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OSCORE-capable Proxies
draft-ietf-core-oscore-capable-proxies-00

Document Type Active Internet-Draft (core WG)
Authors Marco Tiloca , Rikard Höglund
Last updated 2023-10-18
Replaces draft-tiloca-core-oscore-capable-proxies
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draft-ietf-core-oscore-capable-proxies-00
CoRE Working Group                                             M. Tiloca
Internet-Draft                                                R. Höglund
Updates: 8613 (if approved)                                      RISE AB
Intended status: Standards Track                         18 October 2023
Expires: 20 April 2024

                         OSCORE-capable Proxies
               draft-ietf-core-oscore-capable-proxies-00

Abstract

   Object Security for Constrained RESTful Environments (OSCORE) can be
   used to protect CoAP messages end-to-end between two endpoints at the
   application layer, also in the presence of intermediaries such as
   proxies.  This document defines how to use OSCORE for protecting CoAP
   messages also between an origin application endpoint and an
   intermediary, or between two intermediaries.  Also, it defines how to
   secure a CoAP message by applying multiple, nested OSCORE
   protections, e.g., both end-to-end between origin application
   endpoints, as well as between an application endpoint and an
   intermediary or between two intermediaries.  Thus, this document
   updates RFC 8613.  The same approach can be seamlessly used with
   Group OSCORE, for protecting CoAP messages when group communication
   with intermediaries is used.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/oscore-capable-proxies.

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/.

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   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
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   This Internet-Draft will expire on 20 April 2024.

Copyright Notice

   Copyright (c) 2023 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
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  CoAP Group Communication with Proxies . . . . . . . . . .   5
     2.2.  CoAP Observe Notifications over Multicast . . . . . . . .   6
     2.3.  LwM2M Client and External Application Server  . . . . . .   7
     2.4.  LwM2M Gateway . . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Further Use Cases . . . . . . . . . . . . . . . . . . . .   8
   3.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  General Rules on Protecting Options . . . . . . . . . . .  10
     3.2.  Processing an Outgoing Request  . . . . . . . . . . . . .  12
     3.3.  Processing an Incoming Request  . . . . . . . . . . . . .  12
     3.4.  Processing an Outgoing Response . . . . . . . . . . . . .  15
     3.5.  Processing an Incoming Response . . . . . . . . . . . . .  15
   4.  Caching of OSCORE-Protected Responses . . . . . . . . . . . .  16
   5.  Establishment of OSCORE Security Contexts . . . . . . . . . .  17
   6.  CoAP Header Compression with SCHC . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  23
     A.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . . . .  26

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     A.3.  Example 3 . . . . . . . . . . . . . . . . . . . . . . . .  28
     A.4.  Example 4 . . . . . . . . . . . . . . . . . . . . . . . .  30
     A.5.  Example 5 . . . . . . . . . . . . . . . . . . . . . . . .  35
   Appendix B.  State Diagram of the Incoming Request Processing . .  39
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] supports the
   presence of intermediaries, such as forward-proxies and reverse-
   proxies, which assist origin clients by performing requests to origin
   servers on their behalf, and forwarding back the related responses.

   CoAP supports also group communication scenarios
   [I-D.ietf-core-groupcomm-bis], where clients can send a one-to-many
   request targeting all the servers in the group, e.g., by using IP
   multicast.  Like for one-to-one communication, group settings can
   also rely on intermediaries [I-D.tiloca-core-groupcomm-proxy].

   The protocol Object Security for Constrained RESTful Environments
   (OSCORE) [RFC8613] can be used to protect CoAP messages between two
   endpoints at the application layer, especially achieving end-to-end
   security in the presence of (non-trusted) intermediaries.  When CoAP
   group communication is used, the same can be achieved by means of the
   protocol Group OSCORE [I-D.ietf-core-oscore-groupcomm].

   For a number of use cases (see Section 2), it is required and/or
   beneficial that communications are secured also between an
   application endpoint (i.e., a CoAP origin client/server) and an
   intermediary, as well as between two adjacent intermediaries in a
   chain.  This especially applies to the communication leg between the
   CoAP origin client and the adjacent intermediary acting as next hop
   towards the CoAP origin server.

   In such cases, and especially if the origin client already uses
   OSCORE to achieve end-to-end security with the origin server, it
   would be convenient that OSCORE is used also to secure communications
   between the origin client and its next hop.  However, the original
   specification [RFC8613] does not define how OSCORE can be used to
   protect CoAP messages in such communication leg, which would require
   to consider also the intermediary as an "OSCORE endpoint".

   This document fills this gap, and updates [RFC8613] as follows.

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   *  It defines how to use OSCORE for protecting a CoAP message in the
      communication leg between: i) an origin client/server and an
      intermediary; or ii) two adjacent intermediaries in an
      intermediary chain.  That is, besides origin clients/servers, it
      allows also intermediaries to be possible "OSCORE endpoints".

   *  It admits a CoAP message to be secured by multiple, nested OSCORE
      protections applied in sequence, as an "OSCORE-in-OSCORE" process.
      For instance, this is the case when the message is OSCORE-
      protected end-to-end between the origin client and origin server,
      and the result is further OSCORE-protected over the leg between
      the current and next hop (e.g., the origin client and the adjacent
      intermediary acting as next hop towards the origin server).

   This document does not specify any new signaling method to guide the
   message processing on the different endpoints.  In particular, every
   endpoint is always able to understand what steps to take on an
   incoming message depending on the presence of the OSCORE Option, as
   exclusively included or instead combined together with CoAP options
   intended for an intermediary.

   The approach defined in this document can be seamlessly adopted also
   when Group OSCORE is used, for protecting CoAP messages in group
   communication scenarios that rely on intermediaries.

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 CoAP [RFC7252]; OSCORE [RFC8613] and Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].  This document especially builds on
   concepts and mechanics related to intermediaries such as CoAP
   forward-proxies.

   In addition, this document uses the following terms.

   *  Source application endpoint: an origin client producing a request,
      or an origin server producing a response.

   *  Destination application endpoint: an origin server intended to
      consume a request, or an origin client intended to consume a
      response.

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   *  Application endpoint: a source or destination application
      endpoint.

   *  Source OSCORE endpoint: an endpoint protecting a message with
      OSCORE or Group OSCORE.

   *  Destination OSCORE endpoint: an endpoint unprotecting a message
      with OSCORE or Group OSCORE.

   *  OSCORE endpoint: a source/destination OSCORE endpoint.  An OSCORE
      endpoint is not necessarily also an application endpoint with
      respect to a certain message.

   *  Proxy-related options: either of the following (set of) CoAP
      options used for proxying a CoAP request.

      -  The Proxy-Uri Option.  This is relevant when using a forward-
         proxy.

      -  The set of CoAP options comprising the Proxy-Scheme Option
         together with any of the Uri-* Options.  This is relevant when
         using a forward-proxy.

      -  One or more Uri-Path Options, when used not together with the
         Proxy-Scheme Option.  This is relevant when using a reverse-
         proxy.

   *  OSCORE-in-OSCORE: the process by which a message protected with
      (Group) OSCORE is further protected with (Group) OSCORE.  This
      means that, if such a process is used, a successful decryption/
      verification of an OSCORE-protected message might yield an OSCORE-
      protected message.

2.  Use Cases

   The approach defined in this document has been motivated by a number
   of use cases, which are summarized below.

2.1.  CoAP Group Communication with Proxies

   CoAP supports also one-to-many group communication, e.g., over IP
   multicast [I-D.ietf-core-groupcomm-bis], which can be protected end-
   to-end between origin client and origin servers by using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

   This communication model can be assisted by intermediaries such as a
   CoAP forward-proxy or reverse-proxy, which relays a group request to
   the origin servers.  If Group OSCORE is used, the proxy is

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   intentionally not a member of the OSCORE group.  Furthermore,
   [I-D.tiloca-core-groupcomm-proxy] defines a signaling protocol
   between origin client and proxy, to ensure that responses from the
   different origin servers are forwarded back to the origin client
   within a time interval set by the client, and that they can be
   distinguished from one another.

   In particular, it is required that the proxy identifies the origin
   client as allowed-listed, before forwarding a group request to the
   servers (see Section 4 of [I-D.tiloca-core-groupcomm-proxy]).  This
   requires a security association between the origin client and the
   proxy, which would be convenient to provide with a dedicated OSCORE
   Security Context between the two, since the client is possibly using
   also Group OSCORE with the origin servers.

2.2.  CoAP Observe Notifications over Multicast

   The Observe extension for CoAP [RFC7641] allows a client to register
   its interest in "observing" a resource at a server.  The server can
   then send back notification responses upon changes to the resource
   representation, all matching with the original observation request.

   In some applications, such as pub-sub [I-D.ietf-core-coap-pubsub],
   multiple clients are interested to observe the same resource at the
   same server.  Hence, [I-D.ietf-core-observe-multicast-notifications]
   defines a method that allows the server to send a multicast
   notification to all the observer clients at once, e.g., over IP
   multicast.  To this end, the server synchronizes the clients by
   providing them with a common "phantom observation request", against
   which the following multicast notifications will match.

   In case the clients and the server use Group OSCORE for end-to-end
   security and a proxy is also involved, an additional step is required
   (see Section 12 of [I-D.ietf-core-observe-multicast-notifications]).
   That is, clients are in turn required to provide the proxy with the
   obtained "phantom observation request", thus enabling the proxy to
   receive the multicast notifications from the server.

   Therefore, it is preferable to have a security association also
   between each client and the proxy, to especially ensure the integrity
   of that information provided to the proxy (see Section 15.3 of
   [I-D.ietf-core-observe-multicast-notifications]).  Like for the use
   case in Section 2.1, this would be conveniently achieved with a
   dedicated OSCORE Security Context between a client and the proxy,
   since the client is also using Group OSCORE with the origin server.

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2.3.  LwM2M Client and External Application Server

   The Lightweight Machine-to-Machine (LwM2M) protocol [LwM2M-Core]
   enables a LwM2M Client device to securely bootstrap and then register
   at a LwM2M Server, with which it will perform most of its following
   communication exchanges.  As per the transport bindings specification
   of LwM2M [LwM2M-Transport], the LwM2M Client and LwM2M Server can use
   CoAP and OSCORE to secure their communications at the application
   layer, including during the device registration process.

   Furthermore, Section 5.5.1 of [LwM2M-Transport] specifies that:
   "OSCORE MAY also be used between LwM2M endpoint and non-LwM2M
   endpoint, e.g., between an Application Server and a LwM2M Client via
   a LwM2M server.  Both the LwM2M endpoint and non-LwM2M endpoint MUST
   implement OSCORE and be provisioned with an OSCORE Security Context."

   In such a case, the LwM2M Server can practically act as forward-proxy
   between the LwM2M Client and the external Application Server.  At the
   same time, the LwM2M Client and LwM2M Server must continue protecting
   communications on their leg using their Security Context.  Like for
   the use case in Section 2.1, this also allows the LwM2M Server to
   identify the LwM2M Client, before forwarding its request outside the
   LwM2M domain and towards the external Application Server.

2.4.  LwM2M Gateway

   The specification [LwM2M-Gateway] extends the LwM2M architecture by
   defining the LwM2M Gateway functionality.  That is, a LwM2M Server
   can manage end IoT devices "behind" the LwM2M Gateway.  While it is
   outside the scope of such specification, it is possible for the LwM2M
   Gateway to use any suitable protocol with its connected end IoT
   devices, as well as to carry out any required protocol translation.

   Practically, the LwM2M Server can send a request to the LwM2M
   Gateway, asking to forward it to an end IoT device.  With particular
   reference to the CoAP protocol and the related transport binding
   specified in [LwM2M-Transport], the LwM2M Server acting as CoAP
   client sends its request to the LwM2M Gateway acting as CoAP server.

   If CoAP is used in the communication leg between the LwM2M Gateway
   and the end IoT devices, then the LwM2M Gateway fundamentally acts as
   a reverse-proxy (see Section 5.7.3 of [RFC7252]).  That is, in
   addition to its own resources, the LwM2M Gateway serves the resources
   of each end IoT device behind itself, as exposed under a dedicated
   URI-Path.  As per [LwM2M-Gateway], the first URI-Path segment is used
   as "prefix" to identify the specific IoT device, while the remaining
   URI-Path segments specify the target resource at the IoT device.

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   As per Section 7 of [LwM2M-Gateway], message exchanges between the
   LwM2M Server and the L2M2M Gateway are secured using the LwM2M-
   defined technologies, while the LwM2M protocol does not provide end-
   to-end security between the LwM2M Server and the end IoT devices.
   However, the approach defined in this document makes it possible to
   achieve both goals, by allowing the LwM2M Server to use OSCORE for
   protecting a message both end-to-end for the targeted end IoT device
   as well as for the LwM2M Gateway acting as reverse-proxy.

2.5.  Further Use Cases

   The approach defined in this document can be useful also in the
   following use cases relying on a proxy.

   *  A server aware of a suitable cross proxy can rely on it as a
      third-party service, in order to indicate transports for CoAP
      available to that server (see Section 4 of
      [I-D.ietf-core-transport-indication]).

      From a security point of view, it would be convenient if the proxy
      could provide suitable credentials to the client, as a general
      trusted proxy for the system.  At the same time, it can be
      desirable to limit the use of such a proxy to a set of clients
      which have permission to use it, and that the proxy can identify
      through a secure communication association.

      However, in order for OSCORE to be an applicable security
      mechanism for this scenario, OSCORE has to be terminated at the
      proxy.  That is, it would be required for a client and the proxy
      to share a dedicated OSCORE Security Context and to use it for
      protecting their communication leg.

   *  The method specified in [I-D.ietf-core-coap-pm] relies on the
      Performance Measurement Option to enable network telemetry for
      CoAP communications.  This makes it possible to efficiently
      measure Round-Trip Time and message losses, both end-to-end and
      hop-by-hop.  In particular, on-path probes such as intermediary
      proxies can be deployed to perform measurements hop-by-hop.

      When OSCORE is used in deployments including on-path probes, an
      inner Performance Measurement Option is protected end-to-end
      between the two application endpoints and enables end-to-end
      measurements between those.  At the same time, an outer
      Performance Measurement Option allows also hop-by-hop measurements
      to be performed by reying on an on-path probe.

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      Therefore, it is preferable to have a secure association with an
      on-path probe, in order to also ensure the integrity of the hop-
      by-hop measurements exchanged with the probe.

   *  The method specified in [I-D.ietf-ace-coap-est-oscore] enables
      public-key certificate enrollment for Internet of Things
      deployments.  This leverages payload formats defined in Enrollment
      over Secure Transport (EST) [RFC7030], while relying on CoAP for
      message transfer and on OSCORE for message protection.

      In real-world deployments, an EST server issuing public-key
      certificates may reside outside a constrained network that
      includes devices acting as EST clients.  In particular, the EST
      clients are expected to support only CoAP, while the EST server in
      a non-constrained network is expected to support only HTTP.  This
      requires a CoAP-to-HTTP proxy to be deployed between the EST
      clients and the EST server, in order to map CoAP messages with
      HTTP messages across the two networks.

      Even in such a scenario, the EST server and every EST client can
      still effectively use OSCORE to protect their communications end-
      to-end.  At the same time, it is desirable to have an additional
      secure association between the EST client and the CoAP-to-HTTP
      proxy, especially in order for the proxy to identify the EST
      client before forwarding EST messages out of the CoAP boundary of
      the constrained network and towards the EST server.

   *  A proxy may be deployed to act as an entry point to a firewalled
      network, which only authenticated clients can join.  In
      particular, authentication can rely on the used secure
      communication association between a client and the proxy.  If the
      proxy could share a dedicated OSCORE Security Context with each
      client, the proxy can rely on it to identify the client, before
      forwarding its messages to any other member of the firewalled
      network.

   *  The approach defined in this document does not pose a limit to the
      number of OSCORE protections applied to the same CoAP message.
      This enables more privacy-oriented scenarios based on proxy
      chains, where the origin client protects a CoAP request using
      first the OSCORE Security Context shared with the origin server,
      and then the dedicated OSCORE Security Context shared with each of
      the different hops in the chain.  Once received at a chain hop,
      the request would be stripped of the OSCORE protection associated
      with that hop before being forwarded to the next one.

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3.  Message Processing

   As mentioned in Section 1, this document introduces the following two
   main deviations from the original OSCORE specification [RFC8613].

   1.  An "OSCORE endpoint", i.e., a producer/consumer of an OSCORE
       Option can be not only an application endpoint (i.e., an origin
       client or server), but also an intermediary such as a proxy.

       Hence, OSCORE can be used between an origin client/server and a
       proxy, as well as between two proxies in an intermediary chain.

   2.  A CoAP message can be secured by multiple OSCORE protections
       applied in sequence.  Therefore, the final result is a message
       with nested OSCORE protections, as the output of an "OSCORE-in-
       OSCORE" process.  Hence, following a decryption, the resulting
       message might legitimately include an OSCORE Option, and thus
       have in turn to be decrypted.

       The most common case is expected to consider a message protected
       with up to two OSCORE layers, i.e.: i) an inner layer, protecting
       the message end-to-end between the origin client and the origin
       server acting as application endpoints; and ii) an outer layer,
       protecting the message between a certain OSCORE endpoint and the
       other OSCORE endpoint adjacent in the intermediary chain.

       However, a message can also be protected with a higher arbitrary
       number of nested OSCORE layers, e.g., in scenarios relying on a
       longer chain of intermediaries.  For instance, the origin client
       can sequentially apply multiple OSCORE layers to a request, each
       of which to be consumed and removed by one of the intermediaries
       in the chain, until the origin server is reached and it consumes
       the innermost OSCORE layer.

   Appendix A provides a number of examples where the approach defined
   in this document is used to protect message exchanges.

3.1.  General Rules on Protecting Options

   Let us consider a sender endpoint that, when protecting an outgoing
   message, applies the i-th OSCORE layer in sequence, by using the
   OSCORE Security Context shared with another OSCORE endpoint X.

   In addition to the CoAP options specified as class E in [RFC8613] or
   in the document defining them, the sender endpoint MUST encrypt and
   integrity-protect the following CoAP options.  That is, even if they
   are originally specified as class U or class I for OSCORE, such
   options are processed like if they were specified as class E.

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   *  Any CoAP option such that both the following conditions hold.

      1.  The sender endpoint has added the option to the message
          either:

          -  Following a message protection previously performed for an
             OSCORE endpoint different than X; or

          -  Before a message protection previously performed for an
             OSCORE endpoint different than X, where the option was
             treated as Class U or I.

      2.  The option is not intended to be consumed by the other OSCORE
          endpoint X.

      Examples of such CoAP options are:

      -  The OSCORE Option present as the result of the OSCORE layer
         immediately previously applied for an OSCORE endpoint different
         than X, when the sender endpoint is an origin endpoint.

      -  The EDHOC Option defined in [I-D.ietf-core-oscore-edhoc], when
         the sender endpoint is the origin client.

      -  The Request-Hash Option defined in
         [I-D.amsuess-core-cachable-oscore], when X is not an origin
         endpoint).

   *  Any CoAP option such that all the following conditions hold.

      1.  The sender endpoint has added the option to the message.

      2.  The option is intended to be consumed by the other OSCORE
          endpoint X.

      3.  At the other OSCORE endpoint X, the option does not play a
          role in processing the message before having removed the i-th
          OSCORE layer or in removing the i-th OSCORE layer altogether.

      Examples of such CoAP options are:

      -  The Proxy-Uri, Proxy-Scheme, Uri-Host and Uri-Port Options
         defined in [RFC7252].

      -  The Listen-To-Multicast-Notifications Option defined in
         [I-D.ietf-core-observe-multicast-notifications].

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      -  The Multicast-Timeout, Response-Forwarding and Group-ETag
         Options defined in [I-D.tiloca-core-groupcomm-proxy].

   *  Any CoAP option such that the sender endpoint has not added the
      option to the message.

      Examples of such CoAP options are:

      -  The OSCORE Option present as the result of the OSCORE layer
         immediately previously applied for an OSCORE endpoint different
         than X, when the sender endpoint is not an origin endpoint.

      -  The EDHOC Option defined in [I-D.ietf-core-oscore-edhoc], when
         the sender endpoint is not the origin client.

3.2.  Processing an Outgoing Request

   The rules from Section 3.1 apply when processing an outgoing request
   message, with the following addition.

   When an application endpoint applies multiple OSCORE layers in
   sequence to protect an outgoing request, and it uses an OSCORE
   Security Context shared with the other application endpoint, then the
   first OSCORE layer MUST be applied by using that Security Context.

3.3.  Processing an Incoming Request

   Upon receiving a request REQ, the recipient endpoint performs the
   actions described in the following steps.

   1.  If REQ includes proxy-related options, the endpoint moves to step
       2.  Otherwise, the endpoint moves to step 3.

   2.  The endpoint proceeds as defined below, depending on which of the
       two following conditions holds.

       *  REQ includes either the Proxy-Uri Option, or the Proxy-Scheme
          Option together with any of the Uri-* Options.

          If the endpoint is not configured to be a forward-proxy, it
          MUST stop processing the request and MUST respond with a 5.05
          (Proxying Not Supported) error response to (the previous hop
          towards) the origin client, as per Section 5.10.2 of
          [RFC7252].  This may result in protecting the error response
          over that communication leg, as per Section 3.4.

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          Otherwise, the endpoint MUST check whether forwarding this
          request to (the next hop towards) the origin server is an
          authorized operation.  This check can be based, for instance,
          on the specific OSCORE Security Context that the endpoint used
          to decrypt the incoming message, before performing this step.

          In case the authentication check fails, the endpoint MUST stop
          processing the request and MUST respond with a 4.01
          (Unauthorized) error response to (the previous hop towards)
          the origin client, as per Section 5.10.2 of [RFC7252].  This
          may result in protecting the error response over that
          communication leg, as per Section 3.4.

          Otherwise, the endpoint consumes the proxy-related options as
          per Section 5.7.2 of [RFC7252] and forwards REQ to (the next
          hop towards) the origin server, unless differently indicated
          in REQ, e.g., by means of any of its CoAP options.  For
          instance, a forward-proxy does not forward a request that
          includes proxy-related options together with the Listen-To-
          Multicast-Notifications Option (see Section 12 of
          [I-D.ietf-core-observe-multicast-notifications]).

          If the endpoint forwards REQ to (the next hop towards) the
          origin server, this may result in (further) protecting REQ
          over that communication leg, as per Section 3.2.

          After that, the endpoint does not take any further action.

       *  REQ includes one or more Uri-Path Options but not the Proxy-
          Scheme Option.

          If the endpoint is not configured to be a reverse-proxy or its
          resource targeted by the Uri-Path Options is not intended to
          support reverse-proxy functionalities, then the endpoint
          proceeds to step 3.

          Otherwise, the endpoint MUST check whether forwarding this
          request to (the next hop towards) the origin server is an
          authorized operation.  This check can be based, for instance,
          on the specific OSCORE Security Context that the endpoint used
          to decrypt the incoming message, before performing this step.

          In case the authentication check fails, the endpoint MUST stop
          processing the request and MUST respond with a 4.01
          (Unauthorized) error response to (the previous hop towards)
          the origin client, as per Section 5.10.2 of [RFC7252].  This
          may result in protecting the error response over that
          communication leg, as per Section 3.4.

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          Otherwise, the endpoint consumes the Uri-Path options as per
          Section 5.7.3 of [RFC7252], and forwards REQ to (the next hop
          towards) the origin server, unless differently indicated in
          REQ, e.g., by means of any of its CoAP options.

          If the endpoint forwards REQ to (the next hop towards) the
          origin server, this may result in (further) protecting REQ
          over that communication leg, as per Section 3.2.

          After that, the endpoint does not take any further action.

          Note that, when forwarding REQ, the endpoint might not remove
          all the Uri-Path Options originally present, e.g., in case the
          next hop towards the origin server is a further reverse-proxy.

   3.  The endpoint proceeds as defined below, depending on which of the
       two following conditions holds.

       *  REQ does not include an OSCORE Option.

          If the endpoint does not have an application to handle REQ, it
          MUST stop processing the request and MAY respond with a 4.00
          (Bad Request) error response to (the previous hop towards) the
          origin client.  This may result in protecting the error
          response over that communication leg, as per Section 3.4.

          Otherwise, the endpoint delivers REQ to the application.

       *  REQ includes an OSCORE Option.

          If REQ includes any URI-Path Options, the endpoint MUST stop
          processing the request and MAY respond with a 4.00 (Bad
          Request) error response to (the previous hop towards) the
          origin client.  This may result in protecting the error
          response over that communication leg, as per Section 3.4.

          Otherwise, the endpoint decrypts REQ using the OSCORE Security
          Context indicated by the OSCORE Option, i.e., REQ* = dec(REQ).
          After that, the possible presence of an OSCORE Option in the
          decrypted request REQ* is not treated as an error situation.

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          If the OSCORE processing results in an error, the endpoint
          MUST stop processing the request and performs error handling
          as per Section 8.2 of [RFC8613] or Sections 8.2 and 9.4 of
          [I-D.ietf-core-oscore-groupcomm], in case OSCORE or Group
          OSCORE is used, respectively.  In case the endpoint sends an
          error response to (the previous hop towards) the origin
          client, this may result in protecting the error response over
          that communication leg, as per Section 3.4.

          Otherwise, REQ takes REQ*, and the endpoint moves to step 1.

   Appendix B provides an overview of the process defined above as a
   state diagram.

3.4.  Processing an Outgoing Response

   The rules from Section 3.1 apply when processing an outgoing response
   message, with the following additions.

   When an application endpoint applies multiple OSCORE layers in
   sequence to protect an outgoing response, and it uses an OSCORE
   Security Context shared with the other application endpoint, then the
   first OSCORE layer MUST be applied by using that Security Context.

   The sender endpoint protects the response by applying the same OSCORE
   layers that it removed from the corresponding incoming request, but
   in the reverse order than the one they were removed.

   In case the response is an error response, the sender endpoint
   protects it by applying the same OSCORE layers that it successfully
   removed from the corresponding incoming request, but in the reverse
   order than the one they were removed.

3.5.  Processing an Incoming Response

   The recipient endpoint removes the same OSCORE layers that it added
   when protecting the corresponding outgoing request, but in the
   reverse order than the one they were removed.

   When doing so, the possible presence of an OSCORE Option in the
   decrypted response following the removal of an OSCORE layer is not
   treated as an error situation, unless it occurs after having removed
   as many OSCORE layers as were added in the outgoing request.  In such
   a case, the endpoint MUST stop processing the response.

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4.  Caching of OSCORE-Protected Responses

   Although not possible as per the original OSCORE specification
   [RFC8613], cacheability of OSCORE-protected responses at proxies can
   be achieved.  To this end, the approach defined in
   [I-D.amsuess-core-cachable-oscore] can be used, as based on
   Deterministic Requests protected with the pairwise mode of Group
   OSCORE [I-D.ietf-core-oscore-groupcomm] used end-to-end between an
   origin client and an origin server.  The applicability of this
   approach is limited to requests that are safe (in the RESTful sense)
   to process and do not yield side effects at the origin server.

   In particular, this approach requires both the origin client and the
   origin server to have already joined the correct OSCORE group.  Then,
   starting from the same plain CoAP request, different clients in the
   OSCORE group are able to deterministically generate a same request
   protected with Group OSCORE, which is sent to a proxy for being
   forwarded to the origin server.  The proxy can effectively cache the
   resulting OSCORE-protected response from the server, since the same
   plain CoAP request will result again in the same Deterministic
   Request and thus will produce a cache hit.

   When using this approach, the following also applies in addition to
   what is defined in Section 3, when processing incoming messages at a
   proxy that implements caching of responses.

   *  Upon receiving a request from (the previous hop towards) the
      origin client, the proxy checks if specifically the message
      available during the execution of step 2 in Section 3.3 produces a
      cache hit.

      That is, such a message: i) is exactly the one to be forwarded to
      (the next hop towards) the origin server, if no cache hit has
      occurred; and ii) is the result of an OSCORE decryption at the
      proxy, if OSCORE is used on the communication leg between the
      proxy and (the previous hop towards) the origin client.

   *  Upon receiving a response from (the next hop towards) the origin
      server, the proxy first removes the same OSCORE layers that it
      added when protecting the corresponding outgoing request, as
      defined in Section 3.5.

      Then, the proxy stores specifically that resulting response
      message in its cache.  That is, such a message is exactly the one
      to be forwarded to (the previous hop towards) the origin client.

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   The specific rules about serving a request with a cached response are
   defined in Section 5.6 of [RFC7252], as well as in Section 7 of
   [I-D.tiloca-core-groupcomm-proxy] for group communication scenarios.

5.  Establishment of OSCORE Security Contexts

   Like the original OSCORE specification [RFC8613], this document is
   not devoted to any particular approach that two OSCORE endpoints use
   for establishing an OSCORE Security Context.

   At the same time, the following applies, depending on the two peers
   using OSCORE or Group OSCORE [I-D.ietf-core-oscore-groupcomm] to
   protect their communications.

   *  When using OSCORE, the establishment of the OSCORE Security
      Context can rely on the authenticated key establishment protocol
      EDHOC [I-D.ietf-lake-edhoc].

      Assuming the use of OSCORE both between the two origin application
      endpoints as well as between the origin client and the first proxy
      in the chain, it is expected that the origin client first runs
      EDHOC with the first proxy in the chain, and then with the origin
      server through the chain of proxies (see the example in
      Appendix A.4).

      Furthermore, the additional use of the combined EDHOC + OSCORE
      request defined in [I-D.ietf-core-oscore-edhoc] is particularly
      beneficial in this case (see the example in Appendix A.5), and
      especially when relying on a long chain of proxies.

   *  The use of Group OSCORE is expected to be limited between the
      origin applications endpoints, e.g., between the origin client and
      multiple origin servers.  In order to join the same OSCORE group
      and obtain the corresponding Group OSCORE Security Context, those
      endpoints can use the approach defined in
      [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
      for authentication and authorization in constrained environments
      [RFC9200].

      For the purposes of this document, there is no need for a proxy to
      also be a member of the OSCORE group whose Group OSCORE Security
      Context is used by the origin application endpoints for protecting
      communications end-to-end.

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6.  CoAP Header Compression with SCHC

   The method defined in this document enables and results in the
   possible protection of the same CoAP message with multiple, nested
   OSCORE layers.  Especially when this happens, it is desirable to
   compress the header of protected CoAP messages, in order to improve
   performance and ensure that CoAP is usable also in Low-Power Wide-
   Area Networks (LPWANs).

   To this end, it is possible to use the Static Context Header
   Compression and fragmentation (SCHC) framework [RFC8724].  In
   particular, [RFC8824] specifies how to use SCHC for compressing
   headers of CoAP messages, also when messages are protected with
   OSCORE.  As further clarified in [I-D.tiloca-schc-8824-update], the
   SCHC Compression/Decompression is applicable also in the presence of
   CoAP proxies, and especially to the two following cases.

   *  In case OSCORE is not used at all, the SCHC processing occurs hop-
      by-hop, by relying on SCHC Rules that are consistently shared
      between two adjacent hops.

   *  In case OSCORE is used only end-to-end between the application
      endpoints, then an Inner SCHC Compression/Decompression and an
      Outer SCHC Compression/Decompression are performed (see
      Section 7.2 of [RFC8824]).  In particular, the following holds.

      The SCHC processing occurs end-to-end as to the Inner SCHC
      Compression/Decompression.  This relies on Inner SCHC Rules that
      are shared between the two application endpoints, which act as
      OSCORE endpoints and share the used OSCORE Security Context.

      The SCHC processing occurs hop-by-hop as to the Outer SCHC
      Compression/Decompression.  This relies on Outer SCHC Rules that
      are shared between two adjacent hops.

   When using the method defined in this document, and thus enabling
   also an intermediary proxy to be an OSCORE endpoint, the SCHC
   processing above is generalized as specified below.

   When processing an outgoing CoAP message, a sender endpoint proceeds
   as follows.

   *  The sender endpoint performs one Inner SCHC Compression for each
      OSCORE layer applied to the outgoing message.  Each Inner SCHC
      Compression occurs before protecting the message with that OSCORE
      layer, and relies on the SCHC Rules that are shared with the other
      OSCORE endpoint.

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   *  The sender endpoint performs exactly one Outer SCHC Compression.
      This occurs after having performed all the intended OSCORE
      protections of the outgoing message, and relies on the SCHC Rules
      that are shared with the (next hop towards the) recipient
      application endpoint.

   That is, with respect to the SCHC Compression/Decompression
   processing, the following holds.

   *  An Inner SCHC Compression is intended to a recipient OSCORE
      endpoint, which will: first, decrypt an incoming message with the
      OSCORE Security Context shared with the other OSCORE endpoint; and
      then, perform the corresponding Inner SCHC Decompression, by
      relying on the SCHC Rules shared with the other OSCORE endpoint.

   *  An Outer SCHC Compression is intended to the (next hop towards
      the) recipient application endpoint, which will: first, perform a
      corresponding Outer SCHC Decompression on an incoming message, by
      relying on the SCHC Rules shared with the (previous hop towards
      the) recipient application endpoint; then, perform a new Outer
      SCHC Compression on the result, by relying on the SCHC Rules
      shared with the (next hop towards the) recipient application
      endpoint; and, finally, send the result to the (next-hop towards
      the) recipient application endpoint.

   Note that the generalization above does not alter the core approach,
   design choices, and features of the SCHC Compression/Decompression
   applied to CoAP headers.

7.  Security Considerations

   The same security considerations from CoAP [RFC7252] apply to this
   document.  The same security considerations from [RFC8613] and
   [I-D.ietf-core-oscore-groupcomm] apply to this document, when using
   OSCORE or Group OSCORE to protect exchanged messages.

   Further security considerations to take into account are inherited
   from the specifically used CoAP options, extensions, and methods
   employed when relying on OSCORE or Group OSCORE.

   This document does not change the security properties of OSCORE and
   Group OSCORE.  That is, given any two OSCORE endpoints, the method
   defined in this document provides them with the same security
   guarantees that OSCORE and Group OSCORE provide in the case where
   such endpoints are specifically application endpoints.

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8.  IANA Considerations

   This document has no actions for IANA.

9.  References

9.1.  Normative References

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group Object Security for Constrained
              RESTful Environments (Group OSCORE)", Work in Progress,
              Internet-Draft, draft-ietf-core-oscore-groupcomm-20, 2
              September 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-oscore-groupcomm-20>.

   [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/rfc/rfc2119>.

   [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/rfc/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/rfc/rfc8174>.

   [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/rfc/rfc8613>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/rfc/rfc8724>.

   [RFC8824]  Minaburo, A., Toutain, L., and R. Andreasen, "Static
              Context Header Compression (SCHC) for the Constrained
              Application Protocol (CoAP)", RFC 8824,
              DOI 10.17487/RFC8824, June 2021,
              <https://www.rfc-editor.org/rfc/rfc8824>.

9.2.  Informative References

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   [I-D.amsuess-core-cachable-oscore]
              Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
              Progress, Internet-Draft, draft-amsuess-core-cachable-
              oscore-07, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-amsuess-core-
              cachable-oscore-07>.

   [I-D.ietf-ace-coap-est-oscore]
              Selander, G., Raza, S., Furuhed, M., Vučinić, M., and T.
              Claeys, "Protecting EST Payloads with OSCORE", Work in
              Progress, Internet-Draft, draft-ietf-ace-coap-est-oscore-
              02, 9 July 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-ace-coap-est-oscore-02>.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-oscore-16, 6 March
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              ace-key-groupcomm-oscore-16>.

   [I-D.ietf-core-coap-pm]
              Fioccola, G., Zhou, T., Cociglio, M., Bulgarella, F.,
              Nilo, M., and F. Milan, "Constrained Application Protocol
              (CoAP) Performance Measurement Option", Work in Progress,
              Internet-Draft, draft-ietf-core-coap-pm-00, 19 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coap-pm-00>.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keränen, A., and J. Jimenez, "A publish-
              subscribe architecture for the Constrained Application
              Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
              ietf-core-coap-pubsub-12, 13 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coap-pubsub-12>.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              09, 10 July 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-groupcomm-bis-09>.

   [I-D.ietf-core-observe-multicast-notifications]
              Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
              "Observe Notifications as CoAP Multicast Responses", Work
              in Progress, Internet-Draft, draft-ietf-core-observe-

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              multicast-notifications-06, 26 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              observe-multicast-notifications-06>.

   [I-D.ietf-core-oscore-edhoc]
              Palombini, F., Tiloca, M., Höglund, R., Hristozov, S., and
              G. Selander, "Using EDHOC with CoAP and OSCORE", Work in
              Progress, Internet-Draft, draft-ietf-core-oscore-edhoc-09,
              13 October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-oscore-edhoc-09>.

   [I-D.ietf-core-transport-indication]
              Amsüss, C., "CoAP Protocol Indication", Work in Progress,
              Internet-Draft, draft-ietf-core-transport-indication-02,
              13 March 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-transport-indication-02>.

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-22, 25
              August 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lake-edhoc-22>.

   [I-D.tiloca-core-groupcomm-proxy]
              Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
              Communication", Work in Progress, Internet-Draft, draft-
              tiloca-core-groupcomm-proxy-09, 31 August 2023,
              <https://datatracker.ietf.org/doc/html/draft-tiloca-core-
              groupcomm-proxy-09>.

   [I-D.tiloca-schc-8824-update]
              Tiloca, M., Toutain, L., Martinez, I., and A. Minaburo,
              "Clarifications and Updates on using Static Context Header
              Compression (SCHC) for the Constrained Application
              Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
              tiloca-schc-8824-update-01, 10 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-tiloca-schc-
              8824-update-01>.

   [LwM2M-Core]
              Open Mobile Alliance, "Lightweight Machine to Machine
              Technical Specification - Core, Approved Version 1.2, OMA-
              TS-LightweightM2M_Core-V1_2-20201110-A", November 2020,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_2-20201110-A/OMA-TS-LightweightM2M_Core-
              V1_2-20201110-A.pdf>.

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   [LwM2M-Gateway]
              Open Mobile Alliance, "Lightweight Machine to Machine
              Gateway Technical Specification - Approved Version 1.1,
              OMA-TS-LWM2M_Gateway-V1_1-20210518-A", May 2021,
              <https://www.openmobilealliance.org/release/LwM2M_Gateway/
              V1_1-20210518-A/OMA-TS-LWM2M_Gateway-V1_1-20210518-A.pdf>.

   [LwM2M-Transport]
              Open Mobile Alliance, "Lightweight Machine to Machine
              Technical Specification - Transport Bindings, Approved
              Version 1.2, OMA-TS-LightweightM2M_Transport-
              V1_2-20201110-A", November 2020,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_2-20201110-A/OMA-TS-LightweightM2M_Transport-
              V1_2-20201110-A.pdf>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/rfc/rfc7030>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7641>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8742>.

   [RFC9200]  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments Using the OAuth 2.0 Framework
              (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9200>.

Appendix A.  Examples

   This section provides a number of examples where the approach defined
   in this document is used to protect message exchanges.

   The presented examples build on the example shown in Appendix A.1 of
   [RFC8613], and illustrate an origin client requesting the alarm
   status from an origin server, through a forward-proxy.

   The abbreviations "REQ" and "RESP" are used to denote a request
   message and a response message, respectively.

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A.1.  Example 1

   In the example shown in Figure 1, message exchanges are protected
   with OSCORE over the following legs.

   *  End-to-end, between the client and the server, using the OSCORE
      Security Context CTX_C_S.  The client uses the OSCORE Sender ID
      0x5f when using OSCORE with the server.

   *  Between the client and the proxy, using the OSCORE Security
      Context CTX_C_P.  The client uses the OSCORE Sender ID 0x20 when
      using OSCORE with the proxy.

   Client  Proxy  Server
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_S   |       |
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_P   |       |
     |       |       |
     +------>|       |     Code: 0.02 (POST)
     | POST  |       |    Token: 0x8c
     |       |       |   OSCORE: [kid: 0x20, Partial IV: 31]
     |       |       |     0xff
     |       |       |  Payload: {Code: 0.02,
     |       |       |            OSCORE: [kid: 0x5f, Partial IV: 42],
     |       |       |            Uri-Host: example.com,
     |       |       |            Proxy-Scheme: coap,
     |       |       |            0xff,
     |       |       |            {Code: 0.01,
     |       |       |             Uri-Path: "alarm_status"
     |       |       |            } // Encrypted with CTX_C_S
     |       |       |           } // Encrypted with CTX_C_P
     |       |       |
     |     Decrypt   |
     |     REQ with  |
     |     CTX_C_P   |
     |       |       |
     |       +------>|     Code: 0.02 (POST)
     |       | POST  |    Token: 0x7b
     |       |       |   OSCORE: [kid: 0x5f, Partial IV: 42]
     |       |       |     0xff
     |       |       |  Payload: {
     |       |       |            Code: 0.01,
     |       |       |            Uri-Path: "alarm_status"

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     |       |       |           } // Encrypted with CTX_C_S
     |       |       |
     |       |     Decrypt
     |       |     REQ with
     |       |     CTX_C_S
     |       |       |
     |       |     Encrypt
     |       |     RESP with
     |       |     CTX_C_S
     |       |       |
     |       |<------+     Code: 2.04 (Changed)
     |       |  2.04 |    Token: 0x7b
     |       |       |   OSCORE: -
     |       |       |     0xff
     |       |       |  Payload: {Code: 2.05,
     |       |       |            0xff,
     |       |       |            "0"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |
     |     Encrypt   |
     |     RESP with |
     |     CTX_C_P   |
     |       |       |
     |<------+       |     Code: 2.04 (Changed)
     |  2.04 |       |    Token: 0x8c
     |       |       |   OSCORE: -
     |       |       |     0xff
     |       |       |  Payload: {Code: 2.04,
     |       |       |            OSCORE: -,
     |       |       |            0xff,
     |       |       |            {Code: 2.05,
     |       |       |             0xff,
     |       |       |             "0"
     |       |       |            } // Encrypted with CTX_C_S
     |       |       |           } // Encrypted with CTX_C_P
     |       |       |
   Decrypt   |       |
   RESP with |       |
   CTX_C_P   |       |
     |       |       |
   Decrypt   |       |
   RESP with |       |
   CTX_C_S   |       |
     |       |       |

   Square brackets [ ... ] indicate content of compressed COSE object.
   Curly brackets { ... } indicate encrypted data.

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       Figure 1: Use of OSCORE between Client-Server and Client-Proxy

A.2.  Example 2

   In the example shown in Figure 2, message exchanges are protected
   with OSCORE over the following legs.

   *  End-to-end between the client and the server, using the OSCORE
      Security Context CTX_C_S.  The client uses the OSCORE Sender ID
      0x5f when using OSCORE with the server.

   *  Between the proxy and the server, using the OSCORE Security
      Context CTX_P_S.  The proxy uses the OSCORE Sender ID 0xd4 when
      using OSCORE with the server.

 Client  Proxy  Server
   |       |       |
 Encrypt   |       |
 REQ with  |       |
 CTX_C_S   |       |
   |       |       |
   +------>|       |         Code: 0.02 (POST)
   | POST  |       |        Token: 0x8c
   |       |       |     Uri-Host: example.com
   |       |       | Proxy-Scheme: coap
   |       |       |       OSCORE: [kid: 0x5f, Partial IV: 42]
   |       |       |         0xff
   |       |       |      Payload: {Code: 0.01,
   |       |       |                Uri-Path: "alarm_status"
   |       |       |               } // Encrypted with CTX_C_S
   |       |       |
   |     Encrypt   |
   |     REQ with  |
   |     CTX_P_S   |
   |       |       |
   |       +------>|         Code: 0.02 (POST)
   |       | POST  |        Token: 0x7b
   |       |       |       OSCORE: [kid: 0xd4, Partial IV: 31]
   |       |       |         0xff
   |       |       |      Payload: {Code: 0.02,
   |       |       |                OSCORE: [kid: 0x5f, Partial IV: 42],
   |       |       |                0xff,
   |       |       |                {Code: 0.01,
   |       |       |                 Uri-Path: "alarm_status"
   |       |       |                } // Encrypted with CTX_C_S
   |       |       |               } // Encrypted with CTX_P_S
   |       |       |
   |       |     Decrypt

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   |       |     REQ with
   |       |     CTX_P_S
   |       |       |
   |       |     Decrypt
   |       |     REQ with
   |       |     CTX_C_S
   |       |       |
   |       |     Encrypt
   |       |     RESP with
   |       |     CTX_C_S
   |       |       |
   |       |     Encrypt
   |       |     RESP with
   |       |     CTX_P_S
   |       |       |
   |       |<------+         Code: 2.04 (Changed)
   |       |  2.04 |        Token: 0x7b
   |       |       |       OSCORE: -
   |       |       |         0xff
   |       |       |      Payload: {Code: 2.04,
   |       |       |                OSCORE: -,
   |       |       |                0xff,
   |       |       |                {Code: 2.05,
   |       |       |                 0xff,
   |       |       |                 "0"
   |       |       |                } // Encrypted with CTX_C_S
   |       |       |               } // Encrypted with CTX_P_S
   |       |       |
   |     Decrypt   |
   |     RESP with |
   |     CTX_P_S   |
   |       |       |
   |<------+       |         Code: 2.04 (Changed)
   |  2.04 |       |        Token: 0x8c
   |       |       |       OSCORE: -
   |       |       |               0xff
   |       |       |      Payload: {Code: 2.05,
   |       |       |                0xff,
   |       |       |                "0"
   |       |       |               } // Encrypted with CTX_C_S
   |       |       |
 Decrypt   |       |
 RESP with |       |
 CTX_C_S   |       |
   |       |       |

 Square brackets [ ... ] indicate content of compressed COSE object.
 Curly brackets { ... } indicate encrypted data.

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     Figure 2: Use of OSCORE between Client-Server and Proxy-Server

A.3.  Example 3

   In the example shown in Figure 3, message exchanges are protected
   with OSCORE over the following legs.

   *  End-to-end between the client and the server, using the OSCORE
      Security Context CTX_C_S.  The client uses the OSCORE Sender ID
      0x5f when using OSCORE with the server.

   *  Between the client and the proxy, using the OSCORE Security
      Context CTX_C_P.  The client uses the OSCORE Sender ID 0x20 when
      using OSCORE with the proxy.

   *  Between the proxy and the server, using the OSCORE Security
      Context CTX_P_S.  The proxy uses the OSCORE Sender ID 0xd4 when
      using OSCORE with the server.

   Client  Proxy  Server
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_S   |       |
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_P   |       |
     |       |       |
     +------>|       |    Code: 0.02 (POST)
     | POST  |       |   Token: 0x8c
     |       |       |  OSCORE: [kid: 0x20, Partial IV: 31]
     |       |       |    0xff
     |       |       | Payload: {Code: 0.02,
     |       |       |           OSCORE: [kid: 0x5f, Partial IV: 42],
     |       |       |           Uri-Host: example.com,
     |       |       |           Proxy-Scheme: coap,
     |       |       |           0xff,
     |       |       |           {Code: 0.01,
     |       |       |            Uri-Path: "alarm_status"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |          } // Encrypted with CTX_C_P
     |       |       |
     |     Decrypt   |
     |     REQ with  |
     |     CTX_C_P   |
     |       |       |
     |     Encrypt   |

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     |     REQ with  |
     |     CTX_P_S   |
     |       |       |
     |       +------>|    Code: 0.02 (POST)
     |       | POST  |   Token: 0x7b
     |       |       |  OSCORE: [kid: 0xd4, Partial IV: 31]
     |       |       |    0xff
     |       |       | Payload: {Code: 0.02,
     |       |       |           OSCORE: [kid: 0x5f, Partial IV: 42],
     |       |       |           0xff,
     |       |       |           {Code: 0.01,
     |       |       |            Uri-Path: "alarm_status"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |          } // Encrypted with CTX_P_S
     |       |       |
     |       |     Decrypt
     |       |     REQ with
     |       |     CTX_P_S
     |       |       |
     |       |     Decrypt
     |       |     REQ with
     |       |     CTX_C_S
     |       |       |
     |       |     Encrypt
     |       |     RESP with
     |       |     CTX_C_S
     |       |       |
     |       |     Encrypt
     |       |     RESP with
     |       |     CTX_P_S
     |       |       |
     |       |<------+    Code: 2.04 (Changed)
     |       |  2.04 |   Token: 0x7b
     |       |       |  OSCORE: -
     |       |       |    0xff
     |       |       | Payload: {Code: 2.04,
     |       |       |           OSCORE: -,
     |       |       |           0xff,
     |       |       |           {Code: 2.05,
     |       |       |            0xff,
     |       |       |            "0"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |          } // Encrypted with CTX_P_S
     |       |       |
     |     Decrypt   |
     |     RESP with |
     |     CTX_P_S   |
     |       |       |

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     |     Encrypt   |
     |     ERSP with |
     |     CTX_C_P   |
     |       |       |
     |<------+       |    Code: 2.04 (Changed)
     |  2.04 |       |   Token: 0x8c
     |       |       |  OSCORE: -
     |       |       |    0xff
     |       |       | Payload: {Code: 2.04,
     |       |       |           OSCORE: -,
     |       |       |           0xff,
     |       |       |           {Code: 2.05,
     |       |       |            0xff,
     |       |       |            "0"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |          } // Encrypted with CTX_C_P
     |       |       |
   Decrypt   |       |
   RESP with |       |
   CTX_C_P   |       |
     |       |       |
   Decrypt   |       |
   RESP with |       |
   CTX_C_S   |       |
     |       |       |

   Square brackets [ ... ] indicate content of compressed COSE object.
   Curly brackets { ... } indicate encrypted data.

      Figure 3: Use of OSCORE between Client-Server, Client-Proxy and
                                Proxy-Server

A.4.  Example 4

   In the example shown in Figure 4, message exchanges are protected
   over the following legs.

   *  End-to-end, between the client and the server, using the OSCORE
      Security Context CTX_C_S.  The client uses the OSCORE Sender ID
      0x5f when using OSCORE with the server.

   *  Between the client and the proxy, using the OSCORE Security
      Context CTX_C_P.  The client uses the OSCORE Sender ID 0x20 when
      using OSCORE with the proxy.

   The example also shows how the client establishes an OSCORE Security
   Context CTX_C_P with the proxy and CTX_C_S with the server, by using
   the key establishment protocol EDHOC [I-D.ietf-lake-edhoc].

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   Client  Proxy  Server
     |       |       |
     +------>|       |         Code: 0.02 (POST)
     | POST  |       |        Token: 0xf3
     |       |       |     Uri-Path: ".well-known"
     |       |       |     Uri-Path: "edhoc"
     |       |       |         0xff
     |       |       |      Payload: (true, EDHOC message_1)
     |       |       |
     |<------+       |         Code: 2.04 (Changed)
     |  2.04 |       |        Token: 0xf3
     |       |       |         0xff
     |       |       |      Payload: EDHOC message_2
     |       |       |
   Establish |       |
   CTX_C_P   |       |
     |       |       |
     +------>|       |         Code: 0.02 (POST)
     | POST  |       |        Token: 0x82
     |       |       |     Uri-Path: ".well-known"
     |       |       |     Uri-Path: edhoc
     |       |       |         0xff
     |       |       |      Payload: (C_R, EDHOC message_3)
     |       |       |
     |     Establish |
     |     CTX_C_P   |
     |       |       |
     |<------+       |
     |  ACK  |       |
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_P   |       |
     |       |       |
     +------>|       |         Code: 0.02 (POST)
     | POST  |       |        Token: 0xbe
     |       |       |       OSCORE: [kid: 0x20, Partial IV: 00]
     |       |       |         0xff
     |       |       |      Payload: {Code: 0.02,
     |       |       |                Uri-Host: "example.com",
     |       |       |                Uri-Path: ".well-known",
     |       |       |                Uri-Path: "edhoc",
     |       |       |                Proxy-Scheme: "coap",
     |       |       |                0xff,
     |       |       |                (true, EDHOC message_1)
     |       |       |               } // Encrypted with CTX_C_P
     |       |       |
     |     Decrypt   |

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     |     REQ with  |
     |     CTX_C_P   |
     |       |       |
     |       +------>|         Code: 0.02 (POST)
     |       | POST  |        Token: 0xa5
     |       |       |     Uri-Path: ".well-known"
     |       |       |     Uri-Path: "edhoc"
     |       |       |         0xff
     |       |       |      Payload: (true, EDHOC message_1)
     |       |       |
     |       |<------+         Code: 2.04 (Changed)
     |       |  2.04 |        Token: 0xa5
     |       |       |         0xff
     |       |       |      Payload: EDHOC message_2
     |       |       |
     |     Encrypt   |
     |     RESP with |
     |     CTX_C_P   |
     |       |       |
     |<------+       |         Code: 2.04 (Changed)
     |  2.04 |       |        Token: 0xbe
     |       |       |       OSCORE: -
     |       |       |         0xff
     |       |       |      Payload: {Code: 2.04,
     |       |       |                0xff,
     |       |       |                EDHOC message_2
     |       |       |               } // Encrypted with CTX_C_P
     |       |       |
   Establish |       |
   CTX_C_S   |       |
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_P   |       |
     |       |       |
     +------>|       |         Code: 0.02 (POST)
     | POST  |       |        Token: 0xb9
     |       |       |       OSCORE: [kid: 0x20, Partial IV: 01]
     |       |       |         0xff
     |       |       |      Payload: {Code: 0.02,
     |       |       |                Uri-Host: "example.com",
     |       |       |                Uri-Path: ".well-known",
     |       |       |                Uri-Path: "edhoc",
     |       |       |                Proxy-Scheme: "coap",
     |       |       |                0xff,
     |       |       |                (C_R, EDHOC message_3)
     |       |       |               } // Encrypted with CTX_C_P
     |       |       |

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     |     Decrypt   |
     |     REQ with  |
     |     CTX_C_P   |
     |       |       |
     |       +------>|         Code: 0.02 (POST)
     |       | POST  |        Token: 0xdd
     |       |       |     Uri-Path: ".well-known"
     |       |       |     Uri-Path: "edhoc"
     |       |       |         0xff
     |       |       |      Payload: (C_R, EDHOC message_3)
     |       |       |
     |       |     Establish
     |       |     CTX_C_S
     |       |       |
     |       |<------+
     |       |  ACK  |
     |       |       |
     |<------+       |
     |  ACK  |       |
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_S   |       |
     |       |       |
   Encrypt   |       |
   REQ with  |       |
   CTX_C_P   |       |
     |       |       |
     +------>|       |    Code: 0.02 (POST)
     | POST  |       |   Token: 0x8c
     |       |       |  OSCORE: [kid: 0x20, Partial IV: 02]
     |       |       |    0xff
     |       |       | Payload: {Code: 0.02,
     |       |       |           OSCORE: [kid: 0x5f, Partial IV: 00],
     |       |       |           Uri-Host: "example.com",
     |       |       |           Proxy-Scheme: "coap",
     |       |       |           0xff,
     |       |       |           {Code: 0.01,
     |       |       |            Uri-Path: "alarm_status"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |          } // Encrypted with CTX_C_P
     |       |       |
     |     Decrypt   |
     |     REQ with  |
     |     CTX_C_P   |
     |       |       |
     |       +------>|    Code: 0.02 (POST)
     |       | POST  |   Token: 0x7b

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     |       |       |  OSCORE: [kid: 0x5f, Partial IV: 00]
     |       |       |    0xff
     |       |       | Payload: {Code: 0.01,
     |       |       |           Uri-Path: "alarm_status"
     |       |       |          } // Encrypted with CTX_C_S
     |       |       |
     |       |     Decrypt
     |       |     REQ with
     |       |     CTX_C_S
     |       |       |
     |       |     Encrypt
     |       |     RESP with
     |       |     CTX_C_S
     |       |       |
     |       |<------+    Code: 2.04 (Changed)
     |       |  2.04 |   Token: 0x7b
     |       |       |  OSCORE: -
     |       |       |    0xff
     |       |       | Payload: {Code: 2.05,
     |       |       |           0xff,
     |       |       |           "0"
     |       |       |          } // Encrypted with CTX_C_S
     |       |       |
     |     Encrypt   |
     |     RESP with |
     |     CTX_C_P   |
     |       |       |
     |<------+       |    Code: 2.04 (Changed)
     |  2.04 |       |   Token: 0x8c
     |       |       |  OSCORE: -
     |       |       |    0xff
     |       |       | Payload: {Code: 2.04,
     |       |       |           OSCORE: -,
     |       |       |           0xff,
     |       |       |           {Code: 2.05,
     |       |       |            0xff,
     |       |       |            "0"
     |       |       |           } // Encrypted with CTX_C_S
     |       |       |          } // Encrypted with CTX_C_P
     |       |       |
   Decrypt   |       |
   RESP with |       |
   CTX_C_P   |       |
     |       |       |
   Decrypt   |       |
   RESP with |       |
   CTX_C_S   |       |
     |       |       |

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   Square brackets [ ... ] indicate content of compressed COSE object.
   Curly brackets { ... } indicate encrypted data.
   Round brackets (...) indicate a CBOR sequence [RFC 8742].

      Figure 4: Use of OSCORE between Client-Server and Proxy-Server,
          with OSCORE Security Contexts established through EDHOC

A.5.  Example 5

   In the example shown in Figure 5, message exchanges are protected
   over the following legs.

   *  End-to-end, between the client and the server.  The client uses
      the OSCORE Sender ID 0x5f when using OSCORE with the server.

   *  Between the client and the proxy.  The client uses the OSCORE
      Sender ID 0x20 when using OSCORE with the proxy.

   The example also shows how the client establishes an OSCORE Security
   Context CTX_C_P with the proxy and CTX_C_S with the server, by using
   the key establishment protocol EDHOC [I-D.ietf-lake-edhoc].

   In particular, the client relies on the EDHOC + OSCORE request
   defined in [I-D.ietf-core-oscore-edhoc] and denoted as COMB_REQ, in
   order to transport the last EDHOC message_3 and the first OSCORE-
   protected application CoAP request combined together.

 Client  Proxy  Server
   |       |       |
   +------>|       |         Code: 0.02 (POST)
   | POST  |       |        Token: 0xf3
   |       |       |     Uri-Path: ".well-known"
   |       |       |     Uri-Path: "edhoc"
   |       |       |         0xff
   |       |       |      Payload: (true, EDHOC message_1)
   |       |       |
   |<------+       |         Code: 2.04 (Changed)
   |  2.04 |       |        Token: 0xf3
   |       |       |         0xff
   |       |       |      Payload: EDHOC message_2
   |       |       |
 Establish |       |
 CTX_C_P   |       |
   |       |       |
 Encrypt   |       |
 REQ with  |       |
 CTX_C_P   |       |
   |       |       |

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 Prepare   |       |
 COMB_REQ  |       |
 for P     |       |
 from REQ  |       |
   |       |       |
   +------>|       |         Code: 0.02 (POST)
   | POST  |       |        Token: 0x82
   |       |       |       OSCORE: [kid: 0x20, Partial IV: 00]
   |       |       |        EDHOC: -
   |       |       |         0xff
   |       |       |      Payload: EDHOC message_3, // Intended for P
   |       |       |               {Code: 0.02,
   |       |       |                Uri-Host: "example.com",
   |       |       |                Uri-Path: ".well-known",
   |       |       |                Uri-Path: "edhoc",
   |       |       |                Proxy-Scheme: "coap",
   |       |       |                0xff,
   |       |       |                (true, EDHOC message_1)
   |       |       |               } // Encrypted with CTX_C_P
   |       |       |
   |     Establish |
   |     CTX_C_P   |
   |       |       |
   |     Rebuild   |
   |     REQ from  |
   |     COMB_REQ  |
   |       |       |
   |     Decrypt   |
   |     REQ with  |
   |     CTX_C_P   |
   |       |       |
   |       +------>|         Code: 0.02 (POST)
   |       | POST  |        Token: 0xa5
   |       |       |     Uri-Path: ".well-known"
   |       |       |     Uri-Path: "edhoc"
   |       |       |         0xff
   |       |       |      Payload: (true, EDHOC message_1)
   |       |       |
   |       |<------+         Code: 2.04 (Changed)
   |       |  2.04 |        Token: 0xa5
   |       |       |         0xff
   |       |       |      Payload: EDHOC message_2
   |       |       |
   |     Encrypt   |
   |     RESP with |
   |     CTX_C_P   |
   |       |       |
   |<------+       |    Code: 2.04 (Changed)

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   |  2.04 |       |   Token: 0x82
   |       |       |  OSCORE: -
   |       |       |    0xff
   |       |       | Payload: {Code: 2.04,
   |       |       |           0xff,
   |       |       |           EDHOC message_2
   |       |       |          } // Encrypted with CTX_C_P
   |       |       |
 Decrypt   |       |
 RESP with |       |
 CTX_C_P   |       |
   |       |       |
 Establish |       |
 CTX_C_S   |       |
   |       |       |
 Encrypt   |       |
 REQ with  |       |
 CTX_C_S   |       |
   |       |       |
 Prepare   |       |
 COMB_REQ  |       |
 for S     |       |
 from REQ  |       |
   |       |       |
 Encrypt   |       |
 REQ with  |       |
 CTX_C_P   |       |
   |       |       |
   +------>|       |         Code: 0.02 (POST)
   | POST  |       |        Token: 0x83
   |       |       |       OSCORE: [kid: 0x20, Partial IV: 01]
   |       |       |         0xff
   |       |       |      Payload: {Code: 0.02,
   |       |       |                Uri-Host: "example.com",
   |       |       |                OSCORE: [kid: 0x5f, Partial IV: 00],
   |       |       |                EDHOC: -,
   |       |       |                Proxy-Scheme: "coap",
   |       |       |                0xff,
   |       |       |                EDHOC message_3 // Intended for S
   |       |       |                {
   |       |       |                 Code: 0.01,
   |       |       |                 Uri-Path:"alarm_status"
   |       |       |                } // Encrypted with CTX_C_S
   |       |       |               } // Encrypted with CTX_C_P
   |       |       |
   |     Decrypt   |
   |     REQ with  |
   |     CTX_C_P   |

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   |       |       |
   |       +------>|         Code: 0.02 (POST)
   |       | POST  |        Token: 0xa6
   |       |       |       OSCORE: [kid: 0x5f, Partial IV: 00]
   |       |       |        EDHOC: -
   |       |       |         0xff
   |       |       |      Payload: EDHOC message_3 // Intended for S
   |       |       |               {
   |       |       |                Code: 0.01,
   |       |       |                Uri-Path: "alarm_status"
   |       |       |               } // Encrypted with CTX_C_S
   |       |       |
   |       |     Establish
   |       |     CTX_C_S
   |       |       |
   |       |     Rebuild
   |       |     REQ from
   |       |     COMB_REQ
   |       |       |
   |       |     Decrypt
   |       |     REQ with
   |       |     CTX_C_S
   |       |       |
   |       |     Encrypt
   |       |     RESP with
   |       |     CTX_C_S
   |       |       |
   |       |<------+    Code: 2.04 (Changed)
   |       |  2.04 |   Token: 0xa6
   |       |       |  OSCORE: -
   |       |       |    0xff
   |       |       | Payload: {Code: 2.05,
   |       |       |           0xff,
   |       |       |           "0"
   |       |       |          } // Encrypted with CTX_C_S
   |       |       |
   |     Encrypt   |
   |     RESP with |
   |     CTX_C_P   |
   |       |       |
   |<------+       |    Code: 2.04 (Changed)
   |  2.04 |       |   Token: 0x83
   |       |       |  OSCORE: -
   |       |       |    0xff
   |       |       | Payload: {Code: 2.04,
   |       |       |           OSCORE: -,
   |       |       |           0xff,
   |       |       |           {Code: 2.05,

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   |       |       |            0xff,
   |       |       |            "0"
   |       |       |           } // Encrypted with CTX_C_S
   |       |       |          } // Encrypted with CTX_C_P
   |       |       |
 Decrypt   |       |
 RESP with |       |
 CTX_C_P   |       |
   |       |       |
 Decrypt   |       |
 RESP with |       |
 CTX_C_S   |       |
   |       |       |

 Square brackets [ ... ] indicate content of compressed COSE object.
 Curly brackets { ... } indicate encrypted data.
 Round brackets (...) indicate a CBOR sequence [RFC 8742].

    Figure 5: Use of OSCORE between Client-Server and Proxy-Server,
   with OSCORE Security Contexts established through EDHOC using the
                         EDHOC + OSCORE request

Appendix B.  State Diagram of the Incoming Request Processing

   Figure 6 overviews the processing of an incoming request, as
   specified in Section 3.3.  The dotted boxes indicate ending states
   where the processing terminates.

 Original    +-----------------------------------------------+
 incoming -->|        Are there proxy-related options?       |<--------+
 request     +-----------------------------------------------+         |
              |                                           |            |
             YES                                          NO           |
              |                                           |            |
              v                                           v            |
 +-------------+     +---------+    .......... +------------------+    |
 | Is there a  | YES | Am I a  | NO : Return : | Is there an      |    |
 | Proxy-Uri   |---->| forward |--->: 5.05   : | OSCORE Option?   |    |
 | Option?     |     | proxy?  |    :........: +------------------+    |
 +-------------+     +---------+                ^  |        |          |
    |                 ^       |     ..........  |  NO      YES         |
    NO                |      YES    : Return :  |  |        |          |
    |                 |       |     : 4.01   :  |  |        v          |
    |                 |       |     :........:  |  |  +------------+   |
    |                 |       |       ^         |  |  | Are there  |   |
    |                 |       |       |         |  |  | Uri-Path   |   |
    |                YES      |       NO        |  |  | Options?   |   |
    v                 |       v       |         |  |  +------------+   |

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  +--------------------+ +---------------+      |  |   |          |    |
  | Is there the       | | Is forwarding |      |  |  YES         NO   |
  | Proxy-Scheme       | | this request  |      |  |   |          |    |
  | option with the    | | an authorized |      |  |   v          |    |
  | Uri-Host/Uri-Port  | | operation?    |      |  | ..........   |    |
  | options?           | +---------------+      |  | : Return :   |    |
  +--------------------+  ^           |         |  | : 4.00   :   |    |
    |                     |          YES        |  | :........:   |    |
    NO                    |           |         |  |              v    |
    |                     |           |         |  |       +---------+ |
    v                     |           v         |  |       | Decrypt | |
  +--------------+        |  .................  |  |       +---------+ |
  | There are    |        |  : Consume the   :  |  |         |         |
  | Uri-Path     |        |  : proxy-related :  |  |         |         |
  | Options      |        |  : options and   :  |  |         v         |
  | without      |        |  : forward the   :  |  | +----------+      |
  | Proxy-Scheme |        |  : request       :  |  | | Success? |-YES -+
  +--------------+        |  :...............:  |  | +----------+
    |                     |                     |  |      |
    |                     |                     |  |      NO
    |                     |                     |  |      |
    |                     |                     |  |      v
    |                     |                     |  | ................
    |                     |                     |  | : OSCORE error :
    |                     |                     |  | : handling     :
    |                     |                     |  | :..............:
    |                     |                     |  |
    |                     |                     |  v
    |                     |                     | +--------------+
    |                     |                     | | Is there an  |
    |                     |                     | | application? |
    |                     |                     | +--------------+
    |                     |                     |     |        |
    |                     |                     |    YES       NO
    |                     |                     |     |        |
    |                     |                     |     |        v
    |                     |                     |     |      ..........
    |                     |                     |     |      : Return :
    |                     |                     |     |      : 4.00   :
    |                    YES                    |     |      :........:
    v                     |                     |     v
  +------------------------+                    |   ...................
  | Am I a reverse proxy   |                    |   : Deliver the     :
  | using the indicated    |--NO----------------+   : request to the  :
  | resource for proxying? |                        : application     :
  +------------------------+                        :.................:

              Figure 6: Processing of an incoming request.

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Acknowledgments

   The authors sincerely thank Christian Amsüss, Peter Blomqvist, David
   Navarro and Göran 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
   SE-16440 Kista
   Sweden
   Email: marco.tiloca@ri.se

   Rikard Höglund
   RISE AB
   Isafjordsgatan 22
   SE-16440 Kista
   Sweden
   Email: rikard.hoglund@ri.se

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