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Versions: (draft-selander-lake-edhoc) 00 01 02

Network Working Group                                        G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: January 7, 2021                                     Ericsson AB
                                                           July 06, 2020


               Ephemeral Diffie-Hellman Over COSE (EDHOC)
                        draft-ietf-lake-edhoc-00

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 can be kept very low.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 7, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Rationale for EDHOC . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology and Requirements Language . . . . . . . . . .   5
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  EDHOC Overview  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Transport and Message Correlation . . . . . . . . . . . .   8
     3.2.  Authentication Keys and Identities  . . . . . . . . . . .   9
     3.3.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Communication/Negotiation of Protocol Features  . . . . .  11
     3.6.  Auxiliary Data  . . . . . . . . . . . . . . . . . . . . .  12
     3.7.  Ephemeral Public Keys . . . . . . . . . . . . . . . . . .  12
     3.8.  Key Derivation  . . . . . . . . . . . . . . . . . . . . .  12
   4.  EDHOC Authenticated with Asymmetric Keys  . . . . . . . . . .  15
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  17
     4.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  19
     4.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  22
   5.  EDHOC Authenticated with Symmetric Keys . . . . . . . . . . .  25
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  25
     5.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  26
     5.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  27
     5.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  28
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  28
     6.1.  EDHOC Error Message . . . . . . . . . . . . . . . . . . .  28
   7.  Transferring EDHOC and Deriving an OSCORE Context . . . . . .  30
     7.1.  Transferring EDHOC in CoAP  . . . . . . . . . . . . . . .  30
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
     8.1.  Security Properties . . . . . . . . . . . . . . . . . . .  33
     8.2.  Cryptographic Considerations  . . . . . . . . . . . . . .  34
     8.3.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .  35
     8.4.  Unprotected Data  . . . . . . . . . . . . . . . . . . . .  35
     8.5.  Denial-of-Service . . . . . . . . . . . . . . . . . . . .  36
     8.6.  Implementation Considerations . . . . . . . . . . . . . .  36
     8.7.  Other Documents Referencing EDHOC . . . . . . . . . . . .  37
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     9.1.  EDHOC Cipher Suites Registry  . . . . . . . . . . . . . .  37
     9.2.  EDHOC Method Type Registry  . . . . . . . . . . . . . . .  38
     9.3.  The Well-Known URI Registry . . . . . . . . . . . . . . .  39
     9.4.  Media Types Registry  . . . . . . . . . . . . . . . . . .  39
     9.5.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  40



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     9.6.  Expert Review Instructions  . . . . . . . . . . . . . . .  40
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  41
     10.2.  Informative References . . . . . . . . . . . . . . . . .  42
   Appendix A.  Use of CBOR, CDDL and COSE in EDHOC  . . . . . . . .  45
     A.1.  CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . .  45
     A.2.  COSE  . . . . . . . . . . . . . . . . . . . . . . . . . .  46
   Appendix B.  Test Vectors . . . . . . . . . . . . . . . . . . . .  46
     B.1.  Test Vectors for EDHOC Authenticated with Signature Keys
           (x5t) . . . . . . . . . . . . . . . . . . . . . . . . . .  46
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  60
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  60

1.  Introduction

   Security at the application layer provides an attractive option for
   protecting Internet of Things (IoT) deployments, for example where
   transport layer security is not sufficient
   [I-D.hartke-core-e2e-security-reqs] or where the 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].  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].  EDHOC supports
   authentication using pre-shared keys (PSK), raw public keys (RPK),
   and public key certificates.  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.  Note that this document focuses on authentication and key
   establishment: for integration with authorization of resource access,
   refer to [I-D.ietf-ace-oscore-profile].



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   EDHOC is designed to work in highly constrained scenarios making it
   especially suitable for network technologies such as Cellular IoT,
   6TiSCH [I-D.ietf-6tisch-dtsecurity-zerotouch-join], and LoRaWAN
   [LoRa1][LoRa2].  These network technologies are characterized by
   their low throughput, low power consumption, and small frame sizes.
   Compared to the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with ECDH
   and connection ID, the number of bytes in EDHOC + CoAP is less than
   1/4 when PSK authentication is used and less than 1/6 when RPK
   authentication is used, see
   [I-D.ietf-lwig-security-protocol-comparison].  Typical message sizes
   for EDHOC with pre-shared keys, raw public keys with static Diffie-
   Hellman keys, and two different ways to identify X.509 certificates
   with signature keys are shown in Figure 1.  Further reductions of
   message sizes are possible by eliding redundant length indications.

   =====================================================================
                  PSK       RPK       x5t     x5chain
   ---------------------------------------------------------------------
   message_1       38        37        37        37
   message_2       44        46       117       110 + Certificate
   message_3       10        20        91        84 + Certificate
   ---------------------------------------------------------------------
   Total           92       103       245       231 + Certificates
   =====================================================================

                 Figure 1: Typical 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
   [RFC5869].  CBOR [RFC7049] 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,
   formatting of the ephemeral public keys, and key derivation,
   Section 4 specifies EDHOC with signature key and static Diffie-
   Hellman key authentication, Section 5 specifies EDHOC with symmetric
   key authentication, Section 6 specifies the EDHOC error message, and
   Section 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



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   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.  Terminology and Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.





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   Readers are expected to be familiar with the terms and concepts
   described in CBOR [RFC7049], CBOR Sequences [RFC8742], COSE
   [RFC8152], and CDDL [RFC8610].  The Concise Data Definition Language
   (CDDL) is used to express CBOR data structures [RFC7049].  Examples
   of CBOR and CDDL are provided in Appendix A.1.

2.  Background

   EDHOC specifies different authentication methods of the Diffie-
   Hellman key exchange: digital signatures, static Diffie-Hellman keys
   and symmetric keys.  This section outlines the digital signature
   based method.

   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.  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.




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   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 in EDHOC is summarized
   in Appendix A and test vectors including CBOR diagnostic notation are
   given in Appendix B.

3.  EDHOC Overview

   EDHOC consists of three messages (message_1, message_2, message_3)
   that maps directly to the three messages in SIGMA-I, plus an EDHOC
   error message.  EDHOC messages are CBOR Sequences [RFC8742], where
   the first data item (METHOD_CORR) of message_1 is an int specifying



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   the method and the correlation properties of the transport used, see
   Section 3.1.  The method specifies the authentication methods used
   (signature, static DH, symmetric), see Section 9.2.  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 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 algorithms
   (AEAD, hash, etc.) in the selected cipher suite and the connection
   identifiers (C_I, C_R).  EDHOC may be used with the media type
   application/edhoc defined in Section 9.

      Initiator                                             Responder
         |                                                       |
         | ------------------ EDHOC message_1 -----------------> |
         |                                                       |
         | <----------------- EDHOC message_2 ------------------ |
         |                                                       |
         | ------------------ EDHOC message_3 -----------------> |
         |                                                       |
         | <----------- Application Protected Data ------------> |
         |                                                       |

                       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, 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.  It is recommended to transport EDHOC in CoAP payloads, see
   Section 7.

   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



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   very short.  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.

   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 7.1.

3.2.  Authentication Keys and Identities

   The EDHOC message exchange may be authenticated using pre-shared keys
   (PSK), raw public keys (RPK), or public key certificates.  The
   certificates and RPKs can contain signature keys or static Diffie-
   Hellman keys.  In X.509 certificates, signature keys typically have
   key usage "digitalSignature" and Diffie-Hellman keys typically have
   key usage "keyAgreement".  EDHOC assumes the existence of mechanisms
   (certification authority, trusted third party, manual distribution,
   etc.) for distributing authentication keys (public or pre-shared) and
   identities.  Policies are set based on the identity of the other
   party, and parties typically only allow connections from a small
   restricted 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 other party's certificate.  Before running EDHOC
      each party needs at least one CA public key certificate, or just
      the public key, and a set of identities it is allowed to
      communicate with.  Any validated public-key certificate with an
      allowed subject name is accepted.  EDHOC provides proof that the
      other party possesses the private authentication key corresponding
      to the public authentication key in its certificate.  The




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      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.  Before
      running EDHOC, each party need a set of public authentication
      keys/unique associated subject names it is allowed to communicate
      with.  EDHOC provides proof that the other party possesses the
      private authentication key corresponding to the public
      authentication key.

   o  When pre-shared keys are used the information about the other
      party is carried in the PSK identifier field of the protocol,
      ID_PSK.  The purpose of ID_PSK is to facilitate retrieval of the
      pre-shared key, which is used to authenticate and assert trust.
      In this case no other identities or trust anchors are used.

3.3.  Identifiers

   One byte connection and credential identifiers are realistic in many
   scenarios as most constrained devices only have a few keys and
   connections.  In cases where a node only has one connection or key,
   the identifiers may even be the empty byte string.

3.4.  Cipher Suites

   EDHOC cipher suites consist of an ordered set of COSE algorithms: an
   EDHOC AEAD algorithm, an EDHOC hash algorithm, an EDHOC ECDH curve,
   an EDHOC signature algorithm, an EDHOC signature algorithm curve, an
   application AEAD algorithm, and an application hash algorithm from
   the COSE Algorithms and Elliptic Curves registries.  Each cipher
   suite is identified with a pre-defined int label.  This document
   specifies four pre-defined cipher suites.








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      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 different methods use the same cipher suites, but some algorithms
   are not used in some methods.  The EDHOC signature algorithm and the
   EDHOC signature algorithm curve are not used 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 have a list of cipher
   suites it supports.

3.5.  Communication/Negotiation of Protocol Features

   EDHOC allows the communication or negotiation of various protocol
   features during the execution of the protocol.

   o  The Initiator proposes a cipher suite (see Section 3.4), and the
      Responder either accepts or rejects, and may make a counter
      proposal.

   o  The Initiator decides on the correlation parameter corr (see
      Section 3.1).  This is typically given by the transport which the
      Initiator and the Responder have agreed on beforehand.  The
      Responder either accepts or rejects.

   o  The Initiator decides on the method parameter, see Section 9.2.
      The Responder either accepts or rejects.

   o  The Initiator and the Responder decide on the representation of
      the identifier of their respective credentials, ID_CRED_I and
      ID_CRED_R.  The decision is reflected by the label used in the
      CBOR map, see for example Section 4.1.





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3.6.  Auxiliary Data

   In order to reduce round trips and number of messages, 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 is the transport of third-party
   authorization information protected outside of EDHOC
   [I-D.selander-ace-ake-authz].  Another example is the embedding 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 and message_2, respectively.  Protected Auxiliary Data
   (AD_3) may be sent in message_3.

   Since data carried in AD1 and AD2 may not be protected, and the
   content of AD3 is available to both the Initiator and the Responder,
   special considerations need to be made such that the availability of
   the data a) does not violate security and privacy requirements of the
   service which uses this data, and b) does not violate the security
   properties of EDHOC.

3.7.  Ephemeral Public Keys

   The ECDH ephemeral public keys are formatted as a COSE_Key of type
   EC2 or OKP according to Sections 13.1 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
   be used also in the COSE_Key.  If the COSE implementation requires an
   'y' parameter, any of the possible values of the y-coordinate can be
   used, see Appendix C of [RFC6090].  COSE [RFC8152] always use compact
   output for Elliptic Curve Keys of type EC2.

3.8.  Key Derivation

   EDHOC uses HKDF [RFC5869] with the EDHOC hash algorithm in the
   selected cipher suite to derive 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 and IVs produce a
   MAC in message_3 and to derive application specific data.



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   PRK_2e is derived with the following input:

   o  The salt SHALL be the PSK when EDHOC is authenticated with
      symmetric keys, and the empty byte string when EDHOC is
      authenticated with asymmetric keys (signature or static DH).  The
      PSK is used as 'salt' to simplify implementation.  Note that
      [RFC5869] specifies that if the salt is not provided, it is set to
      a string of zeros (see Section 2.2 of [RFC5869]).  For
      implementation purposes, not providing the salt is the same as
      setting the salt to the 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 of [RFC8152].

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

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

   where salt = 0x (the empty byte string) in the asymmetric case and
   salt = PSK in the symmetric case.

   The pseudorandom keys PRK_3e2m and PRK_4x3m are defined as follow:

   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 )




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   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 algorithm
      identifier of the EDHOC AEAD algorithm in the selected cipher
      suite encoded as defined in [RFC8152].  Note that a single fixed
      edhoc_aead_id is used in all invocations of EDHOC-KDF, including
      the derivation of K_2e and invocations of the EDHOC-Exporter.

   o  transcript_hash is a bstr set to one of the transcript hashes
      TH_2, TH_3, or TH_4 as defined in Sections 4.3.1, 4.4.1, and
      3.8.1.

   o  label is a tstr set to the name of the derived key or IV, i.e.
      "K_2m", "IV_2m", "K_2e", "K_2ae", "IV_2ae", "K_3m", "IV_3m",
      "K_3ae", or "IV_2ae".

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

   K_2ae and IV_2ae are derived using the transcript hash TH_2 and the
   pseudorandom key PRK_2e.  K_2m and IV_2m are derived using the
   transcript hash TH_2 and the pseudorandom key PRK_3e2m.  K_3ae and
   IV_3ae are derived using the transcript hash TH_3 and the
   pseudorandom key PRK_3e2m.  K_3m and IV_3m are derived using the
   transcript hash TH_3 and the pseudorandom key PRK_4x3m.  IVs are only
   used if the EDHOC AEAD 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 an
   uint defined by the application.  The label SHALL be different for
   each different exporter value.  The transcript hash TH_4 is a CBOR
   encoded bstr and the input to the hash function is a CBOR Sequence.




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      TH_4 = H( TH_3, CIPHERTEXT_3 )

   where H() is the hash function in the selected cipher suite.  Example
   use of the EDHOC-Exporter is given in Sections 3.8.2 and 7.1.1.

3.8.2.  EDHOC PSK Chaining

   An application using EDHOC may want to derive new PSKs to use for
   authentication in future EDHOC exchanges.  In this case, the new PSK
   and the ID_PSK 'kid_value' parameter SHOULD be derived as follows
   where length is the key length (in bytes) of the EDHOC AEAD
   Algorithm.

      PSK     = EDHOC-Exporter( "EDHOC Chaining PSK", length )
      kid_psk = EDHOC-Exporter( "EDHOC Chaining kid_psk", 4 )

4.  EDHOC Authenticated with Asymmetric Keys

4.1.  Overview

   This section specifies authentication method = 0, 1, 2, and 3, see
   Section 9.2.  EDHOC supports authentication with signature or static
   Diffie-Hellman keys in the form of raw public keys (RPK) and public
   key certificates with the requirements that:

   o  Only the Responder SHALL have access to the Responder's private
      authentication key,

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

   o  The Initiator is able to retrieve the Responder's public
      authentication key using ID_CRED_R,

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

   where the identifiers ID_CRED_I and ID_CRED_R are COSE header_maps,
   i.e. CBOR maps containing COSE Common Header Parameters, see
   Section 3.1 of [RFC8152]).  ID_CRED_I and ID_CRED_R need to contain
   parameters that can identify a public authentication key.  In the
   following paragraph we give some examples of 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.



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   Public key certificates can be identified in different ways.  Several
   header parameters for identifying X.509 certificates are defined in
   [I-D.ietf-cose-x509]:

   o  by a bag of certificates with the 'x5bag' parameter; or

      *  ID_CRED_x = { 32 : COSE_X509 }, for x = I or R,

   o  by a certificate chain with the 'x5chain' parameter;

      *  ID_CRED_x = { 33 : COSE_X509 }, for x = I or R,

   o  by a hash value with the 'x5t' parameter;

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

   o  by a URL with the 'x5u' parameter;

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

   In the first two examples, ID_CRED_I and ID_CRED_R contain the actual
   credential used for authentication.  The purpose of ID_CRED_I and
   ID_CRED_R is to facilitate retrieval of a public authentication key
   and when they do not contain the actual credential, they may be very
   short.  It is RECOMMENDED that they uniquely identify the public
   authentication key as the recipient may otherwise have to try several
   keys.  ID_CRED_I and ID_CRED_R are transported in the ciphertext, see
   Section 4.3.2 and Section 4.4.2.

   The authentication key MUST be a signature key or static Diffie-
   Hellman key.  The Initiator and the Responder MAY use different types
   of authentication keys, e.g. one uses a signature key and the other
   uses a static Diffie-Hellman key.  When using a signature key, the
   authentication is provided 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 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, and the public
   authentication keys are called G_I and G_R, respectively.

   The actual credentials CRED_I and CRED_R are signed or MAC:ed by the
   Initiator and the Responder respectively, see Section 4.4.1 and
   Section 4.3.1.  The Initiator and the Responder MAY use different
   types of credentials, e.g. one uses RPK and the other uses
   certificate.  When the credential is a certificate, CRED_x is end-
   entity certificate (i.e. not the certificate chain) encoded as a CBOR



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   bstr.  When the credential is a COSE_Key, CREX_x is a CBOR map only
   contains specific fields from the COSE_Key.  For COSE_Keys of type
   OKP the CBOR map SHALL only include the parameters 1 (kty), -1 (crv),
   and -2 (x-coordinate).  For COSE_Keys of type EC2 the CBOR map SHALL
   only include the parameters 1 (kty), -1 (crv), -2 (x-coordinate), and
   -3 (y-coordinate).  If the parties have agreed on an identity besides
   the public key, the indentity is included in the CBOR map with the
   label "subject name", otherwise the subject name is the empty text
   string.  The parameters SHALL be encoded in decreasing order with int
   labels first and text string labels last.  An example of CRED_x when
   the RPK contains a 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"
   }

   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.  EDHOC Message 1

4.2.1.  Formatting of Message 1

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









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   message_1 = (
     METHOD_CORR : int,
     SUITES_I : [ selected : suite, supported : 2* suite ] / suite,
     G_X : bstr,
     C_I : bstr_identifier,
     ? AD_1 : bstr,
   )

   suite = int
   bstr_identifier = bsrt / int

   where:

   o  METHOD_CORR = 4 * method + corr, where method = 0, 1, 2, or 3 (see
      Section 9.2) and the correlation parameter corr is chosen based on
      the transport and determines which connection identifiers that are
      omitted (see Section 3.1).

   o  SUITES_I - cipher suites which the Initiator supports in order of
      (decreasing) preference.  The list of supported cipher suites can
      be truncated at the end, as is detailed in the processing steps
      below.  One of the supported cipher suites is selected.  If a
      single supported cipher suite is conveyed then that cipher suite
      is selected and the selected cipher suite is encoded as an int
      instead of an array.

   o  G_X - the ephemeral public key of the Initiator

   o  C_I - variable length connection identifier.  An bstr_identifier
      is a byte string with special encoding.  Byte strings of length
      one is encoded as the corresponding integer - 24, i.e. h'2a' is
      encoded as 18.

   o  AD_1 - bstr containing unprotected opaque auxiliary data

4.2.2.  Initiator Processing 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 to the Responder MAY be truncated such that cipher
      suites which are the 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 least preferred cipher suite in the list
      MUST be included in the list.




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   o  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 4.2.1

4.2.3.  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 suites in SUITES_I are supported.

   o  Pass AD_1 to the security application.

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

4.3.  EDHOC Message 2

4.3.1.  Formatting of Message 2

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

   message_2 = (
     data_2,
     CIPHERTEXT_2 : bstr,
   )



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

   where:

   o  G_Y - the ephemeral public key of the Responder

   o  C_R - variable length connection identifier

4.3.2.  Responder Processing 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 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_2 = H(message_1, data_2) where H()
      is the hash function in the selected cipher suite.  The transcript
      hash TH_2 is a CBOR encoded bstr and the input to the hash
      function is a CBOR Sequence.

   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_2m, IV_2m, and the following parameters:

      *  protected = << ID_CRED_R >>

         +  ID_CRED_R - identifier to facilitate retrieval of CRED_R,
            see Section 4.1

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

         +  CRED_R - bstr containing the credential of the Responder,
            see Section 4.1.

         +  AD_2 = bstr containing opaque unprotected auxiliary data




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      *  plaintext = h''

      COSE constructs the input to the AEAD [RFC5116] 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 is the 'ciphertext' of the inner COSE_Encrypt0.

   o  If the Reponder authenticates with a static Diffie-Hellman key
      (method equals 1 or 3), then Signature_or_MAC_2 is MAC_2.  If the
      Reponder authenticates with a signature key (method equals 0 or
      2), then 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 Responder, and the following parameters:

      *  protected = << ID_CRED_R >>

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

      *  payload = MAC_2

      COSE constructs the input to the Signature Algorithm as:

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

      *  The message M to be signed =

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

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

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

         +  Note that if ID_CRED_R contains a single 'kid' parameter,
            i.e., ID_CRED_R = { 4 : kid_R }, only the byte string kid_R




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            is conveyed in the plaintext encoded as an bstr_identifier,
            see Section 4.1.

      *  CIPHERTEXT_2 = plaintext XOR K_2e

      *  K_2e = EDHOC-KDF( PRK_2e, TH_2, "K_2e", length ), where length
         is the length of the plaintext.

   o  Encode message_2 as a sequence of CBOR encoded data items as
      specified in Section 4.3.1.

4.3.3.  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_I
      and/or other external information such as the CoAP Token and the
      5-tuple.

   o  Decrypt CIPHERTEXT_2.  The decryption process depends on the
      method, see Section 4.3.2.

   o  Verify that the identity of the Responder is among the allowed
      identities for this connection.

   o  Verify Signature_or_MAC_2 using the algorithm in the selected
      cipher suite.  The verification process depends on the method, see
      Section 4.3.2.

   o  Pass AD_2 to the security application.

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

4.4.  EDHOC Message 3

4.4.1.  Formatting of Message 3

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

   message_3 = (
     data_3,
     CIPHERTEXT_3 : bstr,
   )



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   data_3 = (
     ? C_R : bstr_identifier,
   )

4.4.2.  Initiator Processing of Message 3

   The Initiator SHALL compose message_3 as follows:

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

   o  Compute the transcript hash TH_3 = H(TH_2 , CIPHERTEXT_2, data_3)
      where H() is the hash function in the 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 5.3 of
      [RFC8152], with the EDHOC AEAD algorithm in the selected cipher
      suite, K_3m, IV_3m, and the following parameters:

      *  protected = << ID_CRED_I >>

         +  ID_CRED_I - identifier to facilitate retrieval of CRED_I,
            see Section 4.1

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

         +  CRED_I - bstr containing the credential of the Initiator,
            see Section 4.1.

         +  AD_3 = bstr containing opaque protected auxiliary data

      *  plaintext = h''

      COSE constructs the input to the 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 'ciphertext' of the inner COSE_Encrypt0.




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   o  If the Initiator authenticates with a static Diffie-Hellman key
      (method equals 2 or 3), then Signature_or_MAC_3 is MAC_3.  If the
      Initiator authenticates with a signature key (method equals 0 or
      1), then Signature_or_MAC_3 is the 'signature' 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, and the following parameters:

      *  protected = << ID_CRED_I >>

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

      *  payload = MAC_3

      COSE constructs the input to the Signature Algorithm as:

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

      *  The message M to be 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
      [RFC8152], with the EDHOC AEAD algorithm in the selected cipher
      suite, K_3ae, IV_3ae, and 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 the plaintext encoded as an bstr_identifier,
            see Section 4.1.

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

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

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

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

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



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      CIPHERTEXT_3 is the 'ciphertext' of the outer COSE_Encrypt0.

   o  Encode message_3 as a sequence of CBOR encoded data items as
      specified in Section 4.4.1.

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

4.4.3.  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 Token and the
      5-tuple.

   o  Decrypt and verify the outer COSE_Encrypt0 as defined in
      Section 5.3 of [RFC8152], with the EDHOC AEAD algorithm in the
      selected cipher suite, K_3ae, and IV_3ae.

   o  Verify that the identity of the Initiator is among the allowed
      identities for this connection.

   o  Verify Signature_or_MAC_3 using the algorithm in the selected
      cipher suite.  The verification process depends on the method, see
      Section 4.4.2.

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

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

5.  EDHOC Authenticated with Symmetric Keys

5.1.  Overview

   EDHOC supports authentication with pre-shared keys (authentication
   method = 4, see Section 9.2).  The Initiator and the Responder are
   assumed to have a pre-shared key (PSK) with a good amount of
   randomness and the requirement that:



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   o  Only the Initiator and the Responder SHALL have access to the PSK,

   o  The Responder is able to retrieve the PSK using ID_PSK.

   where the identifier ID_PSK is a COSE header_map (i.e. a CBOR map
   containing COSE Common Header Parameters, see [RFC8152]) containing
   COSE header parameter that can identify a pre-shared key.  Pre-shared
   keys are typically stored as COSE_Key objects and identified with a
   'kid' parameter (see [RFC8152]):

   o  ID_PSK = { 4 : kid_psk } , where kid_psk : bstr

   The purpose of ID_PSK is to facilitate retrieval of the PSK and in
   the case a 'kid' parameter is used it may be very short.  It is
   RECOMMENDED that it uniquely identify the PSK as the recipient may
   otherwise have to try several keys.

   EDHOC with symmetric key authentication is illustrated in Figure 5.

   Initiator                                                   Responder
   |           METHOD_CORR, SUITES_I, G_X, C_I, ID_PSK, AD_1           |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |               C_I, G_Y, C_R, AEAD(K_2ae; TH_2, AD_2)              |
   |<------------------------------------------------------------------+
   |                             message_2                             |
   |                                                                   |
   |                    C_R, AEAD(K_3ae; TH_3, AD_3)                   |
   +------------------------------------------------------------------>|
   |                             message_3                             |

      Figure 5: Overview of EDHOC with symmetric key authentication.

   EDHOC with symmetric key authentication is very similar to EDHOC with
   asymmetric authentication.  In the following subsections the
   differences compared to EDHOC with asymmetric authentication are
   described.

5.2.  EDHOC Message 1

5.2.1.  Formatting of Message 1

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






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   message_1 = (
     METHOD_CORR : int,
     SUITES_I : [ selected : suite, supported : 2* suite ] / suite,
     G_X : bstr,
     C_I :  bstr_identifier,
     ID_PSK : header_map / bstr_identifier,
     ? AD_1 : bstr,
   )

   where:

   o  METHOD_CORR = 4 * method + corr, where method = 4 and the
      connection parameter corr is chosen based on the transport and
      determines which connection identifiers that are omitted (see
      Section 3.1).

   o  ID_PSK - identifier to facilitate retrieval of the pre-shared key.
      If ID_PSK contains a single 'kid' parameter, i.e., ID_PSK = { 4 :
      kid_psk }, only the byte string kid_psk is conveyed encoded as an
      bstr_identifier.

5.3.  EDHOC Message 2

5.3.1.  Processing of Message 2

   o  Signature_or_MAC_2 is not used.

   o  The outer COSE_Encrypt0 is computed as defined in Section 5.3 of
      [RFC8152], with the EDHOC AEAD algorithm in the selected cipher
      suite, K_2ae, IV_2ae, and the following parameters.  The protected
      header SHALL be empty.

      *  plaintext = ? AD_2

         +  AD_2 = bstr containing opaque unprotected auxiliary data

      *  external_aad = TH_2

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

      *  Key K = EDHOC-KDF( PRK_2e, TH_2, "K_2ae", length )

      *  Nonce N = EDHOC-KDF( PRK_2e, TH_2, "IV_2ae", length )

      *  Plaintext P = ? AD_2

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




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5.4.  EDHOC Message 3

5.4.1.  Processing of Message 3

   o  Signature_or_MAC_3 is not used.

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

      *  plaintext = ? AD_3

         +  AD_3 = bstr containing opaque protected auxiliary data

      *  external_aad = TH_3

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

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

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

      *  Plaintext P = ? AD_3

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

6.  Error Handling

6.1.  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 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,
   )




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   where:

   o  C_x - 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.

   o  SUITES_R - 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.

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 6 and 7 show
   examples of how the Responder 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 6, the
   Responder supports cipher suite 6 but not the selected cipher suite
   5.

   Initiator                                                   Responder
   |        METHOD_CORR, SUITES_I = [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 6: Example use of error message with SUITES_R.

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







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

           Figure 7: Example use of error message with SUITES_R.

   As the Initiator's list of supported cipher suites and order of
   preference is fixed, and the Responder only accepts message_1 if the
   selected cipher suite is the first cipher suite in SUITES_I that the
   Responder supports, the parties can verify 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 in SUITES_I that the Responder supports, the Responder
   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



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   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 8.  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 8: Transferring EDHOC in CoAP

   The exchange in Figure 8 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 9.  In this case the CoAP Token enables the Responder to
   correlate message_2 and message_3 so the correlation parameter corr =
   2.













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

                   Figure 9: Transferring EDHOC in CoAP

   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].

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.



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

8.  Security Considerations

8.1.  Security Properties

   EDHOC inherits its security properties from the theoretical SIGMA-I
   protocol [SIGMA].  Using the terminology from [SIGMA], EDHOC provides
   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.

   When a Public Key Infrastructure (PKI) is used, EDHOC provides
   identity protection of the Initiator against active attacks and
   identity protection of the Responder against passive attacks.  When
   PKI is not used (kid, x5t) the identity is not sent on the wire and
   EDHOC with asymmetric authentication 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.  EDHOC with symmetric authentication
   does not offer protection of the PSK identifier ID_PSK.

   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
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   but SHOULD be used instead of symmetrical group keys for
   bootstrapping.

   Compromise of the long-term keys (PSK or private authentication 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.  Compromising
   the PSK lets an active attacker impersonate the Initiator in EDHOC
   exchanges with the Responder and impersonate the Responder in EDHOC
   exchanges with the Initiator.  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 and/or
   PSK) leads to compromise of all session keys derived from that
   compromised shared secret.  Compromise of one session key does not
   compromise other session keys.

   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.  In
   EDHOC authenticated with symmetric keys, EDHOC provides KCI
   protection against an attacker having access to the ephemeral secret
   key, but not against an attacker having access to the long-term PSK.
   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, Party U
   could theoretically prove that Party V 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 or PSK
   authentication, both parties can always deny having participated in
   the protocol.

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.



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

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.

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 in the
   asymmetric case, and ID_PSK, AD_1, and ERR_MSG in the symmetric case.
   Using the same ID_PSK or AD_1 in several EDHOC sessions allows
   passive eavesdroppers to correlate the different sessions.  The
   communicating parties may therefore anonymize ID_PSK.  Another
   consideration is that the list of supported cipher suites may 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



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   particular, this applies to AD_1 and ERR_MSG in the asymmetric case,
   and ID_PSK, AD_1, and ERR_MSG in the symmetric case.

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.

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 and the PSK (even though it is used
   as salt) 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



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   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.

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].

9.  IANA Considerations

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]]





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   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]]

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]] |
   |     4 | PSK               | PSK               | [[this document]] |
   +-------+-------------------+-------------------+-------------------+

                          Figure 10: Method Types








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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

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.



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   o  Intended usage: COMMON

   o  Restrictions on usage: N/A

   o  Author: See "Authors' Addresses" section.

   o  Change Controller: IESG

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]]

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.



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10.  References

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-09 (work in progress), March 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-x509-06 (work in progress),
              March 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>.




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

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

   [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>.

   [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>.

10.2.  Informative References

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

   [I-D.hartke-core-e2e-security-reqs]
              Selander, G., Palombini, F., and K. Hartke, "Requirements
              for CoAP End-To-End Security", draft-hartke-core-e2e-
              security-reqs-03 (work in progress), July 2017.

   [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.





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   [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
              (work in progress), June 2020.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-11 (work in progress), June 2020.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-24 (work in progress), March 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-04 (work in progress), March 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 (work in progress), May
              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 (work in progress), March 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>.




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   [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>.







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   [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], CDDL [RFC8610], COSE [RFC8152], and HKDF
   [RFC5869].

A.1.  CBOR and CDDL

   The Concise Binary Object Representation (CBOR) [RFC7049] 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] and [RFC8610].  We recommend
   implementors to get used to CBOR by using the CBOR playground
   [CborMe].















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    Diagnostic          Encoded              Type
    ------------------------------------------------------------------
    1                   0x01                 unsigned integer
    24                  0x1818               unsigned integer
    -24                 0x37                 negative integer
    -25                 0x3818               negative integer
    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, COSE_Sign1, and COSE_KDF_Context 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
   .

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




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   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 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)





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   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.

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




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

   Since this is the asymmetric case, 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 authanticates 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 bytes)
   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



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   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)
   ""

   And because 'x5t' has value certificate are used, ID_CRED_R is the
   following:

   ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R, and since the
   SHA-2 256-bit Hash truncated to 64-bits is used (value -15):

   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

   CRED_R is the certificate encoded as a byte string:

 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

   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 >> ]





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 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:



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  info for K_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'
 ]



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   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:






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 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



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   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_2 = 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)
   ""

   And its credential is a certificate identified by its 'x5t' hash:

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

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

   CRED_I is the certificate encoded as a byte string:




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 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

   Since no opaque auciliary 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.





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   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.












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 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




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   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:





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   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

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, 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



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   John Preuss Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB

   Email: francesca.palombini@ericsson.com









































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