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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: 'TBA' is mentioned on line 316, but not defined == Missing Reference: 'CERTREQ' is mentioned on line 250, but not defined == Outdated reference: A later version (-07) exists of draft-hoffman-c2pq-02 -- Obsolete informational reference (is this intentional?): RFC 2409 (Obsoleted by RFC 4306)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 (~~), 5 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: June 23, 2018 Cisco Systems 6 V. Smyslov 7 ELVIS-PLUS 8 December 20, 2017 10 Postquantum Preshared Keys for IKEv2 11 draft-ietf-ipsecme-qr-ikev2-01 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 June 23, 2018. 43 Copyright Notice 45 Copyright (c) 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 4. Upgrade procedure . . . . . . . . . . . . . . . . . . . . . . 10 66 5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 67 5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 11 68 5.2. Operational Considerations . . . . . . . . . . . . . . . 11 69 5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 12 70 5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 12 71 5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 12 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 75 8.1. Normative References . . . . . . . . . . . . . . . . . . 15 76 8.2. Informational References . . . . . . . . . . . . . . . . 16 77 Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 16 78 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 17 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 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) keys and the SKEYSEED for the child SAs; 109 this secret provides quantum resistance to the IPsec SAs (and any 110 child IKE SAs). We also stir the secret into the SK_pi, SK_pr 111 values; this 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 a 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-ikev2-01 128 o Nits and minor fixes. 130 o prf is replaced with prf+ for the SK_d and SK_pi/r calculations. 132 o Clarified using PPK in case of EAP authentication. 134 o PPK_SUUPORT notification is changed to USE_PPK to better reflect 135 its purpose. 137 draft-ietf-ipsecme-qr-ikev2-00 139 o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr- 140 ikev2-00 that is a WG item. 142 draft-fluhrer-qr-ikev2-05 144 o Nits and editorial fixes. 146 o Made PPK_ID format and PPK Distributions subsection of the PPK 147 section. Also added an Operational Considerations section. 149 o Added comment about Child SA rekey in the Security Considerations 150 section. 152 o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not 153 configured for a responder. 155 o Various text changes and clarifications. 157 o Expanded Security Considerations section to describe some security 158 concerns and how they should be addressed. 160 draft-fluhrer-qr-ikev2-03 162 o Modified how we stir the PPK into the IKEv2 secret state. 164 o Modified how the use of PPKs is negotiated. 166 draft-fluhrer-qr-ikev2-02 168 o Simplified the protocol by stirring in the preshared key into the 169 child SAs; this avoids the problem of having the responder decide 170 which preshared key to use (as it knows the initiator identity at 171 that point); it does mean that someone with a Quantum Computer can 172 recover the initial IKE negotiation. 174 o Removed positive endorsements of various algorithms. Retained 175 warnings about algorithms known to be weak against a Quantum 176 Computer. 178 draft-fluhrer-qr-ikev2-01 180 o Added explicit guidance as to what IKE and IPsec algorithms are 181 quantum resistant. 183 draft-fluhrer-qr-ikev2-00 185 o We switched from using vendor ID's to transmit the additional data 186 to notifications. 188 o We added a mandatory cookie exchange to allow the server to 189 communicate to the client before the initial exchange. 191 o We added algorithm agility by having the server tell the client 192 what algorithm to use in the cookie exchange. 194 o We have the server specify the PPK Indicator Input, which allows 195 the server to make a trade-off between the efficiency for the 196 search of the clients PPK, and the anonymity of the client. 198 o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to 199 transform the nonces during the KDF. 201 1.2. Requirements Language 203 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 204 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 205 document are to be interpreted as described in RFC 2119 [RFC2119]. 207 2. Assumptions 209 We assume that each IKE peer has a list of Postquantum Preshared Keys 210 (PPK) along with their identifiers (PPK_ID), and any potential IKE 211 initiator has a selection of which PPK to use with any specific 212 responder. In addition, implementations have a configurable flag 213 that determines whether this postquantum preshared key is mandatory. 214 This PPK is independent of the preshared key (if any) that the IKEv2 215 protocol uses to perform authentication. The PPK specific 216 configuration that is assumed on each peer consists of the following 217 tuple: 219 Peer, PPK, PPK_ID, mandatory_or_not 221 3. Exchanges 223 If the initiator is configured to use a postquantum preshared key 224 with the responder (whether or not the use of the PPK is mandatory), 225 then he will include a notification USE_PPK in the IKE_SA_INIT 226 request message as follows: 228 Initiator Responder 229 ------------------------------------------------------------------ 230 HDR, SAi1, KEi, Ni, N(USE_PPK) ---> 232 N(USE_PPK) is a status notification payload with the type [TBA]; it 233 has a protocol ID of 0, no SPI and no notification data associated 234 with it. 236 If the initiator needs to resend this initial message with a cookie 237 (because the responder response included a COOKIE notification), then 238 the resend would include the USE_PPK notification if the original 239 message did. 241 If the responder does not support this specification or does not have 242 any PPK configured, then she ignores the received notification and 243 continues with the IKEv2 protocol as normal. Otherwise the responder 244 checks if she has a PPK configured, and if she does, then the 245 responder replies with the IKE_SA_INIT message including a USE_PPK 246 notification in the response: 248 Initiator Responder 249 ------------------------------------------------------------------ 250 <--- HDR, SAr1, KEr, Nr, [CERTREQ], N(USE_PPK) 252 When the initiator receives this reply, he checks whether the 253 responder included the USE_PPK notification. If the responder did 254 not and the flag mandatory_or_not indicates that using PPKs is 255 mandatory for communication with this responder, then the initiator 256 MUST abort the exchange. This situation may happen in case of 257 misconfiguration, when the initiator believes he has a mandatory to 258 use PPK for the responder, while the responder either doesn't support 259 PPKs at all or doesn't have any PPK configured for the initiator. 260 See Section 6 for discussion of the possible impacts of this 261 situation. 263 If the responder did not include the USE_PPK notification and using 264 PPKs for this responder is optional, then the initiator continues 265 with the IKEv2 protocol as normal, without using PPKs. 267 If the responder did include the USE_PPK notification, then the 268 initiator selects a PPK, along with its identifier PPK_ID. Then, she 269 computes this modification of the standard IKEv2 key derivation: 271 SKEYSEED = prf(Ni | Nr, g^ir) 272 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) 273 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } 275 SK_d = prf+ (PPK, SK_d') 276 SK_pi = prf+ (PPK, SK_pi') 277 SK_pr = prf+ (PPK, SK_pr') 279 That is, we use the standard IKEv2 key derivation process except that 280 the three subkeys SK_d, SK_pi, SK_pr are run through the prf+ again, 281 this time using the PPK as the key. Using prf+ construction ensures 282 that it is always possible to get the resulting keys of the same size 283 as the initial ones, even if the underlying prf has output size 284 different from its key size. Note, that at the time this document 285 was written, all prfs defined for use in IKEv2 [IKEV2-IANA-PRFS] had 286 output size equal to the (preferred) key size. For such prfs only 287 the first iteration of prf+ is needed: 289 SK_d = prf (PPK, SK_d' | 0x01) 290 SK_pi = prf (PPK, SK_pi' | 0x01) 291 SK_pr = prf (PPK, SK_pr' | 0x01) 293 The initiator then sends the IKE_AUTH request message, including the 294 PPK_ID value as follows: 296 Initiator Responder 297 ------------------------------------------------------------------ 298 HDR, SK {IDi, [CERT,] [CERTREQ,] 299 [IDr,] AUTH, SAi2, 300 TSi, TSr, N(PPK_IDENTITY)(PPK_ID), [N(NO_PPK_AUTH)]} ---> 302 PPK_IDENTITY is a status notification with the type [TBA]; it has a 303 protocol ID of 0, no SPI and a notification data that consists of the 304 identifier PPK_ID. 306 A situation may happen when the responder has some PPKs, but doesn't 307 have a PPK with the PPK_ID received from the initiator. In this case 308 the responder cannot continue with PPK (in particular, she cannot 309 authenticate the initiator), but she could be able to continue with 310 normal IKEv2 protocol if the initiator provided its authentication 311 data computed as in normal IKEv2, without using PPKs. For this 312 purpose, if using PPKs for communication with this responder is 313 optional for the initiator, then the initiator MAY include a 314 notification NO_PPK_AUTH in the above message. 316 NO_PPK_AUTH is a status notification with the type [TBA]; it has a 317 protocol ID of 0 and no SPI. A notification data consists of the 318 initiator's authentication data computed using SK_pi' (i.e. the data 319 that computed without using PPKs and would normally be placed in the 320 AUTH payload). Authentication Method for computing the 321 authentication data MUST be the same as indicated in the AUTH payload 322 and is not included in the notification. Note that if the initiator 323 decides to include NO_PPK_AUTH notification, then it means that the 324 initiator needs to perform authentication data computation twice that 325 may consume substantial computation power (e.g. if digital signatures 326 are involved). 328 When the responder receives this encrypted exchange, she first 329 computes the values: 331 SKEYSEED = prf(Ni | Nr, g^ir) 332 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } 333 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) 335 She then uses the SK_ei/SK_ai values to decrypt/check the message and 336 then scans through the payloads for the PPK_ID attached to the 337 PPK_IDENTITY notification. If no PPK_IDENTITY notification is found 338 and the peers successfully exchanged USE_PPK notifications in the 339 IKE_SA_INIT exchange, then the responder MUST send back 340 AUTHENTICATION_FAILED notification and then fail the negotiation. 342 If the PPK_IDENTITY notification contains PPK_ID that is not known to 343 the responder or is not configured for use for the identity from IDi 344 payload, then the responder checks whether using PPKs for this 345 initiator is mandatory and whether the initiator included NO_PPK_AUTH 346 notification in the message. If using PPKs is mandatory or no 347 NO_PPK_AUTH notification found, then then the responder MUST send 348 back AUTHENTICATION_FAILED notification and then fail the 349 negotiation. Otherwise (when PPK is optional and the initiator 350 included NO_PPK_AUTH notification) the responder MAY continue regular 351 IKEv2 protocol, except that she uses the data from the NO_PPK_AUTH 352 notification as the authentication data (which usually resides in the 353 AUTH payload), for the purpose of the initiator authentication. 354 Note, that Authentication Method is still indicated in the AUTH 355 payload. 357 This table summarizes the above logic by the responder: 359 Received Received Have PPK 360 USE_PPK NO_PPK_AUTH PPK Mandatory Action 361 ------------------------------------------------------------------ 362 No * No * Standard IKEv2 protocol 363 No * Yes No Standard IKEv2 protocol 364 No * Yes Yes Abort negotiation 365 Yes No No * Abort negotiation 366 Yes Yes No Yes Abort negotiation 367 Yes Yes No No Standard IKEv2 protocol 368 Yes * Yes * Use PPK 370 If PPK is in use, then the responder extracts corresponding PPK and 371 computes the following values: 373 SK_d = prf+ (PPK, SK_d') 374 SK_pi = prf+ (PPK, SK_pi') 375 SK_pr = prf+ (PPK, SK_pr') 377 The responder then continues with the IKE_AUTH exchange (validating 378 the AUTH payload that the initiator included) as usual and sends back 379 a response, which includes the PPK_IDENTITY notification with no data 380 to indicate that the PPK is used in the exchange: 382 Initiator Responder 383 ------------------------------------------------------------------ 384 <-- HDR, SK {IDr, [CERT,] 385 AUTH, SAr2, 386 TSi, TSr, N(PPK_IDENTITY)} 388 When the initiator receives the response, then he checks for the 389 presence of the PPK_IDENTITY notification. If he receives one, he 390 marks the SA as using the configured PPK to generate SK_d, SK_pi, 391 SK_pr (as shown above); if he does not receive one, he MUST either 392 fail the IKE SA negotiation sending the AUTHENTICATION_FAILED 393 notification in the Informational exchange (if the PPK was configured 394 as mandatory), or continue without using the PPK (if the PPK was not 395 configured as mandatory and the initiator included the NO_PPK_AUTH 396 notification in the request). 398 If EAP is used in the IKE_AUTH exchange, then the initiator doesn't 399 include AUTH payload in the first request message, however the 400 responder sends back AUTH payload in the first reply. The peers then 401 exchange AUTH payloads after EAP is successfully completed. As a 402 result, the responder sends AUTH payload twice - in the first 403 IKE_AUTH reply message and in the last one, while the initiator sends 404 AUTH payload only in the last IKE_AUTH request. See more details 405 about EAP authentication in IKEv2 in Section 2.16 of [RFC7296]. 407 The general rule for using PPK in the IKE_AUTH exchange, which covers 408 EAP authentication case too, is that the initiator includes 409 PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request 410 message containing AUTH payload. Therefore, in case of EAP the 411 responder always computes the AUTH payload in the first IKE_AUTH 412 reply message without using PPK (by means of SK_pr'), since PPK_ID is 413 not yet known to the responder. Once the IKE_AUTH request message 414 containing PPK_IDENTITY notification is received, the responder 415 follows rules described above for non-EAP authentication case. 417 Initiator Responder 418 ------------------------------------------------------------------- 419 HDR, SK {IDi, [CERTREQ,] 420 [IDr,] SAi2, 421 TSi, TSr} --> 422 <-- HDR, SK {IDr, [CERT,] AUTH, 423 EAP} 424 HDR, SK {EAP} --> 425 <-- HDR, SK {EAP (success)} 426 HDR, SK {AUTH, 427 N(PPK_IDENTITY)(PPK_ID) 428 [, N(NO_PPK_AUTH)]} --> 429 <-- HDR, SK {AUTH, SAr2, TSi, TSr 430 [, N(PPK_IDENTITY)]} 432 Note, that the IKE_SA_INIT exchange in case of PPK is as described 433 above (including exchange of the USE_PPK notifications), regardless 434 whether EAP is employed in the IKE_AUTH or not. 436 4. Upgrade procedure 438 This algorithm was designed so that someone can introduce PPKs into 439 an existing IKE network without causing network disruption. 441 In the initial phase of the network upgrade, the network 442 administrator would visit each IKE node, and configure: 444 o The set of PPKs (and corresponding PPK_IDs) that this node would 445 need to know. 447 o For each peer that this node would initiate to, which PPK will be 448 used. 450 o That the use of PPK is currently not mandatory. 452 With this configuration, the node will continue to operate with nodes 453 that have not yet been upgraded. This is due to the USE_PPK notify 454 and the NO_PPK_AUTH notify; if the initiator has not been upgraded, 455 he will not send the USE_PPK notify (and so the responder will know 456 that we will not use a PPK). If the responder has not been upgraded, 457 she will not send the USE_PPK notify (and so the initiator will know 458 to not use a PPK). If both peers have been upgraded, but the 459 responder isn't yet configured with the PPK for the initiator, then 460 the responder could do standard IKEv2 protocol if the initiator sent 461 NO_PPK_AUTH notification. If the responder has not been upgraded and 462 properly configured, they will both realize it, and in that case, the 463 link will be quantum secure. 465 As an optional second step, after all nodes have been upgraded, then 466 the administrator may then go back through the nodes, and mark the 467 use of PPK as mandatory. This will not affect the strength against a 468 passive attacker; it would mean that an attacker with a Quantum 469 Computer (which is sufficiently fast to be able to break the (EC)DH 470 in real time would not be able to perform a downgrade attack). 472 5. PPK 474 5.1. PPK_ID format 476 This standard requires that both the initiator and the responder have 477 a secret PPK value, with the responder selecting the PPK based on the 478 PPK_ID that the initiator sends. In this standard, both the 479 initiator and the responder are configured with fixed PPK and PPK_ID 480 values, and do the look up based on PPK_ID value. It is anticipated 481 that later standards will extend this technique to allow dynamically 482 changing PPK values. To facilitate such an extension, we specify 483 that the PPK_ID the initiator sends will have its first octet be the 484 PPK_ID Type value. This document defines two values for PPK_ID Type: 486 o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and 487 the PPK itself) is not specified by this document; it is assumed 488 to be mutually intelligible by both by initiator and the 489 responder. This PPK_ID type is intended for those implementations 490 that choose not to disclose the type of PPK to active attackers. 492 o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the 493 PPK are fixed octet strings; the remaining bytes of the PPK_ID are 494 a configured value. We assume that there is a fixed mapping 495 between PPK_ID and PPK, which is configured locally to both the 496 initiator and the responder. The responder can use to do a look 497 up the passed PPK_ID value to determine the corresponding PPK 498 value. Not all implementations are able to configure arbitrary 499 octet strings; to improve the potential interoperability, it is 500 recommended that, in the PPK_ID_FIXED case, both the PPK and the 501 PPK_ID strings be limited to the base64 character set, namely the 502 64 characters 0-9, A-Z, a-z, + and /. 504 The PPK_ID type value 0 is reserved; values 3-127 are reserved for 505 IANA; values 128-255 are for private use among mutually consenting 506 parties. 508 5.2. Operational Considerations 510 The need to maintain several independent sets of security credentials 511 can significantly complicate security administrators job, and can 512 potentially slow down widespread adoption of this solution. It is 513 anticipated, that administrators will try to simplify their job by 514 decreasing the number of credentials they need to maintain. This 515 section describes some of the considerations for PPK management. 517 5.2.1. PPK Distribution 519 PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are 520 assumed to be configured within the IKE device in an out-of-band 521 fashion. While the method of distribution is a local matter and out 522 of scope of this document or IKEv2, [RFC6030] describes a format for 523 symmetric key exchange. That format could be reused with the Key Id 524 field being the PPK_ID (without the PPK_ID Type octet for a 525 PPK_ID_FIXED), the PPK being the secret, and the algorithm 526 ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as PIN. 528 5.2.2. Group PPK 530 This document doesn't explicitly require that PPK is unique for each 531 pair of peers. If it is the case, then this solution provides full 532 peer authentication, but it also means that each host must have that 533 many independent PPKs, how many peers it is going to communicate 534 with. As the number of hosts grows this will scale badly. 536 Even though it is NOT RECOMMENDED, it is possible to use a single PPK 537 for a group of users. Since each peer uses classical public key 538 cryptography in addition to PPK for key exchange and authentication, 539 members of the group can neither impersonate each other nor read 540 other's traffic, unless they use Quantum Computers to break public 541 key operations. 543 Although it's probably safe to use group PPK in short term, the fact, 544 that the PPK is known to a (potentially large) group of users makes 545 it more susceptible to theft. If an attacker equipped with a Quantum 546 Computer got access to a group PPK, then all the communications 547 inside the group are revealed. 549 5.2.3. PPK-only Authentication 551 If Quantum Computers become a reality, classical public key 552 cryptography will provide little security, so administrators may find 553 it attractive not to use it at all for authentication. This will 554 reduce the number of credentials they need to maintain to PPKs only. 555 Combining group PPK and PPK-only authentication is NOT RECOMMENDED, 556 since in this case any member of the group can impersonate any other 557 member even without help of Quantum Computers. 559 PPK-only authentication can be achieved in IKEv2 if NULL 560 Authentication method [RFC7619] is employed. Without PPK the NULL 561 Authentication method provides no authentication of the peers, 562 however since a PPK is stirred into the SK_pi and the SK_pr, the 563 peers become authenticated if a PPK is in use. Using PPKs MUST be 564 mandatory for the peers if they advertise support for PPK in 565 IKE_SA_INIT and use NULL Authentication. Addtionally, since the 566 peers are authenticated via PPK, the ID Type in the IDi/IDr payloads 567 SHOULD NOT be ID_NULL, despite using NULL Authentication method. 569 6. Security Considerations 571 Quantum computers are able to perform Grover's algorithm; that 572 effectively halves the size of a symmetric key. Because of this, the 573 user SHOULD ensure that the postquantum preshared key used has at 574 least 256 bits of entropy, in order to provide a 128-bit security 575 level. 577 With this protocol, the computed SK_d is a function of the PPK, and 578 assuming that the PPK has sufficient entropy (for example, at least 579 2^256 possible values), then even if an attacker was able to recover 580 the rest of the inputs to the prf function, it would be infeasible to 581 use Grover's algorithm with a Quantum Computer to recover the SK_d 582 value. Similarly, every child SA key is a function of SK_d, hence 583 all the keys for all the child SAs are also quantum resistant 584 (assuming that the PPK was high entropy and secret, and that all the 585 subkeys are sufficiently long). 587 Although this protocol preserves all the security properties of IKEv2 588 against adversaries with conventional computers, it allows an 589 adversary with a Quantum Computer to decrypt all traffic encrypted 590 with the initial IKE SA. In particular, it allows the adversary to 591 recover the identities of both sides. If there is IKE traffic other 592 than the identities that need to be protected against such an 593 adversary, implementations MAY rekey the initial IKE SA immediately 594 after negotiating it to generate a new SKEYSEED with from the 595 postquantum SK_d. This would reduce the amount of data available to 596 an attacker with a Quantum Computer. 598 Alternatively, an initial IKE SA (which is used to exchange 599 identities) can take place, perhaps by using the protocol documented 600 in [RFC6023]. After the childless IKE SA is created, implementations 601 would immediately create a new IKE SA (which is used to exchange 602 everything else) by using a rekey mechanism for IKE SAs. Because the 603 rekeyed IKE SA keys are a function of SK_d, which is a function of 604 the PPK (among other things), traffic protected by that IKE SA is 605 secure against Quantum capable adversaries. 607 If some sensitive information (like keys) is to be transferred over 608 IKE SA, then implementations MUST rekey the initial IKE SA before 609 sending this information to get protection against Quantum Computers. 611 In addition, the policy SHOULD be set to negotiate only quantum- 612 resistant symmetric algorithms; while this RFC doesn't claim to give 613 advise as to what algorithms are secure (as that may change based on 614 future cryptographical results), below is a list of defined IKEv2 and 615 IPsec algorithms that should NOT be used, as they are known not to be 616 quantum resistant 618 o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with 619 key size less than 256 bits. 621 o Any ESP Transform with key size less than 256 bits. 623 o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined 624 to be able to use an arbitrary key size, they convert it into a 625 128-bit key internally. 627 Section 3 requires the initiator to abort the initial exchange if 628 using PPKs is mandatory for it, but the responder didn't include the 629 USE_PPK notification in the response. In this situation when the 630 initiator aborts negotiation he leaves half-open IKE SA on the 631 responder (because IKE_SA_INIT completes successfully from 632 responder's point of view). This half-open SA will eventually expire 633 and be deleted, but if the initiator continues its attempts to create 634 IKE SA with a high enough rate, then the responder may consider it as 635 a Denial-of-Service attack and take some measures (see [RFC8019] for 636 more detail). It is RECOMMENDED that implementations in this 637 situation cache the negative result of negotiation for some time and 638 don't make attempts to create it again for some time, because this is 639 a result of misconfiguration and probably some re-configuration of 640 the peers is needed. 642 If using PPKs is optional for both peers and they authenticate 643 themselves using digital signatures, then an attacker in between, 644 equipped with a Quantum Computer capable of breaking public key 645 operations in real time, is able to mount downgrade attack by 646 removing USE_PPK notification from the IKE_SA_INIT and forging 647 digital signatures in the subsequent exchange. If using PPKs is 648 mandatory for at least one of the peers or PSK is used for 649 authentication, then the attack will be detected and the SA won't be 650 created. 652 If using PPKs is mandatory for the initiator, then an attacker 653 capable to eavesdrop and to inject packets into the network can 654 prevent creating IKE SA by mounting the following attack. The 655 attacker intercepts the the initial request containing the USE_PPK 656 notification and injects the forget response containing no USE_PPK. 657 If the attacker manages to inject this packet before the responder 658 sends a genuine response, then the initiator would abort the 659 exchange. To thwart this kind of attack it is RECOMMENDED, that if 660 using PPKs is mandatory for the initiator and the received response 661 doesn't contain the USE_PPK notification, then the initiator doesn't 662 abort exchange immediately, but instead waits some time for more 663 responses (possibly retransmitting the request). If all the received 664 responses contain no USE_PPK, then the exchange is aborted. 666 7. IANA Considerations 668 This document defines three new Notify Message Types in the "Notify 669 Message Types - Status Types" registry: 671 USE_PPK 672 PPK_IDENTITY 673 NO_PPK_AUTH 675 This document also creates a new IANA registry for the PPK_ID types. 676 The initial values of this registry are: 678 PPK_ID Type Value 679 ----------- ----- 680 Reserved 0 681 PPK_ID_OPAQUE 1 682 PPK_ID_FIXED 2 683 Unassigned 3-127 684 Reserved for private use 128-255 686 Changes and additions to this registry are by Expert Review 687 [RFC5226]. 689 8. References 691 8.1. Normative References 693 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 694 Requirement Levels", BCP 14, RFC 2119, 695 DOI 10.17487/RFC2119, March 1997, 696 . 698 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 699 Kivinen, "Internet Key Exchange Protocol Version 2 700 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 701 2014, . 703 8.2. Informational References 705 [I-D.hoffman-c2pq] 706 Hoffman, P., "The Transition from Classical to Post- 707 Quantum Cryptography", draft-hoffman-c2pq-02 (work in 708 progress), August 2017. 710 [IKEV2-IANA-PRFS] 711 "Internet Key Exchange Version 2 (IKEv2) Parameters, 712 Transform Type 2 - Pseudorandom Function Transform IDs", 713 . 716 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 717 (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, 718 . 720 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 721 IANA Considerations Section in RFCs", RFC 5226, 722 DOI 10.17487/RFC5226, May 2008, 723 . 725 [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A 726 Childless Initiation of the Internet Key Exchange Version 727 2 (IKEv2) Security Association (SA)", RFC 6023, 728 DOI 10.17487/RFC6023, October 2010, 729 . 731 [RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric 732 Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030, 733 October 2010, . 735 [RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication 736 Method in the Internet Key Exchange Protocol Version 2 737 (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015, 738 . 740 [RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange 741 Protocol Version 2 (IKEv2) Implementations from 742 Distributed Denial-of-Service Attacks", RFC 8019, 743 DOI 10.17487/RFC8019, November 2016, 744 . 746 Appendix A. Discussion and Rationale 748 The idea behind this document is that while a Quantum Computer can 749 easily reconstruct the shared secret of an (EC)DH exchange, they 750 cannot as easily recover a secret from a symmetric exchange. This 751 makes the SK_d, and hence the IPsec KEYMAT and any child SA's 752 SKEYSEED, depend on both the symmetric PPK, and also the Diffie- 753 Hellman exchange. If we assume that the attacker knows everything 754 except the PPK during the key exchange, and there are 2^n plausible 755 PPKs, then a Quantum Computer (using Grover's algorithm) would take 756 O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be 757 trivially solved, the attacker still can't recover any key material 758 (except for the SK_ei, SK_er, SK_ai, SK_ar values for the initial IKE 759 exchange) unless they can find the PPK, which is too difficult if the 760 PPK has enough entropy (for example, 256 bits). Note that we do 761 allow an attacker with a Quantum Computer to rederive the keying 762 material for the initial IKE SA; this was a compromise to allow the 763 responder to select the correct PPK quickly. 765 Another goal of this protocol is to minimize the number of changes 766 within the IKEv2 protocol, and in particular, within the cryptography 767 of IKEv2. By limiting our changes to notifications, and translating 768 the nonces, it is hoped that this would be implementable, even on 769 systems that perform much of the IKEv2 processing is in hardware. 771 A third goal was to be friendly to incremental deployment in 772 operational networks, for which we might not want to have a global 773 shared key or quantum resistant IKEv2 is rolled out incrementally. 774 This is why we specifically try to allow the PPK to be dependent on 775 the peer, and why we allow the PPK to be configured as optional. 777 A fourth goal was to avoid violating any of the security goals of 778 IKEv2. 780 Appendix B. Acknowledgements 782 We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett 783 and the rest of the IPSecME Working Group for their feedback and 784 suggestions for the scheme. 786 Authors' Addresses 788 Scott Fluhrer 789 Cisco Systems 791 Email: sfluhrer@cisco.com 793 David McGrew 794 Cisco Systems 796 Email: mcgrew@cisco.com 797 Panos Kampanakis 798 Cisco Systems 800 Email: pkampana@cisco.com 802 Valery Smyslov 803 ELVIS-PLUS 805 Phone: +7 495 276 0211 806 Email: svan@elvis.ru