ACE Working Group                                              S. Gerdes
Internet-Draft                                               O. Bergmann
Intended status: Standards Track                              C. Bormann
Expires: March 10, April 11, 2019                          Universitaet Bremen TZI
                                                             G. Selander
                                                             Ericsson AB
                                                                L. Seitz
                                                               RISE SICS
                                                      September 06,
                                                        October 08, 2018

Datagram Transport Layer Security (DTLS) Profile for Authentication and
            Authorization for Constrained Environments (ACE)
                    draft-ietf-ace-dtls-authorize-04
                    draft-ietf-ace-dtls-authorize-05

Abstract

   This specification defines a profile for delegating that allows constrained servers
   to delegate client authentication and authorization in a constrained environment by
   establishing a Datagram Transport Layer Security (DTLS) channel
   between resource-constrained nodes. authorization.  The protocol
   relies on DTLS for communication security between entities in a
   constrained network using either raw public keys or pre-shared keys.
   A resource-
   constrained node resource-constrained server can use this protocol to delegate
   management of authorization information to a trusted host with less
   severe limitations regarding processing power and memory.

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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on March 10, April 11, 2019.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Resource Access . . . .
   3.  Protocol Flow . . . . . . . . . . . . . . . . .   5
     2.2.  Dynamic Update of Authorization Information . . . . . . .   7
     2.3.  Token Expiration   5
     3.1.  Communication between C and AS  . . . . . . . . . . . . .   5
     3.2.  RawPublicKey Mode . . . . . . .   8
   3.  RawPublicKey Mode . . . . . . . . . . . . .   6
       3.2.1.  DTLS Channel Setup Between C and RS . . . . . . . . .   9
   4.   7
     3.3.  PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.   8
       3.3.1.  DTLS Channel Setup Between C and RS . . . . . . . . .  10
     3.4.  Resource Access . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Updating
   4.  Dynamic Update of Authorization Information . . . . . . . . . . .  13
   5.  Security Considerations  Token Expiration  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Privacy  Security Considerations . . . . . . . . . . . . . . . . . . .  14  15
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
     8.3.  17
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  17  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18  19

1.  Introduction

   This specification defines a profile of the ACE framework
   [I-D.ietf-ace-oauth-authz].  In this profile, a client and a resource
   server use CoAP [RFC7252] over DTLS [RFC6347] to communicate.  The
   client uses obtains an access token, bound to a key (the proof-of-possession
   key) proof-of-
   possession key), from an authorization server to authorize prove its access
   authorization to access protected resources hosted by the resource
   server.  DTLS provides communication security, proof of
   possession, and server authentication.  Optionally  Also, the client and the resource server may also use CoAP over DTLS to communicate are provided by the
   authorization server with the necessary keying material to establish
   a DTLS session.  The communication between client and authorization server.
   server may also be secured with DTLS.  This specification supports the
   DTLS handshake with Raw Public Keys (RPK) [RFC7250] and the DTLS handshake with Pre-
   Shared Pre-Shared Keys
   (PSK) [RFC4279].

   The DTLS RPK handshake [RFC7250] requires the client authentication and server to
   provide proof-of-possession for prove
   that they can use certain keying material.  In the key tied RPK mode, the
   client proves with the DTLS handshake that it can use the RPK bound
   to the access token.
   Here token and the server shows that it can use a certain RPK.  The
   access token needs to must be transferred presented to the resource server
   before server.  For the handshake is initiated, as RPK
   mode, the access token needs to be uploaded to the resource server
   before the handshake is initiated, as described in section Section 5.8.1 of
   draft-ietf-ace-oauth-authz [1].

   The DTLS

   In the PSK handshake [RFC4279] provides mode, client and server show with the proof-of-possession for DTLS handshake that
   they can use the key tied keying material that is bound to the access token.  Furthermore
   To transfer the access token from the client to the psk_identity resource server,
   the "psk_identity" parameter in the DTLS PSK handshake is may be used to transfer
   instead of uploading the access token from the client prior to the resource server. handshake.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with the terms and concepts
   described in [I-D.ietf-ace-oauth-authz]. I-D.ietf-ace-oauth-authz [2].

   The authz-info resource refers to the authz-info endpoint as
   specified in I-D.ietf-ace-oauth-authz [3].

2.  Protocol Overview

   The CoAP-DTLS profile for ACE specifies the transfer of
   authentication information and, if necessary, authorization
   information between the client C (C) and the resource server RS (RS)
   during setup of a DTLS session for CoAP messaging.  It also specifies
   how a Client C can use CoAP over DTLS to retrieve an Access Token access token from the
   authorization server AS (AS) for a protected resource hosted on the
   resource server RS. server.

   This profile requires a Client (C) the client to retrieve an Access Token access token for
   the
   protected resource(s) it wants to access on a Resource Server (RS) RS as specified in [I-D.ietf-ace-oauth-authz]. I-
   D.ietf-ace-oauth-authz [4].  Figure 1 shows the typical message flow
   in this scenario (messages in square brackets are optional):

      C                            RS                   AS
      | [-- Resource Request --->] |                     |
      |                            |                     |
      | [<----- AS Information --] |                     |
      |                            |                     |
      | --- Token Request  ----------------------------> |
      |                            |                     |
      | <---------------------------- Access Token ----- |
      |                           + RS Access Information   |

                   Figure 1: Retrieving an Access Token

   To determine the AS in charge of a resource hosted at the RS, the
   client C MAY
   send an initial Unauthorized Resource Request message to the RS.  The
   RS then denies the request and sends an AS information message
   containing the address of its AS back to the client C as specified in section
   Section 5.1.2 of draft-ietf-
   ace-oauth-authz [2]. draft-ietf-ace-oauth-authz [5].

   Once the client C knows the authorization server's address, it can send
   an Access Token access token request to the token endpoint at the AS as specified
   in [I-D.ietf-ace-oauth-authz]. I-D.ietf-ace-oauth-authz [6].  As the Access Token access token request as well
   as the response may contain confidential data, the communication
   between the client and the authorization server MUST be
   confidentiality-protected and ensure authenticity.  How the mutual
   authentication between the client and the authorization server is
   achieved is out of scope for this document; the client  C may have been
   configured with a public key of the authorization server and have
   been
   registered at the AS via the OAuth 2.0 client registration mechanism
   as outlined in section Section 5.3 of draft-ietf-ace-oauth-authz [3].

   If C wants to use the CoAP RawPublicKey mode as described in
   Section 9 of RFC 7252 [4] it MUST provide a key or key identifier
   within a "cnf" object in the [7].

   The access token request.  If returned by the authorization server AS decides that the request is to can then be authorized it generates
   an access token response for
   used by the client C containing to establish a "profile"
   parameter new DTLS session with the value "coap_dtls" to indicate that this profile
   MUST be used for communication between the client C and the resource
   server.

   For RPK mode, the authorization server also adds a "rs_cnf" parameter
   containing information about the public that is used by the resource
   server (see Section 3).

   For PSK mode, the authorization server adds a "cnf" parameter
   containing information about the shared secret that C can use to
   setup a DTLS session with the resource server (see Section 4).

   The Access Token returned by the authorization server then can be
   used by the client to establish a new DTLS session with the resource
   server.  When the client intends resource
   server.  When the client intends to use asymmetric cryptography in
   the DTLS handshake with the resource server, the client MUST upload
   the Access Token access token to the authz-info resource resource, i.e. the authz-info
   endpoint, on the resource server before starting the DTLS handshake,
   as described in section Section 5.8.1 of draft-ietf-ace-oauth-authz [5]. [8].  If
   only symmetric cryptography is used between the client and the
   resource server, the Access Token access token MAY instead be transferred in the
   DTLS ClientKeyExchange message (see Section 4.1). 3.3.1).

   Figure 2 depicts the common protocol flow for the DTLS profile after
   the client C has retrieved the Access Token access token from the authorization
   server AS.

      C                            RS                   AS
      | [--- Access Token ------>] |                     |
      |                            |                     |
      | <== DTLS channel setup ==> |                     |
      |                            |                     |
      | == Authorized Request ===> |                     |
      |                            |                     |
      | <=== Protected Resource == |                     |

                        Figure 2: Protocol overview

3.  Protocol Flow

   The following sections specify how CoAP is used to interchange
   access-related data between the resource server server, the client and the
   authorization server so that the authorization server can provide the
   client and the resource server with sufficient information to
   establish a secure channel, and convey authorization information
   specific for this communication relationship to the resource server.

   Section 3.1 describes how the communication between C and AS must be
   secured.  Depending on the desired used CoAP security mode, mode (see also
   Section 9 of RFC 7252 [9]), the Client-to-AS request, AS-to-Client
   response and DTLS session establishment carry slightly different
   information.  Section 3 3.2 addresses the use of raw public keys while
   Section 4 3.3 defines how pre-shared keys are used in this profile.

2.1.  Resource Access

   Once a DTLS channel has been established as described in Section 3

3.1.  Communication between C and Section 4, respectively, the client is authorized to AS

   To retrieve an access
   resources covered by the Access Token it has uploaded to the authz-
   info resource hosted by the resource server.

   On token for the resource server side, successful establishment of the DTLS
   channel binds that the client wants to
   access, the client requests an access token, functioning as a proof-
   of-possession associated key.  Any request that token from the resource server
   receives on this channel MUST be checked against these authorization
   rules that are associated with the identity of
   server.  Before C can request the client.  Incoming
   CoAP requests that are not authorized with respect to any Access access token, C and AS must
   establish a secure communication channel.  C must securely have
   obtained keying material to communicate with AS, and C must securely
   have received authorization information intended for C that states
   that AS is authorized to provide keying material concerning RS to C.
   Also, AS must securely have obtained keying material for C, and
   obtained authorization rules approved by the resource owner (RO)
   concerning C and RS that relate to this keying material.  C and AS
   must use their respective keying material for all exchanged messages.
   How the security association between C and AS is established is not
   part of this document.  C and AS MUST ensure the confidentiality,
   integrity and authenticity of all exchanged messages.

   If C is constrained, C and AS should use DTLS to communicate with
   each other.  But C and AS may also use other means to secure their
   communication, e.g., TLS.  The used security protocol must provide
   confidentiality, integrity and authenticity, and enable the client to
   determine if it is the intended recipient of a message, e.g., by
   using an AEAD mechanism.  C must also be able to determine if a
   response from AS belongs to a certain request.  Additionally, the
   protocol must offer replay protection.

3.2.  RawPublicKey Mode

   After C and AS mutually authenticated each other and validated each
   other's authorization, C sends a token request to AS's token
   endpoint.  The client MUST add a "cnf" object carrying either its raw
   public key or a unique identifier for a public key that it has
   previously made known to the authorization server.  To prove that the
   client is in possession of this key, C MUST use the same keying
   material that it uses to secure the communication with AS, e.g., the
   DTLS session.

   An example access token request from the client to the AS is depicted
   in Figure 3.

      POST coaps://as.example.com/token
      Content-Format: application/ace+cbor
      {
        grant_type: client_credentials,
        req_aud:           "tempSensor4711",
        req_cnf: {
          COSE_Key: {
            kty: EC2,
            crv: P-256,
            x:   h'e866c35f4c3c81bb96a1...',
            y:   h'2e25556be097c8778a20...'
          }
        }
      }

            Figure 3: Access Token Request Example for RPK Mode

   The example shows an access token request for the resource identified
   by the string "tempSensor4711" on the authorization server using a
   raw public key.

   AS MUST check if the client that it communicates with is associated
   with the client MUST RPK in the cnf object before issuing an access token to it.
   If AS determines that the request is to be authorized according to
   the respective authorization rules, it generates an access token
   response for C.  The response SHOULD contain a "profile" parameter
   with the value "coap_dtls" to indicate that this profile must be rejected by used
   for communication between the client C and the resource server with 4.01 response as described in Section 5.1.1 of
   draft-ietf-ace-oauth-authz [6].

   Note: server.  The identity of
   response also contains an access token and an "rs_cnf" parameter
   containing information about the client public key that is determined used by the authentication
   process
      during
   resource server.  AS MUST ascertain that the DTLS handshake.  In RPK specified in
   "rs_cnf" belongs to the asymmetric case, resource server that C wants to communicate
   with.  AS MUST protect the public key
      will define integrity of the client's identity, while in token.  If the PSK case, access
   token contains confidential data, AS MUST also protect the
      client's identity is defined by
   confidentiality of the shared secret generated by access token.

   C MUST ascertain that the access token response belongs to a certain
   previously sent access token request, as the request may specify the
      authorization server for this communication.

   The
   resource server SHOULD treat an incoming CoAP with which C wants to communicate.

3.2.1.  DTLS Channel Setup Between C and RS

   Before the client initiates the DTLS handshake with the resource
   server, C MUST send a "POST" request as
   authorized if containing the following holds:

   1.  The message was received on new access token
   to the authz-info resource hosted by the resource server.  If this
   operation yields a secure positive response, the client SHOULD proceed to
   establish a new DTLS channel that has been
       established using with the procedure resource server.  To use the
   RawPublicKey mode, the client MUST specify the public key that AS
   defined in this document.

   2.  The authorization information tied to the sending peer is valid.

   3.  The request is destined for "cnf" field of the access token response in the resource server.

   4.  The resource URI specified
   SubjectPublicKeyInfo structure in the request is covered by DTLS handshake as specified in
   RFC 7250 [10].

   An implementation that supports the
       authorization information.

   5.  The request method is an authorized action on RPK mode of this profile MUST at
   least support the resource ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
   [RFC7251] with
       respect to the authorization information.

   Incoming ed25519 curve (cf.  [RFC8032], [RFC8422]).

   Note:  According to RFC 7252 [11], CoAP requests received on a secure DTLS channel implementations MUST be
   rejected according to [Section 5.1.1 of draft-ietf-ace-oauth-
   authz](https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
   13#section-5.1.1

   1.  with response code 4.03 (Forbidden) when support
      the resource URI
       specified ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and
      the NIST P-256 curve.  As discussed in RFC 7748 [12], new ECC
      curves have been defined recently that are considered superior to
      the request so-called NIST curves.  The curve that is not covered by the authorization
       information, mandatory to
      implement in this specification is said to be efficient and

   2.  with response code 4.05 (Method Not Allowed) when less
      dangerous regarding implementation errors than the resource
       URI specified secp256r1 curve
      mandated in RFC 7252 [13].

   RS MUST check if the request covered by access token is still valid, if RS is the authorization
       information but not
   intended destination, i.e., the requested action.

   The client cannot always know a priori audience, of the token, and if the
   token was issued by an Authorized Resource
   Request will succeed.  If authorized AS.  The access token is
   constructed by the client repeatedly gets error responses
   containing AS Information (cf.  Section 5.1.1 of draft-ietf-ace-
   oauth-authz [7] as response to its requests, it SHOULD request a new
   Access Token from authorization server such that the authorization resource server in order to continue
   communication
   can associate the access token with the resource server.

2.2.  Dynamic Update of Authorization Information Client's public key.  The client can update
   "cnf" claim MUST contain either C's RPK or, if the authorization information stored at key is already
   known by the resource server at any time without changing an established DTLS
   session.  To do so, the Client requests (e.g., from previous communication), a
   reference to this key.  If the authorization server
   a new Access Token for the intended action on has no certain
   knowledge that the respective resource
   and uploads this Access Token Client's key is already known to the authz-info resource on the resource server.

   Figure 3 depicts
   server, the message flow where Client's public key MUST be included in the client C requests access
   token's "cnf" parameter.  If CBOR web tokens [RFC8392] are used as
   recommended in I-D.ietf-ace-oauth-authz [14], unencrypted keys MUST
   be specified using a new
   Access Token after "COSE_Key" object, encrypted keys with a security association between the client
   "COSE_Encrypt0" structure and references to the
   resource server key as "key_id"
   parameters in a CBOR map.  RS has been established using this protocol.  The
   token request MUST specify the key identifier of the existing DTLS
   channel between use the client and keying material in the resource server
   handshake that AS specified in the "kid" rs_cnf parameter of in the Client-to-AS request.  The authorization server MUST
   verify that access
   token.  Thus, the specified "kid" denotes a valid verifier for a proof-
   of-possession ticket that has previously been issued handshake only finishes if C and RS are able to use
   their respective keying material.

3.3.  PreSharedKey Mode

   To retrieve an access token for the
   requesting client.  Otherwise, the Client-to-AS request MUST be
   declined with a resource that the error code "unsupported_pop_key" as defined in
   Section 5.6.3 of draft-ietf-ace-oauth-authz [8].

   When client wants to
   access, the authorization server issues client MAY include a new "cnf" object carrying an identifier
   for a symmetric key in its access token request to update
   existing authorization information it MUST include the specified
   "kid" parameter in this access token.  A resource server MUST
   associate the updated authorization information with any existing
   DTLS session that is identified
   server.  This identifier can be used by this key identifier.

   Note: By associating the access tokens with authorization server to
   determine the identifier of an
      existing DTLS session, shared secret to construct the authorization information can be
      updated without changing proof-of-possession
   token.  AS MUST check if the cryptographic keys identifier refers to a symmetric key
   that was previously generated by AS as a shared secret for the DTLS
   communication between the this client and the resource server, i.e. an
      existing session can be used with updated permissions. server.

   The authorization server MUST determine the authorization rules for
   the C                            RS                   AS
      | <===== DTLS channel =====> |                     |
      |        + Access Token      |                     |
      |                            |                     |
      | --- Token Request  ----------------------------> |
      |                            |                     |
      | <---------------------------- New Access Token - |
      |                               + RS Information   |
      |                            |                     |
      | --- Update /authz-info --> |                     |
      |     New Access Token       |                     |
      |                            |                     |
      | == Authorized Request ===> |                     |
      |                            |                     |
      | <=== Protected Resource == |                     |

              Figure 3: Overview of Dynamic Update Operation

2.3.  Token Expiration

   DTLS sessions that have been established in accordance it communicates with this
   profile are always tied to a specific set of access tokens.  As these
   tokens may become invalid at any time (either because as defined by RO and generate the access
   token has
   expired or accordingly.  If the responsible authorization server has revoked authorizes the
   token),
   client, it returns an AS-to-Client response.  If the session may become useless at some point.  A resource
   server therefore may decide profile
   parameter is present, it is set to terminate existing DTLS sessions after "coap_dtls".  AS MUST ascertain
   that the last valid access token is generated for this session has been deleted.

   As specified in section 5.8.3 of draft-ietf-ace-oauth-authz [9], the resource server that C
   wants to communicate with.  Also, AS MUST notify protect the client with an error response with
   code 4.01 (Unauthorized) for any long running request before
   terminating integrity of
   the session.

   The resource server MAY access token.  If the token contains confidential data such as
   the symmetric key, the confidentiality of the token MUST also keep be
   protected.  Depending on the session alive for some time requested token type and
   respond algorithm in
   the access token request, the authorization server adds access
   Information to incoming requests the response that provides the client with sufficient
   information to setup a 4.01 (Unauthorized) error message
   including DTLS channel with the resource server.  AS Information
   adds a "cnf" parameter to signal the access information carrying a
   "COSE_Key" object that informs the client needs about the symmetric key
   that is to upload be used between C and the resource server.

   An example access token response is illustrated in Figure 4.  In this
   example, the authorization server returns a 2.01 response containing
   a new access token before it can continue using this DTLS session. and information for the client, including the
   symmetric key in the cnf claim.  The
   AS Information information is created transferred as a
   CBOR data structure as specified in section 5.1.2 I-D.ietf-ace-oauth-authz [15].

      2.01 Created
      Content-Format: application/ace+cbor
      Max-Age: 86400
      {
         access_token: h'd08343a10...
         (remainder of draft-
   ietf-ace-oauth-authz [10]. CWT omitted for brevity)
         token_type:   pop,
         alg:          HS256,
         expires_in:   86400,
         profile:      coap_dtls,
         cnf: {
           COSE_Key: {
             kty: symmetric,
             k: h'73657373696f6e6b6579'
           }
         }
      }

                  Figure 4: Example Access Token Response

   The resource server SHOULD add access token also comprises a "kid"
   parameter to the AS Information denoting the identifier of "cnf" claim.  This claim usually
   contains a "COSE_Key" object that carries either the symmetric key
   that it uses internally for this DTLS session.  The client then
   includes this "kid" parameter in
   itself or or a Client-to-AS request key identifier that can be used by the resource server
   to
   retrieve determine the shared secret.  If the access token carries a new
   symmetric key, the access token to MUST be used with this DTLS session.  In
   case the key identifier is already known by encrypted using a
   "COSE_Encrypt0" structure.  The AS MUST use the client (e.g. because
   it was included in keying material
   shared with the RS Information in an AS-to-Client response), to encrypt the "kid" parameter MAY be elided from token.

   Instead of providing the keying material, the AS Information.

   Table 1 updates Figure 2 MAY include a key
   derivation function and a salt in section 5.1.2 of draft-ietf-ace-oauth-
   authz [11] with the new "kid" parameter in accordance with [RFC8152].

              +----------------+----------+-----------------+
              | Parameter name | CBOR Key | Major Type      |
              +----------------+----------+-----------------+
              | kid            | 4        | 2 (byte string) |
              +----------------+----------+-----------------+

                Table 1: Updated AS Information parameters

3.  RawPublicKey Mode

   To retrieve an access token for the resource that enables the client wants
   resource server to
   access, calculate the client requests an Access Token keying material for the
   communication with C from the authorization
   server.  The client MUST add access token.  In this case, the token
   contains a "cnf" object carrying either its raw
   public structure that specifies the key or a unique identifier for a public derivation
   algorithm and the salt that the AS has used to construct the shared
   key.  AS and RS MUST use their shared keying material for the key that it has
   previously made known to
   derivation, and the authorization server.  To prove that key derivation MUST follow Section 11 of RFC 8152
   [16] with parameters as specified here.  The KDF specified in the
   client
   "alg" parameter SHOULD be HKDF-SHA-256.  The salt picked by the AS
   must be uniformly random and is carried in possession the "salt" parameter.

   The fields in the context information "COSE_KDF_Context"
   (Section 11.2 of this key, it RFC 8152 [17]) MUST use have the same public key
   as in certificate message that following values:

   o  AlgorithmID = "ACE-CoAP-DTLS-salt"

   o  PartyUInfo = PartyVInfo = ( null, null, null )
   o  keyDataLength is used to establish a uint equal the DTLS session
   with length of the authorization server. key shared between
      AS and RS in bits

   o  protected MUST be a zero length bstr

   o  other is a zero length bstr

   o  SuppPrivInfo is omitted

   An example Access Token request from the client to the resource
   server "cnf" structure specifying HMAC-based key derivation of a
   symmetric key with SHA-256 as pseudo-random function and a random
   salt value is depicted provided in Figure 4.

      POST coaps://as.example.com/token
      Content-Format: application/cbor
      {
        grant_type:    client_credentials,
        aud:           "tempSensor4711",
        cnf: {
          COSE_Key: 5.

   cnf : {
            kty: EC2,
            crv: P-256,
            x:   h'TODOX',
            y:   h'TODOY'
          }
        }
      kty  : symmetric,
      alg  : HKDF-SHA-256,
      salt : h'eIiOFCa9lObw'
   }

         Figure 4: Access Token Request Example for RPK Mode

   The example shows 5: Key Derivation Specification in an Access Token request for the resource identified
   by the audience string "tempSensor4711"

   A response that declines any operation on the authorization server
   using a raw public key.

   When the authorization server authorizes a request, it will return an requested resource is
   constructed according to Section 5.2 of RFC 6749 [18], (cf.
   Section 5.7.3. of draft-ietf-ace-oauth-authz [19]).

       4.00 Bad Request
       Content-Format: application/ace+cbor
       {
         error: invalid_request
       }

            Figure 6: Example Access Token Response With Reject

3.3.1.  DTLS Channel Setup Between C and RS

   When a "cnf" object in the AS-to-Client response.  Before
   the client initiates receives an access token response from an authorization
   server, C MUST ascertain that the DTLS handshake with access token response belongs to a
   certain previously sent access token request, as the resource server, it
   MUST send a "POST" request containing may
   specify the new Access Token resource server with which C wants to communicate.

   C checks if the
   authz-info resource hosted by payload of the resource server.  If this operation
   yields access token response contains an
   "access_token" parameter and a positive response, "cnf" parameter.  With this
   information the client SHOULD proceed to establish can initiate the establishment of a new DTLS
   channel with the a resource server.  To use raw public key
   mode, DTLS with pre-shared keys,
   the client MUST pass follows the same public PSK key that was used for
   constructing the Access Token with the SubjectPublicKeyInfo structure
   in the DTLS handshake as exchange algorithm specified in [RFC7250].

   An implementation that supports the RPK mode
   Section 2 of this profile MUST at
   least support the ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
   [RFC7251] with the ed25519 curve (cf.  [RFC8032], [RFC8422]).

   Note:  According to [RFC7252], CoAP implementations MUST support the
      ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and the
      NIST P-256 curve.  As discussed in [RFC7748], new ECC curves have
      been defined recently that are considered superior to the so-
      called NIST curves.  The curve that is mandatory to implement in
      this specification is said to be efficient and less dangerous
      regarding implementation errors than RFC 4279 [20] using the secp256r1 curve mandated key conveyed in [RFC7252].

   The Access Token is constructed by the authorization server such that
   the resource server can associate the Access Token with "cnf"
   parameter of the Client's
   public key.  If CBOR web tokens [RFC8392] are used AS response as recommended in
   [I-D.ietf-ace-oauth-authz], PSK when constructing the authorization server MUST include a
   "COSE_Key" object in premaster
   secret.

   In PreSharedKey mode, the "cnf" claim knowledge of the Access Token.  This
   "COSE_Key" object MAY contain a reference to a key for shared secret by the
   client and the client
   that resource server is already known by used for mutual authentication
   between both peers.  Therefore, the resource server (e.g., must be able to
   determine the shared secret from previous
   communication).  If the access token.  Following the
   general ACE authorization server has no certain knowledge
   that framework, the Client's key is already known client can upload the access
   token to the resource server, server's authz-info resource before starting
   the
   Client's public key MUST be included in DTLS handshake.  Alternatively, the Access Token's "cnf"
   parameter.

4.  PreSharedKey Mode

   To retrieve an client MAY provide the most
   recent access token for in the resource that "psk_identity" field of the client wants to
   access,
   ClientKeyExchange message.  To do so, the client MAY include a "cnf" object carrying an identifier
   for a symmetric key in its Access Token request to MUST treat the authorization
   server.  This identifier can be used by
   contents of the authorization server to
   determine "access_token" field from the shared secret to construct AS-to-Client response
   as opaque data and not perform any re-coding.

   Note:  As stated in Section 4.2 of RFC 7925 [21], the proof-of-possession
   token PSK identity
      should be treated as binary data in the Internet of Things space
      and therefore MUST specify not assumed to have a symmetric key human-readable form of any sort.

   If a resource server receives a ClientKeyExchange message that was previously
   generated by
   contains a "psk_identity" with a length greater zero, it uses the authorization server
   contents as a shared secret index for its key store (i.e., treat the
   communication between the client and the contents as key
   identifier).  The resource server.

   Depending on the requested token type and algorithm in server MUST check if it has one or more
   access tokens that are associated with the Access
   Token request, specified key.

   If no key with a matching identifier is found, the authorization resource server adds RS Information to
   MAY process the
   response that provides contents of the client "psk_identity" field as access token
   that is stored with sufficient the authorization information to
   setup a endpoint, before
   continuing the DTLS channel with handshake.  If the resource server.  For symmetric proof-
   of-possession keys (c.f.  [I-D.ietf-ace-oauth-authz]), contents of the client
   must ensure that "psk_identity"
   do not yield a valid access token for the Access Token request requesting client, the DTLS
   session setup is sent over terminated with an "illegal_parameter" DTLS alert
   message.

   Note1:  As a secure
   channel that guarantees authentication, message integrity and
   confidentiality.

   When the authorization resource server authorizes the cannot provide a client it returns an AS-
   to-Client response with the profile parameter set to "coap_dtls" and
   a "cnf" parameter carrying a "COSE_Key" object that contains
      meaningful PSK identity hint in response to the
   symmetric key client's
      ClientHello message, the resource server SHOULD NOT send a
      ServerKeyExchange message.

   Note2:  According to be used between RFC 7252 [22], CoAP implementations MUST support
      the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655].  A client and is
      therefore expected to offer at least this ciphersuite to the
      resource server
   as illustrated in Figure 5.

      2.01 Created
      Content-Format: application/cbor
      Location-Path: /token/asdjbaskd
      {
         access_token: h'd08343a10...
         (remainder server.

   When RS receives an access token, RS MUST check if the access token
   is still valid, if RS is the intended destination, i.e., the audience
   of CWT omitted for brevity)
         token_type:   pop,
         alg:          HS256,
         expires_in:   86400,
         profile:      coap_dtls,
         cnf: {
           COSE_Key: {
             kty: symmetric,
             k: h'73657373696f6e6b6579'
           }
         }
      }

                  Figure 5: Example Access Token response

   In this example, the authorization server returns a 2.01 response
   containing a new Access Token.  The information token, and if the token was issued by an authorized AS.  This
   specification assumes that the access token is transferred as a
   CBOR data structure PoP token as specified
   described in [I-D.ietf-ace-oauth-authz].

   A response that declines any operation on I-D.ietf-ace-oauth-authz [23] unless specifically stated
   otherwise.  Therefore, the requested resource access token is
   constructed according bound to Section 5.2 of RFC 6749 [12], (cf.
   Section 5.7.3 of [I-D.ietf-ace-oauth-authz]).

       4.00 Bad Request
       Content-Format: application/cbor
       {
         error: invalid_request
       }

            Figure 6: Example Access Token response with reject

4.1.  DTLS Channel Setup Between C and RS

   When a client receives an Access Token from an authorization server,
   it checks if symmetric PoP
   key that is used as shared secret between the payload contains an "access_token" parameter client and a
   "cnf" parameter.  With this information the client can initiate
   establishment of a new DTLS channel with a resource
   server.  To use
   DTLS with pre-shared keys,

   While the client follows can retrieve the PSK key exchange
   algorithm specified in Section 2 of [RFC4279] using shared secret from the key conveyed
   in contents of
   the "cnf" parameter of the AS response as PSK when constructing
   the premaster secret.

   In PreSharedKey mode, the knowledge of the shared secret by in the
   client and AS-to-Client response, the resource server is used for mutual authentication
   between both peers.  Therefore,
   uses the resource server must be able information contained in the "cnf" claim of the access token
   to determine the shared actual secret from when no explicit "kid" was provided in
   the Access Token.  Following "psk_identity" field.  If key derivation is used, the
   general ACE authorization framework, RS uses the
   "COSE_KDF_Context" information as described above.

3.4.  Resource Access

   Once a DTLS channel has been established as described in Section 3.2
   and Section 3.3, respectively, the client can upload is authorized to access
   resources covered by the Access
   Token access token it has uploaded to the authz-
   info resource server's authz-info resource before starting
   the DTLS handshake.  Alternatively, the client MAY provide hosted by the most
   recent Access Token in resource server.

   With the "psk_identity" field successful establishment of the
   ClientKeyExchange message.  To do so, DTLS channel, C and RS have
   proven that they can use their respective keying material.  An access
   token that is bound to the client MUST treat client's keying material is associated
   with the
   contents of channel.  Any request that the "access_token" field from resource server receives on
   this channel MUST be checked against these authorization rules.  RS
   MUST check for every request if the AS-to-Client response
   as opaque data and access token is still valid.
   Incoming CoAP requests that are not perform authorized with respect to any re-coding.

   Note: As stated in section 4.2 of [RFC7925],
   access token that is associated with the PSK identity should client MUST be treated rejected by
   the resource server with 4.01 response as binary data described in the Internet of Things space and not
   assumed to have a human-readable form Section 5.1.1
   of any sort.

   If a draft-ietf-ace-oauth-authz [24].

   The resource server receives a ClientKeyExchange SHOULD treat an incoming CoAP request as
   authorized if the following holds:

   1.  The message that
   contains a "psk_identity" with was received on a length greater zero, it uses secure channel that has been
       established using the
   contents as index procedure defined in this document.

   2.  The authorization information tied to the sending client is
       valid.

   3.  The request is destined for its key store (i.e., treat the contents as key
   identifier). resource server.

   4.  The resource URI specified in the request is covered by the
       authorization information.

   5.  The request method is an authorized action on the resource server MUST check if it has one or more
   Access Tokens with
       respect to the authorization information.

   Incoming CoAP requests received on a secure DTLS channel that are associated not
   thus authorized MUST be rejected according to Section 5.8.2 of draft-
   ietf-ace-oauth-authz [25]
   1.  with response code 4.03 (Forbidden) when the resource URI
       specified key.  If no
   valid Access Token is available for this key, in the DTLS session setup
   is terminated with an "illegal_parameter" DTLS alert message.

   If no key with a matching identifier request is found not covered by the resource server authorization
       information, and

   2.  with response code 4.05 (Method Not Allowed) when the resource server MAY process the decoded contents of
       URI specified in the
   "psk_identity" field as access token that is stored with request covered by the authorization
       information endpoint before continuing but not the DTLS
   handshake. requested action.

   The client cannot always know a priori if an Authorized Resource
   Request will succeed.  If the decoded contents client repeatedly gets error responses
   containing AS Information (cf.  Section 5.1.2 of the "psk_identity" do not
   yield draft-ietf-ace-
   oauth-authz [26]) as response to its requests, it SHOULD request a valid
   new access token for the requesting client, from the DTLS
   session setup is terminated with an "illegal_parameter" DTLS alert
   message.

   Note1: As a resource authorization server cannot provide a client with a meaningful
    PSK identity hint in
      response order to continue
   communication with the client's ClientHello message, resource server.

4.  Dynamic Update of Authorization Information

   The client can update the authorization information stored at the
   resource server
      SHOULD NOT send at any time without changing an established DTLS
   session.  To do so, the Client requests a ServerKeyExchange message.

   Note2:  According to [RFC7252], CoAP implementations MUST support new access token from the
   authorization server for the
      ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655].  A client is
      therefore expected to offer at least intended action on the respective
   resource and uploads this ciphersuite access token to the authz-info resource on
   the resource server.

   This specification assumes that

   Figure 7 depicts the Access Token is message flow where the C requests a PoP new access
   token as
   described in [I-D.ietf-ace-oauth-authz] unless specifically stated
   otherwise.  Therefore, the Access Token is bound to after a symmetric PoP
   key that is used as shared secret security association between the client and the
   resource
   server.

   While server has been established using this protocol.  If the
   client can retrieve wants to update the shared secret from authorization information, the contents token
   request MUST specify the key identifier of the "cnf" parameter in existing DTLS channel
   between the AS-to-Client response, client and the resource server
   uses the information contained in the "cnf" claim "kid" parameter of
   the Access Token
   to determine Client-to-AS request.  The authorization server MUST verify that
   the actual secret when no explicit specified "kid" was provided in
   the "psk_identity" field.  Usually, this is done by including denotes a
   "COSE_Key" object carrying either valid verifier for a key proof-of-
   possession token that has previously been encrypted issued to the requesting
   client.  Otherwise, the Client-to-AS request MUST be declined with
   the error code "unsupported_pop_key" as defined in Section 5.6.3 of
   draft-ietf-ace-oauth-authz [27].

   When the authorization server issues a shared secret new access token to update
   existing authorization information, it MUST include the specified
   "kid" parameter in this access token.  A resource server MUST
   associate the updated authorization information with any existing
   DTLS session that is identified by this key identifier.

   Note:  By associating the access tokens with the identifier of an
      existing DTLS session, the authorization information can be
      updated without changing the cryptographic keys for the DTLS
      communication between the authorization server client and the resource server, or a key identifier that i.e. an
      existing session can be used by the resource server
   to lookup the shared secret.

   Instead of the "COSE_Key" object, the authorization server MAY
   include a "COSE_Encrypt" structure to enable the resource server to
   calculate the shared key from the Access Token.  The "COSE_Encrypt"
   structure MUST use the _Direct Key with KDF_ method as described in
   Section 12.1.2 updated permissions.

      C                            RS                   AS
      | <===== DTLS channel =====> |                     |
      |        + Access Token      |                     |
      |                            |                     |
      | --- Token Request  ----------------------------> |
      |                            |                     |
      | <---------------------------- New Access Token - |
      |                           + Access Information   |
      |                            |                     |
      | --- Update /authz-info --> |                     |
      |     New Access Token       |                     |
      |                            |                     |
      | == Authorized Request ===> |                     |
      |                            |                     |
      | <=== Protected Resource == |                     |

              Figure 7: Overview of RFC 8152 [13].  The authorization server MUST
   include a Context information structure carrying a PartyU "nonce"
   parameter carrying the nonce Dynamic Update Operation

5.  Token Expiration

   DTLS sessions that has have been used by the authorization
   server established in accordance with this
   profile are always tied to construct the shared key.

   This specification mandates that a specific set of access tokens.  As these
   tokens may become invalid at least any time (either because the key derivation
   algorithm "HKDF SHA-256" as defined in [RFC8152] MUST be supported.
   This key derivation function is token has
   expired or the default when no "alg" field is
   included in responsible authorization server has revoked the "COSE_Encrypt" structure for
   token), the session may become useless at some point.  A resource server.

4.2.  Updating Authorization Information

   Usually, the authorization information that
   server therefore MUST terminate existing DTLS sessions after the resource server keeps last
   valid access token for a client is updated by uploading a new Access Token as described this session has been deleted.

   As specified in Section 2.2.

   The Client MAY also perform a new DTLS handshake according to
   Section 4.1 that replaces the existing DTLS session.  After
   successful completion 5.8.3 of the DTLS handshake draft-ietf-ace-oauth-authz [28], the
   resource server
   updates MUST notify the existing authorization information client with an error response with
   code 4.01 (Unauthorized) for any long running request before
   terminating the client
   according to session.

   Table 1 updates Figure 2 in Section 5.1.2 of draft-ietf-ace-oauth-
   authz [29] with the new Access Token.

5. "kid" parameter in accordance with [RFC8152].

              +----------------+----------+-----------------+
              | Parameter name | CBOR Key | Major Type      |
              +----------------+----------+-----------------+
              | kid            | 4        | 2 (byte string) |
              +----------------+----------+-----------------+

                Table 1: Updated AS Information parameters

6.  Security Considerations

   This document specifies a profile for the Authentication and
   Authorization for Constrained Environments (ACE) framework
   [I-D.ietf-ace-oauth-authz].  As it follows this framework's general
   approach, the general security and privacy considerations from
   section 6 and section 7 also apply to this profile.

   Constrained devices that use DTLS [RFC6347] are inherently vulnerable
   to Denial of Service (DoS) attacks as the handshake protocol requires
   creation of internal state within the device.  This is specifically
   of concern where an adversary is able to intercept the initial cookie
   exchange and interject forged messages with a valid cookie to
   continue with the handshake.

   [I-D.tiloca-tls-dos-handshake] specifies a TLS extension to prevent
   this type of attack which is applicable especially for constrained
   environments where the authorization server can act as trust anchor.

6.

   The use of multiple access tokens for a single client increases the
   strain on the resource server as it must consider every access token
   and calculate the actual permissions of the client.  Also, tokens may
   contradict each other which may lead the server to enforce wrong
   permissions.  If one of the access tokens expires earlier than
   others, the resulting permissions may offer insufficient protection.
   Developers should avoid using multiple access tokens for a client.

7.  Privacy Considerations

   An unprotected response to an unauthorized request may disclose
   information about the resource server and/or its existing
   relationship with the client.  It is advisable to include as little
   information as possible in an unencrypted response.  When a DTLS
   session between the client and the resource server already exists,
   more detailed information may be included with an error response to
   provide the client with sufficient information to react on that
   particular error.

   Also, unprotected requests to the resource server may reveal
   information about the client, e.g., which resources the client
   attempts to request or the data that the client wants to provide to
   the resource server.  The client should not send confidential data in
   an unprotected request.

   Note that some information might still leak after DTLS session is
   established, due to observable message sizes, the source, and the
   destination addresses.

7.

8.  IANA Considerations

   The following registrations are done for the ACE OAuth Profile
   Registry following the procedure specified in
   [I-D.ietf-ace-oauth-authz].

   Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
   with the RFC number of this specification and delete this paragraph.

   Profile name: coap_dtls

   Profile Description: Profile for delegating client authentication and
   authorization in a constrained environment by establishing a Datagram
   Transport Layer Security (DTLS) channel between resource-constrained
   nodes.

   Profile ID: 1

   Change Controller: IESG

   Reference: [RFC-XXXX]

8.

9.  References

8.1.

9.1.  Normative References

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-13 draft-ietf-ace-oauth-authz-16
              (work in progress), July October 2018.

   [I-D.tiloca-tls-dos-handshake]
              Tiloca, M., Seitz, L., Hoeve, M., and O. Bergmann,
              "Extension for protecting (D)TLS handshakes against Denial
              of Service", draft-tiloca-tls-dos-handshake-02 (work in
              progress), March 2018.

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

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,
              <https://www.rfc-editor.org/info/rfc4279>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <https://www.rfc-editor.org/info/rfc5746>.

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

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

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.

9.2.  Informative References

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655,
              DOI 10.17487/RFC6655, July 2012,
              <https://www.rfc-editor.org/info/rfc6655>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
              <https://www.rfc-editor.org/info/rfc7251>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422,
              DOI 10.17487/RFC8422, August 2018,
              <https://www.rfc-editor.org/info/rfc8422>.

8.3.

9.3.  URIs

   [1] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.8.1
       16#section-5.8.1

   [2] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.1.2 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [3] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.3 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [4] https://tools.ietf.org/html/rfc7252#section-9 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [5] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.8.1
       16#section-5.1.2

   [6] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.1.1 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [7] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.1.1
       16#section-5.3

   [8] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.6.3
       16#section-5.8.1

   [9] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       13#section-5.8.3 https://tools.ietf.org/html/rfc7252#section-9

   [10] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        13#section-5.1.2 https://tools.ietf.org/html/rfc7250

   [11] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        13#section-5.1.2 https://tools.ietf.org/html/rfc7252

   [12] https://tools.ietf.org/html/rfc6749#section-5.2 https://tools.ietf.org/html/rfc7748

   [13] https://tools.ietf.org/html/rfc8152#section-12.1.2 https://tools.ietf.org/html/rfc7252

   [14] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [15] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [16] https://tools.ietf.org/html/rfc8152#section-11

   [17] https://tools.ietf.org/html/rfc8152#section-11.2

   [18] https://tools.ietf.org/html/rfc6749#section-5.2

   [19] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz#section-
        5.7.3

   [20] https://tools.ietf.org/html/rfc4279#section-2

   [21] https://tools.ietf.org/html/rfc7925#section-4.2

   [22] https://tools.ietf.org/html/rfc7252

   [23] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

   [24] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        16#section-5.1.1

   [25] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        16#section-5.8.2

   [26] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        16#section-5.1.2

   [27] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        16#section-5.6.3

   [28] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        16#section-5.8.3

   [29] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
        16#section-5.1.2

Authors' Addresses

   Stefanie Gerdes
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63906
   Email: gerdes@tzi.org
   Olaf Bergmann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63904
   Email: bergmann@tzi.org

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

   Goeran Selander
   Ericsson
   Faroegatan 6
   Kista  164 80
   Sweden AB

   Email: goran.selander@ericsson.com

   Ludwig Seitz
   RISE SICS
   Scheelevaegen 17
   Lund  223 70
   Sweden

   Email: ludwig.seitz@ri.se