Internet DRAFT - draft-ietf-ace-group-oscore-profile
draft-ietf-ace-group-oscore-profile
ACE Working Group M. Tiloca
Internet-Draft R. Höglund
Intended status: Standards Track RISE AB
Expires: 5 September 2024 F. Palombini
Ericsson AB
4 March 2024
The Group Object Security for Constrained RESTful Environments (Group
OSCORE) Profile of the Authentication and Authorization for Constrained
Environments (ACE) Framework
draft-ietf-ace-group-oscore-profile-01
Abstract
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework. The
profile uses Group Object Security for Constrained RESTful
Environments (Group OSCORE) to provide communication security between
a Client and one or multiple Resource Servers that are members of an
OSCORE group. The profile securely binds an OAuth 2.0 Access Token
to the public key of the Client associated with the private key used
by that Client in the OSCORE group. The profile uses Group OSCORE to
achieve server authentication, as well as proof-of-possession for the
Client's public key. Also, it provides proof of the Client's
membership to the OSCORE group by binding the Access Token to
information from the Group OSCORE Security Context, thus allowing the
Resource Server(s) to verify the Client's membership upon receiving a
message protected with Group OSCORE from the Client. Effectively,
the profile enables fine-grained access control paired with secure
group communication, in accordance with the Zero Trust principles.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Authentication and
Authorization for Constrained Environments Working Group mailing list
(ace@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ace/.
Source for this draft and an issue tracker can be found at
https://github.com/ace-wg/ace-group-oscore-profile.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 9
2.2. Access Token Retrieval . . . . . . . . . . . . . . . . . 10
2.3. Access Token Uploading . . . . . . . . . . . . . . . . . 11
2.4. Secure Communication . . . . . . . . . . . . . . . . . . 12
3. Client-AS Communication . . . . . . . . . . . . . . . . . . . 12
3.1. C-to-AS: POST to Token Endpoint . . . . . . . . . . . . . 12
3.1.1. 'context_id' Parameter . . . . . . . . . . . . . . . 18
3.1.2. 'salt_input' Parameter . . . . . . . . . . . . . . . 18
3.1.3. 'client_cred_verify' Parameter . . . . . . . . . . . 18
3.1.4. 'client_cred_verify_mac' Parameter . . . . . . . . . 18
3.2. AS-to-C: Access Token . . . . . . . . . . . . . . . . . . 18
3.2.1. Salt Input Claim . . . . . . . . . . . . . . . . . . 24
3.2.2. Context ID Input Claim . . . . . . . . . . . . . . . 24
4. Client-RS Communication . . . . . . . . . . . . . . . . . . . 24
4.1. C-to-RS POST to authz-info Endpoint . . . . . . . . . . . 25
4.2. RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . . . . 25
4.3. Client-RS Secure Communication . . . . . . . . . . . . . 27
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4.3.1. Client Side . . . . . . . . . . . . . . . . . . . . . 27
4.3.2. Resource Server Side . . . . . . . . . . . . . . . . 27
4.4. Access Rights Verification . . . . . . . . . . . . . . . 28
4.5. Change of Client's Authentication Credential in the
Group . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5. Secure Communication with the AS . . . . . . . . . . . . . . 29
6. Discarding the Security Context . . . . . . . . . . . . . . . 30
7. CBOR Mappings . . . . . . . . . . . . . . . . . . . . . . . . 30
8. Security Considerations . . . . . . . . . . . . . . . . . . . 31
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 32
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
10.1. ACE Profile Registry . . . . . . . . . . . . . . . . . . 32
10.2. OAuth Parameters Registry . . . . . . . . . . . . . . . 33
10.3. OAuth Parameters CBOR Mappings Registry . . . . . . . . 34
10.4. CBOR Web Token (CWT) Claims Registry . . . . . . . . . . 35
10.5. TLS Exporter Label Registry . . . . . . . . . . . . . . 35
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
11.1. Normative References . . . . . . . . . . . . . . . . . . 36
11.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Profile Requirements . . . . . . . . . . . . . . . . 40
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 41
B.1. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 41
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
A number of applications rely on a group communication model where a
Client can access a resource hosted by multiple Resource Servers at
once, e.g., over IP multicast. Typical examples are switching of
luminaries, actuators control, and distribution of software updates.
Secure communication in the group can be achieved by sharing a set of
keying material, which is typically provided upon joining the group.
For some of such applications, it may be just fine to enforce access
control in a straightforward fashion. That is, any Client authorized
to join the group, hence to obtain the group keying material, can be
also implicitly authorized to perform any action at any resource of
any Server in the group. An example of application where such
implicit authorization might serve well is a simple lighting
scenario, where the lightbulbs are the Servers, while the user
account on an app on the user's phone is the Client. In this case,
it might be fine to not require additional authorization evidence
from any user account, if it is acceptable that any current group
member is also authorized to switch on and off any light, or to check
their status.
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However, in different instances of such applications, the approach
above is not desirable, as different group members are intended for
have different access rights to resources of other group members.
For instance, enforcing access control in accordance with a more
fine-grained approach is required in the two following use cases.
As a first case, an application provides control of smart locks
acting as Servers in the group, where: a first type of Client, e.g.,
a user account of a child, is allowed to only query the status of the
smart locks; while a second type of Client, e.g., a user account of a
parent, is allowed to both query and change the status of the smart
locks. Further similar applications concern the enforcement of
different sets of permissions in groups with sensor/actuator devices,
e.g., thermostats acting as Servers. Also, some group members may
even be intended as Servers only. Hence, they must be prevented from
acting as Clients altogether and from accessing resources at other
Servers, especially when attempting to perform non-safe operations.
As a second case, building automation scenarios often rely on Servers
that, under different circumstances, enforce different level of
priority for processing received commands. For instance, BACnet
deployments consider multiple classes of Clients, e.g., a normal
light switch (C1) and an emergency fire panel (C2). Then, a C1
Client is not allowed to override a command from a C2 Client, until
the latter relinquishes control at its higher priority. That is: i)
only C2 Clients should be able to adjust the minimum required level
of priority on the Servers, so rightly locking out C1 Clients if
needed; and ii) when a Server is set to accept only high-priority
commands, only C2 Clients should be able to perform such commands
otherwise allowed also to C1 Clients. Given the different maximum
authority of different Clients, fine-grained access control would
effectively limit the execution of high- and emergency-priority
commands only to devices that are in fact authorized to perform such
actions. Besides, it would prevent a misconfigured or compromised
device from initiating a high-priority command and lock out normal
control.
In the cases above, being a legitimate group member and storing the
group keying material is not supposed to imply any particular access
rights. Instead, access control to the secure group communication
channel and access control to the resource space provided by Servers
in the group should remain logically separated domains.
This is aligned with the Zero Trust paradigm [NIST-800-207], which
focuses on resource protection and builds on the premise that trust
is never granted implicitly, but must be continually evaluated. In
particular, Zero Trust protections involve "minimizing access to
resources (such as data and compute resources and applications/
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services) to only those subjects and assets identified as needing
access as well as continually authenticating and authorizing the
identity and security posture of each access request."
Furthermore, [NIST-800-207] highlights how the Zero Trust goal is to
"prevent unauthorized access to data and services coupled with making
the access control enforcement as granular as possible", to "enforce
least privileges needed to perform the action in the request."
As a step in this direction, one can be tempted to introduce a
different security group for each different set of access rights.
However, this inconveniently results in additional keying material to
distribute and manage. In particular, if the access rights
pertaining to a node change, this requires to evict the node from the
group, after which the node has to join a different group aligned
with its new access rights. Moreover, the keying material of both
groups would have to be renewed for their current members. Overall,
this would have a non negligible impact on operations and
performance.
Instead, a fine-grained yet flexible access control model can be
enforced within the same group, by using the Authentication and
Authorization for Constrained Environments (ACE) framework [RFC9200].
That is, a Client has to first obtain authorization credentials in
the form of an Access Token, and upload it to the Resource Server(s)
in the group before accessing the intended resources.
The ACE framework delegates to separate profile documents how to
secure communications between the Client and the Resource Servers.
However each of the current profiles of ACE defined in
[RFC9202][RFC9203][RFC9431][I-D.ietf-ace-edhoc-oscore-profile] relies
on a security protocol that cannot be used to protect one-to-many
group messages, for example sent over IP multicast.
This document specifies the "coap_group_oscore" profile of the ACE
framework, where a Client uses the Constrained Application Protocol
(CoAP) [RFC7252][I-D.ietf-core-groupcomm-bis] to communicate with one
or multiple Resource Servers that are members of an application group
and share a common set of resources. This profile uses Group Object
Security for Constrained RESTful Environments (Group OSCORE)
[I-D.ietf-core-oscore-groupcomm] as the security protocol to protect
messages exchanged between the Client and the Resource Servers.
Hence, it requires that both the Client and the Resource Servers have
previously joined the same OSCORE group.
That is, this profile describes how access control is enforced for a
Client after it has joined an OSCORE group, to access resources
hosted by other members in that group. The process for joining the
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OSCORE group through the respective Group Manager as defined in
[I-D.ietf-ace-key-groupcomm-oscore] takes place before the process
described in this document, and is out of the scope of this profile.
The Client proves its access to be authorized to the Resource
Server(s) by using an Access Token bound to a key (the proof-of-
possession key). This profile uses Group OSCORE to achieve server
authentication and proof-of-possession for the Client's public key
used in the OSCORE group in question. Note that proof-of-possession
is not achieved through a dedicated protocol element, but instead
after the first message exchange protected with Group OSCORE.
Furthermore, this profile provides proof of the Client's membership
to the OSCORE group, by binding the Access Token to information from
the pre-established Group OSCORE Security Context, as well as to the
Client's authentication credential used in the group and including
the Client's public key. This allows the Resource Server(s) to
verify the Client's group membership upon reception of a message
protected with Group OSCORE from that Client.
OSCORE [RFC8613] specifies how to use COSE [RFC9052][RFC9053] to
secure CoAP messages. Group OSCORE builds on OSCORE to provide
secure group communication, and ensures source authentication: by
means of digital signatures embedded in the protected message (when
using the group mode); or by protecting a message with pairwise
keying material derived from the asymmetric keys of the two peers
exchanging the message (when using the pairwise mode).
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
related to CBOR [RFC8949], COSE [RFC9052][RFC9053], CoAP [RFC7252],
OSCORE [RFC8613], and Group OSCORE [I-D.ietf-core-oscore-groupcomm].
These especially include:
* Group Manager, as the entity responsible for a set of groups where
communications among members are secured with Group OSCORE.
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* Authentication credential, as the set of information associated
with an entity, including that entity's public key and parameters
associated with the public key. Examples of authentication
credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs)
[RFC8392], X.509 certificates [RFC5280], and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert].
Members of an OSCORE group have an associated authentication
credential in the format used within the group. As per
Section 2.4 of [I-D.ietf-core-oscore-groupcomm], an authentication
credential provides the public key as well as the comprehensive
set of information related to the public key algorithm, including,
e.g., the used elliptic curve (when applicable).
Readers are expected to be familiar with the terms and concepts
described in the ACE framework for authentication and authorization
[RFC9200], as well as in the OSCORE profile of ACE [RFC9203]. The
terminology for entities in the considered architecture is defined in
OAuth 2.0 [RFC6749]. In particular, this includes Client (C),
Resource Server (RS), and Authorization Server (AS).
Note that, unless otherwise indicated, the term "endpoint" is used
here following its OAuth definition, aimed at denoting resources such
as /token and /introspect at the AS, and /authz-info at the RS. This
document does not use the CoAP definition of "endpoint", which is "An
entity participating in the CoAP protocol".
Additionally, this document makes use of the following terminology.
* Pairwise-only group: an OSCORE group that uses only the pairwise
mode of Group OSCORE (see Section 9 of
[I-D.ietf-core-oscore-groupcomm]).
Examples throughout this document are expressed in CBOR diagnostic
notation, without the tag and value abbreviations.
2. Protocol Overview
This section provides an overview of this profile, i.e., of how to
use the ACE framework for authentication and authorization [RFC9200]
to secure communications between a Client and one or more Resource
Servers using Group OSCORE [I-D.ietf-core-oscore-groupcomm].
Note that this profile of ACE describes how access control can be
enforced for a node after it has joined an OSCORE group, to access
resources hosted by other members in that group.
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In particular, the process of joining the OSCORE group through the
respective Group Manager as defined in
[I-D.ietf-ace-key-groupcomm-oscore] must take place before the
process described in this document, and is out of the scope of this
profile.
An overview of the protocol flow for this profile is shown in
Figure 1, where it is assumed that both the Resource Servers RS1 and
RS2 are associated with the same Authorization Server AS. It is also
assumed that the Client C, as well as RS1 and RS2 have previously
joined an OSCORE group with Group Identifier (gid) 0xabcd0000, and
that they got assigned Sender ID (sid) 0x00, 0x01, and 0x02 in the
group, respectively. The names of messages coincide with those of
[RFC9200] when applicable, and messages in square brackets are
optional.
C RS1 RS2 AS
| [--- Resource Request --->] | | |
| | | |
| [<----- AS Request -------] | | |
| Creation Hints | | |
| | | |
|-------- POST /token ----------------------------------------------->|
| (aud: "RS1", sid: 0x00, | | |
| gid: 0xabcd0000, ...) | | |
| | | |
| | | |
|<---------------------------------------------- Access Token T1 -----|
| | + Access Information |
| | | |
|---- POST /authz-info ------>| | |
| (access_token: T1) | | |
| | | |
|<------ 2.01 Created --------| | |
| | | |
|-------- POST /token ----------------------------------------------->|
| (aud: "RS2", sid: 0x00, | | |
| gid: 0xabcd0000, ...) | | |
| | | |
| | | |
|<---------------------------------------------- Access Token T2 -----|
| | + Access Information |
| | | |
|----- POST /authz-info ------------------>| |
| (access_token: T2) | | |
| | | |
| | | |
|<------ 2.01 Created ---------------------| |
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| | | |
|-- Group OSCORE Request -+-->| | |
| (kid: 0x00, \ | | |
| gid: 0xabcd0000) \------------->| |
| | | |
| /proof-of-possession/ |
| | | |
| | | |
|<-- Group OSCORE Response ---| | |
| (kid: 0x01) | | |
| | | |
/proof-of-possession/ | | |
| | | |
/Mutual authentication | | |
between C and RS1/ | | |
| | | |
| | | |
|<-- Group OSCORE Response ----------------| |
| (kid: 0x02) | | |
| | | |
/proof-of-possession/ | | |
| | | |
/Mutual authentication | | |
between C and RS2/ | | |
| | | |
| ... | | |
Figure 1: Protocol Overview.
2.1. Pre-Conditions
Using Group OSCORE and this profile requires that both the Client and
the Resource Servers have previously joined the same OSCORE group.
This especially includes the derivation of the Group OSCORE Security
Context and the assignment of unique Sender IDs to use in the group.
Nodes can join the OSCORE group through the respective Group Manager
by using the approach defined in [I-D.ietf-ace-key-groupcomm-oscore],
which is also based on ACE.
After the Client and Resource Servers have joined the group, this
profile provides access control for accessing resources on those
Resource Servers, by securely communicating with Group OSCORE.
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As a pre-requisite for this profile, the Client has to have
successfully joined the OSCORE group where also the Resource Servers
(RSs) are members. Depending on the limited information initially
available, the Client may have to first discover the exact OSCORE
group used by the RSs for the resources of interest, e.g., by using
the approach defined in [I-D.tiloca-core-oscore-discovery].
2.2. Access Token Retrieval
This profile requires that the Client retrieves an Access Token from
the AS for the resource(s) that it wants to access at the RS(s), by
using the /token endpoint as specified in Section 5.8 of [RFC9200].
In general, different RSs can be associated with different ASs, even
if the RSs are members of the same OSCORE group. However, assuming
proper configurations and trust relations, it is possible for
multiple RSs associated with the same AS to be part of a single
audience (i.e., a group-audience, see Section 6.9 of [RFC9200]). In
such a case, the Client can request a single Access Token intended
for the group-audience, hence to all the RSs included therein. A
particular group-audience might be defined as including all the RSs
in the OSCORE group.
In the Access Token Request to the AS, the Client MUST include the
Group Identifier of the OSCORE group and its own Sender ID in that
group. The AS MUST specify these pieces of information in the Access
Token.
Furthermore, in the Access Token Request to the AS, the Client MUST
also include: its own authentication credential used in the OSCORE
group; and a proof-of-possession (PoP) evidence to prove possession
of the corresponding private key. The PoP evidence is computed over
a PoP input uniquely related to the secure communication association
between the Client and the AS. The AS MUST include also the
authentication credential specified by the Client in the Access
Token.
The Access Token Request and Response MUST be confidentiality-
protected and ensure authenticity. In this profile, it is
RECOMMENDED to use OSCORE [RFC8613] between the Client and the AS, to
reduce the number of libraries the client has to support. Other
protocols fulfilling the security requirements defined in Sections 5
and 6 of [RFC9200] MAY alternatively be used, such as TLS [RFC8446]
or DTLS [RFC9147].
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2.3. Access Token Uploading
After having retrieved the Access Token from the AS, the Client
uploads the Access Token to the RS, by sending a POST request to the
/authz-info endpoint and using the mechanisms specified in
Section 5.10 of [RFC9200]. When using this profile, the
communication that C has with the /authz-info endpoint is not
protected.
If the Access Token is valid, the RS replies to the POST request with
a 2.01 (Created) response. Also, the RS associates the received
Access Token with the Group OSCORE Security Context identified by the
Group Identifier specified in the Access Token, following Section 3.2
of [RFC8613]. In practice, the RS maintains a collection of Security
Contexts with associated authorization information, for all the
clients that it is currently communicating with. The authorization
information is a policy that is used as input when processing
requests from those clients.
Finally, the RS stores the association between i) the authorization
information from the Access Token; and ii) the Group Identifier of
the OSCORE group together with the Sender ID and the authentication
credential of the Client in that group. This binds the Access Token
to the Group OSCORE Security Context of the OSCORE group.
Finally, when the Client communicates with the RS using the Group
OSCORE Security Context, the RS verifies that the Client is a
legitimate member of the OSCORE group and especially the exact group
member with the same Sender ID associated with the Access Token.
This occurs when verifying a request protected with Group OSCORE,
since the request includes the Client's Sender ID and either it
embeds a signature computed also over that Sender ID (if protected
with the group mode), or it is protected by means of pairwise
symmetric keying material derived from the asymmetric keys of the two
peers (if protected with the pairwise mode).
The above has considered an Access Token intended for a single RS.
However, as discussed in Section 2.2, an Access Token can be intended
for a group-audience including multiple RSs in the OSCORE group. In
such a case, the Client could efficiently upload the Access Token to
many or all of those RSs at once (e.g., over IP multicast), after
which each RS individually performs the same steps described above.
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2.4. Secure Communication
The Client can send a request protected with Group OSCORE
[I-D.ietf-core-oscore-groupcomm] to the RS. This can be a unicast
request targeting the RS, or a one-to-many group request (e.g., over
IP multicast) targeting the OSCORE group where the RS is also a
member. To this end, the Client uses the Group OSCORE Security
Context already established upon joining the OSCORE group, e.g., by
using the approach defined in [I-D.ietf-ace-key-groupcomm-oscore].
The RS may send a response back to the Client, protecting it by means
of the same Group OSCORE Security Context.
3. Client-AS Communication
This section details the Access Token POST Request that the Client
sends to the /token endpoint of the AS, as well as the related Access
Token Response.
The Access Token MUST be bound to the public key of the Client as
proof-of-possession key (pop-key), which is included in the Client's
authentication credential specified in the 'cnf' claim of the Access
Token.
3.1. C-to-AS: POST to Token Endpoint
The Client-to-AS request is specified in Section 5.8.1 of [RFC9200].
The Client MUST send this POST request to the /token endpoint over a
secure channel that guarantees authentication, message integrity, and
confidentiality.
The POST request is formatted as the analogous Client-to-AS request
in the OSCORE profile of ACE (see Section 3.1 of [RFC9203]), with the
following additional parameters that MUST be included in the payload.
* 'context_id', defined in Section 3.1.1 of this document. This
parameter specifies the Group Identifier (GID), i.e., the ID
Context of an OSCORE group that includes as members both the
Client and the RS(s) in the audience for which the Access Token is
asked to be issued. In particular, the Client wishes to
communicate with the RS(s) in that audience using the Group OSCORE
Security Context associated with that OSCORE group.
* 'salt_input', defined in Section 3.1.2 of this document. This
parameter includes the Sender ID that the Client has in the OSCORE
group whose GID is specified in the 'context_id' parameter above.
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* 'req_cnf', defined in Section 3.1 of [RFC9201]. This parameter
follows the syntax from Section 3.1 of [RFC8747], and its inner
confirmation value specifies the authentication credential that
the Client uses in the OSCORE group. The public key included in
the authentication credential will be used as the pop-key bound to
the Access Token.
At the time of writing this specification, acceptable formats of
authentication credentials in Group OSCORE are CBOR Web Tokens
(CWTs) and CWT Claims Sets (CCSs) [RFC8392], X.509 certificates
[RFC5280], and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert].
Further formats may be available in the future, and would be
acceptable to use as long as they comply with the criteria
compiled in Section 2.4 of [I-D.ietf-core-oscore-groupcomm]. In
particular, an authentication credential has to explicitly include
the public key as well as the comprehensive set of information
related to the public key algorithm, including, e.g., the used
elliptic curve (when applicable).
[ As to CWTs and CCSs, the CWT Confirmation Methods 'kcwt' and
'kccs' are under pending registration requested by draft-ietf-ace-
edhoc-oscore-profile. ]
[ As to X.509 certificates, the CWT Confirmation Methods 'x5bag'
and '5chain' are under pending registration requested by draft-
ietf-ace-edhoc-oscore-profile. ]
[ As to C509 certificates, the CWT Confirmation Methods 'c5b'and
'c5c' are under pending registration requested by draft-ietf-ace-
edhoc-oscore-profile. ]
In addition, the Client computes its proof-of-possession (PoP)
evidence, in order to prove to the AS the possession of its own
private key used in the OSCORE group. This allows the AS to verify
that the Client indeed owns the private key associated with the
public key of the authentication credential that the Client allegedly
uses in the OSCORE group.
To this end, the Client MUST use as PoP input the byte representation
of an information that uniquely represents the secure communication
association between the Client and the AS. It is RECOMMENDED that
the Client uses the following as PoP input.
* If the Client and the AS communicate over TLS 1.2 [RFC5246] or
DTLS 1.2 [RFC6347], the PoP input is an exporter value computed as
defined in Section 4 of [RFC5705], using the following inputs:
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- The exporter label "EXPORTER-ACE-PoP-Input-Client-AS", defined
in Section 10.5 of this document.
- The empty 'context value', i.e., a 'context value' of zero-
length.
- 32 as length value in bytes.
* If the Client and the AS communicate over TLS 1.3 [RFC8446] or
DTLS 1.3 [RFC9147], the PoP input is an exporter value computed as
defined in Section 7.5 of [RFC8446], using the following inputs:
- The exporter label "EXPORTER-ACE-PoP-Input-Client-AS", defined
in Section 10.5 of this document.
- The empty 'context_value', i.e., a 'context_value' of zero-
length.
- 32 as 'key_length' in bytes.
* If the Client and the AS communicate over OSCORE [RFC8613], the
PoP input is the output PRK of an HKDF-Extract step [RFC5869],
i.e., PRK = HMAC-Hash(salt, IKM).
In particular, given the OSCORE Security Context CTX shared
between the Client and the AS, then the following applies.
- 'salt' takes (x1 | x2), where | denotes byte string
concatenation, while x1 and x2 are defined as follows.
o x1 is the binary serialization of a CBOR data item. If CTX
does not specify an OSCORE ID Context, the CBOR data item is
the CBOR simple value null (0xf6). Otherwise, the CBOR data
item is a CBOR byte string, with value the OSCORE ID Context
specified in CTX.
o x2 is the binary serialization of a CBOR byte string. The
value of the CBOR byte string is the OSCORE Sender ID of the
Client, which the Client stores in its Sender Context of CTX
and the AS stores in its Recipient Context of CTX.
- 'IKM' is the OSCORE Master Secret specified in CTX.
- The used HKDF is the HKDF Algorithm specified in CTX.
The following shows an example of input to the HMAC-Hash()
function.
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On the Client side, the OSCORE Security Context shared with the AS
includes:
o ID Context: 0x37cbf3210017a2d3 (8 bytes)
o Sender ID: 0x01 (1 byte)
o Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)
Then, the following holds.
x1 (Raw value) (8 bytes)
0x37cbf3210017a2d3
x1 (CBOR Data Item) (9 bytes)
0x4837cbf3210017a2d3
x2 (Raw value) (1 bytes)
0x01
x2 (CBOR Data Item) (2 bytes)
0x4101
salt (11 bytes)
0x4837cbf3210017a2d34101
IKM (16 bytes)
0x0102030405060708090a0b0c0d0e0f10
After that, the Client computes the PoP evidence as follows.
* If the OSCORE group is not a pairwise-only group, the PoP evidence
MUST be a signature. The Client computes the signature by using
the same private key and signature algorithm it uses for signing
messages in the OSCORE group. The Client's private key is the one
associated with the Client's authentication credential used in the
OSCORE group and specified in the 'req_cnf' parameter above.
* If the OSCORE group is a pairwise-only group, the PoP evidence
MUST be a MAC computed as follows, by using the HKDF Algorithm
HKDF SHA-256, which consists of composing the HKDF-Extract and
HKDF-Expand steps [RFC5869].
MAC = HKDF(salt, IKM, info, L)
The input parameters of HKDF are as follows.
- salt takes as value the empty byte string.
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- IKM is computed as a cofactor Diffie-Hellman shared secret (see
Section 5.7.1.2 of [NIST-800-56A]), using the ECDH algorithm
that is used as Pairwise Key Agreement Algorithm in the OSCORE
group. The Client uses its own Diffie-Hellman private key and
the Diffie-Hellman public key of the AS. For X25519 and X448,
the procedure is described in Section 5 of [RFC7748].
The Client's private key is the one associated with the
Client's authentication credential used in the OSCORE group and
specified in the 'req_cnf' parameter above. The Client may
obtain the Diffie-Hellman public key of the AS during its
registration process at the AS.
- info takes as value the PoP input.
- L is equal to 8, i.e., the size of the MAC, in bytes.
Finally, the Client MUST include one of the two following parameters
in the payload of the POST request to the AS.
* 'client_cred_verify', defined in Section 3.1.3 of this document,
specifying the Client's PoP evidence as a signature, which is
computed as defined above. This parameter MUST be included if and
only if the OSCORE group is not a pairwise-only group.
* 'client_cred_verify_mac', defined in Section 3.1.4 of this
document, specifying the Client's PoP evidence as a MAC, which is
computed as defined above. This parameter MUST be included if and
only if the OSCORE group is a pairwise-only group.
An example of such a request is shown in Figure 2.
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Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"audience" : "tempSensor4711",
"scope" : "read",
"context_id" : h'abcd0000',
"salt_input" : h'00',
"req_cnf" : {
"kccs" : {
"sub" : "42-50-31-FF-EF-37-32-39",
"cnf" : {
"COSE_Key" : {
"kty" : 2,
"crv" : 1,
"x" : h'd7cc072de2205bdc1537a543d53c60a6
acb62eccd890c7fa27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22af
b725d535e515d020731e79a3b4e47120'
}
}
}
},
"client_cred_verify" : h'/...
(signature content omitted for brevity)/'
}
Figure 2: Example C-to-AS POST /token Request for an Access Token
Bound to an Asymmetric Key.
In the example above, the Client specifies that its authentication
credential in the OSCORE group is the CCS shown in Figure 3.
{
"sub" : "42-50-31-FF-EF-37-32-39",
"cnf" : {
"COSE_Key" : {
"kty" : 2,
"crv" : 1,
"x" : h'd7cc072de2205bdc1537a543d53c60a6
acb62eccd890c7fa27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22af
b725d535e515d020731e79a3b4e47120'
}
}
}
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Figure 3: Example of Client Authentication Credential as CWT
Claims Set (CCS).
3.1.1. 'context_id' Parameter
The 'context_id' parameter is an OPTIONAL parameter of the Access
Token Request message defined in Section 5.8.1 of [RFC9200]. This
parameter provides a value that the Client wishes to use with the RS
as a hint for a security context. Its exact content is profile
specific.
3.1.2. 'salt_input' Parameter
The 'salt_input' parameter is an OPTIONAL parameter of the Access
Token Request message defined in Section 5.8.1 of [RFC9200]. This
parameter provides a value that the Client wishes to use as part of a
salt with the RS, for deriving cryptographic keying material. Its
exact content is profile specific.
3.1.3. 'client_cred_verify' Parameter
The 'client_cred_verify' parameter is an OPTIONAL parameter of the
Access Token Request message defined in Section 5.8.1. of [RFC9200].
This parameter provides a signature computed by the Client to prove
the possession of its own private key.
3.1.4. 'client_cred_verify_mac' Parameter
The 'client_cred_verify_mac' parameter is an OPTIONAL parameter of
the Access Token Request message defined in Section 5.8.1. of
[RFC9200]. This parameter provides a Message Authentication Code
(MAC) computed by the Client to prove the possession of its own
private key.
3.2. AS-to-C: Access Token
After having verified the POST request to the /token endpoint and
that the Client is authorized to obtain an Access Token corresponding
to its Access Token Request, the AS MUST verify the proof-of-
possession (PoP) evidence. In particular, the AS proceeds as
follows.
* As PoP input, the AS uses the same value used by the Client in
Section 3.1.
* As public key of the Client, the AS uses the one included in the
authentication credential specified in the 'req_cnf' parameter of
the Access Token Request.
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This requires the AS to support the format of the authentication
credential specified in the 'req_cnf' parameter, i.e., the format
of authentication credential that is used in the OSCORE group
where the Client uses that authentication credential.
Practically, this is not an issue, since an RS supporting this
profile is expected to be registered only at an AS that supports
the formats of authentication credential that the RS supports.
* If the Access Token Request includes the 'client_cred_verify'
parameter, this specifies the PoP evidence as a signature. Then,
the AS verifies the signature by using the public key of the
Client.
This requires the AS to support the signature algorithm and curve
(when applicable) that are used in the OSCORE group where the
Client uses the authentication credential specified in the
'req_cnf' parameter of the Access Token Request. Practically,
this is not an issue, since an RS supporting this profile is
expected to be registered only at an AS that supports the
signature algorithms and curves (when applicable) that the RS
supports.
* If the Access Token Request includes the 'client_cred_verify_mac'
parameter, this specifies the PoP evidence as a Message
Authentication Code (MAC).
Then, the AS recomputes the MAC through the same process taken by
the Client when preparing the value of the
'client_cred_verify_mac' parameter for the Access Token (see
Section 3.1), with the difference that the AS uses its own Diffie-
Hellman private key and the Diffie-Hellman public key of the
Client. The verification succeeds if and only if the recomputed
MAC is equal to the MAC conveyed as PoP evidence in the Access
Token Request.
This requires the AS to support the ECDH algorithm that is used as
Pairwise Key Agreement Algorithm in the OSCORE group where the
Client uses the authentication credential specified in the
'req_cnf' parameter of the Access Token Request. Practically,
this is not an issue, since an RS supporting this profile is
expected to be registered only at an AS that supports the ECDH
algorithms that the RS supports.
If both the 'client_cred_verify' and 'client_cred_verify_mac'
parameters are present, or if the verification of the PoP evidence
fails, the AS considers the Client request invalid.
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If the Client request was invalid or not authorized, the AS returns
an error response as described in Section 5.8.3 of [RFC9200].
If all verifications are successful, the AS responds as defined in
Section 5.8.2 of [RFC9200]. In particular:
* The AS can signal that the use of Group OSCORE is REQUIRED for a
specific Access Token by including the 'ace_profile' parameter
with the value "coap_group_oscore" in the Access Token Response.
The Client MUST use Group OSCORE towards all the Resource Servers
for which this Access Token is valid. Usually, it is assumed that
constrained devices will be pre-configured with the necessary
profile, so that this kind of profile signaling can be omitted.
* The AS MUST NOT include the 'rs_cnf' parameter defined in
[RFC9201]. In general, the AS may not be aware of the
authentication credentials (and public keys included thereof) that
the RSs use in the OSCORE group. Also, the Client is able to
retrieve the authentication credentials of other group members
from the responsible Group Manager, both upon joining the group or
later on as a group member, as defined in
[I-D.ietf-ace-key-groupcomm-oscore].
The AS MUST include the following information as metadata of the
issued Access Token. The use of CBOR web tokens (CWT) as specified
in [RFC8392] is RECOMMENDED.
* The profile "coap_group_oscore". If the Access Token is a CWT,
this is specified in the 'ace_profile' claim of the Access Token,
as per Section 5.10 of [RFC9200].
* The salt input specified in the 'salt_input' parameter of the
Access Token Request. If the Access Token is a CWT, the content
of the 'salt_input' parameter MUST be specified in the
'salt_input' claim of the Access Token, defined in Section 3.2.1
of this document.
* The Context ID input specified in the 'context_id' parameter of
the Access Token Request. If the Access Token is a CWT, the
content of the 'context_id' parameter MUST be specified in the
'contextId_input' claim of the Access Token, defined in
Section 3.2.2 of this document.
* The authentication credential that the client uses in the OSCORE
group and specified in the 'req_cnf' parameter of the Access Token
Request.
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If the Access Token is a CWT, the Client's authentication
credential MUST be specified in the 'cnf' claim, which follows the
syntax from Section 3.1 of [RFC8747]. In particular, the 'cnf'
claim includes the same authentication credential specified in the
'req_cnf' parameter of the Access Token Request (see Section 3.1).
Figure 4 shows an example of such an AS response. The Access Token
has been truncated for readability.
Header: Created (Code=2.01)
Content-Type: "application/ace+cbor"
Payload:
{
"access_token" : h'8343a1010aa2044c53/...
(remainder of CWT omitted for brevity)/',
"ace_profile" : "coap_group_oscore",
"expires_in" : 3600
}
Figure 4: Example AS-to-C Access Token Response with the Group
OSCORE Profile.
Figure 5 shows an example CWT Claims Set, containing the Client's
public key in the group (as pop-key), as specified by the inner
confirmation value in the 'cnf' claim.
{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"exp" : "1360289224",
"scope" : "temperature_g firmware_p",
"cnf" : {
"kccs" : {
"sub" : "42-50-31-FF-EF-37-32-39",
"cnf" : {
"COSE_Key" : {
"kty" : 2,
"crv" : 1,
"x" : h'd7cc072de2205bdc1537a543d53c60a6
acb62eccd890c7fa27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22af
b725d535e515d020731e79a3b4e47120'
}
}
}
"salt_input" : h'00',
"contextId_input" : h'abcd0000'
}
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Figure 5: Example CWT Claims Set with OSCORE Parameters.
The same CWT Claims Set as in Figure 5 and encoded in CBOR is shown
in Figure 6, using the value abbreviations defined in [RFC9200] and
[RFC8747]. The bytes in hexadecimal are reported in the first
column, while their corresponding CBOR meaning is reported after the
"#" sign on the second column, for easiness of readability.
Editor's note: it should be checked (and in case fixed) that the
values used below (which are not yet registered) are the final values
registered by IANA.
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A7 # map(7)
03 # unsigned(3)
76 # text(22)
74656D7053656E736F72496E4C6976696E67526F6F6D
# "tempSensorInLivingRoom"
06 # unsigned(6)
1A 5112D728 # unsigned(1360189224)
04 # unsigned(4)
1A 51145DC8 # unsigned(1360289224)
09 # unsigned(9)
78 18 # text(24)
74656D70657261747572655F67206669726D776172655F70
# "temperature_g firmware_p"
08 # unsigned(8)
A1 # map(1)
05 # unsigned(5)
A2 # map(2)
02 # unsigned(2)
77 # text(23)
34322D35302D33312D46462D45462D33372D33322D3339
# "42-50-31-FF-EF-37-32-39"
08 # unsigned(8)
A1 # map(1)
01 # unsigned(1)
A4 # map(4)
01 # unsigned(1)
02 # unsigned(2)
20 # negative(0)
01 # unsigned(1)
21 # negative(1)
58 20 # bytes(32)
D7CC072DE2205BDC1537A543D53C60A6
ACB62ECCD890C7FA27C9E354089BBE13
22 # negative(2)
58 20 # bytes(32)
F95E1D4B851A2CC80FFF87D8E23F22AF
B725D535E515D020731E79A3B4E47120
18 3C # unsigned(60)
41 # bytes(1)
00
18 3D # unsigned(61)
44 # bytes(4)
ABCD0000
Figure 6: Example CWT Claims Set with OSCORE Parameters, CBOR
Encoded.
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3.2.1. Salt Input Claim
The 'salt_input' claim provides a value that the Client requesting
the Access Token wishes to use as a part of a salt with the RS, e.g.,
for deriving cryptographic material.
This parameter specifies the value of the salt input, encoded as a
CBOR byte string.
3.2.2. Context ID Input Claim
The 'contextId_input' claim provides a value that the Client
requesting the Access Token wishes to use with the RS, as a hint for
a security context.
This parameter specifies the value of the Context ID input, encoded
as a CBOR byte string.
4. Client-RS Communication
This section details the POST request and response to the /authz-info
endpoint between the Client and the RS.
The proof-of-possession required to bind the Access Token to the
Client is explicitly performed when the RS receives and verifies a
request from the Client protected with Group OSCORE, either with the
group mode (see Section 8 of [I-D.ietf-core-oscore-groupcomm]) or
with the pairwise mode (see Section 9 of
[I-D.ietf-core-oscore-groupcomm]).
In particular, the RS uses the Client's public key bound to the
Access Token, either when verifying the signature of the request (if
protected with the group mode), or when verifying the request as
integrity-protected with pairwise keying material derived from the
two peers' authentication credentials and asymmetric keys (if
protected with the pairwise mode). In either case, the RS also
authenticates the Client.
Similarly, when receiving a protected response from the RS, the
Client uses the RS's public key either when verifying the signature
of the response (if protected with the group mode), or when verifying
the response as integrity-protected with pairwise keying material
derived from the two peers' authentication credentials and asymmetric
keys (if protected with the pairwise mode). In either case, the
Client also authenticates the RS. Mutual authentication is only
achieved after the client has successfully verified the protected
response from the RS.
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Therefore, an attacker using a stolen Access Token cannot generate a
valid Group OSCORE message as protected through the Client's private
key, and thus cannot prove possession of the pop-key bound to the
Access Token. Also, if a Client legitimately owns an Access Token
but has not joined the OSCORE group, it cannot generate a valid Group
OSCORE message, as it does not store the necessary keying material
shared among the group members.
Furthermore, a Client C1 is supposed to obtain a valid Access Token
from the AS, as specifying its own authentication credential (and the
public key included thereof) associated with the its own private key
used in the OSCORE group, together with its own Sender ID in that
OSCORE group (see Section 3.1). This allows the RS receiving the
Access Token to verify with the Group Manager of that OSCORE group
whether such a Client indeed has that Sender ID and uses that
authentication credential in the OSCORE group.
As a consequence, a different Client C2, also member of the same
OSCORE group, is not able to impersonate C1, by: i) getting a valid
Access Token, specifying the Sender ID of C1 and a different (made-
up) authentication credential; ii) successfully posting the Access
Token to the RS; and then iii) attempting to communicate using Group
OSCORE impersonating C1, while blaming C1 for the consequences.
4.1. C-to-RS POST to authz-info Endpoint
The Client uploads the Access Token to the /authz-info endpoint of
the RS, as defined in Section 5.10.1 of [RFC9200].
4.2. RS-to-C: 2.01 (Created)
The RS MUST verify the validity of the Access Token as defined in
Section 5.10.1 of [RFC9200], with the following additions.
* The RS MUST check that the claims 'salt_input', 'contextId_input',
and 'cnf' are included in the Access Token.
* The RS considers: the content of the 'contextId_input' claim as
the GID of the OSCORE group; the content of the 'salt_input' claim
as the Sender ID that the Client has in the group; and the inner
confirmation value of the 'cnf' claim as the authentication
credential that the Client uses in the group.
The RS MUST check whether it already stores the authentication
credential specified in the inner confirmation value of the 'cnf'
claim as associated with the pair (GID, Sender ID) above.
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If this is not the case, the RS MUST request the Client's
authentication credential to the Group Manager of the OSCORE group
as described in Section 9.3 of
[I-D.ietf-ace-key-groupcomm-oscore], specifying the Client's
Sender ID in the OSCORE group, i.e., the value of the 'salt_input'
claim. Then, the RS performs the following actions.
- The RS MUST check whether the Client's authentication
credential retrieved from the Group Manager matches the one
retrieved from the inner confirmation value of the 'cnf' claim
of the Access Token.
- The RS MUST check whether the Client's Sender ID provided by
the Group Manager together with the Client's authentication
credential matches the one retrieved from the 'salt_input'
claim of the Access Token.
If any of the checks above fails, the RS MUST consider the Access
Token invalid, and MUST reply to the Client with an error response
code equivalent to the CoAP code 4.00 (Bad Request).
If the Access Token is valid and further checks on its content are
successful, the RS associates the authorization information from the
Access Token with the Group OSCORE Security Context.
In particular, the RS associates the authorization information from
the Access Token with the triple (GID, SaltInput, AuthCred), where
GID is the Group Identifier of the OSCORE group, while SaltInput and
AuthCred are the Sender ID and the authentication credential that the
Client uses in that OSCORE group, respectively.
The RS MUST keep this association up-to-date over time, as the triple
(GID, SaltInput, AuthCred) associated with the Access Token might
change. In particular:
* If the OSCORE group is rekeyed (see Section 3.2 of
[I-D.ietf-core-oscore-groupcomm] and Section 11 of
[I-D.ietf-ace-key-groupcomm-oscore]), the Group Identifier also
changes in the group, and the new one replaces the current 'GID'
value in the triple (GID, SaltInput, AuthCred).
* If the Client requests and obtains a new OSCORE Sender ID from the
Group Manager (see Section 2.6.3.1 of
[I-D.ietf-core-oscore-groupcomm] and Section 9.2 of
[I-D.ietf-ace-key-groupcomm-oscore]), the new Sender ID replaces
the current 'SaltInput' value in the triple (GID, SaltInput,
AuthCred).
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As defined in Section 4.5, a possible change of the Client's
authentication credential requires the Client to upload to the RS a
new Access Token bound to the new authentication credential.
Finally, the RS MUST send a 2.01 (Created) response to the Client, as
defined in Section 5.10.1 of [RFC9200].
4.3. Client-RS Secure Communication
When previously joining the OSCORE group, both the Client and the RS
have already established the related Group OSCORE Security Context to
communicate as group members. Therefore, they can simply start to
securely communicate using Group OSCORE, without deriving any
additional keying material or security association.
If the Client or the RS delete an Access Token (e.g., when the Access
Token has expired or has been revoked), it MUST NOT delete the
related Group OSCORE Security Context.
4.3.1. Client Side
After having received the 2.01 (Created) response from the RS,
following the POST request to the /authz-info endpoint, the Client
starts the communication with the RS, by sending a request protected
with Group OSCORE using the Group OSCORE Security Context
[I-D.ietf-core-oscore-groupcomm].
When communicating with the RS to access the resources as specified
by the authorization information, the Client MUST use the Group
OSCORE Security Context of the pertinent OSCORE group, whose GID was
specified in the 'context_id' parameter of the Access Token Request.
4.3.2. Resource Server Side
After successful validation of the Access Token as defined in
Section 4.2 and after having sent the 2.01 (Created) response, the RS
can start to communicate with the Client using Group OSCORE
[I-D.ietf-core-oscore-groupcomm].
When processing an incoming request protected with Group OSCORE, the
RS MUST consider as valid Client's authentication credential only the
one associated with the stored Access Token. As defined in
Section 4.5, a possible change of the Client's authentication
credential requires the Client to upload to the RS a new Access Token
bound to the new authentication credential.
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For every incoming request, if Group OSCORE verification succeeds,
the verification of access rights is performed as described in
Section 4.4.
If the RS receives a request protected with a Group OSCORE Security
Context CTX, the target resource requires authorization, and the RS
does not store a valid Access Token related to CTX, then the RS MUST
reply with a 4.01 (Unauthorized) error response protected with CTX.
4.4. Access Rights Verification
The RS MUST follow the procedures defined in Section 5.10.2 of
[RFC9200]. If an RS receives a request protected with Group OSCORE
from a Client, the RS processes the request according to
[I-D.ietf-core-oscore-groupcomm].
If the Group OSCORE verification succeeds, and the target resource
requires authorization, the RS retrieves the authorization
information from the Access Token associated with the Group OSCORE
Security Context. Then, the RS MUST verify that the action requested
on the resource is authorized.
If the RS has no valid Access Token for the Client, the RS MUST
reject the request and MUST reply to the Client with a 4.01
(Unauthorized) error response. If the RS has an Access Token for the
Client but no actions are authorized on the target resource, the RS
MUST reject the request and MUST reply to the Client with a 4.03
(Forbidden) error response. If the RS has an Access Token for the
Client but the requested action is not authorized, the RS MUST reject
the request and MUST reply to the Client with a 4.05 (Method Not
Allowed) error response.
4.5. Change of Client's Authentication Credential in the Group
During its membership in the OSCORE group, the Client might change
the authentication credential that it uses in the group. When this
happens, the Client uploads the new authentication credential to the
Group Manager, as defined in Section 9.4 of
[I-D.ietf-ace-key-groupcomm-oscore].
After that, and in order to continue communicating with the RS, the
Client MUST perform the following actions.
1. The Client requests a new Access Token to the AS, as defined in
Section 3. In particular, when sending the Access Token Request
as defined in Section 3.1, the Client specifies:
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* The current Group Identifier of the OSCORE group, as value of
the 'context_id' parameter.
* The current Sender ID that it has in the OSCORE group, as
value of the 'salt_input' parameter.
* The new authentication credential that it uses in the OSCORE
group, as inner confirmation value of the 'req_cnf' parameter.
* The proof-of-possession (PoP) evidence corresponding to the
public key of the new authentication credential, as value of
the 'client_cred_verify' or 'client_cred_verify_mac'
parameter.
2. After receiving the Access Token Response from the AS (see
Section 3.2), the Client performs the same exchanges with the RS
as defined in Section 4.
When receiving the new Access Token, the RS performs the same steps
defined in Section 4.2, with the following addition in case the new
Access Token is successfully verified and stored. The RS also
deletes the old Access Token, i.e., the one whose associated triple
(GID, SaltInput, AuthCred) has the same GID and SaltInput values as
in the triple including the new authentication credential of the
Client and associated with the new Access Token.
5. Secure Communication with the AS
As specified in the ACE framework (see Sections 5.8 and 5.9 of
[RFC9200]), the requesting entity (Client and/or RS) and the AS
communicate via the /token or /introspection endpoint. The use of
CoAP and OSCORE [RFC8613] for this communication is RECOMMENDED in
this profile. Other protocols fulfilling the security requirements
defined in Sections 5 and 6 of [RFC9200] (such as HTTP and DTLS or
TLS) MAY be used instead.
If OSCORE [RFC8613] is used, the requesting entity and the AS are
expected to have a pre-established Security Context in place. How
this Security Context is established is out of the scope of this
profile. Furthermore, the requesting entity and the AS communicate
using OSCORE through the /token endpoint as specified in Section 5.8
of [RFC9200], and through the /introspection endpoint as specified in
Section 5.9 of [RFC9200].
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6. Discarding the Security Context
As members of an OSCORE group, the Client and the RS may
independently leave the group or be forced to, e.g., if compromised
or suspected so. Upon leaving the OSCORE group, the Client or RS
also discards the Group OSCORE Security Context, which may anyway be
renewed by the Group Manager through a group rekeying process (see
Section 3.2 of [I-D.ietf-core-oscore-groupcomm]).
The Client or RS can acquire a new Group OSCORE Security Context, by
re-joining the OSCORE group, e.g., by using the approach defined in
[I-D.ietf-ace-key-groupcomm-oscore]. In such a case, the Client
SHOULD request a new Access Token to be uploaded to the RS.
7. CBOR Mappings
The new parameters defined in this document MUST be mapped to CBOR
types as specified in Figure 7, using the given integer abbreviation
for the map key.
+------------------------+----------+------------+
| Parameter name | CBOR Key | Value Type |
+------------------------+----------+------------+
| context_id | TBD | bstr |
| salt_input | TBD | bstr |
| client_cred_verify | TBD | bstr |
| client_cred_verify_mac | TBD | bstr |
+------------------------+----------+------------+
Figure 7: CBOR Mappings for New Parameters.
The new claims defined in this document MUST be mapped to CBOR types
as specified in Figure 8, using the given integer abbreviation for
the map key.
+-----------------+----------+------------+
| Claim name | CBOR Key | Value Type |
+-----------------+----------+------------+
| salt_input | TBD | bstr |
| contextId_input | TBD | bstr |
+-----------------+----------+------------+
Figure 8: CBOR Mappings for New Claims.
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8. Security Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework [RFC9200].
Thus, the general security considerations from the ACE framework also
apply to this profile.
The proof-of-possession (PoP) key bound to an Access Token is always
an asymmetric key, i.e., the public key included in the
authentication credential that the Client uses in the OSCORE group.
This means that there is never a same shared secret used as PoP key
with possible multiple RSs. Therefore, it is possible and safe for
the AS to issue an Access Token for an audience that includes
multiple RSs (i.e., a group-audience, see Section 6.9 of [RFC9200]).
In such a case, as per Section 6.1 of [RFC9200], the AS has to ensure
the integrity protection of the Access Token by protecting it through
an asymmetric signature. In addition, the used group-audience has to
correctly identify all the RSs that are intended recipients of the
Access Token, and for which the single scope specified in the Access
Token applies. As a particular case, the audience can be the name of
the OSCORE group, if the Access Token is intended for all the RSs in
that group.
Furthermore, this document inherits the general security
considerations about Group OSCORE [I-D.ietf-core-oscore-groupcomm],
as to the specific use of Group OSCORE according to this profile.
Group OSCORE is designed to secure point-to-point as well as point-
to-multipoint communications, providing a secure binding between a
single request and multiple corresponding responses. In particular,
Group OSCORE fulfills the same security requirements of OSCORE.
Group OSCORE ensures source authentication of messages both in group
mode (see Section 8 of [I-D.ietf-core-oscore-groupcomm]) and in
pairwise mode (see Section 9 of [I-D.ietf-core-oscore-groupcomm]).
When protecting an outgoing message in group mode, the sender uses
its private key to compute a digital signature, which is embedded in
the protected message. The group mode can be used to protect
messages sent to multiple recipients (e.g., over IP multicast) or to
a single recipient.
When protecting an outgoing message in pairwise mode, the sender uses
a pairwise symmetric key, as derived from the asymmetric keys of the
two peers exchanging the message. The pairwise mode can be used to
protect only messages intended for a single recipient.
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9. Privacy Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework [RFC9200].
Thus the general privacy considerations from the ACE framework also
apply to this profile.
As this profile uses Group OSCORE, the privacy considerations from
[I-D.ietf-core-oscore-groupcomm] apply to this document as well.
An unprotected response to an unauthorized request may disclose
information about the RS and/or its existing relationship with the
Client. It is advisable to include as little information as possible
in an unencrypted response. However, since both the Client and the
RS share a Group OSCORE Security Context, unauthorized, yet protected
requests are followed by protected responses, which can thus include
more detailed information.
Although it may be encrypted, the Access Token is sent in the clear
to the /authz-info endpoint at the RS. Thus, if the Client uses the
same single Access Token from multiple locations with multiple
Resource Servers, it can risk being tracked through the Access
Token's value.
Note that, even though communications are protected with Group
OSCORE, some information might still leak, due to the observable
size, source address, and destination address of exchanged messages.
10. IANA Considerations
This document has the following actions for IANA.
Note to RFC Editor: Please replace "[RFC-XXXX]" with the RFC number
of this document and delete this paragraph.
10.1. ACE Profile Registry
IANA is asked to add the following entry to the "ACE Profile"
registry within the "Authentication and Authorization for Constrained
Environments (ACE)" registry group, following the procedure specified
in Section 8.8 of [RFC9200].
* Name: coap_group_oscore
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* Description: Profile to secure communications between constrained
nodes using the Authentication and Authorization for Constrained
Environments framework, by enabling authentication and fine-
grained authorization of members of an OSCORE group that use a
pre-established Group OSCORE Security Context to communicate with
Group OSCORE.
* CBOR Value: TBD (value between 1 and 255)
* Reference: [RFC-XXXX]
10.2. OAuth Parameters Registry
IANA is asked to add the following entries to the "OAuth Parameters"
registry, following the procedure specified in Section 11.2 of
[RFC6749].
* Name: "context_id"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.1 of [RFC-XXXX]
* Name: "salt_input"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.2 of [RFC-XXXX]
* Name: "client_cred_verify"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.3 of [RFC-XXXX]
* Name: "client_cred_verify_mac"
* Parameter Usage Location: token request
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* Change Controller: IESG
* Specification Document(s): Section 3.1.4 of [RFC-XXXX]
10.3. OAuth Parameters CBOR Mappings Registry
IANA is asked to add the following entries to the "OAuth Parameters
CBOR Mappings" registry within the "Authentication and Authorization
for Constrained Environments (ACE)" registry group, following the
procedure specified in Section 8.10 of [RFC9200].
* Name: "context_id"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.1 of [RFC-XXXX]
* Name: "salt_input"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.2 of [RFC-XXXX]
* Name: "client_cred_verify"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.3 of [RFC-XXXX]
* Name: "client_cred_verify_mac"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.4 of [RFC-XXXX]
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10.4. CBOR Web Token (CWT) Claims Registry
IANA is asked to add the following entries to the "CBOR Web Token
(CWT) Claims" registry, following the procedure specified in
Section 9.1 of [RFC8392].
* Claim Name: "salt_input"
* Claim Description: Client provided salt input
* JWT Claim Name: "N/A"
* Claim Key: TBD
* Claim Value Type(s): bstr
* Change Controller: IESG
* Specification Document(s): Section 3.2.1 of [RFC-XXXX]
* Claim Name: "contextId_input"
* Claim Description: Client context id input
* JWT Claim Name: "N/A"
* Claim Key: TBD
* Claim Value Type(s): bstr
* Change Controller: IESG
* Specification Document(s): Section 3.2.2 of [RFC-XXXX]
10.5. TLS Exporter Label Registry
IANA is asked to add the following entry to the "TLS Exporter Label"
registry within the "Transport Layer Security (TLS) Parameters"
registry group, following the procedure specified in Section 6 of
[RFC5705] and updated in Section 12 of [RFC8447].
* Value: EXPORTER-ACE-PoP-Input-Client-AS
* DTLS-OK: Y
* Recommended: N
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* Reference: Section 3.1 of [RFC-XXXX]
11. References
11.1. Normative References
[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-oscore-16, 6 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ace-key-groupcomm-oscore-16>.
[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
10, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
groupcomm-bis-10>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-20, 2
September 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-20>.
[NIST-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography - NIST
Special Publication 800-56A, Revision 3", April 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>.
[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/rfc/rfc2119>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/rfc/rfc5246>.
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[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/rfc/rfc5705>.
[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/rfc/rfc5869>.
[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/rfc/rfc6347>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/rfc/rfc6749>.
[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/rfc/rfc7252>.
[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/rfc/rfc7748>.
[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/rfc/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/rfc/rfc8392>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/rfc/rfc8447>.
[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/rfc/rfc8613>.
[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/rfc/rfc8747>.
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[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/rfc/rfc8949>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/rfc/rfc9200>.
[RFC9201] Seitz, L., "Additional OAuth Parameters for Authentication
and Authorization for Constrained Environments (ACE)",
RFC 9201, DOI 10.17487/RFC9201, August 2022,
<https://www.rfc-editor.org/rfc/rfc9201>.
[RFC9203] Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"The Object Security for Constrained RESTful Environments
(OSCORE) Profile of the Authentication and Authorization
for Constrained Environments (ACE) Framework", RFC 9203,
DOI 10.17487/RFC9203, August 2022,
<https://www.rfc-editor.org/rfc/rfc9203>.
11.2. Informative References
[I-D.ietf-ace-edhoc-oscore-profile]
Selander, G., Mattsson, J. P., Tiloca, M., and R. Höglund,
"Ephemeral Diffie-Hellman Over COSE (EDHOC) and Object
Security for Constrained Environments (OSCORE) Profile for
Authentication and Authorization for Constrained
Environments (ACE)", Work in Progress, Internet-Draft,
draft-ietf-ace-edhoc-oscore-profile-03, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
edhoc-oscore-profile-03>.
[I-D.ietf-cose-cbor-encoded-cert]
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
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ietf-cose-cbor-encoded-cert-09, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-cose-
cbor-encoded-cert-09>.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-14, 8 September 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-discovery-14>.
[NIST-800-207]
Rose, S., Borchert, O., Mitchell, S., and S. Connelly,
"Zero Trust Architecture - NIST Special Publication
800-207", August 2020,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-207.pdf>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[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/rfc/rfc8446>.
[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/rfc/rfc9147>.
[RFC9202] Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS)
Profile for Authentication and Authorization for
Constrained Environments (ACE)", RFC 9202,
DOI 10.17487/RFC9202, August 2022,
<https://www.rfc-editor.org/rfc/rfc9202>.
[RFC9431] Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT) and Transport Layer Security (TLS)
Profile of Authentication and Authorization for
Constrained Environments (ACE) Framework", RFC 9431,
DOI 10.17487/RFC9431, July 2023,
<https://www.rfc-editor.org/rfc/rfc9431>.
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Appendix A. Profile Requirements
This appendix lists the specifications of this profile based on the
requirements of the ACE framework, as requested in Appendix C of
[RFC9200].
* Optionally, define new methods for the Client to discover the
necessary permissions and AS for accessing a resource, different
from the one proposed in [RFC9200]: Not specified.
* Optionally, specify new grant types: Not specified.
* Optionally, define the use of client certificates as client
credential type: Not specified.
* Specify the communication protocol the Client and RS must use:
CoAP.
* Specify the security protocol the Client and RS must use to
protect their communication: Group OSCORE, by using a pre-
established Group OSCORE Security Context.
* Specify how the Client and the RS mutually authenticate:
Explicitly, by possession of a common Group OSCORE Security
Context, and by either: usage of digital signatures embedded in
messages, if protected with the group mode of Group OSCORE; or
protection of messages with the pairwise mode of Group OSCORE, by
using pairwise symmetric keys derived from the asymmetric keys of
the two peers exchanging the message. Note that mutual
authentication is not completed before the Client has verified a
Group OSCORE response using the corresponding Group OSCORE
Security Context.
* Specify the proof-of-possession protocol(s) and how to select one,
if several are available. Also specify which key types (e.g.,
symmetric/asymmetric) are supported by a specific proof-of-
possession protocol: Group OSCORE algorithms; asymmetric keys
verified and distributed by a Group Manager.
* Specify a unique ace_profile identifier: coap_group_oscore.
* If introspection is supported, specify the communication and
security protocol for introspection: HTTP/CoAP (+ TLS/DTLS/
OSCORE).
* Specify the communication and security protocol for interactions
between client and AS: HTTP/CoAP (+ TLS/DTLS/OSCORE).
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* Specify if/how the authz-info endpoint is protected, including how
error responses are protected: Not protected.
* Optionally, define other methods of token transport than the
authz-info endpoint: Not defined.
Appendix B. Document Updates
This section is to be removed before publishing as an RFC.
B.1. Version -00 to -01
* Deleting an Access Token does not delete the Group OSCORE Security
Context.
* Distinct computation of the PoP input when C and the AS use (D)TLS
1.2 or 1.3.
* Revised computation of the PoP input when C and the AS use OSCORE.
* Revised computation of the PoP evidence when the OSCORE group is a
pairwise-only group.
* Clarified requirements on the AS for verifying the PoP evidence.
* Renamed the TLS Exporter Label for computing the PoP input.
* Editorial fixes and improvements.
Acknowledgments
Ludwig Seitz contributed as a co-author of initial versions of this
document.
The authors sincerely thank Christian Amsüss, Benjamin Kaduk, John
Preuß Mattsson, Dave Robin, Jim Schaad, and Göran Selander for their
comments and feedback.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
(Grant agreement 952652).
Authors' Addresses
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Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
Rikard Höglund
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: rikard.hoglund@ri.se
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
SE-16440 Stockholm Kista
Sweden
Email: francesca.palombini@ericsson.com
Tiloca, et al. Expires 5 September 2024 [Page 42]
