draft-ietf-dkim-base-03.txt   draft-ietf-dkim-base-04.txt 
DKIM E. Allman DKIM E. Allman
Internet-Draft Sendmail, Inc. Internet-Draft Sendmail, Inc.
Expires: December 27, 2006 J. Callas Expires: January 16, 2007 J. Callas
PGP Corporation PGP Corporation
M. Delany M. Delany
M. Libbey M. Libbey
Yahoo! Inc Yahoo! Inc
J. Fenton J. Fenton
M. Thomas M. Thomas
Cisco Systems, Inc. Cisco Systems, Inc.
June 25, 2006 July 15, 2006
DomainKeys Identified Mail (DKIM) Signatures DomainKeys Identified Mail (DKIM) Signatures
draft-ietf-dkim-base-03 draft-ietf-dkim-base-04
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware 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 have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 41 skipping to change at page 1, line 41
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 27, 2006. This Internet-Draft will expire on January 16, 2007.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key cryptography and authentication framework for email using public-key cryptography and
key server technology to permit verification of the source and key server technology to permit verification of the source and
contents of messages by either Mail Transfer Agents (MTAs) or Mail contents of messages by either Mail Transfer Agents (MTAs) or Mail
User Agents (MUAs). The ultimate goal of this framework is to permit User Agents (MUAs). The ultimate goal of this framework is to permit
a signing domain to assert responsibility for a message, thus proving a signing domain to assert responsibility for a message, thus
and protecting message signer identity and the integrity of the protecting message signer identity and the integrity of the messages
messages they convey while retaining the functionality of Internet they convey while retaining the functionality of Internet email as it
email as it is known today. Proof and protection of email identity, is known today. Protection of email identity may assist in the
including repudiation and non-repudiation, may assist in the global global control of "spam" and "phishing".
control of "spam" and "phishing".
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6
1.2 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 1.2 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Simple Key Management . . . . . . . . . . . . . . . . . . 6
1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . 6
2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 White Space . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7 2.4 Common ABNF Tokens . . . . . . . . . . . . . . . . . . . . 7
2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8 2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 7
2.6 DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8 2.6 DKIM-Quoted-Printable . . . . . . . . . . . . . . . . . . 8
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 9 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 9
3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11 3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 11
3.3 Signing and Verification Algorithms . . . . . . . . . . . 12 3.3 Signing and Verification Algorithms . . . . . . . . . . . 12
3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 13 3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 14
3.5 The DKIM-Signature header field . . . . . . . . . . . . . 18 3.5 The DKIM-Signature header field . . . . . . . . . . . . . 18
3.6 Key Management and Representation . . . . . . . . . . . . 25 3.6 Key Management and Representation . . . . . . . . . . . . 26
3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 30 3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 30
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 31 3.8 Signing by Parent Domains . . . . . . . . . . . . . . . . 32
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 32
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 32 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 32
5.1 Determine if the Email Should be Signed and by Whom . . . 32 5.1 Determine if the Email Should be Signed and by Whom . . . 33
5.2 Select a private-key and corresponding selector 5.2 Select a private-key and corresponding selector
information . . . . . . . . . . . . . . . . . . . . . . . 32 information . . . . . . . . . . . . . . . . . . . . . . . 33
5.3 Normalize the Message to Prevent Transport Conversions . . 33 5.3 Normalize the Message to Prevent Transport Conversions . . 34
5.4 Determine the header fields to Sign . . . . . . . . . . . 33 5.4 Determine the header fields to Sign . . . . . . . . . . . 34
5.5 Compute the Message Hash and Signature . . . . . . . . . . 35 5.5 Compute the Message Hash and Signature . . . . . . . . . . 36
5.6 Insert the DKIM-Signature header field . . . . . . . . . . 36 5.6 Insert the DKIM-Signature header field . . . . . . . . . . 36
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 37 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 37
6.1 Extract Signatures from the Message . . . . . . . . . . . 37 6.1 Extract Signatures from the Message . . . . . . . . . . . 37
6.2 Communicate Verification Results . . . . . . . . . . . . . 42 6.2 Communicate Verification Results . . . . . . . . . . . . . 43
6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 42 6.3 Interpret Results/Apply Local Policy . . . . . . . . . . . 43
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 44 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 44
8. Security Considerations . . . . . . . . . . . . . . . . . . 44 7.1 DKIM-Signature Tag Specifications . . . . . . . . . . . . 44
8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 44 7.2 DKIM-Signature Query Method Registry . . . . . . . . . . . 45
8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 45 7.3 DKIM-Signature Canonicalization Registry . . . . . . . . . 45
8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 45 7.4 _domainkey DNS TXT Record Tag Specifications . . . . . . . 46
8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 46 7.5 DKIM Key Type Registry . . . . . . . . . . . . . . . . . . 47
8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 46 7.6 DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 47
8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 47 8. Security Considerations . . . . . . . . . . . . . . . . . . 47
8.7 Intentionally malformed Key Records . . . . . . . . . . . 47 8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 47
8.8 Intentionally Malformed DKIM-Signature header fields . . . 47 8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 48
8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 48 8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 49
8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 48 8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 49
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 50
9.1 Normative References . . . . . . . . . . . . . . . . . . . 48 8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 51
9.2 Informative References . . . . . . . . . . . . . . . . . . 49 8.7 Intentionally malformed Key Records . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 50 8.8 Intentionally Malformed DKIM-Signature header fields . . . 51
A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 51 8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 51
A.1 The user composes an email . . . . . . . . . . . . . . . . 51 8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 51
A.2 The email is signed . . . . . . . . . . . . . . . . . . . 51 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 52
A.3 The email signature is verified . . . . . . . . . . . . . 52 9.1 Normative References . . . . . . . . . . . . . . . . . . . 52
B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 53 9.2 Informative References . . . . . . . . . . . . . . . . . . 52
B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 53 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 53
B.2 Outsourced Business Functions . . . . . . . . . . . . . . 53 A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 54
B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 54 A.1 The user composes an email . . . . . . . . . . . . . . . . 55
B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 54 A.2 The email is signed . . . . . . . . . . . . . . . . . . . 55
B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 55 A.3 The email signature is verified . . . . . . . . . . . . . 56
B.6 Third-party Message Transmission . . . . . . . . . . . . . 55 B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 57
C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 56 B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 57
D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 57 B.2 Outsourced Business Functions . . . . . . . . . . . . . . 57
E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 58 B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 57
F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 58 B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 58
F.1 Changes since -ietf-02 version . . . . . . . . . . . . . . 58 B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 58
F.2 Changes since -ietf-01 version . . . . . . . . . . . . . . 59 B.6 Third-party Message Transmission . . . . . . . . . . . . . 59
F.3 Changes since -ietf-00 version . . . . . . . . . . . . . . 60 B.7 SMTP Servers for Roaming Users . . . . . . . . . . . . . . 59
F.4 Changes since -allman-01 version . . . . . . . . . . . . . 60 C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 59
F.5 Changes since -allman-00 version . . . . . . . . . . . . . 61 D. MUA Considerations . . . . . . . . . . . . . . . . . . . . . 61
Intellectual Property and Copyright Statements . . . . . . . 62 E. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 62
F. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 62
F.1 Changes since -ietf-03 version . . . . . . . . . . . . . . 62
F.2 Changes since -ietf-02 version . . . . . . . . . . . . . . 63
F.3 Changes since -ietf-01 version . . . . . . . . . . . . . . 64
F.4 Changes since -ietf-00 version . . . . . . . . . . . . . . 65
F.5 Changes since -allman-01 version . . . . . . . . . . . . . 66
F.6 Changes since -allman-00 version . . . . . . . . . . . . . 66
Intellectual Property and Copyright Statements . . . . . . . 67
1. Introduction 1. Introduction
[[Note: text in double square brackets (such as this text) will be [[Note: text in double square brackets (such as this text) will be
deleted before publication.]] deleted before publication.]]
1.1 Overview
DomainKeys Identified Mail (DKIM) defines a mechanism by which email DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key, the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain. possession of the private key for the signing domain.
The approach taken by DKIM differs from previous approaches to The approach taken by DKIM differs from previous approaches to
message signing (e.g. S/MIME [RFC1847], OpenPGP [RFC2440]) in that: message signing (e.g. S/MIME [RFC1847], OpenPGP [RFC2440]) in that:
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neither human recipients nor existing MUA (Mail User Agent) neither human recipients nor existing MUA (Mail User Agent)
software are confused by signature-related content appearing in software are confused by signature-related content appearing in
the message body, the message body,
o there is no dependency on public and private key pairs being o there is no dependency on public and private key pairs being
issued by well-known, trusted certificate authorities, issued by well-known, trusted certificate authorities,
o there is no dependency on the deployment of any new Internet o there is no dependency on the deployment of any new Internet
protocols or services for public key distribution or revocation, protocols or services for public key distribution or revocation,
o it makes no attempt to include encryption as part of the o signature verification failure does not result in rejection of the
mechanism. message,
o no attempt is made to include encryption as part of the mechanism,
o archival is not a design goal.
DKIM: DKIM:
o is compatible with the existing email infrastructure and o is compatible with the existing email infrastructure and
transparent to the fullest extent possible transparent to the fullest extent possible
o requires minimal new infrastructure o requires minimal new infrastructure
o can be implemented independently of clients in order to reduce o can be implemented independently of clients in order to reduce
deployment time deployment time
o does not require the use of a trusted third party (such as a o does not require the use of an additional trusted third party
certificate authority or other entity) which might impose (such as a certificate authority or other entity) which might
significant costs or introduce delays to deployment impose significant costs or introduce delays to deployment
o can be deployed incrementally o can be deployed incrementally
o allows delegation of signing to third parties
o is not intended be used for archival purposes
A "selector" mechanism allows multiple keys per domain, including o allows delegation of signing to third parties
delegation of the right to authenticate a portion of the namespace to
a trusted third party.
1.2 Signing Identity 1.1 Signing Identity
DKIM separates the question of the identity of the signer of the DKIM separates the question of the identity of the signer of the
message from the purported author of the message. In particular, a message from the purported author of the message. In particular, a
signature includes the identity of the signer. Verifiers can use the signature includes the identity of the signer. Verifiers can use the
signing information to decide how they want to process the message. signing information to decide how they want to process the message.
The signing identity is included as part of the signature header The signing identity is included as part of the signature header
field. field.
INFORMATIVE RATIONALE: The signing identity associated with a INFORMATIVE RATIONALE: The signing identity specified by a DKIM
DKIM signature is not required to match an address in any signature is not required to match an address in any particular
particular header field because of the broad methods of header field because of the broad methods of interpretation by
interpretation by recipient mail systems, including MUAs. recipient mail systems, including MUAs.
1.3 Scalability 1.2 Scalability
DKIM is designed to support the extreme scalability requirements DKIM is designed to support the extreme scalability requirements
which characterize the email identification problem. There are which characterize the email identification problem. There are
currently over 70 million domains and a much larger number of currently over 70 million domains and a much larger number of
individual addresses. DKIM seeks to preserve the positive aspects of individual addresses. DKIM seeks to preserve the positive aspects of
the current email infrastructure, such as the ability for anyone to the current email infrastructure, such as the ability for anyone to
communicate with anyone else without introduction. communicate with anyone else without introduction.
1.4 Simple Key Management 1.3 Simple Key Management
DKIM differs from traditional hierarchical public-key systems in that DKIM differs from traditional hierarchical public-key systems in that
no key signing infrastructure is required; the verifier requests the no key signing infrastructure is required; the verifier requests the
public key from the claimed signer directly. public key from the claimed signer directly.
The DNS is proposed as the initial mechanism for publishing public The DNS is proposed as the initial mechanism for publishing public
keys. DKIM is designed to be extensible to other key fetching keys. DKIM is designed to be extensible to other key fetching
services as they become available. services as they become available.
2. Terminology and Definitions 2. Terminology and Definitions
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o "sub-domain" o "sub-domain"
The following definitions are imported from [RFC2822]: The following definitions are imported from [RFC2822]:
o "WSP" (space or tab) o "WSP" (space or tab)
o "FWS" (folding white space) o "FWS" (folding white space)
o "field-name" (name of a header field) o "field-name" (name of a header field)
o "dot-atom" (in the local-part of an email address) o "dot-atom-text" (in the local-part of an email address)
The following tokens are imported from [RFC2045]: The following tokens are imported from [RFC2045]:
o "qp-section" (a single line of quoted-printable-encoded text) o "qp-section" (a single line of quoted-printable-encoded text)
o "hex-octet" (a quoted-printable encoded octet) o "hex-octet" (a quoted-printable encoded octet)
INFORMATIVE NOTE: Be aware that the ABNF in RFC 2045 does not INFORMATIVE NOTE: Be aware that the ABNF in RFC 2045 does not
obey the rules of RFC 4234 and must be interpreted accordingly, obey the rules of RFC 4234 and must be interpreted accordingly,
particularly as regards case folding. particularly as regards case folding.
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Protocol Elements are conceptual parts of the protocol that are not Protocol Elements are conceptual parts of the protocol that are not
specific to either signers or verifiers. The protocol descriptions specific to either signers or verifiers. The protocol descriptions
for signers and verifiers are described in later sections (Signer for signers and verifiers are described in later sections (Signer
Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This Actions (Section 5) and Verifier Actions (Section 6)). NOTE: This
section must be read in the context of those sections. section must be read in the context of those sections.
3.1 Selectors 3.1 Selectors
To support multiple concurrent public keys per signing domain, the To support multiple concurrent public keys per signing domain, the
key namespace is subdivided using "selectors". For example, key namespace is subdivided using "Selectors". For example,
selectors might indicate the names of office locations (e.g., Selectors might indicate the names of office locations (e.g.,
"sanfrancisco", "coolumbeach", and "reykjavik"), the signing date "sanfrancisco", "coolumbeach", and "reykjavik"), the signing date
(e.g., "january2005", "february2005", etc.), or even the individual (e.g., "january2005", "february2005", etc.), or even the individual
user. user.
Selectors are needed to support some important use cases. For Selectors are needed to support some important use cases. For
example: example:
o Domains which want to delegate signing capability for a specific o Domains which want to delegate signing capability for a specific
address for a given duration to a partner, such as an advertising address for a given duration to a partner, such as an advertising
provider or other outsourced function. provider or other out-sourced function.
o Domains which want to allow frequent travelers to send messages o Domains which want to allow frequent travelers to send messages
locally without the need to connect with a particular MSA. locally without the need to connect with a particular MSA.
o "Affinity" domains (e.g., college alumni associations) which o "Affinity" domains (e.g., college alumni associations) which
provide forwarding of incoming mail but which do not operate a provide forwarding of incoming mail but which do not operate a
mail submission agent for outgoing mail. mail submission agent for outgoing mail.
Periods are allowed in selectors and are component separators. If Periods are allowed in Selectors and are component separators. When
keys are stored in DNS, the period defines sub-domain boundaries. keys are retrieved from the DNS, periods in Selectors define DNS
Sub-selectors might be used to combine dates with locations; for label boundaries in a manner similar to the conventional use in
example, "march2005.reykjavik". This can be used to allow delegation domain names. Selector components might be used to combine dates
of a portion of the selector name-space. 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: ABNF:
selector = sub-domain *( "." sub-domain ) selector = sub-domain *( "." sub-domain )
The number of public keys and corresponding selectors for each domain The number of public keys and corresponding Selectors for each domain
are determined by the domain owner. Many domain owners will be are determined by the domain owner. Many domain owners will be
satisfied with just one selector whereas administratively distributed satisfied with just one Selector whereas administratively distributed
organizations may choose to manage disparate selectors and key pairs organizations may choose to manage disparate Selectors and key pairs
in different regions or on different email servers. in different regions or on different email servers.
Beyond administrative convenience, selectors make it possible to Beyond administrative convenience, Selectors make it possible to
seamlessly replace public keys on a routine basis. If a domain seamlessly replace public keys on a routine basis. If a domain
wishes to change from using a public key associated with selector wishes to change from using a public key associated with Selector
"january2005" to a public key associated with selector "january2005" to a public key associated with Selector
"february2005", it merely makes sure that both public keys are "february2005", it merely makes sure that both public keys are
advertised in the public-key repository concurrently for the advertised in the public-key repository concurrently for the
transition period during which email may be in transit prior to transition period during which email may be in transit prior to
verification. At the start of the transition period, the outbound verification. At the start of the transition period, the outbound
email servers are configured to sign with the "february2005" private- email servers are configured to sign with the "february2005" private-
key. At the end of the transition period, the "january2005" public key. At the end of the transition period, the "january2005" public
key is removed from the public-key repository. key is removed from the public-key repository.
While some domains may wish to make selector values well known, INFORMATIVE NOTE: A key may also be revoked as described below.
others will want to take care not to allocate selector names in a way 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. E.g., if per-user that allows harvesting of data by outside parties. E.g., if per-user
keys are issued, the domain owner will need to make the decision as 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 to whether to associate this Selector directly with the user name, or
make it some unassociated random value, such as a fingerprint of the make it some unassociated random value, such as a fingerprint of the
public key. public key.
INFORMATIVE IMPLEMENTERS' NOTE: reusing a selector with a new key INFORMATIVE IMPLEMENTERS' NOTE: reusing a Selector with a new key
(for example, changing the key associated with a user's name) (for example, changing the key associated with a user's name)
makes it impossible to tell the difference between a message that makes it impossible to tell the difference between a message that
didn't verify because the key is no longer valid versus a message didn't verify because the key is no longer valid versus a message
that is actually forged. Signers should not change the key that is actually forged. Signers should not change the key
associated with a selector. When creating a new key, signers associated with a Selector. When creating a new key, signers
should associate it with a new selector. should associate it with a new Selector.
3.2 Tag=Value Lists 3.2 Tag=Value Lists
DKIM uses a simple "tag=value" syntax in several contexts, including DKIM uses a simple "tag=value" syntax in several contexts, including
in messages and domain signature records. in messages and domain signature records.
Values are a series of strings containing either plain text, base64 Values are a series of strings containing either plain text, "base64"
text (as defined in [RFC2045], section 6.8), qp-section (ibid, text (as defined in [RFC2045], section 6.8), "qp-section" (ibid,
section 6.7), or dkim-quoted-printable (as defined above). The name section 6.7), or "dkim-quoted-printable" (as defined above). The
of the tag will determine the encoding of each value; however, no name of the tag will determine the encoding of each value; however,
encoding may include the semicolon (";") character, since that no encoding may include the semicolon (";") character, since that
separates tag-specs. 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: Formally, the syntax rules are:
tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ] tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS] tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tag-name = ALPHA 0*ALNUMPUNC tag-name = ALPHA 0*ALNUMPUNC
tag-value = [ 1*VALCHAR 0*( 1*(WSP / FWS) 1*VALCHAR ) ] tag-value = [ 1*VALCHAR 0*( 1*(WSP / FWS) 1*VALCHAR ) ]
; WSP and FWS prohibited at beginning and end ; WSP and FWS prohibited at beginning and end
VALCHAR = %x21-3A / %x3C-7E VALCHAR = %x21-3A / %x3C-7E
; EXCLAMATION to TILDE except SEMICOLON ; EXCLAMATION to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / "_" ALNUMPUNC = ALPHA / DIGIT / "_"
skipping to change at page 12, line 21 skipping to change at page 12, line 42
Unrecognized tags MUST be ignored. Unrecognized tags MUST be ignored.
Tags that have an empty value are not the same as omitted tags. An 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 omitted tag is treated as having the default value; a tag with an
empty value explicitly designates the empty string as the value. For empty value explicitly designates the empty string as the value. For
example, "g=" does not mean "g=*", even though "g=*" is the default example, "g=" does not mean "g=*", even though "g=*" is the default
for that tag. for that tag.
3.3 Signing and Verification Algorithms 3.3 Signing and Verification Algorithms
DKIM supports multiple key signing/verification algorithms. Two DKIM supports multiple digital signature algorithms. Two algorithms
algorithms are defined by this specification at this time: rsa-sha1, are defined by this specification at this time: rsa-sha1, and rsa-
and rsa-sha256. The rsa-sha256 algorithm is the default if no sha256. The rsa-sha256 algorithm is the default if no algorithm is
algorithm is specified. Verifiers MUST implement both rsa-sha1 and specified. Verifiers MUST implement both rsa-sha1 and rsa-sha256.
rsa-sha256. Signers MUST implement and SHOULD sign using rsa-sha256. Signers MUST implement and SHOULD sign using rsa-sha256.
3.3.1 The rsa-sha1 Signing Algorithm 3.3.1 The rsa-sha1 Signing Algorithm
The rsa-sha1 Signing Algorithm computes a message hash as described The rsa-sha1 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-1 as the hash-alg. That hash is then in Section 3.7 below using SHA-1 as the hash-alg. That hash is then
signed by the signer using the RSA algorithm (defined in PKCS#1 signed by the signer using the RSA algorithm (defined in PKCS#1
version 1.5 [RFC3447]; in particular see section 5.2) with an version 1.5 [RFC3447]) as the crypt-alg and the signer's private key.
exponent of 65537 as the crypt-alg and the signer's private key. The The hash MUST NOT be truncated or converted into any form other than
hash MUST NOT be truncated or converted into any form other than the the native binary form before being signed.
native binary form before being signed.
3.3.2 The rsa-sha256 Signing Algorithm 3.3.2 The rsa-sha256 Signing Algorithm
The rsa-sha256 Signing Algorithm computes a message hash as described The rsa-sha256 Signing Algorithm computes a message hash as described
in Section 3.7 below using SHA-256 as the hash-alg. That hash is in Section 3.7 below using SHA-256 as the hash-alg. That hash is
then signed by the signer using the RSA algorithm (actually PKCS#1 then signed by the signer using the RSA algorithm (actually PKCS#1
version 1.5 [RFC3447]; in particular see section 5.2) with an version 1.5 [RFC3447]) as the crypt-alg and the signer's private key.
exponent of 65537 as the crypt-alg and the signer's private key. The The hash MUST NOT be truncated or converted into any form other than
hash MUST NOT be truncated or converted into any form other than the the native binary form before being signed.
native binary form before being signed.
3.3.3 Other algorithms 3.3.3 Other algorithms
Other algorithms MAY be defined in the future. Verifiers MUST ignore Other algorithms MAY be defined in the future. Verifiers MUST ignore
any signatures using algorithms that they do not understand. any signatures using algorithms that they do not implement.
3.3.4 Key sizes 3.3.4 Key sizes
Selecting appropriate key sizes is a trade-off between cost, Selecting appropriate key sizes is a trade-off between cost,
performance and risk. Since short RSA keys more easily succumb to 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 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 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 keys ranging from 512 bits to 2048 bits, and they MAY be able to
validate signatures with larger keys. Security policies may use the validate signatures with larger keys. Verifier policies may use the
length of the signing key as one metric for determining whether a length of the signing key as one metric for determining whether a
signature is acceptable. signature is acceptable.
Factors that should influence the key size choice include: Factors that should influence the key size choice include:
o The practical constraint that large keys may not fit within a 512 o The practical constraint that large keys may not fit within a 512
byte DNS UDP response packet byte DNS UDP response packet
o The security constraint that keys smaller than 1024 bits are o The security constraint that keys smaller than 1024 bits are
subject to off-line attacks subject to off-line attacks
o Larger keys impose higher CPU costs to verify and sign email 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 o Keys can be replaced on a regular basis, thus their lifetime can
be relatively short be relatively short
o The security goals of this specification are modest compared to o The security goals of this specification are modest compared to
typical goals of public-key systems typical goals of public-key systems
See RFC3766 [RFC3766] for further discussion of selecting key sizes. See [RFC3766] for further discussion of selecting key sizes.
3.4 Canonicalization 3.4 Canonicalization
Empirical evidence demonstrates that some mail servers and relay Empirical evidence demonstrates that some mail servers and relay
systems modify email in transit, potentially invalidating a systems modify email in transit, potentially invalidating a
signature. There are two competing perspectives on such signature. There are two competing perspectives on such
modifications. For most signers, mild modification of email is modifications. For most signers, mild modification of email is
immaterial to the authentication status of the email. For such immaterial to the authentication status of the email. For such
signers a canonicalization algorithm that survives modest in-transit signers a canonicalization algorithm that survives modest in-transit
modification is preferred. modification is preferred.
Other signers demand that any modification of the email, however Other signers demand that any modification of the email, however
minor, result in an authentication failure. These signers prefer a minor, result in a signature verification failure. These signers
canonicalization algorithm that does not tolerate in-transit prefer a canonicalization algorithm that does not tolerate in-transit
modification of the signed email. modification of the signed email.
Some signers may be willing to accept modifications to header fields Some signers may be willing to accept modifications to header fields
that are within the bounds of email standards such as [RFC2822], but that are within the bounds of email standards such as [RFC2822], but
are unwilling to accept any modification to the body of messages. are unwilling to accept any modification to the body of messages.
To satisfy all requirements, two canonicalization algorithms are To satisfy all requirements, two canonicalization algorithms are
defined for each of the header and the body: a "simple" algorithm defined for each of the header and the body: a "simple" algorithm
that tolerates almost no modification and a "relaxed" algorithm that that tolerates almost no modification and a "relaxed" algorithm that
tolerates common modifications such as white-space replacement and tolerates common modifications such as white-space replacement and
header field line re-wrapping. A signer MAY specify either algorithm header field line re-wrapping. A signer MAY specify either algorithm
for header or body when signing an email. If no canonicalization for header or body when signing an email. If no canonicalization
algorithm is specified by the signer, the "simple" algorithm defaults algorithm is specified by the signer, the "simple" algorithm defaults
for both header and body. Verifiers MUST implement both for both header and body. Verifiers MUST implement both
canonicalization algorithms. Further canonicalization algorithms MAY canonicalization algorithms. Note that the header and body may use
be defined in the future; verifiers MUST ignore any signatures that different canonicalization algorithms. Further canonicalization
use unrecognized canonicalization algorithms. algorithms MAY be defined in the future; verifiers MUST ignore any
signatures that use unrecognized canonicalization algorithms.
In all cases, the header fields of the message are presented to the [[WORKING GROUP DISCUSSION POINT: If a message is transmitted
signing algorithm first in the order indicated by the signature using CHUNKING (that is, BDAT instead of the DATA command) and
header field and canonicalized using the indicated algorithm. Only BODY=BINARYMIME [RFC3030] then the body should be treated as a
header fields listed as signed in the signature header field are binary stream, and no canonicalization whatsoever should be done.
included. Note: the signature header field itself is presented at Do we want to leave this for the future, say that canonicalization
the end of the hash, not with the other headers. The CRLF separating is ignored in this circumstance, or add a third "binary" body
the header field from the body is then presented, followed by the canonicalization algorithm? Or something else, of course.]]
canonicalized body. Note that the header and body may use different
canonicalization algorithms.
Canonicalization simply prepares the email for presentation to the Canonicalization simply prepares the email for presentation to the
signing or verification algorithm. It MUST NOT change the signing or verification algorithm. It MUST NOT change the
transmitted data in any way. Canonicalization of header fields and transmitted data in any way. Canonicalization of header fields and
body are described below. body are described below.
NOTE: This section assumes that the message is already in "network NOTE: This section assumes that the message is already in "network
normal" format (e.g., text is ASCII encoded, lines are separated with normal" format (e.g., text is ASCII encoded, lines are separated with
CRLF characters, etc.). See also Section 5.3 for information about CRLF characters, etc.). See also Section 5.3 for information about
normalizing the message. normalizing the message.
skipping to change at page 15, line 26 skipping to change at page 15, line 44
value. value.
o Delete any WSP characters remaining before and after the colon o Delete any WSP characters remaining before and after the colon
separating the header field name from the header field value. The separating the header field name from the header field value. The
colon separator MUST be retained. colon separator MUST be retained.
3.4.3 The "simple" Body Canonicalization Algorithm 3.4.3 The "simple" Body Canonicalization Algorithm
The "simple" body canonicalization algorithm ignores all empty lines The "simple" body canonicalization algorithm ignores all empty lines
at the end of the message body. An empty line is a line of zero at the end of the message body. An empty line is a line of zero
length after removal of the line terminator. It makes no other length after removal of the line terminator. If there is no trailing
changes to the message body. In more formal terms, the "simple" body CRLF on the message, a CRLF is added. It makes no other changes to
canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the the message body. In more formal terms, the "simple" body
body to a single "CRLF". canonicalization algorithm converts "0*CRLF" at the end of the body
to a single "CRLF".
3.4.4 The "relaxed" Body Canonicalization Algorithm 3.4.4 The "relaxed" Body Canonicalization Algorithm
[[This section may be deleted; see discussion below.]] The "relaxed" [[This section may be deleted; see discussion below.]] The "relaxed"
body canonicalization algorithm: body canonicalization algorithm:
o Ignores all white space at the end of lines. Implementations MUST o Ignores all white space at the end of lines. Implementations MUST
NOT remove the CRLF at the end of the line. NOT remove the CRLF at the end of the line.
o Reduces all sequences of WSP within a line to a single SP o Reduces all sequences of WSP within a line to a single SP
character. character.
o Ignores all empty lines at the end of the message body. "Empty o Ignores all empty lines at the end of the message body. "Empty
line" is defined in Section 3.4.3. line" is defined in Section 3.4.3.
[[NON-NORMATIVE DISCUSSION: The authors are undecided whether to [[WORKING GROUP DISCUSSION POINT (ISSUE 1326): Mike Thomas has
leave the "relaxed" body canonicalization algorithm in to the found bare CRs in the wild that are getting converted to CRLF by
specification or delete it entirely. We believe that for the vast some MTAs and thus breaking signatures. Shall we (a) drop
majority of cases, the "simple" body canonicalization algorithm "relaxed" until we can figure out how to do it right and then put
should be sufficient. We simply do not have enough data to know it in as an extension, (b) change "relaxed" to handle this case,
whether to retain the "relaxed" body canonicalization algorithm or probably by having it convert bare CR and LF to CRLF, or (c)
not.]] something else?]]
3.4.5 Body Length Limits 3.4.5 Body Length Limits
A body length count MAY be specified to limit the signature A body length count MAY be specified to limit the signature
calculation to an initial prefix of the body text, measured in calculation to an initial prefix of the body text, measured in
octets. If the body length count is not specified then the entire octets. If the body length count is not specified then the entire
message body is signed and verified. message body is signed.
INFORMATIVE IMPLEMENTATION NOTE: Body length limits could be
useful in increasing signature robustness when sending to a
mailing list that both appends to content sent to it and does not
sign its messages. However, using such limits enables an attack
in which an attacker modifies a message to include content that
solely benefits the attacker. It is possible for the appended
content to completely replace the original content in the end
recipient's eyes and to defeat duplicate message detection
algorithms. To avoid this attack, signers should be wary of using
this tag, and verifiers might wish to ignore the tag or remove
text that appears after the specified content length, perhaps
based on other criteria.
The body length count allows the signer of a message to permit data
to be appended to the end of the body of a signed message. The body
length count is made following the canonicalization algorithm; for
example, any white space ignored by a canonicalization algorithm is
not included as part of the body length count.
INFORMATIVE RATIONALE: This capability is provided because it is INFORMATIVE RATIONALE: This capability is provided because it is
very common for mailing lists to add trailers to messages (e.g., very common for mailing lists to add trailers to messages (e.g.,
instructions how to get off the list). Until those messages are instructions how to get off the list). Until those messages are
also signed, the body length count is a useful tool for the also signed, the body length count is a useful tool for the
verifier since it may as a matter of policy accept messages having verifier since it may as a matter of policy accept messages having
valid signatures with extraneous data. valid signatures with extraneous data.
Signers of MIME messages that include a body length count SHOULD be INFORMATIVE IMPLEMENTATION NOTE: Using body length limits enables
sure that the length extends to the closing MIME boundary string. 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 INFORMATIVE IMPLEMENTATION NOTE: A signer wishing to ensure that
the only acceptable modifications are to add to the MIME postlude the only acceptable modifications are to add to the MIME postlude
would use a body length count encompassing the entire final MIME would use a body length count encompassing the entire final MIME
boundary string, including the final "--CRLF". A signer wishing boundary string, including the final "--CRLF". A signer wishing
to allow additional MIME parts but not modification of existing to allow additional MIME parts but not modification of existing
parts would use a body length count extending through the final parts would use a body length count extending through the final
MIME boundary string, omitting the final "--CRLF". MIME boundary string, omitting the final "--CRLF".
A body length count of zero means that the body is completely A body length count of zero means that the body is completely
unsigned. unsigned.
INFORMATIVE IMPLEMENTATION NOTE: Note that verifiers may choose
to modify their interpretation of messages with unsigned content,
including truncating the unsigned part, refusing to display the
unsigned part to the user, or simply treating the signature as
invalid.
Signers wishing to ensure that no modification of any sort can occur Signers wishing to ensure that no modification of any sort can occur
should specify the "simple" algorithm and omit the body length count. should specify the "simple" canonicalization algorithm for both
header and body and omit the body length count.
3.4.6 Canonicalization Examples (INFORMATIVE) 3.4.6 Canonicalization Examples (INFORMATIVE)
(In the following examples, actual white space is used only for (In the following examples, actual white space is used only for
clarity. The actual input and output text is designated using clarity. The actual input and output text is designated using
bracketed descriptors: "<SP>" for a space character, "<TAB>" for a bracketed descriptors: "<SP>" for a space character, "<TAB>" for a
tab character, and "<CRLF>" for a carriage-return/line-feed sequence. tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
For example, "X <SP> Y" and "X<SP>Y" represent the same three For example, "X <SP> Y" and "X<SP>Y" represent the same three
characters.) characters.)
skipping to change at page 17, line 37 skipping to change at page 18, line 4
<CRLF> <CRLF>
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF> D <SP><TAB><SP> E <CRLF>
<CRLF> <CRLF>
<CRLF> <CRLF>
when canonicalized using relaxed canonicalization for both header and when canonicalized using relaxed canonicalization for both header and
body results in a header reading: body results in a header reading:
a:X <CRLF> a:X <CRLF>
b:Y <SP> Z <CRLF> b:Y <SP> Z <CRLF>
and a body reading: and a body reading:
<SP> C <CRLF> <SP> C <CRLF>
D <SP> E <CRLF> D <SP> E <CRLF>
(postamble)
Example 2: The same message canonicalized using simple Example 2: The same message canonicalized using simple
canonicalization for both header and body results in a header canonicalization for both header and body results in a header
reading: reading:
A: <SP> X <CRLF> A: <SP> X <CRLF>
B <SP> : <SP> Y <TAB><CRLF> B <SP> : <SP> Y <TAB><CRLF>
<TAB> Z <SP><SP><CRLF> <TAB> Z <SP><SP><CRLF>
and a body reading: and a body reading:
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF> D <SP><TAB><SP> E <CRLF>
(postamble)
Example 3: When processed using relaxed header canonicalization and Example 3: When processed using relaxed header canonicalization and
simple body canonicalization, the canonicalized version has a header simple body canonicalization, the canonicalized version has a header
of: of:
a:X <CRLF> a:X <CRLF>
b:Y <SP> Z <CRLF> b:Y <SP> Z <CRLF>
and a body reading: and a body reading:
<SP> C <SP><CRLF> <SP> C <SP><CRLF>
D <SP><TAB><SP> E <CRLF> D <SP><TAB><SP> E <CRLF>
(postamble)
3.5 The DKIM-Signature header field 3.5 The DKIM-Signature header field
The signature of the email is stored in the "DKIM-Signature:" header The signature of the email is stored in the "DKIM-Signature:" header
field. This header field contains all of the signature and key- field. This header field contains all of the signature and key-
fetching data. The DKIM-Signature value is a tag-list as described fetching data. The DKIM-Signature value is a tag-list as described
in Section 3.2. in Section 3.2.
The "DKIM-Signature:" header field SHOULD be treated as though it 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 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. hence SHOULD NOT be reordered and SHOULD be prepended to the message.
In particular, the "DKIM-Signature" header field SHOULD precede the
original email header fields presented to the canonicalization and
signature algorithms.
The "DKIM-Signature:" header field being created or verified is The "DKIM-Signature:" header field being created or verified is
always included in the signature calculation, after the body of the always included in the signature calculation, after the body of the
message; however, when calculating or verifying the signature, the message; however, when calculating or verifying the signature, the
value of the b= tag (signature value) of that DKIM-Signature header value of the b= tag (signature value) of that DKIM-Signature header
field MUST be treated as though it were the null string. Unknown field MUST be treated as though it were an empty string. Unknown
tags in the "DKIM-Signature:" header field MUST be included in the tags in the "DKIM-Signature:" header field MUST be included in the
signature calculation but MUST be otherwise ignored by verifiers. signature calculation but MUST be otherwise ignored by verifiers.
Other "DKIM-Signature:" header fields that are included in the Other "DKIM-Signature:" header fields that are included in the
signature should be treated as normal header fields; in particular, signature should be treated as normal header fields; in particular,
the b= tag is not treated specially. the b= tag is not treated specially.
The encodings for each field type are listed below. Tags described The encodings for each field type are listed below. Tags described
as qp-section are as described in section 6.7 of MIME Part One as qp-section are as described in section 6.7 of MIME Part One
[RFC2045], with the additional conversion of semicolon characters to [RFC2045], with the additional conversion of semicolon characters to
"=3B"; intuitively, this is one line of quoted-printable encoded "=3B"; intuitively, this is one line of quoted-printable encoded
text. Tags described as dkim-quoted-printable are as defined above. text. Tags described as dkim-quoted-printable are as defined in
Section 2.6.
Tags on the DKIM-Signature header field along with their type and Tags on the DKIM-Signature header field along with their type and
requirement status are shown below. Defined tags are described requirement status are shown below. Unrecognized tags MUST be
below. Unrecognized tags MUST be ignored. ignored.
v= Version (MUST be included). This tag defines the version of v= Version (MUST be included). This tag defines the version of this
this specification that applies to the signature record. It MUST specification that applies to the signature record. It MUST have
have the value 0.3. the value 0.4.
ABNF: ABNF:
sig-v-tag = %x76 [FWS] "=" [FWS] "0.3" sig-v-tag = %x76 [FWS] "=" [FWS] "0.4"
INFORMATIVE NOTE: DKIM-Signature version numbers are INFORMATIVE NOTE: DKIM-Signature version numbers are
expected to increase arithmetically as new versions of this expected to increase arithmetically as new versions of this
specification are released. specification are released.
[[INFORMATIVE NOTE: Upon publication, this version number [[INFORMATIVE NOTE: Upon publication, this version number
should be changed to "1", and this note should be deleted.]] should be changed to "1", and this note should be deleted.]]
a= The algorithm used to generate the signature (plain-text; a= The algorithm used to generate the signature (plain-text;
REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256"; REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256";
skipping to change at page 20, line 4 skipping to change at page 20, line 9
this value and MUST be ignored when re-assembling the original this value and MUST be ignored when re-assembling the original
signature. In particular, the signing process can safely insert signature. In particular, the signing process can safely insert
FWS in this value in arbitrary places to conform to line-length FWS in this value in arbitrary places to conform to line-length
limits. See Signer Actions (Section 5) for how the signature is limits. See Signer Actions (Section 5) for how the signature is
computed. computed.
ABNF: ABNF:
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data
sig-b-tag-data = base64string sig-b-tag-data = base64string
bh= The hash of the body part of the message (base64; REQUIRED).
Whitespace is ignored in this value and MUST be ignored when re- bh= The hash of the canonicalized body part of the message as limited
assembling the original signature. In particular, the signing by the "l=" tag (base64; REQUIRED). Whitespace is ignored in
process can safely insert FWS in this value in arbitrary places this value and MUST be ignored when re-assembling the original
to conform to line-length limits. See Section 3.7 for how the signature. In particular, the signing process can safely insert
body hash is computed. 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: ABNF:
sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
sig-bh-tag-data = base64string sig-bh-tag-data = base64string
c= Message canonicalization (plain-text; OPTIONAL, default is c= Message canonicalization (plain-text; OPTIONAL, default is
"simple/simple"). This tag informs the verifier of the type of "simple/simple"). This tag informs the verifier of the type of
canonicalization used to prepare the message for signing. It canonicalization used to prepare the message for signing. It
consists of two names separated by a "slash" (%d47) character, consists of two names separated by a "slash" (%d47) character,
skipping to change at page 20, line 33 skipping to change at page 20, line 39
header and "simple" is used for the body. For example, header and "simple" is used for the body. For example,
"c=relaxed" is treated the same as "c=relaxed/simple". "c=relaxed" is treated the same as "c=relaxed/simple".
ABNF: ABNF:
sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg
["/" sig-c-tag-alg] ["/" sig-c-tag-alg]
sig-c-tag-alg = "simple" / "relaxed" / x-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 x-sig-c-tag-alg = hyphenated-word ; for later extension
d= The domain of the signing entity (plain-text; REQUIRED). This d= The domain of the signing entity (plain-text; REQUIRED). This is
is the domain that will be queried for the public key. This the domain that will be queried for the public key. This domain
domain MUST be the same as or a parent domain of the "i=" tag MUST be the same as or a parent domain of the "i=" tag (the
(the signing identity, as described below). If the "t=s" tag is signing identity, as described below), or it MUST meet the
specified in the key record referenced by the selector in the requirements for parent domain signing described in Section 3.8.
"s=" tag, then the domain in the "d=" tag must be identical to When presented with a signature that does not meet these
the domain specified in the "i=" tag. When presented with a requirement, verifiers MUST consider the signature invalid.
signature that does not meet these requirement, verifiers MUST
consider the signature invalid.
Internationalized domain names MUST be punycode-encoded Internationalized domain names MUST be punycode-encoded
[RFC3492]. [RFC3492].
ABNF: ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] domain-name sig-d-tag = %x64 [FWS] "=" [FWS] domain-name
domain-name = sub-domain 1*("." sub-domain) domain-name = sub-domain 1*("." sub-domain)
; from RFC 2821 Domain, but excluding address-literal ; from RFC 2821 Domain, but excluding address-literal
h= Signed header fields (plain-text, but see description; h= Signed header fields (plain-text, but see description; REQUIRED).
REQUIRED). A colon-separated list of header field names that A colon-separated list of header field names that identify the
identify the header fields presented to the signing algorithm. header fields presented to the signing algorithm. The field MUST
The field MUST contain the complete list of header fields in the contain the complete list of header fields in the order presented
order presented to the signing algorithm. The field MAY contain to the signing algorithm. The field MAY contain names of header
names of header fields that do not exist when signed; nonexistent fields that do not exist when signed; nonexistent header fields
header fields do not contribute to the signature computation do not contribute to the signature computation (that is, they are
(that is, they are treated as the null input, including the treated as the null input, including the header field name, the
header field name, the separating colon, the header field value, separating colon, the header field value, and any CRLF
and any CRLF terminator). The field MUST NOT include the DKIM- terminator). The field MUST NOT include the DKIM-Signature
Signature header field that is being created or verified, but may header field that is being created or verified, but may include
include others. Folding white space (FWS) MAY be included on others. Folding white space (FWS) MAY be included on either side
either side of the colon separator. Header field names MUST be of the colon separator. Header field names MUST be compared
compared against actual header field names in a case insensitive against actual header field names in a case insensitive manner.
manner. This list MUST NOT be empty. See Section 5.4 for a This list MUST NOT be empty. See Section 5.4 for a discussion of
discussion of choosing header fields to sign. choosing header fields to sign.
ABNF: ABNF:
sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name
0*( *FWS ":" *FWS hdr-name ) 0*( *FWS ":" *FWS hdr-name )
hdr-name = field-name hdr-name = field-name
INFORMATIVE EXPLANATION: By "signing" header fields that do INFORMATIVE EXPLANATION: By "signing" header fields that do
not actually exist, a signer can prevent insertion of those not actually exist, a signer can prevent insertion of those
header fields before verification. However, since a signer header fields before verification. However, since a signer
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actual header field with a null value. actual header field with a null value.
i= Identity of the user or agent (e.g., a mailing list manager) on i= Identity of the user or agent (e.g., a mailing list manager) on
behalf of which this message is signed (dkim-quoted-printable; behalf of which this message is signed (dkim-quoted-printable;
OPTIONAL, default is an empty local-part followed by an "@" OPTIONAL, default is an empty local-part followed by an "@"
followed by the domain from the "d=" tag). The syntax is a followed by the domain from the "d=" tag). The syntax is a
standard email address where the local-part MAY be omitted. The 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 domain part of the address MUST be the same as or a subdomain of
the value of the "d=" tag. the value of the "d=" tag.
Internationalized domain names MUST be punycode-encoded
[RFC3492].
ABNF: ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name sig-i-tag = %x69 [FWS] "=" [FWS] [ Local-part ] "@" domain-name
INFORMATIVE NOTE: The local-part of the "i=" tag is optional INFORMATIVE NOTE: The local-part of the "i=" tag is optional
because in some cases a signer may not be able to establish a because in some cases a signer may not be able to establish a
verified individual identity. In such cases, the signer may verified individual identity. In such cases, the signer may
wish to assert that although it is willing to go as far as wish to assert that although it is willing to go as far as
signing for the domain, it is unable or unwilling to commit signing for the domain, it is unable or unwilling to commit
to an individual user name within their domain. It can do so to an individual user name within their domain. It can do so
skipping to change at page 22, line 35 skipping to change at page 22, line 46
complex topic and trust mechanisms are subject to highly complex topic and trust mechanisms are subject to highly
creative attacks. The real-world efficacy of any but the creative attacks. The real-world efficacy of any but the
most basic bindings between the "i=" value and other most basic bindings between the "i=" value and other
identities is not well established, nor is its vulnerability identities is not well established, nor is its vulnerability
to subversion by an attacker. Hence reliance on the use of to subversion by an attacker. Hence reliance on the use of
these options should be strictly limited. In particular it these options should be strictly limited. In particular it
is not at all clear to what extent a typical end-user is not at all clear to what extent a typical end-user
recipient can rely on any assurances that might be made by recipient can rely on any assurances that might be made by
successful use of the "i=" options. successful use of the "i=" options.
l= Body length count (plain-text unsigned decimal integer; l= Body length count (plain-text unsigned decimal integer; OPTIONAL,
OPTIONAL, default is entire body). This tag informs the verifier default is entire body). This tag informs the verifier of the
of the number of octets in the body of the email after number of octets in the body of the email after canonicalization
canonicalization included in the cryptographic hash, starting included in the cryptographic hash, starting from 0 immediately
from 0 immediately following the CRLF preceding the body. This following the CRLF preceding the body. This value MUST NOT be
value MUST NOT be larger than the actual number of octets in the larger than the actual number of octets in the canonicalized
canonicalized message body. message body.
INFORMATIVE IMPLEMENTATION WARNING: Use of the l= tag might INFORMATIVE IMPLEMENTATION WARNING: Use of the l= tag might
allow display of fraudulent content without appropriate allow display of fraudulent content without appropriate
warning to end users. The l= tag is intended for increasing warning to end users. The l= tag is intended for increasing
signature robustness when sending to mailing lists that both signature robustness when sending to mailing lists that both
modify their content and do not sign their messages. modify their content and do not sign their messages.
However, using the l= tag enables attacks in which an However, using the l= tag enables attacks in which an
intermediary with malicious intent modifies a message to intermediary with malicious intent modifies a message to
include content that solely benefits the attacker. It is include content that solely benefits the attacker. It is
possible for the appended content to completely replace the possible for the appended content to completely replace the
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Currently the only valid value is "dns/txt" which defines the DNS Currently the only valid value is "dns/txt" which defines the DNS
TXT record lookup algorithm described elsewhere in this document. TXT record lookup algorithm described elsewhere in this document.
The only option defined for the "dns" query type is "txt", which The only option defined for the "dns" query type is "txt", which
MUST be included. Verifiers and signers MUST support "dns/txt". MUST be included. Verifiers and signers MUST support "dns/txt".
ABNF: ABNF:
sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([FWS] ":" [FWS] sig-q-tag-method) *([FWS] ":" [FWS] sig-q-tag-method)
sig-q-tag-method = "txt/dns" / x-sig-q-tag-type ["/" x-sig-q-tag-args] 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-type = hyphenated-word ; for future extension
x-sig-q-tag-args = qp-hdr-value x-sig-q-tag-args = qp-hdr-value
s= The selector subdividing the namespace for the "d=" (domain) tag
s= The Selector subdividing the namespace for the "d=" (domain) tag
(plain-text; REQUIRED). (plain-text; REQUIRED).
ABNF: ABNF:
sig-s-tag = %x73 [FWS] "=" [FWS] subdomain *( "." sub-domain ) sig-s-tag = %x73 [FWS] "=" [FWS] selector
t= Signature Timestamp (plain-text unsigned decimal integer; t= Signature Timestamp (plain-text unsigned decimal integer;
RECOMMENDED, default is an unknown creation time). The time that RECOMMENDED, default is an unknown creation time). The time that
this signature was created. The format is the number of seconds 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 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 expressed as an unsigned integer in decimal ASCII. This
value is not constrained to fit into a 31- or 32-bit integer. value is not constrained to fit into a 31- or 32-bit integer.
Implementations SHOULD be prepared to handle values up to at Implementations SHOULD be prepared to handle values up to at
least 10^12 (until approximately AD 200,000; this fits into 40 least 10^12 (until approximately AD 200,000; this fits into 40
bits). To avoid denial of service attacks, implementations MAY bits). To avoid denial of service attacks, implementations MAY
skipping to change at page 25, line 4 skipping to change at page 25, line 11
if that time is reliably available; otherwise the current time if that time is reliably available; otherwise the current time
should be used. The value of the "x=" tag MUST be greater than should be used. The value of the "x=" tag MUST be greater than
the value of the "t=" tag if both are present. the value of the "t=" tag if both are present.
INFORMATIVE NOTE: The x= tag is not intended as an anti- INFORMATIVE NOTE: The x= tag is not intended as an anti-
replay defense. replay defense.
ABNF: ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT 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- z= Copied header fields (dkim-quoted-printable, but see description;
separated list of selected header fields present when the message OPTIONAL, default is null). A vertical-bar-separated list of
was signed, including both the field name and value. It is not selected header fields present when the message was signed,
required to include all header fields present at the time of including both the field name and value. It is not required to
signing. This field need not contain the same header fields include all header fields present at the time of signing. This
listed in the "h=" tag. The header field text itself must encode field need not contain the same header fields listed in the "h="
the vertical bar ("|", %x7C) character (i.e., vertical bars in tag. The header field text itself must encode the vertical bar
the z= text are metacharacters, and any actual vertical bar ("|", %x7C) character (i.e., vertical bars in the z= text are
characters in a copied header field must be encoded). Note that metacharacters, and any actual vertical bar characters in a
all white space must be encoded, including white space between copied header field must be encoded). Note that all white space
the colon and the header field value. After encoding, SWSP MAY must be encoded, including white space between the colon and the
be added at arbitrary locations in order to avoid excessively header field value. After encoding, SWSP MAY be added at
long lines; such white space is NOT part of the value of the arbitrary locations in order to avoid excessively long lines;
header field, and MUST be removed before decoding. such white space is NOT part of the value of the header field,
and MUST be removed before decoding.
Verifiers MUST NOT use the header field names or copied values Verifiers MUST NOT use the header field names or copied values
for checking the signature in any way. Copied header field for checking the signature in any way. Copied header field
values are for diagnostic use only. values are for diagnostic use only.
Header fields with characters requiring conversion (perhaps from Header fields with characters requiring conversion (perhaps from
legacy MTAs which are not [RFC2822] compliant) SHOULD be legacy MTAs which are not [RFC2822] compliant) SHOULD be
converted as described in MIME Part Three [RFC2047]. converted as described in MIME Part Three [RFC2047].
ABNF: ABNF:
skipping to change at page 25, line 41 skipping to change at page 25, line 49
sig-z-tag-copy = hdr-name ":" qp-hdr-value sig-z-tag-copy = hdr-name ":" qp-hdr-value
qp-hdr-value = dkim-quoted-printable ; with "|" encoded qp-hdr-value = dkim-quoted-printable ; with "|" encoded
INFORMATIVE EXAMPLE of a signature header field spread across INFORMATIVE EXAMPLE of a signature header field spread across
multiple continuation lines: multiple continuation lines:
DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane; DKIM-Signature: a=rsa-sha256; d=example.net; s=brisbane;
c=simple; q=dns/txt; i=@eng.example.net; t=1117574938; x=1118006938; c=simple; q=dns/txt; i=@eng.example.net; t=1117574938; x=1118006938;
h=from:to:subject:date; h=from:to:subject:date;
z=From:foo@eng.example.net|To:joe@example.com| z=From:foo@eng.example.net|To:joe@example.com|
Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700 Subject:demo=20run|Date:July=205,=202005=203:44:08=20PM=20-0700;
bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR VoG4ZHRNiYzR
3.6 Key Management and Representation 3.6 Key Management and Representation
Signature applications require some level of assurance that the Signature applications require some level of assurance that the
verification public key is associated with the claimed signer. Many verification public key is associated with the claimed signer. Many
applications achieve this by using public key certificates issued by applications achieve this by using public key certificates issued by
a trusted third party. However, DKIM can achieve a sufficient level a trusted third party. However, DKIM can achieve a sufficient level
of security, with significantly enhanced scalability, by simply of security, with significantly enhanced scalability, by simply
having the verifier query the purported signer's DNS entry (or some having the verifier query the purported signer's DNS entry (or some
security-equivalent) in order to retrieve the public key. security-equivalent) in order to retrieve the public key.
DKIM keys can potentially be stored in multiple types of key servers DKIM keys can potentially be stored in multiple types of key servers
and in multiple formats. The storage and format of keys are and in multiple formats. The storage and format of keys are
irrelevant to the remainder of the DKIM algorithm. irrelevant to the remainder of the DKIM algorithm.
Parameters to the key lookup algorithm are the type of the lookup 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 "q=" tag), the domain of the responsible signer (the "d=" tag of
the DKIM-Signature header field), and the selector (the "s=" tag). the DKIM-Signature header field), and the Selector (the "s=" tag).
public_key = dkim_find_key(q_val, d_val, s_val) public_key = dkim_find_key(q_val, d_val, s_val)
This document defines a single binding, using DNS TXT records to This document defines a single binding, using DNS TXT records to
distribute the keys. Other bindings may be defined in the future. distribute the keys. Other bindings may be defined in the future.
3.6.1 Textual Representation 3.6.1 Textual Representation
It is expected that many key servers will choose to present the keys It is expected that many key servers will choose to present the keys
in an otherwise unstructured text format (for example, an XML form in an otherwise unstructured text format (for example, an XML form
would not be considered to be unstructured text for this purpose). would not be considered to be unstructured text for this purpose).
The following definition MUST be used for any DKIM key represented in The following definition MUST be used for any DKIM key represented in
an otherwise unstructured textual form. an otherwise unstructured textual form.
The overall syntax is a key-value-list as described in Section 3.2. The overall syntax is a tag-list as described in Section 3.2. The
The current valid tags are described below. Other tags MAY be current valid tags are described below. Other tags MAY be present
present and MUST be ignored by any implementation that does not and MUST be ignored by any implementation that does not understand
understand them. them.
v= Version of the DKIM key record (plain-text; RECOMMENDED, default v= Version of the DKIM key record (plain-text; RECOMMENDED, default
is "DKIM1"). If specified, this tag MUST be set to "DKIM1" is "DKIM1"). If specified, this tag MUST be set to "DKIM1"
(without the quotes). This tag MUST be the first tag in the (without the quotes). This tag MUST be the first tag in the
response. Responses beginning with a "v=" tag with any other record. Records beginning with a "v=" tag with any other value
value MUST be discarded. MUST be discarded.
ABNF: ABNF:
key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1" key-v-tag = %x76 [FWS] "=" [FWS] "DKIM1"
g= granularity of the key (plain-text; OPTIONAL, default is "*"). g= granularity of the key (plain-text; OPTIONAL, default is "*").
This value MUST match the Local-part of the "i=" tag of the DKIM- 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 Signature header field (or its default value of the empty string
if "i=" is not specified), with a "*" character matching a if "i=" is not specified), with a "*" character matching a
sequence of zero or more arbitrary characters ("wildcarding"). sequence of zero or more arbitrary characters ("wildcarding").
The intent of this tag is to constrain which signing address can The intent of this tag is to constrain which signing address can
legitimately use this selector. An email with a signing address legitimately use this Selector. An email with a signing address
that does not match the value of this tag constitutes a failed that does not match the value of this tag constitutes a failed
verification. Wildcarding allows matching for addresses such as verification. Wildcarding allows matching for addresses such as
"user+*". An empty "g=" value never matches any addresses. "user+*". An empty "g=" value never matches any addresses.
ABNF: ABNF:
key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart
key-g-tag-lpart = [dot-atom] ["*"] [dot-atom] key-g-tag-lpart = [dot-atom-text] ["*"] [dot-atom-text]
[[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a dot- [[NON-NORMATIVE DISCUSSION POINT: "*" is legal in a "dot-
atom. This should probably use a different character for atom-text". This should probably use a different character
wildcarding. Unfortunately, the options are non-mnemonic for wildcarding. Unfortunately, the options are non-mnemonic
(e.g., "@", "(", ":"). Alternatively we could insist on (e.g., "@", "(", ":"). Alternatively we could insist on
escaping a "*" intended as a literal "*" in the address.]] escaping a "*" intended as a literal "*" in the address.]]
h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
allowing all algorithms). A colon-separated list of hash allowing all algorithms). A colon-separated list of hash
algorithms that might be used. Signers and Verifiers MUST algorithms that might be used. Signers and Verifiers MUST
support the "sha1" hash algorithm. support the "sha256" hash algorithm. Verifiers MUST also support
the "sha1" hash algorithm.
ABNF: ABNF:
key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg
0*( [FWS] ":" [FWS] key-h-tag-alg ) 0*( [FWS] ":" [FWS] key-h-tag-alg )
key-h-tag-alg = "sha1" / "sha256" / x-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 x-key-h-tag-alg = hyphenated-word ; for future extension
k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and
verifiers MUST support the "rsa" key type. The "rsa" key type verifiers MUST support the "rsa" key type. The "rsa" key type
indicates that an RSA public key, as defined in [RFC3447], indicates that an ASN.1 DER-encoded [X.660] RSAPublicKey
sections 3.1 and A.1.1, is being used in the p= tag. (Note: the [RFC3447] (see sections 3.1 and A.1.1) is being used in the p=
p= tag further encodes the value using the base64 algorithm.) tag. (Note: the p= tag further encodes the value using the
base64 algorithm.)
ABNF: ABNF:
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type
key-k-tag-type = "rsa" / x-key-k-tag-type key-k-tag-type = "rsa" / x-key-k-tag-type
x-key-k-tag-type = hyphenated-word ; for future extension x-key-k-tag-type = hyphenated-word ; for future extension
[[NON-NORMATIVE DISCUSSION NOTE: In some cases it can be [[NON-NORMATIVE DISCUSSION NOTE: In some cases it can be
hard to separate h= and k=; for example DSA implies that hard to separate h= and k=; for example DSA implies that
SHA-1 will be used. This might be an actual change to the SHA-1 will be used. This might be an actual change to the
spec depending on how we decide to fix this.]] spec depending on how we decide to fix this.]]
n= Notes that might be of interest to a human (qp-section;
OPTIONAL, default is empty). No interpretation is made by any n= Notes that might be of interest to a human (qp-section; OPTIONAL,
program. This tag should be used sparingly in any key server default is empty). No interpretation is made by any program.
mechanism that has space limitations (notably DNS). This tag should be used sparingly in any key server mechanism
that has space limitations (notably DNS).
ABNF: ABNF:
key-n-tag = %x6e [FWS] "=" [FWS] qp-section key-n-tag = %x6e [FWS] "=" [FWS] qp-section
p= Public-key data (base64; REQUIRED). An empty value means that p= Public-key data (base64; REQUIRED). An empty value means that
this public key has been revoked. The syntax and semantics of this public key has been revoked. The syntax and semantics of
this tag value before being encoded in base64 is defined by the this tag value before being encoded in base64 is defined by the
k= tag. k= tag.
ABNF: ABNF:
key-p-tag = %x70 [FWS] "=" [FWS] base64string key-p-tag = %x70 [FWS] "=" [ [FWS] base64string ]
s= Service Type (plain-text; OPTIONAL; default is "*"). A colon- s= Service Type (plain-text; OPTIONAL; default is "*"). A colon-
separated list of service types to which this record applies. separated list of service types to which this record applies.
Verifiers for a given service type MUST ignore this record if the Verifiers for a given service type MUST ignore this record if the
appropriate type is not listed. Currently defined service types appropriate type is not listed. Currently defined service types
are: are:
* matches all service types * matches all service types
email electronic mail (not necessarily limited to SMTP) email electronic mail (not necessarily limited to SMTP)
skipping to change at page 29, line 16 skipping to change at page 29, line 23
y This domain is testing DKIM. Verifiers MUST NOT treat y This domain is testing DKIM. Verifiers MUST NOT treat
messages from signers in testing mode differently from messages from signers in testing mode differently from
unsigned email, even should the signature fail to verify. unsigned email, even should the signature fail to verify.
Verifiers MAY wish to track testing mode results to assist Verifiers MAY wish to track testing mode results to assist
the signer. the signer.
s Any DKIM-Signature header fields using the "i=" tag MUST have 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 same domain value on the right hand side of the "@" in
the "i=" tag and the value of the "d=" tag. That is, the the "i=" tag and the value of the "d=" tag. That is, the
"i=" domain MUST NOT be a subdomain of "d=". "i=" domain MUST NOT be a subdomain of "d=". Use of this
flag is RECOMMENDED unless subdomaining is required.
ABNF: ABNF:
key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag key-t-tag = %x74 [FWS] "=" [FWS] key-t-tag-flag
0*( [FWS] ":" [FWS] key-t-tag-flag ) 0*( [FWS] ":" [FWS] key-t-tag-flag )
key-t-tag-flag = "y" / "s" / x-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 x-key-t-tag-flag = hyphenated-word ; for future extension
Unrecognized flags MUST be ignored. Unrecognized flags MUST be ignored.
3.6.2 DNS binding 3.6.2 DNS binding
A binding using DNS TXT records as a key service is hereby defined. A binding using DNS TXT records as a key service is hereby defined.
All implementations MUST support this binding. All implementations MUST support this binding.
3.6.2.1 Name Space 3.6.2.1 Name Space
All DKIM keys are stored in a subdomain named ""_domainkey"". Given All DKIM keys are stored in a subdomain named "_domainkey". Given a
a DKIM-Signature field with a "d=" tag of ""example.com"" and an "s=" DKIM-Signature field with a "d=" tag of "example.com" and an "s=" tag
tag of ""sample"", the DNS query will be for of "foo.bar", the DNS query will be for
""sample._domainkey.example.com"". "foo.bar._domainkey.example.com".
The value of the "i=" tag is not used by the DNS binding. Wildcard DNS records (e.g., *.bar._domainkey.example.com) MUST 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 3.6.2.2 Resource Record Types for Key Storage
The DNS Resource Record type used is specified by an option to the The DNS Resource Record type used is specified by an option to the
query-type ("q=") tag. The only option defined in this base query-type ("q=") tag. The only option defined in this base
specification is "txt", indicating the use of a TXT RR record. A specification is "txt", indicating the use of a TXT RR record. A
later extension of this standard may define another Resource Record later extension of this standard may define another Resource Record
type, tentatively dubbed "DKK". type.
TXT records are encoded as described in Section 3.6.1. TXT records are encoded as described in Section 3.6.1.
3.7 Computing the Message Hashes 3.7 Computing the Message Hashes
Both signing and verifying message signatures starts with a step of Both signing and verifying message signatures starts with a step of
computing two cryptographic hashes over the message. Signers will computing two cryptographic hashes over the message. Signers will
choose the parameters of the signature as described in Signer Actions choose the parameters of the signature as described in Signer Actions
(Section 5); verifiers will use the parameters specified in the (Section 5); verifiers will use the parameters specified in the
"DKIM-Signature" header field being verified. In the following "DKIM-Signature" header field being verified. In the following
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When calculating the hash on messages that will be transmitted using When calculating the hash on messages that will be transmitted using
base64 or quoted-printable encoding, signers MUST compute the hash base64 or quoted-printable encoding, signers MUST compute the hash
after the encoding. Likewise, the verifier MUST incorporate the after the encoding. Likewise, the verifier MUST incorporate the
values into the hash before decoding the base64 or quoted-printable values into the hash before decoding the base64 or quoted-printable
text. However, the hash MUST be computed before transport level text. However, the hash MUST be computed before transport level
encodings such as SMTP "dot-stuffing." encodings such as SMTP "dot-stuffing."
With the exception of the canonicalization procedure described in With the exception of the canonicalization procedure described in
Section 3.4, the DKIM signing process treats the body of messages as Section 3.4, the DKIM signing process treats the body of messages as
simply a string of characters. DKIM messages MAY be either in plain- simply a string of octets. DKIM messages MAY be either in plain-text
text or in MIME format; no special treatment is afforded to MIME or in MIME format; no special treatment is afforded to MIME content.
content. Message attachments in MIME format MUST be included in the Message attachments in MIME format MUST be included in the content
content which is signed. which is signed.
More formally, the algorithm for the signature is: More formally, the algorithm for the signature is:
body-hash = hash-alg(canon_body) body-hash = hash-alg(canon_body)
header-hash = hash-alg(canon_header || DKIM-SIG) header-hash = hash-alg(canon_header || DKIM-SIG)
signature = sig-alg(header-hash, key) signature = sig-alg(header-hash, key)
where sig-alg is the signature algorithm specified by the "a=" tag, where "sig-alg" is the signature algorithm specified by the "a=" tag,
hash-alg is the hash 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 "canon_header" and "canon_body" are the canonicalized message header
body (respectively) as defined in Section 3.4 (excluding the DKIM- and body (respectively) as defined in Section 3.4 (excluding the
Signature header field), and DKIM-SIG is the canonicalized DKIM- DKIM-Signature header field), and "DKIM-SIG" is the canonicalized
Signature header field sans the signature value itself, but with DKIM-Signature header field sans the signature value itself, but with
body-hash included as the "bh=" tag. "body-hash" included as the "bh=" tag.
INFORMATIVE 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 4. Semantics of Multiple Signatures
A signer that is adding a signature to a message merely creates a new A signer that is adding a signature to a message merely creates a new
DKIM-Signature header, using the usual semantics of the h= option. A DKIM-Signature header, using the usual semantics of the h= option. A
signer MAY sign previously existing DKIM-Signature headers using the signer MAY sign previously existing DKIM-Signature headers using the
method described in section Section 5.4 to sign trace headers. method described in section Section 5.4 to sign trace headers.
Signers should be cognizant that signing DKIM-Signature headers may Signers should be cognizant that signing DKIM-Signature headers may
result in signature failures with intermediaries that do not result in signature failures with intermediaries that do not
recognize that DKIM-Signature's are trace headers and unwittingly recognize that DKIM-Signatures are trace headers and unwittingly
reorder them. reorder them.
When evaluating a message with multiple signatures, a verifier should When evaluating a message with multiple signatures, a verifier should
evaluate signatures independently and on their own merits. For evaluate signatures independently and on their own merits. For
example, a verifier that by policy chooses not to accept signatures example, a verifier that by policy chooses not to accept signatures
with deprecated cryptographic algorithms should consider such with deprecated cryptographic algorithms should consider such
signatures invalid. As with messages with a single signature, signatures invalid. As with messages with a single signature,
verifiers are at liberty to use the presence of valid signatures as verifiers are at liberty to use the presence of valid signatures as
an input to local policy; likewise, the interpretation of multiple an input to local policy; likewise, the interpretation of multiple
valid signatures in combination is a local policy decision of the valid signatures in combination is a local policy decision of the
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cannot be verified. cannot be verified.
5. Signer Actions 5. Signer Actions
The following steps are performed in order by signers. The following steps are performed in order by signers.
5.1 Determine if the Email Should be Signed and by Whom 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 A signer can obviously only sign email for domains for which it has a
private-key and the necessary knowledge of the corresponding public private-key and the necessary knowledge of the corresponding public
key and selector information. However there are a number of other key and Selector information. However there are a number of other
reasons beyond the lack of a private key why a signer could choose reasons beyond the lack of a private key why a signer could choose
not to sign an email. not to sign an email.
A SUBMISSION server MAY sign if the submitter is authenticated by INFORMATIVE NOTE: Signing modules may be incorporated into any
some secure means, e.g., SMTP AUTH. Within a trusted enclave the portion of the mail system as deemed appropriate, including an
signing address MAY be derived from the header field according to MUA, a SUBMISSION server, or an MTA. Wherever implemented,
local signer policy. Within a trusted enclave an MTA MAY do the signers should beware of signing (and thereby asserting
signing. responsibility for) messages that may be problematic. In
particular, within a trusted enclave the signing address might be
derived from the header according to local policy; SUBMISSION
servers might only sign messages from users that are properly
authenticated and authorized.
INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not INFORMATIVE IMPLEMENTER ADVICE: SUBMISSION servers should not
sign Received header fields if the outgoing gateway MTA obfuscates sign Received header fields if the outgoing gateway MTA obfuscates
Received header fields, for example to hide the details of Received header fields, for example to hide the details of
internal topology. internal topology.
A signer MUST NOT sign an email if it is unwilling to be held
responsible for the message; in particular, the signer SHOULD ensure
that the submitter has a bona fide relationship with the signer and
that the submitter has the right to use the address being claimed.
If an email cannot be signed for some reason, it is a local policy If an email cannot be signed for some reason, it is a local policy
decision as to what to do with that email. decision as to what to do with that email.
5.2 Select a private-key and corresponding selector information 5.2 Select a private-key and corresponding selector information
This specification does not define the basis by which a signer should This specification does not define the basis by which a signer should
choose which private-key and selector information to use. Currently, choose which private-key and Selector information to use. Currently,
all selectors are equal as far as this specification is concerned, so all Selectors are equal as far as this specification is concerned, so
the decision should largely be a matter of administrative the decision should largely be a matter of administrative
convenience. Distribution and management of private-keys is also convenience. Distribution and management of private-keys is also
outside the scope of this document. outside the scope of this document.
INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a INFORMATIVE OPERATIONS ADVICE: A signer should not sign with a
private key when the selector containing the corresponding public private key when the Selector containing the corresponding public
key is expected to be removed before the verifier has an key is expected to be revoked or removed before the verifier has
opportunity to validate the signature. The signer should an opportunity to validate the signature. The signer should
anticipate that verifiers may choose to defer validation, perhaps anticipate that verifiers may choose to defer validation, perhaps
until the message is actually read by the final recipient. In until the message is actually read by the final recipient. In
particular, when rotating to a new key-pair, signing should particular, when rotating to a new key-pair, signing should
immediately commence with the new private key and the old public immediately commence with the new private key and the old public
key should be retained for a reasonable validation interval before key should be retained for a reasonable validation interval before
being removed from the key server. being removed from the key server.
5.3 Normalize the Message to Prevent Transport Conversions 5.3 Normalize the Message to Prevent Transport Conversions
Some messages, particularly those using 8-bit characters, are subject Some messages, particularly those using 8-bit characters, are subject
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DKIM algorithm. DKIM algorithm.
Should the message be submitted to the signer with any local encoding Should the message be submitted to the signer with any local encoding
that will be modified before transmission, such conversion to that will be modified before transmission, such conversion to
canonical form MUST be done before signing. In particular, some canonical form MUST be done before signing. In particular, some
systems use local line separator conventions (such as the Unix systems use local line separator conventions (such as the Unix
newline character) internally rather than the SMTP-standard CRLF newline character) internally rather than the SMTP-standard CRLF
sequence. All such local conventions MUST be converted to canonical sequence. All such local conventions MUST be converted to canonical
format before signing. format before signing.
More generally, the signer MUST sign the message as it will be More generally, the signer MUST sign the message as it is expected to
received by the verifier rather than in some local or internal form. be received by the verifier rather than in some local or internal
form.
5.4 Determine the header fields to Sign 5.4 Determine the header fields to Sign
The From header field MUST be signed (that is, included in the h= tag The From header field MUST be signed (that is, included in the h= tag
of the resulting DKIM-Signature header field); any header field that of the resulting DKIM-Signature header field). Signers SHOULD NOT
describes the role of the signer (for example, the Sender or Resent- sign an existing header field likely to be legitimately modified or
From header field if the signature is on behalf of the corresponding removed in transit. In particular, [RFC2821] explicitly permits
address and that address is different from the From address) MUST modification or removal of the "Return-Path" header field in transit.
also be included. The signed header fields SHOULD also include the Signers MAY include any other header fields present at the time of
Subject and Date header fields as well as all MIME header fields. signing at the discretion of the signer.
Signers SHOULD NOT sign an existing header field likely to be
legitimately modified or removed in transit. In particular, INFORMATIVE OPERATIONS NOTE: The choice of which header fields to
[RFC2821] explicitly permits modification or removal of the "Return- sign is non-obvious. One strategy is to sign all existing, non-
Path" header field in transit. Signers MAY include any other header repeatable header fields. An alternative strategy is to sign only
fields present at the time of signing at the discretion of the header fields that are likely to be displayed to or otherwise be
signer. It is RECOMMENDED that all other existing, non-repeatable likely to affect the processing of the message at the receiver. A
header fields be signed. third strategy is to sign only "well known" headers. Note that
verifiers may treat unsigned header fields with extreme
skepticism, including refusing to display them to the end user or
even ignore the signature if it does not cover certain header
fields. For this reason signing fields present in the message
such as Date, Subject, Reply-To, Sender, and all MIME headers is
highly advised.
The DKIM-Signature header field is always implicitly signed and MUST The DKIM-Signature header field is always implicitly signed and MUST
NOT be included in the h= tag except to indicate that other NOT be included in the h= tag except to indicate that other
preexisting signatures are also signed. preexisting signatures are also signed.
Signers MUST sign any header fields that the signers wish to assert
were present at the time of signing. Put another way, verifiers MAY
treat unsigned header fields with extreme skepticism, up to and
including refusing to display them to the end user.
Signers MAY claim to have signed header fields that do not exist Signers MAY claim to have signed header fields that do not exist
(that is, signers MAY include the header field name in the h= tag (that is, signers MAY include the header field name in the h= tag
even if that header field does not exist in the message). When even if that header field does not exist in the message). When
computing the signature, the non-existing header field MUST be computing the signature, the non-existing header field MUST be
treated as the null string (including the header field name, header treated as the null string (including the header field name, header
field value, all punctuation, and the trailing CRLF). field value, all punctuation, and the trailing CRLF).
INFORMATIVE RATIONALE: This allows signers to explicitly assert INFORMATIVE RATIONALE: This allows signers to explicitly assert
the absence of a header field; if that header field is added later the absence of a header field; if that header field is added later
the signature will fail. the signature will fail.
Signers choosing to sign an existing replicated header field (such as Signers choosing to sign an existing header field that occurs more
Received) MUST sign the physically last instance of that header field than once in the message (such as Received) MUST sign the physically
in the header field block. Signers wishing to sign multiple last instance of that header field in the header block. Signers
instances of an existing replicated header field MUST include the wishing to sign multiple instances of such a header field MUST
header field name multiple times in the h= tag of the DKIM-Signature include the header field name multiple times in the h= tag of the
header field, and MUST sign such header fields in order from the DKIM-Signature header field, and MUST sign such header fields in
bottom of the header field block to the top. The signer MAY include order from the bottom of the header field block to the top. The
more header field names than there are actual corresponding header signer MAY include more instances of a header field name in h= than
fields to indicate that additional header fields of that name SHOULD there are actual corresponding header fields to indicate that
NOT be added. additional header fields of that name SHOULD NOT be added.
INFORMATIVE EXAMPLE: INFORMATIVE EXAMPLE:
If the signer wishes to sign two existing Received header fields, If the signer wishes to sign two existing Received header fields,
and the existing header contains: and the existing header contains:
Received: <A> Received: <A>
Received: <B> Received: <B>
Received: <C> Received: <C>
then the resulting DKIM-Signature header field should read: then the resulting DKIM-Signature header field should read:
DKIM-Signature: ... h=Received : Received : ... DKIM-Signature: ... h=Received : Received : ...
and Received header fields <C> and <B> will be signed in that and Received header fields <C> and <B> will be signed in that
order. order.
Signers SHOULD NOT sign header fields that might be replicated Signers should be careful of signing header fields that might have
(either at the time of signing or potentially in the future), with additional instances added later in the delivery process, since such
the exception of trace header fields such as Received. Comment and header fields might be inserted after the signed instance or
non standard header fields (including X-* header fields) are otherwise reordered. Trace header fields (such as Received and DKIM-
permitted by [RFC2822] to be replicated; however, many such header Signature) and Resent-* blocks are the only fields prohibited by
fields are, by convention, not replicated. Signers need to [RFC2822] from being reordered.
understand the implications of signing header fields that might later
be replicated, especially in the face of header field reordering. In
particular, [RFC2822] only requires that trace header fields retain
the original order.
INFORMATIVE RATIONALE: Received: is allowed because these header
fields, as well as Resent-* header fields, are already order-
sensitive.
INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits INFORMATIVE ADMONITION: Despite the fact that [RFC2822] permits
header field blocks to be reordered (with the exception of header fields to be reordered (with the exception of Received
Received header fields), reordering of signed replicated header header fields), reordering of signed header fields with multiple
fields by intermediate MTAs will cause DKIM signatures to be instances by intermediate MTAs will cause DKIM signatures to be
broken; such anti-social behavior should be avoided. broken; such anti-social behavior should be avoided.
INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this INFORMATIVE IMPLEMENTER'S NOTE: Although not required by this
specification, all end-user visible header fields should be signed specification, all end-user visible header fields should be signed
to avoid possible "indirect spamming." For example, if the to avoid possible "indirect spamming." For example, if the
"Subject" header field is not signed, a spammer can resend a "Subject" header field is not signed, a spammer can resend a
previously signed mail, replacing the legitimate subject with a previously signed mail, replacing the legitimate subject with a
one-line spam. one-line spam.
INFORMATIVE NOTE: There has been some discussion that a Sender
Signing Policy include the list of header fields that the signer
always signs. N.B. In theory this is unnecessary, since as long
as the signer really always signs the indicated header fields
there is no possibility of an attacker replaying an existing
message that has such an unsigned header field.
5.5 Compute the Message Hash and Signature 5.5 Compute the Message Hash and Signature
The signer MUST compute the message hash as described in Section 3.7 The signer MUST compute the message hash as described in Section 3.7
and then sign it using the selected public-key algorithm. This will and then sign it using the selected public-key algorithm. This will
result in a DKIM-Signature header field which will include the body result in a DKIM-Signature header field which will include the body
hash and a signature of the header hash, where that header includes hash and a signature of the header hash, where that header includes
the DKIM-Signature header field itself. the DKIM-Signature header field itself.
To avoid possible ambiguity, a signer SHOULD either sign or remove
any preexisting header fields which convey the results of previous
verifications of the message signature prior to preparation for
signing and transmission. Such header fields MUST NOT be signed if
the signer is uncertain of the authenticity of the preexisting header
field, for example, if it is not locally generated or signed by a
previous DKIM-Signature line that the current signer has verified.
Entities such as mailing list managers that implement DKIM and which Entities such as mailing list managers that implement DKIM and which
modify the message or a header field (for example, inserting modify the message or a header field (for example, inserting
unsubscribe information) before retransmitting the message SHOULD unsubscribe information) before retransmitting the message SHOULD
check any existing signature on input and MUST make such check any existing signature on input and MUST make such
modifications before re-signing the message; such signing SHOULD modifications before re-signing the message.
include any prior verification status, if any, that was inserted upon
message receipt.
The signer MAY elect to limit the number of bytes of the body that 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 will be included in the hash and hence signed. The length actually
hashed should be inserted in the "l=" tag of the "DKIM-Signature" hashed should be inserted in the "l=" tag of the "DKIM-Signature"
header field. header field.
INFORMATIVE NOTE: A possible value to include in the "l=" tag
would include the entire length of the message being signed,
thereby allowing intermediate agents to append further information
to the message without breaking the signature (e.g., a mailing
list manager might add unsubscribe information to the body). A
signer wishing to permit such intermediate agents to add another
MIME body part to a "multipart/mixed" message should use a length
that covers the entire presented message except for the trailing
"--CRLF" characters; this is known as the "N-4" approach. Note
that more than four characters may need to be stripped, since
there could be postlude information that needs to be ignored.
5.6 Insert the DKIM-Signature header field 5.6 Insert the DKIM-Signature header field
Finally, the signer MUST insert the "DKIM-Signature:" header field Finally, the signer MUST insert the "DKIM-Signature:" header field
created in the previous step prior to transmitting the email. The created in the previous step prior to transmitting the email. The
"DKIM-Signature" header field MUST be the same as used to compute the "DKIM-Signature" header field MUST be the same as used to compute the
hash as described above, except that the value of the "b=" tag MUST hash as described above, except that the value of the "b=" tag MUST
be the appropriately signed hash computed in the previous step, be the appropriately signed hash computed in the previous step,
signed using the algorithm specified in the "a=" tag of the "DKIM- signed using the algorithm specified in the "a=" tag of the "DKIM-
Signature" header field and using the private key corresponding to Signature" header field and using the private key corresponding to
the selector given in the "s=" tag of the "DKIM-Signature" header the Selector given in the "s=" tag of the "DKIM-Signature" header
field, as chosen above in Section 5.2 field, as chosen above in Section 5.2
The "DKIM-Signature" MUST be inserted before any other DKIM-Signature
The "DKIM-Signature" SHOULD be inserted before any header fields that fields in the header block.
it signs in the header block.
INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this INFORMATIVE IMPLEMENTATION NOTE: The easiest way to achieve this
is to insert the "DKIM-Signature" header field at the beginning of is to insert the "DKIM-Signature" header field at the beginning of
the header block. In particular, it may be placed before any the header block. In particular, it may be placed before any
existing Received header fields. This is consistent with treating existing Received header fields. This is consistent with treating
"DKIM-Signature" as a trace header. "DKIM-Signature" as a trace header.
6. Verifier Actions 6. Verifier Actions
Since a signer MAY expire a public key at any time, it is recommended Since a signer MAY remove or revoke a public key at any time, it is
that verification occur in a timely manner with the most timely place recommended that verification occur in a timely manner with the most
being during acceptance by the border MTA. timely place being during acceptance by the border MTA.
A border or intermediate MTA MAY verify the message signatures and A border or intermediate MTA MAY verify the message signature(s). An
add a verification header field to incoming messages. This MTA who has performed verification MAY communicate the result of that
considerably simplifies things for the user, who can now use an verification by adding a verification header field to incoming
existing mail user agent. Most MUAs have the ability to filter messages. This considerably simplifies things for the user, who can
messages based on message header fields or content; these filters now use an existing mail user agent. Most MUAs have the ability to
would be used to implement whatever policy the user wishes with filter messages based on message header fields or content; these
respect to unsigned mail. filters would be used to implement whatever policy the user wishes
with respect to unsigned mail.
A verifying MTA MAY implement a policy with respect to unverifiable A verifying MTA MAY implement a policy with respect to unverifiable
mail, regardless of whether or not it applies the verification header mail, regardless of whether or not it applies the verification header
field to signed messages. field to signed messages.
Verifiers MUST produce a result that is semantically equivalent to Verifiers MUST produce a result that is semantically equivalent to
applying the following steps in the order listed. In practice, applying the following steps in the order listed. In practice,
several of these steps can be performed in parallel in order to several of these steps can be performed in parallel in order to
improve performance. improve performance.
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For example, one implementation might prefer to try the signatures in For example, one implementation might prefer to try the signatures in
textual order, whereas another might want to prefer signatures by textual order, whereas another might want to prefer signatures by
identities that match the contents of the "From" header field over identities that match the contents of the "From" header field over
other identities. Verifiers MUST NOT attribute ultimate meaning to other identities. Verifiers MUST NOT attribute ultimate meaning to
the order of multiple DKIM-Signature header fields. In particular, the order of multiple DKIM-Signature header fields. In particular,
there is reason to believe that some relays will reorder the header there is reason to believe that some relays will reorder the header
fields in potentially arbitrary ways. fields in potentially arbitrary ways.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as INFORMATIVE IMPLEMENTATION NOTE: Verifiers might use the order as
a clue to signing order in the absence of any other information. a clue to signing order in the absence of any other information.
However, other clues as to the semantics of multiple signatures However, other clues as to the semantics of multiple signatures
must be considered before using ordering. (such as correlating the signing host with Received headers) may
also be considered.
A verifier SHOULD NOT treat a message that has one or more bad A verifier SHOULD NOT treat a message that has one or more bad
signatures and no good signatures differently from a message with no signatures and no good signatures differently from a message with no
signature at all; this is local policy and is beyond the scope of signature at all; such treatment is a matter of local policy and is
this document. beyond the scope of this document.
When a signature successfully verifies, a verifier will either stop When a signature successfully verifies, a verifier will either stop
processing or attempt to verify any other signatures, at the processing or attempt to verify any other signatures, at the
discretion of the implementation. discretion of the implementation. A verifier MAY limit the number of
signatures it tries to avoid denial-of-service attacks.
INFORMATIVE NOTE: An attacker could send messages with large
numbers of faulty signatures, each of which would require a DNS
lookup. This could be an attack on the domain that receives the
message, by slowing down the verifier by requiring it to do large
number of DNS lookups and signature verifications. It could also
be an attack against the domains listed in the signatures,
essentially by enlisting innocent verifiers in launching an attack
against the DNS servers of the actual victim.
In the following description, text reading "return status In the following description, text reading "return status
(explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL") (explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
means that the verifier MUST immediately cease processing that means that the verifier MUST immediately cease processing that
signature. The verifier SHOULD proceed to the next signature, if any signature. The verifier SHOULD proceed to the next signature, if any
is present, and completely ignore the bad signature. If the status is present, and completely ignore the bad signature. If the status
is "PERMFAIL", the signature failed and should not be reconsidered. is "PERMFAIL", the signature failed and should not be reconsidered.
If the status is "TEMPFAIL", the signature could not be verified at If the status is "TEMPFAIL", the signature could not be verified at
this time but may be tried again later. A verifier MAY either defer this time but may be tried again later. A verifier MAY either defer
the message for later processing, perhaps by queueing it locally or the message for later processing, perhaps by queueing it locally or
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If the DKIM-Signature header field does not contain any of the tags If the DKIM-Signature header field does not contain any of the tags
listed as required in Section 3.5 the verifier MUST ignore the DKIM- listed as required in Section 3.5 the verifier MUST ignore the DKIM-
Signature header field and return PERMFAIL (signature missing Signature header field and return PERMFAIL (signature missing
required tag). required tag).
If the "DKIM-Signature" header field does not contain the "i=" tag, If the "DKIM-Signature" header field does not contain the "i=" tag,
the verifier MUST behave as though the value of that tag were "@d", the verifier MUST behave as though the value of that tag were "@d",
where "d" is the value from the "d=" tag. where "d" is the value from the "d=" tag.
Verifiers MUST confirm that the domain specified in the "d=" tag is Verifiers MUST confirm that the domain specified in the "d=" tag is
the same as or a superdomain of the domain part of the "i=" tag. If the same as or a parent domain of the domain part of the "i=" tag.
not, the DKIM-Signature header field MUST be ignored and the verifier If not, the DKIM-Signature header field MUST be ignored and the
should return PERMFAIL (domain mismatch). verifier should return PERMFAIL (domain mismatch).
If the "h=" tag does not include the "From" header field the verifier
MUST ignore the DKIM-Signature header field and return PERMFAIL (From
field not signed).
Verifiers MAY ignore the DKIM-Signature header field and return Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (signature expired) if it contains an "x=" tag and the PERMFAIL (signature expired) if it contains an "x=" tag and the
signature has expired. signature has expired.
Verifiers MAY ignore the DKIM-Signature header field and return
PERMFAIL (unacceptable signature header) for any other reason, for
example, if the signature does not sign header fields that the
verifier views to be essential. As a case in point, if MIME header
fields are not signed, certain attacks may be possible that the
verifier would prefer to avoid.
6.1.2 Get the Public Key 6.1.2 Get the Public Key
The public key for a signature is needed to complete the verification The public key for a signature is needed to complete the verification
process. The process of retrieving the public key depends on the process. The process of retrieving the public key depends on the
query type as defined by the "q=" tag in the "DKIM-Signature:" header query type as defined by the "q=" tag in the "DKIM-Signature:" header
field. Obviously, a public key need only be retrieved if the process field. Obviously, a public key need only be retrieved if the process
of extracting the signature information is completely successful. of extracting the signature information is completely successful.
Details of key management and representation are described in Details of key management and representation are described in
Section 3.6. The verifier MUST validate the key record and MUST Section 3.6. The verifier MUST validate the key record and MUST
ignore any public key records that are malformed. ignore any public key records that are malformed.
When validating a message, a verifier MUST perform the following When validating a message, a verifier MUST perform the following
steps in a manner that is semantically the same as performing them in steps in a manner that is semantically the same as performing them in
the order indicated (in some cases the implementation may parallelize the order indicated (in some cases the implementation may parallelize
or reorder these steps, as long as the semantics remain unchanged): or reorder these steps, as long as the semantics remain unchanged):
1. Retrieve the public key as described in (Section 3.6) using the 1. Retrieve the public key as described in (Section 3.6) using the
domain from the "d=" tag and the selector from the "s=" tag. domain from the "d=" tag and the Selector from the "s=" tag.
2. If the query for the public key fails to respond, the verifier 2. If the query for the public key fails to respond, the verifier
MAY defer acceptance of this email and return TEMPFAIL (key MAY defer acceptance of this email and return TEMPFAIL (key
unavailable). If verification is occuring during the incoming unavailable). If verification is occurring during the incoming
SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
code. Alternatively, the verifier MAY store the message in the code. Alternatively, the verifier MAY store the message in the
local queue for later trial or ignore the signature. Note that local queue for later trial or ignore the signature. Note that
storing a message in the local queue is subject to denial-of- storing a message in the local queue is subject to denial-of-
service attacks. service attacks.
3. If the query for the public key fails because the corresponding 3. If the query for the public key fails because the corresponding
key record does not exist, the verifier MUST immediately return key record does not exist, the verifier MUST immediately return
PERMFAIL (no key for signature). PERMFAIL (no key for signature).
skipping to change at page 41, line 9 skipping to change at page 41, line 34
domain signature. domain signature.
7. If the "h=" tag exists in the public key record and the hash 7. If the "h=" tag exists in the public key record and the hash
algorithm implied by the a= tag in the DKIM-Signature header is algorithm implied by the a= tag in the DKIM-Signature header is
not included in the contents of the "h=" tag, the verifier MUST not included in the contents of the "h=" tag, the verifier MUST
ignore the key record and return PERMFAIL (inappropriate hash ignore the key record and return PERMFAIL (inappropriate hash
algorithm). algorithm).
8. If the public key data (the "p=" tag) is empty then this key has 8. If the public key data (the "p=" tag) is empty then this key has
been revoked and the verifier MUST treat this as a failed been revoked and the verifier MUST treat this as a failed
signature check and return PERMFAIL (key revoked). signature check and return PERMFAIL (key revoked). There is no
defined semantic difference between a key that has been revoked
and a key record that has been removed.
9. If the public key data is not suitable for use with the algorithm 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- and key types defined by the "a=" and "k=" tags in the "DKIM-
Signature" header field, the verifier MUST immediately return Signature" header field, the verifier MUST immediately return
PERMFAIL (inappropriate key algorithm). PERMFAIL (inappropriate key algorithm).
6.1.3 Compute the Verification 6.1.3 Compute the Verification
Given a signer and a public key, verifying a signature consists of Given a signer and a public key, verifying a signature consists of
the following steps. actions semantically equivalent to the following steps.
1. Based on the algorithm defined in the "c=" tag, the body length 1. Based on the algorithm defined in the "c=" tag, the body length
specified in the "l=" tag, and the header field names in the "h=" specified in the "l=" tag, and the header field names in the "h="
tag, create a canonicalized copy of the email as is described in tag, prepare a canonicalized version of the message as is
Section 3.7. When matching header field names in the "h=" tag described in Section 3.7 (note that this version does not
against the actual message header field, comparisons MUST be actually need to be instantiated). When matching header field
case-insensitive. names in the "h=" tag against the actual message header field,
comparisons MUST be case-insensitive.
2. Based on the algorithm indicated in the "a=" tag, compute the 2. Based on the algorithm indicated in the "a=" tag, compute the
message hashes from the canonical copy as described in message hashes from the canonical copy as described in
Section 3.7. Section 3.7.
3. Verify that the hash of the canonicalized message body computed 3. Verify that the hash of the canonicalized message body computed
in the previous step matches the hash value conveyed in the "bh=" in the previous step matches the hash value conveyed in the "bh="
tag. tag.
4. Using the signature conveyed in the "b=" tag, verify the 4. Using the signature conveyed in the "b=" tag, verify the
skipping to change at page 42, line 30 skipping to change at page 43, line 10
the MIME structure. A simple way to achieve this might be to the MIME structure. A simple way to achieve this might be to
append "--CRLF" to any "multipart" message with a body length; if append "--CRLF" to any "multipart" message with a body length; if
the MIME structure is already correctly formed, this will appear the MIME structure is already correctly formed, this will appear
in the postlude and will not be displayed to the end user. in the postlude and will not be displayed to the end user.
6.2 Communicate Verification Results 6.2 Communicate Verification Results
Verifiers wishing to communicate the results of verification to other Verifiers wishing to communicate the results of verification to other
parts of the mail system may do so in whatever manner they see fit. parts of the mail system may do so in whatever manner they see fit.
For example, implementations might choose to add an email header For example, implementations might choose to add an email header
field to the message before passing it on. An example proposal for a field to the message before passing it on. Any such header field
header field is the Authentication-Results header field [ID-AUTH- SHOULD be inserted before any existing DKIM-Signature or preexisting
RES]. Any such header field SHOULD be inserted before any existing authentication status header fields in the header field block.
DKIM-Signature or preexisting authentication status header fields in
the header field block.
INFORMATIVE ADVICE to MUA filter writers: Patterns intended to INFORMATIVE ADVICE to MUA filter writers: Patterns intended to
search for results header fields to visibly mark authenticated search for results header fields to visibly mark authenticated
mail for end users should verify that such header field was added mail for end users should verify that such header field was added
by the appropriate verifying domain and that the verified identity by the appropriate verifying domain and that the verified identity
matches the author identity that will be displayed by the MUA. In matches the author identity that will be displayed by the MUA. In
particular, MUA filters should not be influenced by bogus results particular, MUA filters should not be influenced by bogus results
header fields added by attackers. header fields added by attackers. Verifiers may wish to delete
existing results header fields after verification and before
adding a new header field to circumvent this attack.
6.3 Interpret Results/Apply Local Policy 6.3 Interpret Results/Apply Local Policy
It is beyond the scope of this specification to describe what actions It is beyond the scope of this specification to describe what actions
a verifier system should make, but an authenticated email presents an a verifier system should make, but an authenticated email presents an
opportunity to a receiving system that unauthenticated email cannot. opportunity to a receiving system that unauthenticated email cannot.
Specifically, an authenticated email creates a predictable identifier Specifically, an authenticated email creates a predictable identifier
by which other decisions can reliably be managed, such as trust and by which other decisions can reliably be managed, such as trust and
reputation. Conversely, unauthenticated email lacks a reliable reputation. Conversely, unauthenticated email lacks a reliable
identifier that can be used to assign trust and reputation. It is identifier that can be used to assign trust and reputation. It is
skipping to change at page 43, line 45 skipping to change at page 44, line 25
signed on behalf of any address other than that in the From: header signed on behalf of any address other than that in the From: header
field, the mail system SHOULD take pains to ensure that the actual field, the mail system SHOULD take pains to ensure that the actual
signing identity is clear to the reader. signing identity is clear to the reader.
The verifier MAY treat unsigned header fields with extreme The verifier MAY treat unsigned header fields with extreme
skepticism, including marking them as untrusted or even deleting them skepticism, including marking them as untrusted or even deleting them
before display to the end user. before display to the end user.
While the symptoms of a failed verification are obvious -- the While the symptoms of a failed verification are obvious -- the
signature doesn't verify -- establishing the exact cause can be more signature doesn't verify -- establishing the exact cause can be more
difficult. If a selector cannot be found, is that because the difficult. If a Selector cannot be found, is that because the
selector has been removed or was the value changed somehow in Selector has been removed or was the value changed somehow in
transit? If the signature line is missing is that because it was transit? If the signature line is missing is that because it was
never there, or was it removed by an over-zealous filter? For never there, or was it removed by an over-zealous filter? For
diagnostic purposes, the exact reason why the verification fails diagnostic purposes, the exact reason why the verification fails
SHOULD be made available to the policy module and possibly recorded SHOULD be made available to the policy module and possibly recorded
in the system logs. However in terms of presentation to the end in the system logs. However in terms of presentation to the end
user, the result SHOULD be presented as a simple binary result: user, the result SHOULD be presented as a simple binary result:
either the email is verified or it is not. If the email cannot be either the email is verified or it is not. If the email cannot be
verified, then it SHOULD be rendered the same as all unverified email verified, then it SHOULD be rendered the same as all unverified email
regardless of whether it looks like it was signed or not. regardless of whether it looks like it was signed or not.
7. IANA Considerations 7. IANA Considerations
To avoid conflicts, tag names for the DKIM-Signature header and key DKIM introduces some new namespaces that require IANA registry.
records should be registered with IANA.
Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature [[Missing registries for signature t= (flags) tags, as well as key
header field, and the "h=", "k=", "s=", and "t" tags in key records record s= (service type) and t= (flags).]]
should be registered with IANA for the same reason.
7.1 DKIM-Signature Tag Specifications
A DKIM-Signature provides for a list of tag specifications. IANA is
requested to establish the DKIM Signature Tag Specification Registry,
for tag specifications that can be used in DKIM-Signature fields and
that have been specified in any published RFC.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | RFC |
+------+-----------------+
| 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 in Section 3.5 provides for a list of
query methods.
IANA is requested to establish the DKIM Query Method Registry, for
mechanisms that can be used to retrieve the key that will permit
validation processing of a message signed using DKIM and have been
specified in any published RFC.
The initial entry in the registry comprises:
+------+--------+-----------------+
| TYPE | OPTION | RFC |
+------+--------+-----------------+
| dns | txt | (this document) |
+------+--------+-----------------+
7.3 DKIM-Signature Canonicalization Registry
The "c=" tag-spec, as specified in Section 3.5 provides for a
specifier for canonicalization algorithms for the header and body of
the message.
IANA is requested to establish the DKIM Canonicalization Algorithm
Registry, for algorithms for converting a message into a canonical
form before signing or verifying using DKIM and have been specified
in any published RFC.
The initial entries in the header registry comprise:
+---------+-----------------+
| TYPE | RFC |
+---------+-----------------+
| simple | (this document) |
| relaxed | (this document) |
+---------+-----------------+
The initial entries in the body registry comprise:
+---------+-----------------+
| TYPE | RFC |
+---------+-----------------+
| simple | (this document) |
| relaxed | (this document) |
+---------+-----------------+
7.4 _domainkey DNS TXT Record Tag Specifications
A _domainkey DNS TXT record provides for a list of tag
specifications. IANA is requested to establish the DKIM _domainkey
DNS TXT Tag Specification Registry, for tag specifications that can
be used in DNS TXT Records and that have been specified in any
published RFC.
The initial entries in the registry comprise:
+------+-----------------+
| TYPE | RFC |
+------+-----------------+
| 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 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 in the registry comprises:
+------+---------+
| TYPE | RFC |
+------+---------+
| rsa | RFC3447 |
+------+---------+
7.6 DKIM Hash Algorithms Registry
The "h=" <key-h-tag> list (specified in Section 3.6.1) and 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 the DKIM Hash Algorithms Registry, for
such mechanisms that have been specified in any published RFC.
The initial entries in the registry comprise:
+--------+-----+
| TYPE | RFC |
+--------+-----+
| sha1 | ? |
| sha256 | ? |
+--------+-----+
8. Security Considerations 8. Security Considerations
It has been observed that any mechanism that is introduced which It has been observed that any mechanism that is introduced which
attempts to stem the flow of spam is subject to intensive attack. attempts to stem the flow of spam is subject to intensive attack.
DKIM needs to be carefully scrutinized to identify potential attack DKIM needs to be carefully scrutinized to identify potential attack
vectors and the vulnerability to each. See also [ID-DKIM-THREATS]. vectors and the vulnerability to each. See also [ID-DKIM-THREATS].
8.1 Misuse of Body Length Limits ("l=" Tag) 8.1 Misuse of Body Length Limits ("l=" Tag)
skipping to change at page 44, line 40 skipping to change at page 48, line 15
8.1.1 Addition of new MIME parts to multipart/* 8.1.1 Addition of new MIME parts to multipart/*
If the body length limit does not cover a closing MIME multipart If the body length limit does not cover a closing MIME multipart
section (including the trailing ""--CRLF"" portion), then it is section (including the trailing ""--CRLF"" portion), then it is
possible for an attacker to intercept a properly signed multipart possible for an attacker to intercept a properly signed multipart
message and add a new body part. Depending on the details of the message and add a new body part. Depending on the details of the
MIME type and the implementation of the verifying MTA and the MIME type and the implementation of the verifying MTA and the
receiving MUA, this could allow an attacker to change the information receiving MUA, this could allow an attacker to change the information
displayed to an end user from an apparently trusted source. displayed to an end user from an apparently trusted source.
*** Example appropriate here *** 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 8.1.2 Addition of new HTML content to existing content
Several receiving MUA implementations do not cease display after a Several receiving MUA implementations do not cease display after a
""</html>"" tag. In particular, this allows attacks involving ""</html>"" tag. In particular, this allows attacks involving
overlaying images on top of existing text. overlaying images on top of existing text.
INFORMATIVE EXAMPLE: Appending the following text to an existing, INFORMATIVE EXAMPLE: Appending the following text to an existing,
properly closed message will in many MUAs result in inappropriate properly closed message will in many MUAs result in inappropriate
data being rendered on top of existing, correct data: data being rendered on top of existing, correct data:
skipping to change at page 45, line 19 skipping to change at page 48, line 45
8.2 Misappropriated Private Key 8.2 Misappropriated Private Key
If the private key for a user is resident on their computer and is If the private key for a user is resident on their computer and is
not protected by an appropriately secure mechanism, it is possible not protected by an appropriately secure mechanism, it is possible
for malware to send mail as that user and any other user sharing the 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 same private key. The malware would, however, not be able to
generate signed spoofs of other signers' addresses, which would aid generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user and would limit the in identification of the infected user and would limit the
possibilities for certain types of attacks involving socially- possibilities for certain types of attacks involving socially-
engineered messages. 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 A larger problem occurs if malware on many users' computers obtains
the private keys for those users and transmits them via a covert the private keys for those users and transmits them via a covert
channel to a site where they can be shared. The compromised users channel to a site where they can be shared. The compromised users
would likely not know of the misappropriation until they receive would likely not know of the misappropriation until they receive
"bounce" messages from messages they are purported to have sent. "bounce" messages from messages they are purported to have sent.
Many users might not understand the significance of these bounce Many users might not understand the significance of these bounce
messages and would not take action. messages and would not take action.
One countermeasure is to use a user-entered passphrase to encrypt the One countermeasure is to use a user-entered passphrase to encrypt the
skipping to change at page 48, line 9 skipping to change at page 51, line 38
8.8 Intentionally Malformed DKIM-Signature header fields 8.8 Intentionally Malformed DKIM-Signature header fields
Verifiers MUST be prepared to receive messages with malformed DKIM- Verifiers MUST be prepared to receive messages with malformed DKIM-
Signature header fields, and thoroughly verify the header field Signature header fields, and thoroughly verify the header field
before depending on any of its contents. before depending on any of its contents.
8.9 Information Leakage 8.9 Information Leakage
An attacker could determine when a particular signature was verified An attacker could determine when a particular signature was verified
by using a per-message selector and then monitoring their DNS traffic 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 the key lookup. This would act as the equivalent of a "web bug"
for verification time rather than when the message was read. for verification time rather than when the message was read.
8.10 Remote Timing Attacks 8.10 Remote Timing Attacks
In some cases it may be possible to extract private keys using a In some cases it may be possible to extract private keys using a
remote timing attack [BONEH03]. Implementations should consider remote timing attack [BONEH03]. Implementations should consider
obfuscating the timing to prevent such attacks. obfuscating the timing to prevent such attacks.
9. References 9. References
skipping to change at page 49, line 5 skipping to change at page 52, line 35
Standards (PKCS) #1: RSA Cryptography Specifications Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003. Version 2.1", RFC 3447, February 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Application(IDNA)", for Internationalized Domain Names in Application(IDNA)",
March 2003. March 2003.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
[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 9.2 Informative References
[BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing [BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing
Attacks are Practical", 2003, <http://www.usenix.org/ Attacks are Practical", 2003, <http://www.usenix.org/
publications/library/proceedings/sec03/tech/brumley.html>. publications/library/proceedings/sec03/tech/brumley.html>.
[ID-AUTH-RES]
Kucherawy, M., "Message header field for Indicating Sender
Authentication Status",
draft-kucherawy-sender-auth-header-02 (work in progress),
February 2006.
[ID-DKIM-THREATS] [ID-DKIM-THREATS]
Fenton, J., "Analysis of Threats Motivating DomainKeys Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", draft-fenton-dkim-threats-02 Identified Mail (DKIM)", draft-fenton-dkim-threats-02
(work in progress), April 2006. (work in progress), April 2006.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed, [RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and "Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995. Multipart/Encrypted", RFC 1847, October 1995.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998. "OpenPGP Message Format", RFC 2440, November 1998.
[RFC3766] Orman, H. and P. Hoffman, "Determing Strengths for Public [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths for
Keys Used For Exchanging Symmetric Keys", RFC 3766, Public Keys Used For Exchanging Symmetric Keys", RFC 3766,
April 2004. April 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004. Name System (DNS)", RFC 3833, August 2004.
[RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification", [RFC3851] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 3851, June 1999. RFC 3851, June 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
skipping to change at page 52, line 5 skipping to change at page 55, line 24
We lost the game. Are you hungry yet? We lost the game. Are you hungry yet?
Joe. Joe.
A.2 The email is signed A.2 The email is signed
This email is signed by the example.com outbound email server and now This email is signed by the example.com outbound email server and now
looks like this: looks like this:
DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com; DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com; c=simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID; h=Received : From : To : Subject : Date : Message-ID;
bh=ZSVEYuq4ri3LR9S+qjlzCP+LxvJrIfrOI2g5hxp5+MI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR; VoG4ZHRNiYzR;
Received: from dsl-10.2.3.4.football.example.com [10.2.3.4] Received: from dsl-10.2.3.4.football.example.com [10.2.3.4]
by submitserver.example.com with SUBMISSION; by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT) Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com> From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net> To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready? Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com> Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi. Hi.
We lost the game. Are you hungry yet? We lost the game. Are you hungry yet?
Joe. Joe.
The signing email server requires access to the private-key The signing email server requires access to the private-key
associated with the "brisbane" selector to generate this signature. associated with the "brisbane" Selector to generate this signature.
A.3 The email signature is verified A.3 The email signature is verified
The signature is normally verified by an inbound SMTP server or The signature is normally verified by an inbound SMTP server or
possibly the final delivery agent. However, intervening MTAs can possibly the final delivery agent. However, intervening MTAs can
also perform this verification if they choose to do so. The also perform this verification if they choose to do so. The
verification process uses the domain "example.com" extracted from the verification process uses the domain "example.com" extracted from the
"d=" tag and the selector "brisbane" from the "s=" tag in the "DKIM- "d=" tag and the Selector "brisbane" from the "s=" tag in the "DKIM-
Signature" header field to form the DNS DKIM query for: Signature" header field to form the DNS DKIM query for:
brisbane._domainkey.example.com brisbane._domainkey.example.com
Signature verification starts with the physically last "Received" Signature verification starts with the physically last "Received"
header field, the "From" header field, and so forth, in the order header field, the "From" header field, and so forth, in the order
listed in the "h=" tag. Verification follows with a single CRLF listed in the "h=" tag. Verification follows with a single CRLF
followed by the body (starting with "Hi."). The email is canonically followed by the body (starting with "Hi."). The email is canonically
prepared for verifying with the "simple" method. The result of the prepared for verifying with the "simple" method. The result of the
query and subsequent verification of the signature is stored in the query and subsequent verification of the signature is stored in the
"Authentication-Results" header field line. After successful "Authentication-Results" header field line. After successful
verification, the email looks like this: verification, the email looks like this:
Authentication-Results: shopping.example.net X-Authentication-Results: shopping.example.net
header.from=joe@football.example.com; dkim=pass header.from=joe@football.example.com; dkim=pass
Received: from mout23.football.example.com (192.168.1.1) Received: from mout23.football.example.com (192.168.1.1)
by shopping.example.net with SMTP; by shopping.example.net with SMTP;
Fri, 11 Jul 2003 21:01:59 -0700 (PDT) Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
DKIM-Signature: a=rsa-sha1; s=brisbane; d=example.com; DKIM-Signature: a=rsa-sha256; s=brisbane; d=example.com;
c=simple; q=dns/txt; i=joe@football.example.com; c=simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID; h=Received : From : To : Subject : Date : Message-ID;
bh=ZSVEYuq4ri3LR9S+qjlzCP+LxvJrIfrOI2g5hxp5+MI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR VoG4ZHRNiYzR
Received: from dsl-10.2.3.4.network.example.com [10.2.3.4] Received: from dsl-10.2.3.4.network.example.com [10.2.3.4]
by submitserver.example.com with SUBMISSION; by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT) Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com> From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net> To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready? Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com> Message-ID: <20030712040037.46341.5F8J@football.example.com>
skipping to change at page 53, line 50 skipping to change at page 57, line 25
.forward file or equivalent. In this case messages are typically .forward file or equivalent. In this case messages are typically
forwarded without modification, except for the addition of a Received forwarded without modification, except for the addition of a Received
header field to the message and a change in the Envelope-to address. header field to the message and a change in the Envelope-to address.
In this case, the eventual recipient should be able to verify the In this case, the eventual recipient should be able to verify the
original signature since the signed content has not changed, and original signature since the signed content has not changed, and
attribute the message correctly. attribute the message correctly.
B.2 Outsourced Business Functions B.2 Outsourced Business Functions
Outsourced business functions represent a use case that motivates the Outsourced business functions represent a use case that motivates the
need for selectors (the "s=" signature tag) and granularity (the "g=" need for Selectors (the "s=" signature tag) and granularity (the "g="
key tag). Examples of outsourced business functions are legitimate key tag). Examples of outsourced business functions are legitimate
email marketing providers and corporate benefits providers. In email marketing providers and corporate benefits providers. In
either case, the outsourced function would like to be able to send either case, the outsourced function would like to be able to send
messages using the email domain of the client company. At the same messages using the email domain of the client company. At the same
time, the client may be reluctant to register a key for the provider time, the client may be reluctant to register a key for the provider
that grants the ability to send messages for any address in the that grants the ability to send messages for any address in the
domain. domain.
The outsourcing company can generate a keypair and the client company The outsourcing company can generate a keypair and the client company
can register the public key using a unique selector for a specific can register the public key using a unique Selector for a specific
address such as winter-promotions@example.com by specifying a address such as winter-promotions@example.com by specifying a
granularity of "g=winter-promotions" or "g=*-promotions" (to allow a granularity of "g=winter-promotions" or "g=*-promotions" (to allow a
range of addresses). This would enable the provider to send messages range of addresses). This would enable the provider to send messages
using that specific address and have them verify properly. The using that specific address and have them verify properly. The
client company retains control over the email address because it client company retains control over the email address because it
retains the ability to revoke the key at any time. retains the ability to revoke the key at any time.
B.3 PDAs and Similar Devices B.3 PDAs and Similar Devices
PDAs are one example of the use of multiple keys per user. Suppose PDAs are one example of the use of multiple keys per user. Suppose
skipping to change at page 56, line 7 skipping to change at page 59, line 29
One way this can be handled is to continue to put the reader's email One way this can be handled is to continue to put the reader's email
address in the From header field of the message, but put an address address in the From header field of the message, but put an address
owned by the site into the Sender header field, and sign the message owned by the site into the Sender header field, and sign the message
on behalf of that address. A verifying MTA could accept this and on behalf of that address. A verifying MTA could accept this and
rewrite the From header field to indicate the address that was rewrite the From header field to indicate the address that was
verified, i.e., From: John Doe via news@news-site.com verified, i.e., From: John Doe via news@news-site.com
<jdoe@example.com>. (However, such rewriting must be done after the <jdoe@example.com>. (However, such rewriting must be done after the
verification pass is complete, and will break any later attempts to verification pass is complete, and will break any later attempts to
re-verify.) re-verify.)
B.7 SMTP Servers for Roaming Users
Roaming users may find themselves in circumstances where it is
convenient or necessary to use an SMTP server other than their home
server; examples are IETF conferences and many hotels. In such
circumstances the signature, if any, will be added by a party other
than 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 may be high. If neither of these are possible,
these roaming users will not be able to send mail signed using their
own domain key.
Appendix C. Creating a public key (INFORMATIVE) Appendix C. Creating a public key (INFORMATIVE)
The default signature is an RSA signed SHA256 digest of the complete The default signature is an RSA signed SHA256 digest of the complete
email. For ease of explanation, the openssl command is used to email. For ease of explanation, the openssl command is used to
describe the mechanism by which keys and signatures are managed. One describe the mechanism by which keys and signatures are managed. One
way to generate a 768 bit private-key suitable for DKIM, is to use way to generate a 1024 bit, unencrypted private-key suitable for
openssl like this: DKIM, is to use openssl like this:
$ openssl genrsa -out rsa.private 1024 $ openssl genrsa -out rsa.private 1024
This results in the file rsa.private containing the key information For increased security, the "-passin" parameter can also be added to
similar to this: 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:
-----BEGIN RSA PRIVATE KEY----- -----BEGIN RSA PRIVATE KEY-----
MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC
jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb
to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB
AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX
/1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ /1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ
gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO
n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m
3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/ 3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/
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This results in signature data similar to this when represented in This results in signature data similar to this when represented in
Base64 [MIME] format: Base64 [MIME] format:
aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz
msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl
How this signature is added to the email is discussed elsewhere in How this signature is added to the email is discussed elsewhere in
this document. this document.
The final record entered into a DNS zone file would be:
brisbane IN TXT ("v=DKIM1; p=aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvct"
"qc4rSEFYby9+omKD3pJ/TVxATeTzmsybuW3WZiamb+mvn7f"
"3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl" )
Appendix D. MUA Considerations Appendix D. MUA Considerations
When a DKIM signature is verified, one of the results is a validated When a DKIM signature is verified, one of the results is a validated
signing identity. MUAs might highlight the address associated with signing identity. MUAs might highlight the address associated with
this identity in some way to show the user the address from which the this identity in some way to show the user the address from which the
mail is sent. An MUA might do this with visual cues such as mail is sent. An MUA might do this with visual cues such as
graphics, or it might include the address in an alternate views, or graphics, or it might include the address in an alternate views, or
it might even rewrite the original "From:" address using the verified it might even rewrite the original "From:" address using the verified
information. Some MUAs might want to indicate which headers were information. Some MUAs might want to indicate which headers were
covered in a validated DKIM signature. This might be done with a covered in a validated DKIM signature. This might be done with a
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The DomainKeys specification was a primary source from which this The DomainKeys specification was a primary source from which this
specification has been derived. Further information about DomainKeys specification has been derived. Further information about DomainKeys
is at is at
<http://domainkeys.sourceforge.net/license/patentlicense1-1.html>. <http://domainkeys.sourceforge.net/license/patentlicense1-1.html>.
Appendix F. Edit History Appendix F. Edit History
[[This section to be removed before publication.]] [[This section to be removed before publication.]]
F.1 Changes since -ietf-02 version F.1 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.2 Changes since -ietf-02 version
The following changes were made between draft-ietf-dkim-base-02 and The following changes were made between draft-ietf-dkim-base-02 and
draft-ietf-dkim-base-03: draft-ietf-dkim-base-03:
o Section 5.2: changed key expiration text to be informational; o Section 5.2: changed key expiration text to be informational;
drop "seven day" wording in favor of something vaguer. drop "seven day" wording in favor of something vaguer.
o Don't indicate that the "i=" tag value should be passed to the key 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. lookup service; this can be added as an extension if required.
skipping to change at page 59, line 36 skipping to change at page 64, line 45
may contain the content. may contain the content.
o Use dkim-quoted-printable as the encoding used in z= rather than o Use dkim-quoted-printable as the encoding used in z= rather than
referring to RFC2045, since they are different. referring to RFC2045, since they are different.
o Rewrite description of g= tag in the key record. o Rewrite description of g= tag in the key record.
o Deleted use of Domain in ABNF, which permits address-literals; o Deleted use of Domain in ABNF, which permits address-literals;
define domain-name to act in stead. define domain-name to act in stead.
F.2 Changes since -ietf-01 version F.3 Changes since -ietf-01 version
The following changes were made between draft-ietf-dkim-base-01 and The following changes were made between draft-ietf-dkim-base-01 and
draft-ietf-dkim-base-02: draft-ietf-dkim-base-02:
o Change wording on "x=" tag in DKIM-Signature header field o Change wording on "x=" tag in DKIM-Signature header field
regarding verifier handling of expired signatures from MUST to MAY regarding verifier handling of expired signatures from MUST to MAY
(per 20 April Jabber session). Also, make it clear that received (per 20 April Jabber session). Also, make it clear that received
time is to be preferred over current time if reliably available. time is to be preferred over current time if reliably available.
o Several changes to limit wording that would intrude into verifier o Several changes to limit wording that would intrude into verifier
skipping to change at page 60, line 21 skipping to change at page 65, line 28
o Change "q=dns" query access method to "q=dnstxt" to emphasize the 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 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 will define "q=dnsdkk" to indicate use of a DKK record. (Per 18
May Jabber session.) May Jabber session.)
o Several typos fixed, including removing a paragraph that implied o Several typos fixed, including removing a paragraph that implied
that the DKIM-Signature header field should be hashed with the that the DKIM-Signature header field should be hashed with the
body (it should not). body (it should not).
F.3 Changes since -ietf-00 version F.4 Changes since -ietf-00 version
The following changes were made between draft-ietf-dkim-base-00 and The following changes were made between draft-ietf-dkim-base-00 and
draft-ietf-dkim-base-01: draft-ietf-dkim-base-01:
o Added section 8.9 (Information Leakage). o Added section 8.9 (Information Leakage).
o Replace section 4 (Multiple Signatures) with much less vague text. o Replace section 4 (Multiple Signatures) with much less vague text.
o Fixed ABNF for base64string. o Fixed ABNF for base64string.
skipping to change at page 60, line 45 skipping to change at page 66, line 7
o Changed signing algorithm to use separate hash of the body of the o Changed signing algorithm to use separate hash of the body of the
message; this is represented as the "bh=" tag in the DKIM- message; this is represented as the "bh=" tag in the DKIM-
Signature header field. Signature header field.
o Changed "z=" tag so that it need not have the same header field o Changed "z=" tag so that it need not have the same header field
names as the "h=" tag. names as the "h=" tag.
o Significant wordsmithing. o Significant wordsmithing.
F.4 Changes since -allman-01 version F.5 Changes since -allman-01 version
The following changes were made between draft-allman-dkim-base-01 and The following changes were made between draft-allman-dkim-base-01 and
draft-ietf-dkim-base-00: draft-ietf-dkim-base-00:
o Remove references to Sender Signing Policy document. Such o Remove references to Sender Signing Policy document. Such
consideration is implicitly included in Section 6.3. consideration is implicitly included in Section 6.3.
o Added ABNF for all tags. o Added ABNF for all tags.
o Updated references (still includes some references to expired o Updated references (still includes some references to expired
drafts, notably [ID-AUTH-RES]. drafts, notably ID-AUTH-RES.
o Significant wordsmithing. o Significant wordsmithing.
F.5 Changes since -allman-00 version F.6 Changes since -allman-00 version
The following changes were made between draft-allman-dkim-base-00 and The following changes were made between draft-allman-dkim-base-00 and
draft-allman-dkim-base-01: draft-allman-dkim-base-01:
o Changed "c=" tag to separate out header from body o Changed "c=" tag to separate out header from body
canonicalization. canonicalization.
o Eliminated "nowsp" canonicalization in favor of "relaxed", which o Eliminated "nowsp" canonicalization in favor of "relaxed", which
is somewhat less relaxed (but more secure) than "nowsp". is somewhat less relaxed (but more secure) than "nowsp".
 End of changes. 151 change blocks. 
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