\
Network Working Group                                        G. Selander
Internet-Draft                                         J. Preuß Mattsson
Intended status: Standards Track                            F. Palombini
Expires: 23 April 2022                                          Ericsson
                                                         20 October 2021


               Ephemeral Diffie-Hellman Over COSE (EDHOC)
                        draft-ietf-lake-edhoc-12

Abstract

   This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
   very compact and lightweight authenticated Diffie-Hellman key
   exchange with ephemeral keys.  EDHOC provides mutual authentication,
   forward secrecy, and identity protection.  EDHOC is intended for
   usage in constrained scenarios and a main use case is to establish an
   OSCORE security context.  By reusing COSE for cryptography, CBOR for
   encoding, and CoAP for transport, the additional code size can be
   kept very low.

Discussion Venues

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

   Discussion of this document takes place on the Lightweight
   Authenticated Key Exchange Working Group mailing list
   (lake@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/lake/.

   Source for this draft and an issue tracker can be found at
   https://github.com/lake-wg/edhoc.

Status of This Memo

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

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

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




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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Use of EDHOC  . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Message Size Examples . . . . . . . . . . . . . . . . . .   6
     1.4.  Document Structure  . . . . . . . . . . . . . . . . . . .   6
     1.5.  Terminology and Requirements Language . . . . . . . . . .   6
   2.  EDHOC Outline . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Method  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Connection Identifiers  . . . . . . . . . . . . . . . . .  10
     3.4.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.5.  Authentication Parameters . . . . . . . . . . . . . . . .  12
     3.6.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  18
     3.7.  Ephemeral Public Keys . . . . . . . . . . . . . . . . . .  19
     3.8.  External Authorization Data (EAD) . . . . . . . . . . . .  20
     3.9.  Applicability Statement . . . . . . . . . . . . . . . . .  21
   4.  Key Derivation  . . . . . . . . . . . . . . . . . . . . . . .  22
     4.1.  Extract . . . . . . . . . . . . . . . . . . . . . . . . .  23
     4.2.  Expand  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     4.3.  EDHOC-Exporter  . . . . . . . . . . . . . . . . . . . . .  25
     4.4.  EDHOC-KeyUpdate . . . . . . . . . . . . . . . . . . . . .  26
   5.  Message Formatting and Processing . . . . . . . . . . . . . .  26
     5.1.  Message Processing Outline  . . . . . . . . . . . . . . .  27
     5.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  28
     5.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  29
     5.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  32
     5.5.  EDHOC Message 4 . . . . . . . . . . . . . . . . . . . . .  35
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  37
     6.1.  Success . . . . . . . . . . . . . . . . . . . . . . . . .  38



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     6.2.  Unspecified . . . . . . . . . . . . . . . . . . . . . . .  38
     6.3.  Wrong Selected Cipher Suite . . . . . . . . . . . . . . .  38
   7.  Mandatory-to-Implement Compliance Requirements  . . . . . . .  41
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  42
     8.1.  Security Properties . . . . . . . . . . . . . . . . . . .  42
     8.2.  Cryptographic Considerations  . . . . . . . . . . . . . .  44
     8.3.  Cipher Suites and Cryptographic Algorithms  . . . . . . .  45
     8.4.  Post-Quantum Considerations . . . . . . . . . . . . . . .  46
     8.5.  Unprotected Data  . . . . . . . . . . . . . . . . . . . .  46
     8.6.  Denial-of-Service . . . . . . . . . . . . . . . . . . . .  47
     8.7.  Implementation Considerations . . . . . . . . . . . . . .  47
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
     9.1.  EDHOC Exporter Label Registry . . . . . . . . . . . . . .  49
     9.2.  EDHOC Cipher Suites Registry  . . . . . . . . . . . . . .  49
     9.3.  EDHOC Method Type Registry  . . . . . . . . . . . . . . .  51
     9.4.  EDHOC Error Codes Registry  . . . . . . . . . . . . . . .  51
     9.5.  EDHOC External Authorization Data Registry  . . . . . . .  52
     9.6.  COSE Header Parameters Registry . . . . . . . . . . . . .  52
     9.7.  COSE Header Parameters Registry . . . . . . . . . . . . .  52
     9.8.  COSE Key Common Parameters Registry . . . . . . . . . . .  53
     9.9.  CWT Confirmation Methods Registry . . . . . . . . . . . .  53
     9.10. The Well-Known URI Registry . . . . . . . . . . . . . . .  53
     9.11. Media Types Registry  . . . . . . . . . . . . . . . . . .  54
     9.12. CoAP Content-Formats Registry . . . . . . . . . . . . . .  55
     9.13. Resource Type (rt=) Link Target Attribute Values
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  55
     9.14. Expert Review Instructions  . . . . . . . . . . . . . . .  55
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  56
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  56
     10.2.  Informative References . . . . . . . . . . . . . . . . .  59
   Appendix A.  Use with OSCORE and Transfer over CoAP . . . . . . .  62
     A.1.  Selecting EDHOC Connection Identifier . . . . . . . . . .  62
     A.2.  Deriving the OSCORE Security Context  . . . . . . . . . .  63
     A.3.  Transferring EDHOC over CoAP  . . . . . . . . . . . . . .  64
   Appendix B.  Compact Representation . . . . . . . . . . . . . . .  67
   Appendix C.  Use of CBOR, CDDL and COSE in EDHOC  . . . . . . . .  67
     C.1.  CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . .  68
     C.2.  CDDL Definitions  . . . . . . . . . . . . . . . . . . . .  69
     C.3.  COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  70
   Appendix D.  Applicability Template . . . . . . . . . . . . . . .  72
   Appendix E.  EDHOC Message Deduplication  . . . . . . . . . . . .  73
   Appendix F.  Transports Not Natively Providing Correlation  . . .  74
   Appendix G.  Change Log . . . . . . . . . . . . . . . . . . . . .  74
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  79
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  80

1.  Introduction




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1.1.  Motivation

   Many Internet of Things (IoT) deployments require technologies which
   are highly performant in constrained environments [RFC7228].  IoT
   devices may be constrained in various ways, including memory,
   storage, processing capacity, and power.  The connectivity for these
   settings may also exhibit constraints such as unreliable and lossy
   channels, highly restricted bandwidth, and dynamic topology.  The
   IETF has acknowledged this problem by standardizing a range of
   lightweight protocols and enablers designed for the IoT, including
   the Constrained Application Protocol (CoAP, [RFC7252]), Concise
   Binary Object Representation (CBOR, [RFC8949]), and Static Context
   Header Compression (SCHC, [RFC8724]).

   The need for special protocols targeting constrained IoT deployments
   extends also to the security domain [I-D.ietf-lake-reqs].  Important
   characteristics in constrained environments are the number of round
   trips and protocol message sizes, which if kept low can contribute to
   good performance by enabling transport over a small number of radio
   frames, reducing latency due to fragmentation or duty cycles, etc.
   Another important criteria is code size, which may be prohibitive for
   certain deployments due to device capabilities or network load during
   firmware update.  Some IoT deployments also need to support a variety
   of underlying transport technologies, potentially even with a single
   connection.

   Some security solutions for such settings exist already.  CBOR Object
   Signing and Encryption (COSE, [I-D.ietf-cose-rfc8152bis-struct])
   specifies basic application-layer security services efficiently
   encoded in CBOR.  Another example is Object Security for Constrained
   RESTful Environments (OSCORE, [RFC8613]) which is a lightweight
   communication security extension to CoAP using CBOR and COSE.  In
   order to establish good quality cryptographic keys for security
   protocols such as COSE and OSCORE, the two endpoints may run an
   authenticated Diffie-Hellman key exchange protocol, from which shared
   secret key material can be derived.  Such a key exchange protocol
   should also be lightweight; to prevent bad performance in case of
   repeated use, e.g., due to device rebooting or frequent rekeying for
   security reasons; or to avoid latencies in a network formation
   setting with many devices authenticating at the same time.

   This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
   lightweight authenticated key exchange protocol providing good
   security properties including forward secrecy, identity protection,
   and cipher suite negotiation.  Authentication can be based on raw
   public keys (RPK) or public key certificates and requires the
   application to provide input on how to verify that endpoints are
   trusted.  This specification focuses on referencing instead of



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   transporting credentials to reduce message overhead.  EDHOC does
   currently not support pre-shared key (PSK) authentication as
   authentication with static Diffie-Hellman public keys by reference
   produces equally small message sizes but with much simpler key
   distribution and identity protection.

   EDHOC makes use of known protocol constructions, such as SIGMA
   [SIGMA] and Extract-and-Expand [RFC5869].  EDHOC uses COSE for
   cryptography and identification of credentials (including COSE_Key,
   CWT, CCS, X.509, C509, see Section 3.5.3).  COSE provides crypto
   agility and enables the use of future algorithms and credentials
   targeting IoT.

1.2.  Use of EDHOC

   EDHOC is designed for highly constrained settings making it
   especially suitable for low-power wide area networks [RFC8376] such
   as Cellular IoT, 6TiSCH, and LoRaWAN.  A main objective for EDHOC is
   to be a lightweight authenticated key exchange for OSCORE, i.e., to
   provide authentication and session key establishment for IoT use
   cases such as those built on CoAP [RFC7252].  CoAP is a specialized
   web transfer protocol for use with constrained nodes and networks,
   providing a request/response interaction model between application
   endpoints.  As such, EDHOC is targeting a large variety of use cases
   involving 'things' with embedded microcontrollers, sensors, and
   actuators.

   A typical setting is when one of the endpoints is constrained or in a
   constrained network, and the other endpoint is a node on the Internet
   (such as a mobile phone) or at the edge of the constrained network
   (such as a gateway).  Thing-to-thing interactions over constrained
   networks are also relevant since both endpoints would then benefit
   from the lightweight properties of the protocol.  EDHOC could e.g.,
   be run when a device connects for the first time, or to establish
   fresh keys which are not revealed by a later compromise of the long-
   term keys.  Further security properties are described in Section 8.1.

   EDHOC enables the reuse of the same lightweight primitives as OSCORE:
   CBOR for encoding, COSE for cryptography, and CoAP for transport.  By
   reusing existing libraries, the additional code size can be kept very
   low.  Note that, while CBOR and COSE primitives are built into the
   protocol messages, EDHOC is not bound to a particular transport.
   Transfer of EDHOC messages in CoAP payloads is detailed in
   Appendix A.3.







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1.3.  Message Size Examples

   Compared to the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with ECDHE
   and connection ID, the number of bytes in EDHOC + CoAP can be less
   than 1/6 when RPK authentication is used, see
   [I-D.ietf-lwig-security-protocol-comparison].  Figure 1 shows
   examples of message sizes for EDHOC with different kinds of
   authentication keys and different COSE header parameters for
   identification: static Diffie-Hellman keys or signature keys, either
   in CBOR Web Token (CWT) / CWT Claims Set (CCS) [RFC8392] identified
   by a key identifier using 'kid' [I-D.ietf-cose-rfc8152bis-struct], or
   in X.509 certificates identified by a hash value using 'x5t'
   [I-D.ietf-cose-x509].

         ========================================================
                             Static DH Keys        Signature Keys
                             --------------        --------------
                             kid        x5t        kid        x5t
         --------------------------------------------------------
         message_1            37         37         37         37
         message_2            45         58        102        115
         message_3            19         33         77         90
         --------------------------------------------------------
         Total               101        128        216        242
         ========================================================

                Figure 1: Example of message sizes in bytes.

1.4.  Document Structure

   The remainder of the document is organized as follows: Section 2
   outlines EDHOC authenticated with digital signatures, Section 3
   describes the protocol elements of EDHOC, including formatting of the
   ephemeral public keys, Section 4 specifies the key derivation,
   Section 5 specifies message processing for EDHOC authenticated with
   signature keys or static Diffie-Hellman keys, Section 6 describes the
   error messages, and Appendix A shows how to transfer EDHOC with CoAP
   and establish an OSCORE security context.

1.5.  Terminology and Requirements Language

   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.





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   Readers are expected to be familiar with the terms and concepts
   described in CBOR [RFC8949], CBOR Sequences [RFC8742], COSE
   structures and processing [I-D.ietf-cose-rfc8152bis-struct], COSE
   algorithms [I-D.ietf-cose-rfc8152bis-algs], CWT and CWT Claims Set
   [RFC8392], and CDDL [RFC8610].  The Concise Data Definition Language
   (CDDL) is used to express CBOR data structures [RFC8949].  Examples
   of CBOR and CDDL are provided in Appendix C.1.  When referring to
   CBOR, this specification always refers to Deterministically Encoded
   CBOR as specified in Sections 4.2.1 and 4.2.2 of [RFC8949].  The
   single output from authenticated encryption (including the
   authentication tag) is called "ciphertext", following [RFC5116].

2.  EDHOC Outline

   EDHOC specifies different authentication methods of the Diffie-
   Hellman key exchange: digital signatures and static Diffie-Hellman
   keys.  This section outlines the digital signature-based method.
   Further details of protocol elements and other authentication methods
   are provided in the remainder of this document.

   SIGMA (SIGn-and-MAc) is a family of theoretical protocols with a
   large number of variants [SIGMA].  Like IKEv2 [RFC7296] and (D)TLS
   1.3 [RFC8446], EDHOC authenticated with digital signatures is built
   on a variant of the SIGMA protocol which provides identity protection
   of the initiator (SIGMA-I) against active attackers, and like IKEv2
   [RFC7296], EDHOC implements the MAC-then-Sign variant of the SIGMA-I
   protocol shown in Figure 2.

   Initiator                                                   Responder
   |                                G_X                                |
   +------------------------------------------------------------------>|
   |                                                                   |
   |      G_Y, Enc( ID_CRED_R, Sig( R; MAC( CRED_R, G_X, G_Y ) ) )     |
   |<------------------------------------------------------------------+
   |                                                                   |
   |        AEAD( ID_CRED_I, Sig( I; MAC( CRED_I, G_Y, G_X ) ) )       |
   +------------------------------------------------------------------>|
   |                                                                   |

          Figure 2: MAC-then-Sign variant of the SIGMA-I protocol.

   The parties exchanging messages are called Initiator (I) and
   Responder (R).  They exchange ephemeral public keys, compute a shared
   secret, and derive symmetric application keys used to protect
   application data.

   *  G_X and G_Y are the ECDH ephemeral public keys of I and R,
      respectively.



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   *  CRED_I and CRED_R are the credentials containing the public
      authentication keys of I and R, respectively.

   *  ID_CRED_I and ID_CRED_R are credential identifiers enabling the
      recipient party to retrieve the credential of I and R,
      respectively.

   *  Sig(I; . ) and Sig(R; . ) denote signatures made with the private
      authentication key of I and R, respectively.

   *  Enc(), AEAD(), and MAC() denotes encryption, authenticated
      encryption with additional data, and message authentication code
      using keys derived from the shared secret.

   In order to create a "full-fledged" protocol some additional protocol
   elements are needed.  EDHOC adds:

   *  Transcript hashes (hashes of message data) TH_2, TH_3, TH_4 used
      for key derivation and as additional authenticated data.

   *  Computationally independent keys derived from the ECDH shared
      secret and used for authenticated encryption of different
      messages.

   *  An optional fourth message giving explicit key confirmation to I
      in deployments where no protected application data is sent from R
      to I.

   *  A key material exporter and a key update function with forward
      secrecy.

   *  Verification of a common preferred cipher suite.

   *  Method types and error handling.

   *  Selection of connection identifiers C_I and C_R which may be used
      to identify established keys or protocol state.

   *  Transport of external authorization data.

   EDHOC is designed to encrypt and integrity protect as much
   information as possible, and all symmetric keys are derived using as
   much previous information as possible.  EDHOC is furthermore designed
   to be as compact and lightweight as possible, in terms of message
   sizes, processing, and the ability to reuse already existing CBOR,
   COSE, and CoAP libraries.





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   To simplify for implementors, the use of CBOR and COSE in EDHOC is
   summarized in Appendix C.  Test vectors including CBOR diagnostic
   notation are provided in [I-D.selander-lake-traces].

3.  Protocol Elements

3.1.  General

   The EDHOC protocol consists of three mandatory messages (message_1,
   message_2, message_3) between Initiator and Responder, an optional
   fourth message (message_4), and an error message.  All EDHOC messages
   are CBOR Sequences [RFC8742].  Figure 3 illustrates an EDHOC message
   flow with the optional fourth message as well as the content of each
   message.  The protocol elements in the figure are introduced in
   Section 3 and Section 5.  Message formatting and processing is
   specified in Section 5 and Section 6.

   Application data may be protected using the agreed application
   algorithms (AEAD, hash) in the selected cipher suite (see
   Section 3.6) and the application can make use of the established
   connection identifiers C_I and C_R (see Section 3.3).  EDHOC may be
   used with the media type application/edhoc defined in Section 9.

   The Initiator can derive symmetric application keys after creating
   EDHOC message_3, see Section 4.3.  Protected application data can
   therefore be sent in parallel or together with EDHOC message_3.
   EDHOC message_4 is typically not sent.

   Initiator                                                   Responder
   |                 METHOD, SUITES_I, G_X, C_I, EAD_1                 |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |       G_Y, Enc( ID_CRED_R, Signature_or_MAC_2, EAD_2 ), C_R       |
   |<------------------------------------------------------------------+
   |                             message_2                             |
   |                                                                   |
   |            AEAD( ID_CRED_I, Signature_or_MAC_3, EAD_3 )           |
   +------------------------------------------------------------------>|
   |                             message_3                             |
   |                                                                   |
   |                           AEAD( EAD_4 )                           |
   |<- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
   |                             message_4                             |

       Figure 3: EDHOC Message Flow with the Optional Fourth Message





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3.2.  Method

   The data item METHOD in message_1 (see Section 5.2.1), is an integer
   specifying the authentication method.  EDHOC supports authentication
   with signature or static Diffie-Hellman keys, as defined in the four
   authentication methods: 0, 1, 2, and 3, see Figure 4.  When using a
   static Diffie-Hellman key the authentication is provided by a Message
   Authentication Code (MAC) computed from an ephemeral-static ECDH
   shared secret which enables significant reductions in message sizes.

   The Initiator and the Responder need to have agreed on a single
   method to be used for EDHOC, see Section 3.9.

   +-------+-------------------+-------------------+-------------------+
   | Value | Initiator         | Responder         | Reference         |
   +-------+-------------------+-------------------+-------------------+
   |     0 | Signature Key     | Signature Key     | [[this document]] |
   |     1 | Signature Key     | Static DH Key     | [[this document]] |
   |     2 | Static DH Key     | Signature Key     | [[this document]] |
   |     3 | Static DH Key     | Static DH Key     | [[this document]] |
   +-------+-------------------+-------------------+-------------------+

                           Figure 4: Method Types

3.3.  Connection Identifiers

   EDHOC includes the selection of connection identifiers (C_I, C_R)
   identifying a connection for which keys are agreed.

   Connection identifiers may be used to correlate EDHOC messages and
   facilitate the retrieval of protocol state during EDHOC protocol
   execution (see Section 3.4) or in a subsequent application protocol,
   e.g., OSCORE (see Section 3.3.2).  The connection identifiers do not
   have any cryptographic purpose in EDHOC.

   Connection identifiers in EDHOC are byte strings or integers, encoded
   in CBOR.  One byte connection identifiers (the integers -24 to 23 and
   the empty CBOR byte string h'') are realistic in many scenarios as
   most constrained devices only have a few connections.

3.3.1.  Selection of Connection Identifiers

   C_I and C_R are chosen by I and R, respectively.  The Initiator
   selects C_I and sends it in message_1 for the Responder to use as a
   reference to the connection in communications with the Initiator.
   The Responder selects C_R and sends in message_2 for the Initiator to
   use as a reference to the connection in communications with the
   Responder.



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   If connection identifiers are used by an application protocol for
   which EDHOC establishes keys then the selected connection identifiers
   SHALL adhere to the requirements for that protocol, see Section 3.3.2
   for an example.

3.3.2.  Use of Connection Identifiers with OSCORE

   For OSCORE, the choice of a connection identifier results in the
   endpoint selecting its Recipient ID, see Section 3.1 of [RFC8613],
   for which certain uniqueness requirements apply, see Section 3.3 of
   [RFC8613].  Therefore, the Initiator and the Responder MUST NOT
   select connection identifiers such that it results in same OSCORE
   Recipient ID.  Since the Recipient ID is a byte string and a EDHOC
   connection identifier is either a CBOR byte string or a CBOR integer,
   care must be taken when selecting the connection identifiers and
   converting them to Recipient IDs.  A mapping from EDHOC connection
   identifier to OSCORE Recipient ID is specified in Appendix A.1.

3.4.  Transport

   Cryptographically, EDHOC does not put requirements on the lower
   layers.  EDHOC is not bound to a particular transport layer and can
   even be used in environments without IP.  The transport is
   responsible, where necessary, to handle:

   *  message loss,

   *  message reordering,

   *  message duplication,

   *  fragmentation,

   *  demultiplex EDHOC messages from other types of messages,

   *  denial-of-service protection,

   *  message correlation.

   The Initiator and the Responder need to have agreed on a transport to
   be used for EDHOC, see Section 3.9.










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3.4.1.  Use of Connection Identifiers for EDHOC Message Correlation

   The transport needs to support the correlation between EDHOC messages
   and facilitate the retrieval of protocol state during EDHOC protocol
   execution, including an indication of a message being message_1.  The
   correlation may reuse existing mechanisms in the transport protocol.
   For example, the CoAP Token may be used to correlate EDHOC messages
   in a CoAP response and an associated CoAP request.

   Connection identifiers may be used to correlate EDHOC messages and
   facilitate the retrieval of protocol state during EDHOC protocol
   execution.  EDHOC transports that do not inherently provide
   correlation across all messages of an exchange can send connection
   identifiers along with EDHOC messages to gain that required
   capability, e.g., by prepending the appropriate connection identifier
   (when available from the EDHOC protocol) to the EDHOC message.
   Transport of EDHOC in CoAP payloads is described in Appendix A.3,
   which also shows how to use connection identifiers and message_1
   indication with CoAP.

3.5.  Authentication Parameters

   EDHOC supports various settings for how the other endpoint's
   authentication (public) key is transported, identified, and trusted
   as described in this section.

   The authentication key (see Section 3.5.2) is used in several parts
   of EDHOC:

   1.  as part of the authentication credential included in the
       integrity calculation

   2.  for verification of the Signature_or_MAC field in message_2 and
       message_3 (see Section 5.3.2 and Section 5.4.2)

   3.  in the key derivation (in case of a static Diffie-Hellman key,
       see Section 4).

   The authentication credential (CRED_x) contains, in addition to the
   authentication key, also the authentication key algorithm and
   optionally other parameters such as identity, key usage, expiry,
   issuer, etc. (see Section 3.5.3).  Identical authentication
   credentials need to be established in both endpoints to be able to
   verify integrity.  For many settings it is not necessary to transport
   the authentication credential within EDHOC over constrained links,
   for example, it may be pre-provisioned or acquired out-of-band over
   less constrained links.




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   EDHOC relies on COSE for identification of authentication credentials
   (using ID_CRED_x, see Section 3.5.4) and supports all credential
   types for which COSE header parameters are defined (see
   Section 3.5.3).

   The choice of authentication credential depends also on the trust
   model (see Section 3.5.1).  For example, a certificate or CWT may
   rely on a trusted third party, whereas a CCS or a self-signed
   certificate/CWT may be used when trust in the public key can be
   achieved by other means, or in the case of trust-on-first-use.

   The type of authentication key, authentication credential, and the
   way to identify the credential have a large impact on the message
   size.  For example, the signature_or_MAC field is much smaller with a
   static DH key than with a signature key.  A CCS is much smaller than
   a self-signed certificate/CWT, but if it is possible to reference the
   credential with a COSE header like 'kid', then that is typically much
   smaller than to transport a CCS.

3.5.1.  Identities and trust anchors

   Policies for what connections to allow are typically set based on the
   identity of the other party, and parties typically only allow
   connections from a specific identity or a small restricted set of
   identities.  For example, in the case of a device connecting to a
   network, the network may only allow connections from devices which
   authenticate with certificates having a particular range of serial
   numbers and signed by a particular CA.  On the other hand, the device
   may only be allowed to connect to a network which authenticates with
   a particular public key (information of which may be provisioned,
   e.g., out of band or in the external authorization data, see
   Section 3.8).  The EDHOC implementation or the application must
   enforce information about the intended endpoint, and in particular
   whether it is a specific identity or a set of identities.  Either
   EDHOC passes information about identity to the application for a
   decision, or EDHOC needs to have access to relevant information and
   makes the decision on its own.

   EDHOC assumes the existence of mechanisms (certification authority,
   trusted third party, pre-provisioning, etc.) for specifying and
   distributing authentication credentials.

   *  When a Public Key Infrastructure (PKI) is used with certificates,
      the trust anchor is a Certification Authority (CA) certificate,
      and the identity is the subject whose unique name (e.g., a domain
      name, NAI, or EUI) is included in the endpoint's certificate.  In
      order to run EDHOC each party needs at least one CA public key
      certificate, or just the public key, and a specific identity or



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      set of identities it is allowed to communicate with.  Only
      validated public-key certificates with an allowed subject name, as
      specified by the application, are to be accepted.  EDHOC provides
      proof that the other party possesses the private authentication
      key corresponding to the public authentication key in its
      certificate.  The certification path provides proof that the
      subject of the certificate owns the public key in the certificate.

   *  Similarly, when a PKI is used with CWTs, each party needs to have
      a trusted third party public key as trust anchor to verify the
      end-entity CWTs, and a specific identity or set of identities in
      the 'sub' (subject) claim of the CWT to determine if it is allowed
      to communicate with.  The trusted third party public key can,
      e.g., be stored in a self-signed CWT or in a CCS.

   *  When PKI is not used (CCS, self-signed certificate/CWT), the trust
      anchor is the authentication key of the other party.  In this
      case, the identity is typically directly associated to the
      authentication key of the other party.  For example, the name of
      the subject may be a canonical representation of the public key.
      Alternatively, if identities can be expressed in the form of
      unique subject names assigned to public keys, then a binding to
      identity can be achieved by including both public key and
      associated subject name in the protocol message computation:
      CRED_I or CRED_R may be a self-signed certificate/CWT or CCS
      containing the authentication key and the subject name, see
      Section 3.5.3.  In order to run EDHOC, each endpoint needs a
      specific authentication key/unique associated subject name, or a
      set of public authentication keys/unique associated subject names,
      which it is allowed to communicate with.  EDHOC provides the proof
      that the other party possesses the private authentication key
      corresponding to the public authentication key.

   To prevent misbinding attacks in systems where an attacker can
   register public keys without proving knowledge of the private key,
   SIGMA [SIGMA] enforces a MAC to be calculated over the "identity".
   EDHOC follows SIGMA by calculating a MAC over the whole credential,
   which in case of an X.509 or C509 certificate includes the "subject"
   and "subjectAltName" fields, and in the case of CWT or CCS includes
   the "sub" claim.  While the SIGMA paper only focuses on the identity,
   the same principle is true for other information such as policies
   associated to the public key.









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3.5.2.  Authentication Keys

   The authentication key (i.e. the public key used for authentication)
   MUST be a signature key or static Diffie-Hellman key.  The Initiator
   and the Responder MAY use different types of authentication keys,
   e.g., one uses a signature key and the other uses a static Diffie-
   Hellman key.  The authentication key algorithm needs to be compatible
   with the method and the cipher suite.  The authentication key
   algorithm needs to be compatible with the EDHOC key exchange
   algorithm when static Diffie-Hellman authentication is used, and
   compatible with the EDHOC signature algorithm when signature
   authentication is used.

   Note that for most signature algorithms, the signature is determined
   by the signature algorithm and the authentication key algorithm
   together.  When using static Diffie-Hellman keys the Initiator's and
   Responder's private authentication keys are called I and R,
   respectively, and the public authentication keys are called G_I and
   G_R, respectively.

   For X.509 the authentication key is represented with a
   SubjectPublicKeyInfo field.  For CWT and CCS, the authentication key
   is represented with a 'cnf' claim [RFC8747] containing a COSE_Key
   [I-D.ietf-cose-rfc8152bis-struct].

3.5.3.  Authentication Credentials

   The authentication credentials, CRED_I and CRED_R, contain the public
   authentication key of the Initiator and the Responder, respectively.

   EDHOC relies on COSE for identification of authentication credentials
   (see Section 3.5.4) and supports all credential types for which COSE
   header parameters are defined including X.509 [RFC5280], C509
   [I-D.ietf-cose-cbor-encoded-cert], CWT [RFC8392] and CWT Claims Set
   (CCS) [RFC8392].  When the identified credential is a chain or bag,
   CRED_x is just the end-entity X.509 or C509 certificate / CWT.  In
   X.509 and C509 certificates, signature keys typically have key usage
   "digitalSignature" and Diffie-Hellman public keys typically have key
   usage "keyAgreement".

   CRED_x needs to be defined such that it is identical when used by
   Initiator or Responder.  The Initiator and Responder are expected to
   agree on a specific encoding of the credential, see Section 3.9.  It
   is RECOMMENDED that the COSE 'kid' parameter, when used, refers to a
   specific encoding.  The Initiator and Responder SHOULD use an
   available authentication credential (transported in EDHOC or
   otherwise provisioned) without re-encoding.  If for some reason re-
   encoding of the authentication credential may occur, then a potential



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   common encoding for CBOR based credentials is bytewise lexicographic
   order of their deterministic encodings as specified in Section 4.2.1
   of [RFC8949].

   *  When the authentication credential is an X.509 certificate, CRED_x
      SHALL be the end-entity DER encoded certificate, encoded as a bstr
      [I-D.ietf-cose-x509].

   *  When the authentication credential is a C509 certificate, CRED_x
      SHALL be the end-entity C509Certificate
      [I-D.ietf-cose-cbor-encoded-cert]

   *  When the authentication credential is a COSE_Key in a CWT, CRED_x
      SHALL be the untagged CWT.

   *  When the authentication credential is a COSE_Key but not in a CWT,
      CRED_x SHALL be an untagged CCS.

      -  Naked COSE_Keys are thus dressed as CCS when used in EDHOC,
         which is done by prefixing the COSE_Key with 0xA108A101.

   An example of a CRED_x is shown below:

   {                                              /CCS/
     2 : "42-50-31-FF-EF-37-32-39",               /sub/
     8 : {                                        /cnf/
       1 : {                                      /COSE_Key/
         1 : 1,                                   /kty/
         2 : 0,                                   /kid/
        -1 : 4,                                   /crv/
        -2 : h'b1a3e89460e88d3a8d54211dc95f0b90   /x/
               3ff205eb71912d6db8f4af980d2db83a'
       }
     }
   }

       Figure 5: A CCS Containing an X25519 Static Diffie-Hellman Key
                          and an EUI-64 Identity.

3.5.4.  Identification of Credentials

   ID_CRED_R and ID_CRED_I are transported in message_2 and message_3,
   respectively (see Section 5.3.2 and Section 5.4.2).  They are used to
   identify and optionally transport the authentication keys of the
   Initiator and the Responder, respectively.  ID_CRED_I and ID_CRED_R
   do not have any cryptographic purpose in EDHOC since EDHOC integrity
   protects the authentication credential.  EDHOC relies on COSE for
   identification of authentication credentials and supports all types



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   of COSE header parameters used to identify authentication credentials
   including X.509, C509, CWT and CCS.

   *  ID_CRED_R is intended to facilitate for the Initiator to retrieve
      the Responder's authentication key.

   *  ID_CRED_I is intended to facilitate for the Responder to retrieve
      the Initiator's authentication key.

   ID_CRED_I and ID_CRED_R are COSE header maps and contains one or more
   COSE header parameter.  ID_CRED_I and ID_CRED_R MAY contain different
   header parameters.  The header parameters typically provide some
   information about the format of authentication credential.

   Note that COSE header parameters in ID_CRED_x are used to identify
   the sender's authentication credential.  There is therefore no reason
   to use the "-sender" header parameters, such as x5t-sender, defined
   in Section 3 of [I-D.ietf-cose-x509].  Instead, the corresponding
   parameter without "-sender", such as x5t, SHOULD be used.

   Example: X.509 certificates can be identified by a hash value using
   the 'x5t' parameter:

   *  ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,

   Example: CWT or CCS can be identified by a key identifier using the
   'kid' parameter:

   *  ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or
      R.

   Note that 'kid' is extended to support int values to allow more one-
   byte identifiers (see Section 9.7 and Section 9.8) which may be
   useful in many scenarios since constrained devices only have a few
   keys.  As stated in Section 3.1 of [I-D.ietf-cose-rfc8152bis-struct],
   applications MUST NOT assume that 'kid' values are unique and several
   keys associated with a 'kid' may need to be checked before the
   correct one is found.  Applications might use additional information
   such as 'kid context' or lower layers to determine which key to try
   first.  Applications should strive to make ID_CRED_x as unique as
   possible, since the recipient may otherwise have to try several keys.

   See Appendix C.3 for more examples.








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3.6.  Cipher Suites

   An EDHOC cipher suite consists of an ordered set of algorithms from
   the "COSE Algorithms" and "COSE Elliptic Curves" registries as well
   as the EDHOC MAC length.  Algorithms need to be specified with enough
   parameters to make them completely determined.  EDHOC is currently
   only specified for use with key exchange algorithms of type ECDH
   curves, but any Key Encapsulation Method (KEM), including Post-
   Quantum Cryptography (PQC) KEMs, can be used in method 0, see
   Section 8.4.  Use of other types of key exchange algorithms to
   replace static DH authentication (method 1,2,3) would likely require
   a specification updating EDHOC with new methods.

   EDHOC supports all signature algorithms defined by COSE, including
   PQC signature algorithms such as HSS-LMS.  Just like in TLS 1.3
   [RFC8446] and IKEv2 [RFC7296], a signature in COSE is determined by
   the signature algorithm and the authentication key algorithm
   together, see Section 3.5.2.  The exact details of the authentication
   key algorithm depend on the type of authentication credential.  COSE
   supports different formats for storing the public authentication keys
   including COSE_Key and X.509, which have different names and ways to
   represent the authentication key and the authentication key
   algorithm.

   An EDHOC cipher suite consists of the following parameters:

   *  EDHOC AEAD algorithm

   *  EDHOC hash algorithm

   *  EDHOC MAC length in bytes (Static DH)

   *  EDHOC key exchange algorithm (ECDH curve)

   *  EDHOC signature algorithm

   *  Application AEAD algorithm

   *  Application hash algorithm

   Each cipher suite is identified with a pre-defined int label.










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   EDHOC can be used with all algorithms and curves defined for COSE.
   Implementation can either use any combination of COSE algorithms and
   parameters to define their own private cipher suite, or use one of
   the pre-defined cipher suites.  Private cipher suites can be
   identified with any of the four values -24, -23, -22, -21.  The pre-
   defined cipher suites are listed in the IANA registry (Section 9.2)
   with initial content outlined here:

   *  Cipher suites 0-3, based on AES-CCM, are intended for constrained
      IoT where message overhead is a very important factor.  Note that
      AES-CCM-16-64-128 and AES-CCM-16-64-128 are compatible with the
      IEEE CCM* mode.

      -  Cipher suites 1 and 3 use a larger tag length (128-bit) in
         EDHOC than in the Application AEAD algorithm (64-bit).

   *  Cipher suites 4 and 5, based on ChaCha20, are intended for less
      constrained applications and only use 128-bit tag lengths.

   *  Cipher suite 6, based on AES-GCM, is for general non-constrained
      applications.  It uses high performance algorithms that are widely
      supported.

   *  Cipher suites 24 and 25 are intended for high security
      applications such as government use and financial applications.
      These cipher suites do not share any algorithms.  Cipher suite 24
      is compatible with the CNSA suite [CNSA].

   The different methods (Section 3.2) use the same cipher suites, but
   some algorithms are not used in some methods.  The EDHOC signature
   algorithm is not used in methods without signature authentication.

   The Initiator needs to have a list of cipher suites it supports in
   order of preference.  The Responder needs to have a list of cipher
   suites it supports.  SUITES_I contains cipher suites supported by the
   Initiator, formatted and processed as detailed in Section 5.2.1 to
   secure the cipher suite negotiation.  Examples of cipher suite
   negotiation are given in Section 6.3.2.

3.7.  Ephemeral Public Keys

   EDHOC always uses compact representation of elliptic curve points,
   see Appendix B.  In COSE compact representation is achieved by
   formatting the ECDH ephemeral public keys as COSE_Keys of type EC2 or
   OKP according to Sections 7.1 and 7.2 of
   [I-D.ietf-cose-rfc8152bis-algs], but only including the 'x' parameter
   in G_X and G_Y.  For Elliptic Curve Keys of type EC2, compact
   representation MAY be used also in the COSE_Key.  If the COSE



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   implementation requires an 'y' parameter, the value y = false SHALL
   be used.  COSE always use compact output for Elliptic Curve Keys of
   type EC2.

3.8.  External Authorization Data (EAD)

   In order to reduce round trips and number of messages or to simplify
   processing, external security applications may be integrated into
   EDHOC by transporting authorization related data in the messages.
   One example is third-party identity and authorization information
   protected out of scope of EDHOC [I-D.selander-ace-ake-authz].
   Another example is a certificate enrolment request or the resulting
   issued certificate.

   EDHOC allows opaque external authorization data (EAD) to be sent in
   the EDHOC messages.  External authorization data sent in message_1
   (EAD_1) or message_2 (EAD_2) should be considered unprotected by
   EDHOC, see Section 8.5.  External authorization data sent in
   message_3 (EAD_3) or message_4 (EAD_4) is protected between Initiator
   and Responder.

   External authorization data is a CBOR sequence (see Appendix C.1)
   consisting of one or more (ead_label, ead_value) pairs as defined
   below:

   ead = 1* (
     ead_label : int,
     ead_value : any,
   )

   Applications using external authorization data need to specify
   format, processing, and security considerations and register the
   (ead_label, ead_value) pair, see Section 9.5.  The CDDL type of
   ead_value is determined by the int ead_label and MUST be specified.

   The EAD fields of EDHOC are not intended for generic application
   data.  Since data carried in EAD_1 and EAD_2 fields may not be
   protected, special considerations need to be made such that it does
   not violate security and privacy requirements of the service which
   uses this data.  Moreover, the content in an EAD field may impact the
   security properties provided by EDHOC.  Security applications making
   use of the EAD fields must perform the necessary security analysis.









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3.9.  Applicability Statement

   EDHOC requires certain parameters to be agreed upon between Initiator
   and Responder.  Some parameters can be agreed through the protocol
   execution (specifically cipher suite negotiation, see Section 3.6)
   but other parameters may need to be known out-of-band (e.g., which
   authentication method is used, see Section 3.2).

   The purpose of the applicability statement is to describe the
   intended use of EDHOC to allow for the relevant processing and
   verifications to be made, including things like:

   1.  How the endpoint detects that an EDHOC message is received.  This
       includes how EDHOC messages are transported, for example in the
       payload of a CoAP message with a certain Uri-Path or Content-
       Format; see Appendix A.3.

       *  The method of transporting EDHOC messages may also describe
          data carried along with the messages that are needed for the
          transport to satisfy the requirements of Section 3.4, e.g.,
          connection identifiers used with certain messages, see
          Appendix A.3.

   2.  Authentication method (METHOD; see Section 3.2).

   3.  Profile for authentication credentials (CRED_I, CRED_R; see
       Section 3.5.3), e.g., profile for certificate or CCS, including
       supported authentication key algorithms (subject public key
       algorithm in X.509 or C509 certificate).

   4.  Type used to identify authentication credentials (ID_CRED_I,
       ID_CRED_R; see Section 3.5.4).

   5.  Use and type of external authorization data (EAD_1, EAD_2, EAD_3,
       EAD_4; see Section 3.8).

   6.  Identifier used as identity of endpoint; see Section 3.5.1.

   7.  If message_4 shall be sent/expected, and if not, how to ensure a
       protected application message is sent from the Responder to the
       Initiator; see Section 5.5.

   The applicability statement may also contain information about
   supported cipher suites.  The procedure for selecting and verifying
   cipher suite is still performed as described in Section 5.2.1 and
   Section 6.3, but it may become simplified by this knowledge.

   An example of an applicability statement is shown in Appendix D.



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   For some parameters, like METHOD, ID_CRED_x, type of EAD, the
   receiver is able to verify compliance with applicability statement,
   and if it needs to fail because of incompliance, to infer the reason
   why the protocol failed.

   For other parameters, like CRED_x in the case that it is not
   transported, it may not be possible to verify that incompliance with
   applicability statement was the reason for failure: Integrity
   verification in message_2 or message_3 may fail not only because of
   wrong authentication credential.  For example, in case the Initiator
   uses public key certificate by reference (i.e., not transported
   within the protocol) then both endpoints need to use an identical
   data structure as CRED_I or else the integrity verification will
   fail.

   Note that it is not necessary for the endpoints to specify a single
   transport for the EDHOC messages.  For example, a mix of CoAP and
   HTTP may be used along the path, and this may still allow correlation
   between messages.

   The applicability statement may be dependent on the identity of the
   other endpoint, or other information carried in an EDHOC message, but
   it then applies only to the later phases of the protocol when such
   information is known.  (The Initiator does not know identity of
   Responder before having verified message_2, and the Responder does
   not know identity of the Initiator before having verified message_3.)

   Other conditions may be part of the applicability statement, such as
   target application or use (if there is more than one application/use)
   to the extent that EDHOC can distinguish between them.  In case
   multiple applicability statements are used, the receiver needs to be
   able to determine which is applicable for a given session, for
   example based on URI or external authorization data type.

4.  Key Derivation

   EDHOC uses Extract-and-Expand [RFC5869] with the EDHOC hash algorithm
   in the selected cipher suite to derive keys used in EDHOC and in the
   application.  Extract is used to derive fixed-length uniformly
   pseudorandom keys (PRK) from ECDH shared secrets.  Expand is used to
   derive additional output keying material (OKM) from the PRKs.

   This section defines Extract, Expand and other key derivation
   functions based on these: Expand is used to define EDHOC-KDF and in
   turn EDHOC-Exporter, whereas Extract is used to define EDHOC-
   KeyUpdate.





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4.1.  Extract

   The pseudorandom keys (PRKs) are derived using Extract.

      PRK = Extract( salt, IKM )

   where the input keying material (IKM) and salt are defined for each
   PRK below.

   The definition of Extract depends on the EDHOC hash algorithm of the
   selected cipher suite:

   *  if the EDHOC hash algorithm is SHA-2, then Extract( salt, IKM ) =
      HKDF-Extract( salt, IKM ) [RFC5869]

   *  if the EDHOC hash algorithm is SHAKE128, then Extract( salt, IKM )
      = KMAC128( salt, IKM, 256, "" )

   *  if the EDHOC hash algorithm is SHAKE256, then Extract( salt, IKM )
      = KMAC256( salt, IKM, 512, "" )

4.1.1.  PRK_2e

   PRK_2e is used to derive a keystream to encrypt message_2.  PRK_2e is
   derived with the following input:

   *  The salt SHALL be a zero-length byte string.  Note that [RFC5869]
      specifies that if the salt is not provided, it is set to a string
      of zeros (see Section 2.2 of [RFC5869]).  For implementation
      purposes, not providing the salt is the same as setting the salt
      to the zero-length byte string (0x).

   *  The IKM SHALL be the ephemeral-ephemeral ECDH shared secret G_XY
      (calculated from G_X and Y or G_Y and X) as defined in
      Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs].  The use of G_XY
      gives forward secrecy, in the sense that compromise of the private
      authentication keys does not compromise past session keys.

   Example: Assuming the use of curve25519, the ECDH shared secret G_XY
   is the output of the X25519 function [RFC7748]:

      G_XY = X25519( Y, G_X ) = X25519( X, G_Y )

   Example: Assuming the use of SHA-256 the extract phase of HKDF
   produces PRK_2e as follows:

      PRK_2e = HMAC-SHA-256( salt, G_XY )




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   where salt = 0x (zero-length byte string).

4.1.2.  PRK_3e2m

   PRK_3e2m is used to produce a MAC in message_2 and to encrypt
   message_3.  PRK_3e2m is derived as follows:

   If the Responder authenticates with a static Diffie-Hellman key, then
   PRK_3e2m = Extract( PRK_2e, G_RX ), where G_RX is the ECDH shared
   secret calculated from G_R and X, or G_X and R, else PRK_3e2m =
   PRK_2e.

4.1.3.  PRK_4x3m

   PRK_4x3m is used to produce a MAC in message_3, to encrypt message_4,
   and to derive application specific data.  PRK_4x3m is derived as
   follows:

   If the Initiator authenticates with a static Diffie-Hellman key, then
   PRK_4x3m = Extract( PRK_3e2m, G_IY ), where G_IY is the ECDH shared
   secret calculated from G_I and Y, or G_Y and I, else PRK_4x3m =
   PRK_3e2m.

4.2.  Expand

   The keys, IVs and MACs used in EDHOC are derived from the PRKs using
   Expand, and instantiated with the EDHOC AEAD algorithm in the
   selected cipher suite.

      OKM = EDHOC-KDF( PRK, transcript_hash, label, context, length )
          = Expand( PRK, info, length )

   where info is encoded as the CBOR sequence

   info = (
     transcript_hash : bstr,
     label : tstr,
     context : bstr,
     length : uint,
   )

   where

   *  transcript_hash is a bstr set to one of the transcript hashes
      TH_2, TH_3, or TH_4 as defined in Sections 5.3.1, 5.4.1, and 4.3.

   *  label is a tstr set to the name of the derived key, IV or MAC;
      i.e., "KEYSTREAM_2", "MAC_2", "K_3", "IV_3", or "MAC_3".



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   *  context is a bstr

   *  length is the length of output keying material (OKM) in bytes

   The definition of Expand depends on the EDHOC hash algorithm of the
   selected cipher suite:

   *  if the EDHOC hash algorithm is SHA-2, then Expand( PRK, info,
      length ) = HKDF-Expand( PRK, info, length ) [RFC5869]

   *  if the EDHOC hash algorithm is SHAKE128, then Expand( PRK, info,
      length ) = KMAC128( PRK, info, L, "" )

   *  if the EDHOC hash algorithm is SHAKE256, then Expand( PRK, info,
      length ) = KMAC256( PRK, info, L, "" )

   where L = 8*length, the output length in bits.

   The keys, IVs and MACs are derived as follows:

   *  KEYSTREAM_2 is derived using the transcript hash TH_2 and the
      pseudorandom key PRK_2e.

   *  MAC_2 is derived using the transcript hash TH_2 and the
      pseudorandom key PRK_3e2m.

   *  K_3 and IV_3 are derived using the transcript hash TH_3 and the
      pseudorandom key PRK_3e2m.  IVs are only used if the EDHOC AEAD
      algorithm uses IVs.

   *  MAC_3 is derived using the transcript hash TH_3 and the
      pseudorandom key PRK_4x3m.

   KEYSTREAM_2, K_3, and IV_3 use an empty CBOR byte string h'' as
   context.  MAC_2 and MAC_3 use context as defined in Section 5.3.2 and
   Section 5.4.2, respectively.

4.3.  EDHOC-Exporter

   Application keys and other application specific data can be derived
   using the EDHOC-Exporter interface defined as:

      EDHOC-Exporter(label, context, length)
        = EDHOC-KDF(PRK_4x3m, TH_4, label, context, length)

   where label is a registered tstr from the EDHOC Exporter Label
   registry (Section 9.1), context is a bstr defined by the application,
   and length is a uint defined by the application.  The (label,



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   context) pair must be unique, i.e., a (label, context) MUST NOT be
   used for two different purposes.  However an application can re-
   derive the same key several times as long as it is done in a secure
   way.  For example, in most encryption algorithms the same key kan be
   reused with different nonces.  The context can for example be the
   empty (zero-length) sequence or a single CBOR byte string.

   The transcript hash TH_4 is a CBOR encoded bstr and the input to the
   hash function is a CBOR Sequence.

      TH_4 = H( TH_3, CIPHERTEXT_3 )

   where H() is the hash function in the selected cipher suite.
   Examples of use of the EDHOC-Exporter are given in Section 5.5.2 and
   Appendix A.

   *  K_4 and IV_4 are derived with the EDHOC-Exporter using the empty
      CBOR byte string h'' as context, and labels "EDHOC_K_4" and
      "EDHOC_IV_4", respectively.  IVs are only used if the EDHOC AEAD
      algorithm uses IVs.

4.4.  EDHOC-KeyUpdate

   To provide forward secrecy in an even more efficient way than re-
   running EDHOC, EDHOC provides the function EDHOC-KeyUpdate.  When
   EDHOC-KeyUpdate is called the old PRK_4x3m is deleted and the new
   PRK_4x3m is calculated as a "hash" of the old key using the Extract
   function as illustrated by the following pseudocode:

      EDHOC-KeyUpdate( nonce ):
         PRK_4x3m = Extract( nonce, PRK_4x3m )

   The EDHOC-KeyUpdate takes a nonce as input to guarantee that there
   are no short cycles.  The Initiator and the Responder need to agree
   on the nonce, which can e.g., be a counter or a random number.  While
   the KeyUpdate method provides forward secrecy it does not give as
   strong security properties as re-running EDHOC, see Section 8.

5.  Message Formatting and Processing

   This section specifies formatting of the messages and processing
   steps.  Error messages are specified in Section 6.  Annotated traces
   of EDHOC protocol runs are provided in [I-D.selander-lake-traces].

   An EDHOC message is encoded as a sequence of CBOR data items (CBOR
   Sequence, [RFC8742]).  Additional optimizations are made to reduce
   message overhead.




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   While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0
   structures, only a subset of the parameters is included in the EDHOC
   messages, see Appendix C.3.  The unprotected COSE header in
   COSE_Sign1, and COSE_Encrypt0 (not included in the EDHOC message) MAY
   contain parameters (e.g., 'alg').

5.1.  Message Processing Outline

   This section outlines the message processing of EDHOC.

   For each new/ongoing session, the endpoints are assumed to keep an
   associated protocol state containing identifiers, keying material,
   etc. used for subsequent processing of protocol related data.  The
   protocol state is assumed to be associated to an applicability
   statement (Section 3.9) which provides the context for how messages
   are transported, identified, and processed.

   EDHOC messages SHALL be processed according to the current protocol
   state.  The following steps are expected to be performed at reception
   of an EDHOC message:

   1.  Detect that an EDHOC message has been received, for example by
       means of port number, URI, or media type (Section 3.9).

   2.  Retrieve the protocol state according to the message correlation
       provided by the transport, see Section 3.4.  If there is no
       protocol state, in the case of message_1, a new protocol state is
       created.  The Responder endpoint needs to make use of available
       Denial-of-Service mitigation (Section 8.6).

   3.  If the message received is an error message, then process
       according to Section 6, else process as the expected next message
       according to the protocol state.

   If the processing fails for some reason then, typically, an error
   message is sent, the protocol is discontinued, and the protocol state
   erased.  Further details are provided in the following subsections
   and in Section 6.

   Different instances of the same message MUST NOT be processed in one
   session.  Note that processing will fail if the same message appears
   a second time for EDHOC processing because the state of the protocol
   has moved on and now expects something else.  This assumes that
   message duplication due to re-transmissions is handled by the
   transport protocol, see Section 3.4.  The case when the transport
   does not support message deduplication is addressed in Appendix E.





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5.2.  EDHOC Message 1

5.2.1.  Formatting of Message 1

   message_1 SHALL be a CBOR Sequence (see Appendix C.1) as defined
   below

   message_1 = (
     METHOD : int,
     SUITES_I : suites,
     G_X : bstr,
     C_I : bstr / int,
     ? EAD_1 : ead,
   )

   suites = [ 2* int ] / int

   where:

   *  METHOD - authentication method, see Section 3.2.

   *  SUITES_I - array of cipher suites which the Initiator supports in
      order of preference, starting with the most preferred and ending
      with the cipher suite selected for this session.  If the most
      preferred cipher suite is selected then SUITES_I is encoded as
      that cipher suite, i.e., as an int.  The processing steps are
      detailed below and in Section 6.3.

   *  G_X - the ephemeral public key of the Initiator

   *  C_I - variable length connection identifier

   *  EAD_1 - unprotected external authorization data, see Section 3.8.

5.2.2.  Initiator Processing of Message 1

   The Initiator SHALL compose message_1 as follows:

   *  SUITES_I contains a list of supported cipher suites, in order of
      preference, truncated after the cipher suite selected for this
      session.

      -  The Initiator MUST select its most preferred cipher suite,
         conditioned on what it can assume to be supported by the
         Responder.






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      -  The selected cipher suite MAY be changed between sessions,
         e.g., based on previous error messages (see next bullet), but
         all cipher suites which are more preferred than the selected
         cipher suite in the list MUST be included in SUITES_I.

      -  If the Initiator previously received from the Responder an
         error message with error code 2 (see Section 6.3) indicating
         cipher suites supported by the Responder, then the Initiator
         SHOULD select the most preferred supported cipher suite among
         those (note that error messages are not authenticated and may
         be forged).

      -  The supported cipher suites and the order of preference MUST
         NOT be changed based on previous error messages.

   *  Generate an ephemeral ECDH key pair using the curve in the
      selected cipher suite and format it as a COSE_Key.  Let G_X be the
      'x' parameter of the COSE_Key.

   *  Choose a connection identifier C_I and store it for the length of
      the protocol.

   *  Encode message_1 as a sequence of CBOR encoded data items as
      specified in Section 5.2.1

5.2.3.  Responder Processing of Message 1

   The Responder SHALL process message_1 as follows:

   *  Decode message_1 (see Appendix C.1).

   *  Verify that the selected cipher suite is supported and that no
      prior cipher suite in SUITES_I is supported.

   *  Pass EAD_1 to the security application.

   If any processing step fails, the Responder SHOULD send an EDHOC
   error message back, formatted as defined in Section 6, and the
   session MUST be discontinued.  Sending error messages is essential
   for debugging but MAY e.g., be skipped due to denial-of-service
   reasons, see Section 8.6.  If an error message is sent, the session
   MUST be discontinued.

5.3.  EDHOC Message 2







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5.3.1.  Formatting of Message 2

   message_2 SHALL be a CBOR Sequence (see Appendix C.1) as defined
   below

   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
     C_R : bstr / int,
   )

   where:

   *  G_Y_CIPHERTEXT_2 - the concatenation of G_Y, the ephemeral public
      key of the Responder, and CIPHERTEXT_2

   *  C_R - variable length connection identifier

5.3.2.  Responder Processing of Message 2

   The Responder SHALL compose message_2 as follows:

   *  Generate an ephemeral ECDH key pair using the curve in the
      selected cipher suite and format it as a COSE_Key.  Let G_Y be the
      'x' parameter of the COSE_Key.

   *  Choose a connection identifier C_R and store it for the length of
      the protocol.

   *  Compute the transcript hash TH_2 = H( H(message_1), G_Y, C_R )
      where H() is the hash function in the selected cipher suite.  The
      transcript hash TH_2 is a CBOR encoded bstr and the input to the
      hash function is a CBOR Sequence.  Note that H(message_1) can be
      computed and cached already in the processing of message_1.

   *  Compute MAC_2 = EDHOC-KDF( PRK_3e2m, TH_2, "MAC_2", << ID_CRED_R,
      CRED_R, ? EAD_2 >>, mac_length_2 ).  If the Responder
      authenticates with a static Diffie-Hellman key (method equals 1 or
      3), then mac_length_2 is the EDHOC MAC length given by the
      selected cipher suite.  If the Responder authenticates with a
      signature key (method equals 0 or 2), then mac_length_2 is equal
      to the output size of the EDHOC hash algorithm given by the
      selected cipher suite.

      -  ID_CRED_R - identifier to facilitate retrieval of CRED_R, see
         Section 3.5.4

      -  CRED_R - CBOR item containing the credential of the Responder,
         see Section 3.5.3



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      -  EAD_2 - unprotected external authorization data, see
         Section 3.8

   *  If the Responder authenticates with a static Diffie-Hellman key
      (method equals 1 or 3), then Signature_or_MAC_2 is MAC_2.  If the
      Responder authenticates with a signature key (method equals 0 or
      2), then Signature_or_MAC_2 is the 'signature' field of a
      COSE_Sign1 object as defined in Section 4.4 of
      [I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm of
      the selected cipher suite, the private authentication key of the
      Responder, and the following parameters as input (see
      Appendix C.3):

      -  protected = << ID_CRED_R >>

      -  external_aad = << TH_2, CRED_R, ? EAD_2 >>

      -  payload = MAC_2

   *  CIPHERTEXT_2 is calculated by using the Expand function as a
      binary additive stream cipher.

      -  plaintext = ( ID_CRED_R / bstr / int, Signature_or_MAC_2, ?
         EAD_2 )

         o  If ID_CRED_R contains a single 'kid' parameter, i.e.,
            ID_CRED_R = { 4 : kid_R }, then only the byte string or
            integer kid_R is conveyed in the plaintext encoded
            accordingly as bstr or int.

      -  Compute KEYSTREAM_2 = EDHOC-KDF( PRK_2e, TH_2, "KEYSTREAM_2",
         h'', plaintext_length ), where plaintext_length is the length
         of the plaintext.

      -  CIPHERTEXT_2 = plaintext XOR KEYSTREAM_2

   *  Encode message_2 as a sequence of CBOR encoded data items as
      specified in Section 5.3.1.

5.3.3.  Initiator Processing of Message 2

   The Initiator SHALL process message_2 as follows:

   *  Decode message_2 (see Appendix C.1).

   *  Retrieve the protocol state using the message correlation provided
      by the transport (e.g., the CoAP Token, the 5-tuple, or the
      prepended C_I, see Appendix A.3).



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   *  Decrypt CIPHERTEXT_2, see Section 5.3.2.

   *  Pass EAD_2 to the security application.

   *  Verify that the identity of the Responder is an allowed identity
      for this connection, see Section 3.5.1.

   *  Verify Signature_or_MAC_2 using the algorithm in the selected
      cipher suite.  The verification process depends on the method, see
      Section 5.3.2.

   If any processing step fails, the Initiator SHOULD send an EDHOC
   error message back, formatted as defined in Section 6.  Sending error
   messages is essential for debugging but MAY e.g., be skipped if a
   session cannot be found or due to denial-of-service reasons, see
   Section 8.6.  If an error message is sent, the session MUST be
   discontinued.

5.4.  EDHOC Message 3

5.4.1.  Formatting of Message 3

   message_3 SHALL be a CBOR Sequence (see Appendix C.1) as defined
   below

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

5.4.2.  Initiator Processing of Message 3

   The Initiator SHALL compose message_3 as follows:

   *  Compute the transcript hash TH_3 = H(TH_2, CIPHERTEXT_2) where H()
      is the hash function in the selected cipher suite.  The transcript
      hash TH_3 is a CBOR encoded bstr and the input to the hash
      function is a CBOR Sequence.  Note that H(TH_2, CIPHERTEXT_2) can
      be computed and cached already in the processing of message_2.

   *  Compute MAC_3 = EDHOC-KDF( PRK_4x3m, TH_3, "MAC_3", << ID_CRED_I,
      CRED_I, ? EAD_3 >>, mac_length_3 ).  If the Initiator
      authenticates with a static Diffie-Hellman key (method equals 2 or
      3), then mac_length_3 is the EDHOC MAC length given by the
      selected cipher suite.  If the Initiator authenticates with a
      signature key (method equals 0 or 1), then mac_length_3 is equal
      to the output size of the EDHOC hash algorithm given by the
      selected cipher suite.




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      -  ID_CRED_I - identifier to facilitate retrieval of CRED_I, see
         Section 3.5.4

      -  CRED_I - CBOR item containing the credential of the Initiator,
         see Section 3.5.3

      -  EAD_3 - protected external authorization data, see Section 3.8

   *  If the Initiator authenticates with a static Diffie-Hellman key
      (method equals 2 or 3), then Signature_or_MAC_3 is MAC_3.  If the
      Initiator authenticates with a signature key (method equals 0 or
      1), then Signature_or_MAC_3 is the 'signature' field of a
      COSE_Sign1 object as defined in Section 4.4 of
      [I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm of
      the selected cipher suite, the private authentication key of the
      Initiator, and the following parameters as input (see
      Appendix C.3):

      -  protected = << ID_CRED_I >>

      -  external_aad = << TH_3, CRED_I, ? EAD_3 >>

      -  payload = MAC_3

   *  Compute a COSE_Encrypt0 object as defined in Section 5.3 of
      [I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
      of the selected cipher suite, using the encryption key K_3, the
      initialization vector IV_3, the plaintext P, and the following
      parameters as input (see Appendix C.3):

      -  protected = h''

      -  external_aad = TH_3

      where

      -  K_3 = EDHOC-KDF( PRK_3e2m, TH_3, "K_3", h'', key_length )

         o  key_length - length of the encryption key of the EDHOC AEAD
            algorithm

      -  IV_3 = EDHOC-KDF( PRK_3e2m, TH_3, "IV_3", h'', iv_length )

         o  iv_length - length of the intialization vector of the EDHOC
            AEAD algorithm

      -  P = ( ID_CRED_I / bstr / int, Signature_or_MAC_3, ? EAD_3 )




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         o  If ID_CRED_I contains a single 'kid' parameter, i.e.,
            ID_CRED_I = { 4 : kid_I }, only the byte string or integer
            kid_I is conveyed in the plaintext encoded accordingly as
            bstr or int.

      CIPHERTEXT_3 is the 'ciphertext' of COSE_Encrypt0.

   *  Encode message_3 as a CBOR data item as specified in
      Section 5.4.1.

   Pass the connection identifiers (C_I, C_R) and the application
   algorithms in the selected cipher suite to the application.  The
   application can now derive application keys using the EDHOC-Exporter
   interface, see Section 4.3.

   After sending message_3, the Initiator is assured that no other party
   than the Responder can compute the key PRK_4x3m (implicit key
   authentication).  The Initiator can securely derive application keys
   and send protected application data.  However, the Initiator does not
   know that the Responder has actually computed the key PRK_4x3m and
   therefore the Initiator SHOULD NOT permanently store the keying
   material PRK_4x3m and TH_4, or derived application keys, until the
   Initiator is assured that the Responder has actually computed the key
   PRK_4x3m (explicit key confirmation).  This is similar to waiting for
   acknowledgement (ACK) in a transport protocol.  Explicit key
   confirmation is e.g., assured when the Initiator has verified an
   OSCORE message or message_4 from the Responder.

5.4.3.  Responder Processing of Message 3

   The Responder SHALL process message_3 as follows:

   *  Decode message_3 (see Appendix C.1).

   *  Retrieve the protocol state using the message correlation provided
      by the transport (e.g., the CoAP Token, the 5-tuple, or the
      prepended C_I, see Appendix A.3).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Section 5.3 of
      [I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
      in the selected cipher suite, and the parameters defined in
      Section 5.4.2.

   *  Pass EAD_3 to the security application.

   *  Verify that the identity of the Initiator is an allowed identity
      for this connection, see Section 3.5.1.




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   *  Verify Signature_or_MAC_3 using the algorithm in the selected
      cipher suite.  The verification process depends on the method, see
      Section 5.4.2.

   *  Pass the connection identifiers (C_I, C_R), and the application
      algorithms in the selected cipher suite to the security
      application.  The application can now derive application keys
      using the EDHOC-Exporter interface.

   If any processing step fails, the Responder SHOULD send an EDHOC
   error message back, formatted as defined in Section 6.  Sending error
   messages is essential for debugging but MAY e.g., be skipped if a
   session cannot be found or due to denial-of-service reasons, see
   Section 8.6.  If an error message is sent, the session MUST be
   discontinued.

   After verifying message_3, the Responder is assured that the
   Initiator has calculated the key PRK_4x3m (explicit key confirmation)
   and that no other party than the Responder can compute the key.  The
   Responder can securely send protected application data and store the
   keying material PRK_4x3m and TH_4.

5.5.  EDHOC Message 4

   This section specifies message_4 which is OPTIONAL to support.  Key
   confirmation is normally provided by sending an application message
   from the Responder to the Initiator protected with a key derived with
   the EDHOC-Exporter, e.g., using OSCORE (see Appendix A).  In
   deployments where no protected application message is sent from the
   Responder to the Initiator, the Responder MUST send message_4.  Two
   examples of such deployments:

   1.  When EDHOC is only used for authentication and no application
       data is sent.

   2.  When application data is only sent from the Initiator to the
       Responder.

   Further considerations about when to use message_4 are provided in
   Section 3.9 and Section 8.1.

5.5.1.  Formatting of Message 4

   message_4 SHALL be a CBOR Sequence (see Appendix C.1) as defined
   below






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   message_4 = (
     CIPHERTEXT_4 : bstr,
   )

5.5.2.  Responder Processing of Message 4

   The Responder SHALL compose message_4 as follows:

   *  Compute a COSE_Encrypt0 as defined in Section 5.3 of
      [I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
      of the selected cipher suite, using the encryption key K_4, the
      initialization vector IV_4, the plaintext P, and the following
      parameters as input (see Appendix C.3):

      -  protected = h''

      -  external_aad = TH_4

      where

      -  K_4 = EDHOC-Exporter( "EDHOC_K_4", h'', key_length )

         o  key_length - length of the encryption key of the EDHOC AEAD
            algorithm

      -  IV_4 = EDHOC-Exporter( "EDHOC_IV_4", h'', iv_length )

         o  iv_length - length of the intialization vector of the EDHOC
            AEAD algorithm

      -  P = ( ? EAD_4 )

         o  EAD_4 - protected external authorization data, see
            Section 3.8.

      CIPHERTEXT_4 is the 'ciphertext' of COSE_Encrypt0.

   *  Encode message_4 as a CBOR data item as specified in
      Section 5.5.1.

5.5.3.  Initiator Processing of Message 4

   The Initiator SHALL process message_4 as follows:

   *  Decode message_4 (see Appendix C.1).






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   *  Retrieve the protocol state using the message correlation provided
      by the transport (e.g., the CoAP Token, the 5-tuple, or the
      prepended C_I, see Appendix A.3).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Section 5.3 of
      [I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
      in the selected cipher suite, and the parameters defined in
      Section 5.5.2.

   *  Pass EAD_4 to the security application.

   If any processing step fails, the Responder SHOULD send an EDHOC
   error message back, formatted as defined in Section 6.  Sending error
   messages is essential for debugging but MAY e.g., be skipped if a
   session cannot be found or due to denial-of-service reasons, see
   Section 8.6.  If an error message is sent, the session MUST be
   discontinued.

6.  Error Handling

   This section defines the format for error messages.

   An EDHOC error message can be sent by either endpoint as a reply to
   any non-error EDHOC message.  How errors at the EDHOC layer are
   transported depends on lower layers, which need to enable error
   messages to be sent and processed as intended.

   Errors in EDHOC are fatal.  After sending an error message, the
   sender MUST discontinue the protocol.  The receiver SHOULD treat an
   error message as an indication that the other party likely has
   discontinued the protocol.  But as the error message is not
   authenticated, a received error message might also have been sent by
   an attacker and the receiver MAY therefore try to continue the
   protocol.

   error SHALL be a CBOR Sequence (see Appendix C.1) as defined below

   error = (
     ERR_CODE : int,
     ERR_INFO : any,
   )

                       Figure 6: EDHOC Error Message

   where:






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   *  ERR_CODE - error code encoded as an integer.  The value 0 is used
      for success, all other values (negative or positive) indicate
      errors.

   *  ERR_INFO - error information.  Content and encoding depend on
      error code.

   The remainder of this section specifies the currently defined error
   codes, see Figure 7.  Additional error codes and corresponding error
   information may be specified.

   +----------+---------------+----------------------------------------+
   | ERR_CODE | ERR_INFO Type | Description                            |
   +==========+===============+========================================+
   |        0 | any           | Success                                |
   +----------+---------------+----------------------------------------+
   |        1 | tstr          | Unspecified                            |
   +----------+---------------+----------------------------------------+
   |        2 | suites        | Wrong selected cipher suite            |
   +----------+---------------+----------------------------------------+

                Figure 7: Error Codes and Error Information

6.1.  Success

   Error code 0 MAY be used internally in an application to indicate
   success, e.g., in log files.  ERR_INFO can contain any type of CBOR
   item.  Error code 0 MUST NOT be used as part of the EDHOC message
   exchange flow.

6.2.  Unspecified

   Error code 1 is used for errors that do not have a specific error
   code defined.  ERR_INFO MUST be a text string containing a human-
   readable diagnostic message written in English.  The diagnostic text
   message is mainly intended for software engineers that during
   debugging need to interpret it in the context of the EDHOC
   specification.  The diagnostic message SHOULD be provided to the
   calling application where it SHOULD be logged.

6.3.  Wrong Selected Cipher Suite

   Error code 2 MUST only be used in a response to message_1 in case the
   cipher suite selected by the Initiator is not supported by the
   Responder, or if the Responder supports a cipher suite more preferred
   by the Initiator than the selected cipher suite, see Section 5.2.3.
   ERR_INFO is in this case denoted SUITES_R and is of type suites, see
   Section 5.2.1.  If the Responder does not support the selected cipher



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   suite, then SUITES_R MUST include one or more supported cipher
   suites.  If the Responder supports a cipher suite in SUITES_I other
   than the selected cipher suite (independently of if the selected
   cipher suite is supported or not) then SUITES_R MUST include the
   supported cipher suite in SUITES_I which is most preferred by the
   Initiator.  SUITES_R MAY include a single cipher suite, i.e., be
   encoded as an int.  If the Responder does not support any cipher
   suite in SUITES_I, then it SHOULD include all its supported cipher
   suites in SUITES_R in any order.

6.3.1.  Cipher Suite Negotiation

   After receiving SUITES_R, the Initiator can determine which cipher
   suite to select (if any) for the next EDHOC run with the Responder.

   If the Initiator intends to contact the Responder in the future, the
   Initiator SHOULD remember which selected cipher suite to use until
   the next message_1 has been sent, otherwise the Initiator and
   Responder will likely run into an infinite loop where the Initiator
   selects its most preferred and the Responder sends an error with
   supported cipher suites.  After a successful run of EDHOC, the
   Initiator MAY remember the selected cipher suite to use in future
   EDHOC sessions.  Note that if the Initiator or Responder is updated
   with new cipher suite policies, any cached information may be
   outdated.

6.3.2.  Examples

   Assume that the Initiator supports the five cipher suites 5, 6, 7, 8,
   and 9 in decreasing order of preference.  Figures 8 and 9 show
   examples of how the Initiator can format SUITES_I and how SUITES_R is
   used by Responders to give the Initiator information about the cipher
   suites that the Responder supports.

   In the first example (Figure 8), the Responder supports cipher suite
   6 but not the initially selected cipher suite 5.















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   Initiator                                                   Responder
   |              METHOD, SUITES_I = 5, G_X, C_I, EAD_1                |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                   ERR_CODE = 2, SUITES_R = 6                      |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |             METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1            |
   +------------------------------------------------------------------>|
   |                             message_1                             |

     Figure 8: Example of Responder supporting suite 6 but not suite 5.

   In the second example (Figure 9), the Responder supports cipher
   suites 8 and 9 but not the more preferred (by the Initiator) cipher
   suites 5, 6 or 7.  To illustrate the negotiation mechanics we let the
   Initiator first make a guess that the Responder supports suite 6 but
   not suite 5.  Since the Responder supports neither 5 nor 6, it
   responds with SUITES_R containing the supported suites, after which
   the Initiator selects its most preferred supported suite.  The order
   of cipher suites in SUITES_R does not matter.  (If the Responder had
   supported suite 5, it would have included it in SUITES_R of the
   response, and it would in that case have become the selected suite in
   the second message_1.)

   Initiator                                                   Responder
   |            METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1             |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                  ERR_CODE = 2, SUITES_R = [9, 8]                  |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |           METHOD, SUITES_I = [5, 6, 7, 8], G_X, C_I, EAD_1        |
   +------------------------------------------------------------------>|
   |                             message_1                             |

      Figure 9: Example of Responder supporting suites 8 and 9 but not
                                 5, 6 or 7.









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   Note that the Initiator's list of supported cipher suites and order
   of preference is fixed (see Section 5.2.1 and Section 5.2.2).
   Furthermore, the Responder shall only accept message_1 if the
   selected cipher suite is the first cipher suite in SUITES_I that the
   Responder supports (see Section 5.2.3).  Following this procedure
   ensures that the selected cipher suite is the most preferred (by the
   Initiator) cipher suite supported by both parties.

   If the selected cipher suite is not the first cipher suite which the
   Responder supports in SUITES_I received in message_1, then Responder
   MUST discontinue the protocol, see Section 5.2.3.  If SUITES_I in
   message_1 is manipulated, then the integrity verification of
   message_2 containing the transcript hash TH_2 will fail and the
   Initiator will discontinue the protocol.

7.  Mandatory-to-Implement Compliance Requirements

   An implementation may support only Initiator or only Responder.

   An implementation may support only a single method.  None of the
   methods are mandatory-to-implement.

   Implementations MUST support 'kid' parameters of type int.  None of
   the other COSE header parameters are mandatory-to-implement.

   An implementation may support only a single credential type (CCS,
   CWT, X.509, C509).  None of the credential types are mandatory-to-
   implement.

   Implementations MUST support the EDHOC-Exporter.  Implementations
   SHOULD support EDHOC-KeyUpdate.

   Implementations MAY support message_4.  Error codes 1 and 2 MUST be
   supported.

   Implementations MAY support EAD.

   For many constrained IoT devices it is problematic to support more
   than one cipher suite.  Existing devices can be expected to support
   either ECDSA or EdDSA.  To enable as much interoperability as we can
   reasonably achieve, less constrained devices SHOULD implement both
   cipher suite 0 (AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA, AES-
   CCM-16-64-128, SHA-256) and cipher suite 2 (AES-CCM-16-64-128, SHA-
   256, 8, P-256, ES256, AES-CCM-16-64-128, SHA-256).  Constrained
   endpoints SHOULD implement cipher suite 0 or cipher suite 2.
   Implementations only need to implement the algorithms needed for
   their supported methods.




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8.  Security Considerations

8.1.  Security Properties

   EDHOC inherits its security properties from the theoretical SIGMA-I
   protocol [SIGMA].  Using the terminology from [SIGMA], EDHOC provides
   forward secrecy, mutual authentication with aliveness, consistency,
   and peer awareness.  As described in [SIGMA], peer awareness is
   provided to the Responder, but not to the Initiator.

   As described in [SIGMA], different levels of identity protection is
   provided to the Initiator and the Responder.  EDHOC protects the
   credential identifier of the Initiator against active attacks and the
   credential identifier of the Responder against passive attacks.  The
   roles should be assigned to protect the most sensitive identity/
   identifier, typically that which is not possible to infer from
   routing information in the lower layers.

   Compared to [SIGMA], EDHOC adds an explicit method type and expands
   the message authentication coverage to additional elements such as
   algorithms, external authorization data, and previous messages.  This
   protects against an attacker replaying messages or injecting messages
   from another session.

   EDHOC also adds selection of connection identifiers and downgrade
   protected negotiation of cryptographic parameters, i.e., an attacker
   cannot affect the negotiated parameters.  A single session of EDHOC
   does not include negotiation of cipher suites, but it enables the
   Responder to verify that the selected cipher suite is the most
   preferred cipher suite by the Initiator which is supported by both
   the Initiator and the Responder.

   As required by [RFC7258], IETF protocols need to mitigate pervasive
   monitoring when possible.  EDHOC therefore only supports methods with
   ephemeral Diffie-Hellman and provides a KeyUpdate function for
   lightweight application protocol rekeying with forward secrecy, in
   the sense that compromise of the private authentication keys does not
   compromise past session keys, and compromise of a session key does
   not compromise past session keys.

   While the KeyUpdate method can be used to meet cryptographic limits
   and provide partial protection against key leakage, it provides
   significantly weaker security properties than re-running EDHOC with
   ephemeral Diffie-Hellman.  Even with frequent use of KeyUpdate,
   compromise of one session key compromises all future session keys,
   and an attacker therefore only needs to perform static key
   exfiltration [RFC7624].  Frequently re-running EDHOC with ephemeral
   Diffie-Hellman forces attackers to perform dynamic key exfiltration



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   instead of static key exfiltration [RFC7624].  In the dynamic case,
   the attacker must have continuous interactions with the collaborator,
   which is more complicated and has a higher risk profile than the
   static case.

   To limit the effect of breaches, it is important to limit the use of
   symmetrical group keys for bootstrapping.  EDHOC therefore strives to
   make the additional cost of using raw public keys and self-signed
   certificates as small as possible.  Raw public keys and self-signed
   certificates are not a replacement for a public key infrastructure
   but SHOULD be used instead of symmetrical group keys for
   bootstrapping.

   Compromise of the long-term keys (private signature or static DH
   keys) does not compromise the security of completed EDHOC exchanges.
   Compromising the private authentication keys of one party lets an
   active attacker impersonate that compromised party in EDHOC exchanges
   with other parties but does not let the attacker impersonate other
   parties in EDHOC exchanges with the compromised party.  Compromise of
   the long-term keys does not enable a passive attacker to compromise
   future session keys.  Compromise of the HDKF input parameters (ECDH
   shared secret) leads to compromise of all session keys derived from
   that compromised shared secret.  Compromise of one session key does
   not compromise other session keys.  Compromise of PRK_4x3m leads to
   compromise of all exported keying material derived after the last
   invocation of the EDHOC-KeyUpdate function.

   EDHOC provides a minimum of 64-bit security against online brute
   force attacks and a minimum of 128-bit security against offline brute
   force attacks.  This is in line with IPsec, TLS, and COSE.  To break
   64-bit security against online brute force an attacker would on
   average have to send 4.3 billion messages per second for 68 years,
   which is infeasible in constrained IoT radio technologies.


















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   After sending message_3, the Initiator is assured that no other party
   than the Responder can compute the key PRK_4x3m (implicit key
   authentication).  The Initiator does however not know that the
   Responder has actually computed the key PRK_4x3m.  While the
   Initiator can securely send protected application data, the Initiator
   SHOULD NOT permanently store the keying material PRK_4x3m and TH_4
   until the Initiator is assured that the Responder has actually
   computed the key PRK_4x3m (explicit key confirmation).  Explicit key
   confirmation is e.g., assured when the Initiator has verified an
   OSCORE message or message_4 from the Responder.  After verifying
   message_3, the Responder is assured that the Initiator has calculated
   the key PRK_4x3m (explicit key confirmation) and that no other party
   than the Responder can compute the key.  The Responder can securely
   send protected application data and store the keying material
   PRK_4x3m and TH_4.

   Key compromise impersonation (KCI): In EDHOC authenticated with
   signature keys, EDHOC provides KCI protection against an attacker
   having access to the long-term key or the ephemeral secret key.  With
   static Diffie-Hellman key authentication, KCI protection would be
   provided against an attacker having access to the long-term Diffie-
   Hellman key, but not to an attacker having access to the ephemeral
   secret key.  Note that the term KCI has typically been used for
   compromise of long-term keys, and that an attacker with access to the
   ephemeral secret key can only attack that specific session.

   Repudiation: In EDHOC authenticated with signature keys, the
   Initiator could theoretically prove that the Responder performed a
   run of the protocol by presenting the private ephemeral key, and vice
   versa.  Note that storing the private ephemeral keys violates the
   protocol requirements.  With static Diffie-Hellman key
   authentication, both parties can always deny having participated in
   the protocol.

   Two earlier versions of EDHOC have been formally analyzed [Norrman20]
   [Bruni18] and the specification has been updated based on the
   analysis.

8.2.  Cryptographic Considerations

   The SIGMA protocol requires that the encryption of message_3 provides
   confidentiality against active attackers and EDHOC message_4 relies
   on the use of authenticated encryption.  Hence the message
   authenticating functionality of the authenticated encryption in EDHOC
   is critical: authenticated encryption MUST NOT be replaced by plain
   encryption only, even if authentication is provided at another level
   or through a different mechanism.




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   To reduce message overhead EDHOC does not use explicit nonces and
   instead rely on the ephemeral public keys to provide randomness to
   each session.  A good amount of randomness is important for the key
   generation, to provide liveness, and to protect against interleaving
   attacks.  For this reason, the ephemeral keys MUST NOT be used in
   more than one EDHOC message, and both parties SHALL generate fresh
   random ephemeral key pairs.  Note that an ephemeral key may be used
   to calculate several ECDH shared secrets.  When static Diffie-Hellman
   authentication is used the same ephemeral key is used in both
   ephemeral-ephemeral and ephemeral-static ECDH.

   As discussed in [SIGMA], the encryption of message_2 does only need
   to protect against passive attacker as active attackers can always
   get the Responders identity by sending their own message_1.  EDHOC
   uses the Expand function (typically HKDF-Expand) as a binary additive
   stream cipher.  HKDF-Expand provides better confidentiality than AES-
   CTR but is not often used as it is slow on long messages, and most
   applications require both IND-CCA confidentiality as well as
   integrity protection.  For the encryption of message_2, any speed
   difference is negligible, IND-CCA does not increase security, and
   integrity is provided by the inner MAC (and signature depending on
   method).

   Requirement for how to securely generate, validate, and process the
   ephemeral public keys depend on the elliptic curve.  For X25519 and
   X448, the requirements are defined in [RFC7748].  For secp256r1,
   secp384r1, and secp521r1, the requirements are defined in Section 5
   of [SP-800-56A].  For secp256r1, secp384r1, and secp521r1, at least
   partial public-key validation MUST be done.

8.3.  Cipher Suites and Cryptographic Algorithms

   When using private cipher suite or registering new cipher suites, the
   choice of key length used in the different algorithms needs to be
   harmonized, so that a sufficient security level is maintained for
   certificates, EDHOC, and the protection of application data.  The
   Initiator and the Responder should enforce a minimum security level.

   The hash algorithms SHA-1 and SHA-256/64 (SHA-256 truncated to
   64-bits) SHALL NOT be supported for use in EDHOC except for
   certificate identification with x5t and c5t.  Note that secp256k1 is
   only defined for use with ECDSA and not for ECDH.  Note that some
   COSE algorithms are marked as not recommended in the COSE IANA
   registry.







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8.4.  Post-Quantum Considerations

   As of the publication of this specification, it is unclear when or
   even if a quantum computer of sufficient size and power to exploit
   public key cryptography will exist.  Deployments that need to
   consider risks decades into the future should transition to Post-
   Quantum Cryptography (PQC) in the not-too-distant future.  Many other
   systems should take a slower wait-and-see approach where PQC is
   phased in when the quantum threat is more imminent.  Current PQC
   algorithms have limitations compared to Elliptic Curve Cryptography
   (ECC) and the data sizes would be problematic in many constrained IoT
   systems.

   Symmetric algorithms used in EDHOC such as SHA-256 and AES-CCM-
   16-64-128 are practically secure against even large quantum
   computers.  EDHOC supports all signature algorithms defined by COSE,
   including PQC signature algorithms such as HSS-LMS.  EDHOC is
   currently only specified for use with key exchange algorithms of type
   ECDH curves, but any Key Encapsulation Method (KEM), including PQC
   KEMs, can be used in method 0.  While the key exchange in method 0 is
   specified with terms of the Diffie-Hellman protocol, the key exchange
   adheres to a KEM interface: G_X is then the public key of the
   Initiator, G_Y is the encapsulation, and G_XY is the shared secret.
   Use of PQC KEMs to replace static DH authentication would likely
   require a specification updating EDHOC with new methods.

8.5.  Unprotected Data

   The Initiator and the Responder must make sure that unprotected data
   and metadata do not reveal any sensitive information.  This also
   applies for encrypted data sent to an unauthenticated party.  In
   particular, it applies to EAD_1, ID_CRED_R, EAD_2, and error
   messages.  Using the same EAD_1 in several EDHOC sessions allows
   passive eavesdroppers to correlate the different sessions.  Another
   consideration is that the list of supported cipher suites may
   potentially be used to identify the application.

   The Initiator and the Responder must also make sure that
   unauthenticated data does not trigger any harmful actions.  In
   particular, this applies to EAD_1 and error messages.











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8.6.  Denial-of-Service

   As CoAP provides Denial-of-Service protection in the form of the Echo
   option [I-D.ietf-core-echo-request-tag], EDHOC itself does not
   provide countermeasures against Denial-of-Service attacks.  By
   sending a number of new or replayed message_1 an attacker may cause
   the Responder to allocate state, perform cryptographic operations,
   and amplify messages.  To mitigate such attacks, an implementation
   SHOULD rely on lower layer mechanisms such as the Echo option in CoAP
   that forces the initiator to demonstrate reachability at its apparent
   network address.

   An attacker can also send faked message_2, message_3, message_4, or
   error in an attempt to trick the receiving party to send an error
   message and discontinue the session.  EDHOC implementations MAY
   evaluate if a received message is likely to have been forged by an
   attacker and ignore it without sending an error message or
   discontinuing the session.

8.7.  Implementation Considerations

   The availability of a secure random number generator is essential for
   the security of EDHOC.  If no true random number generator is
   available, a truly random seed MUST be provided from an external
   source and used with a cryptographically secure pseudorandom number
   generator.  As each pseudorandom number must only be used once, an
   implementation needs to get a new truly random seed after reboot, or
   continuously store state in nonvolatile memory, see ([RFC8613],
   Appendix B.1.1) for issues and solution approaches for writing to
   nonvolatile memory.  Intentionally or unintentionally weak or
   predictable pseudorandom number generators can be abused or exploited
   for malicious purposes.  [RFC8937] describes a way for security
   protocol implementations to augment their (pseudo)random number
   generators using a long-term private key and a deterministic
   signature function.  This improves randomness from broken or
   otherwise subverted random number generators.  The same idea can be
   used with other secrets and functions such as a Diffie-Hellman
   function or a symmetric secret and a PRF like HMAC or KMAC.  It is
   RECOMMENDED to not trust a single source of randomness and to not put
   unaugmented random numbers on the wire.

   If ECDSA is supported, "deterministic ECDSA" as specified in
   [RFC6979] MAY be used.  Pure deterministic elliptic-curve signatures
   such as deterministic ECDSA and EdDSA have gained popularity over
   randomized ECDSA as their security do not depend on a source of high-
   quality randomness.  Recent research has however found that
   implementations of these signature algorithms may be vulnerable to
   certain side-channel and fault injection attacks due to their



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   determinism.  See e.g., Section 1 of
   [I-D.mattsson-cfrg-det-sigs-with-noise] for a list of attack papers.
   As suggested in Section 6.1.2 of [I-D.ietf-cose-rfc8152bis-algs] this
   can be addressed by combining randomness and determinism.

   All private keys, symmetric keys, and IVs MUST be secret.
   Implementations should provide countermeasures to side-channel
   attacks such as timing attacks.  Intermediate computed values such as
   ephemeral ECDH keys and ECDH shared secrets MUST be deleted after key
   derivation is completed.

   The Initiator and the Responder are responsible for verifying the
   integrity of certificates.  The selection of trusted CAs should be
   done very carefully and certificate revocation should be supported.
   The private authentication keys MUST be kept secret, only the
   Responder SHALL have access to the Responder's private authentication
   key and only the Initiator SHALL have access to the Initiator's
   private authentication key.

   The Initiator and the Responder are allowed to select the connection
   identifiers C_I and C_R, respectively, for the other party to use in
   the ongoing EDHOC protocol as well as in a subsequent application
   protocol (e.g., OSCORE [RFC8613]).  The choice of connection
   identifier is not security critical in EDHOC but intended to simplify
   the retrieval of the right security context in combination with using
   short identifiers.  If the wrong connection identifier of the other
   party is used in a protocol message it will result in the receiving
   party not being able to retrieve a security context (which will
   terminate the protocol) or retrieve the wrong security context (which
   also terminates the protocol as the message cannot be verified).

   If two nodes unintentionally initiate two simultaneous EDHOC message
   exchanges with each other even if they only want to complete a single
   EDHOC message exchange, they MAY terminate the exchange with the
   lexicographically smallest G_X.  If the two G_X values are equal, the
   received message_1 MUST be discarded to mitigate reflection attacks.
   Note that in the case of two simultaneous EDHOC exchanges where the
   nodes only complete one and where the nodes have different preferred
   cipher suites, an attacker can affect which of the two nodes'
   preferred cipher suites will be used by blocking the other exchange.

   If supported by the device, it is RECOMMENDED that at least the long-
   term private keys are stored in a Trusted Execution Environment (TEE)
   and that sensitive operations using these keys are performed inside
   the TEE.  To achieve even higher security it is RECOMMENDED that
   additional operations such as ephemeral key generation, all
   computations of shared secrets, and storage of the PRK keys can be
   done inside the TEE.  The use of a TEE enforces that code within that



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   environment cannot be tampered with, and that any data used by such
   code cannot be read or tampered with by code outside that
   environment.

9.  IANA Considerations

9.1.  EDHOC Exporter Label Registry

   IANA has created a new registry titled "EDHOC Exporter Label" under
   the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)".  The
   registration procedure is "Expert Review".  The columns of the
   registry are Label, Description, and Reference.  All columns are text
   strings where Label consists only of the printable ASCII characters
   0x21 - 0x7e.  Labels beginning with "PRIVATE" MAY be used for private
   use without registration.  All other label values MUST be registered.
   The initial contents of the registry are:

   Label: EDHOC_K_4
   Description: Key used to protect EDHOC message_4
   Reference: [[this document]]

   Label: EDHOC_IV_4
   Description: IV used to protect EDHOC message_4
   Reference: [[this document]]

   Label: OSCORE_Master_Secret
   Description: Derived OSCORE Master Secret
   Reference: [[this document]]

   Label: OSCORE_Master_Salt
   Description: Derived OSCORE Master Salt
   Reference: [[this document]]

9.2.  EDHOC Cipher Suites Registry

   IANA has created a new registry titled "EDHOC Cipher Suites" under
   the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)".  The
   registration procedure is "Expert Review".  The columns of the
   registry are Value, Array, Description, and Reference, where Value is
   an integer and the other columns are text strings.  The initial
   contents of the registry are:

   Value: -24
   Algorithms: N/A
   Desc: Reserved for Private Use
   Reference: [[this document]]





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   Value: -23
   Algorithms: N/A
   Desc: Reserved for Private Use
   Reference: [[this document]]

   Value: -22
   Algorithms: N/A
   Desc: Reserved for Private Use
   Reference: [[this document]]

   Value: -21
   Algorithms: N/A
   Desc: Reserved for Private Use
   Reference: [[this document]]

   Value: 0
   Array: 10, -16, 8, 4, -8, 10, -16
   Desc: AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 1
   Array: 30, -16, 16, 4, -8, 10, -16
   Desc: AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 2
   Array: 10, -16, 8, 1, -7, 10, -16
   Desc: AES-CCM-16-64-128, SHA-256, 8, P-256, ES256,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 3
   Array: 30, -16, 16, 1, -7, 10, -16
   Desc: AES-CCM-16-128-128, SHA-256, 16, P-256, ES256,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 4
   Array: 24, -16, 16, 4, -8, 24, -16
   Desc: ChaCha20/Poly1305, SHA-256, 16, X25519, EdDSA,
         ChaCha20/Poly1305, SHA-256
   Reference: [[this document]]







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   Value: 5
   Array: 24, -16, 16, 1, -7, 24, -16
   Desc: ChaCha20/Poly1305, SHA-256, 16, P-256, ES256,
         ChaCha20/Poly1305, SHA-256
   Reference: [[this document]]

   Value: 6
   Array: 1, -16, 16, 4, -7, 1, -16
   Desc: A128GCM, SHA-256, 16, X25519, ES256,
         A128GCM, SHA-256
   Reference: [[this document]]

   Value: 24
   Array: 3, -43, 16, 2, -35, 3, -43
   Desc: A256GCM, SHA-384, 16, P-384, ES384,
         A256GCM, SHA-384
   Reference: [[this document]]

   Value: 25
   Array: 24, -45, 16, 5, -8, 24, -45
   Desc: ChaCha20/Poly1305, SHAKE256, 16, X448, EdDSA,
         ChaCha20/Poly1305, SHAKE256
   Reference: [[this document]]

9.3.  EDHOC Method Type Registry

   IANA has created a new registry entitled "EDHOC Method Type" under
   the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)".  The
   registration procedure is "Expert Review".  The columns of the
   registry are Value, Description, and Reference, where Value is an
   integer and the other columns are text strings.  The initial contents
   of the registry are shown in Figure 4.

9.4.  EDHOC Error Codes Registry

   IANA has created a new registry entitled "EDHOC Error Codes" under
   the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)".  The
   registration procedure is "Expert Review".  The columns of the
   registry are ERR_CODE, ERR_INFO Type and Description, where ERR_CODE
   is an integer, ERR_INFO is a CDDL defined type, and Description is a
   text string.  The initial contents of the registry are shown in
   Figure 7.









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9.5.  EDHOC External Authorization Data Registry

   IANA has created a new registry entitled "EDHOC External
   Authorization Data" under the new group name "Ephemeral Diffie-
   Hellman Over COSE (EDHOC)".  The registration procedure is "Expert
   Review".  The columns of the registry are Label, Description, Value
   Type, and Reference, where Label is an integer and the other columns
   are text strings.

9.6.  COSE Header Parameters Registry

   IANA has registered the following entries in the "COSE Header
   Parameters" registry under the group name "CBOR Object Signing and
   Encryption (COSE)".  The value of the 'kcwt' header parameter is a
   COSE Web Token (CWT) [RFC8392], and the value of the 'kccs' header
   parameter is an CWT Claims Set (CCS), see Section 1.5.  The CWT/CCS
   must contain a COSE_Key in a 'cnf' claim [RFC8747].  The Value
   Registry for this item is empty and omitted from the table below.

   +-----------+-------+----------------+---------------------------+
   | Name      | Label | Value Type     | Description               |
   +===========+=======+================+===========================+
   | kcwt      | TBD1  | COSE_Messages  | A CBOR Web Token (CWT)    |
   |           |       |                | containing a COSE_Key in  |
   |           |       |                | a 'cnf' claim             |
   +-----------+-------+----------------+---------------------------+
   | kccs      | TBD2  | map / #6(map)  | A CWT Claims Set (CCS)    |
   |           |       |                | containing a COSE_Key in  |
   |           |       |                | a 'cnf' claim             |
   +-----------+-------+----------------+---------------------------+

9.7.  COSE Header Parameters Registry

   IANA has extended the Value Type of 'kid' in the "COSE Header
   Parameters" registry under the group name "CBOR Object Signing and
   Encryption (COSE)" to also allow the Value Type int.  The resulting
   Value Type is bstr / int.  The Value Registry for this item is empty
   and omitted from the table below.

   +------+-------+------------+----------------+-------------------+
   | Name | Label | Value Type | Description    | Reference         |
   +------+-------+------------+----------------+-------------------+
   | kid  |   4   | bstr / int | Key identifier | [RFC9052]         |
   |      |       |            |                | [[This document]] |
   +------+-------+------------+----------------+-------------------+






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9.8.  COSE Key Common Parameters Registry

   IANA has extended the Value Type of 'kid' in the "COSE Key Common
   Parameters" registry under the group name "CBOR Object Signing and
   Encryption (COSE)" to also allow the Value Type int.  The resulting
   Value Type is bstr / int.  The Value Registry for this item is empty
   and omitted from the table below.

   +------+-------+------------+----------------+-------------------+
   | Name | Label | Value Type | Description    | Reference         |
   +------+-------+------------+----------------+-------------------+
   | kid  |   2   | bstr / int | Key identifi-  | [RFC9052]         |
   |      |       |            | cation value - | [[This document]] |
   |      |       |            | match to kid   |                   |
   |      |       |            | in message     |                   |
   +------+-------+------------+----------------+-------------------+

9.9.  CWT Confirmation Methods Registry

   IANA has extended the Value Type of 'kid' in the "CWT Confirmation
   Methods" registry under the group name "CBOR Web Token (CWT) Claims"
   to also allow the Value Type int.  The incorrect term binary string
   has been corrected to bstr.  The resulting Value Type is bstr / int.
   The new updated content for the 'kid' method is shown in the list
   below.

   *  Confirmation Method Name: kid

   *  Confirmation Method Description: Key Identifier

   *  JWT Confirmation Method Name: kid

   *  Confirmation Key: 3

   *  Confirmation Value Type(s): bstr / int

   *  Change Controller: IESG

   *  Specification Document(s): Section 3.4 of RFC 8747 [[This
      document]]

9.10.  The Well-Known URI Registry

   IANA has added the well-known URI "edhoc" to the "Well-Known URIs"
   registry under the group name "Well-Known URIs".

   *  URI suffix: edhoc




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   *  Change controller: IETF

   *  Specification document(s): [[this document]]

   *  Related information: None

9.11.  Media Types Registry

   IANA has added the media type "application/edhoc" to the "Media
   Types" registry.

   *  Type name: application

   *  Subtype name: edhoc

   *  Required parameters: N/A

   *  Optional parameters: N/A

   *  Encoding considerations: binary

   *  Security considerations: See Section 7 of this document.

   *  Interoperability considerations: N/A

   *  Published specification: [[this document]] (this document)

   *  Applications that use this media type: To be identified

   *  Fragment identifier considerations: N/A

   *  Additional information:

      -  Magic number(s): N/A

      -  File extension(s): N/A

      -  Macintosh file type code(s): N/A

   *  Person & email address to contact for further information: See
      "Authors' Addresses" section.

   *  Intended usage: COMMON

   *  Restrictions on usage: N/A

   *  Author: See "Authors' Addresses" section.




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   *  Change Controller: IESG

9.12.  CoAP Content-Formats Registry

   IANA has added the media type "application/edhoc" to the "CoAP
   Content-Formats" registry under the group name "Constrained RESTful
   Environments (CoRE) Parameters".

   *  Media Type: application/edhoc

   *  Encoding:

   *  ID: TBD42

   *  Reference: [[this document]]

9.13.  Resource Type (rt=) Link Target Attribute Values Registry

   IANA has added the resource type "core.edhoc" to the "Resource Type
   (rt=) Link Target Attribute Values" registry under the group name
   "Constrained RESTful Environments (CoRE) Parameters".

   *  Value: "core.edhoc"

   *  Description: EDHOC resource.

   *  Reference: [[this document]]

   Client applications can use this resource type to discover a server's
   resource for EDHOC, where to send a request for executing the EDHOC
   protocol.

9.14.  Expert Review Instructions

   The IANA Registries established in this document is defined as
   "Expert Review".  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.

   Expert reviewers should take into consideration the following points:











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   *  Clarity and correctness of registrations.  Experts are expected to
      check the clarity of purpose and use of the requested entries.
      Expert needs to make sure the values of algorithms are taken from
      the right registry, when that is required.  Expert should consider
      requesting an opinion on the correctness of registered parameters
      from relevant IETF working groups.  Encodings that do not meet
      these objective of clarity and completeness should not be
      registered.

   *  Experts should take into account the expected usage of fields when
      approving point assignment.  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.

   *  Specifications are recommended.  When specifications are not
      provided, the description provided needs to have sufficient
      information to verify the points above.

10.  References

10.1.  Normative References

   [I-D.ietf-core-echo-request-tag]
              Amsüss, C., Mattsson, J. P., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", Work in Progress,
              Internet-Draft, draft-ietf-core-echo-request-tag-14, 4
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-echo-request-tag-14.txt>.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-12.txt>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-struct-15.txt>.

   [I-D.ietf-cose-x509]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header parameters for carrying and referencing X.509
              certificates", Work in Progress, Internet-Draft, draft-



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              ietf-cose-x509-08, 14 December 2020,
              <https://www.ietf.org/internet-drafts/draft-ietf-cose-
              x509-08.txt>.

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

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [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/info/rfc5280>.

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

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <https://www.rfc-editor.org/info/rfc6090>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.






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   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [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/info/rfc7959>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

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

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

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

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





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

10.2.  Informative References

   [Bruni18]  Bruni, A., Sahl Jørgensen, T., Grønbech Petersen, T., and
              C. Schürmann, "Formal Verification of Ephemeral Diffie-
              Hellman Over COSE (EDHOC)", November 2018,
              <https://www.springerprofessional.de/en/formal-
              verification-of-ephemeral-diffie-hellman-over-cose-
              edhoc/16284348>.

   [CborMe]   Bormann, C., "CBOR Playground", May 2018,
              <http://cbor.me/>.

   [CNSA]     (Placeholder), ., "Commercial National Security Algorithm
              Suite", August 2015,
              <https://apps.nsa.gov/iaarchive/programs/iad-initiatives/
              cnsa-suite.cfm>.

   [I-D.ietf-core-oscore-edhoc]
              Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
              and G. Selander, "Combining EDHOC and OSCORE", Work in
              Progress, Internet-Draft, draft-ietf-core-oscore-edhoc-01,
              12 July 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-oscore-edhoc-01.txt>.

   [I-D.ietf-core-resource-directory]
              Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
              D. Stok, "CoRE Resource Directory", Work in Progress,
              Internet-Draft, draft-ietf-core-resource-directory-28, 7
              March 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-resource-directory-28.txt>.

   [I-D.ietf-cose-cbor-encoded-cert]
              Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
              M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-02, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
              encoded-cert-02.txt>.








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   [I-D.ietf-lake-reqs]
              Vucinic, M., Selander, G., Mattsson, J. P., and D. Garcia-
              Carrillo, "Requirements for a Lightweight AKE for OSCORE",
              Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
              8 June 2020, <https://www.ietf.org/archive/id/draft-ietf-
              lake-reqs-04.txt>.

   [I-D.ietf-lwig-security-protocol-comparison]
              Mattsson, J. P., Palombini, F., and M. Vucinic,
              "Comparison of CoAP Security Protocols", Work in Progress,
              Internet-Draft, draft-ietf-lwig-security-protocol-
              comparison-05, 2 November 2020,
              <https://www.ietf.org/archive/id/draft-ietf-lwig-security-
              protocol-comparison-05.txt>.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
              drafts/draft-ietf-tls-dtls13-43.txt>.

   [I-D.mattsson-cfrg-det-sigs-with-noise]
              Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
              "Deterministic ECDSA and EdDSA Signatures with Additional
              Randomness", Work in Progress, Internet-Draft, draft-
              mattsson-cfrg-det-sigs-with-noise-02, 11 March 2020,
              <https://www.ietf.org/archive/id/draft-mattsson-cfrg-det-
              sigs-with-noise-02.txt>.

   [I-D.selander-ace-ake-authz]
              Selander, G., Mattsson, J. P., Vucinic, M., Richardson,
              M., and A. Schellenbaum, "Lightweight Authorization for
              Authenticated Key Exchange.", Work in Progress, Internet-
              Draft, draft-selander-ace-ake-authz-03, 4 May 2021,
              <https://www.ietf.org/archive/id/draft-selander-ace-ake-
              authz-03.txt>.

   [I-D.selander-lake-traces]
              Selander, G. and J. P. Mattsson, "Traces of EDHOC", Work
              in Progress, Internet-Draft, draft-selander-lake-traces-
              01, 24 September 2021, <https://www.ietf.org/archive/id/
              draft-selander-lake-traces-01.txt>.








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   [Norrman20]
              Norrman, K., Sundararajan, V., and A. Bruni, "Formal
              Analysis of EDHOC Key Establishment for Constrained IoT
              Devices", September 2020,
              <https://arxiv.org/abs/2007.11427>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

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

   [RFC8937]  Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
              and C. Wood, "Randomness Improvements for Security
              Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
              <https://www.rfc-editor.org/info/rfc8937>.

   [SECG]     "Standards for Efficient Cryptography 1 (SEC 1)", May
              2009, <https://www.secg.org/sec1-v2.pdf>.

   [SIGMA]    Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE-
              Protocols (Long version)", June 2003,
              <http://webee.technion.ac.il/~hugo/sigma-pdf.pdf>.

   [SP-800-56A]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key-Establishment
              Schemes Using Discrete Logarithm Cryptography",
              NIST Special Publication 800-56A Revision 3, April 2018,
              <https://doi.org/10.6028/NIST.SP.800-56Ar3>.








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Appendix A.  Use with OSCORE and Transfer over CoAP

   This appendix describes how to select EDHOC connection identifiers
   and derive an OSCORE security context when OSCORE is used with EDHOC,
   and how to transfer EDHOC messages over CoAP.

A.1.  Selecting EDHOC Connection Identifier

   This section specifies a rule for converting from EDHOC connection
   identifier to OSCORE Sender/Recipient ID.  (An identifier is Sender
   ID or Recipient ID depending on from which endpoint is the point of
   view, see Section 3.1 of [RFC8613].)

   *  If the EDHOC connection identifier is numeric, i.e., encoded as a
      CBOR integer on the wire, it is converted to a (naturally byte-
      string shaped) OSCORE Sender/Recipient ID equal to its CBOR
      encoded form.

   For example, a numeric C_R equal to 10 (0x0A in CBOR encoding) is
   converted to a (typically client) Sender ID equal to 0x0A, while a
   numeric C_I equal to -12 (0x2B in CBOR encoding) is converted to a
   (typically client) Sender ID equal to 0x2B.

   *  If the EDHOC connection identifier is byte-valued, hence encoded
      as a CBOR byte string on the wire, it is converted to an OSCORE
      Sender/Recipient ID equal to the byte string.

   For example, a byte-string valued C_R equal to 0xFF (0x41FF in CBOR
   encoding) is converted to a (typically client) Sender ID equal to
   0xFF.

   Two EDHOC connection identifiers are called "equivalent" if and only
   if, by applying the conversion above, they both result in the same
   OSCORE Sender/Recipient ID.  For example, the two EDHOC connection
   identifiers with CBOR encoding 0x0A (numeric) and 0x410A (byte-
   valued) are equivalent since they both result in the same OSCORE
   Sender/Recipient ID 0x0A.

   When EDHOC is used to establish an OSCORE security context, the
   connection identifiers C_I and C_R MUST NOT be equivalent.
   Furthermore, in case of multiple OSCORE security contexts with
   potentially different endpoints, to facilitate retrieval of the
   correct OSCORE security context, an endpoint SHOULD select an EDHOC
   connection identifier that when converted to OSCORE Recipient ID does
   not coincide with its other Recipient IDs.






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A.2.  Deriving the OSCORE Security Context

   This section specifies how to use EDHOC output to derive the OSCORE
   security context.

   After successful processing of EDHOC message_3, Client and Server
   derive Security Context parameters for OSCORE as follows (see
   Section 3.2 of [RFC8613]):

   *  The Master Secret and Master Salt are derived by using the EDHOC-
      Exporter interface, see Section 4.3.

   The EDHOC Exporter Labels for deriving the OSCORE Master Secret and
   the OSCORE Master Salt, are "OSCORE_Master_Secret" and
   "OSCORE_Master_Salt", respectively.

   The context parameter is h'' (0x40), the empty CBOR byte string.

   By default, key_length is the key length (in bytes) of the
   application AEAD Algorithm of the selected cipher suite for the EDHOC
   session.  Also by default, salt_length has value 8.  The Initiator
   and Responder MAY agree out-of-band on a longer key_length than the
   default and on a different salt_length.

 Master Secret = EDHOC-Exporter("OSCORE_Master_Secret", h'', key_length)
 Master Salt   = EDHOC-Exporter("OSCORE_Master_Salt", h'', salt_length)

   *  The AEAD Algorithm is the application AEAD algorithm of the
      selected cipher suite for the EDHOC session.

   *  The HKDF Algorithm is the one based on the application hash
      algorithm of the selected cipher suite for the EDHOC session.  For
      example, if SHA-256 is the application hash algorithm of the
      selected cipher suite, HKDF SHA-256 is used as HKDF Algorithm in
      the OSCORE Security Context.

   *  In case the Client is Initiator and the Server is Responder, the
      Client's OSCORE Sender ID and the Server's OSCORE Sender ID are
      determined from the EDHOC connection identifiers C_R and C_I for
      the EDHOC session, respectively, by applying the conversion in
      Appendix A.1.  The reverse applies in case the Client is the
      Responder and the Server is the Initiator.

   Client and Server use the parameters above to establish an OSCORE
   Security Context, as per Section 3.2.1 of [RFC8613].






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   From then on, Client and Server retrieve the OSCORE protocol state
   using the Recipient ID, and optionally other transport information
   such as the 5-tuple.

A.3.  Transferring EDHOC over CoAP

   This section specifies one instance for how EDHOC can be transferred
   as an exchange of CoAP [RFC7252] messages.  CoAP provides a reliable
   transport that can preserve packet ordering and handle message
   duplication.  CoAP can also perform fragmentation and protect against
   denial-of-service attacks.  The underlying CoAP transport should be
   used in reliable mode, in particular when fragmentation is used, to
   avoid, e.g., situations with hanging endpoints waiting for each
   other.

   By default, the CoAP client is the Initiator and the CoAP server is
   the Responder, but the roles SHOULD be chosen to protect the most
   sensitive identity, see Section 8.  According to this specification,
   EDHOC is transferred in POST requests and 2.04 (Changed) responses to
   the Uri-Path: "/.well-known/edhoc".  An application may define its
   own path that can be discovered, e.g., using resource directory
   [I-D.ietf-core-resource-directory].

   By default, the message flow is as follows: EDHOC message_1 is sent
   in the payload of a POST request from the client to the server's
   resource for EDHOC.  EDHOC message_2 or the EDHOC error message is
   sent from the server to the client in the payload of a 2.04 (Changed)
   response.  EDHOC message_3 or the EDHOC error message is sent from
   the client to the server's resource in the payload of a POST request.
   If needed, an EDHOC error message is sent from the server to the
   client in the payload of a 2.04 (Changed) response.  Alternatively,
   if EDHOC message_4 is used, it is sent from the server to the client
   in the payload of a 2.04 (Changed) response analogously to message_2.

   In order to correlate a message received from a client to a message
   previously sent by the server, messages sent by the client are
   prepended with the CBOR serialization of the connection identifier
   which the server has chosen.  This applies independently of if the
   CoAP server is Responder or Initiator.  For the default case when the
   server is Responder, the prepended connection identifier is C_R, and
   C_I if the server is Initiator.  If message_1 is sent to the server,
   the CBOR simple value "true" (0xf5) is sent in its place (given that
   the server has not selected C_R yet).

   These identifiers are encoded in CBOR and thus self-delimiting.  They
   are sent in front of the actual EDHOC message, and only the part of
   the body following the identifier is used for EDHOC processing.




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   Consequently, the application/edhoc media type does not apply to
   these messages; their media type is unnamed.

   An example of a successful EDHOC exchange using CoAP is shown in
   Figure 10.  In this case the CoAP Token enables correlation on the
   Initiator side, and the prepended C_R enables correlation on the
   Responder (server) side.

             Client    Server
               |          |
               +--------->| Header: POST (Code=0.02)
               |   POST   | Uri-Path: "/.well-known/edhoc"
               |          | Payload: true, EDHOC message_1
               |          |
               |<---------+ Header: 2.04 Changed
               |   2.04   | Content-Format: application/edhoc
               |          | Payload: EDHOC message_2
               |          |
               +--------->| Header: POST (Code=0.02)
               |   POST   | Uri-Path: "/.well-known/edhoc"
               |          | Payload: C_R, EDHOC message_3
               |          |
               |<---------+ Header: 2.04 Changed
               |   2.04   |
               |          |

      Figure 10: Transferring EDHOC in CoAP when the Initiator is CoAP
                                   Client

   The exchange in Figure 10 protects the client identity against active
   attackers and the server identity against passive attackers.

   An alternative exchange that protects the server identity against
   active attackers and the client identity against passive attackers is
   shown in Figure 11.  In this case the CoAP Token enables the
   Responder to correlate message_2 and message_3, and the prepended C_I
   enables correlation on the Initiator (server) side.  If EDHOC
   message_4 is used, C_I is prepended, and it is transported with CoAP
   in the payload of a POST request with a 2.04 (Changed) response.












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             Client    Server
               |          |
               +--------->| Header: POST (Code=0.02)
               |   POST   | Uri-Path: "/.well-known/edhoc"
               |          |
               |<---------+ Header: 2.04 Changed
               |   2.04   | Content-Format: application/edhoc
               |          | Payload: EDHOC message_1
               |          |
               +--------->| Header: POST (Code=0.02)
               |   POST   | Uri-Path: "/.well-known/edhoc"
               |          | Payload: C_I, EDHOC message_2
               |          |
               |<---------+ Header: 2.04 Changed
               |   2.04   | Content-Format: application/edhoc
               |          | Payload: EDHOC message_3
               |          |

      Figure 11: Transferring EDHOC in CoAP when the Initiator is CoAP
                                   Server

   To protect against denial-of-service attacks, the CoAP server MAY
   respond to the first POST request with a 4.01 (Unauthorized)
   containing an Echo option [I-D.ietf-core-echo-request-tag].  This
   forces the initiator to demonstrate its reachability at its apparent
   network address.  If message fragmentation is needed, the EDHOC
   messages may be fragmented using the CoAP Block-Wise Transfer
   mechanism [RFC7959].

   EDHOC does not restrict how error messages are transported with CoAP,
   as long as the appropriate error message can to be transported in
   response to a message that failed (see Section 6).  EDHOC error
   messages transported with CoAP are carried in the payload.

A.3.1.  Transferring EDHOC and OSCORE over CoAP

   When using EDHOC over CoAP for establishing an OSCORE Security
   Context, EDHOC error messages sent as CoAP responses MUST be sent in
   the payload of error responses, i.e., they MUST specify a CoAP error
   response code.  In particular, it is RECOMMENDED that such error
   responses have response code either 4.00 (Bad Request) in case of
   client error (e.g., due to a malformed EDHOC message), or 5.00
   (Internal Server Error) in case of server error (e.g., due to failure
   in deriving EDHOC key material).  The Content-Format of the error
   response MUST be set to application/edhoc.

   A method for combining EDHOC and OSCORE protocols in two round-trips
   is specified in [I-D.ietf-core-oscore-edhoc].



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Appendix B.  Compact Representation

   As described in Section 4.2 of [RFC6090] the x-coordinate of an
   elliptic curve public key is a suitable representative for the entire
   point whenever scalar multiplication is used as a one-way function.
   One example is ECDH with compact output, where only the x-coordinate
   of the computed value is used as the shared secret.

   This section defines a format for compact representation based on the
   Elliptic-Curve-Point-to-Octet-String Conversion defined in
   Section 2.3.3 of [SECG].  Using the notation from [SECG], the output
   is an octet string of length ceil( (log2 q) / 8 ).  See [SECG] for a
   definition of q, M, X, xp, and ~yp.  The steps in Section 2.3.3 of
   [SECG] are replaced by:

   1.  Convert the field element xp to an octet string X of length ceil(
       (log2 q) / 8 ) octets using the conversion routine specified in
       Section 2.3.5 of [SECG].

   2.  Output M = X

   The encoding of the point at infinity is not supported.  Compact
   representation does not change any requirements on validation.  If a
   y-coordinate is required for validation or compatibily with APIs the
   value ~yp SHALL be set to zero.  For such use, the compact
   representation can be transformed into the SECG point compressed
   format by prepending it with the single byte 0x02 (i.e., M = 0x02 ||
   X).

   Using compact representation have some security benefits.  An
   implementation does not need to check that the point is not the point
   at infinity (the identity element).  Similarly, as not even the sign
   of the y-coordinate is encoded, compact representation trivially
   avoids so called "benign malleability" attacks where an attacker
   changes the sign, see [SECG].

Appendix C.  Use of CBOR, CDDL and COSE in EDHOC

   This Appendix is intended to simplify for implementors not familiar
   with CBOR [RFC8949], CDDL [RFC8610], COSE
   [I-D.ietf-cose-rfc8152bis-struct], and HKDF [RFC5869].










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C.1.  CBOR and CDDL

   The Concise Binary Object Representation (CBOR) [RFC8949] is a data
   format designed for small code size and small message size.  CBOR
   builds on the JSON data model but extends it by e.g., encoding binary
   data directly without base64 conversion.  In addition to the binary
   CBOR encoding, CBOR also has a diagnostic notation that is readable
   and editable by humans.  The Concise Data Definition Language (CDDL)
   [RFC8610] provides a way to express structures for protocol messages
   and APIs that use CBOR.  [RFC8610] also extends the diagnostic
   notation.

   CBOR data items are encoded to or decoded from byte strings using a
   type-length-value encoding scheme, where the three highest order bits
   of the initial byte contain information about the major type.  CBOR
   supports several different types of data items, in addition to
   integers (int, uint), simple values, byte strings (bstr), and text
   strings (tstr), CBOR also supports arrays [] of data items, maps {}
   of pairs of data items, and sequences [RFC8742] of data items.  Some
   examples are given below.

   The EDHOC specification sometimes use CDDL names in CBOR dignostic
   notation as in e.g., << ID_CRED_R, ? EAD_2 >>.  This means that EAD_2
   is optional and that ID_CRED_R and EAD_2 should be substituted with
   their values before evaluation.  I.e., if ID_CRED_R = { 4 : h'' } and
   EAD_2 is omitted then << ID_CRED_R, ? EAD_2 >> = << { 4 : h'' } >>,
   which encodes to 0x43a10440.

   For a complete specification and more examples, see [RFC8949] and
   [RFC8610].  We recommend implementors to get used to CBOR by using
   the CBOR playground [CborMe].




















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    Diagnostic          Encoded              Type
    ------------------------------------------------------------------
    1                   0x01                 unsigned integer
    24                  0x1818               unsigned integer
    -24                 0x37                 negative integer
    -25                 0x3818               negative integer
    true                0xf5                 simple value
    h''                 0x40                 byte string
    h'12cd'             0x4212cd             byte string
    '12cd'              0x4431326364         byte string
    "12cd"              0x6431326364         text string
    { 4 : h'cd' }       0xa10441cd           map
    << 1, 2, true >>    0x430102f5           byte string
    [ 1, 2, true ]      0x830102f5           array
    ( 1, 2, true )      0x0102f5             sequence
    1, 2, true          0x0102f5             sequence
    ------------------------------------------------------------------

C.2.  CDDL Definitions

   This sections compiles the CDDL definitions for ease of reference.






























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   suites = [ 2* int ] / int

   ead = 1* (
     ead_label : int,
     ead_value : any,
   )

   message_1 = (
     METHOD : int,
     SUITES_I : suites,
     G_X : bstr,
     C_I : bstr / int,
     ? EAD_1 : ead,
   )

   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
     C_R : bstr / int,
   )

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

   message_4 = (
     CIPHERTEXT_4 : bstr,
   )

   error = (
     ERR_CODE : int,
     ERR_INFO : any,
   )

   info = (
     transcript_hash : bstr,
     label : tstr,
     context : bstr,
     length : uint,
   )

C.3.  COSE

   CBOR Object Signing and Encryption (COSE)
   [I-D.ietf-cose-rfc8152bis-struct] describes how to create and process
   signatures, message authentication codes, and encryption using CBOR.
   COSE builds on JOSE, but is adapted to allow more efficient
   processing in constrained devices.  EDHOC makes use of COSE_Key,
   COSE_Encrypt0, and COSE_Sign1 objects in the message processing:



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   *  ECDH ephemeral public keys of type EC2 or OKP in message_1 and
      message_2 consist of the COSE_Key parameter named 'x', see
      Section 7.1 and 7.2 of [I-D.ietf-cose-rfc8152bis-algs]

   *  The ciphertexts in message_3 and message_4 consist of a subset of
      the single recipient encrypted data object COSE_Encrypt0, which is
      described in Sections 5.2-5.3 of
      [I-D.ietf-cose-rfc8152bis-struct].  The ciphertext is computed
      over the plaintext and associated data, using an encryption key
      and an initialization vector.  The associated data is an
      Enc_structure consisting of protected headers and externally
      supplied data (external_aad).  COSE constructs the input to the
      AEAD [RFC5116] for message_i (i = 3 or 4, see Section 5.4 and
      Section 5.5, respectively) as follows:

      -  Secret key K = K_i

      -  Nonce N = IV_i

      -  Plaintext P for message_i

      -  Associated Data A = [ "Encrypt0", h'', TH_i ]

   *  Signatures in message_2 of method 0 and 2, and in message_3 of
      method 0 and 1, consist of a subset of the single signer data
      object COSE_Sign1, which is described in Sections 4.2-4.4 of
      [I-D.ietf-cose-rfc8152bis-struct].  The signature is computed over
      a Sig_structure containing payload, protected headers and
      externally supplied data (external_aad) using a private signature
      key and verified using the corresponding public signature key.
      For COSE_Sign1, the message to be signed is:

       [ "Signature1", protected, external_aad, payload ]

      where protected, external_aad and payload are specified in
      Section 5.3 and Section 5.4.

   Different header parameters to identify X.509 or C509 certificates by
   reference are defined in [I-D.ietf-cose-x509] and
   [I-D.ietf-cose-cbor-encoded-cert]:

   *  by a hash value with the 'x5t' or 'c5t' parameters, respectively:

      -  ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,

      -  ID_CRED_x = { TBD3 : COSE_CertHash }, for x = I or R;

   *  or by a URI with the 'x5u' or 'c5u' parameters, respectively:



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      -  ID_CRED_x = { 35 : uri }, for x = I or R,

      -  ID_CRED_x = { TBD4 : uri }, for x = I or R.

   When ID_CRED_x does not contain the actual credential, it may be very
   short, e.g., if the endpoints have agreed to use a key identifier
   parameter 'kid':

   *  ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or
      R.

   Note that a COSE header map can contain several header parameters,
   for example { x5u, x5t } or { kid, kid_context }.

   ID_CRED_x MAY also identify the authentication credential by value.
   For example, a certificate chain can be transported in ID_CRED_x with
   COSE header parameter c5c or x5chain, defined in
   [I-D.ietf-cose-cbor-encoded-cert] and [I-D.ietf-cose-x509] and
   credentials of type CWT and CCS can be transported with the COSE
   header parameters registered in Section 9.6.

Appendix D.  Applicability Template

   This appendix contains a rudimentary example of an applicability
   statement, see Section 3.9.

   For use of EDHOC in the XX protocol, the following assumptions are
   made:

   1.  Transfer in CoAP as specified in Appendix A.3 with requests
       expected by the CoAP server (= Responder) at /app1-edh, no
       Content-Format needed.

   2.  METHOD = 1 (I uses signature key, R uses static DH key.)

   3.  CRED_I is an IEEE 802.1AR IDevID encoded as a C509 certificate of
       type 0 [I-D.ietf-cose-cbor-encoded-cert].

       *  R acquires CRED_I out-of-band, indicated in EAD_1.

       *  ID_CRED_I = {4: h''} is a 'kid' with value empty CBOR byte
          string.

   4.  CRED_R is a CCS of type OKP as specified in Section 3.5.3.

       *  The CBOR map has parameters 1 (kty), -1 (crv), and -2
          (x-coordinate).




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       *  ID_CRED_R is {TBD2 : CCS}.  Editor's note: TBD2 is the COSE
          header parameter value of 'kccs', see Section 9.6

   5.  External authorization data is defined and processed as specified
       in [I-D.selander-ace-ake-authz].

   6.  EUI-64 used as identity of endpoint.

   7.  No use of message_4: the application sends protected messages
       from R to I.

Appendix E.  EDHOC Message Deduplication

   EDHOC by default assumes that message duplication is handled by the
   transport, in this section exemplified with CoAP.

   Deduplication of CoAP messages is described in Section 4.5 of
   [RFC7252].  This handles the case when the same Confirmable (CON)
   message is received multiple times due to missing acknowledgement on
   CoAP messaging layer.  The recommended processing in [RFC7252] is
   that the duplicate message is acknowledged (ACK), but the received
   message is only processed once by the CoAP stack.

   Message deduplication is resource demanding and therefore not
   supported in all CoAP implementations.  Since EDHOC is targeting
   constrained environments, it is desirable that EDHOC can optionally
   support transport layers which does not handle message duplication.
   Special care is needed to avoid issues with duplicate messages, see
   Section 5.1.

   The guiding principle here is similar to the deduplication processing
   on CoAP messaging layer: a received duplicate EDHOC message SHALL NOT
   result in a response consisting of another instance of the next EDHOC
   message.  The result MAY be that a duplicate EDHOC response is sent,
   provided it is still relevant with respect the current protocol
   state.  In any case, the received message MUST NOT be processed more
   than once in the same EDHOC session.  This is called "EDHOC message
   deduplication".

   An EDHOC implementation MAY store the previously sent EDHOC message
   to be able to resend it.  An EDHOC implementation MAY keep the
   protocol state to be able to recreate the previously sent EDHOC
   message and resend it.  The previous message or protocol state MUST
   NOT be kept longer than what is required for retransmission, for
   example, in the case of CoAP transport, no longer than the
   EXCHANGE_LIFETIME (see Section 4.8.2 of [RFC7252]).





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   Note that the requirements in Section 5.1 still apply because
   duplicate messages are not processed by the EDHOC state machine:

   *  EDHOC messages SHALL be processed according to the current
      protocol state.

   *  Different instances of the same message MUST NOT be processed in
      one session.

Appendix F.  Transports Not Natively Providing Correlation

   Protocols that do not natively provide full correlation between a
   series of messages can send the C_I and C_R identifiers along as
   needed.

   The transport over CoAP (Appendix A.3) can serve as a blueprint for
   other server-client protocols: The client prepends the C_x which the
   server selected (or, for message 1, the CBOR simple value 'true'
   which is not a valid C_x) to any request message it sends.  The
   server does not send any such indicator, as responses are matched to
   request by the client-server protocol design.

   Protocols that do not provide any correlation at all can prescribe
   prepending of the peer's chosen C_x to all messages.

Appendix G.  Change Log

   RFC Editor: Please remove this appendix.

   *  From -11 to -12:

      -  Clarified applicability to KEMs

      -  Clarified use of COSE header parameters

      -  Updates on MTI

      -  Updated security considerations

      -  New section on PQC

      -  Removed duplicate definition of cipher suites

      -  Explanations of use of COSE moved to Appendix C.3

      -  Updated internal references

   *  From -10 to -11:



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      -  Restructured section on authentication parameters

      -  Changed UCCS to CCS

      -  Changed names and description of COSE header parameters for
         CWT/CCS

      -  Changed several of the KDF and Exporter labels

      -  Removed edhoc_aead_id from info (already in transcript_hash)

      -  Added MTI section

      -  EAD: changed CDDL names and added value type to registry

      -  Updated Figures 1, 2, and 3

      -  Some correction and clarifications

      -  Added core.edhoc to CoRE Resource Type registry

   *  From -09 to -10:

      -  SUITES_I simplified to only contain the selected and more
         preferred suites

      -  Info is a CBOR sequence and context is a bstr

      -  Added kid to UCCS example

      -  Separate header parameters for CWT and UCCS

      -  CWT Confirmation Method kid extended to bstr / int

   *  From -08 to -09:

      -  G_Y and CIPHERTEXT_2 are now included in one CBOR bstr

      -  MAC_2 and MAC_3 are now generated with EDHOC-KDF

      -  Info field "context" is now general and explicit in EDHOC-KDF

      -  Restructured Section 4, Key Derivation

      -  Added EDHOC MAC length to cipher suite for use with static DH

      -  More details on the use of CWT and UCCS




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      -  Restructured and clarified Section 3.5, Authentication
         Parameters

      -  Replaced 'kid2' with extension of 'kid'

      -  EAD encoding now supports multiple ead types in one message

      -  Clarified EAD type

      -  Updated message sizes

      -  Replaced "perfect forward secrecy" with "forward secrecy"

      -  Updated security considerations

      -  Replaced prepended 'null' with 'true' in the CoAP transport of
         message_1

      -  Updated CDDL definitions

      -  Expanded on the use of COSE

   *  From -07 to -08:

      -  Prepended C_x moved from the EDHOC protocol itself to the
         transport mapping

      -  METHOD_CORR renamed to METHOD, corr removed

      -  Removed bstr_identifier and use bstr / int instead; C_x can now
         be int without any implied bstr semantics

      -  Defined COSE header parameter 'kid2' with value type bstr / int
         for use with ID_CRED_x

      -  Updated message sizes

      -  New cipher suites with AES-GCM and ChaCha20 / Poly1305

      -  Changed from one- to two-byte identifier of CNSA compliant
         suite

      -  Separate sections on transport and connection id with further
         sub-structure

      -  Moved back key derivation for OSCORE from draft-ietf-core-
         oscore-edhoc




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      -  OSCORE and CoAP specific processing moved to new appendix

      -  Message 4 section moved to message processing section

   *  From -06 to -07:

      -  Changed transcript hash definition for TH_2 and TH_3

      -  Removed "EDHOC signature algorithm curve" from cipher suite

      -  New IANA registry "EDHOC Exporter Label"

      -  New application defined parameter "context" in EDHOC-Exporter

      -  Changed normative language for failure from MUST to SHOULD send
         error

      -  Made error codes non-negative and 0 for success

      -  Added detail on success error code

      -  Aligned terminology "protocol instance" -> "session"

      -  New appendix on compact EC point representation

      -  Added detail on use of ephemeral public keys

      -  Moved key derivation for OSCORE to draft-ietf-core-oscore-edhoc

      -  Additional security considerations

      -  Renamed "Auxililary Data" as "External Authorization Data"

      -  Added encrypted EAD_4 to message_4

   *  From -05 to -06:

      -  New section 5.2 "Message Processing Outline"

      -  Optional inital byte C_1 = null in message_1

      -  New format of error messages, table of error codes, IANA
         registry

      -  Change of recommendation transport of error in CoAP

      -  Merge of content in 3.7 and appendix C into new section 3.7
         "Applicability Statement"



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      -  Requiring use of deterministic CBOR

      -  New section on message deduplication

      -  New appendix containin all CDDL definitions

      -  New appendix with change log

      -  Removed section "Other Documents Referencing EDHOC"

      -  Clarifications based on review comments

   *  From -04 to -05:

      -  EDHOC-Rekey-FS -> EDHOC-KeyUpdate

      -  Clarification of cipher suite negotiation

      -  Updated security considerations

      -  Updated test vectors

      -  Updated applicability statement template

   *  From -03 to -04:

      -  Restructure of section 1

      -  Added references to C509 Certificates

      -  Change in CIPHERTEXT_2 -> plaintext XOR KEYSTREAM_2 (test
         vector not updated)

      -  "K_2e", "IV_2e" -> KEYSTREAM_2

      -  Specified optional message 4

      -  EDHOC-Exporter-FS -> EDHOC-Rekey-FS

      -  Less constrained devices SHOULD implement both suite 0 and 2

      -  Clarification of error message

      -  Added exporter interface test vector

   *  From -02 to -03:

      -  Rearrangements of section 3 and beginning of section 4



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      -  Key derivation new section 4

      -  Cipher suites 4 and 5 added

      -  EDHOC-EXPORTER-FS - generate a new PRK_4x3m from an old one

      -  Change in CIPHERTEXT_2 -> COSE_Encrypt0 without tag (no change
         to test vector)

      -  Clarification of error message

      -  New appendix C applicability statement

   *  From -01 to -02:

      -  New section 1.2 Use of EDHOC

      -  Clarification of identities

      -  New section 4.3 clarifying bstr_identifier

      -  Updated security considerations

      -  Updated text on cipher suite negotiation and key confirmation

      -  Test vector for static DH

   *  From -00 to -01:

      -  Removed PSK method

      -  Removed references to certificate by value

Acknowledgments

   The authors want to thank Christian Amsuess, Alessandro Bruni,
   Karthikeyan Bhargavan, Timothy Claeys, Martin Disch, Loic Ferreira,
   Theis Groenbech Petersen, Dan Harkins, Klaus Hartke, Russ Housley,
   Stefan Hristozov, Alexandros Krontiris, Ilari Liusvaara, Karl
   Norrman, Salvador Perez, Eric Rescorla, Michael Richardson, Thorvald
   Sahl Joergensen, Jim Schaad, Carsten Schuermann, Ludwig Seitz,
   Stanislav Smyshlyaev, Valery Smyslov, Peter van der Stok, Rene
   Struik, Vaishnavi Sundararajan, Erik Thormarker, Marco Tiloca, Michel
   Veillette, and Malisa Vucinic for reviewing and commenting on
   intermediate versions of the draft.  We are especially indebted to
   Jim Schaad for his continuous reviewing and implementation of
   different versions of the draft.




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   Work on this document has in part been supported by the H2020 project
   SIFIS-Home (grant agreement 952652).

Authors' Addresses

   Göran Selander
   Ericsson AB
   SE-164 80 Stockholm
   Sweden

   Email: goran.selander@ericsson.com


   John Preuß Mattsson
   Ericsson AB
   SE-164 80 Stockholm
   Sweden

   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB
   SE-164 80 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com
























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