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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 5751 (ref. 'SMIME') (Obsoleted by RFC 8551) -- Obsolete informational reference (is this intentional?): RFC 5246 (ref. 'TLS') (Obsoleted by RFC 8446) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 REPUTE Working Group N. Borenstein 3 Internet-Draft Mimecast 4 Intended status: Informational M. Kucherawy 5 Expires: January 13, 2014 6 A. Sullivan, Ed. 7 Dyn, Inc. 8 July 12, 2013 10 A Model for Reputation Reporting 11 draft-ietf-repute-model-07 13 Abstract 15 This document describes a general architecture for a reputation-based 16 service and a model for requesting reputation-related data over the 17 Internet, where "reputation" refers to predictions or expectations 18 about an entity or an identifier such as a domain name. The document 19 roughly follows the recommendations of RFC4101 for describing a 20 protocol model. 22 Status of this Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on January 13, 2014. 39 Copyright Notice 41 Copyright (c) 2013 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 3. High-Level Architecture . . . . . . . . . . . . . . . . . . . 5 59 4. Terminology and Definitions . . . . . . . . . . . . . . . . . 7 60 4.1. Response Set . . . . . . . . . . . . . . . . . . . . . . . 7 61 4.2. Reputon . . . . . . . . . . . . . . . . . . . . . . . . . 8 62 5. Information Represented in a Response Set . . . . . . . . . . 8 63 6. Information Flow in the Reputation Query Protocol . . . . . . 8 64 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 65 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 9 66 8.1. Data In Transit . . . . . . . . . . . . . . . . . . . . . 9 67 8.2. Collection Of Data . . . . . . . . . . . . . . . . . . . . 9 68 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 69 9.1. Biased Reputation Agents . . . . . . . . . . . . . . . . . 10 70 9.2. Malformed Messages . . . . . . . . . . . . . . . . . . . . 10 71 9.3. Further Discussion . . . . . . . . . . . . . . . . . . . . 10 72 10. Informative References . . . . . . . . . . . . . . . . . . . . 10 73 Appendix A. Public Discussion . . . . . . . . . . . . . . . . . . 11 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 76 1. Introduction 78 Historically, many Internet protocols have operated between 79 unauthenticated entities. For example, an email message's author 80 field (From) [MAIL] can contain any display name or address and is 81 not verified by the recipient or other agents along the delivery 82 path. Similarly, a sending email server using [SMTP] trusts that the 83 [DNS] has led it to the intended receiving server. Both kinds of 84 trust are easily betrayed, opening the operation to subversion of 85 some kind, which leads to spam, phishing, and other attacks. 87 In recent years, explicit identity authentication mechanisms have 88 begun to see wider deployment. For example, the [DKIM] protocol 89 permits associating a validated identifier to a message. This 90 association is cryptographically strong, and is an improvement over 91 the prior state of affairs, but it does not distinguish between 92 identifiers of good actors and bad. Even when it is possible to 93 validate the domain name in an author field (e.g. 94 "trustworthy.example.com" in "john.doe@trustworthy.example.com") 95 there is no basis for knowing whether it is associated with a good 96 actor worthy of trust. As a practical matter, both bad actors and 97 good adopt basic authentication mechanisms like DKIM. In fact, bad 98 actors tend to adopt them even more rapidly than the good actors do 99 in the hope that some receivers will confuse identity authentication 100 with identity assessment. The former merely means that the name is 101 being used by its owner or their agent, while the latter makes a 102 statement about the quality of the owner. 104 With the advent of these authentication protocols, it is possible to 105 statisfy the requirement for a mechanism by which mutually trusted 106 parties can exchange assessment information about other actors. For 107 these purposes, we may usefully define "reputation" as "the 108 estimation in which an identifiable actor is held, especially by the 109 community or the Internet public generally". We may call an 110 aggregation of individual assessments "reputation input." 112 While the need for reputation services has been perhaps especially 113 clear in the email world, where abuses are commonplace, other 114 Internet services are coming under attack and may have a similar 115 need. For instance, a reputation mechanism could be useful in rating 116 the security of web sites, the quality of service of an Internet 117 Service Provider (ISP), or an Application Service Provider (ASP). 118 More generally, there are many different opportunities for use of 119 reputation services, such as customer satisfaction at e-commerce 120 sites, and even things unrelated to Internet protocols, such as 121 plumbers, hotels, or books. Just as human beings traditionally rely 122 on the recommendations of trusted parties in the physical world, so 123 too they can be expected to make use of such reputation services in a 124 variety of applications on the Internet. 126 A full trust architecture encompasses a range of actors and 127 activities, to enable an end-to-end service for creating, exchanging, 128 and consuming trust-related information. One component of that is a 129 query mechanism, to permit retrieval of a reputation. Not all such 130 reputation services will need to convey the same information. Some 131 need only produce a basic rating, while others need to provide 132 underlying detail. This is akin to the difference between check 133 approval versus a credit report. 135 An overall reckoning of goodness versus badness can be defined 136 generically, but specific applications are likely to want to describe 137 reputations for multiple attributes: an e-commerce site might be 138 rated on price, speed of delivery, customer service, etc., and might 139 receive very different ratings on each. Therefore, the model defines 140 a generic query mechanism and basic format for reputation retrieval, 141 but allows extensions for each application. 143 Omitted from this model is the means by which a reputation-reporting 144 agent goes about collecting such data and the method for creating an 145 evaluation. The mechanism defined here merely enables asking a 146 question and getting an answer; the remainder of an overall service 147 provided by such a reputation agent is specific to the implementation 148 of that service and is out of scope here. 150 2. Overview 152 The basic premise of this reputation system involves a client that is 153 seeking to evaluate content based on an identifier associated with 154 the content, and a reputation service provider that collects, 155 aggregates, and makes available for consumption, scores based on the 156 collected data. Typically client and service operators enter into 157 some kind of agreement during which some parameters are exchanged 158 such as the location at which the reputation service can be reached, 159 the nature of the reputation data being offered, possibly some client 160 authentication details, and the like. 162 Upon receipt of some content the client operator wishes to evaluate 163 (an Internet message, for example), the client extracts from the 164 content one or more identifiers of interest to be evaluated. 165 Examples of this include the domain name found in the From: field of 166 a message, or the domain name extracted from a valid DomainKeys 167 Identified Mail (DKIM) signature. 169 Next, the goal is to ask the reputation service provider what the 170 reputation of the extracted identifier is. The query will contain 171 the identifier to be evaluated and possibly some context-specific 172 information (such as to establish the context of the query, e.g., an 173 email message) or client-specific information. The client typically 174 folds the data in the response into whatever local evaluation logic 175 it applies to decide what disposition the content deserves. 177 3. High-Level Architecture 179 A reputation mechanism functions as a component of an overall 180 service. A current example is that of an email system that uses 181 DomainKeys Identified Mail (DKIM; see [DKIM]) to affix a stable 182 identifier to a message and then uses that as a basis for evaluation: 184 +-------------+ +------------+ 185 | Author | | Recipient | 186 +-------------+ +------------+ 187 | ^ 188 V | 189 +-------------+ +------------+ 190 | MSA | | MDA | 191 +-------------+ +------------+ 192 | ^ 193 | | 194 | +------------+ 195 | | Handling | 196 | | Filter | 197 | +------------+ 198 | ^ 199 | | 200 | +------------+ +------------+ 201 | | Reputation |<=====>| Identifier | 202 | | Service | | Assessor | 203 | +------------+ +------------+ 204 | ^ 205 V | 206 +----------------------------------------------------------+ 207 | +------------+ Responsible Identifier +------------+ | 208 | | Identifier |. . . . . . . . . . . . . .>| Identifier | | 209 | | Signer | | Verifier | | 210 | +------------+ DKIM Service +------------+ | 211 +----------------------------------------------------------+ 212 | ^ 213 V | 214 +-------------+ /~~~~~~~~~~\ +------+-----+ 215 | MTA |----->( other MTAs )------>| MTA | 216 +-------------+ \~~~~~~~~~~/ +------------+ 217 Figure 1: Actors in a Trust Sequence Using DKIM 219 (See [EMAIL-ARCH] for a general description of the Internet messaging 220 architecture.) In this figure, the solid lines indicate the flow of 221 a message; the dotted line indicates transfer of validated 222 identifiers within the message content; and the double line shows the 223 query and response of the reputation information. 225 Here, the DKIM Service provides one or more stable identifiers that 226 is the basis for the reputation query. On receipt of a message from 227 an MTA, the DKIM Service provides a (possibly empty) set of validated 228 identifiers -- domain names, in this case -- which are the subjects 229 of reputation queries made by the Identity Assessor. The Identity 230 Assessor queries a Reputation Service to determine the reputation of 231 the provided identifiers, and delivers the identifiers and their 232 reputations to the Handling Filter. The Handling Filter makes a 233 decision about whether and how to deliver the message to the 234 recipient based on these and other inputs about the message, possibly 235 including evaluation mechansisms in addition to DKIM. 237 This document outlines the reputation query and response mechanism. 238 It provides the following definitions: 240 o Vocabulary for the current work and work of this type; 242 o The types and content of queries that can be supported; 244 o The extensible range of response information that can be provided; 246 o A query/response protocol; 248 o Query/response transport conventions. 250 It provides an extremely simple query/response model that can be 251 carried over a variety of transports, including the Domain Name 252 System. (Although not typically thought of as a 'transport', the DNS 253 provides generic capabilities and can be thought of as a mechanism 254 for transporting queries and responses that have nothing to do with 255 Internet addresses, such as is one with a DNS BlockList [DNSBL].) 256 Each specification for Repute transport is independent of any other 257 specification. A diagram of the basic query service is found in 258 Figure 2. 260 + . . . . . . . . . . . . . . . . . . . . . . . . . . . . + 261 . Reputation Service . 262 . +------------+ . 263 . | Reputation | . 264 . | Database | . 265 . +------------+ . 266 . | . 267 . V . 268 . +-----------+ Query +----------+ . 269 . | |. . . . . . . . . . . . . .>| | . 270 . | Client | | Server | . 271 . | |< . . . . . . . . . . . . . | | . 272 . +-----+-----+ Response +----------+ . 273 . ^ ^ . 274 + . . . | . . . . . . . . . . . . . . . . . . . | . . . . + 275 V | 276 +-----------+ +-----------+ | 277 | Transport |<-------------->| Transport |<---+ 278 +-----------+ DNS +-----------+ 279 TCP 280 UDP 281 ... 283 Figure 2: Basic Reputation Query Service 285 The precise syntaxes of both the query and response are application- 286 specific. An application of the model defines the parameters 287 available to queries of that type, and also defines the data returned 288 in response to any query. 290 4. Terminology and Definitions 292 This section defines terms used in the rest of the document. 294 4.1. Response Set 296 A "Response Set" comprises those data that are returned in response 297 to a reputation query about a particular entity. The types of data 298 are specific to an application; the data returned in the evaluation 299 of email senders would be different than the reputation data returned 300 about a movie or a baseball player. 302 Response Sets have symbolic names, and these have to be registered 303 with IANA, in the Reputation Applications Registry, to prevent name 304 collisions. IANA registries are created in a separate document. 305 Each definition of a Response Set also needs to define its registry 306 entry. 308 4.2. Reputon 310 A "reputon" is an object that comprises the basic response to a 311 reputation query. It contains the response set relevant to the 312 subject of the query. Its specific encoding is left to documents 313 that implement this model. 315 5. Information Represented in a Response Set 317 The basic information to be represented in the protocol is fairly 318 simple, and includes the following: 320 o the identity of the entity providing the reputation information; 322 o the identity of the entity being rated; 324 o the application context for the query (e.g., email address 325 evaluation); 327 o the overall rating score for that entity; 329 o the level of confidence in the accuracy of that rating; and 331 o the number of data points underlying that score. 333 Beyond this, arbitrary amounts of additional information might be 334 provided for specific uses of the service. The entire collection is 335 the Response Set for that application. The query/response protocol 336 defines a syntax for representing such Response Sets, but each 337 application defines its own Response Set. Thus, the basic information 338 also includes the name of the application for which the reputation 339 data is being expressed. 341 Each application requires its own specification of the Response Set. 342 For example, a specification might be needed for a reputation 343 Response Set for an "email-sending-domain"; the Response Set might 344 include information on how often spam was received from that domain. 345 Additional documents define a [MIME] type for reputation data, and 346 protocols for exchanging such data. 348 6. Information Flow in the Reputation Query Protocol 350 The basic Response Set could be wrapped into a new MIME media type 351 [MIME] or a DNS RR, and transported accordingly. It also could be 352 the integral payload of a purpose-built protocol. For a basic 353 request/response scenario, one entity (the Client) will ask a second 354 entity (the Server) for reputation data about a third entity (the 355 Target), and the second entity will respond with that data. 357 An application might benefit from an extremely lightweight mechanism, 358 supporting constrained queries and responses, while others might need 359 to support larger and more complex responses. 361 7. IANA Considerations 363 This document presents no actions for IANA. 365 [RFC Editor: Please remove this section prior to publication.] 367 8. Privacy Considerations 369 8.1. Data In Transit 371 Some kinds of reputation data are sensitive, and should not be shared 372 publicly. For cases that have such sensitivity, it is imperative to 373 protect the information from unauthorized access and viewing. The 374 model described here neither suggests nor precludes any particular 375 transport mechanism for the data. However, for the purpose of 376 illustration, a reputation service that operates over HTTP might 377 employ any of its well-known mechanisms to solve these problems, 378 which include OpenPGP [OPENPGP], Transport Layer Security [TLS], and 379 S/MIME [SMIME]. 381 8.2. Collection Of Data 383 The basic notion of collection and storage of reputation data is 384 obviously a privacy issue in that the opinions of one party about 385 another are likely to be sensitive. Inadvertent or unauthorized 386 exposure of those data can lead to personal or commercial damage. 388 9. Security Considerations 390 This document introduces an overall protocol model, but no 391 implementation details. As such, the security considerations 392 presented here are very high-level. The detailed analyses of the 393 various specific components of the protocol can be found the 394 documents that instantiate this model. 396 9.1. Biased Reputation Agents 398 As with [VBR], an agent seeking to make use of a reputation reporting 399 service is placing some trust that the service presents an unbiased 400 "opinion" of the object about which reputation is being returned. 401 The result of trusting the data is, presumably, to guide action taken 402 by the reputation client. It follows, then, that bias in the 403 reputation service can adversely affect the client. Clients 404 therefore need to be aware of this possibility and the effect it 405 might have. For example, a biased system returning a reputation 406 about a DNS domain found in email messages could result in the 407 admission of spam, phishing or malware through a mail gateway (by 408 rating the domain name more favourably than warranted) or could 409 result in the needless rejection or delay of mail (by rating the 410 domain more unfavourably than warranted). As a possible mitigation 411 strategy, clients might seek to interact only with reputation 412 services that offer some disclosure of the computation methods for 413 the results they return. Such disclosure and evaluation is beyond 414 the scope of the present document. 416 Similarly, a client placing trust in the results returned by such a 417 service might suffer if the service itself is compromised, returning 418 biased results under the control of an attacker without the knowledge 419 of the agency providing the reputation service. This might result 420 from an attack on the data being returned at the source, or from a 421 man-in-the-middle attack. Protocols, therefore, need to be designed 422 so as to be as resilient against such attacks as possible. 424 9.2. Malformed Messages 426 Both clients and servers of reputation systems need to be resistant 427 to attacks that involve malformed messages, deliberate or otherwise. 428 Malformations can be used to confound clients and servers alike in 429 terms of identifying the party or parties responsible for the content 430 under evaluation. This can result in delivery of undesirable or even 431 dangerous content. 433 9.3. Further Discussion 435 Numerous other topics related to use and management of reputation 436 systems can be found in [I-D.REPUTE-CONSIDERATIONS]. 438 10. Informative References 440 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 441 "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, 442 September 2011. 444 [DNS] Mockapetris, P., "Domain names - implementation and 445 specification", STD 13, RFC 1035, November 1987. 447 [DNSBL] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, 448 February 2010. 450 [EMAIL-ARCH] 451 Crocker, D., "Internet Mail Architecture", RFC 5598, 452 July 2009. 454 [I-D.REPUTE-CONSIDERATIONS] 455 Kucherawy, M., "Operational Considerations Regarding 456 Reputation Services", draft-ietf-repute-considerations 457 (work in progress), November 2012. 459 [MAIL] Resnick, P., "Internet Message Format", RFC 5322, 460 October 2008. 462 [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 463 Extensions (MIME) Part One: Format of Internet Message 464 Bodies", RFC 2045, November 1996. 466 [OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. 467 Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 469 [SMIME] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 470 Mail Extensions (S/MIME) Version 3.2: Message 471 Specification", RFC 5751, January 2010. 473 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 474 October 2008. 476 [TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security 477 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 479 [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By 480 Reference", RFC 5518, April 2009. 482 Appendix A. Public Discussion 484 Public discussion of this suite of documents takes place on the 485 domainrep@ietf.org mailing list. See 486 https://www.ietf.org/mailman/listinfo/domainrep. 488 Authors' Addresses 490 Nathaniel Borenstein 491 Mimecast 492 203 Crescent St., Suite 303 493 Waltham, MA 02453 494 USA 496 Phone: +1 781 996 5340 497 Email: nsb@guppylake.com 499 Murray S. Kucherawy 500 270 Upland Drive 501 San Francisco, CA 94127 502 USA 504 Email: superuser@gmail.com 506 Andrew Sullivan (editor) 507 Dyn, Inc. 508 150 Dow St. 509 Manchester, NH 03101 510 USA 512 Email: asullivan@dyn.com