idnits 2.17.00 (12 Aug 2021) /tmp/idnits11882/draft-ietf-ipsecme-qr-ikev2-08.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 (March 28, 2019) is 1150 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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-07) exists of draft-hoffman-c2pq-04 -- Obsolete informational reference (is this intentional?): RFC 2409 (Obsoleted by RFC 4306)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: September 29, 2019 Cisco Systems 6 V. Smyslov 7 ELVIS-PLUS 8 March 28, 2019 10 Postquantum Preshared Keys for IKEv2 11 draft-ietf-ipsecme-qr-ikev2-08 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 http://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 September 29, 2019. 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 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 16 75 8.1. Normative References . . . . . . . . . . . . . . . . . . 16 76 8.2. Informational References . . . . . . . . . . . . . . . . 16 77 Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 17 78 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 18 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-ikev2-05 128 o Addressed comments received during WGLC. 130 draft-ietf-ipsecme-qr-ikev2-04 132 o Using Group PPK is clarified based on comment from Quynh Dang. 134 draft-ietf-ipsecme-qr-ikev2-03 136 o Editorial changes and minor text nit fixes. 138 o Integrated Tommy P. text suggestions. 140 draft-ietf-ipsecme-qr-ikev2-02 142 o Added note that the PPK is stirred in the initial IKE SA setup 143 only. 145 o Added note about the initiator ignoring any content in the 146 PPK_IDENTITY notification from the responder. 148 o fixed Tero's suggestions from 2/6/1028 150 o Added IANA assigned message types where necessary. 152 o fixed minor text nits 154 draft-ietf-ipsecme-qr-ikev2-01 156 o Nits and minor fixes. 158 o prf is replaced with prf+ for the SK_d and SK_pi/r calculations. 160 o Clarified using PPK in case of EAP authentication. 162 o PPK_SUPPORT notification is changed to USE_PPK to better reflect 163 its purpose. 165 draft-ietf-ipsecme-qr-ikev2-00 167 o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr- 168 ikev2-00 that is a WG item. 170 draft-fluhrer-qr-ikev2-05 172 o Nits and editorial fixes. 174 o Made PPK_ID format and PPK Distributions subsection of the PPK 175 section. Also added an Operational Considerations section. 177 o Added comment about Child SA rekey in the Security Considerations 178 section. 180 o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not 181 configured for a responder. 183 o Various text changes and clarifications. 185 o Expanded Security Considerations section to describe some security 186 concerns and how they should be addressed. 188 draft-fluhrer-qr-ikev2-03 190 o Modified how we stir the PPK into the IKEv2 secret state. 192 o Modified how the use of PPKs is negotiated. 194 o Simplified the protocol by stirring in the preshared key into the 195 child SAs; this avoids the problem of having the responder decide 196 which preshared key to use (as it knows the initiator identity at 197 that point); it does mean that someone with a Quantum Computer can 198 recover the initial IKE negotiation. 200 o Removed positive endorsements of various algorithms. Retained 201 warnings about algorithms known to be weak against a Quantum 202 Computer. 204 draft-fluhrer-qr-ikev2-01 206 o Added explicit guidance as to what IKE and IPsec algorithms are 207 quantum resistant. 209 draft-fluhrer-qr-ikev2-00 211 o We switched from using vendor ID's to transmit the additional data 212 to notifications. 214 o We added a mandatory cookie exchange to allow the server to 215 communicate to the client before the initial exchange. 217 o We added algorithm agility by having the server tell the client 218 what algorithm to use in the cookie exchange. 220 o We have the server specify the PPK Indicator Input, which allows 221 the server to make a trade-off between the efficiency for the 222 search of the clients PPK, and the anonymity of the client. 224 o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to 225 transform the nonces during the KDF. 227 1.2. Requirements Language 229 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 230 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 231 "OPTIONAL" in this document are to be interpreted as described in RFC 232 2119 [RFC2119]. 234 2. Assumptions 236 We assume that each IKE peer has a list of Postquantum Preshared Keys 237 (PPK) along with their identifiers (PPK_ID), and any potential IKE 238 initiator has a selection of which PPK to use with any specific 239 responder. In addition, implementations have a configurable flag 240 that determines whether this postquantum preshared key is mandatory. 241 This PPK is independent of the preshared key (if any) that the IKEv2 242 protocol uses to perform authentication. The PPK specific 243 configuration that is assumed on each peer consists of the following 244 tuple: 246 Peer, PPK, PPK_ID, mandatory_or_not 248 3. Exchanges 250 If the initiator is configured to use a postquantum preshared key 251 with the responder (whether or not the use of the PPK is mandatory), 252 then he will include a notification USE_PPK in the IKE_SA_INIT 253 request message as follows: 255 Initiator Responder 256 ------------------------------------------------------------------ 257 HDR, SAi1, KEi, Ni, N(USE_PPK) ---> 259 N(USE_PPK) is a status notification payload with the type 16435; it 260 has a protocol ID of 0, no SPI and no notification data associated 261 with it. 263 If the initiator needs to resend this initial message with a cookie 264 (because the responder response included a COOKIE notification), then 265 the resend would include the USE_PPK notification if the original 266 message did. 268 If the responder does not support this specification or does not have 269 any PPK configured, then she ignores the received notification and 270 continues with the IKEv2 protocol as normal. Otherwise the responder 271 checks if she has a PPK configured, and if she does, then the 272 responder replies with the IKE_SA_INIT message including a USE_PPK 273 notification in the response: 275 Initiator Responder 276 ------------------------------------------------------------------ 277 <--- HDR, SAr1, KEr, Nr, [CERTREQ,] N(USE_PPK) 279 When the initiator receives this reply, he checks whether the 280 responder included the USE_PPK notification. If the responder did 281 not and the flag mandatory_or_not indicates that using PPKs is 282 mandatory for communication with this responder, then the initiator 283 MUST abort the exchange. This situation may happen in case of 284 misconfiguration, when the initiator believes he has a mandatory to 285 use PPK for the responder, while the responder either doesn't support 286 PPKs at all or doesn't have any PPK configured for the initiator. 288 See Section 6 for discussion of the possible impacts of this 289 situation. 291 If the responder did not include the USE_PPK notification and using a 292 PPK for this particular responder is optional, then the initiator 293 continues with the IKEv2 protocol as normal, without using PPKs. 295 If the responder did include the USE_PPK notification, then the 296 initiator selects a PPK, along with its identifier PPK_ID. Then, she 297 computes this modification of the standard IKEv2 key derivation: 299 SKEYSEED = prf(Ni | Nr, g^ir) 300 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) 301 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } 303 SK_d = prf+ (PPK, SK_d') 304 SK_pi = prf+ (PPK, SK_pi') 305 SK_pr = prf+ (PPK, SK_pr') 307 That is, we use the standard IKEv2 key derivation process except that 308 the three subkeys SK_d, SK_pi, SK_pr are run through the prf+ again, 309 this time using the PPK as the key. Using prf+ construction ensures 310 that it is always possible to get the resulting keys of the same size 311 as the initial ones, even if the underlying PRF has output size 312 different from its key size. Note, that at the time this document 313 was written, all PRFs defined for use in IKEv2 [IKEV2-IANA-PRFS] had 314 output size equal to the (preferred) key size. For such PRFs only 315 the first iteration of prf+ is needed: 317 SK_d = prf (PPK, SK_d' | 0x01) 318 SK_pi = prf (PPK, SK_pi' | 0x01) 319 SK_pr = prf (PPK, SK_pr' | 0x01) 321 Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only 322 during the initial IKE SA setup. It MUST NOT be used when these 323 subkeys are calculated as result of IKE SA rekey, resumption or other 324 similar operation. 326 The initiator then sends the IKE_AUTH request message, including the 327 PPK_ID value as follows: 329 Initiator Responder 330 ------------------------------------------------------------------ 331 HDR, SK {IDi, [CERT,] [CERTREQ,] 332 [IDr,] AUTH, SAi2, 333 TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} ---> 335 PPK_IDENTITY is a status notification with the type 16436; it has a 336 protocol ID of 0, no SPI and a notification data that consists of the 337 identifier PPK_ID. 339 A situation may happen when the responder has some PPKs, but doesn't 340 have a PPK with the PPK_ID received from the initiator. In this case 341 the responder cannot continue with PPK (in particular, she cannot 342 authenticate the initiator), but she could be able to continue with 343 normal IKEv2 protocol if the initiator provided its authentication 344 data computed as in normal IKEv2, without using PPKs. For this 345 purpose, if using PPKs for communication with this responder is 346 optional for the initiator, then the initiator MAY include a 347 notification NO_PPK_AUTH in the above message. 349 NO_PPK_AUTH is a status notification with the type 16437; it has a 350 protocol ID of 0 and no SPI. The Notification Data field contains 351 the initiator's authentication data computed using SK_pi', which has 352 been computed without using PPKs. This is the same data that would 353 normally be placed in the Authentication Data field of an AUTH 354 payload. Since the Auth Method field is not present in the 355 notification, the authentication method used for computing the 356 authentication data MUST be the same as method indicated in the AUTH 357 payload. Note that if the initiator decides to include the 358 NO_PPK_AUTH notification, the initiator needs to perform 359 authentication data computation twice, which may consume computation 360 power (e.g. if digital signatures are involved). 362 When the responder receives this encrypted exchange, she first 363 computes the values: 365 SKEYSEED = prf(Ni | Nr, g^ir) 366 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } 367 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) 369 She then uses the SK_ei/SK_ai values to decrypt/check the message and 370 then scans through the payloads for the PPK_ID attached to the 371 PPK_IDENTITY notification. If no PPK_IDENTITY notification is found 372 and the peers successfully exchanged USE_PPK notifications in the 373 IKE_SA_INIT exchange, then the responder MUST send back 374 AUTHENTICATION_FAILED notification and then fail the negotiation. 376 If the PPK_IDENTITY notification contains PPK_ID that is not known to 377 the responder or is not configured for use for the identity from IDi 378 payload, then the responder checks whether using PPKs for this 379 initiator is mandatory and whether the initiator included NO_PPK_AUTH 380 notification in the message. If using PPKs is mandatory or no 381 NO_PPK_AUTH notification found, then then the responder MUST send 382 back AUTHENTICATION_FAILED notification and then fail the 383 negotiation. Otherwise (when PPK is optional and the initiator 384 included NO_PPK_AUTH notification) the responder MAY continue regular 385 IKEv2 protocol, except that she uses the data from the NO_PPK_AUTH 386 notification as the authentication data (which usually resides in the 387 AUTH payload), for the purpose of the initiator authentication. 388 Note, that Authentication Method is still indicated in the AUTH 389 payload. 391 This table summarizes the above logic for the responder: 393 Received Received Configured PPK is 394 USE_PPK NO_PPK_AUTH with PPK Mandatory Action 395 --------------------------------------------------------------------- 396 No * No * Standard IKEv2 protocol 397 No * Yes No Standard IKEv2 protocol 398 No * Yes Yes Abort negotiation 399 Yes No No * Abort negotiation 400 Yes Yes No Yes Abort negotiation 401 Yes Yes No No Standard IKEv2 protocol 402 Yes * Yes * Use PPK 404 If PPK is in use, then the responder extracts the corresponding PPK 405 and computes the following values: 407 SK_d = prf+ (PPK, SK_d') 408 SK_pi = prf+ (PPK, SK_pi') 409 SK_pr = prf+ (PPK, SK_pr') 411 The responder then continues with the IKE_AUTH exchange (validating 412 the AUTH payload that the initiator included) as usual and sends back 413 a response, which includes the PPK_IDENTITY notification with no data 414 to indicate that the PPK is used in the exchange: 416 Initiator Responder 417 ------------------------------------------------------------------ 418 <-- HDR, SK {IDr, [CERT,] 419 AUTH, SAr2, 420 TSi, TSr, N(PPK_IDENTITY)} 422 When the initiator receives the response, then he checks for the 423 presence of the PPK_IDENTITY notification. If he receives one, he 424 marks the SA as using the configured PPK to generate SK_d, SK_pi, 425 SK_pr (as shown above); the content of the received PPK_IDENTITY (if 426 any) MUST be ignored. If the initiator does not receive the 427 PPK_IDENTITY, he MUST either fail the IKE SA negotiation sending the 428 AUTHENTICATION_FAILED notification in the Informational exchange (if 429 the PPK was configured as mandatory), or continue without using the 430 PPK (if the PPK was not configured as mandatory and the initiator 431 included the NO_PPK_AUTH notification in the request). 433 If EAP is used in the IKE_AUTH exchange, then the initiator doesn't 434 include AUTH payload in the first request message, however the 435 responder sends back AUTH payload in the first reply. The peers then 436 exchange AUTH payloads after EAP is successfully completed. As a 437 result, the responder sends AUTH payload twice - in the first 438 IKE_AUTH reply message and in the last one, while the initiator sends 439 AUTH payload only in the last IKE_AUTH request. See more details 440 about EAP authentication in IKEv2 in Section 2.16 of [RFC7296]. 442 The general rule for using PPK in the IKE_AUTH exchange, which covers 443 EAP authentication case too, is that the initiator includes 444 PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request 445 message containing AUTH payload. Therefore, in case of EAP the 446 responder always computes the AUTH payload in the first IKE_AUTH 447 reply message without using PPK (by means of SK_pr'), since PPK_ID is 448 not yet known to the responder. Once the IKE_AUTH request message 449 containing PPK_IDENTITY notification is received, the responder 450 follows rules described above for non-EAP authentication case. 452 Initiator Responder 453 ---------------------------------------------------------------- 454 HDR, SK {IDi, [CERTREQ,] 455 [IDr,] SAi2, 456 TSi, TSr} --> 457 <-- HDR, SK {IDr, [CERT,] AUTH, 458 EAP} 459 HDR, SK {EAP} --> 460 <-- HDR, SK {EAP (success)} 461 HDR, SK {AUTH, 462 N(PPK_IDENTITY, PPK_ID) 463 [, N(NO_PPK_AUTH)]} --> 464 <-- HDR, SK {AUTH, SAr2, TSi, TSr 465 [, N(PPK_IDENTITY)]} 467 Note, that the IKE_SA_INIT exchange in case of PPK is as described 468 above (including exchange of the USE_PPK notifications), regardless 469 whether EAP is employed in the IKE_AUTH or not. 471 4. Upgrade procedure 473 This algorithm was designed so that someone can introduce PPKs into 474 an existing IKE network without causing network disruption. 476 In the initial phase of the network upgrade, the network 477 administrator would visit each IKE node, and configure: 479 o The set of PPKs (and corresponding PPK_IDs) that this node would 480 need to know. 482 o For each peer that this node would initiate to, which PPK will be 483 used. 485 o That the use of PPK is currently not mandatory. 487 With this configuration, the node will continue to operate with nodes 488 that have not yet been upgraded. This is due to the USE_PPK notify 489 and the NO_PPK_AUTH notify; if the initiator has not been upgraded, 490 he will not send the USE_PPK notify (and so the responder will know 491 that we will not use a PPK). If the responder has not been upgraded, 492 she will not send the USE_PPK notify (and so the initiator will know 493 to not use a PPK). If both peers have been upgraded, but the 494 responder isn't yet configured with the PPK for the initiator, then 495 the responder could do standard IKEv2 protocol if the initiator sent 496 NO_PPK_AUTH notification. If both the responder and initiator have 497 been upgraded and properly configured, they will both realize it, and 498 in that case, the link will be quantum secure. 500 As an optional second step, after all nodes have been upgraded, then 501 the administrator should then go back through the nodes, and mark the 502 use of PPK as mandatory. This will not affect the strength against a 503 passive attacker; it would mean that an attacker with a Quantum 504 Computer (which is sufficiently fast to be able to break the (EC)DH 505 in real time) would not be able to perform a downgrade attack. 507 5. PPK 509 5.1. PPK_ID format 511 This standard requires that both the initiator and the responder have 512 a secret PPK value, with the responder selecting the PPK based on the 513 PPK_ID that the initiator sends. In this standard, both the 514 initiator and the responder are configured with fixed PPK and PPK_ID 515 values, and do the look up based on PPK_ID value. It is anticipated 516 that later standards will extend this technique to allow dynamically 517 changing PPK values. To facilitate such an extension, we specify 518 that the PPK_ID the initiator sends will have its first octet be the 519 PPK_ID Type value. This document defines two values for PPK_ID Type: 521 o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and 522 the PPK itself) is not specified by this document; it is assumed 523 to be mutually intelligible by both by initiator and the 524 responder. This PPK_ID type is intended for those implementations 525 that choose not to disclose the type of PPK to active attackers. 527 o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the 528 PPK are fixed octet strings; the remaining bytes of the PPK_ID are 529 a configured value. We assume that there is a fixed mapping 530 between PPK_ID and PPK, which is configured locally to both the 531 initiator and the responder. The responder can use to do a look 532 up the passed PPK_ID value to determine the corresponding PPK 533 value. Not all implementations are able to configure arbitrary 534 octet strings; to improve the potential interoperability, it is 535 recommended that, in the PPK_ID_FIXED case, both the PPK and the 536 PPK_ID strings be limited to the base64 character set, namely the 537 64 characters 0-9, A-Z, a-z, + and /. 539 The PPK_ID type value 0 is reserved; values 3-127 are reserved for 540 IANA; values 128-255 are for private use among mutually consenting 541 parties. 543 5.2. Operational Considerations 545 The need to maintain several independent sets of security credentials 546 can significantly complicate a security administrator's job, and can 547 potentially slow down widespread adoption of this specification. It 548 is anticipated, that administrators will try to simplify their job by 549 decreasing the number of credentials they need to maintain. This 550 section describes some of the considerations for PPK management. 552 5.2.1. PPK Distribution 554 PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are 555 assumed to be configured within the IKE device in an out-of-band 556 fashion. While the method of distribution is a local matter and out 557 of scope of this document or IKEv2, [RFC6030] describes a format for 558 symmetric key exchange. That format could be reused with the Key Id 559 field being the PPK_ID (without the PPK_ID Type octet for a 560 PPK_ID_FIXED), the PPK being the secret, and algorithm 561 ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as the PIN. 563 5.2.2. Group PPK 565 This document doesn't explicitly require that PPK is unique for each 566 pair of peers. If it is the case, then this solution provides full 567 peer authentication, but it also means that each host must have as 568 many independent PPKs as the peers it is going to communicate with. 569 As the number of peers grows the PPKs will not scale. 571 It is possible to use a single PPK for a group of users. Since each 572 peer uses classical public key cryptography in addition to PPK for 573 key exchange and authentication, members of the group can neither 574 impersonate each other nor read other's traffic, unless they use 575 Quantum Computers to break public key operations. However group 576 members can record other members' traffic and decrypt it later, when 577 they get access to a Quantum Computer. 579 In addition, the fact that the PPK is known to a (potentially large) 580 group of users makes it more susceptible to theft. When an attacker 581 equipped with a Quantum Computer got access to a group PPK, all 582 communications inside the group are revealed. 584 For these reasons using group PPK is NOT RECOMMENDED. 586 5.2.3. PPK-only Authentication 588 If Quantum Computers become a reality, classical public key 589 cryptography will provide little security, so administrators may find 590 it attractive not to use it at all for authentication. This will 591 reduce the number of credentials they need to maintain to PPKs only. 592 Combining group PPK and PPK-only authentication is NOT RECOMMENDED, 593 since in this case any member of the group can impersonate any other 594 member even without help of Quantum Computers. 596 PPK-only authentication can be achieved in IKEv2 if NULL 597 Authentication method [RFC7619] is employed. Without PPK the NULL 598 Authentication method provides no authentication of the peers, 599 however since a PPK is stirred into the SK_pi and the SK_pr, the 600 peers become authenticated if a PPK is in use. Using PPKs MUST be 601 mandatory for the peers if they advertise support for PPK in 602 IKE_SA_INIT and use NULL Authentication. Addtionally, since the 603 peers are authenticated via PPK, the ID Type in the IDi/IDr payloads 604 SHOULD NOT be ID_NULL, despite using the NULL Authentication method. 606 6. Security Considerations 608 Quantum computers are able to perform Grover's algorithm; that 609 effectively halves the size of a symmetric key. Because of this, the 610 user SHOULD ensure that the postquantum preshared key used has at 611 least 256 bits of entropy, in order to provide 128-bit security 612 level. 614 With this protocol, the computed SK_d is a function of the PPK. 615 Assuming that the PPK has sufficient entropy (for example, at least 616 2^256 possible values), then even if an attacker was able to recover 617 the rest of the inputs to the PRF function, it would be infeasible to 618 use Grover's algorithm with a Quantum Computer to recover the SK_d 619 value. Similarly, all keys that are a function of SK_d, which 620 include all Child SAs keys and all keys for subsequent IKE SAs 621 (created when the initial IKE SA is rekeyed), are also quantum 622 resistant (assuming that the PPK was of high enough entropy, and that 623 all the subkeys are sufficiently long). 625 An attacker with a Quantum Computer that can decrypt the initial IKE 626 SA has access to all the information exchanged over it, such as 627 identities of the peers, configuration parameters and all negotiated 628 IPsec SAs information (including traffic selectors), with the 629 exception of the cryptographic keys used by the IPsec SAs which are 630 protected by the PPK. 632 Deployments that treat this information as sensitive or that send 633 other sensitive data (like cryptographic keys) over IKE SA MUST rekey 634 the IKE SA before the sensitive information is sent to ensure this 635 information is protected by the PPK. It is possible to create a 636 childless IKE SA as specified in [RFC6023]. This prevents Child SA 637 configuration information from being transmited in the original IKE 638 SA that is not protected by a PPK. Some information related to IKE 639 SA, that is sent in the IKE_AUTH exchange, such as peer identities, 640 feature notifications, Vendor ID's etc. cannot be hidden from the 641 attack described above, even if the additional IKE SA rekey is 642 performed. 644 In addition, the policy SHOULD be set to negotiate only quantum- 645 resistant symmetric algorithms; while this RFC doesn't claim to give 646 advice as to what algorithms are secure (as that may change based on 647 future cryptographical results), below is a list of defined IKEv2 and 648 IPsec algorithms that should NOT be used, as they are known not to be 649 quantum resistant 651 o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with 652 key size less than 256 bits. 654 o Any ESP Transform with key size less than 256 bits. 656 o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined 657 to be able to use an arbitrary key size, they convert it into a 658 128-bit key internally. 660 Section 3 requires the initiator to abort the initial exchange if 661 using PPKs is mandatory for it, but the responder might not include 662 the USE_PPK notification in the response. In this situation when the 663 initiator aborts negotiation he leaves half-open IKE SA on the 664 responder (because IKE_SA_INIT completes successfully from the 665 responder's point of view). This half-open SA will eventually expire 666 and be deleted, but if the initiator continues its attempts to create 667 IKE SA with a high enough rate, then the responder may consider it as 668 a Denial-of-Service attack and take protection measures (see 669 [RFC8019] for more detail). It is RECOMMENDED that implementations 670 in this situation cache the negative result of negotiation for some 671 time and don't make attempts to create it again for some time, 672 because this is a result of misconfiguration and probably some re- 673 configuration of the peers is needed. 675 If using PPKs is optional for both peers and they authenticate 676 themselves using digital signatures, then an attacker in between, 677 equipped with a Quantum Computer capable of breaking public key 678 operations in real time, is able to mount downgrade attack by 679 removing USE_PPK notification from the IKE_SA_INIT and forging 680 digital signatures in the subsequent exchange. If using PPKs is 681 mandatory for at least one of the peers or PSK is used for 682 authentication, then the attack will be detected and the SA won't be 683 created. 685 If using PPKs is mandatory for the initiator, then an attacker 686 capable to eavesdrop and to inject packets into the network can 687 prevent creating IKE SA by mounting the following attack. The 688 attacker intercepts the initial request containing the USE_PPK 689 notification and injects the forget response containing no USE_PPK. 690 If the attacker manages to inject this packet before the responder 691 sends a genuine response, then the initiator would abort the 692 exchange. To thwart this kind of attack it is RECOMMENDED, that if 693 using PPKs is mandatory for the initiator and the received response 694 doesn't contain the USE_PPK notification, then the initiator doesn't 695 abort the exchange immediately, but instead waits some time for more 696 responses (possibly retransmitting the request). If all the received 697 responses contain no USE_PPK, then the exchange is aborted. 699 7. IANA Considerations 701 This document defines three new Notify Message Types in the "Notify 702 Message Types - Status Types" registry: 704 16435 USE_PPK 705 16436 PPK_IDENTITY 706 16437 NO_PPK_AUTH 708 This document also creates a new IANA registry for the PPK_ID types. 709 The initial values of this registry are: 711 PPK_ID Type Value 712 ----------- ----- 713 Reserved 0 714 PPK_ID_OPAQUE 1 715 PPK_ID_FIXED 2 716 Unassigned 3-127 717 Reserved for private use 128-255 719 Changes and additions to this registry are by Expert Review 720 [RFC8126]. 722 8. References 724 8.1. Normative References 726 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 727 Requirement Levels", BCP 14, RFC 2119, 728 DOI 10.17487/RFC2119, March 1997, . 731 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 732 Kivinen, "Internet Key Exchange Protocol Version 2 733 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 734 2014, . 736 8.2. Informational References 738 [I-D.hoffman-c2pq] 739 Hoffman, P., "The Transition from Classical to Post- 740 Quantum Cryptography", draft-hoffman-c2pq-04 (work in 741 progress), August 2018. 743 [IKEV2-IANA-PRFS] 744 "Internet Key Exchange Version 2 (IKEv2) Parameters, 745 Transform Type 2 - Pseudorandom Function Transform IDs", 746 . 749 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 750 (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, 751 . 753 [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A 754 Childless Initiation of the Internet Key Exchange Version 755 2 (IKEv2) Security Association (SA)", RFC 6023, 756 DOI 10.17487/RFC6023, October 2010, . 759 [RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric 760 Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030, 761 October 2010, . 763 [RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication 764 Method in the Internet Key Exchange Protocol Version 2 765 (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015, 766 . 768 [RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange 769 Protocol Version 2 (IKEv2) Implementations from 770 Distributed Denial-of-Service Attacks", RFC 8019, 771 DOI 10.17487/RFC8019, November 2016, . 774 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 775 Writing an IANA Considerations Section in RFCs", BCP 26, 776 RFC 8126, DOI 10.17487/RFC8126, June 2017, 777 . 779 Appendix A. Discussion and Rationale 781 The idea behind this document is that while a Quantum Computer can 782 easily reconstruct the shared secret of an (EC)DH exchange, they 783 cannot as easily recover a secret from a symmetric exchange. This 784 makes the SK_d, and hence the IPsec KEYMAT and any child SA's 785 SKEYSEED, depend on both the symmetric PPK, and also the Diffie- 786 Hellman exchange. If we assume that the attacker knows everything 787 except the PPK during the key exchange, and there are 2^n plausible 788 PPKs, then a Quantum Computer (using Grover's algorithm) would take 789 O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be 790 trivially solved, the attacker still can't recover any key material 791 (except for the SK_ei, SK_er, SK_ai, SK_ar values for the initial IKE 792 exchange) unless they can find the PPK, which is too difficult if the 793 PPK has enough entropy (for example, 256 bits). Note that we do 794 allow an attacker with a Quantum Computer to rederive the keying 795 material for the initial IKE SA; this was a compromise to allow the 796 responder to select the correct PPK quickly. 798 Another goal of this protocol is to minimize the number of changes 799 within the IKEv2 protocol, and in particular, within the cryptography 800 of IKEv2. By limiting our changes to notifications, and adjusting 801 the SK_d, SK_pi, SK_pr, it is hoped that this would be implementable, 802 even on systems that perform most of the IKEv2 processing in 803 hardware. 805 A third goal was to be friendly to incremental deployment in 806 operational networks, for which we might not want to have a global 807 shared key, or quantum resistant IKEv2 is rolled out incrementally. 808 This is why we specifically try to allow the PPK to be dependent on 809 the peer, and why we allow the PPK to be configured as optional. 811 A fourth goal was to avoid violating any of the security goals of 812 IKEv2. 814 Appendix B. Acknowledgements 816 We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett, 817 Tommy Pauly, Quynh Dang and the rest of the IPSecME Working Group for 818 their feedback and suggestions for the scheme. 820 Authors' Addresses 822 Scott Fluhrer 823 Cisco Systems 825 Email: sfluhrer@cisco.com 827 David McGrew 828 Cisco Systems 830 Email: mcgrew@cisco.com 832 Panos Kampanakis 833 Cisco Systems 835 Email: pkampana@cisco.com 837 Valery Smyslov 838 ELVIS-PLUS 840 Phone: +7 495 276 0211 841 Email: svan@elvis.ru