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Thaler 3 Internet-Draft Microsoft 4 Intended status: Informational July 29, 2016 5 Expires: January 30, 2017 7 Privacy Considerations for IPv6 over Networks of Resource-Constrained 8 Nodes 9 draft-ietf-6lo-privacy-considerations-02 11 Abstract 13 This document discusses how a number of privacy threats apply to 14 technologies designed for IPv6 over networks of resource-constrained 15 nodes, and provides advice to protocol designers on how to address 16 such threats in adaptation layer specifications for IPv6 over such 17 links. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on January 30, 2017. 36 Copyright Notice 38 Copyright (c) 2016 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 2. Amount of Entropy Needed in Global Addresses . . . . . . . . 3 55 3. Potential Approaches . . . . . . . . . . . . . . . . . . . . 4 56 3.1. IEEE-Identifier-Based Addresses . . . . . . . . . . . . . 5 57 3.2. Short Addresses . . . . . . . . . . . . . . . . . . . . . 6 58 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6 59 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 60 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 61 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 62 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 63 7.2. Informative References . . . . . . . . . . . . . . . . . 8 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 66 1. Introduction 68 RFC 6973 [RFC6973] discusses privacy considerations for Internet 69 protocols, and Section 5.2 in particular covers a number of privacy- 70 specific threats. In the context of IPv6 addresses, Section 3 of 71 [RFC7721] provides further elaboration on the applicability of the 72 privacy threats. 74 When interface identifiers (IIDs) are generated without sufficient 75 entropy compared to the link lifetime, devices and users can become 76 vulnerable to the various threats discussed there, including: 78 o Correlation of activities over time, if the same identifier is 79 used for traffic over period of time 81 o Location tracking, if the same interface identifier is used with 82 different prefixes as a device moves between different networks 84 o Device-specific vulnerability exploitation, if the identifier 85 helps identify a vendor or version or protocol and hence suggests 86 what types of attacks to try 88 o Address scanning, which enables all of the above attacks by off- 89 link attackers. (On some Non-Broadcast Multi-Access (NBMA) links 90 where all nodes aren't already privy to all on-link addresses, 91 address scans might also be done by on-link attackers, but in most 92 cases address scans are not an interesting threat from on-link 93 attackers and thus address scans generally apply only to routable 94 addresses.) 96 For example, for links that may last for years, "enough" bits of 97 entropy means at least 46 or so bits (see Section 2 for why) in a 98 routable address; ideally all 64 bits of the IID should be used, 99 although historically some bits have been excluded for reasons 100 discussed in [RFC7421]. Link-local addresses can also be susceptible 101 to the same privacy threats from off-link attackers, since experience 102 shows they are often leaked by upper-layer protocols such as SMTP, 103 SIP, or DNS. 105 For these reasons, [I-D.ietf-6man-default-iids] recommends using an 106 address generation scheme in [RFC7217], rather than addresses 107 generated from a fixed link-layer address. 109 Furthermore, to mitigate the threat of correlation of activities over 110 time on long-lived links, [RFC4941] specifies the notion of a 111 "temporary" address to be used for transport sessions (typically 112 locally-initiated outbound traffic to the Internet) that should not 113 be linkable to a more permanent identifier such as a DNS name, user 114 name, or fixed link-layer address. Indeed, the default address 115 selection rules [RFC6724] now prefer temporary addresses by default 116 for outgoing connections. If a device needs to simultaneously 117 support unlinkable traffic as well as traffic that is linkable to 118 such a stable identifier, this necessitates supporting simultaneous 119 use of multiple addresses per device. 121 2. Amount of Entropy Needed in Global Addresses 123 In terms of privacy threats discussed in [RFC7721], the one with the 124 need for the most entropy is address scans of routable addresses. To 125 mitigate address scans, one needs enough entropy to make the 126 probability of a successful address probe be negligible. Typically 127 this is measured in the length of time it would take to have a 50% 128 probability of getting at least one hit. Address scans often rely on 129 sending a packet such as a TCP SYN or ICMP Echo Request, and 130 determining whether the reply is an ICMP unreachable error (if no 131 host exists with that address) or a TCP response or ICMP Echo Reply 132 (if a host exists), or neither in which case nothing is known for 133 certain. 135 Many privacy-sensitive devices support a "stealth mode" as discussed 136 in Section 5 of [RFC7288] or are behind a network firewall that will 137 drop unsolicited inbound traffic (e.g., TCP SYNs, ICMP Echo Requests, 138 etc.) and thus no TCP RST or ICMP Echo Reply will be sent. In such 139 cases, and when the device does not listen on a well-known TCP or UDP 140 port known to the scanner, the effectiveness of an address scan is 141 limited by the ability to get ICMP unreachable errors, since the 142 attacker can only infer the presence of a host based on the absense 143 of an ICMP unreachable error. 145 Generation of ICMP unreachable errors is typically rate limited to 2 146 per second (the default in routers such as Cisco routers running IOS 147 12.0 or later). Such a rate results in taking about a year to 148 completely scan 26 bits of space. 150 The actual math is as follows. Let 2^N be the number of devices on 151 the subnet. Let 2^M be the size of the space to scan (i.e., M bits 152 of entropy). Let S be the number of scan attempts. The formula for 153 a 50% chance of getting at least one hit in S attempts is: P(at least 154 one success) = 1 - (1 - 2^N/2^M)^S = 1/2. Assuming 2^M >> S, this 155 simplifies to: S * 2^N/2^M = 1/2, giving S = 2^(M-N-1), or M = N + 1 156 + log_2(S). Using a scan rate of 2 per second, this results in the 157 following rule of thumb: 159 Bits of entropy needed = log_2(# devices per link) + log_2(seconds 160 of link lifetime) + 2 162 For example, for a network with at most 2^16 devices on the same 163 long-lived link, and the average lifetime of a link being 8 years 164 (2^28 seconds) or less, this results in a need for at least 46 bits 165 of entropy (16+28+2) so that an address scan would need to be 166 sustained for longer than the lifetime of the link to have a 50% 167 chance of getting a hit. 169 Although 46 bits of entropy may be enough to provide privacy in such 170 cases, 59 or more bits of entropy would be needed if addresses are 171 used to provide security against attacks such as spoofing, as CGAs 172 [RFC3972] and HBAs [RFC5535] do, since attacks are not limited by 173 ICMP rate limiting but by the processing power of the attacker. See 174 those RFCs for more discussion. 176 If, on the other hand, the devices being scanned for respond to 177 unsolicited inbound packets, then the address scan is not limited by 178 the ICMP unreachable rate limit in routers, since an adversary can 179 determine the presence of a host without them. In such cases, more 180 bits of entropy would be needed to provide the same level of 181 protection. 183 3. Potential Approaches 185 The table below shows the number of bits of entropy currently 186 available in various technologies: 188 +---------------+--------------------------+--------------------+ 189 | Technology | Reference | Bits of Entropy | 190 +---------------+--------------------------+--------------------+ 191 | 802.15.4 | [RFC4944] | 16+ or any EUI-64 | 192 | Bluetooth LE | [RFC7668] | 48 | 193 | DECT ULE | [I-D.ietf-6lo-dect-ule] | 40 or any EUI-48 | 194 | MS/TP | [I-D.ietf-6lo-6lobac] | 7 or 64 | 195 | ITU-T G.9959 | [RFC7428] | 8 | 196 | NFC | [I-D.ietf-6lo-nfc] | 6 or ??? | 197 +---------------+--------------------------+--------------------+ 199 Such technologies generally support either IEEE identifiers or so 200 called "Short Addresses", or both, as link layer addresses. We 201 discuss each in turn. 203 3.1. IEEE-Identifier-Based Addresses 205 Some technologies allow the use of IEEE EUI-48 or EUI-64 identifiers, 206 or allow using an arbitrary 64-bit identifier. Using such an 207 identifier to construct IPv6 addresses makes it easy to use the 208 normal LOWPAN_IPHC encoding with stateless compression, allowing such 209 IPv6 addresses to be fully elided in common cases. 211 Global addresses with interface identifiers formed from IEEE 212 identifiers can have insufficient entropy to mitigate address scans 213 unless the IEEE identifier itself has sufficient entropy, and enough 214 bits of entropy are carried over into the IPv6 address to 215 sufficiently mitigate the threats. Privacy threats other than 216 "Correlation over time" can be mitigated using per-network randomized 217 link-layer addresses with enough entropy compared to the link 218 lifetime. A number of such proposals can be found at 219 , and Section 10.8 of 220 [BTCorev4.1] specifies one for Bluetooth. Using routable IPv6 221 addresses derived from such link-layer addresses would be roughly 222 equivalent to those specified in [RFC7217]. 224 Correlation over time (for all addresses, not just routable 225 addresses) can be mitigated if the link-layer address itself changes 226 often enough, such as each time the link is established, if the link 227 lifetime is short. For further discussion, see 228 [I-D.huitema-6man-random-addresses]. 230 Another potential concern is that of efficiency, such as avoiding 231 Duplicate Address Detection (DAD) all together when IPv6 addresses 232 are IEEE-identifier-based. Appendix A of [RFC4429] provides an 233 analysis of address collision probability based on the number of bits 234 of entropy. A simple web search on "duplicate MAC addresses" will 235 show that collisions do happen with MAC addresses, and thus based on 236 the analysis in [RFC4429], using sufficient bits of entropy in random 237 addresses can provide greater protection against collision than using 238 MAC addresses. 240 3.2. Short Addresses 242 A routable IPv6 address with an interface identifier formed from the 243 combination of a "Short Address" and a set of well-known constant 244 bits (such as padding with 0's) lacks sufficient entropy to mitigate 245 address scanning unless the link lifetime is extremely short. 246 Furthermore, an adversary could also use statisical methods to 247 determine the size of the L2 address space and thereby make some 248 inference regarding the underlying technology on a given link, and 249 target further attacks accordingly. 251 When Short Addresses are desired on links that are not guaranteed to 252 have a short enough lifetime, the mechanism for constructing an IPv6 253 interface identifier from a Short Address could be designed to 254 sufficiently mitigate the problem. For example, if all nodes on a 255 given L2 network have a shared secret (such as the key needed to get 256 on the layer-2 network), the 64-bit IID might be generated using a 257 one-way hash that includes (at least) the shared secret together with 258 the Short Address. The use of such a hash would result in the IIDs 259 being spread out among the full range of IID address space, thus 260 mitigating address scans, while still allowing full stateless 261 compression/elision. 263 For long-lived links, "temporary" addresses might even be generated 264 in the same way by (for example) also including in the hash the 265 Version Number from the Authoritative Border Router Option 266 (Section 4.3 of [RFC6775]), if any. This would allow changing 267 temporary addresses whenever the Version Number is changed, even if 268 the set of prefix or context information is unchanged. 270 In summary, any specification using Short Addresses should carefully 271 construct an IID generation mechanism so as to provide sufficient 272 entropy compared to the link lifetime. 274 4. Recommendations 276 The following are recommended for adaptation layer specifications: 278 o Security (privacy) sections should say how address scans are 279 mitigated. An address scan might be mitigated by having a link 280 always be short-lived, or might be mitigated by having a large 281 number of bits of entropy in routable addresses, or some 282 combination. Thus, a specification should explain what the 283 maximum lifetime of a link is in practice, and show how the number 284 of bits of entropy is sufficient given that lifetime. 286 o Technologies should define a way to include sufficient bits of 287 entropy in the IPv6 interface identifier, based on the maximum 288 link lifetime. Specifying that randomized link-layer addresses 289 can be used is one easy way to do so, for technologies that 290 support such identifiers. 292 o Specifications should not simply construct an IPv6 interface 293 identifier by padding a short address with a set of other well- 294 known constant bits, unless the link lifetime is guaranteed to be 295 extremely short. This also applies to link-local addresses if the 296 same short address is used independent of network and is unique 297 enough to allow location tracking. 299 o Specifications should make sure that an IPv6 address can change 300 over long periods of time. For example, the interface identifier 301 might change each time a device connects to the network (if 302 connections are short), or might change each day (if connections 303 can be long). This is necessary to mitigate correlation over 304 time. 306 o If a device can roam between networks, and more than a few bits of 307 entropy exist in the IPv6 interface identifier, then make sure 308 that the interface identifier can vary per network as the device 309 roams. This is necessary to mitigate location tracking. 311 5. IANA Considerations 313 This document has no actions for IANA. 315 6. Security Considerations 317 This entire document is about security considerations and how to 318 specify possible mitigations. 320 7. References 322 7.1. Normative References 324 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 325 Requirement Levels", BCP 14, RFC 2119, 326 DOI 10.17487/RFC2119, March 1997, 327 . 329 7.2. Informative References 331 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", 332 RFC 3972, DOI 10.17487/RFC3972, March 2005, 333 . 335 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) 336 for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, 337 . 339 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 340 Extensions for Stateless Address Autoconfiguration in 341 IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, 342 . 344 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 345 "Transmission of IPv6 Packets over IEEE 802.15.4 346 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 347 . 349 [RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, 350 DOI 10.17487/RFC5535, June 2009, 351 . 353 [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, 354 "Default Address Selection for Internet Protocol Version 6 355 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, 356 . 358 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 359 Bormann, "Neighbor Discovery Optimization for IPv6 over 360 Low-Power Wireless Personal Area Networks (6LoWPANs)", 361 RFC 6775, DOI 10.17487/RFC6775, November 2012, 362 . 364 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 365 Morris, J., Hansen, M., and R. Smith, "Privacy 366 Considerations for Internet Protocols", RFC 6973, 367 DOI 10.17487/RFC6973, July 2013, 368 . 370 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque 371 Interface Identifiers with IPv6 Stateless Address 372 Autoconfiguration (SLAAC)", RFC 7217, 373 DOI 10.17487/RFC7217, April 2014, 374 . 376 [RFC7288] Thaler, D., "Reflections on Host Firewalls", RFC 7288, 377 DOI 10.17487/RFC7288, June 2014, 378 . 380 [RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., 381 Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit 382 Boundary in IPv6 Addressing", RFC 7421, 383 DOI 10.17487/RFC7421, January 2015, 384 . 386 [RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets 387 over ITU-T G.9959 Networks", RFC 7428, 388 DOI 10.17487/RFC7428, February 2015, 389 . 391 [RFC7668] Nieminen, J., Savolainen, T., Isomaki, M., Patil, B., 392 Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low 393 Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015, 394 . 396 [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy 397 Considerations for IPv6 Address Generation Mechanisms", 398 RFC 7721, DOI 10.17487/RFC7721, March 2016, 399 . 401 [I-D.ietf-6man-default-iids] 402 Gont, F., Cooper, A., Thaler, D., and S. (Will), 403 "Recommendation on Stable IPv6 Interface Identifiers", 404 draft-ietf-6man-default-iids-13 (work in progress), July 405 2016. 407 [I-D.ietf-6lo-6lobac] 408 Lynn, K., Martocci, J., Neilson, C., and S. Donaldson, 409 "Transmission of IPv6 over MS/TP Networks", draft-ietf- 410 6lo-6lobac-05 (work in progress), June 2016. 412 [I-D.ietf-6lo-dect-ule] 413 Mariager, P., Petersen, J., Shelby, Z., Logt, M., and D. 414 Barthel, "Transmission of IPv6 Packets over DECT Ultra Low 415 Energy", draft-ietf-6lo-dect-ule-05 (work in progress), 416 May 2016. 418 [I-D.ietf-6lo-nfc] 419 Hong, Y. and J. Youn, "Transmission of IPv6 Packets over 420 Near Field Communication", draft-ietf-6lo-nfc-04 (work in 421 progress), July 2016. 423 [I-D.huitema-6man-random-addresses] 424 Huitema, C., "Implications of Randomized Link Layers 425 Addresses for IPv6 Address Assignment", draft-huitema- 426 6man-random-addresses-03 (work in progress), March 2016. 428 [BTCorev4.1] 429 Bluetooth Special Interest Group, "Bluetooth Core 430 Specification Version 4.1", December 2013, 431 . 434 Author's Address 436 Dave Thaler 437 Microsoft 438 One Microsoft Way 439 Redmond, WA 98052 440 USA 442 Email: dthaler@microsoft.com