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ACE Working Group                                              M. Tiloca
Internet-Draft                                                 RISE SICS
Intended status: Standards Track                                 J. Park
Expires: December 31, 2018                   Universitaet Duisburg-Essen
                                                           June 29, 2018


                      Joining OSCORE groups in ACE
                   draft-tiloca-ace-oscoap-joining-04

Abstract

   This document describes a method to join a group where communications
   are based on CoAP and secured with Object Security for Constrained
   RESTful Environments (OSCORE).  The proposed method delegates the
   authentication and authorization of client nodes that join an OSCORE
   group through a Group Manager server.  This approach builds on the
   ACE framework for Authentication and Authorization, and leverages
   protocol-specific profiles of ACE to achieve communication security,
   proof-of-possession and server authentication.

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 December 31, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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



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   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.  Joining Node to Authorization Server  . . . . . . . . . . . .   6
     3.1.  Authorization Request . . . . . . . . . . . . . . . . . .   6
     3.2.  Authorization Response  . . . . . . . . . . . . . . . . .   7
   4.  Joining Node to Group Manager . . . . . . . . . . . . . . . .   8
     4.1.  Join Request  . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Join Response . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Public Keys of Joining Nodes  . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Object Security for Constrained RESTful Environments (OSCORE)
   [I-D.ietf-core-object-security] is a method for application-layer
   protection of the Constrained Application Protocol (CoAP) [RFC7252],
   using CBOR Object Signing and Encryption (COSE) [RFC8152] and
   enabling end-to-end security of CoAP payload and options.

   As described in [I-D.ietf-core-oscore-groupcomm], OSCORE may be used
   also to protect CoAP group communication over IP multicast [RFC7390].
   This relies on a Group Manager entity, which is responsible for
   managing an OSCORE group, where members exchange CoAP messages
   secured with OSCORE.  In particular, the Group Manager coordinates
   the join process of new group members and can be responsible for
   multiple groups.

   This specification builds on the ACE framework for Authentication and
   Authorization [I-D.ietf-ace-oauth-authz] and defines how a client
   joins an OSCORE group through a resource server acting as Group
   Manager.  The client acting as joining node relies on an Access
   Token, which is bound to a proof-of-possession key and authorizes the
   access to a specific join resource at the Group Manager.  Messages
   exchanged among the participants follow the formats defined in



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   [I-D.palombini-ace-key-groupcomm] for provisioning keying material in
   group communication scenarios.

   In order to achieve communication security, proof-of-possession and
   server authentication, the client and the Group Manager leverage
   protocol-specific profiles of ACE.  These include also possible
   forthcoming profiles that comply with the requirements in Appendix C
   of [I-D.ietf-ace-oauth-authz].

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 the ACE framework for authentication and authorization
   [I-D.ietf-ace-oauth-authz].  The terminology for entities in the
   considered architecture is defined in OAuth 2.0 [RFC6749].  In
   particular, this includes Client (C), Resource Server (RS), and
   Authorization Server (AS).

   Readers are expected to be familiar with the terms and concepts
   related to the CoAP protocol described in [RFC7252][RFC7390].  Note
   that, unless otherwise indicated, the term "endpoint" is used here
   following its OAuth definition, aimed at denoting resources such as
   /token and /introspect at the AS and /authz-info at the RS.  This
   document does not use the CoAP definition of "endpoint", which is "An
   entity participating in the CoAP protocol".

   Readers are expected to be familiar with the terms and concepts for
   protection and processing of CoAP messages through OSCORE
   [I-D.ietf-core-object-security] also in group communication scenarios
   [I-D.ietf-core-oscore-groupcomm].  These include the concept of Group
   Manager, as the entity responsible for a set of groups where
   communications are secured with OSCORE.  In this specification, the
   Group Manager acts as Resource Server.

   This document refers also to the following terminology.

   o  Joining node: a network node intending to join an OSCORE group,
      where communication is based on CoAP [RFC7390] and secured with
      OSCORE as described in [I-D.ietf-core-oscore-groupcomm].






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   o  Join process: the process through which a joining node becomes a
      member of an OSCORE group.  The join process is enforced and
      assisted by the Group Manager responsible for that group.

   o  Join resource: a resource hosted by the Group Manager, associated
      to an OSCORE group under that Group Manager.  A join resource is
      identifiable with the Group Identifier (Gid) of the respective
      group.  A joining node accesses a join resource to start the join
      process and become a member of that group.

   o  Join endpoint: an endpoint at the Group Manager associated to a
      join resource.

   o  Requester: member of an OSCORE group that sends request messages
      to other members of the group.

   o  Listener: member of an OSCORE group that receives request messages
      from other members of the group.  A listener may reply back, by
      sending a response message to the requester which has sent the
      request message.

   o  Pure listener: member of a group that is configured as listener
      and never replies back to requesters after receiving request
      messages.  This corresponds to the term "silent server" used in
      [I-D.ietf-core-oscore-groupcomm].

2.  Protocol Overview

   Group communication for CoAP over IP multicast has been enabled in
   [RFC7390] and can be secured with Object Security for Constrained
   RESTful Environments (OSCORE) [I-D.ietf-core-object-security] as
   described in [I-D.ietf-core-oscore-groupcomm].  A network node
   explicitly joins an OSCORE group, by interacting with the responsible
   Group Manager.  Once registered in the group, the new node can
   securely exchange messages with other group members.

   This specification describes how a network node joins an OSCORE group
   by using the ACE framework for authentication and authorization
   [I-D.ietf-ace-oauth-authz].  With reference to the ACE framework and
   the terminology defined in OAuth 2.0 [RFC6749]:

   o  The Group Manager acts as Resource Server (RS), and hosts one join
      resource for each OSCORE group it manages.  Each join resource is
      exported by a distinct join endpoint.  During the join process,
      the Group Manager provides joining nodes with the parameters and
      keying material for taking part to secure communications in the
      group.




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   o  The joining node acts as Client (C), and requests to join an
      OSCORE group by accessing the related join endpoint at the Group
      Manager.

   o  The Authorization Server (AS) authorizes joining nodes to join
      OSCORE groups under their respective Group Manager.  Multiple
      Group Managers can be associated to the same AS.  The AS MAY
      release Access Tokens for other purposes than joining OSCORE
      groups under registered Group Managers.  For example, the AS may
      also release Access Tokens for accessing resources hosted by
      members of OSCORE groups.

   All communications between the involved entities rely on the CoAP
   protocol and must be secured.

   In particular, communications between the joining node and the Group
   Manager leverage protocol-specific profiles of ACE to achieve
   communication security, proof-of-possession and server
   authentication.  To this end, the AS must signal the specific profile
   to use, consistently with requirements and assumptions defined in the
   ACE framework [I-D.ietf-ace-oauth-authz].

   With reference to the AS, communications between the joining node and
   the AS (/token endpoint) as well as between the Group Manager and the
   AS (/introspect endpoint) can be secured by different means, for
   instance by means of DTLS [RFC6347] or OSCORE
   [I-D.ietf-core-object-security].  Further details on how the AS
   secures communications (with the joining node and the Group Manager)
   depend on the specifically used profile of ACE, and are out of the
   scope of this specification.

   The following steps are performed for joining an OSCORE group.
   Messages exchanged among the participants follow the formats defined
   in [I-D.palombini-ace-key-groupcomm], and are further specified in
   Section 3 and Section 4 of this document.  The Group Manager acts as
   the Key Distribution Center (KDC) referred in
   [I-D.palombini-ace-key-groupcomm].

   1.  The joining node requests an Access Token from the AS, in order
       to access a join resource on the Group Manager and hence join the
       associated OSCORE group (see Section 3).  The joining node will
       start or continue using a secure communication channel with the
       Group Manager, according to the response from the AS.

   2.  The joining node transfers authentication and authorization
       information to the Group Manager by posting the obtained Access
       Token (see Section 4).  After that, a joining node must have a
       secure communication channel established with the Group Manager,



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       before starting to join an OSCORE group under that Group Manager
       (see Section 4).  Possible alternatives to provide a secure
       communication channel include DTLS [RFC6347] and OSCORE
       [I-D.ietf-core-object-security].

   3.  The joining node starts the join process to become a member of
       the OSCORE group, by accessing the related join resource hosted
       by the Group Manager (see Section 4).

   4.  At the end of the join process, the joining node has received
       from the Group Manager the parameters and keying material to
       securely communicate with the other members of the OSCORE group.

   5.  The joining node and the Group Manager maintain the secure
       channel, to support possible future communications.

3.  Joining Node to Authorization Server

   This section describes how the joining node interacts with the AS in
   order to be authorized to join an OSCORE group under a given Group
   Manager.  In particular, it considers a joining node that intends to
   contact that Group Manager for the first time.

   The message exchange between the joining node and the AS consists of
   the messages Authorization Request and Authorization Response defined
   in [I-D.palombini-ace-key-groupcomm].

   In case the specific AS associated to the Group Manager is unknown to
   the joining node, the latter can rely on mechanisms like the
   Unauthorized Resource Request message described in Section 2 of
   [I-D.ietf-ace-dtls-authorize] to discover the correct AS to contact.

3.1.  Authorization Request

   The joining node contacts the AS, in order to request an Access Token
   for accessing the join resource hosted by the Group Manager and
   associated to the OSCORE group.  The Access Token request sent to the
   /token endpoint follows the format of the Authorization Request
   message defined in Section 3.1 of [I-D.palombini-ace-key-groupcomm].
   In particular:

   o  The 'scope' parameter MUST be present and includes:

      *  in the first element, the Group Identifier (Gid) of the group
         to join under the Group Manager.  The value of this identifier
         may not fully coincide with the Gid value currently associated
         to the group, e.g. if the Gid is composed of a variable part




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         such as a Group Epoch (see Appendix C of
         [I-D.ietf-core-oscore-groupcomm]).

      *  in the second element, which MUST be present, the role(s) that
         the joining node intends to have in the group it intends to
         join.  Possible values are: "requester"; "listener"; and "pure
         listener".  Possible combinations are: "requester and
         listener"; and "requester and pure listener".  Multiple roles
         are specified in the form of a CBOR array.

   o  The 'aud' parameter MUST be present and is set to the identifier
      of the Group Manager.

3.2.  Authorization Response

   The AS is responsible for authorizing the joining node to join
   specific OSCORE groups, according to join policies enforced on behalf
   of the respective Group Manager.

   In case of successful authorization, the AS releases an Access Token
   bound to a proof-of-possession key associated to the joining node.

   Then, the AS provides the joining node with the Access Token as part
   of an Access Token response, which follows the format of the
   Authorization Response message defined in Section 3.2 of
   [I-D.palombini-ace-key-groupcomm].

   The 'exp' parameter MUST be present.  Other means for the AS to
   specify the lifetime of Access Tokens are out of the scope of this
   specification.

   The AS must include the 'scope' parameter in the response when the
   value included in the Access Token differs from the one specified by
   the joining node in the request.  In such a case, the second element
   of 'scope' MUST be present and includes the role(s) that the joining
   node is actually authorized to take in the group, encoded as
   specified in Section 3.1 of this document.

   Also, the 'profile' parameter indicates the specific profile of ACE
   to use for securing communications between the joining node and the
   Group Manager (see Section 5.6.4.4 of [I-D.ietf-ace-oauth-authz]).

   In particular, if symmetric keys are used, the AS generates a proof-
   of-possession key, binds it to the Access Token, and provides it to
   the joining node in the 'cnf' parameter of the Access Token response.
   Instead, if asymmetric keys are used, the joining node provides its
   own public key to the AS in the 'cnf' parameter of the Access Token
   request.  Then, the AS uses it as proof-of-possession key bound to



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   the Access Token, and provides the joining node with the Group
   Manager's public key in the 'rs_cnf' parameter of the Access Token
   response.

4.  Joining Node to Group Manager

   First, the joining node posts the Access Token to the /authz-info
   endpoint at the Group Manager, in accordance with the Token post
   defined in Section 3.3 of [I-D.palombini-ace-key-groupcomm].  Then,
   the joining node establishes a secure channel with the Group Manager,
   according to what is specified in the Access Token response and to
   the signalled profile of ACE.

4.1.  Join Request

   Once a secure communication channel with the Group Manager has been
   established, the joining node requests to join the OSCORE group, by
   accessing the related join resource at the Group Manager.

   In particular, the joining node sends to the Group Manager a
   confirmable CoAP request, using the method POST and targeting the
   join endpoint associated to that group.  This join request follows
   the format of the Key Distribution Request message defined in
   Section 4.1 of [I-D.palombini-ace-key-groupcomm].  In particular:

   o  The 'get_pub_keys' parameter is present only if the Group Manager
      is configured to store the public keys of the group members and,
      at the same time, the joining node wants to retrieve such public
      keys during the joining process (see Section 5).  In any other
      case, this parameter MUST NOT be present.

   o  The 'client_cred' parameter, if present, includes the public key
      or certificate of the joining node.  Specifically, it includes the
      public key of the joining node if the Group Manager is configured
      to store the public keys of the group members, or the certificate
      of the joining node otherwise.  This parameter MAY be omitted if:
      i) public keys are used as proof-of-possession keys between the
      joining node and the Group Manager; or ii) the joining node is
      asking to access the group exclusively as pure listener; or iii)
      the Group Manager already acquired this information during a
      previous join process.  In any other case, this parameter MUST be
      present.

   o  The 'pub_keys_repos' parameter MAY be present if the 'client_cred'
      parameter is both present and with value a certificate of the
      joining node.  If present, this parameter contains the list of
      public key repositories storing the certificate of the joining
      node.  In any other case, this parameter MUST NOT be present.



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4.2.  Join Response

   The Group Manager processes the request according to
   [I-D.ietf-ace-oauth-authz].  If this yields a positive outcome, the
   Group Manager updates the group membership by registering the joining
   node as a new member of the OSCORE group.

   Then, the Group Manager replies to the joining node providing the
   information necessary to participate in the group communication.
   This join response follows the format of the Key Distribution success
   Response message defined in Section 4.2 of
   [I-D.palombini-ace-key-groupcomm].  In particular:

   o  The 'key' parameter includes what the joining node needs in order
      to set up the OSCORE Security Context as per Section 2 of
      [I-D.ietf-core-oscore-groupcomm].  In particular:

      *  The 'kty' parameter has value "Symmetric".

      *  The 'k' parameter includes the OSCORE Master Secret.

      *  The 'exp' parameter specifies when the OSCORE Master Secret
         expires.

      *  The 'alg' parameter, if present, has as value the AEAD
         algorithm used in the group.

      *  The 'kid' parameter, if present, has as value the identifier of
         the key in the parameter 'k'.

      *  The 'base IV' parameter, if present, has as value the OSCORE
         Common IV.

      *  The 'clientID' parameter, if present, has as value the OSCORE
         Endpoint ID assigned to the joining node by the Group Manager.
         This parameter is not present if the node joins the group
         exclusively as pure listener, according to what specified in
         the Access Token (see Section 3.2).  In any other case, this
         parameter MUST be present.

      *  The 'serverID' parameter MUST be present and has as value the
         Group Identifier (Gid) currently associated to the group.

      *  The 'kdf' parameter, if present, has as value the KDF algorithm
         used in the group.

      *  The 'slt' parameter, if present, has as value the OSCORE Master
         Salt.



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      *  The 'cs_alg' parameter MUST be present and has as value the
         countersignature algorithm used in the group.

   o  The 'pub_keys' parameter is present only if the 'get_pub_keys'
      parameter was present in the join request.  If present, this
      parameter includes the public keys of the group members that are
      relevant to the joining node.  That is, it includes: i) the public
      keys of the non-pure listeners currently in the group, in case the
      joining node is configured (also) as requester; and ii) the public
      keys of the requesters currently in the group, in case the joining
      node is configured (also) as listener or pure listener.

   o  The 'group_policies' parameter SHOULD be present and includes a
      list of parameters indicating particular policies enforced in the
      group.  For instance, it can indicate the method to achieve
      synchronization of sequence numbers among group members (see
      Appendix E of [I-D.ietf-core-oscore-groupcomm]), as well as the
      rekeying protocol used to renew the keying material in the group
      (see Section 2.1 of [I-D.ietf-core-oscore-groupcomm]).

   o  The 'mgt_key_material' parameter SHOULD be present and includes
      the administrative keying material that the joining node requires
      to participate in the rekeying process led by the Group Manager.
      The exact content and format depend on the specific rekeying
      protocol used in the group.

   Finally, the joining node uses the information received in the join
   response to set up the OSCORE Security Context, as described in
   Section 2 of [I-D.ietf-core-oscore-groupcomm].  From then on, the
   joining node can exchange group messages secured with OSCORE as
   described in Section 4 of [I-D.ietf-core-oscore-groupcomm].

   When the OSCORE Master Secret expires, as specified by 'exp' in the
   'key' parameter of the join response, the node considers the OSCORE
   Security Context also invalid and to be renewed.  A possible approach
   for the node to renew the OSCORE Security Context through the Group
   Manager is described in Section 6 of
   [I-D.palombini-ace-key-groupcomm].

5.  Public Keys of Joining Nodes

   Source authentication of OSCORE messages exchanged within the group
   is ensured by means of digital counter signatures
   [I-D.ietf-core-oscore-groupcomm].  Therefore, group members must be
   able to retrieve each other's public key from a trusted key
   repository, in order to verify the source authenticity of incoming
   group messages.




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   Upon joining an OSCORE group, a joining node is expected to make its
   own public key available to the other group members, either through
   the Group Manager or through another trusted, publicly available, key
   repository.  However, this is not required for a node that joins a
   group exclusively as pure listener.

   As also discussed in Section 6 of [I-D.ietf-core-oscore-groupcomm],
   it is recommended that the Group Manager is configured to store the
   public keys of the group members and to provide them upon request.
   If so, three cases can occur when a new node joins a group.

   o  The Group Manager already acquired the public key of the joining
      node during a previous join process.  In this case, the joining
      node may not provide again its own public key to the Group
      Manager, in order to limit the size of the join request.

   o  The joining node and the Group Manager use an asymmetric proof-of-
      possession key to establish a secure communication channel.  In
      this case, the Group Manager stores the proof-of-possession key
      conveyed in the Access Token as the public key of the joining
      node.

   o  The joining node and the Group Manager use a symmetric proof-of-
      possession key to establish a secure communication channel.  In
      this case, upon performing a join process with that Group Manager
      for the first time, the joining node specifies its own public key
      in the 'client_cred' parameter of the join request targeting the
      join endpoint (see Section 4.1).

   Furthermore, as described in Section 4.1, the joining node may have
   explicitly requested the Group Manager to retrieve the public keys of
   the current group members, i.e. through the 'get_pub_keys' parameter
   in the join request.  In this case, the Group Manager includes also
   such public keys in the 'pub_keys' parameter of the join response
   (see Section 4.2).

   Later on as a group member, the node may need to retrieve the public
   keys of other group members.  A possible approach to do this through
   the Group Manager is described in Section 7 of
   [I-D.palombini-ace-key-groupcomm].

   On the other hand, in case the Group Manager is not configured to
   store public keys of group members, the joining node provides the
   Group Manager with its own certificate in the 'client_cred' parameter
   of the join request targeting the join endpoint (see Section 4.1).
   Then, the Group Manager validates and handles the certificate, for
   instance as described in Appendix D.2 of
   [I-D.ietf-core-oscore-groupcomm].



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6.  Security Considerations

   The method described in this document leverages the following
   management aspects related to OSCORE groups and discussed in the
   sections of [I-D.ietf-core-oscore-groupcomm] referred below.

   o  Management of group keying material (see Section 2.1 of
      [I-D.ietf-core-oscore-groupcomm]).  This includes the need to
      revoke and renew the keying material currently used in the OSCORE
      group, upon changes in the group membership.  In particular,
      renewing the keying material is required upon a new node joining
      the group, in order to preserve backward security.  That is, the
      Group Manager should renew the keying material before completing
      the join process and sending a join response.  Such a join
      response provides the joining node with the updated keying
      material just established in the group.  The Group Manager is
      responsible to enforce rekeying policies and accordingly update
      the keying material in the groups of its competence (see Section 6
      of [I-D.ietf-core-oscore-groupcomm]).

   o  Synchronization of sequence numbers (see Section 5 of
      [I-D.ietf-core-oscore-groupcomm]).  This concerns how a listener
      node that has just joined an OSCORE group can synchronize with the
      sequence number of requesters in the same group.

   o  Provisioning and retrieval of public keys (see Appendix D.2 of
      [I-D.ietf-core-oscore-groupcomm]).  This provides guidelines about
      how to ensure the availability of group members' public keys,
      possibly relying on the Group Manager as trusted key repository
      (see Section 6 of [I-D.ietf-core-oscore-groupcomm]).

   Before sending the join response, the Group Manager should verify
   that the joining node actually owns the associated private key, for
   instance by performing a proof-of-possession challenge-response,
   whose details are out of the scope of this specification.

   Further security considerations are inherited from the ACE framework
   for Authentication and Authorization [I-D.ietf-ace-oauth-authz], as
   well as from the specific profile of ACE signalled by the AS, such as
   [I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-oscore-profile].

7.  IANA Considerations

   This document has no actions for IANA.







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8.  Acknowledgments

   The authors sincerely thank Santiago Aragon, Stefan Beck, Martin
   Gunnarsson, Francesca Palombini, Jim Schaad, Ludwig Seitz, Goeran
   Selander and Peter van der Stok for their comments and feedback.

   The work on this document has been partly supported by the EIT-
   Digital High Impact Initiative ACTIVE.

9.  References

9.1.  Normative References

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-12
              (work in progress), May 2018.

   [I-D.ietf-ace-oscore-profile]
              Seitz, L., Palombini, F., Gunnarsson, M., and G. Selander,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-02 (work in progress), June 2018.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-13 (work in
              progress), June 2018.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Secure group communication for CoAP", draft-ietf-core-
              oscore-groupcomm-02 (work in progress), June 2018.

   [I-D.palombini-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", draft-palombini-ace-key-
              groupcomm-01 (work in progress), June 2018.

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





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   [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>.

   [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

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-03 (work in progress), March 2018.

   [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>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014, <https://www.rfc-
              editor.org/info/rfc7390>.

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

Authors' Addresses

   Marco Tiloca
   RISE SICS
   Isafjordsgatan 22
   Kista  SE-164 29 Stockholm
   Sweden

   Email: marco.tiloca@ri.se







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   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   Essen  45127
   Germany

   Email: ji-ye.park@uni-due.de












































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