DKIM                                                           E. Allman
Internet-Draft                                            Sendmail, Inc.
Expires:  November 23,  December 27, 2006                                    J. Callas
                                                         PGP Corporation
                                                               M. Delany
                                                               M. Libbey
                                                              Yahoo! Inc
                                                               J. Fenton
                                                               M. Thomas
                                                     Cisco Systems, Inc.
                                                            May 22,
                                                           June 25, 2006

              DomainKeys Identified Mail Signatures (DKIM)
                        draft-ietf-dkim-base-02 Signatures
                        draft-ietf-dkim-base-03

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 23, December 27, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

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

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1  Overview . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2  Signing Identity . . . . . . . . . . . . . . . . . . . . .   6
     1.3  Scalability  . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4  Simple Key Management  . . . . . . . . . . . . . . . . . .   6
   2.   Terminology and Definitions  . . . . . . . . . . . . . . . .   6
     2.1  Signers  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.2  Verifiers  . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3  White Space  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.4  Common ABNF Tokens . . . . . . . . . . . . . . . . . . . .   7
     2.5  Imported ABNF Tokens . . . . . . . . . . . . . . . . . . .   8
     2.6  DKIM-Quoted-Printable  . . . . . . . . . . . . . . . . . .   8
   3.   Protocol Elements  . . . . . . . . . . . . . . . . . . . . .   8   9
     3.1  Selectors  . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     3.2  Tag=Value Lists  . . . . . . . . . . . . . . . . . . . . .  10  11
     3.3  Signing and Verification Algorithms  . . . . . . . . . . .  11  12
     3.4  Canonicalization . . . . . . . . . . . . . . . . . . . . .  12  13
     3.5  The DKIM-Signature header field  . . . . . . . . . . . . .  17  18
     3.6  Key Management and Representation  . . . . . . . . . . . .  24  25
     3.7  Computing the Message Hashes . . . . . . . . . . . . . . .  28  30
   4.   Semantics of Multiple Signatures . . . . . . . . . . . . . .  29  31
   5.   Signer Actions . . . . . . . . . . . . . . . . . . . . . . .  30  32
     5.1  Determine if the Email Should be Signed and by Whom  . . .  30  32
     5.2  Select a private-key and corresponding selector
          information  . . . . . . . . . . . . . . . . . . . . . . .  30  32
     5.3  Normalize the Message to Prevent Transport Conversions . .  31  33
     5.4  Determine the header fields to Sign  . . . . . . . . . . .  31  33
     5.5  Compute the Message Hash and Signature . . . . . . . . . .  33  35
     5.6  Insert the DKIM-Signature header field . . . . . . . . . .  34  36
   6.   Verifier Actions . . . . . . . . . . . . . . . . . . . . . .  35  37
     6.1  Extract the Signature Signatures from the Message  . . . . . . . . . .  36
     6.2  Get the Public Key . . . . . . . . . . . . . . . . . . . .  37
     6.3  Compute the Verification . . . . . . . . . . . . . . . . .  38
     6.4
     6.2  Communicate Verification Results . . . . . . . . . . . . .  40
     6.5  42
     6.3  Interpret Results/Apply Local Policy . . . . . . . . . . .  40
     6.6  MUA Considerations . . . . . . . . . . . . . . . . . . . .  41  42
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  42  44
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  42  44
     8.1  Misuse of Body Length Limits ("l=" Tag)  . . . . . . . . .  43  44
     8.2  Misappropriated Private Key  . . . . . . . . . . . . . . .  43  45
     8.3  Key Server Denial-of-Service Attacks . . . . . . . . . . .  44  45
     8.4  Attacks Against DNS  . . . . . . . . . . . . . . . . . . .  44  46
     8.5  Replay Attacks . . . . . . . . . . . . . . . . . . . . . .  45  46
     8.6  Limits on Revoking Keys  . . . . . . . . . . . . . . . . .  46  47
     8.7  Intentionally malformed Key Records  . . . . . . . . . . .  46  47
     8.8  Intentionally Malformed DKIM-Signature header fields . . .  46  47
     8.9  Information Leakage  . . . . . . . . . . . . . . . . . . .  46  48
     8.10   Remote Timing Attacks  . . . . . . . . . . . . . . . . .  46  48
   9.   References . . . . . . . . . . . . . . . . . . . . . . . . .  47  48
     9.1  Normative References . . . . . . . . . . . . . . . . . . .  47  48
     9.2  Informative References . . . . . . . . . . . . . . . . . .  47  49
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  48  50
   A.   Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . .  49  51
     A.1  The user composes an email . . . . . . . . . . . . . . . .  50  51
     A.2  The email is signed  . . . . . . . . . . . . . . . . . . .  50  51
     A.3  The email signature is verified  . . . . . . . . . . . . .  51  52
   B.   Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . .  52  53
     B.1  Simple Message Forwarding  . . . . . . . . . . . . . . . .  52  53
     B.2  Outsourced Business Functions  . . . . . . . . . . . . . .  52  53
     B.3  PDAs and Similar Devices . . . . . . . . . . . . . . . . .  52  54
     B.4  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . .  53  54
     B.5  Affinity Addresses . . . . . . . . . . . . . . . . . . . .  53  55
     B.6  Third-party Message Transmission . . . . . . . . . . . . .  54  55
   C.   Creating a public key (INFORMATIVE)  . . . . . . . . . . . .  54  56
   D.   MUA Considerations . . . . . . . . . . . . . . . . . . . . .  57
   E.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  56
   E.  58
   F.   Edit History . . . . . . . . . . . . . . . . . . . . . . . .  56
     E.1  58
     F.1  Changes since -ietf-02 version . . . . . . . . . . . . . .  58
     F.2  Changes since -ietf-01 version . . . . . . . . . . . . . .  56
     E.2  59
     F.3  Changes since -ietf-00 version . . . . . . . . . . . . . .  57
     E.3  60
     F.4  Changes since -allman-01 version . . . . . . . . . . . . .  57
     E.4  60
     F.5  Changes since -allman-00 version . . . . . . . . . . . . .  58  61
        Intellectual Property and Copyright Statements . . . . . . .  59  62

1.  Introduction

   [[Note:  text in double square brackets (such as this text) will be
   deleted before publication.]]

1.1  Overview

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

   The approach taken by DKIM differs from previous approaches to
   message signing (e.g.  S/MIME [RFC1847], OpenPGP [RFC2440]) in that:

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

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

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

   o  it makes no attempt to include encryption as part of the
      mechanism.

   DKIM:

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

   o  requires minimal new infrastructure

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

   o  does not require the use of a trusted third party (such as a
      certificate authority or other entity) which might impose
      significant costs or introduce delays to deployment

   o  can be deployed incrementally
   o  allows delegation of signing to third parties

   o  is not intended be used for archival purposes

   A "selector" mechanism allows multiple keys per domain, including
   delegation of the right to authenticate a portion of the namespace to
   a trusted third party.

1.2  Signing Identity

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

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

1.3  Scalability

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

1.4  Simple Key Management

   DKIM differs from traditional hierarchical public-key systems in that
   no key signing infrastructure is required; the verifier requests the
   public key from the claimed signer directly.

   The DNS is proposed as the initial mechanism for publishing public
   keys.  DKIM is designed to be extensible to other key fetching
   services as they become available.

2.  Terminology and Definitions

   This section defines terms used in the rest of the document.  Syntax
   descriptions use the form described in Augmented BNF for Syntax
   Specifications [RFC4234].

2.1  Signers

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

2.2  Verifiers

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

2.3  White Space

   There are three forms of white space:

   o  WSP represents simple white space, i.e., a space or a tab
      character, and is inherited from[RFC2822].

   o  SWSP is streaming white space; it adds space, defined as WSP plus the CR and LF
      characters.

   o  FWS, also from [RFC2822], is folding white space.  It allows
      multiple lines separated by CRLF followed by at least one white
      space, to be joined.

   The formal ABNF for SWSP is:

   SWSP =   CR / LF / WSP   ; streaming white space

2.4  Common ABNF Tokens

   The following ABNF tokens are used elsewhere in this document.

   hyphenated-word =  ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
   base64string =     1*(ALPHA / DIGIT / "+" / "/" / "=" / SWSP)

2.5  Imported ABNF Tokens

   The following tokens are imported from other RFCs as noted.  Those
   RFCs should be considered definitive.  However, all tokens having
   names beginning with "obs-" should be excluded from this import, as
   they have been obsoleted and are expected to go away in future
   editions of those RFCs.

   The following tokens are imported from [RFC2821]:

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

   o  Domain (implementation warning:  this permits address-literals)

   o  sub-domain  "sub-domain"

   The following definitions are imported from [RFC2822]:

   o  WSP  "WSP" (space or tab)

   o  FWS  "FWS" (folding white space)

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

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

   The following tokens are imported from [RFC2045]:

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

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

      INFORMATIVE NOTE:  Be aware that the ABNF in RFC 2045 does not
      obey the rules of RFC 4234 and must be interpreted accordingly,
      particularly as regards case folding.

   Other tokens not defined herein are imported from [RFC4234].  These
   are intuitive primitives such as SP, ALPHA, CRLF, etc.

2.6  DKIM-Quoted-Printable

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

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

      INFORMATIVE NOTE:  DKIM-Quoted-Printable differs from Quoted-
      Printable as defined in RFC 2045 in several important ways:

      1.  White space in the input text, including CR and LF, must be
          encoded.  RFC 2045 does not require such encoding, and does
          not permit encoded of CR or LF characters that are part of a
          CRLF line break.

      2.  White space in the encoded text is ignored.  This is to allow
          DKIM-Quoted-Printable to be wrapped as needed in headers.  In
          particular, RFC 2045 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-white-space
          character on the line) does not apply.

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

3.  Protocol Elements

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

3.1  Selectors

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

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

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

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

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

   Periods are allowed in selectors and are component separators.  If
   keys are stored in DNS, the period defines sub-domain boundaries.
   Sub-selectors might be used to combine dates with locations; for
   example, "march2005.reykjavik".  This can be used to allow delegation
   of a portion of the selector name-space.

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

   The number of public keys and 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 may choose to manage disparate selectors and key pairs
   in different regions or on different email servers.

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

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

      INFORMATIVE IMPLEMENTERS' NOTE:  reusing a selector with a new key
      (for example, changing the key associated with a user's name)
      makes it impossible to tell the difference between a message that
      didn't verify because the key is no longer valid versus a message
      that is actually forged.  Signers should not change the key
      associated with a selector.  When creating a new key, signers
      should associate it with a new selector.

3.2  Tag=Value Lists

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

   Values are a series of strings containing either base64 text, plain text, or quoted printable text, as base64
   text (as defined in [RFC2045], section 6.7. 6.8), qp-section (ibid,
   section 6.7), or dkim-quoted-printable (as defined above).  The name
   of the tag will determine the encoding of each value; however, no
   encoding may include the semicolon (";") character, since that
   separates tag-specs.

   Formally, the syntax rules are:
        tag-list  =  tag-spec 0*( ";" tag-spec ) [ ";" ]
        tag-spec  =  [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
        tag-name  =  ALPHA 0*ALNUMPUNC
        tag-value =  0*VALCHAR  [ 1*VALCHAR 0*( 1*(WSP / FWS) 1*VALCHAR ) ]
                             ; SWSP WSP and FWS prohibited at beginning and end
        VALCHAR   =  %9 / %d32 - %d58  %x21-3A / %d60 - %d126 %x3C-7E
                             ; HTAB and SP EXCLAMATION to TILDE except SEMICOLON
        ALNUMPUNC =  ALPHA / DIGIT / "_"

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

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

   Tags with duplicate names MUST NOT be specified within a single tag-
   list.

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

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

   Unrecognized tags MUST be ignored.

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

3.3  Signing and Verification Algorithms

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

3.3.1  The rsa-sha1 Signing Algorithm

   The rsa-sha1 Signing Algorithm computes a message hash as described
   in Section 3.7 below using SHA-1 as the hash-alg.  That hash is then
   signed by the signer using the RSA algorithm (defined in PKCS#1
   version 1.5 [RFC3447]; in particular see section 5.2) with an
   exponent of 65537 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.

3.3.2  The rsa-sha256 Signing Algorithm

   The rsa-sha256 Signing Algorithm computes a message hash as described
   in Section 3.7 below using SHA-256 as the hash-alg.  That hash is
   then signed by the signer using the RSA algorithm (actually PKCS#1
   version 1.5 [RFC3447]; in particular see section 5.2) with an
   exponent of 65537 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.

3.3.3  Other algorithms

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

3.3.4  Key sizes

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

   Factors that should influence the key size choice include:

   o  The practical constraint that large keys may not fit within a 512
      byte DNS UDP response packet

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

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

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

   o  The security goals of this specification are modest compared to
      typical goals of public-key systems

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

3.4  Canonicalization

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

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

   Some signers may be willing to accept modifications to header fields
   that are within the bounds of email standards such as [RFC2822], but
   are unwilling to accept any modification to the body of messages.

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

   In all cases, the header fields of the message are presented to the
   signing algorithm first in the order indicated by the signature
   header field and canonicalized using the indicated algorithm.  Only
   header fields listed as signed in the signature header field are
   included.  Note:  the signature header field itself is presented at
   the end of the hash, not with the other headers.  The CRLF separating
   the header field from the body is then presented, followed by the
   canonicalized body.  Note that the header and body may use different
   canonicalization algorithms.

   Canonicalization simply prepares the email for presentation to the
   signing or verification algorithm.  It MUST NOT change the
   transmitted data in any way.  Canonicalization of header fields and
   body are described below.

   NOTE:  This section assumes that the message is already in "network
   normal" format (e.g., text is ASCII encoded, lines are separated with
   CRLF characters, etc.).  See also Section 5.3 for information about
   normalizing the message.

3.4.1  The "simple" Header Field Canonicalization Algorithm

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

3.4.2  The "relaxed" Header Field Canonicalization Algorithm

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

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

   o  Unfold all header field continuation lines as described in
      [RFC2822]; in particular, lines with terminators embedded in
      continued header field values (that is, CRLF sequences followed by
      WSP) MUST be interpreted without the CRLF.  Implementations MUST
      NOT remove the CRLF at the end of the header field value.

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

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

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

3.4.3  The "simple" Body Canonicalization Algorithm

   The "simple" body canonicalization algorithm ignores all empty lines
   at the end of the message body.  An empty line is a line of zero
   length after removal of the line terminator.  It makes no other
   changes to the message body.  In more formal terms, the "simple" body
   canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the
   body to a single "CRLF".

3.4.4  The "relaxed" Body Canonicalization Algorithm

   [[This section may be deleted; see discussion below.]]  The "relaxed"
   body canonicalization algorithm:

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

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

   o  Ignores all empty lines at the end of the message body.  "Empty
      line" is defined in Section 3.4.3.

      [[NON-NORMATIVE DISCUSSION:  The authors are undecided whether to
      leave the "relaxed" body canonicalization algorithm in to the
      specification or delete it entirely.  We believe that for the vast
      majority of cases, the "simple" body canonicalization algorithm
      should be sufficient.  We simply do not have enough data to know
      whether to retain the "relaxed" body canonicalization algorithm or
      not.]]

3.4.5  Body Length Limits

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

      INFORMATIVE IMPLEMENTATION NOTE:  Body length limits could be
      useful in increasing signature robustness when sending to a
      mailing list that both appends to content sent to it and does not
      sign its messages.  However, using such limits enables an attack
      in which a sender with malicious intent an attacker modifies a message to include content that
      solely benefits the attacker.  It is possible for the appended
      content to completely replace the original content in the end
      recipient's eyes and to defeat duplicate message detection
      algorithms.  To avoid this attack, signers should be wary of using
      this tag, and verifiers might wish to ignore the tag or remove
      text that appears after the specified content length, perhaps
      based on other criteria.

   The body length count allows the signer of a message to permit data
   to be appended to the end of the body of a signed message.  The body
   length count is made following the canonicalization algorithm; for
   example, any white space ignored by a canonicalization algorithm is
   not included as part of the body length count.

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

   Signers of MIME messages that include a body length count SHOULD be
   sure that the length extends to the closing MIME boundary string.

      INFORMATIVE IMPLEMENTATION NOTE:  A signer wishing to ensure that
      the only acceptable modifications are to add to the MIME postlude
      would use a body length count encompassing the entire final MIME
      boundary string, including the final "--CRLF".  A signer wishing
      to allow additional MIME parts but not modification of existing
      parts would use a body length count extending through the final
      MIME boundary string, omitting the final "--CRLF".

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

      INFORMATIVE IMPLEMENTATION NOTE:  Note that verifiers MAY may choose
      to truncate modify their interpretation of messages that have body
   content beyond that specified by with unsigned content,
      including truncating the body length count.
   Alternatively, verifiers MAY ignore signatures that do not cover unsigned part, refusing to display the
   entire message body.
      unsigned part to the user, or simply treating the signature as
      invalid.

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

3.4.6  Example  Canonicalization Examples (INFORMATIVE)

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

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

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

   and a body reading:
       <SP> C <CRLF>
       D <SP> E <CRLF>
   (postamble)

   Example 2:  The same message canonicalized using simple
   canonicalization for both header and body results in a header
   reading:
       A: <SP> X <CRLF>
       B <SP> : <SP> Y <TAB><CRLF>
       <TAB> Z <SP><SP><CRLF>
   and a body reading:
       <SP> C <SP><CRLF>
       D <SP><TAB><SP> E <CRLF>
   (postamble)

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

   and a body reading:
       <SP> C <SP><CRLF>
       D <SP><TAB><SP> E <CRLF>
   (postamble)

3.5  The DKIM-Signature header field

   The signature of the email is stored in the "DKIM-Signature:" header
   field.  This header field contains all of the signature and key-
   fetching data.  The DKIM-Signature value is a tag-list as described
   in Section 3.2.

   The "DKIM-Signature:" header field SHOULD be treated as though it
   were a trace header field as defined in section 3.6 of [RFC2822], and
   hence SHOULD NOT be reordered and SHOULD be prepended to the message.
   In particular, the "DKIM-Signature" header field SHOULD precede the
   original email header fields presented to the canonicalization and
   signature algorithms.

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

   The encodings for each field type are listed below.  Tags described
   as quoted-printable qp-section are as described in section 6.7 of MIME Part One
   [RFC2045], with the additional conversion of semicolon characters to
   "=3B".
   "=3B"; intuitively, this is one line of quoted-printable encoded
   text.  Tags described as dkim-quoted-printable are as defined above.

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

   v=   Version (MUST be included).  This tag defines the version of
       this specification that applies to the signature record.  It MUST
       have the value 0.2. 0.3.

       ABNF:

   sig-v-tag   = %x76 [FWS] "=" [FWS] "0.2" "0.3"

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

           [[INFORMATIVE NOTE:  Upon publication, this version number
           should be changed to "1", and this note should be deleted.]]

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

       ABNF:

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

   b=   The signature data (base64; REQUIRED).  Whitespace is ignored in
       this value and MUST be ignored when re-assembling tthe 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 5) for how the signature is
       computed.

       ABNF:

   sig-b-tag       = %x62 [FWS] "=" [FWS] sig-b-tag-data
   sig-b-tag-data  = base64string
   bh= The hash of the body part of the message (base64; REQUIRED).
       Whitespace is ignored in this value and MUST be ignored when re-
       assembling 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 3.7 for how the
       body hash is computed.

       ABNF:

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

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

       ABNF:

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

   d=   The domain of the signing entity (plain-text; REQUIRED).  This
       is the domain that will be queried for the public key.  This
       domain MUST be the same as or a parent domain of the "i=" tag
       (the signing identity, as described below).  If the "t=s" tag is
       specified in the key record referenced by the selector in the
       "s=" tag, then the domain in the "d=" tag must be identical to
       the domain specified in the "i=" tag.  When presented with a
       signature that does not meet this these requirement, verifiers MUST
       consider the signature invalid.

       Internationalized domain names MUST be punycode-encoded
       [RFC3492].

       ABNF:

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

   h=   Signed header fields (plain-text, but see description;
       REQUIRED).  A colon-separated list of header field names that
       identify the header fields presented to the signing algorithm.
       The field MUST contain the complete list of header fields in the
       order presented to the signing algorithm.  The field MAY contain
       names of header fields that do not exist when signed; nonexistent
       header fields do not contribute to the signature computation
       (that is, they are treated as the null input, includiing including the
       header field name, the separating colon, the header field value,
       and any CRLF terminator).  The field MUST NOT include the DKIM-
       Signature header field that is being created or verified. verified, but may
       include others.  Folding white space (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 5.4 for a
       discussion of choosing header fields to sign.

       ABNF:

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

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

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

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

       ABNF:

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

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

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

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

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

           INFORMATIVE NOTE:  The value of the l= tag is constrained to
           76 decimal digits, which will fit in a 256-bit binary integer
           field.  This constraint is not intended to predict the size
           of future messages, but is intended to remind the implementer
           to check the length of this and all other tags during
           verification.  Implementers may need to limit the actual
           value expressed to a value smaller than 10^76, e.g., to allow
           a message to fit within the available storage space.

       ABNF:

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

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

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

       ABNF:

   sig-q-tag        = %x71 [FWS] "=" [FWS] sig-q-tag-method
                      *([FWS] ":" [FWS] sig-q-tag-method)
   sig-q-tag-method = sig-q-tag-type ["/" sig-q-tag-args]
   sig-q-tag-type   = "dns" "txt/dns" / x-sig-q-tag-type ["/" x-sig-q-tag-args]
   x-sig-q-tag-type = hyphenated-word  ; for future extension
   x-sig-q-tag-args = qp-hdr-value
   s=   The selector subdividing the namespace for the "d=" (domain) tag
       (plain-text; REQUIRED).

       ABNF:

   sig-s-tag    = %x73 [FWS] "=" [FWS] subdomain *( "." sub-domain )

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

       ABNF:

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

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

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

       ABNF:

   sig-x-tag    = %x78 [FWS] "=" [FWS] 1*12DIGIT
   z=   Copied header fields (plain-text, (dkim-quoted-printable, but see
       description; OPTIONAL, default is null).  A vertical-bar-separated vertical-bar-
       separated list of selected header field names and copies of header field values fields present when the message
       was signed. 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.  Copied  The header field values MUST immediately follow text itself must encode
       the vertical bar ("|", %x7C) character (i.e., vertical bars in
       the z= text are metacharacters, and any actual vertical bar
       characters in a copied header field name
       with a colon separator (no must be encoded).  Note that
       all white space permitted).  Header field
       values MUST must be represented as Quoted-Printable [RFC2045] with
       vertical bars, colons, semicolons, and encoded, including white space encoded between
       the colon and the header field value.  After encoding, SWSP MAY
       be added at arbitrary locations in
       addition order to avoid excessively
       long lines; such white space is NOT part of the usual requirements. value of the
       header field, and MUST be removed before decoding.

       Verifiers MUST NOT use the header field names or copied values
       for checking the signature in any way.  Copied header field
       values are for diagnostic use onnly. only.

       Header fields with characters requiring conversion (perhaps from
       legacy MTAs which are not [RFC2822] compliant) SHOULD be
       converted as described in MIME Part Three [RFC2047].

       ABNF:
   sig-z-tag      = %x7A [FWS] "=" [FWS] sig-z-tag-copy
                    *( [FWS] "|" sig-z-tag-copy )
   sig-z-tag-copy = hdr-name ":" [FWS] qp-hdr-value
   qp-hdr-value   = <quoted-printable text with WS, "|", ":",
                    and ";" encoded> dkim-quoted-printable    ; needs to be updated with real definition
                             ; (could be messy) "|" encoded

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

   DKIM-Signature: a=rsa-sha1; a=rsa-sha256; d=example.net; s=brisbane s=brisbane;
      c=simple; q=dns; q=dns/txt; i=@eng.example.net; t=1117574938; x=1118006938;
      h=from:to:subject:date;
      z=From:foo@eng.example.net|To:joe@example.com|
        Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700
      b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
               VoG4ZHRNiYzR

3.6  Key Management and Representation

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

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

   Parameters to the key lookup algorithm are the type of the lookup
   (the "q=" tag), the domain of the responsible signer (the "d=" tag of
   the DKIM-Signature header field), the signing identity (the "i="
   tag), and the selector (the "s=" tag).  The "i=" tag value could be
   ignored by some key services.

   public_key = dkim_find_key(q_val, d_val, i_val, s_val)

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

3.6.1  Textual Representation

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

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

   v=   Version of the DKIM key record (plain-text; RECOMMENDED, default
       is "DKIM1").  If specified, this tag MUST be set to "DKIM1"
       (without the quotes).  This tag MUST be the first tag in the
       response.  Responses beginning with a "v=" tag with any other
       value MUST be discarded.

       ABNF:

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

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

       ABNF:

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

           [[NON-NORMATIVE DISCUSSION POINT:  "*" is legal in a dot-
           atom.  This should probably use a different character for
           wildcarding.  Unfortunately, the options are non-mnemonic
           (e.g., "@", "(", ":").  Alternatively we could insist on
           escaping a "*" intended as a literal "*" in the address.]]

   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 "sha1" hash algorithm.

       ABNF:

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

   k=   Key type (plain-text; OPTIONAL, default is "rsa").  Signers and
       verifiers MUST support the "rsa" key type.  The "rsa" key type
       indicates that an RSA public key, as defined in [RFC3447],
       sections 3.1 and A.1.1, is being used in the p= tag.  (Note:  the
       p= tag further encodes the value using the base64 algorithm.)

       ABNF:

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

           [[NON-NORMATIVE DISCUSSION NOTE:  In some cases it can be
           hard to separate h= and k=; for example DSA implies that
           SHA-1 will be used.  This might be an actual change to the
           spec depending on how we decide to fix this.]]
   n=   Notes that might be of interest to a human (quoted-printable; (qp-section;
       OPTIONAL, default is empty).  No interpretation is made by any
       program.  This tag should be used sparingly in any key server
       mechanism that has space limitations (notably DNS).

       ABNF:

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

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

       ABNF:

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

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

       *   matches all service types

       email   electronic mail (not necessarily limited to SMTP)

       This tag is intended to permit senders signers to constrain the use of
       delegated keys, e.g., where a company is willing to delegate the
       right to send mail in their name to an outsourcer, but not to
       send IM or make VoIP calls.  (This of course presumes that these
       keys are used in other services in the future.)

       ABNF:

   key-s-tag        = %x73 [FWS] "=" [FWS] key-s-tag-type
                       0*( [FWS] ":" [FWS] key-s-tag-type
   key-s-tag-type   = "email" / "*" / x-key-s-tag-type
   x-key-s-tag-type = hyphenated-word   ; for future extension
   t=   Flags, represented as a colon-separated list of names (plain-
       text; OPTIONAL, default is no flags set).  The defined flags are:

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

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

       ABNF:

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

       Unrecognized flags MUST be ignored.

3.6.2  DNS binding

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

3.6.2.1  Name Space

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

   The value of the "i=" tag is not used by the DNS binding.

3.6.2.2  Resource Record Types for Key Storage

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

   TXT records are encoded as described in Section 3.6.1.

3.7  Computing the Message Hashes

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

   The signer or verifier must compute two hashes, one over the body of
   the message and one over the selected header fields of the message.
   Signers MUST compute them in the order shown.  Verifiers MAY compute
   them in any order convenient to the verifier, provided that the
   result is semantically identical to the semantics that would be the
   case had they been computed in this order.

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

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

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

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

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

   When calculating the hash on messages that will be transmitted using
   base64 or quoted-printable encoding, signers MUST compute the hash
   after the encoding.  Likewise, the verifier MUST incorporate the
   values into the hash before decoding the base64 or quoted-printable
   text.  However, the hash MUST be computed before transport level
   encodings such as SMTP "dot-stuffing."

   With the exception of the canonicalization procedure described in
   Section 3.4, the DKIM signing process treats the body of messages as
   simply a string of characters.  DKIM messages MAY be either in plain-
   text or in MIME format; no special treatment is afforded to MIME
   content.  Message attachments in MIME format MUST be included in the
   content which is signed.

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

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

4.  Semantics of Multiple Signatures

   A signer that is adding a signature to a message merely creates a new
   DKIM-Signature header, using the usual semantics of the h= option.  A
   signer MAY sign previously existing DKIM-Signature headers using the
   method described in section Section 5.4 to sign trace headers.
   Signers should be cognizant that signing DKIM-Signature headers may
   result in signature failures with intermediaries that do not
   recognize that DKIM-Signature's are trace headers and unwittingly
   reorder them.

   When evaluating a message with multiple signatures, a receiver verifier should
   evaluate signatures independently and on their own merits.  For
   example, a receiver verifier that by policy chooses not to accept signatures
   with deprecated crypto cryptographic algorithms should consider such
   signatures invalid.  As with messages with a single signature, receievers
   verifiers are at liberty to use the presence of valid signatures as
   an input to local policy; likewise, the interpretation of multiple
   valid signatures in combination is a local policy decision of the receiver.
   verifier.

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

5.  Signer Actions

   The following steps are performed in order by signers.

5.1  Determine if the Email Should be Signed and by Whom

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

   A SUBMISSION server MAY sign if the sender submitter is authenticated by
   some secure means, e.g., SMTP AUTH.  Within a trusted enclave the
   signing address MAY be derived from the header field according to
   local signer policy.  Within a trusted enclave an MTA MAY do the
   signing.

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

   A signer MUST NOT sign an email if it is unwilling to be held
   responsible for the message; in particular, the signer SHOULD ensure
   that the submitter has a bona fide relationship with the signer and
   that the submitter has tthe the right to use the address being claimed.

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

5.2  Select a private-key and corresponding selector information

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

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

5.3  Normalize the Message to Prevent Transport Conversions

   Some messages, particularly those using 8-bit characters, are subject
   to modification during transit, notably conversion to 7-bit form.
   Such conversions will break DKIM signatures.  In order to minimize
   the chances of such breakage, signers SHOULD convert the message to a
   suitable MIME content transfer encoding such as quoted-printable or
   base64 as described in MIME Part One [RFC2045] before signing.  Such
   conversion is outside the scope of DKIM; the actual message SHOULD be
   converted to 7-bit MIME by an MUA or MSA prior to presentation to the
   DKIM algorithm.

   Should the message be submitted to the signer with any local encoding
   that will be modified before transmission, such conversion to
   canonical form MUST be done before signing.  In particular, some
   systems use local line separator conventions (such as the Unix
   newline character) internally rather than the SMTP-standard CRLF
   sequence.  All such local conventions MUST be converted to canonical
   format before signing.

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

5.4  Determine the header fields to Sign

   The From header field MUST be signed (that is, included in the h= tag
   of the resulting DKIM-Signature header field); any header field that
   describes the role of the signer (for example, the Sender or Resent-
   From header field if the signature is on behalf of the corresponding
   address and that address is different from the From address) MUST
   also be included.  The signed header fields SHOULD also include the
   Subjectt
   Subject and Date header fields as well as all MIME header fields.
   Signers SHOULD NOT sign an existing header field likely to be
   legitimately modified or removed in transit.  In particular,
   [RFC2821] explicitly permits modification or removal of the "Return-
   Path" header field in transit.  Signers MAY include any other header
   fields present at the time of signing at the discretion of the
   signer.  It is RECOMMENDED that all other existing, non-repeatable
   header fields be signed.

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

   Signers MUST sign any header fields that the signers wish to assert
   were present at the time of signing.  Put another way, verifiers MAY
   treat unsigned header fields with extreme skepticism, up to and
   including refusing to display them to the end user.

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

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

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

      INFORMATIVE EXAMPLE:

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

   Received: <A>
   Received: <B>
   Received: <C>

      then the resulting DKIM-Signature header field should read:

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

      and Received header fields <C> and <B> will be signed in that
      order.

   Signers SHOULD NOT sign header fields that might be replicated
   (either at the time of signing or potentially in the future), with
   the exception of trace header fields such as Received.  Comment and
   non standard header fields (including X-* header fields) are
   permitted by [RFC2822] to be replicated; however, many such header
   fields are, by convention, not replicated.  Signers need to
   understand the implications of signing header fields that might later
   be replicated, especially in the face of header field reordering.  In
   particular, [RFC2822] only requires that trace header fields retain
   the original order.

      INFORMATIVE RATIONALE:  Received:  is allowed because these header
      fields, as well as Resent-* header fields, are already order-
      sensitive.

      INFORMATIVE ADMONITION:  Despite the fact that [RFC2822] permits
      header field blocks to be reordered (with the exception of
      Received header fields), reordering of signed replicated header
      fields by intermediate MTAs will cause DKIM signatures to be
      broken; such anti-social behavior should be avoided.

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

      INFORMATIVE NOTE:  There has been some discussion that a Sender
      Signing Policy include the list of header fields that the signer
      always signs.  N.B. In theory this is unnecessary, since as long
      as the signer really always signs the indicated header fields
      there is no possibility of an attacker replaying an existing
      message that has such an unsigned header field.

5.5  Compute the Message Hash and Signature

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

   To avoid possible ambiguity, a signer SHOULD either sign or remove
   any preexisting header fields which convey the results of previous
   verifications of the message signature prior to preparation for
   signing and transmission.  Such header fields MUST NOT be signed if
   the signer is uncertain of the authenticity of the preexisting header
   field, for example, if it is not locally generated or signed by a
   previous DKIM-Signature line that the current signer has verified.

   Entities such as mailing list managers that implement DKIM and which
   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; such signing SHOULD
   include any prior verification status, if any, that was inserted upon
   message receipt.

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

      INFORMATIVE NOTE:  A possible value to include in the "l=" tag
      would include the entire length of the message being signed,
      thereby allowing intermediate agents to append further information
      to the message without breaking the signature (e.g., a mailing
      list manager might add unsubscribe information to the body).  A
      signer wishing to permit such intermediate agents to add another
      MIME body part to a "multipart/mixed" message should use a length
      that covers the entire presented message except for the trailing
      "--CRLF" characters; this is known as the "N-4" approach.  Note
      that more than four characters may need to be stripped, since
      there could be postlude information that needs to be ignored.

5.6  Insert the DKIM-Signature header field

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

   The "DKIM-Signature" SHOULD be inserted before any header fields that
   it signs in the header block.

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

6.  Verifier Actions

   Since a signer MAY expire a public key at any time, it is recommended
   that verification occur in a timely manner with the most timely place
   being during acceptance by the border MTA.

   A border or intermediate MTA MAY verify the message signatures and
   add a verification header field to incoming messages.  This
   considerably simplifies things for the user, who can now use an
   existing mail user agent.  Most MUAs have the ability to filter
   messages based on message header fields or content; these filters
   would be used to implement whatever policy the user wishes with
   respect to unsigned mail.

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

6.1  Extract Signatures from the Message

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

      INFORMATIVE IMPLEMENTATION NOTE:  Verifiers might use the order as
      a clue to signing order in the absence of any other information.
      However, other clues as to the semantics of multiple signatures
      must be considered before using ordering.

   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; this is 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.

   In the following description, text reading "return with
   DKIM_STAT_something" 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.  There are two special cases:  DKIM_STAT_PARTIALSIG
   indicates that only a portion of  If the message was actually signed, status
   is "PERMFAIL", the signature failed and
   DKIM_STAT_TEMPFAIL means that should not be reconsidered.
   If the status is "TEMPFAIL", the signature could not be verified at
   this time but should may be tried again later.  In the former case, the
   action a verifier takes is a matter of local policy.  In the latter
   case, a  A verifier MAY either defer
   the message for later processing, perhaps by queueing it local 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 DKIM_STAT_TEMPFAIL TEMPFAIL
   status, the verifier
   SHOULD MAY save the message for later processing.  Note that an
   implementation  The
   "(explanation)" is not constrained to use these status codes; these
   are for explanatory purposes only, and an implementation may define
   fewer or more status codes.

   The order in which signatures are tried normative text; it is a matter of local policy provided solely for
   clarification.

   Verifiers SHOULD ignore any DKIM-Signature header fields where the verifier and is
   signature does not defined here.  A verifier validate.  Verifiers that are prepared to validate
   multiple signature header fields SHOULD NOT
   treat a message proceed to the next signature
   header field, should it exist.  However, verifiers MAY make note of
   the fact that has one or more bad signatures and no good
   signatures differently from a message with no an invalid signature was present for consideration at all;
   again, this is local policy and is beyond the scope a
   later step.

      INFORMATIVE NOTE:  The rationale of this document.

   When requirement is to permit
      messages that have invalid signatures but also a valid signature successfully verifies,
      to work.  For example, a verifier will either stop
   processing or attempt mailing list exploder might opt to verify any other signatures, at the
   discretion of the implementation.

    Verifiers MUST apply leave
      the following steps original submitter signature in place even though the exploder
      knows that it is modifying the order listed.

6.1  Extract message in some way that will break
      that signature, and the Signature from exploder inserts its own signature.  In
      this case the Message

   The signature and associated signing identity is included message should succeed even in the
   value presence of the DKIM-Signature header field.  The order in which
   verifiers try DKIM-Signature header fields is not defined; verifiers
   MAY try signatures in any order they would like.
      known-broken signature.

   For example, one
   implementation might prefer each signature to try be validated, the signatures following steps should be
   performed in textual order,
   whereas another might want such a manner as to prefer signatures by identities produce a result that
   match is
   semantically equivalent to performing them in the contents of indicated order.

6.1.1  Validate the "From" header field over other identities. Signature Header Field

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

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

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

   If the DKIM-Signature header field does not contain any of the tags
   listed as required in Section 3.5 the verifier MUST ignore the DKIM-
   Signature header field and return with DKIM_STAT_SYNTAX. PERMFAIL (signature missing
   required tag).

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

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

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

   Verifiers MUST NOT attribute ultimate meaning to the order of
   multiple DKIM-Signature header fields.  In particular, there is
   reason to believe that some relays will reorder the header fields in
   potentially arbitrary ways.

      INFORMATIVE IMPLEMENTATION NOTE:  Verifiers might use the order as
      a clue to signing order in the absence of any other informaation.
      However, other clues as to the semantics of multiple signatures
      must be considered before using ordering.

   If there are no valid signatures remaining after this step, a
   verifier MUST NOT proceed to the next step.

6.2

6.1.2  Get the Public Key

   The public key for a signature is needed to complete the verification
   process.  The process of retrieving the public key depends on the
   query type as defined by the "q=" tag in the "DKIM-Signature:" header
   field.  Obviously, a public key should 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 3.6.  The verifier MUST validate the key record and MUST
   ignore any public key records that are malformed.

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

   1.  Retrieve the public key as described in (Section 3.6) using the
       domain from the "d=" tag and the selector from the "s=" tag.

   2.  If the query for the public key fails to respond, the verifier
       SHOULD
       MAY defer acceptance of this email and return with
       DKIM_STAT_TEMPFAIL. TEMPFAIL (key
       unavailable).  If verification is occuring 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 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
       with DKIM_STAT_NOKEY.
       PERMFAIL (no key for signature).

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

   5.  If the result returned from the query does not adhere to the
       format defined in this specification, the verifier MUST ignore
       the key record aand and return with DKIM_STAT_NOKEY. PERMFAIL (key syntax error).  Verifiers
       are urged to validate the syntax of key records carefully to
       avoid attempted attacks.

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

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

   8.  If the public key data (the "p=" tag) is empty then this key has
       been revoked and the verifier MUST treat this as a failed
       signature check and return with DKIM_STAT_REVOKED. PERMFAIL (key revoked).

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

6.3
       PERMFAIL (inappropriate key algorithm).

6.1.3  Compute the Verification

   Given a signer and a public key, verifying a signature consists of
   the following steps.

   1.  Based on the algorithm defined in the "c=" tag, the body length
       specified in the "l=" tag, and the header field names in the "h="
       tag, create a canonicalized copy of the email as is described in
       Section 3.7.  When matching header field names in the "h=" tag
       against the actual message header field, comparisons MUST be
       case-insensitive.

   2.  Based on the algorithm indicated in the "a=" tag, compute the
       message hashes from the canonical copy as described in
       Section 3.7.

   3.  Verify that the hash of the canonicalized message body computed
       in the previous step matches the hash value conveyed in the "bh="
       tag.

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

   5.  Otherwise, the signature has correctly verified.

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

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

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

   If a

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

      INFORMATIVE IMPLEMENTATION NOTE:  Verifiers that truncate the body
      at the indicated body length might pass on a malformed MIME
      message if the signer used the "N-4" trick described in the
      informative note in Section 5.5.  Such verifiers may wish to check
      for this case and include a trailing "--CRLF" to avoid breaking
      the MIME structure.  A simple way to achieve this might be to
      append "--CRLF" to any "multipart" message with a body length; if
      the MIME structure is already correctly formed, this will appear
      in the postlude and will not be displayeed displayed to the end user.

6.4

6.2  Communicate Verification Results

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

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

6.5

6.3  Interpret Results/Apply Local Policy

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

   In general verifiers SHOULD NOT reject messages solely on the basis
   of a lack of signature or an unverifiable signature.  However, if the
   verifier does opt to reject such messages, and the verifier runs
   synchronously with the SMTP session and a signature is missing or
   does not verify, the MTA SHOULD reject the message with an error such
   as:

      550 5.7.1 Unsigned messages not accepted

      550 5.7.5 Message signature incorrect

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

      451 4.7.5 Unable to verify signature - key server unavailable

   A temporary failure of the key server or other external service is
   the only condition that should use a 4xx SMTP reply code.  In
   particular, signature verification failures MUST NOT return 4xx SMTP
   replies.

   Oncee

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

      INFORMATIVE NOTE:  If the authentication status actual
   signing identity is clear to be stored in
      the message header field, the Authentication-Results header field
      [ID-AUTH-RES] may be used to convey this information. reader.

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

   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 over-zealous filter?  For
   diagnostic purposes, the exact reason why the verification fails
   SHOULD be recorded in made available to the "Authentication-Results" header field policy module and possibly recorded
   in the system logs.  However in terms of presentation to the end
   user, the result SHOULD be presented as a simple binary result:
   either the email is verified or it is not.  If the email cannot be
   verified, then it SHOULD be rendered the same as all unverified email
   regardless of whether it looks like it was signed or not.

6.6  MUA Considerations

   In order to retain the current semantics and visibility of the From
   header field, verifying mail agents SHOULD take steps to ensure that
   the signing address is prominently visible to the user if it is
   different from the From address.  MUAs MAY visually mark the
   unverified part of the body in a distinctive font or color to the end
   user.

   If MUA implementations that highlight the signed address are not
   available, this MAY be done by the validating MTA or MDA by rewriting
   the From address in a manner which remains compliant with [RFC2822].
   Such modifications MUST be performed after the final verification
   step since they will break the signature.  If performed, the
   rewriting SHOULD include the name of the signer in the address.  For
   example:

   From: John Q. User <user@example.com>

   might be converted to

   From: "John Q. User via <asrg-admin@ietf.org>" <user@example.com>

   This sort of addrress inconsistency is expected for mailing lists, but
   might be otherwise used to mislead the verifier, for example if a
   message supposedly from administration@your-bank.com had a Sender
   address of fraud@badguy.com.

   Under no circumstances should an unsigned header field be displayed
   in any context that might be construed by the end user as having been
   signed.  Notably, unsigned header fields SHOULD be hidden from the
   end user to the extent possible.

   The MUA MAY hide or mark portions of the message body that are not
   signed when using the "l=" tag.

7.  IANA Considerations

   To avoid conflicts, tag names for the DKIM-Signature header and key
   records should be registered with IANA.

   Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature
   header field, and the "h=", "k=", "s=", and "t" tags in key records
   should be registered with IANA for the same reason.

   The DKK RR type must be registered by IANA. reason.

8.  Security Considerations

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

8.1  Misuse of Body Length Limits ("l=" Tag)

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

8.1.1  Addition of new MIME parts to multipart/*

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

   *** Example appropriate here ***

8.1.2  Addition of new HTML content to existing content

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

      INFORMATIVE EXAMPLE:  Appending the following text to an existing,
      properly closed message will in many MUAs result in inappropriate
      data being rendered on top of existing, correct data:

   <div style="position: relative; bottom: 350px; z-index: 2;">
   <img src="http://www.ietf.org/images/ietflogo2e.gif"
     width=578 height=370>
   </div>

8.2  Misappropriateed  Misappropriated Private Key

   If the private key for a user is resident on their computer and is
   not protected by an appropriately secure mechanism, it is possible
   for malware to send mail as that user and any other user sharing the
   same private key.  The malware would, however, not be able to
   generate signed spoofs of other signers' addresses, which would aid
   in identification of the infected user and would limit the
   possibilities for certain types of attacks involving socially-
   engineered messages.

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

   One countermeasure is to use a user-entered passphrase to encrypt the
   private key, although users tend to choose weak passphrases and often
   reuse them for different purposes, possibly allowing an attack
   against DKIM to be extended into other domains.  Nevertheless, the
   decoded private key might be briefly available to compromise by
   malware when it is entered, or might be discovered via keystroke
   logging.  The added complexity of entering a passphrase each time one
   sends a message would also tend to discourage the use of a secure
   passphrase.

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

8.3  Key Server Denial-of-Service Attacks

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

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

8.4  Attacks Against DNS

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

   While the DNS is currently insecure [RFC3833], it is expected that
   the security problems should and will be solved by DNSSEC [RFC4033],
   and all users of the DNS will reap the benefit of that work.

   Secondly, the types of DNS attacks relevant to DKIM are very costly
   and are far less rewarding than DNS attacks on other Internet
   applications.

   To systematically thwart the intent of DKIM, an attacker must conduct
   a very costly and very extensive attack on many parts of the DNS over
   an extended period.  No one knows for sure how attackers will
   respond, however the cost/benefit of conducting prolonged DNS attacks
   of this nature is expected to be uneconomical.

   Finally, DKIM is only intended as a "sufficient" method of proving
   authenticity.  It is not intended to provide strong cryptographic
   proof about authorship or contents.  Other technologies such as
   OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements.

   A second security issue related to the DNS revolves around the
   increased DNS traffic as a consequence of fetching Selector-based
   data as well as fetching signing domain policy.  Widespread
   deployment of DKIM will result in a significant increase in DNS
   queries to the claimed signing domain.  In the case of forgeries on a
   large scale, DNS servers could see a substantial increase in queries.

8.5  Replay Attacks

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

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

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

8.6  Limits on Revoking Keys

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

8.7  Intentionally malformed Key Records

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

8.8  Intentionally Malformed DKIM-Signature header fields

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

8.9  Information Leakage

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

8.10  Remote Timing Attacks

   In some cases it may be possible to extract private keys using a
   remote timing attack [BONEH03].  Implementations should consider
   obfuscating the timing to prevent such attacks.

9.  References

9.1  Normative References

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

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

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

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

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

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Application(IDNA)",
              March 2003.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

9.2  Informative References

   [BONEH03]  Proc. 12th USENIX Security Symposium, "Remote Timing
              Attacks are Practical", 2003, <http://www.usenix.org/
              publications/library/proceedings/sec03/tech/brumley.html>.

   [ID-AUTH-RES]
              Kucherawy, M., "Message header field for Indicating Sender
              Authentication Status",
              draft-kucherawy-sender-auth-header-02 (work in progress),
              February 2006.

   [ID-DKIM-THREATS]
              Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", draft-fenton-dkim-threats-02
              (work in progress), April 2006.

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

   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, November 1998.

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

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

   [RFC3851]  Ramsdell, B., "S/MIME Version 3 Message Specification",
              RFC 3851, June 1999.

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

Authors' Addresses

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

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

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

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

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

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

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

   Email:  mlibbeymail-mailsig@yahoo.com
   URI:

   Jim Fenton
   Cisco Systems, Inc.
   MS SJ-24/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706
   USA

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

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

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

Appendix A.  Example of Use (INFORMATIVE)

   This section shows the complete flow of an email from submission to
   final delivery, demonstrating how the various components fit
   together.

A.1  The user composes an email

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

   Hi.

   We lost the game. Are you hungry yet?

               Joe.

A.2  The email is signed

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

   DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com;
         c=simple; q=dns; q=dns/txt; i=joe@football.example.com;
         h=Received : From : To : Subject : Date : Message-ID;
         b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
           VoG4ZHRNiYzR;
   Received: from dsl-10.2.3.4.football.example.com  [10.2.3.4]
         by submitserver.example.com with SUBMISSION;
         Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
   From: Joe SixPack <joe@football.example.com>
   To: Suzie Q <suzie@shopping.example.net>
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: <20030712040037.46341.5F8J@football.example.com>

   Hi.

   We lost the game. Are you hungry yet?

        Joe.

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

A.3  The email signature is verified

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

   brisbane._domainkey.example.com

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

   Authentication-Results: shopping.example.net
          header.from=joe@football.example.com; dkim=pass
   Received: from mout23.football.example.com (192.168.1.1)
                 by shopping.example.net with SMTP;
                 Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
   DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com;
          c=simple; q=dns; q=dns/txt; i=joe@football.example.com;
          h=Received : From : To : Subject : Date : Message-ID;
          b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
             VoG4ZHRNiYzR
   Received: from dsl-10.2.3.4.network.example.com  [10.2.3.4]
            by submitserver.example.com with SUBMISSION;
            Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
   From: Joe SixPack <joe@football.example.com>
   To: Suzie Q <suzie@shopping.example.net>
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: <20030712040037.46341.5F8J@football.example.com>

   Hi.

   We lost the game. Are you hungry yet?

        Joe.

Appendix B.  Usage Examples (INFORMATIVE)

   Studies in this appendix are for informational purposes only.  In no
   case should these examples be used as guidance when creating an
   implementation.

B.1  Simple Message Forwarding

   In some cases the recipient may request forwarding of email messages
   from the original address tto to another, through the use of a Unix
   .forward file or equivalent.  In this case messages are typically
   forwarded without modification, except for the addition of a Received
   header field to the message and a change in the Envelope-to address.
   In this case, the eventual recipient should be able to verify the
   original signature since the signed content has not changed, and
   attribute the message correctly.

B.2  Outsourced Business Functions

   Outsourced business functions represent a use case that motivates the
   need for selectors (the "s=" signature tag) and granularity (the "g="
   key tag).  Examples of outsourced business functions are legitimate
   email marketing providers and corporate benefits providers.  In
   either case, the outsourced function would like to be able to send
   messages using the email domain of the client company.  At the same
   time, the client may be reluctant to register a key for the provider
   that grants the ability to send messages for any address in the
   domain.

   The outsourcing company can generate a keypair and the client company
   can register the public key using a unique selector for a specific
   address such as winter-promotions@example.com by specifying a
   granularity of "g=winter-promotions" or "g=*-promotions" (to allow a
   range of addresses).  This would enable the provider to send messages
   using that specific address and have them verify properly.  The
   client company retains control over the email address because it
   retains the ability to revoke the key at any time.

B.3  PDAs and Similar Devices

   PDAs are one example of the use of multiple keys per user.  Suppose
   that John Doe wanted to be able to send messages using his corporate
   email address, jdoe@example.com, and the device did not have the
   ability to make a VPN connection to the corporate network.  If the
   device was equipped with a private key registered for
   jdoe@example.com by the administrator of that domain, and appropriate
   software to sign messages, John could send signed messages through
   the outgoing network of the PDA service provider.

B.4  Mailing Lists

   There is a wide range of behavior in forwarders and mailing lists
   (collectively called "forwarders" below), ranging from those which
   make no modification to the message itself (other than to add a
   Received header field and change the envelope information) to those
   which may add header fields, change the Subject header field, add
   content to the body (typically at the end), or reformat the body in
   some manner.

   Forwarders which do noot not modify the body or signed header fields of a
   message with a valid signature may re-sign the message as described
   below.

   Forwarders which make any modification to a message that could result
   in its signature becoming invalid should sign or re-sign using an
   appropriate identification (e.g., mailing-list-name@example.net).
   Since in so doing the (re-)signer is taking responsibility for the
   content of the message, modifying forwarders may elect to forward or
   re-sign only for messages which were received with valid signatures
   or other indications that the messages being signed are not spoofed.

   Forwarders which wish to re-sign a message must apply a Sender header
   field to the message to identify the address being used to sign the
   message and must remove any preexisting Sender header field as
   required by [RFC2822].  The forwarder applies a new DKIM-Signature
   header field with the signature, public key, and related information
   of the forwarder.

B.5  Affinity Addresses

   "Affinity addresses" are email addresses that users employ to have an
   email address that is independent of any changes in email service
   provider they may choose to make.  They are typically associated with
   college alumni associations, professional organizations, and
   recreational organizations with which they expect to have a long-term
   relationship.  These domains usually provide forwarding of incoming
   email, but (currently) usually depend on the user to send outgoing
   messages through their own service provider's MTA.  They usually have
   an associated Web application which authenticates the user and allows
   the forwarding address to be changed.

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

B.6  Third-party Message Transmission

   Third-party message transmission refers to the authorized sending of
   mail by an Internet application on behalf of a user.  For example, a
   website providing news may allow the reader to forward a copy of the
   message to a friend; this is typically done using the reader's email
   address.  This is sometimes referred to as the "Evite problem", named
   after the website of the same name that allows a user to send
   invitations to friends.

   One way this can be handled is to continue to put the reader's email
   address in the From header field of the message, but put an address
   owned by the site into the Sender header field, and sign the message
   on behalf of the
   Sender. that address.  A verifying MTA should could accept this and
   rewrite the From header field to indicate the address that was
   verified, i.e., From:  John Doe via news@news-site.com
   <jdoe@example.com>.  (However, such rewriting must be done after the
   verification pass is complete, and will break any later attempts to
   re-verify.)

Appendix C.  Creating a public key (INFORMATIVE)

   XXX Update to 1024 bit key and SHA-256 and adjust examples
   accordingly.  XXX

   The default signature is an RSA signed SHA1 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 768 bit private-key suitable for DKIM, is to use
   openssl like this:

   $ openssl genrsa -out rsa.private 768 1024

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

             -----BEGIN RSA PRIVATE KEY-----
   MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5
   ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo
   AUsFUq+J6+OprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH
   +Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn
   Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/
   3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew
   ti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs
   FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb
   weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG
   6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U=
    MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC
    jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb
    to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB
    AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX
    /1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ
    gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO
    n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m
    3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/
    eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj
    7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA
    qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf
    eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX
    GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc=
             -----END RSA PRIVATE KEY-----

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

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

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

             -----BEGIN PUBLIC KEY-----
   MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l
   MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E
   XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB
   MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM
   oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R
   tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI
   MmPSPDdQPNUYckcQ2QIDAQAB
             -----END PUBLIC KEY-----

   This public-key data (without the BEGIN and END tags) is placed in
   the DNS.  With the signature, canonical email contents contents, and puublic public
   key, a verifying system can test the validity of the signature.  The
   openssl invocation to verify a signature looks like this:

   openssl dgst -verify rsa.public -sha1 -sha256 -signature signature.file \
           <input.file

   Once a private-key has been generated, the openssl command can be
   used to sign an appropriately prepared email, like this:

   $ openssl dgst -sign rsa.private -sha1 -sha256 <input.file

   This results in signature data similar to this when represented in
   Base64 [MIME] format:

   aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz
   msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl

   How this signature is added to the email is discussed elsewhere in
   this document. email is discussed elsewhere in
   this document.

Appendix D.  MUA Considerations

   When a DKIM signature is verified, one of the results is a validated
   signing identity.  MUAs might highlight the address associated with
   this identity in some way to show the user the address from which the
   mail is sent.  An MUA might do this with visual cues such as
   graphics, or it might include the address in an alternate views, or
   it might even rewrite the original "From:" address using the verified
   information.  Some MUAs might want to indicate which headers were
   covered in a validated DKIM signature.  This might be done with a
   positive indication on the signed headers, it might be done with a
   negative indication on the unsigned headers or visually hiding the
   unsigned headers, or some combination of both.  If an MUA uses visual
   indications for signed headers, the MUA needs to be careful not to
   display unsigned headers in a way that might be construed by the end
   user as having been signed.  If the message has an l= tag whose value
   does not extend to the end of the message, he MUA might also hide or
   mark the portion of the message body that is not signed.

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

Appendix D. E.  Acknowledgements

   The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
   Steve Atkins, Fred Baker, Mark Baugher, Nathaniel Borenstein, Dave
   Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Patrik
   Faltstrom, Duncan Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony
   Hansen, Arvel Hathcock, Amir Herzberg, Paul Hoffman, Craig Hughes,
   Don Johnsen, Harry Katz, Murray S. Kucherawy, Barry Leiba, John
   Levine, Simon Longsdale, David Margrave, Justin Mason, David Mayne,
   Steve Murphy,
   Russell€ Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada,
   Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud,
   Scott Renfro, Eric Rescorla, Dave Rossetti, Hector Santos, the
   Spamhaus.org team, Malte S. Stretz, Robert Sanders, Rand Wacker, and
   Dan Wing for their valuable suggestions and constructive criticism.

   The DomainKeys specification was a primary source from which this
   specification has been derived.  Further information about DomainKeys
   is at
   <http://domainkeys.sourceforge.net/license/patentlicense1-1.html>.

Appendix E. F.  Edit History

   [[This section to be removed before publication.]]

E.1

F.1  Changes since -ietf-02 version

   The following changes were made between draft-ietf-dkim-base-02 and
   draft-ietf-dkim-base-03:

   o  Section 5.2:  changed key expiration text to be informational;
      drop "seven day" wording in favor of something vaguer.

   o  Don't indicate that the "i=" tag value should be passed to the key
      lookup service; this can be added as an extension if required.

   o  Move Section 6.6 (MUA Considerations) to be Appendix D and modify
      it to avoid any hint of normative language.

   o  Soften the DKIM_STAT_ language in section 6 so that it doesn't
      appear normative.  This involved using only PERMFAIL and TEMPFAIL
      as status, with parenthetical explanations.

   o  Restructured section 6 to make it clearer which steps apply on a
      per-signature basis versus a per-message basis.

   o  Clarification of "signing identity" in several places.

   o  Clarification that DKIM-Signature header fields being signed by
      another DKIM-Signature header field should be treated as a normal
      header field (i.e., their "b=" field is unchanged).

   o  Change ABNF on a= tag to separate the public key algorithm from
      the hash algorithm.

   o  Add t=s flag in key record to disallow subdomains in the i= tag
      relative to the d= tag of the DKIM-Signature header field.

   o  Add a new definition for "dkim-quoted-printable", which is a
      simple case of quoted-printable from RFC2045. dkim-quoted-
      printable requires that all white space in the original text be
      escaped, and all unescaped white space in the encoded field should
      be ignored to allow arbitrary wrapping of the header fields which
      may contain the content.

   o  Use dkim-quoted-printable as the encoding used in z= rather than
      referring to RFC2045, since they are different.

   o  Rewrite description of g= tag in the key record.

   o  Deleted use of Domain in ABNF, which permits address-literals;
      define domain-name to act in stead.

F.2  Changes since -ietf-01 version

   The following changes were made between draft-ietf-dkim-base-01 and
   draft-ietf-dkim-base-02:

   o  Change wording on "x=" tag in DKIM-Signature header field
      regarding verifier handling of expired signatures from MUST to MAY
      (per 20 April Jabber session).  Also, make it clear that received
      time is to be preferred over current time if reliably available.

   o  Several changes to limit wording that would intrude into verifier
      policy.  This is largely changing statements such as "...  MUST
      reject the message" to "...  MUST consider the signature invalid."
   o  Drop normative references to ID-DKIM-RR, OpenSSL, PEM, and
      Stringprep.

   o  Change "v=" tag in DKIM-Signature from "MUST NOT" to "MUST"; the
      version number is 0.2 for this draft, with the expectation that
      the first official version will be "v=1".  (Per 18 May Jabber
      session.)

   o  Change "q=dns" query access method to "q=dnstxt" to emphasize the
      use of the TXT record.  The expectation is that a later extension
      will define "q=dnsdkk" to indicate use of a DKK record.  (Per 18
      May Jabber session.)

   o  Several typos fixed, including removing a paragraph that implied
      that the DKIM-Signature header field should be hashed with the
      body (it should not).

E.2

F.3  Changes since -ietf-00 version

   The following changes were made between draft-ietf-dkim-base-00 and
   draft-ietf-dkim-base-01:

   o  Added section 8.9 (Information Leakage).

   o  Replace section 4 (Multiple Signatures) with much less vague text.

   o  Fixed ABNF for base64string.

   o  Added rsa-sha256 signing algorithm.

   o  Expanded several examples.

   o  Changed signing algorithm to use separate hash of the body of the
      message; this is represented as the "bh=" tag in the DKIM-
      Signature header field.

   o  Changed "z=" tag so that it need not have the same header field
      names as the "h=" tag.

   o  Significant wordsmithing.

E.3

F.4  Changes since -allman-01 version

   The following changes were made between draft-allman-dkim-base-01 and
   draft-ietf-dkim-base-00:

   o  Remove references to Sender Signing Policy document.  Such
      consideration is implicitly included in Section 6.5. 6.3.

   o  Added ABNF for all tags.

   o  Updated references (still includes some references to expired
      drafts, notably [ID-AUTH-RES].

   o  Significant wordsmithing.

E.4

F.5  Changes since -allman-00 version

   The following changes were made between draft-allman-dkim-base-00 and
   draft-allman-dkim-base-01:

   o  Changed "c=" tag to separate out header from body
      canonicalization.

   o  Eliminated "nowsp" canonicalization in favor of "relaxed", which
      is somewhat less relaxed (but more secure) than "nowsp".

   o  Moved the (empty) Compliance section to the Sender Signing Policy
      document.

   o  Added several IANA Considerations.

   o  Fixed a number of grammar and formatting errors.

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