REPUTE Working Group N. Borenstein Internet-Draft Mimecast Intended status: Informational M. Kucherawy Expires: May 17, 2013 Cloudmark A. Sullivan, Ed. Dyn, Inc. November 13, 2012 A Model for Reputation Reporting draft-ietf-repute-model-03 Abstract This document describes a general architecture for a reputation-based service and a model for the exchange of reputation information on the Internet. The document roughly follows the recommendations of RFC4101 for describing a protocol model. 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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Expires May 17, 2013 [Page 1] Internet-Draft Reputation Reporting Model November 2012 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. High-Level Architecture . . . . . . . . . . . . . . . . . . . 4 3. Terminology and Definitions . . . . . . . . . . . . . . . . . 6 3.1. Response Set . . . . . . . . . . . . . . . . . . . . . . . 6 4. Information Represented in a Response Set . . . . . . . . . . 6 5. Information Flow in the Protocol . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8.1. Biased Reputation Agents . . . . . . . . . . . . . . . . . 8 8.2. Malformed Messages . . . . . . . . . . . . . . . . . . . . 9 8.3. Further Discussion . . . . . . . . . . . . . . . . . . . . 9 9. Informative References . . . . . . . . . . . . . . . . . . . . 9 Appendix A. Public Discussion . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Borenstein, et al. Expires May 17, 2013 [Page 2] Internet-Draft Reputation Reporting Model November 2012 1. Introduction Historically, many Internet protocols have operated between unauthenticated entities. For example, an email message's author field (From) [MAIL] can contain any display name or address and is not verified by the recipient or other agents along the delivery path. Similarly, a sending email server using [SMTP] trusts that the [DNS] has led it to the intended receiving server. Both kinds of trust are easily betrayed, opening the operation to subversion of some kind, which leads to spam, phishing, and other attacks. In recent years, stronger identity mechanisms have begun to see wider deployment. For example, the [DKIM] protocol permits associating a validated identifier to a message. This association is cryptographically strong, and is an improvement over the prior state of affairs, but it does not distinguish between identifiers of good actors and bad. Even when it is possible to validate the domain name in an author field (e.g. "trustworthy.example.com" in "john.doe@trustworthy.example.com") there is no basis for knowing whether it is associated with a good actor worthy of trust. As a practical matter, both bad actors and good adopt basic authentication mechanisms like DKIM. In fact, bad actors tend to adopt them even more rapidly than the good actors do in the hope that some receivers will confuse identity authentication with identity assessment. The former merely means that the name is being used by its owner or their agent, while the latter makes a statement about the quality of the owner. The added requirement -- which can usefully be undertaken only in the presence of such stronger identity validation -- is for a mechanism by which mutually trusted parties can exchange assessment information about other actors. For these purposes, we may usefully define "reputation" as "the estimation in which an identifiable actor is held, especially by the community or the Internet public generally". We may call an aggregation of individual assessments "reputation information." While the need for reputation information has been perhaps most clear in the email world, where abuses are commonplace, other Internet services are coming under attack and may have a similar need. For instance, a reputation mechanism could be useful in rating the security of web sites; the quality of service of an Internet Service Provider (ISP) or Application Service Provider (ASP); customer satisfaction at e-commerce sites; and even things unrelated to Internet protocols, such as plumbers, hotels, or books. Just as human beings traditionally rely on the recommendations of trusted parties in the physical world, so too they can be expected to make use of such reputation information in a variety of applications on Borenstein, et al. Expires May 17, 2013 [Page 3] Internet-Draft Reputation Reporting Model November 2012 the Internet. A full trust architecture encompasses a range of actors and activities, to enable an end-to-end service for creating and consuming trust-related information. One component of that is a query mechanism, to permit retrieval of reputation information. Not all such reputation services will need to convey the same information. Some need only produce a basic rating, while others need to provide underlying detail. This is akin to the difference between check approval versus a credit report. An overall reckoning of goodness versus badness can be defined generically, but specific applications are likely to want to describe reputations for multiple attributes: an e-commerce site might be rated on price, speed of delivery, customer service, etc., and might receive very different ratings on each. Therefore, a model defines a generic query mechanism and basic format for reputation information, but allows extensions for each application. Omitted from this model is the means by which an reputation-reporting agent goes about collecting such data and the mechanism for creating an evaluation. The mechanism defined here merely enables asking a question and getting an answer; the remainder of an overall service provided by such a reputation agent is specific to the implementation of that service and is out of scope here. 2. High-Level Architecture A reputation mechanism functions as a component of a service, such as that depicted in Figure 1 of [RFC5863], reproduced here: Borenstein, et al. Expires May 17, 2013 [Page 4] Internet-Draft Reputation Reporting Model November 2012 +------+------+ +------+------+ | Author | | Recipient | +------+------+ +------+------+ | ^ | | | +------+------+ | -->| Handling |<-- | -->| Filter |<-- | +-------------+ | ^ V Responsible | +-------------+ Identifier +------+------+ | Responsible |. . . . . . . . . . .>| Identity | | Identity | . . | Assessor | +------+------+ . . +-------------+ | V . ^ ^ V . . | | +------------------.-------.------------------+ | | | +------+------+ . . . > . +-------------+ | | | +------------+ | | Identifier | | Identifier +-|-+ +-+ Assessment | | | Signer +------------>| Validator | | | Databases | | +-------------+ +-------------+ | +------------+ | DKIM Service | +---------------------------------------------+ Figure 1: RFC5683 'Actors in a Trust Sequence Using DKIM' Here, the reputation mechanism is shown only as a query by an Identity Assessor, made to Assessment Databases. This memo outlines the query and response mechanism. It provides the following definitions: o Vocabulary for the current work and work of this type; o The types and content of queries that can be supported; o The extensible range of response information that can be provided; o A query/response protocol; o Query/response transport conventions. It provides an extremely simple query/response model that can be carried over a variety of transports, including the Domain Name System. (Although not typically thought of as a 'transport', the DNS provides generic capabilities and can be thought of as a mechanism for transporting queries and responses that have nothing to do with Borenstein, et al. Expires May 17, 2013 [Page 5] Internet-Draft Reputation Reporting Model November 2012 addresses.) Each specification for Repute transport is independent of any other specification. A diagram of the basic query service is found in Figure 2. +-----------+ Query +----------+ | +. . . . . . . . . . . . . .>| | | Client | | Server | | <. . . . . . . . . . . . . . + | +-----+-----+ Response +-------+--+ | ^ | V | | +------+----+ +-----------+ | | Response | Transport |--------------->| Transport |--+ | Set +-----------+ DNS +-----------+ | TCP V UDP +------------+ ... | Reputation | | Database | +------------+ Figure 2: Basic Reputation Query Service Both the query and response are application-specific. An application of the model defines the parameters available to queries of that type, and also defines the data returned in response to any query. 3. Terminology and Definitions This section defines terms used in the rest of the document. 3.1. Response Set A "Response Set" comprises those data that are returned in response to a reputation query about a particular entity. The types of data are specific to an application; the data returned in the evaluation of email senders would be different than the reputation data returned about a movie or a baseball player. Response Sets have symbolic names, and these have to be registered with IANA to prevent name collisions. IANA registries are created in a separate memo. Each definition of a Response Set needs to define its registry entry. 4. Information Represented in a Response Set The basic information to be represented in the protocol is fairly Borenstein, et al. Expires May 17, 2013 [Page 6] Internet-Draft Reputation Reporting Model November 2012 simple, and includes the following: o the identity of the entity providing the reputation information; o the identity of the entity being rated; o the overall rating score for that entity; o the level of confidence in the accuracy of that rating; and o the number of data points underlying that score. Beyond this, arbitrary amounts of additional information might be provided for specific uses of the service. The entire collection is the Response Set for that application. The query/response protocol defines a syntax for representing such Response Sets, but each application defines its own Response Set. Thus, the basic information also includes the name of the application for which the reputation data is being expressed. Each application requires its own specification of the Response Set. For example, a specification might be needed for a reputation Response Set for an "email-sending-domain"; the Response Set might include information on how often spam was received from that domain. Additional documents define a [MIME] type for reputation data, and protocols for exchanging such data. 5. Information Flow in the Protocol The basic Response Set could be wrapped into a new MIME media type [MIME] or a DNS RR, and transported accordingly. It also could be the integral payload of a purpose-built protocol. For a basic request/response scenario, one entity (the Client) will ask a second entity (the Server) for reputation data about a third entity (the Target), and the second entity will respond with that data. An applications might benefit from an extremely lightweight mechanism, supporting constrained queries and responses, while others might need to support larger and more complex responses. 6. IANA Considerations This memo presents no actions for IANA. Borenstein, et al. Expires May 17, 2013 [Page 7] Internet-Draft Reputation Reporting Model November 2012 7. Privacy Considerations Some kinds of reputation data are sensitive, and should not be shared publicly. For applications that have such sensitivity, it is imperative to pick a transport that will provide the required authentication and authorization mechanisms in order to secure communication and deliver responses correctly according to the proferred credentials. Such transport questions are the province of the application definitions. 8. Security Considerations This memo introduces an overall protocol model, but no implementation details. As such, the security considerations presented here are very high-level. The detailed analyses of the various specific components of the protocol can be found the documents that instantiate this model. 8.1. Biased Reputation Agents As with [VBR], an agent seeking to make use of a reputation reporting service is placing some trust that the service presents an unbiased "opinion" of the object about which reputation is being returned. The result of trusting the data is, presumably, to guide action taken by the reputation client. It follows, then, that bias in the reputation service can adversely affect the client. Clients therefore need to be aware of this possibility and the effect it might have. For example, a biased system returning reputation information about a DNS domain found in email messages could result in the admission of spam, phishing or malware through a mail gateway (by rating the domain name more favourably than warranted) or could result in the needless rejection or delay of mail (by rating the domain more unfavourably than warranted). As a possible mitigation strategy, clients might seek to interact only with reputation services that offer some disclosure of the computation methods for the results they return. Such disclosure and evaluation is beyond the scope of the present memo. Similarly, a client placing trust in the results returned by such a service might suffer if the service itself is compromised, returning biased results under the control of an attacker without the knowledge of the agency providing the reputation service. This might result from an attack on the data being returned at the source, or from a man-in-the-middle attack. Protocols, therefore, need to be designed so as to be as resilient against such attacks as possible. Borenstein, et al. Expires May 17, 2013 [Page 8] Internet-Draft Reputation Reporting Model November 2012 8.2. Malformed Messages Both clients and servers of reputation systems need to be resistant to attacks that involve malformed messages, deliberate or otherwise. Malformations can be used to confound clients and servers alike in terms of identifying the party or parties responsible for the content under evaluation. This can result in delivery of undesirable or even dangerous content. 8.3. Further Discussion Numerous other topics related to use and management of reputation systems can be found in [I-D.REPUTE-CONSIDERATIONS]. 9. Informative References [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. [I-D.REPUTE-CONSIDERATIONS] Kucherawy, M., "Operational Considerations Regarding Reputation Services", draft-ietf-repute-considerations (work in progress), November 2012. [MAIL] Resnick, P., "Internet Message Format", RFC 5322, October 2008. [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, "DomainKeys Identified Mail (DKIM) Development, Deployment, and Operations", RFC 5863, May 2010. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009. Borenstein, et al. Expires May 17, 2013 [Page 9] Internet-Draft Reputation Reporting Model November 2012 Appendix A. Public Discussion Public discussion of this suite of memos takes place on the domainrep@ietf.org mailing list. See https://www.ietf.org/mailman/listinfo/domainrep. Authors' Addresses Nathaniel Borenstein Mimecast 203 Crescent St., Suite 303 Waltham, MA 02453 USA Phone: +1 781 996 5340 Email: nsb@guppylake.com Murray S. Kucherawy Cloudmark 128 King St., 2nd Floor San Francisco, CA 94107 USA Email: superuser@gmail.com Andrew Sullivan (editor) Dyn, Inc. 150 Dow St. Manchester, NH 03101 USA Email: asullivan@dyn.com Borenstein, et al. Expires May 17, 2013 [Page 10]