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Network Working Group                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Informational                              M. Kucherawy
Expires: January 13, 2012                                      Cloudmark
                                                           July 12, 2011

                  DomainKeys Security Tagging (DOSETA)


   DomainKeys Security Tagging (DOSETA) is a component mechanism that
   enables easy development of security-related services, such as for
   authentication or encryption.  It uses self-certifying keys based on
   domain names.  The domain 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 key retrieval.  The DOSETA Signing Template
   creates common framework for a signature of data that are in a
   "header/content" form.  Defining the meaning of a signature is the
   responsibility of the service that incorporates DOSETA.  Data
   security is enforced through the use of cryptographic algorithms.

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 January 13, 2012.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Comments and Issues  . . . . . . . . . . . . . . . . . . .  5
   2.  Framework  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  DOSETA Architecture  . . . . . . . . . . . . . . . . . . .  5
     2.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  DOSETA Library . . . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Normalization for Transport Robustness . . . . . . . . . . 11
     3.2.  Canonicalization . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Tag=Value Parameters . . . . . . . . . . . . . . . . . . . 16
     3.4.  Key Management . . . . . . . . . . . . . . . . . . . . . . 17
     3.5.  Selectors for Keys . . . . . . . . . . . . . . . . . . . . 18
     3.6.  DNS Binding for Key Retrieval  . . . . . . . . . . . . . . 20
     3.7.  Stored Key Data  . . . . . . . . . . . . . . . . . . . . . 21
   4.  DOSETA H/C Signing Template  . . . . . . . . . . . . . . . . . 23
     4.1.  Cryptographic Algorithms . . . . . . . . . . . . . . . . . 23
     4.2.  Signature Data Structure . . . . . . . . . . . . . . . . . 25
     4.3.  Additional Tags  . . . . . . . . . . . . . . . . . . . . . 31
     4.4.  Signature Calculations . . . . . . . . . . . . . . . . . . 31
     4.5.  Signer Actions . . . . . . . . . . . . . . . . . . . . . . 35
     4.6.  Verifier Actions . . . . . . . . . . . . . . . . . . . . . 38
     4.7.  Requirements for Tailoring the Signing Service . . . . . . 45
   5.  Semantics of Multiple Signatures . . . . . . . . . . . . . . . 46
     5.1.  Example Scenarios  . . . . . . . . . . . . . . . . . . . . 46
     5.2.  Interpretation . . . . . . . . . . . . . . . . . . . . . . 47
   6.  DOSETA Claims Registry Definition  . . . . . . . . . . . . . . 48
   7.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 49
     7.1.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 49
     7.2.  Security Considerations  . . . . . . . . . . . . . . . . . 49
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 50
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 51
   Appendix A.  Creating a Public Key . . . . . . . . . . . . . . . . 52
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 53
   Appendix C.  Example -- DKIM Using DOSETA  . . . . . . . . . . . . 53
     C.1.  Signing and Verification Protocol  . . . . . . . . . . . . 53
     C.2.  Extensions to DOSETA Template  . . . . . . . . . . . . . . 54
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 62

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1.  Introduction

   DomainKeys Security Tagging (DOSETA) is a component mechanism
   enabling development of security-related services, such as for
   authentication or encryption; it uses self-certifying keys based on
   domain names [RFC1034].  The domain 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 key retrieval.  The DOSETA
   Signing Template creates common framework for signing data that are
   in a "header/content" form.  Defining the intended meaning of a
   signature is the responsibility of the service that incorporates
   DOSETA.  Data security is enforced through the use of cryptographic

   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 security-related content
      appearing in the Content.

   o  There is no dependency on having 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  Specific security services can be limited to those needed by the
      service using them.


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

   o  requires minimal new infrastructure

   o  can support a variety of implementation configurations, in order
      to reduce deployment time

   o  can be deployed incrementally

   o  allows delegation of signing to third parties

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   DOSETA derives from Domain Keys Identified Mail (DKIM) [RFC5672] and
   has extracted the core portions of the its signing specification
   [DKIMSign], 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 ion mechanism
   using the same DNS-based, self-certified key service as DKIM.

   DOSETA features include:

      Identity:   DOSETA distinguishes the identity of the DOSETA
         signature's producer from that of any other identifier
         associated with the data, such as the data's purported author.
         In particular, the DOSETA header field includes the DOSETA
         Domain Identifier (DDI), per Section 2.2.1.  DOSETA consumers
         can use the DDI to decide how they want to process the data.
         The DDI can be directly included in the attributes of the data
         or can be recorded elsewhere.

         NOTE:   DOSETA does not, itself, specify that the identity it
            uses is 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 DOSETA-based security
            service might choose to add constraints on the choice of
            identifier, such as having it match another identifier that
            is associated with the data.

      Scalability:   DOSETA is designed to easily support the extreme
         scaling requirements that characterize Internet data

      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 under the domain name associated with the
         use of DOSETA directly, rather than requiring consultation of a
         certificate authority.  That is, DOSETA provides 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.

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      Data Integrity:   When DOSETA is used to sign data -- independent
         of the semantics of that signature -- there is a computed hash
         of some or all of the data that ensures detection of changes to
         that data, between the times of signing and verifying.

1.1.  Comments and Issues

   [RFC EDITOR]   Remove this sub-section prior to publication.

   Possible applications:

   o  JSON structure

   o  XMPP message

   o  XML object

   o  vCard

   o  vCal

   o  Web page signing?

   o  Web ad authentication

   o  Handle System

   Discussion Venue:  Discussion of this draft should take place on the
      doseta-discuss mailing list.  It is located at:


2.  Framework

   This section provides the technical background for the remainder of
   the document.

2.1.  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:

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        +--------+       +----------+           +-----------------+
        |  DKIM  |       | MIMEAUTH |           | Message Privacy |
        +---+----+       +-----+----+           +--------+--------+
            |                  |                         |
     ++=====V==================V========++               |
     ||                                 ||               |
     || Header/Content Signing Template ||               |
     ||                                 ||               |
     ++================+================++               |
                       |                                 |
    ||                                                                ||
    ||                      D O S E T A     L I B R A R Y             ||
    || +------------------+ +------------+ +-------------+ +--------+ ||
    || |                  | | Key        | | Parameter   | | Tags   | ||
    || | Canonicalization | | Management | | Format      | | Header | ||
    || |                  | | (DNS)      | | (tag=value) | | Field  | ||
    || +------------------+ +------------+ +-------------+ +--------+ ||
    ||                                                                ||

   DKIM is as specified in [DKIMSign].  MIMEAUTH is an exemplar use of
   DOSETA, specified in [mimeauth].  Message Privacy is a generic term,
   indicating any service that provides encryption; it is expected that
   such a service can use the DOSETA core library, but not take
   advantage of the DOSETA signing template.

   The library comprises:

   Canonicalization:   This ensures common data representation and
      robustness against some forms of data modification during transit.
      It is discussed in Section 3.2 and Section 3.1.

   Key Management:   This covers the mechanisms for discovering and
      obtaining signature key information by a verifier.  It is
      discussed in Section 3.4, Section 3.5, and Section 3.6.

   Format:   This describes a simple syntax for encoding parametric
      information and is discussed in Section 3.3.

   Tags:   These are common parameters for the stored public key record,
      defined in Section 3.7 and the common parameters for the signature
      record that is associated with the signed data, defined in
      Section 4.2.

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2.2.  Terminology

   Within the specification, the label "[TEMPLATE]" is used to indicate
   actions that are required for tailoring the use of DOSETA into a
   specific service.

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

   Additional terms for this document are divided among Identity and

2.2.1.  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.  The primary
      example is a domain name.

   DOSETA Domain Identifier (DDI):   A single domain name that serves as
      an identifier, referring to the DOSETA key owner's identity.  The
      DDI is specified in Section 4.2.  Within this specification, the
      name has only basic domain name semantics; any possible owner-
      specific semantics MUST be provided in the specification that
      incorporates DOSETA.

   Identity Assessor:   A module that consumes DOSETA's payload output.
      The module is dedicated to the assessment of the delivered
      identifier.  Optionally, other DOSETA (and non-DOSETA) values can
      also be delivered to this module as well as to a more general
      message evaluation filtering engine.  However, this additional
      activity is outside the scope of the DOSETA specification.

2.2.2.  Actors

   Producer:   An element in the data handling system that produces a
      cryptographic encoding, on behalf of a domain, is referred to as a
      Producer.  For example, a signer is a type of producer.

   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.  For example, a verifier is a type of

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   Signer:   An element in the data handling system that creates a
      digital signature, on behalf of a domain, is referred to as a
      signer.  This element specifies an actor that is a type of DOSETA
      producer.  The actor might operate through a client, server or
      other agent such as a reputation service.  The core requirement is
      that the data MUST be signed before it leaves the control of the
      signer's administrative domain.

   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 of the data
      or some consuming agent such as a data processing intermediary.

2.3.  Syntax

   This section specifies foundational syntactic constructs used in the
   remainder of the document.

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

2.3.1.  Whitespace

   There are three forms of whitespace:

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

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

      FWS:   is folding whitespace.  It allows multiple lines to be
         joined with each a separated by a sequence having CRLF followed
         by at least one whitespace.

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

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

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   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:

   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

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


   From [RFC5322]:

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

   From [RFC2045]:

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

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      <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,

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

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

   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:

   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:

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   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's library of functional components is distinguished by a DNS-
   based, self-certifying public key mechanism, common data
   normalization and canonicalization algorithms, and a common parameter
   encoding mechanism.

3.1.  Normalization for Transport Robustness

   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.  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.

   In order to minimize the chances of such breakage, signers convert
   the data to a suitable encoding, such as quoted-printable or base64,
   as described in [RFC2045] before signing.  Specification and use of
   such conversions is outside the scope of DOSETA.

   If the data is submitted to a DOSETA process with any local encoding
   that will be modified before transmission, that modification to a
   canonical form MUST be done before DOSETA processing.  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

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   SHOULD be applied to the data actually sent to the recipient(s), not
   just to the version presented to the signing algorithm.

   More generally, a DOSETA producer MUST use the data as it is expected
   to be received by the DOSETA consumer rather than in some local or
   internal form.

3.2.  Canonicalization

   Some data handling systems modify the original data during transit,
   potentially invalidating a cryptographic function.  In some cases,
   mild modification of data can be immaterial to the validity of a
   DOSETA-based service.  In these cases, a canonicalization algorithm
   that survives modest handling modification is preferred.

   In other cases, preservation of the exact, original bits is required;
   even minor modifications need to result in a failure.  Hence a
   canonicalization algorithm is needed that does not tolerate any in-
   transit modification of the data.

   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 presented for canonicalization MUST already be in "network
   normal" format -- text is ASCII encoded, lines are separated with
   CRLF characters, etc.)  See Section 3.1 for information about
   normalizing data.

   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 Internet Mail
         header fields or MIME header fields

      Content:   Lines of ASCII text

   Some DOSETA producers might be willing to accept modifications to
   some portions of the data, but not other portions.  For DOSETA, a
   producer MAY specify one algorithm for the header and another for the

   If no canonicalization algorithm is specified, the "simple" algorithm
   defaults for each part.  DOSETA producers MUST implement both of the

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   base canonicalization algorithms.  Because additional
   canonicalization algorithms might be defined in the future, producers
   MUST ignore any unrecognized canonicalization algorithms.

   Canonicalization simply prepares the data for presentation to the
   DOSETA processing algorithm.

   NOTE:   Canonicalization operates on a copy of the data; it MUST NOT
      change the transmitted data in any way.  Canonicalization of
      distinct data portions is described below.

3.2.1.  Header Canonicalization Algorithms

   This section describes basic entries for the Header Canonicalization
   IANA registry defined in [DKIMSign], , which also applies to DOSETA
   header canonicalization.

   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 processing
      algorithm exactly as they are in the data being processed.  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.  The following steps MUST be applied in order:

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

      *  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.

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      *  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.

3.2.2.  Content Canonicalization Algorithms

   This section describes basic entries for the Message Canonicalization
   IANA registry defined in [DKIMSign], which also applies to DOSETA

   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

      The sha1 value (in base64) for an empty Content (canonicalized to
      a "CRLF") is:
      The sha256 value is:

   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

      B.  Ignore all empty lines at the end of the Content.  "Empty
          line" is defined in Section 3.2.2.  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.)

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      The sha1 value (in base64) for an empty Content (canonicalized to
      a null input) is:
      The sha256 value is:

      NOTE:   The relaxed Content canonicalization algorithm can enable
         certain types of extremely crude "ASCII Art" attacks in which a
         message can be conveyed, by adjusting the spacing between
         words.  If this is a concern, the "simple" Content
         canonicalization algorithm is more appropriate for use.

3.2.3.  Canonicalization Examples

   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

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

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

   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>

3.3.  Tag=Value Parameters

   DOSETA uses a simple "tag=value" parameter syntax in several
   contexts, such as when representing associated cryptographic data and
   domain 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 definition of a tag will determine the specific encoding for its
   associated value.  Unencoded semicolon (";") characters MUST NOT
   occur in the tag value, since that separates tag-specs.

   NOTE:   The "plain text" defined below, as "tag-value", only supports
      use of 7-bit characters.  However, it is likely that support of
      UTF-8 Unicode [UTF8] data will eventually be deemed important.

   Formally the syntax rules are as follows:

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

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   NOTE:   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 interpret a VALCHAR as case-sensitive, unless the specific
   tag description of semantics specifies case insensitivity.

   Tags MUST be unique; 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.

3.4.  Key Management

   Applications require some level of assurance that a producer is
   authorized to use a cited public.  Many applications achieve this by
   using public key certificates issued by a trusted authority.  For
   applications with modest certification requirements, DOSETA achieves
   a sufficient level of security, with excellent scaling properties, by
   simply having the consumer query the purported producer's DNS entry
   (or a supported equivalent) in order to retrieve the public key.  The
   safety of this model is increased by the use of DNSSEC [RFC4033] for
   the key records in the DNS.

   DOSETA keys might be stored in multiple types of key servers and in
   multiple formats.  As long as the key-related information is the same
   and as long as the security properties of key storage and retrieval
   are the same, DOSETA's operation is unaffected by the actual source
   of a key.

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

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

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

      s-val:   The selector of the lookup as specified in the "s"
         parameter (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 between the abstract lookup
   algorithm and a physical instance, using DNS TXT records, per
   Section 3.6.  Other bindings can be defined.

3.5.  Selectors for Keys

   It can be extremely helpful to support multiple DOSETA keys for the
   same domain name.  For example:

      *  Rolling over from one key to another is a common security
         administration requirement; for an operational service this is
         made far easier when the old and new keys are supported

      *  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.

      *  Domains that want to allow frequent travelers to generate
         signed data locally without the need to connect to a particular

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

   To these ends, DOSETA includes a mechanism that supports 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.

   For further administrative convenience, sub-division of selectors is

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   allowed, distinguished as dotted sub-components of the selector name.
   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.

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

   The number of public keys and the corresponding selectors for each
   domain are determined by the domain owner.  Many domain owners will
   be 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.

   As noted, 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.

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      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, reuse of selectors with
         different keys is ill-advised.  A better strategy is to assign
         new keys to new selectors.

3.6.  DNS Binding for Key Retrieval

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

3.6.1.  Namespace

   A DOSETA key is stored in a subdomain named:

   dns-record =  s "._domainkey." d

      s:    is the selector of the lookup as specified in the "s"
         parameter (Section 4.2); use of selectors is discussed in
         Section 3.5

      d:    is the domain of the signature, as specified in the "d"
         parameter (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, with 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.  That said, a new service is certainly
      free to define a new constant and maintain and entirely
      independent set of keys.

   Given a DOSETA-Signature field with a "d" parameter of "example.com"
   and an "s" parameter of "foo.bar", the DNS query will be for:

   Wildcard DNS records (for example, *.bar._domainkey.example.com) do
   not make sense in the context of DOSETA and their presence can be

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   problematic.  Hence DNS wildcards with DOSETA SHOULD NOT be used.
   Note also that wildcards within domains (for example,
   s._domainkey.*.example.com) are not supported by the DNS.

3.6.2.  Resource Record Types for Key Storage

   The DNS Resource Record type used is specified by an option to the
   query-type ("q") parameter.  The only option defined in this base
   specification is "txt", indicating the use of a DNS TXT Resource
   Record (RR), as defined in Section 3.7.  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
   selector name; that is, if there are multiple records in an RRset,
   the results are undefined.

3.7.  Stored Key Data

   This section defines a syntax for encoding stored key data within an
   unstructured environment such as the simple text environment of a DNS
   TXT record.

      [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 3.6.  Use of this means
         sharing the set of public keys with DKIM and other DOSETA-based

   The overall syntax is a tag-list as described in Section 3.3.  The
   base set of valid tags is described below.  Other tags MAY be present
   and MUST be ignored by any implementation that does not understand

      k=    Key type (MAY be include; 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 3.5 and A.1.1) is
         being used in the "p" parameter.  (Note: the "p" parameter

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         further encodes the value using the base64 algorithm.)
         Unrecognized key types MUST be ignored.

   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 (MAY be included;
         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.

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

      p=    Public-key data (MUST be included).  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" parameter.

   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.

         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.

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4.  DOSETA H/C Signing Template

   This section specifies the basic components of a signing mechanism;
   it 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 for DKIM this 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.  To the extent that authentication is meant by the
         presence of a signature, that needs to be specified as part of
         the semantics for the service based upon this template.

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

         The service also MUST define the precise meaning of a

4.1.  Cryptographic Algorithms

   DOSETA supports multiple digital signature algorithms:

      rsa-sha1:   The rsa-sha1 Signing Algorithm computes a message hash
         as described in Section 4.4 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
         than the native binary form before being signed.  The signing
         algorithm SHOULD use a public exponent of 65537.

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      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

   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

      *  The practical constraint that large (for example, 4096 bit)
         keys might not fit within a 512-byte DNS UDP response packet

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

      *  Larger keys impose higher CPU costs to verify and sign data

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

      *  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

   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 3.3.

      [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 structure is part of a sequence of
   structures -- such as being added to an email header -- it SHOULD NOT
   be reordered and SHOULD be pre-pended 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

      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 an exact
         string comparison on this value; for example, "1" is not the
         same as "1.0".

   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 (MUST be
         included).  Verifiers MUST support "rsa-sha1" and "rsa-sha256";
         signers SHOULD sign using "rsa-sha256".  See Section 4.1 for a
         description of algorithms.


   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 (MUST be included).  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 4.5) for
         how the signature is computed.

   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" parameter (MUST be included).  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 4.4 for how the Content hash
         is computed.

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

      c=    Data canonicalization (MAY be included; 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:

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

            sig-c-header:   A value from Header Canonicalization IANA
               registry defined in [DKIMSign]

            sig-c-content:   A value from Message Canonicalization IANA
               registry defined in [DKIMSign]

         These algorithms are described in Section 3.2.  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".

   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

      cl=    A list of semantic claims, asserting the set of "meanings"
         intended by the signer, such as author validity or content
         validity (SHOULD be included; default is "handled"}.  The list
         of supported claims comprises values from the DOSETA Claims
         IANA registry, defined in Section 6 using values listed in
         Section 7.1.2.

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   sig-cl-tag        = %x63 %x6C [FWS] "=" [FWS]
                                           ["/" sig-c-tag-claim]
   sig-c-tag-claim   = hyphenated-word
                          ; per DOSETA Claims Registry

      d=    The DDI doing the signing (MUST be included).  Hence, the
         DDI value is used to form the query for the public key.  The
         DDI 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 DDI 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 consider
         the signature invalid.

         Internationalized domain names MUST be encoded as described in

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


   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 (MUST be included).  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

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         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 4.5.3 for a discussion of choosing Header fields to

   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

      q=    A colon-separated list of query methods used to retrieve the
         public key (MAY be included; 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".

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   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 (MUST be included).

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

      t=    Signature Timestamp (SHOULD be included; 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.

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

      x=    Signature Expiration (SHOULD be included; default is no
         expiration).  The format is the same as in the "t" parameter,
         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

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         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" parameter MUST be greater than the value of the "t"
         parameter if both are present.

         NOTE:   The "x" parameter is not intended as an anti-replay

         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.

   sig-x-tag    = %x78 [FWS] "=" [FWS]

4.3.  Additional Tags

   Some applications can benefit from additional, common functional
   enhancements.  These are defined here, as options to the core

      id=    The local identifier, restricting the scope of the DDI,
         such as to a specific user.  This value is combined with the
         DDI, to specify the identifier to be used for assessment.
         Administrative choices for selectors can provide different keys
         for different local identifiers.  See Section 3.5

   key-id-tag   = %x69 %64 [FWS] "=" [FWS] hyphenated-word

4.4.  Signature Calculations

   Hashing and cryptographic signature algorithms are combined into a
   procedure for computing a digital signature.  Producers will choose
   appropriate parameters for the signing process.  Consumers will use
   the tags that are then passed as an associated DOSETA-Signature
   header field.  Section 4.2.  In the following discussion, the names
   of the tags are parameters in that field.

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   The basic operations for producing a signature are canonicalization,
   hashing and signing.  Canonicalization removes irrelevant variations.
   Hashing produces a very short representation for the data and signing
   produces a unique, protected string to be exchanged.

      NOTE:   Canonicalization (see Section 3.2) prepares a separate
         representation of the data for additional processing; it does
         not affect the original, transmitted data in any way.

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

   The combined hashing and signing algorithms are:

      Content Hash:

         1.  Truncate the content to the length specified in the "l"

         2.  Canonicalize the truncated content, using the algorithm
             specified in the "c" parameter.

         3.  Hash the canonicalized content, using the hashing algorithm
             specified in the "a" parameter.

         4.  Convert the resulting hash to base64 form.

         5.  Signers then insert the base64 result into the "bh"
             parameter of the DOSETA-Signature field; verifiers compare
             their hash with the value in the "bh" parameter.

      Header Hash:

         1.  Select the header fields specified by the "h" parameter.

         2.  Ensure that each field is terminated by a single CRLF.

         3.  Canonicalize each of these fields, using the header
             canonicalization algorithm specified in the "c" parameter.

         4.  Select the DOSETA-Signature field that exists (verifying),
             or will be inserted (signing), in the header.

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         5.  From that field, delete the value portion of the "b"
             parameter, including all surrounding whitespace; that is,
             treat the "b" parameter as containing an empty string.

         6.  Canonicalize the resulting field, using the Header
             canonicalization algorithm specified in the "c" parameter,
             but remove the trailing CRLF, if present.

         7.  Using the hashing algorithm specified in the "a" parameter,
             hash the sequence:

             1.  the canonicalized header fields, in the order specified
                 by the "h" parameter,

             2.  the output from the content hash, and

             3.  the canonicalized DOSETA-Signature field.


         1.  Obtain the relevant key associated with the relevant domain
             and selector; for signing this is a private key; for
             verifying this is a public key.

         2.  Obtain the header hash produced by the previous

         3.  Using the signing algorithm specified in the "a" parameter,
             and the relevant key, process the hash.

   All tags cited in the "h" parameter MUST be included even if they are
   not understood by the verifier.  Note that the DOSETA-Signature field
   is presented to the hash algorithm after the content hash is
   processed, rather than with the rest of the header fields that are
   processed before the content hash.  The DOSETA-Signature header
   structure MUST NOT be cited in its own h= tag.  If present, other
   DOSETA-Signature header fields MAY be cited and included in the
   signature process (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]).

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   With the exception of the canonicalization procedure described in
   Section 3.2, the DOSETA signing process treats the content as a
   simple string of octets.  DOSETA content MAY be either simple lines
   of plain-text or as a MIME object; no special treatment is afforded
   to MIME content.

   Formally, the algorithm for the signature is as follows:

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


      content-hash:   is the output from hashing the content, using

      hash-alg:   is the hashing algorithm specified in the "a"

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

      l-param:   is the length-of-content value of the "l" parameter.

      data-hash:   is the output from using the hash-alg algorithm, to
         hash the header including the DKIM-Signature header, and the
         content hash.

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

      D-SIG:   is the canonicalized DOSETA-Signature field without the
         signature value portion of the parameter, itself; that is, an
         empty parameter value.

      canon-content:   is the canonicalized data content (respectively)
         as defined in Section 3.2 (excluding the DOSETA-Signature

      signature:   is the signature value produced by the signing

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

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

      selector:   is the selector value specified in the "s" parameter.

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

4.5.  Signer Actions

   The following steps are performed in order by signers.

4.5.1.  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

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

4.5.2.  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.

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   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
      interval before being removed from the key server.

4.5.3.  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

   The DOSETA-Signature header field is always implicitly signed and
   MUST NOT be included in the "h" parameter 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"
   parameter 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" parameter is sufficient to

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      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" parameter.

   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.


      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

   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

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      the legitimate subject with a one-line spam.

4.5.4.  Compute the Message Signature

   The signer MUST compute the message hash as described in Section 4.4
   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.

4.5.5.  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" parameter
   MUST be the appropriately signed hash computed in the previous step,
   signed using the algorithm specified in the "a" parameter of the
   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 4.5.2

   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.

4.6.  Verifier Actions

   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

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   verification until the message is accessed by the end user is

   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

   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.

4.6.1.  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.

   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

   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.

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   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.

   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.

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4.6.2.  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.

   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"
   parameter, 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"
   parameter.  If not, the DOSETA-Signature header field MUST be ignored
   and the verifier SHOULD return PERMFAIL (domain mismatch).

   If the "h" parameter does not include the From header field, the
   verifier 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" parameter and the
   signature has expired.

   Verifiers MAY ignore the DOSETA-Signature header field if the domain
   used by the signer in the "d" parameter 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.

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4.6.3.  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" parameter 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 3.4.  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 following order -- in some cases the implementation might
   parallelize or reorder these steps, as long as the semantics remain

   1.  Retrieve the public key as described in Section 3.4 using the
       algorithm in the "q=" tag, the domain from the "d" parameter, and
       the selector from the "s" parameter.

   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".

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   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.

   6.  If the "h" parameter 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" parameter, the
       verifier MUST ignore the key record and return PERMFAIL
       (inappropriate hash algorithm).

   7.  If the public key data (the "p" parameter) 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" parameters in the
       DOSETA-Signature header field, the verifier MUST immediately
       return PERMFAIL (inappropriate key algorithm).

4.6.4.  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" parameter, the Content
       length specified in the "l" parameter, and the header field names
       in the "h" parameter, prepare a canonicalized version of the
       Content as is described in Section 4.4 (note that this version
       does not actually need to be instantiated).  When matching header
       field names in the "h" parameter against the actual message
       header field, comparisons MUST be case-insensitive.

   2.  Based on the algorithm indicated in the "a" parameter, compute
       the message hashes from the canonical copy as described in
       Section 4.4

   3.  Verify that the hash of the canonicalized Content computed in the
       previous step matches the hash value conveyed in the "bh"
       parameter.  If the hash does not match, the verifier SHOULD
       ignore the signature and return PERMFAIL (Content hash did not

   4.  Using the signature conveyed in the "b" parameter, verify the
       signature against the header hash using the mechanism appropriate

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       for the public key algorithm described in the "a" parameter.  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" parameter 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.

4.6.5.  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

      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.

4.6.6.  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

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   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 DDI is not the
   same as the address in the From: header field, the data system SHOULD
   take pains to ensure that the actual DDI is clear to the reader.

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

   While the symptoms of a failed verification are obvious -- the
   signature doesn't verify -- establishing the exact cause can be more
   difficult.  If a selector cannot be found, is that because the
   selector has been removed, or was the value changed somehow in
   transit?  If the signature line is missing, is that because it was
   never there, or was it removed by an overzealous filter?  For
   diagnostic purposes, the exact 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.

4.7.  Requirements for Tailoring the Signing Service

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

      Association:   Specify the means by which the signature field is
         associated with the data it signs.  This identifies the
         specific signing service and the mechanics of attaching the
         signature.  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 or JSON,
         then that also needs to be specified, including the algorithm
         for mapping between the common encoding provided here and the
         new encoding.

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      Semantics:   Define the meaning(s) of a signature that is intended
         by the signer.  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.  A single service can
         support multiple semantics for a signature.  These SHOULD be
         specified using the "cl=" claims mechanism defined in
         Section 4.2 and Section 6 with values listed in Section 7.1.2

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

      Required Fields:   Specify the header fields that MUST, SHOULD or
         MAY be included in the signature calculation.

      Required Algorithms:   Specify the hashing and signing algorithms
         that MUST, SHOULD or MAY be supported by participants in the

5.  Semantics of Multiple Signatures

5.1.  Example Scenarios

   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" parameter (to satisfy strict verifiers) and a second time with a
   limited set of headers and an "l" parameter (in anticipation of
   possible message modifications in route to other verifiers).
   Verifiers could then choose which signature they preferred.

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   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
   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

   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

   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 4.5.3 to sign trace Header

   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.

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   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 instances
      of header fields.  For example, if the message being signed
      already contains three DOSETA-Signature header fields (from the
      bottom, up) A, B, and C, it is possible to sign all of them, A and
      B only, or A only, but not C only, B only, B and C 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

   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 4.6.1 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.  DOSETA Claims Registry Definition

   A registry entry MUST contain:

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      Label:   Specifies a textual name for claim, to be used in the
         "cl=" tag.

      Description:   Explains the semantics of the claim being asserted.

   The registry entries are contained in the IANA DOSETA Claims
   Registry, defined in Section 7.1.2

7.  Considerations

7.1.  IANA Considerations

7.1.1.  DKIM Registries

   DOSETA relies on IANA registration data bases specified by DKIM
   [DKIMSign].  Services that incorporate DOSETA might need to define
   new registries or add to existing ones.

7.1.2.  Claims Registry

   Per [RFC2434], IANA is requested to establish a DOSETA Claims
   Registry, for assertions (claims) that are meant by the presence of
   the DOSETA-based signature that contains the claims.  See Section 6
   for the definition of the columns in the registry table.

   | LABEL       | CLAIM DESCRIPTION                                   |
   | handled     | The signer claims they have had a role in           |
   |             | processing the object.  (This claim is              |
   |             | approximately equivalent to the semantics of DKIM.) |
   | validauth   | If there is a standardized field listing the        |
   |             | purported author of the data, the signer claims     |
   |             | that the value in that field is valid.              |
   | validdata   | The signer claims that all of the data in the       |
   |             | object valid.                                       |
   | validfields | The signer claims that the portions of the object   |
   |             | that are covered by the signature hash are valid.   |

           Table 1: DOSETA Claim Registry (with initial values)

7.2.  Security Considerations

   Any mechanism that attempts to prevent or detect abuse is subject to
   intensive attack.  DOSETA needs to be carefully scrutinized to
   identify potential attack vectors and the vulnerability to each.  See

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   also [RFC4686].

   DOSETA core technology derives from DKIM [DKIMSign].  The Security
   Considerations of that specification applies equally to DOSETA.

   The DOSETA "cl=" claims list provides a list of claimed meanings for
   a DOSETA signature.  An opportunity for security problems comes from
   failing to distinguish between a signer "claim" and claim validity.
   Whether to trust claims made by a signer requires a level of
   assessment beyond DOSETA.

8.  References

8.1.  Normative References

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

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

              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.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434,
              October 1998.

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

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   [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.

8.2.  Informative References

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

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

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII
              Content", RFC 2047, November 1996.

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

   [RFC4870]  Delany, M., "Domain-Based Email Authentication Using
              Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
              May 2007.

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

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   [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.

   [UTF8]     Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, November 2003.

              Crocker, D. and M. Kucherawy, "MIME Content Authentication
              using DOSETA (MIMEAUTH)",
              I-D draft-crocker-doseta-mimeauth, 2011.

Appendix A.  Creating a Public Key

   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:
   -----END RSA PRIVATE KEY-----

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   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-----
   -----END PUBLIC KEY-----

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

   brisbane IN  TXT

Appendix B.  Acknowledgements

   DOSETA is derived from DKIM [DKIMSign].  DKIM is an evolution of
   DomainKeys [RFC4870], which was developed by Mark Delany, then of
   Yahoo!.  In particular, the key management service, based on the DNS,
   and the user-INvisible tagging scheme was developed by him.

Appendix C.  Example -- DKIM Using DOSETA

   This example re-specifies DKIM in terms of DOSETA, while retaining
   bit-level compatibility with the existing DKIM specification

   NOTE:   This section is merely an example.  Any use of normative
      language in this section is strictly for completness of the
      example and has no normative effect on the DOSETA specification.

C.1.  Signing and Verification Protocol

   The DOSETA template specifies TEMPLATE information that is required
   to tailor the signing service:

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      Association:   The DOSETA-Signature data are stored in a
         DKIM-Signature header field that is part of the header of the
         message being signed.  This contains all of the signature and
         key-fetching data, per Section 4.2.

      Semantics:   A DKIM signature means that the owner of the signing
         domain is taking "some" responsibility for the message.  Hence,
         the payload, or output, of DKIM is:

         +  A validated domain name, specifically the d= parameter in
            the DKIM-Signature header field

         +  An indication that its use has been validated

         The nature and extent of a DKIM signer's responsibility can
         vary widely and is beyond the scope of this specification.

      Structural Mapping:   DKIM maps the DOSETA Header processing to an
         email header and the DOSETA Content to an email body, per

      Required Support

         Field Hash:   The basic rule for choosing fields to include is
            to select those fields that constitute the "core" of the
            message content.  Hence, any replay attack will have to
            include these in order to have the signature succeed; but
            with these included, the core of the message is valid, even
            if sent on to new recipients.

            Common examples of fields with addresses and fields with
            textual content related to the body are: From (MUST be
            included), Reply-To, Subject, Date, To, Cc, Resent-Date,
            Resent-From, Resent-To, Resent-Cc, In-Reply-To, References,
            List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-
            Post, List-Owner, List-Archive.

         Required Algorithms:   Use DOSETA defaults.

C.2.  Extensions to DOSETA Template

   This section contains specifications that are added to the basic
   DOSETA H/C Signing Template.

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C.2.1.  Signature Data Structure

   These are DKIM-specific tags:

      i=   The Agent or User Identifier (AUID) on behalf of which the
         SDID is taking responsibility (DOSETA-quoted-printable;
         OPTIONAL, default is an empty <local-part> followed by an "@"
         followed by the domain from the "d=" tag).

         The syntax is a standard email address where the <local-part>
         MAY be omitted.  The domain part of the address MUST be the
         same as, or a subdomain of, the value of the "d=" tag.

         Internationalized domain names MUST be converted using the
         steps listed in Section 4 of [RFC5890] using the "ToASCII"

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

         The AUID is specified as having the same syntax as an email
         address, but is not required to have the same semantics.
         Notably, the domain name is not required to be registered in
         the DNS -- so it might not resolve in a query -- and the
         <local-part> MAY be drawn from a namespace unrelated to any
         mailbox.  The details of the structure and semantics for the
         namespace are determined by the Signer.  Any knowledge or use
         of those details by verifiers or assessors is outside the scope
         of the DOSETA Signing specification.  The Signer MAY choose to
         use the same namespace for its AUIDs as its users' email
         addresses or MAY choose other means of representing its users.
         However, the signer SHOULD use the same AUID for each message
         intended to be evaluated as being within the same sphere of
         responsibility, if it wishes to offer receivers the option of
         using the AUID as a stable identifier that is finer grained
         than the SDID.

         NOTE:   The <local-part> of the "i=" tag is optional because in
            some cases a signer might not be able to establish a
            verified individual identity.  In such cases, the signer
            might wish to assert that although it is willing to go as

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            far as signing for the domain, it is unable or unwilling to
            commit to an individual user name within their domain.  It
            can do so by including the domain part but not the <local-
            part> of the identity.

         NOTE:   Absent public standards for the semantics of an AUID,
            an assessment based on AUID requires a non-standardized

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

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

   sig-l-tag    = %x6c [FWS] "=" [FWS]

         NOTE:   Use of the "l=" tag might allow display of fraudulent
            content without appropriate warning to end users.  The "l="
            tag is intended for increasing signature robustness when
            sending to intermediaries that append data to Content, such
            as mailing lists that both modify their content and do not
            sign their messages.  However, using the "l=" tag enables
            attacks in which an intermediary with malicious intent
            modifies a message to include content that solely benefits

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            the attacker.  It is possible for the appended content to
            completely replace the original content in the end
            recipient's eyes and to defeat duplicate message detection
            algorithms.  Examples are described in Security
            Considerations Section 7.2.  To avoid this attack, signers
            need be extremely wary of using this tag, and verifiers
            might wish to ignore the tag or remove text that appears
            after the specified content length.

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

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

         The Header fields referenced by the "h=" tag refer to the
         fields in the [RFC5322] Header, not to any copied fields in the
         "z=" tag.  Copied header field values are for diagnostic use.

         Header fields with characters requiring conversion (perhaps
         from legacy MTAs that are not [RFC5322] compliant) SHOULD be
         converted as described in MIME Part Three [RFC2047].

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   sig-z-tag      = %x7A [FWS] "=" [FWS]
                    *( "|" [FWS] sig-z-tag-copy )
   sig-z-tag-copy = hdr-name [FWS] ":"

   EXAMPLE of a signature header field spread across multiple
   continuation lines:
   DKIM-Signature: v=1; a=rsa-sha256; d=example.net;
     s=brisbane;   c=simple; q=dns/txt; i=@eng.example.net;
     t=1117574938; x=1118006938;

C.2.1.1.  Content Length Limits

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

   This capability is provided because it is very common for
   intermediate data handling services to add trailers to text (for
   example, instructions how to get off a mailing list).  Until such
   data is signed by the intermediate handler, the text length count can
   be a useful tool for the verifier since it can, as a matter of
   policy, accept messages having valid signatures that do not cover the
   additional data.

   NOTE:   Using text length limits enables an attack in which an
      attacker modifies a message to include content that solely
      benefits the attacker.  It is possible for the appended content to
      completely replace the original content in the end recipient's
      eyes and to defeat duplicate message detection algorithms.  To
      avoid this attack, signers need to be wary of using this tag, and
      verifiers might wish to ignore the tag or remove text that appears
      after the specified content length, perhaps based on other

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

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

   A text length count of zero means that the text is completely

   Creators wishing to ensure that no modification of any sort can occur
   will specify the "simple" canonicalization algorithm for all data
   portions and will and omit the text length counts.

C.2.1.2.  Signature Verification

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

   NOTE:   Verifiers that truncate the Content at the indicated Content
      length might pass on a malformed MIME message if the signer used
      the "N-4" trick (omitting the final "--CRLF") described in the
      informative note in Appendix C.2.1.1.  Such verifiers might wish
      to check for this case and include a trailing "--CRLF" to avoid
      breaking the MIME structure.  A simple way to achieve this might
      be to append "--CRLF" to any "multipart" message with a Content
      length; if the MIME structure is already correctly formed, this
      will appear in the postlude and will not be displayed to the end

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C.2.2.  Stored Key Data

   This section defines additions to the DOSETA Library, concerning
   stored key data.

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

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

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

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

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

         *  matches all service types

         email   electronic mail (not necessarily limited to SMTP)

         This tag is intended to constrain the use of keys for other
         purposes, if use of DOSETA is defined by other services in the


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

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

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         y    This domain is testing DOSETA.  Verifiers MUST NOT treat
            data from signers in testing mode differently from unsigned
            data, even if the signature fails to verify.  Verifiers MAY
            wish to track testing mode results to assist the signer.

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

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

Authors' Addresses

   D. Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.

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

   M. Kucherawy
   128 King St., 2nd Floor
   San Francisco, CA  94107

   Email: msk@cloudmark.com

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