ACE Working Group                                              S. Gerdes
Internet-Draft                                               O. Bergmann
Intended status: Standards Track                              C. Bormann
Expires: January 4, May 3, 2018                             Universitaet Bremen TZI
                                                             G. Selander
                                                                Ericsson
                                                                L. Seitz
                                                               RISE SICS
                                                           July 03,
                                                        October 30, 2017

Datagram Transport Layer Security (DTLS) Profile Profiles for Authentication and
            Authorization for Constrained Environments (ACE)
                    draft-ietf-ace-dtls-authorize-01
                    draft-ietf-ace-dtls-authorize-02

Abstract

   This specification defines a profile two profiles for delegating client
   authentication and authorization in a constrained environment by
   establishing a Datagram Transport Layer Security (DTLS) channel
   between resource-constrained nodes.  The protocol relies on DTLS for
   communication security between entities in a constrained network. network
   using either raw public keys or pre-shared keys.  A
   resource-constrained resource-
   constrained node 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 4, May 3, 2018.

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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.  Unauthorized  Resource Request Message . . . Access . . . . . . .   5
     2.2.  AS Information . . . . . . . . . . . . . .   5
     2.2.  Dynamic Update of Authorization Information . . . . . . .   6
     2.3.  Resource Access .  Token Expiration  . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Dynamic Update of Authorization Information . . . . . . .   8
   3.  RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . . .   9   8
   4.  PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  DTLS Channel Setup Between C and RS . . . . . . . . . . .  11
     4.2.  Updating Authorization Information  . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     5.1.  Unprotected AS Information  . . . . . . . . . . . . . . .  14
     5.2.  Use of Nonces for Replay Protection . . . . . . . . . . .  14
     5.3.
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
     7.3.
     8.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 an access token, bound to a key (the proof-of-possession
   key) to authorize its access to protected resources hosted by the
   resource server.  DTLS provides communication security, proof of
   possession, and server authentication.  Optionally the client and the
   resource server may also use CoAP over DTLS to communicate with the
   authorization server.  This specification supports the DTLS handshake
   with Raw Public Keys (RPK) [RFC7250] and the DTLS PSK handshake with Pre-
   Shared Keys (PSK) [RFC4279].

   The DTLS RPK handshake [RFC7250] requires client authentication to
   provide proof-of-possession for the key tied to the access token.
   Here the access token needs to be transferred to the resource server
   before the handshake is initiated, as described in section 8.1 5.8.1 of
   draft-ietf-ace-oauth-authz. [1]
   draft-ietf-ace-oauth-authz [1].

   The DTLS PSK handshake [RFC4279] provides the proof-of-possession for
   the key tied to the access token.  Furthermore the psk_identity
   parameter in the DTLS PSK handshake is used to transfer the access
   token from the client to the resource server.

   Note: While the scope of this draft is on client and resource server

      communicating using CoAP over DTLS, it is expected that it applies
      also to CoAP over TLS, possibly with minor modifications.
      However, that is out of scope for this version of the draft.

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 RFC 2119 [RFC2119]. 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].

2.  Protocol Overview

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

   This profile requires a Client (C) to retrieve an Access Token for
   the resource(s) it wants to access on a Resource Server (RS) as
   specified in [I-D.ietf-ace-oauth-authz].  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 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 the address of its
   AS back to C.

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

   Once the client C knows AS's the authorization server's address, it can
   send an Access Token request to the
   /token token endpoint at the AS as
   specified in [I-D.ietf-ace-oauth-authz].  If C wants to use the CoAP
   RawPublicKey mode as described in Section 9 of RFC 7252 [2] it MUST
   provide a key or key identifier within a "cnf" object in the token
   request.  If the authorization server AS decides that the request is
   to be authorized it generates an access token response for the client
   C containing a "profile" parameter with the value "coap_dtls" to
   indicate that this profile MUST be used for communication between the
   client C and RS. the resource server.  Is also adds a "cnf" parameter
   with additional data for the establishment of a secure DTLS channel
   between C the client and RS. the resource server.  The semantics of the
   'cnf' parameter depend on the type of key used between C the client and RS,
   the resource server, see Section 3 and Section 4.

   The Access Token returned by AS the authorization server then can be
   used by C the client to establish a new DTLS session with RS. the resource
   server.  When C the client intends to use asymmetric cryptography in
   the DTLS handshake with RS, C the resource server, the client MUST upload
   the Access Token to the "/authz-info" authz-info resource on RS the resource server
   before starting the DTLS handshake, as described in section 8.1 5.8.1 of
   draft-ietf-ace-oauth-authz [3].  If only symmetric cryptography is
   used between C the client and RS, the resource server, the Access Token MAY
   instead be transferred in the DTLS ClientKeyExchange message (see
   Section 4.1).

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

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

                        Figure 2: Protocol overview

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

   Depending on the desired CoAP security mode, the Client-to-AS
   request, AS-to-Client response and DTLS session establishment carry
   slightly different information.  Section 3 addresses the use of raw
   public keys while Section 4 defines how pre-shared keys are used in
   this profile.

2.1.  Unauthorized Resource Request Message

   The optional Unauthorized  Resource Request message is a request for a
   resource hosted by RS for which no proper authorization is granted.
   RS MUST treat any CoAP request for a resource other than "/authz-
   info" as Unauthorized Resource Request message when any of the
   following holds:

   o  The request has been received on an unprotected channel.

   o  RS has no valid access token for the sender of the request
      regarding the requested action on that resource.

   o  RS has a valid access token for the sender of the request, but
      this does not allow the requested action on the requested
      resource.

   Note: These conditions ensure that RS can handle requests
   autonomously once access was granted and a secure channel has been
   established between C and RS.  The resource "/authz-info" is publicly
   accessible to be able to upload new access tokens to RS (cf.
   [I-D.ietf-ace-oauth-authz]).

   Unauthorized Resource Request messages MUST be denied with a client
   error response.  In this response, the Resource Server SHOULD provide
   proper AS Information to enable the Client to request an access token
   from RS's Authorization Server as described in Section 2.2.

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

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

2.2.  AS Information

   The AS Information is sent by RS as a response to an Unauthorized
   Resource Request message (see Section 2.1) to point the sender of the
   Unauthorized Resource Request message to RS's AS.  The AS information
   is a set of attributes containing an absolute URI (see Section 4.3 of
   [RFC3986]) that specifies the AS in charge of RS.

   TBD: We might not want to add more parameters in the AS information
   because
      this would not only reveal too much information about RS's
      capabilities to unauthorized peers but also be of little value as
      C cannot really trust that information anyway.

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

   Figure 3 shows an example for an AS Information message payload using
   CBOR [RFC7049] diagnostic notation.

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

                 Figure 3: AS Information payload example

   In this example, the attribute AS points the receiver of this message
   to the URI "coaps://as.example.com/token" to request access
   permissions.  The originator of the AS Information payload (i.e., RS)
   uses a local clock that is loosely synchronized with a time scale
   common between RS and AS (e.g., wall clock time).  Therefore, it has
   included a parameter "nonce" for replay attack prevention (c.f.
   Section 5.2).

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

   The examples in this document are written in CBOR diagnostic notation
   to improve readability.  Figure 4 illustrates the binary encoding of
   the message payload shown in Figure 3.

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

             Figure 4: AS Information example encoded in CBOR

2.3.  Resource Access

   Once Access

   Once a DTLS channel has been established as described in Section 3
   and Section 4, respectively, C the client is authorized to access
   resources covered by the Access Token it has uploaded to the "/authz-info" authz-
   info resource hosted by RS. the resource server.

   On the resource server side (i.e., RS), side, successful establishment of the DTLS
   channel binds C the client to the access token, functioning as a proof-of-
   possession proof-
   of-possession associated key.  Any request that RS the resource server
   receives on this channel MUST be checked against these authorization
   rules that are associated with the identity of C. the client.  Incoming
   CoAP requests that are not authorized with respect to any Access
   Token that is associated with C the client MUST be rejected by RS the
   resource server with 4.01 response as described in Section 2.1. 5.1.1 of
   draft-ietf-ace-oauth-authz [4].

   Note: The identity of C the client is determined by the authentication
   process
      during the DTLS handshake.  In the asymmetric case, the public key
      will define C's the client's identity, while in the PSK case, C's the
      client's identity is defined by the session key generated by AS the
      authorization server for this communication.

   RS

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

   1.  The message was received on a secure channel that has been
       established using the procedure defined in this document.

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

   3.  The request is destined for RS. the 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 with
       respect to the authorization information.

   Incoming CoAP requests received on a secure DTLS channel 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-
   08#section-5.1.1

   1.  with response code 4.03 (Forbidden) when the resource URI
       specified in the request is not covered by the authorization
       information, and

   2.  with response code 4.05 (Method Not Allowed) when the resource
       URI specified in the request covered by the authorization
       information but not the requested action.

   C

   The client cannot always know a priori if a an Authorized Resource
   Request will succeed.  If C the client repeatedly gets error responses
   containing AS Information messages (cf.  Section 2.2) 5.1.1 of draft-ietf-ace-
   oauth-authz [5] as response to its requests, it SHOULD request a new
   Access Token from AS the authorization server in order to continue
   communication with RS.

2.4. the resource server.

2.2.  Dynamic Update of Authorization Information

   The Client client can update the authorization information stored at RS the
   resource server at any time. time without changing an established DTLS
   session.  To do so, the Client requests from AS the authorization server
   a new Access Token for the intended action on the respective resource
   and uploads this Access Token to the "/authz-info" authz-info resource on RS. the
   resource server.

   Figure 5 3 depicts the message flow where the client C requests a new
   Access Token after a security association between C the client and the
   resource server RS has been established using this protocol.  The
   token request MUST specify the key identifier of the existing DTLS
   channel between the client and the resource server in the "kid"
   parameter of the Client-to-AS request.  The authorization server MUST
   verify that the specified "kid" denotes a valid verifier for a proof-
   of-possession ticket that has previously been issued to the
   requesting client.  Otherwise, the Client-to-AS request MUST be
   declined with a the error code "unsupported_pop_key" as defined in
   Section 5.6.3 of draft-ietf-ace-oauth-authz [6].

   When the authorization server issues a 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 client and the resource server, i.e. an
      existing session can be used with updated permissions.

      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 5: 3: Overview of Dynamic Update Operation

2.3.  Token Expiration

   DTLS sessions that have been established in accordance with this
   profile are always tied to a specific set of access tokens.  As these
   tokens may become invalid at any time (either because the token has
   expired or the responsible authorization server has revoked the
   token), the session may become useless at some point.  A resource
   server therefore may decide to terminate existing DTLS sessions after
   the last valid access token for this session has been deleted.

   As specified in section 5.8.2 of draft-ietf-ace-oauth-authz [7], the
   resource server MUST notify the client with an error response with
   code 4.01 (Unauthorized) for any long running request before
   terminating the session.

   The resource server MAY also keep the session alive for some time and
   respond to incoming requests with a 4.01 (Unauthorized) error message
   including AS Information to signal that the client needs to upload a
   new access token before it can continue using this DTLS session.  The
   AS Information is created as specified in section 5.1.2 of draft-
   ietf-ace-oauth-authz [8].  The resource server SHOULD add a "kid"
   parameter to the AS Information denoting the identifier of the key
   that it uses internally for this DTLS session.  The client then
   includes this "kid" parameter in a Client-to-AS request used to
   retrieve a new access token to be used with this DTLS session.  In
   case the key identifier is already known by the client (e.g. because
   it was included in the RS Information in an AS-to-Client response),
   the "kid" parameter MAY be elided from the AS Information.

   Table 1 updates Figure 2 in section 5.1.2 of draft-ietf-ace-oauth-
   authz [9] 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 C the client wants to
   access,
   C the client requests an Access Token from AS.  C the authorization
   server.  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 AS. the authorization server.

   An example Access Token request from C the client to RS the resource
   server is depicted in Figure 6. 4.

      POST coaps://as.example.com/token
      Content-Format: application/cbor
      {
        grant_type:    client_credentials,
        aud:           "tempSensor4711",
        cnf: {
          COSE_Key: {
            kty: EC2,
            crv: P-256,
            x:   h'TODOX',
            y:   h'TODOY'
          }
        }
      }

            Figure 6: 4: Access Token Request Example for RPK Mode

   The example shows an Access Token request for the resource identified
   by the audience string "tempSensor4711" on the AS authorization server
   using a raw public key.

   When AS the authorization server authorizes a request, it will return an
   Access Token and a "cnf" object in the AS-to-Client response.  Before C
   the client initiates the DTLS handshake with RS, the resource server, it
   MUST send a "POST" request containing the new Access Token to the "/authz-info"
   authz-info resource hosted by RS. the resource server.  If this operation
   yields a positive response, C the client SHOULD proceed to establish a
   new DTLS channel with RS. the resource server.  To use raw public key
   mode, C the client MUST pass the same public key that was used for
   constructing the Access Token with the SubjectPublicKeyInfo structure
   in the DTLS handshake as specified in [RFC7250].

   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.  C the client is therefore expected to offer at
      least this ciphersuite to RS. the resource server.

   The Access Token is constructed by AS the authorization server such that RS
   the resource server can associate the Access Token with the Client's
   public key.  If CBOR web tokens [I-D.ietf-ace-cbor-web-token] are
   used as recommended in [I-D.ietf-ace-oauth-authz], the AS authorization
   server MUST include a "COSE_Key" object in the "cnf" claim of the
   Access Token.  This "COSE_Key" object MAY contain a reference to a
   key for C the client that is already known by RS the resource server
   (e.g., from previous communication).  If the AS authorization server has
   no certain knowledge that the Client's key is already known to RS, the
   resource server, the Client's public key MUST be included in the
   Access Token's "cnf" parameter.

4.  PreSharedKey Mode

   To retrieve an access token for the resource that C the client wants to
   access,
   C the client MAY include a "cnf" object carrying an identifier
   for a symmetric key in its Access Token request to AS. the authorization
   server.  This identifier can be used by AS the authorization server to
   determine the session key to construct the proof-of-
   possession proof-of-possession token
   and therefore MUST specify a symmetric key that was previously
   generated by AS the authorization server as a session key for the
   communication between C the client and RS. the resource server.

   Depending on the requested token type and algorithm in the Access
   Token request, AS the authorization server adds RS Information to the
   response that provides C the client with sufficient information to
   setup a DTLS channel with RS. the resource server.  For symmetric proof-of-possession proof-
   of-possession keys (c.f.  [I-D.ietf-ace-oauth-authz]), C the client
   must ensure that the Access Token request is sent over a secure
   channel that guarantees authentication, message integrity and
   confidentiality.

   When AS the authorization server authorizes C the client it returns an AS-to-Client AS-
   to-Client response with the profile parameter set to "coap_dtls" and
   a "cnf" parameter carrying a "COSE_Key" object that contains the
   symmetric session key to be used between C the client and RS the resource
   server as illustrated in Figure 7. 5.

      2.01 Created
      Content-Format: application/cbor
      Location-Path: /token/asdjbaskd
      Max-Age: 86400
      {
         access_token: b64'SlAV32hkKG ...
         (remainder 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 7: 5: Example Access Token response

   In this example, AS the authorization server returns a 2.01 response
   containing a new Access Token.  The information is transferred as a
   CBOR data structure as specified in [I-D.ietf-ace-oauth-authz].  The
   Max-Age option tells the receiving Client how long this token will be
   valid.

   A response that declines any operation on the requested resource is
   constructed according to Section 5.2 of RFC 6749 [4], [10], (cf.
   Section 5.5.3 5.7.3 of [I-D.ietf-ace-oauth-authz]).

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

            Figure 8: 6: Example Access Token response with reject

4.1.  DTLS Channel Setup Between C and RS

   When C a client receives an Access Token from AS, an authorization server,
   it checks if the payload contains an "access_token" parameter and a
   "cnf" parameter.  With this information C the client can initiate
   establishment of a new DTLS channel with RS. a resource server.  To use
   DTLS with pre-shared keys, C the client follows the PSK key exchange
   algorithm specified in Section 2 of [RFC4279] using the key conveyed
   in the "cnf" parameter of the AS response as PSK when constructing
   the premaster secret.

   In PreSharedKey mode, the knowledge of the session key by C the client
   and RS the resource server is used for mutual authentication between
   both peers.  Therefore, RS the resource server must be able to determine
   the session key from the Access Token.  Following the general ACE
   authorization framework, C the client can upload the Access Token to RS's "/authz-info"
   the resource server's authz-info resource before starting the DTLS
   handshake.  Alternatively, C the client MAY provide the most recent
   base64-encoded Access Token in the "psk_identity" field of the
   ClientKeyExchange message.

   If RS a resource server receives a ClientKeyExchange message that
   contains a "psk_identity" with a length greater zero, it MUST
   base64-decode its contents and check if the "psk_identity" field contains a key
   identifier or Access Token according to the following CDDL
   specification:

   psk_identity = {
     kid => bstr // access_token => bstr
   }

   The identifiers for the map keys "kid" and "access_token" are used
   with use the same meaning resulting byte sequence as in COSE [I-D.ietf-cose-msg] and the ACE
   framework [I-D.ietf-ace-oauth-authz] respectively.  The identifier
   "kid" thus has the value 4 (see [I-D.ietf-cose-msg]), and the
   identifier "access_token" has the value 19, respectively (see
   [I-D.ietf-ace-oauth-authz]).

   If the "psk_identity" field contains a
   index for its key identifier, store (i.e., treat the receiver contents as key identifier).
   The resource server MUST check if it has one or more Access Tokens
   that are associated with the specified key.  If no valid Access Token
   is available for this key, the DTLS session setup is terminated with
   an "illegal_parameter" DTLS alert message.

   If instead no key with a matching identifier is found the resource server the
   resource server MAY process the decoded contents of the
   "psk_identity" field contains an Access Token, it must
   processed in the same way as an Access Token access token that has been uploaded
   to its "/authz-info" resource.  In this case, RS continues processing is stored with the ClientKeyExchange message if
   authorization information endpoint before continuing the DTLS
   handshake.  If the decoded contents of the "psk_identity"
   contained do not
   yield a valid Access Token.  Otherwise, access token for the requesting client, the DTLS
   session setup is terminated with an "illegal_parameter" DTLS alert
   message.

   Note1: As RS a resource server cannot provide C a client with a meaningful
    PSK identity hint in
      response to C's the client's ClientHello message, RS the resource server
      SHOULD NOT send a ServerKeyExchange message.

   Note2:  According to [RFC7252], CoAP implementations MUST support the
      ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655].  C  A client is
      therefore expected to offer at least this ciphersuite to RS. the
      resource server.

   This specification assumes that the Access Token is a PoP token as
   described in [I-D.ietf-ace-oauth-authz] unless specifically stated
   otherwise.  Therefore, the Access Token is bound to a symmetric PoP
   key that is used as session key between C the client and RS. the resource
   server.

   While C the client can retrieve the session key from the contents of
   the "cnf" parameter in the AS-to-Client response, RS the resource server
   uses the information contained in the "cnf" claim of the Access Token
   to determine the actual session key when no explicit "kid" was
   provided in the "psk_identity" field.  Usually, this is done by
   including a "COSE_Key" object carrying either a key that has been
   encrypted with a shared secret between AS the authorization server and RS,
   the resource server, or a key identifier that can be used by RS the
   resource server to lookup the session key.

   Instead of the "COSE_Key" object, AS the authorization server MAY
   include a "COSE_Encrypt" structure to enable RS the resource server to
   calculate the session key from the Access Token.  The "COSE_Encrypt"
   structure MUST use the _Direct Key with KDF_ method as described in
   Section 12.1.2 of draft-ietf-cose-msg
   [5]. RFC 8152 [11].  The AS authorization server MUST
   include a Context information structure carrying a PartyU "nonce"
   parameter carrying the nonce that has been used by AS the authorization
   server to construct the session key.

   This specification mandates that at least the key derivation
   algorithm "HKDF SHA-256" as defined in [I-D.ietf-cose-msg] [RFC8152] MUST be supported.
   This key derivation function is the default when no "alg" field is
   included in the "COSE_Encrypt" structure for RS. the resource server.

4.2.  Updating Authorization Information

   Usually, the authorization information that RS the resource server keeps
   for C a client is updated by uploading a new Access Token as described
   in Section 2.4. 2.2.

   If the security association with RS the resource server still exists and RS
   the resource server has indicated support for session renegotiation
   according to [RFC5746], the new Access Token MAY be used to
   renegotiate the existing DTLS session.  In this case, the Access
   Token is used as "psk_identity" as defined in Section 4.1.  The
   Client MAY also perform a new DTLS handshake according to Section 4.1
   that replaces the existing DTLS session.

   After successful completion of the DTLS handshake RS the resource server
   updates the existing authorization information for C the client
   according to the new Access Token.

5.  Security Considerations

   TODO

5.1.  Unprotected AS Information

   Initially, no secure channel exists to protect

   This document specifies a profile for the communication
   between C Authentication and RS.  Thus, C cannot determine if the AS information
   contained in an unprotected response from RS to an unauthorized
   request (c.f.  Section 2.2) is authentic.  It is therefore advisable
   to provide C with a (possibly hard-coded) list of trustworthy
   authorization servers.  AS information responses referring to a URI
   not listed there would be ignored.

5.2.  Use of Nonces
   Authorization for Replay Protection

   RS may add a nonce to Constrained Environments (ACE) framework
   [I-D.ietf-ace-oauth-authz].  As it follows this framework's general
   approach, the AS Information message sent as a response
   to an unauthorized request general security and privacy considerations from
   section 6 and section 7 also apply to ensure freshness 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 Access Token
   subsequently presented adversary is able to RS.  While intercept the initial cookie
   exchange and interject forged messages with a timestamp of some granularity
   would be sufficient valid cookie to protect against replay attacks, using
   randomized nonce is preferred
   continue with the handshake.

   [I-D.tiloca-tls-dos-handshake] specifies a TLS extension to prevent disclosure
   this type of information
   about RS's internal clock characteristics.

5.3. attack which is applicable especially for constrained
   environments where the authorization server can act as trust anchor.

6.  Privacy Considerations

   An unprotected response to an unauthorized request (c.f.
   Section 2.2) may disclose
   information about RS the resource server and/or its existing
   relationship with C. the client.  It is advisable to include as little
   information as possible in an unencrypted response.  When a DTLS
   session between C the client and RS the resource server already exists,
   more detailed information may be included with an error response to
   provide C the client with sufficient information to react on that
   particular error.

6.

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

7.  IANA Considerations

   This document has no actions

   The following registrations are done for IANA.

7. 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

   Specification Document(s): [RFC-XXXX]

8.  References

7.1.

8.1.  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)", draft-ietf-ace-oauth-
              authz-06
              authz-08 (work in progress), March October 2017.

   [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-01 (work in
              progress), October 2017.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>. <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,
              <http://www.rfc-editor.org/info/rfc4279>.
              <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,
              <http://www.rfc-editor.org/info/rfc5746>.
              <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, <http://www.rfc-editor.org/info/rfc6347>. <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,
              <http://www.rfc-editor.org/info/rfc7252>.

7.2. <https://www.rfc-
              editor.org/info/rfc7252>.

   [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.  Informative References

   [I-D.ietf-ace-cbor-web-token]
              Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-07
              (work in progress), July 2017.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-ietf-core-
              object-security-04 (work in progress), July 2017.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-10 draft-ietf-ace-cbor-web-token-09
              (work in progress), March October 2017.

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

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

   [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>. <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, <http://www.rfc-editor.org/info/rfc7250>. <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,
              <http://www.rfc-editor.org/info/rfc7251>.

7.3.
              <https://www.rfc-editor.org/info/rfc7251>.

8.3.  URIs

   [1] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       06#section-5.7.1
       08#section-5.8.1

   [2] https://tools.ietf.org/html/rfc7252#section-9

   [3] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       03#section-8.1
       08#section-5.8.1

   [4] https://tools.ietf.org/html/rfc6749#section-5.2 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       08#section-5.5.1

   [5] https://tools.ietf.org/html/draft-ietf-cose-msg-23#section-12.1.2 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
       08#section-5.1.1

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

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

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

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

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

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

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

   Email: goran.selander@ericsson.com

   Ludwig Seitz
   RISE SICS
   Scheelevaegen 17
   Lund  223 70
   Sweden

   Email: ludwig.seitz@ri.se