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Versions: (draft-gerdes-ace-dtls-authorize) 00 01

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


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

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 January 4, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   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
     2.1.  Unauthorized Resource Request Message . . . . . . . . . .   5
     2.2.  AS Information  . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Resource Access . . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Dynamic Update of Authorization Information . . . . . . .   8
   3.  RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . . .   9
   4.  PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  DTLS Channel Setup Between C and RS . . . . . . . . . . .  11
     4.2.  Updating Authorization Information  . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     5.1.  Unprotected AS Information  . . . . . . . . . . . . . . .  14
     5.2.  Use of Nonces for Replay Protection . . . . . . . . . . .  14
     5.3.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This specification defines a profile of the ACE framework
   [I-D.ietf-ace-oauth-authz].  In this profile, a client and a resource
   server use CoAP [RFC7252] over DTLS [RFC6347] to communicate.  The
   client uses an access token, bound to a key (the proof-of-possession
   key) to authorize its access to the resource server.  DTLS provides
   communication security, proof of possession, and server
   authentication.  Optionally the client and the resource server may
   also use CoAP over DTLS to communicate with the authorization server.
   This specification supports the DTLS handshake with Raw Public Keys
   (RPK) [RFC7250] and the DTLS PSK handshake [RFC4279].





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

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

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

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

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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

2.  Protocol Overview

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

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












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


                   Figure 1: Retrieving an Access Token

   To determine the AS in charge of a resource hosted at the RS, C MAY
   send an initial Unauthorized Resource Request message to RS.  RS then
   denies the request and sends the address of its AS back to C.

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

   Once C knows AS'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].
   If C wants to use the CoAP RawPublicKey mode as described in
   Section 9 of RFC 7252 [2] it MUST provide a key or key identifier
   within a "cnf" object in the token request.  If AS decides that the
   request is to be authorized it generates an access token response for
   C containing a "profile" parameter with the value "coap_dtls" to
   indicate that this profile MUST be used for communication between C
   and RS.  Is also adds a "cnf" parameter with additional data for the
   establishment of a secure DTLS channel between C and RS.  The
   semantics of the 'cnf' parameter depend on the type of key used
   between C and RS, see Section 3 and Section 4.

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

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






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


                        Figure 2: Protocol overview

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

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

2.1.  Unauthorized Resource Request Message

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

   o  The request has been received on an unprotected channel.

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

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

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





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

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

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

2.2.  AS Information

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

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

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

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

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

                 Figure 3: AS Information payload example




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

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

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

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

             Figure 4: AS Information example encoded in CBOR

2.3.  Resource Access

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

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

   Note: The identity of C is determined by the authentication process





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      during the DTLS handshake.  In the asymmetric case, the public key
      will define C's identity, while in the PSK case, C's identity is
      defined by the session key generated by AS for this communication.

   RS SHOULD treat an incoming CoAP request as authorized if the
   following holds:

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

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

   3.  The request is destined for RS.

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

   5.  The request method is an authorized action on the resource with
       respect to the authorization information.

   Incoming CoAP requests received on a secure DTLS channel MUST be
   rejected

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

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

   C cannot always know a priori if a Authorized Resource Request will
   succeed.  If C repeatedly gets AS Information messages (cf.
   Section 2.2) as response to its requests, it SHOULD request a new
   Access Token from AS in order to continue communication with RS.

2.4.  Dynamic Update of Authorization Information

   The Client can update the authorization information stored at RS at
   any time.  To do so, the Client requests from AS a new Access Token
   for the intended action on the respective resource and uploads this
   Access Token to the "/authz-info" resource on RS.

   Figure 5 depicts the message flow where C requests a new Access Token
   after a security association between C and RS has been established
   using this protocol.





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


              Figure 5: Overview of Dynamic Update Operation

3.  RawPublicKey Mode

   To retrieve an access token for the resource that C wants to access,
   C requests an Access Token from AS.  C MUST add a "cnf" object
   carrying either its raw public key or a unique identifier for a
   public key that it has previously made known to AS.

   An example Access Token request from C to RS is depicted in Figure 6.

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

            Figure 6: Access Token Request Example for RPK Mode

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



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

   Note:  According to [RFC7252], CoAP implementations MUST support the
      ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and the
      NIST P-256 curve.  C is therefore expected to offer at least this
      ciphersuite to RS.

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

4.  PreSharedKey Mode

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

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

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




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      2.01 Created
      Content-Format: application/cbor
      Location-Path: /token/asdjbaskd
      Max-Age: 86400
      {
         access_token: b64'SlAV32hkKG ...
         (remainder of CWT omitted for brevity;
         token_type:   pop,
         alg:          HS256,
         expires_in:   86400,
         profile:      coap_dtls,
         cnf: {
           COSE_Key: {
             kty: symmetric,
             k: h'73657373696f6e6b6579'
           }
         }
      }

                  Figure 7: Example Access Token response

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

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

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

            Figure 8: Example Access Token response with reject

4.1.  DTLS Channel Setup Between C and RS

   When C receives an Access Token from AS, it checks if the payload
   contains an "access_token" parameter and a "cnf" parameter.  With
   this information C can initiate establishment of a new DTLS channel
   with RS.  To use DTLS with pre-shared keys, C 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.




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

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

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

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

   If the "psk_identity" field contains a key identifier, the receiver
   MUST check if it has one or more Access Tokens that are associated
   with the specified key.  If no valid Access Token is available for
   this key, the DTLS session setup is terminated with an
   "illegal_parameter" DTLS alert message.

   If instead the "psk_identity" field contains an Access Token, it must
   processed in the same way as an Access Token that has been uploaded
   to its "/authz-info" resource.  In this case, RS continues processing
   the ClientKeyExchange message if the contents of the "psk_identity"
   contained a valid Access Token.  Otherwise, the DTLS session setup is
   terminated with an "illegal_parameter" DTLS alert message.

   Note1: As RS cannot provide C with a meaningful PSK identity hint in

      response to C's ClientHello message, RS SHOULD NOT send a
      ServerKeyExchange message.

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




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

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

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

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

4.2.  Updating Authorization Information

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

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

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

5.  Security Considerations

   TODO





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5.1.  Unprotected AS Information

   Initially, no secure channel exists to protect the communication
   between C and RS.  Thus, C cannot determine if the AS information
   contained in an unprotected response from RS to an unauthorized
   request (c.f.  Section 2.2) is authentic.  It is therefore advisable
   to provide C with a (possibly hard-coded) list of trustworthy
   authorization servers.  AS information responses referring to a URI
   not listed there would be ignored.

5.2.  Use of Nonces for Replay Protection

   RS may add a nonce to the AS Information message sent as a response
   to an unauthorized request to ensure freshness of an Access Token
   subsequently presented to RS.  While a timestamp of some granularity
   would be sufficient to protect against replay attacks, using
   randomized nonce is preferred to prevent disclosure of information
   about RS's internal clock characteristics.

5.3.  Privacy

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

6.  IANA Considerations

   This document has no actions for IANA.

7.  References

7.1.  Normative References

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-06 (work in progress), March 2017.

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




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

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

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

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

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

7.2.  Informative References

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

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

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

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







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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://www.rfc-editor.org/info/rfc7049>.

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

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

7.3.  URIs

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

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

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

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

   [5] https://tools.ietf.org/html/draft-ietf-cose-msg-23#section-12.1.2

Authors' Addresses

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

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







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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
   Faroegatan 6
   Kista  164 80
   Sweden

   Email: goran.selander@ericsson.com


   Ludwig Seitz
   RISE SICS
   Scheelevaegen 17
   Lund  223 70
   Sweden

   Email: ludwig.seitz@ri.se















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