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ACE Working Group                                            G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: January 4, 2018                                     Ericsson AB
                                                           July 03, 2017


               Ephemeral Diffie-Hellman Over COSE (EDHOC)
                    draft-selander-ace-cose-ecdhe-07

Abstract

   This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
   compact, and lightweight authenticated Diffie-Hellman key exchange
   with ephemeral keys that can be used over any layer.  EDHOC messages
   are encoded with CBOR and COSE, allowing reuse of existing libraries.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 4, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
   3.  EDHOC Overview  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Formatting of the Ephemeral Public Keys . . . . . . . . .   6
     3.2.  Key Derivation  . . . . . . . . . . . . . . . . . . . . .   7
   4.  EDHOC Authenticated with Asymmetric Keys  . . . . . . . . . .   8
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  14
   5.  EDHOC Authenticated with Symmetric Keys . . . . . . . . . . .  16
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . .  16
     5.3.  EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . .  19
     5.4.  EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . .  21
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  22
     6.1.  Error Message Format  . . . . . . . . . . . . . . . . . .  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Media Types Registry  . . . . . . . . . . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     10.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .  27
   Appendix B.  PSK Chaining . . . . . . . . . . . . . . . . . . . .  28
   Appendix C.  EDHOC with CoAP and OSCOAP . . . . . . . . . . . . .  28
     C.1.  Transferring EDHOC in CoAP  . . . . . . . . . . . . . . .  28
     C.2.  Deriving an OSCOAP context from EDHOC . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

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 protocol needs to
   work on 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)
   [I-D.ietf-cose-msg]), which builds on the Concise Binary Object
   Representation (CBOR) [RFC7049].



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   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), an authenticated ECDH protocol using CBOR
   and COSE objects.  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 certificates (Cert).  Note that this document focuses
   on authentication and key establishment: for integration with
   authorization of resource access, refer to
   [I-D.seitz-ace-oscoap-profile].  This document also specifies the
   derivation of shared key material.

   The ECDH exchange and the key derivation follow [SIGMA], NIST SP-
   800-56a [SP-800-56a], and HKDF [RFC5869].  CBOR [RFC7049] and COSE
   [I-D.ietf-cose-msg] are used to implement these standards.

1.1.  Terminology

   This document use the same informational CBOR Data Definition
   Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl] grammar as COSE
   (see Section 1.3 of [I-D.ietf-cose-msg]).  A vertical bar | denotes
   byte string concatenation.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  These
   words may also appear in this document in lowercase, absent their
   normative meanings.

2.  Protocol Overview

   SIGMA (SIGn-and-MAc) is a family of theoretical protocols with a
   large number of variants [SIGMA].  Like IKEv2 and TLS 1.3, EDHOC is
   built on a variant of the SIGMA protocol which provide identity
   protection, and like TLS 1.3, EDHOC implements the SIGMA-I variant as
   Sign-then-MAC.  The SIGMA-I protocol using an AEAD algorithm is shown
   in Figure 1.









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      Party U                                                 Party V
         |                          E_U                          |
         +------------------------------------------------------>|
         |                                                       |
         |         E_V, Enc(K_2; ID_V, Sig(V; E_U, E_V);)        |
         |<------------------------------------------------------+
         |                                                       |
         |           Enc(K_3; ID_U, Sig(U; E_V, E_U);)           |
         +------------------------------------------------------>|
         |                                                       |

              Figure 1: AEAD variant of the SIGMA-I protocol

   The parties exchanging messages are called "U" and "V".  They
   exchange identities and ephemeral public keys, compute the shared
   secret, and derive the keying material.  The messages are signed,
   MACed, and encrypted.

   o  E_U and E_V are the ECDH ephemeral public keys of U and V,
      respectively.

   o  ID_U and ID_V are identifiers for the public keys of U and V,
      respectively.

   o  Sig(U; . ) and S(V; . ) denote signatures made with the private
      key of U and V, respectively.

   o  Enc(K; P; A) denotes AEAD encryption of plaintext P and additional
      authenticated data A using the key K derived from the shared
      secret.  The AEAD MUST NOT be replaced by plain encryption, see
      Section 8.

   As described in Appendix B of [SIGMA], in order to create a "full-
   fledged" protocol some additional protocol elements are needed.
   EDHOC adds:

   o  Explicit session identifiers S_U, S_V different from other
      concurrent session identifiers (EDHOC or other used protocol
      identifier) chosen by U and V, respectively.

   o  Explicit nonces N_U, N_V chosen freshly and anew with each session
      by U and V, respectively.

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

   EDHOC also makes the following additions:




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   o  Negotiation of key derivation, encryption, and signature
      algorithms:

      *  U proposes one or more algorithms of the following kinds:

         +  HKDF

         +  AEAD

         +  Signature verification

         +  Signature generation

      *  V selects one algorithm of each kind

   o  Verification of common preferred ECDH curve:

      *  U lists supported ECDH curves in order of preference

      *  V verifies that the ECDH curve of the ephemeral key is the most
         preferred common curve

   o  Transport of opaque application defined 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 and
   COSE libraries.  EDHOC does not put any requirement on the lower
   layers and can therefore be also be used e.g. in environments without
   IP.

   This paper is organized as follows: Section 3 specifies general
   properties of EDHOC, including formatting of the ephemeral public
   keys and key derivation, Section 4 specifies EDHOC with asymmetric
   key authentication, Section 5 specifies EDHOC with symmetric key
   authentication, and Appendix A provides a wealth of test vectors to
   ease implementation and ensure interoperability.

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.  All EDHOC messages consists of a CBOR array where the
   first element is an int specifying the message type (MSG_TYPE).
   After creating EDHOC message_3, Party U can derive the traffic key
   (master secret) and protected application data can therefore be sent



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   in parallel with EDHOC message_3.  The application data may be
   protected using the negotiated AEAD algorithm and the explicit
   session identifiers S_U and S_V.  EDHOC may be used with the media
   type application/edhoc defined in Section 7.

      Party U                                                 Party V
         |                                                       |
         | ------------------ EDHOC message_1 -----------------> |
         |                                                       |
         | <----------------- EDHOC message_2 ------------------ |
         |                                                       |
         | ------------------ EDHOC message_3 -----------------> |
         |                                                       |
         | <----------- Protected Application Data ------------> |
         |                                                       |

                       Figure 2: EDHOC message flow

   The EDHOC message exchange may be authenticated using pre-shared keys
   (PSK), raw public keys (RPK), or certificates (Cert).  EDHOC assumes
   the existence of mechanisms (certification authority, manual
   distribution, etc.) for binding identities with authentication keys
   (public or pre-shared).  EDHOC with symmetric key authentication is
   very similar to EDHOC with asymmetric key authentication, the
   difference being that information is only MACed, not signed.

   EDHOC also allows opaque application data (APP_1, APP_2, APP_3) to be
   sent in the respective messages.  APP_1 is unprotected, APP_2 is
   protected (encrypted and integrity protected), and APP_3 is protected
   and mutually authenticated.  When EDHOC is used with asymmetric key
   authentication APP_2 is sent to an unauthenticated party, but with
   symmetric key authentication APP_2 is mutually authenticated.

3.1.  Formatting of the Ephemeral Public Keys

   The ECDH ephemeral public key SHALL be formatted as a COSE_Key of
   type EC2 or OKP according to section 13.1 and 13.2 of
   [I-D.ietf-cose-msg].  The curve X25519 is mandatory to implement.
   For Elliptic Curve Keys of type EC2, compact representation and
   compact output as per [RFC6090] SHALL be used, i.e. the 'y' parameter
   SHALL NOT be present in the The COSE_Key object.  COSE
   [I-D.ietf-cose-msg] always use compact output for Elliptic Curve Keys
   of type EC2.








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3.2.  Key Derivation

   Key and IV derivation SHALL be done as specified in Section 11.1 of
   [I-D.ietf-cose-msg] with the following input:

   o  The PRF SHALL be the HKDF [RFC5869] in the ECDH-SS w/ HKDF
      negotiated during the message exchange (HKDF_V).

   o  The secret SHALL be the ECDH shared secret as defined in
      Section 12.4.1 of [I-D.ietf-cose-msg].

   o  The salt SHALL be the PSK when EDHOC is authenticated with
      symmetric keys and nil when EDHOC is authenticated with asymmetric
      keys.

   o  The fields in the context information COSE_KDF_Context SHALL have
      the following values:

      *  AlgorithmID is a tstr as defined below

      *  PartyUInfo = PartyVInfo = ( nil, nil, nil )

      *  keyDataLength is a uint as defined below

      *  protected SHALL be a zero length bstr

      *  other is a bstr SHALL be aad_2, aad_3, or exchange_hash

   where exchange_hash, in non-CDDL notation, is:

   exchange_hash = H( H( message_1 | message_2 ) | message_3 )

   where H() is the hash function in HKDF_V.

   For message_i the key, called K_i, SHALL be derived using other =
   aad_i, where i = 2 or 3.  The key SHALL be derived using AlgorithmID
   set to the name of the negotiated AEAD (AEAD_V), and keyDataLength
   equal to the key length of AEAD_V.

   If the AEAD algorithm requires an IV, then IV_i for message_i SHALL
   be derived using other = aad_i, where i = 2 or 3.  The IV SHALL be
   derived using AlgorithmID = "IV-GENERATION" as specified in section
   12.1.2. of [I-D.ietf-cose-msg], and keyDataLength equal to the IV
   length of AEAD_V.




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   Application specific traffic keys and other data SHALL be derived
   using other = exchange_hash.  AlgorithmID SHALL be a tstr defined by
   the application and SHALL be different for different data being
   derived (an example is given in Appendix C.2). keyDataLength is set
   to the length of the data being derived.

4.  EDHOC Authenticated with Asymmetric Keys

4.1.  Overview

   EDHOC supports authentication with raw public keys (RPK) and
   certificates (Cert) with the requirements that:

   o  Party U SHALL be able to identify Party V's public key using ID_V.

   o  Party V SHALL be able to identify Party U's public key using ID_U.

   ID_U and ID_V SHALL either contain the credential used for
   authentication (e.g. x5bag or x5chain) or uniquely identify the
   credential used for authentication (e.g. x5t), see
   [I-D.schaad-cose-x509].  Party U and V MAY retrieve the other party's
   credential out of band.  Optionally, ID_U and ID_V are complemented
   with the additional parameters HINT_ID_U and HINT_ID_V containing
   information about how to retrieve the credential of Party U and Party
   V, respectively (e.g. x5u), see [I-D.schaad-cose-x509].

   Party U and Party V MAY use different type of credentials, e.g. one
   uses RPK and the other uses Cert.  Party U and Party V MAY use
   different signature algorithms.

   EDHOC with asymmetric key authentication is illustrated in Figure 3.

Party U                                                          Party V
|                      S_U, N_U, E_U, ALG_1, APP_1                     |
+--------------------------------------------------------------------->|
|                               message_1                              |
|                                                                      |
|S_U, S_V, N_V, E_V, ALG_2, Enc(K_2; Sig(V; ID_V, aad_2, APP_2); aad_2)|
|<---------------------------------------------------------------------+
|                               message_2                              |
|                                                                      |
|           S_V, Enc(K_3; Sig(U; ID_U, aad_3, APP_3); aad_3)           |
+--------------------------------------------------------------------->|
|                               message_3                              |

            Figure 3: EDHOC with asymmetric key authentication.





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4.1.1.  Mandatory to Implement Algorithms

   For EDHOC authenticated with asymmetric keys, the COSE algorithms
   ECDH-SS + HKDF-256, AES-CCM-64-64-128, and EdDSA are mandatory to
   implement.

4.2.  EDHOC Message 1

4.2.1.  Formatting of Message 1

   message_1 SHALL be a CBOR array as defined below

   message_1 = [
     MSG_TYPE : int,
     S_U : bstr,
     N_U : bstr,
     E_U : serialized_COSE_Key,
     ECDH-Curves_U : alg_array,
     HKDFs_U : alg_array,
     AEADs_U : alg_array,
     SIGs_V : alg_array,
     SIGs_U : alg_array,
     ? APP_1 : bstr
   ]

   serialized_COSE_Key = bstr .cbor COSE_Key

   alg_array = [ + alg : int / tstr ]

   where:

   o  MSG_TYPE = 1

   o  S_U - variable length session identifier

   o  N_U - 64-bit random nonce

   o  E_U - the ephemeral public key of Party U

   o  ECDH-Curves_U - EC curves for ECDH which Party U supports, in the
      order of decreasing preference

   o  HKDFs_U - supported ECDH-SS w/ HKDF algorithms

   o  AEADs_U - supported AEAD algorithms

   o  SIGs_V - signature algorithms, with which Party U supports
      verification



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   o  SIGs_U - signature algorithms, with which Party U supports signing

   o  APP_1 - bstr containing opaque application data

4.2.2.  Party U Processing of Message 1

   Party U SHALL compose message_1 as follows:

   o  Determine which ECDH curve to use with Party V.  If U previously
      received from Party V an error message to message_1 with
      diagnostic payload identifying an ECDH curve in ECDH-Curves_U,
      then U SHALL retrieve an ephemeral from that curve.  Otherwise the
      first curve in ECDH-Curves_U MUST be used.

   o  Retrieve an ephemeral ECDH key pair generated as specified in
      Section 5 of [SP-800-56a] and format the ephemeral public key E_U
      as a COSE_key as specified in Section 3.1.

   o  Generate the pseudo-random nonce N_U.

   o  Choose a session identifier S_U which is not in use and store it
      for the length of the protocol.  The session identifier SHOULD be
      different from other concurrent session identifiers used by Party
      U.  The session identifier MAY be used with the protocol for which
      EDHOC establishes traffic keys/master secret, in which case S_U
      SHALL be different from the concurrently used session identifiers
      of that protocol.

   o  Format message_1 as specified in Section 4.2.1.

4.2.3.  Party V Processing of Message 1

   Party V SHALL process message_1 as follows:

   o  Verify (OPTIONAL) that N_U has not been received before.

   o  Verify that at least one of each kind of the proposed algorithms
      are supported.

   o  Verify that the ECDH curve used in E_U is supported, and that no
      prior curve in ECDH-Curves_U is supported.

   o  For EC2 curves, validate that E_U is a valid point by verifying
      that there is a solution to the curve definition for the given
      parameter 'x'.

   If any verification step fails, Party V MUST send an EDHOC error
   message back, formatted as defined in Section 6.1, and the protocol



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   MUST be discontinued.  If V does not support the ECDH curve used in
   E_U, but supports another ECDH curves in ECDH-Curves_U, then the
   error message MUST include the following diagnostic payload
   describing the first supported ECDH curve in ECDH-Curves_U:

   ERR_MSG = "Curve not supported; X"

   where X is the first curve in ECDH-Curves_U that V supports,
   encoded as in Table 22 of {{I-D.ietf-cose-msg}}.

   o  Pass APP_1 to the application.

4.3.  EDHOC Message 2

4.3.1.  Formatting of Message 2

   message_2 SHALL be a CBOR array as defined below

   message_2 = [
     data_2,
     COSE_ENC_2 : COSE_Encrypt0
   ]

   data_2 = (
     MSG_TYPE : int,
     S_U : bstr,
     S_V : bstr,
     N_V : bstr,
     E_V : serialized_COSE_Key,
     HKDF_V : int / tstr,
     AEAD_V : int / tstr,
     SIG_V : int / tstr,
     SIG_U : int / tstr
   )

   aad_2 : bstr

   where aad_2, in non-CDDL notation, is:

   aad_2 = H( message_1 | [ data_2 ] )

   where:

   o  MSG_TYPE = 2

   o  S_V - variable length session identifier

   o  N_V - 64-bit random nonce



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   o  E_V - the ephemeral public key of Party V

   o  HKDF_V - a single chosen algorithm from HKDFs_U

   o  AEAD_V - a single chosen algorithm from AEADs_U

   o  SIG_V - a single chosen algorithm from SIGs_V with which Party V
      signs

   o  SIG_U - a single chosen algorithm from SIGs_U with which Party U
      signs

   o  COSE_ENC_2 has the following fields and values:

      *  external_aad = aad_2

      *  plaintext = [ COSE_SIG_V, ? APP_2 ]

   o  COSE_SIG_V is a COSE_Sign1 object with the following fields and
      values:

      *  protected = { abc : ID_V, ? xyz : HINT_ID_V }

      *  detached payload = aad_2, ? APP_2

   o  abc - any COSE map label that can identify a public key, see
      Section 4.1

   o  ID_V - identifier for the public key of Party V

   o  xyz - any COSE map label for information about how to retrieve the
      credential of Party V, see Section 4.1

   o  HINT_ID_V - information about how to retrieve the credential of
      Party V

   o  APP_2 - bstr containing opaque application data

   o  H() - the hash function in HKDF_V

4.3.2.  Party V Processing of Message 2

   Party V SHALL compose message_2 as follows:

   o  Retrieve an ephemeral ECDH key pair generated as specified in
      Section 5 of [SP-800-56a] using same curve as used in E_U.  Format
      the ephemeral public key E_V as a COSE_key as specified in
      Section 3.1.



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   o  Generate the pseudo-random nonce N_V.

   o  Choose a session identifier S_V which is not in use and store it
      for the length of the protocol.  The session identifier SHOULD be
      different from other relevant concurrent session identifiers used
      by Party V.  The session identifier MAY be used with the protocol
      for which EDHOC establishes traffic keys/master secret, in which
      case S_V SHALL be different from the concurrently used session
      identifiers of that protocol.

   o  Select HKDF_V, AEAD_V, SIG_V, and SIG_U from the algorithms
      proposed in HKDFs_U, AEADs_U, SIGs_V, and SIGs_U.

   o  Format message_2 as specified in Section 4.3.1:

      *  COSE_Sign1 is computed as defined in section 4.4 of
         [I-D.ietf-cose-msg], using algorithm SIG_V and the private key
         of Party V.

      *  COSE_Encrypt0 is computed as defined in section 5.3 of
         [I-D.ietf-cose-msg], with AEAD_V, K_2, and IV_2.  The AEAD
         algorithm MUST NOT be replaced by plain encryption, see
         Section 8.

4.3.3.  Party U Processing of Message 2

   Party U SHALL process message_2 as follows:

   o  Use the session identifier S_U to retrieve the protocol state.

   o  Verify that HKDF_V, AEAD_V, SIG_V, and SIG_U were proposed in
      HKDFs_U, AEADs_U, SIGs_V, and SIGs_U.

   o  Verify (OPTIONAL) that N_V has not been received before.

   o  For EC2 curves, validate that E_V is a valid point by verifying
      that there is a solution to the curve definition for the given
      parameter 'x'.

   o  Verify message_2 as specified in Section 4.3.1:

      *  COSE_Encrypt0 is decrypted defined in section 5.3 of
         [I-D.ietf-cose-msg], with AEAD_V, K_2, and IV_2.

      *  COSE_Sign1 is verified as defined in section 4.4 of
         [I-D.ietf-cose-msg], using algorithm SIG_V and the public key
         of Party V.




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   If any verification step fails, Party U MUST send an EDHOC error
   message back, formatted as defined in Section 6.1, and the protocol
   MUST be discontinued.

   o  Pass APP_2 to the application.

4.4.  EDHOC Message 3

4.4.1.  Formatting of Message 3

   message_3 SHALL be a CBOR array as defined below

   message_3 = [
     data_3,
     COSE_ENC_3 : COSE_Encrypt0
   ]

   data_3 = (
     MSG_TYPE : int,
     S_V : bstr
   )

   aad_3 : bstr

   where aad_3, in non-CDDL notation, is:

   aad_3 = H( H( message_1 | message_2 ) | [ data_3 ] )

   where:

   o  MSG_TYPE = 3

   o  COSE_ENC_3 has the following fields and values:

      *  external_aad = aad_3

      *  plaintext = [ COSE_SIG_U, ? APP_3 ]

   o  COSE_SIG_U is a COSE_Sign1 object with the following fields and
      values:

      *  protected = { abc : ID_U, ? xyz : HINT_ID_U }

      *  detached payload = aad_3, ? APP_3

   o  abc - any COSE map label that can identify a public key, see
      Section 4.1




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   o  ID_U - identifier for the public key of Party U

   o  xyz - any COSE map label for information about how to retrieve the
      credential of Party U, see Section 4.1

   o  HINT_ID_V - information about how to retrieve the credential of
      Party U

   o  APP_3 - bstr containing opaque application data

4.4.2.  Party U Processing of Message 3

   Party U SHALL compose message_3 as follows:

   o  Format message_3 as specified in Section 4.4.1:

      *  COSE_Sign1 is computed as defined in section 4.4 of
         [I-D.ietf-cose-msg], using algorithm SIG_U and the private key
         of Party U.

      *  COSE_Encrypt0 is computed as defined in section 5.3 of
         [I-D.ietf-cose-msg], with AEAD_V, K_3, and IV_3.  The AEAD
         algorithm MUST NOT be replaced by plain encryption, see
         Section 8.

4.4.3.  Party V Processing of Message 3

   Party V SHALL process message_3 as follows:

   o  Use the session identifier S_V to retrieve the protocol state.

   o  Verify message_3 as specified in Section 4.4.1:

      *  COSE_Encrypt0 is decrypted as defined in section 5.3 of
         [I-D.ietf-cose-msg], with AEAD_V, K_3, and IV_3.

      *  COSE_Sign1 is verified as defined in section 4.4 of
         [I-D.ietf-cose-msg], using algorithm SIG_U and the public key
         of Party U.

   If any verification step fails, Party V MUST send an EDHOC error
   message back, formatted as defined in Section 6.1, and the protocol
   MUST be discontinued.

   o  Pass APP_3 to the application.






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5.  EDHOC Authenticated with Symmetric Keys

5.1.  Overview

   EDHOC supports authentication with pre-shared keys.  Party U and V
   are assumed to have a pre-shared uniformly random key (PSK) with the
   requirement that:

   o  Party V SHALL be able to identify the PSK using KID.

   KID may optionally contain information about how to retrieve the PSK.

   EDHOC with symmetric key authentication is illustrated in Figure 4.

   Party U                                                       Party V
   |                S_U, N_U, E_U, ALG_1, KID, APP_1                   |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |         S_U, S_V, N_V, E_V, ALG_2, Enc(K_2; APP_2; aad_2)         |
   |<------------------------------------------------------------------+
   |                             message_2                             |
   |                                                                   |
   |                    S_V, Enc(K_3; APP_3; aad_3)                    |
   +------------------------------------------------------------------>|
   |                             message_3                             |

            Figure 4: EDHOC with symmetric key authentication.

5.1.1.  Mandatory to Implement Algorithms

   For EDHOC authenticated with symmetric keys, the COSE algorithms
   ECDH-SS + HKDF-256 and AES-CCM-64-64-128 are mandatory to implement.

5.2.  EDHOC Message 1

5.2.1.  Formatting of Message 1

   message_1 SHALL be a CBOR array as defined below












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   message_1 = [
     data_1
   ]

   data_1 = (
     MSG_TYPE : int,
     S_U : bstr,
     N_U : bstr,
     E_U : serialized_COSE_Key,
     ECDH-Curves_U : alg_array,
     HKDFs_U : alg_array,
     AEADs_U : alg_array,
     KID : bstr,
     ? APP_1 : bstr
   )

   serialized_COSE_Key = bstr .cbor COSE_Key

   alg_array = [ + alg : int / tstr ]

   where:

   o  MSG_TYPE = 4

   o  S_U - variable length session identifier

   o  N_U - 64-bit random nonce

   o  E_U - the ephemeral public key of Party U

   o  ECDH-Curves_U - EC curves for ECDH which Party U supports, in the
      order of decreasing preference

   o  HKDFs_U - supported ECDH-SS w/ HKDF algorithms

   o  AEADs_U - supported AEAD algorithms

   o  KID - identifier of the pre-shared key

   o  APP_1 - bstr containing opaque application data

5.2.2.  Party U Processing of Message 1

   Party U SHALL compose message_1 as follows:

   o  Determine which ECDH curve to use with Party V.  If U previously
      received from Party V an error message to message_1 with
      diagnostic payload identifying an ECDH curve in ECDH-Curves_U,



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      then U SHALL retrieve an ephemeral from that curve.  Otherwise the
      first curve in ECDH-Curves_U MUST be used.

   o  Retrieve an ephemeral ECDH key pair generated as specified in
      Section 5 of [SP-800-56a] and format the ephemeral public key E_U
      as a COSE_key as specified in Section 3.1.

   o  Generate the pseudo-random nonce N_U.

   o  Choose a session identifier S_U which is not in use and store it
      for the length of the protocol.  The session identifier SHOULD be
      different from other relevant concurrent session identifiers used
      by Party U.  The session identifier MAY be used with the protocol
      for which EDHOC establishes traffic keys/master secret, in which
      case S_U SHALL be different from the concurrently used session
      identifiers of that protocol.

   o  Format message_1 as specified in Section 5.2.1.

5.2.3.  Party V Processing of Message 1

   Party V SHALL process message_1 as follows:

   o  Verify (OPTIONAL) that N_U has not been received before.

   o  Verify that at least one of each kind of the proposed algorithms
      are supported.

   o  Verify that the ECDH curve used in E_U is supported, and that no
      prior curve in ECDH-Curves_U is supported.

   o  For EC2 curves, validate that E_U is a valid point by verifying
      that there is a solution to the curve definition for the given
      parameter 'x'.

   If any verification step fails, Party V MUST send an EDHOC error
   message back, formatted as defined in Section 6.1, and the protocol
   MUST be discontinued.  If V does not support the ECDH curve used in
   E_U, but supports another ECDH curves in ECDH-Curves_U, then the
   error message SHOULD include a diagnostic payload describing the
   first supported ECDH curve in ECDH-Curves_U.

   o  Pass APP_1 to the application.








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5.3.  EDHOC Message 2

5.3.1.  Formatting of Message 2

   message_2 SHALL be a CBOR array as defined below

   message_2 = [
     data_2,
     COSE_ENC_2 : COSE_Encrypt0
   ]

   data_2 = (
     MSG_TYPE : int,
     S_U : bstr,
     S_V : bstr,
     N_V : bstr,
     E_V : serialized_COSE_Key,
     HKDF_V : int / tstr,
     AEAD_V : int / tstr
   )

   aad_2 : bstr

   where aad_2, in non-CDDL notation, is:

   aad_2 = H( message_1 | [ data_2 ] )

   where:

   o  MSG_TYPE = 5

   o  S_V - variable length session identifier

   o  N_V - 64-bit random nonce

   o  E_V - the ephemeral public key of Party V

   o  HKDF_V - an single chosen algorithm from HKDFs_U

   o  AEAD_V - an single chosen algorithm from AEADs_U

   o  COSE_ENC_2 has the following fields and values:

      *  external_aad = aad_2

      *  plaintext = ? APP_2

   o  APP_2 - bstr containing opaque application data



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   o  H() - the hash function in HKDF_V

5.3.2.  Party V Processing of Message 2

   Party V SHALL compose message_2 as follows:

   o  Retrieve an ephemeral ECDH key pair generated as specified in
      Section 5 of [SP-800-56a] using same curve as used in E_U.  Format
      the ephemeral public key E_V as a COSE_key as specified in
      Section 3.1.

   o  Generate the pseudo-random nonce N_V.

   o  Choose a session identifier S_V which is not in use and store it
      for the length of the protocol.  The session identifier SHOULD be
      different from other relevant concurrent session identifiers used
      by Party V.  The session identifier MAY be used with the protocol
      for which EDHOC establishes traffic keys/master secret, in which
      case S_V SHALL be different from the concurrently used session
      identifiers of that protocol.

   o  Select HKDF_V and AEAD_V from the algorithms proposed in HKDFs_U
      and AEADs_U.

   o  Format message_2 as specified in Section 5.3.1 where COSE_Encrypt0
      is computed as defined in section 5.3 of [I-D.ietf-cose-msg], with
      AEAD_V, K_2, and IV_2.

5.3.3.  Party U Processing of Message 2

   Party U SHALL process message_2 as follows:

   o  Use the session identifier S_U to retrieve the protocol state.

   o  For EC2 curves, validate that E_V is a valid point by verifying
      that there is a solution to the curve definition for the given
      parameter 'x'.

   o  Verify message_2 as specified in Section 5.3.1 where COSE_Encrypt0
      is decrypted defined in section 5.3 of [I-D.ietf-cose-msg], with
      AEAD_V, K_2, and IV_2.

   If any verification step fails, Party U MUST send an EDHOC error
   message back, formatted as defined in Section 6.1, and the protocol
   MUST be discontinued.

   o  Pass APP_2 to the application.




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5.4.  EDHOC Message 3

5.4.1.  Formatting of Message 3

   message_3 SHALL be a CBOR array as defined below

   message_3 = [
     data_3,
     COSE_ENC_3 : COSE_Encrypt0
   ]

   data_3 = (
     MSG_TYPE : int,
     S_V : bstr
   )

   aad_3 : bstr

   where aad_3, in non-CDDL notation, is:

   aad_3 = H( H( message_1 | message_2 ) | [ data_3 ] )

   where:

   o  MSG_TYPE = 6

   o  COSE_ENC_3 has the following fields and values:

      *  external_aad = aad_3

      *  plaintext = ? APP_3

   o  APP_3 - bstr containing opaque application data

5.4.2.  Party U Processing of Message 3

   Party U SHALL compose message_3 as follows:

   o  Format message_3 as specified in Section 5.4.1 where COSE_Encrypt0
      is computed as defined in section 5.3 of [I-D.ietf-cose-msg], with
      AEAD_V, K_3, and IV_3.

5.4.3.  Party V Processing of Message 3

   Party V SHALL process message_3 as follows:

   o  Use the session identifier S_V to retrieve the protocol state.




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   o  Verify message_3 as specified in Section 5.4.1 where COSE_Encrypt0
      is decrypted and verified as defined in section 5.3 of
      [I-D.ietf-cose-msg], with AEAD_V, K_3, and IV_3.

   If any verification step fails, Party V MUST send an EDHOC error
   message back, formatted as defined in Section 6.1, and the protocol
   MUST be discontinued.

   o  Pass APP_3 to the application.

6.  Error Handling

6.1.  Error Message Format

   This section defines a message format for an EDHOC error message,
   used during the protocol.  This is an error on EDHOC level and is
   independent of the lower layers used.  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 array as defined below

   error = [
     MSG_TYPE : int,
     ? ERR_MSG : tstr
   ]

   where:

   o  MSG_TYPE = 0

   o  ERR_MSG is an optional text string containing the diagnostic
      payload, defined in the same way as in Section 5.5.2 of [RFC7252].

7.  IANA Considerations

7.1.  Media Types Registry

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











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       Type name: application

       Subtype name: edhoc

       Required parameters: N/A

       Optional parameters: N/A

       Encoding considerations: binary

       Security considerations: See Section 7 of this document.

       Interoperability considerations: N/A

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

       Applications that use this media type: To be identified

       Fragment identifier considerations: N/A

       Additional information:

       * Magic number(s): N/A

       * File extension(s): N/A

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

       Person & email address to contact for further information:
          Goeran Selander <goran.selander@ericsson.com>

       Intended usage: COMMON

       Restrictions on usage: N/A

       Author: Goeran Selander <goran.selander@ericsson.com>

       Change Controller: IESG

8.  Security Considerations

   EDHOC builds on the SIGMA-I family of theoretical protocols that
   provides perfect forward secrecy and identity protection with a
   minimal number of messages.  The encryption algorithm of the SIGMA-I
   protocol provides identity protection, but the security of the
   protocol requires the MAC to cover the identity of the signer.  Hence
   the message authenticating functionality of the authenticated
   encryption in EDHOC is critical: authenticated encryption MUST NOT be



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   replaced by plain encryption only, even if authentication is provided
   at another level or through a different mechanism.

   EDHOC adds an explicit message type and expands the message
   authentication coverage to additional elements such as algorithms,
   application data, and previous messages.  EDHOC uses the same Sign-
   then-MAC approach as TLS 1.3.

   EDHOC does not include negotiation of parameters related to the
   ephemeral key, but it enables Party V to verify that the ECDH curve
   used in the protocol is the most preferred curve by U which is
   supported by both U and V.

   Party U and V 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 APP_1 and APP_2 in the asymmetric case, and APP_1 and KID
   in the symmetric case.  The communicating parties may therefore
   anonymize KID.

   Using the same KID or unprotected application data in several EDHOC
   sessions allows passive eavesdroppers to correlate the different
   sessions.  Another consideration is that the list of supported
   algorithms may be used to identify the application.

   Party U and V are allowed to select the session identifiers S_U and
   S_V, respectively, for the other party to use in the ongoing EDHOC
   protocol as well as in a subsequent traffic protection protocol (e.g.
   OSCOAP).  The choice of session identifier is not security critical
   but intended to simplify the retrieval of the right security context
   in combination with using short identifiers.  If the wrong session
   identifier of the other party is used in a protocol message it will
   result in the receiving party not being able to retrieve a security
   context (which will terminate the protocol) or retrieving the wrong
   security context (which also terminates the protocol as the message
   cannot be verified).

   Party U and V must make sure that unprotected data does not trigger
   any harmful actions.  In particular, this applies to APP_1 in the
   asymmetric case, and APP_1 and KID in the symmetric case.  Party V
   should be aware that replays of EDHOC message_1 cannot be detected
   unless previous nonces are stored.

   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.  If ECDSA is supported,
   "deterministic ECDSA" as specified in RFC6979 is RECOMMENDED.



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   Nonces MUST NOT be reused, both parties MUST generate fresh random
   nonces.

   Ephemeral keys SHOULD NOT be reused, both parties SHOULD generate
   fresh random ephemeral key pairs.  Party V MAY reuse the ephemeral
   key to limit the effect of certain DoS attacks.  For example, to
   reduce processing costs in the case of repeated uncompleted protocol
   runs, party V MAY pre-compute its ephemeral key E_V and reuse it for
   a small number of concurrent EDHOC executions, for example until a
   number of EDHOC protocol instances has been successfully completed,
   which triggers party V to pre-compute a new ephemeral key E_V to use
   with subsequent protocol runs.

   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.

   Party U and V 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 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.  Party U
   and V should enforce a minimum security level.

   Note that, depending on the application, the keys established through
   the EDHOC protocol will need to be renewed, in which case the
   communicating parties need to run the protocol again.

   Implementations should provide countermeasures to side-channel
   attacks such as timing attacks.

9.  Acknowledgments

   The authors want to thank Dan Harkins, Ilari Liusvaara, Jim Schaad
   and Ludwig Seitz for reviewing intermediate versions of the draft and
   contributing concrete proposals incorporated in this version.  We are
   especially indebted to Jim Schaad for his continuous reviewing and
   implementation of different versions of the draft.

   TODO: This section should be after Appendices and before Authors'
   addresses according to RFC7322.







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10.  References

10.1.  Normative References

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

   [I-D.schaad-cose-x509]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Headers for carrying and referencing X.509 certificates",
              draft-schaad-cose-x509-01 (work in progress), May 2017.

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

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

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

   [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., and M. Smid,
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography", NIST Special
              Publication 800-56A Revision 2, May 2013,
              <http://dx.doi.org/10.6028/NIST.SP.800-56Ar2>.

10.2.  Informative References

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "CBOR data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-greevenbosch-appsawg-
              cbor-cddl-10 (work in progress), March 2017.





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   [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-02 (work in progress), January 2017.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-06 (work in progress), March 2017.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-ietf-core-
              object-security-04 (work in progress), July 2017.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-10
              (work in progress), March 2017.

   [I-D.seitz-ace-oscoap-profile]
              Seitz, L., Gunnarsson, M., and F. Palombini, "OSCOAP
              profile of ACE", draft-seitz-ace-oscoap-profile-03 (work
              in progress), June 2017.

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

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

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

Appendix A.  Test Vectors

   TODO: This section needs to be updated.







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Appendix B.  PSK Chaining

   An application using EDHOC with symmetric keys may have a security
   policy to change the PSK as a result of successfully completing the
   EDHOC protocol.  In this case, the old PSK SHALL be replaced with a
   new PSK derived using other = exchange_hash, AlgorithmID = "EDHOC PSK
   Chaining" and keyDataLength equal to the key length of AEAD_V, see
   Section 3.2.

Appendix C.  EDHOC with CoAP and OSCOAP

C.1.  Transferring EDHOC in CoAP

   EDHOC can be transferred as an exchange of CoAP [RFC7252] messages,
   with the CoAP client as party U and the CoAP server as party V.  By
   default EDHOC is sent 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].

   In practice, EDHOC message_1 is sent in the payload of a POST request
   from the client to the server's resource for EDHOC.  EDHOC message_2
   or the EDHOC error message is sent from the server to the client in
   the payload of a 2.04 Changed response.  EDHOC message_3 or the EDHOC
   error message is sent from the client to the server's resource in the
   payload of a POST request.  If needed, an EDHOC error message is sent
   from the server to the client in the payload of a 2.04 Changed
   response

   An example of successful EDHOC exchange using CoAP is shown in
   Figure 5.





















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

                   Figure 5: Transferring EDHOC in CoAP

C.2.  Deriving an OSCOAP context from EDHOC

   When EDHOC is use to derive parameters for OSCOAP
   [I-D.ietf-core-object-security], the parties must make sure that the
   EDHOC session identifiers are unique Recipient IDs in OSCOAP.  In
   case that the CoAP client is party U and the CoAP server is party V:

   o  The AEAD Algorithm is AEAD_V, as defined in this document

   o  The KDF algorithm is HKDF_V, as defined in this document

   o  The Client's Sender ID is S_V, as defined in this document

   o  The Server's Sender ID is S_U, as defined in this document

   o  The Master Secret is derived as specified in Section 3.2 of this
      document, with other = exchange_hash, AlgorithmID = "EDHOC OSCOAP
      Master Secret" and keyDataLength equal to the key length of
      AEAD_V.

   o  The Master Salt is derived as specified in Section 3.2 of this
      document, with other = exchange_hash, AlgorithmID = "EDHOC OSCOAP
      Master Salt" and keyDataLength equal to 64 bits.






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Authors' Addresses

   Goeran Selander
   Ericsson AB
   Faeroegatan 6
   Kista  SE-164 80 Stockholm
   Sweden

   Email: goran.selander@ericsson.com


   John Mattsson
   Ericsson AB
   Faeroegatan 6
   Kista  SE-164 80 Stockholm
   Sweden

   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB
   Faeroegatan 6
   Kista  SE-164 80 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com
























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