Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-06

ACE Working Group                                              S. Gerdes
Internet-Draft                                               O. Bergmann
Intended status: Standards Track                              C. Bormann
Expires: September 1, 2019                       Universitaet Bremen TZI
                                                             G. Selander
                                                             Ericsson AB
                                                                L. Seitz
                                                               RISE SICS
                                                       February 28, 2019

Datagram Transport Layer Security (DTLS) Profile for Authentication and
            Authorization for Constrained Environments (ACE)
                    draft-ietf-ace-dtls-authorize-06

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 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
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 1, 2019.

Copyright Notice

   Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Communication between C and AS  . . . . . . . . . . . . .   5
     3.2.  RawPublicKey Mode . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  DTLS Channel Setup Between C and RS . . . . . . . . .   7
     3.3.  PreSharedKey Mode . . . . . . . . . . . . . . . . . . . .   8
       3.3.1.  DTLS Channel Setup Between C and RS . . . . . . . . .  11
     3.4.  Resource Access . . . . . . . . . . . . . . . . . . . . .  12
   4.  Dynamic Update of Authorization Information . . . . . . . . .  13
   5.  Token Expiration  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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 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
   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].

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   The DTLS handshake requires the client and server to prove that they
   can use certain keying material.  In the RPK mode, the client proves
   with the DTLS handshake that it can use the RPK bound to the token
   and the server shows that it can use a certain RPK.  The access token
   must be presented to the resource server.  For the RPK mode, the
   access token needs to be uploaded to the resource server before the
   handshake is initiated, 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.

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

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 C can use CoAP over DTLS to retrieve an access token from the
   authorization server (AS) for a protected resource hosted on the
   resource server.

   This profile requires the client to retrieve an access token for
   protected resource(s) it wants to access on 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):

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

                   Figure 1: Retrieving an Access Token

   To determine the AS in charge of a resource hosted at the RS, C MAY
   send an initial Unauthorized Resource Request message to the RS.  The
   RS then denies the request and sends an AS information message
   containing the address of its AS back to the client 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 AS as specified
   in [I-D.ietf-ace-oauth-authz].  As the access token request as well
   as the response may contain confidential data, the communication
   between the client and the authorization server MUST be
   confidentiality-protected and ensure authenticity.  C may have been
   registered at the AS 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 asymmetric cryptography 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].  If only
   symmetric cryptography is used between the client and the resource
   server, the access token MAY instead be transferred in the DTLS
   ClientKeyExchange message (see Section 3.3.1).

   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.

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      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 C and AS must be
   secured.  Depending on the used CoAP security mode (see also
   Section 9 of [RFC7252], the Client-to-AS request, AS-to-Client
   response 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 C and AS

   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 C can request the access token, C and AS must
   establish a secure communication channel.  C must securely have
   obtained keying material to communicate with AS, and C must securely
   have received authorization information intended for C that states
   that AS is authorized to provide keying material concerning RS to C.
   Also, AS must securely have obtained keying material for C, and
   obtained authorization rules approved by the resource owner (RO)
   concerning C and RS that relate to this keying material.  C and AS
   must use their respective keying material for all exchanged messages.
   How the security association between C and AS is established is not
   part of this document.  C and AS MUST ensure the confidentiality,
   integrity and authenticity of all exchanged messages.

   If C is constrained, C and AS should use DTLS to communicate with
   each other.  But C and AS may also use other means to secure their
   communication, e.g., TLS.  The used security protocol must provide

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   confidentiality, integrity and authenticity, and enable the client to
   determine if it is the intended recipient of a message, e.g., by
   using an AEAD mechanism.  C must also be able to determine if a
   response from AS belongs to a certain request.  Additionally, the
   protocol must offer replay protection.

3.2.  RawPublicKey Mode

   After C and AS mutually authenticated each other and validated each
   other's authorization, C sends a token request to AS's token
   endpoint.  The client MUST add a "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.  To prove that the
   client is in possession of this key, C MUST use the same keying
   material that it uses to secure the communication with AS, e.g., the
   DTLS session.

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

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

            Figure 3: Access Token Request Example for RPK Mode

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

   AS MUST check if the client that it communicates with is associated
   with the RPK in the cnf object before issuing an access token to it.
   If AS determines that the request is to be authorized according to
   the respective authorization rules, it generates an access token
   response for C.  The response SHOULD contain a "profile" parameter
   with the value "coap_dtls" to indicate that this profile must be used
   for communication between the client C and the resource server.  The

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   response also contains an access token and an "rs_cnf" parameter
   containing information about the public key that is used by the
   resource server.  AS MUST ascertain that the RPK specified in
   "rs_cnf" belongs to the resource server that C wants to communicate
   with.  AS MUST protect the integrity of the token.  If the access
   token contains confidential data, AS MUST also protect the
   confidentiality of the access token.

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

3.2.1.  DTLS Channel Setup Between C and RS

   Before the client initiates the DTLS handshake with the resource
   server, C MUST send a "POST" request containing the new access token
   to the authz-info resource hosted by the resource server.  If this
   operation yields a positive response, the client SHOULD proceed to
   establish a new DTLS channel with the resource server.  To use the
   RawPublicKey mode, the client MUST specify the public key that AS
   defined in the "cnf" field of the access token response in the
   SubjectPublicKeyInfo structure in the DTLS handshake as specified in
   [RFC7250].

   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] with the ed25519 curve (cf.  [RFC8032], [RFC8422]).

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

   RS MUST check if the access token is still valid, if RS is the
   intended destination, i.e., the audience, of the token, and if the
   token was issued by an authorized AS.  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 C'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

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   recommended in [I-D.ietf-ace-oauth-authz], keys MUST be encoded as
   specified in [I-D.ietf-ace-cwt-proof-of-possession].  RS MUST use the
   keying material in the handshake that AS specified in the rs_cnf
   parameter in the access token.  Thus, the handshake only finishes if
   C and RS are able to use their respective keying material.

3.3.  PreSharedKey Mode

   To retrieve an access token for the 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.  AS MUST check if the identifier refers to a symmetric key
   that was previously generated by AS as a shared secret for the
   communication between this client and the resource server.

   The authorization server MUST determine the authorization rules for
   the C it communicates with as defined by RO 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".  AS MUST ascertain
   that the access token is generated for the resource server that C
   wants to communicate with.  Also, AS MUST protect the integrity of
   the access token.  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.  AS
   adds a "cnf" parameter to the access information carrying a
   "COSE_Key" object that informs the client about the symmetric key
   that is to be used between C and the resource server.

   An example access token response is illustrated in Figure 4.  In this
   example, the authorization server returns a 2.01 response containing
   a new access token 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].

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      2.01 Created
      Content-Format: application/ace+cbor
      Max-Age: 86400
      {
         access_token: h'd08343a10...
         (remainder of CWT omitted for brevity)
         token_type:   pop,
         expires_in:   86400,
         profile:      coap_dtls,
         cnf: {
           COSE_Key: {
             kty: symmetric,
             alg: TLS_PSK_WITH_AES_128_CCM_8
             kid: h'3d027833fc6267ce',
             k: h'73657373696f6e6b6579'
           }
         }
      }

                  Figure 4: Example Access Token Response

   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 shared with the client.  If the access token
   carries a symmetric key, the access token MUST be encrypted using a
   "COSE_Encrypt0" structure.  The AS MUST use the keying material
   shared with the RS to encrypt the token.

   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]).

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

            Figure 5: 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 AS 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

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   derived from some other secret key shared between the AS 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 AS and resource server.

   In order to generate a new symmetric key to be used by client and
   resource server, the AS generates a key identifier and 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 AS includes the key identifier in
   the "kid" parameter, see Figure 6.  This key identifier enables the
   resource server to calculate the keying material for the
   communication with the client from the access token using the key
   derivation key and following Section 11 of [RFC8152] with parameters
   as specified here.  The KDF to be used needs to be defined by the
   application, for example HKDF-SHA-256.  The key identifier picked by
   the AS needs to be unique for each access token where a unique
   symmetric key is required.

   The fields in the context information "COSE_KDF_Context"
   (Section 11.2 of [RFC8152]) have the following values:

   o  AlgorithmID = "ACE-CoAP-DTLS-key-derivation"

   o  PartyUInfo = PartyVInfo = ( null, null, null )

   o  keyDataLength is a uint equal the length of the symmetric key
      shared between C and RS in bits

   o  protected MUST be a zero length bstr

   o  other is a zero length bstr

   o  SuppPrivInfo is omitted

   The "cnf" structure in the access token is provided in Figure 6.

   cnf : {
     COSE_Key : {
       kty  : symmetric,
       alg  : TLS_PSK_WITH_AES_128_CCM_8,
       kid : h'eIiOFCa9lObw'
     }
   }

              Figure 6: Access Token without Keying Material

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3.3.1.  DTLS Channel Setup Between C and RS

   When a client receives an access token response from an authorization
   server, C 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 C wants to communicate.

   C 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.

   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.  Alternatively, the client MAY provide the most
   recent access token in the "psk_identity" field of the
   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 and not perform any re-coding.

   Note:  As stated in Section 4.2 of [RFC7925], the PSK identity should
      be treated as binary data in the Internet of Things space and not
      assumed to have a human-readable form of any sort.

   If a resource server receives a ClientKeyExchange message that
   contains a "psk_identity" with a length greater zero, it uses the
   contents as index for its key store (i.e., treat the 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 key with a matching identifier is found, the resource server
   MAY process the contents of the "psk_identity" field as access token
   that is stored with the authorization information endpoint, before
   continuing the DTLS handshake.  If the contents of the "psk_identity"
   do not yield a valid access token for the requesting client, the DTLS
   session setup is terminated with an "illegal_parameter" DTLS alert
   message.

   Note1:  As a resource server cannot provide a client with a
      meaningful PSK identity hint in response to the client's

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      ClientHello message, 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].  A client is
      therefore expected to offer at least this ciphersuite to the
      resource server.

   When RS receives an access token, RS MUST check if the access token
   is still valid, if RS is the intended destination, i.e., the audience
   of the token, and if the token was issued by an authorized AS.  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.

   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 RS uses the
   "COSE_KDF_Context" information as described above.

3.4.  Resource Access

   Once a DTLS channel has been established as described in Section 3.2
   and Section 3.3, respectively, the client 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, C and RS 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.  Any request that the resource server receives on
   this channel MUST be checked against these authorization rules.  RS
   MUST check for every request if the access token is still valid.
   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 as described in Section 5.1.1
   of [I-D.ietf-ace-oauth-authz].

   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.

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   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.

   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 cannot always know a priori if an Authorized Resource
   Request will succeed.  It must check the validity of its keying
   material before sending a request or processing a response.  If the
   client repeatedly gets error responses containing AS 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 RS, i.e., the DTLS session
   SHOULD be kept alive until the associated access token has expired.

4.  Dynamic Update of Authorization Information

   The client can 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.

   Figure 7 depicts the message flow where the C 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

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   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 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.

   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 - |
      |                           + Access Information   |
      |                            |                     |
      | --- Update /authz-info --> |                     |
      |     New Access Token       |                     |
      |                            |                     |
      | == Authorized Request ===> |                     |
      |                            |                     |
      | <=== Protected Resource == |                     |

              Figure 7: Overview of Dynamic Update Operation

5.  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

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   server therefore MUST terminate existing DTLS sessions after the last
   valid access token for this session has been deleted.

   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 session.

6.  Security Considerations

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

   Constrained devices that use DTLS [RFC6347] are inherently vulnerable
   to Denial of Service (DoS) attacks as the handshake protocol requires
   creation of internal state within the device.  This is specifically
   of concern where an adversary is able to intercept the initial cookie
   exchange and interject forged messages with a valid cookie to
   continue with the handshake.  A similar issue exists with the
   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 use of multiple access tokens for a single client increases the
   strain on the resource server as it must consider every access token
   and calculate the actual permissions of the client.  Also, tokens may
   contradict each other which may lead the server to enforce wrong
   permissions.  If one of the access tokens expires earlier than
   others, the resulting permissions may offer insufficient protection.
   Developers should avoid using multiple access tokens for a client.

7.  Privacy Considerations

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

   Also, unprotected requests to the resource server may reveal
   information about the client, e.g., which resources the client

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   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.

8.  IANA Considerations

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

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

   Profile name: coap_dtls

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

   Profile ID: 1

   Change Controller: IESG

   Reference: [RFC-XXXX]

9.  References

9.1.  Normative References

   [I-D.ietf-ace-cwt-proof-of-possession]
              Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
              possession-06 (work in progress), February 2019.

   [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-21
              (work in progress), February 2019.

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   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-04 (work in progress), February 2019.

   [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>.

   [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>.

   [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>.

9.2.  Informative References

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

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

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

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

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

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

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

Authors' Addresses

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

   Phone: +49-421-218-63906
   Email: gerdes@tzi.org

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   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
   RISE SICS
   Scheelevaegen 17
   Lund  223 70
   Sweden

   Email: ludwig.seitz@ri.se

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