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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to use 'NOT RECOMMENDED' as an RFC 2119 keyword, but does not include the phrase in its RFC 2119 key words list. -- The document date (June 18, 2018) is 1433 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to lower-maturity documents in RFCs) == Missing Reference: 'CERTREQ' is mentioned on line 267, but not defined == Outdated reference: A later version (-07) exists of draft-hoffman-c2pq-03 -- Obsolete informational reference (is this intentional?): RFC 2409 (Obsoleted by RFC 4306) -- Obsolete informational reference (is this intentional?): RFC 5226 (Obsoleted by RFC 8126) Summary: 0 errors (**), 0 flaws (~~), 4 warnings (==), 3 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: December 20, 2018 Cisco Systems 6 V. Smyslov 7 ELVIS-PLUS 8 June 18, 2018 10 Postquantum Preshared Keys for IKEv2 11 draft-ietf-ipsecme-qr-ikev2-03 13 Abstract 15 The possibility of Quantum Computers pose a serious challenge to 16 cryptography 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 December 20, 2018. 43 Copyright Notice 45 Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . 5 63 2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 6 65 4. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 10 66 5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 67 5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 11 68 5.2. Operational Considerations . . . . . . . . . . . . . . . 12 69 5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 12 70 5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 12 71 5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 13 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 75 8.1. Normative References . . . . . . . . . . . . . . . . . . 16 76 8.2. Informational References . . . . . . . . . . . . . . . . 16 77 Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 17 78 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 17 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 81 1. Introduction 83 It is an open question whether or not it is feasible to build a 84 Quantum Computer (and if so, when one might be implemented), but if 85 it is, many of the cryptographic algorithms and protocols currently 86 in use would be insecure. A Quantum Computer would be able to solve 87 DH and ECDH problems in polynomial time [I-D.hoffman-c2pq], and this 88 would imply that the security of existing IKEv2 [RFC7296] systems 89 would be compromised. IKEv1 [RFC2409], when used with strong 90 preshared keys, is not vulnerable to quantum attacks, because those 91 keys are one of the inputs to the key derivation function. If the 92 preshared key has sufficient entropy and the PRF, encryption and 93 authentication transforms are postquantum secure, then the resulting 94 system is believed to be quantum resistant, that is, invulnerable to 95 an attacker with a Quantum Computer. 97 This document describes a way to extend IKEv2 to have a similar 98 property; assuming that the two end systems share a long secret key, 99 then the resulting exchange is quantum resistant. By bringing 100 postquantum security to IKEv2, this note removes the need to use an 101 obsolete version of the Internet Key Exchange in order to achieve 102 that security goal. 104 The general idea is that we add an additional secret that is shared 105 between the initiator and the responder; this secret is in addition 106 to the authentication method that is already provided within IKEv2. 107 We stir this secret into the SK_d value, which is used to generate 108 the key material (KEYMAT) and the SKEYSEED for the child SAs; this 109 secret provides quantum resistance to the IPsec SAs (and any child 110 IKE SAs). We also stir the secret into the SK_pi, SK_pr values; this 111 allows both sides to detect a secret mismatch cleanly. 113 It was considered important to minimize the changes to IKEv2. The 114 existing mechanisms to do authentication and key exchange remain in 115 place (that is, we continue to do (EC)DH, and potentially PKI 116 authentication if configured). This document does not replace the 117 authentication checks that the protocol does; instead, it is done as 118 a parallel check. 120 1.1. Changes 122 RFC EDITOR PLEASE DELETE THIS SECTION. 124 Changes in this draft in each version iterations. 126 draft-ietf-ipsecme-qr-ikev2-03 128 o Editorial changes and minor text nit fixes. 130 o Integrated Tommy P. text suggestions. 132 draft-ietf-ipsecme-qr-ikev2-02 134 o Added note that the PPK is stirred in the initial IKE SA setup 135 only. 137 o Added note about the initiator ignoring any content in the 138 PPK_IDENTITY notification from the responder. 140 o fixed Tero's suggestions from 2/6/1028 142 o Added IANA assigned message types where necessary. 144 o fixed minor text nits 145 o Nits and minor fixes. 147 o prf is replaced with prf+ for the SK_d and SK_pi/r calculations. 149 o Clarified using PPK in case of EAP authentication. 151 o PPK_SUPPORT notification is changed to USE_PPK to better reflect 152 its purpose. 154 draft-ietf-ipsecme-qr-ikev2-00 156 o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr- 157 ikev2-00 that is a WG item. 159 draft-fluhrer-qr-ikev2-05 161 o Nits and editorial fixes. 163 o Made PPK_ID format and PPK Distributions subsection of the PPK 164 section. Also added an Operational Considerations section. 166 o Added comment about Child SA rekey in the Security Considerations 167 section. 169 o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not 170 configured for a responder. 172 o Various text changes and clarifications. 174 o Expanded Security Considerations section to describe some security 175 concerns and how they should be addressed. 177 draft-fluhrer-qr-ikev2-03 179 o Modified how we stir the PPK into the IKEv2 secret state. 181 o Modified how the use of PPKs is negotiated. 183 draft-fluhrer-qr-ikev2-02 185 o Simplified the protocol by stirring in the preshared key into the 186 child SAs; this avoids the problem of having the responder decide 187 which preshared key to use (as it knows the initiator identity at 188 that point); it does mean that someone with a Quantum Computer can 189 recover the initial IKE negotiation. 191 o Removed positive endorsements of various algorithms. Retained 192 warnings about algorithms known to be weak against a Quantum 193 Computer. 195 draft-fluhrer-qr-ikev2-01 197 o Added explicit guidance as to what IKE and IPsec algorithms are 198 quantum resistant. 200 draft-fluhrer-qr-ikev2-00 202 o We switched from using vendor ID's to transmit the additional data 203 to notifications. 205 o We added a mandatory cookie exchange to allow the server to 206 communicate to the client before the initial exchange. 208 o We added algorithm agility by having the server tell the client 209 what algorithm to use in the cookie exchange. 211 o We have the server specify the PPK Indicator Input, which allows 212 the server to make a trade-off between the efficiency for the 213 search of the clients PPK, and the anonymity of the client. 215 o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to 216 transform the nonces during the KDF. 218 1.2. Requirements Language 220 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 221 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 222 document are to be interpreted as described in RFC 2119 [RFC2119]. 224 2. Assumptions 226 We assume that each IKE peer has a list of Postquantum Preshared Keys 227 (PPK) along with their identifiers (PPK_ID), and any potential IKE 228 initiator has a selection of which PPK to use with any specific 229 responder. In addition, implementations have a configurable flag 230 that determines whether this postquantum preshared key is mandatory. 231 This PPK is independent of the preshared key (if any) that the IKEv2 232 protocol uses to perform authentication. The PPK specific 233 configuration that is assumed on each peer consists of the following 234 tuple: 236 Peer, PPK, PPK_ID, mandatory_or_not 238 3. Exchanges 240 If the initiator is configured to use a postquantum preshared key 241 with the responder (whether or not the use of the PPK is mandatory), 242 then he will include a notification USE_PPK in the IKE_SA_INIT 243 request message as follows: 245 Initiator Responder 246 ------------------------------------------------------------------ 247 HDR, SAi1, KEi, Ni, N(USE_PPK) ---> 249 N(USE_PPK) is a status notification payload with the type 16435; it 250 has a protocol ID of 0, no SPI and no notification data associated 251 with it. 253 If the initiator needs to resend this initial message with a cookie 254 (because the responder response included a COOKIE notification), then 255 the resend would include the USE_PPK notification if the original 256 message did. 258 If the responder does not support this specification or does not have 259 any PPK configured, then she ignores the received notification and 260 continues with the IKEv2 protocol as normal. Otherwise the responder 261 checks if she has a PPK configured, and if she does, then the 262 responder replies with the IKE_SA_INIT message including a USE_PPK 263 notification in the response: 265 Initiator Responder 266 ------------------------------------------------------------------ 267 <--- HDR, SAr1, KEr, Nr, [CERTREQ], N(USE_PPK) 269 When the initiator receives this reply, he checks whether the 270 responder included the USE_PPK notification. If the responder did 271 not and the flag mandatory_or_not indicates that using PPKs is 272 mandatory for communication with this responder, then the initiator 273 MUST abort the exchange. This situation may happen in case of 274 misconfiguration, when the initiator believes he has a mandatory to 275 use PPK for the responder, while the responder either doesn't support 276 PPKs at all or doesn't have any PPK configured for the initiator. 277 See Section 6 for discussion of the possible impacts of this 278 situation. 280 If the responder did not include the USE_PPK notification and using a 281 PPK for this particular responder is optional, then the initiator 282 continues with the IKEv2 protocol as normal, without using PPKs. 284 If the responder did include the USE_PPK notification, then the 285 initiator selects a PPK, along with its identifier PPK_ID. Then, she 286 computes this modification of the standard IKEv2 key derivation: 288 SKEYSEED = prf(Ni | Nr, g^ir) 289 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) 290 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } 292 SK_d = prf+ (PPK, SK_d') 293 SK_pi = prf+ (PPK, SK_pi') 294 SK_pr = prf+ (PPK, SK_pr') 296 That is, we use the standard IKEv2 key derivation process except that 297 the three subkeys SK_d, SK_pi, SK_pr are run through the prf+ again, 298 this time using the PPK as the key. Using prf+ construction ensures 299 that it is always possible to get the resulting keys of the same size 300 as the initial ones, even if the underlying PRF has output size 301 different from its key size. Note, that at the time this document 302 was written, all PRFs defined for use in IKEv2 [IKEV2-IANA-PRFS] had 303 output size equal to the (preferred) key size. For such PRFs only 304 the first iteration of prf+ is needed: 306 SK_d = prf (PPK, SK_d' | 0x01) 307 SK_pi = prf (PPK, SK_pi' | 0x01) 308 SK_pr = prf (PPK, SK_pr' | 0x01) 310 Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only 311 during the initial IKE SA setup. It MUST NOT be used when these 312 subkeys are calculated as result of IKE SA rekey, resumption or other 313 similar operation. 315 The initiator then sends the IKE_AUTH request message, including the 316 PPK_ID value as follows: 318 Initiator Responder 319 ------------------------------------------------------------------ 320 HDR, SK {IDi, [CERT,] [CERTREQ,] 321 [IDr,] AUTH, SAi2, 322 TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} ---> 324 PPK_IDENTITY is a status notification with the type 16436; it has a 325 protocol ID of 0, no SPI and a notification data that consists of the 326 identifier PPK_ID. 328 A situation may happen when the responder has some PPKs, but doesn't 329 have a PPK with the PPK_ID received from the initiator. In this case 330 the responder cannot continue with PPK (in particular, she cannot 331 authenticate the initiator), but she could be able to continue with 332 normal IKEv2 protocol if the initiator provided its authentication 333 data computed as in normal IKEv2, without using PPKs. For this 334 purpose, if using PPKs for communication with this responder is 335 optional for the initiator, then the initiator MAY include a 336 notification NO_PPK_AUTH in the above message. 338 NO_PPK_AUTH is a status notification with the type 16437; it has a 339 protocol ID of 0 and no SPI. The Notification Data field contains 340 the initiator's authentication data computed using SK_pi', which has 341 been computed without using PPKs. This is the same data that would 342 normally be placed in the Authentication Data field of an AUTH 343 payload. Since the Auth Method field is not present in the 344 notification, the authentication method used for computing the 345 authentication data MUST be the same as method indicated in the AUTH 346 payload. Note that if the initiator decides to include the 347 NO_PPK_AUTH notification, the initiator needs to perform 348 authentication data computation twice, which may consume computation 349 power (e.g. if digital signatures are involved). 351 When the responder receives this encrypted exchange, she first 352 computes the values: 354 SKEYSEED = prf(Ni | Nr, g^ir) 355 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } 356 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) 358 She then uses the SK_ei/SK_ai values to decrypt/check the message and 359 then scans through the payloads for the PPK_ID attached to the 360 PPK_IDENTITY notification. If no PPK_IDENTITY notification is found 361 and the peers successfully exchanged USE_PPK notifications in the 362 IKE_SA_INIT exchange, then the responder MUST send back 363 AUTHENTICATION_FAILED notification and then fail the negotiation. 365 If the PPK_IDENTITY notification contains PPK_ID that is not known to 366 the responder or is not configured for use for the identity from IDi 367 payload, then the responder checks whether using PPKs for this 368 initiator is mandatory and whether the initiator included NO_PPK_AUTH 369 notification in the message. If using PPKs is mandatory or no 370 NO_PPK_AUTH notification found, then then the responder MUST send 371 back AUTHENTICATION_FAILED notification and then fail the 372 negotiation. Otherwise (when PPK is optional and the initiator 373 included NO_PPK_AUTH notification) the responder MAY continue regular 374 IKEv2 protocol, except that she uses the data from the NO_PPK_AUTH 375 notification as the authentication data (which usually resides in the 376 AUTH payload), for the purpose of the initiator authentication. 377 Note, that Authentication Method is still indicated in the AUTH 378 payload. 380 This table summarizes the above logic for the responder: 382 Received Received Have PPK 383 USE_PPK NO_PPK_AUTH PPK Mandatory Action 384 ----------------------------------------------------------------- 385 No * No * Standard IKEv2 protocol 386 No * Yes No Standard IKEv2 protocol 387 No * Yes Yes Abort negotiation 388 Yes No No * Abort negotiation 389 Yes Yes No Yes Abort negotiation 390 Yes Yes No No Standard IKEv2 protocol 391 Yes * Yes * Use PPK 393 If PPK is in use, then the responder extracts the corresponding PPK 394 and computes the following values: 396 SK_d = prf+ (PPK, SK_d') 397 SK_pi = prf+ (PPK, SK_pi') 398 SK_pr = prf+ (PPK, SK_pr') 400 The responder then continues with the IKE_AUTH exchange (validating 401 the AUTH payload that the initiator included) as usual and sends back 402 a response, which includes the PPK_IDENTITY notification with no data 403 to indicate that the PPK is used in the exchange: 405 Initiator Responder 406 ------------------------------------------------------------------ 407 <-- HDR, SK {IDr, [CERT,] 408 AUTH, SAr2, 409 TSi, TSr, N(PPK_IDENTITY)} 411 When the initiator receives the response, then he checks for the 412 presence of the PPK_IDENTITY notification. If he receives one, he 413 marks the SA as using the configured PPK to generate SK_d, SK_pi, 414 SK_pr (as shown above); the content of the received PPK_IDENTITY (if 415 any) MUST be ignored. If the initiator does not receive the 416 PPK_IDENTITY, he MUST either fail the IKE SA negotiation sending the 417 AUTHENTICATION_FAILED notification in the Informational exchange (if 418 the PPK was configured as mandatory), or continue without using the 419 PPK (if the PPK was not configured as mandatory and the initiator 420 included the NO_PPK_AUTH notification in the request). 422 If EAP is used in the IKE_AUTH exchange, then the initiator doesn't 423 include AUTH payload in the first request message, however the 424 responder sends back AUTH payload in the first reply. The peers then 425 exchange AUTH payloads after EAP is successfully completed. As a 426 result, the responder sends AUTH payload twice - in the first 427 IKE_AUTH reply message and in the last one, while the initiator sends 428 AUTH payload only in the last IKE_AUTH request. See more details 429 about EAP authentication in IKEv2 in Section 2.16 of [RFC7296]. 431 The general rule for using PPK in the IKE_AUTH exchange, which covers 432 EAP authentication case too, is that the initiator includes 433 PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request 434 message containing AUTH payload. Therefore, in case of EAP the 435 responder always computes the AUTH payload in the first IKE_AUTH 436 reply message without using PPK (by means of SK_pr'), since PPK_ID is 437 not yet known to the responder. Once the IKE_AUTH request message 438 containing PPK_IDENTITY notification is received, the responder 439 follows rules described above for non-EAP authentication case. 441 Initiator Responder 442 ---------------------------------------------------------------- 443 HDR, SK {IDi, [CERTREQ,] 444 [IDr,] SAi2, 445 TSi, TSr} --> 446 <-- HDR, SK {IDr, [CERT,] AUTH, 447 EAP} 448 HDR, SK {EAP} --> 449 <-- HDR, SK {EAP (success)} 450 HDR, SK {AUTH, 451 N(PPK_IDENTITY, PPK_ID) 452 [, N(NO_PPK_AUTH)]} --> 453 <-- HDR, SK {AUTH, SAr2, TSi, TSr 454 [, N(PPK_IDENTITY)]} 456 Note, that the IKE_SA_INIT exchange in case of PPK is as described 457 above (including exchange of the USE_PPK notifications), regardless 458 whether EAP is employed in the IKE_AUTH or not. 460 4. Upgrade procedure 462 This algorithm was designed so that someone can introduce PPKs into 463 an existing IKE network without causing network disruption. 465 In the initial phase of the network upgrade, the network 466 administrator would visit each IKE node, and configure: 468 o The set of PPKs (and corresponding PPK_IDs) that this node would 469 need to know. 471 o For each peer that this node would initiate to, which PPK will be 472 used. 474 o That the use of PPK is currently not mandatory. 476 With this configuration, the node will continue to operate with nodes 477 that have not yet been upgraded. This is due to the USE_PPK notify 478 and the NO_PPK_AUTH notify; if the initiator has not been upgraded, 479 he will not send the USE_PPK notify (and so the responder will know 480 that we will not use a PPK). If the responder has not been upgraded, 481 she will not send the USE_PPK notify (and so the initiator will know 482 to not use a PPK). If both peers have been upgraded, but the 483 responder isn't yet configured with the PPK for the initiator, then 484 the responder could do standard IKEv2 protocol if the initiator sent 485 NO_PPK_AUTH notification. If both the responder and initiator have 486 been upgraded and properly configured, they will both realize it, and 487 in that case, the link will be quantum secure. 489 As an optional second step, after all nodes have been upgraded, then 490 the administrator may then go back through the nodes, and mark the 491 use of PPK as mandatory. This will not affect the strength against a 492 passive attacker; it would mean that an attacker with a Quantum 493 Computer (which is sufficiently fast to be able to break the (EC)DH 494 in real time) would not be able to perform a downgrade attack. 496 5. PPK 498 5.1. PPK_ID format 500 This standard requires that both the initiator and the responder have 501 a secret PPK value, with the responder selecting the PPK based on the 502 PPK_ID that the initiator sends. In this standard, both the 503 initiator and the responder are configured with fixed PPK and PPK_ID 504 values, and do the look up based on PPK_ID value. It is anticipated 505 that later standards will extend this technique to allow dynamically 506 changing PPK values. To facilitate such an extension, we specify 507 that the PPK_ID the initiator sends will have its first octet be the 508 PPK_ID Type value. This document defines two values for PPK_ID Type: 510 o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and 511 the PPK itself) is not specified by this document; it is assumed 512 to be mutually intelligible by both by initiator and the 513 responder. This PPK_ID type is intended for those implementations 514 that choose not to disclose the type of PPK to active attackers. 516 o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the 517 PPK are fixed octet strings; the remaining bytes of the PPK_ID are 518 a configured value. We assume that there is a fixed mapping 519 between PPK_ID and PPK, which is configured locally to both the 520 initiator and the responder. The responder can use to do a look 521 up the passed PPK_ID value to determine the corresponding PPK 522 value. Not all implementations are able to configure arbitrary 523 octet strings; to improve the potential interoperability, it is 524 recommended that, in the PPK_ID_FIXED case, both the PPK and the 525 PPK_ID strings be limited to the base64 character set, namely the 526 64 characters 0-9, A-Z, a-z, + and /. 528 The PPK_ID type value 0 is reserved; values 3-127 are reserved for 529 IANA; values 128-255 are for private use among mutually consenting 530 parties. 532 5.2. Operational Considerations 534 The need to maintain several independent sets of security credentials 535 can significantly complicate a security administrator's job, and can 536 potentially slow down widespread adoption of this specification. It 537 is anticipated, that administrators will try to simplify their job by 538 decreasing the number of credentials they need to maintain. This 539 section describes some of the considerations for PPK management. 541 5.2.1. PPK Distribution 543 PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are 544 assumed to be configured within the IKE device in an out-of-band 545 fashion. While the method of distribution is a local matter and out 546 of scope of this document or IKEv2, [RFC6030] describes a format for 547 symmetric key exchange. That format could be reused with the Key Id 548 field being the PPK_ID (without the PPK_ID Type octet for a 549 PPK_ID_FIXED), the PPK being the secret, and algorithm 550 ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as the PIN. 552 5.2.2. Group PPK 554 This document doesn't explicitly require that PPK is unique for each 555 pair of peers. If it is the case, then this solution provides full 556 peer authentication, but it also means that each host must have as 557 many independent PPKs as the peers it is going to communicate with. 558 As the number of peers grows the PPKs will not scale. 560 Even though it is NOT RECOMMENDED, it is possible to use a single PPK 561 for a group of users. Since each peer uses classical public key 562 cryptography in addition to PPK for key exchange and authentication, 563 members of the group can neither impersonate each other nor read 564 other's traffic, unless they use Quantum Computers to break public 565 key operations. 567 Although it's probably safe to use group PPK, the fact that the PPK 568 is known to a (potentially large) group of users makes it more 569 susceptible to theft. If an attacker equipped with a Quantum 570 Computer got access to a group PPK, then all communications inside 571 the group are revealed. 573 5.2.3. PPK-only Authentication 575 If Quantum Computers become a reality, classical public key 576 cryptography will provide little security, so administrators may find 577 it attractive not to use it at all for authentication. This will 578 reduce the number of credentials they need to maintain to PPKs only. 579 Combining group PPK and PPK-only authentication is NOT RECOMMENDED, 580 since in this case any member of the group can impersonate any other 581 member even without help of Quantum Computers. 583 PPK-only authentication can be achieved in IKEv2 if NULL 584 Authentication method [RFC7619] is employed. Without PPK the NULL 585 Authentication method provides no authentication of the peers, 586 however since a PPK is stirred into the SK_pi and the SK_pr, the 587 peers become authenticated if a PPK is in use. Using PPKs MUST be 588 mandatory for the peers if they advertise support for PPK in 589 IKE_SA_INIT and use NULL Authentication. Addtionally, since the 590 peers are authenticated via PPK, the ID Type in the IDi/IDr payloads 591 SHOULD NOT be ID_NULL, despite using the NULL Authentication method. 593 6. Security Considerations 595 Quantum computers are able to perform Grover's algorithm; that 596 effectively halves the size of a symmetric key. Because of this, the 597 user SHOULD ensure that the postquantum preshared key used has at 598 least 256 bits of entropy, in order to provide 128-bit security 599 level. 601 With this protocol, the computed SK_d is a function of the PPK. 602 Assuming that the PPK has sufficient entropy (for example, at least 603 2^256 possible values), then even if an attacker was able to recover 604 the rest of the inputs to the PRF function, it would be infeasible to 605 use Grover's algorithm with a Quantum Computer to recover the SK_d 606 value. Similarly, every child SA key is a function of SK_d, hence 607 all the keys for all the child SAs are also quantum resistant 608 (assuming that the PPK was of high enough entropy, and that all the 609 subkeys are sufficiently long). 611 Although this protocol preserves all the security properties of IKEv2 612 against adversaries with conventional computers, it allows an 613 adversary with a Quantum Computer to decrypt all traffic encrypted 614 with the initial IKE SA. In particular, it allows the adversary to 615 recover the identities of both sides. If there is IKE traffic other 616 than the identities that need to be protected against such an 617 adversary, implementations MAY rekey the initial IKE SA immediately 618 after negotiating it to generate a new SKEYSEED from the postquantum 619 SK_d. This would reduce the amount of data available to an attacker 620 with a Quantum Computer. 622 If sensitive information (like keys) is to be transferred over IKE 623 SA, then implementations MUST rekey the initial IKE SA before sending 624 this information to get protection against Quantum Computers. 626 Alternatively, an initial IKE SA (which is used to exchange 627 identities) can take place, perhaps by using the protocol documented 628 in [RFC6023]. After the childless IKE SA is created, implementations 629 would immediately create a new IKE SA (which is used to exchange 630 everything else) by using a rekey mechanism for IKE SAs. Because the 631 rekeyed IKE SA keys are a function of SK_d, which is a function of 632 the PPK (among other things), traffic protected by that IKE SA is 633 secure against Quantum capable adversaries. 635 In addition, the policy SHOULD be set to negotiate only quantum- 636 resistant symmetric algorithms; while this RFC doesn't claim to give 637 advice as to what algorithms are secure (as that may change based on 638 future cryptographical results), below is a list of defined IKEv2 and 639 IPsec algorithms that should NOT be used, as they are known not to be 640 quantum resistant 642 o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with 643 key size less than 256 bits. 645 o Any ESP Transform with key size less than 256 bits. 647 o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined 648 to be able to use an arbitrary key size, they convert it into a 649 128-bit key internally. 651 Section 3 requires the initiator to abort the initial exchange if 652 using PPKs is mandatory for it, but the responder might not include 653 the USE_PPK notification in the response. In this situation when the 654 initiator aborts negotiation he leaves half-open IKE SA on the 655 responder (because IKE_SA_INIT completes successfully from the 656 responder's point of view). This half-open SA will eventually expire 657 and be deleted, but if the initiator continues its attempts to create 658 IKE SA with a high enough rate, then the responder may consider it as 659 a Denial-of-Service attack and take protection measures (see 660 [RFC8019] for more detail). It is RECOMMENDED that implementations 661 in this situation cache the negative result of negotiation for some 662 time and don't make attempts to create it again for some time, 663 because this is a result of misconfiguration and probably some re- 664 configuration of the peers is needed. 666 If using PPKs is optional for both peers and they authenticate 667 themselves using digital signatures, then an attacker in between, 668 equipped with a Quantum Computer capable of breaking public key 669 operations in real time, is able to mount downgrade attack by 670 removing USE_PPK notification from the IKE_SA_INIT and forging 671 digital signatures in the subsequent exchange. If using PPKs is 672 mandatory for at least one of the peers or PSK is used for 673 authentication, then the attack will be detected and the SA won't be 674 created. 676 If using PPKs is mandatory for the initiator, then an attacker 677 capable to eavesdrop and to inject packets into the network can 678 prevent creating IKE SA by mounting the following attack. The 679 attacker intercepts the initial request containing the USE_PPK 680 notification and injects the forget response containing no USE_PPK. 681 If the attacker manages to inject this packet before the responder 682 sends a genuine response, then the initiator would abort the 683 exchange. To thwart this kind of attack it is RECOMMENDED, that if 684 using PPKs is mandatory for the initiator and the received response 685 doesn't contain the USE_PPK notification, then the initiator doesn't 686 abort the exchange immediately, but instead waits some time for more 687 responses (possibly retransmitting the request). If all the received 688 responses contain no USE_PPK, then the exchange is aborted. 690 7. IANA Considerations 692 This document defines three new Notify Message Types in the "Notify 693 Message Types - Status Types" registry: 695 16435 USE_PPK 696 16436 PPK_IDENTITY 697 16437 NO_PPK_AUTH 699 This document also creates a new IANA registry for the PPK_ID types. 700 The initial values of this registry are: 702 PPK_ID Type Value 703 ----------- ----- 704 Reserved 0 705 PPK_ID_OPAQUE 1 706 PPK_ID_FIXED 2 707 Unassigned 3-127 708 Reserved for private use 128-255 710 Changes and additions to this registry are by Expert Review 711 [RFC5226]. 713 8. References 714 8.1. Normative References 716 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 717 Requirement Levels", BCP 14, RFC 2119, 718 DOI 10.17487/RFC2119, March 1997, 719 . 721 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 722 Kivinen, "Internet Key Exchange Protocol Version 2 723 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 724 2014, . 726 8.2. Informational References 728 [I-D.hoffman-c2pq] 729 Hoffman, P., "The Transition from Classical to Post- 730 Quantum Cryptography", draft-hoffman-c2pq-03 (work in 731 progress), February 2018. 733 [IKEV2-IANA-PRFS] 734 "Internet Key Exchange Version 2 (IKEv2) Parameters, 735 Transform Type 2 - Pseudorandom Function Transform IDs", 736 . 739 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 740 (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, 741 . 743 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 744 IANA Considerations Section in RFCs", RFC 5226, 745 DOI 10.17487/RFC5226, May 2008, 746 . 748 [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A 749 Childless Initiation of the Internet Key Exchange Version 750 2 (IKEv2) Security Association (SA)", RFC 6023, 751 DOI 10.17487/RFC6023, October 2010, 752 . 754 [RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric 755 Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030, 756 October 2010, . 758 [RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication 759 Method in the Internet Key Exchange Protocol Version 2 760 (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015, 761 . 763 [RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange 764 Protocol Version 2 (IKEv2) Implementations from 765 Distributed Denial-of-Service Attacks", RFC 8019, 766 DOI 10.17487/RFC8019, November 2016, 767 . 769 Appendix A. Discussion and Rationale 771 The idea behind this document is that while a Quantum Computer can 772 easily reconstruct the shared secret of an (EC)DH exchange, they 773 cannot as easily recover a secret from a symmetric exchange. This 774 makes the SK_d, and hence the IPsec KEYMAT and any child SA's 775 SKEYSEED, depend on both the symmetric PPK, and also the Diffie- 776 Hellman exchange. If we assume that the attacker knows everything 777 except the PPK during the key exchange, and there are 2^n plausible 778 PPKs, then a Quantum Computer (using Grover's algorithm) would take 779 O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be 780 trivially solved, the attacker still can't recover any key material 781 (except for the SK_ei, SK_er, SK_ai, SK_ar values for the initial IKE 782 exchange) unless they can find the PPK, which is too difficult if the 783 PPK has enough entropy (for example, 256 bits). Note that we do 784 allow an attacker with a Quantum Computer to rederive the keying 785 material for the initial IKE SA; this was a compromise to allow the 786 responder to select the correct PPK quickly. 788 Another goal of this protocol is to minimize the number of changes 789 within the IKEv2 protocol, and in particular, within the cryptography 790 of IKEv2. By limiting our changes to notifications, and adjusting 791 the SK_d, SK_pi, SK_pr, it is hoped that this would be implementable, 792 even on systems that perform most of the IKEv2 processing in 793 hardware. 795 A third goal was to be friendly to incremental deployment in 796 operational networks, for which we might not want to have a global 797 shared key, or quantum resistant IKEv2 is rolled out incrementally. 798 This is why we specifically try to allow the PPK to be dependent on 799 the peer, and why we allow the PPK to be configured as optional. 801 A fourth goal was to avoid violating any of the security goals of 802 IKEv2. 804 Appendix B. Acknowledgements 806 We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett, 807 Tommy Pauly and the rest of the IPSecME Working Group for their 808 feedback and suggestions for the scheme. 810 Authors' Addresses 812 Scott Fluhrer 813 Cisco Systems 815 Email: sfluhrer@cisco.com 817 David McGrew 818 Cisco Systems 820 Email: mcgrew@cisco.com 822 Panos Kampanakis 823 Cisco Systems 825 Email: pkampana@cisco.com 827 Valery Smyslov 828 ELVIS-PLUS 830 Phone: +7 495 276 0211 831 Email: svan@elvis.ru