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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 EMU Working Group T. Otto 3 Internet-Draft H. Tschofenig 4 Expires: October 21, 2006 Siemens AG 5 April 19, 2006 7 The EAP-TLS-PSK Authentication Protocol 8 draft-otto-emu-eap-tls-psk-00 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on October 21, 2006. 35 Copyright Notice 37 Copyright (C) The Internet Society (2006). 39 Abstract 41 The Extensible Authentication Protocol (EAP), defined in RFC 3748, is 42 a network access authentication framework which provides support for 43 multiple authentication methods. One proposal is EAP-TLS, which 44 relies on the Transport Layer Security (TLS) protocol and allows for 45 certificate-based authentication. This document specifies EAP-TLS- 46 PSK, which also relies on TLS, but allows for shared secret-based 47 authentication. EAP-TLS-PSK supports the pre-shared key ciphersuites 48 specified in RFC 4279. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 1.1. Overview about pre-shared key TLS ciphersuites . . . . . . 3 54 1.2. Requirements notation . . . . . . . . . . . . . . . . . . 4 55 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 56 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6 57 2.1. Overview of the EAP-TLS-PSK Conversation . . . . . . . . . 6 58 2.2. Retry Behavior . . . . . . . . . . . . . . . . . . . . . . 10 59 2.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 10 60 2.4. Identity Verification . . . . . . . . . . . . . . . . . . 12 61 2.5. Key Hierarchy . . . . . . . . . . . . . . . . . . . . . . 13 62 2.6. Ciphersuite and Compression Negotiation . . . . . . . . . 13 63 3. EAP-TLS-PSK Packet Format . . . . . . . . . . . . . . . . . . 15 64 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 65 5. Security Considerations . . . . . . . . . . . . . . . . . . . 18 66 5.1. Mutual Authentication . . . . . . . . . . . . . . . . . . 18 67 5.2. Protected Result Indications . . . . . . . . . . . . . . . 18 68 5.3. Integrity Protection . . . . . . . . . . . . . . . . . . . 18 69 5.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 18 70 5.5. Dictionary Attacks . . . . . . . . . . . . . . . . . . . . 18 71 5.6. Key Derivation . . . . . . . . . . . . . . . . . . . . . . 19 72 5.7. Session Independence . . . . . . . . . . . . . . . . . . . 19 73 5.8. Exposition of the PSK . . . . . . . . . . . . . . . . . . 19 74 5.9. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 20 75 5.10. Channel Binding . . . . . . . . . . . . . . . . . . . . . 20 76 5.11. Fast Reconnect . . . . . . . . . . . . . . . . . . . . . . 20 77 5.12. Identity Protection . . . . . . . . . . . . . . . . . . . 20 78 5.13. Protected Ciphersuite Negotiation . . . . . . . . . . . . 20 79 5.14. Confidentiality . . . . . . . . . . . . . . . . . . . . . 20 80 5.15. Cryptographic Binding . . . . . . . . . . . . . . . . . . 21 81 5.16. Security Claims . . . . . . . . . . . . . . . . . . . . . 21 82 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 83 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 84 7.1. Normative References . . . . . . . . . . . . . . . . . . . 24 85 7.2. Informative References . . . . . . . . . . . . . . . . . . 24 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 87 Intellectual Property and Copyright Statements . . . . . . . . . . 28 89 1. Introduction 91 The Extensible Authentication Protocol (EAP), described in [RFC3748], 92 provides a standard mechanism for support of multiple authentication 93 methods. Through the use of EAP, support for a number of 94 authentication schemes may be added, including smart cards, Kerberos, 95 Public Key, One Time Passwords, and others. 97 In 1998, EAP-TLS ([RFC2716]) was published. It performs mutual 98 authentication based on the Transport Layer Security (TLS) protocol. 99 EAP-TLS allows an EAP peer to take advantage of the protected 100 ciphersuite negotiation, mutual authentication and key management 101 capabilities of the TLS protocol, described in [RFC2246bis]. 102 Nonetheless, EAP-TLS restricts on certificate-based authentication. 104 In December 2005, IETF standardized pre-shared key ciphersuites for 105 TLS [RFC4279]. At IETF 65 in March 2006, the EMU Working Group 106 agreed on leaving EAP-TLS in its current form, i.e. not to enhance it 107 by support of the pre-shared key ciphersuites, but rather to specify 108 a new EAP method for this purpose. 110 This is the rationale for the EAP method specified in this document, 111 called EAP-TLS-PSK. 113 1.1. Overview about pre-shared key TLS ciphersuites 115 The goal of this subsection is to survey the pre-shared key TLS 116 ciphersuites specified in [RFC4279]. These ciphersuites are divided 117 into three sets, which distinguish in the underlying key exchange 118 mechanism and in the way the premaster_secret is computed. The three 119 key exchange mechanisms are henceforth referred to as PSK, DHE_PSK, 120 and RSA_PSK, in compliance with [RFC4279]. 122 Basically, the pre-shared key extensions are realized by adding 123 attributes to the TLS client_key_exchange and TLS server_key_exchange 124 message, or also by changing the semantic of existing attributes. 125 For instance, all three sets extend the the client_key_exchange 126 message by a PSK identity, and the server_key_exchange message by an 127 attribute "PSK identity hint". This attribute is optional, and can 128 be used by the server to send some hint to the client which identity 129 to choose. 131 The three key exchange mechanisms PSK, DHE_PSK and RSA_PSK shall be 132 contrasted as next. 134 PSK 136 These ciphersuites fully relies on symmetric key algorithms for 137 authentication, are thus very efficient and therefore well 138 suited for constrainted environments. where 140 The premaster_secret results from a concatenation of the pre- 141 shared key and its length. 143 DHE_PSK 145 DHE_PSK uses a PSK to authenticate a Diffie-Hellman key 146 exchange. The Diffie-Hellman public keys are exchanged within 147 the server_key_exchange and client_key_exchange messages. The 148 DHE_PSK ciphersuites provide Perfect Forward Secrecy (PFS). 150 The premaster_secret results from a concatenation of the pre- 151 shared key, its length, and the Diffie-Hellman shared secret and 152 its length. 154 RSA_PSK 156 RSA_PSK combines public key authentication of the server (using 157 RSA and certificates) with pre-shared key authentication of the 158 client. The server sends a certificate message to the client 159 which contains his public key. In this sense, RSA_PSK equals 160 TLS ciphersuites with RSA key exchange. However, [RFC4279] does 161 not further specify what the certificates contain. The 162 client_key_exchange message contains next to the PSK identity a 163 parameter "EncryptedPreMasterSecret" in length of 48 byte. The 164 first two octets are the TLS version number, the other 46 byte 165 are random data. For a RSA-based ciphersuite, this value is 166 exactly the TLS premaster_secret. For the pre-shared key 167 ciphersuites with RSA_PSK key exchange, however, the 168 premaster_secret results from a concatenation of its 48-byte 169 value with the pre-shared key and its length. 171 For further information on the respective mechanism, however, please 172 refer to the original specification [RFC4279]. 174 1.2. Requirements notation 176 In this document, several words are used to signify the requirements 177 of the specification. The key words "MUST", "MUST NOT", "REQUIRED", 178 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 179 and "OPTIONAL" in this document are to be interpreted as described in 180 [RFC2119]. 182 1.3. Terminology 184 This document frequently uses the following terms: 186 authenticator: 188 The end of the link initiating EAP authentication. The term 189 authenticator is used in [IEEE-802.1X], and has the same meaning 190 in this document. 192 peer: 194 The end of the link that responds to the authenticator. In 195 [IEEE-802.1X], this end is known as the Supplicant. 197 backend authentication server: 199 A backend authentication server is an entity that provides an 200 authentication service to an authenticator. When used, this 201 server typically executes EAP methods for the authenticator. 202 This terminology is also used in [IEEE-802.1X]. 204 EAP server: 206 The entity that terminates the EAP authentication method with the 207 peer. In the case where no backend authentication server is 208 used, the EAP server is part of the authenticator. In the case 209 where the authenticator operates in pass-through mode, the EAP 210 server is located on the backend authentication server. 212 Master Session Key (MSK): 214 Keying material that is derived between the EAP peer and server 215 and exported by the EAP method. The MSK is at least 64 octets in 216 length. 218 Extended Master Session Key (EMSK): 220 Additional keying material derived between the EAP client and 221 server that is exported by the EAP method. The EMSK is at least 222 64 octets in length. 224 2. Protocol Overview 226 2.1. Overview of the EAP-TLS-PSK Conversation 228 The following figure depicts the EAP-TLS-PSK message flow in the 229 successful case. 231 Authenticating Peer Authenticator / 232 EAP Server 233 ------------------- ------------- 234 <- EAP-Request/ 235 Identity 236 EAP-Response/ 237 Identity (MyID) -> 238 <- EAP-Request/ 239 EAP-Type=EAP-TLS-PSK 240 (TLS Start) 241 EAP-Response/ 242 EAP-Type=EAP-TLS-PSK 243 (TLS client_hello)-> 244 <- EAP-Request/ 245 EAP-Type=EAP-TLS-PSK 246 (TLS server_hello, 247 [TLS certificate,] 248 [TLS server_key_exchange,] 249 TLS server_hello_done) 250 EAP-Response/ 251 EAP-Type=EAP-TLS-PSK 252 (TLS client_key_exchange, 253 TLS change_cipher_spec, 254 TLS finished) -> 256 <- EAP-Request/ 257 EAP-Type=EAP-TLS-PSK 258 (TLS change_cipher_spec, 259 TLS finished) 260 EAP-Response/ 261 EAP-Type=EAP-TLS-PSK -> 262 <- EAP-Success 264 Figure 1: EAP-TLS-PSK message flow 266 As described in [RFC3748], the EAP-TLS-PSK conversation will 267 typically begin with the authenticator and the peer negotiating EAP. 268 The authenticator will then typically send an EAP-Request/Identity 269 packet to the peer, and the peer will respond with an EAP-Response/ 270 Identity packet to the authenticator, containing the peer's userId. 272 From this point forward, while nominally the EAP conversation occurs 273 between the EAP authenticator and the peer, the authenticator MAY act 274 as a passthrough device, with the EAP packets received from the peer 275 being encapsulated for transmission to a backend security server. In 276 the discussion that follows, we will use the term "EAP server" to 277 denote the ultimate endpoint conversing with the peer. 279 Once having received the peer's Identity, the EAP server MUST respond 280 with an EAP-TLS-PSK/Start packet, which is an EAP-Request packet with 281 EAP-Type=EAP-TLS-PSK, the Start (S) bit set, and no data. The EAP- 282 TLS-PSK conversation will then begin, with the peer sending an EAP- 283 Response packet with EAP-Type=EAP-TLS-PSK. The data field of that 284 packet will encapsulate one or more TLS records in TLS record layer 285 format, containing a TLS client_hello handshake message. 287 The current cipher spec for the TLS records will be 288 TLS_NULL_WITH_NULL_NULL and null compression. This current cipher 289 spec remains the same until the change_cipher_spec message signals 290 that subsequent records will have the negotiated attributes for the 291 remainder of the handshake. 293 The client_hello message contains the client's TLS version number, a 294 sessionId, a random number, and a set of ciphersuites supported by 295 the client. The version offered by the client MUST correspond to TLS 296 v1.0 or later. 298 The EAP server will then respond with an EAP-Request packet with EAP- 299 Type=EAP-TLS-PSK. The data field of this packet will encapsulate one 300 or more TLS records. These will contain a TLS server_hello handshake 301 message, possibly followed by TLS server_key_exchange, 302 server_hello_done and/or finished handshake messages, and/or a TLS 303 change_cipher_spec message. The server_hello handshake message 304 contains a TLS version number, another random number, a sessionId, 305 and a ciphersuite. The version offered by the server MUST correspond 306 to TLS v1.0 or later. 308 If the client's sessionId is null or unrecognized by the server, the 309 server MUST choose the sessionId to establish a new session; 310 otherwise, the sessionId will match that offered by the client, 311 indicating a resumption of the previously established session with 312 that sessionID. The server will also choose a ciphersuite from those 313 offered by the client; if the session matches the client's, then the 314 ciphersuite MUST match the one negotiated during the handshake 315 protocol execution that established the session. 317 The purpose of the sessionId within the TLS protocol is to allow for 318 improved efficiency in the case where a client repeatedly attempts to 319 authenticate to an EAP server within a short period of time. 321 As a result, it is left up to the peer whether to attempt to continue 322 a previous session, thus shortening the TLS conversation. Typically 323 the peer's decision will be made based on the time elapsed since the 324 previous authentication attempt to that EAP server. Based on the 325 sessionId chosen by the peer, and the time elapsed since the previous 326 authentication, the EAP server will decide whether to allow the 327 continuation, or whether to choose a new session. 329 In the case where the EAP server and authenticator reside on the same 330 device, then client will only be able to continue sessions when 331 connecting to the same NAS or tunnel server. Should these devices be 332 set up in a rotary or round-robin then it may not be possible for the 333 peer to know in advance the authenticator it will be connecting to, 334 and therefore which sessionId to attempt to reuse. As a result, it 335 is likely that the continuation attempt will fail. In the case where 336 the EAP authentication is remoted then continuation is much more 337 likely to be successful, since multiple NAS devices and tunnel 338 servers will remote their EAP authentications to the same backend 339 authentication server. 341 If the EAP server is resuming a previously established session, then 342 it MUST include only a TLS change_cipher_spec message and a TLS 343 finished handshake message after the server_hello message. The 344 finished message contains the EAP server's authentication response to 345 the peer. If the EAP server is not resuming a previously established 346 session, then it MUST include a TLS server_certificate handshake 347 message, and a server_hello_done handshake message MUST be the last 348 handshake message encapsulated in this EAP-Request packet. 350 In case of a RSA_PSK ciphersuite, the server sends a certificate 351 message. This message MUST contain the server's public key. In 352 accordance to EAP-TLS, the certificate message contains a public key 353 certificate chain for either a key exchange public key (such as an 354 RSA or Diffie-Hellman key exchange public key) or a signature public 355 key (such as an RSA or DSS signature public key). In the latter 356 case, a TLS server_key_exchange handshake message MUST also be 357 included to allow the key exchange to take place. 359 In an EAP-TLS-PSK message exchange, the client will never send a 360 certificate message and certificate_verify message, and the server 361 will never send a certificate_request message. 363 The peer MUST respond to the EAP-Request with an EAP-Response packet 364 of EAP-Type=EAP-TLS-PSK. The data field of this packet will 365 encapsulate one or more TLS records containing a TLS 366 client_key_exchange, change_cipher_spec and and finished handshake 367 message. 369 If the preceding server_hello message sent by the EAP server in the 370 preceding EAP-Request packet indicated the resumption of a previous 371 session, then the peer MUST send only the change_cipher_spec and 372 finished handshake messages. The finished message contains the 373 peer's authentication response to the EAP server. 375 If the preceding server_hello message sent by the EAP server in the 376 preceeding EAP-Request packet did not indicate the resumption of a 377 previous session, then the peer MUST send, in addition to the 378 change_cipher_spec and finished messages, a client_key_exchange 379 message, which completes the exchange of a shared master secret 380 between the peer and the EAP server. 382 If the peer's authentication is unsuccessful, the EAP server SHOULD 383 send an EAP-Request packet with EAP-Type=EAP-TLS-PSK, encapsulating a 384 TLS record containing the appropriate TLS alert message. In 385 particular, if the server does not recognize the PSK identity, it 386 MUST respond with either an "unknown_psk_identity" TLS alert pessage 387 or continue with the protocol and send a TLS "decrypt_error" alert, 388 which stands for an incorrect key. 390 The server SHOULD send a TLS alert message rather immediately 391 terminating the conversation so as to allow the peer to inform the 392 user of the cause of the failure and possibly allow for a restart of 393 the conversation. 395 To ensure that the peer receives the TLS alert message, the EAP 396 server MUST wait for the peer to reply with an EAP-Response packet. 397 The EAP-Response packet sent by the peer MAY encapsulate a TLS 398 client_hello handshake message, in which case the EAP server MAY 399 allow the EAP-TLS-PSK conversation to be restarted, or it MAY contain 400 an EAP-Response packet with EAP-Type=EAP-TLS-PSK and no data, in 401 which case the EAP server MUST send an EAP-Failure packet, and 402 terminate the conversation. It is up to the EAP server whether to 403 allow restarts, and if so, how many times the conversation can be 404 restarted. An EAP server implementing restart capability SHOULD 405 impose a limit on the number of restarts, so as to protect against 406 denial of service attacks. 408 If the peers authenticates successfully, the EAP server MUST respond 409 with an EAP-Request packet with EAP-Type=EAP-TLS-PSK, which includes, 410 in the case of a new TLS session, one or more TLS records containing 411 TLS change_cipher_spec and finished handshke messages. The latter 412 contains the EAP server's authentication response to the peer. The 413 peer will then verify the hash in order to authenticate the EAP 414 server. 416 If the EAP server authenticates unsuccessfully, the peer MAY send an 417 EAP-Response packet of EAP-Type=EAP-TLS-PSK containing a TLS Alert 418 message identifying the reason for the failed authentication. The 419 peer MAY send a TLS alert message rather than immediately terminating 420 the conversation so as to allow the EAP server to log the cause of 421 the error for examination by the system administrator. 423 To ensure that the EAP server receives the TLS alert message, the 424 peer MUST wait for the EAP server to reply before terminating the 425 conversation. The EAP server MUST reply with an EAP-Failure packet 426 since server authentication failure is a terminal condition. 428 If the EAP server authenticates successfully, the peer MUST send an 429 EAP-Response packet of EAP-Type=EAP-TLS-PSK, and no data. The EAP 430 server then MUST respond with an EAP-Success message. 432 2.2. Retry Behavior 434 As with other EAP protocols, the EAP server is responsible for retry 435 behavior. This means that if the EAP server does not receive a reply 436 from the peer, it MUST resend the EAP-Request for which it has not 437 yet received an EAP-Response. However, the peer MUST NOT resend EAP- 438 Response packets without first being prompted by the EAP server. 440 For example, if the initial EAP-TLS-PSK start packet sent by the EAP 441 server were to be lost, then the peer would not receive this packet, 442 and would not respond to it. As a result, the EAP-TLS-PSK start 443 packet would be resent by the EAP server. Once the peer received the 444 EAP-TLS-PSK start packet, it would send an EAP-Response encapsulating 445 the client_hello message. If the EAP-Response were to be lost, then 446 the EAP server would resend the initial EAP-TLS-PSK start, and the 447 peer would resend the EAP-Response. 449 As a result, it is possible that a peer will receive duplicate EAP- 450 Request messages, and may send duplicate EAP-Responses. Both the 451 peer and the EAP server should be engineered to handle this 452 possibility. 454 2.3. Fragmentation 456 A single TLS record may be up to 16384 octets in length, but a TLS 457 message may span multiple TLS records, and a TLS certificate message 458 may in principle be as long as 16MB. The group of EAP-TLS-PSK 459 messages sent in a single round may thus be larger than the PPP MTU 460 size, the maximum RADIUS packet size of 4096 octets, or even the 461 Multilink Maximum Received Reconstructed Unit (MRRU). As described 462 in [RFC1990], the multilink MRRU is negotiated via the Multilink MRRU 463 LCP option, which includes an MRRU length field of two octets, and 464 thus can support MRRUs as large as 64 KB. 466 However, note that in order to protect against reassembly lockup and 467 denial of service attacks, it may be desirable for an implementation 468 to set a maximum size for one such group of TLS messages. Since a 469 typical certificate chain is rarely longer than a few thousand 470 octets, and no other field is likely to be anwhere near as long, a 471 reasonable choice of maximum acceptable message length might be 64 472 KB. 474 If this value is chosen, then fragmentation can be handled via the 475 multilink PPP fragmentation mechanisms described in [RFC1990]. While 476 this is desirable, there may be cases in which multilink or the MRRU 477 LCP option cannot be negotiated. As a result, an EAP-TLS-PSK 478 implementation MUST provide its own support for fragmentation and 479 reassembly. 481 Since EAP is a simple ACK-NAK protocol, fragmentation support can be 482 added in a simple manner. In EAP, fragments that are lost or damaged 483 in transit will be retransmitted, and since sequencing information is 484 provided by the Identifier field in EAP, there is no need for a 485 fragment offset field as is provided in IPv4. 487 EAP-TLS-PSK fragmentation support is provided through addition of a 488 flags octet within the EAP-Response and EAP-Request packets, as well 489 as a TLS Message Length field of four octets. Flags include the 490 Length included (L), More fragments (M), and EAP-TLS-PSK Start (S) 491 bits. The L flag is set to indicate the presence of the four octet 492 TLS Message Length field, and MUST be set for the first fragment of a 493 fragmented TLS message or set of messages. The M flag is set on all 494 but the last fragment. The S flag is set only within the EAP-TLS-PSK 495 start message sent from the EAP server to the peer. The TLS Message 496 Length field is four octets, and provides the total length of the TLS 497 message or set of messages that is being fragmented; this simplifies 498 buffer allocation. 500 When an EAP-TLS-PSK peer receives an EAP-Request packet with the M 501 bit set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS- 502 PSK and no data. This serves as a fragment ACK. The EAP server MUST 503 wait until it receives the EAP-Response before sending another 504 fragment. In order to prevent errors in processing of fragments, the 505 EAP server MUST increment the Identifier field for each fragment 506 contained within an EAP-Request, and the peer MUST include this 507 Identifier value in the fragment ACK contained within the EAP- 508 Response. Retransmitted fragments will contain the same Identifier 509 value. 511 Similarly, when the EAP server receives an EAP-Response with the M 512 bit set, it MUST respond with an EAP-Request with EAP-Type=EAP-TLS- 513 PSK and no data. This serves as a fragment ACK. The EAP peer MUST 514 wait until it receives the EAP-Request before sending another 515 fragment. In order to prevent errors in the processing of fragments, 516 the EAP server MUST use increment the Identifier value for each 517 fragment ACK contained within an EAP-Request, and the peer MUST 518 include this Identifier value in the subsequent fragment contained 519 within an EAP- Response. 521 2.4. Identity Verification 523 As noted in [RFC3748] Section 5.1: It is RECOMMENDED that the 524 Identity Response be used primarily for routing purposes and 525 selecting which EAP method to use. EAP Methods SHOULD include a 526 method-specific mechanism for obtaining the identity, so that they do 527 not have to rely on the Identity Response. 529 As part of the EAP-TLS-PSK authentication, the peer uses the 530 client_key_exchange message to transmit his identity. 532 For RSA_PSK ciphersuites, the server presents a certificate to the 533 peer, which contains his identity. 535 EAP-TLS-PSK therefore provides a mechanism for determining both the 536 peer and server identities. 538 o The peer identity (Peer-ID in is exactly the PSK identity of the 539 client_key_exchange message. 541 o The server identity (Server-ID in is for ciphersuites of type 542 "RSA_PSK" either directly determined from the altSubjectName in 543 the server certificate or by a mapping of the altSubjectName to 544 the Server-ID using a directory service. 545 For ciphersuites of type "PSK" and "DHE_PSK", the server identity 546 is uniquely defined by means of the pre-shared key, which is 547 shared exclusively between an EAP peer and EAP server. 549 For RSA_PSK, the peer MUST verify the validity of the EAP server 550 certificate, and SHOULD also examine the EAP server name presented in 551 the certificate, in order to determine whether the EAP server can be 552 trusted. Please note that in the case where the EAP authentication 553 is remoted that the EAP server will not reside on the same machine as 554 the authenticator, and therefore the name in the EAP server's 555 certificate cannot be expected to match that of the intended 556 destination. In this case, a more appropriate test might be whether 557 the EAP server's certificate is signed by a CA controlling the 558 intended destination and whether the EAP server exists within a 559 target sub-domain. 561 2.5. Key Hierarchy 563 In EAP-TLS-PSK, the MSK, EMSK and IV are derived from the TLS master 564 secret via a one-way function. This ensures that the TLS master 565 secret cannot be derived from the MSK, EMSK or IV unless the one-way 566 function (TLS PRF) is broken. Since the MSK is derived from the the 567 TLS master secret, if the TLS master secret is compromised then the 568 MSK is also compromised. 570 The notation X[A..B] means byte A to B of X. The notation TLS-PRF-X 571 means that the TLS-PRF is iterated as long as possible to generate X 572 byte output. 574 Having this, the EAP-TLS-PSK key derivation procedure looks as 575 follows: 577 MSK = TLS-PRF-128 (master_secret, "client EAP encryption", 578 client.random || server.random)[0..63] 580 EMSK = TLS-PRF-128 (master_secret, "client EAP encryption", 581 client.random || server.random)[64..127] 583 IV = TLS-PRF-64 ("", "client EAP encryption", 584 client.random || server.random)[0..63] 586 The TLS-negotiated ciphersuite is used to set up a protected channel 587 for use in protecting the EAP conversation, keyed by the derived 588 TEKs. The TEK derivation proceeds as follows: 590 master_secret = TLS-PRF-48 (pre_master_secret, "master secret", 591 client.random || server.random) 593 TEK = TLS-PRF-X (master_secret, "key expansion", 594 server.random || client.random) 596 To meet the requirements of [I-D.ietf-eap-keying], EAP-TLS-PSK 597 defines a Method-ID, which is used for computing the Session-ID and 598 key names. In the current version, the Method-ID is set to the 599 concatenation of the server and client Finished messages. The 600 Method-ID uniquely identifies an EAP-TLS-PSK session, because the 601 hashes in the Finished message contain the random values exchanged 602 with the ClientHello- and ServerHello messages as well as the 603 identities of client and EAP server. 605 2.6. Ciphersuite and Compression Negotiation 607 Since TLS supports ciphersuite negotiation, peers completing the TLS 608 negotiation will also have selected a ciphersuite, which includes 609 encryption and hashing methods. Since the ciphersuite negotiated 610 within EAP-TLS-PSK applies only to the EAP conversation, TLS 611 ciphersuite negotiation SHOULD NOT be used to negotiate the 612 ciphersuites used to secure data. 614 EAP-TLS-PSK is intended to be used only with the ciphersuites defined 615 in [RFC4279]. For convenience, these ciphersuites are summarized 616 below. 618 CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0x8A }; 619 CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8B }; 620 CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x8C }; 621 CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x8D }; 622 CipherSuite TLS_DHE_PSK_WITH_RC4_128_SHA = { 0x00, 0x8E }; 623 CipherSuite TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8F }; 624 CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x90 }; 625 CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x91 }; 626 CipherSuite TLS_RSA_PSK_WITH_RC4_128_SHA = { 0x00, 0x92 }; 627 CipherSuite TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x93 }; 628 CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x94 }; 629 CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x95 }; 631 TLS also supports compression as well as ciphersuite negotiation. 632 Since compression negotiated within EAP-TLS-PSK applies only to the 633 EAP conversation, TLS compression negotiation MUST NOT be used to 634 negotiate compression mechanisms to be applied to data. 636 3. EAP-TLS-PSK Packet Format 638 A summary of the EAP TLS Request/Response packet format is shown 639 below. The fields are transmitted from left to right. 641 0 1 2 3 642 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 1 643 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 644 | Code | Identifier | Length | 645 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 646 | Type | Flags | TLS Message Length 647 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 648 | TLS Message Length | TLS Data... 649 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 651 Figure 3: EAP-TLS-PSK packet format 653 Code 655 1 - EAP-TLS-PSK Request (short: Request) 656 2 - EAP-TLS-PSK Response (short: Response) 658 Identifier 660 The identifier field is one octet and aids in matching responses 661 with requests. If the message is of type Response, then the 662 identifier MUST match the Identifier field from the corresponding 663 request. 665 Length 667 The Length field is two octets and indicates the length of the 668 EAP packet including the Code, Identifier, Length, Type, and Data 669 fields. Octets outside the range of the Length field should be 670 treated as Data Link Layer padding and should be ignored on 671 reception. 673 Type 675 TBD - EAP-TLS-PSK 677 Flags 678 0 1 2 3 4 5 6 7 8 679 +-+-+-+-+-+-+-+-+ 680 |L M S R R R R R| 681 +-+-+-+-+-+-+-+-+ 683 L = Length included 684 M = More fragments 685 S = EAP-TLS-PSK start 686 R = Reserved 688 The L bit (length included) is set to indicate the presence of 689 the four octet TLS Message Length field, and MUST be set for the 690 first fragment of a fragmented TLS message or set of messages. 691 The M bit (more fragments) is set on all but the last fragment. 692 The S bit (EAP-TLS-PSK start) is set in an EAP-TLS-PSK Start 693 message. This differentiates the EAP-TLS-PSK Start message from 694 a fragment acknowledgement. Implementations of this 695 specification MUST set the reserved bits to zero, and MUST ignore 696 them on reception. 698 TLS Message Length 700 The TLS Message Length field is four octets, and is present only 701 if the L bit is set. This field provides the total length of the 702 TLS message or set of messages that is being fragmented. 704 TLS Data 706 The TLS data consists of the encapsulated TLS packet in TLS 707 record format. 709 4. IANA Considerations 711 This document requires IANA to allocate a new EAP Type for EAP-TLS- 712 PSK. 714 5. Security Considerations 716 [RFC3748] highlights several attacks that are possible against EAP as 717 EAP does not provide any robust security mechanism. This section 718 discusses the claimed security properties of EAP-TLS-PSK as well as 719 vulnerabilities and security recommendations in the threat model of 720 [RFC3748]. 722 5.1. Mutual Authentication 724 EAP-TLS-PSK provides mutual authentication. This holds for all three 725 modes PSK, DHE_PSK and RSA_PSK. 727 5.2. Protected Result Indications 729 EAP-TLS-PSK does not provide protected result indications. 731 5.3. Integrity Protection 733 EAP-TLS-PSK provides integrity protection thanks to the TLS Finished 734 message, which contains a Message Authentication Code computed over 735 the whole previous conversation. That is, the verification of the 736 Finished message serves as guarantee of the conversation's integrity. 738 5.4. Replay Protection 740 EAP-TLS-PSK provides replay protection of its mutual authentication 741 part thanks to the use of random numbers in the client_hello and 742 server_hello messages. These random numbers are 16 byte long. One 743 expects to have to record 2**64 (i.e. approximately 1.84*10**19) EAP- 744 TLS-PSK successful authentication before an authentication can be 745 replayed. A good source for randomness is cruicial for the security 746 of EAP-TLS-PSK. 748 5.5. Dictionary Attacks 750 For PSK and DHE_PSK, mutual authentication is based on a shared 751 secret. While [RFC4279] does not specify the length of this pre- 752 shared key, EAP-TLS-PSK does so. The pre-shared key MUST be at least 753 16 byte long and have full entropy. For these two modes, it is 754 highly discouraged having derived the pre-shared key from low entropy 755 source, e.g. a password. 757 For RSA_PSK, the pre-shared key SHOULD also be at least 16 byte long. 758 In contrast to PSK and DHE_PSK, the pre-shared key may also be 759 derived from a low entropy source, e.g. a password. This becomes 760 possible because the EAP server authenticates himself through public 761 key techniques and prior than the client. 763 In this version of EAP-TLS-PSK, however, the pre-shared key MUST BE 764 at least 16 byte long and MUST HAVE full entropy. Then, EAP-TLS-PSK 765 is not susceptible for dictionary attacks. 767 5.6. Key Derivation 769 EAP-TLS-PSK supports key derivation according to [RFC3748] and 770 [I-D.ietf-eap-keying]. 772 EAP-TLS-PSK exports keys, namely a 64-byte MSK, a 64-byte EMSK and a 773 64-byte IV. 775 Some remarks regarding the key strength of EAP-TLS-PSK. In case of 776 RSA_PSK, the server's private key size should be chosen accordingly 777 to the length of the pre-shared key. Section 5 in [RFC3766] 778 contrasts symmetric key sizes with their public key counterparts, to 779 obtain roughly the same overall key strength. Based on the table 780 below, a 3072-bit RSA key is required to provide 128-bit equivalent 781 key strength. 783 Attack Resistance RSA or DH Modulus DSA subgroup 784 (bits) size (bits) size (bits) 785 ----------------- ----------------- ------------ 786 70 947 128 787 80 1228 145 788 90 1553 153 789 100 1926 184 790 150 4575 279 791 200 8719 373 792 250 14596 475 794 5.7. Session Independence 796 Thanks to its key derivation mechanisms, EAP-PSK provides session 797 independence: passive attacks (such as capture of the EAP 798 conversation) or active attacks (including compromise of the MSK or 799 EMSK) does not enable compromise of subsequent or prior MSKs or 800 EMSKs. The assumption that the random numbers of the TLS 801 client_hello and server_hello messages are random is central for the 802 security of EAP-TLS-PSK in general and session independance in 803 particular. 805 5.8. Exposition of the PSK 807 EAP-TLS-PSK specifies three sets of ciphersuites, which distinguish 808 in the underlying key derivation mechanism. 810 o The PSK and RSA_PSK ciphersuites do not provide perfect forward 811 secrecy. Compromise of the pre-shared key or the pre-shared key 812 and the server's private key (in case of RSA_PSK) leads to 813 compromise of recorded past sessions. 815 o DHE_PSK ciphersuites provide perfect forward secrecy, if a fresh 816 Diffie-Hellman private key is generated for each handshake. 818 5.9. Fragmentation 820 EAP-TLS-PSK supports fragmentation and reassembly. The mechanism is 821 inherited from EAP-TLS ([RFC2716]). 823 5.10. Channel Binding 825 EAP-TLS-PSK does not provide channel binding. 827 5.11. Fast Reconnect 829 EAP-TLS-PSK relies on the Transport Layer Security protocol, which 830 specifies a fast resumption mode. If peer and server agree on 831 continuing a previously established session, the session's master 832 secret can be re-used. That is, related computations can be omitted, 833 which make the fast resumption mode very efficient. 835 For PSK ciphersuites, the speedup is believed to be minimal, since 836 these ciphersuites rely on symmetric key operations only. It is 837 expected, that DHE_PSK and RSA_PSK may benefit from the fast 838 resumption mode. 840 5.12. Identity Protection 842 EAP-TLS-PSK does not provide user identity protection. The 843 client_key_exchange message contains the peer's identity. This 844 message is sent in plaintext. 846 5.13. Protected Ciphersuite Negotiation 848 Since EAP-TLS-PSK relies on TLS, it also supports ciphersuite 849 negotiation. This is done in a securely manner, because the TLS 850 Finished messages authenticate the whole handshake. Therefore, EAP- 851 TLS-PSK provides protected ciphersuite negotiation. 853 5.14. Confidentiality 855 EAP-TLS-PSK does not support this feature. According to Section 856 7.2.1 of [RFC3748], this feature would mandate for the feature 857 'identity protection', which is also not addressed by EAP-TLS-PSK. 859 5.15. Cryptographic Binding 861 This feature is not applicable for EAP-TLS-PSK. 863 5.16. Security Claims 865 This section provides the security claims required by [RFC3748]. 867 [a] Mechanism. 869 * For PSK and DHE_PSK: Pre-shared key. 871 * For RSA_PSK: Server via Public key, Peer via Pre-shared key. 873 [b] Security claims. EAP-TLS-PSK provides: 875 * Mutual authentication (see Section 5.1) 877 * Integrity protection (see Section 5.3) 879 * Replay protection (see Section 5.4) 881 * Key derivation (see Section 5.6) 883 * Dictionary attack resistance (see Section 5.5) 885 * Session independence (see section Section 5.5) 887 * Fast reconnect (see Section 5.11) 889 * Fragmentation (see Section 5.9) 891 * Protected cipher suite negotiation (see Section 5.13) 893 * Perfect Forward Secrecy (at least partially; see Section 5.6) 895 [c] Key strength. EAP-TLS-PSK provides at least a 16-byte effective 896 key strength. 898 [d] Indication of vulnerabilities. EAP-TLS-PSK does not provide: 900 * Identity protection (see Section 5.12) 902 * Confidentiality (see Section 5.14) 904 * Cryptographic binding (see Section 5.15) 905 * Key agreement: the session key is chosen by the peer (see 906 Section 5.6) 908 * Channel binding (see Section 5.10) 910 6. Acknowledgments 912 The EAP-TLS-PSK specification lends parts of both the EAP-TLS and 913 EAP-PSK specifications. The authors would like to thank Bernard 914 Aboba and Dan Simon and Florent Bersani for their challenging work. 916 7. References 918 7.1. Normative References 920 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 921 Requirement Levels", BCP 14, RFC 2119, March 1997. 923 [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", 924 RFC 2246, January 1999. 926 [RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication 927 Protocol", RFC 2716, October 1999. 929 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 930 Levkowetz, "Extensible Authentication Protocol (EAP)", 931 RFC 3748, June 2004. 933 [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites 934 for Transport Layer Security (TLS)", RFC 4279, 935 December 2005. 937 7.2. Informative References 939 [I-D.ietf-eap-keying] 940 Aboba, B., "Extensible Authentication Protocol (EAP) Key 941 Management Framework", draft-ietf-eap-keying-12 (work in 942 progress), April 2006. 944 [IEEE-802.11] 945 Institute of Electrical and Electronics Engineers, 946 "Standard for Telecommunications and Information Exchange 947 Between Systems - LAN/MAN Specific Requirements - Part 11: 948 Wireless LAN Medium Access Control (MAC) and Physical 949 Layer (PHY) Specifications", IEEE Standard 802.11, 1999. 951 [IEEE-802.11i] 952 Institute of Electrical and Electronics Engineers, 953 "Approved Draft Supplement to Standard for 954 Telecommunications and Information Exchange Between 955 Systems-LAN/MAN Specific Requirements - Part 11: Wireless 956 LAN Medium Access Control (MAC) and Physical Layer (PHY) 957 Specifications: Specification for Enhanced Security", 958 IEEE 802.11i-2004, June 2004. 960 [IEEE-802.16e] 961 Institute of Electrical and Electronics Engineers, 962 "Standard for Local and Metropolitan Area Networks: Part 963 16: Air Interface for Fixed and Mobile Broadband Wireless 964 Access Systems: Amendment for Physical and Medium Access 965 Control Layers for Combined Fixed and Mobile Operations in 966 Licensed Bands" IEEE 802.16e", IEEE 802.16e, August 2005. 968 [IEEE-802.1X] 969 Institute of Electrical and Electronics Engineers, "Local 970 and Metropolitan Area Networks: Port-Based Network Access 971 Control", IEEE Standard 802.1X, September 2001. 973 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 974 April 1992. 976 [RFC1570] Simpson, W., "PPP LCP Extensions", RFC 1570, January 1994. 978 [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, 979 RFC 1661, July 1994. 981 [RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, 982 July 1994. 984 [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. 985 Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, 986 August 1996. 988 [RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption 989 Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998. 991 [RFC2420] Kummert, H., "The PPP Triple-DES Encryption Protocol 992 (3DESE)", RFC 2420, September 1998. 994 [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", 995 RFC 2548, March 1999. 997 [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, 998 W., and G. Zorn, "Point-to-Point Tunneling Protocol", 999 RFC 2637, July 1999. 1001 [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, 1002 G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", 1003 RFC 2661, August 1999. 1005 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 1006 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 1007 RFC 3766, April 2004. 1009 [RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible 1010 Authentication Protocol (EAP) Method Requirements for 1011 Wireless LANs", RFC 4017, March 2005. 1013 [RFC4334] Housley, R. and T. Moore, "Certificate Extensions and 1014 Attributes Supporting Authentication in Point-to-Point 1015 Protocol (PPP) and Wireless Local Area Networks (WLAN)", 1016 RFC 4334, February 2006. 1018 [TLSPSK-Perf] 1019 Fang-Chun Kuo, Hannes Tschofenig, Fabian Meyer, and 1020 Xiaoming Fu, "Comparison Studies between Pre-Shared Key 1021 and Public Key Exchange Mechanisms for Transport Layer 1022 Security (TLS)", IFI-TB-2006-01 URL: http:// 1023 user.informatik.uni-goettingen.de/~fkuo/publications/ 1024 ptls-ifi-tb-2006-01.pdf, January 2006. 1026 Authors' Addresses 1028 Thomas Otto 1029 Siemens AG 1030 Otto-Hahn-Ring 6 1031 Munich 81739 1032 Germany 1034 Email: thomas.g.otto@googlemail.com 1036 Hannes Tschofenig 1037 Siemens AG 1038 Otto-Hahn-Ring 6 1039 Munich 81739 1040 Germany 1042 Email: hannes.tschofenig@siemens.com 1044 Intellectual Property Statement 1046 The IETF takes no position regarding the validity or scope of any 1047 Intellectual Property Rights or other rights that might be claimed to 1048 pertain to the implementation or use of the technology described in 1049 this document or the extent to which any license under such rights 1050 might or might not be available; nor does it represent that it has 1051 made any independent effort to identify any such rights. 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