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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'Realm' is mentioned on line 1173, but not defined == Missing Reference: 'ServerInfo' is mentioned on line 1173, but not defined -- Possible downref: Non-RFC (?) normative reference: ref. 'NIST-DH' ** Downref: Normative reference to an Informational RFC: RFC 2104 ** Downref: Normative reference to an Informational RFC: RFC 6234 ** Downref: Normative reference to an Informational RFC: RFC 7748 Summary: 3 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group T. Aura 3 Internet-Draft Aalto University 4 Intended status: Standards Track M. Sethi 5 Expires: January 4, 2020 Ericsson 6 July 3, 2019 8 Nimble out-of-band authentication for EAP (EAP-NOOB) 9 draft-aura-eap-noob-06 11 Abstract 13 Extensible Authentication Protocol (EAP) provides support for 14 multiple authentication methods. This document defines the EAP-NOOB 15 authentication method for nimble out-of-band (OOB) authentication and 16 key derivation. This EAP method is intended for bootstrapping all 17 kinds of Internet-of-Things (IoT) devices that have a minimal user 18 interface and no pre-configured authentication credentials. The 19 method makes use of a user-assisted one-directional OOB channel 20 between the peer device and authentication server. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on January 4, 2020. 39 Copyright Notice 41 Copyright (c) 2019 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 3. EAP-NOOB protocol . . . . . . . . . . . . . . . . . . . . . . 5 59 3.1. Protocol overview . . . . . . . . . . . . . . . . . . . . 5 60 3.2. Protocol messages and sequences . . . . . . . . . . . . . 8 61 3.2.1. Common handshake in all EAP exchanges . . . . . . . . 8 62 3.2.2. Initial Exchange . . . . . . . . . . . . . . . . . . 10 63 3.2.3. OOB Step . . . . . . . . . . . . . . . . . . . . . . 11 64 3.2.4. Completion Exchange . . . . . . . . . . . . . . . . . 13 65 3.2.5. Waiting Exchange . . . . . . . . . . . . . . . . . . 15 66 3.3. Protocol data fields . . . . . . . . . . . . . . . . . . 16 67 3.3.1. Peer identifier, realm and NAI . . . . . . . . . . . 16 68 3.3.2. Message data fields . . . . . . . . . . . . . . . . . 18 69 3.4. Fast reconnect and rekeying . . . . . . . . . . . . . . . 23 70 3.4.1. Persistent EAP-NOOB association . . . . . . . . . . . 23 71 3.4.2. Reconnect Exchange . . . . . . . . . . . . . . . . . 24 72 3.4.3. User reset . . . . . . . . . . . . . . . . . . . . . 27 73 3.5. Key derivation . . . . . . . . . . . . . . . . . . . . . 28 74 3.6. Error handling . . . . . . . . . . . . . . . . . . . . . 31 75 3.6.1. Invalid messages . . . . . . . . . . . . . . . . . . 33 76 3.6.2. Unwanted peer . . . . . . . . . . . . . . . . . . . . 33 77 3.6.3. State mismatch . . . . . . . . . . . . . . . . . . . 33 78 3.6.4. Negotiation failure . . . . . . . . . . . . . . . . . 33 79 3.6.5. Cryptographic verification failure . . . . . . . . . 34 80 3.6.6. Application-specific failure . . . . . . . . . . . . 34 81 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 82 4.1. Cryptosuites . . . . . . . . . . . . . . . . . . . . . . 35 83 4.2. Message Types . . . . . . . . . . . . . . . . . . . . . . 35 84 4.3. Error codes . . . . . . . . . . . . . . . . . . . . . . . 36 85 4.4. Domain name reservation considerations . . . . . . . . . 37 86 5. Implementation Status . . . . . . . . . . . . . . . . . . . . 38 87 5.1. Implementation with wpa_supplicant and hostapd . . . . . 38 88 5.2. Protocol modeling . . . . . . . . . . . . . . . . . . . . 39 89 6. Security considerations . . . . . . . . . . . . . . . . . . . 39 90 6.1. Authentication principle . . . . . . . . . . . . . . . . 39 91 6.2. Identifying correct endpoints . . . . . . . . . . . . . . 41 92 6.3. Trusted path issues and misbinding attacks . . . . . . . 41 93 6.4. Peer identifiers and attributes . . . . . . . . . . . . . 42 94 6.5. Identity protection . . . . . . . . . . . . . . . . . . . 43 95 6.6. Downgrading threats . . . . . . . . . . . . . . . . . . . 44 96 6.7. Recovery from loss of last message . . . . . . . . . . . 45 97 6.8. EAP security claims . . . . . . . . . . . . . . . . . . . 46 98 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 99 7.1. Normative references . . . . . . . . . . . . . . . . . . 48 100 7.2. Informative references . . . . . . . . . . . . . . . . . 49 101 Appendix A. Exchanges and events per state . . . . . . . . . . . 51 102 Appendix B. Application-specific parameters . . . . . . . . . . 52 103 Appendix C. ServerInfo and PeerInfo contents . . . . . . . . . . 53 104 Appendix D. EAP-NOOB roaming . . . . . . . . . . . . . . . . . . 55 105 Appendix E. OOB message as URL . . . . . . . . . . . . . . . . . 56 106 Appendix F. Example messages . . . . . . . . . . . . . . . . . . 57 107 Appendix G. TODO list . . . . . . . . . . . . . . . . . . . . . 59 108 Appendix H. Version history . . . . . . . . . . . . . . . . . . 59 109 Appendix I. Acknowledgments . . . . . . . . . . . . . . . . . . 61 110 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 61 112 1. Introduction 114 This document describes a method for registration, authentication and 115 key derivation for network-connected ubiquitous computing devices, 116 such as consumer and enterprise appliances that are part of the 117 Internet of Things (IoT). These devices may be off-the-shelf 118 hardware that is sold and distributed without any prior registration 119 or credential-provisioning process. Thus, the device registration in 120 a server database, ownership of the device, and the authentication 121 credentials for both network access and application-level security 122 must all be established at the time of the device deployment. 123 Furthermore, many such devices have only limited user interfaces that 124 could be used for their configuration. Often, the interfaces are 125 limited to either only input (e.g. camera) or output (e.g. display 126 screen). The device configuration is made more challenging by the 127 fact that the devices may exist in large numbers and may have to be 128 deployed or re-configured nimbly based on user needs. 130 More specifically, the devices may have the following 131 characteristics: 133 o no pre-established relation with a specific server or user, 135 o no pre-provisioned device identifier or authentication 136 credentials, 138 o limited user interface and configuration capabilities. 140 Many proprietary OOB configuration methods exits for specific IoT 141 devices. The goal of this specification is to provide an open 142 standard and a generic protocol for bootstrapping the security of 143 network-connected appliances, such as displays, printers, speakers, 144 and cameras. The security bootstrapping in this specification makes 145 use of a user-assisted out-of-band (OOB) channel. The device 146 authentication relies on user having physical access to the device, 147 and the of the key exchange security is based on the assumption that 148 attackers are not able to observe or modify the messages conveyed 149 through the OOB channel. We follow the common approach taken in 150 pairing protocols: performing a Diffie-Hellman key exchange over the 151 insecure network and authenticating the established key with the help 152 of the OOB channel in order to prevent impersonation and man-in-the- 153 middle (MitM) attacks. 155 The solution presented here is intended for devices that have either 156 an input or output interface, such as a camera, microphone, display 157 screen, speakers or blinking LED light, which is able to send or 158 receive dynamically generated messages of tens of bytes in length. 159 Naturally, this solution may not be appropriate for very small 160 sensors or actuators that have no user interface at all or for 161 devices that are inaccessible to the user. We also assume that the 162 OOB channel is at least partly automated (e.g. camera scanning a bar 163 code) and, thus, there is no need to absolutely minimize the length 164 of the data transferred through the OOB channel. This differs, for 165 example, from Bluetooth simple pairing [BluetoothPairing], where it 166 is critical to minimize the length of the manually transferred or 167 compared codes. Since the OOB messages are dynamically generated, we 168 do not support static printed registration codes. This also prevents 169 attacks where a static secret code would be leaked. 171 2. Terminology 173 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 174 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 175 document are to be interpreted as described in [RFC2119]. 177 In addition, this document frequently uses the following terms as 178 they have been defined in [RFC5216]: 180 authenticator The entity initiating EAP authentication. 182 peer The entity that responds to the authenticator. In 183 [IEEE-802.1X], this entity is known as the supplicant. 185 server The entity that terminates the EAP authentication method with 186 the peer. In the case where no backend authentication server 187 is used, the EAP server is part of the authenticator. In the 188 case where the authenticator operates in pass-through mode, the 189 EAP server is located on the backend authentication server. 191 3. EAP-NOOB protocol 193 This section defines the EAP-NOOB protocol. The protocol is a 194 generalized version of the original idea presented by Sethi et al. 195 [Sethi14]. 197 3.1. Protocol overview 199 One EAP-NOOB protocol execution spans multiple EAP conversations, 200 called Exchanges. This is necessary to leave time for the OOB 201 message to be delivered, as will be explained below. 203 The overall protocol starts with the Initial Exchange, in which the 204 server allocates an identifier to the peer, and the server and peer 205 negotiate the protocol version and cryptosuite (i.e. cryptographic 206 algorithm suite), exchange nonces and perform an Ephemeral Elliptic 207 Curve Diffie-Hellman (ECDHE) key exchange. The user-assisted OOB 208 Step then takes place. This step requires only one out-of-band 209 message either from the peer to the server or from the server to the 210 peer. While waiting for the OOB Step action, the peer MAY probe the 211 server by reconnecting to it with EAP-NOOB. If the OOB Step has 212 already taken place, the probe leads to the Completion Exchange, 213 which completes the mutual authentication and key confirmation. On 214 the other hand, if the OOB Step has not yet taken place, the probe 215 leads to the Waiting Exchange, and the peer will perform another 216 probe after a server-defined minimum waiting time. The Initial 217 Exchange and Waiting Exchange always end in EAP-Failure, while the 218 Completion Exchange may result in EAP-Success. Once the peer and 219 server have performed a successful Completion Exchange, both 220 endpoints store the created association in persistent storage, and 221 the OOB Step is not repeated. Thereafter, creation of new temporal 222 keys, ECDHE rekeying, and updates of cryptographic algorithms can be 223 achieved with the Reconnect Exchange. 225 OOB Output/Initial Exchange/ 226 Waiting Exchange 227 .-----. 228 | v 229 .------------------. Initial .------------------. 230 | | Exchange | | 231 .->| 0. Unregistered |---------------->|1. Waiting for OOB| 232 | | | | | 233 | '------------------' '------------------' 234 | | | ^ 235 User Reset Completion | | | 236 | Exchange | OOB OOB 237 |<-------. .-------------------------' Input Reject/ 238 | | | | Initial 239 | | | | Exchange 240 | | v v | 241 | .------------------. Completion .------------------. 242 | | | Exchange | | 243 | | 4. Registered |<----------------| 2. OOB Received | 244 | | | | | 245 | '------------------' '------------------' 246 | | ^ 247 | Mobility/ | 248 | Timeout/ Reconnect 249 | Failure Exchange 250 | | | 251 | v | 252 | .-----------------. 253 | | | 254 '--| 3. Reconnecting | 255 | | 256 '-----------------' 258 Figure 1: EAP-NOOB server-peer association state machine 260 Figure 1 shows the association state machine, which is the same for 261 the server and for the peer. (For readability, only the main state 262 transitions are shown. The complete table of transitions can be 263 found in Appendix A.) When the peer initiates the EAP-NOOB method, 264 the server chooses the ensuing message exchange based on the 265 combination of the server and peer states. The EAP server and peer 266 are initially in the Unregistered state, in which no state 267 information needs to be stored. Before a successful Completion 268 Exchange, the server-peer association state is ephemeral in both the 269 server and peer (ephemeral states 0..2), and either endpoint may 270 cause the protocol to fall back to the Initial Exchange. After the 271 Completion Exchange has resulted in EAP-Success, the association 272 state becomes persistent (persistent states 3..4). Only user reset 273 or memory failure can cause the return of the server or the peer from 274 the persistent states to the ephemeral states and to the Initial 275 Exchange. 277 The server MUST NOT repeat a successful OOB Step with the same peer 278 except if the association with the peer is explicitly reset by the 279 user or lost due to failure of the persistent storage in the server. 280 More specifically, once the association has entered the Registered 281 state, the server MUST NOT delete the association or go back to 282 states 0..2 without explicit user approval. Similarly, the peer MUST 283 NOT repeat the OOB Step unless the user explicitly deletes from the 284 peer the association with the server or resets the peer to the 285 Unregistered state. The server and peer MAY implement user reset of 286 the association by deleting the state data from that endpoint. If an 287 endpoint continues to store data about the association after the user 288 reset, its behavior SHOULD be equivalent to having deleted the 289 association data. 291 It can happen that the peer accidentally or through user reset loses 292 its persistent state and reconnects to the server without a 293 previously allocated peer identifier. In that case, the server MUST 294 treat the peer as a new peer. The server MAY use auxiliary 295 information, such as the PeerInfo field received in the Initial 296 Exchange, to detect multiple associations with the same peer. 297 However, it MUST NOT delete or merge redundant associations without 298 user or application approval because EAP-NOOB internally has no 299 secure way of verifying that the two peers are the same physical 300 device. Similarly, the server might lose the association state 301 because of a memory failure or user reset. In that case, the only 302 way to recover is that the user resets also the peer. 304 A special feature of the EAP-NOOB method is that the server is not 305 assumed to have any a-priori knowledge of the peer. Therefore, the 306 peer initially uses the generic identity string "noob@eap-noob.net" 307 as its network access identifier (NAI). The server then allocates a 308 server-specific identifier to the peer. The generic NAI serves two 309 purposes: firstly, it tells the server that the peer supports and 310 expects the EAP-NOOB method and, secondly, it allows routing of the 311 EAP-NOOB sessions to a specific authentication server in the AAA 312 architecture. 314 EAP-NOOB is an unusual EAP method in that the peer has to have 315 multiple EAP conversations with the server before it can receive EAP- 316 Success. The reason is that, while EAP allows delays between the 317 request-response pairs, e.g. for repeated password entry, the user 318 delays in OOB authentication can be much longer than in password 319 trials. In particular, EAP-NOOB supports also peers with no input 320 capability in the user interface. Since user cannot initiate the 321 protocol in these devices, they have to perform the Initial Exchange 322 opportunistically and hope for the OOB Step to take place within a 323 timeout period (NoobTimeout), which is why the timeout needs to be 324 several minutes rather than seconds. For example, consider a printer 325 (peer) that outputs the OOB message on paper, which is then scanned 326 for the server. To support such high-latency OOB channels, the peer 327 and server perform the Initial Exchange in one EAP conversation, then 328 allow time for the OOB message to be delivered, and later perform the 329 Waiting and Completion Exchanges in different EAP conversations. 331 3.2. Protocol messages and sequences 333 This section defines the EAP-NOOB exchanges, which correspond to EAP 334 conversations. The exchanges start with a common handshake, which 335 determines the type of the following exchange. The common handshake 336 messages and the subsequent messages for each exchange type are 337 listed in the diagrams below. The diagrams also specify the data 338 members present in each message. Each exchange comprises multiple 339 EAP requests-response pairs and ends in either EAP-Failure, 340 indicating that authentication is not (yet) successful, or in EAP- 341 Success. 343 3.2.1. Common handshake in all EAP exchanges 345 All EAP-NOOB exchanges start with common handshake messages. The 346 handshake starts with the identity request and response that are 347 common to all EAP methods. Their purpose is to enable the AAA 348 architecture to route the EAP conversation to the EAP server and to 349 enable the EAP server to select the EAP method. The handshake then 350 continues with one EAP-NOOB request-response pair in which the server 351 discovers the peer identifier used in EAP-NOOB and the peer state. 353 In more detail, each EAP-NOOB exchanges begin with the authenticator 354 sending an EAP-Request/Identity packet to the peer. From this point 355 on, the EAP conversation occurs between the server and the peer, and 356 the authenticator acts as a pass-through device. The peer responds 357 to the authenticator with an EAP-Response/Identity packet, which 358 contains the network access identifier (NAI). The authenticator, 359 acting as a pass-through device, forwards this response and the 360 following EAP conversation between the peer and the AAA architecture. 361 The AAA architecture routes the conversation to a specific AAA server 362 (called "EAP server" or simply "server" in this specification) based 363 on the realm part of the NAI. The server selects the EAP-NOOB method 364 based on the user part of the NAI, as defined in Section 3.3.1. 366 After receiving the EAP-Response/Identity message, the server sends 367 the first EAP-NOOB request (Type=9) to the peer, which responds with 368 the peer identifier (PeerId) and state (PeerState) in the range 0..3. 369 However, the peer SHOULD omit the PeerId from the response (Type=9) 370 when PeerState=0. The server then chooses the EAP-NOOB exchange, 371 i.e. the ensuing message sequence, as explained below. The peer 372 recognizes the exchange based on the message type field (Type) of the 373 next EAP-NOOB request received from the server. 375 The server determines the exchange type based on the combination of 376 the peer and server states as follows (also summarized in Figure 11). 377 If one of the peer and server is in the Unregistered (0) state and 378 the other is in one of the ephemeral states (0..2), the server 379 chooses the Initial Exchange. If one of the peer or server is in the 380 OOB Received (2) state and the other is either in the Waiting for OOB 381 (1) or OOB Received (2) state, the OOB Step has taken place and the 382 server chooses the Completion Exchange. If both the server and peer 383 are in the Waiting for OOB (1) state, the server chooses the Waiting 384 Exchange. If the peer is in the Reconnecting (3) state and the 385 server is in the Registered (4) or Reconnecting (3) state, the server 386 chooses the Reconnect Exchange. All other state combinations are 387 error situations where user action is required, and the server 388 indicates such errors to the peer with the error code 2002 (see 389 Section 3.6.3). Note also that the peer MUST NOT initiate EAP-NOOB 390 when the peer is in Registered (4) state. 392 EAP Peer EAP Server 393 | | 394 |<----------- EAP-Request/Identity -| | 395 | | 396 | | 397 |------------ EAP-Response/Identity -------------->| 398 | (NAI=noob@eap-noob.net) | 399 | | 400 | | 401 |<----------- EAP-Request/EAP-NOOB ----------------| 402 | (Type=9) | 403 | | 404 | | 405 |------------ EAP-Response/EAP-NOOB -------------->| 406 | (Type=9,[PeerId],PeerState=1) | 407 | | 408 | continuing with exchange-specific messages... | 410 Figure 2: Common handshake in all EAP-NOOB exchanges 412 3.2.2. Initial Exchange 414 The Initial Exchange comprises the common handshake and two further 415 EAP-NOOB request-response pairs, one for version, cryptosuite and 416 parameter negotiation and the other for the ECDHE key exchange. The 417 first EAP-NOOB request (Type=1) from the server contains a newly 418 allocated PeerId for the peer and an optional Realm. The server 419 allocates a new PeerId in the Initial Exchange regardless of any old 420 PeerId in the username part of the received NAI. The server also 421 sends in the request a list of the protocol versions (Vers) and 422 cryptosuites (Cryptosuites) it supports, an indicator of the OOB 423 channel directions it supports (Dirs), and a ServerInfo object. The 424 peer chooses one of the versions and cryptosuites. The peer sends a 425 response (Type=1) with the selected protocol version (Verp), the 426 received PeerId, the selected cryptosuite (Cryptosuitep), an 427 indicator of the OOB channel directions selected by the peer (Dirp), 428 and a PeerInfo object. In the second EAP-NOOB request and response 429 (Type=2), the server and peer exchange the public components of their 430 ECDHE keys and nonces (PKs,Ns,PKp,Np). The ECDHE keys MUST be based 431 on the negotiated cryptosuite i.e. Cryptosuitep. The Initial 432 Exchange always ends with EAP-Failure from the server because the 433 authentication cannot yet be completed. 435 EAP Peer EAP Server 436 | ...continuing from common handshake | 437 | | 438 |<----------- EAP-Request/EAP-NOOB ----------------| 439 | (Type=1,Vers,PeerId,[Realm], | 440 | Cryptosuites,Dirs,ServerInfo) | 441 | | 442 | | 443 |------------ EAP-Response/EAP-NOOB -------------->| 444 | (Type=1,Verp,PeerId,Cryptosuitep, | 445 | Dirp,PeerInfo) | 446 | | 447 | | 448 |<----------- EAP-Request/EAP-NOOB ----------------| 449 | (Type=2,PeerId,PKs,Ns,[SleepTime]) | 450 | | 451 | | 452 |------------ EAP-Response/EAP-NOOB -------------->| 453 | (Type=2,PeerId,PKp,Np) | 454 | | 455 | | 456 |<----------- EAP-Failure -------------------------| 457 | | 459 Figure 3: Initial Exchange 461 At the conclusion of the Initial Exchange, both the server and the 462 peer move to the Waiting for OOB (1) state. 464 3.2.3. OOB Step 466 The OOB Step, labeled as OOB Output and OOB Input in Figure 1, takes 467 place after the Initial Exchange. Depending on the negotiated OOB 468 channel direction, the peer or the server outputs the OOB message 469 shown in Figure 4 or Figure 5, respectively. The data fields are the 470 PeerId, the secret nonce Noob, and the cryptographic fingerprint 471 Hoob. The contents of the data fields are defined in Section 3.3.2. 472 The OOB message is delivered to the other endpoint via a user- 473 assisted OOB channel. 475 For brevity, we will use the terms OOB sender and OOB receiver in 476 addition to the already familiar EAP server and EAP peer. If the OOB 477 message is sent in in the server-to-peer direction, the OOB sender is 478 the server and the OOB receiver is the peer. On the other hand, if 479 the OOB message is sent in the peer-to-server direction, the OOB 480 sender is the peer and the OOB receiver is the server. 482 EAP Peer EAP Server 483 | | 484 |=================OOB=============================>| 485 | (PeerId,Noob,Hoob) | 486 | | 488 Figure 4: OOB Step, from peer to EAP server 490 EAP Peer EAP Server 491 | | 492 |<================OOB==============================| 493 | (PeerId,Noob,Hoob) | 494 | | 496 Figure 5: OOB Step, from EAP server to peer 498 The OOB receiver MUST compare the received value of the fingerprint 499 Hoob with a value that it computes locally. If the values are equal, 500 the receiver moves to the OOB Received (2) state. Otherwise, the 501 receiver MUST reject the OOB message. For usability reasons, the OOB 502 receiver SHOULD indicate the acceptance or rejection of the OOB 503 message to the user. The receiver SHOULD reject invalid OOB messages 504 without changing its state, until an application-specific number of 505 invalid messages (OobRetries) has been reached, after which the 506 receiver SHOULD consider it an error and go back to the Unregistered 507 (0) state. 509 The server or peer MAY send multiple OOB messages with different Noob 510 values while in the Waiting for OOB (1) state. The OOB sender SHOULD 511 remember the Noob values until they expire and accept any one of them 512 in the following Completion Exchange. The Noob values sent by the 513 server expire after an application-dependent timeout (NoobTimeout), 514 and the server MUST NOT accept Noob values older than that in the 515 Completion Exchange. The RECOMMENDED value for NoobTimeout is 3600 516 seconds if there are no application-specific reasons for making it 517 shorter or longer. The Noob values sent by the peer expire as 518 defined in Section 3.2.5. 520 The OOB receiver does not accept further OOB messages after it has 521 accepted one and moved to the OOB Received (2) state. However, the 522 receiver MAY buffer redundant OOB messages in case OOB message expiry 523 or similar error detected in the Completion Exchange causes it to 524 return to the Waiting for OOB (1) state. It is RECOMMENED that the 525 OOB receiver notifies the user about redundant OOB messages, but it 526 MAY also discard them silently. 528 The sender will typically generate a new Noob, and therefore a new 529 OOB message, at constant time intervals (NoobInterval). The 530 RECOMMENDED interval is NoobInterval = NoobTimeout / 2, so that the 531 two latest values are always accepted. However, the timing of the 532 Noob generation may also be based on user interaction or on 533 implementation considerations. 535 Even though not recommended (see Section 3.3), this specification 536 allows both directions to be negotiated (Dirp=3) for the OOB channel. 537 In that case, both sides SHOULD output the OOB message, and it is up 538 to the user to deliver one of them. 540 The details of the OOB channel implementation including the message 541 encoding are defined by the application. Appendix E gives an example 542 of how the OOB message can be encoded as a URL that may be embedded 543 in a QR code and NFC tag. 545 3.2.4. Completion Exchange 547 After the Initial Exchange, if both the server and the peer support 548 the peer-to-server direction for the OOB channel, the peer SHOULD 549 initiate the EAP-NOOB method again after an applications-specific 550 waiting time in order to probe for completion of the OOB Step. Also, 551 if both sides support the server-to-peer direction of the OOB 552 exchange and the peer receives the OOB message, it SHOULD initiate 553 the EAP-NOOB method immediately. Depending on the combination of the 554 peer and server states, the server continues with with the Completion 555 Exchange or Waiting Exchange (see Section 3.2.1 on how the server 556 makes this decision). 558 The Completion Exchange comprises the common handshake and one or two 559 further EAP-NOOB request-response pairs. If the peer is in the 560 Waiting for OOB (1) state, the OOB message has been sent in the peer- 561 to-server direction. In that case, only one request-response pair 562 (Type=4) takes place. In the request, the server sends the NoobId 563 value, which the peer uses to identify the exact OOB message received 564 by the server. On the other hand, if the peer is in the OOB Received 565 (2) state, the direction of the OOB message is from server to peer. 566 In that case, two request-response pairs (Type=8 and Type=4) are 567 needed. The purpose of the first request-response pair (Type=8) is 568 that it enables the server to discover NoobId, which identifies the 569 exact OOB message received by the peer. The server returns the same 570 NoobId to the peer in the latter request. 572 In the last and sometimes only request-response pair (Type=4) of the 573 Completion Exchange, the server and peer exchange message 574 authentication codes. Both sides MUST compute the keys Kms and Kmp 575 as defined in Section 3.5 and the message authentication codes MACs 576 and MACp as defined in Section 3.3.2. Both sides MUST compare the 577 received message authentication code with a locally computed value. 578 If the peer finds that it has received the correct value of MACs and 579 the server finds that it has received the correct value of MACp, the 580 Completion Exchange ends in EAP-Success. Otherwise, the endpoint 581 where the comparison fails indicates this with an error message 582 (error code 4001, see Section 3.6.1) and the Completion Exchange ends 583 in EAP-Failure. 585 After successful Completion Exchange, both the server and the peer 586 move to the Registered (4) state. They also derive the output keying 587 material and store the persistent EAP-NOOB association state as 588 defined in Section 3.4 and Section 3.5. 590 It is possible that the OOB message expires before it is received. 591 In that case, the sender of the OOB message no longer recognizes the 592 NoobId that it receives in the Completion Exchange. Another reason 593 why the OOB sender might not recognize the NoobId is if the received 594 OOB message was spoofed and contained an attacker-generated Noob 595 value. The recipient of an unrecognized NoobId indicates this with 596 an error message (error code 2003, see Section 3.6.1) and the 597 Completion Exchange ends in EAP-Failure. The recipient of the error 598 message 2003 moves back to the Waiting for OOB (1) state. This state 599 transition is shown as OOB Reject in Figure 1 (even though it really 600 is a specific type of failed Completion Exchange). The sender of the 601 error message, on the other hand, stays in its previous state. 603 Although it is not expected to occur in practice, poor user interface 604 design could lead to two OOB messages delivered simultaneously, one 605 from the peer to the server and the other from the server to the 606 peer. The server detects this event in the beginning of the 607 Completion Exchange by observing that both the server and peer are in 608 the OOB Received state (2). In that case, as a tiebreaker, the 609 server MUST behave as if only the server-to-peer message had been 610 delivered. 612 EAP Peer EAP Server 613 | ...continuing from common handshake | 614 | | 615 |<----------- [ EAP-Request/EAP-NOOB ] ------------| 616 | (Type=8,PeerId) | 617 | | 618 | | 619 |------------ [ EAP-Response/EAP-NOOB ] ---------->| 620 | (Type=8,PeerId,NoobId) | 621 | | 622 | | 623 |<----------- EAP-Request/EAP-NOOB ----------------| 624 | (Type=4,PeerId,NoobId,MACs) | 625 | | 626 | | 627 |------------ EAP-Response/EAP-NOOB -------------->| 628 | (Type=4,PeerId,MACp) | 629 | | 630 | | 631 |<----------- EAP-Success -------------------------| 632 | | 634 Figure 6: Completion Exchange 636 3.2.5. Waiting Exchange 638 As explained in Section 3.2.4, the peer SHOULD probe the server for 639 completion of the OOB Step. When the combination of the peer and 640 server states indicates that the OOB message has not yet been 641 delivered, the server chooses the Waiting Exchange (see Section 3.2.1 642 on how the server makes this decision). The Waiting Exchange 643 comprises the common handshake and one further request-response pair, 644 and it ends always in EAP-Failure. 646 In order to limit the rate at which peers probe the server, the 647 server MAY send to the peer either in the Initial Exchange or in the 648 Waiting Exchange a minimum time to wait before probing the server 649 again. A peer that has not received an OOB message MUST wait at 650 least the server-specified minimum waiting time in seconds 651 (SleepTime) before initiating EAP again with the same server. The 652 peer uses the latest SleepTime value that it has received in or after 653 the Initial Exchange. If the server has not sent any SleepTime 654 value, the peer SHOULD wait for an application-specified minimum time 655 (SleepTimeDefault). 657 After the Waiting Exchange, the peer MUST discard (from its local 658 ephemeral storage) Noob values that it has sent to the server in OOB 659 messages that are older than the application-defined timeout 660 NoobTimeout (see Section 3.2.3). The peer SHOULD discard such 661 expired Noob values even if the probing failed, e.g. because of 662 failure to connect to the EAP server or incorrect HMAC. The timeout 663 of peer-generated Noob values is defined like this in order to allow 664 the peer to probe the server once after it has waited for the server- 665 specified SleepTime. 667 If the server and peer have negotiated to use only the server-to-peer 668 direction for the OOB channel (Dirp=2), the peer SHOULD nevertheless 669 probe the server. The purpose of this is to keep the server informed 670 about the peers that are still waiting for OOB messages. The server 671 MAY set SleepTime to a high number (3600) to prevent the peer from 672 probing the server frequently. 674 EAP Peer EAP Server 675 | ...continuing from common handshake | 676 | | 677 |<----------- EAP-Request/EAP-NOOB ----------------| 678 | (Type=3,PeerId,[SleepTime]) | 679 | | 680 | | 681 |------------ EAP-Response/EAP-NOOB -------------->| 682 | (Type=3,PeerId) | 683 | | 684 | | 685 |<----------- EAP-Failure -------------------------| 686 | | 688 Figure 7: Waiting Exchange 690 3.3. Protocol data fields 692 This section defines the various identifiers and data fields used in 693 the EAP-NOOB protocol. 695 3.3.1. Peer identifier, realm and NAI 697 The server allocates a new peer identifier (PeerId) for the peer in 698 the Initial Exchange. The peer identifier MUST follow the syntax of 699 the utf8-username specified in [RFC7542]. The server MUST generate 700 the identifiers in such a way that they do not repeat and cannot be 701 guessed by the peer or third parties before the server sends them to 702 the peer in the Initial Exchange. One way to generate the 703 identifiers is to choose a random 16-byte identifier and to base64url 704 encode it without padding [RFC4648] into a 22-character string. 706 Another way to generate the identifiers is to choose a random 707 22-character alphanumeric string. It is RECOMMENDED to not use 708 identifiers longer than this because they result in longer OOB 709 messages. 711 The peer uses the allocated PeerId to identify itself to the server 712 in the subsequent exchanges. It sets the PeerId value in response 713 type 9 as follows. When the peer is in the Unregistered (0) state, 714 it SHOULD omit the PeerId from response type 9. When the peer is in 715 one of the states 1..2, it MUST use the PeerId that the server 716 assigned to it in the latest Initial Exchange. When the peer is in 717 one of the persistent states 3..4, it MUST use the PeerId from its 718 persistent EAP-NOOB association. (The PeerId is written to the 719 association when the peer moves to the Registered (4) state after a 720 Completion Exchange.) 722 The default realm for the peer is "eap-noob.net". However, the user 723 or application MAY provide a different default realm to the peer. 724 Furthermore, the server MAY assign a new realm to the peer in the 725 Initial Exchange or Reconnect Exchange, in the Realm field of 726 response types 1 and 5. The Realm value MUST follow the syntax of 727 the utf8-realm specified in [RFC7542]. When the peer is in the 728 Unregistered (0) state, or when the peer is in one of the states 1..2 729 and the server did not send a Realm in the latest Initial Exchange, 730 the peer MUST use the default realm. When the peer is in one of the 731 states 1..2 and the server sent a Realm in the latest Initial 732 Exchange, the peer MUST use that realm. Finally, when the peer is in 733 one of the persistent states 3..4, it MUST use the Realm from its 734 persistent EAP-NOOB association. (The Realm is written to the 735 association when the peer moves to the Registered (4) state after a 736 Completion Exchange or Reconnect Exchange.) 738 To compose its NAI [RFC7542], the peer concatenates the string 739 "noob@" and the server-assigned realm. When no server-assigned realm 740 is available, the default value is used instead. 742 The purpose of the server-assigned realm is to enable more flexible 743 routing of the EAP sessions over the AAA infrastructure, including 744 roaming scenarios (see Appendix D). Moreover, some Authenticators or 745 AAA servers use the assigned Realm to determine peer-specific 746 connection parameters, such as isolating the peer to a specific VLAN. 747 The possibility to configure a different default realm enables 748 registration of new devices while roaming. It also enables 749 manufacturers to set up their own AAA servers for bootstrapping of 750 new peer devices. 752 The peer's PeerId and Realm are ephemeral until a successful 753 Completion Exchange takes place. Thereafter, the values become parts 754 of the persistent EAP-NOOB association, until the user resets the 755 peer and the server or until a new Realm is assigned in the Reconnect 756 Exchange. 758 3.3.2. Message data fields 760 Table 1 defines the data fields in the protocol messages. The in- 761 band messages are formatted as JSON objects [RFC8259] in UTF-8 762 encoding. The JSON member names are in the left-hand column of the 763 table. 765 +------------------+------------------------------------------------+ 766 | Data field | Description | 767 +------------------+------------------------------------------------+ 768 | Vers, Verp | EAP-NOOB protocol versions supported by the | 769 | | EAP server, and the protocol version chosen by | 770 | | the peer. Vers is a JSON array of unsigned | 771 | | integers, and Verp is an unsigned integer. | 772 | | Example values are "[1]" and "1", | 773 | | respectively. | 774 | | | 775 | PeerId | Peer identifier as defined in Section 3.3.1. | 776 | | | 777 | Realm | Peer realm as defined in Section 3.3.1. | 778 | | | 779 | PeerState | Peer state is an integer in the range 0..4 | 780 | | (see Figure 1). However, only values 0..3 are | 781 | | ever sent in the protocol messages. | 782 | | | 783 | Type | EAP-NOOB message type. The type is an integer | 784 | | in the range 0..9. EAP-NOOB requests and the | 785 | | corresponding responses share the same type | 786 | | value. | 787 | | | 788 | PKs, PKp | The public components of the ECDHE keys of the | 789 | | server and peer. PKs and PKp are sent in the | 790 | | JSON Web Key (JWK) format [RFC7517]. Detailed | 791 | | format of the JWK object is defined by the | 792 | | cryptosuite. | 793 | | | 794 | Cryptosuites, | The identifiers of cryptosuites supported by | 795 | Cryptosuitep | the server and of the cryptosuite selected by | 796 | | the peer. The server-supported cryptosuites in | 797 | | Cryptosuites are formatted as a JSON array of | 798 | | the identifier integers. The server MUST send | 799 | | a nonempty array with no repeating elements, | 800 | | ordered by decreasing priority. The peer MUST | 801 | | respond with exactly one suite in the | 802 | | Cryptosuitep value, formatted as an identifier | 803 | | integer. The registration of cryptosuites is | 804 | | specified in Section 4.1. Example values are | 805 | | "[1]" and "1", respectively. | 806 | | | 807 | Dirs, Dirp | The OOB channel directions supported by the | 808 | | server and the directions selected by the | 809 | | peer. The possible values are 1=peer-to- | 810 | | server, 2=server-to-peer, 3=both directions. | 811 | | | 812 | Dir | The actual direction of the OOB message (1 | 813 | | =peer-to-server, 2=server-to-peer). This value | 814 | | is not sent over any communication channel but | 815 | | it is included in the computation of the | 816 | | cryptographic fingerprint Hoob. | 817 | | | 818 | Ns, Np | 32-byte nonces for the Initial Exchange. | 819 | | | 820 | ServerInfo | This field contains information about the | 821 | | server to be passed from the EAP method to the | 822 | | application layer in the peer. The information | 823 | | is specific to the application or to the OOB | 824 | | channel and it is encoded as a JSON object of | 825 | | at most 500 bytes. It could include, for | 826 | | example, the access-network name and server | 827 | | name or a Uniform Resource Locator (URL) | 828 | | [RFC4266] or some other information that helps | 829 | | the user to deliver the OOB message to the | 830 | | server through the out-of-band channel. | 831 | | | 832 | PeerInfo | This field contains information about the peer | 833 | | to be passed from the EAP method to the | 834 | | application layer in the server. The | 835 | | information is specific to the application or | 836 | | to the OOB channel and it is encoded as a JSON | 837 | | object of at most 500 bytes. It could include, | 838 | | for example, the peer brand, model and serial | 839 | | number, which help the user to distinguish | 840 | | between devices and to deliver the OOB message | 841 | | to the correct peer through the out-of-band | 842 | | channel. | 843 | | | 844 | SleepTime | The number of seconds for which peer MUST NOT | 845 | | start a new execution of the EAP-NOOB method | 846 | | with the authenticator, unless the peer | 847 | | receives the OOB message or the peer is reset | 848 | | by the user. The server can use this field to | 849 | | limit the rate at which peers probe it. | 850 | | SleepTime is an unsigned integer in the range | 851 | | 0..3600. | 852 | | | 853 | Noob | 16-byte secret nonce sent through the OOB | 854 | | channel and used for the session key | 855 | | derivation. The endpoint that received the OOB | 856 | | message uses this secret in the Completion | 857 | | Exchange to authenticate the exchanged key to | 858 | | the endpoint that sent the OOB message. | 859 | | | 860 | Hoob | 16-byte cryptographic fingerprint (i.e. hash | 861 | | value) computed from all the parameters | 862 | | exchanged in the Initial Exchange and in the | 863 | | OOB message. Receiving this fingerprint over | 864 | | the OOB channel guarantees the integrity of | 865 | | the key exchange and parameter negotiation. | 866 | | Hence, it authenticates the exchanged key to | 867 | | the endpoint that receives the OOB message. | 868 | | | 869 | NoobId | 16-byte identifier for the OOB message, | 870 | | computed with a one-way function from the | 871 | | nonce Noob in the message. | 872 | | | 873 | MACs, MACp | Message authentication codes (HMAC) for mutual | 874 | | authentication, key confirmation, and | 875 | | integrity check on the exchanged information. | 876 | | The input to the HMAC is defined below, and | 877 | | the key for the HMAC is defined in Section | 878 | | 3.5. | 879 | | | 880 | Ns2, Np2 | 32-byte Nonces for the Reconnect Exchange. | 881 | | | 882 | KeyingMode | Integer indicating the key derivation method. | 883 | | 0 in the Completion Exchange, and 1..3 in the | 884 | | Reconnect Exchange. | 885 | | | 886 | PKs2, PKp2 | The public components of the ECDHE keys of the | 887 | | server and peer for the Reconnect Exchange. | 888 | | PKp2 and PKs2 are sent in the JSON Web Key | 889 | | (JWK) format [RFC7517]. Detailed format of the | 890 | | JWK object is defined by the cryptosuite. | 891 | | | 892 | MACs2, MACp2 | Message authentication codes (HMAC) for mutual | 893 | | authentication, key confirmation, and | 894 | | integrity check on the Reconnect Exchange. The | 895 | | input to the HMAC is defined below, and the | 896 | | key for the HMAC is defined in Section 3.5. | 897 | | | 898 | ErrorCode | Integer indicating an error condition. Defined | 899 | | in Section 4.3. | 900 | | | 901 | ErrorInfo | Textual error message for logging and | 902 | | debugging purposes. UTF-8 string of at most | 903 | | 500 bytes. | 904 | | | 905 +------------------+------------------------------------------------+ 907 Table 1: Message data fields 909 It is RECOMMENDED for servers to support both OOB channel directions 910 (Dirs=3), unless the type of the OOB channel limits them to one 911 direction (Dirs=1 or Dirs=2). On the other hand, it is RECOMMENDED 912 that the peer selects only one direction (Dirp=1 or Dirp=2) even when 913 both directions (Dirp=3) would be technically possible. The reason 914 is that, if value 3 is negotiated, the user may be presented with two 915 OOB messages, one for each direction, even though only one of them 916 needs to be delivered. This can be confusing to the user. 917 Nevertheless, the EAP-NOOB protocol is designed to cope also with 918 selected value 3, in which case it uses the first delivered OOB 919 message. In the unlikely case of simultaneously delivered OOB 920 messages, the protocol prioritizes the server-to-peer direction. 922 The nonces in the in-band messages (Ns, Np, Ns2, Np2) are 32-byte 923 fresh random byte strings, and the secret nonce Noob is a 16-byte 924 fresh random byte string. All the nonces are generated by the 925 endpoint that sends the message. 927 The fingerprint Hoob and the identifier NoobId are computed with the 928 cryptographic hash function specified in the negotiated cryptosuite 929 and truncated to the 16 leftmost bytes of the output. The message 930 authentication codes (MACs, MACp, MACs2, MACp2) are computed with the 931 HMAC function [RFC2104] based on the same cryptographic hash function 932 and truncated to the 32 leftmost bytes of the output. 934 The inputs to the hash function for computing the fingerprint Hoob 935 and to the HMAC for computing MACs, MACp, MACs2 and MACp2 are JSON 936 arrays containing a fixed number (17) of elements. The array 937 elements MUST be copied to the array verbatim from the sent and 938 received in-band messages. When the element is a JSON object, its 939 members MUST NOT be reordered or re-encoded. Whitespace MUST NOT be 940 added anywhere in the JSON structure. Implementers should check that 941 their JSON library copies the elements as UTF-8 strings and does not 942 modify then in any way, and that it does not add whitespace to the 943 HMAC input. 945 The inputs for computing the fingerprint and message authentication 946 codes are the following: 948 Hoob = H(Dir,Vers,Verp,PeerId,Cryptosuites,Dirs,ServerInfo,Cryptos 949 uitep,Dirp,[Realm],PeerInfo,0,PKs,Ns,PKp,Np,Noob). 951 NoobId = H("NoobId",Noob). 953 MACs = HMAC(Kms; 2,Vers,Verp,PeerId,Cryptosuites,Dirs,ServerInfo,C 954 ryptosuitep,Dirp,[Realm],PeerInfo,0,PKs,Ns,PKp,Np,Noob). 956 MACp = HMAC(Kmp; 1,Vers,Verp,PeerId,Cryptosuites,Dirs,ServerInfo,C 957 ryptosuitep,Dirp,[Realm],PeerInfo,0,PKs,Ns,PKp,Np,Noob). 959 MACs2 = HMAC(Kms2; 2,Vers,Verp,PeerId,Cryptosuites,"",[ServerInfo] 960 ,Cryptosuitep,"",[Realm],[PeerInfo],KeyingMode,[PKs2],Ns2,[PKp2],N 961 p2,"") 963 MACp2 = HMAC(Kmp2; 1,Vers,Verp,PeerId,Cryptosuites,"",[ServerInfo] 964 ,Cryptosuitep,"",[Realm],[PeerInfo],KeyingMode,[PKs2],Ns2,[PKp2],N 965 p2,"") 967 Missing input values are represented by empty strings "" in the 968 array. The values indicated with "" above are always empty strings. 969 Realm is included in the computation of MACs and MACp if it was sent 970 or received in the preceding Initial Exchange. Each of the values in 971 brackets for the computation of Macs2 and Macp2 MUST be included if 972 it was sent or received in the same Reconnect Exchange; otherwise the 973 value is replaced by an empty string "". 975 The parameter Dir indicates the direction in which the OOB message 976 containing the Noob value is being sent (1=peer-to-server, 2=server- 977 to-peer). This field is included in the Hoob input to prevent the 978 user from accidentally delivering the OOB message back to its 979 originator in the rare cases where both OOB directions have been 980 negotiated. The keys (Kms, Kmp, Kms2, Kmp2) for the HMACs are 981 defined in Section 3.5. 983 The nonces (Ns, Np, Ns2, Np2, Noob) and the hash value (NoobId) MUST 984 be base64url encoded [RFC4648] when they are used as input to the 985 cryptograhic functions H or HMAC. These values and the message 986 authentication codes (MACs, MACp, MACs2, MACp2) MUST also be 987 base64url encoded when they are sent in the in-band messages. The 988 values Noob and Hoob in the OOB channel MAY be base64url encoded if 989 that is appropriate for the application and the OOB channel. All 990 base64url encoding is done without padding. The base64url encoded 991 values will naturally consume more space than the number of bytes 992 specified above (22-character string for a 16-byte nonce and 993 43-character string for a 32-byte nonce or message authentication 994 code). In the key derivation in Section 3.5, on the other hand, the 995 unencoded nonces (raw bytes) are used as input to the key derivation 996 function. 998 The ServerInfo and PeerInfo are JSON objects with UTF-8 encoding. 999 The length of either encoded object as a byte array MUST NOT exceed 1000 500 bytes. The format and semantics of these objects MUST be defined 1001 by the application that uses the EAP-NOOB method. 1003 3.4. Fast reconnect and rekeying 1005 EAP-NOOB implements Fast Reconnect ([RFC3748], section 7.2.1) that 1006 avoids repeated use of the user-assisted OOB channel. 1008 The rekeying and the Reconnect Exchange may be needed for several 1009 reasons. New EAP output values MSK and EMSK may be needed because of 1010 mobility or timeout of session keys. Software or hardware failure or 1011 user action may also cause the authenticator, EAP server or peer to 1012 lose its non-persistent state data. The failure would typically be 1013 detected by the peer or authenticator when session keys no longer are 1014 accepted by the other endpoint. Change in the supported cryptosuites 1015 in the EAP server or peer may also cause the need for a new key 1016 exchange. When the EAP server or peer detects any one of these 1017 events, it MUST change from the Registered to Reconnecting state. 1018 These state transitions are labeled Mobility/Timeout/Failure in 1019 Figure 1. The EAP-NOOB method will then perform the Reconnect 1020 Exchange next time when EAP is triggered. 1022 3.4.1. Persistent EAP-NOOB association 1024 To enable rekeying, the EAP server and peer store the session state 1025 in persistent memory after a successful Completion Exchange. This 1026 state data, called "persistent EAP-NOOB association", MUST include at 1027 least the data fields shown in Table 2. They are used for 1028 identifying and authenticating the peer in the Reconnect Exchange. 1029 When a persistent EAP-NOOB association exists, the EAP server and 1030 peer are in the Registered state (4) or Reconnecting state (3), as 1031 shown in Figure 1. 1033 +------------------+------------------------------+-----------------+ 1034 | Data field | Value | Type | 1035 +------------------+------------------------------+-----------------+ 1036 | PeerId | Peer identifier allocated by | UTF-8 string | 1037 | | server | (typically 22 | 1038 | | | bytes) | 1039 | | | | 1040 | Verp | Negotiated protocol version | integer | 1041 | | | | 1042 | Cryptosuitep | Negotiated cryptosuite | integer | 1043 | | | | 1044 | CryptosuitepPrev | Previous cryptosuite | integer | 1045 | (at peer only) | | | 1046 | | | | 1047 | Realm | Optional realm assigned by | UTF-8 string | 1048 | | server (default value is | | 1049 | | "eap-noob.net") | | 1050 | | | | 1051 | Kz | Persistent key material | 32 bytes | 1052 | | | | 1053 | KzPrev (at | Previous Kz value | 32 bytes | 1054 | peer only) | | | 1055 +------------------+------------------------------+-----------------+ 1057 Table 2: Persistent EAP-NOOB association 1059 3.4.2. Reconnect Exchange 1061 The server chooses the Reconnect Exchange when both the peer and the 1062 server are in a persistent state and fast reconnection is needed (see 1063 Section 3.2.1 for details). 1065 The Reconnect Exchange comprises the common handshake and three 1066 further EAP-NOOB request-response pairs, one for cryptosuite and 1067 parameter negotiation, another for the nonce and ECDHE key exchange, 1068 and the last one for exchanging message authentication codes. In the 1069 first request and response (Type=5) the server and peer negotiate a 1070 protocol version and cryptosuite in the same way as in the Initial 1071 Exchange. The server SHOULD NOT offer and the peer MUST NOT accept 1072 protocol versions or cryptosuites that it knows to be weaker than the 1073 one currently in the Cryptosuitep field of the persistent EAP-NOOB 1074 association. The server SHOULD NOT needlessly change the 1075 cryptosuites it offers to the same peer because peer devices may have 1076 limited ability to update their persistent storage. However, if the 1077 peer has different values in the Cryptosuitep and CryptosuitepPrev 1078 fields, it SHOULD also accept offers that are not weaker than 1079 CryptosuitepPrev. Note that Cryptosuitep and CryptosuitePrev from 1080 the persistent EAP-NOOB association are only used to support the 1081 negotiation as described above; all actual cryptographic operations 1082 use the negotiated cryptosuite. The request and response (Type=5) 1083 MAY additionally contain PeerInfo and ServerInfo objects. 1085 The server then determines the KeyingMode (defined in Section 3.5) 1086 based on changes in the negotiated cryptosuite and whether it desires 1087 to achieve forward secrecy or not. The server SHOULD only select 1088 KeyingMode 3 when the negotiated cryptosuite differs from the 1089 Cryptosuitep in the server's persistent EAP-NOOB association, 1090 although it is technically possible to select this values without 1091 changing the cryptosuite. In the second request and response 1092 (Type=6), the server informs the peer about the KeyingMode, and the 1093 server and peer exchange nonces (Ns2, Np2). When KeyingMode is 2 or 1094 3 (rekeying with ECDHE), they also exchange public components of 1095 ECDHE keys (PKs2, PKp2). The server ECDHE key MUST be fresh, i.e. 1096 not previously used with the same peer, and the peer ECDHE key SHOULD 1097 be fresh, i.e. not previously used. 1099 In the third and final request and response (Type=7), the server and 1100 peer exchange message authentication codes. Both sides MUST compute 1101 the keys Kms2 and Kmp2 as defined in Section 3.5 and the message 1102 authentication codes MACs2 and MACp2 as defined in Section 3.3.2. 1103 Both sides MUST compare the received message authentication code with 1104 a locally computed value. 1106 The rules by which the peer compares the received MACs2 are non- 1107 trivial because, in addition to authenticating the current exchange, 1108 MACs2 may confirm the success or failure of a recent cryptosuite 1109 upgrade. The peer processes the final request (Type=7) as follows: 1111 1. The peer first compares the received MACs2 value with one it 1112 computed using the Kz stored in the persistent EAP-NOOB 1113 association. If the received and computed values match, the peer 1114 deletes any data stored in the CryptosuitepPrev and KzPrev fields 1115 of the persistent EAP-NOOB association. It does this because the 1116 received MACs2 confirms that the peer and server share the same 1117 Cryptosuitep and Kz, and any previous values must no longer be 1118 accepted. 1120 2. If, on the other hand, the peer finds that the received MACs2 1121 value does not match the one it computed locally with Kz, the 1122 peer checks whether the KzPrev field in the persistent EAP-NOOB 1123 association stores a key. If it does, the peer repeats the key 1124 derivation (Section 3.5) and local MACs2 computation 1125 (Section 3.3.2) using KzPrev in place of Kz. If this second 1126 computed MACs2 matches the received value, the match indicates 1127 synchronization failure caused by the loss of the last response 1128 (Type=7) in a previously attempted cryptosuite upgrade. In this 1129 case, the peer rolls back that upgrade by overwriting 1130 Cryptosuitep with CryptosuitepPrev and Kz with KzPrev in the 1131 persistent EAP-NOOB association. It also clears the 1132 CryptosuitepPrev and KzPrev fields. 1134 3. If the received MACs2 matched one of the locally computed values, 1135 the peer proceeds to send the final response (Type=7). The peer 1136 also moves to the Registered (4) state. When KeyingMode is 1 or 1137 2, the peer stops here. When KeyingMode is 3, the peer also 1138 updates the persistent EAP-NOOB association with the negotiated 1139 Cryptosuitep and the newly-derived Kz value. To prepare for 1140 possible synchronization failure caused by the loss of the final 1141 response (Type=7) during cryptosuite upgrade, the peer copies the 1142 old Cryptosuitep and Kz values in the persistent EAP-NOOB 1143 association to the CryptosuitepPrev and KzPrev fields. 1145 4. Finally, if the peer finds that the received MACs2 does not match 1146 either of the two values that it computed locally (or one value 1147 if no KzPrev was stored), the peer sends an error message (error 1148 code 4001, see Section 3.6.1), which causes the the Reconnect 1149 Exchange to end in EAP-Failure. 1151 The server rules for processing the final message are simpler than 1152 the peer rules because the server does not store previous keys and it 1153 never rolls back a cryptosuite upgrade. Upon receiving the final 1154 response (Type=7), the server compares the received value of MACp2 1155 with one it computes locally. If the values match, the Reconnect 1156 Exchange ends in EAP-Success. When KeyingMode is 3, the server also 1157 updates Cryptosuitep and Kz in the persistent EAP-NOOB association. 1158 On the other hand, if the server finds that the values do not match, 1159 it sends an error message (error code 4001), and the Reconnect 1160 Exchange ends in EAP-Failure. 1162 The endpoints MAY send updated Realm, ServerInfo and PeerInfo objects 1163 in the Reconnect Exchange. When there is no update to the values, 1164 they SHOULD omit this information from the messages. If the Realm 1165 was sent, each side updates Realm in the persistent EAP-NOOB 1166 association when moving to the Registered (4) state. 1168 EAP Peer EAP Server 1169 | ...continuing from common handshake | 1170 | | 1171 |<----------- EAP-Request/EAP-NOOB ----------------| 1172 | (Type=5,Vers,PeerId,Cryptosuites, | 1173 | [Realm],[ServerInfo]) | 1174 | | 1175 | | 1176 |------------ EAP-Response/EAP-NOOB -------------->| 1177 | (Type=5,Verp,PeerId,Cryptosuitep,[PeerInfo])| 1178 | | 1179 | | 1180 |<----------- EAP-Request/EAP-NOOB ----------------| 1181 | (Type=6,PeerId,KeyingMode,[PKs2],Ns2) | 1182 | | 1183 | | 1184 |------------ EAP-Response/EAP-NOOB -------------->| 1185 | (Type=6,PeerId,[PKp2],Np2) | 1186 | | 1187 | | 1188 |<----------- EAP-Request/EAP-NOOB ----------------| 1189 | (Type=7,PeerId,MACs2) | 1190 | | 1191 | | 1192 |------------ EAP-Response/EAP-NOOB -------------->| 1193 | (Type=7,PeerId,MACp2) | 1194 | | 1195 | | 1196 |<----------- EAP-Success -------------------------| 1197 | | 1199 Figure 8: Reconnect Exchange 1201 3.4.3. User reset 1203 As shown in the association state machine in Figure 1, the only 1204 specified way for the association to return from the Registered state 1205 (4) to the Unregistered state (0) is through user-initiated reset. 1206 After the reset, a new OOB message will be needed to establish a new 1207 association between the EAP server and peer. Typical situations in 1208 which the user reset is required are when the other side has 1209 accidentally lost the persistent EAP-NOOB association data, or when 1210 the peer device is decommissioned. 1212 The server could detect that the peer is in the Registered or 1213 Reconnecting state but the server itself is in one of the ephemeral 1214 states 0..2 (including situations where the server does not recognize 1215 the PeerId). In this case, effort should be made to recover the 1216 persistent server state, for example, from a backup storage - 1217 especially if many peer devices are similarly affected. If that is 1218 not possible, the EAP server SHOULD log the error or notify an 1219 administrator. The only way to continue from such a situation is by 1220 having the user reset the peer device. 1222 On the other hand, if the peer is in any of the ephemeral states 1223 0..2, including the Unregistered state, the server will treat the 1224 peer as a new peer device and allocate a new PeerId to it. The 1225 PeerInfo can be used by the user as a clue to which physical device 1226 has lost its state. However, there is no secure way of matching the 1227 "new" peer with the old PeerId without repeating the OOB Step. This 1228 situation will be resolved when the user performs the OOB Step and, 1229 thus, identifies the physical peer device. The server user interface 1230 MAY support situations where the "new" peer is actually a previously 1231 registered peer that has been reset by a user or otherwise lost its 1232 persistent data. In those cases, the user could choose to merge new 1233 peer identity with the old one in the server. The alternative is to 1234 treat the device just like a new peer. 1236 3.5. Key derivation 1238 EAP-NOOB derives the EAP output values MSK and EMSK and other secret 1239 keying material from the output of an Ephemeral Elliptic Curve 1240 Diffie-Hellman (ECDHE) algorithm following the NIST specification 1241 [NIST-DH]. In NIST terminology, we use a C(2, 0, ECC CDH) scheme, 1242 i.e. two ephemeral keys and no static keys. In the Initial and 1243 Reconnect Exchanges, the server and peer compute the ECDHE shared 1244 secret Z as defined in section 6.1.2.2 of the NIST specification 1245 [NIST-DH]. In the Completion and Reconnect Exchanges, the server and 1246 peer compute the secret keying material from Z with the single-step 1247 key derivation function (KDF) defined in section 5.8.1 of the NIST 1248 specification. The hash function H for KDF is taken from the 1249 negotiated cryptosuite. 1251 +------------+------------------------------------------------------+ 1252 | KeyingMode | Description | 1253 +------------+------------------------------------------------------+ 1254 | 0 | Completion Exchange (always with ECDHE) | 1255 | | | 1256 | 1 | Reconnect Exchange, rekeying without ECDHE | 1257 | | | 1258 | 2 | Reconnect Exchange, rekeying with ECHDE, no change | 1259 | | in cryptosuite | 1260 | | | 1261 | 3 | Reconnect Exchange, rekeying with ECDHE, new | 1262 | | cryptosuite negotiated | 1263 +------------+------------------------------------------------------+ 1265 Table 3: Keying modes 1267 The key derivation has three different modes (KeyingMode), which are 1268 specified in Table 3. Table 4 defines the inputs to KDF in each 1269 KeyingMode. 1271 In the Completion Exchange (KeyingMode=0), the input Z comes from the 1272 preceding Initial exchange. KDF takes some additional inputs 1273 (OtherInfo), for which we use the concatenation format defined in 1274 section 5.8.1.2.1 of the NIST specification [NIST-DH]. OtherInfo 1275 consists of the AlgorithmId, PartyUInfo, PartyVInfo, and SuppPrivInfo 1276 fields. The first three fields are fixed-length bit strings, and 1277 SuppPrivInfo is a variable-length string with a one-byte Datalength 1278 counter. AlgorithmId is the fixed-length 8-byte ASCII string "EAP- 1279 NOOB". The other input values are the server and peer nonces. In 1280 the Completion Exchange, the inputs also include the secret nonce 1281 Noob from the OOB message. 1283 In the simplest form of the Reconnect Exchange (KeyingMode=1), fresh 1284 nonces are exchanged but no ECDHE keys are sent. In this case, input 1285 Z to the KDF is replaced with the shared key Kz from the persistent 1286 EAP-NOOB association. The result is rekeying without the 1287 computational cost of the ECDHE exchange, but also without forward 1288 secrecy. 1290 When forward secrecy is desired in the Reconnect Exchange 1291 (KeyingMode=2 or KeyingMode=3), both nonces and ECDHE keys are 1292 exchanged. Input Z is the fresh shared secret from the ECDHE 1293 exchange with PKs2 and PKp2. The inputs also include the shared 1294 secret Kz from the persistent EAP-NOOB association. This binds the 1295 rekeying output to the previously authenticated keys. 1297 +--------------+--------------+------------------------+------------+ 1298 | KeyingMode | KDF input | Value | Length | 1299 | | field | | (bytes) | 1300 +--------------+--------------+------------------------+------------+ 1301 | 0 | Z | ECDHE shared secret | variable | 1302 | Completion | | from PKs and PKp | | 1303 | | AlgorithmId | "EAP-NOOB" | 8 | 1304 | | PartyUInfo | Np | 32 | 1305 | | PartyVInfo | Ns | 32 | 1306 | | SuppPubInfo | (not allowed) | | 1307 | | SuppPrivInfo | Noob | 16 | 1308 | | | | | 1309 | 1 | Z | Kz | 32 | 1310 | Reconnect, | AlgorithmId | "EAP-NOOB" | 8 | 1311 | rekeying | PartyUInfo | Np2 | 32 | 1312 | without | PartyVInfo | Ns2 | 32 | 1313 | ECDHE | SuppPubInfo | (not allowed) | | 1314 | | SuppPrivInfo | (null) | 0 | 1315 | | | | | 1316 | 2 or 3 | Z | ECDHE shared secret | variable | 1317 | Reconnect, | | from PKs2 and PKp2 | | 1318 | rekeying, | AlgorithmId | "EAP-NOOB" | 8 | 1319 | with ECDHE, | PartyUInfo | Np2 | 32 | 1320 | same or new | PartyVInfo | Ns2 | 32 | 1321 | cryptosuite | SuppPubInfo | (not allowed) | | 1322 | | SuppPrivInfo | Kz | 32 | 1323 +--------------+--------------+------------------------+------------+ 1325 Table 4: Key derivation input 1327 Table 5 defines how the output bytes of KDF are used. In addition to 1328 the EAP output values MSK and EMSK, the server and peer derive 1329 another shared secret key AMSK, which MAY be used for application- 1330 layer security. Further output bytes are used internally by EAP-NOOB 1331 for the message authentication keys (Kms, Kmp, Kms2, Kmp2). 1333 The Completion Exchange (KeyingMode=0) produces the shared secret Kz, 1334 which the server and peer store in the persistent EAP-NOOB 1335 association. When a new cryptosuite is negotiated in the Reconnect 1336 Exchange (KeyingMode=3), it similarly produces a new Kz. In that 1337 case, the server and peer update both the cryptosuite and Kz in the 1338 persistent EAP-NOOB association. Additionally, the peer stores the 1339 previous Cryptosuitep and Kz values in the CryptosuitepPrev and 1340 KzPrev fields of the persistent EAP-NOOB association. 1342 +-----------------+------------------+----------+----------------+ 1343 | KeyingMode | KDF output bytes | Used as | Length (bytes) | 1344 +-----------------+------------------+----------+----------------+ 1345 | 0 | 0..63 | MSK | 64 | 1346 | Completion | 64..127 | EMSK | 64 | 1347 | | 128..191 | AMSK | 64 | 1348 | | 192..223 | MethodId | 32 | 1349 | | 224..255 | Kms | 32 | 1350 | | 256..287 | Kmp | 32 | 1351 | | 288..319 | Kz | 32 | 1352 | | | | | 1353 | 1 or 2 | 0..63 | MSK | 64 | 1354 | Reconnect, | 64..127 | EMSK | 64 | 1355 | rekeying | 128..191 | AMSK | 64 | 1356 | without ECDHE, | 192..223 | MethodId | 32 | 1357 | or with ECDHE | 224..255 | Kms2 | 32 | 1358 | and unchanged | 256..287 | Kmp2 | 32 | 1359 | cryptosuite | | | | 1360 | | | | | 1361 | | | | | 1362 | 3 Reconnect, | 0..63 | MSK | 64 | 1363 | rekeying | 64..127 | EMSK | 64 | 1364 | with ECDHE, | 128..191 | AMSK | 64 | 1365 | new cryptosuite | 192..223 | MethodId | 32 | 1366 | | 224..255 | Kms2 | 32 | 1367 | | 256..287 | Kmp2 | 32 | 1368 | | 288..319 | Kz | 32 | 1369 +-----------------+------------------+----------+----------------+ 1371 Table 5: Key derivation output 1373 Finally, every EAP method must export a Server-Id, Peer-Id and 1374 Session-Id [RFC5247]. In EAP-NOOB, the exported Peer-Id is the 1375 PeerId which the server has assigned to the peer. The exported 1376 Server-Id is a zero-length string (i.e. null string) because EAP-NOOB 1377 neither knows nor assigns any server identifier. The exported 1378 Session-Id is created by concatenating the Type-Code xxx (TBA) with 1379 the MethodId, which is obtained from the KDF output as shown in 1380 Table 5. 1382 3.6. Error handling 1384 Various error conditions in EAP-NOOB are handled by sending an error 1385 notification message (Type=0) instead of the expected next EAP 1386 request or response message. Both the EAP server and the peer may 1387 send the error notification, as shown in Figure 9 and Figure 10. 1388 After sending or receiving an error notification, the server MUST 1389 send an EAP-Failure (as required by [RFC3748] section 4.2). The 1390 notification MAY contain an ErrorInfo field, which is a UTF-8 encoded 1391 text string with a maximum length of 500 bytes. It is used for 1392 sending descriptive information about the error for logging and 1393 debugging purposes. 1395 EAP Peer EAP Server 1396 ... ... 1397 | | 1398 |<----------- EAP-Request/EAP-NOOB ----------------| 1399 | (Type=0,[PeerId],ErrorCode,[ErrorInfo]) | 1400 | | 1401 | | 1402 |<----------- EAP-Failure -------------------------| 1403 | | 1405 Figure 9: Error notification from server to peer 1407 EAP Peer EAP Server 1408 ... ... 1409 | | 1410 |------------ EAP-Response/EAP-NOOB -------------->| 1411 | (Type=0,[PeerId],ErrorCode,[ErrorInfo]) | 1412 | | 1413 | | 1414 |<----------- EAP-Failure -------------------------| 1415 | | 1417 Figure 10: Error notification from peer to server 1419 After the exchange fails due to an error notification, the server and 1420 peer set the association state as follows. In the Initial Exchange, 1421 both the sender and recipient of the error notification MUST set the 1422 association state to the Unregistered (0) state. In the Waiting and 1423 Completion Exchanges, each side MUST remain in its old state as if 1424 the failed exchange had not taken place, with the exception that the 1425 recipient of error code 2003 processes it as specified in 1426 Section 3.2.4. In the Reconnect Exchange, both sides MUST set the 1427 association state to the Reconnecting (3) state. 1429 Errors that occur in the OOB channel are not explicitly notified in- 1430 band. 1432 3.6.1. Invalid messages 1434 If the NAI structure is invalid, the server SHOULD send the error 1435 code 1001 to the peer. The recipient of an EAP-NOOB request or 1436 response SHOULD send the following error codes back to the sender: 1437 1002 if it cannot parse the message as a JSON object or the top-level 1438 JSON object has missing or unrecognized members; 1003 if a data field 1439 has an invalid value, such as an integer out of range, and there is 1440 no more specific error code available; 1004 if the received message 1441 type was unexpected in the current state; 2004 if the PeerId has an 1442 unexpected value; 2003 if the NoobId is not recognized; and 1007 if 1443 the ECDHE key is invalid. 1445 3.6.2. Unwanted peer 1447 The preferred way for the EAP server to rate limit EAP-NOOB 1448 connections from a peer is to use the SleepTime parameter in the 1449 Waiting Exchange. However, if the EAP server receives repeated EAP- 1450 NOOB connections from a peer which apparently should not connect to 1451 this server, the server MAY indicate that the connections are 1452 unwanted by sending the error code 2001. After receiving this error 1453 message, the peer MAY refrain from reconnecting to the same EAP 1454 server and, if possible, both the EAP server and peer SHOULD indicate 1455 this error condition to the user or server administrator. However, 1456 in order to avoid persistent denial of service, the peer is not 1457 required to stop entirely from reconnecting to the server. 1459 3.6.3. State mismatch 1461 In the states indicated by "-" in Figure 11 in Appendix A, user 1462 action is required to reset the association state or to recover it, 1463 for example, from backup storage. In those cases, the server sends 1464 the error code 2002 to the peer. If possible, both the EAP server 1465 and peer SHOULD indicate this error condition to the user or server 1466 administrator. 1468 3.6.4. Negotiation failure 1470 If there is no matching protocol version, the peer sends the error 1471 code 3001 to the server. If there is no matching cryptosuite, the 1472 peer sends the error code 3002 to the server. If there is no 1473 matching OOB direction, the peer sends the error code 3003 to the 1474 server. 1476 In practice, there is no way of recovering from these errors without 1477 software or hardware changes. If possible, both the EAP server and 1478 peer SHOULD indicate these error conditions to the user. 1480 3.6.5. Cryptographic verification failure 1482 If the receiver of the OOB message detects an unrecognized PeerId or 1483 incorrect fingerprint (Hoob) in the OOB message, the receiver MUST 1484 remain in the Waiting for OOB state (1) as if no OOB message was 1485 received. The receiver SHOULD indicate the failure to accept the OOB 1486 message to the user. No in-band error message is sent. 1488 Note that if the OOB message was delivered from the server to the 1489 peer and the peer does not recognize the PeerId, the likely cause is 1490 that the user has unintentionally delivered the OOB message to the 1491 wrong peer device. If possible, the peer SHOULD indicate this to the 1492 user; however, the peer device may not have the capability for many 1493 different error indications to the user and it MAY use the same 1494 indication as in the case of an incorrect fingerprint. 1496 The rationale for the above is that the invalid OOB message could 1497 have been presented to the receiver by mistake or intentionally by a 1498 malicious party and, thus, it should be ignored in the hope that the 1499 honest user will soon deliver a correct OOB message. 1501 If the EAP server or peer detects an incorrect message authentication 1502 code (MACs, MACp, MACs2, MACp2), it sends the error code 4001 to the 1503 other side. As specified in the beginning of Section 3.6, the failed 1504 Completion Exchange will not result in server or peer state changes 1505 while error in the Reconnect Exchange will put both sides to the 1506 Reconnecting (3) state and thus lead to another reconnect attempt. 1508 The rationale for this is that the invalid cryptographic message may 1509 have been spoofed by a malicious party and, thus, it should be 1510 ignored. In particular, a spoofed message on the in-band channel 1511 should not force the honest user to perform the OOB Step again. In 1512 practice, however, the error may be caused by other failures, such as 1513 a software bug. For this reason, the EAP server MAY limit the rate 1514 of peer connections with SleepTime after the above error. Also, 1515 there SHOULD be a way for the user to reset the peer to the 1516 Unregistered state (0), so that the OOB Step can be repeated at the 1517 last resort. 1519 3.6.6. Application-specific failure 1521 Applications MAY define new error messages for failures that are 1522 specific to the application or to one type of OOB channel. They MAY 1523 also use the generic application-specific error code 5001, or the 1524 error codes 5002 and 5004, which have been reserved for indicating 1525 invalid data in the ServerInfo and PeerInfo fields, respectively. 1526 Additionally, anticipating OOB channels that make use of a URL, the 1527 error code 5003 has been reserved for indicating invalid server URL. 1529 4. IANA Considerations 1531 This section provides guidance to the Internet Assigned Numbers 1532 Authority (IANA) regarding registration of values related to the EAP- 1533 NOOB protocol, in accordance with [RFC8126]. 1535 The EAP Method Type number for EAP-NOOB needs to be assigned. 1537 This memo also requires IANA to create new registries as defined in 1538 the following subsections. 1540 4.1. Cryptosuites 1542 Cryptosuites are identified by an integer. Each cryptosuite MUST 1543 specify an ECDHE curve for the key exchange, encoding of the ECDHE 1544 public key as a JWK object, and a cryptographic hash function for the 1545 fingerprint and HMAC computation and key derivation. The hash value 1546 output by the cryptographic hash function MUST be at least 32 bytes 1547 in length. The following suites are defined by EAP-NOOB: 1549 +-------------+-----------------------------------------------------+ 1550 | Cryptosuite | Algorithms | 1551 +-------------+-----------------------------------------------------+ 1552 | 1 | ECDHE curve Curve25519 [RFC7748], public-key format | 1553 | | [RFC7518] Section 6.2.1, hash function SHA-256 | 1554 | | [RFC6234] | 1555 +-------------+-----------------------------------------------------+ 1557 Table 6: EAP-NOOB cryptosuites 1559 An example of Cryptosuite 1 public-key encoded as a JWK object is 1560 given below (line breaks are for readability only). 1562 "jwk":{"kty":"EC","crv":"Curve25519","x":"3p7bfXt9wbTTW2HC7OQ1Nz- 1563 DQ8hbeGdNrfx-FG-IK08"} 1565 Assignment of new values for new cryptosuites MUST be done through 1566 IANA with "Specification Required" and "IESG Approval" as defined in 1567 [RFC8126]. 1569 4.2. Message Types 1571 EAP-NOOB request and response pairs are identified by an integer 1572 Message Type. The following Message Types are defined by EAP-NOOB: 1574 +----------+------------+-------------------------------------------+ 1575 | Message | Used in | Purpose | 1576 | Type | Exchange | | 1577 +----------+------------+-------------------------------------------+ 1578 | 1 | Initial | Version, cryptosuite and parameter | 1579 | | | negotiation | 1580 | | | | 1581 | 2 | Initial | Exchange of ECDHE keys and nonces | 1582 | | | | 1583 | 3 | Waiting | Indication to peer that the server has | 1584 | | | not yet received an OOB message | 1585 | | | | 1586 | 4 | Completion | Authentication and key confirmation with | 1587 | | | HMAC | 1588 | | | | 1589 | 5 | Reconnect | Version, cryptosuite, and parameter | 1590 | | | negotiation | 1591 | | | | 1592 | 6 | Reconnect | Exchange of ECDHE keys and nonces | 1593 | | | | 1594 | 7 | Reconnect | Authentication and key confirmation with | 1595 | | | HMAC | 1596 | | | | 1597 | 8 | Completion | NoobId discovery | 1598 | | | | 1599 | 9 | All | PeerId and PeerState discovery | 1600 | | exchanges | | 1601 | | | | 1602 | 0 | Error | Error notification | 1603 | | | | 1604 +----------+------------+-------------------------------------------+ 1606 Table 7: EAP-NOOB 1608 Assignment of new values for new Message Types MUST be done through 1609 IANA with "Expert Review" as defined in [RFC8126]. 1611 4.3. Error codes 1613 The error codes defined by EAP-NOOB are listed in Table 8. 1615 +------------+----------------------------------------+ 1616 | Error code | Purpose | 1617 +------------+----------------------------------------+ 1618 | 1001 | Invalid NAI | 1619 | 1002 | Invalid message structure | 1620 | 1003 | Invalid data | 1621 | 1004 | Unexpected message type | 1622 | 1007 | Invalid ECDHE key | 1623 | 2001 | Unwanted peer | 1624 | 2002 | State mismatch, user action required | 1625 | 2003 | Unrecognized OOB message identifier | 1626 | 2004 | Unexpected peer identifier | 1627 | 3001 | No mutually supported protocol version | 1628 | 3002 | No mutually supported cryptosuite | 1629 | 3003 | No mutually supported OOB direction | 1630 | 4001 | HMAC verification failure | 1631 | 5001 | Application-specific error | 1632 | 5002 | Invalid server info | 1633 | 5003 | Invalid server URL | 1634 | 5004 | Invalid peer info | 1635 | 6001-6999 | Private and experimental use | 1636 +------------+----------------------------------------+ 1638 Table 8: EAP-NOOB error codes 1640 Assignment of new error codes MUST be done through IANA with 1641 "Specification Required" and "IESG Approval" as defined in [RFC8126], 1642 with the exception of the range 6001-6999, which is reserved for 1643 "Private Use" and "Experimental Use". 1645 4.4. Domain name reservation considerations 1647 "eap-noob.net" should be registered as a special-use domain. The 1648 considerations required by [RFC6761] for registering this special-use 1649 domain name are the following: 1651 o Users: Non-admin users are not expected to encounter this name or 1652 recognize it as special. AAA administrators may need to recognize 1653 the name. 1655 o Application Software: Application software is not expected to 1656 recognize this domain name as special. 1658 o Name Resolution APIs and Libraries: Name resolution APIs and 1659 libraries are not expected to recognize this domain name as 1660 special. 1662 o Caching DNS Servers: Caching servers are not expected to recognize 1663 this domain name as special. 1665 o Authoritative DNS Servers: Authoritative DNS servers MUST respond 1666 to queries for eap-noob.net with NXDOMAIN. 1668 o DNS Server Operators: Except for the authoritative DNS server, 1669 there are no special requirements for the operators. 1671 o DNS Registries/Registrars: There are no special requirements for 1672 DNS registrars. 1674 5. Implementation Status 1676 This section records the status of known implementations of the 1677 protocol defined by this specification at the time of posting of this 1678 Internet-Draft, and is based on a proposal described in [RFC7942]. 1679 The description of implementations in this section is intended to 1680 assist the IETF in its decision processes in progressing drafts to 1681 RFCs. Please note that the listing of any individual implementation 1682 here does not imply endorsement by the IETF. Furthermore, no effort 1683 has been spent to verify the information presented here that was 1684 supplied by IETF contributors. This is not intended as, and must not 1685 be construed to be, a catalog of available implementations or their 1686 features. Readers are advised to note that other implementations may 1687 exist. 1689 5.1. Implementation with wpa_supplicant and hostapd 1691 o Responsible Organization: Aalto University 1693 o Location: 1695 o Coverage: This implementation includes all of the features 1696 described in the current specification. The implementation 1697 supports two dimensional QR codes and NFC as example out-of-band 1698 (OOB) channels 1700 o Level of Maturity: Alpha 1702 o Version compatibility: Version 03 of the draft implemented 1704 o Licensing: BSD 1706 o Contact Information: Tuomas Aura, tuomas.aura@aalto.fi 1708 5.2. Protocol modeling 1710 The current EAP-NOOB specification has been modeled with the mCRL2 1711 formal specification language [mcrl2]. The model 1712 was used mainly for simulating the protocol behavior and for 1714 verifying basic safety and liveness properties as part of the 1715 specification process. For example, we verified the correctness of 1716 the tiebreaking mechanism when two OOB messages are received 1717 simultaneously, one in each direction. We also verified that a man- 1718 in-the-middle attacker cannot cause persistent failure by spoofing a 1719 finite number of messages in the Reconnect Exchange. Additionally, 1720 the protocol has been modeled with the ProVerif [proverif] tool. 1721 This model was used to verify security 1723 properties such as mutual authentication. 1725 6. Security considerations 1727 EAP-NOOB is an authentication and key derivation protocol and, thus, 1728 security considerations can be found in most sections of this 1729 specification. In the following, we explain the protocol design and 1730 highlight some other special considerations. 1732 6.1. Authentication principle 1734 EAP-NOOB establishes a shared secret with an authenticated ECDHE key 1735 exchange. The mutual authentication in EAP-NOOB is based on two 1736 separate features, both conveyed in the OOB message. The first 1737 authentication feature is the secret nonce Noob. The peer and server 1738 use this secret in the Completion Exchange to mutually authenticate 1739 the session key previously created with ECDHE. The message 1740 authentication codes computed with the secret nonce Noob are alone 1741 sufficient for authenticating the key exchange. The second 1742 authentication feature is the integrity-protecting fingerprint Hoob. 1743 Its purpose is to prevent impersonation and man-in-the-middle attacks 1744 even in situations where the attacker is able to eavesdrop the OOB 1745 channel and the nonce Noob is compromised. In some human-assisted 1746 OOB channels, such as sound burst or user-transferred URL, it may be 1747 easier to detect tampering than spying of the OOB message, and such 1748 applications benefit from the second authentication feature. 1750 The additional security provided by the cryptographic fingerprint 1751 Hoob is somewhat intricate to understand. The endpoint that receives 1752 the OOB message uses Hoob to verify the integrity of the ECDHE 1753 exchange. Thus, the OOB receiver can detect impersonation and man- 1754 in-the-middle attacks on the in-band channel. The other endpoint, 1755 however, is not equally protected because the OOB message and 1756 fingerprint are sent only in one direction. Some protection to the 1757 OOB sender is afforded by the fact that the user may notice the 1758 failure of the association at the OOB receiver and therefore reset 1759 the OOB sender. Other device-pairing protocols have solved similar 1760 situations by requiring the user to confirm to the OOB sender that 1761 the association was accepted by the OOB receiver, e.g. by pressing an 1762 "confirm" button on the sender side. Applications MAY implement EAP- 1763 NOOB in this way. Nevertheless, since EAP-NOOB was designed to work 1764 with strictly one-directional OOB communication and the fingerprint 1765 is only the second authentication feature, the EAP-NOOB specification 1766 does not mandate such explicit confirmation to the OOB sender. 1768 To summarize, EAP-NOOB uses the combined protection of the secret 1769 nonce Noob and the cryptographic fingerprint Hoob, both conveyed in 1770 the OOB message. The secret nonce Noob alone is sufficient for 1771 mutual authentication, unless the attacker can eavesdrop it from the 1772 OOB channel. Even if an attacker is able to eavesdrop the secret 1773 nonce Noob, it nevertheless cannot perform a full man-in-the-middle 1774 attack on the in-band channel because the mismatching fingerprint 1775 would alert the OOB receiver, which would reject the OOB message. 1776 The attacker that eavesdropped the secret nonce can impersonate the 1777 OOB receiver to the OOB sender. In this case, the association will 1778 appear to be complete only on the OOB sender side, and such 1779 situations have to be resolved by the user by resetting the OOB 1780 sender to the initial state. 1782 The expected use cases for EAP-NOOB are ones where it replaces a 1783 user-entered access credentials in IoT appliances. In wireless 1784 network access without EAP, the user-entered credential is often a 1785 passphrase that is shared by all the network stations. The advantage 1786 of an EAP-based solution, including EAP-NOOB, is that it establishes 1787 a different master secret for each peer device, which makes the 1788 system more resilient against device compromise than if there were a 1789 common master secret. Additionally, it is possible to revoke the 1790 security association for an individual device on the server side. 1792 Forward secrecy in EAP-NOOB is optional. The Reconnect Exchange in 1793 EAP-NOOB provides forward secrecy only if both the server and peer 1794 send their fresh ECDHE keys. This allows both the server and the 1795 peer to limit the frequency of the costly computation that is 1796 required for forward secrecy. The server MAY adjust the frequency of 1797 its attempts at ECDHE rekeying based on what it knows about the 1798 peer's computational capabilities. 1800 The users delivering the OOB messages will often authenticate 1801 themselves to the EAP server, e.g. by logging into a secure web page. 1802 In this case, the server can reliably associate the peer device with 1803 the user account. Applications that make use of EAP-NOOB can use 1804 this information for configuring the initial owner of the freshly- 1805 registered device. 1807 6.2. Identifying correct endpoints 1809 Potential weaknesses in EAP-NOOB arise from the fact that the user 1810 must identify physically the correct peer device. If the attacker is 1811 able to trick the user into delivering the OOB message to or from the 1812 wrong peer device, the server may create an association with the 1813 wrong peer. This reliance on user in identifying the correct 1814 endpoints is an inherent property of user-assisted out-of-band 1815 authentication. 1817 It is, however, not possible to exploit accidental delivery of the 1818 OOB message to the wrong device when the user makes a mistake. This 1819 is because the wrong peer device would not have prepared for the 1820 attack by performing the Initial Exchange with the server. In 1821 comparison, simpler solutions where the master key is transferred to 1822 the device via the OOB channel are vulnerable to opportunistic 1823 attacks if the user mistakenly delivers the master key to more than 1824 one device. 1826 One mechanism that can mitigate user mistakes is certification of 1827 peer devices. The certificate can convey to the server authentic 1828 identifiers and attributes of the peer device. Compared to a fully 1829 certificate-based authentication, however, EAP-NOOB can be used 1830 without trusted third parties and does not require the user to know 1831 any identifier of the peer device; physical access to the device is 1832 sufficient. 1834 Similarly, the attacker can try to trick the user to deliver the OOB 1835 message to the wrong server, so that the peer device becomes 1836 associated with the wrong server. Since the EAP server is typically 1837 online and accessed through a web user interface, the attack would be 1838 akin to phishing attacks where the user is tricked to accessing the 1839 wrong URL and wrong web page. 1841 6.3. Trusted path issues and misbinding attacks 1843 Another potential threat is spoofed user input or output on the peer 1844 device. When the user is delivering the OOB message to or from the 1845 correct peer device, a trusted path between the user and the peer 1846 device is needed. That is, the user must communicate directly with 1847 an authentic operating system and EAP-NOOB implementation in the peer 1848 device and not with a spoofed user interface. Otherwise, a 1849 Registered device that is under the control of the attacker could 1850 emulate the behavior of an unregistered device. The secure path can 1851 be implemented, for example, by having the user pressing a reset 1852 button to return the device to the Unregistered state and a trusted 1853 UI. The problem with such trusted paths is that they are not 1854 standardized across devices. 1856 Another potential consequence of spoofed UI is the misbinding attack 1857 where the user tries to register the correct but compromised device, 1858 and that device tricks the user into registering another device 1859 instead. For example, a compromised device might have a malicious 1860 full-screen app running, which presents to the user QR codes copied, 1861 in real time, from another device's screen. If the unwitting user 1862 scans the QR code and delivers the OOB message in it to the server, 1863 the wrong device may become registered in the server. Such 1864 misbinding vulnerabilities arise because the user does not have any 1865 secure way of verifying that the in-band cryptographic handshake and 1866 the out-of-band physical access are terminated at the same physical 1867 device. Sethi et al. [Sethi19] analyze the binding threat against 1868 device-pairing protocols and also EAP-NOOB. Essentially, all 1869 protocols where the authentication relies on the user's physical 1870 access to the device are vulnerable to misbinding, including EAP- 1871 NOOB. 1873 A standardized trusted path for communicating directly with the 1874 trusted computing base in a physical device would mitigate the 1875 misbinding threat, but such paths rarely exist in practice. Careful 1876 asset tracking can also prevent most misbinding attacks because the 1877 PeerInfo sent in-band by the wrong device will not match expected 1878 values. Device certification by the manufacturer can further 1879 strengthen the asset tracking. 1881 6.4. Peer identifiers and attributes 1883 The PeerId value in the protocol is a server-allocated identifier for 1884 its association with the peer and SHOULD NOT be shown to the user 1885 because its value is initially ephemeral. Since the PeerId is 1886 allocated by the server and the scope of the identifier is the single 1887 server, the so-called identifier squatting attacks, where a malicious 1888 peer could reserve another peer's identifier, are not possible in 1889 EAP-NOOB. The server SHOULD assign a random or pseudo-random PeerId 1890 to each new peer. It SHOULD NOT select the PeerId based on any peer 1891 characteristics that it may know, such as the peer's link-layer 1892 network address. 1894 User reset or failure in the OOB Step can cause the peer to perform 1895 many Initial Exchanges with the server and to allocate many PeerIds 1896 and to store the ephemeral protocol state for them. The peer will 1897 typically only remember the latest one. EAP-NOOB leaves it to the 1898 implementation to decide when to delete these ephemeral associations. 1899 There is no security reason to delete them early, and the server does 1900 not have any way to verify that the peers are actually the same one. 1901 Thus, it is safest to store the ephemeral states for at least one 1902 day. If the OOB messages are sent only in the server-to-peer 1903 direction, the server SHOULD NOT delete the ephemeral state before 1904 all the related Noob values have expired. 1906 After completion of EAP-NOOB, the server may store the PeerInfo data, 1907 and the user may use it to identify the peer and its properties, such 1908 as the make and model or serial number. A compromised peer could lie 1909 in the PeerInfo that it sends to the server. If the server stores 1910 any information about the peer, it is important that this information 1911 is approved by the user during or after the OOB Step. Without 1912 verification by the user or authentication with vendor certificates 1913 on the application level, the PeerInfo is not authenticated 1914 information and should not be relied on. 1916 One possible use for the PeerInfo field is EAP channel binding 1917 ([RFC3748] Section 7.15). That is, the PeerInfo may include data 1918 items that bind the EAP-NOOB association and exported keys to 1919 properties of the authenticator or the access link, such as the SSID 1920 and BSSID of the wireless network (see Appendix C). 1922 6.5. Identity protection 1924 The PeerInfo field contains identifiers and other information about 1925 the peer device (see Appendix C), and the peer sends this information 1926 in plaintext to the EAP server before the server authentication in 1927 EAP-NOOB has been completed. While the information refers to the 1928 peer device and not directly to the user, it may be better for user 1929 privacy to avoid sending unnecessary information. In the Reconnect 1930 Exchange, the optional PeerInfo SHOULD be omitted unless some 1931 critical data has changed. 1933 Peer devices that randomize their layer-2 address to prevent tracking 1934 can do this whenever the user resets the EAP-NOOB association. 1935 During the lifetime of the association, the PeerId is a unique 1936 identifier that can be used to track the peer in the access network. 1937 Later versions of this specification may consider updating the PeerId 1938 at each Reconnect Exchange. In that case, it is necessary to 1939 consider how the authenticator and access-network administrators can 1940 recognize and blacklist misbehaving peer devices and how to avoid 1941 loss of synchronization between the server and the peer if messages 1942 are lost during the identifier update. 1944 6.6. Downgrading threats 1946 The fingerprint Hoob protects all the information exchanged in the 1947 Initial Exchange, including the cryptosuite negotiation. The message 1948 authentication codes MACs and MACp also protect the same information. 1949 The message authentication codes MACs2 and MACp2 protect information 1950 exchanged during key renegotiation in the Reconnect Exchange. This 1951 prevents downgrading attacks to weaker cryptosuites as long as the 1952 possible attacks take more time than the maximum time allowed for the 1953 EAP-NOOB completion. This is typically the case for recently 1954 discovered cryptanalytic attacks. 1956 As an additional precaution, the EAP server and peer SHOULD check for 1957 downgrading attacks in the Reconnect Exchange. As long as the server 1958 or peer saves any information about the other endpoint, it MUST also 1959 remember the previously negotiated cryptosuite and MUST NOT accept 1960 renegotiation of any cryptosuite that is known to be weaker than the 1961 previous one, such as a deprecated cryptosuite. 1963 Integrity of the direction negotiation cannot be verified in the same 1964 way as the integrity of the cryptosuite negotiation. That is, if the 1965 OOB channel used in an application is critically insecure in one 1966 direction, a man-in-the-middle attacker could modify the negotiation 1967 messages and thereby cause that direction to be used. Applications 1968 that support OOB messages in both directions SHOULD therefore ensure 1969 that the OOB channel has sufficiently strong security in both 1970 directions. While this is a theoretical vulnerability, it could 1971 arise in practice if EAP-NOOB is deployed in unexpected applications. 1972 However, most devices acting as the peer are likely to support only 1973 one direction of exchange, in which case interfering with the 1974 direction negotiation can only prevent the completion of the 1975 protocol. 1977 The long-term shared key material Kz in the persistent EAP-NOOB 1978 association is established with an ECDHE key exchange when the peer 1979 and server are first associated. It is a weaker secret than a 1980 manually configured random shared key because advances in 1981 cryptanalysis against the used ECDHE curve could eventually enable 1982 the attacker to recover Kz. EAP-NOOB protects against such attacks 1983 by allowing cryptosuite upgrades in the Reconnect Exchange and by 1984 updating shared key material Kz whenever the cryptosuite is upgraded. 1985 We do not expect the cryptosuite upgrades to be frequent, but if one 1986 becomes necessary, the upgrade can be made without manual resetting 1987 and reassociation of the peer devices. 1989 6.7. Recovery from loss of last message 1991 The EAP-NOOB Completion Exchange, as well as the Reconnect Exchange 1992 with cryptosuite update, result in a persistent state change that 1993 should take place either on both endpoints or on neither; otherwise, 1994 the result is a state mismatch that requires user action to resolve. 1995 The state mismatch can occur if the final EAP response of the 1996 exchanges is lost. In the Completion Exchange, the loss of the final 1997 response (Type=4) results in the peer moving to Registered (4) state 1998 and creating a persistent EAP-NOOB association while the server stays 1999 in an ephemeral state (1 or 2). In the Reconnect Exchange, the loss 2000 of the final response (Type=7) results in the peer moving to the 2001 Registered (4) state and updating its persistent key material Kz 2002 while the server stays in the Reconnecting (3) state and keeps the 2003 old key material. 2005 The state mismatch is an example of a unavoidable problem in 2006 distributed systems: it is theoretically impossible to guarantee 2007 synchronous state changes in endpoints that communicate 2008 asynchronously. The protocol will always have one critical message 2009 that may get lost, so that one side commits to the state change and 2010 the other side does not. In EAP, the critical message is the final 2011 response from the peer to the server. While the final response is 2012 normally followed by EAP-Success, [RFC3748] section 4.2 states that 2013 the peer MAY assume that the EAP-Success was lost and the 2014 authentication was successful. Furthermore, EAP methods in the peer 2015 do not receive notification of the EAP-Success message from the 2016 parent EAP state machine [RFC4137]. For these reasons, EAP-NOOB on 2017 the peer side commits to a state change already when it sends the 2018 final response. 2020 The best available solution to the loss of the critical message is to 2021 keep trying. EAP retransmission behavior defined in Section 4.3 of 2022 [RFC3748] suggests 3-5 retransmissions. In the absence of an 2023 attacker, this would be sufficient to reduce the probability of 2024 failure to an acceptable level. However, a determined attacker on 2025 the in-band channel can drop the final EAP-Response message and all 2026 subsequent retransmissions. In the Completion Exchange 2027 (KeyingMode=0) and in the Reconnect Exchange with cryptosuite upgrade 2028 (KeyingMode=3), this could result in state mismatch and persistent 2029 denial of service until user resets the peer state. 2031 EAP-NOOB implements its own recovery mechanism that allows unlimited 2032 retries of the Reconnect Exchange. When the DoS attacker eventually 2033 stops dropping packets on the in-band channel, the protocol will 2034 recover. The logic for this recovery mechanism is specified in 2035 Section 3.4.2. 2037 EAP-NOOB does not implement the same kind of retry mechanism in the 2038 Completion Exchange. The reason is that there is always a user 2039 involved in the initial association process, and the user can repeat 2040 the OOB Step to complete the association after the DoS attacker has 2041 left. On the other hand, Reconnect Exchange needs to work without 2042 user involvement. 2044 6.8. EAP security claims 2046 EAP security claims are defined in section 7.2.1 of [RFC3748]. The 2047 security claims for EAP-NOOB are listed in Table 9. 2049 +----------------+--------------------------------------------------+ 2050 | Security | EAP-NOOB claim | 2051 | property | | 2052 +----------------+--------------------------------------------------+ 2053 | Authentication | ECDHE key exchange with out-of-band | 2054 | mechanism | authentication | 2055 | | | 2056 | Protected | yes | 2057 | cryptosuite | | 2058 | negotiation | | 2059 | | | 2060 | Mutual | yes | 2061 | authentication | | 2062 | | | 2063 | Integrity | yes | 2064 | protection | | 2065 | | | 2066 | Replay | yes | 2067 | protection | | 2068 | | | 2069 | Key derivation | yes | 2070 | | | 2071 | Key strength | The specified cryptosuites provide key strength | 2072 | | of at least 128 bits. | 2073 | | | 2074 | Dictionary | yes | 2075 | attack | | 2076 | protection | | 2077 | | | 2078 | Fast reconnect | yes | 2079 | | | 2080 | Cryptographic | not applicable | 2081 | binding | | 2082 | | | 2083 | Session | yes | 2084 | independence | | 2085 | | | 2086 | Fragmentation | no | 2087 | | | 2088 | Channel | yes (The ServerInfo and PeerInfo can be used to | 2089 | binding | convey integrity-protected channel properties | 2090 | | such as network SSID or peer MAC address.) | 2091 +----------------+--------------------------------------------------+ 2093 Table 9: EAP security claims 2095 7. References 2097 7.1. Normative references 2099 [NIST-DH] Barker, E., Chen, L., Roginsky, A., and M. Smid, 2100 "Recommendation for Pair-Wise Key Establishment Schemes 2101 Using Discrete Logarithm Cryptography", NIST Special 2102 Publication 800-56A Revision 2 , May 2013, 2103 . 2106 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 2107 Hashing for Message Authentication", RFC 2104, 2108 DOI 10.17487/RFC2104, February 1997, 2109 . 2111 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2112 Requirement Levels", BCP 14, RFC 2119, 2113 DOI 10.17487/RFC2119, March 1997, 2114 . 2116 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 2117 Levkowetz, Ed., "Extensible Authentication Protocol 2118 (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004, 2119 . 2121 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2122 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 2123 . 2125 [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible 2126 Authentication Protocol (EAP) Key Management Framework", 2127 RFC 5247, DOI 10.17487/RFC5247, August 2008, 2128 . 2130 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms 2131 (SHA and SHA-based HMAC and HKDF)", RFC 6234, 2132 DOI 10.17487/RFC6234, May 2011, 2133 . 2135 [RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names", 2136 RFC 6761, DOI 10.17487/RFC6761, February 2013, 2137 . 2139 [RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517, 2140 DOI 10.17487/RFC7517, May 2015, 2141 . 2143 [RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, 2144 DOI 10.17487/RFC7518, May 2015, 2145 . 2147 [RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542, 2148 DOI 10.17487/RFC7542, May 2015, 2149 . 2151 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 2152 for Security", RFC 7748, DOI 10.17487/RFC7748, January 2153 2016, . 2155 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2156 Writing an IANA Considerations Section in RFCs", BCP 26, 2157 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2158 . 2160 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2161 Interchange Format", STD 90, RFC 8259, 2162 DOI 10.17487/RFC8259, December 2017, 2163 . 2165 7.2. Informative references 2167 [BluetoothPairing] 2168 Bluetooth, SIG, "Simple pairing whitepaper", Technical 2169 report , 2007. 2171 [EUI-48] Institute of Electrical and Electronics Engineers, 2172 "802-2014 IEEE Standard for Local and Metropolitan Area 2173 Networks: Overview and Architecture.", IEEE Standard 2174 802-2014. , June 2014. 2176 [IEEE-802.1X] 2177 Institute of Electrical and Electronics Engineers, "Local 2178 and Metropolitan Area Networks: Port-Based Network Access 2179 Control", IEEE Standard 802.1X-2004. , December 2004. 2181 [mcrl2] Groote, J. and M. Mousavi, "Modeling and analysis of 2182 communicating systems", The MIT press , 2014, 2183 . 2186 [proverif] 2187 Blanchet, B., Smyth, B., Cheval, V., and M. Sylvestre, 2188 "ProVerif 2.00: Automatic Cryptographic Protocol Verifier, 2189 User Manual and Tutorial", The MIT press , 2018, 2190 . 2193 [RFC2904] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L., 2194 Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and 2195 D. Spence, "AAA Authorization Framework", RFC 2904, 2196 DOI 10.17487/RFC2904, August 2000, 2197 . 2199 [RFC4137] Vollbrecht, J., Eronen, P., Petroni, N., and Y. Ohba, 2200 "State Machines for Extensible Authentication Protocol 2201 (EAP) Peer and Authenticator", RFC 4137, 2202 DOI 10.17487/RFC4137, August 2005, 2203 . 2205 [RFC4266] Hoffman, P., "The gopher URI Scheme", RFC 4266, 2206 DOI 10.17487/RFC4266, November 2005, 2207 . 2209 [RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS 2210 Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216, 2211 March 2008, . 2213 [RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running 2214 Code: The Implementation Status Section", BCP 205, 2215 RFC 7942, DOI 10.17487/RFC7942, July 2016, 2216 . 2218 [Sethi14] Sethi, M., Oat, E., Di Francesco, M., and T. Aura, "Secure 2219 Bootstrapping of Cloud-Managed Ubiquitous Displays", 2220 Proceedings of ACM International Joint Conference on 2221 Pervasive and Ubiquitous Computing (UbiComp 2014), pp. 2222 739-750, Seattle, USA , September 2014, 2223 . 2225 [Sethi19] Sethi, M., Peltonen, A., and T. Aura, "Misbinding Attacks 2226 on Secure Device Pairing", 2019, 2227 . 2229 Appendix A. Exchanges and events per state 2231 Figure 11 shows how the EAP server chooses the exchange type 2232 depending on the server and peer states. In the state combinations 2233 marked with hyphen "-", there is no possible exchange and user action 2234 is required to make progress. Note that peer state 4 is omitted from 2235 the table because the peer never connects to the server when the peer 2236 is in that state. The table also shows the handling of errors in 2237 each exchange. A notable detail is that the recipient of error code 2238 2003 moves to state 1. 2240 +--------+---------------------------+------------------------------+ 2241 | peer | exchange chosen by | next peer and | 2242 | states | server | server states | 2243 +========+===========================+==============================+ 2244 | server state: Unregistered (0) | 2245 +--------+---------------------------+------------------------------+ 2246 | 0..2 | Initial Exchange | both 1 (0 on error) | 2247 | 3 | - | no change, notify user | 2248 +--------+---------------------------+------------------------------+ 2249 | server state: Waiting for OOB (1) | 2250 +--------+---------------------------+------------------------------+ 2251 | 0 | Initial Exchange | both 1 (0 on error) | 2252 | 1 | Waiting Exchange | both 1 (no change on error) | 2253 | 2 | Completion Exchange | both 4 (A) | 2254 | 3 | - | no change, notify user | 2255 +--------+---------------------------+------------------------------+ 2256 | server state: OOB Received (2) | 2257 +--------+---------------------------+------------------------------+ 2258 | 0 | Initial Exchange | both 1 (0 on error) | 2259 | 1 | Completion Exchange | both 4 (B) | 2260 | 2 | Completion Exchange | both 4 (A) | 2261 | 3 | - | no change, notify user | 2262 +--------+---------------------------+------------------------------+ 2263 | server state: Reconnecting (3) or Registered (4) | 2264 +--------+---------------------------+------------------------------+ 2265 | 0..2 | - | no change, notify user | 2266 | 3 | Reconnect Exchange | both 4 (3 on error) | 2267 +--------+---------------------------+------------------------------+ 2268 (A) peer to 1 on error 2003, no other changes on error 2269 (B) server to 1 on error 2003, no other changes on error 2271 Figure 11: How server chooses the exchange type 2273 Figure 12 lists the local events that can take place in the server or 2274 peer. Both the server and peer output and accept OOB messages in 2275 association state 1, leading the receiver to state 2. Communication 2276 errors and timeouts in states 0..2 lead back to state 0, while 2277 similar errors in states 3..4 lead to state 3. Application request 2278 for rekeying (e.g. to refresh session keys or to upgrade cryptosuite) 2279 also takes the association from state 3..4 to state 3. User can 2280 always reset the association state to 0. Recovering association 2281 data, e.g. from a backup, leads to state 3. 2283 +--------+---------------------------+------------------------------+ 2284 | server/| possible local events | next state | 2285 | peer | on server and peer | | 2286 | state | | | 2287 +========+===========================+==============================+ 2288 | 1 | OOB Output* | 1 | 2289 | 1 | OOB Input* | 2 (1 on error) | 2290 | 0..2 | Timeout/network failure | 0 | 2291 | 3..4 | Timeout/network failure | 3 | 2292 | 3..4 | Rekeying request | 3 | 2293 | 0..4 | User resets peer state | 0 | 2294 | 0..4 | Association state recovery| 3 | 2295 +--------+---------------------------+------------------------------+ 2297 Figure 12: Local events on server and peer 2299 Appendix B. Application-specific parameters 2301 Table 10 lists OOB channel parameters that need to be specified in 2302 each application that makes use of EAP-NOOB. The list is not 2303 exhaustive and is included for the convenience of implementors only. 2305 +--------------------+----------------------------------------------+ 2306 | Parameter | Description | 2307 +--------------------+----------------------------------------------+ 2308 | OobDirs | Allowed directions of the OOB channel | 2309 | | | 2310 | OobMessageEncoding | How the OOB message data fields are encoded | 2311 | | for the OOB channel | 2312 | | | 2313 | SleepTimeDefault | Default minimum time in seconds that the | 2314 | | peer should sleep before the next Waiting | 2315 | | Exchange | 2316 | | | 2317 | OobRetries | Number of received OOB messages with invalid | 2318 | | Hoob after which the receiver moves to | 2319 | | Unregistered (0) state | 2320 | | | 2321 | NoobTimeout | How many seconds the sender of the OOB | 2322 | | message remembers the sent Noob value. The | 2323 | | RECOMMENDED value is 3600 seconds. | 2324 | | | 2325 | ServerInfoMembers | Required members in ServerInfo | 2326 | | | 2327 | PeerInfoMembers | Required members in PeerInfo | 2328 +--------------------+----------------------------------------------+ 2330 Table 10: OOB channel characteristics 2332 Appendix C. ServerInfo and PeerInfo contents 2334 The ServerInfo and PeerInfo fields in the Initial Exchange and 2335 Reconnect Exchange enable the server and peer, respectively, send 2336 information about themselves to the other endpoint. They contain 2337 JSON objects whose structure may be specified separately for each 2338 application and each type of OOB channel. ServerInfo and PeerInfo 2339 MAY contain auxiliary data needed for the OOB channel messaging and 2340 for EAP channel binding. Table 11 lists some suggested data fields 2341 for ServerInfo. 2343 +----------------+--------------------------------------------------+ 2344 | Data field | Description | 2345 +----------------+--------------------------------------------------+ 2346 | ServerName | String that may be used to aid human | 2347 | | identification of the server. | 2348 | | | 2349 | ServerURL | Prefix string when the OOB message is formatted | 2350 | | as URL, as suggested in Appendix E. | 2351 | | | 2352 | SSIDList | List of wireless network identifier (SSID) | 2353 | | strings used for roaming support, as suggested | 2354 | | in Appendix D. JSON array of UTF-8 encoded SSID | 2355 | | strings. | 2356 | | | 2357 | Base64SSIDList | List of wireless network identifier (SSID) | 2358 | | strings used for roaming support, as suggested | 2359 | | in Appendix D. JSON array of SSIDs, each of | 2360 | | which is base64url encoded without padding. Peer | 2361 | | SHOULD send at most one of the fields SSIDList | 2362 | | and Base64SSIDList in PeerInfo, and the server | 2363 | | SHOULD ignore SSIDList if Base64SSIDList is | 2364 | | included. | 2365 +----------------+--------------------------------------------------+ 2367 Table 11: Suggested ServerInfo data fields 2369 PeerInfo typically contains auxiliary information for identifying and 2370 managing peers on the application level at the server end. Table 12 2371 lists some suggested data fields for PeerInfo. 2373 +--------------+----------------------------------------------------+ 2374 | Data field | Description | 2375 +--------------+----------------------------------------------------+ 2376 | PeerName | String that may be used to aid human | 2377 | | identification of the peer. | 2378 | | | 2379 | Manufacturer | Manufacturer or brand string. | 2380 | | | 2381 | Model | Manufacturer-specified model string. | 2382 | | | 2383 | SerialNumber | Manufacturer-assigned serial number. | 2384 | | | 2385 | MACAddress | Peer link-layer identifier (EUI-48) in the | 2386 | | 12-digit base-16 form [EUI-48]. The string MAY | 2387 | | include additional colon ':' or dash '-' | 2388 | | characters that MUST be ignored by the server. | 2389 | | | 2390 | SSID | Wireless network SSID for channel binding. The | 2391 | | SSID is a UTF-8 string. | 2392 | | | 2393 | Base64SSID | Wireless network SSID for channel binding. The | 2394 | | SSID is base64url encoded. Peer SHOULD send at | 2395 | | most one of the fields SSID and Base64SSID in | 2396 | | PeerInfo, and the server SHOULD ignore SSID if | 2397 | | Base64SSID is included. | 2398 | | | 2399 | BSSID | Wireless network BSSID (EUI-48) in the 12-digit | 2400 | | base-16 form [EUI-48]. The string MAY include | 2401 | | additional colon ':' or dash '-' characters that | 2402 | | MUST be ignored by the server. | 2403 | | | 2404 +--------------+----------------------------------------------------+ 2406 Table 12: Suggested PeerInfo data fields 2408 Appendix D. EAP-NOOB roaming 2410 AAA architectures [RFC2904] allow for roaming of network-connected 2411 appliances that are authenticated over EAP. While the peer is 2412 roaming in a visited network, authentication still takes place 2413 between the peer and an authentication server at its home network. 2414 EAP-NOOB supports such roaming by assigning a Realm to the peer. 2415 After the Realm has been assigned, the peer's NAI enables the visited 2416 network to route the EAP session to the peer's home AAA server. 2418 A peer device that is new or has gone through a hard reset should be 2419 connected first to the home network and establish an EAP-NOOB 2420 association with its home AAA server before it is able to roam. 2422 After that, it can perform the Reconnect Exchange from the visited 2423 network. 2425 Alternatively, the device may provide some method for the user to 2426 configure the Realm of the home network. In that case, the EAP-NOOB 2427 association can be created while roaming. The device will use the 2428 user-assigned Realm in the Initial Exchange, which enables the EAP 2429 messages to be routed correctly to the home AAA server. 2431 While roaming, the device needs to identify the networks where the 2432 EAP-NOOB association can be used to gain network access. For 802.11 2433 access networks, the server MAY send a list of SSID strings in the 2434 ServerInfo JSON object in a member called either SSIDList or 2435 Base64SSIDList. The list is formated as explained in Table 11. If 2436 present, the peer MAY use this list as a hint to determine the 2437 networks where the EAP-NOOB association can be used for access 2438 authorization, in addition to the access network where the Initial 2439 Exchange took place. 2441 Appendix E. OOB message as URL 2443 While EAP-NOOB does not mandate any particular OOB communication 2444 channel, typical OOB channels include graphical displays and emulated 2445 NFC tags. In the peer-to-server direction, it may be convenient to 2446 encode the OOB message as a URL, which is then encoded as a QR code 2447 for displays and printers or as an NDEF record for NFC tags. A user 2448 can then simply scan the QR code or NFC tag and open the URL, which 2449 causes the OOB message to be delivered to the authentication server. 2450 The URL MUST specify the https protocol i.e. secure connection to the 2451 server, so that the man-in-the-middle attacker cannot read or modify 2452 the OOB message. 2454 The ServerInfo in this case includes a JSON member called ServerUrl 2455 of the following format with maximum length of 60 characters: 2457 https://[:]/[] 2459 To this, the peer appends the OOB message fields (PeerId, Noob, Hoob) 2460 as a query string. PeerId is provided to the peer by the server and 2461 might be a 22-character string. The peer base64url encodes, without 2462 padding, the 16-byte values Noob and Hoob into 22-character strings. 2463 The query parameters MAY be in any order. The resulting URL is of 2464 the following format: 2466 https://[:]/[]?P=&N=&H= 2468 The following is an example of a well-formed URL encoding the OOB 2469 message (without line breaks): 2471 https://example.com/Noob?P=ZrD7qkczNoHGbGcN2bN0&N=rMinS0-F4EfCU8D9ljx 2472 X_A&H=QvnMp4UGxuQVFaXPW_14UW 2474 Appendix F. Example messages 2476 The message examples in this section are generated with Curve25519 2477 ECDHE test vectors specified in section 6.1 of [RFC7748] 2478 (server=Alice, peer=Bob). The direction of the OOB channel 2479 negotiated is 1 (peer-to-server). The JSON messages are as follows 2480 (line breaks are for readability only). 2482 ====== Initial Exchange ====== 2484 Identity response: 2485 noob@eap-noob.net 2487 EAP request (type 1): 2488 {"Type":1,"Vers":[1],"PeerId":"07KRU6OgqX0HIeRFldnbSW","Realm":"no 2489 ob.example.com","Cryptosuites":[1],"Dirs":3,"ServerInfo":{"Name":" 2490 Example","Url":"https://noob.example.com/sendOOB"}} 2492 EAP response (type 1): 2493 {"Type":1,"Verp":1,"PeerId":"07KRU6OgqX0HIeRFldnbSW","Cryptosuitep 2494 ":1,"Dirp":1,"PeerInfo":{"Make":"Acme","Type":"None","Serial":"DU- 2495 9999","SSID":"Noob1","BSSID":"6c:19:8f:83:c2:80"}} 2497 EAP request (type 2): 2498 {"Type":2,"PeerId":"07KRU6OgqX0HIeRFldnbSW","PKs":{"kty":"EC","crv 2499 ":"Curve25519","x":"hSDwCYkwp1R0i33ctD73Wg2_Og0mOBr066SpjqqbTmo"}, 2500 "Ns":"PYO7NVd9Af3BxEri1MI6hL8Ck49YxwCjSRPqlC1SPbw","SleepTime":60} 2502 EAP response (type 2): 2503 {"Type":2,"PeerId":"07KRU6OgqX0HIeRFldnbSW","PKp":{"kty":"EC","crv 2504 ":"Curve25519","x":"3p7bfXt9wbTTW2HC7OQ1Nz-DQ8hbeGdNrfx-FG- 2505 IK08"},"Np":"HIvB6g0n2btpxEcU7YXnWB-451ED6L6veQQd6ugiPFU"} 2507 ====== Waiting Exchange ====== 2509 Identity response: 2510 07KRU6OgqX0HIeRFldnbSW+s1@noob.example.com 2512 EAP request (type 3): 2513 {"Type":3,"PeerId":"07KRU6OgqX0HIeRFldnbSW","SleepTime":60} 2515 EAP response (type 3): 2516 {"Type":3,"PeerId":"07KRU6OgqX0HIeRFldnbSW"} 2518 ====== OOB Step ====== 2519 Identity response: 2520 P=07KRU6OgqX0HIeRFldnbSW&N=x3JlolaPciK4Wa6XlMJxtQ&H=faqWz68trUrBTK 2521 AnioZMQA 2523 ====== Completion Exchange ====== 2525 Identity response: 2526 07KRU6OgqX0HIeRFldnbSW+s2@noob.example.com 2528 EAP request (type 8): 2529 {"Type":8,"PeerId":"07KRU6OgqX0HIeRFldnbSW"} 2531 EAP response (type 8): 2532 {"Type":8,"PeerId":"07KRU6OgqX0HIeRFldnbSW","NoobId":"U0OHwYGCS4nE 2533 kzk2TPIE6g"} 2535 EAP request (type 4): 2536 {"Type":4,"PeerId":"07KRU6OgqX0HIeRFldnbSW","NoobId":"U0OHwYGCS4nE 2537 kzk2TPIE6g","MACs":"Y5NfKQkZTbRW3sEFhWy0Bv0ic2wsMnaA6xGqtUmQqmc"} 2539 EAP response (type 4): 2540 {"Type":4,"PeerId":"07KRU6OgqX0HIeRFldnbSW","MACp":"ddY225rN3lYzo7 2541 qZNPStbVO1HRdNnTx0Rit6_8xEh7A"} 2543 ====== Reconnect Exchange ====== 2545 Identity response: 2546 07KRU6OgqX0HIeRFldnbSW+s3@noob.example.com 2548 EAP request (type 5): 2549 {"Type":5,"Vers":[1],"PeerId":"07KRU6OgqX0HIeRFldnbSW","Cryptosuit 2550 es":[1],"Realm":"noob.example.com","ServerInfo":{"Name":"Example", 2551 "Url":"https://noob.example.com/sendOOB"}} 2553 EAP response (type 5): 2554 {"Type":5,"Verp":1,"PeerId":"07KRU6OgqX0HIeRFldnbSW","Cryptosuitep 2555 ":1,"PeerInfo":{"Make":"Acme","Type":"None","Serial":"DU- 2556 9999","SSID":"Noob1","BSSID":"6c:19:8f:83:c2:80"}} 2558 EAP request (type 6): 2559 {"Type":6,"PeerId":"07KRU6OgqX0HIeRFldnbSW","PKs2":{"kty":"EC","cr 2560 v":"Curve25519","x":"hSDwCYkwp1R0i33ctD73Wg2_Og0mOBr066SpjqqbTmo"} 2561 ,"Ns2":"RDLahHBlIgnmL_F_xcynrHurLPkCsrp3G3B_S82WUF4"} 2563 EAP response (type 6): 2564 {"Type":6,"PeerId":"07KRU6OgqX0HIeRFldnbSW","PKp2":{"kty":"EC","cr 2565 v":"Curve25519","x":"3p7bfXt9wbTTW2HC7OQ1Nz-DQ8hbeGdNrfx-FG- 2566 IK08"},"Np2":"jN0_V4P0JoTqwI9VHHQKd9ozUh7tQdc9ABd-j6oTy_4"} 2568 EAP request (type 7): 2569 {"Type":7,"PeerId":"07KRU6OgqX0HIeRFldnbSW","MACs2":"_pXDF4- 2570 7uBKXKqVKKB6U-GP9EDnGCNOMdkyfEQp_iwA"} 2572 EAP response (type 7): 2573 {"Type":7,"PeerId":"07KRU6OgqX0HIeRFldnbSW","MACp2":"qSUH4zA0VzMqU 2574 2O1U-JJTqwGRXGB8i3bggasYL6o1uU"} 2576 Appendix G. TODO list 2578 o Update example messages with request-reponse type 9. 2580 Appendix H. Version history 2582 o Version 01: 2584 * Fixed Reconnection Exchange. 2586 * URL examples. 2588 * Message examples. 2590 * Improved state transition (event) tables. 2592 o Version 02: 2594 * Reworked the rekeying and key derivation. 2596 * Increased internal key lengths and in-band nonce and HMAC 2597 lengths to 32 bytes. 2599 * Less data in the persistent EAP-NOOB association. 2601 * Updated reference [NIST-DH] to Revision 2 (2013). 2603 * Shorter suggested PeerId format. 2605 * Optimized the example of encoding OOB message as URL. 2607 * NoobId in Completion Exchange to differentiate between multiple 2608 valid Noob values. 2610 * List of application-specific parameters in appendix. 2612 * Clarified the equivalence of Unregistered state and no state. 2614 * Peer SHOULD probe the server regardless of the OOB channel 2615 direction. 2617 * Added new error messages. 2619 * Realm is part of the persistent association and can be updated. 2621 * Clarified error handling. 2623 * Updated message examples. 2625 * Explained roaming in appendix. 2627 * More accurate definition of timeout for the Noob nonce. 2629 * Additions to security considerations. 2631 o Version 03: 2633 * Clarified reasons for going to Reconnecting state. 2635 * Included Verp in persistent state. 2637 * Added appendix on suggested ServerInfo and PeerInfo fields. 2639 * Exporting PeerId and SessionId. 2641 * Explicitly specified next state after OOB Step. 2643 * Clarified the processing of an expired OOB message and 2644 unrecognized NoobId. 2646 * Enabled protocol version upgrade in Reconnect Exchange. 2648 * Explained handling of redundant received OOB messages. 2650 * Clarified where raw and base64url encoded values are used. 2652 * Cryptosuite must specify the detailed format of the JWK object. 2654 * Base64url encoding in JSON strings is done without padding. 2656 * Simplified explanation of PeerId, Realm and NAI. 2658 * Added error codes for private and experimental use. 2660 * Updated the security considerations. 2662 o Version 04: 2664 * Recovery from synchronization failure due to lost last 2665 response. 2667 o Version 05: 2669 * Kz identifier added to help recovery from lost last messages. 2671 * Error message codes changed for better structure. 2673 * Improved security considerations section. 2675 o Version 06: 2677 * Kz identifier removed to enable PeerId anonymization in the 2678 future. 2680 * Clarified text on when to use server-assigned realm. 2682 * Send PeerId and PeerState in a separate request-reponse pair, 2683 not in NAI. 2685 * New subsection for the common handshake in all exchanges to 2686 avoid repetition. 2688 Appendix I. Acknowledgments 2690 Aleksi Peltonen modeled the protocol specification with the mCRL2 2691 formal specification language. Shiva Prasad TP and Raghavendra MS 2692 implemented parts of the protocol with wpa_supplicant and hostapd. 2693 Their inputs helped us in improving the specification. 2695 The authors would also like to thank Rhys Smith and Josh Howlett for 2696 providing valuable feedback as well as new use cases and requirements 2697 for the protocol. Thanks to Eric Rescorla, Darshak Thakore, Stefan 2698 Winter and Hannes Tschofenig for interesting discussions in this 2699 problem space. 2701 Authors' Addresses 2703 Tuomas Aura 2704 Aalto University 2705 Aalto 00076 2706 Finland 2708 EMail: tuomas.aura@aalto.fi 2709 Mohit Sethi 2710 Ericsson 2711 Jorvas 02420 2712 Finland 2714 EMail: mohit@piuha.net