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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: July 14, 2020 Ericsson 6 E. Wahlstroem 8 S. Erdtman 9 Spotify AB 10 H. Tschofenig 11 Arm Ltd. 12 January 11, 2020 14 Authentication and Authorization for Constrained Environments (ACE) 15 using the OAuth 2.0 Framework (ACE-OAuth) 16 draft-ietf-ace-oauth-authz-30 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 July 14, 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 . . . . . . . . . . . . . . . . . . 43 105 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. OAuth Extensions Error Registration . . . . . . . . . . . 51 116 8.3. OAuth Error Code CBOR Mappings Registry . . . . . . . . . 51 117 8.4. OAuth Grant Type CBOR Mappings . . . . . . . . . . . . . 51 118 8.5. OAuth Access Token Types . . . . . . . . . . . . . . . . 52 119 8.6. OAuth Access Token Type CBOR Mappings . . . . . . . . . . 52 120 8.6.1. Initial Registry Contents . . . . . . . . . . . . . . 52 121 8.7. ACE Profile Registry . . . . . . . . . . . . . . . . . . 53 122 8.8. OAuth Parameter Registration . . . . . . . . . . . . . . 53 123 8.9. OAuth Parameters CBOR Mappings Registry . . . . . . . . . 53 124 8.10. OAuth Introspection Response Parameter Registration . . . 54 125 8.11. OAuth Token Introspection Response CBOR Mappings Registry 54 126 8.12. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 55 127 8.13. CBOR Web Token Claims . . . . . . . . . . . . . . . . . . 55 128 8.14. Media Type Registrations . . . . . . . . . . . . . . . . 56 129 8.15. CoAP Content-Format Registry . . . . . . . . . . . . . . 57 130 8.16. Expert Review Instructions . . . . . . . . . . . . . . . 57 131 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 58 132 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 59 133 10.1. Normative References . . . . . . . . . . . . . . . . . . 59 134 10.2. Informative References . . . . . . . . . . . . . . . . . 61 135 Appendix A. Design Justification . . . . . . . . . . . . . . . . 64 136 Appendix B. Roles and Responsibilities . . . . . . . . . . . . . 68 137 Appendix C. Requirements on Profiles . . . . . . . . . . . . . . 70 138 Appendix D. Assumptions on AS knowledge about C and RS . . . . . 71 139 Appendix E. Deployment Examples . . . . . . . . . . . . . . . . 71 140 E.1. Local Token Validation . . . . . . . . . . . . . . . . . 72 141 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. 190 Taking energy consumption as an example: At one end there are energy- 191 harvesting or battery powered devices which have a tight power 192 budget, on the other end there are mains-powered devices, and all 193 levels in between. 195 Hence, IoT devices may be very different in terms of available 196 processing and message exchange capabilities and there is a need to 197 support many different authorization use cases [RFC7744]. 199 This specification describes a framework for authentication and 200 authorization in constrained environments (ACE) built on re-use of 201 OAuth 2.0 [RFC6749], thereby extending authorization to Internet of 202 Things devices. This specification contains the necessary building 203 blocks for adjusting OAuth 2.0 to IoT environments. 205 More detailed, interoperable specifications can be found in separate 206 profile specifications. Implementations may claim conformance with a 207 specific profile, whereby implementations utilizing the same profile 208 interoperate while implementations of different profiles are not 209 expected to be interoperable. Some devices, such as mobile phones 210 and tablets, may implement multiple profiles and will therefore be 211 able to interact with a wider range of low end devices. Requirements 212 on profiles are described at contextually appropriate places 213 throughout this specification, and also summarized in Appendix C. 215 2. Terminology 217 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 218 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 219 "OPTIONAL" in this document are to be interpreted as described in BCP 220 14 [RFC2119] [RFC8174] when, and only when, they appear in all 221 capitals, as shown here. 223 Certain security-related terms such as "authentication", 224 "authorization", "confidentiality", "(data) integrity", "message 225 authentication code", and "verify" are taken from [RFC4949]. 227 Since exchanges in this specification are described as RESTful 228 protocol interactions, HTTP [RFC7231] offers useful terminology. 230 Terminology for entities in the architecture is defined in OAuth 2.0 231 [RFC6749] such as client (C), resource server (RS), and authorization 232 server (AS). 234 Note that the term "endpoint" is used here following its OAuth 235 definition, which is to denote resources such as token and 236 introspection at the AS and authz-info at the RS (see Section 5.8.1 237 for a definition of the authz-info endpoint). The CoAP [RFC7252] 238 definition, which is "An entity participating in the CoAP protocol" 239 is not used in this specification. 241 The specifications in this document is called the "framework" or "ACE 242 framework". When referring to "profiles of this framework" it refers 243 to additional specifications that define the use of this 244 specification with concrete transport and communication security 245 protocols (e.g., CoAP over DTLS). 247 We use the term "Access Information" for parameters other than the 248 access token provided to the client by the AS to enable it to access 249 the RS (e.g. public key of the RS, profile supported by RS). 251 We use the term "Authorization Information" to denote all 252 information, including the claims of relevant access tokens, that an 253 RS uses to determine whether an access request should be granted. 255 3. Overview 257 This specification defines the ACE framework for authorization in the 258 Internet of Things environment. It consists of a set of building 259 blocks. 261 The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys 262 widespread deployment. Many IoT devices can support OAuth 2.0 263 without any additional extensions, but for certain constrained 264 settings additional profiling is needed. 266 Another building block is the lightweight web transfer protocol CoAP 267 [RFC7252], for those communication environments where HTTP is not 268 appropriate. CoAP typically runs on top of UDP, which further 269 reduces overhead and message exchanges. While this specification 270 defines extensions for the use of OAuth over CoAP, other underlying 271 protocols are not prohibited from being supported in the future, such 272 as HTTP/2 [RFC7540], Message Queuing Telemetry Transport (MQTT) 273 [MQTT5.0], Bluetooth Low Energy (BLE) [BLE] and QUIC 274 [I-D.ietf-quic-transport]. Note that this document specifies 275 protocol exchanges in terms of RESTful verbs such as GET and POST. 276 Future profiles using protocols that do not support these verbs MUST 277 specify how the corresponding protocol messages are transmitted 278 instead. 280 A third building block is the Concise Binary Object Representation 281 (CBOR) [RFC7049], for encodings where JSON [RFC8259] is not 282 sufficiently compact. CBOR is a binary encoding designed for small 283 code and message size, which may be used for encoding of self 284 contained tokens, and also for encoding payloads transferred in 285 protocol messages. 287 A fourth building block is CBOR Object Signing and Encryption (COSE) 288 [RFC8152], which enables object-level layer security as an 289 alternative or complement to transport layer security (DTLS [RFC6347] 290 or TLS [RFC8446]). COSE is used to secure self-contained tokens such 291 as proof-of-possession (PoP) tokens, which are an extension to the 292 OAuth bearer tokens. The default token format is defined in CBOR web 293 token (CWT) [RFC8392]. Application layer security for CoAP using 294 COSE can be provided with OSCORE [RFC8613]. 296 With the building blocks listed above, solutions satisfying various 297 IoT device and network constraints are possible. A list of 298 constraints is described in detail in [RFC7228] and a description of 299 how the building blocks mentioned above relate to the various 300 constraints can be found in Appendix A. 302 Luckily, not every IoT device suffers from all constraints. The ACE 303 framework nevertheless takes all these aspects into account and 304 allows several different deployment variants to co-exist, rather than 305 mandating a one-size-fits-all solution. It is important to cover the 306 wide range of possible interworking use cases and the different 307 requirements from a security point of view. Once IoT deployments 308 mature, popular deployment variants will be documented in the form of 309 ACE profiles. 311 3.1. OAuth 2.0 313 The OAuth 2.0 authorization framework enables a client to obtain 314 scoped access to a resource with the permission of a resource owner. 315 Authorization information, or references to it, is passed between the 316 nodes using access tokens. These access tokens are issued to clients 317 by an authorization server with the approval of the resource owner. 318 The client uses the access token to access the protected resources 319 hosted by the resource server. 321 A number of OAuth 2.0 terms are used within this specification: 323 The token and introspection Endpoints: 324 The AS hosts the token endpoint that allows a client to request 325 access tokens. The client makes a POST request to the token 326 endpoint on the AS and receives the access token in the response 327 (if the request was successful). 328 In some deployments, a token introspection endpoint is provided by 329 the AS, which can be used by the RS if it needs to request 330 additional information regarding a received access token. The RS 331 makes a POST request to the introspection endpoint on the AS and 332 receives information about the access token in the response. (See 333 "Introspection" below.) 335 Access Tokens: 336 Access tokens are credentials needed to access protected 337 resources. An access token is a data structure representing 338 authorization permissions issued by the AS to the client. Access 339 tokens are generated by the AS and consumed by the RS. The access 340 token content is opaque to the client. 342 Access tokens can have different formats, and various methods of 343 utilization e.g., cryptographic properties) based on the security 344 requirements of the given deployment. 346 Refresh Tokens: 347 Refresh tokens are credentials used to obtain access tokens. 348 Refresh tokens are issued to the client by the authorization 349 server and are used to obtain a new access token when the current 350 access token becomes invalid or expires, or to obtain additional 351 access tokens with identical or narrower scope (such access tokens 352 may have a shorter lifetime and fewer permissions than authorized 353 by the resource owner). Issuing a refresh token is optional at 354 the discretion of the authorization server. If the authorization 355 server issues a refresh token, it is included when issuing an 356 access token (i.e., step (B) in Figure 1). 358 A refresh token in OAuth 2.0 is a string representing the 359 authorization granted to the client by the resource owner. The 360 string is usually opaque to the client. The token denotes an 361 identifier used to retrieve the authorization information. Unlike 362 access tokens, refresh tokens are intended for use only with 363 authorization servers and are never sent to resource servers. In 364 this framework, refresh tokens are encoded in binary instead of 365 strings, if used. 367 Proof of Possession Tokens: 368 A token may be bound to a cryptographic key, which is then used to 369 bind the token to a request authorized by the token. Such tokens 370 are called proof-of-possession tokens (or PoP tokens). 372 The proof-of-possession (PoP) security concept used here assumes 373 that the AS acts as a trusted third party that binds keys to 374 tokens. In the case of access tokens, these so called PoP keys 375 are then used by the client to demonstrate the possession of the 376 secret to the RS when accessing the resource. The RS, when 377 receiving an access token, needs to verify that the key used by 378 the client matches the one bound to the access token. When this 379 specification uses the term "access token" it is assumed to be a 380 PoP access token token unless specifically stated otherwise. 382 The key bound to the token (the PoP key) may use either symmetric 383 or asymmetric cryptography. The appropriate choice of the kind of 384 cryptography depends on the constraints of the IoT devices as well 385 as on the security requirements of the use case. 387 Symmetric PoP key: 388 The AS generates a random symmetric PoP key. The key is either 389 stored to be returned on introspection calls or encrypted and 390 included in the token. The PoP key is also encrypted for the 391 token recipient and sent to the recipient together with the 392 token. 394 Asymmetric PoP key: 395 An asymmetric key pair is generated on the token's recipient 396 and the public key is sent to the AS (if it does not already 397 have knowledge of the recipient's public key). Information 398 about the public key, which is the PoP key in this case, is 399 either stored to be returned on introspection calls or included 400 inside the token and sent back to the requesting party. The 401 consumer of the token can identify the public key from the 402 information in the token, which allows the recipient of the 403 token to use the corresponding private key for the proof of 404 possession. 406 The token is either a simple reference, or a structured 407 information object (e.g., CWT [RFC8392]) protected by a 408 cryptographic wrapper (e.g., COSE [RFC8152]). The choice of PoP 409 key does not necessarily imply a specific credential type for the 410 integrity protection of the token. 412 Scopes and Permissions: 413 In OAuth 2.0, the client specifies the type of permissions it is 414 seeking to obtain (via the scope parameter) in the access token 415 request. In turn, the AS may use the scope response parameter to 416 inform the client of the scope of the access token issued. As the 417 client could be a constrained device as well, this specification 418 defines the use of CBOR encoding, see Section 5, for such requests 419 and responses. 421 The values of the scope parameter in OAuth 2.0 are expressed as a 422 list of space-delimited, case-sensitive strings, with a semantic 423 that is well-known to the AS and the RS. More details about the 424 concept of scopes is found under Section 3.3 in [RFC6749]. 426 Claims: 427 Information carried in the access token or returned from 428 introspection, called claims, is in the form of name-value pairs. 429 An access token may, for example, include a claim identifying the 430 AS that issued the token (via the "iss" claim) and what audience 431 the access token is intended for (via the "aud" claim). The 432 audience of an access token can be a specific resource or one or 433 many resource servers. The resource owner policies influence what 434 claims are put into the access token by the authorization server. 436 While the structure and encoding of the access token varies 437 throughout deployments, a standardized format has been defined 438 with the JSON Web Token (JWT) [RFC7519] where claims are encoded 439 as a JSON object. In [RFC8392], an equivalent format using CBOR 440 encoding (CWT) has been defined. 442 Introspection: 443 Introspection is a method for a resource server to query the 444 authorization server for the active state and content of a 445 received access token. This is particularly useful in those cases 446 where the authorization decisions are very dynamic and/or where 447 the received access token itself is an opaque reference rather 448 than a self-contained token. More information about introspection 449 in OAuth 2.0 can be found in [RFC7662]. 451 3.2. CoAP 453 CoAP is an application layer protocol similar to HTTP, but 454 specifically designed for constrained environments. CoAP typically 455 uses datagram-oriented transport, such as UDP, where reordering and 456 loss of packets can occur. A security solution needs to take the 457 latter aspects into account. 459 While HTTP uses headers and query strings to convey additional 460 information about a request, CoAP encodes such information into 461 header parameters called 'options'. 463 CoAP supports application-layer fragmentation of the CoAP payloads 464 through blockwise transfers [RFC7959]. However, blockwise transfer 465 does not increase the size limits of CoAP options, therefore data 466 encoded in options has to be kept small. 468 Transport layer security for CoAP can be provided by DTLS or TLS 469 [RFC6347][RFC8446] [I-D.ietf-tls-dtls13]. CoAP defines a number of 470 proxy operations that require transport layer security to be 471 terminated at the proxy. One approach for protecting CoAP 472 communication end-to-end through proxies, and also to support 473 security for CoAP over a different transport in a uniform way, is to 474 provide security at the application layer using an object-based 475 security mechanism such as COSE [RFC8152]. 477 One application of COSE is OSCORE [RFC8613], which provides end-to- 478 end confidentiality, integrity and replay protection, and a secure 479 binding between CoAP request and response messages. In OSCORE, the 480 CoAP messages are wrapped in COSE objects and sent using CoAP. 482 This framework RECOMMENDS the use of CoAP as replacement for HTTP for 483 use in constrained environments. For communication security this 484 framework does not make an explicit protocol recommendation, since 485 the choice depends on the requirements of the specific application. 486 DTLS [RFC6347], [I-D.ietf-tls-dtls13] and OSCORE [RFC8613] are 487 mentioned as examples, other protocols fulfilling the requirements 488 from Section 6.5 are also applicable. 490 4. Protocol Interactions 492 The ACE framework is based on the OAuth 2.0 protocol interactions 493 using the token endpoint and optionally the introspection endpoint. 494 A client obtains an access token, and optionally a refresh token, 495 from an AS using the token endpoint and subsequently presents the 496 access token to an RS to gain access to a protected resource. In 497 most deployments the RS can process the access token locally, however 498 in some cases the RS may present it to the AS via the introspection 499 endpoint to get fresh information. These interactions are shown in 500 Figure 1. An overview of various OAuth concepts is provided in 501 Section 3.1. 503 The OAuth 2.0 framework defines a number of "protocol flows" via 504 grant types, which have been extended further with extensions to 505 OAuth 2.0 (such as [RFC7521] and [RFC8628]). What grant types works 506 best depends on the usage scenario and [RFC7744] describes many 507 different IoT use cases but there are two preferred grant types, 508 namely the Authorization Code Grant (described in Section 4.1 of 509 [RFC7521]) and the Client Credentials Grant (described in Section 4.4 510 of [RFC7521]). The Authorization Code Grant is a good fit for use 511 with apps running on smart phones and tablets that request access to 512 IoT devices, a common scenario in the smart home environment, where 513 users need to go through an authentication and authorization phase 514 (at least during the initial setup phase). The native apps 515 guidelines described in [RFC8252] are applicable to this use case. 516 The Client Credential Grant is a good fit for use with IoT devices 517 where the OAuth client itself is constrained. In such a case, the 518 resource owner has pre-arranged access rights for the client with the 519 authorization server, which is often accomplished using a 520 commissioning tool. 522 The consent of the resource owner, for giving a client access to a 523 protected resource, can be provided dynamically as in the traditional 524 OAuth flows, or it could be pre-configured by the resource owner as 525 authorization policies at the AS, which the AS evaluates when a token 526 request arrives. The resource owner and the requesting party (i.e., 527 client owner) are not shown in Figure 1. 529 This framework supports a wide variety of communication security 530 mechanisms between the ACE entities, such as client, AS, and RS. It 531 is assumed that the client has been registered (also called enrolled 532 or onboarded) to an AS using a mechanism defined outside the scope of 533 this document. In practice, various techniques for onboarding have 534 been used, such as factory-based provisioning or the use of 535 commissioning tools. Regardless of the onboarding technique, this 536 provisioning procedure implies that the client and the AS exchange 537 credentials and configuration parameters. These credentials are used 538 to mutually authenticate each other and to protect messages exchanged 539 between the client and the AS. 541 It is also assumed that the RS has been registered with the AS, 542 potentially in a similar way as the client has been registered with 543 the AS. Established keying material between the AS and the RS allows 544 the AS to apply cryptographic protection to the access token to 545 ensure that its content cannot be modified, and if needed, that the 546 content is confidentiality protected. 548 The keying material necessary for establishing communication security 549 between C and RS is dynamically established as part of the protocol 550 described in this document. 552 At the start of the protocol, there is an optional discovery step 553 where the client discovers the resource server and the resources this 554 server hosts. In this step, the client might also determine what 555 permissions are needed to access the protected resource. A generic 556 procedure is described in Section 5.1; profiles MAY define other 557 procedures for discovery. 559 In Bluetooth Low Energy, for example, advertisements are broadcasted 560 by a peripheral, including information about the primary services. 562 In CoAP, as a second example, a client can make a request to "/.well- 563 known/core" to obtain information about available resources, which 564 are returned in a standardized format as described in [RFC6690]. 566 +--------+ +---------------+ 567 | |---(A)-- Token Request ------->| | 568 | | | Authorization | 569 | |<--(B)-- Access Token ---------| Server | 570 | | + Access Information | | 571 | | + Refresh Token (optional) +---------------+ 572 | | ^ | 573 | | Introspection Request (D)| | 574 | Client | (optional) | | 575 | | Response | |(E) 576 | | (optional) | v 577 | | +--------------+ 578 | |---(C)-- Token + Request ----->| | 579 | | | Resource | 580 | |<--(F)-- Protected Resource ---| Server | 581 | | | | 582 +--------+ +--------------+ 584 Figure 1: Basic Protocol Flow. 586 Requesting an Access Token (A): 587 The client makes an access token request to the token endpoint at 588 the AS. This framework assumes the use of PoP access tokens (see 589 Section 3.1 for a short description) wherein the AS binds a key to 590 an access token. The client may include permissions it seeks to 591 obtain, and information about the credentials it wants to use 592 (e.g., symmetric/asymmetric cryptography or a reference to a 593 specific credential). 595 Access Token Response (B): 596 If the AS successfully processes the request from the client, it 597 returns an access token and optionally a refresh token (note that 598 only certain grant types support refresh tokens). It can also 599 return additional parameters, referred to as "Access Information". 600 In addition to the response parameters defined by OAuth 2.0 and 601 the PoP access token extension, this framework defines parameters 602 that can be used to inform the client about capabilities of the 603 RS, e.g. the profiles the RS supports. More information about 604 these parameters can be found in Section 5.6.4. 606 Resource Request (C): 607 The client interacts with the RS to request access to the 608 protected resource and provides the access token. The protocol to 609 use between the client and the RS is not restricted to CoAP. 610 HTTP, HTTP/2, QUIC, MQTT, Bluetooth Low Energy, etc., are also 611 viable candidates. 613 Depending on the device limitations and the selected protocol, 614 this exchange may be split up into two parts: 616 (1) the client sends the access token containing, or 617 referencing, the authorization information to the RS, that may 618 be used for subsequent resource requests by the client, and 620 (2) the client makes the resource access request, using the 621 communication security protocol and other Access Information 622 obtained from the AS. 624 The Client and the RS mutually authenticate using the security 625 protocol specified in the profile (see step B) and the keys 626 obtained in the access token or the Access Information. The RS 627 verifies that the token is integrity protected and originated by 628 the AS. It then compares the claims contained in the access token 629 with the resource request. If the RS is online, validation can be 630 handed over to the AS using token introspection (see messages D 631 and E) over HTTP or CoAP. 633 Token Introspection Request (D): 634 A resource server may be configured to introspect the access token 635 by including it in a request to the introspection endpoint at that 636 AS. Token introspection over CoAP is defined in Section 5.7 and 637 for HTTP in [RFC7662]. 639 Note that token introspection is an optional step and can be 640 omitted if the token is self-contained and the resource server is 641 prepared to perform the token validation on its own. 643 Token Introspection Response (E): 644 The AS validates the token and returns the most recent parameters, 645 such as scope, audience, validity etc. associated with it back to 646 the RS. The RS then uses the received parameters to process the 647 request to either accept or to deny it. 649 Protected Resource (F): 650 If the request from the client is authorized, the RS fulfills the 651 request and returns a response with the appropriate response code. 652 The RS uses the dynamically established keys to protect the 653 response, according to the communication security protocol used. 655 5. Framework 657 The following sections detail the profiling and extensions of OAuth 658 2.0 for constrained environments, which constitutes the ACE 659 framework. 661 Credential Provisioning 662 For IoT, it cannot be assumed that the client and RS are part of a 663 common key infrastructure, so the AS provisions credentials or 664 associated information to allow mutual authentication between 665 client and RS. The resulting security association between client 666 and RS may then also be used to bind these credentials to the 667 access tokens the client uses. 669 Proof-of-Possession 670 The ACE framework, by default, implements proof-of-possession for 671 access tokens, i.e., that the token holder can prove being a 672 holder of the key bound to the token. The binding is provided by 673 the "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession] indicating 674 what key is used for proof-of-possession. If a client needs to 675 submit a new access token, e.g., to obtain additional access 676 rights, they can request that the AS binds this token to the same 677 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 8.15). 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 [I-D.ietf-oauth-token-exchange] is 1013 OPTIONAL to request an 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 8.4 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 8.6, 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 8.7. 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 8.9, 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 8.3. 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 8.11, 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 1601 [I-D.ietf-ace-cwt-proof-of-possession] and the "scope" claim from 1602 [I-D.ietf-oauth-token-exchange] for JWT- and CWT-encoded tokens. In 1603 addition to string encoding specified for the "scope" claim, a binary 1604 encoding MAY be used. The syntax of such an encoding is explicitly 1605 not specified here and left to profiles or applications, specifically 1606 note that a binary encoded scope does not necessarily use the space 1607 character '0x20' to delimit scope-tokens. 1609 If the AS needs to convey a hint to the RS about which profile it 1610 should use to communicate with the client, the AS MAY include an 1611 "ace_profile" claim in the access token, with the same syntax and 1612 semantics as defined in Section 5.6.4.3. 1614 If the client submitted a client-nonce parameter in the access token 1615 request Section 5.6.4.4, the AS MUST include the value of this 1616 parameter in the "cnonce" claim specified here. The "cnonce" claim 1617 uses binary encoding. 1619 5.8.1. The Authorization Information Endpoint 1621 The access token, containing authorization information and 1622 information about the proof-of-possession method used by the client, 1623 needs to be transported to the RS so that the RS can authenticate and 1624 authorize the client request. 1626 This section defines a method for transporting the access token to 1627 the RS using a RESTful protocol such as CoAP. Profiles of this 1628 framework MAY define other methods for token transport. 1630 The method consists of an authz-info endpoint, implemented by the RS. 1631 A client using this method MUST make a POST request to the authz-info 1632 endpoint at the RS with the access token in the payload. The RS 1633 receiving the token MUST verify the validity of the token. If the 1634 token is valid, the RS MUST respond to the POST request with 2.01 1635 (Created). Section Section 5.8.1.1 outlines how an RS MUST proceed 1636 to verify the validity of an access token. 1638 The RS MUST be prepared to store at least one access token for future 1639 use. This is a difference to how access tokens are handled in OAuth 1640 2.0, where the access token is typically sent along with each 1641 request, and therefore not stored at the RS. 1643 This specification RECOMMENDS that an RS stores only one token per 1644 proof-of-possession key, meaning that an additional token linked to 1645 the same key will overwrite any existing token at the RS. The reason 1646 is that this greatly simplifies (constrained) implementations, with 1647 respect to required storage and resolving a request to the applicable 1648 token. 1650 If the payload sent to the authz-info endpoint does not parse to a 1651 token, the RS MUST respond with a response code equivalent to the 1652 CoAP code 4.00 (Bad Request). 1654 The RS MAY make an introspection request to validate the token before 1655 responding to the POST request to the authz-info endpoint, e.g. if 1656 the token is an opaque reference. Some transport protocols may 1657 provide a way to indicate that the RS is busy and the client should 1658 retry after an interval; this type of status update would be 1659 appropriate while the RS is waiting for an introspection response. 1661 Profiles MUST specify whether the authz-info endpoint is protected, 1662 including whether error responses from this endpoint are protected. 1663 Note that since the token contains information that allow the client 1664 and the RS to establish a security context in the first place, mutual 1665 authentication may not be possible at this point. 1667 The default name of this endpoint in an url-path is '/authz-info', 1668 however implementations are not required to use this name and can 1669 define their own instead. 1671 5.8.1.1. Verifying an Access Token 1673 When an RS receives an access token, it MUST verify it before storing 1674 it. The details of token verification depends on various aspects, 1675 including the token encoding, the type of token, the security 1676 protection applied to the token, and the claims. The token encoding 1677 matters since the security wrapper differs between the token 1678 encodings. For example, a CWT token uses COSE while a JWT token uses 1679 JOSE. The type of token also has an influence on the verification 1680 procedure since tokens may be self-contained whereby token 1681 verification may happen locally at the RS while a token-by-reference 1682 requires further interaction with the authorization server, for 1683 example using token introspection, to obtain the claims associated 1684 with the token reference. Self-contained tokens MUST, at a minimum, 1685 be integrity protected but they MAY also be encrypted. 1687 For self-contained tokens the RS MUST process the security protection 1688 of the token first, as specified by the respective token format. For 1689 CWT the description can be found in [RFC8392] and for JWT the 1690 relevant specification is [RFC7519]. This MUST include a 1691 verification that security protection (and thus the token) was 1692 generated by an AS that has the right to issue access tokens for this 1693 RS. 1695 In case the token is communicated by reference the RS needs to obtain 1696 the claims first. When the RS uses token introspection the relevant 1697 specification is [RFC7662] with CoAP transport specified in 1698 Section 5.7. 1700 Errors may happen during this initial processing stage: 1702 o If token or claim verification fails, the RS MUST discard the 1703 token and, if this was an interaction with authz-info, return an 1704 error message with a response code equivalent to the CoAP code 1705 4.01 (Unauthorized). 1707 o If the claims cannot be obtained the RS MUST discard the token 1708 and, in case of an interaction via the authz-info endpoint, return 1709 an error message with a response code equivalent to the CoAP code 1710 4.00 (Bad Request). 1712 Next, the RS MUST verify claims, if present, contained in the access 1713 token. Errors are returned when claim checks fail, in the order of 1714 priority of this list: 1716 iss The issuer claim must identify an AS that has the authority to 1717 issue access tokens for the receiving RS. If that is not the case 1718 the RS MUST discard the token. If this was an interaction with 1719 authz-info, the RS MUST also respond with a response code 1720 equivalent to the CoAP code 4.01 (Unauthorized). 1722 exp The expiration date must be in the future. If that is not the 1723 case the RS MUST discard the token. If this was an interaction 1724 with authz-info the RS MUST also respond with a response code 1725 equivalent to the CoAP code 4.01 (Unauthorized). Note that the RS 1726 has to terminate access rights to the protected resources at the 1727 time when the tokens expire. 1729 aud The audience claim must refer to an audience that the RS 1730 identifies with. If that is not the case the RS MUST discard the 1731 token. If this was an interaction with authz-info, the RS MUST 1732 also respond with a response code equivalent to the CoAP code 4.03 1733 (Forbidden). 1735 scope The RS must recognize value of the scope claim. If that is 1736 not the case the RS MUST discard the token. If this was an 1737 interaction with authz-info, the RS MUST also respond with a 1738 response code equivalent to the CoAP code 4.00 (Bad Request). The 1739 RS MAY provide additional information in the error response, to 1740 clarify what went wrong. 1742 Additional processing may be needed for other claims in a way 1743 specific to a profile or the underlying application. 1745 Note that the Subject (sub) claim cannot always be verified when the 1746 token is submitted to the RS since the client may not have 1747 authenticated yet. Also note that a counter for the expires_in (exi) 1748 claim MUST be initialized when the RS first verifies this token. 1750 Also note that profiles of this framework may define access token 1751 transport mechanisms that do not allow for error responses. 1752 Therefore the error messages specified here only apply if the token 1753 was sent to the authz-info endpoint. 1755 When sending error responses, the RS MAY use the error codes from 1756 Section 3.1 of [RFC6750], to provide additional details to the 1757 client. 1759 5.8.1.2. Protecting the Authorization Information Endpoint 1761 As this framework can be used in RESTful environments, it is 1762 important to make sure that attackers cannot perform unauthorized 1763 requests on the authz-info endpoints, other than submitting access 1764 tokens. 1766 Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT 1767 on the authz-info endpoint and on it's children (if any). 1769 The POST method SHOULD NOT be allowed on children of the authz-info 1770 endpoint. 1772 The RS SHOULD implement rate limiting measures to mitigate attacks 1773 aiming to overload the processing capacity of the RS by repeatedly 1774 submitting tokens. For CoAP-based communication the RS could use the 1775 mechanisms from [RFC8516] to indicate that it is overloaded. 1777 5.8.2. Client Requests to the RS 1779 Before sending a request to an RS, the client MUST verify that the 1780 keys used to protect this communication are still valid. See 1781 Section 5.8.4 for details on how the client determines the validity 1782 of the keys used. 1784 If an RS receives a request from a client, and the target resource 1785 requires authorization, the RS MUST first verify that it has an 1786 access token that authorizes this request, and that the client has 1787 performed the proof-of-possession binding that token to the request. 1789 The response code MUST be 4.01 (Unauthorized) in case the client has 1790 not performed the proof-of-possession, or if RS has no valid access 1791 token for the client. If RS has an access token for the client but 1792 the token does not authorize access for the resource that was 1793 requested, RS MUST reject the request with a 4.03 (Forbidden). If RS 1794 has an access token for the client but it does not cover the action 1795 that was requested on the resource, RS MUST reject the request with a 1796 4.05 (Method Not Allowed). 1798 Note: The use of the response codes 4.03 and 4.05 is intended to 1799 prevent infinite loops where a dumb Client optimistically tries to 1800 access a requested resource with any access token received from AS. 1801 As malicious clients could pretend to be C to determine C's 1802 privileges, these detailed response codes must be used only when a 1803 certain level of security is already available which can be achieved 1804 only when the Client is authenticated. 1806 Note: The RS MAY use introspection for timely validation of an access 1807 token, at the time when a request is presented. 1809 Note: Matching the claims of the access token (e.g., scope) to a 1810 specific request is application specific. 1812 If the request matches a valid token and the client has performed the 1813 proof-of-possession for that token, the RS continues to process the 1814 request as specified by the underlying application. 1816 5.8.3. Token Expiration 1818 Depending on the capabilities of the RS, there are various ways in 1819 which it can verify the expiration of a received access token. Here 1820 follows a list of the possibilities including what functionality they 1821 require of the RS. 1823 o The token is a CWT and includes an "exp" claim and possibly the 1824 "nbf" claim. The RS verifies these by comparing them to values 1825 from its internal clock as defined in [RFC7519]. In this case the 1826 RS's internal clock must reflect the current date and time, or at 1827 least be synchronized with the AS's clock. How this clock 1828 synchronization would be performed is out of scope for this 1829 specification. 1831 o The RS verifies the validity of the token by performing an 1832 introspection request as specified in Section 5.7. This requires 1833 the RS to have a reliable network connection to the AS and to be 1834 able to handle two secure sessions in parallel (C to RS and RS to 1835 AS). 1837 o In order to support token expiration for devices that have no 1838 reliable way of synchronizing their internal clocks, this 1839 specification defines the following approach: The claim "exi" 1840 ("expires in") can be used, to provide the RS with the lifetime of 1841 the token in seconds from the time the RS first receives the 1842 token. For CBOR-based interaction this parameter is encoded as 1843 unsigned integer, while JSON-based interactions encode this as 1844 JSON number. 1846 o Processing this claim requires that the RS does the following: 1848 * For each token the RS receives, that contains an "exi" claim: 1849 Keep track of the time it received that token and revisit that 1850 list regularly to expunge expired tokens. 1852 * Keep track of the identifiers of tokens containing the "exi" 1853 claim that have expired (in order to avoid accepting them 1854 again). 1856 If a token that authorizes a long running request such as a CoAP 1857 Observe [RFC7641] expires, the RS MUST send an error response with 1858 the response code equivalent to the CoAP code 4.01 (Unauthorized) to 1859 the client and then terminate processing the long running request. 1861 5.8.4. Key Expiration 1863 The AS provides the client with key material that the RS uses. This 1864 can either be a common symmetric PoP-key, or an asymmetric key used 1865 by the RS to authenticate towards the client. Since there is 1866 currently no expiration metadata associated to those keys, the client 1867 has no way of knowing if these keys are still valid. This may lead 1868 to situations where the client sends requests containing sensitive 1869 information to the RS using a key that is expired and possibly in the 1870 hands of an attacker, or accepts responses from the RS that are not 1871 properly protected and could possibly have been forged by an 1872 attacker. 1874 In order to prevent this, the client must assume that those keys are 1875 only valid as long as the related access token is. Since the access 1876 token is opaque to the client, one of the following methods MUST be 1877 used to inform the client about the validity of an access token: 1879 o The client knows a default validity time for all tokens it is 1880 using (i.e. how long a token is valid after being issued). This 1881 information could be provisioned to the client when it is 1882 registered at the AS, or published by the AS in a way that the 1883 client can query. 1885 o The AS informs the client about the token validity using the 1886 "expires_in" parameter in the Access Information. 1888 A client that is not able to obtain information about the expiration 1889 of a token MUST NOT use this token. 1891 6. Security Considerations 1893 Security considerations applicable to authentication and 1894 authorization in RESTful environments provided in OAuth 2.0 [RFC6749] 1895 apply to this work. Furthermore [RFC6819] provides additional 1896 security considerations for OAuth which apply to IoT deployments as 1897 well. If the introspection endpoint is used, the security 1898 considerations from [RFC7662] also apply. 1900 The following subsections address issues specific to this document 1901 and it's use in constrained environments. 1903 6.1. Protecting Tokens 1905 A large range of threats can be mitigated by protecting the contents 1906 of the access token by using a digital signature or a keyed message 1907 digest (MAC) or an Authenticated Encryption with Associated Data 1908 (AEAD) algorithm. Consequently, the token integrity protection MUST 1909 be applied to prevent the token from being modified, particularly 1910 since it contains a reference to the symmetric key or the asymmetric 1911 key used for proof-of-possession. If the access token contains the 1912 symmetric key, this symmetric key MUST be encrypted by the 1913 authorization server so that only the resource server can decrypt it. 1914 Note that using an AEAD algorithm is preferable over using a MAC 1915 unless the token needs to be publicly readable. 1917 If the token is intended for multiple recipients (i.e. an audience 1918 that is a group), integrity protection of the token with a symmetric 1919 key, shared between the AS and the recipients, is not sufficient, 1920 since any of the recipients could modify the token undetected by the 1921 other recipients. Therefore a token with a multi-recipient audience 1922 MUST be protected with an asymmetric signature. 1924 It is important for the authorization server to include the identity 1925 of the intended recipient (the audience), typically a single resource 1926 server (or a list of resource servers), in the token. The same 1927 shared secret MUST NOT be used as proof-of-possession key with 1928 multiple resource servers since the benefit from using the proof-of- 1929 possession concept is then significantly reduced. 1931 If clients are capable of doing so, they should frequently request 1932 fresh access tokens, as this allows the AS to keep the lifetime of 1933 the tokens short. This allows the AS to use shorter proof-of- 1934 possession key sizes, which translate to a performance benefit for 1935 the client and for the resource server. Shorter keys also lead to 1936 shorter messages (particularly with asymmetric keying material). 1938 When authorization servers bind symmetric keys to access tokens, they 1939 SHOULD scope these access tokens to a specific permission. 1941 In certain situations it may be necessary to revoke an access token 1942 that is still valid. Client-initiated revocation is specified in 1943 [RFC7009] for OAuth 2.0. Other revocation mechanisms are currently 1944 not specified, as the underlying assumption in OAuth is that access 1945 tokens are issued with a relatively short lifetime. This may not 1946 hold true for disconnected constrained devices, needing access tokens 1947 with relatively long lifetimes, and would therefore necessitate 1948 further standardization work that is out of scope for this document. 1950 6.2. Communication Security 1952 Communication with the authorization server MUST use confidentiality 1953 protection. This step is extremely important since the client or the 1954 RS may obtain the proof-of-possession key from the authorization 1955 server for use with a specific access token. Not using 1956 confidentiality protection exposes this secret (and the access token) 1957 to an eavesdropper thereby completely negating proof-of-possession 1958 security. Profiles MUST specify how communication security according 1959 to the requirements in Section 5 is provided. 1961 Additional protection for the access token can be applied by 1962 encrypting it, for example encryption of CWTs is specified in 1963 Section 5.1 of [RFC8392]. Such additional protection can be 1964 necessary if the token is later transferred over an insecure 1965 connection (e.g. when it is sent to the authz-info endpoint). 1967 Developers MUST ensure that the ephemeral credentials (i.e., the 1968 private key or the session key) are not leaked to third parties. An 1969 adversary in possession of the ephemeral credentials bound to the 1970 access token will be able to impersonate the client. Be aware that 1971 this is a real risk with many constrained environments, since 1972 adversaries can often easily get physical access to the devices. 1973 This risk can also be mitigated to some extent by making sure that 1974 keys are refreshed more frequently. 1976 6.3. Long-Term Credentials 1978 Both clients and RSs have long-term credentials that are used to 1979 secure communications, and authenticate to the AS. These credentials 1980 need to be protected against unauthorized access. In constrained 1981 devices, deployed in publicly accessible places, such protection can 1982 be difficult to achieve without specialized hardware (e.g. secure key 1983 storage memory). 1985 If credentials are lost or compromised, the operator of the affected 1986 devices needs to have procedures to invalidate any access these 1987 credentials give and to revoke tokens linked to such credentials. 1988 The loss of a credential linked to a specific device MUST NOT lead to 1989 a compromise of other credentials not linked to that device, 1990 therefore secret keys used for authentication MUST NOT be shared 1991 between more than two parties. 1993 Operators of clients or RS SHOULD have procedures in place to replace 1994 credentials that are suspected to have been compromised or that have 1995 been lost. 1997 Operators also SHOULD have procedures for decommissioning devices, 1998 that include securely erasing credentials and other security critical 1999 material in the devices being decommissioned. 2001 6.4. Unprotected AS Request Creation Hints 2003 Initially, no secure channel exists to protect the communication 2004 between C and RS. Thus, C cannot determine if the AS Request 2005 Creation Hints contained in an unprotected response from RS to an 2006 unauthorized request (see Section 5.1.2) are authentic. It is 2007 therefore advisable to provide C with a (possibly hard-coded) list of 2008 trustworthy authorization servers, possibly including information 2009 used to authenticate the AS, such as a public key or certificate 2010 fingerprint. AS Request Creation Hints referring to a URI not listed 2011 there would be ignored. 2013 A compromised RS may use the hints to trick a client into contacting 2014 an AS that is not supposed to be in charge of that RS. Since this AS 2015 must be in the hard-coded list of trusted AS no violation of 2016 privileges and or exposure of credentials should happen, since a 2017 trusted AS is expected to refuse requestes for which it is not 2018 applicable and render a corresponding error response. However a 2019 compromised RS may use this to perform a denial of service against a 2020 specific AS, by redirecting a large number of client requests to that 2021 AS. 2023 A compromised client can be made to contact any AS, including 2024 compromised ones. This should not affect the RS, since it is 2025 supposed to keep track of which AS are trusted and have corresponding 2026 credentials to verify the source of access tokens it receives. 2028 6.5. Minimal security requirements for communication 2030 This section summarizes the minimal requirements for the 2031 communication security of the different protocol interactions. 2033 C-AS All communication between the client and the Authorization 2034 Server MUST be encrypted, integrity and replay protected. 2035 Furthermore responses from the AS to the client MUST be bound to 2036 the client's request to avoid attacks where the attacker swaps the 2037 intended response for an older one valid for a previous request. 2038 This requires that the client and the Authorization Server have 2039 previously exchanged either a shared secret or their public keys 2040 in order to negotiate a secure communication. Furthermore the 2041 client MUST be able to determine whether an AS has the authority 2042 to issue access tokens for a certain RS. This can for example be 2043 done through pre-configured lists, or through an online lookup 2044 mechanism that in turn also must be secured. 2046 RS-AS The communication between the Resource Server and the 2047 Authorization Server via the introspection endpoint MUST be 2048 encrypted, integrity and replay protected. Furthermore responses 2049 from the AS to the RS MUST be bound to the RS's request. This 2050 requires that the RS and the Authorization Server have previously 2051 exchanged either a shared secret, or their public keys in order to 2052 negotiate a secure communication. Furthermore the RS MUST be able 2053 to determine whether an AS has the authority to issue access 2054 tokens itself. This is usually configured out of band, but could 2055 also be performed through an online lookup mechanism provided that 2056 it is also secured in the same way. 2058 C-RS The initial communication between the client and the Resource 2059 Server can not be secured in general, since the RS is not in 2060 possession of on access token for that client, which would carry 2061 the necessary parameters. If both parties support DTLS without 2062 client authentication it is RECOMMEND to use this mechanism for 2063 protecting the initial communication. After the client has 2064 successfully transmitted the access token to the RS, a secure 2065 communication protocol MUST be established between client and RS 2066 for the actual resource request. This protocol MUST provide 2067 confidentiality, integrity and replay protection as well as a 2068 binding between requests and responses. This requires that the 2069 client learned either the RS's public key or received a symmetric 2070 proof-of-possession key bound to the access token from the AS. 2071 The RS must have learned either the client's public key or a 2072 shared symmetric key from the claims in the token or an 2073 introspection request. Since ACE does not provide profile 2074 negotiation between C and RS, the client MUST have learned what 2075 profile the RS supports (e.g. from the AS or pre-configured) and 2076 initiate the communication accordingly. 2078 6.6. Token Freshness and Expiration 2080 An RS that is offline faces the problem of clock drift. Since it 2081 cannot synchronize its clock with the AS, it may be tricked into 2082 accepting old access tokens that are no longer valid or have been 2083 compromised. In order to prevent this, an RS may use the nonce-based 2084 mechanism defined in Section 5.1.2 to ensure freshness of an Access 2085 Token subsequently presented to this RS. 2087 Another problem with clock drift is that evaluating the standard 2088 token expiration claim "exp" can give unpredictable results. 2090 Acceptable ranges of clock drift are highly dependent on the concrete 2091 application. Important factors are how long access tokens are valid, 2092 and how critical timely expiration of access token is. 2094 The expiration mechanism implemented by the "exi" claim, based on the 2095 first time the RS sees the token was defined to provide a more 2096 predictable alternative. The "exi" approach has some drawbacks that 2097 need to be considered: 2099 A malicious client may hold back tokens with the "exi" claim in 2100 order to prolong their lifespan. 2102 If an RS loses state (e.g. due to an unscheduled reboot), it may 2103 loose the current values of counters tracking the "exi" claims of 2104 tokens it is storing. 2106 The RS needs to keep state about expired tokens that used "exi" in 2107 order to be sure not to accept it again. Attacker may use this to 2108 deplete the RS's storage resources. 2110 The first drawback is inherent to the deployment scenario and the 2111 "exi" solution. It can therefore not be mitigated without requiring 2112 the the RS be online at times. The second drawback can be mitigated 2113 by regularly storing the value of "exi" counters to persistent 2114 memory. The third problem can be mitigated by the AS, by assigning 2115 identifiers (e.g. 'cti') to the tokens, that include an RS identifier 2116 and a sequence number. The RS would then just have to store the 2117 highest sequence number of an expired token it has seen, thus 2118 limiting the necessary state. 2120 6.7. Combining profiles 2122 There may be use cases were different profiles of this framework are 2123 combined. For example, an MQTT-TLS profile is used between the 2124 client and the RS in combination with a CoAP-DTLS profile for 2125 interactions between the client and the AS. The security of a 2126 profile MUST NOT depend on the assumption that the profile is used 2127 for all the different types of interactions in this framework. 2129 6.8. Unprotected Information 2131 Communication with the authz-info endpoint, as well as the various 2132 error responses defined in this framework, all potentially include 2133 sending information over an unprotected channel. These messages may 2134 leak information to an adversary, or may be manipulated by active 2135 attackers to induce incorrect behavior. For example error responses 2136 for requests to the Authorization Information endpoint can reveal 2137 information about an otherwise opaque access token to an adversary 2138 who has intercepted this token. 2140 As far as error messages are concerned, this framework is written 2141 under the assumption that, in general, the benefits of detailed error 2142 messages outweigh the risk due to information leakage. For 2143 particular use cases, where this assessment does not apply, detailed 2144 error messages can be replaced by more generic ones. 2146 In some scenarios it may be possible to protect the communication 2147 with the authz-info endpoint (e.g. through DTLS with only server-side 2148 authentication). In cases where this is not possible this framework 2149 RECOMMENDS to use encrypted CWTs or tokens that are opaque references 2150 and need to be subjected to introspection by the RS. 2152 If the initial unauthorized resource request message (see 2153 Section 5.1.1) is used, the client MUST make sure that it is not 2154 sending sensitive content in this request. While GET and DELETE 2155 requests only reveal the target URI of the resource, POST and PUT 2156 requests would reveal the whole payload of the intended operation. 2158 Since the client is not authenticated at the point when it is 2159 submitting an access token to the authz-info endpoint, attackers may 2160 be pretending to be a client and trying to trick an RS to use an 2161 obsolete profile that in turn specifies a vulnerable security 2162 mechanism via the authz-info endpoint. Such an attack would require 2163 a valid access token containing an "ace_profile" claim requesting the 2164 use of said obsolete profile. Resource Owners should update the 2165 configuration of their RS's to prevent them from using such obsolete 2166 profiles. 2168 6.9. Identifying audiences 2170 The audience claim as defined in [RFC7519] and the equivalent 2171 "audience" parameter from [I-D.ietf-oauth-token-exchange] are 2172 intentionally vague on how to match the audience value to a specific 2173 RS. This is intended to allow application specific semantics to be 2174 used. This section attempts to give some general guidance for the 2175 use of audiences in constrained environments. 2177 URLs are not a good way of identifying mobile devices that can switch 2178 networks and thus be associated with new URLs. If the audience 2179 represents a single RS, and asymmetric keys are used, the RS can be 2180 uniquely identified by a hash of its public key. If this approach is 2181 used this framework RECOMMENDS to apply the procedure from section 3 2182 of [RFC6920]. 2184 If the audience addresses a group of resource servers, the mapping of 2185 group identifier to individual RS has to be provisioned to each RS 2186 before the group-audience is usable. Managing dynamic groups could 2187 be an issue, if any RS is not always reachable when the groups' 2188 memberships change. Furthermore, issuing access tokens bound to 2189 symmetric proof-of-possession keys that apply to a group-audience is 2190 problematic, as an RS that is in possession of the access token can 2191 impersonate the client towards the other RSs that are part of the 2192 group. It is therefore NOT RECOMMENDED to issue access tokens bound 2193 to a group audience and symmetric proof-of possession keys. 2195 Even the client must be able to determine the correct values to put 2196 into the "audience" parameter, in order to obtain a token for the 2197 intended RS. Errors in this process can lead to the client 2198 inadvertently obtaining a token for the wrong RS. The correct values 2199 for "audience" can either be provisioned to the client as part of its 2200 configuration, or dynamically looked up by the client in some 2201 directory. In the latter case the integrity and correctness of the 2202 directory data must be assured. Note that the "audience" hint 2203 provided by the RS as part of the "AS Request Creation Hints" 2204 Section 5.1.2 is not typically source authenticated and integrity 2205 protected, and should therefore not be treated a trusted value. 2207 6.10. Denial of service against or with Introspection 2209 The optional introspection mechanism provided by OAuth and supported 2210 in the ACE framework allows for two types of attacks that need to be 2211 considered by implementers. 2213 First, an attacker could perform a denial of service attack against 2214 the introspection endpoint at the AS in order to prevent validation 2215 of access tokens. To maintain the security of the system, an RS that 2216 is configured to use introspection MUST NOT allow access based on a 2217 token for which it couldn't reach the introspection endpoint. 2219 Second, an attacker could use the fact that an RS performs 2220 introspection to perform a denial of service attack against that RS 2221 by repeatedly sending tokens to its authz-info endpoint that require 2222 an introspection call. RS can mitigate such attacks by implementing 2223 rate limits on how many introspection requests they perform in a 2224 given time interval for a certain client IP address submitting tokens 2225 to /authz-info. When that limit has been reached, incoming requests 2226 from that address are rejected for a certain amount of time. A 2227 general rate limit on the introspection requests should also be 2228 considered, to mitigate distributed attacks. 2230 7. Privacy Considerations 2232 Implementers and users should be aware of the privacy implications of 2233 the different possible deployments of this framework. 2235 The AS is in a very central position and can potentially learn 2236 sensitive information about the clients requesting access tokens. If 2237 the client credentials grant is used, the AS can track what kind of 2238 access the client intends to perform. With other grants this can be 2239 prevented by the Resource Owner. To do so, the resource owner needs 2240 to bind the grants it issues to anonymous, ephemeral credentials that 2241 do not allow the AS to link different grants and thus different 2242 access token requests by the same client. 2244 The claims contained in a token can reveal privacy sensitive 2245 information about the client and the RS to any party having access to 2246 them (whether by processing the content of a self-contained token or 2247 by introspection). The AS SHOULD be configured to minimize the 2248 information about clients and RSs disclosed in the tokens it issues. 2250 If tokens are only integrity protected and not encrypted, they may 2251 reveal information to attackers listening on the wire, or able to 2252 acquire the access tokens in some other way. In the case of CWTs the 2253 token may, e.g., reveal the audience, the scope and the confirmation 2254 method used by the client. The latter may reveal the identity of the 2255 device or application running the client. This may be linkable to 2256 the identity of the person using the client (if there is a person and 2257 not a machine-to-machine interaction). 2259 Clients using asymmetric keys for proof-of-possession should be aware 2260 of the consequences of using the same key pair for proof-of- 2261 possession towards different RSs. A set of colluding RSs or an 2262 attacker able to obtain the access tokens will be able to link the 2263 requests, or even to determine the client's identity. 2265 An unprotected response to an unauthorized request (see 2266 Section 5.1.2) may disclose information about RS and/or its existing 2267 relationship with C. It is advisable to include as little 2268 information as possible in an unencrypted response. If means of 2269 encrypting communication between C and RS already exist, more 2270 detailed information may be included with an error response to 2271 provide C with sufficient information to react on that particular 2272 error. 2274 8. IANA Considerations 2276 This document creates several registries with a registration policy 2277 of "Expert Review"; guidelines to the experts are given in 2278 Section 8.16. 2280 8.1. ACE Authorization Server Request Creation Hints 2282 This specification establishes the IANA "ACE Authorization Server 2283 Request Creation Hints" registry. The registry has been created to 2284 use the "Expert Review" registration procedure [RFC8126]. It should 2285 be noted that, in addition to the expert review, some portions of the 2286 registry require a specification, potentially a Standards Track RFC, 2287 be supplied as well. 2289 The columns of the registry are: 2291 Name The name of the parameter 2293 CBOR Key CBOR map key for the parameter. Different ranges of values 2294 use different registration policies [RFC8126]. Integer values 2295 from -256 to 255 are designated as Standards Action. Integer 2296 values from -65536 to -257 and from 256 to 65535 are designated as 2297 Specification Required. Integer values greater than 65535 are 2298 designated as Expert Review. Integer values less than -65536 are 2299 marked as Private Use. 2301 Value Type The CBOR data types allowable for the values of this 2302 parameter. 2304 Reference This contains a pointer to the public specification of the 2305 request creation hint abbreviation, if one exists. 2307 This registry will be initially populated by the values in Figure 2. 2308 The Reference column for all of these entries will be this document. 2310 8.2. OAuth Extensions Error Registration 2312 This specification registers the following error values in the OAuth 2313 Extensions Error registry [IANA.OAuthExtensionsErrorRegistry]. 2315 o Error name: "unsupported_pop_key" 2316 o Error usage location: token error response 2317 o Related protocol extension: [this document] 2318 o Change Controller: IESG 2319 o Specification document(s): Section 5.6.3 of [this document] 2321 o Error name: "incompatible_ace_profiles" 2322 o Error usage location: token error response 2323 o Related protocol extension: [this document] 2324 o Change Controller: IESG 2325 o Specification document(s): Section 5.6.3 of [this document] 2327 8.3. OAuth Error Code CBOR Mappings Registry 2329 This specification establishes the IANA "OAuth Error Code CBOR 2330 Mappings" registry. The registry has been created to use the "Expert 2331 Review" registration procedure [RFC8126], except for the value range 2332 designated for private use. 2334 The columns of the registry are: 2336 Name The OAuth Error Code name, refers to the name in Section 5.2. 2337 of [RFC6749], e.g., "invalid_request". 2338 CBOR Value CBOR abbreviation for this error code. Integer values 2339 less than -65536 are marked as "Private Use", all other values use 2340 the registration policy "Expert Review" [RFC8126]. 2341 Reference This contains a pointer to the public specification of the 2342 error code abbreviation, if one exists. 2344 This registry will be initially populated by the values in Figure 10. 2345 The Reference column for all of these entries will be this document. 2347 8.4. OAuth Grant Type CBOR Mappings 2349 This specification establishes the IANA "OAuth Grant Type CBOR 2350 Mappings" registry. The registry has been created to use the "Expert 2351 Review" registration procedure [RFC8126], except for the value range 2352 designated for private use. 2354 The columns of this registry are: 2356 Name The name of the grant type as specified in Section 1.3 of 2357 [RFC6749]. 2359 CBOR Value CBOR abbreviation for this grant type. Integer values 2360 less than -65536 are marked as "Private Use", all other values use 2361 the registration policy "Expert Review" [RFC8126]. 2362 Reference This contains a pointer to the public specification of the 2363 grant type abbreviation, if one exists. 2364 Original Specification This contains a pointer to the public 2365 specification of the grant type, if one exists. 2367 This registry will be initially populated by the values in Figure 11. 2368 The Reference column for all of these entries will be this document. 2370 8.5. OAuth Access Token Types 2372 This section registers the following new token type in the "OAuth 2373 Access Token Types" registry [IANA.OAuthAccessTokenTypes]. 2375 o Type name: "PoP" 2376 o Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see 2377 section 3.3 of [I-D.ietf-ace-oauth-params]. 2378 o HTTP Authentication Scheme(s): N/A 2379 o Change Controller: IETF 2380 o Specification document(s): [this document] 2382 8.6. OAuth Access Token Type CBOR Mappings 2384 This specification established the IANA "OAuth Access Token Type CBOR 2385 Mappings" registry. The registry has been created to use the "Expert 2386 Review" registration procedure [RFC8126], except for the value range 2387 designated for private use. 2389 The columns of this registry are: 2391 Name The name of token type as registered in the OAuth Access Token 2392 Types registry, e.g., "Bearer". 2393 CBOR Value CBOR abbreviation for this token type. Integer values 2394 less than -65536 are marked as "Private Use", all other values use 2395 the registration policy "Expert Review" [RFC8126]. 2396 Reference This contains a pointer to the public specification of the 2397 OAuth token type abbreviation, if one exists. 2398 Original Specification This contains a pointer to the public 2399 specification of the OAuth token type, if one exists. 2401 8.6.1. Initial Registry Contents 2403 o Name: "Bearer" 2404 o Value: 1 2405 o Reference: [this document] 2406 o Original Specification: [RFC6749] 2407 o Name: "PoP" 2408 o Value: 2 2409 o Reference: [this document] 2410 o Original Specification: [this document] 2412 8.7. ACE Profile Registry 2414 This specification establishes the IANA "ACE Profile" registry. The 2415 registry has been created to use the "Expert Review" registration 2416 procedure [RFC8126]. It should be noted that, in addition to the 2417 expert review, some portions of the registry require a specification, 2418 potentially a Standards Track RFC, be supplied as well. 2420 The columns of this registry are: 2422 Name The name of the profile, to be used as value of the profile 2423 attribute. 2424 Description Text giving an overview of the profile and the context 2425 it is developed for. 2426 CBOR Value CBOR abbreviation for this profile name. Different 2427 ranges of values use different registration policies [RFC8126]. 2428 Integer values from -256 to 255 are designated as Standards 2429 Action. Integer values from -65536 to -257 and from 256 to 65535 2430 are designated as Specification Required. Integer values greater 2431 than 65535 are designated as "Expert Review". Integer values less 2432 than -65536 are marked as Private Use. 2433 Reference This contains a pointer to the public specification of the 2434 profile abbreviation, if one exists. 2436 This registry will be initially empty and will be populated by the 2437 registrations from the ACE framework profiles. 2439 8.8. OAuth Parameter Registration 2441 This specification registers the following parameter in the "OAuth 2442 Parameters" registry [IANA.OAuthParameters]: 2444 o Name: "ace_profile" 2445 o Parameter Usage Location: token response 2446 o Change Controller: IESG 2447 o Reference: Section 5.6.2 and Section 5.6.4.3 of [this document] 2449 8.9. OAuth Parameters CBOR Mappings Registry 2451 This specification establishes the IANA "OAuth Parameters CBOR 2452 Mappings" registry. The registry has been created to use the "Expert 2453 Review" registration procedure [RFC8126], except for the value range 2454 designated for private use. 2456 The columns of this registry are: 2458 Name The OAuth Parameter name, refers to the name in the OAuth 2459 parameter registry, e.g., "client_id". 2460 CBOR Key CBOR map key for this parameter. Integer values less than 2461 -65536 are marked as "Private Use", all other values use the 2462 registration policy "Expert Review" [RFC8126]. 2463 Value Type The allowable CBOR data types for values of this 2464 parameter. 2465 Reference This contains a pointer to the public specification of the 2466 OAuth parameter abbreviation, if one exists. 2468 This registry will be initially populated by the values in Figure 12. 2469 The Reference column for all of these entries will be this document. 2471 8.10. OAuth Introspection Response Parameter Registration 2473 This specification registers the following parameter in the OAuth 2474 Token Introspection Response registry 2475 [IANA.TokenIntrospectionResponse]. 2477 o Name: "ace_profile" 2478 o Description: The ACE profile used between client and RS. 2479 o Change Controller: IESG 2480 o Reference: Section 5.7.2 of [this document] 2482 8.11. OAuth Token Introspection Response CBOR Mappings Registry 2484 This specification establishes the IANA "OAuth Token Introspection 2485 Response CBOR Mappings" registry. The registry has been created to 2486 use the "Expert Review" registration procedure [RFC8126], except for 2487 the value range designated for private use. 2489 The columns of this registry are: 2491 Name The OAuth Parameter name, refers to the name in the OAuth 2492 parameter registry, e.g., "client_id". 2493 CBOR Key CBOR map key for this parameter. Integer values less than 2494 -65536 are marked as "Private Use", all other values use the 2495 registration policy "Expert Review" [RFC8126]. 2496 Value Type The allowable CBOR data types for values of this 2497 parameter. 2498 Reference This contains a pointer to the public specification of the 2499 introspection response parameter abbreviation, if one exists. 2501 This registry will be initially populated by the values in Figure 16. 2502 The Reference column for all of these entries will be this document. 2504 Note that the mappings of parameters corresponding to claim names 2505 intentionally coincide with the CWT claim name mappings from 2506 [RFC8392]. 2508 8.12. JSON Web Token Claims 2510 This specification registers the following new claims in the JSON Web 2511 Token (JWT) registry of JSON Web Token Claims 2512 [IANA.JsonWebTokenClaims]: 2514 o Claim Name: "ace_profile" 2515 o Claim Description: The ACE profile a token is supposed to be used 2516 with. 2517 o Change Controller: IESG 2518 o Reference: Section 5.8 of [this document] 2520 o Claim Name: "exi" 2521 o Claim Description: "Expires in". Lifetime of the token in seconds 2522 from the time the RS first sees it. Used to implement a weaker 2523 from of token expiration for devices that cannot synchronize their 2524 internal clocks. 2525 o Change Controller: IESG 2526 o Reference: Section 5.8.3 of [this document] 2528 o Claim Name: "cnonce" 2529 o Claim Description: "client-nonce". A nonce previously provided to 2530 the AS by the RS via the client. Used to verify token freshness 2531 when the RS cannot synchronize its clock with the AS. 2532 o Change Controller: IESG 2533 o Reference: Section 5.8 of [this document] 2535 8.13. CBOR Web Token Claims 2537 This specification registers the following new claims in the "CBOR 2538 Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims]. 2540 o Claim Name: "scope" 2541 o Claim Description: The scope of an access token as defined in 2542 [RFC6749]. 2543 o JWT Claim Name: scope 2544 o Claim Key: TBD (suggested: 9) 2545 o Claim Value Type(s): byte string or text string 2546 o Change Controller: IESG 2547 o Specification Document(s): Section 4.2 of 2548 [I-D.ietf-oauth-token-exchange] 2550 o Claim Name: "ace_profile" 2551 o Claim Description: The ACE profile a token is supposed to be used 2552 with. 2553 o JWT Claim Name: ace_profile 2554 o Claim Key: TBD (suggested: 38) 2555 o Claim Value Type(s): integer 2556 o Change Controller: IESG 2557 o Specification Document(s): Section 5.8 of [this document] 2559 o Claim Name: "exi" 2560 o Claim Description: The expiration time of a token measured from 2561 when it was received at the RS in seconds. 2562 o JWT Claim Name: exi 2563 o Claim Key: TBD (suggested: 40) 2564 o Claim Value Type(s): integer 2565 o Change Controller: IESG 2566 o Specification Document(s): Section 5.8.3 of [this document] 2568 o Claim Name: "cnonce" 2569 o Claim Description: The client-nonce sent to the AS by the RS via 2570 the client. 2571 o JWT Claim Name: cnonce 2572 o Claim Key: TBD (suggested: 39) 2573 o Claim Value Type(s): byte string 2574 o Change Controller: IESG 2575 o Specification Document(s): Section 5.8 of [this document] 2577 8.14. Media Type Registrations 2579 This specification registers the 'application/ace+cbor' media type 2580 for messages of the protocols defined in this document carrying 2581 parameters encoded in CBOR. This registration follows the procedures 2582 specified in [RFC6838]. 2584 Type name: application 2586 Subtype name: ace+cbor 2588 Required parameters: none 2590 Optional parameters: none 2592 Encoding considerations: Must be encoded as CBOR map containing the 2593 protocol parameters defined in [this document]. 2595 Security considerations: See Section 6 of this document. 2597 Interoperability considerations: n/a 2598 Published specification: [this document] 2600 Applications that use this media type: The type is used by 2601 authorization servers, clients and resource servers that support the 2602 ACE framework as specified in [this document]. 2604 Additional information: 2606 Magic number(s): n/a 2608 File extension(s): .ace 2610 Macintosh file type code(s): n/a 2612 Person & email address to contact for further information: 2613 2615 Intended usage: COMMON 2617 Restrictions on usage: None 2619 Author: Ludwig Seitz 2621 Change controller: IESG 2623 8.15. CoAP Content-Format Registry 2625 This specification registers the following entry to the "CoAP 2626 Content-Formats" registry: 2628 Media Type: application/ace+cbor 2630 Encoding: - 2632 ID: TBD (suggested: 19) 2634 Reference: [this document] 2636 8.16. Expert Review Instructions 2638 All of the IANA registries established in this document are defined 2639 to use a registration policy of Expert Review. This section gives 2640 some general guidelines for what the experts should be looking for, 2641 but they are being designated as experts for a reason, so they should 2642 be given substantial latitude. 2644 Expert reviewers should take into consideration the following points: 2646 o Point squatting should be discouraged. Reviewers are encouraged 2647 to get sufficient information for registration requests to ensure 2648 that the usage is not going to duplicate one that is already 2649 registered, and that the point is likely to be used in 2650 deployments. The zones tagged as private use are intended for 2651 testing purposes and closed environments; code points in other 2652 ranges should not be assigned for testing. 2653 o Specifications are needed for the first-come, first-serve range if 2654 they are expected to be used outside of closed environments in an 2655 interoperable way. When specifications are not provided, the 2656 description provided needs to have sufficient information to 2657 identify what the point is being used for. 2658 o Experts should take into account the expected usage of fields when 2659 approving point assignment. The fact that there is a range for 2660 standards track documents does not mean that a standards track 2661 document cannot have points assigned outside of that range. The 2662 length of the encoded value should be weighed against how many 2663 code points of that length are left, the size of device it will be 2664 used on. 2665 o Since a high degree of overlap is expected between these 2666 registries and the contents of the OAuth parameters 2667 [IANA.OAuthParameters] registries, experts should require new 2668 registrations to maintain alignment with parameters from OAuth 2669 that have comparable functionality. Deviation from this alignment 2670 should only be allowed if there are functional differences, that 2671 are motivated by the use case and that cannot be easily or 2672 efficiently addressed by comparable OAuth parameters. 2674 9. Acknowledgments 2676 This document is a product of the ACE working group of the IETF. 2678 Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and 2679 UMA in IoT scenarios, Robert Taylor for his discussion input, and 2680 Malisa Vucinic for his input on the predecessors of this proposal. 2682 Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from 2683 where large parts of the security considerations where copied. 2685 Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for 2686 contributing their work on AS discovery from draft-gerdes-ace-dcaf- 2687 authorize (see Section 5.1). 2689 Thanks to Jim Schaad and Mike Jones for their comprehensive reviews. 2691 Thanks to Benjamin Kaduk for his input on various questions related 2692 to this work. 2694 Thanks to Cigdem Sengul for some very useful review comments. 2696 Ludwig Seitz and Goeran Selander worked on this document as part of 2697 the CelticPlus project CyberWI, with funding from Vinnova. Ludwig 2698 Seitz was also received further funding for this work by Vinnova in 2699 the context of the CelticNext project Critisec. 2701 10. References 2703 10.1. Normative References 2705 [I-D.ietf-ace-cwt-proof-of-possession] 2706 Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 2707 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 2708 Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of- 2709 possession-11 (work in progress), October 2019. 2711 [I-D.ietf-ace-oauth-params] 2712 Seitz, L., "Additional OAuth Parameters for Authorization 2713 in Constrained Environments (ACE)", draft-ietf-ace-oauth- 2714 params-10 (work in progress), January 2020. 2716 [I-D.ietf-oauth-token-exchange] 2717 Jones, M., Nadalin, A., Campbell, B., Bradley, J., and C. 2718 Mortimore, "OAuth 2.0 Token Exchange", draft-ietf-oauth- 2719 token-exchange-19 (work in progress), July 2019. 2721 [IANA.CborWebTokenClaims] 2722 IANA, "CBOR Web Token (CWT) Claims", 2723 . 2726 [IANA.JsonWebTokenClaims] 2727 IANA, "JSON Web Token Claims", 2728 . 2730 [IANA.OAuthAccessTokenTypes] 2731 IANA, "OAuth Access Token Types", 2732 . 2735 [IANA.OAuthExtensionsErrorRegistry] 2736 IANA, "OAuth Extensions Error Registry", 2737 . 2740 [IANA.OAuthParameters] 2741 IANA, "OAuth Parameters", 2742 . 2745 [IANA.TokenIntrospectionResponse] 2746 IANA, "OAuth Token Introspection Response", 2747 . 2750 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2751 Requirement Levels", BCP 14, RFC 2119, 2752 DOI 10.17487/RFC2119, March 1997, 2753 . 2755 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2756 Resource Identifier (URI): Generic Syntax", STD 66, 2757 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2758 . 2760 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 2761 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 2762 . 2764 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 2765 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 2766 January 2012, . 2768 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 2769 RFC 6749, DOI 10.17487/RFC6749, October 2012, 2770 . 2772 [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization 2773 Framework: Bearer Token Usage", RFC 6750, 2774 DOI 10.17487/RFC6750, October 2012, 2775 . 2777 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2778 Specifications and Registration Procedures", BCP 13, 2779 RFC 6838, DOI 10.17487/RFC6838, January 2013, 2780 . 2782 [RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B., 2783 Keranen, A., and P. Hallam-Baker, "Naming Things with 2784 Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013, 2785 . 2787 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object 2788 Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, 2789 October 2013, . 2791 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 2792 Application Protocol (CoAP)", RFC 7252, 2793 DOI 10.17487/RFC7252, June 2014, 2794 . 2796 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 2797 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 2798 . 2800 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 2801 RFC 7662, DOI 10.17487/RFC7662, October 2015, 2802 . 2804 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2805 Writing an IANA Considerations Section in RFCs", BCP 26, 2806 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2807 . 2809 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 2810 RFC 8152, DOI 10.17487/RFC8152, July 2017, 2811 . 2813 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2814 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2815 May 2017, . 2817 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 2818 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 2819 May 2018, . 2821 10.2. Informative References 2823 [BLE] Bluetooth SIG, "Bluetooth Core Specification v5.1", 2824 Section 4.4, January 2019, 2825 . 2828 [I-D.erdtman-ace-rpcc] 2829 Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared- 2830 Key as OAuth client credentials", draft-erdtman-ace- 2831 rpcc-02 (work in progress), October 2017. 2833 [I-D.ietf-quic-transport] 2834 Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed 2835 and Secure Transport", draft-ietf-quic-transport-24 (work 2836 in progress), November 2019. 2838 [I-D.ietf-tls-dtls13] 2839 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 2840 Datagram Transport Layer Security (DTLS) Protocol Version 2841 1.3", draft-ietf-tls-dtls13-34 (work in progress), 2842 November 2019. 2844 [Margi10impact] 2845 Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr, 2846 M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold, 2847 "Impact of Operating Systems on Wireless Sensor Networks 2848 (Security) Applications and Testbeds", Proceedings of 2849 the 19th International Conference on Computer 2850 Communications and Networks (ICCCN), August 2010. 2852 [MQTT5.0] Banks, A., Briggs, E., Borgendale, K., and R. Gupta, "MQTT 2853 Version 5.0", OASIS Standard, March 2019, 2854 . 2857 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 2858 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 2859 . 2861 [RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0 2862 Threat Model and Security Considerations", RFC 6819, 2863 DOI 10.17487/RFC6819, January 2013, 2864 . 2866 [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth 2867 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, 2868 August 2013, . 2870 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2871 Constrained-Node Networks", RFC 7228, 2872 DOI 10.17487/RFC7228, May 2014, 2873 . 2875 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2876 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2877 DOI 10.17487/RFC7231, June 2014, 2878 . 2880 [RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland, 2881 "Assertion Framework for OAuth 2.0 Client Authentication 2882 and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521, 2883 May 2015, . 2885 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 2886 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 2887 DOI 10.17487/RFC7540, May 2015, 2888 . 2890 [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and 2891 P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", 2892 RFC 7591, DOI 10.17487/RFC7591, July 2015, 2893 . 2895 [RFC7641] Hartke, K., "Observing Resources in the Constrained 2896 Application Protocol (CoAP)", RFC 7641, 2897 DOI 10.17487/RFC7641, September 2015, 2898 . 2900 [RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M., 2901 and S. Kumar, "Use Cases for Authentication and 2902 Authorization in Constrained Environments", RFC 7744, 2903 DOI 10.17487/RFC7744, January 2016, 2904 . 2906 [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 2907 the Constrained Application Protocol (CoAP)", RFC 7959, 2908 DOI 10.17487/RFC7959, August 2016, 2909 . 2911 [RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", 2912 BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017, 2913 . 2915 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2916 Interchange Format", STD 90, RFC 8259, 2917 DOI 10.17487/RFC8259, December 2017, 2918 . 2920 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 2921 Authorization Server Metadata", RFC 8414, 2922 DOI 10.17487/RFC8414, June 2018, 2923 . 2925 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2926 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2927 . 2929 [RFC8516] Keranen, A., ""Too Many Requests" Response Code for the 2930 Constrained Application Protocol", RFC 8516, 2931 DOI 10.17487/RFC8516, January 2019, 2932 . 2934 [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 2935 "Object Security for Constrained RESTful Environments 2936 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 2937 . 2939 [RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig, 2940 "OAuth 2.0 Device Authorization Grant", RFC 8628, 2941 DOI 10.17487/RFC8628, August 2019, 2942 . 2944 Appendix A. Design Justification 2946 This section provides further insight into the design decisions of 2947 the solution documented in this document. Section 3 lists several 2948 building blocks and briefly summarizes their importance. The 2949 justification for offering some of those building blocks, as opposed 2950 to using OAuth 2.0 as is, is given below. 2952 Common IoT constraints are: 2954 Low Power Radio: 2956 Many IoT devices are equipped with a small battery which needs to 2957 last for a long time. For many constrained wireless devices, the 2958 highest energy cost is associated to transmitting or receiving 2959 messages (roughly by a factor of 10 compared to AES) 2960 [Margi10impact]. It is therefore important to keep the total 2961 communication overhead low, including minimizing the number and 2962 size of messages sent and received, which has an impact of choice 2963 on the message format and protocol. By using CoAP over UDP and 2964 CBOR encoded messages, some of these aspects are addressed. 2965 Security protocols contribute to the communication overhead and 2966 can, in some cases, be optimized. For example, authentication and 2967 key establishment may, in certain cases where security 2968 requirements allow, be replaced by provisioning of security 2969 context by a trusted third party, using transport or application 2970 layer security. 2972 Low CPU Speed: 2974 Some IoT devices are equipped with processors that are 2975 significantly slower than those found in most current devices on 2976 the Internet. This typically has implications on what timely 2977 cryptographic operations a device is capable of performing, which 2978 in turn impacts, e.g., protocol latency. Symmetric key 2979 cryptography may be used instead of the computationally more 2980 expensive public key cryptography where the security requirements 2981 so allow, but this may also require support for trusted-third- 2982 party-assisted secret key establishment using transport- or 2983 application-layer security. 2984 Small Amount of Memory: 2986 Microcontrollers embedded in IoT devices are often equipped with 2987 only a small amount of RAM and flash memory, which places 2988 limitations on what kind of processing can be performed and how 2989 much code can be put on those devices. To reduce code size, fewer 2990 and smaller protocol implementations can be put on the firmware of 2991 such a device. In this case, CoAP may be used instead of HTTP, 2992 symmetric-key cryptography instead of public-key cryptography, and 2993 CBOR instead of JSON. An authentication and key establishment 2994 protocol, e.g., the DTLS handshake, in comparison with assisted 2995 key establishment, also has an impact on memory and code 2996 footprints. 2998 User Interface Limitations: 3000 Protecting access to resources is both an important security as 3001 well as privacy feature. End users and enterprise customers may 3002 not want to give access to the data collected by their IoT device 3003 or to functions it may offer to third parties. Since the 3004 classical approach of requesting permissions from end users via a 3005 rich user interface does not work in many IoT deployment 3006 scenarios, these functions need to be delegated to user-controlled 3007 devices that are better suitable for such tasks, such as smart 3008 phones and tablets. 3010 Communication Constraints: 3012 In certain constrained settings an IoT device may not be able to 3013 communicate with a given device at all times. Devices may be 3014 sleeping, or just disconnected from the Internet because of 3015 general lack of connectivity in the area, for cost reasons, or for 3016 security reasons, e.g., to avoid an entry point for Denial-of- 3017 Service attacks. 3019 The communication interactions this framework builds upon (as 3020 shown graphically in Figure 1) may be accomplished using a variety 3021 of different protocols, and not all parts of the message flow are 3022 used in all applications due to the communication constraints. 3023 Deployments making use of CoAP are expected, but this framework is 3024 not limited to them. Other protocols such as HTTP, or even 3025 protocols such as Bluetooth Smart communication that do not 3026 necessarily use IP, could also be used. The latter raises the 3027 need for application layer security over the various interfaces. 3029 In the light of these constraints we have made the following design 3030 decisions: 3032 CBOR, COSE, CWT: 3034 This framework RECOMMENDS the use of CBOR [RFC7049] as data 3035 format. Where CBOR data needs to be protected, the use of COSE 3036 [RFC8152] is RECOMMENDED. Furthermore, where self-contained 3037 tokens are needed, this framework RECOMMENDS the use of CWT 3038 [RFC8392]. These measures aim at reducing the size of messages 3039 sent over the wire, the RAM size of data objects that need to be 3040 kept in memory and the size of libraries that devices need to 3041 support. 3043 CoAP: 3045 This framework RECOMMENDS the use of CoAP [RFC7252] instead of 3046 HTTP. This does not preclude the use of other protocols 3047 specifically aimed at constrained devices, like, e.g., Bluetooth 3048 Low Energy (see Section 3.2). This aims again at reducing the 3049 size of messages sent over the wire, the RAM size of data objects 3050 that need to be kept in memory and the size of libraries that 3051 devices need to support. 3053 Access Information: 3055 This framework defines the name "Access Information" for data 3056 concerning the RS that the AS returns to the client in an access 3057 token response (see Section 5.6.2). This aims at enabling 3058 scenarios where a powerful client, supporting multiple profiles, 3059 needs to interact with an RS for which it does not know the 3060 supported profiles and the raw public key. 3062 Proof-of-Possession: 3064 This framework makes use of proof-of-possession tokens, using the 3065 "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession]. A request 3066 parameter "cnf" and a Response parameter "cnf", both having a 3067 value space semantically and syntactically identical to the "cnf" 3068 claim, are defined for the token endpoint, to allow requesting and 3069 stating confirmation keys. This aims at making token theft 3070 harder. Token theft is specifically relevant in constrained use 3071 cases, as communication often passes through middle-boxes, which 3072 could be able to steal bearer tokens and use them to gain 3073 unauthorized access. 3075 Authz-Info endpoint: 3077 This framework introduces a new way of providing access tokens to 3078 an RS by exposing a authz-info endpoint, to which access tokens 3079 can be POSTed. This aims at reducing the size of the request 3080 message and the code complexity at the RS. The size of the 3081 request message is problematic, since many constrained protocols 3082 have severe message size limitations at the physical layer (e.g., 3083 in the order of 100 bytes). This means that larger packets get 3084 fragmented, which in turn combines badly with the high rate of 3085 packet loss, and the need to retransmit the whole message if one 3086 packet gets lost. Thus separating sending of the request and 3087 sending of the access tokens helps to reduce fragmentation. 3089 Client Credentials Grant: 3091 This framework RECOMMENDS the use of the client credentials grant 3092 for machine-to-machine communication use cases, where manual 3093 intervention of the resource owner to produce a grant token is not 3094 feasible. The intention is that the resource owner would instead 3095 pre-arrange authorization with the AS, based on the client's own 3096 credentials. The client can then (without manual intervention) 3097 obtain access tokens from the AS. 3099 Introspection: 3101 This framework RECOMMENDS the use of access token introspection in 3102 cases where the client is constrained in a way that it can not 3103 easily obtain new access tokens (i.e. it has connectivity issues 3104 that prevent it from communicating with the AS). In that case 3105 this framework RECOMMENDS the use of a long-term token, that could 3106 be a simple reference. The RS is assumed to be able to 3107 communicate with the AS, and can therefore perform introspection, 3108 in order to learn the claims associated with the token reference. 3109 The advantage of such an approach is that the resource owner can 3110 change the claims associated to the token reference without having 3111 to be in contact with the client, thus granting or revoking access 3112 rights. 3114 Appendix B. Roles and Responsibilities 3116 Resource Owner 3118 * Make sure that the RS is registered at the AS. This includes 3119 making known to the AS which profiles, token_type, scopes, and 3120 key types (symmetric/asymmetric) the RS supports. Also making 3121 it known to the AS which audience(s) the RS identifies itself 3122 with. 3123 * Make sure that clients can discover the AS that is in charge of 3124 the RS. 3125 * If the client-credentials grant is used, make sure that the AS 3126 has the necessary, up-to-date, access control policies for the 3127 RS. 3129 Requesting Party 3131 * Make sure that the client is provisioned the necessary 3132 credentials to authenticate to the AS. 3133 * Make sure that the client is configured to follow the security 3134 requirements of the Requesting Party when issuing requests 3135 (e.g., minimum communication security requirements, trust 3136 anchors). 3137 * Register the client at the AS. This includes making known to 3138 the AS which profiles, token_types, and key types (symmetric/ 3139 asymmetric) the client. 3141 Authorization Server 3143 * Register the RS and manage corresponding security contexts. 3144 * Register clients and authentication credentials. 3145 * Allow Resource Owners to configure and update access control 3146 policies related to their registered RSs. 3147 * Expose the token endpoint to allow clients to request tokens. 3148 * Authenticate clients that wish to request a token. 3149 * Process a token request using the authorization policies 3150 configured for the RS. 3151 * Optionally: Expose the introspection endpoint that allows RS's 3152 to submit token introspection requests. 3153 * If providing an introspection endpoint: Authenticate RSs that 3154 wish to get an introspection response. 3155 * If providing an introspection endpoint: Process token 3156 introspection requests. 3157 * Optionally: Handle token revocation. 3158 * Optionally: Provide discovery metadata. See [RFC8414] 3159 * Optionally: Handle refresh tokens. 3161 Client 3162 * Discover the AS in charge of the RS that is to be targeted with 3163 a request. 3164 * Submit the token request (see step (A) of Figure 1). 3166 + Authenticate to the AS. 3167 + Optionally (if not pre-configured): Specify which RS, which 3168 resource(s), and which action(s) the request(s) will target. 3169 + If raw public keys (rpk) or certificates are used, make sure 3170 the AS has the right rpk or certificate for this client. 3171 * Process the access token and Access Information (see step (B) 3172 of Figure 1). 3174 + Check that the Access Information provides the necessary 3175 security parameters (e.g., PoP key, information on 3176 communication security protocols supported by the RS). 3177 + Safely store the proof-of-possession key. 3178 + If provided by the AS: Safely store the refresh token. 3179 * Send the token and request to the RS (see step (C) of 3180 Figure 1). 3182 + Authenticate towards the RS (this could coincide with the 3183 proof of possession process). 3184 + Transmit the token as specified by the AS (default is to the 3185 authz-info endpoint, alternative options are specified by 3186 profiles). 3187 + Perform the proof-of-possession procedure as specified by 3188 the profile in use (this may already have been taken care of 3189 through the authentication procedure). 3190 * Process the RS response (see step (F) of Figure 1) of the RS. 3192 Resource Server 3194 * Expose a way to submit access tokens. By default this is the 3195 authz-info endpoint. 3196 * Process an access token. 3198 + Verify the token is from a recognized AS. 3199 + Check the token's integrity. 3200 + Verify that the token applies to this RS. 3201 + Check that the token has not expired (if the token provides 3202 expiration information). 3203 + Store the token so that it can be retrieved in the context 3204 of a matching request. 3206 Note: The order proposed here is not normative, any process 3207 that arrives at an equivalent result can be used. A noteworthy 3208 consideration is whether one can use cheap operations early on 3209 to quickly discard non-applicable or invalid tokens, before 3210 performing expensive cryptographic operations (e.g. doing an 3211 expiration check before verifying a signature). 3213 * Process a request. 3215 + Set up communication security with the client. 3216 + Authenticate the client. 3217 + Match the client against existing tokens. 3218 + Check that tokens belonging to the client actually authorize 3219 the requested action. 3220 + Optionally: Check that the matching tokens are still valid, 3221 using introspection (if this is possible.) 3222 * Send a response following the agreed upon communication 3223 security mechanism(s). 3224 * Safely store credentials such as raw public keys for 3225 authentication or proof-of-possession keys linked to access 3226 tokens. 3228 Appendix C. Requirements on Profiles 3230 This section lists the requirements on profiles of this framework, 3231 for the convenience of profile designers. 3233 o Optionally define new methods for the client to discover the 3234 necessary permissions and AS for accessing a resource, different 3235 from the one proposed in Section 5.1. Section 4 3236 o Optionally specify new grant types. Section 5.2 3237 o Optionally define the use of client certificates as client 3238 credential type. Section 5.3 3239 o Specify the communication protocol the client and RS the must use 3240 (e.g., CoAP). Section 5 and Section 5.6.4.3 3241 o Specify the security protocol the client and RS must use to 3242 protect their communication (e.g., OSCORE or DTLS). This must 3243 provide encryption, integrity and replay protection. 3244 Section 5.6.4.3 3245 o Specify how the client and the RS mutually authenticate. 3246 Section 4 3247 o Specify the proof-of-possession protocol(s) and how to select one, 3248 if several are available. Also specify which key types (e.g., 3249 symmetric/asymmetric) are supported by a specific proof-of- 3250 possession protocol. Section 5.6.4.2 3251 o Specify a unique ace_profile identifier. Section 5.6.4.3 3252 o If introspection is supported: Specify the communication and 3253 security protocol for introspection. Section 5.7 3254 o Specify the communication and security protocol for interactions 3255 between client and AS. This must provide encryption, integrity 3256 protection, replay protection and a binding between requests and 3257 responses. Section 5 and Section 5.6 3259 o Specify how/if the authz-info endpoint is protected, including how 3260 error responses are protected. Section 5.8.1 3261 o Optionally define other methods of token transport than the authz- 3262 info endpoint. Section 5.8.1 3264 Appendix D. Assumptions on AS knowledge about C and RS 3266 This section lists the assumptions on what an AS should know about a 3267 client and an RS in order to be able to respond to requests to the 3268 token and introspection endpoints. How this information is 3269 established is out of scope for this document. 3271 o The identifier of the client or RS. 3272 o The profiles that the client or RS supports. 3273 o The scopes that the RS supports. 3274 o The audiences that the RS identifies with. 3275 o The key types (e.g., pre-shared symmetric key, raw public key, key 3276 length, other key parameters) that the client or RS supports. 3277 o The types of access tokens the RS supports (e.g., CWT). 3278 o If the RS supports CWTs, the COSE parameters for the crypto 3279 wrapper (e.g., algorithm, key-wrap algorithm, key-length) that the 3280 RS supports. 3281 o The expiration time for access tokens issued to this RS (unless 3282 the RS accepts a default time chosen by the AS). 3283 o The symmetric key shared between client and AS (if any). 3284 o The symmetric key shared between RS and AS (if any). 3285 o The raw public key of the client or RS (if any). 3286 o Whether the RS has synchronized time (and thus is able to use the 3287 'exp' claim) or not. 3289 Appendix E. Deployment Examples 3291 There is a large variety of IoT deployments, as is indicated in 3292 Appendix A, and this section highlights a few common variants. This 3293 section is not normative but illustrates how the framework can be 3294 applied. 3296 For each of the deployment variants, there are a number of possible 3297 security setups between clients, resource servers and authorization 3298 servers. The main focus in the following subsections is on how 3299 authorization of a client request for a resource hosted by an RS is 3300 performed. This requires the security of the requests and responses 3301 between the clients and the RS to be considered. 3303 Note: CBOR diagnostic notation is used for examples of requests and 3304 responses. 3306 E.1. Local Token Validation 3308 In this scenario, the case where the resource server is offline is 3309 considered, i.e., it is not connected to the AS at the time of the 3310 access request. This access procedure involves steps A, B, C, and F 3311 of Figure 1. 3313 Since the resource server must be able to verify the access token 3314 locally, self-contained access tokens must be used. 3316 This example shows the interactions between a client, the 3317 authorization server and a temperature sensor acting as a resource 3318 server. Message exchanges A and B are shown in Figure 17. 3320 A: The client first generates a public-private key pair used for 3321 communication security with the RS. 3322 The client sends a CoAP POST request to the token endpoint at the 3323 AS. The security of this request can be transport or application 3324 layer. It is up the the communication security profile to define. 3325 In the example it is assumed that both client and AS have 3326 performed mutual authentication e.g. via DTLS. The request 3327 contains the public key of the client and the Audience parameter 3328 set to "tempSensorInLivingRoom", a value that the temperature 3329 sensor identifies itself with. The AS evaluates the request and 3330 authorizes the client to access the resource. 3331 B: The AS responds with a 2.05 Content response containing the 3332 Access Information, including the access token. The PoP access 3333 token contains the public key of the client, and the Access 3334 Information contains the public key of the RS. For communication 3335 security this example uses DTLS RawPublicKey between the client 3336 and the RS. The issued token will have a short validity time, 3337 i.e., "exp" close to "iat", in order to mitigate attacks using 3338 stolen client credentials. The token includes the claim such as 3339 "scope" with the authorized access that an owner of the 3340 temperature device can enjoy. In this example, the "scope" claim, 3341 issued by the AS, informs the RS that the owner of the token, that 3342 can prove the possession of a key is authorized to make a GET 3343 request against the /temperature resource and a POST request on 3344 the /firmware resource. Note that the syntax and semantics of the 3345 scope claim are application specific. 3346 Note: In this example it is assumed that the client knows what 3347 resource it wants to access, and is therefore able to request 3348 specific audience and scope claims for the access token. 3350 Authorization 3351 Client Server 3352 | | 3353 |<=======>| DTLS Connection Establishment 3354 | | and mutual authentication 3355 | | 3356 A: +-------->| Header: POST (Code=0.02) 3357 | POST | Uri-Path:"token" 3358 | | Content-Format: application/ace+cbor 3359 | | Payload: 3360 | | 3361 B: |<--------+ Header: 2.05 Content 3362 | 2.05 | Content-Format: application/ace+cbor 3363 | | Payload: 3364 | | 3366 Figure 17: Token Request and Response Using Client Credentials. 3368 The information contained in the Request-Payload and the Response- 3369 Payload is shown in Figure 18 Note that the parameter "rs_cnf" from 3370 [I-D.ietf-ace-oauth-params] is used to inform the client about the 3371 resource server's public key. 3373 Request-Payload : 3374 { 3375 "audience" : "tempSensorInLivingRoom", 3376 "client_id" : "myclient", 3377 "req_cnf" : { 3378 "COSE_Key" : { 3379 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3380 "kty" : "EC", 3381 "crv" : "P-256", 3382 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3383 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3384 } 3385 } 3386 } 3388 Response-Payload : 3389 { 3390 "access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...', 3391 "rs_cnf" : { 3392 "COSE_Key" : { 3393 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3394 "kty" : "EC", 3395 "crv" : "P-256", 3396 "x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4', 3397 "y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM' 3398 } 3399 } 3400 } 3402 Figure 18: Request and Response Payload Details. 3404 The content of the access token is shown in Figure 19. 3406 { 3407 "aud" : "tempSensorInLivingRoom", 3408 "iat" : "1563451500", 3409 "exp" : "1563453000", 3410 "scope" : "temperature_g firmware_p", 3411 "cnf" : { 3412 "COSE_Key" : { 3413 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3414 "kty" : "EC", 3415 "crv" : "P-256", 3416 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3417 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3418 } 3419 } 3420 } 3422 Figure 19: Access Token including Public Key of the Client. 3424 Messages C and F are shown in Figure 20 - Figure 21. 3426 C: The client then sends the PoP access token to the authz-info 3427 endpoint at the RS. This is a plain CoAP POST request, i.e., no 3428 transport or application layer security is used between client and 3429 RS since the token is integrity protected between the AS and RS. 3430 The RS verifies that the PoP access token was created by a known 3431 and trusted AS, that it applies to this RS, and that it is valid. 3432 The RS caches the security context together with authorization 3433 information about this client contained in the PoP access token. 3435 Resource 3436 Client Server 3437 | | 3438 C: +-------->| Header: POST (Code=0.02) 3439 | POST | Uri-Path:"authz-info" 3440 | | Payload: 0INDoQEKoQVN ... 3441 | | 3442 |<--------+ Header: 2.04 Changed 3443 | 2.04 | 3444 | | 3446 Figure 20: Access Token provisioning to RS 3447 The client and the RS runs the DTLS handshake using the raw public 3448 keys established in step B and C. 3449 The client sends a CoAP GET request to /temperature on RS over 3450 DTLS. The RS verifies that the request is authorized, based on 3451 previously established security context. 3453 F: The RS responds over the same DTLS channel with a CoAP 2.05 3454 Content response, containing a resource representation as payload. 3456 Resource 3457 Client Server 3458 | | 3459 |<=======>| DTLS Connection Establishment 3460 | | using Raw Public Keys 3461 | | 3462 +-------->| Header: GET (Code=0.01) 3463 | GET | Uri-Path: "temperature" 3464 | | 3465 | | 3466 | | 3467 F: |<--------+ Header: 2.05 Content 3468 | 2.05 | Payload: 3469 | | 3471 Figure 21: Resource Request and Response protected by DTLS. 3473 E.2. Introspection Aided Token Validation 3475 In this deployment scenario it is assumed that a client is not able 3476 to access the AS at the time of the access request, whereas the RS is 3477 assumed to be connected to the back-end infrastructure. Thus the RS 3478 can make use of token introspection. This access procedure involves 3479 steps A-F of Figure 1, but assumes steps A and B have been carried 3480 out during a phase when the client had connectivity to AS. 3482 Since the client is assumed to be offline, at least for a certain 3483 period of time, a pre-provisioned access token has to be long-lived. 3484 Since the client is constrained, the token will not be self contained 3485 (i.e. not a CWT) but instead just a reference. The resource server 3486 uses its connectivity to learn about the claims associated to the 3487 access token by using introspection, which is shown in the example 3488 below. 3490 In the example interactions between an offline client (key fob), an 3491 RS (online lock), and an AS is shown. It is assumed that there is a 3492 provisioning step where the client has access to the AS. This 3493 corresponds to message exchanges A and B which are shown in 3494 Figure 22. 3496 Authorization consent from the resource owner can be pre-configured, 3497 but it can also be provided via an interactive flow with the resource 3498 owner. An example of this for the key fob case could be that the 3499 resource owner has a connected car, he buys a generic key that he 3500 wants to use with the car. To authorize the key fob he connects it 3501 to his computer that then provides the UI for the device. After that 3502 OAuth 2.0 implicit flow can used to authorize the key for his car at 3503 the the car manufacturers AS. 3505 Note: In this example the client does not know the exact door it will 3506 be used to access since the token request is not send at the time of 3507 access. So the scope and audience parameters are set quite wide to 3508 start with, while tailored values narrowing down the claims to the 3509 specific RS being accessed can be provided to that RS during an 3510 introspection step. 3512 A: The client sends a CoAP POST request to the token endpoint at 3513 AS. The request contains the Audience parameter set to "PACS1337" 3514 (PACS, Physical Access System), a value the that identifies the 3515 physical access control system to which the individual doors are 3516 connected. The AS generates an access token as an opaque string, 3517 which it can match to the specific client and the targeted 3518 audience. It furthermore generates a symmetric proof-of- 3519 possession key. The communication security and authentication 3520 between client and AS is assumed to have been provided at 3521 transport layer (e.g. via DTLS) using a pre-shared security 3522 context (psk, rpk or certificate). 3523 B: The AS responds with a CoAP 2.05 Content response, containing 3524 as playload the Access Information, including the access token and 3525 the symmetric proof-of-possession key. Communication security 3526 between C and RS will be DTLS and PreSharedKey. The PoP key is 3527 used as the PreSharedKey. 3529 Note: In this example we are using a symmetric key for a multi-RS 3530 audience, which is not recommended normally (see Section 6.9). 3531 However in this case the risk is deemed to be acceptable, since all 3532 the doors are part of the same physical access control system, and 3533 therefore the risk of a malicious RS impersonating the client towards 3534 another RS is low. 3536 Authorization 3537 Client Server 3538 | | 3539 |<=======>| DTLS Connection Establishment 3540 | | and mutual authentication 3541 | | 3542 A: +-------->| Header: POST (Code=0.02) 3543 | POST | Uri-Path:"token" 3544 | | Content-Format: application/ace+cbor 3545 | | Payload: 3546 | | 3547 B: |<--------+ Header: 2.05 Content 3548 | | Content-Format: application/ace+cbor 3549 | 2.05 | Payload: 3550 | | 3552 Figure 22: Token Request and Response using Client Credentials. 3554 The information contained in the Request-Payload and the Response- 3555 Payload is shown in Figure 23. 3557 Request-Payload: 3558 { 3559 "client_id" : "keyfob", 3560 "audience" : "PACS1337" 3561 } 3563 Response-Payload: 3564 { 3565 "access_token" : b64'VGVzdCB0b2tlbg==', 3566 "cnf" : { 3567 "COSE_Key" : { 3568 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3569 "kty" : "oct", 3570 "alg" : "HS256", 3571 "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE' 3572 } 3573 } 3574 } 3576 Figure 23: Request and Response Payload for C offline 3578 The access token in this case is just an opaque byte string 3579 referencing the authorization information at the AS. 3581 C: Next, the client POSTs the access token to the authz-info 3582 endpoint in the RS. This is a plain CoAP request, i.e., no DTLS 3583 between client and RS. Since the token is an opaque string, the 3584 RS cannot verify it on its own, and thus defers to respond the 3585 client with a status code until after step E. 3586 D: The RS sends the token to the introspection endpoint on the AS 3587 using a CoAP POST request. In this example RS and AS are assumed 3588 to have performed mutual authentication using a pre shared 3589 security context (psk, rpk or certificate) with the RS acting as 3590 DTLS client. 3591 E: The AS provides the introspection response (2.05 Content) 3592 containing parameters about the token. This includes the 3593 confirmation key (cnf) parameter that allows the RS to verify the 3594 client's proof of possession in step F. Note that our example in 3595 Figure 25 assumes a pre-established key (e.g. one used by the 3596 client and the RS for a previous token) that is now only 3597 referenced by its key-identifier 'kid'. 3598 After receiving message E, the RS responds to the client's POST in 3599 step C with the CoAP response code 2.01 (Created). 3601 Resource 3602 Client Server 3603 | | 3604 C: +-------->| Header: POST (T=CON, Code=0.02) 3605 | POST | Uri-Path:"authz-info" 3606 | | Payload: b64'VGVzdCB0b2tlbg==' 3607 | | 3608 | | Authorization 3609 | | Server 3610 | | | 3611 | D: +--------->| Header: POST (Code=0.02) 3612 | | POST | Uri-Path: "introspect" 3613 | | | Content-Format: "application/ace+cbor" 3614 | | | Payload: 3615 | | | 3616 | E: |<---------+ Header: 2.05 Content 3617 | | 2.05 | Content-Format: "application/ace+cbor" 3618 | | | Payload: 3619 | | | 3620 | | 3621 |<--------+ Header: 2.01 Created 3622 | 2.01 | 3623 | | 3625 Figure 24: Token Introspection for C offline 3626 The information contained in the Request-Payload and the Response- 3627 Payload is shown in Figure 25. 3629 Request-Payload: 3630 { 3631 "token" : b64'VGVzdCB0b2tlbg==', 3632 "client_id" : "FrontDoor", 3633 } 3635 Response-Payload: 3636 { 3637 "active" : true, 3638 "aud" : "lockOfDoor4711", 3639 "scope" : "open, close", 3640 "iat" : 1563454000, 3641 "cnf" : { 3642 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk' 3643 } 3644 } 3646 Figure 25: Request and Response Payload for Introspection 3648 The client uses the symmetric PoP key to establish a DTLS 3649 PreSharedKey secure connection to the RS. The CoAP request PUT is 3650 sent to the uri-path /state on the RS, changing the state of the 3651 door to locked. 3652 F: The RS responds with a appropriate over the secure DTLS 3653 channel. 3655 Resource 3656 Client Server 3657 | | 3658 |<=======>| DTLS Connection Establishment 3659 | | using Pre Shared Key 3660 | | 3661 +-------->| Header: PUT (Code=0.03) 3662 | PUT | Uri-Path: "state" 3663 | | Payload: 3664 | | 3665 F: |<--------+ Header: 2.04 Changed 3666 | 2.04 | Payload: 3667 | | 3669 Figure 26: Resource request and response protected by OSCORE 3671 Appendix F. Document Updates 3673 RFC EDITOR: PLEASE REMOVE THIS SECTION. 3675 F.1. Version -21 to 22 3677 o Provided section numbers in references to OAuth RFC. 3678 o Updated IANA mapping registries to only use "Private Use" and 3679 "Expert Review". 3680 o Made error messages optional for RS at token submission since it 3681 may not be able to send them depending on the profile. 3682 o Corrected errors in examples. 3684 F.2. Version -20 to 21 3686 o Added text about expiration of RS keys. 3688 F.3. Version -19 to 20 3690 o Replaced "req_aud" with "audience" from the OAuth token exchange 3691 draft. 3692 o Updated examples to remove unnecessary elements. 3694 F.4. Version -18 to -19 3696 o Added definition of "Authorization Information". 3697 o Explicitly state that ACE allows encoding refresh tokens in binary 3698 format in addition to strings. 3699 o Renamed "AS Information" to "AS Request Creation Hints" and added 3700 the possibility to specify req_aud and scope as hints. 3701 o Added the "kid" parameter to AS Request Creation Hints. 3702 o Added security considerations about the integrity protection of 3703 tokens with multi-RS audiences. 3704 o Renamed IANA registries mapping OAuth parameters to reflect the 3705 mapped registry. 3706 o Added JWT claim names to CWT claim registrations. 3707 o Added expert review instructions. 3708 o Updated references to TLS from 1.2 to 1.3. 3710 F.5. Version -17 to -18 3712 o Added OSCORE options in examples involving OSCORE. 3713 o Removed requirement for the client to send application/cwt, since 3714 the client has no way to know. 3715 o Clarified verification of tokens by the RS. 3716 o Added exi claim CWT registration. 3718 F.6. Version -16 to -17 3720 o Added references to (D)TLS 1.3. 3721 o Added requirement that responses are bound to requests. 3723 o Specify that grant_type is OPTIONAL in C2AS requests (as opposed 3724 to REQUIRED in OAuth). 3725 o Replaced examples with hypothetical COSE profile with OSCORE. 3726 o Added requirement for content type application/ace+cbor in error 3727 responses for token and introspection requests and responses. 3728 o Reworked abbreviation space for claims, request and response 3729 parameters. 3730 o Added text that the RS may indicate that it is busy at the authz- 3731 info resource. 3732 o Added section that specifies how the RS verifies an access token. 3733 o Added section on the protection of the authz-info endpoint. 3734 o Removed the expiration mechanism based on sequence numbers. 3735 o Added reference to RFC7662 security considerations. 3736 o Added considerations on minimal security requirements for 3737 communication. 3738 o Added security considerations on unprotected information sent to 3739 authz-info and in the error responses. 3741 F.7. Version -15 to -16 3743 o Added text the RS using RFC6750 error codes. 3744 o Defined an error code for incompatible token request parameters. 3745 o Removed references to the actors draft. 3746 o Fixed errors in examples. 3748 F.8. Version -14 to -15 3750 o Added text about refresh tokens. 3751 o Added text about protection of credentials. 3752 o Rephrased introspection so that other entities than RS can do it. 3753 o Editorial improvements. 3755 F.9. Version -13 to -14 3757 o Split out the 'aud', 'cnf' and 'rs_cnf' parameters to 3758 [I-D.ietf-ace-oauth-params] 3759 o Introduced the "application/ace+cbor" Content-Type. 3760 o Added claim registrations from 'profile' and 'rs_cnf'. 3761 o Added note on schema part of AS Information Section 5.1.2 3762 o Realigned the parameter abbreviations to push rarely used ones to 3763 the 2-byte encoding size of CBOR integers. 3765 F.10. Version -12 to -13 3767 o Changed "Resource Information" to "Access Information" to avoid 3768 confusion. 3769 o Clarified section about AS discovery. 3770 o Editorial changes 3772 F.11. Version -11 to -12 3774 o Moved the Request error handling to a section of its own. 3775 o Require the use of the abbreviation for profile identifiers. 3776 o Added rs_cnf parameter in the introspection response, to inform 3777 RS' with several RPKs on which key to use. 3778 o Allowed use of rs_cnf as claim in the access token in order to 3779 inform an RS with several RPKs on which key to use. 3780 o Clarified that profiles must specify if/how error responses are 3781 protected. 3782 o Fixed label number range to align with COSE/CWT. 3783 o Clarified the requirements language in order to allow profiles to 3784 specify other payload formats than CBOR if they do not use CoAP. 3786 F.12. Version -10 to -11 3788 o Fixed some CBOR data type errors. 3789 o Updated boilerplate text 3791 F.13. Version -09 to -10 3793 o Removed CBOR major type numbers. 3794 o Removed the client token design. 3795 o Rephrased to clarify that other protocols than CoAP can be used. 3796 o Clarifications regarding the use of HTTP 3798 F.14. Version -08 to -09 3800 o Allowed scope to be byte strings. 3801 o Defined default names for endpoints. 3802 o Refactored the IANA section for briefness and consistency. 3803 o Refactored tables that define IANA registry contents for 3804 consistency. 3805 o Created IANA registry for CBOR mappings of error codes, grant 3806 types and Authorization Server Information. 3807 o Added references to other document sections defining IANA entries 3808 in the IANA section. 3810 F.15. Version -07 to -08 3812 o Moved AS discovery from the DTLS profile to the framework, see 3813 Section 5.1. 3814 o Made the use of CBOR mandatory. If you use JSON you can use 3815 vanilla OAuth. 3816 o Made it mandatory for profiles to specify C-AS security and RS-AS 3817 security (the latter only if introspection is supported). 3818 o Made the use of CBOR abbreviations mandatory. 3820 o Added text to clarify the use of token references as an 3821 alternative to CWTs. 3822 o Added text to clarify that introspection must not be delayed, in 3823 case the RS has to return a client token. 3824 o Added security considerations about leakage through unprotected AS 3825 discovery information, combining profiles and leakage through 3826 error responses. 3827 o Added privacy considerations about leakage through unprotected AS 3828 discovery. 3829 o Added text that clarifies that introspection is optional. 3830 o Made profile parameter optional since it can be implicit. 3831 o Clarified that CoAP is not mandatory and other protocols can be 3832 used. 3833 o Clarified the design justification for specific features of the 3834 framework in appendix A. 3835 o Clarified appendix E.2. 3836 o Removed specification of the "cnf" claim for CBOR/COSE, and 3837 replaced with references to [I-D.ietf-ace-cwt-proof-of-possession] 3839 F.16. Version -06 to -07 3841 o Various clarifications added. 3842 o Fixed erroneous author email. 3844 F.17. Version -05 to -06 3846 o Moved sections that define the ACE framework into a subsection of 3847 the framework Section 5. 3848 o Split section on client credentials and grant into two separate 3849 sections, Section 5.2, and Section 5.3. 3850 o Added Section 5.4 on AS authentication. 3851 o Added Section 5.5 on the Authorization endpoint. 3853 F.18. Version -04 to -05 3855 o Added RFC 2119 language to the specification of the required 3856 behavior of profile specifications. 3857 o Added Section 5.3 on the relation to the OAuth2 grant types. 3858 o Added CBOR abbreviations for error and the error codes defined in 3859 OAuth2. 3860 o Added clarification about token expiration and long-running 3861 requests in Section 5.8.3 3862 o Added security considerations about tokens with symmetric PoP keys 3863 valid for more than one RS. 3864 o Added privacy considerations section. 3865 o Added IANA registry mapping the confirmation types from RFC 7800 3866 to equivalent COSE types. 3868 o Added appendix D, describing assumptions about what the AS knows 3869 about the client and the RS. 3871 F.19. Version -03 to -04 3873 o Added a description of the terms "framework" and "profiles" as 3874 used in this document. 3875 o Clarified protection of access tokens in section 3.1. 3876 o Clarified uses of the "cnf" parameter in section 6.4.5. 3877 o Clarified intended use of Client Token in section 7.4. 3879 F.20. Version -02 to -03 3881 o Removed references to draft-ietf-oauth-pop-key-distribution since 3882 the status of this draft is unclear. 3883 o Copied and adapted security considerations from draft-ietf-oauth- 3884 pop-key-distribution. 3885 o Renamed "client information" to "RS information" since it is 3886 information about the RS. 3887 o Clarified the requirements on profiles of this framework. 3888 o Clarified the token endpoint protocol and removed negotiation of 3889 "profile" and "alg" (section 6). 3890 o Renumbered the abbreviations for claims and parameters to get a 3891 consistent numbering across different endpoints. 3892 o Clarified the introspection endpoint. 3893 o Renamed token, introspection and authz-info to "endpoint" instead 3894 of "resource" to mirror the OAuth 2.0 terminology. 3895 o Updated the examples in the appendices. 3897 F.21. Version -01 to -02 3899 o Restructured to remove communication security parts. These shall 3900 now be defined in profiles. 3901 o Restructured section 5 to create new sections on the OAuth 3902 endpoints token, introspection and authz-info. 3903 o Pulled in material from draft-ietf-oauth-pop-key-distribution in 3904 order to define proof-of-possession key distribution. 3905 o Introduced the "cnf" parameter as defined in RFC7800 to reference 3906 or transport keys used for proof of possession. 3907 o Introduced the "client-token" to transport client information from 3908 the AS to the client via the RS in conjunction with introspection. 3909 o Expanded the IANA section to define parameters for token request, 3910 introspection and CWT claims. 3911 o Moved deployment scenarios to the appendix as examples. 3913 F.22. Version -00 to -01 3915 o Changed 5.1. from "Communication Security Protocol" to "Client 3916 Information". 3917 o Major rewrite of 5.1 to clarify the information exchanged between 3918 C and AS in the PoP access token request profile for IoT. 3920 * Allow the client to indicate preferences for the communication 3921 security protocol. 3922 * Defined the term "Client Information" for the additional 3923 information returned to the client in addition to the access 3924 token. 3925 * Require that the messages between AS and client are secured, 3926 either with (D)TLS or with COSE_Encrypted wrappers. 3927 * Removed dependency on OSCOAP and added generic text about 3928 object security instead. 3929 * Defined the "rpk" parameter in the client information to 3930 transmit the raw public key of the RS from AS to client. 3931 * (D)TLS MUST use the PoP key in the handshake (either as PSK or 3932 as client RPK with client authentication). 3933 * Defined the use of x5c, x5t and x5tS256 parameters when a 3934 client certificate is used for proof of possession. 3935 * Defined "tktn" parameter for signaling for how to transfer the 3936 access token. 3937 o Added 5.2. the CoAP Access-Token option for transferring access 3938 tokens in messages that do not have payload. 3939 o 5.3.2. Defined success and error responses from the RS when 3940 receiving an access token. 3941 o 5.6.:Added section giving guidance on how to handle token 3942 expiration in the absence of reliable time. 3943 o Appendix B Added list of roles and responsibilities for C, AS and 3944 RS. 3946 Authors' Addresses 3948 Ludwig Seitz 3949 Combitech 3950 Djaeknegatan 31 3951 Malmoe 211 35 3952 Sweden 3954 Email: ludwig.seitz@combitech.se 3955 Goeran Selander 3956 Ericsson 3957 Faroegatan 6 3958 Kista 164 80 3959 Sweden 3961 Email: goran.selander@ericsson.com 3963 Erik Wahlstroem 3964 Sweden 3966 Email: erik@wahlstromstekniska.se 3968 Samuel Erdtman 3969 Spotify AB 3970 Birger Jarlsgatan 61, 4tr 3971 Stockholm 113 56 3972 Sweden 3974 Email: erdtman@spotify.com 3976 Hannes Tschofenig 3977 Arm Ltd. 3978 Absam 6067 3979 Austria 3981 Email: Hannes.Tschofenig@arm.com