Internet Engineering Task Force (IETF) F. Palombini

Internet Engineering Task Force (IETF) F. Palombini
Request for Comments: 9668 Ericsson AB
Category: Standards Track M. Tiloca
ISSN: 2070-1721 R. Höglund
RISE AB
S. Hristozov
Eriptic
G. Selander
Ericsson
November 2024

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

Abstract

The lightweight authenticated key exchange protocol Ephemeral Diffie-
Hellman Over COSE (EDHOC) can be run over the Constrained Application
Protocol (CoAP) and used by two peers to establish a Security Context
for the security protocol Object Security for Constrained RESTful
Environments (OSCORE). This document details this use of the EDHOC
protocol by specifying a number of additional and optional
mechanisms, including an optimization approach for combining the
execution of EDHOC with the first 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

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.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
1.1. Terminology
2. EDHOC Overview
3. EDHOC Combined with OSCORE
3.1. EDHOC Option
3.2. Client Processing
3.2.1. Processing of the EDHOC + OSCORE Request
3.2.2. Supporting Block-Wise Transfers
3.3. Server Processing
3.3.1. Processing of the EDHOC + OSCORE Request
3.3.2. Supporting Block-Wise Transfers
3.4. Example of the EDHOC + OSCORE Request
4. Use of EDHOC Connection Identifiers with OSCORE
4.1. Additional Processing of EDHOC Messages
4.1.1. Initiator Processing of Message 1
4.1.2. Responder Processing of Message 2
4.1.3. Initiator Processing of Message 2
5. Extension and Consistency of Application Profiles
6. Web Linking
7. Security Considerations
8. IANA Considerations
8.1. CoAP Option Numbers Registry
8.2. Target Attributes Registry
8.3. EDHOC Authentication Credential Types Registry
8.4. Expert Review Instructions
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses

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.

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], Concise Binary Object Representation (CBOR)
[RFC8949], OSCORE [RFC8613], and EDHOC [RFC9528].

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]).

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
| Derivation
| |
| <---------------- EDHOC Response------------------ |
| Header: 2.04 (Changed) |
| Content-Format: application/edhoc+cbor-seq |
| Payload: EDHOC message_4 |
| |
OSCORE Sec Ctx |
Derivation |
| |
| ---------------- OSCORE Request -----------------> |
| Header: 0.02 (POST) |
| OSCORE: { ... ; kid: C_R } |
| Payload: OSCORE-protected data |
| |
| <--------------- OSCORE Response ----------------- |
| Header: 2.04 (Changed) |
| OSCORE: { ... } |
| Payload: OSCORE-protected data |
| |

Figure 1: Sequential Flow of EDHOC and OSCORE with the Optional
message_4 Included

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.

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 |
+ |
OSCORE Sec Ctx |
Derivation |
| |
| -------------- EDHOC + OSCORE Request ------------> |
| Header: 0.02 (POST) |
| OSCORE: { ... ; kid: C_R } |
| Payload: EDHOC message_3 + OSCORE-protected data |
| |
| EDHOC verification
| +
| OSCORE Sec Ctx
| Derivation
| |
| <--------------- OSCORE Response ------------------ |
| Header: 2.04 (Changed) |
| OSCORE: { ... } |
| Payload: OSCORE-protected data |
| |

Figure 2: EDHOC and OSCORE Combined

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 OSCORE Request from Figure 1, which, in this case, is also
sent to a protected resource with the correct CoAP method and
options intended for accessing that resource.

* EDHOC data consisting of the pair (C_R, EDHOC message_3) required
for completing the EDHOC session transported as follows:

- C_R is the OSCORE Sender ID of the client; hence, it is
transported in the 'kid' field of the OSCORE option (see
Section 6.1 of [RFC8613]). Unlike the sequential workflow
shown in Figure 1, C_R is not transported in the payload of the
EDHOC + OSCORE request.

- EDHOC message_3 is transported in the payload of the EDHOC +
OSCORE request and prepended to the payload of the OSCORE
Request. This is because EDHOC message_3 may be too large to
be included in a CoAP option, e.g., when conveying a large
public key certificate chain in the ID_CRED_I field (see
Section 3.5.3 of [RFC9528]), or when conveying large External
Authorization Data in the EAD_3 field (see Section 3.8 of
[RFC9528]).

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.

3.1. EDHOC Option

This section defines the EDHOC option. This option is used in a CoAP
request to signal that the request payload conveys both an EDHOC
message_3 and OSCORE-protected data combined together.

The EDHOC option has the properties summarized in Table 1, 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 MUST be empty. If any value is sent, the recipient MUST
ignore it. (Future documents may update the definition of the option
by expanding its semantics and specifying admitted values.) 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 |
+=====+===+===+===+===+=======+========+========+=========+
| 21 | x | | | | EDHOC | Empty | 0 | (none) |
+-----+---+---+---+---+-------+--------+--------+---------+

Table 1: The EDHOC Option. C=Critical, U=Unsafe,
N=NoCacheKey, R=Repeatable

The presence of this option means that the message payload also
contains EDHOC data that must be extracted and processed as defined
in Section 3.3 before the rest of the message can be processed.

Figure 3 shows an example of a CoAP message that is transported over
UDP and that contains 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) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Observe Option| OSCORE Option ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EDHOC Option | Other Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Example of a CoAP Message Containing the Combined EDHOC
and OSCORE Data, Signalled by the EDHOC Option and Transported
over UDP

3.2. Client Processing

This section describes the processing on the client side.

3.2.1. Processing of the EDHOC + OSCORE Request

The client prepares an EDHOC + OSCORE request as follows.

Step 1. Compose EDHOC message_3 into EDHOC_MSG_3 as per
Section 5.4.2 of [RFC9528].

Step 2. Establish the new OSCORE Security Context and use it to
encrypt the original CoAP request as per Section 8.1 of
[RFC8613].

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.

Step 3. Build COMB_PAYLOAD as the concatenation of EDHOC_MSG_3 and
OSCORE_PAYLOAD in the order of COMB_PAYLOAD = EDHOC_MSG_3 |
OSCORE_PAYLOAD, where | denotes byte string concatenation
and:

* EDHOC_MSG_3 is the binary encoding of EDHOC message_3
resulting from Step 1. As per Section 5.4.1 of
[RFC9528], EDHOC message_3 consists of one CBOR data item
CIPHERTEXT_3, which is a CBOR byte string. Therefore,
EDHOC_MSG_3 is the binary encoding of CIPHERTEXT_3.

* OSCORE_PAYLOAD is the OSCORE ciphertext of the OSCORE-
protected CoAP request resulting from Step 2.

Step 4. Compose the EDHOC + OSCORE request, as the OSCORE-protected
CoAP request resulting from Step 2, where the payload is
replaced with COMB_PAYLOAD built at Step 3.

Note that the new payload includes EDHOC message_3, but it
does not include the EDHOC connection identifier C_R. As
the client is the EDHOC Initiator, C_R is the OSCORE Sender
ID of the client, which is already specified as the value of
the 'kid' field in the OSCORE option of the request from
Step 2; hence, C_R is specified as the value of the 'kid'
field of the EDHOC + OSCORE request.

Step 5. Include the new EDHOC option defined in Section 3.1 into the
EDHOC + OSCORE request.

The application/cid-edhoc+cbor-seq media type does not apply
to this message, whose media type is unnamed.

Step 6. Send the EDHOC + OSCORE request to the server.

With the same server, the client SHOULD NOT have multiple
simultaneous outstanding interactions (see Section 4.7 of [RFC7252]),
such that they consist of an EDHOC + OSCORE request and their EDHOC
data pertains to the EDHOC session with the same connection
identifier C_R.

An exception might apply for clients that operate under particular
time constraints over particularly unreliable networks, thus raising
the chances to promptly complete the EDHOC execution with the server
through multiple simultaneous EDHOC + OSCORE requests. As discussed
in Section 7, this does not have any impact in terms of security.

3.2.2. Supporting Block-Wise Transfers

If Block-wise transfers [RFC7959] are supported, the client may
fragment the first CoAP application request before protecting it as
an original message with OSCORE as defined in Section 4.1.3.4.1 of
[RFC8613].

In such a case, the OSCORE processing in Step 2 of Section 3.2.1 is
performed on each inner block of the first CoAP application request.
The following also applies.

* The client takes the following additional step between Steps 2 and
3 of Section 3.2.1.

Step 2.1. If the OSCORE-protected request from Step 2 conveys a
non-first inner block of the first CoAP application
request (i.e., the Block1 option processed at Step 2 had
NUM different than 0), then the client skips the
following steps and sends the OSCORE-protected request
to the server. In particular, the client MUST NOT
include the EDHOC option in the OSCORE-protected
request.

* The client takes the following additional step between Steps 3 and
4 of Section 3.2.1.

Step 3.1. If the size of COMB_PAYLOAD exceeds
MAX_UNFRAGMENTED_SIZE (see Section 4.1.3.4.2 of
[RFC8613]), the client MUST stop processing the request
and MUST abandon the Block-wise transfer. Then, the
client can continue by switching to the sequential
workflow shown in Figure 1. That is, the client first
sends EDHOC message_3 prepended by the EDHOC connection
identifier C_R encoded as per Section 3.3 of [RFC9528].
Then, the client sends the OSCORE-protected CoAP request
once the EDHOC execution is completed.

The performance advantage of using the EDHOC + OSCORE request can be
lost when used in combination with Block-wise transfers that rely on
specific parameter values and block sizes. Application policies at
the CoAP client can define when and how to detect whether the
performance advantage is lost. If that is the case, they can also
define whether to appropriately adjust the parameter values and block
sizes or to fall back on the sequential workflow of EDHOC.

3.3. Server Processing

This section describes the processing on the server side.

3.3.1. Processing of the EDHOC + OSCORE Request

In order to process a request containing the EDHOC option, i.e., an
EDHOC + OSCORE request, the server MUST perform the following steps.

Step 1. Check that the EDHOC + OSCORE request includes the OSCORE
option and that the request payload has the format defined
at Step 3 of Section 3.2.1 for COMB_PAYLOAD. If this is not
the case, the server MUST stop processing the request and
MUST reply with a 4.00 (Bad Request) error response.

Step 2. Extract EDHOC message_3 from the payload COMB_PAYLOAD of the
EDHOC + OSCORE request as the first element EDHOC_MSG_3 (see
Step 3 of Section 3.2.1).

Step 3. Take the value of the 'kid' field from the OSCORE option of
the EDHOC + OSCORE request (i.e., the OSCORE Sender ID of
the client), and use it as the EDHOC connection identifier
C_R.

Step 4. Retrieve the correct EDHOC session by using the connection
identifier C_R from Step 3.

If the application profile used in the EDHOC session
specifies that EDHOC message_4 shall be sent, the server
MUST stop the EDHOC processing and consider it failed due to
a client error.

Otherwise, perform the EDHOC processing on the EDHOC
message_3 extracted at Step 2 as per Section 5.4.3 of
[RFC9528] based on the protocol state of the retrieved EDHOC
session.

The application profile used in the EDHOC session is the
same one associated with the EDHOC resource where the server
received the request conveying EDHOC message_1 that started
the session. This is relevant in case the server provides
multiple EDHOC resources that may generally refer to
different application profiles.

Step 5. Establish a new OSCORE Security Context associated with the
client as per Appendix A.1 of [RFC9528] using the EDHOC
output from Step 4.

Step 6. Extract the OSCORE ciphertext from the payload COMB_PAYLOAD
of the EDHOC + OSCORE request as the second element
OSCORE_PAYLOAD (see Step 3 of Section 3.2.1).

Step 7. Rebuild the OSCORE-protected CoAP request as the EDHOC +
OSCORE request, where the payload is replaced with the
OSCORE ciphertext extracted at Step 6. Then, remove the
EDHOC option.

Step 8. Decrypt and verify the OSCORE-protected CoAP request rebuilt
at Step 7 as per Section 8.2 of [RFC8613] by using the
OSCORE Security Context established at Step 5.

When the decrypted request is checked for any critical CoAP
options (as it is during regular CoAP processing), the
presence of an EDHOC option MUST be regarded as an
unprocessed critical option unless it is processed by some
further mechanism.

Step 9. Deliver the CoAP request resulting from Step 8 to the
application.

If Steps 4 (EDHOC processing) and 8 (OSCORE processing) are both
successfully completed, the server MUST reply with an OSCORE-
protected response (see Section 5.4.3 of [RFC9528]). The usage of
EDHOC message_4 as defined in Section 5.5 of [RFC9528] is not
applicable to the approach defined in this document.

If Step 4 (EDHOC processing) fails, the server aborts the session as
per Section 5.4.3 of [RFC9528] and responds with an EDHOC error
message with error code 1, which is formatted as defined in
Section 6.2 of [RFC9528]. The server MUST NOT establish a new OSCORE
Security Context from the present EDHOC session with the client. The
CoAP response conveying the EDHOC error message is not protected with
OSCORE. As per Section 9.5 of [RFC9528], the server has to make sure
that the error message does not reveal sensitive information. The
CoAP response conveying the EDHOC error message MUST have Content-
Format set to application/edhoc+cbor-seq registered in Section 10.9
of [RFC9528].

If Step 4 (EDHOC processing) is successfully completed but Step 8
(OSCORE processing) fails, the same OSCORE error handling as defined
in Section 8.2 of [RFC8613] applies.

3.3.2. Supporting Block-Wise Transfers

If Block-wise transfers [RFC7959] are supported, the server takes the
additional following step before any other in Section 3.3.1.

Step 0. If a Block option is present in the request, then process
the Outer Block options according to [RFC7959] until all
blocks of the request have been received (see
Section 4.1.3.4 of [RFC8613]).

3.4. Example of the EDHOC + OSCORE Request

Figure 4 shows an example of an EDHOC + OSCORE request transported
over UDP. In particular, the example assumes that:

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

* The OSCORE Sender ID of the client is 0x01.

As per Section 3.3.3 of [RFC9528], this straightforwardly
corresponds to the EDHOC connection identifier C_R 0x01.

As per Section 3.3.2 of [RFC9528], when using the sequential flow
shown in Figure 1, the same C_R with a value of 0x01 would be
encoded on the wire as the CBOR integer 1 (0x01 in CBOR encoding)
and prepended to EDHOC message_3 in the payload of the second
EDHOC request.

This results in the following components shown in Figure 4:

OSCORE option value: 0x090001 (3 bytes)

EDHOC option value: - (0 bytes)

EDHOC message_3: 0x52d5535f3147e85f1cfacd9e78abf9e0a81bbf (19 bytes)

OSCORE ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)

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

Figure 4: Example of a Protected CoAP Request Combining EDHOC and
OSCORE Data

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.

4.1. Additional Processing of EDHOC Messages

When using EDHOC to establish an OSCORE Security Context, the client
and server MUST perform the following additional steps during an
EDHOC execution, thus extending Section 5 of [RFC9528].

4.1.1. Initiator Processing of Message 1

The Initiator selects an EDHOC connection identifier C_I as follows.

The Initiator MUST choose a C_I that is neither used in any current
EDHOC session as this peer's EDHOC connection identifier nor the
Recipient ID in a current OSCORE Security Context where the ID
Context is not present.

The chosen C_I SHOULD NOT be the Recipient ID of any current OSCORE
Security Context. Note that, unless the two peers concurrently use
alternative methods to establish OSCORE Security Contexts, this
allows the Responder to always omit the 'kid context' in the OSCORE
option of its messages sent to the Initiator when protecting those
with an OSCORE Security Context where C_I is the Responder's OSCORE
Sender ID (see Section 6.1 of [RFC8613]).

4.1.2. Responder Processing of Message 2

The Responder selects an EDHOC connection identifier C_R as follows.

The Responder MUST choose a C_R that is none of the following:

* used in any current EDHOC session as this peer's EDHOC connection
identifier,

* equal to the EDHOC connection identifier C_I specified in the
EDHOC message_1 of the present EDHOC session, or

* the Recipient ID in a current OSCORE Security Context where the ID
Context is not present.

The chosen C_R SHOULD NOT be the Recipient ID of any current OSCORE
Security Context. Note that, for a reason analogous to the one given
in Section 4.1.1 with C_I, this allows the Initiator to always omit
the 'kid context' in the OSCORE option of its messages sent to the
Responder when protecting those with an OSCORE Security Context where
C_R is the Initiator's OSCORE Sender ID (see Section 6.1 of
[RFC8613]).

4.1.3. Initiator Processing of Message 2

If the EDHOC connection identifier C_I is equal to the EDHOC
connection identifier C_R specified in EDHOC message_2, then the
Initiator MUST abort the session and reply with an EDHOC error
message with error code 1 formatted as defined in Section 6.2 of
[RFC9528].

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

Figure 5: The Web Link

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:

* The Initiator is authenticated with 128-bit security against
online attacks. As per Section 9.1 of [RFC9528], this results
from the combination of the strength of the 64-bit Message
Authentication Code (MAC) in EDHOC message_3 and of the 64-bit MAC
in the Authenticated Encryption with Associated Data (AEAD) of the
first OSCORE-protected CoAP request as rebuilt at Step 7 of
Section 3.3.1.

* The Responder is authenticated with 128-bit security against
online attacks. As per Section 9.1 of [RFC9528], this results
from the combination of the strength of the 64-bit MAC in EDHOC
message_2 and of the 64-bit MAC in the AEAD of the first OSCORE-
protected CoAP response.

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.

8.1. CoAP Option Numbers Registry

IANA has registered the following option number in the "CoAP Option
Numbers" registry within the "Constrained RESTful Environments (CoRE)
Parameters" registry group.

+========+=======+===========+
| Number | Name | Reference |
+========+=======+===========+
| 21 | EDHOC | RFC 9668 |
+--------+-------+-----------+

Table 2: Registration in
the "CoAP Option Numbers"
Registry

8.2. Target Attributes Registry

IANA has registered the following entries in the "Target Attributes"
registry [CORE.Target.Attributes] within the "Constrained RESTful
Environments (CoRE) Parameters" registry group as per [RFC9423]. For
all entries, the Change Controller is IETF and the reference is RFC
9668.

+================+=============================================+
| Attribute Name | Brief Description |
+================+=============================================+
| ed-i | Hint: support for the EDHOC Initiator role |
+----------------+---------------------------------------------+
| ed-r | Hint: support for the EDHOC Responder role |
+----------------+---------------------------------------------+
| ed-method | A supported authentication method for EDHOC |
+----------------+---------------------------------------------+
| ed-csuite | A supported cipher suite for EDHOC |
+----------------+---------------------------------------------+
| ed-cred-t | A supported type of authentication |
| | credential for EDHOC |
+----------------+---------------------------------------------+
| ed-idcred-t | A supported type of authentication |
| | credential identifier for EDHOC |
+----------------+---------------------------------------------+
| ed-ead | A supported External Authorization Data |
| | (EAD) item for EDHOC |
+----------------+---------------------------------------------+
| ed-comb-req | Hint: support for the EDHOC + OSCORE |
| | request |
+----------------+---------------------------------------------+

Table 3: Registrations in the "Target Attributes" Registry

8.3. EDHOC Authentication Credential Types Registry

IANA has created the "EDHOC Authentication Credential Types" registry
within the "Ephemeral Diffie-Hellman Over COSE (EDHOC)" registry
group defined in [RFC9528].

The registration policy is either "Private Use", "Standards Action
with Expert Review", or "Specification Required" per [RFC8126].
"Expert Review" guidelines are provided in Section 8.4.

All assignments according to "Standards Action with Expert Review"
are made on a "Standards Action" basis per Section 4.9 of [RFC8126]
with "Expert Review" additionally required per Section 4.5 of
[RFC8126]. The procedure for early IANA allocation of "standards
track code points" defined in [RFC7120] also applies. When such a
procedure is used, IANA will ask the designated expert(s) to approve
the early allocation before registration. In addition, working group
chairs are encouraged to consult the expert(s) early during the
process outlined in Section 3.1 of [RFC7120].

The columns of this registry are:

Value: This field contains the value used to identify the type of
authentication credential. These values MUST be unique. The
value can be an unsigned integer or a negative integer. Different
ranges of values use different registration policies:

* Integer values from -24 to 23 are designated as "Standards
Action With Expert Review".

* Integer values from -65536 to -25 and from 24 to 65535 are
designated as "Specification Required".

* Integer values smaller than -65536 and greater than 65535 are
marked as "Private Use".

Description: This field contains a short description of the type of
authentication credential.

Reference: This field contains a pointer to the public specification
for the type of authentication credential.

+=======+============================================+===========+
| Value | Description | Reference |
+=======+============================================+===========+
| 0 | CBOR Web Token (CWT) containing a COSE_Key | [RFC8392] |
| | in a 'cnf' claim and possibly other | |
| | claims. CWT is defined in RFC 8392. | |
+-------+--------------------------------------------+-----------+
| 1 | CWT Claims Set (CCS) containing a COSE_Key | [RFC8392] |
| | in a 'cnf' claim and possibly other | |
| | claims. CCS is defined in RFC 8392. | |
+-------+--------------------------------------------+-----------+
| 2 | X.509 certificate | [RFC5280] |
+-------+--------------------------------------------+-----------+

Table 4: Initial Entries in the "EDHOC Authentication
Credential Types" Registry

8.4. Expert Review Instructions

"Standards Action with Expert Review" and "Specification Required"
are two of the registration policies defined for the IANA registry
established in Section 8.3. This section gives some general
guidelines for what the experts should be looking for; however, they
are being designated as experts for a reason, so they should be given
substantial latitude.

Expert reviewers should take into consideration the following points:

* Clarity and correctness of registrations. Experts are expected to
check the clarity of purpose and use of the requested entries.
Experts need to make sure that registered identifiers indicate a
type of authentication credential whose format and encoding is
clearly defined in the corresponding specification. Identifiers
of types of authentication credentials that do not meet these
objectives of clarity and completeness must not be registered.

* Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered and that the point is likely to be used in deployments.
The zones tagged as "Private Use" are intended for testing
purposes and closed environments. Code points in other ranges
should not be assigned for testing.

* Specifications are required for the "Standards Action With Expert
Review" range of point assignment. Specifications should exist
for "Specification Required" ranges, but early assignment before a
specification is available is considered to be permissible. When
specifications are not provided, the description provided needs to
have sufficient information to identify what the point is being
used for.

* Experts should take into account the expected usage of fields when
approving point assignment. Documents published via Standards
Action can also register points outside the Standards Action
range. The length of the encoded value should be weighed against
how many code points of that length are left, the size of device
it will be used on, and the number of code points left that encode
to that size.

9. References

9.1. Normative References

[CORE.Target.Attributes]
IANA, "Target Attributes",
<https://www.iana.org/assignments/core-parameters>.

[COSE.Header.Parameters]
IANA, "COSE Header Parameters",
<https://www.iana.org/assignments/cose>.

[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>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.

[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.

[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>.

[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[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>.

[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.

[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.

[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>.

[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>.

[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/info/rfc9176>.

[RFC9528] Selander, G., Preuß Mattsson, J., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
DOI 10.17487/RFC9528, March 2024,
<https://www.rfc-editor.org/info/rfc9528>.

9.2. Informative References

[RFC9423] Bormann, C., "Constrained RESTful Environments (CoRE)
Target Attributes Registry", RFC 9423,
DOI 10.17487/RFC9423, April 2024,
<https://www.rfc-editor.org/info/rfc9423>.

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

Francesca Palombini
Ericsson AB
Torshamnsgatan 23
SE-164 40 Kista
Sweden
Email: [email protected]

Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-164 40 Kista
Sweden
Email: [email protected]

Rikard Höglund
RISE AB
Isafjordsgatan 22
SE-164 40 Kista
Sweden
Email: [email protected]

Stefan Hristozov
Eriptic
Email: [email protected]

Göran Selander
Ericsson
Email: [email protected]