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Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 4282 (Obsoleted by RFC 7542) -- Obsolete informational reference (is this intentional?): RFC 4306 (Obsoleted by RFC 5996) -- Obsolete informational reference (is this intentional?): RFC 5226 (Obsoleted by RFC 8126) == Outdated reference: A later version (-04) exists of draft-arkko-eap-aka-pfs-03 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Arkko 3 Internet-Draft V. Lehtovirta 4 Obsoletes: 5448 (if approved) V. Torvinen 5 Updates: 4187 (if approved) Ericsson 6 Intended status: Informational P. Eronen 7 Expires: July 21, 2019 Independent 8 January 17, 2019 10 Improved Extensible Authentication Protocol Method for 3rd Generation 11 Authentication and Key Agreement (EAP-AKA') 12 draft-ietf-emu-rfc5448bis-04 14 Abstract 16 The 3rd Generation Authentication and Key Agreement (AKA) is the 17 primary authentication mechanism for devices wishing to access mobile 18 networks. RFC 4187 (EAP-AKA) made the use of this mechanism possible 19 within the Extensible Authentication Protocol (EAP) framework. RFC 20 5448 (EAP-AKA') was an improved version of EAP-AKA. 22 This memo is an update of the specification for EAP-AKA'. This 23 version obsoletes RFC 5448. 25 EAP-AKA' differs from EAP-AKA by providing a key derivation function 26 that binds the keys derived within the method to the name of the 27 access network. The key derivation function has been defined in the 28 3rd Generation Partnership Project (3GPP). EAP-AKA' allows its use 29 in EAP in an interoperable manner. EAP-AKA' is also an algorithm 30 update, as it employs SHA-256 instead of SHA-1 as in EAP-AKA. 32 This version of EAP-AKA' specification specifies the protocol 33 behaviour for 5G deployments as well. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at http://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on July 21, 2019. 51 Copyright Notice 53 Copyright (c) 2019 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (http://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 70 3. EAP-AKA' . . . . . . . . . . . . . . . . . . . . . . . . . . 5 71 3.1. AT_KDF_INPUT . . . . . . . . . . . . . . . . . . . . . . 8 72 3.2. AT_KDF . . . . . . . . . . . . . . . . . . . . . . . . . 11 73 3.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . 13 74 3.4. Hash Functions . . . . . . . . . . . . . . . . . . . . . 15 75 3.4.1. PRF' . . . . . . . . . . . . . . . . . . . . . . . . 15 76 3.4.2. AT_MAC . . . . . . . . . . . . . . . . . . . . . . . 15 77 3.4.3. AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . 15 78 4. Bidding Down Prevention for EAP-AKA . . . . . . . . . . . . . 16 79 5. Peer Identities . . . . . . . . . . . . . . . . . . . . . . . 17 80 5.1. Username Types in EAP-AKA' Identities . . . . . . . . . . 18 81 5.2. Generating Pseudonyms and Fast Re-Authentication 82 Identities . . . . . . . . . . . . . . . . . . . . . . . 18 83 5.3. Identifier Usage in 5G . . . . . . . . . . . . . . . . . 19 84 5.3.1. Key Derivation . . . . . . . . . . . . . . . . . . . 20 85 5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY 86 Attribute . . . . . . . . . . . . . . . . . . . . . . 21 87 6. Exported Parameters . . . . . . . . . . . . . . . . . . . . . 23 88 7. Security Considerations . . . . . . . . . . . . . . . . . . . 24 89 7.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 26 90 7.2. Discovered Vulnerabilities . . . . . . . . . . . . . . . 28 91 7.3. Pervasive Monitoring . . . . . . . . . . . . . . . . . . 30 92 7.4. Security Properties of Binding Network Names . . . . . . 30 93 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 94 8.1. Type Value . . . . . . . . . . . . . . . . . . . . . . . 32 95 8.2. Attribute Type Values . . . . . . . . . . . . . . . . . . 32 96 8.3. Key Derivation Function Namespace . . . . . . . . . . . . 32 98 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 99 9.1. Normative References . . . . . . . . . . . . . . . . . . 32 100 9.2. Informative References . . . . . . . . . . . . . . . . . 34 101 Appendix A. Changes from RFC 5448 . . . . . . . . . . . . . . . 37 102 Appendix B. Changes from RFC 4187 to RFC 5448 . . . . . . . . . 38 103 Appendix C. Changes from Previous Version of This Draft . . . . 38 104 Appendix D. Importance of Explicit Negotiation . . . . . . . . . 39 105 Appendix E. Test Vectors . . . . . . . . . . . . . . . . . . . . 40 106 Appendix F. Contributors . . . . . . . . . . . . . . . . . . . . 44 107 Appendix G. Acknowledgments . . . . . . . . . . . . . . . . . . 45 108 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45 110 1. Introduction 112 The 3rd Generation Authentication and Key Agreement (AKA) is the 113 primary authentication mechanism for devices wishing to access mobile 114 networks. [RFC4187] (EAP-AKA) made the use of this mechanism 115 possible within the Extensible Authentication Protocol (EAP) 116 framework [RFC3748]. 118 [RFC5448] (EAP-AKA') was an improved version of EAP-AKA. This memo 119 is an update of the specification for EAP-AKA'. This version 120 obsoletes RFC 5448. 122 EAP-AKA' is commonly implemented in smart phones and network 123 equipment. It can be used for authentication to gain network access 124 via Wireless LAN networks and, with 5G, also directly to mobile 125 networks. 127 EAP-AKA' differs from EAP-AKA by providing a different key derivation 128 function. This function binds the keys derived within the method to 129 the name of the access network. This limits the effects of 130 compromised access network nodes and keys. EAP-AKA' is also an 131 algorithm update for the used hash functions. 133 The EAP-AKA' method employs the derived keys CK' and IK' from the 134 3GPP specification [TS-3GPP.33.402] and updates the used hash 135 function to SHA-256 [FIPS.180-4]. Otherwise, EAP-AKA' is equivalent 136 to EAP-AKA. Given that a different EAP method type value is used for 137 EAP-AKA and EAP-AKA', a mutually supported method may be negotiated 138 using the standard mechanisms in EAP [RFC3748]. 140 Note that any change of the key derivation must be unambiguous to 141 both sides in the protocol. That is, it must not be possible to 142 accidentally connect old equipment to new equipment and get the 143 key derivation wrong or attempt to use wrong keys without getting 144 a proper error message. See Appendix D for further information. 146 Note also that choices in authentication protocols should be 147 secure against bidding down attacks that attempt to force the 148 participants to use the least secure function. See Section 4 for 149 further information. 151 The changes from RFC 5448 to this specification are as follows: 153 o Update the reference on how the Network Name field is constructed 154 in the protocol. The update ensures that EAP-AKA' is compatible 155 with 5G deployments. RFC 5448 referred to the Release 8 version 156 of [TS-3GPP.24.302] and this update points to the first 5G 157 version, Release 15. 159 o Specify how EAP and EAP-AKA' use identifiers in 5G. Additional 160 identifiers are introduced in 5G, and for interoperability, it is 161 necessary that the right identifiers are used as inputs in the key 162 derivation. In addition, for identity privacy it is important 163 that when privacy-friendly identifiers in 5G are used, no 164 trackable, permanent identifiers are passed in EAP-AKA' either. 166 o Specify session identifiers and other exported parameters, as 167 those were not specified in [RFC5448] despite requirements set 168 forward in [RFC5247] to do so. Also, while [RFC5247] specified 169 session identifiers for EAP-AKA, it only did so for the full 170 authentication case, not for the case of fast re-authentication. 172 o Update the requirements on generating pseudonym usernames and fast 173 re-authentication identities to ensure identity privacy. 175 o Describe what has been learned about any vulnerabilities in AKA or 176 EAP-AKA'. 178 o Describe the privacy and pervasive monitoring considerations 179 related to EAP-AKA'. 181 Some of the updates are small. For instance, for the first update, 182 the reference update does not change the 3GPP specification number, 183 only the version. But this reference is crucial in correct 184 calculation of the keys resulting from running the EAP-AKA' method, 185 so an update of the RFC with the newest version pointer may be 186 warranted. 188 Note: This specification refers only to the 5G specifications. 189 Any further update that affects, for instance, key derivation is 190 something that EAP-AKA' implementations should take into account. 191 Upon such updates there will be a need to both update the 192 specification and the implementations. 194 It is an explicit non-goal of this draft to include any other 195 technical modifications, addition of new features or other changes. 196 The EAP-AKA' base protocol is stable and needs to stay that way. If 197 there are any extensions or variants, those need to be proposed as 198 standalone extensions or even as different authentication methods. 200 The rest of this specification is structured as follows. Section 3 201 defines the EAP-AKA' method. Section 4 adds support to EAP-AKA to 202 prevent bidding down attacks from EAP-AKA'. Section 5 specifies 203 requirements regarding the use of peer identities, including how how 204 EAP-AKA' identifiers are used in 5G context. Section 6 specifies 205 what parameters EAP-AKA' exports out of the method. Section 7 206 explains the security differences between EAP-AKA and EAP-AKA'. 207 Section 8 describes the IANA considerations and Appendix A and 208 Appendix B explains what updates to RFC 5448 EAP-AKA' and RFC 4187 209 EAP-AKA have been made in this specification. Appendix D explains 210 some of the design rationale for creating EAP-AKA' Finally, 211 Appendix E provides test vectors. 213 Editor's Note: The publication of this RFC depends on its 214 normative references [TS-3GPP.24.302] and [TS-3GPP.33.501] 215 reaching a stable status for Release 15, as indicated by 3GPP. 216 This is expected to happen shortly. The RFC Editor should check 217 with the 3GPP liaisons that this has happened. RFC Editor: Please 218 delete this note upon publication of this specification as an RFC. 220 2. Requirements Language 222 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 223 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 224 "OPTIONAL" in this document are to be interpreted as described in BCP 225 14 [RFC2119] [RFC8174] when, and only when, they appear in all 226 capitals, as shown here. 228 3. EAP-AKA' 230 EAP-AKA' is an EAP method that follows the EAP-AKA specification 231 [RFC4187] in all respects except the following: 233 o It uses the Type code 50, not 23 (which is used by EAP-AKA). 235 o It carries the AT_KDF_INPUT attribute, as defined in Section 3.1, 236 to ensure that both the peer and server know the name of the 237 access network. 239 o It supports key derivation function negotiation via the AT_KDF 240 attribute (Section 3.2) to allow for future extensions. 242 o It calculates keys as defined in Section 3.3, not as defined in 243 EAP-AKA. 245 o It employs SHA-256, not SHA-1 [FIPS.180-4] (Section 3.4). 247 Figure 1 shows an example of the authentication process. Each 248 message AKA'-Challenge and so on represents the corresponding message 249 from EAP-AKA, but with EAP-AKA' Type code. The definition of these 250 messages, along with the definition of attributes AT_RAND, AT_AUTN, 251 AT_MAC, and AT_RES can be found in [RFC4187]. 253 Peer Server 254 | EAP-Request/Identity | 255 |<-------------------------------------------------------| 256 | | 257 | EAP-Response/Identity | 258 | (Includes user's Network Access Identifier, NAI) | 259 |------------------------------------------------------->| 260 | +--------------------------------------------------+ 261 | | Server determines the network name and ensures | 262 | | that the given access network is authorized to | 263 | | use the claimed name. The server then runs the | 264 | | AKA' algorithms generating RAND and AUTN, and | 265 | | derives session keys from CK' and IK'. RAND and | 266 | | AUTN are sent as AT_RAND and AT_AUTN attributes, | 267 | | whereas the network name is transported in the | 268 | | AT_KDF_INPUT attribute. AT_KDF signals the used | 269 | | key derivation function. The session keys are | 270 | | used in creating the AT_MAC attribute. | 271 | +--------------------------------------------------+ 272 | EAP-Request/AKA'-Challenge | 273 | (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC)| 274 |<-------------------------------------------------------| 275 +------------------------------------------------------+ | 276 | The peer determines what the network name should be, | | 277 | based on, e.g., what access technology it is using. | | 278 | The peer also retrieves the network name sent by | | 279 | the network from the AT_KDF_INPUT attribute. The | | 280 | two names are compared for discrepancies, and if | | 281 | necessary, the authentication is aborted. Otherwise,| | 282 | the network name from AT_KDF_INPUT attribute is | | 283 | used in running the AKA' algorithms, verifying AUTN | | 284 | from AT_AUTN and MAC from AT_MAC attributes. The | | 285 | peer then generates RES. The peer also derives | | 286 | session keys from CK'/IK'. The AT_RES and AT_MAC | | 287 | attributes are constructed. | | 288 +------------------------------------------------------+ | 289 | EAP-Response/AKA'-Challenge | 290 | (AT_RES, AT_MAC) | 291 |------------------------------------------------------->| 292 | +--------------------------------------------------+ 293 | | Server checks the RES and MAC values received | 294 | | in AT_RES and AT_MAC, respectively. Success | 295 | | requires both to be found correct. | 296 | +--------------------------------------------------+ 297 | EAP-Success | 298 |<-------------------------------------------------------| 300 Figure 1: EAP-AKA' Authentication Process 302 EAP-AKA' can operate on the same credentials as EAP-AKA and employ 303 the same identities. However, EAP-AKA' employs different leading 304 characters than EAP-AKA for the conventions given in Section 4.1.1 of 305 [RFC4187] for International Mobile Subscriber Identifier (IMSI) based 306 usernames. EAP-AKA' MUST use the leading character "6" (ASCII 36 307 hexadecimal) instead of "0" for IMSI-based permanent usernames. All 308 other usage and processing of the leading characters, usernames, and 309 identities is as defined by EAP-AKA [RFC4187]. For instance, the 310 pseudonym and fast re-authentication usernames need to be constructed 311 so that the server can recognize them. As an example, a pseudonym 312 could begin with a leading "7" character (ASCII 37 hexadecimal) and a 313 fast re-authentication username could begin with "8" (ASCII 38 314 hexadecimal). Note that a server that implements only EAP-AKA may 315 not recognize these leading characters. According to Section 4.1.4 316 of [RFC4187], such a server will re-request the identity via the EAP- 317 Request/AKA-Identity message, making obvious to the peer that EAP-AKA 318 and associated identity are expected. 320 3.1. AT_KDF_INPUT 322 The format of the AT_KDF_INPUT attribute is shown below. 324 0 1 2 3 325 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 326 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 327 | AT_KDF_INPUT | Length | Actual Network Name Length | 328 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 329 | | 330 . Network Name . 331 . . 332 | | 333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 335 The fields are as follows: 337 AT_KDF_INPUT 339 This is set to 23. 341 Length 343 The length of the attribute, calculated as defined in [RFC4187], 344 Section 8.1. 346 Actual Network Name Length 347 This is a 2 byte actual length field, needed due to the 348 requirement that the previous field is expressed in multiples of 4 349 bytes per the usual EAP-AKA rules. The Actual Network Name Length 350 field provides the length of the network name in bytes. 352 Network Name 354 This field contains the network name of the access network for 355 which the authentication is being performed. The name does not 356 include any terminating null characters. Because the length of 357 the entire attribute must be a multiple of 4 bytes, the sender 358 pads the name with 1, 2, or 3 bytes of all zero bits when 359 necessary. 361 Only the server sends the AT_KDF_INPUT attribute. The value is sent 362 as specified in [TS-3GPP.24.302] for non-3GPP access networks, and as 363 specified in [TS-3GPP.33.501] for 5G access networks. Per 364 [TS-3GPP.33.402], the server always verifies the authorization of a 365 given access network to use a particular name before sending it to 366 the peer over EAP-AKA'. The value of the AT_KDF_INPUT attribute from 367 the server MUST be non-empty. If it is empty, the peer behaves as if 368 AUTN had been incorrect and authentication fails. See Section 3 and 369 Figure 3 of [RFC4187] for an overview of how authentication failures 370 are handled. 372 Note: Currently, [TS-3GPP.24.302] or [TS-3GPP.33.501] specify 373 separate values. The former specifies what is called "Access 374 Network ID" and the latter specifies what is called "Serving 375 Network Name". However, from an EAP-AKA' perspective both occupy 376 the same field, and need to be distinguishable from each other. 377 Currently specified values are distinguishable, but it would be 378 useful that this be specified explicitly in the 3GPP 379 specifications. 381 In addition, the peer MAY check the received value against its own 382 understanding of the network name. Upon detecting a discrepancy, the 383 peer either warns the user and continues, or fails the authentication 384 process. More specifically, the peer SHOULD have a configurable 385 policy that it can follow under these circumstances. If the policy 386 indicates that it can continue, the peer SHOULD log a warning message 387 or display it to the user. If the peer chooses to proceed, it MUST 388 use the network name as received in the AT_KDF_INPUT attribute. If 389 the policy indicates that the authentication should fail, the peer 390 behaves as if AUTN had been incorrect and authentication fails. 392 The Network Name field contains a UTF-8 string. This string MUST be 393 constructed as specified in [TS-3GPP.24.302] for "Access Network 394 Identity". The string is structured as fields separated by colons 395 (:). The algorithms and mechanisms to construct the identity string 396 depend on the used access technology. 398 On the network side, the network name construction is a configuration 399 issue in an access network and an authorization check in the 400 authentication server. On the peer, the network name is constructed 401 based on the local observations. For instance, the peer knows which 402 access technology it is using on the link, it can see information in 403 a link-layer beacon, and so on. The construction rules specify how 404 this information maps to an access network name. Typically, the 405 network name consists of the name of the access technology, or the 406 name of the access technology followed by some operator identifier 407 that was advertised in a link-layer beacon. In all cases, 408 [TS-3GPP.24.302] is the normative specification for the construction 409 in both the network and peer side. If the peer policy allows running 410 EAP-AKA' over an access technology for which that specification does 411 not provide network name construction rules, the peer SHOULD rely 412 only on the information from the AT_KDF_INPUT attribute and not 413 perform a comparison. 415 If a comparison of the locally determined network name and the one 416 received over EAP-AKA' is performed on the peer, it MUST be done as 417 follows. First, each name is broken down to the fields separated by 418 colons. If one of the names has more colons and fields than the 419 other one, the additional fields are ignored. The remaining 420 sequences of fields are compared, and they match only if they are 421 equal character by character. This algorithm allows a prefix match 422 where the peer would be able to match "", "FOO", and "FOO:BAR" 423 against the value "FOO:BAR" received from the server. This 424 capability is important in order to allow possible updates to the 425 specifications that dictate how the network names are constructed. 426 For instance, if a peer knows that it is running on access technology 427 "FOO", it can use the string "FOO" even if the server uses an 428 additional, more accurate description, e.g., "FOO:BAR", that contains 429 more information. 431 The allocation procedures in [TS-3GPP.24.302] ensure that conflicts 432 potentially arising from using the same name in different types of 433 networks are avoided. The specification also has detailed rules 434 about how a client can determine these based on information available 435 to the client, such as the type of protocol used to attach to the 436 network, beacons sent out by the network, and so on. Information 437 that the client cannot directly observe (such as the type or version 438 of the home network) is not used by this algorithm. 440 The AT_KDF_INPUT attribute MUST be sent and processed as explained 441 above when AT_KDF attribute has the value 1. Future definitions of 442 new AT_KDF values MUST define how this attribute is sent and 443 processed. 445 3.2. AT_KDF 447 AT_KDF is an attribute that the server uses to reference a specific 448 key derivation function. It offers a negotiation capability that can 449 be useful for future evolution of the key derivation functions. 451 The format of the AT_KDF attribute is shown below. 453 0 1 2 3 454 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 456 | AT_KDF | Length | Key Derivation Function | 457 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 459 The fields are as follows: 461 AT_KDF 463 This is set to 24. 465 Length 467 The length of the attribute, MUST be set to 1. 469 Key Derivation Function 471 An enumerated value representing the key derivation function that 472 the server (or peer) wishes to use. Value 1 represents the 473 default key derivation function for EAP-AKA', i.e., employing CK' 474 and IK' as defined in Section 3.3. 476 Servers MUST send one or more AT_KDF attributes in the EAP-Request/ 477 AKA'-Challenge message. These attributes represent the desired 478 functions ordered by preference, the most preferred function being 479 the first attribute. 481 Upon receiving a set of these attributes, if the peer supports and is 482 willing to use the key derivation function indicated by the first 483 attribute, the function is taken into use without any further 484 negotiation. However, if the peer does not support this function or 485 is unwilling to use it, it does not process the received EAP-Request/ 486 AKA'-Challenge in any way except by responding with the EAP-Response/ 487 AKA'-Challenge message that contains only one attribute, AT_KDF with 488 the value set to the selected alternative. If there is no suitable 489 alternative, the peer behaves as if AUTN had been incorrect and 490 authentication fails (see Figure 3 of [RFC4187]). The peer fails the 491 authentication also if there are any duplicate values within the list 492 of AT_KDF attributes (except where the duplication is due to a 493 request to change the key derivation function; see below for further 494 information). 496 Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the 497 peer, the server checks that the suggested AT_KDF value was one of 498 the alternatives in its offer. The first AT_KDF value in the message 499 from the server is not a valid alternative. If the peer has replied 500 with the first AT_KDF value, the server behaves as if AT_MAC of the 501 response had been incorrect and fails the authentication. For an 502 overview of the failed authentication process in the server side, see 503 Section 3 and Figure 2 of [RFC4187]. Otherwise, the server re-sends 504 the EAP-Response/AKA'-Challenge message, but adds the selected 505 alternative to the beginning of the list of AT_KDF attributes and 506 retains the entire list following it. Note that this means that the 507 selected alternative appears twice in the set of AT_KDF values. 508 Responding to the peer's request to change the key derivation 509 function is the only legal situation where such duplication may 510 occur. 512 When the peer receives the new EAP-Request/AKA'-Challenge message, it 513 MUST check that the requested change, and only the requested change, 514 occurred in the list of AT_KDF attributes. If so, it continues with 515 processing the received EAP-Request/AKA'-Challenge as specified in 516 [RFC4187] and Section 3.1 of this document. If not, it behaves as if 517 AT_MAC had been incorrect and fails the authentication. If the peer 518 receives multiple EAP-Request/AKA'-Challenge messages with differing 519 AT_KDF attributes without having requested negotiation, the peer MUST 520 behave as if AT_MAC had been incorrect and fail the authentication. 522 Note that the peer may also request sequence number resynchronization 523 [RFC4187]. This happens after AT_KDF negotiation has already 524 completed. An AKA'-Synchronization-Failure message is sent as a 525 response to the newly received EAP-Request/AKA'-Challenge (the last 526 message of the AT_KDF negotiation). The AKA'-Synchronization-Failure 527 message MUST contain the AUTS parameter as specified in [RFC4187] and 528 a copy the AT_KDF attributes as they appeared in the last message of 529 the AT_KDF negotiation. If the AT_KDF attributes are found to differ 530 from their earlier values, the peer and server MUST behave as if 531 AT_MAC had been incorrect and fail the authentication. 533 3.3. Key Derivation 535 Both the peer and server MUST derive the keys as follows. 537 AT_KDF parameter has the value 1 539 In this case, MK is derived and used as follows: 541 MK = PRF'(IK'|CK',"EAP-AKA'"|Identity) 542 K_encr = MK[0..127] 543 K_aut = MK[128..383] 544 K_re = MK[384..639] 545 MSK = MK[640..1151] 546 EMSK = MK[1152..1663] 548 Here [n..m] denotes the substring from bit n to m. PRF' is a new 549 pseudo-random function specified in Section 3.4. The first 1664 550 bits from its output are used for K_encr (encryption key, 128 551 bits), K_aut (authentication key, 256 bits), K_re (re- 552 authentication key, 256 bits), MSK (Master Session Key, 512 bits), 553 and EMSK (Extended Master Session Key, 512 bits). These keys are 554 used by the subsequent EAP-AKA' process. K_encr is used by the 555 AT_ENCR_DATA attribute, and K_aut by the AT_MAC attribute. K_re 556 is used later in this section. MSK and EMSK are outputs from a 557 successful EAP method run [RFC3748]. 559 IK' and CK' are derived as specified in [TS-3GPP.33.402]. The 560 functions that derive IK' and CK' take the following parameters: 561 CK and IK produced by the AKA algorithm, and value of the Network 562 Name field comes from the AT_KDF_INPUT attribute (without length 563 or padding) . 565 The value "EAP-AKA'" is an eight-characters-long ASCII string. It 566 is used as is, without any trailing NUL characters. 568 Identity is the peer identity as specified in Section 7 of 569 [RFC4187]. 571 When the server creates an AKA challenge and corresponding AUTN, 572 CK, CK', IK, and IK' values, it MUST set the Authentication 573 Management Field (AMF) separation bit to 1 in the AKA algorithm 574 [TS-3GPP.33.102]. Similarly, the peer MUST check that the AMF 575 separation bit is set to 1. If the bit is not set to 1, the peer 576 behaves as if the AUTN had been incorrect and fails the 577 authentication. 579 On fast re-authentication, the following keys are calculated: 581 MK = PRF'(K_re,"EAP-AKA' re-auth"|Identity|counter|NONCE_S) 582 MSK = MK[0..511] 583 EMSK = MK[512..1023] 585 MSK and EMSK are the resulting 512-bit keys, taking the first 1024 586 bits from the result of PRF'. Note that K_encr and K_aut are not 587 re-derived on fast re-authentication. K_re is the re- 588 authentication key from the preceding full authentication and 589 stays unchanged over any fast re-authentication(s) that may happen 590 based on it. The value "EAP-AKA' re-auth" is a sixteen- 591 characters-long ASCII string, again represented without any 592 trailing NUL characters. Identity is the fast re-authentication 593 identity, counter is the value from the AT_COUNTER attribute, 594 NONCE_S is the nonce value from the AT_NONCE_S attribute, all as 595 specified in Section 7 of [RFC4187]. To prevent the use of 596 compromised keys in other places, it is forbidden to change the 597 network name when going from the full to the fast re- 598 authentication process. The peer SHOULD NOT attempt fast re- 599 authentication when it knows that the network name in the current 600 access network is different from the one in the initial, full 601 authentication. Upon seeing a re-authentication request with a 602 changed network name, the server SHOULD behave as if the re- 603 authentication identifier had been unrecognized, and fall back to 604 full authentication. The server observes the change in the name 605 by comparing where the fast re-authentication and full 606 authentication EAP transactions were received at the 607 Authentication, Authorization, and Accounting (AAA) protocol 608 level. 610 AT_KDF has any other value 612 Future variations of key derivation functions may be defined, and 613 they will be represented by new values of AT_KDF. If the peer 614 does not recognize the value, it cannot calculate the keys and 615 behaves as explained in Section 3.2. 617 AT_KDF is missing 619 The peer behaves as if the AUTN had been incorrect and MUST fail 620 the authentication. 622 If the peer supports a given key derivation function but is unwilling 623 to perform it for policy reasons, it refuses to calculate the keys 624 and behaves as explained in Section 3.2. 626 3.4. Hash Functions 628 EAP-AKA' uses SHA-256, not SHA-1 (see [FIPS.180-4]) as in EAP-AKA. 629 This requires a change to the pseudo-random function (PRF) as well as 630 the AT_MAC and AT_CHECKCODE attributes. 632 3.4.1. PRF' 634 The PRF' construction is the same one IKEv2 uses (see Section 2.13 of 635 [RFC4306]). The function takes two arguments. K is a 256-bit value 636 and S is an byte string of arbitrary length. PRF' is defined as 637 follows: 639 PRF'(K,S) = T1 | T2 | T3 | T4 | ... 641 where: 642 T1 = HMAC-SHA-256 (K, S | 0x01) 643 T2 = HMAC-SHA-256 (K, T1 | S | 0x02) 644 T3 = HMAC-SHA-256 (K, T2 | S | 0x03) 645 T4 = HMAC-SHA-256 (K, T3 | S | 0x04) 646 ... 648 PRF' produces as many bits of output as is needed. HMAC-SHA-256 is 649 the application of HMAC [RFC2104] to SHA-256. 651 3.4.2. AT_MAC 653 When used within EAP-AKA', the AT_MAC attribute is changed as 654 follows. The MAC algorithm is HMAC-SHA-256-128, a keyed hash value. 655 The HMAC-SHA-256-128 value is obtained from the 32-byte HMAC-SHA-256 656 value by truncating the output to the first 16 bytes. Hence, the 657 length of the MAC is 16 bytes. 659 Otherwise, the use of AT_MAC in EAP-AKA' follows Section 10.15 of 660 [RFC4187]. 662 3.4.3. AT_CHECKCODE 664 When used within EAP-AKA', the AT_CHECKCODE attribute is changed as 665 follows. First, a 32-byte value is needed to accommodate a 256-bit 666 hash output: 668 0 1 2 3 669 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 670 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 671 | AT_CHECKCODE | Length | Reserved | 672 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 673 | | 674 | Checkcode (0 or 32 bytes) | 675 | | 676 | | 677 | | 678 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 680 Second, the checkcode is a hash value, calculated with SHA-256 681 [FIPS.180-4], over the data specified in Section 10.13 of [RFC4187]. 683 4. Bidding Down Prevention for EAP-AKA 685 As discussed in [RFC3748], negotiation of methods within EAP is 686 insecure. That is, a man-in-the-middle attacker may force the 687 endpoints to use a method that is not the strongest that they both 688 support. This is a problem, as we expect EAP-AKA and EAP-AKA' to be 689 negotiated via EAP. 691 In order to prevent such attacks, this RFC specifies a new mechanism 692 for EAP-AKA that allows the endpoints to securely discover the 693 capabilities of each other. This mechanism comes in the form of the 694 AT_BIDDING attribute. This allows both endpoints to communicate 695 their desire and support for EAP-AKA' when exchanging EAP-AKA 696 messages. This attribute is not included in EAP-AKA' messages as 697 defined in this RFC. It is only included in EAP-AKA messages. This 698 is based on the assumption that EAP-AKA' is always preferable (see 699 Section 7). If during the EAP-AKA authentication process it is 700 discovered that both endpoints would have been able to use EAP-AKA', 701 the authentication process SHOULD be aborted, as a bidding down 702 attack may have happened. 704 The format of the AT_BIDDING attribute is shown below. 706 0 1 2 3 707 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 708 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 709 | AT_BIDDING | Length |D| Reserved | 710 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 712 The fields are as follows: 714 AT_BIDDING 715 This is set to 136. 717 Length 719 The length of the attribute, MUST be set to 1. 721 D 723 This bit is set to 1 if the sender supports EAP-AKA', is willing 724 to use it, and prefers it over EAP-AKA. Otherwise, it should be 725 set to zero. 727 Reserved 729 This field MUST be set to zero when sent and ignored on receipt. 731 The server sends this attribute in the EAP-Request/AKA-Challenge 732 message. If the peer supports EAP-AKA', it compares the received 733 value to its own capabilities. If it turns out that both the server 734 and peer would have been able to use EAP-AKA' and preferred it over 735 EAP-AKA, the peer behaves as if AUTN had been incorrect and fails the 736 authentication (see Figure 3 of [RFC4187]). A peer not supporting 737 EAP-AKA' will simply ignore this attribute. In all cases, the 738 attribute is protected by the integrity mechanisms of EAP-AKA, so it 739 cannot be removed by a man-in-the-middle attacker. 741 Note that we assume (Section 7) that EAP-AKA' is always stronger than 742 EAP-AKA. As a result, there is no need to prevent bidding "down" 743 attacks in the other direction, i.e., attackers forcing the endpoints 744 to use EAP-AKA'. 746 5. Peer Identities 748 EAP-AKA' peer identities are as specified in [RFC4187] Section 4.1, 749 with the addition of some requirements specified in this section. 751 EAP-AKA' includes optional identity privacy support that can be used 752 to hide the cleartext permanent identity and thereby make the 753 subscriber's EAP exchanges untraceable to eavesdroppers. EAP-AKA' 754 can also use the privacy friendly identifiers specified for 5G 755 networks. 757 The permanent identity is usually based on the IMSI. Exposing the 758 IMSI is undesirable, because as a permanent identity it is easily 759 trackable. In addition, since IMSIs may be used in other contexts as 760 well, there would be additional opportunities for such tracking. 762 In EAP-AKA', identity privacy is based on temporary usernames, or 763 pseudonym usernames. These are similar to but separate from the 764 Temporary Mobile Subscriber Identities (TMSI) that are used on 765 cellular networks. 767 5.1. Username Types in EAP-AKA' Identities 769 Section 4.1.1.3 of [RFC4187] specified that there are three types of 770 usernames: permanent, pseudonym, and fast re-authentication 771 usernames. This specification extends this definition as follows. 772 There are four types of usernames: 774 (1) Regular usernames. These are external names given to EAP- 775 AKA'. The regular usernames are further subdivided into to 776 categories: 778 (a) Permanent usernames, for instance IMSI-based usernames. 780 (b) Privacy-friendly temporary usernames, for instance 5G 781 privacy identifiers (see Section 5.3.2 and Section 5.3.2.1. 783 (2) EAP-AKA' pseudonym usernames. For example, 784 2s7ah6n9q@example.com might be a valid pseudonym identity. In 785 this example, 2s7ah6n9q is the pseudonym username. 787 (3) EAP-AKA' fast re-authentication usernames. For example, 788 43953754@example.com might be a valid fast re-authentication 789 identity and 43953754 the fast re-authentication username. 791 The permanent, privacy-friendly temporary, and pseudonym usernames 792 are only used on full authentication, and fast re-authentication 793 usernames only on fast re-authentication. Unlike permanent usernames 794 and pseudonym usernames, privacy friendly temporary usernames and 795 fast re-authentication usernames are one-time identifiers, which are 796 not re-used across EAP exchanges. 798 5.2. Generating Pseudonyms and Fast Re-Authentication Identities 800 As specified by [RFC4187] Section 4.1.1.7, pseudonym usernames and 801 fast re-authentication identities are generated by the EAP server, in 802 an implementation-dependent manner. RFC 4187 provides some general 803 requirements on how these identities are transported, how they map to 804 the NAI syntax, how they are distinguished from each other, and so 805 on. 807 However, to ensure privacy some additional requirements need to be 808 applied. 810 The pseudonym usernames and fast re-authentication identities MUST be 811 generated in a cryptographically secure way so that that it is 812 computationally infeasible for at attacker to differentiate two 813 identities belonging to the same user from two identities belonging 814 to different users. This can be achieved, for instance, by using 815 random or pseudo-random identifiers such as random byte strings or 816 ciphertexts. See also [RFC4086] for guidance on random number 817 generation. 819 Note that the pseudonym and fast re-authentication usernames also 820 MUST NOT include substrings that can be used to relate the username 821 to a particular entity or a particular permanent identity. For 822 instance, the usernames can not include any subscriber-identifying 823 part of an IMSI or other permanent identifier. Similarly, no part of 824 the username can be formed by a fixed mapping that stays the same 825 across multiple different pseudonyms or fast re-authentication 826 identities for the same subscriber. 828 When the identifier used to identify a subscriber in an EAP-AKA' 829 authentication exchange is a privacy-friendly identifier that is used 830 only once, the EAP-AKA' peer MUST NOT use a pseudonym provided in 831 that authentication exchange in subsequent exchanges more than once. 832 To ensure that this does not happen, EAP-AKA' server MAY decline to 833 provide a pseudonym in such authentication exchanges. An important 834 case where such privacy-friendly identifiers are used is in 5G 835 networks (see Section 5.3). 837 5.3. Identifier Usage in 5G 839 In EAP-AKA', the peer identity may be communicated to the server in 840 one of three ways: 842 o As a part of link layer establishment procedures, externally to 843 EAP. 845 o With the EAP-Response/Identity message in the beginning of the EAP 846 exchange, but before the selection of EAP-AKA'. 848 o Transmitted from the peer to the server using EAP-AKA messages 849 instead of EAP-Response/Identity. In this case, the server 850 includes an identity requesting attribute (AT_ANY_ID_REQ, 851 AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the EAP-Request/AKA- 852 Identity message; and the peer includes the AT_IDENTITY attribute, 853 which contains the peer's identity, in the EAP-Response/AKA- 854 Identity message. 856 The identity carried above may be a permanent identity, privacy 857 friendly identity, pseudonym identity, or fast re-authentication 858 identity as defined in this RFC. 860 5G supports the concept of privacy identifiers, and it is important 861 for interoperability that the right type of identifier is used. 863 5G defines the SUbscription Permanent Identifier (SUPI) and 864 SUbscription Concealed Identifier (SUCI) [TS-3GPP.23.501] 865 [TS-3GPP.33.501] [TS-3GPP.23.003]. SUPI is globally unique and 866 allocated to each subscriber. However, it is only used internally in 867 the 5G network, and is privacy sensitive. The SUCI is a privacy 868 preserving identifier containing the concealed SUPI, using public key 869 cryptography to encrypt the SUPI. 871 Given the choice between these two types of identifiers, EAP-AKA' 872 ensures interoperability as follows: 874 o Where identifiers are used within EAP-AKA' -- such as key 875 derivation -- specify what values exactly should be used, to avoid 876 ambiguity (see Section 5.3.1). 878 o Where identifiers are carried within EAP-AKA' packets -- such as 879 in the AT_IDENTITY attribute -- specify which identifiers should 880 be filled in (see Section 5.3.2). 882 In 5G, the normal mode of operation is that identifiers are only 883 transmitted outside EAP. However, in a system involving terminals 884 from many generations and several connectivity options via 5G and 885 other mechanisms, implementations and the EAP-AKA' specification need 886 to prepare for many different situations, including sometimes having 887 to communicate identities within EAP. 889 The following sections clarify which identifiers are used and how. 891 5.3.1. Key Derivation 893 In EAP-AKA', the peer identity is used in the Section 3.3 key 894 derivation formula. 896 If the AT_KDF_INPUT parameter contains the prefix "5G:", the AT_KDF 897 parameter has the value 1, and this authentication is not a fast re- 898 authentication, then the peer identity used in the key derivation 899 MUST be the 5G SUPI for the peer. This rule applies to all full EAP- 900 AKA' authentication processes, even if the peer sent some other 901 identifier at a lower layer or as a response to an EAP Identity 902 Request or if no identity was sent. 904 The identity MUST also be represented in the exact correct format for 905 the key derivation formula to produce correct results. For the SUPI, 906 this format is as defined Section 5.3.1.1. 908 In all other cases, the following applies: 910 The identity used in the key derivation formula MUST be exactly 911 the one sent in EAP-AKA' AT_IDENTITY attribute, if one was sent, 912 regardless of the kind of identity that it may have been. If no 913 AT_IDENTITY was sent, the identity MUST be the exactly the one 914 sent in the generic EAP Identity exchange, if one was made. 915 Again, the identity MUST be used exactly as sent. 917 If no identity was communicated inside EAP, then the identity is 918 the one communicated outside EAP in link layer messaging. 920 In this case, the used identity MUST be the identity most recently 921 communicated by the peer to the network, again regardless of what 922 type of identity it may have been. 924 5.3.1.1. Format of the SUPI 926 A SUPI is either an IMSI or a Network Access Identifier [RFC4282]. 928 When used in EAP-AKA', the format of the SUPI MUST be as specified in 929 [TS-3GPP.23.003] Section 28.7.2, with the semantics defined in 930 [TS-3GPP.23.003] Section 2.2B. Also, in contrast to [RFC5448], in 5G 931 EAP-AKA' does not use the "0" or "6" prefix in front of the entire 932 IMSI. 934 For instance, if the IMSI is 234150999999999 (MCC = 234, MNC = 15), 935 the NAI format for the SUPI takes the form: 937 234150999999999@nai.5gc.mnc015.mcc234.3gppnetwork.org 939 5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY Attribute 941 The EAP authentication option is only available in 5G when the new 5G 942 core network is also in use. However, in other networks an EAP-AKA' 943 peer may be connecting to other types of networks and existing 944 equipment. 946 When the EAP peer is connecting to a 5G access network and uses the 947 5G Non-Access Stratum (NAS) protocol [TS-3GPP.24.501], the EAP server 948 is in a 5G network. The EAP identity exchanges are generally not 949 used in this case, as the identity is already made available on 950 previous link layer exchanges. 952 In this situation, the EAP server SHOULD NOT request an additional 953 identity from the peer. If the peer for some reason receives EAP- 954 Request/Identity or EAP-Request/AKA-Identity messages, the peer 955 should behave as follows. 957 Receive EAP-Request/Identity 959 In this case, the peer SHOULD respond with a EAP-Response/Identity 960 containing the privacy-friendly 5G identifier, the SUCI. The SUCI 961 SHOULD be represented as specified in Section 5.3.2.1. 963 EAP-Request/AKA-Identity with AT_PERMANENT_REQ 965 For privacy reasons, the peer should follow a "conservative" 966 policy and terminate the authentication exchange rather than risk 967 revealing its permanent identity. 969 The peer SHOULD respond with EAP-Response/AKA-Client-Error with 970 the client error code 0, "unable to process packet". 972 EAP-Request/AKA-Identity with AT_FULLAUTH_REQ 974 In this case, the peer SHOULD respond with a EAP-Response/AKA- 975 Identity containing the SUCI. The SUCI SHOULD be represented as 976 specified in Section 5.3.2.1. 978 EAP-Request/AKA-Identity with AT_ANY_ID_REQ 980 If the peer supports fast re-authentication and has a fast re- 981 authentication identity available, the peer SHOULD respond with 982 EAP-Response/AKA-Identity containing the fast re-authentication 983 identity. Otherwise the peer SHOULD respond with a EAP-Response/ 984 AKA-Identity containing the SUCI, and SHOULD represent the SUCI as 985 specified in Section 5.3.2.1. 987 Similarly, if the peer is communicating over a non-3GPP network but 988 carrying EAP inside 5G NAS protocol, it MUST assume that the EAP 989 server is in a 5G network, and again employ the SUCI within EAP. 991 Otherwise, the peer SHOULD employ IMSI, SUPI, or a NAI as it is 992 configured to use. 994 5.3.2.1. Format of the SUCI 996 When used in EAP-AKA', the format of the SUCI MUST be as specified in 997 [TS-3GPP.23.003] Section 28.7.3, with the semantics defined in 998 [TS-3GPP.23.003] Section 2.2B. Also, in contrast to [RFC5448], in 5G 999 EAP-AKA' does not use the "0" or "6" prefix in front of the 1000 identifier. 1002 For instance, assuming the IMSI 234150999999999, where MCC=234, 1003 MNC=15 and MSISN=0999999999, the Routing Indicator 678, and a Home 1004 Network Public Key Identifier of 27, the NAI format for the SUCI 1005 takes the form: 1007 For the null-scheme: 1009 type0.rid678.schid0.userid0999999999@nai.5gc.mnc015. 1010 mcc234.3gppnetwork.org 1012 For the Profile protection scheme: 1014 type0.rid678.schid1.hnkey27.ecckey. 1015 cip.mac@nai.5gc. 1016 mnc015.mcc234.3gppnetwork.org 1018 6. Exported Parameters 1020 The EAP-AKA' Session-Id is the concatenation of the EAP Type Code 1021 (50, one byte) with the contents of the RAND field from the AT_RAND 1022 attribute, followed by the contents of the AUTN field in the AT_AUTN 1023 attribute: 1025 Session-Id = 50 || RAND || AUTN 1027 When using fast re-authentication, the EAP-AKA' Session-Id is the 1028 concatenation of the EAP Type Code (50) with the contents of the 1029 NONCE_S field from the AT_NONCE_S attribute, followed by the contents 1030 of the MAC field from the AT_MAC attribute from EAP-Request/AKA- 1031 Reauthentication: 1033 Session-Id = 50 || NONCE_S || MAC 1035 The Peer-Id is the contents of the Identity field from the 1036 AT_IDENTITY attribute, using only the Actual Identity Length bytes 1037 from the beginning. Note that the contents are used as they are 1038 transmitted, regardless of whether the transmitted identity was a 1039 permanent, pseudonym, or fast EAP re-authentication identity. If no 1040 AT_IDENTITY attribute was exchanged, the exported Peer-Id is the 1041 identity provided from the EAP Identity Response packet. If no EAP 1042 Identity Response was provided either, the exported Peer-Id is null 1043 string (zero length). 1045 The Server-Id is the null string (zero length). 1047 7. Security Considerations 1049 A summary of the security properties of EAP-AKA' follows. These 1050 properties are very similar to those in EAP-AKA. We assume that HMAC 1051 SHA-256 is at least as secure as HMAC SHA-1 (see also [RFC6194]. 1052 This is called the SHA-256 assumption in the remainder of this 1053 section. Under this assumption, EAP-AKA' is at least as secure as 1054 EAP-AKA. 1056 If the AT_KDF attribute has value 1, then the security properties of 1057 EAP-AKA' are as follows: 1059 Protected ciphersuite negotiation 1061 EAP-AKA' has no ciphersuite negotiation mechanisms. It does have 1062 a negotiation mechanism for selecting the key derivation 1063 functions. This mechanism is secure against bidding down attacks. 1064 The negotiation mechanism allows changing the offered key 1065 derivation function, but the change is visible in the final EAP- 1066 Request/AKA'-Challenge message that the server sends to the peer. 1067 This message is authenticated via the AT_MAC attribute, and 1068 carries both the chosen alternative and the initially offered 1069 list. The peer refuses to accept a change it did not initiate. 1070 As a result, both parties are aware that a change is being made 1071 and what the original offer was. 1073 Mutual authentication 1075 Under the SHA-256 assumption, the properties of EAP-AKA' are at 1076 least as good as those of EAP-AKA in this respect. Refer to 1077 [RFC4187], Section 12 for further details. 1079 Integrity protection 1081 Under the SHA-256 assumption, the properties of EAP-AKA' are at 1082 least as good (most likely better) as those of EAP-AKA in this 1083 respect. Refer to [RFC4187], Section 12 for further details. The 1084 only difference is that a stronger hash algorithm, SHA-256, is 1085 used instead of SHA-1. 1087 Replay protection 1089 Under the SHA-256 assumption, the properties of EAP-AKA' are at 1090 least as good as those of EAP-AKA in this respect. Refer to 1091 [RFC4187], Section 12 for further details. 1093 Confidentiality 1094 The properties of EAP-AKA' are exactly the same as those of EAP- 1095 AKA in this respect. Refer to [RFC4187], Section 12 for further 1096 details. 1098 Key derivation 1100 EAP-AKA' supports key derivation with an effective key strength 1101 against brute force attacks equal to the minimum of the length of 1102 the derived keys and the length of the AKA base key, i.e., 128 1103 bits or more. The key hierarchy is specified in Section 3.3. 1105 The Transient EAP Keys used to protect EAP-AKA packets (K_encr, 1106 K_aut, K_re), the MSK, and the EMSK are cryptographically 1107 separate. If we make the assumption that SHA-256 behaves as a 1108 pseudo-random function, an attacker is incapable of deriving any 1109 non-trivial information about any of these keys based on the other 1110 keys. An attacker also cannot calculate the pre-shared secret 1111 from IK, CK, IK', CK', K_encr, K_aut, K_re, MSK, or EMSK by any 1112 practically feasible means. 1114 EAP-AKA' adds an additional layer of key derivation functions 1115 within itself to protect against the use of compromised keys. 1116 This is discussed further in Section 7.4. 1118 EAP-AKA' uses a pseudo-random function modeled after the one used 1119 in IKEv2 [RFC4306] together with SHA-256. 1121 Key strength 1123 See above. 1125 Dictionary attack resistance 1127 Under the SHA-256 assumption, the properties of EAP-AKA' are at 1128 least as good as those of EAP-AKA in this respect. Refer to 1129 [RFC4187], Section 12 for further details. 1131 Fast reconnect 1133 Under the SHA-256 assumption, the properties of EAP-AKA' are at 1134 least as good as those of EAP-AKA in this respect. Refer to 1135 [RFC4187], Section 12 for further details. Note that 1136 implementations MUST prevent performing a fast reconnect across 1137 method types. 1139 Cryptographic binding 1140 Note that this term refers to a very specific form of binding, 1141 something that is performed between two layers of authentication. 1142 It is not the same as the binding to a particular network name. 1143 The properties of EAP-AKA' are exactly the same as those of EAP- 1144 AKA in this respect, i.e., as it is not a tunnel method, this 1145 property is not applicable to it. Refer to [RFC4187], Section 12 1146 for further details. 1148 Session independence 1150 The properties of EAP-AKA' are exactly the same as those of EAP- 1151 AKA in this respect. Refer to [RFC4187], Section 12 for further 1152 details. 1154 Fragmentation 1156 The properties of EAP-AKA' are exactly the same as those of EAP- 1157 AKA in this respect. Refer to [RFC4187], Section 12 for further 1158 details. 1160 Channel binding 1162 EAP-AKA', like EAP-AKA, does not provide channel bindings as 1163 they're defined in [RFC3748] and [RFC5247]. New skippable 1164 attributes can be used to add channel binding support in the 1165 future, if required. 1167 However, including the Network Name field in the AKA' algorithms 1168 (which are also used for other purposes than EAP-AKA') provides a 1169 form of cryptographic separation between different network names, 1170 which resembles channel bindings. However, the network name does 1171 not typically identify the EAP (pass-through) authenticator. See 1172 Section 7.4 for more discussion. 1174 7.1. Privacy 1176 [RFC6973] suggests that the privacy considerations of IETF protocols 1177 be documented. 1179 The confidentiality properties of EAP-AKA' itself have been discussed 1180 above under "Confidentiality". 1182 EAP-AKA' uses several different types of identifiers to identify the 1183 authenticating peer. It is strongly RECOMMENDED to use the privacy- 1184 friendly temporary or hidden identifiers, i.e., the 5G SUCI, 1185 pseudonym usernames, and fast re-authentication usernames. The use 1186 of permanent identifiers such as the IMSI or SUPI may lead to an 1187 ability to track the peer and/or user associated with the peer. The 1188 use of permanent identifiers such as the IMSI or SUPI is strongly NOT 1189 RECOMMENDED. 1191 As discussed in Section 5.3, when authenticating to a 5G network, 1192 only the 5G SUCI identifier should be used. The use of pseudonyms in 1193 this situation is at best limited. In fact, the re-use of the same 1194 pseudonym multiple times will result in a tracking opportunity for 1195 observers that see the pseudonym pass by. To avoid this, the peer 1196 and server need to follow the guidelines given in Section 5.2. 1198 When authenticating to a 5G network, per Section 5.3.1, both the EAP- 1199 AKA' peer and server need employ permanent identifier, SUPI, as an 1200 input to key derivation. However, this use of the SUPI is only 1201 internal and the SUPI need not be communicated in EAP messages. SUCI 1202 MUST NOT be communicated in EAP-AKA' when authenticating to a 5G 1203 network. 1205 While the use of SUCI in 5G networks generally provides identity 1206 privacy, this is not true if the null-scheme encryption is used to 1207 construct the SUCI (see [TS-3GPP.23.501] Annex C). The use of this 1208 scheme turns the use of SUCI equivalent to the use of SUPI or IMSI. 1209 The use of the null scheme is NOT RECOMMENDED where identity privacy 1210 is important. 1212 The use of fast re-authentication identities when authenticating to a 1213 5G network does not have the same problems as the use of pseudonyms, 1214 as long as the 5G authentication server generates the fast re- 1215 authentication identifiers in a proper manner specified in 1216 Section 5.2. 1218 Outside 5G, there is a full choice to use permanent, pseudonym, or 1219 fast re-authentication identifiers: 1221 o A peer that has not yet performed any EAP-AKA' exchanges does not 1222 typically have a pseudonym available. If the peer does not have a 1223 pseudonym available, then the privacy mechanism cannot be used, 1224 and the permanent identity will have to be sent in the clear. 1226 The terminal SHOULD store the pseudonym in non-volatile memory so 1227 that it can be maintained across reboots. An active attacker that 1228 impersonates the network may use the AT_PERMANENT_ID_REQ attribute 1229 ([RFC4187] Section 4.1.2) to learn the subscriber's IMSI. 1230 However, as discussed in [RFC4187] Section 4.1.2, the terminal can 1231 refuse to send the cleartext permanent identity if it believes 1232 that the network should be able to recognize the pseudonym. 1234 o When pseudonyms and fast re-authentication identities are used, 1235 the peer relies on the properly created identifiers by the server. 1237 It is essential that an attacker cannot link a privacy-friendly 1238 identifier to the user in any way or determine that two 1239 identifiers belong to the same user as outlined in Section 5.2. 1240 The pseudonym usernames and fast re-authentication identities MUST 1241 also not be used for other purposes (e.g. in other protocols). 1243 If the peer and server cannot guarantee that 5G SUCI can be used or 1244 pseudonyms will available, generated properly, and maintained 1245 reliably, and identity privacy is required then additional protection 1246 from an external security mechanism such as tunneled EAP methods may 1247 be used. The benefits and the security considerations of using an 1248 external security mechanism with EAP-AKA are beyond the scope of this 1249 document. 1251 Finally, as with other EAP methods, even when privacy-friendly 1252 identifiers or EAP tunneling is used, typically the domain part of an 1253 identifier (e.g., the home operator) is visible to external parties. 1255 7.2. Discovered Vulnerabilities 1257 There have been no published attacks that violate the primary secrecy 1258 or authentication properties defined for Authentication and Key 1259 Agreement (AKA) under the originally assumed trust model. The same 1260 is true of EAP-AKA'. 1262 However, there have been attacks when a different trust model is in 1263 use, with characteristics not originally provided by the design, or 1264 when participants in the protocol leak information to outsiders on 1265 purpose, and there has been some privacy-related attacks. 1267 For instance, the original AKA protocol does not prevent supplying 1268 keys by an insider to a third party as done in, e.g., by Mjolsnes and 1269 Tsay in [MT2012] where a serving network lets an authentication run 1270 succeed, but then misuses the session keys to send traffic on the 1271 authenticated user's behalf. This particular attack is not different 1272 from any on-path entity (such as a router) pretending to send 1273 traffic, but the general issue of insider attacks can be a problem, 1274 particularly in a large group of collaborating operators. 1276 Another class of attacks is the use of tunneling of traffic from one 1277 place to another, e.g., as done by Zhang and Fang in [ZF2005] to 1278 leverage security policy differences between different operator 1279 networks, for instance. To gain something in such an attack, the 1280 attacker needs to trick the user into believing it is in another 1281 location where, for instance, it is not required to encrypt all 1282 payload traffic after encryption. As an authentication mechanism, 1283 EAP-AKA' is not directly affected by most such attacks. EAP-AKA' 1284 network name binding can also help alleviate some of the attacks. In 1285 any case, it is recommended that EAP-AKA' configuration not be 1286 dependent on the location of where a request comes from, unless the 1287 location information can be cryptographically confirmed, e.g., with 1288 the network name binding. 1290 Zhang and Fang also looked at Denial-of-Service attacks [ZF2005]. A 1291 serving network may request large numbers of authentication runs for 1292 a particular subscriber from a home network. While resynchronization 1293 process can help recover from this, eventually it is possible to 1294 exhaust the sequence number space and render the subscriber's card 1295 unusable. This attack is possible for both native AKA and EAP-AKA'. 1296 However, it requires the collaboration of a serving network in an 1297 attack. It is recommended that EAP-AKA' implementations provide 1298 means to track, detect, and limit excessive authentication attempts 1299 to combat this problem. 1301 There has also been attacks related to the use of AKA without the 1302 generated session keys (e.g., [BT2013]). Some of those attacks 1303 relate to the use of originally man-in-the-middle vulnerable HTTP 1304 Digest AKAv1 [RFC3310]. This has since then been corrected in 1305 [RFC4169]. The EAP-AKA' protocol uses session keys and provides 1306 channel binding, and as such, is resistant to the above attacks 1307 except where the protocol participants leak information to outsiders. 1309 Basin et al [Basin2018] have performed formal analysis and concluded 1310 that the AKA protocol would have benefited from additional security 1311 requirements, such as key confirmation. 1313 In the context of pervasive monitoring revelations, there were also 1314 reports of compromised long term pre-shared keys used in SIM and AKA 1315 [Heist2015]. While no protocol can survive the theft of key material 1316 associated with its credentials, there are some things that alleviate 1317 the impacts in such situations. These are discussed further in 1318 Section 7.3. 1320 Arapinis et al ([Arapinis2012]) describe an attack that uses the AKA 1321 resynchronization protocol to attempt to detect whether a particular 1322 subscriber is on a given area. This attack depends on the ability of 1323 the attacker to have a false base station on the given area, and the 1324 subscriber performing at least one authentication between the time 1325 the attack is set up and run. 1327 Finally, while this is not a problem with the protocol itself, bad 1328 implementations may not produce pseudonym usernames or fast re- 1329 authentication identities in a manner that is sufficiently secure. 1330 Recommendations from Section 5.2 need to be followed to avoid this. 1332 7.3. Pervasive Monitoring 1334 As required by [RFC7258], work on IETF protocols needs to consider 1335 the effects of pervasive monitoring and mitigate them when possible. 1337 As described Section 7.2, after the publication of RFC 5448, new 1338 information has come to light regarding the use of pervasive 1339 monitoring techniques against many security technologies, including 1340 AKA-based authentication. 1342 For AKA, these attacks relate to theft of the long-term shared secret 1343 key material stored on the cards. Such attacks are conceivable, for 1344 instance, during the manufacturing process of cards, through coercion 1345 of the card manufacturers, or during the transfer of cards and 1346 associated information to an operator. Since the publication of 1347 reports about such attacks, manufacturing and provisioning processes 1348 have gained much scrutiny and have improved. 1350 In particular, it is crucial that manufacturers limit access to the 1351 secret information and the cards only to necessary systems and 1352 personnel. It is also crucial that secure mechanisms be used to 1353 communicate the secrets between the manufacturer and the operator 1354 that adopts those cards for their customers. 1356 Beyond these operational considerations, there are also technical 1357 means to improve resistance to these attacks. One approach is to 1358 provide Perfect Forwards Secrecy (PFS). This would prevent any 1359 passive attacks merely based on the long-term secrets and observation 1360 of traffic. Such a mechanism can be defined as an backwards- 1361 compatible extension of EAP-AKA', and is pursued separately from this 1362 specification [I-D.arkko-eap-aka-pfs]. Alternatively, EAP-AKA' 1363 authentication can be run inside a PFS-capable tunneled 1364 authentication method. In any case, the use of some PFS-capable 1365 mechanism is recommended. 1367 7.4. Security Properties of Binding Network Names 1369 The ability of EAP-AKA' to bind the network name into the used keys 1370 provides some additional protection against key leakage to 1371 inappropriate parties. The keys used in the protocol are specific to 1372 a particular network name. If key leakage occurs due to an accident, 1373 access node compromise, or another attack, the leaked keys are only 1374 useful when providing access with that name. For instance, a 1375 malicious access point cannot claim to be network Y if it has stolen 1376 keys from network X. Obviously, if an access point is compromised, 1377 the malicious node can still represent the compromised node. As a 1378 result, neither EAP-AKA' nor any other extension can prevent such 1379 attacks; however, the binding to a particular name limits the 1380 attacker's choices, allows better tracking of attacks, makes it 1381 possible to identify compromised networks, and applies good 1382 cryptographic hygiene. 1384 The server receives the EAP transaction from a given access network, 1385 and verifies that the claim from the access network corresponds to 1386 the name that this access network should be using. It becomes 1387 impossible for an access network to claim over AAA that it is another 1388 access network. In addition, if the peer checks that the information 1389 it has received locally over the network-access link layer matches 1390 with the information the server has given it via EAP-AKA', it becomes 1391 impossible for the access network to tell one story to the AAA 1392 network and another one to the peer. These checks prevent some 1393 "lying NAS" (Network Access Server) attacks. For instance, a roaming 1394 partner, R, might claim that it is the home network H in an effort to 1395 lure peers to connect to itself. Such an attack would be beneficial 1396 for the roaming partner if it can attract more users, and damaging 1397 for the users if their access costs in R are higher than those in 1398 other alternative networks, such as H. 1400 Any attacker who gets hold of the keys CK and IK, produced by the AKA 1401 algorithm, can compute the keys CK' and IK' and, hence, the Master 1402 Key (MK) according to the rules in Section 3.3. The attacker could 1403 then act as a lying NAS. In 3GPP systems in general, the keys CK and 1404 IK have been distributed to, for instance, nodes in a visited access 1405 network where they may be vulnerable. In order to reduce this risk, 1406 the AKA algorithm MUST be computed with the AMF separation bit set to 1407 1, and the peer MUST check that this is indeed the case whenever it 1408 runs EAP-AKA'. Furthermore, [TS-3GPP.33.402] requires that no CK or 1409 IK keys computed in this way ever leave the home subscriber system. 1411 The additional security benefits obtained from the binding depend 1412 obviously on the way names are assigned to different access networks. 1413 This is specified in [TS-3GPP.24.302]. See also [TS-3GPP.23.003]. 1414 Ideally, the names allow separating each different access technology, 1415 each different access network, and each different NAS within a 1416 domain. If this is not possible, the full benefits may not be 1417 achieved. For instance, if the names identify just an access 1418 technology, use of compromised keys in a different technology can be 1419 prevented, but it is not possible to prevent their use by other 1420 domains or devices using the same technology. 1422 8. IANA Considerations 1424 IANA should update the Extensible Authentication Protocol (EAP) 1425 Registry and the EAP-AKA and EAP-SIM Parameters so that entries 1426 pointing to RFC 5448 will point to this RFC instead. 1428 8.1. Type Value 1430 EAP-AKA' has the EAP Type value 50 in the Extensible Authentication 1431 Protocol (EAP) Registry under Method Types. Per Section 6.2 of 1432 [RFC3748], this allocation can be made with Designated Expert and 1433 Specification Required. 1435 8.2. Attribute Type Values 1437 EAP-AKA' shares its attribute space and subtypes with EAP-SIM 1438 [RFC4186] and EAP-AKA [RFC4187]. No new registries are needed. 1440 However, a new Attribute Type value (23) in the non-skippable range 1441 has been assigned for AT_KDF_INPUT (Section 3.1) in the EAP-AKA and 1442 EAP-SIM Parameters registry under Attribute Types. 1444 Also, a new Attribute Type value (24) in the non-skippable range has 1445 been assigned for AT_KDF (Section 3.2). 1447 Finally, a new Attribute Type value (136) in the skippable range has 1448 been assigned for AT_BIDDING (Section 4). 1450 8.3. Key Derivation Function Namespace 1452 IANA has also created a new namespace for EAP-AKA' AT_KDF Key 1453 Derivation Function Values. This namespace exists under the EAP-AKA 1454 and EAP-SIM Parameters registry. The initial contents of this 1455 namespace are given below; new values can be created through the 1456 Specification Required policy [RFC8126]. 1458 Value Description Reference 1459 --------- ---------------------- ------------------------------- 1460 0 Reserved [RFC Editor: Refer to this RFC] 1461 1 EAP-AKA' with CK'/IK' [RFC Editor: Refer to this RFC] 1462 2-65535 Unassigned 1464 9. References 1466 9.1. Normative References 1468 [TS-3GPP.23.003] 1469 3GPP, "3rd Generation Partnership Project; Technical 1470 Specification Group Core Network and Terminals; Numbering, 1471 addressing and identification (Release 15)", 3GPP Draft 1472 Technical Specification 23.003, September 2018. 1474 [TS-3GPP.23.501] 1475 3GPP, "3rd Generation Partnership Project; Technical 1476 Specification Group Services and System Aspects; 3G 1477 Security; Security architecture and procedures for 5G 1478 System; (Release 15)", 3GPP Technical Specification 1479 23.501, September 2018. 1481 [TS-3GPP.24.302] 1482 3GPP, "3rd Generation Partnership Project; Technical 1483 Specification Group Core Network and Terminals; Access to 1484 the 3GPP Evolved Packet Core (EPC) via non-3GPP access 1485 networks; Stage 3; (Release 15)", 3GPP Draft Technical 1486 Specification 24.302, September 2018. 1488 [TS-3GPP.24.501] 1489 3GPP, "3rd Generation Partnership Project; Technical 1490 Specification Group Core Network and Terminals; Access to 1491 the 3GPP Evolved Packet Core (EPC) via non-3GPP access 1492 networks; Stage 3; (Release 15)", 3GPP Draft Technical 1493 Specification 24.501, September 2018. 1495 [TS-3GPP.33.102] 1496 3GPP, "3rd Generation Partnership Project; Technical 1497 Specification Group Services and System Aspects; 3G 1498 Security; Security architecture (Release 15)", 3GPP Draft 1499 Technical Specification 33.102, June 2018. 1501 [TS-3GPP.33.402] 1502 3GPP, "3GPP System Architecture Evolution (SAE); Security 1503 aspects of non-3GPP accesses (Release 15)", 3GPP Draft 1504 Technical Specification 33.402, June 2018. 1506 [TS-3GPP.33.501] 1507 3GPP, "3rd Generation Partnership Project; Technical 1508 Specification Group Services and System Aspects; 3G 1509 Security; Security architecture and procedures for 5G 1510 System (Release 15)", 3GPP Draft Technical Specification 1511 33.501, September 2018. 1513 [FIPS.180-4] 1514 National Institute of Standards and Technology, "Secure 1515 Hash Standard", FIPS PUB 180-4, August 2015, 1516 . 1519 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1520 Hashing for Message Authentication", RFC 2104, 1521 DOI 10.17487/RFC2104, February 1997, . 1524 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1525 Requirement Levels", BCP 14, RFC 2119, 1526 DOI 10.17487/RFC2119, March 1997, . 1529 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 1530 Levkowetz, Ed., "Extensible Authentication Protocol 1531 (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004, 1532 . 1534 [RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication 1535 Protocol Method for 3rd Generation Authentication and Key 1536 Agreement (EAP-AKA)", RFC 4187, DOI 10.17487/RFC4187, 1537 January 2006, . 1539 [RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The 1540 Network Access Identifier", RFC 4282, 1541 DOI 10.17487/RFC4282, December 2005, . 1544 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 1545 Writing an IANA Considerations Section in RFCs", BCP 26, 1546 RFC 8126, DOI 10.17487/RFC8126, June 2017, 1547 . 1549 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1550 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1551 May 2017, . 1553 9.2. Informative References 1555 [TS-3GPP.35.208] 1556 3GPP, "3rd Generation Partnership Project; Technical 1557 Specification Group Services and System Aspects; 3G 1558 Security; Specification of the MILENAGE Algorithm Set: An 1559 example algorithm set for the 3GPP authentication and key 1560 generation functions f1, f1*, f2, f3, f4, f5 and f5*; 1561 Document 4: Design Conformance Test Data (Release 14)", 1562 3GPP Technical Specification 35.208, March 2017. 1564 [FIPS.180-1] 1565 National Institute of Standards and Technology, "Secure 1566 Hash Standard", FIPS PUB 180-1, April 1995, 1567 . 1569 [FIPS.180-2] 1570 National Institute of Standards and Technology, "Secure 1571 Hash Standard", FIPS PUB 180-2, August 2002, 1572 . 1575 [RFC3310] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer 1576 Protocol (HTTP) Digest Authentication Using Authentication 1577 and Key Agreement (AKA)", RFC 3310, DOI 10.17487/RFC3310, 1578 September 2002, . 1580 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, 1581 "Randomness Requirements for Security", BCP 106, RFC 4086, 1582 DOI 10.17487/RFC4086, June 2005, . 1585 [RFC4169] Torvinen, V., Arkko, J., and M. Naslund, "Hypertext 1586 Transfer Protocol (HTTP) Digest Authentication Using 1587 Authentication and Key Agreement (AKA) Version-2", 1588 RFC 4169, DOI 10.17487/RFC4169, November 2005, 1589 . 1591 [RFC4186] Haverinen, H., Ed. and J. Salowey, Ed., "Extensible 1592 Authentication Protocol Method for Global System for 1593 Mobile Communications (GSM) Subscriber Identity Modules 1594 (EAP-SIM)", RFC 4186, DOI 10.17487/RFC4186, January 2006, 1595 . 1597 [RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity 1598 Selection Hints for the Extensible Authentication Protocol 1599 (EAP)", RFC 4284, DOI 10.17487/RFC4284, January 2006, 1600 . 1602 [RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) 1603 Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005, 1604 . 1606 [RFC5113] Arkko, J., Aboba, B., Korhonen, J., Ed., and F. Bari, 1607 "Network Discovery and Selection Problem", RFC 5113, 1608 DOI 10.17487/RFC5113, January 2008, . 1611 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1612 IANA Considerations Section in RFCs", RFC 5226, 1613 DOI 10.17487/RFC5226, May 2008, . 1616 [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible 1617 Authentication Protocol (EAP) Key Management Framework", 1618 RFC 5247, DOI 10.17487/RFC5247, August 2008, 1619 . 1621 [RFC5448] Arkko, J., Lehtovirta, V., and P. Eronen, "Improved 1622 Extensible Authentication Protocol Method for 3rd 1623 Generation Authentication and Key Agreement (EAP-AKA')", 1624 RFC 5448, DOI 10.17487/RFC5448, May 2009, 1625 . 1627 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security 1628 Considerations for the SHA-0 and SHA-1 Message-Digest 1629 Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, 1630 . 1632 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 1633 Morris, J., Hansen, M., and R. Smith, "Privacy 1634 Considerations for Internet Protocols", RFC 6973, 1635 DOI 10.17487/RFC6973, July 2013, . 1638 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 1639 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 1640 2014, . 1642 [I-D.arkko-eap-aka-pfs] 1643 Arkko, J., Norrman, K., and V. Torvinen, "Perfect-Forward 1644 Secrecy for the Extensible Authentication Protocol Method 1645 for Authentication and Key Agreement (EAP-AKA' PFS)", 1646 draft-arkko-eap-aka-pfs-03 (work in progress), October 1647 2018. 1649 [Heist2015] 1650 Scahill, J. and J. Begley, "The great SIM heist", February 1651 2015, in https://firstlook.org/theintercept/2015/02/19/ 1652 great-sim-heist/ . 1654 [MT2012] Mjolsnes, S. and J-K. Tsay, "A vulnerability in the UMTS 1655 and LTE authentication and key agreement protocols", 1656 October 2012, in Proceedings of the 6th international 1657 conference on Mathematical Methods, Models and 1658 Architectures for Computer Network Security: computer 1659 network security. 1661 [BT2013] Beekman, J. and C. Thompson, "Breaking Cell Phone 1662 Authentication: Vulnerabilities in AKA, IMS and Android", 1663 August 2013, in 7th USENIX Workshop on Offensive 1664 Technologies, WOOT '13. 1666 [ZF2005] Zhang, M. and Y. Fang, "Breaking Cell Phone 1667 Authentication: Vulnerabilities in AKA, IMS and Android", 1668 March 2005, IEEE Transactions on Wireless Communications, 1669 Vol. 4, No. 2. 1671 [Basin2018] 1672 Basin, D., Dreier, J., Hirsch, L., Radomirovic, S., Sasse, 1673 R., and V. Stettle, "A Formal Analysis of 5G 1674 Authentication", August 2018, arXiv:1806.10360. 1676 [Arapinis2012] 1677 Arapinis, M., Mancini, L., Ritter, E., Ryan, M., Golde, 1678 N., and R. Borgaonkar, "New Privacy Issues in Mobile 1679 Telephony: Fix and Verification", October 2012, CCS'12, 1680 Raleigh, North Carolina, USA. 1682 Appendix A. Changes from RFC 5448 1684 The changes consist first of all, referring to a newer version of 1685 [TS-3GPP.24.302]. The new version includes an updated definition of 1686 the Network Name field, to include 5G. 1688 Secondly, identifier usage for 5G has been specified in Section 5.3. 1689 Also, the requirements on generating pseudonym usernames and fast re- 1690 authentication identities have been updated from the original 1691 definition in RFC 5448, which referenced RFC 4187. See Section 5. 1693 Thirdly, exported parameters for EAP-AKA' have been defined in 1694 Section 6, as required by [RFC5247], including the definition of 1695 those parameters for both full authentication and fast re- 1696 authentication. 1698 The security, privacy, and pervasive monitoring considerations have 1699 been updated or added. See Section 7. 1701 The references to [RFC2119], [RFC5226], [FIPS.180-1] and [FIPS.180-2] 1702 have been updated to their most recent versions and language in this 1703 document changed accordingly. Similarly, references to all 3GPP 1704 technical specifications have been updated to their 5G (Release 15) 1705 versions or otherwise most recent version when there has not been a 1706 5G-related update. 1708 Finally, a number of editorial clarifications have been made. 1710 Appendix B. Changes from RFC 4187 to RFC 5448 1712 The changes to RFC 4187 relate only to the bidding down prevention 1713 support defined in Section 4. In particular, this document does not 1714 change how the Master Key (MK) is calculated in RFC 4187 (it uses CK 1715 and IK, not CK' and IK'); neither is any processing of the AMF bit 1716 added to RFC 4187. 1718 Appendix C. Changes from Previous Version of This Draft 1720 RFC Editor: Please delete this section at the time of publication. 1722 The -00 version of the working group draft is merely a republication 1723 of an earlier individual draft. 1725 The -01 version of the working group draft clarifies updates 1726 relationship to RFC 4187, clarifies language relating to obsoleting 1727 RFC 5448, clarifies when the 3GPP references are expected to be 1728 stable, updates several past references to their more recently 1729 published versions, specifies what identifiers should be used in key 1730 derivation formula for 5G, specifies how to construct the network 1731 name in manner that is compatible with both 5G and previous versions, 1732 and has some minor editorial changes. 1734 The -02 version of the working group draft added specification of 1735 peer identity usage in EAP-AKA', added requirements on the generation 1736 of pseudonym and fast re-authentication identifiers, specified the 1737 format of 5G-identifiers when they are used within EAP-AKA', defined 1738 privacy and pervasive surveillance considerations, clarified when 5G- 1739 related procedures apply, specified what Peer-Id value is exported 1740 when no AT_IDENTITY is exchanged within EAP-AKA', and made a number 1741 of other clarifications and editorial improvements. The security 1742 considerations section also includes a summary of vulnerabilities 1743 brought up in the context of AKA or EAP-AKA', and discusses their 1744 applicability and impacts in EAP-AKA'. 1746 The -03 version of the working group draft corrected some typos, 1747 referred to the 3GPP specifications for the SUPI and SUCI formats, 1748 updated some of the references to newer versions, and reduced the 1749 strength of some of the recommendations in the security 1750 considerations section from keyword level to normal language (as they 1751 are just deployment recommendations). 1753 The -04 version of the working group draft rewrote the abstract and 1754 some of the introduction, corrected some typos, added sentence to the 1755 abstract about obsoleting RFC 5448, clarified the use of the language 1756 when referring to AT_KDF values vs. AT_KDF attribute number, provided 1757 guidance on random number generation, clarified the dangers relating 1758 to the use of permanent user identities such as IMSIs, aligned the 1759 key derivation function/mechanism terminology, aligned the key 1760 derivation/generation terminology, aligned the octet/byte 1761 terminology, clarified the text regarding strength of SHA-256, added 1762 some cross references between sections, instructed IANA to change 1763 registries to point to this RFC rather than RFC 5448, and changed 1764 Pasi's listed affiliation. 1766 Appendix D. Importance of Explicit Negotiation 1768 Choosing between the traditional and revised AKA key derivation 1769 functions is easy when their use is unambiguously tied to a 1770 particular radio access network, e.g., Long Term Evolution (LTE) as 1771 defined by 3GPP or evolved High Rate Packet Data (eHRPD) as defined 1772 by 3GPP2. There is no possibility for interoperability problems if 1773 this radio access network is always used in conjunction with new 1774 protocols that cannot be mixed with the old ones; clients will always 1775 know whether they are connecting to the old or new system. 1777 However, using the new key derivation functions over EAP introduces 1778 several degrees of separation, making the choice of the correct key 1779 derivation functions much harder. Many different types of networks 1780 employ EAP. Most of these networks have no means to carry any 1781 information about what is expected from the authentication process. 1782 EAP itself is severely limited in carrying any additional 1783 information, as noted in [RFC4284] and [RFC5113]. Even if these 1784 networks or EAP were extended to carry additional information, it 1785 would not affect millions of deployed access networks and clients 1786 attaching to them. 1788 Simply changing the key derivation functions that EAP-AKA [RFC4187] 1789 uses would cause interoperability problems with all of the existing 1790 implementations. Perhaps it would be possible to employ strict 1791 separation into domain names that should be used by the new clients 1792 and networks. Only these new devices would then employ the new key 1793 derivation function. While this can be made to work for specific 1794 cases, it would be an extremely brittle mechanism, ripe to result in 1795 problems whenever client configuration, routing of authentication 1796 requests, or server configuration does not match expectations. It 1797 also does not help to assume that the EAP client and server are 1798 running a particular release of 3GPP network specifications. Network 1799 vendors often provide features from future releases early or do not 1800 provide all features of the current release. And obviously, there 1801 are many EAP and even some EAP-AKA implementations that are not 1802 bundled with the 3GPP network offerings. In general, these 1803 approaches are expected to lead to hard-to-diagnose problems and 1804 increased support calls. 1806 Appendix E. Test Vectors 1808 Test vectors are provided below for four different cases. The test 1809 vectors may be useful for testing implementations. In the first two 1810 cases, we employ the MILENAGE algorithm and the algorithm 1811 configuration parameters (the subscriber key K and operator algorithm 1812 variant configuration value OP) from test set 19 in [TS-3GPP.35.208]. 1814 The last two cases use artificial values as the output of AKA, and is 1815 useful only for testing the computation of values within EAP-AKA', 1816 not AKA itself. 1818 Case 1 1820 The parameters for the AKA run are as follows: 1822 Identity: "0555444333222111" 1824 Network name: "WLAN" 1826 RAND: 81e9 2b6c 0ee0 e12e bceb a8d9 2a99 dfa5 1828 AUTN: bb52 e91c 747a c3ab 2a5c 23d1 5ee3 51d5 1830 IK: 9744 871a d32b f9bb d1dd 5ce5 4e3e 2e5a 1832 CK: 5349 fbe0 9864 9f94 8f5d 2e97 3a81 c00f 1834 RES: 28d7 b0f2 a2ec 3de5 1836 Then the derived keys are generated as follows: 1838 CK': 0093 962d 0dd8 4aa5 684b 045c 9edf fa04 1840 IK': ccfc 230c a74f cc96 c0a5 d611 64f5 a76c 1842 K_encr: 766f a0a6 c317 174b 812d 52fb cd11 a179 1844 K_aut: 0842 ea72 2ff6 835b fa20 3249 9fc3 ec23 1845 c2f0 e388 b4f0 7543 ffc6 77f1 696d 71ea 1847 K_re: cf83 aa8b c7e0 aced 892a cc98 e76a 9b20 1848 95b5 58c7 795c 7094 715c b339 3aa7 d17a 1850 MSK: 67c4 2d9a a56c 1b79 e295 e345 9fc3 d187 1851 d42b e0bf 818d 3070 e362 c5e9 67a4 d544 1852 e8ec fe19 358a b303 9aff 03b7 c930 588c 1853 055b abee 58a0 2650 b067 ec4e 9347 c75a 1855 EMSK: f861 703c d775 590e 16c7 679e a387 4ada 1856 8663 11de 2907 64d7 60cf 76df 647e a01c 1857 313f 6992 4bdd 7650 ca9b ac14 1ea0 75c4 1858 ef9e 8029 c0e2 90cd bad5 638b 63bc 23fb 1860 Case 2 1862 The parameters for the AKA run are as follows: 1864 Identity: "0555444333222111" 1866 Network name: "HRPD" 1868 RAND: 81e9 2b6c 0ee0 e12e bceb a8d9 2a99 dfa5 1870 AUTN: bb52 e91c 747a c3ab 2a5c 23d1 5ee3 51d5 1872 IK: 9744 871a d32b f9bb d1dd 5ce5 4e3e 2e5a 1874 CK: 5349 fbe0 9864 9f94 8f5d 2e97 3a81 c00f 1876 RES: 28d7 b0f2 a2ec 3de5 1878 Then the derived keys are generated as follows: 1880 CK': 3820 f027 7fa5 f777 32b1 fb1d 90c1 a0da 1882 IK': db94 a0ab 557e f6c9 ab48 619c a05b 9a9f 1884 K_encr: 05ad 73ac 915f ce89 ac77 e152 0d82 187b 1886 K_aut: 5b4a caef 62c6 ebb8 882b 2f3d 534c 4b35 1887 2773 37a0 0184 f20f f25d 224c 04be 2afd 1889 K_re: 3f90 bf5c 6e5e f325 ff04 eb5e f653 9fa8 1890 cca8 3981 94fb d00b e425 b3f4 0dba 10ac 1892 MSK: 87b3 2157 0117 cd6c 95ab 6c43 6fb5 073f 1893 f15c f855 05d2 bc5b b735 5fc2 1ea8 a757 1894 57e8 f86a 2b13 8002 e057 5291 3bb4 3b82 1895 f868 a961 17e9 1a2d 95f5 2667 7d57 2900 1897 EMSK: c891 d5f2 0f14 8a10 0755 3e2d ea55 5c9c 1898 b672 e967 5f4a 66b4 bafa 0273 79f9 3aee 1899 539a 5979 d0a0 042b 9d2a e28b ed3b 17a3 1900 1dc8 ab75 072b 80bd 0c1d a612 466e 402c 1902 Case 3 1904 The parameters for the AKA run are as follows: 1906 Identity: "0555444333222111" 1908 Network name: "WLAN" 1910 RAND: e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 1912 AUTN: a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 1914 IK: b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 1916 CK: c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 1918 RES: d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 1920 Then the derived keys are generated as follows: 1922 CK': cd4c 8e5c 68f5 7dd1 d7d7 dfd0 c538 e577 1924 IK': 3ece 6b70 5dbb f7df c459 a112 80c6 5524 1926 K_encr: 897d 302f a284 7416 488c 28e2 0dcb 7be4 1928 K_aut: c407 00e7 7224 83ae 3dc7 139e b0b8 8bb5 1929 58cb 3081 eccd 057f 9207 d128 6ee7 dd53 1931 K_re: 0a59 1a22 dd8b 5b1c f29e 3d50 8c91 dbbd 1932 b4ae e230 5189 2c42 b6a2 de66 ea50 4473 1934 MSK: 9f7d ca9e 37bb 2202 9ed9 86e7 cd09 d4a7 1935 0d1a c76d 9553 5c5c ac40 a750 4699 bb89 1936 61a2 9ef6 f3e9 0f18 3de5 861a d1be dc81 1937 ce99 1639 1b40 1aa0 06c9 8785 a575 6df7 1939 EMSK: 724d e00b db9e 5681 87be 3fe7 4611 4557 1940 d501 8779 537e e37f 4d3c 6c73 8cb9 7b9d 1941 c651 bc19 bfad c344 ffe2 b52c a78b d831 1942 6b51 dacc 5f2b 1440 cb95 1552 1cc7 ba23 1944 Case 4 1946 The parameters for the AKA run are as follows: 1948 Identity: "0555444333222111" 1950 Network name: "HRPD" 1952 RAND: e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 1954 AUTN: a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 1956 IK: b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 1958 CK: c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 1960 RES: d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 1962 Then the derived keys are generated as follows: 1964 CK': 8310 a71c e6f7 5488 9613 da8f 64d5 fb46 1966 IK': 5adf 1436 0ae8 3819 2db2 3f6f cb7f 8c76 1968 K_encr: 745e 7439 ba23 8f50 fcac 4d15 d47c d1d9 1970 K_aut: 3e1d 2aa4 e677 025c fd86 2a4b e183 61a1 1971 3a64 5765 5714 63df 833a 9759 e809 9879 1973 K_re: 99da 835e 2ae8 2462 576f e651 6fad 1f80 1974 2f0f a119 1655 dd0a 273d a96d 04e0 fcd3 1976 MSK: c6d3 a6e0 ceea 951e b20d 74f3 2c30 61d0 1977 680a 04b0 b086 ee87 00ac e3e0 b95f a026 1978 83c2 87be ee44 4322 94ff 98af 26d2 cc78 1979 3bac e75c 4b0a f7fd feb5 511b a8e4 cbd0 1981 EMSK: 7fb5 6813 838a dafa 99d1 40c2 f198 f6da 1982 cebf b6af ee44 4961 1054 02b5 08c7 f363 1983 352c b291 9644 b504 63e6 a693 5415 0147 1984 ae09 cbc5 4b8a 651d 8787 a689 3ed8 536d 1986 Appendix F. Contributors 1988 The test vectors in Appendix C were provided by Yogendra Pal and 1989 Jouni Malinen, based on two independent implementations of this 1990 specification. 1992 Jouni Malinen provided suggested text for Section 6. John Mattsson 1993 provided much of the text for Section 7.1. Karl Norrman was the 1994 source of much of the information in Section 7.2. 1996 Appendix G. Acknowledgments 1998 The authors would like to thank Guenther Horn, Joe Salowey, Mats 1999 Naslund, Adrian Escott, Brian Rosenberg, Laksminath Dondeti, Ahmad 2000 Muhanna, Stefan Rommer, Miguel Garcia, Jan Kall, Ankur Agarwal, Jouni 2001 Malinen, John Mattsson, Jesus De Gregorio, Brian Weis, Russ Housley, 2002 Alfred Hoenes, Anand Palanigounder, and Mohit Sethi for their in- 2003 depth reviews and interesting discussions in this problem space. 2005 Authors' Addresses 2007 Jari Arkko 2008 Ericsson 2009 Jorvas 02420 2010 Finland 2012 Email: jari.arkko@piuha.net 2014 Vesa Lehtovirta 2015 Ericsson 2016 Jorvas 02420 2017 Finland 2019 Email: vesa.lehtovirta@ericsson.com 2021 Vesa Torvinen 2022 Ericsson 2023 Jorvas 02420 2024 Finland 2026 Email: vesa.torvinen@ericsson.com 2028 Pasi Eronen 2029 Independent 2030 Finland 2032 Email: pe@iki.fi