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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: November 6, 2021 Ericsson 6 E. Wahlstroem 8 S. Erdtman 9 Spotify AB 10 H. Tschofenig 11 Arm Ltd. 12 May 5, 2021 14 Authentication and Authorization for Constrained Environments (ACE) 15 using the OAuth 2.0 Framework (ACE-OAuth) 16 draft-ietf-ace-oauth-authz-41 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 November 6, 2021. 46 Copyright Notice 48 Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 14 70 5.1. Discovering Authorization Servers . . . . . . . . . . . . 16 71 5.2. Unauthorized Resource Request Message . . . . . . . . . . 16 72 5.3. AS Request Creation Hints . . . . . . . . . . . . . . . . 17 73 5.3.1. The Client-Nonce Parameter . . . . . . . . . . . . . 19 74 5.4. Authorization Grants . . . . . . . . . . . . . . . . . . 20 75 5.5. Client Credentials . . . . . . . . . . . . . . . . . . . 21 76 5.6. AS Authentication . . . . . . . . . . . . . . . . . . . . 21 77 5.7. The Authorization Endpoint . . . . . . . . . . . . . . . 21 78 5.8. The Token Endpoint . . . . . . . . . . . . . . . . . . . 21 79 5.8.1. Client-to-AS Request . . . . . . . . . . . . . . . . 22 80 5.8.2. AS-to-Client Response . . . . . . . . . . . . . . . . 25 81 5.8.3. Error Response . . . . . . . . . . . . . . . . . . . 27 82 5.8.4. Request and Response Parameters . . . . . . . . . . . 28 83 5.8.4.1. Grant Type . . . . . . . . . . . . . . . . . . . 28 84 5.8.4.2. Token Type . . . . . . . . . . . . . . . . . . . 29 85 5.8.4.3. Profile . . . . . . . . . . . . . . . . . . . . . 29 86 5.8.4.4. Client-Nonce . . . . . . . . . . . . . . . . . . 30 87 5.8.5. Mapping Parameters to CBOR . . . . . . . . . . . . . 30 88 5.9. The Introspection Endpoint . . . . . . . . . . . . . . . 31 89 5.9.1. Introspection Request . . . . . . . . . . . . . . . . 32 90 5.9.2. Introspection Response . . . . . . . . . . . . . . . 33 91 5.9.3. Error Response . . . . . . . . . . . . . . . . . . . 34 92 5.9.4. Mapping Introspection Parameters to CBOR . . . . . . 35 93 5.10. The Access Token . . . . . . . . . . . . . . . . . . . . 35 94 5.10.1. The Authorization Information Endpoint . . . . . . . 36 95 5.10.1.1. Verifying an Access Token . . . . . . . . . . . 37 96 5.10.1.2. Protecting the Authorization Information 97 Endpoint . . . . . . . . . . . . . . . . . . . . 39 98 5.10.2. Client Requests to the RS . . . . . . . . . . . . . 40 99 5.10.3. Token Expiration . . . . . . . . . . . . . . . . . . 40 100 5.10.4. Key Expiration . . . . . . . . . . . . . . . . . . . 42 101 6. Security Considerations . . . . . . . . . . . . . . . . . . . 42 102 6.1. Protecting Tokens . . . . . . . . . . . . . . . . . . . . 43 103 6.2. Communication Security . . . . . . . . . . . . . . . . . 44 104 6.3. Long-Term Credentials . . . . . . . . . . . . . . . . . . 44 105 6.4. Unprotected AS Request Creation Hints . . . . . . . . . . 45 106 6.5. Minimal Security Requirements for Communication . 45 107 6.6. Token Freshness and Expiration . . . . . . . . . . . . . 46 108 6.7. Combining Profiles . . . . . . . . . . . . . . . . . . . 47 109 6.8. Unprotected Information . . . . . . . . . . . . . . . . . 47 110 6.9. Identifying Audiences . . . . . . . . . . . . . . . . . . 48 111 6.10. Denial of Service Against or with Introspection . . 48 112 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 49 113 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50 114 8.1. ACE Authorization Server Request Creation Hints . . . . . 50 115 8.2. CoRE Resource Type Registry . . . . . . . . . . . . . . . 51 116 8.3. OAuth Extensions Error Registration . . . . . . . . . . . 51 117 8.4. OAuth Error Code CBOR Mappings Registry . . . . . . . . . 51 118 8.5. OAuth Grant Type CBOR Mappings . . . . . . . . . . . . . 52 119 8.6. OAuth Access Token Types . . . . . . . . . . . . . . . . 52 120 8.7. OAuth Access Token Type CBOR Mappings . . . . . . . . . . 52 121 8.7.1. Initial Registry Contents . . . . . . . . . . . . . . 53 122 8.8. ACE Profile Registry . . . . . . . . . . . . . . . . . . 53 123 8.9. OAuth Parameter Registration . . . . . . . . . . . . . . 54 124 8.10. OAuth Parameters CBOR Mappings Registry . . . . . . . . . 54 125 8.11. OAuth Introspection Response Parameter Registration . . . 54 126 8.12. OAuth Token Introspection Response CBOR Mappings Registry 55 127 8.13. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 55 128 8.14. CBOR Web Token Claims . . . . . . . . . . . . . . . . . . 56 129 8.15. Media Type Registrations . . . . . . . . . . . . . . . . 57 130 8.16. CoAP Content-Format Registry . . . . . . . . . . . . . . 58 131 8.17. Expert Review Instructions . . . . . . . . . . . . . . . 58 132 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 59 133 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 59 134 10.1. Normative References . . . . . . . . . . . . . . . . . . 59 135 10.2. Informative References . . . . . . . . . . . . . . . . . 62 136 Appendix A. Design Justification . . . . . . . . . . . . . . . . 65 137 Appendix B. Roles and Responsibilities . . . . . . . . . . . . . 68 138 Appendix C. Requirements on Profiles . . . . . . . . . . . . . . 71 139 Appendix D. Assumptions on AS Knowledge about C and RS . . . . . 72 140 Appendix E. Differences to OAuth 2.0 . . . . . . . . . . . . . . 72 141 Appendix F. Deployment Examples . . . . . . . . . . . . . . . . 73 142 F.1. Local Token Validation . . . . . . . . . . . . . . . . . 73 143 F.2. Introspection Aided Token Validation . . . . . . . . . . 77 144 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 81 146 1. Introduction 148 Authorization is the process for granting approval to an entity to 149 access a generic resource [RFC4949]. The authorization task itself 150 can best be described as granting access to a requesting client, for 151 a resource hosted on a device, the resource server (RS). This 152 exchange is mediated by one or multiple authorization servers (AS). 153 Managing authorization for a large number of devices and users can be 154 a complex task. 156 While prior work on authorization solutions for the Web and for the 157 mobile environment also applies to the Internet of Things (IoT) 158 environment, many IoT devices are constrained, for example, in terms 159 of processing capabilities, available memory, etc. For such devices 160 the Constrained Application Protocol (CoAP) [RFC7252] can alleviate 161 some resource concerns when used instead of HTTP to implement the 162 communication flows of this specification. 164 Appendix A gives an overview of the constraints considered in this 165 design, and a more detailed treatment of constraints can be found in 166 [RFC7228]. This design aims to accommodate different IoT deployments 167 and thus a continuous range of device and network capabilities. 168 Taking energy consumption as an example: At one end there are energy- 169 harvesting or battery powered devices which have a tight power 170 budget, on the other end there are mains-powered devices, and all 171 levels in between. 173 Hence, IoT devices may be very different in terms of available 174 processing and message exchange capabilities and there is a need to 175 support many different authorization use cases [RFC7744]. 177 This specification describes a framework for authentication and 178 authorization in constrained environments (ACE) built on re-use of 179 OAuth 2.0 [RFC6749], thereby extending authorization to Internet of 180 Things devices. This specification contains the necessary building 181 blocks for adjusting OAuth 2.0 to IoT environments. 183 Profiles of this framework are available in separate specifications, 184 such as [I-D.ietf-ace-dtls-authorize] or 185 [I-D.ietf-ace-oscore-profile]. Such profiles may specify the use of 186 the framework for a specific security protocol and the underlying 187 transports for use in a specific deployment environment to improve 188 interoperability. Implementations may claim conformance with a 189 specific profile, whereby implementations utilizing the same profile 190 interoperate, while implementations of different profiles are not 191 expected to be interoperable. More powerful devices, such as mobile 192 phones and tablets, may implement multiple profiles and will 193 therefore be able to interact with a wider range of constrained 194 devices. Requirements on profiles are described at contextually 195 appropriate places throughout this specification, and also summarized 196 in Appendix C. 198 2. Terminology 200 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 201 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 202 "OPTIONAL" in this document are to be interpreted as described in BCP 203 14 [RFC2119] [RFC8174] when, and only when, they appear in all 204 capitals, as shown here. 206 Certain security-related terms such as "authentication", 207 "authorization", "confidentiality", "(data) integrity", "message 208 authentication code", and "verify" are taken from [RFC4949]. 210 Since exchanges in this specification are described as RESTful 211 protocol interactions, HTTP [RFC7231] offers useful terminology. 213 Terminology for entities in the architecture is defined in OAuth 2.0 214 [RFC6749] such as client (C), resource server (RS), and authorization 215 server (AS). 217 Note that the term "endpoint" is used here following its OAuth 218 definition, which is to denote resources such as token and 219 introspection at the AS and authz-info at the RS (see Section 5.10.1 220 for a definition of the authz-info endpoint). The CoAP [RFC7252] 221 definition, which is "An entity participating in the CoAP protocol" 222 is not used in this specification. 224 The specifications in this document is called the "framework" or "ACE 225 framework". When referring to "profiles of this framework" it refers 226 to additional specifications that define the use of this 227 specification with concrete transport and communication security 228 protocols (e.g., CoAP over DTLS). 230 The term "Access Information" is used for parameters, other than the 231 access token, provided to the client by the AS to enable it to access 232 the RS (e.g. public key of the RS, profile supported by RS). 234 The term "Authorization Information" is used to denote all 235 information, including the claims of relevant access tokens, that an 236 RS uses to determine whether an access request should be granted. 238 3. Overview 240 This specification defines the ACE framework for authorization in the 241 Internet of Things environment. It consists of a set of building 242 blocks. 244 The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys 245 widespread deployment. Many IoT devices can support OAuth 2.0 246 without any additional extensions, but for certain constrained 247 settings additional profiling is needed. 249 Another building block is the lightweight web transfer protocol CoAP 250 [RFC7252], for those communication environments where HTTP is not 251 appropriate. CoAP typically runs on top of UDP, which further 252 reduces overhead and message exchanges. While this specification 253 defines extensions for the use of OAuth over CoAP, other underlying 254 protocols are not prohibited from being supported in the future, such 255 as HTTP/2 [RFC7540], Message Queuing Telemetry Transport (MQTT) 256 [MQTT5.0], Bluetooth Low Energy (BLE) [BLE] and QUIC 257 [I-D.ietf-quic-transport]. Note that this document specifies 258 protocol exchanges in terms of RESTful verbs such as GET and POST. 259 Future profiles using protocols that do not support these verbs MUST 260 specify how the corresponding protocol messages are transmitted 261 instead. 263 A third building block is the Concise Binary Object Representation 264 (CBOR) [RFC8949], for encodings where JSON [RFC8259] is not 265 sufficiently compact. CBOR is a binary encoding designed for small 266 code and message size, which may be used for encoding of self 267 contained tokens, and also for encoding payloads transferred in 268 protocol messages. 270 A fourth building block is CBOR Object Signing and Encryption (COSE) 271 [RFC8152], which enables object-level layer security as an 272 alternative or complement to transport layer security (DTLS [RFC6347] 273 or TLS [RFC8446]). COSE is used to secure self-contained tokens such 274 as proof-of-possession (PoP) tokens, which are an extension to the 275 OAuth bearer tokens. The default token format is defined in CBOR Web 276 Token (CWT) [RFC8392]. Application-layer security for CoAP using 277 COSE can be provided with OSCORE [RFC8613]. 279 With the building blocks listed above, solutions satisfying various 280 IoT device and network constraints are possible. A list of 281 constraints is described in detail in [RFC7228] and a description of 282 how the building blocks mentioned above relate to the various 283 constraints can be found in Appendix A. 285 Luckily, not every IoT device suffers from all constraints. The ACE 286 framework nevertheless takes all these aspects into account and 287 allows several different deployment variants to co-exist, rather than 288 mandating a one-size-fits-all solution. It is important to cover the 289 wide range of possible interworking use cases and the different 290 requirements from a security point of view. Once IoT deployments 291 mature, popular deployment variants will be documented in the form of 292 ACE profiles. 294 3.1. OAuth 2.0 296 The OAuth 2.0 authorization framework enables a client to obtain 297 scoped access to a resource with the permission of a resource owner. 298 Authorization information, or references to it, is passed between the 299 nodes using access tokens. These access tokens are issued to clients 300 by an authorization server with the approval of the resource owner. 301 The client uses the access token to access the protected resources 302 hosted by the resource server. 304 A number of OAuth 2.0 terms are used within this specification: 306 Access Tokens: 307 Access tokens are credentials needed to access protected 308 resources. An access token is a data structure representing 309 authorization permissions issued by the AS to the client. Access 310 tokens are generated by the AS and consumed by the RS. The access 311 token content is opaque to the client. 313 Access tokens can have different formats, and various methods of 314 utilization e.g., cryptographic properties) based on the security 315 requirements of the given deployment. 317 Introspection: 318 Introspection is a method for a resource server or potentially a 319 client, to query the authorization server for the active state and 320 content of a received access token. This is particularly useful 321 in those cases where the authorization decisions are very dynamic 322 and/or where the received access token itself is an opaque 323 reference rather than a self-contained token. More information 324 about introspection in OAuth 2.0 can be found in [RFC7662]. 326 Refresh Tokens: 327 Refresh tokens are credentials used to obtain access tokens. 328 Refresh tokens are issued to the client by the authorization 329 server and are used to obtain a new access token when the current 330 access token expires, or to obtain additional access tokens with 331 identical or narrower scope (such access tokens may have a shorter 332 lifetime and fewer permissions than authorized by the resource 333 owner). Issuing a refresh token is optional at the discretion of 334 the authorization server. If the authorization server issues a 335 refresh token, it is included when issuing an access token (i.e., 336 step (B) in Figure 1). 338 A refresh token in OAuth 2.0 is a string representing the 339 authorization granted to the client by the resource owner. The 340 string is usually opaque to the client. The token denotes an 341 identifier used to retrieve the authorization information. Unlike 342 access tokens, refresh tokens are intended for use only with 343 authorization servers and are never sent to resource servers. In 344 this framework, refresh tokens are encoded in binary instead of 345 strings, if used. 347 Proof of Possession Tokens: 348 A token may be bound to a cryptographic key, which is then used to 349 bind the token to a request authorized by the token. Such tokens 350 are called proof-of-possession tokens (or PoP tokens). 352 The proof-of-possession security concept used here assumes that 353 the AS acts as a trusted third party that binds keys to tokens. 354 In the case of access tokens, these so called PoP keys are then 355 used by the client to demonstrate the possession of the secret to 356 the RS when accessing the resource. The RS, when receiving an 357 access token, needs to verify that the key used by the client 358 matches the one bound to the access token. When this 359 specification uses the term "access token" it is assumed to be a 360 PoP access token unless specifically stated otherwise. 362 The key bound to the token (the PoP key) may use either symmetric 363 or asymmetric cryptography. The appropriate choice of the kind of 364 cryptography depends on the constraints of the IoT devices as well 365 as on the security requirements of the use case. 367 Symmetric PoP key: 368 The AS generates a random symmetric PoP key. The key is either 369 stored to be returned on introspection calls or included in the 370 token. Either the whole token or only the key MUST be 371 encrypted in the latter case. The PoP key is also returned to 372 client together with the token. 374 Asymmetric PoP key: 375 An asymmetric key pair is generated by the client and the 376 public key is sent to the AS (if it does not already have 377 knowledge of the client's public key). Information about the 378 public key, which is the PoP key in this case, is either stored 379 to be returned on introspection calls or included inside the 380 token and sent back to the client. The resource server 381 consuming the token can identify the public key from the 382 information in the token, which allows the client to use the 383 corresponding private key for the proof of possession. 385 The token is either a simple reference, or a structured 386 information object (e.g., CWT [RFC8392]) protected by a 387 cryptographic wrapper (e.g., COSE [RFC8152]). The choice of PoP 388 key does not necessarily imply a specific credential type for the 389 integrity protection of the token. 391 Scopes and Permissions: 392 In OAuth 2.0, the client specifies the type of permissions it is 393 seeking to obtain (via the scope parameter) in the access token 394 request. In turn, the AS may use the scope response parameter to 395 inform the client of the scope of the access token issued. As the 396 client could be a constrained device as well, this specification 397 defines the use of CBOR encoding, see Section 5, for such requests 398 and responses. 400 The values of the scope parameter in OAuth 2.0 are expressed as a 401 list of space-delimited, case-sensitive strings, with a semantic 402 that is well-known to the AS and the RS. More details about the 403 concept of scopes is found under Section 3.3 in [RFC6749]. 405 Claims: 406 Information carried in the access token or returned from 407 introspection, called claims, is in the form of name-value pairs. 408 An access token may, for example, include a claim identifying the 409 AS that issued the token (via the "iss" claim) and what audience 410 the access token is intended for (via the "aud" claim). The 411 audience of an access token can be a specific resource or one or 412 many resource servers. The resource owner policies influence what 413 claims are put into the access token by the authorization server. 415 While the structure and encoding of the access token varies 416 throughout deployments, a standardized format has been defined 417 with the JSON Web Token (JWT) [RFC7519] where claims are encoded 418 as a JSON object. In [RFC8392] the CBOR Web Token (CWT) has been 419 defined as an equivalent format using CBOR encoding. 421 The token and introspection Endpoints: 422 The AS hosts the token endpoint that allows a client to request 423 access tokens. The client makes a POST request to the token 424 endpoint on the AS and receives the access token in the response 425 (if the request was successful). 426 In some deployments, a token introspection endpoint is provided by 427 the AS, which can be used by the RS and potentially the client, if 428 they need to request additional information regarding a received 429 access token. The requesting entity makes a POST request to the 430 introspection endpoint on the AS and receives information about 431 the access token in the response. (See "Introspection" above.) 433 3.2. CoAP 435 CoAP is an application-layer protocol similar to HTTP, but 436 specifically designed for constrained environments. CoAP typically 437 uses datagram-oriented transport, such as UDP, where reordering and 438 loss of packets can occur. A security solution needs to take the 439 latter aspects into account. 441 While HTTP uses headers and query strings to convey additional 442 information about a request, CoAP encodes such information into 443 header parameters called 'options'. 445 CoAP supports application-layer fragmentation of the CoAP payloads 446 through blockwise transfers [RFC7959]. However, blockwise transfer 447 does not increase the size limits of CoAP options, therefore data 448 encoded in options has to be kept small. 450 Transport layer security for CoAP can be provided by DTLS or TLS 451 [RFC6347][RFC8446] [I-D.ietf-tls-dtls13]. CoAP defines a number of 452 proxy operations that require transport layer security to be 453 terminated at the proxy. One approach for protecting CoAP 454 communication end-to-end through proxies, and also to support 455 security for CoAP over a different transport in a uniform way, is to 456 provide security at the application layer using an object-based 457 security mechanism such as COSE [RFC8152]. 459 One application of COSE is OSCORE [RFC8613], which provides end-to- 460 end confidentiality, integrity and replay protection, and a secure 461 binding between CoAP request and response messages. In OSCORE, the 462 CoAP messages are wrapped in COSE objects and sent using CoAP. 464 In this framework the use of CoAP as replacement for HTTP is 465 RECOMMENDED for use in constrained environments. For communication 466 security this framework does not make an explicit protocol 467 recommendation, since the choice depends on the requirements of the 468 specific application. DTLS [RFC6347], [I-D.ietf-tls-dtls13] and 469 OSCORE [RFC8613] are mentioned as examples, other protocols 470 fulfilling the requirements from Section 6.5 are also applicable. 472 4. Protocol Interactions 474 The ACE framework is based on the OAuth 2.0 protocol interactions 475 using the token endpoint and optionally the introspection endpoint. 476 A client obtains an access token, and optionally a refresh token, 477 from an AS using the token endpoint and subsequently presents the 478 access token to an RS to gain access to a protected resource. In 479 most deployments the RS can process the access token locally, however 480 in some cases the RS may present it to the AS via the introspection 481 endpoint to get fresh information. These interactions are shown in 482 Figure 1. An overview of various OAuth concepts is provided in 483 Section 3.1. 485 +--------+ +---------------+ 486 | |---(A)-- Token Request ------->| | 487 | | | Authorization | 488 | |<--(B)-- Access Token ---------| Server | 489 | | + Access Information | | 490 | | + Refresh Token (optional) +---------------+ 491 | | ^ | 492 | | Introspection Request (D)| | 493 | Client | Response | |(E) 494 | | (optional exchange) | | 495 | | | v 496 | | +--------------+ 497 | |---(C)-- Token + Request ----->| | 498 | | | Resource | 499 | |<--(F)-- Protected Resource ---| Server | 500 | | | | 501 +--------+ +--------------+ 503 Figure 1: Basic Protocol Flow. 505 Requesting an Access Token (A): 506 The client makes an access token request to the token endpoint at 507 the AS. This framework assumes the use of PoP access tokens (see 508 Section 3.1 for a short description) wherein the AS binds a key to 509 an access token. The client may include permissions it seeks to 510 obtain, and information about the credentials it wants to use for 511 proof-of-possession (e.g., symmetric/asymmetric cryptography or a 512 reference to a specific key) of the access token. 514 Access Token Response (B): 515 If the request from the client has been successfully verified, 516 authenticated, and authorized, the AS returns an access token and 517 optionally a refresh token. Note that only certain grant types 518 support refresh tokens. The AS can also return additional 519 parameters, referred to as "Access Information". In addition to 520 the response parameters defined by OAuth 2.0 and the PoP access 521 token extension, this framework defines parameters that can be 522 used to inform the client about capabilities of the RS, e.g. the 523 profile the RS supports. More information about these parameters 524 can be found in Section 5.8.4. 526 Resource Request (C): 527 The client interacts with the RS to request access to the 528 protected resource and provides the access token. The protocol to 529 use between the client and the RS is not restricted to CoAP. 530 HTTP, HTTP/2 [RFC7540], QUIC [I-D.ietf-quic-transport], MQTT 531 [MQTT5.0], Bluetooth Low Energy [BLE], etc., are also viable 532 candidates. 534 Depending on the device limitations and the selected protocol, 535 this exchange may be split up into two parts: 537 (1) the client sends the access token containing, or 538 referencing, the authorization information to the RS, that will 539 be used for subsequent resource requests by the client, and 541 (2) the client makes the resource access request, using the 542 communication security protocol and other Access Information 543 obtained from the AS. 545 The client and the RS mutually authenticate using the security 546 protocol specified in the profile (see step B) and the keys 547 obtained in the access token or the Access Information. The RS 548 verifies that the token is integrity protected and originated by 549 the AS. It then compares the claims contained in the access token 550 with the resource request. If the RS is online, validation can be 551 handed over to the AS using token introspection (see messages D 552 and E) over HTTP or CoAP. 554 Token Introspection Request (D): 555 A resource server may be configured to introspect the access token 556 by including it in a request to the introspection endpoint at that 557 AS. Token introspection over CoAP is defined in Section 5.9 and 558 for HTTP in [RFC7662]. 560 Note that token introspection is an optional step and can be 561 omitted if the token is self-contained and the resource server is 562 prepared to perform the token validation on its own. 564 Token Introspection Response (E): 565 The AS validates the token and returns the most recent parameters, 566 such as scope, audience, validity etc. associated with it back to 567 the RS. The RS then uses the received parameters to process the 568 request to either accept or to deny it. 570 Protected Resource (F): 571 If the request from the client is authorized, the RS fulfills the 572 request and returns a response with the appropriate response code. 573 The RS uses the dynamically established keys to protect the 574 response, according to the communication security protocol used. 576 The OAuth 2.0 framework defines a number of "protocol flows" via 577 grant types, which have been extended further with extensions to 578 OAuth 2.0 (such as [RFC7521] and [RFC8628]). What grant type works 579 best depends on the usage scenario and [RFC7744] describes many 580 different IoT use cases but there are two grant types that cover a 581 majority of these scenarios, namely the Authorization Code Grant 582 (described in Section 4.1 of [RFC7521]) and the Client Credentials 583 Grant (described in Section 4.4 of [RFC7521]). The Authorization 584 Code Grant is a good fit for use with apps running on smart phones 585 and tablets that request access to IoT devices, a common scenario in 586 the smart home environment, where users need to go through an 587 authentication and authorization phase (at least during the initial 588 setup phase). The native apps guidelines described in [RFC8252] are 589 applicable to this use case. The Client Credential Grant is a good 590 fit for use with IoT devices where the OAuth client itself is 591 constrained. In such a case, the resource owner has pre-arranged 592 access rights for the client with the authorization server, which is 593 often accomplished using a commissioning tool. 595 The consent of the resource owner, for giving a client access to a 596 protected resource, can be provided dynamically as in the traditional 597 OAuth flows, or it could be pre-configured by the resource owner as 598 authorization policies at the AS, which the AS evaluates when a token 599 request arrives. The resource owner and the requesting party (i.e., 600 client owner) are not shown in Figure 1. 602 This framework supports a wide variety of communication security 603 mechanisms between the ACE entities, such as client, AS, and RS. It 604 is assumed that the client has been registered (also called enrolled 605 or onboarded) to an AS using a mechanism defined outside the scope of 606 this document. In practice, various techniques for onboarding have 607 been used, such as factory-based provisioning or the use of 608 commissioning tools. Regardless of the onboarding technique, this 609 provisioning procedure implies that the client and the AS exchange 610 credentials and configuration parameters. These credentials are used 611 to mutually authenticate each other and to protect messages exchanged 612 between the client and the AS. 614 It is also assumed that the RS has been registered with the AS, 615 potentially in a similar way as the client has been registered with 616 the AS. Established keying material between the AS and the RS allows 617 the AS to apply cryptographic protection to the access token to 618 ensure that its content cannot be modified, and if needed, that the 619 content is confidentiality protected. Confidentiality protection of 620 the access token content would be provided on top of confidentiality 621 protection via a communication security protocol. 623 The keying material necessary for establishing communication security 624 between C and RS is dynamically established as part of the protocol 625 described in this document. 627 At the start of the protocol, there is an optional discovery step 628 where the client discovers the resource server and the resources this 629 server hosts. In this step, the client might also determine what 630 permissions are needed to access the protected resource. A generic 631 procedure is described in Section 5.1; profiles MAY define other 632 procedures for discovery. 634 In Bluetooth Low Energy, for example, advertisements are broadcast by 635 a peripheral, including information about the primary services. In 636 CoAP, as a second example, a client can make a request to "/.well- 637 known/core" to obtain information about available resources, which 638 are returned in a standardized format as described in [RFC6690]. 640 5. Framework 642 The following sections detail the profiling and extensions of OAuth 643 2.0 for constrained environments, which constitutes the ACE 644 framework. 646 Credential Provisioning 647 In constrained environments it cannot be assumed that the client 648 and the RS are part of a common key infrastructure. Therefore, 649 the AS provisions credentials and associated information to allow 650 mutual authentication between the client and the RS. The 651 resulting security association between the client and the RS may 652 then also be used to bind these credentials to the access tokens 653 the client uses. 655 Proof-of-Possession 656 The ACE framework, by default, implements proof-of-possession for 657 access tokens, i.e., that the token holder can prove being a 658 holder of the key bound to the token. The binding is provided by 659 the "cnf" claim [RFC8747] indicating what key is used for proof- 660 of-possession. If a client needs to submit a new access token, 661 e.g., to obtain additional access rights, they can request that 662 the AS binds this token to the same key as the previous one. 664 ACE Profiles 665 The client or RS may be limited in the encodings or protocols it 666 supports. To support a variety of different deployment settings, 667 specific interactions between client and RS are defined in an ACE 668 profile. In ACE framework the AS is expected to manage the 669 matching of compatible profile choices between a client and an RS. 670 The AS informs the client of the selected profile using the 671 "ace_profile" parameter in the token response. 673 OAuth 2.0 requires the use of TLS both to protect the communication 674 between AS and client when requesting an access token; between client 675 and RS when accessing a resource and between AS and RS if 676 introspection is used. In constrained settings TLS is not always 677 feasible, or desirable. Nevertheless it is REQUIRED that the 678 communications named above are encrypted, integrity protected and 679 protected against message replay. It is also REQUIRED that the 680 communicating endpoints perform mutual authentication. Furthermore 681 it MUST be assured that responses are bound to the requests in the 682 sense that the receiver of a response can be certain that the 683 response actually belongs to a certain request. Note that setting up 684 such a secure communication may require some unprotected messages to 685 be exchanged first (e.g. sending the token from the client to the 686 RS). 688 Profiles MUST specify a communication security protocol between 689 client and RS that provides the features required above. Profiles 690 MUST specify a communication security protocol RECOMMENDED to be used 691 between client and AS that provides the features required above. 692 Profiles MUST specify for introspection a communication security 693 protocol RECOMMENDED to be used between RS and AS that provides the 694 features required above. These recommendations enable 695 interoperability between different implementations without the need 696 to define a new profile if the communication between C and AS, or 697 between RS and AS, is protected with a different security protocol 698 complying with the security requirements above. 700 In OAuth 2.0 the communication with the Token and the Introspection 701 endpoints at the AS is assumed to be via HTTP and may use Uri-query 702 parameters. When profiles of this framework use CoAP instead, it is 703 REQUIRED to use of the following alternative instead of Uri-query 704 parameters: The sender (client or RS) encodes the parameters of its 705 request as a CBOR map and submits that map as the payload of the POST 706 request. 708 Profiles that use CBOR encoding of protocol message parameters at the 709 outermost encoding layer MUST use the content format 'application/ 710 ace+cbor'. If CoAP is used for communication, the Content-Format 711 MUST be abbreviated with the ID: 19 (see Section 8.16). 713 The OAuth 2.0 AS uses a JSON structure in the payload of its 714 responses both to client and RS. If CoAP is used, it is REQUIRED to 715 use CBOR [RFC8949] instead of JSON. Depending on the profile, the 716 CBOR payload MAY be enclosed in a non-CBOR cryptographic wrapper. 718 5.1. Discovering Authorization Servers 720 C must discover the AS in charge of RS to determine where to request 721 the access token. To do so, C must 1. find out the AS URI to which 722 the token request message must be sent and 2. MUST validate that the 723 AS with this URI is authorized to provide access tokens for this RS. 725 In order to determine the AS URI, C MAY send an initial Unauthorized 726 Resource Request message to RS. RS then denies the request and sends 727 the address of its AS back to C (see Section 5.2). How C validates 728 the AS authorization is not in scope for this document. C may, e.g., 729 ask its owner if this AS is authorized for this RS. C may also use a 730 mechanism that addresses both problems at once (e.g. by querying a 731 dedicated secure service provided by the client owner) . 733 5.2. Unauthorized Resource Request Message 735 An Unauthorized Resource Request message is a request for any 736 resource hosted by RS for which the client does not have 737 authorization granted. RSes MUST treat any request for a protected 738 resource as an Unauthorized Resource Request message when any of the 739 following hold: 741 o The request has been received on an unsecured channel. 743 o The RS has no valid access token for the sender of the request 744 regarding the requested action on that resource. 746 o The RS has a valid access token for the sender of the request, but 747 that token does not authorize the requested action on the 748 requested resource. 750 Note: These conditions ensure that the RS can handle requests 751 autonomously once access was granted and a secure channel has been 752 established between C and RS. The authz-info endpoint, as part of 753 the process for authorizing to protected resources, is not itself a 754 protected resource and MUST NOT be protected as specified above (cf. 755 Section 5.10.1). 757 Unauthorized Resource Request messages MUST be denied with an 758 "unauthorized_client" error response. In this response, the Resource 759 Server SHOULD provide proper "AS Request Creation Hints" to enable 760 the client to request an access token from RS's AS as described in 761 Section 5.3. 763 The handling of all client requests (including unauthorized ones) by 764 the RS is described in Section 5.10.2. 766 5.3. AS Request Creation Hints 768 The "AS Request Creation Hints" message is sent by an RS as a 769 response to an Unauthorized Resource Request message (see 770 Section 5.2) to help the sender of the Unauthorized Resource Request 771 message acquire a valid access token. The "AS Request Creation 772 Hints" message is a CBOR or JSON map, with an OPTIONAL element "AS" 773 specifying an absolute URI (see Section 4.3 of [RFC3986]) that 774 identifies the appropriate AS for the RS. 776 The message can also contain the following OPTIONAL parameters: 778 o A "audience" element contains an identifier the client should 779 request at the AS, as suggested by the RS. With this parameter, 780 when included in the access token request to the AS, the AS is 781 able to restrict the use of access token to specific RSs. See 782 Section 6.9 for a discussion of this parameter. 784 o A "kid" element containing the key identifier of a key used in an 785 existing security association between the client and the RS. The 786 RS expects the client to request an access token bound to this 787 key, in order to avoid having to re-establish the security 788 association. 790 o A "cnonce" element containing a client-nonce. See Section 5.3.1. 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 796 Request 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 [RFC8949] 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 uses an internal clock 836 that is not synchronized with the clock of the AS. Therefore, it can 837 not reliably verify the expiration time of access tokens it receives. 838 To ensure a certain level of access token freshness nevertheless, the 839 RS has included a "cnonce" parameter (see Section 5.3.1) in the 840 response. (The hex-sequence of the cnonce parameter is encoded in 841 CBOR-based notation in this example.) 843 Figure 4 illustrates the mandatory to use binary encoding of the 844 message payload shown in Figure 3. 846 a4 # map(4) 847 01 # unsigned(1) (=AS) 848 78 1c # text(28) 849 636f6170733a2f2f61732e657861 850 6d706c652e636f6d2f746f6b656e # "coaps://as.example.com/token" 851 05 # unsigned(5) (=audience) 852 76 # text(22) 853 636f6170733a2f2f72732e657861 854 6d706c652e636f6d # "coaps://rs.example.com" 855 09 # unsigned(9) (=scope) 856 66 # text(6) 857 7254656d7043 # "rTempC" 858 18 27 # unsigned(39) (=cnonce) 859 45 # bytes(5) 860 e0a156bb3f # 862 Figure 4: AS Request Creation Hints example encoded in CBOR 864 5.3.1. The Client-Nonce Parameter 866 If the RS does not synchronize its clock with the AS, it could be 867 tricked into accepting old access tokens, that are either expired or 868 have been compromised. In order to ensure some level of token 869 freshness in that case, the RS can use the "cnonce" (client-nonce) 870 parameter. The processing requirements for this parameter are as 871 follows: 873 o An RS sending a "cnonce" parameter in an "AS Request Creation 874 Hints" message MUST store information to validate that a given 875 cnonce is fresh. How this is implemented internally is out of 876 scope for this specification. Expiration of client-nonces should 877 be based roughly on the time it would take a client to obtain an 878 access token after receiving the "AS Request Creation Hints" 879 message, with some allowance for unexpected delays. 881 o A client receiving a "cnonce" parameter in an "AS Request Creation 882 Hints" message MUST include this in the parameters when requesting 883 an access token at the AS, using the "cnonce" parameter from 884 Section 5.8.4.4. 886 o If an AS grants an access token request containing a "cnonce" 887 parameter, it MUST include this value in the access token, using 888 the "cnonce" claim specified in Section 5.10. 890 o An RS that is using the client-nonce mechanism and that receives 891 an access token MUST verify that this token contains a cnonce 892 claim, with a client-nonce value that is fresh according to the 893 information stored at the first step above. If the cnonce claim 894 is not present or if the cnonce claim value is not fresh, the RS 895 MUST discard the access token. If this was an interaction with 896 the authz-info endpoint the RS MUST also respond with an error 897 message using a response code equivalent to the CoAP code 4.01 898 (Unauthorized). 900 5.4. Authorization Grants 902 To request an access token, the client obtains authorization from the 903 resource owner or uses its client credentials as a grant. The 904 authorization is expressed in the form of an authorization grant. 906 The OAuth framework [RFC6749] defines four grant types. The grant 907 types can be split up into two groups, those granted on behalf of the 908 resource owner (password, authorization code, implicit) and those for 909 the client (client credentials). Further grant types have been added 910 later, such as [RFC7521] defining an assertion-based authorization 911 grant. 913 The grant type is selected depending on the use case. In cases where 914 the client acts on behalf of the resource owner, the authorization 915 code grant is recommended. If the client acts on behalf of the 916 resource owner, but does not have any display or has very limited 917 interaction possibilities, it is recommended to use the device code 918 grant defined in [RFC8628]. In cases where the client acts 919 autonomously the client credentials grant is recommended. 921 For details on the different grant types, see section 1.3 of 922 [RFC6749]. The OAuth 2.0 framework provides an extension mechanism 923 for defining additional grant types, so profiles of this framework 924 MAY define additional grant types, if needed. 926 5.5. Client Credentials 928 Authentication of the client is mandatory independent of the grant 929 type when requesting an access token from the token endpoint. In the 930 case of the client credentials grant type, the authentication and 931 grant coincide. 933 Client registration and provisioning of client credentials to the 934 client is out of scope for this specification. 936 The OAuth framework defines one client credential type in section 937 2.3.1 of [RFC6749]: client id and client secret. 938 [I-D.erdtman-ace-rpcc] adds raw-public-key and pre-shared-key to the 939 client credentials types. Profiles of this framework MAY extend with 940 an additional client credentials type using client certificates. 942 5.6. AS Authentication 944 The client credential grant does not, by default, authenticate the AS 945 that the client connects to. In classic OAuth, the AS is 946 authenticated with a TLS server certificate. 948 Profiles of this framework MUST specify how clients authenticate the 949 AS and how communication security is implemented. By default, server 950 side TLS certificates, as defined by OAuth 2.0, are required. 952 5.7. The Authorization Endpoint 954 The OAuth 2.0 authorization endpoint is used to interact with the 955 resource owner and obtain an authorization grant, in certain grant 956 flows. The primary use case for the ACE-OAuth framework is for 957 machine-to-machine interactions that do not involve the resource 958 owner in the authorization flow; therefore, this endpoint is out of 959 scope here. Future profiles may define constrained adaptation 960 mechanisms for this endpoint as well. Non-constrained clients 961 interacting with constrained resource servers can use the 962 specification in section 3.1 of [RFC6749] and the attack 963 countermeasures suggested in section 4.2 of [RFC6819]. 965 5.8. The Token Endpoint 967 In standard OAuth 2.0, the AS provides the token endpoint for 968 submitting access token requests. This framework extends the 969 functionality of the token endpoint, giving the AS the possibility to 970 help the client and RS to establish shared keys or to exchange their 971 public keys. Furthermore, this framework defines encodings using 972 CBOR, as a substitute for JSON. 974 The endpoint may also be exposed over HTTPS as in classical OAuth or 975 even other transports. A profile MUST define the details of the 976 mapping between the fields described below, and these transports. If 977 HTTPS is used, the semantics of Sections 4.1.3 and 4.1.4 of the OAuth 978 2.0 specification MUST be followed (with additions as described 979 below). If the CoAP is some other transport with CBOR payload format 980 is supported, the semantics described in this section 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 SHOULD be '/token'. 990 However, implementations are not required to use this name and can 991 define 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 5.8.1. Client-to-AS Request 1001 The client sends a POST request to the token endpoint at the AS. The 1002 profile MUST specify how the communication is protected. The content 1003 of the request consists of the parameters specified in the relevant 1004 subsection of section 4 of the OAuth 2.0 specification [RFC6749], 1005 depending on the grant type, with the following exceptions and 1006 additions: 1008 o The parameter "grant_type" is OPTIONAL in the context of this 1009 framework (as opposed to REQUIRED in RFC6749). If that parameter 1010 is missing, the default value "client_credentials" is implied. 1012 o The "audience" parameter from [RFC8693] is OPTIONAL to request an 1013 access token bound to a specific audience. 1015 o The "cnonce" parameter defined in Section 5.8.4.4 is REQUIRED if 1016 the RS provided a client-nonce in the "AS Request Creation Hints" 1017 message Section 5.3 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. Note specifically 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 5.8.4.3. 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 CoAP is used then these parameters MUST be provided in a CBOR map, 1043 see Figure 12. 1045 When HTTP is used as a transport then the client makes a request to 1046 the token endpoint, the parameters MUST be encoded as defined in 1047 Appendix B 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 } 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 5.8.2. 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 5.8.3. 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 CoAP, the payload MUST be encoded as a CBOR map, when using 1154 HTTP the encoding is a JSON map as specified in section 5.1 of 1156 [RFC6749]. In both cases the parameters specified in Section 5.1 of 1157 [RFC6749] are used, with the following additions and changes: 1159 ace_profile: 1160 OPTIONAL unless the request included an empty ace_profile 1161 parameter in which case it is MANDATORY. This indicates the 1162 profile that the client MUST use towards the RS. See 1163 Section 5.8.4.3 for the formatting of this parameter. If this 1164 parameter is absent, the AS assumes that the client implicitly 1165 knows which profile to use towards the RS. 1167 token_type: 1168 This parameter is OPTIONAL, as opposed to 'required' in [RFC6749]. 1169 By default implementations of this framework SHOULD assume that 1170 the token_type is "PoP". If a specific use case requires another 1171 token_type (e.g., "Bearer") to be used then this parameter is 1172 REQUIRED. 1174 Furthermore [I-D.ietf-ace-oauth-params] defines additional parameters 1175 that the AS MUST be able to use when responding to a request to the 1176 token endpoint. 1178 Figure 8 summarizes the parameters that can currently be part of the 1179 Access Information. Future extensions may define additional 1180 parameters. 1182 /-------------------+-------------------------------\ 1183 | Parameter name | Specified in | 1184 |-------------------+-------------------------------| 1185 | access_token | RFC 6749 | 1186 | token_type | RFC 6749 | 1187 | expires_in | RFC 6749 | 1188 | refresh_token | RFC 6749 | 1189 | scope | RFC 6749 | 1190 | state | RFC 6749 | 1191 | error | RFC 6749 | 1192 | error_description | RFC 6749 | 1193 | error_uri | RFC 6749 | 1194 | ace_profile | [this document] | 1195 | cnf | [I-D.ietf-ace-oauth-params] | 1196 | rs_cnf | [I-D.ietf-ace-oauth-params] | 1197 \-------------------+-------------------------------/ 1199 Figure 8: Access Information parameters 1201 Figure 9 shows a response containing a token and a "cnf" parameter 1202 with a symmetric proof-of-possession key, which is defined in 1203 [I-D.ietf-ace-oauth-params]. Note that the key identifier 'kid' is 1204 only used to simplify indexing and retrieving the key, and no 1205 assumptions should be made that it is unique in the domains of either 1206 the client or the RS. 1208 Header: Created (Code=2.01) 1209 Content-Format: "application/ace+cbor" 1210 Payload: 1211 { 1212 "access_token" : b64'SlAV32hkKG ... 1213 (remainder of CWT omitted for brevity; 1214 CWT contains COSE_Key in the "cnf" claim)', 1215 "ace_profile" : "coap_dtls", 1216 "expires_in" : "3600", 1217 "cnf" : { 1218 "COSE_Key" : { 1219 "kty" : "Symmetric", 1220 "kid" : b64'39Gqlw', 1221 "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh' 1222 } 1223 } 1224 } 1226 Figure 9: Example AS response with an access token bound to a 1227 symmetric key. 1229 5.8.3. Error Response 1231 The error responses for interactions with the AS are generally 1232 equivalent to the ones defined in Section 5.2 of [RFC6749], with the 1233 following exceptions: 1235 o When using CoAP the payload MUST be encoded as a CBOR map, with 1236 the Content-Format "application/ace+cbor". When using HTTP the 1237 payload is encoded in JSON as specified in section 5.2 of 1238 [RFC6749]. 1240 o A response code equivalent to the CoAP code 4.00 (Bad Request) 1241 MUST be used for all error responses, except for invalid_client 1242 where a response code equivalent to the CoAP code 4.01 1243 (Unauthorized) MAY be used under the same conditions as specified 1244 in Section 5.2 of [RFC6749]. 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" specified 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" specified in 1282 Figure 10. 1284 5.8.4. 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 5.8.4.1. Grant Type 1293 The abbreviations specified in the registry defined in Section 8.5 1294 MUST be used in CBOR encodings instead of the string values defined 1295 in [RFC6749], if CBOR payloads are used. 1297 /--------------------+------------+------------------------\ 1298 | Name | CBOR Value | Original Specification | 1299 |--------------------+------------+------------------------| 1300 | password | 0 | s. 4.3.2 of [RFC6749] | 1301 | authorization_code | 1 | s. 4.1.3 of [RFC6749] | 1302 | client_credentials | 2 | s. 4.4.2 of [RFC6749] | 1303 | refresh_token | 3 | s. 6 of [RFC6749] | 1304 \--------------------+------------+------------------------/ 1306 Figure 11: CBOR abbreviations for common grant types 1308 5.8.4.2. Token Type 1310 The "token_type" parameter, defined in section 5.1 of [RFC6749], 1311 allows the AS to indicate to the client which type of access token it 1312 is receiving (e.g., a bearer token). 1314 This document registers the new value "PoP" for the OAuth Access 1315 Token Types registry, specifying a proof-of-possession token. How 1316 the proof-of-possession by the client to the RS is performed MUST be 1317 specified by the profiles. 1319 The values in the "token_type" parameter MUST use the CBOR 1320 abbreviations defined in the registry specified by Section 8.7, if a 1321 CBOR encoding is used. 1323 In this framework the "pop" value for the "token_type" parameter is 1324 the default. The AS may, however, provide a different value from 1325 those registered in [IANA.OAuthAccessTokenTypes]. 1327 5.8.4.3. Profile 1329 Profiles of this framework MUST define the communication protocol and 1330 the communication security protocol between the client and the RS. 1331 The security protocol MUST provide encryption, integrity and replay 1332 protection. It MUST also provide a binding between requests and 1333 responses. Furthermore profiles MUST define a list of allowed proof- 1334 of-possession methods, if they support proof-of-possession tokens. 1336 A profile MUST specify an identifier that MUST be used to uniquely 1337 identify itself in the "ace_profile" parameter. The textual 1338 representation of the profile identifier is intended for human 1339 readability and for JSON-based interactions, it MUST NOT be used for 1340 CBOR-based interactions. Profiles MUST register their identifier in 1341 the registry defined in Section 8.8. 1343 Profiles MAY define additional parameters for both the token request 1344 and the Access Information in the access token response in order to 1345 support negotiation or signaling of profile specific parameters. 1347 Clients that want the AS to provide them with the "ace_profile" 1348 parameter in the access token response can indicate that by sending a 1349 ace_profile parameter with a null value for CBOR-based interactions, 1350 or an empty string if CBOR is not used, in the access token request. 1352 5.8.4.4. 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 5.3. The parameter is encoded as a byte string for 1357 CBOR-based interactions, and as a string (Base64 encoded binary) if 1358 CBOR is not used. It MUST copy the value from the cnonce parameter 1359 in the "AS Request Creation Hints". 1361 5.8.5. Mapping Parameters to CBOR 1363 If CBOR encoding is used, all OAuth parameters in access token 1364 requests and responses MUST be mapped to CBOR types as specified in 1365 the registry defined by Section 8.10, using the given integer 1366 abbreviation for the map keys. 1368 Note that we have aligned the abbreviations corresponding to claims 1369 with the abbreviations defined in [RFC8392]. 1371 Note also that abbreviations from -24 to 23 have a 1 byte encoding 1372 size in CBOR. We have thus chosen to assign abbreviations in that 1373 range to parameters we expect to be used most frequently in 1374 constrained scenarios. 1376 /-------------------+----------+---------------------\ 1377 | Name | CBOR Key | Value Type | 1378 |-------------------+----------+---------------------| 1379 | access_token | 1 | byte string | 1380 | expires_in | 2 | unsigned integer | 1381 | audience | 5 | text string | 1382 | scope | 9 | text or byte string | 1383 | client_id | 24 | text string | 1384 | client_secret | 25 | byte string | 1385 | response_type | 26 | text string | 1386 | redirect_uri | 27 | text string | 1387 | state | 28 | text string | 1388 | code | 29 | byte string | 1389 | error | 30 | integer | 1390 | error_description | 31 | text string | 1391 | error_uri | 32 | text string | 1392 | grant_type | 33 | unsigned integer | 1393 | token_type | 34 | integer | 1394 | username | 35 | text string | 1395 | password | 36 | text string | 1396 | refresh_token | 37 | byte string | 1397 | ace_profile | 38 | integer | 1398 | cnonce | 39 | byte string | 1399 \-------------------+----------+---------------------/ 1401 Figure 12: CBOR mappings used in token requests and responses 1403 5.9. The Introspection Endpoint 1405 Token introspection [RFC7662] MAY be implemented by the AS, and the 1406 RS. When implemented, it MAY be used by the RS and 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 1416 MUST perform mutual authentication. Finally, the AS SHOULD verify 1417 that the requesting entity has the right to access introspection 1418 information about the provided token. Profiles of this framework 1419 that support introspection MUST specify how authentication and 1420 communication security between the requesting entity and the AS is 1421 implemented. 1423 The default name of this endpoint in an url-path SHOULD be 1424 '/introspect'. However, implementations are not required to use this 1425 name and can define their own instead. 1427 The figures of this section use the CBOR diagnostic notation without 1428 the integer abbreviations for the parameters and their values for 1429 better readability. 1431 5.9.1. Introspection Request 1433 The requesting entity sends a POST request to the introspection 1434 endpoint at the AS. The profile MUST specify how the communication 1435 is protected. If CoAP is used, the payload MUST be encoded as a CBOR 1436 map with a "token" entry containing the access token. Further 1437 optional parameters representing additional context that is known by 1438 the requesting entity to aid the AS in its response MAY be included. 1440 For CoAP-based interaction, all messages MUST use the content type 1441 "application/ace+cbor". For HTTP the encoding defined in section 2.1 1442 of [RFC7662] is used. 1444 The same parameters are required and optional as in Section 2.1 of 1445 [RFC7662]. 1447 For example, Figure 13 shows an RS calling the token introspection 1448 endpoint at the AS to query about an OAuth 2.0 proof-of-possession 1449 token. Note that object security based on OSCORE [RFC8613] is 1450 assumed in this example, therefore the Content-Format is 1451 "application/oscore". Figure 14 shows the decoded payload. 1453 Header: POST (Code=0.02) 1454 Uri-Host: "as.example.com" 1455 Uri-Path: "introspect" 1456 OSCORE: 0x09, 0x05, 0x25 1457 Content-Format: "application/oscore" 1458 Payload: 1459 ... COSE content ... 1461 Figure 13: Example introspection request. 1463 { 1464 "token" : b64'7gj0dXJQ43U', 1465 "token_type_hint" : "PoP" 1466 } 1468 Figure 14: Decoded payload. 1470 5.9.2. Introspection Response 1472 If the introspection request is authorized and successfully 1473 processed, the AS sends a response with the response code equivalent 1474 to the CoAP code 2.01 (Created). If the introspection request was 1475 invalid, not authorized or couldn't be processed the AS returns an 1476 error response as described in Section 5.9.3. 1478 In a successful response, the AS encodes the response parameters in a 1479 map. If CoAP is used, this MUST be encoded as a CBOR map, if HTTP is 1480 used the JSON encoding specified in section 2.2 of [RFC7662] is used. 1481 The map containing the response payload includes the same required 1482 and optional parameters as in Section 2.2 of [RFC7662] with the 1483 following additions: 1485 ace_profile OPTIONAL. This indicates the profile that the RS MUST 1486 use with the client. See Section 5.8.4.3 for more details on the 1487 formatting of this parameter. If this parameter is absent, the AS 1488 assumes that the RS implicitly knows which profile to use towards 1489 the client. 1491 cnonce OPTIONAL. A client-nonce provided to the AS by the client. 1492 The RS MUST verify that this corresponds to the client-nonce 1493 previously provided to the client in the "AS Request Creation 1494 Hints". See Section 5.3 and Section 5.8.4.4. 1496 exi OPTIONAL. The "expires-in" claim associated to this access 1497 token. See Section 5.10.3. 1499 Furthermore [I-D.ietf-ace-oauth-params] defines more parameters that 1500 the AS MUST be able to use when responding to a request to the 1501 introspection endpoint. 1503 For example, Figure 15 shows an AS response to the introspection 1504 request in Figure 13. Note that this example contains the "cnf" 1505 parameter defined in [I-D.ietf-ace-oauth-params]. 1507 Header: Created (Code=2.01) 1508 Content-Format: "application/ace+cbor" 1509 Payload: 1510 { 1511 "active" : true, 1512 "scope" : "read", 1513 "ace_profile" : "coap_dtls", 1514 "cnf" : { 1515 "COSE_Key" : { 1516 "kty" : "Symmetric", 1517 "kid" : b64'39Gqlw', 1518 "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh' 1519 } 1520 } 1521 } 1523 Figure 15: Example introspection response. 1525 5.9.3. Error Response 1527 The error responses for CoAP-based interactions with the AS are 1528 equivalent to the ones for HTTP-based interactions as defined in 1529 Section 2.3 of [RFC7662], with the following differences: 1531 o If content is sent and CoAP is used the payload MUST be encoded as 1532 a CBOR map and the Content-Format "application/ace+cbor" MUST be 1533 used. For HTTP the encoding defined in section 2.3 of [RFC6749] 1534 is used. 1536 o If the credentials used by the requesting entity (usually the RS) 1537 are invalid the AS MUST respond with the response code equivalent 1538 to the CoAP code 4.01 (Unauthorized) and use the required and 1539 optional parameters from Section 2.3 in [RFC7662]. 1541 o If the requesting entity does not have the right to perform this 1542 introspection request, the AS MUST respond with a response code 1543 equivalent to the CoAP code 4.03 (Forbidden). In this case no 1544 payload is returned. 1546 o The parameters "error", "error_description" and "error_uri" MUST 1547 be abbreviated using the codes specified in Figure 12. 1549 o The error codes MUST be abbreviated using the codes specified in 1550 the registry defined by Section 8.4. 1552 Note that a properly formed and authorized query for an inactive or 1553 otherwise invalid token does not warrant an error response by this 1554 specification. In these cases, the authorization server MUST instead 1555 respond with an introspection response with the "active" field set to 1556 "false". 1558 5.9.4. Mapping Introspection Parameters to CBOR 1560 If CBOR is used, the introspection request and response parameters 1561 MUST be mapped to CBOR types as specified in the registry defined by 1562 Section 8.12, using the given integer abbreviation for the map key. 1564 Note that we have aligned abbreviations that correspond to a claim 1565 with the abbreviations defined in [RFC8392] and the abbreviations of 1566 parameters with the same name from Section 5.8.5. 1568 /-------------------+----------+-------------------------\ 1569 | Parameter name | CBOR Key | Value Type | 1570 |-------------------+----------+-------------------------| 1571 | iss | 1 | text string | 1572 | sub | 2 | text string | 1573 | aud | 3 | text string | 1574 | exp | 4 | integer or | 1575 | | | floating-point number | 1576 | nbf | 5 | integer or | 1577 | | | floating-point number | 1578 | iat | 6 | integer or | 1579 | | | floating-point number | 1580 | cti | 7 | byte string | 1581 | scope | 9 | text or byte string | 1582 | active | 10 | True or False | 1583 | token | 11 | byte string | 1584 | client_id | 24 | text string | 1585 | error | 30 | integer | 1586 | error_description | 31 | text string | 1587 | error_uri | 32 | text string | 1588 | token_type_hint | 33 | text string | 1589 | token_type | 34 | integer | 1590 | username | 35 | text string | 1591 | ace_profile | 38 | integer | 1592 | cnonce | 39 | byte string | 1593 | exi | 40 | unsigned integer | 1594 \-------------------+----------+-------------------------/ 1596 Figure 16: CBOR Mappings to Token Introspection Parameters. 1598 5.10. The Access Token 1600 In this framework the use of CBOR Web Token (CWT) as specified in 1601 [RFC8392] is RECOMMENDED. 1603 In order to facilitate offline processing of access tokens, this 1604 document uses the "cnf" claim from [RFC8747] and the "scope" claim 1605 from [RFC8693] for JWT- and CWT-encoded tokens. In addition to 1606 string encoding specified for the "scope" claim, a binary encoding 1607 MAY be used. The syntax of such an encoding is explicitly not 1608 specified here and left to profiles or applications, specifically 1609 note that a binary encoded scope does not necessarily use the space 1610 character '0x20' to delimit scope-tokens. 1612 If the AS needs to convey a hint to the RS about which profile it 1613 should use to communicate with the client, the AS MAY include an 1614 "ace_profile" claim in the access token, with the same syntax and 1615 semantics as defined in Section 5.8.4.3. 1617 If the client submitted a client-nonce parameter in the access token 1618 request Section 5.8.4.4, the AS MUST include the value of this 1619 parameter in the "cnonce" claim specified here. The "cnonce" claim 1620 uses binary encoding. 1622 5.10.1. The Authorization Information Endpoint 1624 The access token, containing authorization information and 1625 information about the proof-of-possession method used by the client, 1626 needs to be transported to the RS so that the RS can authenticate and 1627 authorize the client request. 1629 This section defines a method for transporting the access token to 1630 the RS using a RESTful protocol such as CoAP. Profiles of this 1631 framework MAY define other methods for token transport. 1633 The method consists of an authz-info endpoint, implemented by the RS. 1634 A client using this method MUST make a POST request to the authz-info 1635 endpoint at the RS with the access token in the payload. The CoAP 1636 Content-Format or HTTP Media Type MUST reflect the format of the 1637 token, e.g. application/cwt for CBOR Web Tokens, if no Content-Format 1638 or Media Type is defined for the token format, application/octet- 1639 stream MUST be used. 1641 The RS receiving the token MUST verify the validity of the token. If 1642 the token is valid, the RS MUST respond to the POST request with a 1643 response code equivalent to CoAP's 2.01 (Created). Section 5.10.1.1 1644 outlines how an RS MUST proceed to verify the validity of an access 1645 token. 1647 The RS MUST be prepared to store at least one access token for future 1648 use. This is a difference to how access tokens are handled in OAuth 1649 2.0, where the access token is typically sent along with each 1650 request, and therefore not stored at the RS. 1652 When using this framework it is RECOMMENDED that an RS stores only 1653 one token per proof-of-possession key. This means that an additional 1654 token linked to the same key will supersede any existing token at the 1655 RS, by replacing the corresponding authorization information. The 1656 reason is that this greatly simplifies (constrained) implementations, 1657 with respect to required storage and resolving a request to the 1658 applicable token. The use of multiple access tokens for a single 1659 client increases the strain on the resource server as it must 1660 consider every access token and calculate the actual permissions of 1661 the client. Also, tokens may contradict each other which may lead 1662 the server to enforce wrong permissions. If one of the access tokens 1663 expires earlier than others, the resulting permissions may offer 1664 insufficient protection. 1666 If the payload sent to the authz-info endpoint does not parse to a 1667 token, the RS MUST respond with a response code equivalent to the 1668 CoAP code 4.00 (Bad Request). 1670 The RS MAY make an introspection request to validate the token before 1671 responding to the POST request to the authz-info endpoint, e.g. if 1672 the token is an opaque reference. Some transport protocols may 1673 provide a way to indicate that the RS is busy and the client should 1674 retry after an interval; this type of status update would be 1675 appropriate while the RS is waiting for an introspection response. 1677 Profiles MUST specify whether the authz-info endpoint is protected, 1678 including whether error responses from this endpoint are protected. 1679 Note that since the token contains information that allow the client 1680 and the RS to establish a security context in the first place, mutual 1681 authentication may not be possible at this point. 1683 The default name of this endpoint in an url-path is '/authz-info', 1684 however implementations are not required to use this name and can 1685 define their own instead. 1687 5.10.1.1. Verifying an Access Token 1689 When an RS receives an access token, it MUST verify it before storing 1690 it. The details of token verification depends on various aspects, 1691 including the token encoding, the type of token, the security 1692 protection applied to the token, and the claims. The token encoding 1693 matters since the security protection differs between the token 1694 encodings. For example, a CWT token uses COSE while a JWT token uses 1695 JOSE. The type of token also has an influence on the verification 1696 procedure since tokens may be self-contained whereby token 1697 verification may happen locally at the RS while a token-by-reference 1698 requires further interaction with the authorization server, for 1699 example using token introspection, to obtain the claims associated 1700 with the token reference. Self-contained tokens MUST, at least be 1701 integrity protected but they MAY also be encrypted. 1703 For self-contained tokens the RS MUST process the security protection 1704 of the token first, as specified by the respective token format. For 1705 CWT the description can be found in [RFC8392] and for JWT the 1706 relevant specification is [RFC7519]. This MUST include a 1707 verification that security protection (and thus the token) was 1708 generated by an AS that has the right to issue access tokens for this 1709 RS. 1711 In case the token is communicated by reference the RS needs to obtain 1712 the claims first. When the RS uses token introspection the relevant 1713 specification is [RFC7662] with CoAP transport specified in 1714 Section 5.9. 1716 Errors may happen during this initial processing stage: 1718 o If the verification of the security wrapper fails, or the token 1719 was issued by an AS that does not have the right to issue tokens 1720 for the receiving RS, the RS MUST discard the token and, if this 1721 was an interaction with authz-info, return an error message with a 1722 response code equivalent to the CoAP code 4.01 (Unauthorized). 1724 o If the claims cannot be obtained the RS MUST discard the token 1725 and, in case of an interaction via the authz-info endpoint, return 1726 an error message with a response code equivalent to the CoAP code 1727 4.00 (Bad Request). 1729 Next, the RS MUST verify claims, if present, contained in the access 1730 token. Errors are returned when claim checks fail, in the order of 1731 priority of this list: 1733 iss The issuer claim (if present) must identify the AS that has 1734 produced the security protection for the access token. If that is 1735 not the case the RS MUST discard the token. If this was an 1736 interaction with authz-info, the RS MUST also respond with a 1737 response code equivalent to the CoAP code 4.01 (Unauthorized). 1739 exp The expiration date must be in the future. If that is not the 1740 case the RS MUST discard the token. If this was an interaction 1741 with authz-info the RS MUST also respond with a response code 1742 equivalent to the CoAP code 4.01 (Unauthorized). Note that the RS 1743 has to terminate access rights to the protected resources at the 1744 time when the tokens expire. 1746 aud The audience claim must refer to an audience that the RS 1747 identifies with. If that is not the case the RS MUST discard the 1748 token. If this was an interaction with authz-info, the RS MUST 1749 also respond with a response code equivalent to the CoAP code 4.03 1750 (Forbidden). 1752 scope The RS must recognize value of the scope claim. If that is 1753 not the case the RS MUST discard the token. If this was an 1754 interaction with authz-info, the RS MUST also respond with a 1755 response code equivalent to the CoAP code 4.00 (Bad Request). The 1756 RS MAY provide additional information in the error response, to 1757 clarify what went wrong. 1759 Additional processing may be needed for other claims in a way 1760 specific to a profile or the underlying application. 1762 Note that the Subject (sub) claim cannot always be verified when the 1763 token is submitted to the RS since the client may not have 1764 authenticated yet. Also note that a counter for the expires_in (exi) 1765 claim MUST be initialized when the RS first verifies this token. 1767 Also note that profiles of this framework may define access token 1768 transport mechanisms that do not allow for error responses. 1769 Therefore the error messages specified here only apply if the token 1770 was sent to the authz-info endpoint. 1772 When sending error responses, the RS MAY use the error codes from 1773 Section 3.1 of [RFC6750], to provide additional details to the 1774 client. 1776 5.10.1.2. Protecting the Authorization Information Endpoint 1778 As this framework can be used in RESTful environments, it is 1779 important to make sure that attackers cannot perform unauthorized 1780 requests on the authz-info endpoints, other than submitting access 1781 tokens. 1783 Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT 1784 on the authz-info endpoint and on its children (if any). 1786 The POST method SHOULD NOT be allowed on children of the authz-info 1787 endpoint. 1789 The RS SHOULD implement rate limiting measures to mitigate attacks 1790 aiming to overload the processing capacity of the RS by repeatedly 1791 submitting tokens. For CoAP-based communication the RS could use the 1792 mechanisms from [RFC8516] to indicate that it is overloaded. 1794 5.10.2. Client Requests to the RS 1796 Before sending a request to an RS, the client MUST verify that the 1797 keys used to protect this communication are still valid. See 1798 Section 5.10.4 for details on how the client determines the validity 1799 of the keys used. 1801 If an RS receives a request from a client, and the target resource 1802 requires authorization, the RS MUST first verify that it has an 1803 access token that authorizes this request, and that the client has 1804 performed the proof-of-possession binding that token to the request. 1806 The response code MUST be 4.01 (Unauthorized) in case the client has 1807 not performed the proof-of-possession, or if RS has no valid access 1808 token for the client. If RS has an access token for the client but 1809 the token does not authorize access for the resource that was 1810 requested, RS MUST reject the request with a 4.03 (Forbidden). If RS 1811 has an access token for the client but it does not cover the action 1812 that was requested on the resource, RS MUST reject the request with a 1813 4.05 (Method Not Allowed). 1815 Note: The use of the response codes 4.03 and 4.05 is intended to 1816 prevent infinite loops where a dumb client optimistically tries to 1817 access a requested resource with any access token received from AS. 1818 As malicious clients could pretend to be C to determine C's 1819 privileges, these detailed response codes must be used only when a 1820 certain level of security is already available which can be achieved 1821 only when the client is authenticated. 1823 Note: The RS MAY use introspection for timely validation of an access 1824 token, at the time when a request is presented. 1826 Note: Matching the claims of the access token (e.g., scope) to a 1827 specific request is application specific. 1829 If the request matches a valid token and the client has performed the 1830 proof-of-possession for that token, the RS continues to process the 1831 request as specified by the underlying application. 1833 5.10.3. Token Expiration 1835 Depending on the capabilities of the RS, there are various ways in 1836 which it can verify the expiration of a received access token. Here 1837 follows a list of the possibilities including what functionality they 1838 require of the RS. 1840 o The token is a CWT and includes an "exp" claim and possibly the 1841 "nbf" claim. The RS verifies these by comparing them to values 1842 from its internal clock as defined in [RFC7519]. In this case the 1843 RS's internal clock must reflect the current date and time, or at 1844 least be synchronized with the AS's clock. How this clock 1845 synchronization would be performed is out of scope for this 1846 specification. 1848 o The RS verifies the validity of the token by performing an 1849 introspection request as specified in Section 5.9. This requires 1850 the RS to have a reliable network connection to the AS and to be 1851 able to handle two secure sessions in parallel (C to RS and RS to 1852 AS). 1854 o In order to support token expiration for devices that have no 1855 reliable way of synchronizing their internal clocks, this 1856 specification defines the following approach: The claim "exi" 1857 ("expires in") can be used, to provide the RS with the lifetime of 1858 the token in seconds from the time the RS first receives the 1859 token. This mechanism only works for self-contained tokens, i.e. 1860 CWTs and JWTs. For CWTs this parameter is encoded as unsigned 1861 integer, while JWTs encode this as JSON number. 1863 o Processing this claim requires that the RS does the following: 1865 * For each token the RS receives, that contains an "exi" claim: 1866 Keep track of the time it received that token and revisit that 1867 list regularly to expunge expired tokens. 1869 * Keep track of the identifiers of tokens containing the "exi" 1870 claim that have expired (in order to avoid accepting them 1871 again). In order to avoid an unbounded memory usage growth, 1872 this MUST be implemented in the following way when the "exi" 1873 claim is used: 1875 + When creating the token, the AS MUST add a 'cti' claim ( or 1876 'jti' for JWTs) to the access token. The value of this 1877 claim MUST be created as the binary representation of the 1878 concatenation of the identifier of the RS with a sequence 1879 number counting the tokens containing an 'exi' claim, issued 1880 by this AS for the RS. 1882 + The RS MUST store the highest sequence number of an expired 1883 token containing the "exi" claim that it has seen, and treat 1884 tokens with lower sequence numbers as expired. Note that 1885 this could lead to discarding valid tokens with lower 1886 sequence numbers, if the AS where to issue tokens of 1887 different validity time for the same RS. The assumption is 1888 that typically tokens in such a scenario would all have the 1889 same validity time. 1891 If a token that authorizes a long running request such as a CoAP 1892 Observe [RFC7641] expires, the RS MUST send an error response with 1893 the response code equivalent to the CoAP code 4.01 (Unauthorized) to 1894 the client and then terminate processing the long running request. 1896 5.10.4. Key Expiration 1898 The AS provides the client with key material that the RS uses. This 1899 can either be a common symmetric PoP-key, or an asymmetric key used 1900 by the RS to authenticate towards the client. Since there is 1901 currently no expiration metadata associated to those keys, the client 1902 has no way of knowing if these keys are still valid. This may lead 1903 to situations where the client sends requests containing sensitive 1904 information to the RS using a key that is expired and possibly in the 1905 hands of an attacker, or accepts responses from the RS that are not 1906 properly protected and could possibly have been forged by an 1907 attacker. 1909 In order to prevent this, the client must assume that those keys are 1910 only valid as long as the related access token is. Since the access 1911 token is opaque to the client, one of the following methods MUST be 1912 used to inform the client about the validity of an access token: 1914 o The client knows a default validity time for all tokens it is 1915 using (i.e. how long a token is valid after being issued). This 1916 information could be provisioned to the client when it is 1917 registered at the AS, or published by the AS in a way that the 1918 client can query. 1920 o The AS informs the client about the token validity using the 1921 "expires_in" parameter in the Access Information. 1923 A client that is not able to obtain information about the expiration 1924 of a token MUST NOT use this token. 1926 6. Security Considerations 1928 Security considerations applicable to authentication and 1929 authorization in RESTful environments provided in OAuth 2.0 [RFC6749] 1930 apply to this work. Furthermore [RFC6819] provides additional 1931 security considerations for OAuth which apply to IoT deployments as 1932 well. If the introspection endpoint is used, the security 1933 considerations from [RFC7662] also apply. 1935 The following subsections address issues specific to this document 1936 and it's use in constrained environments. 1938 6.1. Protecting Tokens 1940 A large range of threats can be mitigated by protecting the contents 1941 of the access token by using a digital signature or a keyed message 1942 digest (MAC) or an Authenticated Encryption with Associated Data 1943 (AEAD) algorithm. Consequently, the token integrity protection MUST 1944 be applied to prevent the token from being modified, particularly 1945 since it contains a reference to the symmetric key or the asymmetric 1946 key used for proof-of-possession. If the access token contains the 1947 symmetric key, this symmetric key MUST be encrypted by the 1948 authorization server so that only the resource server can decrypt it. 1949 Note that using an AEAD algorithm is preferable over using a MAC 1950 unless the token needs to be publicly readable. 1952 If the token is intended for multiple recipients (i.e. an audience 1953 that is a group), integrity protection of the token with a symmetric 1954 key, shared between the AS and the recipients, is not sufficient, 1955 since any of the recipients could modify the token undetected by the 1956 other recipients. Therefore a token with a multi-recipient audience 1957 MUST be protected with an asymmetric signature. 1959 It is important for the authorization server to include the identity 1960 of the intended recipient (the audience), typically a single resource 1961 server (or a list of resource servers), in the token. The same 1962 shared secret MUST NOT be used as proof-of-possession key with 1963 multiple resource servers since the benefit from using the proof-of- 1964 possession concept is then significantly reduced. 1966 If clients are capable of doing so, they should frequently request 1967 fresh access tokens, as this allows the AS to keep the lifetime of 1968 the tokens short. This allows the AS to use shorter proof-of- 1969 possession key sizes, which translate to a performance benefit for 1970 the client and for the resource server. Shorter keys also lead to 1971 shorter messages (particularly with asymmetric keying material). 1973 When authorization servers bind symmetric keys to access tokens, they 1974 SHOULD scope these access tokens to a specific permission. 1976 In certain situations it may be necessary to revoke an access token 1977 that is still valid. Client-initiated revocation is specified in 1978 [RFC7009] for OAuth 2.0. Other revocation mechanisms are currently 1979 not specified, as the underlying assumption in OAuth is that access 1980 tokens are issued with a relatively short lifetime. This may not 1981 hold true for disconnected constrained devices, needing access tokens 1982 with relatively long lifetimes, and would therefore necessitate 1983 further standardization work that is out of scope for this document. 1985 6.2. Communication Security 1987 Communication with the authorization server MUST use confidentiality 1988 protection. This step is extremely important since the client or the 1989 RS may obtain the proof-of-possession key from the authorization 1990 server for use with a specific access token. Not using 1991 confidentiality protection exposes this secret (and the access token) 1992 to an eavesdropper thereby completely negating proof-of-possession 1993 security. The requirements for communication security of profiles 1994 are specified in Section 5. 1996 Additional protection for the access token can be applied by 1997 encrypting it, for example encryption of CWTs is specified in 1998 Section 5.1 of [RFC8392]. Such additional protection can be 1999 necessary if the token is later transferred over an insecure 2000 connection (e.g. when it is sent to the authz-info endpoint). 2002 Care must by taken by developers to prevent leakage of the PoP 2003 credentials (i.e., the private key or the symmetric key). An 2004 adversary in possession of the PoP credentials bound to the access 2005 token will be able to impersonate the client. Be aware that this is 2006 a real risk with many constrained environments, since adversaries may 2007 get physical access to the devices and can therefore use physical 2008 extraction techniques to gain access to memory contents. This risk 2009 can be mitigated to some extent by making sure that keys are 2010 refreshed frequently, by using software isolation techniques and by 2011 using hardware security. 2013 6.3. Long-Term Credentials 2015 Both clients and RSs have long-term credentials that are used to 2016 secure communications, and authenticate to the AS. These credentials 2017 need to be protected against unauthorized access. In constrained 2018 devices, deployed in publicly accessible places, such protection can 2019 be difficult to achieve without specialized hardware (e.g. secure key 2020 storage memory). 2022 If credentials are lost or compromised, the operator of the affected 2023 devices needs to have procedures to invalidate any access these 2024 credentials give and to revoke tokens linked to such credentials. 2025 The loss of a credential linked to a specific device MUST NOT lead to 2026 a compromise of other credentials not linked to that device, 2027 therefore secret keys used for authentication MUST NOT be shared 2028 between more than two parties. 2030 Operators of clients or RS SHOULD have procedures in place to replace 2031 credentials that are suspected to have been compromised or that have 2032 been lost. 2034 Operators also SHOULD have procedures for decommissioning devices, 2035 that include securely erasing credentials and other security critical 2036 material in the devices being decommissioned. 2038 6.4. Unprotected AS Request Creation Hints 2040 Initially, no secure channel exists to protect the communication 2041 between C and RS. Thus, C cannot determine if the "AS Request 2042 Creation Hints" contained in an unprotected response from RS to an 2043 unauthorized request (see Section 5.3) are authentic. C therefore 2044 MUST determine if an AS is authorized to provide access tokens for a 2045 certain RS. How this determination is implemented is out of scope 2046 for this document and left to the applications. 2048 6.5. Minimal Security Requirements for Communication 2050 This section summarizes the minimal requirements for the 2051 communication security of the different protocol interactions. 2053 C-AS All communication between the client and the Authorization 2054 Server MUST be encrypted, integrity and replay protected. 2055 Furthermore responses from the AS to the client MUST be bound to 2056 the client's request to avoid attacks where the attacker swaps the 2057 intended response for an older one valid for a previous request. 2058 This requires that the client and the Authorization Server have 2059 previously exchanged either a shared secret or their public keys 2060 in order to negotiate a secure communication. Furthermore the 2061 client MUST be able to determine whether an AS has the authority 2062 to issue access tokens for a certain RS. This can for example be 2063 done through pre-configured lists, or through an online lookup 2064 mechanism that in turn also must be secured. 2066 RS-AS The communication between the Resource Server and the 2067 Authorization Server via the introspection endpoint MUST be 2068 encrypted, integrity and replay protected. Furthermore responses 2069 from the AS to the RS MUST be bound to the RS's request. This 2070 requires that the RS and the Authorization Server have previously 2071 exchanged either a shared secret, or their public keys in order to 2072 negotiate a secure communication. Furthermore the RS MUST be able 2073 to determine whether an AS has the authority to issue access 2074 tokens itself. This is usually configured out of band, but could 2075 also be performed through an online lookup mechanism provided that 2076 it is also secured in the same way. 2078 C-RS The initial communication between the client and the Resource 2079 Server can not be secured in general, since the RS is not in 2080 possession of on access token for that client, which would carry 2081 the necessary parameters. If both parties support DTLS without 2082 client authentication it is RECOMMEND to use this mechanism for 2083 protecting the initial communication. After the client has 2084 successfully transmitted the access token to the RS, a secure 2085 communication protocol MUST be established between client and RS 2086 for the actual resource request. This protocol MUST provide 2087 confidentiality, integrity and replay protection as well as a 2088 binding between requests and responses. This requires that the 2089 client learned either the RS's public key or received a symmetric 2090 proof-of-possession key bound to the access token from the AS. 2091 The RS must have learned either the client's public key or a 2092 shared symmetric key from the claims in the token or an 2093 introspection request. Since ACE does not provide profile 2094 negotiation between C and RS, the client MUST have learned what 2095 profile the RS supports (e.g. from the AS or pre-configured) and 2096 initiate the communication accordingly. 2098 6.6. Token Freshness and Expiration 2100 An RS that is offline faces the problem of clock drift. Since it 2101 cannot synchronize its clock with the AS, it may be tricked into 2102 accepting old access tokens that are no longer valid or have been 2103 compromised. In order to prevent this, an RS may use the nonce-based 2104 mechanism (cnonce) defined in Section 5.3 to ensure freshness of an 2105 Access Token subsequently presented to this RS. 2107 Another problem with clock drift is that evaluating the standard 2108 token expiration claim "exp" can give unpredictable results. 2110 Acceptable ranges of clock drift are highly dependent on the concrete 2111 application. Important factors are how long access tokens are valid, 2112 and how critical timely expiration of access token is. 2114 The expiration mechanism implemented by the "exi" claim, based on the 2115 first time the RS sees the token was defined to provide a more 2116 predictable alternative. The "exi" approach has some drawbacks that 2117 need to be considered: 2119 A malicious client may hold back tokens with the "exi" claim in 2120 order to prolong their lifespan. 2122 If an RS loses state (e.g. due to an unscheduled reboot), it may 2123 lose the current values of counters tracking the "exi" claims of 2124 tokens it is storing. 2126 The first drawback is inherent to the deployment scenario and the 2127 "exi" solution. It can therefore not be mitigated without requiring 2128 the RS be online at times. The second drawback can be mitigated by 2129 regularly storing the value of "exi" counters to persistent memory. 2131 6.7. Combining Profiles 2133 There may be use cases were different profiles of this framework are 2134 combined. For example, an MQTT-TLS profile is used between the 2135 client and the RS in combination with a CoAP-DTLS profile for 2136 interactions between the client and the AS. The security of a 2137 profile MUST NOT depend on the assumption that the profile is used 2138 for all the different types of interactions in this framework. 2140 6.8. Unprotected Information 2142 Communication with the authz-info endpoint, as well as the various 2143 error responses defined in this framework, all potentially include 2144 sending information over an unprotected channel. These messages may 2145 leak information to an adversary, or may be manipulated by active 2146 attackers to induce incorrect behavior. For example error responses 2147 for requests to the Authorization Information endpoint can reveal 2148 information about an otherwise opaque access token to an adversary 2149 who has intercepted this token. 2151 As far as error messages are concerned, this framework is written 2152 under the assumption that, in general, the benefits of detailed error 2153 messages outweigh the risk due to information leakage. For 2154 particular use cases, where this assessment does not apply, detailed 2155 error messages can be replaced by more generic ones. 2157 In some scenarios it may be possible to protect the communication 2158 with the authz-info endpoint (e.g. through DTLS with only server-side 2159 authentication). In cases where this is not possible, it is 2160 RECOMMENDED to use encrypted CWTs or tokens that are opaque 2161 references and need to be subjected to introspection by the RS. 2163 If the initial unauthorized resource request message (see 2164 Section 5.2) is used, the client MUST make sure that it is not 2165 sending sensitive content in this request. While GET and DELETE 2166 requests only reveal the target URI of the resource, POST and PUT 2167 requests would reveal the whole payload of the intended operation. 2169 Since the client is not authenticated at the point when it is 2170 submitting an access token to the authz-info endpoint, attackers may 2171 be pretending to be a client and trying to trick an RS to use an 2172 obsolete profile that in turn specifies a vulnerable security 2173 mechanism via the authz-info endpoint. Such an attack would require 2174 a valid access token containing an "ace_profile" claim requesting the 2175 use of said obsolete profile. Resource Owners should update the 2176 configuration of their RS's to prevent them from using such obsolete 2177 profiles. 2179 6.9. Identifying Audiences 2181 The audience claim as defined in [RFC7519] and the equivalent 2182 "audience" parameter from [RFC8693] are intentionally vague on how to 2183 match the audience value to a specific RS. This is intended to allow 2184 application specific semantics to be used. This section attempts to 2185 give some general guidance for the use of audiences in constrained 2186 environments. 2188 URLs are not a good way of identifying mobile devices that can switch 2189 networks and thus be associated with new URLs. If the audience 2190 represents a single RS, and asymmetric keys are used, the RS can be 2191 uniquely identified by a hash of its public key. If this approach is 2192 used it is RECOMMENDED to apply the procedure from section 3 of 2193 [RFC6920]. 2195 If the audience addresses a group of resource servers, the mapping of 2196 group identifier to individual RS has to be provisioned to each RS 2197 before the group-audience is usable. Managing dynamic groups could 2198 be an issue, if any RS is not always reachable when the groups' 2199 memberships change. Furthermore, issuing access tokens bound to 2200 symmetric proof-of-possession keys that apply to a group-audience is 2201 problematic, as an RS that is in possession of the access token can 2202 impersonate the client towards the other RSs that are part of the 2203 group. It is therefore NOT RECOMMENDED to issue access tokens bound 2204 to a group audience and symmetric proof-of possession keys. 2206 Even the client must be able to determine the correct values to put 2207 into the "audience" parameter, in order to obtain a token for the 2208 intended RS. Errors in this process can lead to the client 2209 inadvertently obtaining a token for the wrong RS. The correct values 2210 for "audience" can either be provisioned to the client as part of its 2211 configuration, or dynamically looked up by the client in some 2212 directory. In the latter case the integrity and correctness of the 2213 directory data must be assured. Note that the "audience" hint 2214 provided by the RS as part of the "AS Request Creation Hints" 2215 Section 5.3 is not typically source authenticated and integrity 2216 protected, and should therefore not be treated a trusted value. 2218 6.10. Denial of Service Against or with Introspection 2220 The optional introspection mechanism provided by OAuth and supported 2221 in the ACE framework allows for two types of attacks that need to be 2222 considered by implementers. 2224 First, an attacker could perform a denial of service attack against 2225 the introspection endpoint at the AS in order to prevent validation 2226 of access tokens. To maintain the security of the system, an RS that 2227 is configured to use introspection MUST NOT allow access based on a 2228 token for which it couldn't reach the introspection endpoint. 2230 Second, an attacker could use the fact that an RS performs 2231 introspection to perform a denial of service attack against that RS 2232 by repeatedly sending tokens to its authz-info endpoint that require 2233 an introspection call. RS can mitigate such attacks by implementing 2234 rate limits on how many introspection requests they perform in a 2235 given time interval for a certain client IP address submitting tokens 2236 to /authz-info. When that limit has been reached, incoming requests 2237 from that address are rejected for a certain amount of time. A 2238 general rate limit on the introspection requests should also be 2239 considered, to mitigate distributed attacks. 2241 7. Privacy Considerations 2243 Implementers and users should be aware of the privacy implications of 2244 the different possible deployments of this framework. 2246 The AS is in a very central position and can potentially learn 2247 sensitive information about the clients requesting access tokens. If 2248 the client credentials grant is used, the AS can track what kind of 2249 access the client intends to perform. With other grants this can be 2250 prevented by the Resource Owner. To do so, the resource owner needs 2251 to bind the grants it issues to anonymous, ephemeral credentials that 2252 do not allow the AS to link different grants and thus different 2253 access token requests by the same client. 2255 The claims contained in a token can reveal privacy sensitive 2256 information about the client and the RS to any party having access to 2257 them (whether by processing the content of a self-contained token or 2258 by introspection). The AS SHOULD be configured to minimize the 2259 information about clients and RSs disclosed in the tokens it issues. 2261 If tokens are only integrity protected and not encrypted, they may 2262 reveal information to attackers listening on the wire, or able to 2263 acquire the access tokens in some other way. In the case of CWTs the 2264 token may, e.g., reveal the audience, the scope and the confirmation 2265 method used by the client. The latter may reveal the identity of the 2266 device or application running the client. This may be linkable to 2267 the identity of the person using the client (if there is a person and 2268 not a machine-to-machine interaction). 2270 Clients using asymmetric keys for proof-of-possession should be aware 2271 of the consequences of using the same key pair for proof-of- 2272 possession towards different RSs. A set of colluding RSs or an 2273 attacker able to obtain the access tokens will be able to link the 2274 requests, or even to determine the client's identity. 2276 An unprotected response to an unauthorized request (see Section 5.3) 2277 may disclose information about RS and/or its existing relationship 2278 with C. It is advisable to include as little information as possible 2279 in an unencrypted response. Even the absolute URI of the AS may 2280 reveal sensitive information about the service that RS provides. 2281 Developers must ensure that the RS does not disclose information that 2282 has an impact on the privacy of the stakeholders in the "AS Request 2283 Creation Hints". They may choose to use a different mechanism for 2284 the discovery of the AS if necessary. If means of encrypting 2285 communication between C and RS already exist, more detailed 2286 information may be included with an error response to provide C with 2287 sufficient information to react on that particular error. 2289 8. IANA Considerations 2291 This document creates several registries with a registration policy 2292 of "Expert Review"; guidelines to the experts are given in 2293 Section 8.17. 2295 8.1. ACE Authorization Server Request Creation Hints 2297 This specification establishes the IANA "ACE Authorization Server 2298 Request Creation Hints" registry. The registry has been created to 2299 use the "Expert Review" registration procedure [RFC8126]. It should 2300 be noted that, in addition to the expert review, some portions of the 2301 registry require a specification, potentially a Standards Track RFC, 2302 be supplied as well. 2304 The columns of the registry are: 2306 Name The name of the parameter 2308 CBOR Key CBOR map key for the parameter. Different ranges of values 2309 use different registration policies [RFC8126]. Integer values 2310 from -256 to 255 are designated as Standards Action. Integer 2311 values from -65536 to -257 and from 256 to 65535 are designated as 2312 Specification Required. Integer values greater than 65535 are 2313 designated as Expert Review. Integer values less than -65536 are 2314 marked as Private Use. 2316 Value Type The CBOR data types allowable for the values of this 2317 parameter. 2319 Reference This contains a pointer to the public specification of the 2320 request creation hint abbreviation, if one exists. 2322 This registry will be initially populated by the values in Figure 2. 2323 The Reference column for all of these entries will be this document. 2325 8.2. CoRE Resource Type Registry 2327 IANA is requested to register a new Resource Type (rt=) Link Target 2328 Attribute in the "Resource Type (rt=) Link Target Attribute Values" 2329 subregistry under the "Constrained RESTful Environments (CoRE) 2330 Parameters" [IANA.CoreParameters] registry: 2332 o Value: "ace.ai" 2333 o Description: ACE-OAuth authz-info endpoint resource. 2334 o Reference: [this document] 2336 Specific ACE-OAuth profiles can use this common resource type for 2337 defining their profile-specific discovery processes. 2339 8.3. OAuth Extensions Error Registration 2341 This specification registers the following error values in the OAuth 2342 Extensions Error registry [IANA.OAuthExtensionsErrorRegistry]. 2344 o Error name: "unsupported_pop_key" 2345 o Error usage location: token error response 2346 o Related protocol extension: [this document] 2347 o Change Controller: IESG 2348 o Specification document(s): Section 5.8.3 of [this document] 2350 o Error name: "incompatible_ace_profiles" 2351 o Error usage location: token error response 2352 o Related protocol extension: [this document] 2353 o Change Controller: IESG 2354 o Specification document(s): Section 5.8.3 of [this document] 2356 8.4. OAuth Error Code CBOR Mappings Registry 2358 This specification establishes the IANA "OAuth Error Code CBOR 2359 Mappings" registry. The registry has been created to use the "Expert 2360 Review" registration procedure [RFC8126], except for the value range 2361 designated for private use. 2363 The columns of the registry are: 2365 Name The OAuth Error Code name, refers to the name in Section 5.2. 2366 of [RFC6749], e.g., "invalid_request". 2367 CBOR Value CBOR abbreviation for this error code. Integer values 2368 less than -65536 are marked as "Private Use", all other values use 2369 the registration policy "Expert Review" [RFC8126]. 2370 Reference This contains a pointer to the public specification of the 2371 error code abbreviation, if one exists. 2373 This registry will be initially populated by the values in Figure 10. 2374 The Reference column for all of these entries will be this document. 2376 8.5. OAuth Grant Type CBOR Mappings 2378 This specification establishes the IANA "OAuth Grant Type CBOR 2379 Mappings" registry. The registry has been created to use the "Expert 2380 Review" registration procedure [RFC8126], except for the value range 2381 designated for private use. 2383 The columns of this registry are: 2385 Name The name of the grant type as specified in Section 1.3 of 2386 [RFC6749]. 2387 CBOR Value CBOR abbreviation for this grant type. Integer values 2388 less than -65536 are marked as "Private Use", all other values use 2389 the registration policy "Expert Review" [RFC8126]. 2390 Reference This contains a pointer to the public specification of the 2391 grant type abbreviation, if one exists. 2392 Original Specification This contains a pointer to the public 2393 specification of the grant type, if one exists. 2395 This registry will be initially populated by the values in Figure 11. 2396 The Reference column for all of these entries will be this document. 2398 8.6. OAuth Access Token Types 2400 This section registers the following new token type in the "OAuth 2401 Access Token Types" registry [IANA.OAuthAccessTokenTypes]. 2403 o Type name: "PoP" 2404 o Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see 2405 section 3.3 of [I-D.ietf-ace-oauth-params]. 2406 o HTTP Authentication Scheme(s): N/A 2407 o Change Controller: IETF 2408 o Specification document(s): [this document] 2410 8.7. OAuth Access Token Type CBOR Mappings 2412 This specification established the IANA "OAuth Access Token Type CBOR 2413 Mappings" registry. The registry has been created to use the "Expert 2414 Review" registration procedure [RFC8126], except for the value range 2415 designated for private use. 2417 The columns of this registry are: 2419 Name The name of token type as registered in the OAuth Access Token 2420 Types registry, e.g., "Bearer". 2422 CBOR Value CBOR abbreviation for this token type. Integer values 2423 less than -65536 are marked as "Private Use", all other values use 2424 the registration policy "Expert Review" [RFC8126]. 2425 Reference This contains a pointer to the public specification of the 2426 OAuth token type abbreviation, if one exists. 2427 Original Specification This contains a pointer to the public 2428 specification of the OAuth token type, if one exists. 2430 8.7.1. Initial Registry Contents 2432 o Name: "Bearer" 2433 o Value: 1 2434 o Reference: [this document] 2435 o Original Specification: [RFC6749] 2437 o Name: "PoP" 2438 o Value: 2 2439 o Reference: [this document] 2440 o Original Specification: [this document] 2442 8.8. ACE Profile Registry 2444 This specification establishes the IANA "ACE Profile" registry. The 2445 registry has been created to use the "Expert Review" registration 2446 procedure [RFC8126]. It should be noted that, in addition to the 2447 expert review, some portions of the registry require a specification, 2448 potentially a Standards Track RFC, be supplied as well. 2450 The columns of this registry are: 2452 Name The name of the profile, to be used as value of the profile 2453 attribute. 2454 Description Text giving an overview of the profile and the context 2455 it is developed for. 2456 CBOR Value CBOR abbreviation for this profile name. Different 2457 ranges of values use different registration policies [RFC8126]. 2458 Integer values from -256 to 255 are designated as Standards 2459 Action. Integer values from -65536 to -257 and from 256 to 65535 2460 are designated as Specification Required. Integer values greater 2461 than 65535 are designated as "Expert Review". Integer values less 2462 than -65536 are marked as Private Use. 2463 Reference This contains a pointer to the public specification of the 2464 profile abbreviation, if one exists. 2466 This registry will be initially empty and will be populated by the 2467 registrations from the ACE framework profiles. 2469 8.9. OAuth Parameter Registration 2471 This specification registers the following parameter in the "OAuth 2472 Parameters" registry [IANA.OAuthParameters]: 2474 o Name: "ace_profile" 2475 o Parameter Usage Location: token response 2476 o Change Controller: IESG 2477 o Reference: Section 5.8.2 and Section 5.8.4.3 of [this document] 2479 8.10. OAuth Parameters CBOR Mappings Registry 2481 This specification establishes the IANA "OAuth Parameters CBOR 2482 Mappings" registry. The registry has been created to use the "Expert 2483 Review" registration procedure [RFC8126], except for the value range 2484 designated for private use. 2486 The columns of this registry are: 2488 Name The OAuth Parameter name, refers to the name in the OAuth 2489 parameter registry, e.g., "client_id". 2490 CBOR Key CBOR map key for this parameter. Integer values less than 2491 -65536 are marked as "Private Use", all other values use the 2492 registration policy "Expert Review" [RFC8126]. 2493 Value Type The allowable CBOR data types for values of this 2494 parameter. 2495 Reference This contains a pointer to the public specification of the 2496 OAuth parameter abbreviation, if one exists. 2498 This registry will be initially populated by the values in Figure 12. 2499 The Reference column for all of these entries will be this document. 2501 8.11. OAuth Introspection Response Parameter Registration 2503 This specification registers the following parameters in the OAuth 2504 Token Introspection Response registry 2505 [IANA.TokenIntrospectionResponse]. 2507 o Name: "ace_profile" 2508 o Description: The ACE profile used between client and RS. 2509 o Change Controller: IESG 2510 o Reference: Section 5.9.2 of [this document] 2512 o Name: "cnonce" 2513 o Description: "client-nonce". A nonce previously provided to the 2514 AS by the RS via the client. Used to verify token freshness when 2515 the RS cannot synchronize its clock with the AS. 2516 o Change Controller: IESG 2517 o Reference: Section 5.9.2 of [this document] 2519 o Name: "exi" 2520 o Description: "Expires in". Lifetime of the token in seconds from 2521 the time the RS first sees it. Used to implement a weaker from of 2522 token expiration for devices that cannot synchronize their 2523 internal clocks. 2524 o Change Controller: IESG 2525 o Reference: Section 5.9.2 of [this document] 2527 8.12. OAuth Token Introspection Response CBOR Mappings Registry 2529 This specification establishes the IANA "OAuth Token Introspection 2530 Response CBOR Mappings" registry. The registry has been created to 2531 use the "Expert Review" registration procedure [RFC8126], except for 2532 the value range designated for private use. 2534 The columns of this registry are: 2536 Name The OAuth Parameter name, refers to the name in the OAuth 2537 parameter registry, e.g., "client_id". 2538 CBOR Key CBOR map key for this parameter. Integer values less than 2539 -65536 are marked as "Private Use", all other values use the 2540 registration policy "Expert Review" [RFC8126]. 2541 Value Type The allowable CBOR data types for values of this 2542 parameter. 2543 Reference This contains a pointer to the public specification of the 2544 introspection response parameter abbreviation, if one exists. 2546 This registry will be initially populated by the values in Figure 16. 2547 The Reference column for all of these entries will be this document. 2549 Note that the mappings of parameters corresponding to claim names 2550 intentionally coincide with the CWT claim name mappings from 2551 [RFC8392]. 2553 8.13. JSON Web Token Claims 2555 This specification registers the following new claims in the JSON Web 2556 Token (JWT) registry of JSON Web Token Claims 2557 [IANA.JsonWebTokenClaims]: 2559 o Claim Name: "ace_profile" 2560 o Claim Description: The ACE profile a token is supposed to be used 2561 with. 2562 o Change Controller: IESG 2563 o Reference: Section 5.10 of [this document] 2564 o Claim Name: "cnonce" 2565 o Claim Description: "client-nonce". A nonce previously provided to 2566 the AS by the RS via the client. Used to verify token freshness 2567 when the RS cannot synchronize its clock with the AS. 2568 o Change Controller: IESG 2569 o Reference: Section 5.10 of [this document] 2571 o Claim Name: "exi" 2572 o Claim Description: "Expires in". Lifetime of the token in seconds 2573 from the time the RS first sees it. Used to implement a weaker 2574 from of token expiration for devices that cannot synchronize their 2575 internal clocks. 2576 o Change Controller: IESG 2577 o Reference: Section 5.10.3 of [this document] 2579 8.14. CBOR Web Token Claims 2581 This specification registers the following new claims in the "CBOR 2582 Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims]. 2584 o Claim Name: "ace_profile" 2585 o Claim Description: The ACE profile a token is supposed to be used 2586 with. 2587 o JWT Claim Name: ace_profile 2588 o Claim Key: TBD (suggested: 38) 2589 o Claim Value Type(s): integer 2590 o Change Controller: IESG 2591 o Specification Document(s): Section 5.10 of [this document] 2593 o Claim Name: "cnonce" 2594 o Claim Description: The client-nonce sent to the AS by the RS via 2595 the client. 2596 o JWT Claim Name: cnonce 2597 o Claim Key: TBD (suggested: 39) 2598 o Claim Value Type(s): byte string 2599 o Change Controller: IESG 2600 o Specification Document(s): Section 5.10 of [this document] 2602 o Claim Name: "exi" 2603 o Claim Description: The expiration time of a token measured from 2604 when it was received at the RS in seconds. 2605 o JWT Claim Name: exi 2606 o Claim Key: TBD (suggested: 40) 2607 o Claim Value Type(s): integer 2608 o Change Controller: IESG 2609 o Specification Document(s): Section 5.10.3 of [this document] 2611 o Claim Name: "scope" 2612 o Claim Description: The scope of an access token as defined in 2613 [RFC6749]. 2614 o JWT Claim Name: scope 2615 o Claim Key: TBD (suggested: 9) 2616 o Claim Value Type(s): byte string or text string 2617 o Change Controller: IESG 2618 o Specification Document(s): Section 4.2 of [RFC8693] 2620 8.15. Media Type Registrations 2622 This specification registers the 'application/ace+cbor' media type 2623 for messages of the protocols defined in this document carrying 2624 parameters encoded in CBOR. This registration follows the procedures 2625 specified in [RFC6838]. 2627 Type name: application 2629 Subtype name: ace+cbor 2631 Required parameters: N/A 2633 Optional parameters: N/A 2635 Encoding considerations: Must be encoded as CBOR map containing the 2636 protocol parameters defined in [this document]. 2638 Security considerations: See Section 6 of [this document] 2640 Interoperability considerations: N/A 2642 Published specification: [this document] 2644 Applications that use this media type: The type is used by 2645 authorization servers, clients and resource servers that support the 2646 ACE framework with CBOR encoding as specified in [this document]. 2648 Fragment identifier considerations: N/A 2650 Additional information: N/A 2652 Person & email address to contact for further information: 2653 2655 Intended usage: COMMON 2657 Restrictions on usage: none 2659 Author: Ludwig Seitz 2660 Change controller: IESG 2662 8.16. CoAP Content-Format Registry 2664 This specification registers the following entry to the "CoAP 2665 Content-Formats" registry: 2667 Media Type: application/ace+cbor 2669 Encoding: - 2671 ID: TBD (suggested: 19) 2673 Reference: [this document] 2675 8.17. Expert Review Instructions 2677 All of the IANA registries established in this document are defined 2678 to use a registration policy of Expert Review. This section gives 2679 some general guidelines for what the experts should be looking for, 2680 but they are being designated as experts for a reason, so they should 2681 be given substantial latitude. 2683 Expert reviewers should take into consideration the following points: 2685 o Point squatting should be discouraged. Reviewers are encouraged 2686 to get sufficient information for registration requests to ensure 2687 that the usage is not going to duplicate one that is already 2688 registered, and that the point is likely to be used in 2689 deployments. The zones tagged as private use are intended for 2690 testing purposes and closed environments; code points in other 2691 ranges should not be assigned for testing. 2692 o Specifications are needed for the first-come, first-serve range if 2693 they are expected to be used outside of closed environments in an 2694 interoperable way. When specifications are not provided, the 2695 description provided needs to have sufficient information to 2696 identify what the point is being used for. 2697 o Experts should take into account the expected usage of fields when 2698 approving point assignment. The fact that there is a range for 2699 standards track documents does not mean that a standards track 2700 document cannot have points assigned outside of that range. The 2701 length of the encoded value should be weighed against how many 2702 code points of that length are left, the size of device it will be 2703 used on. 2704 o Since a high degree of overlap is expected between these 2705 registries and the contents of the OAuth parameters 2706 [IANA.OAuthParameters] registries, experts should require new 2707 registrations to maintain alignment with parameters from OAuth 2708 that have comparable functionality. Deviation from this alignment 2709 should only be allowed if there are functional differences, that 2710 are motivated by the use case and that cannot be easily or 2711 efficiently addressed by comparable OAuth parameters. 2713 9. Acknowledgments 2715 This document is a product of the ACE working group of the IETF. 2717 Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and 2718 UMA in IoT scenarios, Robert Taylor for his discussion input, and 2719 Malisa Vucinic for his input on the predecessors of this proposal. 2721 Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from 2722 where parts of the security considerations where copied. 2724 Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for 2725 contributing their work on AS discovery from draft-gerdes-ace-dcaf- 2726 authorize (see Section 5.1) and the considerations on multiple access 2727 tokens. 2729 Thanks to Jim Schaad and Mike Jones for their comprehensive reviews. 2731 Thanks to Benjamin Kaduk for his input on various questions related 2732 to this work. 2734 Thanks to Cigdem Sengul for some very useful review comments. 2736 Thanks to Carsten Bormann for contributing the text for the CoRE 2737 Resource Type registry. 2739 Thanks to Roman Danyliw for suggesting the Appendix E (including its 2740 contents). 2742 Ludwig Seitz and Goeran Selander worked on this document as part of 2743 the CelticPlus project CyberWI, with funding from Vinnova. Ludwig 2744 Seitz was also received further funding for this work by Vinnova in 2745 the context of the CelticNext project Critisec. 2747 10. References 2749 10.1. Normative References 2751 [I-D.ietf-ace-oauth-params] 2752 Seitz, L., "Additional OAuth Parameters for Authorization 2753 in Constrained Environments (ACE)", draft-ietf-ace-oauth- 2754 params-14 (work in progress), March 2021. 2756 [IANA.CborWebTokenClaims] 2757 IANA, "CBOR Web Token (CWT) Claims", 2758 . 2761 [IANA.CoreParameters] 2762 IANA, "Constrained RESTful Environments (CoRE) 2763 Parameters", . 2766 [IANA.JsonWebTokenClaims] 2767 IANA, "JSON Web Token Claims", 2768 . 2770 [IANA.OAuthAccessTokenTypes] 2771 IANA, "OAuth Access Token Types", 2772 . 2775 [IANA.OAuthExtensionsErrorRegistry] 2776 IANA, "OAuth Extensions Error Registry", 2777 . 2780 [IANA.OAuthParameters] 2781 IANA, "OAuth Parameters", 2782 . 2785 [IANA.TokenIntrospectionResponse] 2786 IANA, "OAuth Token Introspection Response", 2787 . 2790 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2791 Requirement Levels", BCP 14, RFC 2119, 2792 DOI 10.17487/RFC2119, March 1997, 2793 . 2795 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2796 Resource Identifier (URI): Generic Syntax", STD 66, 2797 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2798 . 2800 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 2801 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 2802 January 2012, . 2804 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 2805 RFC 6749, DOI 10.17487/RFC6749, October 2012, 2806 . 2808 [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization 2809 Framework: Bearer Token Usage", RFC 6750, 2810 DOI 10.17487/RFC6750, October 2012, 2811 . 2813 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2814 Specifications and Registration Procedures", BCP 13, 2815 RFC 6838, DOI 10.17487/RFC6838, January 2013, 2816 . 2818 [RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B., 2819 Keranen, A., and P. Hallam-Baker, "Naming Things with 2820 Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013, 2821 . 2823 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 2824 Application Protocol (CoAP)", RFC 7252, 2825 DOI 10.17487/RFC7252, June 2014, 2826 . 2828 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 2829 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 2830 . 2832 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 2833 RFC 7662, DOI 10.17487/RFC7662, October 2015, 2834 . 2836 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2837 Writing an IANA Considerations Section in RFCs", BCP 26, 2838 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2839 . 2841 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 2842 RFC 8152, DOI 10.17487/RFC8152, July 2017, 2843 . 2845 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2846 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2847 May 2017, . 2849 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 2850 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 2851 May 2018, . 2853 [RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J., 2854 and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693, 2855 DOI 10.17487/RFC8693, January 2020, 2856 . 2858 [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 2859 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 2860 Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 2861 2020, . 2863 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object 2864 Representation (CBOR)", STD 94, RFC 8949, 2865 DOI 10.17487/RFC8949, December 2020, 2866 . 2868 10.2. Informative References 2870 [BLE] Bluetooth SIG, "Bluetooth Core Specification v5.1", 2871 Section 4.4, January 2019, 2872 . 2875 [I-D.erdtman-ace-rpcc] 2876 Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared- 2877 Key as OAuth client credentials", draft-erdtman-ace- 2878 rpcc-02 (work in progress), October 2017. 2880 [I-D.ietf-ace-dtls-authorize] 2881 Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and 2882 L. Seitz, "Datagram Transport Layer Security (DTLS) 2883 Profile for Authentication and Authorization for 2884 Constrained Environments (ACE)", draft-ietf-ace-dtls- 2885 authorize-16 (work in progress), March 2021. 2887 [I-D.ietf-ace-oscore-profile] 2888 Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson, 2889 "OSCORE Profile of the Authentication and Authorization 2890 for Constrained Environments Framework", draft-ietf-ace- 2891 oscore-profile-18 (work in progress), April 2021. 2893 [I-D.ietf-quic-transport] 2894 Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed 2895 and Secure Transport", draft-ietf-quic-transport-34 (work 2896 in progress), January 2021. 2898 [I-D.ietf-tls-dtls13] 2899 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 2900 Datagram Transport Layer Security (DTLS) Protocol Version 2901 1.3", draft-ietf-tls-dtls13-43 (work in progress), April 2902 2021. 2904 [Margi10impact] 2905 Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr, 2906 M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold, 2907 "Impact of Operating Systems on Wireless Sensor Networks 2908 (Security) Applications and Testbeds", Proceedings of 2909 the 19th International Conference on Computer 2910 Communications and Networks (ICCCN), August 2010. 2912 [MQTT5.0] Banks, A., Briggs, E., Borgendale, K., and R. Gupta, "MQTT 2913 Version 5.0", OASIS Standard, March 2019, 2914 . 2917 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 2918 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 2919 . 2921 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 2922 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 2923 . 2925 [RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0 2926 Threat Model and Security Considerations", RFC 6819, 2927 DOI 10.17487/RFC6819, January 2013, 2928 . 2930 [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth 2931 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, 2932 August 2013, . 2934 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2935 Constrained-Node Networks", RFC 7228, 2936 DOI 10.17487/RFC7228, May 2014, 2937 . 2939 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2940 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2941 DOI 10.17487/RFC7231, June 2014, 2942 . 2944 [RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland, 2945 "Assertion Framework for OAuth 2.0 Client Authentication 2946 and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521, 2947 May 2015, . 2949 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 2950 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 2951 DOI 10.17487/RFC7540, May 2015, 2952 . 2954 [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and 2955 P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", 2956 RFC 7591, DOI 10.17487/RFC7591, July 2015, 2957 . 2959 [RFC7641] Hartke, K., "Observing Resources in the Constrained 2960 Application Protocol (CoAP)", RFC 7641, 2961 DOI 10.17487/RFC7641, September 2015, 2962 . 2964 [RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M., 2965 and S. Kumar, "Use Cases for Authentication and 2966 Authorization in Constrained Environments", RFC 7744, 2967 DOI 10.17487/RFC7744, January 2016, 2968 . 2970 [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 2971 the Constrained Application Protocol (CoAP)", RFC 7959, 2972 DOI 10.17487/RFC7959, August 2016, 2973 . 2975 [RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", 2976 BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017, 2977 . 2979 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2980 Interchange Format", STD 90, RFC 8259, 2981 DOI 10.17487/RFC8259, December 2017, 2982 . 2984 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 2985 Authorization Server Metadata", RFC 8414, 2986 DOI 10.17487/RFC8414, June 2018, 2987 . 2989 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2990 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2991 . 2993 [RFC8516] Keranen, A., ""Too Many Requests" Response Code for the 2994 Constrained Application Protocol", RFC 8516, 2995 DOI 10.17487/RFC8516, January 2019, 2996 . 2998 [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 2999 "Object Security for Constrained RESTful Environments 3000 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 3001 . 3003 [RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig, 3004 "OAuth 2.0 Device Authorization Grant", RFC 8628, 3005 DOI 10.17487/RFC8628, August 2019, 3006 . 3008 Appendix A. Design Justification 3010 This section provides further insight into the design decisions of 3011 the solution documented in this document. Section 3 lists several 3012 building blocks and briefly summarizes their importance. The 3013 justification for offering some of those building blocks, as opposed 3014 to using OAuth 2.0 as is, is given below. 3016 Common IoT constraints are: 3018 Low Power Radio: 3020 Many IoT devices are equipped with a small battery which needs to 3021 last for a long time. For many constrained wireless devices, the 3022 highest energy cost is associated to transmitting or receiving 3023 messages (roughly by a factor of 10 compared to AES) 3024 [Margi10impact]. It is therefore important to keep the total 3025 communication overhead low, including minimizing the number and 3026 size of messages sent and received, which has an impact of choice 3027 on the message format and protocol. By using CoAP over UDP and 3028 CBOR encoded messages, some of these aspects are addressed. 3029 Security protocols contribute to the communication overhead and 3030 can, in some cases, be optimized. For example, authentication and 3031 key establishment may, in certain cases where security 3032 requirements allow, be replaced by provisioning of security 3033 context by a trusted third party, using transport or application- 3034 layer security. 3036 Low CPU Speed: 3038 Some IoT devices are equipped with processors that are 3039 significantly slower than those found in most current devices on 3040 the Internet. This typically has implications on what timely 3041 cryptographic operations a device is capable of performing, which 3042 in turn impacts, e.g., protocol latency. Symmetric key 3043 cryptography may be used instead of the computationally more 3044 expensive public key cryptography where the security requirements 3045 so allow, but this may also require support for trusted-third- 3046 party-assisted secret key establishment using transport- or 3047 application-layer security. 3048 Small Amount of Memory: 3050 Microcontrollers embedded in IoT devices are often equipped with 3051 only a small amount of RAM and flash memory, which places 3052 limitations on what kind of processing can be performed and how 3053 much code can be put on those devices. To reduce code size, fewer 3054 and smaller protocol implementations can be put on the firmware of 3055 such a device. In this case, CoAP may be used instead of HTTP, 3056 symmetric-key cryptography instead of public-key cryptography, and 3057 CBOR instead of JSON. An authentication and key establishment 3058 protocol, e.g., the DTLS handshake, in comparison with assisted 3059 key establishment, also has an impact on memory and code 3060 footprints. 3062 User Interface Limitations: 3064 Protecting access to resources is both an important security as 3065 well as privacy feature. End users and enterprise customers may 3066 not want to give access to the data collected by their IoT device 3067 or to functions it may offer to third parties. Since the 3068 classical approach of requesting permissions from end users via a 3069 rich user interface does not work in many IoT deployment 3070 scenarios, these functions need to be delegated to user-controlled 3071 devices that are better suitable for such tasks, such as smart 3072 phones and tablets. 3074 Communication Constraints: 3076 In certain constrained settings an IoT device may not be able to 3077 communicate with a given device at all times. Devices may be 3078 sleeping, or just disconnected from the Internet because of 3079 general lack of connectivity in the area, for cost reasons, or for 3080 security reasons, e.g., to avoid an entry point for Denial-of- 3081 Service attacks. 3083 The communication interactions this framework builds upon (as 3084 shown graphically in Figure 1) may be accomplished using a variety 3085 of different protocols, and not all parts of the message flow are 3086 used in all applications due to the communication constraints. 3087 Deployments making use of CoAP are expected, but this framework is 3088 not limited to them. Other protocols such as HTTP, or even 3089 protocols such as Bluetooth Smart communication that do not 3090 necessarily use IP, could also be used. The latter raises the 3091 need for application-layer security over the various interfaces. 3093 In the light of these constraints we have made the following design 3094 decisions: 3096 CBOR, COSE, CWT: 3098 When using this framework, it is RECOMMENDED to use CBOR [RFC8949] 3099 as data format. Where CBOR data needs to be protected, the use of 3100 COSE [RFC8152] is RECOMMENDED. Furthermore, where self-contained 3101 tokens are needed, it is RECOMMENDED to use of CWT [RFC8392]. 3102 These measures aim at reducing the size of messages sent over the 3103 wire, the RAM size of data objects that need to be kept in memory 3104 and the size of libraries that devices need to support. 3106 CoAP: 3108 When using this framework, it is RECOMMENDED to use of CoAP 3109 [RFC7252] instead of HTTP. This does not preclude the use of 3110 other protocols specifically aimed at constrained devices, like, 3111 e.g., Bluetooth Low Energy (see Section 3.2). This aims again at 3112 reducing the size of messages sent over the wire, the RAM size of 3113 data objects that need to be kept in memory and the size of 3114 libraries that devices need to support. 3116 Access Information: 3118 This framework defines the name "Access Information" for data 3119 concerning the RS that the AS returns to the client in an access 3120 token response (see Section 5.8.2). This aims at enabling 3121 scenarios where a powerful client, supporting multiple profiles, 3122 needs to interact with an RS for which it does not know the 3123 supported profiles and the raw public key. 3125 Proof-of-Possession: 3127 This framework makes use of proof-of-possession tokens, using the 3128 "cnf" claim [RFC8747]. A request parameter "cnf" and a Response 3129 parameter "cnf", both having a value space semantically and 3130 syntactically identical to the "cnf" claim, are defined for the 3131 token endpoint, to allow requesting and stating confirmation keys. 3132 This aims at making token theft harder. Token theft is 3133 specifically relevant in constrained use cases, as communication 3134 often passes through middle-boxes, which could be able to steal 3135 bearer tokens and use them to gain unauthorized access. 3137 Authz-Info endpoint: 3139 This framework introduces a new way of providing access tokens to 3140 an RS by exposing a authz-info endpoint, to which access tokens 3141 can be POSTed. This aims at reducing the size of the request 3142 message and the code complexity at the RS. The size of the 3143 request message is problematic, since many constrained protocols 3144 have severe message size limitations at the physical layer (e.g., 3145 in the order of 100 bytes). This means that larger packets get 3146 fragmented, which in turn combines badly with the high rate of 3147 packet loss, and the need to retransmit the whole message if one 3148 packet gets lost. Thus separating sending of the request and 3149 sending of the access tokens helps to reduce fragmentation. 3151 Client Credentials Grant: 3153 In this framework the use of the client credentials grant is 3154 RECOMMENDED for machine-to-machine communication use cases, where 3155 manual intervention of the resource owner to produce a grant token 3156 is not feasible. The intention is that the resource owner would 3157 instead pre-arrange authorization with the AS, based on the 3158 client's own credentials. The client can then (without manual 3159 intervention) obtain access tokens from the AS. 3161 Introspection: 3163 In this framework the use of access token introspection is 3164 RECOMMENDED in cases where the client is constrained in a way that 3165 it can not easily obtain new access tokens (i.e. it has 3166 connectivity issues that prevent it from communicating with the 3167 AS). In that case it is RECOMMENDED to use a long-term token, 3168 that could be a simple reference. The RS is assumed to be able to 3169 communicate with the AS, and can therefore perform introspection, 3170 in order to learn the claims associated with the token reference. 3171 The advantage of such an approach is that the resource owner can 3172 change the claims associated to the token reference without having 3173 to be in contact with the client, thus granting or revoking access 3174 rights. 3176 Appendix B. Roles and Responsibilities 3178 Resource Owner 3180 * Make sure that the RS is registered at the AS. This includes 3181 making known to the AS which profiles, token_type, scopes, and 3182 key types (symmetric/asymmetric) the RS supports. Also making 3183 it known to the AS which audience(s) the RS identifies itself 3184 with. 3185 * Make sure that clients can discover the AS that is in charge of 3186 the RS. 3187 * If the client-credentials grant is used, make sure that the AS 3188 has the necessary, up-to-date, access control policies for the 3189 RS. 3191 Requesting Party 3193 * Make sure that the client is provisioned the necessary 3194 credentials to authenticate to the AS. 3195 * Make sure that the client is configured to follow the security 3196 requirements of the Requesting Party when issuing requests 3197 (e.g., minimum communication security requirements, trust 3198 anchors). 3199 * Register the client at the AS. This includes making known to 3200 the AS which profiles, token_types, and key types (symmetric/ 3201 asymmetric) the client. 3203 Authorization Server 3205 * Register the RS and manage corresponding security contexts. 3206 * Register clients and authentication credentials. 3207 * Allow Resource Owners to configure and update access control 3208 policies related to their registered RSs. 3209 * Expose the token endpoint to allow clients to request tokens. 3210 * Authenticate clients that wish to request a token. 3211 * Process a token request using the authorization policies 3212 configured for the RS. 3213 * Optionally: Expose the introspection endpoint that allows RS's 3214 to submit token introspection requests. 3215 * If providing an introspection endpoint: Authenticate RSs that 3216 wish to get an introspection response. 3217 * If providing an introspection endpoint: Process token 3218 introspection requests. 3219 * Optionally: Handle token revocation. 3220 * Optionally: Provide discovery metadata. See [RFC8414] 3221 * Optionally: Handle refresh tokens. 3223 Client 3225 * Discover the AS in charge of the RS that is to be targeted with 3226 a request. 3227 * Submit the token request (see step (A) of Figure 1). 3229 + Authenticate to the AS. 3231 + Optionally (if not pre-configured): Specify which RS, which 3232 resource(s), and which action(s) the request(s) will target. 3233 + If raw public keys (rpk) or certificates are used, make sure 3234 the AS has the right rpk or certificate for this client. 3235 * Process the access token and Access Information (see step (B) 3236 of Figure 1). 3238 + Check that the Access Information provides the necessary 3239 security parameters (e.g., PoP key, information on 3240 communication security protocols supported by the RS). 3241 + Safely store the proof-of-possession key. 3242 + If provided by the AS: Safely store the refresh token. 3243 * Send the token and request to the RS (see step (C) of 3244 Figure 1). 3246 + Authenticate towards the RS (this could coincide with the 3247 proof of possession process). 3248 + Transmit the token as specified by the AS (default is to the 3249 authz-info endpoint, alternative options are specified by 3250 profiles). 3251 + Perform the proof-of-possession procedure as specified by 3252 the profile in use (this may already have been taken care of 3253 through the authentication procedure). 3254 * Process the RS response (see step (F) of Figure 1) of the RS. 3256 Resource Server 3258 * Expose a way to submit access tokens. By default this is the 3259 authz-info endpoint. 3260 * Process an access token. 3262 + Verify the token is from a recognized AS. 3263 + Check the token's integrity. 3264 + Verify that the token applies to this RS. 3265 + Check that the token has not expired (if the token provides 3266 expiration information). 3267 + Store the token so that it can be retrieved in the context 3268 of a matching request. 3270 Note: The order proposed here is not normative, any process 3271 that arrives at an equivalent result can be used. A noteworthy 3272 consideration is whether one can use cheap operations early on 3273 to quickly discard non-applicable or invalid tokens, before 3274 performing expensive cryptographic operations (e.g. doing an 3275 expiration check before verifying a signature). 3277 * Process a request. 3279 + Set up communication security with the client. 3280 + Authenticate the client. 3281 + Match the client against existing tokens. 3282 + Check that tokens belonging to the client actually authorize 3283 the requested action. 3284 + Optionally: Check that the matching tokens are still valid, 3285 using introspection (if this is possible.) 3286 * Send a response following the agreed upon communication 3287 security mechanism(s). 3288 * Safely store credentials such as raw public keys for 3289 authentication or proof-of-possession keys linked to access 3290 tokens. 3292 Appendix C. Requirements on Profiles 3294 This section lists the requirements on profiles of this framework, 3295 for the convenience of profile designers. 3297 o Optionally define new methods for the client to discover the 3298 necessary permissions and AS for accessing a resource, different 3299 from the one proposed in Section 5.1. Section 4 3300 o Optionally specify new grant types. Section 5.4 3301 o Optionally define the use of client certificates as client 3302 credential type. Section 5.5 3303 o Specify the communication protocol the client and RS the must use 3304 (e.g., CoAP). Section 5 and Section 5.8.4.3 3305 o Specify the security protocol the client and RS must use to 3306 protect their communication (e.g., OSCORE or DTLS). This must 3307 provide encryption, integrity and replay protection. 3308 Section 5.8.4.3 3309 o Specify how the client and the RS mutually authenticate. 3310 Section 4 3311 o Specify the proof-of-possession protocol(s) and how to select one, 3312 if several are available. Also specify which key types (e.g., 3313 symmetric/asymmetric) are supported by a specific proof-of- 3314 possession protocol. Section 5.8.4.2 3315 o Specify a unique ace_profile identifier. Section 5.8.4.3 3316 o If introspection is supported: Specify the communication and 3317 security protocol for introspection. Section 5.9 3318 o Specify the communication and security protocol for interactions 3319 between client and AS. This must provide encryption, integrity 3320 protection, replay protection and a binding between requests and 3321 responses. Section 5 and Section 5.8 3322 o Specify how/if the authz-info endpoint is protected, including how 3323 error responses are protected. Section 5.10.1 3324 o Optionally define other methods of token transport than the authz- 3325 info endpoint. Section 5.10.1 3327 Appendix D. Assumptions on AS Knowledge about C and RS 3329 This section lists the assumptions on what an AS should know about a 3330 client and an RS in order to be able to respond to requests to the 3331 token and introspection endpoints. How this information is 3332 established is out of scope for this document. 3334 o The identifier of the client or RS. 3335 o The profiles that the client or RS supports. 3336 o The scopes that the RS supports. 3337 o The audiences that the RS identifies with. 3338 o The key types (e.g., pre-shared symmetric key, raw public key, key 3339 length, other key parameters) that the client or RS supports. 3340 o The types of access tokens the RS supports (e.g., CWT). 3341 o If the RS supports CWTs, the COSE parameters for the crypto 3342 wrapper (e.g., algorithm, key-wrap algorithm, key-length) that the 3343 RS supports. 3344 o The expiration time for access tokens issued to this RS (unless 3345 the RS accepts a default time chosen by the AS). 3346 o The symmetric key shared between client and AS (if any). 3347 o The symmetric key shared between RS and AS (if any). 3348 o The raw public key of the client or RS (if any). 3349 o Whether the RS has synchronized time (and thus is able to use the 3350 'exp' claim) or not. 3352 Appendix E. Differences to OAuth 2.0 3354 This document adapts OAuth 2.0 to be suitable for constrained 3355 environments. This sections lists the main differences from the 3356 normative requirements of OAuth 2.0. 3358 o Use of TLS -- OAuth 2.0 requires the use of TLS both to protect 3359 the communication between AS and client when requesting an access 3360 token; between client and RS when accessing a resource and between 3361 AS and RS if introspection is used. This framework requires 3362 similar security properties, but does not require that they be 3363 realized with TLS. See Section 5. 3364 o Cardinality of "grant_type" parameter -- In client-to-AS requests 3365 using OAuth 2.0, the "grant_type" parameter is required (per 3366 [RFC6749]). In this framework, this parameter is optional. See 3367 Section 5.8.1. 3368 o Encoding of "scope" parameter -- In client-to-AS requests using 3369 OAuth 2.0, the "scope" parameter is string encoded (per 3370 [RFC6749]). In this framework, this parameter may also be encoded 3371 as a byte string. See Section 5.8.1. 3372 o Cardinality of "token_type" parameter -- in AS-to-client responses 3373 using OAuth 2.0, the token_type parameter is required (per 3375 [RFC6749]). In this framework, this parameter is optional. See 3376 Section 5.8.2. 3377 o Access token retention -- in OAuth 2.0, the access token is sent 3378 with each request to the RS. In this framework, the RS must be 3379 able to store these tokens for later use. See Section 5.10.1. 3381 Appendix F. Deployment Examples 3383 There is a large variety of IoT deployments, as is indicated in 3384 Appendix A, and this section highlights a few common variants. This 3385 section is not normative but illustrates how the framework can be 3386 applied. 3388 For each of the deployment variants, there are a number of possible 3389 security setups between clients, resource servers and authorization 3390 servers. The main focus in the following subsections is on how 3391 authorization of a client request for a resource hosted by an RS is 3392 performed. This requires the security of the requests and responses 3393 between the clients and the RS to be considered. 3395 Note: CBOR diagnostic notation is used for examples of requests and 3396 responses. 3398 F.1. Local Token Validation 3400 In this scenario, the case where the resource server is offline is 3401 considered, i.e., it is not connected to the AS at the time of the 3402 access request. This access procedure involves steps A, B, C, and F 3403 of Figure 1. 3405 Since the resource server must be able to verify the access token 3406 locally, self-contained access tokens must be used. 3408 This example shows the interactions between a client, the 3409 authorization server and a temperature sensor acting as a resource 3410 server. Message exchanges A and B are shown in Figure 17. 3412 A: The client first generates a public-private key pair used for 3413 communication security with the RS. 3414 The client sends a CoAP POST request to the token endpoint at the 3415 AS. The security of this request can be transport or application 3416 layer. It is up the communication security profile to define. In 3417 the example it is assumed that both client and AS have performed 3418 mutual authentication e.g. via DTLS. The request contains the 3419 public key of the client and the Audience parameter set to 3420 "tempSensorInLivingRoom", a value that the temperature sensor 3421 identifies itself with. The AS evaluates the request and 3422 authorizes the client to access the resource. 3424 B: The AS responds with a 2.05 Content response containing the 3425 Access Information, including the access token. The PoP access 3426 token contains the public key of the client, and the Access 3427 Information contains the public key of the RS. For communication 3428 security this example uses DTLS RawPublicKey between the client 3429 and the RS. The issued token will have a short validity time, 3430 i.e., "exp" close to "iat", in order to mitigate attacks using 3431 stolen client credentials. The token includes the claim such as 3432 "scope" with the authorized access that an owner of the 3433 temperature device can enjoy. In this example, the "scope" claim, 3434 issued by the AS, informs the RS that the owner of the token, that 3435 can prove the possession of a key is authorized to make a GET 3436 request against the /temperature resource and a POST request on 3437 the /firmware resource. Note that the syntax and semantics of the 3438 scope claim are application specific. 3439 Note: In this example it is assumed that the client knows what 3440 resource it wants to access, and is therefore able to request 3441 specific audience and scope claims for the access token. 3443 Authorization 3444 Client Server 3445 | | 3446 |<=======>| DTLS Connection Establishment 3447 | | and mutual authentication 3448 | | 3449 A: +-------->| Header: POST (Code=0.02) 3450 | POST | Uri-Path:"token" 3451 | | Content-Format: application/ace+cbor 3452 | | Payload: 3453 | | 3454 B: |<--------+ Header: 2.05 Content 3455 | 2.05 | Content-Format: application/ace+cbor 3456 | | Payload: 3457 | | 3459 Figure 17: Token Request and Response Using Client Credentials. 3461 The information contained in the Request-Payload and the Response- 3462 Payload is shown in Figure 18 Note that the parameter "rs_cnf" from 3463 [I-D.ietf-ace-oauth-params] is used to inform the client about the 3464 resource server's public key. 3466 Request-Payload : 3467 { 3468 "audience" : "tempSensorInLivingRoom", 3469 "client_id" : "myclient", 3470 "req_cnf" : { 3471 "COSE_Key" : { 3472 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3473 "kty" : "EC", 3474 "crv" : "P-256", 3475 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3476 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3477 } 3478 } 3479 } 3481 Response-Payload : 3482 { 3483 "access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...', 3484 "rs_cnf" : { 3485 "COSE_Key" : { 3486 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3487 "kty" : "EC", 3488 "crv" : "P-256", 3489 "x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4', 3490 "y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM' 3491 } 3492 } 3493 } 3495 Figure 18: Request and Response Payload Details. 3497 The content of the access token is shown in Figure 19. 3499 { 3500 "aud" : "tempSensorInLivingRoom", 3501 "iat" : "1563451500", 3502 "exp" : "1563453000", 3503 "scope" : "temperature_g firmware_p", 3504 "cnf" : { 3505 "COSE_Key" : { 3506 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3507 "kty" : "EC", 3508 "crv" : "P-256", 3509 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3510 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3511 } 3512 } 3513 } 3515 Figure 19: Access Token including Public Key of the client. 3517 Messages C and F are shown in Figure 20 - Figure 21. 3519 C: The client then sends the PoP access token to the authz-info 3520 endpoint at the RS. This is a plain CoAP POST request, i.e., no 3521 transport or application-layer security is used between client and 3522 RS since the token is integrity protected between the AS and RS. 3523 The RS verifies that the PoP access token was created by a known 3524 and trusted AS, that it applies to this RS, and that it is valid. 3525 The RS caches the security context together with authorization 3526 information about this client contained in the PoP access token. 3528 Resource 3529 Client Server 3530 | | 3531 C: +-------->| Header: POST (Code=0.02) 3532 | POST | Uri-Path:"authz-info" 3533 | | Payload: 0INDoQEKoQVN ... 3534 | | 3535 |<--------+ Header: 2.04 Changed 3536 | 2.04 | 3537 | | 3539 Figure 20: Access Token provisioning to RS 3540 The client and the RS runs the DTLS handshake using the raw public 3541 keys established in step B and C. 3542 The client sends a CoAP GET request to /temperature on RS over 3543 DTLS. The RS verifies that the request is authorized, based on 3544 previously established security context. 3546 F: The RS responds over the same DTLS channel with a CoAP 2.05 3547 Content response, containing a resource representation as payload. 3549 Resource 3550 Client Server 3551 | | 3552 |<=======>| DTLS Connection Establishment 3553 | | using Raw Public Keys 3554 | | 3555 +-------->| Header: GET (Code=0.01) 3556 | GET | Uri-Path: "temperature" 3557 | | 3558 | | 3559 | | 3560 F: |<--------+ Header: 2.05 Content 3561 | 2.05 | Payload: 3562 | | 3564 Figure 21: Resource Request and Response protected by DTLS. 3566 F.2. Introspection Aided Token Validation 3568 In this deployment scenario it is assumed that a client is not able 3569 to access the AS at the time of the access request, whereas the RS is 3570 assumed to be connected to the back-end infrastructure. Thus the RS 3571 can make use of token introspection. This access procedure involves 3572 steps A-F of Figure 1, but assumes steps A and B have been carried 3573 out during a phase when the client had connectivity to AS. 3575 Since the client is assumed to be offline, at least for a certain 3576 period of time, a pre-provisioned access token has to be long-lived. 3577 Since the client is constrained, the token will not be self contained 3578 (i.e. not a CWT) but instead just a reference. The resource server 3579 uses its connectivity to learn about the claims associated to the 3580 access token by using introspection, which is shown in the example 3581 below. 3583 In the example interactions between an offline client (key fob), an 3584 RS (online lock), and an AS is shown. It is assumed that there is a 3585 provisioning step where the client has access to the AS. This 3586 corresponds to message exchanges A and B which are shown in 3587 Figure 22. 3589 Authorization consent from the resource owner can be pre-configured, 3590 but it can also be provided via an interactive flow with the resource 3591 owner. An example of this for the key fob case could be that the 3592 resource owner has a connected car, he buys a generic key that he 3593 wants to use with the car. To authorize the key fob he connects it 3594 to his computer that then provides the UI for the device. After that 3595 OAuth 2.0 implicit flow can used to authorize the key for his car at 3596 the car manufacturers AS. 3598 Note: In this example the client does not know the exact door it will 3599 be used to access since the token request is not send at the time of 3600 access. So the scope and audience parameters are set quite wide to 3601 start with, while tailored values narrowing down the claims to the 3602 specific RS being accessed can be provided to that RS during an 3603 introspection step. 3605 A: The client sends a CoAP POST request to the token endpoint at 3606 AS. The request contains the Audience parameter set to "PACS1337" 3607 (PACS, Physical Access System), a value the that identifies the 3608 physical access control system to which the individual doors are 3609 connected. The AS generates an access token as an opaque string, 3610 which it can match to the specific client and the targeted 3611 audience. It furthermore generates a symmetric proof-of- 3612 possession key. The communication security and authentication 3613 between client and AS is assumed to have been provided at 3614 transport layer (e.g. via DTLS) using a pre-shared security 3615 context (psk, rpk or certificate). 3616 B: The AS responds with a CoAP 2.05 Content response, containing 3617 as payload the Access Information, including the access token and 3618 the symmetric proof-of-possession key. Communication security 3619 between C and RS will be DTLS and PreSharedKey. The PoP key is 3620 used as the PreSharedKey. 3622 Note: In this example we are using a symmetric key for a multi-RS 3623 audience, which is not recommended normally (see Section 6.9). 3624 However in this case the risk is deemed to be acceptable, since all 3625 the doors are part of the same physical access control system, and 3626 therefore the risk of a malicious RS impersonating the client towards 3627 another RS is low. 3629 Authorization 3630 Client Server 3631 | | 3632 |<=======>| DTLS Connection Establishment 3633 | | and mutual authentication 3634 | | 3635 A: +-------->| Header: POST (Code=0.02) 3636 | POST | Uri-Path:"token" 3637 | | Content-Format: application/ace+cbor 3638 | | Payload: 3639 | | 3640 B: |<--------+ Header: 2.05 Content 3641 | | Content-Format: application/ace+cbor 3642 | 2.05 | Payload: 3643 | | 3645 Figure 22: Token Request and Response using Client Credentials. 3647 The information contained in the Request-Payload and the Response- 3648 Payload is shown in Figure 23. 3650 Request-Payload: 3651 { 3652 "client_id" : "keyfob", 3653 "audience" : "PACS1337" 3654 } 3656 Response-Payload: 3657 { 3658 "access_token" : b64'VGVzdCB0b2tlbg==', 3659 "cnf" : { 3660 "COSE_Key" : { 3661 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3662 "kty" : "oct", 3663 "alg" : "HS256", 3664 "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE' 3665 } 3666 } 3667 } 3669 Figure 23: Request and Response Payload for C offline 3671 The access token in this case is just an opaque byte string 3672 referencing the authorization information at the AS. 3674 C: Next, the client POSTs the access token to the authz-info 3675 endpoint in the RS. This is a plain CoAP request, i.e., no DTLS 3676 between client and RS. Since the token is an opaque string, the 3677 RS cannot verify it on its own, and thus defers to respond the 3678 client with a status code until after step E. 3679 D: The RS sends the token to the introspection endpoint on the AS 3680 using a CoAP POST request. In this example RS and AS are assumed 3681 to have performed mutual authentication using a pre shared 3682 security context (psk, rpk or certificate) with the RS acting as 3683 DTLS client. 3684 E: The AS provides the introspection response (2.05 Content) 3685 containing parameters about the token. This includes the 3686 confirmation key (cnf) parameter that allows the RS to verify the 3687 client's proof of possession in step F. Note that our example in 3688 Figure 25 assumes a pre-established key (e.g. one used by the 3689 client and the RS for a previous token) that is now only 3690 referenced by its key-identifier 'kid'. 3691 After receiving message E, the RS responds to the client's POST in 3692 step C with the CoAP response code 2.01 (Created). 3694 Resource 3695 Client Server 3696 | | 3697 C: +-------->| Header: POST (T=CON, Code=0.02) 3698 | POST | Uri-Path:"authz-info" 3699 | | Payload: b64'VGVzdCB0b2tlbg==' 3700 | | 3701 | | Authorization 3702 | | Server 3703 | | | 3704 | D: +--------->| Header: POST (Code=0.02) 3705 | | POST | Uri-Path: "introspect" 3706 | | | Content-Format: "application/ace+cbor" 3707 | | | Payload: 3708 | | | 3709 | E: |<---------+ Header: 2.05 Content 3710 | | 2.05 | Content-Format: "application/ace+cbor" 3711 | | | Payload: 3712 | | | 3713 | | 3714 |<--------+ Header: 2.01 Created 3715 | 2.01 | 3716 | | 3718 Figure 24: Token Introspection for C offline 3719 The information contained in the Request-Payload and the Response- 3720 Payload is shown in Figure 25. 3722 Request-Payload: 3723 { 3724 "token" : b64'VGVzdCB0b2tlbg==', 3725 "client_id" : "FrontDoor", 3726 } 3728 Response-Payload: 3729 { 3730 "active" : true, 3731 "aud" : "lockOfDoor4711", 3732 "scope" : "open, close", 3733 "iat" : 1563454000, 3734 "cnf" : { 3735 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk' 3736 } 3737 } 3739 Figure 25: Request and Response Payload for Introspection 3741 The client uses the symmetric PoP key to establish a DTLS 3742 PreSharedKey secure connection to the RS. The CoAP request PUT is 3743 sent to the uri-path /state on the RS, changing the state of the 3744 door to locked. 3745 F: The RS responds with a appropriate over the secure DTLS 3746 channel. 3748 Resource 3749 Client Server 3750 | | 3751 |<=======>| DTLS Connection Establishment 3752 | | using Pre Shared Key 3753 | | 3754 +-------->| Header: PUT (Code=0.03) 3755 | PUT | Uri-Path: "state" 3756 | | Payload: 3757 | | 3758 F: |<--------+ Header: 2.04 Changed 3759 | 2.04 | Payload: 3760 | | 3762 Figure 26: Resource request and response protected by OSCORE 3764 Authors' Addresses 3765 Ludwig Seitz 3766 Combitech 3767 Djaeknegatan 31 3768 Malmoe 211 35 3769 Sweden 3771 Email: ludwig.seitz@combitech.com 3773 Goeran Selander 3774 Ericsson 3775 Faroegatan 6 3776 Kista 164 80 3777 Sweden 3779 Email: goran.selander@ericsson.com 3781 Erik Wahlstroem 3782 Sweden 3784 Email: erik@wahlstromstekniska.se 3786 Samuel Erdtman 3787 Spotify AB 3788 Birger Jarlsgatan 61, 4tr 3789 Stockholm 113 56 3790 Sweden 3792 Email: erdtman@spotify.com 3794 Hannes Tschofenig 3795 Arm Ltd. 3796 Absam 6067 3797 Austria 3799 Email: Hannes.Tschofenig@arm.com