Domain-Based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)
draft-delany-domainkeys-base-06
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4870.
|
|
|---|---|---|---|
| Author | Mark Delany | ||
| Last updated | 2018-12-20 (Latest revision 2006-07-26) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Historic | ||
| Formats | |||
| Reviews | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4870 (Historic) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Russ Housley | ||
| Send notices to | MarkD@yahoo-inc.com |
draft-delany-domainkeys-base-06
INTERNET DRAFT Mark Delany
Title: draft-delany-domainkeys-base-06.txt Yahoo! Inc
Expires: 24 January 2007 25 July 2006
Domain-based Email Authentication Using Public Keys
Advertised in the DNS (DomainKeys)
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware have
been or will be disclosed, and any of which he or she becomes aware
will be disclosed, in accordance with Section 6 of BCP 79.
This document may not be modified, and derivative works of it may not
be created, except to publish it as an RFC and to translate it into
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Abstract
"DomainKeys" creates a domain-level authentication framework for email
by using public key technology and the DNS to prove the provenance and
contents of an email.
This document defines a framework for digitally signing email on a
per-domain basis. The ultimate goal of this framework is to
unequivocally prove and protect identity while retaining the semantics
of Internet email as it is known today.
Proof and protection of email identity may assist in the global
control of "spam" and "phishing".
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Purpose of Submission
The DomainKeys specification was a primary source from which the
DomainKeys Identified Mail [DKIM] specification has been derived. The
purpose in submitting this document is as an historical reference for
deployed implementations written prior to the DKIM specification.
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Table of Contents
1. Introduction
1.1 Lack of authentication is damaging Internet email
1.2 Digitally signed email creates credible domain authentication
1.3 Public keys in the DNS
1.4 Initial deployment is likely at the border MTA
1.5 Conveying verification results to MUAs
1.6 Technical minutiae are not completely covered
1.7 Motivation
1.8 Benefits of DomainKeys
1.9 Definitions
1.10 Requirements Notation
2. DomainKeys overview
3. DomainKeys detailed view
3.1 Determining the sending address of an email
3.2 Retrieving the public key given the sending domain
3.2.1 Introducing "selectors"
3.2.2 Public key signing and verification algorithm
3.2.3 Public key representation in the DNS
3.2.4 Key sizes
3.3 Storing the signature in the email header
3.4 Preparation of email for transit and signing
3.4.1 Preparation for transit
3.4.2 Canonicalization for signing
3.4.2.1 The "simple" canonicalization algorithm
3.4.2.2 The "nofws" canonicalization algorithm
3.5 The signing process
3.5.1 Identifying the sending domain
3.5.2 Determining if an email should be signed
3.5.3 Selecting a private key and corresponding selector information
3.5.4 Calculating the signature value
3.5.5 Prepending the "DomainKey-Signature:" header
3.6 Policy statement of sending domain
3.7 The verification process
3.7.1 Presumption that headers are not reordered
3.7.2 Verification should render a binary result
3.7.3 Selecting the most appropriate "DomainKey-Signature:" header
3.7.4 Retrieve the public key based on the signature information
3.7.5 Verify the signature
3.7.6 Retrieving sending domain policy
3.7.7 Applying local policy
3.8 Conveying verification results to MUAs
4. Example of use
4.1 The user composes an email
4.2 The email is signed
4.3 The email signature is verified
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5. Association with a Certificate Authority
5.1 The "DomainKey-X509:" header
6. Topics for discussion
6.1 The benefits of selectors
6.2 Canonicalization of email
6.3 Mailing lists
6.4 Roving users
7. Security Considerations
7.1 DNS
7.1.1 The DNS is not currently secure
7.1.2 DomainKeys creates additional DNS load
7.2 Key Management
7.3 Implementation risks
7.4 Privacy assumptions with forwarding addresses
7.5 Cryptographic processing is computationally intensive
8. The trial
8.1 Goals
8.2 Results of trial
9. Note to implementors regarding TXT records
10. References
10.1 Normative References
10.2 Informative References
Appendix A - Syntax rules for the tag=value format
Acknowledgments
Author's Address
Intellectual Property and Copyright Statements
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1. Introduction
This document proposes an authentication framework for email that
stores public keys in the DNS and digitally signs email on a domain
basis. Separate documents discuss how this framework can be extended
to validate the delivery path of email as well as facilitate per-user
authentication.
1.1 Lack of authentication is damaging Internet email
Authentication of email is not currently widespread. Not only is it
difficult to prove your own identity, it is impossible to prevent
others from abusing your identity.
While most email exchanges do not intrinsically need authentication
beyond context, it is the rampant abuse of identity by "spammers",
"phishers", and their criminal ilk that makes proof necessary. In
other words, authentication is as much about protection as proof.
Importantly, the inability to authenticate email effectively delegates
much of the control of the disposition of inbound email to the sender,
since senders can trivially assume any email address. Creating email
authentication is the first step to returning dispositional control of
email to the recipient.
For the purposes of this document, authentication is seen from a user
perspective, and is intended to answer the question "who sent this
email?" where "who" is the email address the recipient sees and "this
email" is the content that the recipient sees.
1.2 Digitally signing email creates credible domain authentication
DomainKeys combines public key cryptography and the DNS to provide
credible domain-level authentication for email.
When an email claims to originate from a certain domain, DomainKeys
provides a mechanism by which the recipient system can credibly
determine that the email did in fact originate from a person or system
authorized to send email for that domain.
The authentication provided by DomainKeys works in a number of
scenarios in which other authentication systems fail or create complex
operational requirements. These include:
o forwarded email
o distributed sending systems
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o authorized third-party sending
This base definition of DomainKeys is intended to primarily enable
domain-level authenticity; whether a given message is really sent by
the purported user within the domain is outside the scope of the base
definition. Having said that, this specification includes the
possibility that some domains may wish to delegate fine-grained
authentication to individual users.
1.3 Public keys in the DNS
DomainKeys differs from traditional hierarchical public key systems in
that it leverages the DNS for public key management, placing complete
and direct control of key generation and management with the owner of
the domain. That is, if you have control over the DNS for a given
domain, you have control over your DomainKeys for that domain.
The DNS is proposed as the initial mechanism for publishing
public keys. DomainKeys is specifically designed to be extensible to
other key fetching services as they become available.
1.4 Initial deployment is likely at the border MTA
For practical reasons, it is expected that initial implementations of
DomainKeys will be deployed on MTAs that accept or relay email across
administrative or organizational boundaries. There are numerous
advantages to deployment at the border MTA, including:
o a reduction in the number of MTAs that have to be changed to
support an implementation of DomainKeys
o a reduction in the number of MTAs involved in transmitting the
email between a signing system and a verifying system, thus
reducing the number of places that can make accidental changes
to the contents
o removing the need to implement DomainKeys within an internal
email network.
However there is no necessity to deploy DomainKeys at the border as
signing and verifying can effectively occur anywhere from the border
MTA right back to the MUA. In particular the best place to sign an
email for many domains is likely to be at the point of SUBMISSION
where the sender is often authenticated through SMTP AUTH or other
identifying mechanisms.
1.5 Conveying verification results to MUAs
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It follows that testing the authenticity of an email results in some
action based on the results of the test. Oftentimes the action is to
notify the MUA in some way - typically via a header line.
The "Domainkey-Status:" header is defined in this specification for
recording authentication results in the email.
1.6 Technical minutiae are not completely covered
The intent of this specification is to communicate the fundamental
characteristics of DomainKeys for an implementor. However some aspects
are derived from the functionality of the openssl command [OPENSSL]
and, rather than duplicate that documentation, implementors are
expected to understand the mechanics of the openssl command,
sufficient to complete the implementation.
1.7 Motivation
The motivation for DomainKeys is to define a simple, cheap, and
"sufficiently effective" mechanism by which domain owners can control
who has authority to send email using their domain. To this end, the
designers of DomainKeys set out to build a framework which:
o is transparent and compatible with the existing email
infrastructure
o requires no new infrastructure
o can be implemented independently of clients in order to
reduce deployment time
o does not require the use of a central certificate authority
which might impose fees for certificates or introduce delays to
deployment
o can be deployed incrementally
While we believe that DomainKeys meets these criteria, it is by no
means a perfect solution. The current Internet imposes considerable
compromises on any similar scheme, and readers should be careful not
to misinterpret the information provided in this document to imply
that DomainKeys makes stronger credibility statements than it is able
to do.
1.8 Benefits of DomainKeys
As the reader will discover, DomainKeys is solely an authentication
system. It is not a magic bullet for spam, nor is it an authorization
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system, a reputation system, a certification system, or a trust
system.
However, a strong authentication system such as DomainKeys creates an
unimpeachable framework within which comprehensive authorization
systems, reputations systems and their ilk can be developed.
1.9 Definitions
With reference to the following sample email:
Line Data
Number Bytes Content
---- --- --------------------------------------------
01 46 From: "Joe SixPack" <joe@football.example.com>
02 40 To: "Suzie Q" <suzie@shopping.example.net>
03 25 Subject: Is dinner ready?
04 43 Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
05 40 Comment: This comment has a continuation
06 51 because this line begins with folding white space
07 60 Message-ID: <20030712040037.46341@football.example.com>
08 00
09 03 Hi.
10 00
11 37 We lost the game. Are you hungry yet?
12 00
13 04 Joe.
14 00
15 00
Line 01 is the first line of the email and the first line of the
headers.
Line 05 and 06 constitute the "Comment:" header.
Line 06 is a continuation header line.
Line 07 is the last line of the headers.
Line 08 is the empty line that separates the header from the body.
Line 09 is the first line of the body.
Line 10, 12, 14 and 15 are empty lines.
Line 13 is the last non-empty line of the email.
Line 15 is the last line of the body and the last line of the email.
Line 01 to 15 constitute the complete email.
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Line 01 is earlier than line 02 and line 02 is later than line 01.
1.10 Requirements notation
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
NOT", and "MAY" appear capitalized, they are being used to indicate
particular requirements of this specification. A discussion of the
meanings of these terms appears in [RFC2119].
2. DomainKeys overview
Under DomainKeys, a domain owner generates one or more private/public
key-pairs that will be used to sign messages originating from that
domain. The domain owner places the public key in his domain
namespace (i.e., in a DNS record associated with that domain), and
makes the private key available to the outbound email system. When an
email is submitted by an authorized user of that domain, the email
system uses the private key to digitally sign the email associated
with the sending domain. The signature is added as a header to the
email, and the message is transferred to its recipients in the usual
way.
When a message is received with a DomainKey signature header, the
receiving system can verify the signature as follows:
1. Extract the signature and claimed sending domain from the
email.
2. Fetch the public key from the claimed sending domain namespace.
3. Use public key to determine whether the signature of the email
has been generated with the corresponding private key, and thus
whether the email was sent with the authority of the claimed
sending domain.
In the event that an email arrives without a signature or when the
signature verification fails, the receiving system retrieves the
policy of the claimed sending domain to ascertain the preferred
disposition of such email.
Armed with this information, the recipient system can apply local
policy based on the results of the signature test.
3. DomainKeys detailed view
This section discusses the specifics of DomainKeys that are needed to
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create interoperable implementations. This section answers the
following questions:
Given an email, how is the sending domain determined?
How is the public key retrieved for a sending domain?
As email transits the email system, it can potentially go through a
number of changes. Which parts of the email are included in the
signature and how are they protected from such transformations?
How is the signature represented in the email?
If a signature is not present, or a verification fails, how does the
recipient determine the policy intent of the sending domain?
Finally, on verifying the authenticity of an email, how is that
result conveyed to participating MUAs?
While there are many alternative design choices, most lead to
comparable functionality. The overriding selection criteria used to
choose amongst the alternatives are:
o use deployed technology whenever possible
o prefer ease of implementation
o avoid trading risk for excessive flexibility or interoperability
o include basic flexibility
Adherence to these criteria implies that some existing email
implementations will require changes to participate in DomainKeys.
Ultimately some hard choices need to be made regarding which
requirements are more important.
3.1 Determining the sending address of an email
The goal of DomainKeys is to give the recipient confidence that the
email originated from the claimed sender. As with much of Internet
email, agreement over what constitutes the "sender" is no easy
matter. Forwarding systems and mailing lists add serious complications
to an overtly simple question. From the point of view of the
recipient, the authenticity claim should be directed at the domain
most visible to the recipient.
In the first instance, the most visible address is clearly the RFC2822
"From:" address [RFC2822]. Therefore, a conforming email MUST contain
a single "From:" header from which an email address with a domain name
can be extracted.
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A conforming email MAY contain a single RFC2822 "Sender:" header from
which an email address with a domain name can be extracted.
If the email has a valid "From:" and a valid "Sender:" header, then
the signer MUST use the sending address in the "Sender:" header.
If the email has a valid "From:" and no "Sender:" header, then the
signer MUST use the first sending address in the "From:" header.
In all other cases, a signer MUST NOT sign the email. Implementors
should note the an email with a "Sender:" header and no "From:" header
MUST NOT be signed.
The domain name in the sending address constitutes the "sending
domain".
3.2 Retrieving the public key given the sending domain
To avoid namespace conflicts, it is proposed that the DNS namespace
"_domainkey." be reserved within the sending domain for storing
public keys, e.g., if the sending domain is example.net, then the
public keys for that domain are stored in the _domainkey.example.net
namespace.
3.2.1 Introducing "selectors"
To support multiple concurrent public keys per sending domain, the DNS
namespace is further subdivided with "selectors". Selectors are
arbitrary names below the "_domainkey." namespace. A selector value
and length MUST be legal in the DNS namespace and in email headers
with the additional provision that they cannot contain a semicolon.
Examples of namespace using selectors are:
"coolumbeach._domainkey.example.net"
"sebastopol._domainkey.example.net"
"reykjavik._domainkey.example.net"
"default._domainkey.example.net"
and
"2005.pao._domainkey.example.net"
"2005.sql._domainkey.example.net"
"2005.rhv._domainkey.example.net"
Periods are allowed in selectors and are to be treated as component
separators. In the case of DNS queries that means the period defines
sub-domain boundaries.
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The number of public keys and corresponding selectors for each domain
are determined by the domain owner. Many domain owners will be
satisfied with just one selector whereas administratively distributed
organizations may choose to manage disparate selectors and key pairs
in different regions or on different email servers.
Beyond administrative convenience, selectors make it possible to
seamlessly replace public keys on a routine basis. If a domain wishes
to change from using a public key associated with selector "2005" to a
public key associated with selector "2006", it merely makes sure that
both public keys are advertised in the DNS concurrently for the
transition period during which email may be in transit prior to
verification. At the start of the transition period, the outbound
email servers are configured to sign with the "2006" private key. At
the end of the transition period, the "2005" public key is removed
from the DNS.
While some domains may wish to make selector values well known, others
will want to take care not to allocate selector names in a way that
allows harvesting of data by outside parties. E.g., if per-user keys
are issued, the domain owner will need to make the decision as to
whether to make this selector associated directly with the user name,
or make it some unassociated random value, such as the fingerprint of
the public key.
3.2.2 Public key signing and verification algorithm
The default signature is an RSA signed SHA1 digest of the complete
email.
For ease of explanation, the openssl command is used throughout this
document to describe the mechanism by which keys and signatures are
managed.
One way to generate a 768 bit private key suitable for DomainKeys, is
to use openssl like this:
$ openssl genrsa -out rsa.private 768
Which results in the file rsa.private containing the key information
similar to this:
-----BEGIN RSA PRIVATE KEY-----
MIIByQIBAAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5
ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIo
AUsFUq+J6+OprwIDAQABAmBOX0UaLdWWusYzNol++nNZ0RLAtr1/LKMX3tk1MkLH
+Ug13EzB2RZjjDOWlUOY98yxW9/hX05Uc9V5MPo+q2Lzg8wBtyRLqlORd7pfxYCn
Kapi2RPMcR1CxEJdXOkLCFECMQDTO0fzuShRvL8q0m5sitIHlLA/L+0+r9KaSRM/
3WQrmUpV+fAC3C31XGjhHv2EuAkCMQDE5U2nP2ZWVlSbxOKBqX724amoL7rrkUew
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ti9TEjfaBndGKF2yYF7/+g53ZowRkfcCME/xOJr58VN17pejSl1T8Icj88wGNHCs
FDWGAH4EKNwDSMnfLMG4WMBqd9rzYpkvGQIwLhAHDq2CX4hq2tZAt1zT2yYH7tTb
weiHAQxeHe0RK+x/UuZ2pRhuoSv63mwbMLEZAjAP2vy6Yn+f9SKw2mKuj1zLjEhG
6ppw+nKD50ncnPoP322UMxVNG4Eah0GYJ4DLP0U=
-----END RSA PRIVATE KEY-----
Once a private key has been generated, the openssl command can be used
to sign an appropriately prepared email, like this:
$ openssl dgst -sign rsa.private -sha1 <input.file
Which results in signature data similar to this when represented in
Base64 [BASE64] format:
aoiDeX42BB/gP4ScqTdIQJcpAObYr+54yvctqc4rSEFYby9+omKD3pJ/TVxATeTz
msybuW3WZiamb+mvn7f3rhmnozHJ0yORQbnn4qJQhPbbPbWEQKW09AMJbyz/0lsl
How this signature is added to the email is discussed later in this
document.
To extract the public key component from the private key, use openssl
like this:
$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
Which results in the file rsa.public containing the key information
similar to this:
-----BEGIN PUBLIC KEY-----
MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l
MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E
XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB
-----END PUBLIC KEY-----
This public key data is placed in the DNS.
With the signature, canonical email contents and public key, a
verifying system can test the validity of the signature. The openssl
invocation to verify a signature looks like this:
openssl dgst -verify rsa.public -sha1 -signature signature.file <input.file
3.2.3 Public key representation in the DNS
There is currently no standard method defined for storing public keys
in the DNS. As an interim measure, the public key is stored as a TXT
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record derived from a PEM format [PEM], that is, as a Base64
representation of a DER encoded key. Here is an example of a 768 bit
RSA key in PEM form:
-----BEGIN PUBLIC KEY-----
MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l
MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E
XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB
-----END PUBLIC KEY-----
To save scarce DNS packet space and aid extensibility, the PEM
wrapping MUST be removed and the remaining public key data along with
other attributes relevant to DomainKeys functionality are stored as
tag=value pairs separated by semicolons, e.g.:
brisbane._domainkey IN TXT "g=; k=rsa; p=MHww ... IDAQAB"
Verifiers MUST support key sizes of 512, 768, 1024, 1536 and 2048
bits. Signers MUST support at least one of the verifier supported key
sizes.
The current valid tags are:
g = granularity of the key. If present with a non-zero length
value, this value MUST exactly match the local part of the
sending address. This tag is optional.
The intent of this tag is to constrain which sending address
can legitimately use this selector. An email with a sending
address that does not match the value of this tag constitutes
a failed verification.
k = key type (rsa is the default). Signers and verifiers MUST
support the 'rsa' key type. This tag is optional.
n = Notes that may be of interest to a human. No interpretation is
made by any program. This tag is optional.
p = public key data, encoded as a Base64 string. An empty value
means that this public key has been revoked. This tag MUST be
present.
t = a set of flags that define boolean attributes. Valid
attributes are:
y = testing mode. This domain is testing DomainKeys and
unverified email MUST NOT be treated differently from
verified email. Recipient systems MAY wish to track
testing mode results to assist the sender.)
This tag is optional.
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(Syntax rules for the tag=value format are discussed in Appendix A).
Keeping the size of the TXT record to a minimum is important as some
implementations of content and caching DNS servers are reported to
have problems supporting large TXT records. In the example above, the
encoding generates a 182 byte TXT record. That this encoding is less
than 512 bytes is of particular significance as it fits within a
single UDP response packet. With careful selection of query values, a
TXT record can accommodate a 2048 bit key.
For the same size restriction reason, the "n" tag SHOULD be used
sparingly. The most likely use of this tag is to convey a reason why a
public key might have been revoked. In this case set the "n" tag to
the explanation and remove the public key value from the "p" tag.
3.2.4 Key sizes
Selecting appropriate key sizes is a trade-off between cost,
performance and risk. This specification does not define either
minimum or maximum keys sizes - that decision is a matter for each
domain owner.
Factors that should influence this decision include:
o the practical constraint that a 2048 bit key is the largest key
that fits within a 512 byte DNS UDP response packet
o Larger keys impose higher CPU costs to verify and sign email
o Keys can be replaced on a regular basis, thus their lifetime can
be relatively short
o The security goals of this specification are modest compared to
typical goals of public key systems
In general, it is expected that most domain owners will use keys that
are no larger than 1024 bits.
3.3 Storing the signature in the email header
The signature of the email is stored in the "DomainKey-Signature:"
header. This header contains all of the signature and key-fetching
data.
When generating the signed email, the "DomainKey-Signature:" header
MUST precede the original email headers presented to the signature
algorithm.
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The "DomainKey-Signature:" header is not included in the signature
calculation.
For extensibility, the "DomainKey-Signature:" header contains
tag=value pairs separated by semicolons, e.g.:
DomainKey-Signature: a=rsa-sha1; s=brisbane; d=example.net; q=dns; c=simple
The current valid tags are:
a = The algorithm used to generate the signature. The default is
"rsa-sha1", an RSA signed SHA1 digest. Signers and verifiers
MUST support "rsa-sha1".
b = The signature data, encoded as a Base64 string. This tag MUST
be present.
Whitespace is ignored in this value and MUST be removed when
re-assembling the original signature. This is another way of
saying that the signing process can safely insert folding
whitespace in this value to conform to line-length limits.
c = Canonicalization algorithm. The method by which the headers
and content are prepared for presentation to the signing
algorithm. This tag MUST be present. Verifiers MUST support
"simple" and "nofws". Signers MUST support at least one of
the verifier supported algorithms.
d = The domain name of the signing domain. This tag MUST be
present. In conjunction with the selector tag, this domain
forms the basis of the public key query. The value in this tag
MUST match the domain of the sending email address or MUST be
one of the parent domains of the sending email address. Domain
name comparison is case insensitive.
The matching process for this tag is called sub-domain
matching. As the sending email address must be the domain
or sub-domain of the value.
h = A colon separated list of header field names that identify the
headers presented to the signing algorithm. If present, the
value MUST contain the complete list of headers in the order
presented to the signing algorithm.
If present, this tag MUST include the header that was used to
identify the sending domain, ie, the "From:" or "Sender:"
header, thus this tag can never contain an empty value.
If this tag is not present, all headers subsequent to the
signature header are included in the order found in the email.
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A verifier MUST support this tag. A signer MAY support this
tag. If a signer generates this tag it MUST include all email
headers in the original email as a verifier MAY remove or
render suspicious, lines that are not included in the
signature.
In the presence of duplicate headers, a signer may include
duplicate entries in the list of headers in this tag. If a
header is included in this list, a verifier must include all
occurrences of that header, subsequent to the
"DomainKey-Signature:" header in the verification.
If a header identified in this list is not found after the
"DomainKey-Signature:" header in the verification process, a
verifier may "look" for a matching header prior to the
"DomainKey-Signature:" header, however signers should not
rely on this as early experience suggests that most verifiers
do not try to "look" back before the "DomainKey-Signature:"
header.
Whitespace is ignored in this value and header comparisons
are case insensitive.
q = The query method used to retrieve the public key. This tag
MUST be present. Currently the only valid value is "dns" which
defines the DNS lookup algorithm described in this
document. Verifiers and signers MUST support "dns".
s = The selector used to form the query for the public key. This
tag MUST be present. In the DNS query type, this value is
prepended to the "_domainkey." namespace of the sending
domain.
(Syntax rules for the tag=value format are discussed in Appendix A).
Here is an example of a signature header spread across multiple
continuation lines:
DomainKey-Signature: a=rsa-sha1; s=brisbane; d=example.net;
c=simple; q=dns;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR;
Extreme care must be taken to ensure that any new tags added to this
header are defined and used solely for the purpose of fetching and
verifying the signature. Any semantics beyond verification cannot be
trusted as this header is not protected by the signature.
If additional semantics not pertaining directly to signature
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verification are required, they must only be added as subsequent
headers protected by the signature. Semantic additions might include
audit information describing the initial submission.
3.4 Preparation of email for transit and signing
The fundamental purpose of a cryptographic signature is to ensure that
the signed content matches the contents presented for verification.
However, unlike just about every other Internet protocol, the email
content is routinely modified as it enters and transits the email
system.
Fortunately most of the modifications typically made to email can be
predicted and consequently accounted for when signing and verifying.
To maximize the chance of a successful verification, submitted email
should be prepared for transport prior to signing and the data
presented to the signing algorithm is canonicalized to excluded the
most common and minor changes made to email.
3.4.1 Preparation for transit
The SMTP protocol defines a number of potential limitations to email
transport, particularly pertaining to line lengths and eight bit
content.
While the editor has observed that most modern SMTP implementations
accept eight bit email and long lines, some implementations still do
not. Consequently a DomainKeys implementation SHOULD prepare an email
to be suitable for the lowest common denominator of SMTP prior to
presenting the email for signing.
3.4.2 Canonicalization for signing
DomainKeys is initially expected to be deployed at, or close to, the
email borders of an organization rather than in MUAs or SUBMISSION
servers. In other words, the signing and verifying algorithms normally
apply after an email has been packaged, transmogrified and generally
prepared for transmission across the Internet via SMTP and thus the
likelihood of the email being subsequently modified is reduced.
Nonetheless, empirical evidence suggests that some mail servers and
relay systems modify email in transit, potentially invalidating a
signature.
There are two competing perspectives on such modifications. For most
senders, mild modification of email is immaterial to the
authentication status of the email. For such senders a
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canonicalization algorithm that survives modest in-transit
modification is preferred.
For other senders however, any modification of the email - however
minor - results in a desire for the authentication to fail. In other
words, such senders do not want a modified email to be seen as being
authorized by them. These senders prefer a canonicalization algorithm
that does not tolerate in-transit modification of the signed email.
To satisfy both requirements, two canonicalization algorithms are
defined. A "simple" algorithm that tolerates almost no modification
and a "nofws" algorithm that tolerates common modifications as
white-space replacement and header line re-wrapping.
A sender may choose either algorithm when signing an email. A verifier
MUST be able to process email using either algorithm.
Either algorithm can be used in conjunction with the "h" tag in the
"DomainKey-Signature:" header.
Canonicalization simply prepares the email for the signing or
verification algorithm. It does not change the transmitted data in any
way.
3.4.2.1 The "simple" canonicalization algorithm
o Each line of the email is presented to the signing algorithm in
the order it occurs in the complete email, from the first line
of the headers to the last line of the body.
o If the "h" tag is used, only those header lines (and their
continuation lines if any) added to the "h" tag list are
included.
o The "h" tag only constrains header lines. It has no bearing on
body lines, which are always included.
o Remove any local line terminator.
o Append CRLF to the resulting line.
o All trailing empty lines are ignored. An empty line is a line of
zero length after removal of the local line terminator.
If the body consists entirely of empty lines, then the
header/body line is similarly ignored.
3.4.2.2 The "nofws" canonicalization algorithm
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The "No Folding White Space" algorithm (nofws) is more complicated
than the "simple" algorithm for two reasons; folding white space is
removed from all lines and header continuation lines are unwrapped.
o Each line of the email is presented to the signing algorithm in
the order it occurs in the complete email, from the first line
of the headers to the last line of the body.
o Header continuation lines are unwrapped so that header lines are
processed as a single line.
o If the "h" tag is used, only those header lines (and their
continuation lines if any) added to the "h" tag list are
included.
o The "h" tag only constrains header lines. It has no bearing on
body lines, which are are always included.
o For each line in the email, remove all folding white space
characters. Folding white space is defined in RFC2822 as being
the decimal ASCII values 9 (HTAB), 10 (NL), 13 (CR) and 32 (SP).
o Append CRLF to the resulting line.
o Trailing empty lines are ignored. An empty line is a line of
zero length after removal of the local line terminator. Note that
the test for an empty line occurs after removing all folding white
characters.
If the body consists entirely of empty lines, then the
header/body line is similarly ignored.
3.5 The signing process
The previous sections defined the various components and mechanisms
needed to sign an email. This section brings those together to define
the complete process of signing an email.
A signer MUST only sign email from submissions that are authorized to
use the sending address.
Once authorization of the submission has been determined, the signing
process consists of the following steps:
o identifying the sending domain
o determining if an email should be signed
o selecting a private key and corresponding selector information
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o calculating the signature value
o prepending the "DomainKey-Signature:" header
If an email cannot be signed for some reason, it is a local policy
decision as to what to do with that email.
3.5.1 Identifying the sending domain
The sending domain is determined by finding the email address in the
"Sender:" header, or, if the "Sender:" header is not present, the
first email address of the "From:" header is used to determine the
sending domain.
If the email has an invalid "From:" or an invalid "Sender:" header, it
MUST NOT be signed.
If the signer adds the "h" tag to the "DomainKey-Signature:" header,
that tag MUST include the header that was used to determine the
sending domain.
3.5.2 Determining if an email should be signed
A signer can obviously only sign email for domains for which it has a
private key and the necessary knowledge of the corresponding
public key and selector information, however there are a number of
other reasons why a signer may choose not to sign an email.
A signer MUST NOT sign an email if the email submission is not
authorized to use the sending domain.
A signer MUST NOT sign an email that already contains a
"DomainKey-Signature:" header unless a "Sender:" header has been added
that was not included in the original signature. The most obvious case
where this occurs is with mailing lists.
A signer SHOULD NOT remove an existing "DomainKey-Signature:" header.
3.5.3 Selecting a private key and corresponding selector information
This specification does not define the basis by which a signer should
choose which private key and selector information to use. Currently,
all selectors are equal as far as this specification is concerned, so
the decision should largely be a matter of administrative convenience.
3.5.4 Calculating the signature value
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The signer MUST use one of the defined canonicalization algorithms to
present the email to the signing algorithm. Canonicalization is only
used to prepare the email for signing. It does not affect the
transmitted email in any way.
To avoid possible ambiguity, a signing server may choose to remove any
pre-existing "DomainKey-Status:" headers from the email prior to
preparation for signing and transmission.
3.5.5 Prepending the "DomainKey-Signature:" header
The final step in the signing process is that the signer MUST prepend
the "DomainKey-Signature:" header prior to continuing with the process
of transmitting the email.
3.6 Policy statement of sending domain
While the disposition of inbound email is ultimately a matter for the
receiving system, the introduction of authentication in email creates
a need for the sender domain to indicate their signing policy and
preferred disposition of unsigned email. In particular, whether a
domain is participating in DomainKeys, whether it is testing and
whether it signs all outbound email.
The sending domain policy is very simple and is expressed in the
_domainkey TXT record in the DNS of the sending domain. E.g., in the
example.com domain that record is called _domainkey.example.com.
The contents of this TXT record are stored as tag=value pairs
separated by semicolons, e.g.:
_domainkey IN TXT "t=y; o=-; n=notes; r=emailAddress"
All tags are optional. The current valid tags are:
n = Notes that may be of interest to a human. No interpretation is
made by any program.
o = Outbound Signing policy ("-" means that this domain signs all
email, "~" is the default and means that this domain may sign
some email with DomainKeys).
r = A reporting email address. If present, this defines the email
address where invalid verification results are reported. This
tag is primarily intended for early implementors - the content
and frequency of the reports will be defined in a separate
document.
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t = a set of flags that define boolean attributes. Valid
attributes are:
y = testing mode. This domain is testing DomainKeys and
unverified email MUST NOT be treated differently from
verified email. Recipient systems MAY wish to track
testing mode results to assist the sender.)
Note that testing mode cannot be turned off by this tag - thus
policy cannot revert the testing mode setting of a Selector.
This tag is optional.
(Syntax rules for the tag=value format are discussed in Appendix A).
Recipient systems SHOULD only retrieve this policy TXT record to
determine policy when an email fails to verify or does not include a
signature. Recipient systems SHOULD not retrieve this policy TXT
record for email that successfully verifies. Note that "testing mode"
SHOULD also be in the Selector TXT record if the domain owner is
running a DomainKeys test.
If the policy TXT record does not exist, recipient systems MUST
assume the default values.
There is an important implication when a domain states that it signs
all email with the "o=-" setting. Namely that the sending domain
prefers that the recipient system treat unsigned mail with a great
deal of suspicion. Such suspicion could reasonably extend to rejecting
such email. A verifying system MAY reject unverified email if a domain
policy indicates that it signs all email.
Of course nothing compels a recipient MTA to abide by the policy of
the sender. In fact, during the trial a sending domain would want to
be very certain about setting this policy, as processing by recipient
MTAs may be unpredictable. Nonetheless, a domain that states that it
signs all email MUST expect that unverified email may be rejected by
some receiving MTAs.
3.7 The verification process
There is no defined or recommended limit on the lifetime of a selector
and corresponding public key, however it is recommended that
verification occur in a timely manner with the most timely place being
during acceptance or local delivery by the MTA.
Verifying a signature consists of the following three steps:
o Extract the signature information from the headers
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o Retrieve the public key based on the signature information
o Check that the signature verifies against the contents
In the event that any of these steps fails, the sending domain policy
is ascertained to assist in applying local policy.
3.7.1 Presumption that headers are not reordered
Indications from deployment of previous versions of this specification
suggest that the canonicalization algorithms in conjunction with the
"h" tag in the "DomainKey-Signature:" header allows most email to
cryptographically survive intact between signing and verifying.
The one assumption that most of the early deployments make, is that
the headers included in the signature are not reordered prior to
verification.
While nothing in this specification precludes a verifier from
"looking" for a header that may have been reordered, including being
moved to a position prior to the "DomainKey-Signature:" header, such
reordered email is unlikely to be successfully verified by most
implementations.
A second consequence of this assumption - particularly in the presence
of multiple "Domainkey-Signature:" headers - is that the first
DomainKey-Signature: header in the email was the last signature added
to the email and thus is the one to be verified.
3.7.2 Verification should render a binary result
While the symptoms of a failed verification are obvious - the
signature doesn't verify - establishing the exact cause can be more
difficult. If a selector cannot be found, is that because the selector
has been removed or was the value changed somehow in transit? If the
signature line is missing is that because it was never there, or was
it removed by an over-zealous filter?
For diagnostic purposes, the exact reason why the verification fails
SHOULD be recorded, however in terms of presentation to the end user,
the result SHOULD be presented as a simple binary result: either the
email is verified or it is not. If the email cannot be verified, then
it SHOULD be rendered the same as all unverified email regardless of
whether it looks like it was signed or not.
3.7.3 Selecting the most appropriate "DomainKey-Signature:" header
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In most cases a signed email is expected to have just one signature -
that is, one "DomainKey-Signature:" header. However it is entirely
possible that an email can contain multiple signatures. In such cases,
a verifier MUST find the most appropriate signature to use and SHOULD
ignore all other signatures.
The process of finding the most appropriate signature consists of the
following "best match" rules. The rules are to be evaluated in order.
1. Selecting the sending domain
If the email contains a "Sender:" header, the sending domain is
extracted from the "Sender:" address. If this extraction fails,
the email SHALL fail verification.
If no "Sender:" header is present, the sending domain is
extracted from the first address of the "From:" header. If this
extraction fails, the email SHALL fail verification.
2. Domain matching
A signature can only match if the sending domain matches the
"d" tag domain - according to the "d" tag sub-domain matching
rules.
3. "h" tag matching
If the signature contains the "h" tag list of headers, that
list must include the header used to extract the sending
domain in rule 1, above.
4. Most secure signing algorithm
While it is not yet the case, in the event that additional
algorithms are added to this specification, a verifier MUST use
the signature that contains the most secure algorithm - as
defined by the future specification. For current
implementations that means verifiers MUST ignore signatures
that are coded with an unrecognized signing algorithm.
5. Earlier signatures are preferred
If multiple signatures are equal as far as these rules apply,
the signature from the earlier header MUST be used in
preference to later signature headers.
Implementors MUST meticulously validate the format and values in the
"DomainKey-Signature:" header; any inconsistency or unexpected values
MUST result in ignoring that header. Being "liberal in what you
accept" is definitely a bad strategy in this security context.
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In all cases, if a verification fails, the "DomainKey-Status:" header
SHOULD be generated and include a message to help explain the reason
for failure.
3.7.4 Retrieve the public key based on the signature information
The public key is needed to complete the verification process. The
process of retrieving the public key depends on the query type as
defined by the "q" tag in the "DomainKey-Signature:" header
line. Obviously, a public key should only be retrieved if the process
of extracting the signature information is completely successful.
Currently the only valid query type is "dns". The public key retrieval
process for this type is:
1. Using the selector name defined by the "s" tag, the
"_domainkey" namespace and the domain name defined by the "d"
tag, construct and issue the DNS TXT record query string.
E.g., if s=brisbane and d=example.net, the query string is
"brisbane._domainkey.example.net".
2. If the query for the public key fails to respond, the verifier
SHOULD defer acceptance of this email (normally this will be
achieved with a 4XX SMTP response code).
3. If the query for the public key fails because the corresponding
data does not exist, the verifier MUST treat the email as
unverified.
4. If the result returned from the query does not adhere to the
format defined in this specification, the verifier MUST treat
the email as unverified.
5. If the public key data is not suitable for use with the
algorithm type defined by the "a" tag in the
"DomainKey-Signature:" header, the verifier MUST treat the
email as unverified.
Implementors MUST meticulously validate the format and values returned
by the public key query. Any inconsistency or unexpected values MUST
result in an unverified email. Being "liberal in what you accept" is
definitely a bad strategy in this security context.
Latency critical implementations may wish to initiate the public key
query in parallel with calculating the SHA-1 hash as the public key is not
needed until the the final RSA is calculated.
3.7.5 Verify the signature
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Armed with the signature information from the "DomainKey-Signature:"
header and the public key information returned by the query, the
signature of the email can now be verified.
The canonicalization algorithm defined by the "c" tag in the
"DomainKey-Signature:" header defines how the data is prepared for the
verification algorithm and the "a" tag in the same header defines
which verification algorithm to use.
3.7.6 Retrieving sending domain policy
In the event that an email fails to verify, the policy of the sending
domain MUST be consulted. For now that means consulting the _domainkey
TXT record in the DNS of the domain in the sending domain as defined
in 3.5.1. Eg, if example.net is the sending domain, the TXT query is:
_domainkey.example.net
A verifier SHOULD consider the sending domain policy statement and act
accordingly. The range of possibilities is up to the receiver, but it
MAY include rejecting the email.
3.7.7 Applying local policy
After all verification processes are complete the recipient system has
authentication information that can help it decide what to do with the
email.
It is beyond the scope of this specification to describe what actions
a recipient system should make, but an authenticated email presents an
opportunity to a receiving system that unauthenticated email
cannot. Specifically, an authenticated email creates a predictable
identifier by which other decisions can reliably be managed, such as
trust and reputation.
Conversely, unauthenticated email lacks a reliable identifier that can
be used to assign trust and reputation. It is not unreasonable to
treat unauthenticated email as lacking any trust and having no
positive reputation.
3.8 Conveying verification results to MUAs
Apart from the application of automated policy, the result of a
signature verification should be conveyed to the user reading the
email.
Most email clients can be configured to recognize specific headers and
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apply simple rules - e.g., filing into a particular folder. Since
DomainKey signatures are expected to be initially verified at the
border MTA, the results of the verification need to be conveyed to the
email client. This is done with the "DomainKey-Status:" header
line prepended to the email.
The "DomainKey-Status:" header starts with a string that indicate the
result of the verification. Valid values are:
"good" - the signature was verified at the time of testing
"bad" - the signature failed the verification
"no key" - the public key query failed as the key does not exist
"revoked" - the public key query failed as the key has been revoked
"no signature" - this email has no "DomainKey-Signature:" header
"bad format" - the signature or the public key contains unexpected data
"non-participant" - this sending domain has indicated that it does
not participate in DomainKeys.
Verifiers may append additional data that expands on the reason for
the particular status value.
A client SHOULD just look for "good" and assume that all other values
imply that the verification could not be performed for some
reason. Policy action as a consequence of this header is entirely a
local matter.
Here are some examples:
DomainKey-Status: good
DomainKey-Status: bad format
Although it is expected that MTAs will be DomainKey aware before MUAs,
it is nonetheless possible that a DomainKey aware MUA can be fooled by
a spoofed "DomainKey-Status:" header that passes through a
non-DomainKey aware MTA.
If this is perceived to be a serious problem, then it may make sense
to preclude the "good" value and only have values that effectively
demote the email as far as the UA is concerned. That way successful
spoofing attempts can only serve to demote themselves.
4. Example of use
This section shows the complete flow of an email from submission to
final delivery, demonstrating how the various components fit together.
4.1 The user composes an email
From: "Joe SixPack" <joe@football.example.com>
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To: "Suzie Q" <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
4.2 The email is signed
This email is signed by the football.example.com outbound email server
and now looks like this:
DomainKey-Signature: a=rsa-sha1; s=brisbane; d=football.example.com;
c=simple; q=dns;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR;
Received: from dsl-10.2.3.4.football.example.com [10.2.3.4]
by submitserver.football.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: "Joe SixPack" <joe@football.example.com>
To: "Suzie Q" <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
The signing email server requires access to the private key associated
with the "brisbane" selector to generate this signature. Distribution
and management of private keys is outside the scope of this document.
4.3 The email signature is verified
The signature is normally verified by an inbound SMTP server or
possibly the final delivery agent. However, intervening MTAs can also
perform this verification if they choose to do so.
The verification process uses the domain "football.example.com"
extracted from the "From:" header and the selector "brisbane" from the
"DomainKey-Signature:" header to form the DNS TXT query for:
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brisbane._domainkey.football.example.com
Since there is no "h" tag in the "DomainKey-Signature:" header,
signature verification starts with the line following the
"DomainKey-Signature:" line. The email is canonically prepared for
verifying with the "simple" method.
The result of the query and subsequent verification of the signature
is stored in the "DomainKey-Status:" header line. After successful
verification, the email looks like this:
DomainKey-Status: good
from=joe@football.example.com; domainkeys=pass
Received: from mout23.brisbane.football.example.com (192.168.1.1)
by shopping.example.net with SMTP;
Fri, 11 Jul 2003 21:01:59 -0700 (PDT)
DomainKey-Signature: a=rsa-sha1; s=brisbane; d=football.example.com;
c=simple; q=dns;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR;
Received: from dsl-10.2.3.4.network.example.com [10.2.3.4]
by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: "Joe SixPack" <joe@football.example.com>
To: "Suzie Q" <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
5. Association with a Certificate Authority
A fundamental aspect of DomainKeys is that public keys are generated
and advertised by each domain at no additional cost. This
accessibility markedly differs from traditional Public Key
Infrastructures where there is typically a Certificate Authority (CA)
who validates an applicant and issues a signed certificate -
containing their public key - often for a recurring fee.
While CAs do impose costs, they also have the potential to provide
additional value as part of their certification process. Consider
financial institutions, public utilities, law enforcement agencies and
the like. In many cases, such entities justifiably need to
discriminate themselves above and beyond the authentication that
DomainKeys offers.
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Creating a link between DomainKeys and CA issued certificates has the
potential to access additional authentication mechanisms that are more
authoritative than domain owner issued authentication. It is well
beyond the scope of this specification to describe such authorities
apart from defining how the linkage could be achieved with the
"DomainKey-X509:" header.
5.1 The "DomainKey-X509:" header
The "DomainKey-X509:" header provides a link between the public key
used to sign the email and the certificate issued by a CA.
The exact content, syntax and semantics of this header are yet to be
resolved. One possibility is that this header contains an encoding of
the certificate issued by a CA. Another possibility is that this
header contains a URL that points to a certificate issued by a CA.
In either case, this header can only be consulted if the signature
verifies and MUST be part of the content signed by the corresponding
"DomainKey-Signature:" header. Furthermore, it is likely that MUAs
rather than MTAs will confirm that the link to the CA issued
certificate is valid. In part this is because many MUAs already have
built-in capabilities as a consequence of S/MIME [SMIME] and SSL [SSL]
support.
The proof of linkage is made by testing that the public key in the
certificate matches the public key used to sign the email.
An example of a email containing the "DomainKey-X509:" header is:
DomainKey-Signature: a=rsa-sha1; s=statements;
d=largebank.example.com; c=simple; q=dns;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ
VoG4ZHRNiYzR;
DomainKey-X509: https://ca.example.net/largebank.example.com
From: "Large Bank" <statements@largebank.example.com>
To: "Suzie Q" <suzie@shopping.example.net>
Subject: Statement for Account: 1234-5678
...
The format of the retrieved value from the URL is not yet defined, nor
is the determination of valid CAs.
The whole matter of linkage to CA issued certificates is one aspect of
DomainKeys that needs to be resolved with relevant CA and certificate
issuing entities. The primary point is that a link is possible to a
higher authority.
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6. Topics for discussion
6.1 The benefits of selectors
Selectors are at the heart of the flexibility of DomainKeys. A domain
administrator is free to use a single DomainKey for all outbound
mail. Alternatively, they may use many DomainKeys differentiated by
selector and assign each key to different servers.
For example, a large outbound email farm might have a unique DomainKey
for each server, and thus their DNS will advertise potentially
hundreds of keys via their unique selectors.
Another example is a corporate email administrator who might generate a
separate DomainKey for each regional office email server.
In essence, selectors allow a domain owner to distribute authority to
send on behalf of that domain. Combined with the ability to revoke by
removal or TTL expiration, a domain owner has coarse-grained control
over the duration of the distributed authority.
Selectors are particularly useful for domain owners who want to
contract a third-party mailing system to send a particular set of
mail. The domain owner can generate a special key pair and selector
just for this mail-out. The domain owner has to provide the Private
Key and selector to the third party for the life of the
mail-out. However, as soon as the mail-out is completely delivered,
the domain owner can revoke the public key by the simple expedient of
removing the entry from the DNS.
6.2 Canonicalization of email
It is an unfortunate fact that some email software routinely (and
often unnecessarily) transforms email as it transits through the
network. Such transformations conflict with the fundamental purpose of
cryptographic signatures - to detect modifications.
While two canonicalization algorithms are defined in this
specification, the primary goal of "nofws" is to provide a transition
path to "simple". With a mixture of "simple" and "nofws" email, a
receiver can determine which systems are modifying email in ways that
cause the signature to fail and thus provide feedback to the modifying
system.
6.3 Mailing lists
Integrating existing Mailing List Managers (MLMs) into the DomainKeys
authentication system is a complicated areas as the behavior of MLMs is
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highly variable. Essentially there are two types of MLMs under
consideration. Those that modify email to such an extent that
verification of the original content is not possible, and MLMs that
make minimal or no modifications to an email.
MLMs that modify email in a way that causes verification to fail, MUST
pre-pend either a "Sender:" header and preferably a "List-ID:" header
and arrange for the email to be resigned as it is distributed to the
list recipients.
A participating SUBMISSION server can deduce the need to resign such
an email by the presence of a "Sender:" or "List-ID:" header from an
authorized submission.
MLMs that do not modify email in a way that causes verification to
fail, MAY perform the same actions as a modifying MLM.
6.4 Roving users
One scenario that presents a particular problem with any form of email
authentication, including DomainKeys, is the roving user. A user who
is obliged to use a third-party SUBMISSION service when unable to
connect to their own SUBMISSION service. The classic example cited is
a traveling salesperson being redirected to a hotel email server to
send email.
As far as DomainKeys is concerned, email of this nature clearly
originates from an email server that does not have authority to send
on behalf of the domain of the salesperson and is therefore
indistinguishable from a forgery. While DomainKeys does not prescribe
any specific action for such email it is likely that over time, such
email will be treated as second class email.
The typical solution offered to roving users is to submit email via an
authorized server for their domain - perhaps via a VPN or a web
interface or even SMTP AUTH back to a SUBMISSION server.
While these are perfectly acceptable solutions for many, they are not
necessarily solutions that are available or possible for all such
users.
One possible way to address the needs of this contingent of
potentially disenfranchised users, is for the domain to issue per-user
DomainKeys. Per-user DomainKeys are identified by a non-empty "g" tag
value in the corresponding DNS record.
7. Security Considerations
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7.1 DNS
DomainKeys is primarily a security mechanism. Its core purpose is to
make claims about email authentication in a credible way. However,
DomainKeys, like virtually all Internet applications, relies on the
DNS which has well-known security flaws [RFC3833].
7.1.1 The DNS is not currently secure
While the DNS is currently insecure, it is expected that the security
problems should and will be solved by DNSSEC [DNSSEC], and all users
of the DNS will reap the benefit of that work.
Secondly, the types of DNS attacks relevant to DomainKeys are very
costly and are far less rewarding than DNS attacks on other Internet
applications.
To systematically thwart the intent of DomainKeys, an attacker must
conduct a very costly and very extensive attack on many parts of the
DNS over an extended period. No one knows for sure how attackers will
respond, however the cost/benefit of conducting prolonged DNS attacks
of this nature is expected to be uneconomical.
Finally, DomainKeys is only intended as a "sufficient" method of
proving authenticity. It is not intended to provide strong
cryptographic proof about authorship or contents. Other technologies
such as GnuPG and S/MIME address those requirements.
7.1.2 DomainKeys creates additional DNS load
A second security issue related to the DNS revolves around the
increased DNS traffic as a consequence of fetching Selector-based data
as well as fetching sending domain policy. Widespread deployment of
DomainKeys will result in a significant increase in DNS queries to the
claimed sending domain. In the case of forgeries on a large scale, DNS
servers could see a substantial increase in queries.
7.2 Key Management
All public key systems require management of key pairs. Private keys
in particular need to be securely distributed to each signing mail
server and protected on those servers. For those familiar with SSL,
the key management issues are similar to those of managing SSL
certificates. Poor key management may result in unauthorized access to
private keys, which in essence gives unauthorized access to your
identity.
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7.3 Implementation Risks
It is well recognized in cryptographic circles that many security
failures are caused by poor implementations rather than poor
algorithms. For example, early SSL implementations were vulnerable
because the implementors used predictable "random numbers".
While some MTA software already supports various cryptographic
techniques, such as TLS, many do not. This proposal introduces
cryptographic requirements into MTA software which implies a much
higher duty of care to manage the increased risk.
There are numerous articles, books, courses, and consultants that help
programming security applications. Potential implementors are strongly
encouraged to avail themselves of all possible resources to ensure
secure implementations.
7.4 Privacy assumptions with forwarding addresses
Some people believe that they can achieve anonymity by using an email
forwarding service. While this has never been particularly true as
bounces, over-quota messages, vacation messages and web bugs all
conspire to expose IP addresses and domain names associated with the
delivery path, the DNS queries that are required to verify DomainKeys
signature can provide additional information to the sender.
In particular, as mail is forwarded through the mail network, the DNS
queries for the selector will typically identify the DNS cache used by
the forwarding and delivery MTAs.
7.5 Cryptographic processing is computationally intensive
Verifying a signature is computationally significant. Early
indications are that a typical mail server can expect to increase CPU
demands by 8-15 percent. While this increased demand is modest
compared to other common mail processing costs - such as Bayesian
filtering - any increase in resource requirements can make a
denial-of-service attack more effective against a mail system.
A constraining factor of such attacks is that the net computational
cost of verifying is bounded by the maximum key size allowed by this
specification and is essentially linear to the rate at which mail is
accepted by the verifying system. Consequently, the additional
computational cost may augment a denial-of-service attack, but it does
not add a non-linear component to such attacks.
8. The trial
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The Domainkeys protocol was deployed as a trial to better understand
the implications of deploying wide-scale cryptographic email
authentication.
Open Source implementations were made available at various places,
particularly Source Forge [SOURCEFORGE] which includes links to
numerous implementations, both Open Source and commercial.
8.1 Goals
The primary goals of the trial were to:
o understand the operational implications of running a DNS-based
public key system for email
o measure the effectiveness of the canonicalization algorithms
o experiment with possible per-user key deployment models
o fully define the semantics of the "DomainKey-X509:" header
8.2 Results of trial
The DomainKeys trial ran for approximately 2 years in which time
numerous large ISPs and many thousands of smaller domains participated
in signing or verifying with DomainKeys. The low order numbers are
that at least one billion DomainKey signed emails transit the Internet
each day between some 12,000 participating domains.
The operational and development experience of that trial was applied
to DKIM.
9. Note to Implementors regarding TXT records
The DNS is very flexible in that it is possible to have multiple TXT
records for a single name and for those TXT records to contain
multiple strings.
In all cases, implementors of DomainKeys should expect a single TXT
record for any particular name. If multiple TXT records are returned,
the implementation is free to pick any single TXT record as the
authoritative data. In other words, if a name server returns different
TXT records for the same name, it can expect unpredictable results.
Within a single TXT record, implementors should concatenate multiple
strings in the order presented and ignore string boundaries. Note that
a number of popular DNS command-line tools render multiple strings as
separately quoted strings which can be misleading to a novice
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implementor.
10. References
10.1 Normative References
[BASE64] S. Josefsson, Ed., "The Base16, Base32, and Base64
Data Encodings", RFC 3548, July, 2003.
[PEM] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I: Message Encryption and Authentication
Procedures", RFC 1421 February, 1993.
10.2 Informative References
[DKIM] http://www.ietf.org/html.charters/dkim-charter.html
[DNSSEC] http://www.ietf.org/html.charters/dnsext-charter.html
[OPENSSL] http://www.openssl.org
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2822] Resnick, P., Editor, "Internet Message Format", RFC
2822, April 2001.
[RFC3833] Atkins, D., Austein, R., "Threat Analysis of the
Domain Name System (DNS)", RFC 3833, August, 2004.
[SMIME] B. Ramsdell, Editor, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.1 Message
Specification", RFC 3851, July 2004.
[SOURCEFORGE] http://domainkeys.sourceforge.net
[SSL] http://wp.netscape.com/security/techbriefs/ssl.html
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Appendix A - Syntax rules for the tag=value format
A simple tag=value syntax is used to encode data in the response
values for DNS queries as well as headers embedded in emails. This
section summarized the syntactic rules for this encoding:
o A tag=value pair consists of three tokens, a "tag", the "="
character and the "value"
o A tag MUST be one character long and MUST be a lower-case
alphabetic character
o Duplicate tags are not allowed
o A value MUST only consist of characters that are valid in
RFC2822 headers, DNS TXT records and are within the ASCII range
of characters from SPACE (0x20) to TILDE (0x7E)
inclusive. Values MUST NOT contain a semicolon but they may
contain "=" characters.
o A tag=value pair MUST be terminated by a semicolon or the end
of the data
o Values MUST be processed as case sensitive unless the specific
tag description of semantics imply case insensitivity.
o Values MAY be zero bytes long
o Whitespace MAY surround any of the tokens, however whitespace
within a value MUST be retained unless explicitly excluded by
the specific tag description. Currently the only tags that
specifically ignores embedded whitespace are the "b" and "h" tag
in the "DomainKey-Signature:" header.
o Tag=value pairs that represent the default value MAY be included
to aid legibility.
o Unrecognized tags MUST be ignored
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Acknowledgments
The editor wishes to thank Russ Allbery, Eric Allman, Edwin Aoki,
Claus Asmann, Steve Atkins, Jon Callas, Dave Crocker, Michael Cudahy,
Jutta Degener, Timothy Der, Jim Fenton, Duncan Findlay, Phillip
Hallam-Baker, Murray S. Kucherawy, John Levine, Miles Libbey, David
Margrave, Justin Mason, David Mayne, Russell Nelson, Juan Altmayer
Pizzorno, Blake Ramsdell, Scott Renfro, the Spamhaus.org team, Malte
S. Stretz, Robert Sanders, Bradley Taylor, and Rand Wacker for their
valuable suggestions and constructive criticism.
Author's Address
Mark Delany
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Email: markd+domainkeys@yahoo-inc.com
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