predictable identifier
   by which other decisions can reliably be managed, such as trust and
   reputation.  Conversely, unauthenticated email lacks a reliable
   identifier that can be used to assign trust and reputation.  It is
   reasonable to treat unauthenticated email as lacking any trust and
   having no positive reputation.

   In general, verifiers SHOULD NOT reject messages solely on the basis
   of a lack of signature or an unverifiable signature; such rejection
   would cause severe interoperability problems.  However, if the
   verifier does opt to reject such messages (for example, when
   communicating with a peer who, by prior agreement, agrees to only
   send signed messages), and the verifier runs synchronously with the
   SMTP session and a signature is missing or does not verify, the MTA
   SHOULD use a 550/5.7.x reply code.

   If it is not possible to fetch the public key, perhaps because the
   key server is not available, a temporary failure message MAY be
   generated using a 451/4.7.5 reply code, such as:

      451 4.7.5 Unable to verify signature - key server unavailable

   Temporary failures such as inability to access the key server or
   other external service are the only conditions that SHOULD use a 4xx
   SMTP reply code.  In particular, cryptographic signature verification
   failures MUST NOT return 4xx SMTP replies.

   Once the signature has been verified, that information MUST be
   conveyed to higher-level systems (such as explicit allow/whitelists
   and reputation systems) and/or to the end user.  If the message is
   signed on behalf of any address other than that in the From: header
   field, the mail system SHOULD take pains to ensure that the actual
   signing identity is clear to the reader.

   The verifier MAY treat unsigned header fields with extreme
   skepticism, including marking them as untrusted or even deleting them
   before display to the end user.





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   While the symptoms of a failed verification are obvious -- the
   signature doesn't verify -- establishing the exact cause can be more
   difficult.  If a selector cannot be found, is that because the
   selector has been removed, or was the value changed somehow in
   transit?  If the signature line is missing, is that because it was
   never there, or was it removed by an overzealous filter?  For
   diagnostic purposes, the exact reason why the verification fails
   SHOULD be made available to the policy module and possibly recorded
   in the system logs.  If the email cannot be verified, then it SHOULD
   be rendered the same as all unverified email regardless of whether or
   not it looks like it was signed.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IANA Considerations</span>

   DKIM introduces some new namespaces that have been registered with
   IANA.  In all cases, new values are assigned only for values that
   have been documented in a published RFC that has IETF Consensus
   [<a href="./rfc2434" title="">RFC2434</a>].

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  DKIM-Signature Tag Specifications</span>

   A DKIM-Signature provides for a list of tag specifications.  IANA has
   established the DKIM-Signature Tag Specification Registry for tag
   specifications that can be used in DKIM-Signature fields.

               The initial entries in the registry comprise:

                        +------+-----------------+
                        | TYPE | REFERENCE       |
                        +------+-----------------+
                        | v    | (this document) |
                        | a    | (this document) |
                        | b    | (this document) |
                        | bh   | (this document) |
                        | c    | (this document) |
                        | d    | (this document) |
                        | h    | (this document) |
                        | i    | (this document) |
                        | l    | (this document) |
                        | q    | (this document) |
                        | s    | (this document) |
                        | t    | (this document) |
                        | x    | (this document) |
                        | z    | (this document) |
                        +------+-----------------+

         DKIM-Signature Tag Specification Registry Initial Values




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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  DKIM-Signature Query Method Registry</span>

   The "q=" tag-spec (specified in <a href="#section-3.5">Section 3.5</a>) provides for a list of
   query methods.

   IANA has established the DKIM-Signature Query Method Registry for
   mechanisms that can be used to retrieve the key that will permit
   validation processing of a message signed using DKIM.

               The initial entry in the registry comprises:

                    +------+--------+-----------------+
                    | TYPE | OPTION | REFERENCE       |
                    +------+--------+-----------------+
                    | dns  | txt    | (this document) |
                    +------+--------+-----------------+

            DKIM-Signature Query Method Registry Initial Values

<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  DKIM-Signature Canonicalization Registry</span>

   The "c=" tag-spec (specified in <a href="#section-3.5">Section 3.5</a>) provides for a specifier
   for canonicalization algorithms for the header and body of the
   message.

   IANA has established the DKIM-Signature Canonicalization Algorithm
   Registry for algorithms for converting a message into a canonical
   form before signing or verifying using DKIM.

           The initial entries in the header registry comprise:

                       +---------+-----------------+
                       | TYPE    | REFERENCE       |
                       +---------+-----------------+
                       | simple  | (this document) |
                       | relaxed | (this document) |
                       +---------+-----------------+

        DKIM-Signature Header Canonicalization Algorithm Registry
                              Initial Values











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            The initial entries in the body registry comprise:

                       +---------+-----------------+
                       | TYPE    | REFERENCE       |
                       +---------+-----------------+
                       | simple  | (this document) |
                       | relaxed | (this document) |
                       +---------+-----------------+

         DKIM-Signature Body Canonicalization Algorithm Registry
                              Initial Values

<span class="h3"><a class="selflink" id="section-7.4" href="#section-7.4">7.4</a>.  _domainkey DNS TXT Record Tag Specifications</span>

   A _domainkey DNS TXT record provides for a list of tag
   specifications.  IANA has established the DKIM _domainkey DNS TXT Tag
   Specification Registry for tag specifications that can be used in DNS
   TXT Records.

               The initial entries in the registry comprise:

                        +------+-----------------+
                        | TYPE | REFERENCE       |
                        +------+-----------------+
                        | v    | (this document) |
                        | g    | (this document) |
                        | h    | (this document) |
                        | k    | (this document) |
                        | n    | (this document) |
                        | p    | (this document) |
                        | s    | (this document) |
                        | t    | (this document) |
                        +------+-----------------+

         DKIM _domainkey DNS TXT Record Tag Specification Registry
                              Initial Values

<span class="h3"><a class="selflink" id="section-7.5" href="#section-7.5">7.5</a>.  DKIM Key Type Registry</span>

   The "k=" &lt;key-k-tag&gt; (specified in <a href="#section-3.6.1">Section 3.6.1</a>) and the "a=" &lt;sig-
   a-tag-k&gt; (specified in <a href="#section-3.5">Section 3.5</a>) tags provide for a list of
   mechanisms that can be used to decode a DKIM signature.

   IANA has established the DKIM Key Type Registry for such mechanisms.







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               The initial entry in the registry comprises:

                           +------+-----------+
                           | TYPE | REFERENCE |
                           +------+-----------+
                           | rsa  | [<a href="./rfc3447" title="&quot;Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1&quot;">RFC3447</a>] |
                           +------+-----------+

                       DKIM Key Type Initial Values

<span class="h3"><a class="selflink" id="section-7.6" href="#section-7.6">7.6</a>.  DKIM Hash Algorithms Registry</span>

   The "h=" &lt;key-h-tag&gt; (specified in <a href="#section-3.6.1">Section 3.6.1</a>) and the "a=" &lt;sig-
   a-tag-h&gt; (specified in <a href="#section-3.5">Section 3.5</a>) tags provide for a list of
   mechanisms that can be used to produce a digest of message data.

   IANA has established the DKIM Hash Algorithms Registry for such
   mechanisms.

               The initial entries in the registry comprise:

                      +--------+-------------------+
                      | TYPE   | REFERENCE         |
                      +--------+-------------------+
                      | sha1   | [<a href="#ref-FIPS.180-2.2002" title="&quot;Secure Hash Standard&quot;">FIPS.180-2.2002</a>] |
                      | sha256 | [<a href="#ref-FIPS.180-2.2002" title="&quot;Secure Hash Standard&quot;">FIPS.180-2.2002</a>] |
                      +--------+-------------------+

                    DKIM Hash Algorithms Initial Values

<span class="h3"><a class="selflink" id="section-7.7" href="#section-7.7">7.7</a>.  DKIM Service Types Registry</span>

   The "s=" &lt;key-s-tag&gt; tag (specified in <a href="#section-3.6.1">Section 3.6.1</a>) provides for a
   list of service types to which this selector may apply.

   IANA has established the DKIM Service Types Registry for service
   types.

               The initial entries in the registry comprise:

                        +-------+-----------------+
                        | TYPE  | REFERENCE       |
                        +-------+-----------------+
                        | email | (this document) |
                        | *     | (this document) |
                        +-------+-----------------+

                DKIM Service Types Registry Initial Values



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<span class="h3"><a class="selflink" id="section-7.8" href="#section-7.8">7.8</a>.  DKIM Selector Flags Registry</span>

   The "t=" &lt;key-t-tag&gt; tag (specified in <a href="#section-3.6.1">Section 3.6.1</a>) provides for a
   list of flags to modify interpretation of the selector.

   IANA has established the DKIM Selector Flags Registry for additional
   flags.

               The initial entries in the registry comprise:

                        +------+-----------------+
                        | TYPE | REFERENCE       |
                        +------+-----------------+
                        | y    | (this document) |
                        | s    | (this document) |
                        +------+-----------------+

                DKIM Selector Flags Registry Initial Values

<span class="h3"><a class="selflink" id="section-7.9" href="#section-7.9">7.9</a>.  DKIM-Signature Header Field</span>

   IANA has added DKIM-Signature to the "Permanent Message Header
   Fields" registry (see [<a href="./rfc3864" title="&quot;Registration Procedures for Message Header Fields&quot;">RFC3864</a>]) for the "mail" protocol, using this
   document as the reference.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Security Considerations</span>

   It has been observed that any mechanism that is introduced that
   attempts to stem the flow of spam is subject to intensive attack.
   DKIM needs to be carefully scrutinized to identify potential attack
   vectors and the vulnerability to each.  See also [<a href="./rfc4686" title="&quot;Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)&quot;">RFC4686</a>].

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Misuse of Body Length Limits ("l=" Tag)</span>

   Body length limits (in the form of the "l=" tag) are subject to
   several potential attacks.

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>.  Addition of New MIME Parts to Multipart/*</span>

   If the body length limit does not cover a closing MIME multipart
   section (including the trailing "--CRLF" portion), then it is
   possible for an attacker to intercept a properly signed multipart
   message and add a new body part.  Depending on the details of the
   MIME type and the implementation of the verifying MTA and the
   receiving MUA, this could allow an attacker to change the information
   displayed to an end user from an apparently trusted source.





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   For example, if attackers can append information to a "text/html"
   body part, they may be able to exploit a bug in some MUAs that
   continue to read after a "&lt;/html&gt;" marker, and thus display HTML text
   on top of already displayed text.  If a message has a
   "multipart/alternative" body part, they might be able to add a new
   body part that is preferred by the displaying MUA.

<span class="h4"><a class="selflink" id="section-8.1.2" href="#section-8.1.2">8.1.2</a>.  Addition of new HTML content to existing content</span>

   Several receiving MUA implementations do not cease display after a
   ""&lt;/html&gt;"" tag.  In particular, this allows attacks involving
   overlaying images on top of existing text.

      INFORMATIVE EXAMPLE: Appending the following text to an existing,
      properly closed message will in many MUAs result in inappropriate
      data being rendered on top of existing, correct data:
   &lt;div style="position: relative; bottom: 350px; z-index: 2;"&gt;
   &lt;img src="http://www.ietf.org/images/ietflogo2e.gif"
     width=578 height=370&gt;
   &lt;/div&gt;

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Misappropriated Private Key</span>

   If the private key for a user is resident on their computer and is
   not protected by an appropriately secure mechanism, it is possible
   for malware to send mail as that user and any other user sharing the
   same private key.  The malware would not, however, be able to
   generate signed spoofs of other signers' addresses, which would aid
   in identification of the infected user and would limit the
   possibilities for certain types of attacks involving socially
   engineered messages.  This threat applies mainly to MUA-based
   implementations; protection of private keys on servers can be easily
   achieved through the use of specialized cryptographic hardware.

   A larger problem occurs if malware on many users' computers obtains
   the private keys for those users and transmits them via a covert
   channel to a site where they can be shared.  The compromised users
   would likely not know of the misappropriation until they receive
   "bounce" messages from messages they are purported to have sent.
   Many users might not understand the significance of these bounce
   messages and would not take action.

   One countermeasure is to use a user-entered passphrase to encrypt the
   private key, although users tend to choose weak passphrases and often
   reuse them for different purposes, possibly allowing an attack
   against DKIM to be extended into other domains.  Nevertheless, the
   decoded private key might be briefly available to compromise by
   malware when it is entered, or might be discovered via keystroke



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   logging.  The added complexity of entering a passphrase each time one
   sends a message would also tend to discourage the use of a secure
   passphrase.

   A somewhat more effective countermeasure is to send messages through
   an outgoing MTA that can authenticate the submitter using existing
   techniques (e.g., SMTP Authentication), possibly validate the message
   itself (e.g., verify that the header is legitimate and that the
   content passes a spam content check), and sign the message using a
   key appropriate for the submitter address.  Such an MTA can also
   apply controls on the volume of outgoing mail each user is permitted
   to originate in order to further limit the ability of malware to
   generate bulk email.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Key Server Denial-of-Service Attacks</span>

   Since the key servers are distributed (potentially separate for each
   domain), the number of servers that would need to be attacked to
   defeat this mechanism on an Internet-wide basis is very large.
   Nevertheless, key servers for individual domains could be attacked,
   impeding the verification of messages from that domain.  This is not
   significantly different from the ability of an attacker to deny
   service to the mail exchangers for a given domain, although it
   affects outgoing, not incoming, mail.

   A variation on this attack is that if a very large amount of mail
   were to be sent using spoofed addresses from a given domain, the key
   servers for that domain could be overwhelmed with requests.  However,
   given the low overhead of verification compared with handling of the
   email message itself, such an attack would be difficult to mount.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>.  Attacks Against the DNS</span>

   Since the DNS is a required binding for key services, specific
   attacks against the DNS must be considered.

   While the DNS is currently insecure [<a href="./rfc3833" title="&quot;Threat Analysis of the Domain Name System (DNS)&quot;">RFC3833</a>], these security
   problems are the motivation behind DNS Security (DNSSEC) [<a href="./rfc4033" title="&quot;DNS Security Introduction and Requirements&quot;">RFC4033</a>],
   and all users of the DNS will reap the benefit of that work.

   DKIM is only intended as a "sufficient" method of proving
   authenticity.  It is not intended to provide strong cryptographic
   proof about authorship or contents.  Other technologies such as
   OpenPGP [<a href="./rfc2440" title="&quot;OpenPGP Message Format&quot;">RFC2440</a>] and S/MIME [<a href="./rfc3851" title="&quot;S/MIME Version 3 Message Specification&quot;">RFC3851</a>] address those requirements.

   A second security issue related to the DNS revolves around the
   increased DNS traffic as a consequence of fetching selector-based
   data as well as fetching signing domain policy.  Widespread



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   deployment of DKIM will result in a significant increase in DNS
   queries to the claimed signing domain.  In the case of forgeries on a
   large scale, DNS servers could see a substantial increase in queries.

   A specific DNS security issue that should be considered by DKIM
   verifiers is the name chaining attack described in <a href="#section-2.3">Section 2.3</a> of the
   DNS Threat Analysis [<a href="./rfc3833" title="&quot;Threat Analysis of the Domain Name System (DNS)&quot;">RFC3833</a>].  A DKIM verifier, while verifying a
   DKIM-Signature header field, could be prompted to retrieve a key
   record of an attacker's choosing.  This threat can be minimized by
   ensuring that name servers, including recursive name servers, used by
   the verifier enforce strict checking of "glue" and other additional
   information in DNS responses and are therefore not vulnerable to this
   attack.

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>.  Replay Attacks</span>

   In this attack, a spammer sends a message to be spammed to an
   accomplice, which results in the message being signed by the
   originating MTA.  The accomplice resends the message, including the
   original signature, to a large number of recipients, possibly by
   sending the message to many compromised machines that act as MTAs.
   The messages, not having been modified by the accomplice, have valid
   signatures.

   Partial solutions to this problem involve the use of reputation
   services to convey the fact that the specific email address is being
   used for spam and that messages from that signer are likely to be
   spam.  This requires a real-time detection mechanism in order to
   react quickly enough.  However, such measures might be prone to
   abuse, if for example an attacker resent a large number of messages
   received from a victim in order to make them appear to be a spammer.

   Large verifiers might be able to detect unusually large volumes of
   mails with the same signature in a short time period.  Smaller
   verifiers can get substantially the same volume of information via
   existing collaborative systems.

<span class="h3"><a class="selflink" id="section-8.6" href="#section-8.6">8.6</a>.  Limits on Revoking Keys</span>

   When a large domain detects undesirable behavior on the part of one
   of its users, it might wish to revoke the key used to sign that
   user's messages in order to disavow responsibility for messages that
   have not yet been verified or that are the subject of a replay
   attack.  However, the ability of the domain to do so can be limited
   if the same key, for scalability reasons, is used to sign messages
   for many other users.  Mechanisms for explicitly revoking keys on a
   per-address basis have been proposed but require further study as to
   their utility and the DNS load they represent.



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<span class="h3"><a class="selflink" id="section-8.7" href="#section-8.7">8.7</a>.  Intentionally Malformed Key Records</span>

   It is possible for an attacker to publish key records in DNS that are
   intentionally malformed, with the intent of causing a denial-of-
   service attack on a non-robust verifier implementation.  The attacker
   could then cause a verifier to read the malformed key record by
   sending a message to one of its users referencing the malformed
   record in a (not necessarily valid) signature.  Verifiers MUST
   thoroughly verify all key records retrieved from the DNS and be
   robust against intentionally as well as unintentionally malformed key
   records.

<span class="h3"><a class="selflink" id="section-8.8" href="#section-8.8">8.8</a>.  Intentionally Malformed DKIM-Signature Header Fields</span>

   Verifiers MUST be prepared to receive messages with malformed DKIM-
   Signature header fields, and thoroughly verify the header field
   before depending on any of its contents.

<span class="h3"><a class="selflink" id="section-8.9" href="#section-8.9">8.9</a>.  Information Leakage</span>

   An attacker could determine when a particular signature was verified
   by using a per-message selector and then monitoring their DNS traffic
   for the key lookup.  This would act as the equivalent of a "web bug"
   for verification time rather than when the message was read.

<span class="h3"><a class="selflink" id="section-8.10" href="#section-8.10">8.10</a>.  Remote Timing Attacks</span>

   In some cases, it may be possible to extract private keys using a
   remote timing attack [<a href="#ref-BONEH03" title="&quot;Remote Timing Attacks are Practical&quot;">BONEH03</a>].  Implementations should consider
   obfuscating the timing to prevent such attacks.

<span class="h3"><a class="selflink" id="section-8.11" href="#section-8.11">8.11</a>.  Reordered Header Fields</span>

   Existing standards allow intermediate MTAs to reorder header fields.
   If a signer signs two or more header fields of the same name, this
   can cause spurious verification errors on otherwise legitimate
   messages.  In particular, signers that sign any existing DKIM-
   Signature fields run the risk of having messages incorrectly fail to
   verify.

<span class="h3"><a class="selflink" id="section-8.12" href="#section-8.12">8.12</a>.  RSA Attacks</span>

   An attacker could create a large RSA signing key with a small
   exponent, thus requiring that the verification key have a large
   exponent.  This will force verifiers to use considerable computing
   resources to verify the signature.  Verifiers might avoid this attack
   by refusing to verify signatures that reference selectors with public
   keys having unreasonable exponents.



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   In general, an attacker might try to overwhelm a verifier by flooding
   it with messages requiring verification.  This is similar to other
   MTA denial-of-service attacks and should be dealt with in a similar
   fashion.

<span class="h3"><a class="selflink" id="section-8.13" href="#section-8.13">8.13</a>.  Inappropriate Signing by Parent Domains</span>

   The trust relationship described in <a href="#section-3.8">Section 3.8</a> could conceivably be
   used by a parent domain to sign messages with identities in a
   subdomain not administratively related to the parent.  For example,
   the ".com" registry could create messages with signatures using an
   "i=" value in the example.com domain.  There is no general solution
   to this problem, since the administrative cut could occur anywhere in
   the domain name.  For example, in the domain "example.podunk.ca.us"
   there are three administrative cuts (podunk.ca.us, ca.us, and us),
   any of which could create messages with an identity in the full
   domain.

      INFORMATIVE NOTE: This is considered an acceptable risk for the
      same reason that it is acceptable for domain delegation.  For
      example, in the example above any of the domains could potentially
      simply delegate "example.podunk.ca.us" to a server of their choice
      and completely replace all DNS-served information.  Note that a
      verifier MAY ignore signatures that come from an unlikely domain
      such as ".com", as discussed in <a href="#section-6.1.1">Section 6.1.1</a>.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Normative References</span>

   [<a id="ref-FIPS.180-2.2002">FIPS.180-2.2002</a>]  U.S. Department of Commerce, "Secure Hash
                      Standard", FIPS PUB 180-2, August 2002.

   [<a id="ref-ITU.X660.1997">ITU.X660.1997</a>]    "Information Technology - ASN.1 encoding rules:
                      Specification of Basic Encoding Rules (BER),
                      Canonical Encoding Rules (CER) and Distinguished
                      Encoding Rules (DER)", ITU-T Recommendation X.660,
                      1997.

   [<a id="ref-RFC2045">RFC2045</a>]          Freed, N. and N. Borenstein, "Multipurpose
                      Internet Mail Extensions (MIME) Part One: Format
                      of Internet Message Bodies", <a href="./rfc2045">RFC 2045</a>,
                      November 1996.

   [<a id="ref-RFC2047">RFC2047</a>]          Moore, K., "MIME (Multipurpose Internet Mail
                      Extensions) Part Three: Message header field
                      Extensions for Non-ASCII Text", <a href="./rfc2047">RFC 2047</a>,
                      November 1996.



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   [<a id="ref-RFC2119">RFC2119</a>]          Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
                      March 1997.

   [<a id="ref-RFC2821">RFC2821</a>]          Klensin, J., "Simple Mail Transfer Protocol",
                      <a href="./rfc2821">RFC 2821</a>, April 2001.

   [<a id="ref-RFC2822">RFC2822</a>]          Resnick, P., "Internet Message Format", <a href="./rfc2822">RFC 2822</a>,
                      April 2001.

   [<a id="ref-RFC3447">RFC3447</a>]          Jonsson, J. and B. Kaliski, "Public-Key
                      Cryptography Standards (PKCS) #1: RSA Cryptography
                      Specifications Version 2.1", <a href="./rfc3447">RFC 3447</a>,
                      February 2003.

   [<a id="ref-RFC3490">RFC3490</a>]          Faltstrom, P., Hoffman, P., and A. Costello,
                      "Internationalizing Domain Names in Applications
                      (IDNA)", <a href="./rfc3490">RFC 3490</a>, March 2003.

   [<a id="ref-RFC4234">RFC4234</a>]          Crocker, D., Ed. and P. Overell, "Augmented BNF
                      for Syntax Specifications: ABNF", <a href="./rfc4234">RFC 4234</a>,
                      October 2005.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Informative References</span>

   [<a id="ref-BONEH03">BONEH03</a>]          Proc. 12th USENIX Security Symposium, "Remote
                      Timing Attacks are Practical", 2003.

   [<a id="ref-RFC1847">RFC1847</a>]          Galvin, J., Murphy, S., Crocker, S., and N. Freed,
                      "Security Multiparts for MIME: Multipart/Signed
                      and Multipart/Encrypted", <a href="./rfc1847">RFC 1847</a>, October 1995.

   [<a id="ref-RFC2434">RFC2434</a>]          Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc2434">RFC 2434</a>, October 1998.

   [<a id="ref-RFC2440">RFC2440</a>]          Callas, J., Donnerhacke, L., Finney, H., and R.
                      Thayer, "OpenPGP Message Format", <a href="./rfc2440">RFC 2440</a>,
                      November 1998.

   [<a id="ref-RFC3766">RFC3766</a>]          Orman, H. and P. Hoffman, "Determining Strengths
                      for Public Keys Used For Exchanging Symmetric
                      Keys", <a href="./rfc3766">RFC 3766</a>, April 2004.

   [<a id="ref-RFC3833">RFC3833</a>]          Atkins, D. and R. Austein, "Threat Analysis of the
                      Domain Name System (DNS)", <a href="./rfc3833">RFC 3833</a>, August 2004.





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   [<a id="ref-RFC3851">RFC3851</a>]          Ramsdell, B., "S/MIME Version 3 Message
                      Specification", <a href="./rfc3851">RFC 3851</a>, June 1999.

   [<a id="ref-RFC3864">RFC3864</a>]          Klyne, G., Nottingham, M., and J. Mogul,
                      "Registration Procedures for Message Header
                      Fields", <a href="https://www.rfc-editor.org/bcp/bcp90">BCP 90</a>, September 2004.

   [<a id="ref-RFC4033">RFC4033</a>]          Arends, R., Austein, R., Larson, M., Massey, D.,
                      and S. Rose, "DNS Security Introduction and
                      Requirements", <a href="./rfc4033">RFC 4033</a>, March 2005.

   [<a id="ref-RFC4686">RFC4686</a>]          Fenton, J., "Analysis of Threats Motivating
                      DomainKeys Identified Mail (DKIM)", <a href="./rfc4686">RFC 4686</a>,
                      September 2006.

   [<a id="ref-RFC4870">RFC4870</a>]          Delany, M., "Domain-Based Email Authentication
                      Using Public Keys Advertised in the DNS
                      (DomainKeys)", <a href="./rfc4870">RFC 4870</a>, May 2007.

































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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Example of Use (INFORMATIVE)</span>

   This section shows the complete flow of an email from submission to
   final delivery, demonstrating how the various components fit
   together.  The key used in this example is shown in <a href="#appendix-C">Appendix C</a>.

<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>.  The User Composes an Email</span>


   From: Joe SixPack &lt;joe@football.example.com&gt;
   To: Suzie Q &lt;suzie@shopping.example.net&gt;
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: &lt;20030712040037.46341.5F8J@football.example.com&gt;

   Hi.

   We lost the game. Are you hungry yet?

   Joe.































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<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>.  The Email Is Signed</span>

   This email is signed by the example.com outbound email server and now
   looks like this:

   DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
         c=simple/simple; q=dns/txt; i=joe@football.example.com;
         h=Received : From : To : Subject : Date : Message-ID;
         bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
         b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
           4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
           KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
           4bmp/YzhwvcubU4=;
   Received: from client1.football.example.com  [192.0.2.1]
         by submitserver.example.com with SUBMISSION;
         Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
   From: Joe SixPack &lt;joe@football.example.com&gt;
   To: Suzie Q &lt;suzie@shopping.example.net&gt;
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: &lt;20030712040037.46341.5F8J@football.example.com&gt;

   Hi.

   We lost the game. Are you hungry yet?

   Joe.

   The signing email server requires access to the private key
   associated with the "brisbane" selector to generate this signature.

<span class="h3"><a class="selflink" id="appendix-A.3" href="#appendix-A.3">A.3</a>.  The Email Signature Is Verified</span>

   The signature is normally verified by an inbound SMTP server or
   possibly the final delivery agent.  However, intervening MTAs can
   also perform this verification if they choose to do so.  The
   verification process uses the domain "example.com" extracted from the
   "d=" tag and the selector "brisbane" from the "s=" tag in the DKIM-
   Signature header field to form the DNS DKIM query for:

   brisbane._domainkey.example.com

   Signature verification starts with the physically last Received
   header field, the From header field, and so forth, in the order
   listed in the "h=" tag.  Verification follows with a single CRLF
   followed by the body (starting with "Hi.").  The email is canonically
   prepared for verifying with the "simple" method.  The result of the
   query and subsequent verification of the signature is stored (in this



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   example) in the X-Authentication-Results header field line.  After
   successful verification, the email looks like this:

   X-Authentication-Results: shopping.example.net
         header.from=joe@football.example.com; dkim=pass
   Received: from mout23.football.example.com (192.168.1.1)
         by shopping.example.net with SMTP;
         Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
   DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
         c=simple/simple; q=dns/txt; i=joe@football.example.com;
         h=Received : From : To : Subject : Date : Message-ID;
         bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
         b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
           4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
           KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
           4bmp/YzhwvcubU4=;
   Received: from client1.football.example.com  [192.0.2.1]
         by submitserver.example.com with SUBMISSION;
         Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
   From: Joe SixPack &lt;joe@football.example.com&gt;
   To: Suzie Q &lt;suzie@shopping.example.net&gt;
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: &lt;20030712040037.46341.5F8J@football.example.com&gt;

   Hi.

   We lost the game. Are you hungry yet?

   Joe.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Usage Examples (INFORMATIVE)</span>

   DKIM signing and validating can be used in different ways, for
   different operational scenarios.  This Appendix discusses some common
   examples.

      NOTE: Descriptions in this Appendix are for informational purposes
      only.  They describe various ways that DKIM can be used, given
      particular constraints and needs.  In no case are these examples
      intended to be taken as providing explanation or guidance
      concerning DKIM specification details, when creating an
      implementation.








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<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>.  Alternate Submission Scenarios</span>

   In the most simple scenario, a user's MUA, MSA, and Internet
   (boundary) MTA are all within the same administrative environment,
   using the same domain name.  Therefore, all of the components
   involved in submission and initial transfer are related.  However, it
   is common for two or more of the components to be under independent
   administrative control.  This creates challenges for choosing and
   administering the domain name to use for signing, and for its
   relationship to common email identity header fields.

<span class="h4"><a class="selflink" id="appendix-B.1.1" href="#appendix-B.1.1">B.1.1</a>.  Delegated Business Functions</span>

   Some organizations assign specific business functions to discrete
   groups, inside or outside the organization.  The goal, then, is to
   authorize that group to sign some mail, but to constrain what
   signatures they can generate.  DKIM selectors (the "s=" signature
   tag) and granularity (the "g=" key tag) facilitate this kind of
   restricted authorization.  Examples of these outsourced business
   functions are legitimate email marketing providers and corporate
   benefits providers.

   Here, the delegated group needs to be able to send messages that are
   signed, using the email domain of the client company.  At the same
   time, the client often is reluctant to register a key for the
   provider that grants the ability to send messages for arbitrary
   addresses in the domain.

   There are multiple ways to administer these usage scenarios.  In one
   case, the client organization provides all of the public query
   service (for example, DNS) administration, and in another it uses DNS
   delegation to enable all ongoing administration of the DKIM key
   record by the delegated group.

   If the client organization retains responsibility for all of the DNS
   administration, the outsourcing company can generate a key pair,
   supplying the public key to the client company, which then registers
   it in the query service, using a unique selector that authorizes a
   specific From header field Local-part.  For example, a client with
   the domain "example.com" could have the selector record specify
   "g=winter-promotions" so that this signature is only valid for mail
   with a From address of "winter-promotions@example.com".  This would
   enable the provider to send messages using that specific address and
   have them verify properly.  The client company retains control over
   the email address because it retains the ability to revoke the key at
   any time.





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   If the client wants the delegated group to do the DNS administration,
   it can have the domain name that is specified with the selector point
   to the provider's DNS server.  The provider then creates and
   maintains all of the DKIM signature information for that selector.
   Hence, the client cannot provide constraints on the Local-part of
   addresses that get signed, but it can revoke the provider's signing
   rights by removing the DNS delegation record.

<span class="h4"><a class="selflink" id="appendix-B.1.2" href="#appendix-B.1.2">B.1.2</a>.  PDAs and Similar Devices</span>

   PDAs demonstrate the need for using multiple keys per domain.
   Suppose that John Doe wanted to be able to send messages using his
   corporate email address, jdoe@example.com, and his email device did
   not have the ability to make a Virtual Private Network (VPN)
   connection to the corporate network, either because the device is
   limited or because there are restrictions enforced by his Internet
   access provider.  If the device was equipped with a private key
   registered for jdoe@example.com by the administrator of the
   example.com domain, and appropriate software to sign messages, John
   could sign the message on the device itself before transmission
   through the outgoing network of the access service provider.

<span class="h4"><a class="selflink" id="appendix-B.1.3" href="#appendix-B.1.3">B.1.3</a>.  Roaming Users</span>

   Roaming users often find themselves in circumstances where it is
   convenient or necessary to use an SMTP server other than their home
   server; examples are conferences and many hotels.  In such
   circumstances, a signature that is added by the submission service
   will use an identity that is different from the user's home system.

   Ideally, roaming users would connect back to their home server using
   either a VPN or a SUBMISSION server running with SMTP AUTHentication
   on port 587.  If the signing can be performed on the roaming user's
   laptop, then they can sign before submission, although the risk of
   further modification is high.  If neither of these are possible,
   these roaming users will not be able to send mail signed using their
   own domain key.

<span class="h4"><a class="selflink" id="appendix-B.1.4" href="#appendix-B.1.4">B.1.4</a>.  Independent (Kiosk) Message Submission</span>

   Stand-alone services, such as walk-up kiosks and web-based
   information services, have no enduring email service relationship
   with the user, but users occasionally request that mail be sent on
   their behalf.  For example, a website providing news often allows the
   reader to forward a copy of the article to a friend.  This is
   typically done using the reader's own email address, to indicate who
   the author is.  This is sometimes referred to as the "Evite problem",




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   named after the website of the same name that allows a user to send
   invitations to friends.

   A common way this is handled is to continue to put the reader's email
   address in the From header field of the message, but put an address
   owned by the email posting site into the Sender header field.  The
   posting site can then sign the message, using the domain that is in
   the Sender field.  This provides useful information to the receiving
   email site, which is able to correlate the signing domain with the
   initial submission email role.

   Receiving sites often wish to provide their end users with
   information about mail that is mediated in this fashion.  Although
   the real efficacy of different approaches is a subject for human
   factors usability research, one technique that is used is for the
   verifying system to rewrite the From header field, to indicate the
   address that was verified.  For example: From: John Doe via
   news@news-site.com &lt;jdoe@example.com&gt;.  (Note that such rewriting
   will break a signature, unless it is done after the verification pass
   is complete.)

<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>.  Alternate Delivery Scenarios</span>

   Email is often received at a mailbox that has an address different
   from the one used during initial submission.  In these cases, an
   intermediary mechanism operates at the address originally used and it
   then passes the message on to the final destination.  This mediation
   process presents some challenges for DKIM signatures.

<span class="h4"><a class="selflink" id="appendix-B.2.1" href="#appendix-B.2.1">B.2.1</a>.  Affinity Addresses</span>

   "Affinity addresses" allow a user to have an email address that
   remains stable, even as the user moves among different email
   providers.  They are typically associated with college alumni
   associations, professional organizations, and recreational
   organizations with which they expect to have a long-term
   relationship.  These domains usually provide forwarding of incoming
   email, and they often have an associated Web application that
   authenticates the user and allows the forwarding address to be
   changed.  However, these services usually depend on users sending
   outgoing messages through their own service providers' MTAs.  Hence,
   mail that is signed with the domain of the affinity address is not
   signed by an entity that is administered by the organization owning
   that domain.

   With DKIM, affinity domains could use the Web application to allow
   users to register per-user keys to be used to sign messages on behalf
   of their affinity address.  The user would take away the secret half



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   of the key pair for signing, and the affinity domain would publish
   the public half in DNS for access by verifiers.

   This is another application that takes advantage of user-level
   keying, and domains used for affinity addresses would typically have
   a very large number of user-level keys.  Alternatively, the affinity
   domain could handle outgoing mail, operating a mail submission agent
   that authenticates users before accepting and signing messages for
   them.  This is of course dependent on the user's service provider not
   blocking the relevant TCP ports used for mail submission.

<span class="h4"><a class="selflink" id="appendix-B.2.2" href="#appendix-B.2.2">B.2.2</a>.  Simple Address Aliasing (.forward)</span>

   In some cases, a recipient is allowed to configure an email address
   to cause automatic redirection of email messages from the original
   address to another, such as through the use of a Unix .forward file.
   In this case, messages are typically redirected by the mail handling
   service of the recipient's domain, without modification, except for
   the addition of a Received header field to the message and a change
   in the envelope recipient address.  In this case, the recipient at
   the final address' mailbox is likely to be able to verify the
   original signature since the signed content has not changed, and DKIM
   is able to validate the message signature.

<span class="h4"><a class="selflink" id="appendix-B.2.3" href="#appendix-B.2.3">B.2.3</a>.  Mailing Lists and Re-Posters</span>

   There is a wide range of behaviors in services that take delivery of
   a message and then resubmit it.  A primary example is with mailing
   lists (collectively called "forwarders" below), ranging from those
   that make no modification to the message itself, other than to add a
   Received header field and change the envelope information, to those
   that add header fields, change the Subject header field, add content
   to the body (typically at the end), or reformat the body in some
   manner.  The simple ones produce messages that are quite similar to
   the automated alias services.  More elaborate systems essentially
   create a new message.

   A Forwarder that does not modify the body or signed header fields of
   a message is likely to maintain the validity of the existing
   signature.  It also could choose to add its own signature to the
   message.

   Forwarders which modify a message in a way that could make an
   existing signature invalid are particularly good candidates for
   adding their own signatures (e.g., mailing-list-name@example.net).
   Since (re-)signing is taking responsibility for the content of the
   message, these signing forwarders are likely to be selective, and
   forward or re-sign a message only if it is received with a valid



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   signature or if they have some other basis for knowing that the
   message is not spoofed.

   A common practice among systems that are primarily redistributors of
   mail is to add a Sender header field to the message, to identify the
   address being used to sign the message.  This practice will remove
   any preexisting Sender header field as required by [<a href="./rfc2822" title="&quot;Internet Message Format&quot;">RFC2822</a>].  The
   forwarder applies a new DKIM-Signature header field with the
   signature, public key, and related information of the forwarder.

<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  Creating a Public Key (INFORMATIVE)</span>

   The default signature is an RSA signed SHA256 digest of the complete
   email.  For ease of explanation, the openssl command is used to
   describe the mechanism by which keys and signatures are managed.  One
   way to generate a 1024-bit, unencrypted private key suitable for DKIM
   is to use openssl like this:

   $ openssl genrsa -out rsa.private 1024

   For increased security, the "-passin" parameter can also be added to
   encrypt the private key.  Use of this parameter will require entering
   a password for several of the following steps.  Servers may prefer to
   use hardware cryptographic support.

   The "genrsa" step results in the file rsa.private containing the key
   information similar to this:

    -----BEGIN RSA PRIVATE KEY-----
    MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC
    jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb
    to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB
    AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX
    /1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ
    gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO
    n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m
    3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/
    eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj
    7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA
    qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf
    eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX
    GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc=
    -----END RSA PRIVATE KEY-----

   To extract the public-key component from the private key, use openssl
   like this:

   $ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM



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   This results in the file rsa.public containing the key information
   similar to this:

   -----BEGIN PUBLIC KEY-----
   MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM
   oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R
   tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI
   MmPSPDdQPNUYckcQ2QIDAQAB
   -----END PUBLIC KEY-----

   This public-key data (without the BEGIN and END tags) is placed in
   the DNS:

   brisbane IN  TXT  ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ"
                      "KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt"
                      "IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v"
                      "/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi"
                      "tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB")

<span class="h2"><a class="selflink" id="appendix-D" href="#appendix-D">Appendix D</a>.  MUA Considerations</span>

   When a DKIM signature is verified, the processing system sometimes
   makes the result available to the recipient user's MUA.  How to
   present this information to the user in a way that helps them is a
   matter of continuing human factors usability research.  The tendency
   is to have the MUA highlight the address associated with this signing
   identity in some way, in an attempt to show the user the address from
   which the mail was sent.  An MUA might do this with visual cues such
   as graphics, or it might include the address in an alternate view, or
   it might even rewrite the original From address using the verified
   information.  Some MUAs might indicate which header fields were
   protected by the validated DKIM signature.  This could be done with a
   positive indication on the signed header fields, with a negative
   indication on the unsigned header fields, by visually hiding the
   unsigned header fields, or some combination of these.  If an MUA uses
   visual indications for signed header fields, the MUA probably needs
   to be careful not to display unsigned header fields in a way that
   might be construed by the end user as having been signed.  If the
   message has an l= tag whose value does not extend to the end of the
   message, the MUA might also hide or mark the portion of the message
   body that was not signed.

   The aforementioned information is not intended to be exhaustive.  The
   MUA may choose to highlight, accentuate, hide, or otherwise display
   any other information that may, in the opinion of the MUA author, be
   deemed important to the end user.





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<span class="h2"><a class="selflink" id="appendix-E" href="#appendix-E">Appendix E</a>.  Acknowledgements</span>

   The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
   Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin,
   Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman,
   Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto,
   Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur
   Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman,
   Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig
   Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S.
   Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon
   Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau,
   Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada,
   Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud,
   Scott Renfro, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector
   Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert
   Sanders, Rand Wacker, Sam Weiler, and Dan Wing for their valuable
   suggestions and constructive criticism.

   The DomainKeys specification was a primary source from which this
   specification has been derived.  Further information about DomainKeys
   is at [<a href="./rfc4870" title="&quot;Domain-Based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)&quot;">RFC4870</a>].

Authors' Addresses

   Eric Allman
   Sendmail, Inc.
   6425 Christie Ave, Suite 400
   Emeryville, CA  94608
   USA

   Phone: +1 510 594 5501
   EMail: eric+dkim@sendmail.org
   URI:


   Jon Callas
   PGP Corporation
   3460 West Bayshore
   Palo Alto, CA  94303
   USA

   Phone: +1 650 319 9016
   EMail: jon@pgp.com







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   Mark Delany
   Yahoo! Inc
   701 First Avenue
   Sunnyvale, CA  95087
   USA

   Phone: +1 408 349 6831
   EMail: markd+dkim@yahoo-inc.com
   URI:


   Miles Libbey
   Yahoo! Inc
   701 First Avenue
   Sunnyvale, CA  95087
   USA

   EMail: mlibbeymail-mailsig@yahoo.com
   URI:


   Jim Fenton
   Cisco Systems, Inc.
   MS SJ-9/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706
   USA

   Phone: +1 408 526 5914
   EMail: fenton@cisco.com
   URI:


   Michael Thomas
   Cisco Systems, Inc.
   MS SJ-9/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706

   Phone: +1 408 525 5386
   EMail: mat@cisco.com










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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
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Allman, et al.              Standards Track                    [Page 71]
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