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Using Ephemeral Diffie-Hellman Over COSE (EDHOC) with the Constrained Application Protocol (CoAP) and Object Security for Constrained RESTful Environments (OSCORE)
draft-ietf-core-oscore-edhoc-11

Document Type Active Internet-Draft (core WG)
Authors Francesca Palombini , Marco Tiloca , Rikard Höglund , Stefan Hristozov , Göran Selander
Last updated 2024-10-04 (Latest revision 2024-04-09)
Replaces draft-palombini-core-oscore-edhoc
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Using EDHOC with CoAP and OSCORE submitted to IESG for PS
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Details
draft-ietf-core-oscore-edhoc-11
CoRE Working Group                                          F. Palombini
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               M. Tiloca
Expires: 11 October 2024                                      R. Höglund
                                                                 RISE AB
                                                            S. Hristozov
                                                        Fraunhofer AISEC
                                                             G. Selander
                                                                Ericsson
                                                            9 April 2024

 Using Ephemeral Diffie-Hellman Over COSE (EDHOC) with the Constrained
Application Protocol (CoAP) and Object Security for Constrained RESTful
                         Environments (OSCORE)
                    draft-ietf-core-oscore-edhoc-11

Abstract

   The lightweight authenticated key exchange protocol Ephemeral Diffie-
   Hellman Over COSE (EDHOC) can be run over the Constrained Application
   Protocol (CoAP) and used by two peers to establish a Security Context
   for the security protocol Object Security for Constrained RESTful
   Environments (OSCORE).  This document details this use of the EDHOC
   protocol, by specifying a number of additional and optional
   mechanisms.  These especially include an optimization approach for
   combining the execution of EDHOC with the first OSCORE transaction.
   This combination reduces the number of round trips required to set up
   an OSCORE Security Context and to complete an OSCORE transaction
   using that Security Context.

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-edhoc.

Status of This Memo

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  EDHOC Overview  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  EDHOC Combined with OSCORE  . . . . . . . . . . . . . . . . .   8
     3.1.  EDHOC Option  . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Client Processing . . . . . . . . . . . . . . . . . . . .  11
       3.2.1.  Processing of the EDHOC + OSCORE Request  . . . . . .  11
       3.2.2.  Supporting Block-wise . . . . . . . . . . . . . . . .  13
     3.3.  Server Processing . . . . . . . . . . . . . . . . . . . .  13
       3.3.1.  Processing of the EDHOC + OSCORE Request  . . . . . .  13
       3.3.2.  Supporting Block-wise . . . . . . . . . . . . . . . .  15
     3.4.  Example of EDHOC + OSCORE Request . . . . . . . . . . . .  15
   4.  Use of EDHOC Connection Identifiers with OSCORE . . . . . . .  16
     4.1.  Additional Processing of EDHOC Messages . . . . . . . . .  17
       4.1.1.  Initiator Processing of Message 1 . . . . . . . . . .  17
       4.1.2.  Responder Processing of Message 2 . . . . . . . . . .  17
       4.1.3.  Initiator Processing of Message 2 . . . . . . . . . .  18
   5.  Extension and Consistency of Application Profiles . . . . . .  18
   6.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  21

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   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  22
     8.2.  Target Attributes Registry  . . . . . . . . . . . . . . .  23
     8.3.  EDHOC Authentication Credential Types Registry  . . . . .  24
     8.4.  Expert Review Instructions  . . . . . . . . . . . . . . .  25
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  26
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  28
   Appendix A.  Document Updates . . . . . . . . . . . . . . . . . .  28
     A.1.  Version -10 to -11  . . . . . . . . . . . . . . . . . . .  28
     A.2.  Version -09 to -10  . . . . . . . . . . . . . . . . . . .  29
     A.3.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  29
     A.4.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  29
     A.5.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  29
     A.6.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  30
     A.7.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  30
     A.8.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  31
     A.9.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  31
     A.10. Version -01 to -02  . . . . . . . . . . . . . . . . . . .  32
     A.11. Version -00 to -01  . . . . . . . . . . . . . . . . . . .  32
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   Ephemeral Diffie-Hellman Over COSE (EDHOC) [RFC9528] is a lightweight
   authenticated key exchange protocol, especially intended for use in
   constrained scenarios.  In particular, EDHOC messages can be
   transported over the Constrained Application Protocol (CoAP)
   [RFC7252] and used for establishing a Security Context for Object
   Security for Constrained RESTful Environments (OSCORE) [RFC8613].

   This document details the use of the EDHOC protocol with CoAP and
   OSCORE, and specifies a number of additional and optional mechanisms.
   These especially include an optimization approach that combines the
   EDHOC execution with the first OSCORE transaction (see Section 3).
   This allows for a minimum number of two round trips necessary to
   setup the OSCORE Security Context and complete an OSCORE transaction,
   e.g., when an IoT device gets configured in a network for the first
   time.

   This optimization is desirable, since the number of message exchanges
   can have a substantial impact on the latency of conveying the first
   OSCORE request, when using certain radio technologies.

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   Without this optimization, it is not possible to achieve the minimum
   number of two round trips.  This optimization makes it possible,
   since the message_3 of the EDHOC protocol can be made relatively
   small (see Section 1.2 of [RFC9528]), thus allowing additional
   OSCORE-protected CoAP data within target MTU sizes.

   The minimum number of two round trips can be achieved only if the
   default, forward message flow of EDHOC is used, i.e., when a CoAP
   client acts as EDHOC Initiator and a CoAP server acts as EDHOC
   Responder.  The performance advantage of using this optimization can
   be lost when used in combination with Block-wise transfers [RFC7959]
   that rely on specific parameter values and block sizes.

   Furthermore, this document defines a number of parameters
   corresponding to different information elements of an EDHOC
   application profile (see Section 6).  These can be specified as
   target attributes in the link to an EDHOC resource associated with
   that application profile, thus enabling an enhanced discovery of such
   a resource for CoAP clients.

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.

   The reader is expected to be familiar with terms and concepts defined
   in CoAP [RFC7252], CBOR [RFC8949], OSCORE [RFC8613], and EDHOC
   [RFC9528].

2.  EDHOC Overview

   This section is not normative and summarizes what is specified in
   [RFC9528], in particular its Appendix A.2.  Thus, it provides a
   baseline for the enhancements in the subsequent sections.

   The EDHOC protocol specified in [RFC9528] allows two peers to agree
   on a cryptographic secret, in a mutually-authenticated way and by
   using Diffie-Hellman ephemeral keys to achieve forward secrecy.  The
   two peers are denoted as Initiator and Responder, as the one sending
   or receiving the initial EDHOC message_1, respectively.

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   After successful processing of EDHOC message_3, both peers agree on a
   cryptographic secret that can be used to derive further security
   material, and especially to establish an OSCORE Security Context
   [RFC8613].  The Responder can also send an optional EDHOC message_4
   to achieve key confirmation, e.g., in deployments where no protected
   application message is sent from the Responder to the Initiator.

   Appendix A.2 of [RFC9528] specifies how to transfer EDHOC over CoAP.
   That is, the EDHOC data (i.e., the EDHOC message possibly with a
   prepended connection identifier) are transported in the payload of
   CoAP requests and responses.  The default, forward message flow of
   EDHOC consists in the CoAP client acting as Initiator and the CoAP
   server acting as Responder (see Appendix A.2.1 of [RFC9528]).
   Alternatively, the two roles can be reversed, as per the reverse
   message flow of EDHOC (see Appendix A.2.2 of [RFC9528]).  In the rest
   of this document, EDHOC messages are considered to be transferred
   over CoAP.

   Figure 1 shows a successful execution of EDHOC, with a CoAP client
   and a CoAP server running EDHOC as Initiator and Responder,
   respectively.  In particular, it extends Figure 10 from
   Appendix A.2.1 of [RFC9528], by highlighting when the two peers
   perform EDHOC verification and establish the OSCORE Security Context,
   and by adding an exchange of OSCORE-protected CoAP messages after
   completing the EDHOC execution.

   That is, the client sends a POST request to a reserved _EDHOC
   resource_ at the server, by default at the Uri-Path "/.well-known/
   edhoc".  The request payload consists of the CBOR simple value true
   (0xf5) concatenated with EDHOC message_1, which also includes the
   EDHOC connection identifier C_I of the client encoded as per
   Section 3.3 of [RFC9528].  The request has Content-Format
   application/cid-edhoc+cbor-seq.

   This triggers the EDHOC execution at the server, which replies with a
   2.04 (Changed) response.  The response payload consists of EDHOC
   message_2, which also includes the EDHOC connection identifier C_R of
   the server encoded as per Section 3.3 of [RFC9528].  The response has
   Content-Format application/edhoc+cbor-seq.

   Finally, the client sends a POST request to the same EDHOC resource
   used earlier when it sent EDHOC message_1.  The request payload
   consists of the EDHOC connection identifier C_R encoded as per
   Section 3.3 of [RFC9528], concatenated with EDHOC message_3.  The
   request has Content-Format application/cid-edhoc+cbor-seq.

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   After this exchange takes place, and after successful verifications
   as specified in the EDHOC protocol, the client and server can derive
   an OSCORE Security Context, as defined in Appendix A.1 of [RFC9528].
   After that, they can use OSCORE to protect their communications as
   per [RFC8613].  Note that the EDHOC Connection Identifier C_R is used
   as the OSCORE Sender ID of the client (see Appendix A.1 of
   [RFC9528]).  Therefore, C_R is transported in the 'kid' field of the
   OSCORE Option of the OSCORE Request (see Section 6.1 of [RFC8613]).

   The client and server are required to agree in advance on certain
   information and parameters describing how they should use EDHOC.
   These are specified in an application profile associated with the
   EDHOC resource addressed (see Section 3.9 of [RFC9528].

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      CoAP client                                         CoAP server
    (EDHOC Initiator)                                 (EDHOC Responder)
           |                                                    |
           |                                                    |
           | ----------------- EDHOC Request -----------------> |
           |   Header: 0.02 (POST)                              |
           |   Uri-Path: "/.well-known/edhoc"                   |
           |   Content-Format: application/cid-edhoc+cbor-seq   |
           |   Payload: true, EDHOC message_1                   |
           |                                                    |
           | <---------------- EDHOC Response------------------ |
           |       Header: 2.04 (Changed)                       |
           |       Content-Format: application/edhoc+cbor-seq   |
           |       Payload: EDHOC message_2                     |
           |                                                    |
    EDHOC verification                                          |
           |                                                    |
           | ----------------- EDHOC Request -----------------> |
           |   Header: 0.02 (POST)                              |
           |   Uri-Path: "/.well-known/edhoc"                   |
           |   Content-Format: application/cid-edhoc+cbor-seq   |
           |   Payload: C_R, EDHOC message_3                    |
           |                                                    |
           |                                         EDHOC verification
           |                                                    +
           |                                            OSCORE Sec Ctx
           |                                             Derivation
           |                                                    |
           | <---------------- EDHOC Response------------------ |
           |       Header: 2.04 (Changed)                       |
           |       Content-Format: application/edhoc+cbor-seq   |
           |       Payload: EDHOC message_4                     |
           |                                                    |
    OSCORE Sec Ctx                                              |
     Derivation                                                 |
           |                                                    |
           | ---------------- OSCORE Request -----------------> |
           |   Header: 0.02 (POST)                              |
           |   OSCORE: { ... ; kid: C_R }                       |
           |   Payload: OSCORE-protected data                   |
           |                                                    |
           | <--------------- OSCORE Response ----------------- |
           |                 Header: 2.04 (Changed)             |
           |                 OSCORE: { ... }                    |
           |                 Payload: OSCORE-protected data     |
           |                                                    |

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         Figure 1: EDHOC and OSCORE run sequentially.  The optional
                   message_4 is included in this example.

   As shown in Figure 1, this sequential flow where EDHOC is run first
   and then OSCORE is used takes three round trips to complete.

   Section 3 defines an optimization for combining EDHOC with the first
   OSCORE transaction.  This reduces the number of round trips required
   to set up an OSCORE Security Context and to complete an OSCORE
   transaction using that Security Context.

3.  EDHOC Combined with OSCORE

   This section defines an optimization for combining the EDHOC message
   exchange with the first OSCORE transaction, thus minimizing the
   number of round trips between the two peers to the absolute possible
   minimum of two round trips.

   To this end, this approach can be used only if the default, forward
   message flow of EDHOC is used, i.e., when the client acts as
   Initiator and the server acts as Responder.  The same is not possible
   in the case with reversed roles as per the reverse message flow of
   EDHOC.

   When running the sequential flow of Section 2, the client has all the
   information to derive the OSCORE Security Context already after
   receiving EDHOC message_2 and before sending EDHOC message_3.

   Hence, the client can potentially send both EDHOC message_3 and the
   subsequent OSCORE Request at the same time.  On a semantic level,
   this requires sending two REST requests at once, as in Figure 2.

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     CoAP client                                          CoAP server
   (EDHOC Initiator)                                  (EDHOC Responder)
          |                                                     |
          | ------------------ EDHOC Request -----------------> |
          |   Header: 0.02 (POST)                               |
          |   Uri-Path: "/.well-known/edhoc"                    |
          |   Content-Format: application/cid-edhoc+cbor-seq    |
          |   Payload: true, EDHOC message_1                    |
          |                                                     |
          | <----------------- EDHOC Response------------------ |
          |        Header: Changed (2.04)                       |
          |        Content-Format: application/edhoc+cbor-seq   |
          |        Payload: EDHOC message_2                     |
          |                                                     |
   EDHOC verification                                           |
          +                                                     |
    OSCORE Sec Ctx                                              |
      Derivation                                                |
          |                                                     |
          | -------------- EDHOC + OSCORE Request ------------> |
          |   Header: 0.02 (POST)                               |
          |   OSCORE: { ... ; kid: C_R }                        |
          |   Payload: EDHOC message_3 + OSCORE-protected data  |
          |                                                     |
          |                                          EDHOC verification
          |                                                     +
          |                                            OSCORE Sec Ctx
          |                                               Derivation
          |                                                     |
          | <--------------- OSCORE Response ------------------ |
          |                    Header: 2.04 (Changed)           |
          |                    OSCORE: { ... }                  |
          |                    Payload: OSCORE-protected data   |
          |                                                     |

                    Figure 2: EDHOC and OSCORE combined.

   To this end, the specific approach defined in this section consists
   of sending a single EDHOC + OSCORE request, which conveys the pair
   (C_R, EDHOC message_3) within an OSCORE-protected CoAP message.

   That is, the EDHOC + OSCORE request is composed of the following two
   parts combined together in a single CoAP message.  The steps for
   processing the EDHOC + OSCORE request and the two parts combined in
   there are defined in Section 3.2.1 and Section 3.3.1.

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   *  The OSCORE Request from Figure 1, which is also in this case sent
      to a protected resource, with the correct CoAP method and options
      intended for accessing that resource.

   *  EDHOC data consisting of the pair (C_R, EDHOC message_3) required
      for completing the EDHOC session, transported as follows:

      -  C_R is the OSCORE Sender ID of the client and hence transported
         in the 'kid' field of the OSCORE Option (see Section 6.1 of
         [RFC8613]).  Unlike in the sequential workflow shown in
         Figure 1, C_R is thus not transported in the payload of the
         EDHOC + OSCORE request.

      -  EDHOC message_3 is transported in the payload of the EDHOC +
         OSCORE request, prepended to the payload of the OSCORE Request.
         This is because EDHOC message_3 may be too large to be included
         in a CoAP Option, e.g., when conveying a large public key
         certificate chain as ID_CRED_I (see Section 3.5.3 of [RFC9528])
         or when conveying large External Authorization Data as EAD_3
         (see Section 3.8 of [RFC9528]).

   The rest of this section specifies how to transport the data in the
   EDHOC + OSCORE request and their processing order.  In particular,
   the use of this approach is explicitly signalled by including an
   EDHOC Option (see Section 3.1) in the EDHOC + OSCORE request.  The
   processing of the EDHOC + OSCORE request is specified in Section 3.2
   for the client side and in Section 3.3 for the server side.

3.1.  EDHOC Option

   This section defines the EDHOC Option.  The option is used in a CoAP
   request, to signal that the request payload conveys both an EDHOC
   message_3 and OSCORE-protected data, combined together.

   The EDHOC Option has the properties summarized in Table 1, which
   extends Table 4 of [RFC7252].  The option is Critical, Safe-to-
   Forward, and part of the Cache-Key. The option MUST occur at most
   once and MUST be empty.  If any value is sent, the recipient MUST
   ignore it.  (Future documents may update the definition of the
   option, by expanding its semantics and specifying admitted values.)
   The option is intended only for CoAP requests and is of Class U for
   OSCORE [RFC8613].

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        +=====+===+===+===+===+=======+========+========+=========+
        | No. | C | U | N | R | Name  | Format | Length | Default |
        +=====+===+===+===+===+=======+========+========+=========+
        | 21  | x |   |   |   | EDHOC | Empty  | 0      | (none)  |
        +-----+---+---+---+---+-------+--------+--------+---------+

             Table 1: The EDHOC Option.  C=Critical, U=Unsafe,
                         N=NoCacheKey, R=Repeatable

   The presence of this option means that the message payload also
   contains EDHOC data, which must be extracted and processed as defined
   in Section 3.3, before the rest of the message can be processed.

   Figure 3 shows an example of a CoAP message transported over UDP and
   containing both the EDHOC data and the OSCORE ciphertext, using the
   newly defined EDHOC option for signalling.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver| T |  TKL  |      Code     |          Message ID           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Token (if any, TKL bytes) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Observe Option| OSCORE Option ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | EDHOC Option  | Other Options (if any) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1 1 1 1 1 1 1 1| Payload ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 3: Example of CoAP message transported over UDP, combining
       EDHOC data and OSCORE data as signalled with the EDHOC Option.

3.2.  Client Processing

   This section describes the processing on the client side.

3.2.1.  Processing of the EDHOC + OSCORE Request

   The client prepares an EDHOC + OSCORE request as follows.

   1.  Compose EDHOC message_3 into EDHOC_MSG_3, as per Section 5.4.2 of
       [RFC9528].

   2.  Establish the new OSCORE Security Context and use it to encrypt
       the original CoAP request as per Section 8.1 of [RFC8613].

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       Note that the OSCORE ciphertext is not computed over EDHOC
       message_3, which is not protected by OSCORE.  That is, the result
       of this step is the OSCORE Request as in Figure 1.

   3.  Build COMB_PAYLOAD as the concatenation of EDHOC_MSG_3 and
       OSCORE_PAYLOAD in this order: COMB_PAYLOAD = EDHOC_MSG_3 |
       OSCORE_PAYLOAD, where | denotes byte string concatenation and:

       *  EDHOC_MSG_3 is the binary encoding of EDHOC message_3
          resulting from step 1.  As per Section 5.4.1 of [RFC9528],
          EDHOC message_3 consists of one CBOR data item CIPHERTEXT_3,
          which is a CBOR byte string.  Therefore, EDHOC_MSG_3 is the
          binary encoding of CIPHERTEXT_3.

       *  OSCORE_PAYLOAD is the OSCORE ciphertext of the OSCORE-
          protected CoAP request resulting from step 2.

   4.  Compose the EDHOC + OSCORE request, as the OSCORE-protected CoAP
       request resulting from step 2, where the payload is replaced with
       COMB_PAYLOAD built at step 3.

       Note that the new payload includes EDHOC message_3, but it does
       not include the EDHOC connection identifier C_R.  As the client
       is the EDHOC Initiator, C_R is the OSCORE Sender ID of the
       client, which is already specified as 'kid' in the OSCORE Option
       of the request from step 2, hence of the EDHOC + OSCORE request.

   5.  Include the new EDHOC Option defined in Section 3.1 into the
       EDHOC + OSCORE request.

       The application/cid-edhoc+cbor-seq media type does not apply to
       this message, whose media type is unnamed.

   6.  Send the EDHOC + OSCORE request to the server.

   With the same server, the client SHOULD NOT have multiple
   simultaneous outstanding interactions (see Section 4.7 of [RFC7252])
   such that: they consist of an EDHOC + OSCORE request; and their EDHOC
   data pertain to the EDHOC session with the same connection identifier
   C_R.

   (An exception might apply for clients that operate under particular
   time constraints over particularly unreliable networks, thus raising
   the chances to promptly complete the EDHOC execution with the server
   through multiple, simultaneous EDHOC + OSCORE requests.  As discussed
   in Section 7, this does not have any impact in terms of security.)

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3.2.2.  Supporting Block-wise

   If Block-wise [RFC7959] is supported, the client may fragment the
   first application CoAP request before protecting it as an original
   message with OSCORE, as defined in Section 4.1.3.4.1 of [RFC8613].

   In such a case, the OSCORE processing in step 2 of Section 3.2.1 is
   performed on each inner block of the first application CoAP request,
   and the following also applies.

   *  The client takes the additional following step between steps 2 and
      3 of Section 3.2.1.

      A.  If the OSCORE-protected request from step 2 conveys a non-
      first inner block of the first application CoAP request (i.e., the
      Block1 Option processed at step 2 had NUM different than 0), then
      the client skips the following steps and sends the OSCORE-
      protected request to the server.  In particular, the client MUST
      NOT include the EDHOC Option in the OSCORE-protected request.

   *  The client takes the additional following step between steps 3 and
      4 of Section 3.2.1.

      B.  If the size of COMB_PAYLOAD exceeds MAX_UNFRAGMENTED_SIZE (see
      Section 4.1.3.4.2 of [RFC8613]), the client MUST stop processing
      the request and MUST abandon the Block-wise transfer.  Then, the
      client can continue by switching to the sequential workflow shown
      in Figure 1.  That is, the client first sends EDHOC message_3
      prepended by the EDHOC Connection Identifier C_R encoded as per
      Section 3.3 of [RFC9528], and then sends the OSCORE-protected CoAP
      request once the EDHOC execution is completed.

   The performance advantage of using the EDHOC + OSCORE request can be
   lost when used in combination with Block-wise transfers that rely on
   specific parameter values and block sizes.  Application policies at
   the CoAP client can define when and how to detect whether the
   performance advantage is lost, and, if that is the case, whether to
   appropriately adjust the parameter values and block sizes, or instead
   to fall back on the sequential workflow of EDHOC.

3.3.  Server Processing

   This section describes the processing on the server side.

3.3.1.  Processing of the EDHOC + OSCORE Request

   In order to process a request containing the EDHOC option, i.e., an
   EDHOC + OSCORE request, the server MUST perform the following steps.

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   1.  Check that the EDHOC + OSCORE request includes the OSCORE option
       and that the request payload has the format defined at step 3 of
       Section 3.2.1 for COMB_PAYLOAD.  If this is not the case, the
       server MUST stop processing the request and MUST reply with a
       4.00 (Bad Request) error response.

   2.  Extract EDHOC message_3 from the payload COMB_PAYLOAD of the
       EDHOC + OSCORE request, as the first element EDHOC_MSG_3 (see
       step 3 of Section 3.2.1).

   3.  Take the value of 'kid' from the OSCORE option of the EDHOC +
       OSCORE request (i.e., the OSCORE Sender ID of the client), and
       use it as the EDHOC connection identifier C_R.

   4.  Retrieve the correct EDHOC session by using the connection
       identifier C_R from step 3.

       If the application profile used in the EDHOC session specifies
       that EDHOC message_4 shall be sent, the server MUST stop the
       EDHOC processing and consider it failed, as due to a client
       error.

       Otherwise, perform the EDHOC processing on the EDHOC message_3
       extracted at step 2 as per Section 5.4.3 of [RFC9528], based on
       the protocol state of the retrieved EDHOC session.

       The application profile used in the EDHOC session is the same one
       associated with the EDHOC resource where the server received the
       request conveying EDHOC message_1 that started the session.  This
       is relevant in case the server provides multiple EDHOC resources,
       which may generally refer to different application profiles.

   5.  Establish a new OSCORE Security Context associated with the
       client as per Appendix A.1 of [RFC9528], using the EDHOC output
       from step 4.

   6.  Extract the OSCORE ciphertext from the payload COMB_PAYLOAD of
       the EDHOC + OSCORE request, as the second element OSCORE_PAYLOAD
       (see step 3 of Section 3.2.1).

   7.  Rebuild the OSCORE-protected CoAP request, as the EDHOC + OSCORE
       request where the payload is replaced with the OSCORE ciphertext
       extracted at step 6.  Then, remove the EDHOC option.

   8.  Decrypt and verify the OSCORE-protected CoAP request rebuilt at
       step 7, as per Section 8.2 of [RFC8613], by using the OSCORE
       Security Context established at step 5.

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       When the decrypted request is checked for any critical CoAP
       options (as it is during regular CoAP processing), the presence
       of an EDHOC option MUST be regarded as an unprocessed critical
       option, unless it is processed by some further mechanism.

   9.  Deliver the CoAP request resulting from step 8 to the
       application.

   If steps 4 (EDHOC processing) and 8 (OSCORE processing) are both
   successfully completed, the server MUST reply with an OSCORE-
   protected response (see Section 5.4.3 of [RFC9528]).  The usage of
   EDHOC message_4 as defined in Section 5.5 of [RFC9528] is not
   applicable to the approach defined in this document.

   If step 4 (EDHOC processing) fails, the server aborts the session as
   per Section 5.4.3 of [RFC9528] and responds with an EDHOC error
   message with error code 1, formatted as defined in Section 6.2 of
   [RFC9528].  The server MUST NOT establish a new OSCORE Security
   Context from the present EDHOC session with the client.  The CoAP
   response conveying the EDHOC error message is not protected with
   OSCORE.  As per Section 9.5 of [RFC9528], the server has to make sure
   that the error message does not reveal sensitive information.  The
   CoAP response conveying the EDHOC error message MUST have Content-
   Format set to application/edhoc+cbor-seq registered in Section 10.9
   of [RFC9528].

   If step 4 (EDHOC processing) is successfully completed but step 8
   (OSCORE processing) fails, the same OSCORE error handling as defined
   in Section 8.2 of [RFC8613] applies.

3.3.2.  Supporting Block-wise

   If Block-wise [RFC7959] is supported, the server takes the additional
   following step before any other in Section 3.3.1.

   A.  If Block-wise is present in the request, then process the Outer
   Block options according to [RFC7959], until all blocks of the request
   have been received (see Section 4.1.3.4 of [RFC8613]).

3.4.  Example of EDHOC + OSCORE Request

   Figure 4 shows an example of EDHOC + OSCORE Request transported over
   UDP.  In particular, the example assumes that:

   *  The OSCORE Partial IV in use is 0, consistently with the first
      request protected with the new OSCORE Security Context.

   *  The OSCORE Sender ID of the client is 0x01.

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      As per Section 3.3.3 of [RFC9528], this straightforwardly
      corresponds to the EDHOC connection identifier C_R 0x01.

      As per Section 3.3.2 of [RFC9528], when using the sequential flow
      shown in Figure 1, the same C_R with value 0x01 would be encoded
      on the wire as the CBOR integer 1 (0x01 in CBOR encoding), and
      prepended to EDHOC message_3 in the payload of the second EDHOC
      request.

   *  The EDHOC option is registered with CoAP option number 21.

   Note to RFC Editor: Please delete the last bullet point in the
   previous list, since, at the time of publication, the CoAP option
   number will be in fact registered.

   This results in the following components shown in Figure 4:

   *  OSCORE option value: 0x090001 (3 bytes)

   *  EDHOC option value: - (0 bytes)

   *  EDHOC message_3: 0x52d5535f3147e85f1cfacd9e78abf9e0a81bbf (19
      bytes)

   *  OSCORE ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)

             Protected CoAP request (OSCORE message):

                0x44025d1f               ; CoAP 4-byte header
                  00003974               ; Token
                  93 090001              ; OSCORE Option
                  c0                     ; EDHOC Option
                  ff 52d5535f3147e85f1cfacd9e78abf9e0a81bbf
                     612f1092f1776f1c1668b3825e
                (46 bytes)

     Figure 4: Example of CoAP message transported over UDP, combining
       EDHOC data and OSCORE data as signalled with the EDHOC Option.

4.  Use of EDHOC Connection Identifiers with OSCORE

   The OSCORE Sender/Recipient IDs are the EDHOC connection identifiers
   (see Section 3.3.3 of [RFC9528]).  This applies also to the optimized
   workflow defined in Section 3 of this document.

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   Note that, at step 3 of Section 3.3.1, the value of 'kid' in the
   OSCORE Option of the EDHOC + OSCORE request is both the server's
   Recipient ID (i.e., the client's Sender ID) and the EDHOC Connection
   Identifier C_R of the server.

4.1.  Additional Processing of EDHOC Messages

   When using EDHOC to establish an OSCORE Security Context, the client
   and server MUST perform the following additional steps during an
   EDHOC execution, thus extending Section 5 of [RFC9528].

4.1.1.  Initiator Processing of Message 1

   The Initiator selects an EDHOC Connection Identifier C_I as follows.

   The Initiator MUST choose a C_I that is neither used in any current
   EDHOC session as this peer's EDHOC Connection Identifier, nor the
   Recipient ID in a current OSCORE Security Context where the ID
   Context is not present.

   The chosen C_I SHOULD NOT be the Recipient ID of any current OSCORE
   Security Context.  Note that, unless the two peers concurrently use
   alternative methods to establish OSCORE Security Contexts, this
   allows the Responder to always omit the 'kid context' in the OSCORE
   Option of its messages sent to the Initiator, when protecting those
   with an OSCORE Security Context where C_I is the Responder's OSCORE
   Sender ID (see Section 6.1 of [RFC8613]).

4.1.2.  Responder Processing of Message 2

   The Responder selects an EDHOC Connection Identifier C_R as follows.

   The Responder MUST choose a C_R that is neither used in any current
   EDHOC session as this peer's EDHOC Connection Identifier, nor is
   equal to the EDHOC Connection Identifier C_I specified in the EDHOC
   message_1 of the present EDHOC session, nor is the Recipient ID in a
   current OSCORE Security Context where the ID Context is not present.

   The chosen C_R SHOULD NOT be the Recipient ID of any current OSCORE
   Security Context.  Note that, for a reason analogous to the one given
   above with C_I, this allows the Initiator to always omit the 'kid
   context' in the OSCORE Option of its messages sent to the Responder,
   when protecting those with an OSCORE Security Context where C_R is
   the Initiator's OSCORE Sender ID (see Section 6.1 of [RFC8613]).

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4.1.3.  Initiator Processing of Message 2

   If the EDHOC Connection Identifier C_I is equal to the EDHOC
   Connection Identifier C_R specified in EDHOC message_2, then the
   Initiator MUST abort the session and reply with an EDHOC error
   message with error code 1, formatted as defined in Section 6.2 of
   [RFC9528].

5.  Extension and Consistency of Application Profiles

   It is possible to include the information below in the application
   profile referred by the client and server, according to the specified
   consistency rules.

   If the server supports the EDHOC + OSCORE request within an EDHOC
   execution started at a certain EDHOC resource, then the application
   profile associated with that resource SHOULD explicitly specify
   support for the EDHOC + OSCORE request.

   In case the application profile indicates that the server supports
   the optional EDHOC message_4 (see Section 5.5 of [RFC9528]), it is
   still possible to use the optimized workflow based on the EDHOC +
   OSCORE request.  However, this means the server is not going to send
   EDHOC message_4, since it is not applicable to the optimized workflow
   (see Section 3.3.1).

   Also, in case the application profile indicates that the server shall
   send EDHOC message_4, then the application profile MUST NOT specify
   support for the EDHOC + OSCORE request, and there is no point for the
   client to use the optimized workflow, which is bound to fail (see
   Section 3.3.1).

6.  Web Linking

   Section 10.10 of [RFC9528] registers the resource type "core.edhoc",
   which can be used as target attribute in a web link [RFC8288] to an
   EDHOC resource, e.g., using a link-format document [RFC6690].  This
   enables clients to discover the presence of EDHOC resources at a
   server, possibly using the resource type as filter criterion.

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   At the same time, the application profile associated with an EDHOC
   resource provides information describing how the EDHOC protocol can
   be used through that resource.  A client may become aware of the
   application profile, e.g., by obtaining its information elements upon
   discovering the EDHOC resources at the server.  This allows the
   client to discover especially the EDHOC resources whose associated
   application profile denotes a way of using EDHOC which is most
   suitable to the client, e.g., with EDHOC cipher suites or
   authentication methods that the client also supports or prefers.

   That is, while discovering an EDHOC resource, a client can
   contextually obtain relevant pieces of information from the
   application profile associated with that resource.  The resource
   discovery can occur by means of a direct interaction with the server,
   or instead by means of the CoRE Resource Directory [RFC9176], where
   the server may have registered the links to its resources.

   In order to enable the above, this section defines a number of
   parameters, each of which can be optionally specified as a target
   attribute with the same name in the link to the respective EDHOC
   resource, or as filter criteria in a discovery request from the
   client.  When specifying these parameters in a link to an EDHOC
   resource, the target attribute rt="core.edhoc" MUST be included, and
   the same consistency rules defined in Section 5 for the corresponding
   information elements of an application profile MUST be followed.

   The following parameters are defined.

   *  'ed-i', specifying, if present, that the server supports the EDHOC
      Initiator role, hence the reverse message flow of EDHOC.  A value
      MUST NOT be given to this parameter and any present value MUST be
      ignored by the recipient.

   *  'ed-r', specifying, if present, that the server supports the EDHOC
      Responder role, hence the forward message flow of EDHOC.  A value
      MUST NOT be given to this parameter and any present value MUST be
      ignored by the recipient.

   *  'ed-method', specifying an authentication method supported by the
      server.  This parameter MUST specify a single value, which is
      taken from the 'Value' column of the "EDHOC Method Type" registry
      defined in Section 10.3 of [RFC9528].  This parameter MAY occur
      multiple times, with each occurrence specifying an authentication
      method.

   *  'ed-csuite', specifying an EDHOC cipher suite supported by the
      server.  This parameter MUST specify a single value, which is
      taken from the 'Value' column of the "EDHOC Cipher Suites"

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      registry defined in Section 10.2 of [RFC9528].  This parameter MAY
      occur multiple times, with each occurrence specifying a cipher
      suite.

   *  'ed-cred-t', specifying a type of authentication credential
      supported by the server.  This parameter MUST specify a single
      value, which is taken from the 'Value' column of the "EDHOC
      Authentication Credential Types" Registry defined in Section 8.3
      of this document.  This parameter MAY occur multiple times, with
      each occurrence specifying a type of authentication credential.

   *  'ed-idcred-t', specifying a type of identifier supported by the
      server for identifying authentication credentials.  This parameter
      MUST specify a single value, which is taken from the 'Label'
      column of the "COSE Header Parameters" registry
      [COSE.Header.Parameters].  This parameter MAY occur multiple
      times, with each occurrence specifying a type of identifier for
      authentication credentials.

      Note that the values in the 'Label' column of the "COSE Header
      Parameters" registry are strongly typed.  On the contrary, Link
      Format is weakly typed and thus does not distinguish between, for
      instance, the string value "-10" and the integer value -10.  Thus,
      if responses in Link Format are returned, string values which look
      like an integer are not supported.  Therefore, such values MUST
      NOT be used in the 'ed-idcred-t' parameter.

   *  'ed-ead', specifying the support of the server for an External
      Authorization Data (EAD) item (see Section 3.8 of [RFC9528]).
      This parameter MUST specify a single value, which is taken from
      the 'Label' column of the "EDHOC External Authorization Data"
      registry defined in Section 10.5 of [RFC9528].  This parameter MAY
      occur multiple times, with each occurrence specifying the
      ead_label of an EAD item that the server supports.

   *  'ed-comb-req', specifying, if present, that the server supports
      the EDHOC + OSCORE request defined in Section 3.  A value MUST NOT
      be given to this parameter and any present value MUST be ignored
      by the recipient.

   (Future documents may update the definition of the parameters 'ed-i',
   'ed-r', and 'ed-comb-req', by expanding their semantics and
   specifying what they can take as value.)

   The example in Figure 5 shows how a client discovers one EDHOC
   resource at a server, obtaining information elements from the
   respective application profile.  The Link Format notation from
   Section 5 of [RFC6690] is used.

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      REQ: GET /.well-known/core

      RES: 2.05 Content
          </sensors/temp>;osc,
          </sensors/light>;if=sensor,
          </.well-known/edhoc>;rt=core.edhoc;ed-csuite=0;ed-csuite=2;
              ed-method=0;ed-cred-t=1;ed-cred-t=3;ed-idcred-t=4;
              ed-i;ed-r;ed-comb-req

                          Figure 5: The Web Link.

7.  Security Considerations

   The same security considerations from OSCORE [RFC8613] and EDHOC
   [RFC9528] hold for this document.  In addition, the following
   considerations also apply.

   Section 3.2.1 specifies that a client SHOULD NOT have multiple
   outstanding EDHOC + OSCORE requests pertaining to the same EDHOC
   session.  Even if a client did not fulfill this requirement, it would
   not have any impact in terms of security.  That is, the server would
   still not process different instances of the same EDHOC message_3
   more than once in the same EDHOC session (see Section 5.1 of
   [RFC9528]), and would still enforce replay protection of the OSCORE-
   protected request (see Sections 7.4 and 8.2 of [RFC8613]).

   When using the optimized workflow in Figure 2, a minimum of 128-bit
   security against online brute force attacks is achieved after the
   client receives and successfully verifies the first OSCORE-protected
   response (see Sections 9.1 and 9.4 of [RFC9528]).  As an example, if
   EDHOC is used with method 3 (see Section 3.2 of [RFC9528]) and cipher
   suite 2 (see Section 3.6 of [RFC9528]), then the following holds.

   *  The Initiator is authenticated with 128-bit security against
      online attacks.  As per Section 9.1 of [RFC9528], this results
      from the combination of the strength of the 64-bit MAC in EDHOC
      message_3 and of the 64-bit MAC in the AEAD of the first OSCORE-
      protected CoAP request, as rebuilt at step 7 of Section 3.3.1.

   *  The Responder is authenticated with 128-bit security against
      online attacks.  As per Section 9.1 of [RFC9528], this results
      from the combination of the strength of the 64-bit MAC in EDHOC
      message_2 and of the 64-bit MAC in the AEAD of the first OSCORE-
      protected CoAP response.

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   With reference to the sequential workflow in Figure 1, the OSCORE
   request might have to undergo access control checks at the server,
   before being actually executed for accessing the target protected
   resource.  The same MUST hold when the optimized workflow in Figure 2
   is used, i.e., when using the EDHOC + OSCORE request.

   That is, the rebuilt OSCORE-protected application request from step 7
   in Section 3.3.1 MUST undergo the same access control checks that
   would be performed on a traditional OSCORE-protected application
   request sent individually as shown in Figure 1.

   To this end, validated information to perform access control checks
   (e.g., an access token issued by a trusted party) has to be available
   at the server before starting to process the rebuilt OSCORE-protected
   application request.  Such information may have been provided to the
   server separately before starting the EDHOC execution altogether, or
   instead as External Authorization Data during the EDHOC execution
   (see Section 3.8 of [RFC9528]).

   Thus, a successful completion of the EDHOC protocol and the following
   derivation of the OSCORE Security Context at the server do not play a
   role in determining whether the rebuilt OSCORE-protected request is
   authorized to access the target protected resource at the server.

8.  IANA Considerations

   This document has the following actions for IANA.

   Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
   with the RFC number of this specification and delete this paragraph.

8.1.  CoAP Option Numbers Registry

   IANA is asked to enter the following option number to the "CoAP
   Option Numbers" registry within the "CoRE Parameters" registry group.

                      +========+=======+============+
                      | Number | Name  | Reference  |
                      +========+=======+============+
                      | 21     | EDHOC | [RFC-XXXX] |
                      +--------+-------+------------+

                         Table 2: Registrations in
                            CoAP Option Numbers
                                  Registry

   Note to RFC Editor: Please delete this paragraph and all the
   following text within the present Section 8.1.

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   [

   The CoAP option number 21 is consistent with the properties of the
   EDHOC Option defined in Section 3.1, and it allows the EDHOC Option
   to always result in an overall size of 1 byte.  This is because:

   *  The EDHOC option is always empty, i.e., with zero-length value;
      and

   *  Since the OSCORE Option with option number 9 is always present in
      the EDHOC + OSCORE request, the EDHOC Option is encoded with a
      delta equal to at most 12.

   Although the currently unassigned option number 13 would also work
   well for the same reasons in the use case in question, different use
   cases or protocols may make a better use of the option number 13.
   Hence the preference for the option number 21, and why it is _not_
   necessary to register additional option numbers than 21.

   ]

8.2.  Target Attributes Registry

   IANA is asked to register the following entries in the "Target
   Attributes" registry [CORE.Target.Attributes] within the "Constrained
   RESTful Environments (CoRE) Parameters" registry group, as per
   [I-D.ietf-core-target-attr].  For all entries, the Change Controller
   is IETF, and the reference is [RFC-XXXX].

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     +=================+=============================================+
     | Attribute Name: | Brief Description:                          |
     +=================+=============================================+
     | ed-i            | Hint: support for the EDHOC Initiator role  |
     +-----------------+---------------------------------------------+
     | ed-r            | Hint: support for the EDHOC Responder role  |
     +-----------------+---------------------------------------------+
     | ed-method       | A supported authentication method for EDHOC |
     +-----------------+---------------------------------------------+
     | ed-csuite       | A supported cipher suite for EDHOC          |
     +-----------------+---------------------------------------------+
     | ed-cred-t       | A supported type of authentication          |
     |                 | credential for EDHOC                        |
     +-----------------+---------------------------------------------+
     | ed-idcred-t     | A supported type of authentication          |
     |                 | credential identifier for EDHOC             |
     +-----------------+---------------------------------------------+
     | ed-ead          | A supported External Authorization Data     |
     |                 | (EAD) item for EDHOC                        |
     +-----------------+---------------------------------------------+
     | ed-comb-req     | Hint: support for the EDHOC+OSCORE request  |
     +-----------------+---------------------------------------------+

            Table 3: Registrations in Target Attributes Registry

8.3.  EDHOC Authentication Credential Types Registry

   IANA is requested to create a new "EDHOC Authentication Credential
   Types" registry within the "Ephemeral Diffie-Hellman Over COSE
   (EDHOC)" registry group defined in [RFC9528].

   As registration policy, the registry uses either "Standards Action
   with Expert Review", or "Specification Required" per Section 4.6 of
   [RFC8126].  Expert Review guidelines are provided in Section 8.4.

   All assignments according to "Standards Action with Expert Review"
   are made on a "Standards Action" basis per Section 4.9 of [RFC8126],
   with Expert Review additionally required per Section 4.5 of
   [RFC8126].  The procedure for early IANA allocation of Standards
   Track code points defined in [RFC7120] also applies.  When such a
   procedure is used, review and approval by the designated expert are
   also required, in order for the WG chairs to determine that the
   conditions for early allocation are met (see step 2 in Section 3.1 of
   [RFC7120]).

   The columns of this registry are:

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   *  Value: This field contains the value used to identify the type of
      authentication credential.  These values MUST be unique.  The
      value can be an unsigned integer or a negative integer, in the
      range from -65536 to 65535.  Different ranges of values use
      different registration policies [RFC8126]:

      -  Integer values from -24 to 23 are designated as "Standards
         Action With Expert Review".

      -  Integer values from -65536 to -25 and from 24 to 65535 are
         designated as "Specification Required".

      -  Integer values smaller than -65536 and greater than 65535 are
         marked as "Private Use".

   *  Description: This field contains a short description of the type
      of authentication credential.

   *  Reference: This field contains a pointer to the public
      specification for the type of authentication credential.

   Initial entries in this registry are as listed in Table 4.

    +=======+============================================+===========+
    | Value | Description                                | Reference |
    +=======+============================================+===========+
    | 0     | CBOR Web Token (CWT) containing a COSE_Key | [RFC8392] |
    |       | in a 'cnf' claim and possibly other        |           |
    |       | claims.  CWT is defined in RFC 8392.       |           |
    +-------+--------------------------------------------+-----------+
    | 1     | CWT Claims Set (CCS) containing a COSE_Key | [RFC8392] |
    |       | in a 'cnf' claim and possibly other        |           |
    |       | claims.  CCS is defined in RFC 8392.       |           |
    +-------+--------------------------------------------+-----------+
    | 2     | X.509 certificate                          | [RFC5280] |
    +-------+--------------------------------------------+-----------+

          Table 4: Initial Entries in the "EDHOC Authentication
                        Credential Types" Registry

8.4.  Expert Review Instructions

   The IANA registry established in this document is defined as
   "Standards Action with Expert Review" or "Specification Required",
   depending on the range of values for which an assignment is
   requested.  This section gives some general guidelines for what the
   experts should be looking for; but they are being designated as
   experts for a reason, so they should be given substantial latitude.

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   Expert reviewers should take into consideration the following points:

   *  Clarity and correctness of registrations.  Experts are expected to
      check the clarity of purpose and use of the requested entries.
      Experts need to make sure that registered identifiers indicate a
      type of authentication credential whose format and encoding is
      clearly defined in the corresponding specification.  Identifiers
      of types of authentication credentials that do not meet these
      objective of clarity and completeness must not be registered.

   *  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered and that the point is likely to be used in deployments.
      The zones tagged as "Private Use" are intended for testing
      purposes and closed environments.  Code points in other ranges
      should not be assigned for testing.

   *  Specifications are required for the "Standards Action With Expert
      Review" range of point assignment.  Specifications should exist
      for "Specification Required" ranges, but early assignment before a
      specification is available is considered to be permissible.  When
      specifications are not provided, the description provided needs to
      have sufficient information to identify what the point is being
      used for.

   *  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      Standards Track documents does not mean that a Standards Track
      document cannot have points assigned outside of that range.  The
      length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

9.  References

9.1.  Normative References

   [CORE.Target.Attributes]
              IANA, "Target Attributes",
              <https://www.iana.org/assignments/core-parameters/core-
              parameters.xhtml#target-attributes>.

   [COSE.Header.Parameters]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose/cose.xhtml#header-
              parameters>.

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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/rfc/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/rfc/rfc6690>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/rfc/rfc7120>.

   [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>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/rfc/rfc7959>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8288>.

   [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/rfc/rfc8392>.

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   [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>.

   [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/rfc/rfc8949>.

   [RFC9176]  Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/rfc/rfc9176>.

   [RFC9528]  Selander, G., Preuß Mattsson, J., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
              DOI 10.17487/RFC9528, March 2024,
              <https://www.rfc-editor.org/rfc/rfc9528>.

9.2.  Informative References

   [I-D.ietf-core-target-attr]
              Bormann, C., "CoRE Target Attributes Registry", Work in
              Progress, Internet-Draft, draft-ietf-core-target-attr-06,
              11 October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-target-attr-06>.

Appendix A.  Document Updates

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

A.1.  Version -10 to -11

   *  Avoid using quotation marks for CBOR Simple Values.

   *  Early mentioning of the optimization properties.

   *  Less entries of new IANA registry; made their references
      normative.

   *  Clarified meaning of "Standards Action with Expert Review" policy.

   *  Clarifications, simplified phrasing, and editorial improvements.

   *  Updated references.

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A.2.  Version -09 to -10

   *  Expanded acronyms in the document title.

   *  Clarified transport of EDHOC C_R and EDHOC message_3.

   *  Simplified text on the use of EDHOC Connection Identifiers as
      OSCORE Identifiers.

   *  Added the CoAP OSCORE Option in the figures of the EDHOC message
      flows.

   *  Added more pointers to the message processing, now defined in
      dedicated subsections.

   *  Detecting and preventing a loss of performance advantage when
      using Block-wise transfers is for application policies to
      specifiy.

   *  Clarified use of EDHOC application profiles.

   *  Clarified security considerations on the achieved security level.

   *  Fixes and editorial improvements.

A.3.  Version -08 to -09

   *  Clarified meaning of "EDHOC data".

   *  Improved description of entries for the new IANA registry.

   *  Change Controller changed from "IESG" to "IETF".

   *  Editorial: EDHOC Option number denoted as "21" instead of "TBD21".

   *  Fixed references to sections of draft-ietf-lake-edhoc

   *  Clarifications and editorial improvements.

A.4.  Version -07 to -08

   *  Fixes and clarifications from the Shepherd's review.

A.5.  Version -06 to -07

   *  Changed document title.

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   *  The client creates the OSCORE Security Context after creating
      EDHOC message_3.

   *  Revised selection of EDHOC connection identifiers.

   *  Use of "forward message flow" and "reverse message flow".

   *  The payload of the combined request is not a CBOR sequence
      anymore.

   *  EDHOC error messages from the server are not protected with
      OSCORE.

   *  More future-proof error handling on the server side.

   *  Target attribute names prefixed by "ed-".

   *  Defined new target attributes "ed-i" and "ed-r".

   *  Defined single target attribute "ed-ead" signaling supported EAD
      items.

   *  Security consideration on the minimally achieved 128-bit security.

   *  Defined and used the "EDHOC Authentication Credential Types"
      Registry.

   *  High-level sentence replacing the appendix on Block-wise
      performance.

   *  Revised examples.

   *  Editorial improvements.

A.6.  Version -05 to -06

   *  Extended figure on EDHOC sequential workflow.

   *  Revised naming of target attributes.

   *  Clarified semantics of target attributes 'eadx'.

   *  Registration of target attributes.

A.7.  Version -04 to -05

   *  Clarifications on Web Linking parameters.

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   *  Added security considerations.

   *  Revised IANA considerations to focus on the CoAP option number 21.

   *  Guidelines on using Block-wise moved to an appendix.

   *  Editorial improvements.

A.8.  Version -03 to -04

   *  Renamed "applicability statement" to "application profile".

   *  Use the latest Content-Formats.

   *  Use of SHOULD NOT for multiple simultaneous outstanding
      interactions.

   *  No more special conversion from OSCORE ID to EDHOC ID.

   *  Considerations on using Block-wise.

   *  Wed Linking signaling of multiple supported EAD labels.

   *  Added security considerations.

   *  Editorial improvements.

A.9.  Version -02 to -03

   *  Clarifications on transporting EDHOC message_3 in the CoAP
      payload.

   *  At most one simultaneous outstanding interaction as an EDHOC +
      OSCORE request with the same server for the same session with
      connection identifier C_R.

   *  The EDHOC option is removed from the EDHOC + OSCORE request after
      processing the EDHOC data.

   *  Added explicit constraints when selecting a Recipient ID as C_X.

   *  Added processing steps for when Block-wise is used.

   *  Improved error handling on the server.

   *  Improved section on Web Linking.

   *  Updated figures; editorial improvements.

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A.10.  Version -01 to -02

   *  New title, abstract and introduction.

   *  Restructured table of content.

   *  Alignment with latest format of EDHOC messages.

   *  Guideline on ID conversions based on application profile.

   *  Clarifications, extension and consistency on application profile.

   *  Section on web-linking.

   *  RFC8126 terminology in IANA considerations.

   *  Revised Appendix "Checking CBOR Encoding of Numeric Values".

A.11.  Version -00 to -01

   *  Improved background overview of EDHOC.

   *  Added explicit rules for converting OSCORE Sender/Recipient IDs to
      EDHOC connection identifiers following the removal of
      bstr_identifier from EDHOC.

   *  Revised section organization.

   *  Recommended number for EDHOC option changed to 21.

   *  Editorial improvements.

Acknowledgments

   The authors sincerely thank Christian Amsüss, Emmanuel Baccelli,
   Carsten Bormann, Roman Danyliw, Esko Dijk, Joel Halpern, Wes
   Hardaker, Klaus Hartke, John Preuß Mattsson, David Navarro, Shuping
   Peng, Jim Schaad, Jürgen Schönwälder, John Scudder, Orie Steele,
   Gunter Van de Velde, Mališa Vučinić, and Paul Wouters for their
   feedback and comments.

   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

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   Francesca Palombini
   Ericsson
   Email: francesca.palombini@ericsson.com

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden
   Email: marco.tiloca@ri.se

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

   Stefan Hristozov
   Fraunhofer AISEC
   Email: stefan.hristozov@eriptic.com

   Göran Selander
   Ericsson
   Email: goran.selander@ericsson.com

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