Individual submission M. Kucherawy Internet-Draft Cloudmark, Inc. Intended status: Experimental December 28, 2011 Expires: June 30, 2012 DKIM Authorized Third-Party Signers draft-kucherawy-dkim-atps-12 Abstract This experimental specification proposes a modification to Domain Keys Identified Mail (DKIM) allowing advertisement of third-party signature authorizations that are to be interpreted as equivalent to a signature added by the administrative domain of the message's author. 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 June 30, 2012. Copyright Notice Copyright (c) 2011 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 June 30, 2012 [Page 1] Internet-Draft DKIM ATPS Experiment December 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. E-Mail Architecture Terminology . . . . . . . . . . . . . 3 3. Roles and Scope . . . . . . . . . . . . . . . . . . . . . . . 3 4. Queries and Replies . . . . . . . . . . . . . . . . . . . . . 4 4.1. Extension to DKIM . . . . . . . . . . . . . . . . . . . . 4 4.2. ATPS Query Details . . . . . . . . . . . . . . . . . . . . 5 4.3. ATPS Reply Details . . . . . . . . . . . . . . . . . . . . 6 5. Interpretation . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Relationship to ADSP . . . . . . . . . . . . . . . . . . . . . 7 7. Experiment Process . . . . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8.1. ATPS Tag Registry . . . . . . . . . . . . . . . . . . . . 8 8.2. Email Authentication Method Name Registry Update . . . . . 9 8.3. Email Authentication Result Name Registry Update . . . . . 9 8.4. DKIM-Signature Tag Specification Registry . . . . . . . . 10 9. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9.1. False Privacy . . . . . . . . . . . . . . . . . . . . . . 11 9.2. Transient Security Failures . . . . . . . . . . . . . . . 11 9.3. Load on the DNS . . . . . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10.1. Normative References . . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . . 12 Appendix A. Example Query and Reply . . . . . . . . . . . . . . . 13 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 13 Kucherawy Expires June 30, 2012 [Page 2] Internet-Draft DKIM ATPS Experiment December 2011 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. DKIM, however, deliberately makes no binding between the DNS domain of the signer and any other identity found in the message. Despite this, there is an automatic human perception that an author domain signature (one for which the RFC5322.From domain matches the DNS domain of the signer) is more valuable or trustworthy than any other. To enable a third party to apply DKIM signatures to messages, the DKIM specification suggests either delegation of subdomains or private keys to third parties so that the third party can add DKIM signatures that appear to have been added by the Author ADMD. Absent is a protocol by which an Author ADMD can announce that specific DKIM signatures on its mail, which are added by other ADMDs, are to be treated the same as a signature by the Author ADMD itself. This memo presents an experimental mechanism for doing so. ATPS augments the semantics of DKIM by selecting multiple identifiers rather than one. Specifically, it validates the identifier found in the DKIM signature, and then provides the RFC5322.From domain for evaluation. 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 are advised to be familiar with the material and terminology discussed in [MAIL] and [EMAIL-ARCH]. 3. Roles and Scope The context of this protocol involves the following roles: o ADministrative Mail Domain (ADMD)s, whose DNS domain name(s) appear in the RFC5322.From field of a [MAIL] message; Kucherawy Expires June 30, 2012 [Page 3] Internet-Draft DKIM ATPS Experiment December 2011 o Signers, which apply [DKIM] signatures using their own domains, but on behalf of the message Author's ADMD; and o the Verifier, who implements the signature validation procedures described in [DKIM]. An ADMD implements in this protocol if it wishes to announce that a message from it (in the RFC5322.From sense) is to be considered authorized as long as it bears a signature from any in a set of specified DNS 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 transfer described in Appendix B.1.1 of [DKIM]. This is communicated to Verifiers via a new tag in the DKIM signature. A Verifier implements in this protocol if it wishes to ensure that a message bears one or more signatures from sources authorized to sign mail on behalf of the ADMD, and identify for special treatment mail that meets (or does not meet) that criterion. It will do so by treating the signer's authorization on behalf of the author's ADMD to mean that the signer's signature is equivalent to one affixed by the author's ADMD. 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 DKIM [DKIM] signatures contain a "tag=value" sequence. This protocol will add additional tags called "atps" and "atpsh". When the Signer generates a DKIM signature on behalf of an ADMD other than its own, it MUST include an "atps" tag in the signature and include as its value the Author's domain name. For creating records in the DNS, a hash algorithm needs to be selected. The Signer and the ADMD have to agree on which hash is to be used, since the ADMD places a corresponding record in its DNS and the Signer will have to indicate in its generated signatures which hash algorithm the Author ADMD is using. The selected hash algorithm MUST be one registered with IANA as valid for use with DKIM (see Section 7.7 of [DKIM]). The tag name that carries the hash name used is "atpsh". If absent, the Verifier MUST assume "sha1". The formal syntax definition, per [ABNF]: Kucherawy Expires June 30, 2012 [Page 4] Internet-Draft DKIM ATPS Experiment December 2011 dkim-atps-tag = %x61.74.70.73 *WSP "=" *WSP domain-name dkim-atpsh-tag = %x61.74.70.73.68 *WSP "=" *WSP key-h-tag-alg "domain-name" and "key-h-tag-alg" are defined in [DKIM]. The registration for these tags can be found in Section 8. 4.2. ATPS Query Details When a [DKIM] signature including an "atps" tag is successfully verified, and is considered acceptable to the Verifier according to any local policy requirements (which are not discussed here or in [DKIM]), the Verifier compares the domain name in the value of that tag with the one found in the RFC5322.From field of the message. The match MUST be done in a case-insensitive manner. If they do not match, the "atps" tag MUST be ignored. If they do match, the Verifier issues a DNS TXT query, as specified below, looking for confirmation by the Author ADMD that the Signer is authorized by that ADMD to sign mail on its behalf. Where multiple DKIM signatures are present including valid "atps" tags, these queries MAY be done in any order or MAY be done in parallel. Where the RFC5322.From field contains multiple addresses, this process SHOULD be applied if the "atps" tag's value matches any of the domains found in that field. These MAY be done in any order. Note that the algorithm uses hashing, but this is not a security mechanism. See Section 9.1 for discussion. The name for the query is constructed as follows: 1. Select the hash algorithm from the "atpsh" tag in the signature. If none, the default is "sha1". If one is specified but does not appear in the list registered with IANA as one valid for use with DKIM (see Section 7.7 of [DKIM]), abort the query. 2. Extract the value of the "d=" tag from the signature. 3. Convert any upper-case characters in that string to their lower- case equivalents. 4. Feed the resulting string to the selected hash algorithm. 5. Convert the output of the hash to a string of printable ASCII characters by applying base32 encoding as defined in Section 6 of Kucherawy Expires June 30, 2012 [Page 5] Internet-Draft DKIM ATPS Experiment December 2011 [BASE32]. The base32 encoding is used because its output is restricted to characters that are legal for use in labels in the DNS, and evaluates the same way in the DNS whether encoded using uppercase or lowercase characters. 6. Append the string "._atps." 7. Append the domain name found in the "atps" tag of the validated signature. The query's formal syntax definition, per [ABNF]: atps-query = 1*BASE32 %x2e.5f.61.74.70.73.2e domain-name BASE32 = ( ALPHA / %x32-37 ) See Appendix A for an example of a query construction. 4.3. ATPS Reply Details In the descriptions below, the label NOERROR symbolizes DNS response code ("rcode") 0, and NXDOMAIN represents rcode 3. See Section 4.1.1 of [DNS] for further 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 with at least one answer) containing a valid ATPS reply. In this case, the protocol has been satisfied and the Verifier can conclude that the signing domain is authorized by the ADMD to sign its mail. Further queries SHOULD NOT be initiated. o No answer is returned (i.e. [DNS] reply code NXDOMAIN, or NOERROR with no answers), or one or more answers have been returned as described above but none contain a valid ATPS reply. In this case, the Signer has not been authorized to act as a third-party signer for this ADMD, 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 ADMD'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 either NXDOMAIN or NOERROR with no answers, then the Signer was not authorized by the ADMD. Kucherawy Expires June 30, 2012 [Page 6] Internet-Draft DKIM ATPS Experiment December 2011 A valid ATPS reply consists of a sequence of tag-value pairs as described in Section 3.2 of [DKIM]. The following tag and value is the only one currently supported in ATPS records: v: Version (plain-text; REQUIRED) This tag defines the version of this specification that applies to the ATPS record. It MUST have the value "ATPS1". ABNF: atps-v-tag = %x76 [FWS] "=" [FWS] %x41.54.50.53.31 ; FWS is defined in [DKIM] 5. Interpretation For each DKIM signature that verifies (see Section 6 of [DKIM], if a Verifier succeeds in confirming that the Author's ADMD authorized the Signer using this protocol, then the Verifier SHOULD evaluate the message as though the Signer is the Author's ADMD. It MAY also independently evaluate the Signer when determining message disposition. This assertion is based on the fact that the ADMD explicitly endorsed the Signer. Therefore, a module assessing reputation that is based on DKIM signature verification SHOULD apply the reputation of the Author's ADMD domain instead of, or in addition to, that of the Signer domain. 6. Relationship to ADSP [ADSP] defined a protocol by which the owner of an Author Domain can advertise a request to message receivers that messages bearing no valid author signature be treated with suspicion or even discarded. A Verifier implementing both ADSP and ATPS SHOULD treat a message for which the ATPS test described above passes as if the message had a valid Author Domain Signature. That is, a pass of ATPS means a pass for ADSP. 7. Experiment Process The working group that developed DKIM considered a third-party mechanism such as this one to be controversial, in terms of need and practicality, and decided that an alternative mechanism was suffient. However, this was not based on actual experience as there is no specific history on this question. Thus, this experiment was devised. The experimental protocol described here has been implemented as an extension to DKIM in two software products, one of which is open Kucherawy Expires June 30, 2012 [Page 7] Internet-Draft DKIM ATPS Experiment December 2011 source and seeing increasingly wide use. It is included there to allow customers of those systems to make use of it if they believe such third party assertions are useful to the overall DKIM mechanism. Further adoption as part of the experiment is welcome and encouraged. Use of the protocol and anecdotes of how it affects the overall DKIM experience will be collected by those implementers and the author of this memo. Those participating in the experiment are also advised to observe and report the impact of what is discussed in Section 9.3, especially with respect to MTA latency that may be introduced. If the response is substantial and positive, advancement along the Standards Track might be warranted. 8. IANA Considerations Section 8.2, Section 8.3, and Section 8.4, below, specify requested IANA actions. Section 8.1 needs no IANA action at this time. 8.1. ATPS Tag Registry NOTE: NO IANA ACTION IS REQUIRED BY THIS SECTION AT THIS TIME. [RFC EDITOR: Please remove this paragraph before publication.] If this specification is ever moved to the Standards Track, IANA will then be asked to create an Authorized Third Party Signature (ATPS) Tag Registry to enumerate the tags that are valid for use in ATPS records. This section documents that future registry in advance. New registrations or updates MUST be published in accordance with the "Specification Required" guidelines described in [IANA]. Such registry changes MUST contain the following information: 1. Name of the tag being registered or updated 2. The document where the specification is created or updated 3. The status of the tag, one of "current" (tag is in current use), "deprecated" (tag is in current use but its use is discouraged), or "historic" (tag is no longer in use) The registry's sole initial entry is: +-----+--------------+---------+ | Tag | Specified In | Status | +-----+--------------+---------+ | v | [this memo] | current | +-----+--------------+---------+ Kucherawy Expires June 30, 2012 [Page 8] Internet-Draft DKIM ATPS Experiment December 2011 8.2. Email Authentication Method Name Registry Update The following is to be added to the Email Authentication Methods Registry (in the Email Authentication Parameters group) established by [AUTHRES] as per [IANA]: Method: dkim-atps Defined In: [THIS MEMO] ptype: header property: from value: contents of the [MAIL] From: header field, with comments removed 8.3. Email Authentication Result Name Registry Update The following are to be added to the Email Authentication Result Names Registry (in the Email Authentication Parameters group) established by [AUTHRES] as per [IANA]: Code: none Existing/New Code: existing Defined In: [AUTHRES] Auth Method: dkim-atps Meaning: No valid DKIM signatures were found on the message bearing "atps" tags. 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. Kucherawy Expires June 30, 2012 [Page 9] Internet-Draft DKIM ATPS Experiment December 2011 Code: fail Existing/New Code: existing Defined In: [AUTHRES] Auth Method: dkim-atps Meaning: All valid DKIM signatures bearing an "atps" tag either did not reference a domain name found in the RFC5322.From field, or the ATPS test(s) performed failed to confirm a third-party authorization. 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 might 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. 8.4. DKIM-Signature Tag Specification Registry The following are to be added to the DKIM-Signature Tag Speficication Registry (in the DomainKeys Identifie Mail (DKIM) Parameters group) established by [DKIM] as per [IANA]: Kucherawy Expires June 30, 2012 [Page 10] Internet-Draft DKIM ATPS Experiment December 2011 +-------+-------------+---------+ | TYPE | REFERENCE | STATUS | +-------+-------------+---------+ | atps | [THIS MEMO] | current | +-------+-------------+---------+ | atpsh | [THIS MEMO] | current | +-------+-------------+---------+ 9. Security Considerations This section discusses potential security issues related to this experimental protocol. 9.1. False Privacy The fact that the authorized third-party domain name is hashed and then encoded with base32 might give some the false sense that the relationship between the two parties is somehow protected. This is not the case. Indeed, the very purpose of this protocol is to make it possible for such relationships to be discovered, so such an obscuration would make that process more difficult without a shared secret delivered out-of-band to message verifiers (which also adds further complexity. Rather, the hash and encode steps are done merely to convert any third-party domain name to a fixed width in the construction of the DNS query. 9.2. Transient Security Failures Approving a third party signer exposes the ADMD to the risk that the third party signer becomes compromised and then begins to sign malicious or nuisance messages on behalf of the ADMD. This can obviously reflect negatively on the ADMD, and the impact of this can become more severe as automated domain reputation systems are developed and deployed. Thorough vetting and monitoring practices by ADMDs of third party signers will likely need to become the norm. 9.3. 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 as many as one (for the ADSP query) plus the number of signatures on the message (one for each key that is to be verified) plus the number of signatures that validated which also bear an "atps" tag. This is in addition to any PTR and A queries the MTA might 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. Kucherawy Expires June 30, 2012 [Page 11] Internet-Draft DKIM ATPS Experiment December 2011 An alternative to this that has not yet been explored is the storage of the ATPS data at a specific URL tied to the Author's domain name. This would alleviate pressure on the DNS at the expense of requiring the ADMD 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. See Section 8.5 of [DKIM] for further discussion of DNS-related issues. 10. References 10.1. 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] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, September 2011. [DNS] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, October 2008. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. 10.2. 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. [IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing Kucherawy Expires June 30, 2012 [Page 12] Internet-Draft DKIM ATPS Experiment December 2011 an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 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, from "one.example.net" and from "two.example.net", each with an "atps" tag whose value is "example.com", and no "atpsh" tag is present in either. The following actions are taken: 1. A SHA1 hash of "one.example.net" is computed and then converted to printable form using base32 encoding, resulting in the string "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6". 2. A TXT query is issued to "QSP4I4D24CRHOPDZ3O3ZIU2KSGS3X6Z6._atps.example.com". 3. If a valid 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. 4. A SHA1 hash of "two.example.net" is computed and then converted to printable form using base32 encoding, resulting in the string "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX". 5. A TXT query is issued to "ZTZGRRV3F45A4U6HLDKBF3ZCOW4V2AJX._atps.example.com". 6. If a valid 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. 7. As there are no valid signatures left to test, the algorithm stops with an "unknown" result. Appendix B. Acknowledgements The author wishes to acknowledge Dave Crocker, Frank Ellermann, Mark Martinec and Phil Pennock for their review and constructive criticism of this proposal. Kucherawy Expires June 30, 2012 [Page 13] Internet-Draft DKIM ATPS Experiment December 2011 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 June 30, 2012 [Page 14]