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Versions: (draft-gerdes-ace-dtls-authorize) 00 01 02 03 04 05 06 07 08 09 11

ACE Working Group                                              S. Gerdes
Internet-Draft                                               O. Bergmann
Intended status: Standards Track                              C. Bormann
Expires: December 20, 2020                       Universitaet Bremen TZI
                                                             G. Selander
                                                             Ericsson AB
                                                                L. Seitz
                                                               Combitech
                                                           June 18, 2020


Datagram Transport Layer Security (DTLS) Profile for Authentication and
            Authorization for Constrained Environments (ACE)
                    draft-ietf-ace-dtls-authorize-11

Abstract

   This specification defines a profile of the ACE framework that allows
   constrained servers to delegate client authentication and
   authorization.  The protocol relies on DTLS version 1.2 for
   communication security between entities in a constrained network
   using either raw public keys or pre-shared keys.  A 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 20, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Communication Between the Client and the Authorization
           Server  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  RawPublicKey Mode . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  DTLS Channel Setup Between Client and Resource Server   9
     3.3.  PreSharedKey Mode . . . . . . . . . . . . . . . . . . . .  10
       3.3.1.  DTLS Channel Setup Between Client and Resource Server  14
     3.4.  Resource Access . . . . . . . . . . . . . . . . . . . . .  16
   4.  Dynamic Update of Authorization Information . . . . . . . . .  17
   5.  Token Expiration  . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Secure Communication with an Authorization Server . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     7.1.  Reuse of Existing Sessions  . . . . . . . . . . . . . . .  20
     7.2.  Multiple Access Tokens  . . . . . . . . . . . . . . . . .  21
     7.3.  Out-of-Band Configuration . . . . . . . . . . . . . . . .  21
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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 version 1.2 [RFC6347] to
   communicate.  The client obtains an access token, bound to a key (the
   proof-of-possession key), from an authorization server to prove its
   authorization to access protected resources hosted by the resource
   server.  Also, the client and the resource server are provided by the
   authorization server with the necessary keying material to establish



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   a DTLS session.  The communication between client and authorization
   server may also be secured with DTLS.  This specification supports
   DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys
   (PSK) [RFC4279].

   The ACE framework requires that client and server mutually
   authenticate each other before any application data is exchanged.
   DTLS enables mutual authentication if both client and server prove
   their ability to use certain keying material in the DTLS handshake.
   The authorization server assists in this process on the server side
   by incorporating keying material (or information about keying
   material) into the access token, which is considered a "proof of
   possession" token.

   In the RPK mode, the client proves that it can use the RPK bound to
   the token and the server shows that it can use a certain RPK.

   The resource server needs access to the token in order to complete
   this exchange.  For the RPK mode, the client must upload the access
   token to the resource server before initiating the handshake, as
   described in Section 5.8.1 of the ACE framework
   [I-D.ietf-ace-oauth-authz].

   In the PSK mode, client and server show with the DTLS handshake that
   they can use the keying material that is bound to the access token.
   To transfer the access token from the client to the resource server,
   the "psk_identity" parameter in the DTLS PSK handshake may be used
   instead of uploading the token prior to the handshake.

   As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this
   specification uses CBOR web tokens to convey claims within an access
   token issued by the server.  While other formats could be used as
   well, those are out of scope for this document.

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] and in
   [I-D.ietf-ace-oauth-params].

   The authorization information (authz-info) resource refers to the
   authorization information endpoint as specified in



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   [I-D.ietf-ace-oauth-authz].  The term "claim" is used in this
   document with the same semantics as in [I-D.ietf-ace-oauth-authz],
   i.e., it denotes information carried in the access token or returned
   from introspection.

2.  Protocol Overview

   The CoAP-DTLS profile for ACE specifies the transfer of
   authentication information 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 the 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.  As specified in Section 6.7 of
   [I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these
   interactions is completely independent

   This profile requires the client to retrieve an access token for
   protected resource(s) it wants to access on the resource server 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 Request Creation Hints-] |                     |
      |                                |                     |
      | ------- Token Request  ----------------------------> |
      |                                |                     |
      | <---------------------------- Access Token --------- |
      |                               + Access Information   |


                   Figure 1: Retrieving an Access Token

   To determine the authorization server in charge of a resource hosted
   at the resource server, the client can send an initial Unauthorized
   Resource Request message to the resource server.  The resource server
   then denies the request and sends an AS Request Creation Hints
   message containing the address of its authorization server back to
   the client as specified in Section 5.1.2 of
   [I-D.ietf-ace-oauth-authz].

   Once the client knows the authorization server's address, it can send
   an access token request to the token endpoint at the authorization
   server as specified in [I-D.ietf-ace-oauth-authz].  As the access
   token request as well as the response may contain confidential data,



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   the communication between the client and the authorization server
   must be confidentiality-protected and ensure authenticity.  The
   client may have been registered at the authorization server via the
   OAuth 2.0 client registration mechanism as outlined in Section 5.3 of
   [I-D.ietf-ace-oauth-authz].

   The access token returned by the authorization server can then be
   used by the client to establish a new DTLS session with the resource
   server.  When the client intends to use an asymmetric proof-of-
   possession key in the DTLS handshake with the resource server, the
   client MUST upload the access token to the authz-info resource, i.e.
   the authz-info endpoint, on the resource server before starting the
   DTLS handshake, as described in Section 5.8.1 of
   [I-D.ietf-ace-oauth-authz].  In case the client uses a symmetric
   proof-of-possession key in the DTLS handshake, the procedure as above
   MAY be used, or alternatively, the access token MAY instead be
   transferred in the DTLS ClientKeyExchange message (see
   Section 3.3.1).  In any case, DTLS MUST be used in a mode that
   provides replay protection.

   Figure 2 depicts the common protocol flow for the DTLS profile after
   the client 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

3.  Protocol Flow

   The following sections specify how CoAP is used to interchange
   access-related data between the resource 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 the client (C)
   and the authorization server (AS) must be secured.  Depending on the
   used CoAP security mode (see also Section 9 of [RFC7252], the Client-



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   to-AS request, AS-to-Client response (see Section 5.6 of
   [I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry
   slightly different information.  Section 3.2 addresses the use of raw
   public keys while Section 3.3 defines how pre-shared keys are used in
   this profile.

3.1.  Communication Between the Client and the Authorization Server

   To retrieve an access token for the resource that the client wants to
   access, the client requests an access token from the authorization
   server.  Before the client can request the access token, the client
   and the authorization server MUST establish a secure communication
   channel.  This profile assumes that the keying material to secure
   this communication channel has securely been obtained either by
   manual configuration or in an automated provisioning process.  The
   following requirements in alignment with Section 6.5 of
   [I-D.ietf-ace-oauth-authz] therefore must be met:

   o  The client MUST securely have obtained keying material to
      communicate with the authorization server.

   o  Furthermore, the client MUST verify that the authorization server
      is authorized to provide access tokens (including authorization
      information) about the resource server to the client, and that
      this authorization information about the authorization server is
      still valid.

   o  Also, the authorization server MUST securely have obtained keying
      material for the client, and obtained authorization rules approved
      by the resource owner (RO) concerning the client and the resource
      server that relate to this keying material.

   The client and the authorization server MUST use their respective
   keying material for all exchanged messages.  How the security
   association between the client and the authorization server is
   bootstrapped is not part of this document.  The client and the
   authorization server must ensure the confidentiality, integrity and
   authenticity of all exchanged messages within the ACE protocol.

   Section 6 specifies how communication with the authorization server
   is secured.

3.2.  RawPublicKey Mode

   When the client and the resource server use RawPublicKey
   authentication, the procedure is as follows: After the client and the
   authorization server mutually authenticated each other and validated
   each other's authorization, the client sends a token request to the



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   authorization server's token endpoint.  The client MUST add a
   "req_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.  It is RECOMMENDED that the client uses DTLS
   with the same keying material to secure the communication with the
   authorization server, proving possession of the key as part of the
   token request.  Other mechanisms for proving possession of the key
   may be defined in the future.

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

      POST coaps://as.example.com/token
      Content-Format: application/ace+cbor
      Payload:
      {
        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.

   The authorization server MUST check if the client that it
   communicates with is associated with the RPK in the "req_cnf"
   parameter before issuing an access token to it.  If the authorization
   server determines that the request is to be authorized according to
   the respective authorization rules, it generates an access token
   response for the client.  The access token MUST be bound to the RPK
   of the client by means of the "cnf" claim.

   The response MAY contain a "profile" parameter with the value
   "coap_dtls" to indicate that this profile MUST be used for
   communication between the client and the resource server.  The
   "profile" may be specified out-of-band, in which case it does not
   have to be sent.  The response also contains an access token with
   information for the resource server about the client's public key.



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   The authorization server MUST return in its response the parameter
   "rs_cnf" unless it is certain that the client already knows the
   public key of the resource server.  The authorization server MUST
   ascertain that the RPK specified in "rs_cnf" belongs to the resource
   server that the client wants to communicate with.  The authorization
   server MUST protect the integrity of the access token such that the
   resource server can detect unauthorized changes.  If the access token
   contains confidential data, the authorization server MUST also
   protect the confidentiality of the access token.

   The client MUST ascertain that the access token response belongs to a
   certain previously sent access token request, as the request may
   specify the resource server with which the client wants to
   communicate.

   An example access token response from the authorization to the client
   is depicted in Figure 4.  Here, the contents of the "access_token"
   claim have been truncated to improve readability.  Caching proxies
   process the Max-Age option in the CoAP response which has a default
   value of 60 seconds (Section 5.6.1 of [RFC7252]).  The authorization
   server SHOULD adjust the Max-Age option such that it does not exceed
   the "expires_in" parameter to avoid stale responses.

      2.01 Created
      Content-Format: application/ace+cbor
      Max-Age: 3560
      Payload:
      {
        access_token : b64'SlAV32hkKG...
         (remainder of CWT omitted for brevity;
         CWT contains the client's RPK in the cnf claim)',
        expires_in : 3600,
        rs_cnf     : {
          COSE_Key : {
            kty : EC2,
            crv : P-256,
            x   : h'd7cc072de2205bdc1537...',
            y   : h'f95e1d4b851a2cc80fff...'
          }
        }
      }

           Figure 4: Access Token Response Example for RPK Mode








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3.2.1.  DTLS Channel Setup Between Client and Resource Server

   Before the client initiates the DTLS handshake with the resource
   server, the client MUST send a "POST" request containing the obtained
   access token to the authz-info resource hosted by the resource
   server.  After the client receives a confirmation that the resource
   server has accepted the access token, it SHOULD proceed to establish
   a new DTLS channel with the resource server.  The client MUST use its
   correct public key in the DTLS handshake.  If the authorization
   server has specified a "cnf" field in the access token response, the
   client MUST use this key.  Otherwise, the client MUST use the public
   key that it specified in the "req_cnf" of the access token request.
   The client MUST specify this public key in the SubjectPublicKeyInfo
   structure of the DTLS handshake as described in [RFC7250].

   To be consistent with [RFC7252] which allows for shortened MAC tags
   in constrained environments, an implementation that supports the RPK
   mode of this profile MUST at least support the ciphersuite
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251].  As discussed in
   [RFC7748], new ECC curves have been defined recently that are
   considered superior to the so-called NIST curves.  This specification
   therefore mandates implementation support for curve25519 (cf.
   [RFC8032], [RFC8422]) as this curve said to be efficient and less
   dangerous regarding implementation errors than the secp256r1 curve
   mandated in [RFC7252].

   The resource server MUST check if the access token is still valid, if
   the resource server is the intended destination (i.e., the audience)
   of the token, and if the token was issued by an authorized
   authorization server.  The access token is constructed by the
   authorization server such that the resource server can associate the
   access token with the Client's public key.  The "cnf" claim MUST
   contain either the client's RPK or, if the key is already known by
   the resource server (e.g., from previous communication), a reference
   to this key.  If the authorization server has no certain knowledge
   that the Client's key is already known to the resource server, the
   Client's public key MUST be included in the access token's "cnf"
   parameter.  If CBOR web tokens [RFC8392] are used (as recommended in
   [I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in
   [RFC8747].

   The raw public key used in the DTLS handshake with the client MUST
   belong to the resource server.  If the resource server has several
   raw public keys, it needs to determine which key to use.  The
   authorization server can help with this decision by including a "cnf"
   parameter in the access token that is associated with this
   communication.  In this case, the resource server MUST use the




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   information from the "cnf" field to select the proper keying
   material.

   Thus, the handshake only finishes if the client and the resource
   server are able to use their respective keying material.

3.3.  PreSharedKey Mode

   To retrieve an access token for the resource that the client wants to
   access, the client MAY include a "cnf" object carrying an identifier
   for a symmetric key in its access token request to the authorization
   server.  This identifier can be used by the authorization server to
   determine the shared secret to construct the proof-of-possession
   token.  The authorization server MUST check if the identifier refers
   to a symmetric key that was previously generated by the authorization
   server as a shared secret for the communication between this client
   and the resource server.  If no such symmetric key was found, the
   authorization server MUST generate a new symmetric key that is
   returned in its response to the client.

   The authorization server MUST determine the authorization rules for
   the client it communicates with as defined by the resource owner and
   generate the access token accordingly.  If the authorization server
   authorizes the client, it returns an AS-to-Client response.  If the
   profile parameter is present, it is set to "coap_dtls".  The
   authorization server MUST ascertain that the access token is
   generated for the resource server that the client wants to
   communicate with.  Also, the authorization server MUST protect the
   integrity of the access token to ensure that the resource server can
   detect unauthorized changes.  If the token contains confidential data
   such as the symmetric key, the confidentiality of the token MUST also
   be protected.  Depending on the requested token type and algorithm in
   the access token request, the authorization server adds access
   Information to the response that provides the client with sufficient
   information to setup a DTLS channel with the resource server.  The
   authorization server adds a "cnf" parameter to the access information
   carrying a "COSE_Key" object that informs the client about the shared
   secret that is to be used between the client and the resource server.
   To convey the same secret to the resource server, the authorization
   server can include it directly in the access token by means of the
   "cnf" claim or provide sufficient information to enable the resource
   server to derive the shared secret from the access token.  As an
   alternative, the resource server MAY use token introspection to
   retrieve the keying material for this access token directly from the
   authorization server.

   An example access token request for an access token with a symmetric
   proof-of-possession key is illustrated in Figure 5.



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      POST coaps://as.example.com/token
      Content-Format: application/ace+cbor
      Payload:
      {
        audience    : "smokeSensor1807",
      }

   Figure 5: Example Access Token Request, (implicit) symmetric PoP-key

   A corresponding example access token response is illustrated in
   Figure 6.  In this example, the authorization server returns a 2.01
   response containing a new access token (truncated to improve
   readability) and information for the client, including the symmetric
   key in the cnf claim.  The information is transferred as a CBOR data
   structure as specified in [I-D.ietf-ace-oauth-authz].

      2.01 Created
      Content-Format: application/ace+cbor
      Max-Age: 85800
      Payload:
      {
         access_token : h'd08343a10...
         (remainder of CWT omitted for brevity)
         token_type : PoP,
         expires_in : 86400,
         profile    : coap_dtls,
         cnf        : {
           COSE_Key : {
             kty : symmetric,
             kid : h'3d027833fc6267ce',
             k   : h'73657373696f6e6b6579'
           }
         }
      }

        Figure 6: Example Access Token Response, symmetric PoP-key

   The access token also comprises a "cnf" claim.  This claim usually
   contains a "COSE_Key" object that carries either the symmetric key
   itself or a key identifier that can be used by the resource server to
   determine the secret key it shares with the client.  If the access
   token carries a symmetric key, the access token MUST be encrypted
   using a "COSE_Encrypt0" structure.  The authorization server MUST use
   the keying material shared with the resource server to encrypt the
   token.

   The "cnf" structure in the access token is provided in Figure 7.




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   cnf : {
     COSE_Key : {
       kty : symmetric,
       kid : h'3d027833fc6267ce'
     }
   }

              Figure 7: Access Token without Keying Material

   A response that declines any operation on the requested resource is
   constructed according to Section 5.2 of [RFC6749], (cf.
   Section 5.6.3. of [I-D.ietf-ace-oauth-authz]).  Figure 8 shows an
   example for a request that has been rejected due to invalid request
   parameters.

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

            Figure 8: Example Access Token Response With Reject

   The method for how the resource server determines the symmetric key
   from an access token containing only a key identifier is application-
   specific; the remainder of this section provides one example.

   The authorization server and the resource server are assumed to share
   a key derivation key used to derive the symmetric key shared with the
   client from the key identifier in the access token.  The key
   derivation key may be derived from some other secret key shared
   between the authorization server and the resource server.  This key
   needs to be securely stored and processed in the same way as the key
   used to protect the communication between the authorization server
   and the resource server.

   Knowledge of the symmetric key shared with the client must not reveal
   any information about the key derivation key or other secret keys
   shared between the authorization server and resource server.

   In order to generate a new symmetric key to be used by client and
   resource server, the authorization server generates a new key
   identifier which MUST be unique among all key identifiers used by the
   authorization server for this resource server.  The authorization
   server then uses the key derivation key shared with the resource
   server to derive the symmetric key as specified below.  Instead of
   providing the keying material in the access token, the authorization



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   server includes the key identifier in the "kid" parameter, see
   Figure 7.  This key identifier enables the resource server to
   calculate the symmetric key used for the communication with the
   client using the key derivation key and a KDF to be defined by the
   application, for example HKDF-SHA-256.  The key identifier picked by
   the authorization server needs to be unique for each access token
   where a unique symmetric key is required.

   In this example, HKDF consists of the composition of the HKDF-Extract
   and HKDF-Expand steps [RFC5869].  The symmetric key is derived from
   the key identifier, the key derivation key and other data:

   OKM = HKDF(salt, IKM, info, L),

   where:

   o  OKM, the output keying material, is the derived symmetric key

   o  salt is the empty byte string

   o  IKM, the input keying material, is the key derivation key as
      defined above

   o  info is the serialization of a CBOR array consisting of
      ([RFC8610]):

         info = [
           type : tstr,
           L : uint,
           access_token: map
         ]

   where:

   o  type is set to the constant text string "ACE-CoAP-DTLS-key-
      derivation",

   o  L is the size of the symmetric key in bytes,

   o  access_token is the decrypted access_token as transferred from the
      authorization server to the resource server.  The decrypted access
      token usually denotes a CWT claim set represented as CBOR map.

   Use of a unique (per resource server) "kid" and the use of a key
   derivation IKM that is unique per authorization server/resource
   server pair as specified above will ensure that the derived key is
   not shared across multiple clients.  However, to additionally provide
   variation in the derived key across different tokens used by the same



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   client, it is additionally RECOMMENDED to include the "iat" claim and
   either the "exp" or "exi" claims in the access token.

3.3.1.  DTLS Channel Setup Between Client and Resource Server

   When a client receives an access token response from an authorization
   server, the client MUST check if the access token response is bound
   to a certain previously sent access token request, as the request may
   specify the resource server with which the client wants to
   communicate.

   The client checks if the payload of the access token response
   contains an "access_token" parameter and a "cnf" parameter.  With
   this information the client can initiate the establishment of a new
   DTLS channel with a resource server.  To use DTLS with pre-shared
   keys, 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.  To
   be consistent with the recommendations in [RFC7252] a client is
   expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8
   [RFC6655] to the resource server.

   In PreSharedKey mode, the knowledge of the shared secret by the
   client and the resource server is used for mutual authentication
   between both peers.  Therefore, the resource server must be able to
   determine the shared secret from the access token.  Following the
   general ACE authorization framework, the client can upload the access
   token to the resource server's authz-info resource before starting
   the DTLS handshake.  The client then needs to indicate during the
   DTLS handshake which previously uploaded access token it intends to
   use.  To do so, it MUST create a "COSE_Key" structure with the "kid"
   that was conveyed in the "rs_cnf" claim in the token response from
   the authorization server and the key type "symmetric".  This
   structure then is included as the only element in the "cnf" structure
   that is used as value for "psk_identity" as shown in Figure 9.

   { cnf : {
      COSE_Key : {
         kty: symmetric,
         kid : h'3d027833fc6267ce'
       }
     }
   }

         Figure 9: Access token containing a single kid parameter

   As an alternative to the access token upload, the client can provide
   the most recent access token in the "psk_identity" field of the



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   ClientKeyExchange message.  To do so, the client MUST treat the
   contents of the "access_token" field from the AS-to-Client response
   as opaque data as specified in Section 4.2 of [RFC7925] and not
   perform any re-coding.  This allows the resource server to retrieve
   the shared secret directly from the "cnf" claim of the access token.

   If a resource server receives a ClientKeyExchange message that
   contains a "psk_identity" with a length greater than zero, it MUST
   parse the contents of the "psk_identity" field as CBOR data structure
   and process the contents as following:

   o  If the data contains a "cnf" field with a "COSE_Key" structure
      with a "kid", the resource server continues the DTLS handshake
      with the stored key associated with this kid.

   o  If the data comprises additional CWT information, this information
      must be stored as access token for this DTLS association before
      continuing with the DTLS handshake.

   If the contents of the "psk_identity" do not yield sufficient
   information to select a valid access token for the requesting client,
   the resource server aborts the DTLS handshake with an
   "illegal_parameter" alert.

   When the resource server receives an access token, it MUST check if
   the access token is still valid, if the resource server is the
   intended destination (i.e., the audience of the token), and if the
   token was issued by an authorized authorization 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 shared secret between the client and the resource
   server.  The resource server may use token introspection [RFC7662] on
   the access token to retrieve more information about the specific
   token.  The use of introspection is out of scope for this
   specification.

   While the client can retrieve the shared secret from the contents of
   the "cnf" parameter in the AS-to-Client response, the resource server
   uses the information contained in the "cnf" claim of the access token
   to determine the actual secret when no explicit "kid" was provided in
   the "psk_identity" field.  If key derivation is used, the resource
   server uses the "COSE_KDF_Context" information as described above.








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3.4.  Resource Access

   Once a DTLS channel has been established as described in Section 3.2
   or Section 3.3, respectively, the client is authorized to access
   resources covered by the access token it has uploaded to the authz-
   info resource hosted by the resource server.

   With the successful establishment of the DTLS channel, the client and
   the resource server have proven that they can use their respective
   keying material.  An access token that is bound to the client's
   keying material is associated with the channel.  According to
   Section 5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one
   access token for each client.  New access tokens issued by the
   authorization server are supposed to replace previously issued access
   tokens for the respective client.  The resource server therefore must
   have a common understanding with the authorization server how access
   tokens are ordered.

   Any request that the resource server receives on a DTLS channel that
   is tied to an access token via its keying material MUST be checked
   against the authorization rules that can be determined with the
   access token.  The resource server MUST check for every request if
   the access token is still valid.  If the token has expired, the
   resource server MUST remove it.  Incoming CoAP requests that are not
   authorized with respect to any access token that is associated with
   the client MUST be rejected by the resource server with 4.01
   response.  The response SHOULD include AS Request Creation Hints as
   described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz].

   The resource server MUST only accept 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 client is
       valid.

   3.  The request is destined for 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.






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   Incoming CoAP requests received on a secure DTLS channel that are not
   thus authorized MUST be rejected according to Section 5.8.2 of
   [I-D.ietf-ace-oauth-authz]

   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.

   The client MUST ascertain that its keying material is still valid
   before sending a request or processing a response.  If the client
   recently has updated the access token (see Section 4), it must be
   prepared that its request is still handled according to the previous
   authorization rules as there is no strict ordering between access
   token uploads and resource access messages.  See also Section 7.2 for
   a discussion of access token processing.

   If the client gets an error response containing AS Request Creation
   Hints (cf.  Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response
   to its requests, it SHOULD request a new access token from the
   authorization server in order to continue communication with the
   resource server.

   Unauthorized requests that have been received over a DTLS session
   SHOULD be treated as non-fatal by the resource server, i.e., the DTLS
   session SHOULD be kept alive until the associated access token has
   expired.

4.  Dynamic Update of Authorization Information

   Resource servers must only use a new access token to update the
   authorization information for a DTLS session if the keying material
   that is bound to the token is the same that was used in the DTLS
   handshake.  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.

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



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   Figure 10 depicts the message flow where the client requests a new
   access token after a security association between the client and the
   resource server has been established using this protocol.  If the
   client wants to update the authorization information, the token
   request MUST specify the key identifier of the proof-of-possession
   key used for 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 token that has previously
   been 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 [I-D.ietf-ace-oauth-authz].

   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 replace
   the authorization information of any existing DTLS session that is
   identified by this key identifier with the updated authorization
   information.

      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 10: Overview of Dynamic Update Operation

5.  Token Expiration

   The resource server MUST delete access tokens that are no longer
   valid.  DTLS associations that have been setup in accordance with
   this profile are always tied to specific tokens (which may be
   exchanged with a dynamic update as described in Section 4).  As
   tokens may become invalid at any time (e.g., because they have
   expired), the association may become useless at some point.  A
   resource server therefore MUST terminate existing DTLS association



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   after the last access token associated with this association has
   expired.

   As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the
   resource server MUST notify the client with an error response with
   code 4.01 (Unauthorized) for any long running request before
   terminating the association.

6.  Secure Communication with an Authorization Server

   As specified in the ACE framework (Sections 5.6 and 5.7 of
   [I-D.ietf-ace-oauth-authz]), the requesting entity (the resource
   server and/or the client) and the authorization server communicate
   via the token endpoint or introspection endpoint.  The use of CoAP
   and DTLS for this communication is RECOMMENDED in this profile, other
   protocols (such as HTTP and TLS, or CoAP and OSCORE [RFC8613]) MAY be
   used instead.

   How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the
   authorization server are established is out of scope for this
   profile.

   If other means of securing the communication with the authorization
   server are used, the communication security requirements from
   Section 6.2 of [I-D.ietf-ace-oauth-authz] remain applicable.

7.  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 considerations from Section 6 of
   [I-D.ietf-ace-oauth-authz] also apply to this profile.

   The authorization server must ascertain that the keying material for
   the client that it provides to the resource server actually is
   associated with this client.  Malicious clients may hand over access
   tokens containing their own access permissions to other entities.
   This problem cannot be completely eliminated.  Nevertheless, in RPK
   mode it should not be possible for clients to request access tokens
   for arbitrary public keys: if the client can cause the authorization
   server to issue a token for a public key without proving possession
   of the corresponding private key, this allows for identity misbinding
   attacks where the issued token is usable by an entity other than the
   intended one.  The authorization server therefore at some point needs
   to validate that the client can actually use the private key
   corresponding to the client's public key.




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   When using pre-shared keys provisioned by the authorization server,
   the security level depends on the randomness of PSK, and the security
   of the TLS cipher suite and key exchange algorithm.  As this
   specification targets at constrained environments, message payloads
   exchanged between the client and the resource server are expected to
   be small and rare.  CoAP [RFC7252] mandates the implementation of
   cipher suites with abbreviated, 8-byte tags for message integrity
   protection.  For consistency, this profile requires implementation of
   the same cipher suites.  For application scenarios where the cost of
   full-width authentication tags is low compared to the overall amount
   of data being transmitted, the use of cipher suites with 16-byte
   integrity protection tags is preferred.

   The PSK mode of this profile offers a distribution mechanism to
   convey authorization tokens together with a shared secret to a client
   and a server.  As this specification aims at constrained devices and
   uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite
   TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported.  The access
   tokens and the corresponding shared secrets generated by the
   authorization server are expected to be sufficiently short-lived to
   provide similar forward-secrecy properties to using ephemeral Diffie-
   Hellman (DHE) key exchange mechanisms.  For longer-lived access
   tokens, DHE ciphersuites should be used.

   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.  A similar issue exists with the
   unprotected authorization information endpoint where the resource
   server needs to keep valid access tokens until their expiry.
   Adversaries can fill up the constrained resource server's internal
   storage for a very long time with interjected or otherwise retrieved
   valid access tokens.  The protection of access tokens that are stored
   in the authorization information endpoint depends on the keying
   material that is used between the authorization server and the
   resource server: The resource server must ensure that it processes
   only access tokens that are encrypted and integrity-protected by an
   authorization server that is authorized to provide access tokens for
   the resource server.

7.1.  Reuse of Existing Sessions

   To avoid the overhead of a repeated DTLS handshake, [RFC7925]
   recommends session resumption [RFC5077] to reuse session state from
   an earlier DTLS association and thus requires client side
   implementation.  In this specification, the DTLS session is subject



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   to the authorization rules denoted by the access token that was used
   for the initial setup of the DTLS association.  Enabling session
   resumption would require the server to transfer the authorization
   information with the session state in an encrypted SessionTicket to
   the client.  Assuming that the server uses long-lived keying
   material, this could open up attacks due to the lack of forward
   secrecy.  Moreover, using this mechanism, a client can resume a DTLS
   session without proving the possession of the PoP key again.
   Therefore, the use of session resumption is NOT RECOMMENDED for
   resource servers.

   Since renogiation of DTLS associations is prone to attacks as well,
   [RFC7925] requires clients to decline any renogiation attempt.  A
   server that wants to initiate re-keying therefore SHOULD periodically
   force a full handshake.

7.2.  Multiple Access Tokens

   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.

   Even when a single access token per client is used, an attacker could
   compromise the dynamic update mechanism for existing DTLS connections
   by delaying or reordering packets destined for the authz-info
   endpoint.  Thus, the order in which operations occur at the resource
   server (and thus which authorization info is used to process a given
   client request) cannot be guaranteed.  Especially in the presence of
   later-issued access tokens that reduce the client's permissions from
   the initial access token, it is impossible to guarantee that the
   reduction in authorization will take effect prior to the expiration
   of the original token.

7.3.  Out-of-Band Configuration

   To communicate securely, the authorization server, the client and the
   resource server require certain information that must be exchanged
   outside the protocol flow described in this document.  The
   authorization server must have obtained authorization information
   concerning the client and the resource server that is approved by the
   resource owner as well as corresponding keying material.  The
   resource server must have received authorization information approved
   by the resource owner concerning its authorization managers and the
   respective keying material.  The client must have obtained



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   authorization information concerning the authorization server
   approved by its owner as well as the corresponding keying material.
   Also, the client's owner must have approved of the client's
   communication with the resource server.  The client and the
   authorization server must have obtained a common understanding how
   this resource server is identified to ensure that the client obtains
   access token and keying material for the correct resource server.  If
   the client is provided with a raw public key for the resource server,
   it must be ascertained to which resource server (which identifier and
   authorization information) the key is associated.  All authorization
   information and keying material must be kept up to date.

8.  Privacy Considerations

   This privacy considerations from Section 7 of the
   [I-D.ietf-ace-oauth-authz] apply also to this profile.

   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 an authenticated 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.

9.  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





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   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: TBD (suggested: 1)

   Change Controller: IESG

   Reference: [RFC-XXXX]

10.  Acknowledgments

   Thanks to Jim Schaad for his contributions and reviews of this
   document.  Special thanks to Ben Kaduk for his thorough review of
   this document.

   Ludwig Seitz worked on this document as part of the CelticNext
   projects CyberWI, and CRITISEC with funding from Vinnova.

11.  References

11.1.  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-33
              (work in progress), February 2020.

   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-13 (work in progress), April 2020.

   [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>.






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   [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>.

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

   [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>.

   [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>.

   [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>.

   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/info/rfc8747>.



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11.2.  Informative References

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <https://www.rfc-editor.org/info/rfc5077>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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>.

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

   [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>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.







Gerdes, et al.          Expires December 20, 2020              [Page 25]


Internet-Draft                  CoAP-DTLS                      June 2020


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 AB

   Email: goran.selander@ericsson.com


   Ludwig Seitz
   Combitech
   Djaeknegatan 31
   Malmoe  211 35
   Sweden

   Email: ludwig.seitz@combitech.se






Gerdes, et al.          Expires December 20, 2020              [Page 26]


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