draft-ietf-dkim-deployment-09.txt   draft-ietf-dkim-deployment-10.txt 
DomainKeys Identified Mail T. Hansen DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories Internet-Draft AT&T Laboratories
Intended status: Informational E. Siegel Intended status: Informational E. Siegel
Expires: April 29, 2010 Expires: June 21, 2010
P. Hallam-Baker P. Hallam-Baker
Default Deny Security, Inc. Default Deny Security, Inc.
D. Crocker D. Crocker
Brandenburg InternetWorking Brandenburg InternetWorking
October 26, 2009 December 18, 2009
DomainKeys Identified Mail (DKIM) Development, Deployment and Operations DomainKeys Identified Mail (DKIM) Development, Deployment and Operations
draft-ietf-dkim-deployment-09 draft-ietf-dkim-deployment-10
Abstract
DomainKeys Identified Mail (DKIM) allows an organization to claim
responsibility for transmitting a message, in a way that can be
validated by a recipient. The organization can be the author's, the
originating sending site, an intermediary, or one of their agents. A
message can contain multiple signatures, from the same or different
organizations involved with the message. DKIM defines a domain-level
digital signature authentication framework for email, using public
key cryptography, using the domain name service as its key server
technology [RFC4871]. This permits verification of a responsible
organization, as well as the integrity of the message contents. DKIM
will also provide a mechanism that permits potential email signers to
publish information about their email signing practices; this will
permit email receivers to make additional assessments about messages.
DKIM's authentication of email identity can assist in the global
control of "spam" and "phishing". This document provides
implementation, deployment, operational and migration considerations
for DKIM.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material provisions of BCP 78 and BCP 79.
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
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
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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."
skipping to change at page 1, line 40 skipping to change at page 2, line 4
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
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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 April 29, 2010. This Internet-Draft will expire on June 21, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents
publication of this document (http://trustee.ietf.org/license-info). (http://trustee.ietf.org/license-info) in effect on the date of
Please review these documents carefully, as they describe your rights publication of this document. Please review these documents
and restrictions with respect to this document. carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
DomainKeys Identified Mail (DKIM) allows an organization to claim This document may contain material from IETF Documents or IETF
responsibility for transmitting a message, in a way that can be Contributions published or made publicly available before November
validated by a recipient. The organization can be the author's, the 10, 2008. The person(s) controlling the copyright in some of this
originating sending site, an intermediary, or one of their agents. A material may not have granted the IETF Trust the right to allow
message can contain multiple signatures, from the same or different modifications of such material outside the IETF Standards Process.
organizations involved with the message. DKIM defines a domain-level Without obtaining an adequate license from the person(s) controlling
digital signature authentication framework for email, using public the copyright in such materials, this document may not be modified
key cryptography, using the domain name service as its key server outside the IETF Standards Process, and derivative works of it may
technology [RFC4871]. This permits verification of a responsible not be created outside the IETF Standards Process, except to format
organization, as well as the integrity of the message contents. DKIM it for publication as an RFC or to translate it into languages other
will also provide a mechanism that permits potential email signers to than English.
publish information about their email signing practices; this will
permit email receivers to make additional assessments about messages.
DKIM's authentication of email identity can assist in the global
control of "spam" and "phishing". This document provides
implementation, deployment, operational and migration considerations
for DKIM.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Using DKIM as Part of Trust Assessment . . . . . . . . . . . . 5 2. Using DKIM as Part of Trust Assessment . . . . . . . . . . . . 5
2.1. A Systems View of Email Trust Assessment . . . . . . . . . 5 2.1. A Systems View of Email Trust Assessment . . . . . . . . . 5
2.2. Choosing a DKIM Tag for the Assessment Identifier . . . . 7 2.2. Choosing a DKIM Tag for the Assessment Identifier . . . . 7
2.3. Choosing the Signing Domain Name . . . . . . . . . . . . . 9 2.3. Choosing the Signing Domain Name . . . . . . . . . . . . . 9
2.4. Recipient-based Assessments . . . . . . . . . . . . . . . 11 2.4. Recipient-based Assessments . . . . . . . . . . . . . . . 11
2.5. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 12 2.5. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 13
3. DKIM Key Generation, Storage, and Management . . . . . . . . . 14 3. DKIM Key Generation, Storage, and Management . . . . . . . . . 14
3.1. Private Key Management: Deployment and Ongoing 3.1. Private Key Management: Deployment and Ongoing
Operations . . . . . . . . . . . . . . . . . . . . . . . . 15 Operations . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2. Storing Public Keys: DNS Server Software Considerations . 16 3.2. Storing Public Keys: DNS Server Software Considerations . 16
3.3. Per User Signing Key Management Issues . . . . . . . . . . 17 3.3. Per User Signing Key Management Issues . . . . . . . . . . 17
3.4. Third Party Signer Key Management and Selector 3.4. Third Party Signer Key Management and Selector
Administration . . . . . . . . . . . . . . . . . . . . . . 17 Administration . . . . . . . . . . . . . . . . . . . . . . 17
3.5. Key Pair / Selector Lifecycle Management . . . . . . . . . 18 3.5. Key Pair / Selector Lifecycle Management . . . . . . . . . 18
4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1. DNS Records . . . . . . . . . . . . . . . . . . . . . . . 20 4.1. DNS Records . . . . . . . . . . . . . . . . . . . . . . . 20
4.2. Signing Module . . . . . . . . . . . . . . . . . . . . . . 20 4.2. Signing Module . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Signing Policies and Practices . . . . . . . . . . . . . . 21 4.3. Signing Policies and Practices . . . . . . . . . . . . . . 21
5. Verifying . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5. Verifying . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Intended Scope of Use . . . . . . . . . . . . . . . . . . 21 5.1. Intended Scope of Use . . . . . . . . . . . . . . . . . . 21
5.2. Signature Scope . . . . . . . . . . . . . . . . . . . . . 22 5.2. Signature Scope . . . . . . . . . . . . . . . . . . . . . 22
5.3. Design Scope of Use . . . . . . . . . . . . . . . . . . . 22 5.3. Design Scope of Use . . . . . . . . . . . . . . . . . . . 22
5.4. Inbound Mail Filtering . . . . . . . . . . . . . . . . . . 23 5.4. Inbound Mail Filtering . . . . . . . . . . . . . . . . . . 23
5.5. Messages sent through Mailing Lists and other 5.5. Messages sent through Mailing Lists and other
Intermediaries . . . . . . . . . . . . . . . . . . . . . . 23 Intermediaries . . . . . . . . . . . . . . . . . . . . . . 23
5.6. Generation, Transmission and Use of Results Headers . . . 24 5.6. Generation, Transmission and Use of Results Headers . . . 23
6. Taxonomy of Signatures . . . . . . . . . . . . . . . . . . . . 24 6. Taxonomy of Signatures . . . . . . . . . . . . . . . . . . . . 24
6.1. Single Domain Signature . . . . . . . . . . . . . . . . . 25 6.1. Single Domain Signature . . . . . . . . . . . . . . . . . 24
6.2. Parent Domain Signature . . . . . . . . . . . . . . . . . 25 6.2. Parent Domain Signature . . . . . . . . . . . . . . . . . 25
6.3. Third Party Signature . . . . . . . . . . . . . . . . . . 26 6.3. Third Party Signature . . . . . . . . . . . . . . . . . . 26
6.4. Using Trusted Third Party Senders . . . . . . . . . . . . 27 6.4. Using Trusted Third Party Senders . . . . . . . . . . . . 27
6.5. Multiple Signatures . . . . . . . . . . . . . . . . . . . 28 6.5. Multiple Signatures . . . . . . . . . . . . . . . . . . . 28
7. Example Usage Scenarios . . . . . . . . . . . . . . . . . . . 30 7. Example Usage Scenarios . . . . . . . . . . . . . . . . . . . 30
7.1. Author's Organization - Simple . . . . . . . . . . . . . . 30 7.1. Author's Organization - Simple . . . . . . . . . . . . . . 30
7.2. Author's Organization - Differentiated Types of Mail . . . 30 7.2. Author's Organization - Differentiated Types of Mail . . . 30
7.3. Author Domain Signing Practices . . . . . . . . . . . . . 31 7.3. Author Domain Signing Practices . . . . . . . . . . . . . 30
7.4. Delegated Signing . . . . . . . . . . . . . . . . . . . . 32 7.4. Delegated Signing . . . . . . . . . . . . . . . . . . . . 32
7.5. Independent Third Party Service Providers . . . . . . . . 33 7.5. Independent Third Party Service Providers . . . . . . . . 33
7.6. Mail Streams Based on Behavioral Assessment . . . . . . . 34 7.6. Mail Streams Based on Behavioral Assessment . . . . . . . 33
7.7. Agent or Mediator Signatures . . . . . . . . . . . . . . . 34 7.7. Agent or Mediator Signatures . . . . . . . . . . . . . . . 34
8. Usage Considerations . . . . . . . . . . . . . . . . . . . . . 35 8. Usage Considerations . . . . . . . . . . . . . . . . . . . . . 34
8.1. Non-standard Submission and Delivery Scenarios . . . . . . 35 8.1. Non-standard Submission and Delivery Scenarios . . . . . . 34
8.2. Protection of Internal Mail . . . . . . . . . . . . . . . 36 8.2. Protection of Internal Mail . . . . . . . . . . . . . . . 35
8.3. Signature Granularity . . . . . . . . . . . . . . . . . . 36 8.3. Signature Granularity . . . . . . . . . . . . . . . . . . 36
8.4. Email Infrastructure Agents . . . . . . . . . . . . . . . 37 8.4. Email Infrastructure Agents . . . . . . . . . . . . . . . 37
8.5. Mail User Agent . . . . . . . . . . . . . . . . . . . . . 39 8.5. Mail User Agent . . . . . . . . . . . . . . . . . . . . . 39
9. Other Considerations . . . . . . . . . . . . . . . . . . . . . 40 9. Other Considerations . . . . . . . . . . . . . . . . . . . . . 40
9.1. Security Considerations . . . . . . . . . . . . . . . . . 40 9.1. Security Considerations . . . . . . . . . . . . . . . . . 40
9.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 40 9.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 40
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
11. Informative References . . . . . . . . . . . . . . . . . . . . 40 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
11.1. Normative References . . . . . . . . . . . . . . . . . . . 40
11.2. Informative References . . . . . . . . . . . . . . . . . . 41
Appendix A. Migration Strategies . . . . . . . . . . . . . . . . 41 Appendix A. Migration Strategies . . . . . . . . . . . . . . . . 41
A.1. Migrating from DomainKeys . . . . . . . . . . . . . . . . 41 A.1. Migrating from DomainKeys . . . . . . . . . . . . . . . . 41
A.2. Migrating Hash Algorithms . . . . . . . . . . . . . . . . 46 A.2. Migrating Hash Algorithms . . . . . . . . . . . . . . . . 46
A.3. Migrating Signing Algorithms . . . . . . . . . . . . . . . 47 A.3. Migrating Signing Algorithms . . . . . . . . . . . . . . . 47
Appendix B. General Coding Criteria for Cryptographic Appendix B. General Coding Criteria for Cryptographic
Applications . . . . . . . . . . . . . . . . . . . . 48 Applications . . . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
1. Introduction 1. Introduction
DomainKeys Identified Mail (DKIM) allows an organization to claim DomainKeys Identified Mail (DKIM) allows an organization to claim
responsibility for transmitting a message, in a way that can be responsibility for transmitting a message, in a way that can be
validated by a recipient. This document provides practical tips for: validated by a recipient. This document provides practical tips for:
those who are developing DKIM software, mailing list managers, those who are developing DKIM software, mailing list managers,
filtering strategies based on the output from DKIM verification, and filtering strategies based on the output from DKIM verification, and
DNS servers; those who are deploying DKIM software, keys, mailing DNS servers; those who are deploying DKIM software, keys, mailing
list software, and migrating from DomainKeys; and those who are list software, and migrating from DomainKeys [RFC4870]; and those who
responsible for the on-going operations of an email infrastructure are responsible for the on-going operations of an email
that has deployed DKIM. infrastructure that has deployed DKIM.
The document is organized around the key concepts related to DKIM. The document is organized around the key concepts related to DKIM.
Within each section, additional considerations specific to Within each section, additional considerations specific to
development, deployment, or ongoing operations are highlighted where development, deployment, or ongoing operations are highlighted where
appropriate. The possibility of use of DKIM results as input to a appropriate. The possibility of use of DKIM results as input to a
local reputation database is also discussed. local reputation database is also discussed.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119, except that
all uses are to be considered advisory rather than normative.
2. Using DKIM as Part of Trust Assessment 2. Using DKIM as Part of Trust Assessment
2.1. A Systems View of Email Trust Assessment 2.1. A Systems View of Email Trust Assessment
DKIM participates in a trust-oriented enhancement to the Internet's DKIM participates in a trust-oriented enhancement to the Internet's
email service, to facilitate message handling decisions, such as for email service, to facilitate message handling decisions, such as for
delivery and for content display. Trust-oriented message handling delivery and for content display. Trust-oriented message handling
has substantial differences from approaches that consider messages in has substantial differences from the more established approaches that
terms of risk and abuse. With trust, there is a collaborative consider messages in terms of risk and abuse. With trust, there is a
exchange between a willing participant along the sending path and a collaborative exchange between a willing participant along the
willing participant at the recipient site. In contrast, the risk sending path and a willing participant at a recipient site. In
model entails independent action by the recipient site, in the face contrast, the risk model entails independent, unilateral action by
of a potentially unknown, hostile and deceptive sender. This the recipient site, in the face of a potentially unknown, hostile and
translates into a very basic technical difference: In the face of deceptive sender. This translates into a very basic technical
unilateral action by the recipient and even antagonistic efforts by difference: In the face of unilateral action by the recipient and
the sender, risk-oriented mechanisms will be based on heuristics, even antagonistic efforts by the sender, risk-oriented mechanisms are
that is, on guessing. Guessing produces statistical results with be based on heuristics, that is, on guessing. Guessing produces
some false negatives and some false positives. For trust-based statistical results with some false negatives and some false
exchanges, the goal is the deterministic exchange of information. positives. For trust-based exchanges, the goal is the deterministic
For DKIM, that information is the one identifier that represents a exchange of information. For DKIM, that information is the one
stream of mail for which an independent assessment is sought (by the identifier that represents a stream of mail for which an independent
signer.) assessment is sought (by the signer.)
A trust-based service is built upon a validated Responsible A trust-based service is built upon a validated Responsible
Identifier that labels a stream of mail and is controlled by an Identifier that labels a stream of mail and is controlled by an
identity (role, person or organization). The identity is identity (role, person or organization). The identity is
acknowledging some degree of responsibility for the message stream. acknowledging some degree of responsibility for the message stream.
Given a basis for believing that an identifier is being used in an Given a basis for believing that an identifier is being used in an
authorized manner, the recipient site can make and use an assessment authorized manner, the recipient site can make and use an assessment
of the associated identity. An identity can use different of the associated identity. An identity can use different
identifiers, on the assumption that the different streams might identifiers, on the assumption that the different streams might
produce different assessments. For example, even the best-run produce different assessments. For example, even the best-run
skipping to change at page 6, line 26 skipping to change at page 6, line 21
Determining that the identifier's use is valid is quite different Determining that the identifier's use is valid is quite different
from determining that the content of a message is valid. The former from determining that the content of a message is valid. The former
means only that the identifier for the responsible role, person or means only that the identifier for the responsible role, person or
organization has been legitimately associated with a message. The organization has been legitimately associated with a message. The
latter means that the content of the message can be believed and, latter means that the content of the message can be believed and,
typically, that the claimed author of the content is correct. DKIM typically, that the claimed author of the content is correct. DKIM
validates only the presence of the identifier used to sign the validates only the presence of the identifier used to sign the
message. Even when this identifier is validated, DKIM carries no message. Even when this identifier is validated, DKIM carries no
implication that any of the message content, including the implication that any of the message content, including the
RFC5322.From field, is valid. Surprisingly, this limit to the RFC5322.From field [RFC5322], is valid. Surprisingly, this limit to
semantics of a DKIM signature applies even when the validated signing the semantics of a DKIM signature applies even when the validated
identifier is the same domain name as is used in the RFC5322.From signing identifier is the same domain name as is used in the
field! DKIM's only claim about message content is that the content RFC5322.From field! DKIM's only claim about message content is that
cited in the DKIM-Signature: field's h= tag has been delivered the content cited in the DKIM-Signature: field's h= tag has been
without modification. That is, it asserts message content integrity, delivered without modification. That is, it asserts message content
not message content validity. integrity -- between signing and verifying -- not message content
validity.
As shown in Figure 1, this enhancement is a communication between a As shown in Figure 1, this enhancement is a communication between a
responsible role, person or organization that signs the message and a responsible role, person or organization that signs the message and a
recipient organization that assesses its trust in the signer and then recipient organization that assesses its trust in the signer. The
makes handling decisions based on a collection of assessments, of recipient then makes handling decisions based on a collection of
which the DKIM mechanism is only a part. In this model, validation assessments, of which the DKIM mechanism is only a part. In this
is an intermediary step, having the sole task of passing a validated model, as shown in Figure 1, validation is an intermediary step,
Responsible Identifier to the Identity Assessor. The communication having the sole task of passing a validated Responsible Identifier to
is of a single Responsible Identifier that the Responsible Identity the Identity Assessor. The communication is of a single Responsible
wishes to have used by the Identity Assessor. The Identifier is the Identifier that the Responsible Identity wishes to have used by the
sole, formal input and output value of DKIM signing. The Identity Identity Assessor. The Identifier is the sole, formal input and
Assessor uses this single, provided Identifier for consulting output value of DKIM signing. The Identity Assessor uses this
whatever assessment data bases are deemed appropriate by the single, provided Identifier for consulting whatever assessment data
assessing entity. In turn, output from the Identity Assessor is fed bases are deemed appropriate by the assessing entity. In turn,
into a Handling Filter engine that considers a range of factors, output from the Identity Assessor is fed into a Handling Filter
along with this single output value; the range of factors can include engine that considers a range of factors, along with this single
ancillary information from the DKIM validation. output value. The range of factors can include ancillary information
from the DKIM validation.
Identity Assessment covers a range of possible functions. It can be Identity Assessment covers a range of possible functions. It can be
as simple as determining whether the identifier is a member of some as simple as determining whether the identifier is a member of some
list, such as authorized operators or participants in a group that list, such as authorized operators or participants in a group that
might be of interest for recipient assessment. Equally, it can might be of interest for recipient assessment. Equally, it can
indicate a degree of trust (reputation) that is to be afforded the indicate a degree of trust (reputation) that is to be afforded the
actor using that identifier. The extent to which the assessment actor using that identifier. The extent to which the assessment
affects handling of the message is, of course, determined later, by affects handling of the message is, of course, determined later, by
the Handling Filter. the Handling Filter.
skipping to change at page 7, line 27 skipping to change at page 7, line 24
| +------+------+ | +------+------+
| -->| Handling |<-- | -->| Handling |<--
| -->| Filter |<-- | -->| Filter |<--
| +-------------+ | +-------------+
| ^ | ^
V Responsible | V Responsible |
+-------------+ Identifier +------+------+ +-------------+ Identifier +------+------+
| Responsible |. . . . . . . . . . .>| Identity | | Responsible |. . . . . . . . . . .>| Identity |
| Identity | . . | Assessor | | Identity | . . | Assessor |
+------+------+ . . +-------------+ +------+------+ . . +-------------+
| . . ^ ^ | V . ^ ^
V . . | | V . . | |
+------------------.-------.--------------------+ | | +------------------.-------.--------------------+ | |
| +------+------+ . . . . . +-------------+ | | | +-----------+ | +------+------+ . . . > . +-------------+ | | | +-----------+
| | Identifier | | Identifier +--|--+ +--+ Assessment| | | Identifier | | Identifier +--|--+ +--+ Assessment|
| | Signer +------------->| Validator | | | Databases | | | Signer +------------->| Validator | | | Databases |
| +-------------+ +-------------+ | +-----------+ | +-------------+ +-------------+ | +-----------+
| DKIM Service | | DKIM Service |
+-----------------------------------------------+ +-----------------------------------------------+
Figure 1: Actors in a Trust Sequence using DKIM Figure 1: Actors in a Trust Sequence using DKIM
2.2. Choosing a DKIM Tag for the Assessment Identifier 2.2. Choosing a DKIM Tag for the Assessment Identifier
skipping to change at page 8, line 12 skipping to change at page 8, line 9
The salient values include the s=, d= and i= parameters in the DKIM- The salient values include the s=, d= and i= parameters in the DKIM-
Signature: header field. In order to achieve the end-to-end Signature: header field. In order to achieve the end-to-end
determinism needed for this collaborative exchange from the signer to determinism needed for this collaborative exchange from the signer to
the assessor, the core model needs to specify what the signer is the assessor, the core model needs to specify what the signer is
required to provide to the assessor. The Update to RFC4871 [RFC5672] required to provide to the assessor. The Update to RFC4871 [RFC5672]
now specifies: now specifies:
DKIM's primary task is to communicate from the Signer to a DKIM's primary task is to communicate from the Signer to a
recipient-side Identity Assessor a single Signing Domain recipient-side Identity Assessor a single Signing Domain
Identifier (SDID) that refers to a responsible identity. DKIM MAY Identifier (SDID) that refers to a responsible identity. DKIM can
optionally provide a single responsible Agent or User Identifier optionally provide a single responsible Agent or User Identifier
(AUID)... A receive-side DKIM verifier MUST communicate the (AUID)... A receive-side DKIM verifier needs to communicate the
Signing Domain Identifier (d=) to a consuming Identity Assessor Signing Domain Identifier (d=) to a consuming Identity Assessor
module and MAY communicate the User Agent Identifier (i=) if module and can also communicate the User Agent Identifier (i=) if
present.... To the extent that a receiver attempts to intuit any present.... To the extent that a receiver attempts to intuit any
structured semantics for either of the identifiers, this is a structured semantics for either of the identifiers, this is a
heuristic function that is outside the scope of DKIM's heuristic function that is outside the scope of DKIM's
specification and semantics. specification and semantics.
The single, mandatory value that DKIM supplies as its output is: The single, mandatory value that DKIM supplies as its output is:
d= This specifies the "domain of the signing entity." It is a d= This specifies the "domain of the signing entity." It is a
domain name and is combined with the Selector to form a DNS domain name and is combined with the Selector to form a DNS
query... A receive-side DKIM verifier MUST communicate the query... A receive-side DKIM verifier needs to communicate the
Signing Domain Identifier (d=) to a consuming Identity Assessor Signing Domain Identifier (d=) to a consuming Identity Assessor
module and MAY communicate the User Agent Identifier (i=) if module and can also communicate the User Agent Identifier (i=)
present. if present.
The adjunct values are: The adjunct values are:
s= This tag specifies the Selector. It is used to discriminate s= This tag specifies the Selector. It is used to discriminate
among different keys that can be used for the same d= domain among different keys that can be used for the same d= domain
name. As discussed in Section 4.3 of [RFC5585]: "If verifiers name. As discussed in Section 4.3 of [RFC5585]: "If verifiers
were to employ the selector as part of a name assessment were to employ the selector as part of a name assessment
mechanism, then there would be no remaining mechanism for mechanism, then there would be no remaining mechanism for
making a transition from an old, or compromised, key to a new making a transition from an old, or compromised, key to a new
one." Consequently, the Selector is not appropriate for use as one." Consequently, the Selector is not appropriate for use as
skipping to change at page 10, line 30 skipping to change at page 10, line 30
stable pattern of traffic for developing an accurate and reliable stable pattern of traffic for developing an accurate and reliable
assessment. So the differentiation needs to achieve a balance. assessment. So the differentiation needs to achieve a balance.
Generally in a trust system, legitimate signers have an incentive to Generally in a trust system, legitimate signers have an incentive to
pick a small stable set of identities, so that recipients and others pick a small stable set of identities, so that recipients and others
can attribute reputations to them. The set of these identities a can attribute reputations to them. The set of these identities a
receiver trusts is likely to be quite a bit smaller than the set it receiver trusts is likely to be quite a bit smaller than the set it
views as risky. views as risky.
The challenge in using additional layers of sub-domains is whether The challenge in using additional layers of sub-domains is whether
the extra granularity will be useful for the assessor. In fact, the extra granularity will be useful for the assessor. In fact,
potentially excessive levels invite ambiguity: if the assessor does excessive levels invite ambiguity: if the assessor does not take
not take advantage of the added granularity, then what granularity advantage of the added granularity in the entire domain name that is
will it use? That ambiguity would move the use of DKIM back to the provided, they might unilaterally decide to use only some rightmost
realm of heuristics, rather than the deterministic processing that is part of the identifier. The signer cannot know what portion will be
its goal. used. That ambiguity would move the use of DKIM back to the realm of
heuristics, rather than the deterministic processing that is its
goal.
Hence the challenge is to determine a useful scheme for labeling Hence the challenge is to determine a useful scheme for labeling
different traffic streams. The most obvious choices are among different traffic streams. The most obvious choices are among
different types of content and/or different types of authors. different types of content and/or different types of authors.
Although stability is essential, it is likely that the choices will Although stability is essential, it is likely that the choices will
change, over time, so the scheme needs to be flexible. change, over time, so the scheme needs to be flexible.
For those originating message content, the most likely choice of sub- For those originating message content, the most likely choice of sub-
domain naming scheme will by based upon type of content, which can domain naming scheme will by based upon type of content, which can
use content-oriented labels or service-oriented labels. For example: use content-oriented labels or service-oriented labels. For example:
skipping to change at page 13, line 15 skipping to change at page 13, line 23
The discussion focuses on variations in Organizational Trust versus The discussion focuses on variations in Organizational Trust versus
Message Risk, that is, the degree of positive assessment of a DKIM- Message Risk, that is, the degree of positive assessment of a DKIM-
signing organization, and the potential danger present in the message signing organization, and the potential danger present in the message
stream signed by that organization. While it might seem that higher stream signed by that organization. While it might seem that higher
trust automatically means lower risk, the experience with real-world trust automatically means lower risk, the experience with real-world
operations provides examples of every combination of the two factors, operations provides examples of every combination of the two factors,
as shown in Table 1. Only three levels of granularity are listed, in as shown in Table 1. Only three levels of granularity are listed, in
order to keep discussion manageable. This also ensures extensive order to keep discussion manageable. This also ensures extensive
flexibility for each site's detailed choices. flexibility for each site's detailed choices.
+---+---------------------+--------------------+--------------------+ +-------------+-----------------+-----------------+-----------------+
| | Low | Medium | High | | ORG TRUST \ | Low | Medium | High |
| | | | | | MSG RISK | | | |
| | | | | +-------------+-----------------+-----------------+-----------------+
| | | | | | *Low* | Unknown org, | Registered org, | Good Org, Good |
| | | | | | | Few msgs: _Mild | New Identifier: | msgs: _Avoid |
| O | | | | | | filter_ | _Medium filter_ | ;FP(!)_ |
| R | | | | | *Medium* | Unknown org, | Registered org, | Good org, Bad |
| G | | | | | | New Identifier: | Mixed msgs: | msg burst: |
| | | | | | | _Default | _Medium filter_ | _Accept & |
| T | | | | | | filter_ | | Contact_ |
| R | | | | | *High* | Black-Listed | Registered org, | Good org, |
| U | | | | | | org, Bad msgs: | Bad msgs: | Compromised: |
| S | | | | | | _Avoid FN(!)_ | _Strong filter_ | _Fully blocked_ |
| T | | | | +-------------+-----------------+-----------------+-----------------+
| | | | |
| M | | | |
+---+---------------------+--------------------+--------------------+
| * | Unknown org, | Registered org, | Good Org, |
| L | Few msgs: | New Identifier: | Good msgs: |
| o | _Mild filtering_ | _Medium filtering_ | _Avoid FP(!)_ |
| w | | | |
| * | Unknown org, | Registered org, | Good org, Bad msg |
| M | New Identifier: | Mixed msgs: | burst: |
| e | _Default filtering_ | _Medium filtering_ | _Accept & Contact_ |
| d | | | |
| i | | | |
| u | | | |
| * | Black-Listed org, | Registered org, | Good org, |
| H | Bad msgs: | Bad msgs: | Compromised: |
| i | _Avoid FN(!)_ | _Strong filtering_ | _Fully blocked_ |
| g | | | |
| h | | | |
+---+---------------------+--------------------+--------------------+
Table 1: Organizational Trust vs. Message Risk Table 1: Org Trust vs. Msg Risk
The table indicates preferences for different handling of different The table indicates preferences for different handling of different
combinations, such as tuning filtering to avoid False Positives (FP) combinations, such as tuning filtering to avoid False Positives (FP)
or avoiding False Negatives (FN). Perhaps unexpectedly, it also or avoiding False Negatives (FN). IT distinguishes various
lists a case in which the receiving site might wish to deliver characteristics, including: 1) organizations that are unknown, known
problematic mail, rather than redirecting it, but also of course to be good actors and known to be bad actors; and 2) assessment of
contacting the signing organization, seeking resolution of the messages. It includes advice about the degree of filtering that
might be done, and other message disposition. Perhaps unexpectedly,
it also lists a case in which the receiving site might wish to
deliver problematic mail, rather than redirecting it, but also of
course contacting the signing organization, seeking resolution of the
problem. problem.
3. DKIM Key Generation, Storage, and Management 3. DKIM Key Generation, Storage, and Management
By itself, verification of a digital signature only allows the By itself, verification of a digital signature only allows the
verifier to conclude with a very high degree of certainty that the verifier to conclude with a very high degree of certainty that the
signature was created by a party with access to the corresponding signature was created by a party with access to the corresponding
private signing key. It follows that a verifier requires means to private signing key. It follows that a verifier requires means to
(1) obtain the public key for the purpose of verification and (2) (1) obtain the public key for the purpose of verification and (2)
infer useful attributes of the key holder. infer useful attributes of the key holder.
In a traditional Public Key Infrastructure (PKI), the functions of In a traditional Public Key Infrastructure (PKI), the functions of
key distribution and key accreditation are separated. In DKIM key distribution and key accreditation are separated. In DKIM
[RFC4871], these functions are both performed through the DNS. [RFC4871], these functions are both performed through the DNS.
In either case, the ability to infer semantics from a digital In either case, the ability to infer semantics from a digital
signature depends on the assumption that the corresponding private signature depends on the assumption that the corresponding private
key is only accessible to a party with a particular set of key is only accessible to a party with a particular set of
attributes. In traditional PKI, a Trusted Third Party (TTP) vouches attributes. In traditional PKI, a Trusted Third Party (TTP) vouches
that the key holder has been validated with respect to a specified that the key holder has been validated with respect to a specified
set of attributes. The range of attributes that may be attested in set of attributes. The range of attributes that can be attested in
such a scheme is thus limited only to the type of attributes that a such a scheme is thus limited only to the type of attributes that a
TTP can establish effective processes for validating. In DKIM, TTP can establish effective processes for validating. In DKIM,
Trusted Third parties are not employed and the functions of key Trusted Third parties are not employed and the functions of key
distribution and accreditation are combined. distribution and accreditation are combined.
Consequently there are only two types of inference that a signer may Consequently there are only two types of inference that a signer can
make from a key published in a DKIM Key Record: make from a key published in a DKIM Key Record:
1. That a party with the ability to control DNS records within a DNS 1. That a party with the ability to control DNS records within a DNS
zone intends to claim responsibility for messages signed using zone intends to claim responsibility for messages signed using
the corresponding private signature key. the corresponding private signature key.
2. That use of a specific key is restricted to the particular subset 2. That use of a specific key is restricted to the particular subset
of messages identified by the selector. of messages identified by the selector.
The ability to draw any useful conclusion from verification of a The ability to draw any useful conclusion from verification of a
skipping to change at page 15, line 14 skipping to change at page 15, line 7
Ideally we would like to draw a stronger conclusion, that if we Ideally we would like to draw a stronger conclusion, that if we
obtain a DKIM key record from the DNS zone example.com, that the obtain a DKIM key record from the DNS zone example.com, that the
legitimate holder of the DNS zone example.com claims responsibility legitimate holder of the DNS zone example.com claims responsibility
for the signed message. In order for this conclusion to be drawn it for the signed message. In order for this conclusion to be drawn it
is necessary for the verifier to assume that the operational security is necessary for the verifier to assume that the operational security
of the DNS zone and corresponding private key are adequate. of the DNS zone and corresponding private key are adequate.
3.1. Private Key Management: Deployment and Ongoing Operations 3.1. Private Key Management: Deployment and Ongoing Operations
Access to signing keys MUST be carefully managed to prevent use by Access to signing keys needs to be carefully managed to prevent use
unauthorized parties and to minimize the consequences if a compromise by unauthorized parties and to minimize the consequences if a
were to occur. compromise were to occur.
While a DKIM signing key is used to sign messages on behalf of many While a DKIM signing key is used to sign messages on behalf of many
mail users, the signing key itself SHOULD be under direct control of mail users, the signing key itself needs to be under direct control
as few key holders as possible. If a key holder were to leave the of as few key holders as possible. If a key holder were to leave the
organization, all signing keys held by that key holder SHOULD be organization, all signing keys held by that key holder needs to be
withdrawn from service and if appropriate, replaced. withdrawn from service and if appropriate, replaced.
If key management hardware support is available, it SHOULD be used. If key management hardware support is available, it needs to be used.
If keys are stored in software, appropriate file control protections If keys are stored in software, appropriate file control protections
MUST be employed, and any location in which the private key is stored needs to be employed, and any location in which the private key is
in plaintext form SHOULD be excluded from regular backup processes stored in plaintext form needs to be excluded from regular backup
and SHOULD NOT be accessible through any form of network including processes and is best not accessible through any form of network
private local area networks. Auditing software SHOULD be used including private local area networks. Auditing software needs to be
periodically to verify that the permissions on the private key files used periodically to verify that the permissions on the private key
remain secure. files remain secure.
Wherever possible a signature key SHOULD exist in exactly one Wherever possible a signature key needs to exist in exactly one
location and be erased when no longer used. Ideally a signature key location and be erased when no longer used. Ideally a signature key
pair SHOULD be generated as close to the signing point as possible pair needs to be generated as close to the signing point as possible
and only the public key component transferred to another party. If and only the public key component transferred to another party. If
this is not possible, the private key MUST be transported in an this is not possible, the private key needs to be transported in an
encrypted format that protects the confidentiality of the signing encrypted format that protects the confidentiality of the signing
key. A shared directory on a local file system does not provide key. A shared directory on a local file system does not provide
adequate security for distribution of signing keys in plaintext form. adequate security for distribution of signing keys in plaintext form.
Key escrow schemes are not necessary and SHOULD NOT be used. In the Key escrow schemes are not necessary and are best not used. In the
unlikely event of a signing key becoming lost, a new signature key unlikely event of a signing key becoming lost, a new signature key
pair may be generated as easily as recovery from a key escrow scheme. pair can be generated as easily as recovery from a key escrow scheme.
To enable accountability and auditing: To enable accountability and auditing:
o Responsibility for the security of a signing key SHOULD ultimately o Responsibility for the security of a signing key needs to
vest in a single named individual. ultimately vest in a single named individual.
o Where multiple parties are authorized to sign messages, each o Where multiple parties are authorized to sign messages, each
signer SHOULD use a different key to enable accountability and signer needs to use a different key to enable accountability and
auditing. auditing.
Best practices for management of cryptographic keying material Best practices for management of cryptographic keying material
require keying material to be refreshed at regular intervals, require keying material to be refreshed at regular intervals,
particularly where key management is achieved through software. particularly where key management is achieved through software.
While this practice is highly desirable it is of considerably less While this practice is highly desirable it is of considerably less
importance than the requirement to maintain the secrecy of the importance than the requirement to maintain the secrecy of the
corresponding private key. An operational practice in which the corresponding private key. An operational practice in which the
private key is stored in tamper proof hardware and changed once a private key is stored in tamper proof hardware and changed once a
year is considerably more desirable than one in which the signature year is considerably more desirable than one in which the signature
skipping to change at page 16, line 33 skipping to change at page 16, line 26
DNS record management is often operated by an administrative staff DNS record management is often operated by an administrative staff
that is different from those who operate an organization's email that is different from those who operate an organization's email
service. In order to ensure that DKIM DNS records are accurate, this service. In order to ensure that DKIM DNS records are accurate, this
imposes a requirement for careful coordination between the two imposes a requirement for careful coordination between the two
operations groups. If the best practices for private key management operations groups. If the best practices for private key management
described above are observed, such deployment is not a onetime event; described above are observed, such deployment is not a onetime event;
DNS DKIM selectors will be changed over time signing keys are DNS DKIM selectors will be changed over time signing keys are
terminated and replaced. terminated and replaced.
At a minimum, a DNS server that handles queries for DKIM key records At a minimum, a DNS server that handles queries for DKIM key records
MUST allow the server administrators to add free-form TXT records. needs to allow the server administrators to add free-form TXT
It would be better if the DKIM records could be entered using a records. It would be better if the DKIM records could be entered
structured form, supporting the DKIM-specific fields. using a structured form, supporting the DKIM-specific fields.
Ideally DNSSEC [RFC4034] SHOULD be employed in a configuration that Ideally DNSSEC [RFC4034] needs to be employed in a configuration that
provides protection against record insertion attacks and zone provides protection against record insertion attacks and zone
enumeration. In the case that NSEC3 [RFC5155] records are employed enumeration. In the case that NSEC3 [RFC5155] records are employed
to prevent insertion attack, the OPT-OUT flag MUST be set clear. to prevent insertion attack, the OPT-OUT flag needs to be set clear.
3.2.1. Assignment of Selectors 3.2.1. Assignment of Selectors
Selectors are assigned according to the administrative needs of the Selectors are assigned according to the administrative needs of the
signing domain, such as for rolling over to a new key or for signing domain, such as for rolling over to a new key or for
delegating of the right to authenticate a portion of the namespace to delegating of the right to authenticate a portion of the namespace to
a trusted third party. Examples include: a trusted third party. Examples include:
jun2005.eng._domainkey.example.com jun2005.eng._domainkey.example.com
skipping to change at page 17, line 4 skipping to change at page 16, line 45
3.2.1. Assignment of Selectors 3.2.1. Assignment of Selectors
Selectors are assigned according to the administrative needs of the Selectors are assigned according to the administrative needs of the
signing domain, such as for rolling over to a new key or for signing domain, such as for rolling over to a new key or for
delegating of the right to authenticate a portion of the namespace to delegating of the right to authenticate a portion of the namespace to
a trusted third party. Examples include: a trusted third party. Examples include:
jun2005.eng._domainkey.example.com jun2005.eng._domainkey.example.com
widget.promotion._domainkey.example.com widget.promotion._domainkey.example.com
It is intended that assessments of DKIM identities be based on the It is intended that assessments of DKIM identities be based on the
domain name, and not include the selector. While past practice of a domain name, and not include the selector. While past practice of a
signer may permit a verifier to infer additional properties of signer can permit a verifier to infer additional properties of
particular messages from the structure DKIM key selector, unannounced particular messages from the structure DKIM key selector, unannounced
administrative changes such as a change of signing software may cause administrative changes such as a change of signing software can cause
such heuristics to fail at any time. such heuristics to fail at any time.
3.3. Per User Signing Key Management Issues 3.3. Per User Signing Key Management Issues
While a signer may establish business rules, such as issue of While a signer can establish business rules, such as issue of
individual signature keys for each end-user, DKIM makes no provision individual signature keys for each end-user, DKIM makes no provision
for communicating these to other parties. Out of band distribution for communicating these to other parties. Out of band distribution
of such business rules is outside the scope of DKIM. Consequently of such business rules is outside the scope of DKIM. Consequently
there is no means by which external parties may make use of such keys there is no means by which external parties can make use of such keys
to attribute messages with any greater granularity than a DNS domain. to attribute messages with any greater granularity than a DNS domain.
If per-user signing keys are assigned for internal purposes (e.g. If per-user signing keys are assigned for internal purposes (e.g.
authenticating messages sent to an MTA for distribution), the authenticating messages sent to an MTA for distribution), the
following issues need to be considered before using such signatures following issues need to be considered before using such signatures
as an alternative to traditional edge signing at the outbound MTA: as an alternative to traditional edge signing at the outbound MTA:
External verifiers will be unable to make use of the additional External verifiers will be unable to make use of the additional
signature granularity without access to additional information signature granularity without access to additional information
passed out of band with respect to [RFC4871]. passed out of band with respect to [RFC4871].
If the number of user keys is large, the efficiency of local If the number of user keys is large, the efficiency of local
caching of key records by verifiers will be lower. caching of key records by verifiers will be lower.
A large number of end users may be less likely to be able to A large number of end users is be less likely to do an adequate
manage private key data securely on their personal computer than job of managing private key data securely on their personal
an administrator running an edge MTA. computers than is an administrator running an edge MTA.
3.4. Third Party Signer Key Management and Selector Administration 3.4. Third Party Signer Key Management and Selector Administration
A DKIM key record only asserts that the holder of the corresponding A DKIM key record only asserts that the holder of the corresponding
domain name makes a claim of responsibility for messages signed under domain name makes a claim of responsibility for messages signed under
the corresponding key. In some applications, such as bulk mail the corresponding key. In some applications, such as bulk mail
delivery, it is desirable to delegate the ability to make a claim of delivery, it is desirable to delegate use of the key. That is, to
responsibility to another party. In this case the trust relationship allow a third party to sign on behalf of the domain holder. The
is established between the domain holder and the verifier but the trust relationship is still established between the domain holder and
private signature key is held by a third party. the verifier but the private signature key is held by a third party.
Signature keys used by a third party signer SHOULD be kept entirely Signature keys used by a third party signer needs to be kept entirely
separate from those used by the domain holder and other third party separate from those used by the domain holder and other third party
signers. To limit potential exposure of the private key, the signers. To limit potential exposure of the private key, the
signature key pair SHOULD be generated by the third party signer and signature key pair needs to be generated by the third party signer
the public component of the key transmitted to the domain holder, and the public component of the key transmitted to the domain holder,
rather than have the domain holder generate the key pair and transmit rather than have the domain holder generate the key pair and transmit
the private component to the third party signer. the private component to the third party signer.
Domain holders SHOULD adopt a least privilege approach and grant Domain holders needs to adopt a least privilege approach and grant
third party signers the minimum access necessary to perform the third party signers the minimum access necessary to perform the
desired function. Limiting the access granted to Third Party Signers desired function. Limiting the access granted to Third Party Signers
serves to protect the interests of both parties. The domain holder serves to protect the interests of both parties. The domain holder
minimizes its security risk and the Trusted Third Party Signer avoids minimizes its security risk and the Trusted Third Party Signer avoids
unnecessary liability. unnecessary liability.
In the most restrictive case a domain holder maintains full control In the most restrictive case a domain holder maintains full control
over the creation of key records and employs appropriate key record over the creation of key records and employs appropriate key record
restrictions to enforce restrictions on the messages for which the restrictions to enforce restrictions on the messages for which the
third party signer is able to sign. If such restrictions are third party signer is able to sign. If such restrictions are
impractical, the domain holder SHOULD delegate a DNS subzone for impractical, the domain holder needs to delegate a DNS subzone for
publishing key records to the third party signer. The domain holder publishing key records to the third party signer. It is best that
SHOULD NOT allow a third party signer unrestricted access to its DNS the domain holder NOT allow a third party signer unrestricted access
service for the purpose of publishing key records. to its DNS service for the purpose of publishing key records.
3.5. Key Pair / Selector Lifecycle Management 3.5. Key Pair / Selector Lifecycle Management
Deployments SHOULD establish, document and observe processes for Deployments need to establish, document and observe processes for
managing the entire lifecycle of a public key pair. managing the entire lifecycle of an asymmetric key pair.
3.5.1. Example Key Deployment Process 3.5.1. Example Key Deployment Process
When it is determined that a new key pair is required: When it is determined that a new key pair is required:
1. A Key Pair is generated by the signing device. 1. A Key Pair is generated by the signing device.
2. A proposed key selector record is generated and transmitted to 2. A proposed key selector record is generated and transmitted to
the DNS administration infrastructure. the DNS administration infrastructure.
skipping to change at page 19, line 20 skipping to change at page 19, line 12
When it is determined that a private signature key is no longer When it is determined that a private signature key is no longer
required: required:
1. Signer stops using the private key for signature operations. 1. Signer stops using the private key for signature operations.
2. Signer deletes all records of the private key, including in- 2. Signer deletes all records of the private key, including in-
memory copies at the signing device. memory copies at the signing device.
3. Signer notifies the DNS administration infrastructure that the 3. Signer notifies the DNS administration infrastructure that the
signing key is withdrawn from service and that the corresponding signing key is withdrawn from service and that the corresponding
key records may be withdrawn from service at a specified future key records can be withdrawn from service at a specified future
date. date.
4. The DNS administration infrastructure verifies the authenticity 4. The DNS administration infrastructure verifies the authenticity
of the key selector termination request. If accepted, of the key selector termination request. If accepted,
1. The key selector is scheduled for deletion at a future time 1. The key selector is scheduled for deletion at a future time
determined by site policy. determined by site policy.
2. Wait for deletion time to arrive. 2. Wait for deletion time to arrive.
3. The signer either publishes a revocation key selector with an 3. The signer either publishes a revocation key selector with an
empty "p=" field, or deletes the key selector record empty public-key data ("p=") field, or deletes the key
entirely. selector record entirely.
5. As far as the verifier is concerned, there is no functional 5. As far as the verifier is concerned, there is no functional
difference between verifying against a key selector with an empty difference between verifying against a key selector with an empty
"p=" field, and verifying against a missing key selector: both "p=" field, and verifying against a missing key selector: both
result in a failed signature and the signature should be treated result in a failed signature and the signature needs to be
as if it had not been there. However, there is a minor semantic treated as if it had not been there. However, there is a minor
difference: with the empty "p=" field, the signer is explicitly semantic difference: with the empty "p=" field, the signer is
stating that the key has been revoked. The empty "p=" record explicitly stating that the key has been revoked. The empty "p="
provides a gravestone for an old selector, making it less likely record provides a gravestone for an old selector, making it less
that the selector might be accidently reused with a different likely that the selector might be accidentally reused with a
public key. different public key.
4. Signing 4. Signing
Creating messages that have one or more DKIM signatures, requires Creating messages that have one or more DKIM signatures, requires
support in only two outbound email service components: support in only two outbound email service components:
o A DNS Administrative interface that can create and maintain the o A DNS Administrative interface that can create and maintain the
relevant DNS names -- including names with underscores -- and relevant DNS names -- including names with underscores -- and
resource records (RR). resource records (RR).
skipping to change at page 21, line 6 skipping to change at page 20, line 45
handling tradeoffs. One perspective that helps to resolve this handling tradeoffs. One perspective that helps to resolve this
choice is the difference between the increased flexibility, from choice is the difference between the increased flexibility, from
placement at (or close to) the MUA, versus the streamlined placement at (or close to) the MUA, versus the streamlined
administration and operation, that is more easily obtained by administration and operation, that is more easily obtained by
implementing the mechanism "deeper" into the organization's email implementing the mechanism "deeper" into the organization's email
infrastructure, such as at its boundary MTA. infrastructure, such as at its boundary MTA.
Note the discussion in Section 2.2, concerning use of the i= tag. Note the discussion in Section 2.2, concerning use of the i= tag.
The signing module uses the appropriate private key to create one or The signing module uses the appropriate private key to create one or
more signatures. The means by which the signing module obtains the more signatures. (See Section 6.5 for a discussion of multiple
signatures.) The means by which the signing module obtains the
private key(s) is not specified by DKIM. Given that DKIM is intended private key(s) is not specified by DKIM. Given that DKIM is intended
for use during email transit, rather than for long-term storage, it for use during email transit, rather than for long-term storage, it
is expected that keys will be changed regularly. For administrative is expected that keys will be changed regularly. For administrative
convenience, key information SHOULD NOT be hard-coded into software. convenience, it is best not to hard-code key information into
software.
4.3. Signing Policies and Practices 4.3. Signing Policies and Practices
Every organization (ADMD) will have its own policies and practices Every organization (ADMD) will have its own policies and practices
for deciding when to sign messages (message stream) and with what for deciding when to sign messages (message stream) and with what
domain name, selector and key. Examples of particular message domain name, selector and key. Examples of particular message
streams include all mail sent from the ADMD, versus mail from streams include all mail sent from the ADMD, versus mail from
particular types of user accounts, versus mail having particular particular types of user accounts, versus mail having particular
types of content. Given this variability, and the likelihood that types of content. Given this variability, and the likelihood that
signing practices will change over time, it will be useful to have signing practices will change over time, it will be useful to have
skipping to change at page 21, line 37 skipping to change at page 21, line 30
and well might attempt to impose more differential analysis on the and well might attempt to impose more differential analysis on the
recipient than they wish to support. In such cases, they are likely recipient than they wish to support. In such cases, they are likely
to use only a super-name -- right-hand substring -- of the signing to use only a super-name -- right-hand substring -- of the signing
name. When this occurs, the signer will not know what portion is name. When this occurs, the signer will not know what portion is
being used; this then moves DKIM back to the non-deterministic world being used; this then moves DKIM back to the non-deterministic world
of heuristics, rather than the mechanistic world of signer/recipient of heuristics, rather than the mechanistic world of signer/recipient
collaboration that DKIM seeks. collaboration that DKIM seeks.
5. Verifying 5. Verifying
A message recipient may verify a DKIM signature to determine if a A message recipient can verify a DKIM signature to determine if a
claim of responsibility has been made for the message by a trusted claim of responsibility has been made for the message by a trusted
domain. domain.
Access control requires two components: authentication and Access control requires two components: authentication and
authorization. By design, verification of a DKIM signature only authorization. By design, verification of a DKIM signature only
provides the authentication component of an access control decision provides the authentication component of an access control decision
and MUST be combined with additional sources of information such as and needs to be combined with additional sources of information such
reputation data to arrive at an access control decision. as reputation data to arrive at an access control decision.
5.1. Intended Scope of Use 5.1. Intended Scope of Use
DKIM requires that a message with a signature that is found to be DKIM requires that a message with a signature that is found to be
invalid is to be treated as if the message had not been signed at invalid is to be treated as if the message had not been signed at
all. all.
If a DKIM signature fails to verify, it is entirely possible that the If a DKIM signature fails to verify, it is entirely possible that the
message is valid and that either there is a configuration error in message is valid and that either there is a configuration error in
the signer's system (e.g. a missing key record) or that the message the signer's system (e.g. a missing key record) or that the message
was inadvertently modified in transit. It is thus undesirable for was inadvertently modified in transit. It is thus undesirable for
mail infrastructure to treat messages with invalid signatures less mail infrastructure to treat messages with invalid signatures less
favorably than those with no signatures whatsoever. Contrariwise, favorably than those with no signatures whatsoever. Contrariwise,
creation of an invalid signature requires a trivial amount of effort creation of an invalid signature requires a trivial amount of effort
on the part of an attacker. If messages with invalid signatures were on the part of an attacker. If messages with invalid signatures were
to be treated preferentially to messages with no signatures to be treated preferentially to messages with no signatures
whatsoever, attackers will simply add invalid signature blocks to whatsoever, attackers will simply add invalid signature blocks to
gain the preferential treatment. It follows that messages with gain the preferential treatment. It follows that messages with
invalid signatures SHOULD be treated no better and no worse than invalid signatures need to be treated no better and no worse than
those with no signature at all. those with no signature at all.
5.2. Signature Scope 5.2. Signature Scope
As with any other digital signature scheme, verifiers MUST only As with any other digital signature scheme, verifiers need to
consider the part of the message that is inside the scope of the consider only the part of the message that is inside the scope of the
message as being authenticated by the signature. message as being authenticated by the signature.
For example, if the l= option is employed to specify a content length For example, if the l= option is employed to specify a content length
for the scope of the signature, only the part of the message that is for the scope of the signature, only the part of the message that is
within the scope of the content signature would be considered within the scope of the content signature would be considered
authentic. authentic.
5.3. Design Scope of Use 5.3. Design Scope of Use
Public Key cryptography provides an exceptionally high degree of Public Key cryptography provides an exceptionally high degree of
skipping to change at page 22, line 45 skipping to change at page 22, line 38
corresponding to the public key indicated in the signature. corresponding to the public key indicated in the signature.
In order to make useful conclusions from the verification of a valid In order to make useful conclusions from the verification of a valid
digital signature, the verifier is obliged to make assumptions that digital signature, the verifier is obliged to make assumptions that
fall far short of absolute certainty. Consequently, mere validation fall far short of absolute certainty. Consequently, mere validation
of a DKIM signature does not represent proof positive that a valid of a DKIM signature does not represent proof positive that a valid
claim of responsibility was made for it by the indicated party, that claim of responsibility was made for it by the indicated party, that
the message is authentic, or that the message is not abusive. In the message is authentic, or that the message is not abusive. In
particular: particular:
o The legitimate private key holder may have lost control of its o The legitimate private key holder might have lost control of its
private key. private key.
o The legitimate domain holder may have lost control of the DNS o The legitimate domain holder might have lost control of the DNS
server for the zone from which the key record was retrieved. server for the zone from which the key record was retrieved.
o The key record may not have been delivered from the legitimate DNS o The key record might not have been delivered from the legitimate
server for the zone from which the key record was retrieved. DNS server for the zone from which the key record was retrieved.
o Ownership of the DNS zone may have changed. o Ownership of the DNS zone might have changed.
In practice these limitations have little or no impact on the field In practice these limitations have little or no impact on the field
of use for which DKIM is designed but may have a bearing if use is of use for which DKIM is designed but can have a bearing if use is
made of the DKIM message signature format or key retrieval mechanism made of the DKIM message signature format or key retrieval mechanism
in other specifications. in other specifications.
In particular the DKIM key retrieval mechanism is designed for ease In particular the DKIM key retrieval mechanism is designed for ease
of use and deployment rather than to provide a high assurance Public of use and deployment rather than to provide a high assurance Public
Key Infrastructure suitable for purposes that require robust non- Key Infrastructure suitable for purposes that require robust non-
repudiation such as establishing legally binding contracts. repudiation such as establishing legally binding contracts.
Developers seeking to extend DKIM beyond its design application Developers seeking to extend DKIM beyond its design application needs
SHOULD consider replacing or supplementing the DNS key retrieval to consider replacing or supplementing the DNS key retrieval
mechanism with one that is designed to meet the intended purposes. mechanism with one that is designed to meet the intended purposes.
5.4. Inbound Mail Filtering 5.4. Inbound Mail Filtering
DKIM is frequently employed in a mail filtering strategy to avoid DKIM is frequently employed in a mail filtering strategy to avoid
performing content analysis on email originating from trusted performing content analysis on email originating from trusted
sources. Messages that carry a valid DKIM signature from a trusted sources. Messages that carry a valid DKIM signature from a trusted
source may be whitelisted, avoiding the need to perform computation source can be whitelisted, avoiding the need to perform computation
and hence energy intensive content analysis to determine the and hence energy intensive content analysis to determine the
disposition of the message. disposition of the message.
Mail sources may be determined to be trusted by means of previously Mail sources can be determined to be trusted by means of previously
observed behavior and/or reference to external reputation or observed behavior and/or reference to external reputation or
accreditation services. The precise means by which this is accreditation services. The precise means by which this is
accomplished is outside the scope of DKIM. accomplished is outside the scope of DKIM.
5.4.1. Non-Verifying Adaptive Spam Filtering Systems 5.4.1. Non-Verifying Adaptive Spam Filtering Systems
Adaptive (or learning) spam filtering mechanisms that are not capable Adaptive (or learning) spam filtering mechanisms that are not capable
of verifying DKIM signatures SHOULD at minimum be configured to of verifying DKIM signatures need to, at minimum, be configured to
ignore DKIM header data entirely. ignore DKIM header data entirely.
5.5. Messages sent through Mailing Lists and other Intermediaries 5.5. Messages sent through Mailing Lists and other Intermediaries
Intermediaries such as mailing lists pose a particular challenge for Intermediaries such as mailing lists pose a particular challenge for
DKIM implementations as the message processing steps performed by the DKIM implementations, as the message processing steps performed by
intermediary may cause the message content to change in ways that the intermediary can cause the message content to change in ways that
prevent the signature passing verification. prevent the signature passing verification.
Such intermediaries are strongly encouraged to deploy DKIM signing so Such intermediaries are strongly encouraged to deploy DKIM signing so
that a verifiable claim of responsibility remains available to that a verifiable claim of responsibility remains available to
parties attempting to verify the modified message. parties attempting to verify the modified message.
5.6. Generation, Transmission and Use of Results Headers 5.6. Generation, Transmission and Use of Results Headers
In many deployments it is desirable to separate signature In many deployments it is desirable to separate signature
verification from the application relying on the verification. A verification from the application relying on the verification. A
system may choose to relay information indicating the results of its system can choose to relay information indicating the results of its
message authentication efforts using various means; adding a "results message authentication efforts using various means; adding a "results
header" to the message is one such mechanism. [RFC5451] For example, header" to the message is one such mechanism. [RFC5451] For example,
consider the cases where: consider the cases where:
o The application relying on DKIM signature verification is not o The application relying on DKIM signature verification is not
capable of performing the verification. capable of performing the verification.
o The message may be modified after the signature verification is o The message can be modified after the signature verification is
performed. performed.
o The signature key may not be available by the time that the o The signature key can not be available by the time that the
message is read. message is read.
In such cases it is important that the communication link between the In such cases it is important that the communication link between the
signature verifier and the relying application be sufficiently secure signature verifier and the relying application be sufficiently secure
to prevent insertion of a message that carries a bogus results to prevent insertion of a message that carries a bogus results
header. header.
An intermediary that generates results headers SHOULD ensure that An intermediary that generates results headers need to ensure that
relying applications are able to distinguish valid results headers relying applications are able to distinguish valid results headers
issued by the intermediary from those introduced by an attacker. For issued by the intermediary from those introduced by an attacker. For
example, this can be accomplished by signing the results header. At example, this can be accomplished by signing the results header. At
a minimum, results headers on incoming messages SHOULD be removed if a minimum, results headers on incoming messages need to be removed if
they purport to have been issued by the intermediary but cannot be they purport to have been issued by the intermediary but cannot be
verified as authentic. verified as authentic.
Further discussion on trusting the results as relayed from a verifier Further discussion on trusting the results as relayed from a verifier
to something downstream can be found in [RFC5451] to something downstream can be found in [RFC5451]
6. Taxonomy of Signatures 6. Taxonomy of Signatures
As described in section Section 2.1, a DKIM signature tells the As described in section Section 2.1, a DKIM signature tells the
signature verifier that the owner of a particular domain name accepts signature verifier that the owner of a particular domain name accepts
skipping to change at page 26, line 7 skipping to change at page 25, line 48
mail from all subdomains. In this case, the signature chosen would mail from all subdomains. In this case, the signature chosen would
usually be the signature of a parent domain common to all subdomains. usually be the signature of a parent domain common to all subdomains.
For example, mail from marketing.domain.example, For example, mail from marketing.domain.example,
sales.domain.example, and engineering.domain.example might all use a sales.domain.example, and engineering.domain.example might all use a
signature where d=domain.example. signature where d=domain.example.
This approach has the virtue of simplicity, but it is important to This approach has the virtue of simplicity, but it is important to
consider the implications of such a choice. As discussed in consider the implications of such a choice. As discussed in
Section 2.3, if the type of mail sent from the different subdomains Section 2.3, if the type of mail sent from the different subdomains
is significantly different or if there is reason to believe that the is significantly different or if there is reason to believe that the
reputation of the subdomains would differ, then it may be a good idea reputation of the subdomains would differ, then it can be a good idea
to acknowledge this and provide distinct signatures for each of the to acknowledge this and provide distinct signatures for each of the
subdomains (d=marketing.domain.example, sales.domain.example, etc.). subdomains (d=marketing.domain.example, sales.domain.example, etc.).
However, if the mail and reputations are likely to be similar, then However, if the mail and reputations are likely to be similar, then
the simpler approach of using a single common parent domain in the the simpler approach of using a single common parent domain in the
signature may work well. signature can work well.
Another approach to distinguishing the streams using a single DKIM Another approach to distinguishing the streams using a single DKIM
key would be to leverage the AUID [RFC5672] (i= tag) in the DKIM key would be to leverage the AUID [RFC5672] (i= tag) in the DKIM
signature to differentiate the mail streams. For example, marketing signature to differentiate the mail streams. For example, marketing
email would be signed with i=marketing.domain.example and email would be signed with i=@marketing.domain.example and
d=domain.example. d=domain.example.
It's important to remember, however, that under core DKIM semantics It's important to remember, however, that under core DKIM semantics
the AUID is opaque to receivers. That means that it will only be an the AUID is opaque to receivers. That means that it will only be an
effective differentiator if there is an out of band agreement about effective differentiator if there is an out of band agreement about
the i= semantics. the i= semantics.
6.3. Third Party Signature 6.3. Third Party Signature
A signature whose domain does not match the domain of the A signature whose domain does not match the domain of the
skipping to change at page 27, line 20 skipping to change at page 27, line 13
low risk to facilitate the delivery of messages that contain a valid low risk to facilitate the delivery of messages that contain a valid
signature of a domain with a strong positive reputation, independent signature of a domain with a strong positive reputation, independent
of whether or not that domain is associated with the address in the of whether or not that domain is associated with the address in the
RFC5322.From header field of the message. RFC5322.From header field of the message.
Third party signatures encompass a wide range of identities. Some of Third party signatures encompass a wide range of identities. Some of
the more common are: the more common are:
Service Provider: In cases where email is outsourced to an Email Service Provider: In cases where email is outsourced to an Email
Service Provider (ESP), Internet Service Provider (ISP), or other Service Provider (ESP), Internet Service Provider (ISP), or other
type of service provider, that service provider may choose to DKIM type of service provider, that service provider can choose to DKIM
sign outbound mail with either its own identifier -- relying on sign outbound mail with either its own identifier -- relying on
its own, aggregate reputation -- or with a subdomain of the its own, aggregate reputation -- or with a subdomain of the
provider that is unique to the message author but still part of provider that is unique to the message author but still part of
the provider's aggregate reputation. Such service providers may the provider's aggregate reputation. Such service providers can
also encompass delegated business functions such as benefit also encompass delegated business functions such as benefit
management, although these will more often be treated as trusted management, although these will more often be treated as trusted
third party senders (see below). third party senders (see below).
Parent Domain. As discussed above, organizations choosing to apply a Parent Domain. As discussed above, organizations choosing to apply a
parent domain signature to mail originating from subdomains may parent domain signature to mail originating from subdomains can
have their signatures treated as third party by some verifiers, have their signatures treated as third party by some verifiers,
depending on whether or not the "t=s" tag is used to constrain the depending on whether or not the "t=s" tag is used to constrain the
parent signature to apply only to its own specific domain. The parent signature to apply only to its own specific domain. The
default is to consider a parent domain signature valid for its default is to consider a parent domain signature valid for its
subdomains. subdomains.
Reputation Provider: Another possible category of third party Reputation Provider: Another possible category of third party
signature would be the identity of a third party reputation signature would be the identity of a third party reputation
provider. Such a signature would indicate to receivers that the provider. Such a signature would indicate to receivers that the
message was being vouched for by that third party. message was being vouched for by that third party.
skipping to change at page 28, line 34 skipping to change at page 28, line 26
use d=benefits.companyA.example. use d=benefits.companyA.example.
6.4.1. DNS Delegation 6.4.1. DNS Delegation
Another possibility for configuring trusted third party access, as Another possibility for configuring trusted third party access, as
discussed in section 3.4, is to have Company A use DNS delegation and discussed in section 3.4, is to have Company A use DNS delegation and
have the designated subdomain managed directly by the trusted third have the designated subdomain managed directly by the trusted third
party. In this case, Company A would create a subdomain party. In this case, Company A would create a subdomain
benefits.companya.example, and delegate the DNS management of that benefits.companya.example, and delegate the DNS management of that
subdomain to the benefits company so it could maintain its own key subdomain to the benefits company so it could maintain its own key
records. Should revocation become necessary, Company A could simply records. When revocation becomes necessary, Company A could simply
remove the DNS delegation record. remove the DNS delegation record.
6.5. Multiple Signatures 6.5. Multiple Signatures
A simple configuration for DKIM-signed mail is to have a single A simple configuration for DKIM-signed mail is to have a single
signature on a given message. This works well for domains that signature on a given message. This works well for domains that
manage and send all of their own email from single sources, or for manage and send all of their own email from single sources, or for
cases where multiple email streams exist but each has its own unique cases where multiple email streams exist but each has its own unique
key pair. It also represents the case in which only one of the key pair. It also represents the case in which only one of the
participants in an email sequence is able to sign, no matter whether participants in an email sequence is able to sign, no matter whether
it represents the author or one of the operators. it represents the author or one of the operators.
The examples thus far have considered the implications of using The examples thus far have considered the implications of using
different identities in DKIM signatures, but have used only one such different identities in DKIM signatures, but have used only one such
identity for any given message. In some cases, it may make sense to identity for any given message. In some cases, it can make sense to
have more than one identity claiming responsibility for the same have more than one identity claiming responsibility for the same
message. message.
There are a number of situations where applying more than one DKIM There are a number of situations where applying more than one DKIM
signature to the same message might make sense. A few examples are: signature to the same message might make sense. A few examples are:
Companies with multiple subdomain identities: A company that has Companies with multiple subdomain identities: A company that has
multiple subdomains sending distinct categories of mail might multiple subdomains sending distinct categories of mail might
choose to sign with distinct subdomain identities to enable each choose to sign with distinct subdomain identities to enable each
subdomain to manage its own identity. However, it might also want subdomain to manage its own identity. However, it might also want
to provide a common identity that cuts across all of the distinct to provide a common identity that cuts across all of the distinct
subdomains. For example, Company A may sign mail for its sales subdomains. For example, Company A can sign mail for its sales
department with a signature where d=sales.companya.example, and a department with a signature where d=sales.companya.example, and a
second signature where d=companya.example second signature where d=companya.example
Service Providers: Service providers may, as described above, choose Service Providers: Service providers can, as described above, choose
to sign outbound messages with either its own identity or with an to sign outbound messages with either its own identity or with an
identity unique to each of its clients (possibly delegated). identity unique to each of its clients (possibly delegated).
However, it may also do both: sign each outbound message with its However, it can also do both: sign each outbound message with its
own identity as well as with the identity of each individual own identity as well as with the identity of each individual
client. For example, ESP A might sign mail for its client Company client. For example, ESP A might sign mail for its client Company
B with its service provider signature d=espa.example, and a second B with its service provider signature d=espa.example, and a second
client-specific signature where d= either companyb.example, or client-specific signature where d= either companyb.example, or
companyb.espa.example. The existence of the service provider companyb.espa.example. The existence of the service provider
signature could, for example, help cover a new client while it signature could, for example, help cover a new client while it
establishes its own reputation, or help a very small volume client establishes its own reputation, or help a very small volume client
who might never reach a volume threshold sufficient to establish who might never reach a volume threshold sufficient to establish
an individual reputation. an individual reputation.
skipping to change at page 29, line 45 skipping to change at page 29, line 37
make modifications that can invalidate a DKIM signature. make modifications that can invalidate a DKIM signature.
Some forwarders such as mailing lists or "forward article to a Some forwarders such as mailing lists or "forward article to a
friend" services might choose to add their own signatures to friend" services might choose to add their own signatures to
outbound messages to vouch for them having legitimately originated outbound messages to vouch for them having legitimately originated
from the designated service. In this case, the signature would be from the designated service. In this case, the signature would be
added even in the presence of a preexisting signature, and both added even in the presence of a preexisting signature, and both
signatures would be relevant to the verifier. signatures would be relevant to the verifier.
Any forwarder that modifies messages in ways that will break Any forwarder that modifies messages in ways that will break
preexisting DKIM signatures SHOULD always sign its forwarded preexisting DKIM signatures needs to sign its forwarded messages.
messages.
Reputation Providers: Although third party reputation providers Reputation Providers: Although third party reputation providers
today use a variety of protocols to communicate their information today use a variety of protocols to communicate their information
to receivers, it is possible that they, or other organizations to receivers, it is possible that they, or other organizations
willing to put their "seal of approval" on an email stream, might willing to put their "seal of approval" on an email stream, might
choose to use a DKIM signature to do it. In nearly all cases, choose to use a DKIM signature to do it. In nearly all cases,
this "reputation" signature would be in addition to the author or this "reputation" signature would be in addition to the author or
originator signature. originator signature.
One important caveat to the use of multiple signatures is that there One important caveat to the use of multiple signatures is that there
is currently no clear consensus among receivers on how they plan to is currently no clear consensus among receivers on how they plan to
handle them. The opinions range from ignoring all but one signature handle them. The opinions range from ignoring all but one signature
(and the specification of which of them is verified differs from (and the specification of which of them is verified differs from
receiver to receiver), to verifying all signatures present and receiver to receiver), to verifying all signatures present and
applying a weighted blend of the trust assessments for those applying a weighted blend of the trust assessments for those
identifiers, to verifying all signatures present and simply using the identifiers, to verifying all signatures present and simply using the
identifier that represents the most positive trust assessment. It is identifier that represents the most positive trust assessment. It is
likely that the industry will evolve to accept multiple signatures likely that the industry will evolve to accept multiple signatures
using either the second or third of these, but it may take some time using either the second or third of these, but it can take some time
before one approach becomes pervasive. before one approach becomes pervasive.
7. Example Usage Scenarios 7. Example Usage Scenarios
Signatures are created by different types of email actors, based on Signatures are created by different types of email actors, based on
different criteria, such as where the actor operates in the sequence different criteria, such as where the actor operates in the sequence
from author to recipient, whether they want different messages to be from author to recipient, whether they want different messages to be
evaluated under the same reputation or a different one, and so on. evaluated under the same reputation or a different one, and so on.
This section provides some examples of usage scenarios for DKIM This section provides some examples of usage scenarios for DKIM
deployments; the selection is not intended to be exhaustive, but to deployments; the selection is not intended to be exhaustive, but to
skipping to change at page 31, line 9 skipping to change at page 30, line 48
of its outbound mail by using different identifiers. For example, it of its outbound mail by using different identifiers. For example, it
might choose to distinguish marketing mail, billing or transactional might choose to distinguish marketing mail, billing or transactional
mail, and individual corporate email into marketing.companya.example, mail, and individual corporate email into marketing.companya.example,
billing.companya.example, and companya.example, where each category billing.companya.example, and companya.example, where each category
is assigned a unique subdomain and unique signing keys. is assigned a unique subdomain and unique signing keys.
7.3. Author Domain Signing Practices 7.3. Author Domain Signing Practices
7.3.1. Introduction 7.3.1. Introduction
Some domains may decide to sign all of their outgoing mail. If all Some domains might decide to sign all of their outgoing mail. If all
of the legitimate mail for a domain is signed, recipients can be more of the legitimate mail for a domain is signed, recipients can be more
aggressive in their filtering of mail that uses the domain but does aggressive in their filtering of mail that uses the domain but does
not have a valid signature from the domain; in such a configuration, not have a valid signature from the domain; in such a configuration,
the absence of a signature would be more significant than for the the absence of a signature would be more significant than for the
general case. It might be desirable for such domains to be able to general case. It might be desirable for such domains to be able to
advertise their intent to other receivers: this is the topic of advertise their intent to other receivers: this is the topic of
Author Domain Signing Practices (ADSP). Author Domain Signing Practices (ADSP).
Note that ADSP is not for everyone. Sending domains that do not Note that ADSP is not for everyone. Sending domains that do not
control all legitimate outbound mail purporting to be from their control all legitimate outbound mail purporting to be from their
domain (i.e., with an RFC5322.From address in their domain) are domain (i.e., with an RFC5322.From address in their domain) are
likely to experience delivery problems with some percentage of that likely to experience delivery problems with some percentage of that
mail. Administrators evaluating ADSP for their domains SHOULD mail. Administrators evaluating ADSP for their domains needs to
carefully weigh the risk of phishing attacks against the likelihood carefully weigh the risk of phishing attacks against the likelihood
of undelivered mail. of undelivered mail.
This section covers some examples of ADSP usage: for the complete This section covers some examples of ADSP usage: for the complete
specification, see [RFC5617] specification, see [RFC5617]
7.3.2. A Few Definitions 7.3.2. A Few Definitions
In the ADSP specification, an address in the RFC5322.From header In the ADSP specification, an address in the RFC5322.From header
field of a message is defined as an "Author Address", and an "Author field of a message is defined as an "Author Address", and an "Author
skipping to change at page 31, line 48 skipping to change at page 31, line 40
It is important to note that unlike the DKIM specification which It is important to note that unlike the DKIM specification which
makes no correlation between the signature domain and any message makes no correlation between the signature domain and any message
headers, the ADSP specification applies only to the author domain. headers, the ADSP specification applies only to the author domain.
In essence, under ADSP, any non-author signatures are ignored In essence, under ADSP, any non-author signatures are ignored
(treated as if they are not present). (treated as if they are not present).
Signers wishing to publish an Author Domain Signing Practices (ADSP) Signers wishing to publish an Author Domain Signing Practices (ADSP)
[RFC5617] record describing their signing practices will thus want to [RFC5617] record describing their signing practices will thus want to
include an author signature on their outbound mail to avoid ADSP include an author signature on their outbound mail to avoid ADSP
verification failures. For example, if the address in the verification failures.
RFC5322.From is bob@company.example, the SDID value of the author
signature must be company.example.
7.3.3. Some ADSP Examples 7.3.3. Some ADSP Examples
An organization (Company A) may specify its signing practices by An organization (Company A) can specify its signing practices by
publishing an ADSP record with "dkim=all" or "dkim=discardable". In publishing an ADSP record with "dkim=all" or "dkim=discardable". In
order to avoid misdelivery of its mail at receivers that are order to avoid misdelivery of its mail at receivers that are
validating ADSP, Company A MUST first have done an exhaustive validating ADSP, Company A needs to first have done an exhaustive
analysis to determine all sources of outbound mail from its domain analysis to determine all sources of outbound mail from its domain
(companyA.example) and ensure that they all have valid author (companyA.example) and ensure that they all have valid author
signatures from that domain. signatures from that domain.
For example, email with an RFC5322.From address of bob@ For example, email with an RFC5322.From address of bob@
companyA.example MUST have an author signature where the SDID value companyA.example needs to have an author signature where the SDID
is "companyA.example" or it will fail an ADSP validation. value is "companyA.example" or it will fail an ADSP validation.
Note that once an organization publishes an ADSP record using Note that once an organization publishes an ADSP record using
dkim=all or dkim=discardable, any email with an RFC5322.From address dkim=all or dkim=discardable, any email with an RFC5322.From address
that uses the domain where the ADSP record is published that does not that uses the domain where the ADSP record is published that does not
have a valid author signature is at risk of being misdelivered or have a valid author signature is at risk of being misdelivered or
discarded. For example, if a message with an RFC5322.From address of discarded. For example, if a message with an RFC5322.From address of
newsletter@companyA.example has a signature with newsletter@companyA.example has a signature with
d=marketing.companyA.example, that message will fail the ADSP check d=marketing.companyA.example, that message will fail the ADSP check
because the signature would not be considered a valid author because the signature would not be considered a valid author
signature. signature.
skipping to change at page 32, line 40 skipping to change at page 32, line 30
constrained than the semantics of a "pure" DKIM signature, it is constrained than the semantics of a "pure" DKIM signature, it is
important to make sure the nuances are well understood before important to make sure the nuances are well understood before
deploying an ADSP record. The ADSP specification [RFC5617] provides deploying an ADSP record. The ADSP specification [RFC5617] provides
some fairly extensive lookup examples (in Appendix A) and usage some fairly extensive lookup examples (in Appendix A) and usage
examples (in Appendix B). examples (in Appendix B).
In particular, in order to prevent mail from being negatively In particular, in order to prevent mail from being negatively
impacted or even discarded at the receiver, it is essential to impacted or even discarded at the receiver, it is essential to
perform a thorough survey of outbound mail from a domain before perform a thorough survey of outbound mail from a domain before
publishing an ADSP policy of anything stronger than "unknown". This publishing an ADSP policy of anything stronger than "unknown". This
includes mail that might be sent from external sources that may not includes mail that might be sent from external sources that might not
be authorized to use the domain signature, as well as mail that risks be authorized to use the domain signature, as well as mail that risks
modification in transit that might invalidate an otherwise valid modification in transit that might invalidate an otherwise valid
author signature (e.g. mailing lists, courtesy forwarders, and other author signature (e.g. mailing lists, courtesy forwarders, and other
paths that could add or modify headers, or modify the message body). paths that could add or modify headers, or modify the message body).
7.4. Delegated Signing 7.4. Delegated Signing
An organization may choose to outsource certain key services to an An organization might choose to outsource certain key services to an
independent company. For example, Company A might outsource its independent company. For example, Company A might outsource its
benefits management, or Organization B might outsource its marketing benefits management, or Organization B might outsource its marketing
email. email.
If Company A wants to ensure that all of the mail sent on its behalf If Company A wants to ensure that all of the mail sent on its behalf
through the benefits providers email servers shares the Company A through the benefits providers email servers shares the Company A
reputation, as discussed in Section 6.4 it can either publish keys reputation, as discussed in Section 6.4 it can either publish keys
designated for the use of the benefits provider under designated for the use of the benefits provider under
companyA.example (preferably under a designated subdomain of companyA.example (preferably under a designated subdomain of
companyA.example), or it can delegate a subdomain (e.g. companyA.example), or it can delegate a subdomain (e.g.
skipping to change at page 33, line 34 skipping to change at page 33, line 23
the organization that plans to do the signing so that there is no the organization that plans to do the signing so that there is no
need to transfer the private key. In other words, the benefits need to transfer the private key. In other words, the benefits
provider would generate keys for both of the above scenarios. provider would generate keys for both of the above scenarios.
7.5. Independent Third Party Service Providers 7.5. Independent Third Party Service Providers
Another way to manage the service provider configuration would be to Another way to manage the service provider configuration would be to
have the service provider sign the outgoing mail on behalf of its have the service provider sign the outgoing mail on behalf of its
client Company A with its own (provider) identifier. For example, an client Company A with its own (provider) identifier. For example, an
Email Service Provider (ESP A) might want to share its own mailing Email Service Provider (ESP A) might want to share its own mailing
reputation with its clients, and may sign all outgoing mail from its reputation with its clients, and might sign all outgoing mail from
clients with its own d= domain (e.g. d=espa.example). its clients with its own d= domain (e.g. d=espa.example).
Should the ESP want to distinguish among its clients, it has two When the ESP want to distinguish among its clients, it has two
options: options:
o Share the SDID domain, and use the AUID value to distinguish among o Share the SDID domain, and use the AUID value to distinguish among
the clients: e.g. a signature on behalf of client A would have the clients: e.g. a signature on behalf of client A would have
d=espa.example and i=clienta.espa.example (or d=espa.example and i=@clienta.espa.example (or
i=clienta@espa.example) i=clienta@espa.example)
o Extend the SDID domain, so there is a unique value (and subdomain) o Extend the SDID domain, so there is a unique value (and subdomain)
for each client: e.g. a signature on behalf of client A would have for each client: e.g. a signature on behalf of client A would have
d=clienta.espa.example. d=clienta.espa.example.
Note that this scenario and the delegation scenario are not mutually Note that this scenario and the delegation scenario are not mutually
exclusive: in some cases, it may be desirable to sign the same exclusive: in some cases, it can be desirable to sign the same
message with both the ESP and the ESP client identities. message with both the ESP and the ESP client identities.
7.6. Mail Streams Based on Behavioral Assessment 7.6. Mail Streams Based on Behavioral Assessment
An ISP (ISP A) might want to assign signatures to outbound mail from An ISP (ISP A) might want to assign signatures to outbound mail from
its users according to each user's past sending behavior its users according to each user's past sending behavior
(reputation). In other words, the ISP would segment its outbound (reputation). In other words, the ISP would segment its outbound
traffic according to its own assessment of message quality, to aid traffic according to its own assessment of message quality, to aid
recipients in differentiating among these different streams. Since recipients in differentiating among these different streams. Since
the semantics of behavioral assessments are not valid AUID values, the semantics of behavioral assessments are not valid AUID values,
ISP A (ispa.example) may configure subdomains corresponding to the ISP A (ispa.example) can configure subdomains corresponding to the
assessment categories (e.g. good.ispa.example, neutral.ispa.example, assessment categories (e.g. good.ispa.example, neutral.ispa.example,
bad.ispa.example), and use these subdomains in the d= value of the bad.ispa.example), and use these subdomains in the d= value of the
signature. signature.
The signing module may also set the AUID value to have a unique user The signing module can also set the AUID value to have a unique user
id (distinct from the local-part of the user's email address), for id (distinct from the local-part of the user's email address), for
example user3456@neutral.domain.example. Using a userid that is example user3456@neutral.domain.example. Using a userid that is
distinct from a given email alias is useful in environments where a distinct from a given email alias is useful in environments where a
single user might register multiple email aliases. single user might register multiple email aliases.
Note that in this case the AUID values are only partially stable. Note that in this case the AUID values are only partially stable.
They are stable in the sense that a given i= value will always They are stable in the sense that a given i= value will always
represent the same identity, but they are unstable in the sense that represent the same identity, but they are unstable in the sense that
a given user can migrate among the assessment subdomains depending on a given user can migrate among the assessment subdomains depending on
their sending behavior (i.e., the same user might have multiple AUID their sending behavior (i.e., the same user might have multiple AUID
values over the lifetime of a single account). values over the lifetime of a single account).
In this scenario, ISP A may generate as many keys as there are In this scenario, ISP A can generate as many keys as there are
assessment subdomains (SDID values), so that each assessment assessment subdomains (SDID values), so that each assessment
subdomain has its own key. The signing module would then choose its subdomain has its own key. The signing module would then choose its
signing key based on the assessment of the user whose mail was being signing key based on the assessment of the user whose mail was being
signed, and if desired include the user id in the AUID of the signed, and if desired include the user id in the AUID of the
signature. As discussed earlier, the per-user granularity of the signature. As discussed earlier, the per-user granularity of the
AUID may be ignored by many verifiers, so organizations choosing to AUID can be ignored by verifiers; so organizations choosing to use it
use it should not rely on its use for receiver side filtering ought not rely on its use for receiver side filtering results.
results; however, some organizations may also find the information However, some organizations might also find the information useful
useful for their own purposes in processing bounces or abuse reports. for their own purposes in processing bounces or abuse reports.
7.7. Agent or Mediator Signatures 7.7. Agent or Mediator Signatures
Another scenario is that of an agent, usually a re-mailer of some Another scenario is that of an agent, usually a re-mailer of some
kind, that signs on behalf of the service or organization that it kind, that signs on behalf of the service or organization that it
represents. Some examples of agents might be a mailing list manager, represents. Some examples of agents might be a mailing list manager,
or the "forward article to a friend" service that many online or the "forward article to a friend" service that many online
publications offer. In most of these cases, the signature is publications offer. In most of these cases, the signature is
asserting that the message originated with, or was relayed by, the asserting that the message originated with, or was relayed by, the
service asserting responsibility. In general, if the service is service asserting responsibility. In general, if the service is
configured in such a way that its forwarding would break existing configured in such a way that its forwarding would break existing
DKIM signatures, it SHOULD always add its own signature. DKIM signatures, it needs to always add its own signature.
8. Usage Considerations 8. Usage Considerations
8.1. Non-standard Submission and Delivery Scenarios 8.1. Non-standard Submission and Delivery Scenarios
The robustness of DKIM's verification mechanism is based on the fact The robustness of DKIM's verification mechanism is based on the fact
that only authorized signing modules have access to the designated that only authorized signing modules have access to the designated
private key. This has the side effect that email submission and private key. This has the side effect that email submission and
delivery scenarios that originate or relay messages from outside the delivery scenarios that originate or relay messages from outside the
domain of the authorized signing module will not have access to that domain of the authorized signing module will not have access to that
protected private key, and thus will be unable to attach the expected protected private key, and thus will be unable to attach the expected
domain signature to those messages. Such scenarios include mailing domain signature to those messages. Such scenarios include mailing
lists, courtesy forwarders, MTAs at hotels, hotspot networks used by lists, courtesy forwarders, MTAs at hotels, hotspot networks used by
travelling users, and other paths that could add or modify headers, travelling users, and other paths that could add or modify headers,
or modify the message body. or modify the message body.
For example, assume Joe works for Company A and has an email address For example, assume Joe works for Company A and has an email address
joe@companya.example. Joe also has an ISP-1 account joe@companya.example. Joe also has an ISP-1 account
joe@isp1.example.com, and he uses ISP-1's multiple address feature to joe@isp1.example.com, and he uses ISP-1's multiple address feature to
attach his work email joe@companya.example to his ISP-1 account. attach his work email address, joe@companya.example, to email from
When Joe sends email from his ISP-1 account and uses his ISP-1 account. When Joe sends email from his ISP-1 account and
joe@companya.example as his designated RFC5322.From address, that uses joe@companya.example as his designated RFC5322.From address,
email cannot have a signature with d=companya.example because the that email cannot have a signature with d=companya.example because
ISP-1 servers have no access to Company A's private key. In ISP-1's the ISP-1 servers have no access to Company A's private key. In
case it will have an ISP-1 signature, but for some other mail clients ISP-1's case it will have an ISP-1 signature, but for some other mail
offering the same multiple address feature there may be no signature clients offering the same multiple address feature there might be no
at all on the message. signature at all on the message.
Another example might be the use of a forward article to a friend Another example might be the use of a forward article to a friend
service. Most instances of these services today allow someone to service. Most instances of these services today allow someone to
send an article with their email address in the RFC5322.From to their send an article with their email address in the RFC5322.From to their
designated recipient. If Joe used either of his two addresses designated recipient. If Joe used either of his two addresses
(joe@companya.example or joe@isp1.example.com), the forwarder would (joe@companya.example or joe@isp1.example.com), the forwarder would
be equally unable to sign with a corresponding domain. As in the be equally unable to sign with a corresponding domain. As in the
mail client case, the forwarder may either sign as its own domain, or mail client case, the forwarder can either sign as its own domain, or
may put no signature on the message. can put no signature on the message.
A third example is the use of privately configured forwarding. A third example is the use of privately configured forwarding.
Assume that Joe has another account at ISP-2, joe@isp-2.example.com, Assume that Joe has another account at ISP-2, joe@isp-2.example.com,
but he'd prefer to read his ISP-2 mail from his ISP-1 account. He but he'd prefer to read his ISP-2 mail from his ISP-1 account. He
sets up his ISP-2 account to forward all incoming mail to sets up his ISP-2 account to forward all incoming mail to
joe@isp1.example.com. Assume alice@companyb.example sends joe@isp1.example.com. Assume alice@companyb.example sends
joe@isp-2.example.com an email. Depending on how companyb.example joe@isp-2.example.com an email. Depending on how companyb.example
configured its signature, and depending on whether or not ISP-2 configured its signature, and depending on whether or not ISP-2
modifies messages that it forwards, it is possible that when Alice's modifies messages that it forwards, it is possible that when Alice's
message is received in Joe's ISP-1 account the original signature message is received in Joe's ISP-1 account the original signature
skipping to change at page 36, line 37 skipping to change at page 36, line 27
8.3. Signature Granularity 8.3. Signature Granularity
Although DKIM's use of domain names is optimized for a scope of Although DKIM's use of domain names is optimized for a scope of
organization-level signing, it is possible to administer sub-domains organization-level signing, it is possible to administer sub-domains
or otherwise adjust signatures in a way that supports per-user or otherwise adjust signatures in a way that supports per-user
identification. This user level granularity can be specified in two identification. This user level granularity can be specified in two
ways: either by sharing the signing identity and specifying an ways: either by sharing the signing identity and specifying an
extension to the i= value that has a per-user granularity, or by extension to the i= value that has a per-user granularity, or by
creating and signing with unique per-user keys. creating and signing with unique per-user keys.
A subdomain or local part in the i= tag SHOULD be treated as an A subdomain or local part in the i= tag needs to be treated as an
opaque identifier and thus need not correspond directly to a DNS opaque identifier and thus need not correspond directly to a DNS
subdomain or be a specific user address. subdomain or be a specific user address.
The primary way to sign with per-user keys requires each user to have The primary way to sign with per-user keys requires each user to have
a distinct DNS (sub)domain, where each distinct d= value has a key a distinct DNS (sub)domain, where each distinct d= value has a key
published. (It is possible, although not recommended, to publish the published. (It is possible, although not advised, to publish the
same key in more than one distinct domain.) same key in more than one distinct domain.)
It is technically possible to publish per-user keys within a single It is technically possible to publish per-user keys within a single
domain or subdomain by utilizing different selector values. This is domain or subdomain by utilizing different selector values. This is
not recommended and is unlikely to be treated uniquely by Assessors: not advised and is unlikely to be treated uniquely by Assessors: the
the primary purpose of selectors is to facilitate key management, and primary purpose of selectors is to facilitate key management, and the
the DKIM specification recommends against using them in determining DKIM specification recommends against using them in determining or
or assessing identities. assessing identities.
In most cases, it would be impractical to sign email on a per-user In most cases, it would be impractical to sign email on a per-user
granularity. Such an approach would be granularity. Such an approach would be
likely to be ignored: In most cases today, if receivers are likely to be ignored: In most cases today, if receivers are
verifying DKIM signatures they are in general taking the simplest verifying DKIM signatures they are in general taking the simplest
possible approach. In many cases maintaining reputation possible approach. In many cases maintaining reputation
information at a per-user granularity is not interesting to them, information at a per-user granularity is not interesting to them,
in large part because the per-user volume is too small to be in large part because the per-user volume is too small to be
useful or interesting. So even if senders take on the complexity useful or interesting. So even if senders take on the complexity
necessary to support per-user signatures, receivers are unlikely necessary to support per-user signatures, receivers are unlikely
to retain anything more than the base domain reputation. to retain anything more than the base domain reputation.
difficult to manage: Any scheme that involves maintenance of a difficult to manage: Any scheme that involves maintenance of a
significant number of public keys may require infrastructure significant number of public keys might require infrastructure
enhancements or extensive administrative expertise. For domains enhancements or extensive administrative expertise. For domains
of any size, maintaining a valid per-user keypair, knowing when of any size, maintaining a valid per-user keypair, knowing when
keys need to be revoked or added due to user attrition or keys need to be revoked or added due to user attrition or
onboarding, and the overhead of having the signing engine onboarding, and the overhead of having the signing engine
constantly swapping keys can create significant and often constantly swapping keys can create significant and often
unnecessary management complexity. It is also important to note unnecessary management complexity. It is also important to note
that there is no way within the scope of the DKIM specification that there is no way within the scope of the DKIM specification
for a receiver to infer that a sender intends a per-user for a receiver to infer that a sender intends a per-user
granularity. granularity.
As mentioned before, what may make sense, however, is to use the As mentioned before, what might make sense, however, is to use the
infrastructure that enables finer granularity in signatures to infrastructure that enables finer granularity in signatures to
identify segments smaller than a domain but much larger than a per- identify segments smaller than a domain but much larger than a per-
user segmentation. For example, a university might want to segment user segmentation. For example, a university might want to segment
student, staff, and faculty mail into three distinct streams with student, staff, and faculty mail into three distinct streams with
differing reputations. This can be done by creating separate sub- differing reputations. This can be done by creating separate sub-
domains for the desired segments, and either specifying the domains for the desired segments, and either specifying the
subdomains in the i= tag of the DKIM Signature or by adding subdomains in the i= tag of the DKIM Signature or by adding
subdomains to the d= tag and assigning and signing with different subdomains to the d= tag and assigning and signing with different
keys for each subdomain. keys for each subdomain.
skipping to change at page 38, line 6 skipping to change at page 37, line 44
extending the d= domain and publishing individual keys. extending the d= domain and publishing individual keys.
8.4. Email Infrastructure Agents 8.4. Email Infrastructure Agents
It is expected that the most common venue for a DKIM implementation It is expected that the most common venue for a DKIM implementation
will be within the infrastructure of an organization's email service, will be within the infrastructure of an organization's email service,
such as a department or a boundary MTA. What follows are some such as a department or a boundary MTA. What follows are some
general recommendations for the Email Infrastructure. general recommendations for the Email Infrastructure.
Outbound: An MSA or an Outbound MTA used for mail submission Outbound: An MSA or an Outbound MTA used for mail submission
SHOULD ensure that the message sent is in compliance with the needs to ensure that the message sent is in compliance with the
advertised email sending policy. It SHOULD also be able to advertised email sending policy. It needs to also be able to
generate an operator alert if it determines that the email generate an operator alert if it determines that the email
messages do not comply with the published DKIM sending policy. messages do not comply with the published DKIM sending policy.
An MSA SHOULD be aware that some MUAs may add their own An MSA needs to be aware that some MUAs might add their own
signatures. If the MSA needs to perform operations on a signatures. If the MSA needs to perform operations on a
message to make it comply with its email sending policy, if at message to make it comply with its email sending policy, if at
all possible, it SHOULD do so in a way that would not break all possible, it needs to do so in a way that would not break
those signatures. those signatures.
MUAs equipped with the ability to sign SHOULD NOT be MUAs equipped with the ability to sign ought not to be
encouraged. In terms of security, MUAs are generally not under encouraged. In terms of security, MUAs are generally not under
the direct control of those in responsible roles within an the direct control of those in responsible roles within an
organization and are thus more vulnerable to attack and organization and are thus more vulnerable to attack and
compromise, which would expose private signing keys to compromise, which would expose private signing keys to
intruders and thus jeopardize the integrity and reputation of intruders and thus jeopardize the integrity and reputation of
the organization. the organization.
Inbound: When an organization deploys DKIM, it needs to make Inbound: When an organization deploys DKIM, it needs to make
sure that its email infrastructure components that do not have sure that its email infrastructure components that do not have
primary roles in DKIM handling do not modify message in ways primary roles in DKIM handling do not modify message in ways
that prevent subsequent verification. that prevent subsequent verification.
An inbound MTA or an MDA may incorporate an indication of the An inbound MTA or an MDA can incorporate an indication of the
verification results into the message, such as using an verification results into the message, such as using an
Authentication-Results header field. [RFC5451] Authentication-Results header field. [RFC5451]
Intermediaries: An email intermediary is both an inbound and Intermediaries: An email intermediary is both an inbound and
outbound MTA. Each of the requirements outlined in the outbound MTA. Each of the requirements outlined in the
sections relating to MTAs apply. If the intermediary modifies sections relating to MTAs apply. If the intermediary modifies
a message in a way that breaks the signature, the intermediary a message in a way that breaks the signature, the intermediary
+ SHOULD deploy abuse filtering measures on the inbound mail, + needs to deploy abuse filtering measures on the inbound
and mail, and
+ MAY remove all signatures that will be broken + probably also needs to remove all signatures that will be
broken
In addition the intermediary MAY: In addition the intermediary can:
+ Verify the message signature prior to modification. + Verify the message signature prior to modification.
+ Incorporate an indication of the verification results into + Incorporate an indication of the verification results into
the message, such as using an Authentication-Results header the message, such as using an Authentication-Results header
field. [RFC5451] field. [RFC5451]
+ Sign the modified message including the verification results + Sign the modified message including the verification results
(e.g., the Authentication-Results header field). (e.g., the Authentication-Results header field).
8.5. Mail User Agent 8.5. Mail User Agent
The DKIM specification is expected to be used primarily between The DKIM specification is expected to be used primarily between
Boundary MTAs, or other infrastructure components of the originating Boundary MTAs, or other infrastructure components of the originating
and receiving ADMDs. However there is nothing in DKIM that is and receiving ADMDs. However there is nothing in DKIM that is
specific to those venues. In particular, MUAs MAY also support DKIM specific to those venues. In particular, MUAs can also support DKIM
signing and verifying directly. signing and verifying directly.
Outbound: An MUA MAY support signing even if mail is to be Outbound: An MUA can support signing even if mail is to be
relayed through an outbound MSA. In this case the signature relayed through an outbound MSA. In this case the signature
applied by the MUA will be in addition to any signature added applied by the MUA will be in addition to any signature added
by the MSA. However, the warnings in the previous section by the MSA. However, the warnings in the previous section need
should be taken into consideration. to be taken into consideration.
Some user software goes beyond simple user functionality and Some user software goes beyond simple user functionality and
also performs MSA and MTA functions. When this is employed for also performs MSA and MTA functions. When this is employed for
sending directly to a receiving ADMD, the user software SHOULD sending directly to a receiving ADMD, the user software needs
be considered an outbound MTA. to be considered an outbound MTA.
Inbound: An MUA MAY rely on a report of a DKIM signature Inbound: An MUA can rely on a report of a DKIM signature
verification that took place at some point in the inbound MTA/ verification that took place at some point in the inbound MTA/
MDA path (e.g., an Authentication-Results header field), or an MDA path (e.g., an Authentication-Results header field), or an
MUA MAY perform DKIM signature verification directly. A MUA can perform DKIM signature verification directly. A
verifying MUA SHOULD allow for the case where mail has modified verifying MUA needs to allow for the case where mail has
in the inbound MTA path; if a signature fails, the message modified in the inbound MTA path; if a signature fails, the
SHOULD NOT be treated any different than if it did not have a message is to be treated the same as a message which does not
signature. have a signature.
An MUA that looks for an Authentication-Results header field An MUA that looks for an Authentication-Results header field
MUST be configurable to choose which Authentication-Results are needs to be configurable to choose which Authentication-Results
considered trustable. The MUA developer is encouraged to re- are considered trustable. The MUA developer is encouraged to
read the Security Considerations of [RFC5451]. re-read the Security Considerations of [RFC5451].
DKIM requires that all verifiers treat messages with signatures DKIM requires that all verifiers treat messages with signatures
that do not verify as if they are unsigned. that do not verify as if they are unsigned.
If verification in the client is to be acceptable to users, it If verification in the client is to be acceptable to users, it
is essential that successful verification of a signature not is essential that successful verification of a signature not
result in a less than satisfactory user experience compared to result in a less than satisfactory user experience compared to
leaving the message unsigned. The mere presence of a verified leaving the message unsigned. The mere presence of a verified
DKIM signature MUST NOT be used by itself by an MUA to indicate DKIM signature cannot be used by itself by an MUA to indicate
that a message is to be treated better than a message without a that a message is to be treated better than a message without a
verified DKIM signature. However, the fact that a DKIM verified DKIM signature. However, the fact that a DKIM
signature was verified MAY be used as input into a reputation signature was verified can be used as input into a reputation
system (i.e., a whitelist of domains and users) for system (i.e., a whitelist of domains and users) for
presentation of such indicators. presentation of such indicators.
It is common for components of an ADMD's email infrastructure to do It is common for components of an ADMD's email infrastructure to do
violence to a message, such that a DKIM signature might be rendered violence to a message, such that a DKIM signature might be rendered
invalid. Hence, users of MUAs that support DKIM signing and/or invalid. Hence, users of MUAs that support DKIM signing and/or
verifying need a basis for knowing that their associated email verifying need a basis for knowing that their associated email
infrastructure will not break a signature. infrastructure will not break a signature.
9. Other Considerations 9. Other Considerations
skipping to change at page 40, line 27 skipping to change at page 40, line 24
The security considerations of the DKIM protocol are described in the The security considerations of the DKIM protocol are described in the
DKIM base specification [RFC4871]. DKIM base specification [RFC4871].
9.2. IANA Considerations 9.2. IANA Considerations
This document has no considerations for IANA. This document has no considerations for IANA.
10. Acknowledgements 10. Acknowledgements
TBD The effort of the DKIM Working Group is gratefully acknowledged.
11. Informative References 11. References
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 11.1. Normative References
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM) J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007. Signatures", RFC 4871, May 2007.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
Security (DNSSEC) Hashed Authenticated Denial of October 2008.
Existence", RFC 5155, March 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating [RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009. Message Authentication Status", RFC 5451, April 2009.
[RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys [RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
Identified Mail (DKIM) Service Overview", RFC 5585, Identified Mail (DKIM) Service Overview", RFC 5585,
July 2009. July 2009.
[RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine, [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing "DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, August 2009. Practices (ADSP)", RFC 5617, August 2009.
[RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM) [RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM)
Signatures -- Update", RFC 5672, August 2009. Signatures -- Update", RFC 5672, August 2009.
11.2. Informative References
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4870] Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
Appendix A. Migration Strategies Appendix A. Migration Strategies
There are three migration occassions worth noting in particular for There are three migration occasions worth noting in particular for
DKIM: DKIM:
1. Migrating from Domain Keys to DKIM. 1. Migrating from Domain Keys to DKIM.
2. Migrating from a current hash algorithm to a new standardized 2. Migrating from a current hash algorithm to a new standardized
hash algorithm. hash algorithm.
3. Migrating from a current signing algorithm to a new standardized 3. Migrating from a current signing algorithm to a new standardized
signing algorithm. signing algorithm.
skipping to change at page 41, line 46 skipping to change at page 42, line 9
A.1. Migrating from DomainKeys A.1. Migrating from DomainKeys
DKIM replaces the earlier DomainKeys (DK) specification. Selector DKIM replaces the earlier DomainKeys (DK) specification. Selector
files are mostly compatible between the two specifications. files are mostly compatible between the two specifications.
A.1.1. Signers A.1.1. Signers
A signer that currently signs with DK will go through various stages A signer that currently signs with DK will go through various stages
as it migrates to using DKIM, not all of which are required for all as it migrates to using DKIM, not all of which are required for all
signers. The real questions that a signer must ask are: signers. The real questions that a signer needs to ask are:
1. how many receivers or what types of receivers are *only* looking 1. how many receivers or what types of receivers are *only* looking
at the DK signatures and not the DKIM signatures, and at the DK signatures and not the DKIM signatures, and
2. how much does the signer care about those receivers? 2. how much does the signer care about those receivers?
If no one is looking at the DK signature any more, then it's no If no one is looking at the DK signature any more, then it's no
longer necessary to sign with DK. Or if all "large players" are longer necessary to sign with DK. Or if all "large players" are
looking at DKIM in addition to or instead of DK, a signer MAY choose looking at DKIM in addition to or instead of DK, a signer can choose
to stop signing with DK. to stop signing with DK.
With respect to signing policies, a reasonable, initial approach is With respect to signing policies, a reasonable, initial approach is
to use DKIM signatures in the same way as DomainKeys signatures are to use DKIM signatures in the same way as DomainKeys signatures are
already being used. In particular, the same selectors and DNS Key already being used. In particular, the same selectors and DNS Key
Records may be used for both, after verifying that they are Records can be used for both, after verifying that they are
compatible as discussed below. compatible as discussed below.
Each secondary step in all of the following scenarios is to be Each secondary step in all of the following scenarios is to be
prefaced with the gating factor "test, then when comfortable with the prefaced with the gating factor "test, then when comfortable with the
previous step's results, continue". previous step's results, continue".
One migration strategy is to: One migration strategy is to:
o ensure that the current selector DNS key record is compatible with o ensure that the current selector DNS key record is compatible with
both DK and DKIM both DK and DKIM
skipping to change at page 43, line 36 skipping to change at page 43, line 45
Note that when you have separate key records for DK and DKIM, you can Note that when you have separate key records for DK and DKIM, you can
use the same public key for both. use the same public key for both.
A.1.1.1. DNS Selector Key Records A.1.1.1. DNS Selector Key Records
The first step in some of the above scenarios is ensuring that the The first step in some of the above scenarios is ensuring that the
selector DNS key records are compatible for both DK and DKIM. The selector DNS key records are compatible for both DK and DKIM. The
format of the DNS key record was intentionally meant to be backwardly format of the DNS key record was intentionally meant to be backwardly
compatible between the two systems, but not necessarily upwardly compatible between the two systems, but not necessarily upwardly
compatible. DKIM has enhanced the DK DNS key record format by adding compatible. DKIM has enhanced the DK DNS key record format by adding
several optional parameters, which DK must ignore. However, there is several optional parameters, which DK needs to ignore. However,
one critical difference between DK and DKIM DNS key records: the there is one critical difference between DK and DKIM DNS key records:
definitions of the "g" fields: the definitions of the "g" fields:
g= granularity of the key In both DK and DKIM, this is an optional g= granularity of the key In both DK and DKIM, this is an optional
field that is used to constrain which sending address(es) can field that is used to constrain which sending address(es) can
legitimately use this selector. Unfortunately, the treatment of legitimately use this selector. Unfortunately, the treatment of
an empty field ("g=;") is different. DKIM allows wildcards where an empty field ("g=;") is different. DKIM allows wildcards where
DK does not. For DK, an empty field is the same as a missing DK does not. For DK, an empty field is the same as a missing
value, and is treated as allowing any sending address. For DKIM, value, and is treated as allowing any sending address. For DKIM,
an empty field only matches an empty local part. In DKIM, both a an empty field only matches an empty local part. In DKIM, both a
missing value and "g=*;" mean to allow any sending address. missing value and "g=*;" mean to allow any sending address.
Also, in DomainKeys, the g= field is required to match the address
in "From:"/"Sender:", while in DKIM, it is required to match i= .
This might or might not affect transition.
If your DK DNS key record has an empty "g" field in it ("g=;"), If your DK DNS key record has an empty "g" field in it ("g=;"),
your best course of action is to modify the record to remove the your best course of action is to modify the record to remove the
empty field. In that way, the DK semantics will remain the same, empty field. In that way, the DK semantics will remain the same,
and the DKIM semantics will match. and the DKIM semantics will match.
If your DNS key record does not have an empty "g" field in it If your DNS key record does not have an empty "g" field in it
("g=;"), it's probable that the record can be left alone. But your ("g=;"), it's probable that the record can be left alone. But the
best course of action would still be to make sure it has a "v" field. best course of action would still be to make sure that it has a
When the decision is made to stop supporting DomainKeys and to only "v" field. When the decision is made to stop supporting
support DKIM, you MUST verify that the "g" field is compatible with DomainKeys and to only support DKIM, it is important to verify
DKIM, and it SHOULD have "v=DKIM1;" in it. It is highly RECOMMENDED that the "g" field is compatible with DKIM, and typically having
that if you want to use an empty "g" field in your DKIM selector, you "v=DKIM1;" in it. It is strongly encouraged that if use of an
also include the "v" field. empty "g" field in the DKIM selector, include the "v" field.
A.1.1.2. Removing DomainKeys Signatures A.1.1.2. Removing DomainKeys Signatures
The principal use of DomainKeys is at Boundary MTAs. Because no The principal use of DomainKeys is at Boundary MTAs. Because no
operational transition is ever instantaneous, it is advisable to operational transition is ever instantaneous, it is advisable to
continue performing DomainKeys signing until it is determined that continue performing DomainKeys signing until it is determined that
DomainKeys receive-side support is no longer used, or is sufficiently DomainKeys receive-side support is no longer used, or is sufficiently
reduced. That is, a signer SHOULD add a DKIM signature to a message reduced. That is, a signer needs to add a DKIM signature to a
that also has a DomainKeys signature and keep it there until you message that also has a DomainKeys signature and keep it there until
decide it is deemed no longer useful. The signer may do its they decide it is deemed no longer useful. The signer can do its
transitions in a straightforward manner, or more gradually. Note transitions in a straightforward manner, or more gradually. Note
that because digital signatures are not free, there is a cost to that because digital signatures are not free, there is a cost to
performing both signing algorithms, so signing with both algorithms performing both signing algorithms, so signing with both algorithms
should not be needlessly prolonged. ought not be needlessly prolonged.
The tricky part is deciding when DK signatures are no longer The tricky part is deciding when DK signatures are no longer
necessary. The real questions are: how many DomainKeys verifiers are necessary. The real questions are: how many DomainKeys verifiers are
there that do *not* also do DKIM verification, which of those are there that do *not* also do DKIM verification, which of those are
important, and how can you track their usage? Most of the early important, and how can you track their usage? Most of the early
adopters of DK verification have added DKIM verification, but not all adopters of DK verification have added DKIM verification, but not all
yet. If a verifier finds a message with both DK and DKIM, it may yet. If a verifier finds a message with both DK and DKIM, it can
choose to verify both signatures, or just one or the other. choose to verify both signatures, or just one or the other.
Many DNS services offer tracking statistics so it can be determined Many DNS services offer tracking statistics so it can be determined
how often a DNS record has been accessed. By using separate DNS how often a DNS record has been accessed. By using separate DNS
selector key records for your signatures, you can chart the usage of selector key records for your signatures, you can chart the usage of
your records over time, and watch the trends. An additional your records over time, and watch the trends. An additional
distinguishing factor to track would take into account the verifiers distinguishing factor to track would take into account the verifiers
that verify both the DK and DKIM signatures, and discount those from that verify both the DK and DKIM signatures, and discount those from
counts of DK selector usage. When the number for DK selector access counts of DK selector usage. When the number for DK selector access
reaches a low-enough level, that's the time to consider discontinuing reaches a low-enough level, that's the time to consider discontinuing
signing with DK. signing with DK.
Note, this level of rigor is not required. It is perfectly Note, this level of rigor is not required. It is perfectly
reasonable for a DK signer to decide to follow the "flash cut" reasonable for a DK signer to decide to follow the "flash cut"
scenario described above. scenario described above.
A.1.2. Verifiers A.1.2. Verifiers
As a verifier, several issues must be considered: As a verifier, several issues needs to be considered:
A.1.2.1. Should DK signature verification be performed? A.1.2.1. Ought DK signature verification be performed?
At the time of writing, there is still a significant number of sites At the time of writing, there is still a significant number of sites
that are only producing DK signatures. Over time, it is expected that are only producing DK signatures. Over time, it is expected
that this number will go to zero, but it may take several years. So that this number will go to zero, but it might take several years.
it would be prudent for the foreseeable future for a verifier to look So it would be prudent for the foreseeable future for a verifier to
for and verify both DKIM and DK signatures. look for and verify both DKIM and DK signatures.
A.1.2.2. Should both DK and DKIM signatures be evaluated on a single A.1.2.2. Ought both DK and DKIM signatures be evaluated on a single
message? message?
For a period of time, there will be sites that sign with both DK and For a period of time, there will be sites that sign with both DK and
DKIM. A verifier receiving a message that has both types of DKIM. A verifier receiving a message that has both types of
signatures may verify both signatures, or just one. One disadvantage signatures can verify both signatures, or just one. One disadvantage
of verifying both signatures is that signers will have a more of verifying both signatures is that signers will have a more
difficult time deciding how many verifiers are still using their DK difficult time deciding how many verifiers are still using their DK
selectors. One transition strategy is to verify the DKIM signature, selectors. One transition strategy is to verify the DKIM signature,
then only verify the DK signature if the DKIM verification fails. then only verify the DK signature if the DKIM verification fails.
A.1.2.3. DNS Selector Key Records A.1.2.3. DNS Selector Key Records
The format of the DNS key record was intentionally meant to be The format of the DNS key record was intentionally meant to be
backwardly compatible between DK and DKIM, but not necessarily backwardly compatible between DK and DKIM, but not necessarily
upwardly compatible. DKIM has enhanced the DK DNS key record format upwardly compatible. DKIM has enhanced the DK DNS key record format
by adding several optional parameters, which DK must ignore. by adding several optional parameters, which DK needs to ignore.
However, there is one key difference between DK and DKIM DNS key However, there is one key difference between DK and DKIM DNS key
records: the definitions of the g fields: records: the definitions of the g fields:
g= granularity of the key In both DK and DKIM, this is an optional g= granularity of the key In both DK and DKIM, this is an optional
field that is used to constrain which sending address(es) can field that is used to constrain which sending address(es) can
legitimately use this selector. Unfortunately, the treatment of legitimately use this selector. Unfortunately, the treatment of
an empty field ("g=;") is different. For DK, an empty field is an empty field ("g=;") is different. For DK, an empty field is
the same as a missing value, and is treated as allowing any the same as a missing value, and is treated as allowing any
sending address. For DKIM, an empty field only matches an empty sending address. For DKIM, an empty field only matches an empty
local part. local part.
v= version of the selector It is recommended that a DKIM selector v= version of the selector It is advised that a DKIM selector have
have "v=DKIM1;" at its beginning, but it is not required. "v=DKIM1;" at its beginning, but it is not required.
If a DKIM verifier finds a selector record that has an empty "g" If a DKIM verifier finds a selector record that has an empty "g"
field ("g=;") and it does not have a "v" field ("v=DKIM1;") at its field ("g=;") and it does not have a "v" field ("v=DKIM1;") at its
beginning, it is faced with deciding if this record was beginning, it is faced with deciding if this record was
1. from a DK signer that transitioned to supporting DKIM but forgot 1. from a DK signer that transitioned to supporting DKIM but forgot
to remove the "g" field (so that it could be used by both DK and to remove the "g" field (so that it could be used by both DK and
DKIM verifiers), or DKIM verifiers), or
2. from a DKIM signer that truly meant to use the empty "g" field 2. from a DKIM signer that truly meant to use the empty "g" field
but forgot to put in the "v" field. It is RECOMMENDED that you but forgot to put in the "v" field. It is advised that you treat
treat such records using the first interpretation, and treat such such records using the first interpretation, and treat such
records as if the signer did not have a "g" field in the record. records as if the signer did not have a "g" field in the record.
A.2. Migrating Hash Algorithms A.2. Migrating Hash Algorithms
[RFC4871] defines the use of two hash algorithms, SHA-1 and SHA-256. [RFC4871] defines the use of two hash algorithms, SHA-1 and SHA-256.
The security of all hash algorithms is constantly under attack, and The security of all hash algorithms is constantly under attack, and
SHA-1 has already shown weaknesses as of this writing. Migrating SHA-1 has already shown weaknesses as of this writing. Migrating
from SHA-1 to SHA-256 is not an issue, because all verifiers are from SHA-1 to SHA-256 is not an issue, because all verifiers are
already required to support SHA-256. But when it becomes necessary already required to support SHA-256. But when it becomes necessary
to replace SHA-256 with a more secure algorithm, there will be a to replace SHA-256 with a more secure algorithm, there will be a
skipping to change at page 46, line 42 skipping to change at page 47, line 9
algorithms are considered and how important that is to the signers. algorithms are considered and how important that is to the signers.
One strategy is to wait until it's determined that there is a large One strategy is to wait until it's determined that there is a large
enough base of verifiers available that support NEWHASH, and then enough base of verifiers available that support NEWHASH, and then
flash cut to the new algorithm. flash cut to the new algorithm.
Another strategy is to sign with both the old and new hash algorithms Another strategy is to sign with both the old and new hash algorithms
for a period of time. This is particularly useful for testing the for a period of time. This is particularly useful for testing the
new code to support the new hash algorithm, as verifiers will new code to support the new hash algorithm, as verifiers will
continue to accept the signature for the older hash algorithm and continue to accept the signature for the older hash algorithm and
should ignore any signature that fails because the code is slightly ought to ignore any signature that fails because the code is slightly
wrong. Once the signer has determined that the new code is correct wrong. Once the signer has determined that the new code is correct
AND it's determined that there is a large enough base of verifiers AND it's determined that there is a large enough base of verifiers
available that support NEWHASH, the signer can flash cut to the new available that support NEWHASH, the signer can flash cut to the new
algorithm. algorithm.
One advantage migrating hash algorithms has is that the selector can One advantage migrating hash algorithms has is that the selector can
be completely compatible for all hash algorithms. The key selector be completely compatible for all hash algorithms. The key selector
has an optional "h=" field that may be used to list the hash has an optional "h=" field that can be used to list the hash
algorithms being used; it also is used to limit the algorithms that a algorithms being used; it also is used to limit the algorithms that a
verifier will accept. If the signer is not currently using the key- verifier will accept. If the signer is not currently using the key-
selector "h=" field, no change is required. If the signer is selector "h=" field, no change is required. If the signer is
currently using the key-selector "h=" field, NEWHASH will need to be currently using the key-selector "h=" field, NEWHASH will need to be
added to the list, as in "h=sha256:NEWHASH;". (When the signer is no added to the list, as in "h=sha256:NEWHASH;". (When the signer is no
longer using sha256, it can be removed from the "h=" list.) longer using sha256, it can be removed from the "h=" list.)
A.2.2. Verifiers A.2.2. Verifiers
When a new hash algorithm becomes standardized, it is best for a When a new hash algorithm becomes standardized, it is best for a
skipping to change at page 48, line 17 skipping to change at page 48, line 30
NOTE: This section could possibly be changed into a reference to NOTE: This section could possibly be changed into a reference to
something else, such as another rfc. something else, such as another rfc.
Correct implementation of a cryptographic algorithm is a necessary Correct implementation of a cryptographic algorithm is a necessary
but not a sufficient condition for the coding of cryptographic but not a sufficient condition for the coding of cryptographic
applications. Coding of cryptographic libraries requires close applications. Coding of cryptographic libraries requires close
attention to security considerations that are unique to cryptographic attention to security considerations that are unique to cryptographic
applications. applications.
In addition to the usual security coding considerations, such as In addition to the usual security coding considerations, such as
avoiding buffer or integer overflow and underflow, implementers avoiding buffer or integer overflow and underflow, implementers need
should pay close attention to management of cryptographic private to pay close attention to management of cryptographic private keys
keys and session keys, ensuring that these are correctly initialized and session keys, ensuring that these are correctly initialized and
and disposed of. disposed of.
Operating system mechanisms that permit the confidentiality of Operating system mechanisms that permit the confidentiality of
private keys to be protected against other processes should be used private keys to be protected against other processes ought to be used
when available. In particular, great care must be taken when when available. In particular, great care needs to be taken when
releasing memory pages to the operating system to ensure that private releasing memory pages to the operating system to ensure that private
key information is not disclosed to other processes. key information is not disclosed to other processes.
Certain implementations of public key algorithms such as RSA may be Certain implementations of public key algorithms such as RSA can be
vulnerable to a timing analysis attack. vulnerable to a timing analysis attack.
Support for cryptographic hardware providing key management Support for cryptographic hardware providing key management
capabilities is strongly encouraged. In addition to offering capabilities is strongly encouraged. In addition to offering
performance benefits, many cryptographic hardware devices provide performance benefits, many cryptographic hardware devices provide
robust and verifiable management of private keys. robust and verifiable management of private keys.
Fortunately appropriately designed and coded cryptographic libraries Fortunately appropriately designed and coded cryptographic libraries
are available for most operating system platforms under license terms are available for most operating system platforms under license terms
compatible with commercial, open source and free software license compatible with commercial, open source and free software license
 End of changes. 168 change blocks. 
362 lines changed or deleted 371 lines changed or added

This html diff was produced by rfcdiff 1.37b. The latest version is available from http://tools.ietf.org/tools/rfcdiff/