Individual submission M. Kucherawy Internet-Draft Cloudmark, Inc. Intended status: Experimental September 21, 2010 Expires: March 25, 2011 DKIM Authorized Third-Party Signers draft-kucherawy-dkim-atps-00 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 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 25, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Kucherawy Expires March 25, 2011 [Page 1] Internet-Draft DKIM ATPS Experiment September 2010 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 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6.1. Transient Security Failures . . . . . . . . . . . . . . . 8 6.2. Load on the DNS . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . . 9 Appendix A. Example Query and Reply . . . . . . . . . . . . . . . 10 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 11 Kucherawy Expires March 25, 2011 [Page 2] Internet-Draft DKIM ATPS Experiment September 2010 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 acceptable 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 may 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 [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 valid 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. A Verifier participates in this protocol if it wishes to ensure that Kucherawy Expires March 25, 2011 [Page 3] Internet-Draft DKIM ATPS Experiment September 2010 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 flag, or any other value in the tag, 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 [IANA-CONSIDERATIONS]. 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. These MAY be done in any order or in parallel. Where the RFC5322.From field contains multiple addresses, this process MAY be repeated for each domain found in the field, and these MAY be done in any order. The name for the query is constructed as follows: Kucherawy Expires March 25, 2011 [Page 4] Internet-Draft DKIM ATPS Experiment September 2010 1. Extract the value of the "d=" tag from the signature and feed it to the [MD5] hash algorithm. 2. Convert the output of the MD5 hash to a string of 32 hexadecimal digits, two for each byte in the hash output, in order. Where alphabetic characters are needed in the conversion, lowercase characters should be used. 3. Append the string "._atps." 4. Append the domain name found in the RFC5322.From field of the message. The formal syntax definition, per [ABNF]: atps-query = 32*HEXDIG %x2e.5f.61.74.70.73.2e domain-name 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. 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 Kucherawy Expires March 25, 2011 [Page 5] Internet-Draft DKIM ATPS Experiment September 2010 does not satisfy the policy advertised by the Sender, and the action(s) suggested 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. 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 contents removed 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 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] Kucherawy Expires March 25, 2011 [Page 6] Internet-Draft DKIM ATPS Experiment September 2010 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] 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 Kucherawy Expires March 25, 2011 [Page 7] Internet-Draft DKIM ATPS Experiment September 2010 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. 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. 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 two plus twice the number Kucherawy Expires March 25, 2011 [Page 8] Internet-Draft DKIM ATPS Experiment September 2010 of valid signatures on the message plus the number of invalid signatures on the message. 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 resolver, and waiting for that number of queries to return when they are issued in parallel can 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 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. [DKIM] Allman, E., Callas, J., Delany, M., Libbey, 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. [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. 7.2. Informative References [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", Kucherawy Expires March 25, 2011 [Page 9] Internet-Draft DKIM ATPS Experiment September 2010 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". 2. An MD5 hash of "one.example.net" is computed and converted to hexadecimal form. The result of this is the string "5d6794099a6ce988a5a7ec5f4c285a42". 3. A TXT query is issued to "5d6794099a6ce988a5a7ec5f4c285a42._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. An MD5 hash of "two.example.net" is computed and converted to hexadecimal form. The result of this is the string "69ccfaf20790ccf4e348eabc61ed86ad". 6. A TXT query is issued to "69ccfaf20790ccf4e348eabc61ed86ad._atps.example.com". Kucherawy Expires March 25, 2011 [Page 10] Internet-Draft DKIM ATPS Experiment September 2010 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 Kucherawy Expires March 25, 2011 [Page 11]