DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories
Intended status: Informational D. Crocker
Expires: April 25, 2007 Brandenburg InternetWorking
P. Hallam-Baker
VeriSign Inc.
October 22, 2006
DomainKeys Identified Mail (DKIM) Service Overview
draft-ietf-dkim-overview-02
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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.
Note
This document is being discussed on the DKIM mailing list,
ietf-dkim@mipassoc.org.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. The DKIM Value Proposition . . . . . . . . . . . . . . . . . . 6
3. DKIM's Goals . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Treat verification failure as if unsigned. . . . . . . . 8
3.2. Domain-level assurance . . . . . . . . . . . . . . . . . 8
3.3. Incremental adoption . . . . . . . . . . . . . . . . . . 8
3.4. Minimal infrastructure . . . . . . . . . . . . . . . . . 9
3.5. Transparent signature . . . . . . . . . . . . . . . . . . 9
3.6. Security policy . . . . . . . . . . . . . . . . . . . . . 9
4. A Quick Overview of DKIM . . . . . . . . . . . . . . . . . . . 9
4.1. What is a DKIM signature? . . . . . . . . . . . . . . . . 10
4.2. The Selector construct . . . . . . . . . . . . . . . . . 10
4.3. Who validates the signature? . . . . . . . . . . . . . . 10
4.4. What does DKIM NOT do? . . . . . . . . . . . . . . . . . 11
4.5. Does DKIM eliminate anonymity for email? . . . . . . . . 11
4.6. Outline potential DKIM applications . . . . . . . . . . . 11
4.7. What is a DKIM policy? . . . . . . . . . . . . . . . . . 11
5. DKIM Within Existing Internet Email . . . . . . . . . . . . . 12
5.1. Review of Internet Mail Service Architecture . . . . . . 12
5.2. Where to Place DKIM Functions . . . . . . . . . . . . . . 15
5.3. Impact on Email Activities . . . . . . . . . . . . . . . 16
5.4. Migrating from DomainKeys . . . . . . . . . . . . . . . . 18
6. DKIM Service Architecture . . . . . . . . . . . . . . . . . . 19
7. Implementation Considerations . . . . . . . . . . . . . . . . 21
7.1. Development . . . . . . . . . . . . . . . . . . . . . . . 21
7.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 24
7.3. DNS Server . . . . . . . . . . . . . . . . . . . . . . . 24
7.4. Accreditation service . . . . . . . . . . . . . . . . . . 24
8. Deployment . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Signing . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.2. Verifying . . . . . . . . . . . . . . . . . . . . . . . . 26
8.3. Transition strategy . . . . . . . . . . . . . . . . . . . 27
9. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. DNS Signature Record Deployment Considerations . . . . . 28
10. Outline Future Extensions . . . . . . . . . . . . . . . . . . 31
10.1. Introducing a new signing algorithm . . . . . . . . . . . 32
10.2. Possible future signature algorithm choices . . . . . . . 32
10.3. Linkage to Other PKIs . . . . . . . . . . . . . . . . . . 33
10.4. Trusted Third Party Assertions . . . . . . . . . . . . . 33
10.5. Linkage to X.509 Certificates . . . . . . . . . . . . . . 34
10.6. XKMS . . . . . . . . . . . . . . . . . . . . . . . . . . 35
10.7. Verification in the Client . . . . . . . . . . . . . . . 36
10.8. Per user signature . . . . . . . . . . . . . . . . . . . 36
10.9. Encryption . . . . . . . . . . . . . . . . . . . . . . . 37
10.10. Reuse of Key Record . . . . . . . . . . . . . . . . . . . 37
10.11. Use of Policy Record . . . . . . . . . . . . . . . . . . 38
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11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
12. { Misc Text -- Should go elsewhere, if used at all } . . . . . 38
12.1. What Needs To Be Moved Here From the Base Doc? . . . . . 39
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
13.1. References -- Normative . . . . . . . . . . . . . . . . . 39
13.2. Informative References . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
Intellectual Property and Copyright Statements . . . . . . . . . . 41
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1. Introduction
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.
NOTE: 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.
NOTE: A number of sections in this document are just placeholders,
for now.
There have been four other efforts at standardizing an email
signature scheme:
o Privacy Enhanced Mail (PEM) was first published in 1987 [RFC0989]
and eventually transformed into MIME Object Security Services in
1995 [RFC1848]. Today, they are only of historical interest.
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.
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Unlike all four 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.
NOTE: It would be useful to include a citation to a general
discussion about PKI issues, including their long history of
difficulties with respect to the Internet.
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 layers on UDP and key retrieval is
typically achieved in a single round trip.
2. The DKIM Value Proposition
Spam can be understood as two separate problems. The first is the
problem of the 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. 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.
Even without the addition of independent accreditation services, 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 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.
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NOTE: Perhaps add some citations to detailed discussions about spam
and phishing and the role of authentication and accreditation in
fighting them?
DKIM used in conjunction with a real-time blacklist allows phishing
emails to be preemptively blocked. 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.
3. DKIM's 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.
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 trusted third parties (e.g.,
certificate authorities) which might impose significant costs or
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introduce delays to deployment
o can be deployed incrementally
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.
3.1. 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, at least warning the user that the message was unsigned. In
a small number of cases the application went even further 'warning'
the user whenever a signed message was received. This approach has
proved problematic. Hence for DKIM, the guidance is that an email
signature verifier is to treat messages with signatures that fail as
if they were unsigned.
It is highly unlikely that an attacker is going to add a digital
signature to a message unless doing so causes the message to be
treated more favorably than an unsigned one. Any messages that carry
signatures that fail verification are thus much more likely to be a
genuine message that has been damaged in transit than an attempted
forgery. It makes no sense to warn the recipient unless it is known
that the sender always signs email messages and that there is a high
probability that a forgery would be attempted.
3.2. 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.
3.3. Incremental adoption
DKIM can immediately provide benefit 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.
Over time, DKIM adoption might become sufficiently widespread to
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permit special, negative handling of messages that are not signed.
However early benefits do not require this more-stringent
enforcement.
3.4. 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.
3.5. Transparent signature
S/MIME and PGP are 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
for recipients that do not support it.
3.6. Security policy
DKIM is pursuing an incremental innovation, over basic identity
authentication, through the publication of security policies
associated with one or of the identities presented in a message. For
example, a valid DKIM signature allows the signing party to assert
responsibility for a message . For a recipient to interpret an
unsigned message it is necessary to know whether it should expect a
message from that source to be signed and if so the signature
characteristics to expect. It would, therefore, be helpful for a
potential signer to be able to publish whether they sign all of their
message. Once this is published, recipients can choose to handle the
receipt of unsigned messages with added caution.
4. A Quick Overview of DKIM
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
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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.)
4.1. 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
[RFC2822] message.
4.2. The Selector construct
In order to allow a domain name to support multiple, simultaneous
keys, a particular signature is identified by the combination of the
domain name and an added field, called the "selector". Both of these
are coded into the DKIM-Signature header field. 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, rather than include the selector. This
permits the selector to be used only for key administration,
rather than having an effect on reputation assessment.
4.3. Who validates the signature?
After a message has been signed, any agent in the message transit
path can choose to validate the signature. Validation of the
signature, by a later agent in the path, demonstrates 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
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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. Again,
this may be done by any functional component within that environment.
(Notably this means that the signature can be used by the recipient
ADMD's filtering software, rather than requiring the recipient end-
user to make an assessment.)
4.4. What does DKIM NOT do?
DKIM 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.
4.5. 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.
4.6. Outline potential DKIM applications
TBD
4.7. What is a DKIM policy?
TBD
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5. DKIM Within Existing Internet Email
5.1. 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 others, 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)
+--------+
+---------------->| 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.
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5.1.1. 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
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.
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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:
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.
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5.1.2. Field Referencing Convention
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 and <RFC2821.MailFrom> [RFC2821] is the address in the
SMTP "Mail From" command.
5.2. Where to Place DKIM Functions
DKIM associates a "responsible" identity with a message and provides
a means of verifying that the association is legitimate. Deciding
which ADMD shall perform signing or verifying, which identity to
assign and which functional components to use for DKIM processing
depend 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.
5.3. Impact on Email Activities
5.3.1. Resources
Although cryptographic authentication is considered to be
computationally expensive, the real impact of a mechanism like DKIM
is 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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5.3.2. 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.
In spite of these concerns, placing DKIM support into the MUA is
the only way to ensure that a highly mobile user retains its
benefits, in spite of having to send mail through independent
MSAs.
Challenge of mobile users. Server-resident folders -- web or imap --
no problem. Laptop-resident folders, requires remote MSA access or
per-user keying and mobile-author awareness.
Key creation and replacement. Update DNS and signing component
5.3.3. Users
Challenge of mobile users. Server-resident folders -- web or imap --
no problem. Laptop-resident folders, requires remote MSA access or
per-user keying and mobile-author awareness.
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Challenge of mailing lists. Different list styles warrant different
signature policies.
Can be hidden from end-user; used by filter engine. Method and
benefits for displaying to users unknown.
5.4. Migrating from DomainKeys
5.4.1. Signing
DNS Records: DKIM is upward compatible with existing DomainKeys
(DK) DNS records, so that a DKIM module does not automatically
require additional DNS administration! DKIM has enhanced the DK
DNS key record, to permit the addition of several parameters.
Boundary MTA: Enforce signature policies and practices
>
5.4.2. Verifying
DNS Client: TBD
Verifying module: See "Signing Module".
5.4.3. Boundary MTA
Strip "local" signatures that are not local?
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6. DKIM 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, such as for 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
. | | | 1*(Domain, Selector,
. | Key | | Sig)
. | Store | [Internet]
. | | |
. +---+---+ 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 divides between signing the message,
validating the signature, and then performing further decision-making
based upon the validated signature. The component doing the signing
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applies whatever signing policies are in force, including ones that
determine what private key to use. Validation may be performed by
any functional component along the relay and delivery path. The
public key is retrieved, based upon the parameters stored in the
message. The example shows use of the validated identity for
assessing an associated reputation. However it could be applied for
other tasks, such as management tracking of mail.
7. Implementation Considerations
7.1. Development
7.1.1. 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.
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7.1.1.1. 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.
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
CSR for certificate registration are highly desirable.
7.1.1.2. 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 a email
message that does not contain a signature or which contains a
signature that does not validate is forged. Verifiers should treat a
signature that fails to validate as if no signature was present.
7.1.2. Email Handlers
7.1.2.1. Mail User Agent
DKIM is designed to support deployment and use in email components
other than an MUA. However an MUA may also however implement DKIM
features directly.
Outbound: If an MUA is configured to send email directly, rather
than relayed through an outbound MTA, 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 MTA. In this case the signature applied by the MUA may
be in addition to or in place of the MTA signature.
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Inbound: An MUA MAY rely on a report of a DKIM signature
verification that took place at some point in the inbound MTA
path. An MUA MAY perform DKIM signature verification directly.
Such an MUA SHOULD allow for the case where mail is modified in
the inbound MTA path.
7.1.3. Mail Transfer Agent
It is expected that the most common venue for a DKIM implementation
with be a department or a boundary MTA.
Outbound: An 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 that does not support DKIM, it should avoid
modifying messages in ways that prevent verification by other
MTAs, or MUAs to which the message may be forwarded.
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 it SHOULD
* deploy abuse filtering measures on the inbound mail
* remove all signatures that will be broken
In addition the intermediary MAY:
* Verify the message signature prior to modification
* Incorporate an Authentication-Results header to report the
verification result.
* Sign the modified message including the Authentication-Results
header
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7.2. 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.
7.3. DNS Server
TBD
7.4. Accreditation service
TBD
8. Deployment
This section describes the basic steps for introducing DKIM into an
organization's email service operation. The considerations are
divided between an organization's generating DKIM signatures
(Signing) and an organization's processing of messages that contain a
DKIM signature (Verifying).
8.1. Signing
Creating messages that have DKIM signatures requires modification of
only two portions of the email service:
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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, for creating 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.
8.1.1. 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 RR that provides the public key information.
The administrator of the zone containing the relevant domain name
adds this information. DNS administrative software varies
considerably in its abilities to add new (types of) DNS records.
Initial DKIM DNS information is contained within TXT RRs.
8.1.2. 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.
However, note that it is entirely acceptable to have signing and
validation be done by MUAs. 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.
8.1.3. 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-
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domains. Given this variability, and the likelihood that signing
practices will change over time, it will useful to have these
decisions represented in some sort of configuration information,
rather than being more deeply coded into the signing software.
8.2. Verifying
Verification is performed within an ADMD that wishes to make
assessments based upon the domain name used for a DKIM signature.
Any component within the ADMD that handles messages, whether in
transit or being delivered, can be appropriate to do the verifying.
It must communicate the results of verification to another component
within the ADMD that performs the desired 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.
8.2.1. DNS Client
The primary means of publishing DKIM key information, initially, is
through DNS TXT records. Some DNS client software might have
problems obtaining these records. As DNS client software improves
this will not be an concern.
8.2.2. Boundary Enforcement
In order for an assessment module to trust the information it
receives about verification, it is essential to 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,
from within the ADMD, not escape out side of it.
In most environments, the easiest way to enforce this stripping of
verification information is to place modules in the receiving and
sending Boundary MTA(s). For incoming mail, check for known means of
communicating verification information and remove it. The same
applies for outgoing mail.
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8.3. 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
out support for the old algorithm independently.
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.
8.3.1. 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 a new signing
algorithm 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, first with 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 it 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
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8.3.2. 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 B to be
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 chose to evaluate signature records in any order
it chooses, including making use of the signature algorithm for
this purpose.
o The verifier MAY make a determination that Algorithm A does not
offer a useful level of security, that the cost of verifying the
signature is less than the value of doing so.
* In this case the verifier ignores signatures created using the
algorithm A and references to algorithm A in policy records are
treated as if the algorithm is not implemented.
o The verifier MAY decide to add support for additional signature
algorithms at any time.
* The verified MAY add support for algorithm B at any time.
9. Operations
This section describes the basic steps for the continued operation of
email systems that use DKIM. This section discusses keeping DKIM
going, as opposed to getting DKIM started. The primary
considerations are: the upkeep of the selector files, and the
security of the private keys.
9.1. 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:
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o time based
o associations with clusters of servers
o the use of third party signers
o security considerations
9.1.1. 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.
9.1.2. Server Clusters
TBD
9.1.3. 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.
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When deploying a 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
2. add the new selector record to your DNS
3. turn on signing with the new private key
4. optionally back up the old private key in a secure location
5. remove the old private key from your servers
6. after a period of time, remove the old selector
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.
9.1.4. Subdomain Considerations
TBD
9.1.5. 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.
9.1.6. Mailing List Management
[ Note: this section may be controversial. ]
A mailing list often provides facilities to its administrator to
manipulate parts of the mail messages that are flowing through. The
desired goal is that messages flowing through the mailing list will
be verifiable by the recipient, which means that either the mailing
list (or its MSA) must sign the message, or the mailing list must not
perform actions on the messages that will break existing DKIM
signatures. To avoid breaking existing signatures, a mailing list
system has these choices:
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o A mailing list may add its own DKIM signature. If it does this,
it must make sure that it adds its signature after it performs any
content transformations to the message, such as adding a footer to
the body, adding a prefix to the body, modifying the subject
header, etc.
o If a mailing list does not add its own DKIM signature, it must not
modify any existing headers that are listed in an h= parameter of
any existing DKIM-Signature headers, nor may it add any footer
content to the body if there is no l= in any existing DKIM-
Signature headers.
o If a mailing list cannot add its own DKIM signature, and must
modify the headers or body in a way that will break verification
of existing DKIM-Signature headers, it should remove any existing
DKIM-Signature headers.
10. Outline Future Extensions
The design of DKIM is unapologetically focused on overcoming the need
for immediate deployment and achieving ubiquitous use in the near
future. As such, the original design discussions have generally
taken the approach 'if in doubt, leave it out'.
The need to exclude consideration of these features in the near term
is in no way intended to preclude their development at a later date.
Nor is the lack of a specification describing the use of DKIM with a
different PKI infrastructure intended to indicate an intention to
develop similar capabilities within the DKIM framework at a future
date.
DKIM is an example of 'Design for Deployment'. The need to support
rapid deployment has been the overriding priority. It has
traditionally been asserted that deployment of a flawed cryptographic
protocol is an almost unacceptable risk, and that bad security is
worse than no security. Experience demonstrates otherwise.
Informing users that email is insecure does not cause them to modify
their behavior to avoid dependence thereupon. Deployment of flawed
cryptographic security systems such as SSL and WEP has been rectified
far more quickly than deployment of protocols such as IPSEC or DNSSEC
where caution has prevailed.
One possible future for DKIM is that it becomes the starting point
for a new cryptographic infrastructure that eventually replaces
legacy systems including S/MIME and PGP. While this future is
certainly preferable to never achieving ubiquitous deployment of
strong cryptographic security in the Internet, it would certainly
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take a long time to re-invent this particular wheel. Moreover the
deployment of the proposed DKIM enhancements would face political
opposition from the adherents to existing formats to be rendered
historical. A likely outcome of such a strategy is that the existing
two way standards stalemate between S/MIME and PGP would become a
three way stalemate.
Another possible future is that DKIM provides the 'bootstrap' that
enables ubiquitous use of the legacy infrastructure, including the
deployed base of PGP and S/MIME capable email clients and the
existing business infrastructure of commercial Certificate
Authorities. Such a strategy would make use of DKIM in conjunction
with existing PKI standards such as PKIX and XKMS and leverage
features of PGP and S/MIME where appropriate.
10.1. Introducing a new signing algorithm
Regardless of whether extension of the DKIM feature set is desirable,
the need to replace the signature algorithm is practically a
certainty. The RSA signature algorithm at best provides equivalent
security to an 80 bit symmetric cipher when used with a 1024 bit key
[cite]. Extending the key size to 2048 bits improves the cipher
strength to only 112 bit equivalence. Achieving 128 bit security
requires a minimum of 3072 bits. Achieving equivalent cipher
strength to a 192 bit symmetric algorithm requires a prohibitive key
size.
The choice of cryptographic algorithm affects the DKIM algorithm in
two important ways. First there is the difficulty of storing keys in
the DNS. Secondly there is the problem of handling larger
signatures.
The default DNS response packet limit of 512 bytes places an
effective upper bound of 4096 bits on a DKIM key. In practice the
need for packaging, meta-data and the desirability of using DNSSEC to
sign the record reduces the upper bound to no more than 2048 bits.
The size of the DKIM signature itself is a weaker constraint. Even
so, while 1024 and even 2048 bit signatures are likely to be
acceptable in most implementations larger signature sizes may become
prohibitive, particularly since the signature must be Base 64
encoded.
10.2. Possible future signature algorithm choices
ECC cryptography offers a means of implementing public key
cryptography using a key size and signature size that are each only
twice the size of the equivalent symmetric key algorithm.
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While ECC offers clear technical advantages over algorithms based on
the difficulty on solving the discrete log problem in a finite field,
it is not possible at this point to be confident that a means of
applying ECC has been found that is consistent with the position on
intellectual property adopted by the DKIM working group.
The DSA signature algorithm based on the discrete log problem faces
the same key size limitations as RSA. Importantly for the design of
DKIM and DNSSEC however, the signature size is much smaller, the same
size as for ECC algorithms.
It is likely that DSA would have received greater attention during
the design of DSA if key sizes greater than 512 bits and use of hash
functions stronger than SHA-1 had been supported at the time. In
March 2006 a proposed revision of the DSA signature algorithm
answered these objections permitting larger key sizes and specifying
the use of stronger hash functions (including SHA-256 and SHA 512).
While the advantages offered by DSA are not sufficient to warrant an
immediate transition to the new algorithm at this late stage, it is
highly probably that the algorithm will be employed by DNSSEC when
finally deployed.
10.3. Linkage to Other PKIs
The principle limitations in DKIM are the lack of support for end-
user keying, the lack of support for long term verification of
signatures, and the lack of support for trusted third party issued
assertions. Each of these limitations is determined by the key
distribution mechanism rather than the signature format.
Although the DNS infrastructure could in principle be extended to
support these features, this approach would require substantial
modifications that entirely negate the advantage of employing an
existing infrastructure. The point of using DNS is to reuse the DNS
infrastructure, not necessarily the DNS protocol.
The preferred approach to addressing these limitations is to support
use of a PKI infrastructure designed to support these requirements
such as PKIX, PGP or XKMS.
10.4. Trusted Third Party Assertions
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 behavior
of the domain name owner to be predicted may allow the probability
that the message is abusive to be determined without the need for
heuristic content analysis.
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Recipients of large volumes of email can generate reputation data for
email senders internally. Recipients of smaller volumes of messages
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.
Another commonly used technique for evaluating email senders is
accreditation. Most spam sent today is sent by criminals to further
a scheme that is unambiguously illegal. Spam demonstrates an
Internet equivalent of Gresham's law: the bad spam drives out the
merely irritating, the outright criminal drives out the bad. A
message is highly unlikely to be spam if: the email sender can
demonstrate that it is a legitimate business, and that it has
provided a legitimate address where legal process can be served. In
addition the accredited email sender may accept a legally binding
undertaking not to send spam and possibly post a performance bond
that is subject to forfeiture in case of default.
As with reputation data, a high volume email recipient may be in a
position to establish bilateral agreements with high volume senders.
Smaller recipients are not in a position to require accreditation,
nor is it practical for each large sender to accredit every sender.
Trusted Third Party accreditation services allow an email sender to
obtain an accreditation that is recognized by every email recipient
that recognizes the Trusted Third Party.
[Need use of both systems]
[Need means of advertising existence of positive reputation data]
10.5. Linkage to X.509 Certificates
The industry standard for distribution of Trusted Third Party data
tied to a public key is the X.509v3/PKIX standard. X.509 based PKI
is designed to support requirements such as long term archiving of
signatures, end entity signing and Trusted Third Party assertions.
Combining the DKIM signature format with the PKIX PKI infrastructure
provides an equivalent set of capabilities to S/MIME.
Two approaches may be used to inform signature verifiers that an
X.509 certificate has been issued that makes an assertion about the
signing key for a DKIM signature:
1. By means of an attribute in the key record
2. By means of a signature header q= parameter
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Typical commercially issued digital certificates are considerably
larger (1-2 kb) than the 512 byte message size that DNS servers are
required to support as a minimum. Practical large scale PKI
deployment requires support for certificate chains in addition to the
end entity certificate. Publication of a URL for the certificate or
certificate chain is therefore a more appropriate approach than
storing the certificate data itself in the DNS.
The ability to support multiple key distribution mechanisms for the
same key is highly desirable. For example a signer may support DNS
based key distribution for the convenience and efficiency of
transport based DKIM signature verifiers and an X.509 certificate
In other cases a signer may intentionally discourage transport
verification by only providing an X.509 certificate.
An X.509 application of particular interest is the use of DKIM as a
signature format for Secure Internet Letterhead (Letterhead).
Letterhead employs X.509 certificates containing a LOGOTYPE attribute
extension [LOGOTYPE] to identify the certificate subject and/or
issuer to the user by means of a brand image such as a corporate
logo. [PHB-NIST]
10.6. XKMS
XKMS is a key centric PKI that supports registration and location of
keys. XKMS is layered as a Web service and the existence of XKMS
service for a domain is typically advertised by means of a DNS SRV
record.
XKISS, the key location component of XKMS provides a superset of the
capabilities of the DKIM DNS key distribution mechanism. As XKMS is
layered on SOAP over HTTP over TCP/IP the overhead incurred in
retrieving keys through XKMS is substantially higher than the single
UDP request/response of DNS key distribution.
Like X.509 XKMS is designed to key management over time periods of
years and decades rather than days and supports the use of trusted
third parties. XKMS is designed to allow complexity to be
concentrated at the Web service as opposed to the client. A client
interacts with an XKMS service using request of the form 'provide a
key to verify signatures on messages sent by A using protocol B'.
XKMS may also be used as a gateway to one or more PKIs including an
X.509 based PKI that makes use of sophisticated features such as
cross certification. The verifier may at its option rely on the XKMS
service to provide a trusted key or request the complete certificate
path allowing offline verification.
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A signer may notify signature verifiers that a key may be retrieved
using XKMS by means of the q= attribute. The verifier may then
discover the corresponding XKMS service using the SRV mechanism as
set out in the XKMS specification.
10.7. Verification in the Client
The DKIM specification is designed to support edge to edge
authentication. The specification neither supports nor prohibits
verification of DKIM signatures in the client. In particular the
specification does not attempt to define client semantics for
signatures or provide an exhaustive list of user interface security
considerations.
For client based verification to be practical, it is likely that a
client needs to be capable of determining that it is able to receive
messages that do not get clobbered before coming to any conclusions
with respect to unsigned messages.
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.
10.8. Per user signature
Although DKIM is designed to support domain level signatures, the
DKIM core design neither supports nor prohibits use of per user
signatures. A DKIM key record can specify restrictions on the email
addresses it can be used to sign for. These restrictions are not
intended to be exhaustive nor are detailed semantics or security
considerations set out for interpreting per user signatures. The
primary use that this feature is intended to support, is to allow a
company to delegate the signing authority for bulk marketing
communications without the risk of effectively delegating the
authority to sign contracts on behalf of the CEO.
For per user signing keys to provide value beyond this limited use
scenario, it is likely that additional requirements will be
necessary, such as the ability to perform long term validation of the
key. Linkage to some form of PKI is likely to be necessary.
In addition any scheme that involves maintenance of a significant
number of public keys will require infrastructure to support that
management. 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
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security. At a minimum, a key registration protocol must be defined.
10.9. Encryption
DKIM is not designed to support encryption. A strong case can be
made for applying the DKIM style of transparent security, key centric
key management and domain level keying. It is not clear that re-
using the DKIM signature architecture is the best way to achieve this
goal.
The DKIM signature format in particular is designed to allow meta-
data to be attached to a message without modifying the content.
Content encryption by its very nature requires modification of the
message content. The message encryption formats of PGP and S/MIME
both solve the problem of message encryption perfectly adequately;
there is no reason to believe that a new effort in this space would
improve matters.
An architecture of this form would require development of an email
receiver security policy that allows a recipient to state that
encrypted email messages are acceptable and to specify the key
distribution infrastructure(s) by which the necessary encryption keys
may be discovered.
A policy infrastructure of this type is implicit in the XKMS
standard. One drawback to this approach is that policy distribution
and key distribution are conflated, an approach that is satisfactory
for message level encryption schemes such as PGP and S/MIME, but less
satisfactory for transport layer encryption such as SSL.
10.10. Reuse of Key Record
A number of proposals have been made that attempt to reuse the DKIM
key record. Architects considering this approach should be aware of
the advantages and limitations.
As a minimum each of the security considerations listed in the DKIM
specification should be considered and its possible relevance to the
proposed field of use carefully evaluated. Application of a security
mechanism outside the context in which it was originally designed for
is a principle cause of security failures.
DKIM is designed to meet the security needs of an application where
the cost of individual failures is insignificant or small. A single
spam in an email inbox is not a disaster, indeed it is no longer even
an irritation. For the long time spam sufferer who has seen their
inbox filled with hundreds or even thousands of spams, an occasional
failure may even be cause for satisfaction as a reminder of a
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successfully vanquished foe.
One of the chief limitations of using DNS based key records is that
maintenance of DNS records is typically a network operations concern.
If the entity to which the public key corresponds is a network object
(e.g. a mail server), the use of DNS based key management is likely
to be satisfactory. If the entity is not a network related object
(e.g. an end user) a significant degree of pushback from network
administrators should be anticipated.
The design of DKIM is designed for rapid deployment in response to an
immediate need. As such many of the design decisions are not the
decisions that would be taken if the choice were unconstrained by the
limitations of the current legacy DNS. In particular the use of Base
64 encoded public keys distributed through TXT records is not ideal.
Applications that do not face the same constraints as DKIM should
carefully evaluate the feasibility of using the binary key record.
In particular an application predicated on the use of DNSSEC to
authenticate keys should assume support for DKIM binary key records.
10.11. Use of Policy Record
DKIM demonstrates the power of using the DNS to distribute security
policy information. It is not possible to achieve robust security
unless the parties to a conversation know the security capabilities
and expectations of the other.
Any party proposing re-use of the DKIM policy record should carefully
consider whether their needs would be better met by proposing a
flexible security policy architecture in the DKIM style rather than
proposing ad-hoc extensions and variations.
At a minimum any proposal for new security policy formats that make
use of the TXT record should employ a new policy prefix and ensure
that mislabeled and wild-carded policy records are not accidentally
misinterpreted.
11. Acknowledgements
TBD
12. { 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,
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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".
12.1. What Needs To Be Moved Here From the Base Doc?
MUA considerations
key delegation/ 3rd party
13. References
13.1. References -- Normative
[I-D.ietf-dkim-base]
Allman, E., "DomainKeys Identified Mail (DKIM)
Signatures", draft-ietf-dkim-base-05 (work in progress),
August 2006.
[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.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
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13.2. Informative References
[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.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001.
[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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