DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories
Expires: December 27, 2006 D. Crocker
Brandenburg InternetWorking
P. Hallam-Baker
VeriSign Inc.
June 25, 2006
DomainKeys Identified Mail (DKIM) Service Overview
draft-ietf-dkim-overview-01.txt
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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 to permit verification of the source and
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contents of messages by either Mail Transfer Agents (MTAs) or Mail
User Agents (MUAs). The ultimate goal of this framework is to permit
a signing domain to assert responsibility for a message, thus proving
and protecting message sender identity and the integrity of the
messages they convey while retaining the functionality of Internet
email as it is known today. Proof and protection of email identity,
including repudiation and non-repudiation, may assist in the global
control of "spam" and "phishing".
This document provides an overview of DomainKeys Identified Mail and
how it can fit into overall messaging systems, how it relates to
other IETF message signature technologies, implementation and
migration considerations, and outlines potential DKIM applications
and future extensions.
Note
This document is being discussed on the DKIM mailing list,
ietf-dkim@mipassoc.org.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. About This Document . . . . . . . . . . . . . . . . . . . 4
1.2. A Quick Overview of DKIM . . . . . . . . . . . . . . . . . 4
1.3. Outline Potential DKIM Applications . . . . . . . . . . . 7
2. DKIM Within Existing Internet Email . . . . . . . . . . . . . 7
2.1. Review of Internet Mail Service Architecture . . . . . . . 7
2.2. Where to Place DKIM Functions . . . . . . . . . . . . . . 10
2.3. Impact on Email Activities . . . . . . . . . . . . . . . . 11
2.4. Migrating from DomainKeys . . . . . . . . . . . . . . . . 12
2.5. { Misc Text -- Should go elsewhere, if used at all } . . . 13
3. DKIM Within Existing Internet Email . . . . . . . . . . . . . 14
3.1. Review of Internet Mail Service Architecture . . . . . . . 14
3.2. Where to Place DKIM Functions . . . . . . . . . . . . . . 17
3.3. Impact on Email Activities . . . . . . . . . . . . . . . . 17
3.4. Migrating from DomainKeys . . . . . . . . . . . . . . . . 18
3.5. { Misc Text -- Should go elsewhere, if used at all } . . . 19
4. DKIM Service Architecture . . . . . . . . . . . . . . . . . . 21
5. Relationship to previous Message Signature Technologies . . . 23
5.1. Transparent Signature . . . . . . . . . . . . . . . . . . 23
5.2. Treat verification failure as if unsigned. . . . . . . . . 24
5.3. Legacy Client Semantics . . . . . . . . . . . . . . . . . 25
5.4. Key Centric PKI . . . . . . . . . . . . . . . . . . . . . 25
5.5. Domain Level Assurance . . . . . . . . . . . . . . . . . . 27
5.6. Security Policy . . . . . . . . . . . . . . . . . . . . . 27
6. Implementation Considerations . . . . . . . . . . . . . . . . 28
6.1. Development . . . . . . . . . . . . . . . . . . . . . . . 28
6.2. Deployment . . . . . . . . . . . . . . . . . . . . . . . . 29
6.3. Operations . . . . . . . . . . . . . . . . . . . . . . . . 29
7. Outline Future Extensions . . . . . . . . . . . . . . . . . . 30
7.1. Introducing a new signing algorithm . . . . . . . . . . . 31
7.2. Possible future signature algorithm choices . . . . . . . 31
7.3. Transition strategy . . . . . . . . . . . . . . . . . . . 32
7.4. Linkage to Other PKIs . . . . . . . . . . . . . . . . . . 33
7.5. Trusted Third Party Assertions . . . . . . . . . . . . . . 34
7.6. Linkage to X.509 Certificates . . . . . . . . . . . . . . 35
7.7. XKMS . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.8. Verification in the Client . . . . . . . . . . . . . . . . 36
7.9. Per user signature . . . . . . . . . . . . . . . . . . . . 37
7.10. Encryption . . . . . . . . . . . . . . . . . . . . . . . . 37
7.11. Reuse of Key Record . . . . . . . . . . . . . . . . . . . 38
7.12. Use of Policy Record . . . . . . . . . . . . . . . . . . . 38
8. What Needs To Be Moved Here From the Base Doc? . . . . . . . . 39
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
10. Informative References . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
Intellectual Property and Copyright Statements . . . . . . . . . . 42
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1. Introduction
1.1. About This Document
This document provides an overview of DomainKeys Identified Mail
(DKIM). It provides information for: those who are adopting DKIM;
those who are developing DKIM; those who are deploying DKIM; and
those who are considering extending DKIM, either into other areas or
to provide additional features.
This document does not provide information on threats to DKIM or
email, or details on the implementation. Such information can be
found in other RFC documents.[I-D.ietf-dkim-base] [I-D.ietf-dkim-
threats] Nor does this document describe how to solve the world's
problems with spam, phish, virii, worms, joe jobs, etc.
[ NOTE: a number of sections in this document are just placeholders
for now ]
1.2. A Quick Overview of DKIM
1.2.1. Axiom: Ubiquitous Authentication is Good
DKIM builds on previous work in the form of Domain Keys, Identified
Internet Mail, Authenticated Sender, Meta-Mail, etc. The starting
point for all of these technologies, and now DKIM, is the belief that
authenticating email is a good thing to do in and of itself. It has
been pointed out that it is unlikely that RFC 822 [RFC0822] would
pass today without some form of strong authentication mechanism.
DKIM provides such a strong authentication mechanism.
The ultimate goal of DKIM is to achieve a situation where email
authentication is ubiquitous and the unsigned email becomes the
exception the rule, as is the case today. Only when the majority of
Internet email is authenticated is it possible to make interesting
conclusions about the lack of authentication.
It follows then that a new message signature scheme is required to
meet the goal of ubiquitous authentication. In each of the above-
mentioned proposals, several design elements are shared:
o The signature is carried in the message header and does not affect
the message body.
o The signature may include certain headers.
o There is a policy mechanism, either explicit or implicit, that
tells receivers when to expect a signature.
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o Keys are self-created (it is not necessary to buy a certificate).
o Keys are stored in the DNS (it is not necessary to deploy a
separate key server).
The remarkable similarity of these architectural proposals strongly
suggests that this architecture should be the basis for ubiquitous
authentication. DKIM was produced by merging the two previous
proposals of DomainKeys and Identified Internet Mail. Significant
enhancements were then made from that base.
The approach taken by DKIM differs from previous approaches to
message signing in that:
o the message signature is written as a message header field so that
neither human recipients nor existing MUA (Mail User Agent)
software are confused by signature-related content appearing in
the message body,
o there is no dependency on public and private key pairs being
issued by well-known, trusted certificate authorities,
o there is no dependency on the deployment of any new Internet
protocols or services for public key distribution or revocation,
o it makes no attempt to include encryption as part of the
mechanism.
1.2.2. What is the purpose of DKIM?
DKIM lets an organization take responsibility for a message. The
organization taking responsibility is a handler of the message,
either as its originator or as an intermediary. 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
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o does not require the use of trusted third parties (e.g.,
certificate authorities) which might impose significant costs or
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.
1.2.3. What does DKIM do?
DKIM defines a mechanism by which email messages can be
cryptographically signed, permitting a signing domain to claim
responsibility for the introduction of a message into the mail
stream. The responsible organization adds a digital signature to the
message, associating it with a domain name of that organization.
Typically, signing will be done by an 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.)
1.2.4. Who validates the signature?
After a message has been signed, any agent in the message transit
path can choose to validate the signature. 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. 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.)
1.2.5. What does DKIM *not* do?
DKIM does not:
o offer any assertions about the behaviors of the identity doing the
signing.
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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.
1.3. Outline Potential DKIM Applications
TBD
2. DKIM Within Existing Internet Email
2.1. 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)
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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.
2.1.1. 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). 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.
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Basic components of ADMD distinction include:
Edge: Independent transfer services, in networks at the edge of the
Internet Mail service.
User: End-user services. This might be subsumed under the 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:
Enterprise Service Providers:
Operating an organization's internal data and/or mail services.
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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.
2.1.2. 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> is the address in the SMTP
"Mail From" command.
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.
2.2. Where to Place DKIM Functions
Messages may be signed or verified by any functional component within
an ADMD, as that domain wishes to arrange, such as:
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.
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 outermost portion of the MHS, true end-to-end service is
provided, requiring the smallest amount of trust on the rest of the
infrastructure. By signing or verifying at a boundary, the smallest
number of systems need modifying and the signature is subject to the
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smallest amount of handling that can break the signature.
The choice of identity to use might not be obvious. Examples
include:
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 reputation 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 creation or transfer of 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, the choice of component for signing will depend upon both
the identity being used and the tradeoff between flexibility of uses,
versus administrative and operational control. The choice of
component for verification 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.
2.3. Impact on Email Activities
2.3.1. Resources
Although the cryptographic authentication are 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%. Usually, email is i/o-intensive, with unused
computational capacity. So, it is likely that no new hardware will
be required.
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2.3.2. Operations
Administrative cost to deploy, versus expected reduction in the cost
of administration for problem email.
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
2.3.3. 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.
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.
2.4. Migrating from DomainKeys
2.4.1. Signing
2.4.1.1. 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.
2.4.1.2. Signing Module
DKIM uses a different RFC2822 [RFC2822] header field for storing the
signature, in order to distinguish it from DK.
DKIM includes language to make it clear which particular header field
is signed, if there is more than one header field of a given name in
the message.
DKIM allows some values that were scalars in DomainKeys to be colon-
separated arrays. For example, the list of query methods used to
find a key and the "t=" tags (originally testing, now flags).
DKIM permits copying the original version of headers fields and their
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values, to aid in diagnosing signatures that do not survive transit.
DKIM has the ability to limit keys to hash algorithms specified in a
list, in the DNS record.
DKIM allows body length limits, to permit signatures, to survive
transit through some intermediaries, such as some mailing list agents
that add text to the end of the message.
2.4.1.3. Boundary MTA
Enforce signature policies and practises
2.4.2. Verifying
2.4.2.1. DNS Client
2.4.2.2. Verifying module
See "Signing Module".
2.4.2.3. Boundary MTA
Strip "local" signatures that are not local?
2.5. { 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,
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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".
3. DKIM Within Existing Internet Email
3.1. 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 3: 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
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other components for performing message transfer. Consequently, it
is necessary to distinguish administrative boundaries that surround
sets of functional components.
3.1.1. 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). 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 4: 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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3.1.2. 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> is the address in the SMTP
"Mail From" command.
DKIM associates a "responsible" identity with a message and provides
a means of verifying that the association is legitimate. The choices
of what ADMD to have 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.
3.2. Where to Place DKIM Functions
Messages may be signed or verified by any functional component within
an ADMD, as that domain wishes to arrange, such as:
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
and MHS component do signing or verifying, there is no dependence
upon user training or the upgrade of potentially large numbers of
user applications.
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 outermost portion of the MHS, true end-to-end service is
provided, requiring the smallest amount of trust on the rest of the
infrastructure. By signing or verifying at a boundary, the smallest
number of systems need modifying and the signature is subject to the
smallest amount of handling that can break the signature.
Ultimately, deciding where to sign a message is likely to depend upon
a combination of the identity being used, and tradeoffs between
flexibility, control, and administrative ease.
3.3. Impact on Email Activities
3.3.1. Resources
Although the cryptographic authentication are considered to be
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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%. Usually, email is i/o-intensive, with unused
computational capacity. So, it is likely that no new hardware will
be required.
3.3.2. Operations
Administrative cost to deploy, versus expected reduction in the cost
of administration for problem email.
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
3.3.3. 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.
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.
3.4. Migrating from DomainKeys
3.4.1. Signing
3.4.1.1. 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.
3.4.1.2. Signing Module
DKIM uses a different RFC2822 [RFC2822] header field for storing the
signature, in order to distinguish it from DK.
DKIM includes language to make it clear which particular header field
is signed, if there is more than one header field of a given name in
the message.
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DKIM allows some values that were scalars in DomainKeys to be colon-
separated arrays. For example, the list of query methods used to
find a key and the "t=" tags (originally testing, now flags).
DKIM permits copying the original version of headers fields and their
values, to aid in diagnosing signatures that do not survive transit.
DKIM has the ability to limit keys to hash algorithms specified in a
list, in the DNS record.
DKIM allows body length limits, to permit signatures, to survive
transit through some intermediaries, such as some mailing list agents
that add text to the end of the message.
3.4.1.3. Boundary MTA
Enforce signature policies and practises
3.4.2. Verifying
3.4.2.1. DNS Client
3.4.2.2. Verifying module
See "Signing Module".
3.4.2.3. Boundary MTA
Strip "local" signatures that are not local?
3.5. { 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.
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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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4. 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 | | |
| Practises +......>| 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 Practises) |
. . | ...> 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 5: DKIM Service Architecture
Basic message process divides between signing the message, validating
the signature, and the performing further decision-making based upon
the validated signature. The component doing the signing applies
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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 strored 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.
5. Relationship to previous Message Signature Technologies
DKIM is the fifth IETF proposal for an email signature scheme. The
first RFC describing Privacy Enhanced Mail (PEM) was published in
1987 [RFC0989]. The PEM scheme went through a number of revisions
and eventually transformed into MIME Object Security Services in 1995
[RFC1848].
Neither PEM nor MOSS ever achieved significant deployment. PEM
relied on the prior deployment of an extensive PKI predicated on a
rigid hierarchy of Certificate Authorities. By the time it was
understood that this infrastructure assumption was unrealistic the
opportunity to deploy PEM had closed.
Pretty Good Privacy (PGP) was developed by Phil Zimmerman and
released in 1991 and quickly established a significant support base
within the security community. This support base was driven by two
principal factors: superior ease of deployment and an aggressive
marketing campaign assisted by the U.S. government. A working group
was formed within the IETF to continue development of the PGP
protocol as OpenPGP beginning with publication of the original
protocol as an informational RFC in 1996 [RFC1991].
At the same time RSA Security, the holder of the patent rights to the
principle public key cryptography algorithm independently developed
Secure MIME (S/MIME) which employed the recently developed MIME
format to transport a PKCS #7 data object. S/MIME was particularly
attractive for software developers who had already implemented SSL as
much of the code required to support could be reused.
Development of S/MIME and OpenPGP has continued in the IETF since.
While both have achieved a significant user base neither has achieved
ubiquity. In particular it is notable that only a small percentage
of messages on the IETF mailing lists concerned with security are
signed.
5.1. Transparent Signature
The core goals of DKIM require that use of message signatures becomes
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ubiquitous. For this to be possible it must be possible to apply a
signature to any message in any circumstance with a negligible chance
of causing a negative user experience for any recipient regardless of
the legacy email client used.
Experiences from S/MIME and PGP deployment strongly indicate that
this usability goal can only be met if the addition of the signature
leaves the message body unchanged.
S/MIME and PGP are both designed to achieve the highest level of
security possible. The sender of a message is assured that the
confidentiality and/or integrity of the message are protected from
the originating end point machine to the destination end point.
Achieving this level of security naturally places requirements on
both the sender and the receiver. Support for both signature and
encryption causes a subtle but important architectural assumption to
be introduced. Although signature and encryption are complimentary
from a cryptographic point of view their effect is entirely different
from a messaging protocol point of view. A digital signature is
meta-data providing information about the message contents.
Encryption is a transformation of the message content (and possibly
related meta-data).
The recipient of an encrypted email message must as a matter of
course have a specially adapted email client capable of decrypting
the message. Adding a signature to the message does not in principle
create a requirement for the recipient to have a specially adapted
client provided the signature is added in a manner that is ignored by
legacy clients.
In the case of an S/MIME signature the recipient is at a minimum
expected to have a client capable of decoding the MIME multipart/
security format. In practice this means that the recipient must
support S/MIME. OpenPGP allows the use of a signature encapsulation
that is not MIME based. This has the advantage that the message can
be read using a standard email client. The disadvantage with this
approach is that the application of the signature is visible to the
user and thus intrusive.
5.2. Treat verification failure as if unsigned.
PGP and S/MIME were both designed to meet a high standard of
cryptographic excellence. At the time the protocols were designed it
was generally considered that the correct thing for an application to
do in the case of a signature verification failure was to warn the
user that the message was unsigned. In a small number of cases the
application went even further 'warning' the user whenever a signed
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message was received.
This type of user experience has since been severely deprecated. A
user who is constantly bombarded with warning messages is much less
likely to respond appropriately when an important warning message is
received.
While modern messaging infrastructure is considerably friendlier to
the use of digital signatures than in the past even a residual
failure rate of 1% results in unacceptably high support costs when
signatures are used ubiquitously.
It is now generally accepted that the correct semantics for an email
signature verifier to adopt are to treat messages with signatures
that fail as if they are 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.
5.3. Legacy Client Semantics
The deployed base of S/MIME is both a benefit and a burden. While
the S/MIME protocol is in principle capable of extension to support
many of the features of DKIM, the same is not true of the deployed
S/MIME base.
While the S/MIME protocol can in principle support semantics such as
domain level signatures or make use of keys stored in the DNS the
legacy deployed base does not. The behavior of legacy clients
receiving an S/MIME message dependent on the novel semantics is
likely to result in a negative user experience in a significant
number of cases.
5.4. Key Centric PKI
Unlike all four previous IETF email security initiatives, DKIM
employs a key centric, directory based PKI as opposed to a
certificate based PKI in the style of Kohnfelder (X.509) or Zimmerman
(web of trust).
While message syntax and PKI are orthogonal in principle,
implementation practice means that most S/MIME clients only support
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use of keys contained in X.509/PKIX digital certificates.
Although PGP is sometimes held up as an alternative to a certificate
based PKI a PGP key signing is in essence a digital certificate by
another name. There has since been considerable conversion as X.509
has adopted the web of trust principle under the term cross-
certification. The chief distinction between the PGP and PKIX models
is that in the PGP model every user is also (potentially) a trust
provider. In PKIX trust providers are distinct from end-entities.
The Kohnfelder architecture was originally designed to allow use of
public key cryptography before the ubiquitous availability of
networking. A particular benefit of the Kohnfelder architecture is
that Alice can send an encrypted message to Bob when the only
transport available is sending floppy disks through the postal
system. Another benefit of the Kohnfelder architecture is that a
signed message supported by a digital certificate is self supporting
and may be verified years after the fact provided only that the CA
signing key does not become compromised.
The principle weakness in PKIs based on the Kohnfelder architecture
is the difficulty of locating the correct digital key in the absence
of a directory infrastructure. This led Brian LaMacchia, then at MIT
to develop the MIT PGP key server, in effect returning to the
original public key directory model proposed by Whitt Diffe and Marty
Hellman.
XML Key Management Service (XKMS) realizes the key centric PKI model
as a SOAP based Web Service. In the XKMS model the PKI client makes
a request of the form 'provide me with the signing key that Alice
uses with the PGP protocol'.
Although DKIM follows the same architectural model as XKMS, DNS is
used as the transport layer in place of SOAP over HTTP.
The use of DNS significantly reduces the infrastructure requirements
for DKIM as existing DNS servers are used for key distribution. This
approach also has significant performance advantages as DNS is layers
on UDP and key retrieval is typically achieved in a single round
trip. XKMS requires a TCP session to be established and the request
and response messages are significantly larger.
The principle disadvantage of using DNS over XKMS is that the DNS is
a network administration resource designed to answer questions about
current network configuration. While it is quite possible to re-use
the DNS infrastructure to support queries about past and even future
network configurations this is not the core objective of the
infrastructure. The DNS is thus unsuited to supporting any use of
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digital signatures in which long term archival is desirable or the
possibility of repudiation is undesirable.
5.5. Domain Level Assurance
As previously mentioned PGP and S/MIME were designed in the heyday of
the security end-to-end principle. It has since been realized that
the end points with respect to trust are not the same as the end
points with respect to the communication protocol.
When Alice sends a personal message to Bob it is Alice the person,
not the machine Alice happens to be using that is the true trust end
point. When Alice sends a piece of business correspondence to Bob it
is her employer.
The objective of DKIM is to allow parties to accept responsibility
for messages by signing them. While accepting responsibility at the
personal level may be desirable in some circumstances the Internet
now has a billion users. Attempting to achieve accountability in a
population of a billion users is impractical, particularly when the
owner of the domain example.com has the ability to create a
practically unlimited number of accounts within that domain at will.
The logical unit of accountability for DKIM is therefore the DNS
domain name to which the signing key record is bound and not the
individual email user.
5.6. Security Policy
The innovation in DKIM that has no precedent in the previous email
security standards is the publication of a security policy. The
purpose of DKIM is to allow a party to accept responsibility for an
email message by signing it. A message with a signature is treated
as if it is unsigned. 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.
The introduction of security policy allows unsigned messages and
messages that fail signature validation to be subjected to a higher
level of anti-spam filtering or rejected out of hand in circumstances
where the owner of the purported originating domain suggests. For
example a bank concerned at the possibility of phishing attack might
publish a policy stating that all legitimate messages from the domain
are signed.
While the Sender-ID/SPF security policy format allows application to
existing formats including PGP and S/MIME the advantages to
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developing the protocol and security policy in tandem are
considerable. It is not practical to expect to be able to write a
useful sender signing policy for S/MIME or PGP within the constraints
of the 512 byte response message size imposed on the legacy DNS.
6. Implementation Considerations
6.1. Development
What software has to change, to use DKIM?
6.1.1. Signer
The signer needs to add code in the appropriate agent, to perform
signing, and they need to modify their DNS administrative tools to
permit creation of DKIM key records.
6.1.2. Validator
A validator needs to add code to the appropriate agent and then feed
the result into the portion of their system needing it, such as a
filtering engine.
The mere existence of a valid signature does not imply that the mail
is acceptable, such as for delivery. Acceptability requires an
assessment phase. Hence the result of signature validation must be
fed into a vetting mechanism that is part of the validator's filter.
6.1.3. Outbound mail user agent
TBD
6.1.4. Outbound mail transport agent
TBD
6.1.5. DNS Server
TBD
6.1.6. Mailing list manager
TBD
6.1.7. Inbound mail transport agent
TBD
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6.1.8. Inbound mail user agent
TBD
6.1.9. Accreditation service
TBD
6.2. Deployment
6.2.1. Signing
6.2.1.1. DNS Records
add sig key info
6.2.1.2. Signing Module
Delete old signs with same key; add new sig
6.2.1.3. Boundary MTA
Enforce signature policies and practises
6.2.2. Verifying
6.2.2.1. DNS Client
Ensure able to obtain DKIM key sig records
6.2.2.2. Verifying module
Validate sig; channel info to filtering engine. Maybe provide user-
visible info.
6.2.2.3. Boundary MTA
Strip "local" signatures that are not local?
6.3. Operations
6.3.1. DNS Signature Record Deployment Considerations
TBD
6.3.2. Thoughts on Expiring Signatures
TBD
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6.3.3. DNS Policy Record Deployment Considerations
TBD
6.3.4. Subdomain Considerations
TBD
6.3.5. Third party Signature Delegations
TBD
7. 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 is the overriding priority. It has traditionally
been asserted that deployment of a flawed cryptographic protocol is
an almost unacceptable risk, 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 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
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three way stalemate.
Another possible future is that DKIM provides the 'bootstrap' that
enables ubiquitous use of 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.
7.1. 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 as the signature must be Base 64 encoded.
7.2. 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.
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 that is consistent with the position on intellectual
property adopted by the DKIM working group has been found.
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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
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.
7.3. 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.
7.3.1. 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
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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
7.3.2. 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:
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. A verifier MAY chose to evaluate signature records in any
order it chooses, including making use of the signature
algorithm for this purpose.
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.
A. 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.
The verifier MAY decide to add support for additional signature
algorithms at any time.
A. The verified MAY add support for algorithm B at any time.
7.4. 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
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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 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.
7.5. 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.
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 sender which 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 that 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.
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[Need use of both systems]
[Need means of advertising existence of positive reputation data]
7.6. 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
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]
7.7. 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
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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.
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.
7.8. 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 the 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 does not result in a less satisfactory
user experience than leaving the message unsigned.
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7.9. 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 this feature is intended to support is to allow a company
to delegate 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 are 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
security. As a minimum a key registration protocol must be defined.
7.10. 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 and
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 key
distribution infrastructure(s) by which the necessary encryption keys
may be discovered.
A policy infrastructure of this type is implicit in the XKMS
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standard. One drawback to this approach is that policy distribution
an key distribution are conflated, an approach hat is satisfactory
for message level encryption schemes such as PGP and S/MIME but less
satisfactory for transport layer encryption such as SSL.
7.11. Reuse of Key Record
A number of proposals have been made which 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, a reminder of a
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 was 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.
7.12. 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
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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.
8. What Needs To Be Moved Here From the Base Doc?
MUA considerations
key delegation/ 3rd party
9. Acknowledgements
TBD
10. Informative References
[I-D.ietf-dkim-base]
Allman, E., "DomainKeys Identified Mail Signatures
(DKIM)", draft-ietf-dkim-base-02 (work in progress),
May 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.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[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
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Object Security Services", RFC 1848, October 1995.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
Exchange Formats", RFC 1991, August 1996.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
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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.
USA
Email: pbaker@verisign.com
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