DomainKeys Identified Mail T. Hansen
Internet-Draft AT&T Laboratories
Intended status: Informational E. Siegel
Expires: August 13, 2009 Constant Contact, Inc.
P. Hallam-Baker
VeriSign Inc.
D. Crocker
Brandenburg InternetWorking
February 9, 2009
DomainKeys Identified Mail (DKIM) Development, Deployment and Operations
draft-ietf-dkim-deployment-03
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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, using the domain name service as its key server
technology [RFC4871]. This permits verification of a responsible
organization, as well as the integrity of the message contents. DKIM
will also provide a mechanism that permits potential email signers to
publish information about their email signing practices; this will
permit email receivers to make additional assessments 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Using DKIM as Part of Trust Assessment . . . . . . . . . . . . 5
2.1. A Systems View of Email Trust Assessment . . . . . . . . . 5
2.2. Choosing a DKIM Tag for the Assessment Identifier . . . . 7
2.3. Choosing the Signing Domain Name . . . . . . . . . . . . . 9
2.4. Recipient-based Assessments . . . . . . . . . . . . . . . 11
2.5. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 12
3. DKIM Key Generation, Storage, and Management . . . . . . . . . 14
3.1. Private Key Management: Deployment and Ongoing
Operations . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2. Storing Public Keys: DNS Server Software Considerations . 16
3.3. Per User Signing Key Management Issues . . . . . . . . . . 17
3.4. Third Party Signer Key Management and Selector
Administration . . . . . . . . . . . . . . . . . . . . . . 17
3.5. Key Pair / Selector Lifecycle Management . . . . . . . . . 18
4. Signing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1. DNS Records . . . . . . . . . . . . . . . . . . . . . . . 19
4.2. Signing Module . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Signing Policies and Practices . . . . . . . . . . . . . . 20
5. Verifying . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Taxonomy of Signatures . . . . . . . . . . . . . . . . . . . . 21
6.1. Single Domain Signature . . . . . . . . . . . . . . . . . 21
6.2. Parent Domain Signature . . . . . . . . . . . . . . . . . 22
6.3. Third Party Signature . . . . . . . . . . . . . . . . . . 23
6.4. Using Trusted 3rd Party Senders . . . . . . . . . . . . . 24
6.5. Multiple Signatures . . . . . . . . . . . . . . . . . . . 25
7. Example Usage Scenarios . . . . . . . . . . . . . . . . . . . 27
7.1. Author's Organization - Simple . . . . . . . . . . . . . . 27
7.2. Author's Organization - Differentiated Types of Mail . . . 27
7.3. Author Signature . . . . . . . . . . . . . . . . . . . . . 27
7.4. Author Domain Signing Practices . . . . . . . . . . . . . 28
7.5. Delegated Signing . . . . . . . . . . . . . . . . . . . . 28
7.6. Independent Third Party Service Providers . . . . . . . . 28
7.7. Mail Streams Based on Behavioral Assessment . . . . . . . 29
7.8. Agent or Mediator Signatures . . . . . . . . . . . . . . . 29
8. Usage Considerations . . . . . . . . . . . . . . . . . . . . . 30
8.1. Non-standard Submission and Delivery Scenarios . . . . . . 30
8.2. Protection of Internal Mail . . . . . . . . . . . . . . . 31
8.3. Signature Granularity . . . . . . . . . . . . . . . . . . 31
8.4. Email Infrastructure Agents . . . . . . . . . . . . . . . 32
8.5. Mail User Agent . . . . . . . . . . . . . . . . . . . . . 34
9. Other Considerations . . . . . . . . . . . . . . . . . . . . . 35
9.1. Security Considerations . . . . . . . . . . . . . . . . . 35
9.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 35
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
11. Informative References . . . . . . . . . . . . . . . . . . . . 35
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Appendix A. Migrating from DomainKeys . . . . . . . . . . . . . . 37
A.1. Signers . . . . . . . . . . . . . . . . . . . . . . . . . 37
A.2. Verifiers . . . . . . . . . . . . . . . . . . . . . . . . 40
Appendix B. General Coding Criteria for Cryptographic
Applications . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42
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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; and those who are
responsible for the on-going operations of an email infrastructure
that has deployed DKIM.
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.
[[anchor2: MSK: maybe this is a good place to mention the possibility
of collecting verification history for selectors domains as a means
of observing over time behaviour of signers for the purpose of
asserting local reputation]]
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 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 the recipient site. In contrast, the risk
model entails independent 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 must be 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.)
A trust-based service is built upon a validated Responsible
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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, 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 From: field!
DKIM's only claim about message content is that the content cited in
the DKIM-Signature: field's h= tag have been delivered without
modification. That is, it asserts message content integrity, 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 and then
makes handling decisions based on a collection of assessments, of
which the DKIM mechanism is only a part. In this model, 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 data bases are deemed appropriate by the
assessing entity. In turn, output from the Identity Assessor is fed
into a Handling Filter 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
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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 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 . . | |
+------------------.-------.--------------------+ | |
| +------+------+ . . . . . +-------------+ | | | +-------------+
| | 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 receive
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
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-
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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 that the signer MUST
provide the assessor with a single, opaque value that the signer
wishes to have used for assessment. This value MUST be the basis for
DKIM-based assessment. The signer MAY provide the assessor with a
second, opaque value that MAY be used when reporting problems with
the end-to-end DKIM process and MAY be used for additional analysis,
such as by the higher-level Handling Filter. These values are
opaque, in that any internal semantics are known only to the signer
and MUST NOT be assumed by the Assessor, within the confines of
DKIM's formal signing specification. Assessment MUST use a value as
a single, complete and uninterpreted string.
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.
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 [I-D.ietf-dkim-overview]:
"If verifiers were to employ the selector as part of a name
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 "[i]dentity of the
user or agent (e.g., a mailing list manager) on behalf of which
this message is signed." The identity can be in the syntax 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 that 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
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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 sub-domain 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 an Web cookie,
for return to the signing 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 author of
content, versus operator of the mail service, versus operator of a
reputation service. 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 their mail will warrant different levels of trust. For example:
* Mail is sent for different purposes, such as marketing vs.
transactional, and recipients demonstrate different patterns of
acceptance between these.
* 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 vs. free users, or users engaged in
direct correspondence vs. users sending bulk mail.
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* 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=
sub-domain 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 sub-domains is whether
the extra granularity will be useful for the assessor. In fact,
potentially excessive levels invites ambiguity: if the assessor does
not take advantage of the added granularity, then what granularity
will it use? 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.
For those originating message content, the most likely choice of sub-
domain 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
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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 sub-domain 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
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
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,
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 which is not
necessarily related to any other identifier in the message. Hence,
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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 different 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 From: field validity
remain a concern. Hence a signer using a domain name that is
unrelated to the domain name in the 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.
2.5. Filtering
After assessing the signer of a message, each receiving site creates
and tunes its own Handling Filter according to criteria specific for
that site. Still, there are commonalities across sites, and this
section offers a discussion, rather than a specification, of some
types of input to that process and how they can be used.
The discussion focuses on variations in Organizational Trust versus
Message 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 Table 1. Only 3 levels of granularity are listed, in
order to keep discussion manageable. This also ensures extensive
flexibility for each site's detailed choices.
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+---+---------------------+--------------------+--------------------+
| | Low | Medium | High |
| | | | |
| | | | |
| | | | |
| | | | |
| O | | | |
| R | | | |
| G | | | |
| | | | |
| T | | | |
| R | | | |
| U | | | |
| S | | | |
| T | | | |
| | | | |
| M | | | |
+---+---------------------+--------------------+--------------------+
| * | Unknown org, | Registered org, | Good Org, |
| L | Few msgs: | New Identifier: | Good msgs: |
| o | _Mild filtering_ | _Medium filtering_ | _Avoid FP(!)_ |
| w | | | |
| * | Unknown org, | Registered org, | Good org, Bad msg |
| M | New Identifier: | Mixed msgs: | burst: |
| e | _Default filtering_ | _Medium filtering_ | _Accept & Contact_ |
| d | | | |
| i | | | |
| u | | | |
| * | Black-Listed org, | Registered org, | Good org, |
| H | Bad msgs: | Bad msgs: | Compromised: |
| i | _Avoid FN(!)_ | _Strong filtering_ | _Fully blocked_ |
| g | | | |
| h | | | |
+---+---------------------+--------------------+--------------------+
Table 1: Organizational Trust vs. Message Risk
The table indicates preferences for different handling of different
combinations, such as tuning filtering to avoid False Positives (FP)
or avoiding False Negatives (FN). Perhaps unexpectedly, it also
lists a case in which the receiving site might wish to deliver
problematic mail, rather than redirecting it, but also of course
contacting the signing organization, seeking resolution of the
problem.
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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, these
functions are both performed through the DNS [RFC4871] (Allman, E.,
Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas,
"DomainKeys Identified Mail (DKIM) Signatures," May 2007.).
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 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 may 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 may 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 a particular subset
of messages.
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
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
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of the DNS zone and corresponding private key are adequate.
3.1. Private Key Management: Deployment and Ongoing Operations
Access to signing keys must be carefully managed to prevent use by
unauthorized parties and to minimize the consequences should a
compromise occur.
While a DKIM signing key is used to sign messages on behalf of many
mail users, the signing key itself should be under direct control of
as few key-holders as possible. Should a key-holder leave the
organization, all signing keys held by that key holder should be
withdrawn from service and if appropriate, replaced.
If key management hardware support is available, it should be used.
If keys are stored in software, sppropriate file control protections
must be employed and any location in which the private key is stored
in plaintext form should be excluded from regular backup processes
and should not be accessible through any form of network including
private local area networks. Auditing software should be used
periodically to verify that the permissions on the private key files
remain secure.
Wherever possible a signature key should exist in exactly one
location and be erased when no longer used. Ideally a signature key
pair should 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 MUST 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 should not be used. In the
unlikely event of a signing key becomming lost, a new signature key
pair may be generated as easily as recovery from a key escrow scheme.
Responsibility for the security of a signing key should ultimately
vest in a single named individual. Where multiple parties are
authorized to sign messages, each signer should 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,
particular 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
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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 usually 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 signing keys are
terminated and replaced.
At a minimum, a DNS server that handles queries for DKIM key records
must allow the server administrators to add free-form TXT records.
It would be better if the the DKIM records could be entered using a
structured form, supporting the DKIM-specific fields.
Ideally DNSSEC [] should be employed in a configuration that provides
protection against record insertion attacks and zone enumeration. In
the case that NSEC3 [RFC 5155] records are employed to prevent
insertion attack, the OPT-OUT flag must be set clear.
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
delegating of the right to authenticate a portion of the namespace to
a trusted third party. Examples include:
jun2005.eng._domainkey.example.com
widget.promotion._domainkey.example.com
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 may 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 softeware may
cause such heuristics to fail at any time.
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3.3. Per User Signing Key Management Issues
While a signer may establish business rules, such as 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 may 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 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 DKIM-base.
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 may be less likely to be able to
manage private key data securely on their personal computer than
an administrator running an edge MTA.
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 the ability to make a claim of
responsibility to another party. In this case the trust relationship
is 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 should be kept entirely
separate from those used by the domain holder and other third party
signers. As with any other private key, the signature key pair
should 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 should 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 their security risk and the Trusted Third Party Signer
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avoids unnecessary liability.
In the most restrictive case a domain holder maintains full control
over the creation of key records and employ appropriate key record
restrictions to enforce restrictions on the messages for which the
third party signer is able to sign. If such restrictions are
impractical, the domain holder should delegate a DNS subzone for
publishing key records to the third party signer. The domain holder
should not allow a third party signer unrestricted access to their
DNS service for the purpose of publishing key records.
3.5. Key Pair / Selector Lifecycle Management
Deployments should establish, document and observe processes for
managing the entire lifecycle of a public 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 infrasrtructure.
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 published in the DNS.
3. Wait for DNS updates to propagate (if necessary).
4. Report assigned key selector to signing device.
4. Signer verifies correct registration of the key record.
5. Signer begins generating signatures using the new key pair.
6. 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:
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1. Signer stops using the private key for signature operations.
2. Signer deletes all records of the private key, including in-
memory copies at the signing device.
3. Signer notifies the DNS administration infrasrtructure that the
signing key is withdrawn from service and that the corresponding
key records may 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 key selector is deleted
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 is 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
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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
outbound boundary MTAs to the open Internet. However any other
module, including the author's MUA, 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 use of the i= tag.
The signing module uses the appropriate private key to create one or
more 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, key information should not be hard-coded 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
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recipients. Too much granularity is higher administrative overhead
and well might 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.
5. Verifying
To be added.
6. Taxonomy of Signatures
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 d= tag 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 RFC
5322 From 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.
Author signatures are signatures from an authors organization that
have an i= value that matches the From: address of the message.
Under the ADSP specification, an i= value matches a RFC 5322 From
address when the domains of the two match exactly, and if the i=
value contains a local part it also matches the local part of the
From: address exactly.
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Although an author signature might in some cases be proof against
domain name spoofing the RFC 5322 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 nor to the human-
friendly "display-name" or names and addresses used within the body
of the message. That is, it protects only 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
signature where i=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 i=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 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 with 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 may 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 may work well.
Another approach to distinguishing the streams using a single DKIM
key would be to leverage the 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 i= identifer 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 (e.g., the semantics specified in
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ADSP).
6.3. Third Party Signature
A signature whose domain does not match the domain of the RFC 5322
From address is sometimes referred to as a third party signature. In
certain cases even the parent domain signature described above would
be considered a third party signature because it would not be an
exact match for the domain in the 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. 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 (identity in the RFC
5322 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 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 may choose to DKIM
sign outbound mail with either its own identifier -- relying on
its own, aggregate reptutation -- or with a subdomain of the
provider that is unique to the message author but still part of
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the provider's aggregate reputation. Such service providers may
also encompass delegated business functions such as benefit
management, although these will more often be treated as trusted
third party senders (see below).
Parent Domain. As discussed above, organizations choosing to sign
for mail originating from subdomains with a parent domain
signature may also considered to be using 3rd party signatures in
some configurations, depending on whether or not the "t=s" tag is
used to constrain the parent signature to apply to only its own
specific domain. The default is that a parent domain signature is
considered valid for its subdomains.
Reputation Provider: Another possible category of third party
signature would be the identity of a 3rd party reputation
provider. Such a signature would indicate to receivers that the
message was being vouched for by that 3rd party.
6.4. Using Trusted 3rd Party Senders
For most of the cases described so far, there has been an assumption
that the identity doing the signing was responsible for creating and
maintaining their own DKIM signing infrastructure, including their
own keys, and signing with their 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 3rd party's
mail servers, but the signature applied would be that of the
controlling organization.
This can be done by having the 3rd party generate a key pair that is
designated uniquely for use by that trusted 3rd 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 3rd party, they can use a special keypair that
enables the benefits company to sign mail as "companyA.example".
Because the keypair is unique to that trusted 3rd party, it is easy
for Company A to revoke the authorization if necessary by simply
removing the public key from the companyA.example DNS.
In this scenario, it is usually a good idea to limit the specific
identities that can be used by even trusted third parties. The DKIM
g= tag enables a key record to specify one particular From: address
local part that must be specified in the i= tag of the signature: for
example, "g=benefits" would require a signature header tag of
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"i=benefits@companyA.example". It is important to note that although
this distinction will be clear to the verifier it may be invisible to
the recipient: there is no constraint within the DKIM verification
process that constrains that specific i= value to correspond to any
of the other message headers.
A more reliable way of distinguishing the third part mail stream
would be to create a dedicated subdomain (e.g.
benefits.companyA.example) and publish the public key there; the
signature would then use d=benefits.companyA.example.
6.4.1. DNS Delegation
Another possbility for configuring trusted third party access 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. Should revocation become 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 a single source, 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
they represent 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 may make sense to
have more than one identity claiming responsiblity for the same
message.
One important caveat to the use of multiple signatures is that there
is currently no clear consensus amoung 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
identfier that represents the most positive trust assessment. It is
likely that the industry will evolve to accept multiple signatures
using either option two or three, but it may take some time before
that approach becomes pervasive.
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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 subdomain 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 may sign mail for its sales
department with a signature where d=marketing.companya.example,
and a second signature where d=companya.example
Service Providers: Service providers may, as described above, choose
to sign outbound messages with either their own identity or with
an identity unique to each of their clients (possibly delegated).
However, they may also do both: sign each outbound message with
their own identity as well as the identity of each individual
client. For example, ESP A might sign mail for their client
Company B with their 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 signature could, for example, help cover a new
client while they establish their 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, although some forwarders do
make modifications that can invalidate a DKIM signature.
However, some forwarders such as mailing lists or forward article
to a friend services, might choose to add their own signature to
outbound messages to vouch for it having legitimately originated
from the designated service. In this case, the signature would be
added even in the presence of a pre-existing signature, and both
signatures would be relevant to the verifier.
Any forwarder that modifies messages in ways that will break pre-
existing DKIM signatures should always 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,
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this "reputation" signature would be in addition to the author or
originator signature.
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 different, and so on. 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 all 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 a user
identity or some other related information, the i= 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 i= 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
all of its mail, but it wants to differentiate different categories
of its outbound mail by using different identifiers. For example, it
might choose to distinguish marketing mail, billing or transactional
mail, and individual corporate email into marketing.companya.example,
billing.companya.example, and companya.example, where each category
is assigned a unique subdomain and unique signing keys.
7.3. Author Signature
As discussed in Section 6.1, author signatures are a special case of
signatures from an authors organization where at least one signature
on the message has an i= value that matches the From: address of the
message.
Signers wishing to publish an ADSP record describing their signing
practices will want to include an author signature on their outbound
mail to avoid ADSP verification failures. For example, if the
address in the RFC 5322 From is bob@company.example, the d= value of
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the author signature would be company.example, and the i= value would
be either company.example or bob@company.example.
7.4. Author Domain Signing Practices
To be added.
7.5. Delegated Signing
An organization may choose to outsource certain key services to third
party companies. 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 they 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 From: address is not
constrained: it could either be affiliated with the benefits company
(e.g. benefits-admin@benefitprovider.example, or
benefits-provider@benefits.companya.example).
Note that in both of the above scenarios, 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.
7.6. 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 may sign all outgoing mail from its
clients with its own d= domain (e.g. d=espa.example).
Should the ESP want to distinguish among its clients, it has two
options:
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Share the d= domain and use the i= 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)
Extend the d= 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 may be desirable to sign the same
message with both the ESP and the ESP client identities.
7.7. Mail Streams Based on Behavioral Assessment
An ISP (ISP A) might want to assign signatures to outbound mail from
their users according to the users past sending behavior
(reputation). Since the semantics of behavioral assessments arent
allowed as i= values, ISP A (ispa.example) would have to configure
subdomains corresponding the assessment categories (e.g.
good.ispa.example, neutral.ispa.example, bad.ispa.example), and use
these domains as the d= value of the signature.
The signing module can also optionally set the i= value to have a
unique user id (distinct from the users email address local part),
for example user3456@neutral.domain.example. Using a userid 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 i= 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 a given
user can migrate among the assessment subdomains depending on their
sending behavior (i.e., the same user might have multiple i= values
over the lifetime of their account).
In this scenario, ISP A would have to generate as many keys as there
are assessment subdomains (d= values), so that each assessment
subdomain had 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 i= tag of the
signature.
7.8. 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,
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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.
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
travelling 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 a GMail account joe@gmail.com,
and he uses GMails multiple address feature to attach his work email
joe@companya.example to his GMail account. When Joe sends email from
his GMail account and uses joe@companya.example as his designated
From: address, that email cannot have a signature with
d=companya.example because the GMail servers have no access to
Company A's private key. In GMail's case it will have a GMail
signature, but for some other mail clients offering the same multiple
address feature there may 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 RFC 5322 From to
their designated recipient. If Joe used either of his two addresses
(joe@companya.example or joe@gmail.com), the forwarder would be
equally unable to sign with a corresponding domain . As in the mail
client case, the forwarder may either sign as its own domain, or may
put no signature on the message.
A third example is the use of privately configured forwarding.
Assume that Joe has another account at Yahoo, joe@yahoo.com, but he'd
prefer to read his Yahoo mail from his GMail account. He sets up his
Yahoo account to forward all incoming mail to joe@gmail.com. Assume
alice@companyb.example sends joe@yahoo.com an email. Depending on
how companyb.example configured its signature, and depending on
whether or not Yahoo modifies messages that it forwards, it is
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possible that when Alice's message is received in Joe's gmail account
the original signature fails 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 travelling users, and other
scenarios described in the previous section). Such breakage is
particularly relevant in the presence of Author Domain Signing
Practices.
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 sub-domains
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 should be treated as an
opaque identifier and thus need not correspond directly to a DNS sub
domain or to a specific user address
The primary way to sign with per-user keys require that each user
have a distinct DNS (sub)domain, where each distinct d= value has a
key published (it is possible, although not recommended, 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 recommended and is unlikely to be treated uniquely by Identity
Assessors: the primary purpose of selectors is to facilitate key
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management, and the DKIM specification recommends against using them
in determining or assessing identies.
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 may 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 managment complexity. It is also important to note
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.
What may 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 seperate sub-domains 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 it 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
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general recommendations for the Email Infrastructure.
Outbound: An MSA or an Outbound MTA used for mail submission
SHOULD ensure that the message sent is in compliance with the
advertised email sending policy. It SHOULD 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 SHOULD be aware that some MUAs may 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 SHOULD do so in a way that would not break
those signatures.
[[anchor38: MSK: MUAs being able to sign is a security
consideration; MUAs are more prone to vulnerabilities, so an
MUA having direct access to signing keys is a security concern;
general MUA vulnerability came up during the IETF Security
Directorate review of draft-kucherawy-sender-auth-header]]
Inbound: When an organization deploys DKIM, it needs to make
sure that it 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 may incorporate an indication of the
verification results into the message, such as using an
Authentication-Results header field.
[I-D.kucherawy-sender-auth-header]
Intermediaries: An email intermediary is both an inbound and
outbound MTA. Each of the requirements outlined in the
sections relating to MTAs apply. If the intermediary modifies
a message in a way that breaks the signature, the intermediary
+ SHOULD deploy abuse filtering measures on the inbound mail,
and
+ MAY remove all signatures that will be broken
In addition the intermediary MAY:
+ Verify the message signature prior to modification.
+ Incorporate an indication of the verification results into
the message, such as using an Authentication-Results header
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field. [I-D.kucherawy-sender-auth-header]
+ 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 MAY also support DKIM
signing and verifying directly.
Outbound: An MUA MAY support signing even if mail is to be
relayed through an outbound MSA. In this case the signature
applied by the MUA will be in addition to any signature added
by the MSA.
Some user software goes beyond simple user functionality and
also perform MSA and MTA functions. When this is employed for
sending directly to a receiving ADMD, the user software SHOULD
be considered an outbound MTA.
Inbound: An MUA MAY 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 MAY perform DKIM signature verification directly. A
verifying MUA SHOULD allow for the case where mail has modified
in the inbound MTA path; if a signature fails, the message
SHOULD NOT be treated any different than if it did not have a
signature.
An MUA that looks for an Authentication-Results header field
MUST be configurable to choose which Authentication-Results are
considered trustable.
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 MUST NOT by itself be used 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
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signature was verified MAY 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. Other Considerations
9.1. Security Considerations
The security considerations of the DKIM protocol are described in the
DKIM base specification [RFC4871].
9.2. IANA Considerations
This document has no considerations for IANA.
10. Acknowledgements
TBD
11. Informative References
[I-D.ietf-dkim-overview]
Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys
Identified Mail (DKIM) Service Overview",
draft-ietf-dkim-overview-10 (work in progress), July 2008.
[I-D.ietf-dkim-ssp]
Local-part, a., Domain, A., error, r., Allman, E., Fenton,
J., Delany, M., and J. Levine, "DomainKeys Identified Mail
(DKIM) Author Domain Signing Practices (ADSP)",
draft-ietf-dkim-ssp-09 (work in progress), February 2009.
[I-D.ietf-openpgp-rfc2440bis]
Callas, J., "OpenPGP Message Format",
draft-ietf-openpgp-rfc2440bis-22 (work in progress),
April 2007.
[]
Kucherawy, M., "Message Header Field for Indicating
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Message Authentication Status",
draft-kucherawy-sender-auth-header-20 (work in progress),
January 2009.
[RFC0989] Linn, J. and IAB Privacy Task Force, "Privacy enhancement
for Internet electronic mail: Part I: Message encipherment
and authentication procedures", RFC 989, February 1987.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1848] Crocker, S., Galvin, J., Murphy, S., and N. Freed, "MIME
Object Security Services", RFC 1848, October 1995.
[RFC1991] Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
Exchange Formats", RFC 1991, August 1996.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001.
[RFC3164] Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
August 2001.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC4870] Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
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[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
Appendix A. Migrating from DomainKeys
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.
A.1. Signers
A signer that currently signs with DomainKeys (DK) will go through
various stages as they migrate to using DKIM, not all of which are
required for all signers. The real questions that a signer must ask
are:
1. how many receivers or what types of receivers are *only* looking
at the DK signatures and not the DKIM signatures,
2. and 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 there are no more "large
players" looking only at the DK signatures, a signer may choose to
stop signing with DK.
With respect to signing policies, a reasonable, initial approach is
to use DKIM signatures in the same way as DomainKeys signatures are
already being used. In particular, the same selectors and DNS Key
Records may 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
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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
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.
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A.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 must 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.
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.
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 your best
course of action would still be to make sure it has a v= field. When
the decision is made to stop supporting DomainKeys and to only
support DKIM, you MUST verify that the "g" field is compatible with
DKIM, and it SHOULD have "v=DKIM1;" in it. It is highly RECOMMENDED
that if you want to use an empty g= field in your DKIM selector, you
also include the v= field.
A.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 SHOULD add a DKIM signature to a message
that also has a DomainKeys signature and keep it there until you
decide it can go away. The signer may 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 you don't want to be signing with both algorithms for
too long a period.
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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 ones of them do
you care about, 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 may
choose to verify both signatures, or just one or the other.
Many DNS services offer tracking statistics so you can find out how
often a DNS record has been accessed. By using separate DNS selector
key records for your signatures, you can chart the usage 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
your counts of DK selector usage. When the number for DK selector
access reaches a low-enough level, that's the time to consider
stopping your DK signing.
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.
A.2. Verifiers
As a verifier, you are faced with several issues:
A.2.1. Do you verify DK signatures?
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 may 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.2.2. Do you verify both DK and DKIM signatures within 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 may 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.
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A.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 must 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 recommended 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
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 RECOMMENDED 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.
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
should pay close attention to management of cryptographic private
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keys and session keys, ensuring that these are correctly initialized
and disposed of.
Operating system mechanisms that permit the confidentiality of
private keys to be protected against other processes should be used
when available. In particular, great care must be taken when
releasing memory pages to the operating system to ensure that private
key information is not disclosed to other processes.
Certain implementations of public key algorithms such as RSA may be
vulnerable to a timing analysis attack.
Support for cryptographic hardware providing key management
capabilities is strongly encouraged. In addition to offering
performance benefits, many cryptographic hardware devices provide
robust and verifiable management of private keys.
Fortunately appropriately designed and coded cryptographic libraries
are available for most operating system platforms under license terms
compatible with commercial, open source and free software license
terms. Use of standard cryptographic libraries is strongly
encouraged. These have been extensively tested, reduce development
time and support a wide range of cryptographic hardware.
Authors' Addresses
Tony Hansen
AT&T Laboratories
200 Laurel Ave. South
Middletown, NJ 07748
USA
Email: tony+dkimov@maillennium.att.com
Ellen Siegel
Constant Contact, Inc.
1601 Trapelo Rd, Ste 329
Waltham, MA 02451
USA
Email: esiegel@constantcontact.com
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Phillip Hallam-Baker
VeriSign Inc.
Email: pbaker@verisign.com
Dave Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
USA
Email: dcrocker@bbiw.net
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