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".
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
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.
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
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
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.
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.
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.
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:
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.
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.
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
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 <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 sig.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
record derived from a Privacy-Enhanced Mail (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, for example, as in the
following:
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 as follows:
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 as follows:
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.
(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 key sizes -- that decision is a matter for each
domain owner.
Factors that should influence this decision include the following:
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.
The "DomainKey-Signature:" header is not included in the signature
calculation.
For extensibility, the "DomainKey-Signature:" header contains
tag=value pairs separated by semicolons, for example, as in the
following:
DomainKey-Signature: a=rsa-sha1; s=brisbane; d=example.net;
q=dns; c=simple
The current valid tags are as follows:
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
reassembling 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 subdomain
matching, as the sending email address must be the domain
or subdomain 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, i.e., 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.
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
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 exclude 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 8-bit content.
While the editor has observed that most modern SMTP implementations
accept 8-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
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
whitespace replacement and header line rewrapping.
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
The "No Folding Whitespace" algorithm (nofws) is more complicated
than the "simple" algorithm for two reasons; folding whitespace 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 always included.
o For each line in the email, remove all folding whitespace
characters. Folding whitespace is defined in RFC 2822 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 whitespace 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
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 Whether 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
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. For example,
in the example.com domain, the record is called
_domainkey.example.com.
The contents of this TXT record are stored as tag=value pairs
separated by semicolons, for example, as in the following:
_domainkey IN TXT "t=y; o=-; n=notes; r=emailAddress"
All tags are optional. The current valid tags are as follows:
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 implementers -- the
content and frequency of the reports will be defined in a
separate document.
t = a set of flags that define boolean attributes. Valid
attributes are as follows:
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 signature information from the headers
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 overzealous 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 or not it looks like it was signed.
3.7.3. Selecting the Most Appropriate "DomainKey-Signature:" Header
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 subdomain 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.
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 as follows:
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.
For example, 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 final RSA is calculated.
3.7.5. Verify the Signature
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 Section 3.5.1. For example, 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 take, 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 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 as follows:
"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>
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 are 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:
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.
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.
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" <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'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.
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.
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
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.
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
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.
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
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
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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