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

Document Type Active Internet-Draft (core WG)
Authors Francesca Palombini  , Marco Tiloca  , Rikard Höglund  , Stefan Hristozov  , Göran Selander 
Last updated 2021-04-13 (latest revision 2021-04-01)
Replaces draft-palombini-core-oscore-edhoc
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CoRE Working Group                                          F. Palombini
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               M. Tiloca
Expires: 3 October 2021                                      R. Hoeglund
                                                                 RISE AB
                                                            S. Hristozov
                                                        Fraunhofer AISEC
                                                             G. Selander
                                                                Ericsson
                                                            1 April 2021

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

Abstract

   This document defines an optimization approach 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 to complete an OSCORE transaction using that
   Security Context.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 3 October 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   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 Option  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  EDHOC Combined with OSCORE  . . . . . . . . . . . . . . . . .   6
     4.1.  Client Processing . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Server Processing . . . . . . . . . . . . . . . . . . . .   7
   5.  Example of EDHOC + OSCORE Request . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  10
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This document defines an optimization approach 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.

   This optimization is desirable, since the number of protocol round
   trips impacts the minimum number of flights, which in turn can have a
   substantial impact on the latency of conveying the first OSCORE
   request, when using certain radio technologies.

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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.

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 [RFC8949], CBOR sequences [RFC8742], OSCORE
   [RFC8613] and EDHOC [I-D.ietf-lake-edhoc].

2.  Background

   EDHOC is a 3-message key exchange protocol.  Section 7.2 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; instead, the case of the
   Initiator acting as CoAP Server cannot be optimized by using this
   approach.

   That is, the CoAP Client sends a POST request containing 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 EDHOC message_2.  Finally, the CoAP
   Client sends EDHOC message_3, as a CoAP POST request to the same
   resource used for EDHOC message_1.  The Content-Format of these CoAP
   messages may be 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 7.2.1 of
   [I-D.ietf-lake-edhoc].  Then, they are ready to use OSCORE.

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

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         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 as follows.  Already after
   receiving EDHOC message_2 and before sending EDHOC message_3, the
   CoAP Client has all the information needed to derive the OSCORE
   Security Context.

   This means that the Client can potentially send at the same time both
   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, as defined
   in Section 4.

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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 Option

   This section defines the EDHOC Option, used in a CoAP request to
   signal that the request combines EDHOC message_3 and OSCORE protected
   data.

   The EDHOC Option has the properties summarized in Figure 3, which
   extends Table 4 of [RFC7252].  The option is Critical, Safe-to-
   Forward, and part of the Cache-Key. The option MUST occur at most
   once and is always empty.  If any value is sent, the value is simply
   ignored.  The option is intended only for CoAP requests and is of
   Class U for OSCORE [RFC8613].

       +-------+---+---+---+---+-------+--------+--------+---------+
       | No.   | C | U | N | R | Name  | Format | Length | Default |
       +-------+---+---+---+---+-------+--------+--------+---------+
       | TBD13 | x |   |   |   | EDHOC | Empty  |   0    | (none)  |
       +-------+---+---+---+---+-------+--------+--------+---------+
              C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                        Figure 3: The EDHOC Option.

   The presence of this option means that the message payload contains
   also EDHOC data, that must be extracted and processed as defined in
   Section 4.2, before the rest of the message can be processed.

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   Figure 4 shows the format of a CoAP message containing both the EDHOC
   data and the OSCORE ciphertext, using the newly defined EDHOC option
   for signalling.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver| T |  TKL  |      Code     |          Message ID           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Token (if any, TKL bytes) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  OSCORE option  |   EDHOC option  | other options (if any) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1 1 1 1 1 1 1 1|    Payload
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 4: CoAP message for EDHOC and OSCORE combined - signalled
                           with the EDHOC Option

4.  EDHOC Combined with OSCORE

   The approach defined in this specification consists of sending EDHOC
   message_3 inside an OSCORE protected CoAP message.

   The resulting EDHOC + OSCORE 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 EDHOC message_3.

   Since 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
   has to be transported through the CoAP payload.

   The use of this approach is explicitly signalled by including an
   EDHOC Option (see Section 3) in the EDHOC + OSCORE request.

4.1.  Client Processing

   The Client prepares an EDHOC + OSCORE request as follows.

   1.  Compose EDHOC message_3 as per Section 5.4.2 of
       [I-D.ietf-lake-edhoc].

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       Since the Client is the EDHOC Initiator and the used Correlation
       Method is 1 (see Section 3.2.4 of [I-D.ietf-lake-edhoc]), the
       EDHOC message_3 always includes the Connection Identifier C_R and
       CIPHERTEXT_3.  Note that C_R is the OSCORE Sender ID of the
       Client, encoded as a bstr_identifier (see Section 5.1 of
       [I-D.ietf-lake-edhoc]).

   2.  Encrypt the original CoAP request as per Section 8.1 of
       [RFC8613], using the new OSCORE Security Context established
       after receiving EDHOC message_2.

       Note that the OSCORE ciphertext is not computed over EDHOC
       message_3, which is not protected by OSCORE.  That is, the result
       of this step is the OSCORE Request as in Figure 1.

   3.  Build a CBOR sequence [RFC8742] composed of two CBOR byte strings
       in the following order.

       *  The first CBOR byte string is the CIPHERTEXT_3 of the EDHOC
          message_3 resulting from step 3.

       *  The second CBOR byte string has as value the OSCORE ciphertext
          of the OSCORE protected CoAP request resulting from step 2.

   4.  Compose the EDHOC + OSCORE request, as the OSCORE protected CoAP
       request resulting from step 2, where the payload is replaced with
       the CBOR sequence built at step 3.

   5.  Signal the usage of this approach within the EDHOC + OSCORE
       request, by including the new EDHOC Option defined in Section 3.

4.2.  Server Processing

   When receiving an EDHOC + OSCORE request, the Server performs the
   following steps.

   1.  Check the presence of the EDHOC option defined in Section 3, to
       determine that the received request is an EDHOC + OSCORE request.
       If this is the case, the Server continues with the steps defined
       below.

   2.  Extract CIPHERTEXT_3 from the payload of the EDHOC + OSCORE
       request, as the first CBOR byte string in the CBOR sequence.

   3.  Rebuild EDHOC message_3, as a CBOR sequence composed of two CBOR
       byte strings in the following order.

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       *  The first CBOR byte string is the 'kid' of the Client
          indicated in the OSCORE option of the EDHOC + OSCORE request,
          encoded as a bstr_identifier (see Section 5.1 of
          [I-D.ietf-lake-edhoc]).

       *  The second CBOR byte string is the CIPHERTEXT_3 retrieved at
          step 2.

   4.  Perform the EDHOC processing on the EDHOC message_3 rebuilt at
       step 3, including verifications, and the OSCORE Security Context
       derivation, as per Section 5.4.3 and Section 7.2.1 of
       [I-D.ietf-lake-edhoc], respectively.

   5.  Extract the OSCORE ciphertext from the payload of the EDHOC +
       OSCORE request, as the value of the second CBOR byte string in
       the CBOR sequence.

   6.  Rebuild the OSCORE protected CoAP request as the EDHOC + OSCORE
       request, where the payload is replaced with the OSCORE ciphertext
       resulting from step 5.

   7.  Decrypt and verify the OSCORE protected CoAP request resulting
       from step 6, as per Section 8.2 of [RFC8613], by using the new
       OSCORE Security Context established at step 4.

   8.  Process the CoAP request resulting from step 7.

   If steps 4 (EDHOC processing) and 7 (OSCORE processing) are both
   successfully completed, the Server MUST reply with an OSCORE
   protected response, in order for the Client to achieve key
   confirmation (see Section 5.4.2 of [I-D.ietf-lake-edhoc]).  The usage
   of EDHOC message_4 as defined in Section 7.1 of [I-D.ietf-lake-edhoc]
   is not applicable to the approach defined in this specification.

   If step 4 (EDHOC processing) fails, the server discontinues the
   protocol as per Section 5.4.3 of [I-D.ietf-lake-edhoc] and sends an
   EDHOC error message, formatted as defined in Section 6.1 of
   [I-D.ietf-lake-edhoc].  In particular, the CoAP response conveying
   the EDHOC error message:

   *  MUST have Content-Format set to application/edhoc defined in
      Section 9.5 of [I-D.ietf-lake-edhoc].

   *  MUST specify a CoAP error response code, i.e. 4.00 (Bad Request)
      in case of client error (e.g. due to a malformed EDHOC message_3),
      or 5.00 (Internal Server Error) in case of server error (e.g. due
      to failure in deriving EDHOC key material).

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   If step 4 (EDHOC processing) is successfully completed but step 7
   (OSCORE processing) fails, the same OSCORE error handling applies as
   defined in Section 8.2 of [RFC8613].

5.  Example of EDHOC + OSCORE Request

   An example based on the OSCORE test vector from Appendix C.4 of
   [RFC8613] and the EDHOC test vector from Appendix B.2 of
   [I-D.ietf-lake-edhoc] is given in Figure 5.  In particular, the
   example assumes that:

   *  The used OSCORE Partial IV is 0, consistently with the first
      request protected with the new OSCORE Security Context.

   *  The OSCORE Sender ID of the Client is 0x20.  This corresponds to
      the EDHOC Connection Identifier C_R, which is encoded as the
      bstr_identifier 0x08 in EDHOC message_3.

   *  The EDHOC option is registered with CoAP option number 13.

       o  OSCORE option value: 0x090020 (3 bytes)

       o  EDHOC option value: - (0 bytes)

       o  C_R: 0x20 (1 byte)

       o  CIPHERTEXT_3: 0x5253c3991999a5ffb86921e99b607c067770e0
          (19 bytes)

       o  EDHOC message_3: 0x08 5253c3991999a5ffb86921e99b607c067770e0
          (20 bytes)

       o  OSCORE ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)

       From there:

       o  Protected CoAP request (OSCORE message):

          0x44025d1f               ; CoAP 4-byte header
            00003974               ; Token
            39 6c6f63616c686f7374  ; Uri-Host Option: "localhost"
            63 090020              ; OSCORE Option
            40                     ; EDHOC Option
            ff 5253c3991999a5ffb86921e99b607c067770e0
               4d612f1092f1776f1c1668b3825e
          (57 bytes)

      Figure 5: Example of CoAP message with EDHOC and OSCORE combined

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6.  Security Considerations

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

   TODO (more considerations)

7.  IANA Considerations

   This document has the following actions for IANA.

7.1.  CoAP Option Numbers Registry

   IANA is asked to enter the following option numbers to the "CoAP
   Option Numbers" registry defined in [RFC7252] within the "CoRE
   Parameters" registry.

   [

   The CoAP option numbers 13 and 21 are both consistent with the
   properties of the EDHOC Option defined in Section 3, and they both
   allow the EDHOC Option to always result in an overall size of 1 byte.
   This is because:

   *  The EDHOC option is always empty, i.e. with zero-length value; and

   *  Since the OSCORE option with option number 9 is always present in
      the CoAP request, the EDHOC option would be encoded with a maximum
      delta of 4 or 12, depending on its option number being 13 or 21.

   At the time of writing, the CoAP option numbers 13 and 21 are both
   unassigned in the "CoAP Option Numbers" registry, as first available
   and consistent option numbers for the EDHOC option.

   ]

                  +--------+-------+-------------------+
                  | Number | Name  |     Reference     |
                  +--------+-------+-------------------+
                  | TBD13  | EDHOC | [[this document]] |
                  +--------+-------+-------------------+

8.  Normative References

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   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-05, 22
              February 2021, <https://www.ietf.org/archive/id/draft-
              ietf-lake-edhoc-05.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>.

   [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>.

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

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

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; and by the H2020 project SIFIS-Home
   (Grant agreement 952652).

Authors' Addresses

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   Francesca Palombini
   Ericsson

   Email: francesca.palombini@ericsson.com

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden

   Email: marco.tiloca@ri.se

   Rikard Hoeglund
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden

   Email: rikard.hoglund@ri.se

   Stefan Hristozov
   Fraunhofer AISEC

   Email: stefan.hristozov@aisec.fraunhofer.de

   Goeran Selander
   Ericsson

   Email: goran.selander@ericsson.com

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