Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)
draft-ietf-ace-oauth-authz-25

ACE Working Group                                               L. Seitz
Internet-Draft                                                      SICS
Intended status: Standards Track                             G. Selander
Expires: December 12, 2016                                      Ericsson
                                                           E. Wahlstroem
                                                        Nexus Technology
                                                              S. Erdtman
                                                              Spotify AB
                                                           H. Tschofenig
                                                                ARM Ltd.
                                                           June 10, 2016


  Authentication and Authorization for Constrained Environments (ACE)
                     draft-ietf-ace-oauth-authz-02

Abstract

   This specification defines the ACE framework for authentication and
   authorization in Internet of Things (IoT) deployments.  The ACE
   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 limitations of IoT devices require it,
   profiles and extensions are provided.

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 December 12, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   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  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Protocol Interactions . . . . . . . . . . . . . . . . . . . .   9
   5.  Framework . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  The 'Token' Resource  . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Client-to-AS Request  . . . . . . . . . . . . . . . . . .  14
     6.2.  AS-to-Client Response . . . . . . . . . . . . . . . . . .  17
     6.3.  Error Response  . . . . . . . . . . . . . . . . . . . . .  18
     6.4.  New Request and Response Parameters . . . . . . . . . . .  18
       6.4.1.  Grant Type  . . . . . . . . . . . . . . . . . . . . .  19
       6.4.2.  Token Type and Algorithms . . . . . . . . . . . . . .  19
       6.4.3.  Profile . . . . . . . . . . . . . . . . . . . . . . .  20
       6.4.4.  Confirmation  . . . . . . . . . . . . . . . . . . . .  20
     6.5.  Mapping parameters to CBOR  . . . . . . . . . . . . . . .  22
   7.  The 'Introspect' Resource . . . . . . . . . . . . . . . . . .  22
     7.1.  RS-to-AS Request  . . . . . . . . . . . . . . . . . . . .  23
     7.2.  AS-to-RS Response . . . . . . . . . . . . . . . . . . . .  23
     7.3.  Error Response  . . . . . . . . . . . . . . . . . . . . .  24
     7.4.  Client Token  . . . . . . . . . . . . . . . . . . . . . .  25
     7.5.  Mapping Introspection parameters to CBOR  . . . . . . . .  26
   8.  The Access Token  . . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  The 'Authorization Information' Resource  . . . . . . . .  27
     8.2.  Token Expiration  . . . . . . . . . . . . . . . . . . . .  28
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  OAuth Introspection Response Parameter Registration  . .  29
     10.2.  OAuth Parameter Registration . . . . . . . . . . . . . .  30
     10.3.  OAuth Access Token Types . . . . . . . . . . . . . . . .  30
     10.4.  Token Type Mappings  . . . . . . . . . . . . . . . . . .  30
       10.4.1.  Registration Template  . . . . . . . . . . . . . . .  30
       10.4.2.  Initial Registry Contents  . . . . . . . . . . . . .  31
     10.5.  JSON Web Token Claims  . . . . . . . . . . . . . . . . .  31
     10.6.  ACE Profile Registry . . . . . . . . . . . . . . . . . .  31



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       10.6.1.  Registration Template  . . . . . . . . . . . . . . .  31
     10.7.  OAuth Parameter Mappings Registry  . . . . . . . . . . .  32
       10.7.1.  Registration Template  . . . . . . . . . . . . . . .  32
       10.7.2.  Initial Registry Contents  . . . . . . . . . . . . .  32
     10.8.  Introspection Resource CBOR Mappings Registry  . . . . .  34
       10.8.1.  Registration Template  . . . . . . . . . . . . . . .  35
       10.8.2.  Initial Registry Contents  . . . . . . . . . . . . .  35
     10.9.  CoAP Option Number Registration  . . . . . . . . . . . .  37
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  37
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     12.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  Design Justification . . . . . . . . . . . . . . . .  40
   Appendix B.  Roles and Responsibilites  . . . . . . . . . . . . .  42
   Appendix C.  Deployment Examples  . . . . . . . . . . . . . . . .  44
     C.1.  Local Token Validation  . . . . . . . . . . . . . . . . .  44
     C.2.  Introspection Aided Token Validation  . . . . . . . . . .  48
   Appendix D.  Document Updates . . . . . . . . . . . . . . . . . .  51
     D.1.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  52
     D.2.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  52
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  53

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

   We envision that end consumers and enterprises will manage access to
   resources on, or produced by, Internet of Things (IoT) devices in the
   same style as they do today with data, services and applications on
   the Web or with their mobile devices.  This desire will increase with
   the number of exposed services and capabilities provided by
   applications hosted on the IoT devices.

   While prior work on authorization solutions for the Web and for the
   mobile environment also applies to the 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 makes use of CoAP [RFC7252].

   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:



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   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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD 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].

   Since we describe exchanges 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
   /introspect at the AS and /authz-info at the RS.  The CoAP [RFC7252]
   definition, which is "An entity participating in the CoAP protocol"
   is not used in this memo.

   Since this specification focuses on the problem of access control to
   resources, we simplify the actors by assuming that the client
   authorization server (CAS) functionality is not stand-alone but



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   subsumed by either the authorization server or the client (see
   section 2.2 in [I-D.ietf-ace-actors]).

3.  Overview

   This specification describes the ACE framework for authorization in
   the Internet of Things consisting 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, we do envision further
   underlying protocols to be supported in the future, such as MQTT or
   QUIC.

   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 CoAP POST
   parameters and CoAP responses.

   A fourth building block is the compact CBOR-based secure message
   format COSE [I-D.ietf-cose-msg], 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
   [I-D.ietf-oauth-pop-architecture], which is an extension to the OAuth
   access tokens, and "client tokens" which are defined in this
   framework (see Section 7.4).  The default access 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.selander-ace-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



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   mandating a one-size-fits-all solution.  We believe this 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
   form of ACE profiles.

   In the subsections below we provide further details about the
   different building blocks.

3.1.  OAuth 2.0

   The OAuth 2.0 authorization framework enables a client to obtain
   limited 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 introspect Endpoints:

      The AS hosts the /token endpoint that allows a client to request
      access tokens.  The client makes a POST request to the /token
      endpoint on the AS and receives the access token in the response
      (if the request was successful).

      The token introspection endpoint, /introspect, is used by the RS
      when requesting additional information regarding a received access
      token.  The RS makes a POST request to /introspect on the AS and
      receives information about the access token contain 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 authorization server and consumed by
      the resource server.  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.




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   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 tokens (or PoP tokens)
      [I-D.ietf-oauth-pop-architecture].

      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 included in
      the access token.  When this specification uses the term "access
      token" it is assumed to be a PoP token unless specifically stated
      otherwise.

      The key bound to the access token (aka PoP key) may be based on
      symmetric as well as on asymmetric cryptography.  The appropriate
      choice of security 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,
         encrypts it for the RS and includes it inside an 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 then 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 protected against modifications using a MAC or
      a digital signature, which is added by the AS.  The choice of PoP
      key does not necessarily imply a specific credential type for the
      integrity protection of the token.  More information about PoP
      tokens can be found in [I-D.ietf-oauth-pop-architecture].

   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 request.
      In turn, the AS may use the scope response parameter to inform the



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      client of the scope of the access token issued.  As the client
      could be a constrained device as well, this specification uses
      CBOR encoded messages for CoAP, defined in Section 5, to request
      scopes and to be informed what scopes the access token was
      actually authorized for by the AS.

      The values of the scope parameter 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, called claims, is in the
      form of type-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.

      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 a 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 need to take the
   latter aspects into account.






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   While HTTP uses headers and query-strings to convey additional
   information about a request, CoAP encodes such information in so-
   called 'options'.

   CoAP supports application-layer fragmentation of the CoAP payloads
   through blockwise transfers [I-D.ietf-core-block].  However, block-
   wise 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 which requires 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
   different transport in a uniform way, is to provide security on
   application layer using an object-based security mechanism such as
   CBOR Encoded Message Syntax [I-D.ietf-cose-msg].

   One application of COSE is OSCOAP [I-D.selander-ace-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.

4.  Protocol Interactions

   The ACE framework is based on the OAuth 2.0 protocol interactions
   using the /token and /introspect endpoints.  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.  The RS, after receiving an access token, may present it to
   the AS via the /introspect endpoint to get information about the
   access token.  In other deployments the RS may process the access
   token locally without the need to contact an AS.  These interactions
   are shown in Figure 1.  An overview of various OAuth concepts is
   provided in Section 3.1.

   The consent of the resource owner, for giving a client access to a
   protected resource, can be pre-configured authorization policies or
   dynamically at the time when the request is sent.  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.  We
   assume 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



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   been used, such as factory-based provisioning or the use of
   commissioning tools.  Regardless of the onboarding technique, this
   registration procedure implies that the client and the AS share
   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.  The
   detailed procedures for this discovery process may be defined in an
   ACE profile and depend on the protocols being used and the specific
   deployment environment.

   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 makes a request to
   "/.well-known/core" to obtain information about available resources,
   which are returned in a standardized format as described in
   [RFC6690].



















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

                      Figure 1: Basic Protocol Flow.

   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 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 also returns various parameters,
      referred as "Client Information".  In addition to the response
      parameters defined by OAuth 2.0 and the PoP token extension,
      further response parameters, such as information on which profile
      the client should use with the resource server(s).  More
      information about these parameters can be found in in Section 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.




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      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 client 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 client 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.

   Token Introspection Request (D):

      A resource server may be configured to introspect the access token
      by including it in a request to the /introspect endpoint at that
      AS.  Token introspection over CoAP is defined in Section 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.

   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.



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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 we cannot generally assume 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 authenticated token holder is bound
      to the token.  The binding is provided by the "cnf" claim
      indicating what key is used for mutual authentication.  If clients
      need to update a token, e.g. to get additional rights, they can
      request that the AS binds the new access token to the same
      credential as the previous token.

   ACE Profile Negotiation

      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.  The ACE framework supports the negotiation of different
      ACE profiles between client and AS using the "profile" parameter
      in the token request and token response.


   OAuth 2.0 requires the use of TLS both to protect the communication
   between AS and client when requesting an access token and between AS
   and RS for introspection.  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 are expected to specify the details of how this 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
   resources at the AS is assumed to be via HTTP and may use Uri-query



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   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 MUST be "application/cbor" in that case.

   The OAuth 2.0 AS uses a JSON structure in the payload of its
   responses both to client and RS.  This framework RECOMMENDS the use
   of CBOR [RFC7049] instead.  The requesting device can explicitly
   request this encoding by setting the CoAP Accept option in the
   request to "application/cbor".

6.  The 'Token' Resource

   In plain OAuth 2.0 the AS provides the /token resource for submitting
   access token requests.  This framework extends the functionality of
   the /token resource, giving the AS the possibility to help client and
   RS to establish shared keys or to exchange their public keys.

   Communication between the client and the token resource at the AS
   MUST be integrity protected and encrypted.  Furthermore AS and client
   MUST perform mutual authentication.  Profiles of this framework are
   expected to specify how authentication and communication security is
   implemented.

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

6.1.  Client-to-AS Request

   When requesting an access token from the AS, the client MAY include
   the following parameters in the request in addition to the ones
   required or optional according to the OAuth 2.0 specification
   [RFC6749]:

   token_type
      OPTIONAL.  See Section 6.4 for more details.

   alg
      OPTIONAL.  See Section 6.4 for more details.

   profile
      OPTIONAL.  This indicates the profile that the client would like
      to use with the RS.  See Section 6.4 for more details on the
      formatting of this parameter.  If the RS cannot support the
      requested profile, the AS MUST reply with an error message.




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   cnf
      OPTIONAL.  This field contains information about a public key the
      client would like to bind to the access token.  If the client
      requests an asymmetric proof-of-possession algorithm, but does not
      provide a public key, the AS MUST respond with an error message.
      See Section 6.4 for more details on the formatting of the 'cnf'
      parameter.

   These new parameters are optional in the case where the AS has prior
   knowledge of the capabilities of the client, otherwise these
   parameters are required.  This prior knowledge may, for example, be
   set by the use of a dynamic client registration protocol exchange
   [RFC7591].

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

   Figure 2 shows a request for a token with a symmetric proof-of-
   possession key.

   Header: POST (Code=0.02)
   Uri-Host: "server.example.com"
   Uri-Path: "token"
   Content-Type: "application/cbor"
   Payload:
   {
     "grant_type" : "client_credentials",
     "aud" : "tempSensor4711",
     "client_id" : "myclient",
     "client_secret" : b64'FWRUVGZUZmZFRkWSRlVGhA',
     "token_type" : "pop",
     "alg" : "HS256",
     "profile" : "coap_dtls"
   }

    Figure 2: Example request for an access token bound to a symmetric
                                   key.

   Figure 3 shows a request for a token with an asymmetric proof-of-
   possession key.











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   Header: POST (Code=0.02)
   Uri-Host: "server.example.com"
   Uri-Path: "token"
   Content-Type: "application/cbor"
   Payload:
   {
     "grant_type" : "token",
     "aud" : "lockOfDoor0815",
     "client_id" : "myclient",
     "token_type" : "pop",
     "alg" : "ES256",
     "profile" : "coap_oscoap"
     "cnf" : {
       "COSE_Key" : {
         "kty" : "EC",
         "kid" : h'11',
         "crv" : "P-256",
         "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
         "y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
       }
     }
   }

   Figure 3: Example request for an access token bound to an asymmetric
                                   key.

   Figure 4 shows a request for a token where a previously communicated
   proof-of-possession key is only referenced.























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   Header: POST (Code=0.02)
   Uri-Host: "server.example.com"
   Uri-Path: "token"
   Content-Type: "application/cbor"
   Payload:
   {
     "grant_type" : "client_credentials",
     "aud" : "valve424",
     "scope" : "read",
     "client_id" : "myclient",
     "token_type" : "pop",
     "alg" : "ES256",
     "profile" : "coap_oscoap"
     "cnf" : {
       "kid" : b64'6kg0dXJM13U'
     }
   }

       Figure 4: Example request for an access token bound to a key
                                reference.

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 a PoP token for the indicated
   audience and scopes (if any), the AS issues an access token.  If
   client authentication failed or is invalid, the authorization server
   returns an error response as described in Section 6.3.

   The following parameters may also be part of a successful response in
   addition to those defined in section 5.1 of [RFC6749]:

   profile
      REQUIRED.  This indicates the profile that the client MUST use
      towards the RS.  See Section 6.4 for the formatting of this
      parameter.

   cnf
      REQUIRED.  This field contains information about the proof-of
      possession key for this access token.  See Section 6.4 for the
      formatting of this parameter.

   Note that the access token can also contains a 'cnf' claim, however,
   these two values are consumed by different parties.  The access token
   is created by the AS and processed by the RS (and opaque to the
   client) whereas the Client Information is created by the AS and
   processed by the client; it is never forwarded to the resource
   server.



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

   Figure 5 shows a response containing a token and a 'cnf' parameter
   with a symmetric proof-of-possession key.

   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)',
     "token_type" : "pop",
     "alg" : "HS256",
     "expires_in" : "3600",
     "profile" : "coap_dtls"
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : b64'39Gqlw',
         "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
       }
     }
   }

       Figure 5: Example AS response with an access token bound to a
                              symmetric key.

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: The
   Content-Type MUST be set to "application/cbor", the payload MUST be
   encoded in a CBOR map and the CoAP response code 4.00 Bad Request
   MUST be used unless specified otherwise.

6.4.  New Request and Response Parameters

   This section defines parameters that can be used in access token
   requests and responses, as well as abbreviations for more compact
   encoding of existing parameters and common values.








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6.4.1.  Grant Type

   The abbreviations in Figure 6 MAY be used in CBOR encodings instead
   of the string values defined in [RFC6749].

             /--------------------+----------+--------------\
             | grant_type         | CBOR Key | Major Type   |
             |--------------------+----------+--------------|
             | password           |    0     |     0 (uint) |
             | authorization_code |    1     |     0        |
             | client_credentials |    2     |     0        |
             | refresh_token      |    3     |     0        |
             \--------------------+----------+--------------/

            Figure 6: CBOR abbreviations for common grant types

6.4.2.  Token Type and Algorithms

   To allow clients to indicate support for specific token types and
   respective algorithms they need to interact with the AS.  They can
   either provide this information out-of-band or via the 'token_type'
   and 'alg' parameter in the client request.

   The value in the 'alg' parameter together with value from the
   'token_type' parameter allow the client to indicate the supported
   algorithms for a given token type.  The token type refers to the
   specification used by the client to interact with the resource server
   to demonstrate possession of the key.  The 'alg' parameter provides
   further information about the algorithm, such as whether a symmetric
   or an asymmetric crypto-system is used.  Hence, a client supporting a
   specific token type also knows how to populate the values to the
   'alg' parameter.

   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 is specified by the 'alg'
   parameter.  Profiles of this framework are responsible for defining
   values for the 'alg' parameter together with the corresponding proof-
   of-possession mechanisms.

   The values in the 'alg' parameter are case-sensitive.  If the client
   supports more than one algorithm then each individual value MUST be
   separated by a space.








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6.4.3.  Profile

   The "profile" parameter identifies the communication protocol and the
   communication security protocol between the client and the RS.

   An initial set of profile identifiers and their CBOR encodings are
   specified in Figure 7.  Profiles using other combinations of
   protocols are expected to define their own profile identifiers.

           /--------------------+----------+--------------\
           | Profile identifier | CBOR Key | Major Type   |
           |--------------------+----------+--------------|
           | http_tls           |    0     |     0 (uint) |
           | coap_dtls          |    1     |     0        |
           | coap_oscoap        |    2     |     0        |
           \--------------------+----------+--------------/

           Figure 7: Profile identifiers and their CBOR mappings

   Profiles MAY define additional parameters for both the token request
   and the client information in the access token response in order to
   support negotioation or signalling of profile specific parameters.

6.4.4.  Confirmation

   The "cnf" parameter identifies or provides the key used for proof-of-
   possession.  This framework extends the definition of 'cnf' from
   [RFC7800] by defining CBOR/COSE encodings and the use of 'cnf' for
   transporting keys in the client information.

   A CBOR encoded payload MAY contain the 'cnf' parameter with the
   following contents:

   COSE_Key  In this case the 'cnf' parameter contains the proof-of-
      possession key to be used by the client.  An example is shown in
      Figure 8.

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

         Figure 8: Confirmation parameter containing a public key



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   COSE_Encrypted  In this case the 'cnf' parameter contains an
      encrypted symmetriic key destined for the client.  The client is
      assumed to be able to decrypt the cihpertext of this parameter.
      The parameter is encoded as COSE_Encrypted object wrapping a
      COSE_Key object.  Figure 9 shows an example of this type of
      encoding.

   "cnf" : {
     "COSE_Encrypted" : {
       993(
         [ h'a1010a' # protected header : {"alg" : "AES-CCM-16-64-128"}
           "iv" : b64'ifUvZaHFgJM7UmGnjA',  # unprotected header
          b64'WXThuZo6TMCaZZqi6ef/8WHTjOdGk8kNzaIhIQ' # ciphertext
         ]
       )
     }
   }

   Figure 9: Confirmation paramter containing an encrypted symmetric key

      The ciphertext here could e.g. contain a symmetric key as in
      Figure 10.

   {
     "kty" : "Symmetric",
     "kid" : b64'39Gqlw',
     "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
   }

        Figure 10: Example plaintext of an encrypted cnf parameter


   Key Identifier  In this case the 'cnf' parameter references a key
      that is assumed to be previously known by the recipient.  This
      allows clients that perform repeated requests for an access token
      for the same audience but e.g. with different scopes to omit key
      transport in the access token, token request and token response.
      Figure 11 shows such an example.

   "cnf" : {
     "kid" : b64'39Gqlw'
   }

      Figure 11: A Confirmation parameter with just a key identifier







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6.5.  Mapping parameters to CBOR

   All OAuth parameters in access token requests and responses are
   mapped to CBOR types as follows and are given an integer key value to
   save space.

           /-------------------+----------+-----------------\
           | Parameter name    | CBOR Key | Major Type      |
           |-------------------+----------+-----------------|
           | client_id         | 1        | 3 (text string) |
           | client_secret     | 2        | 2 (byte string) |
           | response_type     | 3        | 3               |
           | redirect_uri      | 4        | 3               |
           | scope             | 5        | 3               |
           | state             | 6        | 3               |
           | code              | 7        | 2               |
           | error_description | 8        | 3               |
           | error_uri         | 9        | 3               |
           | grant_type        | 10       | 0 (unit)        |
           | access_token      | 11       | 3               |
           | token_type        | 12       | 0               |
           | expires_in        | 13       | 0               |
           | username          | 14       | 3               |
           | password          | 15       | 3               |
           | refresh_token     | 16       | 3               |
           | alg               | 17       | 3               |
           | cnf               | 18       | 5 (map)         |
           | aud               | 19       | 3               |
           | profile           | 20       | 0               |
           \---------------+--------------+-----------------/

              Figure 12: CBOR mappings used in token requests

7.  The 'Introspect' Resource

   Token introspection [RFC7662] is 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 CoAP and CBOR.

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




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

7.1.  RS-to-AS Request

   The RS sends a CoAP POST request to the introspection resource at the
   AS, with payload sent as "application/cbor" data.  The payload is a
   CBOR map with a 'token' parameter containing the access token along
   with optional parameters representing additional context that is
   known by the RS to aid the AS in its response.

   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
   resource at the AS to query about an OAuth 2.0 proof-of-possession
   token.

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

                 Figure 13: Example introspection request.

7.2.  AS-to-RS Response

   The AS responds with a CBOR object in "application/cbor" format with
   the same required and optional parameters as in section 2.2. of RFC
   7662 [RFC7662] with the following additions:

   alg
      OPTIONAL.  See Section 6.4 for more details.

   cnf
      OPTIONAL.  This field contains information about the proof-of-
      possession key that binds the client to the access token.  See
      Section 6.4 for more details on the formatting of the 'cnf'
      parameter.

   profile




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      OPTIONAL.  This indicates the profile that the RS MUST use with
      the client.  See Section 6.4 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 7.4 for more details.

   For example, Figure 14 shows an AS response to the introspection
   request in Figure 13.

   Header: Created Code=2.01)
   Content-Type: "application/cbor"
   Payload:
   {
     "active" : true,
     "scope" : "read",
     "token_type" : "pop",
     "alg" : "HS256",
     "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.

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 to "application/
      cbor", and the payload MUST be encoded in a CBOR map.
   o  If the credentials used by the RS are invalid the AS MUST respond
      with the CoAP response code 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 the CoAP response code 4.03
      (Forbidden).  In this case no payload is returned.



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   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".

7.4.  Client Token

   EDITORIAL NOTE: We have tentatively introduced this concept and would
   specifically like feedback if this is viewed as a useful addition to
   the framework.

   In cases where the client has limited connectivity and is requesting
   access to a previously unknown resource servers, using a long term
   token, there are situations where it would be beneficial to relay the
   proof-of-possession key and other relevant information from the AS to
   the client through the RS.  The client_token parameter is designed to
   carry such information, and is intended to be used as described in
   Figure 15.

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

               Figure 15: Use of the client_token parameter.

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

   cnf
      REQUIRED.  Contains information about the proof-of-possession key
      the client is to use with its access token.  See Section 6.4.4.




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   token_type
      OPTIONAL.  See Section 6.4.2.

   alg
      OPTIONAL.  See Section 6.4.2.

   profile
      REQUIRED.  See Section 6.4.3.

   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 6.4.3.

   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.

7.5.  Mapping Introspection parameters to CBOR

   The introspection request and response parameters are mapped to CBOR
   types as follows and are given an integer key value to save space.


























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             /----------------+----------+-----------------\
             | Parameter name | CBOR Key | Major Type      |
             |----------------+----------+-----------------|
             | active         | 1        | 0 (uint)        |
             | username       | 2        | 3 (text string) |
             | client_id      | 3        | 3               |
             | scope          | 4        | 3               |
             | token_type     | 5        | 3               |
             | exp            | 6        | 6 tag value 1   |
             | iat            | 7        | 6 tag value 1   |
             | nbf            | 8        | 6 tag value 1   |
             | sub            | 9        | 3               |
             | aud            | 10       | 3               |
             | iss            | 11       | 3               |
             | jti            | 12       | 3               |
             | alg            | 13       | 3               |
             | cnf            | 14       | 5 (map)         |
             | aud            | 15       | 3               |
             | client_token   | 16       | 3               |
             | rs_cnf         | 17       | 5               |
             \----------------+----------+-----------------/

        Figure 16: CBOR Mappings to Token Introspection Parameters.

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 specfifies the "scope" claim for access tokens that explicitly
   encodes the scope of a given access token.  This claim follows the
   same encoding rules as defined in section 3.3 of [RFC6749].  The
   meaning of a specific scope value is application specific and
   expected to be known to the RS running that application.

8.1.  The 'Authorization Information' Resource

   The access token, containing authorization information and
   information of the key used by the client, is 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 CoAP that MAY be used.  An ACE profile MAY define other
   methods for token transport.

   This method REQUIRES the RS to implement an /authz-info resource.  A
   client using this method MUST make a POST request to /authz-info on
   the RS with the access token in the payload.  The RS receiving the



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   token MUST verify the validity of the token.  If the token is valid,
   the RS MUST respond to the POST request with 2.04 (Changed).

   If the token is not valid, the RS MUST respond with error 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 error code 4.03
   (Forbidden).

   The RS MAY make an introspection request to validate the token before
   responding to the POST /authz-info request.  If the introspection
   response contains a client token (Section 7.4) then this token SHALL
   be included in the payload of the 2.04 (Changed) response.

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.  We list
   the possibilities here including what functionality they require of
   the RS.

   o  The token is a CWT/JWT and includes a '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 must have a real time chip (RTC) or some other way of reliably
      measuring time.
   o  The RS verifies the validity of the token by performing an
      introspection request as specified in Section 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/JWT.  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.

9.  Security Considerations

   The entire document is about security.  Security considerations
   applicable to authentication and authorization in RESTful
   environments provided in OAuth 2.0 [RFC6749] apply to this work, as



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   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.  Finally
   [I-D.ietf-oauth-pop-architecture] discusses security and privacy
   threats as well as mitigation measures for Proof-of-Possession
   tokens.

10.  IANA Considerations

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

10.1.  OAuth Introspection Response Parameter Registration

   This specification registers the following parameters in the OAuth
   introspection response parameters

   o  Name: "alg"
   o  Description: Algorithm to use with PoP key, as defined in PoP
      token specification,
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Name: "cnf"
   o  Description: Key to use to prove the right to use an access token,
      as defined in [RFC7800].
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Name: "aud"
   o  Description: reference to intended receiving RS, as defined in PoP
      token specification.
   o  Change Controller: IESG
   o  Specification Document(s): 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  Specification Document(s): this document

   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  Specification Document(s): this document





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10.2.  OAuth Parameter Registration

   This specification registers the following parameters in the OAuth
   Parameters Registry

   o  Name: "alg"
   o  Description: Algorithm to use with PoP key, as defined in PoP
      token specification,
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "profile"
   o  Parameter usage location: token request, and token response
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Name: "cnf"
   o  Description: Key to use to prove the right to use an access token,
      as defined in [RFC7800].
   o  Change Controller: IESG
   o  Specification Document(s): this document

10.3.  OAuth Access Token Types

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

   o  Name: "PoP"
   o  Description: A proof-of-possession token.
   o  Change Controller: IESG
   o  Specification Document(s): this document

10.4.  Token Type Mappings

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

10.4.1.  Registration Template

   Token Type:
      Name of token type as registered in the OAuth token type registry
      e.g.  "Bearer".
   Mapped value:
      Integer representation for the token type value.  The key value
      MUST be an integer in the range of 1 to 65536.
   Change Controller:





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      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.
   Specification Document(s):
      Reference to the document or documents that specify the
      parameter,preferably including URIs that can be used to retrieve
      copies of the documents.  An indication of the relevant sections
      may also be included but is not required.

10.4.2.  Initial Registry Contents

   o  Parameter name: "Bearer"
   o  Mapped value: 1
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "pop"
   o  Mapped value: 2
   o  Change Controller: IESG
   o  Specification Document(s): this document

10.5.  JSON Web Token Claims

   This specification registers the following new claim in the JSON Web
   Token (JWT) registry.

   o  Claim Name: "scope"
   o  Claim Description: The scope of an access token as defined in
      [RFC6749].
   o  Change Controller: IESG
   o  Specification Document(s): this document

10.6.  ACE Profile Registry

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

10.6.1.  Registration Template

   Profile name:
      Name of the profile to be included in the profile attribute.
   Profile description:
      Text giving an over view of the profile and the context it is
      developed for.
   Profile ID:
      Integer value to identify the profile.  The value MUST be an
      integer in the range of 1 to 65536.
   Change Controller:



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      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.
   Specification Document(s):
      Reference to the document or documents that specify the
      parameter,preferably including URIs that can be used to retrieve
      copies of the documents.  An indication of the relevant sections
      may also be included but is not required.

10.7.  OAuth Parameter Mappings Registry

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

10.7.1.  Registration Template

   Parameter name:
      OAuth Parameter name, refers to the name in the OAuth parameter
      registry e.g. "client_id".
   CBOR key value:
      Key value for the claim.  The key value MUST be an integer in the
      range of 1 to 65536.
   Change Controller:
      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.
   Specification Document(s):
      Reference to the document or documents that specify the
      parameter,preferably including URIs that can be used to retrieve
      copies of the documents.  An indication of the relevant sections
      may also be included but is not required.

10.7.2.  Initial Registry Contents

   o  Parameter name: "client_id"
   o  CBOR key value: 1
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "client_secret"
   o  CBOR key value: 2
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "response_type"
   o  CBOR key value: 3
   o  Change Controller: IESG



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   o  Specification Document(s): this document

   o  Parameter name: "redirect_uri"
   o  CBOR key value: 4
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "scope"
   o  CBOR key value: 5
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "state"
   o  CBOR key value: 6
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "code"
   o  CBOR key value: 7
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "error_description"
   o  CBOR key value: 8
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "error_uri"
   o  CBOR key value: 9
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "grant_type"
   o  CBOR key value: 10
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "access_token"
   o  CBOR key value: 11
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "token_type"
   o  CBOR key value: 12
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "expires_in"



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   o  CBOR key value: 13
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "username"
   o  CBOR key value: 14
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "password"
   o  CBOR key value: 15
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "refresh_token"
   o  CBOR key value: 16
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "alg"
   o  CBOR key value: 17
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "cnf"
   o  CBOR key value: 18
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "aud"
   o  CBOR key value: 19
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "profile"
   o  CBOR key value: 20
   o  Change Controller: IESG
   o  Specification Document(s): this document

10.8.  Introspection Resource CBOR Mappings Registry

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







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10.8.1.  Registration Template

   Response parameter name:
      Name of the response parameter as defined in the "OAuth Token
      Introspection Response" registry e.g. "active".
   CBOR key value:
      Key value for the claim.  The key value MUST be an integer in the
      range of 1 to 65536.
   Change Controller:
      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.
   Specification Document(s):
      Reference to the document or documents that specify the
      parameter,preferably including URIs that can be used to retrieve
      copies of the documents.  An indication of the relevant sections
      may also be included but is not required.

10.8.2.  Initial Registry Contents

   o  Response parameter name: "active"
   o  CBOR key value: 1
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "username"
   o  CBOR key value: 2
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "client_id"
   o  CBOR key value: 3
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "scope"
   o  CBOR key value: 4
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "token_type"
   o  CBOR key value: 5
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "exp"
   o  CBOR key value: 6
   o  Change Controller: IESG



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   o  Specification Document(s): this document

   o  Response parameter name: "iat"
   o  CBOR key value: 7
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "nbf"
   o  CBOR key value: 8
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "sub"
   o  CBOR key value: 9
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "aud"
   o  CBOR key value: 10
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "iss"
   o  CBOR key value: 11
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Response parameter name: "jti"
   o  CBOR key value: 12
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "alg"
   o  CBOR key value: 13
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "cnf"
   o  CBOR key value: 14
   o  Change Controller: IESG
   o  Specification Document(s): this document

   o  Parameter name: "aud"
   o  CBOR key value: 15
   o  Change Controller: IESG
   o  Specification Document(s): this document





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10.9.  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].

   Name

      Access-Token
   Number

      TBD
   Reference

      [This document].
   Meaning in Request

      Contains an Access Token according to [This document] containing
      access permissions of the client.
   Meaning in Response

      Not used in response
   Safe-to-Forward

      TBD
   Format

      Based on the observer the format is perceived differently.  Opaque
      data to the client and CWT or reference token to the RS.
   Length

      Less then 255 bytes

11.  Acknowledgments

   We would like to thank 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 ACRE proposal
   [I-D.seitz-ace-core-authz] which was one source of inspiration for
   this work.  Finally, we would like to thank the ACE working group in
   general for their feedback.

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







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12.  References

12.1.  Normative References

   [I-D.ietf-ace-cbor-web-token]
              Wahlstroem, E., Jones, M., and H. Tschofenig, "CBOR Web
              Token (CWT)", draft-ietf-ace-cbor-web-token-00 (work in
              progress), May 2016.

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Encoded Message Syntax", draft-ietf-
              cose-msg-12 (work in progress), May 2016.

   [I-D.selander-ace-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-selander-ace-
              object-security-04 (work in progress), March 2016.

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

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://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,
              <http://www.rfc-editor.org/info/rfc7252>.

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

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,
              <http://www.rfc-editor.org/info/rfc7800>.

12.2.  Informative References

   [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-03 (work in
              progress), March 2016.



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   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",
              draft-ietf-core-block-20 (work in progress), April 2016.

   [I-D.ietf-oauth-pop-architecture]
              Hunt, P., Richer, J., Mills, W., Mishra, P., and H.
              Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security
              Architecture", draft-ietf-oauth-pop-architecture-07 (work
              in progress), December 2015.

   [I-D.seitz-ace-core-authz]
              Seitz, L., Selander, G., and M. Vucinic, "Authorization
              for Constrained RESTful Environments", draft-seitz-ace-
              core-authz-00 (work in progress), June 2015.

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

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

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

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://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,
              <http://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, <http://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, <http://www.rfc-editor.org/info/rfc7159>.







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   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7519>.

   [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,
              <http://www.rfc-editor.org/info/rfc7591>.

   [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,
              <http://www.rfc-editor.org/info/rfc7744>.

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.  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



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      so allows be replaced by provisioning of security context by a
      trusted third party, using transport or application layer
      security.

   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 to perform, 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 do
      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




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      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
      of different protocols, and not all parts of the message flow are
      used in all applications due to the communication constraints.
      While we envision deployments to make use of CoAP we explicitly
      want to support HTTP, HTTP/2 or specific protocols, such as
      Bluetooth Smart communication, which does not necessarily use IP.
      The latter raises the need for application layer security over the
      various interfaces.

Appendix B.  Roles and Responsibilites

   Resource Owner

      *  Make sure that the RS is registered at the AS.
      *  Make sure that clients can discover the AS which is in charge
         of the RS.
      *  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.

   Authorization Server

      *  Register RS and manage corresponding security contexts.
      *  Register clients and including authentication credentials.
      *  Allow Resource Owners to configure and update access control
         policies related to their registered RS'
      *  Expose a service that allows clients to request tokens.
      *  Authenticate clients that wishes to request a token.
      *  Process a token requests against the authorization policies
         configured for the RS.
      *  Expose a service that allows RS's to submit token introspection
         requests.
      *  Authenticate RS's that wishes to get an introspection response.
      *  Process token introspection requests.
      *  Optionally: Handle token revocation.



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   Client

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

         +  Authenticate towards the AS.
         +  Specify which RS, which resource(s), and which action(s) the
            request(s) will target.
         +  Specify preferences for communication security
         +  If raw public key (rpk) or certificate is used, make sure
            the AS has the right rpk or certificate for this client.
      *  Process the access token and client information (B)

         +  Check that the token has the right format (e.g.  CWT).
         +  Check that the client 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 (C)

         +  Authenticate towards the RS (this could coincide with the
            proof of possession process).
         +  Transmit the token as specified by the AS (default is to an
            authorization information resource, alternative options are
            as a CoAP option or in the DTLS handshake).
         +  Perform the proof-of-possession procedure as specified for
            the type of used token (this may already have been taken
            care of through the authentication procedure).
      *  Process the RS response (F) requirements of the Requesting
         Party, when issuing requests (e.g. minimum communication
         security requirements, trust anchors).
      *  Register the client at the AS.

   Resource Server

      *  Expose a way to submit access tokens.
      *  Process an access token.

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



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         +  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
            (if this is possible.
      *  Send a response following the agreed upon communication
         security.

Appendix C.  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 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.

C.1.  Local Token Validation

   In this scenario we consider the case where the resource server is
   offline, 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.

   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 /token at the AS.  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.




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      B: The AS responds with a PoP token and client information.  The
      PoP token contains the public key of the client, and the client
      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
      since it, that cannot do introspection to check the tokens current
      validity.  The token includes the claim "aif" with the authorized
      access that an owner of the temperature device can enjoy.  The
      'aif' 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: In this example we assume 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.

            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
       |  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.
















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   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",
     "cnf" : {
       "COSE_Key" : {
         "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
         "kty" : "EC",
         "crv" : "P-256",
         "x"   : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
         "y"   : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
       }
     }
   }

             Figure 18: Request and Response Payload Details.

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

   {
     "aud" : "tempSensorInLivingRoom",
     "iat" : "1360189224",
     "exp" : "1360289224",
     "aif" :  [["/temperature", 0], ["/firmware", 2]],
     "cnf" : {
       "jwk" : {
         "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.




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      C: The client then sends the PoP token to the /authz-info resource
      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 AS and RS.  The RS verifies that
      the PoP 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 token.


              Resource
    Client     Server
       |         |
   C:  +-------->| Header: POST (Code=0.02)
       |  POST   | Uri-Path:"authz-info"
       |         | Payload: SlAV32hkKG ...
       |         |
       |<--------+ Header: 2.01 Created
       |  2.01   |
       |         |

                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.
      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.






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C.2.  Introspection Aided Token Validation

   In this deployment scenario we assume that a client is not be able to
   access the AS at the time of the access request.  Since the RS is,
   however, connected to the back-end infrastructure it 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.
   The resource server may use its online connectivity to validate the
   access token with the authorization server, which is shown in the
   example below.

   In the example we show the interactions between an offline client
   (key fob), a resource server (online lock), and an authorization
   server.  We assume 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
   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 is 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 /token 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 on opaque string, which it can match to the specific client, a
      targeted audience and a symmetric key.
      B: The AS responds with the an access token and client
      information, the latter containing a symmetric key.  Communication
      security between C and RS will be DTLS and PreSharedKey.  The PoP
      key being used as the PreSharedKey.





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            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.

   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.





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      C: Next, the client POSTs the access token to the /authz-info
      resource 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 /introspect resource on the
      AS.  Introspection assumes a secure connection between the AS and
      the RS, e.g. using transport of application layer security, which
      is not detailed in this example.
      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 Code 2.01 Created.


              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>
       |         |          |
       |         |
   C:  |<--------+ 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.






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   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

      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 RS changing 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 D.  Document Updates







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D.1.  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, /introspect 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 posession.
   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.

D.2.  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 token request profile for IoT.

      *  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.



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Authors' Addresses

   Ludwig Seitz
   SICS
   Scheelevaegen 17
   Lund  223 70
   SWEDEN

   Email: ludwig@sics.se


   Goeran Selander
   Ericsson
   Faroegatan 6
   Kista  164 80
   SWEDEN

   Email: goran.selander@ericsson.com


   Erik Wahlstroem
   Nexus Technology
   Telefonvagen 26
   Hagersten  126 26
   Sweden

   Email: erik.wahlstrom@nexusgroup.com


   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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