wdiff draft-ietf-dkim-base-07.txt draft-ietf-dkim-base-08.txt

DKIM E. Allman
Internet-Draft Sendmail, Inc.
Expires: June 7, July 22, 2007 J. Callas
PGP Corporation
M. Delany
M. Libbey
Yahoo! Inc
J. Fenton
M. Thomas
Cisco Systems, Inc.
December 4, 2006
January 18, 2007

DomainKeys Identified Mail (DKIM) Signatures
draft-ietf-dkim-base-07
draft-ietf-dkim-base-08

Status of this Memo

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
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This Internet-Draft will expire on June 7, July 22, 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".

Requirements Language

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

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6
1.2 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Simple Key Management . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions . . . . . . . . . . . . . . . . 6
2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7 6
2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7
2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8
2.6 DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 9
3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11
3.3 Signing and Verification Algorithms . . . . . . . . . . . 12
3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 14
3.5 The DKIM-Signature header field . . . . . . . . . . . . . 18
3.6 Key Management and Representation . . . . . . . . . . . . 25 26
3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 30
3.8 Signing by Parent Domains . . . . . . . . . . . . . . . . 32
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 32 33
4.1 Example Scenarios . . . . . . . . . . . . . . . . . . . . 33
4.2 Interpretation . . . . . . . . . . . . . . . . . . . . . . 34
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 33 35
5.1 Determine if the Email Should be Signed and by Whom . . . 33 35
5.2 Select a private-key Private Key and corresponding selector
information Corresponding Selector
Information . . . . . . . . . . . . . . . . . . . . . . . 33 36
5.3 Normalize the Message to Prevent Transport Conversions . . 34 36
5.4 Determine the header fields Header Fields to Sign . . . . . . . . . . . 34 37
5.5 Compute the Message Hash and Signature . . . . . . . . . . 36 40
5.6 Insert the DKIM-Signature header field Header Field . . . . . . . . . . 37 40
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 37 41
6.1 Extract Signatures from the Message . . . . . . . . . . . 38 41
6.2 Communicate Verification Results . . . . . . . . . . . . . 43 47
6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 44 47
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 45 49
7.1 DKIM-Signature Tag Specifications . . . . . . . . . . . . 45 49
7.2 DKIM-Signature Query Method Registry . . . . . . . . . . . 46 49
7.3 DKIM-Signature Canonicalization Registry . . . . . . . . . 46 50
7.4 _domainkey DNS TXT Record Tag Specifications . . . . . . . 47 50
7.5 DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 48 51
7.6 DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 48 51
7.7 DKIM Service Types Registry . . . . . . . . . . . . . . . 48 52
7.8 DKIM Selector Flags Registry . . . . . . . . . . . . . . . 49 52
7.9 DKIM-Signature Header Field . . . . . . . . . . . . . . . 49 53
8. Security Considerations . . . . . . . . . . . . . . . . . . 49 53
8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 49 53
8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 50 54
8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 51 54
8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 51 55
8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 52 55
8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 53 56
8.7 Intentionally malformed Key Records . . . . . . . . . . . 53 56
8.8 Intentionally Malformed DKIM-Signature header fields . . . 53 56
8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 53 57
8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 53 57
8.11 Reordered Header Fields . . . . . . . . . . . . . . . . 54 57
8.12 RSA Attacks . . . . . . . . . . . . . . . . . . . . . . 57
8.13 Inappropriate Signing by Parent Domains . . . . . . . . 57
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 54 58
9.1 Normative References . . . . . . . . . . . . . . . . . . . 54 58
9.2 Informative References . . . . . . . . . . . . . . . . . . 55 59
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 55 59
A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 57 61
A.1 The user composes an email . . . . . . . . . . . . . . . . 57 61
A.2 The email is signed . . . . . . . . . . . . . . . . . . . 57 61
A.3 The email signature is verified . . . . . . . . . . . . . 58 62
B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 59 63
B.1 Alternate Submission Scenarios . . . . . . . . . . . . . . 59 64
B.2 Alternate Delivery Scenarios . . . . . . . . . . . . . . . 62 66
C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 64 68
D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 65 70
E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 66 70
F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 66 71
F.1 Changes since -ietf-06 -ietf-07 version . . . . . . . . . . . . . . 67 71
F.2 Changes since -ietf-05 -ietf-06 version . . . . . . . . . . . . . . 67 72
F.3 Changes since -ietf-04 -ietf-05 version . . . . . . . . . . . . . . 68 73
F.4 Changes since -ietf-03 -ietf-04 version . . . . . . . . . . . . . . 68 73
F.5 Changes since -ietf-02 -ietf-03 version . . . . . . . . . . . . . . 69 74
F.6 Changes since -ietf-01 -ietf-02 version . . . . . . . . . . . . . . 70 75
F.7 Changes since -ietf-00 -ietf-01 version . . . . . . . . . . . . . . 71 76
F.8 Changes since -allman-01 -ietf-00 version . . . . . . . . . . . . . 71 . 76
F.9 Changes since -allman-00 -allman-01 version . . . . . . . . . . . . . 72 77
F.10 Changes since -allman-00 version . . . . . . . . . . . . 77
Intellectual Property and Copyright Statements . . . . . . . 73 79

1. Introduction

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

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

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

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

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

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

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

o allows delegation of signing to third parties.

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

1.3 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 adminstration 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].

2.1 Signers

Elements in the mail system that sign messages are 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 White Space

There are three forms of white space:

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

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

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

The formal ABNF for 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 / "+" / "/" / LWSP)
[ "=" *LWSP LWSP [ "=" *LWSP LWSP ] ]

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 lower case characters permitted) representing
the hexadecimal-encoded integer value of that character. All control
characters (those with values < %x20), eight-bit characters (values >
%x7F), and the characters DEL (%x7F), SPACE (%x20), and semicolon
(";", %x3B) MUST be encoded. Note that all white space, including
SPACE, CR and LF characters, MUST be encoded. After encoding, FWS
MAY be added at arbitrary locations in order to avoid excessively
long lines; such white space is NOT part of the value, and MUST be
removed before decoding.

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

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

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

2. White space in the encoded text is ignored. This is to allow
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-white-space
character on the line) does not apply.

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

3. Protocol Elements

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

3.1 Selectors

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

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

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

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

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

Periods are allowed in Selectors and are component separators. 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 name-space.

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

The number of public keys and corresponding Selectors for each domain
are determined by the domain owner. Many domain owners will be
satisfied with just one Selector whereas administratively distributed
organizations may choose to manage disparate Selectors and key pairs
in different regions or on different email servers.

Beyond administrative convenience, Selectors make it possible to
seamlessly replace public keys on a routine basis. If a domain
wishes to change from using a public key associated with Selector
"january2005" to a public key associated with Selector
"february2005", it merely makes sure that both public keys are
advertised in the public-key repository concurrently for the
transition period during which email may be in transit prior to
verification. At the start of the transition period, the outbound
email servers are configured to sign with the "february2005" private- 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
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. Signers
SHOULD NOT change the key associated with a Selector. When creating
a new key, For this reason, signers SHOULD associate it with a are ill-advised
to reuse selectors for new Selector. 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 to not preclude the use of UTF8-encoded text in tag=value
lists.

Formally, the syntax rules are:
tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tag-name = ALPHA 0*ALNUMPUNC
tag-value = [ 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.

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 [SHA] 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.

INFORMATIVE IMPLEMENTATION WARNING: Low-valued exponents should
be avoided, as they are believed to be subject to attack.

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 [SHA] 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:

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

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 white-space replacement and
header field line re-wrapping. A signer MAY specify either algorithm
for header or body when signing an email. If no canonicalization
algorithm is specified by the signer, the "simple" algorithm defaults
for both header and body. Verifiers MUST implement both
canonicalization algorithms. 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
transmitted data in any way. Canonicalization of header fields and
body are described below.

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

3.4.1 The "simple" Header Canonicalization Algorithm

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

3.4.2 The "relaxed" Header Canonicalization Algorithm

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

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

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

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

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

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

3.4.3 The "simple" Body Canonicalization Algorithm

The "simple" body canonicalization algorithm ignores all empty lines
at the end of the message body. An empty line is a line of zero
length after removal of the line terminator. If there is no body or
no trailing CRLF on the message, message body, a CRLF is added. It makes no
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 white space at the end of lines. Implementations MUST
NOT remove the CRLF at the end of the line.

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

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

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 then 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
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 white space 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 white space 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>

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.

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 0.5. 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] "0.5"

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

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

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

ABNF:

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

b= The signature data (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when re-assembling the original
signature. In particular, the signing process can safely insert
FWS in this value in arbitrary places to conform to line-length
limits. See Signer Actions (Section 5) for how the signature is
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 re-assembling the original
signature. In particular, the signing process can safely insert
FWS in this value in arbitrary places to conform to line-length
limits. See Section 3.7 for how the body hash is computed.

ABNF:

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

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

ABNF:

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

d= The domain of the signing entity (plain-text; REQUIRED). This is
the domain that will be queried for the public key. This domain
MUST be the same as or a parent domain of the "i=" tag (the
signing identity, as described below), 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.

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 white space (FWS) MAY be included on either side
of the colon separator. Header field names MUST be compared
against actual header field names in a case insensitive manner.
This list MUST NOT be empty. See Section 5.4 for a discussion of
choosing header fields to sign.

ABNF:

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

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

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

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

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

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

INFORMATIVE NOTE: The value of the l= tag is constrained to
76 decimal digits, which will fit in a 256-bit binary integer
field. digits. This constraint is not intended to
predict the size of future messages, 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. 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 depends on the type and specified
options. If there are multiple query mechanisms listed, the
choice of query mechanism MUST NOT change the interpretation of
the signature. Implementations MUST use the recognized query
mechanisms in the order presented.

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

ABNF:

sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([FWS] ":" [FWS] sig-q-tag-method)
sig-q-tag-method = "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.

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 white space
must be encoded, including white space between the colon and the
header field value. After encoding, LWSP MAY be added at
arbitrary locations in order to avoid excessively long lines;
such white space is NOT part of the value of the header field,
and MUST be removed before decoding.

Verifiers MUST NOT use the

The header field names or copied values
for checking fields referenced by the signature h= tag refer to the fields in
the 2822 header of the message, not to any way. copied fields in the
z= tag. Copied header field values are for diagnostic use only. use.

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

ABNF:
sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy
*( [FWS] "|" sig-z-tag-copy )
sig-z-tag-copy = hdr-name ":" qp-hdr-value
qp-hdr-value = dkim-quoted-printable ; with "|" encoded

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

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 responsible signer (the "d=" tag of the DKIM-Signature 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.

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").
The intent of this tag is to constrain which signing address can
legitimately use this Selector. An email with a signing address that does not
match the value of this tag constitutes a failed verification. Wildcarding allows matching
The intent of this tag is to constrain which signing address can
legitimately use this Selector, for addresses such as
"user+*". An empty "g=" value never 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] ]

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

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

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 [X.660] 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 is 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 ]

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

* matches all service types

email electronic mail (not necessarily limited to SMTP)

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

ABNF:

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

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

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

3.6.2 DNS binding

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

3.6.2.1 Name Space

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

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

In hash step 1, the signer or verifier 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: DKIM-
Signature: header field.

In hash step 2, the signer or verifier 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." "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
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
which is signed.

More formally, the algorithm for the signature is:
body-hash = hash-alg(canon_body)
header-hash = hash-alg(canon_header || DKIM-SIG)
signature = 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 which are not intended for use with subdomains SHOULD specify
the "s" flag in the t= tag.

4. Semantics of Multiple Signatures

A signer

4.1 Example Scenarios

There are many reasons that is adding a signature to a message merely creates might have multiple signatures.
For example, a new
DKIM-Signature header, using the usual semantics of 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 h= option. future found to be
insufficiently strong, and DKIM usage transitions to SHA-1024. A
signer MAY might immediately sign previously existing DKIM-Signature header fields using the method described in section Section 5.4 newer algorithm, but
continue to sign trace
header fields.

INFORMATIVE NOTE: Signers should be cognizant that signing DKIM-
Signature header fields may result in signature failures using the older algorithm for interoperability
with
intermediaries 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 DKIM-Signature header
fields are trace header fields signature and unwittingly reorder them, thus
breaking such signatures.

INFORMATIVE NOTE: If a header field with multiple instances is
signed, those header fields are always signed from use the bottom up.
Thus, it is older
algorithm; newer verifiers could use either signature at their
option, and all other things being equal might not possible even attempt to sign only specific DKIM-Signature
header fields. For example, if
verify the message being signed already
contains three DKIM-Signature header fields A, B, and C, it is
possible to other signature.

Similarly, a signer might sign a message including all of them, B and C only, or C only, but not A
only, B only, A headers and B only, or A no
"l=" tag (to satisfy strict verifiers) and C only.

When evaluating a message second time with multiple signatures, a verifier should
evaluate signatures independently
limited set of headers and on their own merits. For
example, a verifier that by policy chooses not an "l=" tag (in anticipation of possible
message modifications in route to accept signatures
with deprecated cryptographic algorithms should consider such
signatures invalid. As with messages with other verifiers). Verifiers could
then choose which signature they preferred.

INFORMATIVE EXAMPLE: A verifier might receive a single signature,
verifiers are at liberty to use message with two
signatures, one covering more of the presence message than the other. If
the signature covering more of valid signatures as
an input to local policy; likewise, the interpretation message verified, then the
verifier could make one set of multiple
valid signatures in combination is a local policy decision of the
verifier.

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

5. Signer Actions

The following steps are performed in order by signers.

5.1 Determine if the Email Should be Signed and by Whom

A signer can obviously only sign email for domains for which it has a
private-key and the necessary knowledge signature covering less of the corresponding public
key and Selector information. However there are a number of other
reasons beyond message verified,
the lack of verifier might make a private key why different set of policy decisions.

Of course, a signer could choose
not message might also have multiple signatures because it
passed through multiple signers. A common case is expected to sign an email.

INFORMATIVE NOTE: Signing modules may be incorporated into any
portion
that 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 signed message that may be problematic. In
particular, within passes through a trusted enclave the signing address mailing list that also
signs all messages. Assuming both of those signatures verify, a
recipient might be
derived from choose to accept the header according message if either of those
signatures were known to local policy; SUBMISSION
servers might only sign messages come from users that are properly
authenticated and authorized. trusted sources.

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

If an email cannot be signed for some reason, it is a local policy
decision whitelist mailing
lists to which they have subscribed and which have acceptable
anti-abuse policies so as to what accept messages sent to do with that email.

5.2 Select a private-key and corresponding selector information

This specification does not define the basis by which a signer should
choose which private-key list
even from unknown authors. They might also subscribe to less
trusted mailing lists (e.g., those without anti-abuse protection)
and Selector information be willing to use. Currently, accept 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 messages from specific authors, but
insist on doing additional abuse scanning for other messages.

Another related example of this document. multiple signers might be forwarding
services, such as those commonly associated with academic alumni
sites.

INFORMATIVE OPERATIONS ADVICE: EXAMPLE: A signer should not sign with recipient might have an address at
alumni.example.edu, a
private key when the Selector containing the corresponding public
key site that has anti-abuse protection that is expected to be revoked or removed before
somewhat less effective than the verifier has
an opportunity to validate recipient would prefer. Such a
recipient might have specific authors whose messages would be
trusted absolutely, but messages from unknown authors which had
passed the signature. The forwarder's scrutiny would have only medium trust.

4.2 Interpretation

A 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 adding a signature to a message merely creates a new key pair, signing should
immediately commence with the new private key and
DKIM-Signature header, using the old public
key should be retained for a reasonable validation interval before
being removed from usual semantics of the key server.

5.3 Normalize h= option. A
signer MAY sign previously existing DKIM-Signature header fields
using the Message method described in section Section 5.4 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 sign trace
header fields.

INFORMATIVE NOTE: Signers 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 cognizant that signing DKIM-
Signature header fields may result in signature failures with any local encoding
intermediaries that will be modified before transmission, do not recognize that modification to
canonical [RFC2822] form MUST be done before signing. In particular,
bare CR or LF characters (used by some systems as 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 local line
separator convention) MUST be converted to the SMTP-standard CRLF
sequence before the message header field with multiple instances is signed. Any conversion of this sort
SHOULD be applied to the message actually sent to
signed, those header fields are always signed from the recipient(s), bottom up.
Thus, it is not just to the version presented possible to the signing algorithm.

More generally, the signer MUST sign only specific DKIM-Signature
header fields. For example, if the message as it is expected to
be received by the verifier rather than in some local or internal
form.

5.4 Determine the being signed already
contains three DKIM-Signature header fields A, B, and C, it is
possible to Sign

The From header field MUST be signed (that is, included in the h= tag sign all of the resulting 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). 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
sign an existing remove any DKIM-Signature header field likely to fields from
messages they are signing, even if they know that the signatures
cannot be legitimately modified or
removed 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 transit. In particular, [RFC2821] explicitly permits
modification or removal any order of
their choice; for example, some verifiers might choose to process
signatures corresponding to the "Return-Path" header From field in transit.
Signers MAY include any other the message header fields
before other signatures. See Section 6.1 for more information about
signature choices.

Verifiers SHOULD ignore failed signatures as though they were not
present at in the time of
signing at message. Verifiers SHOULD continue to check
signatures until a signature successfully verifies to the discretion
satisfaction 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 verifier. To limit potential denial-of-service
attacks, verifiers MAY limit the total number of signatures they will
attempt to sign only
header fields that verify.

5. Signer Actions

The following steps are likely to be displayed to or otherwise performed in order by signers.

5.1 Determine if the Email Should be
likely to affect Signed and by Whom

A signer can obviously only sign email for domains for which it has a
private key and the processing necessary knowledge of the message at corresponding public
key and Selector information. However there are a number of other
reasons beyond the receiver. A
third strategy is lack of a private key why a signer could choose
not to sign only "well known" headers. Note that
verifiers an email.

INFORMATIVE NOTE: Signing modules may treat unsigned header fields with extreme
skepticism, including refusing to display them to be incorporated into any
portion of the end user mail system as deemed appropriate, including an
MUA, a SUBMISSION server, or
even ignore 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 signature if it does not cover certain header
fields. For this reason signing fields present in address might be
derived from the message
such as Date, Subject, Reply-To, Sender, header according to local policy; SUBMISSION
servers might only sign messages from users that are properly
authenticated and all MIME authorized.

INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not
sign Received header fields is highly advised.

The DKIM-Signature if the outgoing gateway MTA obfuscates
Received header field is always implicitly signed and MUST
NOT be included in fields, for example to hide the h= tag except details of
internal topology.

If an email cannot be signed for some reason, it is a local policy
decision as to indicate that other
preexisting signatures are also signed.

Signers MAY claim what to have signed header fields that do not exist
(that is, signers MAY include the header field name in the h= tag
even if with that header field email.

5.2 Select a Private Key and Corresponding Selector Information

This specification does not exist in the message). When
computing the signature, the non-existing header field MUST be
treated as the null string (including define the header field name, header
field value, all punctuation, basis by which a signer should
choose which private key and the trailing CRLF).

INFORMATIVE RATIONALE: This allows signers Selector information to explicitly assert
the absence of a header field; if that header field use. Currently,
all Selectors are equal as far as this specification is added later concerned, so
the signature will fail.

INFORMATIVE NOTE: A header field name need only decision should largely be listed once
more than the actual number of that header field in a message at
the time matter of signing in order to prevent any further additions.
For example, if there administrative
convenience. Distribution and management of private keys is a single "Comments" header field at also
outside the
time scope of signing, listing "Comments" twice in this document.

INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
private key when the h= tag is
sufficient to prevent any number of Comments header fields from
being appended; it is not necessary (but Selector containing the corresponding public
key is legal) expected to list
"Comments" three be revoked or more times in removed before the h= tag.

Signers choosing to sign verifier has
an existing header field opportunity to validate the signature. The signer should
anticipate that occurs more
than once in verifiers may choose to defer validation, perhaps
until the message (such as Received) MUST sign the physically
last instance of that header field in is actually read by the header block. Signers
wishing final recipient. In
particular, when rotating to sign multiple instances of such a header field MUST
include the header field name multiple times in the h= tag of new key pair, signing should
immediately commence with the
DKIM-Signature header field, new private key and MUST sign such header fields in
order the old public
key should be retained for a reasonable validation interval before
being removed from the bottom of key server.

5.3 Normalize the header field block Message to the top. The
signer MAY include more instances of a header field name in h= than
there Prevent Transport Conversions

Some messages, particularly those using 8-bit characters, are actual corresponding header fields subject
to indicate that
additional header fields modification during transit, notably conversion to 7-bit form.
Such conversions will break DKIM signatures. In order to minimize
the chances of that name such breakage, signers SHOULD NOT be added.

INFORMATIVE EXAMPLE:

If convert the signer wishes message 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 a
suitable MIME content transfer encoding such as quoted-printable or
base64 as described in that
order.

Signers should be careful MIME Part One [RFC2045] before signing. Such
conversion is outside the scope of signing header fields that might have
additional instances added later in DKIM; the delivery process, since such
header fields might actual message SHOULD be inserted after the signed instance
converted to 7-bit MIME by an MUA or
otherwise reordered. Trace header fields (such as Received and DKIM-
Signature) and Resent-* blocks are MSA prior to presentation to the only fields prohibited by
[RFC2822] from being reordered.

INFORMATIVE ADMONITION: Despite
DKIM algorithm.

If the fact that [RFC2822] permits
header fields message is submitted to be reordered (with the exception of Received
header fields), reordering of signed header fields signer with multiple
instances by intermediate MTAs any local encoding
that will cause DKIM signatures to be
broken; such anti-social behavior should modified before transmission, that modification to
canonical [RFC2822] form MUST be avoided.

INFORMATIVE IMPLEMENTER'S NOTE: Although not required done before signing. In particular,
bare CR or LF characters (used by this
specification, all end-user visible header fields should some systems as a local line
separator convention) MUST be signed converted to avoid possible "indirect spamming." For example, if the
"Subject" header field SMTP-standard CRLF
sequence before the message is not signed, a spammer can resend a
previously signed mail, replacing signed. Any conversion of this sort
SHOULD be applied to the legitimate subject with a
one-line spam.

5.5 Compute message actually sent to the recipient(s),
not just to the version presented to the signing algorithm.

More generally, the Message Hash and Signature

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

Entities such as mailing list managers that implement DKIM and which
modify the message some local or a internal
form.

5.4 Determine the Header Fields to Sign

The From 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 signed (that is, included in the hash and hence signed. The length actually
hashed should be inserted in the "l=" h= tag
of the "DKIM-Signature"
header field.

5.6 Insert the resulting 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" field). Signers SHOULD NOT
sign an existing header field MUST be the same as used likely 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 legitimately modified or
removed in the "a=" tag transit. In particular, [RFC2821] explicitly permits
modification or removal of the "DKIM-
Signature" "Return-Path" 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" MUST be inserted before transit.
Signers MAY include any other DKIM-Signature header fields in present at the header block. time of
signing at the discretion of the signer.

INFORMATIVE IMPLEMENTATION OPERATIONS NOTE: The easiest way choice of which header fields to achieve this
sign is non-obvious. One strategy 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 sign all existing, non-
repeatable header fields. This An alternative strategy is consistent with treating
"DKIM-Signature" as a trace to sign only
header field.

6. Verifier Actions

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

A border are likely to be displayed to or intermediate MTA MAY verify otherwise be
likely to affect the processing of the message signature(s). An
MTA who has performed verification MAY communicate at the result of receiver. A
third strategy is to sign only "well known" headers. Note that
verification by adding a verification
verifiers may treat unsigned header field fields with extreme
skepticism, including refusing to display them to incoming
messages. This considerably simplifies things for the user, who can
now use an existing mail end user agent. Most MUAs have or
even ignore the ability to
filter messages based on message signature if it does not cover certain header
fields. For this reason signing fields or content; these
filters would be used to implement whatever policy present in 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 message
such as Date, Subject, Reply-To, Sender, and all MIME header
fields is highly advised.

The DKIM-Signature header field to is always implicitly signed messages.

Verifiers and MUST produce a result that is semantically equivalent to
applying the following steps in the order listed. In practice,
several of these steps can
NOT be performed in parallel included in order to
improve performance.

6.1 Extract Signatures from the Message

The order in which verifiers try DKIM-Signature h= tag except to indicate that other
preexisting signatures are also signed.

Signers MAY claim to have signed header fields is that do not
defined; verifiers exist
(that is, signers MAY try signatures in any order they would like.
For example, one implementation might prefer to try include the signatures header field name in
textual order, whereas another might want to prefer signatures by
identities the h= tag
even if that match header field does not exist in the contents of message). When
computing the "From" signature, the non-existing header field over
other identities. Verifiers MUST NOT attribute ultimate meaning 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 order absence of multiple DKIM-Signature a header fields. In particular,
there is reason to believe field; if that some relays header field is added later
the signature will reorder fail.

INFORMATIVE NOTE: A header field name need only be listed once
more than the actual number of that header
fields field in potentially arbitrary ways.

INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as a clue to message at
the time of signing order in order to prevent any further additions.
For example, if there is a single "Comments" header field at the absence
time of any other information.
However, other clues as to signing, listing "Comments" twice in the semantics h= tag is
sufficient to prevent any number of multiple signatures
(such as correlating Comments header fields from
being appended; it is not necessary (but is legal) to list
"Comments" three or more times in the signing host with Received h= tag.

Signers choosing to sign an existing header fields)
may also be considered.

A verifier SHOULD NOT treat a message field that has one or occurs more bad
signatures and no good signatures differently from a
than once in the message with no
signature at all; such treatment is a matter (such as Received) MUST sign the physically
last instance of local policy and is
beyond that header field in the scope header block. Signers
wishing to sign multiple instances of this document.

When a signature successfully verifies, such a verifier will either stop
processing or attempt to verify any other signatures, at header field MUST
include the
discretion header field name multiple times in the h= tag of the implementation. A verifier MAY limit
DKIM-Signature header field, and MUST sign such header fields in
order from the number bottom of
signatures it tries the header field block to avoid denial-of-service attacks.

INFORMATIVE NOTE: An attacker could send messages with large
numbers of faulty signatures, each the top. The
signer MAY include more instances of which would require a DNS
lookup and header field name in h= than
there are actual corresponding CPU time header fields to verify the message. This
could be an attack on the domain indicate that receives the message, by
slowing down the verifier by requiring it to do large number
additional header fields of
DNS lookups and/or signature verifications. It could also that name SHOULD NOT be an
attack against the domains listed in the signatures, essentially
by enlisting innocent verifiers in launching an attack against added.

INFORMATIVE EXAMPLE:

If the
DNS servers of signer wishes to sign two existing Received header fields,
and the actual victim.

In existing header contains:

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

then the following description, text reading "return status
(explanation)" (where "status" is one 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 "PERMFAIL" or "TEMPFAIL")
means signing header fields that might have
additional instances added later in the verifier MUST immediately cease processing that
signature. The verifier SHOULD proceed to delivery process, since such
header fields might be inserted after the next signature, if any
is present, signed instance or
otherwise reordered. Trace header fields (such as Received) and completely ignore the bad signature. If
Resent-* blocks are the status 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 "PERMFAIL", error-prone.

INFORMATIVE ADMONITION: Despite the signature failed and should not fact that [RFC2822] permits
header fields to be reconsidered.
If reordered (with the status is "TEMPFAIL", the signature could not exception of Received
header fields), reordering of signed header fields with multiple
instances by intermediate MTAs will cause DKIM signatures to be verified at
this time but may
broken; such anti-social behavior should be tried again later. A verifier MAY either defer
the message for later processing, perhaps avoided.

INFORMATIVE IMPLEMENTER'S NOTE: Although not required by queueing it locally or
issuing a 451/4.7.5 SMTP reply, or try another signature; this
specification, all end-user visible header fields should be signed
to avoid possible "indirect spamming." For example, if no good
signature the
"Subject" header field is found and any not signed, a spammer can resend a
previously signed mail, replacing the legitimate subject with a
one-line spam.

5.4.1 Recommended Signature Content

In order to maximize compatibility with a variety of verifiers, it is
recommended that signers follow the signatures resulted practices outlined in this
section when signing a TEMPFAIL
status, message. However, these are generic
recommendations applying to the verifier MAY save 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 message for later processing. The
"(explanation)" recommended header fields is not normative text; it is provided solely for
clarification.

Verifiers SHOULD ignore any DKIM-Signature signed (with
the exception of From, which must always be signed) or if one or more
of the disrecommended header fields where is signed. Note that verifiers
do have the
signature does not validate. Verifiers option of ignoring signatures that are prepared to validate
multiple signature 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 proceed to the next signature
header field, should it exist. However, verifiers MAY make note of be included in the fact that an invalid signature was signature, if
they are 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 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 some way that will break
that signature, and the exploder inserts its own signature. In
this case the message should succeed even signature:

o Return-Path

o Received

o Comments, Keywords

o Bcc, Resent-Bcc

o DKIM-Signature

Optional header fields (those not mentioned above) normally SHOULD
NOT be included in the presence signature, because of the
known-broken signature.

For each signature potential for
additional header fields of the same name to be validated, legitimately added or
reordered prior to verification. There are likely to be legitimate
exceptions to this rule, because of the following steps should wide variety of application-
specific header fields which may be
performed in such a manner as applied to produce a result that is
semantically equivalent message, some of
which are unlikely to performing them in the indicated order.

6.1.1 Validate be duplicated, modified, or reordered.

Signers SHOULD include all or nearly all of the Signature Header Field

Implementers MUST meticulously validate body content when
specifying the format and values body length count (l= tag) in the
DKIM-Signature header field; any inconsistency or unexpected values
MUST cause the header field to be completely ignored signature. In
particular, signers SHOULD NOT specify a body length of 0 since this
may be interpreted as a meaningless signature by some verifiers.

5.5 Compute the Message Hash and Signature

The signer MUST compute the verifier
to return PERMFAIL (signature syntax error). Being "liberal in what
you accept" is definitely a bad strategy message hash as described in this security context.
Note however that this does not include Section 3.7
and then sign it using the existence of unknown tags selected public-key algorithm. This will
result in a DKIM-Signature header field, field which are explicitly permitted.

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

INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of
course, choose to also verify signatures generated by older
versions a signature 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 hash, where that header includes
the DKIM-
Signature DKIM-Signature header field and return PERMFAIL (signature missing
required tag).

INFORMATIONAL NOTE: The tags listed itself.

Entities such as required in Section 3.5
are "v=", "a=", "b=", "bh=", "d=", "h=", and "s=". Should there
be a conflict between this note mailing list managers that implement DKIM and Section 3.5, Section 3.5 is
normative.

If which
modify the "DKIM-Signature" message or a header field does not contain the "i=" tag, (for example, inserting
unsubscribe information) before retransmitting the verifier message SHOULD
check any existing signature on input and MUST behave as though make such
modifications before re-signing the value 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 were "@d",
where "d" is of the value from "DKIM-Signature"
header field.

5.6 Insert the "d=" tag.

Verifiers DKIM-Signature Header Field

Finally, the signer MUST confirm that insert the domain specified "DKIM-Signature:" header field
created in the "d=" tag is previous step prior to transmitting the email. The
"DKIM-Signature" header field MUST be the same as or a parent domain of used to compute the domain part of
hash as described above, except that the "i=" tag.
If not, value of the DKIM-Signature header field "b=" tag MUST
be ignored and the
verifier should return PERMFAIL (domain mismatch).

If appropriately signed hash computed in the "h=" tag does not include previous step,
signed using the "From" header field algorithm specified in the verifier
MUST ignore "a=" tag of the DKIM-Signature "DKIM-
Signature" header field and return PERMFAIL (From
field not signed).

Verifiers MAY ignore using the DKIM-Signature header field and return
PERMFAIL (signature expired) if it contains an "x=" tag and private key corresponding to
the
signature has expired.

Verifiers MAY ignore Selector given in the DKIM-Signature "s=" tag of the "DKIM-Signature" header field and return
PERMFAIL (unacceptable signature header) for
field, as chosen above in Section 5.2

The "DKIM-Signature" MUST be inserted before any other reason, for
example, if the signature does not sign header DKIM-Signature
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 header block.

INFORMATIVE IMPLEMENTATION NOTE: The easiest way to avoid.

6.1.2 Get achieve this
is to insert the Public Key

The public key for a signature is needed to complete "DKIM-Signature" header field at the verification
process. The process beginning of retrieving the public key depends on
the
query type header block. In particular, it may be placed before any
existing Received header fields. This is consistent with treating
"DKIM-Signature" as defined by the "q=" tag in the "DKIM-Signature:" a trace header field. Obviously,

6. Verifier Actions

Since a signer MAY remove or revoke 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 at any public key records time, it is
recommended that are malformed.

When validating a message, a verifier MUST perform the following
steps verification occur in a manner that is semantically the same as performing them in timely manner. In many
configurations, the order indicated (in some cases most timely place is during acceptance by the implementation may parallelize
border MTA 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 shortly thereafter. In particular, deferring
verification until the query for message is accessed by the public key fails to respond, end user is
discouraged.

A border or intermediate MTA MAY verify the verifier message signature(s). An
MTA who has performed verification MAY defer acceptance communicate the result of this email and return TEMPFAIL (key
unavailable). If that
verification is occurring during the incoming
SMTP session, this MAY be achieved with by adding a 451/4.7.5 SMTP reply
code. Alternatively, verification header field to incoming
messages. This considerably simplifies things for the verifier MAY store user, who can
now use an existing mail user agent. Most MUAs have the ability to
filter messages based on message in the
local queue for later trial header fields or ignore content; these
filters would be used to implement whatever policy the signature. Note that
storing user wishes
with respect to unsigned mail.

A verifying MTA MAY implement a message in the local queue is subject policy with respect to denial-of-
service attacks.

3. If the query for the public key fails because the corresponding
key record does unverifiable
mail, regardless of whether or not exist, it applies the verifier verification header
field to signed messages.

Verifiers 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 produce a result that is semantically equivalent to
applying the key records performing following steps in the remainder order listed. In practice,
several of these steps on each
record at the discretion of can be performed in parallel in order to
improve performance.

6.1 Extract Signatures from the implementer. Message

The order of the
key records in which verifiers try DKIM-Signature header fields is unspecified. If the verifier chooses not
defined; verifiers MAY try signatures in any order they would like.
For example, one implementation might prefer to cycle
through the key records, then try the "return ..." wording signatures in the
remainder of this section means "try the next key record, if any;
if none, return to try
textual order, whereas another signature in the usual way."

5. If the result returned from the query does not adhere might want to prefer signatures by
identities that match the
format defined in this specification, the verifier MUST ignore contents of the key record and return PERMFAIL (key syntax error). "From" header field over
other identities. Verifiers
are urged MUST NOT attribute ultimate meaning to validate
the syntax order of key records carefully to
avoid attempted attacks. multiple DKIM-Signature header fields. In particular, the verifier MUST ignore
keys with a version code ("v=" tag)
there is reason to believe that they do not implement.

6. If some relays will reorder the "g=" tag header
fields in potentially arbitrary ways.

INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the public key does not match the Local-part
of the "i=" tag order as
a clue to signing order in the message signature header field, the
verifier MUST ignore the key record and return PERMFAIL
(inapplicable key). If the Local-part absence of any other information.
However, other clues as to the "i=" tag on the
message signature is not present, the g= tag must be * (valid for
all addresses in the domain) or semantics of multiple signatures
(such as correlating the entire g= tag must signing host with Received header fields)
may also be omitted
(which defaults to "g=*"), otherwise the considered.

A verifier MUST ignore the
key record and return PERMFAIL (inapplicable key). Other than
this test, verifiers SHOULD NOT treat a message signed that has one or more bad
signatures and no good signatures differently from a message with no
signature at all; such treatment is a key
record having matter of local policy and is
beyond the scope of this document.

When a g= tag signature successfully verifies, a verifier will either stop
processing or attempt to verify any differently than one without; in
particular, verifiers SHOULD NOT prefer messages that seem other signatures, at the
discretion of the implementation. A verifier MAY limit the number of
signatures it tries to
have an individual signature by virtue avoid denial-of-service attacks.

INFORMATIVE NOTE: An attacker could send messages with large
numbers of faulty signatures, each of which would require a g= tag versus a
domain signature.

7. If DNS
lookup and corresponding CPU time to verify the "h=" tag exists in message. This
could be an attack on the public key record and domain that receives the hash
algorithm implied message, by
slowing down the verifier by requiring it to do large number of
DNS lookups and/or signature verifications. It could also be an
attack against the a= tag domains listed in the DKIM-Signature header
field is not included signatures, essentially
by enlisting innocent verifiers in launching an attack against the contents
DNS servers of the "h=" tag, actual victim.

In the following description, text reading "return status
(explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
means that the verifier MUST ignore immediately cease processing that
signature. The verifier SHOULD proceed to the key record next signature, if any
is present, and return PERMFAIL
(inappropriate hash algorithm).

8. completely ignore the bad signature. If the public key data (the "p=" tag) status
is empty then this key has
been revoked and "PERMFAIL", the verifier MUST treat this as a failed signature check failed and return PERMFAIL (key revoked). There should not be reconsidered.
If the status is no
defined semantic difference between "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 key that has been revoked 451/4.7.5 SMTP reply, or try another signature; if no good
signature is found and any of the signatures resulted in a key record that has been removed.

9. If TEMPFAIL
status, the public key data verifier MAY save the message for later processing. The
"(explanation)" is not suitable normative text; it is provided solely for use with the algorithm
and key types defined by
clarification.

Verifiers SHOULD ignore any DKIM-Signature header fields where the "a=" and "k=" tags in
signature does not validate. Verifiers that are prepared to validate
multiple signature header fields SHOULD proceed to the "DKIM-
Signature" next signature
header field, should it exist. However, verifiers MAY make note of
the verifier MUST immediately return
PERMFAIL (inappropriate key algorithm).

6.1.3 Compute the Verification

Given a signer and fact that an invalid signature was present for consideration at a public key, verifying
later step.

INFORMATIVE NOTE: The rationale of this requirement is to permit
messages that have invalid signatures but also a valid signature consists of
actions semantically equivalent
to work. For example, a mailing list exploder might opt to leave
the following steps.

1. Based on the algorithm defined original submitter signature in place even though the "c=" tag, exploder
knows that it is modifying the body length
specified message in the "l=" tag, some way that will break
that signature, and the header field names exploder inserts its own signature. In
this case the message should succeed even in the "h="
tag, prepare a canonicalized version presence of the message as is
described in Section 3.7 (note that this version does not
actually need
known-broken signature.

For each signature to be instantiated). When matching header field
names in the "h=" tag against validated, the actual message header field,
comparisons MUST following steps should be case-insensitive.

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

3. Verify to produce a result that the hash of the canonicalized message body computed is
semantically equivalent to performing them in the previous step matches indicated order.

6.1.1 Validate the hash value conveyed Signature Header Field

Implementers MUST meticulously validate the format and values in the "bh="
tag. If
DKIM-Signature header field; any inconsistency or unexpected values
MUST cause the hash does not match, header field to be completely ignored and the verifier SHOULD ignore the
signature and
to return PERMFAIL (body hash did not verify).

4. Using the signature conveyed (signature syntax error). Being "liberal in the "b=" tag, verify the
signature against the header hash using the mechanism appropriate
for the public key algorithm described what
you accept" is definitely a bad strategy in the "a=" tag. If the
signature this security context.
Note however that this does not validate, include the verifier SHOULD existence of unknown tags
in a DKIM-Signature header field, which are explicitly permitted.

Verifiers MUST ignore the
signature DKIM-Signature header fields with a "v=" tag
that is inconsistent with this specification and return PERMFAIL (signature did not verify).

5. Otherwise, the signature has correctly verified.
(incompatible version).

INFORMATIVE IMPLEMENTER'S IMPLEMENTATION NOTE: Implementations might wish An implementation may, of
course, choose to
initiate the public-key query also verify signatures generated by older
versions of this specification.

If any tag listed as "required" in parallel with calculating Section 3.5 is omitted from the
hash as
DKIM-Signature header field, the public key verifier MUST ignore the DKIM-
Signature header field and return PERMFAIL (signature missing
required tag).

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 needed until contain the final decryption "i=" tag,
the verifier MUST behave as though the value of that tag were "@d",
where "d" is
calculated. Implementations may also verify the signature on value from the
message header before validating "d=" tag.

Verifiers MUST confirm that the message hash listed domain specified in the "bh=" "d=" tag in is
the same as or a parent domain of the domain part of the "i=" tag.
If not, the DKIM-Signature header field matches that of MUST be ignored and the actual message body; however, if
verifier should return PERMFAIL (domain mismatch).

If the body hash "h=" tag does not match, include the entire signature must be considered to have failed.

A body length specified in "From" header field the "l=" 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 of and the
signature limits has expired.

Verifiers MAY ignore the
number of bytes of the body passed to DKIM-Signature header field and return
PERMFAIL (unacceptable signature header) for any other reason, for
example, if the verification algorithm.
All data beyond that limit is signature does not validated by DKIM. Hence,
verifiers might treat a message sign header fields that contains bytes beyond the
indicated body length with suspicion, such as by truncating the
message at
verifier views to be essential. As a case in point, if MIME header
fields are not signed, certain attacks may be possible that the indicated body length, declaring
verifier would prefer to avoid.

6.1.2 Get the Public Key

The public key for a signature invalid
(e.g., by returning PERMFAIL (unsigned content)), or conveying is needed to complete the
partial verification to
process. The process of retrieving the policy module.

INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate public key depends on the body
at
query type as defined by the indicated body length might pass on "q=" tag in the "DKIM-Signature:" header
field. Obviously, a malformed MIME
message public key need only be retrieved if the signer used the "N-4" trick (omitting process
of extracting the final
"--CRLF") signature information is completely successful.
Details of key management and representation are 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 3.6. The verifier MUST validate the MIME structure. A simple
way to achieve this might be to append "--CRLF" to key record and MUST
ignore any "multipart"
message with public key records that are malformed.

When validating a body length; if message, a verifier MUST perform the MIME structure following
steps in a manner that is already
correctly formed, this will appear semantically the same as performing them in
the postlude and will not be
displayed to order indicated (in some cases the end user.

6.2 Communicate Verification Results

Verifiers wishing to communicate implementation may parallelize
or reorder these steps, as long as the results of verification to other
parts of semantics remain unchanged):

1. Retrieve the mail system may do so public key as described in whatever manner they see fit.
For example, implementations might choose to add an email header
field to (Section 3.6) using the message before passing it on. Any such header field
SHOULD be inserted before any existing DKIM-Signature or preexisting
authentication status header fields
algorithm 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 "q=" tag, the appropriate verifying domain from the "d=" tag, and that the verified identity
matches
Selector from the author identity that will be displayed by "s=" tag.

2. If the MUA. In
particular, MUA filters should not be influenced by bogus results
header fields added by attackers. To circumvent this attack,
verifiers may wish query for the public key fails to delete existing results header fields after
verification respond, the verifier
MAY defer acceptance of this email and before adding a new header field.

6.3 Interpret Results/Apply Local Policy

It return TEMPFAIL (key
unavailable). If verification is beyond occurring during the scope of incoming
SMTP session, this specification to describe what actions MAY be achieved with a 451/4.7.5 SMTP reply
code. Alternatively, the 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 MAY store the basis
of a lack of signature message in the
local queue for later trial or an unverifiable ignore the signature. However, if Note that
storing a message in the
verifier does opt local queue is subject to reject such messages, and denial-of-
service attacks.

3. If the verifier runs
synchronously with query for the SMTP session and a signature is missing or public key fails because the corresponding
key record does not verify, the MTA SHOULD reject exist, the message with an error such
as:

550 5.7.1 Unsigned messages not accepted

550 5.7.5 Message signature incorrect verifier MUST immediately return
PERMFAIL (no key for signature).

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

451 4.7.5 Unable to verify signature - returns multiple key server unavailable

Temporary failures such as inability to access records, the
verifier may choose one of the key server records or
other external service are may cycle through
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/white lists
and reputation systems) and/or to the end user. If key records performing the message is
signed on behalf remainder of any address other than that in these steps on each
record at the From: header
field, discretion of the mail system SHOULD take pains to ensure that implementer. The order of the actual
signing identity
key records is clear to unspecified. If the reader.

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

While key records, then the symptoms "return ..." wording in the
remainder of a failed verification are obvious -- this section means "try the next key record, if any;
if none, return to try another signature doesn't verify -- establishing in the exact cause can be more
difficult. usual way."

5. If a Selector cannot be found, is that because the
Selector has been removed or was the value changed somehow in
transit? If result returned from the signature line is missing is that because it was
never there, or was it removed by an over-zealous filter? For
diagnostic purposes, query does not adhere to the exact reason why
format defined in this specification, the verification fails
SHOULD be made available to verifier MUST ignore
the policy module key record and possibly recorded
in return PERMFAIL (key syntax error). Verifiers
are urged to validate the system logs. However in terms syntax of presentation key records carefully to
avoid attempted attacks. In particular, the end
user, the result SHOULD be presented as verifier MUST ignore
keys with a simple binary result:
either the email is verified or it is not. version code ("v=" tag) that they do not implement.

6. If the email cannot be
verified, then it SHOULD be rendered "g=" tag in the same as all unverified email
regardless public key does not match the Local-part
of whether it looks like it was signed or not.

7. IANA Considerations

DKIM introduces some new namespaces that require IANA registry.

7.1 DKIM-Signature Tag Specifications

A DKIM-Signature provides for a list 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 specifications. IANA on the
message signature is
requested to establish not present, the DKIM Signature Tag Specification Registry,
for g= tag specifications that can must be used * (valid for
all addresses in DKIM-Signature fields 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 seem to
have been specified an individual signature by virtue of a g= tag versus a
domain signature.

7. If the "h=" tag exists in any published RFC.

The initial entries the public key record and the hash
algorithm implied by the a= tag 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) |
+------+-----------------+

7.2 DKIM-Signature Query Method Registry

The "q=" tag-spec, as specified header
field is not included in Section 3.5 provides for a list the contents of
query methods.

IANA is requested to establish the DKIM Query Method Registry, for
mechanisms that can be used to retrieve "h=" tag, the
verifier MUST ignore the key that will permit
validation processing of a message signed using DKIM record and have return PERMFAIL
(inappropriate hash algorithm).

8. If the public key data (the "p=" tag) is empty then this key has
been
specified in any published RFC.

The initial entry in revoked and the registry comprises:

+------+--------+-----------------+
| TYPE | OPTION | REFERENCE |
+------+--------+-----------------+
| dns | txt | (this document) |
+------+--------+-----------------+

7.3 DKIM-Signature Canonicalization Registry

The "c=" tag-spec, verifier MUST treat this as specified in Section 3.5 provides for a
specifier for canonicalization algorithms for the header failed
signature check and body of
the message.

IANA return PERMFAIL (key revoked). There is requested to establish the DKIM Canonicalization Algorithm
Registry, for algorithms for converting no
defined semantic difference between a message into key that has been revoked
and a canonical
form before signing or verifying using DKIM and have key record that has been specified
in any published RFC.

The initial entries 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 registry comprise:

The initial entries 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 registry comprise:

7.4 _domainkey DNS TXT Record Tag Specifications

A _domainkey DNS TXT record provides for length
specified in the "l=" tag, and the header field names in the "h="
tag, prepare a list canonicalized version of tag
specifications. IANA the message as is requested
described in Section 3.7 (note that this version does not
actually need to establish be instantiated). When matching header field
names in the DKIM _domainkey
DNS TXT Tag Specification Registry, for "h=" tag specifications that can against the actual message header field,
comparisons MUST be used in DNS TXT Records and that have been specified case-insensitive.

2. Based on the algorithm indicated in any
published RFC.

The initial entries the "a=" tag, compute the
message hashes from the canonical copy as described in
Section 3.7.

3. Verify that 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) |
+------+-----------------+

7.5 DKIM Key Type Registry

The "k=" <key-k-tag> (as specified in Section 3.6.1) and the "a="
<sig-a-tag-k> (Section 3.5) tags provide for a list hash of mechanisms
that can be used to decode a DKIM signature.

IANA is requested to establish the DKIM Key Type Registry, for such
mechanisms that have been specified in any published RFC.

The initial entry canonicalized message body computed
in the registry comprises:

+------+-----------+
| TYPE | REFERENCE |
+------+-----------+
| rsa | [RFC3447] |
+------+-----------+

7.6 DKIM Hash Algorithms Registry

The "h=" <key-h-tag> list (specified previous step matches the hash value conveyed in Section 3.6.1) 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 "a="
<sig-a-tag-h> (Section 3.5) provide for a list of mechanisms that can
be used to produce a digest of message data.

IANA is requested to establish signature conveyed in the DKIM Hash Algorithms Registry, "b=" tag, verify the
signature against the header hash using the mechanism appropriate
for
such mechanisms that have been specified in any published RFC.

The initial entries the public key algorithm described in the registry comprise:

+--------+-----------+
| TYPE | REFERENCE |
+--------+-----------+
| sha1 | [SHA] |
| sha256 | [SHA] |
+--------+-----------+

7.7 DKIM Service Types Registry

The "s=" <key-s-tag> list (specified in Section 3.6.1) provides for a
list of service types to which this selector may apply.

IANA is requested "a=" tag. If the
signature does not validate, the verifier SHOULD ignore the
signature and return PERMFAIL (signature did not verify).

5. Otherwise, the signature has correctly verified.

INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to establish
initiate the DKIM Service Types Registry, for
service types that have been specified in any published RFC.

The initial entries public-key query in parallel with calculating the registry comprise:

+-------+-----------------+
| TYPE | REFERENCE |
+-------+-----------------+
| email | (this document) |
| * | (this document) |
+-------+-----------------+

7.8 DKIM Selector Flags Registry

The "t=" <key-t-tag> list (specified in Section 3.6.1) provides for a
list of flags to modify interpretation of
hash as the selector.

IANA public key is requested to establish not needed until the DKIM Selector Flags Registry, for
additional flags final decryption is
calculated. Implementations may also verify the signature on the
message header before validating that have been specified the message hash listed in any published RFC.

The initial entries
the "bh=" tag in the registry comprise:

+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| y | (this document) |
| s | (this document) |
+------+-----------------+

7.9 DKIM-Signature Header Field

IANA is requested to add DKIM-Signature to header field matches that of
the "Permanent Header
Messages" registry for actual message body; however, if the "mail" protocol, using this document as body hash does not match,
the Reference.

8. Security Considerations

It has been observed that any mechanism that is introduced which
attempts to stem the flow of spam is subject to intensive attack.
DKIM needs to entire signature must be carefully scrutinized considered to identify potential attack
vectors and have failed.

A body length specified in the vulnerability to each. See also [RFC4686].

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

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

8.1.1 Addition bytes of new MIME parts to multipart/*

If the body length passed to the verification algorithm.
All data beyond that limit does is not cover validated by DKIM. Hence,
verifiers might treat a closing MIME multipart
section (including message that contains bytes beyond the
indicated body length with suspicion, such as by truncating the trailing ""--CRLF"" portion), then it is
possible for an attacker to intercept a properly signed multipart
message and add a new at the indicated body part. Depending on length, declaring the details of 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 type and
message if the implementation of signer used the verifying MTA and "N-4" trick (omitting the
receiving MUA, final
"--CRLF") described in the informative note in Section 3.4.5.
Such verifiers may wish to check for this could allow an attacker case and include a
trailing "--CRLF" to change avoid breaking the information
displayed MIME structure. A simple
way to achieve this might be to an end user from an apparently trusted source.

For example, if an attacker can append information "--CRLF" to any "multipart"
message with a "text/html" body part, they may be able to exploit a bug length; if the MIME structure is already
correctly formed, this will appear in some MUAs that
continue to read after a "</html>" marker, the postlude and thus display HTML text
on top of already displayed text. If a message has a "multipart/
alternative" body part, they might will not be able
displayed to add a new body part
that is preferred by the displaying MTA.

8.1.2 Addition end user.

6.2 Communicate Verification Results

Verifiers wishing to communicate the results of new HTML content verification 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 other
parts of existing text.

INFORMATIVE EXAMPLE: Appending the following text mail system may do so in whatever manner they see fit.
For example, implementations might choose to add an existing,
properly closed email header
field to the message will in many MUAs result before passing it on. Any such header field
SHOULD be inserted before any existing DKIM-Signature or preexisting
authentication status header fields 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 header field block.

INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
search for malware results header fields to send visibly mark authenticated
mail as for end users should verify that user and any other user sharing such header field was added
by the
same private key. The malware would, however, 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 able influenced by bogus results
header fields added by attackers. To circumvent this attack,
verifiers may wish to
generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user delete existing results header fields after
verification and would limit before adding a new header field.

6.3 Interpret Results/Apply Local Policy

It is beyond the
possibilities for certain types scope of attacks involving socially-
engineered messages. This threat applies mainly this specification to MUA-based
implementations; protection of private keys on servers 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 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 managed, such as trust and transmits them via a covert
channel to
reputation. Conversely, unauthenticated email lacks a site where they reliable
identifier that can be shared. The compromised users
would likely not know of the misappropriation until they receive
"bounce" messages from messages they are purported used to have sent.
Many users might not understand the significance of these bounce
messages assign trust and would not take action.

One countermeasure reputation. It is
reasonable to use a user-entered passphrase to encrypt the
private key, although users tend to choose weak passphrases treat unauthenticated email as lacking any trust and often
reuse them for different purposes, possibly allowing
having no positive reputation.

In general verifiers SHOULD NOT reject messages solely on the basis
of a lack of signature or an attack
against DKIM to be extended into other domains. Nevertheless, unverifiable signature; such rejection
would cause severe interoperability problems. However, if the
decoded private key might be briefly available
verifier does opt to compromise by
malware reject such messages (for example, when it is entered, or might be discovered via keystroke
logging. The added complexity of entering a passphrase each time one
sends
communicating with a message would also tend peer who, by prior agreement, agrees to discourage 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 of a secure
passphrase.

A somewhat more effective countermeasure 550/5.7.x reply code, for example:

550 5.7.1 Unsigned messages not accepted

550 5.7.5 Message signature incorrect

If it is not possible to send messages through
an outgoing MTA that can authenticate the submitter using existing
techniques (e.g., SMTP Authentication), possibly validate fetch the message
itself (e.g., verify that public key, perhaps because the header
key server is legitimate and that the
content passes not available, a spam content check), and sign the 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 appropriate for server unavailable

Temporary failures such as inability to access the submitter address. Such an MTA can also
apply controls on key server or
other external service are 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), only conditions that SHOULD use a 4xx
SMTP reply code. In particular, cryptographic signature verification
failures MUST NOT return 4xx SMTP replies.

Once the number of servers signature has been verified, that would need to information MUST be attacked
conveyed to higher level systems (such as explicit allow/white lists
and reputation systems) and/or to
defeat this mechanism on an Internet-wide basis is very large.
Nevertheless, key servers for individual domains could be attacked,
impeding the verification end user. If the message is
signed on behalf of messages from any address other than that domain. This is not
significantly different from in the ability of an attacker to deny
service to From: header
field, the mail exchangers for a given domain, although it
affects outgoing, not incoming, mail.

A variation on this attack is system SHOULD take pains to ensure that if a very large amount of mail
were the actual
signing identity is clear to be sent using spoofed addresses from a given domain, the key
servers for that domain could be overwhelmed reader.

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

While the symptoms of a failed verification compared with handling of are obvious -- the
email message itself, such an attack would
signature doesn't verify -- establishing the exact cause can be difficult to mount.

8.4 Attacks Against DNS

Since DNS is more
difficult. If a required binding for key services, specific attacks
against DNS must Selector cannot be considered.

While the DNS is currently insecure [RFC3833], it found, is expected that because the security problems should and will be solved by DNSSEC [RFC4033],
and all users of
Selector has been removed or was the DNS will reap value changed somehow in
transit? If the benefit of signature line is missing is that work.

Secondly, the types of DNS attacks relevant to DKIM are very costly
and are far less rewarding than DNS attacks on other Internet
protocols. because it was
never there, or was it removed by an over-zealous filter? For example, attacking A records (to force users to a
phishing site) is likely to be a more lucrative reason to poison DNS
caches. None
diagnostic purposes, the less, exact reason why the security of DKIM is strongly tied verification fails
SHOULD be made available to the
security of DNS.

To systematically thwart the intent of DKIM, an attacker must conduct
a very costly policy module and very extensive attack on many parts of possibly recorded
in the DNS over
an extended period. No one knows for sure how attackers will
respond, however system logs. If the cost/benefit of conducting prolonged DNS attacks
of this nature is expected to email cannot be uneconomical.

Finally, verified, then it SHOULD
be rendered the same as all unverified email regardless of whether it
looks like it was signed or not.

7. IANA Considerations

DKIM is introduces some new namespaces that require IANA registry. In
all cases, new values are assigned only intended as for Standards Track RFCs
approved by the IESG.

7.1 DKIM-Signature Tag Specifications

A DKIM-Signature provides for a "sufficient" method list of proving
authenticity. It tag specifications. IANA is not intended
requested 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 establish the
increased DNS traffic as a consequence of fetching Selector-based
data as well as fetching signing domain policy. Widespread
deployment of DKIM will result Signature Tag Specification Registry,
for tag specifications that can be used in a significant increase DKIM-Signature fields and
that have been specified in any published RFC.

The initial entries in DNS
queries to the claimed signing domain. In the case of forgeries on a
large scale, DNS servers could see registry comprise:

+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| v | (this document) |
| a substantial increase | (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) |
+------+-----------------+

7.2 DKIM-Signature Query Method Registry

The "q=" tag-spec, as specified in queries.

8.5 Replay Attacks

In this attack, a spammer sends Section 3.5 provides for a message list of
query methods.

IANA is requested to establish the DKIM Query Method Registry, for
mechanisms that can be spammed used to an
accomplice, which results in retrieve the key that will permit
validation processing of a message being signed by the
originating MTA. using DKIM and have been
specified in any published RFC.

The accomplice resends the message, including initial entry in the
original signature, to registry comprises:

+------+--------+-----------------+
| TYPE | OPTION | REFERENCE |
+------+--------+-----------------+
| dns | txt | (this document) |
+------+--------+-----------------+

7.3 DKIM-Signature Canonicalization Registry

The "c=" tag-spec, as specified in Section 3.5 provides for a large number
specifier for canonicalization algorithms for the header and body of recipients, possibly by
sending
the message message.

IANA is requested to many compromised machines that act as MTAs.
The messages, not having been modified by establish the accomplice, DKIM Canonicalization Algorithm
Registry, for algorithms for converting a message into a canonical
form before signing or verifying using DKIM and have valid
signatures.

Partial solutions to this problem involve been specified
in any published RFC.

The initial entries in the use header registry comprise:

The initial entries in the body registry comprise:

7.4 _domainkey DNS TXT Record Tag Specifications

A _domainkey DNS TXT record provides for a list of reputation
services tag
specifications. IANA is requested to convey establish the fact DKIM _domainkey
DNS TXT Tag Specification Registry, for tag specifications that the specific email address is being can
be used for spam, in DNS TXT Records and that messages from that signer are likely to be
spam. This requires a real-time detection mechanism have been specified 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 any
published RFC.

The initial entries 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 registry comprise:

7.5 DKIM Key Type Registry

The "k=" <key-k-tag> (as specified in a short time period. Smaller
verifiers can get substantially the same volume information via
existing collaborative systems.

8.6 Limits on Revoking Keys

When a large domain detects undesirable behavior on the part of one
of its users, it might wish to revoke Section 3.6.1) and the key used to sign that
user's messages in order to disavow responsibility "a="
<sig-a-tag-k> (Section 3.5) tags provide for messages which
have not yet been verified or which are the subject of a replay
attack. However, the ability list of the domain to do so mechanisms
that 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 decode a
per-address basis have been proposed but require further study as DKIM signature.

IANA is requested to
their utility and establish the DNS load they represent.

8.7 Intentionally malformed DKIM Key Records

It is possible Type Registry, for an attacker to publish key records such
mechanisms that have been specified in any published RFC.

The initial entry in DNS which
are intentionally malformed, with the intent of causing a denial-of-
service attack on a non-robust verifier implementation. registry comprises:

+------+-----------+
| TYPE | REFERENCE |
+------+-----------+
| rsa | [RFC3447] |
+------+-----------+

7.6 DKIM Hash Algorithms Registry

The attacker
could then cause a verifier to read "h=" <key-h-tag> list (specified in Section 3.6.1) and the malformed key record by
sending "a="
<sig-a-tag-h> (Section 3.5) provide for a message to one list of its users referencing the malformed
record in a (not necessarily valid) signature. Verifiers MUST
thoroughly verify all key records retrieved from DNS and be robust
against intentionally as well as unintentionally malformed key
records.

8.8 Intentionally Malformed DKIM-Signature header fields

Verifiers MUST mechanisms that can
be prepared used 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 produce a per-message Selector and then monitoring their DNS traffic
for the key lookup. This would act as the equivalent digest 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 data.

IANA is requested to extract private keys using a
remote timing attack [BONEH03]. Implementations should consider
obfuscating establish the timing to prevent DKIM Hash Algorithms Registry, for
such attacks.

8.11 Reordered Header Fields

Existing standards mechanisms that have been specified in any published RFC.

The initial entries in the registry comprise:

+--------+-----------+
| TYPE | REFERENCE |
+--------+-----------+
| sha1 | [SHA] |
| sha256 | [SHA] |
+--------+-----------+

7.7 DKIM Service Types Registry

The "s=" <key-s-tag> list (specified in Section 3.6.1) provides for a
list of service types to which this selector may apply.

IANA is requested to establish the DKIM Service Types Registry, for
service types that have been specified in any published RFC.

The initial entries in the registry comprise:

+-------+-----------------+
| TYPE | REFERENCE |
+-------+-----------------+
| email | (this document) |
| * | (this document) |
+-------+-----------------+

7.8 DKIM Selector Flags Registry

The "t=" <key-t-tag> list (specified in Section 3.6.1) provides for a
list of flags to modify interpretation of the selector.

IANA is requested to establish the DKIM Selector Flags Registry, for
additional flags that have been specified in any published RFC.

The initial entries in the registry comprise:

+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| y | (this document) |
| s | (this document) |
+------+-----------------+

7.9 DKIM-Signature Header Field

IANA is requested to add DKIM-Signature to the "Permanent Header
Messages" registry for the "mail" protocol, using this document as
the Reference.

8. Security Considerations

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

For example, if an attacker 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 MTA.

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, however, not be able to
generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user and would limit the
possibilities for certain types of attacks involving socially-
engineered messages. 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
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 DNS

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

While the DNS is currently insecure [RFC3833], these security
problems are the motivation behind 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
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 which 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 information via
existing collaborative systems.

8.6 Limits on Revoking Keys

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

8.7 Intentionally malformed Key Records

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

8.8 Intentionally Malformed DKIM-Signature header fields

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

8.9 Information Leakage

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

8.10 Remote Timing Attacks

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

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. 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.

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.

9. References

9.1 Normative References

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

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

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

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

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

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

[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
March 2003.

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

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

[X.660] "ITU-T Recommendation X.660 Information Technology - ASN.1
encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER)", 1997.

9.2 Informative References

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

[RFC-DK] "DomainKeys specification (to be published with this
RFC)", 2005.

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

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

[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.

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

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

[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.

Authors' Addresses

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

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

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

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

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

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

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

Email: mlibbeymail-mailsig@yahoo.com
URI:

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

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

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

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

Appendix A. Example of Use (INFORMATIVE)

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

A.1 The user composes an email

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

Hi.

We lost the game. Are you hungry yet?

Joe.

A.2 The email is signed

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

DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
bh=jpltwNFTq83Bkjt/Y2ekyqr/+i296daNkFZSdaz8VCY=;
b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B
7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt
zEn8/OHpTCcdLOJaTp8/mKz69/RpatVBas2OqWas7jrlaLGfHdBkt
Hs6fxOzzAB7Wro=;
Received: from dsl-10.2.3.4.football.example.com [10.2.3.4] 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 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
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: a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
bh=jpltwNFTq83Bkjt/Y2ekyqr/+i296daNkFZSdaz8VCY=;
b=bnUoMBPJ5wBigyZG2V4OG2JxLWJATkSkb9Ig+8OAu3cE2x/er+B
7Tp1a1kEwZKdOtlTHlvF4JKg6RZUbN5urRJoaiD4RiSbf8D6fmMHt
zEn8/OHpTCcdLOJaTp8/mKz69/RpatVBas2OqWas7jrlaLGfHdBkt
Hs6fxOzzAB7Wro=;
Received: from dsl-10.2.3.4.network.example.com [10.2.3.4] 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.

INFORMATIVE NOTE: The key used to compute this signature is shown
in Appendix C.

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.

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 on-going 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.

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 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 the user occasionally requests 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",
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 which
authenticates the user and allows the forwarding address to be
changed. However these services usually depend on the user's sending
outgoing messages through their own service provider's MTA. 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
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
which make no modification to the message itself, other than to add a
Received header field and change the envelope information, to those
which 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 produces messages that are quite similar to
the automated alias services. More elaborate systems essentially
create a new message.

A Forwarder which 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 only those messages which are received with a
valid signature or some other basis for knowing that the messages
being signed is not spoofed.

A common practice among systems that are primarily re-distributors 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 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:

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

$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
This results in the file rsa.public containing the key information
similar to this:

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

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

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

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

This results in signature data similar to this when represented in
Base64 [MIME] format: as a
base64string:

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

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

The final record entered into a DNS zone file would be:

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 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, or with a negative
indication on the unsigned header fields or 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 address associated with this signing
identity in some way, in an attempt MUA author, be
deemed important to show the user end user.

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 Faltstrom, 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, 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 address 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 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
specification has been derived. Further information about DomainKeys
is at [RFC-DK].

Appendix F. Edit History

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

F.1 Changes since -ietf-07 version

The following changes were made between draft-ietf-dkim-base-07 and
draft-ietf-dkim-base-08:

o Drop reference to "trusted third party" in section 1; it was
redundant with a
positive indication on existing bullet points and created confusion.

o Drop the signed header fields, wording on re-using keys from normative to an operational
note.

o Add a sentence to 3.4.3 to clarify how empty or missing bodies are
canonicalized.

o Section 3.5, t= tag: note that a verifier MAY ignore signatures
with a negative
indication on timestamp in the unsigned header fields or by visually hiding future.

o Section 3.5, z= tag: dropped the
unsigned header fields, or some combination MUST NOT wording.

o Clarify the description of these. If an MUA uses
visual indications g= in section 3.6.1.

o Mention that n= is not intended for signed header fields, the MUA probably needs end-user use in section 3.6.1.

o Modify wording of s= in section 3.6.1 so as to be careful not imply possible
future uses.

o Add a reference to display unsigned header fields RFC 2821 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 section 3.7 to the end of the
message, the MUA might also hide or mark the portion describe dot-
stuffing.

o Fairly extensive update of the message
body that was not signed.

The aforementioned information section 4 as requested during IESG
review.

o DKIM-Signature is not intended a "trace header field" as defined by RFC
2822 (section 5.4).

o Add sentence in section 5.4 to be exhaustive. The
MUA may choose discourage signing of existing
DKIM-Signature header fields.

o Added section 5.4.1 describing recommended headers for signing per
IESG review.

o Dropped comment about presenting binary result to highlight, accentuate, hide, or otherwise display
any other information end user in
section 6.3 (out of scope, and by IESG request).

o Clarify that SMTP-level rejects are discouraged, but that may, in the opinion of if they
are used they should use the MUA author, be
deemed important indicated reply codes (section 6.3).

o Add text to the end user.

Appendix E. Acknowledgements

The authors wish section 7 (IANA Considerations) 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, Patrik Faltstrom, Mark Fanto, Stephen Farrell, Duncan
Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony Hansen, Sam
Hartman, Arvel Hathcock, Amir Herzberg, Paul Hoffman, Russ Housley,
Craig Hughes, Don Johnsen, Harry Katz, Murray S. Kucherawy, Barry
Leiba, John Levine, Simon Longsdale, David Margrave, Justin Mason,
David Mayne, Steve Murphy, Russell Nelson, Dave Oran, 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 make it clear that
registry updates require a primary source from which this
specification has been derived. Further information about DomainKeys
is at [RFC-DK].

Appendix F. Edit History

[[This standards-track document.

o Rewrote section 8.4 per request by Security Area review.

o Add sentence in section 8.11 to be removed before publication.]]

F.1 emphasize that signing existing
DKIM-Signature header fields may result in incorrect validation
failures, as requested by Security Area review.

o Added section 8.14 (RSA Attacks) based on DNS-dir review from
Olafur Gu[eth]mundsson.

o Added section 8.15 (Inappropriate Signing by Parent Domains).

F.2 Changes since -ietf-06 version

The following changes were made between draft-ietf-dkim-base-06 and
draft-ietf-dkim-base-07:

o Added section 8.11 regarding header reordering.

o Added informative note to section 3.3 regarding use of sha256.

o Added informative rationale to section 3.6.1, "p=", regarding key
revocation.

o Added second informative note in section 4 regarding signing
multiple DKIM-Signature header fields.

o Minor modification of the second informative note in section 6.1
regarding DoS attacks.

o Added explicit mention of v= to section 6.1.2, step 5.

o Updated paragraph 3 of section 8.4 regarding DNS attacks.

o Added section 7.9 (DKIM-Signature IANA Registry) per IANA request.

F.2

F.3 Changes since -ietf-05 version

The following changes were made between draft-ietf-dkim-base-05 and
draft-ietf-dkim-base-06:

o Fix an error in an example in Appendix C.

o Substantial updates to Appendixes B and D.

o Clarify ABNF for tag-value.

o Changed SWSP (streaming white space) to LWSP (linear white space
from RFC 4234); LWSP makes it clear that white space is required
after a CRLF.

o Add normative reference to SHA1/SHA256 FIPS publication 180-2.

o Several minor edits based on AD Review.

o Move discussion of not re-using a selector (i.e., changing the
public key for a single selector) from informational to normative.

o Assorted wordsmithing based on external review.

F.3

F.4 Changes since -ietf-04 version

The following changes were made between draft-ietf-dkim-base-04 and
draft-ietf-dkim-base-05:

o Clarified definition of "plain text" in section 3.2 (issue 1316).

o Added some clarification about multiple listings of non-existent
header field names in h= in section 5.4 (issue 1316).

o Finished filling out IANA registries in section 7 (issue 1320).

o Clarified handling of bare CR and LF in section 5.3 (issue 1326).

o Listed the required tags in section 6.1.1 as an informational note
(issue 1330).

o Changed IDNA reference from 3492 to 3490 (issue 1331).

o Changed the reference for WSP to 4234; changed the definition of
SWSP to exclude bare CR and LF (issue 1332).

F.4

F.5 Changes since -ietf-03 version

The following changes were made between draft-ietf-dkim-base-03 and
draft-ietf-dkim-base-04:

o Re-worded Abstract to avoid use of "prove" and "non-repudiation".

o Use dot-atom-text instead of dot-atom to avoid inclusion of CFWS.

o Capitalize Selector throughout.

o Add discussion of plain text, mentioning informatively that
implementors should plan for eventual 8-bit requirements.

o Drop RSA requirement of exponent of 65537 (not required, since it
is already in the key) and clarify the key format.

o Drop SHOULD that DKIM-Signature should precede header fields that
it signs.

o Mention that wildcard DNS records MUST NOT be used for selector
records.

o Add section 3.8 to clarify the t=s flag.

o Change the list of header fields that MUST be signed to include
only From.

o Require that verifier check that From is in the list of signed
header fields.

o Drop all reference to draft-kucherawy-sender-auth-header draft.

o Substantially expand Section 7 (IANA Considerations) to include
initial registries.

o Add section B.7 (use case: SMTP Servers for Roaming Users).

o Add several examples; update some others.

o Considerable minor editorial updating to clarify language, delete
redundant text, fix spelling errors, etc.

Still to be resolved:

o How does "simple" body canonicalization interact with BINARYMIME
data?
o Deal with "relaxed" body canonicalizations, especially in regard
to bare CRs and NLs.

o How to handle "*" in g= dot-atom-text (which allows "*" as a
literal character).

o The IANA Considerations need to be completed and cleaned up.

F.5

F.6 Changes since -ietf-02 version

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

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

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

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

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

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

o Clarification of "signing identity" in several places.

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

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

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

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

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

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

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

F.6

F.7 Changes since -ietf-01 version

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

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

o Several changes to limit wording that would intrude into verifier
policy. This is largely changing statements such as "... MUST
reject the message" to "... MUST consider the signature invalid."

o Drop normative references to ID-DKIM-RR, OpenSSL, PEM, and
Stringprep.

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

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

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

F.7

F.8 Changes since -ietf-00 version

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

o Added section 8.9 (Information Leakage).

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

o Fixed ABNF for base64string.

o Added rsa-sha256 signing algorithm.

o Expanded several examples.

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

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

o Significant wordsmithing.

F.8

F.9 Changes since -allman-01 version

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

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

o Added ABNF for all tags.

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

o Significant wordsmithing.

F.9

F.10 Changes since -allman-00 version

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

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

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

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

o Added several IANA Considerations.

o Fixed a number of grammar and formatting errors.

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