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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: October 18, 2021 Ericsson 6 E. Wahlstroem 8 S. Erdtman 9 Spotify AB 10 H. Tschofenig 11 Arm Ltd. 12 April 16, 2021 14 Authentication and Authorization for Constrained Environments (ACE) 15 using the OAuth 2.0 Framework (ACE-OAuth) 16 draft-ietf-ace-oauth-authz-39 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 October 18, 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 . . . . . . . . . . . . . 39 99 5.10.3. Token Expiration . . . . . . . . . . . . . . . . . . 40 100 5.10.4. Key Expiration . . . . . . . . . . . . . . . . . . . 42 101 6. Security Considerations . . . . . . . . . . . . . . . . . . . 42 102 6.1. Protecting Tokens . . . . . . . . . . . . . . . . . . . . 42 103 6.2. Communication Security . . . . . . . . . . . . . . . . . 43 104 6.3. Long-Term Credentials . . . . . . . . . . . . . . . . . . 44 105 6.4. Unprotected AS Request Creation Hints . . . . . . . . . . 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 seciton 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. 1660 If the payload sent to the authz-info endpoint does not parse to a 1661 token, the RS MUST respond with a response code equivalent to the 1662 CoAP code 4.00 (Bad Request). 1664 The RS MAY make an introspection request to validate the token before 1665 responding to the POST request to the authz-info endpoint, e.g. if 1666 the token is an opaque reference. Some transport protocols may 1667 provide a way to indicate that the RS is busy and the client should 1668 retry after an interval; this type of status update would be 1669 appropriate while the RS is waiting for an introspection response. 1671 Profiles MUST specify whether the authz-info endpoint is protected, 1672 including whether error responses from this endpoint are protected. 1673 Note that since the token contains information that allow the client 1674 and the RS to establish a security context in the first place, mutual 1675 authentication may not be possible at this point. 1677 The default name of this endpoint in an url-path is '/authz-info', 1678 however implementations are not required to use this name and can 1679 define their own instead. 1681 5.10.1.1. Verifying an Access Token 1683 When an RS receives an access token, it MUST verify it before storing 1684 it. The details of token verification depends on various aspects, 1685 including the token encoding, the type of token, the security 1686 protection applied to the token, and the claims. The token encoding 1687 matters since the security protection differs between the token 1688 encodings. For example, a CWT token uses COSE while a JWT token uses 1689 JOSE. The type of token also has an influence on the verification 1690 procedure since tokens may be self-contained whereby token 1691 verification may happen locally at the RS while a token-by-reference 1692 requires further interaction with the authorization server, for 1693 example using token introspection, to obtain the claims associated 1694 with the token reference. Self-contained tokens MUST, at least be 1695 integrity protected but they MAY also be encrypted. 1697 For self-contained tokens the RS MUST process the security protection 1698 of the token first, as specified by the respective token format. For 1699 CWT the description can be found in [RFC8392] and for JWT the 1700 relevant specification is [RFC7519]. This MUST include a 1701 verification that security protection (and thus the token) was 1702 generated by an AS that has the right to issue access tokens for this 1703 RS. 1705 In case the token is communicated by reference the RS needs to obtain 1706 the claims first. When the RS uses token introspection the relevant 1707 specification is [RFC7662] with CoAP transport specified in 1708 Section 5.9. 1710 Errors may happen during this initial processing stage: 1712 o If the verification of the security wrapper fails, or the token 1713 was issued by an AS that does not have the right to issue tokens 1714 for the receiving RS, the RS MUST discard the token and, if this 1715 was an interaction with authz-info, return an error message with a 1716 response code equivalent to the CoAP code 4.01 (Unauthorized). 1718 o If the claims cannot be obtained the RS MUST discard the token 1719 and, in case of an interaction via the authz-info endpoint, return 1720 an error message with a response code equivalent to the CoAP code 1721 4.00 (Bad Request). 1723 Next, the RS MUST verify claims, if present, contained in the access 1724 token. Errors are returned when claim checks fail, in the order of 1725 priority of this list: 1727 iss The issuer claim (if present) must identify the AS that has 1728 produced the security protection for the access token. If that is 1729 not the case the RS MUST discard the token. If this was an 1730 interaction with authz-info, the RS MUST also respond with a 1731 response code equivalent to the CoAP code 4.01 (Unauthorized). 1733 exp The expiration date must be in the future. If that is not the 1734 case the RS MUST discard the token. If this was an interaction 1735 with authz-info the RS MUST also respond with a response code 1736 equivalent to the CoAP code 4.01 (Unauthorized). Note that the RS 1737 has to terminate access rights to the protected resources at the 1738 time when the tokens expire. 1740 aud The audience claim must refer to an audience that the RS 1741 identifies with. If that is not the case the RS MUST discard the 1742 token. If this was an interaction with authz-info, the RS MUST 1743 also respond with a response code equivalent to the CoAP code 4.03 1744 (Forbidden). 1746 scope The RS must recognize value of the scope claim. If that is 1747 not the case the RS MUST discard the token. If this was an 1748 interaction with authz-info, the RS MUST also respond with a 1749 response code equivalent to the CoAP code 4.00 (Bad Request). The 1750 RS MAY provide additional information in the error response, to 1751 clarify what went wrong. 1753 Additional processing may be needed for other claims in a way 1754 specific to a profile or the underlying application. 1756 Note that the Subject (sub) claim cannot always be verified when the 1757 token is submitted to the RS since the client may not have 1758 authenticated yet. Also note that a counter for the expires_in (exi) 1759 claim MUST be initialized when the RS first verifies this token. 1761 Also note that profiles of this framework may define access token 1762 transport mechanisms that do not allow for error responses. 1763 Therefore the error messages specified here only apply if the token 1764 was sent to the authz-info endpoint. 1766 When sending error responses, the RS MAY use the error codes from 1767 Section 3.1 of [RFC6750], to provide additional details to the 1768 client. 1770 5.10.1.2. Protecting the Authorization Information Endpoint 1772 As this framework can be used in RESTful environments, it is 1773 important to make sure that attackers cannot perform unauthorized 1774 requests on the authz-info endpoints, other than submitting access 1775 tokens. 1777 Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT 1778 on the authz-info endpoint and on its children (if any). 1780 The POST method SHOULD NOT be allowed on children of the authz-info 1781 endpoint. 1783 The RS SHOULD implement rate limiting measures to mitigate attacks 1784 aiming to overload the processing capacity of the RS by repeatedly 1785 submitting tokens. For CoAP-based communication the RS could use the 1786 mechanisms from [RFC8516] to indicate that it is overloaded. 1788 5.10.2. Client Requests to the RS 1790 Before sending a request to an RS, the client MUST verify that the 1791 keys used to protect this communication are still valid. See 1792 Section 5.10.4 for details on how the client determines the validity 1793 of the keys used. 1795 If an RS receives a request from a client, and the target resource 1796 requires authorization, the RS MUST first verify that it has an 1797 access token that authorizes this request, and that the client has 1798 performed the proof-of-possession binding that token to the request. 1800 The response code MUST be 4.01 (Unauthorized) in case the client has 1801 not performed the proof-of-possession, or if RS has no valid access 1802 token for the client. If RS has an access token for the client but 1803 the token does not authorize access for the resource that was 1804 requested, RS MUST reject the request with a 4.03 (Forbidden). If RS 1805 has an access token for the client but it does not cover the action 1806 that was requested on the resource, RS MUST reject the request with a 1807 4.05 (Method Not Allowed). 1809 Note: The use of the response codes 4.03 and 4.05 is intended to 1810 prevent infinite loops where a dumb client optimistically tries to 1811 access a requested resource with any access token received from AS. 1812 As malicious clients could pretend to be C to determine C's 1813 privileges, these detailed response codes must be used only when a 1814 certain level of security is already available which can be achieved 1815 only when the client is authenticated. 1817 Note: The RS MAY use introspection for timely validation of an access 1818 token, at the time when a request is presented. 1820 Note: Matching the claims of the access token (e.g., scope) to a 1821 specific request is application specific. 1823 If the request matches a valid token and the client has performed the 1824 proof-of-possession for that token, the RS continues to process the 1825 request as specified by the underlying application. 1827 5.10.3. Token Expiration 1829 Depending on the capabilities of the RS, there are various ways in 1830 which it can verify the expiration of a received access token. Here 1831 follows a list of the possibilities including what functionality they 1832 require of the RS. 1834 o The token is a CWT and includes an "exp" claim and possibly the 1835 "nbf" claim. The RS verifies these by comparing them to values 1836 from its internal clock as defined in [RFC7519]. In this case the 1837 RS's internal clock must reflect the current date and time, or at 1838 least be synchronized with the AS's clock. How this clock 1839 synchronization would be performed is out of scope for this 1840 specification. 1842 o The RS verifies the validity of the token by performing an 1843 introspection request as specified in Section 5.9. This requires 1844 the RS to have a reliable network connection to the AS and to be 1845 able to handle two secure sessions in parallel (C to RS and RS to 1846 AS). 1848 o In order to support token expiration for devices that have no 1849 reliable way of synchronizing their internal clocks, this 1850 specification defines the following approach: The claim "exi" 1851 ("expires in") can be used, to provide the RS with the lifetime of 1852 the token in seconds from the time the RS first receives the 1853 token. This mechanism only works for self-contained tokens, i.e. 1854 CWTs and JWTs. For CWTs this parameter is encoded as unsigned 1855 integer, while JWTs encode this as JSON number. 1857 o Processing this claim requires that the RS does the following: 1859 * For each token the RS receives, that contains an "exi" claim: 1860 Keep track of the time it received that token and revisit that 1861 list regularly to expunge expired tokens. 1863 * Keep track of the identifiers of tokens containing the "exi" 1864 claim that have expired (in order to avoid accepting them 1865 again). In order to avoid an unbounded memory usage growth, 1866 this MUST be implemented in the following way when the "exi" 1867 claim is used: 1869 + When creating the token, the AS MUST add a 'cti' claim ( or 1870 'jti' for JWTs) to the access token. The value of this 1871 claim MUST be created as the binary representation of the 1872 concatenation of the identifier of the RS with a sequence 1873 number counting the tokens containing an 'exi' claim, issued 1874 by this AS for the RS. 1876 + The RS MUST store the highest sequence number of an expired 1877 token containing the "exi" claim that it has seen, and treat 1878 tokens with lower sequence numbers as expired. Note that 1879 this could lead to discarding valid tokens with lower 1880 sequence numbers, if the AS where to issue tokens of 1881 different validity time for the same RS. The assumption is 1882 that typically tokens in such a scenario would all have the 1883 same validity time. 1885 If a token that authorizes a long running request such as a CoAP 1886 Observe [RFC7641] expires, the RS MUST send an error response with 1887 the response code equivalent to the CoAP code 4.01 (Unauthorized) to 1888 the client and then terminate processing the long running request. 1890 5.10.4. Key Expiration 1892 The AS provides the client with key material that the RS uses. This 1893 can either be a common symmetric PoP-key, or an asymmetric key used 1894 by the RS to authenticate towards the client. Since there is 1895 currently no expiration metadata associated to those keys, the client 1896 has no way of knowing if these keys are still valid. This may lead 1897 to situations where the client sends requests containing sensitive 1898 information to the RS using a key that is expired and possibly in the 1899 hands of an attacker, or accepts responses from the RS that are not 1900 properly protected and could possibly have been forged by an 1901 attacker. 1903 In order to prevent this, the client must assume that those keys are 1904 only valid as long as the related access token is. Since the access 1905 token is opaque to the client, one of the following methods MUST be 1906 used to inform the client about the validity of an access token: 1908 o The client knows a default validity time for all tokens it is 1909 using (i.e. how long a token is valid after being issued). This 1910 information could be provisioned to the client when it is 1911 registered at the AS, or published by the AS in a way that the 1912 client can query. 1914 o The AS informs the client about the token validity using the 1915 "expires_in" parameter in the Access Information. 1917 A client that is not able to obtain information about the expiration 1918 of a token MUST NOT use this token. 1920 6. Security Considerations 1922 Security considerations applicable to authentication and 1923 authorization in RESTful environments provided in OAuth 2.0 [RFC6749] 1924 apply to this work. Furthermore [RFC6819] provides additional 1925 security considerations for OAuth which apply to IoT deployments as 1926 well. If the introspection endpoint is used, the security 1927 considerations from [RFC7662] also apply. 1929 The following subsections address issues specific to this document 1930 and it's use in constrained environments. 1932 6.1. Protecting Tokens 1934 A large range of threats can be mitigated by protecting the contents 1935 of the access token by using a digital signature or a keyed message 1936 digest (MAC) or an Authenticated Encryption with Associated Data 1937 (AEAD) algorithm. Consequently, the token integrity protection MUST 1938 be applied to prevent the token from being modified, particularly 1939 since it contains a reference to the symmetric key or the asymmetric 1940 key used for proof-of-possession. If the access token contains the 1941 symmetric key, this symmetric key MUST be encrypted by the 1942 authorization server so that only the resource server can decrypt it. 1943 Note that using an AEAD algorithm is preferable over using a MAC 1944 unless the token needs to be publicly readable. 1946 If the token is intended for multiple recipients (i.e. an audience 1947 that is a group), integrity protection of the token with a symmetric 1948 key, shared between the AS and the recipients, is not sufficient, 1949 since any of the recipients could modify the token undetected by the 1950 other recipients. Therefore a token with a multi-recipient audience 1951 MUST be protected with an asymmetric signature. 1953 It is important for the authorization server to include the identity 1954 of the intended recipient (the audience), typically a single resource 1955 server (or a list of resource servers), in the token. The same 1956 shared secret MUST NOT be used as proof-of-possession key with 1957 multiple resource servers since the benefit from using the proof-of- 1958 possession concept is then significantly reduced. 1960 If clients are capable of doing so, they should frequently request 1961 fresh access tokens, as this allows the AS to keep the lifetime of 1962 the tokens short. This allows the AS to use shorter proof-of- 1963 possession key sizes, which translate to a performance benefit for 1964 the client and for the resource server. Shorter keys also lead to 1965 shorter messages (particularly with asymmetric keying material). 1967 When authorization servers bind symmetric keys to access tokens, they 1968 SHOULD scope these access tokens to a specific permission. 1970 In certain situations it may be necessary to revoke an access token 1971 that is still valid. Client-initiated revocation is specified in 1972 [RFC7009] for OAuth 2.0. Other revocation mechanisms are currently 1973 not specified, as the underlying assumption in OAuth is that access 1974 tokens are issued with a relatively short lifetime. This may not 1975 hold true for disconnected constrained devices, needing access tokens 1976 with relatively long lifetimes, and would therefore necessitate 1977 further standardization work that is out of scope for this document. 1979 6.2. Communication Security 1981 Communication with the authorization server MUST use confidentiality 1982 protection. This step is extremely important since the client or the 1983 RS may obtain the proof-of-possession key from the authorization 1984 server for use with a specific access token. Not using 1985 confidentiality protection exposes this secret (and the access token) 1986 to an eavesdropper thereby completely negating proof-of-possession 1987 security. The requirements for communication security of profiles 1988 are specified in Section 5. 1990 Additional protection for the access token can be applied by 1991 encrypting it, for example encryption of CWTs is specified in 1992 Section 5.1 of [RFC8392]. Such additional protection can be 1993 necessary if the token is later transferred over an insecure 1994 connection (e.g. when it is sent to the authz-info endpoint). 1996 Care must by taken by developers to prevent leakage of the PoP 1997 credentials (i.e., the private key or the symmetric key). An 1998 adversary in possession of the PoP credentials bound to the access 1999 token will be able to impersonate the client. Be aware that this is 2000 a real risk with many constrained environments, since adversaries may 2001 get physical access to the devices and can therefore use phyical 2002 extraction techniques to gain access to memory contents. This risk 2003 can be mitigated to some extent by making sure that keys are 2004 refreshed frequently, by using software isolation techniques and by 2005 using hardware security. 2007 6.3. Long-Term Credentials 2009 Both clients and RSs have long-term credentials that are used to 2010 secure communications, and authenticate to the AS. These credentials 2011 need to be protected against unauthorized access. In constrained 2012 devices, deployed in publicly accessible places, such protection can 2013 be difficult to achieve without specialized hardware (e.g. secure key 2014 storage memory). 2016 If credentials are lost or compromised, the operator of the affected 2017 devices needs to have procedures to invalidate any access these 2018 credentials give and to revoke tokens linked to such credentials. 2019 The loss of a credential linked to a specific device MUST NOT lead to 2020 a compromise of other credentials not linked to that device, 2021 therefore secret keys used for authentication MUST NOT be shared 2022 between more than two parties. 2024 Operators of clients or RS SHOULD have procedures in place to replace 2025 credentials that are suspected to have been compromised or that have 2026 been lost. 2028 Operators also SHOULD have procedures for decommissioning devices, 2029 that include securely erasing credentials and other security critical 2030 material in the devices being decommissioned. 2032 6.4. Unprotected AS Request Creation Hints 2034 Initially, no secure channel exists to protect the communication 2035 between C and RS. Thus, C cannot determine if the "AS Request 2036 Creation Hints" contained in an unprotected response from RS to an 2037 unauthorized request (see Section 5.3) are authentic. C therefore 2038 MUST determine if an AS is authorized to provide access tokens for a 2039 certain RS. How this determination is implemented is out of scope 2040 for this document and left to the applications. 2042 6.5. Minimal Security Requirements for Communication 2044 This section summarizes the minimal requirements for the 2045 communication security of the different protocol interactions. 2047 C-AS All communication between the client and the Authorization 2048 Server MUST be encrypted, integrity and replay protected. 2049 Furthermore responses from the AS to the client MUST be bound to 2050 the client's request to avoid attacks where the attacker swaps the 2051 intended response for an older one valid for a previous request. 2052 This requires that the client and the Authorization Server have 2053 previously exchanged either a shared secret or their public keys 2054 in order to negotiate a secure communication. Furthermore the 2055 client MUST be able to determine whether an AS has the authority 2056 to issue access tokens for a certain RS. This can for example be 2057 done through pre-configured lists, or through an online lookup 2058 mechanism that in turn also must be secured. 2060 RS-AS The communication between the Resource Server and the 2061 Authorization Server via the introspection endpoint MUST be 2062 encrypted, integrity and replay protected. Furthermore responses 2063 from the AS to the RS MUST be bound to the RS's request. This 2064 requires that the RS and the Authorization Server have previously 2065 exchanged either a shared secret, or their public keys in order to 2066 negotiate a secure communication. Furthermore the RS MUST be able 2067 to determine whether an AS has the authority to issue access 2068 tokens itself. This is usually configured out of band, but could 2069 also be performed through an online lookup mechanism provided that 2070 it is also secured in the same way. 2072 C-RS The initial communication between the client and the Resource 2073 Server can not be secured in general, since the RS is not in 2074 possession of on access token for that client, which would carry 2075 the necessary parameters. If both parties support DTLS without 2076 client authentication it is RECOMMEND to use this mechanism for 2077 protecting the initial communication. After the client has 2078 successfully transmitted the access token to the RS, a secure 2079 communication protocol MUST be established between client and RS 2080 for the actual resource request. This protocol MUST provide 2081 confidentiality, integrity and replay protection as well as a 2082 binding between requests and responses. This requires that the 2083 client learned either the RS's public key or received a symmetric 2084 proof-of-possession key bound to the access token from the AS. 2085 The RS must have learned either the client's public key or a 2086 shared symmetric key from the claims in the token or an 2087 introspection request. Since ACE does not provide profile 2088 negotiation between C and RS, the client MUST have learned what 2089 profile the RS supports (e.g. from the AS or pre-configured) and 2090 initiate the communication accordingly. 2092 6.6. Token Freshness and Expiration 2094 An RS that is offline faces the problem of clock drift. Since it 2095 cannot synchronize its clock with the AS, it may be tricked into 2096 accepting old access tokens that are no longer valid or have been 2097 compromised. In order to prevent this, an RS may use the nonce-based 2098 mechanism (cnonce) defined in Section 5.3 to ensure freshness of an 2099 Access Token subsequently presented to this RS. 2101 Another problem with clock drift is that evaluating the standard 2102 token expiration claim "exp" can give unpredictable results. 2104 Acceptable ranges of clock drift are highly dependent on the concrete 2105 application. Important factors are how long access tokens are valid, 2106 and how critical timely expiration of access token is. 2108 The expiration mechanism implemented by the "exi" claim, based on the 2109 first time the RS sees the token was defined to provide a more 2110 predictable alternative. The "exi" approach has some drawbacks that 2111 need to be considered: 2113 A malicious client may hold back tokens with the "exi" claim in 2114 order to prolong their lifespan. 2116 If an RS loses state (e.g. due to an unscheduled reboot), it may 2117 lose the current values of counters tracking the "exi" claims of 2118 tokens it is storing. 2120 The first drawback is inherent to the deployment scenario and the 2121 "exi" solution. It can therefore not be mitigated without requiring 2122 the RS be online at times. The second drawback can be mitigated by 2123 regularly storing the value of "exi" counters to persistent memory. 2125 6.7. Combining Profiles 2127 There may be use cases were different profiles of this framework are 2128 combined. For example, an MQTT-TLS profile is used between the 2129 client and the RS in combination with a CoAP-DTLS profile for 2130 interactions between the client and the AS. The security of a 2131 profile MUST NOT depend on the assumption that the profile is used 2132 for all the different types of interactions in this framework. 2134 6.8. Unprotected Information 2136 Communication with the authz-info endpoint, as well as the various 2137 error responses defined in this framework, all potentially include 2138 sending information over an unprotected channel. These messages may 2139 leak information to an adversary, or may be manipulated by active 2140 attackers to induce incorrect behavior. For example error responses 2141 for requests to the Authorization Information endpoint can reveal 2142 information about an otherwise opaque access token to an adversary 2143 who has intercepted this token. 2145 As far as error messages are concerned, this framework is written 2146 under the assumption that, in general, the benefits of detailed error 2147 messages outweigh the risk due to information leakage. For 2148 particular use cases, where this assessment does not apply, detailed 2149 error messages can be replaced by more generic ones. 2151 In some scenarios it may be possible to protect the communication 2152 with the authz-info endpoint (e.g. through DTLS with only server-side 2153 authentication). In cases where this is not possible, it is 2154 RECOMMENDED to use encrypted CWTs or tokens that are opaque 2155 references and need to be subjected to introspection by the RS. 2157 If the initial unauthorized resource request message (see 2158 Section 5.2) is used, the client MUST make sure that it is not 2159 sending sensitive content in this request. While GET and DELETE 2160 requests only reveal the target URI of the resource, POST and PUT 2161 requests would reveal the whole payload of the intended operation. 2163 Since the client is not authenticated at the point when it is 2164 submitting an access token to the authz-info endpoint, attackers may 2165 be pretending to be a client and trying to trick an RS to use an 2166 obsolete profile that in turn specifies a vulnerable security 2167 mechanism via the authz-info endpoint. Such an attack would require 2168 a valid access token containing an "ace_profile" claim requesting the 2169 use of said obsolete profile. Resource Owners should update the 2170 configuration of their RS's to prevent them from using such obsolete 2171 profiles. 2173 6.9. Identifying Audiences 2175 The audience claim as defined in [RFC7519] and the equivalent 2176 "audience" parameter from [RFC8693] are intentionally vague on how to 2177 match the audience value to a specific RS. This is intended to allow 2178 application specific semantics to be used. This section attempts to 2179 give some general guidance for the use of audiences in constrained 2180 environments. 2182 URLs are not a good way of identifying mobile devices that can switch 2183 networks and thus be associated with new URLs. If the audience 2184 represents a single RS, and asymmetric keys are used, the RS can be 2185 uniquely identified by a hash of its public key. If this approach is 2186 used it is RECOMMENDED to apply the procedure from section 3 of 2187 [RFC6920]. 2189 If the audience addresses a group of resource servers, the mapping of 2190 group identifier to individual RS has to be provisioned to each RS 2191 before the group-audience is usable. Managing dynamic groups could 2192 be an issue, if any RS is not always reachable when the groups' 2193 memberships change. Furthermore, issuing access tokens bound to 2194 symmetric proof-of-possession keys that apply to a group-audience is 2195 problematic, as an RS that is in possession of the access token can 2196 impersonate the client towards the other RSs that are part of the 2197 group. It is therefore NOT RECOMMENDED to issue access tokens bound 2198 to a group audience and symmetric proof-of possession keys. 2200 Even the client must be able to determine the correct values to put 2201 into the "audience" parameter, in order to obtain a token for the 2202 intended RS. Errors in this process can lead to the client 2203 inadvertently obtaining a token for the wrong RS. The correct values 2204 for "audience" can either be provisioned to the client as part of its 2205 configuration, or dynamically looked up by the client in some 2206 directory. In the latter case the integrity and correctness of the 2207 directory data must be assured. Note that the "audience" hint 2208 provided by the RS as part of the "AS Request Creation Hints" 2209 Section 5.3 is not typically source authenticated and integrity 2210 protected, and should therefore not be treated a trusted value. 2212 6.10. Denial of Service Against or with Introspection 2214 The optional introspection mechanism provided by OAuth and supported 2215 in the ACE framework allows for two types of attacks that need to be 2216 considered by implementers. 2218 First, an attacker could perform a denial of service attack against 2219 the introspection endpoint at the AS in order to prevent validation 2220 of access tokens. To maintain the security of the system, an RS that 2221 is configured to use introspection MUST NOT allow access based on a 2222 token for which it couldn't reach the introspection endpoint. 2224 Second, an attacker could use the fact that an RS performs 2225 introspection to perform a denial of service attack against that RS 2226 by repeatedly sending tokens to its authz-info endpoint that require 2227 an introspection call. RS can mitigate such attacks by implementing 2228 rate limits on how many introspection requests they perform in a 2229 given time interval for a certain client IP address submitting tokens 2230 to /authz-info. When that limit has been reached, incoming requests 2231 from that address are rejected for a certain amount of time. A 2232 general rate limit on the introspection requests should also be 2233 considered, to mitigate distributed attacks. 2235 7. Privacy Considerations 2237 Implementers and users should be aware of the privacy implications of 2238 the different possible deployments of this framework. 2240 The AS is in a very central position and can potentially learn 2241 sensitive information about the clients requesting access tokens. If 2242 the client credentials grant is used, the AS can track what kind of 2243 access the client intends to perform. With other grants this can be 2244 prevented by the Resource Owner. To do so, the resource owner needs 2245 to bind the grants it issues to anonymous, ephemeral credentials that 2246 do not allow the AS to link different grants and thus different 2247 access token requests by the same client. 2249 The claims contained in a token can reveal privacy sensitive 2250 information about the client and the RS to any party having access to 2251 them (whether by processing the content of a self-contained token or 2252 by introspection). The AS SHOULD be configured to minimize the 2253 information about clients and RSs disclosed in the tokens it issues. 2255 If tokens are only integrity protected and not encrypted, they may 2256 reveal information to attackers listening on the wire, or able to 2257 acquire the access tokens in some other way. In the case of CWTs the 2258 token may, e.g., reveal the audience, the scope and the confirmation 2259 method used by the client. The latter may reveal the identity of the 2260 device or application running the client. This may be linkable to 2261 the identity of the person using the client (if there is a person and 2262 not a machine-to-machine interaction). 2264 Clients using asymmetric keys for proof-of-possession should be aware 2265 of the consequences of using the same key pair for proof-of- 2266 possession towards different RSs. A set of colluding RSs or an 2267 attacker able to obtain the access tokens will be able to link the 2268 requests, or even to determine the client's identity. 2270 An unprotected response to an unauthorized request (see Section 5.3) 2271 may disclose information about RS and/or its existing relationship 2272 with C. It is advisable to include as little information as possible 2273 in an unencrypted response. Even the absolute URI of the AS may 2274 reveal sensitive information about the service that RS provides. 2275 Developers must ensure that the RS does not disclose information that 2276 has an impact on the privacy of the stakeholders in the "AS Request 2277 Creation Hints". They may choose to use a different mechanism for 2278 the discovery of the AS if necessary. If means of encrypting 2279 communication between C and RS already exist, more detailed 2280 information may be included with an error response to provide C with 2281 sufficient information to react on that particular error. 2283 8. IANA Considerations 2285 This document creates several registries with a registration policy 2286 of "Expert Review"; guidelines to the experts are given in 2287 Section 8.17. 2289 8.1. ACE Authorization Server Request Creation Hints 2291 This specification establishes the IANA "ACE Authorization Server 2292 Request Creation Hints" registry. The registry has been created to 2293 use the "Expert Review" registration procedure [RFC8126]. It should 2294 be noted that, in addition to the expert review, some portions of the 2295 registry require a specification, potentially a Standards Track RFC, 2296 be supplied as well. 2298 The columns of the registry are: 2300 Name The name of the parameter 2302 CBOR Key CBOR map key for the parameter. Different ranges of values 2303 use different registration policies [RFC8126]. Integer values 2304 from -256 to 255 are designated as Standards Action. Integer 2305 values from -65536 to -257 and from 256 to 65535 are designated as 2306 Specification Required. Integer values greater than 65535 are 2307 designated as Expert Review. Integer values less than -65536 are 2308 marked as Private Use. 2310 Value Type The CBOR data types allowable for the values of this 2311 parameter. 2313 Reference This contains a pointer to the public specification of the 2314 request creation hint abbreviation, if one exists. 2316 This registry will be initially populated by the values in Figure 2. 2317 The Reference column for all of these entries will be this document. 2319 8.2. CoRE Resource Type Registry 2321 IANA is requested to register a new Resource Type (rt=) Link Target 2322 Attribute in the "Resource Type (rt=) Link Target Attribute Values" 2323 subregistry under the "Constrained RESTful Environments (CoRE) 2324 Parameters" [IANA.CoreParameters] registry: 2326 o Value: "ace.ai" 2327 o Description: ACE-OAuth authz-info endpoint resource. 2328 o Reference: [this document] 2330 Specific ACE-OAuth profiles can use this common resource type for 2331 defining their profile-specific discovery processes. 2333 8.3. OAuth Extensions Error Registration 2335 This specification registers the following error values in the OAuth 2336 Extensions Error registry [IANA.OAuthExtensionsErrorRegistry]. 2338 o Error name: "unsupported_pop_key" 2339 o Error usage location: token error response 2340 o Related protocol extension: [this document] 2341 o Change Controller: IESG 2342 o Specification document(s): Section 5.8.3 of [this document] 2344 o Error name: "incompatible_ace_profiles" 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 8.4. OAuth Error Code CBOR Mappings Registry 2352 This specification establishes the IANA "OAuth Error Code CBOR 2353 Mappings" registry. The registry has been created to use the "Expert 2354 Review" registration procedure [RFC8126], except for the value range 2355 designated for private use. 2357 The columns of the registry are: 2359 Name The OAuth Error Code name, refers to the name in Section 5.2. 2360 of [RFC6749], e.g., "invalid_request". 2361 CBOR Value CBOR abbreviation for this error code. Integer values 2362 less than -65536 are marked as "Private Use", all other values use 2363 the registration policy "Expert Review" [RFC8126]. 2364 Reference This contains a pointer to the public specification of the 2365 error code abbreviation, if one exists. 2367 This registry will be initially populated by the values in Figure 10. 2368 The Reference column for all of these entries will be this document. 2370 8.5. OAuth Grant Type CBOR Mappings 2372 This specification establishes the IANA "OAuth Grant Type CBOR 2373 Mappings" registry. The registry has been created to use the "Expert 2374 Review" registration procedure [RFC8126], except for the value range 2375 designated for private use. 2377 The columns of this registry are: 2379 Name The name of the grant type as specified in Section 1.3 of 2380 [RFC6749]. 2381 CBOR Value CBOR abbreviation for this grant type. Integer values 2382 less than -65536 are marked as "Private Use", all other values use 2383 the registration policy "Expert Review" [RFC8126]. 2384 Reference This contains a pointer to the public specification of the 2385 grant type abbreviation, if one exists. 2386 Original Specification This contains a pointer to the public 2387 specification of the grant type, if one exists. 2389 This registry will be initially populated by the values in Figure 11. 2390 The Reference column for all of these entries will be this document. 2392 8.6. OAuth Access Token Types 2394 This section registers the following new token type in the "OAuth 2395 Access Token Types" registry [IANA.OAuthAccessTokenTypes]. 2397 o Type name: "PoP" 2398 o Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see 2399 section 3.3 of [I-D.ietf-ace-oauth-params]. 2400 o HTTP Authentication Scheme(s): N/A 2401 o Change Controller: IETF 2402 o Specification document(s): [this document] 2404 8.7. OAuth Access Token Type CBOR Mappings 2406 This specification established the IANA "OAuth Access Token Type CBOR 2407 Mappings" registry. The registry has been created to use the "Expert 2408 Review" registration procedure [RFC8126], except for the value range 2409 designated for private use. 2411 The columns of this registry are: 2413 Name The name of token type as registered in the OAuth Access Token 2414 Types registry, e.g., "Bearer". 2416 CBOR Value CBOR abbreviation for this token type. Integer values 2417 less than -65536 are marked as "Private Use", all other values use 2418 the registration policy "Expert Review" [RFC8126]. 2419 Reference This contains a pointer to the public specification of the 2420 OAuth token type abbreviation, if one exists. 2421 Original Specification This contains a pointer to the public 2422 specification of the OAuth token type, if one exists. 2424 8.7.1. Initial Registry Contents 2426 o Name: "Bearer" 2427 o Value: 1 2428 o Reference: [this document] 2429 o Original Specification: [RFC6749] 2431 o Name: "PoP" 2432 o Value: 2 2433 o Reference: [this document] 2434 o Original Specification: [this document] 2436 8.8. ACE Profile Registry 2438 This specification establishes the IANA "ACE Profile" registry. The 2439 registry has been created to use the "Expert Review" registration 2440 procedure [RFC8126]. It should be noted that, in addition to the 2441 expert review, some portions of the registry require a specification, 2442 potentially a Standards Track RFC, be supplied as well. 2444 The columns of this registry are: 2446 Name The name of the profile, to be used as value of the profile 2447 attribute. 2448 Description Text giving an overview of the profile and the context 2449 it is developed for. 2450 CBOR Value CBOR abbreviation for this profile name. Different 2451 ranges of values use different registration policies [RFC8126]. 2452 Integer values from -256 to 255 are designated as Standards 2453 Action. Integer values from -65536 to -257 and from 256 to 65535 2454 are designated as Specification Required. Integer values greater 2455 than 65535 are designated as "Expert Review". Integer values less 2456 than -65536 are marked as Private Use. 2457 Reference This contains a pointer to the public specification of the 2458 profile abbreviation, if one exists. 2460 This registry will be initially empty and will be populated by the 2461 registrations from the ACE framework profiles. 2463 8.9. OAuth Parameter Registration 2465 This specification registers the following parameter in the "OAuth 2466 Parameters" registry [IANA.OAuthParameters]: 2468 o Name: "ace_profile" 2469 o Parameter Usage Location: token response 2470 o Change Controller: IESG 2471 o Reference: Section 5.8.2 and Section 5.8.4.3 of [this document] 2473 8.10. OAuth Parameters CBOR Mappings Registry 2475 This specification establishes the IANA "OAuth Parameters CBOR 2476 Mappings" registry. The registry has been created to use the "Expert 2477 Review" registration procedure [RFC8126], except for the value range 2478 designated for private use. 2480 The columns of this registry are: 2482 Name The OAuth Parameter name, refers to the name in the OAuth 2483 parameter registry, e.g., "client_id". 2484 CBOR Key CBOR map key for this parameter. Integer values less than 2485 -65536 are marked as "Private Use", all other values use the 2486 registration policy "Expert Review" [RFC8126]. 2487 Value Type The allowable CBOR data types for values of this 2488 parameter. 2489 Reference This contains a pointer to the public specification of the 2490 OAuth parameter abbreviation, if one exists. 2492 This registry will be initially populated by the values in Figure 12. 2493 The Reference column for all of these entries will be this document. 2495 8.11. OAuth Introspection Response Parameter Registration 2497 This specification registers the following parameters in the OAuth 2498 Token Introspection Response registry 2499 [IANA.TokenIntrospectionResponse]. 2501 o Name: "ace_profile" 2502 o Description: The ACE profile used between client and RS. 2503 o Change Controller: IESG 2504 o Reference: Section 5.9.2 of [this document] 2506 o Name: "cnonce" 2507 o Description: "client-nonce". A nonce previously provided to the 2508 AS by the RS via the client. Used to verify token freshness when 2509 the RS cannot synchronize its clock with the AS. 2510 o Change Controller: IESG 2511 o Reference: Section 5.9.2 of [this document] 2513 o Name: "exi" 2514 o Description: "Expires in". Lifetime of the token in seconds from 2515 the time the RS first sees it. Used to implement a weaker from of 2516 token expiration for devices that cannot synchronize their 2517 internal clocks. 2518 o Change Controller: IESG 2519 o Reference: Section 5.9.2 of [this document] 2521 8.12. OAuth Token Introspection Response CBOR Mappings Registry 2523 This specification establishes the IANA "OAuth Token Introspection 2524 Response CBOR Mappings" registry. The registry has been created to 2525 use the "Expert Review" registration procedure [RFC8126], except for 2526 the value range designated for private use. 2528 The columns of this registry are: 2530 Name The OAuth Parameter name, refers to the name in the OAuth 2531 parameter registry, e.g., "client_id". 2532 CBOR Key CBOR map key for this parameter. Integer values less than 2533 -65536 are marked as "Private Use", all other values use the 2534 registration policy "Expert Review" [RFC8126]. 2535 Value Type The allowable CBOR data types for values of this 2536 parameter. 2537 Reference This contains a pointer to the public specification of the 2538 introspection response parameter abbreviation, if one exists. 2540 This registry will be initially populated by the values in Figure 16. 2541 The Reference column for all of these entries will be this document. 2543 Note that the mappings of parameters corresponding to claim names 2544 intentionally coincide with the CWT claim name mappings from 2545 [RFC8392]. 2547 8.13. JSON Web Token Claims 2549 This specification registers the following new claims in the JSON Web 2550 Token (JWT) registry of JSON Web Token Claims 2551 [IANA.JsonWebTokenClaims]: 2553 o Claim Name: "ace_profile" 2554 o Claim Description: The ACE profile a token is supposed to be used 2555 with. 2556 o Change Controller: IESG 2557 o Reference: Section 5.10 of [this document] 2558 o Claim Name: "cnonce" 2559 o Claim Description: "client-nonce". A nonce previously provided to 2560 the AS by the RS via the client. Used to verify token freshness 2561 when the RS cannot synchronize its clock with the AS. 2562 o Change Controller: IESG 2563 o Reference: Section 5.10 of [this document] 2565 o Claim Name: "exi" 2566 o Claim Description: "Expires in". Lifetime of the token in seconds 2567 from the time the RS first sees it. Used to implement a weaker 2568 from of token expiration for devices that cannot synchronize their 2569 internal clocks. 2570 o Change Controller: IESG 2571 o Reference: Section 5.10.3 of [this document] 2573 8.14. CBOR Web Token Claims 2575 This specification registers the following new claims in the "CBOR 2576 Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims]. 2578 o Claim Name: "ace_profile" 2579 o Claim Description: The ACE profile a token is supposed to be used 2580 with. 2581 o JWT Claim Name: ace_profile 2582 o Claim Key: TBD (suggested: 38) 2583 o Claim Value Type(s): integer 2584 o Change Controller: IESG 2585 o Specification Document(s): Section 5.10 of [this document] 2587 o Claim Name: "cnonce" 2588 o Claim Description: The client-nonce sent to the AS by the RS via 2589 the client. 2590 o JWT Claim Name: cnonce 2591 o Claim Key: TBD (suggested: 39) 2592 o Claim Value Type(s): byte string 2593 o Change Controller: IESG 2594 o Specification Document(s): Section 5.10 of [this document] 2596 o Claim Name: "exi" 2597 o Claim Description: The expiration time of a token measured from 2598 when it was received at the RS in seconds. 2599 o JWT Claim Name: exi 2600 o Claim Key: TBD (suggested: 40) 2601 o Claim Value Type(s): integer 2602 o Change Controller: IESG 2603 o Specification Document(s): Section 5.10.3 of [this document] 2605 o Claim Name: "scope" 2606 o Claim Description: The scope of an access token as defined in 2607 [RFC6749]. 2608 o JWT Claim Name: scope 2609 o Claim Key: TBD (suggested: 9) 2610 o Claim Value Type(s): byte string or text string 2611 o Change Controller: IESG 2612 o Specification Document(s): Section 4.2 of [RFC8693] 2614 8.15. Media Type Registrations 2616 This specification registers the 'application/ace+cbor' media type 2617 for messages of the protocols defined in this document carrying 2618 parameters encoded in CBOR. This registration follows the procedures 2619 specified in [RFC6838]. 2621 Type name: application 2623 Subtype name: ace+cbor 2625 Required parameters: N/A 2627 Optional parameters: N/A 2629 Encoding considerations: Must be encoded as CBOR map containing the 2630 protocol parameters defined in [this document]. 2632 Security considerations: See Section 6 of [this document] 2634 Interoperability considerations: N/A 2636 Published specification: [this document] 2638 Applications that use this media type: The type is used by 2639 authorization servers, clients and resource servers that support the 2640 ACE framework with CBOR encoding as specified in [this document]. 2642 Fragment identifier considerations: N/A 2644 Additional information: N/A 2646 Person & email address to contact for further information: 2647 2649 Intended usage: COMMON 2651 Restrictions on usage: none 2653 Author: Ludwig Seitz 2654 Change controller: IESG 2656 8.16. CoAP Content-Format Registry 2658 This specification registers the following entry to the "CoAP 2659 Content-Formats" registry: 2661 Media Type: application/ace+cbor 2663 Encoding: - 2665 ID: TBD (suggested: 19) 2667 Reference: [this document] 2669 8.17. Expert Review Instructions 2671 All of the IANA registries established in this document are defined 2672 to use a registration policy of Expert Review. This section gives 2673 some general guidelines for what the experts should be looking for, 2674 but they are being designated as experts for a reason, so they should 2675 be given substantial latitude. 2677 Expert reviewers should take into consideration the following points: 2679 o Point squatting should be discouraged. Reviewers are encouraged 2680 to get sufficient information for registration requests to ensure 2681 that the usage is not going to duplicate one that is already 2682 registered, and that the point is likely to be used in 2683 deployments. The zones tagged as private use are intended for 2684 testing purposes and closed environments; code points in other 2685 ranges should not be assigned for testing. 2686 o Specifications are needed for the first-come, first-serve range if 2687 they are expected to be used outside of closed environments in an 2688 interoperable way. When specifications are not provided, the 2689 description provided needs to have sufficient information to 2690 identify what the point is being used for. 2691 o Experts should take into account the expected usage of fields when 2692 approving point assignment. The fact that there is a range for 2693 standards track documents does not mean that a standards track 2694 document cannot have points assigned outside of that range. The 2695 length of the encoded value should be weighed against how many 2696 code points of that length are left, the size of device it will be 2697 used on. 2698 o Since a high degree of overlap is expected between these 2699 registries and the contents of the OAuth parameters 2700 [IANA.OAuthParameters] registries, experts should require new 2701 registrations to maintain alignment with parameters from OAuth 2702 that have comparable functionality. Deviation from this alignment 2703 should only be allowed if there are functional differences, that 2704 are motivated by the use case and that cannot be easily or 2705 efficiently addressed by comparable OAuth parameters. 2707 9. Acknowledgments 2709 This document is a product of the ACE working group of the IETF. 2711 Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and 2712 UMA in IoT scenarios, Robert Taylor for his discussion input, and 2713 Malisa Vucinic for his input on the predecessors of this proposal. 2715 Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from 2716 where large parts of the security considerations where copied. 2718 Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for 2719 contributing their work on AS discovery from draft-gerdes-ace-dcaf- 2720 authorize (see Section 5.1). 2722 Thanks to Jim Schaad and Mike Jones for their comprehensive reviews. 2724 Thanks to Benjamin Kaduk for his input on various questions related 2725 to this work. 2727 Thanks to Cigdem Sengul for some very useful review comments. 2729 Thanks to Carsten Bormann for contributing the text for the CoRE 2730 Resource Type registry. 2732 Thanks to Roman Danyliw for suggesting the Appendix E (including its 2733 contents). 2735 Ludwig Seitz and Goeran Selander worked on this document as part of 2736 the CelticPlus project CyberWI, with funding from Vinnova. Ludwig 2737 Seitz was also received further funding for this work by Vinnova in 2738 the context of the CelticNext project Critisec. 2740 10. References 2742 10.1. Normative References 2744 [I-D.ietf-ace-oauth-params] 2745 Seitz, L., "Additional OAuth Parameters for Authorization 2746 in Constrained Environments (ACE)", draft-ietf-ace-oauth- 2747 params-14 (work in progress), March 2021. 2749 [IANA.CborWebTokenClaims] 2750 IANA, "CBOR Web Token (CWT) Claims", 2751 . 2754 [IANA.CoreParameters] 2755 IANA, "Constrained RESTful Environments (CoRE) 2756 Parameters", . 2759 [IANA.JsonWebTokenClaims] 2760 IANA, "JSON Web Token Claims", 2761 . 2763 [IANA.OAuthAccessTokenTypes] 2764 IANA, "OAuth Access Token Types", 2765 . 2768 [IANA.OAuthExtensionsErrorRegistry] 2769 IANA, "OAuth Extensions Error Registry", 2770 . 2773 [IANA.OAuthParameters] 2774 IANA, "OAuth Parameters", 2775 . 2778 [IANA.TokenIntrospectionResponse] 2779 IANA, "OAuth Token Introspection Response", 2780 . 2783 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2784 Requirement Levels", BCP 14, RFC 2119, 2785 DOI 10.17487/RFC2119, March 1997, 2786 . 2788 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2789 Resource Identifier (URI): Generic Syntax", STD 66, 2790 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2791 . 2793 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 2794 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 2795 January 2012, . 2797 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 2798 RFC 6749, DOI 10.17487/RFC6749, October 2012, 2799 . 2801 [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization 2802 Framework: Bearer Token Usage", RFC 6750, 2803 DOI 10.17487/RFC6750, October 2012, 2804 . 2806 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2807 Specifications and Registration Procedures", BCP 13, 2808 RFC 6838, DOI 10.17487/RFC6838, January 2013, 2809 . 2811 [RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B., 2812 Keranen, A., and P. Hallam-Baker, "Naming Things with 2813 Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013, 2814 . 2816 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 2817 Application Protocol (CoAP)", RFC 7252, 2818 DOI 10.17487/RFC7252, June 2014, 2819 . 2821 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 2822 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 2823 . 2825 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 2826 RFC 7662, DOI 10.17487/RFC7662, October 2015, 2827 . 2829 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2830 Writing an IANA Considerations Section in RFCs", BCP 26, 2831 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2832 . 2834 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 2835 RFC 8152, DOI 10.17487/RFC8152, July 2017, 2836 . 2838 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2839 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2840 May 2017, . 2842 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 2843 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 2844 May 2018, . 2846 [RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J., 2847 and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693, 2848 DOI 10.17487/RFC8693, January 2020, 2849 . 2851 [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 2852 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 2853 Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 2854 2020, . 2856 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object 2857 Representation (CBOR)", STD 94, RFC 8949, 2858 DOI 10.17487/RFC8949, December 2020, 2859 . 2861 10.2. Informative References 2863 [BLE] Bluetooth SIG, "Bluetooth Core Specification v5.1", 2864 Section 4.4, January 2019, 2865 . 2868 [I-D.erdtman-ace-rpcc] 2869 Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared- 2870 Key as OAuth client credentials", draft-erdtman-ace- 2871 rpcc-02 (work in progress), October 2017. 2873 [I-D.ietf-ace-dtls-authorize] 2874 Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and 2875 L. Seitz, "Datagram Transport Layer Security (DTLS) 2876 Profile for Authentication and Authorization for 2877 Constrained Environments (ACE)", draft-ietf-ace-dtls- 2878 authorize-16 (work in progress), March 2021. 2880 [I-D.ietf-ace-oscore-profile] 2881 Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson, 2882 "OSCORE Profile of the Authentication and Authorization 2883 for Constrained Environments Framework", draft-ietf-ace- 2884 oscore-profile-18 (work in progress), April 2021. 2886 [I-D.ietf-quic-transport] 2887 Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed 2888 and Secure Transport", draft-ietf-quic-transport-34 (work 2889 in progress), January 2021. 2891 [I-D.ietf-tls-dtls13] 2892 Rescorla, E., Tschofenig, H., and N. Modadugu, "The 2893 Datagram Transport Layer Security (DTLS) Protocol Version 2894 1.3", draft-ietf-tls-dtls13-41 (work in progress), 2895 February 2021. 2897 [Margi10impact] 2898 Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr, 2899 M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold, 2900 "Impact of Operating Systems on Wireless Sensor Networks 2901 (Security) Applications and Testbeds", Proceedings of 2902 the 19th International Conference on Computer 2903 Communications and Networks (ICCCN), August 2010. 2905 [MQTT5.0] Banks, A., Briggs, E., Borgendale, K., and R. Gupta, "MQTT 2906 Version 5.0", OASIS Standard, March 2019, 2907 . 2910 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 2911 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 2912 . 2914 [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link 2915 Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, 2916 . 2918 [RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0 2919 Threat Model and Security Considerations", RFC 6819, 2920 DOI 10.17487/RFC6819, January 2013, 2921 . 2923 [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth 2924 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, 2925 August 2013, . 2927 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 2928 Constrained-Node Networks", RFC 7228, 2929 DOI 10.17487/RFC7228, May 2014, 2930 . 2932 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 2933 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 2934 DOI 10.17487/RFC7231, June 2014, 2935 . 2937 [RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland, 2938 "Assertion Framework for OAuth 2.0 Client Authentication 2939 and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521, 2940 May 2015, . 2942 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 2943 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 2944 DOI 10.17487/RFC7540, May 2015, 2945 . 2947 [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and 2948 P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", 2949 RFC 7591, DOI 10.17487/RFC7591, July 2015, 2950 . 2952 [RFC7641] Hartke, K., "Observing Resources in the Constrained 2953 Application Protocol (CoAP)", RFC 7641, 2954 DOI 10.17487/RFC7641, September 2015, 2955 . 2957 [RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M., 2958 and S. Kumar, "Use Cases for Authentication and 2959 Authorization in Constrained Environments", RFC 7744, 2960 DOI 10.17487/RFC7744, January 2016, 2961 . 2963 [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 2964 the Constrained Application Protocol (CoAP)", RFC 7959, 2965 DOI 10.17487/RFC7959, August 2016, 2966 . 2968 [RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", 2969 BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017, 2970 . 2972 [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data 2973 Interchange Format", STD 90, RFC 8259, 2974 DOI 10.17487/RFC8259, December 2017, 2975 . 2977 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 2978 Authorization Server Metadata", RFC 8414, 2979 DOI 10.17487/RFC8414, June 2018, 2980 . 2982 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 2983 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 2984 . 2986 [RFC8516] Keranen, A., ""Too Many Requests" Response Code for the 2987 Constrained Application Protocol", RFC 8516, 2988 DOI 10.17487/RFC8516, January 2019, 2989 . 2991 [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 2992 "Object Security for Constrained RESTful Environments 2993 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 2994 . 2996 [RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig, 2997 "OAuth 2.0 Device Authorization Grant", RFC 8628, 2998 DOI 10.17487/RFC8628, August 2019, 2999 . 3001 Appendix A. Design Justification 3003 This section provides further insight into the design decisions of 3004 the solution documented in this document. Section 3 lists several 3005 building blocks and briefly summarizes their importance. The 3006 justification for offering some of those building blocks, as opposed 3007 to using OAuth 2.0 as is, is given below. 3009 Common IoT constraints are: 3011 Low Power Radio: 3013 Many IoT devices are equipped with a small battery which needs to 3014 last for a long time. For many constrained wireless devices, the 3015 highest energy cost is associated to transmitting or receiving 3016 messages (roughly by a factor of 10 compared to AES) 3017 [Margi10impact]. It is therefore important to keep the total 3018 communication overhead low, including minimizing the number and 3019 size of messages sent and received, which has an impact of choice 3020 on the message format and protocol. By using CoAP over UDP and 3021 CBOR encoded messages, some of these aspects are addressed. 3022 Security protocols contribute to the communication overhead and 3023 can, in some cases, be optimized. For example, authentication and 3024 key establishment may, in certain cases where security 3025 requirements allow, be replaced by provisioning of security 3026 context by a trusted third party, using transport or application- 3027 layer security. 3029 Low CPU Speed: 3031 Some IoT devices are equipped with processors that are 3032 significantly slower than those found in most current devices on 3033 the Internet. This typically has implications on what timely 3034 cryptographic operations a device is capable of performing, which 3035 in turn impacts, e.g., protocol latency. Symmetric key 3036 cryptography may be used instead of the computationally more 3037 expensive public key cryptography where the security requirements 3038 so allow, but this may also require support for trusted-third- 3039 party-assisted secret key establishment using transport- or 3040 application-layer security. 3041 Small Amount of Memory: 3043 Microcontrollers embedded in IoT devices are often equipped with 3044 only a small amount of RAM and flash memory, which places 3045 limitations on what kind of processing can be performed and how 3046 much code can be put on those devices. To reduce code size, fewer 3047 and smaller protocol implementations can be put on the firmware of 3048 such a device. In this case, CoAP may be used instead of HTTP, 3049 symmetric-key cryptography instead of public-key cryptography, and 3050 CBOR instead of JSON. An authentication and key establishment 3051 protocol, e.g., the DTLS handshake, in comparison with assisted 3052 key establishment, also has an impact on memory and code 3053 footprints. 3055 User Interface Limitations: 3057 Protecting access to resources is both an important security as 3058 well as privacy feature. End users and enterprise customers may 3059 not want to give access to the data collected by their IoT device 3060 or to functions it may offer to third parties. Since the 3061 classical approach of requesting permissions from end users via a 3062 rich user interface does not work in many IoT deployment 3063 scenarios, these functions need to be delegated to user-controlled 3064 devices that are better suitable for such tasks, such as smart 3065 phones and tablets. 3067 Communication Constraints: 3069 In certain constrained settings an IoT device may not be able to 3070 communicate with a given device at all times. Devices may be 3071 sleeping, or just disconnected from the Internet because of 3072 general lack of connectivity in the area, for cost reasons, or for 3073 security reasons, e.g., to avoid an entry point for Denial-of- 3074 Service attacks. 3076 The communication interactions this framework builds upon (as 3077 shown graphically in Figure 1) may be accomplished using a variety 3078 of different protocols, and not all parts of the message flow are 3079 used in all applications due to the communication constraints. 3080 Deployments making use of CoAP are expected, but this framework is 3081 not limited to them. Other protocols such as HTTP, or even 3082 protocols such as Bluetooth Smart communication that do not 3083 necessarily use IP, could also be used. The latter raises the 3084 need for application-layer security over the various interfaces. 3086 In the light of these constraints we have made the following design 3087 decisions: 3089 CBOR, COSE, CWT: 3091 When using this framework, it is RECOMMENDED to use CBOR [RFC8949] 3092 as data format. Where CBOR data needs to be protected, the use of 3093 COSE [RFC8152] is RECOMMENDED. Furthermore, where self-contained 3094 tokens are needed, it is RECOMMENDED to use of CWT [RFC8392]. 3095 These measures aim at reducing the size of messages sent over the 3096 wire, the RAM size of data objects that need to be kept in memory 3097 and the size of libraries that devices need to support. 3099 CoAP: 3101 When using this framework, it is RECOMMENDED to use of CoAP 3102 [RFC7252] instead of HTTP. This does not preclude the use of 3103 other protocols specifically aimed at constrained devices, like, 3104 e.g., Bluetooth Low Energy (see Section 3.2). This aims again at 3105 reducing the size of messages sent over the wire, the RAM size of 3106 data objects that need to be kept in memory and the size of 3107 libraries that devices need to support. 3109 Access Information: 3111 This framework defines the name "Access Information" for data 3112 concerning the RS that the AS returns to the client in an access 3113 token response (see Section 5.8.2). This aims at enabling 3114 scenarios where a powerful client, supporting multiple profiles, 3115 needs to interact with an RS for which it does not know the 3116 supported profiles and the raw public key. 3118 Proof-of-Possession: 3120 This framework makes use of proof-of-possession tokens, using the 3121 "cnf" claim [RFC8747]. A request parameter "cnf" and a Response 3122 parameter "cnf", both having a value space semantically and 3123 syntactically identical to the "cnf" claim, are defined for the 3124 token endpoint, to allow requesting and stating confirmation keys. 3125 This aims at making token theft harder. Token theft is 3126 specifically relevant in constrained use cases, as communication 3127 often passes through middle-boxes, which could be able to steal 3128 bearer tokens and use them to gain unauthorized access. 3130 Authz-Info endpoint: 3132 This framework introduces a new way of providing access tokens to 3133 an RS by exposing a authz-info endpoint, to which access tokens 3134 can be POSTed. This aims at reducing the size of the request 3135 message and the code complexity at the RS. The size of the 3136 request message is problematic, since many constrained protocols 3137 have severe message size limitations at the physical layer (e.g., 3138 in the order of 100 bytes). This means that larger packets get 3139 fragmented, which in turn combines badly with the high rate of 3140 packet loss, and the need to retransmit the whole message if one 3141 packet gets lost. Thus separating sending of the request and 3142 sending of the access tokens helps to reduce fragmentation. 3144 Client Credentials Grant: 3146 In this framework the use of the client credentials grant is 3147 RECOMMENDED for machine-to-machine communication use cases, where 3148 manual intervention of the resource owner to produce a grant token 3149 is not feasible. The intention is that the resource owner would 3150 instead pre-arrange authorization with the AS, based on the 3151 client's own credentials. The client can then (without manual 3152 intervention) obtain access tokens from the AS. 3154 Introspection: 3156 In this framework the use of access token introspection is 3157 RECOMMENDED in cases where the client is constrained in a way that 3158 it can not easily obtain new access tokens (i.e. it has 3159 connectivity issues that prevent it from communicating with the 3160 AS). In that case it is RECOMMENDED to use a long-term token, 3161 that could be a simple reference. The RS is assumed to be able to 3162 communicate with the AS, and can therefore perform introspection, 3163 in order to learn the claims associated with the token reference. 3164 The advantage of such an approach is that the resource owner can 3165 change the claims associated to the token reference without having 3166 to be in contact with the client, thus granting or revoking access 3167 rights. 3169 Appendix B. Roles and Responsibilities 3171 Resource Owner 3173 * Make sure that the RS is registered at the AS. This includes 3174 making known to the AS which profiles, token_type, scopes, and 3175 key types (symmetric/asymmetric) the RS supports. Also making 3176 it known to the AS which audience(s) the RS identifies itself 3177 with. 3178 * Make sure that clients can discover the AS that is in charge of 3179 the RS. 3180 * If the client-credentials grant is used, make sure that the AS 3181 has the necessary, up-to-date, access control policies for the 3182 RS. 3184 Requesting Party 3186 * Make sure that the client is provisioned the necessary 3187 credentials to authenticate to the AS. 3188 * Make sure that the client is configured to follow the security 3189 requirements of the Requesting Party when issuing requests 3190 (e.g., minimum communication security requirements, trust 3191 anchors). 3192 * Register the client at the AS. This includes making known to 3193 the AS which profiles, token_types, and key types (symmetric/ 3194 asymmetric) the client. 3196 Authorization Server 3198 * Register the RS and manage corresponding security contexts. 3199 * Register clients and authentication credentials. 3200 * Allow Resource Owners to configure and update access control 3201 policies related to their registered RSs. 3202 * Expose the token endpoint to allow clients to request tokens. 3203 * Authenticate clients that wish to request a token. 3204 * Process a token request using the authorization policies 3205 configured for the RS. 3206 * Optionally: Expose the introspection endpoint that allows RS's 3207 to submit token introspection requests. 3208 * If providing an introspection endpoint: Authenticate RSs that 3209 wish to get an introspection response. 3210 * If providing an introspection endpoint: Process token 3211 introspection requests. 3212 * Optionally: Handle token revocation. 3213 * Optionally: Provide discovery metadata. See [RFC8414] 3214 * Optionally: Handle refresh tokens. 3216 Client 3218 * Discover the AS in charge of the RS that is to be targeted with 3219 a request. 3220 * Submit the token request (see step (A) of Figure 1). 3222 + Authenticate to the AS. 3224 + Optionally (if not pre-configured): Specify which RS, which 3225 resource(s), and which action(s) the request(s) will target. 3226 + If raw public keys (rpk) or certificates are used, make sure 3227 the AS has the right rpk or certificate for this client. 3228 * Process the access token and Access Information (see step (B) 3229 of Figure 1). 3231 + Check that the Access Information provides the necessary 3232 security parameters (e.g., PoP key, information on 3233 communication security protocols supported by the RS). 3234 + Safely store the proof-of-possession key. 3235 + If provided by the AS: Safely store the refresh token. 3236 * Send the token and request to the RS (see step (C) of 3237 Figure 1). 3239 + Authenticate towards the RS (this could coincide with the 3240 proof of possession process). 3241 + Transmit the token as specified by the AS (default is to the 3242 authz-info endpoint, alternative options are specified by 3243 profiles). 3244 + Perform the proof-of-possession procedure as specified by 3245 the profile in use (this may already have been taken care of 3246 through the authentication procedure). 3247 * Process the RS response (see step (F) of Figure 1) of the RS. 3249 Resource Server 3251 * Expose a way to submit access tokens. By default this is the 3252 authz-info endpoint. 3253 * Process an access token. 3255 + Verify the token is from a recognized AS. 3256 + Check the token's integrity. 3257 + Verify that the token applies to this RS. 3258 + Check that the token has not expired (if the token provides 3259 expiration information). 3260 + Store the token so that it can be retrieved in the context 3261 of a matching request. 3263 Note: The order proposed here is not normative, any process 3264 that arrives at an equivalent result can be used. A noteworthy 3265 consideration is whether one can use cheap operations early on 3266 to quickly discard non-applicable or invalid tokens, before 3267 performing expensive cryptographic operations (e.g. doing an 3268 expiration check before verifying a signature). 3270 * Process a request. 3272 + Set up communication security with the client. 3273 + Authenticate the client. 3274 + Match the client against existing tokens. 3275 + Check that tokens belonging to the client actually authorize 3276 the requested action. 3277 + Optionally: Check that the matching tokens are still valid, 3278 using introspection (if this is possible.) 3279 * Send a response following the agreed upon communication 3280 security mechanism(s). 3281 * Safely store credentials such as raw public keys for 3282 authentication or proof-of-possession keys linked to access 3283 tokens. 3285 Appendix C. Requirements on Profiles 3287 This section lists the requirements on profiles of this framework, 3288 for the convenience of profile designers. 3290 o Optionally define new methods for the client to discover the 3291 necessary permissions and AS for accessing a resource, different 3292 from the one proposed in Section 5.1. Section 4 3293 o Optionally specify new grant types. Section 5.4 3294 o Optionally define the use of client certificates as client 3295 credential type. Section 5.5 3296 o Specify the communication protocol the client and RS the must use 3297 (e.g., CoAP). Section 5 and Section 5.8.4.3 3298 o Specify the security protocol the client and RS must use to 3299 protect their communication (e.g., OSCORE or DTLS). This must 3300 provide encryption, integrity and replay protection. 3301 Section 5.8.4.3 3302 o Specify how the client and the RS mutually authenticate. 3303 Section 4 3304 o Specify the proof-of-possession protocol(s) and how to select one, 3305 if several are available. Also specify which key types (e.g., 3306 symmetric/asymmetric) are supported by a specific proof-of- 3307 possession protocol. Section 5.8.4.2 3308 o Specify a unique ace_profile identifier. Section 5.8.4.3 3309 o If introspection is supported: Specify the communication and 3310 security protocol for introspection. Section 5.9 3311 o Specify the communication and security protocol for interactions 3312 between client and AS. This must provide encryption, integrity 3313 protection, replay protection and a binding between requests and 3314 responses. Section 5 and Section 5.8 3315 o Specify how/if the authz-info endpoint is protected, including how 3316 error responses are protected. Section 5.10.1 3317 o Optionally define other methods of token transport than the authz- 3318 info endpoint. Section 5.10.1 3320 Appendix D. Assumptions on AS Knowledge about C and RS 3322 This section lists the assumptions on what an AS should know about a 3323 client and an RS in order to be able to respond to requests to the 3324 token and introspection endpoints. How this information is 3325 established is out of scope for this document. 3327 o The identifier of the client or RS. 3328 o The profiles that the client or RS supports. 3329 o The scopes that the RS supports. 3330 o The audiences that the RS identifies with. 3331 o The key types (e.g., pre-shared symmetric key, raw public key, key 3332 length, other key parameters) that the client or RS supports. 3333 o The types of access tokens the RS supports (e.g., CWT). 3334 o If the RS supports CWTs, the COSE parameters for the crypto 3335 wrapper (e.g., algorithm, key-wrap algorithm, key-length) that the 3336 RS supports. 3337 o The expiration time for access tokens issued to this RS (unless 3338 the RS accepts a default time chosen by the AS). 3339 o The symmetric key shared between client and AS (if any). 3340 o The symmetric key shared between RS and AS (if any). 3341 o The raw public key of the client or RS (if any). 3342 o Whether the RS has synchronized time (and thus is able to use the 3343 'exp' claim) or not. 3345 Appendix E. Differences to OAuth 2.0 3347 This document adapts OAuth 2.0 to be suitable for constrained 3348 environments. This sections lists the main differences from the 3349 normative requirements of OAuth 2.0. 3351 o Use of TLS -- OAuth 2.0 requires the use of TLS both to protect 3352 the communication between AS and client when requesting an access 3353 token; between client and RS when accessing a resource and between 3354 AS and RS if introspection is used. This framework requires 3355 similar security properties, but does not require that they be 3356 realized with TLS. See Section 5. 3357 o Cardinality of "grant_type" parameter -- In client-to-AS requests 3358 using OAuth 2.0, the "grant_type" parameter is required (per 3359 [RFC6749]). In this framework, this parameter is optional. See 3360 Section 5.8.1. 3361 o Encoding of "scope" parameter -- In client-to-AS requests using 3362 OAuth 2.0, the "scope" parameter is string encoded (per 3363 [RFC6749]). In this framework, this parameter may also be encoded 3364 as a byte string. See Section 5.8.1. 3365 o Cardinality of "token_type" parameter -- in AS-to-client responses 3366 using OAuth 2.0, the token_type parameter is required (per 3368 [RFC6749]). In this framework, this parameter is optional. See 3369 Section 5.8.2. 3370 o Access token retention -- in OAuth 2.0, the access token is sent 3371 with each request to the RS. In this framework, the RS must be 3372 able to store these tokens for later use. See Section 5.10.1. 3374 Appendix F. Deployment Examples 3376 There is a large variety of IoT deployments, as is indicated in 3377 Appendix A, and this section highlights a few common variants. This 3378 section is not normative but illustrates how the framework can be 3379 applied. 3381 For each of the deployment variants, there are a number of possible 3382 security setups between clients, resource servers and authorization 3383 servers. The main focus in the following subsections is on how 3384 authorization of a client request for a resource hosted by an RS is 3385 performed. This requires the security of the requests and responses 3386 between the clients and the RS to be considered. 3388 Note: CBOR diagnostic notation is used for examples of requests and 3389 responses. 3391 F.1. Local Token Validation 3393 In this scenario, the case where the resource server is offline is 3394 considered, i.e., it is not connected to the AS at the time of the 3395 access request. This access procedure involves steps A, B, C, and F 3396 of Figure 1. 3398 Since the resource server must be able to verify the access token 3399 locally, self-contained access tokens must be used. 3401 This example shows the interactions between a client, the 3402 authorization server and a temperature sensor acting as a resource 3403 server. Message exchanges A and B are shown in Figure 17. 3405 A: The client first generates a public-private key pair used for 3406 communication security with the RS. 3407 The client sends a CoAP POST request to the token endpoint at the 3408 AS. The security of this request can be transport or application 3409 layer. It is up the communication security profile to define. In 3410 the example it is assumed that both client and AS have performed 3411 mutual authentication e.g. via DTLS. The request contains the 3412 public key of the client and the Audience parameter set to 3413 "tempSensorInLivingRoom", a value that the temperature sensor 3414 identifies itself with. The AS evaluates the request and 3415 authorizes the client to access the resource. 3417 B: The AS responds with a 2.05 Content response containing the 3418 Access Information, including the access token. The PoP access 3419 token contains the public key of the client, and the Access 3420 Information contains the public key of the RS. For communication 3421 security this example uses DTLS RawPublicKey between the client 3422 and the RS. The issued token will have a short validity time, 3423 i.e., "exp" close to "iat", in order to mitigate attacks using 3424 stolen client credentials. The token includes the claim such as 3425 "scope" with the authorized access that an owner of the 3426 temperature device can enjoy. In this example, the "scope" claim, 3427 issued by the AS, informs the RS that the owner of the token, that 3428 can prove the possession of a key is authorized to make a GET 3429 request against the /temperature resource and a POST request on 3430 the /firmware resource. Note that the syntax and semantics of the 3431 scope claim are application specific. 3432 Note: In this example it is assumed that the client knows what 3433 resource it wants to access, and is therefore able to request 3434 specific audience and scope claims for the access token. 3436 Authorization 3437 Client Server 3438 | | 3439 |<=======>| DTLS Connection Establishment 3440 | | and mutual authentication 3441 | | 3442 A: +-------->| Header: POST (Code=0.02) 3443 | POST | Uri-Path:"token" 3444 | | Content-Format: application/ace+cbor 3445 | | Payload: 3446 | | 3447 B: |<--------+ Header: 2.05 Content 3448 | 2.05 | Content-Format: application/ace+cbor 3449 | | Payload: 3450 | | 3452 Figure 17: Token Request and Response Using Client Credentials. 3454 The information contained in the Request-Payload and the Response- 3455 Payload is shown in Figure 18 Note that the parameter "rs_cnf" from 3456 [I-D.ietf-ace-oauth-params] is used to inform the client about the 3457 resource server's public key. 3459 Request-Payload : 3460 { 3461 "audience" : "tempSensorInLivingRoom", 3462 "client_id" : "myclient", 3463 "req_cnf" : { 3464 "COSE_Key" : { 3465 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3466 "kty" : "EC", 3467 "crv" : "P-256", 3468 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3469 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3470 } 3471 } 3472 } 3474 Response-Payload : 3475 { 3476 "access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...', 3477 "rs_cnf" : { 3478 "COSE_Key" : { 3479 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3480 "kty" : "EC", 3481 "crv" : "P-256", 3482 "x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4', 3483 "y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM' 3484 } 3485 } 3486 } 3488 Figure 18: Request and Response Payload Details. 3490 The content of the access token is shown in Figure 19. 3492 { 3493 "aud" : "tempSensorInLivingRoom", 3494 "iat" : "1563451500", 3495 "exp" : "1563453000", 3496 "scope" : "temperature_g firmware_p", 3497 "cnf" : { 3498 "COSE_Key" : { 3499 "kid" : b64'1Bg8vub9tLe1gHMzV76e8', 3500 "kty" : "EC", 3501 "crv" : "P-256", 3502 "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU', 3503 "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0' 3504 } 3505 } 3506 } 3508 Figure 19: Access Token including Public Key of the client. 3510 Messages C and F are shown in Figure 20 - Figure 21. 3512 C: The client then sends the PoP access token to the authz-info 3513 endpoint at the RS. This is a plain CoAP POST request, i.e., no 3514 transport or application-layer security is used between client and 3515 RS since the token is integrity protected between the AS and RS. 3516 The RS verifies that the PoP access token was created by a known 3517 and trusted AS, that it applies to this RS, and that it is valid. 3518 The RS caches the security context together with authorization 3519 information about this client contained in the PoP access token. 3521 Resource 3522 Client Server 3523 | | 3524 C: +-------->| Header: POST (Code=0.02) 3525 | POST | Uri-Path:"authz-info" 3526 | | Payload: 0INDoQEKoQVN ... 3527 | | 3528 |<--------+ Header: 2.04 Changed 3529 | 2.04 | 3530 | | 3532 Figure 20: Access Token provisioning to RS 3533 The client and the RS runs the DTLS handshake using the raw public 3534 keys established in step B and C. 3535 The client sends a CoAP GET request to /temperature on RS over 3536 DTLS. The RS verifies that the request is authorized, based on 3537 previously established security context. 3539 F: The RS responds over the same DTLS channel with a CoAP 2.05 3540 Content response, containing a resource representation as payload. 3542 Resource 3543 Client Server 3544 | | 3545 |<=======>| DTLS Connection Establishment 3546 | | using Raw Public Keys 3547 | | 3548 +-------->| Header: GET (Code=0.01) 3549 | GET | Uri-Path: "temperature" 3550 | | 3551 | | 3552 | | 3553 F: |<--------+ Header: 2.05 Content 3554 | 2.05 | Payload: 3555 | | 3557 Figure 21: Resource Request and Response protected by DTLS. 3559 F.2. Introspection Aided Token Validation 3561 In this deployment scenario it is assumed that a client is not able 3562 to access the AS at the time of the access request, whereas the RS is 3563 assumed to be connected to the back-end infrastructure. Thus the RS 3564 can make use of token introspection. This access procedure involves 3565 steps A-F of Figure 1, but assumes steps A and B have been carried 3566 out during a phase when the client had connectivity to AS. 3568 Since the client is assumed to be offline, at least for a certain 3569 period of time, a pre-provisioned access token has to be long-lived. 3570 Since the client is constrained, the token will not be self contained 3571 (i.e. not a CWT) but instead just a reference. The resource server 3572 uses its connectivity to learn about the claims associated to the 3573 access token by using introspection, which is shown in the example 3574 below. 3576 In the example interactions between an offline client (key fob), an 3577 RS (online lock), and an AS is shown. It is assumed that there is a 3578 provisioning step where the client has access to the AS. This 3579 corresponds to message exchanges A and B which are shown in 3580 Figure 22. 3582 Authorization consent from the resource owner can be pre-configured, 3583 but it can also be provided via an interactive flow with the resource 3584 owner. An example of this for the key fob case could be that the 3585 resource owner has a connected car, he buys a generic key that he 3586 wants to use with the car. To authorize the key fob he connects it 3587 to his computer that then provides the UI for the device. After that 3588 OAuth 2.0 implicit flow can used to authorize the key for his car at 3589 the car manufacturers AS. 3591 Note: In this example the client does not know the exact door it will 3592 be used to access since the token request is not send at the time of 3593 access. So the scope and audience parameters are set quite wide to 3594 start with, while tailored values narrowing down the claims to the 3595 specific RS being accessed can be provided to that RS during an 3596 introspection step. 3598 A: The client sends a CoAP POST request to the token endpoint at 3599 AS. The request contains the Audience parameter set to "PACS1337" 3600 (PACS, Physical Access System), a value the that identifies the 3601 physical access control system to which the individual doors are 3602 connected. The AS generates an access token as an opaque string, 3603 which it can match to the specific client and the targeted 3604 audience. It furthermore generates a symmetric proof-of- 3605 possession key. The communication security and authentication 3606 between client and AS is assumed to have been provided at 3607 transport layer (e.g. via DTLS) using a pre-shared security 3608 context (psk, rpk or certificate). 3609 B: The AS responds with a CoAP 2.05 Content response, containing 3610 as payload the Access Information, including the access token and 3611 the symmetric proof-of-possession key. Communication security 3612 between C and RS will be DTLS and PreSharedKey. The PoP key is 3613 used as the PreSharedKey. 3615 Note: In this example we are using a symmetric key for a multi-RS 3616 audience, which is not recommended normally (see Section 6.9). 3617 However in this case the risk is deemed to be acceptable, since all 3618 the doors are part of the same physical access control system, and 3619 therefore the risk of a malicious RS impersonating the client towards 3620 another RS is low. 3622 Authorization 3623 Client Server 3624 | | 3625 |<=======>| DTLS Connection Establishment 3626 | | and mutual authentication 3627 | | 3628 A: +-------->| Header: POST (Code=0.02) 3629 | POST | Uri-Path:"token" 3630 | | Content-Format: application/ace+cbor 3631 | | Payload: 3632 | | 3633 B: |<--------+ Header: 2.05 Content 3634 | | Content-Format: application/ace+cbor 3635 | 2.05 | Payload: 3636 | | 3638 Figure 22: Token Request and Response using Client Credentials. 3640 The information contained in the Request-Payload and the Response- 3641 Payload is shown in Figure 23. 3643 Request-Payload: 3644 { 3645 "client_id" : "keyfob", 3646 "audience" : "PACS1337" 3647 } 3649 Response-Payload: 3650 { 3651 "access_token" : b64'VGVzdCB0b2tlbg==', 3652 "cnf" : { 3653 "COSE_Key" : { 3654 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk', 3655 "kty" : "oct", 3656 "alg" : "HS256", 3657 "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE' 3658 } 3659 } 3660 } 3662 Figure 23: Request and Response Payload for C offline 3664 The access token in this case is just an opaque byte string 3665 referencing the authorization information at the AS. 3667 C: Next, the client POSTs the access token to the authz-info 3668 endpoint in the RS. This is a plain CoAP request, i.e., no DTLS 3669 between client and RS. Since the token is an opaque string, the 3670 RS cannot verify it on its own, and thus defers to respond the 3671 client with a status code until after step E. 3672 D: The RS sends the token to the introspection endpoint on the AS 3673 using a CoAP POST request. In this example RS and AS are assumed 3674 to have performed mutual authentication using a pre shared 3675 security context (psk, rpk or certificate) with the RS acting as 3676 DTLS client. 3677 E: The AS provides the introspection response (2.05 Content) 3678 containing parameters about the token. This includes the 3679 confirmation key (cnf) parameter that allows the RS to verify the 3680 client's proof of possession in step F. Note that our example in 3681 Figure 25 assumes a pre-established key (e.g. one used by the 3682 client and the RS for a previous token) that is now only 3683 referenced by its key-identifier 'kid'. 3684 After receiving message E, the RS responds to the client's POST in 3685 step C with the CoAP response code 2.01 (Created). 3687 Resource 3688 Client Server 3689 | | 3690 C: +-------->| Header: POST (T=CON, Code=0.02) 3691 | POST | Uri-Path:"authz-info" 3692 | | Payload: b64'VGVzdCB0b2tlbg==' 3693 | | 3694 | | Authorization 3695 | | Server 3696 | | | 3697 | D: +--------->| Header: POST (Code=0.02) 3698 | | POST | Uri-Path: "introspect" 3699 | | | Content-Format: "application/ace+cbor" 3700 | | | Payload: 3701 | | | 3702 | E: |<---------+ Header: 2.05 Content 3703 | | 2.05 | Content-Format: "application/ace+cbor" 3704 | | | Payload: 3705 | | | 3706 | | 3707 |<--------+ Header: 2.01 Created 3708 | 2.01 | 3709 | | 3711 Figure 24: Token Introspection for C offline 3712 The information contained in the Request-Payload and the Response- 3713 Payload is shown in Figure 25. 3715 Request-Payload: 3716 { 3717 "token" : b64'VGVzdCB0b2tlbg==', 3718 "client_id" : "FrontDoor", 3719 } 3721 Response-Payload: 3722 { 3723 "active" : true, 3724 "aud" : "lockOfDoor4711", 3725 "scope" : "open, close", 3726 "iat" : 1563454000, 3727 "cnf" : { 3728 "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk' 3729 } 3730 } 3732 Figure 25: Request and Response Payload for Introspection 3734 The client uses the symmetric PoP key to establish a DTLS 3735 PreSharedKey secure connection to the RS. The CoAP request PUT is 3736 sent to the uri-path /state on the RS, changing the state of the 3737 door to locked. 3738 F: The RS responds with a appropriate over the secure DTLS 3739 channel. 3741 Resource 3742 Client Server 3743 | | 3744 |<=======>| DTLS Connection Establishment 3745 | | using Pre Shared Key 3746 | | 3747 +-------->| Header: PUT (Code=0.03) 3748 | PUT | Uri-Path: "state" 3749 | | Payload: 3750 | | 3751 F: |<--------+ Header: 2.04 Changed 3752 | 2.04 | Payload: 3753 | | 3755 Figure 26: Resource request and response protected by OSCORE 3757 Authors' Addresses 3758 Ludwig Seitz 3759 Combitech 3760 Djaeknegatan 31 3761 Malmoe 211 35 3762 Sweden 3764 Email: ludwig.seitz@combitech.se 3766 Goeran Selander 3767 Ericsson 3768 Faroegatan 6 3769 Kista 164 80 3770 Sweden 3772 Email: goran.selander@ericsson.com 3774 Erik Wahlstroem 3775 Sweden 3777 Email: erik@wahlstromstekniska.se 3779 Samuel Erdtman 3780 Spotify AB 3781 Birger Jarlsgatan 61, 4tr 3782 Stockholm 113 56 3783 Sweden 3785 Email: erdtman@spotify.com 3787 Hannes Tschofenig 3788 Arm Ltd. 3789 Absam 6067 3790 Austria 3792 Email: Hannes.Tschofenig@arm.com