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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 NETWORK WORKING GROUP N. Williams 3 Internet-Draft Sun 4 Intended status: Standards Track M. Richardson 5 Expires: February 7, 2009 SSW 6 August 6, 2008 8 Better-Than-Nothing-Security: An Unauthenticated Mode of IPsec 9 draft-ietf-btns-core-07.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on February 7, 2009. 36 Copyright Notice 38 Copyright (C) The IETF Trust (2008). 40 Abstract 42 This document specifies how to use the Internet Key Exchange (IKE) 43 protocols, such as IKEv1 and IKEv2, to setup "unauthenticated" 44 security associations (SAs) for use with the IPsec Encapsulating 45 Security Payload (ESP) and the IPsec Authentication Header (AH). No 46 changes to IKEv2 bits-on-the-wire are required, but Peer 47 Authorization Database (PAD) and Security Policy Database (SPD) 48 extensions are specified. Unauthenticated IPsec is herein referred 49 to by its popular acronym, "BTNS" (Better Than Nothing Security). 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 54 1.1. Conventions used in this document . . . . . . . . . . . . . 3 55 2. BTNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 3. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . 6 57 3.1. Example #1: A security gateway . . . . . . . . . . . . . . . 6 58 3.2. Example #2: A mixed end-system . . . . . . . . . . . . . . . 8 59 3.3. Example #3: A BTNS-only system . . . . . . . . . . . . . . . 9 60 3.4. Miscellaneous comments . . . . . . . . . . . . . . . . . . . 10 61 4. Security Considerations . . . . . . . . . . . . . . . . . . 11 62 4.1. Connection-Latching and Channel Binding . . . . . . . . . . 11 63 4.2. Leap-of-Faith (LoF) for BTNS . . . . . . . . . . . . . . . . 11 64 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12 65 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 66 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 67 7.1. Normative References . . . . . . . . . . . . . . . . . . . . 14 68 7.2. Informative References . . . . . . . . . . . . . . . . . . . 14 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15 70 Intellectual Property and Copyright Statements . . . . . . . 16 72 1. Introduction 74 Here we describe how to establish unauthenticated IPsec SAs using 75 IKEv2 [RFC4306] and unauthenticated public keys. No new on-the-wire 76 protocol elements are added to IKEv2. 78 The [RFC4301] processing model is assumed. 80 This document does not define an opportunistic BTNS mode of IPsec 81 whereby nodes may fallback to unprotected IP when their peers do not 82 support IKEv2, nor does it describe "leap-of-faith" modes, or 83 "connection latching." 85 See [I-D.ietf-btns-prob-and-applic] for the applicability and uses of 86 BTNS and definitions of these terms. 88 This document describes BTNS in terms of IKEv2 and [RFC4301]'s 89 concepts. There is no reason why the same methods cannot be used 90 with IKEv1 [RFC2408] [RFC2409] and [RFC2401], however, those 91 specifications do not include the PAD concepts, and therefore it may 92 not be possible to implement BTNS on all compliant RFC2401 93 implementations. 95 1.1. Conventions used in this document 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 99 document are to be interpreted as described in [RFC2119]. 101 2. BTNS 103 The IPsec processing model is hereby modified as follows: 105 o A new ID type is added, 'PUBLICKEY'; IDs of this type have public 106 keys as values. This ID type is not used on the wire. 108 o PAD entries that match on PUBLICKEY IDs are referred to as "BTNS 109 PAD entries." All other PAD entries are referred to as "non-BTNS 110 PAD entries." 112 o BTNS PAD entries may match on specific peer PUBLICKEY IDs (or 113 public key fingerprints), or on all peer public keys. The latter 114 is referred to as the "wildcard BTNS PAD entry." 116 o BTNS PAD entries MUST logically (see below) follow all other PAD 117 entries (the PAD being an ordered list). 119 o At most one wildcard BTNS PAD entry may appear in the PAD, and, if 120 present, MUST be the last entry in the PAD (see below). 122 o Any peer that uses an IKEv2 AUTH method involving a digital 123 signature (made with a private key to a public key cryptosystem) 124 may match a BTNS PAD entry, provided that it matches no non-BTNS 125 PAD entries. Suitable AUTH methods as of August 2007 are: RSA 126 Digital Signature (method #1) and DSS Digital Signature (method 127 #3); see [RFC4306], section 3.8. 129 o A BTNS capable implementation of IPsec will first search the PAD 130 for non-BTNS entries matching a peer's ID. If no matching non- 131 BTNS PAD entries are found then the peer's ID MUST then be coerced 132 to be of 'PUBLICKEY' type with the peer's public key as its value 133 and the PAD is then searched again for matching BTNS PAD entries. 134 This ensures that BTNS PAD entries logically follow non-BTNS PAD 135 entries. A single PAD search that preserves these semantics is 136 allowed. 138 o A peer that matches a BTNS PAD entry is referred to as a "BTNS 139 peer." Such a peer is "authenticated" by verifying that the 140 signature in its IKEv2 AUTH payload with the public key from the 141 peer's CERT payload. 143 o Of course, if no matching PAD entry is found, then the IKE SA is 144 rejected as usual. 146 o A new flag for SPD entries: 'BTNS_OK'. Traffic to/from peers that 147 match the BTNS PAD entry will match only SPD entries that have the 148 BTNS_OK flag set. The SPD may be searched by address or by ID (of 149 type PUBLICKEY, for BTNS peers), as per the IPsec processing model 150 [RFC4301]; searching by ID in this case requires creation of SPD 151 entries that are bound to public key values (this could be used to 152 build "leap-of-faith" [I-D.ietf-btns-prob-and-applic] Section 4.2 153 behaviour, for example). 155 Nodes MUST reject IKE_SA proposals from peers that match non-BTNS PAD 156 entries but fail to authenticate properly. 158 Nodes wishing to be treated as BTNS nodes by their peers MUST include 159 bare public key CERT payloads. Currently only bare RSA public key 160 CERT payloads are defined, which means that BTNS works only with RSA 161 public keys at this time (see "Raw RSA Key" in section 3.6 of 162 [RFC4306]). Nodes MAY also include any number of certificates that 163 bind the same public key. These certificates need not to have been 164 pre-shared with their peers (e.g., because ephermal, self-signed). 165 RSA keys for use in BTNS may be generated at any time, but 166 "connection latching" [I-D.ietf-btns-connection-latching] requires 167 that they remain constant between IKEv2 exchanges that are used to 168 establish SAs for latched connections. 170 To preserve standard IPsec access control semantics: 172 o BTNS PAD entries MUST logically follow all non-BTNS PAD entries 174 o the wildcard BTNS PAD entry MUST be the last entry in the PAD, 175 logically 177 o the wildcard BTNS PAD entry MUST have ID constraints that do not 178 logically overlap those of other PAD entries. 180 As described above, the logical PAD ordering requirements can easily 181 be implemented by searching the PAD twice at peer authentication 182 time: once using the peer-asserted ID, and if that fails, once using 183 the peer's public key as a PUBLICKEY ID. A single pass 184 implementation that meets this requirement is permitted. 186 The BTNS entry ID constraint non-overlap requirement can easily be 187 implemented by searching the PAD twice: once when BTNS peers 188 authenticate, and again when BTNS peers negotiate child SAs. In the 189 first pass the PAD is searched for a matching PAD entry as described 190 above, and in the second it is searched to make sure that BTNS peers' 191 asserted child SA traffic selectors do not conflict with non-BTNS PAD 192 entries. Single pass implementations that preserve these semantics 193 are feasible. 195 3. Usage Scenarios 197 In order to explain the above rules a number of scenarios will be 198 examined. The goal here is to persuade the reader that the above 199 rules are both sufficient and necessary. 201 This section is informative only. 203 To explain the scenarios a reference diagram describing an example 204 network will be used. It is as follows: 206 [Q] [R] 207 AS1 . . AS2 208 [A]----+----[SG-A].......+....+.......[SG-B]-------[B] 209 ...... \ 210 ..PI.. ----[btns-B] 211 ...... 212 [btns-C].....+....+.......[btns-D] 214 Figure 1: Reference Network Diagram 216 In this diagram, there are six end-nodes: A, B, C and D. Two of the 217 systems are security gateways: SG-A, SG-B, protecting networks on 218 which [A] and [B] reside. There is a node [Q] which is IPsec and 219 BTNS capable, and node [R] is a simple node, with no IPsec or BTNS 220 capability. Nodes [C] and [D] are BTNS capable. 222 Nodes [C] and [Q] have fixed addresses. Node [D] has a non-fixed 223 address. 225 We will examine how these various nodes communicate with node SG-A, 226 and/or how SG-A rejects communications with some such nodes. In the 227 first example, we examine SG-A's point of view. In the second 228 example we look at Q's point of view. In the third example we look 229 at C's point of view. 231 PI is the Public Internet ("The Wild"). 233 3.1. Example #1: A security gateway 235 The machine that we will care in this example is [SG-A], a firewall 236 device of some kind which we wish to configure to respond to BTNS 237 connections from [C]. 239 SG-A has the following PAD and SPD entries: 241 Child SA 243 Rule Remote ID IDs allowed SPD Search by 244 ---- --------- ----------- ------------- 245 1 by-IP 246 2 by-IP 247 3 PUBLICKEY:any ANY by-IP 249 The last entry is the BTNS entry. 251 Figure 2: SG-A PAD table 253 Note that [SG-A]'s PAD entry has one and only one wildcard PAD entry: 254 the BTNS catch-all PAD entry as the last entry, as described in 255 Section 2. 257 and are from [RFC4301] section 258 4.4.3 260 Rule Local Remote Next Layer BTNS Action 261 addr addr Protocol ok 262 ---- ----- ------ ---------- ---- ----------------------- 263 1 [A] [R] ANY N/A BYPASS 264 2 [A] [Q] ANY no PROTECT(ESP,tunnel,AES, 265 SHA256) 266 3 [A] B-net ANY no PROTECT(ESP,tunnel,AES, 267 SHA256) 268 4 [A] ANY ANY yes PROTECT(ESP,transport, 269 integr+conf) 271 Figure 3: [SG-A] SPD table 273 The processing by [SG-A] of SA establishment attempts by various 274 peers is as follows: 276 o [Q] does not match PAD entry #1, but does match PAD entry #2; PAD 277 processing stops, then the SPD is searched by [Q]'s ID to find 278 entry #2; CHILD SAs are then allowed that have [SG-A]'s and [Q]'s 279 addresses as the end-point addresses. 281 o [SG-B] matches PAD entry #1; PAD processing stops, then the SPD is 282 searched by [SG-B]'s ID to find SPD entry #3; CHILD SAs are then 283 allowed that have [SG-A]'s address and any addresses from B's 284 network as the end-point addresses. 286 o [R] does not initiate any IKE SAs; its traffic to [A] is bypassed 287 by SPD entry #1. 289 o [C] does not match PAD entries #1 or #2, but does match entry #3, 290 the BTNS wildcard PAD entry; the SPD is searched by [C]'s address 291 and SPD entry #4 is matched. CHILD SAs are then allowed that have 292 [SG-A]'s address and [C]'s address as the end-point addresses 293 provided that [C]'s address is neither [Q]'s nor any of [B]'s (see 294 Section 2). See the last bullet item below. 296 o A rogue BTNS node attempting to assert [Q]'s or [B]'s addresses 297 will either match the PAD entries for [Q] or [B] and fail to 298 authenticate as [Q] or [B], in which case they are rejected, or 299 they will match PAD entry #3 but will not be allowed to create 300 CHILD SAs with [Q]'s or [B]'s addresses as traffic selectors. 302 o A rogue BTNS node attempting to establish an SA whereby the rogue 303 node asserts [C]'s address will succeed at establishing such an 304 SA. Protection for [C] requires additional bindings of [C]'s 305 specific BTNS ID (that is, its public key) to its traffic flows 306 through connection-latching and channel binding, or leap-of-faith, 307 none of which are described here. 309 3.2. Example #2: A mixed end-system 311 [Q] is an NFSv4 server. 313 [Q] is a native IPsec implementation, and it's NFSv4 implementation 314 is IPsec-aware. 316 [Q] wants to protect all traffic with [A]. [Q] also wants to protect 317 NFSv4 traffice with all peers. It's PAD and SPD are configured as 318 follows: 320 Child SA 321 Rule Remote ID IDs allowed SPD Search by 322 ---- --------- ----------- ------------- 323 1 <[A]'s ID> <[A]'s address> by-IP 324 2 PUBLICKEY:any ANY by-IP 326 The last entry is the BTNS entry. 328 Figure 4: [Q] PAD table 330 Rule Local Remote Next Layer BTNS Action 331 addr addr Protocol ok 332 ---- ----- ------ ---------- ---- ----------------------- 333 1 [Q] [A] ANY no PROTECT(ESP,tunnel,AES, 334 SHA256) 335 2 [Q] ANY ANY yes PROTECT(ESP,transport, 336 with integr+conf) 337 port 2049 339 Figure 5: [Q] SPD table 341 The same analysis shown above in Section 3.1 applies here with 342 respect to [SG-A], [C] and rogue peers. The second SPD entry permits 343 any BTNS capable node to negotiate a port-specific SA to port 2049, 344 the port on which NFSv4 runs. Additionally [SG-B] is treated as a 345 BTNS peer as it is not known to [Q], and therefore any host behind 346 [SG-B] can access the NFSv4 service on [Q]. As [Q] has no formal 347 relationship with [SG-B], rogues can impersonate [B] (i.e., assert 348 [B]'s addresses). 350 3.3. Example #3: A BTNS-only system 352 [C] supports only BTNS and wants to use BTNS to protect NFSv4 353 traffic. It's PAD and SPD are configured as follows: 355 Child SA 356 Rule Remote ID IDs allowed SPD Search by 357 ---- --------- ----------- ------------- 358 1 PUBLICKEY:any ANY by-IP 360 The last (and only) entry is the BTNS entry. 362 Figure 6: Q PAD table 364 Rule Local Remote Next Layer BTNS Action 365 addr addr Protocol ok 366 ---- ----- ------ ---------- ---- ----------------------- 367 1 [C] ANY ANY yes PROTECT(ESP,transport, 368 with integr+conf) 369 port 370 2049 372 2 [C] ANY ANY N/A BYPASS 374 Figure 7: SG-A SPD table 376 The analysis from Section 3.1 applies as follows: 378 o Communication with [Q] on port 2049 matches SPD entry number 1. 379 This causes [C] to initiate an IKEv2 exchange with [Q]. The PAD 380 entry on [C] causes it to not care what identity [Q] asserts. 381 Further authentication (and channel binding) could occur within 382 the NFSv4 protocol. 384 o Communication with [A], [B] or any other internet machine 385 (including [Q]), occurs in the clear, so long as it is not on port 386 2049. 388 o All analysis about rogue BTNS nodes applies, but they can only 389 assert SAs for port 2049. 391 3.4. Miscellaneous comments 393 If [SG-A] were not BTNS-capable then it would not have PAD and SPD 394 entries #3 and #4, respectively in example #1. Then [C] would be 395 rejected as usual under the standard IPsec model [RFC4301]. 397 Similarly, if [Q] were not BTNS-capable then it would not have PAD 398 and SPD entries #2 in example #2. Then [C] would be rejected as 399 usual under the standard IPsec model [RFC4301]. 401 4. Security Considerations 403 Unauthenticated security association negotiation is subject to MITM 404 attacks and should be used with care. Where security infrastructures 405 are lacking this may indeed be better than nothing. 407 Use with applications that bind authentication at higher network 408 layers to secure channels at lower layers may provide one secure way 409 to use unauthenticated IPsec, but this is not specified herein. 411 The BTNS PAD entry must be last and its child SA ID constraints must 412 be non-overlapping with any other PAD entry, as described in section 413 2, in order to ensure that no BTNS peer can impersonate another IPsec 414 non-BTNS peer. 416 4.1. Connection-Latching and Channel Binding 418 BTNS is subject to MITM attacks. One way to protect against MITM 419 attacks subsequent to initial communications is to use "connection 420 latching" [I-D.ietf-btns-connection-latching]. In connection 421 latching, ULPs cooperate with IPsec to bind discrete packet flows to 422 sequences of similar SAs. Connection latching requires native IPsec 423 implementations. 425 MITMs can be detected by using application-layer authentication 426 frameworks and/or mechanisms, such as the GSS-API [RFC2743], with 427 channel binding [RFC5056]. IPsec "channels" are nothing other than 428 latched connnections. 430 4.2. Leap-of-Faith (LoF) for BTNS 432 "Leap of faith" is the term generally used when a user accepts the 433 assertion that a given key identifies a peer on the first 434 communication, despite a lack of strong evidence for that assertion, 435 and then remembers this association for future communications. 436 Specifically this is a common mode of operation for Secure Shell 437 [RFC4251] client. When a server is encountered for the first time 438 the Secure Shell client may ask the user whether to accept the 439 server's public key. If so, records the server's name (as given by 440 the user) and public key in a database. 442 Leap of faith can work in a similar way for BTNS nodes, but it is 443 currently still being designed and specified by the IETF BTNS WG. 445 5. IANA Considerations 447 This document has no IANA considerations, neither seeking to create 448 new registrations nor new registries. (The new ID type is not used 449 on the wire, therefore it need not be assigned a number from the IANA 450 IKEv2 Identification Payload ID Types registry.) 452 6. Acknowledgements 454 Thanks for the following reviewers: Stephen Kent 456 7. References 458 7.1. Normative References 460 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 461 Requirement Levels", BCP 14, RFC 2119, March 1997. 463 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 464 Internet Protocol", RFC 4301, December 2005. 466 7.2. Informative References 468 [I-D.ietf-btns-connection-latching] 469 Williams, N., "IPsec Channels: Connection Latching", 470 draft-ietf-btns-connection-latching-07 (work in progress), 471 April 2008. 473 [I-D.ietf-btns-prob-and-applic] 474 Touch, J., Black, D., and Y. Wang, "Problem and 475 Applicability Statement for Better Than Nothing Security 476 (BTNS)", draft-ietf-btns-prob-and-applic-07 (work in 477 progress), July 2008. 479 [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the 480 Internet Protocol", RFC 2401, November 1998. 482 [RFC2408] Maughan, D., Schneider, M., and M. Schertler, "Internet 483 Security Association and Key Management Protocol 484 (ISAKMP)", RFC 2408, November 1998. 486 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 487 (IKE)", RFC 2409, November 1998. 489 [RFC2743] Linn, J., "Generic Security Service Application Program 490 Interface Version 2, Update 1", RFC 2743, January 2000. 492 [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) 493 Protocol Architecture", RFC 4251, January 2006. 495 [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 496 RFC 4306, December 2005. 498 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 499 Channels", RFC 5056, November 2007. 501 Authors' Addresses 503 Nicolas Williams 504 Sun Microsystems 505 5300 Riata Trace Ct 506 Austin, TX 78727 507 US 509 Email: Nicolas.Williams@sun.com 511 Michael C. Richardson 512 Sandelman Software Works 513 470 Dawson Avenue 514 Ottawa, ON K1Z 5V7 515 CA 517 Email: mcr@sandelman.ottawa.on.ca 518 URI: http://www.sandelman.ottawa.on.ca/ 520 Full Copyright Statement 522 Copyright (C) The IETF Trust (2008). 524 This document is subject to the rights, licenses and restrictions 525 contained in BCP 78, and except as set forth therein, the authors 526 retain all their rights. 528 This document and the information contained herein are provided on an 529 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 530 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 531 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 532 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 533 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 534 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 536 Intellectual Property 538 The IETF takes no position regarding the validity or scope of any 539 Intellectual Property Rights or other rights that might be claimed to 540 pertain to the implementation or use of the technology described in 541 this document or the extent to which any license under such rights 542 might or might not be available; nor does it represent that it has 543 made any independent effort to identify any such rights. 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