DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories
Intended status: Informational D. Crocker
Expires: September 5, 2007 Brandenburg InternetWorking
P. Hallam-Baker
VeriSign Inc.
March 4, 2007
DomainKeys Identified Mail (DKIM) Message Signing Service Overview
draft-ietf-dkim-overview-04
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Abstract
DomainKeys Identified Mail (DKIM) associates a "responsible" identity
with a message and provides a means of verifying that the association
is legitimate.[I-D.ietf-dkim-base]. DKIM defines a domain-level
authentication framework for email using public-key cryptography and
key server technology. This permits verifying the source or
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intermediary for a message, as well as the contents of messages. The
ultimate goal of this framework is to permit a signing domain to
assert responsibility for a message, thus proving and protecting the
identity associated with the message and the integrity of the
messages itself, while retaining the functionality of Internet email
as it is known today. Such protection of email identity, may assist
in the global control of "spam" and "phishing". This document
provides an overview of DKIM and describes how it can fit into a
messaging service, how it relates to other IETF message signature
technologies. It also includes implementation and migration
considerations.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. The DKIM Value Proposition . . . . . . . . . . . . . . . . 5
1.3. Phishing . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Conventions . . . . . . . . . . . . . . . . . . . . . . . 6
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Treat verification failure as if unsigned. . . . . . . . . 7
2.2. Domain-level assurance . . . . . . . . . . . . . . . . . . 7
2.3. Incremental adoption . . . . . . . . . . . . . . . . . . . 7
2.4. Minimal infrastructure . . . . . . . . . . . . . . . . . . 8
2.5. Transparent signature . . . . . . . . . . . . . . . . . . 8
2.6. Security policy . . . . . . . . . . . . . . . . . . . . . 9
3. Function and Use . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. What is a DKIM signature? . . . . . . . . . . . . . . . . 9
3.2. The Selector construct . . . . . . . . . . . . . . . . . . 9
3.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. What does DKIM NOT do? . . . . . . . . . . . . . . . . . . 10
3.5. Does DKIM eliminate anonymity for email? . . . . . . . . . 11
4. DKIM Within Existing Internet Email . . . . . . . . . . . . . 11
4.1. Review of Internet Mail Service Architecture . . . . . . . 11
4.2. Where to Place DKIM Functions . . . . . . . . . . . . . . 14
4.3. Impact on Email Activities . . . . . . . . . . . . . . . . 15
4.4. Migrating from DomainKeys . . . . . . . . . . . . . . . . 17
5. Service Architecture . . . . . . . . . . . . . . . . . . . . . 18
6. Development . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Coding Criteria for Cryptographic Applications . . . . . . 20
6.2. Email Infrastructure Agents . . . . . . . . . . . . . . . 21
6.3. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 22
6.4. DNS Server . . . . . . . . . . . . . . . . . . . . . . . . 23
7. Deployment . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2. Verifying . . . . . . . . . . . . . . . . . . . . . . . . 24
7.3. Mail User Agent . . . . . . . . . . . . . . . . . . . . . 25
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7.4. Transition strategy . . . . . . . . . . . . . . . . . . . 25
8. On-going Operations . . . . . . . . . . . . . . . . . . . . . 27
8.1. DNS Signature Record Deployment Considerations . . . . . . 27
8.2. Private Key Management . . . . . . . . . . . . . . . . . . 29
8.3. Mailing list manager developers . . . . . . . . . . . . . 29
9. Example Uses . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.1. Protection of Internal Mail . . . . . . . . . . . . . . . 30
9.2. Recipient-based Assessments . . . . . . . . . . . . . . . 30
9.3. DKIM Support in the Client . . . . . . . . . . . . . . . . 31
9.4. Per user signature . . . . . . . . . . . . . . . . . . . . 31
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
11. { Misc Text -- Should go elsewhere, if used at all } . . . . . 32
12. Informative References . . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
Intellectual Property and Copyright Statements . . . . . . . . . . 35
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1. Introduction
DomainKeys Identified Mail (DKIM) associates a "responsible" identity
with a message and provides a means of verifying that the association
is legitimate.[I-D.ietf-dkim-base]. DKIM defines a domain-level
authentication framework for email using public-key cryptography and
key server technology. This permits verifying the source or
intermediary for a message, as well as the contents of messages. The
ultimate goal of this framework is to permit a signing domain to
assert responsibility for a message, thus proving and protecting the
identity associated with the message and the integrity of the
messages itself, while retaining the functionality of Internet email
as it is known today. Such protection of email identity, may assist
in the global control of "spam" and "phishing". This document
provides an overview of DKIM and describes how it can fit into a
messaging service, how it relates to other IETF message signature
technologies. It also includes implementation and migration
considerations.
This document provides an overview of DomainKeys Identified Mail
(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 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 [I-D.ietf-dkim-base] and [I-D.ietf-dkim-threats],
respectively. The document assumes a background in basic network
security technology and services.
It must be stressed that neither this document nor DKIM attempt to
provide solutions to the world's problems with spam, phish, virii,
worms, joe jobs, etc. DKIM creates one basic tool in what needs to
be a large arsenal of tools, for improving the safety of Internet
mail. 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. Rather, it is a basic introduction to the
technology and its deployment.
1.1. Background
There have been four other efforts at standardizing an email
signature scheme:
o Privacy Enhanced Mail (PEM) was first published in 1987 [RFC0989]
o PEM eventually transformed into MIME Object Security Services in
1995 [RFC1848]. Today, these two are only of historical interest.
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o Pretty Good Privacy (PGP) was developed by Phil Zimmerman and
first released in 1991.[RFC1991] A later version was standardized
as OpenPGP. [RFC3156]
o RSA Security, the holder of the patent rights to the principle
public key cryptography algorithm, 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 has achieved ubiquity in
deployment or use and their goals differ from those of DKIM.
In principle the S/MIME protocol can support semantics such as domain
level signatures or make use of keys stored in the DNS. However the
currently deployed base does not and modifying it to do so would
require extensive effort.
Unlike previous IETF email security initiatives, DKIM employs a key
centric Public Key Infrastructure (PKI) as opposed to one that is
based on a certificate in the style of Kohnfelder (X.509) or
Zimmerman (web of trust). That is, the owner of a key asserts its
validity, rather than relying on 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
validating signature service.
Further, DKIM's PKI is supported as additional information records to
the existing Domain Name Service, rather than requiring deployment of
a new query infrastructure. This approach also has significant
performance advantages as DNS is layered on UDP and key retrieval is
typically achieved in a single round trip.
1.2. The DKIM Value Proposition
Spam can be understood as two separate problems. The first is the
problem of companies that are inappropriately aggressive, in sending
out unsolicited marketing email. This accounts for, perhaps, 5% of
the spam volume and is in any case usually handled by existing spam
filters. [N.B. need a reference for the 5%.] The second problem is
rogue spam -- often involving criminal activities -- mostly sent from
coerced botnets of compromised machines. For this latter set of
mail, the origins of a message are often falsely stated.
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.
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For messages that produce a valid signature, it will be possible to
make affirmative trust assessments. For messages using identities
that are typically signed, it will be possible to detect some types
of phishing emails and block them.
1.3. Phishing
The value of a cousin domain, that could be mistaken for the
legitimate domain, is significantly reduced if the number of emails
that can be successfully sent from it is small.
Phishing attacks are typically made against trusted brands, that is,
names that are closely affiliated with well-known organizations. A
DKIM-based accreditation service can enforce a basic separation
between domains used by such known organizations and domains used by
others.
Receivers who successfully validate a signature can use information
about the signer as part of a program to limit spam, spoofing,
phishing, or other undesirable behavior, although the DKIM
specification itself does not prescribe any specific actions by the
recipient.
1.4. Conventions
In this document, references to structured fields of a message use a
two-part dotted notation. The first part cites the document that
contains the specification for the field and the second is the name
of the field. Hence <RFC2822.From> is the From field in an email
content header [RFC2822] and <RFC2821.MailFrom> is the address in the
SMTP "Mail From" command. [RFC2821]
This document is being discussed on the DKIM mailing list,
ietf-dkim@mipassoc.org.
2. Goals
DKIM lets an organization take responsibility for a message. The
organization taking responsibility typically is a handler of the
message, either as its originator or as an intermediary. It can also
be an independent service, providing assistance to a handler of the
message. Their reputation 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. This means that
their reputation is at stake.
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By design, DKIM purposely:
o is compatible with the existing email infrastructure and
transparent to the fullest extent possible
o requires minimal new infrastructure
o can be implemented independently of clients in order to reduce
deployment time
o does not require the use of new trusted third parties (e.g.,
certificate authorities) that might impose significant costs or
introduce delays to deployment
o can be deployed incrementally, with separate deployment of signers
and verifiers in either order
o allows delegation of signing to third parties
o is not intended be used for archival purposes
DKIM authentication provides one link in a chain of responsibility,
hopefully leading to better accountability by the senders.
2.1. Treat verification failure as if unsigned.
PGP and S/MIME were both designed for strong cryptographic
protection. This included treating validation failure as message
failure. For DKIM, the guidance is that an email 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.
2.2. Domain-level assurance
PGP and S/MIME apply the end-to-end principle in terms of individual
originators and recipients, notably using full email addresses. DKIM
seeks accountability at the more coarse grain of an organization or,
perhaps, a department. A deployed construct that enables this
granularity is the domain name, to which the signing key record is
bound.
2.3. Incremental adoption
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.
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Although it is envisioned that this mechanism will call upon
independent assessment services, they are not essential in order to
obtain initial benefit. For example DKIM allows pair-wise sets of
(possibly large) 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 spam filters. In effect the email receiver is
using their set of known relationships to generate their own
accreditation/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.
Over time, DKIM adoption might become sufficiently widespread to
permit special, negative handling of messages that are not signed.
However early benefits do not require this more-stringent
enforcement.
2.4. Minimal infrastructure
DKIM can be implemented 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
infrastructure, rather than part of a more localized operation.
Similarly, DKIM's reliance on the Domain Name Service greatly reduces
the amount of new administrative infrastructure that must be deployed
over the open Internet.
Even with use of the DNS, one challenge is that it is usually
operated by different administrative staff than those who operate an
organization's email service. In order to ensure that DKIM DNS
records are accurate, this imposes a requirement for careful
coordination between the two operations groups.
2.5. Transparent signature
S/MIME and PGP both modify the message body. Hence, their presence
is visible to all email recipients and their user software must be
able to process the associated constructs. In order to facilitate
incremental adoption, DKIM is designed to be transparent to
recipients that do not support it.
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2.6. Security policy
DKIM separates basic authentication from policy. An authenticated
identity may be subject to a variety of processing policies, either
ad hoc or standardized. The only policy inherent in a DKIM signature
is that the signer is asserting (some) responsibility for the
message. Other possible policies to consider developing include
asserting a relationship between the signing identity and the author
(RFC 2822 From) domain identity, as well as whether to tread unsigned
message with that From domain as problematic. It would, therefore,
be helpful for a potential signer to be able to publish various
policies, to permit a receiver to know more about the signer's
practices
3. Function and Use
DKIM has a very constrained set of capabilities, primarily targeting
email while it is in transit, from an originator to one or more
recipients. DKIM defines a mechanism by which email messages can be
cryptographically signed, permitting a signing domain to claim
responsibility for the presence of a message in the mail stream. A
responsible organization adds a digital signature to the message,
associating it with a domain name of that organization. Typically,
signing will be done by a service agent within the authority of the
message originator's Administrative Management Domain (ADMD).
(Signing might be performed by any of the functional components in
that environment, including a Mail User Agent (MUA), a Mail
Submission Agent (MSA), or an Internet Boundary MTA. DKIM also
permits signing to be performed by authorized third-parties.)
3.1. What is a DKIM signature?
A 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. They then use a private key to cryptographically encode
this information, along with other signing parameters. The signature
information is placed into a new header field of the [RFC2822]
message.
3.2. The Selector construct
For a single domain, DKIM permits use of multiple signing keys and/or
multiple signers. To do this, DKIM identifies a particular signature
as a combination of the domain name and an added field, called the
"selector". Both of these are coded into the DKIM-Signature header
field.
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Selectors are assigned according to the administrative needs of the
signing domain, such as for rolling over to a new key or for
delegating of the right to authenticate a portion of the namespace to
a trusted third party.
Examples include: jun2005.eng._domainkey.example.com
widget.promotion._domainkey.example.com
NOTE: It is intended that assessments of DKIM identities be
based on the domain name, and not include the selector. This
permits the selector to be used only for key administration,
rather than having an effect on reputation assessment.
3.3. Validation
After a message has been signed, any agent in the message transit
path can choose to validate the signature, to determine that the
signing identity took responsibility for the message. Message
recipients can verify the signature by querying 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, validation will be
done by an agent in the ADMD of the message recipient.
3.4. What does DKIM NOT do?
A DKIM signature does not:
o offer any assertions about the behaviors of the identity doing the
signing.
o prescribe any specific actions for receivers to take upon
successful (or unsuccessful) signature validation.
o provide protection after message delivery.
o protect against re-sending (replay of) a message that already has
a valid signature; therefore a transit intermediary or a recipient
can re-post the message in such a way that the signature would
remain valid, although the new recipient(s) would not have been
specified by the originator.
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3.5. Does DKIM eliminate anonymity for email?
The ability to send a message that does not identify its author is
considered to be a valuable quality of the current email system. It
turns out that DKIM is particularly helpful to this goal, because a
DKIM signature will typically be used to identity an email system
operator, rather than a content author. Knowing that a mail
definitely came from example.com does not threaten the anonymity of
the user, if it is still possible to obtain effectively anonymous
accounts at example.com and other web mail providers.
4. DKIM Within Existing Internet Email
4.1. Review of Internet Mail Service Architecture
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
and delivering it to one or more other users, creating a virtual MUA-
to-MUA exchange environment. The first MTA is called the Mail
Submission Agent (MSA) and the final MTA is called the Mail Delivery
Agent (MDA)
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+--------+
+---------------->| User |
| +--------+
| ^
+--------+ | +--------+ .
| User +--+--------->| User | .
+--------+ | +--------+ .
. | ^ .
. | +--------+ . .
. +-->| User | . .
. +--------+ . .
. ^ . .
. . . .
V . . .
+---+----------------+------+------+---+
| . . . . |
| +...............>+ . . |
| . . . |
| +......................>+ . |
| . . |
| +.............................>+ |
| |
| Mail Handling Service (MHS) |
+--------------------------------------+
Figure 1: Basic Internet Mail Service Model
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.
4.1.1. Administrative Actors
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, a department operating a local
Relay, an IT department operating an enterprise Relay and an ISP
operating a public shared email service. 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
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activities within the ADMD, versus those involving interactions with
other ADMDs.
Basic components of ADMD distinction 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.
+-------+ +-------+ +-------+
| ADMD1 | | ADMD3 | | ADMD4 |
| ----- | | ----- | | ----- |
| | +---------------------->| | | |
| User | | |-Edge--+--->|-User |
| | | | +--->| | | |
| V | | | +-------+ +-------+
| Edge--+---+ |
| | | +---------+ |
+-------+ | | ADMD2 | |
| | ----- | |
| | | |
+--->|-Transit-+---+
| |
+---------+
Figure 2: ADministrative Management Domains (ADMD) Example
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 an ADMD are subject to the policies of that domain.
Common ADMD examples are:
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Enterprise Service Providers: Operating an organization's
internal data and/or mail services.
Internet Service Providers: Operating 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: Operating email services, such as for
end-users, or mailing lists.
4.2. Where to Place DKIM Functions
Deciding which ADMD shall perform signing or verifying, which
identity to assign and which functional components to use for DKIM
processing depends upon the nature of the trust/reputation that is of
interest and the most convenient or efficient way to use it.
Messages may be signed or verified by any functional component within
an ADMD, as that domain wishes to arrange. Examples include:
Outbound: MUA, MSA or boundary MTA.
Inbound: Boundary MTA, MDA or MUA.
By having an MUA do the signing or verifying, there is no dependence
upon implementation by an email service infrastructure. By having an
MHS component do signing or verifying, there is no dependence upon
user training or the upgrade of potentially large numbers of user
applications.
For implementation by an ADMD's email service operators, perhaps the
most obvious choices within the MHS are the MSA or MDA, and the
outbound or inbound (boundary) MTA. By signing or verifying at the
MSA and MDA, respectively, this outermost portion of the MHS provides
true end-to-end service, and requires the smallest amount of trust of
the intervening service infrastructure. By signing or verifying at a
boundary, the smallest number of systems need modifying within an
ADMD and the signature is subject to the smallest amount of handling
that can break the signature. Note, however, that this will
eliminate DKIM signing for mail that stays within the ADMD.
The choice of identity to use might not be obvious. Examples
include:
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Author: The domain associated with the RFC2822.From field
provides basic authorization for the author to generate mail.
Because the organization controlling that domain is closest to
the author, they well might be in the best position to offer
their reputation as a basis for asserting that the content is
"safe".
Operator: Email recipient services have long-used the IP Address
of a client SMTP server as the basis for assessing whether to
permit relay or delivery of a message. These Addresses
identify the operator of an email service, rather than
necessarily indicating the author of messages being sent by
that service. Use of an operator's domain name is a natural
extension of this model.
Third-party: An independent service might wish to certify an
author, a message or an operator, by providing its own
signature to a message. Hence, evaluation of the message will
be based upon the identity of that third-party, rather than any
of the identities involved in creating or transferring the
message. Indeed, this model is already emerging among a number
of reputation-vetting services and is similar to the way a
credit card permits a customer to make purchases, based upon
the reputation of the credit card company -- and its
willingness to issue the card.
Ultimately, deciding where to sign a message will depend upon both
the identity being used and tradeoffs among flexibility of uses,
administrative control, and operational control. Deciding where to
verify a message will depend upon the intended use and similar
concerns about flexibility and control. A typical choice for
reputation-related verification will be to place the signature
verification function "close" to the message-filtering engine.
4.3. Impact on Email Activities
4.3.1. Resources
Although public-key cryptographic authentication is considered to be
computationally expensive, the real impact of a mechanism like DKIM
can be remarkably small. Direct impact on CPU-load has been measured
to be 10-15%. Mail handling tends to be I/O-intensive, so that
dedicated email platforms tend to have unused computational capacity.
It is therefore likely that no new hardware will be required for
these systems to be able to add DKIM support.
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4.3.2. Operations
The costs to deploy, administer and operate DKIM vary greatly,
depending upon the placement of DKIM-related modules. The greatest
flexibility comes from placing the modules as close as possible to
the end user. However this also imposes the greatest costs. The
most common scenarios are likely to be:
Boundary MTA: Here, DKIM is used only for email external to the
ADMD. Administration and operation is the simplest, but could
cause problems for mobile users who are associated with the
organization but must send mail using facilities that are
independent of their home ADMD. It also provides no assistance
for inter-departmental accountability within the ADMD..
MSA/MDA (Department): Placing DKIM support at the points of
submission and delivery increases the deployment costs but
still keeps control within the ADMD's operational staff. It
avoids the considerable, added costs of having to enhance all
of the MUAs. This does not improve the lot of mobile users who
submit from independent MSAs, but does provide full protection
within the ADMD.
MUA: Obviously this can provide the most complete protection,
but at the cost of considerable added administrative effort.
Worse, there is extensive evidence that email infrastructure
services often perform changes to message content that can
break a message signature. Examples include transformation by
the MSA to ensure that the message is in full conformance with
Internet standards and transformation by Boundary MTAs, to
ensure conformance with organizational policies about external
communications.
4.3.3. Mobile Users
Mobile users often must post messages through MSAs that are not under
the control of the user's home ADMD. Placing DKIM signing into the
MUA is the only way to ensure that a highly mobile user retains all
of its benefits, in spite of having to send mail through these
independent MSAs. However this leads to the administrative overhead
of having a DKIM DNS key selector record for each mobile user. For
mail reading, no changes are needed.
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4.3.4. Mailing Lists
A mailing list takes delivery of a message and reposts it, usually
with significant changes to the message. This will often break a
DKIM signature, although DKIM has some features to survive simple
mailing list modifications. For mailing lists that impose
substantial changes, it will be the responsibility of the list
operator to add their own DKIM signature.
4.4. Migrating from DomainKeys
4.4.1. Signing
DNS Records: DKIM is upward compatible with existing DomainKeys
(DK) [I-D.delany-domainkeys-base] DNS records, so that a DKIM
module does not automatically require additional DNS
administration! However, it should be noted that DKIM has
enhanced the DomainKeys DNS key record format by adding several
optional parameters.
Boundary MTA: The principle use of DomainKeys is at Boundary
MTAs. Because no operational transition is ever instantaneous,
a signer SHOULD add a DKIM signature to a message that has a
DomainKeys signature, rather than replacing it, until such time
as DomainKeys receive-side support is sufficiently reduced.
With respect to signing policies, a reasonable, initial
approach is to use DKIM signatures in the same way as
DomainKeys signatures are already being use.
4.4.2. Verifying
DNS Client: DNS queries for the DKIM key record, use the same
Domain Name naming conventions and the same basic record
format. No changes to the DNS client should be required.
Verifying module: See "Signing Module".
4.4.3. Boundary MTA
Independent of whether a Boundary MTA is performing general message
filtering, a helpful practice is to have it check for DKIM signatures
that purport to be made with a domain name that belongs to the ADMD
of the Boundary MTA. If the signature does not validate, the MTA MAY
decide to delete the signature.
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5. Service Architecture
DKIM provides an end-to-end service for signing and verifying
messages that are in transit. It is divided into components that can
be performed using different, external services (e.g., key
retrieval), although the basic DKIM operation provides an initial
set.
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|
| - RFC2822 Message
V
+---------------------------------------------+
+-----------+ | ORIGINATING OR RELAYING ADMD |
| Signer | | ============================ |
| Practices +......>| SIGN MESSAGE |
+-----+-----+ | ...> ADD A SIGNATURE HEADER FIELD |
. ...>| . GET (Domain, Selector, Priv-Key) |
. / | ... COMPUTE SIGNATURE |
. V +----------------+----------------------------+
. +-------+ | - Message
. | Key | | 1*(Domain, Selector,
. | Store | [Internet] Sig)
. +---+---+ V
. . +---------------------------------------------+
. . | RELAYING OR DELIVERING ADMD |
. . | =========================== |
. . | VERIFY MESSAGE (Verifier Practices) |
. . | ...> VERIFY A SIGNATURE HEADER FIELD |
. . | . GET A SIGNATURE |
. \...>| . LOOKUP PUB-KEY (Domain, Selector) |
. | . VERIFY SIGNATURE VALUE |
. | ... EVALUATE SIGNATURE CONSTRAINTS |
. +-------------------+-------------------------+
. | - Verified Domain(s)
. V - [Report]
. +---------------------------------------------+
\............>| MESSAGE DISPOSITION |
............>| SIGNER PRACTICES |
/ | REPUTATION |
+-----+------+ +---------------------------------------------+
| Reputation |
+------------+>
Figure 3: DKIM Service Architecture
Basic message processing is divided between signing the message,
validating the signature, and then making further decisions based
upon the validated signature. The signing component applies whatever
signing policies are in force, including ones that determine which
private key to use. Validation may be performed by any functional
component along the relay and delivery path. Validators retrieve the
public key based upon the parameters stored in the message. The
figure shows using the validated identity to assess an associated
reputation, but it could be applied to other tasks, such as
management tracking of mail.
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6. Development
6.1. Coding Criteria for Cryptographic Applications
Correct implementation of a cryptographic algorithm is a necessary
but not a sufficient condition for coding of cryptographic
applications. Coding of cryptographic libraries requires close
attention to security considerations that are unique to cryptographic
applications.
In addition to the usual security coding considerations, such as
avoiding buffer or integer overflow and underflow, implementers
should pay close attention to management of cryptographic private
keys and session keys, ensuring that these are correctly initialized
and disposed of.
Operating system mechanisms that permit the confidentiality of
private keys to be protected against other processes SHOULD be used
when available. In particular, great care MUST be taken when
releasing memory pages to the operating system to ensure that private
key information is not disclosed to other processes.
On multiple processor and dual core architectures, certain
implementations of public key algorithms such as RSA may be
vulnerable to a timing analysis attack.
Support for cryptographic hardware providing key management
capabilities is strongly encouraged. In addition to offering
performance benefits, many cryptographic hardware devices provide
robust and verifiable management of private keys.
Fortunately appropriately designed and coded cryptographic libraries
are available for most operating system platforms under license terms
compatible with commercial, open source and free software license
terms. Use of standard cryptographic libraries is strongly
encouraged. These have been extensively tested, reduce development
time and support a wide range of cryptographic hardware.
6.1.1. Signer
Signer implementations SHOULD provide a convenient means of
generating DNS key records corresponding to the signer configuration.
Support for automatic insertion of key records into the DNS is also
highly desirable. If supported however such mechanism(s) MUST be
properly authenticated.
Means of verifying that the signer configuration is compatible with
the signature policy is also highly desirable.
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Disclosure of a private signature key component to a third party
allows that third party to impersonate the sender. Protection of
private signature key data is therefore a critical concern. Signers
SHOULD support use of cryptographic hardware providing key management
features.
The import and export of private keys, and the ability to generate a
Certificate Signing Request (CSR) for certificate registration are
highly desirable.
6.1.2. Verifier
Verifiers SHOULD treat the result of the verification step as an
input to the message evaluation process rather than as providing a
final decision. The knowledge that a message is authentically sent
by a domain does not say much about the legitimacy of the message,
unless the characteristics of the domain claiming responsibility are
known.
In particular, verifiers SHOULD NOT automatically assume that an
email message that does not contain a signature, or that contains a
signature that does not validate, is forged. Verifiers should treat
a signature that fails to validate the same as if no signature were
present.
6.2. Email Infrastructure Agents
It is expected that the most common venue for a DKIM implementation
will be within the infrastructure of an organization's email service,
such as a department or a boundary MTA.
Outbound: An MSA or Outbound MTA should allow for automatic
verification of the MTA configuration such that the MTA can
generate an operator alert if it determines that it is (1) an
edge MTA, (2) configured to send email messages that do not
comply with the published DKIM sending policy.
An outbound MTA should be aware that users may employ MUAs that
add their own signatures and be prepared to take steps
necessary to ensure that the message sent is in compliance with
the advertised email sending policy.
Inbound: An inbound MTA or an MDA that does not support DKIM
should avoid modifying messages in ways that prevent
verification by other MTAs, or MUAs to which the message may be
forwarded.
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Intermediaries: An email intermediary is both an inbound and
outbound MTA. Each of the requirements outlined in the
sections relating to MTAs apply. If the intermediary modifies
a message in a way that breaks the signature, the intermediary
+ SHOULD deploy abuse filtering measures on the inbound mail,
and
+ MAY remove all signatures that will be broken
In addition the intermediary MAY:
* Verify the message signature prior to modification.
* Incorporate an indication of the verification results into the
message, such as using an Authentication-Results header
[I-D.kucherawy-sender-auth-header].
* Sign the modified message including the verification results
(e.g., the Authentication-Results header).
6.3. Filtering
Developers of filtering schemes designed to accept DKIM
authentication results as input should be aware that their
implementations will be subject to counter-attack by email abusers.
The efficacy of a filtering scheme cannot therefore be determined by
reference to static test vectors alone; resistance to counter attack
must also be considered.
Naive learning algorithms that only consider the presence or absence
of a valid DKIM signature are vulnerable to an attack in which a
spammer or other malefactor signs all their mail, thus creating a
large negative value for presence of a DKIM signature in the hope of
discouraging widespread use.
If heuristic algorithms are employed, they should be trained on
feature sets that sufficiently reveal the internal structure of the
DKIM responses. In particular the algorithm should consider the
domains purporting to claim responsibility for the signature, rather
than the existence of a signature or not.
Unless a scheme can correlate the DKIM signature with accreditation
or reputation data, the presence of a DKIM signature SHOULD be
ignored.
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6.4. DNS Server
At a minimum, a DNS server that handles queries for DKIM key records
must perform server administrators to add free-form TXT records. It
would, of course, be better for provide them with a structured form,
to support the DKIM-specific fields.
7. Deployment
This section describes the basic steps for introducing DKIM into an
organization's email service operation. The considerations are
divided between the generating DKIM signatures (Signing) and the
processing of messages that contain a DKIM signature (Verifying).
7.1. Signing
Creating messages that have DKIM signatures requires modification of
only two portions of the email service:
o Addition of relevant DNS information.
o Addition of the signature by a trusted module within the
organization's email handling service.
The signing module uses the appropriate private key to create a
signature. The means by which the signing module obtains this key is
not specified by DKIM. Given that DKIM is intended for use during
email transit, rather than for long-term storage, it is expected that
keys will be changed regularly. Clearly this means that key
information should not be hard-coded into software.
7.1.1. DNS Records
A receiver attempting to validate a DKIM signature must obtain the
public key that is associated with the signature for that message.
The DKIM-Signature header in the message will specify the basic
domain name doing the signing and the selector to be used for the
specific public key. Hence, the relevant
<selector>._domainkeys.<domain-name> DNS record needs to contain a
DKIM-related resource record (RR) that provides the public key
information.
The administrator of the zone containing the relevant domain name
adds this information. Initial DKIM DNS information is contained
within TXT RRs. DNS administrative software varies considerably in
its abilities to add new types of DNS records.
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7.1.2. Signing Module
The module doing signing can be placed anywhere within an
organization's trusted Administrative Management Domain (ADMD);
common choices are expected to be department-level posting and
delivering agents, as well as boundary MTAs to the open Internet.
(Note that it is entirely acceptable for MUAs to perform signing and
validation.) Hence the choice among the modules depends upon
software development and administrative overhead tradeoffs. One
perspective that helps resolve this choice is the difference between
the flexibility of use by systems at (or close to) the MUA, versus
the centralized control that is more easily obtained by implementing
the mechanism "deeper" into the organization's email infrastructure,
such as at its boundary MTA.
7.1.3. Signing Policies and Practices
Every organization (ADMD) will have its own policies and practices
for deciding when to sign messages and with what domain name and key
(selector). Examples include signing all mail, signing mail from
particular email addresses, or signing mail from particular sub-
domains. Given this variability, and the likelihood that signing
practices will change over time, it will be useful to have these
decisions represented in some sort of configuration information,
rather than being more deeply coded into the signing software.
7.2. Verifying
Verification is performed within an ADMD that wishes to make
assessments based upon the DKIM signature's domain name. Any
component within the ADMD that handles messages, whether in transit
or being delivered, can do the verifying and subsequent assessments.
Verification and assessment might occur within the same software
mechanism, such as a Boundary MTA, or an MDA. Or they may be
separated, such as having verification performed by the Boundary MTA
and assessment performed by the MDA.
As with the signing process, choice of service venues for
verification and assessment -- such as filtering or presentation to
the recipient user -- depend on trade-offs for flexibility, control,
and operational ease. An added concern is that the linkage between
verification and assessment entails essential trust: the assessment
module MUST have a strong basis for believing that the verification
information is correct.
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7.2.1. DNS Client
The primary means of publishing DKIM key information, initially, is
initially through DNS TXT records. Some DNS client software might
have problems obtaining these records; as DNS client software
improves this will not be a concern.
7.2.2. Boundary Enforcement
In order for an assessment module to trust the information it
receives about verification (e.g., Authentication-Results headers),
it MUST eliminate verification information originating from outside
the ADMD in which the assessment mechanism operates. As a matter of
friendly practice, it is equally important to make sure that
verification information generated within the ADMD not escape outside
of it.
In most environments, the easiest way to enforce this is to place
modules in the receiving and sending Boundary MTA(s) that strip any
existing verification information.
7.3. Mail User Agent
DKIM is designed to support deployment and use in email components
other than an MUA. However an MUA MAY also implement DKIM features
directly.
Outbound: If an MUA is configured to send email directly, rather
than relayed through an outbound MSA, the MUA SHOULD be
considered as if it were an outbound MTA for the purposes of
DKIM. An MUA MAY support signing even if mail is to be relayed
through an outbound MSA. In this case the signature applied by
the MUA may be in addition to any MSA signature.
Inbound: An MUA MAY rely on a report of a DKIM signature
verification that took place at some point in the inbound MTA
path (e.g., an Authentication-Results header), or an MUA MAY
perform DKIM signature verification directly. A verifying MUA
SHOULD allow for the case where mail is modified in the inbound
MTA path.
7.4. Transition strategy
Deployment of a new signature algorithm without a 'flag day' requires
a transition strategy such that signers and verifiers can phase in
support for the new algorithm independently, and (if necessary) phase
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out support for the old algorithm independently.
[Note: this section assumes that a security policy mechanism exists.
It is subject to change.]
DKIM achieves these requirements through two features: First a signed
message may contain multiple signatures created by the same signer.
Secondly the security policy layer allows the signing algorithms in
use to be advertised, thus preventing a downgrade attack.
7.4.1. Signer transition strategy
Let the old signing algorithm be A, the new signing algorithm be B.
The sequence of events by which a Signer may introduce the new
signing algorithm B, without disruption of service to legacy
verifiers, is as follows:
1. Signer signs with algorithm A
A. Signer advertises that it signs with algorithm A
2. Signer signs messages twice, with both algorithm A and algorithm
B
A. The signer tests new signing configuration
B. Signer advertises that it signs with algorithm A and
algorithm B
3. Signer determines that support for Algorithm A is no longer
necessary
4. Signer determines that support for algorithm A is to be withdrawn
A. Signer removes advertisement for Algorithm A
B. Signer waits for cached copies of earlier signature policy to
expire
C. Signer stops signing with Algorithm A
7.4.2. Verifier transition strategy
The actions of the verifier are independent of the signer's actions
and do not need to be performed in a particular sequence. The
verifier may make a decision to cease accepting algorithm A without
first deploying support for algorithm B. Similarly a verifier may be
upgraded to support algorithm B without requiring algorithm A to be
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withdrawn. The decisions of the verifier must make are therefore:
o The verifier MAY change the degree of confidence associated with
any signature at any time, including determining that a given
signature algorithm provides a limited assurance of authenticity
at a given key strength.
* A verifier MAY evaluate signature records in any order it
chooses, including using the signature algorithm to choose the
order.
o The verifier MAY make a determination that Algorithm A does not
offer a useful level of security, or that the cost of verifying
the signature is less than the value of doing so.
* In this case the verifier would ignore signatures created using
algorithm A and references to algorithm A in policy records
would be treated as if the algorithm were not implemented.
o The verifier MAY decide to add support for additional signature
algorithms at any time.
* The verifier MAY add support for algorithm B at any time.
8. On-going Operations
This section describes the basic steps for operation of email systems
that use DKIM, after the capability has initially been deployed. The
primary considerations are:
o the upkeep of the selector files, and
o the security of the private keys.
8.1. DNS Signature Record Deployment Considerations
The key point to remember is that the DNS DKIM selectors WILL and
SHOULD be changed over time. Some reasons for changing DKIM
selectors are well understood; others are still theoretical. There
are several schemes that may be used to determine the policies for
changing DKIM selectors:
o time based
o associations with clusters of servers
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o the use of third party signers
o security considerations
8.1.1. Time Basis and Security Considerations
The reason for changing the selector periodically is usually related
to the security exposure of a system. When the potential exposure of
the private keys associated with the DKIM selector have reached
sufficient levels, the selector should be changed. (It is unclear
currently what kinds of metrics can be used to aid in deciding when
the exposure has reached sufficient levels to warrant changing the
selector.)
For example,
o Selectors should be changed more frequently on systems that are
widely exposed, than on systems that are less widely exposed. For
example, a gateway system that has numerous externally-accessible
services running on it, is more widely exposed than one that ONLY
runs a mail server.
o Selectors should be changed more frequently on operating systems
that are under wide attack.
o While the use of DKIM information is transient, keys with
sufficient exposure do become stale and should be changed.
o Whenever you make a substantial system change, such as bringing up
a new server, or making a major operating system change, you
should consider changing the selector.
o Whenever there is either suspicion or evidence of the compromise
of the system or the private keys, you should change the selector.
8.1.2. Deploying New Selectors
A primary consideration in changing the selector is remembering to
change it. It needs to be a standard part of the operational staff
Methods and Procedures for your systems. If they are separate, both
the mail team and the DNS team will be involved in deploying new
selectors.
When deploying a new selector, it needs to be phased in:
1. generate the new public / private key pair and create a selector
record with the public key in it
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2. add the new selector record to your DNS
3. verify that the new selector record can be used to verify
signatures
4. turn on signing with the new private key
5. remove the old private key from your servers
6. after a period of time, remove the old selector from your DNS
The time an unused selector should be kept in the DNS system is
dependent on the reason it's being changed. If the private key has
definitely been exposed, the corresponding selector should be removed
immediately. Otherwise, longer periods are allowable.
8.1.3. Subdomain Considerations
A Domain Name is the basis for making differential quality
assessments about a DKIM-signed message. It is reasonable for a
single organization to have a variety of very different activities,
which warrant a variety of very different assessments. A convenient
way to distinguish among such activities is to sign with different
domain names. That is, the organization should sign with sub-domain
names that are used for different organizational activities.
8.1.4. Third party Signature Delegations
Allowing third parties to sign email from your domain opens your
system security to include the security of the third party's systems.
At a minimum, you should not allow the third parties to use the same
selector and private key as your main mail system. It is recommended
that each third party be given their own private key and selector.
This limits the exposure for any given private key, and minimizes the
impact if any given private key were exposed.
8.2. Private Key Management
The permissions of private key files must be carefully managed. If
key management hardware support is available, it should be used.
Auditing software should be used to periodically verify that the
permissions on the private key files remain secure. [ Expand this
section? ]
8.3. Mailing list manager developers
A mailing list often provides facilities to its administrator to
manipulate parts of the mail messages that flow through the list.
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The desired goal is that messages flowing through the mailing list
will be verifiable by the recipient as being from the list, or
failing that, as being from the individual list members.
In most cases, the list and/or its mail host SHOULD add its own DKIM
signature to list mail. This could be done in the list management
software, in an outgoing MSA or MTA, or both. List management
software often makes modifications to messages that will break
incoming signatures, such as adding subject tags, adding message
headers or footers, and adding, deleting, or reordering MIME parts.
By adding its own signature after these modifications, the list
provides a valid recognizable signature for list recipients.
In some cases, mailing list software is simple enough that signatures
on incoming messages will still be valid after being remailed by the
list. It is still preferable that the list sign its mail so that
recipients can distinguish mail sent through the list from mail sent
directly by list members, but in the absence of a list signature, a
recipient may be able to recognize and use the signatures of list
members known to the recipient.
9. Example Uses
A DKIM signature tells the signature verifier that the owner of a
particular domain name accepts responsibility for the message.
Combining this information with information that allows the history
of the domain name owner to be assessed may allow processing the
message, based on the probability that the message is likely to be
trustworthy, or not, without the need for heuristic content analysis.
9.1. Protection of Internal Mail
One identity is particularly amenable to easy and accurate
assessment: The organization's own identity! Members of an
organization tend to trust messages that purport to be from within
that organization. However Internet Mail does not provide a
straightforward means of determining whether such mail is, in fact,
from within the organization. DKIM can be used to remedy this
exposure. If the organization signs all of its mail, then its
boundary MTAs can look for mail purporting to be from the
organization but which does not contain a valid signature. Such mail
can be presumed to be spurious.
9.2. Recipient-based Assessments
Recipients of large volumes of email can generate reputation data for
email senders internally. Recipients of smaller volumes of messages
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are likely to need to acquire reputation data from a third party. In
either case the use of reputation data is intrinsically limited to
email senders that have established a prior history of sending
messages.
In fact, an email receiving service may be in a position to establish
bilateral agreements with particular senders, such as business
partners or trusted bulk sending services. Although it is not
practical for each recipient to accredit every sender, definition of
core networks of explicit trust can be quite useful.
9.2.1. Third-party Assessments
For scaling efficiency, it is appealing to have Trusted Third Party
assessment services, to allow an email sender to obtain a single
assessment that is then recognized by every email recipient that
recognizes the Trusted Third Party.
9.3. DKIM Support in the Client
The DKIM specification is expected to be used primarily between
Boundary MTAs, or other infrastructure components of the originating
and receiving ADMDs. However there is nothing in DKIM that is
specific to those venues. In particular, it should be possible to
support signing and validating in MUAs.
However, it is comment for components of an ADMD's email
infrastructure to do violence to message, so as to render a DKIM
signature invalid. Hence, users of MUAs that support DKIM signing
and/or validating need a basis for knowing that their associated
email infrastructure will maintain signature validity.
DKIM requires that all verifiers treat messages with signatures that
do not verify as if they are unsigned. If verification in the client
is to be acceptable to users, it is also essential that successful
verification of a signature not result in a less than satisfactory
user experience compared to leaving the message unsigned.
9.4. Per user signature
Although DKIM's use of domain names is optimized for a scope of
organization-level signing, it is possible to have sub-domains and/or
selectors be administered in a way that supports per-user signing.
NOTE: As stated earlier, it is important to distinguish between use
of selectors, for differential administration of keys, versus sub-
domains, for differential reputations.
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As a constraint on an authorized DKIM signing agent, their associated
key record can specify restrictions on the email addresses permitted
to be signed with that domain and key. A typical intent of this
feature is to allow a company to delegate the signing authority for
bulk marketing communications without the risk of effectively
delegating the authority to sign messages purporting to come from the
domain-owning organization's CEO.
NOTE: Any scheme that involves maintenance of a significant number
of public keys is likely to require infrastructure enhancements,
to support that management. For example, a system in which the
end user is required to generate a public key pair and transmit it
to the DNS administrator out of band is not likely to meet
acceptance criteria for either usability or security.
10. Acknowledgements
TBD
11. { Misc Text -- Should go elsewhere, if used at all }
DKIM permits the signing identity to be different from the identities
used for the author or the initial posting agent. This is essential,
for example, to enable support of signing by authorized third-
parties, and to permit signatures by email providers who are
otherwise independent of the domain name of the message author.
DKIM permits restricting the use of a signature key to 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 MUST explicitly list the headers that are
signed. This is an improvement because it requires the signer to use
the more conservative (likely to verify correctly) mechanism and
makes it considerably more robust against the handling of
intermediary MTAs.
DKIM self-signs the DKIM-Signature header field, to protect against
its being modified.
In order to survive the vagaries of different email transfer systems,
mechanisms like DKIM must evaluate the message data in some canonical
form, such as treating a string of spaces as tabs as if they were a
single space. DKIM has added the "relaxed" canonicalization in place
of "nofws".
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MUA UI considerations
key delegation/ 3rd party
12. Informative References
[I-D.delany-domainkeys-base]
Delany, M., "Domain-based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)",
draft-delany-domainkeys-base-06 (work in progress),
July 2006.
[I-D.ietf-dkim-base]
Allman, E., "DomainKeys Identified Mail (DKIM)
Signatures", draft-ietf-dkim-base-10 (work in progress),
February 2007.
[I-D.ietf-dkim-threats]
Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", draft-ietf-dkim-threats-03 (work
in progress), May 2006.
[I-D.kucherawy-sender-auth-header]
Kucherawy, M., "Message Header for Indicating Sender
Authentication Status",
draft-kucherawy-sender-auth-header-04 (work in progress),
February 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.
[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.
[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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[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
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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