Network Working Group M. Delany
Request for Comments: 4870 Yahoo! Inc
Obsoleted By: 4871 May 2007
Category: Historic
Domain-Based Email Authentication Using Public Keys
Advertised in the DNS (DomainKeys)
Status of This Memo
This memo defines a Historic Document for the Internet community. It
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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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Table of Contents
1. Introduction ....................................................3
1.1. Lack of Authentication Is Damaging Internet Email ..........3
1.2. Digitally Signing Email Creates Credible Domain
Authentication .............................................4
1.3. Public Keys in the DNS .....................................4
1.4. Initial Deployment Is Likely at the Border MTA .............5
1.5. Conveying Verification Results to MUAs .....................5
1.6. Technical Minutiae Are Not Completely Covered ..............5
1.7. Motivation .................................................6
1.8. Benefits of DomainKeys .....................................6
1.9. Definitions ................................................7
1.10. Requirements Notation .....................................8
2. DomainKeys Overview .............................................8
3. DomainKeys Detailed View ........................................8
3.1. Determining the Sending Address of an Email ................9
3.2. Retrieving the Public Key Given the Sending Domain ........10
3.2.1. Introducing "selectors" ............................10
3.2.2. Public Key Signing and Verification Algorithm ......11
3.2.3. Public key Representation in the DNS ...............13
3.2.4. Key Sizes ..........................................14
3.3. Storing the Signature in the Email Header .................15
3.4. Preparation of Email for Transit and Signing ..............17
3.4.1. Preparation for Transit ............................18
3.4.2. Canonicalization for Signing .......................18
3.4.2.1. The "simple" Canonicalization Algorithm ...19
3.4.2.2. The "nofws" Canonicalization Algorithm ....19
3.5. The Signing Process .......................................20
3.5.1. Identifying the Sending Domain .....................20
3.5.2. Determining Whether an Email Should Be Signed ......21
3.5.3. Selecting a Private Key and Corresponding
Selector Information ...............................21
3.5.4. Calculating the Signature Value ....................21
3.5.5. Prepending the "DomainKey-Signature:" Header .......21
3.6. Policy Statement of Sending Domain ........................22
3.7. The Verification Process ..................................23
3.7.1. Presumption that Headers Are Not Reordered .........24
3.7.2. Verification Should Render a Binary Result .........24
3.7.3. Selecting the Most Appropriate
"DomainKey-Signature:" Header ......................24
3.7.4. Retrieve the Public Key Based on the
Signature Information ..............................26
3.7.5. Verify the Signature ...............................27
3.7.6. Retrieving Sending Domain Policy ...................27
3.7.7. Applying Local Policy ..............................27
3.8. Conveying Verification Results to MUAs ....................27
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4. Example of Use .................................................29
4.1. The User Composes an Email ................................29
4.2. The Email Is Signed .......................................29
4.3. The Email Signature Is Verified ...........................30
5. Association with a Certificate Authority .......................31
5.1. The "DomainKey-X509:" Header ..............................31
6. Topics for Discussion ..........................................32
6.1. The Benefits of Selectors .................................32
6.2. Canonicalization of Email .................................33
6.3. Mailing Lists .............................................33
6.4. Roving Users ..............................................33
7. Security Considerations ........................................34
7.1. DNS .......................................................34
7.1.1. The DNS Is Not Currently Secure ....................34
7.1.2. DomainKeys Creates Additional DNS Load .............35
7.2. Key Management ............................................35
7.3. Implementation Risks ......................................35
7.4. Privacy Assumptions with Forwarding Addresses .............35
7.5. Cryptographic Processing Is Computationally Intensive .....36
8. The Trial ......................................................36
8.1. Goals .....................................................36
8.2. Results of Trial ..........................................37
9. Note to Implementors Regarding TXT Records .....................37
10. References ....................................................37
10.1. Normative References .....................................37
10.2. Informative References ...................................38
Appendix A - Syntax Rules for the Tag=Value Format .............39
Acknowledgments ................................................40
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.
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.
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.
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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 the following:
o forwarded email
o distributed sending systems
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
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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 Mail Transfer Agents (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 Mail User Agent (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
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
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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 that:
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
that 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 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.
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1.9. Definitions
With reference to the following sample email:
Line Data
Number Bytes Content
---- --- --------------------------------------------
01 46 From: "Joe SixPack"
02 40 To: "Suzie Q"
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.
Lines 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.
Lines 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.
Lines 01 to 15 constitute the complete email.
Line 01 is earlier than line 02, and line 02 is later than line 01.
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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
create interoperable implementations. This section answers the
following questions:
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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 among the alternatives are as follows:
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 RFC
2822 "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 RFC 2822 "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 that 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 namespaces using selectors are as follows:
"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 subdomain boundaries.
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The number of public keys and corresponding selectors for each domain
is 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. For example, 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
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which results in the file rsa.private containing the key information
similar to this:
-----BEGIN RSA PRIVATE KEY-----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-----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
To: "Suzie Q"
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"
To: "Suzie Q"
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 are outside the scope of
this document.
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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:
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"
To: "Suzie Q"
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.
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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.
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 Secure/Multipurpose
Internet Mail Extensions (S/MIME) [SMIME] and Secure Socket Layer
(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.
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An example of an 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"
To: "Suzie Q"
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's and
certificate-issuing entities. The primary point is that a link is
possible to a higher authority.
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, the domain administrator 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 Time to Live (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.
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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 area, as the behavior of MLMs
is 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 those that
make minimal or no modifications to an email.
MLMs that modify email in a way that causes verification to fail MUST
prepend a "Sender:" header and SHOULD prepend a "List-ID:" header
prior to signing for distribution to list recipients.
A participating SUBMISSION server can deduce the need to re-sign 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 the user's own SUBMISSION service. The classic
example cited is a traveling salesperson being redirected to a hotel
email server to send email.
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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 Virtual
Private Network (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
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 DNS Security (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
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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.
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 that 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.
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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
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
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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 implementor.
10. References
10.1. Normative References
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[PEM] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I: Message Encryption and Authentication
Procedures", RFC 1421 February, 1993.
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10.2. Informative References
[DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[DNSSEC] http://www.ietf.org/html.charters/dnsext-charter.html
[OPENSSL] http://www.openssl.org
[RFC2822] Resnick, P., Editor, "Internet Message Format", RFC
2822, April 2001.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the
Domain Name System (DNS)", RFC 3833, August 2004.
[SMIME] Ramsdell, B., Ed., "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 lowercase
alphabetic character
o Duplicate tags are not allowed
o A value MUST only consist of characters that are valid in RFC
2822 headers and 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 ignore embedded whitespace are the "b" and "h"
tags 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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contained in BCP 78, and except as set forth therein, the authors
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RFC, FYI, BCP