Individual submission M. Kucherawy
Internet-Draft Cloudmark, Inc.
Intended status: Standards Track June 17, 2010
Expires: December 19, 2010
Authentication-Results Registration For Differentiating Among
Cryptographic Results
draft-kucherawy-authres-header-b-04
Abstract
This memo updates the registry of properties in Authentication-
Results: message header fields to allow a multiple-result report to
distinguish among one or more cryptographic signatures on a message,
thus associating specific results with the signatures they represent.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6.1. Improvement . . . . . . . . . . . . . . . . . . . . . . . . 4
6.2. Result Forgeries . . . . . . . . . . . . . . . . . . . . . 5
6.3. New Schemes with Small Signatures . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Authentication-Results Examples . . . . . . . . . . . 6
A.1. Multiple DKIM Signatures with One Failure . . . . . . . . . 7
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
[AUTHRES] defined a new header field for electronic mail messages
that presents the results of a message authentication effort in a
machine-readable format. Absent from that specification was the
means by which the results of two cryptographic signatures, such as
those provided by [DKIM], can both have results reported in an
unambiguous manner.
Fortunately, [AUTHRES] created IANA registries of reporting
properties, enabling an easy remedy for this problem. This memo thus
registers an additional reporting property allowing a result to be
associated with a specific digital signature.
2. 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].
3. Discussion
A message can contain multiple signatures of a common sender
authentication mechanism, such as [DKIM]. For example, a DKIM signer
could apply signatures using two or more different message
canonicalization algorithms to determine the resistance of each to
being broken in transit.
By applying supported "ptype.property" combinations (cf. the ABNF in
[AUTHRES]), a result can be associated with a given signature
provided the signatures are all unique within one of the registered
values (e.g. all of them had unique "header.d" or "header.i" values).
This is not guaranteed, however; a single signing agent might have
practical reasons for affixing multiple signatures with the same "d="
values while varying other signature parameters. This means one
could get a "dkim=pass" and "dkim=fail" result simultaneously on
verification which is clearly ambiguous.
It is thus necessary either to create or to identify a signature
attribute guaranteed to be unique, such that it is possible to
unambiguously associate a result with the signature to which it
refers.
Collisions during general use of SHA1 and SHA256 are uncommon (see
[HASH-ATTACKS]) and RSA key signing mechanisms are resilient to
producing common substrings. Thus, the actual digital signature for
a cryptographic signing of the message is an ideal property for such
a unique identification. It is not however necessary to include the
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entire digital signature in an [AUTHRES] header field just to
identify which result goes with signature; since the signatures will
almost always be substantially different, it is anticipated that only
the first several bytes of a signature will be needed for
disambiguating results.
4. Definition
This memo adds to the "Email Authentication Method Name Registry",
created by IANA upon publication of [AUTHRES], the "header.b"
reporting item. The value associated with this item in the header
field MUST be at least the first eight characters of the digital
signature (the "b=" tag from a DKIM-Signature) for which a result is
being relayed, and MUST be long enough to be unique among the results
being reported. Where the total length of the digital signature is
fewer than eight characters, the entire signature MUST be included.
Matching of the value of this item against the signature itself MUST
be case-sensitive.
If an evaluating agent observes that, despite the use of this
disambiguating tag, unequal authentication results are offered about
the same signature from the same trusted authserv-id, that agent
SHOULD ignore all such results.
5. IANA Considerations
Per [IANA-CONSIDERATIONS], the following item is added to the "Email
Authentication Method Name Registry":
+------------+----------+--------+----------------+-----------------+
| Method | Defined | ptype | property | value |
+------------+----------+--------+----------------+-----------------+
| dkim | RFC4871 | header | b | full or partial |
| | | | | value of |
| | | | | signature "b" |
| | | | | tag |
+------------+----------+--------+----------------+-----------------+
6. Security Considerations
[AUTHRES] discussed general security considerations regarding the use
of this header field. The following new security considerations
apply when adding or processing this new ptype/property combination:
6.1. Improvement
Rather than introducing a new security issue, this can be seen to fix
a security weakness of the original specification: Useful information
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can now be obtained from results that could previously have been
ambiguous and thus obscured or, worse, misinterpreted.
6.2. Result Forgeries
An attacker could copy a valid signature and add it to a message in
transit, modifying some portion of it. This could cause two results
to be provided for the same "header.b" value even if the entire "b="
string is used in an attempt to differentiate the results. This
attack could cause an ambiguous result to be relayed and possibly
neutralize any benefit given to a "pass" result that would have
otherwise occurred, possibly impacting the delivery of valid
messages.
It is worth noting, however, that a false negative ("fail") can be
generated in this way, but it is extremely difficult to create a
false positive ("pass") through such an attack. Thus, a cautious
implementation could discard the false negative in that instance.
6.3. New Schemes with Small Signatures
Should a new signing scheme be introduced with a signature whose
length is less than eight characters, Section 4 specifies that the
entire signature must be used. The obvious concern in such a case
would be that the signature scheme is itself prone to collisions,
making the the value reported by this field not useful. In such
cases, the risk is created by the likelihood of collisions and not by
this mechanism; furthermore, Section 4 recommends the results be
ignored if that were to occur, preventing the application of an
ambiguous result.
7. References
7.1. Normative References
[AUTHRES] Kucherawy, M., "Message Header Field for
Indicating Message Authentication Status",
RFC 5451, April 2009.
[DKIM] Allman, E., Callas, J., Delany, M., Libbey,
M., Fenton, J., and M. Thomas, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 4871,
May 2007.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
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7.2. Informative References
[HASH-ATTACKS] Hoffman, P. and B. Schneier, "Attacks on
Cryptographic Hashes in Internet Protocols",
RFC 4270, November 2005.
[IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 5226, May 2008.
Appendix A. Authentication-Results Examples
This section presents an example of the use of this new item header
field to indicate unambiguous authentication results.
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A.1. Multiple DKIM Signatures with One Failure
A message that had two DKIM signatures applied by the same domain,
one of which failed:
Authentication-Results: mail-router.example.net;
dkim=pass (good signature) header.d=newyork.example.com
header.b=oINEO8hg;
dkim=fail (bad signature) header.d=newyork.example.com
header.b=EToRSuvU
Received: from newyork.example.com
(newyork.example.com [192.0.2.250])
by mail-router.example.net (8.11.6/8.11.6)
for <recipient@example.net>
with ESMTP id i7PK0sH7021929;
Fri, Feb 15 2002 17:19:22 -0800
DKIM-Signature: v=1; a=rsa-sha256; s=rashani;
d=newyork.example.com;
t=1188964191; c=relaxed/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3=
DKIM-Signature: v=1; a=rsa-sha256; s=rashani;
d=newyork.example.com;
t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: meetings@example.net
Message-Id: <12345.abc@newyork.example.com>
Subject: here's a sample
Example 1: Header field reporting results from multiple signatures
added at initial signing
Here we see an example of a message that was signed twice at by the
author's ADMD. One signature used "relaxed" header canonicalization
and the other used "simple" header canonicalization; both used
"simple" body canonicalization.
Presumably due to a change in one of the five header fields covered
by the two signatures, the former signature failed to verify while
the latter passed. In particular, the "relaxed" header
canonicalization of [DKIM] is resilient to changes in whitespace in
the header while "simple" is not, and the latter is the one that
failed in this example.
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The item registered by this memo allows an evaluation module to
determine which DKIM result goes with which signature. Without the
"header.b" portion of the result, it is unclear which one passed and
which one failed.
Appendix B. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Dave Crocker, Tony Hansen,
Eliot Lear, S. Moonesamy, Alessandro Vesely.
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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