DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories
Intended status: Informational D. Crocker
Expires: May 21, 2008 Brandenburg InternetWorking
P. Hallam-Baker
VeriSign Inc.
November 18, 2007
DomainKeys Identified Mail (DKIM) Service Overview
draft-ietf-dkim-overview-07
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Abstract
DomainKeys Identified Mail (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. DKIM defines
a domain-level digital signature authentication framework for email,
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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 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Prior Work . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Venue . . . . . . . . . . . . . . . . . . . . . 6
2. Internet Mail Background . . . . . . . . . . . . . . . . . . . 6
2.1. Administrative Management Domain (ADMD) . . . . . . . . . 6
2.2. DKIM Placement within an ADMD . . . . . . . . . . . . . . 8
3. The DKIM Value Proposition . . . . . . . . . . . . . . . . . . 9
4. DKIM Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Functional Goals . . . . . . . . . . . . . . . . . . . . . 10
4.2. Operational Goals . . . . . . . . . . . . . . . . . . . . 11
5. DKIM Function . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. The Basic Signing Service . . . . . . . . . . . . . . . . 13
5.2. Characteristics of a DKIM signature . . . . . . . . . . . 13
5.3. The Selector construct . . . . . . . . . . . . . . . . . . 14
5.4. Verification . . . . . . . . . . . . . . . . . . . . . . . 14
6. Service Architecture . . . . . . . . . . . . . . . . . . . . . 14
6.1. Administration and Maintenance . . . . . . . . . . . . . . 16
6.2. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3. Verifying . . . . . . . . . . . . . . . . . . . . . . . . 17
6.4. Unverified or Unsigned Mail . . . . . . . . . . . . . . . 17
6.5. Evaluating . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
10. Informative References . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
DomainKeys Identified Mail (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. 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 messages. Such protection of
email identity can assist in the global control of "spam" and
"phishing".
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 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 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
of a system that sent the message. The Address is obtained via
underlying Internet information mechanisms and is therefore trusted
to be accurate. Besides having some known security weaknesses, the
use of Addresses present a number of functional and operational
problems. Consequently there is an industry desire to use a more
stable value that has better correspondence to organizational
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boundaries. Domain Names are viewed as often satisfying this need.
There have been four previous efforts at standardizing an Internet
email signature scheme:
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 historical
interest.
o Pretty Good Privacy (PGP) was developed by Phil Zimmermann and
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
transport a PKCS #7 data object. [RFC3851]
Development of S/MIME and OpenPGP has continued. While both have
achieved a significant user base, neither have achieved ubiquity in
deployment or use, and their goals differ from those of DKIM.
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
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 of
those other 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 the styles
of Kohnfelder (X.509) or Zimmermann (web of trust). For DKIM, the
owner of a key asserts its validity, rather than relying on the key
having a broader semantic implication of the assertion, such as a
quality assessment of the key's owner. DKIM treats quality
assessment as 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 highly
reliable distributed operation. Second, the technical aspect of the
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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 DNS, DKIM avoids these
growing pains.
1.2. 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.
2. Internet Mail Background
Internet Mail has a simple 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.
2.1. 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
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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 1 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 1: ADministrative Management Domains (ADMD) Example
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In Figure 1, 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:
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.
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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 can make decisions about
further handling of the message, based upon assessments of that
identity.
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. Determine whether a known identity is trusted
The role of DKIM is in the first two of these; DKIM is an enabler for
the third.
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. One point of leverage used by attackers is
either to use a legitimate domain name, without 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 assessment
service can enforce a basic separation between domains used by such
known organizations and domains used by others.
DKIM signatures can be created by a direct handler of a message,
either as its originator or as an intermediary. It can also be
created by an independent service, providing assistance to a handler
of the message. Whoever does the signing chooses the domain name to
be used as the basis for later assessments. Hence, reputation
associated with that domain name is the basis for evaluating whether
to trust the message for delivery. The owner of the domain name
being used for a DKIM signature is declaring that they are
accountable for the message.
DKIM is a value-added feature for email. Mail that is not signed by
DKIM is handled in the same way as it was, before DKIM was defined;
it continues to be evaluated by established analysis and filtering
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techniques. Over time, widespread DKIM adoption could permit more
strict handling of messages that are not signed. However early
benefits do not require this and probably do not warrant this.
It is important 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 a component like
DKIM, 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 the signing.
o Does not prescribe any specific actions for receivers 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
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. 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. Functional Goals
4.1.1. 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 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 the signing key record is bound.
Further DKIM signing and/or validating can be implemented anywhere
along the transit path, rather than only in the end systems or only
in the boundary MTA.
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4.1.2. Allow delegation of signing to independent 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 visible to end users.
4.1.3. Distinguish the core authentication mechanism from its
derivative 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 this core service. One such mechanism might
assert a relationship between the signing identity and the author, as
specified in the From header field's domain identity[RFC2822].
Another might specify how to treat an unsigned message with that From
field domain.
4.1.4. Retain ability to have anonymous 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 email system operator to be authenticated, rather than the
content author. 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. Operational Goals
4.2.1. 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. 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 is not to take a message that has a broken
signature and treat it any differently than if the signature weren't
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there.
4.2.2. 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. 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. Permit incremental adoption for incremental 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, 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. Minimize the amount of required infrastructure
A new service, or an enhancement to an existing service, requires
adoption by some number of systems, 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 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 pair of participants
exchanging mail. Similarly, DKIM's reliance on the Domain Name
System greatly reduces the amount of new administrative
infrastructure that is need, across the open Internet.
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4.2.5. Permit wide range of deployment 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.
5. DKIM Function
DKIM has a very constrained set of capabilities, primarily targeting
email while it is in 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 not
signed, DKIM will permit the domain name of the author to be used for
obtaining information about their signing practices.
5.1. 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 to signing
messages for particular types of services, such as only for email.
This is 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.
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. 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
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parameters. Signature information is placed into a new [RFC2822]
header field of the message.
5.3. The Selector construct
The key for a signature is associated with a domain name, as
specified in the d= DKIM-Signature header field parameter. That
domain name is the complete identity used for making assessments
about the signer. 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
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. Verification
After a message has been signed, any agent in the message transit
path can verify the 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 of the message recipient.
6. Service Architecture
The DKIM service is divided into components that can be performed
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using different, external services, such as for key retrieval.
However the basic DKIM signing specification defines an initial set
of these services, in order to ensure a basic level of
interoperability.
|
|- RFC2822 Message
V
+------------------------------------+
| ORIGINATING OR RELAYING ADMD (MSA) |
| |
+..>| Sign Message |
. +--------------+---------------------+
. |
.private |
+---+---+ |
| Key | | +-----------+
| Store | [Internet] | Sender |
+---+---+ | | Practices |
.public | +-----+-----+
. V .
. +-----------------------------------+ .
. | RELAYING OR DELIVERING ADMD (MDA) | .
. | | .
. | Message Signed? | .
. +-------+----------------+----------+ .
. |yes |no .
. V V .
. +-----------+ +-----------+ .
+.....>| Verify | +-->| Check |<.......+
| Signature | | | Practices |<.......+
+---+-----+-+ | +---+-------+ .
| | | | .
| +---+ | .
pass| fail | .
V | +-----+-----+
+--------+ | | Local |
+.......>| Assess | | | Sender |
. | Signer | | | Practices |
. +---+----+ | +-----------+
. assessment| |
. +------+ +------+
. | |
+-+-----------+ V V
| Reputation/ | +-----------+
|Accreditation| | Message |
| Info | | Filtering |
+-----+-------+ | Engine |
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+-----------+>
Figure 2: DKIM Service Architecture
As shown in Figure 2, 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 practices is retrieved remotely and/or locally, and that
information is passed to the message filtering system.
Note: Figure 2 does not show the effects on the message-handling
flow when multiple signatures or third-party signatures are
present.
6.1. 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 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
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create and maintain it.
Reputation/Accreditation "Reputation/Accreditation" provides
quality-assessment information that is associated with a domain
name, and comes in many forms and from many sources. DKIM does
not define these 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 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.
6.3. 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.
6.4. 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 signature (a signature associated with the originator of
the message as opposed to a signature associated with an
intermediary) prompt a query for any published "sender practices"
information, as an aid in determining whether the sender information
has been used without authorization.
6.5. Evaluating
The Figure 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, other than the expectation that
DKIM-related information is 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 is practices
7. Security Considerations
TBD
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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.
[]
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
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.
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[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.
[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.
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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