draft-ietf-dkim-overview-12.txt   rfc5585.txt 
DomainKeys Identified Mail T. Hansen Network Working Group T. Hansen
Internet-Draft AT&T Laboratories Request for Comments: 5585 AT&T Laboratories
Intended status: Informational D. Crocker Category: Informational D. Crocker
Expires: November 30, 2009 Brandenburg InternetWorking Brandenburg InternetWorking
P. Hallam-Baker P. Hallam-Baker
May 29, 2009 Default Deny Security, Inc.
DomainKeys Identified Mail (DKIM) Service Overview DomainKeys Identified Mail (DKIM) Service Overview
draft-ietf-dkim-overview-12
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Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract Abstract
This document provides an overview of the DomainKeys Identified Mail This document provides an overview of the DomainKeys Identified Mail
(DKIM) service and describes how it can fit into a messaging service. (DKIM) service and describes how it can fit into a messaging service.
It also describes how DKIM relates to other IETF message signature It also describes how DKIM relates to other IETF message signature
technologies. It is intended for those who are adopting, developing, technologies. It is intended for those who are adopting, developing,
or deploying DKIM. DKIM allows an organization to take or deploying DKIM. DKIM allows an organization to take
responsibility for transmitting a message, in a way that can be responsibility for transmitting a message, in a way that can be
verified by a recipient. The organization can be the author's, the verified by a recipient. The organization can be the author's, the
originating sending site, an intermediary, or one of their agents. A originating sending site, an intermediary, or one of their agents. A
message can contain multiple signatures, from the same or different message can contain multiple signatures from the same or different
organizations involved with the message. DKIM defines a domain-level organizations involved with the message. DKIM defines a domain-level
digital signature authentication framework for email, using public- digital signature authentication framework for email, using public-
key cryptography, with the domain name service as its key server key cryptography, with the domain name service as its key server
technology [RFC4871]. This permits verification of a responsible technology (RFC 4871). This permits verification of a responsible
organization, as well as the integrity of the message contents. DKIM organization, as well as the integrity of the message contents. DKIM
also enables a mechanism that permits potential email signers to also enables a mechanism that permits potential email signers to
publish information about their email signing practices; this will publish information about their email signing practices; this will
permit email receivers to make additional assessments about messages. permit email receivers to make additional assessments about messages.
DKIM's authentication of email identity can assist in the global DKIM's authentication of email identity can assist in the global
control of "spam" and "phishing". control of "spam" and "phishing".
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction ....................................................3
1.1. DKIM's Scope . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. DKIM's Scope ...............................................4
1.2. Prior Work . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Prior Work .................................................5
1.3. Internet Mail Background . . . . . . . . . . . . . . . . . 7 1.3. Internet Mail Background ...................................6
1.4. Discussion Venue . . . . . . . . . . . . . . . . . . . . . 7 2. The DKIM Value Proposition ......................................6
2. The DKIM Value Proposition . . . . . . . . . . . . . . . . . . 8 2.1. Identity Verification ......................................7
2.1. Identity Verification . . . . . . . . . . . . . . . . . . 8 2.2. Enabling Trust Assessments .................................7
2.2. Enabling Trust Assessments . . . . . . . . . . . . . . . . 9 2.3. Establishing Message Validity ..............................8
2.3. Establishing Message Validity . . . . . . . . . . . . . . 10 3. DKIM Goals ......................................................8
3. DKIM Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1. Functional Goals ...........................................9
3.1. Functional Goals . . . . . . . . . . . . . . . . . . . . . 10 3.2. Operational Goals .........................................10
3.2. Operational Goals . . . . . . . . . . . . . . . . . . . . 11 4. DKIM Function ..................................................12
4. DKIM Function . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. Basic Signing .............................................12
4.1. Basic Signing . . . . . . . . . . . . . . . . . . . . . . 13 4.2. Characteristics of a DKIM Signature .......................12
4.2. Characteristics of a DKIM Signature . . . . . . . . . . . 14 4.3. The Selector Construct ....................................13
4.3. The Selector Construct . . . . . . . . . . . . . . . . . . 14 4.4. Verification ..............................................13
4.4. Verification . . . . . . . . . . . . . . . . . . . . . . . 14 4.5. Sub-Domain Assessment .....................................13
4.5. Sub-Domain Assessment . . . . . . . . . . . . . . . . . . 15 5. Service Architecture ...........................................14
5. Service Architecture . . . . . . . . . . . . . . . . . . . . . 15 5.1. Administration and Maintenance ............................15
5.1. Administration and Maintenance . . . . . . . . . . . . . . 17 5.2. Signing ...................................................16
5.2. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.3. Verifying .................................................16
5.3. Verifying . . . . . . . . . . . . . . . . . . . . . . . . 18 5.4. Unverified or Unsigned Mail ...............................16
5.4. Unverified or Unsigned Mail . . . . . . . . . . . . . . . 18 5.5. Assessing .................................................17
5.5. Assessing . . . . . . . . . . . . . . . . . . . . . . . . 18 5.6. DKIM Processing within an ADMD ............................17
5.6. DKIM Processing within an ADMD . . . . . . . . . . . . . . 19 6. Considerations .................................................17
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 19 6.1. Security Considerations ...................................17
6.1. Security Considerations . . . . . . . . . . . . . . . . . 19 6.2. Acknowledgements ..........................................17
6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 19 7. Informative References .........................................18
6.3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 19 Appendix A. Internet Mail Background .............................20
7. Informative References . . . . . . . . . . . . . . . . . . . . 20 A.1. Core Model ................................................20
Appendix A. Internet Mail Background . . . . . . . . . . . . . . 21 A.2. Trust Boundaries ..........................................20
A.1. Core Model . . . . . . . . . . . . . . . . . . . . . . . . 21 Index .............................................................22
A.2. Trust Boundaries . . . . . . . . . . . . . . . . . . . . . 21
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
This document provides a description of the architecture and This document provides a description of the architecture and
functionality for DomainKeys Identified Mail (DKIM), that is, the functionality for DomainKeys Identified Mail (DKIM), that is, the
core mechanism for signing and verifying messages. It is intended core mechanism for signing and verifying messages. It is intended
for those who are adopting, developing, or deploying DKIM. It will for those who are adopting, developing, or deploying DKIM. It will
also be helpful for those who are considering extending DKIM, either also be helpful for those who are considering extending DKIM, either
into other areas of use or to support additional features. This into other areas of use or to support additional features. This
overview does not provide information on threats to DKIM or email, or overview does not provide information on threats to DKIM or email or
details on the protocol specifics, which can be found in [RFC4686] details on the protocol specifics, which can be found in [RFC4686]
and [RFC4871], respectively. Because the scope of this Overview is and [RFC4871], respectively. Because the scope of this overview is
restricted to the technical details of signing and verifying using restricted to the technical details of signing and verifying using
DKIM, it does not explore operational issues, the details of services DKIM, it does not explore operational issues, the details of services
that DKIM uses or that, in turn, use DKIM. Nor does it discuss that DKIM uses, or those that, in turn, use DKIM. Nor does it
services that build upon DKIM for enforcement of policies or discuss services that build upon DKIM for enforcement of policies or
assessments. The document assumes a background in basic email and assessments. The document assumes a background in basic email and
network security technology and services. network security technology and services.
DKIM allows an organization to take responsibility for a message, in DKIM allows an organization to take responsibility for a message in a
a way that can be verified by a recipient. The organization can be a way that can be verified by a recipient. The organization can be a
direct handler of the message, such as the author's, the originating direct handler of the message, such as the author's, the originating
sending site's or an intermediary's along the transit path. However sending site's, or an intermediary's along the transit path.
it can also be and indirect handler, such as an independent service However, it can also be an indirect handler, such as an independent
that is providing assistance to a direct handler. DKIM defines a service that is providing assistance to a direct handler. DKIM
domain-level digital signature authentication framework for email defines a domain-level digital signature authentication framework for
through the use of public-key cryptography and using the domain name email through the use of public-key cryptography and using the domain
service as its key server technology. [RFC4871] It permits name service as its key server technology [RFC4871]. It permits
verification of the signer of a message, as well as the integrity of verification of the signer of a message, as well as the integrity of
its contents. DKIM will also provide a mechanism that permits its contents. DKIM will also provide a mechanism that permits
potential email signers to publish information about their email potential email signers to publish information about their email
signing practices; this will permit email receivers to make signing practices; this will permit email receivers to make
additional assessments of unsigned messages. DKIM's authentication additional assessments of unsigned messages. DKIM's authentication
of email identity can assist in the global control of "spam" and of email identity can assist in the global control of "spam" and
"phishing". "phishing".
Neither this document nor DKIM attempts to provide solutions to the Neither this document nor DKIM attempts to provide solutions to the
world's problems with spam, phishing, viruses, worms, joe jobs, etc. world's problems with spam, phishing, viruses, worms, joe jobs, etc.
DKIM provides one basic tool, in what needs to be a large arsenal, DKIM provides one basic tool, in what needs to be a large arsenal,
for improving basic trust in the Internet mail service. However by for improving basic trust in the Internet mail service. However, by
itself, DKIM is not sufficient to that task and this overview does itself, DKIM is not sufficient to that task and this overview does
not pursue the issues of integrating DKIM into these larger efforts, not pursue the issues of integrating DKIM into these larger efforts,
beyond a simple reference within a system diagram. Rather, it is a beyond a simple reference within a system diagram. Rather, it is a
basic introduction to the technology and its use. basic introduction to the technology and its use.
1.1. DKIM's Scope 1.1. DKIM's Scope
A person or organization has an "identity" -- that is, a A person or organization has an "identity" -- that is, a
constellation of characteristics that distinguish them from any other constellation of characteristics that distinguish them from any other
identity. Associated with this abstraction can be a label used as a identity. Associated with this abstraction can be a label used as a
reference, or "identifier". This is the distinction between a thing reference, or "identifier". This is the distinction between a thing
and the name of the thing. DKIM uses a domain name as an identifier, and the name of the thing. DKIM uses a domain name as an identifier,
to refer to the identity of a responsible person or organization. In to refer to the identity of a responsible person or organization. In
DKIM, this identifier is called the Signing Domain IDentifier (SDID) DKIM, this identifier is called the Signing Domain IDentifier (SDID)
and is contained in the DKIM-Signature header fields "d=" tag. Note and is contained in the DKIM-Signature header fields "d=" tag. Note
that the same identity can have multiple identifiers. that the same identity can have multiple identifiers.
A DKIM signature can be created by a direct handler of a message, A DKIM signature can be created by a direct handler of a message,
such as the message's author or an intermediary. A signature also such as the message's author or by an intermediary. A signature also
can be created by an independent service that is providing assistance can be created by an independent service that is providing assistance
to a handler of the message. Whoever does the signing chooses the to a handler of the message. Whoever does the signing chooses the
SDID to be used as the basis for later assessments. Hence, the SDID to be used as the basis for later assessments. Hence, the
reputation associated with that domain name might be an additional reputation associated with that domain name might be an additional
basis for evaluating whether to trust the message for delivery. The basis for evaluating whether to trust the message for delivery. The
owner of the SDID is declaring that they accept responsibility for owner of the SDID is declaring that they accept responsibility for
the message and can thus be held accountable for it. the message and can thus be held accountable for it.
DKIM is intended as a value-added feature for email. Mail that is DKIM is intended as a value-added feature for email. Mail that is
not signed by DKIM is handled in the same way as it was before DKIM not signed by DKIM is handled in the same way as it was before DKIM
was defined. The message will be evaluated by established analysis was defined. The message will be evaluated by established analysis
and filtering techniques. (A signing policy can provide additional and filtering techniques. (A signing policy can provide additional
information for that analysis and filtering.) Over time, widespread information for that analysis and filtering.) Over time, widespread
DKIM adoption could permit more strict handling of messages that are DKIM adoption could permit stricter handling of messages that are not
not signed. However early benefits do not require this and probably signed. However, early benefits do not require this and probably do
do not warrant this. not warrant this.
DKIM has a narrow scope. It is an enabling technology, intended for DKIM has a narrow scope. It is an enabling technology, intended for
use in the larger context of determining message legitimacy. This use in the larger context of determining message legitimacy. This
larger context is complex, so it is easy to assume that a component larger context is complex, so it is easy to assume that a component
like DKIM, which actually provides only a limited service, instead like DKIM, which actually provides only a limited service, instead
satisfies the broader set of requirements. satisfies the broader set of requirements.
By itself, a DKIM signature: By itself, a DKIM signature:
o Does not authenticate or verify the contents of the message header o Does not authenticate or verify the contents of the message header
skipping to change at page 5, line 51 skipping to change at page 5, line 8
integrity between the time of signing and the time of verifying. integrity between the time of signing and the time of verifying.
o Does not offer any assertions about the behaviors of the signer. o Does not offer any assertions about the behaviors of the signer.
o Does not prescribe any specific actions for receivers to take upon o Does not prescribe any specific actions for receivers to take upon
successful signature verification. successful signature verification.
o Does not provide protection after signature verification. o Does not provide protection after signature verification.
o Does not protect against re-sending (replay of) a message that o Does not protect against re-sending (replay of) a message that
already has a verified signature; therefore a transit intermediary already has a verified signature; therefore, a transit
or a recipient can re-post the message -- that is, post it as a intermediary or a recipient can re-post the message -- that is,
new message -- with the original signature remaining verifiable, post it as a new message -- with the original signature remaining
even though the new recipient(s) might be different from those who verifiable, even though the new recipient(s) might be different
were originally specified by the author. from those who were originally specified by the author.
1.2. Prior Work 1.2. Prior Work
Historically, the IP Address of the system that directly sent the Historically, the IP Address of the system that directly sent the
message -- that is, the previous email "hop" -- has been treated as message -- that is, the previous email "hop" -- has been treated as
an identity to use for making assessments. For example, see an identity to use for making assessments. For example, see
[RFC4408], [RFC4406] and [RFC4407] for some current uses of the [RFC4408], [RFC4406], and [RFC4407] for some current uses of the
sending system's IP address. The IP Address is obtained via sending system's IP Address. The IP Address is obtained via
underlying Internet information mechanisms and is therefore trusted underlying Internet information mechanisms and is therefore trusted
to be accurate. Besides having some known security weaknesses, the to be accurate. Besides having some known security weaknesses, the
use of addresses presents a number of functional and operational use of addresses presents a number of functional and operational
problems. Consequently there is a widespread desire to use an problems. Consequently, there is a widespread desire to use an
identifier that has better correspondence to organizational identifier that has better correspondence to organizational
boundaries. Domain names can satisfy this need. boundaries. Domain names can satisfy this need.
There have been four previous IETF Internet Mail signature standards. There have been four previous IETF Internet Mail signature standards.
Their goals have differed from those of DKIM. PEM and MOSS are only Their goals have differed from those of DKIM. PEM and MOSS are only
of historical interest. of historical interest.
o Privacy Enhanced Mail (PEM) was first published in 1987. o Privacy Enhanced Mail (PEM) was first published in 1987 [RFC0989].
[RFC0989]
o Pretty Good Privacy (PGP) was developed by Phil Zimmermann and o Pretty Good Privacy (PGP) was developed by Phil Zimmermann and
first released in 1991. A later version was standardized as first released in 1991. A later version was standardized as
OpenPGP. [RFC1991] [RFC2440] [RFC3156] [RFC4880] OpenPGP [RFC1991] [RFC2440] [RFC3156] [RFC4880].
o PEM eventually transformed into MIME Object Security Services o PEM eventually transformed into MIME Object Security Services
(MOSS) in 1995. [RFC1848] (MOSS) in 1995 [RFC1848].
o RSA Security independently developed Secure MIME (S/MIME) to o RSA Security independently developed Secure MIME (S/MIME) to
transport a PKCS #7 data object. It was standardized as transport a Public Key Cryptographic System (PKCS) #7 data object.
[RFC3851]. It was standardized as [RFC3851].
Development of both S/MIME and OpenPGP has continued. While each has Development of both S/MIME and OpenPGP has continued. While each has
achieved a significant user base, neither one has achieved ubiquity achieved a significant user base, neither one has achieved ubiquity
in deployment or use. in deployment or use.
To the extent that other message-signing services might have been To the extent that other message-signing services might have been
adapted to do the job that DKIM is designed to perform, it was felt adapted to do the job that DKIM is designed to perform, it was felt
that re-purposing any of those would be more problematic than that repurposing any of those would be more problematic than creating
creating a separate service. That said, DKIM only uses cryptographic a separate service. That said, DKIM only uses cryptographic
components that have a long history, including use within some of components that have a long history, including use within some of
those other messaging security services. those other messaging security services.
DKIM is differentiated by its reliance on an identifier that is DKIM is differentiated by its reliance on an identifier that is
specific to DKIM use. specific to DKIM use.
DKIM also has a distinctive approach for distributing and vouching DKIM also has a distinctive approach for distributing and vouching
for keys. It uses a key-centric public key management scheme, rather for keys. It uses a key-centric, public-key management scheme,
than the more typical approaches based on a certificate in the styles rather than the more typical approaches based on a certificate in the
of Kohnfelder (X.509) [Kohnfelder] or Zimmermann (web of trust) styles of Kohnfelder (X.509) [Kohnfelder] or Zimmermann (web of
[WebofTrust]. For DKIM, the owner of the SDID asserts the validity trust) [WebofTrust]. For DKIM, the owner of the SDID asserts the
of a key, rather than having the validity of the key attested to by a validity of a key, rather than having the validity of the key
trusted third party, often including other assertions, such as a attested to by a trusted third party, often including other
quality assessment of the key's owner. DKIM treats quality assertions, such as a quality assessment of the key's owner. DKIM
assessment as an independent, value-added service, beyond the initial treats quality assessment as an independent, value-added service,
work of deploying a signature verification service. beyond the initial work of deploying a signature verification
service.
Further, DKIM's key management is provided by adding information Further, DKIM's key management is provided by adding information
records to the existing Domain Name System (DNS) [RFC1034], rather records to the existing Domain Name System (DNS) [RFC1034], rather
than requiring deployment of a new query infrastructure. This than requiring deployment of a new query infrastructure. This
approach has significant operational advantages. First, it avoids approach has significant operational advantages. First, it avoids
the considerable barrier of creating a new global infrastructure; the considerable barrier of creating a new global infrastructure;
hence it leverages a global base of administrative experience and hence, it leverages a global base of administrative experience and
highly reliable distributed operation. Second, the technical aspect highly reliable distributed operation. Second, the technical aspect
of the DNS is already known to be efficient. Any new service would of the DNS is already known to be efficient. Any new service would
have to undergo a period of gradual maturation, with potentially have to undergo a period of gradual maturation, with potentially
problematic early-stage behaviors. By (re-)using the DNS, DKIM problematic early-stage behaviors. By (re-)using the DNS, DKIM
avoids these growing pains. avoids these growing pains.
1.3. Internet Mail Background 1.3. Internet Mail Background
The basic Internet Email service has evolved extensively over its The basic Internet email service has evolved extensively over its
several decades of continuous operation. Its modern architecture several decades of continuous operation. Its modern architecture
comprises a number of specialized components. A discussion about comprises a number of specialized components. A discussion about
Mail User Agents (MUA), Mail Handling Services (MHS), Mail Transfer Mail User Agents (MUAs), Mail Handling Services (MHSs), Mail Transfer
Agents (MTA), Mail Submission Agents (MSA), Mail Delivery Agents Agents (MTAs), Mail Submission Agents (MSAs), Mail Delivery Agents
(MDA), Mail Service Providers (MSP), Administrative Management (MDAs), Mail Service Providers (MSPs), Administrative Management
Domains (ADMDs), Mediators, and their relationships can be found in Domains (ADMDs), Mediators, and their relationships can be found in
Appendix A. Appendix A.
1.4. Discussion Venue
NOTE TO RFC EDITOR: This "Discussion Venue" section is to be
removed prior to publication.
This document is being discussed on the DKIM mailing list,
ietf-dkim@mipassoc.org.
1.4.1. Changes to document
In addition to simple wordsmithing, the following substantive changes
were made:
Service Arch figure and text: (per Allman) Existing figure and text
carries vestigial references to role of MSA and MDA. New text
switches focus to ADMD more completely and merely cites possible
functional modules within them.
Identity vs. Identifier: Added text in Scope to define terms and
their relationship.
Message Validity: Added section discussing restricted implication
of this.
2. The DKIM Value Proposition 2. The DKIM Value Proposition
The nature and origins of a message often are falsely stated. Such The nature and origins of a message often are falsely stated. Such
misrepresentations may be employed for legitimate reasons or for misrepresentations may be employed for legitimate or nefarious
nefarious reasons. DKIM provides a foundation for distinguishing reasons. DKIM provides a foundation for distinguishing legitimate
legitimate mail, and thus a means of associating a verifiable mail, and thus a means of associating a verifiable identifier with a
identifier with a message. Given the presence of that identifier, a message. Given the presence of that identifier, a receiver can make
receiver can make decisions about further handling of the message, decisions about further handling of the message, based upon
based upon assessments of the identity that is associated with the assessments of the identity that is associated with the identifier.
identifier.
Receivers who successfully verify a signature can use information Receivers who successfully verify a signature can use information
about the signer as part of a program to limit spam, spoofing, about the signer as part of a program to limit spam, spoofing,
phishing, or other undesirable behavior. DKIM does not, itself, phishing, or other undesirable behaviors. DKIM does not, itself,
prescribe any specific actions by the recipient; rather it is an prescribe any specific actions by the recipient; rather, it is an
enabling technology for services that do. enabling technology for services that do.
These services will typically: These services will typically:
1. Determine a verified identity as taking responsibility for the 1. Determine a verified identity as taking responsibility for the
message, if possible. message, if possible.
2. Evaluate the trustworthiness of this/these identities. 2. Evaluate the trustworthiness of this/these identities.
The role of DKIM is to perform the first of these; DKIM is an enabler The role of DKIM is to perform the first of these; DKIM is an enabler
for the second. for the second.
2.1. Identity Verification 2.1. Identity Verification
Consider an attack made against an organization or against customers Consider an attack made against an organization or against customers
of an organization. The name of the organization is linked to a of an organization. The name of the organization is linked to
particular Internet domain names (identifiers). Attackers can particular Internet domain names (identifiers). Attackers can
leverage either using a legitimate domain name, without leverage using either a legitimate domain name, one without
authorization, or a "cousin" name that is similar to one that is authorization, or a "cousin" name that is similar to one that is
legitimate, but is not controlled by the target organization. An legitimate, but is not controlled by the target organization. An
assessment service that uses DKIM can differentiate between a domain assessment service that uses DKIM can differentiate between a domain
(SDID) used by a known organization and a domain used by others. As (SDID) used by a known organization and a domain used by others. As
such, DKIM performs the positive step of identifying messages such, DKIM performs the positive step of identifying messages
associated with verifiable identities, rather than the negative step associated with verifiable identities, rather than the negative step
of identifying messages with problematic use of identities. Whether of identifying messages with problematic use of identities. Whether
a verified identity belongs to a Good Actor or a Bad Actor is a a verified identity belongs to a Good Actor or a Bad Actor is a
question for later stages of assessment. question for later stages of assessment.
2.2. Enabling Trust Assessments 2.2. Enabling Trust Assessments
Email receiving services are faced with a basic decision: Whether to Email receiving services are faced with a basic decision: whether to
accept and deliver a newly-arrived message to the indicated accept and deliver a newly arrived message to the indicated
recipient? That is, does the receiving service trust that the recipient? That is, does the receiving service trust that the
message is sufficiently "safe" to be viewed? For the modern message is sufficiently "safe" to be viewed? For the modern
Internet, most receiving services have an elaborate engine that Internet, most receiving services have an elaborate engine that
formulates this quality assessment. These engines take a variety of formulates this quality assessment. These engines take a variety of
information as input to the decision, such as from reputation lists information as input to the decision, such as from reputation lists
and accreditation services. As the engine processes information, it and accreditation services. As the engine processes information, it
raises or lowers its trust assessment for the message. raises or lowers its trust assessment for the message.
In order to formulate reputation information, an accurate, stable In order to formulate reputation information, an accurate, stable
identifier is needed. Otherwise, the information might not pertain identifier is needed. Otherwise, the information might not pertain
to the identified organization's own actions. When using an IP to the identified organization's own actions. When using an IP
Address, accuracy is based on the belief that the underlying Internet Address, accuracy is based on the belief that the underlying Internet
infrastructure supplies an accurate address. When using domain based infrastructure supplies an accurate address. When using domain-based
reputation data, some other form of verification is needed, since it reputation data, some other form of verification is needed, since it
is not supplied independently by the infrastructure. is not supplied independently by the infrastructure.
DKIM satisfies this requirement by declaring a valid "responsible" DKIM satisfies this requirement by declaring a valid "responsible"
identity -- referenced through the SDID -- about which the engine can identity -- referenced through the SDID -- about which the engine can
make quality assessments and by using a digital signature to ensure make quality assessments and by using a digital signature to ensure
that use of the identifier is authorized. However by itself, a valid that use of the identifier is authorized. However, by itself, a
DKIM signature neither lowers nor raises the level of trust valid DKIM signature neither lowers nor raises the level of trust
associated with the message, but it enables other mechanisms to be associated with the message, but it enables other mechanisms to be
used for doing so. used for doing so.
An organization might build upon its use of DKIM by publishing An organization might build upon its use of DKIM by publishing
information about its Signing Practices (SP). This could permit information about its Signing Practices (SP). This could permit
detecting some messages that purport to be associated with a domain, detecting some messages that purport to be associated with a domain,
but which are not. As such, an SP can cause the trust assessment to but which are not. As such, an SP can cause the trust assessment to
be reduced, or leave it unchanged. be reduced, or leave it unchanged.
2.3. Establishing Message Validity 2.3. Establishing Message Validity
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email transfer infrastructure. That is, there can be multiple email email transfer infrastructure. That is, there can be multiple email
relaying hops between signing and verifying. Hence, it defines a relaying hops between signing and verifying. Hence, it defines a
mechanism that only needs to be supported by the signer and the mechanism that only needs to be supported by the signer and the
verifier, rather than any of the functional components along the verifier, rather than any of the functional components along the
handling path. This motivates functional goals about the handling path. This motivates functional goals about the
authentication itself and operational goals about its integration authentication itself and operational goals about its integration
with the rest of the Internet email service. with the rest of the Internet email service.
3.1. Functional Goals 3.1. Functional Goals
3.1.1. Use Domain-level granularity for assurance 3.1.1. Use Domain-Level Granularity for Assurance
DKIM provides accountability at the coarse granularity of an DKIM provides accountability at the coarse granularity of an
organization or, perhaps, a department. An existing construct that organization or, perhaps, a department. An existing construct that
enables this granularity is the Domain Name [RFC1034]. DKIM binds a enables this granularity is the Domain Name [RFC1034]. DKIM binds a
signing key record to a Domain Name, as the SDID. Further benefits signing key record to a Domain Name as the SDID. Further benefits of
of using domain names include simplifying key management, enabling using domain names include simplifying key management, enabling
signing by the infrastructure as opposed to the MUA, and reducing signing by the infrastructure as opposed to the MUA, and reducing
privacy concerns. privacy concerns.
Contrast this with OpenPGP and S/MIME, which associate verification Contrast this with OpenPGP and S/MIME, which associate verification
with individual authors, using their full email addresses. with individual authors, using their full email addresses.
3.1.2. Implementation Locality 3.1.2. Implementation Locality
Any party, anywhere along the transit path can implement DKIM Any party, anywhere along the transit path, can implement DKIM
signing. Its use is not confined to particular systems, such as the signing. Its use is not confined to particular systems, such as the
author's MUA or the inbound boundary MTA, and there can be more than author's MUA or the inbound boundary MTA, and there can be more than
one signature per message. one signature per message.
3.1.3. Allow delegation of signing to independent parties 3.1.3. Allow Delegation of Signing to Independent Parties
Different parties have different roles in the process of email Different parties have different roles in the process of email
exchange. Some are easily visible to end users and others are exchange. Some are easily visible to end users and others are
primarily visible to operators of the service. DKIM was designed to primarily visible to operators of the service. DKIM was designed to
support signing by any of these different parties and to permit them support signing by any of these different parties and to permit them
to sign with any domain name that they deem appropriate (and for to sign with any domain name that they deem appropriate (and for
which they hold authorized signing keys.) As an example an which they hold authorized signing keys). As an example, an
organization that creates email content often delegates portions of organization that creates email content often delegates portions of
its processing or transmission to an outsourced group. DKIM supports its processing or transmission to an outsourced group. DKIM supports
this mode of activity, in a manner that is not normally visible to this mode of activity, in a manner that is not normally visible to
end users. Similarly, a reputation provider can delegate a signing end users. Similarly, a reputation provider can delegate a signing
key for a domain under the control of the provider, to be used by an key for a domain under the control of the provider, to be used by an
organization the provider is prepared to vouch for. organization for which the provider is prepared to vouch.
3.1.4. Distinguish the core authentication mechanism from its 3.1.4. Distinguish the Core Authentication Mechanism from Its
derivative uses Derivative Uses
An authenticated identity can be subject to a variety of assessment An authenticated identity can be subject to a variety of assessment
policies, either ad hoc or standardized. DKIM separates basic policies, either ad hoc or standardized. DKIM separates basic
authentication from assessment. The only semantics inherent to a authentication from assessment. The only semantics inherent to a
DKIM signature are that the signer is asserting some kind of DKIM signature are that the signer is asserting some kind of
responsibility for the message. Any interpretation of this kind of responsibility for the message. Any interpretation of this kind of
responsibility is the job of services building on DKIM, but the responsibility is the job of services building on DKIM, but the
details are beyond the scope of that core. One such mechanism might details are beyond the scope of that core. One such mechanism might
assert a relationship between the SDID and the author, as specified assert a relationship between the SDID and the author, as specified
in the rfc5322.From: header field's domain identity. Another might in the rfc5322.From: header field's domain identity. Another might
specify how to treat an unsigned message with that rfc5322.From: specify how to treat an unsigned message with that rfc5322.From:
field domain. field domain.
3.1.5. Retain ability to have anonymous email 3.1.5. Retain Ability to Have Anonymous Email
The ability to send a message that does not identify its author is The ability to send a message that does not identify its author is
considered to be a valuable quality of the current email service that considered to be a valuable quality of the current email service that
needs to be retained. DKIM is compatible with this goal since it needs to be retained. DKIM is compatible with this goal since it
permits authentication of the email system operator, rather than the permits authentication of the email system operator, rather than the
content author. If it is possible to obtain effectively anonymous content author. If it is possible to obtain effectively anonymous
accounts at example.com, knowing that a message definitely came from accounts at example.com, knowing that a message definitely came from
example.com does not threaten the anonymity of the user who authored example.com does not threaten the anonymity of the user who authored
it. it.
3.2. Operational Goals 3.2. Operational Goals
3.2.1. Make presence of signature transparent to non-supporting 3.2.1. Make Presence of Signature Transparent to Non-Supporting
recipients Recipients
In order to facilitate incremental adoption, DKIM is designed to be In order to facilitate incremental adoption, DKIM is designed to be
transparent to recipients that do not support it. A DKIM signature transparent to recipients that do not support it. A DKIM signature
does not "get in the way" for such recipients. does not "get in the way" for such recipients.
Contrast this with S/MIME and OpenPGP, which modify the message body. Contrast this with S/MIME and OpenPGP, which modify the message body.
Hence, their presence is potentially visible to email recipients, Hence, their presence is potentially visible to email recipients,
whose user software needs to process the associated constructs. whose user software needs to process the associated constructs.
3.2.2. Treat verification failure the same as no signature present 3.2.2. Treat Verification Failure the Same as No Signature Present
DKIM must also be transparent to existing assessment mechanisms. DKIM must also be transparent to existing assessment mechanisms.
Consequently, a DKIM signature verifier is to treat messages with Consequently, a DKIM signature verifier is to treat messages with
signatures that fail as if they were unsigned. Hence the message signatures that fail as if they were unsigned. Hence, the message
will revert to normal handling, through the receiver's existing will revert to normal handling, through the receiver's existing
filtering mechanisms. Thus, DKIM specifies that an assessing site is filtering mechanisms. Thus, DKIM specifies that an assessing site is
not to take a message that has a broken signature and treat it any not to take a message that has a broken signature and treat it any
differently than if the signature weren't there. differently than if the signature weren't there.
Contrast this with OpenPGP and S/MIME, which were designed for strong Contrast this with OpenPGP and S/MIME, which were designed for strong
cryptographic protection. This included treating verification cryptographic protection. This included treating verification
failure as message failure. failure as message failure.
3.2.3. Permit incremental adoption for incremental benefit 3.2.3. Permit Incremental Adoption for Incremental Benefit
DKIM can be used by any two organizations that exchange email and DKIM can be used by any two organizations that exchange email and
implement DKIM; it does not require adoption within the open implement DKIM; it does not require adoption within the open
Internet's email infrastructure. In the usual manner of "network Internet's email infrastructure. In the usual manner of "network
effects", the benefits of DKIM increase as its adoption increases. effects", the benefits of DKIM increase as its adoption increases.
Although this mechanism can be used in association with independent Although this mechanism can be used in association with independent
assessment services, such services are not essential in order to assessment services, such services are not essential in order to
obtain initial benefit. For example DKIM allows (possibly large) obtain initial benefit. For example, DKIM allows (possibly large)
pairwise sets of email providers and spam filtering companies to pairwise sets of email providers and spam filtering companies to
distinguish mail that is associated with a known organization, versus distinguish mail that is associated with a known organization, versus
mail that might deceptively purport to have the affiliation. This in mail that might deceptively purport to have the affiliation. This in
turn allows the development of "whitelist" schemes whereby turn allows the development of "whitelist" schemes whereby
authenticated mail from a known source with good reputation is authenticated mail from a known source with good reputation is
allowed to bypass some anti-abuse filters. allowed to bypass some anti-abuse filters.
In effect the email receiver can use their set of known relationships In effect, the email receiver can use their set of known
to generate their own reputation data. This works particularly well relationships to generate their own reputation data. This works
for traffic between large sending providers and large receiving particularly well for traffic between large sending providers and
providers. However it also works well for any operator, public or large receiving providers. However, it also works well for any
private, that has mail traffic dominated by exchanges among a stable operator, public or private, that has mail traffic dominated by
set of organizations. exchanges among a stable set of organizations.
Management of email delivery problems currently represents a Management of email delivery problems currently represents a
significant pain point for email administrators at every point on the significant pain point for email administrators at every point on the
mail transit path. Administrators who have deployed DKIM mail transit path. Administrators who have deployed DKIM
verification have an incentive to encourage senders (who might verification have an incentive to encourage senders (who might
subsequently complain that their email is not being delivered) to use subsequently complain that their email is not being delivered) to use
DKIM signatures. DKIM signatures.
3.2.4. Minimize the amount of required infrastructure 3.2.4. Minimize the Amount of Required Infrastructure
In order to allow early adopters to gain early benefit, DKIM makes no In order to allow early adopters to gain early benefit, DKIM makes no
changes to the core Internet Mail service and, instead, can provide a changes to the core Internet Mail service and, instead, can provide a
useful benefit for any individual pair of signers and verifiers who useful benefit for any individual pair of signers and verifiers who
are exchanging mail. Similarly, DKIM's reliance on the Domain Name are exchanging mail. Similarly, DKIM's reliance on the Domain Name
System greatly reduces the amount of new administrative System greatly reduces the amount of new administrative
infrastructure that is needed across the open Internet. infrastructure that is needed across the open Internet.
3.2.5. Permit a wide range of deployment choices 3.2.5. Permit a Wide Range of Deployment Choices
DKIM can be deployed at a variety of places within an organization's DKIM can be deployed at a variety of places within an organization's
email service. This affords flexibility in terms of who administers email service. This affords flexibility in terms of who administers
its use, as well as what traffic carries a DKIM signature. For its use, as well as what traffic carries a DKIM signature. For
example, employing DKIM at an outbound boundary MTA will mean that it example, employing DKIM at an outbound boundary MTA will mean that it
is administered by the organization's central IT department and that is administered by the organization's central IT department and that
internal messages are not signed. internal messages are not signed.
4. DKIM Function 4. DKIM Function
DKIM has a very constrained set of capabilities, primarily targeting DKIM has a very constrained set of capabilities, primarily targeting
email while it is in transit from an author to a set of recipients. email while it is in transit from an author to a set of recipients.
It associates verifiable information with a message, especially a It associates verifiable information with a message, especially a
responsible identity. When a message does not have a valid signature responsible identity. When a message does not have a valid signature
associated with the author, a DKIM SP will permit the domain name of associated with the author, a DKIM SP will permit the domain name of
the author to be used for obtaining information about their signing the author to be used for obtaining information about their signing
practices. practices.
4.1. Basic Signing 4.1. Basic Signing
With the DKIM signature mechanism, a signer chooses a SDID, performs With the DKIM signature mechanism, a signer chooses an SDID, performs
digital signing on the message, and adds the signature information digital signing on the message, and adds the signature information
using a DKIM header field. A verifier obtains the domain name and using a DKIM header field. A verifier obtains the domain name and
the "selector" from the DKIM header field, obtains the public key the "selector" from the DKIM header field, obtains the public key
associated with the name, and verifies the signature. associated with the name, and verifies the signature.
DKIM permits any domain name to be used as the SDID, and supports DKIM permits any domain name to be used as the SDID, and supports
extensible choices for various algorithms. As is typical for extensible choices for various algorithms. As is typical for
Internet standards, there is a core set of algorithms that all Internet standards, there is a core set of algorithms that all
implementations are required to support, in order to guarantee basic implementations are required to support, in order to guarantee basic
interoperability. interoperability.
DKIM permits restricting the use of a signature key to signing DKIM permits restricting the use of a signature key to signing
messages for particular types of services, such as only for a single messages for particular types of services, such as only for a single
source of email. This is intended to be helpful when delegating source of email. This is intended to be helpful when delegating
signing authority, such as to a particular department or to a third- signing authority, such as to a particular department or to a third-
party outsourcing service. party outsourcing service.
With DKIM the signer explicitly lists the headers that are signed, With DKIM, the signer explicitly lists the headers that are signed,
such as From:, Date: and Subject:. By choosing the minimal set of such as From:, Date:, and Subject:. By choosing the minimal set of
headers needed, the signature is likely to be considerably more headers needed, the signature is likely to be considerably more
robust against the handling vagaries of intermediary MTAs. robust against the handling vagaries of intermediary MTAs.
4.2. Characteristics of a DKIM Signature 4.2. Characteristics of a DKIM Signature
A DKIM signature applies to the message body and selected header A DKIM signature applies to the message body and selected header
fields. The signer computes a hash of the selected header fields and fields. The signer computes a hash of the selected header fields and
another hash of the body. The signer then uses a private key to another hash of the body. The signer then uses a private key to
cryptographically encode this information, along with other signing cryptographically encode this information, along with other signing
parameters. Signature information is placed into DKIM-Signature:, a parameters. Signature information is placed into DKIM-Signature:, a
new [RFC5322] message header field. new [RFC5322] message header field.
4.3. The Selector Construct 4.3. The Selector Construct
The key for a signature is associated with an SDID. That domain name The key for a signature is associated with an SDID. That domain name
provides the complete identity used for making assessments about the provides the complete identity used for making assessments about the
signer. (The DKIM specification does not give any guidance on how to signer. (The DKIM specification does not give any guidance on how to
do an assessment.) However this name is not sufficient for making a do an assessment.) However, this name is not sufficient for making a
DNS query to obtain the key needed to verify the signature. DNS query to obtain the key needed to verify the signature.
A single SDID can have multiple signing keys and/or multiple A single SDID can have multiple signing keys and/or multiple
potential signers. To support this, DKIM identifies a particular potential signers. To support this, DKIM identifies a particular
signature as using a combination of the SDID and an added field, signature as using a combination of the SDID and an added field,
called the "selector", specified in a separate DKIM-Signature: header called the "selector", specified in a separate DKIM-Signature: header
field parameter. field parameter.
NOTE: The semantics of the selector (if any) are strictly reserved NOTE: The semantics of the selector (if any) are strictly reserved
to the signer and is to be treated as an opaque string by all to the signer and is to be treated as an opaque string by all
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mechanism for making a transition from an old, or compromised, key mechanism for making a transition from an old, or compromised, key
to a new one. to a new one.
4.4. Verification 4.4. Verification
After a message has been signed, any agent in the message transit After a message has been signed, any agent in the message transit
path can verify the signature to determine that the owner of the SDID path can verify the signature to determine that the owner of the SDID
took responsibility for the message. Message recipients can verify took responsibility for the message. Message recipients can verify
the signature by querying the DNS for the signer's domain directly, the signature by querying the DNS for the signer's domain directly,
to retrieve the appropriate public key, and thereby confirm that the to retrieve the appropriate public key, and thereby confirm that the
message was signed to by a party in possession of the private key for message was signed by a party in possession of the private key for
the SDID. Typically, verification will be done by an agent in the the SDID. Typically, verification will be done by an agent in the
Administrative Management Domain (ADMD) of the message recipient. Administrative Management Domain (ADMD) of the message recipient.
4.5. Sub-Domain Assessment 4.5. Sub-Domain Assessment
Signers often need to support multiple assessments about their Signers often need to support multiple assessments about their
organization, such as to distinguish one type of message from organization, such as to distinguish one type of message from
another, or one portion of the organization from another. To permit another, or one portion of the organization from another. To permit
assessments that are independent, one method is for an organization assessments that are independent, one method is for an organization
to use different sub-domains as the SDID tag, such as to use different sub-domains as the SDID tag, such as
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. | +-------+-------+ . . | +-------+-------+ .
. | | . . | | .
. | V . . | V .
+----+--------+ | +-----------+ +------+-----+ +----+--------+ | +-----------+ +------+-----+
|Reputation/ | | | Message | | Local Info | |Reputation/ | | | Message | | Local Info |
|Accreditation| +----------->| Filtering | | on Sender | |Accreditation| +----------->| Filtering | | on Sender |
|Info | | Engine | | Practices | |Info | | Engine | | Practices |
+-------------+ +-----------+ +------------+ +-------------+ +-----------+ +------------+
Figure 1: DKIM Service Architecture Figure 1: DKIM Service Architecture
As shown in Figure 1, basic message processing is divided between a As shown in Figure 1, basic message processing is divided between a
signing Administrative Management Domain (ADMD) and a verifying ADMD. signing Administrative Management Domain (ADMD) and a verifying ADMD.
At its simplest, this is between the Originating ADMD and the At its simplest, this is between the originating ADMD and the
delivering ADMD, but can involve other ADMDs in the handling path. delivering ADMD, but can involve other ADMDs in the handling path.
Signing: Signing is performed by an authorized module within the signing: Signing is performed by an authorized module within the
signing ADMD and uses private information from the Key Store, as signing ADMD and uses private information from the Key Store, as
discussed below. Within the originating ADMD, this might be discussed below. Within the originating ADMD, this might be
performed by the MUA, MSA or an MTA. performed by the MUA, MSA, or an MTA.
verifying: verifying is performed by an authorized module within verifying: verifying is performed by an authorized module within
the verifying ADMD. Within a delivering ADMD, verifying might be the verifying ADMD. Within a delivering ADMD, verifying might be
performed by an MTA, MDA or MUA. The module verifies the performed by an MTA, MDA, or MUA. The module verifies the
signature or determines whether a particular signature was signature or determines whether a particular signature was
required. Verifying the signature uses public information from required. Verifying the signature uses public information from
the Key Store. If the signature passes, reputation information is the Key Store. If the signature passes, reputation information is
used to assess the signer and that information is passed to the used to assess the signer and that information is passed to the
message filtering system. If the signature fails or there is no message filtering system. If the signature fails or there is no
signature using the author's domain, information about signing signature using the author's domain, information about signing
practices related to the author can be retrieved remotely and/or practices related to the author can be retrieved remotely and/or
locally, and that information is passed to the message filtering locally, and that information is passed to the message filtering
system. system.
If message has more than one valid signature, the order in which the If a message has more than one valid signature, the order in which
signers are assessed and the interactions among the assessments are the signers are assessed and the interactions among the assessments
not defined by the DKIM specification. are not defined by the DKIM specification.
5.1. Administration and Maintenance 5.1. Administration and Maintenance
A number of tables and services are used to provide external A number of tables and services are used to provide external
information. Each of these introduces administration and maintenance information. Each of these introduces administration and maintenance
requirements. requirements.
Key Store: DKIM uses public/private (asymmetric) key cryptography. Key Store: DKIM uses public-/private-key (asymmetric) cryptography.
The signer users a private key and the verifier uses the The signer users a private key and the verifier uses the
corresponding public key. The current DKIM Signing specification corresponding public key. The current DKIM Signing specification
provides for querying the Domain Names Service (DNS), to permit a provides for querying the Domain Names Service (DNS), to permit a
verifier to obtain the public key. The signing organization verifier to obtain the public key. The signing organization
therefore needs to have a means of adding a key to the DNS, for therefore needs to have a means of adding a key to the DNS, for
every selector/SDID combination. Further, the signing every selector/SDID combination. Further, the signing
organization needs policies for distributing and revising keys. organization needs policies for distributing and revising keys.
Reputation/Accreditation: If a message contains a valid signature, Reputation/Accreditation: If a message contains a valid signature,
then the verifier can evaluate the associated domain name's then the verifier can evaluate the associated domain name's
reputation, in order to determine appropriate delivery or display reputation, in order to determine appropriate delivery or display
options for that message. Quality assessment information, which options for that message. Quality assessment information, which
is associated with a domain name, comes in many forms and from is associated with a domain name, comes in many forms and from
many sources. DKIM does not define assessment services. Its many sources. DKIM does not define assessment services. Its
relevance to them is to provide a verified domain name, upon which relevance to them is to provide a verified domain name, upon which
assessments can be made. assessments can be made.
Signing Practices (SP): Separate from determining the validity of a Signing Practices (SP): Separate from determining the validity of a
signature, and separate from assessing the reputation of the signature, and separate from assessing the reputation of the
organization that is associated with the signed identity, there is organization that is associated with the signed identity, there is
an the opportunity to determine any organizational practices an opportunity to determine any organizational practices
concerning a domain name. Practices can range widely. They can concerning a domain name. Practices can range widely. They can
be published by the owner of the domain or they can be maintained be published by the owner of the domain or they can be maintained
by the evaluating site. They can pertain to the use of the domain by the evaluating site. They can pertain to the use of the domain
name, such as whether it is used for signing messages, whether all name, such as whether it is used for signing messages, whether all
mail having that domain name in the author rfc5322.From: header mail having that domain name in the author rfc5322.From: header
field is signed, or even whether the domain owner recommends field is signed, or even whether the domain owner recommends
discarding messages in the absence of an appropriate signature. discarding messages in the absence of an appropriate signature.
The statements of practice are made at the level of a domain name, The statements of practice are made at the level of a domain name,
and are distinct from assessments made about particular messages, and are distinct from assessments made about particular messages,
as occur in a Message Filtering Engine. Such assessments of as occur in a Message Filtering Engine. Such assessments of
practices can provide useful input for the Message Filtering practices can provide useful input for the Message Filtering
Engine's determination of message handling. As practices are Engine's determination of message handling. As practices are
defined, each domain name owner needs to consider what information defined, each domain name owner needs to consider what information
to publish. The nature and degree of checking practices, if any to publish. The nature and degree of checking practices, if any
is performed, is optional to the evaluating site and is strictly a are performed, is optional to the evaluating site and is strictly
matter of local policy. a matter of local policy.
5.2. Signing 5.2. Signing
Signing can be performed by a component of the ADMD that creates the Signing can be performed by a component of the ADMD that creates the
message, and/or within any ADMD along the relay path. The signer message, and/or within any ADMD along the relay path. The signer
uses the appropriate private key that is associated with the SDID. uses the appropriate private key that is associated with the SDID.
5.3. Verifying 5.3. Verifying
Verification can be performed by any functional component along the Verification can be performed by any functional component along the
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Messages lacking a valid author signature (a signature associated Messages lacking a valid author signature (a signature associated
with the author of the message as opposed to a signature associated with the author of the message as opposed to a signature associated
with an intermediary) can prompt a query for any published "signing with an intermediary) can prompt a query for any published "signing
practices" information, as an aid in determining whether the author practices" information, as an aid in determining whether the author
information has been used without authorization. information has been used without authorization.
5.5. Assessing 5.5. Assessing
Figure 1 shows the verified identity as being used to assess an Figure 1 shows the verified identity as being used to assess an
associated reputation, but it could be applied for other tasks, such associated reputation, but it could be applied to other tasks, such
as management tracking of mail. Local policy guidelines may cause as management tracking of mail. Local policy guidelines may cause
signing practices to be checked or the message may be sent directly signing practices to be checked or the message may be sent directly
to the message filtering engine. to the message Filtering Engine.
A popular use of reputation information is as input to a filtering A popular use of reputation information is as input to a Filtering
engine that decides whether to deliver -- and possibly whether to Engine that decides whether to deliver -- and possibly whether to
specially mark -- a message. Filtering engines have become complex specially mark -- a message. Filtering Engines have become complex
and sophisticated. Their details are outside of the scope of DKIM, and sophisticated. Their details are outside of the scope of DKIM,
other than the expectation that the verified identity produced by other than the expectation that the verified identity produced by
DKIM can accumulate its own reputation, and will be added to the DKIM can accumulate its own reputation, and will be added to the
varied soup of rules used by the engines. The rules can cover signed varied soup of rules used by the engines. The rules can cover signed
messages and can deal with unsigned messages from a domain, if the messages and can deal with unsigned messages from a domain, if the
domain has published information about its practices. domain has published information about its practices.
5.6. DKIM Processing within an ADMD 5.6. DKIM Processing within an ADMD
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 infrastructures of the authoring organization's will be within the infrastructures of the authoring organization's
outbound service and the receiving organization's inbound service, outbound service and the receiving organization's inbound service,
such as a department or a boundary MTA. DKIM can be implemented in such as a department or a boundary MTA. DKIM can be implemented in
an author's or recipient's MUA, but this is expected to be less an author's or recipient's MUA, but this is expected to be less
typical, since it has higher administration and support costs. typical, since it has higher administration and support costs.
A Mediator is an MUA that receives a message and can re-post a A Mediator is an MUA that receives a message and can repost a
modified version of it, such as to a mailing list. A DKIM signature modified version of it, such as to a mailing list. A DKIM signature
can survive some types of modifications through this process. can survive some types of modifications through this process.
Furthermore the Mediator can add its own signature. This can be Furthermore, the Mediator can add its own signature. This can be
added by the Mediator software itself, or by any outbound component added by the Mediator software itself, or by any outbound component
in the Mediator's ADMD. in the Mediator's ADMD.
6. Considerations 6. Considerations
6.1. Security Considerations 6.1. Security Considerations
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], with [RFC4686] as their basis. DKIM base specification [RFC4871], with [RFC4686] as their basis.
6.2. IANA Considerations 6.2. Acknowledgements
There are no actions for IANA.
NOTE TO RFC EDITOR: This section is to be removed prior to
publication.
6.3. Acknowledgements
Many people contributed to the development of the DomainKeys Many people contributed to the development of the DomainKeys
Identified Mail and the effort of the DKIM Working Group is Identified Mail and the effort of the DKIM Working Group is
gratefully acknowledged. In particular, we would like to thank Jim gratefully acknowledged. In particular, we would like to thank Jim
Fenton for his extensive feedback diligently provided on every Fenton for his extensive feedback diligently provided on every
version of this document. version of this document.
7. Informative References 7. Informative References
[Kohnfelder] [Kohnfelder] Kohnfelder, L., "Towards a Practical Public-key
Kohnfelder, L., "Towards a Practical Public-key
Cryptosystem", May 1978. Cryptosystem", May 1978.
[RFC0989] Linn, J. and IAB Privacy Task Force, "Privacy enhancement [RFC0989] Linn, J. and IAB Privacy Task Force, "Privacy
for Internet electronic mail: Part I: Message encipherment enhancement for Internet electronic mail: Part I:
and authentication procedures", RFC 989, February 1987. Message encipherment and authentication procedures",
RFC 989, February 1987.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and
STD 13, RFC 1034, November 1987. facilities", STD 13, RFC 1034, November 1987.
[RFC1848] Crocker, S., Galvin, J., Murphy, S., and N. Freed, "MIME [RFC1113] Linn, J., "Privacy enhancement for Internet electronic
Object Security Services", RFC 1848, October 1995. mail: Part I - message encipherment and authentication
procedures", RFC 1113, August 1989.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message [RFC1848] Crocker, S., Galvin, J., Murphy, S., and N. Freed,
Exchange Formats", RFC 1991, August 1996. "MIME Object Security Services", RFC 1848,
October 1995.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP
Message Exchange Formats", RFC 1991, August 1996.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer, [RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998. "OpenPGP Message Format", RFC 2440, November 1998.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler, [RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001. "MIME Security with OpenPGP", RFC 3156, August 2001.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification", Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004. RFC 3851, July 2004.
[RFC4406] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", [RFC4406] Lyon, J. and M. Wong, "Sender ID: Authenticating
RFC 4406, April 2006. E-Mail", RFC 4406, April 2006.
[RFC4407] Lyon, J., "Purported Responsible Address in E-Mail [RFC4407] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, April 2006. Messages", RFC 4407, April 2006.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006. RFC 4408, April 2006.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006. Identified Mail (DKIM)", RFC 4686, September 2006.
[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.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and
Thayer, "OpenPGP Message Format", RFC 4880, November 2007. R. Thayer, "OpenPGP Message Format", RFC 4880,
November 2007.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008. October 2008.
[WebofTrust] [WebofTrust] Network Associates, Inc. and its Affiliated Companies,
Wikipedia, "Web of Trust", "How PGP works, in Introduction to Cryptography", 1999,
URL http://en.wikipedia.org/wiki/Web_of_trust, <http://www.pgpi.org/doc/pgpintro/>.
<http://en.wikipedia.org/wiki/Web_of_trust>.
Appendix A. Internet Mail Background Appendix A. Internet Mail Background
A.1. Core Model A.1. Core Model
Internet Mail is split between the user world, in the form of Mail Internet Mail is split between the user world, in the form of Mail
User Agents (MUA), and the transmission world, in the form of the User Agents (MUA), and the transmission world, in the form of the
Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA). Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA).
The MHS is responsible for accepting a message from one user, the The MHS is responsible for accepting a message from one user, the
author, and delivering it to one or more other users, the recipients. author, and delivering it to one or more other users, the recipients.
This creates a virtual MUA-to-MUA exchange environment. The first This creates a virtual MUA-to-MUA exchange environment. The first
component of the MHS is called the Mail Submission Agent (MSA) and component of the MHS is called the Mail Submission Agent (MSA) and
the last is called the Mail Delivery Agent (MDA). the last is called the Mail Delivery Agent (MDA).
An email Mediator is both an inbound MDA and outbound MSA. It takes An email Mediator is both an inbound MDA and outbound MSA. It takes
delivery of a message, makes changes appropriate to its service, and delivery of a message, makes changes appropriate to its service, and
then re-posts it for further distribution. Typically the new message then reposts it for further distribution. Typically, the new message
will retain the original rfc5322.From: header field. A mailing list will retain the original rfc5322.From: header field. A mailing list
is a common example of a Mediator. is a common example of a Mediator.
The modern Internet Mail service is marked by many independent The modern Internet Mail service is marked by many independent
operators, many different components for providing users with service operators, many different components for providing users with service
and many other components for performing message transfer. and many other components for performing message transfer.
Consequently, it is necessary to distinguish administrative Consequently, it is necessary to distinguish administrative
boundaries that surround sets of functional components, which are boundaries that surround sets of functional components, which are
subject to coherent operational policies. subject to coherent operational policies.
As elaborated on below, every MSA is a candidate for signing using As elaborated on below, every MSA is a candidate for signing using
DKIM, and every MDA is a candidate for doing DKIM verification. DKIM, and every MDA is a candidate for doing DKIM verification.
A.2. Trust Boundaries A.2. Trust Boundaries
Operation of Internet Mail services is apportioned to different Operation of Internet Mail services is apportioned to different
providers (or operators). Each can be composed of an independent providers (or operators). Each can be composed of an independent
ADministrative Management Domain (ADMD). An ADMD operates with an ADministrative Management Domain (ADMD). An ADMD operates with an
independent set of policies and interacts with other ADMDs according independent set of policies and interacts with other ADMDs according
to differing types and amounts of trust. Examples include an end- to differing types and amounts of trust. Examples include an end
user operating a desktop client that connects to an independent email user operating a desktop client that connects to an independent email
service, a department operating a submission agent or a local Relay, service, a department operating a submission agent or a local Relay,
an organization's IT group that operates enterprise Relays, and an an organization's IT group that operates enterprise Relays, and an
ISP operating a public shared email service. ISP operating a public shared email service.
Each of these can be configured into many combinations of Each of these can be configured into many combinations of
administrative and operational relationships, with each ADMD administrative and operational relationships, with each ADMD
potentially having a complex arrangement of functional components. potentially having a complex arrangement of functional components.
Figure 2 depicts the relationships among ADMDs. Perhaps the most Figure 2 depicts the relationships among ADMDs. Perhaps the most
salient aspect of an ADMD is the differential trust that determines salient aspect of an ADMD is the differential trust that determines
skipping to change at page 22, line 49 skipping to change at page 21, line 41
+--------+ | | ADMD#2 | | +--------+ | | ADMD#2 | |
| | ------ | | | | ------ | |
| | | | | | | |
+--->|-Transit--+---+ +--->|-Transit--+---+
| | | |
+----------+ +----------+
Figure 2: ADministrative Management Domains (ADMD) Example Figure 2: ADministrative Management Domains (ADMD) Example
In Figure 2, ADMD numbers 1 and 2 are candidates for doing DKIM In Figure 2, ADMD numbers 1 and 2 are candidates for doing DKIM
signing, and ADMD numbers 2, 3 and 4 are candidates for doing DKIM signing, and ADMD numbers 2, 3, and 4 are candidates for doing DKIM
verification. verification.
The distinction between Transit network and Edge network transfer The distinction between Transit network and Edge network transfer
services is primarily significant because it highlights the need for services is primarily significant because it highlights the need for
concern over interaction and protection between independent concern over interaction and protection between independent
administrations. The interactions between functional components administrations. The interactions between functional components
within a single ADMD are subject to the policies of that domain. within a single ADMD are subject to the policies of that domain.
Although any pair of ADMDs can arrange for whatever policies they Although any pair of ADMDs can arrange for whatever policies they
wish, Internet Mail is designed to permit inter-operation without wish, Internet Mail is designed to permit inter-operation without
prior arrangement. prior arrangement.
skipping to change at page 23, line 32 skipping to change at page 22, line 28
Internet Service Providers: Internet Service Providers:
Operators of underlying data communication services that, in Operators of underlying data communication services that, in
turn, are used by one or more Relays and Users. It is not turn, are used by one or more Relays and Users. It is not
necessarily their job to perform email functions, but they necessarily their job to perform email functions, but they
can, instead, provide an environment in which those can, instead, provide an environment in which those
functions can be performed. functions can be performed.
Mail Service Providers: Mail Service Providers:
Operators of email services, such as for end-users, or Operators of email services, such as for end users, or
mailing lists. mailing lists.
Index Index
A A
ADMD 7 ADMD 6
Administrative Management Domain 7 Administrative Management Domain 6
assessment 9 assessment 7
D D
DKIM-Signature 14 DKIM-Signature 12-13
DNS 7, 14-17 DNS 6, 13-15
I I
identifier 5-6, 8 identifier 4-8
identity 4-6, 8-9, 11, 13-14 identity 3-7, 9, 12
infrastructure 6-7, 9-10, 12-13, 19 infrastructure 5-6, 8-11, 17
M M
Mail Delivery Agent 7 Mail Delivery Agent 6
Mail Handling Service 7 Mail Handling Service 6
Mail Service Provider 7 Mail Service Provider 6
Mail Submission Agent 7 Mail Submission Agent 6
Mail Transfer Agent 7 Mail Transfer Agent 6
Mail User Agent 7 Mail User Agent 6
MDA 7 MDA 6
MHS 7 MHS 6
MIME Object Security Services 6 MIME Object Security Services 5
MOSS 6 MOSS 5
MSA 7 MSA 6
MSP 7 MSP 6
MTA 7 MTA 6
MUA 7 MUA 6
O O
OpenPGP 6 OpenPGP 5
P P
PEM 6 PEM 5
PGP 6 PGP 5
Pretty Good Privacy 6 Pretty Good Privacy 5
Privacy Enhanced Mail 6 Privacy Enhanced Mail 5
S S
S/MIME 6 S/MIME 5
T T
trust 4, 9-10, 21 trust 3, 7-8, 20
V V
verification 5, 8, 10, 12, 15, 18, 21-22 verification 4, 7-8, 10-11, 13, 16, 20-21
W W
Web of Trust 6 Web of Trust 6
X X
X.509 6 X.509 6
Authors' Addresses Authors' Addresses
Tony Hansen Tony Hansen
AT&T Laboratories AT&T Laboratories
200 Laurel Ave. 200 Laurel Ave.
Middletown, NJ 07748 Middletown, NJ 07748
USA USA
Email: tony+dkimov@maillennium.att.com EMail: tony+dkimov@maillennium.att.com
Dave Crocker Dave Crocker
Brandenburg InternetWorking Brandenburg InternetWorking
675 Spruce Dr. 675 Spruce Dr.
Sunnyvale, CA 94086 Sunnyvale, CA 94086
USA USA
Email: dcrocker@bbiw.net EMail: dcrocker@bbiw.net
Phillip Hallam-Baker Phillip Hallam-Baker
Default Deny Security, Inc.
Email: phillip@hallambaker.com EMail: phillip@hallambaker.com
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