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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IAB T. Hardie, Ed. 3 Internet-Draft March 07, 2016 4 Intended status: Informational 5 Expires: September 8, 2016 7 Confidentiality in the Face of Pervasive Surveillance 8 draft-iab-privsec-confidentiality-mitigations-04 10 Abstract 12 The IAB has published [RFC7624] in response to several revelations of 13 pervasive attack on Internet communications. In this document we 14 survey the mitigations to those threats which are currently available 15 or which might plausibly be deployed. We discuss these primarily in 16 the context of Internet protocol design, focusing on robustness to 17 pervasive monitoring and avoidance of unwanted cross-mitigation 18 impacts. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on September 8, 2016. 37 Copyright Notice 39 Copyright (c) 2016 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 56 3. Available Mitigations . . . . . . . . . . . . . . . . . . . . 4 57 4. Interplay among Mitigations . . . . . . . . . . . . . . . . . 9 58 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 59 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 60 7. Contributors {Contributors} . . . . . . . . . . . . . . . . . 10 61 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 62 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 63 8.2. Informative References . . . . . . . . . . . . . . . . . 11 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 66 1. Introduction 68 To ensure that the Internet can be trusted by users, it is necessary 69 for the Internet technical community to address the vulnerabilities 70 exploited in the attacks document in [RFC7258] and the threats 71 described in [RFC7624]. The goal of this document is to describe 72 more precisely the mitigations available for those threats and to lay 73 out the interactions among them should they be deployed in 74 combination. 76 2. Terminology 78 This document makes extensive use of standard security and privacy 79 terminology; see [RFC4949] and [RFC6973]. Terms used from [RFC6973] 80 include Eavesdropper, Observer, Initiator, Intermediary, Recipient, 81 Attack (in a privacy context), Correlation, Fingerprint, Traffic 82 Analysis, and Identifiability (and related terms). In addition, we 83 use a few terms that are specific to the attacks discussed in this 84 document. Note especially that "passive" and "active" below do not 85 refer to the effort used to mount the attack; a "passive attack" is 86 any attack that accesses a flow but does not modify it, while an 87 "active attack" is any attack that modifies a flow. Some passive 88 attacks involve active interception and modifications of devices, 89 rather than simple access to the medium. The introduced terms are: 91 Pervasive Attack: An attack on Internet communications that makes 92 use of access at a large number of points in the network, or 93 otherwise provides the attacker with access to a large amount of 94 Internet traffic; see [RFC7258]. 96 Passive Pervasive Attack: An eavesdropping attack undertaken by a 97 pervasive attacker, in which the packets in a traffic stream 98 between two endpoints are intercepted, but in which the attacker 99 does not modify the packets in the traffic stream between two 100 endpoints, modify the treatment of packets in the traffic stream 101 (e.g. delay, routing), or add or remove packets in the traffic 102 stream. Passive pervasive attacks are undetectable from the 103 endpoints. Equivalent to passive wiretapping as defined in 104 [RFC4949]; we use an alternate term here since the methods 105 employed are wider than those implied by the word "wiretapping", 106 including the active compromise of intermediate systems. 108 Active Pervasive Attack: An attack undertaken by a pervasive 109 attacker, which in addition to the elements of a passive pervasive 110 attack, also includes modification, addition, or removal of 111 packets in a traffic stream, or modification of treatment of 112 packets in the traffic stream. Active pervasive attacks provide 113 more capabilities to the attacker at the risk of possible 114 detection at the endpoints. Equivalent to active wiretapping as 115 defined in [RFC4949]. 117 Observation: Information collected directly from communications by 118 an eavesdropper or observer. For example, the knowledge that 119 sent a message to via SMTP 120 taken from the headers of an observed SMTP message would be an 121 observation. 123 Inference: Information derived from analysis of information 124 collected directly from communications by an eavesdropper or 125 observer. For example, the knowledge that a given web page was 126 accessed by a given IP address, by comparing the size in octets of 127 measured network flow records to fingerprints derived from known 128 sizes of linked resources on the web servers involved, would be an 129 inference. 131 Collaborator: An entity that is a legitimate participant in a 132 communication, and provides information about that communication 133 to an attacker. Collaborators may either deliberately or 134 unwittingly cooperate with the attacker, in the latter case 135 because the attacker has subverted the collaborator through 136 technical, social, or other means. 138 Key Exfiltration: The transmission of cryptographic keying material 139 for an encrypted communication from a collaborator, deliberately 140 or unwittingly, to an attacker. 142 Content Exfiltration: The transmission of the content of a 143 communication from a collaborator, deliberately or unwittingly, to 144 an attacker. 146 Data Minimization: With respect to protocol design, refers to the 147 practice of only exposing the minimum amount of data or metadata 148 necessary for the task supported by that protocol to the other 149 endpoint(s) and/or devices along the path. 151 3. Available Mitigations 153 Given the threat model laid out in [RFC7624], how should the Internet 154 technical community respond to pervasive attack? The cost and risk 155 considerations discussed in it provide a guide to responses. Namely, 156 responses to passive attack should close off avenues for those 157 attacks that are safe, scalable, and cheap, forcing the attacker to 158 mount attacks that expose it to higher cost and risk. Protocols and 159 security measures protecting against active attacks must also limit 160 the impact of compromise and malfeasance by avoiding systems which 161 grant universal credentials. 163 In this section, we discuss a collection of high-level approaches to 164 mitigating pervasive attacks. These approaches are not meant to be 165 exhaustive, but rather to provide general guidance to protocol 166 designers in creating protocols that are resistant to pervasive 167 attack. 169 Many of these are basic tools which already exist. As Edward Snowden 170 put it, "properly implemented strong crypto systems are one of the 171 few things you can rely on". The task for the Internet community is 172 to ensure that applications are able to use the strong crypto and 173 other mitigations already available- and that these are properly 174 implemented and commonly turned on. Some of this work will require 175 architectural changes to applications, e. g., in order to limit the 176 information that is exposed to servers. In many other cases, 177 however, the need is simply to make the best use we can of the 178 cryptographic tools we have. 180 +--------------------------+----------------------------------------+ 181 | Attack Class | High-level mitigations | 182 +--------------------------+----------------------------------------+ 183 | Passive observation | Encryption for confidentiality | 184 | | | 185 | Passive inference | Path differentiation | 186 | | | 187 | Active | Authentication, monitoring | 188 | | | 189 | Metadata Analysis | Data Minimization | 190 | | | 191 | Static key exfiltration | Encryption with per-session state | 192 | | (PFS) | 193 | | | 194 | Dynamic key exfiltration | Transparency, validation of end | 195 | | systems | 196 | | | 197 | Content exfiltration | Object encryption, distributed systems | 198 +--------------------------+----------------------------------------+ 200 Figure 1: Table of Mitigations 202 The traditional mitigation to passive attack is to render content 203 unintelligible to the attacker by applying encryption, for example, 204 by using TLS or IPsec [RFC5246][RFC4301]. Even without 205 authentication, encryption will prevent a passive attacker from being 206 able to read the encrypted content. Exploiting unauthenticated 207 encryption requires an active attack (man in the middle); with 208 authentication, a key exfiltration attack is required. For 209 cryptographic systems providing forward secrecy, even exfiltration of 210 long-term keys will not compromise data captured under session keys 211 used before the exfiltration. 213 The additional capabilities of a pervasive passive attacker, however, 214 require some changes in how protocol designers evaluate what 215 information is encrypted. In addition to directly collecting 216 unencrypted data, a pervasive passive attacker can also make 217 inferences about the content of encrypted messages based on what is 218 observable. For example, if a user typically visits a particular set 219 of web sites, then a pervasive passive attacker observing all of the 220 user's behavior can track the user based on the hosts the user 221 communicates with, even if the user changes IP addresses, and even if 222 all of the connections are encrypted. 224 Thus, in designing protocols to be resistant to pervasive passive 225 attacks, protocol designers should consider what information is left 226 unencrypted in the protocol, and how that information might be 227 correlated with other traffic. Some of the data left unencrypted may 228 be considered "metadata" within the context of a single protocol, as 229 it provides adjunct information used for delivery or display, rather 230 than the data directly created or consumed by protocol users. This 231 does not mean it is not useful to attackers, however, and when this 232 metadata is not protected by encryption it may leak substantial 233 amounts of information. Data minimization strategies should thus be 234 applied to any data left unencrypted, whether it be payload or 235 metadata. Information that cannot be encrypted or omited should be 236 be dissociated from other information. For example, the TOR[TOR] 237 overlay routing network anonymizes IP addresses by using multi-hop 238 onion routing. 240 As with traditional, limited active attacks, a basic mitigation to 241 pervasive active attack is to enable the endpoints of a communication 242 to authenticate each other over the encrypted channel. However, 243 attackers that can mount pervasive active attacks can often subvert 244 the authorities on which authentication systems rely. Thus, in order 245 to make authentication systems more resilient to pervasive attack, it 246 is beneficial to monitor these authorities to detect misbehavior that 247 could enable active attack. For example, DANE and Certificate 248 Transparency both provide mechanisms for detecting when a CA has 249 issued a certificate for a domain name without the authorization of 250 the holder of that domain name [RFC6962][RFC6698]. Other systems may 251 use external notaries to detect certificate authority mismatch (e.g. 252 Convergence [Convergence]). 254 While encryption and authentication protect the security of 255 individual sessions, these sessions may still leak information, such 256 as IP addresses or server names, that a pervasive attacker can use to 257 correlate sessions and derive additional information about the 258 target. Thus, pervasive attack highlights the need for anonymization 259 technologies, which make correlation more difficult. Typical 260 approaches to anonymization against traffic analysis include: 262 o Aggregation: Routing sessions for many endpoints through a common 263 mid-point (e.g, an HTTP proxy). The midpoint appears as the 264 origin of the communication when traffic analysis is conducted 265 from points after it, so individual sources cannot be 266 distinguished. If traffic analysis is being conducted prior to 267 the mid-point, all flows appear to be destined to the same point, 268 which leaks very little information. Even when traffic analysis 269 is being performed both before and after the mid-point, 270 simultaneous connections may make it difficult to corelate the 271 traffic going into and out of the mid-point. For this to be 272 effective as a mitigation, traffic to the mid-point must be 273 encrypted and traffic from the mid-point should be. 275 o Onion routing: Routing a session through several mid-points, 276 rather than directly end-to-end, with encryption that guarantees 277 that each node can only see the previous and next hops. This 278 ensures that the source and destination of a communication are 279 never revealed simultaneously. 281 o Multi-path: Routing different sessions via different paths (even 282 if they originate from the same endpoint). This reduces the 283 probability that the same attacker will be able to collect many 284 sessions or associate them with the same individual. If, for 285 example, a device has both a cellular and 802.11 interface, 286 routing some traffic across the cellular network and other traffic 287 over the 802.11 interface means that traffic analysis conducted 288 only with one network will be incomplete. Even if conducted in 289 both, it may be more difficult for the attacker to associate the 290 traffic in each network with the other. For this to be effective 291 as a mitigation, signalling protocols which gather and transmit 292 data about multiple interfaces (such as SIP) must be encrypted to 293 avoid the information being used in cross-corelation. 295 An encrypted, authenticated session is safe from content-monitoring 296 attacks in which neither end collaborates with the attacker, but can 297 still be subverted by the endpoints. The most common ciphersuites 298 used for HTTPS today, for example, are based on using RSA encryption 299 in such a way that if an attacker has the private key, the attacker 300 can derive the session keys from passive observation of a session. 301 These ciphersuites are thus vulnerable to a static key exfiltration 302 attack - if the attacker obtains the server's private key once, then 303 they can decrypt all past and future sessions for that server. 305 Static key exfiltration attacks are prevented by including ephemeral, 306 per-session secret information in the keys used for a session. Most 307 IETF security protocols include modes of operation that have this 308 property. These modes are known in the literature under the heading 309 "perfect forward secrecy" (PFS) because even if an adversary has all 310 of the secrets for one session, the next session will use new, 311 different secrets and the attacker will not be able to decrypt it. 312 The Internet Key Exchange (IKE) protocol used by IPsec supports PFS 313 by default [RFC4306], and TLS supports PFS via the use of specific 314 ciphersuites [RFC5246]. 316 Dynamic key exfiltration cannot be prevented by protocol means. By 317 definition, any secrets that are used in the protocol will be 318 transmitted to the attacker and used to decrypt what the protocol 319 encrypts. Likewise, no technical means will stop a willing 320 collaborator from sharing keys with an attacker. However, this 321 attack model also covers "unwitting collaborators", whose technical 322 resources are collaborating with the attacker without their owners' 323 knowledge. This could happen, for example, if flaws are built into 324 products or if malware is injected later on. 326 Standards can also define protocols that provide greater or lesser 327 opportunity for dynamic key exfiltration. Collaborators engaging in 328 key exfiltration through a standard protocol will need to use covert 329 channels in the protocol to leak information that can be used by the 330 attacker to recover the key. Such use of covert channels has been 331 demonstrated for SSL, TLS, and SSH. Any protocol bits that can be 332 freely set by the collaborator can be used as a covert channel, 333 including, for example, TCP options or unencrypted traffic sent 334 before a STARTTLS message in SMTP or XMPP. Protocol designers should 335 consider what covert channels their protocols expose, and how those 336 channels can be exploited to exfiltrate key information. 338 Content exfiltration has some similarity to the dynamic exfiltration 339 case, in that nothing can prevent a collaborator from revealing what 340 they know, and the mitigations against becoming an unwitting 341 collaborator apply. In this case, however, applications can limit 342 what the collaborator is able to reveal. For example, the S/MIME and 343 PGP systems for secure email both deny intermediate servers access to 344 certain parts of the message [RFC5750][RFC2015]. Even if a server 345 were to provide an attacker with full access, the attacker would 346 still not be able to read the protected parts of the message. 348 Mechanisms like S/MIME and PGP are often referred to as "end-to- 349 end"security mechanisms, as opposed to "hop-by-hop" or "end-to- 350 middle" mechanisms like the use of SMTP over TLS. These two 351 different mechanisms address different types of attackers: Hop-by-hop 352 mechanisms protect from attackers on the wire (passive or active), 353 while end-to-end mechansims protect against attackers within 354 intermediate nodes as well as those on the wire. Even end-to-end 355 mechanisms are not complete protection in themselves, as intermediate 356 nodes can still access some metadata. For example: 358 o Two users messaging via Facebook over HTTPS are protected against 359 passive and active attackers in the network between the users and 360 Facebook. However, if Facebook is a collaborator in an 361 exfiltration attack, their communications can still be monitored. 362 They would need to encrypt their messages end-to-end in order to 363 protect themselves against this risk. 365 o Two users exchanging PGP-protected email have protected the 366 content of their exchange from network attackers and intermediate 367 servers, but the header information (e. g., To and From addresses) 368 is unnecessarily exposed to passive and active attackers that can 369 see communications among the mail agents handling the email 370 messages. These mail agents need to use hop-by-hop encryption and 371 traffic analysis mitigation to address this risk. 373 Mechanisms such as S/MIME and PGP are also known as "object-based" 374 security mechanisms (as opposed to "communications security" 375 mechanisms), since they operate at the level of objects, rather than 376 communications sessions. Such secure object can be safely handled by 377 intermediaries in order to realize, for example, store and forward 378 messaging. In the examples above, the encrypted instant messages or 379 email messages would be the secure objects. 381 The mitigations to the content exfiltration case regard participants 382 in the protocol as potential passive attackers themselves, and apply 383 the mitigations discussed above with regard to passive attack. 384 Information that is not necessary for these participants to fulfill 385 their role in the protocol can be encrypted, and other information 386 can be anonymized. 388 The tools that we currently have have not generally been designed 389 with mitigation in mind, so they may need elaboration or adjustment 390 to be completely suitable. The next section examines one common 391 reason for such adjustment: managing the integration of one 392 mitigation with the environment in which it is deployed. 394 4. Interplay among Mitigations 396 One of the key considerations in selecting mitigations is how to 397 manage the interplay among different mechanisms. Care must be taken 398 to avoid situations where a mitigation is rendered fruitless because 399 of a different mitigation which is working at a different time scale 400 or with a different aim. 402 As an example, there is work in progress in IEEE 802 to standardize a 403 method for the randomization of MAC Addresses. This work aims to 404 enable a mitigation in which the MAC address varies as the device 405 connects to different networks, or connects at different times. In 406 theory, the randomization will mitigate tracking by MAC address. 407 However, the randomization will be defeated if the adversary can link 408 the randomized MAC address to other identifiers such as the interface 409 identifier used in IPv6 addresses, the unique identifiers used in 410 DHCP or DHCPv6, or unique identifiers used in various link-local 411 discovery protocols. 413 For mitigations which rely on aggregation to separate the origin of 414 traffic from its destination, care must be taken that the protocol 415 mechanics do not expose origin IP through secondary means. 416 [I-D.ietf-dnsop-edns-client-subnet] for example, documents a method 417 to carry the IP address or subnet of a querying party through a 418 recursive resolver to an authoritative resolver. Even with a 419 truncated IP address, this mechanism increases the likelihood that a 420 pervasive monitor would be able to associate query traffic and 421 responses. If a client wished to ensure that its traffic did not 422 expose this data, it would need to require that its stub resolver 423 emit any privacy-sensitive queries with a source NETMASK set to 0, as 424 detailed in Section 5.1 of [I-D.ietf-dnsop-edns-client-subnet]. 425 Given that setting this only occasionally might also be used a signal 426 to observors, any client wishing to have any privacy sensitive 427 traffic would, in essence have to emit this for every query. While 428 this would succeed at providing the required privacy, given the 429 mechanism proposed, it would also mean no split-DNS adjustments in 430 response would be possible for the privacy sensitive client. 432 5. IANA Considerations 434 This memo makes no request of IANA. 436 6. Security Considerations 438 This memorandum describes a series of mitigations to the attacks 439 described in [RFC7258]. No such list could possibly be 440 comprehensive, nor is the attack therein described the only possible 441 attack. 443 7. Contributors {Contributors} 445 This document is derived in part from the work initially done on the 446 Perpass mailing list and at the STRINT workshop. Work from Brian 447 Trammell, Bruce Schneier, Christian Huitema, Cullen Jennings, Daniel 448 Borkmann, and Richard Barnes is incorporated here, as are ideas and 449 commentary from Jeff Hodges, Phillip Hallam-Baker, and Stephen 450 Farrell. 452 8. References 454 8.1. Normative References 456 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 457 Requirement Levels", BCP 14, RFC 2119, 458 DOI 10.17487/RFC2119, March 1997, 459 . 461 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 462 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 463 . 465 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 466 Morris, J., Hansen, M., and R. Smith, "Privacy 467 Considerations for Internet Protocols", RFC 6973, 468 DOI 10.17487/RFC6973, July 2013, 469 . 471 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 472 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 473 2014, . 475 [RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., 476 Trammell, B., Huitema, C., and D. Borkmann, 477 "Confidentiality in the Face of Pervasive Surveillance: A 478 Threat Model and Problem Statement", RFC 7624, 479 DOI 10.17487/RFC7624, August 2015, 480 . 482 8.2. Informative References 484 [Convergence] 485 M Marlinspike, ., "Convergence Project", August 2011, 486 . 488 [I-D.ietf-dnsop-edns-client-subnet] 489 Contavalli, C., Gaast, W., tale, t., and W. Kumari, 490 "Client Subnet in DNS Queries", draft-ietf-dnsop-edns- 491 client-subnet-06 (work in progress), December 2015. 493 [RFC2015] Elkins, M., "MIME Security with Pretty Good Privacy 494 (PGP)", RFC 2015, DOI 10.17487/RFC2015, October 1996, 495 . 497 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 498 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 499 December 2005, . 501 [RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) 502 Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005, 503 . 505 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 506 (TLS) Protocol Version 1.2", RFC 5246, 507 DOI 10.17487/RFC5246, August 2008, 508 . 510 [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 511 Mail Extensions (S/MIME) Version 3.2 Certificate 512 Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010, 513 . 515 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 516 of Named Entities (DANE) Transport Layer Security (TLS) 517 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 518 2012, . 520 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 521 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 522 . 524 [STRINT] S Farrell, ., "Strint Workshop Report", April 2014, 525 . 528 [TOR] The Tor Project, "Tor", 2013, 529 . 531 Author's Address 533 Ted Hardie (editor) 535 Email: ted.ietf@gmail.com