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CoRE Working Group                                          F. Palombini
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               M. Tiloca
Expires: 14 January 2021                                     R. Hoeglund
                                                                 RISE AB
                                                            S. Hristozov
                                                        Fraunhofer AISEC
                                                             G. Selander
                                                                Ericsson
                                                            13 July 2020


                       Combining EDHOC and OSCORE
                  draft-palombini-core-oscore-edhoc-00

Abstract

   This document defines possible optimization approaches for combining
   the lightweight authenticated key exchange protocol EDHOC run over
   CoAP with the first subsequent OSCORE transaction.  This combination
   reduces the number of round trips required to set up an OSCORE
   Security Context and complete an OSCORE transaction using that
   context.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/EricssonResearch/oscore-edhoc
   (https://github.com/EricssonResearch/oscore-edhoc).

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 14 January 2021.



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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 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  EDHOC in OSCORE . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Signalling in a New EDHOC Option  . . . . . . . . . . . .   6
     3.2.  Signalling in the OSCORE Option . . . . . . . . . . . . .   6
   4.  OSCORE in EDHOC . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Signalling in a New EDHOC+OSCORE Option . . . . . . . . .   8
     4.2.  Signalling Based on the Number of Elements in the
           Payload . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document presents possible optimization approaches to combine
   the lightweight authenticated key exchange protocol EDHOC
   [I-D.ietf-lake-edhoc], when running over CoAP [RFC7252], with the
   first subsequent OSCORE [RFC8613] transaction.

   This allows for a minimum number of round trips necessary to setup
   the OSCORE Security Context and complete an OSCORE transaction, for
   example when an IoT device gets configured in a network for the first
   time.

   The number of protocol round trips impacts the minimum number of
   flights, which can have a substantial impact on performance with
   certain radio technologies as discussed in Section 2.11 of
   [I-D.ietf-lake-reqs].



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   Without this optimization, it is not possible, not even in theory, to
   achieve the minimum number of flights.  This optimization makes it
   possible also in practice, since the last message of the EDHOC
   protocol can be made relatively small (see Section 1 of
   [I-D.ietf-lake-edhoc]), thus allowing additional OSCORE protected
   CoAP data within target MTU sizes [I-D.ietf-lake-reqs].

   The goal of this document is to provide details on different
   alternatives for transporting and processing the necessary data,
   gather opinions on the different approaches, and select only one of
   those.

1.1.  Terminology

   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.

   The reader is expected to be familiar with terms and concepts defined
   in CoAP [RFC7252], CBOR [I-D.ietf-cbor-7049bis], OSCORE [RFC8613] and
   EDHOC [I-D.ietf-lake-edhoc].

2.  Background

   EDHOC is a 3-message key exchange protocol.  Section 7.1 of
   [I-D.ietf-lake-edhoc] specifies how to transport EDHOC over CoAP: the
   EDHOC data (referred to as "EDHOC messages") are transported in the
   payload of CoAP requests and responses.

   This draft deals with the case of the Initiator acting as CoAP Client
   and the Responder acting as CoAP Server.  (The case of the Initiator
   acting as CoAP server cannot be optimized in this way.)  That is, the
   CoAP Client sends a POST request containing the EDHOC message 1 to a
   reserved resource at the CoAP Server.  This triggers the EDHOC
   exchange on the CoAP Server, which replies with a 2.04 (Changed)
   Response containing the EDHOC message 2.  Finally, the EDHOC message
   3 is sent by the CoAP Client in a CoAP POST request to the same
   resource used for the EDHOC message 1.  The Content-Format of these
   CoAP messages is set to "application/edhoc".

   After this exchange takes place, and after successful verifications
   specified in the EDHOC protocol, the Client and Server derive the
   OSCORE Security Context, as specified in Section 7.1.1 of
   [I-D.ietf-lake-edhoc].  Then, they are ready to use OSCORE.





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   This sequential way of running EDHOC and then OSCORE is specified in
   Figure 1.  As shown in the figure, this mechanism is executed in 3
   round trips.

         CoAP Client                                  CoAP Server
              | ------------- EDHOC message_1 ------------> |
              |                                             |
              | <------------ EDHOC message_2 ------------- |
              |                                             |
      EDHOC verification                                    |
              |                                             |
              | ------------- EDHOC message_3 ------------> |
              |                                             |
              |                                    EDHOC verification
              |                                             |
      OSCORE Sec Ctx                                OSCORE Sec Ctx
        Derivation                                    Derivation
              |                                             |
              | -------------- OSCORE Request ------------> |
              |                                             |
              | <------------ OSCORE Response ------------- |
              |                                             |

                Figure 1: EDHOC and OSCORE run sequentially

   The number of roundtrips can be minimized: after receiving the EDHOC
   message 2, the CoAP Client has all the information needed to derive
   the OSCORE Security Context before sending the EDHOC message 3.

   This means that the Client can potentially send at the same time both
   the EDHOC message 3 and the subsequent OSCORE Request.  On a semantic
   level, this approach practically requires to send two separate REST
   requests at the same time.

   The high level message flow of running EDHOC and OSCORE combined is
   shown in Figure 2.

   Defining the specific details of how to transport the data and of
   their processing order is the goal of this specification.












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         CoAP Client                                  CoAP Server
              | ------------- EDHOC message_1 ------------> |
              |                                             |
              | <------------ EDHOC message_2 ------------- |
              |                                             |
      EDHOC verification +                                  |
        OSCORE Sec Ctx                                      |
          Derivation                                        |
              |                                             |
              | ------------- EDHOC message_3 ------------> |
              |              + OSCORE Request               |
              |                                             |
              |                                  EDHOC verification +
              |                                     OSCORE Sec Ctx
              |                                        Derivation
              |                                             |
              | <------------ OSCORE Response ------------- |
              |                                             |

                    Figure 2: EDHOC and OSCORE combined

3.  EDHOC in OSCORE

   The first approach consists in sending the EDHOC message 3 inside an
   OSCORE message (i.e., an OSCORE protected CoAP message).

   The request is in practice the OSCORE Request from Figure 1, sent to
   a protected resource and with the correct CoAP method and options,
   with the addition that it also transports the EDHOC message 3.

   As the EDHOC message 3 may be too large to be included in a CoAP
   Option, e.g. if containing a large public key certificate chain, it
   would have to be transported in the CoAP payload.

   The payload of the request is formatted as a CBOR sequence
   [I-D.ietf-lake-reqs] of two CBOR wrapped items: the EDHOC message 3
   and the OSCORE ciphertext, in this order.

   Note that the OSCORE ciphertext is not computed over the EDHOC
   message 3, which is not protected by OSCORE.  That is, the client
   first prepares the OSCORE Request as in Figure 1.  Then, it reformats
   the payload to include also the EDHOC message 3, as defined above.

   The usage of this approach is indicated by a signalling information,
   which can be either a new EDHOC option (see Section 3.1) or the
   OSCORE option with a particular Flag Bit set (see Section 3.2).





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   When receiving such a request, the Server needs to perform the
   following processing, in addition to the EDHOC, OSCORE and CoAP
   processing:

   1.  Check the signalling information to identify that this is an
       OSCORE + EDHOC request.

   2.  Extract the EDHOC message 3 from the payload.

   3.  Execute the EDHOC processing, including verifications and OSCORE
       Security Context derivation.

   4.  Decrypt and verify the remaining OSCORE protected CoAP request as
       defined by OSCORE.

   5.  Process the CoAP request.

   The following sections expand on the 2 ways of signalling that the
   EDHOC message is transported in the OSCORE message.

3.1.  Signalling in a New EDHOC Option

   One way to signal that the Server is to extract and process the EDHOC
   message 3 before the OSCORE message is processed is to define a new
   CoAP Option, called the EDHOC Option.

   This Option being present means that the message contains EDHOC data
   in the payload, that must be extracted and processed before the rest
   of the message can be processed.

   In particular, the EDHOC message is to be extracted from the CoAP
   payload, as the CBOR wrapped first element of a CBOR sequence.

   The Option is critical, Safe-to-Forward, and part of the Cache-Key.

   The Option value is always empty.  If any value is sent, the value is
   simply discarded.

   The Option must occur at most once.

   The Option is of Class U for OSCORE.

3.2.  Signalling in the OSCORE Option

   Another way to signal that the EDHOC message is to be extracted from
   the CoAP payload as the CBOR wrapped first element of a CBOR
   sequence, and that the processing defined in Section 3 is to be
   executed, is to use one of the OSCORE Flag Bits.



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   Bit Position: 8

   Name: EDHOC

   Description: Set to 1 if the payload is a sequence of EDHOC data and
   OSCORE payload.

   Reference: this document

4.  OSCORE in EDHOC

   Instead of transporting the EDHOC message inside an OSCORE message,
   the second approach consists in transporting the OSCORE protected
   data in an EDHOC message.

   The request is in practice the CoAP POST Request containing the EDHOC
   message 3 from Figure 1, sent to the unprotected resource reserved to
   EDHOC processing, with the addition that it also transports the
   OSCORE Option and ciphertext of the original OSCORE Request.

   The OSCORE Option and ciphertext contain all the information to
   reconstruct the original OSCORE Request, including CoAP method,
   options and payload.

   The payload is formatted as a CBOR sequence of three CBOR wrapped
   items: the EDHOC message 3, the OSCORE Option and the OSCORE
   ciphertext, in this order.

   Note that the OSCORE ciphertext is not computed over the EDHOC
   message 3, which is not protected by OSCORE.  That is, the client
   first prepares the OSCORE protected CoAP Request.  Then, it adds the
   OSCORE option and ciphertext to the payload of the EDHOC request to
   send, as defined above.

   The usage of this approach is indicated by a signalling information,
   which can be either a new EDHOC+OSCORE option (see Section 4.1) or
   the particular structure of the request payload (see Section 4.2).

   When receiving such a request, the Server needs to execute the
   following processing, in addition to the EDHOC, OSCORE and CoAP
   processing:

   1.  Check the signalling information to identify that this is an
       EDHOC + OSCORE request.

   2.  Extract the EDHOC message 3 from the payload.





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   3.  Execute the EDHOC processing, including verifications and OSCORE
       Security Context derivation.

   4.  Extract the OSCORE Option value and ciphertext from the payload
       and reconstruct the OSCORE protected CoAP Request.

   5.  Decrypt and verify the reconstructed OSCORE protected CoAP
       request as defined by OSCORE.

   6.  Process the CoAP request.

   Compared to the approach in Section 3, this processing requires one
   more step, as the Server must build the OSCORE protected CoAP request
   from the payload before being able to process it.

4.1.  Signalling in a New EDHOC+OSCORE Option

   One way to signal that the Server is to build and process the OSCORE
   protected CoAP request after the EDHOC processing is to define a new
   CoAP Option, called the EDHOC+OSCORE Option.

   This Option being present (either in a request or response) means
   that the message contains an OSCORE option value and ciphertext in
   the payload, that must be extracted and processed after the EDHOC
   processing.

   The OSCORE option and ciphertext are to be extracted from the CoAP
   payload as the CBOR wrapped second and third element of a CBOR
   sequence.

   The Option is critical, Safe-to-Forward, and part of the Cache-Key.

   The Option value is always empty.  If any value is sent, the value is
   simply discarded.

   The Option must occur at most once.

   The Option is of Class U for OSCORE.

4.2.  Signalling Based on the Number of Elements in the Payload

   Another way to signal this approach and to mandate that the Server is
   to build and process the OSCORE protected CoAP request after the
   EDHOC processing is to set up pre-determined policies on both the
   Client and Server.






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   A Client may be set up to support at the same time receiving only the
   EDHOC message 3 or both the EDHOC message 3 and the OSCORE Option and
   ciphertext in the request.  The Client would be able to distinguish
   the two cases based on the number of CBOR elements in the payload,
   and process the message accordingly.

5.  Security Considerations

   The same security considerations from OSCORE [RFC8613] and EDHOC
   [I-D.ietf-lake-edhoc] hold for this document.

   TODO (more considerations)

6.  IANA Considerations

   This document has no IANA actions.

7.  Normative References

   [I-D.ietf-cbor-7049bis]
              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", Work in Progress, Internet-Draft,
              draft-ietf-cbor-7049bis-14, 16 June 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-cbor-
              7049bis-14.txt>.

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", Work in Progress,
              Internet-Draft, draft-ietf-lake-edhoc-00, 6 July 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-lake-
              edhoc-00.txt>.

   [I-D.ietf-lake-reqs]
              Vucinic, M., Selander, G., Mattsson, J., and D. Garcia-
              Carillo, "Requirements for a Lightweight AKE for OSCORE",
              Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
              8 June 2020, <http://www.ietf.org/internet-drafts/draft-
              ietf-lake-reqs-04.txt>.

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







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   [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>.

   [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>.

   [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>.

Acknowledgments

   The authors sincerely thank Christian Amsuess, Klaus Hartke, Jim
   Schaad and Malisa Vucinic for their feedback and comments in the
   discussion leading up to this draft.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC.

Authors' Addresses

   Francesca Palombini
   Ericsson

   Email: francesca.palombini@ericsson.com


   Marco Tiloca
   RISE AB

   Email: marco.tiloca@ri.se


   Rikard Hoeglund
   RISE AB

   Email: rikard.hoglund@ri.se


   Stefan Hristozov
   Fraunhofer AISEC

   Email: stefan.hristozov@aisec.fraunhofer.de




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   Goeran Selander
   Ericsson

   Email: goran.selander@ericsson.com















































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