wdiff rfc9202v1.txt rfc9202.txt

Internet Engineering Task Force (IETF) S. Gerdes
Request for Comments: 9202 O. Bergmann
Category: Standards Track C. Bormann
ISSN: 2070-1721 Universität Bremen TZI
G. Selander
Ericsson AB
L. Seitz
Combitech
March
August 2022

Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE)

Abstract

This specification defines a profile of the Authentication and
Authorization for Constrained Environments (ACE) framework that
allows constrained servers to delegate client authentication and
authorization. The protocol relies on DTLS version 1.2 or later for
communication security between entities in a constrained network
using either raw public keys or pre-shared keys. A resource-
constrained server can use this protocol to delegate management of
authorization information to a trusted host with less-severe
limitations regarding processing power and memory.

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/rfc9202.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Terminology
2. Protocol Overview
3. Protocol Flow
3.1. Communication between the Client and the Authorization
Server
3.2. Raw Public Key Mode
3.2.1. Access Token Retrieval from the Authorization Server
3.2.2. DTLS Channel Setup between the Client and Resource
Server
3.3. PreSharedKey Pre-shared Key Mode
3.3.1. Access Token Retrieval from the Authorization Server
3.3.2. DTLS Channel Setup between the Client and Resource
Server
3.4. Resource Access
4. Dynamic Update of Authorization Information
5. Token Expiration
6. Secure Communication with an Authorization Server
7. Security Considerations
7.1. Reuse of Existing Sessions
7.2. Multiple Access Tokens
7.3. Out-of-Band Configuration
8. Privacy Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

This specification defines a profile of the ACE framework [RFC9200].
In this profile, a client (C) and a resource server (RS) use the
Constrained Application Protocol (CoAP) [RFC7252] over DTLS version
1.2 [RFC6347] to communicate. This specification uses DTLS 1.2
terminology, but later versions such as DTLS 1.3 [RFC9147] can be
used instead. The client obtains an access token bound to a key (the proof-of-
possession
proof-of-possession (PoP) key) from an authorization server (AS) to
prove its authorization to access protected resources hosted by the
resource server. Also, the client and the resource server are
provided by the authorization server with the necessary keying
material to establish a DTLS session. The communication between the
client and authorization server may also be secured with DTLS. This
specification supports DTLS with raw public keys (RPKs) [RFC7250] and
with pre-shared keys (PSKs) [RFC4279]. How token introspection
[RFC7662] is performed between the RS and AS is out of scope for this
specification.

The ACE framework requires that the client and server mutually
authenticate each other before any application data is exchanged.
DTLS enables mutual authentication if both the client and server
prove their ability to use certain keying material in the DTLS
handshake. The authorization server assists in this process on the
server side by incorporating keying material (or information about
keying material) into the access token, which is considered a "proof-
of-possession" proof-
of-possession token.

In the RPK mode, the client proves that it can use the RPK bound to
the token and the server shows that it can use a certain RPK.

The resource server needs access to the token in order to complete
this exchange. For the RPK mode, the client must upload the access
token to the resource server before initiating the handshake, as
described in Section 5.10.1 of the ACE framework [RFC9200].

In the PSK mode, the client and server show with the DTLS handshake
that they can use the keying material that is bound to the access
token. To transfer the access token from the client to the resource
server, the psk_identity parameter in the DTLS PSK handshake may be
used instead of uploading the token prior to the handshake.

As recommended in Section 5.8 of [RFC9200], this specification uses
Concise Binary Object Representation (CBOR) web tokens to convey
claims within an access token issued by the server. While other
formats could be used as well, those are out of scope for this
document.

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.

Readers are expected to be familiar with the terms and concepts
described in [RFC9200] and [RFC9201].

The authorization information (authz-info) resource refers to the
authorization information endpoint, as specified in [RFC9200]. The
term claim is used in this document with the same semantics as in
[RFC9200], i.e., it denotes information carried in the access token
or returned from introspection.

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.

2. Protocol Overview

The CoAP-DTLS profile for ACE specifies the transfer of
authentication information and, if necessary, authorization
information between the client (C) and the resource server (RS)
during setup of a DTLS session for CoAP messaging. It also specifies
how the client can use CoAP over DTLS to retrieve an access token
from the authorization server (AS) for a protected resource hosted on
the resource server. As specified in Section 6.7 of [RFC9200], use
of DTLS for one or both of these interactions is completely
independent.

This profile requires the client to retrieve an access token for the
protected resource(s) it wants to access on the resource server, as
specified in [RFC9200]. Figure 1 shows the typical message flow in
this scenario (messages in square brackets are optional):

-->

C RS AS
| [---- Resource Request ------>]| |
| | |
| [<-AS Request Creation Hints-] | |
| | |
| ------- Token Request ----------------------------> |
| | |
| <---------------------------- Access Token --------- |
| + Access Information |

Figure 1: Retrieving an Access Token

To determine the authorization server in charge of a resource hosted
at the resource server, the client can send an initial Unauthorized
Resource Request message to the resource server. The resource server
then denies the request and sends an AS Request Creation Hints
message containing the address of its authorization server back to
the client, as specified in Section 5.3 of [RFC9200].

Once the client knows the authorization server's address, it can send
an access token request to the token endpoint at the authorization
server, as specified in [RFC9200]. As the access token request and
the response may contain confidential data, the communication between
the client and the authorization server must be confidentiality
protected and ensure authenticity. The client is expected to have
been registered at the authorization server, as outlined in Section 4
of [RFC9200].

The access token returned by the authorization server can then be
used by the client to establish a new DTLS session with the resource
server. When the client intends to use an asymmetric proof-of-
possession key in the DTLS handshake with the resource server, the
client MUST upload the access token to the authz-info resource, i.e.,
the authz-info endpoint, on the resource server before starting the
DTLS handshake, as described in Section 5.10.1 of [RFC9200]. In case
the client uses a symmetric proof-of-possession key in the DTLS
handshake, the procedure above MAY be used, or alternatively the
access token MAY instead be transferred in the DTLS ClientKeyExchange
message (see Section 3.3.2). In any case, DTLS MUST be used in a
mode that provides replay protection.

Figure 2 depicts the common protocol flow for the DTLS profile after
the client has retrieved the access token from the authorization
server (AS).

Figure 2: Protocol Overview

3. Protocol Flow

The following sections specify how CoAP is used to interchange
access-related data between the resource server, the client, and the
authorization server so that the authorization server can provide the
client and the resource server with sufficient information to
establish a secure channel and convey authorization information
specific for this communication relationship to the resource server.

Section 3.1 describes how the communication between the client (C)
and the authorization server (AS) must be secured. Depending on the
used
CoAP security mode used (see also Section 9 of [RFC7252]), the
client-to-AS request, AS-to-client response, and DTLS session
establishment carry slightly different information. Section 3.2
addresses the use of raw public keys, while Section 3.3 defines how
pre-shared keys are used in this profile.

3.1. Communication between the Client and the Authorization Server

To retrieve an access token for the resource that the client wants to
access, the client requests an access token from the authorization
server. Before the client can request the access token, the client
and the authorization server MUST establish a secure communication
channel. This profile assumes that the keying material to secure
this communication channel has securely been obtained either by
manual configuration or in an automated provisioning process. The
following requirements, in alignment with Section 6.5 of [RFC9200],
therefore must be met:

* The client MUST securely have obtained keying material to
communicate with the authorization server.

* Furthermore, the client MUST verify that the authorization server
is authorized to provide access tokens (including authorization
information) about the resource server to the client and that this
authorization information about the authorization server is still
valid.

* Also, the authorization server MUST securely have obtained keying
material for the client and obtained authorization rules approved
by the resource owner (RO) concerning the client and the resource
server that relate to this keying material.

The client and the authorization server MUST use their respective
keying material for all exchanged messages. How the security
association between the client and the authorization server is
bootstrapped is not part of this document. The client and the
authorization server must ensure the confidentiality, integrity, and
authenticity of all exchanged messages within the ACE protocol.

Section 6 specifies how communication with the authorization server
is secured.

3.2. Raw Public Key Mode

When the client uses raw public key authentication, the procedure is
as described in the following.

3.2.1. Access Token Retrieval from the Authorization Server

After the client and the authorization server mutually authenticated
each other and validated each other's authorization, the client sends
a token request to the authorization server's token endpoint. The
client MUST add a req_cnf object carrying either its raw public key
or a unique identifier for a public key that it has previously made
known to the authorization server. It is RECOMMENDED that the client
uses DTLS with the same keying material to secure the communication
with the authorization server, proving possession of the key as part
of the token request. Other mechanisms for proving possession of the
key may be defined in the future.

An example access token request from the client to the authorization
server is depicted in Figure 3.

POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
/ grant_type / 33 : client_credentials, / client_credentials / 2,
/ audience / 5 : "tempSensor4711",
/ req_cnf / 4 : {
/ COSE_Key / 1 : {
/ kty / 1 : EC2, / EC2 / 2,
/ crv / -1 : P-256, / P-256 / 1,
/ x / -2 : h'e866c35f4c3c81bb96a1...', h'e866c35f4c3c81bb96a1/.../',
/ y / -3 : h'2e25556be097c8778a20...' h'2e25556be097c8778a20/.../'
}
}
}

Figure 3: Access Token Request Example for RPK Mode

The example shows an access token request for the resource identified
by the string "tempSensor4711" on the authorization server using a
raw public key.

The authorization server MUST check if the client that it
communicates with is associated with the RPK in the req_cnf parameter
before issuing an access token to it. If the authorization server
determines that the request is to be authorized according to the
respective authorization rules, it generates an access token response
for the client. The access token MUST be bound to the RPK of the
client by means of the cnf claim.

The response MUST contain an ace_profile parameter if the ace_profile
parameter in the request is empty and MAY contain this parameter
otherwise (see Section 5.8.2 of [RFC9200]). This parameter is set to
coap_dtls to indicate that this profile MUST be used for
communication between the client and the resource server. The
response also contains an access token with information for the
resource server about the client's public key. The authorization
server MUST return in its response the parameter rs_cnf unless it is
certain that the client already knows the public key of the resource
server. The authorization server MUST ascertain that the RPK
specified in rs_cnf belongs to the resource server that the client
wants to communicate with. The authorization server MUST protect the
integrity of the access token such that the resource server can
detect unauthorized changes. If the access token contains
confidential data, the authorization server MUST also protect the
confidentiality of the access token.

The client MUST ascertain that the access token response belongs to a
certain, previously sent access token request, as the request may
specify the resource server with which the client wants to
communicate.

An example access token response from the authorization server to the
client is depicted in Figure 4. Here, the contents of the
access_token claim have been truncated to improve readability. For
the client, the response comprises Access Information that contains
the server's public key in the rs_cnf parameter. Caching proxies
process the Max-Age option in the CoAP response, which has a default
value of 60 seconds (Section 5.6.1 of [RFC7252]). The authorization
server SHOULD adjust the Max-Age option such that it does not exceed
the expires_in parameter to avoid stale responses.

2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3560
Payload:
{
/ access_token / 1 : b64'SlAV32hkKG... b64'SlAV32hk'/...
(remainder of CWT omitted for brevity;
CWT contains the client's RPK in the cnf claim)', claim)/,
/ expires_in / 2 : 3600,
/ rs_cnf / 41 : {
/ COSE_Key / 1 : {
/ kty / 1 : EC2, / EC2 / 2,
/ crv / -1 : P-256, / P-256 / 1,
/ x / -2 : h'd7cc072de2205bdc1537...', h'd7cc072de2205bdc1537/.../',
/ y / -3 : h'f95e1d4b851a2cc80fff...' h'f95e1d4b851a2cc80fff/.../'
}
}
}

Figure 4: Access Token Response Example for RPK Mode

3.2.2. DTLS Channel Setup between the Client and Resource Server

Before the client initiates the DTLS handshake with the resource
server, the client MUST send a POST request containing the obtained
access token to the authz-info resource hosted by the resource
server. After the client receives a confirmation that the resource
server has accepted the access token, it proceeds to establish a new
DTLS channel with the resource server. The client MUST use its
correct public key in the DTLS handshake. If the authorization
server has specified a cnf field in the access token response, the
client MUST use this key. Otherwise, the client MUST use the public
key that it specified in the req_cnf of the access token request.
The client MUST specify this public key in the SubjectPublicKeyInfo
structure of the DTLS handshake, as described in [RFC7250].

If the client does not have the keying material belonging to the
public key, the client MAY try to send an access token request to the
AS, where it the client specifies its public key in the req_cnf
parameter. If the AS still specifies a public key in the response
that the client does not have, the client SHOULD re-register with the
authorization server to establish a new client public key. This
process is out of scope for this document.

To be consistent with [RFC7252], which allows for shortened MAC Message
Authentication Code (MAC) tags in constrained environments, an
implementation that supports the RPK mode of this profile MUST at
least support the cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
[RFC7251]. As discussed in [RFC7748], new Elliptic Curve
Cryptography (ECC) curves have been defined recently that are
considered superior to the so-called NIST curves. Implementations of
this profile MUST therefore implement support for curve25519
(cf. [RFC8032], [RFC8422]), as this curve is said to be efficient and
less dangerous, regarding implementation errors, than the secp256r1
curve mandated in [RFC7252].

The resource server MUST check if the access token is still valid, if
the resource server is the intended destination (i.e., the audience)
of the token, and if the token was issued by an authorized
authorization server (see also Section 5.10.1.1 of [RFC9200]). The
access token is constructed by the authorization server such that the
resource server can associate the access token with the client's
public key. The cnf claim MUST contain either the client's RPK or,
if the key is already known by the resource server (e.g., from
previous communication), a reference to this key. If the
authorization server has no certain knowledge that the client's key
is already known to the resource server, the client's public key MUST
be included in the access token's cnf parameter. If CBOR web tokens
[RFC8392] are used (as recommended in [RFC9200]), keys MUST be
encoded,
encoded as specified in [RFC8747]. A resource server MUST have the
capacity to store one access token for every proof-of-possession key
of every authorized client.

The raw public key used in the DTLS handshake with the client MUST
belong to the resource server. If the resource server has several
raw public keys, it needs to determine which key to use. The
authorization server can help with this decision by including a cnf
parameter in the access token that is associated with this
communication. In this case, the resource server MUST use the
information from the cnf field to select the proper keying material.

Thus, the handshake only finishes if the client and the resource
server are able to use their respective keying material.

3.3. PreSharedKey Pre-shared Key Mode

When the client uses pre-shared key authentication, the procedure is
as described in the following.

3.3.1. Access Token Retrieval from the Authorization Server

To retrieve an access token for the resource that the client wants to
access, the client MAY include a cnf req_cnf object carrying an
identifier for a symmetric key in its access token request to the
authorization server. This identifier can be used by the
authorization server to determine the shared secret to construct the
proof-of-possession token. The authorization server MUST check if
the identifier refers to a symmetric key that was previously
generated by the authorization server as a shared secret for the
communication between this client and the resource server. If no
such symmetric key was found, the authorization server MUST generate
a new symmetric key that is returned in its response to the client.

The authorization server MUST determine the authorization rules for
the client it communicates with, as defined by the resource owner,
and generate the access token accordingly. If the authorization
server authorizes the client, it returns an AS-to-client response.
If the ace_profile parameter is present, it is set to coap_dtls. The
authorization server MUST ascertain that the access token is
generated for the resource server that the client wants to
communicate with. Also, the authorization server MUST protect the
integrity of the access token to ensure that the resource server can
detect unauthorized changes. If the token contains confidential
data, such as the symmetric key, the confidentiality of the token
MUST also be protected. Depending on the requested token type and
algorithm in the access token request, the authorization server adds
Access Information to the response that provides the client with
sufficient information to set up a DTLS channel with the resource
server. The authorization server adds a cnf parameter to the Access
Information carrying a COSE_Key object that informs the client about
the shared secret that is to be used between the client and the
resource server. To convey the same secret to the resource server,
the authorization server can include it directly in the access token
by means of the cnf claim or provide sufficient information to enable
the resource server to derive the shared secret from the access
token. As an alternative, the resource server MAY use token
introspection to retrieve the keying material for this access token
directly from the authorization server.

An example access token request for an access token with a symmetric
proof-of-possession key is illustrated in Figure 5.

POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
/ audience / 5 : "smokeSensor1807", "smokeSensor1807"
}

Figure 5: Example Access Token Request, (Implicit) Symmetric PoP Key

A corresponding example access token response is illustrated in
Figure 6. In this example, the authorization server returns a 2.01
response containing a new access token (truncated to improve
readability) and information for the client, including the symmetric
key in the cnf claim. The information is transferred as a CBOR data
structure as specified in [RFC9200].

2.01 Created
Content-Format: application/ace+cbor
Max-Age: 85800
Payload:
{
/ access_token / 1 : h'd08343a10... h'd08343a1/...
(remainder of CWT omitted for brevity) brevity)/',
/ token_type / 34 : PoP, / PoP / 2,
/ expires_in / 2 : 86400,
profile
/ ace_profile / 38 : coap_dtls, / coap_dtls / 1,
/ cnf / 8 : {
/ COSE_Key / 1 : {
/ kty / 1 : symmetric, / symmetric / 4,
/ kid / 2 : h'3d027833fc6267ce',
/ k / -1 : h'73657373696f6e6b6579'
}
}
}

Figure 6: Example Access Token Response, Symmetric PoP Key

The access token also comprises a cnf claim. This claim usually
contains a COSE_Key object [RFC8152] that carries either the
symmetric key itself or a key identifier that can be used by the
resource server to determine the secret key it shares with the
client. If the access token carries a symmetric key, the access
token MUST be encrypted using a COSE_Encrypt0 structure (see
Section 7.1 of [RFC8392]). The authorization server MUST use the
keying material shared with the resource server to encrypt the token.

The cnf structure in the access token is provided in Figure 7.

/ cnf / 8 : {
/ COSE_Key / 1 : {
/ kty / 1 : symmetric, / symmetric / 4,
/ kid / 2 : h'3d027833fc6267ce'
}
}

Figure 7: Access Token without Keying Material

A response that declines any operation on the requested resource is
constructed according to Section 5.2 of [RFC6749] (cf. Section 5.8.3
of [RFC9200]). Figure 8 shows an example for a request that has been
rejected due to invalid request parameters.

4.00 Bad Request
Content-Format: application/ace+cbor
Payload:
{
/ error / 30 : / invalid_request / 1
}

Figure 8: Example Access Token Response with Reject

The method for how the resource server determines the symmetric key
from an access token containing only a key identifier is application
specific; the remainder of this section provides one example.

The authorization server and the resource server are assumed to share
a key derivation key used to derive the symmetric key shared with the
client from the key identifier in the access token. The key
derivation key may be derived from some other secret key shared
between the authorization server and the resource server. This key
needs to be securely stored and processed in the same way as the key
used to protect the communication between the authorization server
and the resource server.

Knowledge of the symmetric key shared with the client must not reveal
any information about the key derivation key or other secret keys
shared between the authorization server and resource server.

In order to generate a new symmetric key to be used by the client and
resource server, the authorization server generates a new key
identifier that MUST be unique among all key identifiers used by the
authorization server for this resource server. The authorization
server then uses the key derivation key shared with the resource
server to derive the symmetric key, as specified below. Instead of
providing the keying material in the access token, the authorization
server includes the key identifier in the kid parameter (see
Figure 7). This key identifier enables the resource server to
calculate the symmetric key used for the communication with the
client using the key derivation key and a key derivation function
(KDF) to be defined by the application, for example, HKDF-SHA-256.
The key identifier picked by the authorization server MUST be unique
for each access token where a unique symmetric key is required.

In this example, the HMAC-based key derivation function (HKDF)
consists of the composition of the HKDF-Extract and HKDF-Expand steps
[RFC5869]. The symmetric key is derived from the key identifier, the
key derivation key, and other data:

OKM = HKDF(salt, IKM, info, L),

where:

* OKM, the output keying material, is the derived symmetric key

* salt is the empty byte string

* IKM, the input keying material, is the key derivation key, as
defined above

* info is the serialization of a CBOR array consisting of [RFC8610]:

info = [
type : tstr,
L : uint,
access_token:
access_token : bytes
]

where:

- type is set to the constant text string "ACE-CoAP-DTLS-key-
derivation"

- L is the size of the symmetric key in bytes

- access_token is the content of the access_token field, as
transferred from the authorization server to the resource
server.

All CBOR data types are encoded in CBOR using preferred serialization
and deterministic encoding, as specified in Section 4 of [RFC8949].
In particular, this implies that the type and L components use the
minimum length encoding. The content of the access_token field is
treated as opaque data for the purpose of key derivation.

Use of a unique (per-resource-server) kid and the use of a key
derivation IKM that MUST be unique per AS/RS pair, as specified
above, will ensure that the derived key is not shared across multiple
clients. However, to provide variation in the derived key across
different tokens used by the same client, it is additionally
RECOMMENDED to include the "iat" iat claim and either the "exp" exp or "exi" exi claims
in the access token.

3.3.2. DTLS Channel Setup between the Client and Resource Server

When a client receives an access token response from an authorization
server, the client MUST check if the access token response is bound
to a certain, previously sent access token request, as the request
may specify the resource server with which the client wants to
communicate.

The client checks if the payload of the access token response
contains an access_token parameter and a cnf parameter. With this
information, the client can initiate the establishment of a new DTLS
channel with a resource server. To use DTLS with pre-shared keys,
the client follows the PSK key exchange algorithm specified in
Section 2 of [RFC4279], using the key conveyed in the cnf parameter
of the AS response as a PSK when constructing the premaster secret.
To be consistent with the recommendations in [RFC7252], a client in
the PSK mode MUST support the cipher suite TLS_PSK_WITH_AES_128_CCM_8
[RFC6655].

In PreSharedKey mode, the knowledge of the shared secret by the
client and the resource server is used for mutual authentication
between both peers. Therefore, the resource server must be able to
determine the shared secret from the access token. Following the
general ACE authorization framework, the client can upload the access
token to the resource server's authz-info resource before starting
the DTLS handshake. The client then needs to indicate during the
DTLS handshake which previously uploaded access token it intends to
use. To do so, it MUST create a COSE_Key structure with the kid that
was conveyed in the rs_cnf claim in the token response from the
authorization server and the key type symmetric. This structure then
is included as the only element in the cnf structure whose CBOR
serialization is used as value for psk_identity, as shown in
Figure 9.

{ / cnf / 8 : {
/ COSE_Key / 1 : {
kty: symmetric,
/ kty / 1 : / symmetric / 4,
/ kid / 2 : h'3d027833fc6267ce'
}
}
}

Figure 9: Access Token Containing a Single kid Parameter

The actual CBOR serialization for the data structure from Figure 9 as
a sequence of bytes in hexadecimal notation will be:

A1 08 A1 01 A2 01 04 02 48 3D 02 78 33 FC 62 67 CE

As an alternative to the access token upload, the client can provide
the most recent access token in the psk_identity field of the
ClientKeyExchange message. To do so, the client MUST treat the
contents of the access_token field from the AS-to-client response as
opaque data, as specified in Section 4.2 of [RFC7925], and not
perform any recoding. This allows the resource server to retrieve
the shared secret directly from the cnf claim of the access token.

DTLS 1.3 [RFC9147] does not use the ClientKeyExchange message; for
DTLS 1.3, the access token is placed in the identity field of a
PSKIdentity within the PreSharedKeyExtension of the ClientHello.

If a resource server receives a ClientKeyExchange message that
contains a psk_identity with a length greater than zero, it MUST
parse the contents of the psk_identity field as a CBOR data structure
and process the contents as following:

* If the data contains a cnf field with a COSE_Key structure with a
kid, the resource server continues the DTLS handshake with the
associated key that corresponds to this kid.

* If the data comprises additional CWT information, this information
must be stored as an access token for this DTLS association before
continuing with the DTLS handshake.

If the contents of the psk_identity do not yield sufficient
information to select a valid access token for the requesting client,
the resource server aborts the DTLS handshake with an
illegal_parameter alert.

When the resource server receives an access token, it MUST check if
the access token is still valid, if the resource server is the
intended destination (i.e., the audience of the token), and if the
token was issued by an authorized authorization server. This
specification implements access tokens as proof-of-possession tokens.
Therefore, the access token is bound to a symmetric PoP key that is
used as a shared secret between the client and the resource server.
A resource server MUST have the capacity to store one access token
for every proof-of-possession key of every authorized client. The
resource server may use token introspection [RFC7662] on the access
token to retrieve more information about the specific token. The use
of introspection is out of scope for this specification.

While the client can retrieve the shared secret from the contents of
the cnf parameter in the AS-to-client response, the resource server
uses the information contained in the cnf claim of the access token
to determine the actual secret when no explicit kid was provided in
the psk_identity field. If key derivation is used, the cnf claim
MUST contain a kid parameter to be used by the server as the IKM for
key derivation, as described above.

3.4. Resource Access

Once a DTLS channel has been established, established as described in either
Sections 3.2 and or 3.3, respectively, the client is authorized to access
resources covered by the access token it has uploaded to the authz-
info resource that is hosted by the resource server.

With the successful establishment of the DTLS channel, the client and
the resource server have proven that they can use their respective
keying material. An access token that is bound to the client's
keying material is associated with the channel. According to
Section 5.10.1 of [RFC9200], there should be only one access token
for each client. New access tokens issued by the authorization
server SHOULD replace previously issued access tokens for the
respective client. The resource server therefore needs a common
understanding with the authorization server about how access tokens
are ordered. The authorization server may, e.g., specify a cti claim
for the access token (see Section 5.9.4 5.9.2 of [RFC9200]) to employ a
strict order.

Any request that the resource server receives on a DTLS channel that
is tied to an access token via its keying material MUST be checked
against the authorization rules that can be determined with the
access token. The resource server MUST check for every request if
the access token is still valid. If the token has expired, the
resource server MUST remove it. Incoming CoAP requests that are not
authorized with respect to any access token that is associated with
the client MUST be rejected by the resource server with a 4.01
response. The response SHOULD include AS Request Creation Hints, as
described in Section 5.2 of [RFC9200].

The resource server MUST NOT accept an incoming CoAP request as
authorized if any of the following fails:

1. The message was received on a secure channel that has been
established using the procedure defined in this document.

2. The authorization information tied to the sending client is
valid.

3. The request is destined for the resource server.

4. The resource URI specified in the request is covered by the
authorization information.

5. The request method is an authorized action on the resource with
respect to the authorization information.

Incoming CoAP requests received on a secure DTLS channel that are not
thus authorized MUST be rejected according to Section 5.10.1.1 5.10.2 of
[RFC9200]:

1. with response code 4.03 (Forbidden) when the resource URI
specified in the request is not covered by the authorization
information and

2. with response code 4.05 (Method Not Allowed) when the resource
URI specified in the request is covered by the authorization
information but not the requested action.

The client MUST ascertain that its keying material is still valid
before sending a request or processing a response. If the client
recently has updated the access token (see Section 4), it must be
prepared that its request is still handled according to the previous
authorization rules, as there is no strict ordering between access
token uploads and resource access messages. See also Section 7.2 for
a discussion of access token processing.

If the client gets an error response containing AS Request Creation
Hints (cf. Section 5.3 of [RFC9200]) as a response to its requests,
it SHOULD request a new access token from the authorization server in
order to continue communication with the resource server.

Unauthorized requests that have been received over a DTLS session
SHOULD be treated as nonfatal by the resource server, i.e., the DTLS
session SHOULD be kept alive until the associated access token has
expired.

4. Dynamic Update of Authorization Information

Resource servers must only use a new access token to update the
authorization information for a DTLS session if the keying material
that is bound to the token is the same that was used in the DTLS
handshake. By associating the access tokens with the identifier of
an existing DTLS session, the authorization information can be
updated without changing the cryptographic keys for the DTLS
communication between the client and the resource server, i.e., an
existing session can be used with updated permissions.

The client can therefore update the authorization information stored
at the resource server at any time without changing an established
DTLS session. To do so, the client requests a new access token from
the authorization server for the intended action on the respective
resource and uploads this access token to the authz-info resource on
the resource server.

Figure 10 depicts the message flow where the client requests a new
access token after a security association between the client and the
resource server has been established using this protocol. If the
client wants to update the authorization information, the token
request MUST specify the key identifier of the proof-of-possession
key used for the existing DTLS channel between the client and the
resource server in the kid parameter of the client-to-AS request.
The authorization server MUST verify that the specified kid denotes a
valid verifier for a proof-of-possession token that has previously
been issued to the requesting client. Otherwise, the client-to-AS
request MUST be declined with the error code unsupported_pop_key, as
defined in Section 5.8.3 of [RFC9200].

When the authorization server issues a new access token to update
existing authorization information, it MUST include the specified kid
parameter in this access token. A resource server MUST replace the
authorization information of any existing DTLS session that is
identified by this key identifier with the updated authorization
information.

C RS AS
| <===== DTLS channel =====> | |
| + Access Token | |
| | |
| --- Token Request ----------------------------> |
| | |
| <---------------------------- New Access Token - |
| + Access Information |
| | |
| --- Update /authz-info --> | |
| New Access Token | |
| | |
| == Authorized Request ===> | |
| | |
| <=== Protected Resource == | |

Figure 10: Overview of Dynamic Update Operation

5. Token Expiration

The resource server MUST delete access tokens that are no longer
valid. DTLS associations that have been set up in accordance with
this profile are always tied to specific tokens (which may be
exchanged with a dynamic update, as described in Section 4). As
tokens may become invalid at any time (e.g., because they have
expired), the association may become useless at some point. A
resource server therefore MUST terminate existing DTLS association
after the last access token associated with this association has
expired.

As specified in Section 5.10.3 of [RFC9200], the resource server MUST
notify the client with an error response with code 4.01
(Unauthorized) for any long-running request before terminating the
association.

6. Secure Communication with an Authorization Server

As specified in the ACE framework (Sections 5.8 and 5.9 of
[RFC9200]), the requesting entity (the resource server and/or the
client) and the authorization server communicate via the token
endpoint or introspection endpoint. The use of CoAP and DTLS for
this communication is RECOMMENDED in this profile. Other protocols
fulfilling the security requirements defined in Section 5 of
[RFC9200] MAY be used instead.

How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the
authorization server are established is out of scope for this
profile.

If other means of securing the communication with the authorization
server are used, the communication security requirements from
Section 6.2 of [RFC9200] remain applicable.

7. Security Considerations

This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework [RFC9200].
As it follows this framework's general approach, the general security
considerations from Section 6 of [RFC9200] also apply to this
profile.

The authorization server must ascertain that the keying material for
the client that it provides to the resource server actually is
associated with this client. Malicious clients may hand over access
tokens containing their own access permissions to other entities.
This problem cannot be completely eliminated. Nevertheless, in RPK
mode, it should not be possible for clients to request access tokens
for arbitrary public keys; if the client can cause the authorization
server to issue a token for a public key without proving possession
of the corresponding private key, this allows for identity misbinding
attacks, where the issued token is usable by an entity other than the
intended one. At some point, the authorization server therefore
needs to validate that the client can actually use the private key
corresponding to the client's public key.

When using pre-shared keys provisioned by the authorization server,
the security level depends on the randomness of PSKs and the security
of the TLS cipher suite and key exchange algorithm. As this
specification targets constrained environments, message payloads
exchanged between the client and the resource server are expected to
be small and rare. CoAP [RFC7252] mandates the implementation of
cipher suites with abbreviated, 8-byte tags for message integrity
protection. For consistency, this profile requires implementation of
the same cipher suites. For application scenarios where the cost of
full-width authentication tags is low compared to the overall amount
of data being transmitted, the use of cipher suites with 16-byte
integrity protection tags is preferred.

The PSK mode of this profile offers a distribution mechanism to
convey authorization tokens together with a shared secret to a client
and a server. As this specification aims at constrained devices and
uses CoAP [RFC7252] as the transfer protocol, at least the cipher
suite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The
access tokens and the corresponding shared secrets generated by the
authorization server are expected to be sufficiently short-lived to
provide similar forward-secrecy properties to using ephemeral Diffie-
Hellman (DHE) key exchange mechanisms. For longer-lived access
tokens, DHE cipher suites should be used, i.e., cipher suites of the
form TLS_DHE_PSK_* or TLS_ECDHE_PSK_*.

Constrained devices that use DTLS [RFC6347] [RFC9147] are inherently
vulnerable to Denial of Service (DoS) attacks, as the handshake
protocol requires creation of an internal state within the device. This
is specifically of concern where an adversary is able to intercept
the initial cookie exchange and interject forged messages with a
valid cookie to continue with the handshake. A similar issue exists
with the unprotected authorization information endpoint when the
resource server needs to keep valid access tokens for a long time.
Adversaries could fill up the constrained resource server's internal
storage for a very long time with interjected intercepted or otherwise retrieved
valid access tokens. To mitigate against this, the resource server
should set a time boundary until an access token that has not been
used until then will be deleted.

The protection of access tokens that are stored in the authorization
information endpoint depends on the keying material that is used
between the authorization server and the resource server; the
resource server must ensure that it processes only access tokens that
are (encrypted and) integrity-protected integrity protected (and encrypted) by an authorization server
that is authorized to provide access tokens for the resource server.

7.1. Reuse of Existing Sessions

To avoid the overhead of a repeated DTLS handshake, [RFC7925]
recommends session resumption [RFC8446] to reuse session state from
an earlier DTLS association and thus requires client-side
implementation. In this specification, the DTLS session is subject
to the authorization rules denoted by the access token that was used
for the initial setup of the DTLS association. Enabling session
resumption would require the server to transfer the authorization
information with the session state in an encrypted SessionTicket to
the client. Assuming that the server uses long-lived keying
material, this could open up attacks due to the lack of forward
secrecy. Moreover, using this mechanism, a client can resume a DTLS
session without proving the possession of the PoP key again.
Therefore, session resumption should be used only in combination with
reasonably short-lived PoP keys.

Since renegotiation of DTLS associations is prone to attacks as well,
[RFC7925] requires that clients decline any renegotiation attempt. A
server that wants to initiate rekeying therefore SHOULD periodically
force a full handshake.

7.2. Multiple Access Tokens

Developers

Implementers SHOULD avoid using multiple access tokens for a client
(see also Section 5.10.1 of [RFC9200]).

Even when a single access token per client is used, an attacker could
compromise the dynamic update mechanism for existing DTLS connections
by delaying or reordering packets destined for the authz-info
endpoint. Thus, the order in which operations occur at the resource
server (and thus which authorization info is used to process a given
client request) cannot be guaranteed. Especially in the presence of
later-issued access tokens that reduce the client's permissions from
the initial access token, it is impossible to guarantee that the
reduction in authorization will take effect prior to the expiration
of the original token.

7.3. Out-of-Band Configuration

To communicate securely, the authorization server, the client, and
the resource server require certain information that must be
exchanged outside the protocol flow described in this document. The
authorization server must have obtained authorization information
concerning the client and the resource server that is approved by the
resource owner, as well as corresponding keying material. The
resource server must have received authorization information approved
by the resource owner concerning its authorization managers and the
respective keying material. The client must have obtained
authorization information concerning the authorization server
approved by its owner, as well as the corresponding keying material.
Also, the client's owner must have approved of the client's
communication with the resource server. The client and the
authorization server must have obtained a common understanding about
how this resource server is identified to ensure that the client
obtains access tokens and keying material for the correct resource
server. If the client is provided with a raw public key for the
resource server, it must be ascertained to which resource server
(which identifier and authorization information) the key is
associated. All authorization information and keying material must
be kept up to date.

8. Privacy Considerations

This privacy considerations from Section 7 of [RFC9200] apply also to
this profile.

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 a DTLS
session between an authenticated client and the resource server
already exists, more detailed information MAY be included with an
error response to provide the client with sufficient information to
react on that particular error.

Also, unprotected requests to the resource server may reveal
information about the client, e.g., which resources the client
attempts to request or the data that the client wants to provide to
the resource server. The client SHOULD NOT send confidential data in
an unprotected request.

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

9. IANA Considerations

The following registration has been made in the "ACE Profiles"
registry, following the procedure specified in [RFC9200].

Name: coap_dtls
Description: Profile for delegating client Authentication and
Authorization for Constrained Environments by establishing a
Datagram Transport Layer Security (DTLS) channel between resource-
constrained nodes.
CBOR Value: 1
Reference: RFC 9202

10. References

10.1. Normative References

[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>.

[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[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>.

[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.

[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
<https://www.rfc-editor.org/info/rfc7251>.

[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>.

[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.

[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.

[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>.

[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.

[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>.

[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>.

[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>.

[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) Using the OAuth 2.0
Framework (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200,
March
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, March August 2022,
<https://www.rfc-editor.org/info/rfc9201>.

10.2. Informative References

[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>.

[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012,
<https://www.rfc-editor.org/info/rfc6655>.

[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.

[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.

[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.

[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>.

[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>.

Acknowledgments

Special thanks to Jim Schaad for his contributions and reviews of
this document and to Ben Kaduk for his thorough reviews of this
document. Thanks also to Paul Kyzivat for his review. The authors
also would like to thank Marco Tiloca for his contributions.

Ludwig Seitz worked on this document as part of the CelticNext
projects CyberWI and CRITISEC with funding from Vinnova.

Authors' Addresses

Stefanie Gerdes
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org

Olaf Bergmann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63904
Email: bergmann@tzi.org

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org

Göran Selander
Ericsson AB
Email: goran.selander@ericsson.com

Ludwig Seitz
Combitech
Djäknegatan 31
SE-211 35 Malmö
Sweden
Email: ludwig.seitz@combitech.com