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Smyslov 3 Internet-Draft ELVIS-PLUS 4 Intended status: Standards Track February 4, 2020 5 Expires: August 7, 2020 7 Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum 8 Security 9 draft-smyslov-ipsecme-ikev2-qr-alt-01 11 Abstract 13 An IKEv2 extension defined in [I-D.ietf-ipsecme-qr-ikev2] allows 14 IPsec traffic to be protected against someone storing VPN 15 communications today and decrypting it later, when (and if) quantum 16 computers are available. However, this protection doesn't cover an 17 initial IKEv2 SA, which might be unacceptable in some scenarios. 18 This specification defines an alternative way get the same protection 19 against quantum computers, which allows to extend it on the initial 20 IKEv2 SA. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on August 7, 2020. 39 Copyright Notice 41 Copyright (c) 2020 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3 58 3. Alternative Approach Description . . . . . . . . . . . . . . 3 59 4. Computing IKE SA Keys . . . . . . . . . . . . . . . . . . . . 5 60 5. Comparison of the Conventional and the Alternative Approaches 6 61 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 62 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 63 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 64 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 65 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 66 9.2. Informative References . . . . . . . . . . . . . . . . . 7 67 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 69 1. Introduction 71 The Internet Key Exchange Protocol version 2, defined in [RFC7296], 72 is used in the IPsec architecture to perform authenticated key 73 exchange. [I-D.ietf-ipsecme-qr-ikev2] defines an extension of IKEv2 74 for protecting today's VPN traffic against future quantum computers. 75 At the time this extension was being developed, it was a consensus in 76 the IPSECME WG that only IPsec traffic needs to have such a 77 protection. It was believed that no sensitive information is 78 transferred over IKE SA and extending the protection to also cover 79 IKE SA traffic would require serious modifications to core IKEv2 80 protocol, that contradicted to one of the goals to minimize such 81 changes. For the cases when this protection is needed it was 82 suggested to immediately rekey IKE SA once it is created. 84 In some situations it is desirable to have this protection for IKE SA 85 from the very beginning, when an initial IKE SA is created. An 86 example of such situation is Group Key Management protocol using 87 IKEv2, defined in [I-D.yeung-g-ikev2]. In this protocol session keys 88 are transferred from Group Controller / Key Server (GCKS) to Group 89 Members (GM) immediately once an initial IKE SA is created. While it 90 is possible to postpone transfer of the keys until the IKE SA is 91 rekeyed (and [I-D.yeung-g-ikev2] specifies how to do it), the needed 92 sequence of actions introduces an additional delay and adds 93 unnecessary complexity to the protocol. 95 Since [I-D.ietf-ipsecme-qr-ikev2] was written, a new IKE_INTERMEDIATE 96 exchange for IKEv2 was defined in 98 [I-D.ietf-ipsecme-ikev2-intermediate]. While the primary motivation 99 for developing this exchange was to allow multiple key exchanges to 100 be used in IKEv2 (which is defined in 101 [I-D.ietf-ipsecme-ikev2-multiple-ke]), the IKE_INTERMEDIATE exchange 102 itself can be used for other purposes too. 104 This specification makes use of the IKE_INTERMEDIATE exchange to 105 define an alternative approach to [I-D.ietf-ipsecme-qr-ikev2], which 106 allows getting protection against quantum computers for initial IKE 107 SA. 109 2. Terminology and Notation 111 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 112 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 113 "OPTIONAL" in this document are to be interpreted as described in BCP 114 14 [RFC2119] [RFC8174] when, and only when, they appear in all 115 capitals, as shown here. 117 We will use a term Conventional Approach in the content of using PPK 118 to refer to the [I-D.ietf-ipsecme-qr-ikev2] and a term Alternative 119 Approach to refer to this specification. 121 3. Alternative Approach Description 123 IKE initiator who supports the IKE_INTERMEDIATE exchange and wants to 124 use PPK includes both the INTERMEDIATE_EXCHANGE_SUPPORTED and the 125 USE_PPK notifications in the IKE_SA_INIT request. If responder 126 supports the IKE_INTERMEDIATE exchange and is willing to use PPK, it 127 includes both these notifications in the response. 129 Initiator Responder 130 ------------------------------------------------------------------ 131 HDR, SAi1, KEi, Ni, 132 N(INTERMEDIATE_EXCHANGE_SUPPORTED), 133 N(USE_PPK) ---> 134 <--- HDR, SAr1, KEr, Nr, [CERTREQ,] 135 N(INTERMEDIATE_EXCHANGE_SUPPORTED), 136 N(USE_PPK) 138 If the responder returned both these notifications, then the 139 initiator MAY choose to use the IKE_INTERMEDIATE exchange to 140 negotiate PPK identity with the responder. Note, that it is up to 141 the initiator whether to use the alternative or conventional 142 approaches, i.e. whether to to send PPK identity in the 143 IKE_INTERMEDIATE exchange or in the IKE_AUTH exchange, as defined in 144 the [I-D.ietf-ipsecme-qr-ikev2]. 146 If the initiator decides to use alternative approach, it includes one 147 or more PPK_IDENTITY notification containing PPK identities the 148 initiator believes are appropriate for the IKE SA being created, into 149 the IKE_INTERMEDIATE request. If a series of the IKE_INTERMEDIATE 150 exchanges takes place, the PPK_IDENTITY notification(s) MUST be sent 151 in the last one, i.e. in the IKE_INTERMEDIATE exchange immediately 152 preceding the IKE_AUTH exchange. If the last IKE_INTERMEDIATE 153 exchange contains other payloads aimed for some other purpose, then 154 the notification(s) MAY be piggybacked with these payloads. 156 Initiator Responder 157 ------------------------------------------------------------------ 158 HDR, SK { ... N(PPK_IDENTITY, PPK_ID_1) 159 [, N(PPK_IDENTITY, PPK_ID_2)] ... 160 [, N(PPK_IDENTITY, PPK_ID_n)]} ---> 162 Depending on the responder's capabilities and policy the following 163 situations are possible. 165 If the responder doesn't support the alternative approach, it will 166 ignore the received PPK_IDENTITY notification(s) and won't include 167 any additional notifications in the response. If the responder 168 doesn't have any of the PPKs which IDs were sent by the initiator, 169 then it MUST behave as if it doesn't support the alternative 170 approach, i.e. include no additional notifications in the response. 172 Initiator Responder 173 ------------------------------------------------------------------ 174 <--- HDR, SK { ... } 176 In this case the initiator cannot make an initial IKE SA to be a 177 quantum computer resistant, so if this is a requirement for the 178 initiator, then it MUST abort creating IKE SA. Otherwise, the 179 initiator continues with the IKE_AUTH exchange and tries to use PPK 180 as described in [I-D.ietf-ipsecme-qr-ikev2]. 182 If the responder supports this extension and is configured with one 183 of the PPKs which IDs were sent by the initiator, then the responder 184 chooses one of these PPKs and returns back its identity in the 185 PPK_IDENTITY notification. 187 Initiator Responder 188 ------------------------------------------------------------------ 189 <--- HDR, SK { ... N(PPK_IDENTITY, PPK_ID_i)} 191 In this case the IKE_AUTH exchange is performed as defined in 192 [RFC7296], so that neither PPK_IDENTITY nor NO_PPK_AUTH notifications 193 are sent, since it's already known which PPK to use. The keys for 194 the IKE SA are computed using PPK, as described in Section 4. 196 If the responder returns PPK identity that was not suggested by the 197 initiator, then the initiator must treat this as a fatal error and 198 MUST abort the IKE SA establishment. 200 Since the responder selects PPK before it knows identity of the 201 initiator, a situation may occur, when the responder agrees to use 202 some PPK in the IKE_INTERMEDIATE exchange, but later discovers during 203 the IKE_AUTH exchange that this particular PPK is not associated with 204 the initiator's identity in its local policy. Note, that the 205 responder does have this PPK, but it is just not listed among the 206 PPKs for using with this initiator. In this case the responder 207 SHOULD abort negotiation and return back the AUTHENTICATION_FAILED 208 notification to be consistent with its policy. However, if using PPK 209 with this initiator is marked optional in the local policy, then the 210 responder MAY continue creating IKE SA using the negotiated "wrong" 211 PPK. 213 4. Computing IKE SA Keys 215 Once the PPK is negotiated in the last IKE_INTERMEDIATE exchange, the 216 IKE SA keys are recalculated. Note that if the IKE SA keys are also 217 recalculated as the result of the other actions performed in the 218 IKE_INTERMEDIATE exchange (for example, as defined in 219 [I-D.ietf-ipsecme-ikev2-multiple-ke], then applying PPK MUST be done 220 after all of them, so that recalculating IKE SA keys with PPK is the 221 last action before they are used in the IKE_AUTH exchange. 223 The IKE SA keys are computed as follows. A new SKEYSEED' value is 224 computed using the negotiated PPK and the most recently computed SK_d 225 key. Note, that PPK is applied to SK_d exactly how specified in 226 [I-D.ietf-ipsecme-qr-ikev2], and the result is used as SKEYSEED'. 228 SKEYSEED' = prf+ (PPK, SK_d) 230 Then the SKEYSEED' is used to recalculate all SK_* keys as defined in 231 Section 2.14 of [RFC7296]. 233 {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr} 234 = prf+ (SKEYSEED', Ni | Nr | SPIi | SPIr ) 236 In the formula above Ni and Nr are nonces from the IKE_SA_INIT 237 exchange and SPIi, SPIr - SPIs of the IKE SA being creted. 239 The resulting keys are then used in the IKE_AUTH exchange and in the 240 created IKE SA. 242 5. Comparison of the Conventional and the Alternative Approaches 244 This specification isn't intended to be a replacement for 245 [I-D.ietf-ipsecme-qr-ikev2]. Instead, it is supposed to be used in 246 situations where the conventional approach has a significant 247 shortcomings. However, if the partners support both approaches, then 248 the alternative approach MAY also be used in situations where 249 convenient approach suffices. 251 The alternative approach has the following advantages: 253 1. The main advantage of the alternative approach is that it allows 254 an initial IKE SA to be protected against quantum computers. 255 This is important for those IKE extensions which transfer 256 sensitive information, e.g. cryptographic keys, over initial IKE 257 SA. The prominent example of such extensions is 258 [I-D.yeung-g-ikev2]. 260 2. Using the alternative approach allows the initiator to specify 261 several appropriate PPKs and the responder to choose one of them. 262 This feature could simplify PPK rollover. 264 3. With the alternative approach there is no need for the initiator 265 to calculate the content of the AUTH payload twice (with and 266 without PPK) to support a situation when using PPK is optional 267 for both sides. 269 The main disadvantage of the alternative approach is that it requires 270 an additional round trip (the IKE_INTERMEDIATE exchange) to set up 271 IKE SA. However, if the IKE_INTERMEDIATE exchange has to be used for 272 some other purposes in any case, then PPK stuff can be piggybacked 273 with other payloads, thus eliminating this penalty. 275 6. Security Considerations 277 Security considerations of using Post-quantum Preshared Keys in the 278 IKEv2 protocol are discussed in [I-D.ietf-ipsecme-qr-ikev2]. This 279 specification defines an alternative way of exchanging PPK identity 280 information. 282 7. IANA Considerations 284 This specification makes no request to IANA. 286 8. Acknowledgements 288 The author would like to thank Paul Wouters for valuable comments. 290 9. References 292 9.1. Normative References 294 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 295 Requirement Levels", BCP 14, RFC 2119, 296 DOI 10.17487/RFC2119, March 1997, 297 . 299 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 300 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 301 May 2017, . 303 [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. 304 Kivinen, "Internet Key Exchange Protocol Version 2 305 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 306 2014, . 308 [I-D.ietf-ipsecme-qr-ikev2] 309 Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov, 310 "Mixing Preshared Keys in IKEv2 for Post-quantum 311 Security", draft-ietf-ipsecme-qr-ikev2-11 (work in 312 progress), January 2020. 314 [I-D.ietf-ipsecme-ikev2-intermediate] 315 Smyslov, V., "Intermediate Exchange in the IKEv2 316 Protocol", draft-ietf-ipsecme-ikev2-intermediate-03 (work 317 in progress), December 2019. 319 9.2. Informative References 321 [I-D.yeung-g-ikev2] 322 Weis, B. and V. Smyslov, "Group Key Management using 323 IKEv2", draft-yeung-g-ikev2-16 (work in progress), July 324 2019. 326 [I-D.ietf-ipsecme-ikev2-multiple-ke] 327 Tjhai, C., Tomlinson, M., grbartle@cisco.com, g., Fluhrer, 328 S., Geest, D., Garcia-Morchon, O., and V. Smyslov, 329 "Multiple Key Exchanges in IKEv2", draft-ietf-ipsecme- 330 ikev2-multiple-ke-00 (work in progress), January 2020. 332 Author's Address 334 Valery Smyslov 335 ELVIS-PLUS 336 PO Box 81 337 Moscow (Zelenograd) 124460 338 RU 340 Phone: +7 495 276 0211 341 Email: svan@elvis.ru