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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to lower-maturity documents in RFCs) == Missing Reference: 'RFC-XXXX' is mentioned on line 1093, but not defined == Outdated reference: A later version (-46) exists of draft-ietf-ace-oauth-authz-41 == Outdated reference: A later version (-16) exists of draft-ietf-ace-oauth-params-15 ** Obsolete normative reference: RFC 6347 (Obsoleted by RFC 9147) ** Downref: Normative reference to an Informational RFC: RFC 7251 ** Obsolete normative reference: RFC 8152 (Obsoleted by RFC 9052, RFC 9053) Summary: 3 errors (**), 0 flaws (~~), 5 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACE Working Group S. Gerdes 3 Internet-Draft O. Bergmann 4 Intended status: Standards Track C. Bormann 5 Expires: 6 December 2021 Universität Bremen TZI 6 G. Selander 7 Ericsson AB 8 L. Seitz 9 Combitech 10 4 June 2021 12 Datagram Transport Layer Security (DTLS) Profile for Authentication and 13 Authorization for Constrained Environments (ACE) 14 draft-ietf-ace-dtls-authorize-18 16 Abstract 18 This specification defines a profile of the ACE framework that allows 19 constrained servers to delegate client authentication and 20 authorization. The protocol relies on DTLS version 1.2 for 21 communication security between entities in a constrained network 22 using either raw public keys or pre-shared keys. A resource- 23 constrained server can use this protocol to delegate management of 24 authorization information to a trusted host with less severe 25 limitations regarding processing power and memory. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on 6 December 2021. 44 Copyright Notice 46 Copyright (c) 2021 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 51 license-info) in effect on the date of publication of this document. 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. Code Components 54 extracted from this document must include Simplified BSD License text 55 as described in Section 4.e of the Trust Legal Provisions and are 56 provided without warranty as described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 61 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 62 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 63 3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 6 64 3.1. Communication Between the Client and the Authorization 65 Server . . . . . . . . . . . . . . . . . . . . . . . . . 6 66 3.2. Raw Public Key Mode . . . . . . . . . . . . . . . . . . . 7 67 3.2.1. Access Token Retrieval from the Authorization 68 Server . . . . . . . . . . . . . . . . . . . . . . . 7 69 3.2.2. DTLS Channel Setup Between Client and Resource 70 Server . . . . . . . . . . . . . . . . . . . . . . . 9 71 3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 10 72 3.3.1. Access Token Retrieval from the Authorization 73 Server . . . . . . . . . . . . . . . . . . . . . . . 11 74 3.3.2. DTLS Channel Setup Between Client and Resource 75 Server . . . . . . . . . . . . . . . . . . . . . . . 15 76 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 17 77 4. Dynamic Update of Authorization Information . . . . . . . . . 19 78 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 20 79 6. Secure Communication with an Authorization Server . . . . . . 20 80 7. Security Considerations . . . . . . . . . . . . . . . . . . . 21 81 7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 23 82 7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 23 83 7.3. Out-of-Band Configuration . . . . . . . . . . . . . . . . 23 84 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 24 85 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 86 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25 87 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 88 11.1. Normative References . . . . . . . . . . . . . . . . . . 25 89 11.2. Informative References . . . . . . . . . . . . . . . . . 27 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 92 1. Introduction 94 This specification defines a profile of the ACE framework 95 [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource 96 server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to 97 communicate. This specification uses DTLS 1.2 terminology, but later 98 versions such as DTLS 1.3 can be used instead. The client obtains an 99 access token, bound to a key (the proof-of-possession key), from an 100 authorization server to prove its authorization to access protected 101 resources hosted by the resource server. Also, the client and the 102 resource server are provided by the authorization server with the 103 necessary keying material to establish a DTLS session. The 104 communication between client and authorization server may also be 105 secured with DTLS. This specification supports DTLS with Raw Public 106 Keys (RPK) [RFC7250] and with Pre-Shared Keys (PSK) [RFC4279]. How 107 token introspection [RFC7662] is performed between RS and AS is out 108 of scope for this specification. 110 The ACE framework requires that client and server mutually 111 authenticate each other before any application data is exchanged. 112 DTLS enables mutual authentication if both client and server prove 113 their ability to use certain keying material in the DTLS handshake. 114 The authorization server assists in this process on the server side 115 by incorporating keying material (or information about keying 116 material) into the access token, which is considered a "proof of 117 possession" token. 119 In the RPK mode, the client proves that it can use the RPK bound to 120 the token and the server shows that it can use a certain RPK. 122 The resource server needs access to the token in order to complete 123 this exchange. For the RPK mode, the client must upload the access 124 token to the resource server before initiating the handshake, as 125 described in Section 5.10.1 of the ACE framework 126 [I-D.ietf-ace-oauth-authz]. 128 In the PSK mode, client and server show with the DTLS handshake that 129 they can use the keying material that is bound to the access token. 130 To transfer the access token from the client to the resource server, 131 the "psk_identity" parameter in the DTLS PSK handshake may be used 132 instead of uploading the token prior to the handshake. 134 As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this 135 specification uses CBOR web tokens to convey claims within an access 136 token issued by the server. While other formats could be used as 137 well, those are out of scope for this document. 139 1.1. Terminology 141 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 142 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 143 "OPTIONAL" in this document are to be interpreted as described in BCP 144 14 [RFC2119] [RFC8174] when, and only when, they appear in all 145 capitals, as shown here. 147 Readers are expected to be familiar with the terms and concepts 148 described in [I-D.ietf-ace-oauth-authz] and in 149 [I-D.ietf-ace-oauth-params]. 151 The authorization information (authz-info) resource refers to the 152 authorization information endpoint as specified in 153 [I-D.ietf-ace-oauth-authz]. The term "claim" is used in this 154 document with the same semantics as in [I-D.ietf-ace-oauth-authz], 155 i.e., it denotes information carried in the access token or returned 156 from introspection. 158 2. Protocol Overview 160 The CoAP-DTLS profile for ACE specifies the transfer of 161 authentication information and, if necessary, authorization 162 information between the client (C) and the resource server (RS) 163 during setup of a DTLS session for CoAP messaging. It also specifies 164 how the client can use CoAP over DTLS to retrieve an access token 165 from the authorization server (AS) for a protected resource hosted on 166 the resource server. As specified in Section 6.7 of 167 [I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these 168 interactions is completely independent. 170 This profile requires the client to retrieve an access token for 171 protected resource(s) it wants to access on the resource server as 172 specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical 173 message flow in this scenario (messages in square brackets are 174 optional): 176 C RS AS 177 | [---- Resource Request ------>]| | 178 | | | 179 | [<-AS Request Creation Hints-] | | 180 | | | 181 | ------- Token Request ----------------------------> | 182 | | | 183 | <---------------------------- Access Token --------- | 184 | + Access Information | 186 Figure 1: Retrieving an Access Token 188 To determine the authorization server in charge of a resource hosted 189 at the resource server, the client can send an initial Unauthorized 190 Resource Request message to the resource server. The resource server 191 then denies the request and sends an AS Request Creation Hints 192 message containing the address of its authorization server back to 193 the client as specified in Section 5.3 of [I-D.ietf-ace-oauth-authz]. 195 Once the client knows the authorization server's address, it can send 196 an access token request to the token endpoint at the authorization 197 server as specified in [I-D.ietf-ace-oauth-authz]. As the access 198 token request as well as the response may contain confidential data, 199 the communication between the client and the authorization server 200 must be confidentiality-protected and ensure authenticity. The 201 client is expected to have been registered at the authorization 202 server as outlined in Section 4 of [I-D.ietf-ace-oauth-authz]. 204 The access token returned by the authorization server can then be 205 used by the client to establish a new DTLS session with the resource 206 server. When the client intends to use an asymmetric proof-of- 207 possession key in the DTLS handshake with the resource server, the 208 client MUST upload the access token to the authz-info resource, i.e. 209 the authz-info endpoint, on the resource server before starting the 210 DTLS handshake, as described in Section 5.10.1 of 211 [I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric 212 proof-of-possession key in the DTLS handshake, the procedure as above 213 MAY be used, or alternatively, the access token MAY instead be 214 transferred in the DTLS ClientKeyExchange message (see 215 Section 3.3.2). In any case, DTLS MUST be used in a mode that 216 provides replay protection. 218 Figure 2 depicts the common protocol flow for the DTLS profile after 219 the client has retrieved the access token from the authorization 220 server, AS. 222 C RS AS 223 | [--- Access Token ------>] | | 224 | | | 225 | <== DTLS channel setup ==> | | 226 | | | 227 | == Authorized Request ===> | | 228 | | | 229 | <=== Protected Resource == | | 231 Figure 2: Protocol overview 233 3. Protocol Flow 235 The following sections specify how CoAP is used to interchange 236 access-related data between the resource server, the client and the 237 authorization server so that the authorization server can provide the 238 client and the resource server with sufficient information to 239 establish a secure channel, and convey authorization information 240 specific for this communication relationship to the resource server. 242 Section 3.1 describes how the communication between the client (C) 243 and the authorization server (AS) must be secured. Depending on the 244 used CoAP security mode (see also Section 9 of [RFC7252], the Client- 245 to-AS request, AS-to-Client response and DTLS session establishment 246 carry slightly different information. Section 3.2 addresses the use 247 of raw public keys while Section 3.3 defines how pre-shared keys are 248 used in this profile. 250 3.1. Communication Between the Client and the Authorization Server 252 To retrieve an access token for the resource that the client wants to 253 access, the client requests an access token from the authorization 254 server. Before the client can request the access token, the client 255 and the authorization server MUST establish a secure communication 256 channel. This profile assumes that the keying material to secure 257 this communication channel has securely been obtained either by 258 manual configuration or in an automated provisioning process. The 259 following requirements in alignment with Section 6.5 of 260 [I-D.ietf-ace-oauth-authz] therefore must be met: 262 * The client MUST securely have obtained keying material to 263 communicate with the authorization server. 265 * Furthermore, the client MUST verify that the authorization server 266 is authorized to provide access tokens (including authorization 267 information) about the resource server to the client, and that 268 this authorization information about the authorization server is 269 still valid. 271 * Also, the authorization server MUST securely have obtained keying 272 material for the client, and obtained authorization rules approved 273 by the resource owner (RO) concerning the client and the resource 274 server that relate to this keying material. 276 The client and the authorization server MUST use their respective 277 keying material for all exchanged messages. How the security 278 association between the client and the authorization server is 279 bootstrapped is not part of this document. The client and the 280 authorization server must ensure the confidentiality, integrity and 281 authenticity of all exchanged messages within the ACE protocol. 283 Section 6 specifies how communication with the authorization server 284 is secured. 286 3.2. Raw Public Key Mode 288 When the client uses raw public key authentication, the procedure is 289 as described in the following. 291 3.2.1. Access Token Retrieval from the Authorization Server 293 After the client and the authorization server mutually authenticated 294 each other and validated each other's authorization, the client sends 295 a token request to the authorization server's token endpoint. The 296 client MUST add a "req_cnf" object carrying either its raw public key 297 or a unique identifier for a public key that it has previously made 298 known to the authorization server. It is RECOMMENDED that the client 299 uses DTLS with the same keying material to secure the communication 300 with the authorization server, proving possession of the key as part 301 of the token request. Other mechanisms for proving possession of the 302 key may be defined in the future. 304 An example access token request from the client to the authorization 305 server is depicted in Figure 3. 307 POST coaps://as.example.com/token 308 Content-Format: application/ace+cbor 309 Payload: 310 { 311 grant_type : client_credentials, 312 audience : "tempSensor4711", 313 req_cnf : { 314 COSE_Key : { 315 kty : EC2, 316 crv : P-256, 317 x : h'e866c35f4c3c81bb96a1...', 318 y : h'2e25556be097c8778a20...' 319 } 320 } 321 } 323 Figure 3: Access Token Request Example for RPK Mode 325 The example shows an access token request for the resource identified 326 by the string "tempSensor4711" on the authorization server using a 327 raw public key. 329 The authorization server MUST check if the client that it 330 communicates with is associated with the RPK in the "req_cnf" 331 parameter before issuing an access token to it. If the authorization 332 server determines that the request is to be authorized according to 333 the respective authorization rules, it generates an access token 334 response for the client. The access token MUST be bound to the RPK 335 of the client by means of the "cnf" claim. 337 The response MUST contain an "ace_profile" parameter if 338 the"ace_profile" parameter in the request is empty, and MAY contain 339 this parameter otherwise (see Section 5.8.2 of 340 [I-D.ietf-ace-oauth-authz]). This parameter is set to "coap_dtls" to 341 indicate that this profile MUST be used for communication between the 342 client and the resource server. The response also contains an access 343 token with information for the resource server about the client's 344 public key. The authorization server MUST return in its response the 345 parameter "rs_cnf" unless it is certain that the client already knows 346 the public key of the resource server. The authorization server MUST 347 ascertain that the RPK specified in "rs_cnf" belongs to the resource 348 server that the client wants to communicate with. The authorization 349 server MUST protect the integrity of the access token such that the 350 resource server can detect unauthorized changes. If the access token 351 contains confidential data, the authorization server MUST also 352 protect the confidentiality of the access token. 354 The client MUST ascertain that the access token response belongs to a 355 certain previously sent access token request, as the request may 356 specify the resource server with which the client wants to 357 communicate. 359 An example access token response from the authorization server to the 360 client is depicted in Figure 4. Here, the contents of the 361 "access_token" claim have been truncated to improve readability. The 362 response comprises access information for the client that contains 363 the server's public key in the "rs_cnf" parameter. Caching proxies 364 process the Max-Age option in the CoAP response which has a default 365 value of 60 seconds (Section 5.6.1 of [RFC7252]). The authorization 366 server SHOULD adjust the Max-Age option such that it does not exceed 367 the "expires_in" parameter to avoid stale responses. 369 2.01 Created 370 Content-Format: application/ace+cbor 371 Max-Age: 3560 372 Payload: 373 { 374 access_token : b64'SlAV32hkKG... 375 (remainder of CWT omitted for brevity; 376 CWT contains the client's RPK in the cnf claim)', 377 expires_in : 3600, 378 rs_cnf : { 379 COSE_Key : { 380 kty : EC2, 381 crv : P-256, 382 x : h'd7cc072de2205bdc1537...', 383 y : h'f95e1d4b851a2cc80fff...' 384 } 385 } 386 } 388 Figure 4: Access Token Response Example for RPK Mode 390 3.2.2. DTLS Channel Setup Between Client and Resource Server 392 Before the client initiates the DTLS handshake with the resource 393 server, the client MUST send a "POST" request containing the obtained 394 access token to the authz-info resource hosted by the resource 395 server. After the client receives a confirmation that the resource 396 server has accepted the access token, it proceeds to establish a new 397 DTLS channel with the resource server. The client MUST use its 398 correct public key in the DTLS handshake. If the authorization 399 server has specified a "cnf" field in the access token response, the 400 client MUST use this key. Otherwise, the client MUST use the public 401 key that it specified in the "req_cnf" of the access token request. 402 The client MUST specify this public key in the SubjectPublicKeyInfo 403 structure of the DTLS handshake as described in [RFC7250]. 405 If the client does not have the keying material belonging to the 406 public key, the client MAY try to send an access token request to the 407 AS where it specifies its public key in the "req_cnf" parameter. If 408 the AS still specifies a public key in the response that the client 409 does not have, the client SHOULD re-register with the authorization 410 server to establish a new client public key. This process is out of 411 scope for this document. 413 To be consistent with [RFC7252], which allows for shortened MAC tags 414 in constrained environments, an implementation that supports the RPK 415 mode of this profile MUST at least support the cipher suite 416 TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in 418 [RFC7748], new ECC curves have been defined recently that are 419 considered superior to the so-called NIST curves. Implementations of 420 this profile therefore MUST implement support for curve25519 (cf. 421 [RFC8032], [RFC8422]) as this curve said to be efficient and less 422 dangerous regarding implementation errors than the secp256r1 curve 423 mandated in [RFC7252]. 425 The resource server MUST check if the access token is still valid, if 426 the resource server is the intended destination (i.e., the audience) 427 of the token, and if the token was issued by an authorized 428 authorization server (see also section 5.10.1.1 of 429 [I-D.ietf-ace-oauth-authz]). The access token is constructed by the 430 authorization server such that the resource server can associate the 431 access token with the Client's public key. The "cnf" claim MUST 432 contain either the client's RPK or, if the key is already known by 433 the resource server (e.g., from previous communication), a reference 434 to this key. If the authorization server has no certain knowledge 435 that the Client's key is already known to the resource server, the 436 Client's public key MUST be included in the access token's "cnf" 437 parameter. If CBOR web tokens [RFC8392] are used (as recommended in 438 [I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in 439 [RFC8747]. A resource server MUST have the capacity to store one 440 access token for every proof-of-possession key of every authorized 441 client. 443 The raw public key used in the DTLS handshake with the client MUST 444 belong to the resource server. If the resource server has several 445 raw public keys, it needs to determine which key to use. The 446 authorization server can help with this decision by including a "cnf" 447 parameter in the access token that is associated with this 448 communication. In this case, the resource server MUST use the 449 information from the "cnf" field to select the proper keying 450 material. 452 Thus, the handshake only finishes if the client and the resource 453 server are able to use their respective keying material. 455 3.3. PreSharedKey Mode 457 When the client uses pre-shared key authentication, the procedure is 458 as described in the following. 460 3.3.1. Access Token Retrieval from the Authorization Server 462 To retrieve an access token for the resource that the client wants to 463 access, the client MAY include a "cnf" object carrying an identifier 464 for a symmetric key in its access token request to the authorization 465 server. This identifier can be used by the authorization server to 466 determine the shared secret to construct the proof-of-possession 467 token. The authorization server MUST check if the identifier refers 468 to a symmetric key that was previously generated by the authorization 469 server as a shared secret for the communication between this client 470 and the resource server. If no such symmetric key was found, the 471 authorization server MUST generate a new symmetric key that is 472 returned in its response to the client. 474 The authorization server MUST determine the authorization rules for 475 the client it communicates with as defined by the resource owner and 476 generate the access token accordingly. If the authorization server 477 authorizes the client, it returns an AS-to-Client response. If the 478 "ace_profile" parameter is present, it is set to "coap_dtls". The 479 authorization server MUST ascertain that the access token is 480 generated for the resource server that the client wants to 481 communicate with. Also, the authorization server MUST protect the 482 integrity of the access token to ensure that the resource server can 483 detect unauthorized changes. If the token contains confidential data 484 such as the symmetric key, the confidentiality of the token MUST also 485 be protected. Depending on the requested token type and algorithm in 486 the access token request, the authorization server adds access 487 Information to the response that provides the client with sufficient 488 information to setup a DTLS channel with the resource server. The 489 authorization server adds a "cnf" parameter to the access information 490 carrying a "COSE_Key" object that informs the client about the shared 491 secret that is to be used between the client and the resource server. 492 To convey the same secret to the resource server, the authorization 493 server can include it directly in the access token by means of the 494 "cnf" claim or provide sufficient information to enable the resource 495 server to derive the shared secret from the access token. As an 496 alternative, the resource server MAY use token introspection to 497 retrieve the keying material for this access token directly from the 498 authorization server. 500 An example access token request for an access token with a symmetric 501 proof-of-possession key is illustrated in Figure 5. 503 POST coaps://as.example.com/token 504 Content-Format: application/ace+cbor 505 Payload: 506 { 507 audience : "smokeSensor1807", 508 } 510 Figure 5: Example Access Token Request, (implicit) symmetric PoP-key 512 A corresponding example access token response is illustrated in 513 Figure 6. In this example, the authorization server returns a 2.01 514 response containing a new access token (truncated to improve 515 readability) and information for the client, including the symmetric 516 key in the cnf claim. The information is transferred as a CBOR data 517 structure as specified in [I-D.ietf-ace-oauth-authz]. 519 2.01 Created 520 Content-Format: application/ace+cbor 521 Max-Age: 85800 522 Payload: 523 { 524 access_token : h'd08343a10... 525 (remainder of CWT omitted for brevity) 526 token_type : PoP, 527 expires_in : 86400, 528 profile : coap_dtls, 529 cnf : { 530 COSE_Key : { 531 kty : symmetric, 532 kid : h'3d027833fc6267ce', 533 k : h'73657373696f6e6b6579' 534 } 535 } 536 } 538 Figure 6: Example Access Token Response, symmetric PoP-key 540 The access token also comprises a "cnf" claim. This claim usually 541 contains a "COSE_Key" object [RFC8152] that carries either the 542 symmetric key itself or a key identifier that can be used by the 543 resource server to determine the secret key it shares with the 544 client. If the access token carries a symmetric key, the access 545 token MUST be encrypted using a "COSE_Encrypt0" structure (see 546 section 7.1 of [RFC8392]). The authorization server MUST use the 547 keying material shared with the resource server to encrypt the token. 549 The "cnf" structure in the access token is provided in Figure 7. 551 cnf : { 552 COSE_Key : { 553 kty : symmetric, 554 kid : h'3d027833fc6267ce' 555 } 556 } 558 Figure 7: Access Token without Keying Material 560 A response that declines any operation on the requested resource is 561 constructed according to Section 5.2 of [RFC6749], (cf. 562 Section 5.8.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an 563 example for a request that has been rejected due to invalid request 564 parameters. 566 4.00 Bad Request 567 Content-Format: application/ace+cbor 568 Payload: 569 { 570 error : invalid_request 571 } 573 Figure 8: Example Access Token Response With Reject 575 The method for how the resource server determines the symmetric key 576 from an access token containing only a key identifier is application- 577 specific; the remainder of this section provides one example. 579 The authorization server and the resource server are assumed to share 580 a key derivation key used to derive the symmetric key shared with the 581 client from the key identifier in the access token. The key 582 derivation key may be derived from some other secret key shared 583 between the authorization server and the resource server. This key 584 needs to be securely stored and processed in the same way as the key 585 used to protect the communication between the authorization server 586 and the resource server. 588 Knowledge of the symmetric key shared with the client must not reveal 589 any information about the key derivation key or other secret keys 590 shared between the authorization server and resource server. 592 In order to generate a new symmetric key to be used by client and 593 resource server, the authorization server generates a new key 594 identifier which MUST be unique among all key identifiers used by the 595 authorization server for this resource server. The authorization 596 server then uses the key derivation key shared with the resource 597 server to derive the symmetric key as specified below. Instead of 598 providing the keying material in the access token, the authorization 599 server includes the key identifier in the "kid" parameter, see 600 Figure 7. This key identifier enables the resource server to 601 calculate the symmetric key used for the communication with the 602 client using the key derivation key and a KDF to be defined by the 603 application, for example HKDF-SHA-256. The key identifier picked by 604 the authorization server MUST be unique for each access token where a 605 unique symmetric key is required. 607 In this example, HKDF consists of the composition of the HKDF-Extract 608 and HKDF-Expand steps [RFC5869]. The symmetric key is derived from 609 the key identifier, the key derivation key and other data: 611 OKM = HKDF(salt, IKM, info, L), 613 where: 615 * OKM, the output keying material, is the derived symmetric key 617 * salt is the empty byte string 619 * IKM, the input keying material, is the key derivation key as 620 defined above 622 * info is the serialization of a CBOR array consisting of 623 ([RFC8610]): 625 info = [ 626 type : tstr, 627 L : uint, 628 access_token: bytes 629 ] 631 where: 633 * type is set to the constant text string "ACE-CoAP-DTLS-key- 634 derivation", 636 * L is the size of the symmetric key in bytes, 638 * access_token is the content of the "access_token" field as 639 transferred from the authorization server to the resource server. 641 All CBOR data types are encoded in CBOR using preferred serialization 642 and deterministic encoding as specified in Section 4 of [RFC8949]. 643 This implies in particular that the "type" and "L" components use the 644 minimum length encoding. The content of the "access_token" field is 645 treated as opaque data for the purpose of key derivation. 647 Use of a unique (per resource server) "kid" and the use of a key 648 derivation IKM that MUST be unique per authorization server/resource 649 server pair as specified above will ensure that the derived key is 650 not shared across multiple clients. However, to provide variation in 651 the derived key across different tokens used by the same client, it 652 is additionally RECOMMENDED to include the "iat" claim and either the 653 "exp" or "exi" claims in the access token. 655 3.3.2. DTLS Channel Setup Between Client and Resource Server 657 When a client receives an access token response from an authorization 658 server, the client MUST check if the access token response is bound 659 to a certain previously sent access token request, as the request may 660 specify the resource server with which the client wants to 661 communicate. 663 The client checks if the payload of the access token response 664 contains an "access_token" parameter and a "cnf" parameter. With 665 this information the client can initiate the establishment of a new 666 DTLS channel with a resource server. To use DTLS with pre-shared 667 keys, the client follows the PSK key exchange algorithm specified in 668 Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter 669 of the AS response as PSK when constructing the premaster secret. To 670 be consistent with the recommendations in [RFC7252], a client in the 671 PSK mode MUST support the cipher suite TLS_PSK_WITH_AES_128_CCM_8 672 [RFC6655]. 674 In PreSharedKey mode, the knowledge of the shared secret by the 675 client and the resource server is used for mutual authentication 676 between both peers. Therefore, the resource server must be able to 677 determine the shared secret from the access token. Following the 678 general ACE authorization framework, the client can upload the access 679 token to the resource server's authz-info resource before starting 680 the DTLS handshake. The client then needs to indicate during the 681 DTLS handshake which previously uploaded access token it intends to 682 use. To do so, it MUST create a "COSE_Key" structure with the "kid" 683 that was conveyed in the "rs_cnf" claim in the token response from 684 the authorization server and the key type "symmetric". This 685 structure then is included as the only element in the "cnf" structure 686 whose CBOR serialization is used as value for "psk_identity" as shown 687 in Figure 9. 689 { cnf : { 690 COSE_Key : { 691 kty: symmetric, 692 kid : h'3d027833fc6267ce' 693 } 694 } 695 } 697 Figure 9: Access token containing a single kid parameter 699 The actual CBOR serialization for the data structure from Figure 9 as 700 sequence of bytes in hexadecimal notation will be: 702 A1 08 A1 01 A2 01 04 02 48 3D 02 78 33 FC 62 67 CE 704 As an alternative to the access token upload, the client can provide 705 the most recent access token in the "psk_identity" field of the 706 ClientKeyExchange message. To do so, the client MUST treat the 707 contents of the "access_token" field from the AS-to-Client response 708 as opaque data as specified in Section 4.2 of [RFC7925] and not 709 perform any re-coding. This allows the resource server to retrieve 710 the shared secret directly from the "cnf" claim of the access token. 712 If a resource server receives a ClientKeyExchange message that 713 contains a "psk_identity" with a length greater than zero, it MUST 714 parse the contents of the "psk_identity" field as CBOR data structure 715 and process the contents as following: 717 * If the data contains a "cnf" field with a "COSE_Key" structure 718 with a "kid", the resource server continues the DTLS handshake 719 with the associated key that corresponds to this kid. 721 * If the data comprises additional CWT information, this information 722 must be stored as an access token for this DTLS association before 723 continuing with the DTLS handshake. 725 If the contents of the "psk_identity" do not yield sufficient 726 information to select a valid access token for the requesting client, 727 the resource server aborts the DTLS handshake with an 728 "illegal_parameter" alert. 730 When the resource server receives an access token, it MUST check if 731 the access token is still valid, if the resource server is the 732 intended destination (i.e., the audience of the token), and if the 733 token was issued by an authorized authorization server. This 734 specification implements access tokens as proof-of-possession tokens. 735 Therefore, the access token is bound to a symmetric PoP key that is 736 used as shared secret between the client and the resource server. A 737 resource server MUST have the capacity to store one access token for 738 every proof-of-possession key of every authorized client. The 739 resource server may use token introspection [RFC7662] on the access 740 token to retrieve more information about the specific token. The use 741 of introspection is out of scope for this specification. 743 While the client can retrieve the shared secret from the contents of 744 the "cnf" parameter in the AS-to-Client response, the resource server 745 uses the information contained in the "cnf" claim of the access token 746 to determine the actual secret when no explicit "kid" was provided in 747 the "psk_identity" field. If key derivation is used, the "cnf" claim 748 MUST contain a "kid" parameter to be used by the server as the IKM 749 for key derivation as described above. 751 3.4. Resource Access 753 Once a DTLS channel has been established as described in Section 3.2 754 or Section 3.3, respectively, the client is authorized to access 755 resources covered by the access token it has uploaded to the authz- 756 info resource hosted by the resource server. 758 With the successful establishment of the DTLS channel, the client and 759 the resource server have proven that they can use their respective 760 keying material. An access token that is bound to the client's 761 keying material is associated with the channel. According to 762 Section 5.10.1 of [I-D.ietf-ace-oauth-authz], there should be only 763 one access token for each client. New access tokens issued by the 764 authorization server SHOULD replace previously issued access tokens 765 for the respective client. The resource server therefore needs a 766 common understanding with the authorization server how access tokens 767 are ordered. The authorization server may, e.g., specify a "cti" 768 claim for the access token (see Section 5.9.4 of 769 [I-D.ietf-ace-oauth-authz]) to employ a strict order. 771 Any request that the resource server receives on a DTLS channel that 772 is tied to an access token via its keying material MUST be checked 773 against the authorization rules that can be determined with the 774 access token. The resource server MUST check for every request if 775 the access token is still valid. If the token has expired, the 776 resource server MUST remove it. Incoming CoAP requests that are not 777 authorized with respect to any access token that is associated with 778 the client MUST be rejected by the resource server with 4.01 779 response. The response SHOULD include AS Request Creation Hints as 780 described in Section 5.2 of [I-D.ietf-ace-oauth-authz]. 782 The resource server MUST NOT accept an incoming CoAP request as 783 authorized if any of the following fails: 785 1. The message was received on a secure channel that has been 786 established using the procedure defined in this document. 788 2. The authorization information tied to the sending client is 789 valid. 791 3. The request is destined for the resource server. 793 4. The resource URI specified in the request is covered by the 794 authorization information. 796 5. The request method is an authorized action on the resource with 797 respect to the authorization information. 799 Incoming CoAP requests received on a secure DTLS channel that are not 800 thus authorized MUST be rejected according to Section 5.10.1.1 of 801 [I-D.ietf-ace-oauth-authz] 803 1. with response code 4.03 (Forbidden) when the resource URI 804 specified in the request is not covered by the authorization 805 information, and 807 2. with response code 4.05 (Method Not Allowed) when the resource 808 URI specified in the request covered by the authorization 809 information but not the requested action. 811 The client MUST ascertain that its keying material is still valid 812 before sending a request or processing a response. If the client 813 recently has updated the access token (see Section 4), it must be 814 prepared that its request is still handled according to the previous 815 authorization rules as there is no strict ordering between access 816 token uploads and resource access messages. See also Section 7.2 for 817 a discussion of access token processing. 819 If the client gets an error response containing AS Request Creation 820 Hints (cf. Section 5.3 of [I-D.ietf-ace-oauth-authz] as response to 821 its requests, it SHOULD request a new access token from the 822 authorization server in order to continue communication with the 823 resource server. 825 Unauthorized requests that have been received over a DTLS session 826 SHOULD be treated as non-fatal by the resource server, i.e., the DTLS 827 session SHOULD be kept alive until the associated access token has 828 expired. 830 4. Dynamic Update of Authorization Information 832 Resource servers must only use a new access token to update the 833 authorization information for a DTLS session if the keying material 834 that is bound to the token is the same that was used in the DTLS 835 handshake. By associating the access tokens with the identifier of 836 an existing DTLS session, the authorization information can be 837 updated without changing the cryptographic keys for the DTLS 838 communication between the client and the resource server, i.e. an 839 existing session can be used with updated permissions. 841 The client can therefore update the authorization information stored 842 at the resource server at any time without changing an established 843 DTLS session. To do so, the client requests a new access token from 844 the authorization server for the intended action on the respective 845 resource and uploads this access token to the authz-info resource on 846 the resource server. 848 Figure 10 depicts the message flow where the client requests a new 849 access token after a security association between the client and the 850 resource server has been established using this protocol. If the 851 client wants to update the authorization information, the token 852 request MUST specify the key identifier of the proof-of-possession 853 key used for the existing DTLS channel between the client and the 854 resource server in the "kid" parameter of the Client-to-AS request. 855 The authorization server MUST verify that the specified "kid" denotes 856 a valid verifier for a proof-of-possession token that has previously 857 been issued to the requesting client. Otherwise, the Client-to-AS 858 request MUST be declined with the error code "unsupported_pop_key" as 859 defined in Section 5.8.3 of [I-D.ietf-ace-oauth-authz]. 861 When the authorization server issues a new access token to update 862 existing authorization information, it MUST include the specified 863 "kid" parameter in this access token. A resource server MUST replace 864 the authorization information of any existing DTLS session that is 865 identified by this key identifier with the updated authorization 866 information. 868 C RS AS 869 | <===== DTLS channel =====> | | 870 | + Access Token | | 871 | | | 872 | --- Token Request ----------------------------> | 873 | | | 874 | <---------------------------- New Access Token - | 875 | + Access Information | 876 | | | 877 | --- Update /authz-info --> | | 878 | New Access Token | | 879 | | | 880 | == Authorized Request ===> | | 881 | | | 882 | <=== Protected Resource == | | 884 Figure 10: Overview of Dynamic Update Operation 886 5. Token Expiration 888 The resource server MUST delete access tokens that are no longer 889 valid. DTLS associations that have been setup in accordance with 890 this profile are always tied to specific tokens (which may be 891 exchanged with a dynamic update as described in Section 4). As 892 tokens may become invalid at any time (e.g., because they have 893 expired), the association may become useless at some point. A 894 resource server therefore MUST terminate existing DTLS association 895 after the last access token associated with this association has 896 expired. 898 As specified in Section 5.10.3 of [I-D.ietf-ace-oauth-authz], the 899 resource server MUST notify the client with an error response with 900 code 4.01 (Unauthorized) for any long running request before 901 terminating the association. 903 6. Secure Communication with an Authorization Server 905 As specified in the ACE framework (Sections 5.8 and 5.9 of 906 [I-D.ietf-ace-oauth-authz]), the requesting entity (the resource 907 server and/or the client) and the authorization server communicate 908 via the token endpoint or introspection endpoint. The use of CoAP 909 and DTLS for this communication is RECOMMENDED in this profile. 910 Other protocols fulfilling the security requirements defined in 911 Section 5 of [I-D.ietf-ace-oauth-authz] MAY be used instead. 913 How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the 914 authorization server are established is out of scope for this 915 profile. 917 If other means of securing the communication with the authorization 918 server are used, the communication security requirements from 919 Section 6.2 of [I-D.ietf-ace-oauth-authz] remain applicable. 921 7. Security Considerations 923 This document specifies a profile for the Authentication and 924 Authorization for Constrained Environments (ACE) framework 925 [I-D.ietf-ace-oauth-authz]. As it follows this framework's general 926 approach, the general security considerations from Section 6 of 927 [I-D.ietf-ace-oauth-authz] also apply to this profile. 929 The authorization server must ascertain that the keying material for 930 the client that it provides to the resource server actually is 931 associated with this client. Malicious clients may hand over access 932 tokens containing their own access permissions to other entities. 933 This problem cannot be completely eliminated. Nevertheless, in RPK 934 mode it should not be possible for clients to request access tokens 935 for arbitrary public keys: if the client can cause the authorization 936 server to issue a token for a public key without proving possession 937 of the corresponding private key, this allows for identity misbinding 938 attacks where the issued token is usable by an entity other than the 939 intended one. The authorization server therefore at some point needs 940 to validate that the client can actually use the private key 941 corresponding to the client's public key. 943 When using pre-shared keys provisioned by the authorization server, 944 the security level depends on the randomness of PSK, and the security 945 of the TLS cipher suite and key exchange algorithm. As this 946 specification targets at constrained environments, message payloads 947 exchanged between the client and the resource server are expected to 948 be small and rare. CoAP [RFC7252] mandates the implementation of 949 cipher suites with abbreviated, 8-byte tags for message integrity 950 protection. For consistency, this profile requires implementation of 951 the same cipher suites. For application scenarios where the cost of 952 full-width authentication tags is low compared to the overall amount 953 of data being transmitted, the use of cipher suites with 16-byte 954 integrity protection tags is preferred. 956 The PSK mode of this profile offers a distribution mechanism to 957 convey authorization tokens together with a shared secret to a client 958 and a server. As this specification aims at constrained devices and 959 uses CoAP [RFC7252] as transfer protocol, at least the cipher suite 960 TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access 961 tokens and the corresponding shared secrets generated by the 962 authorization server are expected to be sufficiently short-lived to 963 provide similar forward-secrecy properties to using ephemeral Diffie- 964 Hellman (DHE) key exchange mechanisms. For longer-lived access 965 tokens, DHE cipher suites should be used, i.e., cipher suites of the 966 form TLS_DHE_PSK_*. 968 Constrained devices that use DTLS [RFC6347] are inherently vulnerable 969 to Denial of Service (DoS) attacks as the handshake protocol requires 970 creation of internal state within the device. This is specifically 971 of concern where an adversary is able to intercept the initial cookie 972 exchange and interject forged messages with a valid cookie to 973 continue with the handshake. A similar issue exists with the 974 unprotected authorization information endpoint when the resource 975 server needs to keep valid access tokens for a long time. 976 Adversaries could fill up the constrained resource server's internal 977 storage for a very long time with interjected or otherwise retrieved 978 valid access tokens. To mitigate against this, the resource server 979 should set a time boundary until an access token that has not been 980 used until then will be deleted. 982 The protection of access tokens that are stored in the authorization 983 information endpoint depends on the keying material that is used 984 between the authorization server and the resource server: The 985 resource server must ensure that it processes only access tokens that 986 are (encrypted and) integrity-protected by an authorization server 987 that is authorized to provide access tokens for the resource server. 989 7.1. Reuse of Existing Sessions 991 To avoid the overhead of a repeated DTLS handshake, [RFC7925] 992 recommends session resumption [RFC8446] to reuse session state from 993 an earlier DTLS association and thus requires client side 994 implementation. In this specification, the DTLS session is subject 995 to the authorization rules denoted by the access token that was used 996 for the initial setup of the DTLS association. Enabling session 997 resumption would require the server to transfer the authorization 998 information with the session state in an encrypted SessionTicket to 999 the client. Assuming that the server uses long-lived keying 1000 material, this could open up attacks due to the lack of forward 1001 secrecy. Moreover, using this mechanism, a client can resume a DTLS 1002 session without proving the possession of the PoP key again. 1003 Therefore, session resumption should be used only in combination with 1004 reasonably short-lived PoP keys. 1006 Since renegotiation of DTLS associations is prone to attacks as well, 1007 [RFC7925] requires clients to decline any renegotiation attempt. A 1008 server that wants to initiate re-keying therefore SHOULD periodically 1009 force a full handshake. 1011 7.2. Multiple Access Tokens 1013 Developers SHOULD avoid using multiple access tokens for a client 1014 (see also section 5.10.1 of [I-D.ietf-ace-oauth-authz]). 1016 Even when a single access token per client is used, an attacker could 1017 compromise the dynamic update mechanism for existing DTLS connections 1018 by delaying or reordering packets destined for the authz-info 1019 endpoint. Thus, the order in which operations occur at the resource 1020 server (and thus which authorization info is used to process a given 1021 client request) cannot be guaranteed. Especially in the presence of 1022 later-issued access tokens that reduce the client's permissions from 1023 the initial access token, it is impossible to guarantee that the 1024 reduction in authorization will take effect prior to the expiration 1025 of the original token. 1027 7.3. Out-of-Band Configuration 1029 To communicate securely, the authorization server, the client and the 1030 resource server require certain information that must be exchanged 1031 outside the protocol flow described in this document. The 1032 authorization server must have obtained authorization information 1033 concerning the client and the resource server that is approved by the 1034 resource owner as well as corresponding keying material. The 1035 resource server must have received authorization information approved 1036 by the resource owner concerning its authorization managers and the 1037 respective keying material. The client must have obtained 1038 authorization information concerning the authorization server 1039 approved by its owner as well as the corresponding keying material. 1040 Also, the client's owner must have approved of the client's 1041 communication with the resource server. The client and the 1042 authorization server must have obtained a common understanding how 1043 this resource server is identified to ensure that the client obtains 1044 access token and keying material for the correct resource server. If 1045 the client is provided with a raw public key for the resource server, 1046 it must be ascertained to which resource server (which identifier and 1047 authorization information) the key is associated. All authorization 1048 information and keying material must be kept up to date. 1050 8. Privacy Considerations 1052 This privacy considerations from Section 7 of the 1053 [I-D.ietf-ace-oauth-authz] apply also to this profile. 1055 An unprotected response to an unauthorized request may disclose 1056 information about the resource server and/or its existing 1057 relationship with the client. It is advisable to include as little 1058 information as possible in an unencrypted response. When a DTLS 1059 session between an authenticated client and the resource server 1060 already exists, more detailed information MAY be included with an 1061 error response to provide the client with sufficient information to 1062 react on that particular error. 1064 Also, unprotected requests to the resource server may reveal 1065 information about the client, e.g., which resources the client 1066 attempts to request or the data that the client wants to provide to 1067 the resource server. The client SHOULD NOT send confidential data in 1068 an unprotected request. 1070 Note that some information might still leak after DTLS session is 1071 established, due to observable message sizes, the source, and the 1072 destination addresses. 1074 9. IANA Considerations 1076 The following registrations are done for the ACE OAuth Profile 1077 Registry following the procedure specified in 1078 [I-D.ietf-ace-oauth-authz]. 1080 Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" 1081 with the RFC number of this specification and delete this paragraph. 1083 Profile name: coap_dtls 1084 Profile Description: Profile for delegating client authentication and 1085 authorization in a constrained environment by establishing a Datagram 1086 Transport Layer Security (DTLS) channel between resource-constrained 1087 nodes. 1089 Profile ID: TBD (suggested: 1) 1091 Change Controller: IESG 1093 Reference: [RFC-XXXX] 1095 10. Acknowledgments 1097 Special thanks to Jim Schaad for his contributions and reviews of 1098 this document and to Ben Kaduk for his thorough reviews of this 1099 document. Thanks also to Paul Kyzivat for his review. The authors 1100 also would like to thank Marco Tiloca for his contributions. 1102 Ludwig Seitz worked on this document as part of the CelticNext 1103 projects CyberWI, and CRITISEC with funding from Vinnova. 1105 11. References 1107 11.1. Normative References 1109 [I-D.ietf-ace-oauth-authz] 1110 Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and 1111 H. Tschofenig, "Authentication and Authorization for 1112 Constrained Environments (ACE) using the OAuth 2.0 1113 Framework (ACE-OAuth)", Work in Progress, Internet-Draft, 1114 draft-ietf-ace-oauth-authz-41, 6 May 2021, 1115 . 1118 [I-D.ietf-ace-oauth-params] 1119 Seitz, L., "Additional OAuth Parameters for Authorization 1120 in Constrained Environments (ACE)", Work in Progress, 1121 Internet-Draft, draft-ietf-ace-oauth-params-15, 6 May 1122 2021, . 1125 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1126 Requirement Levels", BCP 14, RFC 2119, 1127 DOI 10.17487/RFC2119, March 1997, 1128 . 1130 [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key 1131 Ciphersuites for Transport Layer Security (TLS)", 1132 RFC 4279, DOI 10.17487/RFC4279, December 2005, 1133 . 1135 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1136 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 1137 January 2012, . 1139 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 1140 RFC 6749, DOI 10.17487/RFC6749, October 2012, 1141 . 1143 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 1144 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 1145 Transport Layer Security (TLS) and Datagram Transport 1146 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 1147 June 2014, . 1149 [RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES- 1150 CCM Elliptic Curve Cryptography (ECC) Cipher Suites for 1151 TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014, 1152 . 1154 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1155 Application Protocol (CoAP)", RFC 7252, 1156 DOI 10.17487/RFC7252, June 2014, 1157 . 1159 [RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer 1160 Security (TLS) / Datagram Transport Layer Security (DTLS) 1161 Profiles for the Internet of Things", RFC 7925, 1162 DOI 10.17487/RFC7925, July 2016, 1163 . 1165 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 1166 RFC 8152, DOI 10.17487/RFC8152, July 2017, 1167 . 1169 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1170 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1171 May 2017, . 1173 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 1174 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 1175 May 2018, . 1177 [RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic 1178 Curve Cryptography (ECC) Cipher Suites for Transport Layer 1179 Security (TLS) Versions 1.2 and Earlier", RFC 8422, 1180 DOI 10.17487/RFC8422, August 2018, 1181 . 1183 [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 1184 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 1185 Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 1186 2020, . 1188 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object 1189 Representation (CBOR)", STD 94, RFC 8949, 1190 DOI 10.17487/RFC8949, December 2020, 1191 . 1193 11.2. Informative References 1195 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 1196 Key Derivation Function (HKDF)", RFC 5869, 1197 DOI 10.17487/RFC5869, May 2010, 1198 . 1200 [RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for 1201 Transport Layer Security (TLS)", RFC 6655, 1202 DOI 10.17487/RFC6655, July 2012, 1203 . 1205 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 1206 RFC 7662, DOI 10.17487/RFC7662, October 2015, 1207 . 1209 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 1210 for Security", RFC 7748, DOI 10.17487/RFC7748, January 1211 2016, . 1213 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 1214 Signature Algorithm (EdDSA)", RFC 8032, 1215 DOI 10.17487/RFC8032, January 2017, 1216 . 1218 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1219 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1220 . 1222 [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data 1223 Definition Language (CDDL): A Notational Convention to 1224 Express Concise Binary Object Representation (CBOR) and 1225 JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, 1226 June 2019, . 1228 Authors' Addresses 1230 Stefanie Gerdes 1231 Universität Bremen TZI 1232 Postfach 330440 1233 D-28359 Bremen 1234 Germany 1236 Phone: +49-421-218-63906 1237 Email: gerdes@tzi.org 1239 Olaf Bergmann 1240 Universität Bremen TZI 1241 Postfach 330440 1242 D-28359 Bremen 1243 Germany 1245 Phone: +49-421-218-63904 1246 Email: bergmann@tzi.org 1248 Carsten Bormann 1249 Universität Bremen TZI 1250 Postfach 330440 1251 D-28359 Bremen 1252 Germany 1254 Phone: +49-421-218-63921 1255 Email: cabo@tzi.org 1257 Göran Selander 1258 Ericsson AB 1260 Email: goran.selander@ericsson.com 1261 Ludwig Seitz 1262 Combitech 1263 Djäknegatan 31 1264 SE-211 35 Malmö 1265 Sweden 1267 Email: ludwig.seitz@combitech.com