DKIM                                                           E. Allman
Internet-Draft                                            Sendmail, Inc.
Expires:  August 6, 2006                                       J. Callas
                                                         PGP Corporation
                                                               M. Delany
                                                               M. Libbey
                                                              Yahoo! Inc
                                                               J. Fenton
                                                               M. Thomas
                                                     Cisco Systems, Inc.
                                                        February 2, 2006

              DomainKeys Identified Mail Signatures (DKIM)

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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

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

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 6, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   DomainKeys Identified Mail (DKIM) defines a domain-level

Allman, et al.           Expires August 6, 2006                 [Page 1]

Internet-Draft                DKIM Signing                 February 2006

   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 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",
   document are to be interpreted as described in [RFC2119].

Allman, et al.           Expires August 6, 2006                 [Page 2]

Internet-Draft                DKIM Signing                 February 2006

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  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2   Verifiers  . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3   White Space  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.4   Common ABNF Tokens . . . . . . . . . . . . . . . . . . . .  7
     2.5   Imported ABNF Tokens . . . . . . . . . . . . . . . . . . .  7
   3.  Protocol Elements  . . . . . . . . . . . . . . . . . . . . . .  8
     3.1   Selectors  . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2   Tag=Value Format for DKIM header fields  . . . . . . . . . 10
     3.3   Signing and Verification Algorithms  . . . . . . . . . . . 10
     3.4   Canonicalization . . . . . . . . . . . . . . . . . . . . . 12
     3.5   The DKIM-Signature header field  . . . . . . . . . . . . . 16
     3.6   Key Management and Representation  . . . . . . . . . . . . 22
     3.7   Computing the Message Hash . . . . . . . . . . . . . . . . 26
   4.  Semantics of Multiple Signatures . . . . . . . . . . . . . . . 28
   5.  Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 28
     5.1   Determine if the Email Should be Signed and by Whom  . . . 29
     5.2   Select a private-key and corresponding selector
           information  . . . . . . . . . . . . . . . . . . . . . . . 29
     5.3   Normalize the Message to Prevent Transport Conversions . . 29
     5.4   Determine the header fields to Sign  . . . . . . . . . . . 30
     5.5   Compute the Message Hash . . . . . . . . . . . . . . . . . 32
     5.6   Insert the DKIM-Signature header field . . . . . . . . . . 32
   6.  Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 33
     6.1   Extract the Signature from the Message . . . . . . . . . . 33
     6.2   Get the Public Key . . . . . . . . . . . . . . . . . . . . 34
     6.3   Compute the Verification . . . . . . . . . . . . . . . . . 35
     6.4   Insert the Authentication-Results Header Field . . . . . . 36
     6.5   Interpret Results/Apply Local Policy . . . . . . . . . . . 37
     6.6   MUA Considerations . . . . . . . . . . . . . . . . . . . . 38
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 39
     8.1   Misuse of Body Length Limits ("l=" Tag)  . . . . . . . . . 39
     8.2   Misappropriated Private Key  . . . . . . . . . . . . . . . 40
     8.3   Key Server Denial-of-Service Attacks . . . . . . . . . . . 41
     8.4   Attacks Against DNS  . . . . . . . . . . . . . . . . . . . 41
     8.5   Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 42
     8.6   Limits on Revoking Keys  . . . . . . . . . . . . . . . . . 42
     8.7   Intentionally malformed Key Records  . . . . . . . . . . . 42
     8.8   Intentionally Malformed DKIM-Signature header fields . . . 43
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Allman, et al.           Expires August 6, 2006                 [Page 3]

Internet-Draft                DKIM Signing                 February 2006

     9.1   Normative References . . . . . . . . . . . . . . . . . . . 43
     9.2   Informative References . . . . . . . . . . . . . . . . . . 44
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45
   A.  Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . . 46
     A.1   The user composes an email . . . . . . . . . . . . . . . . 46
     A.2   The email is signed  . . . . . . . . . . . . . . . . . . . 46
     A.3   The email signature is verified  . . . . . . . . . . . . . 47
   B.  Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . . 48
     B.1   Simple Message Forwarding  . . . . . . . . . . . . . . . . 48
     B.2   Outsourced Business Functions  . . . . . . . . . . . . . . 48
     B.3   PDAs and Similar Devices . . . . . . . . . . . . . . . . . 49
     B.4   Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 49
     B.5   Affinity Addresses . . . . . . . . . . . . . . . . . . . . 50
     B.6   Third-party Message Transmission . . . . . . . . . . . . . 50
   C.  Creating a public key (INFORMATIVE)  . . . . . . . . . . . . . 51
   D.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 52
   E.  Edit History . . . . . . . . . . . . . . . . . . . . . . . . . 52
     E.1   Changes since -allman-01 version . . . . . . . . . . . . . 52
     E.2   Changes since -allman-00 version . . . . . . . . . . . . . 53
       Intellectual Property and Copyright Statements . . . . . . . . 54

Allman, et al.           Expires August 6, 2006                 [Page 4]

Internet-Draft                DKIM Signing                 February 2006

1.  Introduction

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 to the message header fields 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


   o  is transparent to and compatible with the existing email

   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.

   A "selector" mechanism allows multiple keys per domain, including

Allman, et al.           Expires August 6, 2006                 [Page 5]

Internet-Draft                DKIM Signing                 February 2006

   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 signer of the message from the
   purported author of the message.  In particular, a signature includes
   the identity of the signer.  Recipients can use the signing
   information to decide how they want to process the message.

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

1.3  Scalability

   The email identification problem is characterized by extreme
   scalability requirements.  There are currently over 70 million
   domains and a much larger number of individual addresses.  It is
   important 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

Allman, et al.           Expires August 6, 2006                 [Page 6]

Internet-Draft                DKIM Signing                 February 2006

   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 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 / "-")
   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.

   The following tokens are imported from [RFC2821]:

Allman, et al.           Expires August 6, 2006                 [Page 7]

Internet-Draft                DKIM Signing                 February 2006

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

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

   o  sub-domain

   The following definitions are imported from [RFC2822]:

   o  WSP (space or tab)

   o  FWS (folding white space)

   o  field-name (name of a header field)

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

   The following tokens are imported from [RFC2045]:

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

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

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

      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.

Allman, et al.           Expires August 6, 2006                 [Page 8]

Internet-Draft                DKIM Signing                 February 2006

   Selectors are needed to support some important use cases.  For

   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.

        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 this selector associated directly with the user
   name, or make it some unassociated random value, such as the
   fingerprint of the public key.

Allman, et al.           Expires August 6, 2006                 [Page 9]

Internet-Draft                DKIM Signing                 February 2006

3.2  Tag=Value Format for DKIM header fields

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

   Values are a series of strings containing either base64 text, plain
   text, or quoted printable text, as defined in [RFC2045], section 6.7.
   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 =  *VALCHAR    ; SWSP prohibited at beginning and end
        VALCHAR   =  %9 / %d32 - %d58 / %d60 - %d126
                             ; HTAB and SP to TILDE except SEMICOLON
        ALNUMPUNC =  ALPHA / DIGIT / "_"

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

   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-

   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.  The only

Allman, et al.           Expires August 6, 2006                [Page 10]

Internet-Draft                DKIM Signing                 February 2006

   algorithm defined by this specification at this time is rsa-sha1,
   which is the default if no algorithm is specified and which MUST be
   supported by all implementations.

3.3.1  The rsa-sha1 Signing Algorithm

   The rsa-sha1 Signing Algorithm computes a SHA-1 hash of the message
   header field and body as described in Section 3.7 below.  That hash
   is then encrypted by the signer using the RSA algorithm (actually
   PKCS#1 version 1.5 [RFC3447]) 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.

   More formally, the algorithm for the signature using rsa-sha1 is:
       RSA(SHA1(canon_message || DKIM-SIG), key)

   where canon_message is the canonicalized message header and body as
   defined in Section 3.4 and DKIM-SIG is the canonicalized DKIM-
   Signature header field sans the signature value itself.

3.3.2  Other algorithms

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

3.3.3  Key sizes

   Selecting appropriate key sizes is a trade-off between cost,
   performance and risk.  All implementations MUST support keys of sizes
   512, 768, 1024, 1536 and 2048 bits and MAY support larger keys.

   Factors that should influence the key size choice include:

   o  The practical constraint that a 2048 bit key is the largest key
      that fits within a 512 byte DNS UDP response packet

   o  The security constraint that keys smaller than 1024 bits are
      subject to brute force 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

Allman, et al.           Expires August 6, 2006                [Page 11]

Internet-Draft                DKIM Signing                 February 2006

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.  A verifier MUST be able to process email
   using either canonicalization algorithm.  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.  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.

Allman, et al.           Expires August 6, 2006                [Page 12]

Internet-Draft                DKIM Signing                 February 2006

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.

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:

   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

   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.

      [NON-NORMATIVE DOCUMENTATION NOTE:  The only difference between
      "relaxed" header field canonicalization and "nowsp" listed in the
      previous version of this draft is that nowsp reduces all strings
      of streaming white space to zero characters while "relaxed"
      reduces strings of white space to one space.]

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" to a single "CRLF"

Allman, et al.           Expires August 6, 2006                [Page 13]

Internet-Draft                DKIM Signing                 February 2006

   at the end of the body.

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

   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 retain the "relaxed" body canonicalization algorithm or

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.  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 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.

   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

Allman, et al.           Expires August 6, 2006                [Page 14]

Internet-Draft                DKIM Signing                 February 2006

   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

   Note that verifiers MAY choose to reject or truncate messages that
   have body content beyond that specified by the body length count.

   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

   Assuming a "c=relaxes/relaxed" canonicalization algorithm, a message
       A: <SP> X <CRLF>
       B <SP> : <SP> Y <TAB><CRLF>
        <TAB> Z <SP><SP><CRLF>
       <SP> C <SP><CRLF>
       D <SP><TAB><SP> E <CRLF>
   when canonicalized using "relaxed" for both header and body results

Allman, et al.           Expires August 6, 2006                [Page 15]

Internet-Draft                DKIM Signing                 February 2006


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 kept in blocks 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 is always included in the
   signature calculation, after the body of the message; however, when
   calculating or verifying the signature, the b= (signature value)
   value MUST be treated as though it were the null string.  Unknown
   tags MUST be signed but MUST be otherwise ignored by verifiers.

   The encodings for each field type are listed below.  Tags described
   as quoted-printable are as described in section 6.7 of MIME Part One
   [RFC2045], with the additional conversion of semicolon characters to

   Tags on the DKIM-Signature header field along with their type and
   requirement status are shown below.  Valid tags are:

   v=   Version (MUST NOT be included).  This tag is reserved for future
       use to indicate a possible new, incompatible version of the
       specification.  It MUST NOT be included in the DKIM-Signature
       header field.


   sig-v-tag =

Allman, et al.           Expires August 6, 2006                [Page 16]

Internet-Draft                DKIM Signing                 February 2006

   a=   The algorithm used to generate the signature (plain-text;
       REQUIRED).  Signers and verifiers MUST support "rsa-sha1", an
       RSA-signed SHA-1 digest.  See Section 3.3 for a description of

           INFORMATIVE RATIONALE:  The authors understand that SHA-1 has
           been theoretically compromised.  However, viable attacks
           require the attacker to choose both sets of input text; given
           a preexisting input (a "preimaging" attack), it is still hard
           to determine another input that produces an SHA-1 collision,
           and the chance that such input would be of value to an
           attacker is minimal.  Also, there is broad library support
           for SHA-1, whereas alternatives such as SHA-256 are just
           emerging.  Finally, DKIM is not intended to have legal- or
           military-grade requirements.  There is nothing inherent in
           using SHA-1 here other than implementer convenience.  See
           for a discussion of the security issues.


   sig-a-tag       = %x61 [FWS] "=" [FWS] sig-a-tag-alg
   sig-a-tag-alg   = "rsa-sha1" / x-sig-a-tag-alg
   x-sig-a-tag-alg = hyphenated-word   ; for later extension

   b=   The signature data (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 Signer Actions (Section 5) for how the signature is


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

   c=   Body 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, "relaxed"
       is treated the same as "relaxed/simple".

Allman, et al.           Expires August 6, 2006                [Page 17]

Internet-Draft                DKIM Signing                 February 2006


   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.
       When presented with a signature that does not meet this
       requirement, verifiers MUST either ignore the signature or reject
       the message.


   sig-d-tag       = %x64 [FWS] "=" [FWS] Domain

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


   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
           cannot possibly know what header fields might be created in
           the future, and that some MUAs might present header fields

Allman, et al.           Expires August 6, 2006                [Page 18]

Internet-Draft                DKIM Signing                 February 2006

           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;
       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 is optional.  If the
       signing domain is unable or unwilling to commit to an individual
       user name within their domain, the local-part of the address MUST
       be omitted.  If the local-part of the address is omitted or the
       "i=" tag is not present, the key used to sign MUST be valid for
       any address in the domain.  The domain part of the address MUST
       be the same as or a subdomain of the value of the "d=" tag.


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

           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, described in another document [XREF-TBD].
           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 count (plain-text decimal integer; OPTIONAL, default is
       entire body).  This tag informs the verifier of the number of
       bytes in the body of the email after canonicalization included in
       the cryptographic hash, starting from 0 immediately following the
       CRLF preceding the body.

Allman, et al.           Expires August 6, 2006                [Page 19]

Internet-Draft                DKIM Signing                 February 2006

           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.


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

   q=   A colon-separated list of query methods used to retrieve the
       public key (plain-text; OPTIONAL, default is "dns").  Each query
       method is of the form "type[/options]", where the syntax and
       semantics of the options depends on the type.  If there are
       multiple query mechanisms listed, the choice of query mechanism
       MUST NOT change the interpretation of the signature.  Currently
       the only valid value is "dns" which defines the DNS lookup
       algorithm described elsewhere in this document.  No options are
       defined for the "dns" query type, but the string "dns" MAY have a
       trailing "/" character.  Verifiers and signers MUST support

           INFORMATIVE RATIONALE:  Explicitly allowing a trailing "/" on
           "dns" allows for the possibility of adding options later and
           makes it clear that matching of the query type must terminate
           on either "/" or end of string.


   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" / x-sig-q-tag-type
   x-sig-q-tag-type = hyphenated-word  ; for future extension
   x-sig-q-tag-args = qp-hdr-value

Allman, et al.           Expires August 6, 2006                [Page 20]

Internet-Draft                DKIM Signing                 February 2006

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


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

   t=   Signature Timestamp (plain-text; RECOMMENDED, default is an
       unknown creation time).  The time that this signature was
       created.  The format is the standard Unix seconds-since-1970.
       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.


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

   x=   Signature Expiration (plain-text; RECOMMENDED, default is no
       expiration).  Signature expiration in seconds-since-1970 format
       as an absolute date, not as a time delta from the signing
       timestamp.  Signatures MUST NOT be considered valid if the
       current time at the verifier is past the expiration date.  The
       value is expressed as an unsigned integer in decimal ASCII, with
       the same contraints on the value in the "t=" tag.

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


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

   z=   Copied header fields (plain-text, but see description; OPTIONAL,
       default is null).  A vertical-bar-separated list of header field
       names and copies of header field values 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.  Copied header field values MUST
       immediately follow the header field name with a colon separator
       (no white space permitted).  Header field values MUST be
       represented as Quoted-Printable [RFC2045] with vertical bars,

Allman, et al.           Expires August 6, 2006                [Page 21]

Internet-Draft                DKIM Signing                 February 2006

       colons, semicolons, and white space encoded in addition to the
       usual requirements.

       Verifiers MUST NOT use the copied header field values for
       verification should they be present in the h= field.  Copied
       header field values are for forensic use 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].

   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>
                             ; needs to be updated with real definition
                             ; (could be messy)

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

   DKIM-Signature: a=rsa-sha1;; s=brisbane
      c=simple; q=dns;; t=1117574938; x=1118006938;

3.6  Key Management and Representation

   DKIM keys do not require third party signatures by Certificate
   Authorities in order to be trusted, since the public key is retrieved
   directly from the signer.

   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.

Allman, et al.           Expires August 6, 2006                [Page 22]

Internet-Draft                DKIM Signing                 February 2006

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

   This document defines a single binding, using DNS to distribute the

3.6.1  Textual Representation

   It is expected that many key servers will choose to present the keys
   in a text format.  The following definition MUST be used for any DKIM
   key represented in textual form.

   The overall syntax is a key-value-list as described above.  The
   current valid tags are:

   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.


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

   g=   granularity of the key (plain-text; OPTIONAL, default is "*").
       This value MUST match the local part of the signing address, with
       a "*" character acting as a wildcard.  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.


   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.]]

Allman, et al.           Expires August 6, 2006                [Page 23]

Internet-Draft                DKIM Signing                 February 2006

   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.


   key-h-tag       = %x68 [FWS] "=" [FWS] key-h-tag-alg
                     0*( [FWS] ":" [FWS] key-h-tag-alg )
   key-h-tag-alg   = "sha1" / 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.)


   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;
       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).


   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.

Allman, et al.           Expires August 6, 2006                [Page 24]

Internet-Draft                DKIM Signing                 February 2006


   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

       *   matches all service types

       email   electronic mail (not necessarily limited to SMTP)

       This tag is intended to permit senders 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.)


   key-s-tag        = %x73 [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 mode results to assist
           the signer.


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

Allman, et al.           Expires August 6, 2006                [Page 25]

Internet-Draft                DKIM Signing                 February 2006

       Unrecognized flags MUST be ignored.

3.6.2  DNS binding

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

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

   The value of the "i=" tag is not used by the DNS binding.  Resource Record Types for Key Storage

   [[This section needs to be fleshed out.  ACTUALLY:  will be addressed
   in another document.]]

   Two RR types are used:  DKK and TXT.

   The DKK RR is expected to be a non-text, binary representation
   intended to allow the largest possible keys to be represented and
   transmitted in a UDP DNS packet.  Details of this RR are described in

   TXT records are encoded as described in Section 3.6.1.

   Verifiers SHOULD search for a DKK RR first, if possible, followed by
   a TXT RR.  If the verifier is unable to search for a DKK RR or a DKK
   RR is not found, the verifier MUST search for a TXT RR.

3.7  Computing the Message Hash

   Both signing and verifying message signatures starts with a step of
   computing a cryptographic hash of 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.

   The signer or verifier MUST pass the following to the hash algorithm
   in the indicated order.  Note that canonicalization (Section 3.4) is
   only used to prepare the email for signing or verifying; it does not

Allman, et al.           Expires August 6, 2006                [Page 26]

Internet-Draft                DKIM Signing                 February 2006

   affect the transmitted email in any way.

   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.

   2.  A single CRLF.

   3.  The message body, canonicalized using the body canonicalization
       algorithm specified in the "c=" tag, and truncated to the length
       specified in the "l=" tag.

   4.  A single CRLF.

   5.  The "DKIM-Signature" header field that exists (verifying) or will
       be inserted (signing) in the message, with the content 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.

   After the body is processed, a single CRLF followed by the "DKIM-
   Signature" header field being created or verified is presented to the
   algorithm.  The value portion of the "b=" tag (that is, the portion
   after the "=" sign) must be treated as though it were empty, and the
   header field must be canonicalized according to the algorithm that is
   specified in the "c=" tag.  Any final CRLF on the "DKIM-Signature"
   header field MUST NOT be included in the signature computation.

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

   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-

Allman, et al.           Expires August 6, 2006                [Page 27]

Internet-Draft                DKIM Signing                 February 2006

   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.

4.  Semantics of Multiple Signatures

   Considerable energy has been spent discussing the desirability and
   semantics of multiple DKIM signatures in a single message,
   particularly in a "re-sending" scenario such as a mailing list.  On
   the one hand, discarding existing signature header fields loses
   information which could prove to be valuable in the future.  On the
   other hand, since header fields are known to be re-ordered in transit
   by at least some MTAs, determining the most interesting signature
   header field is non-trivial.

   Further confusion could occur with multiple signatures added at the
   same logical "depth".  For example, a signer could choose to sign
   using multiple signing or canonicalization algorithms.  There is no a
   priori way to determine that two signatures are alternatives versus
   nested in a re-sending scenario.

      [[DOCUMENTATION NOTE:  ISSUE:  downgrade attacks by removal of sig
      headers that use stronger algorithms.]]

   Also, many agents are expected to break existing signatures (e.g., a
   mailing list that modifies Subject header fields or adds unsubscribe
   information to the end of the message).  Retaining signature
   information that is known to be bad could create more problems than
   it solves.

   For these reasons, multiple signatures are not prohibited but are
   left undefined.

      INFORMATIVE IMPLEMENTATION GUIDANCE:  Agents that forward mail
      without modification could decide whether to add another signature
      or simply retain an existing signatures.  Agents that are known to
      break existing signatures may leave the existing signature or
      delete it.  Agents that re-sign messages that are already signed
      should verify the previous signature and should probably refuse to
      sign any critical information that failed a signature

5.  Signer Actions

   The following steps are performed in order by signers.

Allman, et al.           Expires August 6, 2006                [Page 28]

Internet-Draft                DKIM Signing                 February 2006

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 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.

      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 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

   A signer SHOULD NOT sign with a key that is expected to expire within
   seven days; that is, when rotating to a new key, signing should
   immediately commence with the new key and the old key SHOULD be
   retained for at least seven days before being removed from the key

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

Allman, et al.           Expires August 6, 2006                [Page 29]

Internet-Draft                DKIM Signing                 February 2006

   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
   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 have the
   verifiers treat as trusted.  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).

Allman, et al.           Expires August 6, 2006                [Page 30]

Internet-Draft                DKIM Signing                 February 2006

      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.


      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

   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-

Allman, et al.           Expires August 6, 2006                [Page 31]

Internet-Draft                DKIM Signing                 February 2006


      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

   The signer MUST compute the message hash as described in Section 3.7
   and then sign it using the selected public-key algorithm.

   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 the 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.

5.6  Insert the DKIM-Signature header field

   Finally, the signer MUST insert the "DKIM-Signature:" header field
   prior to transmitting the email.  The "DKIM-Signature" header field

Allman, et al.           Expires August 6, 2006                [Page 32]

Internet-Draft                DKIM Signing                 February 2006

   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.  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.

   Verifiers apply the following steps in the order listed.

6.1  Extract the Signature from the Message

   The signature and associated signing identity is included in the
   value of the DKIM-Signature header field.

   Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag.
   Existence of such a tag indicates a new, incompatible version of the
   DKIM-Signature header field.

   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",

Allman, et al.           Expires August 6, 2006                [Page 33]

Internet-Draft                DKIM Signing                 February 2006

   where "d" is the value from the "d=" tag (which MUST exist).

   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.

   Implementers MUST meticulously validate the format and values in the
   "DKIM-Signature:" header field; any inconsistency or unexpected
   values MUST result in an unverified email.  Being "liberal in what
   you accept" is definitely a bad strategy in this security context.
   Note however that this does not include the existence of unknown tags
   in a "DKIM-Signature" header field, which are explicitly permitted.

   Verifiers MUST 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 field 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.

   Since there can be multiple signatures in a message, a verifier
   SHOULD ignore an invalid signature (regardless if caused by a
   syntactic or semantic problem) and try other signatures.  A verifier
   MAY choose to treat a message with one or more invalid signatures and
   no valid signatures with more suspicion than a message with no
   signature at all.

6.2  Get the Public Key

   The public key 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 line.
   Obviously, a public key should 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:

   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 defer acceptance of this email (normally this will be

Allman, et al.           Expires August 6, 2006                [Page 34]

Internet-Draft                DKIM Signing                 February 2006

       achieved with a 451/4.7.5 SMTP reply code).

   3.  If the query for the public key fails because the corresponding
       RR does not exist, the verifier MUST ignore the signature.

   4.  If the result returned from the query does not adhere to the
       format defined in this specification, the verifier MUST ignore
       the signature.

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

   6.  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 "h=" tag, the verifier MUST ignore the

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

   8.  If the public key data is not suitable for use with the algorithm
       type defined by the "a=" tag in the "DKIM-Signature" header
       field, the verifier MUST ignore the signature.

   If the signature is to be ignored, verifiers SHOULD search for
   another signature in the message.

6.3  Compute the Verification

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

   o  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-

Allman, et al.           Expires August 6, 2006                [Page 35]

Internet-Draft                DKIM Signing                 February 2006

   o  Based on the algorithm indicated in the "a=" tag,

      *  Compute the message hash from the canonical copy as described
         in Section 3.7.

      *  Decrypt the signature using the signer's public key.

   o  Compare the decrypted signature to the message hash.  If they are
      identical, the hash computed by the signer must be the same as the
      hash computed by the verifier, and hence the signature verifies;
      otherwise, the signature fails.

      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

   Verifiers MUST 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 body length is specified in the "l=" tag of the signature,
   verifiers MUST only verify the number of bytes indicated in the body
   length.  Verifiers MAY decide to treat a message containing bytes
   beyond the indicated body length with suspicion.  Verifiers MAY
   truncate the message at the indicated body length or reject the
   signature outright.

6.4  Insert the Authentication-Results Header Field

   Verifiers wishing to communicate the results of verification via an
   email header field SHOULD use the Authentication-Results header field
   [ID-AUTH-RES].  The Authentication-Results header field SHOULD be
   inserted before any existing DKIM-Signature or Authentication-Results
   header fields in the header field block.

Allman, et al.           Expires August 6, 2006                [Page 36]

Internet-Draft                DKIM Signing                 February 2006

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

6.5  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.

   If the verifying MTA is capable of verifying the public key of the
   signer and check the signature on the message synchronously with the
   SMTP session and such signature is missing or does not verify the MTA
   MAY 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

   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
   authentication status is to be stored in the message header field,
   the Authentication-Results header field [ID-AUTH-RES] SHOULD be used
   to convey this information.  If the message is signed on behalf of
   any address other than that in the From:  header field, the mail
   system SHOULD take pains to ensure that the actual signing identity
   is clear to the reader.

   The verifier MAY treat unsigned header fields with extreme

Allman, et al.           Expires August 6, 2006                [Page 37]

Internet-Draft                DKIM Signing                 February 2006

   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 the "Authentication-Results" header field and
   possibly 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

   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

   From: John Q. User <>

   might be converted to

   From: "John Q. User via <>" <>

   This sort of address inconsistency is expected for mailing lists, but
   might be otherwise used to mislead the verifier, for example if a
   message supposedly from had a Sender
   address of

   Under no circumstances should an unsigned header field be displayed

Allman, et al.           Expires August 6, 2006                [Page 38]

Internet-Draft                DKIM Signing                 February 2006

   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

   Use of the _domainkey prefix in DNS records will require registration
   by IANA.

   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.

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

Allman, et al.           Expires August 6, 2006                [Page 39]

Internet-Draft                DKIM Signing                 February 2006

   ""</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=""
     width=578 height=370>

8.2  Misappropriated Private Key

   If the private key for a user is resident on their computer and is
   not protected by an appropriately secure passphrase, 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

   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

Allman, et al.           Expires August 6, 2006                [Page 40]

Internet-Draft                DKIM Signing                 February 2006

   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, 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

   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

Allman, et al.           Expires August 6, 2006                [Page 41]

Internet-Draft                DKIM Signing                 February 2006

   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

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

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

Allman, et al.           Expires August 6, 2006                [Page 42]

Internet-Draft                DKIM Signing                 February 2006


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.

9.  References

9.1  Normative References

              Kucherawy, M., "Message header field for Indicating Sender
              Authentication Status",
              draft-kucherawy-sender-auth-header-02 (work in progress),
              May 2005.

              "DKIM Key Resource Records (To be written)",
              draft-dkim-dkk-rr-xx (work in progress), 2005.

   [ID-SHA]   Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", draft-eastlake-sha2-02 (work in
              progress), January 2006.

   [OPENSSL]  Team, C&D., "OpenSSL Documents",

   [RFC1421]  Linn, J., "Privacy Enhancement for Internet Electronic
              Mail: Part I: Message Encryption and Authentication
              Procedures", RFC 1421, February 1993.

   [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.

Allman, et al.           Expires August 6, 2006                [Page 43]

Internet-Draft                DKIM Signing                 February 2006

   [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.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.

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

9.2  Informative References

              Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", draft-fenton-dkim-threats-02
              (work in progress), December 2005.

   [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.

   [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.

Allman, et al.           Expires August 6, 2006                [Page 44]

Internet-Draft                DKIM Signing                 February 2006

Authors' Addresses

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

   Phone:  +1 510 594 5501

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

   Phone:  +1 650 319 9016

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

   Phone:  +1 408 349 6831

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


Allman, et al.           Expires August 6, 2006                [Page 45]

Internet-Draft                DKIM Signing                 February 2006

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

   Phone:  +1 408 526 5914

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

   Phone:  +1 408 525 5386

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

A.1  The user composes an email

   From: Joe SixPack <>
   To: Suzie Q <>
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: <>


   We lost the game. Are you hungry yet?


A.2  The email is signed

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

Allman, et al.           Expires August 6, 2006                [Page 46]

Internet-Draft                DKIM Signing                 February 2006

   DKIM-Signature: a=rsa-sha1; s=brisbane;;
         c=simple; q=dns;;
         h=Received : From : To : Subject : Date : Message-ID;
   Received: from  []
         by with SUBMISSION;
         Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
   From: Joe SixPack <>
   To: Suzie Q <>
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: <>


   We lost the game. Are you hungry yet?


   The signing email server requires access to the private-key
   associated with the "brisbane" selector to generate this signature.
   Distribution and management of private-keys is outside the scope of
   this document.

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 "" 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:

   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:

Allman, et al.           Expires August 6, 2006                [Page 47]

Internet-Draft                DKIM Signing                 February 2006

; dkim=pass
   Received: from (
                 by with SMTP;
                 Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
   DKIM-Signature: a=rsa-sha1; s=brisbane;;
          c=simple; q=dns;;
          h=Received : From : To : Subject : Date : Message-ID;
   Received: from  []
            by with SUBMISSION;
            Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
   From: Joe SixPack <>
   To: Suzie Q <>
   Subject: Is dinner ready?
   Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
   Message-ID: <>


   We lost the game. Are you hungry yet?


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

B.1  Simple Message Forwarding

   In some cases the recipient may request forwarding of email messages
   from the original address 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="

Allman, et al.           Expires August 6, 2006                [Page 48]

Internet-Draft                DKIM Signing                 February 2006

   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

   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 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,, 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 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 not modify the body or signed header fields of a
   message with a valid signature may re-sign the message as described

   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.,
   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

Allman, et al.           Expires August 6, 2006                [Page 49]

Internet-Draft                DKIM Signing                 February 2006

   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 field of the message, but put an address owned by
   the site into the Sender field, and sign the message on behalf of the
   Sender.  A verifying MTA should accept this and rewrite the From
   field to indicate the address that was verified, i.e., From:  John

Allman, et al.           Expires August 6, 2006                [Page 50]

Internet-Draft                DKIM Signing                 February 2006

   Doe via <>.

Appendix C.  Creating a public key (INFORMATIVE)

   The default signature is an RSA signed SHA1 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

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

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

   This public-key data (without the BEGIN and END tags) is placed in
   the DNS.  With the signature, canonical email contents and public

Allman, et al.           Expires August 6, 2006                [Page 51]

Internet-Draft                DKIM Signing                 February 2006

   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 -signature signature.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 <input.file

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


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

Appendix D.  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, Don Johnsen, Harry Katz,
   Murray S. Kucherawy, Barry Leiba, John Levine, Simon Longsdale, David
   Margrave, Justin Mason, David Mayne, Steve Murphy, Russell Nelson,
   Dave Oran, Shamim Pirzada, Juan Altmayer Pizzorno, Sanjay Pol, Blake
   Ramsdell, Christian Renaud, Scott Renfro, Dave Rossetti, the 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

Appendix E.  Edit History

E.1  Changes since -allman-01 version

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

Allman, et al.           Expires August 6, 2006                [Page 52]

Internet-Draft                DKIM Signing                 February 2006

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

   o  Added ABNF for all tags.

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

   o  Significant wordsmithing.

E.2  Changes since -allman-00 version

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

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

   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

   o  Added several IANA Considerations.

   o  Fixed a number of grammar and formatting errors.

Allman, et al.           Expires August 6, 2006                [Page 53]

Internet-Draft                DKIM Signing                 February 2006

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Allman, et al.           Expires August 6, 2006                [Page 54]

Internet-Draft                DKIM Signing                 February 2006


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Allman, et al.           Expires August 6, 2006                [Page 55]