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Versions: 00 01

ACE Working Group                                              M. Tiloca
Internet-Draft                                               R. Hoeglund
Intended status: Standards Track                                L. Seitz
Expires: May 7, 2020                                             RISE AB
                                                            F. Palombini
                                                             Ericsson AB
                                                       November 04, 2019


    Group OSCORE Profile of the Authentication and Authorization for
                   Constrained Environments Framework
                draft-tiloca-ace-group-oscore-profile-01

Abstract

   This document specifies a profile for the Authentication and
   Authorization for Constrained Environments (ACE) framework.  The
   profile uses Object Security for Constrained RESTful Environments
   (OSCORE) and/or Group OSCORE to provide communication security
   between a Client and (a group of) Resource Server(s).  Furthermore,
   the profile uses (Group) OSCORE to provide server authentication, and
   OSCORE to achieve proof-of-possession for a key owned by the Client
   and bound to an OAuth 2.0 Access Token.  Also, the profile provides
   proof-of-group-membership for the Client, by securely binding the
   pre-established Group OSCORE Security Context to the pairwise OSCORE
   Security Context newly established with the Resource Server.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on May 7, 2020.








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Copyright Notice

   Copyright (c) 2019 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
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Access Token Retrieval  . . . . . . . . . . . . . . . . .   8
     2.3.  Access Token Posting  . . . . . . . . . . . . . . . . . .   9
     2.4.  Secure Communication  . . . . . . . . . . . . . . . . . .  10
   3.  Client-AS Communication . . . . . . . . . . . . . . . . . . .  10
     3.1.  C-to-AS: POST to Token Endpoint . . . . . . . . . . . . .  11
       3.1.1.  'context_id' Parameter  . . . . . . . . . . . . . . .  13
       3.1.2.  'salt' Parameter  . . . . . . . . . . . . . . . . . .  14
       3.1.3.  'client_cred' Parameter . . . . . . . . . . . . . . .  14
       3.1.4.  'client_cred_verify' Parameter  . . . . . . . . . . .  14
     3.2.  AS-to-C: Access Token . . . . . . . . . . . . . . . . . .  14
       3.2.1.  Client Credential Claim . . . . . . . . . . . . . . .  19
   4.  Client-RS Communication . . . . . . . . . . . . . . . . . . .  20
     4.1.  C-to-RS POST to authz-info Endpoint . . . . . . . . . . .  21
     4.2.  RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . . . .  21
     4.3.  OSCORE Setup - Client Side  . . . . . . . . . . . . . . .  22
     4.4.  OSCORE Setup - Resource Server Side . . . . . . . . . . .  23
     4.5.  Access Rights Verification  . . . . . . . . . . . . . . .  25
   5.  Secure Communication with the AS  . . . . . . . . . . . . . .  26
   6.  Discarding the Security Context . . . . . . . . . . . . . . .  26
   7.  CBOR Mappings . . . . . . . . . . . . . . . . . . . . . . . .  27
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  27
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  ACE Profile Registry . . . . . . . . . . . . . . . . . .  28
     10.2.  OAuth Parameters Registry  . . . . . . . . . . . . . . .  28
     10.3.  OAuth Parameters CBOR Mappings Registry  . . . . . . . .  29
     10.4.  CBOR Web Token Claims Registry . . . . . . . . . . . . .  30



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   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     11.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Appendix A.  Profile Requirements . . . . . . . . . . . . . . . .  33
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   A number of applications rely on a group communication model, where a
   Client can access a resource shared by multiple Resource Servers at
   once, e.g. over IP multicast.  Typical examples are switching of
   luminaries, actuators control, and distribution of software updates.
   Secure communication in the group can be achieved by sharing a set of
   key material, which is typically provided upon joining the group.

   For some instances of such applications, it may be just fine to
   enforce access control in a straightforward and plain fashion.  That
   is, it is assumed that any Client authorized to join the group and to
   get the group key material, is also implicitly authorized as a group
   member to perform any action at any resource of any Server in the
   group.  An example of an application where such implicit
   authorization might be used is a lighting scenario, where the
   lightbulbs are the Servers, while the user account on an app on the
   user's phone is the Client.  In this case, it might be fine to not
   require additional authorization evidence from any user account, if
   it is acceptable that any current group member is also authorized to
   switch on and off any light, or to check their status.

   However, in different instances of such applications, the approach
   above is not desirable, as different group members are intended to
   have different access rights to resources of other group members.
   For instance, a more fine-grained authorization approach is required
   in the two following use cases.

   As a first case, an application provides control of smart locks
   acting as Servers in the group, where: a first type of Client, e.g. a
   user account of a child, is allowed to only query the status of the
   smart locks; whereas a second type of Client, e.g. a user account of
   a parent, is allowed to both query and change the status of the smart
   locks.  Further similar applications concern the enforcement of
   different sets of permissions in groups with sensor/actuator devices,
   e.g. thermostats, acting as Servers.  In some settings, some group
   members may even be intended as servers only, hence they must be
   prevented from acting as Clients altogether and from accessing
   resources at other Servers.  For example, in a group of lightbulbs,
   typically none of them should be able to switch on and off other
   lightbulbs in the group.



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   As a second case, building automation scenarios often rely on Servers
   that, under different circumstances, enforce different level of
   priority for processing received commands.  For instance, BACnet
   deployments consider multiple classes of Clients, e.g. a normal light
   switch (C1) and an emergency fire panel (C2).  Then, a C1 Client is
   not allowed to override a command from a C2 Client, until the latter
   relinquishes control at its higher priority.  That is: i) only C2
   Clients should be able to adjust the minimum required level of
   priority on the Servers, so rightly locking out C1 Clients if needed;
   and ii) when a Server is set to accept only high-priority commands,
   only C2 Clients should be able to perform such commands otherwise
   allowed also to C1 Clients.  Given the different maximum authority of
   different Clients, fine-grained access control would effectively
   limit the execution of high- and emergency-priority commands only to
   devices that are in fact authorized to do so.  Besides, it would
   prevent a misconfigured or compromised device from initiating a high-
   priority command and lock out normal control.

   Hence, in the cases discussed above, being a legitimate group member
   and having obtained the group key material is not supposed to imply
   any particular access rights.  Thus, a more fine-grained access
   control model has to be enforced, e.g. by using the Authentication
   and Authorization for Constrained Environments (ACE) framework
   [I-D.ietf-ace-oauth-authz].  That is, a Client has to first obtain
   authorization credentials in the form of an Access Token, and post it
   to the Resource Server(s) in the group before accessing the intended
   resources.

   The ACE framework delegates to separate profile documents how to
   secure communications between the Client and the Resource Server.
   However each of the current profiles of ACE defined in
   [I-D.ietf-ace-oscore-profile] [I-D.ietf-ace-dtls-authorize]
   [I-D.ietf-ace-mqtt-tls-profile] admits a single security protocol
   that cannot be used to protect group messages sent over IP multicast.

   This document specifies a profile of ACE, where a Client uses CoAP
   [RFC7252] to communicate to a single Resource Server, or CoAP over IP
   multicast [RFC7390][I-D.dijk-core-groupcomm-bis] to communicate to
   multiple Resource Servers that are members of a group and share a
   common set of resources.  This profile uses two complementary
   security protocols to provide secure communication between the Client
   and the Resource Server(s).

   That is, this document defines the use of either Object Security for
   Constrained RESTful Environments (OSCORE) [RFC8613] or Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] to protect unicast requests
   addressed to a single Resource Server, as well as possible responses.
   Additionally, it defines the use of Group OSCORE to protect multicast



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   requests sent to a group of Resource Servers, as well as possible
   individual responses.  The Client and the Resource Servers need to
   have already joined an OSCORE group, for instance by using the
   approach defined in [I-D.ietf-ace-key-groupcomm-oscore], which is
   also based on ACE.

   The Client authorizes its access to the Resource Server by using an
   Access Token, which is bound to a key (the proof-of-possession key).
   This profile uses OSCORE to achieve proof of possession, and OSCORE
   or Group OSCORE to achieve server authentication.  Furthermore, this
   profile provides proof of Client's membership to the correct OSCORE
   group, by securely binding the pre-established Group OSCORE Security
   Context to the pairwise OSCORE Security Context newly established
   between the Client and the Resource Server.

   OSCORE specifies how to use CBOR Object Signing and Encryption (COSE)
   [RFC8152] to secure CoAP messages.  Group OSCORE builds on OSCORE to
   support group communication, and ensures source authentication by
   means of digital countersignatures embedded in protected messages.

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
   related to the CoAP protocol [RFC7252], as well as related to the
   protection and processing of CoAP messages through OSCORE [RFC8613],
   also in group communication scenarios through Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].  These include the concept of Group
   Manager, as the entity responsible for a set of groups where
   communications among members are secured with Group OSCORE.

   This document also refers to "pairwise OSCORE Security Context", i.e.
   an OSCORE Security Context established between only one Client and
   one Resource Server, and used to communicate with OSCORE [RFC8613].

   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], as well as in the OSCORE profile of ACE
   [I-D.ietf-ace-oscore-profile].  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).




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   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".

2.  Protocol Overview

   This section provides an overview on how to use the ACE framework for
   authentication and authorization [I-D.ietf-ace-oauth-authz] to secure
   communications between a Client and a (set of) Resource Server(s)
   using OSCORE [RFC8613] and/or Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

   An overview of the protocol flow for this profile is shown in
   Figure 1.  In the figure, it is assumed that C, RS1 and RS2 have
   previously joined an OSCORE group with Group Identifier (gid)
   "abcd0000", and got assigned Sender ID (sid) "0", "1" and "2" in the
   group, respectively.  It is also assumed that both RS1 and RS2 are
   associated with the same AS.  For simplicity, the figure does not
   show the preliminary phase where C, R1 and R2 join the OSCORE group.






























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C                            RS1           RS2                          AS
| [--- Resource Request --->] |             |                            |
|                             |             |                            |
| [<--- AS Information -----] |             |                            |
|                             |             |                            |
|-------- POST /token -------------------------------------------------->|
|  (aud: RS1, sid: 0, gid: abcd0000, ... )  |                            |
|                             |             |                            |
|<---------------------------------- Access Token + RS Information ------|
|                             |  (aud: RS1, sid: 0, gid: abcd0000, ... ) |
|---- POST /authz-info ------>|             |                            |
|    (access_token, N1)       |             |                            |
|                             |             |                            |
|<--- 2.01 Created (N2) ------|             |                            |
|                             |             |                            |
/Pairwise OSCORE Sec  /Pairwise OSCORE Sec  |                            |
 Context Derivation/   Context Derivation/  |                            |
|                             |             |                            |
|-------- POST /token -------------------------------------------------->|
|  (aud: RS2, sid: 0, gid: abcd0000, ... )  |                            |
|                             |             |                            |
|<---------------------------------- Access Token + RS Information ------|
|                             |  (aud: RS2, sid: 0, gid: abcd0000, ... ) |
|                             |             |                            |
|----- POST /authz-info ------------------->|                            |
|     (access_token, N1')      |            |                            |
|                              |            |                            |
|<--- 2.01 Created (N2') -------------------|                            |
|                              |            |                            |
/Pairwise OSCORE Sec           |  /Pairwise OSCORE Sec                   |
 Context Derivation/           |   Context Derivation/                   |
|                              |            |                            |
|------ OSCORE Request ------->|            |                            |
|    ?(abcd0000, N1, N2)       |            |                            |
|                              |            |                            |
|<----- OSCORE Response -------|            |                            |
|                              |            |                            |
|-- Group OSCORE Request --+-->|            |                            |
| (kid: 0, gid: abcd0000)  \--------------->|                            |
|                              |            |                            |
|<--- Group OSCORE Response ---|            |                            |
|          (kid: 1)            |            |                            |
|                              |            |                            |
|<--- Group OSCORE Response ----------------|                            |
|          (kid: 2)            |            |                            |
|             ...              |            |                            |

                       Figure 1: Protocol Overview.



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2.1.  Pre-Conditions

   Using Group OSCORE requires both the Client and the Resource Servers
   to have previously joined an OSCORE group.  This especially includes
   the derivation of the Group OSCORE Security Context and the
   assignment of unique Sender IDs to use in the group.  Nodes may join
   the OSCORE group through the respective Group Manager by using the
   approach defined in [I-D.ietf-ace-key-groupcomm-oscore], which is
   also based on ACE.

   As a pre-requisite for this profile, the Client has to have
   successfully joined the OSCORE group where also the Resource Servers
   (RSs) are members.  Depending on the limited information initially
   available, the Client may have to first discover the exact OSCORE
   group used by the RSs for the resources of interest, e.g. by using
   the approach defined in [I-D.tiloca-core-oscore-discovery].

2.2.  Access Token Retrieval

   This profile requires that the Client retrieves an Access Token from
   the AS for the resource(s) it wants to access on each of the RSs,
   using the /token endpoint, as specified in Section 5.6 of
   [I-D.ietf-ace-oauth-authz].  In a general case, it can be assumed
   that different RSs are associated to different ASs, even if the RSs
   are members of a same OSCORE group.

   In the Access Token request to the AS, the Client MUST include the
   Group Identifier of the OSCORE group and its own Sender ID in that
   group.  The AS MUST include these pieces of information in the Access
   Token and in the Access Token response to the Client.

   Furthermore, in the Access Token request to the AS, the Client MUST
   also include: its own public key, associated to the private signing
   key used in the OSCORE group; and a signature computed with such
   private key, over a quantity uniquely related to the secure
   communication association between the Client and the AS.  Finally,
   the AS MUST include the public key indicated by the client in the
   Access Token.

   To gain knowledge of the AS in charge of a resource hosted at a RS,
   the Client MAY first send an initial Unauthorized Resource Request
   message to that RS.  Then, the RS denies the request and replies to
   the Client by specifying the address of its AS, as defined in
   Section 5.1 of [I-D.ietf-ace-oauth-authz].  The Access Token request
   and response MUST be confidentiality-protected and ensure
   authenticity.  This profile RECOMMENDS the use of OSCORE between the
   Client and the AS, but TLS [RFC5246][RFC8446] or DTLS
   [RFC6347][I-D.ietf-tls-dtls13] MAY be used additionally or instead.



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2.3.  Access Token Posting

   After having retrieved the Access Token from the AS, the Client
   generates a nonce N1 and posts both the Access Token and N1 to the
   RS, using the /authz-info endpoint and mechanisms specified in
   Section 5.8 of [I-D.ietf-ace-oauth-authz] and Content-Format =
   application/ace+cbor.

   If the Access Token is valid, the RS replies to this POST request
   with a 2.01 (Created) response with Content-Format = application/
   ace+cbor, which contains a nonce N2 in a CBOR map.  Also, the RS sets
   the ID Context in the pairwise OSCORE Security Context (see Section 3
   of [RFC8613]) to the Group Identifier of the OSCORE group specified
   in the Access Token, concatenated with N1 concatenated with N2.

   Then, the RS derives the complete pairwise OSCORE Security Context
   associated with the received Access Token, following Section 3.2 of
   [RFC8613].  During the derivation process, the RS uses the ID Context
   above, the nonces N1 and N2, and the parameters in the Access Token.
   The derivation process uses also the Master Secret of the OSCORE
   group, that the RS knows as a group member, as well as the Sender ID
   of the Client in the OSCORE group, which is specified in the Access
   Token.  This ensures that the pairwise OSCORE Security Context is
   securely bound to the Group OSCORE Security Context of the OSCORE
   group.

   Finally, the RS stores the association between i) the authorization
   information from the Access Token; and ii) the Group Identifier of
   the OSCORE group together with the Sender ID and the public key of
   the Client in that group.

   After having received the nonce N2, the Client sets the ID Context in
   its pairwise OSCORE Security Context (see Section 3 of [RFC8613]) to
   the Group Identifier of the OSCORE group concatenated with N1
   concatenated with N2.  Then, the Client derives the complete pairwise
   OSCORE Security Context, following Section 3.2 of [RFC8613].  During
   the derivation process, the Client uses the ID Context above, the
   nonces N1 and N2, plus the parameters received from the AS.  The
   derivation process uses also the Master Secret of the OSCORE group,
   that the Client knows as a group member, as well as its own Sender ID
   in the OSCORE group.

   When the Client communicates with the RS using the pairwise OSCORE
   Security Context, the RS achieves proof-of-possession of the
   credentials bound to the Access Token.  Also, the RS verifies that
   the Client is a legitimate member of the OSCORE group.





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   Finally, when the Client communicates with the RS using the Group
   OSCORE Security Context, the RS verifies that the Client is the exact
   group member with the same Sender ID associated to the Access Token.
   This occurs when verifying a request protected with Group OSCORE,
   since it embeds a countersignature computed also over the Client's
   Sender ID included in the message.

2.4.  Secure Communication

   The Client can send a request protected with OSCORE to the RS.  This
   message may contain the ID Context value, i.e. the Group Identifier
   of the OSCORE group concatenated with N1 concatenated with N2.  If
   the request is correctly verified, then the RS stores the pairwise
   OSCORE Security Context, and uses it to protect the possible
   response, as well as further communications with the Client, until
   the Access Token expires.  This pairwise OSCORE Security Context is
   discarded if the same Access Token is re-used to successfully derive
   a new pairwise OSCORE Security Context.  Once the Client has received
   a valid secure response, it does not continue to include the ID
   Context value in following requests.

   As discussed in Section 2 of [I-D.ietf-ace-oscore-profile], the use
   of random nonces N1 and N2 during the exchange between the Client and
   the RS prevents the reuse of AEAD nonces and keys with different
   messages, in case of re-derivation of the pairwise OSCORE Security
   Context both for Clients and Resource Servers from an old non-expired
   Access Token, e.g. in case of reboot of either the Client or the RS.

   Furthermore, the Client can send a request protected with Group
   OSCORE [I-D.ietf-core-oscore-groupcomm].  This can be a unicast
   request addressed to the RS, or a multicast request addressed to the
   OSCORE group where the RS is also a member.  To this end, the Client
   uses the Group OSCORE Security Context already established upon
   joining the OSCORE group, e.g. by using the approach defined in
   [I-D.ietf-ace-key-groupcomm-oscore].  The RS may send a response back
   to the Client, protecting it by means of the same Group OSCORE
   Security Context.

3.  Client-AS Communication

   This section details the Access Token POST Request that the Client
   sends to the /token endpoint of the AS, as well as the related Access
   Token response.

   Section 3.2 of [RFC8613] defines how to derive a pairwise OSCORE
   Security Context based on a shared Master Secret and a set of other
   parameters, established between the OSCORE client and server.




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   The Client receives these pieces of information from the AS during
   the exchange described in this section.  In particular, the proof-of-
   possession key (pop-key) provisioned by the AS MUST be used to build
   the Master Secret in OSCORE (see Section 4.3 and Section 4.4).

3.1.  C-to-AS: POST to Token Endpoint

   The Client-to-AS request is specified in Section 5.6.1 of
   [I-D.ietf-ace-oauth-authz].  The Client MUST send this POST request
   to the /token endpoint over a secure channel that guarantees
   authentication, message integrity and confidentiality.

   The POST request is formatted as the analogous Client-to-AS request
   in the OSCORE profile of ACE (see Section 3.1 of
   [I-D.ietf-ace-oscore-profile]), with the following additional
   parameters that MUST be included in the payload.

   o  'context_id', defined in Section 3.1.1 of this specification.
      This parameter includes the Group ID of the OSCORE group that the
      Client has previously joined and wants to use to communicate with
      the RS.

   o  'salt', defined in Section 3.1.2 of this specification.  This
      parameter includes the Sender ID that the Client has received in
      the OSCORE group, whose identifier is indicated in the
      'context_id' parameter above, upon previously joining it.  That
      is, its value is the Sender ID that the Client uses to communicate
      in the OSCORE group, whereas it does not relate to the Sender ID
      to be assigned for use in the pairwise OSCORE Security Context
      with the RS.

   o  'client_cred', defined in Section 3.1.3 of this specification.
      This parameter includes the public key associated to the signing
      private key that the Client uses in the OSCORE group, whose
      identifier is indicated in the 'context_id' parameter above.

   o  'client_cred_verify', defined in Section 3.1.4 of this
      specification.  This parameter includes a signature computed by
      the Client, by using the private key associated to the public key
      in the 'client_cred' parameter above.  This allows the AS to
      verify that the Client indeed owns the private key associated to
      the public key in 'client_cred', as its alleged identity
      credential within the OSCORE group.  The information to be signed
      MUST be the byte representation of a quantity that uniquely
      represents the secure communication association between the Client
      and the AS.  It is RECOMMENDED that the Client considers the
      following as information to sign.




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      *  If the Client and the AS communicate over (D)TLS, the
         information to sign is an exporter value computed as defined in
         Section 7.5 of [RFC8446], for which a common exporter label has
         to be agreed between the Client and the AS.

      *  If the Client and the AS communicate over OSCORE, the
         information to sign is the output PRK of a HKDF-Extract step
         [RFC5869], i.e. PRK = HMAC-Hash(salt, IKM).  In particular,
         'salt' takes (x1 | x2), where x1 is the ID Context of the
         OSCORE Security Context between the Client and the AS, x2 is
         the Sender ID of the Client in that Context, and | denotes byte
         string concatenation.  Also, 'IKM' is the OSCORE Master Secret
         of the OSCORE Security Context between the Client and the AS.
         The HKDF MUST be one of the HMAC-based HKDF [RFC5869]
         algorithms defined for COSE [RFC8152].  HKDF SHA-256 is
         mandatory to implement.

   An example of such a request, in CBOR diagnostic notation without the
   tag and value abbreviations is reported in Figure 2.

        Header: POST (Code=0.02)
        Uri-Host: "as.example.com"
        Uri-Path: "token"
        Content-Format: "application/ace+cbor"
        Payload:
        {
          "audience" : "tempSensor4711",
          "scope" : "read",
          "salt" : h'00',
          "context_id" : h'abcd0000',
          "client_cred" : {
            "COSE_Key" : {
              "kty" : EC2,
              "crv" : P-256,
              "x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
                      27c9e354089bbe13',
              "y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
                      731e79a3b4e47120'
            }
          },
          "client_cred_verify" : h'...'
        }

     Figure 2: Example C-to-AS POST /token request for an Access Token
                         bound to a symmetric key.

   Later on, the Client may want to update its current access rights,
   without changing the existing pairwise OSCORE Security Context with



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   the RS.  In this case, like in the OSCORE profile of ACE (see
   Section 3.1 of [I-D.ietf-ace-oscore-profile]), the Client MUST
   include in its POST request to the /token endpoint a req_cnf object,
   where the 'kid' field carries the Client's identifier, that was
   assigned by the AS as per Section 3.2.  That is, the Client's
   identifier is the value of the 'clientId' parameter in the OSCORE
   Security Context object of the 'cnf' parameter, in the AS-to-C Access
   Token response providing the original Access Token (see Section 3.2).

   The AS can use this identifier to determine the shared secret for
   preparing the proof-of-possession Access Token.  Therefore, the
   received value MUST identify a symmetric key that was previously
   generated by the AS, as a shared secret for communications between
   the Client and the RS.  In particular, the AS MUST verify that the
   received value identifies a proof-of-possession key and Access Token
   that have previously been issued to the requesting Client.  If that
   is not the case, the Client-to-AS request MUST be declined with the
   error code 'invalid_request', as defined in Section 5.6.3 of
   [I-D.ietf-ace-oauth-authz].

   This POST request for updating the rights of an Access Token MUST NOT
   include the parameters 'salt', 'context_id', 'client_cred' and
   'client_cred_verify'.

   An example of such a request, in CBOR diagnostic notation without the
   tag and value abbreviations is reported in Figure 3.

        Header: POST (Code=0.02)
        Uri-Host: "as.example.com"
        Uri-Path: "token"
        Content-Format: "application/ace+cbor"
        Payload:
        {
          "audience" : "tempSensor4711",
          "scope" : "read",
          "req_cnf" : {
            "kid" : 'myclient'
          }
        }

   Figure 3: Example C-to-AS POST /token request for updating rights to
                 an Access Token bound to a symmetric key.

3.1.1.  'context_id' Parameter

   The 'context_id' parameter is an OPTIONAL parameter of the Access
   Token request message defined in Section 5.6.1. of
   [I-D.ietf-ace-oauth-authz].  This parameter provides a value that the



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   Client wishes to use with the RS as a hint for a security context.
   Its exact content is profile specific.

3.1.2.  'salt' Parameter

   The 'salt' parameter is an OPTIONAL parameter of the Access Token
   request message defined in Section 5.6.1. of
   [I-D.ietf-ace-oauth-authz].  This parameter provides a value that the
   Client wishes to use as salt with the RS, for deriving cryptographic
   key material.  Its exact content is profile specific.

3.1.3.  'client_cred' Parameter

   The 'client_cred' parameter is an OPTIONAL parameter of the Access
   Token request message defined in Section 5.6.1. of
   [I-D.ietf-ace-oauth-authz].  This parameter provides an asymmetric
   key that the Client wishes to use as its own public key, but which is
   not used as proof-of-possession key.

   This parameter follows the syntax of the 'cnf' claim from Section 3.1
   of [I-D.ietf-ace-cwt-proof-of-possession] when including Value Type
   "COSE_Key" (1) and specifying an asymmetric key.  Alternative Value
   Types defined in future specifications are fine to consider if
   indicating a non-encrypted asymmetric key.

3.1.4.  'client_cred_verify' Parameter

   The 'client_cred_verify' parameter is an OPTIONAL parameter of the
   Access Token request message defined in Section 5.6.1. of
   [I-D.ietf-ace-oauth-authz].  This parameter provides a signature
   computed by the Client to prove the possession of its own private
   key.

3.2.  AS-to-C: Access Token

   After having verified the POST request to the /token endpoint and
   that the Client is authorized to obtain an Access Token corresponding
   to its Access Token request, the AS MUST verify the signature in the
   'client_cred_verify' parameter, by using the public key specified in
   the 'client_cred' parameter.  If the verification fails, the AS
   considers the Client request invalid.  The AS does not perform this
   operation when asked to update a previously released Access Token.

   If all verifications are successful, the AS responds as defined in
   Section 5.6.2 of [I-D.ietf-ace-oauth-authz].  If the Client request
   was invalid, or not authorized, the AS returns an error response as
   described in Section 5.6.3 of [I-D.ietf-ace-oauth-authz].




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   The AS can signal that the use of OSCORE and Group OSCORE is REQUIRED
   for a specific Access Token by including the 'profile' parameter with
   the value "coap_group_oscore" in the Access Token response.  This
   means that the Client MUST use OSCORE and/or Group OSCORE towards all
   the Resource Servers for which this Access Token is valid.

   In particular, the Client MUST follow Section 4.3 to derive the
   pairwise OSCORE Security Context to use for communications with the
   RS.  Additionally, the Client has already established the related
   Group OSCORE Security Context to communicate with members of the
   OSCORE group, upon previously joining that group.

   Usually, it is assumed that constrained devices will be pre-
   configured with the necessary profile, so that this kind of profile
   negotiation can be omitted.

   The Access Token response to the Client is analogous to the one in
   the OSCORE profile of ACE, as described in Section 3.2 of
   [I-D.ietf-ace-oscore-profile].  In particular, the AS provides also
   the following additional information in the OSCORE_Security_Context
   object, which is defined in Section 3.2.1 of
   [I-D.ietf-ace-oscore-profile] and included in the 'cnf' parameter
   (see Section 3.2 of [I-D.ietf-ace-oauth-params]) of the Access Token
   response.

   o  The 'salt' field in the OSCORE_Security_Context object MUST be
      present.  The field MUST contain the value of the 'salt' parameter
      from the Access Token request received from the Client.

   o  The 'contextId' field in the OSCORE_Security_Context object MUST
      be present.  The field MUST contain the value of the 'context_id'
      parameter from the Access Token request received from the Client.

   The same parameters MUST be included as metadata of the issued Access
   Token.  This profile RECOMMENDS the use of CBOR web tokens (CWT) as
   specified in [RFC8392].  If the Access Token is a CWT, the same
   OSCORE_Security_Context structure considered above MUST be placed in
   the 'cnf' claim of the Access Token.

   Furthermore, the AS MUST include also the public key of the client
   specified in the 'client_cred' parameter of the Token Request as
   metadata of the issued Access Token.  If the Access Token is a CWT,
   the content of the 'client_cred' parameter MUST be placed in the
   'client_cred' claim of the Access Token, defined in Section 3.2.1 of
   this specification.

   As discussed in Section 3.2 of [I-D.ietf-ace-oscore-profile],
   collisions of client identifiers may appear in the RS, in case a



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   resource is associated to multiple ASs.  In such a case, the RS needs
   to have a mechanism in place to disambiguate identifiers or mitigate
   the effect of their collision.

   Figure 4 shows an example of such an AS response, in CBOR diagnostic
   notation without the tag and value abbreviations.

        Header: Created (Code=2.01)
        Content-Type: "application/ace+cbor"
        Payload:
        {
          "access_token" : h'a5037674656d7053656e73 ...'
           (remainder of access token omitted for brevity),
          "profile" : "coap_group_oscore",
          "expires_in" : 3600,
          "cnf" : {
            "OSCORE_Security_Context" : {
              "alg" : "AES-CCM-16-64-128",
              "clientId" : b64'qA',
              "serverId" : b64'Qg',
              "ms" : h'f9af838368e353e78888e1426bd94e6f',
              "salt" : h'00',
              "context_id" : h'abcd0000'
            }
          }
        }

   Figure 4: Example AS-to-C Access Token response with the Group OSCORE
                                 profile.

   Figure 5 shows an example CWT, containing the necessary OSCORE
   parameters in the 'cnf' claim, in CBOR diagnostic notation without
   tag and value abbreviations.


















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        {
          "aud" : "tempSensorInLivingRoom",
          "iat" : "1360189224",
          "exp" : "1360289224",
          "scope" :  "temperature_g firmware_p",
          "cnf" : {
            "OSCORE_Security_Context" : {
              "alg" : "AES-CCM-16-64-128",
              "clientId" : 'client',
              "serverId" : 'server',
              "ms" : h'f9af838368e353e78888e1426bd94e6f',
              "salt" : h'00',
              "context_id" : h'abcd0000'
          },
          "client_cred" : {
            "COSE_Key" : {
              "kty" : EC2,
              "crv" : P-256,
              "x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
                      27c9e354089bbe13',
              "y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
                      731e79a3b4e47120'
            }
          }
        }

       Figure 5: Example CWT with OSCORE parameters (CBOR diagnostic
                                notation).

   The same CWT as in Figure 5 and encoded in CBOR is shown in Figure 6,
   using the value abbreviations defined in [I-D.ietf-ace-oauth-authz]
   and [I-D.ietf-ace-cwt-proof-of-possession], and with 12 as value
   abbreviation for the 'client_cred' claim.

   NOTE: it should be checked (and in case fixed) that the values used
   below (which are not yet registered) are the final values registered
   in IANA.

   A5                                      # map(5)
      03                                   # unsigned(3)
      76                                   # text(22)
         74656D7053656E736F72496E4C6976696E67526F6F6D
                                           # "tempSensorInLivingRoom"
      06                                   # unsigned(6)
      1A 5112D728                          # unsigned(1360189224)
      04                                   # unsigned(4)
      1A 51145DC8                          # unsigned(1360289224)
      09                                   # unsigned(9)



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      78 18                                # text(24)
         74656D70657261747572655F67206669726D776172655F70
                                           # "temperature_g firmware_p"
      08                                   # unsigned(8)
      A1                                   # map(1)
         04                                # unsigned(4)
         A6                                # map(6)
            05                             # unsigned(5)
            0A                             # unsigned(10)
            02                             # unsigned(2)
            46                             # bytes(6)
               636C69656E74                # "client"
            03                             # unsigned(3)
            46                             # bytes(6)
               736572766572                # "server"
            01                             # unsigned(1)
            50                             # bytes(16)
               F9AF838368E353E78888E1426BD94E6F
            06                             # unsigned(6)
            41                             # bytes(1)
               00
            07                             # unsigned(7)
            44                             # bytes(4)
               ABCD0000
      0C                                   # unsigned(12)
      A1                                   # map(1)
         01                                # unsigned(1)
         A4                                # map(4)
            01                             # unsigned(1)
            02                             # unsigned(2)
            20                             # negative(0)
            01                             # unsigned(1)
            21                             # negative(1)
            58 20                          # bytes(32)
               D7CC072DE2205BDC1537A543D53C60A6ACB62ECCD890C7FA27C9
               E354089BBE13
            22                             # negative(2)
            58 20                          # bytes(32)
               F95E1D4B851A2CC80FFF87D8E23F22AFB725D535E515D020731E
               79A3B4E47120

               Figure 6: Example CWT with OSCORE parameters.

   If the Client has requested an update to its access rights with
   reference to the same pairwise OSCORE Security Context, which is
   valid and authorized, the AS MUST omit the 'cnf' parameter in the
   Access Token response, MUST omit the 'client_cred' claim in the
   Access Token, and MUST include the Client identifier in the 'kid'



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   field of the 'cnf' claim of the Access Token.  The Client identifier
   needs to be provisioned, in order for the RS to identify the
   previously generated pairwise OSCORE Security Context.

   Figure 7 shows an example of such an AS response, in CBOR diagnostic
   notation without the tag and value abbreviations.

        Header: Created (Code=2.01)
        Content-Type: "application/ace+cbor"
        Payload:
        {
          "access_token" : h'a5037674656d7053656e73 ...'
           (remainder of access token omitted for brevity),
          "profile" : "coap_group_oscore",
          "expires_in" : 3600
        }

   Figure 7: Example AS-to-C Access Token response with the Group OSCORE
                   profile, for update of access rights.

   Figure 8 shows an example CWT, containing the necessary OSCORE
   parameters in the 'cnf' claim for update of access rights, in CBOR
   diagnostic notation without tag and value abbreviations.

        {
          "aud" : "tempSensorInLivingRoom",
          "iat" : "1360189224",
          "exp" : "1360289224",
          "scope" :  "temperature_h",
          "cnf" : {
            "kid" : b64'qA'
          }
        }

     Figure 8: Example CWT with OSCORE parameters for update of access
                                  rights.

3.2.1.  Client Credential Claim

   The 'client_cred' claim provides an asymmetric key that the Client
   owning the Access Token wishes to use as its own public key, but
   which is not used as proof-of-possession key.

   This parameter follows the syntax of the 'cnf' claim from Section 3.1
   of [I-D.ietf-ace-cwt-proof-of-possession] when including Value Type
   "COSE_Key" (1) and specifying an asymmetric key.  Alternative Value
   Types defined in future specifications are fine to consider if
   indicating a non-encrypted asymmetric key.



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4.  Client-RS Communication

   This section details the POST request and response to the /authz-info
   endpoint between the Client and the RS.  With respect to the
   exchanged messages and their content, the Client and the RS perform
   as defined in Section 4 of the OSCORE profile of ACE
   [I-D.ietf-ace-oscore-profile].

   That is, the Client generates a nonce N1 and posts it to the RS,
   together with the Access Token that includes the material provisioned
   by the AS.  Then, the RS generates a nonce N2, and derives a pairwise
   OSCORE Security Context as described Section 3.2 of [RFC8613].  In
   particular, it uses the two nonces established with the Client and
   two shared secrets, together with additional pieces of information
   specified in the Access Token.

   The proof-of-possession required to bind the Access Token to the
   Client is implicitly performed by generating the pairwise OSCORE
   Security Context using the pop-key as part of the OSCORE Master
   Secret, for both the Client and the RS.  In addition, the derivation
   of the pairwise OSCORE Security Context takes as input also
   information related to the OSCORE group, i.e. the Master Secret and
   Group Identifier of the group, as well as the Sender ID of the Client
   in the group.  Hence, the derived pairwise OSCORE Security Context is
   also securely bound to the Group OSCORE Security Context of the
   OSCORE Group.

   Therefore, an attacker using a stolen Access Token cannot generate a
   valid pairwise OSCORE Security Context and thus cannot prove
   possession of the pop-key.  Also, if a Client legitimately owns an
   Access Token but has not joined the OSCORE group, that Client cannot
   generate a valid pairwise OSCORE Security Context either, since it
   lacks the Master Secret used in the OSCORE group.

   Finally, a Client C1 is supposed to obtain a valid Access Token from
   the AS, as including the public key associated to its own signing key
   used in the OSCORE group, together with its own Sender ID in that
   OSCORE group (see Section 3.1).  This makes it possible for the RS
   receiving an Access Token to verify with the Group Manager of that
   OSCORE group whether such a Client has indeed that Sender ID and that
   public key in the OSCORE group.

   As a consequence, a different Client C2, also member of the same
   OSCORE group, is not able to impersonate C1, by: i) getting a valid
   Access Token, specifying the Sender ID of C1 and a different (made-
   up) public key; ii) successfully posting the Access Token to RS; and
   then iii) using only the pairwise OSCORE Security Context to




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   communicate with RS to legitimately perform authorized actions, while
   blaming C1 for the consequences.

4.1.  C-to-RS POST to authz-info Endpoint

   The Client MUST generate a nonce N1 and post it to the /authz-info
   endpoint of the RS together with the Access Token, as defined in
   Section 4.1 of the OSCORE profile of ACE
   [I-D.ietf-ace-oscore-profile].

   The same recommendations, considerations and behaviors defined in
   Section 4.1 of [I-D.ietf-ace-oscore-profile] hold for this
   specification.

4.2.  RS-to-C: 2.01 (Created)

   The RS MUST verify the validity of the Access Token as defined in
   Section 4.2 of the OSCORE profile of ACE
   [I-D.ietf-ace-oscore-profile], with the following additions.

   o  The RS checks that the OSCORE_Security_Context object in the 'cnf'
      claim of the Access Token includes the 'salt' parameter.

   o  The RS checks that the OSCORE_Security_Context object in the 'cnf'
      claim of the Access Token includes the 'context_id' parameter.
      Also, the RS checks that the value of the 'context_id' parameter
      coincides with the one of the group identifier of the OSCORE group
      associated to the resources targeted by the scope in the Access
      Token.

   o  The RS checks that the 'client_cred' claim is included in the
      Access Token, unless this is intended to update and supersede an
      active Access Token for that same Client.  The RS considers the
      content of the 'client_cred' claim (if present) as the public key
      associated to the signing private key that the Client uses in the
      OSCORE group, which is identified by the 'context_id' parameter
      above.  The RS MAY additionally request the Group Manager of the
      OSCORE group for the public key of that Client, as described in
      [I-D.ietf-ace-key-groupcomm-oscore].  In such a case, the RS MUST
      check that the key retrieved from the Group Manager matches the
      one retrieved from the 'client_cred' claim.

   If any of the checks above fails, the RS MUST consider the Access
   Token non valid, and MUST respond to the Client with an error
   response code equivalent to the CoAP code 4.00 (Bad Request).

   If the Access Token is valid and further checks on its content are
   successful, the RS MUST generate a nonce N2 and include it in the



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   2.01 (Created) response to the Client, as defined in Section 4.2 of
   the OSCORE profile of ACE [I-D.ietf-ace-oscore-profile].

   Further recommendations, considerations and behaviors defined in
   Section 4.2 of [I-D.ietf-ace-oscore-profile] hold for this
   specification.

4.3.  OSCORE Setup - Client Side

   Once having received the 2.01 (Created) response from the RS,
   following the POST request to the authz-info endpoint, the Client
   MUST extract the nonce N2 from the 'cnonce' parameter and the client
   identifier from the 'clientId' parameter (if present) in the CBOR map
   in the payload of the response.

   Note that, if present in the 2.01 (Created) response, the 'clientId'
   parameter supersedes the analogous parameter possibly provided by the
   AS to C in Section 3.2.  Also, note that this identifier is used by C
   as Sender ID in the pairwise OSCORE Security Context to be
   established with the RS, and is different as well as unrelated to the
   Sender ID of C in the OSCORE group.

   Then, the Client performs the following actions, in order to set up
   and fully derive the pairwise OSCORE Security Context for
   communicating with the RS.

   o  The Client MUST set the ID Context of the pairwise OSCORE Security
      Context as the concatenation of: the Group Identifier GID of the
      OSCORE group; the nonce N1; and the nonce N2.  The concatenation
      occurs in this order: ID Context = GID | N1 | N2, where | denotes
      byte string concatenation.

   o  The Client MUST set the Master Salt of the pairwise OSCORE
      Security Context as the concatenation of MSalt, N1, N2 and GMSalt,
      where: i) MSalt is the Sender ID that the Client has in the OSCORE
      group; while ii) GMSalt is the (optional) Master Salt in the Group
      OSCORE Security Context, which is known to the Client as a member
      of the OSCORE group.  The concatenation occurs in this order:
      Master Salt = MSalt | N1 | N2 | GMSalt, where | denotes byte
      string concatenation.

   o  The Client MUST set the Master Secret of the pairwise OSCORE
      Security Context to the concatenation of MSec and GMSec, where: i)
      MSec is the value of the 'ms' parameter in the
      OSCORE_Security_Context object of the 'cnf' parameter, received
      from the AS in Section 3.2; while ii) GMSec is the Master Secret
      of the Group OSCORE Security Context, which is known to the Client
      as a member of the OSCORE group.



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   o  The Client MUST set the Recipient ID as indicated in the
      corresponding parameter received from the AS in Section 3.2, i.e.
      to the value of the 'serverId' parameter in the
      OSCORE_Security_Context object of the 'cnf' parameter.

   o  The Client MUST set the AEAD Algorithm, HKDF, and Replay Window as
      indicated in the corresponding parameters received from the AS in
      Section 3.2, if present in the OSCORE_Security_Context object of
      the 'cnf' parameter.  In case these parameters are omitted, the
      default values are used as described in Section 3.2 of [RFC8613].

   o  The client MUST set the Sender ID as indicated in the 'clientId'
      parameter from the 2.01 (Created) response, if present.
      Otherwise, the Client MUST set the Sender ID as indicated in the
      response from the AS in Section 3.2, i.e. to the value of the
      'clientId' parameter in the OSCORE_Security_Context object of the
      'cnf' parameter.

   Finally, the client MUST derive the complete pairwise OSCORE Security
   Context following Section 3.2.1 of [RFC8613].

   From then on, when communicating with the RS to access the resources
   as specified by the authorization information, the Client MUST use
   the newly established pairwise OSCORE Security Context or the Group
   OSCORE Security Context of the OSCORE Group where both the Client and
   the RS are members.

   If any of the expected parameters is missing (e.g. any of the
   mandatory parameters from the AS, or 'clientId' both from the AS and
   in the 2.01 (Created) response from the RS), the Client MUST stop the
   exchange, and MUST NOT derive the pairwise OSCORE Security Context.
   The Client MAY restart the exchange, to get the correct security
   material.

   Then, the Client uses this pairwise OSCORE Security Context to send
   requests to RS protected with OSCORE.  In the first request sent to
   the RS, the Client MAY include the kid context if the application
   needs to, with value the ID Context, i.e. GID concatenated with N1
   concatenated with N2.  Besides, the Client uses the Group OSCORE
   Security Context for protecting unicast requests to the RS, or
   multicast requests to the OSCORE group including also the RS.

4.4.  OSCORE Setup - Resource Server Side

   After validation of the Access Token as defined in Section 4.2 and
   after sending the 2.01 (Created) response, the RS performs the
   following actions, in order to set up and fully derive the pairwise
   OSCORE Security Context created to communicate with the Client.



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   o  The RS MUST set the ID Context of the pairwise OSCORE Security
      Context as the concatenation of: the Group Identifier GID of the
      OSCORE group; the nonce N1; and the nonce N2.  The concatenation
      occurs in this order: ID Context = GID | N1 | N2, where | denotes
      byte string concatenation.

   o  The RS MUST set the Master Salt of the pairwise OSCORE Security
      Context as the concatenation of MSalt, N1, N2 and GMSalt, where:
      i) MSalt is the Sender ID that the Client has in the OSCORE group,
      as specified in the 'salt' parameter in the
      OSCORE_Security_Context object of the 'cnf' claim, included in the
      Access Token received from the Client (see Section 4.1); while ii)
      GMSalt is the (optional) Master Salt in the Group OSCORE Security
      Context, which is known to the RS as a member of the OSCORE group.
      The concatenation occurs in this order: Master Salt = MSalt | N1 |
      N2 | GMSalt, where | denotes byte string concatenation.

   o  The RS MUST set the Master Secret of the pairwise OSCORE Security
      Context to the concatenation of MSec and GMSec, where: i) MSec is
      the value of the 'ms' parameter in the OSCORE_Security_Context
      object of the 'cnf' claim, included in the Access Token received
      from the Client (see Section 4.1); while ii) GMSec is the Master
      Secret of the Group OSCORE Security Context, which is known to the
      RS as a member of the OSCORE group.

   o  The RS MUST set the Sender ID of the pairwise OSCORE Security
      Context from the corresponding parameter received from the Client
      in the Access Token (see Section 4.1), i.e. to the value of the
      'serverId' parameter in the OSCORE_Security_Context object of the
      'cnf' claim.

   o  The RS MUST set the Recipient ID of the pairwise OSCORE Security
      Context from either what it indicated in the 2.01 (Created)
      response if included (see Section 4.2 of
      [I-D.ietf-ace-oscore-profile]), or from the corresponding
      parameter received from the Client in the Access Token (see
      Section 4.1), i.e. to the value of the 'clientId' parameter in the
      OSCORE_Security_Context object of the 'cnf' claim.

   o  The RS MUST set the AEAD Algorithm, HKDF, and Replay Window from
      the corresponding parameters received from the Client in the
      Access Token (see Section 4.1), if present in the
      OSCORE_Security_Context object of the 'cnf' claim.  In case these
      parameters are omitted, the default values are used as described
      in Section 3.2 of [RFC8613].

   Finally, the RS MUST derive the complete pairwise OSCORE Security
   Context following Section 3.2.1 of [RFC8613].



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   Once having completed the derivation above, the RS MUST associate the
   authorization information from the Access Token with the just
   established pairwise OSCORE Security Context.

   Furthermore, the RS MUST associate the authorization information from
   the Access Token with the tuple (GID, MSalt, PKey), where GID is the
   Group Identifier of the OSCORE Group, while MSalt and PKey are the
   Sender ID and the public key that the Client has in that OSCORE
   group, respectively.  The RS MUST keep this association up-to-date
   over time.

   Then, the RS uses this pairwise OSCORE Security Context to verify
   requests from and send responses to the Client protected with OSCORE,
   when this Security Context is used.  If OSCORE verification fails,
   error responses are used, as specified in Section 8 of [RFC8613].
   Besides, the RS uses the Group OSCORE Security Context to verify
   (multicast) requests from and send responses to the Client protected
   with Group OSCORE.  If Group OSCORE verification fails, error
   responses are used, as specified in Section 6 of
   [I-D.ietf-core-oscore-groupcomm].  Additionally, for every incoming
   request, if OSCORE or Group OSCORE verification succeeds, the
   verification of access rights is performed as described in
   Section 4.5.

   After the expiration of the Access Token related to a pairwise OSCORE
   Security Context and to a Group OSCORE Security Context, the RS MUST
   NOT use the pairwise OSCORE Security Context and MUST respond with an
   unprotected 4.01 (Unauthorized) error message.  Also, if the Client
   uses the Group OSCORE Security Context to send a request for any
   resource intended for OSCORE group members and that requires an
   active Access Token, the RS MUST respond with a 4.01 (Unauthorized)
   error message protected with the Group OSCORE Security Context.

4.5.  Access Rights Verification

   The RS MUST follow the procedures defined in Section 5.8.2 of
   [I-D.ietf-ace-oauth-authz].  If an RS receives an OSCORE-protected
   request from a Client, the RS processes it according to [RFC8613].
   If an RS receives a Group OSCORE-protected request from a Client, the
   RS processes it according to [I-D.ietf-core-oscore-groupcomm].

   If the OSCORE or Group OSCORE verification succeeds, and the target
   resource requires authorization, the RS retrieves the authorization
   information from the Access Token associated to the pairwise OSCORE
   Security Context and to the Group OSCORE Security Context.  Then, the
   RS MUST verify that the authorization information covers the resource
   and the action requested by the Client.




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   The response code MUST be 4.01 (Unauthorized) in case the Client has
   not used either of the two Security Contexts associated with the
   Access Token, or if the RS has no valid Access Token for the Client.
   If the RS has an Access Token for the Client but not for the resource
   that was requested, the RS MUST reject the request with a 4.03
   (Forbidden).  If the RS has an Access Token for the Client but it
   does not cover the action that was requested on the resource, the RS
   MUST reject the request with a 4.05 (Method Not Allowed).

5.  Secure Communication with the AS

   As specified in the ACE framework (Section 5.7 of
   [I-D.ietf-ace-oauth-authz]), the requesting entity (RS and/or Client)
   and the AS communicate via the /introspection or /token endpoint.
   The use of CoAP and OSCORE for this communication is RECOMMENDED in
   this profile.  Other protocols (such as HTTP and DTLS or TLS) MAY be
   used instead.

   If OSCORE is used, the requesting entity and the AS are expected to
   have pre-established security contexts in place.  How these security
   contexts are established is out of the scope of this profile.
   Furthermore the requesting entity and the AS communicate using OSCORE
   ([RFC8613]) through the /introspection endpoint as specified in
   Section 5.7 of [I-D.ietf-ace-oauth-authz], and through the /token
   endpoint as specified in Section 5.6 of [I-D.ietf-ace-oauth-authz].

6.  Discarding the Security Context

   The Client and the RS MUST follow what is defined in Section 6 of
   [I-D.ietf-ace-oscore-profile] about discarding the pairwise OSCORE
   Security Context.

   As members of an OSCORE Group, the Client and the RS may
   independently leave the group or be forced to, e.g. if compromised or
   suspected so.  Upon leaving the OSCORE group, the Client or RS also
   discards the Group OSCORE Security Context, which may anyway be
   renewed by the Group Manager through a group rekeying process (see
   Section 2.1 of [I-D.ietf-core-oscore-groupcomm]).

   The Client or RS can acquire a new Group OSCORE Security Context, by
   re-joining the OSCORE group, e.g. by using the approach defined in
   [I-D.ietf-ace-key-groupcomm-oscore].  In such a case, the Client
   SHOULD request a new Access Token and post it to the RS, in order to
   establish a new pairwise OSCORE Security Context and bind it to the
   Group OSCORE Security Context obtained upon re-joining the group.






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7.  CBOR Mappings

   The new parameters and claims defined in this document MUST be mapped
   to CBOR types as specified in Figure 9, using the given integer
   abbreviation for the map key.

              /--------------------+----------+------------\
              |   Parameter name   | CBOR Key | Value Type |
              |--------------------+----------+------------|
              | context_id         | TBD1     | bstr       |
              | salt               | TBD2     | bstr       |
              | client_cred        | TBD3     | map        |
              | client_cred_verify | TBD4     | bstr       |
              \--------------------+----------+------------/

                Figure 9: CBOR mappings for new parameters.

8.  Security Considerations

   This document specifies a profile for the Authentication and
   Authorization for Constrained Environments (ACE) framework
   [I-D.ietf-ace-oauth-authz].  Thus the general security considerations
   from the ACE framework also apply to this profile.

   This specification inherits the security considerations from the
   OSCORE profile of ACE [I-D.ietf-ace-oscore-profile].  Also, the
   general security considerations about OSCORE [RFC8613] hold for this
   document, as to the specific use of OSCORE according to this profile.

   Furthermore, the general security considerations about Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] hold for this document, as to the
   specific use of Group OSCORE according to this profile.

   Group OSCORE is designed to secure point-to-point as well as point-
   to-multipoint communications, providing a secure binding between a
   single request and multiple corresponding responses.  In particular,
   Group OSCORE fulfills the same security requirements of OSCORE, for
   group requests and responses.  To ensure source authentication of
   messages, Group OSCORE uses digital countersignatures that group
   members embed in their own transmitted messages.

9.  Privacy Considerations

   This document specifies a profile for the Authentication and
   Authorization for Constrained Environments (ACE) framework
   [I-D.ietf-ace-oauth-authz].  Thus the general privacy considerations
   from the ACE framework also apply to this profile.




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   As this profile uses OSCORE and Group OSCORE, the privacy
   considerations from [RFC8613] and [I-D.ietf-core-oscore-groupcomm]
   apply to this document as well.

   This profile also inherits the privacy considerations from the OSCORE
   profile of ACE [I-D.ietf-ace-oscore-profile].

10.  IANA Considerations

   This document has the following actions for IANA.

10.1.  ACE Profile Registry

   IANA is asked to enter the following value into the "ACE Profile"
   Registry defined in Section 8.7 of [I-D.ietf-ace-oauth-authz].

   o  Profile name: coap_group_oscore

   o  Profile Description: Profile to secure communications between
      constrained nodes using the Authentication and Authorization for
      Constrained Environments framework by: i) establishing a Pairwise
      OSCORE Security Context and enabling OSCORE communication between
      two members of an OSCORE group; ii) enabling authentication and
      fine-grained authorization of members of an OSCORE group, that use
      a pre-established Group OSCORE Security Context to communicate
      with Group OSCORE.

   o  Profile ID: TBD (value between 1 and 255)

   o  Change Controller: IESG

   o  Specification Document(s): [[this document]]

10.2.  OAuth Parameters Registry

   IANA is asked to enter the following values into the "OAuth
   Parameters" Registry defined in Section 11.2 of [RFC6749].

   o  Name: "context_id"

   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Reference: Section 3.1.1 of [[this document]]

   o  Name: "salt"




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   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Reference: Section 3.1.2 of [[this document]]

   o  Name: "client_cred"

   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Reference: Section 3.1.3 of [[this document]]

   o  Name: "client_cred_verify"

   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Reference: Section 3.1.4 of [[this document]]

10.3.  OAuth Parameters CBOR Mappings Registry

   IANA is asked to enter the following values into the "OAuth
   Parameters CBOR Mappings" Registry defined in Section 8.9 of
   [I-D.ietf-ace-oauth-authz].

   o  Name: "context_id"

   o  CBOR Key: TBD1

   o  Change Controller: IESG

   o  Reference: Section 3.1.1 of [[this document]]

   o  Name: "salt"

   o  CBOR Key: TBD2

   o  Change Controller: IESG

   o  Reference: Section 3.1.2 of [[this document]]

   o  Name: "client_cred"

   o  CBOR Key: TBD3




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   o  Change Controller: IESG

   o  Reference: Section 3.1.3 of [[this document]]

   o  Name: "client_cred_verify"

   o  CBOR Key: TBD4

   o  Change Controller: IESG

   o  Reference: Section 3.1.4 of [[this document]]

10.4.  CBOR Web Token Claims Registry

   IANA is asked to enter the following values into the "CBOR Web Token
   Claims" Registry defined in Section 9.1 of [RFC8392].

   o  Claim Name: "client_cred"

   o  Claim Description: Client Credential

   o  JWT Claim Name: "N/A"

   o  Claim Key: TBD5

   o  Claim Value Type(s): map

   o  Change Controller: IESG

   o  Specification Document(s): Section 3.2.1 of [[this document]]

11.  References

11.1.  Normative References

   [I-D.dijk-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              dijk-core-groupcomm-bis-01 (work in progress), July 2019.

   [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-11 (work in progress), October 2019.






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   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-02 (work in progress), July 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-25
              (work in progress), October 2019.

   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-05 (work in progress), March 2019.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-08 (work in progress), July 2019.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-05 (work in progress),
              July 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>.

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

   [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>.




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   [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>.

   [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>.

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

11.2.  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-08 (work in progress), April 2019.

   [I-D.ietf-ace-mqtt-tls-profile]
              Sengul, C., Kirby, A., and P. Fremantle, "MQTT-TLS profile
              of ACE", draft-ietf-ace-mqtt-tls-profile-02 (work in
              progress), November 2019.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-33 (work in progress), October
              2019.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", draft-tiloca-
              core-oscore-discovery-03 (work in progress), July 2019.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.





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

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Appendix A.  Profile Requirements

   This appendix lists the specifications on this profile based on the
   requirements of the ACE framework, as requested in Appendix C of
   [I-D.ietf-ace-oauth-authz].

   o  (Optional) discovery process of how the Client finds the right AS
      for an RS it wants to send a request to: Not specified.

   o  Communication protocol the Client and the RS must use: CoAP.

   o  Security protocol(s) the Client and RS must use: OSCORE, i.e.
      establishment of a pairwise OSCORE Security Context and exchange
      of secure messages; and/or Group OSCORE, i.e. exchange of secure
      messages by using a pre-established Group OSCORE Security Context.

   o  How the Client and the RS mutually authenticate: Implicitly by
      possession of a common OSCORE Security Context (when using
      OSCORE); and/or explicitly, by possession of a common Group OSCORE
      Security Context and usage of digital countersignatures (when
      using Group OSCORE).

   o  Content-format of the protocol messages: "application/ace+cbor".

   o  Proof-of-Possession protocol(s) and how to select one; which key
      types (e.g. symmetric/asymmetric) supported: OSCORE algorithms;
      pre-established symmetric keys.

   o  profile identifier: coap_group_oscore

   o  (Optional) how the RS talks to the AS for introspection: HTTP/CoAP
      (+ TLS/DTLS/OSCORE).

   o  How the client talks to the AS for requesting a token: HTTP/CoAP
      (+ TLS/DTLS/OSCORE).



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   o  How/if the authz-info endpoint is protected: Security protocol
      above.

   o  (Optional) other methods of token transport than the authz-info
      endpoint: no.

Acknowledgments

   The authors sincerely thank Dave Robin, Jim Schaad and Goeran
   Selander for their comments and feedback.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC.

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: marco.tiloca@ri.se


   Rikard Hoeglund
   RISE AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: rikard.hoglund@ri.se


   Ludwig Seitz
   RISE AB
   Scheelevagen 17
   Lund  SE-22370 Lund
   Sweden

   Email: ludwig.seitz@ri.se










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   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   Kista  SE-16440 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com












































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