rfc5863
Internet Engineering Task Force (IETF) T. Hansen
Request for Comments: 5863 AT&T Laboratories
Category: Informational E. Siegel
ISSN: 2070-1721 Consultant
P. Hallam-Baker
Default Deny Security, Inc.
D. Crocker
Brandenburg InternetWorking
May 2010
DomainKeys Identified Mail (DKIM)
Development, Deployment, and Operations
Abstract
DomainKeys Identified Mail (DKIM) allows an organization to claim
responsibility for transmitting a message, in a way that can be
validated by a recipient. The organization can be the author's, the
originating sending site, an intermediary, or one of their agents. A
message can contain multiple signatures, from the same or different
organizations involved with the message. DKIM defines a domain-level
digital signature authentication framework for email, using public
key cryptography and using the domain name service as its key server
technology. This permits verification of a responsible organization,
as well as the integrity of the message content. DKIM will also
provide a mechanism that permits potential email signers to publish
information about their email signing practices; this will permit
email receivers to make additional assessments about messages.
DKIM's authentication of email identity can assist in the global
control of "spam" and "phishing". This document provides
implementation, deployment, operational, and migration considerations
for DKIM.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
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Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5863.
Copyright Notice
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Table of Contents
1. Introduction ....................................................4
2. Using DKIM as Part of Trust Assessment ..........................4
2.1. A Systems View of Email Trust Assessment ...................4
2.2. Choosing a DKIM Tag for the Assessment Identifier ..........6
2.3. Choosing the Signing Domain Name ...........................8
2.4. Recipient-Based Assessments ...............................10
2.5. Filtering .................................................12
3. DKIM Key Generation, Storage, and Management ...................15
3.1. Private Key Management: Deployment and Ongoing
Operations ................................................16
3.2. Storing Public Keys: DNS Server Software Considerations ...17
3.3. Per-User Signing Key Management Issues ....................18
3.4. Third-Party Signer Key Management and Selector
Administration ............................................19
3.5. Key Pair / Selector Life Cycle Management .................19
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4. Signing ........................................................21
4.1. DNS Records ...............................................21
4.2. Signing Module ............................................21
4.3. Signing Policies and Practices ............................22
5. Verifying ......................................................23
5.1. Intended Scope of Use .....................................23
5.2. Signature Scope ...........................................23
5.3. Design Scope of Use .......................................24
5.4. Inbound Mail Filtering ....................................24
5.5. Messages Sent through Mailing Lists and Other
Intermediaries ............................................25
5.6. Generation, Transmission, and Use of Results Headers ......25
6. Taxonomy of Signatures .........................................26
6.1. Single Domain Signature ...................................26
6.2. Parent Domain Signature ...................................27
6.3. Third-Party Signature .....................................27
6.4. Using Trusted Third-Party Senders .........................29
6.5. Multiple Signatures .......................................30
7. Example Usage Scenarios ........................................31
7.1. Author's Organization - Simple ............................32
7.2. Author's Organization - Differentiated Types of Mail ......32
7.3. Author Domain Signing Practices ...........................32
7.4. Delegated Signing .........................................34
7.5. Independent Third-Party Service Providers .................35
7.6. Mail Streams Based on Behavioral Assessment ...............35
7.7. Agent or Mediator Signatures ..............................36
8. Usage Considerations ...........................................36
8.1. Non-Standard Submission and Delivery Scenarios ............36
8.2. Protection of Internal Mail ...............................37
8.3. Signature Granularity .....................................38
8.4. Email Infrastructure Agents ...............................39
8.5. Mail User Agent ...........................................40
9. Security Considerations ........................................41
10. Acknowledgements ..............................................41
11. References ....................................................42
11.1. Normative References .....................................42
11.2. Informative References ...................................42
Appendix A. Migration Strategies .................................43
A.1. Migrating from DomainKeys .................................43
A.2. Migrating Hash Algorithms .................................48
A.3. Migrating Signing Algorithms ..............................49
Appendix B. General Coding Criteria for Cryptographic
Applications .........................................50
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1. Introduction
DomainKeys Identified Mail (DKIM) allows an organization to claim
responsibility for transmitting a message, in a way that can be
validated by a recipient. This document provides practical tips for
those who are developing DKIM software, mailing list managers,
filtering strategies based on the output from DKIM verification, and
DNS servers; those who are deploying DKIM software, keys, mailing
list software, and migrating from DomainKeys [RFC4870]; and those who
are responsible for the ongoing operations of an email infrastructure
that has deployed DKIM.
The reader is encouraged to read the DKIM Service Overview document
[RFC5585] before this document. More detailed guidance about DKIM
and Author Domain Signing Practices (ADSP) can also be found in the
protocol specifications [RFC4871], [RFC5617], and [RFC5672].
The document is organized around the key concepts related to DKIM.
Within each section, additional considerations specific to
development, deployment, or ongoing operations are highlighted where
appropriate. The possibility of the use of DKIM results as input to
a local reputation database is also discussed.
2. Using DKIM as Part of Trust Assessment
2.1. A Systems View of Email Trust Assessment
DKIM participates in a trust-oriented enhancement to the Internet's
email service, to facilitate message handling decisions, such as for
delivery and for content display. Trust-oriented message handling
has substantial differences from the more established approaches that
consider messages in terms of risk and abuse. With trust, there is a
collaborative exchange between a willing participant along the
sending path and a willing participant at a recipient site. In
contrast, the risk model entails independent, unilateral action by
the recipient site, in the face of a potentially unknown, hostile,
and deceptive sender. This translates into a very basic technical
difference: in the face of unilateral action by the recipient and
even antagonistic efforts by the sender, risk-oriented mechanisms are
based on heuristics, that is, on guessing. Guessing produces
statistical results with some false negatives and some false
positives. For trust-based exchanges, the goal is the deterministic
exchange of information. For DKIM, that information is the one
identifier that represents a stream of mail for which an independent
assessment is sought (by the signer).
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A trust-based service is built upon a validated Responsible
Identifier that labels a stream of mail and is controlled by an
identity (role, person, or organization). The identity is
acknowledging some degree of responsibility for the message stream.
Given a basis for believing that an identifier is being used in an
authorized manner, the recipient site can make and use an assessment
of the associated identity. An identity can use different
identifiers, on the assumption that the different streams might
produce different assessments. For example, even the best-run
marketing campaigns will tend to produce some complaints that can
affect the reputation of the associated identifier, whereas a stream
of transactional messages is likely to have a more pristine
reputation.
Determining that the identifier's use is valid is quite different
from determining that the content of a message is valid. The former
means only that the identifier for the responsible role, person, or
organization has been legitimately associated with a message. The
latter means that the content of the message can be believed and,
typically, that the claimed author of the content is correct. DKIM
validates only the presence of the identifier used to sign the
message. Even when this identifier is validated, DKIM carries no
implication that any of the message content, including the
RFC5322.From field [RFC5322], is valid. Surprisingly, this limit to
the semantics of a DKIM signature applies even when the validated
signing identifier is the same domain name as is used in the
RFC5322.From field! DKIM's only claim about message content is that
the content cited in the DKIM-Signature: field's h= tag has been
delivered without modification. That is, it asserts message content
integrity -- between signing and verifying -- not message content
validity.
As shown in Figure 1, this enhancement is a communication between a
responsible role, person, or organization that signs the message and
a recipient organization that assesses its trust in the signer. The
recipient then makes handling decisions based on a collection of
assessments, of which the DKIM mechanism is only a part. In this
model, as shown in Figure 1, validation is an intermediary step,
having the sole task of passing a validated Responsible Identifier to
the Identity Assessor. The communication is of a single Responsible
Identifier that the Responsible Identity wishes to have used by the
Identity Assessor. The Identifier is the sole, formal input and
output value of DKIM signing. The Identity Assessor uses this
single, provided Identifier for consulting whatever assessment
databases are deemed appropriate by the assessing entity. In turn,
output from the Identity Assessor is fed into a Handling Filter
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engine that considers a range of factors, along with this single
output value. The range of factors can include ancillary information
from the DKIM validation.
Identity Assessment covers a range of possible functions. It can be
as simple as determining whether the identifier is a member of some
list, such as authorized operators or participants in a group that
might be of interest for recipient assessment. Equally, it can
indicate a degree of trust (reputation) that is to be afforded the
actor using that identifier. The extent to which the assessment
affects the handling of the message is, of course, determined later,
by the Handling Filter.
+------+------+ +------+------+
| Author | | Recipient |
+------+------+ +------+------+
| ^
| |
| +------+------+
| -->| Handling |<--
| -->| Filter |<--
| +-------------+
| ^
V Responsible |
+-------------+ Identifier +------+------+
| Responsible |. . . . . . . . . . .>| Identity |
| Identity | . . | Assessor |
+------+------+ . . +-------------+
| V . ^ ^
V . . | |
+------------------.-------.--------------------+ | |
| +------+------+ . . . > . +-------------+ | | | +-----------+
| | Identifier | | Identifier +--|--+ +--+ Assessment|
| | Signer +------------->| Validator | | | Databases |
| +-------------+ +-------------+ | +-----------+
| DKIM Service |
+-----------------------------------------------+
Figure 1: Actors in a Trust Sequence Using DKIM
2.2. Choosing a DKIM Tag for the Assessment Identifier
The signer of a message needs to be able to provide precise data and
know what that data will mean upon delivery to the Assessor. If
there is ambiguity in the choice that will be made on the recipient
side, then the sender cannot know what basis for assessment will be
used. DKIM has three values that specify identification information
and it is easy to confuse their use, although only one defines the
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formal input and output of DKIM, with the other two being used for
internal protocol functioning and adjunct purposes, such as auditing
and debugging.
The salient values include the s=, d= and i= parameters in the DKIM-
Signature: header field. In order to achieve the end-to-end
determinism needed for this collaborative exchange from the signer to
the assessor, the core model needs to specify what the signer is
required to provide to the assessor. The update to RFC 4871
[RFC5672] specifies:
DKIM's primary task is to communicate from the Signer to a
recipient-side Identity Assessor a single Signing Domain
Identifier (SDID) that refers to a responsible identity. DKIM MAY
optionally provide a single responsible Agent or User Identifier
(AUID)... A receive-side DKIM verifier MUST communicate the
Signing Domain Identifier (d=) to a consuming Identity Assessor
module and MAY communicate the User Agent Identifier (i=) if
present.... To the extent that a receiver attempts to intuit any
structured semantics for either of the identifiers, this is a
heuristic function that is outside the scope of DKIM's
specification and semantics.
The single, mandatory value that DKIM supplies as its output is:
d= This specifies the "domain of the signing entity". It is a
domain name and is combined with the selector to form a DNS
query. A receive-side DKIM verifier needs to communicate the
Signing Domain Identifier (d=) to a consuming Identity Assessor
module and can also communicate the User Agent Identifier (i=)
if present.
The adjunct values are:
s= This tag specifies the selector. It is used to discriminate
among different keys that can be used for the same d= domain
name. As discussed in Section 4.3 of [RFC5585], "If verifiers
were to employ the selector as part of an assessment mechanism,
then there would be no remaining mechanism for making a
transition from an old, or compromised, key to a new one".
Consequently, the selector is not appropriate for use as part
or all of the identifier used to make assessments.
i= This tag is optional and provides the "[t]he Agent or User
Identifier (AUID) on behalf of which the SDID is taking
responsibility" [RFC5672]. The identity can be in the syntax
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of an entire email address or only a domain name. The domain
name can be the same as for d= or it can be a sub-name of the
d= name.
NOTE: Although the i= identity has the syntax of an email
address, it is not required to have those semantics. That is,
"the identity of the user" need not be the same as the user's
mailbox. For example, the signer might wish to use i= to
encode user-related audit information, such as how they were
accessing the service at the time of message posting.
Therefore, it is not possible to conclude anything from the i=
string's (dis)similarity to email addresses elsewhere in the
header.
So, i= can have any of these properties:
* Be a valid domain when it is the same as d=
* Appear to be a subdomain of d= but might not even exist
* Look like a mailbox address but might have different semantics
and therefore not function as a valid email address
* Be unique for each message, such as indicating access details
of the user for the specific posting
This underscores why the tag needs to be treated as being opaque,
since it can represent any semantics, known only to the signer.
Hence, i= serves well as a token that is usable like a Web cookie,
for return to the signing Administrative Management Domain (ADMD) --
such as for auditing and debugging. Of course in some scenarios the
i= string might provide a useful adjunct value for additional
(heuristic) processing by the Handling Filter.
2.3. Choosing the Signing Domain Name
A DKIM signing entity can serve different roles, such as being the
author of content, the operator of the mail service, or the operator
of a reputation service that also provides signing services on behalf
of its customers. In these different roles, the basis for
distinguishing among portions of email traffic can vary. For an
entity creating DKIM signatures, it is likely that different portions
of its mail will warrant different levels of trust. For example:
* Mail is sent for different purposes, such as marketing versus
transactional, and recipients demonstrate different patterns of
acceptance between these.
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* For an operator of an email service, there often are distinct
sub-populations of users warranting different levels of trust
or privilege, such as paid versus free users, or users engaged
in direct correspondence versus users sending bulk mail.
* Mail originating outside an operator's system, such as when it
is redistributed by a mailing-list service run by the operator,
will warrant a different reputation from mail submitted by
users authenticated with the operator.
It is therefore likely to be useful for a signer to use different d=
subdomain names, for different message traffic streams, so that
receivers can make differential assessments. However, too much
differentiation -- that is, too fine a granularity of signing domains
-- makes it difficult for the receiver to discern a sufficiently
stable pattern of traffic for developing an accurate and reliable
assessment. So the differentiation needs to achieve a balance.
Generally, in a trust system, legitimate signers have an incentive to
pick a small stable set of identities, so that recipients and others
can attribute reputations to them. The set of these identities a
receiver trusts is likely to be quite a bit smaller than the set it
views as risky.
The challenge in using additional layers of subdomains is whether the
extra granularity will be useful for the Assessor. In fact,
excessive levels invite ambiguity: if the Assessor does not take
advantage of the added granularity in the entire domain name that is
provided, they might unilaterally decide to use only some rightmost
part of the identifier. The signer cannot know what portion will be
used. That ambiguity would move the use of DKIM back to the realm of
heuristics, rather than the deterministic processing that is its
goal.
Hence, the challenge is to determine a useful scheme for labeling
different traffic streams. The most obvious choices are among
different types of content and/or different types of authors.
Although stability is essential, it is likely that the choices will
change, over time, so the scheme needs to be flexible.
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For those originating message content, the most likely choice of
subdomain naming scheme will by based upon type of content, which can
use content-oriented labels or service-oriented labels. For example:
transaction.example.com
newsletter.example.com
bugreport.example.com
support.example.com
sales.example.com
marketing.example.com
where the choices are best dictated by whether they provide the
Identity Assessor with the ability to discriminate usefully among
streams of mail that demonstrate significantly different degrees of
recipient acceptance or safety. Again, the danger in providing too
fine a granularity is that related message streams that are labeled
separately will not benefit from an aggregate reputation.
For those operating messaging services on behalf of a variety of
customers, an obvious scheme to use has a different subdomain label
for each customer. For example:
widgetco.example.net
moviestudio.example.net
bigbank.example.net
However, it can also be appropriate to label by the class of service
or class of customer, such as:
premier.example.net
free.example.net
certified.example.net
Prior to using domain names for distinguishing among sources of data,
IP Addresses have been the basis for distinction. Service operators
typically have done this by dedicating specific outbound IP Addresses
to specific mail streams -- typically to specific customers. For
example, a university might want to distinguish mail from the
administration, versus mail from the student dorms. In order to make
the adoption of a DKIM-based service easier, it can be reasonable to
translate the same partitioning of traffic, using domain names in
place of the different IP Addresses.
2.4. Recipient-Based Assessments
DKIM gives the recipient site's Identity Assessor a verifiable
identifier to use for analysis. Although the mechanism does not make
claims that the signer is a Good Actor or a Bad Actor, it does make
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it possible to know that use of the identifier is valid. This is in
marked contrast with schemes that do not have authentication.
Without verification, it is not possible to know whether the
identifier -- whether taken from the RFC5322.From field, the
RFC5321.MailFrom command, or the like -- is being used by an
authorized agent. DKIM solves this problem. Hence, with DKIM, the
Assessor can know that two messages with the same DKIM d= identifier
are, in fact, signed by the same person or organization. This
permits a far more stable and accurate assessment of mail traffic
using that identifier.
DKIM is distinctive, in that it provides an identifier that is not
necessarily related to any other identifier in the message. Hence,
the signer might be the author's ADMD, one of the operators along the
transit path, or a reputation service being used by one of those
handling services. In fact, a message can have multiple signatures,
possibly by any number of these actors.
As discussed above, the choice of identifiers needs to be based on
differences that the signer thinks will be useful for the recipient
Assessor. Over time, industry practices establish norms for these
choices.
Absent such norms, it is best for signers to distinguish among
streams that have significant differences, while consuming the
smallest number of identifiers possible. This will limit the
burden on recipient Assessors.
A common view about a DKIM signature is that it carries a degree of
assurance about some or all of the message contents, and in
particular, that the RFC5322.From field is likely to be valid. In
fact, DKIM makes assurances only about the integrity of the data and
not about its validity. Still, presumptions of the RFC5322.From
field validity remain a concern. Hence, a signer using a domain name
that is unrelated to the domain name in the RFC5322.From field can
reasonably expect that the disparity will warrant some curiosity, at
least until signing by independent operators has produced some
established practice among recipient Assessors.
With the identifier(s) supplied by DKIM, the Assessor can consult an
independent assessment service about the entity associated with the
identifier(s). Another possibility is that the Assessor can develop
its own reputation rating for the identifier(s). That is, over time,
the Assessor can observe the stream of messages associated with the
identifier(s) developing a reaction to associated content. For
example, if there is a high percentage of user complaints regarding
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signed mail with a d= value of "widgetco.example.net", the Assessor
might include that fact in the vector of data it provides to the
Handling Filter. This is also discussed briefly in Section 5.4.
2.5. Filtering
The assessment of the signing identifier is given to a Handling
Filter that is defined by local policies, according to a potentially
wide range of different factors and weightings. This section
discusses some of the kinds of choices and weightings that are
plausible and the differential actions that might be performed.
Because authenticated domain names represent a collaborative sequence
between signer and Assessor, actions can sometimes reasonably include
contacting the signer.
The discussion focuses on variations in Organizational Trust versus
Message Stream Risk, that is, the degree of positive assessment of a
DKIM-signing organization, and the potential danger present in the
message stream signed by that organization. While it might seem that
higher trust automatically means lower risk, the experience with
real-world operations provides examples of every combination of the
two factors, as shown in Figure 2. For each axis, only three levels
of granularity are listed, in order to keep discussion manageable.
In real-world filtering engines, finer-grained distinctions are
typically needed, and there typically are more axes. For example,
there are different types of risk, so that an engine might
distinguish between spam risk versus virus risk and take different
actions based on which type of problematic content is present. For
spam, the potential damage from a false negative is small, whereas
the damage from a false positive is high. For a virus, the potential
danger from a false negative is extremely high, while the likelihood
of a false positive when using modern detection tools is extremely
low. However, for the discussion here, "risk" is taken as a single
construct.
The DKIM d= identifier is independent of any other identifier in a
message and can be a subdomain of the name owned by the signer. This
permits the use of fine-grained and stable distinctions between
different types of message streams, such as between transactional
messages and marketing messages from the same organization. Hence,
the use of DKIM might permit a richer filtering model than has
typically been possible for mail-receiving engines.
Note that the realities of today's public Internet Mail environment
necessitate having a baseline handling model that is quite
suspicious. Hence, "strong" filtering rules really are the starting
point, as indicated for the UNKNOWN cell.
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The table indicates differential handling for each combination, such
as how aggressive or broad-based the filtering could be.
Aggressiveness affects the types of incorrect assessments that are
likely. So, the table distinguishes various characteristics,
including: 1) whether an organization is unknown, known to be good
actors, or known to be bad actors; and 2) the assessment of messages.
It includes advice about the degree of filtering that might be done,
and other message disposition. Perhaps unexpectedly, it also lists a
case in which the receiving site might wish to deliver problematic
mail, rather than redirecting or deleting it. The site might also
wish to contact the signing organization and seek resolution of the
problem.
+-------------+-----------------------------------------------+
| S T R E A M * O R G A N I Z A T I O N A L T R U S T |
| R I S K * Low Medium High |
| +***************+***************+***************+
| Low * BENIGN: | DILIGENT: | PRISTINE |
| * Moderate | Mild | Accept |
| * filter | filter | |
| +---------------+---------------+---------------+
| Medium * UNKNOWN: | TYPICAL: | PROTECTED: |
| * Strong | Targeted | Accept & |
| * filter | filter | Contact |
| +---------------+---------------+---------------+
| High * MALICIOUS: | NEGLIGENT: | COMPROMISED: |
| * Block & | Block | Block & |
| * Counter | | Contact |
+-------------+---------------+---------------+---------------+
Figure 2: Trust versus Risk Handling Tradeoffs Example
[LEGEND]
AXES
Stream Risk: This is a measure of the recent history of a message
stream and the severity of problems it has presented.
Organizational Trust: This combines longer-term history about
possible stream problems from that organization, and its
responsiveness to problem handling.
CELLS (indicating reasonable responses)
Labels for the cells are meant as a general assessment of an
organization producing that type of mail stream under that
circumstance.
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Benign: There is some history of sending good messages, with very
few harmful messages having been received. This stream
warrants filtering that does not search for problems very
aggressively, in order to reduce the likelihood of false
positives.
Diligent: The stream has had a limited degree of problems and the
organization is consistently successful at controlling their
abuse issues and in a timely manner.
Pristine: There is a history of a clean message stream with no
problems, from an organization with an excellent reputation.
So, the filter primarily needs to ensure that messages are
delivered; catching stray problem messages is a lesser concern.
In other words, the paramount concern, here, is false
positives.
-----
Unknown: There is no history with the organization. Apply an
aggressive level of "naive" filtering, given the nature of the
public email environment.
Typical: The stream suffers significant abuse issues and the
organization has demonstrated a record of having difficulties
resolving them in a timely manner, in spite of legitimate
efforts. Unfortunately, this is the typical case for service
providers with an easy and open subscription policy.
Protected: An organization with a good history and/or providing
an important message stream for the receiving site is subject
to a local policy that messages are not allowed to be blocked,
but the stream is producing a problematic stream. The receiver
delivers messages, but works quickly with the organization to
resolve the matter.
-----
Malicious: A persistently problematic message stream is coming
from an organization that appears to contribute to the problem.
The stream will be blocked, but the organization's role is
sufficiently troubling to warrant following up with others in
the anti-abuse or legal communities, to constrain or end their
impact.
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Negligent: A persistently problematic message stream is coming
from an organization that does not appear to be contributing to
the problem, but also does not appear to be working to
eliminate it. At the least, the stream needs to be blocked.
Compromised: An organization with a good history has a stream
that changes and becomes too problematic to be delivered. The
receiver blocks the stream and works quickly with the
organization to resolve the matter.
3. DKIM Key Generation, Storage, and Management
By itself, verification of a digital signature only allows the
verifier to conclude with a very high degree of certainty that the
signature was created by a party with access to the corresponding
private signing key. It follows that a verifier requires means to
(1) obtain the public key for the purpose of verification and (2)
infer useful attributes of the key holder.
In a traditional Public Key Infrastructure (PKI), the functions of
key distribution and key accreditation are separated. In DKIM
[RFC4871], these functions are both performed through the DNS.
In either case, the ability to infer semantics from a digital
signature depends on the assumption that the corresponding private
key is only accessible to a party with a particular set of
attributes. In a traditional PKI, a Trusted Third Party (TTP)
vouches that the key holder has been validated with respect to a
specified set of attributes. The range of attributes that can be
attested in such a scheme is thus limited only to the type of
attributes that a TTP can establish effective processes for
validating. In DKIM, TTPs are not employed and the functions of key
distribution and accreditation are combined.
Consequently, there are only two types of inference that a signer can
make from a key published in a DKIM key record:
1. That a party with the ability to control DNS records within a DNS
zone intends to claim responsibility for messages signed using
the corresponding private signature key.
2. That use of a specific key is restricted to the particular subset
of messages identified by the selector.
The ability to draw any useful conclusion from verification of a
digital signature relies on the assumption that the corresponding
private key is only accessible to a party with a particular set of
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attributes. In the case of DKIM, this means that the party that
created the corresponding DKIM key record in the specific zone
intended to claim responsibility for the signed message.
Ideally, we would like to draw a stronger conclusion, that if we
obtain a DKIM key record from the DNS zone example.com, that the
legitimate holder of the DNS zone example.com claims responsibility
for the signed message. In order for this conclusion to be drawn, it
is necessary for the verifier to assume that the operational security
of the DNS zone and corresponding private key are adequate.
3.1. Private Key Management: Deployment and Ongoing Operations
Access to signing keys needs to be carefully managed to prevent use
by unauthorized parties and to minimize the consequences if a
compromise were to occur.
While a DKIM signing key is used to sign messages on behalf of many
mail users, the signing key itself needs to be under direct control
of as few key holders as possible. If a key holder were to leave the
organization, all signing keys held by that key holder need to be
withdrawn from service and, if appropriate, replaced.
If key management hardware support is available, it needs to be used.
If keys are stored in software, appropriate file control protections
need to be employed, and any location in which the private key is
stored in plaintext form needs to be excluded from regular backup
processes and is best not accessible through any form of network
including private local area networks. Auditing software needs to be
used periodically to verify that the permissions on the private key
files remain secure.
Wherever possible, a signature key needs to exist in exactly one
location and be erased when no longer used. Ideally, a signature key
pair needs to be generated as close to the signing point as possible,
and only the public key component transferred to another party. If
this is not possible, the private key needs to be transported in an
encrypted format that protects the confidentiality of the signing
key. A shared directory on a local file system does not provide
adequate security for distribution of signing keys in plaintext form.
Key escrow schemes are not necessary and are best not used. In the
unlikely event of a signing key becoming lost, a new signature key
pair can be generated as easily as recovery from a key escrow scheme.
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To enable accountability and auditing:
o Responsibility for the security of a signing key needs to
ultimately vest in a single named individual.
o Where multiple parties are authorized to sign messages, each
signer needs to use a different key to enable accountability and
auditing.
Best practices for management of cryptographic keying material
require keying material to be refreshed at regular intervals,
particularly where key management is achieved through software.
While this practice is highly desirable, it is of considerably less
importance than the requirement to maintain the secrecy of the
corresponding private key. An operational practice in which the
private key is stored in tamper-proof hardware and changed once a
year is considerably more desirable than one in which the signature
key is changed on an hourly basis but maintained in software.
3.2. Storing Public Keys: DNS Server Software Considerations
In order to use DKIM, a DNS domain holder requires (1) the ability to
create the necessary DKIM DNS records and (2) sufficient operational
security controls to prevent insertion of spurious DNS records by an
attacker.
DNS record management is often operated by an administrative staff
that is different from 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. If the best practices for private key management
described above are observed, such deployment is not a one-time
event; DNS DKIM selectors will be changed over time as signing keys
are terminated and replaced.
At a minimum, a DNS server that handles queries for DKIM key records
needs to allow the server administrators to add free-form TXT
records. It would be better if the DKIM records could be entered
using a structured form, supporting the DKIM-specific fields.
Ideally, DNS Security (DNSSEC) [RFC4034] needs to be employed in a
configuration that provides protection against record insertion
attacks and zone enumeration. In the case that NextSECure version 3
(NSEC3) [RFC5155] records are employed to prevent insertion attack,
the OPT-OUT flag needs to be clear. (See [RFC5155] section 6 for
details.)
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3.2.1. Assignment of Selectors
Selectors are assigned according to the administrative needs of the
signing domain, such as for rolling over to a new key or for the
delegation of the right to authenticate a portion of the namespace to
a TTP. Examples include:
jun2005.eng._domainkey.example.com
widget.promotion._domainkey.example.com
It is intended that assessments of DKIM identities be based on the
domain name, and not include the selector. While past practice of a
signer can permit a verifier to infer additional properties of
particular messages from the structure DKIM key selector, unannounced
administrative changes such as a change of signing software can cause
such heuristics to fail at any time.
3.3. Per-User Signing Key Management Issues
While a signer can establish business rules, such as the issue of
individual signature keys for each end-user, DKIM makes no provision
for communicating these to other parties. Out-of-band distribution
of such business rules is outside the scope of DKIM. Consequently,
there is no means by which external parties can make use of such keys
to attribute messages with any greater granularity than a DNS domain.
If per-user signing keys are assigned for internal purposes (e.g.,
authenticating messages sent to an MTA (Mail Transfer Agent) for
distribution), the following issues need to be considered before
using such signatures as an alternative to traditional edge signing
at the outbound MTA:
External verifiers will be unable to make use of the additional
signature granularity without access to additional information
passed out of band with respect to [RFC4871].
If the number of user keys is large, the efficiency of local
caching of key records by verifiers will be lower.
A large number of end users is be less likely to do an adequate
job of managing private key data securely on their personal
computers than is an administrator running an edge MTA.
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3.4. Third-Party Signer Key Management and Selector Administration
A DKIM key record only asserts that the holder of the corresponding
domain name makes a claim of responsibility for messages signed under
the corresponding key. In some applications, such as bulk mail
delivery, it is desirable to delegate use of the key. That is, to
allow a third party to sign on behalf of the domain holder. The
trust relationship is still established between the domain holder and
the verifier, but the private signature key is held by a third party.
Signature keys used by a third-party signer need to be kept entirely
separate from those used by the domain holder and other third-party
signers. To limit potential exposure of the private key, the
signature key pair needs to be generated by the third-party signer
and the public component of the key transmitted to the domain holder,
rather than have the domain holder generate the key pair and transmit
the private component to the third-party signer.
Domain holders needs to adopt a least-privilege approach and grant
third-party signers the minimum access necessary to perform the
desired function. Limiting the access granted to third-party signers
serves to protect the interests of both parties. The domain holder
minimizes its security risk and the TTP signer avoids unnecessary
liability.
In the most restrictive case, domain holders maintain full control
over the creation of key records. They can employ appropriate key
record restrictions to enforce limits on the messages for which the
third-party signer is able to sign. If such restrictions are
impractical, the domain holder needs to delegate a DNS subzone for
publishing key records to the third-party signer. It is best that
the domain holder NOT allow a third-party signer unrestricted access
to its DNS service for the purpose of publishing key records.
3.5. Key Pair / Selector Life Cycle Management
Deployments need to establish, document, and observe processes for
managing the entire life cycle of an asymmetric key pair.
3.5.1. Example Key Deployment Process
When it is determined that a new key pair is required:
1. A Key Pair is generated by the signing device.
2. A proposed key selector record is generated and transmitted to
the DNS administration infrastructure.
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3. The DNS administration infrastructure verifies the authenticity
of the key selector registration request. If accepted:
1. A key selector is assigned.
2. The corresponding key record is published in the DNS.
3. Wait for DNS updates to propagate (if necessary).
4. Report assigned key selector to signing device.
4. The signer verifies correct registration of the key record.
5. The signer begins generating signatures using the new key pair.
6. The signer terminates any private keys that are no longer
required due to issue of replacement.
3.5.2. Example Key Termination Process
When it is determined that a private signature key is no longer
required:
1. The signer stops using the private key for signature operations.
2. The signer deletes all records of the private key, including in-
memory copies at the signing device.
3. The signer notifies the DNS administration infrastructure that
the signing key is withdrawn from service and that the
corresponding key records can be withdrawn from service at a
specified future date.
4. The DNS administration infrastructure verifies the authenticity
of the key selector termination request. If accepted,
1. The key selector is scheduled for deletion at a future time
determined by site policy.
2. Wait for deletion time to arrive.
3. The signer either publishes a revocation key selector with an
empty public-key data (p=) field, or deletes the key selector
record entirely.
5. As far as the verifier is concerned, there is no functional
difference between verifying against a key selector with an empty
p= field, and verifying against a missing key selector: both
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result in a failed signature and the signature needs to be
treated as if it had not been there. However, there is a minor
semantic difference: with the empty p= field, the signer is
explicitly stating that the key has been revoked. The empty p=
record provides a gravestone for an old selector, making it less
likely that the selector might be accidentally reused with a
different public key.
4. Signing
Creating messages that have one or more DKIM signatures requires
support in only two outbound email service components:
o A DNS Administrative interface that can create and maintain the
relevant DNS names -- including names with underscores -- and
resource records (RR).
o A trusted module, called the signing module, which is within the
organization's outbound email handling service and which creates
and adds the DKIM-Signature: header field(s) to the message.
If the module creates more than one signature, there needs to be the
appropriate means of telling it which one(s) to use. If a large
number of names are used for signing, it will help to have the
administrative tool support a batch-processing mode.
4.1. DNS Records
A receiver attempting to verify a DKIM signature obtains the public
key that is associated with the signature for that message. The
DKIM-Signature: header in the message contains the d= tag with the
basic domain name doing the signing and serving as output to the
Identity Assessor and the s= tag with the selector that is added to
the name, for finding the specific public key. Hence, the relevant
<selector>._domainkey.<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. Initial DKIM DNS information is contained
within TXT RRs. DNS administrative software varies considerably in
its abilities to support DKIM names, such as with underscores, and to
add new types of DNS information.
4.2. Signing Module
The module doing signing can be placed anywhere within an
organization's trusted Administrative Management Domain (ADMD);
obvious choices include department-level posting agents, as well as
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outbound boundary MTAs to the open Internet. However, any other
module, including the author's MUA (Mail User Agent), is potentially
acceptable, as long as the signature survives any remaining handling
within the ADMD. Hence, the choice among the modules depends upon
software development, administrative overhead, security exposures,
and transit-handling tradeoffs. One perspective that helps to
resolve this choice is the difference between the increased
flexibility, from placement at (or close to) the MUA, versus the
streamlined administration and operation that is more easily obtained
by implementing the mechanism "deeper" into the organization's email
infrastructure, such as at its boundary MTA.
Note the discussion in Section 2.2 concerning the use of the i= tag.
The signing module uses the appropriate private key to create one or
more signatures. (See Section 6.5 for a discussion of multiple
signatures.) The means by which the signing module obtains the
private key(s) 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. For administrative
convenience, it is best not to hard-code key information into
software.
4.3. Signing Policies and Practices
Every organization (ADMD) will have its own policies and practices
for deciding when to sign messages (message stream) and with what
domain name, selector, and key. Examples of particular message
streams include all mail sent from the ADMD versus mail from
particular types of user accounts versus mail having particular types
of content. Given this variability, and the likelihood that signing
practices will change over time, it will be useful to have these
decisions represented through run-time configuration information,
rather than being hard-coded into the signing software.
As noted in Section 2.3, the choice of signing name granularity
requires balancing administrative convenience and utility for
recipients. Too much granularity is higher administrative overhead
and might well attempt to impose more differential analysis on the
recipient than they wish to support. In such cases, they are likely
to use only a super-name -- right-hand substring -- of the signing
name. When this occurs, the signer will not know what portion is
being used; this then moves DKIM back to the non-deterministic world
of heuristics, rather than the mechanistic world of signer/recipient
collaboration that DKIM seeks.
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5. Verifying
A message recipient can verify a DKIM signature to determine if a
claim of responsibility has been made for the message by a trusted
domain.
Access control requires two components: authentication and
authorization. By design, verification of a DKIM signature only
provides the authentication component of an access control decision
and needs to be combined with additional sources of information such
as reputation data to arrive at an access control decision.
5.1. Intended Scope of Use
DKIM requires that a message with a signature that is found to be
invalid is to be treated as if the message had not been signed at
all.
If a DKIM signature fails to verify, it is entirely possible that the
message is valid and that either there is a configuration error in
the signer's system (e.g., a missing key record) or that the message
was inadvertently modified in transit. It is thus undesirable for
mail infrastructure to treat messages with invalid signatures less
favorably than those with no signatures whatsoever. Contrariwise,
creation of an invalid signature requires a trivial amount of effort
on the part of an attacker. If messages with invalid signatures were
to be treated preferentially to messages with no signatures
whatsoever, attackers will simply add invalid signature blocks to
gain the preferential treatment. It follows that messages with
invalid signatures need to be treated no better and no worse than
those with no signature at all.
5.2. Signature Scope
As with any other digital signature scheme, verifiers need to
consider only the part of the message that is inside the scope of the
message as being authenticated by the signature.
For example, if the l= option is employed to specify a content length
for the scope of the signature, only the part of the message that is
within the scope of the content signature would be considered
authentic.
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5.3. Design Scope of Use
Public key cryptography provides an exceptionally high degree of
assurance, bordering on absolute certainty, that the party that
created a valid digital signature had access to the private key
corresponding to the public key indicated in the signature.
In order to make useful conclusions from the verification of a valid
digital signature, the verifier is obliged to make assumptions that
fall far short of absolute certainty. Consequently, mere validation
of a DKIM signature does not represent proof positive that a valid
claim of responsibility was made for it by the indicated party, that
the message is authentic, or that the message is not abusive. In
particular:
o The legitimate private key holder might have lost control of its
private key.
o The legitimate domain holder might have lost control of the DNS
server for the zone from which the key record was retrieved.
o The key record might not have been delivered from the legitimate
DNS server for the zone from which the key record was retrieved.
o Ownership of the DNS zone might have changed.
In practice, these limitations have little or no impact on the field
of use for which DKIM is designed, but they can have a bearing if use
is made of the DKIM message signature format or key retrieval
mechanism in other specifications.
In particular, the DKIM key retrieval mechanism is designed for ease
of use and deployment rather than to provide a high assurance Public
Key Infrastructure suitable for purposes that require robust non-
repudiation such as establishing legally binding contracts.
Developers seeking to extend DKIM beyond its design application need
to consider replacing or supplementing the DNS key retrieval
mechanism with one that is designed to meet the intended purposes.
5.4. Inbound Mail Filtering
DKIM is frequently employed in a mail filtering strategy to avoid
performing content analysis on email originating from trusted
sources. Messages that carry a valid DKIM signature from a trusted
source can be whitelisted, avoiding the need to perform computation
and hence energy-intensive content analysis to determine the
disposition of the message.
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Mail sources can be determined to be trusted by means of previously
observed behavior and/or reference to external reputation or
accreditation services. The precise means by which this is
accomplished is outside the scope of DKIM.
5.4.1. Non-Verifying Adaptive Spam Filtering Systems
Adaptive (or learning) spam filtering mechanisms that are not capable
of verifying DKIM signatures need to, at minimum, be configured to
ignore DKIM header data entirely.
5.5. Messages Sent through Mailing Lists and Other Intermediaries
Intermediaries, such as mailing lists, pose a particular challenge
for DKIM implementations, as the message processing steps performed
by the intermediary can cause the message content to change in ways
that prevent the signature passing verification.
Such intermediaries are strongly encouraged to deploy DKIM signing so
that a verifiable claim of responsibility remains available to
parties attempting to verify the modified message.
5.6. Generation, Transmission, and Use of Results Headers
In many deployments, it is desirable to separate signature
verification from the application relying on the verification. A
system can choose to relay information indicating the results of its
message authentication efforts using various means; adding a "results
header" to the message is one such mechanism [RFC5451]. For example,
consider the cases where:
o The application relying on DKIM signature verification is not
capable of performing the verification.
o The message can be modified after the signature verification is
performed.
o The signature key cannot be available by the time that the message
is read.
In such cases, it is important that the communication link between
the signature verifier and the relying application be sufficiently
secure to prevent insertion of a message that carries a bogus results
header.
An intermediary that generates results headers need to ensure that
relying applications are able to distinguish valid results headers
issued by the intermediary from those introduced by an attacker. For
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example, this can be accomplished by signing the results header. At
a minimum, results headers on incoming messages need to be removed if
they purport to have been issued by the intermediary but cannot be
verified as authentic.
Further discussion on trusting the results as relayed from a verifier
to something downstream can be found in [RFC5451].
6. Taxonomy of Signatures
As described in Section 2.1, a DKIM signature tells the signature
verifier that the owner of a particular domain name accepts some
responsibility for the message. It does not, in and of itself,
provide any information about the trustworthiness or behavior of that
identity. What it does provide is a verified identity to which such
behavioral information can be associated, so that those who collect
and use such information can be assured that it truly pertains to the
identity in question.
This section lays out a taxonomy of some of the different identities,
or combinations of identities, that might usefully be represented by
a DKIM signature.
6.1. Single Domain Signature
Perhaps the simplest case is when an organization signs its own
outbound email using its own domain in the SDID [RFC5672] of the
signature. For example, Company A would sign the outbound mail from
its employees with d=companyA.example.
In the most straightforward configuration, the addresses in the
RFC5322.From field would also be in the companyA.example domain, but
that direct correlation is not required.
A special case of the single domain signature is an author signature
as defined by the Author Domain Signing Practices specification
[RFC5617]. Author signatures are signatures from an author's
organization that have an SDID value that matches that of an
RFC5322.From address of the signed message.
Although an author signature might, in some cases, be proof against
spoofing the domain name of the RFC5322.From address, it is important
to note that the DKIM and ADSP validation apply only to the exact
address string and not to look-alike addresses or to the human-
friendly "display-name" or names and addresses used within the body
of the message. That is, it only protects against the misuse of a
precise address string within the RFC5322.From field and nothing
else. For example, a message from bob@domain.example with a valid
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signature where d=d0main.example would fail an ADSP check because the
signature domain, however similar, is distinct; however, a message
from bob@d0main.example with a valid signature where d=d0main.example
would pass an ADSP check, even though to a human it might be obvious
that d0main.example is likely a malicious attempt to spoof the domain
domain.example. This example highlights that ADSP, like DKIM, is
only able to validate a signing identifier: it still requires some
external process to attach a meaningful reputation to that
identifier.
6.2. Parent Domain Signature
Another approach that might be taken by an organization with multiple
active subdomains is to apply the same (single) signature domain to
mail from all subdomains. In this case, the signature chosen would
usually be the signature of a parent domain common to all subdomains.
For example, mail from marketing.domain.example,
sales.domain.example, and engineering.domain.example might all use a
signature where d=domain.example.
This approach has the virtue of simplicity, but it is important to
consider the implications of such a choice. As discussed in
Section 2.3, if the type of mail sent from the different subdomains
is significantly different or if there is reason to believe that the
reputation of the subdomains would differ, then it can be a good idea
to acknowledge this and provide distinct signatures for each of the
subdomains (d=marketing.domain.example, sales.domain.example, etc.).
However, if the mail and reputations are likely to be similar, then
the simpler approach of using a single common parent domain in the
signature can work well.
Another approach to distinguishing the streams using a single DKIM
key would be to leverage the AUID [RFC5672] (i= tag) in the DKIM
signature to differentiate the mail streams. For example, marketing
email would be signed with i=@marketing.domain.example and
d=domain.example.
It's important to remember, however, that under core DKIM semantics,
the AUID is opaque to receivers. That means that it will only be an
effective differentiator if there is an out-of-band agreement about
the i= semantics.
6.3. Third-Party Signature
A signature whose domain does not match the domain of the
RFC5322.From address is sometimes referred to as a third-party
signature. In certain cases, even the parent domain signature
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described above would be considered a third-party signature because
it would not be an exact match for the domain in the RFC5322.From
address.
Although there is often heated debate about the value of third party
signatures, it is important to note that the DKIM specification
attaches no particular significance to the identity in a DKIM
signature ([RFC4871], [RFC5672]). The identity specified within the
signature is the identity that is taking responsibility for the
message, and it is only the interpretation of a given receiver that
gives one identity more or less significance than another. In
particular, most independent reputation services assign trust based
on the specific identifier string, not its "role": in general they
make no distinction between, for example, an author signature and a
third-party signature.
For some, a signature unrelated to the author domain (the domain in
the RFC5322.From address) is less valuable because there is an
assumption that the presence of an author signature guarantees that
the use of the address in the RFC5322.From header is authorized.
For others, that relevance is tied strictly to the recorded
behavioral data assigned to the identity in question, i.e., its trust
assessment or reputation. The reasoning here is that an identity
with a good reputation is unlikely to maintain that good reputation
if it is in the habit of vouching for messages that are unwanted or
abusive; in fact, doing so will rapidly degrade its reputation so
that future messages will no longer benefit from it. It is therefore
low risk to facilitate the delivery of messages that contain a valid
signature of a domain with a strong positive reputation, independent
of whether or not that domain is associated with the address in the
RFC5322.From header field of the message.
Third-party signatures encompass a wide range of identities. Some of
the more common are:
Service Provider: In cases where email is outsourced to an Email
Service Provider (ESP), Internet Service Provider (ISP), or other
type of service provider, that service provider can choose to
DKIM-sign outbound mail with either its own identifier -- relying
on its own, aggregate reputation -- or with a subdomain of the
provider that is unique to the message author but still part of
the provider's aggregate reputation. Such service providers can
also encompass delegated business functions such as benefit
management, although these will more often be treated as trusted
third-party senders (see below).
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Parent Domain: As discussed above, organizations choosing to apply a
parent-domain signature to mail originating from subdomains can
have their signatures treated as third party by some verifiers,
depending on whether or not the "t=s" tag is used to constrain the
parent signature to apply only to its own specific domain. The
default is to consider a parent-domain signature valid for its
subdomains.
Reputation Provider: Another possible category of third-party
signature would be the identity of a third-party reputation
provider. Such a signature would indicate to receivers that the
message was being vouched for by that third party.
6.4. Using Trusted Third-Party Senders
For most of the cases described so far, there has been an assumption
that the signing agent was responsible for creating and maintaining
its own DKIM signing infrastructure, including its own keys, and
signing with its own identity.
A different model arises when an organization uses a trusted third-
party sender for certain key business functions, but still wants that
email to benefit from the organization's own identity and reputation.
In other words, the mail would come out of the trusted third party's
mail servers, but the signature applied would be that of the
controlling organization.
This can be done by having the third party generate a key pair that
is designated uniquely for use by that trusted third party and
publishing the public key in the controlling organization's DNS
domain, thus enabling the third party to sign mail using the
signature of the controlling organization. For example, if Company A
outsources its employee benefits to a third party, it can use a
special key pair that enables the benefits company to sign mail as
"companyA.example". Because the key pair is unique to that trusted
third party, it is easy for Company A to revoke the authorization if
necessary by simply removing the public key from the companyA.example
DNS.
A more cautious approach would be to create a dedicated subdomain
(e.g., benefits.companyA.example) to segment the outsourced mail
stream, and to publish the public key there; the signature would then
use d=benefits.companyA.example.
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6.4.1. DNS Delegation
Another possibility for configuring trusted third-party access, as
discussed in Section 3.4, is to have Company A use DNS delegation and
have the designated subdomain managed directly by the trusted third
party. In this case, Company A would create a subdomain
benefits.companya.example, and delegate the DNS management of that
subdomain to the benefits company so it could maintain its own key
records. When revocation becomes necessary, Company A could simply
remove the DNS delegation record.
6.5. Multiple Signatures
A simple configuration for DKIM-signed mail is to have a single
signature on a given message. This works well for domains that
manage and send all of their own email from single sources, or for
cases where multiple email streams exist but each has its own unique
key pair. It also represents the case in which only one of the
participants in an email sequence is able to sign, no matter whether
it represents the author or one of the operators.
The examples thus far have considered the implications of using
different identities in DKIM signatures, but have used only one such
identity for any given message. In some cases, it can make sense to
have more than one identity claiming responsibility for the same
message.
There are a number of situations where applying more than one DKIM
signature to the same message might make sense. A few examples are:
Companies with multiple subdomain identities: A company that has
multiple subdomains sending distinct categories of mail might
choose to sign with distinct subdomain identities to enable each
subdomain to manage its own identity. However, it might also want
to provide a common identity that cuts across all of the distinct
subdomains. For example, Company A can sign mail for its sales
department with a signature where d=sales.companya.example and a
second signature where d=companya.example
Service Providers: A service provider can, as described above,
choose to sign outbound messages with either its own identity or
an identity unique to each of its clients (possibly delegated).
However, it can also do both: sign each outbound message with its
own identity as well as with the identity of each individual
client. For example, ESP A might sign mail for its client Company
B with its service provider signature d=espa.example, and a second
client-specific signature where d= either companyb.example or
companyb.espa.example. The existence of the service provider
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signature could, for example, help cover a new client while it
establishes its own reputation, or help a very small volume client
who might never reach a volume threshold sufficient to establish
an individual reputation.
Forwarders: Forwarded mail poses a number of challenges to email
authentication. DKIM is relatively robust in the presence of
forwarders as long as the signature is designed to avoid message
parts that are likely to be modified; however, some forwarders do
make modifications that can invalidate a DKIM signature.
Some forwarders such as mailing lists or "forward article to a
friend" services might choose to add their own signatures to
outbound messages to vouch for them having legitimately originated
from the designated service. In this case, the signature would be
added even in the presence of a preexisting signature, and both
signatures would be relevant to the verifier.
Any forwarder that modifies messages in ways that will break
preexisting DKIM signatures needs to sign its forwarded messages.
Reputation Providers: Although third-party reputation providers
today use a variety of protocols to communicate their information
to receivers, it is possible that they, or other organizations
willing to put their "seal of approval" on an email stream, might
choose to use a DKIM signature to do it. In nearly all cases,
this "reputation" signature would be in addition to the author or
originator signature.
One important caveat to the use of multiple signatures is that there
is currently no clear consensus among receivers on how they plan to
handle them. The opinions range from ignoring all but one signature
(and the specification of which of them is verified differs from
receiver to receiver), to verifying all signatures present and
applying a weighted blend of the trust assessments for those
identifiers, to verifying all signatures present and simply using the
identifier that represents the most positive trust assessment. It is
likely that the industry will evolve to accept multiple signatures
using either the second or third of these, but it can take some time
before one approach becomes pervasive.
7. Example Usage Scenarios
Signatures are created by different types of email actors, based on
different criteria, such as where the actor operates in the sequence
from author to recipient, whether they want different messages to be
evaluated under the same reputation or a different one, and so on.
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This section provides some examples of usage scenarios for DKIM
deployments; the selection is not intended to be exhaustive but to
illustrate a set of key deployment considerations.
7.1. Author's Organization - Simple
The simplest DKIM configuration is to have some mail from a given
organization (Company A) be signed with the same d= value (e.g.,
d=companya.example). If there is a desire to associate additional
information, the AUID [RFC5672] value can become
uniqueID@companya.example, or @uniqueID.companya.example.
In this scenario, Company A need only generate a single signing key
and publish it under their top-level domain (companya.example); the
signing module would then tailor the AUID value as needed at signing
time.
7.2. Author's Organization - Differentiated Types of Mail
A slight variation of the one signature case is where Company A signs
some of its mail, but it wants to differentiate among categories of
its outbound mail by using different identifiers. For example, it
might choose to distinguish marketing, billing or transactional, and
individual corporate email into marketing.companya.example,
billing.companya.example, and companya.example, respectively, where
each category is assigned a unique subdomain and unique signing keys.
7.3. Author Domain Signing Practices
7.3.1. Introduction
Some domains might decide to sign all of their outgoing mail. If all
of the legitimate mail for a domain is signed, recipients can be more
aggressive in their filtering of mail that uses the domain but does
not have a valid signature from the domain; in such a configuration,
the absence of a signature would be more significant than for the
general case. It might be desirable for such domains to be able to
advertise their intent to other receivers: this is the topic of
Author Domain Signing Practices (ADSP).
Note that ADSP is not for everyone. Sending domains that do not
control all legitimate outbound mail purporting to be from their
domain (i.e., with an RFC5322.From address in their domain) are
likely to experience delivery problems with some percentage of that
mail. Administrators evaluating ADSP for their domains needs to
carefully weigh the risk of phishing attacks against the likelihood
of undelivered mail.
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This section covers some examples of ADSP usage. For the complete
specification, see [RFC5617].
7.3.2. A Few Definitions
In the ADSP specification, an address in the RFC5322.From header
field of a message is defined as an "Author Address", and an "Author
Domain" is defined as anything to the right of the '@' in an author
address.
An "Author Signature" is thus any valid signature where the value of
the SDID matches an author domain in the message.
It is important to note that unlike the DKIM specification, which
makes no correlation between the signature domain and any message
headers, the ADSP specification applies only to the author domain.
In essence, under ADSP, any non-author signatures are ignored
(treated as if they are not present).
Signers wishing to publish an Author Domain Signing Practices (ADSP)
[RFC5617] record describing their signing practices will thus want to
include an author signature on their outbound mail to avoid ADSP
verification failures.
7.3.3. Some ADSP Examples
An organization (Company A) can specify its signing practices by
publishing an ADSP record with "dkim=all" or "dkim=discardable". In
order to avoid misdelivery of its mail at receivers that are
validating ADSP, Company A needs to first have done an exhaustive
analysis to determine all sources of outbound mail from its domain
(companyA.example) and ensure that they all have valid author
signatures from that domain.
For example, email with an RFC5322.From address of bob@
companyA.example needs to have an author signature where the SDID
value is "companyA.example" or it will fail an ADSP validation.
Note that once an organization publishes an ADSP record using
dkim=all or dkim=discardable, any email with an RFC5322.From address
that uses the domain where the ADSP record is published that does not
have a valid author signature is at risk of being misdelivered or
discarded. For example, if a message with an RFC5322.From address of
newsletter@companyA.example has a signature with
d=marketing.companyA.example, that message will fail the ADSP check
because the signature would not be considered a valid author
signature.
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Because the semantics of an ADSP author signature are more
constrained than the semantics of a "pure" DKIM signature, it is
important to make sure the nuances are well understood before
deploying an ADSP record. The ADSP specification [RFC5617] provides
some fairly extensive lookup examples (in Appendix A) and usage
examples (in Appendix B).
In particular, in order to prevent mail from being negatively
impacted or even discarded at the receiver, it is essential to
perform a thorough survey of outbound mail from a domain before
publishing an ADSP policy of anything stronger than "unknown". This
includes mail that might be sent from external sources that might not
be authorized to use the domain signature, as well as mail that risks
modification in transit that might invalidate an otherwise valid
author signature (e.g., mailing lists, courtesy forwarders, and other
paths that could add or modify headers or modify the message body).
7.4. Delegated Signing
An organization might choose to outsource certain key services to an
independent company. For example, Company A might outsource its
benefits management, or Organization B might outsource its marketing
email.
If Company A wants to ensure that all of the mail sent on its behalf
through the benefits providers email servers shares the Company A
reputation, as discussed in Section 6.4, it can either publish keys
designated for the use of the benefits provider under
companyA.example (preferably under a designated subdomain of
companyA.example), or it can delegate a subdomain (e.g.,
benefits.companyA.example) to the provider and enable the provider to
generate the keys and manage the DNS for the designated subdomain.
In both of these cases, mail would be physically going out of the
benefit provider's mail servers with a signature of, e.g.,
d=benefits.companya.example. Note that the RFC5322.From address is
not constrained: it could be affiliated with either the benefits
company (e.g., benefits-admin@benefitprovider.example, or
benefits-provider@benefits.companya.example) or the companyA domain.
Note that in both of the above scenarios, as discussed in
Section 3.4, security concerns dictate that the keys be generated by
the organization that plans to do the signing so that there is no
need to transfer the private key. In other words, the benefits
provider would generate keys for both of the above scenarios.
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7.5. Independent Third-Party Service Providers
Another way to manage the service provider configuration would be to
have the service provider sign the outgoing mail on behalf of its
client, Company A, with its own (provider) identifier. For example,
an Email Service Provider (ESP A) might want to share its own mailing
reputation with its clients, and might sign all outgoing mail from
its clients with its own d= domain (e.g., d=espa.example).
When the ESP wants to distinguish among its clients, it has two
options:
o Share the SDID domain and use the AUID value to distinguish among
the clients, e.g., a signature on behalf of client A would have
d=espa.example and i=@clienta.espa.example (or
i=clienta@espa.example).
o Extend the SDID domain, so there is a unique value (and subdomain)
for each client, e.g., a signature on behalf of client A would
have d=clienta.espa.example.
Note that this scenario and the delegation scenario are not mutually
exclusive. In some cases, it can be desirable to sign the same
message with both the ESP and the ESP client identities.
7.6. Mail Streams Based on Behavioral Assessment
An ISP (ISP A) might want to assign signatures to outbound mail from
its users according to each user's past sending behavior
(reputation). In other words, the ISP would segment its outbound
traffic according to its own assessment of message quality, to aid
recipients in differentiating among these different streams. Since
the semantics of behavioral assessments are not valid AUID values,
ISP A (ispa.example) can configure subdomains corresponding to the
assessment categories (e.g., good.ispa.example, neutral.ispa.example,
bad.ispa.example), and use these subdomains in the d= value of the
signature.
The signing module can also set the AUID value to have a unique user
ID (distinct from the local-part of the user's email address), for
example, user3456@neutral.domain.example. Using a user ID that is
distinct from a given email alias is useful in environments where a
single user might register multiple email aliases.
Note that in this case, the AUID values are only partially stable.
They are stable in the sense that a given i= value will always
represent the same identity, but they are unstable in the sense that
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a given user can migrate among the assessment subdomains depending on
their sending behavior (i.e., the same user might have multiple AUID
values over the lifetime of a single account).
In this scenario, ISP A can generate as many keys as there are
assessment subdomains (SDID values), so that each assessment
subdomain has its own key. The signing module would then choose its
signing key based on the assessment of the user whose mail was being
signed, and if desired, include the user ID in the AUID of the
signature. As discussed earlier, the per-user granularity of the
AUID can be ignored by verifiers; so organizations choosing to use it
ought not rely on its use for receiver side filtering results.
However, some organizations might also find the information useful
for their own purposes in processing bounces or abuse reports.
7.7. Agent or Mediator Signatures
Another scenario is that of an agent, usually a re-mailer of some
kind, that signs on behalf of the service or organization that it
represents. Some examples of agents might be a mailing list manager,
or the "forward article to a friend" service that many online
publications offer. In most of these cases, the signature is
asserting that the message originated with, or was relayed by, the
service asserting responsibility. In general, if the service is
configured in such a way that its forwarding would break existing
DKIM signatures, it needs to always add its own signature.
8. Usage Considerations
8.1. Non-Standard Submission and Delivery Scenarios
The robustness of DKIM's verification mechanism is based on the fact
that only authorized signing modules have access to the designated
private key. This has the side effect that email submission and
delivery scenarios that originate or relay messages from outside the
domain of the authorized signing module will not have access to that
protected private key, and thus will be unable to attach the expected
domain signature to those messages. Such scenarios include mailing
lists, courtesy forwarders, MTAs at hotels, hotspot networks used by
traveling users, and other paths that could add or modify headers, or
modify the message body.
For example, assume Joe works for Company A and has an email address
joe@companya.example. Joe also has an ISP-1 account
joe@isp1.example.com, and he uses ISP-1's multiple address feature to
attach his work email address, joe@companya.example, to email from
his ISP-1 account. When Joe sends email from his ISP-1 account and
uses joe@companya.example as his designated RFC5322.From address,
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that email cannot have a signature with d=companya.example because
the ISP-1 servers have no access to Company A's private key. In
ISP-1's case, it will have an ISP-1 signature, but for some other
mail clients offering the same multiple address feature there might
be no signature at all on the message.
Another example might be the use of a forward article to a friend
service. Most instances of these services today allow someone to
send an article with their email address in the RFC5322.From to their
designated recipient. If Joe used either of his two addresses
(joe@companya.example or joe@isp1.example.com), the forwarder would
be equally unable to sign with a corresponding domain. As in the
mail client case, the forwarder can either sign as its own domain or
put no signature on the message.
A third example is the use of privately configured forwarding.
Assume that Joe has another account at ISP-2, joe@isp-2.example.com,
but he'd prefer to read his ISP-2 mail from his ISP-1 account. He
sets up his ISP-2 account to forward all incoming mail to
joe@isp1.example.com. Assume alice@companyb.example sends
joe@isp-2.example.com an email. Depending on how companyb.example
configured its signature, and depending on whether or not ISP-2
modifies messages that it forwards, it is possible that when Alice's
message is received in Joe's ISP-1 account, the original signature
will fail verification.
8.2. Protection of Internal Mail
One identity is particularly amenable to easy and accurate
assessment: the organization's own identity. Members of an
organization tend to trust messages that purport to be from within
that organization. However, Internet Mail does not provide a
straightforward means of determining whether such mail is, in fact,
from within the organization. DKIM can be used to remedy this
exposure. If the organization signs all of its mail, then its
boundary MTAs can look for mail purporting to be from the
organization that does not contain a verifiable signature.
Such mail can, in most cases, be presumed to be spurious. However,
domain managers are advised to consider the ways that mail processing
can modify messages in ways that will invalidate an existing DKIM
signature: mailing lists, courtesy forwarders, and other paths that
could add or modify headers or modify the message body (e.g., MTAs at
hotels, hotspot networks used by traveling users, and other scenarios
described in the previous section). Such breakage is particularly
relevant in the presence of Author Domain Signing Practices.
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8.3. Signature Granularity
Although DKIM's use of domain names is optimized for a scope of
organization-level signing, it is possible to administer subdomains
or otherwise adjust signatures in a way that supports per-user
identification. This user-level granularity can be specified in two
ways: either by sharing the signing identity and specifying an
extension to the i= value that has a per-user granularity or by
creating and signing with unique per-user keys.
A subdomain or local part in the i= tag needs to be treated as an
opaque identifier and thus need not correspond directly to a DNS
subdomain or be a specific user address.
The primary way to sign with per-user keys requires each user to have
a distinct DNS (sub)domain, where each distinct d= value has a key
published. (It is possible, although not advised, to publish the
same key in more than one distinct domain.)
It is technically possible to publish per-user keys within a single
domain or subdomain by utilizing different selector values. This is
not advised and is unlikely to be treated uniquely by Assessors: the
primary purpose of selectors is to facilitate key management, and the
DKIM specification recommends against using them in determining or
assessing identities.
In most cases, it would be impractical to sign email on a per-user
granularity. Such an approach would be
likely to be ignored: In most cases today, if receivers are
verifying DKIM signatures, they are in general taking the simplest
possible approach. In many cases, maintaining reputation
information at a per-user granularity is not interesting to them,
in large part because the per-user volume is too small to be
useful or interesting. So even if senders take on the complexity
necessary to support per-user signatures, receivers are unlikely
to retain anything more than the base domain reputation.
difficult to manage: Any scheme that involves maintenance of a
significant number of public keys might require infrastructure
enhancements or extensive administrative expertise. For domains
of any size, maintaining a valid per-user keypair, knowing when
keys need to be revoked or added due to user attrition or
onboarding, and the overhead of having the signing engine
constantly swapping keys can create significant and often
unnecessary management complexity. It is also important to note
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that there is no way within the scope of the DKIM specification
for a receiver to infer that a sender intends a per-user
granularity.
As mentioned before, what might make sense, however, is to use the
infrastructure that enables finer granularity in signatures to
identify segments smaller than a domain but much larger than a per-
user segmentation. For example, a university might want to segment
student, staff, and faculty mail into three distinct streams with
differing reputations. This can be done by creating separate
subdomains for the desired segments, and either specifying the
subdomains in the i= tag of the DKIM Signature or by adding
subdomains to the d= tag and assigning and signing with different
keys for each subdomain.
For those who choose to represent user-level granularity in
signatures, the performance and management considerations above
suggest that it would be more effective to do so by specifying a
local part or subdomain extension in the i= tag rather than by
extending the d= domain and publishing individual keys.
8.4. Email Infrastructure Agents
It is expected that the most common venue for a DKIM implementation
will be within the infrastructure of an organization's email service,
such as a department or a boundary MTA. What follows are some
general recommendations for the Email Infrastructure.
Outbound: An MSA (Mail Submission Agent) or an outbound MTA used
for mail submission needs to ensure that the message sent is in
compliance with the advertised email sending policy. It needs
to also be able to generate an operator alert if it determines
that the email messages do not comply with the published DKIM
sending policy.
An MSA needs to be aware that some MUAs might add their own
signatures. If the MSA needs to perform operations on a
message to make it comply with its email sending policy, if at
all possible, it needs to do so in a way that would not break
those signatures.
MUAs equipped with the ability to sign ought not to be
encouraged. In terms of security, MUAs are generally not under
the direct control of those in responsible roles within an
organization and are thus more vulnerable to attack and
compromise, which would expose private signing keys to
intruders and thus jeopardize the integrity and reputation of
the organization.
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Inbound: When an organization deploys DKIM, it needs to make
sure that its email infrastructure components that do not have
primary roles in DKIM handling do not modify message in ways
that prevent subsequent verification.
An inbound MTA or an MDA can incorporate an indication of the
verification results into the message, such as using an
Authentication-Results header field [RFC5451].
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.
+ needs to deploy abuse filtering measures on the inbound
mail, and
+ probably also needs to remove all signatures that will be
broken.
In addition, the intermediary can:
+ verify the message signature prior to modification.
+ incorporate an indication of the verification results into
the message, such as using an Authentication-Results header
field [RFC5451].
+ sign the modified message including the verification results
(e.g., the Authentication-Results header field).
8.5. Mail User Agent
The DKIM specification is expected to be used primarily between
Boundary MTAs, or other infrastructure components of the originating
and receiving ADMDs. However, there is nothing in DKIM that is
specific to those venues. In particular, MUAs can also support DKIM
signing and verifying directly.
Outbound: An MUA can support signing even if mail is to be
relayed through an outbound MSA. In this case, the signature
applied by the MUA will be in addition to any signature added
by the MSA. However, the warnings in the previous section need
to be taken into consideration.
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Some user software goes beyond simple user functionality and
also performs MSA and MTA functions. When this is employed for
sending directly to a receiving ADMD, the user software needs
to be considered an outbound MTA.
Inbound: An MUA can rely on a report of a DKIM signature
verification that took place at some point in the inbound MTA/
MDA path (e.g., an Authentication-Results header field), or an
MUA can perform DKIM signature verification directly. A
verifying MUA needs to allow for the case where mail has been
modified in the inbound MTA path; if a signature fails, the
message is to be treated the same as a message that does not
have a signature.
An MUA that looks for an Authentication-Results header field
needs to be configurable to choose which Authentication-Results
header fields are considered trustable. The MUA developer is
encouraged to re-read the Security Considerations of [RFC5451].
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 essential that successful verification of a signature not
result in a less than satisfactory user experience compared to
leaving the message unsigned. The mere presence of a verified
DKIM signature cannot be used by itself by an MUA to indicate
that a message is to be treated better than a message without a
verified DKIM signature. However, the fact that a DKIM
signature was verified can be used as input into a reputation
system (i.e., a whitelist of domains and users) for
presentation of such indicators.
It is common for components of an ADMD's email infrastructure to do
violence to a message, such that a DKIM signature might be rendered
invalid. Hence, users of MUAs that support DKIM signing and/or
verifying need a basis for knowing that their associated email
infrastructure will not break a signature.
9. Security Considerations
The security considerations of the DKIM protocol are described in the
DKIM base specification [RFC4871].
10. Acknowledgements
The effort of the DKIM Working Group is gratefully acknowledged.
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11. References
11.1. Normative References
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
Identified Mail (DKIM) Service Overview", RFC 5585,
July 2009.
[RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, August 2009.
[RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM)
Signatures -- Update", RFC 5672, August 2009.
11.2. Informative References
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4870] Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
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Appendix A. Migration Strategies
There are three migration occasions worth noting in particular for
DKIM:
1. Migrating from DomainKeys to DKIM.
2. Migrating from a current hash algorithm to a new standardized
hash algorithm.
3. Migrating from a current signing algorithm to a new standardized
signing algorithm.
The case of deploying a new key selector record is described
elsewhere (Section 3.5).
As with any migration, the steps required will be determined by who
is doing the migration and their assessment of:
o the users of what they are generating, or
o the providers of what they are consuming.
Signers and verifiers have different considerations.
A.1. Migrating from DomainKeys
DKIM replaces the earlier DomainKeys (DK) specification. Selector
files are mostly compatible between the two specifications.
A.1.1. Signers
A signer that currently signs with DK will go through various stages
as it migrates to using DKIM, not all of which are required for all
signers. The real questions that a signer needs to ask are:
1. how many receivers or what types of receivers are *only* looking
at the DK signatures and not the DKIM signatures, and
2. how much does the signer care about those receivers?
If no one is looking at the DK signature any more, then it's no
longer necessary to sign with DK. Or if all "large players" are
looking at DKIM in addition to or instead of DK, a signer can choose
to stop signing with DK.
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With respect to signing policies, a reasonable, initial approach is
to use DKIM signatures in the same way that DomainKeys signatures are
already being used. In particular, the same selectors and DNS key
records can be used for both, after verifying that they are
compatible as discussed below.
Each secondary step in all of the following scenarios is to be
prefaced with the gating factor "test, then when comfortable with the
previous step's results, continue".
One migration strategy is to:
o ensure that the current selector DNS key record is compatible with
both DK and DKIM
o sign messages with both DK and DKIM signatures
o when it's decided that DK signatures are no longer necessary, stop
signing with DK
Another migration strategy is to:
o add a new selector DNS key record only for DKIM signatures
o sign messages with both DK (using the old DNS key record) and DKIM
signatures (using the new DNS key record)
o when it's decided that DK signatures are no longer necessary, stop
signing with DK
o eventually remove the old DK selector DNS record
A combined migration strategy is to:
o ensure that the current selector DNS key record is compatible with
both DK and DKIM
o start signing messages with both DK and DKIM signatures
o add a new selector DNS key record for DKIM signatures
o switch the DKIM signatures to use the new selector
o when it's decided that DK signatures are no longer necessary, stop
signing with DK
o eventually remove the old DK selector DNS record
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Another migration strategy is to:
o add a new selector DNS key record for DKIM signatures
o do a flash cut and replace the DK signatures with DKIM signatures
o eventually remove the old DK selector DNS record
Another migration strategy is to:
o ensure that the current selector DNS key record is compatible with
both DK and DKIM
o do a flash cut and replace the DK signatures with DKIM signatures
Note that when you have separate key records for DK and DKIM, you can
use the same public key for both.
A.1.1.1. DNS Selector Key Records
The first step in some of the above scenarios is ensuring that the
selector DNS key records are compatible for both DK and DKIM. The
format of the DNS key record was intentionally meant to be backwardly
compatible between the two systems, but not necessarily upwardly
compatible. DKIM has enhanced the DK DNS key record format by adding
several optional parameters, which DK needs to ignore. However,
there is one critical difference between DK and DKIM DNS key records.
The definitions of the "g" fields:
g= granularity of the key: In both DK and DKIM, this is an optional
field that is used to constrain which sending address(es) can
legitimately use this selector. Unfortunately, the treatment of
an empty field ("g=;") is different. DKIM allows wildcards where
DK does not. For DK, an empty field is the same as a missing
value, and is treated as allowing any sending address. For DKIM,
an empty field only matches an empty local part. In DKIM, both a
missing value and "g=*;" mean to allow any sending address.
Also, in DomainKeys, the "g" field is required to match the
address in "From:"/"Sender:", while in DKIM, it is required to
match i=. This might or might not affect transition.
If your DK DNS key record has an empty "g" field in it ("g=;"),
your best course of action is to modify the record to remove the
empty field. In that way, the DK semantics will remain the same,
and the DKIM semantics will match.
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If your DNS key record does not have an empty "g" field in it
("g=;"), it's probable that the record can be left alone. But the
best course of action would still be to make sure that it has a
"v" field. When the decision is made to stop supporting
DomainKeys and to only support DKIM, it is important to verify
that the "g" field is compatible with DKIM, and typically having
"v=DKIM1;" in it. It is strongly encouraged that if use of an
empty "g" field in the DKIM selector, include the "v" field.
A.1.1.2. Removing DomainKeys Signatures
The principal use of DomainKeys is at boundary MTAs. Because no
operational transition is ever instantaneous, it is advisable to
continue performing DomainKeys signing until it is determined that
DomainKeys receive-side support is no longer used, or is sufficiently
reduced. That is, a signer needs to add a DKIM signature to a
message that also has a DomainKeys signature and keep it there until
they decide it is deemed no longer useful. The signer can do its
transitions in a straightforward manner, or more gradually. Note
that because digital signatures are not free, there is a cost to
performing both signing algorithms, so signing with both algorithms
ought not be needlessly prolonged.
The tricky part is deciding when DK signatures are no longer
necessary. The real questions are: how many DomainKeys verifiers are
there that do *not* also do DKIM verification, which of those are
important, and how can you track their usage? Most of the early
adopters of DK verification have added DKIM verification, but not all
yet. If a verifier finds a message with both DK and DKIM, it can
choose to verify both signatures, or just one or the other.
Many DNS services offer tracking statistics so it can be determined
how often a DNS record has been accessed. By using separate DNS
selector key records for your signatures, you can chart the use of
your records over time, and watch the trends. An additional
distinguishing factor to track would take into account the verifiers
that verify both the DK and DKIM signatures, and discount those from
counts of DK selector usage. When the number for DK selector access
reaches a low-enough level, that's the time to consider discontinuing
signing with DK.
Note, this level of rigor is not required. It is perfectly
reasonable for a DK signer to decide to follow the "flash cut"
scenario described above.
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A.1.2. Verifiers
As a verifier, several issues need to be considered:
A.1.2.1. Ought DK signature verification be performed?
At the time of writing, there is still a significant number of sites
that are only producing DK signatures. Over time, it is expected
that this number will go to zero, but it might take several years.
So it would be prudent for the foreseeable future for a verifier to
look for and verify both DKIM and DK signatures.
A.1.2.2. Ought both DK and DKIM signatures be evaluated on a single
message?
For a period of time, there will be sites that sign with both DK and
DKIM. A verifier receiving a message that has both types of
signatures can verify both signatures, or just one. One disadvantage
of verifying both signatures is that signers will have a more
difficult time deciding how many verifiers are still using their DK
selectors. One transition strategy is to verify the DKIM signature,
then only verify the DK signature if the DKIM verification fails.
A.1.2.3. DNS Selector Key Records
The format of the DNS key record was intentionally meant to be
backwardly compatible between DK and DKIM, but not necessarily
upwardly compatible. DKIM has enhanced the DK DNS key record format
by adding several optional parameters, which DK needs to ignore.
However, there is one key difference between DK and DKIM DNS key
records. The definitions of the g fields:
g= granularity of the key: In both DK and DKIM, this is an optional
field that is used to constrain which sending address(es) can
legitimately use this selector. Unfortunately, the treatment of
an empty field ("g=;") is different. For DK, an empty field is
the same as a missing value, and is treated as allowing any
sending address. For DKIM, an empty field only matches an empty
local part.
v= version of the selector It is advised that a DKIM selector have
"v=DKIM1;" at its beginning, but it is not required.
If a DKIM verifier finds a selector record that has an empty "g"
field ("g=;") and it does not have a "v" field ("v=DKIM1;") at its
beginning, it is faced with deciding if this record was:
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1. from a DK signer that transitioned to supporting DKIM but forgot
to remove the "g" field (so that it could be used by both DK and
DKIM verifiers); or
2. from a DKIM signer that truly meant to use the empty "g" field
but forgot to put in the "v" field. It is advised that you treat
such records using the first interpretation, and treat such
records as if the signer did not have a "g" field in the record.
A.2. Migrating Hash Algorithms
[RFC4871] defines the use of two hash algorithms: SHA-1 and SHA-256.
The security of all hash algorithms is constantly under attack, and
SHA-1 has already shown weaknesses as of this writing. Migrating
from SHA-1 to SHA-256 is not an issue, because all verifiers are
already required to support SHA-256. But when it becomes necessary
to replace SHA-256 with a more secure algorithm, there will be a
migratory period. In the following, "NEWHASH" is used to represent a
new hash algorithm. Section 4.1 of [RFC4871] briefly discusses this
scenario.
A.2.1. Signers
As with migrating from DK to DKIM, migrating hash algorithms is
dependent on the signer's best guess as to the utility of continuing
to sign with the older algorithms and the expected support for the
newer algorithm by verifiers. The utility of continuing to sign with
the older algorithms is also based on how broken the existing hash
algorithms are considered and how important that is to the signers.
One strategy is to wait until it's determined that there is a large
enough base of verifiers available that support NEWHASH, and then
flash cut to the new algorithm.
Another strategy is to sign with both the old and new hash algorithms
for a period of time. This is particularly useful for testing the
new code to support the new hash algorithm, as verifiers will
continue to accept the signature for the older hash algorithm and
ought to ignore any signature that fails because the code is slightly
wrong. Once the signer has determined that the new code is correct
AND it's determined that there is a large enough base of verifiers
available that support NEWHASH, the signer can flash cut to the new
algorithm.
One advantage migrating hash algorithms has is that the selector can
be completely compatible for all hash algorithms. The key selector
has an optional "h=" field that can be used to list the hash
algorithms being used; it also is used to limit the algorithms that a
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verifier will accept. If the signer is not currently using the key
selector "h=" field, no change is required. If the signer is
currently using the key selector "h=" field, NEWHASH will need to be
added to the list, as in "h=sha256:NEWHASH;". (When the signer is no
longer using SHA-256, it can be removed from the "h=" list.)
A.2.2. Verifiers
When a new hash algorithm becomes standardized, it is best for a
verifier to start supporting it as quickly as possible.
A.3. Migrating Signing Algorithms
[RFC4871] defines the use of the RSA signing algorithm. Similar to
hashes, signing algorithms are constantly under attack, and when it
becomes necessary to replace RSA with a newer signing algorithm,
there will be a migratory period. In the following, "NEWALG" is used
to represent a new signing algorithm.
A.3.1. Signers
As with the other migration issues discussed above, migrating signing
algorithms is dependent on the signer's best guess as to the utility
of continuing to sign with the older algorithms and the expected
support for the newer algorithm by verifiers. The utility of
continuing to sign with the older algorithms is also based on how
broken the existing signing algorithms are considered and how
important that is to the signers.
As before, the two basic strategies are to 1) wait until there is
sufficient base of verifiers available that support NEWALG and then
do a flash cut to NEWALG, and 2) use a phased approach by signing
with both the old and new algorithms before removing support for the
old algorithm.
It is unlikely that a new algorithm would be able to use the same
public key as "rsa", so using the same selector DNS record for both
algorithms' keys is ruled out. Therefore, in order to use the new
algorithm, a new DNS selector record would need to be deployed in
parallel with the existing DNS selector record for the existing
algorithm. The new DNS selector record would specify a different
"k=" value to reflect the use of NEWALG.
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A.3.2. Verifiers
When a new hash algorithm becomes standardized, it is best for a
verifier to start supporting it as quickly as possible.
Appendix B. General Coding Criteria for Cryptographic Applications
NOTE: This section could possibly be changed into a reference to
something else, such as another RFC.
Correct implementation of a cryptographic algorithm is a necessary
but not a sufficient condition for the 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 need
to 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 ought to be used
when available. In particular, great care needs to be taken when
releasing memory pages to the operating system to ensure that private
key information is not disclosed to other processes.
Certain implementations of public key algorithms such as RSA can 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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Authors' Addresses
Tony Hansen
AT&T Laboratories
200 Laurel Ave. South
Middletown, NJ 07748
USA
EMail: tony+dkimov@maillennium.att.com
Ellen Siegel
Consultant
EMail: dkim@esiegel.net
Phillip Hallam-Baker
Default Deny Security, Inc.
EMail: phillip@hallambaker.com
Dave Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
USA
EMail: dcrocker@bbiw.net
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ERRATA
