Profiling EDHOC for CoAP and OSCORE

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 12 January 2023.¶

Copyright Notice

Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

Ephemeral Diffie-Hellman Over COSE (EDHOC) [I-D.ietf-lake-edhoc] is a lightweight authenticated key exchange protocol, especially intended for use in constrained scenarios. In particular, EDHOC messages can be transported over the Constrained Application Protocol (CoAP) [RFC7252] and used for establishing a Security Context for Object Security for Constrained RESTful Environments (OSCORE) [RFC8613].¶

This document profiles this use of the EDHOC protocol, and specifies a number of additional and optional mechanisms. These especially include an optimization approach, that combines the EDHOC execution with the first subsequent OSCORE transaction (see Section 3). This allows for a minimum number of round trips necessary to setup the OSCORE Security Context and complete an OSCORE transaction, e.g., 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 on 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.¶

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 1.2 of [I-D.ietf-lake-edhoc]), thus allowing additional OSCORE-protected CoAP data within target MTU sizes.¶

Furthermore, this document defines a number of parameters corresponding to different information elements of an EDHOC application profile (see Section 7). These can be specified as target attributes in the link to an EDHOC resource associated with that application profile, thus enabling an enhanced discovery of such resource for CoAP clients.¶

2. EDHOC Overview

The EDHOC protocol allows two peers to agree on a cryptographic secret, in a mutually-authenticated way and by using Diffie-Hellman ephemeral keys to achieve forward secrecy. The two peers are denoted as Initiator and Responder, as the one sending or receiving the initial EDHOC message_1, respectively.¶

After successful processing of EDHOC message_3, both peers agree on a cryptographic secret that can be used to derive further security material, and especially to establish an OSCORE Security Context [RFC8613]. The Responder can also send an optional EDHOC message_4 to achieve key confirmation, e.g., in deployments where no protected application message is sent from the Responder to the Initiator.¶

Appendix A.2 of [I-D.ietf-lake-edhoc] specifies how to transfer EDHOC over CoAP. That is, the EDHOC data (referred to as "EDHOC messages") are transported in the payload of CoAP requests and responses. The default message flow consists in the CoAP Client acting as Initiator and the CoAP Server acting as Responder. Alternatively, the two roles can be reversed. In the rest of this document, EDHOC messages are considered to be transferred over CoAP.¶

Figure 1 shows a CoAP Client and Server running EDHOC as Initiator and Responder, respectively. That is, the Client sends a POST request to a reserved EDHOC resource at the Server, by default at the Uri-Path "/.well-known/edhoc". The request payload consists of the CBOR simple value "true" (0xf5) concatenated with EDHOC message_1, which also includes the EDHOC connection identifier C_I of the Client represented as per Section 3.3 of [I-D.ietf-lake-edhoc]. The Content-Format of the request may be set to application/cid-edhoc+cbor-seq.¶

This triggers the EDHOC exchange at the Server, which replies with a 2.04 (Changed) response. The response payload consists of EDHOC message_2, which also includes the EDHOC connection identifier C_R of the Server represented as per Section 3.3 of [I-D.ietf-lake-edhoc]. The Content-Format of the response may be set to application/edhoc+cbor-seq.¶

Finally, the Client sends a POST request to the same EDHOC resource used earlier to send EDHOC message_1. The request payload consists of the EDHOC connection identifier C_R represented as per Section 3.3 of [I-D.ietf-lake-edhoc], concatenated with EDHOC message_3. The Content-Format of the request may be set to application/cid-edhoc+cbor-seq.¶

After this exchange takes place, and after successful verifications as specified in the EDHOC protocol, the Client and Server can derive an OSCORE Security Context, as defined in Appendix A.1 of [I-D.ietf-lake-edhoc]. After that, they can use OSCORE to protect their communications as per [RFC8613].¶

The Client and Server are required to agree in advance on certain information and parameters describing how they should use EDHOC. These are specified in an application profile see Section 3.9 of [I-D.ietf-lake-edhoc], associated with the used EDHOC resource.¶

   CoAP Client                                         CoAP Server
(EDHOC Initiator)                                   (EDHOC Responder)
        |                                                    |
        |                                                    |
        | ----------------- EDHOC Request -----------------> |
        |   Header: 0.02 (POST)                              |
        |   Uri-Path: "/.well-known/edhoc"                   |
        |   Content-Format: application/cid-edhoc+cbor-seq   |
        |   Payload: true, EDHOC message_1                   |
        |                                                    |
        | <---------------- EDHOC Response------------------ |
        |       Header: 2.04 (Changed)                       |
        |       Content-Format: application/edhoc+cbor-seq   |
        |       Payload: EDHOC message_2                     |
        |                                                    |
EDHOC verification                                           |
        |                                                    |
        | ----------------- EDHOC Request -----------------> |
        |   Header: 0.02 (POST)                              |
        |   Uri-Path: "/.well-known/edhoc"                   |
        |   Content-Format: application/cid-edhoc+cbor-seq   |
        |   Payload: C_R, EDHOC message_3                    |
        |                                                    |
        |                                           EDHOC verification
        |                                                    +
OSCORE Sec Ctx                                        OSCORE Sec Ctx
  Derivation                                            Derivation
        |                                                    |
        | ---------------- OSCORE Request -----------------> |
        |   Header: 0.02 (POST)                              |
        |   Payload: OSCORE-protected data                   |
        |                                                    |
        | <--------------- OSCORE Response ----------------- |
        |                 Header: 2.04 (Changed)             |
        |                 Payload: OSCORE-protected data     |
        |                                                    |

As shown in Figure 1, this purely-sequential flow where EDHOC is run first and then OSCORE is used takes three round trips to complete.¶

Section 3 defines an optimization for combining EDHOC with the first subsequent OSCORE transaction. This reduces the number of round trips required to set up an OSCORE Security Context and to complete an OSCORE transaction using that Security Context.¶

3. EDHOC Combined with OSCORE

This section defines an optimization for combining the EDHOC exchange with the first subsequent OSCORE transaction, thus minimizing the number of round trips between the two peers.¶

This approach can be used only if the default EDHOC message flow is used, i.e., when the Client acts as Initiator and the Server acts as Responder, while it cannot be used in the case with reversed roles.¶

When running the purely-sequential flow of Section 2, the Client has all the information to derive the OSCORE Security Context already after receiving EDHOC message_2 and before sending EDHOC message_3.¶

Hence, the Client can potentially send both EDHOC message_3 and the subsequent OSCORE Request at the same time. On a semantic level, this requires sending two REST requests at once, as in Figure 2.¶

   CoAP Client                                          CoAP Server
(EDHOC Initiator)                                    (EDHOC Responder)
        |                                                     |
        | ------------------ EDHOC Request -----------------> |
        |   Header: 0.02 (POST)                               |
        |   Uri-Path: "/.well-known/edhoc"                    |
        |   Content-Format: application/cid-edhoc+cbor-seq    |
        |   Payload: true, EDHOC message_1                    |
        |                                                     |
        | <----------------- EDHOC Response------------------ |
        |        Header: Changed (2.04)                       |
        |        Content-Format: application/edhoc+cbor-seq   |
        |        Payload: EDHOC message_2                     |
        |                                                     |
EDHOC verification                                            |
        +                                                     |
  OSCORE Sec Ctx                                              |
    Derivation                                                |
        |                                                     |
        | ------------- EDHOC + OSCORE Request -------------> |
        |   Header: 0.02 (POST)                               |
        |   Payload: EDHOC message_3 + OSCORE-protected data  |
        |                                                     |
        |                                            EDHOC verification
        |                                                     +
        |                                             OSCORE Sec Ctx
        |                                                Derivation
        |                                                     |
        | <--------------- OSCORE Response ------------------ |
        |                    Header: 2.04 (Changed)           |
        |                    Payload: OSCORE-protected data   |
        |                                                     |

To this end, the specific approach defined in this section consists of sending a single EDHOC + OSCORE request, which conveys the pair (C_R, EDHOC message_3) within an OSCORE-protected CoAP message.¶

That is, the EDHOC + OSCORE request is in practice the OSCORE Request from Figure 1, as still sent to a protected resource and with the correct CoAP method and options intended for accessing that resource. At the same time, the EDHOC + OSCORE request also transports the pair (C_R, EDHOC message_3) required for completing the EDHOC exchange. Note that C_R is not transported precisely in the request payload.¶

Since EDHOC message_3 may be too large to be included in a CoAP Option, e.g., if conveying a protected large public key certificate chain as ID_CRED_I (see Section 3.5.3 of [I-D.ietf-lake-edhoc]) or if conveying protected External Authorization Data as EAD_3 (see Section 3.8 of [I-D.ietf-lake-edhoc]), EDHOC message_3 has to be transported in the CoAP payload of the EDHOC + OSCORE request.¶

The rest of this section specifies how to transport the data in the EDHOC + OSCORE request and their processing order. In particular, the use of this approach is explicitly signalled by including an EDHOC Option (see Section 3.1) in the EDHOC + OSCORE request. The processing of the EDHOC + OSCORE request is specified in Section 3.2 for the Client side and in Section 3.3 for the Server side.¶

+-------+---+---+---+---+-------+--------+--------+---------+
| No.   | C | U | N | R | Name  | Format | Length | Default |
+-------+---+---+---+---+-------+--------+--------+---------+
| TBD21 | x |   |   |   | EDHOC | Empty  |   0    | (none)  |
+-------+---+---+---+---+-------+--------+--------+---------+
       C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

o  OSCORE option value: 0x090001 (3 bytes)

o  EDHOC option value: - (0 bytes)

o  EDHOC message_3: 0x52d5535f3147e85f1cfacd9e78abf9e0a81bbf (19 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 090001              ; OSCORE Option
     c0                     ; EDHOC Option
     ff 52d5535f3147e85f1cfacd9e78abf9e0a81bbf
        4d612f1092f1776f1c1668b3825e
   (57 bytes)

4. Use of EDHOC Connection Identifiers with OSCORE

Section 3.3.3 of [I-D.ietf-lake-edhoc] defines the straightforward mapping from an EDHOC connection identifier to an OSCORE Sender/Recipient ID. That is, an EDHOC identifier and the corresponding OSCORE Sender/Recipient ID are both byte strings with the same value.¶

Therefore, the conversion from an OSCORE Sender/Recipient ID to an EDHOC identifier is equally straightforward. In particular, at step 3 of Section 3.3, the value of 'kid' in the OSCORE Option of the EDHOC + OSCORE request is both the Server's Recipient ID (i.e., the Client's Sender ID) as well as the EDHOC Connection Identifier C_R of the Server.¶

5. Extension and Consistency of Application Profiles

The application profile referred by the Client and Server can include the information elements introduced below, in accordance with the specified consistency rules.¶

If the Server supports the EDHOC + OSCORE request within an EDHOC execution started at a certain EDHOC resource, then the application profile associated with that resource:¶

• MUST NOT specify that EDHOC message_4 shall be sent.¶
• SHOULD explicitly specify support for the EDHOC + OSCORE request.¶

6. Considerations on Using Block-wise

This section provides guidelines and recommendations for Clients supporting both the EDHOC + OSCORE request defined in this document as well as Block-wise [RFC7959].¶

The following especially considers a Client that may perform only "inner" Block-wise, but not "outer" Block-wise operations. That is, the considered Client does not (further) split an OSCORE-protected request like an intermediary (e.g., a proxy) might do. This is the typical case for OSCORE endpoints (see Section 4.1.3.4 of [RFC8613]).¶

The rest of this section refers to the following notation.¶

• SIZE_APP: the size in bytes of the application data to be included in a CoAP request. When Block-wise is used, this is referred to as the "body" to be fragmented into blocks.¶
• SIZE_EDHOC: the size in bytes of EDHOC message_3, if this is sent as part of the EDHOC + OSCORE request. Otherwise, the size of EDHOC message_3 plus the size in bytes of the EDHOC Connection Identifier C_R, represented as per Section 3.3 of [I-D.ietf-lake-edhoc].¶
• SIZE_MTU: the maximum amount of transmittable bytes before having to use Block-wise. This is, for example, 64 KiB as maximum datagram size when using UDP, or 1280 bytes as the maximum size for an IPv6 MTU.¶
• SIZE_OH: the size in bytes of the overall overhead due to all the communication layers underlying the application. This takes into account also the overhead introduced by the OSCORE processing.¶
• LIMIT = (SIZE_MTU - SIZE_OH): the practical maximum size in bytes to be considered by the application before using Block-wise.¶
• SIZE_BLOCK: the size in bytes of inner blocks.¶
• ceil(): the ceiling function.¶

7. Web Linking

Section 9.10 of [I-D.ietf-lake-edhoc] registers the resource type "core.edhoc", which can be used as target attribute in a web link [RFC8288] to an EDHOC resource, e.g., using a link-format document [RFC6690]. This enables Clients to discover the presence of EDHOC resources at a Server, possibly using the resource type as filter criterion.¶

At the same time, the application profile associated with an EDHOC resource provides a number of information describing how the EDHOC protocol can be used through that resource. While a Client may become aware of the application profile through several means, it would be convenient to obtain its information elements upon discovering the EDHOC resources at the Server. This might aim at discovering especially the EDHOC resources whose associated application profile denotes a way of using EDHOC which is most suitable to the Client, e.g., with EDHOC cipher suites or authentication methods that the Client also supports or prefers.¶

That is, it would be convenient that a Client discovering an EDHOC resource contextually obtains relevant pieces of information from the application profile associated with that resource. The resource discovery can occur by means of a direct interaction with the Server, or instead by means of the CoRE Resource Directory [RFC9176], where the Server may have registered the links to its resources.¶

In order to enable the above, this section defines a number of parameters, each of which can be optionally specified as a target attribute with the same name in the link to the respective EDHOC resource, or as filter criteria in a discovery request from the Client. When specifying these parameters in a link to an EDHOC resource, the target attribute rt="core.edhoc" MUST be included, and the same consistency rules defined in Section 5 for the corresponding information elements of an application profile MUST be followed.¶

The following parameters are defined.¶

• 'method', specifying an authentication method supported by the Server. This parameter MUST specify a single value, which is taken from the 'Value' column of the "EDHOC Method Type" registry defined in Section 9.3 of [I-D.ietf-lake-edhoc]. This parameter MAY occur multiple times, with each occurrence specifying a different authentication method.¶
• 'csuite', specifying an EDHOC cipher suite supported by the Server. This parameter MUST specify a single value, which is taken from the 'Value' column of the "EDHOC Cipher Suites" registry defined in Section 9.2 of [I-D.ietf-lake-edhoc]. This parameter MAY occur multiple times, with each occurrence specifying a different cipher suite.¶
• 'cred_t', specifying a type of authentication credential supported by the Server. This parameter MAY occur multiple times, with each occurrence specifying a different authentication credential type. Possible values are: "x509", for X.509 certificate [RFC5280]; "c509", for C509 certificate [I-D.ietf-cose-cbor-encoded-cert]; "cwt" for CWT [RFC8392]; "ccs" for CWT Claims Set (CCS) [RFC8392].¶

• 'idcred_t', specifying the type of identifiers supported by the Server for identifying authentication credentials. This parameter MUST specify a single value, which is taken from the 'Label' column of the "COSE Headers Parameters" registry [COSE.Header.Parameters]. This parameter MAY occur multiple times, with each occurrence specifying a different type of identifier for authentication credentials.¶

  Note that the values in the 'Label' column of the "COSE Headers Parameters" registry are strongly typed. On the contrary, Link Format is weakly typed and thus does not distinguish between, for instance, the string value "-10" and the integer value -10. Thus, if responses in Link Format are returned, string values which look like an integer are not supported. Therefore, such values MUST NOT be used in the 'idcred_t' parameter.¶

• 'ead_1', 'ead_2', 'ead_3' and 'ead_4', specifying, if present, that the Server supports the use of External Authorization Data EAD_1, EAD_2, EAD_3 and EAD_4, respectively (see Section 3.8 of [I-D.ietf-lake-edhoc]). For each of these parameters, the following applies.¶

  • It MAY occur multiple times, with its presence denoting support from the server for the respective external authorization data.¶
  • Each occurrence specifies a value taken from the 'Label' column of the "EDHOC External Authorization Data" registry defined in Section 9.5 of [I-D.ietf-lake-edhoc], thus denoting support from the server for that particular type of external authorization data.¶
• 'comb_req', specifying, if present, that the server supports the EDHOC + OSCORE request defined in Section 3. A value MUST NOT be given to this parameter and any present value MUST be ignored by parsers.¶

The example in Figure 6 shows how a Client discovers two EDHOC resources at a Server, obtaining information elements from the respective application profiles. The Link Format notation from Section 5 of [RFC6690] is used.¶

REQ: GET /.well-known/core

RES: 2.05 Content
    </sensors/temp>;osc,
    </sensors/light>;if="sensor",
    </edhoc/resA>;rt="core.edhoc";csuite="0";csuite="2";method="0";
    cred_t="c509";cred_t="ccs";idcred_t="4";comb_req,
    </edhoc/resB>;rt="core.edhoc";csuite="0";csuite="2";method="0";
    method="3";cred_t="c509";cred_t="x509";idcred_t="34"

8. Security Considerations

The same security considerations from OSCORE [RFC8613] and EDHOC [I-D.ietf-lake-edhoc] hold for this document. In addition, the following considerations also apply.¶

Section 3.2 defines that a Client SHOULD NOT have multiple outstanding EDHOC + OSCORE requests pertaining to the same EDHOC session. Even if a Client did not fulfill this requirement, it would not have any impact in terms of security. That is, the Server would still not process different instances of the same EDHOC message_3 more than once in the same EDHOC session (see Section 5.1 of [I-D.ietf-lake-edhoc]), and would still enforce replay protection of the OSCORE-protected request (see Section 7.4 of [RFC8613]).¶

TODO: more considerations¶

9. IANA Considerations

This document has the following actions for IANA.¶

Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" with the RFC number of this specification and delete this paragraph.¶

+--------+-------+------------+
| Number | Name  | Reference  |
+--------+-------+------------+
| TBD21  | EDHOC | [RFC-XXXX] |
+--------+-------+------------+

Appendix A. Document Updates

RFC Editor: Please remove this section.¶

Acknowledgments

The authors sincerely thank Christian Amsüss, Esko Dijk, Klaus Hartke, Jim Schaad and Mališa Vučinić for their feedback and comments.¶

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