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Versions: 00

DKIM                                                     D. Crocker, Ed.
Internet-Draft                               Brandenburg InternetWorking
Intended status: Standards Track                       M. Kucherawy, Ed.
Expires: July 18, 2011                                         Cloudmark
                                                        January 14, 2011


                  DomainKeys Security Tagging (DOSETA)
                      draft-crocker-dkim-doseta-00

Abstract

   DomainKeys Security Tagging (DOSETA) is a component mechanism that
   enables development of a security-related service, such as
   authentication or encryption, with keys based on domain names; the
   name owner can be any actor involved in the handling of the data,
   such as the author's organization, a server operator or one of their
   agents.  The DOSETA Library provides a collection of common
   capabilities, including canonicalization, parameter tagging, and
   retrieval of self-certified keys.  The DOSETA Signing Template
   affixes a signature to data that is in a "header/content" form.
   Defining the meaning of the signature is the responsibility of the
   service that incorporates DOSETA.  The signature is validated through
   a cryptographic signature and querying the signer's domain directly,
   to retrieve the appropriate public key.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 18, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  DOSETA Features  . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  DOSETA Architecture  . . . . . . . . . . . . . . . . . . .  6
   2.  Terminology and Definitions {rfc4871bis-02 2, subset}  . . . .  6
     2.1.  Identity . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Actors . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  DOSETA Library . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Canonicalization {rfc4871bis-02 3.4} . . . . . . . . . . . 11
     3.2.  Tag=Value Parameters {rfc4871bis-02 3.2} . . . . . . . . . 15
     3.3.  Key Management {rfc4871bis-02 3.6} . . . . . . . . . . . . 16
     3.4.  Selectors for Keys {rfc4871bis-02 3.1} . . . . . . . . . . 17
     3.5.  DNS Binding for Key Retrieval {rfc4871bis-02 3.6.2}  . . . 19
     3.6.  Stored Key Data {rfc4871bis-02 3.6.1, subset, with
           preface} . . . . . . . . . . . . . . . . . . . . . . . . . 20
   4.  DOSETA H/C Signing Template  . . . . . . . . . . . . . . . . . 22
     4.1.  Cryptographic Algorithms {rfc4871bis-02 3.3, subset} . . . 22
     4.2.  Signature Data Structure {rfc4871bis-02 3.5, subset} . . . 24
     4.3.  Signature Calculation {rfc4871bis-02 3.7, reorganized
           and clarified} . . . . . . . . . . . . . . . . . . . . . . 30
     4.4.  Signer Actions {rfc4871bis-02 5} . . . . . . . . . . . . . 33
     4.5.  Verifier Actions {rfc4871bis-02 6} . . . . . . . . . . . . 37
     4.6.  Requirements for Tailoring the Signing Service {added} . . 44
     4.7.  Signing by Parent Domains {rfc4871bis-02 3.8}  . . . . . . 44
   5.  Semantics of Multiple Signatures {rfc4871bis-02 4} . . . . . . 45
     5.1.  Example Scenarios {rfc4871bis-02 4.1}  . . . . . . . . . . 45
     5.2.  Interpretation {rfc4871bis-02 4.2} . . . . . . . . . . . . 46
   6.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 47
     6.1.  IANA Considerations {rfc4871bis-02 7, subset}  . . . . . . 47
     6.2.  Security Considerations {rfc4871bis-02 8, subset}  . . . . 50
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 56
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 57
   Appendix A.  Creating a Public Key {rfc4871bis-02 C} . . . . . . . 58
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 59



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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 59


















































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1.  Introduction

   DomainKeys Security Tagging (DOSETA) is a component mechanism that
   enables development of a security-related service, such as
   authentication or encryption, with keys based on domain names
   [RFC1034]; the name owner can be any actor involved in the handling
   of the data, such as the author's organization, a server operator or
   one of their agents.  The DOSETA Library provides a collection of
   common capabilities, including canonicalization, parameter tagging,
   and retrieval of self-certified keys.  The DOSETA Signing Template
   affixes a signature to data that is in a "header/content" form.
   Defining the meaning of the signature is the responsibility of the
   service that incorporates DOSETA.  The signature is validated through
   a cryptographic signature and querying the signer's domain directly,
   to retrieve the appropriate public key.

   The approach taken by DOSETA differs from previous approaches to data
   signing (such as, Secure/Multipurpose Internet Mail Extensions
   (S/MIME) [RFC1847], OpenPGP [RFC4880]) in that:

   o  the message signature can be packaged independently of the data it
      is signing, so that neither human viewers of the data nor existing
      data handling software is confused by signature-related content
      appearing in the Content;

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

   DOSETA:

   o  enables compatibility with the existing data handling
      infrastructure and transparent to the fullest extent possible;

   o  requires minimal new infrastructure;

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

   o  can be deployed incrementally;

   o  allows delegation of signing to third parties.

   DOSETA is taken directly from the original DKIM Signing specification



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   [RFC4871], [RFC5672].  It has merely extracted the core portions of
   the specification, so that they can be applied to other security-
   related services.  For example, the core could support a DKIM-like
   signing service for web pages, and it could support a data encryption
   mechanism using the same DNS-based, self-certified key service as
   DKIM.

   NOTE:   Much of the text for this draft is taken from the DKIM
      working group draft-ietf-DKIM-rfc4871bis-02 revision.  Sections in
      this document cross reference their source with the notation:
   {rfc4871bis-02 xx}
      where "xx" indicates the section number.  It might also indicate
      that a subset is taken, such as when a portion is applied to the
      DKIM-over-DOSETA draft and a portion to the DOSETA draft.  In some
      cases the base text also has been enhanced.

1.1.  DOSETA Features

   DOSETA features include:



      Identity:   DOSETA separates the question of the identity of the
         DOSETA Creator from any other associated identifier, such as
         the data's purported author.  In particular, a DOSETA header
         includes the identity of the signer.  DOSETA Consumers can use
         the DOSETA identity information to decide how they want to
         process the data.  That identity can be included in the
         attributes of the data or recorded elsewhere.

         NOTE:   DOSETA does not, iiself, require that the identity it
            uses be required to match any other associated identifier.
            Those other identifiers already carry their own semantics
            which might conflict with the use of the identifier needed
            by DOSETA.  However a particular service that incorporates
            DNS might choose to add constraints on the choice of
            identifier, such as having it match another identifier
            associated with the data.

      Scalability:   DOSETA is designed to support the extreme
         scalability requirements that characterize Internet data
         identification.

      Key Management:   DOSETA 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.  Hence DOSETA provides



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         self-certifying keys.

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

      Data Integrity:   A DOSETA signature associates its identifier
         with the computed hash of some or all of the data in order to
         prevent the re-use of the signature with different data.
         Verifying the signature asserts that the hashed data has not
         changed since it was signed, and asserts nothing else about
         "protecting" the end-to-end integrity of the data.

      Assessment:   For identity and authentication related processes,
         this phase integrates the validation output for determing
         further action.

1.2.  DOSETA Architecture

   As component technology, DOSETA is meant to be incorporated into a
   service.  This specification provides an underlying set of common
   features and a template for using them to provide a signing service,
   such as for authenticating an identifier.  Hence, the pieces can be
   depicted as follows, with DKIM being shown as a specific service that
   incorporates DOSETA:
        +--------+                              +-----------------+
        |  DKIM  |                              | Message Privacy |
        +---+----+                              +--------+--------+
            |                                            |
      +-----V---------------------------+                |
      | Header/Content Signing Template |                |
      +----------------+----------------+                |
                       |                                 |
    ...................V.................................V.............
    .                     D O S E T A     L I B R A R Y               .
    .+------------------+ +------------+ +-------------+ +-----------+.
    .|                  | |    Key     | |  Common     | | Signature |.
    .| Canonicalization | | Management | |  Parameters | |  Header   |.
    .|                  | |   (DNS)    | | (tab=value) | |           |.
    .+------------------+ +------------+ +-------------+ +-----------+.
    ...................................................................


2.  Terminology and Definitions {rfc4871bis-02 2, subset}

   This section defines terms used in the rest of the document.




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   Within the specification, the label "TEMPLATE" is used to indicate
   actions that are required to incorporate DOSETA into a service.

   Syntax descriptions use Augmented BNF (ABNF) [RFC5234].

   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 [RFC2119].

2.1.  Identity

   Identity:   A person, role, or organization.  In the context of
      DOSETA, examples include author, author's organization, an ISP
      along the handling path, an independent trust assessment service,
      and a data processing intermediary operator.

   Identifier:   A label that refers to an identity.

   Signing Domain Identifier (SDID):   A single domain name that refers
      to the identity owning the key to be used for DOSETA processing.
      The name has only basic domain name semantics; any possible owner-
      specific semantics are outside the scope of DOSETA.  It is
      specified in Section 4.2.

2.2.  Actors

   Creator:   An element in the data handling system that produces a
      cryptographic encoding, on behalf of a domain, is referred to as a
      Creator.

   Consumer:   An element in the data handling system that processes an
      existing cryptographic encoding, on behalf of a domain, is
      referred to as a Consumer.

   Signer:   An element in the data handling system that signs messages
      on behalf of a domain is referred to as a signer.  This element
      specifies the organization acting as a type of DOSETA Creator.  It
      might be a client, server or other agent such as a reputation
      service.  The key issue is that the data MUST be signed before it
      leaves the administrative domain of the signer.

   Verifier:   An element in the data handling system that verifies
      signatures is referred to as a verifier.  This element is a
      Consumer of a signing service.  It might be a client, server, or
      other agent, such as a reputation service.  In most cases it is
      expected that a verifier will be close to an end user (Creator) of
      the data or some consuming agent such as a data processing
      intermediary.



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2.3.  Syntax

2.3.1.  Whitespace

   There are three forms of whitespace:



      WSP:   represents simple whitespace, that is, a space or a tab
         character (formal definition in [RFC5234]).

      LWSP:   is linear whitespace, defined as WSP plus CRLF (formal
         definition in [RFC5234]).

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

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



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

   The definition of FWS is identical to that in [RFC5322] except for
   the exclusion of obs-FWS.

2.3.2.  Common ABNF Tokens

   The following tokens are used in this document:



      ABNF:
   hyphenated-word =  ALPHA
                      [ *(ALPHA / DIGIT / "-")
                      (ALPHA / DIGIT) ]
   ALPHADIGITPS    =  (ALPHA / DIGIT / "+" / "/")
   base64string    =  ALPHADIGITPS *([FWS] ALPHADIGITPS)
                      [ [FWS] "=" [ [FWS] "=" ] ]
   hdr-name        =  field-name
   qp-hdr-value    =  D-quoted-printable
                           ; with "|" encoded





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2.3.3.  Imported ABNF Tokens

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

   From [RFC5321]:



      <local-part>   (implementation warning: this permits quoted
         strings)

      <sub-domain>

   From [RFC5322]:



      <field-name>   (name of a header field)

   From [RFC2045]:



      <qp-section>   a single line of quoted-printable-encoded text

      <hex-octet>   a quoted-printable encoded octet)

   NOTE:   Be aware that the ABNF in [RFC2045] does not obey the rules
      of [RFC5234] and MUST be interpreted accordingly, particularly as
      regards case folding.

   Other tokens not defined herein are imported from [RFC5234].  These
   are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
   etc.

2.3.4.  D-Quoted-Printable

   The D-Quoted-Printable encoding syntax resembles that described in
   Quoted-Printable [RFC2045], Section 6.7:

   o  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.

   o  All control characters (those with values < %x20), 8-bit
      characters (values > %x7F), and the characters DEL (%x7F), SPACE



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      (%x20), and semicolon (";", %x3B) MUST be encoded.

   o  All whitespace, including SPACE, CR, and LF characters, MUST be
      encoded.

   o  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.

   The formal syntax for D-Quoted-Printable is:



      ABNF:
   D-quoted-printable =  *(FWS / hex-octet / D-safe-char)
                            ; hex-octet is from RFC2045
   D-safe-char        =  %x21-3A / %x3C / %x3E-7E
                            ; '!' - ':', '<', '>' - '~'
                            ; Characters not listed as "mail-safe"
                            ; in [RFC2049] are also not
                            ; recommended.

   D-Quoted-Printable differs from Quoted-Printable as defined in
   [RFC2045] in several important ways:

   1.  Whitespace in the input text, including CR and LF, MUST be
       encoded.  [RFC2045] 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 D-Quoted-Printable to be wrapped as needed.  In
       particular, [RFC2045] 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.  D-Quoted-Printable does not require that encoded lines be no more
       than 76 characters long (although there might be other
       requirements depending on the context in which the encoded text
       is being used).


3.  DOSETA Library

   DOSETA is distinguished by a DNS-based, self-certifying public key
   mechanism, common canonicalization algorithms, and a common parameter



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   encoding mechanism.

3.1.  Canonicalization {rfc4871bis-02 3.4}

   Some data handling systems modify the original data, potentially
   invalidating a cryptographic function, such as with a signature.  For
   most signers, mild modification of data is immaterial to validation
   of the DOSETA domain name's use.  For such signers, a
   canonicalization algorithm that survives modest handling modification
   is preferred.

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

   To satisfy basic requirements, two canonicalization algorithms are
   defined: a "simple" algorithm that tolerates almost no modification
   and a "relaxed" algorithm that tolerates common modifications such as
   whitespace replacement and data line rewrapping.

   Data handling systems sometimes treat different portions of text
   differentially and might be subject to more or less likelihood of
   breaking a signature.  DOSETA currently covers two types of data:



      Header:   Attribute:value sets, in the style of an Internet Mail
         header fields

      Content:   Lines of ASCII text

   Some DOSETA Creators might be willing to accept modifications to some
   portions of the data, but not other portions.  For DOSETA, a Creator
   MAY specify either algorithm for one portion of the data and another
   for a different portion when signing the data.

   If no canonicalization algorithm is specified, the "simple" algorithm
   defaults for all of the data.  DOSETA Creators MUST implement both
   canonicalization algorithms.  Because further canonicalization
   algorithms might be defined in the future, Creators MUST ignore any
   signatures that use unrecognized canonicalization algorithms.

   Canonicalization simply prepares the data for presentation to the
   signing or verification algorithm.  It MUST NOT change the
   transmitted data in any way.  Canonicalization of distinct data
   portions is described below.




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   NOTE:   This section assumes that data is already in "network normal"
      format (text is ASCII encoded, lines are separated with CRLF
      characters, etc.).  See also Section 4.4.3 for information about
      normalizing the message.

3.1.1.  Header Canonicalization Algorithms

   This section specifies initial entry for the IANA registry defined in
   Table 2.

   simple:   The "simple" header canonicalization algorithm is for a set
      of "attribute:value" textual data structures, such as email header
      fields [RFC5322].  It does not change the original 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.

   relaxed:   The "relaxed" header canonicalization algorithm is for a
      set of "attribute:value" textual data structures, such as email
      header fields [RFC5322].  It does not change the original Header
      fields in any way.  It MUST apply the following steps in order:

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

      *  Unfold all header field continuation lines as described in
         [RFC5322]; 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.

      *  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.

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

      *  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.








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3.1.2.  Content Canonicalization Algorithms

   This section specifies initial entry for the IANA registry defined in
   Table 3.

   simple:   The "simple" Content canonicalization algorithm is for
      lines of ASCII text, such as occur in the body of email [RFC5322].
      It ignores all empty lines at the end of the Content.  An empty
      line is a line of zero length after removal of the line
      terminator.  If there is no Content or no trailing CRLF on the
      Content, a CRLF is added.  It makes no other changes to the
      Content.  In more formal terms, the "simple" Content
      canonicalization algorithm converts "0*CRLF" at the end of the
      Content to a single "CRLF".

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

      The sha1 value (in base64) for an empty Content (canonicalized to
      a "CRLF") is:
   uoq1oCgLlTqpdDX/iUbLy7J1Wic=
      The sha256 value is:
   frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY=

   relaxed:   The "relaxed" Content canonicalization algorithm is for
      lines of ASCII text, such as occur in the body of email [RFC5322].
      It MUST apply the following steps (a) and (b) in order:

      A.  Reduce whitespace:

          +  Ignore all whitespace at the end of lines.  Implementations
             MUST NOT remove the CRLF at the end of the line.

          +  Reduce all sequences of WSP within a line to a single SP
             character.

      B.  Ignore all empty lines at the end of the Content.  "Empty
          line" is defined in Section 3.4.3.  If the Content is non-
          empty, but does not end with a CRLF, a CRLF is added.  (For
          email, this is only possible when using extensions to SMTP or
          non-SMTP transport mechanisms.)

      The sha1 value (in base64) for an empty Content (canonicalized to
      a null input) is:
   2jmj7l5rSw0yVb/vlWAYkK/YBwk=
      The sha256 value is:
   47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=



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

3.1.3.  Canonicalization Examples (3.4.6}

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

   Example 1: An email message reading:
   A: <SP> X <CRLF>
   B <SP> : <SP> Y <HTAB><CRLF>
                   <HTAB> Z <SP><SP><CRLF>
   <CRLF>
   <SP> C <SP><CRLF>
   D <SP><HTAB><SP> E <CRLF>
   <CRLF>
   <CRLF>

   when canonicalized using relaxed canonicalization for both header and
   Content results in a header reading:
   a:X <CRLF>
   b:Y <SP> Z <CRLF>

   and a Content reading:
   <SP> C <CRLF>
   D <SP> E <CRLF>

   Example 2: The same message canonicalized using simple
   canonicalization for both header and Content results in a header
   reading:
   A: <SP> X <CRLF>
   B <SP> : <SP> Y <HTAB><CRLF>
          <HTAB> Z <SP><SP><CRLF>

   and a Content reading:
   <SP> C <SP><CRLF>
   D <SP><HTAB><SP> E <CRLF>






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   Example 3: When processed using relaxed header canonicalization and
   simple Content canonicalization, the canonicalized version has a
   header of:
   a:X <CRLF>
   b:Y <SP> Z <CRLF>

   and a Content reading:
   <SP> C <SP><CRLF>
   D <SP><HTAB><SP> E <CRLF>

3.2.  Tag=Value Parameters {rfc4871bis-02 3.2}

   DOSETA uses a simple "tag=value" syntax in several contexts,
   including to represent associated cryptographic data and domain
   cryptographic key records.

   Values are a series of strings containing either plain text, "base64"
   text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid,
   Section 6.7), or "D-quoted-printable" (as defined in Section 2.6).
   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.

   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.

   Formally the syntax rules are as follows:



      ABNF:
   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



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   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.

3.3.  Key Management {rfc4871bis-02 3.6}

   Applications require some level of assurance that a cited public key
   is associated with the Creator using it.  Many applications achieve
   this by using public key certificates issued by a trusted third
   party.  For applications with modest certification requirements,
   DOSETA achieves a sufficient level of security, with significantly
   enhanced scalability, by simply having the Consumer query the
   purported Creator's DNS entry (or a supported equivalent) in order to
   retrieve the public key.

   DOSETA keys might 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 DOSETA algorithm.



      The key lookup algorithm is:
   public-key = D-find-key(q-val, d-val, s-val)
      where:

      q-val:   The type of the lookup, as specified in the "q=" tag
         (Section 4.2)

      d-val:   The domain of the signature, as specified in the "d=" tag
         (Section 4.2)







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      s-val:   The selector of the lookup as specified in the "s=" tag
         (Section 4.2)

      D-find-key:   A function that uses q-val to determine the specific
         details for accessing the desired stored Key record.

   This document defines a single binding, using DNS TXT records to
   distribute the keys, per Section 3.5.  Other bindings might be
   defined in the future.

3.4.  Selectors for Keys {rfc4871bis-02 3.1}

   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 (for example,
   "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
   (for example, "january2005", "february2005", etc.), or even an
   individual user.

   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 generate signed
      data locally without the need to connect to a particular server.

   o  "Affinity" domains (such as, college alumni associations) that
      provide data forwarding, but that do not operate a data
      origination agent for outgoing data.

   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



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   satisfied with just one selector, whereas administratively
   distributed organizations might choose to manage disparate selectors
   and key pairs in different regions or on different 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 data might be in transit prior to
   verification.  At the start of the transition period, the outbound
   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.



      NOTE:   A key can 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 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 might 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
   name of a registered end-user, or make it some unassociated random
   value, such as a fingerprint of the public key.



      NOTE:   The ability to reuse 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 data that didn't
         verify because the key is no longer valid versus a data 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.







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3.5.  DNS Binding for Key Retrieval {rfc4871bis-02 3.6.2}

   This section defines a binding using DNS TXT records as a key
   service.  All implementations MUST support this binding.

3.5.1.  Namespace

   A DOSETA key is stored in a subdomain named:



      ABNF:
   dns-record =  s "._domainkey." d
      where:

      s:    is the selector of the lookup as specified in the "s=" tag
         (Section 4.2)

      d:    is the domain of the signature, as specified in the "d=" tag
         (Section 4.2)

   NOTE:   The string constant "_domainkey" is used to mark a sub-tree
      that contains unified DOSETA key information.  This string is a
      constant, rather than being a different string for different key-
      based services, in the view that keys are agnostic about the
      service they are used for.  That is, there is no semantic or
      security benefit in having a different constant string for
      different key services.

   Given a DOSETA-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

   Wildcard DNS records (for example, *.bar._domainkey.example.com) do
   not make sense in this context and SHOULD not be used.  Note also
   that wildcards within domains (for example,
   s._domainkey.*.example.com) are not supported by the DNS.

3.5.2.  Resource Record Types for Key Storage

   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 might 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



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   selector name; that is, if there are multiple records in an RRset,
   the results are undefined.

   TXT RRs are encoded as described in Section 3.6.

3.6.  Stored Key Data {rfc4871bis-02 3.6.1, subset, with preface}

   This section defines a syntax for encoding stored key data within an
   unstructured environment such as simple text.



      TEMPLATE:   (Key Retrieval) A service that incorporates DOSETA MAY
         define the specific mechanism by which Consumers can obtain
         associated public keys.  This might be as easy as referencing
         an existing key management system or it might require a new set
         of conventions.

         Absent an explicit specification for key retrieval, the default
         mechanism is specified in Section 3.5.

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



      v= Version of the DOSETA 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] %x44 %x4B %x49 %x4D %x31

         NOTE:   The version string "DKIM1" is retained from the
            original DKIM specification, in order to preserve the
            installed base of records.  In addition, there is no
            functional benefit in defining a new string, since the key
            record is not application-specific.







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      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 [ITU-X660-1997]
         RSAPublicKey [RFC3447] (see Sections Section 3.4 and A.1.1) is
         being used in the "p=" tag.  (Note: the "p=" tag further
         encodes the value using the base64 algorithm.)  Unrecognized
         key types MUST be ignored.





            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.



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

         NOTE:   If a private key has been compromised or otherwise
            disabled (for example, an outsourcing contract has been
            terminated), a signer might want to explicitly state that it
            knows about the selector, but also have all queries using
            that selector result in a failed verification.  Verifiers
            SHOULD ignore any DOSETA-Signature header fields with a
            selector referencing a revoked key.




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         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.


4.  DOSETA H/C Signing Template

   This section specifies the basic components of a signing mechanism,
   which is similar to the one defined for DKIM.  This template for a
   signing service can be mapped to a two-part -- header/content -- data
   model.  As with DKIM it separates specification of the signer's
   identity from any other identifiers that might be associated with
   that data.



      NOTE:   The use of hashing and signing algorithms by DOSETA
         inherently provides a degree of data integrity protection,
         between the signing and verifying steps.  However it does not
         necessarily "authenticate" the data that is signed.  For
         example, it does not inherently validate the accuracy of the
         data or declare that the signer is the author or owner of the
         data.

      TEMPLATE:   (Header/Content Mapping) The service incorporating
         this mechanism MUST define the precise mappings onto the
         template provided in this section.  (Note that data lacking a
         header component might still be possible to cast in a header/
         content form, where the header comprises on the DOSETA
         Signature information.)

         The service also MUST define the precise meaning of a
         signature.

4.1.  Cryptographic Algorithms {rfc4871bis-02 3.3, subset}

   DOSETA supports multiple digital signature algorithms:



      rsa-sha1:   The rsa-sha1 Signing Algorithm computes a message hash
         as described in Section 4.3 below using SHA-1 [FIPS-180-2-2002]
         as a hashing algorithm.  That hash is then signed by the signer
         using the RSA algorithm (defined in PKCS#1 version 1.5
         [RFC3447]) as the crypt-alg and the signer's private key.  The
         hash MUST NOT be truncated or converted into any form other



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         than the native binary form before being signed.  The signing
         algorithm SHOULD use a public exponent of 65537.

      rsa-sha256:   The rsa-sha256 Signing Algorithm computes a message
         hash as described in [RFC5451] below using SHA-256
         [FIPS-180-2-2002] as the hash-alg.  That hash is then signed by
         the signer using the RSA algorithm (defined in PKCS#1 version
         1.5 [RFC3447]) 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.

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

   Signers MUST implement and SHOULD sign using rsa-sha256.  Verifiers
   MUST implement rsa-sha256.

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

   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 might use
   the length of the signing key as one metric for determining whether a
   signature is acceptable.

   Factors that ought to influence the key size choice include the
   following:

   o  The practical constraint that large (for example, 4096 bit) keys
      might 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 data

   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



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   See [RFC3766] for further discussion on selecting key sizes.

4.2.  Signature Data Structure {rfc4871bis-02 3.5, subset}

   A signature of data is stored into an data structure associated with
   the signed data.  This structure contains all of the signature and
   key-fetching data.  This DOSETA-Signature structure is a tag-list as
   defined in Section 3.2.



      TEMPLATE:   (Signature Association) A service that incorporates
         DOSETA MUST define the exact means by which the Signature
         structure is associated with the data.

   When the DOSETA-Signature is part of a sequence of structures -- such
   as being added to an email header -- it SHOULD NOT be reordered and
   SHOULD be prepended to the message.  (This is the same handling as is
   given to email trace Header fields, defined in Section 3.6 of
   [RFC5322].)

   The tags are specified below.  Tags described as <qp-section> are
   encoded as described in Section 6.7 of MIME Part One [RFC2045], with
   the additional conversion of semicolon characters to "=3B";
   intuitively, this is one line of quoted-printable encoded text.  The
   D-quoted-printable syntax is defined in Section 2.3.4.

   Tags on the DOSETA-Signature structure 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"








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         NOTE:   DOSETA-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 Section 4.1 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

      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 (Section 4.4) 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 Content (body), 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 Section 4.3 for how the Content hash
         is computed.





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            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 Content canonicalization
         algorithms respectively:



            ABNF:
   sig-bh-tag      = %x63 [FWS] "=" [FWS] sig-c-header "/" sig-c-content
            where:

            sig-c-header:   A value from IANA registry defined in
               Table 2

            sig-c-content:   A value from IANA registry defined in
               Table 3

         These algorithms are described in Section 3.1.  If only one
         algorithm is named, that algorithm is used for the header and
         "simple" is used for the Content.  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 SDID doing the signing (plain-text; REQUIRED).  Hence,
         the SDID value is used to form the query for the public key.
         The SDID MUST correspond to a valid DNS name under which the
         DOSETA key record is published.  The conventions and semantics
         used by a signer to create and use a specific SDID are outside
         the scope of the DOSETA Signing specification, as is any use of
         those conventions and semantics.  When presented with a
         signature that does not meet these requirements, verifiers MUST



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         consider the signature invalid.

         Internationalized domain names MUST be encoded as described in
         [RFC5890].





            TEMPLATE:   (Semantics) The service incorporating DOSETA
               MUST define the semantics of a signature.





            ABNF:

   sig-d-tag       = %x64 [FWS] "=" [FWS] domain-name
   domain-name     = sub-domain 1*("." sub-domain)
                        ; from RFC 5321 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 DOSETA Signature header field that is being
         created or verified, but might 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 Section 4.4.4 for a discussion of
         choosing Header fields to sign.











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

         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 all
         possible Header fields that might be created in the future, the
         security of this solution is not total.

         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.

      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.  Unrecognized query
         mechanisms MUST be ignored.

         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".





            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).






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            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.  Ideally verification time is when a
         message is first received at the administrative domain of the
         verifier; 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.



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

         NOTE:   Due to clock drift, the receiver's notion of when to
            consider the signature expired might not match exactly when
            the sender is expecting.  Receivers MAY add a 'fudge factor'
            to allow for such possible drift.



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

4.3.  Signature Calculation {rfc4871bis-02 3.7, reorganized and
      clarified}

   Hashing and Cryptographic algorithms are combined into a procedure
   for computing a digital signature.

   Creators will choose appropriate parameters for the signing process;
   Creators will use the tags that are then passed as an associated
   DOSETA Header field.  Section 4.2 defines the contents of the Header
   field.

   In the following discussion, the names of the tags in the Header
   field that either exist (when consuming) or will be created (when
   creating) are used.



      NOTE:   Canonicalization (see Section 3.1) prepares a separate
         representation of the data for additional processing; it does
         not affect the transmitted data in any way.

   Creators MUST compute hashes in the order defined.  Consumers MAY
   compute them in any order convenient to the Creator, provided that
   the result is semantically identical to the semantics that would be
   the case had they been computed in this order.

   The combined hashing algorithm is:



      Step 1:   The Creator/Creator hashes the Content portion,
         canonicalized using the Content 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.  Signers then insert it into the "bh=" tag of
         the DOSETA-Signature field; verifiers compare their hash with
         the value in the "bh=" tag.

      Step 2:   Pass the following to a second hash algorithm in the
         indicated order:





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         +  Complete attribute:value 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 field MUST be terminated with a single CRLF.

         +  The DOSETA-Signature field that exists (verifying), or will
            be inserted (signing), in the Header, with the value of the
            "b=" signature data tag (including all surrounding
            whitespace) deleted (that is, treated as the empty string),
            canonicalized using the Header canonicalization algorithm
            specified in the "c=" tag, and without a trailing CRLF.

         +  The hash produced in Step 1.

   When calculating or verifying a signature, the value of the "b=" tag
   (signature value) of that DOSETA-Signature structure MUST be treated
   as though it were an empty string.  Unknown tags in the
   DOSETA-Signature header field MUST be included in the signature
   calculation but MUST be otherwise ignored by verifiers.  Other
   DOSETA-Signature tags that are included in the signature SHOULD be
   treated as normal tags; in particular, the "b=" tag is not treated
   specially.

   All tags and their values in the DOSETA-Signature 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 DOSETA-Signature field MUST be
   presented to the hash algorithm after the Content. rather than with
   the rest of the Header fields and MUST be canonicalized as specified
   in the "c=" (canonicalization) tag.  The DOSETA-Signature header
   structure MUST NOT be included in its own h= tag, although other
   DOSETA-Signature Header fields MAY be signed (see Section 5).

   When calculating the hash on data that will be transmitted using
   additional encoding, such as base64 or quoted-printable, 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 [RFC5321]).

   With the exception of the canonicalization procedure described in
   Section 3.1, the DOSETA signing process treats the Content as simply
   a string of octets.  DOSETA Content MAY be either in plain-text or in
   MIME format; no special treatment is afforded to MIME content.




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   More formally, the algorithm for the signature is as follows:



      ABNF:
   content-hash =  bh-hash-alg (canon-content, l-param)
   data-hash    =  a-hash-alg (h-headers, D-SIG, content-hash)
   signature    =  sig-alg (d-domain, selector, data-hash)

   where:



      content-hash:   is the output of a function to hash the data
         Content.

      bh-hash-alg:   is the hashing algorithm specified in the "bh"
         parameter.

      canon-body:   is a canonicalized representation of the body.

      l-param:   is the value of the l= length parameter.

      data-hash:   is the output from hashing the header, the
         DOSETA-Signature header, and the Content hash.

      a-hash-alg:   is the hash algorithm specified by the "a=" tag,

      h-headers:   is the list of headers to be signed, as specified in
         the h= parameter.

      D-SIG:   is the canonicalized DOSETA-Signature field sans the
         signature value itself.

      canon-content:   is the canonicalized data Content(respectively)
         as defined in Section 3.1 (excluding the DOSETA-Signature
         field).

      signature:   is the signature value produced by the signing
         algorithm.

      sig-alg:   is the signature algorithm specified by the "a=" tag,

      d-domain:   is the domain name specified in the d= parameter.







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      selector:   is the value of the s= selector parameter

   NOTE:   Many digital signature APIs provide both hashing and
      application of the RSA private key using last two steps in the
      algorithm would probably be combined into a single call that would
      perform both the "a-hash-alg" and the "sig-alg".

4.4.  Signer Actions {rfc4871bis-02 5}

   The following steps are performed in order by signers.

4.4.1.  Determine Whether the Data Should Be Signed and by Whom

   A signer can obviously only sign data using 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 the data.

   NOTE:   Signing modules can be incorporated into any portion of a
      service, as deemed appropriate, including end-systems, servers and
      intermediaries.  Wherever implemented, signers need to beware of
      the semantics of signing data.  An example of how this can be
      problematic is that within a trusted enclave the signing address
      might be derived from the data according to local policy; the
      derivation is based on local trust rather than explicit
      validation.

   If the data cannot be signed for some reason, the disposition of that
   data is a local policy decision.

4.4.2.  Select a Private Key and Corresponding Selector Information

   This specification does not define the basis by which a signer ought
   to choose which private key and selector information to use.
   Currently, all selectors are equal, with respect to this
   specification.  So the choices ought to largely be a matter of
   administrative convenience.  Distribution and management of private
   keys is also outside the scope of this document.

   NOTE:   A signer SHOULD use a private key with an associated selector
      record that is expected to still be valid by time the verifier is
      likely to have an opportunity to validate the signature.  The
      signer SHOULD anticipate that verifiers can 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, but
      the old public key SHOULD be retained for a reasonable validation



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      interval before being removed from the key server.

4.4.3.  Normalize the Message to Prevent Transport Conversions

   Some messages, particularly those using 8-bit characters, are subject
   to modification during transit, notably from conversion to 7-bit
   form.  Such conversions will break DOSETA signatures.  In order to
   minimize the chances of such breakage, signers SHOULD convert the
   data to a suitable encoding, such as quoted-printable or base64, as
   described in [RFC2045] before signing.  Such conversion is outside
   the scope of DOSETA; the data SHOULD be converted to 7-bit prior to
   presentation to the DOSETA signing procedure.

   Similarly, data that is not compliant with its associated standard,
   might be subject to corrective efforts intermediaries.  See Section 8
   of [RFC4409] for examples of changes that are commonly made to email.
   Such "corrections" might break DOSETA signatures or have other
   undesirable effects.

   If the data is submitted to the signer with any local encoding that
   will be modified before transmission, that modification to a
   canonical form MUST be done before signing.  For Text data in
   particular, bare CR or LF characters (used by some systems as a local
   line separator convention) MUST be converted to the CRLF sequences
   before the data is signed.  Any conversion of this sort SHOULD be
   applied to the data actually sent to the recipient(s), not just to
   the version presented to the signing algorithm.

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

4.4.4.  Determine the Header Fields to Sign

   Signers SHOULD NOT sign an existing header field that is likely to be
   legitimately modified or removed in transit.  Signers MAY include any
   other Header fields present at the time of signing at the discretion
   of the signer.

   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 Content at the receiver.  A third
      strategy is to sign only "well known" headers.  Note that
      verifiers might 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.



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   The DOSETA-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).

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

   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.

   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
   DOSETA-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.

      EXAMPLE:

      If the signer wishes to sign two existing Received Header fields,
      and the existing header contains:
   Received: <A>
   Received: <B>
   Received: <c>

      then the resulting DOSETA-Signature header field ought to read:

   DKIM-Signature: ... h=Received : Received :...
      and Received Header fields <C> and <B> will be signed in that
      order.



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   Signers need to 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 [RFC5322] from
   being reordered.  In particular, since DOSETA-Signature Header fields
   might be reordered by some intermediate MTAs, signing existing
   DOSETA-Signature Header fields is error-prone.

   NOTE:   Despite the fact that [RFC5322] 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 DOSETA signatures to be broken; such
      anti-social behavior ought to be avoided.

   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.

4.4.5.  Compute the Message Signature

   The signer MUST compute the message hash as described in Section 4.3
   and then sign it using the selected public-key algorithm.  This will
   result in a DOSETA-Signature header field that will include the
   Content hash and a signature of the header hash, where that header
   includes the DOSETA-Signature header field itself.

   Entities such as mailing list managers that implement DOSETA 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 Content that
   will be included in the hash and hence signed.  The length actually
   hashed SHOULD be inserted in the "l=" tag of the DOSETA-Signature
   header field.

4.4.6.  Insert the DOSETA-Signature Header Field

   Finally, the signer MUST insert the DOSETA-Signature header field
   created in the previous step prior to transmitting the data.  The
   DOSETA-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



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   DOSETA-Signature header field and using the private key corresponding
   to the selector given in the "s=" tag of the DOSETA-Signature header
   field, as chosen above in Section 4.4.2

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

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

4.5.  Verifier Actions {rfc4871bis-02 6}

   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 server MAY verify the data signature(s).  An
   server that has performed verification MAY communicate the result of
   that verification by adding a verification header field to incoming
   data.

   A verifying server MAY implement a policy with respect to
   unverifiable data, 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.

4.5.1.  Extract Signatures from the Message

   The order in which verifiers try DOSETA-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 DOSETA-Signature Header fields.  In particular, there is
   reason to believe that some relays will reorder the Header fields in
   potentially arbitrary ways.




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   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) might 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.

   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 might 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 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 DOSETA-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, if it exists.  However, verifiers MAY make note of the
   fact that an invalid signature was present for consideration at a
   later step.




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   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 ought to succeed even in the presence of the
      known-broken signature.

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

4.5.2.  Validate the Signature Header Field

   Implementers MUST meticulously validate the format and values in the
   DOSETA-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 DOSETA-Signature header field, which are explicitly permitted.
   Verifiers MUST ignore DOSETA-Signature Header fields with a "v=" tag
   that is inconsistent with this specification and return PERMFAIL
   (incompatible version).

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

   If any tag listed as "required" in Section 4.2 is omitted from the
   DOSETA-Signature header field, the verifier MUST ignore the
   DOSETA-Signature header field and return PERMFAIL (signature missing
   required tag).

   NOTE:   The tags listed as required in Section 4.2 are "v=", "a=",
      "b=", "bh=", "d=", "h=", and "s=".  Should there be a conflict
      between this note and Section 4.2, is normative.

   If the DOSETA-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 DOSETA-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



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   MUST ignore the DOSETA-Signature header field and return PERMFAIL
   (From field not signed).

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

   Verifiers MAY ignore the DOSETA-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" might be ignored.  The list of unacceptable domains
   SHOULD be configurable.

   Verifiers MAY ignore the DOSETA-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 might be possible that the
   verifier would prefer to avoid.

4.5.3.  Get the Public Key

   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 DOSETA-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
   Section 3.3.  The verifier MUST validate the key record and MUST
   ignore any public key records that are malformed.

   NOTE:  The use of wildcard TXT records in the DNS will produce a
      response to a DOSETA query that is unlikely to be valid DOSETA key
      record.  This problem applies to many other types of queries, and
      client software that processes DNS responses needs to take this
      problem into account.

   When validating a message, a verifier MUST perform the following
   steps in a manner that is semantically the same as performing them in
   the order indicated -- in some cases the implementation might
   parallelize or reorder these steps, as long as the semantics remain
   unchanged:

   1.  Retrieve the public key as described in Section 3.3 using the
       algorithm in the "q=" tag, the domain from the "d=" tag, and the
       selector from the "s=" tag.





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   2.  If the query for the public key fails to respond, the verifier
       MAY defer acceptance of this data 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 might choose one of the key records or might 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 "h=" tag exists in the public key record and the hash
       algorithm implied by the a= tag in the DOSETA-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).

   7.  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.

   8.  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
       DOSETA-Signature header field, the verifier MUST immediately
       return PERMFAIL (inappropriate key algorithm).






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4.5.4.  Compute the Verification

   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 Content
       length specified in the "l=" tag, and the header field names in
       the "h=" tag, prepare a canonicalized version of the Content as
       is described in Section 4.3 (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
       Section 4.3

   3.  Verify that the hash of the canonicalized Content 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 (Content 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).

   5.  Otherwise, the signature has correctly verified.

   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
      might also verify the signature on the message header before
      validating that the message hash listed in the "bh=" tag in the
      DOSETA-Signature header field matches that of the actual Content;
      however, if the Content hash does not match, the entire signature
      MUST be considered to have failed.

4.5.5.  Communicate Verification Results

   Verifiers wishing to communicate the results of verification to other
   parts of the data handling system can do so in whatever manner they
   see fit.  For example, implementations might choose to add a Header
   field to the data before passing it on.  Any such header field SHOULD
   be inserted before any existing DOSETA-Signature or preexisting
   verification status Header fields in the header field block.  The
   Authentication-Results: header field ([RFC5451]) MAY be used for this



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   purpose.

      Patterns intended to search for results Header fields to visibly
      mark authenticated data for end users SHOULD verify that the
      header field was added by the appropriate verifying domain.  In
      particular, filters SHOULD NOT be influenced by bogus results
      header fields added by attackers.  To circumvent this attack,
      verifiers SHOULD delete existing results Header fields after
      verification and before adding a new header field.

4.5.6.  Interpret Results/Apply Local Policy

   It is beyond the scope of this specification to describe what actions
   an Assessment phase will take, but data with a verified DOSETA
   signature presents an opportunity to an Assessor that unsigned data
   does not.  Specifically, signed data creates a predictable identifier
   by which other decisions can reliably be managed, such as trust and
   reputation.  Conversely, unsigned data typically lacks a reliable
   identifier that can be used to assign trust and reputation.  It is
   usually reasonable to treat unsigned data as lacking any trust and
   having no positive reputation.

   In general, verifiers SHOULD NOT reject data 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 data.

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

   Once the signature has been verified, that information MUST be
   conveyed to the Assessor (such as an explicit allow/whitelist and
   reputation system) and/or to the end user.  If the SDID is not the
   same as the address in the From: header field, the data system SHOULD
   take pains to ensure that the actual SDID is clear to the reader.

   The verifier MAY treat unsigned Header fields with extreme
   skepticism, including marking them as untrusted or even deleting
   them.

   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



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   diagnostic purposes, the exact nature of a verification failure
   SHOULD be made available to the policy module and possibly recorded
   in the system logs.  If the data cannot be verified, then it SHOULD
   be rendered the same as all unverified data regardless of whether or
   not it looks like it was signed.

4.6.  Requirements for Tailoring the Signing Service {added}

   This generic template requires additional details, to define a
   specific service:



      D-Signature Association:   Specify the means by which a
         DOSETA-Signature header field is associated with the data it
         signs.  For example, DKIM uses the DKIM-Signature email header
         field.  If the header field is encoded differently than defined
         for the DOSETA generic service, such as in XML, then that also
         needs to be specified including the algorithm for mapping
         between the common encoding provided here and the new encoding.

      Semantics Signaling:   The means by which a Consumer is to know
         what semantics apply must be indicated.  This is likely to be
         the same means by which the Consumer knows what specific
         service is being used.  For example, different service-specific
         DOSETA-Signature header fields might be used.  "DKIM-Signature"
         signals a name-affixing service, while "DKVM-Signature" might
         signal a message validation service.

      Semantics:   Define the meaning of a signature.  Note that exactly
         the same algorithms might be used for very different semantics.
         One might merely affix an identifier to some data, in a
         verifiable fashion, while the same set of mechanisms might
         separately be defined as authenticating the validity of that
         data.

      Header/Content Mapping:   The mappings between the generic service
         and data of a particular service needs to be defined.  For
         example, with DKIM Header maps to the email header and Content
         maps to the email body.

4.7.  Signing by Parent Domains {rfc4871bis-02 3.8}

   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;



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   for example, a key record for the domain example.com can be used to
   verify messages where the AUID ("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 domain of the AUID.  If the referenced key record
   contains the "s" flag as part of the "t=" tag, the domain of the AUID
   ("i=" flag) MUST be the same as that of the SDID (d=) domain.  If
   this flag is absent, the domain of the AUID MUST be the same as, or a
   subdomain of, the SDID.


5.  Semantics of Multiple Signatures {rfc4871bis-02 4}

5.1.  Example Scenarios {rfc4871bis-02 4.1}

   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.

   EXAMPLE:   Suppose SHA-256 is in the future found to be
      insufficiently strong, and DOSETA 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 DOSETA-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.

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



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   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.

   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 (for example, 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.

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

   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.

5.2.  Interpretation {rfc4871bis-02 4.2}

   A signer that is adding a signature to a message merely creates a new
   DOSETA-Signature header, using the usual semantics of the h= option.
   A signer MAY sign previously existing DOSETA-Signature Header fields
   using the method described in Section 4.4.4 to sign trace Header
   fields.

   NOTE:   Signers need to be cognizant that signing DOSETA-Signature
      Header fields might result in verification failures due to
      modifications by intermediaries, such as their reordering
      DOSETA-Signature header fields.  For this reason, signing existing
      DOSETA-Signature Header fields is unadvised, albeit legal.

   NOTE:   If a header field with multiple instances is signed, those
      Header fields are always signed from the "bottom" up (from last to
      first).  Thus, it is not possible to sign only specific
      DOSETA-Signature Header fields.  For example, if the message being
      signed already contains three DOSETA-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 DOSETA-Signature header field using
   different parameters.  For example, during a transition period a
   signer might want to produce signatures using two different hash



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   algorithms.

   Signers SHOULD NOT remove any DOSETA-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
   their choice; for example, some verifiers might choose to process
   signatures corresponding to the From field in the message header
   before other signatures.  See Section 4.5.1 for more information
   about signature choices.

   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.


6.  Considerations

6.1.  IANA Considerations {rfc4871bis-02 7, subset}

   This section carries forward the IANA registrations made by DKIM
   [RFC4871].  The original DKIM language has been retained, including
   the use of "DKIM" in the name of these registries, in order to
   provide continuity from the original specification.

   In all cases, new values are assigned only for values that have been
   documented in a published RFC that has IETF Consensus [RFC5226].

6.1.1.  DKIM-Signature Tag Specifications

   NOTE:   The content of a DOSETA-Signature structure match those in a
      DKIM-Signature header field.

   A DKIM-Signature provides for a list of tag specifications.  IANA has
   established the DKIM-Signature Tag Specification Registry for tag



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   specifications that can be used in DKIM-Signature fields.

   The initial entries in the registry comprise:

                  +------+------------------------------+
                  | TYPE | REFERENCE                    |
                  +------+------------------------------+
                  |   v  | (this document, Section 4.2) |
                  |   a  | (this document, Section 4.2) |
                  |   b  | (this document, Section 4.2) |
                  |  bh  | (this document, Section 4.2) |
                  |   c  | (this document, Section 4.2) |
                  |   d  | (this document, Section 4.2) |
                  |   h  | (this document, Section 4.2) |
                  |   q  | (this document, Section 4.2) |
                  |   s  | (this document, Section 4.2) |
                  |   t  | (this document, Section 4.2) |
                  |   x  | (this document, Section 4.2) |
                  +------+------------------------------+

     Table 1: DKIM-Signature Tag Specification Registry Initial Values

6.1.2.  DKIM-Signature Query Method Registry

   The "q=" tag-spec (specified in Section 4.2) 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, Section 4.2, "q=") |
          +------+--------+------------------------------------+

         DKIM&nbhy;Signature Query Method Registry Initial Values

6.1.3.  DKIM-Signature Canonicalization Registry

   The "c=" tag-spec (specified in Section 4.2) provides for a specifier
   for canonicalization algorithms for the header and Content.

   IANA has established the DKIM-Signature Canonicalization Algorithm
   Registry for algorithms for converting a message into a canonical



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   form before signing or verifying using DKIM.

   The initial entries in the Header registry comprise:

               +---------+--------------------------------+
               |   TYPE  | REFERENCE                      |
               +---------+--------------------------------+
               |  simple | (this document, Section 3.1.1) |
               | relaxed | (this document, Section 3.1.1) |
               +---------+--------------------------------+

    Table 2: DKIM-Signature Header Canonicalization Algorithm Registry
                              Initial Values

   The initial entries in the Content (Body) registry comprise:

               +---------+--------------------------------+
               |   TYPE  | REFERENCE                      |
               +---------+--------------------------------+
               |  simple | (this document, Section 3.1.2) |
               | relaxed | (this document, Section 3.1.2) |
               +---------+--------------------------------+

     Table 3: DKIM-Signature Body Canonicalization Algorithm Registry
                              Initial Values

6.1.4.  _domainkey DNS TXT Record Tag Specifications

   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, Section 3.6) |
                  |   k  | (this document, Section 3.6) |
                  |   n  | (this document, Section 3.6) |
                  |   p  | (this document, Section 3.6) |
                  +------+------------------------------+

        DOSETA _domainkey DNS TXT Record Tag Specification Registry
                              Initial Values





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6.1.5.  DKIM Key Type Registry

   The "k=" <key-k-tag> (specified in Section 3.6) and the "a=" <sig-
   a-tag-k> (specified in Section 4.2) 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.

   The initial entry in the registry comprises:

                           +------+-----------+
                           | TYPE | REFERENCE |
                           +------+-----------+
                           |  rsa | [RFC3447] |
                           +------+-----------+

                       DKIM Key Type Initial Values

6.1.6.  DKIM Hash Algorithms Registry

   The "h=" <key-h-tag> (specified in Section 3.6) and the "a=" <sig-
   a-tag-h> (specified in Section 4.2) 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  | [FIPS-180-2-2002] |
                      | sha256 | [FIPS-180-2-2002] |
                      +--------+-------------------+

                    DKIM Hash Algorithms Initial Values

6.2.  Security Considerations {rfc4871bis-02 8, subset}

   It has been observed that any mechanism that is introduced that
   attempts to stem the flow of spam is subject to intensive attack.
   DOSETA needs to be carefully scrutinized to identify potential attack
   vectors and the vulnerability to each.  See also [RFC4686].







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6.2.1.  Misappropriated Private Key

   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 DOSETA 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
   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 (for example, SMTP Authentication), possibly validate the
   message itself (for example, 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.

6.2.2.  Key Server Denial-of-Service Attacks

   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



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   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.

6.2.3.  Attacks Against the DNS

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

   While the DNS is currently insecure [RFC3833], these security
   problems are the motivation behind DNS Security (DNSSEC) [RFC4033],
   and all users of the DNS will reap the benefit of that work.

   DOSETA 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 [RFC4880] and S/MIME [RFC5751] 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
   deployment of DOSETA 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 ought to be considered by DOSETA
   verifiers is the name chaining attack described in Section 2.3 of
   [RFC3833].  A DOSETA verifier, while verifying a DOSETA-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.

6.2.4.  Replay Attacks

   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



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   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.

6.2.5.  Limits on Revoking Keys

   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.

6.2.6.  Intentionally Malformed Key Records

   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.

6.2.7.  Intentionally Malformed DOSETA-Signature Header Fields

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







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6.2.8.  Information Leakage

   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.

6.2.9.  Remote Timing Attacks

   In some cases it might be possible to extract private keys using a
   remote timing attack [BONEH03].  Implementations SHOULD obfuscate the
   timing to prevent such attacks.

6.2.10.  Reordered Header Fields

   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
   DOSETA-Signature fields run the risk of having messages incorrectly
   fail to verify.

6.2.11.  RSA Attacks

   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.

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

6.2.12.  Inappropriate Signing by Parent Domains

   The trust relationship described in Section 4.7 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.



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   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 Section 4.5.2.

6.2.13.  Attacks Involving Addition of Header Fields

   Many email implementations do not enforce [RFC5322] with strictness.
   As discussed in Section 4.4.3 DOSETA processing is predicated on a
   valid mail message as its input.  However, DOSETA implementers need
   to be aware of the potential effect of having loose enforcement by
   email components interacting with DOSETA modules.

   For example, a message with multiple From: Header fields violates
   Section 3.6 of [RFC5322].  With the intent of providing a better user
   experience, many agents tolerate these violations and deliver the
   message anyway.  An MUA then might elect to render to the user the
   value of the last, or "top", From: field.  This might also be done
   simply out of the expectation that there is only one, where a "find
   first" algorithm would have the same result.  Such code in an MUA can
   be exploited to fool the user if it is also known that the other
   From: field is the one checked by arriving message filters.  Such is
   the case with DOSETA; although the From: field MUST be signed, a
   malformed message bearing more than one From: field might only have
   the first ("bottom") one signed, in an attempt to show the message
   with some "DOSETA passed" annotation while also rendering the From:
   field that was not authenticated.  (This can also be taken as a
   demonstration that DOSETA is not designed to support author
   validation.)

   To resist this specific attack the signed header field list can
   include an additional reference for each field that was present at
   signing.  For example, a proper message with one From: field could be
   signed using "h=From:From:..."  Due to the way header fields are
   canonicalized for input to the hash function, the extra field
   references will prevent instances of the cited fields from being
   added after signing, as doing so would render the signature invalid.

   The From: field is used above to illustrate this issue, but it is
   only one of > several fields that Section 3.6 of [RFC5322] constrains
   in this way.  In reality any agent that forgives malformations, or is
   careless about identifying which parts of a message were
   authenticated, is open to exploitation.





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7.  References

7.1.  Normative References

   [FIPS-180-2-2002]
              U.S. Department of Commerce, "Secure Hash Standard", FIPS
              PUB 180-2, August 2002.

   [ITU-X660-1997]
              "Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", 1997.

   [RFC1034]  Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
              RFC 1034, November 1987.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, November 1996.

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

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, January 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5890]  Klensin, J., "Internationalizing Domain Names in
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.







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7.2.  Informative References

   [BONEH03]  "Remote Timing Attacks are Practical", Proceedings  12th
              USENIX Security Symposium, 2003.

   [RFC1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
              "Security Multiparts for MIME: Multipart/Signed and
              Multipart/Encrypted", RFC 1847, October 1995.

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, April 2004.

   [RFC3833]  Atkins, D. and R. Austein, "Threat Analysis of the Domain
              Name System (DNS)", RFC 3833, August 2004.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, September 2006.

   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 4880, November 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, April 2009.

   [RFC5672]  Crocker, D., Ed., "RFC 4871 DomainKeys Identified Mail
              (DKIM) Signatures: Update", RFC 5672, August 2009.

   [RFC5751]  Ramsdell, B., "Secure/Multipurpose Internet Mail



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              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 5751, January 2010.


Appendix A.  Creating a Public Key {rfc4871bis-02 C}

   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
   DOSETA 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 might 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

   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-----




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   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")


Appendix B.  Acknowledgements

   DOSETA is derived from DKIM [RFC4871].


Authors' Addresses

   D. Crocker (editor)
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale
   USA

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net
   URI:   http://bbiw.net


   M. Kucherawy (editor)
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   USA

   Email: msk@cloudmark.com















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