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

DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories
Intended status: Informational D. Crocker
Expires: May 21, August 14, 2008 Brandenburg InternetWorking
P. Hallam-Baker
VeriSign Inc.
November 18, 2007
February 11, 2008

DomainKeys Identified Mail (DKIM) Service Overview
draft-ietf-dkim-overview-07
draft-ietf-dkim-overview-08

Status of this Memo

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This Internet-Draft will expire on May 21, August 14, 2008.

Abstract

This document provides an overview of the DomainKeys Identified Mail
(DKIM) service and describes how it can fit into a messaging service.
It also describes how DKIM relates to other IETF message signature
technologies. It is intended for those who are adopting, developing,
or deploying DKIM. DKIM allows an organization to take
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 agent's. agents.
An organization may use one or more domain names to accomplish this.
DKIM defines a domain-level digital signature authentication
framework for email, using public-key cryptography and key server technology. This
permits verifying the signer of a message, as well as the integrity
of its contents. DKIM accomplishes this by defining a domain-level
authentication framework for email using public-key cryptography and
key server
technology [RFC4871]. This permits verifying a message source, an
intermediary, or one of their agents, as well as the integrity of its
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 of unsigned about messages. Such protection of email identity can
assist in the global control of "spam" and "phishing". This document provides an overview of the DKIM service
and describes how it can fit into a messaging service. It also
describes how DKIM relates to other IETF message signature
technologies. It is intended for those who are adopting, developing,
or deploying DKIM.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3
1.1. Prior Work . DKIM's Scope . . . . . . . . . . . . . . . . . . . . . . . 4 3
1.2. Discussion Venue Prior Work . . . . . . . . . . . . . . . . . . . . . 6
2. . . . 4
1.3. Internet Mail Background . . . . . . . . . . . . . . . . . 6
1.4. Discussion Venue . . . . . . . . . . . . 6
2.1. Administrative Management Domain (ADMD) . . . . . . . . . 6
2.2.
2. The DKIM Placement within an ADMD Value Proposition . . . . . . . . . . . . . . . . 8
3. The DKIM Value Proposition . . 6
2.1. Identity Verification . . . . . . . . . . . . . . . . 9
4. . . 6
2.2. Enabling Trust Assessments . . . . . . . . . . . . . . . . 7
3. DKIM Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. 7
3.1. Functional Goals . . . . . . . . . . . . . . . . . . . . . 10
4.2. 7
3.2. Operational Goals . . . . . . . . . . . . . . . . . . . . 11
5. 9
4. DKIM Function . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. 10
4.1. The Basic Signing Service . . . . . . . . . . . . . . . . 13
5.2. 11
4.2. Characteristics of a DKIM signature . . . . . . . . . . . 13
5.3. 11
4.3. The Selector construct . . . . . . . . . . . . . . . . . . 14
5.4. 11
4.4. Verification . . . . . . . . . . . . . . . . . . . . . . . 14
6. 12
5. Service Architecture . . . . . . . . . . . . . . . . . . . . . 14
6.1. 13
5.1. Administration and Maintenance . . . . . . . . . . . . . . 16
6.2. 15
5.2. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3. 16
5.3. Verifying . . . . . . . . . . . . . . . . . . . . . . . . 17
6.4. 16
5.4. Unverified or Unsigned Mail . . . . . . . . . . . . . . . 17
6.5. Evaluating 16
5.5. Assessing . . . . . . . . . . . . . . . . . . . . . . . . 17
7. 16
5.6. DKIM Placement within an ADMD . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
10. 17
9. Informative References . . . . . . . . . . . . . . . . . . . . 18 17
Appendix A. Internet Mail Background . . . . . . . . . . . . . . 19
A.1. Administrative Management Domain (ADMD) . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 21
Intellectual Property and Copyright Statements . . . . . . . . . . 20 23

1. Introduction

This document provides a description of the architecture and
functionality for DomainKeys Identified Mail (DKIM) (DKIM). It is intended
for those who are adopting, developing, or deploying DKIM. It also
will be helpful for those who are considering extending DKIM, either
into other areas of use or to support additional features. This
Overview does not provide information on threats to DKIM or email, or
details on the protocol specifics, which can be found in [RFC4686]
and [RFC4871], respectively. The document assumes a background in
basic email and network security technology and services.

DKIM allows an organization to take responsibility for 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 agent's. agents. DKIM defines a domain-level digital signature
authentication framework for email,
using email through the use of public-key
cryptography and key server technology. This [RFC4871] It permits
verifying the signer of a message, as well as the integrity of its
contents. DKIM accomplishes this by defining a domain-level
authentication framework for email using public-key cryptography and
key server technology [RFC4871]. This permits verifying a message
source, an intermediary, or one of their agents, as well as the
integrity of its 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 of unsigned messages. Such protection of email identity
can assist in the global control of "spam" and "phishing".

This

Neither this document provides a description of DKIM's architecture and
functionality. It is intended for those who are adopting,
developing, or deploying DKIM. It also will be helpful for those who
are considering extending DKIM, either into other areas of use or nor DKIM attempts to
support additional features. This Overview does not provide
information on threats to DKIM or email, or details on the protocol
specifics, which can be found in [RFC4871] and [RFC4686],
respectively. The document assumes a background in basic network
security technology and services.

Neither this document nor DKIM attempt to provide solutions solutions to the
world's problems with spam, phishing, virii, worms, joe jobs, etc.
DKIM provides one basic tool, in what needs to be a large arsenal,
for improving basic trust in the Internet mail service. However by
itself, DKIM is not sufficient to that task and this Overview does
not pursue the issues of integrating DKIM into these larger efforts,
beyond a simple reference within a system diagram. Rather, it is a
basic introduction to the technology and its use.

1.1. Prior Work

Historical email assessment based on identity has used the IP Address DKIM's Scope

DKIM signatures can be created by a direct handler of a system message,
either as its author or as an intermediary. It can also be created
by an independent service that sent is providing assistance to a handler
of the message. The Address is obtained via
underlying Internet information mechanisms and is therefore trusted Whoever does the signing chooses the domain name to
be accurate. Besides having some known security weaknesses, used as the
use of Addresses present a number of functional and operational
problems. Consequently there basis for later assessments. Hence, the reputation
associated with that domain name is an industry desire additional basis for
evaluating whether to use trust the message for delivery. The owner of
the domain name being used for a more
stable value DKIM signature is declaring that has better correspondence to organizational
boundaries. Domain Names are viewed as often satisfying this need.

There
they have been four previous efforts at standardizing an Internet
email signature scheme:

o Privacy Enhanced some degree of accountability for the message.

DKIM is a value-added feature for email. Mail (PEM) was first published in 1987.
[RFC0989]

o PEM eventually transformed into MIME Object Security Services
(MOSS) that is not signed by
DKIM is handled in 1995. [RFC1848] Today, these two are only of historical
interest.

o Pretty Good Privacy (PGP) the same way as it was developed by Phil Zimmermann and
first released in 1991. [RFC1991] A later version before DKIM was
standardized as OpenPGP. [RFC2440] [RFC3156]
[I-D.ietf-openpgp-rfc2440bis]

o RSA Security independently developed Secure MIME (S/MIME) to
transport a PKCS #7 data object. [RFC3851]

Development of S/MIME defined.

The message will be evaluated by established analysis and OpenPGP has continued. While both have
achieved a significant user base, neither have achieved ubiquity in
deployment or use, filtering
techniques. (A signing policy may provide additional information for
that analysis and their goals differ from those filtering.) Over time, widespread DKIM adoption
could permit more strict handling of DKIM.

To the extent messages that other message-signing services might have been
adapted to are not signed.
However early benefits do the job that DKIM not require this and probably do not
warrant this.

It is designed important to perform, it was felt
that re-purposing any of those would be more problematic than
creating a separate service. That said, DKIM uses security algorithm
components that have a long history, including use within some clear about the narrow scope of
those other messaging security services.

DKIM has a distinctive approach for distributing and vouching for
keys. DKIM's
capabilities. It uses a key-centric Public Key Infrastructure (PKI) rather
than the more typical approaches based on a certificate is an enabling technology, intended for use in the styles
of Kohnfelder (X.509) or Zimmermann (web
larger context of trust). For determining message legitimacy. This larger
context is complex, so it is easy to assume that a component like
DKIM, the
owner of which actually provides only a key asserts its validity, rather than relying on limited service, instead
satisfies the key
having a broader semantic implication set of requirements.

A DKIM signature:

o Does not offer any assertions about the assertion, such as a
quality assessment behaviors of the key's owner. DKIM treats quality
assessment as an independent, value-added service, beyond identity
doing the initial
work of deploying a verifying signature service.

Further, DKIM's PKI is provided by adding information records signing.

o Does not prescribe any specific actions for receivers to the
existing Domain Name System (DNS) [RFC1034], rather than requiring
deployment of take upon
successful signature verification.

o Does not provide protection after signature verification.

o Does not protect against re-sending (replay of) a new query infrastructure. This approach message that
already has
significant operational advantages. First, it avoids the
considerable barrier of creating a new global infrastructure; hence
it leverages verified signature; therefore a global base of administrative experience and highly
reliable distributed operation. Second, transit intermediary
or a recipient can re-post the technical aspect of message in such a way that the
signature would remain verifiable, although the
DNS is already known to be efficient. Any new service recipient(s)
would not have to
undergo a period of gradual maturation, with potentially problematic
early-stage behaviors. By (re-)using been specified by the DNS, DKIM avoids these
growing pains. author.

1.2. Discussion Venue

NOTE TO RFC EDITOR: This "Discussion Venue" section is to be
removed prior to publication.

This document is being discussed Prior Work

Historically, email delivery assessment decisions have been based on
an identity that used the DKIM mailing list,
ietf-dkim@mipassoc.org.

2. Internet Mail Background

Internet Mail has a simple split between the user world, in the form IP Address of Mail User Agents (MUA), and the transmission world, in the form of system that directly sent
the Mail Handling Service (MHS) composed of Mail Transfer Agents
(MTA). The MHS is responsible for accepting a message from one user, (that is, the author, and delivering it to one previous email "hop"), [RFC4408] or more other users, on the
recipients. This creates a virtual MUA-to-MUA exchange environment.
message content (e.g. [RFC4406] and [RFC4407]). The first component of the MHS IP Address is called the Mail Submission Agent
(MSA)
obtained via underlying Internet information mechanisms and the last is called
therefore trusted to be accurate. Besides having some known security
weaknesses, the Mail Delivery Agent (MDA).

An email Mediator is both an inbound MDA and outbound MSA. It takes
delivery use of addresses presents a message and re-posts it for further distribution,
retaining the original From header field. A mailing list is a common
example number of a Mediator

The modern Internet Mail service is marked by many independent
operators, many different components for providing users with service functional and many other components for performing message transfer.
Consequently, it
operational problems. Consequently there is necessary an industry desire to distinguish administrative
boundaries
use an identifier that surround sets of functional components, which are
subject has better correspondence to coherent operational policies.

As elaborated on below, every MSA is a candidate for signing using
DKIM, and every MDA is a candidate for doing DKIM verification.

2.1. Administrative Management organizational
boundaries. Domain (ADMD)

Operation of names are viewed as often satisfying this need.

There have been four previous IETF efforts at standardizing an
Internet Mail services is apportioned to different
providers (or operators). Each can be composed email signature scheme. Their goals have differed from
those of an independent
ADministrative Management Domain (ADMD). An ADMD operates with an
independent set DKIM.

o Privacy Enhanced Mail (PEM) was first published in 1987.
[RFC0989]

o PEM eventually transformed into MIME Object Security Services
(MOSS) in 1995. [RFC1848] Today, these two are only of policies and interacts with other ADMDs according
to differing types historical
interest.

o Pretty Good Privacy (PGP) was developed by Phil Zimmermann and amounts of trust. Examples include: an end-
user operating their desktop client that connects
first released in 1991. [RFC1991] A later version was
standardized as OpenPGP. [RFC2440] [RFC3156]
[I-D.ietf-openpgp-rfc2440bis]

o RSA Security independently developed Secure MIME (S/MIME) to an independent
email service, a department operating a submission agent or a local
Relay, an organization's IT group that operates enterprise Relays,
and an ISP operating
transport a public shared email service.

Each of these can be configured into many combinations PKCS #7 data object. [RFC3851]

Development of
administrative both S/MIME and operational relationships, with OpenPGP has continued. While each ADMD
potentially having has
achieved a complex arrangement of functional components.
Figure 1 depicts significant user base, neither one has achieved ubiquity
in deployment or use.

To the relationships among ADMDs. Perhaps extent that other message-signing services might have been
adapted to do the most
salient aspect of an ADMD job that DKIM is the differential trust designed to perform, it was felt
that determines
its policies for activities re-purposing any of those would be more problematic than
creating a separate service. That said, DKIM uses security algorithm
components that have a long history, including use within the ADMD, versus some of
those involving
interactions with other ADMDs.

Basic types of ADMDs include:

Edge: Independent transfer services, messaging security services.

DKIM has a distinctive approach for distributing and vouching for
keys. It uses a key-centric Public Key Infrastructure (PKI) rather
than the more typical approaches based on a certificate in networks at the edge styles
of Kohnfelder (X.509) [Kohnfelder] or Zimmermann (web of trust). For
DKIM, the Internet Mail service.

User: End-user services. These might be subsumed under an Edge
service, owner of a domain name asserts the validity of a key,
rather than relying on the key having a broader semantic implication
of the assertion, such as is common for web-based email access.

Transit: These are Mail Service Providers (MSP) offering value-
added capabilities for Edge ADMDs, such a quality assessment of the key's owner.
DKIM treats quality assessment as aggregation an independent, value-added
service, beyond the initial work of deploying a verifying signature
service.

Further, DKIM's PKI is provided by adding information records to the
existing Domain Name System (DNS) [RFC1034], rather than requiring
deployment of a new query infrastructure. This approach has
significant operational advantages. First, it avoids the
considerable barrier of creating a new global infrastructure; hence
it leverages a global base of administrative experience and
filtering.

Note that Transit services are quite different from packet-level
transit highly
reliable distributed operation. Whereas end-to-end packet transfers usually go
through intermediate routers, email exchange across Second, the open Internet
is often directly between technical aspect of the Edge ADMDs, at
DNS is already known to be efficient. Any new service would have to
undergo a period of gradual maturation, with potentially problematic
early-stage behaviors. By (re-)using the email level.

+--------+ +--------+ +--------+
| ADMD#1 | | ADMD#3 | | ADMD#4 |
| ------ | | ------ | | ------ |
| | +----------------------->| | | |
| DNS, DKIM avoids these
growing pains.

1.3. Internet Mail Background

The basic Internet Email service has evolved extensively over its
several decades of continuous operation. Its modern architecture
comprises a number of specialized components. A discussion about
Mail User | | |--Edge--+--->|--User |
| | | | +--->| | | |
| V | | | +--------+ +--------+
| Edge---+---+ |
| | | +----------+ |
+--------+ | | ADMD#2 | |
| | ------ | |
| | | |
+--->|-Transit--+---+
| |
+----------+

Figure 1: ADministrative Agents (MUA), Mail Handling Services (MHS), Mail Transfer
Agents (MTA), Mail Submission Agents (MSA), Mail Delivery Agents
(MDA), Mail Service Providers (MSP), Administrative Management
Domains (ADMD) Example
In Figure 1, ADMD numbers 1 (ADMDs), and 2 are candidates for doing their relationships can be found in 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
signing, and ADMD numbers 2, 3 and 4 are candidates for doing mailing list,
ietf-dkim@mipassoc.org.

2. The DKIM
verification. Value Proposition

The distinction between Transit network nature and Edge network transfer
services is primarily significant because it highlights the need origins of a message are often falsely stated. DKIM
provides a foundation for
concern over interaction distinguishing legitimate mail, and protection between independent
administrations. The interactions between functional components
within thus a single ADMD are subject to
means of associating a verifiable identifier with a message. Given
the policies presence of that domain.
Although any pair of ADMDs can arrange for whatever policies they
wish, Internet Mail is designed to permit inter-operation without
prior arrangement.

Common ADMD examples are:

Enterprise Service Providers:

Operators of an organization's internal data and/or mail
services.

Internet Service Providers:

Operators of underlying data communication services that, in
turn, are used by one or more Relays and Users. It is not
necessarily their job to perform email functions, but they
can, instead, provide an environment in which those
functions can be performed.

Mail Service Providers:

Operators of email services, such as for end-users, or
mailing lists.

2.2. DKIM Placement within an ADMD

It is expected that the most common venue for a DKIM implementation
will be within the infrastructures of the originating organization's
outbound service and the receiving organization's inbound service,
such as a department or a boundary MTA. DKIM can be implemented in
an author's or recipient MUA, but this is expected to be less
typical, since it has higher administration and support costs.

A Mediator, such as a mailing list, often can re-post a message
without breaking the DKIM signature. Furthermore it can add its own
signature. This can be added by the Mediator software itself, or by
any outbound component in the Mediator's ADMD.

3. The DKIM Value Proposition

The nature and origins of a message are often falsely stated. As a
foundation for distinguishing legitimate mail, DKIM provides a means
of associating a verifiable identity with a message. Given the
presence of that identity, a receiver identifier, a receiver can make decisions about
further handling of the message, based upon assessments of the
identity that
identity. is associated with the identifier.

Receivers who successfully verify a signature can use information
about the signer as part of a program to limit spam, spoofing,
phishing, or other undesirable behavior. DKIM does not, itself,
prescribe any specific actions by the recipient; rather it is an
enabling technology for services that do.

These services will typically:

1. Verify an identity

2. Determine whether the identity is known or unknown

3. a verified identity, if possible.

2. Determine whether a known identity is trusted trusted.

The role of DKIM is in to perform the first two of these; DKIM is an enabler
for the third.

An second.

2.1. Identity Verification

Consider an attack is made against an organization or against customers
of an organization. The name of the organization is linked to
particular Internet domain names. names (identifiers). One point of
leverage used by for attackers is either to use a legitimate domain name,
without authorization authorization, or to use a "cousin" name that is similar to
one that is legitimate, but is not controlled by the target
organization. A DKIM-based An assessment service that uses DKIM can enforce a basic separation differentiate
between domains used by such known organizations and domains used by
others. As such, DKIM signatures can be created by a direct handler performs the positive step of identifying
messages associated with verifiable identities, rather than the
negative step of identifying messages with problematic use of
identities. Whether a message,
either as its originator or as an intermediary. It can also be
created by an independent service, providing assistance verified identity belongs to a handler Good Actor or a
Bad Actor becomes a later step of assessment.

2.2. Enabling Trust Assessments

Email receiving services are faced with a basic decision: Should they
deliver a newly-arrived message to the message. Whoever indicated recipient? That is,
does the signing chooses receiving service trust that the domain name message is sufficiently
"safe" to be used viewed? For the modern Internet, most receiving
services have an elaborate engine that formulates this quality
assessment. These engines take a variety of information as input to
the basis for later assessments. Hence, decision, such as from reputation
associated with that domain name is lists and accreditation
services. As the basis for evaluating whether
to engine processes information, it raises or lowers
its trust the message assessment for delivery. The owner of the domain name
being used for a message.

DKIM signature is provides additional information to this process by declaring that they are
accountable for a
valid "responsible" identity about which the message.

DKIM is engine can make quality
assessments. By itself, a value-added feature for email. Mail that is not signed by valid DKIM is handled in signature neither lowers nor
raises the same way as it was, before DKIM was defined; level of trust associated with the message, but it continues enables
other mechanisms to be evaluated used for doing so.

An organization might build upon its use of DKIM by established analysis and filtering
techniques. Over time, widespread DKIM adoption publishing
information about its Signing Practices (SP). This could permit more
strict handling of
detecting some messages that are not signed. However early
benefits do not require this and probably do not warrant this.

It is important purport to be clear about the narrow scope of DKIM's
capabilities. It is an enabling technology, intended for use in the
larger context of determining message legitimacy. This larger
context is complex, so it is easy to assume that associated with a component like
DKIM, domain,
but which actually provides only a limited service, instead
satisfies the broader set of requirements. A DKIM signature:

o Does not offer any assertions about the behaviors of the identity
doing are not. As such, an SP can cause the signing.

o Does not prescribe any specific actions for receivers trust assessment to take upon
successful signature verification.

o Does not provide protection after signature verification.

o Does not protect against re-sending (replay of) a message that
already has a verified signature; therefore a transit intermediary
be reduced, or a recipient can re-post the message in such a way that the
signature would remain verifiable, although the new recipient(s)
would not have been specified by the author.

4. leave it unchanged.

3. DKIM Goals

DKIM adds an end-to-end authentication mechanism to the existing
email transfer infrastructure. This motivates functional goals about
the authentication itself and operational goals about its integration
with the rest of the Internet email service.

4.1.

3.1. Functional Goals

4.1.1.

3.1.1. Use Domain-level granularity for assurance.

OpenPGP and S/MIME provide end-to-end validation in terms of
individual originators and recipients, notably using full email
addresses, whereas assurance

DKIM seeks accountability at the more coarse granularity of an
organization or, perhaps, a department. An existing Internet service
construct that enables this granularity is the Domain Name [RFC1034], to which [RFC1034].
DKIM binds the signing key record is bound. to the Domain Name. Further
benefits of using domain names include simplifying key management,
enabling signing by the infrastructure as opposed to the MUA, and
potential privacy issues.

Contrast this with OpenPGP and S/MIME, which provide end-to-end
validation in terms of individual authors, notably using full email
addresses.

3.1.2. Implementation Locality

DKIM signing and/or validating can be implemented anywhere along the
transit path, rather than only in the end systems or only in the a
boundary MTA.

4.1.2.

3.1.3. Allow delegation of signing to independent parties. parties

Different parties have different roles in the process of email
exchange. Some are easily visible to end users and others are
primarily visible to operators of the service. DKIM needs to support
signing by any of these different parties and needs to permit them to
sign with any domain name that they deem appropriate (and for which
they are authorized.) As an example an organization that creates
email content often delegates portions of its processing or
transmission to an outsourced group. DKIM supports this mode of
activity, in a manner that is not normally visible to end users.

4.1.3.

3.1.4. Distinguish the core authentication mechanism from its
derivative uses. uses

An authenticated identity can be subject to a variety of processing
policies, either ad hoc or standardized. The only semantics inherent
to a DKIM signature is that the signer is asserting (some)
responsibility for the message. All other mechanisms and meanings
are independent of built on this core service. One such mechanism might assert a
relationship between the signing identity and the author, as
specified in the From From: header field's domain identity[RFC2822].
Another might specify how to treat an unsigned message with that From
From: field domain.

4.1.4.

3.1.5. Retain ability to have anonymous email. email

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
needs to be retained. DKIM is compatible with this goal since it
permits an authentication of the email system operator to be authenticated, operator, rather than the
content author. Knowing If it is possible to obtain effectively anonymous
accounts at example.com, knowing that a message definitely came from
example.com does not threaten the anonymity of the user who authored
it, if it is still possible to obtain effectively anonymous accounts
at example.com.

4.2.
it.

3.2. Operational Goals

4.2.1.

3.2.1. Treat verification failure the same as no signature present.

OpenPGP and S/MIME were designed for strong cryptographic protection.
This included treating verification failure as message failure. present

As a sub-goal to the requirement for transparency, a DKIM signature
verifier is to treat messages with signatures that fail as if they
were unsigned. Hence the message will revert to normal handling,
through the receiver's existing filtering mechanisms. Thus, DKIM
specifies that a sender an assessing site is not to take a message that has a
broken signature and treat it any differently than if the signature
weren't there.

4.2.2.

Contrast this with OpenPGP and S/MIME, which were designed for strong
cryptographic protection. This included treating verification
failure as message failure.

3.2.2. Make signatures transparent to non-supporting recipients.

S/MIME and OpenPGP modify the message body. Hence, their presence is
potentially visible to email recipients, whose user software needs to
process the associated constructs. recipients

In order to facilitate incremental adoption, DKIM is designed to be
transparent to recipients that do not support it. A DKIM signature
does not "get in the way" for such recipients.

4.2.3.

Contrast this with S/MIME and OpenPGP, which modify the message body.
Hence, their presence is potentially visible to email recipients,
whose user software needs to process the associated constructs.

3.2.3. Permit incremental adoption for incremental benefit. benefit

DKIM can immediately provide benefits between any two organizations
that exchange email and implement DKIM. In the usual manner of
"network effects", the benefits of DKIM increase dramatically as its
adoption increases.

Although it is envisioned that this mechanism will call upon
independent services to aid in the assessment of DKIM results, they
are not essential in order to obtain initial benefit. For example
DKIM allows (possibly large) pair-wise sets of email providers and
spam filtering companies to distinguish mail that is associated with
a known organization, organization from mail that might deceptively purport to have
the affiliation. This in turn allows the development of "whitelist"
schemes whereby authenticated mail from a known source with good
reputation is allowed to bypass some anti-abuse filters.

In effect the email receiver is using their set of known
relationships to generate their own reputation data. This works
particularly well for traffic between large sending providers and
large receiving providers. However it also works well for any
operator, public or private, that has mail traffic dominated by
exchanges among a stable set of organizations.

4.2.4.

3.2.4. Minimize the amount of required infrastructure

A new service, or an enhancement to an existing service, requires
adoption by some number in a critical mass of systems, system components, before it can be
useful. The greater the number of required adopters, the higher the
adoption barrier. This becomes particularly serious when adoption is
required by independent, intermediary -- that is, infrastructure --
service providers. 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 useful benefit for any signer/verifier individual pair of participants
signers and verifiers who are exchanging mail. Similarly, DKIM's
reliance on the Domain Name System greatly reduces the amount of new
administrative infrastructure that is need, needed across the open
Internet.

4.2.5.

3.2.5. Permit wide range of deployment choices. choices

DKIM can be deployed at a variety of places within an organization's
email service. This permits the organization to choose how much or
how little they want DKIM to be part of their service, rather than
part of a more localized operation.

4. DKIM Function

DKIM has a very constrained set of capabilities, primarily targeting
email while it is in transit, transit from an author to a set of recipients.
It creates the ability to associate verifiable information with a
message, especially a responsible identity. When a message is does not
signed,
have a valid signature associated with the author, DKIM SP will
permit the domain name of the author to be used for obtaining
information about their signing practices.

5.1.

4.1. The Basic Signing Service

With the DKIM signature mechanism, a signer chooses a signing
identity based on their domain name, performs digital signing on the
message, and records signature information in a DKIM header field. A
verifier obtains the domain name and the "selector" from the DKIM
header field, queries for a public key associated with the name, and
verifies the signature.

DKIM permits any domain name to be used for signing, and supports
extensible choices for various algorithms. As is typical for
Internet standards, there is a core set of algorithms that all
implementations are required to support, in order to guarantee basic
interoperability.

DKIM permits restricting the use of a signature key (by using s=) to
signing messages for particular types of services, such as only for
email. This is intended to be helpful when delegating signing
authority, such as to a particular department or to a third-party
outsourcing service.

With DKIM the signer explicitly lists the headers that are signed. signed,
such as From:, Date: and Subject:. By choosing the minimal set of
headers needed, the signature is likely to be considerably more
robust against the handling vagaries of intermediary MTAs.

5.2.

4.2. Characteristics of a DKIM signature

A DKIM signature covers the message body and selected header 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
cryptographically encode this information, along with other signing
parameters. Signature information is placed into a new [RFC2822]
header field of the message.

5.3.

4.3. The Selector construct

The key for a signature is associated with a domain name, as
specified in the d= DKIM-Signature header field parameter. parameters. That
domain name, or the domain name is or address in the i= parameter,
provide the complete identity used for making assessments about the
signer. (The DKIM specification does not give any guidance on how to
do an assessment.) However this name is not sufficient for making a
DNS query to obtain the key needed to verify the signature.

A single domain can use multiple signing keys and/or multiple
potential signers. To support this, DKIM identifies a particular
signature as a combination of the domain name and an added field,
called the "selector", coded into separate DKIM-Signature header
field parameters.

NOTE: The selector is not intended to be part of the domain name
that is used for making assessments. Rather, the selector is
strictly reserved for use in administering keys that are
associated with the domain name. If the selector becomes part of
a name assessment mechanism, then there is no remaining mechanism
for making a transition from an old, or compromised, key to a new
one.

Signers often need to support multiple assessments about their
organization, such as to distinguish one type of message from
another, or one portion of the organization from another. To permit
assessments that are independent, one method is for an organization
to use different sub-domains in the "d=" parameter, such as
"transaction.example.com" versus "newsletter.example.com", or
"productA.example.com" versus "productB.example.com".

5.4.

4.4. Verification

After a message has been signed, any agent in the message transit
path can verify the signature, signature to determine that the signing identity
took responsibility for the message. Message recipients can verify
the signature by querying the DNS for the signer's domain directly,
to retrieve the appropriate public key, and thereby confirm that the
message was attested to by a party in possession of the private key
for the signing domain. Typically, verification will be done by an
agent in the ADMD Administrative Management Domain (ADMD) of the message
recipient.

5. Service Architecture

The DKIM service is divided into components that can be are performed using
different, external services, such as for key retrieval.
However the retrieval and relaying
email. The basic DKIM signing specification defines an initial set
of these services, services (using DNS and SMTP), in order to ensure a basic
level of interoperability.

|
|- RFC2822 Message
V
+--------+ +------------------------------------+
| Private| | ORIGINATING OR RELAYING ADMD (MSA) |
| |
+..>| Key |.>| Sign Message |
.
| Store | +--------------+---------------------+
.
+--------+ |
.private
(paired) |
+---+---+
+--------+ | +-----------+
| Key Public | | +-----------+ | Store Remote |
| Key | [Internet] | Sender |
+---+---+
| Store | | | Practices |
.public
+----+---+ | +-----+-----+
. V .
. +-----------------------------------+ .
. | RELAYING OR DELIVERING ADMD (MDA) | .
. | | .
. | Message Signed? | .
. +-------+----------------+----------+ +--------+---------------+----------+ .
. |yes |no .
. V V | .
. +-----------+ +-----------+ +------------+ | .
+.....>| Verify +----+ | +-->| Check |<.......+
| Signature .
| Signatures | | Practices |<.......+
+---+-----+-+ | +---+-------+ .
| |
+-----+------+ | | .
| +---+ | .
pass| fail fail| | .
V | +-----+-----+
+--------+ | .
+--------+ | Local | .
+.......>| Assess | | | Sender | .
. | Signer | V V .
. +---+----+ +-------+ .
. | / Check \<............+
. +------>/ Signing \
. | / Practices | \<..........+
. +---+----+ | +-----------+ +-------+-------+ .
. assessment| | | . +------+ +------+
. | |
+-+-----------+ V V
| Reputation/ .
+---+---------+ | +-----------+
|Accreditation| +------+-----+
|Reputation/ | | | Message | | Local Info | |
|Accreditation| +------>| Filtering |
+-----+-------+ | on Sender |
|Info | | Engine |
+-----------+> | Practices |
+-------------+ +-----------+ +------------+

Figure 2: 1: DKIM Service Architecture

As shown in Figure 2, 1, basic message processing is divided between the
MSA and the MDA.

The MSA The MSA signs the message, using private information from
the Key Store.

The MDA The MDA verifies the signature or determines whether a
signature was required. Verifying the signature uses public
information from the Key Store. If the signature passes,
reputation information is used to asses the signer and that
information is passed to the message filtering system. If the
signature fails or there is no signature, information about the
sender's
related signing practices is retrieved remotely and/or locally,
and that information is passed to the message filtering system.

Note: Figure 2 1 does not show the effects on the message-handling
flow message handling
when multiple signatures or third-party non-author signatures are present.

6.1.

5.1. Administration and Maintenance

A number of tables and services are used to provide external
information. Each of these introduces administration and maintenance
requirements.

Key Store DKIM uses public/private (asymmetric) key technology. The
signer users a private key and the validator uses the
corresponding public key. The current DKIM signing specification
provides for querying the Domain Names Service (DNS), to permit a
validator to obtain the public key. The signing organization
therefore must have a means of adding a key to the DNS, for every
selector/domain-name combination. Further, the signing
organization needs policies for distributing and revising keys.

Sender Practices

Reputation/Accreditation If a message contains a valid signature,
then the verifier can evaluate the associated domain name's
reputation. If
a message does not contain a valid signature, that fact could be
useful, if the verifier can discover information about the DKIM-
related practices of one of the agents purportedly involved with
the message, such as the domain listed in the author's FROM header
field. Such information might come from tables developed through
private agreement or from standards-based mechanisms. As they are
defined, each domain name owner will need to consider what
information to publish through the mechanism and then will need to
create and maintain it.

Reputation/Accreditation "Reputation/Accreditation" provides
quality-assessment information that Quality-assessment information, which is associated
with a domain name, and comes in many forms and from many sources.
DKIM does not define these assessment services. It's relevance to them
is to provide a validated domain name, upon which assessments can
be made.

6.2. Signing

Signing can be performed by Practices (SP) Separate from determining the validity of a component
signature, and separate from assessing the reputation of the ADMD
organization that creates is associated with the
message, and/or within signed identity, there is
an the opportunity to determine any ADMD along organizational practices
concerning a domain name. Practices can range widely. They can
be published by the relay path. The signer
uses owner of the appropriate private key.

6.3. Verifying

Verification domain or they can be performed maintained
by any functional component along the
relay and delivery path. Verifiers retrieve evaluating site. They can pertain to the public key based
upon use of the parameters stored domain
name, such as whether it is used for signing messages, whether all
mail having that domain name in the message.

6.4. Unverified author From: header field is
signed, or whether such mail is to be discarded in the absence of
an appropriate signature. The statements of practice are made at
the level of a domain name, and are distinct from assessments made
about particular messages, as occur in a Message Filtering Engine.
Such assessments of practices can provide useful input for the
Message Filtering Engine's determination of message handling. As
practices are defined, each domain name owner needs to consider
what information to publish. The nature and degree of checking
practices, if any is performed, is optional to the evaluating site
and is strictly a matter of local policy.

5.2. Signing

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
uses the appropriate private key.

5.3. Verifying

Verification can be performed by any functional component along the
relay and delivery path. Verifiers retrieve the public key based
upon the parameters stored in the message.

5.4. Unverified or Unsigned Mail

Note that a failed signature causes the message to be treated in the
same manner as one that is unsigned. Messages lacking a valid
originator author
signature (a signature associated with the originator author of the message as
opposed to a signature associated with an intermediary) can prompt a
query for any published "sender "signing practices" information, as an aid in
determining whether the sender author information has been used without
authorization.

6.5. Evaluating

The

5.5. Assessing

Figure 1 shows the verified identity as being used to assess an
associated reputation, but it could be applied for other tasks, such
as management tracking of mail. A popular use of reputation
information is as input to a filtering engine that decides whether to
deliver -- and possibly whether to specially mark -- a message.
Filtering engines have become complex and sophisticated. Their
details are outside of DKIM's scope, the scope of DKIM, other than the expectation
that
DKIM-related information is the validated identity produced by DKIM will be added to the
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
domain has published information about its practices.

5.6. DKIM Placement within an ADMD

It is practices

7. Security Considerations

TBD

8. IANA Considerations

TBD

9. Acknowledgements

TBD

10. Informative References

[I-D.ietf-openpgp-rfc2440bis]
Callas, J., "OpenPGP Message Format",
draft-ietf-openpgp-rfc2440bis-22 (work in progress),
April 2007.

[I-D.kucherawy-sender-auth-header]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status",
draft-kucherawy-sender-auth-header-09 (work in progress),
November 2007.

[RFC0989] Linn, J. and IAB Privacy Task Force, "Privacy enhancement expected that the most common venue for Internet electronic mail: Part I: Message encipherment a DKIM implementation
will be within the infrastructures of the authoring organization's
outbound service and authentication procedures", RFC 989, February 1987.

[RFC1034] Mockapetris, P., "Domain the receiving organization's inbound service,
such as a department or a boundary MTA. DKIM can be implemented in
an author's or recipient MUA, but this is expected to be less
typical, since it has higher administration and support costs.

6. Security Considerations

The security considerations of the DKIM protocol are described in the
DKIM base specification [RFC4871].

7. IANA Considerations

There are no actions for IANA.

NOTE TO RFC EDITOR: This section may be removed prior to
publication.

8. Acknowledgements

Many people contributed to the development of the DomainKeys
Identified Mail and the efforts of the DKIM Working Group is
gratefully acknowledged. In particular, we would like to thank Jim
Fenton for his extensive feedback diligently provided on every
version of this document.

9. Informative References

[I-D.ietf-openpgp-rfc2440bis]
Callas, J., "OpenPGP Message Format",
draft-ietf-openpgp-rfc2440bis-22 (work in progress),
April 2007.

[I-D.kucherawy-sender-auth-header]
Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status",
draft-kucherawy-sender-auth-header-11 (work in progress),
February 2008.

[Kohnfelder]
Kohnfelder, L., "Towards a Practical Public-key
Cryptosystem", May 1978.

[RFC0989] Linn, J. and IAB Privacy Task Force, "Privacy enhancement
for Internet electronic mail: Part I: Message encipherment
and authentication procedures", RFC 989, February 1987.

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.

[RFC1848] Crocker, S., Galvin, J., Murphy, S., and N. Freed, "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,
"OpenPGP Message Format", RFC 2440, November 1998.

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

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

[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001.

[RFC3164] Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
August 2001.

[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.

[RFC4406] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, April 2006.

[RFC4407] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, April 2006.

[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.

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

[RFC4870] Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.

[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.

Appendix A. Internet Mail Background

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
Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA).
The MHS is responsible for accepting a message from one user, the
author, and delivering it to one or more other users, the recipients.
This creates a virtual MUA-to-MUA exchange environment. The first
component of the MHS is called the Mail Submission Agent (MSA) and
the last is called the Mail Delivery Agent (MDA).

An email Mediator is both an inbound MDA and outbound MSA. It takes
delivery of a message and re-posts it for further distribution,
retaining the original From: header field. A mailing list is a
common example of a Mediator.

The modern Internet Mail service is marked by many independent
operators, many different components for providing users with service
and many other components for performing message transfer.
Consequently, it is necessary to distinguish administrative
boundaries that surround sets of functional components, which are
subject to coherent operational policies.

As elaborated on below, every MSA is a candidate for signing using
DKIM, and every MDA is a candidate for doing DKIM verification.

A.1. Administrative Management Domain (ADMD)

Operation of Internet Mail services is apportioned to different
providers (or operators). Each can be composed of an independent
ADministrative Management Domain (ADMD). An ADMD operates with an
independent set of policies and interacts with other ADMDs according
to differing types and amounts of trust. Examples include: an end-
user operating their desktop client that connects to an independent
email service, a department operating a submission agent or a local
Relay, an organization's IT group that operates enterprise Relays,
and an ISP operating a public shared email service.

Each of these can be configured into many combinations of
administrative and operational relationships, with each ADMD
potentially having a complex arrangement of functional components.
Figure 2 depicts the relationships among ADMDs. Perhaps the most
salient aspect of an ADMD is the differential trust that determines
its policies for activities within the ADMD, versus those involving
interactions with other ADMDs.

Basic types of ADMDs include:

Edge: Independent transfer services, in networks at the edge of
the Internet Mail service.

User: End-user services. These might be subsumed under an Edge
service, such as is common for web-based email access.

Transit: These are Mail Service Providers (MSP) offering value-
added capabilities for Edge ADMDs, such as aggregation and
filtering.

Note that Transit services are quite different from packet-level
transit operation. Whereas end-to-end packet transfers usually go
through intermediate routers, email exchange across the open Internet
is often directly between the Edge ADMDs, at the email level.

+--------+ +--------+ +--------+
| ADMD#1 | | ADMD#3 | | ADMD#4 |
| ------ | | ------ | | ------ |
| | +----------------------->| | | |
| User | | |--Edge--+--->|--User |
| | | | +--->| | | |
| V | | | +--------+ +--------+
| Edge---+---+ |
| | | +----------+ |
+--------+ | | ADMD#2 | |
| | ------ | |
| | | |
+--->|-Transit--+---+
| |
+----------+

Figure 2: ADministrative Management Domains (ADMD) Example

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
verification.

The distinction between Transit network and Edge network transfer
services is primarily significant because it highlights the need for
concern over interaction and protection between independent
administrations. The interactions between functional components
within a single ADMD are subject to the policies of that domain.
Although any pair of ADMDs can arrange for whatever policies they
wish, Internet Mail is designed to permit inter-operation without
prior arrangement.

Common ADMD examples are:

Enterprise Service Providers:

Operators of an organization's internal data and/or mail
services.

Internet Service Providers:

Mail Service Providers:

Operators of email services, such as for end-users, or
mailing lists.

Authors' Addresses

Tony Hansen
AT&T Laboratories
200 Laurel Ave.
Middletown, NJ 07748
USA

Email: tony+dkimov@maillennium.att.com
Dave Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
USA

Email: dcrocker@bbiw.net

Phillip Hallam-Baker
VeriSign Inc.

Email: pbaker@verisign.com

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