draft-ietf-dkim-base-01.txt   draft-ietf-dkim-base-02.txt 
DKIM E. Allman DKIM E. Allman
Internet-Draft Sendmail, Inc. Internet-Draft Sendmail, Inc.
Expires: October 15, 2006 J. Callas Expires: November 23, 2006 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.
April 13, 2006 May 22, 2006
DomainKeys Identified Mail Signatures (DKIM) DomainKeys Identified Mail Signatures (DKIM)
draft-ietf-dkim-base-01 draft-ietf-dkim-base-02
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
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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 October 15, 2006. This Internet-Draft will expire on November 23, 2006.
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
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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 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6 1.2 Signing Identity . . . . . . . . . . . . . . . . . . . . . 6
1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6 1.4 Simple Key Management . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 6 2. Terminology and Definitions . . . . . . . . . . . . . . . . 6
2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 Signers . . . . . . . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . . 7 2.5 Imported ABNF Tokens . . . . . . . . . . . . . . . . . . . 8
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8 3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . 8
3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 Selectors . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10 3.2 Tag=Value Lists . . . . . . . . . . . . . . . . . . . . . 10
3.3 Signing and Verification Algorithms . . . . . . . . . . . 11 3.3 Signing and Verification Algorithms . . . . . . . . . . . 11
3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 12 3.4 Canonicalization . . . . . . . . . . . . . . . . . . . . . 12
3.5 The DKIM-Signature header field . . . . . . . . . . . . . 16 3.5 The DKIM-Signature header field . . . . . . . . . . . . . 17
3.6 Key Management and Representation . . . . . . . . . . . . 23 3.6 Key Management and Representation . . . . . . . . . . . . 24
3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 27 3.7 Computing the Message Hashes . . . . . . . . . . . . . . . 28
4. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 29 4. Semantics of Multiple Signatures . . . . . . . . . . . . . . 29
5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . . 30 5. Signer Actions . . . . . . . . . . . . . . . . . . . . . . . 30
5.1 Determine if the Email Should be Signed and by Whom . . . 30 5.1 Determine if the Email Should be Signed and by Whom . . . 30
5.2 Select a private-key and corresponding selector 5.2 Select a private-key and corresponding selector
information . . . . . . . . . . . . . . . . . . . . . . . 30 information . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 Normalize the Message to Prevent Transport Conversions . . 31 5.3 Normalize the Message to Prevent Transport Conversions . . 31
5.4 Determine the header fields to Sign . . . . . . . . . . . 31 5.4 Determine the header fields to Sign . . . . . . . . . . . 31
5.5 Compute the Message Hash and Signature . . . . . . . . . . 33 5.5 Compute the Message Hash and Signature . . . . . . . . . . 33
5.6 Insert the DKIM-Signature header field . . . . . . . . . . 34 5.6 Insert the DKIM-Signature header field . . . . . . . . . . 34
6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . . 35 6. Verifier Actions . . . . . . . . . . . . . . . . . . . . . . 35
6.1 Extract the Signature from the Message . . . . . . . . . . 35 6.1 Extract the Signature from the Message . . . . . . . . . . 36
6.2 Get the Public Key . . . . . . . . . . . . . . . . . . . . 36 6.2 Get the Public Key . . . . . . . . . . . . . . . . . . . . 37
6.3 Compute the Verification . . . . . . . . . . . . . . . . . 37 6.3 Compute the Verification . . . . . . . . . . . . . . . . . 38
6.4 Communicate Verification Results . . . . . . . . . . . . . 39 6.4 Communicate Verification Results . . . . . . . . . . . . . 40
6.5 Interpret Results/Apply Local Policy . . . . . . . . . . . 39 6.5 Interpret Results/Apply Local Policy . . . . . . . . . . . 40
6.6 MUA Considerations . . . . . . . . . . . . . . . . . . . . 40 6.6 MUA Considerations . . . . . . . . . . . . . . . . . . . . 41
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 42
8. Security Considerations . . . . . . . . . . . . . . . . . . . 41 8. Security Considerations . . . . . . . . . . . . . . . . . . 42
8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 42 8.1 Misuse of Body Length Limits ("l=" Tag) . . . . . . . . . 43
8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 42 8.2 Misappropriated Private Key . . . . . . . . . . . . . . . 43
8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 43 8.3 Key Server Denial-of-Service Attacks . . . . . . . . . . . 44
8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 43 8.4 Attacks Against DNS . . . . . . . . . . . . . . . . . . . 44
8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 44 8.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 45
8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 44 8.6 Limits on Revoking Keys . . . . . . . . . . . . . . . . . 46
8.7 Intentionally malformed Key Records . . . . . . . . . . . 45 8.7 Intentionally malformed Key Records . . . . . . . . . . . 46
8.8 Intentionally Malformed DKIM-Signature header fields . . . 45 8.8 Intentionally Malformed DKIM-Signature header fields . . . 46
8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 45 8.9 Information Leakage . . . . . . . . . . . . . . . . . . . 46
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45 8.10 Remote Timing Attacks . . . . . . . . . . . . . . . . . 46
9.1 Normative References . . . . . . . . . . . . . . . . . . . 45 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
9.2 Informative References . . . . . . . . . . . . . . . . . . 46 9.1 Normative References . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47 9.2 Informative References . . . . . . . . . . . . . . . . . . 47
A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . . 48 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
A.1 The user composes an email . . . . . . . . . . . . . . . . 49 A. Example of Use (INFORMATIVE) . . . . . . . . . . . . . . . . 49
A.2 The email is signed . . . . . . . . . . . . . . . . . . . 49 A.1 The user composes an email . . . . . . . . . . . . . . . . 50
A.3 The email signature is verified . . . . . . . . . . . . . 50 A.2 The email is signed . . . . . . . . . . . . . . . . . . . 50
B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . . 51 A.3 The email signature is verified . . . . . . . . . . . . . 51
B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 51 B. Usage Examples (INFORMATIVE) . . . . . . . . . . . . . . . . 52
B.2 Outsourced Business Functions . . . . . . . . . . . . . . 51 B.1 Simple Message Forwarding . . . . . . . . . . . . . . . . 52
B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 51 B.2 Outsourced Business Functions . . . . . . . . . . . . . . 52
B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 52 B.3 PDAs and Similar Devices . . . . . . . . . . . . . . . . . 52
B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 52 B.4 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 53
B.6 Third-party Message Transmission . . . . . . . . . . . . . 53 B.5 Affinity Addresses . . . . . . . . . . . . . . . . . . . . 53
C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . . 53 B.6 Third-party Message Transmission . . . . . . . . . . . . . 54
D. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 55 C. Creating a public key (INFORMATIVE) . . . . . . . . . . . . 54
E. Edit History . . . . . . . . . . . . . . . . . . . . . . . . . 55 D. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56
E.1 Changes since -ietf-00 version . . . . . . . . . . . . . . 55 E. Edit History . . . . . . . . . . . . . . . . . . . . . . . . 56
E.2 Changes since -allman-01 version . . . . . . . . . . . . . 56 E.1 Changes since -ietf-01 version . . . . . . . . . . . . . . 56
E.3 Changes since -allman-00 version . . . . . . . . . . . . . 56 E.2 Changes since -ietf-00 version . . . . . . . . . . . . . . 57
Intellectual Property and Copyright Statements . . . . . . . . 57 E.3 Changes since -allman-01 version . . . . . . . . . . . . . 57
E.4 Changes since -allman-00 version . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . 59
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 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:
o the message signature is written to the message header fields so o the message signature is written as a message header field so that
that 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 it makes no attempt to include encryption as part of the
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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 a trusted third party (such as a
certificate authority or other entity) which might impose certificate authority or other entity) which might impose
significant costs or introduce delays to deployment 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 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 A "selector" mechanism allows multiple keys per domain, including
delegation of the right to authenticate a portion of the namespace to delegation of the right to authenticate a portion of the namespace to
a trusted third party. a trusted third party.
1.2 Signing Identity 1.2 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.
INFORMATIVE RATIONALE: The signing address associated with a DKIM INFORMATIVE RATIONALE: The signing address associated with a DKIM
signature is not required to match a particular header field signature is not required to match a particular header field
because of the broad methods of interpretation by recipient mail because of the broad methods of interpretation by recipient mail
systems, including MUAs. systems, including MUAs.
1.3 Scalability 1.3 Scalability
The email identification problem is characterized by extreme DKIM is designed to support the extreme scalability requirements
scalability requirements. There are currently over 70 million which characterize the email identification problem. There are
domains and a much larger number of individual addresses. It is currently over 70 million domains and a much larger number of
important to preserve the positive aspects of the current email individual addresses. DKIM seeks to preserve the positive aspects of
infrastructure, such as the ability for anyone to communicate with the current email infrastructure, such as the ability for anyone to
anyone else without introduction. communicate with anyone else without introduction.
1.4 Simple Key Management 1.4 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.
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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.
INFORMATIVE IMPLEMENTERS' NOTE: reusing a selector with a new key
(for example, changing the key associated with a user's name)
makes it impossible to tell the difference between a message that
didn't verify because the key is no longer valid versus a message
that is actually forged. Signers should not change the key
associated with a selector. When creating a new key, signers
should associate it with a new selector.
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 outsourced 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.
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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, 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 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 make this selector associated directly with the user to whether to make this selector associated directly with the user
name, or make it some unassociated random value, such as a name, or make it some unassociated random value, such as a
fingerprint of the public key. fingerprint of the public key.
INFORMATIVE IMPLEMENTERS' NOTE: reusing a selector with a new key
(for example, changing the key associated with a user's name)
makes it impossible to tell the difference between a message that
didn't verify because the key is no longer valid versus a message
that is actually forged. Signers should not change the key
associated with a selector. When creating a new key, signers
should associate it with a new selector.
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, domain signature records, and policy records. in messages, domain signature records, and policy records.
Values are a series of strings containing either base64 text, plain Values are a series of strings containing either base64 text, plain
text, or quoted printable text, as defined in [RFC2045], section 6.7. text, or quoted printable text, as defined in [RFC2045], section 6.7.
The name of the tag will determine the encoding of each value; The name of the tag will determine the encoding of each value;
however, no encoding may include the semicolon (";") character, since however, no encoding may include the semicolon (";") character, since
that separates tag-specs. that separates tag-specs.
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DKIM supports multiple key signing/verification algorithms. Two DKIM supports multiple key signing/verification algorithms. Two
algorithms are defined by this specification at this time: rsa-sha1, algorithms are defined by this specification at this time: rsa-sha1,
and rsa-sha256. The rsa-sha256 algorithm is the default if no and rsa-sha256. The rsa-sha256 algorithm is the default if no
algorithm is specified. Verifiers MUST implement both rsa-sha1 and algorithm is specified. Verifiers MUST implement both rsa-sha1 and
rsa-sha256. Signers MUST implement and SHOULD sign using rsa-sha256. 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
encrypted 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]) as the crypt-alg and the signer's private key. version 1.5 [RFC3447]; in particular see section 5.2) with an
The hash MUST NOT be truncated or converted into any form other than exponent of 65537 as the crypt-alg and the signer's private key. The
the native binary form before being signed. hash MUST NOT be truncated or converted into any form other than the
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 encrypted 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]) as the crypt-alg and the signer's private key. version 1.5 [RFC3447]; in particular see section 5.2) with an
The hash MUST NOT be truncated or converted into any form other than exponent of 65537 as the crypt-alg and the signer's private key. The
the native binary form before being signed. hash MUST NOT be truncated or converted into any form other than the
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 understand.
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
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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. Further canonicalization algorithms MAY
be defined in the future; verifiers MUST ignore any signatures that be defined in the future; verifiers MUST ignore any signatures that
use unrecognized canonicalization algorithms. use unrecognized canonicalization algorithms.
In all cases, the header fields of the message are presented to the In all cases, the header fields of the message are presented to the
signing algorithm first in the order indicated by the signature signing algorithm first in the order indicated by the signature
header field and canonicalized using the indicated algorithm. Only header field and canonicalized using the indicated algorithm. Only
header fields listed as signed in the signature header field are header fields listed as signed in the signature header field are
included. The CRLF separating the header field from the body is then included. Note: the signature header field itself is presented at
presented, followed by the canonicalized body. Note that the header the end of the hash, not with the other headers. The CRLF separating
and body may use different canonicalization algorithms. the header field from the body is then presented, followed by the
canonicalized body. Note that the header and body may use different
canonicalization algorithms.
Canonicalization simply prepares the email for presentation to the 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 14, line 5 skipping to change at page 14, line 16
character. WSP characters here include those before and after a character. WSP characters here include those before and after a
line folding boundary. line folding boundary.
o Delete all WSP characters at the end of each unfolded header field o Delete all WSP characters at the end of each unfolded header field
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.
[NON-NORMATIVE DOCUMENTATION NOTE: The only difference between
"relaxed" header field canonicalization and "nowsp" listed in the
previous version of this draft is that nowsp reduces all strings
of streaming white space to zero characters while "relaxed"
reduces strings of white space to one space.]
3.4.3 The "simple" Body Canonicalization Algorithm 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. It makes no other
changes to the message body. In more formal terms, the "simple" body changes to the message body. In more formal terms, the "simple" body
canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the canonicalization algorithm reduces "CRLF 0*CRLF" at the end of the
body to a single "CRLF". body to a single "CRLF".
3.4.4 The "relaxed" Body Canonicalization Algorithm 3.4.4 The "relaxed" Body Canonicalization Algorithm
skipping to change at page 15, line 42 skipping to change at page 15, line 47
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.
Note that verifiers MAY choose to reject or truncate messages that Note that verifiers MAY choose to truncate messages that have body
have body content beyond that specified by the body length count. content beyond that specified by the body length count.
Alternatively, verifiers MAY ignore signatures that do not cover the
entire message body.
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" algorithm and omit the body length count.
3.4.6 Example 3.4.6 Example
(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.)
A message reading: Example 1: A message 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>
<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: body results in a header reading:
a:X <CRLF> a:X <CRLF>
b:Y <SP> Z <CRLF> b:Y <SP> Z <CRLF>
<CRLF>
and a body reading:
<SP> C <CRLF> <SP> C <CRLF>
D <SP> E <CRLF> D <SP> E <CRLF>
(postamble)
The same message canonicalized using simple canonicalization for both Example 2: The same message canonicalized using simple
header and body results in: canonicalization for both header and body results in a header
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>
<CRLF>
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
simple body canoniccalization, the canonicalized version has a header
of:
When processed using relaxed header canonicalization and simple body
canonicalization, the canonicalized version reads:
a:X <CRLF> a:X <CRLF>
b:Y <SP> Z <CRLF> b:Y <SP> Z <CRLF>
<CRLF>
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 signaturee 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 In particular, the "DKIM-Signature" header field SHOULD precede the
original email header fields presented to the canonicalization and original email header fields presented to the canonicalization and
signature algorithms. signature algorithms.
skipping to change at page 17, line 26 skipping to change at page 17, line 43
The encodings for each field type are listed below. Tags described The encodings for each field type are listed below. Tags described
as quoted-printable are as described in section 6.7 of MIME Part One as quoted-printable 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". "=3B".
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. Defined tags are described
below. Unrecognized tags MUST be ignored. below. Unrecognized tags MUST be ignored.
v= Version (MUST NOT be included). This tag is reserved for future v= Version (MUST be included). This tag defines the version of
use to indicate a possible new, incompatible version of the this specification that applies to the signature record. It MUST
specification. It MUST NOT be included in the DKIM-Signature have the value 0.2.
header field.
ABNF: ABNF:
sig-v-tag = sig-v-tag = %x76 [FWS] "=" [FWS] "0.2"
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";
signers SHOULD sign using "rsa-sha256". See Section 3.3 for a signers SHOULD sign using "rsa-sha256". See Section 3.3 for a
description of algorithms. description of algorithms.
ABNF: ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = "rsa-sha1" / "rsa-sha256" / x-sig-a-tag-alg sig-a-tag-alg = "rsa-sha1" / "rsa-sha256" / x-sig-a-tag-alg
x-sig-a-tag-alg = hyphenated-word ; for later extension x-sig-a-tag-alg = hyphenated-word ; for later extension
skipping to change at page 18, line 4 skipping to change at page 18, line 14
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";
signers SHOULD sign using "rsa-sha256". See Section 3.3 for a signers SHOULD sign using "rsa-sha256". See Section 3.3 for a
description of algorithms. description of algorithms.
ABNF: ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = "rsa-sha1" / "rsa-sha256" / x-sig-a-tag-alg sig-a-tag-alg = "rsa-sha1" / "rsa-sha256" / x-sig-a-tag-alg
x-sig-a-tag-alg = hyphenated-word ; for later extension x-sig-a-tag-alg = hyphenated-word ; for later extension
b= The signature data (base64; REQUIRED). Whitespace is ignored in b= The signature data (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when re-assembling the original this value and MUST be ignored when re-assembling tthe 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-tagg-data = base64string sig-b-tag-data = base64string
bh= The hash of the body part of the message (base64; REQUIRED). bh= The hash of the body part of the message (base64; REQUIRED).
Whitespace is ignored in this value and MUST be ignored when re- Whitespace is ignored in this value and MUST be ignored when re-
assembling the original signature. In particular, the signing assembling the original signature. In particular, the signing
process can safely insert FWS in this value in arbitrary places process can safely insert FWS in this value in arbitrary places
to conform to line-length limits. See Section 3.7 for how the to conform to line-length limits. See Section 3.7 for how the
body hash is computed. body hash is computed.
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
skipping to change at page 18, line 45 skipping to change at page 19, line 15
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 the domain that will be queried for the public key. This is the domain that will be queried for the public key. This
domain MUST be the same as or a parent domain of the "i=" tag domain MUST be the same as or a parent domain of the "i=" tag
(the signing identity, as described below). When presented with (the signing identity, as described below). When presented with
a signature that does not meet this requirement, verifiers MUST a signature that does not meet this requirement, verifiers MUST
either ignore the signature or reject the message. consider the signature invalid.
ABNF: ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] Domain sig-d-tag = %x64 [FWS] "=" [FWS] Domain
h= Signed header fields (plain-text, but see description; h= Signed header fields (plain-text, but see description;
REQUIRED). A colon-separated list of header field names that REQUIRED). A colon-separated list of header field names that
identify the header fields presented to the signing algorithm. identify the header fields presented to the signing algorithm.
The field MUST contain the complete list of header fields in the The field MUST contain the complete list of header fields in the
order presented to the signing algorithm. The field MAY contain order presented to the signing algorithm. The field MAY contain
names of header fields that do not exist when signed; nonexistent names of header fields that do not exist when signed; nonexistent
header fields do not contribute to the signature computation header fields do not contribute to the signature computation
(that is, they are treated as the null input, including the (that is, they are treated as the null input, includiing the
header field name, the separating colon, the header field value, header field name, the separating colon, the header field value,
and any CRLF terminator). The field MUST NOT include the DKIM- and any CRLF terminator). The field MUST NOT include the DKIM-
Signature header field that is being created or verified. Signature header field that is being created or verified.
Folding white space (FWS) MAY be included on either side of the Folding white space (FWS) MAY be included on either side of the
colon separator. Header ffield names MUST be compared against colon separator. Header field names MUST be compared against
actual header field names in a case insensitive manner. This actual header field names in a case insensitive manner. This
list MUST NOT be empty. See Section 5.4 for a discussion of list MUST NOT be empty. See Section 5.4 for a 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
skipping to change at page 20, line 22 skipping to change at page 20, line 34
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
by including the domain part but not the local-part of the by including the domain part but not the local-part of the
identity. identity.
INFORMATIVE DISCUSSION: This document does not require the INFORMATIVE DISCUSSION: This document does not require the
value of the "i=" tag to match the identity in any message value of the "i=" tag to match the identity in any message
header field fields. This is considered to be a verifier header field fields. This is considered to be a verifier
policy issue, described in another document [XREF-TBD]. policy issue. Constraints between the value of the "i=" tag
Constraints between the value of the "i=" tag and other and other identities in other header fields seek to apply
identities in other header fields seek to apply basic basic authentication into the semantics of trust associated
aauthentication into the semantics of trust associated with a with a role such as content author. Trust is a broad and
role such as content author. Trust is a broad and complex complex topic and trust mechanisms are subject to highly
topic and trust mechanisms are subject to highly creative creative attacks. The real-world efficacy of any but the
attacks. The real-world efficacy of any but the most basic most basic bindings between the "i=" value and other
bindings between the "i=" value and other identities is not identities is not well established, nor is its vulnerability
well established, nor is its vulnerability to subversion by to subversion by an attacker. Hence reliance on the use of
an attacker. Hence reliance on the use of these options these options should be strictly limited. In particular it
should be strictly limited. In particular it is not at all is not at all clear to what extent a typical end-user
clear to what extent a typical end-user recipient can rely on recipient can rely on any assurances that might be made by
any assurances that might be made by successful use of the successful use of the "i=" options.
"i=" options.
l= Body count (plain-text decimal integer; OPTIONAL, default is l= Body count (plain-text decimal integer; OPTIONAL, default is
entire body). This tag informs the verifier of the number of entire body). This tag informs the verifier of the number of
bytes in the body of the email after canonicalization included in bytes in the body of the email after canonicalization included in
the cryptographic hash, starting from 0 immediately following the the cryptographic hash, starting from 0 immediately following the
CRLF preceding the body. CRLF preceding the 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
skipping to change at page 21, line 19 skipping to change at page 21, line 33
To avoid this attack, signers should be extremely wary of To avoid this attack, signers should be extremely wary of
using this tag, and verifiers might wish to ignore the tag or using this tag, and verifiers might wish to ignore the tag or
remove text that appears after the specified content length. remove text that appears after the specified content length.
ABNF: ABNF:
sig-l-tag = %x6c [FWS] "=" [FWS] 1*DIGIT sig-l-tag = %x6c [FWS] "=" [FWS] 1*DIGIT
q= A colon-separated list of query methods used to retrieve the q= A colon-separated list of query methods used to retrieve the
public key (plain-text; OPTIONAL, default is "dns"). Each query public key (plain-text; OPTIONAL, default is "dns/txt"). Each
method is of the form "type[/options]", where the syntax and query method is of the form "type[/options]", where the syntax
semantics of the options depends on the type. If there are and semantics of the options depends on the type and specified
multiple query mechanisms listed, the choice of query mechanism options. If there are multiple query mechanisms listed, the
MUST NOT change the interpretation of the signature. Currently choice of query mechanism MUST NOT change the interpretation of
the only valid value iis "dns" which defines the DNS lookup the signature. Implementations MUST use the recognized query
algorithm described elsewhere in this document. No options are mechanisms in the order presented.
defined for the "dns" query type, but the string "dns" MAY have a
trailing "/" character. Verifiers and signers MUST support
"dns".
INFORMATIVE RATIONALE: Explicitly allowing a trailing "/" on Currently the only valid value is "dns/txt" which defines the DNS
"dns" allows for the possibility of adding options later and TXT record lookup algorithm described elsewhere in this document.
makes it clear that matching of the query type must terminate The only option defined for the "dns" query type is "txt", which
on either "/" or end of string. 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 = sig-q-tag-type ["/" sig-q-tag-args] sig-q-tag-method = sig-q-tag-type ["/" sig-q-tag-args]
sig-q-tag-type = "dns" / x-sig-q-tag-type sig-q-tag-type = "dns" / x-sig-q-tag-type
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] Domain sig-s-tag = %x73 [FWS] "=" [FWS] subdomain *( "." sub-domain )
t= Signature Timestamp (plain-text; RECOMMENDED, default is an
t= Signature Timestamp (pplain-text; RECOMMENDED, default is an
unknown creation time). The time that this signature was unknown creation time). The time that this signature was
created. The format is the number of seconds since 00:00:00 on created. The format is the number of seconds since 00:00:00 on
January 1, 1970 in the UTC time zone. The value is expressed as January 1, 1970 in the UTC time zone. The value is expressed as
an unsigned integer in decimal ASCII. This value is not an unsigned integer in decimal ASCII. This value is not
constrained to fit into a 31- or 32-bit integer. Implementations constrained to fit into a 31- or 32-bit integer. Implementations
SHOULD be prepared to handle values up to at least 10^12 (until SHOULD be prepared to handle values up to at least 10^12 (until
approximately AD 200,000; this fits into 40 bits). To avoid approximately AD 200,000; this fits into 40 bits). To avoid
denial of service attacks, implementations MAY consider any value denial of service attacks, implementations MAY consider any value
longer than 12 digits to be infinite. longer than 12 digits to be infinite.
ABNF: ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
x= Signature Expiration (plain-text; RECOMMENDED, default is no x= Signature Expiration (plain-text; RECOMMENDED, default is no
expiration). The format is the same as in the "t=" tag, expiration). The format is the same as in the "t=" tag,
represented as an absolute date, not as a time delta from the represented as an absolute date, not as a time delta from the
signing timestamp. Signatures MUST NOT be considered valid if signing timestamp. The value is expressed as an unsigned integer
the current time at the verifier is past the expiration date. in decimal ASCII, with the same contraints on the value in the
The value is expressed as an unsigned integer in decimal ASCII, "t=" tag. Signatures MAY be considered invalid if the
with the same contraints on the value in the "t=" tag. The value verification time at the verifier is past the expiration date.
of the "x=" tag MUST be greater than the value of the "t=" tag if The verification time should be the time that the message was
both are present. first received at the administrative domain of the verifier if
that time is reliably available; otherwise the current time
should be used. The value of the "x=" tag MUST be greater than
the value of the "t=" tag if both are present.
INFORMATIVE NOTE: The x= tag is not intended as an anti- 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 (plain-text, but see description; OPTIONAL, z= Copied header fields (plain-text, but see description; OPTIONAL,
default is null). A vertical-bar-separated list of header field default is null). A vertical-bar-separated list of selected
names and copies of header field values that identify the header header field names and copies of header field values present when
fields present when the message was signed. This field need not the message was signed. It is not required to include all header
fields present at the time of signing. This field need not
contain the same header fields listed in the "h=" tag. Copied contain the same header fields listed in the "h=" tag. Copied
header field values MUST immediately follow the header field name header field values MUST immediately follow the header field name
with a colon separator (no white space permitted). Header field with a colon separator (no white space permitted). Header field
values MUST be represented as Quoted-Printable [RFC2045] with values MUST be represented as Quoted-Printable [RFC2045] with
vertical bars, colons, semicolons, and white space encoded in vertical bars, colons, semicolons, and white space encoded in
addition to the usual requirements. addition to the usual requirements.
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 onnly.
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:
sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy sig-z-tag = %x7A [FWS] "=" [FWS] sig-z-tag-copy
*( [FWS] "|" sig-z-tag-copy ) *( [FWS] "|" sig-z-tag-copy )
sig-z-tag-copy = hdr-name ":" [FWS] qp-hdr-value sig-z-tag-copy = hdr-name ":" [FWS] qp-hdr-value
qp-hdr-value = <quoted-printable text with WS, "|", ":", qp-hdr-value = <quoted-printable text with WS, "|", ":",
skipping to change at page 23, line 29 skipping to change at page 23, line 45
; needs to be updated with real definition ; needs to be updated with real definition
; (could be messy) ; (could be messy)
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-sha1; d=example.net; s=brisbane DKIM-Signature: a=rsa-sha1; d=example.net; s=brisbane
c=simple; q=dns; i=@eng.example.net; t=1117574938; x=1118006938; c=simple; q=dns; 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
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
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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), the signing identity (the "i=" the DKIM-Signature header field), the signing identity (the "i="
tag), and the selector (the "s=" tag). The "i=" tag value could be tag), and the selector (the "s=" tag). The "i=" tag value could be
ignored by some key services. ignored by some key services.
public_key = dkim_find_key(q_val, d_val, i_val, s_val) public_key = dkim_find_key(q_val, d_val, i_val, s_val)
This document defines a single binding, using DNS to distribute the This document defines a single binding, using DNS TXT records to
keys. 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 MMUST 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 key-value-list as described in Section 3.2.
The current valid tags are described below. Other tags MAY be The current valid tags are described below. Other tags MAY be
present and MUST be ignored by any implementation that does not present and MUST be ignored by any implementation that does not
understand them. understand 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
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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] ["*"] [dot-atom]
[[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. This should probably use a different character for
wildcarding. Unfortunately, the options are non-mnemonic 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= Accceptable 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 "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
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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)
This tag is intended to permit senders to constrain the use of This tag is intended to permit senders to constrain the use of
delegated keys, e.g., where a company is willing to delegate the delegated keys, e.g., where a company is willing to delegate the
right to send mail in their name to an outsourcer, but not to right to send mail in their name to an outsourcer, but not to
send IM or make VoIP calls. (This of courrse presumes that these send IM or make VoIP calls. (This of course presumes that these
keys are used in other services in the future.) keys are used in other services in the future.)
ABNF: ABNF:
key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type key-s-tag = %x73 [FWS] "=" [FWS] key-s-tag-type
0*( [FWS] ":" [FWS] key-s-tag-type
key-s-tag-type = "email" / "*" / x-key-s-tag-type key-s-tag-type = "email" / "*" / x-key-s-tag-type
x-key-s-tag-type = hyphenated-word ; for future extension x-key-s-tag-type = hyphenated-word ; for future extension
t= Flags, represented as a colon-separated list of names (plain- t= Flags, represented as a colon-separated list of names (plain-
text; OPTIONAL, default is no flags set). The defined flags are: text; OPTIONAL, default is no flags set). The defined flags are:
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 modee results to assist
the signer. the signer.
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" / x-key-t-tag-flag key-t-tag-flag = "y" / 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 as a key service is hereby defined. All A binding using DNS TXT records as a key service is hereby defined.
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 DKIM-Signature field with a "d=" tag of ""example.com"" and an "s=" a DKIM-Signature field with a "d=" tag of ""example.com"" and an "s="
tag of ""sample"", the DNS query will be for tag of ""sample"", the DNS query will be for
""sample._domainkey.example.com"". ""sample._domainkey.example.com"".
The value of the "i=" tag is not used by the DNS binding. The value of the "i=" tag is not used by the DNS binding.
3.6.2.2 Resource Record Types for Key Storage 3.6.2.2 Resource Record Types for Key Storage
[[This section needs to be fleshed out. ACTUALLY: will be addressed The DNS Resource Record type used is specified by an option to the
in another document.]] query-type ("q=") tag. The only option defined in this base
specification is "/txt", indicating the use of a TXT RR record. A
Two RR types are used: DKK and TXT. later extension of this standard may define another Resource Record
type, tentatively dubbed "DKK".
The DKK RR is expected to be a non-text, binary representation
intended to allow the largest possible keys to be represented and
transmitted in a UDP DNS packet. Details of this RR are described in
[ID-DKIM-RR].
TXT records are encoded as described in Section 3.6.1. TXT records are encoded as described in Section 3.6.1.
Verifiers SHOULD search for a DKK RR first, if possible, followed by
a TXT RR. If the verifier is unable to search for a DKK RR or a DKK
RR is not found, the verifier MUST search for a TXT RR.
3.7 Computing the Message 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 hash 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
discussion, the names of the tags in the "DKIM-Signature" header discussion, the names of the tags in the "DKIM-Signature" header
field which either exists (when verifying) or will be created (when field which either exists (when verifying) or will be created (when
signing) are used. Note that canonicalization (Section 3.4) is only signing) are used. Note that canonicalization (Section 3.4) is only
used to prepare the email for signing or verifying; it does not used to prepare the email for signing or verifying; it does not
affect the transmitted email in any way. affect the transmitted email in any way.
The signer or verifier must compute two hashes, one over the body of The signer or verifier must compute two hashes, one over the body of
the message and one over the header of the message. Signers MUST the message and one over the header of the message. Signers MUST
compute them in the order shown. Verifiers MAY compute them in any compute them in the order shown. Verifiers MAY compute them in any
order convenient to the verifier, provided that the result is order convenient to the verifier, provided that the result is
semantically identical to the semantics that would be the case had semantically identical to the semantics that would be the case had
they been computed in this order. they been computed in this order.
In hash step 1, the signer or verifier MUST hash the message body, In hash step 1, the signer or verifier MUST hash the message body,
canonicalized using the header canonicalization algorithm specified canonicalized using the body canonicalization algorithm specified in
in the "c=" tag and truncated to the length specified in the "l=" the "c=" tag and truncated to the length specified in the "l=" tag.
tag. That hash value is then converted to base64 form and inserted That hash value is then converted to base64 form and inserted into
into the "XXX=" tag of the DKIM-Signature: header field. the "bh=" tag of the DKIMM-Signature: header field.
In hash step 2, the signer or verifier MUST pass the following to the In hash step 2, the signer or verifier MUST pass the following to the
hash algorithm in the indicated order. hash algorithm in the indicated order.
1. The header fields specified by the "h=" tag, in the order 1. The header fields specified by the "h=" tag, in the order
specified in that tag, and canonicalized using the header specified in that tag, and canonicalized using the header
canonicalization algorithm specified in the "c=" tag. Each canonicalization algorithm specified in the "c=" tag. Each
header field must be terminated with a single CRLF. header field must be terminated with a single CRLF.
2. The "DKIM-Signature" header field that exists (verifying) or will 2. The "DKIM-Signature" header field that exists (verifying) or will
be inserted (signing) in the message, with the value of the "b=" be inserted (signing) in the message, with the value of the "b="
tag deleted (i.e., treated as the empty string), canonicalized tag deleted (i.e., treated as the empty string), canonicalized
using the header canonicalization algorithm specified in the "c=" using the header canonicalization algorithm specified in the "c="
tag, and without a trailing CRLF. tag, and without a trailing CRLF.
After the body is processed, a single CRLF followed by the "DKIM-
Signature" header field being created or verified is presented to the
algorithm. The value portion of the "b=" tag (that is, the portion
after the "=" sign) must be treated as though it were empty, and the
header field must be canonicalized according to the algorithm that is
specified in the "c=" tag. Any final CRLF on the "DKIM-Signature"
header field MUST NOT be included in the signature computation.
All tags and their values in the DKIM-Signature header field are All tags and their values in the DKIM-Signature header field are
included in the cryptographic hash with the sole exception of the included in the cryptographic hash with the sole exception of the
value portion of the "b=" (signature) tag, which MUST be treated as value portion of the "b=" (signature) tag, which MUST be treated as
the null string. All tags MUST be included even if they might not be the null string. All tags MUST be included even if they might not be
understood by the verifier. The header field MUST be presented to understood by the verifier. The header field MUST be presented to
the hash algorithm after the body of the message rather than with the the hash algorithm after the body of the message rather than with the
rest of the headder fields and MUST be canonicalized as specified in rest of the header fields and MUST be canonicalized as specified in
the "c=" (canonicalization) tag. The DKIM-Signature header field the€ "c=" (canonicalization) tag. The DKIM-Signature header field
MUST NOT be included in its own h= tag. MUST NOT be included in its own h= tag.
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 characters. DKIM messages MAY be either in plain-
text or in MIME format; no special treatment is afforded to MIME text or in MIME format; no special treatment is afforded to MIME
content. Message attachments in MIME format MUST be included in the content. Message attachments in MIME format MUST be included in the
content which is signed. content 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 = crypt-alg(hash-alg(canon_header || DKIM-SIG), key) header-hash = hash-alg(canon_header || DKIM-SIG)
signature = crypt-alg(header-hash, key)
where crypt-alg is the encryption algorithm specified by the "a=" where crypt-alg is the encryption algorithm specified by the "a="
tag, hash-alg is the hash algorithm specified by the "a=" tag, 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 and
body (respectively) as defined in Section 3.4 (excluding the DKIM- body (respectively) as defined in Section 3.4 (excluding the DKIM-
Signature header field), and DKIM-SIG is the canonicalized DKIM- Signature header field), and DKIM-SIG is the canonicalized DKIM-
Signature header field sans the signature value itself, but with Signature header field sans the signature value itself, but with
body-hash included as the "bh=" tag. body-hash included as the "bh=" 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 NN to sign trace headers. Signers should method described in section Section 5.4 to sign trace headers.
be cognizant that signing DKIM-Signature headers may result in Signers should be cognizant that signing DKIM-Signature headers may
signature failures with intermediaries that do not recognize that result in signature failures with intermediaries that do not
DKIM-Signature's are trace headers and unwittingly reorder them. recognize that DKIM-Signature's are trace headers and unwittingly
reorder them.
When evaluating a message with multiple signatures, a receiver should When evaluating a message with multiple signatures, a receiver should
evaluate signatures independently and on their own merits. For evaluate signatures independently and on their own merits. For
example, a receiver that by policy chooses not to accept signatures example, a receiver that by policy chooses not to accept signatures
with deprecated crypto algorithms should consider such signatures with deprecated crypto algorithms should consider such signatures
invalid. As with messages with a single signature, receievers are at invalid. As with messages with a single signature, receievers are at
liberty to use the presence of valid signatures as an input to local liberty to use the presence of valid signatures as an input to local
policy; likewise, the interpretation of multiple valid signatures in policy; likewise, the interpretation of multiple valid signatures in
combination is a local policy decision of the receiver. combination is a local policy decision of the receiver.
Signers SHOULD NOT remove any DKIM-Signature headers from messages Signers SHOULD NOT remove any DKIM-Signature header fields from
they are signing, even if they know that the headers cannot be messages they are signing, even if they know that the signatures
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
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signer policy. Within a trusted enclave an MTA MAY do the signing. signer policy. Within a trusted enclave an MTA MAY do the signing.
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 A signer MUST NOT sign an email if it is unwilling to be held
responsible for the message; in particular, the signer SHOULD ensure 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 a bona fide relationship with the signer and
that the submitter has the right to use the address being claimed. that the submitter has tthe 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
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A signer SHOULD NOT sign with a key that is expected to expire within A signer SHOULD NOT sign with a key that is expected to expire within
seven days; that is, when rotating to a new key, signing should seven days; that is, when rotating to a new key, signing should
immediately commence with the new key and the old key SHOULD be immediately commence with the new key and the old key SHOULD be
retained for at least seven days before being removed from the key retained for at least seven days before being removed from the key
server. 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
to modification during transitt, notably conversion to 7-bit form. to modification during transit, notably conversion to 7-bit form.
Such conversions will break DKIM signatures. In order to minimize Such conversions will break DKIM signatures. In order to minimize
the chances of such breakage, signers SHOULD convert the message to a the chances of such breakage, signers SHOULD convert the message to a
suitable MIME content transfer encoding such as quoted-printable or suitable MIME content transfer encoding such as quoted-printable or
base64 as described in MIME Part One [RFC2045] before signing. Such base64 as described in MIME Part One [RFC2045] before signing. Such
conversion is outside the scope of DKIM; the actual message SHOULD be conversion is outside the scope of DKIM; the actual message SHOULD be
converted to 7-bit MIME by an MUA or MSA prior to presentation to the converted to 7-bit MIME by an MUA or MSA prior to presentation to the
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
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received by the verifier rather than in some local or internal form. 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); any header field that
describes the role of the signer (for example, the Sender or Resent- describes the role of the signer (for example, the Sender or Resent-
From header field if the signature is on behalf of the corresponding From header field if the signature is on behalf of the corresponding
address and that address is different from the From address) MUST address and that address is different from the From address) MUST
also be included. The signed header fields SHOULD also include the also be included. The signed header fields SHOULD also include the
Subject and Date header fields as well as all MIME header fields. Subjectt and Date header fields as well as all MIME header fields.
Signers SHOULD NOT sign an existing header field likely to be Signers SHOULD NOT sign an existing header field likely to be
legitimately modified or removed in transit. In particular, legitimately modified or removed in transit. In particular,
[RFC2821] explicitly permits modification or removal of the "Return- [RFC2821] explicitly permits modification or removal of the "Return-
Path" header field in transit. Signers MAY include any other header Path" header field in transit. Signers MAY include any other header
fields present at the time of signing at the discretion of the fields present at the time of signing at the discretion of the
signer. It is RECOMMENDED that all other existing, non-repeatable signer. It is RECOMMENDED that all other existing, non-repeatable
header fields be signed. header fields be signed.
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 Signers MUST sign any header fields that the signers wish to assert
were present at the time of signing. Put another way, verifiers MAY were present at the time of signing. Put another way, verifiers MAY
treat unsigned header fields with extreme skepticism, up to and treat unsigned header fields with extreme skepticism, up to and
including refusing to display them to the end user. 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=D 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 replicated header field (such as
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the original order. the original order.
INFORMATIVE RATIONALE: Received: is allowed because these header INFORMATIVE RATIONALE: Received: is allowed because these header
fields, as well as Resent-* header fields, are already order- fields, as well as Resent-* header fields, are already order-
sensitive. 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 field blocks to be reordered (with the exception of
Received header fields), reordering of signed replicated header Received header fields), reordering of signed replicated header
fields by intermediate MTAs will cause DKIM signatures to be fields by intermediate MTAs will cause DKIM signatures to be
broken; such anti-social behavior shoulld 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 INFORMATIVE NOTE: There has been some discussion that a Sender
Signing Policy include the list of header fields that the signer Signing Policy include the list of header fields that the signer
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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 INFORMATIVE NOTE: A possible value to include in the "l=" tag
would include the entire length of the message being signed, would include the entire length of the message being signed,
thereby allowing intermediate agents to append further information thereby allowing intermediate agents to append further information
to the message without breaking the signature (e.g., a mailing to the message without breaking the signature (e.g., a mailing
list manager might add unsubscribe innformation to the body). A list manager might add unsubscribe information to the body). A
signer wishing to permit such intermediate agents to add another signer wishing to permit such intermediate agents to add another
MIME body part to a "multipart/mixed" message should use a length MIME body part to a "multipart/mixed" message should use a length
that covers the entire presented message except for the trailing that covers the entire presented message except for the trailing
"--CRLF" characters; this is known as the "N-4" approach. Note "--CRLF" characters; this is known as the "N-4" approach. Note
that more than four characters may need to be stripped, since that more than four characters may need to be stripped, since
there could be postlude information that needs to be ignored. 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
skipping to change at page 35, line 23 skipping to change at page 35, line 25
considerably simplifies things for the user, who can now use an considerably simplifies things for the user, who can now use an
existing mail user agent. Most MUAs have the ability to filter existing mail user agent. Most MUAs have the ability to filter
messages based on message header fields or content; these filters messages based on message header fields or content; these filters
would be used to implement whatever policy the user wishes with would be used to implement whatever policy the user wishes with
respect to unsigned mail. 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 apply the following steps in the order listed. In In the following description, text reading "return with
many cases these steps say that a "DKIM-Signature" header field must DKIM_STAT_something" means that the verifier MUST immediately cease
be ignored, e.g., because it is malformed or because the signature processing that signature. The verifier SHOULD proceed to the next
verification failed. In such cases verifiers SSHOULD proceed to the signature, if any is present, and completely ignore the bad
next signature, and treat the message as verified if any signature signature. There are two special cases: DKIM_STAT_PARTIALSIG
succeeded, ignoring the bad signatures. The order in which indicates that only a portion of the message was actually signed, and
signatures are tried is a matter of local policy for the verifier and DKIM_STAT_TEMPFAIL means that the signature could not be verified at
is not defined here. A verifier MAY treat a message that has one or this time but should be tried again later. In the former case, the
more bad signatures and no good signatures differently from a message action a verifier takes is a matter of local policy. In the latter
with no signature at all; again, this is local policy and is beyond case, a verifier MAY either defer the message for later processing,
the scope of this document. perhaps by queueing it local or issuing a 451/4.7.5 SMTP reply, or
try another signature; if no good signature is found and any of the
signatures resulted in a DKIM_STAT_TEMPFAIL status, the verifier
SHOULD save the message for later processing. Note that an
implementation is not constrained to use these status codes; these
are for explanatory purposes only, and an implementation may define
fewer or more status codes.
The order in which signatures are tried is a matter of local policy
for the verifier and is not defined here. A verifier SHOULD NOT
treat a message that has one or more bad signatures and no good
signatures differently from a message with no signature at all;
again, this is local policy and is beyond the scope of this document.
When a signature successfully verifies, a verifier will either stop
processing or attempt to verify any other signatures, at the
discretion of the implementation.
Verifiers MUST apply the following steps in the order listed.
6.1 Extract the Signature from the Message 6.1 Extract the Signature from the Message
The signature and associated signing identity is included in the The signature and associated signing identity is included in the
value of the DKIM-Signature header field. value of the DKIM-Signature header field. The order in which
verifiers try DKIM-Signature header fields is not defined; verifiers
MAY try signatures in any order they would like. For example, one
implementation might prefer to try the signatures in textual order,
whereas another might want to prefer signatures by identities that
match the contents of the "From" header field over other identities.
Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag. Implementers MUST meticulously validate the format and values in the
Existence of such a tag indicates a new, incompatible version of the DKIM-Signature header field; any inconsistency or unexpected values
DKIM-Signature header field. MUST cause the header field to be completely ignored and the verifier
to return with DKIM_STAT_SYNTAX. Being "liberal in what you accept"
is definitely a bad strategy in this security context. Note however
that this does not include the existence of unknown tags in a DKIM-
Signature header field, which are explicitly permitted.
Verifiers MUST ignore DKIM-Signature header fields with a "v=" tag
that is inconsistent with this specification and return with
DKIM_STAT_INCOMPAT.
INFORMATIVE IMPLEMENTATION NOTE: An implementation may, of
course, choose to also verify signatures generated by older
versions of this specification.
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-
Signature header field and return with DKIM_STAT_SYNTAX.
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 (which MUST exist). 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 superdomain of the domain part of the "i=" tag. If
not, the DKIM-Signature header field MUST be ignored. not, the DKIM-Signature header field MUST be ignored and the verifier
should return with DKIM_STAT_SYNTAX.
Implementers MUST meticulously validate the format and values in the Verifiers MAY ignore the DKIM-Signature header field and return with
"DKIM-Signature:" header field; any inconsistency or unexpected DKIM_STAT_EXPIRED if it contains an "x=" tag and the signature has
values MUST cause the header field to be completely ignored. Being expired.
"liberal in what you accept" is definitely a bad strategy in this
security context. Note however that this does not include the
existence of unknown tags in a "DKIM-Signature" header field, which
are explicitly permitted.
Verifiers MUST NOT attribute ultimate meaning to the order of Verifiers MUST NOT attribute ultimate meaning to the order of
multiple DKIM-Signature header fields. In particular, there is multiple DKIM-Signature header fields. In particular, there is
reason to believe that some relays will reorder the header field in reason to believe that some relays will reorder the header fields in
potentially arbitrary ways. 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 informaation.
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. must be considered before using ordering.
Since there can be multiple signatures in a message, a verifier If there are no valid signatures remaining after this step, a
SHOULD ignore an invalid signature (regardless if caused by a verifier MUST NOT proceed to the next step.
syntactic or semantic problem) and try other signatures. A verifier
MAY choose to treat a message with one or more invalid signatures and
no valid signatures with more suspicion than a message with no
signature at all.
6.2 Get the Public Key 6.2 Get the Public Key
The public key is needed to complete the verificatiion process. The The public key is needed to complete the verification process. The
process of retrieving the public key depends on the query type as process of retrieving the public key depends on the query type as
defined by the "q=" tag in the "DKIM-Signature:" header field line. defined by the "q=" tag in the "DKIM-Signature:" header field.
Obviously, a public key should only be retrieved if the process of Obviously, a public key should only be retrieved if the process of
extracting the signature information is completely successful. 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
SHOULD defer acceptance of this email (normally this will be SHOULD defer acceptance of this email and return with
achieved with a 451/4.7.5 SMTP reply code). DKIM_STAT_TEMPFAIL. If verification is occuring during the
incoming SMTP session, this MAY be achieved with a 451/4.7.5 SMTP
reply code. Alternatively, the verifier MAY store the message in
the local queue for later trial or ignore the signature. Note
that storing a message in the local queue is subject to denial-
of-service attacks.
3. If the query for the public key fails because the corresponding 3. If the query for the public key fails because the corresponding
RR does not exist, the verifier MUST ignore the signature. key record does not exist, the verifier MUST immediately return
with DKIM_STAT_NOKEY.
4. If the result returned from the query does not adhere to the 4. If the query for the public key returns multiple key records, the
verifier may choose one of the key records or may cycle through
the key records performing the remainder of these steps on each
record at the discretion of the implementer. The order of the
key records is unspecified. If the verifier chooses to cycle
through the key records, then the "return with ..." wording in
the remainder of this section means "try the next key record, if
any; if none, try the next DKIM-Signature header field."
5. If the result returned from the query does not adhere to the
format defined in this specification, the verifier MUST ignore format defined in this specification, the verifier MUST ignore
the signature. the key record aand return with DKIM_STAT_NOKEY. Verifiers are
urged to validate the syntax of key records carefully to avoid
attempted attacks.
5. If the "g=" tag in the public key does not match the local part 6. If the "g=" tag in the public key does not match the local part
of the "i=" tag on the message signature, the verifier MUST of the "i=" tag on the message signature, the verifier MUST
ignore the signature. If the local part of the "i=" tag on the ignore the key record and return with DKIM_STAT_INAPPLICABLE. If
message signature is not present, the g= tag must be * (valid for the local part of the "i=" tag on the message signature is not
all addresses in the domain) or not present (which defaults to present, the g= tag must be * (valid for all addresses in the
*), otherwise the verifier MUST ignore the signature. Other than domain) or not present (which defaults to *), otherwise the
this test, verifiers MUST NOT treat a message signed with a key verifier MUST ignore the key record and return with
record having a g= tag any differently than one without; in DKIM_STAT_INAPPLICABLE. Other than this test, verifiers SHOULD
particular, verifiers MUST NOT prefer messages that seem to have NOT treat a message signed with a key record having a g= tag any
an individual signature by virtue of a g= tag vs. a domain differently than one without; in particular, verifiers SHOULD NOT
signature. prefer messages that seem to have an individual signature by
virtue of a g= tag vs. a domain signature.
6. 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 "h=" tag, the verifier MUST ignore the not included in the contents of the "h=" tag, the verifier MUST
signature. ignore the key record and return with DKIM_STAT_INAPPLICABLE.
7. 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. signature check and return with DKIM_STAT_REVOKED.
8. If the public key data is not suitable for use with the algorithm
type defined by the "a=" tag in the "DKIM-Signature" header
field, the verifier MUST ignore the signature.
If the signature is to be ignored, verifiers SHOULD search for 9. If the public key data is not suitable for use with the algorithm
another signature in the message. and key types defined by the "a=" and "k=" tags in the "DKIM-
Signature" header field, the verifier MUST immediately return
with DKIM_STAT_INAPPLICABLE.
6.3 Compute the Verification 6.3 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. 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, create a canonicalized copy of the email as is described in
Section 3.7. When matching header field names in the "h=" tag Section 3.7. When matching header field names in the "h=" tag
against the actual message header field, comparisons MUST be against the actual message header field, comparisons MUST be
case-insensitive. case-insensitive.
2. Based on the algorithm indicated in the "a=" tag, 2. Based on the algorithm indicated in the "a=" tag, compute the
message hashes from the canonical copy as described in
* Compute the message hashes from the canonical copy as Section 3.7.
described in Section 3.7.
* Decrypt the signature using the signer's public key.
3. Compare the decrypted signature to the message hash. If they are 3. Using the signature conveyed in the "b=" tag, verify the
identical, the hash computed by the signer must be the same as signature against the header hash using the mechanism appropriate
the hash computed by the verifier, and hence the signature for the public key algorithm described in the "a=" tag. If the
verifies; otherwise, the signature fails. signature does not validatee, the verifier SHOULD ignore the
signature and return with DKIM_STAT_INVALIDSIG.
INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to INFORMATIVE IMPLEMENTER'S NOTE: Implementations might wish to
initiate the public-key query in parallel with calculating the initiate the public-key query in parallel with calculating the
hash as the public key is not needed until the final decryption is hash as the public key is not needed until the final decryption is
calculated. Implementations may also verify the signature on the calculated. Implementations may also verify the signature on the
message header before validating that the message hash listed in message header before validating that the message hash listed in
the "bh=" tag in the DKIM-Signature header field matches that of the "bh=" tag in the DKIM-Signature header field matches that of
the actual message body; however, if the body hash does match, the the actual message body; however, if the body hash does not match,
entire signature must be considered to have failed. the entire signature must be considered to have failed.
Verifiers SHOULD ignore any DKIM-Signature header fields where the Verifiers SHOULD ignore any DKIM-Signature header fields where the
signature does not validate. Verifiers that are prepared to validate signature does not validate. Verifiers that are prepared to validate
multiple signature header fields SHOULD proceed to the next signature multiple signature header fields SHOULD proceed to the next signature
header field, should it exist. However, verifiers MAY make note of header field, should it exist. However, verifiers MAY make note of
the fact that an invalid signature was present for consideration at a the fact that an invalid signature was present for consideration at a
later step. later step.
INFORMATIVE NOTE: The rationale of this requirement is to permit INFORMATIVE NOTE: The rationale of this requirement is to permit
messages that have invalid signatures but also a valid signature messages that have invalid signatures but also a valid signature
to work. For example, a mailing list exploder might opt to leave to work. For example, a mailing list exploder might opt to leave
the original submitter signature in place even though the exploder the original submitter signature in place even though the exploder
knows that it is modifying the message in some way that will break knows that it is modifying the message in some way that will break
that signature, and the exploder inserts its own signature. In that signature, and the exploder inserts its own signature. In
this case the message should succeed eveen in the presence of the this case the message should succeed even in the presence of the
known-broken signature. known-broken signature.
If a body length is specified in the "l=" tag of the signature, If a body length is specified in the "l=" tag of the signature,
verifiers MUST only verify the number of bytes indicated in the body verifiers MUST only verify the number of bytes indicated in the body
length. Verifiers MAY decide to treat a message containing bytes length. Verifiers MAY decide to treat a message containing bytes
beyond the indicated body length with suspicion. Verifiers MAY beyond the indicated body length with suspicion. Verifiers MAY
truncate the message at the indicated body length or reject the truncate the message at the indicated body length, reject the
signature outright. signature outright, or convey the partial verification to the policy
module using DKIM_STAT_PARTIALSIG.
INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body INFORMATIVE IMPLEMENTATION NOTE: Verifiers that truncate the body
at the indicated body length might pass on a malformed MIME at the indicated body length might pass on a malformed MIME
message if the signer used the "N-4" trick described in the message if the signer used the "N-4" trick described in the
informative note inSection 5.5. Such verifiers may wish to check informative note inSection 5.5. Such verifiers may wish to check
for this case and include a trailing "--CRLF" to avoid breaking for this case and include a trailing "--CRLF" to avoid breaking
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 displayeed to the end user.
6.4 Communicate Verification Results 6.4 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. An example proposal for a
header field is the Authentication-Results header field [ID-AUTH- header field is the Authentication-Results header field [ID-AUTH-
RES]. Any such header field SHOULD be inserted before any existing RES]. Any such header field SHOULD be inserted before any existing
DKIM-Signature or Authentication-Results header fields in the header DKIM-Signature or preexisting authentication status header fields in
field block. 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 sender identity that will be displayed by the MUA. In matches the sender identity that will be displayed by the MUA. In
particular, MUA patterns 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.
6.5 Interpret Results/Apply Local Policy 6.5 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
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6.5 Interpret Results/Apply Local Policy 6.5 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
reasonable to treat unauthenticated email as lacking any trust and reasonable to treat unauthenticated email as lacking any trust and
having no positive reputation. having no positive reputation.
If the verifying MTA is capable of verifying the public key of the In general verifiers SHOULD NOT reject messages solely on the basis
signer and check the signature on the message synchronously with the of a lack of signature or an unverifiable signature. However, if the
SMTP session and such signature is missing or does not verify the MTA verifier does opt to reject such messages, and the verifier runs
MAY reject the message with an error such as: synchronously with the SMTP session and a signature is missing or
does not verify, the MTA SHOULD reject the message with an error such
as:
550 5.7.1 Unsigned messages not accepted 550 5.7.1 Unsigned messages not accepted
550 5.7.5 Message signature incorrect 550 5.7.5 Message signature incorrect
If it is not possible to fetch the public key, perhaps because the If it is not possible to fetch the public key, perhaps because the
key server is not available, a temporary failure message MAY be key server is not available, a temporary failure message MAY be
generated, such as: generated, such as:
451 4.7.5 Unable to verify signature - key server unavailable 451 4.7.5 Unable to verify signature - key server unavailable
Once the signature has been verified, that information MUST be A temporary failure of the key server or other external service is
the only condition that should use a 4xx SMTP reply code. In
particular, signature verification failures MUST NOT return 4xx SMTP
replies.
Oncee the signature has been verified, that information MUST be
conveyed to higher level systems (such as explicit allow/white lists conveyed to higher level systems (such as explicit allow/white lists
and reputation systems) and/or to the end user. If the message is and reputation systems) and/or to the end user. If the message is
signed on behalf of any address other than that in the From: header 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.
INFORMATIVE NOTE: If the authentication status is to be stored in INFORMATIVE NOTE: If the authentication status is to be stored in
the message header field, the Authentication-Results header field the message header field, the Authentication-Results header field
[ID-AUTH-RES] may be used to convey this information. [ID-AUTH-RES] may be used to convey this information.
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step since they will break the signature. If performed, the step since they will break the signature. If performed, the
rewriting SHOULD include the name of the signer in the address. For rewriting SHOULD include the name of the signer in the address. For
example: example:
From: John Q. User <user@example.com> From: John Q. User <user@example.com>
might be converted to might be converted to
From: "John Q. User via <asrg-admin@ietf.org>" <user@example.com> From: "John Q. User via <asrg-admin@ietf.org>" <user@example.com>
This sort of address inconsistency is expected for mailing lists, but This sort of addrress inconsistency is expected for mailing lists, but
might be otherwise used to mislead the verifier, for example if a might be otherwise used to mislead the verifier, for example if a
message supposedly from administration@your-bank.com had a Sender message supposedly from administration@your-bank.com had a Sender
address of fraud@badguy.com. address of fraud@badguy.com.
Under no circumstances should an unsigned header field be displayed Under no circumstances should an unsigned header field be displayed
in any context that might be construed by the end user as having been in any context that might be construed by the end user as having been
signed. Notably, unsigned header fields SHOULD be hidden from the signed. Notably, unsigned header fields SHOULD be hidden from the
end user to the extent possible. end user to the extent possible.
The MUA MAY hide or mark portions of the message body that are not The MUA MAY hide or mark portions of the message body that are not
signed when using the "l=" tag. signed when using the "l=" tag.
7. IANA Considerations 7. IANA Considerations
Use of the _domainkey prefix in DNS records will require registration
by IANA.
To avoid conflicts, tag names for the DKIM-Signature header and key To avoid conflicts, tag names for the DKIM-Signature header and key
records should be registered with IANA. records should be registered with IANA.
Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature Tag values for the "a=", "c=", and "q=" tags in the DKIM-Signature
header field, and the "h=", "k=", "s=", and "t" tags in key records header field, and the "h=", "k=", "s=", and "t" tags in key records
should be registered with IANA for the same reason. should be registered with IANA for the same reason.
The DKK RR type must be registered by IANA. The DKK RR type must be registered by IANA.
8. Security Considerations 8. Security Considerations
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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:
<div style="position: relative; bottom: 350px; z-index: 2;"> <div style="position: relative; bottom: 350px; z-index: 2;">
<img src="http://www.ietf.org/images/ietflogo2e.gif" <img src="http://www.ietf.org/images/ietflogo2e.gif"
width=578 height=370> width=578 height=370>
</div> </div>
8.2 Misappropriated Private Key 8.2 Misappropriateed 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.
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generate bulk email. generate bulk email.
8.3 Key Server Denial-of-Service Attacks 8.3 Key Server Denial-of-Service Attacks
Since the key servers are distributed (potentially separate for each Since the key servers are distributed (potentially separate for each
domain), the number of servers that would need to be attacked to domain), the number of servers that would need to be attacked to
defeat this mechanism on an Internet-wide basis is very large. defeat this mechanism on an Internet-wide basis is very large.
Nevertheless, key servers for individual domains could be attacked, Nevertheless, key servers for individual domains could be attacked,
impeding the verification of messages from that domain. This is not impeding the verification of messages from that domain. This is not
significantly different from the ability of an attacker to deny significantly different from the ability of an attacker to deny
service to the mail exchangers for a given domain, although it service to the mail exchangers for a given domain, aalthough it
affects outgoing, not incoming, mail. affects outgoing, not incoming, mail.
A variation on this attack is that if a very large amount of mail A variation on this attack is that if a very large amount of mail
were to be sent using spoofed addresses from a given domain, the key were to be sent using spoofed addresses from a given domain, the key
servers for that domain could be overwhelmed with requests. However, servers for that domain could be overwhelmed with requests. However,
given the low overhead of verification compared with handling of the given the low overhead of verification compared with handling of the
email message itself, such an attack would be difficult to mount. email message itself, such an attack would be difficult to mount.
8.4 Attacks Against DNS 8.4 Attacks Against DNS
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respond, however the cost/benefit of conducting prolonged DNS attacks respond, however the cost/benefit of conducting prolonged DNS attacks
of this nature is expected to be uneconomical. of this nature is expected to be uneconomical.
Finally, DKIM is only intended as a "sufficient" method of proving Finally, DKIM is only intended as a "sufficient" method of proving
authenticity. It is not intended to provide strong cryptographic authenticity. It is not intended to provide strong cryptographic
proof about authorship or contents. Other technologies such as proof about authorship or contents. Other technologies such as
OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements. OpenPGP [RFC2440] and S/MIME [RFC3851] address those requirements.
A second security issue related to the DNS revolves around the A second security issue related to the DNS revolves around the
increased DNS traffic as a consequence of fetching Selector-based increased DNS traffic as a consequence of fetching Selector-based
data as well as fetching siggning domain policy. Widespread data as well as fetching signing domain policy. Widespread
deployment of DKIM will result in a significant increase in DNS deployment of DKIM will result in a significant increase in DNS
queries to the claimed signing domain. In the case of forgeries on a queries to the claimed signing domain. In the case of forgeries on a
large scale, DNS servers could see a substantial increase in queries. large scale, DNS servers could see a substantial increase in queries.
8.5 Replay Attacks 8.5 Replay Attacks
In this attack, a spammer sends a message to be spammed to an In this attack, a spammer sends a message to be spammed to an
accomplice, which results in the message being signed by the accomplice, which results in the message being signed by the
originating MTA. The accomplice resends the message, including the originating MTA. The accomplice resends the message, including the
original signature, to a large number of recipients, possibly by original signature, to a large number of recipients, possibly by
sending the message to many compromised machines that act as MTAs. sending the message to many compromised machines that act as MTAs.
The messages, not having been modified by the accomplice, have valid The messages, not having been modified by the accomplice, have valid
signatures. signatures.
Partial solutions to this problem involve the use of reputation Partial solutions to this problem involve the use of reputation
services to convey the fact that the specific email address is being services to convey the fact that the specific email address is being
used for spam, and that messages from that signer are likely to be used for spam, and that messages from that signer are likely to be
spam. This requires a real-time detection mechanism in order to spam. This requires a real-time detection mechanissm in order to
react quickly enough. However, such measures might be prone to react quickly enough. However, such measures might be prone to
abuse, if for example an attacker resent a large number of messages abuse, if for example an attacker resent a large number of messages
received from a victim in order to make them appear to be a spammer. received from a victim in order to make them appear to be a spammer.
Large verifiers might be able to detect unusually large volumes of Large verifiers might be able to detect unusually large volumes of
mails with the same signature in a short time period. Smaller mails with the same signature in a short time period. Smaller
verifiers can get substantially the same volume information via verifiers can get substantially the same volume information via
existing collaborative systems. existing collaborative systems.
8.6 Limits on Revoking Keys 8.6 Limits on Revoking Keys
skipping to change at page 45, line 22 skipping to change at page 46, line 26
8.7 Intentionally malformed Key Records 8.7 Intentionally malformed Key Records
It is possible for an attacker to publish key records in DNS which It is possible for an attacker to publish key records in DNS which
are intentionally malformed, with the intent of causing a denial-of- are intentionally malformed, with the intent of causing a denial-of-
service attack on a non-robust verifier implementation. The attacker service attack on a non-robust verifier implementation. The attacker
could then cause a verifier to read the malformed key record by could then cause a verifier to read the malformed key record by
sending a message to one of its users referencing the malformed sending a message to one of its users referencing the malformed
record in a (not necessarily valid) signature. Verifiers MUST record in a (not necessarily valid) signature. Verifiers MUST
thoroughly verify all key records retrieved from DNS and be robust thoroughly verify all key records retrieved from DNS and be robust
against intentionally as well as unintentiionally malformed key against intentionally as well as unintentionally malformed key
records. records.
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.
9. References 8.10 Remote Timing Attacks
9.1 Normative References
[ID-DKIM-RR]
"DKIM Key Resource Records (To be written)",
draft-dkim-dkk-rr-xx (work in progress), 2005.
[ID-SHA] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms In some cases it may be possible to extract private keys using a
(SHA and HMAC-SHA)", draft-eastlake-sha2-02 (work in remote timing attack [BONEH03]. Implementations should consider
progress), January 2006. obfuscating the timing to prevent such attacks.
[OPENSSL] Team, C&D., "OpenSSL Documents", 9. References
http://www.openssl.org/docs/,
<http://www.openssl.org/docs/>.
[RFC1421] Linn, J., "Privacy Enhancement for Internet Electronic 9.1 Normative References
Mail: Part I: Message Encryption and Authentication
Procedures", RFC 1421, February 1993.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996. Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message header field Extensions for Non-ASCII Part Three: Message header field Extensions for Non-ASCII
Text", RFC 2047, November 1996. Text", RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
skipping to change at page 46, line 31 skipping to change at page 47, line 28
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001. April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, [RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001. April 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
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.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
9.2 Informative References 9.2 Informative References
[BONEH03] Proc. 12th USENIX Security Symposium, "Remote Timing
Attacks are Practical", 2003, <http://www.usenix.org/
publications/library/proceedings/sec03/tech/brumley.html>.
[ID-AUTH-RES] [ID-AUTH-RES]
Kucherawy, M., "Message header field for Indicating Sender Kucherawy, M., "Message header field for Indicating Sender
Authentication Status", Authentication Status",
draft-kucherawy-sender-auth-header-02 (work in progress), draft-kucherawy-sender-auth-header-02 (work in progress),
May 2005. 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), December 2005. (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] "", 2005. [RFC3766] Orman, H. and P. Hoffman, "Determing Strengths for Public
Keys Used For Exchanging Symmetric Keys", RFC 3766,
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",
RFC 4033, March 2005. RFC 4033, March 2005.
skipping to change at page 49, line 7 skipping to change at page 50, line 7
Email: mat@cisco.com Email: mat@cisco.com
Appendix A. Example of Use (INFORMATIVE) Appendix A. Example of Use (INFORMATIVE)
This section shows the complete flow of an email from submission to This section shows the complete flow of an email from submission to
final delivery, demonstrating how the various components fit final delivery, demonstrating how the various components fit
together. together.
A.1 The user composes an email A.1 The user composes an email
From: Joe SixPack <joe@foootball.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.
skipping to change at page 49, line 44 skipping to change at page 50, line 44
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 thhe 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._dkim.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:
skipping to change at page 51, line 14 skipping to change at page 52, line 14
Appendix B. Usage Examples (INFORMATIVE) Appendix B. Usage Examples (INFORMATIVE)
Studies in this appendix are for informational purposes only. In no Studies in this appendix are for informational purposes only. In no
case should these examples be used as guidance when creating an case should these examples be used as guidance when creating an
implementation. implementation.
B.1 Simple Message Forwarding B.1 Simple Message Forwarding
In some cases the recipient may request forwarding of email messages In some cases the recipient may request forwarding of email messages
from the original address to another, through the use of a Unix from the original address tto another, through the use of a Unix
.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
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B.4 Mailing Lists B.4 Mailing Lists
There is a wide range of behavior in forwarders and mailing lists There is a wide range of behavior in forwarders and mailing lists
(collectively called "forwarders" below), ranging from those which (collectively called "forwarders" below), ranging from those which
make no modification to the message itself (other than to add a make no modification to the message itself (other than to add a
Received header field and change the envelope information) to those Received header field and change the envelope information) to those
which may add header fields, change the Subject header field, add which may add header fields, change the Subject header field, add
content to the body (typically at the end), or reformat the body in content to the body (typically at the end), or reformat the body in
some manner. some manner.
Forwarders which do not modify the body or signed header fields of a Forwarders which do noot modify the body or signed header fields of a
message with a valid signature may re-sign the message as described message with a valid signature may re-sign the message as described
below. below.
Forwarders which make any modification to a message that could result Forwarders which make any modification to a message that could result
in its signature becoming invalid should sign or re-sign using an in its signature becoming invalid should sign or re-sign using an
appropriate identification (e.g., mailing-list-name@example.net). appropriate identification (e.g., mailing-list-name@example.net).
Since in so doing the (re-)signer is taking responsibility for the Since in so doing the (re-)signer is taking responsibility for the
content of the message, modifying forwarders may elect to forward or content of the message, modifying forwarders may elect to forward or
re-sign only for messages which were received with valid signatures re-sign only for messages which were received with valid signatures
or other indications that the messages being signed are not spoofed. or other indications that the messages being signed are not spoofed.
skipping to change at page 52, line 44 skipping to change at page 53, line 44
B.5 Affinity Addresses B.5 Affinity Addresses
"Affinity addresses" are email addresses that users employ to have an "Affinity addresses" are email addresses that users employ to have an
email address that is independent of any changes in email service email address that is independent of any changes in email service
provider they may choose to make. They are typically associated with provider they may choose to make. They are typically associated with
college alumni associations, professional organizations, and college alumni associations, professional organizations, and
recreational organizations with which they expect to have a long-term recreational organizations with which they expect to have a long-term
relationship. These domains usually provide forwarding of incoming relationship. These domains usually provide forwarding of incoming
email, but (currently) usually depend on the user to send outgoing email, but (currently) usually depend on the user to send outgoing
messages through their own service provider's MTA. They usually have messages through their own service provider's MTA. They usually have
an aassociated Web application which authenticates the user and allows an associated Web application which authenticates the user and allows
the forwarding address to be changed. the forwarding address to be changed.
With DKIM, affinity domains could use the Web application to allow With DKIM, affinity domains could use the Web application to allow
users to register their own public keys to be used to sign messages users to register their own public keys to be used to sign messages
on behalf of their affinity address. This is another application on behalf of their affinity address. This is another application
that takes advantage of user-level keying, and domains used for that takes advantage of user-level keying, and domains used for
affinity addresses would typically have a very large number of user- affinity addresses would typically have a very large number of user-
level keys. Alternatively, the affinity domain could handle outgoing level keys. Alternatively, the affinity domain could handle outgoing
mail, operating a mail submission agent that authenticates users mail, operating a mail submission agent that authenticates users
before accepting and signing messages for them. This is of course before accepting and signing messages for them. This is of course
skipping to change at page 54, line 8 skipping to change at page 55, line 8
openssl like this: openssl like this:
$ openssl genrsa -out rsa.private 768 $ openssl genrsa -out rsa.private 768
This results in the file rsa.private containing the key information This results in the file rsa.private containing the key information
similar to this: similar to this:
-----BEGIN RSA PRIVATE KEY----- -----BEGIN RSA PRIVATE KEY-----
MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5 MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5
ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo
AUsFUq+J6+OpprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH AUsFUq+J6+OprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH
+Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn +Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn
Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/ Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/
3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew 3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew
ti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs ti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs
FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb
weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG
6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U= 6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U=
-----END RSA PRIVATE KEY----- -----END RSA PRIVATE KEY-----
To extract the public-key component from the private-key, use openssl To extract the public-key component from the private-key, use openssl
skipping to change at page 54, line 33 skipping to change at page 55, line 33
This results in the file rsa.public containing the key information This results in the file rsa.public containing the key information
similar to this: similar to this:
-----BEGIN PUBLIC KEY----- -----BEGIN PUBLIC KEY-----
MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l
MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E
XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB
-----END PUBLIC KEY----- -----END PUBLIC KEY-----
This public-key data (without the BEGIN and END tags) is placed in This public-key data (without the BEGIN and END tags) is placed in
the DNS. With the signature, canonical email contents and public the DNS. With the signature, canonical email contents and puublic
key, a verifying system can test the validity of the signature. The key, a verifying system can test the validity of the signature. The
openssl invocation to verify a signature looks like this: openssl invocation to verify a signature looks like this:
openssl dgst -verify rsa.public -sha1 -signature signature.file \ openssl dgst -verify rsa.public -sha1 -signature signature.file \
<input.file <input.file
Once a private-key has been generated, the openssl command can be Once a private-key has been generated, the openssl command can be
used to sign an appropriately prepared email, like this: used to sign an appropriately prepared email, like this:
$ openssl dgst -sign rsa.private -sha1 <input.file $ openssl dgst -sign rsa.private -sha1 <input.file
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Appendix D. Acknowledgements Appendix D. Acknowledgements
The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann, The authors wish to thank Russ Allbery, Edwin Aoki, Claus Assmann,
Steve Atkins, Fred Baker, Mark Baugher, Nathaniel Borenstein, Dave Steve Atkins, Fred Baker, Mark Baugher, Nathaniel Borenstein, Dave
Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Patrik Crocker, Michael Cudahy, Dennis Dayman, Jutta Degener, Patrik
Faltstrom, Duncan Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony Faltstrom, Duncan Findlay, Elliot Gillum, Phillip Hallam-Baker, Tony
Hansen, Arvel Hathcock, Amir Herzberg, Craig Hughes, Don Johnsen, Hansen, Arvel Hathcock, Amir Herzberg, Craig Hughes, Don Johnsen,
Harry Katz, Murray S. Kucherawy, Barry Leiba, John Levine, Simon Harry Katz, Murray S. Kucherawy, Barry Leiba, John Levine, Simon
Longsdale, David Margrave, Justin Mason, David Mayne, Steve Murphy, Longsdale, David Margrave, Justin Mason, David Mayne, Steve Murphy,
Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer Russell Nelson, Dave Oran, Doug Otis, Shamim Pirzada, Juan Altmayer
Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro, Pizzorno, Sanjay Pol, Blake Ramsdell, Christian Renaud, Scott Renfro,
Dave Rossetti, Hector Santos, the Spamhaus..org team, Malte S. Stretz, Eric Rescorla, Dave Rossetti, Hector Santos, the Spamhaus.org team,
Robert Sanders, Rand Wacker, and Dan Wing for their valuable Malte S. Stretz, Robert Sanders, Rand Wacker, and Dan Wing for their
suggestions and constructive criticism. valuable suggestions and constructive criticism.
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 E. Edit History Appendix E. Edit History
[[This section to be removed before publication.]] [[This section to be removed before publication.]]
E.1 Changes since -ietf-00 version E.1 Changes since -ietf-01 version
The following changes were made between draft-ietf-dkim-base-01 and
draft-ietf-dkim-base-02:
o Change wording on "x=" tag in DKIM-Signature header field
regarding verifier handling of expired signatures from MUST to MAY
(per 20 April Jabber session). Also, make it clear that received
time is to be preferred over current time if reliably available.
o Several changes to limit wording that would intrude into verifier
policy. This is largely changing statements such as "... MUST
reject the message" to "... MUST consider the signature invalid."
o Drop normative references to ID-DKIM-RR, OpenSSL, PEM, and
Stringprep.
o Change "v=" tag in DKIM-Signature from "MUST NOT" to "MUST"; the
version number is 0.2 for this draft, with the expectation that
the first official version will be "v=1". (Per 18 May Jabber
session.)
o Change "q=dns" query access method to "q=dnstxt" to emphasize the
use of the TXT record. The expectation is that a later extension
will define "q=dnsdkk" to indicate use of a DKK record. (Per 18
May Jabber session.)
o Several typos fixed, including removing a paragraph that implied
that the DKIM-Signature header field should be hashed with the
body (it should not).
E.2 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.
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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.
E.2 Changes since -allman-01 version E.3 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 6.5. consideration is implicitly included in Section 6.5.
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.
E.3 Changes since -allman-00 version E.4 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".
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o Added several IANA Considerations. o Added several IANA Considerations.
o Fixed a number of grammar and formatting errors. o Fixed a number of grammar and formatting errors.
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might oor might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
 End of changes. 134 change blocks. 
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