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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 (July 2, 2018) is 1419 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to lower-maturity documents in RFCs) == Missing Reference: 'CERTREQ' is mentioned on line 273, but not defined == Outdated reference: A later version (-07) exists of draft-hoffman-c2pq-03 -- Obsolete informational reference (is this intentional?): RFC 2409 (Obsoleted by RFC 4306)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: January 3, 2019 Cisco Systems 6 V. Smyslov 7 ELVIS-PLUS 8 July 2, 2018 10 Postquantum Preshared Keys for IKEv2 11 draft-ietf-ipsecme-qr-ikev2-04 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 January 3, 2019. 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 (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-ikev2-04 128 o Using Group PPK is clarified based on comment from Quynh Dang. 130 draft-ietf-ipsecme-qr-ikev2-03 132 o Editorial changes and minor text nit fixes. 134 o Integrated Tommy P. text suggestions. 136 draft-ietf-ipsecme-qr-ikev2-02 138 o Added note that the PPK is stirred in the initial IKE SA setup 139 only. 141 o Added note about the initiator ignoring any content in the 142 PPK_IDENTITY notification from the responder. 144 o fixed Tero's suggestions from 2/6/1028 145 o Added IANA assigned message types where necessary. 147 o fixed minor text nits 149 draft-ietf-ipsecme-qr-ikev2-01 151 o Nits and minor fixes. 153 o prf is replaced with prf+ for the SK_d and SK_pi/r calculations. 155 o Clarified using PPK in case of EAP authentication. 157 o PPK_SUPPORT notification is changed to USE_PPK to better reflect 158 its purpose. 160 draft-ietf-ipsecme-qr-ikev2-00 162 o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr- 163 ikev2-00 that is a WG item. 165 draft-fluhrer-qr-ikev2-05 167 o Nits and editorial fixes. 169 o Made PPK_ID format and PPK Distributions subsection of the PPK 170 section. Also added an Operational Considerations section. 172 o Added comment about Child SA rekey in the Security Considerations 173 section. 175 o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not 176 configured for a responder. 178 o Various text changes and clarifications. 180 o Expanded Security Considerations section to describe some security 181 concerns and how they should be addressed. 183 draft-fluhrer-qr-ikev2-03 185 o Modified how we stir the PPK into the IKEv2 secret state. 187 o Modified how the use of PPKs is negotiated. 189 draft-fluhrer-qr-ikev2-02 191 o Simplified the protocol by stirring in the preshared key into the 192 child SAs; this avoids the problem of having the responder decide 193 which preshared key to use (as it knows the initiator identity at 194 that point); it does mean that someone with a Quantum Computer can 195 recover the initial IKE negotiation. 197 o Removed positive endorsements of various algorithms. Retained 198 warnings about algorithms known to be weak against a Quantum 199 Computer. 201 draft-fluhrer-qr-ikev2-01 203 o Added explicit guidance as to what IKE and IPsec algorithms are 204 quantum resistant. 206 draft-fluhrer-qr-ikev2-00 208 o We switched from using vendor ID's to transmit the additional data 209 to notifications. 211 o We added a mandatory cookie exchange to allow the server to 212 communicate to the client before the initial exchange. 214 o We added algorithm agility by having the server tell the client 215 what algorithm to use in the cookie exchange. 217 o We have the server specify the PPK Indicator Input, which allows 218 the server to make a trade-off between the efficiency for the 219 search of the clients PPK, and the anonymity of the client. 221 o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to 222 transform the nonces during the KDF. 224 1.2. Requirements Language 226 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 227 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 228 document are to be interpreted as described in RFC 2119 [RFC2119]. 230 2. Assumptions 232 We assume that each IKE peer has a list of Postquantum Preshared Keys 233 (PPK) along with their identifiers (PPK_ID), and any potential IKE 234 initiator has a selection of which PPK to use with any specific 235 responder. In addition, implementations have a configurable flag 236 that determines whether this postquantum preshared key is mandatory. 237 This PPK is independent of the preshared key (if any) that the IKEv2 238 protocol uses to perform authentication. The PPK specific 239 configuration that is assumed on each peer consists of the following 240 tuple: 242 Peer, PPK, PPK_ID, mandatory_or_not 244 3. Exchanges 246 If the initiator is configured to use a postquantum preshared key 247 with the responder (whether or not the use of the PPK is mandatory), 248 then he will include a notification USE_PPK in the IKE_SA_INIT 249 request message as follows: 251 Initiator Responder 252 ------------------------------------------------------------------ 253 HDR, SAi1, KEi, Ni, N(USE_PPK) ---> 255 N(USE_PPK) is a status notification payload with the type 16435; it 256 has a protocol ID of 0, no SPI and no notification data associated 257 with it. 259 If the initiator needs to resend this initial message with a cookie 260 (because the responder response included a COOKIE notification), then 261 the resend would include the USE_PPK notification if the original 262 message did. 264 If the responder does not support this specification or does not have 265 any PPK configured, then she ignores the received notification and 266 continues with the IKEv2 protocol as normal. Otherwise the responder 267 checks if she has a PPK configured, and if she does, then the 268 responder replies with the IKE_SA_INIT message including a USE_PPK 269 notification in the response: 271 Initiator Responder 272 ------------------------------------------------------------------ 273 <--- HDR, SAr1, KEr, Nr, [CERTREQ], N(USE_PPK) 275 When the initiator receives this reply, he checks whether the 276 responder included the USE_PPK notification. If the responder did 277 not and the flag mandatory_or_not indicates that using PPKs is 278 mandatory for communication with this responder, then the initiator 279 MUST abort the exchange. This situation may happen in case of 280 misconfiguration, when the initiator believes he has a mandatory to 281 use PPK for the responder, while the responder either doesn't support 282 PPKs at all or doesn't have any PPK configured for the initiator. 283 See Section 6 for discussion of the possible impacts of this 284 situation. 286 If the responder did not include the USE_PPK notification and using a 287 PPK for this particular responder is optional, then the initiator 288 continues with the IKEv2 protocol as normal, without using PPKs. 290 If the responder did include the USE_PPK notification, then the 291 initiator selects a PPK, along with its identifier PPK_ID. Then, she 292 computes this modification of the standard IKEv2 key derivation: 294 SKEYSEED = prf(Ni | Nr, g^ir) 295 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) 296 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } 298 SK_d = prf+ (PPK, SK_d') 299 SK_pi = prf+ (PPK, SK_pi') 300 SK_pr = prf+ (PPK, SK_pr') 302 That is, we use the standard IKEv2 key derivation process except that 303 the three subkeys SK_d, SK_pi, SK_pr are run through the prf+ again, 304 this time using the PPK as the key. Using prf+ construction ensures 305 that it is always possible to get the resulting keys of the same size 306 as the initial ones, even if the underlying PRF has output size 307 different from its key size. Note, that at the time this document 308 was written, all PRFs defined for use in IKEv2 [IKEV2-IANA-PRFS] had 309 output size equal to the (preferred) key size. For such PRFs only 310 the first iteration of prf+ is needed: 312 SK_d = prf (PPK, SK_d' | 0x01) 313 SK_pi = prf (PPK, SK_pi' | 0x01) 314 SK_pr = prf (PPK, SK_pr' | 0x01) 316 Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only 317 during the initial IKE SA setup. It MUST NOT be used when these 318 subkeys are calculated as result of IKE SA rekey, resumption or other 319 similar operation. 321 The initiator then sends the IKE_AUTH request message, including the 322 PPK_ID value as follows: 324 Initiator Responder 325 ------------------------------------------------------------------ 326 HDR, SK {IDi, [CERT,] [CERTREQ,] 327 [IDr,] AUTH, SAi2, 328 TSi, TSr, N(PPK_IDENTITY, PPK_ID), [N(NO_PPK_AUTH)]} ---> 330 PPK_IDENTITY is a status notification with the type 16436; it has a 331 protocol ID of 0, no SPI and a notification data that consists of the 332 identifier PPK_ID. 334 A situation may happen when the responder has some PPKs, but doesn't 335 have a PPK with the PPK_ID received from the initiator. In this case 336 the responder cannot continue with PPK (in particular, she cannot 337 authenticate the initiator), but she could be able to continue with 338 normal IKEv2 protocol if the initiator provided its authentication 339 data computed as in normal IKEv2, without using PPKs. For this 340 purpose, if using PPKs for communication with this responder is 341 optional for the initiator, then the initiator MAY include a 342 notification NO_PPK_AUTH in the above message. 344 NO_PPK_AUTH is a status notification with the type 16437; it has a 345 protocol ID of 0 and no SPI. The Notification Data field contains 346 the initiator's authentication data computed using SK_pi', which has 347 been computed without using PPKs. This is the same data that would 348 normally be placed in the Authentication Data field of an AUTH 349 payload. Since the Auth Method field is not present in the 350 notification, the authentication method used for computing the 351 authentication data MUST be the same as method indicated in the AUTH 352 payload. Note that if the initiator decides to include the 353 NO_PPK_AUTH notification, the initiator needs to perform 354 authentication data computation twice, which may consume computation 355 power (e.g. if digital signatures are involved). 357 When the responder receives this encrypted exchange, she first 358 computes the values: 360 SKEYSEED = prf(Ni | Nr, g^ir) 361 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } 362 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) 364 She then uses the SK_ei/SK_ai values to decrypt/check the message and 365 then scans through the payloads for the PPK_ID attached to the 366 PPK_IDENTITY notification. If no PPK_IDENTITY notification is found 367 and the peers successfully exchanged USE_PPK notifications in the 368 IKE_SA_INIT exchange, then the responder MUST send back 369 AUTHENTICATION_FAILED notification and then fail the negotiation. 371 If the PPK_IDENTITY notification contains PPK_ID that is not known to 372 the responder or is not configured for use for the identity from IDi 373 payload, then the responder checks whether using PPKs for this 374 initiator is mandatory and whether the initiator included NO_PPK_AUTH 375 notification in the message. If using PPKs is mandatory or no 376 NO_PPK_AUTH notification found, then then the responder MUST send 377 back AUTHENTICATION_FAILED notification and then fail the 378 negotiation. Otherwise (when PPK is optional and the initiator 379 included NO_PPK_AUTH notification) the responder MAY continue regular 380 IKEv2 protocol, except that she uses the data from the NO_PPK_AUTH 381 notification as the authentication data (which usually resides in the 382 AUTH payload), for the purpose of the initiator authentication. 383 Note, that Authentication Method is still indicated in the AUTH 384 payload. 386 This table summarizes the above logic for the responder: 388 Received Received Have PPK 389 USE_PPK NO_PPK_AUTH PPK Mandatory Action 390 ----------------------------------------------------------------- 391 No * No * Standard IKEv2 protocol 392 No * Yes No Standard IKEv2 protocol 393 No * Yes Yes Abort negotiation 394 Yes No No * Abort negotiation 395 Yes Yes No Yes Abort negotiation 396 Yes Yes No No Standard IKEv2 protocol 397 Yes * Yes * Use PPK 399 If PPK is in use, then the responder extracts the corresponding PPK 400 and computes the following values: 402 SK_d = prf+ (PPK, SK_d') 403 SK_pi = prf+ (PPK, SK_pi') 404 SK_pr = prf+ (PPK, SK_pr') 406 The responder then continues with the IKE_AUTH exchange (validating 407 the AUTH payload that the initiator included) as usual and sends back 408 a response, which includes the PPK_IDENTITY notification with no data 409 to indicate that the PPK is used in the exchange: 411 Initiator Responder 412 ------------------------------------------------------------------ 413 <-- HDR, SK {IDr, [CERT,] 414 AUTH, SAr2, 415 TSi, TSr, N(PPK_IDENTITY)} 417 When the initiator receives the response, then he checks for the 418 presence of the PPK_IDENTITY notification. If he receives one, he 419 marks the SA as using the configured PPK to generate SK_d, SK_pi, 420 SK_pr (as shown above); the content of the received PPK_IDENTITY (if 421 any) MUST be ignored. If the initiator does not receive the 422 PPK_IDENTITY, he MUST either fail the IKE SA negotiation sending the 423 AUTHENTICATION_FAILED notification in the Informational exchange (if 424 the PPK was configured as mandatory), or continue without using the 425 PPK (if the PPK was not configured as mandatory and the initiator 426 included the NO_PPK_AUTH notification in the request). 428 If EAP is used in the IKE_AUTH exchange, then the initiator doesn't 429 include AUTH payload in the first request message, however the 430 responder sends back AUTH payload in the first reply. The peers then 431 exchange AUTH payloads after EAP is successfully completed. As a 432 result, the responder sends AUTH payload twice - in the first 433 IKE_AUTH reply message and in the last one, while the initiator sends 434 AUTH payload only in the last IKE_AUTH request. See more details 435 about EAP authentication in IKEv2 in Section 2.16 of [RFC7296]. 437 The general rule for using PPK in the IKE_AUTH exchange, which covers 438 EAP authentication case too, is that the initiator includes 439 PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request 440 message containing AUTH payload. Therefore, in case of EAP the 441 responder always computes the AUTH payload in the first IKE_AUTH 442 reply message without using PPK (by means of SK_pr'), since PPK_ID is 443 not yet known to the responder. Once the IKE_AUTH request message 444 containing PPK_IDENTITY notification is received, the responder 445 follows rules described above for non-EAP authentication case. 447 Initiator Responder 448 ---------------------------------------------------------------- 449 HDR, SK {IDi, [CERTREQ,] 450 [IDr,] SAi2, 451 TSi, TSr} --> 452 <-- HDR, SK {IDr, [CERT,] AUTH, 453 EAP} 454 HDR, SK {EAP} --> 455 <-- HDR, SK {EAP (success)} 456 HDR, SK {AUTH, 457 N(PPK_IDENTITY, PPK_ID) 458 [, N(NO_PPK_AUTH)]} --> 459 <-- HDR, SK {AUTH, SAr2, TSi, TSr 460 [, N(PPK_IDENTITY)]} 462 Note, that the IKE_SA_INIT exchange in case of PPK is as described 463 above (including exchange of the USE_PPK notifications), regardless 464 whether EAP is employed in the IKE_AUTH or not. 466 4. Upgrade procedure 468 This algorithm was designed so that someone can introduce PPKs into 469 an existing IKE network without causing network disruption. 471 In the initial phase of the network upgrade, the network 472 administrator would visit each IKE node, and configure: 474 o The set of PPKs (and corresponding PPK_IDs) that this node would 475 need to know. 477 o For each peer that this node would initiate to, which PPK will be 478 used. 480 o That the use of PPK is currently not mandatory. 482 With this configuration, the node will continue to operate with nodes 483 that have not yet been upgraded. This is due to the USE_PPK notify 484 and the NO_PPK_AUTH notify; if the initiator has not been upgraded, 485 he will not send the USE_PPK notify (and so the responder will know 486 that we will not use a PPK). If the responder has not been upgraded, 487 she will not send the USE_PPK notify (and so the initiator will know 488 to not use a PPK). If both peers have been upgraded, but the 489 responder isn't yet configured with the PPK for the initiator, then 490 the responder could do standard IKEv2 protocol if the initiator sent 491 NO_PPK_AUTH notification. If both the responder and initiator have 492 been upgraded and properly configured, they will both realize it, and 493 in that case, the link will be quantum secure. 495 As an optional second step, after all nodes have been upgraded, then 496 the administrator may then go back through the nodes, and mark the 497 use of PPK as mandatory. This will not affect the strength against a 498 passive attacker; it would mean that an attacker with a Quantum 499 Computer (which is sufficiently fast to be able to break the (EC)DH 500 in real time) would not be able to perform a downgrade attack. 502 5. PPK 504 5.1. PPK_ID format 506 This standard requires that both the initiator and the responder have 507 a secret PPK value, with the responder selecting the PPK based on the 508 PPK_ID that the initiator sends. In this standard, both the 509 initiator and the responder are configured with fixed PPK and PPK_ID 510 values, and do the look up based on PPK_ID value. It is anticipated 511 that later standards will extend this technique to allow dynamically 512 changing PPK values. To facilitate such an extension, we specify 513 that the PPK_ID the initiator sends will have its first octet be the 514 PPK_ID Type value. This document defines two values for PPK_ID Type: 516 o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and 517 the PPK itself) is not specified by this document; it is assumed 518 to be mutually intelligible by both by initiator and the 519 responder. This PPK_ID type is intended for those implementations 520 that choose not to disclose the type of PPK to active attackers. 522 o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the 523 PPK are fixed octet strings; the remaining bytes of the PPK_ID are 524 a configured value. We assume that there is a fixed mapping 525 between PPK_ID and PPK, which is configured locally to both the 526 initiator and the responder. The responder can use to do a look 527 up the passed PPK_ID value to determine the corresponding PPK 528 value. Not all implementations are able to configure arbitrary 529 octet strings; to improve the potential interoperability, it is 530 recommended that, in the PPK_ID_FIXED case, both the PPK and the 531 PPK_ID strings be limited to the base64 character set, namely the 532 64 characters 0-9, A-Z, a-z, + and /. 534 The PPK_ID type value 0 is reserved; values 3-127 are reserved for 535 IANA; values 128-255 are for private use among mutually consenting 536 parties. 538 5.2. Operational Considerations 540 The need to maintain several independent sets of security credentials 541 can significantly complicate a security administrator's job, and can 542 potentially slow down widespread adoption of this specification. It 543 is anticipated, that administrators will try to simplify their job by 544 decreasing the number of credentials they need to maintain. This 545 section describes some of the considerations for PPK management. 547 5.2.1. PPK Distribution 549 PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are 550 assumed to be configured within the IKE device in an out-of-band 551 fashion. While the method of distribution is a local matter and out 552 of scope of this document or IKEv2, [RFC6030] describes a format for 553 symmetric key exchange. That format could be reused with the Key Id 554 field being the PPK_ID (without the PPK_ID Type octet for a 555 PPK_ID_FIXED), the PPK being the secret, and algorithm 556 ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as the PIN. 558 5.2.2. Group PPK 560 This document doesn't explicitly require that PPK is unique for each 561 pair of peers. If it is the case, then this solution provides full 562 peer authentication, but it also means that each host must have as 563 many independent PPKs as the peers it is going to communicate with. 564 As the number of peers grows the PPKs will not scale. 566 It is possible to use a single PPK for a group of users. Since each 567 peer uses classical public key cryptography in addition to PPK for 568 key exchange and authentication, members of the group can neither 569 impersonate each other nor read other's traffic, unless they use 570 Quantum Computers to break public key operations. However group 571 members can record other members' traffic and decrypt it later, when 572 they get access to a Quantum Computer. 574 In addition, the fact that the PPK is known to a (potentially large) 575 group of users makes it more susceptible to theft. When an attacker 576 equipped with a Quantum Computer got access to a group PPK, all 577 communications inside the group are revealed. 579 For these reasons using group PPK is NOT RECOMMENDED. 581 5.2.3. PPK-only Authentication 583 If Quantum Computers become a reality, classical public key 584 cryptography will provide little security, so administrators may find 585 it attractive not to use it at all for authentication. This will 586 reduce the number of credentials they need to maintain to PPKs only. 587 Combining group PPK and PPK-only authentication is NOT RECOMMENDED, 588 since in this case any member of the group can impersonate any other 589 member even without help of Quantum Computers. 591 PPK-only authentication can be achieved in IKEv2 if NULL 592 Authentication method [RFC7619] is employed. Without PPK the NULL 593 Authentication method provides no authentication of the peers, 594 however since a PPK is stirred into the SK_pi and the SK_pr, the 595 peers become authenticated if a PPK is in use. Using PPKs MUST be 596 mandatory for the peers if they advertise support for PPK in 597 IKE_SA_INIT and use NULL Authentication. Addtionally, since the 598 peers are authenticated via PPK, the ID Type in the IDi/IDr payloads 599 SHOULD NOT be ID_NULL, despite using the NULL Authentication method. 601 6. Security Considerations 603 Quantum computers are able to perform Grover's algorithm; that 604 effectively halves the size of a symmetric key. Because of this, the 605 user SHOULD ensure that the postquantum preshared key used has at 606 least 256 bits of entropy, in order to provide 128-bit security 607 level. 609 With this protocol, the computed SK_d is a function of the PPK. 610 Assuming that the PPK has sufficient entropy (for example, at least 611 2^256 possible values), then even if an attacker was able to recover 612 the rest of the inputs to the PRF function, it would be infeasible to 613 use Grover's algorithm with a Quantum Computer to recover the SK_d 614 value. Similarly, every child SA key is a function of SK_d, hence 615 all the keys for all the child SAs are also quantum resistant 616 (assuming that the PPK was of high enough entropy, and that all the 617 subkeys are sufficiently long). 619 Although this protocol preserves all the security properties of IKEv2 620 against adversaries with conventional computers, it allows an 621 adversary with a Quantum Computer to decrypt all traffic encrypted 622 with the initial IKE SA. In particular, it allows the adversary to 623 recover the identities of both sides. If there is IKE traffic other 624 than the identities that need to be protected against such an 625 adversary, implementations MAY rekey the initial IKE SA immediately 626 after negotiating it to generate a new SKEYSEED from the postquantum 627 SK_d. This would reduce the amount of data available to an attacker 628 with a Quantum Computer. 630 If sensitive information (like keys) is to be transferred over IKE 631 SA, then implementations MUST rekey the initial IKE SA before sending 632 this information to get protection against Quantum Computers. 634 Alternatively, an initial IKE SA (which is used to exchange 635 identities) can take place, perhaps by using the protocol documented 636 in [RFC6023]. After the childless IKE SA is created, implementations 637 would immediately create a new IKE SA (which is used to exchange 638 everything else) by using a rekey mechanism for IKE SAs. Because the 639 rekeyed IKE SA keys are a function of SK_d, which is a function of 640 the PPK (among other things), traffic protected by that IKE SA is 641 secure against Quantum capable adversaries. 643 In addition, the policy SHOULD be set to negotiate only quantum- 644 resistant symmetric algorithms; while this RFC doesn't claim to give 645 advice as to what algorithms are secure (as that may change based on 646 future cryptographical results), below is a list of defined IKEv2 and 647 IPsec algorithms that should NOT be used, as they are known not to be 648 quantum resistant 650 o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with 651 key size less than 256 bits. 653 o Any ESP Transform with key size less than 256 bits. 655 o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined 656 to be able to use an arbitrary key size, they convert it into a 657 128-bit key internally. 659 Section 3 requires the initiator to abort the initial exchange if 660 using PPKs is mandatory for it, but the responder might not include 661 the USE_PPK notification in the response. In this situation when the 662 initiator aborts negotiation he leaves half-open IKE SA on the 663 responder (because IKE_SA_INIT completes successfully from the 664 responder's point of view). This half-open SA will eventually expire 665 and be deleted, but if the initiator continues its attempts to create 666 IKE SA with a high enough rate, then the responder may consider it as 667 a Denial-of-Service attack and take protection measures (see 668 [RFC8019] for more detail). It is RECOMMENDED that implementations 669 in this situation cache the negative result of negotiation for some 670 time and don't make attempts to create it again for some time, 671 because this is a result of misconfiguration and probably some re- 672 configuration of the peers is needed. 674 If using PPKs is optional for both peers and they authenticate 675 themselves using digital signatures, then an attacker in between, 676 equipped with a Quantum Computer capable of breaking public key 677 operations in real time, is able to mount downgrade attack by 678 removing USE_PPK notification from the IKE_SA_INIT and forging 679 digital signatures in the subsequent exchange. If using PPKs is 680 mandatory for at least one of the peers or PSK is used for 681 authentication, then the attack will be detected and the SA won't be 682 created. 684 If using PPKs is mandatory for the initiator, then an attacker 685 capable to eavesdrop and to inject packets into the network can 686 prevent creating IKE SA by mounting the following attack. The 687 attacker intercepts the initial request containing the USE_PPK 688 notification and injects the forget response containing no USE_PPK. 689 If the attacker manages to inject this packet before the responder 690 sends a genuine response, then the initiator would abort the 691 exchange. To thwart this kind of attack it is RECOMMENDED, that if 692 using PPKs is mandatory for the initiator and the received response 693 doesn't contain the USE_PPK notification, then the initiator doesn't 694 abort the exchange immediately, but instead waits some time for more 695 responses (possibly retransmitting the request). If all the received 696 responses contain no USE_PPK, then the exchange is aborted. 698 7. IANA Considerations 700 This document defines three new Notify Message Types in the "Notify 701 Message Types - Status Types" registry: 703 16435 USE_PPK 704 16436 PPK_IDENTITY 705 16437 NO_PPK_AUTH 707 This document also creates a new IANA registry for the PPK_ID types. 708 The initial values of this registry are: 710 PPK_ID Type Value 711 ----------- ----- 712 Reserved 0 713 PPK_ID_OPAQUE 1 714 PPK_ID_FIXED 2 715 Unassigned 3-127 716 Reserved for private use 128-255 718 Changes and additions to this registry are by Expert Review 719 [RFC5226]. 721 8. References 723 8.1. Normative References 725 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 726 Requirement Levels", BCP 14, RFC 2119, 727 DOI 10.17487/RFC2119, March 1997, . 730 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 731 Kivinen, "Internet Key Exchange Protocol Version 2 732 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 733 2014, . 735 8.2. Informational References 737 [I-D.hoffman-c2pq] 738 Hoffman, P., "The Transition from Classical to Post- 739 Quantum Cryptography", draft-hoffman-c2pq-03 (work in 740 progress), February 2018. 742 [IKEV2-IANA-PRFS] 743 "Internet Key Exchange Version 2 (IKEv2) Parameters, 744 Transform Type 2 - Pseudorandom Function Transform IDs", 745 . 748 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 749 (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, 750 . 752 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 753 IANA Considerations Section in RFCs", RFC 5226, 754 DOI 10.17487/RFC5226, May 2008, . 757 [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A 758 Childless Initiation of the Internet Key Exchange Version 759 2 (IKEv2) Security Association (SA)", RFC 6023, 760 DOI 10.17487/RFC6023, October 2010, . 763 [RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric 764 Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030, 765 October 2010, . 767 [RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication 768 Method in the Internet Key Exchange Protocol Version 2 769 (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015, 770 . 772 [RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange 773 Protocol Version 2 (IKEv2) Implementations from 774 Distributed Denial-of-Service Attacks", RFC 8019, 775 DOI 10.17487/RFC8019, November 2016, . 778 Appendix A. Discussion and Rationale 780 The idea behind this document is that while a Quantum Computer can 781 easily reconstruct the shared secret of an (EC)DH exchange, they 782 cannot as easily recover a secret from a symmetric exchange. This 783 makes the SK_d, and hence the IPsec KEYMAT and any child SA's 784 SKEYSEED, depend on both the symmetric PPK, and also the Diffie- 785 Hellman exchange. If we assume that the attacker knows everything 786 except the PPK during the key exchange, and there are 2^n plausible 787 PPKs, then a Quantum Computer (using Grover's algorithm) would take 788 O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be 789 trivially solved, the attacker still can't recover any key material 790 (except for the SK_ei, SK_er, SK_ai, SK_ar values for the initial IKE 791 exchange) unless they can find the PPK, which is too difficult if the 792 PPK has enough entropy (for example, 256 bits). Note that we do 793 allow an attacker with a Quantum Computer to rederive the keying 794 material for the initial IKE SA; this was a compromise to allow the 795 responder to select the correct PPK quickly. 797 Another goal of this protocol is to minimize the number of changes 798 within the IKEv2 protocol, and in particular, within the cryptography 799 of IKEv2. By limiting our changes to notifications, and adjusting 800 the SK_d, SK_pi, SK_pr, it is hoped that this would be implementable, 801 even on systems that perform most of the IKEv2 processing in 802 hardware. 804 A third goal was to be friendly to incremental deployment in 805 operational networks, for which we might not want to have a global 806 shared key, or quantum resistant IKEv2 is rolled out incrementally. 807 This is why we specifically try to allow the PPK to be dependent on 808 the peer, and why we allow the PPK to be configured as optional. 810 A fourth goal was to avoid violating any of the security goals of 811 IKEv2. 813 Appendix B. Acknowledgements 815 We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett, 816 Tommy Pauly, Quynh Dang and the rest of the IPSecME Working Group for 817 their feedback and suggestions for the scheme. 819 Authors' Addresses 821 Scott Fluhrer 822 Cisco Systems 824 Email: sfluhrer@cisco.com 826 David McGrew 827 Cisco Systems 829 Email: mcgrew@cisco.com 831 Panos Kampanakis 832 Cisco Systems 834 Email: pkampana@cisco.com 836 Valery Smyslov 837 ELVIS-PLUS 839 Phone: +7 495 276 0211 840 Email: svan@elvis.ru