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Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-ietf-lake-edhoc-14

Document Type Active Internet-Draft (lake WG)
Authors Göran Selander , John Preuß Mattsson , Francesca Palombini
Last updated 2022-05-18
Replaces draft-selander-lake-edhoc
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draft-ietf-lake-edhoc-14
Network Working Group                                        G. Selander
Internet-Draft                                         J. Preuß Mattsson
Intended status: Standards Track                            F. Palombini
Expires: 19 November 2022                                       Ericsson
                                                             18 May 2022

               Ephemeral Diffie-Hellman Over COSE (EDHOC)
                        draft-ietf-lake-edhoc-14

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.

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

   This Internet-Draft will expire on 19 November 2022.

Copyright Notice

   Copyright (c) 2022 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

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Message Size Examples . . . . . . . . . . . . . . . . . .   5
     1.3.  Document Structure  . . . . . . . . . . . . . . . . . . .   6
     1.4.  Terminology and Requirements Language . . . . . . . . . .   6
   2.  EDHOC Outline . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Method  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Connection Identifiers  . . . . . . . . . . . . . . . . .  10
     3.4.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  Authentication Parameters . . . . . . . . . . . . . . . .  13
     3.6.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  17
     3.7.  Ephemeral Public Keys . . . . . . . . . . . . . . . . . .  19
     3.8.  External Authorization Data (EAD) . . . . . . . . . . . .  19
     3.9.  Application Profile . . . . . . . . . . . . . . . . . . .  20
   4.  Key Derivation  . . . . . . . . . . . . . . . . . . . . . . .  22
     4.1.  Keys for EDHOC Message Processing . . . . . . . . . . . .  22
     4.2.  Keys for EDHOC Applications . . . . . . . . . . . . . . .  25
   5.  Message Formatting and Processing . . . . . . . . . . . . . .  27
     5.1.  Message Processing Outline  . . . . . . . . . . . . . . .  27
     5.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  28
     5.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  30
     5.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  32
     5.5.  EDHOC Message 4 . . . . . . . . . . . . . . . . . . . . .  36
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  37
     6.1.  Success . . . . . . . . . . . . . . . . . . . . . . . . .  39
     6.2.  Unspecified Error . . . . . . . . . . . . . . . . . . . .  39
     6.3.  Wrong Selected Cipher Suite . . . . . . . . . . . . . . .  39
   7.  Compliance Requirements . . . . . . . . . . . . . . . . . . .  42
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  43
     8.1.  Security Properties . . . . . . . . . . . . . . . . . . .  43
     8.2.  Cryptographic Considerations  . . . . . . . . . . . . . .  45
     8.3.  Cipher Suites and Cryptographic Algorithms  . . . . . . .  47
     8.4.  Post-Quantum Considerations . . . . . . . . . . . . . . .  47
     8.5.  Unprotected Data and Privacy  . . . . . . . . . . . . . .  48
     8.6.  Updated Internet Threat Model Considerations  . . . . . .  48
     8.7.  Denial-of-Service . . . . . . . . . . . . . . . . . . . .  49
     8.8.  Implementation Considerations . . . . . . . . . . . . . .  49
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  51
     9.1.  EDHOC Exporter Label Registry . . . . . . . . . . . . . .  52
     9.2.  EDHOC Cipher Suites Registry  . . . . . . . . . . . . . .  52

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     9.3.  EDHOC Method Type Registry  . . . . . . . . . . . . . . .  53
     9.4.  EDHOC Error Codes Registry  . . . . . . . . . . . . . . .  54
     9.5.  EDHOC External Authorization Data Registry  . . . . . . .  54
     9.6.  COSE Header Parameters Registry . . . . . . . . . . . . .  54
     9.7.  The Well-Known URI Registry . . . . . . . . . . . . . . .  54
     9.8.  Media Types Registry  . . . . . . . . . . . . . . . . . .  55
     9.9.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  57
     9.10. Resource Type (rt=) Link Target Attribute Values
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  57
     9.11. Expert Review Instructions  . . . . . . . . . . . . . . .  57
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  58
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  58
     10.2.  Informative References . . . . . . . . . . . . . . . . .  61
   Appendix A.  Use with OSCORE and Transfer over CoAP . . . . . . .  65
     A.1.  Deriving the OSCORE Security Context  . . . . . . . . . .  65
     A.2.  Transferring EDHOC over CoAP  . . . . . . . . . . . . . .  66
   Appendix B.  Compact Representation . . . . . . . . . . . . . . .  70
   Appendix C.  Use of CBOR, CDDL and COSE in EDHOC  . . . . . . . .  70
     C.1.  CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . .  71
     C.2.  CDDL Definitions  . . . . . . . . . . . . . . . . . . . .  72
     C.3.  COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  73
   Appendix D.  Authentication Related Verifications . . . . . . . .  75
     D.1.  Validating the Authentication Credential  . . . . . . . .  76
     D.2.  Identities  . . . . . . . . . . . . . . . . . . . . . . .  76
     D.3.  Certification Path and Trust Anchors  . . . . . . . . . .  77
     D.4.  Revocation Status . . . . . . . . . . . . . . . . . . . .  78
     D.5.  Trust-on-first-use  . . . . . . . . . . . . . . . . . . .  78
   Appendix E.  Use of External Authorization Data . . . . . . . . .  78
   Appendix F.  Application Profile Example  . . . . . . . . . . . .  80
   Appendix G.  EDHOC Message Deduplication  . . . . . . . . . . . .  80
   Appendix H.  Transports Not Natively Providing Correlation  . . .  82
   Appendix I.  Change Log . . . . . . . . . . . . . . . . . . . . .  82
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  90
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  90

1.  Introduction

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

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   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 keying 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 emphasizes the possibility to reference
   rather than to transport credentials in order to reduce message
   overhead, but the latter is also supported.  EDHOC does not currently
   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,
   CBOR Web Token (CWT), CWT Claims Set (CCS), X.509, and CBOR encoded

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   X.509 (C509) certificates, see Section 3.5.2).  COSE provides crypto
   agility and enables the use of future algorithms and credential types
   targeting IoT.

   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] involving 'things' with
   embedded microcontrollers, sensors, and actuators.  By reusing the
   same lightweight primitives as OSCORE (CBOR, COSE, CoAP) 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.

   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).  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.

1.2.  Message Size Examples

   Compared to the DTLS 1.3 handshake [RFC9147] with ECDHE and
   connection ID, the EDHOC message size when transferred in 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 EDHOC message sizes based on the assumptions in Section 2
   of [I-D.ietf-lwig-security-protocol-comparison], comparing different
   kinds of authentication keys and 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].

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         ========================================================
                             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: Examples of EDHOC message sizes in bytes.

1.3.  Document Structure

   The remainder of the document is organized as follows: Section 2
   outlines EDHOC authenticated with signature keys, 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.

1.4.  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.

   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 the Concise Data Definition Language (CDDL,
   [RFC8610]), which is used to express CBOR data structures.  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].

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2.  EDHOC Outline

   EDHOC specifies different authentication methods of the ephemeral
   Diffie-Hellman key exchange: signature keys and static Diffie-Hellman
   keys.  This section outlines the signature key 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][RFC9147], EDHOC authenticated with signature keys is
   built on a variant of the SIGMA protocol which provides identity
   protection of the initiator (SIGMA-I) against active attackers, and
   like IKEv2, 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 used by
                                   EDHOC.

   The parties exchanging messages are called Initiator (I) and
   Responder (R).  They exchange ephemeral public keys, compute a shared
   secret key PRK_out, 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.

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

   *  ID_CRED_I and ID_CRED_R are used to identify and optionally
      transport the credentials of the Initiator and the Responder,
      respectively.

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

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   *  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 key confirmation to I in
      deployments where no protected application data is sent from R to
      I.

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

   *  Verification of the selected cipher suite.

   *  Method types and error handling.

   *  Selection of connection identifiers C_I and C_R which may be used
      in EDHOC to identify 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.

   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.ietf-lake-traces].

3.  Protocol Elements

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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], and are deterministically encoded.
   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 are 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+cbor-seq defined in
   Section 9.8.

   The Initiator can derive symmetric application keys after creating
   EDHOC message_3, see Section 4.2.1.  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 including 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.

         +-------------+--------------------+--------------------+
         | Method Type | Initiator          | Responder          |
         |       Value | Authentication Key | Authentication Key |
         +-------------+--------------------+--------------------+
         |           0 | Signature Key      | Signature Key      |
         |           1 | Signature Key      | Static DH Key      |
         |           2 | Static DH Key      | Signature Key      |
         |           3 | Static DH Key      | Static DH Key      |
         +-------------+--------------------+--------------------+

               Figure 4: Authentication Keys for Method Types

   EDHOC does not have a dedicated message field to indicate protocol
   version.  Breaking changes to EDHOC can be introduced by specifying
   and registering new methods.

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 execution
   (see Section 3.4) or in subsequent applications of EDHOC, e.g., in
   OSCORE (see Section 3.3.3).  The connection identifiers do not have
   any cryptographic purpose in EDHOC except facilitating the retrieval
   of security data associated to the protocol state.

   Connection identifiers in EDHOC are CBOR byte strings.  Since most
   constrained devices only have a few connections, short identifiers
   are desirable in many cases.  However, except for the empty byte
   string h'', which encodes as one byte (0x40), all byte strings are
   CBOR encoded as two or more bytes.  Therefore EDHOC specifies certain
   byte strings to be represented as CBOR ints on the wire, see
   Section 3.3.2.

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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 it in message_2 for the Initiator
   to use as a reference to the connection in communications with the
   Responder.

   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 3.3.3
   for an example.

3.3.2.  Representation of Byte String Identifiers

   To allow identifiers with minimal overhead on the wire, certain byte
   strings are defined to have integer representations.

   The integers with one-byte CBOR encoding are -24, ..., 23, see
   Figure 5.  This correspondence between integers and byte strings is a
   natural mapping between the byte strings with CBOR diagnostic
   notation h'00', h'01', ..., h'37' (except h'18', h'19', ..., h'1F')
   and integers which are CBOR encoded as one byte.

  Integer:               -24   -23  ...   -2    -1     0     1  ...   23
  CBOR encoding (1 byte): 37    36  ...   21    20    00    01  ...   17

                 Figure 5: One-Byte CBOR Encoded Integers

   The byte strings which coincide with a one-byte CBOR encoding of an
   integer MUST be represented by the CBOR encoding of that integer.
   Other byte strings are encoded as normal CBOR byte strings.

   For example:

   *  h'21' is represented by 0x21 (CBOR encoding of the integer -2),
      not by 0x4121.

   *  h'0D' is represented by 0x0D (CBOR encoding of the integer 13),
      not by 0x410D.

   *  h'18' is represented by 0x4118.

   *  h'38' is represented by 0x4138.

   *  h'ABCD' is represented by 0x42ABCD.

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   One way to view this representation of byte strings is as a transport
   encoding: A byte string which parses as a CBOR int in the range -24,
   ..., 23 is just copied directly into the message, a byte string which
   doesn't is encoded as a CBOR bstr during transport.

3.3.3.  Use of Connection Identifiers with OSCORE

   For OSCORE, the choice of 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 connection identifier is a byte string, it
   is converted to an OSCORE Recipient ID equal to the byte string.

   For example, a C_I equal to 0xFF is converted to a (typically client)
   Responder ID equal to 0xFF; a C_R equal to 0x21 is converted to a
   (typically server) Responder ID equal to 0x21.  Note that the
   representation of connection identifiers as CBOR byte strings or CBOR
   ints in EDHOC messages as described in Section 3.3.2 has no impact on
   this mapping.

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.  In addition to transport of
   messages including errors, 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 and security context
   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/security context during
   EDHOC protocol execution.  Transports that do not inherently provide
   correlation across all EDHOC 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.2, 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 may be transported, identified, and
   trusted.

   EDHOC performs the following authentication related operations:

   *  EDHOC transports information about credentials in ID_CRED_I and
      ID_CRED_R (described in Section 3.5.3).  Based on this
      information, the authentication credentials CRED_I and CRED_R
      (described in Section 3.5.2) can be obtained.  EDHOC may also
      transport certain authentication related information as External
      Authorization Data (see Section 3.8).

   *  EDHOC uses the authentication credentials in two ways (see
      Section 5.3.2 and Section 5.4.2):

      -  The authentication credential is input to the integrity
         verification using the MAC fields.

      -  The authentication key of the authentication credential is used
         with the Signature_or_MAC field to verify proof-of-possession
         of the private key.

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   Other authentication related verifications are out of scope for
   EDHOC, and is the responsibility of the application.  In particular,
   the authentication credential needs to be validated in the context of
   the connection for which EDHOC is used, see Appendix D.  EDHOC MUST
   allow the application to read received information about credential
   (ID_CRED_R, ID_CRED_I).  EDHOC MUST have access to the authentication
   key and the authentication credential.

   Note that the type of authentication key, authentication credential,
   and the identification of 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 in turn much smaller than a CCS.

3.5.1.  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 (see Section 3.6).  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 denoted I and R,
   respectively, and the public authentication keys are denoted G_I and
   G_R, respectively.

   For X.509 certificates the authentication key is represented with a
   SubjectPublicKeyInfo field.  For CWT and CCS (see Section 3.5.2)) the
   authentication key is represented with a 'cnf' claim [RFC8747]
   containing a COSE_Key [I-D.ietf-cose-rfc8152bis-struct].

3.5.2.  Authentication Credentials

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

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   EDHOC relies on COSE for identification of credentials (see
   Section 3.5.3), for example X.509 certificates [RFC5280], C509
   certificates [I-D.ietf-cose-cbor-encoded-cert], CWTs [RFC8392] and
   CWT Claims Sets (CCS) [RFC8392].  When the identified credential is a
   chain or a bag, the authentication credential CRED_x is just the end
   entity X.509 or C509 certificate / CWT.

   Since CRED_R is used in the integrity verification, see
   Section 5.3.2, it needs to be specified such that it is identical
   when used by Initiator or Responder.  Similarly for CRED_I, see
   Section 5.4.2.  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 to
   identify the authentication credential, 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 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 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 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:

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   {                                              /CCS/
     2 : "42-50-31-FF-EF-37-32-39",               /sub/
     8 : {                                        /cnf/
       1 : {                                      /COSE_Key/
         1 : 1,                                   /kty/
         2 : h'00',                               /kid/
        -1 : 4,                                   /crv/
        -2 : h'b1a3e89460e88d3a8d54211dc95f0b90   /x/
               3ff205eb71912d6db8f4af980d2db83a'
       }
     }
   }

         Figure 6: CWT Claims Set (CCS) containing an X25519 static
                 Diffie-Hellman key and an EUI-64 identity.

3.5.3.  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 credentials:

   *  ID_CRED_R is intended to facilitate for the Initiator to retrieve
      the authentication credential CRED_R and the authentication key of
      R.

   *  ID_CRED_I is intended to facilitate for the Responder to retrieve
      the authentication credential CRED_I and the authentication key of
      I.

   ID_CRED_x may contain the authentication credential CRED_x, but for
   many settings it is not necessary to transport the authentication
   credential within EDHOC, for example, it may be pre-provisioned or
   acquired out-of-band over less constrained links.  ID_CRED_I and
   ID_CRED_R do not have any cryptographic purpose in EDHOC since the
   authentication credentials are integrity protected.

   EDHOC relies on COSE for identification of credentials and supports
   all credential types for which COSE header parameters are defined
   including X.509 certificates ([I-D.ietf-cose-x509]), C509
   certificates ([I-D.ietf-cose-cbor-encoded-cert]), CWT (Section 9.6)
   and CWT Claims Set (CCS) (Section 9.6).

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

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   Note that COSE header parameters in ID_CRED_x are used to identify
   the sender's 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.

   The value of a COSE 'kid' parameter is a byte string.  To allow one-
   byte encodings of ID_CRED_x with key identifiers 'kid', which is
   useful in scenarios with only a few keys, the integer representation
   of identifiers in Section 3.3.2 MUST be applied.  For details, see
   Section 5.3.2 and Section 5.4.2.

   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.

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.  All algorithm names and definitions follows
   from COSE [I-D.ietf-cose-rfc8152bis-algs].  Note that COSE sometimes
   uses peculiar names such as ES256 for ECDSA with SHA-256, A128 for
   AES-128, and Ed25519 for the curve edwards25519.  Algorithms need to
   be specified with enough parameters to make them completely
   determined.  The MAC length MUST be at least 8 bytes.  Any
   cryptographic algorithm used in the COSE header parameters in ID_CRED
   is selected independently of the cipher suite.  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

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   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.  Just like
   in (D)TLS 1.3 [RFC8446][RFC9147] and IKEv2 [RFC7296], a signature in
   COSE is determined by the signature algorithm and the authentication
   key algorithm together, see Section 3.5.1.  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 use 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 integer label.

   EDHOC can be used with all algorithms and curves defined for COSE.
   Implementations 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).

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   *  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 consists of high performance algorithms that are
      widely used in non-constrained applications.

   *  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
      consists of algorithms from 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

   The ephemeral public keys in EDHOC (G_X and G_Y) use 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
   implementation requires a '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 the number of messages or to
   simplify processing, external security applications may be integrated
   into EDHOC by transporting authorization related data in the
   messages.

   EDHOC allows opaque external authorization data (EAD) to be sent in
   each of the four EDHOC messages (EAD_1, EAD_2, EAD_3, EAD_4).

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   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 : bstr,
   )

   A security application using external authorization data need to
   register an ead_label, specify the ead_value format for each message
   (see Section 9.5), and describe processing and security
   considerations.

   The EAD fields of EDHOC must not be used for generic application
   data.  Examples of the use of EAD is provided in Appendix E.

3.9.  Application Profile

   EDHOC requires certain parameters to be agreed upon between Initiator
   and Responder.  Some parameters can be negotiated through the
   protocol execution (specifically, cipher suite, see Section 3.6) but
   other parameters are only communicated and may not be negotiated
   (e.g., which authentication method is used, see Section 3.2).  Yet
   other parameters need to be known out-of-band.

   The purpose of an application profile 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.2.

       *  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.2.

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

   3.  Profile for authentication credentials (CRED_I, CRED_R; see
       Section 3.5.2), e.g., profile for certificate or CCS, including
       supported authentication key algorithms (subject public key
       algorithm in X.509 or C509 certificate).

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   4.  Type used to identify credentials (ID_CRED_I, ID_CRED_R; see
       Section 3.5.3).

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

   6.  Identifier used as the identity of the endpoint; see
       Appendix D.2.

   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 application profile may also contain information about supported
   cipher suites.  The procedure for selecting and verifying a 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 application profile is shown in Appendix F.

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

   For other parameters, like the profile of CRED_x in the case that it
   is not transported, it may not be possible to verify that
   incompliance with the application profile was the reason for failure:
   Integrity verification in message_2 or message_3 may fail not only
   because of wrong 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 application profile 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 the identity of
   the Responder before having verified message_2, and the Responder
   does not know the identity of the Initiator before having verified
   message_3.)

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   Other conditions may be part of the application profile, 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 application profiles 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

4.1.  Keys for EDHOC Message Processing

   EDHOC uses Extract-and-Expand [RFC5869] with the EDHOC hash algorithm
   in the selected cipher suite to derive keys used in message
   processing.  This section defines Extract (Section 4.1.1) and Expand
   (Section 4.1.2), and how to use them to derive PRK_out
   (Section 4.1.3) which is the shared secret key resulting from a
   successful EDHOC exchange.

   Extract is used to derive fixed-length uniformly pseudorandom keys
   (PRK) from ECDH shared secrets.  Expand is used to define EDHOC-KDF
   for generating MACs and for deriving output keying material (OKM)
   from PRKs.

   In EDHOC a specific message is protected with a certain pseudorandom
   key, but how the key is derived depends on the method as detailed in
   Section 5.

4.1.1.  Extract

   The pseudorandom keys (PRKs) used for EDHOC message processing 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, "" )

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   The rest of the section defines the pseudo-random keys PRK_2e,
   PRK_3e2m and PRK_4e3m; their use is shown in Figure 7.

4.1.1.1.  PRK_2e

   The pseudo-random key 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 )

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

4.1.1.2.  PRK_3e2m

   The pseudo-random key PRK_3e2m is derived as follows:

   If the Responder authenticates with a static Diffie-Hellman key, then
   PRK_3e2m = Extract( SALT_3e2m, G_RX ), where

   *  SALT_3e2m is derived from PRK_2e, see Section 4.1.2, and

   *  G_RX is the ECDH shared secret calculated from G_R and X, or G_X
      and R (the Responder's private authentication key, see
      Section 3.5.1),

   else PRK_3e2m = PRK_2e.

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4.1.1.3.  PRK_4e3m

   The pseudo-random key PRK_4e3m is derived as follows:

   If the Initiator authenticates with a static Diffie-Hellman key, then
   PRK_4e3m = Extract( SALT_4e3m, G_IY ), where

   *  SALT_4e3m is derived from PRK_3e2m, see Section 4.1.2, and

   *  G_IY is the ECDH shared secret calculated from G_I and Y, or G_Y
      and I (the Initiator's private authentication key, see
      Section 3.5.1),

   else PRK_4e3m = PRK_3e2m.

4.1.2.  Expand and EDHOC-KDF

   The output keying material (OKM) - including keys, IVs, and salts -
   are derived from the PRKs using the EDHOC-KDF, which is defined
   through Expand:

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

   where info is encoded as the CBOR sequence

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

   where

   *  label is a uint

   *  context is a bstr

   *  length is the length of OKM in bytes

   When EDHOC-KDF is used to derive OKM for EDHOC message processing,
   then context includes one of the transcript hashes TH_2, TH_3, or
   TH_4 defined in Sections 5.3.2 and 5.4.2.

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

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   *  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.

   Figure 7 lists derivations made with EDHOC-KDF during message
   processing.  How the output keying material is used is specified in
   Section 5.

   KEYSTREAM_2   = EDHOC-KDF( PRK_2e,   0, TH_2,      plaintext_length )
   SALT_3e2m     = EDHOC-KDF( PRK_2e,   1, TH_2,      hash_length )
   MAC_2         = EDHOC-KDF( PRK_3e2m, 2, context_2, mac_length_2 )
   K_3           = EDHOC-KDF( PRK_3e2m, 3, TH_3,      key_length )
   IV_3          = EDHOC-KDF( PRK_3e2m, 4, TH_3,      iv_length )
   SALT_4e3m     = EDHOC-KDF( PRK_3e2m, 5, TH_3,      hash_length )
   MAC_3         = EDHOC-KDF( PRK_4e3m, 6, context_3, mac_length_3 )
   PRK_out       = EDHOC-KDF( PRK_4e3m, 7, TH_4,      hash_length )
   K_4           = EDHOC-KDF( PRK_4e3m, 8, TH_4,      key_length )
   IV_4          = EDHOC-KDF( PRK_4e3m, 9, TH_4,      iv_length )

                 Figure 7: Key derivations using EDHOC-KDF.

4.1.3.  PRK_out

   The pseudo-random key PRK_out, derived as shown in Figure 7, is the
   only secret key shared between Initiator and Responder that needs to
   be stored after a successful EDHOC exchange, see Section 5.4.  Keys
   for applications are derived from PRK_out, see Section 4.2.1.

4.2.  Keys for EDHOC Applications

   This section defines EDHOC-Exporter and EDHOC-KeyUpdate in terms of
   EDHOC-KDF and PRK_out.

4.2.1.  EDHOC-Exporter

   Keying material for the application can be derived using the EDHOC-
   Exporter interface defined as:

      EDHOC-Exporter(label, context, length)
        = EDHOC-KDF(PRK_exporter, label, context, length)

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   where

   *  label is a registered uint from the EDHOC Exporter Label registry
      (Section 9.1)

   *  context is a bstr defined by the application

   *  length is a uint defined by the application

   *  PRK_exporter is derived from PRK_out:

   PRK_exporter  = EDHOC-KDF( PRK_out, 10, h'', hash_length )

   where hash_length denotes the length of the hash function output in
   bytes, as specified by the COSE hash algorithm definition.

   PRK_exporter MUST be derived anew if PRK_out is updated, in
   particular if EDHOC-KeyUpdate is used, see Section 4.2.2.

   The (label, 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
   can be reused with different nonces.  The context can for example be
   the empty CBOR byte string.

   Examples of use of the EDHOC-Exporter are given in Appendix A.

4.2.2.  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_out is deleted and the new
   PRK_out is calculated as a "hash" of the old key using the Expand
   function as illustrated by the following pseudocode:

      EDHOC-KeyUpdate( context ):
         PRK_out = EDHOC-KDF( PRK_out, 11, context, hash_length )

   where hash_length denotes the length of the hash function output in
   bytes, as specified by the COSE hash algorithm definition.

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   The EDHOC-KeyUpdate takes a context as input to enable binding of the
   updated PRK_out to some event that triggered the keyUpdate.  The
   Initiator and the Responder need to agree on the context, which can,
   e.g., be a counter or a pseudo-random number such as a hash.  The
   Initiator and the Responder also need to cache the old PRK_out until
   it has verfied that the other endpoint has the correct new PRK_out.
   [I-D.ietf-core-oscore-key-update] describes key update for OSCORE
   using EDHOC-KeyUpdate.

   While this key update 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.ietf-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.

   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 application profile
   (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).

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   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 8.7).

   3.  If the message received is an error message, then process it
       according to Section 6, else process it 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 in the same session 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 G.

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 / -24..23,
     ? 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, the first cipher suite in network byte order
      is the most preferred by I, the last is the one selected by I for

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      this session.  If the most preferred cipher suite is selected then
      SUITES_I contains only that cipher suite and is encoded 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.  Note that connection
      identifiers are byte strings but certain values are represented as
      integers in the message, see Section 3.3.2.

   *  EAD_1 - external authorization data, see Section 3.8.

5.2.2.  Initiator Processing of Message 1

   The Initiator SHALL compose message_1 as follows:

   *  Construct SUITES_I complying with the definition in
      Section 5.2.1}, and furthermore:

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

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

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

      -  The Initiator MUST NOT change the supported cipher suites and
         the order of preference in SUITES_I 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.

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   *  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.

   *  If EAD_1 is present then make it available to the application for
      EAD processing.

   If any processing step fails, the Responder MUST send an EDHOC error
   message back, formatted as defined in Section 6, and the session MUST
   be discontinued.

5.3.  EDHOC Message 2

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 / -24..23,
   )

   where:

   *  G_Y_CIPHERTEXT_2 - the concatenation of G_Y (i.e., the ephemeral
      public key of the Responder) and CIPHERTEXT_2.

   *  C_R - variable length connection identifier.  Note that connection
      identifiers are byte strings but certain values are represented as
      integers in the message, see Section 3.3.2.

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.

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

   *  Compute the transcript hash TH_2 = H( G_Y, C_R, H(message_1) )
      where H() is the EDHOC hash algorithm of 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 as in Section 4.1.2 with context_2 = << ID_CRED_R,
      TH_2, CRED_R, ? EAD_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 the retrieval of CRED_R,
         see Section 3.5.3

      -  CRED_R - CBOR item containing the authentication credential of
         the Responder, see Section 3.5.2

      -  EAD_2 - 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, computed as specified 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
      for an overview of COSE and Appendix C.1 for notation):

      -  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 over the following plaintext:

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      -  PLAINTEXT_2 = ( ? PAD, ID_CRED_R / bstr / -24..23,
         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 kid_R
            is conveyed in the plaintext, represented as described in
            Section 3.3.2.

         o  PAD = 1*true is padding that may be used to hide the length
            of the unpadded plaintext

      -  Compute KEYSTREAM_2 as in Section 4.1.2, where plaintext_length
         is the length of PLAINTEXT_2.

      -  CIPHERTEXT_2 = PLAINTEXT_2 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.2).

   *  Decrypt CIPHERTEXT_2, see Section 5.3.2, and discard padding, if
      present.

   *  Make ID_CRED_R and EAD_2 (if present) available to the application
      for authentication- and EAD processing.

   *  Obtain the authentication credential (CRED_R) and the
      authentication key of R from the application (or by other means).

   *  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 Responder MUST send an EDHOC error
   message back, formatted as defined in Section 6, and the session MUST
   be discontinued.

5.4.  EDHOC Message 3

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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, PLAINTEXT_2) where H()
      is the EDHOC hash algorithm of 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, PLAINTEXT_2)
      can be computed and cached already in the processing of message_2.

   *  Compute MAC_3 as in Section 4.1.2, with context_3 = << ID_CRED_I,
      TH_3, CRED_I, ? EAD_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.

      -  ID_CRED_I - identifier to facilitate the retrieval of CRED_I,
         see Section 3.5.3

      -  CRED_I - CBOR item containing the authentication credential of
         the Initiator, see Section 3.5.2

      -  EAD_3 - 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, computed as specified 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 >>

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

      -  payload = MAC_3

   *  Compute a COSE_Encrypt0 object as defined in Sections 5.2 and 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 (if used by the AEAD
      algorithm), the plaintext PLAINTEXT_3, and the following
      parameters as input (see Appendix C.3):

      -  protected = h''

      -  external_aad = TH_3

      -  K_3 and IV_3 are defined in Section 4.1.2, with

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

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

      -  PLAINTEXT_3 = ( ? PAD, ID_CRED_I / bstr / -24..23,
         Signature_or_MAC_3, ? EAD_3 )

         o  If ID_CRED_I contains a single 'kid' parameter, i.e.,
            ID_CRED_I = { 4 : kid_I }, then only the byte string kid_I
            is conveyed in the plaintext, represented as described in
            Section 3.3.2.

         o  PAD = 1*true is padding that may be used to hide the length
            of the unpadded plaintext

      CIPHERTEXT_3 is the 'ciphertext' of COSE_Encrypt0.

   *  Compute the transcript hash TH_4 = H(TH_3, PLAINTEXT_3) where H()
      is the EDHOC hash algorithm of the selected cipher suite.  The
      transcript hash TH_4 is a CBOR encoded bstr and the input to the
      hash function is a CBOR Sequence.

   *  Calculate PRK_out as defined in Figure 7.  The Initiator can now
      derive application keys using the EDHOC-Exporter interface, see
      Section 4.2.1.

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

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   *  Make the connection identifiers (C_I, C_R) and the application
      algorithms in the selected cipher suite available to the
      application.

   The Initiator SHOULD NOT persistently store PRK_out or application
   keys until the Initiator has verified message_4 or a message
   protected with a derived application key, such as an OSCORE message,
   from the Responder.  This is similar to waiting for acknowledgement
   (ACK) in a transport protocol.

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_R, see Appendix A.2).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
      and 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.  Discard padding, if present.

   *  Make ID_CRED_I and EAD_3 (if present) available to the application
      for authentication- and EAD processing.

   *  Obtain the authentication credential (CRED_I) and the
      authentication key of I from the application (or by other means).

   *  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.

   *  Make the connection identifiers (C_I, C_R) and the application
      algorithms in the selected cipher suite available to the
      application.

   After verifying message_3, the Responder can compute PRK_out, see
   Section 4.1.3, derive application keys using the EDHOC-Exporter
   interface, see Section 4.2.1, persistently store the keying material,
   and send protected application data.

   If any processing step fails, the Responder MUST send an EDHOC error
   message back, formatted as defined in Section 6, and the session MUST
   be discontinued.

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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, message_4 MUST be supported and MUST be
   used.  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

   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 Sections 5.2 and 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 (if used by the AEAD algorithm), the
      plaintext PLAINTEXT_4, and the following parameters as input (see
      Appendix C.3):

      -  protected = h''

      -  external_aad = TH_4

      -  K_4 and IV_4 are defined in Section 4.1.2, with

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

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

      -  PLAINTEXT_4 = ( ? PAD, ? EAD_4 )

         o  PAD = 1*true is padding that may be used to hide the length
            of the unpadded plaintext.

         o  EAD_4 - 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).

   *  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.2).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
      and 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.  Discard padding, if present.

   *  Make EAD_4 (if present) available to the application for EAD
      processing.

   If any processing step fails, the Responder MUST send an EDHOC error
   message back, formatted as defined in Section 6, and the session MUST
   be discontinued.

   After verifying message_4, the Initiator is assured that the
   Responder has calculated the key PRK_out (key confirmation) and that
   no other party can derive the key.

6.  Error Handling

   This section defines the format for error messages, and the
   processing associated to the currently defined error codes.
   Additional error codes may be registered, see Section 9.4.

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   There are many kinds of errors that can occur during EDHOC
   processing.  As in CoAP, an error can be triggered by errors in the
   received message or internal errors in the recieving endpoint.
   Except for processing and formatting errors, it is up to the
   implementation when to send an error message.  Sending error messages
   is essential for debugging but MAY be skipped if, for example, a
   session cannot be found or due to denial-of-service reasons, see
   Section 8.7.  Errors messages in EDHOC are always 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.

   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.

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

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

                       Figure 8: EDHOC error message.

   where:

   *  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 9.  Additional error codes and corresponding error
   information may be specified.

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

     Figure 9: Error codes and error information included in the EDHOC
                               error message.

6.1.  Success

   Error code 0 MAY be used internally in an application to indicate
   success, i.e., as a standard value in case of no error, e.g., in
   status reporting or log files.  ERR_INFO can contain any type of CBOR
   item, the content is out of scope for this specification.  Error code
   0 MUST NOT be used as part of the EDHOC message exchange flow.  If an
   endpoint receives an error message with error code 0, then it MUST
   discontinue the protocol and MUST NOT send an error message.

6.2.  Unspecified Error

   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, for example "Method
   not supported".  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 when replying 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
   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

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   suite in SUITES_I, then it SHOULD include all its supported cipher
   suites in SUITES_R.

   In contrast to SUITES_I, the order of the cipher suites in SUITES_R
   has no significance.

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.

   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 the
   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.

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 10 and 11 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 10), 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 10: Example of an Initiator supporting suites 5, 6, 7, 8,
          and 9 in decreasing order of preference, and a Responder
       supporting suite 6 but not suite 5.  The Responder rejects the
       first message_1 with an error indicating support for suite 6.
      The Initiator also supports suite 6, and therefore selects suite
       6 in the second message_1.  The initiator prepends in SUITES_I
     the selected suite 6 with the more preferred suites, in this case
        suite 5, to mitigate a potential attack on the cipher suite
                                negotiation.

   In the second example (Figure 11), 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.  (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                             |

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      Figure 11: Example of an Initiator supporting suites 5, 6, 7, 8,
          and 9 in decreasing order of preference, and a Responder
        supporting suites 8 and 9 but not 5, 6 or 7.  The Responder
      rejects the first message_1 with an error indicating support for
        suites 8 and 9 (in any order).  The Initiator also supports
     suites 8 and 9, and prefers suite 8, so therefore selects suite 8
      in the second message_1.  The initiator prepends in SUITES_I the
        selected suite 8 with the more preferred suites in order of
         preference, in this case suites 5, 6 and 7, to mitigate a
             potential attack on the cipher suite negotiation.

7.  Compliance Requirements

   In the absence of an application profile specifying otherwise:

   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.  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 (ERR_CODE) 1 and
   2 MUST be supported.

   Implementations MAY support EAD.

   Implementations MAY support padding of plaintext when sending
   messages.  Implementations MUST support padding of plaintext when
   receiving messages, i.e. MUST be able to parse padded messages.

   Implementations MUST support cipher suite 2 and 3.  Cipher suites 2
   (AES-CCM-16-64-128, SHA-256, 8, P-256, ES256, AES-CCM-16-64-128, SHA-
   256) and 3 (AES-CCM-16-128-128, SHA-256, 16, P-256, ES256, AES-CCM-
   16-64-128, SHA-256) only differ in size of the MAC length, so
   supporting one or both of these is no essential difference.
   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 are
   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.  An
   active attacker can get the credential identifier of the Responder by
   eavesdropping on the destination address used for transporting
   message_1 and send its own message_1 to the same address.  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,

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   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
   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_out leads to
   compromise of all keying material derived with the EDHOC-Exporter
   since the last invocation (if any) of the EDHOC-KeyUpdate function.

   Based on the cryptographic algorithms requirements Section 8.3, 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.  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.  A forgery against a 64-bit MAC in EDHOC breaks the
   security of all future application data, while a forgery against a
   64-bit MAC in the subsequent application protocol (e.g., OSCORE
   [RFC8613]) typically only breaks the security of the data in the
   forged packet.

   After sending message_3, the Initiator is assured that no other party
   than the Responder can compute the key PRK_out.  While the Initiator
   can securely send protected application data, the Initiator SHOULD
   NOT persistently store the keying material PRK_out until the
   Initiator has verified an OSCORE message or message_4 from the
   Responder.  After verifying message_3, the Responder is assured that

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   an honest Initiator has computed the key PRK_out.  The Responder can
   securely derive and store the keying material PRK_out, and send
   protected application data.

   External authorization data sent in message_1 (EAD_1) or message_2
   (EAD_2) should be considered unprotected by EDHOC, see Section 8.5.
   EAD_2 is encrypted but the Responder has not yet authenticated the
   Initiator.  External authorization data sent in message_3 (EAD_3) or
   message_4 (EAD_4) is protected between Initiator and Responder by the
   protocol, but note that EAD fields may be used by the application
   before the message verification is completed, see Section 3.8.
   Designing a secure mechanism that uses EAD is not necessarily
   straightforward.  This document only provides the EAD transport
   mechanism, but the problem of agreeing on the surrounding context and
   the meaning of the information passed to and from the application
   remains.  Any new uses of EAD should be subject to careful review.

   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: If an endpoint authenticates with a signature, the other
   endpoint can prove that the endpoint performed a run of the protocol
   by presenting the data being signed as well as the signature itself.
   With static Diffie-Hellman key authentication, the authenticating
   endpoint can 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 relies 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 Responder's identity by sending their own message_1.  EDHOC
   uses the Expand function (typically HKDF-Expand) as a binary additive
   stream cipher which is proven secure as long as the expand function
   is a PRF.  HKDF-Expand is not often used as a stream cipher 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).

   Requirements 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.

   As noted in Section 12 of [I-D.ietf-cose-rfc8152bis-struct] the use
   of a single key for multiple algorithms is strongly disencouraged
   unless proven secure by a dedicated cryptographic analysis.  In
   particular this recommendation applies to using the same private key
   for static Diffie-Hellman authentication and digital signature
   authentication.  A preliminary conjecture is that a minor change to
   EDHOC may be sufficient to fit the analysis of secure shared
   signature and ECDH key usage in [Degabriele11] and [Thormarker21].

   So-called selfie attacks are mitigated as long as the Initiator does
   not have its own identity in the set of Responder identities it is
   allowed to communicate with.  In trust on first use (TOFU) use cases
   the Initiator should verify that the the Responder's identity is not
   equal to its own.  Any future EHDOC methods using e.g., pre-shared
   keys might need to mitigate this in other ways.

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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 output size of the EDHOC hash algorithm MUST be at least
   256-bits, i.e., 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.  For security
   considerations of SHA-1, see [RFC6194].  As EDHOC integrity protects
   the whole authentication credential, the choice of hash algorithm in
   x5t and c5t does not affect security and it is RECOMMENDED to use the
   same hash algorithm as in the cipher suite but with as much
   truncation as possible, i.e, when the EDHOC hash algorithm is SHA-256
   it is RECOMMENDED to use SHA-256/64 in x5t and c5t.  The EDHOC MAC
   length MUST be at least 8 bytes and the tag length of the EDHOC AEAD
   algorithm MUST be at least 64-bits.  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.

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.

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8.5.  Unprotected Data and Privacy

   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.

   An attacker observing network traffic may use connection identifiers
   sent in clear in EDHOC or the subsequent application protocol to
   correlate packets sent on different paths or at different times.  The
   attacker may use this information for traffic flow analysis or to
   track an endpoint.  Application protocols using connection
   identifiers from EDHOC SHOULD provide mechanisms to update the
   connection identifier and MAY provide mechanisms to issue several
   simultaneously active connection identifiers.  See [RFC9000] for a
   non-constrained example of such mechanisms.

8.6.  Updated Internet Threat Model Considerations

   Since the publication of [RFC3552] there has been an increased
   awareness of the need to protect against endpoints that are
   compromised, malicious, or whose interests simply do not align with
   the interests of users
   [I-D.arkko-arch-internet-threat-model-guidance].  [RFC7624] describes
   an updated threat model for Internet confidentiality, see
   Section 8.1.  [I-D.arkko-arch-internet-threat-model-guidance] further
   expands the threat model.  Implementations and users SHOULD consider
   these threat models.  In particular, even data sent protected to the
   other endpoint such as ID_CRED and EAD can be used for tracking, see
   Section 2.7 of [I-D.arkko-arch-internet-threat-model-guidance].

   The fields ID_CRED_I, ID_CRED_R, EAD_2, EAD_3, and EAD_4 have
   variable length and information regarding the length may leak to an
   attacker.  An passive attacker may e.g., be able to differentiating
   endpoints using identifiers of different length.  To mitigate this
   information leakage an inmplementation may ensure that the fields
   have fixed length or use padding.  An implementation may e.g., only
   use fix length identifiers like 'kid' of length 1.  Alternatively
   padding may be used to hide the true length of e.g., certificates by
   value in 'x5chain' or 'c5c'.

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8.7.  Denial-of-Service

   EDHOC itself does not provide countermeasures against Denial-of-
   Service attacks.  In particular, 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.  For instance, when EDHOC is transferred as an exchange
   of CoAP messages, the CoAP server can use the Echo option defined in
   [RFC9175] which forces the CoAP client 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.

8.8.  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 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 unique input to the pseudorandom number generator
   after reboot, or continuously store state in nonvolatile memory.
   Appendix B.1.1 in [RFC8613] describes 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.

   Implementations might consider deriving secret and non-secret
   randomness from different PNRG/PRF/KDF instances to limit the damage
   if the PNRG/PRF/KDF turns out to be fundamentally broken.  NIST
   generally forbids deriving secret and non-secret randomness from the
   same KDF instance, but this decision has been criticized by Krawczyk
   [HKDFpaper] and doing so is common practice.  In addition to IVs,
   other examples are the challenge in EAP-TTLS, the RAND in 3GPP AKAs,

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   and the Session-Id in EAP-TLS 1.3.  Note that part of KEYSTREAM_2 is
   also non-secret randomness as it is known or predictable to an
   attacker.  As explained by Krawczyk, if any attack is mitigated by
   the NIST requirement it would mean that the KDF is fully broken and
   would have to be replaced anyway.

   For many constrained IoT devices it is problematic to support several
   crypto primitives.  Existing devices can be expected to support
   either ECDSA or EdDSA.  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
   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 2.1.1 of [I-D.ietf-cose-rfc8152bis-algs] this
   can be addressed by combining randomness and determinism.

   Appendix D of [I-D.ietf-lwig-curve-representations] describes how
   Montgomery curves such as X25519 and X448 and (twisted) Edwards
   curves as curves such as Ed25519 and Ed448 can mapped to and from
   short-Weierstrass form for implementation on platforms that
   accelerate elliptic curve group operations in short-Weierstrass form.

   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 and validity of certificates.  The selection of trusted CAs
   should be done very carefully and certificate revocation should be
   supported.  The choice of revocation mechanism is left to the
   application.  For example, in case of X.509 certificates, Certificate
   Revocation Lists [RFC5280] or OCSP [RFC6960] may be used.
   Verification of validity may require the use of a Real-Time Clock
   (RTC).  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 its 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

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   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.  Note that in cases where several
   EDHOC exchanges with different parameter sets (method, COSE headers,
   etc.) are used, an attacker can affect which of the parameter sets
   that will be used by blocking some of the parameter sets.

   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 aims at preventing code within
   that environment to be tampered with, and preventing data used by
   such code to be read or tampered with by code outside that
   environment.

   Note that HKDF-Expand has a relativly small maximum output length of
   255 * hash_length.  This means that when when SHA-256 is used as hash
   algorithm, message_2 cannot be longer than 8160 bytes.

   The sequence of transcript hashes in EHDOC (TH_2, TH_3, TH_4) do not
   make use of a so called running hash, this is a design choice as
   running hashes are often not supported on constrained platforms.

   When parsing a received EDHOC message, implementations MUST terminate
   the protocol if the message does not comply with the CDDL for that
   message.  It is RECOMMENDED to terminate the protocol if the received
   EDHOC message is not deterministic CBOR.

9.  IANA Considerations

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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 and Description.  Label is a uint.  Description is
   a text string.  The initial contents of the registry are:

   Label: 0
   Description: Derived OSCORE Master Secret

   Label: 1
   Description: Derived OSCORE Master Salt

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 and Description, 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

   Value: -23
   Algorithms: N/A
   Desc: Reserved for Private Use

   Value: -22
   Algorithms: N/A
   Desc: Reserved for Private Use

   Value: -21
   Algorithms: N/A
   Desc: Reserved for Private Use

   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

   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

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   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

   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

   Value: 4
   Array: 24, -16, 16, 4, -8, 24, -16
   Desc: ChaCha20/Poly1305, SHA-256, 16, X25519, EdDSA,
         ChaCha20/Poly1305, SHA-256

   Value: 5
   Array: 24, -16, 16, 1, -7, 24, -16
   Desc: ChaCha20/Poly1305, SHA-256, 16, P-256, ES256,
         ChaCha20/Poly1305, SHA-256

   Value: 6
   Array: 1, -16, 16, 4, -7, 1, -16
   Desc: A128GCM, SHA-256, 16, X25519, ES256,
         A128GCM, SHA-256

   Value: 24
   Array: 3, -43, 16, 2, -35, 3, -43
   Desc: A256GCM, SHA-384, 16, P-384, ES384,
         A256GCM, SHA-384

   Value: 25
   Array: 24, -45, 16, 5, -8, 24, -45
   Desc: ChaCha20/Poly1305, SHAKE256, 16, X448, EdDSA,
         ChaCha20/Poly1305, SHAKE256

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 "Specification Required".  The columns of
   the registry are Value, Initiator Authentication Key, and Responder
   Authentication Key, where Value is an integer and the other columns
   are text strings describing the authentication keys.  The initial
   contents of the registry are shown in Figure 4.

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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 9.

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
   "Specification Required".  The columns of the registry are Label,
   Message, Description, 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 a CWT Claims Set (CCS), see Section 1.4.  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.  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

9.8.  Media Types Registry

   IANA has added the media types "application/edhoc+cbor-seq" and
   "application/cid-edhoc+cbor-seq" to the "Media Types" registry.

9.8.1.  application/edhoc+cbor-seq Media Type Registration

   *  Type name: application

   *  Subtype name: edhoc+cbor-seq

   *  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

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   *  Author: See "Authors' Addresses" section.

   *  Change Controller: IESG

9.8.2.  application/cid-edhoc+cbor-seq Media Type Registration

   *  Type name: application

   *  Subtype name: cid-edhoc+cbor-seq

   *  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.

   *  Change Controller: IESG

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9.9.  CoAP Content-Formats Registry

   IANA has added the media types "application/edhoc+cbor-seq" and
   "application/cid-edhoc+cbor-seq" to the "CoAP Content-Formats"
   registry under the group name "Constrained RESTful Environments
   (CoRE) Parameters".

+--------------------------------+----------+------+-------------------+
| Media Type                     | Encoding | ID   | Reference         |
+--------------------------------+----------+------+-------------------+
| application/edhoc+cbor-seq     | -        | TBD5 | [[this document]] |
| application/cid-edhoc+cbor-seq | -        | TBD6 | [[this document]] |
+--------------------------------+----------+------+-------------------+

                  Figure 12: CoAP Content-Format IDs

9.10.  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]]

9.11.  Expert Review Instructions

   The IANA Registries established in this document are 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:

   *  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.

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   *  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-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-
              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>.

   [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
              2002, <https://www.rfc-editor.org/info/rfc3279>.

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   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [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>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [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>.

   [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.

   [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.
              Zuniga, "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>.

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

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

   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/info/rfc9175>.

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>.

   [Degabriele11]
              Degabriele, J.P., Lehmann, A., Paterson, K.G., Smart,
              N.P., and M. Strefler, "On the Joint Security of
              Encryption and Signature in EMV", December 2011,
              <https://eprint.iacr.org/2011/615>.

   [HKDFpaper]
              Krawczyk, H., "Cryptographic Extraction and Key
              Derivation: The HKDF Scheme", May 2010,
              <https://eprint.iacr.org/2010/264.pdf>.

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   [I-D.arkko-arch-internet-threat-model-guidance]
              Arkko, J. and S. Farrell, "Internet Threat Model
              Guidance", Work in Progress, Internet-Draft, draft-arkko-
              arch-internet-threat-model-guidance-00, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-arkko-arch-
              internet-threat-model-guidance-00.txt>.

   [I-D.ietf-core-oscore-edhoc]
              Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
              and G. Selander, "Profiling EDHOC for CoAP and OSCORE",
              Work in Progress, Internet-Draft, draft-ietf-core-oscore-
              edhoc-03, 7 March 2022, <https://www.ietf.org/archive/id/
              draft-ietf-core-oscore-edhoc-03.txt>.

   [I-D.ietf-core-oscore-key-update]
              Höglund, R. and M. Tiloca, "Key Update for OSCORE
              (KUDOS)", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-key-update-01, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              key-update-01.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-03, 10 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
              encoded-cert-03.txt>.

   [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-lake-traces]
              Selander, G. and J. P. Mattsson, "Traces of EDHOC", Work
              in Progress, Internet-Draft, draft-ietf-lake-traces-00, 25
              November 2021, <https://www.ietf.org/archive/id/draft-
              ietf-lake-traces-00.txt>.

   [I-D.ietf-lwig-curve-representations]
              Struik, R., "Alternative Elliptic Curve Representations",
              Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
              representations-23, 21 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-lwig-curve-
              representations-23.txt>.

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   [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-rats-eat]
              Lundblade, L., Mandyam, G., and J. O'Donoghue, "The Entity
              Attestation Token (EAT)", Work in Progress, Internet-
              Draft, draft-ietf-rats-eat-12, 24 February 2022,
              <https://www.ietf.org/archive/id/draft-ietf-rats-eat-
              12.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-04, 15 February 2022,
              <https://www.ietf.org/archive/id/draft-mattsson-cfrg-det-
              sigs-with-noise-04.txt>.

   [I-D.selander-ace-ake-authz]
              Selander, G., Mattsson, J. P., Vučinić, M., Richardson,
              M., and A. Schellenbaum, "Lightweight Authorization for
              Authenticated Key Exchange.", Work in Progress, Internet-
              Draft, draft-selander-ace-ake-authz-05, 18 April 2022,
              <https://www.ietf.org/archive/id/draft-selander-ace-ake-
              authz-05.txt>.

   [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>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <https://www.rfc-editor.org/info/rfc6194>.

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   [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>.

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,
              <https://www.rfc-editor.org/info/rfc8366>.

   [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>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

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

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

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   [SIGMA]    Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE-
              Protocols (Long version)", June 2003,
              <https://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>.

   [Thormarker21]
              Thormarker, E., "On using the same key pair for Ed25519
              and an X25519 based KEM", April 2021,
              <https://eprint.iacr.org/2021/509.pdf>.

Appendix A.  Use with OSCORE and Transfer over CoAP

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

A.1.  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 4.2.1.

   The EDHOC Exporter Labels for deriving the OSCORE Master Secret and
   the OSCORE Master Salt, are the uints 0 and 1, respectively.

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

   By default, oscore_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, oscore_salt_length has value 8.  The
   Initiator and Responder MAY agree out-of-band on a longer
   oscore_key_length than the default and on a different
   oscore_salt_length.

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   Master Secret = EDHOC-Exporter( 0, h'', oscore_key_length )
   Master Salt   = EDHOC-Exporter( 1, h'', oscore_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
      Section 3.3.3.  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].

   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.2.  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.  Client applications can use the
   resource type "core.edhoc" to discover a server's EDHOC resource,
   i.e., where to send a request for executing the EDHOC protocol, see
   Section 9.10.  According to this specification, EDHOC is transferred
   in POST requests and 2.04 (Changed) responses to the Uri-Path:
   "/.well-known/edhoc", see Section 9.7.  An application may define its
   own path that can be discovered, e.g., using a resource directory
   [RFC9176].

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   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 the response, in
   the former case with response code 2.04 (Changed), in the latter with
   response code as specified in Appendix A.2.1.  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 EDHOC message_4 is used, or in
   case of an error message, it is sent from the server to the client in
   the payload of the response, with response codes analogously to
   message_2.  In case of an error message in response to message_4, it
   is sent analogously to errors in response to message_2.

   In order for the server to correlate a message received from a client
   to a message previously sent in the same EDHOC session over CoAP,
   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, message_3 is
      sent from the client prepended with the identifier C_R.  In this
      case message_1 is also sent by the client, and to indicate that
      this is a new EDHOC session it is prepended with a dummy
      identifier, the CBOR simple value "true" (0xf5), since the server
      has not selected C_R yet.  See Figure 13.

   *  In the case when the server is Initiator, message_2 (and
      message_4, if present) is sent from the client prepended with the
      identifier C_I.  See Figure 14.

   The prepended identifiers are encoded in CBOR and thus self-
   delimiting.  The integer representation of identifiers described in
   Section 3.3.2 is used, when applicable.  They are sent in front of
   the actual EDHOC message to keep track of messages in an EDHOC
   session, and only the part of the body following the identifier is
   used for EDHOC processing.  In particular, the connection identifiers
   within the EDHOC messages are not impacted by the prepended
   identifiers.

   The application/edhoc+cbor-seq media type does not apply to these
   messages; their media type is application/cid-edhoc+cbor-seq.

   An example of a successful EDHOC exchange using CoAP is shown in
   Figure 13.  In this case the CoAP Token enables correlation on the
   Initiator side, and the prepended C_R enables correlation on the
   Responder (server) side.

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       Client    Server
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          | Content-Format: application/cid-edhoc+cbor-seq
         |          | Payload: true, EDHOC message_1
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_2
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          | Content-Format: application/cid-edhoc+cbor-seq
         |          | Payload: C_R, EDHOC message_3
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_4
         |          |

         Figure 13: Example of transferring EDHOC in CoAP when the
      Initiator is CoAP client.  The optional message_4 is included in
         this example, without which that message needs no payload.

   The exchange in Figure 13 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 14.  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+cbor-seq
         |          | Payload: EDHOC message_1
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          | Content-Format: application/cid-edhoc+cbor-seq
         |          | Payload: C_I, EDHOC message_2
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_3
         |          |

         Figure 14: Example of 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 [RFC9175].  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.2.1.  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 keying material).  The Content-Format of the error
   response MUST be set to application/edhoc+cbor-seq, see Section 9.9.

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   A method for combining EDHOC and OSCORE protocols in two round-trips
   is specified in [I-D.ietf-core-oscore-edhoc].  That specification
   also contains conversion from OSCORE Sender/Recipient IDs to EDHOC
   connection identifiers, web-linking and target attributes for
   discovering of EDHOC resources.

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].  In EDHOC, compact representation is used
   for the ephemeral public keys (G_X and G_Y), see Section 3.7.  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 compatibility 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 diagnostic
   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 : bstr,
   )

   message_1 = (
     METHOD : int,
     SUITES_I : suites,
     G_X : bstr,
     C_I : bstr / -24..23,
     ? EAD_1 : ead,
   )

   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
     C_R : bstr / -24..23,
   )

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

   message_4 = (
     CIPHERTEXT_4 : bstr,
   )

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

   info = (
     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 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.  Authentication Related Verifications

   EDHOC performs certain authentication related operations, see
   Section 3.5, but in general it is necessary to make additional
   verifications beyond EDHOC message processing.  What verifications
   are needed depend on the deployment, in particular the trust model
   and the security policies, but most commonly it can be expressed in
   terms of verifications of credential content.

   EDHOC assumes the existence of mechanisms (certification authority or
   other trusted third party, pre-provisioning, etc.) for generating and
   distributing authentication credentials and other credentials, as
   well as the existence of trust anchors (CA certificates, trusted
   public keys, etc.).  For example, a public key certificate or CWT may
   rely on a trusted third party whose public key is pre-provisioned,
   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, see Appendix D.5.

   In this section we provide some examples of such verifications.
   These verifications are the responsibility of the application but may
   be implemented as part of an EDHOC library.

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D.1.  Validating the Authentication Credential

   The authentication credential may contain, in addition to the
   authentication key, other parameters that needs to be verified.  For
   example:

   *  In X.509 and C509 certificates, signature keys typically have key
      usage "digitalSignature" and Diffie-Hellman public keys typically
      have key usage "keyAgreement" [RFC3279][RFC8410].

   *  In X.509 and C509 certificates validity is expressed using Not
      After and Not Before.  In CWT and CCS, the "exp" and "nbf" claims
      have similar meanings.

D.2.  Identities

   The application must decide on allowing a connection or not depending
   on the intended endpoint, and in particular whether it is a specific
   identity or a set of identities.  To prevent misbinding attacks, the
   identity of the endpoint is included in a MAC verified through the
   protocol.  More details and examples are provided in this section.

   Policies for what connections to allow are typically set based on the
   identity of the other endpoint, and endpoints 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.  Conversely, a device may only
   be allowed to connect to a network which authenticates with a
   particular public key.

   *  When a Public Key Infrastructure (PKI) is used with certificates,
      the identity is the subject whose unique name, e.g., a domain
      name, a Network Access Identifier (NAI), or an Extended Unique
      Identifier (EUI), is included in the endpoint's certificate.

   *  Similarly, when a PKI is used with CWTs, the identity is the
      subject identified by the relevant claim(s), such as 'sub'
      (subject).

   *  When PKI is not used (e.g., CCS, self-signed certificate/CWT) the
      identity is typically directly associated to the authentication
      key of the other party.  For example, if identities can be
      expressed in the form of unique subject names assigned to public
      keys, then a binding to identity is achieved by including both
      public key and associated subject name in the authentication
      credential: CRED_I or CRED_R may be a self-signed certificate/CWT

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      or CCS containing the authentication key and the subject name, see
      Section 3.5.2.  Each endpoint thus needs to know the specific
      authentication key/unique associated subject name, or set of
      public authentication keys/unique associated subject names, which
      it is allowed to communicate with.

   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
   authentication 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.)

D.3.  Certification Path and Trust Anchors

   When a Public Key Infrastructure (PKI) is used with certificates, the
   trust anchor is a Certification Authority (CA) certificate.  Each
   party needs at least one CA public key certificate, or just the CA
   public key.  The certification path contains proof that the subject
   of the certificate owns the public key in the certificate.  Only
   validated public-key certificates are to be accepted.

   Similarly, when a PKI is used with CWTs, each party needs to have at
   least one trusted third party public key as trust anchor to verify
   the end entity CWTs.  The trusted third party public key can, e.g.,
   be stored in a self-signed CWT or in a CCS.

   The signature of the authentication credential needs to be verified
   with the public key of the issuer.  X.509 and C509 certificates
   includes the "Issuer" field.  In CWT and CCS, the "iss" claim has a
   similar meaning.  The public key is either a trust anchor or the
   public key in another valid and trusted credential in a certification
   path from trust anchor to authentication credential.

   Similar verifications as made with the authentication credential (see
   Appendix D.1) are also needed for the other credentials in the
   certification path.

   When PKI is not used (CCS, self-signed certificate/CWT), the trust
   anchor is the authentication key of the other party, in which case
   there is no certification path.

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D.4.  Revocation Status

   The application may need to verify that the credentials are not
   revoked, see Section 8.8.  Some use cases may be served by short-
   lived credentials, for example, where the validity of the credential
   is on par with with the interval between revocation checks.  But, in
   general, credential life time and revokation checking are
   complementary measures to control credential status.  Revocation
   information may be transported as External Authentication Data (EAD),
   see Appendix E.

D.5.  Trust-on-first-use

   TBD

Appendix E.  Use of External Authorization Data

   In order to reduce the number of messages and round trips, or to
   simplify processing, external security applications may be integrated
   into EDHOC by transporting external authorization related data (EAD)
   in the messages.

   The EAD format is specified in Section 3.8, this section contains
   examples and further details of how EAD may be used with an
   appropriate accompanying specification.

   *  One example is third-party assisted authorization, requested with
      EAD_1, and an authorization artifact ("voucher", cf. [RFC8366])
      returned in EAD_2, see [I-D.selander-ace-ake-authz].

   *  Another example is remote attestation, requested in EAD_2, and an
      Entity Attestation Token (EAT, [I-D.ietf-rats-eat]) returned in
      EAD_3.

   *  A third example is certificate enrolment, where a Certificate
      Signing Request (CSR, [RFC2986]) is included EAD_3, and the issued
      public key certificate (X.509 [RFC5280], C509
      [I-D.ietf-cose-cbor-encoded-cert]) or a reference thereof is
      returned in EAD_4.

   External authorization data should be considered unprotected by
   EDHOC, and the protection of EAD is the responsibility of the
   security application (third party authorization, remote attestation,
   certificate enrolment, etc.).  The security properties of the EAD
   fields (after EDHOC processing) are discussed in Section 8.1.

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   The content of the EAD field may be used in the EDHOC processing of
   the message in which they are contained.  For example, authentication
   related information like assertions and revocation information,
   transported in EAD fields may provide input about trust anchors or
   validity of credentials relevant to the authentication processing.
   The EAD fields (like ID_CRED fields) are therefore made available to
   the application before the message is verified, see details of
   message processing in Section 5.  In the first example above, a
   voucher in EAD_2 made available to the application can enable the
   Initiator to verify the identity or public key of the Responder
   before verifying the signature.  An application allowing EAD fields
   containing authentication information thus may need to handle
   authentication related verifications associated with EAD processing.

   Conversely, the security application may need to wait for EDHOC
   message verification to complete.  In the third example above, the
   validation of a CSR carried in EAD_3 is not started by the Responder
   before EDHOC has successfully verified message_3 and proven the
   possession of the private key of the Initiator.

   The security application may reuse EDHOC protocol fields which
   therefore need to be available to the application.  For example, the
   security application may use the same crypto algorithms as in the
   EDHOC session and therefore needs access to the selected cipher suite
   (or the whole SUITES_I).  The application may use the ephemeral
   public keys G_X and G_Y, as ephemeral keys or as nonces, see
   [I-D.selander-ace-ake-authz].

   The processing of (ead_label, ead_value) by the security application
   needs to be described in the specification where the ead_label is
   registered, see Section 9.5, including the ead_value for each message
   and actions in case of errors.  An application may support multiple
   security applications that make use of EAD, which may result in
   multiple (ead_label, ead_value) pairs in one EAD field, see
   Section 3.8.  Any dependencies on security applications with
   previously registered EAD fields needs to be documented, and the
   processing needs to consider their simultaneous use.

   Since data carried in EAD may not be protected, or be processed by
   the application before the EDHOC message is verified, special
   considerations need to be made such that it does not violate security
   and privacy requirements of the service which uses this data, see
   Section 8.5.  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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Appendix F.  Application Profile Example

   This appendix contains a rudimentary example of an application
   profile, see Section 3.9.

   For use of EDHOC with application X the following assumptions are
   made:

   1.  Transfer in CoAP as specified in Appendix A.2 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 3.5.2.

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

       *  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 is used as the identity of the endpoint (see example in
       Section 3.5.2).

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

Appendix G.  EDHOC Message Deduplication

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

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   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 do 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 another instance of the next EDHOC message.  The result MAY
   be that a duplicate next EDHOC message is sent, provided it is still
   relevant with respect to 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.

   In principle, if the EDHOC implementation would deterministically
   regenerate the identical EDHOC message previously sent, it would be
   possible to instead store the protocol state to be able to recreate
   and resend the previously sent EDHOC message.  However, even if the
   protocol state is fixed, the message generation may introduce
   differences which compromises security.  For example, in the
   generation of message_3, if I is performing a (non-deterministic)
   ECDSA signature (say, method 0 or 1, cipher suite 2 or 3) then
   PLAINTEXT_3 is randomized, but K_3 and IV_3 are the same, leading to
   a key and nonce reuse.

   The EDHOC implementation MUST NOT store previous protocol state and
   regenerate an EDHOC message if there is a risk that the same key and
   IV are used for two (or more) distinct messages.

   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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Appendix H.  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.2) 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 I.  Change Log

   RFC Editor: Please remove this appendix.

   *  From -13 to -14

      -  Merge of section 1.1 and 1.2

      -  Connection and key identifiers restricted to be byte strings

      -  Representation of byte strings as one-byte CBOR ints (-24..23)

      -  Simplified mapping between EDHOC and OSCORE identifiers

      -  Rewrite of 3.5

         o  Clarification of authentication related operations performed
            by EDHOC

         o  Authentication related verifications, including old section
            3.5.1, moved to new appendix D

      -  Rewrite of 3.8

         o  Move content about use of EAD to new appendix E

         o  ead_value changed to bstr

      -  EDHOC-KDF updated

         o  transcript_hash argument removed

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         o  TH included in context argument

         o  label argument is now type uint, all labels replaced

      -  Key schedule updated

         o  New salts derived to avoid reuse of same key with expand and
            extract

         o  PRK_4x3m renamed PRK_4e3m

         o  K_4 and IV_4 derived from PRK_4e3m

         o  New PRK: PRK_out derived from PRK_4e3m and TH_4

         o  Clarified main output of EDHOC is the shared secret PRK_out

         o  Exporter defined by EDHOC-KDF and new PRK PRK_exporter
            derived from PRK_out

         o  Key update defined by Expand instead of Extract

      -  All applications of EDHOC-KDF in one place

      -  Update of processing

         o  EAD and ID_CRED passed to application when available

         o  identity verification and credential retrieval omitted in
            protocol description

         o  Transcript hash defined by plaintext messages instead of
            ciphertext

         o  Changed order of input to TH_2

         o  Removed general G_X checking against selfie-attacks

      -  Support for padding of plaintext

      -  Updated compliance requirements

      -  Updated security considerations

         o  Updated and more clear requirements on MAC length

         o  Clarification of key confirmation

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         o  Forbid use of same key for signature and static DH

      -  Updated appendix on message deduplication

      -  Clarifications of

         o  connection identifiers

         o  cipher suites, including negotiation

         o  EAD

         o  Error messages

      -  Updated media types

      -  Applicability template renamed application profile

      -  Editorials

   *  From -12 to -13

      -  no changes

   *  From -12:

      -  Shortened labels to derive OSCORE key and salt

      -  ead_value changed to bstr

      -  Removed general G_X checking against selfie-attacks

      -  Updated and more clear requirements on MAC length

      -  Clarifications from Kathleen, Stephen, Marco, Sean, Stefan,

      -  Authentication Related Verifications moved to appendix

      -  Updated MTI section and cipher suite

      -  Updated security considerations

   *  From -11 to -12:

      -  Clarified applicability to KEMs

      -  Clarified use of COSE header parameters

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      -  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:

      -  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

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   *  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

      -  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

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      -  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

      -  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

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   *  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"

      -  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

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      -  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

      -  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

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Acknowledgments

   The authors want to thank Christian Amsuess, Alessandro Bruni,
   Karthikeyan Bhargavan, Carsten Bormann, Timothy Claeys, Martin Disch,
   Stephen Farrell, Loic Ferreira, Theis Groenbech Petersen, Felix
   Guenther, Dan Harkins, Klaus Hartke, Russ Housley, Stefan Hristozov,
   Marc Ilunga, Charlie Jacomme, Elise Klein, Steve Kremer, Alexandros
   Krontiris, Ilari Liusvaara, Kathleen Moriarty, David Navarro, Karl
   Norrman, Salvador Perez, Maiwenn Racouchot, 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, Sean Turner, Michel Veillette, and Malisa Vu&#269;ini&#263;
   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.

   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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