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Versions: (draft-tschofenig-ace-oauth-iot) 00 01 02 03 04 05 06 07 09

ACE Working Group                                               L. Seitz
Internet-Draft                                                 RISE SICS
Intended status: Standards Track                             G. Selander
Expires: May 20, 2018                                           Ericsson
                                                           E. Wahlstroem
                                                        (no affiliation)
                                                              S. Erdtman
                                                              Spotify AB
                                                           H. Tschofenig
                                                                ARM Ltd.
                                                       November 16, 2017


  Authentication and Authorization for Constrained Environments (ACE)
                     draft-ietf-ace-oauth-authz-09

Abstract

   This specification defines a framework for authentication and
   authorization in Internet of Things (IoT) environments.  The
   framework is based on a set of building blocks including OAuth 2.0
   and CoAP, thus making a well-known and widely used authorization
   solution suitable for IoT devices.  Existing specifications are used
   where possible, but where the constraints of IoT devices require it,
   extensions are added and profiles are defined.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 20, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  CoAP  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Protocol Interactions . . . . . . . . . . . . . . . . . . . .  10
   5.  Framework . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Discovering Authorization Servers . . . . . . . . . . . .  14
       5.1.1.  Unauthorized Resource Request Message . . . . . . . .  15
       5.1.2.  AS Information  . . . . . . . . . . . . . . . . . . .  16
     5.2.  Authorization Grants  . . . . . . . . . . . . . . . . . .  17
     5.3.  Client Credentials  . . . . . . . . . . . . . . . . . . .  17
     5.4.  AS Authentication . . . . . . . . . . . . . . . . . . . .  18
     5.5.  The Authorization Endpoint  . . . . . . . . . . . . . . .  18
     5.6.  The Token Endpoint  . . . . . . . . . . . . . . . . . . .  18
       5.6.1.  Client-to-AS Request  . . . . . . . . . . . . . . . .  19
       5.6.2.  AS-to-Client Response . . . . . . . . . . . . . . . .  21
       5.6.3.  Error Response  . . . . . . . . . . . . . . . . . . .  24
       5.6.4.  Request and Response Parameters . . . . . . . . . . .  24
         5.6.4.1.  Audience  . . . . . . . . . . . . . . . . . . . .  25
         5.6.4.2.  Grant Type  . . . . . . . . . . . . . . . . . . .  25
         5.6.4.3.  Token Type  . . . . . . . . . . . . . . . . . . .  25
         5.6.4.4.  Profile . . . . . . . . . . . . . . . . . . . . .  25
         5.6.4.5.  Confirmation  . . . . . . . . . . . . . . . . . .  26
       5.6.5.  Mapping parameters to CBOR  . . . . . . . . . . . . .  26
     5.7.  The 'Introspect' Endpoint . . . . . . . . . . . . . . . .  27
       5.7.1.  RS-to-AS Request  . . . . . . . . . . . . . . . . . .  28
       5.7.2.  AS-to-RS Response . . . . . . . . . . . . . . . . . .  28
       5.7.3.  Error Response  . . . . . . . . . . . . . . . . . . .  29
       5.7.4.  Client Token  . . . . . . . . . . . . . . . . . . . .  30
       5.7.5.  Mapping Introspection parameters to CBOR  . . . . . .  32
     5.8.  The Access Token  . . . . . . . . . . . . . . . . . . . .  32
       5.8.1.  The 'Authorization Information' Endpoint  . . . . . .  33
       5.8.2.  Token Expiration  . . . . . . . . . . . . . . . . . .  34
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
     6.1.  Unprotected AS Information  . . . . . . . . . . . . . . .  36



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     6.2.  Use of Nonces for Replay Protection . . . . . . . . . . .  36
     6.3.  Combining profiles  . . . . . . . . . . . . . . . . . . .  36
     6.4.  Error responses . . . . . . . . . . . . . . . . . . . . .  36
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  37
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     8.1.  Authorization Server Information  . . . . . . . . . . . .  37
     8.2.  OAuth Error Code CBOR Mappings Registry . . . . . . . . .  38
     8.3.  OAuth Grant Type CBOR Mappings  . . . . . . . . . . . . .  38
     8.4.  OAuth Access Token Types  . . . . . . . . . . . . . . . .  39
     8.5.  OAuth Token Type CBOR Mappings  . . . . . . . . . . . . .  39
       8.5.1.  Initial Registry Contents . . . . . . . . . . . . . .  40
     8.6.  ACE OAuth Profile Registry  . . . . . . . . . . . . . . .  40
     8.7.  OAuth Parameter Registration  . . . . . . . . . . . . . .  40
     8.8.  OAuth CBOR Parameter Mappings Registry  . . . . . . . . .  41
     8.9.  OAuth Introspection Response Parameter Registration . . .  41
     8.10. Introspection Endpoint CBOR Mappings Registry . . . . . .  42
     8.11. JSON Web Token Claims . . . . . . . . . . . . . . . . . .  42
     8.12. CBOR Web Token Claims . . . . . . . . . . . . . . . . . .  42
     8.13. CoAP Option Number Registration . . . . . . . . . . . . .  43
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  43
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  44
     10.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Design Justification . . . . . . . . . . . . . . . .  47
   Appendix B.  Roles and Responsibilities . . . . . . . . . . . . .  51
   Appendix C.  Requirements on Profiles . . . . . . . . . . . . . .  53
   Appendix D.  Assumptions on AS knowledge about C and RS . . . . .  54
   Appendix E.  Deployment Examples  . . . . . . . . . . . . . . . .  54
     E.1.  Local Token Validation  . . . . . . . . . . . . . . . . .  54
     E.2.  Introspection Aided Token Validation  . . . . . . . . . .  58
   Appendix F.  Document Updates . . . . . . . . . . . . . . . . . .  62
     F.1.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  62
     F.2.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  62
     F.3.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  63
     F.4.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  63
     F.5.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  63
     F.6.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  64
     F.7.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  64
     F.8.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  64
     F.9.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  64
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  65

1.  Introduction

   Authorization is the process for granting approval to an entity to
   access a resource [RFC4949].  The authorization task itself can best
   be described as granting access to a requesting client, for a
   resource hosted on a device, the resource server (RS).  This exchange



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   is mediated by one or multiple authorization servers (AS).  Managing
   authorization for a large number of devices and users can be a
   complex task.

   While prior work on authorization solutions for the Web and for the
   mobile environment also applies to the Internet of Things (IoT)
   environment, many IoT devices are constrained, for example, in terms
   of processing capabilities, available memory, etc.  For web
   applications on constrained nodes, this specification RECOMMENDS the
   use of CoAP [RFC7252] as replacement for HTTP.

   A detailed treatment of constraints can be found in [RFC7228], and
   the different IoT deployments present a continuous range of device
   and network capabilities.  Taking energy consumption as an example:
   At one end there are energy-harvesting or battery powered devices
   which have a tight power budget, on the other end there are mains-
   powered devices, and all levels in between.

   Hence, IoT devices may be very different in terms of available
   processing and message exchange capabilities and there is a need to
   support many different authorization use cases [RFC7744].

   This specification describes a framework for authentication and
   authorization in constrained environments (ACE) built on re-use of
   OAuth 2.0 [RFC6749], thereby extending authorization to Internet of
   Things devices.  This specification contains the necessary building
   blocks for adjusting OAuth 2.0 to IoT environments.

   More detailed, interoperable specifications can be found in profiles.
   Implementations may claim conformance with a specific profile,
   whereby implementations utilizing the same profile interoperate while
   implementations of different profiles are not expected to be
   interoperable.  Some devices, such as mobile phones and tablets, may
   implement multiple profiles and will therefore be able to interact
   with a wider range of low end devices.  Requirements on profiles are
   described at contextually appropriate places throughout this
   specification, and also summarized in Appendix C.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   Certain security-related terms such as "authentication",
   "authorization", "confidentiality", "(data) integrity", "message
   authentication code", and "verify" are taken from [RFC4949].



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   Since exchanges in this specification are described as RESTful
   protocol interactions, HTTP [RFC7231] offers useful terminology.

   Terminology for entities in the architecture is defined in OAuth 2.0
   [RFC6749] and [I-D.ietf-ace-actors], such as client (C), resource
   server (RS), and authorization server (AS).

   Note that the term "endpoint" is used here following its OAuth
   definition, which is to denote resources such as token and
   introspection at the AS and authz-info at the RS (see Section 5.8.1
   for a definition of the authz-info endpoint).  The CoAP [RFC7252]
   definition, which is "An entity participating in the CoAP protocol"
   is not used in this specification.

   Since this specification focuses on the problem of access control to
   resources, the actors has been simplified by assuming that the client
   authorization server (CAS) functionality is not stand-alone but
   subsumed by either the authorization server or the client (see
   section 2.2 in [I-D.ietf-ace-actors]).

   The specifications in this document is called the "framework" or "ACE
   framework".  When referring to "profiles of this framework" it refers
   to additional specifications that define the use of this
   specification with concrete transport, and communication security
   protocols (e.g., CoAP over DTLS).

   We use the term "RS Information" for parameters describing
   characteristics of the RS (e.g. public key) that the AS provides to
   the client.

3.  Overview

   This specification defines the ACE framework for authorization in the
   Internet of Things environment.  It consists of a set of building
   blocks.

   The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys
   widespread deployment.  Many IoT devices can support OAuth 2.0
   without any additional extensions, but for certain constrained
   settings additional profiling is needed.

   Another building block is the lightweight web transfer protocol CoAP
   [RFC7252], for those communication environments where HTTP is not
   appropriate.  CoAP typically runs on top of UDP, which further
   reduces overhead and message exchanges.  While this specification
   defines extensions for the use of OAuth over CoAP, other underlying
   protocols are not prohibited from being supported in the future, such
   as HTTP/2, MQTT, BLE and QUIC.



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   A third building block is CBOR [RFC7049], for encodings where JSON
   [RFC7159] is not sufficiently compact.  CBOR is a binary encoding
   designed for small code and message size, which may be used for
   encoding of self contained tokens, and also for encoding payload
   transferred in protocol messages.

   A fourth building block is the compact CBOR-based secure message
   format COSE [RFC8152], which enables application layer security as an
   alternative or complement to transport layer security (DTLS [RFC6347]
   or TLS [RFC5246]).  COSE is used to secure self-contained tokens such
   as proof-of-possession (PoP) tokens, which is an extension to the
   OAuth tokens, and "client tokens" which are defined in this framework
   (see Section 5.7.4).  The default token format is defined in CBOR web
   token (CWT) [I-D.ietf-ace-cbor-web-token].  Application layer
   security for CoAP using COSE can be provided with OSCOAP
   [I-D.ietf-core-object-security].

   With the building blocks listed above, solutions satisfying various
   IoT device and network constraints are possible.  A list of
   constraints is described in detail in RFC 7228 [RFC7228] and a
   description of how the building blocks mentioned above relate to the
   various constraints can be found in Appendix A.

   Luckily, not every IoT device suffers from all constraints.  The ACE
   framework nevertheless takes all these aspects into account and
   allows several different deployment variants to co-exist, rather than
   mandating a one-size-fits-all solution.  It is important to cover the
   wide range of possible interworking use cases and the different
   requirements from a security point of view.  Once IoT deployments
   mature, popular deployment variants will be documented in the form of
   ACE profiles.

3.1.  OAuth 2.0

   The OAuth 2.0 authorization framework enables a client to obtain
   scoped access to a resource with the permission of a resource owner.
   Authorization information, or references to it, is passed between the
   nodes using access tokens.  These access tokens are issued to clients
   by an authorization server with the approval of the resource owner.
   The client uses the access token to access the protected resources
   hosted by the resource server.

   A number of OAuth 2.0 terms are used within this specification:

   The token and introspection Endpoints:
      The AS hosts the token endpoint that allows a client to request
      access tokens.  The client makes a POST request to the token




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      endpoint on the AS and receives the access token in the response
      (if the request was successful).
      In some deployments, a token introspection endpoint is provided by
      the AS, which can be used by the RS if it needs to request
      additional information regarding a received access token.  The RS
      makes a POST request to the introspection endpoint on the AS and
      receives information about the access token in the response.  (See
      "Introspection" below.)


   Access Tokens:
      Access tokens are credentials needed to access protected
      resources.  An access token is a data structure representing
      authorization permissions issued by the AS to the client.  Access
      tokens are generated by the AS and consumed by the RS.  The access
      token content is opaque to the client.

      Access tokens can have different formats, and various methods of
      utilization (e.g., cryptographic properties) based on the security
      requirements of the given deployment.


   Proof of Possession Tokens:
      An access token may be bound to a cryptographic key, which is then
      used by an RS to authenticate requests from a client.  Such tokens
      are called proof-of-possession access tokens (or PoP access
      tokens).

      The proof-of-possession (PoP) security concept assumes that the AS
      acts as a trusted third party that binds keys to access tokens.
      These so called PoP keys are then used by the client to
      demonstrate the possession of the secret to the RS when accessing
      the resource.  The RS, when receiving an access token, needs to
      verify that the key used by the client matches the one bound to
      the access token.  When this specification uses the term "access
      token" it is assumed to be a PoP access token token unless
      specifically stated otherwise.

      The key bound to the access token (the PoP key) may use either
      symmetric or asymmetric cryptography.  The appropriate choice of
      the kind of cryptography depends on the constraints of the IoT
      devices as well as on the security requirements of the use case.



      Symmetric PoP key:
         The AS generates a random symmetric PoP key.  The key is either
         stored to be returned on introspection calls or encrypted and



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         included in the access token.  The PoP key is also encrypted
         for the client and sent together with the access token to the
         client.


      Asymmetric PoP key:
         An asymmetric key pair is generated on the client and the
         public key is sent to the AS (if it does not already have
         knowledge of the client's public key).  Information about the
         public key, which is the PoP key in this case, is either stored
         to be returned on introspection calls or included inside the
         access token and sent back to the requesting client.  The RS
         can identify the client's public key from the information in
         the token, which allows the client to use the corresponding
         private key for the proof of possession.

      The access token is either a simple reference, or a structured
      information object (e.g., CWT [I-D.ietf-ace-cbor-web-token]),
      protected by a cryptographic wrapper (e.g., COSE [RFC8152]).  The
      choice of PoP key does not necessarily imply a specific credential
      type for the integrity protection of the token.


   Scopes and Permissions:
      In OAuth 2.0, the client specifies the type of permissions it is
      seeking to obtain (via the scope parameter) in the access token
      request.  In turn, the AS may use the scope response parameter to
      inform the client of the scope of the access token issued.  As the
      client could be a constrained device as well, this specification
      uses CBOR encoding as data format, defined in Section 5, to
      request scopes and to be informed what scopes the access token
      actually authorizes.

      The values of the scope parameter in OAuth 2.0 are expressed as a
      list of space-delimited, case-sensitive strings, with a semantic
      that is well-known to the AS and the RS.  More details about the
      concept of scopes is found under Section 3.3 in [RFC6749].


   Claims:
      Information carried in the access token or returned from
      introspection, called claims, is in the form of name-value pairs.
      An access token may, for example, include a claim identifying the
      AS that issued the token (via the "iss" claim) and what audience
      the access token is intended for (via the "aud" claim).  The
      audience of an access token can be a specific resource or one or
      many resource servers.  The resource owner policies influence what
      claims are put into the access token by the authorization server.



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      While the structure and encoding of the access token varies
      throughout deployments, a standardized format has been defined
      with the JSON Web Token (JWT) [RFC7519] where claims are encoded
      as a JSON object.  In [I-D.ietf-ace-cbor-web-token], an equivalent
      format using CBOR encoding (CWT) has been defined.


   Introspection:
      Introspection is a method for a resource server to query the
      authorization server for the active state and content of a
      received access token.  This is particularly useful in those cases
      where the authorization decisions are very dynamic and/or where
      the received access token itself is an opaque reference rather
      than a self-contained token.  More information about introspection
      in OAuth 2.0 can be found in [RFC7662].

3.2.  CoAP

   CoAP is an application layer protocol similar to HTTP, but
   specifically designed for constrained environments.  CoAP typically
   uses datagram-oriented transport, such as UDP, where reordering and
   loss of packets can occur.  A security solution needs to take the
   latter aspects into account.

   While HTTP uses headers and query strings to convey additional
   information about a request, CoAP encodes such information into
   header parameters called 'options'.

   CoAP supports application-layer fragmentation of the CoAP payloads
   through blockwise transfers [RFC7959].  However, blockwise transfer
   does not increase the size limits of CoAP options, therefore data
   encoded in options has to be kept small.

   Transport layer security for CoAP can be provided by DTLS 1.2
   [RFC6347] or TLS 1.2 [RFC5246].  CoAP defines a number of proxy
   operations that require transport layer security to be terminated at
   the proxy.  One approach for protecting CoAP communication end-to-end
   through proxies, and also to support security for CoAP over a
   different transport in a uniform way, is to provide security at the
   application layer using an object-based security mechanism such as
   COSE [RFC8152].

   One application of COSE is OSCOAP [I-D.ietf-core-object-security],
   which provides end-to-end confidentiality, integrity and replay
   protection, and a secure binding between CoAP request and response
   messages.  In OSCOAP, the CoAP messages are wrapped in COSE objects
   and sent using CoAP.




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   This framework RECOMMENDS the use of CoAP as replacement for HTTP.

4.  Protocol Interactions

   The ACE framework is based on the OAuth 2.0 protocol interactions
   using the token endpoint and optionally the introspection endpoint.
   A client obtains an access token from an AS using the token endpoint
   and subsequently presents the access token to a RS to gain access to
   a protected resource.  In most deployments the RS can process the
   access token locally, however in some cases the RS may present it to
   the AS via the introspection endpoint to get fresh information.
   These interactions are shown in Figure 1.  An overview of various
   OAuth concepts is provided in Section 3.1.

   The OAuth 2.0 framework defines a number of "protocol flows" via
   grant types, which have been extended further with extensions to
   OAuth 2.0 (such as RFC 7521 [RFC7521] and
   [I-D.ietf-oauth-device-flow]).  What grant types works best depends
   on the usage scenario and RFC 7744 [RFC7744] describes many different
   IoT use cases but there are two preferred grant types, namely the
   Authorization Code Grant (described in Section 4.1 of [RFC7521]) and
   the Client Credentials Grant (described in Section 4.4 of [RFC7521]).
   The Authorization Code Grant is a good fit for use with apps running
   on smart phones and tablets that request access to IoT devices, a
   common scenario in the smart home environment, where users need to go
   through an authentication and authorization phase (at least during
   the initial setup phase).  The native apps guidelines described in
   [I-D.ietf-oauth-native-apps] are applicable to this use case.  The
   Client Credential Grant is a good fit for use with IoT devices where
   the OAuth client itself is constrained.  In such a case, the resource
   owner has pre-arranged access rights for the client with the
   authorization server, which is often accomplished using a
   commissioning tool.

   The consent of the resource owner, for giving a client access to a
   protected resource, can be provided dynamically as in the traditional
   OAuth flows, or it could be pre-configured by the resource owner as
   authorization policies at the AS, which the AS evaluates when a token
   request arrives.  The resource owner and the requesting party (i.e.,
   client owner) are not shown in Figure 1.

   This framework supports a wide variety of communication security
   mechanisms between the ACE entities, such as client, AS, and RS.  It
   is assumed that the client has been registered (also called enrolled
   or onboarded) to an AS using a mechanism defined outside the scope of
   this document.  In practice, various techniques for onboarding have
   been used, such as factory-based provisioning or the use of
   commissioning tools.  Regardless of the onboarding technique, this



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   provisioning procedure implies that the client and the AS exchange
   credentials and configuration parameters.  These credentials are used
   to mutually authenticate each other and to protect messages exchanged
   between the client and the AS.

   It is also assumed that the RS has been registered with the AS,
   potentially in a similar way as the client has been registered with
   the AS.  Established keying material between the AS and the RS allows
   the AS to apply cryptographic protection to the access token to
   ensure that its content cannot be modified, and if needed, that the
   content is confidentiality protected.

   The keying material necessary for establishing communication security
   between C and RS is dynamically established as part of the protocol
   described in this document.

   At the start of the protocol, there is an optional discovery step
   where the client discovers the resource server and the resources this
   server hosts.  In this step, the client might also determine what
   permissions are needed to access the protected resource.  A generic
   procedure is described in Section 5.1, profiles MAY define other
   procedures for discovery.

   In Bluetooth Low Energy, for example, advertisements are broadcasted
   by a peripheral, including information about the primary services.
   In CoAP, as a second example, a client can make a request to "/.well-
   known/core" to obtain information about available resources, which
   are returned in a standardized format as described in [RFC6690].

   +--------+                               +---------------+
   |        |---(A)-- Token Request ------->|               |
   |        |                               | Authorization |
   |        |<--(B)-- Access Token ---------|    Server     |
   |        |       + RS Information        |               |
   |        |                               +---------------+
   |        |                                      ^ |
   |        |            Introspection Request  (D)| |
   | Client |                  (optional)          | |
   |        |             Response + Client Token  | |(E)
   |        |                  (optional)          | v
   |        |                               +--------------+
   |        |---(C)-- Token + Request ----->|              |
   |        |                               |   Resource   |
   |        |<--(F)-- Protected Resource ---|    Server    |
   |        |                               |              |
   +--------+                               +--------------+

                      Figure 1: Basic Protocol Flow.



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   Requesting an Access Token (A):
      The client makes an access token request to the token endpoint at
      the AS.  This framework assumes the use of PoP access tokens (see
      Section 3.1 for a short description) wherein the AS binds a key to
      an access token.  The client may include permissions it seeks to
      obtain, and information about the credentials it wants to use
      (e.g., symmetric/asymmetric cryptography or a reference to a
      specific credential).


   Access Token Response (B):
      If the AS successfully processes the request from the client, it
      returns an access token.  It can also return additional
      parameters, referred to as "RS Information".  In addition to the
      response parameters defined by OAuth 2.0 and the PoP access token
      extension, this framework defines parameters that can be used to
      inform the client about capabilities of the RS.  More information
      about these parameters can be found in Section 5.6.4.


   Resource Request (C):
      The client interacts with the RS to request access to the
      protected resource and provides the access token.  The protocol to
      use between the client and the RS is not restricted to CoAP.
      HTTP, HTTP/2, QUIC, MQTT, Bluetooth Low Energy, etc., are also
      viable candidates.

      Depending on the device limitations and the selected protocol,
      this exchange may be split up into two parts:

         (1) the client sends the access token containing, or
         referencing, the authorization information to the RS, that may
         be used for subsequent resource requests by the client, and
         (2) the client makes the resource access request, using the
         communication security protocol and other RS Information
         obtained from the AS.

      The Client and the RS mutually authenticate using the security
      protocol specified in the profile (see step B) and the keys
      obtained in the access token or the RS Information or the client
      token.  The RS verifies that the token is integrity protected by
      the AS and compares the claims contained in the access token with
      the resource request.  If the RS is online, validation can be
      handed over to the AS using token introspection (see messages D
      and E) over HTTP or CoAP, in which case the different parts of
      step C may be interleaved with introspection.





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   Token Introspection Request (D):
      A resource server may be configured to introspect the access token
      by including it in a request to the introspection endpoint at that
      AS.  Token introspection over CoAP is defined in Section 5.7 and
      for HTTP in [RFC7662].

      Note that token introspection is an optional step and can be
      omitted if the token is self-contained and the resource server is
      prepared to perform the token validation on its own.


   Token Introspection Response (E):
      The AS validates the token and returns the most recent parameters,
      such as scope, audience, validity etc. associated with it back to
      the RS.  The RS then uses the received parameters to process the
      request to either accept or to deny it.  The AS can additionally
      return information that the RS needs to pass on to the client in
      the form of a client token.  The latter is used to establish keys
      for mutual authentication between client and RS, when the client
      has no direct connectivity to the AS, see Section 5.7.4 for
      details.


   Protected Resource (F):
      If the request from the client is authorized, the RS fulfills the
      request and returns a response with the appropriate response code.
      The RS uses the dynamically established keys to protect the
      response, according to used communication security protocol.

5.  Framework

   The following sections detail the profiling and extensions of OAuth
   2.0 for constrained environments, which constitutes the ACE
   framework.

   Credential Provisioning
      For IoT, it cannot be assumed that the client and RS are part of a
      common key infrastructure, so the AS provisions credentials or
      associated information to allow mutual authentication.  These
      credentials need to be provided to the parties before or during
      the authentication protocol is executed, and may be re-used for
      subsequent token requests.


   Proof-of-Possession
      The ACE framework, by default, implements proof-of-possession for
      access tokens, i.e., that the token holder can prove being a
      holder of the key bound to the token.  The binding is provided by



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      the "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession] indicating
      what key is used for proof-of-possession.  If a client needs to
      submit a new access token e.g., to obtain additional access
      rights, they can request that the AS binds this token to the same
      key as the previous one.


   ACE Profiles
      The client or RS may be limited in the encodings or protocols it
      supports.  To support a variety of different deployment settings,
      specific interactions between client and RS are defined in an ACE
      profile.  In ACE framework the AS is expected to manage the
      matching of compatible profile choices between a client and an RS.
      The AS informs the client of the selected profile using the
      "profile" parameter in the token response.

   OAuth 2.0 requires the use of TLS both to protect the communication
   between AS and client when requesting an access token; between client
   and RS when accessing a resource and between AS and RS if
   introspection is used.  In constrained settings TLS is not always
   feasible, or desirable.  Nevertheless it is REQUIRED that the data
   exchanged with the AS is encrypted and integrity protected.  It is
   furthermore REQUIRED that the AS and the endpoint communicating with
   it (client or RS) perform mutual authentication.

   Profiles MUST specify how mutual authentication is done, depending
   e.g.  on the communication protocol and the credentials used by the
   client or the RS.

   In OAuth 2.0 the communication with the Token and the Introspection
   endpoints at the AS is assumed to be via HTTP and may use Uri-query
   parameters.  This framework RECOMMENDS to use CoAP instead and
   RECOMMENDS the use of the following alternative instead of Uri-query
   parameters: The sender (client or RS) encodes the parameters of its
   request as a CBOR map and submits that map as the payload of the POST
   request.  The Content-format depends on the security applied to the
   content and MUST be specified by the profile that is used.

   The OAuth 2.0 AS uses a JSON structure in the payload of its
   responses both to client and RS.  This framework REQUIRES the use of
   CBOR [RFC7049] instead.  Depending on the profile, the CBOR payload
   MAY be enclosed in a non-CBOR cryptographic wrapper.

5.1.  Discovering Authorization Servers

   In order to determine the AS in charge of a resource hosted at the
   RS, C MAY send an initial Unauthorized Resource Request message to




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   RS.  RS then denies the request and sends the address of its AS back
   to C.

   Instead of the initial Unauthorized Resource Request message, C MAY
   look up the desired resource in a resource directory (cf.
   [I-D.ietf-core-resource-directory]).

5.1.1.  Unauthorized Resource Request Message

   The optional Unauthorized Resource Request message is a request for a
   resource hosted by RS for which no proper authorization is granted.
   RS MUST treat any request for a protected resource as Unauthorized
   Resource Request message when any of the following holds:

   o  The request has been received on an unprotected channel.
   o  RS has no valid access token for the sender of the request
      regarding the requested action on that resource.
   o  RS has a valid access token for the sender of the request, but
      this does not allow the requested action on the requested
      resource.

   Note: These conditions ensure that RS can handle requests
   autonomously once access was granted and a secure channel has been
   established between C and RS.  The authz-info endpoint MUST NOT be
   protected as specified above, in order to allow clients to upload
   access tokens to RS (cf.  Section 5.8.1).

   Unauthorized Resource Request messages MUST be denied with a client
   error response.  In this response, the Resource Server SHOULD provide
   proper AS Information to enable the Client to request an access token
   from RS's AS as described in Section 5.1.2.

   The response code MUST be 4.01 (Unauthorized) in case the sender of
   the Unauthorized Resource Request message is not authenticated, or if
   RS has no valid access token for C.  If RS has an access token for C
   but not for the resource that C has requested, RS MUST reject the
   request with a 4.03 (Forbidden).  If RS has an access token for C but
   it does not cover the action C requested on the resource, RS MUST
   reject the request with a 4.05 (Method Not Allowed).

   Note: The use of the response codes 4.03 and 4.05 is intended to
   prevent infinite loops where a dumb Client optimistically tries to
   access a requested resource with any access token received from AS.
   As malicious clients could pretend to be C to determine C's
   privileges, these detailed response codes must be used only when a
   certain level of security is already available which can be achieved
   only when the Client is authenticated.




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5.1.2.  AS Information

   The AS Information is sent by RS as a response to an Unauthorized
   Resource Request message (see Section 5.1.1) to point the sender of
   the Unauthorized Resource Request message to RS's AS.  The AS
   information is a set of attributes containing an absolute URI (see
   Section 4.3 of [RFC3986]) that specifies the AS in charge of RS.

   The message MAY also contain a nonce generated by RS to ensure
   freshness in case that the RS and AS do not have synchronized clocks.

   Figure 2 summarizes the parameters that may be part of the AS
   Information.

           /-------+----------+-----------------\
           | Name  | CBOR Key | Major Type      |
           |-------+----------+-----------------|
           | AS    |     0    | 3 (text string) |
           | nonce |     5    | 2 (byte string) |
           \-------+----------+-----------------/

                    Figure 2: AS Information parameters

   Figure 3 shows an example for an AS Information message payload using
   CBOR [RFC7049] diagnostic notation, using the parameter names instead
   of the CBOR keys for better human readability.

       4.01 Unauthorized
       Content-Format: application/ace+cbor
       {AS: "coaps://as.example.com/token",
        nonce: h'e0a156bb3f'}

                 Figure 3: AS Information payload example

   In this example, the attribute AS points the receiver of this message
   to the URI "coaps://as.example.com/token" to request access
   permissions.  The originator of the AS Information payload (i.e., RS)
   uses a local clock that is loosely synchronized with a time scale
   common between RS and AS (e.g., wall clock time).  Therefore, it has
   included a parameter "nonce" for replay attack prevention.

   Note: There is an ongoing discussion how freshness of      access
   tokens
      can be achieved in constrained environments.  This specification
      for now assumes that RS and AS do not have a common understanding
      of time that allows RS to achieve its security objectives without
      explicitly adding a nonce.




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   Figure 4 illustrates the mandatory to use binary encoding of the
   message payload shown in Figure 3.

   a2                                   # map(2)
       00                               # unsigned(0) (=AS)
       78 1c                            # text(28)
          636f6170733a2f2f61732e657861
          6d706c652e636f6d2f746f6b656e  # "coaps://as.example.com/token"
       05                               # unsigned(5) (=nonce)
       45                               # bytes(5)
          e0a156bb3f

             Figure 4: AS Information example encoded in CBOR

5.2.  Authorization Grants

   To request an access token, the client obtains authorization from the
   resource owner or uses its client credentials as grant.  The
   authorization is expressed in the form of an authorization grant.

   The OAuth framework defines four grant types.  The grant types can be
   split up into two groups, those granted on behalf of the resource
   owner (password, authorization code, implicit) and those for the
   client (client credentials).

   The grant type is selected depending on the use case.  In cases where
   the client acts on behalf of the resource owner, authorization code
   grant is recommended.  If the client acts on behalf of the resource
   owner, but does not have any display or very limited interaction
   possibilities it is recommended to use the device code grant defined
   in [I-D.ietf-oauth-device-flow].  In cases where the client does not
   act on behalf of the resource owner, client credentials grant is
   recommended.

   For details on the different grant types, see the OAuth 2.0 framework
   [RFC6749].  The OAuth 2.0 framework provides an extension mechanism
   for defining additional grant types so profiles of this framework MAY
   define additional grant types, if needed.

5.3.  Client Credentials

   Authentication of the client is mandatory independent of the grant
   type when requesting the access token from the token endpoint.  In
   the case of client credentials grant type, the authentication and
   grant coincide.

   Client registration and provisioning of client credentials to the
   client is out of scope for this specification.



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   The OAuth framework [RFC6749] defines one client credential type,
   client id and client secret.  [I-D.erdtman-ace-rpcc] adds raw-public-
   key and pre-shared-key to the client credentials types.  Profiles of
   this framework MAY extend with additional client credentials client
   certificates.

5.4.  AS Authentication

   Client credential does not, by default, authenticate the AS that the
   client connects to.  In classic OAuth, the AS is authenticated with a
   TLS server certificate.

   Profiles of this framework MUST specify how clients authenticate the
   AS and how communication security is implemented, otherwise server
   side TLS certificates, as defined by OAuth 2.0, are required.

5.5.  The Authorization Endpoint

   The authorization endpoint is used to interact with the resource
   owner and obtain an authorization grant in certain grant flows.
   Since it requires the use of a user agent (i.e., browser), it is not
   expected that these types of grant flow will be used by constrained
   clients.  This endpoint is therefore out of scope for this
   specification.  Implementations should use the definition and
   recommendations of [RFC6749] and [RFC6819].

   If clients involved cannot support HTTP and TLS, profiles MAY define
   mappings for the authorization endpoint.

5.6.  The Token Endpoint

   In standard OAuth 2.0, the AS provides the token endpoint for
   submitting access token requests.  This framework extends the
   functionality of the token endpoint, giving the AS the possibility to
   help the client and RS to establish shared keys or to exchange their
   public keys.  Furthermore, this framework defines encodings using
   CBOR, as a substitute for JSON.

   For the AS to be able to issue a token, the client MUST be
   authenticated and present a valid grant for the scopes requested.
   Profiles of this framework MUST specify how the AS authenticates the
   client and how the communication between client and AS is protected.

   The default name of this endpoint in an url-path is 'token', however
   implementations are not required to use this name and can define
   their own instead.





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   The figures of this section use CBOR diagnostic notation without the
   integer abbreviations for the parameters or their values for
   illustrative purposes.  Note that implementations MUST use the
   integer abbreviations and the binary CBOR encoding.

5.6.1.  Client-to-AS Request

   The client sends a POST request to the token endpoint at the AS.  The
   profile MUST specify the Content-Type and wrapping of the payload.
   The content of the request consists of the parameters specified in
   section 4 of the OAuth 2.0 specification [RFC6749], encoded as a CBOR
   map, where the "scope" parameter can additionally be formatted as a
   byte array, in order to allow compact encoding of complex scope
   structures.

   In addition to these parameters, this framework defines the following
   parameters for requesting an access token from a token endpoint:

   aud
      OPTIONAL.  Specifies the audience for which the client is
      requesting an access token.  If this parameter is missing, it is
      assumed that the client and the AS have a pre-established
      understanding of the audience that an access token should address.
      If a client submits a request for an access token without
      specifying an "aud" parameter, and the AS does not have an
      implicit understanding of the "aud" value for this client, then
      the AS MUST respond with an error message using a response code
      equivalent to the CoAP response code 4.00 (Bad Request).

   cnf
      OPTIONAL.  This field contains information about the key the
      client would like to bind to the access token for proof-of-
      possession.  It is RECOMMENDED that an AS reject a request
      containing a symmetric key value in the 'cnf' field, since the AS
      is expected to be able to generate better symmetric keys than a
      potentially constrained client.  See Section 5.6.4.5 for more
      details on the formatting of the 'cnf' parameter.

   The following examples illustrate different types of requests for
   proof-of-possession tokens.

   Figure 5 shows a request for a token with a symmetric proof-of-
   possession key.  Note that in this example it is assumed that
   transport layer communication security is used, therefore the
   Content-Type is "application/cbor".  The content is displayed in CBOR
   diagnostic notation, without abbreviations for better readability.





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   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   Content-Type: "application/cbor"
   Payload:
   {
     "grant_type" : "client_credentials",
     "client_id" : "myclient",
     "aud" : "tempSensor4711"
    }

    Figure 5: Example request for an access token bound to a symmetric
                                   key.

   Figure 6 shows a request for a token with an asymmetric proof-of-
   possession key.  Note that in this example COSE is used to provide
   object-security, therefore the Content-Type is "application/cose".

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   Content-Type: "application/cose"
   Payload:
     16(  # COSE_ENCRYPTED
         [ h'a1010a', # protected header: {"alg" : "AES-CCM-16-64-128"}
         {5 : b64'ifUvZaHFgJM7UmGnjA'},  # unprotected header, IV
         b64'WXThuZo6TMCaZZqi6ef/8WHTjOdGk8kNzaIhIQ' # ciphertext
         ]
     )


   Decrypted payload:
   {
     "grant_type" : "client_credentials",
     "client_id" : "myclient",
     "cnf" : {
       "COSE_Key" : {
         "kty" : "EC",
         "kid" : h'11',
         "crv" : "P-256",
         "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
         "y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
       }
     }
   }

        Figure 6: Example token request bound to an asymmetric key.




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   Figure 7 shows a request for a token where a previously communicated
   proof-of-possession key is only referenced.  Note that a transport
   layer based communication security profile is assumed in this
   example, therefore the Content-Type is "application/cbor".  Also note
   that the client performs a password based authentication in this
   example by submitting its client_secret (see section 2.3.1. of
   [RFC6749]).

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   Content-Type: "application/cbor"
   Payload:
   {
     "grant_type" : "client_credentials",
     "client_id" : "myclient",
     "client_secret" : "mysecret234",
     "aud" : "valve424",
     "scope" : "read",
     "cnf" : {
       "kid" : b64'6kg0dXJM13U'
     }
   }

       Figure 7: Example request for an access token bound to a key
                                reference.

5.6.2.  AS-to-Client Response

   If the access token request has been successfully verified by the AS
   and the client is authorized to obtain an access token corresponding
   to its access token request, the AS sends a response with the
   response code equivalent to the CoAP response code 2.01 (Created).
   If client request was invalid, or not authorized, the AS returns an
   error response as described in Section 5.6.3.

   Note that the AS decides which token type and profile to use when
   issuing a successful response.  It is assumed that the AS has prior
   knowledge of the capabilities of the client and the RS (see
   Appendix D.  This prior knowledge may, for example, be set by the use
   of a dynamic client registration protocol exchange [RFC7591].

   The content of the successful reply is the RS Information.  It MUST
   be encoded as CBOR map, containing parameters as specified in section
   5.1 of [RFC6749].  In addition to these parameters, the following
   parameters are also part of a successful response:





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   profile  OPTIONAL.  This indicates the profile that the client MUST
      use towards the RS.  See Section 5.6.4.4 for the formatting of
      this parameter.

      . If this parameter is absent, the AS assumes that the client
      implicitly knows which profile to use towards the RS.
   cnf  REQUIRED if the token type is "pop" and a symmetric key is used.
      MUST NOT be present otherwise.  This field contains the symmetric
      proof-of-possession key the client is supposed to use.  See
      Section 5.6.4.5 for details on the use of this parameter.
   rs_cnf  OPTIONAL if the token type is "pop" and asymmetric keys are
      used.  MUST NOT be present otherwise.  This field contains
      information about the public key used by the RS to authenticate.
      See Section 5.6.4.5 for details on the use of this parameter.  If
      this parameter is absent, the AS assumes that the client already
      knows the public key of the RS.
   token_type  OPTIONAL.  By default implementations of this framework
      SHOULD assume that the token_type is "pop".  If a specific use
      case requires another token_type (e.g., "Bearer") to be used then
      this parameter is REQUIRED.

   Note that if CBOR Web Tokens [I-D.ietf-ace-cbor-web-token] are used,
   the access token can also contain a "cnf" claim
   [I-D.ietf-ace-cwt-proof-of-possession].  This claim is however
   consumed by a different party.  The access token is created by the AS
   and processed by the RS (and opaque to the client) whereas the RS
   Information is created by the AS and processed by the client; it is
   never forwarded to the resource server.

   Figure 8 summarizes the parameters that may be part of the RS
   Information.




















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           /-------------------+-----------------\
           | Parameter name    | Specified in    |
           |-------------------+-----------------|
           | access_token      |  RFC 6749       |
           | token_type        |  RFC 6749       |
           | expires_in        |  RFC 6749       |
           | refresh_token     |  RFC 6749       |
           | scope             |  RFC 6749       |
           | state             |  RFC 6749       |
           | error             |  RFC 6749       |
           | error_description |  RFC 6749       |
           | error_uri         |  RFC 6749       |
           | profile           | [this document] |
           | cnf               | [this document] |
           | rs_cnf            | [this document] |
           \-------------------+-----------------/

                    Figure 8: RS Information parameters

   Figure 9 shows a response containing a token and a "cnf" parameter
   with a symmetric proof-of-possession key.  Note that transport layer
   security is assumed in this example, therefore the Content-Type is
   "application/cbor".

   Header: Created (Code=2.01)
   Content-Type: "application/cbor"
   Payload:
   {
     "access_token" : b64'SlAV32hkKG ...
      (remainder of CWT omitted for brevity;
      CWT contains COSE_Key in the "cnf" claim)',
     "profile" : "coap_dtls",
     "expires_in" : "3600",
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : b64'39Gqlw',
         "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
       }
     }
   }

       Figure 9: Example AS response with an access token bound to a
                              symmetric key.







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5.6.3.  Error Response

   The error responses for CoAP-based interactions with the AS are
   equivalent to the ones for HTTP-based interactions as defined in
   section 5.2 of [RFC6749], with the following differences:

   o  The Content-Type MUST be specified by the communication security
      profile used between client and AS.  The raw payload before being
      processed by the communication security protocol MUST be encoded
      as a CBOR map.
   o  A response code equivalent to the CoAP code 4.00 (Bad Request)
      MUST be used for all error responses, except for invalid_client
      where a response code equivalent to the CoAP code 4.01
      (Unauthorized) MAY be used under the same conditions as specified
      in section 5.2 of [RFC6749].
   o  The parameters "error", "error_description" and "error_uri" MUST
      be abbreviated using the codes specified in Figure 12.
   o  The error code (i.e., value of the "error" parameter) MUST be
      abbreviated as specified in Figure 10.

           /------------------------+----------\
           | Name                   | CBOR Key |
           |------------------------+----------|
           | invalid_request        |     0    |
           | invalid_client         |     1    |
           | invalid_grant          |     2    |
           | unauthorized_client    |     3    |
           | unsupported_grant_type |     4    |
           | invalid_scope          |     5    |
           | unsupported_pop_key    |     6    |
           \------------------------+----------/

           Figure 10: CBOR abbreviations for common error codes

   In addition to the error responses defined in OAuth 2.0, the
   following behavior MUST be implemented by the AS: If the client
   submits an asymmetric key in the token request that the RS cannot
   process, the AS MUST reject that request with a response code
   equivalent to the CoAP code 4.00 (Bad Request) including the error
   code "unsupported_pop_key" defined in Figure 10.

5.6.4.  Request and Response Parameters

   This section provides more detail about the new parameters that can
   be used in access token requests and responses, as well as
   abbreviations for more compact encoding of existing parameters and
   common parameter values.




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5.6.4.1.  Audience

   This parameter specifies for which audience the client is requesting
   a token.  It should be encoded as CBOR text string (major type 3).
   The formatting and semantics of these strings are application
   specific.

5.6.4.2.  Grant Type

   The abbreviations in Figure 11 MUST be used in CBOR encodings instead
   of the string values defined in [RFC6749].

           /--------------------+----------+------------------------\
           | Name               | CBOR Key | Original Specification |
           |--------------------+----------+------------------------|
           | password           |     0    |       RFC6749          |
           | authorization_code |     1    |       RFC6749          |
           | client_credentials |     2    |       RFC6749          |
           | refresh_token      |     3    |       RFC6749          |
           \--------------------+----------+------------------------/

           Figure 11: CBOR abbreviations for common grant types

5.6.4.3.  Token Type

   The token_type parameter is defined in [RFC6749], allowing the AS to
   indicate to the client which type of access token it is receiving
   (e.g., a bearer token).

   This document registers the new value "pop" for the OAuth Access
   Token Types registry, specifying a Proof-of-Possession token.  How
   the proof-of-possession is performed MUST be specified by the
   profiles.

   The values in the "token_type" parameter MUST be CBOR text strings
   (major type 3).

   In this framework token type "pop" MUST be assumed by default if the
   AS does not provide a different value.

5.6.4.4.  Profile

   Profiles of this framework MUST define the communication protocol and
   the communication security protocol between the client and the RS.
   The security protocol MUST provide encryption, integrity and replay
   protection.  Furthermore profiles MUST define proof-of-possession
   methods, if they support proof-of-possession tokens.




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   A profile MUST specify an identifier that can be used to uniquely
   identify itself in the "profile" parameter.

   Profiles MAY define additional parameters for both the token request
   and the RS Information in the access token response in order to
   support negotiation or signaling of profile specific parameters.

5.6.4.5.  Confirmation

   The "cnf" parameter identifies or provides the key used for proof-of-
   possession, while the "rs_cnf" parameter provides the raw public key
   of the RS.  Both parameters use the same formatting and semantics as
   the "cnf" claim specified in [I-D.ietf-ace-cwt-proof-of-possession].

   In addition to the use as a claim in a CWT, the "cnf" parameter is
   used in the following contexts with the following meaning:

   o  In the token request C -> AS, to indicate the client's raw public
      key, or the key-identifier of a previously established key between
      C and RS.
   o  In the token response AS -> C, to indicate the symmetric key
      generated by the AS for proof-of-possession.
   o  In the introspection response AS -> RS, to indicate the proof-of-
      possession key bound to the introspected token.
   o  In the client token AS -> RS -> C, to indicate the proof-of-
      possession key bound to the access token.

   Note that the COSE_Key structure in a "cnf" claim or parameter may
   contain an "alg" or "key_ops" parameter.  If such parameters are
   present, a client MUST NOT use a key that is not compatible with the
   profile or proof-of-possession algorithm according to those
   parameters.  An RS MUST reject a proof-of-possession using such a
   key.

5.6.5.  Mapping parameters to CBOR

   All OAuth parameters in access token requests and responses MUST be
   mapped to CBOR types as specified in Figure 12, using the given
   integer abbreviation for the key.

   Note that we have aligned these abbreviations with the claim
   abbreviations defined in [I-D.ietf-ace-cbor-web-token].









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           /-------------------+----------+---------------------\
           | Name              | CBOR Key | Major Type          |
           |-------------------+----------+---------------------|
           | aud               | 3        | text string         |
           | client_id         | 8        | text string         |
           | client_secret     | 9        | byte string         |
           | response_type     | 10       | text string         |
           | redirect_uri      | 11       | text string         |
           | scope             | 12       | text or byte string |
           | state             | 13       | text string         |
           | code              | 14       | byte string         |
           | error             | 15       | text string         |
           | error_description | 16       | text string         |
           | error_uri         | 17       | text string         |
           | grant_type        | 18       | unsigned integer    |
           | access_token      | 19       | text string         |
           | token_type        | 20       | unsigned integer    |
           | expires_in        | 21       | unsigned integer    |
           | username          | 22       | text string         |
           | password          | 23       | text string         |
           | refresh_token     | 24       | text string         |
           | cnf               | 25       | map                 |
           | profile           | 26       | text string         |
           | rs_cnf            | 31       | map                 |
           \-------------------+----------+---------------------/

              Figure 12: CBOR mappings used in token requests

5.7.  The 'Introspect' Endpoint

   Token introspection [RFC7662] can be OPTIONALLY provided by the AS,
   and is then used by the RS and potentially the client to query the AS
   for metadata about a given token e.g., validity or scope.  Analogous
   to the protocol defined in RFC 7662 [RFC7662] for HTTP and JSON, this
   section defines adaptations to more constrained environments using
   CBOR and leaving the choice of the application protocol to the
   profile.

   Communication between the RS and the introspection endpoint at the AS
   MUST be integrity protected and encrypted.  Furthermore AS and RS
   MUST perform mutual authentication.  Finally the AS SHOULD verify
   that the RS has the right to access introspection information about
   the provided token.  Profiles of this framework that support
   introspection MUST specify how authentication and communication
   security between RS and AS is implemented.






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   The default name of this endpoint in an url-path is 'introspect',
   however implementations are not required to use this name and can
   define their own instead.

   The figures of this section uses CBOR diagnostic notation without the
   integer abbreviations for the parameters or their values for better
   readability.

   Note that supporting introspection is OPTIONAL for implementations of
   this framework.

5.7.1.  RS-to-AS Request

   The RS sends a POST request to the introspection endpoint at the AS,
   the profile MUST specify the Content-Type and wrapping of the
   payload.  The payload MUST be encoded as a CBOR map with a "token"
   parameter containing either the access token or a reference to the
   token (e.g., the cti).  Further optional parameters representing
   additional context that is known by the RS to aid the AS in its
   response MAY be included.

   The same parameters are required and optional as in section 2.1 of
   RFC 7662 [RFC7662].

   For example, Figure 13 shows a RS calling the token introspection
   endpoint at the AS to query about an OAuth 2.0 proof-of-possession
   token.  Note that object security based on COSE is assumed in this
   example, therefore the Content-Type is "application/cose+cbor".

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "introspect"
   Content-Type: "application/cose+cbor"
   Payload:
   {
     "token" : b64'7gj0dXJQ43U',
     "token_type_hint" : "pop"
   }

                 Figure 13: Example introspection request.

5.7.2.  AS-to-RS Response

   If the introspection request is authorized and successfully
   processed, the AS sends a response with the response code equivalent
   to the CoAP code 2.01 (Created).  If the introspection request was
   invalid, not authorized or couldn't be processed the AS returns an
   error response as described in Section 5.7.3.



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   In a successful response, the AS encodes the response parameters in a
   CBOR map including with the same required and optional parameters as
   in section 2.2. of RFC 7662 [RFC7662] with the following additions:

   cnf  OPTIONAL.  This field contains information about the proof-of-
      possession key that binds the client to the access token.  See
      Section 5.6.4.5 for more details on the use of the "cnf"
      parameter.
   profile  OPTIONAL.  This indicates the profile that the RS MUST use
      with the client.  See Section 5.6.4.4 for more details on the
      formatting of this parameter.
   client_token  OPTIONAL.  This parameter contains information that the
      RS MUST pass on to the client.  See Section 5.7.4 for more
      details.

   For example, Figure 14 shows an AS response to the introspection
   request in Figure 13.  Note that transport layer security is assumed
   in this example, therefore the Content-Type is "application/cbor".

   Header: Created Code=2.01)
   Content-Type: "application/cbor"
   Payload:
   {
     "active" : true,
     "scope" : "read",
     "profile" : "coap_dtls",
     "client_token" : b64'2QPhg0OhAQo ...
     (remainder of client token omitted for brevity)',
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : b64'39Gqlw',
         "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
       }
     }
   }

                Figure 14: Example introspection response.

5.7.3.  Error Response

   The error responses for CoAP-based interactions with the AS are
   equivalent to the ones for HTTP-based interactions as defined in
   section 2.3 of [RFC7662], with the following differences:

   o  If content is sent, the Content-Type MUST be set according to the
      specification of the communication security profile, and the
      content payload MUST be encoded as a CBOR map.



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   o  If the credentials used by the RS are invalid the AS MUST respond
      with the response code equivalent to the CoAP code 4.01
      (Unauthorized) and use the required and optional parameters from
      section 5.2 in RFC 6749 [RFC6749].
   o  If the RS does not have the right to perform this introspection
      request, the AS MUST respond with a response code equivalent to
      the CoAP code 4.03 (Forbidden).  In this case no payload is
      returned.
   o  The parameters "error", "error_description" and "error_uri" MUST
      be abbreviated using the codes specified in Figure 12.
   o  The error codes MUST be abbreviated using the codes specified in
      Figure 10.

   Note that a properly formed and authorized query for an inactive or
   otherwise invalid token does not warrant an error response by this
   specification.  In these cases, the authorization server MUST instead
   respond with an introspection response with the "active" field set to
   "false".

5.7.4.  Client Token

   In cases where the client has limited connectivity and needs to get
   access to a previously unknown resource servers, this framework
   suggests the following OPTIONAL approach: The client is pre-
   configured with a long-term access token, which is not self-contained
   (i.e. it is only a reference to a token at the AS) when it is
   commissioned.  When the client then tries to access a RS it transmits
   this access token.  The RS then performs token introspection to learn
   what access this token grants.  In the introspection response, the AS
   also relays information for the client, such as the proof-of-
   possession key, through the RS.  The RS passes on this Client Token
   to the client in response to the submission of the token.

   The client_token parameter is designed to carry such information, and
   is intended to be used as described in Figure 15.
















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                     Resource       Authorization
    Client            Server           Server
       |                |                |
       |                |                |
   C:  +--------------->|                |
       |  POST          |                |
       |  Access Token  |                |
       |            D:  +--------------->|
       |                | Introspection  |
       |                |    Request     |
       |                |                |
       |            E:  +<---------------+
       |                | Introspection  |
       |                |   Response     |
       |                | + Client Token |
       |<---------------+                |
       |  2.01 Created  |                |
       | + Client Token |

               Figure 15: Use of the client_token parameter.

   The client token is a COSE_Encrypted object, containing as payload a
   CBOR map with the following claims:

   cnf  REQUIRED if the token type is "pop", OPTIONAL otherwise.
      Contains information about the proof-of-possession key the client
      is to use with its access token.  See Section 5.6.4.5.
   token_type  OPTIONAL.  See Section 5.6.4.3.
   profile  REQUIRED.  See Section 5.6.4.4.
   rs_cnf  OPTIONAL.  Contains information about the key that the RS
      uses to authenticate towards the client.  If the key is symmetric
      then this claim MUST NOT be part of the Client Token, since this
      is the same key as the one specified through the "cnf" claim.
      This claim uses the same encoding as the "cnf" parameter.  See
      Section 5.6.4.4.

   The AS encrypts this token using a key shared between the AS and the
   client, so that only the client can decrypt it and access its
   payload.  How this key is established is out of scope of this
   framework, however it can be established at the same time at which
   the client's long term token is created.

   An RS that is configured to perform introspection, MUST do so
   immediately after receiving an access token, in order to be able to
   return a potential client token to the client.  This does not
   preclude the RS to perform additional introspection asynchronously,
   e.g., when the token is later used.




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5.7.5.  Mapping Introspection parameters to CBOR

   The introspection request and response parameters MUST be mapped to
   CBOR types as specified in Figure 16, using the given integer
   abbreviation for the key.

   Note that we have aligned these abbreviations with the claim
   abbreviations defined in [I-D.ietf-ace-cbor-web-token].

       /-----------------+----------+-----------------------\
       | Parameter name  | CBOR Key | Major Type            |
       |-----------------+----------+-----------------------|
       | iss             | 1        | text string           |
       | sub             | 2        | text string           |
       | aud             | 3        | text string           |
       | exp             | 4        | Epoch-based date/time |
       | nbf             | 5        | Epoch-based date/time |
       | iat             | 6        | Epoch-based date/time |
       | cti             | 7        | byte string           |
       | client_id       | 8        | text string           |
       | scope           | 12       | text OR byte string   |
       | token_type      | 20       | text string           |
       | username        | 22       | text string           |
       | cnf             | 25       | map                   |
       | profile         | 26       | unsigned integer      |
       | token           | 27       | text string           |
       | token_type_hint | 28       | text string           |
       | active          | 29       | unsigned integer      |
       | client_token    | 30       | byte string           |
       | rs_cnf          | 31       | map                   |
       \-----------------+----------+-----------------------/

        Figure 16: CBOR Mappings to Token Introspection Parameters.

5.8.  The Access Token

   This framework RECOMMENDS the use of CBOR web token (CWT) as
   specified in [I-D.ietf-ace-cbor-web-token].

   In order to facilitate offline processing of access tokens, this
   draft uses the "cnf" claim from
   [I-D.ietf-ace-cwt-proof-of-possession] and specifies the "scope"
   claim for both JSON and CBOR web tokens.

   The "scope" claim explicitly encodes the scope of a given access
   token.  This claim follows the same encoding rules as defined in
   section 3.3 of [RFC6749], but in addition implementers MAY use byte
   arrays as scope values, to achieve compact encoding of large scope



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   elements.  The meaning of a specific scope value is application
   specific and expected to be known to the RS running that application.

5.8.1.  The 'Authorization Information' Endpoint

   The access token, containing authorization information and
   information about the key used by the client, needs to be transported
   to the RS so that the RS can authenticate and authorize the client
   request.

   This section defines a method for transporting the access token to
   the RS using a RESTful protocol such as CoAP.  Profiles of this
   framework MAY define other methods for token transport.

   The method consists of an authz-info endpoint, implemented by the RS.
   A client using this method MUST make a POST request to the authz-info
   endpoint at the RS with the access token in the payload.  The RS
   receiving the token MUST verify the validity of the token.  If the
   token is valid, the RS MUST respond to the POST request with 2.01
   (Created).  This response MAY contain an identifier of the token
   (e.g., the cti for a CWT) as a payload, in order to allow the client
   to refer to the token.

   The RS MUST be prepared to store at least one access token for future
   use.  This is a difference to how access tokens are handled in OAuth
   2.0, where the access token is typically sent along with each
   request, and therefore not stored at the RS.

   If the token is not valid, the RS MUST respond with a response code
   equivalent to the CoAP code 4.01 (Unauthorized).  If the token is
   valid but the audience of the token does not match the RS, the RS
   MUST respond with a response code equivalent to the CoAP code 4.03
   (Forbidden).  If the token is valid but is associated to claims that
   the RS cannot process (e.g., an unknown scope) the RS MUST respond
   with a response code equivalent to the CoAP code 4.00 (Bad Request).
   In the latter case the RS MAY provide additional information in the
   error response, in order to clarify what went wrong.

   The RS MAY make an introspection request to validate the token before
   responding to the POST request to the authz-info endpoint.  If the
   introspection response contains a client token (Section 5.7.4) then
   this token SHALL be included in the payload of the 2.01 (Created)
   response.

   Profiles MUST specify how the authz-info endpoint is protected.  Note
   that since the token contains information that allow the client and
   the RS to establish a security context in the first place, mutual
   authentication may not be possible at this point.



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   The default name of this endpoint in an url-path is 'authz-info',
   however implementations are not required to use this name and can
   define their own instead.

5.8.2.  Token Expiration

   Depending on the capabilities of the RS, there are various ways in
   which it can verify the validity of a received access token.  Here
   follows a list of the possibilities including what functionality they
   require of the RS.

   o  The token is a CWT and includes an "exp" claim and possibly the
      "nbf" claim.  The RS verifies these by comparing them to values
      from its internal clock as defined in [RFC7519].  In this case the
      RS's internal clock must reflect the current date and time, or at
      least be synchronized with the AS's clock.  How this clock
      synchronization would be performed is out of scope for this
      specification.
   o  The RS verifies the validity of the token by performing an
      introspection request as specified in Section 5.7.  This requires
      the RS to have a reliable network connection to the AS and to be
      able to handle two secure sessions in parallel (C to RS and AS to
      RS).
   o  The RS and the AS both store a sequence number linked to their
      common security association.  The AS increments this number for
      each access token it issues and includes it in the access token,
      which is a CWT.  The RS keeps track of the most recently received
      sequence number, and only accepts tokens as valid, that are in a
      certain range around this number.  This method does only require
      the RS to keep track of the sequence number.  The method does not
      provide timely expiration, but it makes sure that older tokens
      cease to be valid after a certain number of newer ones got issued.
      For a constrained RS with no network connectivity and no means of
      reliably measuring time, this is the best that can be achieved.

   If a token that authorizes a long running request such as a CoAP
   Observe [RFC7641] expires, the RS MUST send an error response with
   the response code 4.01 Unauthorized to the client and then terminate
   processing the long running request.

6.  Security Considerations

   Security considerations applicable to authentication and
   authorization in RESTful environments provided in OAuth 2.0 [RFC6749]
   apply to this work, as well as the security considerations from
   [I-D.ietf-ace-actors].  Furthermore [RFC6819] provides additional
   security considerations for OAuth which apply to IoT deployments as
   well.



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   A large range of threats can be mitigated by protecting the contents
   of the access token by using a digital signature or a keyed message
   digest (MAC) or an Authenticated Encryption with Associated Data
   (AEAD) algorithm.  Consequently, the token integrity protection MUST
   be applied to prevent the token from being modified, particularly
   since it contains a reference to the symmetric key or the asymmetric
   key.  If the access token contains the symmetric key, this symmetric
   key MUST be encrypted by the authorization server so that only the
   resource server can decrypt it.  Note that using an AEAD algorithm is
   preferable over using a MAC unless the message needs to be publicly
   readable.

   It is important for the authorization server to include the identity
   of the intended recipient (the audience), typically a single resource
   server (or a list of resource servers), in the token.  Using a single
   shared secret with multiple resource servers to simplify key
   management is NOT RECOMMENDED since the benefit from using the proof-
   of-possession concept is significantly reduced.

   The authorization server MUST offer confidentiality protection for
   any interactions with the client.  This step is extremely important
   since the client may obtain the proof-of-possession key from the
   authorization server for use with a specific access token.  Not using
   confidentiality protection exposes this secret (and the access token)
   to an eavesdropper thereby completely negating proof-of-possession
   security.  Profiles MUST specify how confidentiality protection is
   provided, and additional protection can be applied by encrypting the
   token, for example encryption of CWTs is specified in section 5.1 of
   [I-D.ietf-ace-cbor-web-token].

   Developers MUST ensure that the ephemeral credentials (i.e., the
   private key or the session key) are not leaked to third parties.  An
   adversary in possession of the ephemeral credentials bound to the
   access token will be able to impersonate the client.  Be aware that
   this is a real risk with many constrained environments, since
   adversaries can often easily get physical access to the devices.

   Clients can at any time request a new proof-of-possession capable
   access token.  If clients have that capability, the AS can keep the
   lifetime of the access token and the associated proof-of-possession
   key short and therefore use shorter proof-of-possession key sizes,
   which translate to a performance benefit for the client and for the
   resource server.  Shorter keys also lead to shorter messages
   (particularly with asymmetric keying material).

   When authorization servers bind symmetric keys to access tokens, they
   SHOULD scope these access tokens to a specific permissions.
   Furthermore access tokens using symmetric keys for proof-of-



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   possession SHOULD NOT be targeted at an audience that contains more
   than one RS, since otherwise any RS in the audience that receives
   that access token can impersonate the client towards the other
   members of the audience.

6.1.  Unprotected AS Information

   Initially, no secure channel exists to protect the communication
   between C and RS.  Thus, C cannot determine if the AS information
   contained in an unprotected response from RS to an unauthorized
   request (c.f.  Section 5.1.2) is authentic.  It is therefore
   advisable to provide C with a (possibly hard-coded) list of
   trustworthy authorization servers.  AS information responses
   referring to a URI not listed there would be ignored.

6.2.  Use of Nonces for Replay Protection

   RS may add a nonce to the AS Information message sent as a response
   to an unauthorized request to ensure freshness of an Access Token
   subsequently presented to RS.  While a timestamp of some granularity
   would be sufficient to protect against replay attacks, using
   randomized nonce is preferred to prevent disclosure of information
   about RS's internal clock characteristics.

6.3.  Combining profiles

   There may exist reasonable use cases where implementers want to
   combine different profiles of this framework, e.g., using an MQTT
   profile between client and RS, while using a DTLS profile for
   interactions between client and AS.  Profiles should be designed in a
   way that the security of a protocol interaction does not depend on
   the specific security mechanisms used in other protocol interactions.

6.4.  Error responses

   The various error responses defined in this framework may leak
   information to an adversary.  For example errors responses for
   requests to the Authorization Information endpoint can reveal
   information about an otherwise opaque access token to an adversary
   who has intercepted this token.  This framework is written under the
   assumption that, in general, the benefits of detailed error messages
   outweigh the risk due to information leakage.  For particular use
   cases, where this assessment does not apply, detailed error messages
   can be replaced by more generic ones.







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7.  Privacy Considerations

   Implementers and users should be aware of the privacy implications of
   the different possible deployments of this framework.

   The AS is in a very central position and can potentially learn
   sensitive information about the clients requesting access tokens.  If
   the client credentials grant is used, the AS can track what kind of
   access the client intends to perform.  With other grants this can be
   prevented by the Resource Owner.  To do so, the resource owner needs
   to bind the grants it issues to anonymous, ephemeral credentials that
   do not allow the AS to link different grants and thus different
   access token requests by the same client.

   If access tokens are only integrity protected and not encrypted, they
   may reveal information to attackers listening on the wire, or able to
   acquire the access tokens in some other way.  In the case of CWTs the
   token may e.g., reveal the audience, the scope and the confirmation
   method used by the client.  The latter may reveal the identity of the
   device or application running the client.  This may be linkable to
   the identity of the person using the client (if there is a person and
   not a machine-to-machine interaction).

   Clients using asymmetric keys for proof-of-possession should be aware
   of the consequences of using the same key pair for proof-of-
   possession towards different RSs.  A set of colluding RSs or an
   attacker able to obtain the access tokens will be able to link the
   requests, or even to determine the client's identity.

   An unprotected response to an unauthorized request (c.f.
   Section 5.1.2) may disclose information about RS and/or its existing
   relationship with C.  It is advisable to include as little
   information as possible in an unencrypted response.  Means of
   encrypting communication between C and RS already exist, more
   detailed information may be included with an error response to
   provide C with sufficient information to react on that particular
   error.

8.  IANA Considerations

   This specification registers new parameters for OAuth and establishes
   registries for mappings to CBOR abbreviations.

8.1.  Authorization Server Information

   A new registry will be requested from IANA, entitled "Authorization
   Server Information".  The registry is to be created as Expert Review
   Required.



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   The columns of this table are:

   Name  The name of the parameter
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this parameter name.  Registration in the table is based on the
      value of the mapping requested.  Integer values between 1 and 255
      are designated as Standards Track Document required.  Integer
      values from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Major Type  The CBOR major type allowable for the values of this
      parameter.
   Reference  This contains a pointer to the public specification of the
      grant type abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 2.
   The Reference column for all of these entries will be this document.

8.2.  OAuth Error Code CBOR Mappings Registry

   A new registry will be requested from IANA, entitled "OAuth Error
   Code CBOR Mappings Registry".  The registry is to be created as
   Expert Review Required.

   The columns of this table are:

   Name  The OAuth Error Code name, refers to the name in section 5.2.
      of [RFC6749] e.g., "invalid_request".
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this error code.  Registration in the table is based on the value
      of the mapping requested.  Integer values between 1 and 255 are
      designated as Standards Track Document required.  Integer values
      from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Reference  This contains a pointer to the public specification of the
      grant type abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 10.
   The Reference column for all of these entries will be this document.

8.3.  OAuth Grant Type CBOR Mappings

   A new registry will be requested from IANA, entitled "OAuth Grant
   Type CBOR Mappings".  The registry is to be created as Expert Review
   Required.

   The columns of this table are:





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   Name  The name of the grant type as specified in Section 1.3 of
      [RFC6749].
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this grant type.  Registration in the table is based on the value
      of the mapping requested.  Integer values between 1 and 255 are
      designated as Standards Track Document required.  Integer values
      from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Reference  This contains a pointer to the public specification of the
      grant type abbreviation, if one exists.
   Original Specification  This contains a pointer to the public
      specification of the grant type, if one exists.

   This registry will be initially populated by the values in Figure 11.
   The Reference column for all of these entries will be this document.

8.4.  OAuth Access Token Types

   This specification registers the following new token type in the
   OAuth Access Token Types Registry

   o  Name: "PoP"
   o  Change Controller: IETF
   o  Reference: [this document]

8.5.  OAuth Token Type CBOR Mappings

   A new registry will be requested from IANA, entitled "Token Type CBOR
   Mappings".  The registry is to be created as Expert Review Required.

   The columns of this table are:

   Name  The name of token type as registered in the OAuth Access Token
      Types registry e.g., "Bearer".
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this access token type.  Registration in the table is based on the
      value of the mapping requested.  Integer values between 1 and 255
      are designated as Standards Track Document required.  Integer
      values from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Reference  This contains a pointer to the public specification of the
      OAuth token type abbreviation, if one exists.
   Original Specification  This contains a pointer to the public
      specification of the grant type, if one exists.







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8.5.1.  Initial Registry Contents

   o  Name: "Bearer"
   o  Value: 1
   o  Reference: [this document]
   o  Original Specification: [RFC6749]

   o  Name: "pop"
   o  Value: 2
   o  Reference: [this document]
   o  Original Specification: [this document]

8.6.  ACE OAuth Profile Registry

   A new registry will be requested from IANA, entitled "ACE Profile
   Registry".  The registry is to be created as Expert Review Required.

   The columns of this table are:

   Name  The name of the profile, to be used as value of the profile
      attribute.
   Description  Text giving an overview of the profile and the context
      it is developed for.
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this profile name.  Registration in the table is based on the
      value of the mapping requested.  Integer values between 1 and 255
      are designated as Standards Track Document required.  Integer
      values from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Reference  This contains a pointer to the public specification of the
      profile abbreviation, if one exists.

8.7.  OAuth Parameter Registration

   This specification registers the following parameters in the OAuth
   Parameters Registry

   o  Name: "profile"
   o  Parameter Usage Location: token request, token response
   o  Change Controller: IESG
   o  Reference: Section 5.6.4.4 of [this document]

   o  Name: "cnf"
   o  Parameter Usage Location: token request, token response
   o  Change Controller: IESG
   o  Reference: Section 5.6.4.5 of [this document]

   o  Name: "rs_cnf"



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   o  Parameter Usage Location: token response
   o  Change Controller: IESG
   o  Reference: Section 5.6.4.5 of [this document]

8.8.  OAuth CBOR Parameter Mappings Registry

   A new registry will be requested from IANA, entitled "Token Endpoint
   CBOR Mappings Registry".  The registry is to be created as Expert
   Review Required.

   The columns of this table are:

   Name  The OAuth Parameter name, refers to the name in the OAuth
      parameter registry e.g., "client_id".
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this parameter.  Registration in the table is based on the value
      of the mapping requested.  Integer values between 1 and 255 are
      designated as Standards Track Document required.  Integer values
      from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Major Type  The allowable CBOR data types for values of this
      parameter.
   Reference  This contains a pointer to the public specification of the
      grant type abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 12.
   The Reference column for all of these entries will be this document.

   Note that these mappings intentionally coincide with the CWT claim
   name mappings from [I-D.ietf-ace-cbor-web-token].

8.9.  OAuth Introspection Response Parameter Registration

   This specification registers the following parameters in the OAuth
   Token Introspection Response registry.

   o  Name: "cnf"
   o  Description: Key to prove the right to use an access token,
      formatted as specified in [I-D.ietf-ace-cwt-proof-of-possession].
   o  Change Controller: IESG
   o  Reference: Section 5.7.2 of [this document]

   o  Name: "profile"
   o  Description: The communication and communication security profile
      used between client and RS, as defined in ACE profiles.
   o  Change Controller: IESG
   o  Reference: Section 5.7.2 of [this document]




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   o  Name: "client_token"
   o  Description: Information that the RS MUST pass to the client e.g.,
      about the proof-of-possession keys.
   o  Change Controller: IESG
   o  Reference: Section 5.7.2 of [this document]

8.10.  Introspection Endpoint CBOR Mappings Registry

   A new registry will be requested from IANA, entitled "Introspection
   Endpoint CBOR Mappings Registry".  The registry is to be created as
   Expert Review Required.

   The columns of this table are:

   Name  The OAuth Parameter name, refers to the name in the OAuth
      parameter registry e.g., "client_id".
   CBOR Key  The unsigned integer value (CBOR major type 0) abbreviating
      this parameter.  Registration in the table is based on the value
      of the mapping requested.  Integer values between 1 and 255 are
      designated as Standards Track Document required.  Integer values
      from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as private use.
   Major Type  The allowable CBOR data types for values of this
      parameter.
   Reference  This contains a pointer to the public specification of the
      grant type abbreviation, if one exists.

   This registry will be initially populated by the values in Figure 16.
   The Reference column for all of these entries will be this document.

8.11.  JSON Web Token Claims

   This specification registers the following new claims in the JSON Web
   Token (JWT) registry of JSON Web Token Claims:

   o  Claim Name: "scope"
   o  Claim Description: The scope of an access token as defined in
      [RFC6749].
   o  Change Controller: IESG
   o  Reference: Section 5.8 of [this document]

8.12.  CBOR Web Token Claims

   This specification registers the following new claims in the CBOR Web
   Token (CWT) registry of CBOR Web Token Claim:s

   o  Claim Name: "scope"




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   o  Claim Description: The scope of an access token as defined in
      [RFC6749].
   o  JWT Claim Name: N/A
   o  Claim Key: 12
   o  Claim Value Type(s): 0 (uint), 2 (byte string), 3 (text string)
   o  Change Controller: IESG
   o  Specification Document(s): Section 5.8 of [this document]

8.13.  CoAP Option Number Registration

   This section registers the "Access-Token" CoAP Option Number in the
   "CoRE Parameters" sub-registry "CoAP Option Numbers" in the manner
   described in [RFC7252].

   o  Name: "Access-Token"
   o  Number: TBD
   o  Reference: [this document].
   o  Meaning in Request: Contains an Access Token according to [this
      document] containing access permissions of the client.
   o  Meaning in Response: Not used in response.
   o  Safe-to-Forward: Yes
   o  Format: Based on the observer the format is perceived differently.
      Opaque data to the client and CWT or reference token to the RS.
   o  Length: Less than 255 bytes

9.  Acknowledgments

   This document is a product of the ACE working group of the IETF.

   Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and
   UMA in IoT scenarios, Robert Taylor for his discussion input, and
   Malisa Vucinic for his input on the predecessors of this proposal.

   Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from
   where large parts of the security considerations where copied.

   Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for
   contributing their work on AS discovery from draft-gerdes-ace-dcaf-
   authorize (see Section 5.1).

   Ludwig Seitz and Goeran Selander worked on this document as part of
   the CelticPlus project CyberWI, with funding from Vinnova.

10.  References







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10.1.  Normative References

   [I-D.ietf-ace-cbor-web-token]
              Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-09
              (work in progress), October 2017.

   [I-D.ietf-ace-cwt-proof-of-possession]
              Jones, M., Seitz, L., Selander, G., Wahlstroem, E.,
              Erdtman, S., and H. Tschofenig, "Proof-of-Possession Key
              Semantics for CBOR Web Tokens (CWTs)", draft-ietf-ace-cwt-
              proof-of-possession-01 (work in progress), October 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014, <https://www.rfc-
              editor.org/info/rfc7252>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

10.2.  Informative References

   [I-D.erdtman-ace-rpcc]
              Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared-
              Key as OAuth client credentials", draft-erdtman-ace-
              rpcc-02 (work in progress), October 2017.





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   [I-D.ietf-ace-actors]
              Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
              architecture for authorization in constrained
              environments", draft-ietf-ace-actors-06 (work in
              progress), November 2017.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-06 (work in
              progress), October 2017.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-12 (work in progress), October 2017.

   [I-D.ietf-oauth-device-flow]
              Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Device Flow for Browserless and Input
              Constrained Devices", draft-ietf-oauth-device-flow-07
              (work in progress), October 2017.

   [I-D.ietf-oauth-discovery]
              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-07 (work in progress), September 2017.

   [I-D.ietf-oauth-native-apps]
              Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              draft-ietf-oauth-native-apps-12 (work in progress), June
              2017.

   [Margi10impact]
              Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr,
              M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold,
              "Impact of Operating Systems on Wireless Sensor Networks
              (Security) Applications and Testbeds", Proceedings of
              the 19th International Conference on Computer
              Communications and Networks (ICCCN), 2010 August.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.







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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
              editor.org/info/rfc5246>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013, <https://www.rfc-
              editor.org/info/rfc6819>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014, <https://www.rfc-
              editor.org/info/rfc7228>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
              editor.org/info/rfc7231>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/info/rfc7521>.







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   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,
              <https://www.rfc-editor.org/info/rfc7591>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015, <https://www.rfc-
              editor.org/info/rfc7641>.

   [RFC7744]  Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M.,
              and S. Kumar, "Use Cases for Authentication and
              Authorization in Constrained Environments", RFC 7744,
              DOI 10.17487/RFC7744, January 2016, <https://www.rfc-
              editor.org/info/rfc7744>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016, <https://www.rfc-
              editor.org/info/rfc7959>.

Appendix A.  Design Justification

   This section provides further insight into the design decisions of
   the solution documented in this document.  Section 3 lists several
   building blocks and briefly summarizes their importance.  The
   justification for offering some of those building blocks, as opposed
   to using OAuth 2.0 as is, is given below.

   Common IoT constraints are:

   Low Power Radio:

      Many IoT devices are equipped with a small battery which needs to
      last for a long time.  For many constrained wireless devices, the
      highest energy cost is associated to transmitting or receiving
      messages (roughly by a factor of 10 compared to e.g.  AES)
      [Margi10impact].  It is therefore important to keep the total
      communication overhead low, including minimizing the number and
      size of messages sent and received, which has an impact of choice
      on the message format and protocol.  By using CoAP over UDP and
      CBOR encoded messages, some of these aspects are addressed.
      Security protocols contribute to the communication overhead and
      can, in some cases, be optimized.  For example, authentication and
      key establishment may, in certain cases where security
      requirements allow, be replaced by provisioning of security
      context by a trusted third party, using transport or application
      layer security.



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   Low CPU Speed:

      Some IoT devices are equipped with processors that are
      significantly slower than those found in most current devices on
      the Internet.  This typically has implications on what timely
      cryptographic operations a device is capable of performing, which
      in turn impacts e.g., protocol latency.  Symmetric key
      cryptography may be used instead of the computationally more
      expensive public key cryptography where the security requirements
      so allows, but this may also require support for trusted third
      party assisted secret key establishment using transport or
      application layer security.
   Small Amount of Memory:

      Microcontrollers embedded in IoT devices are often equipped with
      small amount of RAM and flash memory, which places limitations
      what kind of processing can be performed and how much code can be
      put on those devices.  To reduce code size fewer and smaller
      protocol implementations can be put on the firmware of such a
      device.  In this case, CoAP may be used instead of HTTP, symmetric
      key cryptography instead of public key cryptography, and CBOR
      instead of JSON.  Authentication and key establishment protocol,
      e.g., the DTLS handshake, in comparison with assisted key
      establishment also has an impact on memory and code.

   User Interface Limitations:

      Protecting access to resources is both an important security as
      well as privacy feature.  End users and enterprise customers may
      not want to give access to the data collected by their IoT device
      or to functions it may offer to third parties.  Since the
      classical approach of requesting permissions from end users via a
      rich user interface does not work in many IoT deployment
      scenarios, these functions need to be delegated to user-controlled
      devices that are better suitable for such tasks, such as smart
      phones and tablets.

   Communication Constraints:

      In certain constrained settings an IoT device may not be able to
      communicate with a given device at all times.  Devices may be
      sleeping, or just disconnected from the Internet because of
      general lack of connectivity in the area, for cost reasons, or for
      security reasons, e.g., to avoid an entry point for Denial-of-
      Service attacks.

      The communication interactions this framework builds upon (as
      shown graphically in Figure 1) may be accomplished using a variety



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      of different protocols, and not all parts of the message flow are
      used in all applications due to the communication constraints.
      Deployments making use of CoAP are expected, but not limited to,
      other protocols such as HTTP, HTTP/2 or other specific protocols,
      such as Bluetooth Smart communication, that do not necessarily use
      IP could also be used.  The latter raises the need for application
      layer security over the various interfaces.

   In the light of these constraints we have made the following design
   decisions:

   CBOR, COSE, CWT:

      This framework REQUIRES the use of CBOR [RFC7049] as data format.
      Where CBOR data needs to be protected, the use of COSE [RFC8152]
      is RECOMMENDED.  Furthermore where self-contained tokens are
      needed, this framework RECOMMENDS the use of CWT
      [I-D.ietf-ace-cbor-web-token].  These measures aim at reducing the
      size of messages sent over the wire, the RAM size of data objects
      that need to be kept in memory and the size of libraries that
      devices need to support.

   CoAP:

      This framework RECOMMENDS the use of CoAP [RFC7252] instead of
      HTTP.  This does not preclude the use of other protocols
      specifically aimed at constrained devices, like e.g.  Bluetooth
      Low energy (see Section 3.2).  This aims again at reducing the
      size of messages sent over the wire, the RAM size of data objects
      that need to be kept in memory and the size of libraries that
      devices need to support.

   RS Information:

      This framework defines the name "RS Information" for data
      concerning the RS that the AS returns to the client in an access
      token response (see Section 5.6.2).  This includes the "profile"
      and the "rs_cnf" parameters.  This aims at enabling scenarios,
      where a powerful client, supporting multiple profiles, needs to
      interact with a RS for which it does not know the supported
      profiles and the raw public key.

   Proof-of-Possession:

      This framework makes use of proof-of-possession tokens, using the
      "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession].  A
      semantically and syntactically identical request and response
      parameter is defined for the token endpoint, to allow requesting



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      and stating confirmation keys.  This aims at making token theft
      harder.  Token theft is specifically relevant in constrained use
      cases, as communication often passes through middle-boxes, which
      could be able to steal bearer tokens and use them to gain
      unauthorized access.


   Auth-Info endpoint:

      This framework introduces a new way of providing access tokens to
      a RS by exposing a authz-info endpoint, to which access tokens can
      be POSTed.  This aims at reducing the size of the request message
      and the code complexity at the RS.  The size of the request
      message is problematic, since many constrained protocols have
      severe message size limitations at the physical layer (e.g. in the
      order of 100 bytes).  This means that larger packets get
      fragmented, which in turn combines badly with the high rate of
      packet loss, and the need to retransmit the whole message if one
      packet gets lost.  Thus separating sending of the request and
      sending of the access tokens helps to reduce fragmentation.

   Client Credentials Grant:

      This framework RECOMMENDS the use of the client credentials grant
      for machine-to-machine communication use cases, where manual
      intervention of the resource owner to produce a grant token is not
      feasible.  The intention is that the resource owner would instead
      pre-arrange authorization with the AS, based on the client's own
      credentials.  The client can the (without manual intervention)
      obtain access tokens from the AS.

   Introspection:

      This framework RECOMMENDS the use of access token introspection in
      cases where the client is constrained in a way that it can not
      easily obtain new access tokens (i.e. it has connectivity issues
      that prevent it from communicating with the AS).  In that case
      this framework RECOMMENDS the use of a long-term token, that could
      be a simple reference.  The RS is assumed to be able to
      communicate with the AS, and can therefore perform introspection,
      in order to learn the claims associated with the token reference.
      The advantage of such an approach is that the resource owner can
      change the claims associated to the token reference without having
      to be in contact with the client, thus granting or revoking access
      rights.

   Client Token:




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      In cases where the client is constrained and does not have
      connectivity to the AS, and furthermore does not have a previous
      security relation to the RS that it needs to communicate with,
      this framework proposes the use of "client tokens".  A client
      token is a data object obtained from the AS by the RS, during
      access token introspection.  The RS passes the client token on to
      the client.  It contains information that allows the client to
      perform the proof of possession for its access token and to
      authenticate the RS (e.g. with it's public key).

Appendix B.  Roles and Responsibilities

   Resource Owner

      *  Make sure that the RS is registered at the AS.  This includes
         making known to the AS which profiles, token_types, scopes, and
         key types (symmetric/asymmetric) the RS supports.  Also making
         it known to the AS which audience(s) the RS identifies itself
         with.
      *  Make sure that clients can discover the AS that is in charge of
         the RS.
      *  If the client-credentials grant is used, make sure that the AS
         has the necessary, up-to-date, access control policies for the
         RS.

   Requesting Party

      *  Make sure that the client is provisioned the necessary
         credentials to authenticate to the AS.
      *  Make sure that the client is configured to follow the security
         requirements of the Requesting Party when issuing requests
         (e.g., minimum communication security requirements, trust
         anchors).
      *  Register the client at the AS.  This includes making known to
         the AS which profiles, token_types, and key types (symmetric/
         asymmetric) the client.

   Authorization Server

      *  Register the RS and manage corresponding security contexts.
      *  Register clients and authentication credentials.
      *  Allow Resource Owners to configure and update access control
         policies related to their registered RSs.
      *  Expose the token endpoint to allow clients to request tokens.
      *  Authenticate clients that wish to request a token.
      *  Process a token request using the authorization policies
         configured for the RS.




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      *  Optionally: Expose the introspection endpoint that allows RS's
         to submit token introspection requests.
      *  If providing an introspection endpoint: Authenticate RSs that
         wish to get an introspection response.
      *  If providing an introspection endpoint: Process token
         introspection requests.
      *  Optionally: Handle token revocation.
      *  Optionally: Provide discovery metadata.  See
         [I-D.ietf-oauth-discovery]

   Client

      *  Discover the AS in charge of the RS that is to be targeted with
         a request.
      *  Submit the token request (see step (A) of Figure 1).

         +  Authenticate to the AS.
         +  Optionally (if not pre-configured): Specify which RS, which
            resource(s), and which action(s) the request(s) will target.
         +  If raw public keys (rpk) or certificates are used, make sure
            the AS has the right rpk or certificate for this client.
      *  Process the access token and RS Information (see step (B) of
         Figure 1).

         +  Check that the RS Information provides the necessary
            security parameters (e.g., PoP key, information on
            communication security protocols supported by the RS).
      *  Send the token and request to the RS (see step (C) of
         Figure 1).

         +  Authenticate towards the RS (this could coincide with the
            proof of possession process).
         +  Transmit the token as specified by the AS (default is to the
            authz-info endpoint, alternative options are specified by
            profiles).
         +  Perform the proof-of-possession procedure as specified by
            the profile in use (this may already have been taken care of
            through the authentication procedure).
      *  Process the RS response (see step (F) of Figure 1) of the RS.

   Resource Server

      *  Expose a way to submit access tokens.  By default this is the
         authz-info endpoint.
      *  Process an access token.

         +  Verify the token is from a recognized AS.
         +  Verify that the token applies to this RS.



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         +  Check that the token has not expired (if the token provides
            expiration information).
         +  Check the token's integrity.
         +  Store the token so that it can be retrieved in the context
            of a matching request.
      *  Process a request.

         +  Set up communication security with the client.
         +  Authenticate the client.
         +  Match the client against existing tokens.
         +  Check that tokens belonging to the client actually authorize
            the requested action.
         +  Optionally: Check that the matching tokens are still valid,
            using introspection (if this is possible.)
      *  Send a response following the agreed upon communication
         security.

Appendix C.  Requirements on Profiles

   This section lists the requirements on profiles of this framework,
   for the convenience of profile designers.

   o  Specify the communication protocol the client and RS the must use
      (e.g., CoAP).  Section 5 and Section 5.6.4.4
   o  Specify the security protocol the client and RS must use to
      protect their communication (e.g., OSCOAP or DTLS over CoAP).
      This must provide encryption, integrity and replay protection.
      Section 5.6.4.4
   o  Specify how the client and the RS mutually authenticate.
      Section 4
   o  Specify the Content-format of the protocol messages (e.g.,
      "application/cbor" or "application/cose+cbor").  Section 4
   o  Specify the proof-of-possession protocol(s) and how to select one,
      if several are available.  Also specify which key types (e.g.,
      symmetric/asymmetric) are supported by a specific proof-of-
      possession protocol.  Section 5.6.4.3
   o  Specify a unique profile identifier.  Section 5.6.4.4
   o  If introspection is supported: Specify the communication and
      security protocol for introspection.Section 5.7
   o  Specify the communication and security protocol for interactions
      between client and AS.  Section 5.6
   o  Specify how/if the authz-info endpoint is protected.
      Section 5.8.1
   o  Optionally define other methods of token transport than the authz-
      info endpoint.  Section 5.8.1






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Appendix D.  Assumptions on AS knowledge about C and RS

   This section lists the assumptions on what an AS should know about a
   client and a RS in order to be able to respond to requests to the
   token and introspection endpoints.  How this information is
   established is out of scope for this document.

   o  The identifier of the client or RS.
   o  The profiles that the client or RS supports.
   o  The scopes that the RS supports.
   o  The audiences that the RS identifies with.
   o  The key types (e.g., pre-shared symmetric key, raw public key, key
      length, other key parameters) that the client or RS supports.
   o  The types of access tokens the RS supports (e.g., CWT).
   o  If the RS supports CWTs, the COSE parameters for the crypto
      wrapper (e.g., algorithm, key-wrap algorithm, key-length).
   o  The expiration time for access tokens issued to this RS (unless
      the RS accepts a default time chosen by the AS).
   o  The symmetric key shared between client or RS and AS (if any).
   o  The raw public key of the client or RS (if any).

Appendix E.  Deployment Examples

   There is a large variety of IoT deployments, as is indicated in
   Appendix A, and this section highlights a few common variants.  This
   section is not normative but illustrates how the framework can be
   applied.

   For each of the deployment variants, there are a number of possible
   security setups between clients, resource servers and authorization
   servers.  The main focus in the following subsections is on how
   authorization of a client request for a resource hosted by a RS is
   performed.  This requires the security of the requests and responses
   between the clients and the RS to consider.

   Note: CBOR diagnostic notation is used for examples of requests and
   responses.

E.1.  Local Token Validation

   In this scenario, the case where the resource server is offline is
   considered, i.e., it is not connected to the AS at the time of the
   access request.  This access procedure involves steps A, B, C, and F
   of Figure 1.

   Since the resource server must be able to verify the access token
   locally, self-contained access tokens must be used.




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   This example shows the interactions between a client, the
   authorization server and a temperature sensor acting as a resource
   server.  Message exchanges A and B are shown in Figure 17.

      A: The client first generates a public-private key pair used for
      communication security with the RS.
      The client sends the POST request to the token endpoint at the AS.
      The security of this request can be transport or application
      layer.  It is up the the communication security profile to define.
      In the example transport layer identification of the AS is done
      and the client identifies with client_id and client_secret as in
      classic OAuth.  The request contains the public key of the client
      and the Audience parameter set to "tempSensorInLivingRoom", a
      value that the temperature sensor identifies itself with.  The AS
      evaluates the request and authorizes the client to access the
      resource.
      B: The AS responds with a PoP access token and RS Information.
      The PoP access token contains the public key of the client, and
      the RS Information contains the public key of the RS.  For
      communication security this example uses DTLS RawPublicKey between
      the client and the RS.  The issued token will have a short
      validity time, i.e., "exp" close to "iat", to protect the RS from
      replay attacks.  The token includes the claim such as "scope" with
      the authorized access that an owner of the temperature device can
      enjoy.  In this example, the "scope" claim, issued by the AS,
      informs the RS that the owner of the token, that can prove the
      possession of a key is authorized to make a GET request against
      the /temperature resource and a POST request on the /firmware
      resource.  Note that the syntax and semantics of the scope claim
      are application specific.
      Note: In this example it is assumed that the client knows what
      resource it wants to access, and is therefore able to request
      specific audience and scope claims for the access token.


















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            Authorization
     Client    Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   to identify the AS
       |         |
   A:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"token"
       |         | Content-Type: application/cbor
       |         | Payload: <Request-Payload>
       |         |
   B:  |<--------+ Header: 2.05 Content
       |  2.05   | Content-Type: application/cbor
       |         | Payload: <Response-Payload>
       |         |

      Figure 17: Token Request and Response Using Client Credentials.

   The information contained in the Request-Payload and the Response-
   Payload is shown in Figure 18.  Note that a transport layer security
   based communication security profile is used in this example,
   therefore the Content-Type is "application/cbor".

   Request-Payload :
   {
     "grant_type" : "client_credentials",
     "aud" : "tempSensorInLivingRoom",
     "client_id" : "myclient",
     "client_secret" : "qwerty"
   }

   Response-Payload :
   {
     "access_token" : b64'SlAV32hkKG ...',
     "token_type" : "pop",
     "csp" : "DTLS",
     "rs_cnf" : {
       "COSE_Key" : {
         "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
         "kty" : "EC",
         "crv" : "P-256",
         "x"   : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
         "y"   : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
       }
     }
   }

             Figure 18: Request and Response Payload Details.



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   The content of the access token is shown in Figure 19.

   {
     "aud" : "tempSensorInLivingRoom",
     "iat" : "1360189224",
     "exp" : "1360289224",
     "scope" :  "temperature_g firmware_p",
     "cnf" : {
       "COSE_Key" : {
         "kid" : b64'1Bg8vub9tLe1gHMzV76e8',
         "kty" : "EC",
         "crv" : "P-256",
         "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
         "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
       }
     }
   }

        Figure 19: Access Token including Public Key of the Client.

   Messages C and F are shown in Figure 20 - Figure 21.

      C: The client then sends the PoP access token to the authz-info
      endpoint at the RS.  This is a plain CoAP request, i.e., no
      transport or application layer security between client and RS,
      since the token is integrity protected between the AS and RS.  The
      RS verifies that the PoP access token was created by a known and
      trusted AS, is valid, and responds to the client.  The RS caches
      the security context together with authorization information about
      this client contained in the PoP access token.


              Resource
    Client     Server
       |         |
   C:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"authz-info"
       |         | Payload: SlAV32hkKG ...
       |         |
       |<--------+ Header: 2.04 Changed
       |  2.04   |
       |         |

                Figure 20: Access Token provisioning to RS
      The client and the RS runs the DTLS handshake using the raw public
      keys established in step B and C.





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      The client sends the CoAP request GET to /temperature on RS over
      DTLS.  The RS verifies that the request is authorized, based on
      previously established security context.
      F: The RS responds with a resource representation over DTLS.

              Resource
    Client     Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   using Raw Public Keys
       |         |
       +-------->| Header: GET (Code=0.01)
       | GET     | Uri-Path: "temperature"
       |         |
       |         |
       |         |
   F:  |<--------+ Header: 2.05 Content
       | 2.05    | Payload: <sensor value>
       |         |

        Figure 21: Resource Request and Response protected by DTLS.

E.2.  Introspection Aided Token Validation

   In this deployment scenario it is assumed that a client is not able
   to access the AS at the time of the access request, whereas the RS is
   assumed to be connected to the back-end infrastructure.  Thus the RS
   can make use of token introspection.  This access procedure involves
   steps A-F of Figure 1, but assumes steps A and B have been carried
   out during a phase when the client had connectivity to AS.

   Since the client is assumed to be offline, at least for a certain
   period of time, a pre-provisioned access token has to be long-lived.
   Since the client is constrained, the token will not be self contained
   (i.e. not a CWT) but instead just a reference.  The resource server
   uses its connectivity to learn about the claims associated to the
   access token by using introspection, which is shown in the example
   below.

   In the example interactions between an offline client (key fob), a RS
   (online lock), and an AS is shown.  It is assumed that there is a
   provisioning step where the client has access to the AS.  This
   corresponds to message exchanges A and B which are shown in
   Figure 22.

   Authorization consent from the resource owner can be pre-configured,
   but it can also be provided via an interactive flow with the resource
   owner.  An example of this for the key fob case could be that the



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   resource owner has a connected car, he buys a generic key that he
   wants to use with the car.  To authorize the key fob he connects it
   to his computer that then provides the UI for the device.  After that
   OAuth 2.0 implicit flow can used to authorize the key for his car at
   the the car manufacturers AS.

   Note: In this example the client does not know the exact door it will
   be used to access since the token request is not send at the time of
   access.  So the scope and audience parameters are set quite wide to
   start with and new values different form the original once can be
   returned from introspection later on.

      A: The client sends the request using POST to the token endpoint
      at AS.  The request contains the Audience parameter set to
      "PACS1337" (PACS, Physical Access System), a value the that the
      online door in question identifies itself with.  The AS generates
      an access token as an opaque string, which it can match to the
      specific client, a targeted audience and a symmetric key.  The
      security is provided by identifying the AS on transport layer
      using a pre shared security context (psk, rpk or certificate) and
      then the client is identified using client_id and client_secret as
      in classic OAuth.
      B: The AS responds with the an access token and RS Information,
      the latter containing a symmetric key.  Communication security
      between C and RS will be DTLS and PreSharedKey.  The PoP key is
      used as the PreSharedKey.


            Authorization
    Client     Server
       |         |
       |         |
   A:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"token"
       |         | Content-Type: application/cbor
       |         | Payload: <Request-Payload>
       |         |
   B:  |<--------+ Header: 2.05 Content
       |         | Content-Type: application/cbor
       |  2.05   | Payload: <Response-Payload>
       |         |

      Figure 22: Token Request and Response using Client Credentials.

   The information contained in the Request-Payload and the Response-
   Payload is shown in Figure 23.





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   Request-Payload:
   {
     "grant_type" : "client_credentials",
     "aud" : "lockOfDoor4711",
     "client_id" : "keyfob",
     "client_secret" : "qwerty"
   }

   Response-Payload:
   {
     "access_token" : b64'SlAV32hkKG ...'
     "token_type" : "pop",
     "csp" : "DTLS",
     "cnf" : {
       "COSE_Key" : {
         "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
         "kty" : "oct",
         "alg" : "HS256",
         "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
       }
     }
   }

           Figure 23: Request and Response Payload for C offline

   The access token in this case is just an opaque string referencing
   the authorization information at the AS.

      C: Next, the client POSTs the access token to the authz-info
      endpoint in the RS.  This is a plain CoAP request, i.e., no DTLS
      between client and RS.  Since the token is an opaque string, the
      RS cannot verify it on its own, and thus defers to respond the
      client with a status code until after step E.
      D: The RS forwards the token to the introspection endpoint on the
      AS.  Introspection assumes a secure connection between the AS and
      the RS, e.g., using transport of application layer security.  In
      the example AS is identified using pre shared security context
      (psk, rpk or certificate) while RS is acting as client and is
      identified with client_id and client_secret.
      E: The AS provides the introspection response containing
      parameters about the token.  This includes the confirmation key
      (cnf) parameter that allows the RS to verify the client's proof of
      possession in step F.
      After receiving message E, the RS responds to the client's POST in
      step C with the CoAP response code 2.01 (Created).






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              Resource
     Client    Server
       |         |
   C:  +-------->| Header: POST (T=CON, Code=0.02)
       |  POST   | Uri-Path:"authz-info"
       |         | Content-Type: "application/cbor"
       |         | Payload: b64'SlAV32hkKG ...''
       |         |
       |         |     Authorization
       |         |       Server
       |         |          |
       |      D: +--------->| Header: POST (Code=0.02)
       |         |  POST    | Uri-Path: "introspect"
       |         |          | Content-Type: "application/cbor"
       |         |          | Payload: <Request-Payload>
       |         |          |
       |      E: |<---------+ Header: 2.05 Content
       |         |  2.05    | Content-Type: "application/cbor"
       |         |          | Payload: <Response-Payload>
       |         |          |
       |         |
       |<--------+ Header: 2.01 Created
       |  2.01   |
       |         |

               Figure 24: Token Introspection for C offline
      The information contained in the Request-Payload and the Response-
      Payload is shown in Figure 25.

   Request-Payload:
   {
     "token" : b64'SlAV32hkKG...',
     "client_id" : "FrontDoor",
     "client_secret" : "ytrewq"
   }

   Response-Payload:
   {
     "active" : true,
     "aud" : "lockOfDoor4711",
     "scope" : "open, close",
     "iat" : 1311280970,
     "cnf" : {
       "kid" : b64'JDLUhTMjU2IiwiY3R5Ijoi ...'
     }
   }

         Figure 25: Request and Response Payload for Introspection



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      The client uses the symmetric PoP key to establish a DTLS
      PreSharedKey secure connection to the RS.  The CoAP request PUT is
      sent to the uri-path /state on the RS, changing the state of the
      door to locked.
      F: The RS responds with a appropriate over the secure DTLS
      channel.

              Resource
     Client    Server
       |         |
       |<=======>| DTLS Connection Establishment
       |         |   using Pre Shared Key
       |         |
       +-------->| Header: PUT (Code=0.03)
       | PUT     | Uri-Path: "state"
       |         | Payload: <new state for the lock>
       |         |
   F:  |<--------+ Header: 2.04 Changed
       | 2.04    | Payload: <new state for the lock>
       |         |

       Figure 26: Resource request and response protected by OSCOAP

Appendix F.  Document Updates

F.1.  Version -08 to -09

   o  Allowed scope to be byte arrays.
   o  Defined default names for endpoints.
   o  Refactored the IANA section for briefness and consistency.
   o  Refactored tables that define IANA registry contents for
      consistency.
   o  Created IANA registry for CBOR mappings of error codes, grant
      types and Authorization Server Information.
   o  Added references to other document sections defining IANA entries
      in the IANA section.

F.2.  Version -07 to -08

   o  Moved AS discovery from the DTLS profile to the framework, see
      Section 5.1.
   o  Made the use of CBOR mandatory.  If you use JSON you can use
      vanilla OAuth.
   o  Made it mandatory for profiles to specify C-AS security and RS-AS
      security (the latter only if introspection is supported).
   o  Made the use of CBOR abbreviations mandatory.
   o  Added text to clarify the use of token references as an
      alternative to CWTs.



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   o  Added text to clarify that introspection must not be delayed, in
      case the RS has to return a client token.
   o  Added security considerations about leakage through unprotected AS
      discovery information, combining profiles and leakage through
      error responses.
   o  Added privacy considerations about leakage through unprotected AS
      discovery.
   o  Added text that clarifies that introspection is optional.
   o  Made profile parameter optional since it can be implicit.
   o  Clarified that CoAP is not mandatory and other protocols can be
      used.
   o  Clarified the design justification for specific features of the
      framework in appendix A.
   o  Clarified appendix E.2.
   o  Removed specification of the "cnf" claim for CBOR/COSE, and
      replaced with references to [I-D.ietf-ace-cwt-proof-of-possession]

F.3.  Version -06 to -07

   o  Various clarifications added.
   o  Fixed erroneous author email.

F.4.  Version -05 to -06

   o  Moved sections that define the ACE framework into a subsection of
      the framework Section 5.
   o  Split section on client credentials and grant into two separate
      sections, Section 5.2, and Section 5.3.
   o  Added Section 5.4 on AS authentication.
   o  Added Section 5.5 on the Authorization endpoint.

F.5.  Version -04 to -05

   o  Added RFC 2119 language to the specification of the required
      behavior of profile specifications.
   o  Added Section 5.3 on the relation to the OAuth2 grant types.
   o  Added CBOR abbreviations for error and the error codes defined in
      OAuth2.
   o  Added clarification about token expiration and long-running
      requests in Section 5.8.2
   o  Added security considerations about tokens with symmetric pop keys
      valid for more than one RS.
   o  Added privacy considerations section.
   o  Added IANA registry mapping the confirmation types from RFC 7800
      to equivalent COSE types.
   o  Added appendix D, describing assumptions about what the AS knows
      about the client and the RS.




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F.6.  Version -03 to -04

   o  Added a description of the terms "framework" and "profiles" as
      used in this document.
   o  Clarified protection of access tokens in section 3.1.
   o  Clarified uses of the "cnf" parameter in section 6.4.5.
   o  Clarified intended use of Client Token in section 7.4.

F.7.  Version -02 to -03

   o  Removed references to draft-ietf-oauth-pop-key-distribution since
      the status of this draft is unclear.
   o  Copied and adapted security considerations from draft-ietf-oauth-
      pop-key-distribution.
   o  Renamed "client information" to "RS information" since it is
      information about the RS.
   o  Clarified the requirements on profiles of this framework.
   o  Clarified the token endpoint protocol and removed negotiation of
      "profile" and "alg" (section 6).
   o  Renumbered the abbreviations for claims and parameters to get a
      consistent numbering across different endpoints.
   o  Clarified the introspection endpoint.
   o  Renamed token, introspection and authz-info to "endpoint" instead
      of "resource" to mirror the OAuth 2.0 terminology.
   o  Updated the examples in the appendices.

F.8.  Version -01 to -02

   o  Restructured to remove communication security parts.  These shall
      now be defined in profiles.
   o  Restructured section 5 to create new sections on the OAuth
      endpoints token, introspection and authz-info.
   o  Pulled in material from draft-ietf-oauth-pop-key-distribution in
      order to define proof-of-possession key distribution.
   o  Introduced the "cnf" parameter as defined in RFC7800 to reference
      or transport keys used for proof of possession.
   o  Introduced the "client-token" to transport client information from
      the AS to the client via the RS in conjunction with introspection.
   o  Expanded the IANA section to define parameters for token request,
      introspection and CWT claims.
   o  Moved deployment scenarios to the appendix as examples.

F.9.  Version -00 to -01

   o  Changed 5.1. from "Communication Security Protocol" to "Client
      Information".
   o  Major rewrite of 5.1 to clarify the information exchanged between
      C and AS in the PoP access token request profile for IoT.



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      *  Allow the client to indicate preferences for the communication
         security protocol.
      *  Defined the term "Client Information" for the additional
         information returned to the client in addition to the access
         token.
      *  Require that the messages between AS and client are secured,
         either with (D)TLS or with COSE_Encrypted wrappers.
      *  Removed dependency on OSCOAP and added generic text about
         object security instead.
      *  Defined the "rpk" parameter in the client information to
         transmit the raw public key of the RS from AS to client.
      *  (D)TLS MUST use the PoP key in the handshake (either as PSK or
         as client RPK with client authentication).
      *  Defined the use of x5c, x5t and x5tS256 parameters when a
         client certificate is used for proof of possession.
      *  Defined "tktn" parameter for signaling for how to transfer the
         access token.
   o  Added 5.2. the CoAP Access-Token option for transferring access
      tokens in messages that do not have payload.
   o  5.3.2.  Defined success and error responses from the RS when
      receiving an access token.
   o  5.6.:Added section giving guidance on how to handle token
      expiration in the absence of reliable time.
   o  Appendix B Added list of roles and responsibilities for C, AS and
      RS.

Authors' Addresses

   Ludwig Seitz
   RISE SICS
   Scheelevaegen 17
   Lund  223 70
   Sweden

   Email: ludwig.seitz@ri.se


   Goeran Selander
   Ericsson
   Faroegatan 6
   Kista  164 80
   Sweden

   Email: goran.selander@ericsson.com







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   Erik Wahlstroem
   (no affiliation)
   Sweden

   Email: erik@wahlstromtekniska.se


   Samuel Erdtman
   Spotify AB
   Birger Jarlsgatan 61, 4tr
   Stockholm  113 56
   Sweden

   Email: erdtman@spotify.com


   Hannes Tschofenig
   ARM Ltd.
   Hall in Tirol  6060
   Austria

   Email: Hannes.Tschofenig@arm.com





























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