idnits 2.17.00 (12 Aug 2021) /tmp/idnits14010/draft-ietf-ipsecme-qr-ikev2-09.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (November 27, 2019) is 906 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-07) exists of draft-hoffman-c2pq-05 -- Obsolete informational reference (is this intentional?): RFC 2409 (Obsoleted by RFC 4306) Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force S. Fluhrer 3 Internet-Draft D. McGrew 4 Intended status: Standards Track P. Kampanakis 5 Expires: May 30, 2020 Cisco Systems 6 V. Smyslov 7 ELVIS-PLUS 8 November 27, 2019 10 Postquantum Preshared Keys for IKEv2 11 draft-ietf-ipsecme-qr-ikev2-09 13 Abstract 15 The possibility of Quantum Computers poses a serious challenge to 16 cryptographic algorithms deployed widely today. IKEv2 is one example 17 of a cryptosystem that could be broken; someone storing VPN 18 communications today could decrypt them at a later time when a 19 Quantum Computer is available. It is anticipated that IKEv2 will be 20 extended to support quantum-secure key exchange algorithms; however 21 that is not likely to happen in the near term. To address this 22 problem before then, this document describes an extension of IKEv2 to 23 allow it to be resistant to a Quantum Computer, by using preshared 24 keys. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on May 30, 2020. 43 Copyright Notice 45 Copyright (c) 2019 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 6 63 2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 6 64 3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 6 65 4. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 11 66 5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 67 5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 12 68 5.2. Operational Considerations . . . . . . . . . . . . . . . 13 69 5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 13 70 5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 13 71 5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 14 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 75 8.1. Normative References . . . . . . . . . . . . . . . . . . 17 76 8.2. Informational References . . . . . . . . . . . . . . . . 17 77 Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 18 78 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 19 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 81 1. Introduction 83 Recent achievements in developing Quantum Computers demonstrate that 84 it is probably feasible to build a cryptographically significant one. 85 If such a computer is implemented, many of the cryptographic 86 algorithms and protocols currently in use would be insecure. A 87 Quantum Computer would be able to solve DH and ECDH problems in 88 polynomial time [I-D.hoffman-c2pq], and this would imply that the 89 security of existing IKEv2 [RFC7296] systems would be compromised. 90 IKEv1 [RFC2409], when used with strong preshared keys, is not 91 vulnerable to quantum attacks, because those keys are one of the 92 inputs to the key derivation function. If the preshared key has 93 sufficient entropy and the PRF, encryption and authentication 94 transforms are quantum-secure, then the resulting system is believed 95 to be quantum resistant, that is, invulnerable to an attacker with a 96 Quantum Computer. 98 This document describes a way to extend IKEv2 to have a similar 99 property; assuming that the two end systems share a long secret key, 100 then the resulting exchange is quantum resistant. By bringing 101 postquantum security to IKEv2, this note removes the need to use an 102 obsolete version of the Internet Key Exchange in order to achieve 103 that security goal. 105 The general idea is that we add an additional secret that is shared 106 between the initiator and the responder; this secret is in addition 107 to the authentication method that is already provided within IKEv2. 108 We stir this secret into the SK_d value, which is used to generate 109 the key material (KEYMAT) and the SKEYSEED for the child SAs; this 110 secret provides quantum resistance to the IPsec SAs (and any child 111 IKE SAs). We also stir the secret into the SK_pi, SK_pr values; this 112 allows both sides to detect a secret mismatch cleanly. 114 It was considered important to minimize the changes to IKEv2. The 115 existing mechanisms to do authentication and key exchange remain in 116 place (that is, we continue to do (EC)DH, and potentially PKI 117 authentication if configured). This document does not replace the 118 authentication checks that the protocol does; instead, it is done as 119 a parallel check. 121 1.1. Changes 123 RFC EDITOR PLEASE DELETE THIS SECTION. 125 Changes in this draft in each version iterations. 127 draft-ietf-ipsecme-qr-ikev2-09 129 o Addresses issues raised in AD review. 131 draft-ietf-ipsecme-qr-ikev2-08 133 o Editorial changes. 135 draft-ietf-ipsecme-qr-ikev2-07 137 o Editorial changes. 139 draft-ietf-ipsecme-qr-ikev2-06 141 o Editorial changes. 143 o Addressed comments received during WGLC. 145 draft-ietf-ipsecme-qr-ikev2-04 147 o Using Group PPK is clarified based on comment from Quynh Dang. 149 draft-ietf-ipsecme-qr-ikev2-03 151 o Editorial changes and minor text nit fixes. 153 o Integrated Tommy P. text suggestions. 155 draft-ietf-ipsecme-qr-ikev2-02 157 o Added note that the PPK is stirred in the initial IKE SA setup 158 only. 160 o Added note about the initiator ignoring any content in the 161 PPK_IDENTITY notification from the responder. 163 o fixed Tero's suggestions from 2/6/1028 165 o Added IANA assigned message types where necessary. 167 o fixed minor text nits 169 draft-ietf-ipsecme-qr-ikev2-01 171 o Nits and minor fixes. 173 o prf is replaced with prf+ for the SK_d and SK_pi/r calculations. 175 o Clarified using PPK in case of EAP authentication. 177 o PPK_SUPPORT notification is changed to USE_PPK to better reflect 178 its purpose. 180 draft-ietf-ipsecme-qr-ikev2-00 182 o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr- 183 ikev2-00 that is a WG item. 185 draft-fluhrer-qr-ikev2-05 187 o Nits and editorial fixes. 189 o Made PPK_ID format and PPK Distributions subsection of the PPK 190 section. Also added an Operational Considerations section. 192 o Added comment about Child SA rekey in the Security Considerations 193 section. 195 o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not 196 configured for a responder. 198 o Various text changes and clarifications. 200 o Expanded Security Considerations section to describe some security 201 concerns and how they should be addressed. 203 draft-fluhrer-qr-ikev2-03 205 o Modified how we stir the PPK into the IKEv2 secret state. 207 o Modified how the use of PPKs is negotiated. 209 draft-fluhrer-qr-ikev2-02 211 o Simplified the protocol by stirring in the preshared key into the 212 child SAs; this avoids the problem of having the responder decide 213 which preshared key to use (as it knows the initiator identity at 214 that point); it does mean that someone with a Quantum Computer can 215 recover the initial IKE negotiation. 217 o Removed positive endorsements of various algorithms. Retained 218 warnings about algorithms known to be weak against a Quantum 219 Computer. 221 draft-fluhrer-qr-ikev2-01 223 o Added explicit guidance as to what IKE and IPsec algorithms are 224 quantum resistant. 226 draft-fluhrer-qr-ikev2-00 228 o We switched from using vendor ID's to transmit the additional data 229 to notifications. 231 o We added a mandatory cookie exchange to allow the server to 232 communicate to the client before the initial exchange. 234 o We added algorithm agility by having the server tell the client 235 what algorithm to use in the cookie exchange. 237 o We have the server specify the PPK Indicator Input, which allows 238 the server to make a trade-off between the efficiency for the 239 search of the clients PPK, and the anonymity of the client. 241 o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to 242 transform the nonces during the KDF. 244 1.2. Requirements Language 246 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 247 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 248 "OPTIONAL" in this document are to be interpreted as described in BCP 249 14 [RFC2119] [RFC8174] when, and only when, they appear in all 250 capitals, as shown here. 252 2. Assumptions 254 We assume that each IKE peer has a list of Postquantum Preshared Keys 255 (PPK) along with their identifiers (PPK_ID), and any potential IKE 256 initiator selects which PPK to use with any specific responder. In 257 addition, implementations have a configurable flag that determines 258 whether this postquantum preshared key is mandatory. This PPK is 259 independent of the preshared key (if any) that the IKEv2 protocol 260 uses to perform authentication (because the preshared key in IKEv2 is 261 not used for any key derivation, and thus doesn't protect against 262 Quantum Computers). The PPK specific configuration that is assumed 263 to be on each node consists of the following tuple: 265 Peer, PPK, PPK_ID, mandatory_or_not 267 3. Exchanges 269 If the initiator is configured to use a postquantum preshared key 270 with the responder (whether or not the use of the PPK is mandatory), 271 then it will include a notification USE_PPK in the IKE_SA_INIT 272 request message as follows: 274 Initiator Responder 275 ------------------------------------------------------------------ 276 HDR, SAi1, KEi, Ni, N(USE_PPK) ---> 278 N(USE_PPK) is a status notification payload with the type 16435; it 279 has a protocol ID of 0, no SPI and no notification data associated 280 with it. 282 If the initiator needs to resend this initial message with a cookie 283 (because the responder response included a COOKIE notification), then 284 the resend would include the USE_PPK notification if the original 285 message did. 287 If the responder does not support this specification or does not have 288 any PPK configured, then it ignores the received notification and 289 continues with the IKEv2 protocol as normal. Otherwise the responder 290 replies with the IKE_SA_INIT message including a USE_PPK notification 291 in the response: 293 Initiator Responder 294 ------------------------------------------------------------------ 295 <--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(USE_PPK) 297 When the initiator receives this reply, it checks whether the 298 responder included the USE_PPK notification. If the responder did 299 not and the flag mandatory_or_not indicates that using PPKs is 300 mandatory for communication with this responder, then the initiator 301 MUST abort the exchange. This situation may happen in case of 302 misconfiguration, when the initiator believes it has a mandatory to 303 use PPK for the responder, while the responder either doesn't support 304 PPKs at all or doesn't have any PPK configured for the initiator. 305 See Section 6 for discussion of the possible impacts of this 306 situation. 308 If the responder did not include the USE_PPK notification and using a 309 PPK for this particular responder is optional, then the initiator 310 continues with the IKEv2 protocol as normal, without using PPKs. 312 If the responder did include the USE_PPK notification, then the 313 initiator selects a PPK, along with its identifier PPK_ID. Then, it 314 computes this modification of the standard IKEv2 key derivation: 316 SKEYSEED = prf(Ni | Nr, g^ir) 317 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) 318 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } 320 SK_d = prf+ (PPK, SK_d') 321 SK_pi = prf+ (PPK, SK_pi') 322 SK_pr = prf+ (PPK, SK_pr') 324 That is, we use the standard IKEv2 key derivation process except that 325 the three subkeys SK_d, SK_pi, SK_pr are run through the prf+ again, 326 this time using the PPK as the key. Using prf+ construction ensures 327 that it is always possible to get the resulting keys of the same size 328 as the initial ones, even if the underlying PRF has output size 329 different from its key size. Note, that at the time this document 330 was written, all PRFs defined for use in IKEv2 [IKEV2-IANA-PRFS] had 331 output size equal to the (preferred) key size. For such PRFs only 332 the first iteration of prf+ is needed: 334 SK_d = prf (PPK, SK_d' | 0x01) 335 SK_pi = prf (PPK, SK_pi' | 0x01) 336 SK_pr = prf (PPK, SK_pr' | 0x01) 338 Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only 339 during the initial IKE SA setup. It MUST NOT be used when these 340 subkeys are calculated as result of IKE SA rekey, resumption or other 341 similar operation. 343 The initiator then sends the IKE_AUTH request message, including the 344 PPK_ID value as follows: 346 Initiator Responder 347 ------------------------------------------------------------------ 348 HDR, SK {IDi, [CERT,] [CERTREQ,] 349 [IDr,] AUTH, SAi2, 350 TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} ---> 352 PPK_IDENTITY is a status notification with the type 16436; it has a 353 protocol ID of 0, no SPI and a notification data that consists of the 354 identifier PPK_ID. 356 A situation may happen when the responder has some PPKs, but doesn't 357 have a PPK with the PPK_ID received from the initiator. In this case 358 the responder cannot continue with PPK (in particular, it cannot 359 authenticate the initiator), but the responder could be able to 360 continue with normal IKEv2 protocol if the initiator provided its 361 authentication data computed as in normal IKEv2, without using PPKs. 362 For this purpose, if using PPKs for communication with this responder 363 is optional for the initiator, then the initiator MAY include a 364 notification NO_PPK_AUTH in the above message. 366 NO_PPK_AUTH is a status notification with the type 16437; it has a 367 protocol ID of 0 and no SPI. The Notification Data field contains 368 the initiator's authentication data computed using SK_pi', which has 369 been computed without using PPKs. This is the same data that would 370 normally be placed in the Authentication Data field of an AUTH 371 payload. Since the Auth Method field is not present in the 372 notification, the authentication method used for computing the 373 authentication data MUST be the same as method indicated in the AUTH 374 payload. Note that if the initiator decides to include the 375 NO_PPK_AUTH notification, the initiator needs to perform 376 authentication data computation twice, which may consume computation 377 power (e.g. if digital signatures are involved). 379 When the responder receives this encrypted exchange, it first 380 computes the values: 382 SKEYSEED = prf(Ni | Nr, g^ir) 383 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } 384 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) 386 The responder then uses the SK_ei/SK_ai values to decrypt/check the 387 message and then scans through the payloads for the PPK_ID attached 388 to the PPK_IDENTITY notification. If no PPK_IDENTITY notification is 389 found and the peers successfully exchanged USE_PPK notifications in 390 the IKE_SA_INIT exchange, then the responder MUST send back 391 AUTHENTICATION_FAILED notification and then fail the negotiation. 393 If the PPK_IDENTITY notification contains a PPK_ID that is not known 394 to the responder or is not configured for use for the identity from 395 IDi payload, then the responder checks whether using PPKs for this 396 initiator is mandatory and whether the initiator included NO_PPK_AUTH 397 notification in the message. If using PPKs is mandatory or no 398 NO_PPK_AUTH notification is found, then then the responder MUST send 399 back AUTHENTICATION_FAILED notification and then fail the 400 negotiation. Otherwise (when PPK is optional and the initiator 401 included NO_PPK_AUTH notification) the responder MAY continue regular 402 IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH 403 notification as the authentication data (which usually resides in the 404 AUTH payload), for the purpose of the initiator authentication. 405 Note, that Authentication Method is still indicated in the AUTH 406 payload. 408 This table summarizes the above logic for the responder: 410 Received Received Configured PPK is 411 USE_PPK NO_PPK_AUTH with PPK Mandatory Action 412 --------------------------------------------------------------------- 413 No * No * Standard IKEv2 protocol 414 No * Yes No Standard IKEv2 protocol 415 No * Yes Yes Abort negotiation 416 Yes No No * Abort negotiation 417 Yes Yes No Yes Abort negotiation 418 Yes Yes No No Standard IKEv2 protocol 419 Yes * Yes * Use PPK 421 If PPK is in use, then the responder extracts the corresponding PPK 422 and computes the following values: 424 SK_d = prf+ (PPK, SK_d') 425 SK_pi = prf+ (PPK, SK_pi') 426 SK_pr = prf+ (PPK, SK_pr') 428 The responder then continues with the IKE_AUTH exchange (validating 429 the AUTH payload that the initiator included) as usual and sends back 430 a response, which includes the PPK_IDENTITY notification with no data 431 to indicate that the PPK is used in the exchange: 433 Initiator Responder 434 ------------------------------------------------------------------ 435 <-- HDR, SK {IDr, [CERT,] 436 AUTH, SAr2, 437 TSi, TSr, N(PPK_IDENTITY)} 439 When the initiator receives the response, then it checks for the 440 presence of the PPK_IDENTITY notification. If it receives one, it 441 marks the SA as using the configured PPK to generate SK_d, SK_pi, 442 SK_pr (as shown above); the content of the received PPK_IDENTITY (if 443 any) MUST be ignored. If the initiator does not receive the 444 PPK_IDENTITY, it MUST either fail the IKE SA negotiation sending the 445 AUTHENTICATION_FAILED notification in the Informational exchange (if 446 the PPK was configured as mandatory), or continue without using the 447 PPK (if the PPK was not configured as mandatory and the initiator 448 included the NO_PPK_AUTH notification in the request). 450 If EAP is used in the IKE_AUTH exchange, then the initiator doesn't 451 include AUTH payload in the first request message, however the 452 responder sends back AUTH payload in the first reply. The peers then 453 exchange AUTH payloads after EAP is successfully completed. As a 454 result, the responder sends AUTH payload twice - in the first 455 IKE_AUTH reply message and in the last one, while the initiator sends 456 AUTH payload only in the last IKE_AUTH request. See more details 457 about EAP authentication in IKEv2 in Section 2.16 of [RFC7296]. 459 The general rule for using PPK in the IKE_AUTH exchange, which covers 460 EAP authentication case too, is that the initiator includes 461 PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request 462 message containing AUTH payload. Therefore, in case of EAP the 463 responder always computes the AUTH payload in the first IKE_AUTH 464 reply message without using PPK (by means of SK_pr'), since PPK_ID is 465 not yet known to the responder. Once the IKE_AUTH request message 466 containing the PPK_IDENTITY notification is received, the responder 467 follows the rules described above for the non-EAP authentication 468 case. 470 Initiator Responder 471 ---------------------------------------------------------------- 472 HDR, SK {IDi, [CERTREQ,] 473 [IDr,] SAi2, 474 TSi, TSr} --> 475 <-- HDR, SK {IDr, [CERT,] AUTH, 476 EAP} 477 HDR, SK {EAP} --> 478 <-- HDR, SK {EAP (success)} 479 HDR, SK {AUTH, 480 N(PPK_IDENTITY, PPK_ID) 481 [, N(NO_PPK_AUTH)]} --> 482 <-- HDR, SK {AUTH, SAr2, TSi, TSr 483 [, N(PPK_IDENTITY)]} 485 Note that the diagram above shows both the cases when the responder 486 uses PPK and when it chooses not to use it (provided the initiator 487 has included NO_PPK_AUTH notification), and thus the responder's 488 PPK_IDENTITY notification is marked as optional. Also, note that the 489 IKE_SA_INIT exchange in case of PPK is as described above (including 490 exchange of the USE_PPK notifications), regardless whether EAP is 491 employed in the IKE_AUTH or not. 493 4. Upgrade procedure 495 This algorithm was designed so that someone can introduce PPKs into 496 an existing IKE network without causing network disruption. 498 In the initial phase of the network upgrade, the network 499 administrator would visit each IKE node, and configure: 501 o The set of PPKs (and corresponding PPK_IDs) that this node would 502 need to know. 504 o For each peer that this node would initiate to, which PPK will be 505 used. 507 o That the use of PPK is currently not mandatory. 509 With this configuration, the node will continue to operate with nodes 510 that have not yet been upgraded. This is due to the USE_PPK 511 notification and the NO_PPK_AUTH notification; if the initiator has 512 not been upgraded, it will not send the USE_PPK notification (and so 513 the responder will know that the peers will not use a PPK). If the 514 responder has not been upgraded, it will not send the USE_PPK 515 notification (and so the initiator will know to not use a PPK). If 516 both peers have been upgraded, but the responder isn't yet configured 517 with the PPK for the initiator, then the responder could do standard 518 IKEv2 protocol if the initiator sent NO_PPK_AUTH notification. If 519 both the responder and initiator have been upgraded and properly 520 configured, they will both realize it, and the Child SAs will be 521 quantum-secure. 523 As an optional second step, after all nodes have been upgraded, then 524 the administrator should then go back through the nodes, and mark the 525 use of PPK as mandatory. This will not affect the strength against a 526 passive attacker; it would mean that an attacker with a Quantum 527 Computer (which is sufficiently fast to be able to break the (EC)DH 528 in real time) would not be able to perform a downgrade attack. 530 5. PPK 532 5.1. PPK_ID format 534 This standard requires that both the initiator and the responder have 535 a secret PPK value, with the responder selecting the PPK based on the 536 PPK_ID that the initiator sends. In this standard, both the 537 initiator and the responder are configured with fixed PPK and PPK_ID 538 values, and do the look up based on PPK_ID value. It is anticipated 539 that later standards will extend this technique to allow dynamically 540 changing PPK values. To facilitate such an extension, we specify 541 that the PPK_ID the initiator sends will have its first octet be the 542 PPK_ID Type value. This document defines two values for PPK_ID Type: 544 o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and 545 the PPK itself) is not specified by this document; it is assumed 546 to be mutually intelligible by both by initiator and the 547 responder. This PPK_ID type is intended for those implementations 548 that choose not to disclose the type of PPK to active attackers. 550 o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the 551 PPK are fixed octet strings; the remaining bytes of the PPK_ID are 552 a configured value. We assume that there is a fixed mapping 553 between PPK_ID and PPK, which is configured locally to both the 554 initiator and the responder. The responder can use the PPK_ID to 555 look up the corresponding PPK value. Not all implementations are 556 able to configure arbitrary octet strings; to improve the 557 potential interoperability, it is recommended that, in the 558 PPK_ID_FIXED case, both the PPK and the PPK_ID strings be limited 559 to the base64 character set, namely the 64 characters 0-9, A-Z, 560 a-z, + and /. 562 The PPK_ID type value 0 is reserved; values 3-127 are reserved for 563 IANA; values 128-255 are for private use among mutually consenting 564 parties. 566 5.2. Operational Considerations 568 The need to maintain several independent sets of security credentials 569 can significantly complicate a security administrator's job, and can 570 potentially slow down widespread adoption of this specification. It 571 is anticipated, that administrators will try to simplify their job by 572 decreasing the number of credentials they need to maintain. This 573 section describes some of the considerations for PPK management. 575 5.2.1. PPK Distribution 577 PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are 578 assumed to be configured within the IKE device in an out-of-band 579 fashion. While the method of distribution is a local matter and out 580 of scope of this document or IKEv2, [RFC6030] describes a format for 581 for the transport and provisioning of symmetric keys. That format 582 could be reused using the PIN profile (defined in Section 10.2 of 583 [RFC6030]) with the "Id" attribute of the element being the 584 PPK_ID (without the PPK_ID Type octet for a PPK_ID_FIXED) and the 585 element containing the PPK. 587 5.2.2. Group PPK 589 This document doesn't explicitly require that PPK is unique for each 590 pair of peers. If it is the case, then this solution provides full 591 peer authentication, but it also means that each host must have as 592 many independent PPKs as the peers it is going to communicate with. 593 As the number of peers grows the PPKs will not scale. 595 It is possible to use a single PPK for a group of users. Since each 596 peer uses classical public key cryptography in addition to PPK for 597 key exchange and authentication, members of the group can neither 598 impersonate each other nor read other's traffic, unless they use 599 Quantum Computers to break public key operations. However group 600 members can record any traffic they have access to that comes from 601 other group members and decrypt it later, when they get access to a 602 Quantum Computer. 604 In addition, the fact that the PPK is known to a (potentially large) 605 group of users makes it more susceptible to theft. When an attacker 606 equipped with a Quantum Computer gets access to a group PPK, all 607 communications inside the group are revealed. 609 For these reasons using group PPK is NOT RECOMMENDED. 611 5.2.3. PPK-only Authentication 613 If Quantum Computers become a reality, classical public key 614 cryptography will provide little security, so administrators may find 615 it attractive not to use it at all for authentication. This will 616 reduce the number of credentials they need to maintain to PPKs only. 617 Combining group PPK and PPK-only authentication is NOT RECOMMENDED, 618 since in this case any member of the group can impersonate any other 619 member even without help of Quantum Computers. 621 PPK-only authentication can be achieved in IKEv2 if the NULL 622 Authentication method [RFC7619] is employed. Without PPK the NULL 623 Authentication method provides no authentication of the peers, 624 however since a PPK is stirred into the SK_pi and the SK_pr, the 625 peers become authenticated if a PPK is in use. Using PPKs MUST be 626 mandatory for the peers if they advertise support for PPK in 627 IKE_SA_INIT and use NULL Authentication. Addtionally, since the 628 peers are authenticated via PPK, the ID Type in the IDi/IDr payloads 629 SHOULD NOT be ID_NULL, despite using the NULL Authentication method. 631 6. Security Considerations 633 Quantum computers are able to perform Grover's algorithm; that 634 effectively halves the size of a symmetric key. Because of this, the 635 user SHOULD ensure that the postquantum preshared key used has at 636 least 256 bits of entropy, in order to provide 128 bits of security. 638 With this protocol, the computed SK_d is a function of the PPK. 639 Assuming that the PPK has sufficient entropy (for example, at least 640 2^256 possible values), then even if an attacker was able to recover 641 the rest of the inputs to the PRF function, it would be infeasible to 642 use Grover's algorithm with a Quantum Computer to recover the SK_d 643 value. Similarly, all keys that are a function of SK_d, which 644 include all Child SAs keys and all keys for subsequent IKE SAs 645 (created when the initial IKE SA is rekeyed), are also quantum 646 resistant (assuming that the PPK was of high enough entropy, and that 647 all the subkeys are sufficiently long). 649 An attacker with a Quantum Computer that can decrypt the initial IKE 650 SA has access to all the information exchanged over it, such as 651 identities of the peers, configuration parameters and all negotiated 652 IPsec SAs information (including traffic selectors), with the 653 exception of the cryptographic keys used by the IPsec SAs which are 654 protected by the PPK. 656 Deployments that treat this information as sensitive or that send 657 other sensitive data (like cryptographic keys) over IKE SA MUST rekey 658 the IKE SA before the sensitive information is sent to ensure this 659 information is protected by the PPK. It is possible to create a 660 childless IKE SA as specified in [RFC6023]. This prevents Child SA 661 configuration information from being transmited in the original IKE 662 SA that is not protected by a PPK. Some information related to IKE 663 SA, that is sent in the IKE_AUTH exchange, such as peer identities, 664 feature notifications, Vendor ID's etc. cannot be hidden from the 665 attack described above, even if the additional IKE SA rekey is 666 performed. 668 In addition, the policy SHOULD be set to negotiate only quantum- 669 resistant symmetric algorithms; while this RFC doesn't claim to give 670 advice as to what algorithms are secure (as that may change based on 671 future cryptographical results), below is a list of defined IKEv2 and 672 IPsec algorithms that should NOT be used, as they are known not to be 673 quantum resistant 675 o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with 676 key size less than 256 bits. 678 o Any ESP Transform with key size less than 256 bits. 680 o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined 681 to be able to use an arbitrary key size, they convert it into a 682 128-bit key internally. 684 Section 3 requires the initiator to abort the initial exchange if 685 using PPKs is mandatory for it, but the responder does not include 686 the USE_PPK notification in the response. In this situation, when 687 the initiator aborts negotiation it leaves a half-open IKE SA on the 688 responder (because IKE_SA_INIT completes successfully from the 689 responder's point of view). This half-open SA will eventually expire 690 and be deleted, but if the initiator continues its attempts to create 691 IKE SA with a high enough rate, then the responder may consider it as 692 a Denial-of-Service attack and take protection measures (see 693 [RFC8019] for more detail). In this situation, it is RECOMMENDED 694 that the initiator caches the negative result of the negotiation for 695 some time and doesn't make attempts to create it again for some time, 696 because this is a result of misconfiguration and probably some re- 697 configuration of the peers is needed. 699 If using PPKs is optional for both peers and they authenticate 700 themselves using digital signatures, then an attacker in between, 701 equipped with a Quantum Computer capable of breaking public key 702 operations in real time, is able to mount downgrade attack by 703 removing USE_PPK notification from the IKE_SA_INIT and forging 704 digital signatures in the subsequent exchange. If using PPKs is 705 mandatory for at least one of the peers or PSK is used for 706 authentication, then the attack will be detected and the SA won't be 707 created. 709 If using PPKs is mandatory for the initiator, then an attacker able 710 to eavesdrop and to inject packets into the network can prevent 711 creating an IKE SA by mounting the following attack. The attacker 712 intercepts the initial request containing the USE_PPK notification 713 and injects a forged response containing no USE_PPK. If the attacker 714 manages to inject this packet before the responder sends a genuine 715 response, then the initiator would abort the exchange. To thwart 716 this kind of attack it is RECOMMENDED, that if using PPKs is 717 mandatory for the initiator and the received response doesn't contain 718 the USE_PPK notification, then the initiator doesn't abort the 719 exchange immediately, but instead waits some time for more responses 720 (possibly retransmitting the request). If all the received responses 721 contain no USE_PPK, then the exchange is aborted. 723 If using PPK is optional for both peers, then in case of 724 misconfiguration (e.g. mismatched PPK_ID) the IKE SA will be created 725 without protection against Quantum Computers. It is advised that if 726 PPK was configured, but was not used for a particular IKE SA, then 727 implementations SHOULD audit this event. 729 7. IANA Considerations 731 This document defines three new Notify Message Types in the "Notify 732 Message Types - Status Types" registry: 734 16435 USE_PPK 735 16436 PPK_IDENTITY 736 16437 NO_PPK_AUTH 738 This document also creates a new IANA registry for the PPK_ID types. 739 The initial values of this registry are: 741 PPK_ID Type Value 742 ----------- ----- 743 Reserved 0 744 PPK_ID_OPAQUE 1 745 PPK_ID_FIXED 2 746 Unassigned 3-127 747 Reserved for private use 128-255 749 Changes and additions to this registry are by Expert Review 750 [RFC8126]. 752 8. References 754 8.1. Normative References 756 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 757 Requirement Levels", BCP 14, RFC 2119, 758 DOI 10.17487/RFC2119, March 1997, 759 . 761 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 762 Kivinen, "Internet Key Exchange Protocol Version 2 763 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 764 2014, . 766 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 767 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 768 May 2017, . 770 8.2. Informational References 772 [I-D.hoffman-c2pq] 773 Hoffman, P., "The Transition from Classical to Post- 774 Quantum Cryptography", draft-hoffman-c2pq-05 (work in 775 progress), May 2019. 777 [IKEV2-IANA-PRFS] 778 "Internet Key Exchange Version 2 (IKEv2) Parameters, 779 Transform Type 2 - Pseudorandom Function Transform IDs", 780 . 783 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 784 (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, 785 . 787 [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A 788 Childless Initiation of the Internet Key Exchange Version 789 2 (IKEv2) Security Association (SA)", RFC 6023, 790 DOI 10.17487/RFC6023, October 2010, 791 . 793 [RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric 794 Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030, 795 October 2010, . 797 [RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication 798 Method in the Internet Key Exchange Protocol Version 2 799 (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015, 800 . 802 [RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange 803 Protocol Version 2 (IKEv2) Implementations from 804 Distributed Denial-of-Service Attacks", RFC 8019, 805 DOI 10.17487/RFC8019, November 2016, 806 . 808 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 809 Writing an IANA Considerations Section in RFCs", BCP 26, 810 RFC 8126, DOI 10.17487/RFC8126, June 2017, 811 . 813 Appendix A. Discussion and Rationale 815 The idea behind this document is that while a Quantum Computer can 816 easily reconstruct the shared secret of an (EC)DH exchange, they 817 cannot as easily recover a secret from a symmetric exchange. This 818 document makes the SK_d, and hence the IPsec KEYMAT and any child 819 SA's SKEYSEED, depend on both the symmetric PPK, and also the Diffie- 820 Hellman exchange. If we assume that the attacker knows everything 821 except the PPK during the key exchange, and there are 2^n plausible 822 PPKs, then a Quantum Computer (using Grover's algorithm) would take 823 O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be 824 trivially solved, the attacker still can't recover any key material 825 (except for the SK_ei, SK_er, SK_ai and SK_ar values for the initial 826 IKE exchange) unless they can find the PPK, which is too difficult if 827 the PPK has enough entropy (for example, 256 bits). Note that we do 828 allow an attacker with a Quantum Computer to rederive the keying 829 material for the initial IKE SA; this was a compromise to allow the 830 responder to select the correct PPK quickly. 832 Another goal of this protocol is to minimize the number of changes 833 within the IKEv2 protocol, and in particular, within the cryptography 834 of IKEv2. By limiting our changes to notifications, and only 835 adjusting the SK_d, SK_pi, SK_pr, it is hoped that this would be 836 implementable, even on systems that perform most of the IKEv2 837 processing in hardware. 839 A third goal was to be friendly to incremental deployment in 840 operational networks, for which we might not want to have a global 841 shared key, or quantum resistant IKEv2 is rolled out incrementally. 842 This is why we specifically try to allow the PPK to be dependent on 843 the peer, and why we allow the PPK to be configured as optional. 845 A fourth goal was to avoid violating any of the security properties 846 provided by IKEv2. 848 Appendix B. Acknowledgements 850 We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett, 851 Tommy Pauly, Quynh Dang and the rest of the IPSecME Working Group for 852 their feedback and suggestions for the scheme. 854 Authors' Addresses 856 Scott Fluhrer 857 Cisco Systems 859 Email: sfluhrer@cisco.com 861 David McGrew 862 Cisco Systems 864 Email: mcgrew@cisco.com 866 Panos Kampanakis 867 Cisco Systems 869 Email: pkampana@cisco.com 871 Valery Smyslov 872 ELVIS-PLUS 874 Phone: +7 495 276 0211 875 Email: svan@elvis.ru