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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: November 14, 2020 Universitaet Bremen TZI 6 G. Selander 7 Ericsson AB 8 L. Seitz 9 Combitech 10 May 13, 2020 12 Datagram Transport Layer Security (DTLS) Profile for Authentication and 13 Authorization for Constrained Environments (ACE) 14 draft-ietf-ace-dtls-authorize-10 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 November 14, 2020. 44 Copyright Notice 46 Copyright (c) 2020 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 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5 65 3.1. Communication Between the Client and the Authorization 66 Server . . . . . . . . . . . . . . . . . . . . . . . . . 6 67 3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6 68 3.2.1. DTLS Channel Setup Between Client and Resource Server 9 69 3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 10 70 3.3.1. DTLS Channel Setup Between Client and Resource Server 14 71 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 15 72 4. Dynamic Update of Authorization Information . . . . . . . . . 17 73 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 18 74 6. Secure Communication with an Authorization Server . . . . . . 18 75 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 76 7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 20 77 7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 21 78 7.3. Out-of-Band Configuration . . . . . . . . . . . . . . . . 21 79 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 80 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 81 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 82 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 83 11.1. Normative References . . . . . . . . . . . . . . . . . . 23 84 11.2. Informative References . . . . . . . . . . . . . . . . . 24 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 87 1. Introduction 89 This specification defines a profile of the ACE framework 90 [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource 91 server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to 92 communicate. The client obtains an access token, bound to a key (the 93 proof-of-possession key), from an authorization server to prove its 94 authorization to access protected resources hosted by the resource 95 server. Also, the client and the resource server are provided by the 96 authorization server with the necessary keying material to establish 97 a DTLS session. The communication between client and authorization 98 server may also be secured with DTLS. This specification supports 99 DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys 100 (PSK) [RFC4279]. 102 The ACE framework requires that client and server mutually 103 authenticate each other before any application data is exchanged. 104 DTLS enables mutual authentication if both client and server prove 105 their ability to use certain keying material in the DTLS handshake. 106 The authorization server assists in this process on the server side 107 by incorporating keying material (or information about keying 108 material) into the access token, which is considered a "proof of 109 possession" token. 111 In the RPK mode, the client proves that it can use the RPK bound to 112 the token and the server shows that it can use a certain RPK. 114 The resource server needs access to the token in order to complete 115 this exchange. For the RPK mode, the client must upload the access 116 token to the resource server before initiating the handshake, as 117 described in Section 5.8.1 of the ACE framework 118 [I-D.ietf-ace-oauth-authz]. 120 In the PSK mode, client and server show with the DTLS handshake that 121 they can use the keying material that is bound to the access token. 122 To transfer the access token from the client to the resource server, 123 the "psk_identity" parameter in the DTLS PSK handshake may be used 124 instead of uploading the token prior to the handshake. 126 As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this 127 specification uses CBOR web tokens to convey claims within an access 128 token issued by the server. While other formats could be used as 129 well, those are out of scope for this document. 131 1.1. Terminology 133 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 134 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 135 "OPTIONAL" in this document are to be interpreted as described in BCP 136 14 [RFC2119] [RFC8174] when, and only when, they appear in all 137 capitals, as shown here. 139 Readers are expected to be familiar with the terms and concepts 140 described in [I-D.ietf-ace-oauth-authz] and in 141 [I-D.ietf-ace-oauth-params]. 143 The authorization information (authz-info) resource refers to the 144 authorization information endpoint as specified in 146 [I-D.ietf-ace-oauth-authz]. The term "claim" is used in this 147 document with the same semantics as in [I-D.ietf-ace-oauth-authz], 148 i.e., it denotes information carried in the access token or returned 149 from introspection. 151 2. Protocol Overview 153 The CoAP-DTLS profile for ACE specifies the transfer of 154 authentication information and, if necessary, authorization 155 information between the client (C) and the resource server (RS) 156 during setup of a DTLS session for CoAP messaging. It also specifies 157 how the client can use CoAP over DTLS to retrieve an access token 158 from the authorization server (AS) for a protected resource hosted on 159 the resource server. As specified in Section 6.7 of 160 [I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these 161 interactions is completely independent 163 This profile requires the client to retrieve an access token for 164 protected resource(s) it wants to access on the resource server as 165 specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical 166 message flow in this scenario (messages in square brackets are 167 optional): 169 C RS AS 170 | [---- Resource Request ------>]| | 171 | | | 172 | [<-AS Request Creation Hints-] | | 173 | | | 174 | ------- Token Request ----------------------------> | 175 | | | 176 | <---------------------------- Access Token --------- | 177 | + Access Information | 179 Figure 1: Retrieving an Access Token 181 To determine the authorization server in charge of a resource hosted 182 at the resource server, the client can send an initial Unauthorized 183 Resource Request message to the resource server. The resource server 184 then denies the request and sends an AS Request Creation Hints 185 message containing the address of its authorization server back to 186 the client as specified in Section 5.1.2 of 187 [I-D.ietf-ace-oauth-authz]. 189 Once the client knows the authorization server's address, it can send 190 an access token request to the token endpoint at the authorization 191 server as specified in [I-D.ietf-ace-oauth-authz]. As the access 192 token request as well as the response may contain confidential data, 193 the communication between the client and the authorization server 194 must be confidentiality-protected and ensure authenticity. The 195 client may have been registered at the authorization server via the 196 OAuth 2.0 client registration mechanism as outlined in Section 5.3 of 197 [I-D.ietf-ace-oauth-authz]. 199 The access token returned by the authorization server can then be 200 used by the client to establish a new DTLS session with the resource 201 server. When the client intends to use an asymmetric proof-of- 202 possession key in the DTLS handshake with the resource server, the 203 client MUST upload the access token to the authz-info resource, i.e. 204 the authz-info endpoint, on the resource server before starting the 205 DTLS handshake, as described in Section 5.8.1 of 206 [I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric 207 proof-of-possession key in the DTLS handshake, the procedure as above 208 MAY be used, or alternatively, the access token MAY instead be 209 transferred in the DTLS ClientKeyExchange message (see 210 Section 3.3.1). In any case, DTLS MUST be used in a mode that 211 provides replay protection. 213 Figure 2 depicts the common protocol flow for the DTLS profile after 214 the client has retrieved the access token from the authorization 215 server, AS. 217 C RS AS 218 | [--- Access Token ------>] | | 219 | | | 220 | <== DTLS channel setup ==> | | 221 | | | 222 | == Authorized Request ===> | | 223 | | | 224 | <=== Protected Resource == | | 226 Figure 2: Protocol overview 228 3. Protocol Flow 230 The following sections specify how CoAP is used to interchange 231 access-related data between the resource server, the client and the 232 authorization server so that the authorization server can provide the 233 client and the resource server with sufficient information to 234 establish a secure channel, and convey authorization information 235 specific for this communication relationship to the resource server. 237 Section 3.1 describes how the communication between the client (C) 238 and the authorization server (AS) must be secured. Depending on the 239 used CoAP security mode (see also Section 9 of [RFC7252], the Client- 240 to-AS request, AS-to-Client response (see Section 5.6 of 241 [I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry 242 slightly different information. Section 3.2 addresses the use of raw 243 public keys while Section 3.3 defines how pre-shared keys are used in 244 this profile. 246 3.1. Communication Between the Client and the Authorization Server 248 To retrieve an access token for the resource that the client wants to 249 access, the client requests an access token from the authorization 250 server. Before the client can request the access token, the client 251 and the authorization server MUST establish a secure communication 252 channel. This profile assumes that the keying material to secure 253 this communication channel has securely been obtained either by 254 manual configuration or in an automated provisioning process. The 255 following requirements in alignment with Section 6.5 of 256 [I-D.ietf-ace-oauth-authz] therefore must be met: 258 o The client MUST securely have obtained keying material to 259 communicate with the authorization server. 261 o Furthermore, the client MUST verify that the authorization server 262 is authorized to provide access tokens (including authorization 263 information) about the resource server to the client, and that 264 this authorization information about the authorization server is 265 still valid. 267 o Also, the authorization server MUST securely have obtained keying 268 material for the client, and obtained authorization rules approved 269 by the resource owner (RO) concerning the client and the resource 270 server that relate to this keying material. 272 The client and the authorization server MUST use their respective 273 keying material for all exchanged messages. How the security 274 association between the client and the authorization server is 275 bootstrapped is not part of this document. The client and the 276 authorization server must ensure the confidentiality, integrity and 277 authenticity of all exchanged messages within the ACE protocol. 279 Section Section 6 specifies how communication with the authorization 280 server is secured. 282 3.2. RawPublicKey Mode 284 When the client and the resource server use RawPublicKey 285 authentication, the procedure is as follows: After the client and the 286 authorization server mutually authenticated each other and validated 287 each other's authorization, the client sends a token request to the 288 authorization server's token endpoint. The client MUST add a 289 "req_cnf" object carrying either its raw public key or a unique 290 identifier for a public key that it has previously made known to the 291 authorization server. It is RECOMMENDED that the client uses DTLS 292 with the same keying material to secure the communication with the 293 authorization server, proving possession of the key as part of the 294 token request. Other mechanisms for proving possession of the key 295 may be defined in the future. 297 An example access token request from the client to the authorization 298 server is depicted in Figure 3. 300 POST coaps://as.example.com/token 301 Content-Format: application/ace+cbor 302 Payload: 303 { 304 grant_type : client_credentials, 305 req_aud : "tempSensor4711", 306 req_cnf : { 307 COSE_Key : { 308 kty : EC2, 309 crv : P-256, 310 x : h'e866c35f4c3c81bb96a1...', 311 y : h'2e25556be097c8778a20...' 312 } 313 } 314 } 316 Figure 3: Access Token Request Example for RPK Mode 318 The example shows an access token request for the resource identified 319 by the string "tempSensor4711" on the authorization server using a 320 raw public key. 322 The authorization server MUST check if the client that it 323 communicates with is associated with the RPK in the "req_cnf" 324 parameter before issuing an access token to it. If the authorization 325 server determines that the request is to be authorized according to 326 the respective authorization rules, it generates an access token 327 response for the client. The access token MUST be bound to the RPK 328 of the client by means of the "cnf" claim. 330 The response MAY contain a "profile" parameter with the value 331 "coap_dtls" to indicate that this profile MUST be used for 332 communication between the client and the resource server. The 333 "profile" may be specified out-of-band, in which case it does not 334 have to be sent. The response also contains an access token with 335 information for the resource server about the client's public key. 337 The authorization server MUST return in its response the parameter 338 "rs_cnf" unless it is certain that the client already knows the 339 public key of the resource server. The authorization server MUST 340 ascertain that the RPK specified in "rs_cnf" belongs to the resource 341 server that the client wants to communicate with. The authorization 342 server MUST protect the integrity of the access token such that the 343 resource server can detect unauthorized changes. If the access token 344 contains confidential data, the authorization server MUST also 345 protect the confidentiality of the access token. 347 The client MUST ascertain that the access token response belongs to a 348 certain previously sent access token request, as the request may 349 specify the resource server with which the client wants to 350 communicate. 352 An example access token response from the authorization to the client 353 is depicted in Figure 4. Here, the contents of the "access_token" 354 claim have been truncated to improve readability. Caching proxies 355 process the Max-Age option in the CoAP response which has a default 356 value of 60 seconds (Section 5.6.1 of [RFC7252]). The authorization 357 server SHOULD adjust the Max-Age option such that it does not exceed 358 the "expires_in" parameter to avoid stale responses. 360 2.01 Created 361 Content-Format: application/ace+cbor 362 Max-Age: 3560 363 Payload: 364 { 365 access_token : b64'SlAV32hkKG... 366 (remainder of CWT omitted for brevity; 367 CWT contains the client's RPK in the cnf claim)', 368 expires_in : 3600, 369 rs_cnf : { 370 COSE_Key : { 371 kty : EC2, 372 crv : P-256, 373 x : h'd7cc072de2205bdc1537...', 374 y : h'f95e1d4b851a2cc80fff...' 375 } 376 } 377 } 379 Figure 4: Access Token Response Example for RPK Mode 381 3.2.1. DTLS Channel Setup Between Client and Resource Server 383 Before the client initiates the DTLS handshake with the resource 384 server, the client MUST send a "POST" request containing the obtained 385 access token to the authz-info resource hosted by the resource 386 server. After the client receives a confirmation that the resource 387 server has accepted the access token, it SHOULD proceed to establish 388 a new DTLS channel with the resource server. The client MUST use its 389 correct public key in the DTLS handshake. If the authorization 390 server has specified a "cnf" field in the access token response, the 391 client MUST use this key. Otherwise, the client MUST use the public 392 key that it specified in the "req_cnf" of the access token request. 393 The client MUST specify this public key in the SubjectPublicKeyInfo 394 structure of the DTLS handshake as described in [RFC7250]. 396 To be consistent with [RFC7252] which allows for shortened MAC tags 397 in constrained environments, an implementation that supports the RPK 398 mode of this profile MUST at least support the ciphersuite 399 TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in 400 [RFC7748], new ECC curves have been defined recently that are 401 considered superior to the so-called NIST curves. This specification 402 therefore mandates implementation support for curve25519 (cf. 403 [RFC8032], [RFC8422]) as this curve said to be efficient and less 404 dangerous regarding implementation errors than the secp256r1 curve 405 mandated in [RFC7252]. 407 The resource server MUST check if the access token is still valid, if 408 the resource server is the intended destination (i.e., the audience) 409 of the token, and if the token was issued by an authorized 410 authorization server. The access token is constructed by the 411 authorization server such that the resource server can associate the 412 access token with the Client's public key. The "cnf" claim MUST 413 contain either the client's RPK or, if the key is already known by 414 the resource server (e.g., from previous communication), a reference 415 to this key. If the authorization server has no certain knowledge 416 that the Client's key is already known to the resource server, the 417 Client's public key MUST be included in the access token's "cnf" 418 parameter. If CBOR web tokens [RFC8392] are used as recommended in 419 [I-D.ietf-ace-oauth-authz], keys MUST be encoded as specified in 420 [RFC8747]. The resource server MUST use its own raw public key in 421 the DTLS handshake with the client. If the resource server has 422 several raw public keys, it must already know which key it is 423 supposed to use with this client. How this is achieved is not part 424 of this profile. 426 The resource server MUST use the keying material that the 427 authorizations server has specified in the "cnf" parameter in the 428 access token for the DTLS handshake with the client. Thus, the 429 handshake only finishes if the client and the resource server are 430 able to use their respective keying material. 432 3.3. PreSharedKey Mode 434 To retrieve an access token for the resource that the client wants to 435 access, the client MAY include a "cnf" object carrying an identifier 436 for a symmetric key in its access token request to the authorization 437 server. This identifier can be used by the authorization server to 438 determine the shared secret to construct the proof-of-possession 439 token. The authorization server MUST check if the identifier refers 440 to a symmetric key that was previously generated by the authorization 441 server as a shared secret for the communication between this client 442 and the resource server. If no such symmetric key was found, the 443 authorization server MUST generate a new symmetric key that is 444 returned in its response to the client. 446 The authorization server MUST determine the authorization rules for 447 the client it communicates with as defined by the resource owner and 448 generate the access token accordingly. If the authorization server 449 authorizes the client, it returns an AS-to-Client response. If the 450 profile parameter is present, it is set to "coap_dtls". The 451 authorization server MUST ascertain that the access token is 452 generated for the resource server that the client wants to 453 communicate with. Also, the authorization server MUST protect the 454 integrity of the access token to ensure that the resource server can 455 detect unauthorized changes. If the token contains confidential data 456 such as the symmetric key, the confidentiality of the token MUST also 457 be protected. Depending on the requested token type and algorithm in 458 the access token request, the authorization server adds access 459 Information to the response that provides the client with sufficient 460 information to setup a DTLS channel with the resource server. The 461 authorization server adds a "cnf" parameter to the access information 462 carrying a "COSE_Key" object that informs the client about the shared 463 secret that is to be used between the client and the resource server. 464 To convey the same secret to the resource server, the authorization 465 server either includes it directly in the access token by means of 466 the "cnf" claim or it provides sufficient information to enable the 467 resource server to derive the key from the access token using key 468 derivation. 470 An example access token request for an access token with a symmetric 471 proof-of-possession key is illustrated in Figure 5. 473 POST coaps://as.example.com/token 474 Content-Format: application/ace+cbor 475 Payload: 476 { 477 audience : "smokeSensor1807", 478 } 480 Figure 5: Example Access Token Request, (implicit) symmetric PoP-key 482 A corresponding example access token response is illustrated in 483 Figure 6. In this example, the authorization server returns a 2.01 484 response containing a new access token (truncated to improve 485 readability) and information for the client, including the symmetric 486 key in the cnf claim. The information is transferred as a CBOR data 487 structure as specified in [I-D.ietf-ace-oauth-authz]. 489 2.01 Created 490 Content-Format: application/ace+cbor 491 Max-Age: 85800 492 Payload: 493 { 494 access_token : h'd08343a10... 495 (remainder of CWT omitted for brevity) 496 token_type : PoP, 497 expires_in : 86400, 498 profile : coap_dtls, 499 cnf : { 500 COSE_Key : { 501 kty : symmetric, 502 kid : h'3d027833fc6267ce', 503 k : h'73657373696f6e6b6579' 504 } 505 } 506 } 508 Figure 6: Example Access Token Response, symmetric PoP-key 510 The access token also comprises a "cnf" claim. This claim usually 511 contains a "COSE_Key" object that carries either the symmetric key 512 itself or a key identifier that can be used by the resource server to 513 determine the secret key it shares with the client. If the access 514 token carries a symmetric key, the access token MUST be encrypted 515 using a "COSE_Encrypt0" structure. The authorization server MUST use 516 the keying material shared with the resource server to encrypt the 517 token. 519 The "cnf" structure in the access token is provided in Figure 7. 521 cnf : { 522 COSE_Key : { 523 kty : symmetric, 524 kid : h'3d027833fc6267ce' 525 } 526 } 528 Figure 7: Access Token without Keying Material 530 A response that declines any operation on the requested resource is 531 constructed according to Section 5.2 of [RFC6749], (cf. 532 Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an 533 example for a request that has been rejected due to invalid request 534 parameters. 536 4.00 Bad Request 537 Content-Format: application/ace+cbor 538 Payload: 539 { 540 error : invalid_request 541 } 543 Figure 8: Example Access Token Response With Reject 545 The method for how the resource server determines the symmetric key 546 from an access token containing only a key identifier is application- 547 specific; the remainder of this section provides one example. 549 The authorization server and the resource server are assumed to share 550 a key derivation key used to derive the symmetric key shared with the 551 client from the key identifier in the access token. The key 552 derivation key may be derived from some other secret key shared 553 between the authorization server and the resource server. This key 554 needs to be securely stored and processed in the same way as the key 555 used to protect the communication between the authorization server 556 and the resource server. 558 Knowledge of the symmetric key shared with the client must not reveal 559 any information about the key derivation key or other secret keys 560 shared between the authorization server and resource server. 562 In order to generate a new symmetric key to be used by client and 563 resource server, the authorization server generates a new key 564 identifier which MUST be unique among all key identifiers used by the 565 authorization server. The authorization server then uses the key 566 derivation key shared with the resource server to derive the 567 symmetric key as specified below. Instead of providing the keying 568 material in the access token, the authorization server includes the 569 key identifier in the "kid" parameter, see Figure 7. This key 570 identifier enables the resource server to calculate the symmetric key 571 used for the communication with the client using the key derivation 572 key and a KDF to be defined by the application, for example HKDF-SHA- 573 256. The key identifier picked by the authorization server needs to 574 be unique for each access token where a unique symmetric key is 575 required. 577 In this example, HKDF consists of the composition of the HKDF-Extract 578 and HKDF-Expand steps [RFC5869]. The symmetric key is derived from 579 the key identifier, the key derivation key and other data: 581 OKM = HKDF(salt, IKM, info, L), 583 where: 585 o OKM, the output keying material, is the derived symmetric key 587 o salt is the empty byte string 589 o IKM, the input keying material, is the key derivation key as 590 defined above 592 o info is the serialization of a CBOR array consisting of 593 ([RFC8610]): 595 info = [ 596 type : tstr, 597 L : uint, 598 access_token: map 599 ] 601 where: 603 o type is set to the constant text string "ACE-CoAP-DTLS-key- 604 derivation", 606 o L is the size of the symmetric key in bytes, 608 o access_token is the decrypted access_token as transferred from the 609 authorization server to the resource server. 611 To ensure uniqueness of the derived shared secret, the authorization 612 server SHOULD generate a sufficiently random kid value and include 613 the claims "iat" and either "exp" or "exi" in the access token. 615 3.3.1. DTLS Channel Setup Between Client and Resource Server 617 When a client receives an access token response from an authorization 618 server, the client MUST check if the access token response is bound 619 to a certain previously sent access token request, as the request may 620 specify the resource server with which the client wants to 621 communicate. 623 The client checks if the payload of the access token response 624 contains an "access_token" parameter and a "cnf" parameter. With 625 this information the client can initiate the establishment of a new 626 DTLS channel with a resource server. To use DTLS with pre-shared 627 keys, the client follows the PSK key exchange algorithm specified in 628 Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter 629 of the AS response as PSK when constructing the premaster secret. To 630 be consistent with the recommendations in [RFC7252] a client is 631 expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 632 [RFC6655] to the resource server. 634 In PreSharedKey mode, the knowledge of the shared secret by the 635 client and the resource server is used for mutual authentication 636 between both peers. Therefore, the resource server must be able to 637 determine the shared secret from the access token. Following the 638 general ACE authorization framework, the client can upload the access 639 token to the resource server's authz-info resource before starting 640 the DTLS handshake. The client then needs to indicate during the 641 DTLS handshake which previously uploaded access token it intends to 642 use. To do so, it MUST create a "COSE_Key" structure with the "kid" 643 that was conveyed in the "rs_cnf" claim in the token response from 644 the authorization server and the key type "symmetric". This 645 structure then is included as the only element in the "cnf" structure 646 that is used as value for "psk_identity" as shown in Figure 9. 648 { cnf : { 649 COSE_Key : { 650 kty: symmetric, 651 kid : h'3d027833fc6267ce' 652 } 653 } 654 } 656 Figure 9: Access token containing a single kid parameter 658 As an alternative to the access token upload, the client can provide 659 the most recent access token in the "psk_identity" field of the 660 ClientKeyExchange message. To do so, the client MUST treat the 661 contents of the "access_token" field from the AS-to-Client response 662 as opaque data as specified in Section 4.2 of [RFC7925] and not 663 perform any re-coding. This allows the resource server to retrieve 664 the shared secret directly from the "cnf" claim of the access token. 666 If a resource server receives a ClientKeyExchange message that 667 contains a "psk_identity" with a length greater than zero, it MUST 668 process the contents of the "psk_identity" field as access token that 669 is stored with the authorization information endpoint, before 670 continuing the DTLS handshake. If the contents of the "psk_identity" 671 do not yield a valid access token for the requesting client, the 672 resource server aborts the DTLS handshake with an "illegal_parameter" 673 alert. 675 When the resource server receives an access token, it MUST check if 676 the access token is still valid, if the resource server is the 677 intended destination (i.e., the audience of the token), and if the 678 token was issued by an authorized authorization server. This 679 specification assumes that the access token is a PoP token as 680 described in [I-D.ietf-ace-oauth-authz] unless specifically stated 681 otherwise. Therefore, the access token is bound to a symmetric PoP 682 key that is used as shared secret between the client and the resource 683 server. The resource server may use token introspection [RFC7662] on 684 the access token to retrieve more information about the specific 685 token. The use of introspection is out of scope for this 686 specification. 688 While the client can retrieve the shared secret from the contents of 689 the "cnf" parameter in the AS-to-Client response, the resource server 690 uses the information contained in the "cnf" claim of the access token 691 to determine the actual secret when no explicit "kid" was provided in 692 the "psk_identity" field. If key derivation is used, the resource 693 server uses the "COSE_KDF_Context" information as described above. 695 3.4. Resource Access 697 Once a DTLS channel has been established as described in Section 3.2 698 or Section 3.3, respectively, the client is authorized to access 699 resources covered by the access token it has uploaded to the authz- 700 info resource hosted by the resource server. 702 With the successful establishment of the DTLS channel, the client and 703 the resource server have proven that they can use their respective 704 keying material. An access token that is bound to the client's 705 keying material is associated with the channel. According to section 706 5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one access 707 token for each client. New access tokens issued by the authorization 708 server are supposed to replace previously issued access tokens for 709 the respective client. The resource server therefore must have a 710 common understanding with the authorization server how access tokens 711 are ordered. 713 Any request that the resource server receives on a DTLS channel that 714 is tied to an access token via its keying material MUST be checked 715 against the authorization rules that can be determined with the 716 access token. The resource server MUST check for every request if 717 the access token is still valid. If the token has expired, the 718 resource server MUST remove it. Incoming CoAP requests that are not 719 authorized with respect to any access token that is associated with 720 the client MUST be rejected by the resource server with 4.01 721 response. The response MAY include AS Request Creation Hints as 722 described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz]. 724 The resource server MUST only accept an incoming CoAP request as 725 authorized if the following holds: 727 1. The message was received on a secure channel that has been 728 established using the procedure defined in this document. 730 2. The authorization information tied to the sending client is 731 valid. 733 3. The request is destined for the resource server. 735 4. The resource URI specified in the request is covered by the 736 authorization information. 738 5. The request method is an authorized action on the resource with 739 respect to the authorization information. 741 Incoming CoAP requests received on a secure DTLS channel that are not 742 thus authorized MUST be rejected according to Section 5.8.2 of 743 [I-D.ietf-ace-oauth-authz] 745 1. with response code 4.03 (Forbidden) when the resource URI 746 specified in the request is not covered by the authorization 747 information, and 749 2. with response code 4.05 (Method Not Allowed) when the resource 750 URI specified in the request covered by the authorization 751 information but not the requested action. 753 The client MUST ascertain that its keying material is still valid 754 before sending a request or processing a response. If the client 755 gets an error response containing AS Request Creation Hints (cf. 756 Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response to its 757 requests, it SHOULD request a new access token from the authorization 758 server in order to continue communication with the resource server. 760 Unauthorized requests that have been received over a DTLS session 761 SHOULD be treated as non-fatal by the resource server, i.e., the DTLS 762 session SHOULD be kept alive until the associated access token has 763 expired. 765 4. Dynamic Update of Authorization Information 767 Resource servers must only use a new access token to update the 768 authorization information for a DTLS session if the keying material 769 that is bound to the token is the same that was used in the DTLS 770 handshake. By associating the access tokens with the identifier of 771 an existing DTLS session, the authorization information can be 772 updated without changing the cryptographic keys for the DTLS 773 communication between the client and the resource server, i.e. an 774 existing session can be used with updated permissions. 776 The client can therefore update the authorization information stored 777 at the resource server at any time without changing an established 778 DTLS session. To do so, the client requests a new access token from 779 the authorization server for the intended action on the respective 780 resource and uploads this access token to the authz-info resource on 781 the resource server. 783 Figure 10 depicts the message flow where the client requests a new 784 access token after a security association between the client and the 785 resource server has been established using this protocol. If the 786 client wants to update the authorization information, the token 787 request MUST specify the key identifier of the proof-of-possession 788 key used for the existing DTLS channel between the client and the 789 resource server in the "kid" parameter of the Client-to-AS request. 790 The authorization server MUST verify that the specified "kid" denotes 791 a valid verifier for a proof-of-possession token that has previously 792 been issued to the requesting client. Otherwise, the Client-to-AS 793 request MUST be declined with the error code "unsupported_pop_key" as 794 defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz]. 796 When the authorization server issues a new access token to update 797 existing authorization information, it MUST include the specified 798 "kid" parameter in this access token. A resource server MUST replace 799 the authorization information of any existing DTLS session that is 800 identified by this key identifier with the updated authorization 801 information. 803 C RS AS 804 | <===== DTLS channel =====> | | 805 | + Access Token | | 806 | | | 807 | --- Token Request ----------------------------> | 808 | | | 809 | <---------------------------- New Access Token - | 810 | + Access Information | 811 | | | 812 | --- Update /authz-info --> | | 813 | New Access Token | | 814 | | | 815 | == Authorized Request ===> | | 816 | | | 817 | <=== Protected Resource == | | 819 Figure 10: Overview of Dynamic Update Operation 821 5. Token Expiration 823 The resource server MUST delete access tokens that are no longer 824 valid. DTLS associations that have been setup in accordance with 825 this profile are always tied to specific tokens (which may be 826 exchanged with a dynamic update as described in Section 4). As 827 tokens may become invalid at any time (e.g., because they have 828 expired), the association may become useless at some point. A 829 resource server therefore MUST terminate existing DTLS association 830 after the last access token associated with this association has 831 expired. 833 As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the 834 resource server MUST notify the client with an error response with 835 code 4.01 (Unauthorized) for any long running request before 836 terminating the association. 838 6. Secure Communication with an Authorization Server 840 As specified in the ACE framework (sections 5.6 and 5.7 of 841 [I-D.ietf-ace-oauth-authz]), the requesting entity (the resource 842 server and/or the client) and the authorization server communicate 843 via the token endpoint or introspection endpoint. The use of CoAP 844 and DTLS for this communication is RECOMMENDED in this profile, other 845 protocols (such as HTTP and TLS, or CoAP and OSCORE [RFC8613]) MAY be 846 used instead. 848 How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the 849 authorization server are established is out of scope for this 850 profile. 852 If other means of securing the communication with the authorization 853 server are used, the communication security requirements from 854 Section 6.2 of [I-D.ietf-ace-oauth-authz] remain applicable. 856 7. Security Considerations 858 This document specifies a profile for the Authentication and 859 Authorization for Constrained Environments (ACE) framework 860 [I-D.ietf-ace-oauth-authz]. As it follows this framework's general 861 approach, the general security considerations from section 6 of 862 [I-D.ietf-ace-oauth-authz] also apply to this profile. 864 The authorization server must ascertain that the keying material for 865 the client that it provides to the resource server actually is 866 associated with this client. Malicious clients may hand over access 867 tokens containing their own access permissions to other entities. 868 This problem cannot be completely eliminated. Nevertheless, in RPK 869 mode it should not be possible for clients to request access tokens 870 for arbitrary public keys, since that would allow the client to relay 871 tokens without the need to share its own credentials with others. 872 The authorization server therefore at some point needs to validate 873 that the client can actually use the private key corresponding to the 874 client's public key. 876 When using pre-shared keys provisioned by the authorization server, 877 the security level depends on the randomness of PSK, and the security 878 of the TLS cipher suite and key exchange algorithm. As this 879 specification targets at constrained environments, message payloads 880 exchanged between the client and the resource server are expected to 881 be small and rare. CoAP [RFC7252] mandates the implementation of 882 cipher suites with abbreviated, 8-byte tags for message integrity 883 protection. For consistency, this profile requires implementation of 884 the same cipher suites. For application scenarios where the cost of 885 full-width authentication tags is low compared to the overall amount 886 of data being transmitted, the use of cipher suites with 16-byte 887 integrity protection tags is preferred. 889 The PSK mode of this profile offers a distribution mechanism to 890 convey authorization tokens together with a shared secret to a client 891 and a server. As this specification aims at constrained devices and 892 uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite 893 TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access 894 tokens and the corresponding shared secrets generated by the 895 authorization server are expected to be sufficiently short-lived to 896 provide similar forward-secrecy properties to using ephemeral Diffie- 897 Hellman (DHE) key exchange mechanisms. For longer-lived access 898 tokens, DHE ciphersuites should be used. 900 Constrained devices that use DTLS [RFC6347] are inherently vulnerable 901 to Denial of Service (DoS) attacks as the handshake protocol requires 902 creation of internal state within the device. This is specifically 903 of concern where an adversary is able to intercept the initial cookie 904 exchange and interject forged messages with a valid cookie to 905 continue with the handshake. A similar issue exists with the 906 unprotected authorization information endpoint where the resource 907 server needs to keep valid access tokens until their expiry. 908 Adversaries can fill up the constrained resource server's internal 909 storage for a very long time with interjected or otherwise retrieved 910 valid access tokens. The protection of access tokens that are stored 911 in the authorization information endpoint depends on the keying 912 material that is used between the authorization server and the 913 resource server: The resource server must ensure that it processes 914 only access tokens that are encrypted and integrity-protected by an 915 authorization server that is authorized to provide access tokens for 916 the resource server. 918 7.1. Reuse of Existing Sessions 920 To avoid the overhead of a repeated DTLS handshake, [RFC7925] 921 recommends session resumption [RFC5077] to reuse session state from 922 an earlier DTLS association and thus requires client side 923 implementation. In this specification, the DTLS session is subject 924 to the authorization rules denoted by the access token that was used 925 for the initial setup of the DTLS association. Enabling session 926 resumption would require the server to transfer the authorization 927 information with the session state in an encrypted SessionTicket to 928 the client. Assuming that the server uses long-lived keying 929 material, this could open up attacks due to the lack of forward 930 secrecy. Moreover, using this mechanism, a client can resume a DTLS 931 session without proving the possession of the PoP key again. 932 Therefore, the use of session resumption is NOT RECOMMENDED for 933 resource servers. 935 Since renogiation of DTLS associations is prone to attacks as well, 936 [RFC7925] requires clients to decline any renogiation attempt. A 937 server that wants to initiate re-keying therefore SHOULD periodically 938 force a full handshake. 940 7.2. Multiple Access Tokens 942 The use of multiple access tokens for a single client increases the 943 strain on the resource server as it must consider every access token 944 and calculate the actual permissions of the client. Also, tokens may 945 contradict each other which may lead the server to enforce wrong 946 permissions. If one of the access tokens expires earlier than 947 others, the resulting permissions may offer insufficient protection. 948 Developers SHOULD avoid using multiple access tokens for a client. 950 Even when a single access token per client is used, an attacker could 951 compromise the dynamic update mechanism for existing DTLS connections 952 by delaying or reordering packets destined for the authz-info 953 endpoint. Thus, the order in which operations occur at the resource 954 server (and thus which authorization info is used to process a given 955 client request) cannot be guaranteed. Especially in the presence of 956 later-issued access tokens that reduce the client's permissions from 957 the initial access token, it is impossible to guarantee that the 958 reduction in authorization will take effect prior to the expiration 959 of the original token. 961 7.3. Out-of-Band Configuration 963 To communicate securely, the authorization server, the client and the 964 resource server require certain information that must be exchanged 965 outside the protocol flow described in this document. The 966 authorization server must have obtained authorization information 967 concerning the client and the resource server that is approved by the 968 resource owner as well as corresponding keying material. The 969 resource server must have received authorization information approved 970 by the resource owner concerning its authorization managers and the 971 respective keying material. The client must have obtained 972 authorization information concerning the authorization server 973 approved by its owner as well as the corresponding keying material. 974 Also, the client's owner must have approved of the client's 975 communication with the resource server. The client and the 976 authorization server must have obtained a common understanding how 977 this resource server is identified to ensure that the client obtains 978 access token and keying material for the correct resource server. If 979 the client is provided with a raw public key for the resource server, 980 it must be ascertained to which resource server (which identifier and 981 authorization information) the key is associated. All authorization 982 information and keying material must be kept up to date. 984 8. Privacy Considerations 986 This privacy considerations from section 7 of the 987 [I-D.ietf-ace-oauth-authz] apply also to this profile. 989 An unprotected response to an unauthorized request may disclose 990 information about the resource server and/or its existing 991 relationship with the client. It is advisable to include as little 992 information as possible in an unencrypted response. When a DTLS 993 session between an authenticated client and the resource server 994 already exists, more detailed information MAY be included with an 995 error response to provide the client with sufficient information to 996 react on that particular error. 998 Also, unprotected requests to the resource server may reveal 999 information about the client, e.g., which resources the client 1000 attempts to request or the data that the client wants to provide to 1001 the resource server. The client SHOULD NOT send confidential data in 1002 an unprotected request. 1004 Note that some information might still leak after DTLS session is 1005 established, due to observable message sizes, the source, and the 1006 destination addresses. 1008 9. IANA Considerations 1010 The following registrations are done for the ACE OAuth Profile 1011 Registry following the procedure specified in 1012 [I-D.ietf-ace-oauth-authz]. 1014 Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" 1015 with the RFC number of this specification and delete this paragraph. 1017 Profile name: coap_dtls 1019 Profile Description: Profile for delegating client authentication and 1020 authorization in a constrained environment by establishing a Datagram 1021 Transport Layer Security (DTLS) channel between resource-constrained 1022 nodes. 1024 Profile ID: TBD (suggested: 1) 1026 Change Controller: IESG 1028 Reference: [RFC-XXXX] 1030 10. Acknowledgments 1032 Thanks to Jim Schaad for his contributions and reviews of this 1033 document. Special thanks to Ben Kaduk for his thorough review of 1034 this document. 1036 Ludwig Seitz worked on this document as part of the CelticNext 1037 projects CyberWI, and CRITISEC with funding from Vinnova. 1039 11. References 1041 11.1. Normative References 1043 [I-D.ietf-ace-oauth-authz] 1044 Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and 1045 H. Tschofenig, "Authentication and Authorization for 1046 Constrained Environments (ACE) using the OAuth 2.0 1047 Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-33 1048 (work in progress), February 2020. 1050 [I-D.ietf-ace-oauth-params] 1051 Seitz, L., "Additional OAuth Parameters for Authorization 1052 in Constrained Environments (ACE)", draft-ietf-ace-oauth- 1053 params-13 (work in progress), April 2020. 1055 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1056 Requirement Levels", BCP 14, RFC 2119, 1057 DOI 10.17487/RFC2119, March 1997, 1058 . 1060 [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key 1061 Ciphersuites for Transport Layer Security (TLS)", 1062 RFC 4279, DOI 10.17487/RFC4279, December 2005, 1063 . 1065 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1066 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 1067 January 2012, . 1069 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 1070 RFC 6749, DOI 10.17487/RFC6749, October 2012, 1071 . 1073 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 1074 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 1075 Transport Layer Security (TLS) and Datagram Transport 1076 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 1077 June 2014, . 1079 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1080 Application Protocol (CoAP)", RFC 7252, 1081 DOI 10.17487/RFC7252, June 2014, 1082 . 1084 [RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer 1085 Security (TLS) / Datagram Transport Layer Security (DTLS) 1086 Profiles for the Internet of Things", RFC 7925, 1087 DOI 10.17487/RFC7925, July 2016, 1088 . 1090 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 1091 RFC 8152, DOI 10.17487/RFC8152, July 2017, 1092 . 1094 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1095 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1096 May 2017, . 1098 [RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic 1099 Curve Cryptography (ECC) Cipher Suites for Transport Layer 1100 Security (TLS) Versions 1.2 and Earlier", RFC 8422, 1101 DOI 10.17487/RFC8422, August 2018, 1102 . 1104 [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 1105 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 1106 Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 1107 2020, . 1109 11.2. Informative References 1111 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1112 "Transport Layer Security (TLS) Session Resumption without 1113 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1114 January 2008, . 1116 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 1117 Key Derivation Function (HKDF)", RFC 5869, 1118 DOI 10.17487/RFC5869, May 2010, 1119 . 1121 [RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for 1122 Transport Layer Security (TLS)", RFC 6655, 1123 DOI 10.17487/RFC6655, July 2012, 1124 . 1126 [RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES- 1127 CCM Elliptic Curve Cryptography (ECC) Cipher Suites for 1128 TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014, 1129 . 1131 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 1132 RFC 7662, DOI 10.17487/RFC7662, October 2015, 1133 . 1135 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 1136 for Security", RFC 7748, DOI 10.17487/RFC7748, January 1137 2016, . 1139 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 1140 Signature Algorithm (EdDSA)", RFC 8032, 1141 DOI 10.17487/RFC8032, January 2017, 1142 . 1144 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 1145 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 1146 May 2018, . 1148 [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data 1149 Definition Language (CDDL): A Notational Convention to 1150 Express Concise Binary Object Representation (CBOR) and 1151 JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, 1152 June 2019, . 1154 [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 1155 "Object Security for Constrained RESTful Environments 1156 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 1157 . 1159 Authors' Addresses 1161 Stefanie Gerdes 1162 Universitaet Bremen TZI 1163 Postfach 330440 1164 Bremen D-28359 1165 Germany 1167 Phone: +49-421-218-63906 1168 Email: gerdes@tzi.org 1169 Olaf Bergmann 1170 Universitaet Bremen TZI 1171 Postfach 330440 1172 Bremen D-28359 1173 Germany 1175 Phone: +49-421-218-63904 1176 Email: bergmann@tzi.org 1178 Carsten Bormann 1179 Universitaet Bremen TZI 1180 Postfach 330440 1181 Bremen D-28359 1182 Germany 1184 Phone: +49-421-218-63921 1185 Email: cabo@tzi.org 1187 Goeran Selander 1188 Ericsson AB 1190 Email: goran.selander@ericsson.com 1192 Ludwig Seitz 1193 Combitech 1194 Djaeknegatan 31 1195 Malmoe 211 35 1196 Sweden 1198 Email: ludwig.seitz@combitech.se