Individual submission M. Kucherawy
Internet-Draft Cloudmark, Inc.
Intended status: Experimental September 29, 2010
Expires: April 2, 2011
DKIM Authorized Third-Party Signers
draft-kucherawy-dkim-atps-01
Abstract
This memo proposes an experimental proposal to supplement Domain Keys
Identified Mail (DKIM) and Author Domain Signing Practices (ADSP)
allowing advertisement of third-party signature authorizations on
behalf of an email originator.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 2, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. E-Mail Architecture Terminology . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Queries and Replies . . . . . . . . . . . . . . . . . . . . . 4
4.1. Extension to ADSP . . . . . . . . . . . . . . . . . . . . 4
4.2. ATPS Query Details . . . . . . . . . . . . . . . . . . . . 4
4.3. ATPS Reply Details . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. Email Authentication Method Name Registry Update . . . . . 6
5.2. Email Authentication Result Name Registry Update . . . . . 6
5.3. ADSP Specification Tag Registry . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Transient Security Failures . . . . . . . . . . . . . . . 9
6.2. Load on the DNS . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Example Query and Reply . . . . . . . . . . . . . . . 10
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 11
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1. Introduction
[DKIM] defines a mechanism for transparent domain-level signing of
messages for the purpose of declaring that a particular
Administrative Mail Domain (ADMD) takes some responsibility for a
message. [ADSP] creates a mechanism by which an ADMD can declare
that mail from it is expected to include a valid DKIM signature from
that same ADMD, and mail that does not should be considered suspect
or even discarded by receivers.
Absent is a mechanism by which an ADMD can announce that signatures
on its mail from other ADMDs should also be considered authentic by
verifiers. This memo presents an experimental mechanism for doing
so.
The results of this experiment are intended to yield statistics or
other information as to the efficacy of this or similar models, and
may lead to either evolution of this work toward the Standards Track
or to its abandonment. We anticipate at least a few interoperating
implementations in short order so that the experiment can begin.
2. Definitions
2.1. Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
2.2. E-Mail Architecture Terminology
Readers should be familiar with the material and terminology
discussed in [MAIL] and [EMAIL-ARCH].
3. Discussion
Participation in this experiment is divided into two parties:
Senders, whose domains appear in the RFC5322.From field of a [MAIL]
message, and Verifiers, who implement the validation procedures
described in both [DKIM] and [ADSP].
A Sender participates in this protocol if it wishes to announce that
a message from it (in the RFC5322.From sense) should be considered
authentic as long as it bears a signature from any in a list of
specified domains. One might, for example, wish to delegate signing
authority for its DNS domain to an approved messaging service
provider without doing the work of key exchange described in Appendix
B.1.1 of [DKIM].
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A Verifier participates in this protocol if it wishes to ensure that
a message bears one or more signatures from sources approved to sign
mail on behalf of the Sender, and identify for special treatment mail
that does not meet that criterion.
A Verifier with this interest has presumably already implemented ADSP
and is therefore making one DNS query for each DKIM signature (to
retrieve the matching public key) on the message, and one to check
for any published ADSP record. Given this, it seems to make sense
that the ADSP record is the right place to declare that this
extension is in use. Therefore, a tag indicating this extension will
be added to ADSP.
4. Queries and Replies
This section describes in detail the queries issued, the replies
received, and how they should be interpreted and applied.
4.1. Extension to ADSP
[ADSP] replies contain a "tag=value" sequence, but only one tag
("dkim") is currently defined.
This protocol will add an additional tag "atps" whose value is "y" if
this extension is in use. The absence of this tag, or its presence
with any other value, MUST be ignored and the Verifier MUST act as if
this protocol is not in use.
The formal syntax definition, per [ABNF]:
adsp-atps-tag = %x61.74.70.73 *WSP "=" *WSP %x79
The registration for this tag can be found in Section 5.
4.2. ATPS Query Details
If a Sender announces via the ADSP extension that it is using this
protocol and the Verifier is also participating, then the Verifier
issues a TXT query to the DNS to a specific name looking for
confirmation by the sender that a particular third party signature is
authorized by the Sender. The query SHOULD be repeated for every
domain found in the "d=" tag of a [DKIM] signature on the message
that verified is not otherwise considered unacceptable to the
Verifier for policy reasons. These MAY be done in any order or MAY
be done in parallel.
Where the RFC5322.From field contains multiple addresses, this
process MAY be repeated for each domain found in the field, and these
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MAY be done in any order.
The name for the query is constructed as follows:
1. Extract the value of the "d=" tag from the signature and feed it
to the [SHA1] hash algorithm.
2. Convert any upper-case characters in that string to their lower-
case equivalents.
3. Convert the output of the SHA1 hash to a string of 32 hexadecimal
digits by applying base32 encoding as defined in Section 6 of
[BASE32].
4. Append the string "._atps."
5. Append the domain name found in the RFC5322.From field of the
message.
The query's formal syntax definition, per [ABNF]:
atps-query = 32*BASE32 %x2e.5f.61.74.70.73.2e domain-name
BASE32 = ( ALPHA / %x32-37 )
The "domain-name" is as defined in Section 3.5 of [DKIM].
See Appendix A for an example of a query construction.
Since the size of a [DNS] query is limited to 255 bytes, the size of
"domain-name" in the ABNF above is constrained to 216 bytes.
4.3. ATPS Reply Details
At this time, only three possibilities need to be identified in this
specification:
o An answer is returned (i.e. [DNS] reply code NOERROR). In this
case, the protocol has been satisfied and the Verifier may
conclude that the signing domain is authorized by the Sender to
sign its mail. Further queries SHOULD NOT be initiated. There is
at present no need to parse the content (if any) but this is
likely to be needed as this protocol is extended.
o No answer is returned (i.e. [DNS] reply code NXDOMAIN). In this
case, the "d=" domain has not been authorized to act as a third-
party signer for this Sender, and thus the Verifier MUST continue
to the next query.
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o An error is returned (i.e. any other [DNS] reply code). It is no
longer possible to determine whether or not this message satisfies
the Sender's list of authorized third-party signers. The Verifier
SHOULD stop processing and defer the message for later processing,
such as requesting temporary failure code from the MTA.
If all queries are completed and return NXDOMAIN, then the message
does not satisfy the policy advertised by the Sender, and the
action(s) specified by the "dkim" tag in the ADSP reply SHOULD be
applied.
5. IANA Considerations
No actions are required by IANA at this time. The following need
only be applied if and when this specification reaches the Standards
Track.
5.1. Email Authentication Method Name Registry Update
The following should be added to the Email Authentication Method Name
Registry established by [AUTHRES] as per [IANA-CONSIDERATIONS]:
Method dkim-atps
Defined In [THIS MEMO]
ptype header
property from
value contents of the [MAIL] From: header field, with comments
removed
5.2. Email Authentication Result Name Registry Update
The following should be added to the Email Authentication Result Name
Registry established by [AUTHRES] as per [IANA-CONSIDERATIONS]:
Code none
Existing/New Code existing
Defined In [AUTHRES]
Auth Method dkim-atps
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Meaning No DKIM Author Domain Signing Practices (ADSP) record was
published, or the ADSP record did not indicate ATPS was in use, or
ATPS was not performed by the verifier.
Code pass
Existing/New Code existing
Defined In [AUTHRES]
Auth Method dkim-atps
Meaning An ATPS evaluation was performed and a valid signature from
an authorized third-party was found on the message.
Code unknown
Existing/New Code existing
Defined In [ADSP]
Auth Method dkim-atps
Meaning An ATPS evaluation was performed, no valid signature from an
authorized third-party was found on the message, and the published
ADSP was "unknown".
Code fail
Existing/New Code existing
Defined In [AUTHRES]
Auth Method dkim-atps
Meaning An ATPS evaluation was performed, no valid signature from an
authorized third-party was found on the message, and the published
ADSP was "all".
Code discard
Existing/New Code existing
Defined In [ADSP]
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Auth Method dkim-atps
Meaning An ATPS evaluation was performed, no valid signature from an
authorized third-party was found on the message, and the published
ADSP was "discardable".
Code temperror
Existing/New Code existing
Defined In [AUTHRES]
Auth Method dkim-atps
Meaning An ATPS evaluation could not be completed due to some error
that is likely transient in nature, such as a temporary DNS error.
A later attempt may produce a final result.
Code permerror
Existing/New Code existing
Defined In [AUTHRES]
Auth Method dkim-atps
Meaning An ATPS evaluation could not be completed due to some error
that is not likely transient in nature, such as a permanent DNS
error. A later attempt is unlikely to produce a final result.
5.3. ADSP Specification Tag Registry
The following should be added to the ADSP Specification Tag Registry
established by [ADSP] as per [IANA-CONSIDERATIONS]:
+------+-------------+
| TYPE | REFERENCE |
+------+-------------+
| atps | [THIS MEMO] |
+------+-------------+
6. Security Considerations
This section discusses potential security issues related to this
experimental protocol.
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6.1. Transient Security Failures
Approving a third party signer exposes the Sender to the risk that
the third party signer becomes compromised and then begins to sign
malicious or nuisance messages on behalf of the Sender. This can
obviously reflect negatively on the Sender, and the impact of this
can become more severe as automated domain reputation systems are
developed and deployed. Thorough vetting and monitoring practices by
Senders of third party signers will likely need to become the norm.
6.2. Load on the DNS
A Verifier participating in DKIM, ADSP and ATPS will now issue a
number of TXT queries to the DNS equal to one (for the ADSP query)
plus twice the number of valid signatures on the message (one for
each key, one for an ATPS record) plus the number of invalid
signatures on the message (one for each key). This is in addition to
any PTR and A queries the MTA may issue at the time the actual
message relaying or delivery is initiated. Obviously this can be
burdensome on the DNS at both ends, and waiting for that number of
queries to return when they are issued in parallel could trigger
timeouts in the MTA.
An alternative to this that has not yet been explored is the storage
of the ATPS data at a specific URL tied to the Sender's domain name.
This would alleviate pressure on the DNS at the expense of requiring
the Sender to operate a web server (which has its own security
implications) and the addition of the establishment of a TCP
connection. Moreover, the Verifier would be well advised to
implement caching of this data to prevent ATPS from being used as a
denial-of-service vector.
7. References
7.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68,
RFC 5234, January 2008.
[AUTHRES] Kucherawy, M., "Message Header Field for
Indicating Message Authentication Status",
RFC 5451, April 2009.
[BASE32] Josefsson, S., "The Base16, Base32, and Base64
Data Encodings", RFC 4648, October 2006.
[DKIM] Allman, E., Callas, J., Delany, M., Libbey,
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M., Fenton, J., and M. Thomas, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 4871,
May 2007.
[DNS] Mockapetris, P., "Domain names -
implementation and specification", STD 13,
RFC 1035, November 1987.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
[SHA1] U.S. Department of Commerce, "Secure Hash
Standard", FIPS PUB 180-2, August 2002.
7.2. Informative References
[ADSP] Allman, E., Fenton, J., Delany, M., and J.
Levine, "DomainKeys Identified Mail (DKIM)
Author Domain Signing Practices (ADSP)",
RFC 5617, August 2009.
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture",
RFC 5598, October 2008.
[IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 5226, May 2008.
[MAIL] Resnick, P., Ed., "Internet Message Format",
RFC 5322, October 2008.
Appendix A. Example Query and Reply
This section presents an example of the use of this protocol to query
for a third-party authorization and discusses the interpretation of
the result.
Presume a message for which the RFC5322.From domain is "example.com",
and it bears two valid signatures, one from "one.example.net" and one
from "two.example.net". The following actions are taken:
1. A TXT query is made, per [ADSP], to
"_adsp._domainkey.example.com" to query for its Author Domain
Signing Practices. If no valid reply is returned or the reply
does not contain an "atps" tag with value "y", the algorithm
stops with [AUTHRES] result "none".
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2. A SHA1 hash of "one.example.net" is computed and then converted
to printable form using base32 encoding, resulting in the string
"QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6".
3. A TXT query is issued to
"QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6._atps.example.com".
4. If a reply arrives, the algorithm stops with [AUTHRES] result
"pass". If a DNS error code other than NXDOMAIN is returned, the
algorithm stops with a result of "temperror" or "permerror" as
appropriate.
5. A SHA1 hash of "two.example.net" is computed and then converted
to printable form using base32 encoding, resulting in the string
"ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX".
6. A TXT query is issued to
"ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX._atps.example.com".
7. If a reply arrives, the algorithm stops with [AUTHRES] result
"pass". If a DNS error code other than NXDOMAIN is returned, the
algorithm stops with a result of "temperror" or "permerror" as
appropriate.
8. As there are no valid signatures left to test, the algorithm
stops with a result of "unknown", "fail" or "discard" as per the
value on the ADSP "dkim" tag.
Appendix B. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: (names)
The author also wishes to acknowledge Doug Otis and Daniel Black for
their original draft upon which this work was based.
Author's Address
Murray S. Kucherawy
Cloudmark, Inc.
128 King St., 2nd Floor
San Francisco, CA 94107
US
Phone: +1 415 946 3800
EMail: msk@cloudmark.com
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