Internet Engineering Task Force (IETF) F. Palombini

Internet Engineering Task Force (IETF) F. Palombini
Request for Comments: 9203 Ericsson AB
Category: Standards Track L. Seitz
ISSN: 2070-1721 Combitech
G. Selander
Ericsson AB
M. Gunnarsson
RISE
August 2022

The Object Security for Constrained RESTful Environments (OSCORE)
Profile of the Authentication and Authorization for Constrained
Environments (ACE) Framework

Abstract

This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework. It
utilizes Object Security for Constrained RESTful Environments
(OSCORE) to provide communication security and proof-of-possession
for a key owned by the client and bound to an OAuth 2.0 access token.

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/rfc9203.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.

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in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Terminology
2. Protocol Overview
3. Client-AS Communication
3.1. C-to-AS: POST to Token Endpoint
3.2. AS-to-C: Access Token
3.2.1. The OSCORE_Input_Material
4. Client-RS Communication
4.1. C-to-RS: POST to authz-info Endpoint
4.1.1. The Nonce 1 Parameter
4.1.2. The ace_client_recipientid Parameter
4.2. RS-to-C: 2.01 (Created)
4.2.1. The Nonce 2 Parameter
4.2.2. The ace_server_recipientid Parameter
4.3. OSCORE Setup
4.4. Access Rights Verification
5. Secure Communication with AS
6. Discarding the Security Context
7. Security Considerations
8. Privacy Considerations
9. IANA Considerations
9.1. ACE Profile Registry
9.2. OAuth Parameters Registry
9.3. OAuth Parameters CBOR Mappings Registry
9.4. OSCORE Security Context Parameters Registry
9.5. CWT Confirmation Methods Registry
9.6. JWT Confirmation Methods Registry
9.7. Expert Review Instructions
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Profile Requirements
Acknowledgments
Authors' Addresses

1. Introduction

This document specifies the coap_oscore profile of the ACE framework
[RFC9200]. In this profile, a client (C) and a resource server (RS)
use the Constrained Application Protocol (CoAP) [RFC7252] to
communicate. The client uses an access token, bound to a symmetric
key (the proof-of-possession (PoP) key) to authorize its access to
the resource server. Note that this profile uses a symmetric-crypto-
based scheme, where the symmetric secret is used as input material
for keying material derivation. In order to provide communication
security and PoP, the client and resource server use Object Security
for Constrained RESTful Environments (OSCORE) as defined in
[RFC8613]. Note that the PoP is not achieved through a dedicated
protocol element but rather occurs after the first message exchange
using OSCORE.

OSCORE specifies how to use CBOR Object Signing and Encryption (COSE)
[RFC9052] [RFC9053] to secure CoAP messages. Note that OSCORE can be
used to secure CoAP messages, as well as HTTP and combinations of
HTTP and CoAP; a profile of ACE similar to the one described in this
document, with the difference of using HTTP instead of CoAP as the
communication protocol, could be specified analogously to this one.

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.

Certain security-related terms such as "authentication",
"authorization", "confidentiality", "(data) integrity", "Message
Authentication Code (MAC)", "Hash-based Message Authentication Code
(HMAC)", and "verify" are taken from [RFC4949].

RESTful terminology follows HTTP [RFC9110].

Readers are expected to be familiar with the terms and concepts
defined in OSCORE [RFC8613], such as "security context" and
"Recipient ID".

Terminology for entities in the architecture is defined in OAuth 2.0
[RFC6749], such as client (C), resource server (RS), and
authorization server (AS). It is assumed in this document that a
given resource on a specific RS is associated to a unique AS.

Concise Binary Object Representation (CBOR) [RFC8949] and Concise
Data Definition Language (CDDL) [RFC8610] are used in this document.
CDDL predefined type names, especially "bstr" for CBOR byte strings
and "tstr" for CBOR text strings, are used extensively in this
document.

Note that the term "endpoint" is used as in [RFC9200], following its
OAuth definition, which is to denote resources such as token and
introspect at the AS and authz-info at the RS. The CoAP definition,
which is "[a]n entity participating in the CoAP protocol" [RFC7252],
is not used in this document.

Throughout this document, examples for CBOR data items are expressed
in CBOR extended diagnostic notation as defined in Section 8 of
[RFC8949] and Appendix G of [RFC8610] ("diagnostic notation"), unless
noted otherwise. We often use diagnostic notation comments to
provide a textual representation of the numeric parameter names and
values.

In this document, the term "base64-encoded" refers to URL-Safe base64
encoding (see Section 5 of [RFC4648]) without padding.

2. Protocol Overview

This section gives an overview of how to use the ACE Framework
[RFC9200] to secure the communication between a client and a resource
server using OSCORE [RFC8613]. The parameters needed by the client
to negotiate the use of this profile with the AS, as well as the
OSCORE setup process, are described in detail in the following
sections.

The RS maintains a collection of OSCORE security contexts with
associated authorization information for all the clients that it is
communicating with. The authorization information is maintained as
policy that is used as input to processing requests from those
clients.

This profile requires a client to retrieve an access token from the
AS for the resource it wants to access on an RS, by sending an access
token request to the token endpoint, as specified in Section 5.8 of
[RFC9200]. The access token request and response MUST be
confidentiality protected and ensure authenticity. The use of OSCORE
between the client and AS is RECOMMENDED in this profile, to reduce
the number of libraries the client has to support, but other
protocols fulfilling the security requirements defined in Section 5
of [RFC9200] MAY alternatively be used, such as TLS [RFC8446] or DTLS
[RFC9147].

Once the client has retrieved the access token, it generates a nonce
N1, as defined in this document (see Section 4.1.1). The client also
generates its own OSCORE Recipient ID, ID1 (see Section 3.1 of
[RFC8613]), for use with the keying material associated to the RS.
The client posts the token, N1, and its Recipient ID to the RS using
the authz-info endpoint and mechanisms specified in Section 5.8 of
[RFC9200] and Content-Format = application/ace+cbor. When using this
profile, the communication with the authz-info endpoint is not
protected, except for the update of access rights.

If the access token is valid, the RS replies to this request with a
2.01 (Created) response with Content-Format = application/ace+cbor,
which contains a nonce N2 and its newly generated OSCORE Recipient
ID, ID2, for use with the keying material associated to the client.
Moreover, the server concatenates the input salt received in the
token, N1, and N2 to obtain the Master Salt of the OSCORE security
context (see Section 3 of [RFC8613]). The RS then derives the
complete security context associated with the received token from the
Master Salt; the OSCORE Recipient ID generated by the client (set as
its OSCORE Sender ID); its own OSCORE Recipient ID; plus the
parameters received in the access token from the AS, following
Section 3.2 of [RFC8613].

In a similar way, after receiving the nonce N2, the client
concatenates the input salt, N1, and N2 to obtain the Master Salt of
the OSCORE security context. The client then derives the complete
security context from the Master Salt; the OSCORE Recipient ID
generated by the RS (set as its OSCORE Sender ID); its own OSCORE
Recipient ID; plus the parameters received from the AS.

Finally, the client starts the communication with the RS by sending a
request protected with OSCORE to the RS. If the request is
successfully verified, the server stores the complete security
context state that is ready for use in protecting messages and uses
it in the response, and in further communications with the client,
until token deletion due to, for example, expiration. This security
context is discarded when a token (whether the same or a different
one) is used to successfully derive a new security context for that
client.

The use of nonces N1 and N2 during the exchange prevents the reuse of
an Authenticated Encryption with Associated Data (AEAD) nonce/key
pair for two different messages. Reuse might otherwise occur when
the client and RS derive a new security context from an existing
(non-expired) access token, as might occur when either party has just
rebooted, and that might lead to loss of both confidentiality and
integrity. Instead, by using the exchanged nonces N1 and N2 as part
of the Master Salt, the request to the authz-info endpoint posting
the same token results in a different security context, by OSCORE
construction, since even though the Master Secret, Sender ID, and
Recipient ID are the same, the Master Salt is different (see
Section 3.2.1 of [RFC8613]). If the exchanged nonces were reused, a
node reusing a non-expired old token would be susceptible to on-path
attackers provoking the creation of an OSCORE message using an old
AEAD key and nonce.

After the whole message exchange has taken place, the client can
contact the AS to request an update of its access rights, sending a
similar request to the token endpoint that also includes an
identifier so that the AS can find the correct OSCORE Input Material
it has previously shared with the client. This specific identifier,
encoded as a byte string, is assigned by the AS to be unique in the
sets of its OSCORE Input Materials, and it is not used as input
material to derive the full OSCORE security context.

An overview of the profile flow for the OSCORE profile is given in
Figure 1. The names of messages coincide with those of [RFC9200]
when applicable.

C RS AS
| | |
| ----- POST /token ----------------------------> |
| | |
| <---------------------------- Access Token ----- |
| + Access Information |
| ---- POST /authz-info ---> | |
| (access_token, N1, ID1) | |
| | |
| <- 2.01 Created (N2, ID2)- | |
| | |
/Sec Context /Sec Context |
derivation/ derivation/ |
| | |
| ---- OSCORE Request -----> | |
| | |
| /proof-of-possession |
| Sec Context storage/ |
| | |
| <--- OSCORE Response ----- | |
| | |
/proof-of-possession | |
Sec Context storage/ | |
| | |
| ---- OSCORE Request -----> | |
| | |
| <--- OSCORE Response ----- | |
| | |
| ... | |

Figure 1: Protocol Overview

3. Client-AS Communication

The following subsections describe the details of the POST request
and response to the token endpoint between the client and AS.
Section 3.2 of [RFC8613] defines how to derive a security context
based on a shared Master Secret and a set of other parameters,
established between the client and server, which the client receives
from the AS in this exchange. The PoP key included in the response
from the AS MUST be used as a Master Secret in OSCORE.

3.1. C-to-AS: POST to Token Endpoint

The client-to-AS request is specified in Section 5.8.1 of [RFC9200].

The client must send this POST request to the token endpoint over a
secure channel that guarantees authentication, message integrity, and
confidentiality (see Section 5).

An example of such a request is shown in Figure 2.

Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: application/ace+cbor
Payload:
{
/ audience / 5 : "tempSensor4711",
/ scope / 9 : "read"
}

Figure 2: Example C-to-AS POST /token Request for an Access Token
Bound to a Symmetric Key

If the client wants to update its access rights without changing an
existing OSCORE security context, it MUST include a req_cnf object in
its POST request to the token endpoint, with the kid field carrying a
CBOR byte string containing the OSCORE Input Material identifier
(assigned as discussed in Section 3.2). This identifier, together
with other information such as audience (see Section 5.8.1 of
[RFC9200]), can be used by the AS to determine the shared secret
bound to the proof-of-possession token; therefore, it MUST identify a
symmetric key that was previously generated by the AS as a shared
secret for the communication between the client and the RS. The AS
MUST verify that the received value identifies a proof-of-possession
key that has previously been issued to the requesting client. If
that is not the case, the client-to-AS request MUST be declined with
the error code invalid_request as defined in Section 5.8.3 of
[RFC9200].

An example of such a request is shown in Figure 3.

Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: application/ace+cbor
Payload:
{
/ audience / 5 : "tempSensor4711",
/ scope / 9 : "write",
/ req_cnf / 4 : {
/ kid / 3 : h'01'
}
}

Figure 3: Example C-to-AS POST /token Request for Updating Rights
to an Access Token Bound to a Symmetric Key

3.2. AS-to-C: Access Token

After verifying the POST request to the token endpoint and that the
client is authorized to obtain an access token corresponding to its
access token request, the AS responds as defined in Section 5.8.2 of
[RFC9200]. If the client request was invalid, or not authorized, the
AS returns an error response as described in Section 5.8.3 of
[RFC9200].

The AS can signal that the use of OSCORE is REQUIRED for a specific
access token by including the ace_profile parameter with the value
coap_oscore in the access token response. This means that the client
MUST use OSCORE towards all resource servers for which this access
token is valid, and follow Section 4.3 to derive the security context
to run OSCORE. Usually, it is assumed that constrained devices will
be preconfigured with the necessary profile, so that this kind of
profile signaling can be omitted.

Moreover, the AS MUST send the following data:

* a Master Secret

* an identifier of the OSCORE Input Material

Additionally, the AS MAY send the following data, in the same
response.

* a context identifier

* an AEAD algorithm

* an HMAC-based key derivation function (HKDF) algorithm [RFC5869].
It is specified by the HMAC algorithm value; see Section 3.1 of
[RFC9053].

* a salt

* the OSCORE version number

This data is transported in the OSCORE_Input_Material. The
OSCORE_Input_Material is a CBOR map object, defined in Section 3.2.1.
This object is transported in the cnf parameter of the access token
response, as defined in Section 3.2 of [RFC9201], as the value of a
field named osc, which is registered in Sections 9.5 and 9.6.

The AS MAY assign an identifier to the context (context identifier).
This identifier is used as ID Context in the OSCORE context as
described in Section 3.1 of [RFC8613]. If assigned, these parameters
MUST be communicated as the contextId field in the
OSCORE_Input_Material. The application needs to consider that this
identifier is sent in the clear and may reveal information about the
endpoints, as mentioned in Section 12.8 of [RFC8613].

The Master Secret and the identifier of the OSCORE_Input_Material
MUST be communicated as the ms and id field in the osc field in the
cnf parameter of the access token response. If included, the
following are sent: the AEAD algorithm in the alg parameter in the
OSCORE_Input_Material; the HKDF algorithm in the hkdf parameter of
the OSCORE_Input_Material; a salt in the salt parameter of the
OSCORE_Input_Material; and the OSCORE version in the version
parameter of the OSCORE_Input_Material.

The same parameters MUST be included in the claims associated with
the access token. The OSCORE Master Secret MUST be encrypted by the
authorization server so that only the resource server can decrypt it
(see Section 6.1 of [RFC9200]). The use of a CBOR Web Token (CWT)
protected with COSE_Encrypt/COSE_Encrypt0 as specified in [RFC8392]
is RECOMMENDED in this profile. If the token is a CWT, the same
OSCORE_Input_Material structure defined above MUST be placed in the
osc field of the cnf claim of this token.

The AS MUST send a different OSCORE_Input_Material (and therefore
different access tokens) to different authorized clients, in order
for the RS to differentiate between clients.

Figure 4 shows an example of an AS response. The access token has
been truncated for readability.

Header: Created (Code=2.01)
Content-Type: application/ace+cbor
Payload:
{
/ access_token / 1 : h'8343a1010aa2044c53/...
(remainder of access token (CWT) omitted for brevity)/',
/ ace_profile / 38 : / coap_oscore / 2,
/ expires_in / 2 : 3600,
/ cnf / 8 : {
/ osc / 4 : {
/ id / 0 : h'01',
/ ms / 2 : h'f9af838368e353e78888e1426bd94e6f'
}
}
}

Figure 4: Example AS-to-C Access Token Response with an OSCORE
Profile

Figure 5 shows an example CWT Claims Set, including the relevant
OSCORE parameters in the cnf claim.

{
/ aud / 3 : "tempSensorInLivingRoom",
/ iat / 6 : 1360189224,
/ exp / 4 : 1360289224,
/ scope / 9 : "temperature_g firmware_p",
/ cnf / 8 : {
/ osc / 4 : {
/ id / 0 : h'01',
/ ms / 2 : h'f9af838368e353e78888e1426bd94e6f'
}
}
}

Figure 5: Example CWT Claims Set with OSCORE Parameters

The same CWT Claims Set as in Figure 5, using the value abbreviations
defined in [RFC9200] and [RFC8747] and encoded in CBOR, is shown in
Figure 6. The bytes in hexadecimal are reported in the first column,
while their corresponding CBOR meaning is reported after the # sign
on the second column, for readability.

A5 # map(5)
03 # unsigned(3)
76 # text(22)
74656D7053656E736F72496E4C6976696E67526F6F6D
# "tempSensorInLivingRoom"
06 # unsigned(6)
1A 5112D728 # unsigned(1360189224)
04 # unsigned(4)
1A 51145DC8 # unsigned(1360289224)
09 # unsigned(9)
78 18 # text(24)
74656D70657261747572655F67206669726D776172655F70
# "temperature_g firmware_p"
08 # unsigned(8)
A1 # map(1)
04 # unsigned(4)
A2 # map(2)
00 # unsigned(0)
41 # bytes(1)
01
02 # unsigned(2)
50 # bytes(16)
F9AF838368E353E78888E1426BD94E6F

Figure 6: Example CWT Claims Set with OSCORE Parameters Using
CBOR Encoding

If the client has requested an update to its access rights using the
same OSCORE security context, which is valid and authorized, the AS
MUST omit the cnf parameter in the response and MUST carry the OSCORE
Input Material identifier in the kid field in the cnf claim of the
token. This identifier needs to be included in the token in order
for the RS to identify the correct OSCORE Input Material.

Figure 7 shows an example of such an AS response. The access token
has been truncated for readability.

Header: Created (Code=2.01)
Content-Type: application/ace+cbor
Payload:
{
/ access_token / 1 : h'8343a1010aa2044c53/ ...
(remainder of access token (CWT) omitted for brevity)/',
/ ace_profile / 38 : / coap_oscore / 2,
/ expires_in / 2 : 3600
}

Figure 7: Example AS-to-C Access Token Response with an OSCORE
Profile for the Update of Access Rights

Figure 8 shows an example CWT Claims Set that contains the necessary
OSCORE parameters in the cnf claim for the update of access rights.

{
/ aud / 3 : "tempSensorInLivingRoom",
/ iat / 6 : 1360189224,
/ exp / 4 : 1360289224,
/ scope / 9 : "temperature_h",
/ cnf / 8 : {
/ kid / 3 : h'01'
}
}

Figure 8: Example CWT Claims Set with OSCORE Parameters for the
Update of Access Rights

3.2.1. The OSCORE_Input_Material

An OSCORE_Input_Material is an object that represents the input
material to derive an OSCORE security context, i.e., the local set of
information elements necessary to carry out the cryptographic
operations in OSCORE (Section 3.1 of [RFC8613]). In particular, the
OSCORE_Input_Material is defined to be serialized and transported
between nodes, as specified by this document, but it can also be used
by other specifications if needed. The OSCORE_Input_Material can be
encoded as either a JSON object or a CBOR map. The set of common
parameters that can appear in an OSCORE_Input_Material can be found
in the IANA "OSCORE Security Context Parameters" registry
(Section 9.4), defined for extensibility, and the initial set of
parameters defined in this document is specified below. All
parameters are optional. Table 1 provides a summary of the
OSCORE_Input_Material parameters defined in this section.

+===========+=======+==========+===================+===============+
| name | CBOR | CBOR | registry | description |
| | label | type | | |
+===========+=======+==========+===================+===============+
| id | 0 | byte | | OSCORE Input |
| | | string | | Material |
| | | | | identifier |
+-----------+-------+----------+-------------------+---------------+
| version | 1 | unsigned | | OSCORE |
| | | integer | | version |
+-----------+-------+----------+-------------------+---------------+
| ms | 2 | byte | | OSCORE Master |
| | | string | | Secret value |
+-----------+-------+----------+-------------------+---------------+
| hkdf | 3 | text | [COSE.Algorithms] | OSCORE HKDF |
| | | string / | values (HMAC- | value |
| | | integer | based) | |
+-----------+-------+----------+-------------------+---------------+
| alg | 4 | text | [COSE.Algorithms] | OSCORE AEAD |
| | | string / | values (AEAD) | Algorithm |
| | | integer | | value |
+-----------+-------+----------+-------------------+---------------+
| salt | 5 | byte | | an input to |
| | | string | | OSCORE Master |
| | | | | Salt value |
+-----------+-------+----------+-------------------+---------------+
| contextId | 6 | byte | | OSCORE ID |
| | | string | | Context value |
+-----------+-------+----------+-------------------+---------------+

Table 1: OSCORE_Input_Material Parameters

id: This parameter identifies the OSCORE_Input_Material and is
encoded as a byte string. In JSON, the id value is a
base64-encoded byte string. In CBOR, the id type is a byte
string, and it has label 0.

version: This parameter identifies the OSCORE version number, which
is an unsigned integer. For more information about this field,
see Section 5.4 of [RFC8613]. In JSON, the version value is an
integer. In CBOR, the version type is an integer, and it has
label 1.

ms: This parameter identifies the OSCORE Master Secret value, which
is a byte string. For more information about this field, see
Section 3.1 of [RFC8613]. In JSON, the ms value is a
base64-encoded byte string. In CBOR, the ms type is byte string,
and it has label 2.

hkdf: This parameter identifies the OSCORE HKDF Algorithm. For more
information about this field, see Section 3.1 of [RFC8613]. The
values used MUST be registered in the IANA "COSE Algorithms"
registry (see [COSE.Algorithms]) and MUST be HMAC-based HKDF
algorithms (see Section 3.1 of [RFC9053]). The value can be
either the integer or the text-string value of the HMAC-based HKDF
algorithm in the "COSE Algorithms" registry. In JSON, the hkdf
value is a case-sensitive ASCII string or an integer. In CBOR,
the hkdf type is a text string or integer, and it has label 3.

alg: This parameter identifies the OSCORE AEAD Algorithm. For more
information about this field, see Section 3.1 of [RFC8613]. The
values used MUST be registered in the IANA "COSE Algorithms"
registry (see [COSE.Algorithms]) and MUST be AEAD algorithms. The
value can be either the integer or the text-string value of the
HMAC-based HKDF algorithm in the "COSE Algorithms" registry. In
JSON, the alg value is a case-sensitive ASCII string or an
integer. In CBOR, the alg type is a text string or integer, and
it has label 4.

salt: This parameter identifies an input to the OSCORE Master Salt
value, which is a byte string. For more information about this
field, see Section 3.1 of [RFC8613]. In JSON, the salt value is a
base64-encoded byte string. In CBOR, the salt type is a byte
string, and it has label 5.

contextId: This parameter identifies the security context as a byte
string. This identifier is used as OSCORE ID Context. For more
information about this field, see Section 3.1 of [RFC8613]. In
JSON, the contextID value is a base64-encoded byte string. In
CBOR, the contextID type is a byte string, and it has label 6.

An example of JSON OSCORE_Input_Material is given in Figure 9.

"osc" : {
"alg" : "AES-CCM-16-64-128",
"id" : "AQ",
"ms" : "-a-Dg2jjU-eIiOFCa9lObw"
}

Figure 9: Example JSON OSCORE_Input_Material

The CDDL grammar describing the CBOR OSCORE_Input_Material is shown
in Figure 10.

OSCORE_Input_Material = {
? 0 => bstr, ; id
? 1 => int, ; version
? 2 => bstr, ; ms
? 3 => tstr / int, ; hkdf
? 4 => tstr / int, ; alg
? 5 => bstr, ; salt
? 6 => bstr, ; contextId
* (int / tstr) => any
}

Figure 10: CDDL Grammar of the OSCORE_Input_Material

4. Client-RS Communication

The following subsections describe the details of the POST request
and response to the authz-info endpoint between the client and RS.
The client generates a nonce N1 and an identifier ID1 that is unique
in the sets of its own Recipient IDs and posts them together with the
token that includes the materials (e.g., OSCORE parameters) received
from the AS to the RS. The RS then generates a nonce N2 and an
identifier ID2 that is unique in the sets of its own Recipient IDs
and uses Section 3.2 of [RFC8613] to derive a security context based
on a shared Master Secret, the two exchanged nonces, and the two
identifiers, established between the client and server. The
exchanged nonces and identifiers are encoded as a CBOR byte string if
CBOR is used and as a base64 string if JSON is used. This security
context is used to protect all future communication between the
client and RS using OSCORE, as long as the access token is valid.

Note that the RS and client authenticate each other by generating the
shared OSCORE security context using the PoP key as the Master
Secret. An attacker posting a valid token to the RS will not be able
to generate a valid OSCORE security context and thus will not be able
to prove possession of the PoP key. Additionally, the mutual
authentication is only achieved after the client has successfully
verified a response from the RS protected with the generated OSCORE
security context.

4.1. C-to-RS: POST to authz-info Endpoint

The client MUST generate a nonce value N1 that is very unlikely to
have been previously used with the same input keying material. The
use of a 64-bit long random number as the nonce's value is
RECOMMENDED in this profile. The client MUST store the nonce N1 as
long as the response from the RS is not received and the access token
related to it is still valid (to the best of the client's knowledge).

The client generates its own Recipient ID, ID1, for the OSCORE
security context that it is establishing with the RS. By generating
its own Recipient ID, the client makes sure that it does not collide
with any of its Recipient IDs, nor with any other identifier ID1 if
the client is executing this exchange with a different RS at the same
time.

The client MUST use CoAP and the authorization information resource
as described in Section 5.8.1 of [RFC9200] to transport the token,
N1, and ID1 to the RS.

Note that the use of the payload and the Content-Format is different
from what is described in Section 5.8.1 of [RFC9200], which only
transports the token without any CBOR wrapping. In this profile, the
client MUST wrap the token, N1, and ID1 in a CBOR map. The client
MUST use the Content-Format application/ace+cbor defined in
Section 8.16 of [RFC9200]. The client MUST include the access token
using the access_token parameter; N1 using the nonce1 parameter
defined in Section 4.1.1; and ID1 using the ace_client_recipientid
parameter defined in Section 4.1.2.

The communication with the authz-info endpoint does not have to be
protected, except for the update of access rights case described
below.

Note that a client may be required to repost the access token in
order to complete a request, since an RS may delete a stored access
token (and associated security context) at any time, for example, due
to all storage space being consumed. This situation is detected by
the client when it receives an AS Request Creation Hints response.
Reposting the same access token will result in deriving a new OSCORE
security context to be used with the RS, as different exchanged
nonces will be used.

The client may also choose to repost the access token in order to
update its OSCORE security context. In that case, the client and the
RS will exchange newly generated nonces, renegotiate identifiers, and
derive new keying material. The client and RS might decide to keep
the same identifiers or renew them during the renegotiation.

Figure 11 shows an example of the request sent from the client to the
RS. The access token has been truncated for readability.

Header: POST (Code=0.02)
Uri-Host: "rs.example.com"
Uri-Path: "authz-info"
Content-Format: application/ace+cbor
Payload:
{
/ access_token / 1 : h'8343a1010aa2044c53/...
(remainder of access token (CWT) omitted for brevity)/',
/ nonce1 / 40 : h'018a278f7faab55a',
/ ace_client_recipientid / 43 : h'1645'
}

Figure 11: Example C-to-RS POST /authz-info Request Using CWT

If the client has already posted a valid token, has already
established a security association with the RS, and wants to update
its access rights, the client can do so by posting the new token
(retrieved from the AS and containing the update of access rights) to
the /authz-info endpoint. The client MUST protect the request using
the OSCORE security context established during the first token
exchange. The client MUST only send the access_token field in the
CBOR map in the payload; no nonce or identifier is sent. After
proper verification (see Section 4.2), the RS will replace the old
token with the new one, maintaining the same security context.

4.1.1. The Nonce 1 Parameter

The nonce1 parameter MUST be sent from the client to the RS, together
with the access token, if the ACE profile used is coap_oscore, and
the message is not an update of access rights, protected with an
existing OSCORE security context. The parameter is encoded as a byte
string for CBOR-based interactions and as a string (base64-encoded
binary) for JSON-based interactions. This parameter is registered in
Section 9.2.

4.1.2. The ace_client_recipientid Parameter

The ace_client_recipientid parameter MUST be sent from the client to
the RS, together with the access token, if the ACE profile used is
coap_oscore, and the message is not an update of access rights,
protected with an existing OSCORE security context. The parameter is
encoded as a byte string for CBOR-based interactions and as a string
(base64-encoded binary) for JSON-based interactions. This parameter
is registered in Section 9.2.

4.2. RS-to-C: 2.01 (Created)

The RS MUST follow the procedures defined in Section 5.8.1 of
[RFC9200]: the RS must verify the validity of the token. If the
token is valid, the RS must respond to the POST request with 2.01
(Created). If the token is valid but is associated to claims that
the RS cannot process (e.g., an unknown scope), or if any of the
expected parameters are missing (e.g., any of the mandatory
parameters from the AS or the identifier ID1), or if any parameters
received in the osc field are unrecognized, the RS must respond with
an error response code equivalent to the CoAP code 4.00 (Bad
Request). In the latter two cases, the RS may provide additional
information in the error response, in order to clarify what went
wrong. The RS may make an introspection request (see Section 5.9.1
of [RFC9200]) to validate the token before responding to the POST
request to the authz-info endpoint.

Additionally, the RS MUST generate a nonce N2 that is very unlikely
to have been previously used with the same input keying material and
its own Recipient ID, ID2. The RS makes sure that ID2 does not
collide with any of its Recipient IDs. The RS MUST ensure that ID2
is different from the value received in the ace_client_recipientid
parameter. The RS sends N2 and ID2 within the 2.01 (Created)
response. The payload of the 2.01 (Created) response MUST be a CBOR
map containing the nonce2 parameter defined in Section 4.2.1, set to
N2, and the ace_server_recipientid parameter defined in
Section 4.2.2, set to ID2. The use of a 64-bit long random number as
the nonce's value is RECOMMENDED in this profile. The RS MUST use
the Content-Format application/ace+cbor defined in Section 8.16 of
[RFC9200].

Figure 12 shows an example of the response sent from the RS to the
client.

Header: Created (Code=2.01)
Content-Format: application/ace+cbor
Payload:
{
/ nonce2 / 42 : h'25a8991cd700ac01',
/ ace_server_recipientid / 44 : h'0000'
}

Figure 12: Example RS-to-C 2.01 (Created) Response

As specified in Section 5.8.3 of [RFC9200], the RS must notify the
client with an error response with code 4.01 (Unauthorized) for any
long running request before terminating the session, when the access
token expires.

If the RS receives the token in an OSCORE-protected message, it means
that the client is requesting an update of access rights. The RS
MUST ignore any nonce and identifiers in the request, if any were
sent. The RS MUST check that the kid of the cnf claim of the new
access token matches the identifier of the OSCORE Input Material of
the context used to protect the message. If that is the case, the RS
MUST overwrite the old token and associate the new token to the
security context identified by the kid value in the cnf claim. The
RS MUST respond with a 2.01 (Created) response protected with the
same security context, with no payload. If any verification fails,
the RS MUST respond with a 4.01 (Unauthorized) error response.

As specified in Section 5.8.1 of [RFC9200], when receiving an updated
access token with updated authorization information from the client
(see Section 3.1), it is recommended that the RS overwrites the
previous token; that is, only the latest authorization information in
the token received by the RS is valid. This simplifies the process
needed by the RS to keep track of authorization information for a
given client.

4.2.1. The Nonce 2 Parameter

The nonce2 parameter MUST be sent from the RS to the client if the
ACE profile used is coap_oscore and the message is not a response to
an update of access rights, protected with an existing OSCORE
security context. The parameter is encoded as a byte string for
CBOR-based interactions and as a string (base64-encoded binary) for
JSON-based interactions. This parameter is registered in Section 9.2

4.2.2. The ace_server_recipientid Parameter

The ace_server_recipientid parameter MUST be sent from the RS to the
client if the ACE profile used is coap_oscore and the message is not
a response to an update of access rights, protected with an existing
OSCORE security context. The parameter is encoded as a byte string
for CBOR-based interactions and as a string (base64-encoded binary)
for JSON-based interactions. This parameter is registered in
Section 9.2

4.3. OSCORE Setup

Once the 2.01 (Created) response is received from the RS, following
the POST request to authz-info endpoint, the client MUST extract the
bstr nonce N2 from the nonce2 parameter in the CBOR map in the
payload of the response. Then, the client MUST set the Master Salt
of the security context created to communicate with the RS to the
concatenation of salt, N1, and N2 in this order: Master Salt = salt |
N1 | N2, where | denotes byte string concatenation, salt is the CBOR
byte string received from the AS in Section 3.2, and N1 and N2 are
the two nonces encoded as CBOR byte strings. An example of Master
Salt construction using CBOR encoding is given in Figure 13.

N1, N2, and input salt expressed in CBOR diagnostic notation:
nonce1 = h'018a278f7faab55a'
nonce2 = h'25a8991cd700ac01'
input salt = h'f9af838368e353e78888e1426bd94e6f'

N1, N2, and input salt as CBOR encoded byte strings:
nonce1 = 0x48018a278f7faab55a
nonce2 = 0x4825a8991cd700ac01
input salt = 0x50f9af838368e353e78888e1426bd94e6f

Master Salt = 0x50 f9af838368e353e78888e1426bd94e6f
48 018a278f7faab55a 48 25a8991cd700ac01

Figure 13: Example of Master Salt Construction Using CBOR Encoding

If JSON is used instead of CBOR, the Master Salt of the security
context is the base64 encoding of the concatenation of the same
parameters, each of them prefixed by their size, encoded in 1 byte.
When using JSON, the nonces and input salt have a maximum size of 255
bytes. An example of Master Salt construction using base64 encoding
is given in Figure 14.

N1, N2, and input salt values:
nonce1 = 0x018a278f7faab55a (8 bytes)
nonce2 = 0x25a8991cd700ac01 (8 bytes)
input salt = 0xf9af838368e353e78888e1426bd94e6f (16 bytes)

Input to base64 encoding: 0x10 f9af838368e353e78888e1426bd94e6f
08 018a278f7faab55a 08 25a8991cd700ac01

Master Salt = b64'EPmvg4No41PniIjhQmvZTm8IAYonj3+qtVoIJaiZHNcArAE='

Figure 14: Example of Master Salt Construction Using Base64 Encoding

The client MUST set the Sender ID to the ace_server_recipientid
received in Section 4.2 and set the Recipient ID to the
ace_client_recipientid sent in Section 4.1. The client MUST set the
Master Secret from the parameter received from the AS in Section 3.2.
The client MUST set the AEAD algorithm, ID Context, HKDF, and OSCORE
version from the parameters received from the AS in Section 3.2, if
present. In case an optional parameter is omitted, the default value
SHALL be used as described in Sections 3.2 and 5.4 of [RFC8613].
After that, the client MUST derive the complete security context
following Section 3.2.1 of [RFC8613]. From this point on, the client
MUST use this security context to communicate with the RS when
accessing the resources as specified by the authorization
information.

If any of the expected parameters are missing (e.g., any of the
mandatory parameters from the AS or the RS), or if
ace_client_recipientid equals ace_server_recipientid (and as a
consequence, the Sender and Recipient Keys derived would be equal;
see Section 3.3 of [RFC8613]), then the client MUST stop the exchange
and MUST NOT derive the security context. The client MAY restart the
exchange, to get the correct security material.

The client then uses this security context to send requests to the RS
using OSCORE.

After sending the 2.01 (Created) response, the RS MUST set the Master
Salt of the security context created to communicate with the client
to the concatenation of salt, N1, and N2 in the same way described
above. An example of Master Salt construction using CBOR encoding is
given in Figure 13 and using base64 encoding is given in Figure 14.
The RS MUST set the Sender ID from the ace_client_recipientid
received in Section 4.1 and set the Recipient ID from the
ace_server_recipientid sent in Section 4.2. The RS MUST set the
Master Secret from the parameter received from the AS and forwarded
by the client in the access token in Section 4.1 after validation of
the token as specified in Section 4.2. The RS MUST set the AEAD
algorithm, ID Context, HKDF, and OSCORE version from the parameters
received from the AS and forwarded by the client in the access token
in Section 4.1 after validation of the token as specified in
Section 4.2, if present. In case an optional parameter is omitted,
the default value SHALL be used as described in Sections 3.2 and 5.4
of [RFC8613]. After that, the RS MUST derive the complete security
context following Section 3.2.1 of [RFC8613] and MUST associate this
security context with the authorization information from the access
token.

The RS then uses this security context to verify requests and send
responses to the client using OSCORE. If OSCORE verification fails,
error responses are used, as specified in Section 8 of [RFC8613].
Additionally, if OSCORE verification succeeds, the verification of
access rights is performed as described in Section 4.4. The RS MUST
NOT use the security context after the related token has expired and
MUST respond with an unprotected 4.01 (Unauthorized) error message to
requests received that correspond to a security context with an
expired token.

Note that the ID Context can be assigned by the AS, communicated and
set in both the RS and client after the exchange specified in this
profile is executed. Subsequently, the client and RS can update
their ID Context by running a mechanism such as the one defined in
Appendix B.2 of [RFC8613] if they both support it and are configured
to do so. In that case, the ID Context in the OSCORE security
context will not match the contextId parameter of the corresponding
OSCORE_Input_Material. Running Appendix B.2 results in the keying
material being updated in the security contexts of the client and RS;
this same result can also be achieved by the client reposting the
access token to the unprotected /authz-info endpoint at the RS, as
described in Section 4.1, but without updating the ID Context.

4.4. Access Rights Verification

The RS MUST follow the procedures defined in Section 5.8.2 of
[RFC9200]: if an RS receives an OSCORE-protected request from a
client, then the RS processes it according to [RFC8613]. If OSCORE
verification succeeds, and the target resource requires
authorization, the RS retrieves the authorization information using
the access token associated to the security context. The RS then
must verify that the authorization information covers the resource
and the action requested.

5. Secure Communication with AS

As specified in the ACE framework (Section 5.9 of [RFC9200]), the
requesting entity (RS and/or client) and the AS communicates via the
introspection or token endpoint. The use of CoAP and OSCORE
[RFC8613] for this communication is RECOMMENDED in this profile;
other protocols fulfilling the security requirements defined in
Section 5 of [RFC9200] (such as HTTP and DTLS or TLS) MAY be used
instead.

If OSCORE is used, the requesting entity and the AS are expected to
have preestablished security contexts in place. How these security
contexts are established is out of scope for this profile.
Furthermore, the requesting entity and the AS communicate through the
introspection endpoint as specified in Section 5.9 of [RFC9200] and
through the token endpoint as specified in Section 5.8 of [RFC9200].

6. Discarding the Security Context

There are a number of scenarios where a client or RS needs to discard
the OSCORE security context and acquire a new one.

The client MUST discard the current security context associated with
an RS when any of the following occurs:

* the sequence number space ends.

* the access token associated with the context becomes invalid due
to, for example, expiration.

* the client receives a number of 4.01 Unauthorized responses to
OSCORE requests using the same security context. The exact number
needs to be specified by the application.

* the client receives a new nonce in the 2.01 (Created) response
(see Section 4.2) to a POST request to the authz-info endpoint,
when reposting a (non-expired) token associated to the existing
context.

The RS MUST discard the current security context associated with a
client when any of the following occurs:

* the sequence number space ends.

* the access token associated with the context expires.

* the client has successfully replaced the current security context
with a newer one by posting an access token to the unprotected
/authz-info endpoint at the RS, e.g., by reposting the same token,
as specified in Section 4.1.

Whenever one more access token is successfully posted to the RS, and
a new security context is derived between the client and RS, messages
in transit that were protected with the previous security context
might not pass verification, as the old context is discarded. That
means that messages sent shortly before the client posts one more
access tokens to the RS might not successfully reach the destination.
Analogously, implementations may want to cancel CoAP observations at
the RS registered before the security context is replaced, or
conversely, they will need to implement a mechanism to ensure that
those observations are to be protected with the newly derived
security context.

7. Security Considerations

This document specifies a profile for the ACE framework [RFC9200].
Thus, the general security considerations from the framework also
apply to this profile.

Furthermore, the general security considerations of OSCORE [RFC8613]
also apply to this specific use of the OSCORE protocol.

As previously stated, the proof of possession in this profile is
performed by both parties verifying that they have established the
same security context, as specified in Section 4.3, which means that
both the OSCORE request and the OSCORE response passes verification.
RS authentication requires both that the client trusts the AS and
that the OSCORE response from the RS passes verification.

OSCORE is designed to secure point-to-point communication, providing
a secure binding between the request and the response(s). Thus, the
basic OSCORE protocol is not intended for use in point-to-multipoint
communication (e.g., multicast, publish-subscribe). Implementers of
this profile should make sure that their use case corresponds to the
expected use of OSCORE, to prevent weakening the security assurances
provided by OSCORE.

Since the use of nonces N1 and N2 during the exchange guarantees
uniqueness of AEAD keys and nonces, it is REQUIRED that the exchanged
nonces are not reused with the same input keying material even in
case of reboots. The exchange of 64-bit random nonces is RECOMMENDED
in this document. Considering the birthday paradox, the average
collision for each nonce will happen after 2^32 messages, which is
considerably more token provisionings than would be expected for
intended applications. If applications use something else, such as a
counter, they need to guarantee that reboot and loss of state on
either node does not provoke reuse. If that is not guaranteed, nodes
are susceptible to reuse of AEAD (nonce, key) pairs, especially since
an on-path attacker can cause the use of a previously exchanged
client nonce N1 for security context establishment by replaying the
corresponding client-to-server message.

In this profile, it is RECOMMENDED that the RS maintains a single
access token for each client. The use of multiple access tokens for
a single client increases the strain on the resource server as it
must consider every access token and calculate the actual permissions
of the client. Also, tokens indicating different or disjoint
permissions from each other may lead the server to enforce wrong
permissions. If one of the access tokens expires earlier than
others, the resulting permissions may offer insufficient protection.
Developers SHOULD avoid using multiple access tokens for the same
client.

If a single OSCORE Input Material is used with multiple RSs, the RSs
can impersonate the client to one of the other RSs and impersonate
another RS to the client. If a Master Secret is used with several
clients, the clients can impersonate RS to one of the other clients.
Similarly, if symmetric keys are used to integrity protect the token
between AS and RS and the token can be used with multiple RSs, the
RSs can impersonate AS to one of the other RSs. If the token key is
used for any other communication between the RSs and AS, the RSs can
impersonate each other to the AS.

8. Privacy Considerations

This document specifies a profile for the ACE framework [RFC9200].
Thus, the general privacy considerations from the framework also
apply to this profile.

As this document uses OSCORE, the privacy considerations from
[RFC8613] apply here as well.

An unprotected response to an unauthorized request may disclose
information about the resource server and/or its existing
relationship with the client. It is advisable to include as little
information as possible in an unencrypted response. When an OSCORE
security context already exists between the client and the resource
server, more detailed information may be included.

The token is sent in the clear to the authz-info endpoint, so if a
client uses the same single token from multiple locations with
multiple resource servers, it can risk being tracked by the token's
value even when the access token is encrypted.

The nonces exchanged in the request and response to the authz-info
endpoint are also sent in the clear, so using random nonces is best
for privacy (as opposed to, e.g., a counter, which might leak some
information about the client).

The identifiers used in OSCORE, negotiated between the client and RS,
are privacy sensitive (see Section 12.8 of [RFC8613]) and could
reveal information about the client, or they may be used for
correlating requests from one client.

Note that some information might still leak after OSCORE is
established, due to observable message sizes, the source, and the
destination addresses.

9. IANA Considerations

9.1. ACE Profile Registry

The following registration has been made in the "ACE Profiles"
registry following the procedure specified in Section 8.8 of
[RFC9200]:

Name: coap_oscore
Description: Profile for using OSCORE to secure communication
between constrained nodes using the Authentication and
Authorization for Constrained Environments framework.
CBOR Value: 2
Reference: RFC 9203

9.2. OAuth Parameters Registry

The following registrations have been made in the "OAuth Parameters"
registry [IANA.OAuthParameters] following the procedure specified in
Section 11.2 of [RFC6749]:

Parameter name: nonce1
Parameter usage location: client-rs request
Change Controller: IETF
Specification Document(s): RFC 9203

Parameter name: nonce2
Parameter usage location: rs-client response
Change Controller: IETF
Specification Document(s): RFC 9203

Parameter name: ace_client_recipientid
Parameter usage location: client-rs request
Change Controller: IETF
Specification Document(s): RFC 9203

Parameter name: ace_server_recipientid
Parameter usage location: rs-client response
Change Controller: IETF
Specification Document(s): RFC 9203

9.3. OAuth Parameters CBOR Mappings Registry

The following registrations have been made in the "OAuth Parameters
CBOR Mappings" registry following the procedure specified in
Section 8.10 of [RFC9200]:

Name: nonce1
CBOR Key: 40
Value Type: bstr
Reference: RFC 9203

Name: nonce2
CBOR Key: 42
Value Type: bstr
Reference: RFC 9203

Name: ace_client_recipientid
CBOR Key: 43
Value Type: bstr
Reference: RFC 9203

Name: ace_server_recipientid
CBOR Key: 44
Value Type: bstr
Reference: RFC 9203

9.4. OSCORE Security Context Parameters Registry

IANA has created a new registry entitled "OSCORE Security Context
Parameters". The registration procedure depends on the range of CBOR
label values, following [RFC8126]. Guidelines for the experts are
provided in Section 9.7.

The columns of the registry are:

Name: The JSON name requested (e.g., "ms"). Because a core goal of
this document is for the resulting representations to be compact,
it is RECOMMENDED that the name be short. This name is case
sensitive. Names may not match other registered names in a case-
insensitive manner unless the designated experts determine that
there is a compelling reason to allow an exception. The name is
not used in the CBOR encoding.

CBOR Label: The value to be used to identify this name. Map key
labels MUST be unique. The label can be a positive integer, a
negative integer, or a string. Integer values between -256 and
255 and strings of length 1 are designated as Standards Track
document required. Integer values from -65536 to -257 and from
256 to 65535 and strings of length 2 are designated as
Specification Required. Integer values greater than 65535 and
strings of length greater than 2 are designated as Expert Review.
Integer values less than -65536 are marked as Private Use.

CBOR Type: This field contains the CBOR type for the field.

Registry: This field denotes the registry that values may come from,
if one exists.

Description: This field contains a brief description for the field.

Reference: This contains a pointer to the public specification for
the field, if one exists.

This registry has been initially populated by the values in Table 1.
The Reference column for all of these entries is this document.

9.5. CWT Confirmation Methods Registry

The following registration has been made in the "CWT Confirmation
Methods" registry [IANA.CWTConfirmationMethods] following the
procedure specified in Section 7.2.1 of [RFC8747]:

Confirmation Method Name: osc
Confirmation Method Description: OSCORE_Input_Material carrying the
parameters for using OSCORE per-message security with implicit key
confirmation
JWT Confirmation Method Name: osc
Confirmation Key: 4
Confirmation Value Type(s): map
Change Controller: IETF
Specification Document(s): Section 3.2.1 of RFC 9203

9.6. JWT Confirmation Methods Registry

The following registration has been made in the "JWT Confirmation
Methods" registry [IANA.JWTConfirmationMethods] following the
procedure specified in Section 6.2.1 of [RFC7800]:

Confirmation Method Value: osc
Confirmation Method Description: OSCORE_Input_Material carrying the
parameters for using OSCORE per-message security with implicit key
confirmation
Change Controller: IETF
Specification Document(s): Section 3.2.1 of RFC 9203

9.7. Expert Review Instructions

The IANA registry established in this document is defined to use the
Expert Review registration policy. This section gives some general
guidelines for what the experts should be looking for, but 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:

* 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 Track range of point
assignment. Specifications should exist for specification
required ranges, but early assignment before a specification is
available is considered to be permissible. Specifications are
needed for the First Come First Served range if they are expected
to be used outside of closed environments in an interoperable way.
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. The fact that there is a range for
Standards Track documents does not mean that a Standards Track
document cannot have points assigned outside of that 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.

10. References

10.1. Normative References

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

[IANA.CWTConfirmationMethods]
IANA, "CWT Confirmation Methods",
<https://www.iana.org/assignments/cwt>.

[IANA.JWTConfirmationMethods]
IANA, "JWT Confirmation Methods",
<https://www.iana.org/assignments/jwt>.

[IANA.OAuthParameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.

[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>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.

[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[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>.

[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.

[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>.

[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.

[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/info/rfc9053>.

[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/info/rfc9200>.

[RFC9201] Seitz, L., "Additional OAuth Parameters for Authentication
and Authorization for Constrained Environments (ACE)",
RFC 9201, DOI 10.17487/RFC9201, August 2022,
<https://www.rfc-editor.org/info/rfc9201>.

10.2. Informative References

[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.

[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.

[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>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.

[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.

Appendix A. Profile Requirements

This section lists the specifications of this profile based on the
requirements of the framework, as requested in Appendix C of
[RFC9200].

* Optionally, define new methods for the client to discover the
necessary permissions and AS for accessing a resource, different
from the one proposed in [RFC9200]: Not specified

* Optionally, specify new grant types: Not specified

* Optionally, define the use of client certificates as client
credential type: Not specified

* Specify the communication protocol the client and RS must use:
CoAP

* Specify the security protocol the client and RS must use to
protect their communication: OSCORE

* Specify how the client and the RS mutually authenticate:
Implicitly by possession of a common OSCORE security context.
Note that the mutual authentication is not completed before the
client has verified an OSCORE response using this security
context.

* Specify the proof-of-possession protocol(s) and how to select one,
if several are available. Also specify which key types (e.g.,
symmetric/asymmetric) are supported by a specific proof-of-
possession protocol: OSCORE algorithms; preestablished symmetric
keys

* Specify a unique ace_profile identifier: coap_oscore

* If introspection is supported, specify the communication and
security protocol for introspection: HTTP/CoAP (+ TLS/DTLS/OSCORE)

* Specify the communication and security protocol for interactions
between client and AS: HTTP/CoAP (+ TLS/DTLS/OSCORE)

* Specify if/how the authz-info endpoint is protected, including how
error responses are protected: Not protected

* Optionally, define methods of token transport other than the
authz-info endpoint: Not defined

Acknowledgments

The authors wish to thank Jim Schaad and Marco Tiloca for the
substantial input to this document, as well as Carsten Bormann, Elwyn
Davies, Linda Dunbar, Roman Danyliw, Martin Duke, Lars Eggert, Murray
Kucherawy, and Zaheduzzaman Sarker for their reviews and feedback.
Special thanks to the responsible area director Benjamin Kaduk for
his extensive review and contributed text. Ludwig Seitz worked on
this document as part of the CelticNext projects CyberWI and CRITISEC
with funding from Vinnova. The work on this document has been partly
supported also by the H2020 project SIFIS-Home (Grant agreement
952652).

Authors' Addresses

Francesca Palombini
Ericsson AB
Email: [email protected]

Ludwig Seitz
Combitech
Djäknegatan 31
SE-211 35 Malmö
Sweden
Email: [email protected]

Göran Selander
Ericsson AB
Email: [email protected]

Martin Gunnarsson
RISE
Scheelevägen 17
SE-22370 Lund
Sweden
Email: [email protected]