RFC 9668: Using Ephemeral Diffie-Hellman Over COSE (EDHOC) with the Constrained Application Protocol (CoAP) and Object Security for Constrained RESTful Environments (OSCORE)

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9668.¶

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1. Introduction

Ephemeral Diffie-Hellman Over COSE (EDHOC) [RFC9528] is a lightweight authenticated key exchange protocol that is specifically 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 details the use of the EDHOC protocol with CoAP and OSCORE and specifies a number of additional and optional mechanisms. These include an optimization approach that combines the EDHOC execution with the first OSCORE transaction (see Section 3). This allows for a minimum number of two round trips necessary to set up the OSCORE Security Context and complete an OSCORE transaction, e.g., when an Internet of Things (IoT) device gets configured in a network for the first time.¶

This optimization is desirable since the number of message exchanges 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 to achieve the minimum number of two round trips. This optimization makes it possible since the message_3 of the EDHOC protocol can be made relatively small (see Section 1.2 of [RFC9528]), thus allowing additional OSCORE-protected CoAP data within target MTU sizes.¶

The minimum number of two round trips can be achieved only if the default forward message flow of EDHOC is used, i.e., when a CoAP client acts as EDHOC Initiator and a CoAP server acts as EDHOC Responder. The performance advantage of using this optimization can be lost when used in combination with Block-wise transfers [RFC7959] that rely on specific parameter values and block sizes.¶

Furthermore, this document defines a number of parameters corresponding to different information elements of an EDHOC application profile (see Section 6). These parameters 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 a resource for CoAP clients.¶

2. EDHOC Overview

This section is not normative and summarizes what is specified in [RFC9528] (specifically Appendix A.2 of [RFC9528]). Thus, it provides a baseline for the enhancements in the subsequent sections.¶

The EDHOC protocol specified in [RFC9528] allows two peers to agree on a cryptographic secret in a mutually-authenticated way and achieves forward secrecy by using Diffie-Hellman ephemeral keys. The two peers are denoted as the "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 establish an OSCORE Security Context [RFC8613]. The Responder can also send an optional EDHOC message_4 in order for the Initiator 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 [RFC9528] specifies how to transfer EDHOC over CoAP. That is, the EDHOC data (i.e., the EDHOC message possibly with a prepended connection identifier) is transported in the payload of CoAP requests and responses. The default forward message flow of EDHOC consists in the CoAP client acting as Initiator and the CoAP server acting as Responder (see Appendix A.2.1 of [RFC9528]). Alternatively, the two roles can be reversed as per the reverse message flow of EDHOC (see Appendix A.2.2 of [RFC9528]). In the rest of this document, EDHOC messages are considered to be transferred over CoAP.¶

Figure 1 shows a successful execution of EDHOC, with a CoAP client and a CoAP server running EDHOC as Initiator and Responder, respectively. In particular, it extends Figure 10 from Appendix A.2.1 of [RFC9528] by highlighting when the two peers perform EDHOC verification and establish the OSCORE Security Context, and by adding an exchange of OSCORE-protected CoAP messages after completing the EDHOC execution.¶

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 encoded as per Section 3.3 of [RFC9528]. The request has Content-Format application/cid-edhoc+cbor-seq.¶

This triggers the EDHOC execution 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 encoded as per Section 3.3 of [RFC9528]. The response has Content-Format application/edhoc+cbor-seq.¶

Finally, the client sends a POST request to the same EDHOC resource used earlier when it sent EDHOC message_1. The request payload consists of the EDHOC connection identifier C_R encoded as per Section 3.3 of [RFC9528] concatenated with EDHOC message_3. The request has Content-Format 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 [RFC9528]. After that, the client and server can use OSCORE to protect their communications as per [RFC8613]. Note that the EDHOC connection identifier C_R is used as the OSCORE Sender ID of the client (see Appendix A.1 of [RFC9528]). Therefore, C_R is transported in the 'kid' field of the OSCORE option of the OSCORE Request (see Section 6.1 of [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 associated with the EDHOC resource addressed (see Section 3.9 of [RFC9528]).¶

The sequential flow of EDHOC and OSCORE (where EDHOC runs first and OSCORE is used after) takes three round trips to complete, as shown in Figure 1.¶

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

3. EDHOC Combined with OSCORE

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

To this end, this approach can be used only if the default forward message flow of EDHOC is used, i.e., when the client acts as Initiator and the server acts as Responder. The same is not possible in the case with reversed roles as per the reverse message flow of EDHOC.¶

When running the 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 shown in Figure 2.¶

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 composed of the following two parts combined together in a single CoAP message. The steps for processing the EDHOC + OSCORE request and the two parts combined in the request itself are defined in Sections 3.2.1 and 3.3.1.¶

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 (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.¶

 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) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Observe Option| OSCORE Option ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EDHOC Option  | Other Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   0x44025d1f               ; CoAP 4-byte Header
     00003974               ; Token
     93 090001              ; OSCORE Option
     c0                     ; EDHOC Option
     ff 52d5535f3147e85f1cfacd9e78abf9e0a81bbf
        612f1092f1776f1c1668b3825e
   (46 bytes)

4. Use of EDHOC Connection Identifiers with OSCORE

The OSCORE Sender/Recipient IDs are the EDHOC connection identifiers (see Section 3.3.3 of [RFC9528]). This applies also to the optimized workflow defined in Section 3 of this document.¶

Note that the value of the 'kid' field in the OSCORE option of the EDHOC + OSCORE request is both the server's Recipient ID (i.e., the client's Sender ID) and the EDHOC connection identifier C_R of the server at Step 3 of Section 3.3.1.¶

5. Extension and Consistency of Application Profiles

It is possible to include the information below in the application profile referred by the client and server according to 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 SHOULD explicitly specify support for the EDHOC + OSCORE request.¶

In the case where the application profile indicates that the server supports the optional EDHOC message_4 (see Section 5.5 of [RFC9528]), it is still possible to use the optimized workflow based on the EDHOC + OSCORE request. However, this means that the server is not going to send EDHOC message_4 since it is not applicable to the optimized workflow (see Section 3.3.1).¶

Also, in the case where the application profile indicates that the server shall send EDHOC message_4, the application profile MUST NOT specify support for the EDHOC + OSCORE request. There is no point for the client to use the optimized workflow that is bound to fail (see Section 3.3.1).¶

6. Web Linking

Section 10.10 of [RFC9528] 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 a filter criterion.¶

At the same time, the application profile associated with an EDHOC resource provides information describing how the EDHOC protocol can be used through that resource. A client may become aware of the application profile, e.g., by obtaining its information elements upon discovering the EDHOC resources at the server. This allows the client to discover the EDHOC resources whose associated application profile denotes a way of using EDHOC that is most suitable to the client, e.g., with EDHOC cipher suites or authentication methods that the client also supports or prefers.¶

That is, while discovering an EDHOC resource, a client can contextually obtain 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 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 criterion 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.¶

'ed-i':

If present, specifies that the server supports the EDHOC Initiator role, hence the reverse message flow of EDHOC. A value MUST NOT be given to this parameter and any present value MUST be ignored by the recipient.¶

'ed-r':

If present, specifies that the server supports the EDHOC Responder role, hence the forward message flow of EDHOC. A value MUST NOT be given to this parameter and any present value MUST be ignored by the recipient.¶

'ed-method':

Specifies 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 10.3 of [RFC9528]. This parameter MAY occur multiple times, with each occurrence specifying an authentication method.¶

'ed-csuite':

Specifies 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 10.2 of [RFC9528]. This parameter MAY occur multiple times, with each occurrence specifying a cipher suite.¶

'ed-cred-t':

Specifies a type of authentication credential supported by the server. This parameter MUST specify a single value, which is taken from the 'Value' column of the "EDHOC Authentication Credential Types" Registry defined in Section 8.3 of this document. This parameter MAY occur multiple times, with each occurrence specifying a type of authentication credential.¶

'ed-idcred-t':

Specifies a type of identifier 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 Header Parameters" registry [COSE.Header.Parameters]. This parameter MAY occur multiple times, with each occurrence specifying a type of identifier for authentication credentials.¶

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

'ed-ead':

Specifies the support of the server for an External Authorization Data (EAD) item (see Section 3.8 of [RFC9528]). This parameter MUST specify a single value, which is taken from the 'Label' column of the "EDHOC External Authorization Data" registry defined in Section 10.5 of [RFC9528]. This parameter MAY occur multiple times, with each occurrence specifying the ead_label of an EAD item that the server supports.¶

'ed-comb-req':

If present, specifies 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 the recipient.¶

Future documents may update the definition of the parameters 'ed-i', 'ed-r', and 'ed-comb-req' by expanding their semantics and specifying what they can take as value.¶

The example in Figure 5 shows how a client discovers one EDHOC resource at a server and obtains information elements from the respective application profile. The CoRE 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,
    </.well-known/edhoc>;rt=core.edhoc;ed-csuite=0;ed-csuite=2;
        ed-method=0;ed-cred-t=0;ed-cred-t=1;ed-idcred-t=4;
        ed-i;ed-r;ed-comb-req

7. Security Considerations

The same security considerations from OSCORE [RFC8613] and EDHOC [RFC9528] hold for this document. In addition, the following considerations apply.¶

Section 3.2.1 specifies 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 [RFC9528]) and would still enforce replay protection of the OSCORE-protected request (see Sections 7.4 and 8.2 of [RFC8613]).¶

When using the optimized workflow in Figure 2, a minimum of 128-bit security against online brute-force attacks is achieved after the client receives and successfully verifies the first OSCORE-protected response (see Sections 9.1 and 9.4 of [RFC9528]). As an example, if EDHOC is used with method 3 (see Section 3.2 of [RFC9528]) and cipher suite 2 (see Section 3.6 of [RFC9528]), then the following holds:¶

With reference to the sequential workflow in Figure 1, the OSCORE request might have to undergo access-control checks at the server before being actually executed for accessing the target protected resource. The same MUST hold when the optimized workflow in Figure 2 is used, i.e., when using the EDHOC + OSCORE request.¶

That is, the rebuilt OSCORE-protected application request from Step 7 in Section 3.3.1 MUST undergo the same access-control checks that would be performed on a traditional OSCORE-protected application request sent individually as shown in Figure 1.¶

To this end, validated information to perform access-control checks (e.g., an access token issued by a trusted party) has to be available at the server before starting to process the rebuilt OSCORE-protected application request. Such information may have been provided to the server separately before starting the EDHOC execution altogether, or instead as External Authorization Data during the EDHOC execution (see Section 3.8 of [RFC9528]).¶

Thus, a successful completion of the EDHOC protocol and the following derivation of the OSCORE Security Context at the server do not play a role in determining whether the rebuilt OSCORE-protected request is authorized to access the target protected resource at the server.¶

8. IANA Considerations

This document has the following actions for IANA.¶

Acknowledgments

The authors sincerely thank Christian Amsüss, Emmanuel Baccelli, Carsten Bormann, Roman Danyliw, Esko Dijk, Joel Halpern, Wes Hardaker, Klaus Hartke, John Preuß Mattsson, David Navarro, Shuping Peng, Jim Schaad, Jürgen Schönwälder, John Scudder, Orie Steele, Gunter Van de Velde, Mališa Vučinić, and Paul Wouters for their feedback and comments.¶

The work on this document has been partly supported by the Sweden's Innovation Agency VINNOVA and the Celtic-Next project CRITISEC, and by the H2020 project SIFIS-Home (Grant agreement 952652).¶

Authors' Addresses