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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 4 Intended status: Standards Track H. Tschofenig 5 Expires: April 26, 2007 Siemens Networks GmbH & Co KG 6 October 23, 2006 8 The EAP-TLS-PSK Authentication Protocol 9 draft-otto-emu-eap-tls-psk-01.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on April 26, 2007. 36 Copyright Notice 38 Copyright (C) The Internet Society (2006). 40 Abstract 42 The Extensible Authentication Protocol (EAP), defined in RFC 3748, is 43 a network access authentication framework which provides support for 44 multiple authentication methods. One proposal is EAP-TLS, which 45 relies on the Transport Layer Security (TLS) protocol and allows for 46 certificate-based authentication. This document specifies EAP-TLS- 47 PSK, which also relies on TLS, but allows for shared secret-based 48 authentication. EAP-TLS-PSK supports the pre-shared key ciphersuites 49 specified in RFC 4279. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 54 1.1. Overview about pre-shared key TLS ciphersuites . . . . . . 4 55 1.2. Requirements notation . . . . . . . . . . . . . . . . . . 5 56 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 57 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7 58 2.1. Overview of the EAP-TLS-PSK Conversation . . . . . . . . . 7 59 2.2. Retry Behavior . . . . . . . . . . . . . . . . . . . . . . 11 60 2.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 11 61 2.4. Identity Verification . . . . . . . . . . . . . . . . . . 13 62 2.5. Key Hierarchy . . . . . . . . . . . . . . . . . . . . . . 14 63 2.6. Ciphersuite and Compression Negotiation . . . . . . . . . 15 64 3. EAP-TLS-PSK Packet Format . . . . . . . . . . . . . . . . . . 16 65 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 66 5. Security Considerations . . . . . . . . . . . . . . . . . . . 19 67 5.1. Mutual Authentication . . . . . . . . . . . . . . . . . . 19 68 5.2. Protected Result Indications . . . . . . . . . . . . . . . 19 69 5.3. Integrity Protection . . . . . . . . . . . . . . . . . . . 19 70 5.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 19 71 5.5. Dictionary Attacks . . . . . . . . . . . . . . . . . . . . 19 72 5.6. Key Derivation . . . . . . . . . . . . . . . . . . . . . . 20 73 5.7. Session Independence . . . . . . . . . . . . . . . . . . . 20 74 5.8. Exposition of the PSK . . . . . . . . . . . . . . . . . . 20 75 5.9. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 21 76 5.10. Channel Binding . . . . . . . . . . . . . . . . . . . . . 21 77 5.11. Fast Reconnect . . . . . . . . . . . . . . . . . . . . . . 21 78 5.12. Identity Protection . . . . . . . . . . . . . . . . . . . 21 79 5.13. Protected Ciphersuite Negotiation . . . . . . . . . . . . 21 80 5.14. Confidentiality . . . . . . . . . . . . . . . . . . . . . 21 81 5.15. Cryptographic Binding . . . . . . . . . . . . . . . . . . 22 82 5.16. Security Claims . . . . . . . . . . . . . . . . . . . . . 22 83 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 84 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 85 7.1. Normative References . . . . . . . . . . . . . . . . . . . 25 86 7.2. Informative References . . . . . . . . . . . . . . . . . . 25 87 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 88 Intellectual Property and Copyright Statements . . . . . . . . . . 29 90 1. Introduction 92 The Extensible Authentication Protocol (EAP), described in [RFC3748], 93 provides a standard mechanism for support of multiple authentication 94 methods. Through the use of EAP, support for a number of 95 authentication schemes may be added, including smart cards, Kerberos, 96 Public Key, One Time Passwords, and others. 98 In 1998, EAP-TLS ([RFC2716]) was published. It performs mutual 99 authentication based on the Transport Layer Security (TLS) protocol. 100 EAP-TLS allows an EAP peer to take advantage of the protected 101 ciphersuite negotiation, mutual authentication and key management 102 capabilities of the TLS protocol, described in [RFC2246bis]. 103 Nonetheless, EAP-TLS restricts on certificate-based authentication. 105 In December 2005, IETF standardized pre-shared key ciphersuites for 106 TLS [RFC4279]. At IETF 65 in March 2006, the EMU Working Group 107 agreed on leaving EAP-TLS in its current form, i.e. not to enhance it 108 by support of the pre-shared key ciphersuites, but rather to specify 109 a new EAP method for this purpose. 111 This is the rationale for the EAP method specified in this document, 112 called EAP-TLS-PSK. 114 1.1. Overview about pre-shared key TLS ciphersuites 116 The goal of this subsection is to survey the pre-shared key TLS 117 ciphersuites specified in [RFC4279]. These ciphersuites are divided 118 into three sets, which distinguish in the underlying key exchange 119 mechanism and in the way the premaster_secret is computed. The three 120 key exchange mechanisms are henceforth referred to as PSK, DHE_PSK, 121 and RSA_PSK, in compliance with [RFC4279]. 123 Basically, the pre-shared key extensions are realized by adding 124 attributes to the TLS client_key_exchange and TLS server_key_exchange 125 message, or also by changing the semantic of existing attributes. 126 For instance, all three sets extend the the client_key_exchange 127 message by a PSK identity, and the server_key_exchange message by an 128 attribute "PSK identity hint". This attribute is optional, and can 129 be used by the server to send some hint to the client which identity 130 to choose. 132 The three key exchange mechanisms PSK, DHE_PSK and RSA_PSK shall be 133 contrasted as next. 135 PSK 137 These ciphersuites fully relies on symmetric key algorithms for 138 authentication, are thus very efficient and therefore well 139 suited for constrainted environments. where 141 The premaster_secret results from a concatenation of the pre- 142 shared key and its length. 144 DHE_PSK 146 DHE_PSK uses a PSK to authenticate a Diffie-Hellman key 147 exchange. The Diffie-Hellman public keys are exchanged within 148 the server_key_exchange and client_key_exchange messages. The 149 DHE_PSK ciphersuites provide Perfect Forward Secrecy (PFS). 151 The premaster_secret results from a concatenation of the pre- 152 shared key, its length, and the Diffie-Hellman shared secret and 153 its length. 155 RSA_PSK 157 RSA_PSK combines public key authentication of the server (using 158 RSA and certificates) with pre-shared key authentication of the 159 client. The server sends a certificate message to the client 160 which contains his public key. In this sense, RSA_PSK equals 161 TLS ciphersuites with RSA key exchange. However, [RFC4279] does 162 not further specify what the certificates contain. The 163 client_key_exchange message contains next to the PSK identity a 164 parameter "EncryptedPreMasterSecret" in length of 48 byte. The 165 first two octets are the TLS version number, the other 46 byte 166 are random data. For a RSA-based ciphersuite, this value is 167 exactly the TLS premaster_secret. For the pre-shared key 168 ciphersuites with RSA_PSK key exchange, however, the 169 premaster_secret results from a concatenation of its 48-byte 170 value with the pre-shared key and its length. 172 For further information on the respective mechanism, however, please 173 refer to the original specification [RFC4279]. 175 1.2. Requirements notation 177 In this document, several words are used to signify the requirements 178 of the specification. The key words "MUST", "MUST NOT", "REQUIRED", 179 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 180 and "OPTIONAL" in this document are to be interpreted as described in 181 [RFC2119]. 183 1.3. Terminology 185 This document frequently uses the following terms: 187 authenticator: 189 The end of the link initiating EAP authentication. The term 190 authenticator is used in [IEEE-802.1X], and has the same meaning 191 in this document. 193 peer: 195 The end of the link that responds to the authenticator. In 196 [IEEE-802.1X], this end is known as the Supplicant. 198 backend authentication server: 200 A backend authentication server is an entity that provides an 201 authentication service to an authenticator. When used, this 202 server typically executes EAP methods for the authenticator. 203 This terminology is also used in [IEEE-802.1X]. 205 EAP server: 207 The entity that terminates the EAP authentication method with the 208 peer. In the case where no backend authentication server is 209 used, the EAP server is part of the authenticator. In the case 210 where the authenticator operates in pass-through mode, the EAP 211 server is located on the backend authentication server. 213 Master Session Key (MSK): 215 Keying material that is derived between the EAP peer and server 216 and exported by the EAP method. The MSK is at least 64 octets in 217 length. 219 Extended Master Session Key (EMSK): 221 Additional keying material derived between the EAP client and 222 server that is exported by the EAP method. The EMSK is at least 223 64 octets in length. 225 2. Protocol Overview 227 2.1. Overview of the EAP-TLS-PSK Conversation 229 The following figure depicts the EAP-TLS-PSK message flow in the 230 successful case. 232 Authenticating Peer Authenticator / 233 EAP Server 234 ------------------- ------------- 235 <- EAP-Request/ 236 Identity 237 EAP-Response/ 238 Identity (MyID) -> 239 <- EAP-Request/ 240 EAP-Type=EAP-TLS-PSK 241 (TLS Start) 242 EAP-Response/ 243 EAP-Type=EAP-TLS-PSK 244 (TLS client_hello)-> 245 <- EAP-Request/ 246 EAP-Type=EAP-TLS-PSK 247 (TLS server_hello, 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, there will never the client will 360 never send a certificate and certificate_verify message, and the 361 server 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 MSK is divided into two halves, corresponding to the "Peer to 571 Authenticator Encryption Key" (Enc- RECV-Key, 32 octets) and 572 "Authenticator to Peer Encryption Key" (Enc- SEND-Key, 32 octets). 574 The EMSK is also divided into two halves, corresponding to the "Peer 575 to Authenticator Authentication Key" (Auth-RECV-Key, 32 octets) and 576 "Authenticator to Peer Authentication Key" (Auth-SEND-Key, 32 577 octets). The IV is a 64 octet quantity that is a known value; octets 578 0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and 579 Octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV. 581 The key derivation scheme is as follows. The notation X[A..B] means 582 byte A to B of X. The notation TLS-PRF-X means that the TLS-PRF is 583 iterated as long as possible to generate X byte output. 585 MSK = TLS-PRF-128 (master_secret, "client EAP encryption", 586 client.random || server.random)[0..63] 588 EMSK = TLS-PRF-128 (master_secret, "client EAP encryption", 589 client.random || server.random)[64..127] 591 IV = TLS-PRF-64 ("", "client EAP encryption", 592 client.random || server.random)[0..63] 594 The TLS-negotiated ciphersuite is used to set up a protected channel 595 for use in protecting the EAP conversation, keyed by the derived 596 TEKs. The TEK derivation proceeds as follows: 598 master_secret = TLS-PRF-48(pre_master_secret, "master secret", 599 client.random || server.random) 601 TEK = TLS-PRF-X(master_secret, "key expansion", 602 server.random || client.random) 604 To meet the requirements of [I-D.ietf-eap-keying] EAP-TLS-PSK defines 605 a Method-ID, which is used for computing a session-ID and key names. 606 In the current version, the Method-ID is set to the concatenation of 607 the server and client Finished messages. The Method-ID uniquely 608 identifies an EAP-TLS-PSK session, because the Hashes in the Finished 609 message contain the random values exchanged with the ClientHello- and 610 ServerHello messages as well as the identities of client and EAP 611 server. 613 2.6. Ciphersuite and Compression Negotiation 615 Since TLS supports ciphersuite negotiation, peers completing the TLS 616 negotiation will also have selected a ciphersuite, which includes 617 encryption and hashing methods. Since the ciphersuite negotiated 618 within EAP-TLS-PSK applies only to the EAP conversation, TLS 619 ciphersuite negotiation SHOULD NOT be used to negotiate the 620 ciphersuites used to secure data. 622 EAP-TLS-PSK is intended to be used only with the ciphersuites defined 623 in [RFC4279]. For convenience, these ciphersuites are summarized 624 below. 626 CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0x8A }; 627 CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8B }; 628 CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x8C }; 629 CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x8D }; 630 CipherSuite TLS_DHE_PSK_WITH_RC4_128_SHA = { 0x00, 0x8E }; 631 CipherSuite TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8F }; 632 CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x90 }; 633 CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x91 }; 634 CipherSuite TLS_RSA_PSK_WITH_RC4_128_SHA = { 0x00, 0x92 }; 635 CipherSuite TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x93 }; 636 CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x94 }; 637 CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x95 }; 639 TLS also supports compression as well as ciphersuite negotiation. 640 Since compression negotiated within EAP-TLS-PSK applies only to the 641 EAP conversation, TLS compression negotiation MUST NOT be used to 642 negotiate compression mechanisms to be applied to data. 644 3. EAP-TLS-PSK Packet Format 646 A summary of the EAP TLS Request/Response packet format is shown 647 below. The fields are transmitted from left to right. 649 0 1 2 3 650 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 1 651 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 652 | Code | Identifier | Length | 653 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 654 | Type | Flags | TLS Message Length 655 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 656 | TLS Message Length | TLS Data... 657 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 659 Figure 3: EAP-TLS-PSK packet format 661 Code 663 1 - EAP-TLS-PSK Request (short: Request) 664 2 - EAP-TLS-PSK Response (short: Response) 666 Identifier 668 The identifier field is one octet and aids in matching responses 669 with requests. If the message is of type Response, then the 670 identifier MUST match the Identifier field from the corresponding 671 request. 673 Length 675 The Length field is two octets and indicates the length of the 676 EAP packet including the Code, Identifier, Length, Type, and Data 677 fields. Octets outside the range of the Length field should be 678 treated as Data Link Layer padding and should be ignored on 679 reception. 681 Type 683 TBD - EAP-TLS-PSK 685 Flags 686 0 1 2 3 4 5 6 7 8 687 +-+-+-+-+-+-+-+-+ 688 |L M S R R R R R| 689 +-+-+-+-+-+-+-+-+ 691 L = Length included 692 M = More fragments 693 S = EAP-TLS-PSK start 694 R = Reserved 696 The L bit (length included) is set to indicate the presence of 697 the four octet TLS Message Length field, and MUST be set for the 698 first fragment of a fragmented TLS message or set of messages. 699 The M bit (more fragments) is set on all but the last fragment. 700 The S bit (EAP-TLS-PSK start) is set in an EAP-TLS-PSK Start 701 message. This differentiates the EAP-TLS-PSK Start message from 702 a fragment acknowledgement. Implementations of this 703 specification MUST set the reserved bits to zero, and MUST ignore 704 them on reception. 706 TLS Message Length 708 The TLS Message Length field is four octets, and is present only 709 if the L bit is set. This field provides the total length of the 710 TLS message or set of messages that is being fragmented. 712 TLS Data 714 The TLS data consists of the encapsulated TLS packet in TLS 715 record format. 717 4. IANA Considerations 719 This document requires IANA to allocate a new EAP Type for EAP-TLS- 720 PSK. 722 5. Security Considerations 724 [RFC3748] highlights several attacks that are possible against EAP as 725 EAP does not provide any robust security mechanism. This section 726 discusses the claimed security properties of EAP-TLS-PSK as well as 727 vulnerabilities and security recommendations in the threat model of 728 [RFC3748]. 730 5.1. Mutual Authentication 732 EAP-TLS-PSK provides mutual authentication. This holds for all three 733 modes PSK, DHE_PSK and RSA_PSK. 735 5.2. Protected Result Indications 737 EAP-TLS-PSK does not provide protected result indications. 739 5.3. Integrity Protection 741 EAP-TLS-PSK provides integrity protection thanks to the TLS Finished 742 message, which contains a Message Authentication Code computed over 743 the whole previous conversation. That is, the verification of the 744 Finished message serves as guarantee of the conversation's integrity. 746 5.4. Replay Protection 748 EAP-TLS-PSK provides replay protection of its mutual authentication 749 part thanks to the use of random numbers in the client_hello and 750 server_hello messages. These random numbers are 16 byte long. One 751 expects to have to record 2**64 (i.e. approximately 1.84*10**19) EAP- 752 TLS-PSK successful authentication before an authentication can be 753 replayed. A good source for randomness is cruicial for the security 754 of EAP-TLS-PSK. 756 5.5. Dictionary Attacks 758 For PSK and DHE_PSK, mutual authentication is based on a shared 759 secret. While [RFC4279] does not specify the length of this pre- 760 shared key, EAP-TLS-PSK does so. The pre-shared key MUST be at least 761 16 byte long and have full entropy. For these two modes, it is 762 highly discouraged having derived the pre-shared key from low entropy 763 source, e.g. a password. 765 For RSA_PSK, the shared secret must also be at least 16 byte long. 766 In contrast to PSK and DHE_PSK modes, the shared secret may be also 767 derived from a low entropy source, e.g. a password. This becomes 768 possible because the server authentications with public-key 769 techniques first. 771 In this draft version, however, the following assertion is made. If 772 the shared secret is at least 16 byte long and has full entropy, EAP- 773 TLS-PSK is not susceptible for dictionary attacks. 775 5.6. Key Derivation 777 EAP-TLS-PSK supports key derivation according to [RFC3748] and [I.D.- 778 Keying]. 780 EAP-TLS-PSK exports keys, namely a 64-byte MSK, a 64-byte EMSK and a 781 64-byte IV. 783 Some remarks regarding the key strength. In case of RSA_PSK 784 ciphersuites, the server's private key size should be chosen 785 accordingly to the length of the pre-shared key. Section 5 in 786 [RFC3766] contrasts symmetric key sizes with their public key 787 counterparts, to obtain roughly the same overall key strength. Based 788 on the table below, a 3072-bit RSA key is required to provide 128-bit 789 equivalent key strength. 791 Attack Resistance RSA or DH Modulus DSA subgroup 792 (bits) size (bits) size (bits) 793 ----------------- ----------------- ------------ 794 70 947 128 795 80 1228 145 796 90 1553 153 797 100 1926 184 798 150 4575 279 799 200 8719 373 800 250 14596 475 802 5.7. Session Independence 804 Thanks to its key derivation mechanisms, EAP-PSK provides session 805 independence: passive attacks (such as capture of the EAP 806 conversation) or active attacks (including compromise of the MSK or 807 EMSK) does not enable compromise of subsequent or prior MSKs or 808 EMSKs. The assumption that the random numbers of the TLS 809 client_hello and server_hello messages are random is central for the 810 security of EAP-TLS-PSK in general and session independance in 811 particular. 813 5.8. Exposition of the PSK 815 EAP-TLS-PSK specifies three sets of ciphersuites, which distinguish 816 in the underlying key derivation mechanism. 818 o The PSK and RSA_PSK ciphersuites does not provide perfect forward 819 secrecy. Compromise of the pre-shared key or the pre-shared key 820 and the server's private key (in case of RSA_PSK) leads to 821 compromise of recorded past sessions. 823 o DHE_PSK ciphersuites provide perfect forward secrecy, if a fresh 824 Diffie-Hellman private key is generated for each handshake. 826 5.9. Fragmentation 828 EAP-TLS-PSK supports fragmentation and reassembly. The mechanism is 829 inherited from EAP-TLS ([RFC2716]). 831 5.10. Channel Binding 833 EAP-TLS-PSK does not provide channel binding. 835 5.11. Fast Reconnect 837 EAP-TLS-PSK relies on the Transport Layer Security protocol, which 838 specifies a fast resumption mode. If peer and server agree on 839 continuing a previously established session, the session's master 840 secret can be re-used. That is, related computations can be omitted, 841 which make the fast resumption mode very efficient. 843 For PSK ciphersuites, the speedup is believed to be minimal, since 844 these ciphersuites rely on symmetric key operations only. It is 845 expected, that DHE_PSK and RSA_PSK may benefit from the fast 846 resumption mode. 848 5.12. Identity Protection 850 EAP-TLS-PSK does not provide user identity protection. The 851 client_key_exchange message contains the peer's identity. This 852 message is sent in plaintext. 854 5.13. Protected Ciphersuite Negotiation 856 Since EAP-TLS-PSK relies on TLS, it also supports ciphersuite 857 negotiation. This is done in a securely manner, because the TLS 858 Finished messages authenticate the whole handshake. Therefore, EAP- 859 TLS-PSK provides protected ciphersuite negotiation. 861 5.14. Confidentiality 863 EAP-TLS-PSK does not support this feature. According to Section 864 7.2.1 of [RFC3748], this feature would mandate for the feature 865 'identity protection', which is also not addressed by EAP-TLS-PSK. 867 5.15. Cryptographic Binding 869 This feature is not applicable for EAP-TLS-PSK. 871 5.16. Security Claims 873 This section provides the security claims required by [RFC3748]. 875 [a] Mechanism. 877 * For PSK and DHE_PSK: Pre-shared key. 879 * For RSA_PSK: Server via Public key, Peer via Pre-shared key. 881 [b] Security claims. EAP-TLS-PSK provides: 883 * Mutual authentication (see Section 5.1) 885 * Integrity protection (see Section 5.3) 887 * Replay protection (see Section 5.4) 889 * Key derivation (see Section 5.6) 891 * Dictionary attack resistance (see Section 5.5) 893 * Session independence (see section Section 5.5) 895 * Fast reconnect (see Section 5.11) 897 * Fragmentation (see Section 5.9) 899 * Protected cipher suite negotiation (see Section 5.13) 901 * Perfect Forward Secrecy (at least partially; see Section 5.6) 903 [c] Key strength. EAP-TLS-PSK provides at least a 16-byte effective 904 key strength. 906 [d] Indication of vulnerabilities. EAP-TLS-PSK does not provide: 908 * Identity protection (see Section 5.12) 910 * Confidentiality (see Section 5.14) 912 * Cryptographic binding (see Section 5.15) 913 * Key agreement: the session key is chosen by the peer (see 914 Section 5.6) 916 * Channel binding (see Section 5.10) 918 6. Acknowledgments 920 The authors would like to thank Bernard Aboba and Dan Simon for 921 adopting parts of their EAP-TLS specification, and Florent Bersani 922 for lending parts of the EAP-PSK specification. 924 7. References 926 7.1. Normative References 928 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 929 Requirement Levels", BCP 14, RFC 2119, March 1997. 931 [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", 932 RFC 2246, January 1999. 934 [RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication 935 Protocol", RFC 2716, October 1999. 937 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 938 Levkowetz, "Extensible Authentication Protocol (EAP)", 939 RFC 3748, June 2004. 941 [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites 942 for Transport Layer Security (TLS)", RFC 4279, 943 December 2005. 945 7.2. Informative References 947 [I-D.ietf-eap-keying] 948 Aboba, B., "Extensible Authentication Protocol (EAP) Key 949 Management Framework", draft-ietf-eap-keying-14 (work in 950 progress), June 2006. 952 [IEEE-802.11] 953 Institute of Electrical and Electronics Engineers, 954 "Standard for Telecommunications and Information Exchange 955 Between Systems - LAN/MAN Specific Requirements - Part 11: 956 Wireless LAN Medium Access Control (MAC) and Physical 957 Layer (PHY) Specifications", IEEE Standard 802.11, 1999. 959 [IEEE-802.11i] 960 Institute of Electrical and Electronics Engineers, 961 "Approved Draft Supplement to Standard for 962 Telecommunications and Information Exchange Between 963 Systems-LAN/MAN Specific Requirements - Part 11: Wireless 964 LAN Medium Access Control (MAC) and Physical Layer (PHY) 965 Specifications: Specification for Enhanced Security", 966 IEEE 802.11i-2004, June 2004. 968 [IEEE-802.16e] 969 Institute of Electrical and Electronics Engineers, 970 "Standard for Local and Metropolitan Area Networks: Part 971 16: Air Interface for Fixed and Mobile Broadband Wireless 972 Access Systems: Amendment for Physical and Medium Access 973 Control Layers for Combined Fixed and Mobile Operations in 974 Licensed Bands" IEEE 802.16e", IEEE 802.16e, August 2005. 976 [IEEE-802.1X] 977 Institute of Electrical and Electronics Engineers, "Local 978 and Metropolitan Area Networks: Port-Based Network Access 979 Control", IEEE Standard 802.1X, September 2001. 981 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 982 April 1992. 984 [RFC1570] Simpson, W., "PPP LCP Extensions", RFC 1570, January 1994. 986 [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, 987 RFC 1661, July 1994. 989 [RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, 990 July 1994. 992 [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. 993 Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, 994 August 1996. 996 [RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption 997 Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998. 999 [RFC2420] Kummert, H., "The PPP Triple-DES Encryption Protocol 1000 (3DESE)", RFC 2420, September 1998. 1002 [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", 1003 RFC 2548, March 1999. 1005 [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, 1006 W., and G. Zorn, "Point-to-Point Tunneling Protocol", 1007 RFC 2637, July 1999. 1009 [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, 1010 G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", 1011 RFC 2661, August 1999. 1013 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 1014 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 1015 RFC 3766, April 2004. 1017 [RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible 1018 Authentication Protocol (EAP) Method Requirements for 1019 Wireless LANs", RFC 4017, March 2005. 1021 [RFC4334] Housley, R. and T. Moore, "Certificate Extensions and 1022 Attributes Supporting Authentication in Point-to-Point 1023 Protocol (PPP) and Wireless Local Area Networks (WLAN)", 1024 RFC 4334, February 2006. 1026 [TLSPSK-Perf] 1027 Fang-Chun Kuo, Hannes Tschofenig, Fabian Meyer, and 1028 Xiaoming Fu, "Comparison Studies between Pre-Shared Key 1029 and Public Key Exchange Mechanisms for Transport Layer 1030 Security (TLS)", IFI-TB-2006-01 URL: http:// 1031 user.informatik.uni-goettingen.de/~fkuo/publications/ 1032 ptls-ifi-tb-2006-01.pdf, January 2006. 1034 Authors' Addresses 1036 Thomas Otto 1038 Email: thomas.g.otto@googlemail.com 1040 Hannes Tschofenig 1041 Siemens Networks GmbH & Co KG 1042 Otto-Hahn-Ring 6 1043 Munich 81739 1044 Germany 1046 Email: hannes.tschofenig@siemens.com 1048 Full Copyright Statement 1050 Copyright (C) The Internet Society (2006). 1052 This document is subject to the rights, licenses and restrictions 1053 contained in BCP 78, and except as set forth therein, the authors 1054 retain all their rights. 1056 This document and the information contained herein are provided on an 1057 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1058 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 1059 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 1060 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 1061 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1062 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1064 Intellectual Property 1066 The IETF takes no position regarding the validity or scope of any 1067 Intellectual Property Rights or other rights that might be claimed to 1068 pertain to the implementation or use of the technology described in 1069 this document or the extent to which any license under such rights 1070 might or might not be available; nor does it represent that it has 1071 made any independent effort to identify any such rights. Information 1072 on the procedures with respect to rights in RFC documents can be 1073 found in BCP 78 and BCP 79. 1075 Copies of IPR disclosures made to the IETF Secretariat and any 1076 assurances of licenses to be made available, or the result of an 1077 attempt made to obtain a general license or permission for the use of 1078 such proprietary rights by implementers or users of this 1079 specification can be obtained from the IETF on-line IPR repository at 1080 http://www.ietf.org/ipr. 1082 The IETF invites any interested party to bring to its attention any 1083 copyrights, patents or patent applications, or other proprietary 1084 rights that may cover technology that may be required to implement 1085 this standard. Please address the information to the IETF at 1086 ietf-ipr@ietf.org. 1088 Acknowledgment 1090 Funding for the RFC Editor function is provided by the IETF 1091 Administrative Support Activity (IASA).