Network Working Group                                          E. Allman
Request for Comments: 4871                                Sendmail, Inc.
Obsoletes: 4870                                                J. Callas
Category: Standards Track                                PGP Corporation
                                                               M. Delany
                                                               M. Libbey
                                                              Yahoo! Inc
                                                               J. Fenton
                                                               M. Thomas
                                                     Cisco Systems, Inc.
                                                                May 2007


              DomainKeys Identified Mail (DKIM) Signatures

Status of This Memo

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

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

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

Table of Contents

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

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

1.  Introduction

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

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

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

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

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

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

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

   o  message archiving is not a design goal.

   DKIM:

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

   o  requires minimal new infrastructure;

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

   o  can be deployed incrementally;

   o  allows delegation of signing to third parties.

1.1.  Signing Identity

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

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

1.2.  Scalability

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

1.3.  Simple Key Management

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

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

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].

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

2.1.  Signers

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

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.  Whitespace

   There are three forms of whitespace:

   o  WSP represents simple whitespace, i.e., a space or a tab character
      (formal definition in [RFC4234]).

   o  LWSP is linear whitespace, defined as WSP plus CRLF (formal
      definition in [RFC4234]).

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

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

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

   The definition of FWS is identical to that in [RFC2822] except for
   the exclusion of obs-FWS.

2.4.  Common ABNF Tokens

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

2.5.  Imported ABNF Tokens

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

   The following tokens are imported from [RFC2821]:

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

   o  "sub-domain"

   The following tokens are imported from [RFC2822]:

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

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

   The following tokens are imported from [RFC2045]:

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

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

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

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

2.6.  DKIM-Quoted-Printable

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

   ABNF:

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

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

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

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

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

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