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ACE Working Group                                              M. Tiloca
Internet-Draft                                               R. Hoeglund
Intended status: Standards Track                                 RISE AB
Expires: August 26, 2021                                        L. Seitz
                                                               Combitech
                                                            F. Palombini
                                                             Ericsson AB
                                                       February 22, 2021


    Group OSCORE Profile of the Authentication and Authorization for
                   Constrained Environments Framework
                draft-tiloca-ace-group-oscore-profile-05

Abstract

   This document specifies a profile for the Authentication and
   Authorization for Constrained Environments (ACE) framework.  The
   profile uses Group OSCORE to provide communication security between a
   Client and a (set of) Resource Server(s) as members of an OSCORE
   Group.  The profile securely binds an OAuth 2.0 Access Token with the
   public key of the Client associated to the signing private key used
   in the OSCORE group.  The profile uses Group OSCORE to achieve server
   authentication, as well as proof-of-possession for the Client's
   public key.  Also, it provides proof of the Client's membership to
   the correct OSCORE group, by binding the Access Token to information
   from the Group OSCORE Security Context, thus allowing the Resource
   Server(s) to verify the Client's membership upon receiving a message
   protected with Group OSCORE from the Client.

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 August 26, 2021.





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Copyright Notice

   Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Access Token Retrieval  . . . . . . . . . . . . . . . . .   9
     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 . . . . . . . . . . . . .  10
       3.1.1.  'context_id' Parameter  . . . . . . . . . . . . . . .  12
       3.1.2.  'salt_input' Parameter  . . . . . . . . . . . . . . .  12
       3.1.3.  'client_cred_verify' Parameter  . . . . . . . . . . .  13
     3.2.  AS-to-C: Access Token . . . . . . . . . . . . . . . . . .  13
       3.2.1.  Salt Input Claim  . . . . . . . . . . . . . . . . . .  16
       3.2.2.  Context ID Input Claim  . . . . . . . . . . . . . . .  17
   4.  Client-RS Communication . . . . . . . . . . . . . . . . . . .  17
     4.1.  C-to-RS POST to authz-info Endpoint . . . . . . . . . . .  18
     4.2.  RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . . . .  18
     4.3.  Client-RS Secure Communication  . . . . . . . . . . . . .  19
       4.3.1.  Client Side . . . . . . . . . . . . . . . . . . . . .  19
       4.3.2.  Resource Server Side  . . . . . . . . . . . . . . . .  20
     4.4.  Access Rights Verification  . . . . . . . . . . . . . . .  20
     4.5.  Change of Client's Public Key in the Group  . . . . . . .  21
   5.  Secure Communication with the AS  . . . . . . . . . . . . . .  21
   6.  Discarding the Security Context . . . . . . . . . . . . . . .  22
   7.  CBOR Mappings . . . . . . . . . . . . . . . . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     10.1.  ACE Profile Registry . . . . . . . . . . . . . . . . . .  24
     10.2.  OAuth Parameters Registry  . . . . . . . . . . . . . . .  25



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     10.3.  OAuth Parameters CBOR Mappings Registry  . . . . . . . .  25
     10.4.  CBOR Web Token Claims Registry . . . . . . . . . . . . .  26
     10.5.  TLS Exporter Label Registry  . . . . . . . . . . . . . .  27
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     11.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  Dual Mode (Group OSCORE & OSCORE)  . . . . . . . . .  31
     A.1.  Protocol Overview . . . . . . . . . . . . . . . . . . . .  32
       A.1.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . .  34
       A.1.2.  Access Token Posting  . . . . . . . . . . . . . . . .  34
       A.1.3.  Setup of the Pairwise OSCORE Security Context . . . .  34
       A.1.4.  Secure Communication  . . . . . . . . . . . . . . . .  35
     A.2.  Client-AS Communication . . . . . . . . . . . . . . . . .  36
       A.2.1.  C-to-AS: POST to Token Endpoint . . . . . . . . . . .  36
       A.2.2.  AS-to-C: Access Token . . . . . . . . . . . . . . . .  39
     A.3.  Client-RS Communication . . . . . . . . . . . . . . . . .  45
       A.3.1.  C-to-RS POST to authz-info Endpoint . . . . . . . . .  46
       A.3.2.  RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . .  47
       A.3.3.  OSCORE Setup - Client Side  . . . . . . . . . . . . .  48
       A.3.4.  OSCORE Setup - Resource Server Side . . . . . . . . .  51
       A.3.5.  Access Rights Verification  . . . . . . . . . . . . .  53
       A.3.6.  Change of Client's Public Key in the Group  . . . . .  53
     A.4.  Secure Communication with the AS  . . . . . . . . . . . .  54
     A.5.  Discarding the Security Context . . . . . . . . . . . . .  54
     A.6.  CBOR Mappings . . . . . . . . . . . . . . . . . . . . . .  55
     A.7.  Security Considerations . . . . . . . . . . . . . . . . .  55
     A.8.  Privacy Considerations  . . . . . . . . . . . . . . . . .  55
   Appendix B.  Profile Requirements . . . . . . . . . . . . . . . .  56
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  57
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  57

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 of such applications, it may be just fine to enforce access
   control in a straightforward fashion.  That is, any Client authorized
   to join the group, hence to get the group key material, can be also
   implicitly authorized to perform any action at any resource of any
   Server in the group.  An example of application where such implicit
   authorization might be used is a simple 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



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   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.
   That is, access control to the secure group communication channel and
   access control to the resource space provided by servers in the group
   should remain logically separated domains.  For instance, a more
   fine-grained 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; while 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.  Also, 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, especially when attempting to perform non-safe operations.

   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.

   In the cases above, being a legitimate group member and owning the
   group key material is not supposed to imply any particular access
   rights.  Also, introducing a different security group for each
   different set of access rights would result in additional key
   material to distribute and manage.  In particular, if the access
   rights for a single node change, this would require to evict that



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   node from the current group, followed by that node joining a
   different group aligned with its new access rights.  Moreover, the
   key material of both groups would have to be renewed for their
   current members.  Overall, this would have a non negligible impact on
   operations and performance in the system.

   A fine-grained access control model can be rather enforced within a
   same group, 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] or CoAP over IP multicast [I-D.ietf-core-groupcomm-bis] to
   communicate to one or multiple Resource Servers, which are members of
   an application group and share a common set of resources.  This
   profile uses Group OSCORE [I-D.ietf-core-oscore-groupcomm] as the
   security protocol to protect messages exchanged between the Client
   and the Resource Servers.  Hence, it requires that both the Client
   and the Resource Servers have previously joined the same OSCORE
   group.

   That is, this profile describes how access control is enforced for a
   Client after it has joined an OSCORE group, to access resources at
   other members in that group.  The process for joining the OSCORE
   group through the respective Group Manager as defined in
   [I-D.ietf-ace-key-groupcomm-oscore] takes place before the process
   described in this document, and is out of the scope of this profile.

   The Client proves its access to be authorized to the Resource Server
   by using an Access Token, which is bound to a key (the proof-of-
   possession key).  This profile uses Group OSCORE to achieve server
   authentication, as well as proof-of-possession for the Client's
   public key associated to the signing private key used in an OSCORE
   group.  Note that the proof of possession is not done by a dedicated
   protocol element, but rather occurs after the first Group OSCORE
   exchange.  Furthermore, this profile provides proof of the Client's
   membership to the correct OSCORE group, by binding the Access Token
   to the Client's public key and information from the pre-established
   Group OSCORE Security Context, thus allowing the Resource Server to



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   verify this upon reception of a messages protected with Group OSCORE
   from the Client.

   OSCORE [RFC8613] specifies how to use COSE
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] to
   secure CoAP messages.  Group OSCORE builds on OSCORE to provide
   secure group communication, and ensures source authentication either:
   by means of digital counter signatures embedded in protected messages
   (in group mode); by protecting messages with pairwise key material
   derived from the asymmetric keys of the two peers exchanging the
   message (in pairwise mode).

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 CBOR [RFC8949], COSE
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs],
   CoAP [RFC7252], OSCORE [RFC8613] and 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.

   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).

   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 of this profile, i.e. on how to use
   the ACE framework for authentication and authorization
   [I-D.ietf-ace-oauth-authz] to secure communications between a Client




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   and a (set of) Resource Server(s) using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

   Note that this profile of ACE describes how access control can be
   enforced for a node after it has joined an OSCORE group, to access
   resources at other members in that group.

   In particular, the process for joining the OSCORE group through the
   respective Group Manager as defined in
   [I-D.ietf-ace-key-groupcomm-oscore] must take place before the
   process described in this document, and is out of the scope of this
   profile.

   An overview of the protocol flow for this profile is shown in
   Figure 1.  In the figure, it is assumed that both RS1 and RS2 are
   associated with the same AS.  It is also 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.  The names of messages coincide with those of
   [I-D.ietf-ace-oauth-authz] when applicable.

C                            RS1          RS2                         AS
| [--- Resource Request --->] |            |                           |
|                             |            |                           |
| [<----  AS Request ------]  |            |                           |
|       Creation Hints        |            |                           |
|                             |            |                           |
|-------- POST /token ------------------------------------------------>|
|  (aud: RS1, sid: 0, gid: abcd0000, ...)  |                           |
|                             |            |                           |
|<--------------------------------- Access Token + RS Information -----|
|                             | (aud: RS1, sid: 0, gid: abcd0000, ...) |
|                             |            |                           |
|---- POST /authz-info ------>|            |                           |
|      (access_token)         |            |                           |
|                             |            |                           |
|<--- 2.01 Created -----------|            |                           |
|                             |            |                           |
|-------- POST /token ------------------------------------------------>|
|  (aud: RS2, sid: 0, gid: abcd0000, ...)  |                           |
|                             |            |                           |
|<--------------------------------- Access Token + RS Information -----|
|                             | (aud: RS2, sid: 0, gid: abcd0000, ...) |
|                             |            |                           |
|----- POST /authz-info ------------------>|                           |
|       (access_token)        |            |                           |
|                             |            |                           |
|<--- 2.01 Created ------------------------|                           |



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|                             |            |                           |
|-- Group OSCORE Request -+-->|            |                           |
| (kid: 0, gid: abcd0000)  \-------------->|                           |
|                             |            |                           |
|                          /proof-of-possession/                       |
|                             |            |                           |
|<--- Group OSCORE Response --|            |                           |
|          (kid: 1)           |            |                           |
|                             |            |                           |
/proof-of-possession/         |            |                           |
|                             |            |                           |
/Mutual authentication        |            |                           |
 between C and RS1/           |            |                           |
|                             |            |                           |
|                             |            |                           |
|<--- Group OSCORE Response ---------------|                           |
|          (kid: 2)           |            |                           |
|                             |            |                           |
/proof-of-possession/         |            |                           |
|                             |            |                           |
/Mutual authentication        |            |                           |
 between C and RS2/           |            |                           |
|                             |            |                           |
|             ...             |            |                           |

                       Figure 1: Protocol Overview.

2.1.  Pre-Conditions

   Using Group OSCORE and this profile requires both the Client and the
   Resource Servers to have previously joined the same 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.

   After the Client and Resource Servers have joined the group, this
   profile provides access control for accessing resources on those
   Resource Servers, by securely communicating with Group OSCORE.

   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].




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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 5.8 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 specify these pieces of information in the Access
   Token, included 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.  The AS MUST
   include also the public key indicated by the Client in the Access
   Token.

   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, to reduce the number of
   libraries the client has to support.  Other protocols fulfilling the
   security requirements defined in Section 5 of
   [I-D.ietf-ace-oauth-authz] (such as TLS [RFC8446] or DTLS
   [RFC6347][I-D.ietf-tls-dtls13]) MAY be used additionally or instead.

2.3.  Access Token Posting

   After having retrieved the Access Token from the AS, the Client posts
   the Access Token to the RS, using the /authz-info endpoint and
   mechanisms specified in Section 5.10 of [I-D.ietf-ace-oauth-authz],
   as well as Content-Format = application/ace+cbor.  When using this
   profile, the communication with the /authz-info endpoint is not
   protected.

   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.  Also, the RS associates the received Access Token with the
   Group OSCORE Security Context identified by the Group Identifier
   specified in the Access Token, following Section 3.2 of [RFC8613].
   In practice, the RS maintains a collection of Security Contexts with
   associated authorization information, for all the clients that it is
   currently communicating with, and the authorization information is a
   policy used as input when processing requests from those clients.



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   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.  This binds the Access Token with the Group
   OSCORE Security Context of the OSCORE group.

   Finally, when the Client communicates with the RS using the Group
   OSCORE Security Context, the RS verifies that the Client is a
   legitimate member of the OSCORE group and especially 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 counter signature computed also over the Client's Sender ID
   included in the message.

2.4.  Secure Communication

   The Client can send a request protected with Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] to the RS.  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.

   The Access Token MUST be bound to the public key of the client as
   proof-of-possession key (pop-key), by means of the 'cnf' claim.

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

   The Client-to-AS request is specified in Section 5.8.1 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.





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   o  'context_id', defined in Section 3.1.1 of this specification.
      This parameter specifies the Group Identifier (GID), i.e. the Id
      Context of an OSCORE group where the Client and the RS are
      currently members.  In particular, the Client wishes to
      communicate with the RS using the Group OSCORE Security Context
      associated to that OSCORE group.

   o  'salt_input', defined in Section 3.1.2 of this specification.
      This parameter includes the Sender ID that the Client has in the
      OSCORE group whose GID is specified in the 'context_id' parameter
      above.

   o  'req_cnf', defined in Section 3.1 of [I-D.ietf-ace-oauth-params].
      This parameter includes the public key associated to the signing
      private key that the Client uses in the OSCORE group whose GID is
      specified in the 'context_id' parameter above.  This public key
      will be used as the pop-key bound to the Access Token.

   o  'client_cred_verify', defined in Section 3.1.3 of this
      specification.  This parameter includes a signature computed by
      the Client, by using the private key associated to the public key
      in the 'req_cnf' parameter above.  This allows the AS to verify
      that the Client indeed owns the private key associated to that
      public key, 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.

      *  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].  In particular, the exporter label
         MUST be 'EXPORTER-ACE-Sign-Challenge-Client-AS' defined in
         Section 10.5 of this specification, together with an empty
         'context_value', and 32 bytes as 'key_length'.

      *  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 [I-D.ietf-cose-rfc8152bis-algs].
         HKDF SHA-256 is mandatory to implement.



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   An example of such a request, with payload 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",
          "context_id" : h'abcd0000',
          "salt_input" : h'00',
          "req_cnf" : {
            "COSE_Key" : {
              "kty" : EC2,
              "crv" : P-256,
              "x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
                      27c9e354089bbe13',
              "y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
                      731e79a3b4e47120'
            }
          },
          "client_cred_verify" : h'...'
          (signature content omitted for brevity)
        }

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

3.1.1.  'context_id' Parameter

   The 'context_id' parameter is an OPTIONAL parameter of the Access
   Token request message defined in Section 5.8.1. of
   [I-D.ietf-ace-oauth-authz].  This parameter provides a value that the
   Client wishes to use with the RS as a hint for a security context.
   Its exact content is profile specific.

3.1.2.  'salt_input' Parameter

   The 'salt_input' parameter is an OPTIONAL parameter of the Access
   Token request message defined in Section 5.8.1. of
   [I-D.ietf-ace-oauth-authz].  This parameter provides a value that the
   Client wishes to use as part of a salt with the RS, for deriving
   cryptographic key material.  Its exact content is profile specific.





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3.1.3.  'client_cred_verify' Parameter

   The 'client_cred_verify' parameter is an OPTIONAL parameter of the
   Access Token request message defined in Section 5.8.1. 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 'req_cnf' parameter.  If the verification fails, the AS considers
   the Client request invalid.

   If all verifications are successful, the AS responds as defined in
   Section 5.8.2 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 5.8.3 of [I-D.ietf-ace-oauth-authz].

   The AS can signal that the use of 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.  The Client
   MUST use Group OSCORE towards all the Resource Servers for which this
   Access Token is valid.  Usually, it is assumed that constrained
   devices will be pre-configured with the necessary profile, so that
   this kind of profile signaling can be omitted.

   The AS MUST include the following information as metadata of the
   issued Access Token.  The use of CBOR web tokens (CWT) as specified
   in [RFC8392] is RECOMMENDED.

   o  The same parameter 'profile' included in the Token Response to the
      Client.

   o  The salt input specified in the 'salt_input' parameter of the
      Token Request.  If the Access Token is a CWT, the content of the
      'salt_input' parameter MUST be placed in the 'salt_input' claim of
      the Access Token, defined in Section 3.2.1 of this specification.

   o  The Context Id input specified in the 'context_id' parameter of
      the Token Request.  If the Access Token is a CWT, the content of
      the 'context_id' parameter MUST be placed in the 'contextId_input'
      claim of the Access Token, defined in Section 3.2.2 of this
      specification.




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   o  The public key that the client uses in the OSCORE group and
      specified in the 'req_cnf' parameter of the Token request.  If the
      Access Token is a CWT, the public key MUST be specified in the
      'cnf' claim, which follows the syntax from Section 3.1 of
      [RFC8747] 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.

   Figure 3 shows an example of such an AS response, with payload in
   CBOR diagnostic notation without the tag and value abbreviations.

        Header: Created (Code=2.01)
        Content-Type: "application/ace+cbor"
        Payload:
        {
          "access_token" : h'8343a1010aa2044c53 ...'
           (remainder of CWT omitted for brevity),
          "profile" : "coap_group_oscore",
          "expires_in" : 3600
        }

   Figure 3: Example AS-to-C Access Token response with the Group OSCORE
                                 profile.

   Figure 4 shows an example CWT Claims Set, containing the Client's
   public key in the group (as pop-key) 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" : {
            "COSE_Key" : {
              "kty" : EC2,
              "crv" : P-256,
              "x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
                      27c9e354089bbe13',
              "y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
                      731e79a3b4e47120'
          },
          "salt_input" : h'00',
          "contextId_input" : h'abcd0000'
        }

       Figure 4: Example CWT Claims Set with OSCORE parameters (CBOR
                           diagnostic notation).

   The same CWT Claims Set as in Figure 4 and encoded in CBOR is shown
   in Figure 5, using the value abbreviations defined in
   [I-D.ietf-ace-oauth-authz] and [RFC8747].  The bytes in hexadecimal
   are reported in the first column, while their corresponding CBOR
   meaning is reported after the '#' sign on the second column, for
   easiness of readability.

   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.




















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   A7                                      # map(7)
      03                                   # unsigned(3)
      76                                   # text(22)
         74656D7053656E736F72496E4C6976696E67526F6F6D
      06                                   # unsigned(6)
      1A 5112D728                          # unsigned(1360189224)
      04                                   # unsigned(4)
      1A 51145DC8                          # unsigned(1360289224)
      09                                   # unsigned(9)
      78 18                                # text(24)
         74656D70657261747572655F67206669726D776172655F70
      08                                   # unsigned(8)
      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
      18 3C                                # unsigned(60)
      41                                   # bytes(1)
         00
      18 3D                                # unsigned(61)
      44                                   # bytes(4)
         ABCD0000

       Figure 5: Example CWT Claims Set with OSCORE parameters, CBOR
                                 encoded.

3.2.1.  Salt Input Claim

   The 'salt_input' claim provides a value that the Client requesting
   the Access Token wishes to use as a part of a salt with the RS, e.g.
   for deriving cryptographic material.

   This parameter specifies the value of the salt input, encoded as a
   CBOR byte string.






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3.2.2.  Context ID Input Claim

   The 'contextId_input' claim provides a value that the Client
   requesting the Access Token wishes to use with the RS, as a hint for
   a security context.

   This parameter specifies the value of the Context ID input, encoded
   as a CBOR byte string.

4.  Client-RS Communication

   This section details the POST request and response to the /authz-info
   endpoint between the Client and the RS.

   The proof-of-possession required to bind the Access Token to the
   Client is explicitly performed when the RS receives and verifies a
   request from the Client protected with Group OSCORE, either with the
   group mode (see Section 8 of [I-D.ietf-core-oscore-groupcomm]) or
   with the pairwise mode (see Section 9 of
   [I-D.ietf-core-oscore-groupcomm]).

   In particular, the RS uses the Client's public key bound to the
   Access Token, either when verifying the counter signature of the
   request (if protected with the group mode), or when verifying the
   request as integrity-protected with pairwise key material derived
   from the two peers' asymmetric keys (if protected with the pairwise
   mode).  In either case, the RS also authenticates the Client.

   Similarly, when receiving a protected response from the RS, the
   Client uses the RS's public key either when verifying the counter
   signature of the response (if protected with the group mode), or when
   verifying the response as integrity-protected with pairwise key
   material derived from the two peers' asymmetric keys (if protected
   with the pairwise mode).  In either case, the Client also
   authenticates the RS.  Mutual authentication is only achieved after
   the client has successfully verified the Group OSCORE protected
   response from the RS.

   Therefore, an attacker using a stolen Access Token cannot generate a
   valid Group OSCORE message signed with the Client's private key, and
   thus cannot prove possession of the pop-key bound to the Access
   Token.  Also, if a Client legitimately owns an Access Token but has
   not joined the OSCORE group, it cannot generate a valid Group OSCORE
   message, as it does not own the necessary key material shared among
   the group members.

   Furthermore, a Client C1 is supposed to obtain a valid Access Token
   from the AS, as including the public key associated to its own



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   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) attempting to communicate using Group OSCORE impersonating
   C1, while blaming C1 for the consequences.

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

   The Client posts the Access Token to the /authz-info endpoint of the
   RS, as defined in Section 5.10.1 of [I-D.ietf-ace-oauth-authz].

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

   The RS MUST verify the validity of the Access Token as defined in
   Section 5.10.1 of [I-D.ietf-ace-oauth-authz], with the following
   additions.

   o  The RS checks that the claims 'salt_input', 'contextId_input' and
      'cnf' are included in the Access Token.

   o  The RS considers the content of the 'cnf' claim as the public key
      associated to the signing private key of the Client in the OSCORE
      group, whose GID is specified in the 'contextId_input' claim.

      If it does not already store that public key, the RS MUST request
      it to the Group Manager of the OSCORE group as described in
      [I-D.ietf-ace-key-groupcomm-oscore], specifying the Sender ID of
      that Client in the OSCORE group, i.e. the value of the
      'salt_input' claim above.  The RS MUST check that the key
      retrieved from the Group Manager matches the one retrieved from
      the 'cnf' claim.  When doing so, the 'kid' parameter of the
      COSE_Key, if present, MUST NOT be considered for the comparison.

   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 associates the authorization information from the
   Access Token with the Group OSCORE Security Context.




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   In particular, the RS associates the authorization information from
   the Access Token with the 3-tuple (GID, SaltInput, PubKey), where GID
   is the Group Identifier of the OSCORE Group, while SaltInput and
   PubKey are the Sender ID and the public key that the Client uses in
   that OSCORE group, respectively.  These can be retrieved from the
   'contextId_input', 'salt_input' and 'cnf' claims of the Access Token,
   respectively.

   The RS MUST keep this association up-to-date over time, as the
   3-tuple (GID, SaltInput, PubKey) associated to the Access Token might
   change.  In particular:

   o  If the OSCORE group is rekeyed (see Section 3.1 of
      [I-D.ietf-core-oscore-groupcomm] and Section 18 of
      [I-D.ietf-ace-key-groupcomm-oscore]), the Group Identifier also
      changes in the group, and the new one replaces the current 'GID'
      value in the 3-tuple.

   o  If the Client requests and obtains a new OSCORE Sender ID from the
      Group Manager (see Section 3.1 of [I-D.ietf-core-oscore-groupcomm]
      and Section 9 of [I-D.ietf-ace-key-groupcomm-oscore]), the new
      Sender ID replaces the current 'SaltInput' value in the 3-tuple.

   Finally, the RS MUST send a 2.01 (Created) response to the Client, as
   defined in Section 5.10.1 of [I-D.ietf-ace-oauth-authz].

4.3.  Client-RS Secure Communication

   When previously joining the OSCORE group, both the Client and RS have
   already established the related Group OSCORE Security Context to
   communicate as group members.  Therefore, they can simply start to
   securely communicate using Group OSCORE, without deriving any
   additional key material or security association.

4.3.1.  Client Side

   After having received the 2.01 (Created) response from the RS,
   following the POST request to the authz-info endpoint, the Client
   starts the communication with the RS, by sending a request protected
   with Group OSCORE using the Group OSCORE Security Context
   [I-D.ietf-core-oscore-groupcomm].

   When communicating with the RS to access the resources as specified
   by the authorization information, the Client MUST use the Group
   OSCORE Security Context of the OSCORE group, whose GID was specified
   in the 'context_id' parameter of the Token request.





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4.3.2.  Resource Server Side

   After successful validation of the Access Token as defined in
   Section 4.2 and after having sent the 2.01 (Created) response, the RS
   can start to communicate with the Client using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

   When processing an incoming request protected with Group OSCORE, the
   RS MUST consider as valid public key of the Client only the public
   key specified in the stored Access Token.  As defined in Section 4.5,
   a possible change of public key requires the Client to upload to the
   RS a new Access Token bound to the new public key.

   Additionally, for every incoming request, if Group OSCORE
   verification succeeds, the verification of access rights is performed
   as described in Section 4.4.

   After the expiration of the Access Token related to a Group OSCORE
   Security Context, 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.

4.4.  Access Rights Verification

   The RS MUST follow the procedures defined in Section 5.10.2 of
   [I-D.ietf-ace-oauth-authz].  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 Group OSCORE verification succeeds, and the target resource
   requires authorization, the RS retrieves the authorization
   information from the Access Token associated to the Group OSCORE
   Security Context.  Then, the RS MUST verify that the action requested
   on the resource is authorized.

   The response code MUST be 4.01 (Unauthorized) if the RS has no valid
   Access Token for the Client.  If the RS has an Access Token for the
   Client but no actions are authorized on the target resource, the RS
   MUST reject the request with a 4.03 (Forbidden).  If the RS has an
   Access Token for the Client but the requested action is not
   authorized, the RS MUST reject the request with a 4.05 (Method Not
   Allowed).







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4.5.  Change of Client's Public Key in the Group

   During its membership in the OSCORE group, the client might change
   the public key it uses in the group.  When this happens, the Client
   uploads the new public key to the Group Manager, as defined in
   Section 11 of [I-D.ietf-ace-key-groupcomm-oscore].

   After that, and in order to continue communicating with the RS, the
   Client MUST perform the following actions.

   1.  The Client requests a new Access Token to the AS, as defined in
       Section 3.  In particular, when sending the POST request as
       defined in Section 3.1, the Client indicates:

       *  The current Group Identifier of the OSCORE group, as value of
          the 'context_id' parameter.

       *  The current Sender ID it has in the OSCORE group, as value of
          the 'salt_input' parameter.

       *  The new public key it uses in the OSCORE group, as value of
          the 'req_cnf' parameter.

       *  The proof-of-possession signature corresponding to the new
          public key, as value of the 'client_cred_verify' parameter.

   2.  After receiving the response from the AS (see Section 3.2), the
       Client performs the same exchanges with the RS as defined in
       Section 4.

   When receiving the new Access Token, the RS performs the same steps
   defined in Section 4.2, with the following addition, in case the new
   Access Token is successfully verified and stored.  The RS also
   deletes the old Access Token, i.e. the one whose associated 3-tuple
   has the same GID and SaltInput values as in the 3-tuple including the
   new public key of the Client and associated to the new Access Token.

5.  Secure Communication with the AS

   As specified in the ACE framework (Sections 5.8 and 5.9 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 [RFC8613] for this communication is
   RECOMMENDED in this profile.  Other protocols fulfilling the security
   requirements defined in Section 5 of [I-D.ietf-ace-oauth-authz] (such
   as HTTP and DTLS or TLS) MAY be used instead.





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   If OSCORE [RFC8613] is used, the requesting entity and the AS are
   expected to have a pre-established Security Context in place.  How
   this Security Context is established is out of the scope of this
   profile.  Furthermore, the requesting entity and the AS communicate
   using OSCORE through the /introspection endpoint as specified in
   Section 5.9 of [I-D.ietf-ace-oauth-authz], and through the /token
   endpoint as specified in Section 5.8 of [I-D.ietf-ace-oauth-authz].

6.  Discarding the 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 3.1 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.

7.  CBOR Mappings

   The new parameters defined in this document MUST be mapped to CBOR
   types as specified in Figure 6, using the given integer abbreviation
   for the map key.

              /--------------------+----------+------------\
              | Parameter name     | CBOR Key | Value Type |
              |--------------------+----------+------------|
              | context_id         | TBD1     | bstr       |
              | salt_input         | TBD2     | bstr       |
              | client_cred_verify | TBD3     | bstr       |
              \--------------------+----------+------------/

                Figure 6: CBOR mappings for new parameters.

   The new claims defined in this document MUST be mapped to CBOR types
   as specified in Figure 7, using the given integer abbreviation for
   the map key.










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                /-----------------+----------+------------\
                | Claim name      | CBOR Key | Value Type |
                |-----------------+----------+------------|
                | salt_input      | TBD4     | bstr       |
                | contextId_input | TBD5     | bstr       |
                \-----------------+----------+------------/

                  Figure 7: CBOR mappings for new claims.

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 general security considerations about
   Group OSCORE [I-D.ietf-core-oscore-groupcomm], 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.

   Group OSCORE ensures source authentication of messages both in group
   mode (see Section 8 of [I-D.ietf-core-oscore-groupcomm]) and in
   pairwise mode (see Section 9 of [I-D.ietf-core-oscore-groupcomm]).

   When protecting an outgoing message in group mode, the sender uses
   its private key to compute a digital counter signature, which is
   embedded in the protected message.  The group mode can be used to
   protect messages sent over multicast to multiple recipients, or sent
   over unicast to one recipient.

   When protecting an outgoing message in pairwise mode, the sender uses
   a pairwise symmetric key, as derived from the asymmetric keys of the
   two peers exchanging the message.  The pairwise mode can be used to
   protect only messages sent over unicast to one recipient.

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

   An unprotected response to an unauthorized request may disclose
   information about the RS and/or its existing relationship with the
   Client.  It is advisable to include as little information as possible
   in an unencrypted response.  However, since both the Client and the
   RS share a Group OSCORE Security Context, unauthorized, yet protected
   requests are followed by protected responses, which can thus include
   more detailed information.

   Although it may be encrypted, the Access Token is sent in the clear
   to the /authz-info endpoint at the RS.  Thus, if the Client uses the
   same single Access Token from multiple locations with multiple
   Resource Servers, it can risk being tracked through the Access
   Token's value.

   Note that, even though communications are protected with Group
   OSCORE, some information might still leak, due to the observable
   size, source address and destination address of exchanged messages.

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 8.8 of [I-D.ietf-ace-oauth-authz].

   o  Name: coap_group_oscore

   o  Description: Profile to secure communications between constrained
      nodes using the Authentication and Authorization for Constrained
      Environments framework, by 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.  Optionally, the dual mode defined in Appendix A
      additionally establishes a pairwise OSCORE Security Context, and
      thus also enables OSCORE communication between two members of the
      OSCORE group.

   o  CBOR Value: TBD (value between 1 and 255)

   o  Reference: [[this document]]






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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  Specification Document(s): Section 3.1.1 of [[this document]]


   o  Name: "salt_input"

   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Specification Document(s): Section 3.1.2 of [[this document]]


   o  Name: "client_cred_verify"

   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Specification Document(s): Section 3.1.3 of [[this document]]


   o  Name: "client_cred"

   o  Parameter Usage Location: token request

   o  Change Controller: IESG

   o  Specification Document(s): Appendix A.2.1.1 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 8.10 of
   [I-D.ietf-ace-oauth-authz].

   o  Name: "context_id"




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   o  CBOR Key: TBD1

   o  Value Type: bstr

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


   o  Name: "salt_input"

   o  CBOR Key: TBD2

   o  Value Type: bstr

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


   o  Name: "client_cred_verify"

   o  CBOR Key: TBD3

   o  Value Type: bstr

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


   o  Name: "client_cred"

   o  CBOR Key: TBD6

   o  Value Type: bstr

   o  Reference: Appendix A.2.1.1 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: "salt_input"

   o  Claim Description: Client provided salt input

   o  JWT Claim Name: "N/A"

   o  Claim Key: TBD4

   o  Claim Value Type(s): bstr




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   o  Change Controller: IESG

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


   o  Claim Name: "contextId_input"

   o  Claim Description: Client context id input

   o  JWT Claim Name: "N/A"

   o  Claim Key: TBD5

   o  Claim Value Type(s): bstr

   o  Change Controller: IESG

   o  Specification Document(s): Section 3.2.2 of [[this document]]


   o  Claim Name: "client_cred"

   o  Claim Description: Client Credential

   o  JWT Claim Name: "N/A"

   o  Claim Key: TBD7

   o  Claim Value Type(s): map

   o  Change Controller: IESG

   o  Specification Document(s): Appendix A.2.2.2 of [[this document]]

10.5.  TLS Exporter Label Registry

   IANA is asked to register the following entry in the "TLS Exporter
   Label" Registry defined in Section 6 of [RFC5705] and updated in
   Section 12 of [RFC8447].

   o  Value: EXPORTER-ACE-Sign-Challenge-Client-AS

   o  DTLS-OK: Y

   o  Recommended: N

   o  Reference: [[this document]] (Section 3.1)




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11.  References

11.1.  Normative References

   [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-10 (work in progress), February 2021.

   [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-37
              (work in progress), February 2021.

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

   [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-16 (work in progress), January 2021.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              ietf-core-groupcomm-bis-03 (work in progress), February
              2021.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J., and
              J. Park, "Group OSCORE - Secure Group Communication for
              CoAP", draft-ietf-core-oscore-groupcomm-11 (work in
              progress), February 2021.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
              (work in progress), September 2020.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", draft-ietf-cose-rfc8152bis-
              struct-15 (work in progress), February 2021.



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   [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>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [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>.

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
              <https://www.rfc-editor.org/info/rfc6920>.

   [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>.

   [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>.

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/info/rfc8447>.

   [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>.







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

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

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-15 (work in progress), January 2021.

   [I-D.ietf-ace-mqtt-tls-profile]
              Sengul, C. and A. Kirby, "Message Queuing Telemetry
              Transport (MQTT)-TLS profile of Authentication and
              Authorization for Constrained Environments (ACE)
              Framework", draft-ietf-ace-mqtt-tls-profile-10 (work in
              progress), December 2020.

   [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-dtls13-41 (work in progress),
              February 2021.

   [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-08 (work in progress), February
              2021.

   [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>.

   [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>.






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Appendix A.  Dual Mode (Group OSCORE & OSCORE)

   This appendix defines the dual mode of this profile, which allows
   using both OSCORE [RFC8613] and Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] as security protocols, by still
   relying on a single Access Token.

   That is, the dual mode of this profile specifies how a Client uses
   CoAP [RFC7252] to communicate to a single Resource Server, or CoAP
   over IP multicast [I-D.ietf-core-groupcomm-bis] to communicate to
   multiple Resource Servers that are members of a group and share a
   common set of resources.

   In particular, the dual mode of this profile uses two complementary
   security protocols to provide secure communication between the Client
   and the Resource Server(s).  That is, it defines the use of either
   OSCORE or Group OSCORE 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 requests sent
   to a group of Resource Servers, as well as possible individual
   responses.  Like in the main mode of this profile, the Client and the
   Resource Servers need to have already joined the same 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 proves its access to be authorized to the Resource Server
   by using an Access Token, which is bound to a key (the proof-of-
   possession key).  This profile mode uses OSCORE to achieve proof of
   possession, and OSCORE or Group OSCORE to achieve server
   authentication.

   Unlike in the main mode of this profile, where a public key is used
   as pop-key, this dual mode uses OSCORE-related, symmetric key
   material as pop-key instead.  Furthermore, this dual mode 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.

   In addition to the terminology used for the main mode of this
   profile, the rest of this appendix refers also to "pairwise OSCORE
   Security Context" as to an OSCORE Security Context established
   between only one Client and one Resource Server, and used to
   communicate with OSCORE [RFC8613].







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A.1.  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].

   Just as for main mode of this profile overviewed in Section 2, the
   process for joining the OSCORE group through the respective Group
   Manager as defined in [I-D.ietf-ace-key-groupcomm-oscore] must take
   place before the process described in the rest of this section, and
   is out of the scope of this profile.

   An overview of the protocol flow for the dual mode of this profile is
   shown in Figure 8.  In the figure, it is assumed that both RS1 and
   RS2 are associated with the same AS.  It is also 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.  The names of messages coincide with
   those of [I-D.ietf-ace-oauth-authz] when applicable.

C                            RS1          RS2                         AS
| [--- Resource Request --->] |            |                           |
|                             |            |                           |
| [<----  AS Request ------]  |            |                           |
|       Creation Hints        |            |                           |
|                             |            |                           |
|-------- POST /token ------------------------------------------------>|
|  (aud: RS1, sid: 0, gid: abcd0000, ...)  |                           |
|                             |            |                           |
|<--------------------------------- Access Token + RS Information -----|
|                             | (aud: RS1, sid: 0, gid: abcd0000, ...) |
|                             |            |                           |
|---- POST /authz-info ------>|            |                           |
|    (access_token, N1, ID1)  |            |                           |
|                             |            |                           |
|<-- 2.01 Created (N2, ID2) --|            |                           |
|                             |            |                           |
/Pairwise Sec         /Pairwise Sec        |                           |
 Context Derivation/   Context Derivation/ |                           |
|                             |            |                           |
|-------- POST /token ------------------------------------------------>|
|  (aud: RS2, sid: 0, gid: abcd0000, ...)  |                           |
|                             |            |                           |
|<--------------------------------- Access Token + RS Information -----|
|                             | (aud: RS2, sid: 0, gid: abcd0000, ...) |
|                             |            |                           |



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|----- POST /authz-info ------------------>|                           |
|   (access_token, N1', ID1') |            |                           |
|                             |            |                           |
|<-- 2.01 Created (N2', ID2')--------------|                           |
|                             |            |                           |
/Pairwise Sec                 |   /Pairwise Sec                        |
 Context Derivation/          |    Context Derivation/                 |
|                             |            |                           |
|------ OSCORE Request ------>|            |                           |
|         (kid: ID2)          |            |                           |
|                             |            |                           |
|          /Proof-of-possession;           |                           |
|           Pairwise Sec Context storage/  |                           |
|                             |            |                           |
|<----- OSCORE Response ------|            |                           |
|                             |            |                           |
/Proof-of-possession;         |            |                           |
 Paiwise Sec Context storage/ |            |                           |
|                             |            |                           |
/Mutual authentication        |            |                           |
 between C and RS1            |            |                           |
 (as OSCORE peers)/           |            |                           |
|                             |            |                           |
|                             |            |                           |
|-- Group OSCORE Request -+-->|            |                           |
| (kid: 0, gid: abcd0000)  \-------------->|                           |
|                             |            |                           |
|<--- Group OSCORE Response --|            |                           |
|          (kid: 1)           |            |                           |
|                             |            |                           |
/Mutual authentication        |            |                           |
 between C and RS1            |            |                           |
 (as group members)/          |            |                           |
|                             |            |                           |
|<--- Group OSCORE Response ---------------|                           |
|          (kid: 2)           |            |                           |
|                             |            |                           |
/Mutual authentication        |            |                           |
 between C and RS2            |            |                           |
 (as group members)/          |            |                           |
|                             |            |                           |
|             ...             |            |                           |

                       Figure 8: Protocol Overview.







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A.1.1.  Pre-Conditions

   The same pre-conditions for the main mode of this profile (see
   Section 2.1) hold for the dual mode described in this appendix.

A.1.2.  Access Token Posting

   After having retrieved the Access Token from the AS, the Client
   generates a nonce N1 and an identifier ID1 unique in the sets of its
   own Recipient IDs from its pairwise OSCORE Security Contexts.  The
   client then posts both the Access Token, N1 and its chosen ID to the
   RS, using the /authz-info endpoint and mechanisms specified in
   Section 5.10 of [I-D.ietf-ace-oauth-authz] and Content-Format =
   application/ace+cbor.  When using the dual mode of this profile, the
   communication with the authz-info endpoint is not protected, except
   for update of access rights.  Note that, when using the dual mode,
   this request can alternatively be protected with Group OSCORE, using
   the Group OSCORE Security Context paired with the pairwise OSCORE
   Security Context originally established with the first Access Token
   posting.

   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 in a CBOR map contains a nonce N2 and an identifier
   ID2 unique in the sets of its own Recipient IDs from its pairwise
   OSCORE Security Contexts.

A.1.3.  Setup of the Pairwise OSCORE Security Context

   After sending the 2.01 (Created) response, the RS sets the ID Context
   of 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, concatenated with
   the value in the contextId parameter of the OSCORE_Input_Material
   provided in the 'cnf' claim of the Access Token.

   Then, the RS derives the complete pairwise OSCORE Security Context
   associated with the received Access Token, following Section 3.2 of
   [RFC8613].  In practice, the RS maintains a collection of Security
   Contexts with associated authorization information, for all the
   clients that it is currently communicating with, and the
   authorization information is a policy used as input when processing
   requests from those clients.

   During the derivation process, the RS uses: the ID Context above; the
   nonces N1 and N2; the identifier ID1 received from the Client, set as
   its own OSCORE Sender ID; the identifier ID2 provided to the Client,
   set as its Recipient ID for the Client; and the parameters in the



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   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, concatenated with the value in the contextId
   parameter of the OSCORE_Input_Material provided in the 'cnf'
   parameter of the Access Token response from the AS.  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; the identifier ID1 provided to the RS, set as
   its own Recipient ID for the RS; the identifier ID2 received from the
   RS, set as its own OSCORE Sender ID; and 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.

A.1.4.  Secure Communication

   Other than starting the communication with the RS using Group OSCORE
   as described in Section 4.3, the Client can send to the RS a request
   protected with OSCORE, using the pairwise OSCORE Security Context.

   If the request is successfully 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 is deleted, due to e.g. expiration.  This pairwise
   OSCORE Security Context is discarded when an Access Token (whether
   the same or different) is used to successfully derive a new pairwise
   OSCORE Security Context.




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   As discussed in Section 7 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.  Reuse might otherwise occur when Client and RS derive a
   new pairwise OSCORE Security Context from an existing (non-expired)
   Access Token, e.g. in case of reboot of either the Client or the RS,
   and might lead to loss of both confidentiality and integrity.

   Additionally, just as per the main mode of this profile (see
   Section 4.3), the Client and RS can also securely communicate by
   protecting messages with Group OSCORE, using the Group OSCORE
   Security Context already established upon joining the OSCORE group.

A.2.  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, which
   the client receives from the AS in this exchange.

   The proof-of-possession key (pop-key) received from the AS in this
   exchange MUST be used to build the Master Secret in OSCORE (see
   Appendix A.3.3 and Appendix A.3.4).

A.2.1.  C-to-AS: POST to Token Endpoint

   The Client-to-AS request is specified in Section 5.8.1 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 main mode of this profile (see Section 3.1), with the
   following modifications.

   o  The parameter 'req_cnf' MUST NOT be included in the payload.

   o  The parameter 'client_cred', defined in Appendix A.2.1.1 of this
      specification, MUST be included in the payload.  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.





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   o  The signature included in the parameter 'client_cred_verify' is
      computed by using the private key associated to the public key in
      the 'client_cred' parameter above.

   An example of such a request, with payload in CBOR diagnostic
   notation without the tag and value abbreviations is reported in
   Figure 9.

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

     Figure 9: 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
   the RS.  In this case, the Client MUST include in its POST request to
   the /token endpoint a 'req_cnf' parameter, defined in Section 3.1 of
   [I-D.ietf-ace-oauth-params], which MUST include a 'kid' field, as
   defined in Section 3.1 of [RFC8747].  The 'kid' field has as value a
   CBOR byte string containing the OSCORE_Input_Material Identifier
   (assigned as discussed in Appendix A.2.2).

   This identifier, together with other information such as audience,
   can be used by the AS to determine the shared secret bound to the
   proof-of-possession Access Token and therefore MUST identify a
   symmetric key that was previously generated by the AS as a shared



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   secret for the communication between the Client and the RS.  The AS
   MUST verify that the received value identifies a proof-of-possession
   key that has 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 5.8.3 of
   [I-D.ietf-ace-oauth-authz].

   This POST request for updating the access rights of an Access Token
   SHOULD NOT include the parameters 'salt_input', 'context_id',
   'client_cred' and 'client_cred_verify'.  An exception is the case
   defined in Appendix A.3.6, where the Client, following a change of
   public key in the OSCORE group, requests a new Access Token
   associated to the new public key, while still without changing the
   existing pairwise OSCORE Security Context with the RS.

   An example of such a request, with payload in CBOR diagnostic
   notation without the tag and value abbreviations is reported in
   Figure 10.

        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" : h'01'
          }
        }

   Figure 10: Example C-to-AS POST /token request for updating rights to
                 an Access Token bound to a symmetric key.

A.2.1.1.  'client_cred' Parameter

   The 'client_cred' parameter is an OPTIONAL parameter of the Access
   Token request message defined in Section 5.8.1. 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 [RFC8747] 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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A.2.2.  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 5.8.2 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 5.8.3 of [I-D.ietf-ace-oauth-authz].

   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 Appendix A.3.3 to derive the
   pairwise OSCORE Security Context to use for communications with the
   RS.  Instead, 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
   signaling can be omitted.

   In contrast with the main mode of this profile, 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 an OSCORE_Input_Material 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.

   The AS MUST send different OSCORE_Input_Material (and therefore
   different Access Tokens) to different authorized clients, in order
   for the RS to differentiate between clients.

   In the issued Access Token, the AS MUST include as metadata the same
   information as defined in the main mode of this profile (see
   Section 3.2) with the following modifications.





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   o  The public key that the client uses in the OSCORE group and
      specified in the 'client_cred' parameter of the Token request (see
      Appendix A.2.1) MUST also be included in the Access Token.  If the
      Access Token is a CWT, the AS MUST include it in the 'client_cred'
      claim of the Access Token, defined in Appendix A.2.2.2 of this
      specification.

   o  The OSCORE_Input_Material specified in the 'cnf' parameter of the
      Access Token response MUST also be included in the Access Token.
      If the Access Token is a CWT, the same OSCORE_Input_Material
      included in the 'cnf' parameter of the Access Token response MUST
      be included in the 'osc' field (see Section 9.5 of
      [I-D.ietf-ace-oscore-profile]) of the 'cnf' claim of the Access
      Token.

   Figure 11 shows an example of such an AS response, with payload in
   CBOR diagnostic notation without the tag and value abbreviations.

        Header: Created (Code=2.01)
        Content-Type: "application/ace+cbor"
        Payload:
        {
          "access_token" : h'8343a1010aa2044c53 ...'
           (remainder of CWT omitted for brevity),
          "profile" : "coap_group_oscore",
          "expires_in" : 3600,
          "cnf" : {
            "osc" : {
              "alg" : AES-CCM-16-64-128,
              "id"  : h'01',
              "ms" : h'f9af838368e353e78888e1426bd94e6f',
              "salt" : h'1122',
              "contextId" : h'99'
            }
          }
        }

      Figure 11: Example AS-to-C Access Token response with the Group
                              OSCORE profile.

   Figure 12 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" : {
            "osc" : {
              "alg" : AES-CCM-16-64-128,
              "id"  : h'01',
              "ms" : h'f9af838368e353e78888e1426bd94e6f',
              "salt" : h'1122',
              "contextId" : h'99'
          },
          "salt_input" : h'00',
          "contextId_input" : h'abcd0000',
          "client_cred" : {
            "COSE_Key" : {
              "kty" : EC2,
              "crv" : P-256,
              "x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
                      27c9e354089bbe13',
              "y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
                      731e79a3b4e47120'
            }
          }
        }

      Figure 12: Example CWT with OSCORE parameters (CBOR diagnostic
                                notation).

   The same CWT as in Figure 12 and encoded in CBOR is shown in
   Figure 13, using the value abbreviations defined in
   [I-D.ietf-ace-oauth-authz] and [RFC8747].

   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.

   A8                                      # map(8)
      03                                   # unsigned(3)
      76                                   # text(22)
         74656D7053656E736F72496E4C6976696E67526F6F6D
      06                                   # unsigned(6)
      1A 5112D728                          # unsigned(1360189224)
      04                                   # unsigned(4)
      1A 51145DC8                          # unsigned(1360289224)
      09                                   # unsigned(9)
      78 18                                # text(24)



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         74656D70657261747572655F67206669726D776172655F70
      08                                   # unsigned(8)
      A1                                   # map(1)
         04                                # unsigned(4)
         A5                                # map(5)
            04                             # unsigned(4)
            0A                             # unsigned(10)
            00                             # unsigned(0)
            41                             # bytes(1)
               01                          # "\x01"
            02                             # unsigned(2)
            50                             # bytes(16)
               F9AF838368E353E78888E1426BD94E6F
            05                             # unsigned(5)
            42                             # bytes(2)
               1122                        # "\x11\""
            06                             # unsigned(6)
            41                             # bytes(1)
               99                          # "\x99"
      18 3C                                # unsigned(60)
      41                                   # bytes(1)
         00
      18 3D                                # unsigned(61)
      44                                   # bytes(4)
         ABCD0000
      18 3E                                # unsigned(62)
      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 13: Example CWT with OSCORE parameters.

   If the Client has requested an update to its access rights using the
   same pairwise OSCORE Security Context, which is valid and authorized,
   the AS MUST omit the 'cnf' parameter in the response to the client.




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   Instead, the updated Access Token conveyed in the AS-to-C response
   MUST include a 'cnf' claim specifying a 'kid' field, as defined in
   Section 3.1 of [RFC8747].  The response from the AS MUST carry the
   OSCORE Input Material identifier in the 'kid' field within the 'cnf'
   claim of the Access Token.  That is, the 'kid' field is a CBOR byte
   string, with value the same value of the 'kid' field of the 'req_cnf'
   parameter from the C-to-AS request for updating rights to the Access
   Token (see Figure 10).  This information needs to be included in the
   Access Token, in order for the RS to identify the previously
   generated pairwise OSCORE Security Context.

   Figure 14 shows an example of such an AS response, with payload in
   CBOR diagnostic notation without the tag and value abbreviations.
   The Access Token has been truncated for readability.

        Header: Created (Code=2.01)
        Content-Type: "application/ace+cbor"
        Payload:
        {
          "access_token" : h'8343a1010aa2044c53 ...'
           (remainder of CWT omitted for brevity),
          "profile" : "coap_group_oscore",
          "expires_in" : 3600
        }

      Figure 14: Example AS-to-C Access Token response with the Group
               OSCORE profile, for update of access rights.

   Figure 15 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" : h'01'
          }
        }

    Figure 15: Example CWT with OSCORE parameters for update of access
                                  rights.







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A.2.2.1.  Public Key Hash as Client Credential

   As a possible optimization to limit the size of the Access Token, the
   AS may specify as value of the 'client_cred' claim simply the hash of
   the Client's public key.  The specifically used hash-function MUST be
   collision-resistant on byte-strings, and MUST be selected from the
   "Named Information Hash Algorithm" Registry defined in Section 9.4 of
   [RFC6920].

   In particular, the AS provides the Client with an Access Token as
   defined in Appendix A.2.2, with the following differences.

   The AS prepares INPUT_HASH as follows, with | denoting byte string
   concatenation.

   o  If the public key has COSE Key Type OKP, INPUT_HASH = i, where 'i'
      is the x-parameter of the COSE_Key specified in the 'client_cred'
      parameter of the Token request, encoded as a CBOR byte string.

   o  If the public key has COSE Key Type EC2, INPUT_HASH = (i_1 | i_2),
      where 'i_1' and 'i_2' are the x-parameter and y-parameter of the
      COSE_Key specified in the 'client_cred' parameter of the Token
      request, respectively, each encoded as a CBOR byte string.

   o  If the public key has COSE Key Type RSA, INPUT_HASH = (i_1 | i_2),
      where 'i_1' and 'i_2' are the n-parameter and e-parameter of the
      COSE_Key specified in the 'client_cred' parameter of the Token
      request, respectively, each encoded as a CBOR byte string.

   Then, the AS hashes INPUT_HASH according to the procedure described
   in [RFC6920], with the output OUTPUT_HASH in binary format, as
   described in Section 6 of [RFC6920].

   Finally, the AS includes a single entry within the 'client_cred'
   claim of the Access Token.  This entry has label "kid" (3) defined in
   Section 3.1 of [RFC8747], and value a CBOR byte string wrapping
   OUTPUT_HASH.

   Upon receiving the Access Token, the RS processes it according to
   Appendix A.3.2, with the following differences.

   The RS considers the content of the 'client_cred' claim as the hash
   of 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.

   The RS MAY additionally request the Group Manager of the OSCORE group
   for the public key of that Client, as described in



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   [I-D.ietf-ace-key-groupcomm-oscore], specifying as Sender ID of that
   Client in the OSCORE group the value of the 'salt_input' claim
   included in the Access Token.

   In such a case, the RS MUST check that the hash of the key retrieved
   from the Group Manager matches the hash retrieved from the
   'client_cred' claim of the Access Token.  The RS MUST calculate the
   hash using the same method as the AS described above, and using the
   same hash function.  The hash function used can be determined from
   the information conveyed in the 'client_cred' claim, as the procedure
   described in [RFC6920] also encodes the used hash function as
   metadata of the hash value.

A.2.2.2.  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 [RFC8747] 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.

A.3.  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: an identifier ID1 unique in the sets of its own
   Recipient IDs from its pairwise OSCORE Security Contexts; and the
   Access Token that includes the material provisioned by the AS.

   Then, the RS generates a nonce N2, and an identifier ID2 unique in
   the sets of its own Recipient IDs from its pairwise OSCORE Security
   Contexts.  After that, the RS derives a pairwise OSCORE Security
   Context as described in Section 3.2 of [RFC8613].  In particular, it
   uses the two nonces and the two identifiers established with the
   Client, as well as two shared secrets together with additional pieces
   of information specified in the Access Token.

   Both the client and the RS generate the pairwise OSCORE Security
   Context using the pop-key as part of the OSCORE Master Secret.  In



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   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.  Thus, the proof-of-possession
   required to bind the Access Token to the Client occurs after the
   first OSCORE message exchange.

   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.

   Besides, just as in the main mode (see Section 4), the RS is able to
   verify whether the Client has indeed the claimed Sender ID and public
   key in the OSCORE group.

A.3.1.  C-to-RS POST to authz-info Endpoint

   The Client MUST generate a nonce N1, an OSCORE Recipient ID (ID1),
   and post them 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.

   If the Client has already posted a valid Access Token, has already
   established a pairwise OSCORE Security Context with the RS, and wants
   to update its access rights, the Client can do so by posting the new
   Access Token (retrieved from the AS and specifying the updated set of
   access rights) to the /authz-info endpoint.

   The Client MUST protect the request using either the pairwise OSCORE
   Security Context established during the first Access Token exchange,
   or the Group OSCORE Security Context associated to that pairwise
   OSCORE Security Context.

   In either case, the Client MUST only send the Access Token in the
   payload, i.e. no nonce or identifier are sent.  After proper
   verification (see Section 4.2 of [I-D.ietf-ace-oscore-profile]), the
   new Access Token will supersede the old one at the RS, by replacing
   the corresponding authorization information.  At the same time, the
   RS will maintain the same pairwise OSCORE Security Context and Group




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   OSCORE Security Context, as now both associated to the new Access
   Token.

A.3.2.  RS-to-C: 2.01 (Created)

   The RS MUST verify the validity of the Access Token as defined in
   Section 4.2, with the following modifications.

   o  If the POST request to /authz-info is not protected, the RS checks
      that the 'cnf' claim is included in the Access Token and that it
      contains an OSCORE_Input_Material object.  Also, the RS checks
      that the 'salt_input', 'client_cred' and 'contextId_input' claims
      are included in the Access Token.

   o  If the POST request to /authz-info is protected with the pairwise
      OSCORE Security Context shared with the Client or with the Group
      OSCORE Security Context of the OSCORE group, i.e. the Client is
      requesting an update of access rights, the RS checks that the
      'cnf' claim is included in the Access Token and that it contains
      only the 'kid' field.

   o  If the 'salt_input', 'client_cred' and 'contextId_input' claims
      are included in the Access Token, the RS extracts the content of
      'client_cred'.  Then, the RS considers that content as the public
      key associated to the signing private key of the Client in the
      OSCORE group, whose GID is specified in the 'contextId_input'
      claim.  The RS can compare this public key with the public key of
      the claimed Client retrieved from the Group Manager (see
      Section 4.2).

   If any of the checks 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 proceeds as follows.

   In case the POST request to /authz-info was not protected, the RS
   MUST generate a nonce N2, an OSCORE Recipient ID (ID2), and include
   them in the 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].

   Instead, in case the POST request to /authz-info was protected, the
   RS MUST ignore any nonce and identifiers in the request, if any was
   sent.  Then, the RS MUST check that the 'kid' field of the 'cnf'
   claim in the new Access Token matches the identifier of the OSCORE
   Input Material of a pairwise OSCORE Security Context such that:



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   o  The pairwise OSCORE Security Context was used to protect the
      request, if this was protected with OSCORE; or

   o  The pairwise OSCORE Security Context is bound to the Group OSCORE
      Security Context used to protect the request, if this was
      protected with Group OSCORE.

   If either verification is successful, the new Access Token supersedes
   the old one at the RS.  Besides, the RS associates the new Access
   Token to the same pairwise OSCORE Security Context identified above,
   as also bound to a Group OSCORE Security Context.  The RS MUST
   respond with a 2.01 (Created) response with no payload, protected
   with the same Security Context used to protect the request.  In
   particular, no new pairwise OSCORE Security Context is established
   between the Client and the RS.  If any verification fails, the RS
   MUST respond with a 4.01 (Unauthorized) error response.

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

A.3.3.  OSCORE Setup - Client Side

   Once having received the 2.01 (Created) response from the RS,
   following an unprotected POST request to the /authz-info endpoint,
   the Client MUST extract the nonce N2 from the 'nonce2' parameter, and
   the Client identifier from the 'ace_server_recipientid' parameter in
   the CBOR map of the response payload.  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: i) GID, i.e. the Group Identifier
      of the OSCORE group, as specified by the Client in the
      'context_id' parameter of the Client-to-AS request; ii) the nonce
      N1; iii) the nonce N2; and iv) CID, i.e. the value in the
      contextId parameter of the OSCORE_Input_Material provided in the
      'cnf' parameter of the Access Token response from the AS.  The
      concatenation occurs in this order: ID Context = GID | N1 | N2 |
      CID, where | denotes byte string concatenation.

   o  The Client MUST set the updated Master Salt of the pairwise OSCORE
      Security Context as the concatenation of SaltInput, MSalt, the



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      nonce N1, the nonce N2 and GMSalt, where: i) SaltInput is the
      Sender ID that the Client has in the OSCORE group, which is known
      to the Client as a member of the OSCORE group; ii) MSalt is the
      (optional) Master Salt in the pairwise OSCORE Security Context
      (received from the AS in the Token); and iii) 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 = SaltInput |
      MSalt | N1 | N2 | GMSalt, where | denotes byte string
      concatenation.  Optional values, if not specified, are not
      included in the concatenation.  The five parameters SaltInput,
      MSalt, N1, N2 and GMSalt are to be concatenated as encoded CBOR
      byte strings.  An example of Master Salt construction using CBOR
      encoding is given in Figure 16.

   SaltInput, MSalt, N1, N2 and GMSalt, in CBOR diagnostic notation:
         SaltInput = h'00'
         MSalt = h'f9af838368e353e78888e1426bd94e6f'
         N1 = h'018a278f7faab55a'
         N2 = h'25a8991cd700ac01'
         GMSalt = h'99'

   SaltInput, MSalt, N1, N2 and GMSalt, as CBOR encoded byte strings:
         SaltInput = 0x4100
         MSalt = 0x50f9af838368e353e78888e1426bd94e6f
         N1 = 0x48018a278f7faab55a
         N2 = 0x4825a8991cd700ac01
         GMSalt = 0x4199

   Master Salt = 0x41 00
                   50 f9af838368e353e78888e1426bd94e6f
                   48 018a278f7faab55a
                   48 25a8991cd700ac01
                   41 99

    Figure 16: Example of Master Salt construction using CBOR encoding.

   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_Input_Material of the 'cnf' parameter, received from the AS
      in Appendix A.2.2; 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.

   o  The Client MUST set the Recipient ID as ace_client_recipientid,
      sent as described in Appendix A.3.1.




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   o  The Client MUST set the Sender ID as ace_server_recipientid,
      received as described in Appendix A.3.1.

   o  The Client MUST set the AEAD Algorithm, ID Context, HKDF, and
      OSCORE Version as indicated in the corresponding parameters
      received from the AS in Appendix A.2.2, if present in the
      OSCORE_Input_Material of the 'cnf' parameter.  In case these
      parameters are omitted, the default values SHALL be used as
      described in Section 3.2 and 5.4 of [RFC8613].

   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 the RS), or if
   ace_client_recipientid equals ace_server_recipientid (and as a
   consequence the Sender and Recipient Keys derived would be equal, see
   Section 3.3 of [RFC8613]), then 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 input material.

   The Client can use this pairwise OSCORE Security Context to send
   requests to the RS protected with OSCORE.  Besides, the Client can
   use the Group OSCORE Security Context for protecting unicast requests
   to the RS, or multicast requests to the OSCORE group including also
   the RS.  Mutual authentication as group members is only achieved
   after the client has successfully verified the Group OSCORE protected
   response from the RS.  Similarly, mutual authentication as OSCORE
   peers is only achieved after the client has successfully verified the
   OSCORE protected response from the RS, using the pairwise OSCORE
   Security Context.

   Note that the ID Context of the pairwise OSCORE Security Context can
   be assigned by the AS, communicated and set in both the RS and Client
   after the exchange specified in this profile is executed.
   Subsequently, the Client and RS can update their ID Context by
   running a mechanism such as the one defined in Appendix B.2 of
   [RFC8613] if they both support it and are configured to do so.  In
   that case, the ID Context in the pairwise OSCORE Security Context
   will not match the "contextId" parameter of the corresponding
   OSCORE_Input_Material.  Running the procedure in Appendix B.2 of
   [RFC8613] results in the keying material in the pairwise OSCORE



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   Security Contexts of the Client and RS being updated.  The Client can
   achieve the same result by re-posting the Access Token to the
   unprotected /authz-info endpoint at the RS, as described in
   Section 4.1 of [I-D.ietf-ace-oscore-profile], although without
   updating the ID Context.

A.3.4.  OSCORE Setup - Resource Server Side

   After validation of the Access Token as defined in Appendix A.3.2 and
   after sending the 2.01 (Created) response to an unprotected POST
   request to the /authz-info endpoint, 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.

   o  The RS MUST set the ID Context of the pairwise OSCORE Security
      Context as the concatenation of: i) GID, i.e. the Group Identifier
      of the OSCORE group, as specified in the 'contextId' parameter of
      the OSCORE_Input_Material, in the 'cnf' claim of the Access Token
      received from the Client (see Appendix A.3.1); ii) the nonce N1;
      iii) the nonce N2; and iv) CID which is the value in the contextId
      parameter of the OSCORE_Input_Material provided in the 'cnf' claim
      of the Access Token.  The concatenation occurs in this order: ID
      Context = GID | N1 | N2 | CID, where | denotes byte string
      concatenation.

   o  The RS MUST set the new Master Salt of the pairwise OSCORE
      Security Context as the concatenation of SaltInput, MSalt, the
      nonce N1, the nonce N2 and GMSalt, where: i) SaltInput is the
      Sender ID that the Client has in the OSCORE group, as specified in
      the 'salt_input' claim included in the Access Token received from
      the Client (see Appendix A.3.1); ii) MSalt is the (optional)
      Master Salt in the pairwise OSCORE Security Context as specified
      in the 'salt' parameter in the OSCORE_Input_Material of the 'cnf'
      claim, included in the Access Token received from the Client; and
      iii) 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
      = SaltInput | MSalt | N1 | N2 | GMSalt, where | denotes byte
      string concatenation.  Optional values, if not specified, are not
      included in the concatenation.  The same considerations for
      building the Master Salt, considering the inputs as encoded CBOR
      byte strings as in Figure 16, hold also for the RS.

   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_Input_Material of
      the 'cnf' claim, included in the Access Token received from the
      Client (see Appendix A.3.1); while ii) GMSec is the Master Secret



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      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 Recipient ID as ace_server_recipientid, sent
      as described in Appendix A.3.2.

   o  The RS MUST set the Sender ID as ace_client_recipientid, received
      as described in Appendix A.3.2.

   o  The RS MUST set the AEAD Algorithm, ID Context, HKDF, and OSCORE
      Version from the corresponding parameters received from the Client
      in the Access Token (see Appendix A.3.1), if present in the
      OSCORE_Input_Material of the 'cnf' claim.  In case these
      parameters are omitted, the default values SHALL be used as
      described in Section 3.2 and 5.4 of [RFC8613].

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

   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, as
   defined in Section 4.2, the RS MUST associate the authorization
   information from the Access Token with the Group OSCORE Security
   Context.

   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.  When processing an incoming request protected
   with Group OSCORE, the RS MUST consider as valid public key of the
   Client only the public key specified in the stored Access Token.  As
   defined in Appendix A.3.6, a change of public key in the group
   requires the Client to upload to the RS a new Access Token,
   specifying the new public key in the 'client_cred' claim.

   If Group OSCORE verification fails, error responses are used, as
   specified in Section 8 and Section 9 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
   Appendix A.3.5.





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   After the deletion of the Access Token related to a pairwise OSCORE
   Security Context and to a Group OSCORE Security Context, due to e.g.
   expiration, the RS MUST NOT use the pairwise OSCORE Security Context.
   The RS MUST respond with an unprotected 4.01 (Unauthorized) error
   message to received requests that correspond to a pairwise OSCORE
   Security Context with a deleted Access Token.  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.

   The same considerations, related to the value of the ID Context
   changing, as in Appendix A.3.3 hold also for the RS.

A.3.5.  Access Rights Verification

   The RS MUST follow the procedures defined in Section 4.4.

   Additionally, if the RS receives an OSCORE-protected request from a
   Client, the RS processes it according to [RFC8613].

   If the 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 action requested on the resource is authorized.

   The response code MUST be 4.01 (Unauthorized) if the RS has no valid
   Access Token for the Client.

A.3.6.  Change of Client's Public Key in the Group

   During its membership in the OSCORE group, the client might change
   the public key it uses in the group.  When this happens, the Client
   uploads the new public key to the Group Manager, as defined in
   Section 11 of [I-D.ietf-ace-key-groupcomm-oscore].

   After that, the Client may still have an Access Token previously
   uploaded to the RS, which is not expired yet and still valid to the
   best of the Client's knowledge.  Then, in order to continue
   communicating with the RS, the Client MUST perform the following
   actions.

   1.  The Client requests a new Access Token to the AS, as defined in
       Appendix A.2.1 for the update of access rights, i.e. with the
       'req_cnf' parameter including only a 'kid' field.  In particular,
       when sending the POST request to the AS, the Client indicates:




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       *  The current Group Identifier of the OSCORE group, as value of
          the 'context_id' parameter.

       *  The current Sender ID it has in the OSCORE group, as value of
          the 'salt_input' parameter.

       *  The new public key it has in the OSCORE group, as value of the
          'client_cred' parameter.

       *  The proof-of-possession signature corresponding to the new
          public key, as value of the 'client_cred_verify' parameter.

       *  The same current or instead new set of access rights, as value
          of the 'scope' parameter.

   2.  After receiving the response from the AS (see Appendix A.2.2),
       the Client performs the same exchanges with the RS as defined in
       Appendix A.3, with the following difference: the POST request to
       /authz-info for uploading the new Access Token MUST be protected
       with the pairwise OSCORE Security Context shared with the RS.

   When receiving the new Access Token, the RS performs the same steps
   defined in Appendix A.3.2.  In particular, no new pairwise OSCORE
   Security Context is established between the Client and the RS.

A.4.  Secure Communication with the AS

   The same considerations for secure communication with the AS as
   defined in Section 5 hold.

A.5.  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.

   Additionally, they MUST follow what is defined in the main mode of
   the profile (see Section 6), with respect to the Group OSCORE
   Security Context.

   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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A.6.  CBOR Mappings

   The new parameters defined in this document MUST be mapped to CBOR
   types as specified in Figure 6, with the following addition, using
   the given integer abbreviation for the map key.

                /----------------+----------+------------\
                | Parameter name | CBOR Key | Value Type |
                |----------------+----------+------------|
                | client_cred    | TBD6     | map        |
                \----------------+----------+------------/

               Figure 17: CBOR mappings for new parameters.

   The new claims defined in this document MUST be mapped to CBOR types
   as specified in Figure 7, with the following addition, using the
   given integer abbreviation for the map key.

                 /--------------+----------+------------\
                 | Claim name   | CBOR Key | Value Type |
                 |--------------+----------+------------|
                 | client_cred  | TBD7     | map        |
                 \--------------+----------+------------/

                 Figure 18: CBOR mappings for new claims.

A.7.  Security Considerations

   The dual mode of this profile inherits the security considerations
   from the main mode (see Section 8), as well as from the security
   considerations of the OSCORE profile of ACE
   [I-D.ietf-ace-oscore-profile].  Also, the security considerations
   about OSCORE [RFC8613] hold for the dual mode of this profile, as to
   the specific use of OSCORE.

A.8.  Privacy Considerations

   The same privacy considerations as defined in the main mode of this
   profile apply (see Section 9).

   In addition, as this profile mode also uses OSCORE, the privacy
   considerations from [RFC8613] apply as well, as to the specific use
   of OSCORE.

   Furthermore, this profile mode inherits the privacy considerations
   from the OSCORE profile of ACE [I-D.ietf-ace-oscore-profile].





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Appendix B.  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: Group OSCORE,
      i.e. exchange of secure messages by using a pre-established Group
      OSCORE Security Context.  The optional dual mode defined in
      Appendix A additionally uses OSCORE, i.e. establishment of a
      pairwise OSCORE Security Context and exchange of secure messages.

   o  How the Client and the RS mutually authenticate: Explicitly, by
      possession of a common Group OSCORE Security Context, and by
      either: usage of digital counter signatures embedded in messages,
      if protected with the group mode of Group OSCORE; or protection of
      messages with the pairwise mode of Group OSCORE, by using pairwise
      symmetric keys, derived from the asymmetric keys of the two peers
      exchanging the message.  Note that the mutual authentication is
      not completed before the Client has verified an OSCORE or a Group
      OSCORE response using the corresponding security context.

   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: Group OSCORE
      algorithms; distributed and verified asymmetric keys.  In the
      optional dual mode defined in Appendix A: 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).

   o  How/if the authz-info endpoint is protected: Not protected.

   o  (Optional) other methods of token transport than the authz-info
      endpoint: Not specified.




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Acknowledgments

   The authors sincerely thank Benjamin Kaduk, John Mattsson, 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; and by the H2020 project SIFIS-Home
   (Grant agreement 952652).

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
   Combitech
   Djaeknegatan 31
   Malmoe  SE-21135 Malmoe
   Sweden

   Email: ludwig.seitz@combitech.se


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

   Email: francesca.palombini@ericsson.com





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