Network Working Group                                        G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: May 6, June 21, 2021                                       Ericsson AB
                                                       November 02,
                                                       December 18, 2020

               Ephemeral Diffie-Hellman Over COSE (EDHOC)
                        draft-ietf-lake-edhoc-02
                        draft-ietf-lake-edhoc-03

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,
   perfect 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 footprint size can be kept very low.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Rationale for EDHOC . . . . . . . . . . . . . . . . . . .   4   5
     1.2.  Use of EDHOC  . . . . . . . . . . . . . . . . . . . . . .   5   6
     1.3.  Terminology and Requirements Language . . . . . . . . . .   6
   2.  Background  .  EDHOC Outline . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  EDHOC Overview  .  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Transport and Message Correlation  General . . . . . . . . . . . .   9 . . . . . . . . . . . . .   8
     3.2.  Authentication Keys  Method and Identities  . Correlation  . . . . . . . . . .  10
     3.3.  Identifiers . . . . . . .   9
     3.3.  Authentication Parameters . . . . . . . . . . . . . . . .  11
     3.4.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  11  14
     3.5.  Communication/Negotiation of Protocol Features  Ephemeral Public Keys . . . . .  12
     3.6.  Auxiliary Data . . . . . . . . . . . . .  16
     3.6.  Auxiliary Data  . . . . . . . .  13
     3.7.  Ephemeral Public Keys . . . . . . . . . . . . .  16
     3.7.  Communication of Protocol Features  . . . . .  13
     3.8.  Key Derivation . . . . . .  17
   4.  Key Derivation  . . . . . . . . . . . . . . .  13
   4.  EDHOC Authenticated with Asymmetric Keys . . . . . . . .  17
     4.1.  EDHOC-Exporter Interface  . .  16
     4.1.  Overview . . . . . . . . . . . . . .  19
   5.  Message Formatting and Processing . . . . . . . . . .  16
     4.2.  Encoding of Public Authentication Key Identifiers . . . .  16
     4.3.  20
     5.1.  Encoding of bstr_identifier . . . . . . . . . . . . . . .  18
     4.4.  20
     5.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  18
     4.5.  21
     5.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  20
     4.6.  23
     5.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  23
   5.  26
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  27
     5.1.  29
     6.1.  EDHOC Error Message . . . . . . . . . . . . . . . . . . .  27
   6.  29
   7.  Transferring EDHOC and Deriving an OSCORE Context . . . . . .  29
     6.1.  32
     7.1.  Transferring EDHOC in CoAP  . . . . . . . . . . . . . . .  29
   7.  32
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  32
     7.1.  35
     8.1.  Security Properties . . . . . . . . . . . . . . . . . . .  32
     7.2.  35
     8.2.  Cryptographic Considerations  . . . . . . . . . . . . . .  33
     7.3.  36
     8.3.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  34
     7.4.  37
     8.4.  Unprotected Data  . . . . . . . . . . . . . . . . . . . .  34
     7.5.  37
     8.5.  Denial-of-Service . . . . . . . . . . . . . . . . . . . .  35
     7.6.  38
     8.6.  Implementation Considerations . . . . . . . . . . . . . .  35
     7.7.  38
     8.7.  Other Documents Referencing EDHOC . . . . . . . . . . . .  36
   8.  39
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
     8.1.  39
     9.1.  EDHOC Cipher Suites Registry  . . . . . . . . . . . . . .  36
     8.2.  39
     9.2.  EDHOC Method Type Registry  . . . . . . . . . . . . . . .  37
     8.3.  41
     9.3.  The Well-Known URI Registry . . . . . . . . . . . . . . .  37
     8.4.  41
     9.4.  Media Types Registry  . . . . . . . . . . . . . . . . . .  38
     8.5.  41
     9.5.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  39
     8.6.  42
     9.6.  Expert Review Instructions  . . . . . . . . . . . . . . .  39
   9.  42
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     9.1.  43
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     9.2.  43
     10.2.  Informative References . . . . . . . . . . . . . . . . .  41  45
   Appendix A.  Use of CBOR, CDDL and COSE in EDHOC  . . . . . . . .  44  47
     A.1.  CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . .  44  47
     A.2.  COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  45  48
   Appendix B.  Test Vectors . . . . . . . . . . . . . . . . . . . .  45  48
     B.1.  Test Vectors for EDHOC Authenticated with Signature Keys
           (x5t) . . . . . . . . . . . . . . . . . . . . . . . . . .  45  48
     B.2.  Test Vectors for EDHOC Authenticated with Static Diffie-
           Hellman Keys  . . . . . . . . . . . . . . . . . . . . . .  60  63
   Appendix C.  Applicability Statement Template . . . . . . . . . .  76
     C.1.  Use of EDHOC in the XX Protocol . . . . . . . . . . . . .  76
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  73  77
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  73  77

1.  Introduction

   Security at the application layer provides an attractive option for
   protecting Internet of Things (IoT) deployments, for example where
   protection needs to work over a variety of underlying protocols.  IoT
   devices may be constrained in various ways, including memory,
   storage, processing capacity, and energy [RFC7228].  A method for
   protecting individual messages at the application layer suitable for
   constrained devices, is provided by CBOR Object Signing and
   Encryption (COSE) [RFC8152]), which builds on the Concise Binary
   Object Representation (CBOR) [RFC7049]. [RFC8949].  Object Security for
   Constrained RESTful Environments (OSCORE) [RFC8613] is a method for
   application-layer protection of the Constrained Application Protocol
   (CoAP), using COSE.

   In order for a communication session to provide forward secrecy, the
   communicating parties can run an Elliptic Curve Diffie-Hellman (ECDH)
   key exchange protocol with ephemeral keys, from which shared key
   material can be derived.  This document specifies Ephemeral Diffie-
   Hellman Over COSE (EDHOC), a lightweight key exchange protocol
   providing perfect forward secrecy and identity protection.
   Authentication is based on credentials established out of band, e.g.
   from a trusted third party, such as an Authorization Server as
   specified by [I-D.ietf-ace-oauth-authz].  The construction provided
   by EDHOC can be applied to authenticate raw public keys (RPK) and
   public key certificates.  This version of the protocol is focusing on
   RPK and certificates by reference which is the initial focus for the
   LAKE WG (see Section 2.2 of [I-D.ietf-lake-reqs]).

   After successful completion of the EDHOC protocol, application keys
   and other application specific data can be derived using the EDHOC-
   Exporter interface.  A main use case for EDHOC is to establish an
   OSCORE security context.  EDHOC uses COSE for cryptography, CBOR for
   encoding, and CoAP for transport.  By reusing existing libraries, the
   additional code footprint can be kept very low.

   EDHOC is designed for highly constrained settings making it
   especially suitable for low-power wide area networks [RFC8376] such
   as Cellular IoT, 6TiSCH, and LoRaWAN.  Compared to the DTLS 1.3
   handshake [I-D.ietf-tls-dtls13] with ECDH and connection ID, the
   number of bytes in EDHOC + CoAP can be less than 1/6 when RPK
   authentication is used, see
   [I-D.ietf-lwig-security-protocol-comparison].  Figure 1 shows two
   examples of message sizes for EDHOC with different kinds of
   authentication keys and different COSE header parameters for
   identification: static Diffie-Hellman keys identified by 'kid'
   [RFC8152], and X.509 signature certificates identified by a hash
   value using 'x5t' [I-D.ietf-cose-x509].  Further reductions of
   message sizes are possible, for example by eliding redundant length
   indications.

                    =================================
                                        kid       x5t
                    ---------------------------------
                    message_1            37        37
                    message_2            46       117
                    message_3            20        91
                    ----------------------------------
                    Total               103       245
                    =================================

               Figure 1: Example of message sizes in bytes.

   The ECDH exchange and the key derivation follow known protocol
   constructions such as [SIGMA], NIST SP-800-56A [SP-800-56A], and HKDF
   Extract-and-Expand [RFC5869].  CBOR [RFC7049] [RFC8949] and COSE [RFC8152] are
   used to implement these standards.  The use of COSE provides crypto
   agility and enables use of future algorithms and headers designed for
   constrained IoT.

   This document is organized as follows: Section 2 describes how EDHOC
   authenticated with digital signatures builds on SIGMA-I, Section 3
   specifies general properties of EDHOC, including message flow, flow and
   formatting of the ephemeral public keys, and Section 4 specifies the key
   derivation, Section 4 5 specifies EDHOC with signature key and static Diffie-
   Hellman
   Diffie-Hellman key authentication, Section 5 6 specifies the EDHOC
   error message, and Section 6 7 describes how EDHOC can be transferred
   in CoAP and used to establish an OSCORE security context.

1.1.  Rationale for EDHOC

   Many constrained IoT systems today do not use any security at all,
   and when they do, they often do not follow best practices.  One
   reason is that many current security protocols are not designed with
   constrained IoT in mind.  Constrained IoT systems often deal with
   personal information, valuable business data, and actuators
   interacting with the physical world.  Not only do such systems need
   security and privacy, they often need end-to-end protection with
   source authentication and perfect forward secrecy.  EDHOC and OSCORE
   [RFC8613] enables security following current best practices to
   devices and systems where current security protocols are impractical.

   EDHOC is optimized for small message sizes and can therefore be sent
   over a small number of radio frames.  The message size of a key
   exchange protocol may have a large impact on the performance of an
   IoT deployment, especially in constrained environments.  For example,
   in a network bootstrapping setting a large number of devices turned
   on in a short period of time may result in large latencies caused by
   parallel key exchanges.  Requirements on network formation time in
   constrained environments can be translated into key exchange
   overhead.  In network technologies with duty cycle, each additional
   frame significantly increases the latency even if no other devices
   are transmitting.

   Power consumption for wireless devices is highly dependent on message
   transmission, listening, and reception.  For devices that only send a
   few bytes occasionally, the battery lifetime may be impacted by a
   heavy key exchange protocol.  A key exchange may need to be executed
   more than once, e.g. due to a device rebooting or for security
   reasons such as perfect forward secrecy.

   EDHOC is adapted to primitives and protocols designed for the
   Internet of Things: EDHOC is built on CBOR and COSE which enables
   small message overhead and efficient parsing in constrained devices.
   EDHOC is not bound to a particular transport layer, but it is
   recommended to transport the EDHOC message in CoAP payloads.  EDHOC
   is not bound to a particular communication security protocol but
   works off-the-shelf with OSCORE [RFC8613] providing the necessary
   input parameters with required properties.  Maximum code complexity
   (ROM/Flash) is often a constraint in many devices and by reusing
   already existing libraries, the additional code footprint for EDHOC +
   OSCORE can be kept very low.

1.2.  Use of EDHOC

   EDHOC is designed as a lightweight AKE for OSCORE, i.e. to provide
   authentication and session key establishment for IoT use cases such
   as those built on CoAP [RFC7252].  CoAP is a specialized web transfer
   protocol for use with constrained nodes and networks, providing a
   request/response interaction model between application endpoints.  As
   such, EDHOC is targeting a large variety of use cases involving
   'things' with embedded microcontrollers, sensors and actuators.

   A typical setting is when one of the endpoints is constrained or in a
   constrained network, and the other endpoint is a node on the Internet
   (such as a mobile phone) or at the edge of the constrained network
   (such as a gateway).  Thing-to-thing interactions over constrained
   networks are also relevant since both endpoints would then benefit
   from the lightweight properties of the protocol.  EDHOC could e.g. be
   run when a device/device(s) connect(s) for the first time, or to
   establish fresh keys which are not compromised by a later compromise
   of the long-term keys.  (Further security properties are described in
   Section 7.1.) 8.1.)

1.3.  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 [RFC7049], [RFC8949], CBOR Sequences [RFC8742], COSE
   [RFC8152], and CDDL [RFC8610].  The Concise Data Definition Language
   (CDDL) is used to express CBOR data structures [RFC7049]. [RFC8949].  Examples
   of CBOR and CDDL are provided in Appendix A.1.

2.  Background  EDHOC Outline

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

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

     Initiator                                               Responder
        |                          G_X                            |
        +-------------------------------------------------------->|
        |                                                         |
        |  G_Y, AEAD( K_2; ID_CRED_R, Sig(R; CRED_R, G_X, G_Y) )  |
        |<--------------------------------------------------------+
        |                                                         |
        |     AEAD( K_3; ID_CRED_I, Sig(I; CRED_I, G_Y, G_X) )    |
        +-------------------------------------------------------->|
        |                                                         |

    Figure 2: Authenticated encryption variant of the SIGMA-I protocol.

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

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

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

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

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

   o  AEAD(K; . ) denotes authenticated encryption with additional data
      using a key K derived from the shared secret.

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

   o  Explicit connection identifiers C_I, C_R chosen by I and R,
      respectively, enabling the recipient to find the protocol state.

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

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

   o  Verification of a common preferred cipher suite:

      *  The Initiator lists supported cipher suites in order of
         preference

      *  The Responder verifies that the selected cipher suite is the
         first supported cipher suite

   o  Method types and error handling.

   o  Transport of opaque auxiliary data.

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

   To simplify for implementors, the use of CBOR and COSE in EDHOC is
   summarized in Appendix A and test vectors including CBOR diagnostic
   notation are given in Appendix B.

3.  EDHOC Overview  Protocol Elements

3.1.  General

   EDHOC consists of three messages (message_1, message_2, message_3)
   that maps directly to the three messages in SIGMA-I,
   between Initiator and Responder, plus an EDHOC error message.  EDHOC
   messages are CBOR Sequences [RFC8742], where see Figure 3.  The protocol
   elements in the first data item (METHOD_CORR) of message_1 is an int specifying figure are introduced in the method following sections.
   Message formatting and the correlation properties of the transport used, see processing is specified in Section 3.1.  The method specifies the authentication methods used
   (signature, static DH), see 5 and
   Section 8.2. 6.  An implementation may support only Initiator or Responder.  An implementation may support
   only a single method.  The Initiator and the Responder need to have
   agreed on a single method to be used for EDHOC.

   While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0
   structures, only a subset of the parameters
   Responder.

   Application data is included in the EDHOC
   messages.  The unprotected COSE header in COSE_Sign1, and
   COSE_Encrypt0 (not included in the EDHOC message) MAY contain
   parameters (e.g. 'alg').  After creating EDHOC message_3, the
   Initiator can derive symmetric application keys, and application protected data can therefore be sent in parallel with EDHOC
   message_3.  The application may protect data using the agreed application algorithms
   (AEAD, hash, etc.) hash) in the selected cipher suite (see Section 3.4) and the
   application can make use of the established connection identifiers (C_I, C_R).
   C_I and C_R (see Section 3.2.4).  EDHOC may be used with the media
   type application/edhoc defined in Section 8. 9.

   The Initiator can derive symmetric application keys after creating
   EDHOC message_3, see Section 4.1.  Application protected data can
   therefore be sent in parallel with EDHOC message_3, optionally in the
   same CoAP message [I-D.palombini-core-oscore-edhoc].

   Initiator                                                   Responder
   |               METHOD_CORR, SUITES_I, G_X, C_I, AD_1               |
   +------------------------------------------------------------------>|
   | ------------------ EDHOC                             message_1 -----------------> |                             |
   |                                                                   | <----------------- EDHOC message_2 ------------------ |
   |   C_I, G_Y, C_R, Enc(K_2e; ID_CRED_R, Signature_or_MAC_2, AD_2)   |
   |<------------------------------------------------------------------+
   | ------------------ EDHOC message_3 ----------------->                             message_2                             |
   |                                                                   |
   | <----------- Application Protected Data ------------>       C_R, AEAD(K_3ae; ID_CRED_I, Signature_or_MAC_3, AD_3)       |
   +------------------------------------------------------------------>|
   |                             message_3                             |

                       Figure 3: EDHOC message flow

3.1.  Transport and Message Correlation

   Cryptographically, EDHOC does not put requirements on the lower
   layers.  EDHOC is not bound to a particular transport layer, Flow

3.2.  Method and can
   be used in environments without IP. Correlation

   The transport first data item of message_1, METHOD_CORR (see Section 5.2.1), is responsible to
   handle message loss, reordering, message duplication, fragmentation,
   an integer specifying the method and denial the correlation properties of service protection,
   the transport, which are described in this section.

3.2.1.  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.  (Method 0 corresponds to the case outlined in
   Section 2 where necessary. both Initiator and Responder authenticate with
   signature keys.)

   An implementation may support only a single method.  The Initiator
   and the Responder need to have agreed on a transport single method to be used
   for
   EDHOC.  It is recommended to transport EDHOC in CoAP payloads, EDHOC, see
   Section 6. Appendix C.

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

                          Figure 4: Method Types

3.2.2.  Connection Identifiers

   EDHOC includes connection identifiers (C_I, C_R) to correlate
   messages.  The connection identifiers C_I and C_R do not have any
   cryptographic purpose in EDHOC.  They contain information
   facilitating retrieval of the protocol state and may therefore be
   very short.  One byte connection identifiers are realistic in many
   scenarios as most constrained devices only have a few connections.
   In cases where a node only has one connection, the identifiers may
   even be the empty byte string.

   The connection identifier MAY be used with an application protocol
   (e.g.  OSCORE) for which EDHOC establishes keys, in which case the
   connection identifiers SHALL adhere to the requirements for that
   protocol.  Each party choses a connection identifier it desires the
   other party to use in outgoing messages.  (For OSCORE this results in
   the endpoint selecting its Recipient ID, see Section 3.1 of
   [RFC8613]).

   If the transport

3.2.3.  Transport

   Cryptographically, EDHOC does not put requirements on the lower
   layers.  EDHOC is not bound to a particular transport layer, and can
   be used in environments without IP.  The transport is responsible to
   handle message loss, reordering, message duplication, fragmentation,
   and denial of service protection, where necessary.

   The Initiator and the Responder need to have agreed on a transport to
   be used for EDHOC, see Appendix C.  It is recommended to transport
   EDHOC in CoAP payloads, see Section 7.

3.2.4.  Message Correlation

   If the transport provides a mechanism for correlating messages, some
   of the connection identifiers may be omitted.  There are four cases:

   o  corr = 0, the transport does not provide a correlation mechanism.

   o  corr = 1, the transport provides a correlation mechanism that
      enables the Responder to correlate message_2 and message_1.

   o  corr = 2, the transport provides a correlation mechanism that
      enables the Initiator to correlate message_3 and message_2.

   o  corr = 3, the transport provides a correlation mechanism that
      enables both parties to correlate all three messages.

   For example, if the key exchange is transported over CoAP, the CoAP
   Token can be used to correlate messages, see Section 6.1.

3.2. 7.1.

3.3.  Authentication Parameters

3.3.1.  Authentication Keys and Identities

   The EDHOC message exchange may authentication key MUST be authenticated using raw public keys
   (RPK) or public a signature key certificates. or static Diffie-
   Hellman key.  The certificates Initiator and RPKs can
   contain the Responder MAY use different types
   of authentication keys, e.g. one uses a signature keys or key and the other
   uses a static Diffie-Hellman keys.  In X.509
   certificates, key.  When using a signature keys typically have key, the
   authentication is provided by a signature.  When using a static
   Diffie-Hellman key usage
   "digitalSignature" and the authentication is provided by a Message
   Authentication Code (MAC) computed from an ephemeral-static ECDH
   shared secret which enables significant reductions in message sizes.
   The MAC is implemented with an AEAD algorithm.  When using a static
   Diffie-Hellman keys typically the Initiator's and Responder's private
   authentication keys are called I and R, respectively, and the public
   authentication keys are called G_I and G_R, respectively.

   o  Only the Responder SHALL have key usage
   "keyAgreement". access to the Responder's private
      authentication key.

   o  Only the Initiator SHALL have access to the Initiator's private
      authentication key.

3.3.2.  Identities

   EDHOC assumes the existence of mechanisms (certification authority,
   trusted third party, manual distribution, etc.) for specifying and
   distributing authentication keys and identities.  Policies are set
   based on the identity of the other party, and parties typically only
   allow connections from a specific identity or a small restricted set
   of identities.  For example, in the case of a device connecting to a
   network, the network may only allow connections from devices which
   authenticate with certificates having a particular range of serial
   numbers in the subject field and signed by a particular CA.  On the
   other side, the device may only be allowed to connect to a network
   which authenticate with a particular public key (information of which
   may be provisioned, e.g., out of band or in the Auxiliary Data, see
   Section 3.6).

   The EDHOC implementation must be able to receive and enforce
   information from the application about what is the intended peer
   endpoint, and in particular whether it is a specific identity or a
   set of identities.

   o  When a Public Key Infrastructure (PKI) is used, the trust anchor
      is a Certification Authority (CA) certificate, and the identity is
      the subject whose unique name (e.g. a domain name, NAI, or EUI) is
      included in the endpoint's certificate.  Before running EDHOC each
      party needs at least one CA public key certificate, or just the
      public key, and a specific identity or set of identities it is
      allowed to communicate with.  Only validated public-key
      certificates with an allowed subject name, as specified by the
      application, are to be accepted.  EDHOC provides proof that the
      other party possesses the private authentication key corresponding
      to the public authentication key in its certificate.  The
      certification path provides proof that the subject of the
      certificate owns the public key in the certificate.

   o  When public keys are used but not with a PKI (RPK, self-signed
      certificate), the trust anchor is the public authentication key of
      the other party.  In this case, the identity is typically directly
      associated to the public authentication key of the other party.
      For example, the name of the subject may be a canonical
      representation of the public key.  Alternatively, if identities
      can be expressed in the form of unique subject names assigned to
      public keys, then a binding to identity can be achieved by
      including both public key and associated subject name in the
      protocol message computation: CRED_I or CRED_R may be a self-
      signed certificate or COSE_Key containing the public
      authentication key and the subject name, see Figure 2. Section 3.3.3.
      Before running EDHOC, each endpoint needs a specific public
      authentication key/unique associated subject name, or a set of
      public authentication keys/unique associated subject names, which
      it is allowed to communicate with.  EDHOC provides proof that the
      other party possesses the private authentication key corresponding
      to the public authentication key.

3.3.  Identifiers

   One byte connection

3.3.3.  Authentication Credentials

   The authentication credentials, CRED_I and credential identifiers are realistic in many
   scenarios as most constrained devices only have a few keys CRED_R, contain the public
   authentication key of the Initiator and
   connections.  In cases where a node only has one connection or key, the identifiers may even be Responder, respectively.
   The Initiator and the empty byte string.

3.4.  Cipher Suites

   EDHOC cipher suites consist of an ordered set Responder MAY use different types of COSE algorithms: an
   EDHOC AEAD algorithm, an EDHOC hash algorithm, an EDHOC ECDH curve,
   an EDHOC signature algorithm, an EDHOC signature algorithm curve,
   credentials, e.g. one uses an
   application AEAD algorithm, RPK and an application hash algorithm from the COSE Algorithms and Elliptic Curves registries.  Each cipher
   suite is identified with other uses a pre-defined int label.  This document
   specifies four pre-defined cipher suites.

      0. ( 10, -16, 4, -8, 6, 10, -16 )
         (AES-CCM-16-64-128, SHA-256, X25519, EdDSA, Ed25519,
          AES-CCM-16-64-128, SHA-256)

      1. ( 30, -16, 4, -8, 6, 10, -16 )
         (AES-CCM-16-128-128, SHA-256, X25519, EdDSA, Ed25519,
          AES-CCM-16-64-128, SHA-256)

      2. ( 10, -16, 1, -7, 1, 10, -16 )
         (AES-CCM-16-64-128, SHA-256, P-256, ES256, P-256,
          AES-CCM-16-64-128, SHA-256)

      3. ( 30, -16, 1, -7, 1, 10, -16 )
         (AES-CCM-16-128-128, SHA-256, P-256, ES256, P-256,
          AES-CCM-16-64-128, SHA-256) public key
   certificate.

   The different methods use the same cipher suites, but some algorithms credentials CRED_I and CRED_R are not used in some methods.  The EDHOC signature algorithm signed or MAC:ed (depending on
   method) by the Initiator and the
   EDHOC signature algorithm curve are not used Responder, respectively, see
   Section 5.4 and Section 5.3.

   When the credential is methods without
   signature authentication.

   The Initiator need to have a list of cipher suites it supports in
   order of preference.  The Responder need to certificate, CRED_x is an end-entity
   certificate (i.e. not the certificate chain) encoded as a CBOR bstr.
   In X.509 certificates, signature keys typically have key usage
   "digitalSignature" and Diffie-Hellman keys typically have key usage
   "keyAgreement"

   When the credential is a list of cipher
   suites it supports.

3.5.  Communication/Negotiation COSE_Key, CRED_x is a CBOR map only
   containing specific fields from the COSE_Key:

   o  For COSE_Keys of Protocol Features

   EDHOC allows type OKP the communication or negotiation CBOR map SHALL only include the
      parameters 1 (kty), -1 (crv), and -2 (x-coordinate).

   o  For COSE_Keys of various protocol
   features during type EC2 the execution CBOR map SHALL only include the
      parameters 1 (kty), -1 (crv), -2 (x-coordinate), and -3
      (y-coordinate).

   To prevent misbinding attacks in systems where an attacker can
   register public keys without proving knowledge of the protocol.

   o  The Initiator proposes private key,
   SIGMA [SIGMA] enforces a cipher suite (see Section 3.4), and MAC to be calculated over the
      Responder either accepts or rejects, "Identity",
   which in case of a X.509 certificate would be the 'subject' and may make
   'subjectAltName' fields.  EDHOC follows SIGMA by calculating a counter
      proposal.

   o  The Initiator decides MAC
   over the whole certificate.  While SIGMA paper only focuses on the correlation parameter corr (see
      Section 3.1).  This is typically given by
   identity, the transport which same principle is true for any information such as
   policies connected to the
      Initiator and public key.

   If the Responder parties have agreed on beforehand.  The
      Responder either accepts or rejects.

   o  The Initiator decides on an identity besides the method parameter, see Section 8.2.
      The Responder either accepts or rejects.

   o  The Initiator and public key, the Responder decide on
   identity is included in the representation of CBOR map with the identifier of their respective credentials, ID_CRED_I and
      ID_CRED_R.  The decision label "subject name",
   otherwise the subject name is reflected by the label used empty text string.  The parameters
   SHALL be encoded in the
      CBOR map, see for example Section 4.2.

3.6.  Auxiliary Data

   In decreasing order to reduce round trips with int labels first and number text
   string labels last.  An example of messages, CRED_x when the RPK contains an
   X25519 static Diffie-Hellman key and in some
   cases also streamline processing, certain security applications may
   be integrated into EDHOC by transporting auxiliary data together with the messages.  One example parties have agreed on an
   EUI-64 identity is the transport shown below:

   CRED_x = {
     1:  1,
    -1:  4,
    -2:  h'b1a3e89460e88d3a8d54211dc95f0b90
           3ff205eb71912d6db8f4af980d2db83a',
    "subject name" : "42-50-31-FF-EF-37-32-39"
   }

3.3.4.  Identification of third-party
   authorization information protected outside Credentials

   ID_CRED_I and ID_CRED_R are identifiers of EDHOC
   [I-D.selander-ace-ake-authz].  Another example is the embedding public authentication
   keys of a
   certificate enrolment request or a newly issued certificate.

   EDHOC allows opaque auxiliary data (AD) to be sent in the EDHOC
   messages.  Unprotected Auxiliary Data (AD_1, AD_2) may be sent in
   message_1 Initiator and message_2, the Responder, respectively.  Protected Auxiliary Data
   (AD_3) may be sent in message_3.

   Since data carried in AD_1  ID_CRED_I and AD_2 may
   ID_CRED_R do not be protected, and the
   content of AD_3 have any cryptographic purpose in EDHOC.

   o  ID_CRED_R is available intended to both facilitate for the Initiator and to retrieve
      the Responder,
   special considerations need Responder's public authentication key.

   o  ID_CRED_I is intended to be made such that facilitate for the availability of Responder to retrieve
      the data a) does not violate security Initiator's public authentication key.

   The identifiers ID_CRED_I and privacy requirements of the
   service which uses this data, and b) does not violate ID_CRED_R are COSE header_maps, i.e.
   CBOR maps containing COSE Common Header Parameters, see Section 3.1
   of [RFC8152]).  In the security
   properties following we give some examples of EDHOC.

3.7.  Ephemeral Public Keys

   The ECDH ephemeral COSE
   header_maps.

   Raw public keys are formatted most optimally stored as a COSE_Key of type
   EC2 or OKP according to Sections 13.1 objects and 13.2 of [RFC8152], but only
   the 'x' parameter is included in the EDHOC messages.  For Elliptic
   Curve Keys of type EC2, compact representation as per [RFC6090] MAY
   identified with a 'kid' parameter:

   o  ID_CRED_x = { 4 : kid_x }, where kid_x : bstr, for x = I or R.

   Public key certificates can be used also identified in different ways.  Header
   parameters for identifying X.509 certificates are defined in
   [I-D.ietf-cose-x509], for example:

   o  by a hash value with the COSE_Key.  If the COSE implementation requires an
   'y' parameter, any of 'x5t' parameter;

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

   o  by a URL with the possible values of 'x5u' parameter;

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

   ID_CRED_x MAY contain the y-coordinate can be
   used, see Appendix C of [RFC6090].  COSE [RFC8152] always use compact
   output actual credential used for Elliptic Curve Keys of type EC2.

3.8.  Key Derivation

   EDHOC uses HKDF [RFC5869] with authentication,
   CRED_x.  It is RECOMMENDED that they uniquely identify the EDHOC hash algorithm in public
   authentication key as the
   selected cipher suite recipient may otherwise have to derive try several
   keys.  HKDF-Extract is used to derive
   fixed-length uniformly pseudorandom keys (PRK) from ECDH shared
   secrets.  HKDF-Expand is used to derive additional output keying
   material (OKM) from the PRKs.  The PRKs are derived using HKDF-
   Extract [RFC5869].

      PRK = HKDF-Extract( salt, IKM )

   PRK_2e is used to derive key and IV to encrypt message_2.  PRK_3e2m
   is used to derive keys and IVs produce a MAC in message_2 and to
   encrypt message_3.  PRK_4x3m is used to derive keys  ID_CRED_I and IVs produce a
   MAC ID_CRED_R are transported in message_3 and to derive application specific data.

   PRK_2e is derived with the following input:

   o  The salt SHALL be the empty byte string.  Note that [RFC5869]
      specifies that if the salt is not provided, it is set to a string
      of zeros (see ciphertext, see
   Section 2.2 of [RFC5869]).  For implementation
      purposes, 5.4 and Section 5.3.

   When ID_CRED_x does not providing the salt is contain the same actual credential it may be very
   short.  One byte credential identifiers are realistic in many
   scenarios as setting most constrained devices only have a few keys.  In cases
   where a node only has one key, the salt
      to identifier may even be the empty
   byte string.

   o  The input keying material (IKM) SHALL be the ECDH shared secret
      G_XY (calculated from G_X and Y or G_Y and X) as defined in
      Section 12.4.1

3.4.  Cipher Suites

   An EDHOC cipher suite consists of [RFC8152].

   Example: Assuming the use an ordered set of SHA-256 COSE code points
   from the extract phase of HKDF
   produces PRK_2e as follows:

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

   where salt = 0x (the empty byte string).

   The pseudorandom keys PRK_3e2m "COSE Algorithms" and PRK_4x3m are defined as follow: "COSE Elliptic Curves" registries:

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

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

   Example: Assuming the use of curve25519, the ECDH shared secrets
   G_XY, G_RX, and G_IY are the outputs of the X25519 function
   [RFC7748]:

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

   The keys and IVs used in EDHOC are derived from PRK using HKDF-Expand
   [RFC5869] where the EDHOC-KDF is instantiated with the  EDHOC AEAD algorithm in the selected cipher suite.

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

   where info is the CBOR encoding of
   info = [
      edhoc_aead_id : int / tstr,
      transcript_hash : bstr,
      label : tstr,
      length : uint
   ]

   where

   o  edhoc_aead_id is an int or tstr containing the  EDHOC hash algorithm
      identifier of the

   o  EDHOC ECDH curve
   o  EDHOC signature algorithm

   o  EDHOC signature algorithm curve

   o  Application AEAD algorithm in the selected

   o  Application hash algorithm

   Each cipher suite encoded as defined in [RFC8152].  Note that a single fixed
      edhoc_aead_id is identified with a pre-defined int label.

   EDHOC can be used in with all invocations of EDHOC-KDF, including
      the derivation of K_2e algorithms and invocations of the EDHOC-Exporter.

   o  transcript_hash is a bstr set to curves defined for COSE.
   Implementation can either use one of the transcript hashes
      TH_2, TH_3, pre-defined cipher suites
   (Section 9.1) or TH_4 as defined in Sections 4.5.1, 4.6.1, and
      3.8.1.

   o  label is a tstr set use any combination of COSE algorithms to the name define
   their own private cipher suite.  Private cipher suites can be
   identified with any of the derived key or IV, i.e.
      "K_2m", "IV_2m", "K_2e", "K_3m", "IV_3m", "K_3ae", or "IV_3ae".

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

   K_2m and IV_2m four values -24, -23, -22, -21.

   The following cipher suites are derived using the transcript hash TH_2 and the
   pseudorandom key PRK_3e2m.  K_3ae and IV_3ae for constrained IoT where message
   overhead is a very important factor:

      0. ( 10, -16, 4, -8, 6, 10, -16 )
         (AES-CCM-16-64-128, SHA-256, X25519, EdDSA, Ed25519,
          AES-CCM-16-64-128, SHA-256)

      1. ( 30, -16, 4, -8, 6, 10, -16 )
         (AES-CCM-16-128-128, SHA-256, X25519, EdDSA, Ed25519,
          AES-CCM-16-64-128, SHA-256)

      2. ( 10, -16, 1, -7, 1, 10, -16 )
         (AES-CCM-16-64-128, SHA-256, P-256, ES256, P-256,
          AES-CCM-16-64-128, SHA-256)

      3. ( 30, -16, 1, -7, 1, 10, -16 )
         (AES-CCM-16-128-128, SHA-256, P-256, ES256, P-256,
          AES-CCM-16-64-128, SHA-256)

   The following cipher suite is for general non-constrained
   applications.  It uses very high performance algorithms that also are derived using the
   transcript hash TH_3 and the pseudorandom key PRK_3e2m.  K_3m
   widely supported:

      4. ( 1, -16, 4, -7, 1, 1, -16 )
         (A128GCM, SHA-256, X25519, ES256, P-256,
          A128GCM, SHA-256)

   The following cipher suite is for high security application such as
   government use and
   IV_3m are derived using financial applications.  It is compatible with the transcript hash TH_3 and
   CNSA suite [CNSA].

      5. ( 3, -43, 2, -35, 2, 3, -43 )
         (A256GCM, SHA-384, P-384, ES384, P-384,
          A256GCM, SHA-384)

   The different methods use the pseudorandom
   key PRK_4x3m.  IVs same cipher suites, but some algorithms
   are only not used if the in some methods.  The EDHOC AEAD signature algorithm uses
   IVs.

3.8.1.  EDHOC-Exporter Interface

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

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

   where label is a tstr defined by the application and length is a uint
   defined by the application.  The label SHALL be different for each
   different exporter value.
   EDHOC signature algorithm curve are not used in methods without
   signature authentication.

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

      TH_4 = H( TH_3, CIPHERTEXT_3 )

   where H() is the hash function in the selected cipher suite.  Example
   use list of the EDHOC-Exporter is given in Sections 6.1.1.

4.  EDHOC Authenticated with Asymmetric Keys

4.1.  Overview

   This section specifies authentication method = 0, 1, 2, and 3, see
   Section 8.2.  EDHOC cipher suites it supports authentication with signature or static
   Diffie-Hellman keys in the form
   order of raw public keys (RPK) and public
   key certificates with the requirements that:

   o  Only the preference.  The Responder SHALL have access needs to the Responder's private
      authentication key,

   o  Only the Initiator SHALL have access to a list of cipher
   suites it supports.  SUITES_I is a CBOR array containing cipher
   suites that the Initiator's private
      authentication key,

   o  The Initiator supports.  SUITES_I is able formatted and
   processed as detailed in Section 5.2.1 to retrieve secure the Responder's public
      authentication key using ID_CRED_R,

   o cipher suite
   negotation.

3.5.  Ephemeral Public Keys

   The Responder is able to retrieve the Initiator's ECDH ephemeral public
      authentication key using ID_CRED_I,

   where ID_CRED_I and ID_CRED_R keys are the identifiers formatted as a COSE_Key of the public
   authentication keys.  Their encoding is specified in Section 4.2.

   Initiator                                                   Responder
   |               METHOD_CORR, SUITES_I, G_X, C_I, AD_1               |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |   C_I, G_Y, C_R, Enc(K_2e; ID_CRED_R, Signature_or_MAC_2, AD_2)   |
   |<------------------------------------------------------------------+
   |                             message_2                             |
   |                                                                   |
   |       C_R, AEAD(K_3ae; ID_CRED_I, Signature_or_MAC_3, AD_3)       |
   +------------------------------------------------------------------>|
   |                             message_3                             |

      Figure 4: Overview of EDHOC with asymmetric key authentication.

4.2.  Encoding of Public Authentication Key Identifiers

   The identifiers ID_CRED_I type
   EC2 or OKP according to Sections 13.1 and ID_CRED_R are COSE header_maps, i.e.
   CBOR maps containing COSE Common Header Parameters, see Section 3.1 13.2 of [RFC8152]).  ID_CRED_I [RFC8152], but only
   the 'x' parameter is included G_X and ID_CRED_R need to contain parameters
   that can identify a public authentication key.  In G_Y.  For Elliptic Curve Keys
   of type EC2, compact representation as per [RFC6090] MAY be used also
   in the following
   paragraph we give some examples COSE_Key.  If the COSE implementation requires an 'y'
   parameter, any of the possible COSE header parameters
   used.

   Raw public keys are most optimally stored as COSE_Key objects and
   identified with a 'kid' parameter:

   o  ID_CRED_x = { 4 : kid_x }, where kid_x : bstr, for x = I or R.

   Public key certificates values of the y-coordinate can be identified in different ways.  Header
   parameters
   used, see Appendix C of [RFC6090].  COSE [RFC8152] always use compact
   output for identifying X.509 certificates are defined Elliptic Curve Keys of type EC2.

3.6.  Auxiliary Data

   In order to reduce round trips and number of messages, and in
   [I-D.ietf-cose-x509], for example:

   o some
   cases also streamline processing, certain security applications may
   be integrated into EDHOC by a hash value transporting auxiliary data together with
   the 'x5t' parameter;

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

   o  by a URL with messages.  One example is the 'x5u' parameter;

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

   The purpose transport of ID_CRED_I and ID_CRED_R third-party
   authorization information protected outside of EDHOC
   [I-D.selander-ace-ake-authz].  Another example is to facilitate retrieval the embedding of a public authentication key and when they do not contain
   certificate enrolment request or a newly issued certificate.

   EDHOC allows opaque auxiliary data (AD) to be sent in the actual
   credential, they EDHOC
   messages.  Unprotected Auxiliary Data (AD_1, AD_2) may be very short.  ID_CRED_I sent in
   message_1 and message_2, respectively.  Protected Auxiliary Data
   (AD_3) may be sent in message_3.

   Since data carried in AD_1 and AD_2 may not be protected, and ID_CRED_R MAY
   contain the actual credential used for authentication.  It
   content of AD_3 is
   RECOMMENDED that they uniquely identify available to both the public authentication key
   as Initiator and the recipient may otherwise have Responder,
   special considerations need to try several keys.  ID_CRED_I be made such that the availability of
   the data a) does not violate security and ID_CRED_R are transported in privacy requirements of the ciphertext, see Section 4.5.2
   service which uses this data, and Section 4.6.2.

   The authentication key MUST be a signature key b) does not violate the security
   properties of EDHOC.

3.7.  Communication of Protocol Features

   EDHOC allows the communication or static Diffie-
   Hellman key.  The Initiator and negotiation of various protocol
   features during the Responder MAY use different types execution of authentication keys, e.g. one uses the protocol.

   o  The Initiator proposes a signature key cipher suite (see Section 3.4), and the other
   uses a static Diffie-Hellman key.  When using
      Responder either accepts or rejects, and may make a signature key, counter
      proposal.

   o  The Initiator decides on the
   authentication correlation parameter corr (see
      Section 3.2.4).  This is provided typically given by a signature.  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 transport which enables significant reductions in message sizes.
   The MAC is implemented with an AEAD algorithm.  When using a static
   Diffie-Hellman keys
      the Initiator's and Responder's private
   authentication keys are called I and R, respectively, Initiator and the public
   authentication keys are called G_I and G_R, respectively. Responder have agreed on beforehand.  The actual credentials CRED_I and CRED_R are signed
      Responder either accepts or MAC:ed by the rejects.

   o  The Initiator and decides on the Responder respectively, method parameter, see Section 4.6.1 and
   Section 4.5.1. Figure 4.  The
      Responder either accepts or rejects.

   o  The Initiator and the Responder MAY use different
   types decide on the representation of
      the identifier of their respective credentials, e.g. one uses RPK ID_CRED_I and the other uses
   certificate.  When the credential is a certificate, CRED_x
      ID_CRED_R.  The decision is end-
   entity certificate (i.e. not reflected by the label used in the certificate chain) encoded as a
      CBOR
   bstr.  When map, see for example Section 3.3.4.

   Editor's note: This section needs to be aligned with Appendix C.

4.  Key Derivation

   EDHOC uses Extract-and-Expand [RFC5869] with the credential EDHOC hash algorithm
   in the selected cipher suite to derive keys.  Extract is a COSE_Key, CRED_x used to
   derive fixed-length uniformly pseudorandom keys (PRK) from ECDH
   shared secrets.  Expand is a CBOR map only
   contains specific fields used to derive additional output keying
   material (OKM) from the COSE_Key.  For COSE_Keys of type
   OKP the CBOR map SHALL only include PRKs.  The PRKs are derived using Extract.

      PRK = Extract( salt, IKM )

   If the parameters 1 (kty), -1 (crv),
   and -2 (x-coordinate).  For COSE_Keys of type EC2 EDHOC hash algorithm is SHA-2, then Extract( salt, IKM ) =
   HKDF-Extract( salt, IKM ) [RFC5869].  If the CBOR map SHALL
   only include EDHOC hash algorithm is
   SHAKE128, then Extract( salt, IKM ) = KMAC128( salt, IKM, 256, "" ).
   If the parameters 1 (kty), -1 (crv), -2 (x-coordinate), EDHOC hash algorithm is SHAKE256, then Extract( salt, IKM ) =
   KMAC256( salt, IKM, 512, "" ).

   PRK_2e is used to derive key and
   -3 (y-coordinate).  If IV to encrypt message_2.  PRK_3e2m
   is used to derive keys and IVs produce a MAC in message_2 and to
   encrypt message_3.  PRK_4x3m is used to derive keys and IVs produce a
   MAC in message_3 and to derive application specific data.

   PRK_2e is derived with the parties have agreed on an identity besides following input:

   o  The salt SHALL be the public key, empty byte string.  Note that [RFC5869]
      specifies that if the indentity salt is included in not provided, it is set to a string
      of zeros (see Section 2.2 of [RFC5869]).  For implementation
      purposes, not providing the CBOR map with salt is the
   label "subject name", otherwise same as setting the subject name is salt
      to the empty text byte string.

   o  The parameters input keying material (IKM) SHALL be encoded in decreasing order with int
   labels first the ECDH shared secret
      G_XY (calculated from G_X and text string labels last.  An example Y or G_Y and X) as defined in
      Section 12.4.1 of CRED_x when [RFC8152].

   Example: Assuming the RPK contains an X25519 static Diffie-Hellman key and the parties
   have agreed on an EUI-64 identity is shown below:

   CRED_x = {
     1:  1,
    -1:  4,
    -2:  h'b1a3e89460e88d3a8d54211dc95f0b90
           3ff205eb71912d6db8f4af980d2db83a',
    "subject name" : "42-50-31-FF-EF-37-32-39"
   }

4.3.  Encoding use of bstr_identifier

   A bstr_identifier is a special encoding for byte strings, used
   throughout SHA-256 the protocol.

   Byte strings extract phase of length greater than one are encoded HKDF
   produces PRK_2e as CBOR follows:

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

   where salt = 0x (the empty byte
   strings.  Byte strings of length one string).

   The pseudorandom keys PRK_3e2m and PRK_4x3m are encoded defined as follow:

   o  If the corresponding
   integer - 24.

   For example, the byte string h'59e9' encoded as Responder authenticates with a bstr_identifier is
   equal to h'59e9', while the byte string h'2a' static Diffie-Hellman key,
      then PRK_3e2m = Extract( PRK_2e, G_RX ), where G_RX is encoded as the
   integer 18.

   The CDDL definition of ECDH
      shared secret calculated from G_R and X, or G_X and R, else
      PRK_3e2m = PRK_2e.

   o  If the bstr_identifier Initiator authenticates with a static Diffie-Hellman key,
      then PRK_4x3m = Extract( PRK_3e2m, G_IY ), where G_IY is given below:

   bstr_identifier the ECDH
      shared secret calculated from G_I and Y, or G_Y and I, else
      PRK_4x3m = bstr / int

   Note that, despite what could be interpreted by PRK_3e2m.

   Example: Assuming the CDDL definition
   only, bstr_identifier once decoded use of curve25519, the ECDH shared secrets
   G_XY, G_RX, and G_IY are always byte strings.

4.4.  EDHOC Message 1

4.4.1.  Formatting the outputs of Message 1

   message_1 SHALL be a CBOR Sequence (see Appendix A.1) as defined
   below
   message_1 the X25519 function
   [RFC7748]:

      G_XY = (
     METHOD_CORR : int,
     SUITES_I : [ selected : suite, supported : 2* suite ] / suite, X25519( Y, G_X : bstr,
     C_I : bstr_identifier,
     ? AD_1 : bstr, )

   suite = int

   where:

   o  METHOD_CORR = 4 * method + corr, where method = 0, 1, 2, or 3 (see
      Section 8.2) X25519( X, G_Y )

   The keys and IVs used in EDHOC are derived from PRK using Expand
   [RFC5869] where the correlation parameter corr EDHOC-KDF is chosen based on
      the transport and determines which connection identifiers that are
      omitted (see Section 3.1).

   o  SUITES_I - cipher suites which instantiated with the Initiator supports EDHOC AEAD
   algorithm in order of
      (decreasing) preference.  The list of supported the selected cipher suites can
      be truncated at suite.

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

   where info is the end, as CBOR encoding of
   info = [
      edhoc_aead_id : int / tstr,
      transcript_hash : bstr,
      label : tstr,
      length : uint
   ]

   where

   o  edhoc_aead_id is detailed in an int or tstr containing the processing steps
      below.  One algorithm
      identifier of the supported cipher suites is selected.  If a
      single supported cipher suite is conveyed then that cipher suite
      is selected and EDHOC AEAD algorithm in the selected cipher
      suite is encoded as an int
      instead defined in [RFC8152].  Note that a single fixed
      edhoc_aead_id is used in all invocations of an array.

   o  G_X - EDHOC-KDF, including
      the ephemeral public key derivation of K_2e and invocations of the Initiator EDHOC-Exporter.

   o  C_I - variable length connection identifier, encoded as  transcript_hash is a
      bstr_identifier (see Section 4.3).

   o  AD_1 - bstr containing unprotected opaque auxiliary data

4.4.2.  Initiator Processing set to one of Message 1

   The Initiator SHALL compose message_1 as follows:

   o  The supported cipher suites and the order of preference MUST NOT
      be changed based on previous error messages.  However, the list
      SUITES_I sent transcript hashes
      TH_2, TH_3, or TH_4 as defined in Sections 5.3.1, 5.4.1, and 4.1.

   o  label is a tstr set to the Responder MAY be truncated such that cipher
      suites which are the least preferred are omitted.  The amount name of
      truncation MAY be changed between sessions, e.g. based on previous
      error messages (see next bullet), but all cipher suites which are
      more preferred than the least preferred cipher suite in the list
      MUST be included in the list. derived key or IV, i.e.
      "K_2m", "IV_2m", "K_2e", "IV_2e", "K_3m", "IV_3m", "K_3ae", or
      "IV_3ae".

   o  Determine the cipher suite to use with  length is the Responder length of output keying material (OKM) in message_1. bytes

   If the Initiator previously received from the Responder an error
      message to a message_1 with diagnostic payload identifying a
      cipher suite that EDHOC hash algorithm is SHA-2, then Expand( PRK, info, length
   ) = HKDF-Expand( PRK, info, length ) [RFC5869].  If the Initiator supports, EDHOC hash
   algorithm is SHAKE128, then Expand( PRK, info, length ) = KMAC128(
   PRK, info, L, "" ).  If the Initiator SHALL
      use that cipher suite.  Otherwise EDHOC hash algorithm is SHAKE256, then
   Expand( PRK, info, length ) = KMAC256( PRK, info, L, "" ).

   K_2e and IV_2e are derived using the first supported (i.e. transcript hash TH_2 and the
      most preferred) cipher suite in SUITES_I MUST be used.

   o  Generate an ephemeral ECDH
   pseudorandom key pair as specified in Section 5 of
      [SP-800-56A] PRK_2e.  K_2m and IV_2m are derived using the curve in the selected cipher suite
   transcript hash TH_2 and
      format it as a COSE_Key.  Let G_X be the 'x' parameter of the
      COSE_Key.

   o  Choose a connection identifier C_I pseudorandom key PRK_3e2m.  K_3ae and store it for the length of
   IV_3ae are derived using the protocol.

   o  Encode message_1 as a sequence of CBOR encoded data items as
      specified in Section 4.4.1

4.4.3.  Responder Processing of Message 1

   The Responder SHALL process message_1 as follows:

   o  Decode message_1 (see Appendix A.1).

   o  Verify that transcript hash TH_3 and the selected cipher suite is supported
   pseudorandom key PRK_3e2m.  K_3m and that no
      prior cipher suites in SUITES_I IV_3m are supported.

   o  Pass AD_1 to derived using the security application.

   If any verification step fails,
   transcript hash TH_3 and the pseudorandom key PRK_4x3m.  IVs are only
   used if the Initiator MUST send an EDHOC
   error message back, formatted as AEAD algorithm uses IVs.

4.1.  EDHOC-Exporter Interface

   Application keys and other application specific data can be derived
   using the EDHOC-Exporter interface defined in Section 5, as:

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

   where label is a tstr defined by the application and length is a uint
   defined by the
   protocol MUST application.  The label SHALL be discontinued.  If different for each
   different exporter value.  The transcript hash TH_4 is a CBOR encoded
   bstr and the Responder does not support input to the
   selected cipher suite, then SUITES_R MUST include one or more
   supported cipher suites.  If hash function is a CBOR Sequence.

      TH_4 = H( TH_3, CIPHERTEXT_3 )

   where H() is the Responder does not support hash function in the selected cipher suite, but supports another cipher suite in SUITES_I,
   then SUITES_R MUST include suite.  Example
   use of the first supported cipher suite EDHOC-Exporter is given in
   SUITES_I.

4.5.  EDHOC Message 2

4.5.1.  Formatting of Message 2

   message_2 Sections 7.1.1.

   To provide forward secrecy in an even more efficient way than re-
   running EDHOC, EDHOC provides the function EDHOC-Exporter-FS.  When
   EHDOC-Exporter-FS is called the old PRK_4x3m is deleted and data_2 SHALL be CBOR Sequences (see Appendix A.1) the new
   PRk_4x3m is calculated as
   defined below

   message_2 = (
     data_2,
     CIPHERTEXT_2 : bstr,
   )
   data_2 = (
     ? C_I : bstr_identifier,
     G_Y : bstr,
     C_R : bstr_identifier,
   )

   where:

   o  G_Y - a "hash" of the ephemeral public old key of using the Responder

   o  C_R - variable length connection identifier, encoded Extract
   function as a
      bstr_identifier (see Section 4.3).

4.5.2.  Responder illustrated by the following pseudocode:

      EHDOC-Exporter-FS( nonce ):
         PRK_4x3m = Extract( [ "TH_4", nonce ], PRK_4x3m )

5.  Message Formatting and Processing

   This section specifies formatting of Message 2

   The Responder SHALL compose message_2 as follows:

   o  If corr (METHOD_CORR mod 4) equals 1 or 3, C_I is omitted,
      otherwise C_I is not omitted.

   o  Generate an ephemeral ECDH key pair as the messages and processing
   steps.  Error messages are specified in Section 5 of
      [SP-800-56A] using the curve in the selected cipher suite and
      format it 6.

   An EDHOC message is encoded as a COSE_Key.  Let G_Y be the 'x' parameter sequence of CBOR data (CBOR
   Sequence, [RFC8742]).  Additional optimizations are made to reduce
   message overhead.

   While EDHOC uses the
      COSE_Key.

   o  Choose a connection identifier C_R COSE_Key, COSE_Sign1, and store it for the length COSE_Encrypt0
   structures, only a subset of the protocol.

   o  Compute the transcript hash TH_2 = H(message_1, data_2) where H() parameters is the hash function included in the selected cipher suite. EDHOC
   messages.  The transcript
      hash TH_2 is a CBOR encoded bstr unprotected COSE header in COSE_Sign1, and
   COSE_Encrypt0 (not included in the input to the hash
      function is a EDHOC message) MAY contain
   parameters (e.g. 'alg').

5.1.  Encoding of bstr_identifier

   Byte strings are encoded in CBOR Sequence.

   o  Compute an inner COSE_Encrypt0 as defined two or more bytes, whereas
   integers in Section 5.3 of
      [RFC8152], with the EDHOC AEAD algorithm interval -24 to 23 are encoded in CBOR as one byte.

   bstr_identifier is a special encoding of byte strings, used
   throughout the selected cipher
      suite, K_2m, IV_2m, and the following parameters:

      *  protected = << ID_CRED_R >>

         +  ID_CRED_R - identifier protocol to facilitate retrieval of CRED_R,
            see Section 4.2

      *  external_aad = << TH_2, CRED_R, ? AD_2 >>

         +  CRED_R - bstr containing enable the credential encoding of the Responder,
            see Section 4.2.

         +  AD_2 = bstr containing opaque unprotected auxiliary data

      *  plaintext = h''

      COSE constructs the input to the AEAD [RFC5116] shortest byte
   strings as follows:

      *  Key K = EDHOC-KDF( PRK_3e2m, TH_2, "K_2m", length )

      *  Nonce N = EDHOC-KDF( PRK_3e2m, TH_2, "IV_2m", length )

      *  Plaintext P = 0x (the empty string)

      *  Associated data A =

         [ "Encrypt0", << ID_CRED_R >>, << TH_2, CRED_R, ? AD_2 >> ]

      MAC_2 integers that only require one byte of CBOR encoding.

   The bstr_identifier encoding is defined as follows: Byte strings in
   the 'ciphertext' of interval h'00' to h'2f' are encoded as the inner COSE_Encrypt0.

   o  If corresponding integer
   minus 24, which are all represented by one byte CBOR ints.  Other
   byte strings are encoded as CBOR byte strings.

   For example, the Reponder authenticates with byte string h'59e9' encoded as a static Diffie-Hellman key
      (method equals 1 or 3), then Signature_or_MAC_2 bstr_identifier is MAC_2.  If
   equal to h'59e9', while the
      Reponder authenticates with a signature key (method equals 0 or
      2), then Signature_or_MAC_2 byte string h'2a' is encoded as the 'signature'
   integer 18.

   The CDDL definition of the bstr_identifier is given below:

   bstr_identifier = bstr / int

   Note that, despite what could be interpreted by the CDDL definition
   only, bstr_identifier once decoded are always byte strings.

5.2.  EDHOC Message 1

5.2.1.  Formatting of Message 1

   message_1 SHALL be a COSE_Sign1
      object CBOR Sequence (see Appendix A.1) as defined in Section 4.4 of [RFC8152] using the signature
      algorithm in the
   below

   message_1 = (
     METHOD_CORR : int,
     SUITES_I : [ selected cipher : suite, the private authentication
      key of the Responder, and the following parameters:

      *  protected supported : 2* suite ] / suite,
     G_X : bstr,
     C_I : bstr_identifier,
     ? AD_1 : bstr,
   )

   suite = << ID_CRED_R >>

      *  external_aad int

   where:

   o  METHOD_CORR = << TH_2, CRED_R, ? AD_2 >> 4 *  payload method + corr, where method = MAC_2

      COSE constructs 0, 1, 2, or 3 (see
      Figure 4) and the input to correlation parameter corr is chosen based on
      the Signature Algorithm as:

      * transport and determines which connection identifiers that are
      omitted (see Section 3.2.4).

   o  SUITES_I - cipher suites which the Initiator supports in order of
      (decreasing) preference.  The key list of supported cipher suites can
      be truncated at the end, as is detailed in the private authentication key processing steps
      below.  One of the Responder.

      * supported cipher suites is selected.  The message M to be signed =

         [ "Signature1", << ID_CRED_R >>, << TH_2, CRED_R, ? AD_2 >>,
         MAC_2 ]

   o  CIPHERTEXT_2
      selected suite is the ciphertext resulting from XOR encrypting a
      plaintext with first suite in the following common parameters:

      *  plaintext = ( ID_CRED_R / bstr_identifier, Signature_or_MAC_2,
         ? AD_2 )

         +  Note that if ID_CRED_R contains SUITES_I CBOR array.  If
      a single 'kid' parameter,
            i.e., ID_CRED_R = { 4 : kid_R }, only the byte string kid_R supported cipher suite is conveyed in then that cipher suite
      is selected and the plaintext selected cipher suite is encoded as a bstr_identifier,
            see Section 4.2 and Section 4.3.

      *  CIPHERTEXT_2 = plaintext XOR K_2e

      *  K_2e = EDHOC-KDF( PRK_2e, TH_2, "K_2e", length ), where length
         is an int
      instead of an array.

   o  G_X - the length ephemeral public key of the plaintext. Initiator

   o  Encode message_2 as a sequence of CBOR  C_I - variable length connection identifier, encoded data items as
      specified in a
      bstr_identifier (see Section 4.5.1.

4.5.3. 5.1).

   o  AD_1 - bstr containing unprotected opaque auxiliary data

5.2.2.  Initiator Processing of Message 2 1

   The Initiator SHALL process message_2 compose message_1 as follows:

   o  Decode message_2 (see Appendix A.1).

   o  Retrieve the protocol state using the connection identifier C_I
      and/or other external information such as the CoAP Token  The supported cipher suites and the
      5-tuple.

   o  Decrypt CIPHERTEXT_2.  The decryption process depends order of preference MUST NOT
      be changed based on previous error messages.  However, the
      method, see Section 4.5.2.

   o  Verify list
      SUITES_I sent to the Responder MAY be truncated such that cipher
      suites which are the identity least preferred are omitted.  The amount of
      truncation MAY be changed between sessions, e.g. based on previous
      error messages (see next bullet), but all cipher suites which are
      more preferred than the Responder is an allowed identity
      for this connection, see Section 3.2.

   o  Verify Signature_or_MAC_2 using the algorithm least preferred cipher suite in the selected
      cipher suite.  The verification process depends on list
      MUST be included in the method, see
      Section 4.5.2. list.

   o  Pass AD_2  Determine the cipher suite to use with the Responder in message_1.
      If the Initiator previously received from the Responder an error
      message to a message_1 with diagnostic payload identifying a
      cipher suite that the Initiator supports, then the Initiator SHALL
      use that cipher suite.  Otherwise the first supported (i.e. the
      most preferred) cipher suite in SUITES_I MUST be used.

   o  Generate an ephemeral ECDH key pair as specified in Section 5 of
      [SP-800-56A] 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.

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

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

5.2.3.  Responder Processing of Message 1

   The Responder SHALL process message_1 as follows:

   o  Decode message_1 (see Appendix A.1).

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

   o  Pass AD_1 to the security application.

   If any verification step fails, the Responder MUST send an EDHOC
   error message back, formatted as defined in Section 5, 6, and the
   protocol MUST be discontinued.

4.6.  If the Responder does not support the
   selected cipher suite, then SUITES_R MUST include one or more
   supported cipher suites.  If the Responder does not support the
   selected cipher suite, but supports another cipher suite in SUITES_I,
   then SUITES_R MUST include the first supported cipher suite in
   SUITES_I.

5.3.  EDHOC Message 3

4.6.1. 2

5.3.1.  Formatting of Message 3

   message_3 2

   message_2 and data_3 data_2 SHALL be CBOR Sequences (see Appendix A.1) as
   defined below

   message_3

   message_2 = (
     data_3,
     CIPHERTEXT_3
     data_2,
     CIPHERTEXT_2 : bstr,
   )
   data_3

   data_2 = (
     ? C_I : bstr_identifier,
     G_Y : bstr,
     C_R : bstr_identifier,
   )

4.6.2.  Initiator Processing

   where:

   o  G_Y - the ephemeral public key of Message 3

   The Initiator SHALL compose message_3 as follows: the Responder

   o  If corr (METHOD_CORR  C_R - variable length connection identifier, encoded as a
      bstr_identifier (see Section 5.1).

5.3.2.  Responder Processing of Message 2

   The Responder SHALL compose message_2 as follows:

   o  If corr (METHOD_CORR mod 4) equals 2 1 or 3, C_R C_I is omitted,
      otherwise C_R C_I is not omitted.

   o  Generate an ephemeral ECDH key pair as specified in Section 5 of
      [SP-800-56A] 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.

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

   o  Compute the transcript hash TH_3 TH_2 = H(TH_2 , CIPHERTEXT_2, data_3) H(message_1, data_2) where H()
      is the hash function in the the selected cipher suite.  The transcript
      hash TH_3 TH_2 is a CBOR encoded bstr and the input to the hash
      function is a CBOR Sequence.

   o  Compute an inner COSE_Encrypt0 as defined in Section 5.3 of
      [RFC8152], with the EDHOC AEAD algorithm in the selected cipher
      suite, K_3m, IV_3m, K_2m, IV_2m, and the following parameters:

      *  protected = << ID_CRED_I ID_CRED_R >>

         +  ID_CRED_I  ID_CRED_R - identifier to facilitate retrieval of CRED_I, CRED_R,
            see Section 4.2 3.3.4

      *  external_aad = << TH_3, CRED_I, TH_2, CRED_R, ? AD_3 AD_2 >>

         +  CRED_I  CRED_R - bstr containing the credential of the Initiator, Responder,
            see Section 4.2. 3.3.4.

         +  AD_3  AD_2 = bstr containing opaque protected unprotected auxiliary data

      *  plaintext = h''

      COSE constructs the input to the AEAD [RFC5116] as follows:

      *  Key K = EDHOC-KDF( PRK_4x3m, TH_3, "K_3m", PRK_3e2m, TH_2, "K_2m", length )

      *  Nonce N = EDHOC-KDF( PRK_4x3m, TH_3, "IV_3m", PRK_3e2m, TH_2, "IV_2m", length )

      *  Plaintext P = 0x (the empty string)

      *  Associated data A =

         [ "Encrypt0", << ID_CRED_I ID_CRED_R >>, << TH_3, CRED_I, TH_2, CRED_R, ? AD_3 AD_2 >> ]

      MAC_3

      MAC_2 is the 'ciphertext' of the inner COSE_Encrypt0.

   o  If the Initiator Responder authenticates with a static Diffie-Hellman key
      (method equals 2 1 or 3), then Signature_or_MAC_3 Signature_or_MAC_2 is MAC_3. MAC_2.  If the
      Initiator
      Responder authenticates with a signature key (method equals 0 or
      1),
      2), then Signature_or_MAC_3 Signature_or_MAC_2 is the 'signature' of a COSE_Sign1
      object as defined in Section 4.4 of [RFC8152] using the signature
      algorithm in the selected cipher suite, the private authentication
      key of the Initiator, Responder, and the following parameters:

      *  protected = << ID_CRED_I ID_CRED_R >>

      *  external_aad = << TH_3, CRED_I, TH_2, CRED_R, ? AD_3 AD_2 >>

      *  payload = MAC_3 MAC_2
      COSE constructs the input to the Signature Algorithm as:

      *  The key is the private authentication key of the Initiator. Responder.

      *  The message M to be signed =

         [ "Signature1", << ID_CRED_I ID_CRED_R >>, << TH_3, CRED_I, TH_2, CRED_R, ? AD_3 AD_2 >>,
         MAC_3
         MAC_2 ]

   o  Compute an outer COSE_Encrypt0 as defined in Section 5.3 of
      [RFC8152], with the EDHOC AEAD algorithm in the selected cipher
      suite, K_3ae, IV_3ae, K_2e, IV_2e, and the following parameters.  The protected
      header SHALL be empty.

      *  external_aad = TH_3

      *  plaintext = ( ID_CRED_I ID_CRED_R / bstr_identifier, Signature_or_MAC_3, Signature_or_MAC_2,
         ? AD_3 AD_2 )

         +  Note that if ID_CRED_I ID_CRED_R contains a single 'kid' parameter,
            i.e., ID_CRED_I ID_CRED_R = { 4 : kid_I kid_R }, only the byte string kid_I kid_R
            is conveyed in the plaintext encoded as a bstr_identifier,
            see Section 4.2 3.3.4 and Section 4.3. 5.1.

      COSE constructs the input to the AEAD [RFC5116] as follows:

      *  Key K = EDHOC-KDF( PRK_3e2m, TH_3, "K_3ae", PRK_2e, TH_2, "K_2e", length )

      *  Nonce N = EDHOC-KDF( PRK_3e2m, TH_3, "IV_3ae", PRK_2e, TH_2, "IV_2e", length )

      *  Plaintext P = ( ID_CRED_I ID_CRED_R / bstr_identifier,
         Signature_or_MAC_3,
         Signature_or_MAC_2, ? AD_3 AD_2 )

      *  Associated data A = [ "Encrypt0", h'', TH_3 TH_2 ]
      CIPHERTEXT_3

      CIPHERTEXT_2 is the 'ciphertext' of the outer COSE_Encrypt0. COSE_Encrypt0 with
      the tag removed.

   o  Encode message_3 message_2 as a sequence of CBOR encoded data items as
      specified in Section 4.6.1.

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

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

4.6.3.  Responder Processing of Message 3

   The Responder SHALL process message_3 as follows:

   o  Decode message_3 (see Appendix A.1).

   o  Retrieve the protocol state using 5.3.1.

5.3.3.  Initiator Processing of Message 2

   The Initiator SHALL process message_2 as follows:

   o  Decode message_2 (see Appendix A.1).

   o  Retrieve the protocol state using the connection identifier C_R C_I
      and/or other external information such as the CoAP Token and the
      5-tuple.

   o  Decrypt and verify the CIPHERTEXT_2 by computing an outer COSE_Encrypt0 as
      defined in see Section 5.3 of [RFC8152], 5.3.2 and XORing CIPHERTEXT_2 with the EDHOC AEAD algorithm in
      'ciphertext' of the
      selected cipher suite, K_3ae, and IV_3ae. outer COSE_Encrypt0 with the tag removed.

   o  Verify that the identity of the Initiator Responder is an allowed identity
      for this connection, see Section 3.2. 3.3.

   o  Verify Signature_or_MAC_3 Signature_or_MAC_2 using the algorithm in the selected
      cipher suite.  The verification process depends on the method, see
      Section 4.6.2. 5.3.2.

   o  Pass AD_3, the connection identifiers (C_I, C_R), and the
      application algorithms in the selected cipher suite AD_2 to the security application.  The application can now derive application
      keys using the EDHOC-Exporter interface.

   If any verification step fails, the Responder MUST send an EDHOC
   error message back, formatted as defined in Section 5, 6, and the
   protocol MUST be discontinued.

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

5.  Error Handling

5.1.

5.4.  EDHOC Error Message

   This section defines a message format for the EDHOC error message,
   used during the protocol.  An EDHOC error message can be sent by both
   parties as a reply to any non-error EDHOC message.  After sending an
   error message, the protocol MUST be discontinued.  Errors at the
   EDHOC layer are sent as normal successful messages in the lower
   layers (e.g.  CoAP POST and 2.04 Changed).  An advantage of using
   such a construction is to avoid issues created by usage 3

5.4.1.  Formatting of cross
   protocol proxies (e.g.  UDP to TCP).

   error Message 3

   message_3 and data_3 SHALL be a CBOR Sequence Sequences (see Appendix A.1) as
   defined below

   error

   message_3 = (
     ? C_x : bstr_identifier,
     ERR_MSG
     data_3,
     CIPHERTEXT_3 : tstr, bstr,
   )

   data_3 = (
     ? SUITES_R : [ supported C_R : 2* suite ] / suite, bstr_identifier,
   )

   where:

   o  C_x - variable length connection identifier, encoded

5.4.2.  Initiator Processing of Message 3

   The Initiator SHALL compose message_3 as a
      bstr_identifier (see Section 4.3). follows:

   o  If error is sent by the
      Responder and corr (METHOD_CORR mod 4) equals 0 or 2 then C_x is
      set to C_I, else if error is sent by the Initiator and corr
      (METHOD_CORR mod 4) equals 0 or 1 then C_x 3, C_R is set to C_R, else C_x omitted,
      otherwise C_R is not omitted.

   o  ERR_MSG - text string containing  Compute the diagnostic payload, defined transcript hash TH_3 = H(TH_2 , CIPHERTEXT_2, data_3)
      where H() is the hash function in the same way the selected cipher suite.
      The transcript hash TH_3 is a CBOR encoded bstr and the input to
      the hash function is a CBOR Sequence.

   o  Compute an inner COSE_Encrypt0 as defined in Section 5.5.2 5.3 of [RFC7252].  ERR_MSG MAY be
      a 0-length text string.

   o  SUITES_R - cipher suites from SUITES_I or
      [RFC8152], with the EDHOC AEAD algorithm in the selected cipher suites
      registry that
      suite, K_3m, IV_3m, and the Responder supports.  SUITES_R MUST only be
      included in replies following parameters:

      *  protected = << ID_CRED_I >>

         +  ID_CRED_I - identifier to message_1.  If a single supported cipher
      suite is conveyed then facilitate retrieval of CRED_I,
            see Section 3.3.4

      *  external_aad = << TH_3, CRED_I, ? AD_3 >>

         +  CRED_I - bstr containing the supported cipher suite is encoded as an
      int instead credential of an array.

   After receiving SUITES_R, the Initiator can determine which selected
   cipher suite Initiator,
            see Section 3.3.4.

         +  AD_3 = bstr containing opaque protected auxiliary data

      *  plaintext = h''

      COSE constructs the input to use for the next EDHOC run with AEAD [RFC5116] as follows:

      *  Key K = EDHOC-KDF( PRK_4x3m, TH_3, "K_3m", length )

      *  Nonce N = EDHOC-KDF( PRK_4x3m, TH_3, "IV_3m", length )

      *  Plaintext P = 0x (the empty string)

      *  Associated data A =

         [ "Encrypt0", << ID_CRED_I >>, << TH_3, CRED_I, ? AD_3 >> ]

      MAC_3 is the Responder. 'ciphertext' of the inner COSE_Encrypt0.

   o  If the Initiator intends to contact authenticates with a static Diffie-Hellman key
      (method equals 2 or 3), then Signature_or_MAC_3 is MAC_3.  If the Responder
      Initiator authenticates with a signature key (method equals 0 or
      1), then Signature_or_MAC_3 is the 'signature' of a COSE_Sign1
      object as defined in Section 4.4 of [RFC8152] using the future, signature
      algorithm in the
   Initiator SHOULD remember which selected cipher suite to use until suite, the next message_1 has been sent, otherwise private authentication
      key of the Initiator Initiator, and
   Responder will likely run into an infinite loop.  After a successful
   run of EDHOC, the Initiator MAY remember following parameters:

      *  protected = << ID_CRED_I >>

      *  external_aad = << TH_3, CRED_I, ? AD_3 >>

      *  payload = MAC_3

      COSE constructs the selected cipher suite input to
   use in future EDHOC runs.  Note that if the Initiator or Responder Signature Algorithm as:

      *  The key is
   updated with new cipher suite policies, any cached information may the private authentication key of the Initiator.

      *  The message M to be
   outdated.

5.1.1.  Example Use signed =

         [ "Signature1", << ID_CRED_I >>, << TH_3, CRED_I, ? AD_3 >>,
         MAC_3 ]

   o  Compute an outer COSE_Encrypt0 as defined in Section 5.3 of EDHOC Error Message
      [RFC8152], with SUITES_R

   Assuming that the Initiator supports EDHOC AEAD algorithm in the five selected cipher suites 5, 6, 7,
   8,
      suite, K_3ae, IV_3ae, and 9 the following parameters.  The protected
      header SHALL be empty.

      *  external_aad = TH_3

      *  plaintext = ( ID_CRED_I / bstr_identifier, Signature_or_MAC_3,
         ? AD_3 )

         +  Note that if ID_CRED_I contains a single 'kid' parameter,
            i.e., ID_CRED_I = { 4 : kid_I }, only the byte string kid_I
            is conveyed in decreasing order of preference, Figures 5 and 6 show
   examples of how the Responder can truncate SUITES_I plaintext encoded as a bstr_identifier,
            see Section 3.3.4 and how SUITES_R
   is used by Section 5.1.

      COSE constructs the Responder input to give the Initiator information about the
   cipher suites that the Responder supports.  In Figure 5, the
   Responder supports cipher suite 6 but not the selected cipher suite
   5.

   Initiator                                                   Responder
   |        METHOD_CORR, SUITES_I AEAD [RFC5116] as follows:

      *  Key K = [5, 5, 6, 7], G_X, C_I, AD_1       |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                     C_I, ERR_MSG, SUITES_R = 6                    |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |         METHOD_CORR, SUITES_I = [6, 5, 6], G_X, C_I, AD_1         |
   +------------------------------------------------------------------>|
   |                             message_1                             |

           Figure 5: Example use of error message with SUITES_R.

   In Figure 6, the Responder supports cipher suite 7 but not cipher
   suites 5 and 6.

   Initiator                                                   Responder
   |         METHOD_CORR, SUITES_I EDHOC-KDF( PRK_3e2m, TH_3, "K_3ae", length )

      *  Nonce N = [5, 5, 6], G_X, C_I, AD_1         |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                  C_I, ERR_MSG, SUITES_R EDHOC-KDF( PRK_3e2m, TH_3, "IV_3ae", length )

      *  Plaintext P = [7, 9]                  |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |        METHOD_CORR, SUITES_I ( ID_CRED_I / bstr_identifier,
         Signature_or_MAC_3, ? AD_3 )

      *  Associated data A = [7, 5, 6, 7], G_X, C_I, AD_1       |
   +------------------------------------------------------------------>|
   |                             message_1                             |

           Figure 6: Example use of error message with SUITES_R.

   As [ "Encrypt0", h'', TH_3 ]

      CIPHERTEXT_3 is the Initiator's list 'ciphertext' of supported cipher suites and order the outer COSE_Encrypt0.

   o  Encode message_3 as a sequence of
   preference is fixed, CBOR encoded data items as
      specified in Section 5.4.1.

   Pass the connection identifiers (C_I, C_R) and the Responder only accepts message_1 if application
   algorithms in the selected cipher suite is to the first cipher suite in SUITES_I application.  The
   application can now derive application keys using the EDHOC-Exporter
   interface.

   After sending message_3, the Initiator is assured that no other party
   than the Responder supports, the parties can verify compute the key PRK_4x3m (implicit key
   authentication).  The Initiator does however not know that the selected cipher
   suite is
   Responder has actually computed the most preferred (by key PRK_4x3m.  While the Initiator) cipher suite supported
   by both parties.  If
   Initiator can securely send protected application data, the selected cipher suite is not Initiator
   SHOULD NOT store the first
   cipher suite in SUITES_I keying material PRK_4x3m and TH_4 until the
   Initiator is assured that the Responder supports, has actually computed the Responder
   will discontinue key
   PRK_4x3m (explicit key confirmation).  Explicit key confirmation is
   e.g. assured when the protocol.

6.  Transferring EDHOC and Deriving Initiator has verified an OSCORE Context

6.1.  Transferring EDHOC in CoAP

   It is recommended to transport EDHOC as an exchange message from
   the Responder.

5.4.3.  Responder Processing of Message 3

   The Responder SHALL process message_3 as follows:

   o  Decode message_3 (see Appendix A.1).

   o  Retrieve the protocol state using the connection identifier C_R
      and/or other external information such as the CoAP [RFC7252]
   messages.  CoAP is a reliable transport that can preserve packet
   ordering Token and handle message duplication.  CoAP can also perform
   fragmentation the
      5-tuple.

   o  Decrypt and protect against denial verify the outer COSE_Encrypt0 as defined in
      Section 5.3 of service attacks.  It is
   recommended to carry [RFC8152], with the EDHOC messages AEAD algorithm in Confirmable messages,
   especially if fragmentation is used.

   By default, the CoAP client is the Initiator
      selected cipher suite, K_3ae, and IV_3ae.

   o  Verify that the CoAP server identity of the Initiator is an allowed identity
      for this connection, see Section 3.3.

   o  Verify Signature_or_MAC_3 using the Responder, but algorithm in the roles SHOULD be chosen to protect selected
      cipher suite.  The verification process depends on the most
   sensitive identity, method, see
      Section 7.  By default, EDHOC is transferred
   in POST requests 5.4.2.

   o  Pass AD_3, the connection identifiers (C_I, C_R), and 2.04 (Changed) responses to the Uri-Path:
   "/.well-known/edhoc", but an
      application may define its own path that algorithms in the selected cipher suite to the
      security application.  The application can be discovered e.g. now derive application
      keys using resource directory
   [I-D.ietf-core-resource-directory].

   By default, the EDHOC-Exporter interface.

   If any verification step fails, the Responder MUST send an EDHOC
   error message flow is back, formatted as follows: EDHOC message_1 is sent defined in Section 6, and the payload of a POST request from
   protocol MUST be discontinued.

   After verifying message_3, the client to Responder is assured that the server's
   resource for EDHOC.  EDHOC message_2 or
   Initiator has calculated the EDHOC error message is
   sent from key PRK_4x3m (explicit key confirmation)
   and that no other party than the server to Responder can compute the client in key.  The
   Responder can securely send protected application data and store the payload of
   keying material PRK_4x3m and TH_4.

6.  Error Handling

6.1.  EDHOC Error Message

   This section defines a 2.04 (Changed)
   response. message format for the EDHOC message_3 or error message,
   used during the protocol.  An EDHOC error message is can be sent from
   the client to the server's resource in the payload of by both
   parties as a POST request.
   If needed, an reply to any non-error EDHOC message.  After sending an
   error message is sent from message, the server to protocol MUST be discontinued.  Errors at the
   client
   EDHOC layer are sent as normal successful messages in the payload of a lower
   layers (e.g.  CoAP POST and 2.04 (Changed) response. Changed).  An example advantage of a successful EDHOC exchange using CoAP
   such a construction is shown in
   Figure 7.  In this case the CoAP Token enables the Initiator to
   correlate message_1 and message_2 so the correlation parameter corr =
   1.

             Client    Server
               | avoid issues created by usage of cross
   protocol proxies (e.g.  UDP to TCP).

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

   error = (
     ? C_x : bstr_identifier,
     ERR_MSG : tstr,
     ? SUITES_R : [ supported : 2* suite ] / suite,
   )

   where:

   o  C_x - (optional) variable length connection identifier, encoded as
      a bstr_identifier (see Section 5.1).  If error is sent by the
      Responder and corr (METHOD_CORR mod 4) equals 0 or 2 then C_x is
      set to C_I, else if error is sent by the Initiator and corr
      (METHOD_CORR mod 4) equals 0 or 1 then C_x is set to C_R, else C_x
      is omitted.

   o  ERR_MSG - text string containing the diagnostic payload, defined
      in the same way as in Section 5.5.2 of [RFC7252].  ERR_MSG MAY be
      a 0-length text string.  This text string is mandatory and
      characteristic for error messages, which enables the receiver to
      distinguish between a normal message and an error message of the
      protocol.

   o  SUITES_R - (optional) cipher suites from SUITES_I or the EDHOC
      cipher suites registry that the Responder supports.  SUITES_R MUST
      only be included in replies to message_1.  If a single supported
      cipher suite is conveyed then the supported cipher suite is
      encoded as an int instead of an array.

   After receiving SUITES_R, the Initiator can determine which selected
   cipher suite to use 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.  After a successful
   run of EDHOC, the Initiator MAY remember the selected cipher suite to
   use in future EDHOC runs.  Note that if the Initiator or Responder is
   updated with new cipher suite policies, any cached information may be
   outdated.

6.1.1.  Example Use of EDHOC Error Message with SUITES_R

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

   In Figure 5, the Responder supports cipher suite 6 but not the
   initially selected cipher suite 5.

   Initiator                                                   Responder
   |            METHOD_CORR, SUITES_I = 5, G_X, C_I, AD_1             |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                     C_I, ERR_MSG, SUITES_R = 6                    |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |         METHOD_CORR, SUITES_I = [6, 5, 6], G_X, C_I, AD_1         |
   +------------------------------------------------------------------>|
   |                             message_1                             |

           Figure 5: Example use of error message with SUITES_R.

   In Figure 6, the Responder supports cipher suite 7 and 9 but not the
   more preferred (by the Initiator) cipher suites 5 and 6.  The order
   of cipher suites in SUITES_R does not matter.

   Initiator                                                   Responder
   |            METHOD_CORR, SUITES_I = 5, G_X, C_I, AD_1              |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                  C_I, ERR_MSG, SUITES_R = [9, 7]                  |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |        METHOD_CORR, SUITES_I = [7, 5, 6, 7], G_X, C_I, AD_1       |
   +------------------------------------------------------------------>|
   |                             message_1                             |

           Figure 6: Example use of error message with SUITES_R.

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

   If the selected cipher suite is not the first cipher suite which the
   Responder supports in SUITES_I received in message_1, then Responder
   MUST discontinue the protocol, see Section 5.2.3.  If SUITES_I in
   message_1 is manipulated then the integrity verification of message_2
   containing the transcript hash TH_2 = H( message_1, data_2 ) will
   fail and the Initiator will discontinue the protocol.

7.  Transferring EDHOC and Deriving an OSCORE Context

7.1.  Transferring EDHOC in CoAP

   It is recommended to transport EDHOC as an exchange of CoAP [RFC7252]
   messages.  CoAP is a reliable transport that can preserve packet
   ordering and handle message duplication.  CoAP can also perform
   fragmentation and protect against denial of service attacks.  It is
   recommended to carry the EDHOC messages in Confirmable messages,
   especially if fragmentation is used.

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

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

   An example of a successful EDHOC exchange using CoAP is shown in
   Figure 7.  In this case the CoAP Token enables the Initiator to
   correlate message_1 and message_2 so the correlation parameter corr =
   1.

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

      Figure 7: Transferring EDHOC in CoAP when the Initiator is CoAP
                                  Client

   The exchange in Figure 7 protects the client identity against active
   attackers and the server identity against passive attackers.  An
   alternative exchange that protects the server identity against active
   attackers and the client identity against passive attackers is shown
   in Figure 8.  In this case the CoAP Token enables the Responder to
   correlate message_2 and message_3 so the correlation parameter corr =
   2.

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

      Figure 8: Transferring EDHOC in CoAP when the Initiator is CoAP
                                  Server

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

6.1.1.

7.1.1.  Deriving an OSCORE Context from EDHOC

   When EDHOC is used to derive parameters for OSCORE [RFC8613], the
   parties make sure that the EDHOC connection identifiers are unique,
   i.e. C_R MUST NOT be equal to C_I.  The CoAP client and server MUST
   be able to retrieve the OSCORE protocol state using its chosen
   connection identifier and optionally other information such as the
   5-tuple.  In case that the CoAP client is the Initiator and the CoAP
   server is the Responder:

   o  The client's OSCORE Sender ID is C_R and the server's OSCORE
      Sender ID is C_I, as defined in this document

   o  The AEAD Algorithm and the hash algorithm are the application AEAD
      and hash algorithms in the selected cipher suite.

   o  The Master Secret and Master Salt are derived as follows where
      length is the key length (in bytes) of the application AEAD
      Algorithm.

      Master Secret = EDHOC-Exporter( "OSCORE Master Secret", length )
      Master Salt   = EDHOC-Exporter( "OSCORE Master Salt", 8 )

7.

8.  Security Considerations

7.1.

8.1.  Security Properties

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

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

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

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

   As required by [RFC7258], IETF protocols need to mitigate pervasive
   monitoring when possible.  One way to mitigate pervasive monitoring
   is to use a key exchange that provides perfect forward secrecy.
   EDHOC therefore only supports methods with perfect forward secrecy.
   To limit the effect of breaches, it is important to limit the use of
   symmetrical group keys for bootstrapping.  EDHOC therefore strives to
   make the additional cost of using raw public keys and self-signed
   certificates as small as possible.  Raw public keys and self-signed
   certificates are not a replacement for a public key infrastructure,
   but SHOULD be used instead of symmetrical group keys for
   bootstrapping.

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

   If supported by the device, it is RECOMMENDED that at least the long-
   term private keys is stored in a Trusted Execution Environment (TEE)
   and that sensitive operations using these keys are performed inside
   the TEE.  To achieve even higher security additional operation such
   as ephemeral key generation, all computations of shared secrets, and
   storage of the PRK keys can be done inside the TEE.  Optimally, the
   whole EDHOC protocol can be implemented inside the TEE.  Typically an
   adversary with physical access to a device can be assumed to gain
   access to all information outside of the TEE, but none of the
   information inside the TEE.

   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 protocol run.

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

7.2.

8.2.  Cryptographic Considerations

   The security of the SIGMA protocol requires the MAC to be bound to
   the identity of the signer.  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.  EDHOC implements SIGMA-I using the same Sign-
   then-MAC approach as TLS 1.3.

   To reduce message overhead EDHOC does not use explicit nonces and
   instead rely on the ephemeral public keys to provide randomness to
   each session.  A good amount of randomness is important for the key
   generation, to provide liveness, and to protect against interleaving
   attacks.  For this reason, the ephemeral keys MUST NOT be reused, and
   both parties SHALL generate fresh random ephemeral key pairs.

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

   The data rates in many IoT deployments are very limited.  Given that
   the application keys are protected as well as the long-term
   authentication keys they can often be used for years or even decades
   before the cryptographic limits are reached.  If the application keys
   established through EDHOC need to be renewed, the communicating
   parties can derive application keys with other labels or run EDHOC
   again.

7.3.

8.3.  Cipher Suites

   Cipher suite number 0 (AES-CCM-16-64-128, SHA-256, X25519, EdDSA,
   Ed25519, AES-CCM-16-64-128, SHA-256) is mandatory to implement.
   Implementations only need to implement the algorithms needed for
   their supported methods.  For many constrained IoT devices it is
   problematic to support more than one cipher suites, so some
   deployments with P-256 may not support the mandatory cipher suite.
   This is not a problem for local deployments.

   The HMAC algorithm HMAC 256/64 (HMAC w/ SHA-256 truncated to 64 bits)
   SHALL NOT be supported for use in EDHOC.

7.4.

8.4.  Unprotected Data

   The Initiator and the Responder must make sure that unprotected data
   and metadata do not reveal any sensitive information.  This also
   applies for encrypted data sent to an unauthenticated party.  In
   particular, it applies to AD_1, ID_CRED_R, AD_2, and ERR_MSG.  Using
   the same AD_1 in several EDHOC sessions allows passive eavesdroppers
   to correlate the different sessions.  Another consideration is that
   the list of supported cipher suites may potentially be used to
   identify the application.

   The Initiator and the Responder must also make sure that
   unauthenticated data does not trigger any harmful actions.  In
   particular, this applies to AD_1 and ERR_MSG.

7.5.

8.5.  Denial-of-Service

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

7.6.

8.6.  Implementation Considerations

   The availability of a secure pseudorandom number generator and truly
   random seeds are essential for the security of EDHOC.  If no true
   random number generator is available, a truly random seed must be
   provided from an external source.  As each pseudorandom number must
   only be used once, an implementation need to get a new truly random
   seed after reboot, or continuously store state in nonvolatile memory,
   see ([RFC8613], Appendix B.1.1) for issues and solution approaches
   for writing to nonvolatile memory.  If ECDSA is supported,
   "deterministic ECDSA" as specified in [RFC6979] is RECOMMENDED.

   The referenced processing instructions in [SP-800-56A] must be
   complied with, including deleting the intermediate computed values
   along with any ephemeral ECDH secrets after the key derivation is
   completed.  The ECDH shared secret, keys, and IVs MUST be secret.
   Implementations should provide countermeasures to side-channel
   attacks such as timing attacks.  Depending on the selected curve, the
   parties should perform various validations of each other's public
   keys, see e.g.  Section 5 of [SP-800-56A].

   The Initiator and the Responder are responsible for verifying the
   integrity of certificates.  The selection of trusted CAs should be
   done very carefully and certificate revocation should be supported.
   The private authentication keys MUST be kept secret.

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

   The Responder MUST finish the verification step of message_3 before
   passing AD_3 to the application.

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

7.7.

8.7.  Other Documents Referencing EDHOC

   EDHOC has been analyzed in several other documents.  A formal
   verification of EDHOC was done in [SSR18], an analysis of EDHOC for
   certificate enrollment was done in [Kron18], the use of EDHOC in
   LoRaWAN is analyzed in [LoRa1] and [LoRa2], the use of EDHOC in IoT
   bootstrapping is analyzed in [Perez18], and the use of EDHOC in
   6TiSCH is described in [I-D.ietf-6tisch-dtsecurity-zerotouch-join].

8.

9.  IANA Considerations

8.1.

9.1.  EDHOC Cipher Suites Registry

   IANA has created a new registry titled "EDHOC Cipher Suites" under
   the new heading "EDHOC".  The registration procedure is "Expert
   Review".  The columns of the registry are Value, Array, Description,
   and Reference, where Value is an integer and the other columns are
   text strings.  The initial contents of the registry are:

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

   Value: -23
   Algorithms: N/A
   Desc: Reserved for Private Use
   Reference: [[this document]]
   Value: -22
   Algorithms: N/A
   Desc: Reserved for Private Use
   Reference: [[this document]]

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

   Value: 0
   Array: 10, 5, 4, -8, 6, 10, 5
   Desc: AES-CCM-16-64-128, SHA-256, X25519, EdDSA, Ed25519,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 1
   Array: 30, 5, 4, -8, 6, 10, 5
   Desc: AES-CCM-16-128-128, SHA-256, X25519, EdDSA, Ed25519,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 2
   Array: 10, 5, 1, -7, 1, 10, 5
   Desc: AES-CCM-16-64-128, SHA-256, P-256, ES256, P-256,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

   Value: 3
   Array: 30, 5, 1, -7, 1, 10, 5
   Desc: AES-CCM-16-128-128, SHA-256, P-256, ES256, P-256,
         AES-CCM-16-64-128, SHA-256
   Reference: [[this document]]

8.2.

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

   Value: 5
   Array: 3, -43, 2, -35, 2, 3, -43
   Desc: A256GCM, SHA-384, P-384, ES384, P-384,
         A256GCM, SHA-384
   Reference: [[this document]]

9.2.  EDHOC Method Type Registry

   IANA has created a new registry titled "EDHOC Method Type" under the
   new heading "EDHOC".  The registration procedure is "Expert Review".
   The columns of the registry are Value, Description, and Reference,
   where Value is an integer and the other columns are text strings.
   The initial contents of the registry are:

   +-------+-------------------+-------------------+-------------------+
   | Value | Initiator         | Responder         | Reference         |
   +-------+-------------------+-------------------+-------------------+
   |     0 | Signature Key     | Signature Key     | [[this document]] |
   |     1 | Signature Key     | Static DH Key     | [[this document]] |
   |     2 | Static DH Key     | Signature Key     | [[this document]] |
   |     3 | Static DH Key     | Static DH Key     | [[this document]] |
   +-------+-------------------+-------------------+-------------------+ is shown in Figure 9: Method Types

8.3. 4.

9.3.  The Well-Known URI Registry

   IANA has added the well-known URI 'edhoc' to the Well-Known URIs
   registry.

   o  URI suffix: edhoc

   o  Change controller: IETF

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

   o  Related information: None

8.4.

9.4.  Media Types Registry

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

   o  Type name: application

   o  Subtype name: edhoc

   o  Required parameters: N/A

   o  Optional parameters: N/A

   o  Encoding considerations: binary

   o  Security considerations: See Section 7 of this document.

   o  Interoperability considerations: N/A

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

   o  Applications that use this media type: To be identified

   o  Fragment identifier considerations: N/A

   o  Additional information:

      *  Magic number(s): N/A

      *  File extension(s): N/A

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

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

   o  Intended usage: COMMON

   o  Restrictions on usage: N/A

   o  Author: See "Authors' Addresses" section.

   o  Change Controller: IESG

8.5.

9.5.  CoAP Content-Formats Registry

   IANA has added the media type 'application/edhoc' to the CoAP
   Content-Formats registry.

   o  Media Type: application/edhoc

   o  Encoding:

   o  ID: TBD42

   o  Reference: [[this document]]

8.6.

9.6.  Expert Review Instructions

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

   Expert reviewers should take into consideration the following points:

   o  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's required.  Expert should consider
      requesting an opinion on the correctness of registered parameters
      from relevant IETF working groups.  Encodings that do not meet
      these objective of clarity and completeness should not be
      registered.

   o  Experts should take into account the expected usage of fields when
      approving point assignment.  The length of the encoded value
      should be weighed against how many code points of that length are
      left, the size of device it will be used on, and the number of
      code points left that encode to that size.

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

9.

10.  References

9.1.

10.1.  Normative References

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", draft-ietf-core-echo-
              request-tag-10
              request-tag-11 (work in progress), July November 2020.

   [I-D.ietf-cose-x509]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header parameters for carrying and referencing X.509
              certificates", draft-ietf-cose-x509-07 draft-ietf-cose-x509-08 (work in progress),
              September
              December 2020.

   [I-D.ietf-lake-reqs]
              Vucinic, M., Selander, G., Mattsson, J., and D. Garcia-
              Carillo, "Requirements for a Lightweight AKE for OSCORE",
              draft-ietf-lake-reqs-04 (work in progress), June 2020.

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

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

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

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

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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

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

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

   [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>.

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

9.2.

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

10.2.  Informative References

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

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

   [I-D.ietf-6tisch-dtsecurity-zerotouch-join]
              Richardson, M., "6tisch Zero-Touch Secure Join protocol",
              draft-ietf-6tisch-dtsecurity-zerotouch-join-04 (work in
              progress), July 2019.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-35 draft-ietf-ace-oauth-authz-36
              (work in progress), June November 2020.

   [I-D.ietf-core-resource-directory]
              Amsuess, C., Shelby, Z., Koster, M., Bormann, C., and P.
              Stok, "CoRE Resource Directory", draft-ietf-core-resource-
              directory-26 (work in progress), November 2020.

   [I-D.ietf-lwig-security-protocol-comparison]
              Mattsson, J., Palombini, F., and M. Vucinic, "Comparison
              of CoAP Security Protocols", draft-ietf-lwig-security-
              protocol-comparison-05 (work in progress), November 2020.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 draft-ietf-tls-dtls13-39 (work in progress), May
              November 2020.

   [I-D.palombini-core-oscore-edhoc]
              Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
              and G. Selander, "Combining EDHOC and OSCORE", draft-
              palombini-core-oscore-edhoc-01 (work in progress),
              November 2020.

   [I-D.selander-ace-ake-authz]
              Selander, G., Mattsson, J., Vucinic, M., Richardson, M.,
              and A. Schellenbaum, "Lightweight Authorization for
              Authenticated Key Exchange.", draft-selander-ace-ake-
              authz-01
              authz-02 (work in progress), March November 2020.

   [Kron18]   Krontiris, A., "Evaluation of Certificate Enrollment over
              Application Layer Security", May 2018,
              <https://www.nada.kth.se/~ann/exjobb/
              alexandros_krontiris.pdf>.

   [LoRa1]    Sanchez-Iborra, R., Sanchez-Gomez, J., Perez, S.,
              Fernandez, P., Santa, J., Hernandez-Ramos, J., and A.
              Skarmeta, "Enhancing LoRaWAN Security through a
              Lightweight and Authenticated Key Management Approach",
              June 2018,
              <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6021899/pdf/
              sensors-18-01833.pdf>.

   [LoRa2]    Sanchez-Iborra, R., Sanchez-Gomez, J., Perez, S.,
              Fernandez, P., Santa, J., Hernandez-Ramos, J., and A.
              Skarmeta, "Internet Access for LoRaWAN Devices Considering
              Security Issues", June 2018,
              <https://ants.inf.um.es/~josesanta/doc/GIoTS1.pdf>.

   [Perez18]  Perez, S., Garcia-Carrillo, D., Marin-Lopez, R.,
              Hernandez-Ramos, J., Marin-Perez, R., and A. Skarmeta,
              "Architecture of security association establishment based
              on bootstrapping technologies for enabling critical IoT
              K", October 2018, <http://www.anastacia-
              h2020.eu/publications/Architecture_of_security_association
              _establishment_based_on_bootstrapping_technologies_for_ena
              bling_critical_IoT_infrastructures.pdf>.

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

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

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

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

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

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

   [SSR18]    Bruni, A., Sahl Joergensen, T., Groenbech Petersen, T.,
              and C. Schuermann, "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>.

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

   This Appendix is intended to simplify for implementors not familiar
   with CBOR [RFC7049], [RFC8949], CDDL [RFC8610], COSE [RFC8152], and HKDF
   [RFC5869].

A.1.  CBOR and CDDL

   The Concise Binary Object Representation (CBOR) [RFC7049] [RFC8949] is a data
   format designed for small code size and small message size.  CBOR
   builds on the JSON data model but extends it by e.g. encoding binary
   data directly without base64 conversion.  In addition to the binary
   CBOR encoding, CBOR also has a diagnostic notation that is readable
   and editable by humans.  The Concise Data Definition Language (CDDL)
   [RFC8610] provides a way to express structures for protocol messages
   and APIs that use CBOR.  [RFC8610] also extends the diagnostic
   notation.

   CBOR data items are encoded to or decoded from byte strings using a
   type-length-value encoding scheme, where the three highest order bits
   of the initial byte contain information about the major type.  CBOR
   supports several different types of data items, in addition to
   integers (int, uint), simple values (e.g. null), 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.  For a complete specification
   and more examples, see [RFC7049] [RFC8949] and [RFC8610].  We recommend
   implementors to get used to CBOR by using the CBOR playground
   [CborMe].

    Diagnostic          Encoded              Type
    ------------------------------------------------------------------
    1                   0x01                 unsigned integer
    24                  0x1818               unsigned integer
    -24                 0x37                 negative integer
    -25                 0x3818               negative integer
    null                0xf6                 simple value
    h'12cd'             0x4212cd             byte string
    '12cd'              0x4431326364         byte string
    "12cd"              0x6431326364         text string
    { 4 : h'cd' }       0xa10441cd           map
    << 1, 2, null >>    0x430102f6           byte string
    [ 1, 2, null ]      0x830102f6           array
    ( 1, 2, null )      0x0102f6             sequence
    1, 2, null          0x0102f6             sequence
    ------------------------------------------------------------------

A.2.  COSE

   CBOR Object Signing and Encryption (COSE) [RFC8152] 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.

Appendix B.  Test Vectors

   This appendix provides detailed test vectors to ease implementation
   and ensure interoperability.  In addition to hexadecimal, all CBOR
   data items and sequences are given in CBOR diagnostic notation.  The
   test vectors use the default mapping to CoAP where the Initiator acts
   as CoAP client (this means that corr = 1).

   A more extensive test vector suite covering more combinations of
   authentication method used between Initiator and Responder and
   related code to generate them can be found at
   https://github.com/EricssonResearch/EDHOC/tree/master/Test%20Vectors https://github.com/
   lake-wg/edhoc/tree/master/test-vectors .

B.1.  Test Vectors for EDHOC Authenticated with Signature Keys (x5t)

   EDHOC with signature authentication and X.509 certificates is used.
   In this test vector, the hash value 'x5t' is used to identify the
   certificate.

   method (Signature Authentication)
   0

   CoAP is used as transport and the Initiator acts as CoAP client:

   corr (the Initiator can correlate message_1 and message_2)
   1

   From there, METHOD_CORR has the following value:

   METHOD_CORR (4 * method + corr) (int)
   1

   No unprotected opaque auxiliary data is sent in the message
   exchanges.

   The list of supported cipher suites of the Initiator in order of
   preference is the following:

   Supported Cipher Suites (4 bytes)
   00 01 02 03

   The cipher suite selected by the Initiator is the most preferred:

   Selected Cipher Suite (int)
   0

   The mandatory-to-implement cipher suite 0 is supported by both the
   Initiator and the Responder, see Section 7.3. 8.3.

B.1.1.  Message_1

 X (Initiator's ephemeral private key) (32 bytes)
 8f 78 1a 09 53 72 f8 5b 6d 9f 61 09 ae 42 26 11 73 4d 7d bf a0 06 9a 2d
 f2 93 5b b2 e0 53 bf 35

 G_X (Initiator's ephemeral public key) (32 bytes)
 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6 ec 07 6b ba
 02 59 d9 04 b7 ec 8b 0c

   The Initiator chooses a connection identifier C_I:

   Connection identifier chosen by Initiator (0 bytes)

   Since no unprotected opaque auxiliary data is sent in the message
   exchanges:

   AD_1 (0 bytes)

   Since the list of supported cipher suites needs to contain the
   selected cipher suite, the initiator truncates the list of supported
   cipher suites to one cipher suite only, 00.

   Because one single selected cipher suite is conveyed, it is encoded
   as an int instead of an array:

   SUITES_I (int)
   0

   With SUITES_I = 0, message_1 is constructed, as the CBOR Sequence of
   the CBOR data items above.

  message_1 =
  (
    1,
    0,
    h'898ff79a02067a16ea1eccb90fa52246f5aa4dd6ec076bba0259d904b7ec8b0c',
    h''
  )

 message_1 (CBOR Sequence) (37 bytes)
 01 00 58 20 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6
 ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c 40

B.1.2.  Message_2

   Since METHOD_CORR mod 4 equals 1, C_I is omitted from data_2.

 Y (Responder's ephemeral private key) (32 bytes)
 fd 8c d8 77 c9 ea 38 6e 6a f3 4f f7 e6 06 c4 b6 4c a8 31 c8 ba 33 13 4f
 d4 cd 71 67 ca ba ec da

 G_Y (Responder's ephemeral public key) (32 bytes)
 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52
 81 75 4c 5e bc af 30 1e

   From G_X and Y or from G_Y and X the ECDH shared secret is computed:

 G_XY (ECDH shared secret) (32 bytes)
 2b b7 fa 6e 13 5b c3 35 d0 22 d6 34 cb fb 14 b3 f5 82 f3 e2 e3 af b2 b3
 15 04 91 49 5c 61 78 2b

   The key and nonce for calculating the ciphertext are calculated as
   follows, as specified in Section 3.8. 4.

   HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).

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

   Salt is the empty byte string.

   salt (0 bytes)

   From there, PRK_2e is computed:

 PRK_2e (32 bytes)
 ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
 d8 2f be b7 99 71 39 4a

 SK_R (Responders's private authentication key) (32 bytes)
 df 69 27 4d 71 32 96 e2 46 30 63 65 37 2b 46 83 ce d5 38 1b fc ad cd 44
 0a 24 c3 91 d2 fe db 94

   Since neither the Initiator nor the Responder authenticates with a
   static Diffie-Hellman key, PRK_3e2m = PRK_2e

 PRK_3e2m (32 bytes)
 ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
 d8 2f be b7 99 71 39 4a

   The Responder chooses a connection identifier C_R.

   Connection identifier chosen by Responder (1 byte)
   2b

   Note that since C_R is a byte string of length one, it is encoded as
   the corresponding integer subtracted by 24 (see bstr_identifier in
   Section 4.3). 5.1).  Thus 0x2b = 43, 43 - 24 = 19, and 19 in CBOR encoding
   is equal to 0x13.

   C_R (1 byte)
   13

   Data_2 is constructed, as the CBOR Sequence of G_Y and C_R.

  data_2 =
  (
    h'71a3d599c21da18902a1aea810b2b6382ccd8d5f9bf0195281754c5ebcaf301e',
    h'13'
  )

 data_2 (CBOR Sequence) (35 bytes)
 58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0
 19 52 81 75 4c 5e bc af 30 1e 13

   From data_2 and message_1, compute the input to the transcript hash
   TH_2 = H( message_1, data_2 ), as a CBOR Sequence of these 2 data
   items.

 Input to calculate TH_2 (CBOR Sequence) (72 bytes)
 01 00 58 20 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6
 ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c 40 58 20 71 a3 d5 99 c2 1d a1 89 02
 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52 81 75 4c 5e bc af 30 1e 13

   And from there, compute the transcript hash TH_2 = SHA-256(
   message_1, data_2 )

 TH_2 (32 bytes)
 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60
 9d e4 f6 a1 76 0d 6c f7

   The Responder's subject name is the empty string:

   Responders's subject name (text string)
   ""

   CRED_R is the certificate (X509_R) encoded as a CBOR byte string:

 X509_R (110 bytes)
 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9
 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50
 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b
 5a 6a f3 1e 80 20 9a 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb
 8e 3f 9a 32 a5 08 59 ec d0 bf cf f2 c2 18

 CRED_R (112 bytes)
 58 6e 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e
 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e
 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6
 18 0b 5a 6a f3 1e 80 20 9a 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d
 0f fb 8e 3f 9a 32 a5 08 59 ec d0 bf cf f2 c2 18

   And because certificates are identified by a hash value with the
   'x5t' parameter, ID_CRED_R is the following:

   ID_CRED_R = { 34 : COSE_CertHash }. In this example, the hash
   algorithm used is SHA-2 256-bit with hash truncated to 64-bits (value
   -15).  The hash value is calculated over the certificate X509_R.

   ID_CRED_R =
   {
     34: [-15, h'FC79990F2431A3F5']
   }

   ID_CRED_R (14 bytes)
   a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3 f5

   Since no unprotected opaque auxiliary data is sent in the message
   exchanges:

   AD_2  (0 bytes)

   The Plaintext is defined as the empty string:

   P_2m (0 bytes)

   The Enc_structure is defined as follows: [ "Encrypt0",
   << ID_CRED_R >>, << TH_2, CRED_R >> ]

 A_2m =
 [
   "Encrypt0",
   h'A11822822E48FC79990F2431A3F5',
   h'5820B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF
   7586E47624DC9CDC6824B2A4C52E95EC9D6B0534B71C2B49E4BF9031500CEE6869979
   C297BB5A8B381E98DB714108415E5C50DB78974C271579B01633A3EF6271BE5C225EB
   28F9CF6180B5A6AF31E80209A085CFBF95F3FDCF9B18B693D6C0E0D0FFB8E3F9A32A5
   0859ECD0BFCFF2C218'
   ]

   Which encodes to the following byte string to be used as Additional
   Authenticated Data:

 A_2m (CBOR-encoded) (173 bytes)
 83 68 45 6e 63 72 79 70 74 30 4e a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3
 f5 58 92 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a
 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 58 6e 47 62 4d c9 cd c6 82 4b 2a
 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2
 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16
 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b 5a 6a f3 1e 80 20 9a 08 5c
 fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb 8e 3f 9a 32 a5 08 59 ec d0
 bf cf f2 c2 18

   info for K_2m is defined as follows:

  info for K_2m =
  [
    10,
    h'B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF7',
    "K_2m",
    16
  ]

   Which as a CBOR encoded data item is:

 info for K_2m (CBOR-encoded) (42 bytes)
 84 0a 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47
 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 64 4b 5f 32 6d 10

   From these parameters, K_2m is computed.  Key K_2m is the output of
   HKDF-Expand(PRK_3e2m, info, L), where L is the length of K_2m, so 16
   bytes.

   K_2m (16 bytes)
   b7 48 6a 94 a3 6c f6 9e 67 3f c4 57 55 ee 6b 95

   info for IV_2m is defined as follows:

  info for IV_2m =
  [
    10,
    h'B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF7',
    "IV_2m",
    13
  ]

   Which as a CBOR encoded data item is:

 info for IV_2m (CBOR-encoded) (43 bytes)
 84 0a 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47
 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 65 49 56 5f 32 6d 0d

   From these parameters, IV_2m is computed.  IV_2m is the output of
   HKDF-Expand(PRK_3e2m, info, L), where L is the length of IV_2m, so 13
   bytes.

   IV_2m (13 bytes)
   c5 b7 17 0e 65 d5 4f 1a e0 5d 10 af 56

   Finally, COSE_Encrypt0 is computed from the parameters above.

   o  protected header = CBOR-encoded ID_CRED_R
   o  external_aad = A_2m

   o  empty plaintext = P_2m

   MAC_2 (8 bytes)
   cf 99 99 ae 75 9e c0 d8

   To compute the Signature_or_MAC_2, the key is the private
   authentication key of the Responder and the message M_2 to be signed
   = [ "Signature1", << ID_CRED_R >>, << TH_2, CRED_R, ? AD_2 >>, MAC_2
   ]

 M_2 =
 [
   "Signature1",
   h'A11822822E48FC79990F2431A3F5',
   h'5820B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF
   7586E47624DC9CDC6824B2A4C52E95EC9D6B0534B71C2B49E4BF9031500CEE6869979
   C297BB5A8B381E98DB714108415E5C50DB78974C271579B01633A3EF6271BE5C225EB
   28F9CF6180B5A6AF31E80209A085CFBF95F3FDCF9B18B693D6C0E0D0FFB8E3F9A32A5
   0859ECD0BFCFF2C218',
   h'CF9999AE759EC0D8'
 ]

   Which as a CBOR encoded data item is:

 M_2 (184 bytes)
 84 6a 53 69 67 6e 61 74 75 72 65 31 4e a1 18 22 82 2e 48 fc 79 99 0f 24
 31 a3 f5 58 92 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e
 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 58 6e 47 62 4d c9 cd c6 82
 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99
 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79
 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b 5a 6a f3 1e 80 20 9a
 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb 8e 3f 9a 32 a5 08 59
 ec d0 bf cf f2 c2 18 48 cf 99 99 ae 75 9e c0 d8

   From there Signature_or_MAC_2 is a signature (since method = 0):

 Signature_or_MAC_2 (64 bytes)
 45 47 81 ec ef eb b4 83 e6 90 83 9d 57 83 8d fe 24 a8 cf 3f 66 42 8a a0
 16 20 4a 22 61 84 4a f8 4f 98 b8 c6 83 4f 38 7f dd 60 6a 29 41 3a dd e3
 a2 07 74 02 13 74 01 19 6f 6a 50 24 06 6f ac 0e

   CIPHERTEXT_2 is the ciphertext resulting from XOR encrypting a
   plaintext constructed from the following parameters and the key K_2e.

   o  plaintext = CBOR Sequence of the items ID_CRED_R and
      Singature_or_MAC_2, in this order.

   The plaintext is the following:

 P_2e (CBOR Sequence) (80 bytes)
 a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3 f5 58 40 45 47 81 ec ef eb b4 83
 e6 90 83 9d 57 83 8d fe 24 a8 cf 3f 66 42 8a a0 16 20 4a 22 61 84 4a f8
 4f 98 b8 c6 83 4f 38 7f dd 60 6a 29 41 3a dd e3 a2 07 74 02 13 74 01 19
 6f 6a 50 24 06 6f ac 0e

   K_2e = HKDF-Expand( PRK, info, length ), where length is the length
   of the plaintext, so 80.

  info for K_2e =
  [
    10,
    h'B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF7',
    "K_2e",
    80
  ]

   Which as a CBOR encoded data item is:

 info for K_2e (CBOR-encoded) (43 bytes)
 84 0a 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47
 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 64 4b 5f 32 65 18 50

   From there, K_2e is computed:

 K_2e (80 bytes)
 38 cd 1a 83 89 6d 43 af 3d e8 39 35 27 42 0d ac 7d 7a 76 96 7e 85 74 58
 26 bb 39 e1 76 21 8d 7e 5f e7 97 60 14 c9 ed ba c0 58 ee 18 cd 57 71 80
 a4 4d de 0b 83 00 fe 8e 09 66 9a 34 d6 3e 3a e6 10 12 26 ab f8 5c eb 28
 05 dc 00 13 d1 78 2a 20

   Using the parameters above, the ciphertext CIPHERTEXT_2 can be
   computed:

 CIPHERTEXT_2 (80 bytes)
 99 d5 38 01 a7 25 bf d6 a4 e7 1d 04 84 b7 55 ec 38 3d f7 7a 91 6e c0 db
 c0 2b ba 7c 21 a2 00 80 7b 4f 58 5f 72 8b 67 1a d6 78 a4 3a ac d3 3b 78
 eb d5 66 cd 00 4f c6 f1 d4 06 f0 1d 97 04 e7 05 b2 15 52 a9 eb 28 ea 31
 6a b6 50 37 d7 17 86 2e

   message_2 is the CBOR Sequence of data_2 and CIPHERTEXT_2, in this
   order:

 message_2 =
 (
  data_2,
  h'99d53801a725bfd6a4e71d0484b755ec383df77a916ec0dbc02bba7c21a200807b4f
 585f728b671ad678a43aacd33b78ebd566cd004fc6f1d406f01d9704e705b21552a9eb
 28ea316ab65037d717862e'
 )

   Which as a CBOR encoded data item is:

 message_2 (CBOR Sequence) (117 bytes)
 58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0
 19 52 81 75 4c 5e bc af 30 1e 13 58 50 99 d5 38 01 a7 25 bf d6 a4 e7 1d
 04 84 b7 55 ec 38 3d f7 7a 91 6e c0 db c0 2b ba 7c 21 a2 00 80 7b 4f 58
 5f 72 8b 67 1a d6 78 a4 3a ac d3 3b 78 eb d5 66 cd 00 4f c6 f1 d4 06 f0
 1d 97 04 e7 05 b2 15 52 a9 eb 28 ea 31 6a b6 50 37 d7 17 86 2e

B.1.3.  Message_3

   Since corr equals 1, C_R is not omitted from data_3.

 SK_I (Initiator's private authentication key) (32 bytes)
 2f fc e7 a0 b2 b8 25 d3 97 d0 cb 54 f7 46 e3 da 3f 27 59 6e e0 6b 53 71
 48 1d c0 e0 12 bc 34 d7

   HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).

   PRK_4x3m = HMAC-SHA-256 (PRK_3e2m, G_IY)

 PRK_4x3m (32 bytes)
 ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
 d8 2f be b7 99 71 39 4a

   data 3 is equal to C_R.

   data_3 (CBOR Sequence) (1 bytes)
   13

   From data_3, CIPHERTEXT_2, and TH_2, compute the input to the
   transcript hash TH_3 = H(TH_2 , CIPHERTEXT_2, data_3), as a CBOR
   Sequence of these 3 data items.

 Input to calculate TH_3 (CBOR Sequence) (117 bytes)
 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca
 fb 60 9d e4 f6 a1 76 0d 6c f7 58 50 99 d5 38 01 a7 25 bf d6 a4 e7 1d 04
 84 b7 55 ec 38 3d f7 7a 91 6e c0 db c0 2b ba 7c 21 a2 00 80 7b 4f 58 5f
 72 8b 67 1a d6 78 a4 3a ac d3 3b 78 eb d5 66 cd 00 4f c6 f1 d4 06 f0 1d
 97 04 e7 05 b2 15 52 a9 eb 28 ea 31 6a b6 50 37 d7 17 86 2e 13
   And from there, compute the transcript hash TH_3 = SHA-256(TH_2 ,
   CIPHERTEXT_2, data_3)

 TH_3 (32 bytes)
 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd
 b3 2e 4a 27 ce 93 58 da

   The initiator's subject name is the empty string:

   Initiator's subject name (text string)
   ""

   CRED_I is the certificate (X509_I) encoded as a CBOR byte string:

 X509_I (101 bytes)
 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79
 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60
 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37
 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87
 ec 3f f2 45 b7

 CRED_I (103 bytes)
 58 65 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f
 fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01
 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7
 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44
 2b 87 ec 3f f2 45 b7

   And because certificates are identified by a hash value with the
   'x5t' parameter, ID_CRED_I is the following:

   ID_CRED_I = { 34 : COSE_CertHash }. In this example, the hash
   algorithm used is SHA-2 256-bit with hash truncated to 64-bits (value
   -15).  The hash value is calculated over the certificate X509_I.

   ID_CRED_I =
   {
     34: [-15, h'FC79990F2431A3F5']
   }

   ID_CRED_I (14 bytes)
   a1 18 22 82 2e 48 5b 78 69 88 43 9e bc f2

   Since no opaque auxiliary data is exchanged:

   AD_3 (0 bytes)

   The Plaintext of the COSE_Encrypt is the empty string:

   P_3m (0 bytes)

   The external_aad is the CBOR Sequence od CRED_I and TH_3, in this
   order:

 A_3m (CBOR-encoded) (164 bytes)
 83 68 45 6e 63 72 79 70 74 30 4e a1 18 22 82 2e 48 5b 78 69 88 43 9e bc
 f2 58 89 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39
 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 58 65 fa 34 b2 2a 9c a4 a1 e1 29
 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1
 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d
 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37 00 16 b8 96 5b db 20 74 bf
 f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7

   Info for K_3m is computed as follows:

  info for K_3m =
  [
    10,
    h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA',
    "K_3m",
    16
  ]

   Which as a CBOR encoded data item is:

 info for K_3m (CBOR-encoded) (42 bytes)
 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e
 f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 64 4b 5f 33 6d 10

   From these parameters, K_3m is computed.  Key K_3m is the output of
   HKDF-Expand(PRK_4x3m, info, L), where L is the length of K_2m, so 16
   bytes.

   K_3m (16 bytes)
   3d bb f0 d6 01 03 26 e8 27 3f c6 c6 c3 b0 de cd

   Nonce IV_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L
   = 13 bytes.

   Info for IV_3m is defined as follows:

  info for IV_3m =
  [
    10,
    h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA',
    "IV_3m",
    13
  ]

   Which as a CBOR encoded data item is:

 info for IV_3m (CBOR-encoded) (43 bytes)
 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e
 f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 65 49 56 5f 33 6d 0d

   From these parameters, IV_3m is computed:

   IV_3m (13 bytes)
   10 b6 f4 41 4a 2c 91 3c cd a1 96 42 e3

   MAC_3 is the ciphertext of the COSE_Encrypt0:

   MAC_3 (8 bytes)
   5e ef b8 85 98 3c 22 d9

   Since the method = 0, Signature_or_Mac_3 is a signature:

   o  The message M_3 to be signed = [ "Signature1", << ID_CRED_I >>,
      << TH_3, CRED_I >>, MAC_3 ]

   o  The signing key is the private authentication key of the
      Initiator.

 M_3 =
 [
   "Signature1",
   h'A11822822E485B786988439EBCF2',
   h'5820A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358D
   A5865FA34B22A9CA4A1E12924EAE1D1766088098449CB848FFC795F88AFC49CBE8AFD
   D1BA009F21675E8F6C77A4A2C30195601F6F0A0852978BD43D28207D44486502FF7BD
   DA632C788370016B8965BDB2074BFF82E5A20E09BEC21F8406E86442B87EC3FF245
   B7',
   h'5EEFB885983C22D9']

   Which as a CBOR encoded data item is:

 M_3 (175 bytes)
 84 6a 53 69 67 6e 61 74 75 72 65 31 4e a1 18 22 82 2e 48 5b 78 69 88 43
 9e bc f2 58 89 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92
 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 58 65 fa 34 b2 2a 9c a4 a1
 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a
 fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b
 d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37 00 16 b8 96 5b db 20
 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7 48 5e
 ef b8 85 98 3c 22 d9

   From there, the signature can be computed:

 Signature_or_MAC_3 (64 bytes)
 b3 31 76 33 fa eb c7 f4 24 9c f3 ab 95 96 fd ae 2b eb c8 e7 27 5d 39 9f
 42 00 04 f3 76 7b 88 d6 0f fe 37 dc f3 90 a0 00 d8 5a b0 ad b0 d7 24 e3
 a5 7c 4d fe 24 14 a4 1e 79 78 91 b9 55 35 89 06

   Finally, the outer COSE_Encrypt0 is computed.

   The Plaintext is the following CBOR Sequence: plaintext = ( ID_CRED_I
   , Signature_or_MAC_3 )

 P_3ae (CBOR Sequence) (80 bytes)
 a1 18 22 82 2e 48 5b 78 69 88 43 9e bc f2 58 40 b3 31 76 33 fa eb c7 f4
 24 9c f3 ab 95 96 fd ae 2b eb c8 e7 27 5d 39 9f 42 00 04 f3 76 7b 88 d6
 0f fe 37 dc f3 90 a0 00 d8 5a b0 ad b0 d7 24 e3 a5 7c 4d fe 24 14 a4 1e
 79 78 91 b9 55 35 89 06

   The Associated data A is the following: Associated data A = [
   "Encrypt0", h'', TH_3 ]

 A_3ae (CBOR-encoded) (45 bytes)
 83 68 45 6e 63 72 79 70 74 30 40 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5
 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da

   Key K_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).

   info is defined as follows:

  info for K_3ae =
  [
    10,
    h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA',
    "K_3ae",
    16
  ]

   Which as a CBOR encoded data item is:

 info for K_3ae (CBOR-encoded) (43 bytes)
 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e
 f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 65 4b 5f 33 61 65 10

   L is the length of K_3ae, so 16 bytes.

   From these parameters, K_3ae is computed:

   K_3ae (16 bytes)
   58 b5 2f 94 5b 30 9d 85 4c a7 36 cd 06 a9 62 95

   Nonce IV_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).

   info is defined as follows:

  info for IV_3ae =
  [
    10,
    h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA',
    "IV_3ae",
    13
  ]

   Which as a CBOR encoded data item is:

 info for IV_3ae (CBOR-encoded) (44 bytes)
 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e
 f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 66 49 56 5f 33 61 65 0d

   L is the length of IV_3ae, so 13 bytes.

   From these parameters, IV_3ae is computed:

   IV_3ae (13 bytes)
   cf a9 a5 85 58 10 d6 dc e9 74 3c 3b c3

   Using the parameters above, the ciphertext CIPHERTEXT_3 can be
   computed:

 CIPHERTEXT_3 (88 bytes)
 2d 88 ff 86 da 47 48 2c 0d fa 55 9a c8 24 a4 a7 83 d8 70 c9 db a4 78 05
 e8 aa fb ad 69 74 c4 96 46 58 65 03 fa 9b bf 3e 00 01 2c 03 7e af 56 e4
 5e 30 19 20 83 9b 81 3a 53 f6 d4 c5 57 48 0f 6c 79 7d 5b 76 f0 e4 62 f5
 f5 7a 3d b6 d2 b5 0c 32 31 9f 34 0f 4a c5 af 9a

   From the parameter above, message_3 is computed, as the CBOR Sequence
   of the following items: (C_R, CIPHERTEXT_3).

   message_3 =
   (
     h'13',
     h''
   )

   Which encodes to the following byte string:

 message_3 (CBOR Sequence) (91 bytes)
 13 58 58 2d 88 ff 86 da 47 48 2c 0d fa 55 9a c8 24 a4 a7 83 d8 70 c9 db
 a4 78 05 e8 aa fb ad 69 74 c4 96 46 58 65 03 fa 9b bf 3e 00 01 2c 03 7e
 af 56 e4 5e 30 19 20 83 9b 81 3a 53 f6 d4 c5 57 48 0f 6c 79 7d 5b 76 f0
 e4 62 f5 f5 7a 3d b6 d2 b5 0c 32 31 9f 34 0f 4a c5 af 9a

B.2.  Test Vectors for EDHOC Authenticated with Static Diffie-Hellman
      Keys

   EDHOC with static Diffie-Hellman keys is used.

   method (Static DH Based Authentication)
   3

   CoAP is used as transport and the Initiator acts as CoAP client:

   corr (the Initiator can correlate message_1 and message_2)
   1

   From there, METHOD_CORR has the following value:

   METHOD_CORR (4 * method + corr) (int)
   13

   No unprotected opaque auxiliary data is sent in the message
   exchanges.

   The list of supported cipher suites of the Initiator in order of
   preference is the following:

   Supported Cipher Suites (4 bytes)
   00 01 02 03

   The cipher suite selected by the Initiator is the most preferred:

   Selected Cipher Suite (int)
   0

   The mandatory-to-implement cipher suite 0 is supported by both the
   Initiator and the Responder, see Section 7.3. 8.3.

B.2.1.  Message_1

 X (Initiator's ephemeral private key) (32 bytes)
 ae 11 a0 db 86 3c 02 27 e5 39 92 fe b8 f5 92 4c 50 d0 a7 ba 6e ea b4 ad
 1f f2 45 72 f4 f5 7c fa

 G_X (Initiator's ephemeral public key) (32 bytes)
 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80 a5 38 a4 44
 ee 9e 2b 57 e2 44 1a 7c

   The Initiator chooses a connection identifier C_I:

   Connection identifier chosen by Initiator (1 bytes)
   16

   Note that since C_I is a byte strings of length one, it is encoded as
   the corresponding integer - 24 (see bstr_identifier in Section 4.3), 5.1),
   i.e. 0x16 = 22, 22 - 24 = -2, and -2 in CBOR encoding is equal to
   0x21.

   C_I (1 byte)
   21

   Since no unprotected opaque auxiliary data is sent in the message
   exchanges:

   AD_1 (0 bytes)

   Since the list of supported cipher suites needs to contain the
   selected cipher suite, the initiator truncates the list of supported
   cipher suites to one cipher suite only, 00.

   Because one single selected cipher suite is conveyed, it is encoded
   as an int instead of an array:

   SUITES_I (int)
   0

   With SUITES_I = 0, message_1 is constructed, as the CBOR Sequence of
   the CBOR data items above.

  message_1 =
  (
    13,
    0,
    h'8D3EF56D1B750A4351D68AC250A0E883790EFC80A538A444EE9E2B57E2441A7C',
    -2
  )

 message_1 (CBOR Sequence) (37 bytes)
 0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80
 a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21

B.2.2.  Message_2

   Since METHOD_CORR mod 4 equals 1, C_I is omitted from data_2.

 Y (Responder's ephemeral private key) (32 bytes)
 c6 46 cd dc 58 12 6e 18 10 5f 01 ce 35 05 6e 5e bc 35 f4 d4 cc 51 07 49
 a3 a5 e0 69 c1 16 16 9a

 G_Y (Responder's ephemeral public key) (32 bytes)
 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33
 01 04 70 69 45 1b af 35

   From G_X and Y or from G_Y and X the ECDH shared secret is computed:

 G_XY (ECDH shared secret) (32 bytes)
 de fc 2f 35 69 10 9b 3d 1f a4 a7 3d c5 e2 fe b9 e1 15 0d 90 c2 5e e2 f0
 66 c2 d8 85 f4 f8 ac 4e

   The key and nonce for calculating the ciphertext are calculated as
   follows, as specified in Section 3.8. 4.

   HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).

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

   Salt is the empty byte string.

   salt (0 bytes)

   From there, PRK_2e is computed:

 PRK_2e (32 bytes)
 93 9f cb 05 6d 2e 41 4f 1b ec 61 04 61 99 c2 c7 63 d2 7f 0c 3d 15 fa 16
 71 fa 13 4e 0d c5 a0 4d

 SK_R (Responders's private authentication key) (32 bytes)
 bb 50 1a ac 67 b9 a9 5f 97 e0 ed ed 6b 82 a6 62 93 4f bb fc 7a d1 b7 4c
 1f ca d6 6a 07 94 22 d0 c5 a0 4d

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

 R (Responder's private authentication key) (32 bytes)
 bb 50 1a ac 67 b9 a9 5f 97 e0 ed ed 6b 82 a6 62 93 4f bb fc 7a d1 b7 4c
 1f ca d6 6a 07 94 22 d0
 G_R (Responder's public authentication key) (32 bytes)
 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4 09 66 ac 6b cb 62 20 51
 b8 46 59 18 4d 5d 9a 32

   From the Responder's authentication key and the Initiator's ephemeral
   key (see Appendix B.2.1), the ECDH shared secret G_RX is calculated.

 G_RX (ECDH shared secret) (32 bytes)
 21 c7 ef f4 fb 69 fa 4b 67 97 d0 58 84 31 5d 84 11 a3 fd a5 4f 6d ad a6
 1d 4f cd 85 e7 90 66 68

 PRK_3e2m (32 bytes)
 75 07 7c 69 1e 35 01 2d 48 bc 24 c8 4f 2b ab 89 f5 2f ac 03 fe dd 81 3e
 43 8c 93 b1 0b 39 93 07

   The Responder chooses a connection identifier C_R.

   Connection identifier chosen by Responder (1 byte)
   20

   Note that since C_R is a byte strings of length one, it is encoded as
   the corresponding integer - 24 (see bstr_identifier in Section 4.3), 5.1),
   i.e. 0x20 = 32, 32 - 24 = 8, and 8 in CBOR encoding is equal to 0x08.

   C_R (1 byte)
   08

   Data_2 is constructed, as the CBOR Sequence of G_Y and C_R.

  data_2 =
  (
    h'52FBA0BDC8D953DD86CE1AB2FD7C05A4658C7C30AFDBFC3301047069451BAF35',
    08
  )

 data_2 (CBOR Sequence) (35 bytes)
 58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db
 fc 33 01 04 70 69 45 1b af 35 08

   From data_2 and message_1, compute the input to the transcript hash
   TH_2 = H( message_1, data_2 ), as a CBOR Sequence of these 2 data
   items.

 Input to calculate TH_2 (CBOR Sequence) (72 bytes)
 0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80
 a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21 58 20 52 fb a0 bd c8 d9 53 dd 86
 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33 01 04 70 69 45 1b af 35 08
   And from there, compute the transcript hash TH_2 = SHA-256(
   message_1, data_2 )

 TH_2 (32 bytes)
 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49
 1a 52 e6 83 6a 25 64 ff

   The Responder's subject name is the empty string:

   Responders's subject name (text string)
   ""

   ID_CRED_R is the following:

   ID_CRED_R =
   {
     4: h'07'
   }

   ID_CRED_R (4 bytes)
   a1 04 41 07

   CRED_R is the following COSE_Key:

{
  1: 1,
  -1: 4,
  -2: h'A3FF263595BEB377D1A0CE1D04DAD2D40966AC6BCB622051B84659184D5D9A32',
  "subject name": ""
}

   Which encodes to the following byte string:

 CRED_R (54 bytes)
 a4 01 01 20 04 21 58 20 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4
 09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32 6c 73 75 62 6a 65 63 74
 20 6e 61 6d 65 60

   Since no unprotected opaque auxiliary data is sent in the message
   exchanges:

   AD_2  (0 bytes)

   The Plaintext is defined as the empty string:

   P_2m (0 bytes)
   The Enc_structure is defined as follows: [ "Encrypt0",
   << ID_CRED_R >>, << TH_2, CRED_R >> ]

A_2m =
[
  "Encrypt0",
  h'A1044107',
  h'58206A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FFA401012004215820A3FF263595BEB377D1A0CE1D04DAD2D40966AC6BCB622051B84659184D5D9A326C7375626A656374206E616D6560'
]

   Which encodes to the following byte string to be used as Additional
   Authenticated Data:

 A_2m (CBOR-encoded) (105 bytes)
 83 68 45 6e 63 72 79 70 74 30 44 a1 04 41 07 58 58 58 20 6a 28 78 e8 4b
 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a
 25 64 ff a4 01 01 20 04 21 58 20 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04
 da d2 d4 09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32 6c 73 75 62 6a
 65 63 74 20 6e 61 6d 65 60

   info for K_2m is defined as follows:

  info for K_2m =
  [
    10,
    h'6A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FF',
    "K_2m",
    16
  ]

   Which as a CBOR encoded data item is:

 info for K_2m (CBOR-encoded) (42 bytes)
 84 0a 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30
 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 64 4b 5f 32 6d 10

   From these parameters, K_2m is computed.  Key K_2m is the output of
   HKDF-Expand(PRK_3e2m, info, L), where L is the length of K_2m, so 16
   bytes.

   K_2m (16 bytes)
   81 2a 48 87 d1 90 ff ed 2b 10 0b a7 a5 c2 5e 67

   info for IV_2m is defined as follows:

  info for IV_2m =
  [
    10,
    h'6A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FF',
    "IV_2m",
    13
  ]

   Which as a CBOR encoded data item is:

 info for IV_2m (CBOR-encoded) (43 bytes)
 84 0a 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30
 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 65 49 56 5f 32 6d 0d

   From these parameters, IV_2m is computed.  IV_2m is the output of
   HKDF-Expand(PRK_3e2m, info, L), where L is the length of IV_2m, so 13
   bytes.

   IV_2m (13 bytes)
   92 3c 0f 94 31 51 5b 69 21 30 49 2b 7f

   Finally, COSE_Encrypt0 is computed from the parameters above.

   o  protected header = CBOR-encoded ID_CRED_R

   o  external_aad = A_2m

   o  empty plaintext = P_2m

   MAC_2 (8 bytes)
   64 21 0d 2e 18 b9 28 cd

   From there Signature_or_MAC_2 is the MAC (since method = 3):

   Signature_or_MAC_2 (8 bytes)
   64 21 0d 2e 18 b9 28 cd

   CIPHERTEXT_2 is the ciphertext resulting from XOR encrypting a
   plaintext constructed from the following parameters and the key K_2e.

   o  plaintext = CBOR Sequence of the items ID_CRED_R and the CBOR
      encoded Signature_or_MAC_2, in this order.  Note that since
      ID_CRED_R contains a single 'kid' parameter, i.e., ID_CRED_R = { 4
      : kid_R }, only the byte string kid_R is conveyed in the plaintext
      encoded as a bstr_identifier. kid_R is encoded as the
      corresponding integer - 24 (see bstr_identifier in Section 4.3), 5.1),
      i.e. 0x07 = 7, 7 - 24 = -17, and -17 in CBOR encoding is equal to
      0x30.

   The plaintext is the following:

   P_2e (CBOR Sequence) (10 bytes)
   30 48 64 21 0d 2e 18 b9 28 cd

   K_2e = HKDF-Expand( PRK, info, length ), where length is the length
   of the plaintext, so 80. 10.

  info for K_2e =
  [
    10,
    h'6A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FF',
    "K_2e",
    10
  ]

   Which as a CBOR encoded data item is:

 info for K_2e (CBOR-encoded) (42 bytes)
 84 0a 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30
 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 64 4b 5f 32 65 0a

   From there, K_2e is computed:

   K_2e (10 bytes)
   ec be 9a bd 5f 62 3a fc 65 26

   Using the parameters above, the ciphertext CIPHERTEXT_2 can be
   computed:

   CIPHERTEXT_2 (10 bytes)
   dc f6 fe 9c 52 4c 22 45 4d eb

   message_2 is the CBOR Sequence of data_2 and CIPHERTEXT_2, in this
   order:

   message_2 =
   (
    data_2,
    h'DCF6FE9C524C22454DEB'
   )

   Which as a CBOR encoded data item is:

 message_2 (CBOR Sequence) (46 bytes)
 58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db
 fc 33 01 04 70 69 45 1b af 35 08 4a dc f6 fe 9c 52 4c 22 45 4d eb

B.2.3.  Message_3

   Since corr equals 1, C_R is not omitted from data_3.

 SK_I (Initiator's private authentication key) (32 bytes)
 2b be a6 55 c2 33 71 c3 29 cf bd 3b 1f 02 c6 c0 62 03 38 37 b8 b5 90 99
 a4 43 6f 66 60 81 b0 8e

 G_I (Initiator's public authentication key) (32 bytes)
 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c aa 4f 4e 7a bb 83 5e c3
 0f 1d e8 8a db 96 ff 71

   HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).

   From the Initiator's authentication key and the Responder's ephemeral
   key (see Appendix B.2.2), the ECDH shared secret G_IY is calculated.

 G_IY (ECDH shared secret) (32 bytes)
 cb ff 8c d3 4a 81 df ec 4c b6 5d 9a 57 2e bd 09 64 45 0c 78 56 3d a4 98
 1d 80 d3 6c 8b 1a 75 2a

   PRK_4x3m = HMAC-SHA-256 (PRK_3e2m, G_IY).

 PRK_4x3m (32 bytes)
 ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
 d8 2f be b7 99 71 39 4a

   data 3 is equal to C_R.

   data_3 (CBOR Sequence) (1 bytes)
   08

   From data_3, CIPHERTEXT_2, and TH_2, compute the input to the
   transcript hash TH_3 = H(TH_2 , CIPHERTEXT_2, data_3), as a CBOR
   Sequence of these 3 data items.

 Input to calculate TH_3 (CBOR Sequence) (46 bytes)
 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af
 9c 49 1a 52 e6 83 6a 25 64 ff 4a dc f6 fe 9c 52 4c 22 45 4d eb 08

   And from there, compute the transcript hash TH_3 = SHA-256(TH_2 ,
   CIPHERTEXT_2, data_3)

 TH_3 (32 bytes)
 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07
 03 ee d9 c4 a1 bc b6 11

   The initiator's subject name is the empty string:

   Initiator's subject name (text string)
   ""

   And its credential is:

   ID_CRED_I =
   {
     4: h'24'
   }

   ID_CRED_I (4 bytes)
   a1 04 41 24

   CRED_I is the following COSE_Key:

{
  1: 1,
  -1: 4,
   -2: h'2C440CC121F8D7F24C3B0E41AEDAFE9CAA4F4E7ABB835EC30F1DE88ADB96FF71',
   "subject name": ""
 }

   Which encodes to the following byte string:

 CRED_I (54 bytes)
 a4 01 01 20 04 21 58 20 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c
 aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71 6c 73 75 62 6a 65 63 74
 20 6e 61 6d 65 60

   Since no opaque auxiliary data is exchanged:

   AD_3 (0 bytes)

   The Plaintext of the COSE_Encrypt is the empty string:

   P_3m (0 bytes)

   The external_aad is the CBOR Sequence of CRED_I and TH_3, in this
   order:

 A_3m (CBOR-encoded) (105 bytes)
 83 68 45 6e 63 72 79 70 74 30 44 a1 04 41 24 58 58 58 20 51 dd 22 43 a6
 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1
 bc b6 11 a4 01 01 20 04 21 58 20 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae
 da fe 9c aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71 6c 73 75 62 6a
 65 63 74 20 6e 61 6d 65 60

   Info for K_3m is computed as follows:

  info for K_3m =
  [
    10,
    h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611',
    "K_3m",
    16
  ]

   Which as a CBOR encoded data item is:

 info for K_3m (CBOR-encoded) (42 bytes)
 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc
 e4 80 16 07 03 ee d9 c4 a1 bc b6 11 64 4b 5f 33 6d 10

   From these parameters, K_3m is computed.  Key K_3m is the output of
   HKDF-Expand(PRK_4x3m, info, L), where L is the length of K_2m, so 16
   bytes.

   K_3m (16 bytes)
   84 85 31 8a a3 08 6f d5 86 7a 02 8e 99 e2 40 30

   Nonce IV_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L
   = 13 bytes.

   Info for IV_3m is defined as follows:

  info for IV_3m =
  [
    10,
    h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611',
    "IV_3m",
    13
  ]

   Which as a CBOR encoded data item is:

 info for IV_3m (CBOR-encoded) (43 bytes)
 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc
 e4 80 16 07 03 ee d9 c4 a1 bc b6 11 65 49 56 5f 33 6d 0d

   From these parameters, IV_3m is computed:

   IV_3m (13 bytes)
   1e 10 5b 88 50 0e d5 ae b0 5d 00 6b ea

   MAC_3 is the ciphertext of the COSE_Encrypt0:

   MAC_3 (8 bytes)
   1f b7 5a c1 aa d2 34 25

   Since the method = 3, Signature_or_Mac_3 is the MAC_3:

   Signature_or_MAC_3 (8 bytes)
   1f b7 5a c1 aa d2 34 25

   Finally, the outer COSE_Encrypt0 is computed.

   The Plaintext is the following CBOR Sequence: plaintext = ( ID_CRED_I
   , Signature_or_MAC_3 ).  Note that since ID_CRED_I contains a single
   'kid' parameter, i.e., ID_CRED_I = { 4 : kid_I }, only the byte
   string kid_I is conveyed in the plaintext encoded as a
   bstr_identifier. kid_I is encoded as the corresponding integer - 24
   (see bstr_identifier in Section 4.3), 5.1), i.e. 0x24 = 36, 36 - 24 = 12,
   and 12 in CBOR encoding is equal to 0x0c.

   P_3ae (CBOR Sequence) (10 bytes)
   0c 48 1f b7 5a c1 aa d2 34 25

   The Associated data A is the following: Associated data A = [
   "Encrypt0", h'', TH_3 ]

 A_3ae (CBOR-encoded) (45 bytes)
 83 68 45 6e 63 72 79 70 74 30 40 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53
 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11

   Key K_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).

   info is defined as follows:

  info for K_3ae =
  [
    10,
    h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611',
    "K_3ae",
    16
  ]

   Which as a CBOR encoded data item is:

 info for K_3ae (CBOR-encoded) (43 bytes)
 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc
 e4 80 16 07 03 ee d9 c4 a1 bc b6 11 65 4b 5f 33 61 65 10

   L is the length length of K_3ae, so 16 bytes.

   From these parameters, K_3ae is computed:

   K_3ae (16 bytes)
   bf 29 0b 7e e0 4b 86 5d e1 01 0a 81 1b 36 00 64

   Nonce IV_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).

   info is defined as follows:

  info for IV_3ae =
  [
    10,
    h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611',
    "IV_3ae",
    13
  ]

   Which as a CBOR encoded data item is:

 info for IV_3ae (CBOR-encoded) (44 bytes)
 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc
 e4 80 16 07 03 ee d9 c4 a1 bc b6 11 66 49 56 5f 33 61 65 0d

   L is the length of IV_3ae, so 13 bytes.

   From these parameters, IV_3ae is computed:

   IV_3ae (13 bytes)
   0e 74 45 0a fc ec e9 73 af 64 e9 4d 46

   Using the parameters above, the ciphertext CIPHERTEXT_3 can be
   computed:

   CIPHERTEXT_3 (18 bytes)
   53 c3 99 19 99 a5 ff b8 69 21 e9 9b 60 7c 06 77 70 e0

   From the parameter above, message_3 is computed, as the CBOR Sequence
   of the following items: (C_R, CIPHERTEXT_3).

   message_3 =
   (
     h'08',
     h'53C3991999A5FFB86921E99B607C067770E0'
   )

   Which encodes to the following byte string:

   message_3 (CBOR Sequence) (20 bytes)
   08 52 53 c3 99 19 99 a5 ff b8 69 21 e9 9b 60 7c 06 77 70 e0

Appendix C.  Applicability Statement Template

   EDHOC requires certain parameters to be agreed upon between Initiator
   and Responder.  A cipher suite is negotiated with the protocol, but
   certain other parameters need to be agreed beforehand:

   1.  Method and correlation of underlying transport messages
       (METHOD_CORR, see Section 3.2.1 and Section 3.2.4).

   2.  Type of authentication credentials (CRED_I, CRED_R, see
       Section 3.3.4).

   3.  Type for identifying authentication credentials (ID_CRED_I,
       ID_CRED_R, see Section 3.3.4).

   4.  Type and use of Auxiliary Data AD_1, AD_2, AD_3 (see
       Section 3.6).

   5.  Identifier used as identity of endpoint (see Section 3.3).

   An example of an applicability statement is shown in the next
   section.

   Note that for some of K_3ae, so 16 bytes.

   From these the parameters, K_3ae is computed:

   K_3ae (16 bytes)
   bf 29 0b 7e e0 4b 86 5d e1 01 0a 81 1b 36 00 64

   Nonce IV_3ae like METHOD_CORR, ID_CRED_x,
   type of AD_x, the receiver is able to assert whether it supports the output
   parameter or not and thus, if it fails, to infer why.

   For other parameters, like type of HKDF-Expand(PRK_3e2m, info, L).

   info authentication credential, it may
   be more difficult to detect if the receiver got the wrong type since
   the credential is defined as follows:

  info for IV_3ae =
  [
    10,
    h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611',
    "IV_3ae",
    13
  ]

   Which as not necessarily transported, and a CBOR encoded data item is:

 info for IV_3ae (CBOR-encoded) (44 bytes)
 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc
 e4 80 16 07 03 ee d9 c4 a1 bc b6 11 66 49 56 5f 33 61 65 0d

   L is failed integrity
   of the length received message may be caused by other circumstances.  For
   example in the case of IV_3ae, so 13 bytes.

   From these parameters, IV_3ae public key certificates there is a large
   variety of profiles and alternative encodings, which the
   applicability statement needs to nail down.

   Note also that it is computed:

   IV_3ae (13 bytes)
   0e 74 45 0a fc ec e9 73 af 64 e9 4d 46

   Using not always necessary for the parameters above, endpoints to agree
   on the ciphertext CIPHERTEXT_3 can transport for the EDHOC messages.  For example, a mix of CoAP
   and HTTP may be
   computed:

   CIPHERTEXT_3 (18 bytes)
   53 c3 99 19 99 a5 ff b8 69 21 e9 9b 60 7c 06 77 70 e0

   From used along the parameter above, message_3 is computed, as path and still allow correlation
   between message_1 and message_2.

C.1.  Use of EDHOC in the CBOR Sequence XX Protocol

   For use of EDHOC in the following items: (C_R, CIPHERTEXT_3).

   message_3 =
   (
     h'08',
     h'53C3991999A5FFB86921E99B607C067770E0'
   )

   Which encodes to XX protocol, the following assumptions are
   made on the parameters.

   o  METHOD_CORR = 5

      *  method = 1 (I uses signature key, R uses static DH key.)

      *  corr = 1 (CoAP Token or other transport data enables
         correlation between message_1 and message_2.)

   o  CRED_I is an 802.1AR IDevID encoded as a CBOR Certificate of type
      0

      *  R acquires CRED_I out-of-band, indicated in AD_1

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

   message_3 (CBOR Sequence) (20 bytes)
   08 52 53 c3 99 19 99 a5 ff b8 69 21 e9 9b 60 7c 06 77 70 e0 string

   o  CRED_R is a COSE_Key of type OKP as specified in Section 3.3.4.

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

   o  ID_CRED_R = CRED_R

   o  AD_1 contains Auxiliary Data of type A (TBD)

   o  AD_2 contains Auxiliary Data of type B (TBD)

   Auxiliary Data is processed as specified in
   [I-D.ietf-ace-oauth-authz].

   o  Need to specify use of C_I/C_R ? (TBD)

Acknowledgments

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

Authors' Addresses
   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com

   John Preuss Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com

   Francesca Palombini
   Ericsson AB

   Email: francesca.palombini@ericsson.com