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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 (October 15, 2017) is 1679 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: 'TBA' is mentioned on line 292, but not defined == Missing Reference: 'CERTREQ' is mentioned on line 237, but not defined == Outdated reference: A later version (-07) exists of draft-hoffman-c2pq-01 -- Obsolete informational reference (is this intentional?): RFC 2409 (Obsoleted by RFC 4306)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: April 18, 2018 Cisco Systems 6 V. Smyslov 7 ELVIS-PLUS 8 October 15, 2017 10 Postquantum Preshared Keys for IKEv2 11 draft-ietf-ipsecme-qr-ikev2-00 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 April 18, 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 . . . . . . . . . . . . . . . . . . . . . . 9 66 5. PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 67 5.1. PPK_ID format . . . . . . . . . . . . . . . . . . . . . . 9 68 5.2. Operational Considerations . . . . . . . . . . . . . . . 10 69 5.2.1. PPK Distribution . . . . . . . . . . . . . . . . . . 10 70 5.2.2. Group PPK . . . . . . . . . . . . . . . . . . . . . . 11 71 5.2.3. PPK-only Authentication . . . . . . . . . . . . . . . 11 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 75 8.1. Normative References . . . . . . . . . . . . . . . . . . 14 76 8.2. Informational References . . . . . . . . . . . . . . . . 14 77 Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 15 78 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 16 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 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 Changes in this draft in each version iterations. 124 draft-ietf-ipsecme-qr-ikev2-00 126 o Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr- 127 ikev2-00 that is a WG item. 129 draft-fluhrer-qr-ikev2-05 131 o Nits and editorial fixes. 133 o Made PPK_ID format and PPK Distributions subsection of the PPK 134 section. Also added an Operational Considerations section. 136 o Added comment about Child SA rekey in the Security Considerations 137 section. 139 o Added NO_PPK_AUTH to solve the cases where a PPK_ID is not 140 configured for a responder. 142 o Various text changes and clarifications. 144 o Expanded Security Considerations section to describe some security 145 concerns and how they should be addressed. 147 draft-fluhrer-qr-ikev2-03 149 o Modified how we stir the PPK into the IKEv2 secret state. 151 o Modified how the use of PPKs is negotiated. 153 draft-fluhrer-qr-ikev2-02 155 o Simplified the protocol by stirring in the preshared key into the 156 child SAs; this avoids the problem of having the responder decide 157 which preshared key to use (as it knows the initiator identity at 158 that point); it does mean that someone with a Quantum Computer can 159 recover the initial IKE negotiation. 161 o Removed positive endorsements of various algorithms. Retained 162 warnings about algorithms known to be weak against a Quantum 163 Computer. 165 draft-fluhrer-qr-ikev2-01 167 o Added explicit guidance as to what IKE and IPsec algorithms are 168 quantum resistant. 170 draft-fluhrer-qr-ikev2-00 172 o We switched from using vendor ID's to transmit the additional data 173 to notifications. 175 o We added a mandatory cookie exchange to allow the server to 176 communicate to the client before the initial exchange. 178 o We added algorithm agility by having the server tell the client 179 what algorithm to use in the cookie exchange. 181 o We have the server specify the PPK Indicator Input, which allows 182 the server to make a trade-off between the efficiency for the 183 search of the clients PPK, and the anonymity of the client. 185 o We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to 186 transform the nonces during the KDF. 188 1.2. Requirements Language 190 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 191 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 192 document are to be interpreted as described in RFC 2119 [RFC2119]. 194 2. Assumptions 196 We assume that each IKE peer has a list of Postquantum Preshared Keys 197 (PPK) along with their identifiers (PPK_ID), and any potential IKE 198 initiator has a selection of which PPK to use with any specific 199 responder. In addition, implementations have a configurable flag 200 that determines whether this postquantum preshared key is mandatory. 201 This PPK is independent of the preshared key (if any) that the IKEv2 202 protocol uses to perform authentication. The PPK specific 203 configuration that is assumed on each peer consists of the following 204 tuple: 206 Peer, PPK, PPK_ID, mandatory_or_not 208 3. Exchanges 210 If the initiator is configured to use a postquantum preshared key 211 with the responder (whether or not the use of the PPK is mandatory), 212 then it will include a notification PPK_SUPPORT in the initial 213 exchange as follows: 215 Initiator Responder 216 ------------------------------------------------------------------ 217 HDR, SAi1, KEi, Ni, N(PPK_SUPPORT) ---> 219 N(PPK_SUPPORT) is a status notification payload with the type [TBA]; 220 it has a protocol ID of 0, no SPI and no notification data associated 221 with it. 223 If the initiator needs to resend this initial message with a cookie 224 (because the responder response included a COOKIE notification), then 225 the resend would include the PPK_SUPPORT notification if the original 226 message did. 228 If the responder does not support this specification or does not have 229 any PPK configured, then it ignores the received notification and 230 continues with the IKEv2 protocol as normal. Otherwise the responder 231 checks if it has a PPK configured, and if it does, then the responder 232 replies with the IKEv2 initial exchange including a PPK_SUPPORT 233 notification in the response: 235 Initiator Responder 236 ------------------------------------------------------------------ 237 <--- HDR, SAr1, KEr, Nr, [CERTREQ], N(PPK_SUPPORT) 239 When the initiator receives this reply, it checks whether the 240 responder included the PPK_SUPPORT notification. If the responder 241 did not and the flag mandatory_or_not indicates that using PPKs is 242 mandatory for communication with this responder, then the initiator 243 MUST abort the exchange. This situation may happen in case of 244 misconfiguration, when the initiator believes it has a mandatory to 245 use PPK for the responder, while the responder either doesn't support 246 PPKs at all or doesn't have any PPK configured for the initiator. 247 See Section 6 for discussion of the possible impacts of this 248 situation. 250 If the responder did not include the PPK_SUPPORT notification and 251 using PPKs for this responder is optional, then the initiator 252 continues with the IKEv2 protocol as normal, without using PPKs. 254 If the responder did include the PPK_SUPPORT notification, then the 255 initiator selects a PPK, along with its identifier PPK_ID. Then, it 256 computes this modification of the standard IKEv2 key derivation: 258 SKEYSEED = prf(Ni | Nr, g^ir) 259 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' ) 260 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr } 261 SK_d = prf(PPK, SK_d') 262 SK_pi = prf(PPK, SK_pi') 263 SK_pr = prf(PPK, SK_pr') 265 That is, we use the standard IKEv2 key derivation process except that 266 the three subkeys SK_d, SK_pi, SK_pr are run through the prf again, 267 this time using the PPK as the key. 269 The initiator then sends the initial encrypted message, including the 270 PPK_ID value as follows: 272 Initiator Responder 273 ------------------------------------------------------------------ 274 HDR, SK {IDi, [CERT,] [CERTREQ,] 275 [IDr,] AUTH, SAi2, 276 TSi, TSr, N(PPK_IDENTITY)(PPK_ID), [N(NO_PPK_AUTH)]} ---> 278 PPK_IDENTITY is a status notification with the type [TBA]; it has a 279 protocol ID of 0, no SPI and a notification data that consists of the 280 identifier PPK_ID. 282 A situation may happen when the responder has some PPKs, but doesn't 283 have a PPK with the PPK_ID received from the initiator. In this case 284 the responder cannot continue with PPK (in particular, it cannot 285 authenticate the initiator), but it could be able to continue with 286 normal IKEv2 protocol if the initiator provided its authentication 287 data computed as in normal IKEv2, without using PPKs. For this 288 purpose, if using PPKs for communication with this responder is 289 optional for the initiator, then the initiator MAY include a 290 notification NO_PPK_AUTH in the above message. 292 NO_PPK_AUTH is a status notification with the type [TBA]; it has a 293 protocol ID of 0 and no SPI. A notification data consists of the 294 initiator's authentication data computed using SK_pi' (i.e. the data 295 that computed without using PPKs and would normally be placed in the 296 AUTH payload). Authentication Method for computing the 297 authentication data MUST be the same as indicated in the AUTH payload 298 and is not included in the notification. Note that if the initiator 299 decides to include NO_PPK_AUTH notification, then it means that the 300 initiator needs to perform authentication data computation twice that 301 may consume substantial computation power (e.g. if digital signatures 302 are involved). 304 When the responder receives this encrypted exchange, it first 305 computes the values: 307 SKEYSEED = prf(Ni | Nr, g^ir) 308 {SK_d' | SK_ai | SK_ar | SK_ei | SK_er | SK_pi' | SK_pr' } 309 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) 311 It then uses the SK_ei/SK_ai values to decrypt/check the message and 312 then scans through the payloads for the PPK_ID attached to the 313 PPK_IDENTITY notification. If no PPK_IDENTITY notification is found 314 and the peers successfully exchanged PPK_SUPPORT notifications in the 315 initial exchange, then the responder MUST send back 316 AUTHENTICATION_FAILED notification and then fail the negotiation. 318 If the PPK_IDENTITY notification contains PPK_ID that is not known to 319 the responder or is not configured for use for the identity from IDi 320 payload, then the responder checks whether using PPKs for this 321 initiator is mandatory and whether the initiator included NO_PPK_AUTH 322 notification in the message. If using PPKs is mandatory or no 323 NO_PPK_AUTH notification found, then then the responder MUST send 324 back AUTHENTICATION_FAILED notification and then fail the 325 negotiation. Otherwise (when PPK is optional and the initiator 326 included NO_PPK_AUTH notification) the responder MAY continue regular 327 IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH 328 notification as the authentication data (which usually resides in the 329 AUTH payload), for the purpose of the initiator authentication. 331 Note, that Authentication Method is still indicated in the AUTH 332 payload. 334 This table summarizes the above logic by the responder: 336 Received Received Have PPK 337 PPK_SUPPORT NO_PPK_AUTH PPK Mandatory Action 338 ------------------------------------------------------------------ 339 No * No * Standard IKEv2 protocol 340 No * Yes No Standard IKEv2 protocol 341 No * Yes Yes Abort negotiation 342 Yes No No * Abort negotiation 343 Yes Yes No Yes Abort negotiation 344 Yes Yes No No Standard IKEv2 protocol 345 Yes * Yes * Use PPK 347 If PPK is in use, then the responder extracts corresponding PPK and 348 computes the following values: 350 SK_d = prf(PPK, SK_d') 351 SK_pi = prf(PPK, SK_pi') 352 SK_pr = prf(PPK, SK_pr') 354 The responder then continues with the exchange (validating the AUTH 355 payload that the initiator included) as usual and sends back a 356 response, which includes the PPK_IDENTITY notification with no data 357 to indicate that the PPK is used in the exchange: 359 Initiator Responder 360 ------------------------------------------------------------------ 361 <-- HDR, SK {IDr, [CERT,] 362 AUTH, SAr2, 363 TSi, TSr, N(PPK_IDENTITY)} 365 When the initiator receives the response, then it checks for the 366 presence of the PPK_IDENTITY notification. If it receives one, it 367 marks the SA as using the configured PPK to generate SK_d, SK_pi, 368 SK_pr (as shown above); if it does not receive one, it MUST either 369 fail the IKE SA negotiation sending the AUTHENTICATION_FAILED 370 notification in the Informational exchange (if the PPK was configured 371 as mandatory), or continue without using the PPK (if the PPK was not 372 configured as mandatory and the initiator included the NO_PPK_AUTH 373 notification in the request). 375 4. Upgrade procedure 377 This algorithm was designed so that someone can introduce PPKs into 378 an existing IKE network without causing network disruption. 380 In the initial phase of the network upgrade, the network 381 administrator would visit each IKE node, and configure: 383 o The set of PPKs (and corresponding PPK_IDs) that this node would 384 need to know. 386 o For each peer that this node would initiate to, which PPK will be 387 used. 389 o That the use of PPK is currently not mandatory. 391 With this configuration, the node will continue to operate with nodes 392 that have not yet been upgraded. This is due to the PPK_SUPPORT 393 notify and the NO_PPK_AUTH notify; if the initiator has not been 394 upgraded, it will not send the PPK_SUPPORT notify (and so the 395 responder will know that we will not use a PPK). If the responder 396 has not been upgraded, it will not send the PPK_SUPPORT notify (and 397 so the initiator will know to not use a PPK). If both peers have 398 been upgraded, but the responder isn't yet configured with the PPK 399 for the initiator, then the responder could do standard IKEv2 400 protocol if the initiator sent NO_PPK_AUTH notification. If the 401 responder has not been upgraded and properly configured, they will 402 both realize it, and in that case, the link will be quantum secure. 404 As an optional second step, after all nodes have been upgraded, then 405 the administrator may then go back through the nodes, and mark the 406 use of PPK as mandatory. This will not affect the strength against a 407 passive attacker; it would mean that an attacker with a Quantum 408 Computer (which is sufficiently fast to be able to break the (EC)DH 409 in real time would not be able to perform a downgrade attack). 411 5. PPK 413 5.1. PPK_ID format 415 This standard requires that both the initiator and the responder have 416 a secret PPK value, with the responder selecting the PPK based on the 417 PPK_ID that the initiator sends. In this standard, both the 418 initiator and the responder are configured with fixed PPK and PPK_ID 419 values, and do the look up based on PPK_ID value. It is anticipated 420 that later standards will extend this technique to allow dynamically 421 changing PPK values. To facilitate such an extension, we specify 422 that the PPK_ID the initiator sends will have its first octet be the 423 PPK_ID Type value. This document defines two values for PPK_ID Type: 425 o PPK_ID_OPAQUE (1) - for this type the format of the PPK_ID (and 426 the PPK itself) is not specified by this document; it is assumed 427 to be mutually intelligible by both by initiator and the 428 responder. This PPK_ID type is intended for those implementations 429 that choose not to disclose the type of PPK to active attackers. 431 o PPK_ID_FIXED (2) - in this case the format of the PPK_ID and the 432 PPK are fixed octet strings; the remaining bytes of the PPK_ID are 433 a configured value. We assume that there is a fixed mapping 434 between PPK_ID and PPK, which is configured locally to both the 435 initiator and the responder. The responder can use to do a look 436 up the passed PPK_ID value to determine the corresponding PPK 437 value. Not all implementations are able to configure arbitrary 438 octet strings; to improve the potential interoperability, it is 439 recommended that, in the PPK_ID_FIXED case, both the PPK and the 440 PPK_ID strings be limited to the base64 character set, namely the 441 64 characters 0-9, A-Z, a-z, + and /. 443 The PPK_ID type value 0 is reserved; values 3-127 are reserved for 444 IANA; values 128-255 are for private use among mutually consenting 445 parties. 447 5.2. Operational Considerations 449 The need to maintain several independent sets of security credentials 450 can significantly complicate security administrators job, and can 451 potentially slow down widespread adoption of this solution. It is 452 anticipated, that administrators will try to simplify their job by 453 decreasing the number of credentials they need to maintain. This 454 section describes some of the considerations for PPK management. 456 5.2.1. PPK Distribution 458 PPK_IDs of the type PPK_ID_FIXED (and the corresponding PPKs) are 459 assumed to be configured within the IKE device in an out-of-band 460 fashion. While the method of distribution is a local matter and out 461 of scope of this document or IKEv2, [RFC6030] describes a format for 462 symmetric key exchange. That format could be reused with the Key Id 463 field being the PPK_ID (without the PPK_ID Type octet for a 464 PPK_ID_FIXED), the PPK being the secret, and the algorithm 465 ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as PIN. 467 5.2.2. Group PPK 469 This document doesn't explicitly require that PPK is unique for each 470 pair of peers. If it is the case, then this solution provides full 471 peer authentication, but it also means that each host must have that 472 many independent PPKs, how many peers it is going to communicate 473 with. As the number of hosts grows this will scale badly. 475 Even though it is NOT RECOMMENDED, it is possible to use a single PPK 476 for a group of users. Since each peer uses classical public key 477 cryptography in addition to PPK for key exchange and authentication, 478 members of the group can neither impersonate each other nor read 479 other's traffic, unless they use Quantum Computers to break public 480 key operations. 482 Although it's probably safe to use group PPK in short term, the fact, 483 that the PPK is known to a (potentially large) group of users makes 484 it more susceptible to theft. If an attacker equipped with a Quantum 485 Computer got access to a group PPK, then all the communications 486 inside the group are revealed. 488 5.2.3. PPK-only Authentication 490 If Quantum Computers become a reality, classical public key 491 cryptography will provide little security, so administrators may find 492 it attractive not to use it at all for authentication. This will 493 reduce the number of credentials they need to maintain to PPKs only. 494 Combining group PPK and PPK-only authentication is NOT RECOMMENDED, 495 since in this case any member of the group can impersonate any other 496 member even without help of Quantum Computers. 498 PPK-only authentication can be achieved in IKEv2 if NULL 499 Authentication method [RFC7619] is employed. Without PPK the NULL 500 Authentication method provides no authentication of the peers, 501 however since a PPK is stirred into the SK_pi and the SK_pr, the 502 peers become authenticated if a PPK is in use. Using PPKs MUST be 503 mandatory for the peers if they advertise support for PPK in initial 504 exchange and use NULL Authentication. Addtionally, since the peers 505 are authenticated via PPK, the ID Type in the IDi/IDr payloads SHOULD 506 NOT be ID_NULL, despite using NULL Authentication method. 508 6. Security Considerations 510 Quantum computers are able to perform Grover's algorithm; that 511 effectively halves the size of a symmetric key. Because of this, the 512 user SHOULD ensure that the postquantum preshared key used has at 513 least 256 bits of entropy, in order to provide a 128-bit security 514 level. 516 With this protocol, the computed SK_d is a function of the PPK, and 517 assuming that the PPK has sufficient entropy (for example, at least 518 2^256 possible values), then even if an attacker was able to recover 519 the rest of the inputs to the prf function, it would be infeasible to 520 use Grover's algorithm with a Quantum Computer to recover the SK_d 521 value. Similarly, every child SA key is a function of SK_d, hence 522 all the keys for all the child SAs are also quantum resistant 523 (assuming that the PPK was high entropy and secret, and that all the 524 subkeys are sufficiently long). 526 Although this protocol preserves all the security properties of IKEv2 527 against adversaries with conventional computers, it allows an 528 adversary with a Quantum Computer to decrypt all traffic encrypted 529 with the initial IKE SA. In particular, it allows the adversary to 530 recover the identities of both sides. If there is IKE traffic other 531 than the identities that need to be protected against such an 532 adversary, implementations MAY rekey the initial IKE SA immediately 533 after negotiating it to generate a new SKEYSEED with from the 534 postquantum SK_d. This would reduce the amount of data available to 535 an attacker with a Quantum Computer. 537 Alternatively, an initial IKE SA (which is used to exchange 538 identities) can take place, perhaps by using the protocol documented 539 in [RFC6023]. After the childless IKE SA is created, implementations 540 would immediately create a new IKE SA (which is used to exchange 541 everything else) by using a rekey mechanism for IKE SAs. Because the 542 rekeyed IKE SA keys are a function of SK_d, which is a function of 543 the PPK (among other things), traffic protected by that IKE SA is 544 secure against Quantum capable adversaries. 546 If some sensitive information (like keys) is to be transferred over 547 IKE SA, then implementations MUST rekey the initial IKE SA before 548 sending this information to get protection against Quantum Computers. 550 In addition, the policy SHOULD be set to negotiate only quantum- 551 resistant symmetric algorithms; while this RFC doesn't claim to give 552 advise as to what algorithms are secure (as that may change based on 553 future cryptographical results), below is a list of defined IKEv2 and 554 IPsec algorithms that should NOT be used, as they are known not to be 555 quantum resistant 557 o Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with 558 key size less than 256 bits. 560 o Any ESP Transform with key size less than 256 bits. 562 o PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined 563 to be able to use an arbitrary key size, they convert it into a 564 128-bit key internally. 566 Section 3 requires the initiator to abort the initial exchange if 567 using PPKs is mandatory for it, but the responder didn't include the 568 PPK_SUPPORT notification in the response. In this situation when the 569 initiator aborts negotiation it leaves half-open IKE SA on the 570 responder (because the initial exchange completes successfully from 571 responder's point of view). This half-open SA will eventually expire 572 and be deleted, but if the initiator continues its attempts to create 573 IKE SA with a high enough rate, then the responder may consider it as 574 a Denial-of-Service attack and take some measures (see [RFC8019] for 575 more detail). It is RECOMMENDED that implementations in this 576 situation cache the negative result of negotiation for some time and 577 don't make attempts to create it again for some time, because this is 578 a result of misconfiguration and probably some re-configuration of 579 the peers is needed. 581 If using PPKs is optional for both peers and they authenticate 582 themselves using digital signatures, then an attacker in between, 583 equipped with a Quantum Computer capable of breaking public key 584 operations in real time, is able to mount downgrade attack by 585 removing PPK_SUPPORT notification from the initial exchange and 586 forging digital signatures in the subsequent exchange. If using PPKs 587 is mandatory for at least one of the peers or PSK is used for 588 authentication, then the attack will be detected and the SA won't be 589 created. 591 If using PPKs is mandatory for the initiator, then an attacker 592 capable to eavesdrop and to inject packets into the network can 593 prevent creating IKE SA by mounting the following attack. The 594 attacker intercepts the the initial request containing the 595 PPK_SUPPORT notification and injects the forget response containing 596 no PPK_SUPPORT. If the attacker manages to inject this packet before 597 the responder sends a genuine response, then the initiator would 598 abort the exchange. To thwart this kind of attack it is RECOMMENDED, 599 that if using PPKs is mandatory for the initiator and the received 600 response doesn't contain the PPK_SUPPORT notification, then the 601 initiator doesn't abort exchange immediately, but instead waits some 602 time for more responses (possibly retransmitting the request). If 603 all the received responses contain no PPK_SUPPORT, then the exchange 604 is aborted. 606 7. IANA Considerations 608 This document defines three new Notify Message Types in the "Notify 609 Message Types - Status Types" registry: 611 PPK_SUPPORT 612 PPK_IDENTITY 613 NO_PPK_AUTH 615 This document also creates a new IANA registry for the PPK_ID types. 616 The initial values of this registry are: 618 PPK_ID Type Value 619 ----------- ----- 620 Reserved 0 621 PPK_ID_OPAQUE 1 622 PPK_ID_FIXED 2 623 Unassigned 3-127 624 Reserved for private use 128-255 626 Changes and additions to this registry are by Expert Review 627 [RFC5226]. 629 8. References 631 8.1. Normative References 633 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 634 Requirement Levels", BCP 14, RFC 2119, 635 DOI 10.17487/RFC2119, March 1997, 636 . 638 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 639 Kivinen, "Internet Key Exchange Protocol Version 2 640 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 641 2014, . 643 8.2. Informational References 645 [I-D.hoffman-c2pq] 646 Hoffman, P., "The Transition from Classical to Post- 647 Quantum Cryptography", draft-hoffman-c2pq-01 (work in 648 progress), July 2017. 650 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange 651 (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998, 652 . 654 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 655 IANA Considerations Section in RFCs", RFC 5226, 656 DOI 10.17487/RFC5226, May 2008, 657 . 659 [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A 660 Childless Initiation of the Internet Key Exchange Version 661 2 (IKEv2) Security Association (SA)", RFC 6023, 662 DOI 10.17487/RFC6023, October 2010, 663 . 665 [RFC6030] Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric 666 Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030, 667 October 2010, . 669 [RFC7619] Smyslov, V. and P. Wouters, "The NULL Authentication 670 Method in the Internet Key Exchange Protocol Version 2 671 (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015, 672 . 674 [RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange 675 Protocol Version 2 (IKEv2) Implementations from 676 Distributed Denial-of-Service Attacks", RFC 8019, 677 DOI 10.17487/RFC8019, November 2016, 678 . 680 Appendix A. Discussion and Rationale 682 The idea behind this document is that while a Quantum Computer can 683 easily reconstruct the shared secret of an (EC)DH exchange, they 684 cannot as easily recover a secret from a symmetric exchange. This 685 makes the SK_d, and hence the IPsec KEYMAT and any child SA's 686 SKEYSEED, depend on both the symmetric PPK, and also the Diffie- 687 Hellman exchange. If we assume that the attacker knows everything 688 except the PPK during the key exchange, and there are 2^n plausible 689 PPKs, then a Quantum Computer (using Grover's algorithm) would take 690 O(2^(n/2)) time to recover the PPK. So, even if the (EC)DH can be 691 trivially solved, the attacker still can't recover any key material 692 (except for the SK_ei, SK_er, SK_ai, SK_ar values for the initial IKE 693 exchange) unless they can find the PPK, which is too difficult if the 694 PPK has enough entropy (for example, 256 bits). Note that we do 695 allow an attacker with a Quantum Computer to rederive the keying 696 material for the initial IKE SA; this was a compromise to allow the 697 responder to select the correct PPK quickly. 699 Another goal of this protocol is to minimize the number of changes 700 within the IKEv2 protocol, and in particular, within the cryptography 701 of IKEv2. By limiting our changes to notifications, and translating 702 the nonces, it is hoped that this would be implementable, even on 703 systems that perform much of the IKEv2 processing is in hardware. 705 A third goal was to be friendly to incremental deployment in 706 operational networks, for which we might not want to have a global 707 shared key or quantum resistant IKEv2 is rolled out incrementally. 708 This is why we specifically try to allow the PPK to be dependent on 709 the peer, and why we allow the PPK to be configured as optional. 711 A fourth goal was to avoid violating any of the security goals of 712 IKEv2. 714 Appendix B. Acknowledgements 716 We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett 717 and the rest of the ipsecme Working Group for their feedback and 718 suggestions for the scheme. 720 Authors' Addresses 722 Scott Fluhrer 723 Cisco Systems 725 Email: sfluhrer@cisco.com 727 David McGrew 728 Cisco Systems 730 Email: mcgrew@cisco.com 732 Panos Kampanakis 733 Cisco Systems 735 Email: pkampana@cisco.com 737 Valery Smyslov 738 ELVIS-PLUS 740 Phone: +7 495 276 0211 741 Email: svan@elvis.ru