draft-ietf-ace-dtls-authorize-16.txt   draft-ietf-ace-dtls-authorize-17.txt 
ACE Working Group S. Gerdes ACE Working Group S. Gerdes
Internet-Draft O. Bergmann Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann Intended status: Standards Track C. Bormann
Expires: 9 September 2021 Universität Bremen TZI Expires: 12 November 2021 Universität Bremen TZI
G. Selander G. Selander
Ericsson AB Ericsson AB
L. Seitz L. Seitz
Combitech Combitech
8 March 2021 11 May 2021
Datagram Transport Layer Security (DTLS) Profile for Authentication and Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE) Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-16 draft-ietf-ace-dtls-authorize-17
Abstract Abstract
This specification defines a profile of the ACE framework that allows This specification defines a profile of the ACE framework that allows
constrained servers to delegate client authentication and constrained servers to delegate client authentication and
authorization. The protocol relies on DTLS version 1.2 for authorization. The protocol relies on DTLS version 1.2 for
communication security between entities in a constrained network communication security between entities in a constrained network
using either raw public keys or pre-shared keys. A resource- using either raw public keys or pre-shared keys. A resource-
constrained server can use this protocol to delegate management of constrained server can use this protocol to delegate management of
authorization information to a trusted host with less severe authorization information to a trusted host with less severe
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 September 2021. This Internet-Draft will expire on 12 November 2021.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Simplified BSD License. provided without warranty as described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Communication Between the Client and the Authorization 3.1. Communication Between the Client and the Authorization
Server . . . . . . . . . . . . . . . . . . . . . . . . . 6 Server . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 7 3.2. Raw Public Key Mode . . . . . . . . . . . . . . . . . . . 7
3.2.1. Access Token Retrieval from the Authorization 3.2.1. Access Token Retrieval from the Authorization
Server . . . . . . . . . . . . . . . . . . . . . . . 7 Server . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. DTLS Channel Setup Between Client and Resource 3.2.2. DTLS Channel Setup Between Client and Resource
Server . . . . . . . . . . . . . . . . . . . . . . . 9 Server . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 10 3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 10
3.3.1. Access Token Retrieval from the Authorization 3.3.1. Access Token Retrieval from the Authorization
Server . . . . . . . . . . . . . . . . . . . . . . . 10 Server . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.2. DTLS Channel Setup Between Client and Resource 3.3.2. DTLS Channel Setup Between Client and Resource
Server . . . . . . . . . . . . . . . . . . . . . . . 14 Server . . . . . . . . . . . . . . . . . . . . . . . 15
3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 16 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 17
4. Dynamic Update of Authorization Information . . . . . . . . . 18 4. Dynamic Update of Authorization Information . . . . . . . . . 18
5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 19 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 20
6. Secure Communication with an Authorization Server . . . . . . 20 6. Secure Communication with an Authorization Server . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 21 7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 22
7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 22 7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 22
7.3. Out-of-Band Configuration . . . . . . . . . . . . . . . . 22 7.3. Out-of-Band Configuration . . . . . . . . . . . . . . . . 23
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . 24 11.1. Normative References . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . 26 11.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
This specification defines a profile of the ACE framework This specification defines a profile of the ACE framework
[I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource
server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to
communicate. The client obtains an access token, bound to a key (the communicate. This specification uses DTLS 1.2 terminology, but later
proof-of-possession key), from an authorization server to prove its versions such as DTLS 1.3 can be used instead. The client obtains an
authorization to access protected resources hosted by the resource access token, bound to a key (the proof-of-possession key), from an
server. Also, the client and the resource server are provided by the authorization server to prove its authorization to access protected
authorization server with the necessary keying material to establish resources hosted by the resource server. Also, the client and the
a DTLS session. The communication between client and authorization resource server are provided by the authorization server with the
server may also be secured with DTLS. This specification supports necessary keying material to establish a DTLS session. The
DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys communication between client and authorization server may also be
(PSK) [RFC4279]. secured with DTLS. This specification supports DTLS with Raw Public
Keys (RPK) [RFC7250] and with Pre-Shared Keys (PSK) [RFC4279]. How
token introspection [RFC7662] is performed between RS and AS is out
of scope for this specification.
The ACE framework requires that client and server mutually The ACE framework requires that client and server mutually
authenticate each other before any application data is exchanged. authenticate each other before any application data is exchanged.
DTLS enables mutual authentication if both client and server prove DTLS enables mutual authentication if both client and server prove
their ability to use certain keying material in the DTLS handshake. their ability to use certain keying material in the DTLS handshake.
The authorization server assists in this process on the server side The authorization server assists in this process on the server side
by incorporating keying material (or information about keying by incorporating keying material (or information about keying
material) into the access token, which is considered a "proof of material) into the access token, which is considered a "proof of
possession" token. possession" token.
In the RPK mode, the client proves that it can use the RPK bound to 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 token and the server shows that it can use a certain RPK.
The resource server needs access to the token in order to complete The resource server needs access to the token in order to complete
this exchange. For the RPK mode, the client must upload the access this exchange. For the RPK mode, the client must upload the access
token to the resource server before initiating the handshake, as token to the resource server before initiating the handshake, as
described in Section 5.8.1 of the ACE framework described in Section 5.10.1 of the ACE framework
[I-D.ietf-ace-oauth-authz]. [I-D.ietf-ace-oauth-authz].
In the PSK mode, client and server show with the DTLS handshake that In the PSK mode, client and server show with the DTLS handshake that
they can use the keying material that is bound to the access token. 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, To transfer the access token from the client to the resource server,
the "psk_identity" parameter in the DTLS PSK handshake may be used the "psk_identity" parameter in the DTLS PSK handshake may be used
instead of uploading the token prior to the handshake. instead of uploading the token prior to the handshake.
As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this
specification uses CBOR web tokens to convey claims within an access specification uses CBOR web tokens to convey claims within an access
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2. Protocol Overview 2. Protocol Overview
The CoAP-DTLS profile for ACE specifies the transfer of The CoAP-DTLS profile for ACE specifies the transfer of
authentication information and, if necessary, authorization authentication information and, if necessary, authorization
information between the client (C) and the resource server (RS) information between the client (C) and the resource server (RS)
during setup of a DTLS session for CoAP messaging. It also specifies 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 how the client can use CoAP over DTLS to retrieve an access token
from the authorization server (AS) for a protected resource hosted on from the authorization server (AS) for a protected resource hosted on
the resource server. As specified in Section 6.7 of the resource server. As specified in Section 6.7 of
[I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these [I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these
interactions is completely independent interactions is completely independent.
This profile requires the client to retrieve an access token for This profile requires the client to retrieve an access token for
protected resource(s) it wants to access on the resource server as protected resource(s) it wants to access on the resource server as
specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical
message flow in this scenario (messages in square brackets are message flow in this scenario (messages in square brackets are
optional): optional):
C RS AS C RS AS
| [---- Resource Request ------>]| | | [---- Resource Request ------>]| |
| | | | | |
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| <---------------------------- Access Token --------- | | <---------------------------- Access Token --------- |
| + Access Information | | + Access Information |
Figure 1: Retrieving an Access Token Figure 1: Retrieving an Access Token
To determine the authorization server in charge of a resource hosted To determine the authorization server in charge of a resource hosted
at the resource server, the client can send an initial Unauthorized at the resource server, the client can send an initial Unauthorized
Resource Request message to the resource server. The resource server Resource Request message to the resource server. The resource server
then denies the request and sends an AS Request Creation Hints then denies the request and sends an AS Request Creation Hints
message containing the address of its authorization server back to message containing the address of its authorization server back to
the client as specified in Section 5.1.2 of the client as specified in Section 5.3 of [I-D.ietf-ace-oauth-authz].
[I-D.ietf-ace-oauth-authz].
Once the client knows the authorization server's address, it can send Once the client knows the authorization server's address, it can send
an access token request to the token endpoint at the authorization an access token request to the token endpoint at the authorization
server as specified in [I-D.ietf-ace-oauth-authz]. As the access server as specified in [I-D.ietf-ace-oauth-authz]. As the access
token request as well as the response may contain confidential data, token request as well as the response may contain confidential data,
the communication between the client and the authorization server the communication between the client and the authorization server
must be confidentiality-protected and ensure authenticity. The must be confidentiality-protected and ensure authenticity. The
client may have been registered at the authorization server via the client is expected to have been registered at the authorization
OAuth 2.0 client registration mechanism as outlined in Section 5.3 of server as outlined in Section 4 of [I-D.ietf-ace-oauth-authz].
[I-D.ietf-ace-oauth-authz].
The access token returned by the authorization server can then be The access token returned by the authorization server can then be
used by the client to establish a new DTLS session with the resource used by the client to establish a new DTLS session with the resource
server. When the client intends to use an asymmetric proof-of- server. When the client intends to use an asymmetric proof-of-
possession key in the DTLS handshake with the resource server, the possession key in the DTLS handshake with the resource server, the
client MUST upload the access token to the authz-info resource, i.e. client MUST upload the access token to the authz-info resource, i.e.
the authz-info endpoint, on the resource server before starting the the authz-info endpoint, on the resource server before starting the
DTLS handshake, as described in Section 5.8.1 of DTLS handshake, as described in Section 5.10.1 of
[I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric [I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric
proof-of-possession key in the DTLS handshake, the procedure as above proof-of-possession key in the DTLS handshake, the procedure as above
MAY be used, or alternatively, the access token MAY instead be MAY be used, or alternatively, the access token MAY instead be
transferred in the DTLS ClientKeyExchange message (see transferred in the DTLS ClientKeyExchange message (see
Section 3.3.2). In any case, DTLS MUST be used in a mode that Section 3.3.2). In any case, DTLS MUST be used in a mode that
provides replay protection. provides replay protection.
Figure 2 depicts the common protocol flow for the DTLS profile after Figure 2 depicts the common protocol flow for the DTLS profile after
the client has retrieved the access token from the authorization the client has retrieved the access token from the authorization
server, AS. server, AS.
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The following sections specify how CoAP is used to interchange The following sections specify how CoAP is used to interchange
access-related data between the resource server, the client and the access-related data between the resource server, the client and the
authorization server so that the authorization server can provide the authorization server so that the authorization server can provide the
client and the resource server with sufficient information to client and the resource server with sufficient information to
establish a secure channel, and convey authorization information establish a secure channel, and convey authorization information
specific for this communication relationship to the resource server. specific for this communication relationship to the resource server.
Section 3.1 describes how the communication between the client (C) Section 3.1 describes how the communication between the client (C)
and the authorization server (AS) must be secured. Depending on the and the authorization server (AS) must be secured. Depending on the
used CoAP security mode (see also Section 9 of [RFC7252], the Client- used CoAP security mode (see also Section 9 of [RFC7252], the Client-
to-AS request, AS-to-Client response (see Section 5.6 of to-AS request, AS-to-Client response and DTLS session establishment
[I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry carry slightly different information. Section 3.2 addresses the use
slightly different information. Section 3.2 addresses the use of raw of raw public keys while Section 3.3 defines how pre-shared keys are
public keys while Section 3.3 defines how pre-shared keys are used in used in this profile.
this profile.
3.1. Communication Between the Client and the Authorization Server 3.1. Communication Between the Client and the Authorization Server
To retrieve an access token for the resource that the client wants to To retrieve an access token for the resource that the client wants to
access, the client requests an access token from the authorization access, the client requests an access token from the authorization
server. Before the client can request the access token, the client server. Before the client can request the access token, the client
and the authorization server MUST establish a secure communication and the authorization server MUST establish a secure communication
channel. This profile assumes that the keying material to secure channel. This profile assumes that the keying material to secure
this communication channel has securely been obtained either by this communication channel has securely been obtained either by
manual configuration or in an automated provisioning process. The manual configuration or in an automated provisioning process. The
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The client and the authorization server MUST use their respective The client and the authorization server MUST use their respective
keying material for all exchanged messages. How the security keying material for all exchanged messages. How the security
association between the client and the authorization server is association between the client and the authorization server is
bootstrapped is not part of this document. The client and the bootstrapped is not part of this document. The client and the
authorization server must ensure the confidentiality, integrity and authorization server must ensure the confidentiality, integrity and
authenticity of all exchanged messages within the ACE protocol. authenticity of all exchanged messages within the ACE protocol.
Section 6 specifies how communication with the authorization server Section 6 specifies how communication with the authorization server
is secured. is secured.
3.2. RawPublicKey Mode 3.2. Raw Public Key Mode
When the client uses RawPublicKey authentication, the procedure is as When the client uses raw public key authentication, the procedure is
described in the following. as described in the following.
3.2.1. Access Token Retrieval from the Authorization Server 3.2.1. Access Token Retrieval from the Authorization Server
After the client and the authorization server mutually authenticated After the client and the authorization server mutually authenticated
each other and validated each other's authorization, the client sends each other and validated each other's authorization, the client sends
a token request to the authorization server's token endpoint. The a token request to the authorization server's token endpoint. The
client MUST add a "req_cnf" object carrying either its raw public key 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 or a unique identifier for a public key that it has previously made
known to the authorization server. It is RECOMMENDED that the client known to the authorization server. It is RECOMMENDED that the client
uses DTLS with the same keying material to secure the communication uses DTLS with the same keying material to secure the communication
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key may be defined in the future. key may be defined in the future.
An example access token request from the client to the authorization An example access token request from the client to the authorization
server is depicted in Figure 3. server is depicted in Figure 3.
POST coaps://as.example.com/token POST coaps://as.example.com/token
Content-Format: application/ace+cbor Content-Format: application/ace+cbor
Payload: Payload:
{ {
grant_type : client_credentials, grant_type : client_credentials,
req_aud : "tempSensor4711", audience : "tempSensor4711",
req_cnf : { req_cnf : {
COSE_Key : { COSE_Key : {
kty : EC2, kty : EC2,
crv : P-256, crv : P-256,
x : h'e866c35f4c3c81bb96a1...', x : h'e866c35f4c3c81bb96a1...',
y : h'2e25556be097c8778a20...' y : h'2e25556be097c8778a20...'
} }
} }
} }
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raw public key. raw public key.
The authorization server MUST check if the client that it The authorization server MUST check if the client that it
communicates with is associated with the RPK in the "req_cnf" communicates with is associated with the RPK in the "req_cnf"
parameter before issuing an access token to it. If the authorization parameter before issuing an access token to it. If the authorization
server determines that the request is to be authorized according to server determines that the request is to be authorized according to
the respective authorization rules, it generates an access token the respective authorization rules, it generates an access token
response for the client. The access token MUST be bound to the RPK response for the client. The access token MUST be bound to the RPK
of the client by means of the "cnf" claim. of the client by means of the "cnf" claim.
The response MAY contain a "profile" parameter with the value The response MUST contain an "ace_profile" parameter if
"coap_dtls" to indicate that this profile MUST be used for the"ace_profile" parameter in the request is empty, and MAY contain
communication between the client and the resource server. The this parameter otherwise (see Section 5.8.2 of
"profile" may be specified out-of-band, in which case it does not [I-D.ietf-ace-oauth-authz]). This parameter is set to "coap_dtls" to
have to be sent. The response also contains an access token with indicate that this profile MUST be used for communication between the
information for the resource server about the client's public key. client and the resource server. The response also contains an access
The authorization server MUST return in its response the parameter token with information for the resource server about the client's
"rs_cnf" unless it is certain that the client already knows the public key. The authorization server MUST return in its response the
public key of the resource server. The authorization server MUST 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 ascertain that the RPK specified in "rs_cnf" belongs to the resource
server that the client wants to communicate with. The authorization server that the client wants to communicate with. The authorization
server MUST protect the integrity of the access token such that the server MUST protect the integrity of the access token such that the
resource server can detect unauthorized changes. If the access token resource server can detect unauthorized changes. If the access token
contains confidential data, the authorization server MUST also contains confidential data, the authorization server MUST also
protect the confidentiality of the access token. protect the confidentiality of the access token.
The client MUST ascertain that the access token response belongs to a The client MUST ascertain that the access token response belongs to a
certain previously sent access token request, as the request may certain previously sent access token request, as the request may
specify the resource server with which the client wants to specify the resource server with which the client wants to
communicate. communicate.
An example access token response from the authorization server to the An example access token response from the authorization server to the
client is depicted in Figure 4. Here, the contents of the client is depicted in Figure 4. Here, the contents of the
"access_token" claim have been truncated to improve readability. "access_token" claim have been truncated to improve readability. The
Caching proxies process the Max-Age option in the CoAP response which response comprises access information for the client that contains
has a default value of 60 seconds (Section 5.6.1 of [RFC7252]). The the server's public key in the "rs_cnf" parameter. Caching proxies
authorization server SHOULD adjust the Max-Age option such that it process the Max-Age option in the CoAP response which has a default
does not exceed the "expires_in" parameter to avoid stale responses. 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 2.01 Created
Content-Format: application/ace+cbor Content-Format: application/ace+cbor
Max-Age: 3560 Max-Age: 3560
Payload: Payload:
{ {
access_token : b64'SlAV32hkKG... access_token : b64'SlAV32hkKG...
(remainder of CWT omitted for brevity; (remainder of CWT omitted for brevity;
CWT contains the client's RPK in the cnf claim)', CWT contains the client's RPK in the cnf claim)',
expires_in : 3600, expires_in : 3600,
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} }
Figure 4: Access Token Response Example for RPK Mode Figure 4: Access Token Response Example for RPK Mode
3.2.2. DTLS Channel Setup Between Client and Resource Server 3.2.2. DTLS Channel Setup Between Client and Resource Server
Before the client initiates the DTLS handshake with the resource Before the client initiates the DTLS handshake with the resource
server, the client MUST send a "POST" request containing the obtained server, the client MUST send a "POST" request containing the obtained
access token to the authz-info resource hosted by the resource access token to the authz-info resource hosted by the resource
server. After the client receives a confirmation that the resource server. After the client receives a confirmation that the resource
server has accepted the access token, it SHOULD proceed to establish server has accepted the access token, it proceeds to establish a new
a new DTLS channel with the resource server. The client MUST use its DTLS channel with the resource server. The client MUST use its
correct public key in the DTLS handshake. If the authorization correct public key in the DTLS handshake. If the authorization
server has specified a "cnf" field in the access token response, the server has specified a "cnf" field in the access token response, the
client MUST use this key. Otherwise, the client MUST use the public 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. key that it specified in the "req_cnf" of the access token request.
The client MUST specify this public key in the SubjectPublicKeyInfo The client MUST specify this public key in the SubjectPublicKeyInfo
structure of the DTLS handshake as described in [RFC7250]. structure of the DTLS handshake as described in [RFC7250].
To be consistent with [RFC7252] which allows for shortened MAC tags 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 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 tags
in constrained environments, an implementation that supports the RPK in constrained environments, an implementation that supports the RPK
mode of this profile MUST at least support the ciphersuite mode of this profile MUST at least support the cipher suite
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in
[RFC7748], new ECC curves have been defined recently that are [RFC7748], new ECC curves have been defined recently that are
considered superior to the so-called NIST curves. This specification considered superior to the so-called NIST curves. Implementations of
therefore mandates implementation support for curve25519 (cf. this profile therefore MUST implement support for curve25519 (cf.
[RFC8032], [RFC8422]) as this curve said to be efficient and less [RFC8032], [RFC8422]) as this curve said to be efficient and less
dangerous regarding implementation errors than the secp256r1 curve dangerous regarding implementation errors than the secp256r1 curve
mandated in [RFC7252]. mandated in [RFC7252].
The resource server MUST check if the access token is still valid, if The resource server MUST check if the access token is still valid, if
the resource server is the intended destination (i.e., the audience) the resource server is the intended destination (i.e., the audience)
of the token, and if the token was issued by an authorized of the token, and if the token was issued by an authorized
authorization server. The access token is constructed by the authorization server (see also section 5.10.1.1 of
[I-D.ietf-ace-oauth-authz]). The access token is constructed by the
authorization server such that the resource server can associate the authorization server such that the resource server can associate the
access token with the Client's public key. The "cnf" claim MUST 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 contain either the client's RPK or, if the key is already known by
the resource server (e.g., from previous communication), a reference the resource server (e.g., from previous communication), a reference
to this key. If the authorization server has no certain knowledge to this key. If the authorization server has no certain knowledge
that the Client's key is already known to the resource server, the 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" Client's public key MUST be included in the access token's "cnf"
parameter. If CBOR web tokens [RFC8392] are used (as recommended in parameter. If CBOR web tokens [RFC8392] are used (as recommended in
[I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in [I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in
[RFC8747]. A resource server MUST have the capacity to store one [RFC8747]. A resource server MUST have the capacity to store one
skipping to change at page 11, line 9 skipping to change at page 11, line 23
to a symmetric key that was previously generated by the authorization to a symmetric key that was previously generated by the authorization
server as a shared secret for the communication between this client server as a shared secret for the communication between this client
and the resource server. If no such symmetric key was found, the and the resource server. If no such symmetric key was found, the
authorization server MUST generate a new symmetric key that is authorization server MUST generate a new symmetric key that is
returned in its response to the client. returned in its response to the client.
The authorization server MUST determine the authorization rules for The authorization server MUST determine the authorization rules for
the client it communicates with as defined by the resource owner and the client it communicates with as defined by the resource owner and
generate the access token accordingly. If the authorization server generate the access token accordingly. If the authorization server
authorizes the client, it returns an AS-to-Client response. If the authorizes the client, it returns an AS-to-Client response. If the
profile parameter is present, it is set to "coap_dtls". The "ace_profile" parameter is present, it is set to "coap_dtls". The
authorization server MUST ascertain that the access token is authorization server MUST ascertain that the access token is
generated for the resource server that the client wants to generated for the resource server that the client wants to
communicate with. Also, the authorization server MUST protect the communicate with. Also, the authorization server MUST protect the
integrity of the access token to ensure that the resource server can integrity of the access token to ensure that the resource server can
detect unauthorized changes. If the token contains confidential data detect unauthorized changes. If the token contains confidential data
such as the symmetric key, the confidentiality of the token MUST also such as the symmetric key, the confidentiality of the token MUST also
be protected. Depending on the requested token type and algorithm in be protected. Depending on the requested token type and algorithm in
the access token request, the authorization server adds access the access token request, the authorization server adds access
Information to the response that provides the client with sufficient Information to the response that provides the client with sufficient
information to setup a DTLS channel with the resource server. The information to setup a DTLS channel with the resource server. The
skipping to change at page 12, line 27 skipping to change at page 12, line 43
kty : symmetric, kty : symmetric,
kid : h'3d027833fc6267ce', kid : h'3d027833fc6267ce',
k : h'73657373696f6e6b6579' k : h'73657373696f6e6b6579'
} }
} }
} }
Figure 6: Example Access Token Response, symmetric PoP-key Figure 6: Example Access Token Response, symmetric PoP-key
The access token also comprises a "cnf" claim. This claim usually The access token also comprises a "cnf" claim. This claim usually
contains a "COSE_Key" object that carries either the symmetric key contains a "COSE_Key" object [RFC8152] that carries either the
itself or a key identifier that can be used by the resource server to symmetric key itself or a key identifier that can be used by the
determine the secret key it shares with the client. If the access resource server to determine the secret key it shares with the
token carries a symmetric key, the access token MUST be encrypted client. If the access token carries a symmetric key, the access
using a "COSE_Encrypt0" structure. The authorization server MUST use token MUST be encrypted using a "COSE_Encrypt0" structure (see
the keying material shared with the resource server to encrypt the section 7.1 of [RFC8392]). The authorization server MUST use the
token. keying material shared with the resource server to encrypt the token.
The "cnf" structure in the access token is provided in Figure 7. The "cnf" structure in the access token is provided in Figure 7.
cnf : { cnf : {
COSE_Key : { COSE_Key : {
kty : symmetric, kty : symmetric,
kid : h'3d027833fc6267ce' kid : h'3d027833fc6267ce'
} }
} }
Figure 7: Access Token without Keying Material Figure 7: Access Token without Keying Material
A response that declines any operation on the requested resource is A response that declines any operation on the requested resource is
constructed according to Section 5.2 of [RFC6749], (cf. constructed according to Section 5.2 of [RFC6749], (cf.
Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an Section 5.8.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an
example for a request that has been rejected due to invalid request example for a request that has been rejected due to invalid request
parameters. parameters.
4.00 Bad Request 4.00 Bad Request
Content-Format: application/ace+cbor Content-Format: application/ace+cbor
Payload: Payload:
{ {
error : invalid_request error : invalid_request
} }
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All CBOR data types are encoded in CBOR using preferred serialization All CBOR data types are encoded in CBOR using preferred serialization
and deterministic encoding as specified in Section 4 of [RFC8949]. and deterministic encoding as specified in Section 4 of [RFC8949].
This implies in particular that the "type" and "L" components use the This implies in particular that the "type" and "L" components use the
minimum length encoding. The content of the "access_token" field is minimum length encoding. The content of the "access_token" field is
treated as opaque data for the purpose of key derivation. treated as opaque data for the purpose of key derivation.
Use of a unique (per resource server) "kid" and the use of a key Use of a unique (per resource server) "kid" and the use of a key
derivation IKM that MUST be unique per authorization server/resource derivation IKM that MUST be unique per authorization server/resource
server pair as specified above will ensure that the derived key is server pair as specified above will ensure that the derived key is
not shared across multiple clients. However, to additionally provide not shared across multiple clients. However, to provide variation in
variation in the derived key across different tokens used by the same the derived key across different tokens used by the same client, it
client, it is additionally RECOMMENDED to include the "iat" claim and is additionally RECOMMENDED to include the "iat" claim and either the
either the "exp" or "exi" claims in the access token. "exp" or "exi" claims in the access token.
3.3.2. DTLS Channel Setup Between Client and Resource Server 3.3.2. DTLS Channel Setup Between Client and Resource Server
When a client receives an access token response from an authorization When a client receives an access token response from an authorization
server, the client MUST check if the access token response is bound server, the client MUST check if the access token response is bound
to a certain previously sent access token request, as the request may to a certain previously sent access token request, as the request may
specify the resource server with which the client wants to specify the resource server with which the client wants to
communicate. communicate.
The client checks if the payload of the access token response The client checks if the payload of the access token response
contains an "access_token" parameter and a "cnf" parameter. With contains an "access_token" parameter and a "cnf" parameter. With
this information the client can initiate the establishment of a new this information the client can initiate the establishment of a new
DTLS channel with a resource server. To use DTLS with pre-shared DTLS channel with a resource server. To use DTLS with pre-shared
keys, the client follows the PSK key exchange algorithm specified in keys, the client follows the PSK key exchange algorithm specified in
Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter
of the AS response as PSK when constructing the premaster secret. To of the AS response as PSK when constructing the premaster secret. To
be consistent with the recommendations in [RFC7252] a client is be consistent with the recommendations in [RFC7252], a client in the
expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 PSK mode MUST support the cipher suite TLS_PSK_WITH_AES_128_CCM_8
[RFC6655] to the resource server. [RFC6655].
In PreSharedKey mode, the knowledge of the shared secret by the In PreSharedKey mode, the knowledge of the shared secret by the
client and the resource server is used for mutual authentication client and the resource server is used for mutual authentication
between both peers. Therefore, the resource server must be able to between both peers. Therefore, the resource server must be able to
determine the shared secret from the access token. Following the determine the shared secret from the access token. Following the
general ACE authorization framework, the client can upload the access general ACE authorization framework, the client can upload the access
token to the resource server's authz-info resource before starting token to the resource server's authz-info resource before starting
the DTLS handshake. The client then needs to indicate during the the DTLS handshake. The client then needs to indicate during the
DTLS handshake which previously uploaded access token it intends to DTLS handshake which previously uploaded access token it intends to
use. To do so, it MUST create a "COSE_Key" structure with the "kid" use. To do so, it MUST create a "COSE_Key" structure with the "kid"
skipping to change at page 16, line 35 skipping to change at page 16, line 49
resource server MUST have the capacity to store one access token for resource server MUST have the capacity to store one access token for
every proof-of-possession key of every authorized client. The every proof-of-possession key of every authorized client. The
resource server may use token introspection [RFC7662] on the access resource server may use token introspection [RFC7662] on the access
token to retrieve more information about the specific token. The use token to retrieve more information about the specific token. The use
of introspection is out of scope for this specification. of introspection is out of scope for this specification.
While the client can retrieve the shared secret from the contents of While the client can retrieve the shared secret from the contents of
the "cnf" parameter in the AS-to-Client response, the resource server the "cnf" parameter in the AS-to-Client response, the resource server
uses the information contained in the "cnf" claim of the access token uses the information contained in the "cnf" claim of the access token
to determine the actual secret when no explicit "kid" was provided in to determine the actual secret when no explicit "kid" was provided in
the "psk_identity" field. If key derivation is used, the resource the "psk_identity" field. If key derivation is used, the "cnf" claim
server uses the "COSE_KDF_Context" information as described above. MUST contain a "kid" parameter to be used by the server as the IKM
for key derivation as described above.
3.4. Resource Access 3.4. Resource Access
Once a DTLS channel has been established as described in Section 3.2 Once a DTLS channel has been established as described in Section 3.2
or Section 3.3, respectively, the client is authorized to access or Section 3.3, respectively, the client is authorized to access
resources covered by the access token it has uploaded to the authz- resources covered by the access token it has uploaded to the authz-
info resource hosted by the resource server. info resource hosted by the resource server.
With the successful establishment of the DTLS channel, the client and With the successful establishment of the DTLS channel, the client and
the resource server have proven that they can use their respective 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. An access token that is bound to the client's
keying material is associated with the channel. According to keying material is associated with the channel. According to
Section 5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one Section 5.10.1 of [I-D.ietf-ace-oauth-authz], there should be only
access token for each client. New access tokens issued by the one access token for each client. New access tokens issued by the
authorization server SHOULD replace previously issued access tokens authorization server SHOULD replace previously issued access tokens
for the respective client. The resource server therefore needs a for the respective client. The resource server therefore needs a
common understanding with the authorization server how access tokens common understanding with the authorization server how access tokens
are ordered. The authorization server may, e.g., specify a "cti" are ordered. The authorization server may, e.g., specify a "cti"
claim for the access token (see Section 5.8.3 of claim for the access token (see Section 5.9.4 of
[I-D.ietf-ace-oauth-authz]) to employ a strict order. [I-D.ietf-ace-oauth-authz]) to employ a strict order.
Any request that the resource server receives on a DTLS channel that 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 is tied to an access token via its keying material MUST be checked
against the authorization rules that can be determined with the against the authorization rules that can be determined with the
access token. The resource server MUST check for every request if access token. The resource server MUST check for every request if
the access token is still valid. If the token has expired, the the access token is still valid. If the token has expired, the
resource server MUST remove it. Incoming CoAP requests that are not resource server MUST remove it. Incoming CoAP requests that are not
authorized with respect to any access token that is associated with authorized with respect to any access token that is associated with
the client MUST be rejected by the resource server with 4.01 the client MUST be rejected by the resource server with 4.01
response. The response SHOULD include AS Request Creation Hints as response. The response SHOULD include AS Request Creation Hints as
described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz]. described in Section 5.2 of [I-D.ietf-ace-oauth-authz].
The resource server MUST only accept an incoming CoAP request as The resource server MUST NOT accept an incoming CoAP request as
authorized if the following holds: authorized if any of the following fails:
1. The message was received on a secure channel that has been 1. The message was received on a secure channel that has been
established using the procedure defined in this document. established using the procedure defined in this document.
2. The authorization information tied to the sending client is 2. The authorization information tied to the sending client is
valid. valid.
3. The request is destined for the resource server. 3. The request is destined for the resource server.
4. The resource URI specified in the request is covered by the 4. The resource URI specified in the request is covered by the
authorization information. authorization information.
5. The request method is an authorized action on the resource with 5. The request method is an authorized action on the resource with
respect to the authorization information. respect to the authorization information.
Incoming CoAP requests received on a secure DTLS channel that are not Incoming CoAP requests received on a secure DTLS channel that are not
thus authorized MUST be rejected according to Section 5.8.2 of thus authorized MUST be rejected according to Section 5.10.1.1 of
[I-D.ietf-ace-oauth-authz] [I-D.ietf-ace-oauth-authz]
1. with response code 4.03 (Forbidden) when the resource URI 1. with response code 4.03 (Forbidden) when the resource URI
specified in the request is not covered by the authorization specified in the request is not covered by the authorization
information, and information, and
2. with response code 4.05 (Method Not Allowed) when the resource 2. with response code 4.05 (Method Not Allowed) when the resource
URI specified in the request covered by the authorization URI specified in the request covered by the authorization
information but not the requested action. information but not the requested action.
The client MUST ascertain that its keying material is still valid The client MUST ascertain that its keying material is still valid
before sending a request or processing a response. If the client before sending a request or processing a response. If the client
recently has updated the access token (see Section 4), it must be recently has updated the access token (see Section 4), it must be
prepared that its request is still handled according to the previous prepared that its request is still handled according to the previous
authorization rules as there is no strict ordering between access authorization rules as there is no strict ordering between access
token uploads and resource access messages. See also Section 7.2 for token uploads and resource access messages. See also Section 7.2 for
a discussion of access token processing. a discussion of access token processing.
If the client gets an error response containing AS Request Creation If the client gets an error response containing AS Request Creation
Hints (cf. Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response Hints (cf. Section 5.3 of [I-D.ietf-ace-oauth-authz] as response to
to its requests, it SHOULD request a new access token from the its requests, it SHOULD request a new access token from the
authorization server in order to continue communication with the authorization server in order to continue communication with the
resource server. resource server.
Unauthorized requests that have been received over a DTLS session Unauthorized requests that have been received over a DTLS session
SHOULD be treated as non-fatal by the resource server, i.e., the DTLS SHOULD be treated as non-fatal by the resource server, i.e., the DTLS
session SHOULD be kept alive until the associated access token has session SHOULD be kept alive until the associated access token has
expired. expired.
4. Dynamic Update of Authorization Information 4. Dynamic Update of Authorization Information
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access token after a security association between the client and the access token after a security association between the client and the
resource server has been established using this protocol. If the resource server has been established using this protocol. If the
client wants to update the authorization information, the token client wants to update the authorization information, the token
request MUST specify the key identifier of the proof-of-possession request MUST specify the key identifier of the proof-of-possession
key used for the existing DTLS channel between the client and the key used for the existing DTLS channel between the client and the
resource server in the "kid" parameter of the Client-to-AS request. resource server in the "kid" parameter of the Client-to-AS request.
The authorization server MUST verify that the specified "kid" denotes The authorization server MUST verify that the specified "kid" denotes
a valid verifier for a proof-of-possession token that has previously a valid verifier for a proof-of-possession token that has previously
been issued to the requesting client. Otherwise, the Client-to-AS been issued to the requesting client. Otherwise, the Client-to-AS
request MUST be declined with the error code "unsupported_pop_key" as request MUST be declined with the error code "unsupported_pop_key" as
defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz]. defined in Section 5.8.3 of [I-D.ietf-ace-oauth-authz].
When the authorization server issues a new access token to update When the authorization server issues a new access token to update
existing authorization information, it MUST include the specified existing authorization information, it MUST include the specified
"kid" parameter in this access token. A resource server MUST replace "kid" parameter in this access token. A resource server MUST replace
the authorization information of any existing DTLS session that is the authorization information of any existing DTLS session that is
identified by this key identifier with the updated authorization identified by this key identifier with the updated authorization
information. information.
C RS AS C RS AS
| <===== DTLS channel =====> | | | <===== DTLS channel =====> | |
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The resource server MUST delete access tokens that are no longer The resource server MUST delete access tokens that are no longer
valid. DTLS associations that have been setup in accordance with valid. DTLS associations that have been setup in accordance with
this profile are always tied to specific tokens (which may be this profile are always tied to specific tokens (which may be
exchanged with a dynamic update as described in Section 4). As exchanged with a dynamic update as described in Section 4). As
tokens may become invalid at any time (e.g., because they have tokens may become invalid at any time (e.g., because they have
expired), the association may become useless at some point. A expired), the association may become useless at some point. A
resource server therefore MUST terminate existing DTLS association resource server therefore MUST terminate existing DTLS association
after the last access token associated with this association has after the last access token associated with this association has
expired. expired.
As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the As specified in Section 5.10.3 of [I-D.ietf-ace-oauth-authz], the
resource server MUST notify the client with an error response with resource server MUST notify the client with an error response with
code 4.01 (Unauthorized) for any long running request before code 4.01 (Unauthorized) for any long running request before
terminating the association. terminating the association.
6. Secure Communication with an Authorization Server 6. Secure Communication with an Authorization Server
As specified in the ACE framework (Sections 5.6 and 5.7 of As specified in the ACE framework (Sections 5.8 and 5.9 of
[I-D.ietf-ace-oauth-authz]), the requesting entity (the resource [I-D.ietf-ace-oauth-authz]), the requesting entity (the resource
server and/or the client) and the authorization server communicate server and/or the client) and the authorization server communicate
via the token endpoint or introspection endpoint. The use of CoAP via the token endpoint or introspection endpoint. The use of CoAP
and DTLS for this communication is RECOMMENDED in this profile. and DTLS for this communication is RECOMMENDED in this profile.
Other protocols fulfilling the security requirements defined in Other protocols fulfilling the security requirements defined in
Section 5 of [I-D.ietf-ace-oauth-authz] MAY be used instead. Section 5 of [I-D.ietf-ace-oauth-authz] MAY be used instead.
How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the
authorization server are established is out of scope for this authorization server are established is out of scope for this
profile. profile.
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cipher suites with abbreviated, 8-byte tags for message integrity cipher suites with abbreviated, 8-byte tags for message integrity
protection. For consistency, this profile requires implementation of protection. For consistency, this profile requires implementation of
the same cipher suites. For application scenarios where the cost of the same cipher suites. For application scenarios where the cost of
full-width authentication tags is low compared to the overall amount full-width authentication tags is low compared to the overall amount
of data being transmitted, the use of cipher suites with 16-byte of data being transmitted, the use of cipher suites with 16-byte
integrity protection tags is preferred. integrity protection tags is preferred.
The PSK mode of this profile offers a distribution mechanism to The PSK mode of this profile offers a distribution mechanism to
convey authorization tokens together with a shared secret to a client convey authorization tokens together with a shared secret to a client
and a server. As this specification aims at constrained devices and and a server. As this specification aims at constrained devices and
uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite uses CoAP [RFC7252] as transfer protocol, at least the cipher suite
TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access
tokens and the corresponding shared secrets generated by the tokens and the corresponding shared secrets generated by the
authorization server are expected to be sufficiently short-lived to authorization server are expected to be sufficiently short-lived to
provide similar forward-secrecy properties to using ephemeral Diffie- provide similar forward-secrecy properties to using ephemeral Diffie-
Hellman (DHE) key exchange mechanisms. For longer-lived access Hellman (DHE) key exchange mechanisms. For longer-lived access
tokens, DHE ciphersuites should be used. tokens, DHE cipher suites should be used, i.e., cipher suites of the
form TLS_DHE_PSK_*.
Constrained devices that use DTLS [RFC6347] are inherently vulnerable Constrained devices that use DTLS [RFC6347] are inherently vulnerable
to Denial of Service (DoS) attacks as the handshake protocol requires to Denial of Service (DoS) attacks as the handshake protocol requires
creation of internal state within the device. This is specifically creation of internal state within the device. This is specifically
of concern where an adversary is able to intercept the initial cookie of concern where an adversary is able to intercept the initial cookie
exchange and interject forged messages with a valid cookie to exchange and interject forged messages with a valid cookie to
continue with the handshake. A similar issue exists with the continue with the handshake. A similar issue exists with the
unprotected authorization information endpoint when the resource unprotected authorization information endpoint when the resource
server needs to keep valid access tokens for a long time. server needs to keep valid access tokens for a long time.
Adversaries could fill up the constrained resource server's internal Adversaries could fill up the constrained resource server's internal
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The protection of access tokens that are stored in the authorization The protection of access tokens that are stored in the authorization
information endpoint depends on the keying material that is used information endpoint depends on the keying material that is used
between the authorization server and the resource server: The between the authorization server and the resource server: The
resource server must ensure that it processes only access tokens that resource server must ensure that it processes only access tokens that
are (encrypted and) integrity-protected by an authorization server are (encrypted and) integrity-protected by an authorization server
that is authorized to provide access tokens for the resource server. that is authorized to provide access tokens for the resource server.
7.1. Reuse of Existing Sessions 7.1. Reuse of Existing Sessions
To avoid the overhead of a repeated DTLS handshake, [RFC7925] To avoid the overhead of a repeated DTLS handshake, [RFC7925]
recommends session resumption [RFC5077] to reuse session state from recommends session resumption [RFC8446] to reuse session state from
an earlier DTLS association and thus requires client side an earlier DTLS association and thus requires client side
implementation. In this specification, the DTLS session is subject implementation. In this specification, the DTLS session is subject
to the authorization rules denoted by the access token that was used to the authorization rules denoted by the access token that was used
for the initial setup of the DTLS association. Enabling session for the initial setup of the DTLS association. Enabling session
resumption would require the server to transfer the authorization resumption would require the server to transfer the authorization
information with the session state in an encrypted SessionTicket to information with the session state in an encrypted SessionTicket to
the client. Assuming that the server uses long-lived keying the client. Assuming that the server uses long-lived keying
material, this could open up attacks due to the lack of forward material, this could open up attacks due to the lack of forward
secrecy. Moreover, using this mechanism, a client can resume a DTLS secrecy. Moreover, using this mechanism, a client can resume a DTLS
session without proving the possession of the PoP key again. session without proving the possession of the PoP key again.
Therefore, the use of session resumption is NOT RECOMMENDED for Therefore, session resumption should be used only in combination with
resource servers. reasonably short-lived PoP keys.
Since renegotiation of DTLS associations is prone to attacks as well, Since renegotiation of DTLS associations is prone to attacks as well,
[RFC7925] requires clients to decline any renogiation attempt. A [RFC7925] requires clients to decline any renegotiation attempt. A
server that wants to initiate re-keying therefore SHOULD periodically server that wants to initiate re-keying therefore SHOULD periodically
force a full handshake. force a full handshake.
7.2. Multiple Access Tokens 7.2. Multiple Access Tokens
The use of multiple access tokens for a single client increases the Developers SHOULD avoid using multiple access tokens for a client
strain on the resource server as it must consider every access token (see also section 5.10.1 of [I-D.ietf-ace-oauth-authz]).
and calculate the actual permissions of the client. Also, tokens may
contradict each other which 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 a client.
Even when a single access token per client is used, an attacker could Even when a single access token per client is used, an attacker could
compromise the dynamic update mechanism for existing DTLS connections compromise the dynamic update mechanism for existing DTLS connections
by delaying or reordering packets destined for the authz-info by delaying or reordering packets destined for the authz-info
endpoint. Thus, the order in which operations occur at the resource endpoint. Thus, the order in which operations occur at the resource
server (and thus which authorization info is used to process a given server (and thus which authorization info is used to process a given
client request) cannot be guaranteed. Especially in the presence of client request) cannot be guaranteed. Especially in the presence of
later-issued access tokens that reduce the client's permissions from later-issued access tokens that reduce the client's permissions from
the initial access token, it is impossible to guarantee that the the initial access token, it is impossible to guarantee that the
reduction in authorization will take effect prior to the expiration reduction in authorization will take effect prior to the expiration
skipping to change at page 24, line 26 skipping to change at page 24, line 45
11. References 11. References
11.1. Normative References 11.1. Normative References
[I-D.ietf-ace-oauth-authz] [I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0 Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, Internet-Draft, Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
draft-ietf-ace-oauth-authz-36, 16 November 2020, draft-ietf-ace-oauth-authz-41, 6 May 2021,
<http://www.ietf.org/internet-drafts/draft-ietf-ace-oauth- <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-36.txt>. authz-41.txt>.
[I-D.ietf-ace-oauth-params] [I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", Work in Progress, in Constrained Environments (ACE)", Work in Progress,
Internet-Draft, draft-ietf-ace-oauth-params-13, 28 April Internet-Draft, draft-ietf-ace-oauth-params-15, 6 May
2020, <http://www.ietf.org/internet-drafts/draft-ietf-ace- 2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
oauth-params-13.txt>. oauth-params-15.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)", Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005, RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>. <https://www.rfc-editor.org/info/rfc4279>.
skipping to change at page 25, line 39 skipping to change at page 26, line 13
<https://www.rfc-editor.org/info/rfc7925>. <https://www.rfc-editor.org/info/rfc7925>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017, RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>. <https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/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 [RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422, Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018, DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>. <https://www.rfc-editor.org/info/rfc8422>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>. 2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949, Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020, DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>. <https://www.rfc-editor.org/info/rfc8949>.
11.2. Informative References 11.2. Informative References
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>. <https://www.rfc-editor.org/info/rfc5869>.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for [RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012, DOI 10.17487/RFC6655, July 2012,
<https://www.rfc-editor.org/info/rfc6655>. <https://www.rfc-editor.org/info/rfc6655>.
skipping to change at page 26, line 40 skipping to change at page 27, line 10
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>. 2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032, Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017, DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>. <https://www.rfc-editor.org/info/rfc8032>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
May 2018, <https://www.rfc-editor.org/info/rfc8392>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/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>.
Authors' Addresses Authors' Addresses
Stefanie Gerdes Stefanie Gerdes
Universität Bremen TZI Universität Bremen TZI
Postfach 330440 Postfach 330440
D-28359 Bremen D-28359 Bremen
Germany Germany
Phone: +49-421-218-63906 Phone: +49-421-218-63906
Email: gerdes@tzi.org Email: gerdes@tzi.org
skipping to change at page 27, line 38 skipping to change at page 28, line 4
Email: bergmann@tzi.org Email: bergmann@tzi.org
Carsten Bormann Carsten Bormann
Universität Bremen TZI Universität Bremen TZI
Postfach 330440 Postfach 330440
D-28359 Bremen D-28359 Bremen
Germany Germany
Phone: +49-421-218-63921 Phone: +49-421-218-63921
Email: cabo@tzi.org Email: cabo@tzi.org
Göran Selander Göran Selander
Ericsson AB Ericsson AB
Email: goran.selander@ericsson.com Email: goran.selander@ericsson.com
Ludwig Seitz Ludwig Seitz
Combitech Combitech
Djäknegatan 31 Djäknegatan 31
SE-211 35 Malmö SE-211 35 Malmö
Sweden Sweden
Email: ludwig.seitz@combitech.se Email: ludwig.seitz@combitech.com
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