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rfc:rfc4871

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".

Allman, et al. Standards Track [Page 1] RFC 4871 DKIM Signatures May 2007

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

Allman, et al. Standards Track [Page 2] RFC 4871 DKIM Signatures May 2007

   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

Allman, et al. Standards Track [Page 3] RFC 4871 DKIM Signatures May 2007

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.

Allman, et al. Standards Track [Page 4] RFC 4871 DKIM Signatures May 2007

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].

Allman, et al. Standards Track [Page 5] RFC 4871 DKIM Signatures May 2007

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] ] ]

Allman, et al. Standards Track [Page 6] RFC 4871 DKIM Signatures May 2007

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.

Allman, et al. Standards Track [Page 7] RFC 4871 DKIM Signatures May 2007

 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).

3. Protocol Elements

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

3.1. Selectors

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

Allman, et al. Standards Track [Page 8] RFC 4871 DKIM Signatures May 2007

 Selectors are needed to support some important use cases.  For
 example:
 o  Domains that want to delegate signing capability for a specific
    address for a given duration to a partner, such as an advertising
    provider or other outsourced function.
 o  Domains that want to allow frequent travelers to send messages
    locally without the need to connect with a particular MSA.
 o  "Affinity" domains (e.g., college alumni associations) that
    provide forwarding of incoming mail, but that do not operate a
    mail submission agent for outgoing mail.
 Periods are allowed in selectors and are component separators.  When
 keys are retrieved from the DNS, periods in selectors define DNS
 label boundaries in a manner similar to the conventional use in
 domain names.  Selector components might be used to combine dates
 with locations, for example, "march2005.reykjavik".  In a DNS
 implementation, this can be used to allow delegation of a portion of
 the selector namespace.
 ABNF:
    selector =   sub-domain *( "." sub-domain )
 The number of public keys and corresponding selectors for each domain
 is determined by the domain owner.  Many domain owners will be
 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.
    INFORMATIVE NOTE: A key may also be revoked as described below.
    The distinction between revoking and removing a key selector
    record is subtle.  When phasing out keys as described above, a
    signing domain would probably simply remove the key record after

Allman, et al. Standards Track [Page 9] RFC 4871 DKIM Signatures May 2007

    the transition period.  However, a signing domain could elect to
    revoke the key (but maintain the key record) for a further period.
    There is no defined semantic difference between a revoked key and
    a removed key.
 While some domains 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.  For example, if
 per-user keys are issued, the domain owner will need to make the
 decision as to whether to associate this selector directly with the
 user name, or make it some unassociated random value, such as a
 fingerprint of the public key.
    INFORMATIVE OPERATIONS 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.  For this reason, signers are ill-advised
    to reuse selectors for new keys.  A better strategy is to assign
    new keys to new selectors.

3.2. Tag=Value Lists

 DKIM uses a simple "tag=value" syntax in several contexts, including
 in messages and domain signature records.
 Values are a series of strings containing either plain text, "base64"
 text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid,
 Section 6.7), or "dkim-quoted-printable" (as defined in Section 2.6).
 The name of the tag will determine the encoding of each value.
 Unencoded semicolon (";") characters MUST NOT occur in the tag value,
 since that separates tag-specs.
    INFORMATIVE IMPLEMENTATION NOTE: Although the "plain text" defined
    below (as "tag-value") only includes 7-bit characters, an
    implementation that wished to anticipate future standards would be
    advised not to preclude the use of UTF8-encoded text in tag=value
    lists.

Allman, et al. Standards Track [Page 10] RFC 4871 DKIM Signatures May 2007

 Formally, the syntax rules are as follows:
      tag-list  =  tag-spec 0*( ";" tag-spec ) [ ";" ]
      tag-spec  =  [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
      tag-name  =  ALPHA 0*ALNUMPUNC
      tag-value =  [ tval 0*( 1*(WSP / FWS) tval ) ]
                        ; WSP and FWS prohibited at beginning and end
      tval      =  1*VALCHAR
      VALCHAR   =  %x21-3A / %x3C-7E
                        ; EXCLAMATION to TILDE except SEMICOLON
      ALNUMPUNC =  ALPHA / DIGIT / "_"
 Note that WSP is allowed anywhere around tags.  In particular, any
 WSP after the "=" and any WSP before the terminating ";" is not part
 of the value; however, WSP inside the value is significant.
 Tags MUST be interpreted in a case-sensitive manner.  Values MUST be
 processed as case sensitive unless the specific tag description of
 semantics specifies case insensitivity.
 Tags with duplicate names MUST NOT occur within a single tag-list; if
 a tag name does occur more than once, the entire tag-list is invalid.
 Whitespace within a value MUST be retained unless explicitly excluded
 by the specific tag description.
 Tag=value pairs that represent the default value MAY be included to
 aid legibility.
 Unrecognized tags MUST be ignored.
 Tags that have an empty value are not the same as omitted tags.  An
 omitted tag is treated as having the default value; a tag with an
 empty value explicitly designates the empty string as the value.  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 digital signature algorithms.  Two algorithms
 are defined by this specification at this time: rsa-sha1 and rsa-
 sha256.  The rsa-sha256 algorithm is the default if no algorithm is
 specified.  Verifiers MUST implement both rsa-sha1 and rsa-sha256.
 Signers MUST implement and SHOULD sign using rsa-sha256.

Allman, et al. Standards Track [Page 11] RFC 4871 DKIM Signatures May 2007

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

3.3.1. The rsa-sha1 Signing Algorithm

 The rsa-sha1 Signing Algorithm computes a message hash as described
 in Section 3.7 below using SHA-1 [FIPS.180-2.2002] as the hash-alg.
 That hash is then signed by the signer using the RSA algorithm
 (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the
 signer's private key.  The hash MUST NOT be truncated or converted
 into any form other than the native binary form before being signed.
 The signing algorithm SHOULD use a public exponent of 65537.

3.3.2. The rsa-sha256 Signing Algorithm

 The rsa-sha256 Signing Algorithm computes a message hash as described
 in Section 3.7 below using SHA-256 [FIPS.180-2.2002] as the hash-alg.
 That hash is then signed by the signer using the RSA algorithm
 (defined in PKCS#1 version 1.5 [RFC3447]) as the crypt-alg and the
 signer's private key.  The hash MUST NOT be truncated or converted
 into any form other than the native binary form before being signed.

3.3.3. Key Sizes

 Selecting appropriate key sizes is a trade-off between cost,
 performance, and risk.  Since short RSA keys more easily succumb to
 off-line attacks, signers MUST use RSA keys of at least 1024 bits for
 long-lived keys.  Verifiers MUST be able to validate signatures with
 keys ranging from 512 bits to 2048 bits, and they MAY be able to
 validate signatures with larger keys.  Verifier policies may use the
 length of the signing key as one metric for determining whether a
 signature is acceptable.
 Factors that should influence the key size choice include the
 following:
 o  The practical constraint that large (e.g., 4096 bit) keys may not
    fit within a 512-byte DNS UDP response packet
 o  The security constraint that keys smaller than 1024 bits are
    subject to off-line attacks
 o  Larger keys impose higher CPU costs to verify and sign email

Allman, et al. Standards Track [Page 12] RFC 4871 DKIM Signatures May 2007

 o  Keys can be replaced on a regular basis, thus their lifetime can
    be relatively short
 o  The security goals of this specification are modest compared to
    typical goals of other systems that employ digital signatures
 See [RFC3766] for further discussion on selecting key sizes.

3.3.4. Other Algorithms

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

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 a signature verification 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 whitespace replacement and
 header field line rewrapping.  A signer MAY specify either algorithm
 for header or body when signing an email.  If no canonicalization
 algorithm is specified by the signer, the "simple" algorithm defaults
 for both header and body.  Verifiers MUST implement both
 canonicalization algorithms.  Note that the header and body may use
 different canonicalization algorithms.  Further canonicalization
 algorithms MAY be defined in the future; verifiers MUST ignore any
 signatures that use unrecognized canonicalization algorithms.
 Canonicalization simply prepares the email for presentation to the
 signing or verification algorithm.  It MUST NOT change the

Allman, et al. Standards Track [Page 13] RFC 4871 DKIM Signatures May 2007

 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 (text is ASCII encoded, lines are separated with CRLF
 characters, etc.).  See also Section 5.3 for information about
 normalizing the message.

3.4.1. The "simple" Header Canonicalization Algorithm

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

3.4.2. The "relaxed" Header 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
    lowercase.  For example, convert "SUBJect: AbC" to "subject: AbC".
 o  Unfold all header field continuation lines as described in
    [RFC2822]; in particular, lines with terminators embedded in
    continued header field values (that is, CRLF sequences followed by
    WSP) MUST be interpreted without the CRLF.  Implementations MUST
    NOT remove the CRLF at the end of the header field value.
 o  Convert all sequences of one or more WSP characters to a single SP
    character.  WSP characters here include those before and after a
    line folding boundary.
 o  Delete all WSP characters at the end of each unfolded header field
    value.
 o  Delete any WSP characters remaining before and after the colon
    separating the header field name from the header field value.  The
    colon separator MUST be retained.

3.4.3. The "simple" Body Canonicalization Algorithm

 The "simple" body canonicalization algorithm ignores all empty lines
 at the end of the message body.  An empty line is a line of zero
 length after removal of the line terminator.  If there is no body or
 no trailing CRLF on the message body, a CRLF is added.  It makes no

Allman, et al. Standards Track [Page 14] RFC 4871 DKIM Signatures May 2007

 other changes to the message body.  In more formal terms, the
 "simple" body canonicalization algorithm converts "0*CRLF" at the end
 of the body to a single "CRLF".
 Note that a completely empty or missing body is canonicalized as a
 single "CRLF"; that is, the canonicalized length will be 2 octets.

3.4.4. The "relaxed" Body Canonicalization Algorithm

 The "relaxed" body canonicalization algorithm:
 o  Ignores all whitespace at the end of lines.  Implementations MUST
    NOT remove the CRLF at the end of the line.
 o  Reduces all sequences of WSP within a line to a single SP
    character.
 o  Ignores all empty lines at the end of the message body.  "Empty
    line" is defined in Section 3.4.3.
    INFORMATIVE NOTE: It should be noted that the relaxed body
    canonicalization algorithm may enable certain types of extremely
    crude "ASCII Art" attacks where a message may be conveyed by
    adjusting the spacing between words.  If this is a concern, the
    "simple" body canonicalization algorithm should be used instead.

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, measured in
 octets.  If the body length count is not specified, the entire
 message body is signed.
    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.
    INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables
    an attack in which an attacker modifies a message to include
    content that solely benefits the attacker.  It is possible for the
    appended content to completely replace the original content in the
    end recipient's eyes and to defeat duplicate message detection
    algorithms.  To avoid this attack, signers should be wary of using

Allman, et al. Standards Track [Page 15] RFC 4871 DKIM Signatures May 2007

    this tag, and verifiers might wish to ignore the tag or remove
    text that appears after the specified content length, perhaps
    based on other criteria.
 The body length count allows the signer of a message to permit data
 to be appended to the end of the body of a signed message.  The body
 length count MUST be calculated following the canonicalization
 algorithm; for example, any whitespace ignored by a canonicalization
 algorithm is not included as part of the body length count.  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".  Note that this
    only works for some MIME types, e.g., multipart/mixed but not
    multipart/signed.
 A body length count of zero means that the body is completely
 unsigned.
 Signers wishing to ensure that no modification of any sort can occur
 should specify the "simple" canonicalization algorithm for both
 header and body and omit the body length count.

3.4.6. Canonicalization Examples (INFORMATIVE)

 In the following examples, actual whitespace is used only for
 clarity.  The actual input and output text is designated using
 bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a
 tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
 For example, "X <SP> Y" and "X<SP>Y" represent the same three
 characters.
 Example 1: A message reading:
     A: <SP> X <CRLF>
     B <SP> : <SP> Y <HTAB><CRLF>
     <HTAB> Z <SP><SP><CRLF>
     <CRLF>
     <SP> C <SP><CRLF>
     D <SP><HTAB><SP> E <CRLF>
     <CRLF>
     <CRLF>

Allman, et al. Standards Track [Page 16] RFC 4871 DKIM Signatures May 2007

 when canonicalized using relaxed canonicalization for both header and
 body results in a header reading:
     a:X <CRLF>
     b:Y <SP> Z <CRLF>
 and a body reading:
     <SP> C <CRLF>
     D <SP> E <CRLF>
 Example 2: The same message canonicalized using simple
 canonicalization for both header and body results in a header
 reading:
     A: <SP> X <CRLF>
     B <SP> : <SP> Y <HTAB><CRLF>
     <HTAB> Z <SP><SP><CRLF>
 and a body reading:
     <SP> C <SP><CRLF>
     D <SP><HTAB><SP> E <CRLF>
 Example 3: When processed using relaxed header canonicalization and
 simple body canonicalization, the canonicalized version has a header
 of:
     a:X <CRLF>
     b:Y <SP> Z <CRLF>
 and a body reading:
     <SP> C <SP><CRLF>
     D <SP><HTAB><SP> E <CRLF>

3.5. The DKIM-Signature Header Field

 The signature of the email is stored in the DKIM-Signature header
 field.  This header field contains all of the signature and key-
 fetching data.  The DKIM-Signature value is a tag-list as described
 in Section 3.2.
 The DKIM-Signature header field SHOULD be treated as though it were a
 trace header field as defined in Section 3.6 of [RFC2822], and hence
 SHOULD NOT be reordered and SHOULD be prepended to the message.

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 The DKIM-Signature header field being created or verified is always
 included in the signature calculation, after the rest of the header
 fields being signed; however, when calculating or verifying the
 signature, the value of the "b=" tag (signature value) of that DKIM-
 Signature header field MUST be treated as though it were an empty
 string.  Unknown tags in the DKIM-Signature header field MUST be
 included in the signature calculation but MUST be otherwise ignored
 by verifiers.  Other DKIM-Signature header fields that are included
 in the signature should be treated as normal header fields; in
 particular, the "b=" tag is not treated specially.
 The encodings for each field type are listed below.  Tags described
 as qp-section are encoded as described in Section 6.7 of MIME Part
 One [RFC2045], with the additional conversion of semicolon characters
 to "=3B"; intuitively, this is one line of quoted-printable encoded
 text.  The dkim-quoted-printable syntax is defined in Section 2.6.
 Tags on the DKIM-Signature header field along with their type and
 requirement status are shown below.  Unrecognized tags MUST be
 ignored.
 v=  Version (MUST be included).  This tag defines the version of this
     specification that applies to the signature record.  It MUST have
     the value "1".  Note that verifiers must do a string comparison
     on this value; for example, "1" is not the same as "1.0".
 ABNF:
     sig-v-tag   = %x76 [FWS] "=" [FWS] "1"
         INFORMATIVE NOTE: DKIM-Signature version numbers are expected
         to increase arithmetically as new versions of this
         specification are released.
 a=  The algorithm used to generate the signature (plain-text;
     REQUIRED).  Verifiers MUST support "rsa-sha1" and "rsa-sha256";
     signers SHOULD sign using "rsa-sha256".  See Section 3.3 for a
     description of algorithms.
 ABNF:
     sig-a-tag       = %x61 [FWS] "=" [FWS] sig-a-tag-alg
     sig-a-tag-alg   = sig-a-tag-k "-" sig-a-tag-h
     sig-a-tag-k     = "rsa" / x-sig-a-tag-k
     sig-a-tag-h     = "sha1" / "sha256" / x-sig-a-tag-h
     x-sig-a-tag-k   = ALPHA *(ALPHA / DIGIT)   ; for later extension
     x-sig-a-tag-h   = ALPHA *(ALPHA / DIGIT)   ; for later extension

Allman, et al. Standards Track [Page 18] RFC 4871 DKIM Signatures May 2007

 b=  The signature data (base64; REQUIRED).  Whitespace is ignored in
     this value and MUST be ignored when reassembling the original
     signature.  In particular, the signing process can safely insert
     FWS in this value in arbitrary places to conform to line-length
     limits.  See Signer Actions (Section 5) for how the signature is
     computed.
 ABNF:
     sig-b-tag       = %x62 [FWS] "=" [FWS] sig-b-tag-data
     sig-b-tag-data  = base64string
 bh= The hash of the canonicalized body part of the message as limited
     by the "l=" tag (base64; REQUIRED).  Whitespace is ignored in
     this value and MUST be ignored when reassembling the original
     signature.  In particular, the signing process can safely insert
     FWS in this value in arbitrary places to conform to line-length
     limits.  See Section 3.7 for how the body hash is computed.
 ABNF:
     sig-bh-tag      = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
     sig-bh-tag-data = base64string
 c=  Message canonicalization (plain-text; OPTIONAL, default is
     "simple/simple").  This tag informs the verifier of the type of
     canonicalization used to prepare the message for signing.  It
     consists of two names separated by a "slash" (%d47) character,
     corresponding to the header and body canonicalization algorithms
     respectively.  These algorithms are described in Section 3.4.  If
     only one algorithm is named, that algorithm is used for the
     header and "simple" is used for the body.  For example,
     "c=relaxed" is treated the same as "c=relaxed/simple".
 ABNF:
     sig-c-tag       = %x63 [FWS] "=" [FWS] sig-c-tag-alg
                   ["/" sig-c-tag-alg]
     sig-c-tag-alg   = "simple" / "relaxed" / x-sig-c-tag-alg
     x-sig-c-tag-alg = hyphenated-word    ; for later extension
 d=  The domain of the signing entity (plain-text; REQUIRED).  This is
     the domain that will be queried for the public key.  This domain
     MUST be the same as or a parent domain of the "i=" tag (the
     signing identity, as described below), or it MUST meet the
     requirements for parent domain signing described in Section 3.8.
     When presented with a signature that does not meet these
     requirement, verifiers MUST consider the signature invalid.

Allman, et al. Standards Track [Page 19] RFC 4871 DKIM Signatures May 2007

 Internationalized domain names MUST be encoded as described in
     [RFC3490].
 ABNF:
     sig-d-tag       = %x64 [FWS] "=" [FWS] domain-name
     domain-name     = sub-domain 1*("." sub-domain)
              ; from RFC 2821 Domain, but excluding address-literal
 h=  Signed header fields (plain-text, but see description; REQUIRED).
     A colon-separated list of header field names that identify the
     header fields presented to the signing algorithm.  The field MUST
     contain the complete list of header fields in the order presented
     to the signing algorithm.  The field MAY contain names of header
     fields that do not exist when signed; nonexistent header fields
     do not contribute to the signature computation (that is, they are
     treated as the null input, including the header field name, the
     separating colon, the header field value, and any CRLF
     terminator).  The field MUST NOT include the DKIM-Signature
     header field that is being created or verified, but may include
     others.  Folding whitespace (FWS) MAY be included on either side
     of the colon separator.  Header field names MUST be compared
     against actual header field names in a case-insensitive manner.
     This list MUST NOT be empty.  See Section 5.4 for a discussion of
     choosing header fields to sign.
 ABNF:
     sig-h-tag       = %x68 [FWS] "=" [FWS] hdr-name
                   0*( *FWS ":" *FWS hdr-name )
     hdr-name        = field-name
     INFORMATIVE EXPLANATION: By "signing" header fields that do not
         actually exist, a signer can prevent insertion of those
         header fields before verification.  However, since a signer
         cannot possibly know what header fields might be created in
         the future, and that some MUAs might present header fields
         that are embedded inside a message (e.g., as a message/rfc822
         content type), the security of this solution is not total.
     INFORMATIVE EXPLANATION: The exclusion of the header field name
         and colon as well as the header field value for non-existent
         header fields prevents an attacker from inserting an actual
         header field with a null value.

Allman, et al. Standards Track [Page 20] RFC 4871 DKIM Signatures May 2007

 i=  Identity of the user or agent (e.g., a mailing list manager) on
     behalf of which this message is signed (dkim-quoted-printable;
     OPTIONAL, default is an empty Local-part followed by an "@"
     followed by the domain from the "d=" tag).  The syntax is a
     standard email address where the Local-part MAY be omitted.  The
     domain part of the address MUST be the same as or a subdomain of
     the value of the "d=" tag.
 Internationalized domain names MUST be converted using the steps
     listed in Section 4 of [RFC3490] using the "ToASCII" function.
 ABNF:
     sig-i-tag =   %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name
     INFORMATIVE NOTE: The Local-part of the "i=" tag is optional
         because in some cases a signer may not be able to establish a
         verified individual identity.  In such cases, the signer may
         wish to assert that although it is willing to go as far as
         signing for the domain, it is unable or unwilling to commit
         to an individual user name within their domain.  It can do so
         by including the domain part but not the Local-part of the
         identity.
     INFORMATIVE DISCUSSION: This document does not require the value
         of the "i=" tag to match the identity in any message header
         fields.  This is considered to be a verifier policy issue.
         Constraints between the value of the "i=" tag and other
         identities in other header fields seek to apply basic
         authentication into the semantics of trust associated with a
         role such as content author.  Trust is a broad and complex
         topic and trust mechanisms are subject to highly creative
         attacks.  The real-world efficacy of any but the most basic
         bindings between the "i=" value and other identities is not
         well established, nor is its vulnerability to subversion by
         an attacker.  Hence reliance on the use of these options
         should be strictly limited.  In particular, it is not at all
         clear to what extent a typical end-user recipient can rely on
         any assurances that might be made by successful use of the
         "i=" options.
 l=  Body length count (plain-text unsigned decimal integer; OPTIONAL,
     default is entire body).  This tag informs the verifier of the
     number of octets in the body of the email after canonicalization
     included in the cryptographic hash, starting from 0 immediately
     following the CRLF preceding the body.  This value MUST NOT be
     larger than the actual number of octets in the canonicalized
     message body.

Allman, et al. Standards Track [Page 21] RFC 4871 DKIM Signatures May 2007

     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 attacks in
         which an intermediary with malicious intent modifies a
         message to include content that solely benefits the attacker.
         It is possible for the appended content to completely replace
         the original content in the end recipient's eyes and to
         defeat duplicate message detection algorithms.  Examples are
         described in Security Considerations (Section 8).  To avoid
         this attack, signers should be extremely wary of using this
         tag, and verifiers might wish to ignore the tag or remove
         text that appears after the specified content length.
     INFORMATIVE NOTE: The value of the "l=" tag is constrained to 76
         decimal digits.  This constraint is not intended to predict
         the size of future messages or to require implementations to
         use an integer representation large enough to represent the
         maximum possible value, but is intended to remind the
         implementer to check the length of this and all other tags
         during verification and to test for integer overflow when
         decoding the value.  Implementers may need to limit the
         actual value expressed to a value smaller than 10^76, e.g.,
         to allow a message to fit within the available storage space.
 ABNF:
 sig-l-tag    = %x6c [FWS] "=" [FWS] 1*76DIGIT
 q=  A colon-separated list of query methods used to retrieve the
     public key (plain-text; OPTIONAL, default is "dns/txt").  Each
     query method is of the form "type[/options]", where the syntax
     and semantics of the options depend on the type and specified
     options.  If there are multiple query mechanisms listed, the
     choice of query mechanism MUST NOT change the interpretation of
     the signature.  Implementations MUST use the recognized query
     mechanisms in the order presented.
 Currently, the only valid value is "dns/txt", which defines the DNS
     TXT record lookup algorithm described elsewhere in this document.
     The only option defined for the "dns" query type is "txt", which
     MUST be included.  Verifiers and signers MUST support "dns/txt".

Allman, et al. Standards Track [Page 22] RFC 4871 DKIM Signatures May 2007

 ABNF:
     sig-q-tag        = %x71 [FWS] "=" [FWS] sig-q-tag-method
                    *([FWS] ":" [FWS] sig-q-tag-method)
     sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
                    ["/" x-sig-q-tag-args]
     x-sig-q-tag-type = hyphenated-word  ; for future extension
     x-sig-q-tag-args = qp-hdr-value
 s=  The selector subdividing the namespace for the "d=" (domain) tag
     (plain-text; REQUIRED).
 ABNF:
     sig-s-tag    = %x73 [FWS] "=" [FWS] selector
 t=  Signature Timestamp (plain-text unsigned decimal integer;
     RECOMMENDED, default is an unknown creation time).  The time that
     this signature was created.  The format is the number of seconds
     since 00:00:00 on January 1, 1970 in the UTC time zone.  The
     value is expressed as an unsigned integer in decimal ASCII.  This
     value is not constrained to fit into a 31- or 32-bit integer.
     Implementations SHOULD be prepared to handle values up to at
     least 10^12 (until approximately AD 200,000; this fits into 40
     bits).  To avoid denial-of-service attacks, implementations MAY
     consider any value longer than 12 digits to be infinite.  Leap
     seconds are not counted.  Implementations MAY ignore signatures
     that have a timestamp in the future.
 ABNF:
     sig-t-tag    = %x74 [FWS] "=" [FWS] 1*12DIGIT
 x=  Signature Expiration (plain-text unsigned decimal integer;
     RECOMMENDED, default is no expiration).  The format is the same
     as in the "t=" tag, represented as an absolute date, not as a
     time delta from the signing timestamp.  The value is expressed as
     an unsigned integer in decimal ASCII, with the same constraints
     on the value in the "t=" tag.  Signatures MAY be considered
     invalid if the verification time at the verifier is past the
     expiration date.  The verification time should be the time that
     the message was first received at the administrative domain of
     the verifier if that time is reliably available; otherwise the
     current time should be used.  The value of the "x=" tag MUST be
     greater than the value of the "t=" tag if both are present.

Allman, et al. Standards Track [Page 23] RFC 4871 DKIM Signatures May 2007

     INFORMATIVE NOTE: The "x=" tag is not intended as an anti-replay
         defense.
 ABNF:
     sig-x-tag    = %x78 [FWS] "=" [FWS] 1*12DIGIT
 z=  Copied header fields (dkim-quoted-printable, but see description;
     OPTIONAL, default is null).  A vertical-bar-separated list of
     selected header fields present when the message was signed,
     including both the field name and value.  It is not required to
     include all header fields present at the time of signing.  This
     field need not contain the same header fields listed in the "h="
     tag.  The header field text itself must encode the vertical bar
     ("|", %x7C) character (i.e., vertical bars in the "z=" text are
     metacharacters, and any actual vertical bar characters in a
     copied header field must be encoded).  Note that all whitespace
     must be encoded, including whitespace between the colon and the
     header field value.  After encoding, FWS MAY be added at
     arbitrary locations in order to avoid excessively long lines;
     such whitespace is NOT part of the value of the header field, and
     MUST be removed before decoding.
 The header fields referenced by the "h=" tag refer to the fields in
     the RFC 2822 header of the message, not to any copied fields in
     the "z=" tag.  Copied header field values are for diagnostic use.
 Header fields with characters requiring conversion (perhaps from
     legacy MTAs that are not [RFC2822] compliant) SHOULD be converted
     as described in MIME Part Three [RFC2047].
 ABNF:
     sig-z-tag      = %x7A [FWS] "=" [FWS] sig-z-tag-copy
                  *( [FWS] "|" sig-z-tag-copy )
 sig-z-tag-copy = hdr-name ":" qp-hdr-value
 qp-hdr-value   = dkim-quoted-printable    ; with "|" encoded
    INFORMATIVE EXAMPLE of a signature header field spread across
    multiple continuation lines:

Allman, et al. Standards Track [Page 24] RFC 4871 DKIM Signatures May 2007

 DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane;
    c=simple; q=dns/txt; i=@eng.example.net;
    t=1117574938; x=1118006938;
    h=from:to:subject:date;
    z=From:foo@eng.example.net|To:joe@example.com|
      Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
    bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
    b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
             VoG4ZHRNiYzR

3.6. Key Management and Representation

 Signature applications require some level of assurance that the
 verification public key is associated with the claimed signer.  Many
 applications achieve this by using public key certificates issued by
 a trusted third party.  However, DKIM can achieve a sufficient level
 of security, with significantly enhanced scalability, by simply
 having the verifier query the purported signer's DNS entry (or some
 security-equivalent) in order to retrieve the public key.
 DKIM keys can potentially be stored in multiple types of key servers
 and in multiple formats.  The storage and format of keys are
 irrelevant to the remainder of the DKIM algorithm.
 Parameters to the key lookup algorithm are the type of the lookup
 (the "q=" tag), the domain of the signer (the "d=" tag of the DKIM-
 Signature header field), and the selector (the "s=" tag).
     public_key = dkim_find_key(q_val, d_val, s_val)
 This document defines a single binding, using DNS TXT records to
 distribute the keys.  Other bindings may be defined in the future.

3.6.1. Textual Representation

 It is expected that many key servers will choose to present the keys
 in an otherwise unstructured text format (for example, an XML form
 would not be considered to be unstructured text for this purpose).
 The following definition MUST be used for any DKIM key represented in
 an otherwise unstructured textual form.
 The overall syntax is a tag-list as described in Section 3.2.  The
 current valid tags are described below.  Other tags MAY be present
 and MUST be ignored by any implementation that does not understand
 them.

Allman, et al. Standards Track [Page 25] RFC 4871 DKIM Signatures May 2007

 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
     record.  Records beginning with a "v=" tag with any other value
     MUST be discarded.  Note that verifiers must do a string
     comparison on this value; for example, "DKIM1" is not the same as
     "DKIM1.0".
     ABNF:
     key-v-tag    = %x76 [FWS] "=" [FWS] "DKIM1"
 g=  Granularity of the key (plain-text; OPTIONAL, default is "*").
     This value MUST match the Local-part of the "i=" tag of the DKIM-
     Signature header field (or its default value of the empty string
     if "i=" is not specified), with a single, optional "*" character
     matching a sequence of zero or more arbitrary characters
     ("wildcarding").  An email with a signing address that does not
     match the value of this tag constitutes a failed verification.
     The intent of this tag is to constrain which signing address can
     legitimately use this selector, for example, when delegating a
     key to a third party that should only be used for special
     purposes.  Wildcarding allows matching for addresses such as
     "user+*" or "*-offer".  An empty "g=" value never matches any
     addresses.
 ABNF:
     key-g-tag       = %x67 [FWS] "=" [FWS] key-g-tag-lpart
     key-g-tag-lpart = [dot-atom-text] ["*" [dot-atom-text] ]
 h=  Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
     allowing all algorithms).  A colon-separated list of hash
     algorithms that might be used.  Signers and Verifiers MUST
     support the "sha256" hash algorithm.  Verifiers MUST also support
     the "sha1" hash algorithm.
     ABNF:
     key-h-tag       = %x68 [FWS] "=" [FWS] key-h-tag-alg
                   0*( [FWS] ":" [FWS] key-h-tag-alg )
     key-h-tag-alg   = "sha1" / "sha256" / x-key-h-tag-alg
     x-key-h-tag-alg = hyphenated-word   ; for future extension

Allman, et al. Standards Track [Page 26] RFC 4871 DKIM Signatures May 2007

 k=  Key type (plain-text; OPTIONAL, default is "rsa").  Signers and
     verifiers MUST support the "rsa" key type.  The "rsa" key type
     indicates that an ASN.1 DER-encoded [ITU.X660.1997] RSAPublicKey
     [RFC3447] (see Sections 3.1 and A.1.1) is being used in the "p="
     tag.  (Note: the "p=" tag further encodes the value using the
     base64 algorithm.)
     ABNF:
     key-k-tag        = %x76 [FWS] "=" [FWS] key-k-tag-type
     key-k-tag-type   = "rsa" / x-key-k-tag-type
     x-key-k-tag-type = hyphenated-word   ; for future extension
 n=  Notes that might be of interest to a human (qp-section; OPTIONAL,
     default is empty).  No interpretation is made by any program.
     This tag should be used sparingly in any key server mechanism
     that has space limitations (notably DNS).  This is intended for
     use by administrators, not end users.
 ABNF:
     key-n-tag    = %x6e [FWS] "=" [FWS] qp-section
 p=  Public-key data (base64; REQUIRED).  An empty value means that
     this public key has been revoked.  The syntax and semantics of
     this tag value before being encoded in base64 are defined by the
     "k=" tag.
         INFORMATIVE RATIONALE: If a private key has been compromised
         or otherwise disabled (e.g., an outsourcing contract has been
         terminated), a signer might want to explicitly state that it
         knows about the selector, but all messages using that
         selector should fail verification.  Verifiers should ignore
         any DKIM-Signature header fields with a selector referencing
         a revoked key.
 ABNF:
     key-p-tag    = %x70 [FWS] "=" [ [FWS] base64string ]
     INFORMATIVE NOTE: A base64string is permitted to include white
         space (FWS) at arbitrary places; however, any CRLFs must be
         followed by at least one WSP character.  Implementors and
         administrators are cautioned to ensure that selector TXT
         records conform to this specification.

Allman, et al. Standards Track [Page 27] RFC 4871 DKIM Signatures May 2007

 s=  Service Type (plain-text; OPTIONAL; default is "*").  A colon-
     separated list of service types to which this record applies.
     Verifiers for a given service type MUST ignore this record if the
     appropriate type is not listed.  Currently defined service types
     are as follows:
  • matches all service types
     email   electronic mail (not necessarily limited to SMTP)
     This tag is intended to constrain the use of keys for other
     purposes, should use of DKIM be defined by other services in the
     future.
 ABNF:
     key-s-tag        = %x73 [FWS] "=" [FWS] key-s-tag-type
                     0*( [FWS] ":" [FWS] key-s-tag-type )
     key-s-tag-type   = "email" / "*" / x-key-s-tag-type
     x-key-s-tag-type = hyphenated-word   ; for future extension
 t=  Flags, represented as a colon-separated list of names (plain-
     text; OPTIONAL, default is no flags set).  The defined flags are
     as follows:
     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.
     s   Any DKIM-Signature header fields using the "i=" tag MUST have
         the same domain value on the right-hand side of the "@" in
         the "i=" tag and the value of the "d=" tag.  That is, the
         "i=" domain MUST NOT be a subdomain of "d=".  Use of this
         flag is RECOMMENDED unless subdomaining is required.
 ABNF:
     key-t-tag        = %x74 [FWS] "=" [FWS] key-t-tag-flag
                    0*( [FWS] ":" [FWS] key-t-tag-flag )
     key-t-tag-flag   = "y" / "s" / x-key-t-tag-flag
     x-key-t-tag-flag = hyphenated-word   ; for future extension
 Unrecognized flags MUST be ignored.

Allman, et al. Standards Track [Page 28] RFC 4871 DKIM Signatures May 2007

3.6.2. DNS Binding

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

3.6.2.1. Namespace

 All DKIM keys are stored in a subdomain named "_domainkey".  Given a
 DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag
 of "foo.bar", the DNS query will be for
 "foo.bar._domainkey.example.com".
    INFORMATIVE OPERATIONAL NOTE: Wildcard DNS records (e.g.,
    *.bar._domainkey.example.com) do not make sense in this context
    and should not be used.  Note also that wildcards within domains
    (e.g., s._domainkey.*.example.com) are not supported by the DNS.

3.6.2.2. Resource Record Types for Key Storage

 The DNS Resource Record type used is specified by an option to the
 query-type ("q=") tag.  The only option defined in this base
 specification is "txt", indicating the use of a TXT Resource Record
 (RR).  A later extension of this standard may define another RR type.
 Strings in a TXT RR MUST be concatenated together before use with no
 intervening whitespace.  TXT RRs MUST be unique for a particular
 selector name; that is, if there are multiple records in an RRset,
 the results are undefined.
 TXT RRs are encoded as described in Section 3.6.1.

3.7. Computing the Message Hashes

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

Allman, et al. Standards Track [Page 29] RFC 4871 DKIM Signatures May 2007

 Signers MUST compute them in the order shown.  Verifiers MAY compute
 them in any order convenient to the verifier, provided that the
 result is semantically identical to the semantics that would be the
 case had they been computed in this order.
 In hash step 1, the signer/verifier MUST hash the message body,
 canonicalized using the body canonicalization algorithm specified in
 the "c=" tag and then truncated to the length specified in the "l="
 tag.  That hash value is then converted to base64 form and inserted
 into (signers) or compared to (verifiers) the "bh=" tag of the DKIM-
 Signature header field.
 In hash step 2, the signer/verifier MUST pass the following to the
 hash algorithm in the indicated order.
 1.  The header fields specified by the "h=" tag, in the order
     specified in that tag, and canonicalized using the header
     canonicalization algorithm specified in the "c=" tag.  Each
     header field MUST be terminated with a single CRLF.
 2.  The DKIM-Signature header field that exists (verifying) or will
     be inserted (signing) in the message, with the value of the "b="
     tag deleted (i.e., treated as the empty string), canonicalized
     using the header canonicalization algorithm specified in the "c="
     tag, and without a trailing CRLF.
 All tags and their values in the DKIM-Signature header field are
 included in the cryptographic hash with the sole exception of the
 value portion of the "b=" (signature) tag, which MUST be treated as
 the null string.  All tags MUST be included even if they might not be
 understood by the verifier.  The header field MUST be presented to
 the hash algorithm after the body of the message rather than with the
 rest of the header fields and MUST be canonicalized as specified in
 the "c=" (canonicalization) tag.  The DKIM-Signature header field
 MUST NOT be included in its own h= tag, although other DKIM-Signature
 header fields MAY be signed (see Section 4).
 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" (the modification of lines
 beginning with a "." to avoid confusion with the SMTP end-of-message
 marker, as specified in [RFC2821]).
 With the exception of the canonicalization procedure described in
 Section 3.4, the DKIM signing process treats the body of messages as

Allman, et al. Standards Track [Page 30] RFC 4871 DKIM Signatures May 2007

 simply a string of octets.  DKIM messages MAY be either in plain-text
 or in MIME format; no special treatment is afforded to MIME content.
 Message attachments in MIME format MUST be included in the content
 that is signed.
 More formally, the algorithm for the signature is as follows:
     body-hash = hash-alg(canon_body)
     header-hash = hash-alg(canon_header || DKIM-SIG)
     signature = sig-alg(header-hash, key)
 where "sig-alg" is the signature algorithm specified by the "a=" tag,
 "hash-alg" is the hash algorithm specified by the "a=" tag,
 "canon_header" and "canon_body" are the canonicalized message header
 and body (respectively) as defined in Section 3.4 (excluding the
 DKIM-Signature header field), and "DKIM-SIG" is the canonicalized
 DKIM-Signature header field sans the signature value itself, but with
 "body-hash" included as the "bh=" tag.
    INFORMATIVE IMPLEMENTERS' NOTE: Many digital signature APIs
    provide both hashing and application of the RSA private key using
    a single "sign()" primitive.  When using such an API, the last two
    steps in the algorithm would probably be combined into a single
    call that would perform both the "hash-alg" and the "sig-alg".

3.8. Signing by Parent Domains

 In some circumstances, it is desirable for a domain to apply a
 signature on behalf of any of its subdomains without the need to
 maintain separate selectors (key records) in each subdomain.  By
 default, private keys corresponding to key records can be used to
 sign messages for any subdomain of the domain in which they reside;
 e.g., a key record for the domain example.com can be used to verify
 messages where the signing identity ("i=" tag of the signature) is
 sub.example.com, or even sub1.sub2.example.com.  In order to limit
 the capability of such keys when this is not intended, the "s" flag
 may be set in the "t=" tag of the key record to constrain the
 validity of the record to exactly the domain of the signing identity.
 If the referenced key record contains the "s" flag as part of the
 "t=" tag, the domain of the signing identity ("i=" flag) MUST be the
 same as that of the d= domain.  If this flag is absent, the domain of
 the signing identity MUST be the same as, or a subdomain of, the d=
 domain.  Key records that are not intended for use with subdomains
 SHOULD specify the "s" flag in the "t=" tag.

Allman, et al. Standards Track [Page 31] RFC 4871 DKIM Signatures May 2007

4. Semantics of Multiple Signatures

4.1. Example Scenarios

 There are many reasons why a message might have multiple signatures.
 For example, a given signer might sign multiple times, perhaps with
 different hashing or signing algorithms during a transition phase.
    INFORMATIVE EXAMPLE: Suppose SHA-256 is in the future found to be
    insufficiently strong, and DKIM usage transitions to SHA-1024.  A
    signer might immediately sign using the newer algorithm, but
    continue to sign using the older algorithm for interoperability
    with verifiers that had not yet upgraded.  The signer would do
    this by adding two DKIM-Signature header fields, one using each
    algorithm.  Older verifiers that did not recognize SHA-1024 as an
    acceptable algorithm would skip that signature and use the older
    algorithm; newer verifiers could use either signature at their
    option, and all other things being equal might not even attempt to
    verify the other signature.
 Similarly, a signer might sign a message including all headers and no
 "l=" tag (to satisfy strict verifiers) and a second time with a
 limited set of headers and an "l=" tag (in anticipation of possible
 message modifications in route to other verifiers).  Verifiers could
 then choose which signature they preferred.
    INFORMATIVE EXAMPLE: A verifier might receive a message with two
    signatures, one covering more of the message than the other.  If
    the signature covering more of the message verified, then the
    verifier could make one set of policy decisions; if that signature
    failed but the signature covering less of the message verified,
    the verifier might make a different set of policy decisions.
 Of course, a message might also have multiple signatures because it
 passed through multiple signers.  A common case is expected to be
 that of a signed message that passes through a mailing list that also
 signs all messages.  Assuming both of those signatures verify, a
 recipient might choose to accept the message if either of those
 signatures were known to come from trusted sources.
    INFORMATIVE EXAMPLE: Recipients might choose to whitelist mailing
    lists to which they have subscribed and that have acceptable anti-
    abuse policies so as to accept messages sent to that list even
    from unknown authors.  They might also subscribe to less trusted
    mailing lists (e.g., those without anti-abuse protection) and be
    willing to accept all messages from specific authors, but insist
    on doing additional abuse scanning for other messages.

Allman, et al. Standards Track [Page 32] RFC 4871 DKIM Signatures May 2007

 Another related example of multiple signers might be forwarding
 services, such as those commonly associated with academic alumni
 sites.
    INFORMATIVE EXAMPLE: A recipient might have an address at
    members.example.org, a site that has anti-abuse protection that is
    somewhat less effective than the recipient would prefer.  Such a
    recipient might have specific authors whose messages would be
    trusted absolutely, but messages from unknown authors that had
    passed the forwarder's scrutiny would have only medium trust.

4.2. Interpretation

 A signer that is adding a signature to a message merely creates a new
 DKIM-Signature header, using the usual semantics of the h= option.  A
 signer MAY sign previously existing DKIM-Signature header fields
 using the method described in Section 5.4 to sign trace header
 fields.
    INFORMATIVE NOTE: Signers should be cognizant that signing DKIM-
    Signature header fields may result in signature failures with
    intermediaries that do not recognize that DKIM-Signature header
    fields are trace header fields and unwittingly reorder them, thus
    breaking such signatures.  For this reason, signing existing DKIM-
    Signature header fields is unadvised, albeit legal.
    INFORMATIVE NOTE: If a header field with multiple instances is
    signed, those header fields are always signed from the bottom up.
    Thus, it is not possible to sign only specific DKIM-Signature
    header fields.  For example, if the message being signed already
    contains three DKIM-Signature header fields A, B, and C, it is
    possible to sign all of them, B and C only, or C only, but not A
    only, B only, A and B only, or A and C only.
 A signer MAY add more than one DKIM-Signature header field using
 different parameters.  For example, during a transition period a
 signer might want to produce signatures using two different hash
 algorithms.
 Signers SHOULD NOT remove any DKIM-Signature header fields from
 messages they are signing, even if they know that the signatures
 cannot be verified.
 When evaluating a message with multiple signatures, a verifier SHOULD
 evaluate signatures independently and on their own merits.  For
 example, a verifier that by policy chooses not to accept signatures
 with deprecated cryptographic algorithms would consider such
 signatures invalid.  Verifiers MAY process signatures in any order of

Allman, et al. Standards Track [Page 33] RFC 4871 DKIM Signatures May 2007

 their choice; for example, some verifiers might choose to process
 signatures corresponding to the From field in the message header
 before other signatures.  See Section 6.1 for more information about
 signature choices.
    INFORMATIVE IMPLEMENTATION NOTE: Verifier attempts to correlate
    valid signatures with invalid signatures in an attempt to guess
    why a signature failed are ill-advised.  In particular, there is
    no general way that a verifier can determine that an invalid
    signature was ever valid.
 Verifiers SHOULD ignore failed signatures as though they were not
 present in the message.  Verifiers SHOULD continue to check
 signatures until a signature successfully verifies to the
 satisfaction of the verifier.  To limit potential denial-of-service
 attacks, verifiers MAY limit the total number of signatures they will
 attempt to verify.

5. Signer Actions

 The following steps are performed in order by signers.

5.1. Determine Whether 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.
    INFORMATIVE NOTE: Signing modules may be incorporated into any
    portion of the mail system as deemed appropriate, including an
    MUA, a SUBMISSION server, or an MTA.  Wherever implemented,
    signers should beware of signing (and thereby asserting
    responsibility for) messages that may be problematic.  In
    particular, within a trusted enclave the signing address might be
    derived from the header according to local policy; SUBMISSION
    servers might only sign messages from users that are properly
    authenticated and authorized.
    INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not sign
    Received header fields if the outgoing gateway MTA obfuscates
    Received header fields, for example, to hide the details of
    internal topology.
 If an email cannot be signed for some reason, it is a local policy
 decision as to what to do with that email.

Allman, et al. Standards Track [Page 34] RFC 4871 DKIM Signatures May 2007

5.2. Select a Private Key and Corresponding Selector Information

 This specification does not define the basis by which a signer should
 choose which private key and selector information to use.  Currently,
 all selectors are equal as far as this specification is concerned, so
 the decision should largely be a matter of administrative
 convenience.  Distribution and management of private keys is also
 outside the scope of this document.
    INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
    private key when the selector containing the corresponding public
    key is expected to be revoked or removed before the verifier has
    an opportunity to validate the signature.  The signer should
    anticipate that verifiers may choose to defer validation, perhaps
    until the message is actually read by the final recipient.  In
    particular, when rotating to a new key pair, signing should
    immediately commence with the new private key and the old public
    key should be retained for a reasonable validation interval before
    being removed from the key server.

5.3. Normalize the Message to Prevent Transport Conversions

 Some messages, particularly those using 8-bit characters, are subject
 to modification during transit, notably conversion to 7-bit form.
 Such conversions will break DKIM signatures.  In order to minimize
 the chances of such breakage, signers SHOULD convert the message to a
 suitable MIME content transfer encoding such as quoted-printable or
 base64 as described in MIME Part One [RFC2045] before signing.  Such
 conversion is outside the scope of DKIM; the actual message SHOULD be
 converted to 7-bit MIME by an MUA or MSA prior to presentation to the
 DKIM algorithm.
 If the message is submitted to the signer with any local encoding
 that will be modified before transmission, that modification to
 canonical [RFC2822] form MUST be done before signing.  In particular,
 bare CR or LF characters (used by some systems as a local line
 separator convention) MUST be converted to the SMTP-standard CRLF
 sequence before the message is signed.  Any conversion of this sort
 SHOULD be applied to the message actually sent to the recipient(s),
 not just to the version presented to the signing algorithm.
 More generally, the signer MUST sign the message as it is expected to
 be received by the verifier rather than in some local or internal
 form.

Allman, et al. Standards Track [Page 35] RFC 4871 DKIM Signatures May 2007

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).  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.
    INFORMATIVE OPERATIONS NOTE: The choice of which header fields to
    sign is non-obvious.  One strategy is to sign all existing, non-
    repeatable header fields.  An alternative strategy is to sign only
    header fields that are likely to be displayed to or otherwise be
    likely to affect the processing of the message at the receiver.  A
    third strategy is to sign only "well known" headers.  Note that
    verifiers may treat unsigned header fields with extreme
    skepticism, including refusing to display them to the end user or
    even ignoring the signature if it does not cover certain header
    fields.  For this reason, signing fields present in the message
    such as Date, Subject, Reply-To, Sender, and all MIME header
    fields are highly advised.
 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 MAY claim to have signed header fields that do not exist
 (that is, signers MAY include the header field name in the "h=" tag
 even if that header field does not exist in the message).  When
 computing the signature, the non-existing header field MUST be
 treated as the null string (including the header field name, header
 field value, all punctuation, and the trailing CRLF).
    INFORMATIVE RATIONALE: This allows signers to explicitly assert
    the absence of a header field; if that header field is added later
    the signature will fail.
    INFORMATIVE NOTE: A header field name need only be listed once
    more than the actual number of that header field in a message at
    the time of signing in order to prevent any further additions.
    For example, if there is a single Comments header field at the
    time of signing, listing Comments twice in the "h=" tag is
    sufficient to prevent any number of Comments header fields from
    being appended; it is not necessary (but is legal) to list
    Comments three or more times in the "h=" tag.

Allman, et al. Standards Track [Page 36] RFC 4871 DKIM Signatures May 2007

 Signers choosing to sign an existing header field that occurs more
 than once in the message (such as Received) MUST sign the physically
 last instance of that header field in the header block.  Signers
 wishing to sign multiple instances of such a header field MUST
 include the header field name multiple times in the h= tag of the
 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 instances of a header field name in h= than
 there are actual corresponding header fields to indicate that
 additional header fields of that name SHOULD NOT be added.
    INFORMATIVE EXAMPLE:
    If the signer wishes to sign two existing Received header fields,
    and the existing header contains:
     Received: <A>
     Received: <B>
     Received: <C>
    then the resulting DKIM-Signature header field should read:
     DKIM-Signature: ... h=Received : Received : ...
    and Received header fields <C> and <B> will be signed in that
    order.
 Signers should be careful of signing header fields that might have
 additional instances added later in the delivery process, since such
 header fields might be inserted after the signed instance or
 otherwise reordered.  Trace header fields (such as Received) and
 Resent-* blocks are the only fields prohibited by [RFC2822] from
 being reordered.  In particular, since DKIM-Signature header fields
 may be reordered by some intermediate MTAs, signing existing DKIM-
 Signature header fields is error-prone.
    INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
    header fields to be reordered (with the exception of Received
    header fields), reordering of signed header fields with multiple
    instances by intermediate MTAs will cause 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.

Allman, et al. Standards Track [Page 37] RFC 4871 DKIM Signatures May 2007

5.5. Recommended Signature Content

 In order to maximize compatibility with a variety of verifiers, it is
 recommended that signers follow the practices outlined in this
 section when signing a message.  However, these are generic
 recommendations applying to the general case; specific senders may
 wish to modify these guidelines as required by their unique
 situations.  Verifiers MUST be capable of verifying signatures even
 if one or more of the recommended header fields is not signed (with
 the exception of From, which must always be signed) or if one or more
 of the disrecommended header fields is signed.  Note that verifiers
 do have the option of ignoring signatures that do not cover a
 sufficient portion of the header or body, just as they may ignore
 signatures from an identity they do not trust.
 The following header fields SHOULD be included in the signature, if
 they are present in the message being signed:
 o  From (REQUIRED in all signatures)
 o  Sender, Reply-To
 o  Subject
 o  Date, Message-ID
 o  To, Cc
 o  MIME-Version
 o  Content-Type, Content-Transfer-Encoding, Content-ID, Content-
    Description
 o  Resent-Date, Resent-From, Resent-Sender, Resent-To, Resent-Cc,
    Resent-Message-ID
 o  In-Reply-To, References
 o  List-Id, List-Help, List-Unsubscribe, List-Subscribe, List-Post,
    List-Owner, List-Archive
 The following header fields SHOULD NOT be included in the signature:
 o  Return-Path
 o  Received
 o  Comments, Keywords

Allman, et al. Standards Track [Page 38] RFC 4871 DKIM Signatures May 2007

 o  Bcc, Resent-Bcc
 o  DKIM-Signature
 Optional header fields (those not mentioned above) normally SHOULD
 NOT be included in the signature, because of the potential for
 additional header fields of the same name to be legitimately added or
 reordered prior to verification.  There are likely to be legitimate
 exceptions to this rule, because of the wide variety of application-
 specific header fields that may be applied to a message, some of
 which are unlikely to be duplicated, modified, or reordered.
 Signers SHOULD choose canonicalization algorithms based on the types
 of messages they process and their aversion to risk.  For example,
 e-commerce sites sending primarily purchase receipts, which are not
 expected to be processed by mailing lists or other software likely to
 modify messages, will generally prefer "simple" canonicalization.
 Sites sending primarily person-to-person email will likely prefer to
 be more resilient to modification during transport by using "relaxed"
 canonicalization.
 Signers SHOULD NOT use "l=" unless they intend to accommodate
 intermediate mail processors that append text to a message.  For
 example, many mailing list processors append "unsubscribe"
 information to message bodies.  If signers use "l=", they SHOULD
 include the entire message body existing at the time of signing in
 computing the count.  In particular, signers SHOULD NOT specify a
 body length of 0 since this may be interpreted as a meaningless
 signature by some verifiers.

5.6. Compute the Message Hash and Signature

 The signer MUST compute the message hash as described in Section 3.7
 and then sign it using the selected public-key algorithm.  This will
 result in a DKIM-Signature header field that will include the body
 hash and a signature of the header hash, where that header includes
 the DKIM-Signature header field itself.
 Entities such as mailing list managers that implement DKIM and that
 modify the message or a header field (for example, inserting
 unsubscribe information) before retransmitting the message SHOULD
 check any existing signature on input and MUST make such
 modifications before re-signing the message.
 The signer MAY elect to limit the number of bytes of the body that
 will be included in the hash and hence signed.  The length actually
 hashed should be inserted in the "l=" tag of the DKIM-Signature
 header field.

Allman, et al. Standards Track [Page 39] RFC 4871 DKIM Signatures May 2007

5.7. Insert the DKIM-Signature Header Field

 Finally, the signer MUST insert the DKIM-Signature header field
 created in the previous step prior to transmitting the email.  The
 DKIM-Signature header field MUST be the same as used to compute the
 hash as described above, except that the value of the "b=" tag MUST
 be the appropriately signed hash computed in the previous step,
 signed using the algorithm specified in the "a=" tag of the DKIM-
 Signature header field and using the private key corresponding to the
 selector given in the "s=" tag of the DKIM-Signature header field, as
 chosen above in Section 5.2
 The DKIM-Signature header field MUST be inserted before any other
 DKIM-Signature fields in the header block.
    INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this
    is to insert the DKIM-Signature header field at the beginning of
    the header block.  In particular, it may be placed before any
    existing Received header fields.  This is consistent with treating
    DKIM-Signature as a trace header field.

6. Verifier Actions

 Since a signer MAY remove or revoke a public key at any time, it is
 recommended that verification occur in a timely manner.  In many
 configurations, the most timely place is during acceptance by the
 border MTA or shortly thereafter.  In particular, deferring
 verification until the message is accessed by the end user is
 discouraged.
 A border or intermediate MTA MAY verify the message signature(s).  An
 MTA who has performed verification MAY communicate the result of that
 verification by adding 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 MUST produce a result that is semantically equivalent to
 applying the following steps in the order listed.  In practice,
 several of these steps can be performed in parallel in order to
 improve performance.

Allman, et al. Standards Track [Page 40] RFC 4871 DKIM Signatures May 2007

6.1. Extract Signatures from the Message

 The order in which verifiers try DKIM-Signature header fields is not
 defined; verifiers MAY try signatures in any order they like.  For
 example, one implementation might try the signatures in textual
 order, whereas another might try signatures by identities that match
 the contents of the From header field before trying other signatures.
 Verifiers MUST NOT attribute ultimate meaning to the order of
 multiple DKIM-Signature header fields.  In particular, there is
 reason to believe that some relays will reorder the header fields in
 potentially arbitrary ways.
    INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as
    a clue to signing order in the absence of any other information.
    However, other clues as to the semantics of multiple signatures
    (such as correlating the signing host with Received header fields)
    may also be considered.
 A verifier SHOULD NOT treat a message that has one or more bad
 signatures and no good signatures differently from a message with no
 signature at all; such treatment is a matter of local policy and is
 beyond the scope of this document.
 When a signature successfully verifies, a verifier will either stop
 processing or attempt to verify any other signatures, at the
 discretion of the implementation.  A verifier MAY limit the number of
 signatures it tries to avoid denial-of-service attacks.
    INFORMATIVE NOTE: An attacker could send messages with large
    numbers of faulty signatures, each of which would require a DNS
    lookup and corresponding CPU time to verify the message.  This
    could be an attack on the domain that receives the message, by
    slowing down the verifier by requiring it to do a large number of
    DNS lookups and/or signature verifications.  It could also be an
    attack against the domains listed in the signatures, essentially
    by enlisting innocent verifiers in launching an attack against the
    DNS servers of the actual victim.
 In the following description, text reading "return status
 (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
 means that the verifier MUST immediately cease processing that
 signature.  The verifier SHOULD proceed to the next signature, if any
 is present, and completely ignore the bad signature.  If the status
 is "PERMFAIL", the signature failed and should not be reconsidered.
 If the status is "TEMPFAIL", the signature could not be verified at
 this time but may be tried again later.  A verifier MAY either defer
 the message for later processing, perhaps by queueing it locally or
 issuing a 451/4.7.5 SMTP reply, or try another signature; if no good

Allman, et al. Standards Track [Page 41] RFC 4871 DKIM Signatures May 2007

 signature is found and any of the signatures resulted in a TEMPFAIL
 status, the verifier MAY save the message for later processing.  The
 "(explanation)" is not normative text; it is provided solely for
 clarification.
 Verifiers SHOULD ignore any 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.
 For each signature to be validated, the following steps should be
 performed in such a manner as to produce a result that is
 semantically equivalent to performing them in the indicated order.

6.1.1. Validate the Signature Header Field

 Implementers MUST meticulously validate the format and values in the
 DKIM-Signature header field; any inconsistency or unexpected values
 MUST cause the header field to be completely ignored and the verifier
 to return PERMFAIL (signature syntax error).  Being "liberal in what
 you accept" is definitely a bad strategy in this security context.
 Note however that this does not include the existence of unknown tags
 in a DKIM-Signature header field, which are explicitly permitted.
 Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag
 that is inconsistent with this specification and return PERMFAIL
 (incompatible version).
    INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of course,
    choose to also verify signatures generated by older versions of
    this specification.
 If any tag listed as "required" in Section 3.5 is omitted from the
 DKIM-Signature header field, the verifier MUST ignore the DKIM-
 Signature header field and return PERMFAIL (signature missing
 required tag).

Allman, et al. Standards Track [Page 42] RFC 4871 DKIM Signatures May 2007

    INFORMATIONAL NOTE: The tags listed as required in Section 3.5 are
    "v=", "a=", "b=", "bh=", "d=", "h=", and "s=".  Should there be a
    conflict between this note and Section 3.5, Section 3.5 is
    normative.
 If the DKIM-Signature header field does not contain the "i=" tag, the
 verifier MUST behave as though the value of that tag were "@d", where
 "d" is the value from the "d=" tag.
 Verifiers MUST confirm that the domain specified in the "d=" tag is
 the same as or a parent domain of the domain part of the "i=" tag.
 If not, the DKIM-Signature header field MUST be ignored and the
 verifier should return PERMFAIL (domain mismatch).
 If the "h=" tag does not include the From header field, the verifier
 MUST ignore the DKIM-Signature header field and return PERMFAIL (From
 field not signed).
 Verifiers MAY ignore the DKIM-Signature header field and return
 PERMFAIL (signature expired) if it contains an "x=" tag and the
 signature has expired.
 Verifiers MAY ignore the DKIM-Signature header field if the domain
 used by the signer in the "d=" tag is not associated with a valid
 signing entity.  For example, signatures with "d=" values such as
 "com" and "co.uk" may be ignored.  The list of unacceptable domains
 SHOULD be configurable.
 Verifiers MAY ignore the DKIM-Signature header field and return
 PERMFAIL (unacceptable signature header) for any other reason, for
 example, if the signature does not sign header fields that the
 verifier views to be essential.  As a case in point, if MIME header
 fields are not signed, certain attacks may be possible that the
 verifier would prefer to avoid.

6.1.2. Get the Public Key

 The public key for a signature is needed to complete the verification
 process.  The process of retrieving the public key depends on the
 query type as defined by the "q=" tag in the DKIM-Signature header
 field.  Obviously, a public key need only be retrieved if the process
 of extracting the signature information is completely successful.
 Details of key management and representation are described in
 Section 3.6.  The verifier MUST validate the key record and MUST
 ignore any public key records that are malformed.
 When validating a message, a verifier MUST perform the following
 steps in a manner that is semantically the same as performing them in

Allman, et al. Standards Track [Page 43] RFC 4871 DKIM Signatures May 2007

 the order indicated (in some cases, the implementation may
 parallelize or reorder these steps, as long as the semantics remain
 unchanged):
 1.  Retrieve the public key as described in Section 3.6 using the
     algorithm in the "q=" tag, 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
     MAY defer acceptance of this email and return TEMPFAIL (key
     unavailable).  If verification is occurring during the incoming
     SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
     code.  Alternatively, the verifier MAY store the message in the
     local queue for later trial or ignore the signature.  Note that
     storing a message in the local queue is subject to denial-of-
     service attacks.
 3.  If the query for the public key fails because the corresponding
     key record does not exist, the verifier MUST immediately return
     PERMFAIL (no key for signature).
 4.  If the query for the public key returns multiple key records, the
     verifier may choose one of the key records or may cycle through
     the key records performing the remainder of these steps on each
     record at the discretion of the implementer.  The order of the
     key records is unspecified.  If the verifier chooses to cycle
     through the key records, then the "return ..." wording in the
     remainder of this section means "try the next key record, if any;
     if none, return to try another signature in the usual way".
 5.  If the result returned from the query does not adhere to the
     format defined in this specification, the verifier MUST ignore
     the key record and return PERMFAIL (key syntax error).  Verifiers
     are urged to validate the syntax of key records carefully to
     avoid attempted attacks.  In particular, the verifier MUST ignore
     keys with a version code ("v=" tag) that they do not implement.
 6.  If the "g=" tag in the public key does not match the Local-part
     of the "i=" tag in the message signature header field, the
     verifier MUST ignore the key record and return PERMFAIL
     (inapplicable key).  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 the entire g= tag must be
     omitted (which defaults to "g=*"), otherwise the verifier MUST
     ignore the key record and return PERMFAIL (inapplicable key).
     Other than this test, verifiers SHOULD NOT treat a message signed
     with a key record having a "g=" tag any differently than one
     without; in particular, verifiers SHOULD NOT prefer messages that

Allman, et al. Standards Track [Page 44] RFC 4871 DKIM Signatures May 2007

     seem to have an individual signature by virtue of a "g=" tag
     versus a domain signature.
 7.  If the "h=" tag exists in the public key record and the hash
     algorithm implied by the a= tag in the DKIM-Signature header
     field is not included in the contents of the "h=" tag, the
     verifier MUST ignore the key record and return PERMFAIL
     (inappropriate hash algorithm).
 8.  If the public key data (the "p=" tag) is empty, then this key has
     been revoked and the verifier MUST treat this as a failed
     signature check and return PERMFAIL (key revoked).  There is no
     defined semantic difference between a key that has been revoked
     and a key record that has been removed.
 9.  If the public key data is not suitable for use with the algorithm
     and key types defined by the "a=" and "k=" tags in the DKIM-
     Signature header field, the verifier MUST immediately return
     PERMFAIL (inappropriate key algorithm).

6.1.3. Compute the Verification

 Given a signer and a public key, verifying a signature consists of
 actions semantically equivalent to the following steps.
 1.  Based on the algorithm defined in the "c=" tag, the body length
     specified in the "l=" tag, and the header field names in the "h="
     tag, prepare a canonicalized version of the message as is
     described in Section 3.7 (note that this version does not
     actually need to be instantiated).  When matching header field
     names in the "h=" tag against the actual message header field,
     comparisons MUST be case-insensitive.
 2.  Based on the algorithm indicated in the "a=" tag, compute the
     message hashes from the canonical copy as described in
     Section 3.7.
 3.  Verify that the hash of the canonicalized message body computed
     in the previous step matches the hash value conveyed in the "bh="
     tag.  If the hash does not match, the verifier SHOULD ignore the
     signature and return PERMFAIL (body hash did not verify).
 4.  Using the signature conveyed in the "b=" tag, verify the
     signature against the header hash using the mechanism appropriate
     for the public key algorithm described in the "a=" tag.  If the
     signature does not validate, the verifier SHOULD ignore the
     signature and return PERMFAIL (signature did not verify).

Allman, et al. Standards Track [Page 45] RFC 4871 DKIM Signatures May 2007

 5.  Otherwise, the signature has correctly verified.
    INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to
    initiate the public-key query in parallel with calculating the
    hash as the public key is not needed until the final decryption is
    calculated.  Implementations may also verify the signature on the
    message header before validating that the message hash listed in
    the "bh=" tag in the DKIM-Signature header field matches that of
    the actual message body; however, if the body hash does not match,
    the entire signature must be considered to have failed.
 A body length specified in the "l=" tag of the signature limits the
 number of bytes of the body passed to the verification algorithm.
 All data beyond that limit is not validated by DKIM.  Hence,
 verifiers might treat a message that contains bytes beyond the
 indicated body length with suspicion, such as by truncating the
 message at the indicated body length, declaring the signature invalid
 (e.g., by returning PERMFAIL (unsigned content)), or conveying the
 partial verification to the policy module.
    INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
    at the indicated body length might pass on a malformed MIME
    message if the signer used the "N-4" trick (omitting the final
    "--CRLF") described in the informative note in Section 3.4.5.
    Such verifiers may wish to check for this case and include a
    trailing "--CRLF" to avoid breaking the MIME structure.  A simple
    way to achieve this might be to append "--CRLF" to any "multipart"
    message with a body length; if the MIME structure is already
    correctly formed, this will appear in the postlude and will not be
    displayed to the end user.

6.2. Communicate Verification Results

 Verifiers wishing to communicate the results of verification to other
 parts of the mail system may do so in whatever manner they see fit.
 For example, implementations might choose to add an email header
 field to the message before passing it on.  Any such header field
 SHOULD be inserted before any existing DKIM-Signature or preexisting
 authentication status header fields in the header field block.
    INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
    search for results header fields to visibly mark authenticated
    mail for end users should verify that such header field was added
    by the appropriate verifying domain and that the verified identity
    matches the author identity that will be displayed by the MUA.  In
    particular, MUA filters should not be influenced by bogus results

Allman, et al. Standards Track [Page 46] RFC 4871 DKIM Signatures May 2007

    header fields added by attackers.  To circumvent this attack,
    verifiers may wish to delete existing results header fields after
    verification and before adding a new header field.

6.3. Interpret Results/Apply Local Policy

 It is beyond the scope of this specification to describe what actions
 a verifier system should make, but an authenticated email presents an
 opportunity to a receiving system that unauthenticated email cannot.
 Specifically, an authenticated email creates a predictable identifier
 by which other decisions can reliably be managed, such as trust and
 reputation.  Conversely, unauthenticated email lacks a reliable
 identifier that can be used to assign trust and reputation.  It is
 reasonable to treat unauthenticated email as lacking any trust and
 having no positive reputation.
 In general, verifiers SHOULD NOT reject messages solely on the basis
 of a lack of signature or an unverifiable signature; such rejection
 would cause severe interoperability problems.  However, if the
 verifier does opt to reject such messages (for example, when
 communicating with a peer who, by prior agreement, agrees to only
 send signed messages), and the verifier runs synchronously with the
 SMTP session and a signature is missing or does not verify, the MTA
 SHOULD use a 550/5.7.x reply code.
 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 using a 451/4.7.5 reply code, such as:
    451 4.7.5 Unable to verify signature - key server unavailable
 Temporary failures such as inability to access the key server or
 other external service are the only conditions that SHOULD use a 4xx
 SMTP reply code.  In particular, cryptographic signature verification
 failures MUST NOT return 4xx SMTP replies.
 Once the signature has been verified, that information MUST be
 conveyed to higher-level systems (such as explicit allow/whitelists
 and reputation systems) and/or to the end user.  If the message is
 signed on behalf of any address other than that in the From: header
 field, the mail system SHOULD take pains to ensure that the actual
 signing identity is clear to the reader.
 The verifier MAY treat unsigned header fields with extreme
 skepticism, including marking them as untrusted or even deleting them
 before display to the end user.

Allman, et al. Standards Track [Page 47] RFC 4871 DKIM Signatures May 2007

 While the symptoms of a failed verification are obvious -- the
 signature doesn't verify -- establishing the exact cause can be more
 difficult.  If a selector cannot be found, is that because the
 selector has been removed, or was the value changed somehow in
 transit?  If the signature line is missing, is that because it was
 never there, or was it removed by an overzealous filter?  For
 diagnostic purposes, the exact reason why the verification fails
 SHOULD be made available to the policy module and possibly recorded
 in the system logs.  If the email cannot be verified, then it SHOULD
 be rendered the same as all unverified email regardless of whether or
 not it looks like it was signed.

7. IANA Considerations

 DKIM introduces some new namespaces that have been registered with
 IANA.  In all cases, new values are assigned only for values that
 have been documented in a published RFC that has IETF Consensus
 [RFC2434].

7.1. DKIM-Signature Tag Specifications

 A DKIM-Signature provides for a list of tag specifications.  IANA has
 established the DKIM-Signature Tag Specification Registry for tag
 specifications that can be used in DKIM-Signature fields.
             The initial entries in the registry comprise:
                      +------+-----------------+
                      | TYPE | REFERENCE       |
                      +------+-----------------+
                      | v    | (this document) |
                      | a    | (this document) |
                      | b    | (this document) |
                      | bh   | (this document) |
                      | c    | (this document) |
                      | d    | (this document) |
                      | h    | (this document) |
                      | i    | (this document) |
                      | l    | (this document) |
                      | q    | (this document) |
                      | s    | (this document) |
                      | t    | (this document) |
                      | x    | (this document) |
                      | z    | (this document) |
                      +------+-----------------+
       DKIM-Signature Tag Specification Registry Initial Values

Allman, et al. Standards Track [Page 48] RFC 4871 DKIM Signatures May 2007

7.2. DKIM-Signature Query Method Registry

 The "q=" tag-spec (specified in Section 3.5) provides for a list of
 query methods.
 IANA has established the DKIM-Signature Query Method Registry for
 mechanisms that can be used to retrieve the key that will permit
 validation processing of a message signed using DKIM.
             The initial entry in the registry comprises:
                  +------+--------+-----------------+
                  | TYPE | OPTION | REFERENCE       |
                  +------+--------+-----------------+
                  | dns  | txt    | (this document) |
                  +------+--------+-----------------+
          DKIM-Signature Query Method Registry Initial Values

7.3. DKIM-Signature Canonicalization Registry

 The "c=" tag-spec (specified in Section 3.5) provides for a specifier
 for canonicalization algorithms for the header and body of the
 message.
 IANA has established the DKIM-Signature Canonicalization Algorithm
 Registry for algorithms for converting a message into a canonical
 form before signing or verifying using DKIM.
         The initial entries in the header registry comprise:
                     +---------+-----------------+
                     | TYPE    | REFERENCE       |
                     +---------+-----------------+
                     | simple  | (this document) |
                     | relaxed | (this document) |
                     +---------+-----------------+
      DKIM-Signature Header Canonicalization Algorithm Registry
                            Initial Values

Allman, et al. Standards Track [Page 49] RFC 4871 DKIM Signatures May 2007

          The initial entries in the body registry comprise:
                     +---------+-----------------+
                     | TYPE    | REFERENCE       |
                     +---------+-----------------+
                     | simple  | (this document) |
                     | relaxed | (this document) |
                     +---------+-----------------+
       DKIM-Signature Body Canonicalization Algorithm Registry
                            Initial Values

7.4. _domainkey DNS TXT Record Tag Specifications

 A _domainkey DNS TXT record provides for a list of tag
 specifications.  IANA has established the DKIM _domainkey DNS TXT Tag
 Specification Registry for tag specifications that can be used in DNS
 TXT Records.
             The initial entries in the registry comprise:
                      +------+-----------------+
                      | TYPE | REFERENCE       |
                      +------+-----------------+
                      | v    | (this document) |
                      | g    | (this document) |
                      | h    | (this document) |
                      | k    | (this document) |
                      | n    | (this document) |
                      | p    | (this document) |
                      | s    | (this document) |
                      | t    | (this document) |
                      +------+-----------------+
       DKIM _domainkey DNS TXT Record Tag Specification Registry
                            Initial Values

7.5. DKIM Key Type Registry

 The "k=" <key-k-tag> (specified in Section 3.6.1) and the "a=" <sig-
 a-tag-k> (specified in Section 3.5) tags provide for a list of
 mechanisms that can be used to decode a DKIM signature.
 IANA has established the DKIM Key Type Registry for such mechanisms.

Allman, et al. Standards Track [Page 50] RFC 4871 DKIM Signatures May 2007

             The initial entry in the registry comprises:
                         +------+-----------+
                         | TYPE | REFERENCE |
                         +------+-----------+
                         | rsa  | [RFC3447] |
                         +------+-----------+
                     DKIM Key Type Initial Values

7.6. DKIM Hash Algorithms Registry

 The "h=" <key-h-tag> (specified in Section 3.6.1) and the "a=" <sig-
 a-tag-h> (specified in Section 3.5) tags provide for a list of
 mechanisms that can be used to produce a digest of message data.
 IANA has established the DKIM Hash Algorithms Registry for such
 mechanisms.
             The initial entries in the registry comprise:
                    +--------+-------------------+
                    | TYPE   | REFERENCE         |
                    +--------+-------------------+
                    | sha1   | [FIPS.180-2.2002] |
                    | sha256 | [FIPS.180-2.2002] |
                    +--------+-------------------+
                  DKIM Hash Algorithms Initial Values

7.7. DKIM Service Types Registry

 The "s=" <key-s-tag> tag (specified in Section 3.6.1) provides for a
 list of service types to which this selector may apply.
 IANA has established the DKIM Service Types Registry for service
 types.
             The initial entries in the registry comprise:
                      +-------+-----------------+
                      | TYPE  | REFERENCE       |
                      +-------+-----------------+
                      | email | (this document) |
                      | *     | (this document) |
                      +-------+-----------------+
              DKIM Service Types Registry Initial Values

Allman, et al. Standards Track [Page 51] RFC 4871 DKIM Signatures May 2007

7.8. DKIM Selector Flags Registry

 The "t=" <key-t-tag> tag (specified in Section 3.6.1) provides for a
 list of flags to modify interpretation of the selector.
 IANA has established the DKIM Selector Flags Registry for additional
 flags.
             The initial entries in the registry comprise:
                      +------+-----------------+
                      | TYPE | REFERENCE       |
                      +------+-----------------+
                      | y    | (this document) |
                      | s    | (this document) |
                      +------+-----------------+
              DKIM Selector Flags Registry Initial Values

7.9. DKIM-Signature Header Field

 IANA has added DKIM-Signature to the "Permanent Message Header
 Fields" registry (see [RFC3864]) for the "mail" protocol, using this
 document as the reference.

8. Security Considerations

 It has been observed that any mechanism that is introduced that
 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 [RFC4686].

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.

Allman, et al. Standards Track [Page 52] RFC 4871 DKIM Signatures May 2007

 For example, if attackers can append information to a "text/html"
 body part, they may be able to exploit a bug in some MUAs that
 continue to read after a "</html>" marker, and thus display HTML text
 on top of already displayed text.  If a message has a
 "multipart/alternative" body part, they might be able to add a new
 body part that is preferred by the displaying MUA.

8.1.2. Addition of new HTML content to existing content

 Several receiving MUA implementations do not cease display after a
 ""</html>"" tag.  In particular, this allows attacks involving
 overlaying images on top of existing text.
    INFORMATIVE EXAMPLE: Appending the following text to an existing,
    properly closed message will in many MUAs result in inappropriate
    data being rendered on top of existing, correct data:
 <div style="position: relative; bottom: 350px; z-index: 2;">
 <img src="http://www.ietf.org/images/ietflogo2e.gif"
   width=578 height=370>
 </div>

8.2. Misappropriated Private Key

 If the private key for a user is resident on their computer and is
 not protected by an appropriately secure mechanism, it is possible
 for malware to send mail as that user and any other user sharing the
 same private key.  The malware would not, however, be able to
 generate signed spoofs of other signers' addresses, which would aid
 in identification of the infected user and would limit the
 possibilities for certain types of attacks involving socially
 engineered messages.  This threat applies mainly to MUA-based
 implementations; protection of private keys on servers can be easily
 achieved through the use of specialized cryptographic hardware.
 A larger problem occurs if malware on many users' computers obtains
 the private keys for those users and transmits them via a covert
 channel to a site where they can be shared.  The compromised users
 would likely not know of the misappropriation until they receive
 "bounce" messages from messages they are purported to have sent.
 Many users might not understand the significance of these bounce
 messages and would not take action.
 One countermeasure is to use a user-entered passphrase to encrypt the
 private key, although users tend to choose weak passphrases and often
 reuse them for different purposes, possibly allowing an attack
 against 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

Allman, et al. Standards Track [Page 53] RFC 4871 DKIM Signatures May 2007

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

8.3. Key Server Denial-of-Service Attacks

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

 Since the DNS is a required binding for key services, specific
 attacks against the DNS must be considered.
 While the DNS is currently insecure [RFC3833], these security
 problems are the motivation behind DNS Security (DNSSEC) [RFC4033],
 and all users of the DNS will reap the benefit of that work.
 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

Allman, et al. Standards Track [Page 54] RFC 4871 DKIM Signatures May 2007

 deployment of DKIM will result in a significant increase in DNS
 queries to the claimed signing domain.  In the case of forgeries on a
 large scale, DNS servers could see a substantial increase in queries.
 A specific DNS security issue that should be considered by DKIM
 verifiers is the name chaining attack described in Section 2.3 of the
 DNS Threat Analysis [RFC3833].  A DKIM verifier, while verifying a
 DKIM-Signature header field, could be prompted to retrieve a key
 record of an attacker's choosing.  This threat can be minimized by
 ensuring that name servers, including recursive name servers, used by
 the verifier enforce strict checking of "glue" and other additional
 information in DNS responses and are therefore not vulnerable to this
 attack.

8.5. Replay Attacks

 In this attack, a spammer sends a message to be spammed to an
 accomplice, which results in the message being signed by the
 originating MTA.  The accomplice resends the message, including the
 original signature, to a large number of recipients, possibly by
 sending the message to many compromised machines that act as MTAs.
 The messages, not having been modified by the accomplice, have valid
 signatures.
 Partial solutions to this problem involve the use of reputation
 services to convey the fact that the specific email address is being
 used for spam and that messages from that signer are likely to be
 spam.  This requires a real-time detection mechanism in order to
 react quickly enough.  However, such measures might be prone to
 abuse, if for example an attacker resent a large number of messages
 received from a victim in order to make them appear to be a spammer.
 Large verifiers might be able to detect unusually large volumes of
 mails with the same signature in a short time period.  Smaller
 verifiers can get substantially the same volume of information via
 existing collaborative systems.

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 that
 have not yet been verified or that are the subject of a replay
 attack.  However, the ability of the domain to do so can be limited
 if the same key, for scalability reasons, is used to sign messages
 for many other users.  Mechanisms for explicitly revoking keys on a
 per-address basis have been proposed but require further study as to
 their utility and the DNS load they represent.

Allman, et al. Standards Track [Page 55] RFC 4871 DKIM Signatures May 2007

8.7. Intentionally Malformed Key Records

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

8.8. Intentionally Malformed DKIM-Signature Header Fields

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

8.9. Information Leakage

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

8.10. Remote Timing Attacks

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

8.11. Reordered Header Fields

 Existing standards allow intermediate MTAs to reorder header fields.
 If a signer signs two or more header fields of the same name, this
 can cause spurious verification errors on otherwise legitimate
 messages.  In particular, signers that sign any existing DKIM-
 Signature fields run the risk of having messages incorrectly fail to
 verify.

8.12. RSA Attacks

 An attacker could create a large RSA signing key with a small
 exponent, thus requiring that the verification key have a large
 exponent.  This will force verifiers to use considerable computing
 resources to verify the signature.  Verifiers might avoid this attack
 by refusing to verify signatures that reference selectors with public
 keys having unreasonable exponents.

Allman, et al. Standards Track [Page 56] RFC 4871 DKIM Signatures May 2007

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

8.13. Inappropriate Signing by Parent Domains

 The trust relationship described in Section 3.8 could conceivably be
 used by a parent domain to sign messages with identities in a
 subdomain not administratively related to the parent.  For example,
 the ".com" registry could create messages with signatures using an
 "i=" value in the example.com domain.  There is no general solution
 to this problem, since the administrative cut could occur anywhere in
 the domain name.  For example, in the domain "example.podunk.ca.us"
 there are three administrative cuts (podunk.ca.us, ca.us, and us),
 any of which could create messages with an identity in the full
 domain.
    INFORMATIVE NOTE: This is considered an acceptable risk for the
    same reason that it is acceptable for domain delegation.  For
    example, in the example above any of the domains could potentially
    simply delegate "example.podunk.ca.us" to a server of their choice
    and completely replace all DNS-served information.  Note that a
    verifier MAY ignore signatures that come from an unlikely domain
    such as ".com", as discussed in Section 6.1.1.

9. References

9.1. Normative References

 [FIPS.180-2.2002]  U.S. Department of Commerce, "Secure Hash
                    Standard", FIPS PUB 180-2, August 2002.
 [ITU.X660.1997]    "Information Technology - ASN.1 encoding rules:
                    Specification of Basic Encoding Rules (BER),
                    Canonical Encoding Rules (CER) and Distinguished
                    Encoding Rules (DER)", ITU-T Recommendation X.660,
                    1997.
 [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.

Allman, et al. Standards Track [Page 57] RFC 4871 DKIM Signatures May 2007

 [RFC2119]          Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [RFC2821]          Klensin, J., "Simple Mail Transfer Protocol",
                    RFC 2821, April 2001.
 [RFC2822]          Resnick, P., "Internet Message Format", RFC 2822,
                    April 2001.
 [RFC3447]          Jonsson, J. and B. Kaliski, "Public-Key
                    Cryptography Standards (PKCS) #1: RSA Cryptography
                    Specifications Version 2.1", RFC 3447,
                    February 2003.
 [RFC3490]          Faltstrom, P., Hoffman, P., and A. Costello,
                    "Internationalizing Domain Names in Applications
                    (IDNA)", RFC 3490, March 2003.
 [RFC4234]          Crocker, D., Ed. and P. Overell, "Augmented BNF
                    for Syntax Specifications: ABNF", RFC 4234,
                    October 2005.

9.2. Informative References

 [BONEH03]          Proc. 12th USENIX Security Symposium, "Remote
                    Timing Attacks are Practical", 2003.
 [RFC1847]          Galvin, J., Murphy, S., Crocker, S., and N. Freed,
                    "Security Multiparts for MIME: Multipart/Signed
                    and Multipart/Encrypted", RFC 1847, October 1995.
 [RFC2434]          Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 2434, October 1998.
 [RFC2440]          Callas, J., Donnerhacke, L., Finney, H., and R.
                    Thayer, "OpenPGP Message Format", RFC 2440,
                    November 1998.
 [RFC3766]          Orman, H. and P. Hoffman, "Determining Strengths
                    for Public Keys Used For Exchanging Symmetric
                    Keys", RFC 3766, April 2004.
 [RFC3833]          Atkins, D. and R. Austein, "Threat Analysis of the
                    Domain Name System (DNS)", RFC 3833, August 2004.

Allman, et al. Standards Track [Page 58] RFC 4871 DKIM Signatures May 2007

 [RFC3851]          Ramsdell, B., "S/MIME Version 3 Message
                    Specification", RFC 3851, June 1999.
 [RFC3864]          Klyne, G., Nottingham, M., and J. Mogul,
                    "Registration Procedures for Message Header
                    Fields", BCP 90, September 2004.
 [RFC4033]          Arends, R., Austein, R., Larson, M., Massey, D.,
                    and S. Rose, "DNS Security Introduction and
                    Requirements", RFC 4033, March 2005.
 [RFC4686]          Fenton, J., "Analysis of Threats Motivating
                    DomainKeys Identified Mail (DKIM)", RFC 4686,
                    September 2006.
 [RFC4870]          Delany, M., "Domain-Based Email Authentication
                    Using Public Keys Advertised in the DNS
                    (DomainKeys)", RFC 4870, May 2007.

Allman, et al. Standards Track [Page 59] RFC 4871 DKIM Signatures May 2007

Appendix A. Example of Use (INFORMATIVE)

 This section shows the complete flow of an email from submission to
 final delivery, demonstrating how the various components fit
 together.  The key used in this example is shown in Appendix C.

A.1. The User Composes an Email

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

Allman, et al. Standards Track [Page 60] RFC 4871 DKIM Signatures May 2007

A.2. The Email Is Signed

 This email is signed by the example.com outbound email server and now
 looks like this:
 DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
       c=simple/simple; q=dns/txt; i=joe@football.example.com;
       h=Received : From : To : Subject : Date : Message-ID;
       bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
       b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
         4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
         KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
         4bmp/YzhwvcubU4=;
 Received: from client1.football.example.com  [192.0.2.1]
       by submitserver.example.com with SUBMISSION;
       Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
 From: Joe SixPack <joe@football.example.com>
 To: Suzie Q <suzie@shopping.example.net>
 Subject: Is dinner ready?
 Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
 Message-ID: <20030712040037.46341.5F8J@football.example.com>
 Hi.
 We lost the game. Are you hungry yet?
 Joe.
 The signing email server requires access to the private key
 associated with the "brisbane" selector to generate this signature.

A.3. The Email Signature Is Verified

 The signature is normally verified by an inbound SMTP server or
 possibly the final delivery agent.  However, intervening MTAs can
 also perform this verification if they choose to do so.  The
 verification process uses the domain "example.com" extracted from the
 "d=" tag and the selector "brisbane" from the "s=" tag in the DKIM-
 Signature header field to form the DNS DKIM query for:
 brisbane._domainkey.example.com
 Signature verification starts with the physically last Received
 header field, the From header field, and so forth, in the order
 listed in the "h=" tag.  Verification follows with a single CRLF
 followed by the body (starting with "Hi.").  The email is canonically
 prepared for verifying with the "simple" method.  The result of the
 query and subsequent verification of the signature is stored (in this

Allman, et al. Standards Track [Page 61] RFC 4871 DKIM Signatures May 2007

 example) in the X-Authentication-Results header field line.  After
 successful verification, the email looks like this:
 X-Authentication-Results: shopping.example.net
       header.from=joe@football.example.com; dkim=pass
 Received: from mout23.football.example.com (192.168.1.1)
       by shopping.example.net with SMTP;
       Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
 DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
       c=simple/simple; q=dns/txt; i=joe@football.example.com;
       h=Received : From : To : Subject : Date : Message-ID;
       bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
       b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
         4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
         KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
         4bmp/YzhwvcubU4=;
 Received: from client1.football.example.com  [192.0.2.1]
       by submitserver.example.com with SUBMISSION;
       Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
 From: Joe SixPack <joe@football.example.com>
 To: Suzie Q <suzie@shopping.example.net>
 Subject: Is dinner ready?
 Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
 Message-ID: <20030712040037.46341.5F8J@football.example.com>
 Hi.
 We lost the game. Are you hungry yet?
 Joe.

Appendix B. Usage Examples (INFORMATIVE)

 DKIM signing and validating can be used in different ways, for
 different operational scenarios.  This Appendix discusses some common
 examples.
    NOTE: Descriptions in this Appendix are for informational purposes
    only.  They describe various ways that DKIM can be used, given
    particular constraints and needs.  In no case are these examples
    intended to be taken as providing explanation or guidance
    concerning DKIM specification details, when creating an
    implementation.

Allman, et al. Standards Track [Page 62] RFC 4871 DKIM Signatures May 2007

B.1. Alternate Submission Scenarios

 In the most simple scenario, a user's MUA, MSA, and Internet
 (boundary) MTA are all within the same administrative environment,
 using the same domain name.  Therefore, all of the components
 involved in submission and initial transfer are related.  However, it
 is common for two or more of the components to be under independent
 administrative control.  This creates challenges for choosing and
 administering the domain name to use for signing, and for its
 relationship to common email identity header fields.

B.1.1. Delegated Business Functions

 Some organizations assign specific business functions to discrete
 groups, inside or outside the organization.  The goal, then, is to
 authorize that group to sign some mail, but to constrain what
 signatures they can generate.  DKIM selectors (the "s=" signature
 tag) and granularity (the "g=" key tag) facilitate this kind of
 restricted authorization.  Examples of these outsourced business
 functions are legitimate email marketing providers and corporate
 benefits providers.
 Here, the delegated group needs to be able to send messages that are
 signed, using the email domain of the client company.  At the same
 time, the client often is reluctant to register a key for the
 provider that grants the ability to send messages for arbitrary
 addresses in the domain.
 There are multiple ways to administer these usage scenarios.  In one
 case, the client organization provides all of the public query
 service (for example, DNS) administration, and in another it uses DNS
 delegation to enable all ongoing administration of the DKIM key
 record by the delegated group.
 If the client organization retains responsibility for all of the DNS
 administration, the outsourcing company can generate a key pair,
 supplying the public key to the client company, which then registers
 it in the query service, using a unique selector that authorizes a
 specific From header field Local-part.  For example, a client with
 the domain "example.com" could have the selector record specify
 "g=winter-promotions" so that this signature is only valid for mail
 with a From address of "winter-promotions@example.com".  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.

Allman, et al. Standards Track [Page 63] RFC 4871 DKIM Signatures May 2007

 If the client wants the delegated group to do the DNS administration,
 it can have the domain name that is specified with the selector point
 to the provider's DNS server.  The provider then creates and
 maintains all of the DKIM signature information for that selector.
 Hence, the client cannot provide constraints on the Local-part of
 addresses that get signed, but it can revoke the provider's signing
 rights by removing the DNS delegation record.

B.1.2. PDAs and Similar Devices

 PDAs demonstrate the need for using multiple keys per domain.
 Suppose that John Doe wanted to be able to send messages using his
 corporate email address, jdoe@example.com, and his email device did
 not have the ability to make a Virtual Private Network (VPN)
 connection to the corporate network, either because the device is
 limited or because there are restrictions enforced by his Internet
 access provider.  If the device was equipped with a private key
 registered for jdoe@example.com by the administrator of the
 example.com domain, and appropriate software to sign messages, John
 could sign the message on the device itself before transmission
 through the outgoing network of the access service provider.

B.1.3. Roaming Users

 Roaming users often find themselves in circumstances where it is
 convenient or necessary to use an SMTP server other than their home
 server; examples are conferences and many hotels.  In such
 circumstances, a signature that is added by the submission service
 will use an identity that is different from the user's home system.
 Ideally, roaming users would connect back to their home server using
 either a VPN or a SUBMISSION server running with SMTP AUTHentication
 on port 587.  If the signing can be performed on the roaming user's
 laptop, then they can sign before submission, although the risk of
 further modification is high.  If neither of these are possible,
 these roaming users will not be able to send mail signed using their
 own domain key.

B.1.4. Independent (Kiosk) Message Submission

 Stand-alone services, such as walk-up kiosks and web-based
 information services, have no enduring email service relationship
 with the user, but users occasionally request that mail be sent on
 their behalf.  For example, a website providing news often allows the
 reader to forward a copy of the article to a friend.  This is
 typically done using the reader's own email address, to indicate who
 the author is.  This is sometimes referred to as the "Evite problem",

Allman, et al. Standards Track [Page 64] RFC 4871 DKIM Signatures May 2007

 named after the website of the same name that allows a user to send
 invitations to friends.
 A common way this is handled is to continue to put the reader's email
 address in the From header field of the message, but put an address
 owned by the email posting site into the Sender header field.  The
 posting site can then sign the message, using the domain that is in
 the Sender field.  This provides useful information to the receiving
 email site, which is able to correlate the signing domain with the
 initial submission email role.
 Receiving sites often wish to provide their end users with
 information about mail that is mediated in this fashion.  Although
 the real efficacy of different approaches is a subject for human
 factors usability research, one technique that is used is for the
 verifying system to rewrite the From header field, to indicate the
 address that was verified.  For example: From: John Doe via
 news@news-site.com <jdoe@example.com>.  (Note that such rewriting
 will break a signature, unless it is done after the verification pass
 is complete.)

B.2. Alternate Delivery Scenarios

 Email is often received at a mailbox that has an address different
 from the one used during initial submission.  In these cases, an
 intermediary mechanism operates at the address originally used and it
 then passes the message on to the final destination.  This mediation
 process presents some challenges for DKIM signatures.

B.2.1. Affinity Addresses

 "Affinity addresses" allow a user to have an email address that
 remains stable, even as the user moves among different email
 providers.  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, and they often have an associated Web application that
 authenticates the user and allows the forwarding address to be
 changed.  However, these services usually depend on users sending
 outgoing messages through their own service providers' MTAs.  Hence,
 mail that is signed with the domain of the affinity address is not
 signed by an entity that is administered by the organization owning
 that domain.
 With DKIM, affinity domains could use the Web application to allow
 users to register per-user keys to be used to sign messages on behalf
 of their affinity address.  The user would take away the secret half

Allman, et al. Standards Track [Page 65] RFC 4871 DKIM Signatures May 2007

 of the key pair for signing, and the affinity domain would publish
 the public half in DNS for access by verifiers.
 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.2.2. Simple Address Aliasing (.forward)

 In some cases, a recipient is allowed to configure an email address
 to cause automatic redirection of email messages from the original
 address to another, such as through the use of a Unix .forward file.
 In this case, messages are typically redirected by the mail handling
 service of the recipient's domain, without modification, except for
 the addition of a Received header field to the message and a change
 in the envelope recipient address.  In this case, the recipient at
 the final address' mailbox is likely to be able to verify the
 original signature since the signed content has not changed, and DKIM
 is able to validate the message signature.

B.2.3. Mailing Lists and Re-Posters

 There is a wide range of behaviors in services that take delivery of
 a message and then resubmit it.  A primary example is with mailing
 lists (collectively called "forwarders" below), ranging from those
 that make no modification to the message itself, other than to add a
 Received header field and change the envelope information, to those
 that add header fields, change the Subject header field, add content
 to the body (typically at the end), or reformat the body in some
 manner.  The simple ones produce messages that are quite similar to
 the automated alias services.  More elaborate systems essentially
 create a new message.
 A Forwarder that does not modify the body or signed header fields of
 a message is likely to maintain the validity of the existing
 signature.  It also could choose to add its own signature to the
 message.
 Forwarders which modify a message in a way that could make an
 existing signature invalid are particularly good candidates for
 adding their own signatures (e.g., mailing-list-name@example.net).
 Since (re-)signing is taking responsibility for the content of the
 message, these signing forwarders are likely to be selective, and
 forward or re-sign a message only if it is received with a valid

Allman, et al. Standards Track [Page 66] RFC 4871 DKIM Signatures May 2007

 signature or if they have some other basis for knowing that the
 message is not spoofed.
 A common practice among systems that are primarily redistributors of
 mail is to add a Sender header field to the message, to identify the
 address being used to sign the message.  This practice will 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.

Appendix C. Creating a Public Key (INFORMATIVE)

 The default signature is an RSA signed SHA256 digest of the complete
 email.  For ease of explanation, the openssl command is used to
 describe the mechanism by which keys and signatures are managed.  One
 way to generate a 1024-bit, unencrypted private key suitable for DKIM
 is to use openssl like this:
 $ openssl genrsa -out rsa.private 1024
 For increased security, the "-passin" parameter can also be added to
 encrypt the private key.  Use of this parameter will require entering
 a password for several of the following steps.  Servers may prefer to
 use hardware cryptographic support.
 The "genrsa" step results in the file rsa.private containing the key
 information similar to this:
  1. —-BEGIN RSA PRIVATE KEY—–

MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC

  jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb
  to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB
  AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX
  /1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ
  gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO
  n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m
  3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/
  eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj
  7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA
  qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf
  eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX
  GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc=
  -----END RSA PRIVATE KEY-----
 To extract the public-key component from the private key, use openssl
 like this:
 $ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM

Allman, et al. Standards Track [Page 67] RFC 4871 DKIM Signatures May 2007

 This results in the file rsa.public containing the key information
 similar to this:
  1. —-BEGIN PUBLIC KEY—–

MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM

 oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R
 tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI
 MmPSPDdQPNUYckcQ2QIDAQAB
 -----END PUBLIC KEY-----
 This public-key data (without the BEGIN and END tags) is placed in
 the DNS:
 brisbane IN  TXT  ("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ"
                    "KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt"
                    "IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v"
                    "/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi"
                    "tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB")

Appendix D. MUA Considerations

 When a DKIM signature is verified, the processing system sometimes
 makes the result available to the recipient user's MUA.  How to
 present this information to the user in a way that helps them is a
 matter of continuing human factors usability research.  The tendency
 is to have the MUA highlight the address associated with this signing
 identity in some way, in an attempt to show the user the address from
 which the mail was sent.  An MUA might do this with visual cues such
 as graphics, or it might include the address in an alternate view, or
 it might even rewrite the original From address using the verified
 information.  Some MUAs might indicate which header fields were
 protected by the validated DKIM signature.  This could be done with a
 positive indication on the signed header fields, with a negative
 indication on the unsigned header fields, by visually hiding the
 unsigned header fields, or some combination of these.  If an MUA uses
 visual indications for signed header fields, the MUA probably needs
 to be careful not to display unsigned header fields in a way that
 might be construed by the end user as having been signed.  If the
 message has an l= tag whose value does not extend to the end of the
 message, the MUA might also hide or mark the portion of the message
 body that was not signed.
 The aforementioned information is not intended to be exhaustive.  The
 MUA may choose to highlight, accentuate, hide, or otherwise display
 any other information that may, in the opinion of the MUA author, be
 deemed important to the end user.

Allman, et al. Standards Track [Page 68] RFC 4871 DKIM Signatures May 2007

Appendix E. Acknowledgements

 The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
 Steve Atkins, Rob Austein, Fred Baker, Mark Baugher, Steve Bellovin,
 Nathaniel Borenstein, Dave Crocker, Michael Cudahy, Dennis Dayman,
 Jutta Degener, Frank Ellermann, Patrik Faeltstroem, Mark Fanto,
 Stephen Farrell, Duncan Findlay, Elliot Gillum, Olafur
 Gu[eth]mundsson, Phillip Hallam-Baker, Tony Hansen, Sam Hartman,
 Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley, Craig
 Hughes, Cullen Jennings, Don Johnsen, Harry Katz, Murray S.
 Kucherawy, Barry Leiba, John Levine, Charles Lindsey, Simon
 Longsdale, David Margrave, Justin Mason, David Mayne, Thierry Moreau,
 Steve Murphy, Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada,
 Juan Altmayer Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud,
 Scott Renfro, Neil Rerup, Eric Rescorla, Dave Rossetti, Hector
 Santos, Jim Schaad, the Spamhaus.org team, Malte S. Stretz, Robert
 Sanders, Rand Wacker, Sam Weiler, 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 [RFC4870].

Authors' Addresses

 Eric Allman
 Sendmail, Inc.
 6425 Christie Ave, Suite 400
 Emeryville, CA  94608
 USA
 Phone: +1 510 594 5501
 EMail: eric+dkim@sendmail.org
 URI:
 Jon Callas
 PGP Corporation
 3460 West Bayshore
 Palo Alto, CA  94303
 USA
 Phone: +1 650 319 9016
 EMail: jon@pgp.com

Allman, et al. Standards Track [Page 69] RFC 4871 DKIM Signatures May 2007

 Mark Delany
 Yahoo! Inc
 701 First Avenue
 Sunnyvale, CA  95087
 USA
 Phone: +1 408 349 6831
 EMail: markd+dkim@yahoo-inc.com
 URI:
 Miles Libbey
 Yahoo! Inc
 701 First Avenue
 Sunnyvale, CA  95087
 USA
 EMail: mlibbeymail-mailsig@yahoo.com
 URI:
 Jim Fenton
 Cisco Systems, Inc.
 MS SJ-9/2
 170 W. Tasman Drive
 San Jose, CA  95134-1706
 USA
 Phone: +1 408 526 5914
 EMail: fenton@cisco.com
 URI:
 Michael Thomas
 Cisco Systems, Inc.
 MS SJ-9/2
 170 W. Tasman Drive
 San Jose, CA  95134-1706
 Phone: +1 408 525 5386
 EMail: mat@cisco.com

Allman, et al. Standards Track [Page 70] RFC 4871 DKIM Signatures May 2007

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Allman, et al. Standards Track [Page 71]

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