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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Kucherawy Expires December 19, 2010 [Page 1] Internet-Draft Auth-Results header.b Registration June 2010 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 Kucherawy Expires December 19, 2010 [Page 2] Internet-Draft Auth-Results header.b Registration June 2010 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 Kucherawy Expires December 19, 2010 [Page 3] Internet-Draft Auth-Results header.b Registration June 2010 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 Kucherawy Expires December 19, 2010 [Page 4] Internet-Draft Auth-Results header.b Registration June 2010 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. Kucherawy Expires December 19, 2010 [Page 5] Internet-Draft Auth-Results header.b Registration June 2010 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. Kucherawy Expires December 19, 2010 [Page 6] Internet-Draft Auth-Results header.b Registration June 2010 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 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. Kucherawy Expires December 19, 2010 [Page 7] Internet-Draft Auth-Results header.b Registration June 2010 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 Kucherawy Expires December 19, 2010 [Page 8]