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Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) -- Obsolete informational reference (is this intentional?): RFC 4306 (Obsoleted by RFC 5996) Summary: 2 errors (**), 0 flaws (~~), 1 warning (==), 8 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 Updates: 4187 (if approved) Ericsson 5 Intended status: Informational P. Eronen 6 Expires: May 22, 2009 Nokia 7 November 18, 2008 9 Improved Extensible Authentication Protocol Method for 3rd Generation 10 Authentication and Key Agreement (EAP-AKA') 11 draft-arkko-eap-aka-kdf-10 13 Status of this Memo 15 By submitting this Internet-Draft, each author represents that any 16 applicable patent or other IPR claims of which he or she is aware 17 have been or will be disclosed, and any of which he or she becomes 18 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on May 22, 2009. 38 Abstract 40 This specification defines a new EAP method, EAP-AKA', a small 41 revision of the EAP-AKA method. The change is a new key derivation 42 function that binds the keys derived within the method to the name of 43 the access network. The new key derivation mechanism has been 44 defined in the 3rd Generation Partnership Project (3GPP). This 45 specification allows its use in EAP in an interoperable manner. In 46 addition, EAP-AKA' employs SHA-256 instead of SHA-1. 48 This specification also updates RFC 4187 EAP-AKA to prevent bidding 49 down attacks from EAP-AKA'. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2. Requirements language . . . . . . . . . . . . . . . . . . . . 4 55 3. EAP-AKA' . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 3.1. AT_KDF_INPUT . . . . . . . . . . . . . . . . . . . . . . . 6 57 3.2. AT_KDF . . . . . . . . . . . . . . . . . . . . . . . . . . 8 58 3.3. Key Generation . . . . . . . . . . . . . . . . . . . . . . 10 59 3.4. Hash Functions . . . . . . . . . . . . . . . . . . . . . . 12 60 3.4.1. PRF' . . . . . . . . . . . . . . . . . . . . . . . . . 12 61 3.4.2. AT_MAC . . . . . . . . . . . . . . . . . . . . . . . . 12 62 3.4.3. AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . . 12 63 4. Bidding Down Prevention for EAP-AKA . . . . . . . . . . . . . 13 64 5. Security Considerations . . . . . . . . . . . . . . . . . . . 14 65 5.1. Security Properties of Binding Network Names . . . . . . . 17 66 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 67 6.1. Type Value . . . . . . . . . . . . . . . . . . . . . . . . 18 68 6.2. Attribute Type Values . . . . . . . . . . . . . . . . . . 18 69 6.3. Key Derivation Function Namespace . . . . . . . . . . . . 19 70 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 71 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 72 8.1. Normative References . . . . . . . . . . . . . . . . . . . 19 73 8.2. Informative References . . . . . . . . . . . . . . . . . . 20 74 Appendix A. Changes from RFC 4187 . . . . . . . . . . . . . . . . 21 75 Appendix B. Importance of Explicit Negotiation . . . . . . . . . 21 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 77 Intellectual Property and Copyright Statements . . . . . . . . . . 23 79 1. Introduction 81 This specification defines a new Extensible Authentication Protocol 82 (EAP)[RFC3748] method, EAP-AKA', a small revision of the EAP-AKA 83 method originally defined in [RFC4187]. What is new in EAP-AKA' is 84 that it has a new key derivation function specified in [3GPP.33.402]. 85 This function binds the keys derived within the method to the name of 86 the access network. This limits the effects of compromised access 87 network nodes and keys. This specification defines the EAP 88 encapsulation for AKA when the new key derivation mechanism is in 89 use. 91 3GPP has defined a number of applications for the revised AKA 92 mechanism, some based on native encapsulation of AKA over 3GPP radio 93 access networks and others based on the use of EAP. 95 For making the new key derivation mechanisms usable in EAP-AKA 96 additional protocol mechanisms are necessary. Given that RFC 4187 97 calls for the use of CK (the encryption key) and IK (the integrity 98 key) from AKA, existing implementations continue to use these. Any 99 change of the key derivation must be unambiguous to both sides in the 100 protocol. That is, it must not be possible to accidentally connect 101 old equipment to new equipment and get the key derivation wrong or 102 attempt to use wrong keys without getting a proper error message. 103 The change must also be secure against bidding down attacks that 104 attempt to force the participants to use the least secure mechanism. 106 This specification therefore introduces a variant of the EAP-AKA 107 method, called EAP-AKA'. This method can employ the derived keys CK' 108 and IK' from the 3GPP specification and updates the used hash 109 function to SHA-256 [FIPS.180-2.2002]. But it is otherwise 110 equivalent to RFC 4187. Given that a different EAP method Type value 111 is used for EAP-AKA and EAP-AKA', a mutually supported method may be 112 negotiated using the standard mechanisms in EAP [RFC3748]. 114 Note: Appendix B explains why it is important to be explicit about 115 the change of semantics for the keys, and why other approaches 116 would lead to severe interoperability problems. 118 The rest of this specification is structured as follows. Section 3 119 defines the EAP-AKA' method. Section 4 adds support to EAP-AKA to 120 prevent bidding down attacks from EAP-AKA'. Section 5 explains the 121 security differences between EAP-AKA and EAP-AKA'. Section 6 122 describes the IANA considerations and Appendix A explains what 123 updates to RFC 4187 EAP-AKA have been made in this specification. 124 Finally, Appendix B explains some of the design rationale for 125 creating EAP-AKA'. 127 Editor's Note: The publication of this RFC depends on its 128 normative references [3GPP.24.302], [3GPP.33.102], and 129 [3GPP.33.402] from 3GPP reaching their final Release 8 status at 130 3GPP. This is expected to happen shortly. The RFC Editor should 131 check with the 3GPP liaisons that this has happened. RFC Editor: 132 Please delete this note upon publication of this specification as 133 an RFC. 135 2. Requirements language 137 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 138 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 139 document are to be interpreted as described in [RFC2119]. 141 3. EAP-AKA' 143 EAP-AKA' is a new EAP method that follows the EAP-AKA specification 144 [RFC4187] in all respects except the following: 146 o It uses the Type code TBA1 BY IANA, not 23 which is used by EAP- 147 AKA. 149 o It carries the AT_KDF_INPUT attribute, as defined in Section 3.1 150 to ensure that both the peer and server know the name of the 151 access network. 153 o It supports key derivation function negotiation via the AT_KDF 154 attribute (Section 3.2), to allow for future extensions. 156 o It calculates keys as defined in Section 3.3, not as defined in 157 EAP-AKA. 159 o It employs SHA-256 [FIPS.180-2.2002], not SHA-1 [FIPS.180-1.1995] 160 (Section 3.4). 162 Figure 1 shows an example of the authentication process. Each 163 message AKA'-Challenge and so on represents the corresponding message 164 from EAP-AKA, but with EAP-AKA' Type code. The definition of these 165 messages, along with the definition of attributes AT_RAND, AT_AUTN, 166 AT_MAC, and AT_RES can be found in [RFC4187]. 168 Peer Server 169 | EAP-Request/Identity | 170 |<------------------------------------------------------| 171 | | 172 | EAP-Response/Identity | 173 | (Includes user's Network Access Identifier, NAI) | 174 |------------------------------------------------------>| 175 | +-------------------------------------------------+ 176 | | Server determines the network name and ensures | 177 | | that the given access network is authorized to | 178 | | use the claimed name. The server then runs the | 179 | | AKA' algorithms generating RAND and AUTN, | 180 | | derives session keys from CK' and IK'. RAND and | 181 | | AUTN are sent as AT_RAND and AT_AUTN attributes,| 182 | | whereas the network name is transported in the | 183 | | AT_KDF_INPUT attribute. AT_KDF signals the used | 184 | | key derivation function. The session keys are | 185 | | used in creating the AT_MAC attribute. | 186 | +-------------------------------------------------+ 187 | EAP-Request/AKA'-Challenge | 188 | (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC)| 189 |<------------------------------------------------------| 190 +-----------------------------------------------------+ | 191 | The peer determines what the network name should be,| | 192 | based on, e.g., what access technology it is using.| | 193 | The peer also retrieves the network name sent by | | 194 | the network from the AT_KDF_INPUT attribute. The | | 195 | two names are compared for discrepancies, and if | | 196 | necessary, the authentication is aborted. Otherwise,| | 197 | the network name from AT_KDF_INPUT attribute is | | 198 | used in running the AKA' algorithms, verifying AUTN | | 199 | from AT_AUTN and MAC from AT_MAC attributes. The | | 200 | peer then generates RES. The peer also derives | | 201 | session keys from CK'/IK'. The AT_RES and AT_MAC | | 202 | attributes are constructed. | | 203 +-----------------------------------------------------+ | 204 | EAP-Response/AKA'-Challenge | 205 | (AT_RES, AT_MAC) | 206 |------------------------------------------------------>| 207 | +-------------------------------------------------+ 208 | | Server checks the RES and MAC values received | 209 | | in AT_RES and AT_MAC, respectively. Success | 210 | | requires both to be found correct. | 211 | +-------------------------------------------------+ 212 | EAP-Success | 213 |<------------------------------------------------------| 215 Figure 1: EAP-AKA' Authentication Process 217 3.1. AT_KDF_INPUT 219 The format of the AT_KDF_INPUT attribute is shown below. 221 0 1 2 3 222 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 1 223 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 224 | AT_KDF_INPUT | Length | Actual Network Name Length | 225 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 226 | | 227 . Network Name . 228 . . 229 | | 230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 232 The fields are as follows: 234 AT_KDF_INPUT 236 This is set to TBA2 BY IANA. 238 Length 240 The length of the attribute, calculated as defined in [RFC4187] 241 Section 8.1. 243 Actual Network Name Length 245 This a 2-byte actual length field, needed due to the requirement 246 that the previous field is expressed in multiples of 4 bytes per 247 the usual EAP-AKA rules. The Actual Network Name Length field 248 provides the length of the Network Name in bytes. 250 Network Name 252 This field contains the network name of the access network for 253 which the authentication is being performed. The name does not 254 include any terminating null characters. Because the length of 255 the entire attribute must be a multiple of 4 bytes, the sender 256 pads the name with one, two, or three bytes of all zero bits when 257 necessary. 259 Only the server sends the AT_KDF_INPUT attribute. Per [3GPP.33.402], 260 the server always verifies the authorization of a given access 261 network to use a particular name before sending it to the peer over 262 EAP-AKA'. The value of the AT_KDF_INPUT attribute from the server 263 MUST be non-empty. If it is empty, the peer behaves as if AUTN had 264 been incorrect and authentication fails. See Section 3 and Figure 3 265 of [RFC4187] for an overview of how authentication failures are 266 handled. 268 In addition, the peer MAY check the received value against its own 269 understanding of the network name. Upon detecting a discrepancy, the 270 peer either warns the user and continues, or fails the authentication 271 process. More specifically, the peer SHOULD have a configurable 272 policy which it can follow under these circumstances. If the policy 273 indicates that it can continue, the peer SHOULD log a warning message 274 or display it to the user. If the peer chooses to proceed, it MUST 275 use the network name as received in the AT_KDF_INPUT attribute. If 276 the policy indicates that the authentication should fail, the peer 277 behaves as if AUTN had been incorrect and authentication fails. 279 The Network Name field contains an UTF-8 string. This string MUST be 280 constructed as specified in [3GPP.24.302] for "Access Network 281 Identity". The string is structured as fields separated by colons 282 (:). The algorithms and mechanisms to construct the identity string 283 depend on the used access technology. 285 On the network side, the network name construction is a configuration 286 issue in an access network and an authorization check in the 287 authentication server. On the peer, the network name is constructed 288 based on the local observations. For instance, the peer knows which 289 access technology it is using on the link, it can see information in 290 a link layer beacon, and so on. The construction rules specify how 291 this information maps to an access network name. Typically, the 292 network name consists of the name of the access technology, or name 293 of the access technology followed by some operator identifier that 294 was advertised in a link layer beacon. In all cases [3GPP.24.302] is 295 the normative specification for the construction in both the network 296 and peer side. If the peer policy allows running EAP-AKA' over an 297 access technology for which that specification does not provide 298 network name construction rules, the peer SHOULD rely only on the 299 information from the AT_KDF_INPUT attribute and not perform a 300 comparison. 302 If a comparison of the locally determined network name and the one 303 received over EAP-AKA' is performed on the peer, it MUST be done as 304 follows. First, each name is broken down to the fields separated by 305 colon. If one of the names has more colons and fields than the other 306 one, the additional fields are ignored. The remaining sequences of 307 fields are compared, and they match only if they are equal character 308 by character. This algorithm allows a prefix match where the peer 309 would be able to match "", "FOO", and "FOO:BAR" against the value 310 "FOO:BAR" received from the server. This capability is important in 311 order to allow possible updates to the specifications that dictate 312 how the network names are constructed. For instance, if a peer knows 313 that it is running on access technology "FOO" it can use the string 314 "FOO" even if the server uses an additional, more accurate 315 description "FOO:BAR" that contains more information. 317 The allocation procedures in [3GPP.24.302] ensure that conflicts 318 potentially arising from using the same name in different types of 319 networks are avoided. The specification also has detailed rules 320 about how a client can determine these based on information available 321 to the client, such as the type of protocol used to attach to the 322 network, beacons sent out by the network, and so on. Information 323 that the client cannot directly observe (such as the type or version 324 of the home network) is not used by this algorithm. 326 The AT_KDF_INPUT attribute MUST be sent and processed as explained 327 above when AT_KDF attribute has the value 1. Future definitions of 328 new AT_KDF values MUST define how this attribute is sent and 329 processed. 331 3.2. AT_KDF 333 AT_KDF is an attribute that the server uses to reference a specific 334 key derivation function. It offers a negotiation capability that can 335 be useful for future evolution of the key derivation functions. 337 The format of the AT_KDF attribute is shown below. 339 0 1 2 3 340 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 1 341 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 342 | AT_KDF | Length | Key Derivation Function | 343 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 345 The fields are as follows: 347 AT_KDF 349 This is set to TBA3 BY IANA. 351 Length 353 The length of the attribute, MUST be set to 1. 355 Key Derivation Function 357 An enumerated value representing the key derivation function that 358 the server (or peer) wishes to use. Value 1 represents the 359 default key derivation function for EAP-AKA', i.e., employing CK' 360 and IK' as defined in Section 3.3. 362 Servers MUST send one or more AT_KDF attributes in the EAP-Request/ 363 AKA'-Challenge message. These attributes represent the desired 364 functions ordered by preference, the most preferred function being 365 the first attribute. 367 Upon receiving a set of these attributes, if the peer supports and is 368 willing to use the key derivation function indicated by the first 369 attribute, the function is taken into use without any further 370 negotiation. However, if the peer does not support this function or 371 is unwilling to use it, it responds with the EAP-Response/ 372 AKA'-Challenge message that contains only one attribute, AT_KDF with 373 the value set to the selected alternative. If there is no suitable 374 alternative, the peer behaves as if AUTN had been incorrect and 375 authentication fails (see Figure 3 of [RFC4187]). The peer fails the 376 authentication also if there are any duplicate values within the list 377 of AT_KDF attributes (except where the duplication is due to a 378 request to change the key derivation function; see below for further 379 information). 381 Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the 382 peer, the server checks that the suggested AT_KDF value was one of 383 the alternatives in its offer. The first AT_KDF value in the message 384 from the server is not a valid alternative. If the peer has replied 385 with the first AT_KDF value, the server behaves as if AT_MAC of the 386 response had been incorrect and fails the authentication. For an 387 overview of the failed authentication process in the server side, see 388 Section 3 and Figure 2 in [RFC4187]. Otherwise, the server re-sends 389 the EAP-Response/AKA'-Challenge message, but adds the selected 390 alternative to the beginning of the list of AT_KDF attributes, and 391 retains the entire list following it. Note that this means that the 392 selected alternative appears twice in the set of AT_KDF values. 393 Responding to the peer's request to change the key derivation 394 function is the only legal situation where such duplication may 395 occur. 397 When the peer receives the new EAP-Request/AKA'-Challenge message, it 398 MUST check that the requested change, and only the requested change 399 occurred in the list of AT_KDF attributes. If yes, it continues. If 400 not, it behaves as if AT_MAC had been incorrect and fails the 401 authentication. If the peer receives multiple EAP-Request/ 402 AKA'-Challenge messages with differing AT_KDF attributes without 403 having requested negotiation, the peer MUST behave as if AT_MAC had 404 been incorrect and fail the authentication. 406 3.3. Key Generation 408 Both the peer and server MUST derive the keys as follows. 410 AT_KDF set to 1 412 In this case MK is derived and used as follows: 414 MK = PRF'(IK'|CK',"EAP-AKA'"|Identity) 415 K_encr = MK[0..127] 416 K_aut = MK[128..383] 417 K_re = MK[384..639] 418 MSK = MK[640..1151] 419 EMSK = MK[1152..1663] 421 Here [n..m] denotes the substring from bit n to m. PRF' is a new 422 pseudo random function specified in Section 3.4. The 1664 first 423 bits from its output are used for K_encr (encryption key, 128 424 bits), K_aut (authentication key, 256 bits), K_re (re- 425 authentication key, 256 bits), MSK (Master Session Key, 512 bits) 426 and EMSK (Extended Master Session Key, 512 bits). These keys are 427 used by the subsequent EAP-AKA' process. K_encr is used by the 428 AT_ENCR_DATA attribute, and K_aut by the AT_MAC attribute. K_re 429 is used later in this section. MSK and EMSK are outputs from a 430 successful EAP method run [RFC3748]. 432 IK' and CK' are derived as specified in [3GPP.33.402]. The 433 functions that derive IK' and CK' take the following parameters: 434 CK and IK produced by the AKA algorithm, and value of the Network 435 Name field (without length or padding) from AT_KDF_INPUT. 437 The value "EAP-AKA'" is an eight characters long ASCII string. It 438 is used as is, without any trailing NUL characters. 440 Identity is the peer identity as specified in Section 7 of 441 [RFC4187]. 443 When the server creates an AKA challenge and corresponding AUTN, 444 CK, CK', IK, and IK' values, it MUST set the AMF separation bit to 445 1 in the AKA algorithm [3GPP.33.102]. Similarly, the peer MUST 446 check that the AMF separation bit set is to 1. If the bit is not 447 set to 1, the peer behaves as if the AUTN had been incorrect and 448 fails the authentication. 450 On fast re-authentication, the following keys are calculated: 452 MK = PRF'(K_re,"EAP-AKA' re-auth"|Identity|counter|NONCE_S) 453 MSK = MK[0..511] 454 EMSK = MK[512..1023] 456 MSK and EMSK are the resulting 512 bit keys, taking the first 1024 457 bits from the result of PRF'. Note that K_encr and K_aut are not 458 re-derived on fast re-authentication. K_re is the re- 459 authentication key from the preceding full authentication and 460 stays unchanged over any fast re-authentication(s) that may happen 461 based on it. The value "EAP-AKA' re-auth" is a sixteen characters 462 long ASCII string, again represented without any trailing NUL 463 characters. Identity is the fast re-authentication identity, 464 counter is the value from the AT_COUNTER attribute, NONCE_S is the 465 nonce value from the AT_NONCE_S attribute, all as specified in 466 Section 7 of [RFC4187]. To prevent the use of compromised keys on 467 other places, it is forbidden to change the network name when 468 going from the full to the fast re-authentication process. The 469 peer SHOULD NOT attempt fast re-authentication when it knows that 470 the network name in the current access network is different from 471 the one in the initial, full authentication. Upon seeing a re- 472 authentication request with a changed network name, the server 473 SHOULD behave as if the re-authentication identifier had been 474 unrecognized and fall back to full authentication. The server 475 observes the change in the name by comparing where the fast re- 476 authentication and full authentication EAP transactions were 477 received from at the Authentication, Authorization, and Accounting 478 (AAA) protocol level. 480 AT_KDF has any other value 482 Future variations of key derivation functions may be defined, and 483 they will be represented by new values of AT_KDF. If the peer 484 does not recognize the value it cannot calculate the keys and 485 behaves as explained in Section 3.2. 487 AT_KDF is missing 489 The peer behaves as if the AUTN had been incorrect and MUST fail 490 the authentication. 492 If the peer supports a given key derivation function but is unwilling 493 to perform it for policy reasons, it refuses to calculate the keys 494 and behaves as explained in Section 3.2. 496 3.4. Hash Functions 498 EAP-AKA' uses SHA-256 [FIPS.180-2.2002], not SHA-1 [FIPS.180-1.1995] 499 as in EAP-AKA. This requires a change to the pseudo random function 500 (PRF) as well as the AT_MAC and AT_CHECKCODE attributes. 502 3.4.1. PRF' 504 The PRF' construction is the same one as IKEv2 uses (see Section 2.13 505 in [RFC4306]). The function takes two arguments. K is a 256 bit 506 value and S is an octet string of arbitrary length. PRF' is defined 507 as follows: 509 PRF'(K,S) = T1 | T2 | T3 | T4 | ... 511 where: 512 T1 = HMAC-SHA-256 (K, S | 0x01) 513 T2 = HMAC-SHA-256 (K, T1 | S | 0x02) 514 T3 = HMAC-SHA-256 (K, T2 | S | 0x03) 515 T4 = HMAC-SHA-256 (K, T3 | S | 0x04) 516 ... 518 PRF' produces as many bits of output as is needed. HMAC-SHA-256 is 519 the application of HMAC [RFC2104] to SHA-256. 521 3.4.2. AT_MAC 523 When used within EAP-AKA', the AT_MAC attribute is changed as 524 follows. The MAC algorithm is HMAC-SHA-256-128, a keyed hash value. 525 The HMAC-SHA-256-128 value is obtained from the 32-byte HMAC-SHA-256 526 value by truncating the output to the first 16 bytes. Hence, the 527 length of the MAC is 16 bytes. 529 Otherwise the use of AT_MAC in EAP-AKA' follows Section 10.15 of 530 [RFC4187]. 532 3.4.3. AT_CHECKCODE 534 When used within EAP-AKA', the AT_CHECKCODE attribute is changed as 535 follows. First, a 32 byte value is needed to accommodate a 256 bit 536 hash output: 538 0 1 2 3 539 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 1 540 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 541 | AT_CHECKCODE | Length | Reserved | 542 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 543 | | 544 | Checkcode (0 or 32 bytes) | 545 | | 546 | | 547 | | 548 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 550 Second, the checkcode is a hash value, calculated with SHA-256 551 [FIPS.180-2.2002], over the data specified in Section 10.13 of 552 [RFC4187]. 554 4. Bidding Down Prevention for EAP-AKA 556 As discussed in [RFC3748], negotiation of methods within EAP is 557 insecure. That is, a man-in-the-middle attacker may force the 558 endpoints to use a method that is not the strongest one they both 559 support. This is a problem, as we expect EAP-AKA and EAP-AKA' to be 560 negotiated via EAP. 562 In order to prevent such attacks, this RFC specifies a new mechanism 563 for EAP-AKA that allows the endpoints to securely discover the 564 capabilities of each other. This mechanism comes in the form of the 565 AT_BIDDING attribute. This allows both endpoints to communicate 566 their desire and support for EAP-AKA' when exchanging EAP-AKA 567 messages. This attribute is not included in EAP-AKA' messages as 568 defined in this RFC. It is only included in EAP-AKA messages. This 569 is based on the assumption that EAP-AKA' is always preferable (see 570 Section 5). If during the EAP-AKA authentication process it is 571 discovered that both endpoints would have been able to use EAP-AKA', 572 the authentication process SHOULD be aborted, as a bidding down 573 attack may have happened. 575 The format of the AT_BIDDING attribute is shown below. 577 0 1 2 3 578 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 1 579 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 580 | AT_BIDDING | Length |D| Reserved | 581 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 583 The fields are as follows: 585 AT_BIDDING 587 This is set to TBA4 BY IANA. 589 Length 591 The length of the attribute, MUST be set to 1. 593 D 595 This bit is set to 1 if the sender does support EAP-AKA', is 596 willing to use it, and prefers it over EAP-AKA. Otherwise it 597 should be set to 0. 599 Reserved 601 This field MUST be set to zero when sent and ignored on receipt. 603 The server sends this attribute in the EAP-Request/AKA-Challenge 604 message. If the peer supports EAP-AKA', it compares the received 605 value to its own capabilities. If it turns out that both the server 606 and peer would have been able to use EAP-AKA' and preferred it over 607 EAP-AKA, the peer behaves as if AUTN had been incorrect, and fails 608 the authentication (see Figure 3 of [RFC4187]). A peer not 609 supporting EAP-AKA' will simply ignore this attribute. In all cases, 610 the attribute is protected by the integrity mechanisms of EAP-AKA, so 611 it cannot be removed by a man-in-the-middle attacker. 613 Note that we assume (Section 5) that EAP-AKA' is always stronger than 614 EAP-AKA. As a result, there is no need to prevent bidding "down" 615 attacks in the other direction, i.e., attackers forcing the endpoints 616 to use EAP-AKA'. 618 5. Security Considerations 620 A summary of the security properties of EAP-AKA' follows. These 621 properties are very similar to those in EAP-AKA. We assume that SHA- 622 256 is at least as secure as SHA-1. This is called the SHA-256 623 assumption in the remainder of this section. Under this assumption 624 EAP-AKA' is at least as secure as EAP-AKA. 626 If the AT_KDF attribute has value 1, then the security properties of 627 EAP-AKA' are as follows: 629 Protected ciphersuite negotiation 631 EAP-AKA' has no ciphersuite negotiation mechanisms. It does have 632 a negotiation mechanism for selecting the key derivation 633 functions. This mechanism is secure against bidding down attacks. 634 The negotiation mechanism allows changing the offered key 635 derivation function, but the change is visible in the final EAP- 636 Request/AKA'-Challenge message that the server sends to the peer. 637 This message is authenticated via the AT_MAC attribute, and 638 carries both the chosen alternative and the initially offered 639 list. The peer refuses to accept a change it did not initiate. 640 As a result, both parties are aware that a change is being made 641 and what the original offer was. 643 Mutual authentication 645 Under the SHA-256 assumption, the properties of EAP-AKA' are at 646 least as good as those of EAP-AKA in this respect. Refer to 647 [RFC4187] Section 12 for further details. 649 Integrity protection 651 Under the SHA-256 assumption, the properties of EAP-AKA' are at 652 least as good (most likely better) as those of EAP-AKA in this 653 respect. Refer to [RFC4187] Section 12 for further details. The 654 only difference is that a stronger hash algorithm, SHA-256 is used 655 instead of SHA-1. 657 Replay protection 659 Under the SHA-256 assumption, the properties of EAP-AKA' are at 660 least as good as those of EAP-AKA in this respect. Refer to 661 [RFC4187] Section 12 for further details. 663 Confidentiality 665 The properties of EAP-AKA' are exactly the same as those of EAP- 666 AKA in this respect. Refer to [RFC4187] Section 12 for further 667 details. 669 Key derivation 671 EAP-AKA' supports key derivation with an effective key strength 672 against brute force attacks equal to the minimum of the length of 673 the derived keys and the length of the AKA base key, i.e. 128-bits 674 or more. The key hierarchy is specified in Section 3.3. 676 The Transient EAP Keys used to protect EAP-AKA packets (K_encr, 677 K_aut, K_re), the MSK, and the EMSK are cryptographically 678 separate. If we make the assumption that SHA-256 behaves as a 679 pseudo-random function, an attacker is incapable of deriving any 680 non-trivial information about any of these keys based on the other 681 keys. An attacker also cannot calculate the pre-shared secret 682 from IK, CK, IK', CK', K_encr, K_aut, K_re, MSK, or the EMSK by 683 any practically feasible means. 685 EAP-AKA' adds an additional layer of key derivation functions 686 within itself to protect against the use of compromised keys. 687 This is discussed further in Section 5.1. 689 EAP-AKA' uses a pseudo random function modeled after the one used 690 in IKEv2 [RFC4306] together with SHA-256. 692 Key strength 694 See above. 696 Dictionary attack resistance 698 Under the SHA-256 assumption, the properties of EAP-AKA' are at 699 least as good as those of EAP-AKA in this respect. Refer to 700 [RFC4187] Section 12 for further details. 702 Fast reconnect 704 Under the SHA-256 assumption, the properties of EAP-AKA' are at 705 least as good as those of EAP-AKA in this respect. Refer to 706 [RFC4187] Section 12 for further details. Note that 707 implementations MUST prevent performing a fast reconnect across 708 method types. 710 Cryptographic binding 712 Note that this term refers to a very specific form of binding, 713 something that is performed between two layers of authentication. 714 It is not the same as the binding to a particular network name. 715 The properties of EAP-AKA' are exactly as those of EAP-AKA in this 716 respect, i.e., as it is not a tunnel method this property is not 717 applicable to it. Refer to [RFC4187] Section 12 for further 718 details. 720 Session independence 722 The properties of EAP-AKA' are exactly the same as those of EAP- 723 AKA in this respect. Refer to [RFC4187] Section 12 for further 724 details. 726 Fragmentation 728 The properties of EAP-AKA' are exactly the same as those of EAP- 729 AKA in this respect. Refer to [RFC4187] Section 12 for further 730 details. 732 Channel binding 734 EAP-AKA', like EAP-AKA, does not provide channel bindings as 735 they're defined in [RFC3748] and [RFC5247]. New skippable 736 attributes can be used to add channel binding support in the 737 future, if required. 739 However, including the network name field in the AKA' algorithms 740 (which are also used for other purposes than EAP-AKA') does 741 provide a form of cryptographic separation between different 742 network names, which resembles channel bindings. However, the 743 network name does not typically identify the EAP (pass-through) 744 authenticator. See the following section for more discussion. 746 5.1. Security Properties of Binding Network Names 748 The ability of EAP-AKA' to bind the network name into the used keys 749 provides some additional protection against key leakage to 750 inappropriate parties. The keys used in the protocol are specific to 751 a particular network name. If key leakage occurs due to an accident, 752 access node compromise, or another attack, the leaked keys are only 753 useful when providing access with that name. For instance, a 754 malicious access point cannot claim to be network Y if has stolen 755 keys from network X. Obviously, if an access point is compromised, 756 the malicious node can still represent the compromised node. As a 757 result, neither EAP-AKA' or any other extension can prevent such 758 attacks, but the binding to a particular name limits the attacker's 759 choices, allows better tracking of attacks, makes it possible to 760 identify compromised networks, and applies good cryptographic 761 hygiene. 763 The server receives the EAP transaction from a given access network, 764 and verifies that the claim from the access network corresponds to 765 the name that this access network should be using. It becomes 766 impossible for an access network to claim over AAA that it is another 767 access network. In addition, if the peer checks that the information 768 it has received locally over the network access link layer matches 769 with the information the server has given it via EAP-AKA', it becomes 770 impossible for the access network to tell one story to the AAA 771 network and another one to the peer. These checks prevent some 772 "lying NAS" (Network Access Server) attacks. For instance, a roaming 773 partner, R, might claim that it is the home network H in an effort to 774 lure peers to connect to itself. Such an attack would be beneficial 775 for the roaming partner if it can attract more users, and damaging 776 for the users if their access costs in R are higher than those in 777 other alternative networks, such as H. 779 Any attacker who gets hold of the keys CK, IK produced by the AKA 780 algorithm can compute the keys CK', IK' and hence the master key MK 781 according to the rules in Section 3.3. The attacker could then act 782 as a lying NAS. In 3GPP systems in general, the keys CK and IK have 783 been distributed to, for instance, nodes in a visited access network 784 where they may be vulnerable. In order to reduce this risk the AKA 785 algorithm MUST be computed with the AMF separation bit set to 1, and 786 the peer MUST check that this is indeed the case whenever it runs 787 EAP-AKA'. Furthermore, [3GPP.33.402] requires that no CK, IK keys 788 computed in this way ever leave the home subscriber system. 790 The additional security benefits obtained from the binding depend 791 obviously on the way names are assigned to different access networks. 792 This is specified in [3GPP.24.302]. See also [3GPP.23.003]. 793 Ideally, the names allow separating each different access technology, 794 each different access network, and each different NAS within a 795 domain. If this is not possible, the full benefits may not be 796 achieved. For instance, if the names identify just an access 797 technology, use of compromised keys in a different technology can be 798 prevented, but it is not possible to prevent their use by other 799 domains or devices using the same technology. 801 6. IANA Considerations 803 6.1. Type Value 805 EAP-AKA' has the EAP Type value TBA1 BY IANA in the Extensible 806 Authentication Protocol (EAP) Registry under Method Types. Per 807 [RFC3748] Section 6.2, this allocation can be made with Designated 808 Expert and Specification Required. 810 6.2. Attribute Type Values 812 EAP-AKA' shares its attribute space and subtypes with EAP-SIM 813 [RFC4186] and EAP-AKA [RFC4186]. No new registries are needed. 815 However, a new Attribute Type value (TBA2) in the non-skippable range 816 needs to be assigned for AT_KDF_INPUT (Section 3.1) in the EAP-AKA 817 and EAP-SIM Parameters registry under Attribute Types. 819 Also, a new Attribute Type value (TBA3) in the non-skippable range 820 needs to be assigned for AT_KDF (Section 3.2). 822 Finally, a new Attribute Type value (TBA4) in the skippable range 823 needs to be assigned for AT_BIDDING (Section 4). 825 6.3. Key Derivation Function Namespace 827 IANA also needs to create a new namespace for EAP-AKA' AT_KDF Key 828 Derivation Function values. This namespace can exist under the EAP- 829 AKA and EAP-SIM Parameters registry. The initial contents of this 830 namespace are given below; new values can be created through 831 Specification Required policy [RFC5226]. 833 Value Description Reference 834 --------- ---------------------- --------------- 835 0 Reserved 836 1 EAP-AKA' with CK'/IK' [this document] 837 2-65535 Unassigned 839 7. Acknowledgments 841 The authors would like to thank Guenther Horn, Joe Salowey, Mats 842 Naslund, Adrian Escott, Brian Rosenberg, Laksminath Dondeti, Ahmad 843 Muhanna, Stefan Rommer, Miguel Garcia, Jan Kall, Ankur Agarwal, Jouni 844 Malinen, Brian Weis, Russ Housley, and Alfred Hoenes for their in- 845 depth reviews and interesting discussions in this problem space. 847 8. References 849 8.1. Normative References 851 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 852 Requirement Levels", BCP 14, RFC 2119, March 1997. 854 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 855 Hashing for Message Authentication", RFC 2104, 856 February 1997. 858 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 859 Levkowetz, "Extensible Authentication Protocol (EAP)", 860 RFC 3748, June 2004. 862 [RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication 863 Protocol Method for 3rd Generation Authentication and Key 864 Agreement (EAP-AKA)", RFC 4187, January 2006. 866 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 867 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 868 May 2008. 870 [3GPP.24.302] 871 3GPP, "3rd Generation Partnership Project; Technical 872 Specification Group Core Network and Terminals; Access to 873 the 3GPP Evolved Packet Core (EPC) via non-3GPP access 874 networks; Stage 3; (Release 8)", 3GPP Draft Technical 875 Specification 24.302 v 1.0.0, September 2008. 877 [3GPP.33.102] 878 3GPP, "3rd Generation Partnership Project; Technical 879 Specification Group Services and System Aspects; 3G 880 Security; Security architecture (Release 8)", 3GPP Draft 881 Technical Specification 33.102 v 8.0.0, June 2008. 883 [3GPP.33.402] 884 3GPP, "3GPP System Architecture Evolution (SAE); Security 885 aspects of non-3GPP accesses; Release 8", 3GPP Draft 886 Technical Specification 33.402 v 8.0.0, June 2008. 888 [FIPS.180-2.2002] 889 National Institute of Standards and Technology, "Secure 890 Hash Standard", FIPS PUB 180-2, August 2002, . 893 8.2. Informative References 895 [RFC4186] Haverinen, H. and J. Salowey, "Extensible Authentication 896 Protocol Method for Global System for Mobile 897 Communications (GSM) Subscriber Identity Modules (EAP- 898 SIM)", RFC 4186, January 2006. 900 [RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity 901 Selection Hints for the Extensible Authentication Protocol 902 (EAP)", RFC 4284, January 2006. 904 [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", 905 RFC 4306, December 2005. 907 [RFC5113] Arkko, J., Aboba, B., Korhonen, J., and F. Bari, "Network 908 Discovery and Selection Problem", RFC 5113, January 2008. 910 [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible 911 Authentication Protocol (EAP) Key Management Framework", 912 RFC 5247, August 2008. 914 [3GPP.23.003] 915 3GPP, "3rd Generation Partnership Project; Technical 916 Specification Group Core Network and Terminals; Numbering, 917 addressing and identification (Release 8)", 3GPP Draft 918 Technical Specification 23.003 v 8.0.0, June 2008. 920 [FIPS.180-1.1995] 921 National Institute of Standards and Technology, "Secure 922 Hash Standard", FIPS PUB 180-1, April 1995, 923 . 925 Appendix A. Changes from RFC 4187 927 The changes to RFC 4187 relate only to the bidding down prevention 928 support defined in Section 4. In particular, this document does not 929 change how the Master Key (MK) is calculated in RFC 4187 (it uses CK 930 and IK, not CK' and IK'); neither is any processing of the AMF bit 931 added to RFC 4187. 933 Appendix B. Importance of Explicit Negotiation 935 Choosing between the traditional and revised AKA key derivation 936 functions is easy when their use is unambiguously tied to a 937 particular radio access network, e.g Long Term Evolution (LTE) as 938 defined by 3GPP or evolved High Rate Packet Data (eHRPD) as defined 939 by 3GPP2. There is no possibility for interoperability problems if 940 this radio access network is always used in conjunction with new 941 protocols that cannot be mixed with the old ones; clients will always 942 know whether they are connecting to the old or new system. 944 However, using the new key derivation functions over EAP introduces 945 several degrees of separation, making the choice of the correct key 946 derivation functions much harder. Many different types of networks 947 employ EAP. Most of these networks have no means to carry any 948 information about what is expected from the authentication process. 949 EAP itself is severely limited in carrying any additional 950 information, as noted in [RFC4284] and [RFC5113]. Even if these 951 networks or EAP were extended to carry additional information, it 952 would not affect millions of deployed access networks and clients 953 attaching to them. 955 Simply changing the key derivation functions that EAP-AKA [RFC4187] 956 uses would cause interoperability problems with all of the existing 957 implementations. Perhaps it would be possible to employ strict 958 separation into domain names that should be used by the new clients 959 and networks. Only these new devices would then employ the new key 960 derivation mechanism. While this can be made to work for specific 961 cases, it would be an extremely brittle mechanism, ripe to result in 962 problems whenever client configuration, routing of authentication 963 requests, or server configuration does not match expectations. It 964 also does not help to assume that the EAP client and server are 965 running a particular release of 3GPP network specifications. Network 966 vendors often provide features from the future releases early or do 967 not provide all features of the current release. And obviously, 968 there are many EAP and even some EAP-AKA implementations that are not 969 bundled with the 3GPP network offerings. In general, these 970 approaches are expected to lead to hard-to-diagnose problems and 971 increased support calls. 973 Authors' Addresses 975 Jari Arkko 976 Ericsson 977 Jorvas 02420 978 Finland 980 Email: jari.arkko@piuha.net 982 Vesa Lehtovirta 983 Ericsson 984 Jorvas 02420 985 Finland 987 Email: vesa.lehtovirta@ericsson.com 989 Pasi Eronen 990 Nokia Research Center 991 P.O. Box 407 992 FIN-00045 Nokia Group 993 Finland 995 Email: pasi.eronen@nokia.com 997 Full Copyright Statement 999 Copyright (C) The IETF Trust (2008). 1001 This document is subject to the rights, licenses and restrictions 1002 contained in BCP 78, and except as set forth therein, the authors 1003 retain all their rights. 1005 This document and the information contained herein are provided on an 1006 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1007 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 1008 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 1009 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 1010 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1011 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1013 Intellectual Property 1015 The IETF takes no position regarding the validity or scope of any 1016 Intellectual Property Rights or other rights that might be claimed to 1017 pertain to the implementation or use of the technology described in 1018 this document or the extent to which any license under such rights 1019 might or might not be available; nor does it represent that it has 1020 made any independent effort to identify any such rights. 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