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Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-ietf-lake-edhoc-23

Document Type Active Internet-Draft (lake WG)
Authors Göran Selander , John Preuß Mattsson , Francesca Palombini
Last updated 2024-01-29 (Latest revision 2024-01-22)
Replaces draft-selander-lake-edhoc
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IANA expert review comments I had a short interaction with the authors, clarifying one thing I didn’t understand. Fundamentally, the two registration requests in Section 10.6 are appropriate, each with an integer label to be assigned out of the Standards Action space (e.g., 13 and 14). I have asked for a slight update of the registration request based on the following observations: * The Value Type for the kccs parameter is listed as `map/#6(map)`. This appears to be an incomplete specification, waiting for a tag to be assigned for a CCS in draft-ietf-rats-uccs. Since EDHOC seems to be completing faster than that draft, and the alternative that includes the tag for CCS also is not strongly required, I propose to change the Value Type to just `map`. * The supplementary information about both kccs and kcwt in the paragraph preceding the registry template is unlikely to be picked up by people consulting just the registry. I have requested making the “description” column more complete, in particular also pointing to RFC 8392 as the source of the “CWT” and “CCS” definitions. * As an editorial observation, the description "A CBOR Web Token (CWT)/... containing a COSE_Key in a 'cnf’ claim” could be read as saying that only that one claim is to be contained in the token. Section 3.5.3.1 clarifies this by also saying "There may be any number of additional claims present in the CWT/CCS.”. Adding an “and possibly others” or similar to the description column might make this information more accessible to readers coming from the registry. I have discussed the first two items with the authors, who agree with the suggestions, and expect them to be open to the third one as well. Grüße, Carsten
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Details
draft-ietf-lake-edhoc-23
LAKE Working Group                                           G. Selander
Internet-Draft                                         J. Preuß Mattsson
Intended status: Standards Track                            F. Palombini
Expires: 25 July 2024                                           Ericsson
                                                         22 January 2024

               Ephemeral Diffie-Hellman Over COSE (EDHOC)
                        draft-ietf-lake-edhoc-23

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 25 July 2024.

Copyright Notice

   Copyright (c) 2024 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  . . . . . . . . . . . . . . . . . . . . . . .   4
     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 . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Method  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Connection Identifiers  . . . . . . . . . . . . . . . . .  10
     3.4.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  Authentication Parameters . . . . . . . . . . . . . . . .  14
     3.6.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  20
     3.7.  Ephemeral Public Keys . . . . . . . . . . . . . . . . . .  21
     3.8.  External Authorization Data (EAD) . . . . . . . . . . . .  22
     3.9.  Application Profile . . . . . . . . . . . . . . . . . . .  24
   4.  Key Derivation  . . . . . . . . . . . . . . . . . . . . . . .  25
     4.1.  Keys for EDHOC Message Processing . . . . . . . . . . . .  26
     4.2.  Keys for EDHOC Applications . . . . . . . . . . . . . . .  29
   5.  Message Formatting and Processing . . . . . . . . . . . . . .  30
     5.1.  EDHOC Message Processing Outline  . . . . . . . . . . . .  31
     5.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  32
     5.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  33
     5.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  36
     5.5.  EDHOC Message 4 . . . . . . . . . . . . . . . . . . . . .  39
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  41
     6.1.  Success . . . . . . . . . . . . . . . . . . . . . . . . .  42
     6.2.  Unspecified Error . . . . . . . . . . . . . . . . . . . .  42
     6.3.  Wrong Selected Cipher Suite . . . . . . . . . . . . . . .  43
     6.4.  Unknown Credential Referenced . . . . . . . . . . . . . .  45
   7.  EDHOC Message Deduplication . . . . . . . . . . . . . . . . .  46
   8.  Compliance Requirements . . . . . . . . . . . . . . . . . . .  47
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  47
     9.1.  Security Properties . . . . . . . . . . . . . . . . . . .  48
     9.2.  Cryptographic Considerations  . . . . . . . . . . . . . .  51
     9.3.  Cipher Suites and Cryptographic Algorithms  . . . . . . .  53
     9.4.  Post-Quantum Considerations . . . . . . . . . . . . . . .  53
     9.5.  Unprotected Data and Privacy  . . . . . . . . . . . . . .  54
     9.6.  Updated Internet Threat Model Considerations  . . . . . .  55
     9.7.  Denial-of-Service . . . . . . . . . . . . . . . . . . . .  55
     9.8.  Implementation Considerations . . . . . . . . . . . . . .  56
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  59

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     10.1.  EDHOC Exporter Label Registry  . . . . . . . . . . . . .  59
     10.2.  EDHOC Cipher Suites Registry . . . . . . . . . . . . . .  60
     10.3.  EDHOC Method Type Registry . . . . . . . . . . . . . . .  62
     10.4.  EDHOC Error Codes Registry . . . . . . . . . . . . . . .  63
     10.5.  EDHOC External Authorization Data Registry . . . . . . .  63
     10.6.  COSE Header Parameters Registry  . . . . . . . . . . . .  64
     10.7.  The Well-Known URI Registry  . . . . . . . . . . . . . .  65
     10.8.  Media Types Registry . . . . . . . . . . . . . . . . . .  65
     10.9.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  67
     10.10. Resource Type (rt=) Link Target Attribute Values
             Registry  . . . . . . . . . . . . . . . . . . . . . . .  67
     10.11. Expert Review Instructions . . . . . . . . . . . . . . .  67
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  68
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  68
     11.2.  Informative References . . . . . . . . . . . . . . . . .  71
   Appendix A.  Use with OSCORE and Transfer over CoAP . . . . . . .  76
     A.1.  Deriving the OSCORE Security Context  . . . . . . . . . .  76
     A.2.  Transferring EDHOC over CoAP  . . . . . . . . . . . . . .  78
   Appendix B.  Compact Representation . . . . . . . . . . . . . . .  82
   Appendix C.  Use of CBOR, CDDL, and COSE in EDHOC . . . . . . . .  83
     C.1.  CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . .  84
     C.2.  CDDL Definitions  . . . . . . . . . . . . . . . . . . . .  85
     C.3.  COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  86
   Appendix D.  Authentication Related Verifications . . . . . . . .  88
     D.1.  Validating the Authentication Credential  . . . . . . . .  88
     D.2.  Identities  . . . . . . . . . . . . . . . . . . . . . . .  89
     D.3.  Certification Path and Trust Anchors  . . . . . . . . . .  90
     D.4.  Revocation Status . . . . . . . . . . . . . . . . . . . .  91
     D.5.  Unauthenticated Operation . . . . . . . . . . . . . . . .  91
   Appendix E.  Use of External Authorization Data . . . . . . . . .  91
   Appendix F.  Application Profile Example  . . . . . . . . . . . .  93
   Appendix G.  Long PLAINTEXT_2 . . . . . . . . . . . . . . . . . .  94
   Appendix H.  EDHOC_KeyUpdate  . . . . . . . . . . . . . . . . . .  95
   Appendix I.  Example Protocol State Machine . . . . . . . . . . .  96
     I.1.  Initiator State Machine . . . . . . . . . . . . . . . . .  96
     I.2.  Responder State Machine . . . . . . . . . . . . . . . . .  98
   Appendix J.  Change Log . . . . . . . . . . . . . . . . . . . . . 100
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 112
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 112

1.  Introduction

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1.1.  Motivation

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

   The need for special protocols targeting constrained IoT deployments
   extends also to the security domain [I-D.ietf-lake-reqs].  Important
   characteristics in constrained environments are the number of round
   trips and protocol message sizes, which if kept low can contribute to
   good performance by enabling transport over a small number of radio
   frames, reducing latency due to fragmentation or duty cycles, etc.
   Another important criterion is code size, which may be prohibitively
   large 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, [RFC9052]) 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 supports the referencing of credentials

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   in order to reduce message overhead, but credentials may
   alternatively be embedded in the messages.  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], the Noise XX pattern [Noise], 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 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 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

   Examples of EDHOC message sizes are shown in Figure 1, using
   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' [RFC9052], or in X.509 certificates
   identified by a hash value using 'x5t' [RFC9360].  As a comparison,
   in the case of RPK authentication, the EDHOC message size when
   transferred in CoAP can be less than 1/7 of the DTLS 1.3 handshake
   [RFC9147] with ECDHE and connection ID, see Section 2 of
   [I-D.ietf-iotops-security-protocol-comparison].

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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, Section 7 describes EDHOC support for transport that does
   not handle message duplication, and Section 8 lists compliance
   requirements.  Note that normative text is also used in appendices,
   in particular Appendix A.

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 [RFC9052], COSE algorithms [RFC9053], 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-
   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 in IKEv2 [RFC7296] and (D)TLS
   1.3 [RFC8446][RFC9147], EDHOC authenticated with signature keys is
   built on a variant of the SIGMA protocol, SIGMA-I, which provides
   identity protection against active attacks on the party initiating
   the protocol.  Also like IKEv2, EDHOC implements the MAC-then-Sign
   variant of the SIGMA-I protocol.  The message flow (excluding an
   optional fourth message) is 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 method 0.

   The parties exchanging messages in an EDHOC session are called
   Initiator (I) and Responder (R), where the Initiator sends message_1
   (see Section 3).  They exchange ephemeral public keys, compute a
   shared secret session 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.

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

   *  Enc(), AEAD(), and MAC() denotes encryption, authenticated
      encryption with additional data, and message authentication code -
      crypto algorithms applied with keys derived from one or more
      shared secrets calculated during the protocol.

   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.

   *  Secure negotiation of cipher suite.

   *  Method types, error handling, and padding.

   *  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.  Symmetric keys and random material used in
   EDHOC are derived using EDHOC_KDF with as much previous information
   as possible, see Figure 8.  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.  Like in (D)TLS, authentication is the responsibility
   of the application.  EDHOC identifies (and optionally transports)
   authentication credentials, and provides proof-of-possession of the
   private authentication key.

   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].

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3.  Protocol Elements

3.1.  General

   The EDHOC protocol consists of three mandatory messages (message_1,
   message_2, message_3), an optional fourth message (message_4), and an
   error message, between an Initiator (I) and a Responder (R).  The odd
   messages are sent by I, the even by R.  Both I and R can send error
   messages.  The roles have slightly different security properties
   which should be considered when the roles are assigned, see
   Section 9.1.  All EDHOC messages are CBOR Sequences [RFC8742], and
   are defined to be deterministically encoded CBOR as specified in
   Section 4.2.1 of [RFC8949].  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).  Media types
   that may be used for EDHOC are defined in Section 10.8.

   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( C_R, ID_CRED_R, Signature_or_MAC_2, EAD_2 )       |
   |<------------------------------------------------------------------+
   |                             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.
   Note that also in the static Diffie-Hellman based authentication
   methods there is an ephemeral-ephemeral Diffie-Hellman key exchange.

   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 the
   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 an EDHOC session
   (see Section 3.4), or may be used in applications of EDHOC, e.g., in
   OSCORE (see Section 3.3.3).  The connection identifiers do not have
   any cryptographic purpose in EDHOC and only facilitate the retrieval
   of security data associated with the protocol state.

   Connection identifiers in EDHOC are intrinsically byte strings.  Most
   constrained devices only have a few connections for which short
   identifiers may be sufficient.  In some cases minimum length
   identifiers are necessary to comply with overhead requirements.
   However, CBOR byte strings - with the exception of the empty byte

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   string h’’ which encodes as one byte (0x40) - are encoded as two or
   more bytes.  To enable one-byte encoding of certain byte strings
   while maintaining CBOR encoding, EDHOC represents certain identifiers
   as CBOR integers on the wire, see Section 3.3.2.

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 communication 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 used in connection identifiers and credential identifiers
   (see Section Section 3.5.3) are defined to have integer
   representations.

   The integers with one-byte CBOR encoding are -24, ..., 23, see
   Figure 5.

   Integer:  -24  -23  ... -11  ...  -2   -1    0    1  ...  15  ...  23
   Encoding:  37   36  ...  2A  ...  21   20   00   01  ...  0F  ...  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 simply encoded as CBOR byte strings.

   For example:

   *  0x21 is represented by 0x21 (CBOR encoding of the integer -2), not
      by 0x4121 (CBOR encoding of the byte string 0x21).

   *  0x0D is represented by 0x0D (CBOR encoding of the integer 13), not
      by 0x410D (CBOR encoding of the byte string 0x0D).

   *  0x18 is represented by 0x4118 (CBOR encoding of the byte string
      0x18).

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   *  0x38 is represented by 0x4138 (CBOR encoding of the byte string
      0x38).

   *  0xABCD is represented by 0x42ABCD (CBOR encoding of the byte
      string 0xABCD).

   One may view this representation of byte strings as a transport
   encoding: a byte string which parses as the one-byte CBOR encoding of
   an integer (i.e., integer in the interval -24, ..., 23) is just
   copied directly into the message, a byte string which does not is
   encoded as a CBOR byte string during transport.

   Implementation Note: When implementing the byte string identifier
   representation, it can in some programming languages help to define a
   new type, or other data structure, which (in its user facing API)
   behaves like a byte string, but when serializing to CBOR produces a
   CBOR byte string or a CBOR integer depending on its value.

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 the 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.

   Examples:

   *  A connection identifier 0xFF (represented in the EDHOC message as
      0x41FF, see Section 3.3.2) is converted to the OSCORE Recipient ID
      0xFF.

   *  A connection identifier 0x21 (represented in the EDHOC message as
      0x21, see Section 3.3.2) is converted to the OSCORE Recipient ID
      0x21.

3.4.  Transport

   Cryptographically, EDHOC does not put requirements on the underlying
   layers.  Received messages are processed as the expected next message
   according to protocol state, see Section 5.  If processing fails for
   any reason then, typically, an error message is attempted to be sent
   and the EDHOC session is aborted.

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   EDHOC is not bound to a particular transport layer and can even be
   used in environments without IP.  Ultimately, the application is free
   to choose how to transport EDHOC messages including errors.  In order
   to avoid unnecessary message processing or protocol termination, it
   is RECOMMENDED to use reliable transport, such as CoAP in reliable
   mode, which is the default transport, see Appendix A.2.  In general,
   the transport SHOULD handle:

   *  message loss,

   *  message duplication, see Section 7 for an alternative,

   *  flow control,

   *  congestion control,

   *  fragmentation and reassembly,

   *  demultiplexing EDHOC messages from other types of messages,

   *  denial-of-service mitigation,

   *  message correlation, see Section 3.4.1.

   EDHOC does not require error free transport since a change in message
   content is detected through the transcript hashes in a subsequent
   integrity verification, see Section 5.  The transport does not
   require additional means to handle message reordering because of the
   lockstep processing of EDHOC.

   EDHOC is designed to enable an authenticated key exchange with small
   messages, where the minimum message sizes are of the order
   illustrated in the first column of Figure 1.  There is no maximum
   message size specified by the protocol; this is for example dependent
   on the size of authentication credentials (if they are transported,
   see Section 3.5).

   The use of transport is specified in the application profile, which
   in particular may specify limitations in message sizes, see
   Section 3.9.

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3.4.1.  EDHOC Message Correlation

   Correlation between EDHOC messages is needed to facilitate the
   retrieval of protocol state and security context during an EDHOC
   session.  It is also helpful for the Responder to get an indication
   that a received EDHOC message is the beginning of a new EDHOC
   session, such that no existing protocol state or security context
   needs to be retrieved.

   Correlation may be based on existing mechanisms in the transport
   protocol, for example, the CoAP Token may be used to correlate EDHOC
   messages in a CoAP response and in an associated CoAP request.  The
   connection identifiers may also be used to correlate EDHOC messages.

   If correlation between consecutive messages is not provided by other
   means then the transport binding SHOULD mandate prepending of an
   appropriate connection identifier (when available from the EDHOC
   protocol) to the EDHOC message.  If message_1 indication is not
   provided by other means, then the transport binding SHOULD mandate
   prepending of message_1 with the CBOR simple value true (0xf5).

   Transport of EDHOC in CoAP payloads is described in Appendix A.2,
   including how to use connection identifiers and message_1 indication
   with CoAP.  A similar construction is possible for other client-
   server protocols.  Protocols that do not provide any correlation at
   all can prescribe prepending of the peer's connection identifier to
   all messages.

   Note that correlation between EDHOC messages may be obtained without
   transport support or connection identifiers, for example if the
   endpoints only accept a single instance of the protocol at a time,
   and execute conditionally on a correct sequence of messages.

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).

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   *  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.

   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 CWT Claims
   Set (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 selected 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 as I and R,
   respectively, and the public authentication keys are denoted G_I and
   G_R, respectively.

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   For X.509 certificates the authentication key is represented by a
   SubjectPublicKeyInfo field.  For CWT and CCS (see Section 3.5.2)) the
   authentication key is represented by a 'cnf' claim [RFC8747]
   containing a COSE_Key [RFC9052].  In EDHOC, a raw public key (RPK) is
   an authentication key encoded as a COSE_Key wrapped in a CCS.

3.5.2.  Authentication Credentials

   The authentication credentials, CRED_I and CRED_R, contains the
   public authentication key of the Initiator and the Responder,
   respectively.  We use the notation CRED_x to refer to CRED_I or
   CRED_R.  Requirements on CRED_x applies both to CRED_I and to CRED_R.
   The authentication credential typically also contains other
   parameters that needs to be verified by the application, see
   Appendix D, and in particular information about the identity
   ("subject") of the endpoint to prevent misbinding attacks, see
   Appendix D.2.

   EDHOC relies on COSE for identification of credentials (see
   Section 3.5.3), for example X.509 certificates [RFC9360], 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.  In the choice between chain
   or bag it is RECOMMENDED to use a chain, since the certificates in a
   bag are unordered and may contain self-signed and extraneous
   certificates, which can add complexity to the process of extracting
   the end entity certificate.  The Initiator and the Responder MAY use
   different types of authentication credentials, e.g., one uses an RPK
   and the other uses a public key certificate.

   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
   the specific encoding of the authentication credentials, see
   Section 3.9.  It is RECOMMENDED that the COSE 'kid' parameter, when
   used to identify the authentication credential, refers to a such a
   specific encoding of the authentication credential.  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 an authentication credential passed
   by reference may occur, then a potential common encoding for CBOR
   based credentials is deterministically encoded CBOR as specified in
   Sections 4.2.1 and 4.2.2 of [RFC8949].

   *  When the authentication credential is an X.509 certificate, CRED_x
      SHALL be the DER encoded certificate, encoded as a bstr [RFC9360].

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   *  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 CWT including a COSE_Key,
      CRED_x SHALL be the untagged CWT.

   *  When the authentication credential includes a COSE_Key but is not
      in a CWT, CRED_x SHALL be an untagged CWT Claims Set (CCS).  This
      is how RPKs are encoded, see Figure 6 for an example.

      -  Naked COSE_Keys are thus dressed as CCS when used in EDHOC, in
         its simplest form by prefixing the COSE_Key with 0xA108A101 (a
         map with a 'cnf' claim).  In that case the resulting
         authentication credential contains no other identity than the
         public key itself, see Appendix D.2.

   An example of CRED_x is shown below:

   {                                              /CCS/
     2 : "42-50-31-FF-EF-37-32-39",               /sub/
     8 : {                                        /cnf/
       1 : {                                      /COSE_Key/
         1 : 1,                                   /kty/
         2 : 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

   The ID_CRED fields, 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.  We use the notation ID_CRED_x to refer to ID_CRED_I
   or ID_CRED_R.  Requirements on ID_CRED_x applies both to ID_CRED_I
   and to ID_CRED_R.  The ID_CRED fields are used to identify and
   optionally transport credentials:

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

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

   ID_CRED_x may contain the authentication credential CRED_x, for x = I
   or R, 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 ([RFC9360]), C509 certificates
   ([I-D.ietf-cose-cbor-encoded-cert]), CWT (see Section 3.5.3.1) and
   CWT Claims Set (see Section 3.5.3.1).

   ID_CRED_I and ID_CRED_R are of type COSE header_map, as defined in
   Section 3 of [RFC9052], and contains one or more COSE header
   parameters.  If a map contains several header paramerers, the labels
   do not need to be sorted in bytewise lexicographic order.  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.

   Example: X.509 certificates can be identified by a hash value using
   the 'x5t' parameter, see Section 2 of [RFC9360]:

   *  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, see Section 3.1 of [RFC9052]:

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

   Note that COSE header parameters in ID_CRED_x are used to identify
   the message sender's credential.  There is therefore no reason to use
   the "-sender" header parameters, such as x5t-sender, defined in
   Section 3 of [RFC9360].  Instead, the corresponding parameter without
   "-sender", such as x5t, SHOULD be used.

   As stated in Section 3.1 of [RFC9052], 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.

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   See Appendix C.3 for more examples.

3.5.3.1.  COSE Header Parameters for CWT and CWT Claims Set

   This document registers two new COSE header parameters 'kcwt' and
   'kccs' for use with CBOR Web Token (CWT, [RFC8392]) and CWT Claims
   Set (CCS, [RFC8392]), respectively.  The CWT/CCS MUST contain a
   COSE_Key in a 'cnf' claim [RFC8747].  There may be any number of
   additional claims present in the CWT/CCS.

   CWTs sent in 'kcwt' are protected using a MAC or a signature and are
   similar to a certificate (when with public key cryptography) or a
   Kerberos ticket (when used with symmetric key cryptography).  CCSs
   sent in 'kccs' are not protected and are therefore similar to raw
   public keys or self-signed certificates.

   Security considerations for 'kcwt' and 'kccs' are made in
   Section 9.8.

3.5.3.2.  Compact Encoding of ID_CRED Fields for 'kid'

   To comply with the LAKE message size requirements, see
   [I-D.ietf-lake-reqs], two optimizations are made for the case when
   ID_CRED_x, for x = I or R, contains a single 'kid' parameter.

   1.  The CBOR map { 4 : kid_x } is replaced by the byte string kid_x.

   2.  The representation of identifiers specified in Section 3.3.2 is
       applied to kid_x.

   These optimizations MUST be applied if and only if ID_CRED_x = { 4 :
   kid_x } and ID_CRED_x in PLAINTEXT_y of message_y, y = 2 or 3, see
   Section 5.3.2 and Section 5.4.2.  Note that these optimizations are
   not applied to instances of ID_CRED_x which have no impact on message
   size, e.g., context_y, or the COSE protected header.  Examples:

   *  For ID_CRED_x = { 4 : h'FF' }, the encoding in PLAINTEXT_y is not
      the CBOR map 0xA10441FF but the CBOR byte string h'FF', i.e.,
      0x41FF.

   *  For ID_CRED_x = { 4 : h'21' }, the encoding in PLAINTEXT_y is
      neither the CBOR map 0xA1044121, nor the CBOR byte string h'21',
      i.e., 0x4121, but the CBOR integer 0x21.

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3.6.  Cipher Suites

   An EDHOC cipher suite consists of an ordered set of algorithms from
   the "COSE Algorithms" and "COSE Elliptic Curves" registries as well
   as the EDHOC MAC length.  All algorithm names and definitions follow
   from COSE algorithms [RFC9053].  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 EDHOC MAC length MUST be at least 8 bytes.  Any cryptographic
   algorithm used in the COSE header parameters in ID_CRED fields is
   selected independently of the selected 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
   Section 9.4.  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.

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   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 10.2)
   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-16-128-128 are compatible with the
      IEEE CCM* mode.

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

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

   *  Cipher suite 6, based on AES-GCM, is for general non-constrained
      applications.  It 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 1.0 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 [RFC9053], 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.  COSE always uses compact output for

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   Elliptic Curve Keys of type EC2.  If the COSE implementation requires
   a 'y' parameter, the value y = false or a calculated y-coordinate can
   be used, see Appendix B.

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 processing of external authorization data (EAD) to be
   defined in a separate specification, and sent in dedicated fields of
   the four EDHOC messages (EAD_1, EAD_2, EAD_3, EAD_4).  EAD is opaque
   data to EDHOC.

   Each EAD field, EAD_x for x = 1, 2, 3 or 4, is a CBOR sequence (see
   Appendix C.1) consisting of one or more EAD items.  An EAD item ead
   is a CBOR sequence of an ead_label and an optional ead_value, see
   Figure 7 and Appendix C.2 for the CDDL definitions.

   ead = (
     ead_label : int,
     ? ead_value : bstr,
   )

                            Figure 7: EAD item.

   A security application may register one or more EAD labels, see
   Section 10.5, and specify the associated processing and security
   considerations.  The IANA registry contains the absolute value of the
   ead_label, |ead_label|; the same ead_value applies independently of
   sign of ead_label.

   An EAD item can be either critical or non-critical, determined by the
   sign of the ead_label in the EAD item transported in the EAD field.
   A negative value indicates that the EAD item is critical and a non-
   negative value indicates that the EAD item is non-critical.

   If an endpoint receives a critical EAD item it does not recognize, or
   a critical EAD item that contains information that it cannot process,
   then the endpoint MUST send an EDHOC error message back as defined in
   Section 6, and the EDHOC session MUST be aborted.  The EAD item
   specification defines the error processing.  A non-critical EAD item
   can be ignored.

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   The security application registering a new EAD item needs to describe
   under what conditions the EAD item is critical or non-critical, and
   thus whether the ead_label is used with negative or positive sign.
   ead_label = 0 is used for padding, see Section 3.8.1.

   The security application may define multiple uses of certain EAD
   items, e.g., the same EAD item may be used in different EDHOC
   messages.  Multiple occurrences of an EAD item in one EAD field may
   also be specified, but the criticality of the repeated EAD item is
   expected to be the same.

   The EAD fields of EDHOC MUST only be used with registered EAD items,
   see Section 10.5.  Examples of the use of EAD are provided in
   Appendix E.

3.8.1.  Padding

   EDHOC message_1 and the plaintext of message_2, message_3 and
   message_4 can be padded with the use of the corresponding EAD_x
   field, for x = 1, 2, 3, 4.  Padding in EAD_1 mitigates amplification
   attacks (see Section 9.7), and padding in EAD_2, EAD_3, and EAD_4
   hides the true length of the plaintext (see Section 9.6).  Padding
   MUST be ignored and discarded by the receiving application.

   Padding is obtained by using an EAD item with ead_label = 0 and a
   (pseudo-)randomly generated byte string of appropriate length as
   ead_value, noting that the ead_label and the CBOR encoding of
   ead_value also add bytes.  Examples:

   *  One byte padding (optional ead_value omitted):

      -  EAD_x = 0x00

   *  Two bytes padding, using the empty byte string (0x40) as
      ead_value:

      -  EAD_x = 0x0040

   *  Three bytes padding, constructed from the pseudorandomly generated
      ead_value 0xe9 encoded as byte string:

      -  EAD_x = 0x0041e9

   Multiple occurrences of EAD items with ead_label = 0 are allowed.
   Certain padding lengths require the use of at least two such EAD
   items.

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   Note that padding is non-critical because the intended behaviour when
   receiving is to ignore it.

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 to ensure successful
   completion, e.g., whether message_4 is used or not.  The application
   decides which endpoint is Initiator and which is Responder.

   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).

   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.

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   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.  EDHOC
   messages can be processed without the application profile, i.e., the
   EDHOC messages includes information about the type and length of all
   fields.

   An example of an application profile is shown in Appendix F.

   For some parameters, like METHOD, type of ID_CRED field or EAD, the
   receiver of an EDHOC message is able to verify compliance with the
   application profile, and if it needs to fail because of lack of
   compliance, to infer the reason why the EDHOC session failed.

   For other encodings, like the profiling of CRED_x in the case that it
   is not transported, it may not be possible to verify that lack of
   compliance with the application profile was the reason for failure:
   Integrity verification in message_2 or message_3 may fail not only
   because of a wrong credential.  For example, in case the Initiator
   uses a 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.)

   Other conditions may be part of the application profile, such as what
   is the 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 EDHOC
   session, for example based on URI to which the EDHOC message is sent,
   or external authorization data type.

4.  Key Derivation

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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 EDHOC_Extract (Section 4.1.1) and
   EDHOC_Expand (Section 4.1.2), and how to use them to derive PRK_out
   (Section 4.1.3) which is the shared secret session key resulting from
   a completed EDHOC session.

   EDHOC_Extract is used to derive fixed-length uniformly pseudorandom
   keys (PRK) from ECDH shared secrets.  EDHOC_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 authentication method
   (Section 3.2) as detailed in Section 5.

4.1.1.  EDHOC_Extract

   The pseudorandom keys (PRKs) used for EDHOC message processing are
   derived using EDHOC_Extract:

      PRK = EDHOC_Extract( salt, IKM )

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

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

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

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

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

   where the Keccak message authentication code (KMAC) is specified in
   [SP800-185].

   The rest of the section defines the pseudorandom keys PRK_2e,
   PRK_3e2m and PRK_4e3m; their use is shown in Figure 8.  The index of
   a PRK indicates its use or in what message protection operation it is
   involved.  For example, PRK_3e2m is involved in the encryption of
   message 3 and in calculating the MAC of message 2.

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4.1.1.1.  PRK_2e

   The pseudorandom key PRK_2e is derived with the following input:

   *  The salt SHALL be TH_2.

   *  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 [RFC9053].  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( TH_2, G_XY )

4.1.1.2.  PRK_3e2m

   The pseudorandom key PRK_3e2m is derived as follows:

   If the Responder authenticates with a static Diffie-Hellman key, then
   PRK_3e2m = EDHOC_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.

4.1.1.3.  PRK_4e3m

   The pseudorandom key PRK_4e3m is derived as follows:

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

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

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   *  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.  EDHOC_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 EDHOC_Expand:

      OKM = EDHOC_KDF( PRK, info_label, context, length )
          = EDHOC_Expand( PRK, info, length )

   where info is encoded as the CBOR sequence

   info = (
     info_label : int,
     context : bstr,
     length : uint,
   )

   where

   *  info_label is an int

   *  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 EDHOC_Expand depends on the EDHOC hash algorithm of
   the selected cipher suite:

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

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

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

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

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   Figure 8 lists derivations made with EDHOC_KDF, where

   *  hash_length - length of output size of the EDHOC hash algorithm of
      the selected cipher suite

   *  key_length - length of the encryption key of the EDHOC AEAD
      algorithm of the selected cipher suite

   *  iv_length - length of the initialization vector of the EDHOC AEAD
      algorithm of the selected cipher suite

   Further details of the key derivation and how the output keying
   material is used are 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 )
   PRK_exporter  = EDHOC_KDF( PRK_out, 10, h'',       hash_length )

     Figure 8: Key derivations using EDHOC_KDF. h'' is CBOR diagnostic
                 notation for the empty byte string, 0x40.

4.1.3.  PRK_out

   The pseudorandom key PRK_out, derived as shown in Figure 8, is the
   output session key of a completed EDHOC session.

   Keys for applications are derived using EDHOC_Exporter (see
   Section 4.2.1) from PRK_exporter, which in turn is derived from
   PRK_out as shown in Figure 8.  For the purpose of generating
   application keys, it is sufficient to store PRK_out or PRK_exporter.
   (Note that the word "store" used here does not imply that the
   application has access to the plaintext PRK_out since that may be
   reserved for code within a Trusted Execution Environment, see
   Section 9.8).

4.2.  Keys for EDHOC Applications

   This section defines EDHOC_Exporter in terms of EDHOC_KDF and
   PRK_exporter.  A key update function is defined in Appendix H.

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4.2.1.  EDHOC_Exporter

   Keying material for the application can be derived using the
   EDHOC_Exporter interface defined as:

      EDHOC_Exporter(exporter_label, context, length)
        = EDHOC_KDF(PRK_exporter, exporter_label, context, length)

   where

   *  exporter_label is a registered uint from the EDHOC_Exporter Label
      registry (Section 10.1)

   *  context is a bstr defined by the application

   *  length is a uint defined by the application

   The (exporter_label, context) pair used in EDHOC_Exporter must be
   unique, i.e., an (exporter_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 securely.  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.

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 sessions 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.  In order to recreate the COSE object,
   the recipient endpoint may need to add parameters to the COSE headers
   not included in the EDHOC message, for example the parameter 'alg' to
   COSE_Sign1 or COSE_Encrypt0.

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5.1.  EDHOC Message Processing Outline

   For each new/ongoing EDHOC 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 with 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).

   2.  Retrieve the protocol state according to the message correlation,
       see Section 3.4.1.  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 9.7).

   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.

   The message processing steps SHALL be processed in order, unless
   otherwise stated.  If the processing fails for some reason then,
   typically, an error message is sent, the EDHOC session is aborted,
   and the protocol state erased.  When the composition and sending of
   one message is completed and before the next message is received,
   error messages SHALL NOT be sent.

   After having successfully processed the last message (message_3 or
   message_4 depending on application profile) the EDHOC session is
   completed, after which no error messages are sent and EDHOC session
   output MAY be maintained even if error messages are received.
   Further details are provided in the following subsections and in
   Section 6.

   Different instances of the same message MUST NOT be processed in one
   EDHOC session.  Note that processing will fail if the same message
   appears a second time for EDHOC processing in the same EDHOC session
   because the state of the protocol has moved on and now expects
   something else.  Message deduplication MUST be done by the transport
   protocol (see Section 3.4) or, if not supported by the transport, as
   described in Section 7.

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5.2.  EDHOC Message 1

5.2.1.  Formatting of Message 1

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

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

   suites = [ 2* int ] / int
   EAD_1 = 1* ead

   where:

   *  METHOD - authentication method, see Section 3.2.

   *  SUITES_I - array of cipher suites which the Initiator supports
      constructed as specified in Section 5.2.2.

   *  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 Composition of Message 1

   The processing steps are detailed below and in Section 6.3.

   The Initiator SHALL compose message_1 as follows:

   *  Construct SUITES_I as an array of cipher suites supported by I in
      order of preference by I with the first cipher suite in the array
      being the most preferred by I, and the last being the one selected
      by I for this EDHOC session.  If the cipher suite most preferred
      by I is selected then SUITES_I contains only that cipher suite and
      is encoded as an int.  All cipher suites, if any, preferred by I
      over the selected one MUST be included.  (See also Section 6.3.)

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      -  The selected suite is based on what the Initiator can assume to
         be supported by the Responder; if the Initiator previously
         received from the Responder an error message with error code 2
         containing SUITES_R (see Section 6.3) indicating 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 may be forged).

      -  The Initiator MUST NOT change its order of preference for
         cipher suites, and MUST NOT omit a cipher suite preferred to
         the selected one because of previous error messages received
         from the Responder.

   *  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 during the EDHOC
      session.

   *  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 in the following order:

   *  Decode message_1 (see Appendix C.1).

   *  Process message_1, in particular verify that the selected cipher
      suite is supported and that no prior cipher suite as ordered in
      SUITES_I is supported.

   *  If all processing completed successfully, and if EAD_1 is present,
      then make it available to the application for EAD processing.

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

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

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   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
   )

   where:

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

5.3.2.  Responder Composition of Message 2

   The Responder SHALL compose message_2 as follows:

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

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

   *  Compute the transcript hash TH_2 = H( G_Y, H(message_1) ) where
      H() is the EDHOC hash algorithm of the selected cipher suite.  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 = << C_R,
      ID_CRED_R, TH_2, CRED_R, ? EAD_2 >> (see Appendix C.1 for
      notation)

      -  If the Responder authenticates with a static Diffie-Hellman key
         (method equals 1 or 3), then mac_length_2 is the EDHOC MAC
         length of the selected cipher suite.  If the Responder
         authenticates with a signature key (method equals 0 or 2), then
         mac_length_2 is equal to hash_length.

      -  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.

      -  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

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   *  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
      [RFC9053] 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 with a binary additive stream cipher,
      using a keystream generated with EDHOC_Expand, and the following
      plaintext:

      -  PLAINTEXT_2 = ( C_R, 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 the compact encoding is
            applied, see Section 3.5.3.2.

         o  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.

      -  Compute KEYSTREAM_2 as in Section 4.1.2, where plaintext_length
         is the length of PLAINTEXT_2.  For the case of plaintext_length
         exceeding the EDHOC_KDF output size, see Appendix G.

      -  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 in the following order:

   *  Decode message_2 (see Appendix C.1).

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   *  Retrieve the protocol state using available message correlation
      (e.g., the CoAP Token, the 5-tuple, or the prepended C_I, see
      Section 3.4.1).

   *  Decrypt CIPHERTEXT_2, see Section 5.3.2.

   *  If all processing completed successfully, then make ID_CRED_R and
      (if present) EAD_2 available to the application for
      authentication- and EAD processing.  When and how to perform
      authentication is up to the application.

   *  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.  Make the result of the verification available to
      the application.

   If any processing step fails, then the Initiator MUST send an EDHOC
   error message back as defined in Section 6, and the EDHOC session
   MUST be aborted.

5.4.  EDHOC Message 3

5.4.1.  Formatting of Message 3

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

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

5.4.2.  Initiator Composition of Message 3

   The Initiator SHALL compose message_3 as follows:

   *  Compute the transcript hash TH_3 = H(TH_2, PLAINTEXT_2, CRED_R)
      where H() is the EDHOC hash algorithm of the selected cipher
      suite.  The input to the hash function is a CBOR Sequence.  Note
      that TH_3 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 >>

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      -  If the Initiator authenticates with a static Diffie-Hellman key
         (method equals 2 or 3), then mac_length_3 is the EDHOC MAC
         length of the selected cipher suite.  If the Initiator
         authenticates with a signature key (method equals 0 or 1), then
         mac_length_3 is equal to hash_length.

      -  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
      [RFC9052] using the signature algorithm of the selected cipher
      suite, the private authentication key of the Initiator, and the
      following parameters as input (see Appendix C.3):

      -  protected = << ID_CRED_I >>

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

      -  payload = MAC_3

   *  Compute a COSE_Encrypt0 object as defined in Sections 5.2 and 5.3
      of [RFC9052], 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

      -  PLAINTEXT_3 = ( 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 the compact encoding is
            applied, see Section 3.5.3.2.

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      CIPHERTEXT_3 is the 'ciphertext' of COSE_Encrypt0.

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

   *  Calculate PRK_out as defined in Figure 8.  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.

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

   After creating message_3, the Initiator can compute PRK_out, see
   Section 4.1.3, and derive application keys using the EDHOC_Exporter
   interface, see Section 4.2.1.  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 and the application has
   authenticated the Responder.  This is similar to waiting for an
   acknowledgment (ACK) in a transport protocol.  The Initiator SHOULD
   NOT send protected application data until the application has
   authenticated the Responder.

5.4.3.  Responder Processing of Message 3

   The Responder SHALL process message_3 in the following order:

   *  Decode message_3 (see Appendix C.1).

   *  Retrieve the protocol state using available message correlation
      (e.g., the CoAP Token, the 5-tuple, or the prepended C_R, see
      Section 3.4.1).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
      and 5.3 of [RFC9052], with the EDHOC AEAD algorithm in the
      selected cipher suite, and the parameters defined in
      Section 5.4.2.

   *  If all processing completed successfully, then make ID_CRED_I and
      (if present) EAD_3 available to the application for
      authentication- and EAD processing.  When and how to perform
      authentication is up to the application.

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   *  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 result of the verification available to
      the application.

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

   After processing message_3, the Responder can compute PRK_out, see
   Section 4.1.3, and derive application keys using the EDHOC_Exporter
   interface, see Section 4.2.1.  The Responder SHOULD NOT persistently
   store PRK_out or application keys until the application has
   authenticated the Initiator.  The Responder SHOULD NOT send protected
   application data until the application has authenticated the
   Initiator.

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

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 are:

   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 9.1.

5.5.1.  Formatting of Message 4

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

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   message_4 = (
     CIPHERTEXT_4 : bstr,
   )

5.5.2.  Responder Composition 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
      [RFC9052], 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

      -  PLAINTEXT_4 = ( ? EAD_4 )

         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 available message correlation
      (e.g., the CoAP Token, the 5-tuple, or the prepended C_I, see
      Section 3.4.1).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
      and 5.3 of [RFC9052], with the EDHOC AEAD algorithm in the
      selected cipher suite, and the parameters defined in
      Section 5.5.2.

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

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   If any processing step fails, then the Initiator MUST send an EDHOC
   error message back as defined in Section 6, and the EDHOC session
   MUST be aborted.

   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 with the currently defined error codes.
   Additional error codes may be registered, see Section 10.4.

   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 receiving endpoint.  Except for processing and
   formatting errors, it is up to the application when to send an error
   message.  Sending error messages is essential for debugging but MAY
   be skipped if, for example, an EDHOC session cannot be found or due
   to denial-of-service reasons, see Section 9.7.  Error messages in
   EDHOC are always fatal.  After sending an error message, the sender
   MUST abort the EDHOC session.  The receiver SHOULD treat an error
   message as an indication that the other party likely has aborted the
   EDHOC session.  But since error messages might be forged, the
   receiver MAY try to continue the EDHOC session.

   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 9: EDHOC error message.

   where:

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

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

   The remainder of this section specifies the currently defined error
   codes, see Figure 10.  Additional error codes and corresponding error
   information may be specified.

   +----------+---------------+----------------------------------------+
   | ERR_CODE | ERR_INFO Type | Description                            |
   +==========+===============+========================================+
   |        0 |               | This value is reserved                 |
   +----------+---------------+----------------------------------------+
   |        1 | tstr          | Unspecified error                      |
   +----------+---------------+----------------------------------------+
   |        2 | suites        | Wrong selected cipher suite            |
   +----------+---------------+----------------------------------------+
   |        3 | true          | Unknown credential referenced          |
   +----------+---------------+----------------------------------------+

            Figure 10: EDHOC error codes and error information.

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.  Error code 0 MUST NOT be used as part
   of the EDHOC message exchange.  If an endpoint receives an error
   message with error code 0, then it MUST abort the EDHOC session 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 which SHOULD be 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.

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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.
   In this case, ERR_INFO = 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, in which case
   it is encoded as an int.  If the Responder does not support any
   cipher suite in SUITES_I, then it SHOULD include all its supported
   cipher suites in SUITES_R.

   In 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 cipher suite and the Responder sends an
   error with supported cipher suites.  After a completed EDHOC session,
   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 also 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.
   For examples, see Section 6.3.2.

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   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 abort the EDHOC session, 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 abort the EDHOC session.

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 11 and 12 show two
   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 Example 1 (Figure 11), the Responder supports cipher suite 6 but
   not the initially selected cipher suite 5.  The Responder rejects the
   first message_1 with an error indicating support for suite 6 in
   SUITES_R.  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.

   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 11: Cipher suite negotiation example 1.

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   In Example 2 (Figure 12), 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 rejects the
   first message_1 with an error indicating support for suites 8 and 9
   in SUITES_R (in any order).  The Initiator also supports suites 8 and
   9, and prefers suite 8, so 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.

   Note 1.  If the Responder had supported suite 5, then the first
   message_1 would not have been accepted either, since the Responder
   observes that suite 5 is more preferred by the Initiator than the
   selected suite 6.  In that case the Responder would have included
   suite 5 in SUITES_R of the response, and it would then have become
   the selected and only suite in the second message_1.

   Note 2.  For each message_1 the Initiator MUST generate a new
   ephemeral ECDH key pair matching the selected cipher suite.

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

               Figure 12: Cipher suite negotiation example 2.

6.4.  Unknown Credential Referenced

   Error code 3 is used for errors due to a received credential
   identifier (ID_CRED_R in message_2 or ID_CRED_I message_3) containing
   a reference to a credential which the receiving endpoint does not
   have access to.  The intent with this error code is that the endpoint
   who sent the credential identifier should for the next EDHOC session
   try another credential identifier supported according to the
   application profile.

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   For example, an application profile could list x5t and x5chain as
   supported credential identifiers, and state that x5t should be used
   if it can be assumed that the X.509 certificate is available at the
   receiving side.  This error code thus enables the certificate chain
   to be sent only when needed, bearing in mind that error messages are
   not protected so an adversary can try to cause unnecessary large
   credential identifiers.

   For the error code 3, the error information SHALL be the CBOR simple
   value true (0xf5).  Error code 3 MUST NOT be used when the received
   credential identifier type is not supported.

7.  EDHOC Message Deduplication

   EDHOC by default assumes that message duplication is handled by the
   transport, in this section exemplified with CoAP, see Appendix A.2.

   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 acknowledgment on
   the 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 the 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

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   differences which compromise 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]).

8.  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 MAY support message_4.  Error codes (ERR_CODE) 1 and
   2 MUST be supported.

   Implementations MUST support EAD.

   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 the size of the MAC length, so
   supporting one or both of these is not significantly different.
   Implementations only need to implement the algorithms needed for
   their supported methods.

9.  Security Considerations

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9.1.  Security Properties

   EDHOC has similar security properties as can be expected from the
   theoretical SIGMA-I protocol [SIGMA] and the Noise XX pattern
   [Noise], which are similar to methods 0 and 3, respectively.  Proven
   security properties are detailed in the security analysis
   publications referenced at the end of this section.

   Using the terminology from [SIGMA], EDHOC provides forward secrecy,
   mutual authentication with aliveness, consistency, and peer
   awareness.  As described in [SIGMA], message_3 provides peer
   awareness to the Responder while message_4 provides peer awareness to
   the Initiator.  By including the authentication credentials in the
   transcript hash, EDHOC protects against Duplicate Signature Key
   Selection (DSKS)-like identity mis-binding attack that the MAC-then-
   Sign variant of SIGMA-I is otherwise vulnerable to.

   As described in [SIGMA], different levels of identity protection are
   provided to the Initiator and the Responder.  EDHOC provides identity
   protection of the Initiator against active attacks and identity
   protection 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 then sending 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.

   EDHOC messages might change in transit due to a noisy channel or
   through modification by an attacker.  Changes in message_1 and
   message_2 (except Signature_or_MAC_2 when the signature scheme is not
   strongly unforgeable) are detected when verifying Signature_or_MAC_2.
   Changes to not strongly unforgeable Signature_or_MAC_2, and message_3
   are detected when verifying CIPHERTEXT_3.  Changes to message_4 are
   detected when verifying CIPHERTEXT_4.

   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 plaintext
   messages.  This protects against an attacker replaying messages or
   injecting messages from another EDHOC session.

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   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, and to abort the EDHOC session if
   not.

   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 key update function (see
   Appendix H) for lightweight application protocol rekeying.  Either of
   these provides forward secrecy, in the sense that compromise of the
   private authentication keys does not compromise past session keys
   (PRK_out), and compromise of a session key does not compromise past
   session keys.  Frequently re-running EDHOC with ephemeral Diffie-
   Hellman forces attackers to perform dynamic key exfiltration where
   the attacker must have continuous interactions with the collaborator,
   which is a significant sustained attack.

   To limit the effect of breaches, it is important to limit the use of
   symmetric 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 symmetric group keys for bootstrapping.

   Compromise of the long-term keys (private signature or static DH
   keys) does not compromise the security of completed EDHOC sessions.
   Compromising the private authentication keys of one party lets an
   active attacker impersonate that compromised party in EDHOC sessions
   with other parties but does not let the attacker impersonate other
   parties in EDHOC sessions with the compromised party.  Compromise of
   the long-term keys does not enable a passive attacker to compromise
   future session keys (PRK_out).  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.

   Based on the cryptographic algorithms requirements Section 9.3, 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

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   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.

   As the EDHOC session is aborted when verification fails, the security
   against online attacks is given by the sum of the strength of the
   verified signatures and MACs (including MAC in AEAD).  As an example,
   if EDHOC is used with method 3, cipher suite 2, and message_4, the
   Responder is authenticated with 128-bit security against online
   attacks (the sum of the 64-bit MACs in message_2 and message_4).  The
   same principle applies for MACs in an application protocol keyed by
   EDHOC as long as EDHOC is rerun when verification of the first MACs
   in the application protocol fails.  As an example, if EDHOC with
   method 3 and cipher suite 2 is used as in Figure 2 of
   [I-D.ietf-core-oscore-edhoc], 128-bit mutual authentication against
   online attacks can be achieved after completion of the first OSCORE
   request and response.

   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 message_4 or a message protected with a
   derived application key, such as an OSCORE message, from the
   Responder.  After verifying message_3, the Responder is assured that
   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 9.5.
   EAD_2 is encrypted but the Responder has not yet authenticated the
   Initiator and the encryption does not provide confidentiality against
   active attacks.

   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.

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   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 EDHOC 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.

   Earlier versions of EDHOC have been formally analyzed [Bruni18]
   [Norrman20] [CottierPointcheval22] [Jacomme23] [GuentherIlunga22] and
   the specification has been updated based on the analysis.

9.2.  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.

   To reduce message overhead EDHOC does not use explicit nonces and
   instead relies on the ephemeral public keys to provide randomness to
   each EDHOC 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 EDHOC_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

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   confidentiality with indistinguishability under chosen ciphertext
   (IND-CCA) 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
   public keys depend on the elliptic curve.  For X25519 and X448, the
   requirements are defined in [RFC7748].  For X25519 and X448, the
   check for all-zero output as specified in Section 6 of [RFC7748] MUST
   be done.  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.

   The same authentication credential MAY be used for both the Initiator
   and Responder roles.  As noted in Section 12 of [RFC9052] the use of
   a single key for multiple algorithms is strongly discouraged 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].

   The property that a completed EDHOC session implies that another
   identity has been active is upheld 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,
   see Appendix D.5, the Initiator should verify that the Responder's
   identity is not equal to its own.  Any future EDHOC methods using
   e.g., pre-shared keys might need to mitigate this in other ways.
   However, an active attacker can gain information about the set of
   identities an Initiator is willing to communicate with.  If the
   Initiator is willing to communicate with all identities except its
   own an attacker can determine that a guessed Initiator identity is
   correct.  To not leak any long-term identifiers, using a freshly
   generated authentication key as identity in each initial TOFU session
   is RECOMMENDED.

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   NIST SP 800-56A [SP-800-56A] 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, 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.  The more recent NIST SP 800-108
   [SP-800-108] aligns with [HKDFpaper] and states that for a secure
   KDF, the revelation of one portion of the derived keying material
   must not degrade the security of any other portion of that keying
   material.

9.3.  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
   authentication credentials, the EDHOC session, 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 credentials, the choice of hash algorithm in
   x5t and c5t does not affect security, and using the same hash
   algorithm as in the cipher suite, but with as much truncation as
   possible, is RECOMMENDED.  That is, when the EDHOC hash algorithm is
   SHA-256, using SHA-256/64 in x5t and c5t is RECOMMENDED.  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.

9.4.  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.

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   Symmetric algorithms used in EDHOC such as SHA-256 and AES-CCM-
   16-64-128 are practically secure against even large quantum
   computers.  Two of NIST's security levels for quantum-resistant
   public-key cryptography are based on AES-128 and SHA-256.  Quantum
   computer will likely be expensive, slow due to heavy error
   correction, and Grover’s algorithm, which is proven to be optimal,
   cannot effectively be parallelized.  Grover’s algorithm will provide
   little or no advantage in attacking AES, and AES-128 will remain
   secure for decades to come [NISTPQC].

   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.

9.5.  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.  Note that
   even if ead_value is encrypted outside of EDHOC, the ead_labels in
   EAD_1 is revealed to passive attackers and the ead_labels in EAD_2 is
   revealed to active attackers.  Another consideration is that the list
   of supported cipher suites may potentially be used to identify the
   application.  The Initiator and the Responder must also make sure
   that unauthenticated data does not trigger any harmful actions.  In
   particular, this applies to EAD_1 and error messages.

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   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 identifiers and MAY provide mechanisms to issue several
   simultaneously active connection identifiers.  See [RFC9000] for a
   non-constrained example of such mechanisms.  Connection identifiers
   can e.g., be chosen randomly among the set of unused 1-byte
   connection identifiers.  Connection identity privacy mechanisms are
   only useful when there are not fixed identifiers such as IP address
   or MAC address in the lower layers.

9.6.  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 9.1.  [I-D.arkko-arch-internet-threat-model-guidance] further
   expands the threat model.  Implementations and users should take
   these threat models into account and consider actions to reduce the
   risk of tracking by other endpoints.  In particular, even data sent
   protected to the other endpoint such as ID_CRED fields and EAD fields
   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.  A passive attacker may, e.g., be able to differentiate
   endpoints using identifiers of different length.  To mitigate this
   information leakage an implementation may ensure that the fields have
   fixed length or use padding.  An implementation may, e.g., only use
   fixed length identifiers like 'kid' of length 1.  Alternatively,
   padding may be used (see Section 3.8.1) to hide the true length of,
   e.g., certificates by value in 'x5chain' or 'c5c'.

9.7.  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 make use of available
   lower layer mechanisms.  For instance, when EDHOC is transferred as

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   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.  To avoid an additional
   roundtrip the Initiator can reduce the amplification factor by
   padding message_1, i.e., using EAD_1, see Section 3.8.1.  Note that
   while the Echo option mitigates some resource exhaustion aspects of
   spoofing, it does not protect against a distributed denial-of-service
   attack made by real, potentially compromised, clients.  Similarly,
   limiting amplification only reduces the impact, which still may be
   significant because of a large number of clients engaged in the
   attack.

   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 abort the EDHOC 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 aborting
   the EDHOC session.

9.8.  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.

   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

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   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.irtf-cfrg-det-sigs-with-noise] for a list of attack papers.  As
   suggested in Section 2.1.1 of [RFC9053] 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 be 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.  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.  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.  Verification of validity may
   require the use of a Real-Time Clock (RTC).  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.

   Similar considerations as for certificates are needed for CWT/CCS.
   The endpoints are responsible for verifying the integrity and
   validity of CWT/CCS, and to handle revocation.  The application needs
   to determine what trust anchors are relevant, and have a well-defined
   trust-establishment process.  A self-signed certificate/CWT or CCS
   appearing in the protocol cannot be a trigger to modify the set of
   trust anchors.  One common way for a new trust anchor to be added to
   (or removed from) a device is by means firmware upgrade.  See
   [RFC9360] for a longer discussion on trust and validation in
   constrained devices.

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   Just like for certificates, the contents of the COSE header
   parameters 'kcwt' and 'kccs' defined in Section 10.6 must be
   processed as untrusted input.  Endpoints that intend to rely on the
   assertions made by a CWT/CCS obtained from any of these methods need
   to validate the contents.  For 'kccs', which enables transport of raw
   public keys, the data structure used does not include any protection
   or verification data. 'kccs' may be used for unauthenticated
   operations, e.g. trust on first use, with the limitations and caveats
   entailed, see Appendix D.5.

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

   If two nodes unintentionally initiate two simultaneous EDHOC sessions
   with each other even if they only want to complete a single EDHOC
   session, they MAY abort the EDHOC session with the lexicographically
   smallest G_X.  Note that in cases where several EDHOC sessions with
   different parameter sets (method, COSE headers, etc.) are used, an
   attacker can affect which parameter set 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,
   see for example [RFC9397]) 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.

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   Note that HKDF-Expand has a relatively small maximum output length of
   255 ⋅ hash_length, where hash_length is the output size in bytes of
   the EDHOC hash algorithm of the selected cipher suite.  This means
   that when SHA-256 is used as hash algorithm, PLAINTEXT_2 cannot be
   longer than 8160 bytes.  This is probably not a limitation for most
   intended applications, but to be able to support for example long
   certificate chains or large external authorization data, there is a
   backwards compatible method specified in Appendix G.

   The sequence of transcript hashes in EDHOC (TH_2, TH_3, TH_4) does
   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 abort the
   EDHOC session if the message does not comply with the CDDL for that
   message.  Implementations are not required to support non-
   deterministic encodings and MAY abort the EDHOC session if the
   received EDHOC message is not encoded using deterministic CBOR.
   Implementations MUST abort the EDHOC session if validation of a
   received public key fails or if any cryptographic field has the wrong
   length.

10.  IANA Considerations

   This Section gives IANA Considerations and, unless otherwise noted,
   conforms with [RFC8126].

10.1.  EDHOC Exporter Label Registry

   IANA is requested to create a new registry under the new registry
   group "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name: EDHOC Exporter Label

   Reference: [[this document]]

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   +-------------+------------------------------+-------------------+
   | Label       | Description                  | Reference         |
   +=============+==============================+===================+
   | 0           | Derived OSCORE Master Secret | [[this document]] |
   +-------------+------------------------------+-------------------+
   | 1           | Derived OSCORE Master Salt   | [[this document]] |
   +-------------+------------------------------+-------------------+
   | 2-22        | Unassigned                   |                   |
   +-------------+------------------------------+-------------------+
   | 23          | Reserved                     | [[this document]] |
   +-------------+------------------------------+-------------------+
   | 24-32767    | Unassigned                   |                   |
   +-------------+------------------------------+-------------------+
   | 32768-65535 | Private Use                  |                   |
   +-------------+------------------------------+-------------------+

                      Figure 13: EDHOC exporter label.

   +-------------+-------------------------------------+
   | Range       | Registration Procedures             |
   +=============+=====================================+
   | 0 to 23     | Standards Action                    |
   +-------------+-------------------------------------+
   | 24 to 32767 | Expert Review                       |
   +-------------+-------------------------------------+

10.2.  EDHOC Cipher Suites Registry

   IANA is requested to create a new registry under the new registry
   group "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name: EDHOC Cipher Suites

   Reference: [[this document]]

   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
   Array: N/A
   Description: Private Use
   Reference: [[this document]]

   Value: -23
   Array: N/A
   Description: Private Use
   Reference: [[this document]]

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   Value: -22
   Array: N/A
   Description: Private Use
   Reference: [[this document]]

   Value: -21
   Array: N/A
   Description: Private Use
   Reference: [[this document]]

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

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

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

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

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

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

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

   Value: 23
   Reserved
   Reference: [[this document]]

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

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

   +----------------+-------------------------------------+
   | Range          | Registration Procedures             |
   +================+=====================================+
   | -65536 to -25  | Specification Required              |
   +----------------+-------------------------------------+
   | -20 to 23      | Standards Action with Expert Review |
   +----------------+-------------------------------------+
   | 24 to 65535    | Specification Required              |
   +----------------+-------------------------------------+

10.3.  EDHOC Method Type Registry

   IANA is requested to create a new registry under the new registry
   group "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name: EDHOC Method Type

   Reference: [[this document]]

   The columns of the registry are Value, Initiator Authentication Key,
   and Responder Authentication Key, and Reference, where Value is an
   integer and the key columns are text strings describing the
   authentication keys.

   The initial contents of the registry are shown in Figure 4.  Method
   23 is Reserved.

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   +----------------+-------------------------------------+
   | Range          | Registration Procedures             |
   +================+=====================================+
   | -65536 to -25  | Specification Required              |
   +----------------+-------------------------------------+
   | -24 to 23      | Standards Action with Expert Review |
   +----------------+-------------------------------------+
   | 24 to 65535    | Specification Required              |
   +----------------+-------------------------------------+

10.4.  EDHOC Error Codes Registry

   IANA is requested to create a new registry under the new registry
   group "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name: EDHOC Error Codes

   Reference: [[this document]]

   The columns of the registry are ERR_CODE, ERR_INFO Type, Description,
   and Reference, 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 10.  Error code 23 is Reserved.

   +----------------+-------------------------------------+
   | Range          | Registration Procedures             |
   +================+=====================================+
   | -65536 to -25  | Expert Review                       |
   +----------------+-------------------------------------+
   | -24 to 23      | Standards Action                    |
   +----------------+-------------------------------------+
   | 24 to 65535    | Expert Review                       |
   +----------------+-------------------------------------+

10.5.  EDHOC External Authorization Data Registry

   IANA is requested to create a new registry under the new registry
   group "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name: EDHOC External Authorization Data

   Reference: [[this document]]

   The columns of the registry are Name, Label, Description, and
   Reference, where Label is a non-negative integer and the other
   columns are text strings.  The initial contents of the registry is
   shown in Figure 14.  EAD label 23 is Reserved.

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   +-----------+-------+------------------------+-------------------+
   | Name      | Label | Description            | Reference         |
   +===========+=======+========================+===================+
   | Padding   |   0   | Randomly generated     | [[this document]] |
   |           |       | CBOR byte string       | Section 3.8.1     |
   +-----------+-------+------------------------+-------------------+

                           Figure 14: EAD labels.

   +-------------+-------------------------------------+
   | Range       | Registration Procedures             |
   +=============+=====================================+
   | 0 to 23     | Standards Action with Expert Review |
   +-------------+-------------------------------------+
   | 24 to 65535 | Specification Required              |
   +-------------+-------------------------------------+

10.6.  COSE Header Parameters Registry

   IANA is requested to register the following entries in the "COSE
   Header Parameters" registry under the registry group "CBOR Object
   Signing and Encryption (COSE)" (see Figure 15): 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 and    |
   |      |       |               | possibly other claims. CWT is      |
   |      |       |               | defined in RFC 8392. COSE_Messages |
   |      |       |               | is defined in RFC 9052.            |
   +------+-------+---------------+------------------------------------+
   | kccs | TBD2  | map           | A CWT Claims Set (CCS) containing  |
   |      |       |               | a COSE_Key in a 'cnf' claim and    |
   |      |       |               | possibly other claims. CCS is      |
   |      |       |               | defined in RFC 8392.               |
   +------+-------+---------------+------------------------------------+

                  Figure 15: COSE header parameter labels.

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10.7.  The Well-Known URI Registry

   IANA is requested to add the well-known URI "edhoc" to the "Well-
   Known URIs" registry.

   *  URI suffix: edhoc

   *  Change controller: IETF

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

   *  Related information: None

10.8.  Media Types Registry

   IANA is requested to add the media types "application/edhoc+cbor-seq"
   and "application/cid-edhoc+cbor-seq" to the "Media Types" registry.

10.8.1.  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

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   *  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

10.8.2.  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

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   *  Author: See "Authors' Addresses" section.

   *  Change Controller: IESG

10.9.  CoAP Content-Formats Registry

   IANA is requested to add the media types "application/edhoc+cbor-seq"
   and "application/cid-edhoc+cbor-seq" to the "CoAP Content-Formats"
   registry under the registry group "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 16: CoAP Content-Format IDs

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

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

   *  Value: "core.edhoc"

   *  Description: EDHOC resource.

   *  Reference: [[this document]]

10.11.  Expert Review Instructions

   The IANA Registries established in this document are defined as
   "Expert Review", "Specification Required" or "Standards Action with
   Expert Review".  This section gives some general guidelines for what
   the experts should be looking for, but they are being designated as
   experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

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

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

   *  Even for "Expert Review" specifications are recommended.  When
      specifications are not provided for a request where Expert Review
      is the assignment policy, the description provided needs to have
      sufficient information to verify the code points above.

11.  References

11.1.  Normative References

   [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>.

   [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>.

   [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>.

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   [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>.

   [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>.

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

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

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

   [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>.

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   [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>.

   [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>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/info/rfc9053>.

   [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>.

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   [RFC9360]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header Parameters for Carrying and Referencing X.509
              Certificates", RFC 9360, DOI 10.17487/RFC9360, February
              2023, <https://www.rfc-editor.org/info/rfc9360>.

11.2.  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,
              <https://cbor.me/>.

   [CNSA]     NSA, "Commercial National Security Algorithm Suite",
              August 2015, <https://en.wikipedia.org/wiki/
              Commercial_National_Security_Algorithm_Suite>.

   [CottierPointcheval22]
              Cottier, B. and D. Pointcheval, "Security Analysis of the
              EDHOC protocol", September 2022,
              <https://arxiv.org/abs/2209.03599>.

   [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>.

   [GuentherIlunga22]
              Günther, F. and M. Ilunga, "Careful with MAc-then-SIGn: A
              Computational Analysis of the EDHOC Lightweight
              Authenticated Key Exchange Protocol", December 2022,
              <https://eprint.iacr.org/2022/1705>.

   [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://datatracker.ietf.org/doc/html/draft-arkko-arch-
              internet-threat-model-guidance-00>.

   [I-D.ietf-core-oscore-edhoc]
              Palombini, F., Tiloca, M., Höglund, R., Hristozov, S., and
              G. Selander, "Using Ephemeral Diffie-Hellman Over COSE
              (EDHOC) with the Constrained Application Protocol (CoAP)
              and Object Security for Constrained RESTful Environments
              (OSCORE)", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-edhoc-10, 29 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              oscore-edhoc-10>.

   [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-06, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              oscore-key-update-06>.

   [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-07, 20 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              cbor-encoded-cert-07>.

   [I-D.ietf-iotops-security-protocol-comparison]
              Mattsson, J. P., Palombini, F., and M. Vučinić,
              "Comparison of CoAP Security Protocols", Work in Progress,
              Internet-Draft, draft-ietf-iotops-security-protocol-
              comparison-03, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-iotops-
              security-protocol-comparison-03>.

   [I-D.ietf-lake-reqs]
              Vučinić, M., Selander, G., Mattsson, J. P., and D. Garcia-
              Carillo, "Requirements for a Lightweight AKE for OSCORE",
              Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
              8 June 2020, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lake-reqs-04>.

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   [I-D.ietf-lake-traces]
              Selander, G., Mattsson, J. P., Serafin, M., Tiloca, M.,
              and M. Vučinić, "Traces of EDHOC", Work in Progress,
              Internet-Draft, draft-ietf-lake-traces-08, 22 September
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              lake-traces-08>.

   [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://datatracker.ietf.org/doc/html/draft-ietf-lwig-
              curve-representations-23>.

   [I-D.ietf-rats-eat]
              Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
              Wallace, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-25, 15
              January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-eat-25>.

   [I-D.irtf-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-irtf-
              cfrg-det-sigs-with-noise-00, 8 August 2022,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              det-sigs-with-noise-00>.

   [I-D.selander-lake-authz]
              Selander, G., Mattsson, J. P., Vučinić, M., Richardson,
              M., and A. Schellenbaum, "Lightweight Authorization using
              Ephemeral Diffie-Hellman Over COSE", Work in Progress,
              Internet-Draft, draft-selander-lake-authz-03, 7 July 2023,
              <https://datatracker.ietf.org/doc/html/draft-selander-
              lake-authz-03>.

   [Jacomme23]
              Jacomme, C., Klein, E., Kremer, S., and M. Racouchot, "A
              comprehensive, formal and automated analysis of the EDHOC
              protocol", October 2022,
              <https://hal.inria.fr/hal-03810102/>.

   [NISTPQC]  "Post-Quantum Cryptography FAQs", August 2023,
              <https://csrc.nist.gov/Projects/post-quantum-cryptography/
              faqs>.

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   [Noise]    Perrin, T., "The Noise Protocol Framework, Revision 34",
              July 2018, <https://noiseprotocol.org/noise.html>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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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   [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>.

   [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>.

   [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>.

   [RFC9397]  Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", RFC 9397, DOI 10.17487/RFC9397, July 2023,
              <https://www.rfc-editor.org/info/rfc9397>.

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

   [SIGMA]    Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE-
              Protocols", June 2003,
              <https://www.iacr.org/cryptodb/archive/2003/
              CRYPTO/1495/1495.pdf>.

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   [SP-800-108]
              Chen, L., "Recommendation for Key Derivation Using
              Pseudorandom Functions", NIST Special Publication 800-108
              Revision 1, August 2022,
              <https://doi.org/10.6028/NIST.SP.800-108r1>.

   [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>.

   [SP800-185]
              John Kelsey, Shu-jen Chang, and Ray Perlner, "SHA-3
              Derived Functions cSHAKE, KMAC, TupleHash and
              ParallelHash", NIST Special Publication 800-185, December
              2016, <https://doi.org/10.6028/NIST.SP.800-185>.

   [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
   EDHOC is used to key OSCORE, and how to transfer EDHOC messages over
   CoAP.  The use of CoAP or OSCORE with EDHOC is optional, but if you
   are using CoAP or OSCORE, then certain normative requirements apply
   as detailed in the subsections.

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 SHALL be 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.

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      -  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 shorter or
         longer than the default oscore_salt_length.

      Master Secret = EDHOC_Exporter( 0, h'', oscore_key_length )
      Master Salt   = EDHOC_Exporter( 1, h'', oscore_salt_length )

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

   *  The HKDF Algorithm SHALL be 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.

   *  The relationship between identifiers in OSCORE and EDHOC is
      specified in Section 3.3.3.  The OSCORE Sender ID and Recipient ID
      SHALL be determined by the EDHOC connection identifiers C_R and
      C_I for the EDHOC session as shown in Figure 17.

       +-----------------+------------------+---------------------+
       |                 | OSCORE Sender ID | OSCORE Recipient ID |
       +=================+==================+=====================+
       | EDHOC Initiator |        C_R       |         C_I         |
       +-----------------+------------------+---------------------+
       | EDHOC Responder |        C_I       |         C_R         |
       +-----------------+------------------+---------------------+

           Figure 17: Usage of connection identifiers in OSCORE.

   Client and Server SHALL 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.

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A.2.  Transferring EDHOC over CoAP

   This section specifies how EDHOC can be transferred as an exchange of
   CoAP [RFC7252] messages.  CoAP provides a reliable transport that can
   preserve packet ordering, provides flow and congestion control, and
   handles message duplication.  CoAP can also perform fragmentation and
   mitigate certain 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.

   EDHOC may run with the Initiator either being CoAP client or CoAP
   server.  We denote the former by the "forward message flow" (see
   Appendix A.2.1) and the latter by the "reverse message flow" (see
   Appendix A.2.2).  By default, we assume the forward message flow, but
   the roles SHOULD be chosen to protect the most sensitive identity,
   see Section 9.

   According to this specification, EDHOC is transferred in POST
   requests to the Uri-Path: "/.well-known/edhoc" (see Section 10.7),
   and 2.04 (Changed) responses.  An application may define its own path
   that can be discovered, e.g., using a resource directory [RFC9176].
   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 10.10.  An alternative
   transfer of the forward message flow is specified in
   [I-D.ietf-core-oscore-edhoc].

   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 SHALL be prepended with the CBOR
   serialization of the connection identifier which the server has
   selected, see Section 3.4.1.  This applies both to the forward and
   the reverse message flows.  To indicate a new EDHOC session in the
   forward message flow, message_1 SHALL be prepended with the CBOR
   simple value true (0xf5).  Even if CoAP is carried over a reliable
   transport protocol such as TCP, the prepending of identifiers
   specified here SHALL be practiced to enable interoperability
   independent of how CoAP is transported.

   The prepended identifiers are encoded in CBOR and thus self-
   delimiting.  The representation of identifiers described in
   Section 3.3.2 SHALL be used.  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.

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   An EDHOC message has media type application/edhoc+cbor-seq, whereas
   an EDHOC message prepended by a connection identifier has media type
   application/cid-edhoc+cbor-seq, see Section 10.9.

   To mitigate certain 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 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 error messages need to be transported in response to a message
   that failed (see Section 6).  EDHOC error messages transported with
   CoAP are carried in the payload.

   Note that the transport over CoAP can serve as a blueprint for other
   client-server protocols:

   *  The client prepends the connection identifier selected by the
      server (or, for message_1, the CBOR simple value true) 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.

A.2.1.  The Forward Message Flow

   In the forward message flow the CoAP client is the Initiator and the
   CoAP server is the Responder.  This flow protects the client identity
   against active attackers and the server identity against passive
   attackers.

   In the forward message flow, the CoAP Token enables correlation on
   the Initiator (client) side, and the prepended C_R enables
   correlation on the Responder (server) side.

   *  EDHOC message_1 is sent in the payload of a POST request from the
      client to the server's resource for EDHOC, prepended with the
      identifier true (0xf5) indicating a new EDHOC session.

   *  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.2.3.

   *  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,
      prepended with the connection identifier C_R.

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   *  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 sent in response to message_4, it is sent analogously to
      error message sent in response to message_2.

   An example of a completed EDHOC session over CoAP in the forward
   message flow is shown in Figure 18.

       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 18: Example of the forward message flow.

   The forward message flow of EDHOC can be combined with an OSCORE
   exchange in a total of two round-trips, see
   [I-D.ietf-core-oscore-edhoc].

A.2.2.  The Reverse Message Flow

   In the reverse message flow the CoAP client is the Responder and the
   CoAP server is the Initiator.  This flow protects the server identity
   against active attackers and the client identity against passive
   attackers.

   In the reverse message flow, the CoAP Token enables correlation on
   the Responder (client) side, and the prepended C_I enables
   correlation on the Initiator (server) side.

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   *  To trigger a new EDHOC session, the client makes an empty POST
      request to the server's resource for EDHOC.

   *  EDHOC message_1 is sent from the server to the client in the
      payload of the response with response code 2.04 (Changed).

   *  EDHOC message_2 or the EDHOC error message is sent from the client
      to the server's resource in the payload of a POST request,
      prepended with the connection identifier C_I.

   *  EDHOC message_3 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.2.3.

   *  If EDHOC message_4 is used, or in case of an error message, it is
      sent from the client to the server's resource in the payload of a
      POST request, prepended with the connection identifier C_I.  In
      case of an error message sent in response to message_4, it is sent
      analogously to an error message sent in response to message_2.

   An example of a completed EDHOC session over CoAP in the reverse
   message flow is shown in Figure 19.

       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 19: Example of the reverse message flow.

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A.2.3.  Errors in EDHOC over CoAP

   When using EDHOC over CoAP, 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 10.9.

Appendix B.  Compact Representation

   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].

   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.

   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 ), where
   ceil(x) is the smallest integer not less than x.  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, see Section 9.2.  Using compact representation has 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].

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   The following may be needed for validation or compatibility with APIs
   that do not support compact representation or do not support the full
   [SECG] format:

   *  If a compressed y-coordinate is required, then the value ~yp set
      to zero can be used.  The compact representation described above
      can in such a case be transformed into the SECG point compressed
      format by prepending it with the single byte 0x02 (i.e., M =
      0x02 || X).

   *  If an uncompressed y-coordinate is required, then a y-coordinate
      has to be calculated following Section 2.3.4 of [SECG] or
      Appendix C of [RFC6090].  Any of the square roots (see [SECG] or
      [RFC6090]) can be used.  The uncompressed SECG format is M =
      0x04 || X || Y.

   For example: The curve P-256 has the parameters (using the notation
   in [RFC6090])

   *  p = 2^256 − 2^224 + 2^192 + 2^96 − 1

   *  a = -3

   *  b = 410583637251521421293261297800472684091144410159937255
      54835256314039467401291

   Given an example x:

   *  x = 115792089183396302095546807154740558443406795108653336
      398970697772788799766525

   we can calculate y as the square root w = (x^3 + a ⋅ x + b)^((p +
   1)/4) (mod p)

   *  y = 834387180070192806820075864918626005281451259964015754
      16632522940595860276856

   Note that this does not guarantee that (x, y) is on the correct
   elliptic curve.  A full validation according to Section 5.6.2.3.3 of
   [SP-800-56A] can be achieved by also checking that 0 ≤ x < p and that
   y^2 ≡ x^3 + a ⋅ x + b (mod p).

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

   This Appendix is intended to help implementors not familiar with CBOR
   [RFC8949], CDDL [RFC8610], COSE [RFC9052], 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 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.  We also make use of the occurrence
   symbol "*", like in e.g., 2* int, meaning two or more CBOR integers.

   For a complete specification and more examples, see [RFC8949] and
   [RFC8610].  We recommend implementors 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
        -----------------------------------------------------------

                Figure 20: Examples of use of CBOR and CDDL.

C.2.  CDDL Definitions

   This section compiles the CDDL definitions for ease of reference.

   suites = [ 2* int ] / int

   ead = (
     ead_label : int,
     ? ead_value : bstr,
   )

   EAD_1 = 1* ead
   EAD_2 = 1* ead
   EAD_3 = 1* ead
   EAD_4 = 1* ead

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

   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
   )

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   PLAINTEXT_2 = (
     C_R,
     ID_CRED_R : map / bstr / -24..23,
     Signature_or_MAC_2 : bstr,
     ? EAD_2,
   )

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

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

   message_4 = (
     CIPHERTEXT_4 : bstr,
   )

   PLAINTEXT_4 = (
     ? EAD_4,
   )

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

   info = (
     info_label : int,
     context : bstr,
     length : uint,
   )

C.3.  COSE

   CBOR Object Signing and Encryption (COSE) [RFC9052] 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:

   *  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 [RFC9053]

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   *  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 [RFC9052].  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
      [RFC9052].  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 [RFC9360] 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:

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

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

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   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 : kid_x }, where kid_x : kid, for x = I or R.  For
      further optimization, see Section 3.5.3.

   Note that ID_CRED_x 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 an ID_CRED field with COSE
   header parameter c5c or x5chain, defined in
   [I-D.ietf-cose-cbor-encoded-cert] and [RFC9360] and credentials of
   type CWT and CCS can be transported with the COSE header parameters
   registered in Section 10.6.

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.  Which verifications
   that 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.

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:

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   *  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 in 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 with 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
      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.

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   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
   with 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 9.8.  Some use cases may be served by short-
   lived credentials, for example, where the validity of the credential
   is on par with the interval between revocation checks.  But, in
   general, credential lifetime and revocation checking are
   complementary measures to control credential status.  Revocation
   information may be transported as External Authentication Data (EAD),
   see Appendix E.

D.5.  Unauthenticated Operation

   EDHOC might be used without authentication by allowing the Initiator
   or Responder to communicate with any identity except its own.  Note
   that EDHOC without mutual authentication is vulnerable to active on-
   path attacks and therefore unsafe for general use.  However, it is
   possible to later establish a trust relationship with an unknown or
   not-yet-trusted endpoint.  Some examples:

   *  The EDHOC authentication credential can be verified out-of-band at
      a later stage.

   *  The EDHOC session key can be bound to an identity out-of-band at a
      later stage.

   *  Trust on first use (TOFU) can be used to verify that several EDHOC
      connections are made to the same identity.  TOFU combined with
      proximity is a common IoT deployment model which provides good
      security if done correctly.  Note that secure proximity based on
      short range wireless technology requires very low signal strength
      or very low latency.

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 related external authorization 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-lake-authz].

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   *  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 9.1.

   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-lake-authz].

   The processing of the EAD item (ead_label, ? ead_value) by the
   security application needs to be described in the specification where
   the ead_label is registered, see Section 10.5, including the optional
   ead_value for each message and actions in case of errors.  An

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   application may support multiple security applications that make use
   of EAD, which may result in multiple EAD items in one EAD field, see
   Section 3.8.  Any dependencies on security applications with
   previously registered EAD items 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 9.5.  The content in an EAD item may impact the security
   properties provided by EDHOC.  Security applications making use of
   the EAD items must perform the necessary security analysis.

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 the 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 10.6

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

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   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.  Long PLAINTEXT_2

   By the definition of encryption of PLAINTEXT_2 with KEYSTREAM_2, it
   is limited to lengths of PLAINTEXT_2 not exceeding the output of
   EDHOC_KDF, see Section 4.1.2.  If the EDHOC hash algorithm is SHA-2
   then HKDF-Expand is used, which limits the length of the EDHOC_KDF
   output to 255 ⋅ hash_length, where hash_length is the length of the
   output of the EDHOC hash algorithm given by the cipher suite.  For
   example, with SHA-256 as EDHOC hash algorithm, the length of the hash
   output is 32 bytes and the maximum length of PLAINTEXT_2 is 255 ⋅ 32
   = 8160 bytes.

   While PLAINTEXT_2 is expected to be much shorter than 8 kB for the
   intended use cases, it seems nevertheless prudent to specify a
   solution for the event that this should turn out to be a limitation.

   A potential work-around is to use a cipher suite with a different
   hash function.  In particular, the use of KMAC removes all practical
   limitations in this respect.

   This section specifies a solution which works with any hash function,
   by making use of multiple invocations of HKDF-Expand and negative
   values of info_label.

   Consider the PLAINTEXT_2 partitioned in parts P(i) of length equal to
   M = 255 ⋅ hash_length, except possibly the last part P(last) which
   has 0 < length ≤ M.

   PLAINTEXT_2 = P(0) | P(1) | ... | P(last)

   where | indicates concatenation.

   The object is to define a matching KEYSTREAM_2 of the same length and
   perform the encryption in the same way as defined in Section 5.3.2:

   CIPHERTEXT_2 = PLAINTEXT_2 XOR KEYSTREAM_2

   Define the keystream as:

   KEYSTREAM_2 = OKM(0) | OKM(1)  | ... | OKM(last)

   where

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   OKM(i) = EDHOC_KDF( PRK_2e, -i, TH_2, length(P(i)) )

   Note that if length(PLAINTEXT_2) ≤ M then P(0) = PLAINTEXT_2 and the
   definition of KEYSTREAM_2 = OKM(0) coincides with Figure 8.

   This describes the processing of the Responder when sending
   message_2.  The Initiator makes the same calculations when receiving
   message_2, but interchanging PLAINTEXT_2 and CIPHERTEXT_2.

   An application profile may specify if it supports or not the method
   described in this appendix.

Appendix H.  EDHOC_KeyUpdate

   To provide forward secrecy in an even more efficient way than re-
   running EDHOC, this section specifies the optional function
   EDHOC_KeyUpdate in terms of EDHOC_KDF and PRK_out.

   When EDHOC_KeyUpdate is called, a new PRK_out is calculated as a
   "hash" of the old PRK_out using the EDHOC_Expand function as
   illustrated by the following pseudocode.  The change of PRK_out
   causes a change to PRK_exporter which enables the derivation of new
   application keys superseding the old ones, using EDHOC_Exporter, see
   Section 4.2.1.

    EDHOC_KeyUpdate( context ):
       new PRK_out = EDHOC_KDF( old PRK_out, 11, context, hash_length )
       new PRK_exporter = EDHOC_KDF( new PRK_out, 10, h'', hash_length )

   where hash_length denotes the output size in bytes of the EDHOC hash
   algorithm of the selected cipher suite.

   The EDHOC_KeyUpdate takes a context as input to enable binding of the
   updated PRK_out to some event that triggered the key update.  The
   Initiator and the Responder need to agree on the context, which can,
   e.g., be a counter, a pseudorandom number, or a hash.  To provide
   forward secrecy the old PRK_out and keys derived from it (old
   PRK_exporter and old application keys) must be deleted as soon as
   they are not needed.  When to delete the old keys and how to verify
   that they are not needed is up to the application.

   An application using EDHOC_KeyUpdate needs to store PRK_out.
   Compromise of PRK_out leads to compromise of all keying material
   derived with the EDHOC_Exporter since the last invocation of the
   EDHOC_KeyUpdate function.

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   While this key update method provides forward secrecy it does not
   give as strong security properties as re-running EDHOC.
   EDHOC_KeyUpdate can be used to meet cryptographic limits and provide
   partial protection against key leakage, but it provides significantly
   weaker security properties than re-running EDHOC with ephemeral
   Diffie-Hellman.  Even with frequent use of EDHOC_KeyUpdate,
   compromise of one session key compromises all future session keys,
   and an attacker therefore only needs to perform static key
   exfiltration [RFC7624], which is less complicated and has a lower
   risk profile than the dynamic case, see Section 9.1.

   A similar method to do key update for OSCORE is KUDOS, see
   [I-D.ietf-core-oscore-key-update].

Appendix I.  Example Protocol State Machine

   This appendix describes an example protocol state machine for the
   Initiator and for the Responder.  States are denoted in all capitals
   and parentheses denote actions taken only in some circumstances.

   Note that this state machine is just an example, and that details of
   processing are omitted, for example:

   *  When error messages are being sent (with one exception)

   *  How credentials and EAD are processed by EDHOC and the application
      in the RCVD state

   *  What verifications are made, which includes not only MACs and
      signatures

I.1.  Initiator State Machine

   The Initiator sends message_1, triggering the state machine to
   transition from START to WAIT_M2, and waits for message_2.

   If the incoming message is an error message then the Initiator
   transitions from WAIT_M2 to ABORTED.  In case of error code 2 (Wrong
   Selected Cipher Suite), the Initiator remembers the supported cipher
   suites for this particular Responder and transitions from ABORTED to
   START.  The message_1 that the Initiator subsequently sends takes
   into account the cipher suites supported by the Responder.

   Upon receiving a non-error message, the Initiator transitions from
   WAIT_M2 to RCVD_M2 and processes the message.  If a processing error
   occurs on message_2, then the Initiator transitions from RCVD_M2 to
   ABORTED.  In case of successful processing of message_2, the
   Initiator transitions from RCVD_M2 to VRFD_M2.

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   The Initiator prepares and processes message_3 for sending.  If any
   processing error is encountered, the Initiator transitions from
   VRFD_M2 to ABORTED.  If message_3 is successfully sent, the Initiator
   transitions from VRFD_M2 to COMPLETED.

   If the application profile includes message_4, then the Initiator
   waits for message_4.  If the incoming message is an error message
   then the Initiator transitions from COMPLETED to ABORTED.  Upon
   receiving a non-error message, the Initiator transitions from
   COMPLETED (="WAIT_M4") to RCVD_M4 and processes the message.  If a
   processing error occurs on message_4, then the Initiator transitions
   from RCVD_M4 to ABORTED.  In case of successful processing of
   message_4, the Initiator transitions from RCVD_M4 to PERSISTED
   (="VRFD_M4").

   If the application profile does not include message_4, then the
   Initiator waits for an incoming application message.  If the
   decryption and verification of the application message is successful,
   then the Initiator transitions from COMPLETED to PERSISTED.

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        +- - - - - - - - - -> START
        |                       |
                                | Send message_1
        |                       |
              Receive error     v
    ABORTED <---------------- WAIT_M2
        ^                       |
        |                       | Receive message_2
        |                       |
        |    Processing error   v
        +-------------------- RCVD_M2
        ^                       |
        |                       | Verify message_2
        |                       |
        |    Processing error   v
        +-------------------- VRFD_M2
        ^                       |
        |                       | Send message_3
        |                       |
        |    (Receive error)    v
        +-------------------- COMPLETED ----------------+
        ^                       |                       |
        |                       | (Receive message_4)   |
        |                       |                       |
        |   (Processing error)  v                       | (Verify
        +------------------- (RCVD_M4)                  |  application
                                |                       |  message)
                                | (Verify message_4)    |
                                |                       |
                                v                       |
                              PERSISTED <---------------+

I.2.  Responder State Machine

   Upon receiving message_1, the Responder transitions from START to
   RCVD_M1.

   If a processing error occurs on message_1, the Responder transitions
   from RCVD_M1 to ABORTED.  This includes sending error message with
   error code 2 (Wrong Selected Cipher Suite) if the selected cipher
   suite in message_1 is not supported.  In case of successful
   processing of message_1, the Responder transitions from RCVD_M1 to
   VRFD_M1.

   The Responder prepares and processes message_2 for sending.  If any
   processing error is encountered, the Responder transitions from
   VRFD_M1 to ABORTED.  If message_2 is successfully sent, the Initiator
   transitions from VRFD_M2 to WAIT_M3, and waits for message_3.

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   If the incoming message is an error message then the Responder
   transitions from WAIT_M3 to ABORTED.

   Upon receiving message_3, the Responder transitions from WAIT_M3 to
   RCVD_M3.  If a processing error occurs on message_3, the Responder
   transitions from RCVD_M3 to ABORTED.  In case of successful
   processing of message_3, the Responder transitions from RCVD_M3 to
   COMPLETED (="VRFD_M3").

   If the application profile includes message_4, the Responder prepares
   and processes message_4 for sending.  If any processing error is
   encountered, the Responder transitions from COMPLETED to ABORTED.

   If message_4 is successfully sent, or if the application profile does
   not include message_4, the Responder transitions from COMPLETED to
   PERSISTED.

                                        START
                                          |
                                          | Receive message_1
                                          |
                       Processing error   v
              ABORTED <---------------- RCVD_M1
                  ^                       |
                  |                       | Verify message_1
                  |                       |
                  |    Processing error   v
                  +-------------------- VRFD_M1
                  ^                       |
                  |                       | Send message_2
                  |                       |
                  |     Receive error     v
                  +-------------------- WAIT_M3
                  ^                       |
                  |                       | Receive message_3
                  |                       |
                  |    Processing error   v
                  +-------------------- RCVD_M3
                  ^                       |
                  |                       | Verify message_3
                  |                       |
                  |   (Processing error)  v
                  +------------------- COMPLETED
                                          |
                                          | (Send message_4)
                                          |
                                          v
                                       PERSISTED

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Appendix J.  Change Log

   RFC Editor: Please remove this appendix.

   *  From -21 to -22

      -  Normative text on transport capabilities.

   *  From -20 to -21

      -  Recommendation to use chain instead of bag

      -  Improved text about

         o  denial-of-service

         o  deriving secret and non-secret randomness from the same KDF
            instance

         o  practical security against quantum computers

      -  Clarifications, including

         o  several updates section 3.4.  Transport

         o  descriptions in COSE IANA registration

         o  encoding in Figure 5, reading of Figure 17

      -  Removed term "dummy"

      -  Harmonizing captions

      -  Updated references

      -  Acknowledgments

   *  From -19 to -20

      -  C_R encrypted in message_2

      -  C_R removed from TH_2

      -  Error code for unknown referenced credential

      -  Error code 0 (success) explicitly reserved

      -  Message deduplication section moved from appendix to body

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      -  Terminology

         o  discontinued -> aborted

         o  protocol run / exchange -> session

      -  Clarifications, in particular

         o  when to derive application keys

         o  the role of the application for authentication

      -  Security considerations for kccs and kcwt

      -  Updated references

   *  From -18 to -19

      -  Clarifications:

         o  Relation to SIGMA

         o  Role of Static DH

         o  Initiator and Responder roles

         o  Transport properties

         o  Construction of SUITES_I

         o  Message correlation, new subsection 3.4.1, replacing former
            appendix H

         o  Role of description about long PLAINTEXT_2

         o  ead_label and ead_value

         o  Message processing (Section 5)

         o  Padding

         o  Cipher suite negotiation example

      -  Other updates:

         o  Improved and stricter normative text in Appendix A

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         o  Naming and separate sections for the two message flows in
            Appendix A: Forward/Reverse message flow,

         o  Table index style captions

         o  Aligning with COSE terminology: header map -> header_map

         o  Aligning terminology, use of "_" instead of "-"

         o  Prefixing "EDHOC_" to functions

         o  Updated list of security analysis papers

         o  New appendix with example state machine

         o  Acknowledgements

         o  Language improvements by native English speakers

         o  Updated IANA section with registration procedures

         o  New and updated references

         o  Removed appendix H

   *  From -17 to -18

      -  Padding realised as EAD with ead_label = 0, PAD fields removed

      -  Revised EAD syntax; ead is now EAD item; ead_value is now
         optional

      -  Clarifications of

         o  Identifier representation

         o  Authentication credentials

         o  RPK

         o  Encoding of ID_CRED with kid

         o  Representation of public keys, y-coordinate of ephemeral
            keys and validation

         o  Processing after completed protocol

         o  Making verifications available to the application

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         o  Relation between EDHOC and OSCORE identifiers

      -  Terminology alignment in particular session / protocol;
         discontinue / terminate

      -  Updated CDDL

      -  Additional unicode encodings

      -  Large number of nits from WGLC

   *  From -16 to -17

      -  EDHOC-KeyUpdate moved to appendix

      -  Updated peer awareness properties based on SIGMA

      -  Clarify use of random connection identifiers

      -  Editorials related to appendix about messages with long
         PLAINTEXT_2

      -  Updated acknowledgments (have we forgotten someone else? please
         send email)

   *  From -15 to -16

      -  TH_2 used as salt in the derivation of PRK_2e

      -  CRED_R/CRED_I included in TH_3/TH_4

      -  Distinguish label used in info, exporter or elsewhere

      -  New appendix for optional handling arbitrarily large message_2

         o  info_label type changed to int to support this

      -  Updated security considerations

      -  Implementation note about identifiers which are bstr/int

      -  Clarifications, especifically about compact representation

      -  Type bug fix in CDDL section

   *  From -14 to -15

      -  Connection identifiers and key identifiers are now byte strings

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         o  Represented as CBOR bstr in the EDHOC message

            +  Unless they happen to encode a one-byte CBOR int

         o  More examples

      -  EAD updates and details

         o  Definition of EAD item

         o  Definition of critical / non-critical EAD item

      -  New section in Appendix D: Unauthenticated Operation

      -  Clarifications

         o  Lengths used in EDHOC-KDF

         o  Key derivation from PRK_out

            +  EDHOC-KeyUpdate and EDHOC-Exporter

         o  Padding

      -  Security considerations

         o  When a change in a message is detected

         o  Confidentiality in case of active attacks

         o  Connection identifiers should be unpredictable

         o  Maximum length of message_2

      -  Minor bugs

   *  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

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         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

         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

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         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

         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

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      -  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

      -  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

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      -  Added core.edhoc to CoRE Resource Type registry

   *  From -09 to -10:

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

      -  Info is a CBOR sequence and context is a bstr

      -  Added kid to UCCS example

      -  Separate header parameters for CWT and UCCS

      -  CWT Confirmation Method kid extended to bstr / int

   *  From -08 to -09:

      -  G_Y and CIPHERTEXT_2 are now included in one CBOR bstr

      -  MAC_2 and MAC_3 are now generated with EDHOC-KDF

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

      -  Restructured Section 4, Key Derivation

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

      -  More details on the use of CWT and UCCS

      -  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

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      -  Expanded on the use of COSE

   *  From -07 to -08:

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

      -  METHOD_CORR renamed to METHOD, corr removed

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

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

      -  Updated message sizes

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

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

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

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

      -  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

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

      -  New appendix on compact EC point representation

      -  Added detail on use of ephemeral public keys

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

      -  Additional security considerations

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

      -  Added encrypted EAD_4 to message_4

   *  From -05 to -06:

      -  New section 5.2 "Message Processing Outline"

      -  Optional inital byte C_1 = null in message_1

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

      -  Change of recommendation transport of error in CoAP

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

      -  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

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      -  Updated test vectors

      -  Updated applicability statement template

   *  From -03 to -04:

      -  Restructure of section 1

      -  Added references to C509 Certificates

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

      -  "K_2e", "IV_2e" -> KEYSTREAM_2

      -  Specified optional message 4

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

      -  Less constrained devices SHOULD implement both suite 0 and 2

      -  Clarification of error message

      -  Added exporter interface test vector

   *  From -02 to -03:

      -  Rearrangements of section 3 and beginning of section 4

      -  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

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      -  New section 4.3 clarifying bstr_identifier

      -  Updated security considerations

      -  Updated text on cipher suite negotiation and key confirmation

      -  Test vector for static DH

   *  From -00 to -01:

      -  Removed PSK method

      -  Removed references to certificate by value

Acknowledgments

   The authors want to thank Christian Amsüss, Alessandro Bruni,
   Karthikeyan Bhargavan, Carsten Bormann, Timothy Claeys, Baptiste
   Cottier, Roman Danyliw, Martin Disch, Martin Duke, Donald Eastlake,
   Lars Eggert, Stephen Farrell, Loïc Ferreira, Theis Grønbech Petersen,
   Felix Günther, Dan Harkins, Klaus Hartke, Russ Housley, Stefan
   Hristozov, Marc Ilunga, Charlie Jacomme, Elise Klein, Erik Kline,
   Steve Kremer, Alexandros Krontiris, Ilari Liusvaara, Rafa Marín-
   López, Kathleen Moriarty, David Navarro, Karl Norrman, Salvador
   Pérez, Radia Perlman, David Pointcheval, Maïwenn Racouchot, Eric
   Rescorla, Michael Richardson, Thorvald Sahl Jørgensen, Zaheduzzaman
   Sarker, Jim Schaad, Michael Scharf, Carsten Schürmann, John Scudder,
   Ludwig Seitz, Brian Sipos, Stanislav Smyshlyaev, Valery Smyslov,
   Peter van der Stok, Rene Struik, Vaishnavi Sundararajan, Erik
   Thormarker, Marco Tiloca, Sean Turner, Michel Veillette, Mališa
   Vučinić, Paul Wouters, and Lei Yan for reviewing and commenting on
   intermediate versions of the draft.  We are especially indebted to
   the late Jim Schaad for his continuous reviewing and implementation
   of early versions of this and other drafts.

   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

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   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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