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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACE Working Group L. Seitz 3 Internet-Draft Combitech 4 Intended status: Standards Track G. Selander 5 Expires: December 25, 2020 Ericsson 6 E. Wahlstroem 8 S. Erdtman 9 Spotify AB 10 H. Tschofenig 11 Arm Ltd. 12 June 23, 2020 14 Authentication and Authorization for Constrained Environments (ACE) 15 using the OAuth 2.0 Framework (ACE-OAuth) 16 draft-ietf-ace-oauth-authz-34 18 Abstract 20 This specification defines a framework for authentication and 21 authorization in Internet of Things (IoT) environments called ACE- 22 OAuth. The framework is based on a set of building blocks including 23 OAuth 2.0 and the Constrained Application Protocol (CoAP), thus 24 transforming a well-known and widely used authorization solution into 25 a form suitable for IoT devices. Existing specifications are used 26 where possible, but extensions are added and profiles are defined to 27 better serve the IoT use cases. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at https://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on December 25, 2020. 46 Copyright Notice 48 Copyright (c) 2020 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (https://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 64 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6 66 3.1. OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 7 67 3.2. CoAP . . . . . . . . . . . . . . . . . . . . . . . . . . 10 68 4. Protocol Interactions . . . . . . . . . . . . . . . . . . . . 11 69 5. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 15 70 5.1. Discovering Authorization Servers . . . . . . . . . . . . 16 71 5.1.1. Unauthorized Resource Request Message . . . . . . . . 16 72 5.1.2. AS Request Creation Hints . . . . . . . . . . . . . . 17 73 5.1.2.1. The Client-Nonce Parameter . . . . . . . . . . . 19 74 5.2. Authorization Grants . . . . . . . . . . . . . . . . . . 20 75 5.3. Client Credentials . . . . . . . . . . . . . . . . . . . 20 76 5.4. AS Authentication . . . . . . . . . . . . . . . . . . . . 21 77 5.5. The Authorization Endpoint . . . . . . . . . . . . . . . 21 78 5.6. The Token Endpoint . . . . . . . . . . . . . . . . . . . 21 79 5.6.1. Client-to-AS Request . . . . . . . . . . . . . . . . 22 80 5.6.2. AS-to-Client Response . . . . . . . . . . . . . . . . 25 81 5.6.3. Error Response . . . . . . . . . . . . . . . . . . . 27 82 5.6.4. Request and Response Parameters . . . . . . . . . . . 28 83 5.6.4.1. Grant Type . . . . . . . . . . . . . . . . . . . 28 84 5.6.4.2. Token Type . . . . . . . . . . . . . . . . . . . 29 85 5.6.4.3. Profile . . . . . . . . . . . . . . . . . . . . . 29 86 5.6.4.4. Client-Nonce . . . . . . . . . . . . . . . . . . 30 87 5.6.5. Mapping Parameters to CBOR . . . . . . . . . . . . . 30 88 5.7. The Introspection Endpoint . . . . . . . . . . . . . . . 31 89 5.7.1. Introspection Request . . . . . . . . . . . . . . . . 32 90 5.7.2. Introspection Response . . . . . . . . . . . . . . . 33 91 5.7.3. Error Response . . . . . . . . . . . . . . . . . . . 34 92 5.7.4. Mapping Introspection parameters to CBOR . . . . . . 35 93 5.8. The Access Token . . . . . . . . . . . . . . . . . . . . 35 94 5.8.1. The Authorization Information Endpoint . . . . . . . 36 95 5.8.1.1. Verifying an Access Token . . . . . . . . . . . . 37 96 5.8.1.2. Protecting the Authorization Information 97 Endpoint . . . . . . . . . . . . . . . . . . . . 39 98 5.8.2. Client Requests to the RS . . . . . . . . . . . . . . 39 99 5.8.3. Token Expiration . . . . . . . . . . . . . . . . . . 40 100 5.8.4. Key Expiration . . . . . . . . . . . . . . . . . . . 41 101 6. Security Considerations . . . . . . . . . . . . . . . . . . . 42 102 6.1. Protecting Tokens . . . . . . . . . . . . . . . . . . . . 42 103 6.2. Communication Security . . . . . . . . . . . . . . . . . 43 104 6.3. Long-Term Credentials . . . . . . . . . . . . . . . . . . 44 105 6.4. Unprotected AS Request Creation Hints . . . . . . . . . . 44 106 6.5. Minimal security requirements for communication . 45 107 6.6. Token Freshness and Expiration . . . . . . . . . . . . . 46 108 6.7. Combining profiles . . . . . . . . . . . . . . . . . . . 47 109 6.8. Unprotected Information . . . . . . . . . . . . . . . . . 47 110 6.9. Identifying audiences . . . . . . . . . . . . . . . . . . 48 111 6.10. Denial of service against or with Introspection . . 48 112 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 49 113 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50 114 8.1. ACE Authorization Server Request Creation Hints . . . . . 50 115 8.2. CoRE Resource Type registry . . . . . . . . . . . . . . . 51 116 8.3. OAuth Extensions Error Registration . . . . . . . . . . . 51 117 8.4. OAuth Error Code CBOR Mappings Registry . . . . . . . . . 51 118 8.5. OAuth Grant Type CBOR Mappings . . . . . . . . . . . . . 52 119 8.6. OAuth Access Token Types . . . . . . . . . . . . . . . . 52 120 8.7. OAuth Access Token Type CBOR Mappings . . . . . . . . . . 52 121 8.7.1. Initial Registry Contents . . . . . . . . . . . . . . 53 122 8.8. ACE Profile Registry . . . . . . . . . . . . . . . . . . 53 123 8.9. OAuth Parameter Registration . . . . . . . . . . . . . . 54 124 8.10. OAuth Parameters CBOR Mappings Registry . . . . . . . . . 54 125 8.11. OAuth Introspection Response Parameter Registration . . . 54 126 8.12. OAuth Token Introspection Response CBOR Mappings Registry 55 127 8.13. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 55 128 8.14. CBOR Web Token Claims . . . . . . . . . . . . . . . . . . 56 129 8.15. Media Type Registrations . . . . . . . . . . . . . . . . 57 130 8.16. CoAP Content-Format Registry . . . . . . . . . . . . . . 58 131 8.17. Expert Review Instructions . . . . . . . . . . . . . . . 58 132 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 59 133 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 59 134 10.1. Normative References . . . . . . . . . . . . . . . . . . 59 135 10.2. Informative References . . . . . . . . . . . . . . . . . 62 136 Appendix A. Design Justification . . . . . . . . . . . . . . . . 65 137 Appendix B. Roles and Responsibilities . . . . . . . . . . . . . 68 138 Appendix C. Requirements on Profiles . . . . . . . . . . . . . . 71 139 Appendix D. Assumptions on AS knowledge about C and RS . . . . . 71 140 Appendix E. Deployment Examples . . . . . . . . . . . . . . . . 72 141 E.1. Local Token Validation . . . . . . . . . . . . . . . . . 72 142 E.2. Introspection Aided Token Validation . . . . . . . . . . 76 143 Appendix F. Document Updates . . . . . . . . . . . . . . . . . . 80 144 F.1. Version -21 to 22 . . . . . . . . . . . . . . . . . . . . 81 145 F.2. Version -20 to 21 . . . . . . . . . . . . . . . . . . . . 81 146 F.3. Version -19 to 20 . . . . . . . . . . . . . . . . . . . . 81 147 F.4. Version -18 to -19 . . . . . . . . . . . . . . . . . . . 81 148 F.5. Version -17 to -18 . . . . . . . . . . . . . . . . . . . 81 149 F.6. Version -16 to -17 . . . . . . . . . . . . . . . . . . . 81 150 F.7. Version -15 to -16 . . . . . . . . . . . . . . . . . . . 82 151 F.8. Version -14 to -15 . . . . . . . . . . . . . . . . . . . 82 152 F.9. Version -13 to -14 . . . . . . . . . . . . . . . . . . . 82 153 F.10. Version -12 to -13 . . . . . . . . . . . . . . . . . . . 82 154 F.11. Version -11 to -12 . . . . . . . . . . . . . . . . . . . 83 155 F.12. Version -10 to -11 . . . . . . . . . . . . . . . . . . . 83 156 F.13. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 83 157 F.14. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 83 158 F.15. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 83 159 F.16. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 84 160 F.17. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 84 161 F.18. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 84 162 F.19. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 85 163 F.20. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 85 164 F.21. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 85 165 F.22. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 86 166 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 86 168 1. Introduction 170 Authorization is the process for granting approval to an entity to 171 access a generic resource [RFC4949]. The authorization task itself 172 can best be described as granting access to a requesting client, for 173 a resource hosted on a device, the resource server (RS). This 174 exchange is mediated by one or multiple authorization servers (AS). 175 Managing authorization for a large number of devices and users can be 176 a complex task. 178 While prior work on authorization solutions for the Web and for the 179 mobile environment also applies to the Internet of Things (IoT) 180 environment, many IoT devices are constrained, for example, in terms 181 of processing capabilities, available memory, etc. For web 182 applications on constrained nodes, this specification RECOMMENDS the 183 use of the Constrained Application Protocol (CoAP) [RFC7252] as 184 replacement for HTTP. 186 Appendix A gives an overview of the constraints considered in this 187 design, and a more detailed treatment of constraints can be found in 188 [RFC7228]. This design aims to accommodate different IoT deployments 189 and thus a continuous range of device and network capabilities. 191 Taking energy consumption as an example: At one end there are energy- 192 harvesting or battery powered devices which have a tight power 193 budget, on the other end there are mains-powered devices, and all 194 levels in between. 196 Hence, IoT devices may be very different in terms of available 197 processing and message exchange capabilities and there is a need to 198 support many different authorization use cases [RFC7744]. 200 This specification describes a framework for authentication and 201 authorization in constrained environments (ACE) built on re-use of 202 OAuth 2.0 [RFC6749], thereby extending authorization to Internet of 203 Things devices. This specification contains the necessary building 204 blocks for adjusting OAuth 2.0 to IoT environments. 206 More detailed, interoperable specifications can be found in separate 207 profile specifications. Implementations may claim conformance with a 208 specific profile, whereby implementations utilizing the same profile 209 interoperate while implementations of different profiles are not 210 expected to be interoperable. Some devices, such as mobile phones 211 and tablets, may implement multiple profiles and will therefore be 212 able to interact with a wider range of low end devices. Requirements 213 on profiles are described at contextually appropriate places 214 throughout this specification, and also summarized in Appendix C. 216 2. Terminology 218 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 219 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 220 "OPTIONAL" in this document are to be interpreted as described in BCP 221 14 [RFC2119] [RFC8174] when, and only when, they appear in all 222 capitals, as shown here. 224 Certain security-related terms such as "authentication", 225 "authorization", "confidentiality", "(data) integrity", "message 226 authentication code", and "verify" are taken from [RFC4949]. 228 Since exchanges in this specification are described as RESTful 229 protocol interactions, HTTP [RFC7231] offers useful terminology. 231 Terminology for entities in the architecture is defined in OAuth 2.0 232 [RFC6749] such as client (C), resource server (RS), and authorization 233 server (AS). 235 Note that the term "endpoint" is used here following its OAuth 236 definition, which is to denote resources such as token and 237 introspection at the AS and authz-info at the RS (see Section 5.8.1 238 for a definition of the authz-info endpoint). The CoAP [RFC7252] 239 definition, which is "An entity participating in the CoAP protocol" 240 is not used in this specification. 242 The specifications in this document is called the "framework" or "ACE 243 framework". When referring to "profiles of this framework" it refers 244 to additional specifications that define the use of this 245 specification with concrete transport and communication security 246 protocols (e.g., CoAP over DTLS). 248 We use the term "Access Information" for parameters other than the 249 access token provided to the client by the AS to enable it to access 250 the RS (e.g. public key of the RS, profile supported by RS). 252 We use the term "Authorization Information" to denote all 253 information, including the claims of relevant access tokens, that an 254 RS uses to determine whether an access request should be granted. 256 3. Overview 258 This specification defines the ACE framework for authorization in the 259 Internet of Things environment. It consists of a set of building 260 blocks. 262 The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys 263 widespread deployment. Many IoT devices can support OAuth 2.0 264 without any additional extensions, but for certain constrained 265 settings additional profiling is needed. 267 Another building block is the lightweight web transfer protocol CoAP 268 [RFC7252], for those communication environments where HTTP is not 269 appropriate. CoAP typically runs on top of UDP, which further 270 reduces overhead and message exchanges. While this specification 271 defines extensions for the use of OAuth over CoAP, other underlying 272 protocols are not prohibited from being supported in the future, such 273 as HTTP/2 [RFC7540], Message Queuing Telemetry Transport (MQTT) 274 [MQTT5.0], Bluetooth Low Energy (BLE) [BLE] and QUIC 275 [I-D.ietf-quic-transport]. Note that this document specifies 276 protocol exchanges in terms of RESTful verbs such as GET and POST. 277 Future profiles using protocols that do not support these verbs MUST 278 specify how the corresponding protocol messages are transmitted 279 instead. 281 A third building block is the Concise Binary Object Representation 282 (CBOR) [RFC7049], for encodings where JSON [RFC8259] is not 283 sufficiently compact. CBOR is a binary encoding designed for small 284 code and message size, which may be used for encoding of self 285 contained tokens, and also for encoding payloads transferred in 286 protocol messages. 288 A fourth building block is CBOR Object Signing and Encryption (COSE) 289 [RFC8152], which enables object-level layer security as an 290 alternative or complement to transport layer security (DTLS [RFC6347] 291 or TLS [RFC8446]). COSE is used to secure self-contained tokens such 292 as proof-of-possession (PoP) tokens, which are an extension to the 293 OAuth bearer tokens. The default token format is defined in CBOR web 294 token (CWT) [RFC8392]. Application layer security for CoAP using 295 COSE can be provided with OSCORE [RFC8613]. 297 With the building blocks listed above, solutions satisfying various 298 IoT device and network constraints are possible. A list of 299 constraints is described in detail in [RFC7228] and a description of 300 how the building blocks mentioned above relate to the various 301 constraints can be found in Appendix A. 303 Luckily, not every IoT device suffers from all constraints. The ACE 304 framework nevertheless takes all these aspects into account and 305 allows several different deployment variants to co-exist, rather than 306 mandating a one-size-fits-all solution. It is important to cover the 307 wide range of possible interworking use cases and the different 308 requirements from a security point of view. Once IoT deployments 309 mature, popular deployment variants will be documented in the form of 310 ACE profiles. 312 3.1. OAuth 2.0 314 The OAuth 2.0 authorization framework enables a client to obtain 315 scoped access to a resource with the permission of a resource owner. 316 Authorization information, or references to it, is passed between the 317 nodes using access tokens. These access tokens are issued to clients 318 by an authorization server with the approval of the resource owner. 319 The client uses the access token to access the protected resources 320 hosted by the resource server. 322 A number of OAuth 2.0 terms are used within this specification: 324 The token and introspection Endpoints: 325 The AS hosts the token endpoint that allows a client to request 326 access tokens. The client makes a POST request to the token 327 endpoint on the AS and receives the access token in the response 328 (if the request was successful). 329 In some deployments, a token introspection endpoint is provided by 330 the AS, which can be used by the RS if it needs to request 331 additional information regarding a received access token. The RS 332 makes a POST request to the introspection endpoint on the AS and 333 receives information about the access token in the response. (See 334 "Introspection" below.) 336 Access Tokens: 337 Access tokens are credentials needed to access protected 338 resources. An access token is a data structure representing 339 authorization permissions issued by the AS to the client. Access 340 tokens are generated by the AS and consumed by the RS. The access 341 token content is opaque to the client. 343 Access tokens can have different formats, and various methods of 344 utilization e.g., cryptographic properties) based on the security 345 requirements of the given deployment. 347 Refresh Tokens: 348 Refresh tokens are credentials used to obtain access tokens. 349 Refresh tokens are issued to the client by the authorization 350 server and are used to obtain a new access token when the current 351 access token becomes invalid or expires, or to obtain additional 352 access tokens with identical or narrower scope (such access tokens 353 may have a shorter lifetime and fewer permissions than authorized 354 by the resource owner). Issuing a refresh token is optional at 355 the discretion of the authorization server. If the authorization 356 server issues a refresh token, it is included when issuing an 357 access token (i.e., step (B) in Figure 1). 359 A refresh token in OAuth 2.0 is a string representing the 360 authorization granted to the client by the resource owner. The 361 string is usually opaque to the client. The token denotes an 362 identifier used to retrieve the authorization information. Unlike 363 access tokens, refresh tokens are intended for use only with 364 authorization servers and are never sent to resource servers. In 365 this framework, refresh tokens are encoded in binary instead of 366 strings, if used. 368 Proof of Possession Tokens: 369 A token may be bound to a cryptographic key, which is then used to 370 bind the token to a request authorized by the token. Such tokens 371 are called proof-of-possession tokens (or PoP tokens). 373 The proof-of-possession (PoP) security concept used here assumes 374 that the AS acts as a trusted third party that binds keys to 375 tokens. In the case of access tokens, these so called PoP keys 376 are then used by the client to demonstrate the possession of the 377 secret to the RS when accessing the resource. The RS, when 378 receiving an access token, needs to verify that the key used by 379 the client matches the one bound to the access token. When this 380 specification uses the term "access token" it is assumed to be a 381 PoP access token token unless specifically stated otherwise. 383 The key bound to the token (the PoP key) may use either symmetric 384 or asymmetric cryptography. The appropriate choice of the kind of 385 cryptography depends on the constraints of the IoT devices as well 386 as on the security requirements of the use case. 388 Symmetric PoP key: 389 The AS generates a random symmetric PoP key. The key is either 390 stored to be returned on introspection calls or encrypted and 391 included in the token. The PoP key is also encrypted for the 392 token recipient and sent to the recipient together with the 393 token. 395 Asymmetric PoP key: 396 An asymmetric key pair is generated on the token's recipient 397 and the public key is sent to the AS (if it does not already 398 have knowledge of the recipient's public key). Information 399 about the public key, which is the PoP key in this case, is 400 either stored to be returned on introspection calls or included 401 inside the token and sent back to the requesting party. The 402 consumer of the token can identify the public key from the 403 information in the token, which allows the recipient of the 404 token to use the corresponding private key for the proof of 405 possession. 407 The token is either a simple reference, or a structured 408 information object (e.g., CWT [RFC8392]) protected by a 409 cryptographic wrapper (e.g., COSE [RFC8152]). The choice of PoP 410 key does not necessarily imply a specific credential type for the 411 integrity protection of the token. 413 Scopes and Permissions: 414 In OAuth 2.0, the client specifies the type of permissions it is 415 seeking to obtain (via the scope parameter) in the access token 416 request. In turn, the AS may use the scope response parameter to 417 inform the client of the scope of the access token issued. As the 418 client could be a constrained device as well, this specification 419 defines the use of CBOR encoding, see Section 5, for such requests 420 and responses. 422 The values of the scope parameter in OAuth 2.0 are expressed as a 423 list of space-delimited, case-sensitive strings, with a semantic 424 that is well-known to the AS and the RS. More details about the 425 concept of scopes is found under Section 3.3 in [RFC6749]. 427 Claims: 428 Information carried in the access token or returned from 429 introspection, called claims, is in the form of name-value pairs. 430 An access token may, for example, include a claim identifying the 431 AS that issued the token (via the "iss" claim) and what audience 432 the access token is intended for (via the "aud" claim). The 433 audience of an access token can be a specific resource or one or 434 many resource servers. The resource owner policies influence what 435 claims are put into the access token by the authorization server. 437 While the structure and encoding of the access token varies 438 throughout deployments, a standardized format has been defined 439 with the JSON Web Token (JWT) [RFC7519] where claims are encoded 440 as a JSON object. In [RFC8392], an equivalent format using CBOR 441 encoding (CWT) has been defined. 443 Introspection: 444 Introspection is a method for a resource server to query the 445 authorization server for the active state and content of a 446 received access token. This is particularly useful in those cases 447 where the authorization decisions are very dynamic and/or where 448 the received access token itself is an opaque reference rather 449 than a self-contained token. More information about introspection 450 in OAuth 2.0 can be found in [RFC7662]. 452 3.2. CoAP 454 CoAP is an application layer protocol similar to HTTP, but 455 specifically designed for constrained environments. CoAP typically 456 uses datagram-oriented transport, such as UDP, where reordering and 457 loss of packets can occur. A security solution needs to take the 458 latter aspects into account. 460 While HTTP uses headers and query strings to convey additional 461 information about a request, CoAP encodes such information into 462 header parameters called 'options'. 464 CoAP supports application-layer fragmentation of the CoAP payloads 465 through blockwise transfers [RFC7959]. However, blockwise transfer 466 does not increase the size limits of CoAP options, therefore data 467 encoded in options has to be kept small. 469 Transport layer security for CoAP can be provided by DTLS or TLS 470 [RFC6347][RFC8446] [I-D.ietf-tls-dtls13]. CoAP defines a number of 471 proxy operations that require transport layer security to be 472 terminated at the proxy. One approach for protecting CoAP 473 communication end-to-end through proxies, and also to support 474 security for CoAP over a different transport in a uniform way, is to 475 provide security at the application layer using an object-based 476 security mechanism such as COSE [RFC8152]. 478 One application of COSE is OSCORE [RFC8613], which provides end-to- 479 end confidentiality, integrity and replay protection, and a secure 480 binding between CoAP request and response messages. In OSCORE, the 481 CoAP messages are wrapped in COSE objects and sent using CoAP. 483 This framework RECOMMENDS the use of CoAP as replacement for HTTP for 484 use in constrained environments. For communication security this 485 framework does not make an explicit protocol recommendation, since 486 the choice depends on the requirements of the specific application. 487 DTLS [RFC6347], [I-D.ietf-tls-dtls13] and OSCORE [RFC8613] are 488 mentioned as examples, other protocols fulfilling the requirements 489 from Section 6.5 are also applicable. 491 4. Protocol Interactions 493 The ACE framework is based on the OAuth 2.0 protocol interactions 494 using the token endpoint and optionally the introspection endpoint. 495 A client obtains an access token, and optionally a refresh token, 496 from an AS using the token endpoint and subsequently presents the 497 access token to an RS to gain access to a protected resource. In 498 most deployments the RS can process the access token locally, however 499 in some cases the RS may present it to the AS via the introspection 500 endpoint to get fresh information. These interactions are shown in 501 Figure 1. An overview of various OAuth concepts is provided in 502 Section 3.1. 504 The OAuth 2.0 framework defines a number of "protocol flows" via 505 grant types, which have been extended further with extensions to 506 OAuth 2.0 (such as [RFC7521] and [RFC8628]). What grant types works 507 best depends on the usage scenario and [RFC7744] describes many 508 different IoT use cases but there are two preferred grant types, 509 namely the Authorization Code Grant (described in Section 4.1 of 510 [RFC7521]) and the Client Credentials Grant (described in Section 4.4 511 of [RFC7521]). The Authorization Code Grant is a good fit for use 512 with apps running on smart phones and tablets that request access to 513 IoT devices, a common scenario in the smart home environment, where 514 users need to go through an authentication and authorization phase 515 (at least during the initial setup phase). The native apps 516 guidelines described in [RFC8252] are applicable to this use case. 517 The Client Credential Grant is a good fit for use with IoT devices 518 where the OAuth client itself is constrained. In such a case, the 519 resource owner has pre-arranged access rights for the client with the 520 authorization server, which is often accomplished using a 521 commissioning tool. 523 The consent of the resource owner, for giving a client access to a 524 protected resource, can be provided dynamically as in the traditional 525 OAuth flows, or it could be pre-configured by the resource owner as 526 authorization policies at the AS, which the AS evaluates when a token 527 request arrives. The resource owner and the requesting party (i.e., 528 client owner) are not shown in Figure 1. 530 This framework supports a wide variety of communication security 531 mechanisms between the ACE entities, such as client, AS, and RS. It 532 is assumed that the client has been registered (also called enrolled 533 or onboarded) to an AS using a mechanism defined outside the scope of 534 this document. In practice, various techniques for onboarding have 535 been used, such as factory-based provisioning or the use of 536 commissioning tools. Regardless of the onboarding technique, this 537 provisioning procedure implies that the client and the AS exchange 538 credentials and configuration parameters. These credentials are used 539 to mutually authenticate each other and to protect messages exchanged 540 between the client and the AS. 542 It is also assumed that the RS has been registered with the AS, 543 potentially in a similar way as the client has been registered with 544 the AS. Established keying material between the AS and the RS allows 545 the AS to apply cryptographic protection to the access token to 546 ensure that its content cannot be modified, and if needed, that the 547 content is confidentiality protected. 549 The keying material necessary for establishing communication security 550 between C and RS is dynamically established as part of the protocol 551 described in this document. 553 At the start of the protocol, there is an optional discovery step 554 where the client discovers the resource server and the resources this 555 server hosts. In this step, the client might also determine what 556 permissions are needed to access the protected resource. A generic 557 procedure is described in Section 5.1; profiles MAY define other 558 procedures for discovery. 560 In Bluetooth Low Energy, for example, advertisements are broadcasted 561 by a peripheral, including information about the primary services. 563 In CoAP, as a second example, a client can make a request to "/.well- 564 known/core" to obtain information about available resources, which 565 are returned in a standardized format as described in [RFC6690]. 567 +--------+ +---------------+ 568 | |---(A)-- Token Request ------->| | 569 | | | Authorization | 570 | |<--(B)-- Access Token ---------| Server | 571 | | + Access Information | | 572 | | + Refresh Token (optional) +---------------+ 573 | | ^ | 574 | | Introspection Request (D)| | 575 | Client | (optional) | | 576 | | Response | |(E) 577 | | (optional) | v 578 | | +--------------+ 579 | |---(C)-- Token + Request ----->| | 580 | | | Resource | 581 | |<--(F)-- Protected Resource ---| Server | 582 | | | | 583 +--------+ +--------------+ 585 Figure 1: Basic Protocol Flow. 587 Requesting an Access Token (A): 588 The client makes an access token request to the token endpoint at 589 the AS. This framework assumes the use of PoP access tokens (see 590 Section 3.1 for a short description) wherein the AS binds a key to 591 an access token. The client may include permissions it seeks to 592 obtain, and information about the credentials it wants to use 593 (e.g., symmetric/asymmetric cryptography or a reference to a 594 specific credential). 596 Access Token Response (B): 597 If the AS successfully processes the request from the client, it 598 returns an access token and optionally a refresh token (note that 599 only certain grant types support refresh tokens). It can also 600 return additional parameters, referred to as "Access Information". 601 In addition to the response parameters defined by OAuth 2.0 and 602 the PoP access token extension, this framework defines parameters 603 that can be used to inform the client about capabilities of the 604 RS, e.g. the profiles the RS supports. More information about 605 these parameters can be found in Section 5.6.4. 607 Resource Request (C): 608 The client interacts with the RS to request access to the 609 protected resource and provides the access token. The protocol to 610 use between the client and the RS is not restricted to CoAP. 611 HTTP, HTTP/2, QUIC, MQTT, Bluetooth Low Energy, etc., are also 612 viable candidates. 614 Depending on the device limitations and the selected protocol, 615 this exchange may be split up into two parts: 617 (1) the client sends the access token containing, or 618 referencing, the authorization information to the RS, that may 619 be used for subsequent resource requests by the client, and 621 (2) the client makes the resource access request, using the 622 communication security protocol and other Access Information 623 obtained from the AS. 625 The Client and the RS mutually authenticate using the security 626 protocol specified in the profile (see step B) and the keys 627 obtained in the access token or the Access Information. The RS 628 verifies that the token is integrity protected and originated by 629 the AS. It then compares the claims contained in the access token 630 with the resource request. If the RS is online, validation can be 631 handed over to the AS using token introspection (see messages D 632 and E) over HTTP or CoAP. 634 Token Introspection Request (D): 635 A resource server may be configured to introspect the access token 636 by including it in a request to the introspection endpoint at that 637 AS. Token introspection over CoAP is defined in Section 5.7 and 638 for HTTP in [RFC7662]. 640 Note that token introspection is an optional step and can be 641 omitted if the token is self-contained and the resource server is 642 prepared to perform the token validation on its own. 644 Token Introspection Response (E): 645 The AS validates the token and returns the most recent parameters, 646 such as scope, audience, validity etc. associated with it back to 647 the RS. The RS then uses the received parameters to process the 648 request to either accept or to deny it. 650 Protected Resource (F): 651 If the request from the client is authorized, the RS fulfills the 652 request and returns a response with the appropriate response code. 653 The RS uses the dynamically established keys to protect the 654 response, according to the communication security protocol used. 656 5. Framework 658 The following sections detail the profiling and extensions of OAuth 659 2.0 for constrained environments, which constitutes the ACE 660 framework. 662 Credential Provisioning 663 For IoT, it cannot be assumed that the client and RS are part of a 664 common key infrastructure, so the AS provisions credentials or 665 associated information to allow mutual authentication between 666 client and RS. The resulting security association between client 667 and RS may then also be used to bind these credentials to the 668 access tokens the client uses. 670 Proof-of-Possession 671 The ACE framework, by default, implements proof-of-possession for 672 access tokens, i.e., that the token holder can prove being a 673 holder of the key bound to the token. The binding is provided by 674 the "cnf" claim [RFC8747] indicating what key is used for proof- 675 of-possession. If a client needs to submit a new access token, 676 e.g., to obtain additional access rights, they can request that 677 the AS binds this token to the same key as the previous one. 679 ACE Profiles 680 The client or RS may be limited in the encodings or protocols it 681 supports. To support a variety of different deployment settings, 682 specific interactions between client and RS are defined in an ACE 683 profile. In ACE framework the AS is expected to manage the 684 matching of compatible profile choices between a client and an RS. 685 The AS informs the client of the selected profile using the 686 "ace_profile" parameter in the token response. 688 OAuth 2.0 requires the use of TLS both to protect the communication 689 between AS and client when requesting an access token; between client 690 and RS when accessing a resource and between AS and RS if 691 introspection is used. In constrained settings TLS is not always 692 feasible, or desirable. Nevertheless it is REQUIRED that the 693 communications named above are encrypted, integrity protected and 694 protected against message replay. It is also REQUIRED that the 695 communicating endpoints perform mutual authentication. Furthermore 696 it MUST be assured that responses are bound to the requests in the 697 sense that the receiver of a response can be certain that the 698 response actually belongs to a certain request. Note that setting up 699 such a secure communication may require some unprotected messages to 700 be exchanged first (e.g. sending the token from the client to the 701 RS). 703 Profiles MUST specify a communication security protocol that provides 704 the features required above. 706 In OAuth 2.0 the communication with the Token and the Introspection 707 endpoints at the AS is assumed to be via HTTP and may use Uri-query 708 parameters. When profiles of this framework use CoAP instead, it is 709 REQUIRED to use of the following alternative instead of Uri-query 710 parameters: The sender (client or RS) encodes the parameters of its 711 request as a CBOR map and submits that map as the payload of the POST 712 request. 714 Profiles that use CBOR encoding of protocol message parameters at the 715 outermost encoding layer MUST use the media format 'application/ 716 ace+cbor'. If CoAP is used for communication, the Content-Format 717 MUST be abbreviated with the ID: 19 (see Section 8.16). 719 The OAuth 2.0 AS uses a JSON structure in the payload of its 720 responses both to client and RS. If CoAP is used, it is REQUIRED to 721 use CBOR [RFC7049] instead of JSON. Depending on the profile, the 722 CBOR payload MAY be enclosed in a non-CBOR cryptographic wrapper. 724 5.1. Discovering Authorization Servers 726 In order to determine the AS in charge of a resource hosted at the 727 RS, C MAY send an initial Unauthorized Resource Request message to 728 RS. RS then denies the request and sends the address of its AS back 729 to C. 731 Instead of the initial Unauthorized Resource Request message, other 732 discovery methods may be used, or the client may be pre-provisioned 733 with an RS-to-AS mapping. 735 5.1.1. Unauthorized Resource Request Message 737 An Unauthorized Resource Request message is a request for any 738 resource hosted by RS for which the client does not have 739 authorization granted. RSes MUST treat any request for a protected 740 resource as an Unauthorized Resource Request message when any of the 741 following hold: 743 o The request has been received on an unprotected channel. 745 o The RS has no valid access token for the sender of the request 746 regarding the requested action on that resource. 748 o The RS has a valid access token for the sender of the request, but 749 that token does not authorize the requested action on the 750 requested resource. 752 Note: These conditions ensure that the RS can handle requests 753 autonomously once access was granted and a secure channel has been 754 established between C and RS. The authz-info endpoint, as part of 755 the process for authorizing to protected resources, is not itself a 756 protected resource and MUST NOT be protected as specified above (cf. 757 Section 5.8.1). 759 Unauthorized Resource Request messages MUST be denied with an 760 "unauthorized_client" error response. In this response, the Resource 761 Server SHOULD provide proper AS Request Creation Hints to enable the 762 Client to request an access token from RS's AS as described in 763 Section 5.1.2. 765 The handling of all client requests (including unauthorized ones) by 766 the RS is described in Section 5.8.2. 768 5.1.2. AS Request Creation Hints 770 The AS Request Creation Hints message is sent by an RS as a response 771 to an Unauthorized Resource Request message (see Section 5.1.1) to 772 help the sender of the Unauthorized Resource Request message acquire 773 a valid access token. The AS Request Creation Hints message is a 774 CBOR map, with a MANDATORY element "AS" specifying an absolute URI 775 (see Section 4.3 of [RFC3986]) that identifies the appropriate AS for 776 the RS. 778 The message can also contain the following OPTIONAL parameters: 780 o A "audience" element containing a suggested audience that the 781 client should request at the AS. 783 o A "kid" element containing the key identifier of a key used in an 784 existing security association between the client and the RS. The 785 RS expects the client to request an access token bound to this 786 key, in order to avoid having to re-establish the security 787 association. 789 o A "cnonce" element containing a client-nonce. See 790 Section 5.1.2.1. 792 o A "scope" element containing the suggested scope that the client 793 should request towards the AS. 795 Figure 2 summarizes the parameters that may be part of the AS Request 796 Creation Hints. 798 /-----------+----------+---------------------\ 799 | Name | CBOR Key | Value Type | 800 |-----------+----------+---------------------| 801 | AS | 1 | text string | 802 | kid | 2 | byte string | 803 | audience | 5 | text string | 804 | scope | 9 | text or byte string | 805 | cnonce | 39 | byte string | 806 \-----------+----------+---------------------/ 808 Figure 2: AS Request Creation Hints 810 Note that the schema part of the AS parameter may need to be adapted 811 to the security protocol that is used between the client and the AS. 812 Thus the example AS value "coap://as.example.com/token" might need to 813 be transformed to "coaps://as.example.com/token". It is assumed that 814 the client can determine the correct schema part on its own depending 815 on the way it communicates with the AS. 817 Figure 3 shows an example for an AS Request Creation Hints message 818 payload using CBOR [RFC7049] diagnostic notation, using the parameter 819 names instead of the CBOR keys for better human readability. 821 4.01 Unauthorized 822 Content-Format: application/ace+cbor 823 Payload : 824 { 825 "AS" : "coaps://as.example.com/token", 826 "audience" : "coaps://rs.example.com" 827 "scope" : "rTempC", 828 "cnonce" : h'e0a156bb3f' 829 } 831 Figure 3: AS Request Creation Hints payload example 833 In the example above, the response parameter "AS" points the receiver 834 of this message to the URI "coaps://as.example.com/token" to request 835 access tokens. The RS sending this response (i.e., RS) uses an 836 internal clock that is only loosely synchronized with the clock of 837 the AS. Therefore it can not reliably verify the expiration time of 838 access tokens it receives. To ensure a certain level of access token 839 freshness nevetheless, the RS has included a "cnonce" parameter (see 840 Section 5.1.2.1) in the response. 842 Figure 4 illustrates the mandatory to use binary encoding of the 843 message payload shown in Figure 3. 845 a4 # map(4) 846 01 # unsigned(1) (=AS) 847 78 1c # text(28) 848 636f6170733a2f2f61732e657861 849 6d706c652e636f6d2f746f6b656e # "coaps://as.example.com/token" 850 05 # unsigned(5) (=audience) 851 76 # text(22) 852 636f6170733a2f2f72732e657861 853 6d706c652e636f6d # "coaps://rs.example.com" 854 09 # unsigned(9) (=scope) 855 66 # text(6) 856 7254656d7043 # "rTempC" 857 18 27 # unsigned(39) (=cnonce) 858 45 # bytes(5) 859 e0a156bb3f # 861 Figure 4: AS Request Creation Hints example encoded in CBOR 863 5.1.2.1. The Client-Nonce Parameter 865 If the RS does not synchronize its clock with the AS, it could be 866 tricked into accepting old access tokens, that are either expired or 867 have been compromised. In order to ensure some level of token 868 freshness in that case, the RS can use the "cnonce" (client-nonce) 869 parameter. The processing requirements for this parameter are as 870 follows: 872 o An RS sending a "cnonce" parameter in an AS Request Creation Hints 873 message MUST store information to validate that a given cnonce is 874 fresh. How this is implemented internally is out of scope for 875 this specification. Expiration of client-nonces should be based 876 roughly on the time it would take a client to obtain an access 877 token after receiving the AS Request Creation Hints message, with 878 some allowance for unexpected delays. 880 o A client receiving a "cnonce" parameter in an AS Request Creation 881 Hints message MUST include this in the parameters when requesting 882 an access token at the AS, using the "cnonce" parameter from 883 Section 5.6.4.4. 885 o If an AS grants an access token request containing a "cnonce" 886 parameter, it MUST include this value in the access token, using 887 the "cnonce" claim specified in Section 5.8. 889 o An RS that is using the client-nonce mechanism and that receives 890 an access token MUST verify that this token contains a cnonce 891 claim, with a client-nonce value that is fresh according to the 892 information stored at the first step above. If the cnonce claim 893 is not present or if the cnonce claim value is not fresh, the RS 894 MUST discard the access token. If this was an interaction with 895 the authz-info endpoint the RS MUST also respond with an error 896 message using a response code equivalent to the CoAP code 4.01 897 (Unauthorized). 899 5.2. Authorization Grants 901 To request an access token, the client obtains authorization from the 902 resource owner or uses its client credentials as a grant. The 903 authorization is expressed in the form of an authorization grant. 905 The OAuth framework [RFC6749] defines four grant types. The grant 906 types can be split up into two groups, those granted on behalf of the 907 resource owner (password, authorization code, implicit) and those for 908 the client (client credentials). Further grant types have been added 909 later, such as [RFC7521] defining an assertion-based authorization 910 grant. 912 The grant type is selected depending on the use case. In cases where 913 the client acts on behalf of the resource owner, the authorization 914 code grant is recommended. If the client acts on behalf of the 915 resource owner, but does not have any display or has very limited 916 interaction possibilities, it is recommended to use the device code 917 grant defined in [RFC8628]. In cases where the client acts 918 autonomously the client credentials grant is recommended. 920 For details on the different grant types, see section 1.3 of 921 [RFC6749]. The OAuth 2.0 framework provides an extension mechanism 922 for defining additional grant types, so profiles of this framework 923 MAY define additional grant types, if needed. 925 5.3. Client Credentials 927 Authentication of the client is mandatory independent of the grant 928 type when requesting an access token from the token endpoint. In the 929 case of the client credentials grant type, the authentication and 930 grant coincide. 932 Client registration and provisioning of client credentials to the 933 client is out of scope for this specification. 935 The OAuth framework defines one client credential type in section 936 2.3.1 of [RFC6749]: client id and client secret. 937 [I-D.erdtman-ace-rpcc] adds raw-public-key and pre-shared-key to the 938 client credentials types. Profiles of this framework MAY extend with 939 an additional client credentials type using client certificates. 941 5.4. AS Authentication 943 The client credential grant does not, by default, authenticate the AS 944 that the client connects to. In classic OAuth, the AS is 945 authenticated with a TLS server certificate. 947 Profiles of this framework MUST specify how clients authenticate the 948 AS and how communication security is implemented. By default, server 949 side TLS certificates, as defined by OAuth 2.0, are required. 951 5.5. The Authorization Endpoint 953 The OAuth 2.0 authorization endpoint is used to interact with the 954 resource owner and obtain an authorization grant, in certain grant 955 flows. The primary use case for the ACE-OAuth framework is for 956 machine-to-machine interactions that do not involve the resource 957 owner in the authorization flow; therefore, this endpoint is out of 958 scope here. Future profiles may define constrained adaptation 959 mechanisms for this endpoint as well. Non-constrained clients 960 interacting with constrained resource servers can use the 961 specification in section 3.1 of [RFC6749] and the attack 962 countermeasures suggested in section 4.2 of [RFC6819]. 964 5.6. The Token Endpoint 966 In standard OAuth 2.0, the AS provides the token endpoint for 967 submitting access token requests. This framework extends the 968 functionality of the token endpoint, giving the AS the possibility to 969 help the client and RS to establish shared keys or to exchange their 970 public keys. Furthermore, this framework defines encodings using 971 CBOR, as a substitute for JSON. 973 The endpoint may, however, be exposed over HTTPS as in classical 974 OAuth or even other transports. A profile MUST define the details of 975 the mapping between the fields described below, and these transports. 976 If HTTPS is used, JSON or CBOR payloads may be supported. If JSON 977 payloads are used, the semantics of Section 4 of the OAuth 2.0 978 specification MUST be followed (with additions as described below). 980 If CBOR payload is supported, the semantics described below MUST be 981 followed. 983 For the AS to be able to issue a token, the client MUST be 984 authenticated and present a valid grant for the scopes requested. 985 Profiles of this framework MUST specify how the AS authenticates the 986 client and how the communication between client and AS is protected, 987 fulfilling the requirements specified in Section 5. 989 The default name of this endpoint in an url-path is '/token', however 990 implementations are not required to use this name and can define 991 their own instead. 993 The figures of this section use CBOR diagnostic notation without the 994 integer abbreviations for the parameters or their values for 995 illustrative purposes. Note that implementations MUST use the 996 integer abbreviations and the binary CBOR encoding, if the CBOR 997 encoding is used. 999 5.6.1. Client-to-AS Request 1001 The client sends a POST request to the token endpoint at the AS. The 1002 profile MUST specify how the communication is protected. The content 1003 of the request consists of the parameters specified in the relevant 1004 subsection of section 4 of the OAuth 2.0 specification [RFC6749], 1005 depending on the grant type, with the following exceptions and 1006 additions: 1008 o The parameter "grant_type" is OPTIONAL in the context of this 1009 framework (as opposed to REQUIRED in RFC6749). If that parameter 1010 is missing, the default value "client_credentials" is implied. 1012 o The "audience" parameter from [RFC8693] is OPTIONAL to request an 1013 access token bound to a specific audience. 1015 o The "cnonce" parameter defined in Section 5.6.4.4 is REQUIRED if 1016 the RS provided a client-nonce in the "AS Request Creation Hints" 1017 message Section 5.1.2 1019 o The "scope" parameter MAY be encoded as a byte string instead of 1020 the string encoding specified in section 3.3 of [RFC6749], in 1021 order allow compact encoding of complex scopes. The syntax of 1022 such a binary encoding is explicitly not specified here and left 1023 to profiles or applications, specifically note that a binary 1024 encoded scope does not necessarily use the space character '0x20' 1025 to delimit scope-tokens. 1027 o The client can send an empty (null value) "ace_profile" parameter 1028 to indicate that it wants the AS to include the "ace_profile" 1029 parameter in the response. See Section 5.6.4.3. 1031 o A client MUST be able to use the parameters from 1032 [I-D.ietf-ace-oauth-params] in an access token request to the 1033 token endpoint and the AS MUST be able to process these additional 1034 parameters. 1036 The default behavior, is that the AS generates a symmetric proof-of- 1037 possession key for the client. In order to use an asymmetric key 1038 pair or to re-use a key previously established with the RS, the 1039 client is supposed to use the "req_cnf" parameter from 1040 [I-D.ietf-ace-oauth-params]. 1042 If CBOR is used then these parameters MUST be encoded as a CBOR map. 1044 When HTTP is used as a transport then the client makes a request to 1045 the token endpoint by sending the parameters using the "application/ 1046 x-www-form-urlencoded" format with a character encoding of UTF-8 in 1047 the HTTP request entity-body, as defined in section 3.2 of [RFC6749]. 1049 The following examples illustrate different types of requests for 1050 proof-of-possession tokens. 1052 Figure 5 shows a request for a token with a symmetric proof-of- 1053 possession key. The content is displayed in CBOR diagnostic 1054 notation, without abbreviations for better readability. 1056 Header: POST (Code=0.02) 1057 Uri-Host: "as.example.com" 1058 Uri-Path: "token" 1059 Content-Format: "application/ace+cbor" 1060 Payload: 1061 { 1062 "client_id" : "myclient", 1063 "audience" : "tempSensor4711" 1064 } 1066 Figure 5: Example request for an access token bound to a symmetric 1067 key. 1069 Figure 6 shows a request for a token with an asymmetric proof-of- 1070 possession key. Note that in this example OSCORE [RFC8613] is used 1071 to provide object-security, therefore the Content-Format is 1072 "application/oscore" wrapping the "application/ace+cbor" type 1073 content. The OSCORE option has a decoded interpretation appended in 1074 parentheses for the reader's convenience. Also note that in this 1075 example the audience is implicitly known by both client and AS. 1076 Furthermore note that this example uses the "req_cnf" parameter from 1077 [I-D.ietf-ace-oauth-params]. 1079 Header: POST (Code=0.02) 1080 Uri-Host: "as.example.com" 1081 Uri-Path: "token" 1082 OSCORE: 0x09, 0x05, 0x44, 0x6C 1083 (h=0, k=1, n=001, partialIV= 0x05, kid=[0x44, 0x6C]) 1084 Content-Format: "application/oscore" 1085 Payload: 1086 0x44025d1 ... (full payload omitted for brevity) ... 68b3825e 1088 Decrypted payload: 1089 { 1090 "client_id" : "myclient", 1091 "req_cnf" : { 1092 "COSE_Key" : { 1093 "kty" : "EC", 1094 "kid" : h'11', 1095 "crv" : "P-256", 1096 "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8', 1097 "y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4' 1098 } 1099 } 1100 } 1102 Figure 6: Example token request bound to an asymmetric key. 1104 Figure 7 shows a request for a token where a previously communicated 1105 proof-of-possession key is only referenced using the "req_cnf" 1106 parameter from [I-D.ietf-ace-oauth-params]. 1108 Header: POST (Code=0.02) 1109 Uri-Host: "as.example.com" 1110 Uri-Path: "token" 1111 Content-Format: "application/ace+cbor" 1112 Payload: 1113 { 1114 "client_id" : "myclient", 1115 "audience" : "valve424", 1116 "scope" : "read", 1117 "req_cnf" : { 1118 "kid" : b64'6kg0dXJM13U' 1119 } 1120 }W 1122 Figure 7: Example request for an access token bound to a key 1123 reference. 1125 Refresh tokens are typically not stored as securely as proof-of- 1126 possession keys in requesting clients. Proof-of-possession based 1127 refresh token requests MUST NOT request different proof-of-possession 1128 keys or different audiences in token requests. Refresh token 1129 requests can only use to request access tokens bound to the same 1130 proof-of-possession key and the same audience as access tokens issued 1131 in the initial token request. 1133 5.6.2. AS-to-Client Response 1135 If the access token request has been successfully verified by the AS 1136 and the client is authorized to obtain an access token corresponding 1137 to its access token request, the AS sends a response with the 1138 response code equivalent to the CoAP response code 2.01 (Created). 1139 If client request was invalid, or not authorized, the AS returns an 1140 error response as described in Section 5.6.3. 1142 Note that the AS decides which token type and profile to use when 1143 issuing a successful response. It is assumed that the AS has prior 1144 knowledge of the capabilities of the client and the RS (see 1145 Appendix D). This prior knowledge may, for example, be set by the 1146 use of a dynamic client registration protocol exchange [RFC7591]. If 1147 the client has requested a specific proof-of-possession key using the 1148 "req_cnf" parameter from [I-D.ietf-ace-oauth-params], this may also 1149 influence which profile the AS selects, as it needs to support the 1150 use of the key type requested the client. 1152 The content of the successful reply is the Access Information. When 1153 using CBOR payloads, the content MUST be encoded as a CBOR map, 1154 containing parameters as specified in Section 5.1 of [RFC6749], with 1155 the following additions and changes: 1157 ace_profile: 1158 OPTIONAL unless the request included an empty ace_profile 1159 parameter in which case it is MANDATORY. This indicates the 1160 profile that the client MUST use towards the RS. See 1161 Section 5.6.4.3 for the formatting of this parameter. If this 1162 parameter is absent, the AS assumes that the client implicitly 1163 knows which profile to use towards the RS. 1165 token_type: 1166 This parameter is OPTIONAL, as opposed to 'required' in [RFC6749]. 1167 By default implementations of this framework SHOULD assume that 1168 the token_type is "PoP". If a specific use case requires another 1169 token_type (e.g., "Bearer") to be used then this parameter is 1170 REQUIRED. 1172 Furthermore [I-D.ietf-ace-oauth-params] defines additional parameters 1173 that the AS MUST be able to use when responding to a request to the 1174 token endpoint. 1176 Figure 8 summarizes the parameters that can currently be part of the 1177 Access Information. Future extensions may define additional 1178 parameters. 1180 /-------------------+-------------------------------\ 1181 | Parameter name | Specified in | 1182 |-------------------+-------------------------------| 1183 | access_token | RFC 6749 | 1184 | token_type | RFC 6749 | 1185 | expires_in | RFC 6749 | 1186 | refresh_token | RFC 6749 | 1187 | scope | RFC 6749 | 1188 | state | RFC 6749 | 1189 | error | RFC 6749 | 1190 | error_description | RFC 6749 | 1191 | error_uri | RFC 6749 | 1192 | ace_profile | [this document] | 1193 | cnf | [I-D.ietf-ace-oauth-params] | 1194 | rs_cnf | [I-D.ietf-ace-oauth-params] | 1195 \-------------------+-------------------------------/ 1197 Figure 8: Access Information parameters 1199 Figure 9 shows a response containing a token and a "cnf" parameter 1200 with a symmetric proof-of-possession key, which is defined in 1201 [I-D.ietf-ace-oauth-params]. Note that the key identifier 'kid' is 1202 only used to simplify indexing and retrieving the key, and no 1203 assumptions should be made that it is unique in the domains of either 1204 the client or the RS. 1206 Header: Created (Code=2.01) 1207 Content-Format: "application/ace+cbor" 1208 Payload: 1209 { 1210 "access_token" : b64'SlAV32hkKG ... 1211 (remainder of CWT omitted for brevity; 1212 CWT contains COSE_Key in the "cnf" claim)', 1213 "ace_profile" : "coap_dtls", 1214 "expires_in" : "3600", 1215 "cnf" : { 1216 "COSE_Key" : { 1217 "kty" : "Symmetric", 1218 "kid" : b64'39Gqlw', 1219 "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh' 1220 } 1221 } 1222 } 1224 Figure 9: Example AS response with an access token bound to a 1225 symmetric key. 1227 5.6.3. Error Response 1229 The error responses for CoAP-based interactions with the AS are 1230 generally equivalent to the ones for HTTP-based interactions as 1231 defined in Section 5.2 of [RFC6749], with the following exceptions: 1233 o When using CBOR the raw payload before being processed by the 1234 communication security protocol MUST be encoded as a CBOR map. 1236 o A response code equivalent to the CoAP code 4.00 (Bad Request) 1237 MUST be used for all error responses, except for invalid_client 1238 where a response code equivalent to the CoAP code 4.01 1239 (Unauthorized) MAY be used under the same conditions as specified 1240 in Section 5.2 of [RFC6749]. 1242 o The Content-Format (for CoAP-based interactions) or media type 1243 (for HTTP-based interactions) "application/ace+cbor" MUST be used 1244 for the error response. 1246 o The parameters "error", "error_description" and "error_uri" MUST 1247 be abbreviated using the codes specified in Figure 12, when a CBOR 1248 encoding is used. 1250 o The error code (i.e., value of the "error" parameter) MUST be 1251 abbreviated as specified in Figure 10, when a CBOR encoding is 1252 used. 1254 /---------------------------+-------------\ 1255 | Name | CBOR Values | 1256 |---------------------------+-------------| 1257 | invalid_request | 1 | 1258 | invalid_client | 2 | 1259 | invalid_grant | 3 | 1260 | unauthorized_client | 4 | 1261 | unsupported_grant_type | 5 | 1262 | invalid_scope | 6 | 1263 | unsupported_pop_key | 7 | 1264 | incompatible_ace_profiles | 8 | 1265 \---------------------------+-------------/ 1267 Figure 10: CBOR abbreviations for common error codes 1269 In addition to the error responses defined in OAuth 2.0, the 1270 following behavior MUST be implemented by the AS: 1272 o If the client submits an asymmetric key in the token request that 1273 the RS cannot process, the AS MUST reject that request with a 1274 response code equivalent to the CoAP code 4.00 (Bad Request) 1275 including the error code "unsupported_pop_key" defined in 1276 Figure 10. 1278 o If the client and the RS it has requested an access token for do 1279 not share a common profile, the AS MUST reject that request with a 1280 response code equivalent to the CoAP code 4.00 (Bad Request) 1281 including the error code "incompatible_ace_profiles" defined in 1282 Figure 10. 1284 5.6.4. Request and Response Parameters 1286 This section provides more detail about the new parameters that can 1287 be used in access token requests and responses, as well as 1288 abbreviations for more compact encoding of existing parameters and 1289 common parameter values. 1291 5.6.4.1. Grant Type 1293 The abbreviations specified in the registry defined in Section 8.5 1294 MUST be used in CBOR encodings instead of the string values defined 1295 in [RFC6749], if CBOR payloads are used. 1297 /--------------------+------------+------------------------\ 1298 | Name | CBOR Value | Original Specification | 1299 |--------------------+------------+------------------------| 1300 | password | 0 | [RFC6749] | 1301 | authorization_code | 1 | [RFC6749] | 1302 | client_credentials | 2 | [RFC6749] | 1303 | refresh_token | 3 | [RFC6749] | 1304 \--------------------+------------+------------------------/ 1306 Figure 11: CBOR abbreviations for common grant types 1308 5.6.4.2. Token Type 1310 The "token_type" parameter, defined in section 5.1 of [RFC6749], 1311 allows the AS to indicate to the client which type of access token it 1312 is receiving (e.g., a bearer token). 1314 This document registers the new value "PoP" for the OAuth Access 1315 Token Types registry, specifying a proof-of-possession token. How 1316 the proof-of-possession by the client to the RS is performed MUST be 1317 specified by the profiles. 1319 The values in the "token_type" parameter MUST use the CBOR 1320 abbreviations defined in the registry specified by Section 8.7, if a 1321 CBOR encoding is used. 1323 In this framework the "pop" value for the "token_type" parameter is 1324 the default. The AS may, however, provide a different value. 1326 5.6.4.3. Profile 1328 Profiles of this framework MUST define the communication protocol and 1329 the communication security protocol between the client and the RS. 1330 The security protocol MUST provide encryption, integrity and replay 1331 protection. It MUST also provide a binding between requests and 1332 responses. Furthermore profiles MUST define a list of allowed proof- 1333 of-possession methods, if they support proof-of-possession tokens. 1335 A profile MUST specify an identifier that MUST be used to uniquely 1336 identify itself in the "ace_profile" parameter. The textual 1337 representation of the profile identifier is intended for human 1338 readability and for JSON-based interactions, it MUST NOT be used for 1339 CBOR-based interactions. Profiles MUST register their identifier in 1340 the registry defined in Section 8.8. 1342 Profiles MAY define additional parameters for both the token request 1343 and the Access Information in the access token response in order to 1344 support negotiation or signaling of profile specific parameters. 1346 Clients that want the AS to provide them with the "ace_profile" 1347 parameter in the access token response can indicate that by sending a 1348 ace_profile parameter with a null value (for CBOR-based interactions) 1349 or an empty string (for JSON based interactions) in the access token 1350 request. 1352 5.6.4.4. Client-Nonce 1354 This parameter MUST be sent from the client to the AS, if it 1355 previously received a "cnonce" parameter in the AS Request Creation 1356 Hints Section 5.1.2. The parameter is encoded as a byte string for 1357 CBOR-based interactions, and as a string (Base64 encoded binary) for 1358 JSON-based interactions. It MUST copy the value from the cnonce 1359 parameter in the AS Request Creation Hints. 1361 5.6.5. Mapping Parameters to CBOR 1363 If CBOR encoding is used, all OAuth parameters in access token 1364 requests and responses MUST be mapped to CBOR types as specified in 1365 the registry defined by Section 8.10, using the given integer 1366 abbreviation for the map keys. 1368 Note that we have aligned the abbreviations corresponding to claims 1369 with the abbreviations defined in [RFC8392]. 1371 Note also that abbreviations from -24 to 23 have a 1 byte encoding 1372 size in CBOR. We have thus chosen to assign abbreviations in that 1373 range to parameters we expect to be used most frequently in 1374 constrained scenarios. 1376 /-------------------+----------+---------------------\ 1377 | Name | CBOR Key | Value Type | 1378 |-------------------+----------+---------------------| 1379 | access_token | 1 | byte string | 1380 | expires_in | 2 | unsigned integer | 1381 | audience | 5 | text string | 1382 | scope | 9 | text or byte string | 1383 | client_id | 24 | text string | 1384 | client_secret | 25 | byte string | 1385 | response_type | 26 | text string | 1386 | redirect_uri | 27 | text string | 1387 | state | 28 | text string | 1388 | code | 29 | byte string | 1389 | error | 30 | unsigned integer | 1390 | error_description | 31 | text string | 1391 | error_uri | 32 | text string | 1392 | grant_type | 33 | unsigned integer | 1393 | token_type | 34 | unsigned integer | 1394 | username | 35 | text string | 1395 | password | 36 | text string | 1396 | refresh_token | 37 | byte string | 1397 | ace_profile | 38 | unsigned integer | 1398 | cnonce | 39 | byte string | 1399 \-------------------+----------+---------------------/ 1401 Figure 12: CBOR mappings used in token requests and responses 1403 5.7. The Introspection Endpoint 1405 Token introspection [RFC7662] can be OPTIONALLY provided by the AS, 1406 and is then used by the RS and potentially the client to query the AS 1407 for metadata about a given token, e.g., validity or scope. Analogous 1408 to the protocol defined in [RFC7662] for HTTP and JSON, this section 1409 defines adaptations to more constrained environments using CBOR and 1410 leaving the choice of the application protocol to the profile. 1412 Communication between the requesting entity and the introspection 1413 endpoint at the AS MUST be integrity protected and encrypted. The 1414 communication security protocol MUST also provide a binding between 1415 requests and responses. Furthermore the two interacting parties MUST 1416 perform mutual authentication. Finally the AS SHOULD verify that the 1417 requesting entity has the right to access introspection information 1418 about the provided token. Profiles of this framework that support 1419 introspection MUST specify how authentication and communication 1420 security between the requesting entity and the AS is implemented. 1422 The default name of this endpoint in an url-path is '/introspect', 1423 however implementations are not required to use this name and can 1424 define their own instead. 1426 The figures of this section uses CBOR diagnostic notation without the 1427 integer abbreviations for the parameters or their values for better 1428 readability. 1430 Note that supporting introspection is OPTIONAL for implementations of 1431 this framework. 1433 5.7.1. Introspection Request 1435 The requesting entity sends a POST request to the introspection 1436 endpoint at the AS. The profile MUST specify how the communication 1437 is protected. If CBOR is used, the payload MUST be encoded as a CBOR 1438 map with a "token" entry containing the access token. Further 1439 optional parameters representing additional context that is known by 1440 the requesting entity to aid the AS in its response MAY be included. 1442 For CoAP-based interaction, all messages MUST use the content type 1443 "application/ace+cbor", while for HTTP-based interactions the 1444 equivalent media type "application/ace+cbor" MUST be used. 1446 The same parameters are required and optional as in Section 2.1 of 1447 [RFC7662]. 1449 For example, Figure 13 shows an RS calling the token introspection 1450 endpoint at the AS to query about an OAuth 2.0 proof-of-possession 1451 token. Note that object security based on OSCORE [RFC8613] is 1452 assumed in this example, therefore the Content-Format is 1453 "application/oscore". Figure 14 shows the decoded payload. 1455 Header: POST (Code=0.02) 1456 Uri-Host: "as.example.com" 1457 Uri-Path: "introspect" 1458 OSCORE: 0x09, 0x05, 0x25 1459 Content-Format: "application/oscore" 1460 Payload: 1461 ... COSE content ... 1463 Figure 13: Example introspection request. 1465 { 1466 "token" : b64'7gj0dXJQ43U', 1467 "token_type_hint" : "PoP" 1468 } 1470 Figure 14: Decoded payload. 1472 5.7.2. Introspection Response 1474 If the introspection request is authorized and successfully 1475 processed, the AS sends a response with the response code equivalent 1476 to the CoAP code 2.01 (Created). If the introspection request was 1477 invalid, not authorized or couldn't be processed the AS returns an 1478 error response as described in Section 5.7.3. 1480 In a successful response, the AS encodes the response parameters in a 1481 map including with the same required and optional parameters as in 1482 Section 2.2 of [RFC7662] with the following addition: 1484 ace_profile OPTIONAL. This indicates the profile that the RS MUST 1485 use with the client. See Section 5.6.4.3 for more details on the 1486 formatting of this parameter. 1488 cnonce OPTIONAL. A client-nonce provided to the AS by the client. 1489 The RS MUST verify that this corresponds to the client-nonce 1490 previously provided to the client in the AS Request Creation 1491 Hints. See Section 5.1.2 and Section 5.6.4.4. 1493 exi OPTIONAL. The "expires-in" claim associated to this access 1494 token. See Section 5.8.3. 1496 Furthermore [I-D.ietf-ace-oauth-params] defines more parameters that 1497 the AS MUST be able to use when responding to a request to the 1498 introspection endpoint. 1500 For example, Figure 15 shows an AS response to the introspection 1501 request in Figure 13. Note that this example contains the "cnf" 1502 parameter defined in [I-D.ietf-ace-oauth-params]. 1504 Header: Created (Code=2.01) 1505 Content-Format: "application/ace+cbor" 1506 Payload: 1507 { 1508 "active" : true, 1509 "scope" : "read", 1510 "ace_profile" : "coap_dtls", 1511 "cnf" : { 1512 "COSE_Key" : { 1513 "kty" : "Symmetric", 1514 "kid" : b64'39Gqlw', 1515 "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh' 1516 } 1517 } 1518 } 1520 Figure 15: Example introspection response. 1522 5.7.3. Error Response 1524 The error responses for CoAP-based interactions with the AS are 1525 equivalent to the ones for HTTP-based interactions as defined in 1526 Section 2.3 of [RFC7662], with the following differences: 1528 o If content is sent and CBOR is used the payload MUST be encoded as 1529 a CBOR map and the Content-Format "application/ace+cbor" MUST be 1530 used. 1532 o If the credentials used by the requesting entity (usually the RS) 1533 are invalid the AS MUST respond with the response code equivalent 1534 to the CoAP code 4.01 (Unauthorized) and use the required and 1535 optional parameters from Section 5.2 in [RFC6749]. 1537 o If the requesting entity does not have the right to perform this 1538 introspection request, the AS MUST respond with a response code 1539 equivalent to the CoAP code 4.03 (Forbidden). In this case no 1540 payload is returned. 1542 o The parameters "error", "error_description" and "error_uri" MUST 1543 be abbreviated using the codes specified in Figure 12. 1545 o The error codes MUST be abbreviated using the codes specified in 1546 the registry defined by Section 8.4. 1548 Note that a properly formed and authorized query for an inactive or 1549 otherwise invalid token does not warrant an error response by this 1550 specification. In these cases, the authorization server MUST instead 1551 respond with an introspection response with the "active" field set to 1552 "false". 1554 5.7.4. Mapping Introspection parameters to CBOR 1556 If CBOR is used, the introspection request and response parameters 1557 MUST be mapped to CBOR types as specified in the registry defined by 1558 Section 8.12, using the given integer abbreviation for the map key. 1560 Note that we have aligned abbreviations that correspond to a claim 1561 with the abbreviations defined in [RFC8392] and the abbreviations of 1562 parameters with the same name from Section 5.6.5. 1564 /-------------------+----------+-------------------------\ 1565 | Parameter name | CBOR Key | Value Type | 1566 |-------------------+----------+-------------------------| 1567 | iss | 1 | text string | 1568 | sub | 2 | text string | 1569 | aud | 3 | text string | 1570 | exp | 4 | integer or | 1571 | | | floating-point number | 1572 | nbf | 5 | integer or | 1573 | | | floating-point number | 1574 | iat | 6 | integer or | 1575 | | | floating-point number | 1576 | cti | 7 | byte string | 1577 | scope | 9 | text or byte string | 1578 | active | 10 | True or False | 1579 | token | 11 | byte string | 1580 | client_id | 24 | text string | 1581 | error | 30 | unsigned integer | 1582 | error_description | 31 | text string | 1583 | error_uri | 32 | text string | 1584 | token_type_hint | 33 | text string | 1585 | token_type | 34 | text string | 1586 | username | 35 | text string | 1587 | ace_profile | 38 | unsigned integer | 1588 | cnonce | 39 | byte string | 1589 | exi | 40 | unsigned integer | 1590 \-------------------+----------+-------------------------/ 1592 Figure 16: CBOR Mappings to Token Introspection Parameters. 1594 5.8. The Access Token 1596 This framework RECOMMENDS the use of CBOR web token (CWT) as 1597 specified in [RFC8392]. 1599 In order to facilitate offline processing of access tokens, this 1600 document uses the "cnf" claim from [RFC8747] and the "scope" claim 1601 from [RFC8693] for JWT- and CWT-encoded tokens. In addition to 1602 string encoding specified for the "scope" claim, a binary encoding 1603 MAY be used. The syntax of such an encoding is explicitly not 1604 specified here and left to profiles or applications, specifically 1605 note that a binary encoded scope does not necessarily use the space 1606 character '0x20' to delimit scope-tokens. 1608 If the AS needs to convey a hint to the RS about which profile it 1609 should use to communicate with the client, the AS MAY include an 1610 "ace_profile" claim in the access token, with the same syntax and 1611 semantics as defined in Section 5.6.4.3. 1613 If the client submitted a client-nonce parameter in the access token 1614 request Section 5.6.4.4, the AS MUST include the value of this 1615 parameter in the "cnonce" claim specified here. The "cnonce" claim 1616 uses binary encoding. 1618 5.8.1. The Authorization Information Endpoint 1620 The access token, containing authorization information and 1621 information about the proof-of-possession method used by the client, 1622 needs to be transported to the RS so that the RS can authenticate and 1623 authorize the client request. 1625 This section defines a method for transporting the access token to 1626 the RS using a RESTful protocol such as CoAP. Profiles of this 1627 framework MAY define other methods for token transport. 1629 The method consists of an authz-info endpoint, implemented by the RS. 1630 A client using this method MUST make a POST request to the authz-info 1631 endpoint at the RS with the access token in the payload. The RS 1632 receiving the token MUST verify the validity of the token. If the 1633 token is valid, the RS MUST respond to the POST request with 2.01 1634 (Created). Section Section 5.8.1.1 outlines how an RS MUST proceed 1635 to verify the validity of an access token. 1637 The RS MUST be prepared to store at least one access token for future 1638 use. This is a difference to how access tokens are handled in OAuth 1639 2.0, where the access token is typically sent along with each 1640 request, and therefore not stored at the RS. 1642 This specification RECOMMENDS that an RS stores only one token per 1643 proof-of-possession key, meaning that an additional token linked to 1644 the same key will overwrite any existing token at the RS. The reason 1645 is that this greatly simplifies (constrained) implementations, with 1646 respect to required storage and resolving a request to the applicable 1647 token. 1649 If the payload sent to the authz-info endpoint does not parse to a 1650 token, the RS MUST respond with a response code equivalent to the 1651 CoAP code 4.00 (Bad Request). 1653 The RS MAY make an introspection request to validate the token before 1654 responding to the POST request to the authz-info endpoint, e.g. if 1655 the token is an opaque reference. Some transport protocols may 1656 provide a way to indicate that the RS is busy and the client should 1657 retry after an interval; this type of status update would be 1658 appropriate while the RS is waiting for an introspection response. 1660 Profiles MUST specify whether the authz-info endpoint is protected, 1661 including whether error responses from this endpoint are protected. 1662 Note that since the token contains information that allow the client 1663 and the RS to establish a security context in the first place, mutual 1664 authentication may not be possible at this point. 1666 The default name of this endpoint in an url-path is '/authz-info', 1667 however implementations are not required to use this name and can 1668 define their own instead. 1670 5.8.1.1. Verifying an Access Token 1672 When an RS receives an access token, it MUST verify it before storing 1673 it. The details of token verification depends on various aspects, 1674 including the token encoding, the type of token, the security 1675 protection applied to the token, and the claims. The token encoding 1676 matters since the security wrapper differs between the token 1677 encodings. For example, a CWT token uses COSE while a JWT token uses 1678 JOSE. The type of token also has an influence on the verification 1679 procedure since tokens may be self-contained whereby token 1680 verification may happen locally at the RS while a token-by-reference 1681 requires further interaction with the authorization server, for 1682 example using token introspection, to obtain the claims associated 1683 with the token reference. Self-contained tokens MUST, at a minimum, 1684 be integrity protected but they MAY also be encrypted. 1686 For self-contained tokens the RS MUST process the security protection 1687 of the token first, as specified by the respective token format. For 1688 CWT the description can be found in [RFC8392] and for JWT the 1689 relevant specification is [RFC7519]. This MUST include a 1690 verification that security protection (and thus the token) was 1691 generated by an AS that has the right to issue access tokens for this 1692 RS. 1694 In case the token is communicated by reference the RS needs to obtain 1695 the claims first. When the RS uses token introspection the relevant 1696 specification is [RFC7662] with CoAP transport specified in 1697 Section 5.7. 1699 Errors may happen during this initial processing stage: 1701 o If token or claim verification fails, the RS MUST discard the 1702 token and, if this was an interaction with authz-info, return an 1703 error message with a response code equivalent to the CoAP code 1704 4.01 (Unauthorized). 1706 o If the claims cannot be obtained the RS MUST discard the token 1707 and, in case of an interaction via the authz-info endpoint, return 1708 an error message with a response code equivalent to the CoAP code 1709 4.00 (Bad Request). 1711 Next, the RS MUST verify claims, if present, contained in the access 1712 token. Errors are returned when claim checks fail, in the order of 1713 priority of this list: 1715 iss The issuer claim must identify an AS that has the authority to 1716 issue access tokens for the receiving RS. If that is not the case 1717 the RS MUST discard the token. If this was an interaction with 1718 authz-info, the RS MUST also respond with a response code 1719 equivalent to the CoAP code 4.01 (Unauthorized). 1721 exp The expiration date must be in the future. If that is not the 1722 case the RS MUST discard the token. If this was an interaction 1723 with authz-info the RS MUST also respond with a response code 1724 equivalent to the CoAP code 4.01 (Unauthorized). Note that the RS 1725 has to terminate access rights to the protected resources at the 1726 time when the tokens expire. 1728 aud The audience claim must refer to an audience that the RS 1729 identifies with. If that is not the case the RS MUST discard the 1730 token. If this was an interaction with authz-info, the RS MUST 1731 also respond with a response code equivalent to the CoAP code 4.03 1732 (Forbidden). 1734 scope The RS must recognize value of the scope claim. If that is 1735 not the case the RS MUST discard the token. If this was an 1736 interaction with authz-info, the RS MUST also respond with a 1737 response code equivalent to the CoAP code 4.00 (Bad Request). The 1738 RS MAY provide additional information in the error response, to 1739 clarify what went wrong. 1741 Additional processing may be needed for other claims in a way 1742 specific to a profile or the underlying application. 1744 Note that the Subject (sub) claim cannot always be verified when the 1745 token is submitted to the RS since the client may not have 1746 authenticated yet. Also note that a counter for the expires_in (exi) 1747 claim MUST be initialized when the RS first verifies this token. 1749 Also note that profiles of this framework may define access token 1750 transport mechanisms that do not allow for error responses. 1751 Therefore the error messages specified here only apply if the token 1752 was sent to the authz-info endpoint. 1754 When sending error responses, the RS MAY use the error codes from 1755 Section 3.1 of [RFC6750], to provide additional details to the 1756 client. 1758 5.8.1.2. Protecting the Authorization Information Endpoint 1760 As this framework can be used in RESTful environments, it is 1761 important to make sure that attackers cannot perform unauthorized 1762 requests on the authz-info endpoints, other than submitting access 1763 tokens. 1765 Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT 1766 on the authz-info endpoint and on it's children (if any). 1768 The POST method SHOULD NOT be allowed on children of the authz-info 1769 endpoint. 1771 The RS SHOULD implement rate limiting measures to mitigate attacks 1772 aiming to overload the processing capacity of the RS by repeatedly 1773 submitting tokens. For CoAP-based communication the RS could use the 1774 mechanisms from [RFC8516] to indicate that it is overloaded. 1776 5.8.2. Client Requests to the RS 1778 Before sending a request to an RS, the client MUST verify that the 1779 keys used to protect this communication are still valid. See 1780 Section 5.8.4 for details on how the client determines the validity 1781 of the keys used. 1783 If an RS receives a request from a client, and the target resource 1784 requires authorization, the RS MUST first verify that it has an 1785 access token that authorizes this request, and that the client has 1786 performed the proof-of-possession binding that token to the request. 1788 The response code MUST be 4.01 (Unauthorized) in case the client has 1789 not performed the proof-of-possession, or if RS has no valid access 1790 token for the client. If RS has an access token for the client but 1791 the token does not authorize access for the resource that was 1792 requested, RS MUST reject the request with a 4.03 (Forbidden). If RS 1793 has an access token for the client but it does not cover the action 1794 that was requested on the resource, RS MUST reject the request with a 1795 4.05 (Method Not Allowed). 1797 Note: The use of the response codes 4.03 and 4.05 is intended to 1798 prevent infinite loops where a dumb Client optimistically tries to 1799 access a requested resource with any access token received from AS. 1800 As malicious clients could pretend to be C to determine C's 1801 privileges, these detailed response codes must be used only when a 1802 certain level of security is already available which can be achieved 1803 only when the Client is authenticated. 1805 Note: The RS MAY use introspection for timely validation of an access 1806 token, at the time when a request is presented. 1808 Note: Matching the claims of the access token (e.g., scope) to a 1809 specific request is application specific. 1811 If the request matches a valid token and the client has performed the 1812 proof-of-possession for that token, the RS continues to process the 1813 request as specified by the underlying application. 1815 5.8.3. Token Expiration 1817 Depending on the capabilities of the RS, there are various ways in 1818 which it can verify the expiration of a received access token. Here 1819 follows a list of the possibilities including what functionality they 1820 require of the RS. 1822 o The token is a CWT and includes an "exp" claim and possibly the 1823 "nbf" claim. The RS verifies these by comparing them to values 1824 from its internal clock as defined in [RFC7519]. In this case the 1825 RS's internal clock must reflect the current date and time, or at 1826 least be synchronized with the AS's clock. How this clock 1827 synchronization would be performed is out of scope for this 1828 specification. 1830 o The RS verifies the validity of the token by performing an 1831 introspection request as specified in Section 5.7. This requires 1832 the RS to have a reliable network connection to the AS and to be 1833 able to handle two secure sessions in parallel (C to RS and RS to 1834 AS). 1836 o In order to support token expiration for devices that have no 1837 reliable way of synchronizing their internal clocks, this 1838 specification defines the following approach: The claim "exi" 1839 ("expires in") can be used, to provide the RS with the lifetime of 1840 the token in seconds from the time the RS first receives the 1841 token. For CBOR-based interaction this parameter is encoded as 1842 unsigned integer, while JSON-based interactions encode this as 1843 JSON number. 1845 o Processing this claim requires that the RS does the following: 1847 * For each token the RS receives, that contains an "exi" claim: 1848 Keep track of the time it received that token and revisit that 1849 list regularly to expunge expired tokens. 1851 * Keep track of the identifiers of tokens containing the "exi" 1852 claim that have expired (in order to avoid accepting them 1853 again). In order to avoid an unbounded memory usage growth, 1854 this MUST be implemented in the following way when the "exi" 1855 claim is used: 1857 + When creating the token, the AS MUST add a 'cti' claim ( or 1858 'jti' for JWTs) to the access token. The value of this 1859 claim MUST be created as the binary representation of the 1860 concatenation of the identifier of the RS with a sequence 1861 number counting the tokens containing an 'exi' claim, issued 1862 by this AS for the RS. 1864 + The RS MUST store the highest sequence number of an expired 1865 token containing the "exi" claim that it has seen, and treat 1866 tokens with lower sequence numbers as expired. 1868 If a token that authorizes a long running request such as a CoAP 1869 Observe [RFC7641] expires, the RS MUST send an error response with 1870 the response code equivalent to the CoAP code 4.01 (Unauthorized) to 1871 the client and then terminate processing the long running request. 1873 5.8.4. Key Expiration 1875 The AS provides the client with key material that the RS uses. This 1876 can either be a common symmetric PoP-key, or an asymmetric key used 1877 by the RS to authenticate towards the client. Since there is 1878 currently no expiration metadata associated to those keys, the client 1879 has no way of knowing if these keys are still valid. This may lead 1880 to situations where the client sends requests containing sensitive 1881 information to the RS using a key that is expired and possibly in the 1882 hands of an attacker, or accepts responses from the RS that are not 1883 properly protected and could possibly have been forged by an 1884 attacker. 1886 In order to prevent this, the client must assume that those keys are 1887 only valid as long as the related access token is. Since the access 1888 token is opaque to the client, one of the following methods MUST be 1889 used to inform the client about the validity of an access token: 1891 o The client knows a default validity time for all tokens it is 1892 using (i.e. how long a token is valid after being issued). This 1893 information could be provisioned to the client when it is 1894 registered at the AS, or published by the AS in a way that the 1895 client can query. 1897 o The AS informs the client about the token validity using the 1898 "expires_in" parameter in the Access Information. 1900 A client that is not able to obtain information about the expiration 1901 of a token MUST NOT use this token. 1903 6. Security Considerations 1905 Security considerations applicable to authentication and 1906 authorization in RESTful environments provided in OAuth 2.0 [RFC6749] 1907 apply to this work. Furthermore [RFC6819] provides additional 1908 security considerations for OAuth which apply to IoT deployments as 1909 well. If the introspection endpoint is used, the security 1910 considerations from [RFC7662] also apply. 1912 The following subsections address issues specific to this document 1913 and it's use in constrained environments. 1915 6.1. Protecting Tokens 1917 A large range of threats can be mitigated by protecting the contents 1918 of the access token by using a digital signature or a keyed message 1919 digest (MAC) or an Authenticated Encryption with Associated Data 1920 (AEAD) algorithm. Consequently, the token integrity protection MUST 1921 be applied to prevent the token from being modified, particularly 1922 since it contains a reference to the symmetric key or the asymmetric 1923 key used for proof-of-possession. If the access token contains the 1924 symmetric key, this symmetric key MUST be encrypted by the 1925 authorization server so that only the resource server can decrypt it. 1926 Note that using an AEAD algorithm is preferable over using a MAC 1927 unless the token needs to be publicly readable. 1929 If the token is intended for multiple recipients (i.e. an audience 1930 that is a group), integrity protection of the token with a symmetric 1931 key, shared between the AS and the recipients, is not sufficient, 1932 since any of the recipients could modify the token undetected by the 1933 other recipients. Therefore a token with a multi-recipient audience 1934 MUST be protected with an asymmetric signature. 1936 It is important for the authorization server to include the identity 1937 of the intended recipient (the audience), typically a single resource 1938 server (or a list of resource servers), in the token. The same 1939 shared secret MUST NOT be used as proof-of-possession key with 1940 multiple resource servers since the benefit from using the proof-of- 1941 possession concept is then significantly reduced. 1943 If clients are capable of doing so, they should frequently request 1944 fresh access tokens, as this allows the AS to keep the lifetime of 1945 the tokens short. This allows the AS to use shorter proof-of- 1946 possession key sizes, which translate to a performance benefit for 1947 the client and for the resource server. Shorter keys also lead to 1948 shorter messages (particularly with asymmetric keying material). 1950 When authorization servers bind symmetric keys to access tokens, they 1951 SHOULD scope these access tokens to a specific permission. 1953 In certain situations it may be necessary to revoke an access token 1954 that is still valid. Client-initiated revocation is specified in 1955 [RFC7009] for OAuth 2.0. Other revocation mechanisms are currently 1956 not specified, as the underlying assumption in OAuth is that access 1957 tokens are issued with a relatively short lifetime. This may not 1958 hold true for disconnected constrained devices, needing access tokens 1959 with relatively long lifetimes, and would therefore necessitate 1960 further standardization work that is out of scope for this document. 1962 6.2. Communication Security 1964 Communication with the authorization server MUST use confidentiality 1965 protection. This step is extremely important since the client or the 1966 RS may obtain the proof-of-possession key from the authorization 1967 server for use with a specific access token. Not using 1968 confidentiality protection exposes this secret (and the access token) 1969 to an eavesdropper thereby completely negating proof-of-possession 1970 security. Profiles MUST specify how communication security according 1971 to the requirements in Section 5 is provided. 1973 Additional protection for the access token can be applied by 1974 encrypting it, for example encryption of CWTs is specified in 1975 Section 5.1 of [RFC8392]. Such additional protection can be 1976 necessary if the token is later transferred over an insecure 1977 connection (e.g. when it is sent to the authz-info endpoint). 1979 Developers MUST ensure that the ephemeral credentials (i.e., the 1980 private key or the session key) are not leaked to third parties. An 1981 adversary in possession of the ephemeral credentials bound to the 1982 access token will be able to impersonate the client. Be aware that 1983 this is a real risk with many constrained environments, since 1984 adversaries can often easily get physical access to the devices. 1985 This risk can also be mitigated to some extent by making sure that 1986 keys are refreshed more frequently. 1988 6.3. Long-Term Credentials 1990 Both clients and RSs have long-term credentials that are used to 1991 secure communications, and authenticate to the AS. These credentials 1992 need to be protected against unauthorized access. In constrained 1993 devices, deployed in publicly accessible places, such protection can 1994 be difficult to achieve without specialized hardware (e.g. secure key 1995 storage memory). 1997 If credentials are lost or compromised, the operator of the affected 1998 devices needs to have procedures to invalidate any access these 1999 credentials give and to revoke tokens linked to such credentials. 2000 The loss of a credential linked to a specific device MUST NOT lead to 2001 a compromise of other credentials not linked to that device, 2002 therefore secret keys used for authentication MUST NOT be shared 2003 between more than two parties. 2005 Operators of clients or RS SHOULD have procedures in place to replace 2006 credentials that are suspected to have been compromised or that have 2007 been lost. 2009 Operators also SHOULD have procedures for decommissioning devices, 2010 that include securely erasing credentials and other security critical 2011 material in the devices being decommissioned. 2013 6.4. Unprotected AS Request Creation Hints 2015 Initially, no secure channel exists to protect the communication 2016 between C and RS. Thus, C cannot determine if the AS Request 2017 Creation Hints contained in an unprotected response from RS to an 2018 unauthorized request (see Section 5.1.2) are authentic. It is 2019 therefore advisable to provide C with a (possibly hard-coded) list of 2020 trustworthy authorization servers, possibly including information 2021 used to authenticate the AS, such as a public key or certificate 2022 fingerprint. AS Request Creation Hints referring to a URI not listed 2023 there would be ignored. 2025 A compromised RS may use the hints to trick a client into contacting 2026 an AS that is not supposed to be in charge of that RS. Since this AS 2027 must be in the hard-coded list of trusted AS no violation of 2028 privileges and or exposure of credentials should happen, since a 2029 trusted AS is expected to refuse requestes for which it is not 2030 applicable and render a corresponding error response. However a 2031 compromised RS may use this to perform a denial of service against a 2032 specific AS, by redirecting a large number of client requests to that 2033 AS. 2035 A compromised client can be made to contact any AS, including 2036 compromised ones. This should not affect the RS, since it is 2037 supposed to keep track of which AS are trusted and have corresponding 2038 credentials to verify the source of access tokens it receives. 2040 6.5. Minimal security requirements for communication 2042 This section summarizes the minimal requirements for the 2043 communication security of the different protocol interactions. 2045 C-AS All communication between the client and the Authorization 2046 Server MUST be encrypted, integrity and replay protected. 2047 Furthermore responses from the AS to the client MUST be bound to 2048 the client's request to avoid attacks where the attacker swaps the 2049 intended response for an older one valid for a previous request. 2050 This requires that the client and the Authorization Server have 2051 previously exchanged either a shared secret or their public keys 2052 in order to negotiate a secure communication. Furthermore the 2053 client MUST be able to determine whether an AS has the authority 2054 to issue access tokens for a certain RS. This can for example be 2055 done through pre-configured lists, or through an online lookup 2056 mechanism that in turn also must be secured. 2058 RS-AS The communication between the Resource Server and the 2059 Authorization Server via the introspection endpoint MUST be 2060 encrypted, integrity and replay protected. Furthermore responses 2061 from the AS to the RS MUST be bound to the RS's request. This 2062 requires that the RS and the Authorization Server have previously 2063 exchanged either a shared secret, or their public keys in order to 2064 negotiate a secure communication. Furthermore the RS MUST be able 2065 to determine whether an AS has the authority to issue access 2066 tokens itself. This is usually configured out of band, but could 2067 also be performed through an online lookup mechanism provided that 2068 it is also secured in the same way. 2070 C-RS The initial communication between the client and the Resource 2071 Server can not be secured in general, since the RS is not in 2072 possession of on access token for that client, which would carry 2073 the necessary parameters. If both parties support DTLS without 2074 client authentication it is RECOMMEND to use this mechanism for 2075 protecting the initial communication. After the client has 2076 successfully transmitted the access token to the RS, a secure 2077 communication protocol MUST be established between client and RS 2078 for the actual resource request. This protocol MUST provide 2079 confidentiality, integrity and replay protection as well as a 2080 binding between requests and responses. This requires that the 2081 client learned either the RS's public key or received a symmetric 2082 proof-of-possession key bound to the access token from the AS. 2083 The RS must have learned either the client's public key or a 2084 shared symmetric key from the claims in the token or an 2085 introspection request. Since ACE does not provide profile 2086 negotiation between C and RS, the client MUST have learned what 2087 profile the RS supports (e.g. from the AS or pre-configured) and 2088 initiate the communication accordingly. 2090 6.6. Token Freshness and Expiration 2092 An RS that is offline faces the problem of clock drift. Since it 2093 cannot synchronize its clock with the AS, it may be tricked into 2094 accepting old access tokens that are no longer valid or have been 2095 compromised. In order to prevent this, an RS may use the nonce-based 2096 mechanism defined in Section 5.1.2 to ensure freshness of an Access 2097 Token subsequently presented to this RS. 2099 Another problem with clock drift is that evaluating the standard 2100 token expiration claim "exp" can give unpredictable results. 2102 Acceptable ranges of clock drift are highly dependent on the concrete 2103 application. Important factors are how long access tokens are valid, 2104 and how critical timely expiration of access token is. 2106 The expiration mechanism implemented by the "exi" claim, based on the 2107 first time the RS sees the token was defined to provide a more 2108 predictable alternative. The "exi" approach has some drawbacks that 2109 need to be considered: 2111 A malicious client may hold back tokens with the "exi" claim in 2112 order to prolong their lifespan. 2114 If an RS loses state (e.g. due to an unscheduled reboot), it may 2115 loose the current values of counters tracking the "exi" claims of 2116 tokens it is storing. 2118 The first drawback is inherent to the deployment scenario and the 2119 "exi" solution. It can therefore not be mitigated without requiring 2120 the the RS be online at times. The second drawback can be mitigated 2121 by regularly storing the value of "exi" counters to persistent 2122 memory. 2124 6.7. Combining profiles 2126 There may be use cases were different profiles of this framework are 2127 combined. For example, an MQTT-TLS profile is used between the 2128 client and the RS in combination with a CoAP-DTLS profile for 2129 interactions between the client and the AS. The security of a 2130 profile MUST NOT depend on the assumption that the profile is used 2131 for all the different types of interactions in this framework. 2133 6.8. Unprotected Information 2135 Communication with the authz-info endpoint, as well as the various 2136 error responses defined in this framework, all potentially include 2137 sending information over an unprotected channel. These messages may 2138 leak information to an adversary, or may be manipulated by active 2139 attackers to induce incorrect behavior. For example error responses 2140 for requests to the Authorization Information endpoint can reveal 2141 information about an otherwise opaque access token to an adversary 2142 who has intercepted this token. 2144 As far as error messages are concerned, this framework is written 2145 under the assumption that, in general, the benefits of detailed error 2146 messages outweigh the risk due to information leakage. For 2147 particular use cases, where this assessment does not apply, detailed 2148 error messages can be replaced by more generic ones. 2150 In some scenarios it may be possible to protect the communication 2151 with the authz-info endpoint (e.g. through DTLS with only server-side 2152 authentication). In cases where this is not possible this framework 2153 RECOMMENDS to use encrypted CWTs or tokens that are opaque references 2154 and need to be subjected to introspection by the RS. 2156 If the initial unauthorized resource request message (see 2157 Section 5.1.1) is used, the client MUST make sure that it is not 2158 sending sensitive content in this request. While GET and DELETE 2159 requests only reveal the target URI of the resource, POST and PUT 2160 requests would reveal the whole payload of the intended operation. 2162 Since the client is not authenticated at the point when it is 2163 submitting an access token to the authz-info endpoint, attackers may 2164 be pretending to be a client and trying to trick an RS to use an 2165 obsolete profile that in turn specifies a vulnerable security 2166 mechanism via the authz-info endpoint. Such an attack would require 2167 a valid access token containing an "ace_profile" claim requesting the 2168 use of said obsolete profile. Resource Owners should update the 2169 configuration of their RS's to prevent them from using such obsolete 2170 profiles. 2172 6.9. Identifying audiences 2174 The audience claim as defined in [RFC7519] and the equivalent 2175 "audience" parameter from [RFC8693] are intentionally vague on how to 2176 match the audience value to a specific RS. This is intended to allow 2177 application specific semantics to be used. This section attempts to 2178 give some general guidance for the use of audiences in constrained 2179 environments. 2181 URLs are not a good way of identifying mobile devices that can switch 2182 networks and thus be associated with new URLs. If the audience 2183 represents a single RS, and asymmetric keys are used, the RS can be 2184 uniquely identified by a hash of its public key. If this approach is 2185 used this framework RECOMMENDS to apply the procedure from section 3 2186 of [RFC6920]. 2188 If the audience addresses a group of resource servers, the mapping of 2189 group identifier to individual RS has to be provisioned to each RS 2190 before the group-audience is usable. Managing dynamic groups could 2191 be an issue, if any RS is not always reachable when the groups' 2192 memberships change. Furthermore, issuing access tokens bound to 2193 symmetric proof-of-possession keys that apply to a group-audience is 2194 problematic, as an RS that is in possession of the access token can 2195 impersonate the client towards the other RSs that are part of the 2196 group. It is therefore NOT RECOMMENDED to issue access tokens bound 2197 to a group audience and symmetric proof-of possession keys. 2199 Even the client must be able to determine the correct values to put 2200 into the "audience" parameter, in order to obtain a token for the 2201 intended RS. Errors in this process can lead to the client 2202 inadvertently obtaining a token for the wrong RS. The correct values 2203 for "audience" can either be provisioned to the client as part of its 2204 configuration, or dynamically looked up by the client in some 2205 directory. In the latter case the integrity and correctness of the 2206 directory data must be assured. Note that the "audience" hint 2207 provided by the RS as part of the "AS Request Creation Hints" 2208 Section 5.1.2 is not typically source authenticated and integrity 2209 protected, and should therefore not be treated a trusted value. 2211 6.10. Denial of service against or with Introspection 2213 The optional introspection mechanism provided by OAuth and supported 2214 in the ACE framework allows for two types of attacks that need to be 2215 considered by implementers. 2217 First, an attacker could perform a denial of service attack against 2218 the introspection endpoint at the AS in order to prevent validation 2219 of access tokens. To maintain the security of the system, an RS that 2220 is configured to use introspection MUST NOT allow access based on a 2221 token for which it couldn't reach the introspection endpoint. 2223 Second, an attacker could use the fact that an RS performs 2224 introspection to perform a denial of service attack against that RS 2225 by repeatedly sending tokens to its authz-info endpoint that require 2226 an introspection call. RS can mitigate such attacks by implementing 2227 rate limits on how many introspection requests they perform in a 2228 given time interval for a certain client IP address submitting tokens 2229 to /authz-info. When that limit has been reached, incoming requests 2230 from that address are rejected for a certain amount of time. A 2231 general rate limit on the introspection requests should also be 2232 considered, to mitigate distributed attacks. 2234 7. Privacy Considerations 2236 Implementers and users should be aware of the privacy implications of 2237 the different possible deployments of this framework. 2239 The AS is in a very central position and can potentially learn 2240 sensitive information about the clients requesting access tokens. If 2241 the client credentials grant is used, the AS can track what kind of 2242 access the client intends to perform. With other grants this can be 2243 prevented by the Resource Owner. To do so, the resource owner needs 2244 to bind the grants it issues to anonymous, ephemeral credentials that 2245 do not allow the AS to link different grants and thus different 2246 access token requests by the same client. 2248 The claims contained in a token can reveal privacy sensitive 2249 information about the client and the RS to any party having access to 2250 them (whether by processing the content of a self-contained token or 2251 by introspection). The AS SHOULD be configured to minimize the 2252 information about clients and RSs disclosed in the tokens it issues. 2254 If tokens are only integrity protected and not encrypted, they may 2255 reveal information to attackers listening on the wire, or able to 2256 acquire the access tokens in some other way. In the case of CWTs the 2257 token may, e.g., reveal the audience, the scope and the confirmation 2258 method used by the client. The latter may reveal the identity of the 2259 device or application running the client. This may be linkable to 2260 the identity of the person using the client (if there is a person and 2261 not a machine-to-machine interaction). 2263 Clients using asymmetric keys for proof-of-possession should be aware 2264 of the consequences of using the same key pair for proof-of- 2265 possession towards different RSs. A set of colluding RSs or an 2266 attacker able to obtain the access tokens will be able to link the 2267 requests, or even to determine the client's identity. 2269 An unprotected response to an unauthorized request (see 2270 Section 5.1.2) may disclose information about RS and/or its existing 2271 relationship with C. It is advisable to include as little 2272 information as possible in an unencrypted response. If means of 2273 encrypting communication between C and RS already exist, more 2274 detailed information may be included with an error response to 2275 provide C with sufficient information to react on that particular 2276 error. 2278 8. IANA Considerations 2280 This document creates several registries with a registration policy 2281 of "Expert Review"; guidelines to the experts are given in 2282 Section 8.17. 2284 8.1. ACE Authorization Server Request Creation Hints 2286 This specification establishes the IANA "ACE Authorization Server 2287 Request Creation Hints" registry. The registry has been created to 2288 use the "Expert Review" registration procedure [RFC8126]. It should 2289 be noted that, in addition to the expert review, some portions of the 2290 registry require a specification, potentially a Standards Track RFC, 2291 be supplied as well. 2293 The columns of the registry are: 2295 Name The name of the parameter 2297 CBOR Key CBOR map key for the parameter. Different ranges of values 2298 use different registration policies [RFC8126]. Integer values 2299 from -256 to 255 are designated as Standards Action. Integer 2300 values from -65536 to -257 and from 256 to 65535 are designated as 2301 Specification Required. Integer values greater than 65535 are 2302 designated as Expert Review. Integer values less than -65536 are 2303 marked as Private Use. 2305 Value Type The CBOR data types allowable for the values of this 2306 parameter. 2308 Reference This contains a pointer to the public specification of the 2309 request creation hint abbreviation, if one exists. 2311 This registry will be initially populated by the values in Figure 2. 2312 The Reference column for all of these entries will be this document. 2314 8.2. CoRE Resource Type registry 2316 IANA is requested to register a new Resource Type (rt=) Link Target 2317 Attribute in the "Resource Type (rt=) Link Target Attribute Values" 2318 subregistry under the "Constrained RESTful Environments (CoRE) 2319 Parameters" [IANA.CoreParameters] registry: 2321 rt="ace.ai". This resource type describes an ACE-OAuth authz-info 2322 endpoint resource. 2324 Specific ACE-OAuth profiles can use this common resource type for 2325 defining their profile-specific discovery processes. 2327 8.3. OAuth Extensions Error Registration 2329 This specification registers the following error values in the OAuth 2330 Extensions Error registry [IANA.OAuthExtensionsErrorRegistry]. 2332 o Error name: "unsupported_pop_key" 2333 o Error usage location: token error response 2334 o Related protocol extension: [this document] 2335 o Change Controller: IESG 2336 o Specification document(s): Section 5.6.3 of [this document] 2338 o Error name: "incompatible_ace_profiles" 2339 o Error usage location: token error response 2340 o Related protocol extension: [this document] 2341 o Change Controller: IESG 2342 o Specification document(s): Section 5.6.3 of [this document] 2344 8.4. OAuth Error Code CBOR Mappings Registry 2346 This specification establishes the IANA "OAuth Error Code CBOR 2347 Mappings" registry. The registry has been created to use the "Expert 2348 Review" registration procedure [RFC8126], except for the value range 2349 designated for private use. 2351 The columns of the registry are: 2353 Name The OAuth Error Code name, refers to the name in Section 5.2. 2354 of [RFC6749], e.g., "invalid_request". 2355 CBOR Value CBOR abbreviation for this error code. Integer values 2356 less than -65536 are marked as "Private Use", all other values use 2357 the registration policy "Expert Review" [RFC8126]. 2358 Reference This contains a pointer to the public specification of the 2359 error code abbreviation, if one exists. 2361 This registry will be initially populated by the values in Figure 10. 2362 The Reference column for all of these entries will be this document. 2364 8.5. OAuth Grant Type CBOR Mappings 2366 This specification establishes the IANA "OAuth Grant Type CBOR 2367 Mappings" registry. The registry has been created to use the "Expert 2368 Review" registration procedure [RFC8126], except for the value range 2369 designated for private use. 2371 The columns of this registry are: 2373 Name The name of the grant type as specified in Section 1.3 of 2374 [RFC6749]. 2375 CBOR Value CBOR abbreviation for this grant type. Integer values 2376 less than -65536 are marked as "Private Use", all other values use 2377 the registration policy "Expert Review" [RFC8126]. 2378 Reference This contains a pointer to the public specification of the 2379 grant type abbreviation, if one exists. 2380 Original Specification This contains a pointer to the public 2381 specification of the grant type, if one exists. 2383 This registry will be initially populated by the values in Figure 11. 2384 The Reference column for all of these entries will be this document. 2386 8.6. OAuth Access Token Types 2388 This section registers the following new token type in the "OAuth 2389 Access Token Types" registry [IANA.OAuthAccessTokenTypes]. 2391 o Type name: "PoP" 2392 o Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see 2393 section 3.3 of [I-D.ietf-ace-oauth-params]. 2394 o HTTP Authentication Scheme(s): N/A 2395 o Change Controller: IETF 2396 o Specification document(s): [this document] 2398 8.7. OAuth Access Token Type CBOR Mappings 2400 This specification established the IANA "OAuth Access Token Type CBOR 2401 Mappings" registry. The registry has been created to use the "Expert 2402 Review" registration procedure [RFC8126], except for the value range 2403 designated for private use. 2405 The columns of this registry are: 2407 Name The name of token type as registered in the OAuth Access Token 2408 Types registry, e.g., "Bearer". 2410 CBOR Value CBOR abbreviation for this token type. Integer values 2411 less than -65536 are marked as "Private Use", all other values use 2412 the registration policy "Expert Review" [RFC8126]. 2413 Reference This contains a pointer to the public specification of the 2414 OAuth token type abbreviation, if one exists. 2415 Original Specification This contains a pointer to the public 2416 specification of the OAuth token type, if one exists. 2418 8.7.1. Initial Registry Contents 2420 o Name: "Bearer" 2421 o Value: 1 2422 o Reference: [this document] 2423 o Original Specification: [RFC6749] 2425 o Name: "PoP" 2426 o Value: 2 2427 o Reference: [this document] 2428 o Original Specification: [this document] 2430 8.8. ACE Profile Registry 2432 This specification establishes the IANA "ACE Profile" registry. The 2433 registry has been created to use the "Expert Review" registration 2434 procedure [RFC8126]. It should be noted that, in addition to the 2435 expert review, some portions of the registry require a specification, 2436 potentially a Standards Track RFC, be supplied as well. 2438 The columns of this registry are: 2440 Name The name of the profile, to be used as value of the profile 2441 attribute. 2442 Description Text giving an overview of the profile and the context 2443 it is developed for. 2444 CBOR Value CBOR abbreviation for this profile name. Different 2445 ranges of values use different registration policies [RFC8126]. 2446 Integer values from -256 to 255 are designated as Standards 2447 Action. Integer values from -65536 to -257 and from 256 to 65535 2448 are designated as Specification Required. Integer values greater 2449 than 65535 are designated as "Expert Review". Integer values less 2450 than -65536 are marked as Private Use. 2451 Reference This contains a pointer to the public specification of the 2452 profile abbreviation, if one exists. 2454 This registry will be initially empty and will be populated by the 2455 registrations from the ACE framework profiles. 2457 8.9. OAuth Parameter Registration 2459 This specification registers the following parameter in the "OAuth 2460 Parameters" registry [IANA.OAuthParameters]: 2462 o Name: "ace_profile" 2463 o Parameter Usage Location: token response 2464 o Change Controller: IESG 2465 o Reference: Section 5.6.2 and Section 5.6.4.3 of [this document] 2467 8.10. OAuth Parameters CBOR Mappings Registry 2469 This specification establishes the IANA "OAuth Parameters CBOR 2470 Mappings" registry. The registry has been created to use the "Expert 2471 Review" registration procedure [RFC8126], except for the value range 2472 designated for private use. 2474 The columns of this registry are: 2476 Name The OAuth Parameter name, refers to the name in the OAuth 2477 parameter registry, e.g., "client_id". 2478 CBOR Key CBOR map key for this parameter. Integer values less than 2479 -65536 are marked as "Private Use", all other values use the 2480 registration policy "Expert Review" [RFC8126]. 2481 Value Type The allowable CBOR data types for values of this 2482 parameter. 2483 Reference This contains a pointer to the public specification of the 2484 OAuth parameter abbreviation, if one exists. 2486 This registry will be initially populated by the values in Figure 12. 2487 The Reference column for all of these entries will be this document. 2489 8.11. OAuth Introspection Response Parameter Registration 2491 This specification registers the following parameters in the OAuth 2492 Token Introspection Response registry 2493 [IANA.TokenIntrospectionResponse]. 2495 o Name: "ace_profile" 2496 o Description: The ACE profile used between client and RS. 2497 o Change Controller: IESG 2498 o Reference: Section 5.7.2 of [this document] 2500 o Name: "cnonce" 2501 o Description: "client-nonce". A nonce previously provided to the 2502 AS by the RS via the client. Used to verify token freshness when 2503 the RS cannot synchronize its clock with the AS. 2504 o Change Controller: IESG 2505 o Reference: Section 5.7.2 of [this document] 2507 o Name: "exi" 2508 o Description: "Expires in". Lifetime of the token in seconds from 2509 the time the RS first sees it. Used to implement a weaker from of 2510 token expiration for devices that cannot synchronize their 2511 internal clocks. 2512 o Change Controller: IESG 2513 o Reference: Section 5.7.2 of [this document] 2515 8.12. OAuth Token Introspection Response CBOR Mappings Registry 2517 This specification establishes the IANA "OAuth Token Introspection 2518 Response CBOR Mappings" registry. The registry has been created to 2519 use the "Expert Review" registration procedure [RFC8126], except for 2520 the value range designated for private use. 2522 The columns of this registry are: 2524 Name The OAuth Parameter name, refers to the name in the OAuth 2525 parameter registry, e.g., "client_id". 2526 CBOR Key CBOR map key for this parameter. Integer values less than 2527 -65536 are marked as "Private Use", all other values use the 2528 registration policy "Expert Review" [RFC8126]. 2529 Value Type The allowable CBOR data types for values of this 2530 parameter. 2531 Reference This contains a pointer to the public specification of the 2532 introspection response parameter abbreviation, if one exists. 2534 This registry will be initially populated by the values in Figure 16. 2535 The Reference column for all of these entries will be this document. 2537 Note that the mappings of parameters corresponding to claim names 2538 intentionally coincide with the CWT claim name mappings from 2539 [RFC8392]. 2541 8.13. JSON Web Token Claims 2543 This specification registers the following new claims in the JSON Web 2544 Token (JWT) registry of JSON Web Token Claims 2545 [IANA.JsonWebTokenClaims]: 2547 o Claim Name: "ace_profile" 2548 o Claim Description: The ACE profile a token is supposed to be used 2549 with. 2550 o Change Controller: IESG 2551 o Reference: Section 5.8 of [this document] 2552 o Claim Name: "cnonce" 2553 o Claim Description: "client-nonce". A nonce previously provided to 2554 the AS by the RS via the client. Used to verify token freshness 2555 when the RS cannot synchronize its clock with the AS. 2556 o Change Controller: IESG 2557 o Reference: Section 5.8 of [this document] 2559 o Claim Name: "exi" 2560 o Claim Description: "Expires in". Lifetime of the token in seconds 2561 from the time the RS first sees it. Used to implement a weaker 2562 from of token expiration for devices that cannot synchronize their 2563 internal clocks. 2564 o Change Controller: IESG 2565 o Reference: Section 5.8.3 of [this document] 2567 8.14. CBOR Web Token Claims 2569 This specification registers the following new claims in the "CBOR 2570 Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims]. 2572 o Claim Name: "ace_profile" 2573 o Claim Description: The ACE profile a token is supposed to be used 2574 with. 2575 o JWT Claim Name: ace_profile 2576 o Claim Key: TBD (suggested: 38) 2577 o Claim Value Type(s): integer 2578 o Change Controller: IESG 2579 o Specification Document(s): Section 5.8 of [this document] 2581 o Claim Name: "cnonce" 2582 o Claim Description: The client-nonce sent to the AS by the RS via 2583 the client. 2584 o JWT Claim Name: cnonce 2585 o Claim Key: TBD (suggested: 39) 2586 o Claim Value Type(s): byte string 2587 o Change Controller: IESG 2588 o Specification Document(s): Section 5.8 of [this document] 2590 o Claim Name: "exi" 2591 o Claim Description: The expiration time of a token measured from 2592 when it was received at the RS in seconds. 2593 o JWT Claim Name: exi 2594 o Claim Key: TBD (suggested: 40) 2595 o Claim Value Type(s): integer 2596 o Change Controller: IESG 2597 o Specification Document(s): Section 5.8.3 of [this document] 2599 o Claim Name: "scope" 2600 o Claim Description: The scope of an access token as defined in 2601 [RFC6749]. 2602 o JWT Claim Name: scope 2603 o Claim Key: TBD (suggested: 42) 2604 o Claim Value Type(s): byte string or text string 2605 o Change Controller: IESG 2606 o Specification Document(s): Section 4.2 of [RFC8693] 2608 8.15. Media Type Registrations 2610 This specification registers the 'application/ace+cbor' media type 2611 for messages of the protocols defined in this document carrying 2612 parameters encoded in CBOR. This registration follows the procedures 2613 specified in [RFC6838]. 2615 Type name: application 2617 Subtype name: ace+cbor 2619 Required parameters: N/A 2621 Optional parameters: N/A 2623 Encoding considerations: Must be encoded as CBOR map containing the 2624 protocol parameters defined in [this document]. 2626 Security considerations: See Section 6 of [this document] 2628 Interoperability considerations: N/A 2630 Published specification: [this document] 2632 Applications that use this media type: The type is used by 2633 authorization servers, clients and resource servers that support the 2634 ACE framework as specified in [this document]. 2636 Fragment identifier considerations: N/A 2638 Additional information: N/A 2640 Person & email address to contact for further information: 2641 2643 Intended usage: COMMON 2645 Restrictions on usage: none 2647 Author: Ludwig Seitz 2648 Change controller: IESG 2650 8.16. CoAP Content-Format Registry 2652 This specification registers the following entry to the "CoAP 2653 Content-Formats" registry: 2655 Media Type: application/ace+cbor 2657 Encoding: - 2659 ID: TBD (suggested: 19) 2661 Reference: [this document] 2663 8.17. Expert Review Instructions 2665 All of the IANA registries established in this document are defined 2666 to use a registration policy of Expert Review. This section gives 2667 some general guidelines for what the experts should be looking for, 2668 but they are being designated as experts for a reason, so they should 2669 be given substantial latitude. 2671 Expert reviewers should take into consideration the following points: 2673 o Point squatting should be discouraged. Reviewers are encouraged 2674 to get sufficient information for registration requests to ensure 2675 that the usage is not going to duplicate one that is already 2676 registered, and that the point is likely to be used in 2677 deployments. The zones tagged as private use are intended for 2678 testing purposes and closed environments; code points in other 2679 ranges should not be assigned for testing. 2680 o Specifications are needed for the first-come, first-serve range if 2681 they are expected to be used outside of closed environments in an 2682 interoperable way. When specifications are not provided, the 2683 description provided needs to have sufficient information to 2684 identify what the point is being used for. 2685 o Experts should take into account the expected usage of fields when 2686 approving point assignment. The fact that there is a range for 2687 standards track documents does not mean that a standards track 2688 document cannot have points assigned outside of that range. The 2689 length of the encoded value should be weighed against how many 2690 code points of that length are left, the size of device it will be 2691 used on. 2692 o Since a high degree of overlap is expected between these 2693 registries and the contents of the OAuth parameters 2694 [IANA.OAuthParameters] registries, experts should require new 2695 registrations to maintain alignment with parameters from OAuth 2696 that have comparable functionality. Deviation from this alignment 2697 should only be allowed if there are functional differences, that 2698 are motivated by the use case and that cannot be easily or 2699 efficiently addressed by comparable OAuth parameters. 2701 9. Acknowledgments 2703 This document is a product of the ACE working group of the IETF. 2705 Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and 2706 UMA in IoT scenarios, Robert Taylor for his discussion input, and 2707 Malisa Vucinic for his input on the predecessors of this proposal. 2709 Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from 2710 where large parts of the security considerations where copied. 2712 Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for 2713 contributing their work on AS discovery from draft-gerdes-ace-dcaf- 2714 authorize (see Section 5.1). 2716 Thanks to Jim Schaad and Mike Jones for their comprehensive reviews. 2718 Thanks to Benjamin Kaduk for his input on various questions related 2719 to this work. 2721 Thanks to Cigdem Sengul for some very useful review comments. 2723 Thanks to Carsten Bormann for contributing the text for the CoRE 2724 Resource Type registry. 2726 Ludwig Seitz and Goeran Selander worked on this document as part of 2727 the CelticPlus project CyberWI, with funding from Vinnova. Ludwig 2728 Seitz was also received further funding for this work by Vinnova in 2729 the context of the CelticNext project Critisec. 2731 10. References 2733 10.1. Normative References 2735 [I-D.ietf-ace-oauth-params] 2736 Seitz, L., "Additional OAuth Parameters for Authorization 2737 in Constrained Environments (ACE)", draft-ietf-ace-oauth- 2738 params-13 (work in progress), April 2020. 2740 [IANA.CborWebTokenClaims] 2741 IANA, "CBOR Web Token (CWT) Claims", 2742 . 2745 [IANA.CoreParameters] 2746 IANA, "Constrained RESTful Environments (CoRE) 2747 Parameters", . 2750 [IANA.JsonWebTokenClaims] 2751 IANA, "JSON Web Token Claims", 2752 . 2754 [IANA.OAuthAccessTokenTypes] 2755 IANA, "OAuth Access Token Types", 2756 . 2759 [IANA.OAuthExtensionsErrorRegistry] 2760 IANA, "OAuth Extensions Error Registry", 2761 . 2764 [IANA.OAuthParameters] 2765 IANA, "OAuth Parameters", 2766 . 2769 [IANA.TokenIntrospectionResponse] 2770 IANA, "OAuth Token Introspection Response", 2771 . 2774 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2775 Requirement Levels", BCP 14, RFC 2119, 2776 DOI 10.17487/RFC2119, March 1997, 2777 . 2779 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2780 Resource Identifier (URI): Generic Syntax", STD 66, 2781 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2782 . 2784 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 2785 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 2786 . 2788 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 2789 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 2790 January 2012, . 2792 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 2793 RFC 6749, DOI 10.17487/RFC6749, October 2012, 2794 . 2796 [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization 2797 Framework: Bearer Token Usage", RFC 6750, 2798 DOI 10.17487/RFC6750, October 2012, 2799 . 2801 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2802 Specifications and Registration Procedures", BCP 13, 2803 RFC 6838, DOI 10.17487/RFC6838, January 2013, 2804 . 2806 [RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B., 2807 Keranen, A., and P. Hallam-Baker, "Naming Things with 2808 Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013, 2809 . 2811 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object 2812 Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, 2813 October 2013, . 2815 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 2816 Application Protocol (CoAP)", RFC 7252, 2817 DOI 10.17487/RFC7252, June 2014, 2818 . 2820 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 2821 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 2822 . 2824 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 2825 RFC 7662, DOI 10.17487/RFC7662, October 2015, 2826 . 2828 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2829 Writing an IANA Considerations Section in RFCs", BCP 26, 2830 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2831 . 2833 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 2834 RFC 8152, DOI 10.17487/RFC8152, July 2017, 2835 . 2837 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2838 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2839 May 2017, . 2841 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 2842 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 2843 May 2018, . 2845 [RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J., 2846 and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693, 2847 DOI 10.17487/RFC8693, January 2020, 2848 . 2850 [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 2851 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 2852 Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 2853 2020, . 2855 10.2. Informative References 2857 [BLE] Bluetooth SIG, "Bluetooth Core Specification v5.1", 2858 Section 4.4, January 2019, 2859 . 2862 [I-D.erdtman-ace-rpcc] 2863 Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared- 2864 Key as OAuth client credentials", draft-erdtman-ace- 2865 rpcc-02 (work in progress), October 2017. 2867 [I-D.ietf-quic-transport] 2868 Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed 2869 and Secure Transport", draft-ietf-quic-transport-29 (work 2870 in progress), June 2020. 2872 [I-D.ietf-tls-dtls13] 2873 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 2874 Datagram Transport Layer Security (DTLS) Protocol Version 2875 1.3", draft-ietf-tls-dtls13-38 (work in progress), May 2876 2020. 2878 [Margi10impact] 2879 Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr, 2880 M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold, 2881 "Impact of Operating Systems on Wireless Sensor Networks 2882 (Security) Applications and Testbeds", Proceedings of 2883 the 19th International Conference on Computer 2884 Communications and Networks (ICCCN), August 2010. 2886 [MQTT5.0] Banks, A., Briggs, E., Borgendale, K., and R. Gupta, "MQTT 2887 Version 5.0", OASIS Standard, March 2019, 2888 . 2891 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 2892 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 2893 . 2895 [RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0 2896 Threat Model and Security Considerations", RFC 6819, 2897 DOI 10.17487/RFC6819, January 2013, 2898 . 2900 [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth 2901 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, 2902 August 2013, . 2904 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2905 Constrained-Node Networks", RFC 7228, 2906 DOI 10.17487/RFC7228, May 2014, 2907 . 2909 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2910 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2911 DOI 10.17487/RFC7231, June 2014, 2912 . 2914 [RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland, 2915 "Assertion Framework for OAuth 2.0 Client Authentication 2916 and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521, 2917 May 2015, . 2919 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 2920 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 2921 DOI 10.17487/RFC7540, May 2015, 2922 . 2924 [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and 2925 P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", 2926 RFC 7591, DOI 10.17487/RFC7591, July 2015, 2927 . 2929 [RFC7641] Hartke, K., "Observing Resources in the Constrained 2930 Application Protocol (CoAP)", RFC 7641, 2931 DOI 10.17487/RFC7641, September 2015, 2932 . 2934 [RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M., 2935 and S. Kumar, "Use Cases for Authentication and 2936 Authorization in Constrained Environments", RFC 7744, 2937 DOI 10.17487/RFC7744, January 2016, 2938 . 2940 [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 2941 the Constrained Application Protocol (CoAP)", RFC 7959, 2942 DOI 10.17487/RFC7959, August 2016, 2943 . 2945 [RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", 2946 BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017, 2947 . 2949 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2950 Interchange Format", STD 90, RFC 8259, 2951 DOI 10.17487/RFC8259, December 2017, 2952 . 2954 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 2955 Authorization Server Metadata", RFC 8414, 2956 DOI 10.17487/RFC8414, June 2018, 2957 . 2959 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2960 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2961 . 2963 [RFC8516] Keranen, A., ""Too Many Requests" Response Code for the 2964 Constrained Application Protocol", RFC 8516, 2965 DOI 10.17487/RFC8516, January 2019, 2966 . 2968 [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 2969 "Object Security for Constrained RESTful Environments 2970 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 2971 . 2973 [RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig, 2974 "OAuth 2.0 Device Authorization Grant", RFC 8628, 2975 DOI 10.17487/RFC8628, August 2019, 2976 . 2978 Appendix A. Design Justification 2980 This section provides further insight into the design decisions of 2981 the solution documented in this document. Section 3 lists several 2982 building blocks and briefly summarizes their importance. The 2983 justification for offering some of those building blocks, as opposed 2984 to using OAuth 2.0 as is, is given below. 2986 Common IoT constraints are: 2988 Low Power Radio: 2990 Many IoT devices are equipped with a small battery which needs to 2991 last for a long time. For many constrained wireless devices, the 2992 highest energy cost is associated to transmitting or receiving 2993 messages (roughly by a factor of 10 compared to AES) 2994 [Margi10impact]. It is therefore important to keep the total 2995 communication overhead low, including minimizing the number and 2996 size of messages sent and received, which has an impact of choice 2997 on the message format and protocol. By using CoAP over UDP and 2998 CBOR encoded messages, some of these aspects are addressed. 2999 Security protocols contribute to the communication overhead and 3000 can, in some cases, be optimized. For example, authentication and 3001 key establishment may, in certain cases where security 3002 requirements allow, be replaced by provisioning of security 3003 context by a trusted third party, using transport or application 3004 layer security. 3006 Low CPU Speed: 3008 Some IoT devices are equipped with processors that are 3009 significantly slower than those found in most current devices on 3010 the Internet. This typically has implications on what timely 3011 cryptographic operations a device is capable of performing, which 3012 in turn impacts, e.g., protocol latency. Symmetric key 3013 cryptography may be used instead of the computationally more 3014 expensive public key cryptography where the security requirements 3015 so allow, but this may also require support for trusted-third- 3016 party-assisted secret key establishment using transport- or 3017 application-layer security. 3018 Small Amount of Memory: 3020 Microcontrollers embedded in IoT devices are often equipped with 3021 only a small amount of RAM and flash memory, which places 3022 limitations on what kind of processing can be performed and how 3023 much code can be put on those devices. To reduce code size, fewer 3024 and smaller protocol implementations can be put on the firmware of 3025 such a device. In this case, CoAP may be used instead of HTTP, 3026 symmetric-key cryptography instead of public-key cryptography, and 3027 CBOR instead of JSON. An authentication and key establishment 3028 protocol, e.g., the DTLS handshake, in comparison with assisted 3029 key establishment, also has an impact on memory and code 3030 footprints. 3032 User Interface Limitations: 3034 Protecting access to resources is both an important security as 3035 well as privacy feature. End users and enterprise customers may 3036 not want to give access to the data collected by their IoT device 3037 or to functions it may offer to third parties. Since the 3038 classical approach of requesting permissions from end users via a 3039 rich user interface does not work in many IoT deployment 3040 scenarios, these functions need to be delegated to user-controlled 3041 devices that are better suitable for such tasks, such as smart 3042 phones and tablets. 3044 Communication Constraints: 3046 In certain constrained settings an IoT device may not be able to 3047 communicate with a given device at all times. Devices may be 3048 sleeping, or just disconnected from the Internet because of 3049 general lack of connectivity in the area, for cost reasons, or for 3050 security reasons, e.g., to avoid an entry point for Denial-of- 3051 Service attacks. 3053 The communication interactions this framework builds upon (as 3054 shown graphically in Figure 1) may be accomplished using a variety 3055 of different protocols, and not all parts of the message flow are 3056 used in all applications due to the communication constraints. 3057 Deployments making use of CoAP are expected, but this framework is 3058 not limited to them. Other protocols such as HTTP, or even 3059 protocols such as Bluetooth Smart communication that do not 3060 necessarily use IP, could also be used. The latter raises the 3061 need for application layer security over the various interfaces. 3063 In the light of these constraints we have made the following design 3064 decisions: 3066 CBOR, COSE, CWT: 3068 This framework RECOMMENDS the use of CBOR [RFC7049] as data 3069 format. Where CBOR data needs to be protected, the use of COSE 3070 [RFC8152] is RECOMMENDED. Furthermore, where self-contained 3071 tokens are needed, this framework RECOMMENDS the use of CWT 3072 [RFC8392]. These measures aim at reducing the size of messages 3073 sent over the wire, the RAM size of data objects that need to be 3074 kept in memory and the size of libraries that devices need to 3075 support. 3077 CoAP: 3079 This framework RECOMMENDS the use of CoAP [RFC7252] instead of 3080 HTTP. This does not preclude the use of other protocols 3081 specifically aimed at constrained devices, like, e.g., Bluetooth 3082 Low Energy (see Section 3.2). This aims again at reducing the 3083 size of messages sent over the wire, the RAM size of data objects 3084 that need to be kept in memory and the size of libraries that 3085 devices need to support. 3087 Access Information: 3089 This framework defines the name "Access Information" for data 3090 concerning the RS that the AS returns to the client in an access 3091 token response (see Section 5.6.2). This aims at enabling 3092 scenarios where a powerful client, supporting multiple profiles, 3093 needs to interact with an RS for which it does not know the 3094 supported profiles and the raw public key. 3096 Proof-of-Possession: 3098 This framework makes use of proof-of-possession tokens, using the 3099 "cnf" claim [RFC8747]. A request parameter "cnf" and a Response 3100 parameter "cnf", both having a value space semantically and 3101 syntactically identical to the "cnf" claim, are defined for the 3102 token endpoint, to allow requesting and stating confirmation keys. 3103 This aims at making token theft harder. Token theft is 3104 specifically relevant in constrained use cases, as communication 3105 often passes through middle-boxes, which could be able to steal 3106 bearer tokens and use them to gain unauthorized access. 3108 Authz-Info endpoint: 3110 This framework introduces a new way of providing access tokens to 3111 an RS by exposing a authz-info endpoint, to which access tokens 3112 can be POSTed. This aims at reducing the size of the request 3113 message and the code complexity at the RS. The size of the 3114 request message is problematic, since many constrained protocols 3115 have severe message size limitations at the physical layer (e.g., 3116 in the order of 100 bytes). This means that larger packets get 3117 fragmented, which in turn combines badly with the high rate of 3118 packet loss, and the need to retransmit the whole message if one 3119 packet gets lost. Thus separating sending of the request and 3120 sending of the access tokens helps to reduce fragmentation. 3122 Client Credentials Grant: 3124 This framework RECOMMENDS the use of the client credentials grant 3125 for machine-to-machine communication use cases, where manual 3126 intervention of the resource owner to produce a grant token is not 3127 feasible. The intention is that the resource owner would instead 3128 pre-arrange authorization with the AS, based on the client's own 3129 credentials. The client can then (without manual intervention) 3130 obtain access tokens from the AS. 3132 Introspection: 3134 This framework RECOMMENDS the use of access token introspection in 3135 cases where the client is constrained in a way that it can not 3136 easily obtain new access tokens (i.e. it has connectivity issues 3137 that prevent it from communicating with the AS). In that case 3138 this framework RECOMMENDS the use of a long-term token, that could 3139 be a simple reference. The RS is assumed to be able to 3140 communicate with the AS, and can therefore perform introspection, 3141 in order to learn the claims associated with the token reference. 3142 The advantage of such an approach is that the resource owner can 3143 change the claims associated to the token reference without having 3144 to be in contact with the client, thus granting or revoking access 3145 rights. 3147 Appendix B. Roles and Responsibilities 3149 Resource Owner 3151 * Make sure that the RS is registered at the AS. This includes 3152 making known to the AS which profiles, token_type, scopes, and 3153 key types (symmetric/asymmetric) the RS supports. Also making 3154 it known to the AS which audience(s) the RS identifies itself 3155 with. 3156 * Make sure that clients can discover the AS that is in charge of 3157 the RS. 3158 * If the client-credentials grant is used, make sure that the AS 3159 has the necessary, up-to-date, access control policies for the 3160 RS. 3162 Requesting Party 3164 * Make sure that the client is provisioned the necessary 3165 credentials to authenticate to the AS. 3166 * Make sure that the client is configured to follow the security 3167 requirements of the Requesting Party when issuing requests 3168 (e.g., minimum communication security requirements, trust 3169 anchors). 3170 * Register the client at the AS. This includes making known to 3171 the AS which profiles, token_types, and key types (symmetric/ 3172 asymmetric) the client. 3174 Authorization Server 3176 * Register the RS and manage corresponding security contexts. 3177 * Register clients and authentication credentials. 3178 * Allow Resource Owners to configure and update access control 3179 policies related to their registered RSs. 3180 * Expose the token endpoint to allow clients to request tokens. 3181 * Authenticate clients that wish to request a token. 3182 * Process a token request using the authorization policies 3183 configured for the RS. 3184 * Optionally: Expose the introspection endpoint that allows RS's 3185 to submit token introspection requests. 3186 * If providing an introspection endpoint: Authenticate RSs that 3187 wish to get an introspection response. 3188 * If providing an introspection endpoint: Process token 3189 introspection requests. 3190 * Optionally: Handle token revocation. 3191 * Optionally: Provide discovery metadata. See [RFC8414] 3192 * Optionally: Handle refresh tokens. 3194 Client 3196 * Discover the AS in charge of the RS that is to be targeted with 3197 a request. 3198 * Submit the token request (see step (A) of Figure 1). 3200 + Authenticate to the AS. 3201 + Optionally (if not pre-configured): Specify which RS, which 3202 resource(s), and which action(s) the request(s) will target. 3203 + If raw public keys (rpk) or certificates are used, make sure 3204 the AS has the right rpk or certificate for this client. 3205 * Process the access token and Access Information (see step (B) 3206 of Figure 1). 3208 + Check that the Access Information provides the necessary 3209 security parameters (e.g., PoP key, information on 3210 communication security protocols supported by the RS). 3211 + Safely store the proof-of-possession key. 3212 + If provided by the AS: Safely store the refresh token. 3213 * Send the token and request to the RS (see step (C) of 3214 Figure 1). 3216 + Authenticate towards the RS (this could coincide with the 3217 proof of possession process). 3218 + Transmit the token as specified by the AS (default is to the 3219 authz-info endpoint, alternative options are specified by 3220 profiles). 3221 + Perform the proof-of-possession procedure as specified by 3222 the profile in use (this may already have been taken care of 3223 through the authentication procedure). 3224 * Process the RS response (see step (F) of Figure 1) of the RS. 3226 Resource Server 3228 * Expose a way to submit access tokens. By default this is the 3229 authz-info endpoint. 3230 * Process an access token. 3232 + Verify the token is from a recognized AS. 3233 + Check the token's integrity. 3234 + Verify that the token applies to this RS. 3235 + Check that the token has not expired (if the token provides 3236 expiration information). 3237 + Store the token so that it can be retrieved in the context 3238 of a matching request. 3240 Note: The order proposed here is not normative, any process 3241 that arrives at an equivalent result can be used. A noteworthy 3242 consideration is whether one can use cheap operations early on 3243 to quickly discard non-applicable or invalid tokens, before 3244 performing expensive cryptographic operations (e.g. doing an 3245 expiration check before verifying a signature). 3247 * Process a request. 3249 + Set up communication security with the client. 3250 + Authenticate the client. 3251 + Match the client against existing tokens. 3252 + Check that tokens belonging to the client actually authorize 3253 the requested action. 3254 + Optionally: Check that the matching tokens are still valid, 3255 using introspection (if this is possible.) 3256 * Send a response following the agreed upon communication 3257 security mechanism(s). 3258 * Safely store credentials such as raw public keys for 3259 authentication or proof-of-possession keys linked to access 3260 tokens. 3262 Appendix C. Requirements on Profiles 3264 This section lists the requirements on profiles of this framework, 3265 for the convenience of profile designers. 3267 o Optionally define new methods for the client to discover the 3268 necessary permissions and AS for accessing a resource, different 3269 from the one proposed in Section 5.1. Section 4 3270 o Optionally specify new grant types. Section 5.2 3271 o Optionally define the use of client certificates as client 3272 credential type. Section 5.3 3273 o Specify the communication protocol the client and RS the must use 3274 (e.g., CoAP). Section 5 and Section 5.6.4.3 3275 o Specify the security protocol the client and RS must use to 3276 protect their communication (e.g., OSCORE or DTLS). This must 3277 provide encryption, integrity and replay protection. 3278 Section 5.6.4.3 3279 o Specify how the client and the RS mutually authenticate. 3280 Section 4 3281 o Specify the proof-of-possession protocol(s) and how to select one, 3282 if several are available. Also specify which key types (e.g., 3283 symmetric/asymmetric) are supported by a specific proof-of- 3284 possession protocol. Section 5.6.4.2 3285 o Specify a unique ace_profile identifier. Section 5.6.4.3 3286 o If introspection is supported: Specify the communication and 3287 security protocol for introspection. Section 5.7 3288 o Specify the communication and security protocol for interactions 3289 between client and AS. This must provide encryption, integrity 3290 protection, replay protection and a binding between requests and 3291 responses. Section 5 and Section 5.6 3292 o Specify how/if the authz-info endpoint is protected, including how 3293 error responses are protected. Section 5.8.1 3294 o Optionally define other methods of token transport than the authz- 3295 info endpoint. Section 5.8.1 3297 Appendix D. Assumptions on AS knowledge about C and RS 3299 This section lists the assumptions on what an AS should know about a 3300 client and an RS in order to be able to respond to requests to the 3301 token and introspection endpoints. How this information is 3302 established is out of scope for this document. 3304 o The identifier of the client or RS. 3305 o The profiles that the client or RS supports. 3306 o The scopes that the RS supports. 3307 o The audiences that the RS identifies with. 3308 o The key types (e.g., pre-shared symmetric key, raw public key, key 3309 length, other key parameters) that the client or RS supports. 3311 o The types of access tokens the RS supports (e.g., CWT). 3312 o If the RS supports CWTs, the COSE parameters for the crypto 3313 wrapper (e.g., algorithm, key-wrap algorithm, key-length) that the 3314 RS supports. 3315 o The expiration time for access tokens issued to this RS (unless 3316 the RS accepts a default time chosen by the AS). 3317 o The symmetric key shared between client and AS (if any). 3318 o The symmetric key shared between RS and AS (if any). 3319 o The raw public key of the client or RS (if any). 3320 o Whether the RS has synchronized time (and thus is able to use the 3321 'exp' claim) or not. 3323 Appendix E. Deployment Examples 3325 There is a large variety of IoT deployments, as is indicated in 3326 Appendix A, and this section highlights a few common variants. This 3327 section is not normative but illustrates how the framework can be 3328 applied. 3330 For each of the deployment variants, there are a number of possible 3331 security setups between clients, resource servers and authorization 3332 servers. The main focus in the following subsections is on how 3333 authorization of a client request for a resource hosted by an RS is 3334 performed. This requires the security of the requests and responses 3335 between the clients and the RS to be considered. 3337 Note: CBOR diagnostic notation is used for examples of requests and 3338 responses. 3340 E.1. Local Token Validation 3342 In this scenario, the case where the resource server is offline is 3343 considered, i.e., it is not connected to the AS at the time of the 3344 access request. This access procedure involves steps A, B, C, and F 3345 of Figure 1. 3347 Since the resource server must be able to verify the access token 3348 locally, self-contained access tokens must be used. 3350 This example shows the interactions between a client, the 3351 authorization server and a temperature sensor acting as a resource 3352 server. Message exchanges A and B are shown in Figure 17. 3354 A: The client first generates a public-private key pair used for 3355 communication security with the RS. 3356 The client sends a CoAP POST request to the token endpoint at the 3357 AS. The security of this request can be transport or application 3358 layer. It is up the the communication security profile to define. 3360 In the example it is assumed that both client and AS have 3361 performed mutual authentication e.g. via DTLS. The request 3362 contains the public key of the client and the Audience parameter 3363 set to "tempSensorInLivingRoom", a value that the temperature 3364 sensor identifies itself with. The AS evaluates the request and 3365 authorizes the client to access the resource. 3366 B: The AS responds with a 2.05 Content response containing the 3367 Access Information, including the access token. The PoP access 3368 token contains the public key of the client, and the Access 3369 Information contains the public key of the RS. For communication 3370 security this example uses DTLS RawPublicKey between the client 3371 and the RS. The issued token will have a short validity time, 3372 i.e., "exp" close to "iat", in order to mitigate attacks using 3373 stolen client credentials. The token includes the claim such as 3374 "scope" with the authorized access that an owner of the 3375 temperature device can enjoy. In this example, the "scope" claim, 3376 issued by the AS, informs the RS that the owner of the token, that 3377 can prove the possession of a key is authorized to make a GET 3378 request against the /temperature resource and a POST request on 3379 the /firmware resource. Note that the syntax and semantics of the 3380 scope claim are application specific. 3381 Note: In this example it is assumed that the client knows what 3382 resource it wants to access, and is therefore able to request 3383 specific audience and scope claims for the access token. 3385 Authorization 3386 Client Server 3387 | | 3388 |<=======>| DTLS Connection Establishment 3389 | | and mutual authentication 3390 | | 3391 A: +-------->| Header: POST (Code=0.02) 3392 | POST | Uri-Path:"token" 3393 | | Content-Format: application/ace+cbor 3394 | | Payload: 3395 | | 3396 B: |<--------+ Header: 2.05 Content 3397 | 2.05 | Content-Format: application/ace+cbor 3398 | | Payload: 3399 | | 3401 Figure 17: Token Request and Response Using Client Credentials. 3403 The information contained in the Request-Payload and the Response- 3404 Payload is shown in Figure 18 Note that the parameter "rs_cnf" from 3405 [I-D.ietf-ace-oauth-params] is used to inform the client about the 3406 resource server's public key. 3408 Request-Payload : 3409 { 3410 "audience" : "tempSensorInLivingRoom", 3411 "client_id" : "myclient", 3412 "req_cnf" : { 3413 "COSE_Key" : { 3414 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3415 "kty" : "EC", 3416 "crv" : "P-256", 3417 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3418 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3419 } 3420 } 3421 } 3423 Response-Payload : 3424 { 3425 "access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...', 3426 "rs_cnf" : { 3427 "COSE_Key" : { 3428 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3429 "kty" : "EC", 3430 "crv" : "P-256", 3431 "x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4', 3432 "y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM' 3433 } 3434 } 3435 } 3437 Figure 18: Request and Response Payload Details. 3439 The content of the access token is shown in Figure 19. 3441 { 3442 "aud" : "tempSensorInLivingRoom", 3443 "iat" : "1563451500", 3444 "exp" : "1563453000", 3445 "scope" : "temperature_g firmware_p", 3446 "cnf" : { 3447 "COSE_Key" : { 3448 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3449 "kty" : "EC", 3450 "crv" : "P-256", 3451 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3452 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3453 } 3454 } 3455 } 3457 Figure 19: Access Token including Public Key of the Client. 3459 Messages C and F are shown in Figure 20 - Figure 21. 3461 C: The client then sends the PoP access token to the authz-info 3462 endpoint at the RS. This is a plain CoAP POST request, i.e., no 3463 transport or application layer security is used between client and 3464 RS since the token is integrity protected between the AS and RS. 3465 The RS verifies that the PoP access token was created by a known 3466 and trusted AS, that it applies to this RS, and that it is valid. 3467 The RS caches the security context together with authorization 3468 information about this client contained in the PoP access token. 3470 Resource 3471 Client Server 3472 | | 3473 C: +-------->| Header: POST (Code=0.02) 3474 | POST | Uri-Path:"authz-info" 3475 | | Payload: 0INDoQEKoQVN ... 3476 | | 3477 |<--------+ Header: 2.04 Changed 3478 | 2.04 | 3479 | | 3481 Figure 20: Access Token provisioning to RS 3482 The client and the RS runs the DTLS handshake using the raw public 3483 keys established in step B and C. 3484 The client sends a CoAP GET request to /temperature on RS over 3485 DTLS. The RS verifies that the request is authorized, based on 3486 previously established security context. 3488 F: The RS responds over the same DTLS channel with a CoAP 2.05 3489 Content response, containing a resource representation as payload. 3491 Resource 3492 Client Server 3493 | | 3494 |<=======>| DTLS Connection Establishment 3495 | | using Raw Public Keys 3496 | | 3497 +-------->| Header: GET (Code=0.01) 3498 | GET | Uri-Path: "temperature" 3499 | | 3500 | | 3501 | | 3502 F: |<--------+ Header: 2.05 Content 3503 | 2.05 | Payload: 3504 | | 3506 Figure 21: Resource Request and Response protected by DTLS. 3508 E.2. Introspection Aided Token Validation 3510 In this deployment scenario it is assumed that a client is not able 3511 to access the AS at the time of the access request, whereas the RS is 3512 assumed to be connected to the back-end infrastructure. Thus the RS 3513 can make use of token introspection. This access procedure involves 3514 steps A-F of Figure 1, but assumes steps A and B have been carried 3515 out during a phase when the client had connectivity to AS. 3517 Since the client is assumed to be offline, at least for a certain 3518 period of time, a pre-provisioned access token has to be long-lived. 3519 Since the client is constrained, the token will not be self contained 3520 (i.e. not a CWT) but instead just a reference. The resource server 3521 uses its connectivity to learn about the claims associated to the 3522 access token by using introspection, which is shown in the example 3523 below. 3525 In the example interactions between an offline client (key fob), an 3526 RS (online lock), and an AS is shown. It is assumed that there is a 3527 provisioning step where the client has access to the AS. This 3528 corresponds to message exchanges A and B which are shown in 3529 Figure 22. 3531 Authorization consent from the resource owner can be pre-configured, 3532 but it can also be provided via an interactive flow with the resource 3533 owner. An example of this for the key fob case could be that the 3534 resource owner has a connected car, he buys a generic key that he 3535 wants to use with the car. To authorize the key fob he connects it 3536 to his computer that then provides the UI for the device. After that 3537 OAuth 2.0 implicit flow can used to authorize the key for his car at 3538 the the car manufacturers AS. 3540 Note: In this example the client does not know the exact door it will 3541 be used to access since the token request is not send at the time of 3542 access. So the scope and audience parameters are set quite wide to 3543 start with, while tailored values narrowing down the claims to the 3544 specific RS being accessed can be provided to that RS during an 3545 introspection step. 3547 A: The client sends a CoAP POST request to the token endpoint at 3548 AS. The request contains the Audience parameter set to "PACS1337" 3549 (PACS, Physical Access System), a value the that identifies the 3550 physical access control system to which the individual doors are 3551 connected. The AS generates an access token as an opaque string, 3552 which it can match to the specific client and the targeted 3553 audience. It furthermore generates a symmetric proof-of- 3554 possession key. The communication security and authentication 3555 between client and AS is assumed to have been provided at 3556 transport layer (e.g. via DTLS) using a pre-shared security 3557 context (psk, rpk or certificate). 3558 B: The AS responds with a CoAP 2.05 Content response, containing 3559 as playload the Access Information, including the access token and 3560 the symmetric proof-of-possession key. Communication security 3561 between C and RS will be DTLS and PreSharedKey. The PoP key is 3562 used as the PreSharedKey. 3564 Note: In this example we are using a symmetric key for a multi-RS 3565 audience, which is not recommended normally (see Section 6.9). 3566 However in this case the risk is deemed to be acceptable, since all 3567 the doors are part of the same physical access control system, and 3568 therefore the risk of a malicious RS impersonating the client towards 3569 another RS is low. 3571 Authorization 3572 Client Server 3573 | | 3574 |<=======>| DTLS Connection Establishment 3575 | | and mutual authentication 3576 | | 3577 A: +-------->| Header: POST (Code=0.02) 3578 | POST | Uri-Path:"token" 3579 | | Content-Format: application/ace+cbor 3580 | | Payload: 3581 | | 3582 B: |<--------+ Header: 2.05 Content 3583 | | Content-Format: application/ace+cbor 3584 | 2.05 | Payload: 3585 | | 3587 Figure 22: Token Request and Response using Client Credentials. 3589 The information contained in the Request-Payload and the Response- 3590 Payload is shown in Figure 23. 3592 Request-Payload: 3593 { 3594 "client_id" : "keyfob", 3595 "audience" : "PACS1337" 3596 } 3598 Response-Payload: 3599 { 3600 "access_token" : b64'VGVzdCB0b2tlbg==', 3601 "cnf" : { 3602 "COSE_Key" : { 3603 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3604 "kty" : "oct", 3605 "alg" : "HS256", 3606 "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE' 3607 } 3608 } 3609 } 3611 Figure 23: Request and Response Payload for C offline 3613 The access token in this case is just an opaque byte string 3614 referencing the authorization information at the AS. 3616 C: Next, the client POSTs the access token to the authz-info 3617 endpoint in the RS. This is a plain CoAP request, i.e., no DTLS 3618 between client and RS. Since the token is an opaque string, the 3619 RS cannot verify it on its own, and thus defers to respond the 3620 client with a status code until after step E. 3621 D: The RS sends the token to the introspection endpoint on the AS 3622 using a CoAP POST request. In this example RS and AS are assumed 3623 to have performed mutual authentication using a pre shared 3624 security context (psk, rpk or certificate) with the RS acting as 3625 DTLS client. 3626 E: The AS provides the introspection response (2.05 Content) 3627 containing parameters about the token. This includes the 3628 confirmation key (cnf) parameter that allows the RS to verify the 3629 client's proof of possession in step F. Note that our example in 3630 Figure 25 assumes a pre-established key (e.g. one used by the 3631 client and the RS for a previous token) that is now only 3632 referenced by its key-identifier 'kid'. 3633 After receiving message E, the RS responds to the client's POST in 3634 step C with the CoAP response code 2.01 (Created). 3636 Resource 3637 Client Server 3638 | | 3639 C: +-------->| Header: POST (T=CON, Code=0.02) 3640 | POST | Uri-Path:"authz-info" 3641 | | Payload: b64'VGVzdCB0b2tlbg==' 3642 | | 3643 | | Authorization 3644 | | Server 3645 | | | 3646 | D: +--------->| Header: POST (Code=0.02) 3647 | | POST | Uri-Path: "introspect" 3648 | | | Content-Format: "application/ace+cbor" 3649 | | | Payload: 3650 | | | 3651 | E: |<---------+ Header: 2.05 Content 3652 | | 2.05 | Content-Format: "application/ace+cbor" 3653 | | | Payload: 3654 | | | 3655 | | 3656 |<--------+ Header: 2.01 Created 3657 | 2.01 | 3658 | | 3660 Figure 24: Token Introspection for C offline 3661 The information contained in the Request-Payload and the Response- 3662 Payload is shown in Figure 25. 3664 Request-Payload: 3665 { 3666 "token" : b64'VGVzdCB0b2tlbg==', 3667 "client_id" : "FrontDoor", 3668 } 3670 Response-Payload: 3671 { 3672 "active" : true, 3673 "aud" : "lockOfDoor4711", 3674 "scope" : "open, close", 3675 "iat" : 1563454000, 3676 "cnf" : { 3677 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk' 3678 } 3679 } 3681 Figure 25: Request and Response Payload for Introspection 3683 The client uses the symmetric PoP key to establish a DTLS 3684 PreSharedKey secure connection to the RS. The CoAP request PUT is 3685 sent to the uri-path /state on the RS, changing the state of the 3686 door to locked. 3687 F: The RS responds with a appropriate over the secure DTLS 3688 channel. 3690 Resource 3691 Client Server 3692 | | 3693 |<=======>| DTLS Connection Establishment 3694 | | using Pre Shared Key 3695 | | 3696 +-------->| Header: PUT (Code=0.03) 3697 | PUT | Uri-Path: "state" 3698 | | Payload: 3699 | | 3700 F: |<--------+ Header: 2.04 Changed 3701 | 2.04 | Payload: 3702 | | 3704 Figure 26: Resource request and response protected by OSCORE 3706 Appendix F. Document Updates 3708 RFC EDITOR: PLEASE REMOVE THIS SECTION. 3710 F.1. Version -21 to 22 3712 o Provided section numbers in references to OAuth RFC. 3713 o Updated IANA mapping registries to only use "Private Use" and 3714 "Expert Review". 3715 o Made error messages optional for RS at token submission since it 3716 may not be able to send them depending on the profile. 3717 o Corrected errors in examples. 3719 F.2. Version -20 to 21 3721 o Added text about expiration of RS keys. 3723 F.3. Version -19 to 20 3725 o Replaced "req_aud" with "audience" from the OAuth token exchange 3726 draft. 3727 o Updated examples to remove unnecessary elements. 3729 F.4. Version -18 to -19 3731 o Added definition of "Authorization Information". 3732 o Explicitly state that ACE allows encoding refresh tokens in binary 3733 format in addition to strings. 3734 o Renamed "AS Information" to "AS Request Creation Hints" and added 3735 the possibility to specify req_aud and scope as hints. 3736 o Added the "kid" parameter to AS Request Creation Hints. 3737 o Added security considerations about the integrity protection of 3738 tokens with multi-RS audiences. 3739 o Renamed IANA registries mapping OAuth parameters to reflect the 3740 mapped registry. 3741 o Added JWT claim names to CWT claim registrations. 3742 o Added expert review instructions. 3743 o Updated references to TLS from 1.2 to 1.3. 3745 F.5. Version -17 to -18 3747 o Added OSCORE options in examples involving OSCORE. 3748 o Removed requirement for the client to send application/cwt, since 3749 the client has no way to know. 3750 o Clarified verification of tokens by the RS. 3751 o Added exi claim CWT registration. 3753 F.6. Version -16 to -17 3755 o Added references to (D)TLS 1.3. 3756 o Added requirement that responses are bound to requests. 3758 o Specify that grant_type is OPTIONAL in C2AS requests (as opposed 3759 to REQUIRED in OAuth). 3760 o Replaced examples with hypothetical COSE profile with OSCORE. 3761 o Added requirement for content type application/ace+cbor in error 3762 responses for token and introspection requests and responses. 3763 o Reworked abbreviation space for claims, request and response 3764 parameters. 3765 o Added text that the RS may indicate that it is busy at the authz- 3766 info resource. 3767 o Added section that specifies how the RS verifies an access token. 3768 o Added section on the protection of the authz-info endpoint. 3769 o Removed the expiration mechanism based on sequence numbers. 3770 o Added reference to RFC7662 security considerations. 3771 o Added considerations on minimal security requirements for 3772 communication. 3773 o Added security considerations on unprotected information sent to 3774 authz-info and in the error responses. 3776 F.7. Version -15 to -16 3778 o Added text the RS using RFC6750 error codes. 3779 o Defined an error code for incompatible token request parameters. 3780 o Removed references to the actors draft. 3781 o Fixed errors in examples. 3783 F.8. Version -14 to -15 3785 o Added text about refresh tokens. 3786 o Added text about protection of credentials. 3787 o Rephrased introspection so that other entities than RS can do it. 3788 o Editorial improvements. 3790 F.9. Version -13 to -14 3792 o Split out the 'aud', 'cnf' and 'rs_cnf' parameters to 3793 [I-D.ietf-ace-oauth-params] 3794 o Introduced the "application/ace+cbor" Content-Type. 3795 o Added claim registrations from 'profile' and 'rs_cnf'. 3796 o Added note on schema part of AS Information Section 5.1.2 3797 o Realigned the parameter abbreviations to push rarely used ones to 3798 the 2-byte encoding size of CBOR integers. 3800 F.10. Version -12 to -13 3802 o Changed "Resource Information" to "Access Information" to avoid 3803 confusion. 3804 o Clarified section about AS discovery. 3805 o Editorial changes 3807 F.11. Version -11 to -12 3809 o Moved the Request error handling to a section of its own. 3810 o Require the use of the abbreviation for profile identifiers. 3811 o Added rs_cnf parameter in the introspection response, to inform 3812 RS' with several RPKs on which key to use. 3813 o Allowed use of rs_cnf as claim in the access token in order to 3814 inform an RS with several RPKs on which key to use. 3815 o Clarified that profiles must specify if/how error responses are 3816 protected. 3817 o Fixed label number range to align with COSE/CWT. 3818 o Clarified the requirements language in order to allow profiles to 3819 specify other payload formats than CBOR if they do not use CoAP. 3821 F.12. Version -10 to -11 3823 o Fixed some CBOR data type errors. 3824 o Updated boilerplate text 3826 F.13. Version -09 to -10 3828 o Removed CBOR major type numbers. 3829 o Removed the client token design. 3830 o Rephrased to clarify that other protocols than CoAP can be used. 3831 o Clarifications regarding the use of HTTP 3833 F.14. Version -08 to -09 3835 o Allowed scope to be byte strings. 3836 o Defined default names for endpoints. 3837 o Refactored the IANA section for briefness and consistency. 3838 o Refactored tables that define IANA registry contents for 3839 consistency. 3840 o Created IANA registry for CBOR mappings of error codes, grant 3841 types and Authorization Server Information. 3842 o Added references to other document sections defining IANA entries 3843 in the IANA section. 3845 F.15. Version -07 to -08 3847 o Moved AS discovery from the DTLS profile to the framework, see 3848 Section 5.1. 3849 o Made the use of CBOR mandatory. If you use JSON you can use 3850 vanilla OAuth. 3851 o Made it mandatory for profiles to specify C-AS security and RS-AS 3852 security (the latter only if introspection is supported). 3853 o Made the use of CBOR abbreviations mandatory. 3855 o Added text to clarify the use of token references as an 3856 alternative to CWTs. 3857 o Added text to clarify that introspection must not be delayed, in 3858 case the RS has to return a client token. 3859 o Added security considerations about leakage through unprotected AS 3860 discovery information, combining profiles and leakage through 3861 error responses. 3862 o Added privacy considerations about leakage through unprotected AS 3863 discovery. 3864 o Added text that clarifies that introspection is optional. 3865 o Made profile parameter optional since it can be implicit. 3866 o Clarified that CoAP is not mandatory and other protocols can be 3867 used. 3868 o Clarified the design justification for specific features of the 3869 framework in appendix A. 3870 o Clarified appendix E.2. 3871 o Removed specification of the "cnf" claim for CBOR/COSE, and 3872 replaced with references to [RFC8747] 3874 F.16. Version -06 to -07 3876 o Various clarifications added. 3877 o Fixed erroneous author email. 3879 F.17. Version -05 to -06 3881 o Moved sections that define the ACE framework into a subsection of 3882 the framework Section 5. 3883 o Split section on client credentials and grant into two separate 3884 sections, Section 5.2, and Section 5.3. 3885 o Added Section 5.4 on AS authentication. 3886 o Added Section 5.5 on the Authorization endpoint. 3888 F.18. Version -04 to -05 3890 o Added RFC 2119 language to the specification of the required 3891 behavior of profile specifications. 3892 o Added Section 5.3 on the relation to the OAuth2 grant types. 3893 o Added CBOR abbreviations for error and the error codes defined in 3894 OAuth2. 3895 o Added clarification about token expiration and long-running 3896 requests in Section 5.8.3 3897 o Added security considerations about tokens with symmetric PoP keys 3898 valid for more than one RS. 3899 o Added privacy considerations section. 3900 o Added IANA registry mapping the confirmation types from RFC 7800 3901 to equivalent COSE types. 3903 o Added appendix D, describing assumptions about what the AS knows 3904 about the client and the RS. 3906 F.19. Version -03 to -04 3908 o Added a description of the terms "framework" and "profiles" as 3909 used in this document. 3910 o Clarified protection of access tokens in section 3.1. 3911 o Clarified uses of the "cnf" parameter in section 6.4.5. 3912 o Clarified intended use of Client Token in section 7.4. 3914 F.20. Version -02 to -03 3916 o Removed references to draft-ietf-oauth-pop-key-distribution since 3917 the status of this draft is unclear. 3918 o Copied and adapted security considerations from draft-ietf-oauth- 3919 pop-key-distribution. 3920 o Renamed "client information" to "RS information" since it is 3921 information about the RS. 3922 o Clarified the requirements on profiles of this framework. 3923 o Clarified the token endpoint protocol and removed negotiation of 3924 "profile" and "alg" (section 6). 3925 o Renumbered the abbreviations for claims and parameters to get a 3926 consistent numbering across different endpoints. 3927 o Clarified the introspection endpoint. 3928 o Renamed token, introspection and authz-info to "endpoint" instead 3929 of "resource" to mirror the OAuth 2.0 terminology. 3930 o Updated the examples in the appendices. 3932 F.21. Version -01 to -02 3934 o Restructured to remove communication security parts. These shall 3935 now be defined in profiles. 3936 o Restructured section 5 to create new sections on the OAuth 3937 endpoints token, introspection and authz-info. 3938 o Pulled in material from draft-ietf-oauth-pop-key-distribution in 3939 order to define proof-of-possession key distribution. 3940 o Introduced the "cnf" parameter as defined in RFC7800 to reference 3941 or transport keys used for proof of possession. 3942 o Introduced the "client-token" to transport client information from 3943 the AS to the client via the RS in conjunction with introspection. 3944 o Expanded the IANA section to define parameters for token request, 3945 introspection and CWT claims. 3946 o Moved deployment scenarios to the appendix as examples. 3948 F.22. Version -00 to -01 3950 o Changed 5.1. from "Communication Security Protocol" to "Client 3951 Information". 3952 o Major rewrite of 5.1 to clarify the information exchanged between 3953 C and AS in the PoP access token request profile for IoT. 3955 * Allow the client to indicate preferences for the communication 3956 security protocol. 3957 * Defined the term "Client Information" for the additional 3958 information returned to the client in addition to the access 3959 token. 3960 * Require that the messages between AS and client are secured, 3961 either with (D)TLS or with COSE_Encrypted wrappers. 3962 * Removed dependency on OSCOAP and added generic text about 3963 object security instead. 3964 * Defined the "rpk" parameter in the client information to 3965 transmit the raw public key of the RS from AS to client. 3966 * (D)TLS MUST use the PoP key in the handshake (either as PSK or 3967 as client RPK with client authentication). 3968 * Defined the use of x5c, x5t and x5tS256 parameters when a 3969 client certificate is used for proof of possession. 3970 * Defined "tktn" parameter for signaling for how to transfer the 3971 access token. 3972 o Added 5.2. the CoAP Access-Token option for transferring access 3973 tokens in messages that do not have payload. 3974 o 5.3.2. Defined success and error responses from the RS when 3975 receiving an access token. 3976 o 5.6.:Added section giving guidance on how to handle token 3977 expiration in the absence of reliable time. 3978 o Appendix B Added list of roles and responsibilities for C, AS and 3979 RS. 3981 Authors' Addresses 3983 Ludwig Seitz 3984 Combitech 3985 Djaeknegatan 31 3986 Malmoe 211 35 3987 Sweden 3989 Email: ludwig.seitz@combitech.se 3990 Goeran Selander 3991 Ericsson 3992 Faroegatan 6 3993 Kista 164 80 3994 Sweden 3996 Email: goran.selander@ericsson.com 3998 Erik Wahlstroem 3999 Sweden 4001 Email: erik@wahlstromstekniska.se 4003 Samuel Erdtman 4004 Spotify AB 4005 Birger Jarlsgatan 61, 4tr 4006 Stockholm 113 56 4007 Sweden 4009 Email: erdtman@spotify.com 4011 Hannes Tschofenig 4012 Arm Ltd. 4013 Absam 6067 4014 Austria 4016 Email: Hannes.Tschofenig@arm.com