Group OSCORE Profile of the Authentication and Authorization for Constrained Environments Framework
draft-tiloca-ace-group-oscore-profile-08
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Marco Tiloca , Rikard Höglund , Ludwig Seitz , Francesca Palombini | ||
| Last updated | 2022-03-07 | ||
| Stream | (None) | ||
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| Send notices to | (None) |
draft-tiloca-ace-group-oscore-profile-08
ACE Working Group M. Tiloca
Internet-Draft R. Höglund
Intended status: Standards Track RISE AB
Expires: 8 September 2022 L. Seitz
Combitech
F. Palombini
Ericsson AB
7 March 2022
Group OSCORE Profile of the Authentication and Authorization for
Constrained Environments Framework
draft-tiloca-ace-group-oscore-profile-08
Abstract
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework. The
profile uses Group OSCORE to provide communication security between a
Client and a (set of) Resource Server(s) as members of an OSCORE
Group. The profile securely binds an OAuth 2.0 Access Token with the
public key of the Client associated with the private key used 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 correct 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.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Constrained RESTful
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://gitlab.com/crimson84/draft-tiloca-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 . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 9
2.2. Access Token Retrieval . . . . . . . . . . . . . . . . . 10
2.3. Access Token Posting . . . . . . . . . . . . . . . . . . 11
2.4. Secure Communication . . . . . . . . . . . . . . . . . . 11
3. Client-AS Communication . . . . . . . . . . . . . . . . . . . 12
3.1. C-to-AS: POST to Token Endpoint . . . . . . . . . . . . . 12
3.1.1. 'context_id' Parameter . . . . . . . . . . . . . . . 15
3.1.2. 'salt_input' Parameter . . . . . . . . . . . . . . . 15
3.1.3. 'client_cred_verify' Parameter . . . . . . . . . . . 16
3.1.4. 'client_cred_verify_mac' Parameter . . . . . . . . . 16
3.2. AS-to-C: Access Token . . . . . . . . . . . . . . . . . . 16
3.2.1. Salt Input Claim . . . . . . . . . . . . . . . . . . 20
3.2.2. Context ID Input Claim . . . . . . . . . . . . . . . 21
4. Client-RS Communication . . . . . . . . . . . . . . . . . . . 21
4.1. C-to-RS POST to authz-info Endpoint . . . . . . . . . . . 22
4.2. RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . . . . 22
4.3. Client-RS Secure Communication . . . . . . . . . . . . . 24
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4.3.1. Client Side . . . . . . . . . . . . . . . . . . . . . 24
4.3.2. Resource Server Side . . . . . . . . . . . . . . . . 24
4.4. Access Rights Verification . . . . . . . . . . . . . . . 25
4.5. Change of Client's Authentication Credential in the
Group . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5. Secure Communication with the AS . . . . . . . . . . . . . . 26
6. Discarding the Security Context . . . . . . . . . . . . . . . 26
7. CBOR Mappings . . . . . . . . . . . . . . . . . . . . . . . . 27
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
10.1. ACE Profile Registry . . . . . . . . . . . . . . . . . . 29
10.2. OAuth Parameters Registry . . . . . . . . . . . . . . . 29
10.3. OAuth Parameters CBOR Mappings Registry . . . . . . . . 31
10.4. CBOR Web Token Claims Registry . . . . . . . . . . . . . 32
10.5. TLS Exporter Label Registry . . . . . . . . . . . . . . 33
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Dual Mode (Group OSCORE & OSCORE) . . . . . . . . . 37
A.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 38
A.1.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . 40
A.1.2. Access Token Posting . . . . . . . . . . . . . . . . 40
A.1.3. Setup of the Pairwise OSCORE Security Context . . . . 41
A.1.4. Secure Communication . . . . . . . . . . . . . . . . 42
A.2. Client-AS Communication . . . . . . . . . . . . . . . . . 42
A.2.1. C-to-AS: POST to Token Endpoint . . . . . . . . . . . 43
A.2.2. AS-to-C: Access Token . . . . . . . . . . . . . . . . 45
A.3. Client-RS Communication . . . . . . . . . . . . . . . . . 53
A.3.1. C-to-RS POST to authz-info Endpoint . . . . . . . . . 54
A.3.2. RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . . 54
A.3.3. OSCORE Setup - Client Side . . . . . . . . . . . . . 56
A.3.4. OSCORE Setup - Resource Server Side . . . . . . . . . 58
A.3.5. Access Rights Verification . . . . . . . . . . . . . 61
A.3.6. Change of Client's Authentication Credential in the
Group . . . . . . . . . . . . . . . . . . . . . . . . 61
A.4. Secure Communication with the AS . . . . . . . . . . . . 62
A.5. Discarding the Security Context . . . . . . . . . . . . . 62
A.6. CBOR Mappings . . . . . . . . . . . . . . . . . . . . . . 63
A.7. Security Considerations . . . . . . . . . . . . . . . . . 63
A.8. Privacy Considerations . . . . . . . . . . . . . . . . . 64
Appendix B. Profile Requirements . . . . . . . . . . . . . . . . 64
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 65
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65
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1. Introduction
A number of applications rely on a group communication model, where a
Client can access a resource shared 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 get 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 be used 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.
However, in different instances of such applications, the approach
above is not desirable, as different group members are intended to
have different access rights to resources of other group members.
That is, 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. For instance, 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)
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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 do so.
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. Also, introducing a different security group for each
different set of access rights would result in additional keying
material to distribute and manage. In particular, if the access
rights for a single node change, this would require to evict that
node from the current group, followed by that node joining 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 in the system.
A fine-grained access control model can be rather enforced within a
same group, by using the Authentication and Authorization for
Constrained Environments (ACE) framework [I-D.ietf-ace-oauth-authz].
That is, a Client has to first obtain authorization credentials in
the form of an Access Token, and post 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 Server.
However each of the current profiles of ACE defined in
[I-D.ietf-ace-oscore-profile] [I-D.ietf-ace-dtls-authorize]
[I-D.ietf-ace-mqtt-tls-profile] admits a single security protocol
that cannot be used to protect group messages sent over IP multicast.
This document specifies the "coap_group_oscore" profile of the ACE
framework, where a Client uses CoAP [RFC7252] or CoAP over IP
multicast [I-D.ietf-core-groupcomm-bis] to communicate to one or
multiple Resource Servers, which are members of an application group
and share a common set of resources. This profile uses 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.
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That is, this profile describes how access control is enforced for a
Client after it has joined an OSCORE group, to access resources at
other members in that group. The process for joining the 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
by using an Access Token, which is bound to a key (the proof-of-
possession key). This profile uses Group OSCORE to achieve server
authentication, as well as proof-of-possession for the Client's
public key used in the OSCORE group in question. Note that the proof
of possession is not done by a dedicated protocol element, but rather
occurs after the first Group OSCORE exchange.
Furthermore, this profile provides proof of the Client's membership
to the correct OSCORE group, by binding the Access Token to the
Client's authentication credential used in the group and including
the Client's public public key, as well as to information from the
pre-established Group OSCORE Security Context. This allows the
Resource Server 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
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] 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 protected messages (in group
mode); or by protecting messages with pairwise keying material
derived from the asymmetric keys of the two peers exchanging the
message (in 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
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs],
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 [RFC7925] and C509 certificates
[I-D.ietf-cose-cbor-encoded-cert].
Members of an OSCORE group have an associated authentication
credential in the format used in the group. As per Section 2.3 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
[I-D.ietf-ace-oauth-authz], as well as in the OSCORE profile of ACE
[I-D.ietf-ace-oscore-profile]. 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.
* Equivalent COSE Key: a COSE Key built from an authentication
credential used in the OSCORE group. The equivalent COSE Key
preserves all the main information elements from the
authentication credential, in particular the key coordinates and
the comprehensive set of information related to the public key
algorithm, including, e.g., the used elliptic curve (when
applicable).
* Pairwise-only group: an OSCORE group that uses only the pairwise
mode (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.
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2. Protocol Overview
This section provides an overview of this profile, i.e., on how to
use the ACE framework for authentication and authorization
[I-D.ietf-ace-oauth-authz] to secure communications between a Client
and a (set of) Resource Server(s) 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 at other members in that group.
In particular, the process for 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. In the figure, it is assumed that both RS1 and RS2 are
associated with the same AS. It is also assumed that C, RS1 and RS2
have previously joined an OSCORE group with Group Identifier (gid)
"abcd0000", and got assigned Sender ID (sid) "0", "1" and "2" in the
group, respectively. The names of messages coincide with those of
[I-D.ietf-ace-oauth-authz] when applicable.
C RS1 RS2 AS
| [--- Resource Request -->] | | |
| | | |
| [<---- AS Request ------] | | |
| Creation Hints | | |
| | | |
|-------- POST /token ----------------------------------------------->|
| (aud: RS1, sid: 0, gid: abcd0000, ...) | |
| | | |
| | | |
|<-------------------------------- Access Token + RS Information -----|
| | (aud: RS1, sid: 0, gid: abcd0000, ...) |
| | | |
|---- POST /authz-info ----->| | |
| (access_token) | | |
| | | |
|<--- 2.01 Created ----------| | |
| | | |
|-------- POST /token ----------------------------------------------->|
| (aud: RS2, sid: 0, gid: abcd0000, ...) | |
| | | |
| | | |
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| | | |
|<-------------------------------- Access Token + RS Information -----|
| | (aud: RS2, sid: 0, gid: abcd0000, ...) |
| | | |
|----- POST /authz-info ----------------->| |
| (access_token) | | |
| | | |
| | | |
|<--- 2.01 Created -----------------------| |
| | | |
|-- Group OSCORE Request -+->| | |
| (kid: 0, gid: abcd0000) \-------------->| |
| | | |
| /proof-of-possession/ |
| | | |
| | | |
|<-- Group OSCORE Response --| | |
| (kid: 1) | | |
| | | |
/proof-of-possession/ | | |
| | | |
/Mutual authentication | | |
between C and RS1/ | | |
| | | |
| | | |
|<-- Group OSCORE Response ---------------| |
| (kid: 2) | | |
| | | |
/proof-of-possession/ | | |
| | | |
/Mutual authentication | | |
between C and RS2/ | | |
| | | |
| ... | | |
Figure 1: Protocol Overview.
2.1. Pre-Conditions
Using Group OSCORE and this profile requires both the Client and the
Resource Servers to 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 may 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.
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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.
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) it wants to access on each of the RSs,
using the /token endpoint, as specified in Section 5.8 of
[I-D.ietf-ace-oauth-authz]. In a general case, it can be assumed
that different RSs are associated with different ASs, even if the RSs
are members of a same 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, included in the Access Token response to the Client.
Furthermore, in the Access Token request to the AS, the Client MUST
also include: its own public key used in the OSCORE group; and a
proof-of-possession (PoP) evidence to proof 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 public
key indicated by the Client in the Access Token.
The Access Token request and response MUST be confidentiality-
protected and ensure authenticity. This profile RECOMMENDS the use
of OSCORE 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
[I-D.ietf-ace-oauth-authz] MAY alternatively be used, such as TLS
[RFC8446] or DTLS [RFC6347][I-D.ietf-tls-dtls13].
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2.3. Access Token Posting
After having retrieved the Access Token from the AS, the Client posts
the Access Token to the RS, using the /authz-info endpoint and
mechanisms specified in Section 5.10 of [I-D.ietf-ace-oauth-authz],
as well as Content-Format = application/ace+cbor. When using this
profile, the communication with the /authz-info endpoint is not
protected.
If the Access Token is valid, the RS replies to this POST request
with a 2.01 (Created) response with Content-Format = application/
ace+cbor. 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, and the authorization information is a
policy 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
with 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).
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 addressed to the RS, or a multicast request addressed to 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.
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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), by means of the 'cnf' claim.
3.1. C-to-AS: POST to Token Endpoint
The Client-to-AS request is specified in Section 5.8.1 of
[I-D.ietf-ace-oauth-authz]. 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
[I-D.ietf-ace-oscore-profile]), 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 where the Client and the RS are
currently members. In particular, the Client wishes to
communicate with the RS 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.
* 'req_cnf', defined in Section 3.1 of [I-D.ietf-ace-oauth-params].
This parameter follows the syntax from Section 3.1 of [RFC8747]
when including Value Type "COSE_Key" (1) and specifying an
asymmetric key. In particular, the specified public key is the
COSE Key equivalent to the authentication credential that the
Client uses in the OSCORE group. The specified public key will be
used as the pop-key bound to the Access Token.
Alternative Value Types defined in future specifications are fine
to consider, if indicating a non-encrypted asymmetric key or full-
fledged autentication credential.
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In addition, the Client computes its proof-of-possession (PoP)
evidence, in order to prove possession of its own private key used in
the OSCORE group to the AS. This allows the AS to verify that the
Client indeed owns the private key associated with that public key,
as its alleged identity credential within the OSCORE group.
To this end, the Client MUST use as PoP input the byte representation
of a quantity that uniquely represents the secure communication
association between the Client and the AS. It is RECOMMENDED that
the Client considers the following as PoP input.
* If the Client and the AS communicate over (D)TLS, the PoP input is
an exporter value computed as defined in Section 7.5 of [RFC8446].
In particular, the exporter label MUST be 'EXPORTER-ACE-Sign-
Challenge-Client-AS' defined in Section 10.5 of this document,
together with an empty 'context_value', and 32 bytes as
'key_length'.
* If the Client and the AS communicate over OSCORE, the PoP input is
the output PRK of a HKDF-Extract step [RFC5869], i.e., PRK = HMAC-
Hash(salt, IKM). In particular, 'salt' takes (x1 | x2), where x1
is the ID Context of the OSCORE Security Context between the
Client and the AS, x2 is the Sender ID of the Client in that
Security Context, and | denotes byte string concatenation. Also,
'IKM' is the OSCORE Master Secret of the OSCORE Security Context
between the Client and the AS.
The HKDF MUST be one of the HMAC-based HKDF [RFC5869] algorithms
defined for COSE [I-D.ietf-cose-rfc8152bis-algs]. The Client and
AS may agree on the HKDF algorithm to use during the Client's
registration at the AS. HKDF SHA-256 is mandatory to implement.
Then, 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 private key corresponds to the
authentication credential used in the OSCORE group, for which the
equivalent COSE Key is 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)
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The input parameters of HKDF are as follows.
- salt takes as value the empty byte string.
- IKM is computed as a cofactor Diffie-Hellman shared secret, see
Section 5.7.1.2 of [NIST-800-56A], using an ECDH algorithm pre-
agreed between Client and AS. 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 corresponds to the Client's
authentication credential used in the OSCORE group, for which
the equivalent COSE Key is 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.
The Client and AS may agree on the ECDH algorithm to use during
the Client's registration at the AS. The ECDH-SS + HKDF-256
algorithm specified in Section 6.3.1 of
[I-D.ietf-cose-rfc8152bis-algs] is mandatory to implement.
- 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" : {
"COSE_Key" : {
"kty" : EC2,
"crv" : P-256,
"x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
731e79a3b4e47120'
}
},
"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.
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
[I-D.ietf-ace-oauth-authz]. 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
[I-D.ietf-ace-oauth-authz]. 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.
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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
[I-D.ietf-ace-oauth-authz]. 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
[I-D.ietf-ace-oauth-authz]. 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 considered by the Client
in Section 3.1.
* As public key of the Client, the AS uses the one specified in the
'req_cnf' parameter of the Access Token request.
* 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.
* 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.
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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.
If the Client request was invalid, or not authorized, the AS returns
an error response as described in Section 5.8.3 of
[I-D.ietf-ace-oauth-authz].
If all verifications are successful, the AS responds as defined in
Section 5.8.2 of [I-D.ietf-ace-oauth-authz]. 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
[I-D.ietf-ace-oauth-params]. 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 placed in the 'ace_profile' claim of the Access Token, as
per Section 5.10 of [I-D.ietf-ace-oauth-authz].
* The salt input specified in the 'salt_input' parameter of the
Token Request. If the Access Token is a CWT, the content of the
'salt_input' parameter MUST be placed 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 Token Request. If the Access Token is a CWT, the content of
the 'context_id' parameter MUST be placed in the 'contextId_input'
claim of the Access Token, defined in Section 3.2.2 of this
document.
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* The public key that the client uses in the OSCORE group and
specified in the 'req_cnf' parameter of the Token request.
If the Access Token is a CWT, the public key MUST be specified in
the 'cnf' claim, which follows the syntax from Section 3.1 of
[RFC8747] when including Value Type "COSE_Key" (1) and specifying
an asymmetric key. In particular, the 'cnf' claim includes the
same COSE Key specified in the 'req_cnf' parameter of the Token
Request, i.e., the COSE Key equivalent to the authentication
credential that the Client uses in the OSCORE group.
Alternative Value Types defined in future specifications are fine
to consider, if indicating a non-encrypted asymmetric key or full-
fledged autentication credential.
Figure 3 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 3: Example AS-to-C Access Token response with the Group
OSCORE profile.
Figure 4 shows an example CWT Claims Set, containing the Client's
public key in the group (as pop-key) in the 'cnf' claim.
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{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"exp" : "1360289224",
"scope" : "temperature_g firmware_p",
"cnf" : {
"COSE_Key" : {
"kty" : EC2,
"crv" : P-256,
"x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
731e79a3b4e47120'
},
"salt_input" : h'00',
"contextId_input" : h'abcd0000'
}
Figure 4: Example CWT Claims Set with OSCORE parameters.
The same CWT Claims Set as in Figure 4 and encoded in CBOR is shown
in Figure 5, using the value abbreviations defined in
[I-D.ietf-ace-oauth-authz] 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.
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
06 # unsigned(6)
1A 5112D728 # unsigned(1360189224)
04 # unsigned(4)
1A 51145DC8 # unsigned(1360289224)
09 # unsigned(9)
78 18 # text(24)
74656D70657261747572655F67206669726D776172655F70
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)
D7CC072DE2205BDC1537A543D53C60A6ACB62ECCD890C7FA27C9
E354089BBE13
22 # negative(2)
58 20 # bytes(32)
F95E1D4B851A2CC80FFF87D8E23F22AFB725D535E515D020731E
79A3B4E47120
18 3C # unsigned(60)
41 # bytes(1)
00
18 3D # unsigned(61)
44 # bytes(4)
ABCD0000
Figure 5: Example CWT Claims Set with OSCORE parameters, CBOR
encoded.
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.
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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.
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.
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Furthermore, a Client C1 is supposed to obtain a valid Access Token
from the AS, as including the public key associated with 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
an Access Token to verify with the Group Manager of that OSCORE group
whether such a Client has indeed that Sender ID and an authentication
credential including that public key 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) public key; ii) successfully posting the Access Token to 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 posts the Access Token to the /authz-info endpoint of the
RS, as defined in Section 5.10.1 of [I-D.ietf-ace-oauth-authz].
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 [I-D.ietf-ace-oauth-authz], 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 content
of the 'cnf' claim as the COSE Key equivalent to the
authentication credential that the Client uses in the group.
The RS MUST check whether it already stores an authentication
credential associated with the pair (GID, Sender ID) above, such
that the COSE Key specified in the 'cnf' claim is its equivalent
COSE Key.
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 10 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.
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- The RS MUST check whether the Client's authentication
credential retrieved from the Group Manager is such that the
COSE Key specified in the 'cnf' claim of the Access Token is
its equivalent COSE Key.
- The RS MUST check that 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 non valid, and MUST respond 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 3-tuple (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
3-tuple (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 20 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 3-tuple.
* If the Client requests and obtains a new OSCORE Sender ID from the
Group Manager (see Section 2.5.3.1 of
[I-D.ietf-core-oscore-groupcomm] and Section 9 of
[I-D.ietf-ace-key-groupcomm-oscore]), the new Sender ID replaces
the current 'SaltInput' value in the 3-tuple.
Finally, the RS MUST send a 2.01 (Created) response to the Client, as
defined in Section 5.10.1 of [I-D.ietf-ace-oauth-authz].
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4.3. Client-RS Secure Communication
When previously joining the OSCORE group, both the Client and 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.
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 OSCORE group, whose GID was specified
in the 'context_id' parameter of the 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 to the stored Access Token. As defined in
Section 4.5, a possible change of authentication credential requires
the Client to upload to the RS a new Access Token bound to the new
authentication credential.
Additionally, for every incoming request, if Group OSCORE
verification succeeds, the verification of access rights is performed
as described in Section 4.4.
After the expiration of the Access Token related to a Group OSCORE
Security Context, if the Client uses the Group OSCORE Security
Context to send a request for any resource intended for OSCORE group
members and that requires an active Access Token, the RS MUST respond
with a 4.01 (Unauthorized) error message protected with the Group
OSCORE Security Context.
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4.4. Access Rights Verification
The RS MUST follow the procedures defined in Section 5.10.2 of
[I-D.ietf-ace-oauth-authz]. If an RS receives a Group OSCORE-
protected request from a Client, the RS processes it 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.
The response code MUST be 4.01 (Unauthorized) if the RS has no valid
Access Token for the Client. 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 with a 4.03 (Forbidden). If the RS has an
Access Token for the Client but the requested action is not
authorized, the RS MUST reject the request with a 4.05 (Method Not
Allowed).
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 it uses in the group. When this
happens, the Client uploads the new authentication credential to the
Group Manager, as defined in Section 11 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 POST request as
defined in Section 3.1, the Client indicates:
* The current Group Identifier of the OSCORE group, as value of
the 'context_id' parameter.
* The current Sender ID it has in the OSCORE group, as value of
the 'salt_input' parameter.
* The public key of the new authentication credential it uses in
the OSCORE group, as value of the 'req_cnf' parameter. In
particular, the specified public key is the COSE Key
equivalent to the new authentication credential that the
Client uses in the OSCORE group.
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* 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 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 3-tuple
has the same GID and SaltInput values as in the 3-tuple 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
[I-D.ietf-ace-oauth-authz]), the requesting entity (RS and/or Client)
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
[I-D.ietf-ace-oauth-authz] (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 /introspection endpoint as specified in
Section 5.9 of [I-D.ietf-ace-oauth-authz], and through the /token
endpoint as specified in Section 5.8 of [I-D.ietf-ace-oauth-authz].
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]).
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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 and post it to the RS.
7. CBOR Mappings
The new parameters defined in this document MUST be mapped to CBOR
types as specified in Figure 6, 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 6: CBOR mappings for new parameters.
The new claims defined in this document MUST be mapped to CBOR types
as specified in Figure 7, using the given integer abbreviation for
the map key.
/-----------------+----------+------------\
| Claim name | CBOR Key | Value Type |
|-----------------+----------+------------|
| salt_input | TBD | bstr |
| contextId_input | TBD | bstr |
\-----------------+----------+------------/
Figure 7: CBOR mappings for new claims.
8. Security Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. 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 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 whose audience
comprises multiple RSs.
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In such a case, as per Section 6.1 of [I-D.ietf-ace-oauth-authz], the
AS has to ensure the integrity protection of the Access Token by
protecting it through an asymmetric signature. In addition, the used
audience has to correctly identify all the RSs that are intended
recipients of the Access Token. As a particular case, the audience
can be the name of the OSCORE group, if the Access Token is intended
to 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, for
group requests and responses.
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 over multicast to multiple recipients, or sent over
unicast to one 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 to one recipient.
9. Privacy Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. 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.
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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.
10.1. ACE Profile Registry
IANA is asked to add the following entry to the "ACE Profile"
registry defined in Section 8.8 of [I-D.ietf-ace-oauth-authz].
* Name: coap_group_oscore
* 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. Optionally, the dual mode defined in Appendix A
additionally establishes a pairwise OSCORE Security Context, and
thus also enables OSCORE communication between two members of the
OSCORE group.
* CBOR Value: TBD (value between 1 and 255)
* Reference: [[this document]]
10.2. OAuth Parameters Registry
IANA is asked to add the following entries to the "OAuth Parameters"
registry.
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* Name: "context_id"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.1 of [[this document]]
* Name: "salt_input"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.2 of [[this document]]
* Name: "client_cred_verify"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.3 of [[this document]]
* Name: "client_cred_verify_mac"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Section 3.1.4 of [[this document]]
* Name: "client_cred"
* Parameter Usage Location: token request
* Change Controller: IESG
* Specification Document(s): Appendix A.2.1.1 of [[this document]]
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10.3. OAuth Parameters CBOR Mappings Registry
IANA is asked to add the following entries to the "OAuth Parameters
CBOR Mappings" registry defined in Section 8.10 of
[I-D.ietf-ace-oauth-authz].
* Name: "context_id"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.1 of [[this document]]
* Name: "salt_input"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.2 of [[this document]]
* Name: "client_cred_verify"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.3 of [[this document]]
* Name: "client_cred_verify_mac"
* CBOR Key: TBD
* Value Type: bstr
* Reference: Section 3.1.4 of [[this document]]
* Name: "client_cred"
* CBOR Key: TBD
* Value Type: bstr
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* Reference: Appendix A.2.1.1 of [[this document]]
10.4. CBOR Web Token Claims Registry
IANA is asked to add the following entries to the "CBOR Web Token
Claims" registry.
* 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 [[this document]]
* 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 [[this document]]
* Claim Name: "client_cred"
* Claim Description: Client Credential
* JWT Claim Name: "N/A"
* Claim Key: TBD
* Claim Value Type(s): map
* Change Controller: IESG
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* Specification Document(s): Appendix A.2.2.2 of [[this document]]
10.5. TLS Exporter Label Registry
IANA is asked to add the following entry to the "TLS Exporter Label"
registry defined in Section 6 of [RFC5705] and updated in Section 12
of [RFC8447].
* Value: EXPORTER-ACE-Sign-Challenge-Client-AS
* DTLS-OK: Y
* Recommended: N
* Reference: [[this document]] (Section 3.1)
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-13, 7 March
2022, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
groupcomm-oscore-13.txt>.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
draft-ietf-ace-oauth-authz-46, 8 November 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-46.txt>.
[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", Work in Progress,
Internet-Draft, draft-ietf-ace-oauth-params-16, 7
September 2021, <https://www.ietf.org/archive/id/draft-
ietf-ace-oauth-params-16.txt>.
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[I-D.ietf-ace-oscore-profile]
Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
"OSCORE Profile of the Authentication and Authorization
for Constrained Environments Framework", Work in Progress,
Internet-Draft, draft-ietf-ace-oscore-profile-19, 6 May
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
oscore-profile-19.txt>.
[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-
06, 7 March 2022, <https://www.ietf.org/archive/id/draft-
ietf-core-groupcomm-bis-06.txt>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group OSCORE - Secure Group Communication
for CoAP", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-groupcomm-14, 7 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-core-oscore-
groupcomm-14.txt>.
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-algs-12.txt>.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-struct-15.txt>.
[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>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/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/info/rfc5869>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
<https://www.rfc-editor.org/info/rfc6920>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[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/info/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/info/rfc8613>.
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[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
11.2. Informative References
[I-D.ietf-ace-dtls-authorize]
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)", Work in Progress,
Internet-Draft, draft-ietf-ace-dtls-authorize-18, 4 June
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
dtls-authorize-18.txt>.
[I-D.ietf-ace-mqtt-tls-profile]
Sengul, C. and A. Kirby, "Message Queuing Telemetry
Transport (MQTT)-TLS profile of Authentication and
Authorization for Constrained Environments (ACE)
Framework", Work in Progress, Internet-Draft, draft-ietf-
ace-mqtt-tls-profile-15, 1 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-ace-mqtt-tls-
profile-15.txt>.
[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-
ietf-cose-cbor-encoded-cert-03, 10 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
encoded-cert-03.txt>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
drafts/draft-ietf-tls-dtls13-43.txt>.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsuess, C., and P. V. D. Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
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Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-11, 7 March 2022,
<https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
discovery-11.txt>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Appendix A. Dual Mode (Group OSCORE & OSCORE)
This appendix defines the dual mode of this profile, which allows
using both OSCORE [RFC8613] and Group OSCORE
[I-D.ietf-core-oscore-groupcomm] as security protocols, by still
relying on a single Access Token.
That is, the dual mode of this profile specifies how a Client uses
CoAP [RFC7252] to communicate to a single Resource Server, or CoAP
over IP multicast [I-D.ietf-core-groupcomm-bis] to communicate to
multiple Resource Servers that are members of a group and share a
common set of resources.
In particular, the dual mode of this profile uses two complementary
security protocols to provide secure communication between the Client
and the Resource Server(s). That is, it defines the use of either
OSCORE or Group OSCORE to protect unicast requests addressed to a
single Resource Server, as well as possible responses. Additionally,
it defines the use of Group OSCORE to protect multicast requests sent
to a group of Resource Servers, as well as possible individual
responses. Like in the main mode of this profile, the Client and the
Resource Servers need to have already joined the same OSCORE group,
for instance by using the approach defined in
[I-D.ietf-ace-key-groupcomm-oscore], which is also based on ACE.
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The Client proves its access to be authorized to the Resource Server
by using an Access Token, which is bound to a key (the proof-of-
possession key). This profile mode uses OSCORE to achieve proof of
possession, and OSCORE or Group OSCORE to achieve server
authentication.
Unlike in the main mode of this profile, where a public key is used
as pop-key, this dual mode uses OSCORE-related, symmetric keying
material as pop-key instead. Furthermore, this dual mode provides
proof of Client's membership to the correct OSCORE group, by securely
binding the pre-established Group OSCORE Security Context to the
pairwise OSCORE Security Context newly established between the Client
and the Resource Server.
In addition to the terminology used for the main mode of this
profile, the rest of this appendix refers also to "pairwise OSCORE
Security Context" as to an OSCORE Security Context established
between only one Client and one Resource Server, and used to
communicate with OSCORE [RFC8613].
A.1. Protocol Overview
This section provides an overview on how to use the ACE framework for
authentication and authorization [I-D.ietf-ace-oauth-authz] to secure
communications between a Client and a (set of) Resource Server(s)
using OSCORE [RFC8613] and/or Group OSCORE
[I-D.ietf-core-oscore-groupcomm].
Just as for main mode of this profile overviewed in Section 2, the
process for 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 the rest of this section, and
is out of the scope of this profile.
An overview of the protocol flow for the dual mode of this profile is
shown in Figure 8. In the figure, it is assumed that both RS1 and
RS2 are associated with the same AS. It is also assumed that C, RS1
and RS2 have previously joined an OSCORE group with Group Identifier
(gid) "abcd0000", and got assigned Sender ID (sid) "0", "1" and "2"
in the group, respectively. The names of messages coincide with
those of [I-D.ietf-ace-oauth-authz] when applicable.
C RS1 RS2 AS
| [--- Resource Request -->] | | |
| | | |
| [<---- AS Request ------] | | |
| Creation Hints | | |
| | | |
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|-------- POST /token ----------------------------------------------->|
| (aud: RS1, sid: 0, gid: abcd0000, ...) | |
| | | |
|<-------------------------------- Access Token + RS Information -----|
| | (aud: RS1, sid: 0, gid: abcd0000, ...) |
| | | |
|---- POST /authz-info ----->| | |
| (access_token, N1, ID1) | | |
| | | |
|<-- 2.01 Created (N2, ID2) -| | |
| | | |
/Pairwise /Pairwise | |
Security Context Security Context | |
Derivation/ Derivation/ | |
| | | |
|-------- POST /token ----------------------------------------------->|
| (aud: RS2, sid: 0, gid: abcd0000, ...) | |
| | | |
|<-------------------------------- Access Token + RS Information -----|
| | (aud: RS2, sid: 0, gid: abcd0000, ...) |
| | | |
|---- POST /authz-info ------------------>| |
| (access_token, N1', ID1') | | |
| | | |
|<-- 2.01 Created (N2', ID2')-------------| |
| | | |
/Pairwise Security | /Pairwise Security |
Context Derivation/ | Context Derivation/ |
| | | |
|----- OSCORE Request ------>| | |
| (kid: ID2) | | |
| | | |
| | | |
| | | |
| | | |
| /Proof-of-possession; | |
| Pairwise Security | |
| Context storage/ | |
| | | |
|<---- OSCORE Response ------| | |
| | | |
/Proof-of-possession; | | |
Pairwise Security | | |
Context storage/ | | |
| | | |
/Mutual authentication | | |
between C and RS1 | | |
(as OSCORE peers)/ | | |
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| | | |
| | | |
|- Group OSCORE Request -+-->| | |
| (kid: 0, gid: abcd0000) \-------------->| |
| | | |
|<-- Group OSCORE Response --| | |
| (kid: 1) | | |
| | | |
/Mutual authentication | | |
between C and RS1 | | |
(as group members)/ | | |
| | | |
|<-- Group OSCORE Response ---------------| |
| (kid: 2) | | |
| | | |
/Mutual authentication | | |
between C and RS2 | | |
(as group members)/ | | |
| | | |
| ... | | |
Figure 8: Protocol Overview.
A.1.1. Pre-Conditions
The same pre-conditions for the main mode of this profile (see
Section 2.1) hold for the dual mode described in this appendix.
A.1.2. Access Token Posting
After having retrieved the Access Token from the AS, the Client
generates a nonce N1 and an identifier ID1 unique in the sets of its
own Recipient IDs from its pairwise OSCORE Security Contexts. The
client then posts both the Access Token, N1 and its chosen ID to the
RS, using the /authz-info endpoint and mechanisms specified in
Section 5.10 of [I-D.ietf-ace-oauth-authz] and Content-Format =
application/ace+cbor.
When using the dual mode of this profile, the communication with the
authz-info endpoint is not protected, except for update of access
rights. Note that, when using the dual mode, this request can
alternatively be protected with Group OSCORE, using the Group OSCORE
Security Context paired with the pairwise OSCORE Security Context
originally established with the first Access Token posting.
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If the Access Token is valid, the RS replies to this POST request
with a 2.01 (Created) response with Content-Format = application/
ace+cbor, which in a CBOR map contains a nonce N2 and an identifier
ID2 unique in the sets of its own Recipient IDs from its pairwise
OSCORE Security Contexts.
A.1.3. Setup of the Pairwise OSCORE Security Context
After sending the 2.01 (Created) response, the RS sets the ID Context
of the pairwise OSCORE Security Context (see Section 3 of [RFC8613])
to the Group Identifier of the OSCORE group specified in the Access
Token, concatenated with N1, concatenated with N2, concatenated with
the value in the contextId parameter of the OSCORE_Input_Material
provided in the 'cnf' claim of the Access Token.
Then, the RS derives the complete pairwise OSCORE Security Context
associated with the received 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, and the
authorization information is a policy used as input when processing
requests from those clients.
During the derivation process, the RS uses: the ID Context above; the
exchanged nonces N1 and N2; the identifier ID1 received from the
Client, set as its own OSCORE Sender ID; the identifier ID2 provided
to the Client, set as its Recipient ID for the Client; and the
parameters in the Access Token. The derivation process uses also the
Master Secret of the OSCORE group, that the RS knows as a group
member, as well as the Sender ID of the Client in the OSCORE group,
which is specified in the Access Token. This ensures that the
pairwise OSCORE Security Context is securely bound to the Group
OSCORE Security Context of the OSCORE group.
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.
After having received the nonce N2, the Client sets the ID Context in
its pairwise OSCORE Security Context (see Section 3 of [RFC8613]) to
the Group Identifier of the OSCORE group, concatenated with N1,
concatenated with N2, concatenated with the value in the contextId
parameter of the OSCORE_Input_Material provided in the 'cnf'
parameter of the Access Token response from the AS. Then, the Client
derives the complete pairwise OSCORE Security Context, following
Section 3.2 of [RFC8613].
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During the derivation process, the Client uses: the ID Context above,
the exchanged nonces N1 and N2; the identifier ID1 provided to the
RS, set as its own Recipient ID for the RS; the identifier ID2
received from the RS, set as its own OSCORE Sender ID; and the
parameters received from the AS. The derivation process uses also
the Master Secret of the OSCORE group, that the Client knows as a
group member, as well as its own Sender ID in the OSCORE group.
When the Client communicates with the RS using the pairwise OSCORE
Security Context, the RS achieves proof-of-possession of the
credentials bound to the Access Token. Also, the RS verifies that
the Client is a legitimate member of the OSCORE group.
A.1.4. Secure Communication
Other than starting the communication with the RS using Group OSCORE
as described in Section 4.3, the Client can send to the RS a request
protected with OSCORE, using the pairwise OSCORE Security Context.
If the request is successfully verified, then the RS stores the
pairwise OSCORE Security Context, and uses it to protect the possible
response, as well as further communications with the Client, until
the Access Token is deleted, due to, for example, expiration. This
pairwise OSCORE Security Context is discarded when an Access Token
(whether the same or different) is used to successfully derive a new
pairwise OSCORE Security Context.
As discussed in Section 7 of [I-D.ietf-ace-oscore-profile], the use
of random nonces N1 and N2 during the exchange between the Client and
the RS prevents the reuse of an Authenticated Encryption with
Associated Data (AEAD) nonce/key pair for two different messages.
Reuse might otherwise occur when Client and RS derive a new pairwise
OSCORE Security Context from an existing (non-expired) Access Token,
e.g., in case of reboot of either the Client or the RS, and might
lead to loss of both confidentiality and integrity.
Additionally, just as per the main mode of this profile (see
Section 4.3), the Client and RS can also securely communicate by
protecting messages with Group OSCORE, using the Group OSCORE
Security Context already established upon joining the OSCORE group.
A.2. 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.
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Section 3.2 of [RFC8613] defines how to derive a pairwise OSCORE
Security Context based on a shared Master Secret and a set of other
parameters, established between the OSCORE client and server, which
the client receives from the AS in this exchange.
The proof-of-possession key (pop-key) received from the AS in this
exchange MUST be used to build the Master Secret in OSCORE (see
Appendix A.3.3 and Appendix A.3.4).
A.2.1. C-to-AS: POST to Token Endpoint
The Client-to-AS request is specified in Section 5.8.1 of
[I-D.ietf-ace-oauth-authz]. 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 main mode of this profile (see Section 3.1), with the
following modifications.
* The parameter 'req_cnf' MUST NOT be included in the payload.
* The parameter 'client_cred', defined in Appendix A.2.1.1 of this
document, MUST be included in the payload. This parameter
specifies the public key that the Client uses in the OSCORE group,
whose identifier is indicated in the 'context_id' parameter. In
particular, the specified public key is the COSE Key equivalent to
the authentication credential that the Client uses in the OSCORE
group.
* The proof-of-possession (PoP) evidence included in the
'client_cred_verify' or 'client_cred_verify_mac' parameter is
computed by using the Client's private key associated with the
public key in the 'client_cred' parameter above.
An example of such a request is shown in Figure 9.
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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',
"client_cred" : {
"COSE_Key" : {
"kty" : EC2,
"crv" : P-256,
"x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
731e79a3b4e47120'
}
},
"client_cred_verify" : h'...'
(signature content omitted for brevity),
}
Figure 9: Example C-to-AS POST /token request for an Access Token
bound to a symmetric key.
Later on, the Client may want to update its current access rights,
without changing the existing pairwise OSCORE Security Context with
the RS. In this case, the Client MUST include in its POST request to
the /token endpoint a 'req_cnf' parameter, defined in Section 3.1 of
[I-D.ietf-ace-oauth-params], which MUST include a 'kid' field, as
defined in Section 3.1 of [RFC8747]. The 'kid' field has as value a
CBOR byte string containing the OSCORE_Input_Material Identifier
(assigned as discussed in Appendix A.2.2).
This identifier, together with other information such as audience,
can be used by the AS to determine the shared secret bound to the
proof-of-possession Access Token and therefore MUST identify a
symmetric key that was previously generated by the AS as a shared
secret for the communication between the Client and the RS. The AS
MUST verify that the received value identifies a proof-of-possession
key that has previously been issued to the requesting Client. If
that is not the case, the Client-to-AS request MUST be declined with
the error code "invalid_request" as defined in Section 5.8.3 of
[I-D.ietf-ace-oauth-authz].
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This POST request for updating the access rights of an Access Token
SHOULD NOT include the parameters 'salt_input', 'context_id',
'client_cred' and 'client_cred_verify'. An exception is the case
defined in Appendix A.3.6, where the Client, following a change of
authentication credential in the OSCORE group, requests a new Access
Token associated with the public key of the new authentication
credential, while still without changing the existing pairwise OSCORE
Security Context with the RS.
An example of such a request is shown in Figure 10.
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"audience" : "tempSensor4711",
"scope" : "read",
"req_cnf" : {
"kid" : h'01'
}
}
Figure 10: Example C-to-AS POST /token request for updating
rights to an Access Token bound to a symmetric key.
A.2.1.1. 'client_cred' Parameter
The 'client_cred' parameter is an OPTIONAL parameter of the Access
Token request message defined in Section 5.8.1. of
[I-D.ietf-ace-oauth-authz]. This parameter provides an asymmetric
key that the Client wishes to use as its own public key, but which is
not used as proof-of-possession key.
This parameter follows the syntax of the 'cnf' claim from Section 3.1
of [RFC8747] when including Value Type "COSE_Key" (1) and specifying
an asymmetric key. Alternative Value Types defined in future
specifications are fine to consider if indicating a non-encrypted
asymmetric key.
A.2.2. 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.
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In particular, the AS proceeds as defined in Section 3.2, with the
difference that it uses the public key specified in the 'client_cred'
parameter as public key of the Client.
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. The AS does not
perform this operation when asked to update a previously released
Access Token.
If all verifications are successful, the AS responds as defined in
Section 5.8.2 of [I-D.ietf-ace-oauth-authz]. If the Client request
was invalid, or not authorized, the AS returns an error response as
described in Section 5.8.3 of [I-D.ietf-ace-oauth-authz].
The AS can signal that the use of OSCORE and 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.
This means that the Client MUST use OSCORE and/or Group OSCORE
towards all the Resource Servers for which this Access Token is
valid.
In particular, the Client MUST follow Appendix A.3.3 to derive the
pairwise OSCORE Security Context to use for communications with the
RS. Instead, the Client has already established the related Group
OSCORE Security Context to communicate with members of the OSCORE
group, upon previously joining that group.
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.
In contrast with the main mode of this profile, the Access Token
response to the Client is analogous to the one in the OSCORE profile
of ACE, as described in Section 3.2 of [I-D.ietf-ace-oscore-profile].
In particular, the AS provides an OSCORE_Input_Material object, which
is defined in Section 3.2.1 of [I-D.ietf-ace-oscore-profile] and
included in the 'cnf' parameter (see Section 3.2 of
[I-D.ietf-ace-oauth-params]) of the Access Token response.
The AS MUST send different OSCORE_Input_Material (and therefore
different Access Tokens) to different authorized clients, in order
for the RS to differentiate between clients.
In the issued Access Token, the AS MUST include as metadata the same
information as defined in the main mode of this profile (see
Section 3.2) with the following modifications.
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* The public key that the client uses in the OSCORE group and
specified in the 'client_cred' parameter of the Token request (see
Appendix A.2.1) MUST also be included in the Access Token.
If the Access Token is a CWT, the AS MUST include it in the
'client_cred' claim of the Access Token, defined in
Appendix A.2.2.2 of this document. In particular, the
'client_cred' claim includes the same COSE Key specified in the
'client_cred' parameter of the Token Request, i.e., the COSE Key
equivalent to the authentication credential that the Client uses
in the OSCORE group.
* The OSCORE_Input_Material specified in the 'cnf' parameter of the
Access Token response MUST also be included in the Access Token.
If the Access Token is a CWT, the same OSCORE_Input_Material
included in the 'cnf' parameter of the Access Token response MUST
be included in the 'osc' field of the 'cnf' claim of the Access
Token (see Section 3.2 of [I-D.ietf-ace-oscore-profile]).
Figure 11 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,
"cnf" : {
"osc" : {
"alg" : AES-CCM-16-64-128,
"id" : h'01',
"ms" : h'f9af838368e353e78888e1426bd94e6f',
"salt" : h'1122',
"contextId" : h'99'
}
}
}
Figure 11: Example AS-to-C Access Token response with the Group
OSCORE profile.
Figure 12 shows an example CWT, containing the necessary OSCORE
parameters in the 'cnf' claim.
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{
"aud" : "tempSensorInLivingRoom",
"iat" : 1360189224,
"exp" : 1360289224,
"scope" : "temperature_g firmware_p",
"cnf" : {
"osc" : {
"alg" : AES-CCM-16-64-128,
"id" : h'01',
"ms" : h'f9af838368e353e78888e1426bd94e6f',
"salt" : h'1122',
"contextId" : h'99'
},
"salt_input" : h'00',
"contextId_input" : h'abcd0000',
"client_cred" : {
"COSE_Key" : {
"kty" : EC2,
"crv" : P-256,
"x" : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa
27c9e354089bbe13',
"y" : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020
731e79a3b4e47120'
}
}
}
Figure 12: Example CWT with OSCORE parameters.
The same CWT as in Figure 12 and encoded in CBOR is shown in
Figure 13, using the value abbreviations defined in
[I-D.ietf-ace-oauth-authz] and [RFC8747].
NOTE: it should be checked (and in case fixed) that the values used
below (which are not yet registered) are the final values registered
in IANA.
A8 # map(8)
03 # unsigned(3)
76 # text(22)
74656D7053656E736F72496E4C6976696E67526F6F6D
06 # unsigned(6)
1A 5112D728 # unsigned(1360189224)
04 # unsigned(4)
1A 51145DC8 # unsigned(1360289224)
09 # unsigned(9)
78 18 # text(24)
74656D70657261747572655F67206669726D776172655F70
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08 # unsigned(8)
A1 # map(1)
04 # unsigned(4)
A5 # map(5)
04 # unsigned(4)
0A # unsigned(10)
00 # unsigned(0)
41 # bytes(1)
01 # "\x01"
02 # unsigned(2)
50 # bytes(16)
F9AF838368E353E78888E1426BD94E6F
05 # unsigned(5)
42 # bytes(2)
1122 # "\x11\""
06 # unsigned(6)
41 # bytes(1)
99 # "\x99"
18 3C # unsigned(60)
41 # bytes(1)
00
18 3D # unsigned(61)
44 # bytes(4)
ABCD0000
18 3E # unsigned(62)
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)
D7CC072DE2205BDC1537A543D53C60A6ACB62ECCD890C7FA27C9
E354089BBE13
22 # negative(2)
58 20 # bytes(32)
F95E1D4B851A2CC80FFF87D8E23F22AFB725D535E515D020731E
79A3B4E47120
Figure 13: Example CWT with OSCORE parameters.
If the Client has requested an update to its access rights using the
same pairwise OSCORE Security Context, which is valid and authorized,
the AS MUST omit the 'cnf' parameter in the response to the client.
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Instead, the updated Access Token conveyed in the AS-to-C response
MUST include a 'cnf' claim specifying a 'kid' field, as defined in
Section 3.1 of [RFC8747]. The response from the AS MUST carry the
OSCORE Input Material identifier in the 'kid' field within the 'cnf'
claim of the Access Token. That is, the 'kid' field is a CBOR byte
string, with value the same value of the 'kid' field of the 'req_cnf'
parameter from the C-to-AS request for updating rights to the Access
Token (see Figure 10). This information needs to be included in the
Access Token, in order for the RS to identify the previously
generated pairwise OSCORE Security Context.
Figure 14 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),
"profile" : "coap_group_oscore",
"expires_in" : 3600
}
Figure 14: Example AS-to-C Access Token response with the Group
OSCORE profile, for update of access rights.
Figure 15 shows an example CWT, containing the necessary OSCORE
parameters in the 'cnf' claim for update of access rights.
{
"aud" : "tempSensorInLivingRoom",
"iat" : 1360189224,
"exp" : 1360289224,
"scope" : "temperature_h",
"cnf" : {
"kid" : h'01'
}
}
Figure 15: Example CWT with OSCORE parameters for update of
access rights.
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A.2.2.1. Public Key Hash as Client Credential
As a possible optimization to limit the size of the Access Token, the
AS may specify as value of the 'client_cred' claim simply the hash of
the Client's public key, i.e., the hash of the COSE Key K equivalent
to the authentication credential that the Client uses in the OSCORE
group.
The specifically used hash-function MUST be collision-resistant on
byte-strings, and MUST be selected from the "Named Information Hash
Algorithm" Registry defined in Section 9.4 of [RFC6920].
In particular, the AS provides the Client with an Access Token as
defined in Appendix A.2.2, with the following differences.
The AS prepares INPUT_HASH as follows, with | denoting byte string
concatenation.
* If the equivalent COSE Key K has COSE Key Type OKP, INPUT_HASH =
i, where 'i' is the x-parameter of the COSE_Key specified in the
'client_cred' parameter of the Token request, encoded as a CBOR
byte string.
* If the equivalent COSE Key K has COSE Key Type EC2, INPUT_HASH =
(i_1 | i_2), where 'i_1' and 'i_2' are the x-parameter and
y-parameter of the COSE_Key specified in the 'client_cred'
parameter of the Token request, respectively, each encoded as a
CBOR byte string.
* If the equivalent COSE Key K has COSE Key Type RSA, INPUT_HASH =
(i_1 | i_2), where 'i_1' and 'i_2' are the n-parameter and
e-parameter of the COSE_Key specified in the 'client_cred'
parameter of the Token request, respectively, each encoded as a
CBOR byte string.
Then, the AS hashes INPUT_HASH according to the procedure described
in [RFC6920], with the output OUTPUT_HASH in binary format, as
described in Section 6 of [RFC6920].
Finally, the AS includes a single entry within the 'client_cred'
claim of the Access Token. This entry has label "kid" (3) defined in
Section 3.1 of [RFC8747], and value a CBOR byte string wrapping
OUTPUT_HASH.
Upon receiving the Access Token, the RS processes it according to
Appendix A.3.2, with the following differences.
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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 content of the
'client_cred' claim as the hash RECEIVED_HASH of a COSE Key
equivalent to the authentication credential that the Client uses in
the group.
The RS MUST check whether it already stores an authentication
credential associated with the pair (GID, Sender ID) above, such that
the recomputed hash NEW_HASH of its equivalent COSE Key matches with
RECEIVED_HASH from the 'client_cred' claim.
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 10 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 RECEIVED_HASH matches with the
recomputed hash NEW_HASH of a COSE Key equivalent to the Client's
authentication credential retrieved from the Group Manager.
* The RS MUST check that 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.
The RS MUST calculate NEW_HASH using the same method used by the AS
described above, and using the same hash function. The hash function
to use can be determined from the information conveyed in the
'client_cred' claim, as the procedure described in [RFC6920] also
encodes the used hash function as metadata of the hash value.
A.2.2.2. Client Credential Claim
The 'client_cred' claim provides an asymmetric key that the Client
owning the Access Token wishes to use as its own public key, but
which is not used as proof-of-possession key.
This parameter follows the syntax of the 'cnf' claim from Section 3.1
of [RFC8747] when including Value Type "COSE_Key" (1) and specifying
an asymmetric key. Alternative Value Types defined in future
specifications are fine to consider, if indicating a non-encrypted
asymmetric key or full-fledged autentication credential.
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A.3. Client-RS Communication
This section details the POST request and response to the /authz-info
endpoint between the Client and the RS. With respect to the
exchanged messages and their content, the Client and the RS perform
as defined in the OSCORE profile of ACE (see Section 4 of
[I-D.ietf-ace-oscore-profile]).
That is, the Client generates a nonce N1 and posts it to the RS,
together with: an identifier ID1 unique in the sets of its own
Recipient IDs from its pairwise OSCORE Security Contexts; and the
Access Token that includes the material provisioned by the AS.
Then, the RS generates a nonce N2, and an identifier ID2 unique in
the sets of its own Recipient IDs from its pairwise OSCORE Security
Contexts. After that, the RS derives a pairwise OSCORE Security
Context as described in Section 3.2 of [RFC8613]. In particular, it
uses the two exchanged nonces and the two identifiers established
with the Client, as well as two shared secrets together with
additional pieces of information specified in the Access Token.
Both the client and the RS generate the pairwise OSCORE Security
Context using the pop-key as part of the OSCORE Master Secret. In
addition, the derivation of the pairwise OSCORE Security Context
takes as input also information related to the OSCORE group, i.e.,
the Master Secret and Group Identifier of the group, as well as the
Sender ID of the Client in the group. Hence, the derived pairwise
OSCORE Security Context is also securely bound to the Group OSCORE
Security Context of the OSCORE Group. Thus, the proof-of-possession
required to bind the Access Token to the Client occurs after the
first OSCORE message exchange.
Therefore, an attacker using a stolen Access Token cannot generate a
valid pairwise OSCORE Security Context and thus cannot prove
possession of the pop-key. Also, if a Client legitimately owns an
Access Token but has not joined the OSCORE group, that Client cannot
generate a valid pairwise OSCORE Security Context either, since it
lacks the Master Secret used in the OSCORE group.
Besides, just as in the main mode (see Section 4), the RS is able to
verify whether the Client has indeed the claimed Sender ID and
authentication credential in the OSCORE group.
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A.3.1. C-to-RS POST to authz-info Endpoint
The Client MUST generate a nonce N1, an OSCORE Recipient ID (ID1),
and post them to the /authz-info endpoint of the RS together with the
Access Token, as defined in the OSCORE profile of ACE (see
Section 4.1 of [I-D.ietf-ace-oscore-profile]).
The same recommendations, considerations and behaviors defined in
Section 4.1 of [I-D.ietf-ace-oscore-profile] hold.
If the Client has already posted a valid Access Token, has already
established a pairwise OSCORE Security Context with the RS, and wants
to update its access rights, the Client can do so by posting the new
Access Token (retrieved from the AS and specifying the updated set of
access rights) to the /authz-info endpoint.
The Client MUST protect the request using either the pairwise OSCORE
Security Context established during the first Access Token exchange,
or the Group OSCORE Security Context associated with that pairwise
OSCORE Security Context.
In either case, the Client MUST only send the Access Token in the
payload, i.e., no nonce or identifier are sent. After proper
verification (see Section 4.2 of [I-D.ietf-ace-oscore-profile]), the
new Access Token will supersede the old one at the RS, by replacing
the corresponding authorization information. At the same time, the
RS will maintain the same pairwise OSCORE Security Context and Group
OSCORE Security Context, as now both associated with the new Access
Token.
A.3.2. RS-to-C: 2.01 (Created)
The RS MUST verify the validity of the Access Token as defined in
Section 4.2, with the following modifications.
* If the POST request to /authz-info is not protected, the RS checks
that the 'cnf' claim is included in the Access Token and that it
contains an OSCORE_Input_Material object. Also, the RS checks
that the 'salt_input', 'client_cred' and 'contextId_input' claims
are included in the Access Token.
* If the POST request to /authz-info is protected with the pairwise
OSCORE Security Context shared with the Client or with the Group
OSCORE Security Context of the OSCORE group, i.e., the Client is
requesting an update of access rights, the RS checks that the
'cnf' claim is included in the Access Token and that it contains
only the 'kid' field.
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* If the 'salt_input', 'client_cred' and 'contextId_input' claims
are included in the Access Token, the RS extracts the content of
'client_cred'. Then, the RS considers that content as the COSE
Key equivalent to the authentication credential that the Client
uses in the group, whose GID is specified in the 'contextId_input'
claim. The RS can compare this public key with the actual COSE
Key equivalent to the authentication credential of the claimed
Client, retrieved from its local storage or from the Group Manager
(see Section 4.2).
If any of the checks fails, the RS MUST consider the Access Token non
valid, and MUST respond 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 proceeds as follows.
In case the POST request to /authz-info was not protected, the RS
MUST generate a nonce N2, an OSCORE Recipient ID (ID2), and include
them in the 2.01 (Created) response to the Client, as defined in the
OSCORE profile of ACE (see Section 4.2 of
[I-D.ietf-ace-oscore-profile]).
Instead, in case the POST request to /authz-info was protected, the
RS MUST ignore any nonce and identifiers in the request, if any was
sent. Then, the RS MUST check that the 'kid' field of the 'cnf'
claim in the new Access Token matches the identifier of the OSCORE
Input Material of a pairwise OSCORE Security Context such that:
* The pairwise OSCORE Security Context was used to protect the
request, if this was protected with OSCORE; or
* The pairwise OSCORE Security Context is bound to the Group OSCORE
Security Context used to protect the request, if this was
protected with Group OSCORE.
If either verification is successful, the new Access Token supersedes
the old one at the RS. Besides, the RS associates the new Access
Token to the same pairwise OSCORE Security Context identified above,
as also bound to a Group OSCORE Security Context. The RS MUST
respond with a 2.01 (Created) response with no payload, protected
with the same Security Context used to protect the request. In
particular, no new pairwise OSCORE Security Context is established
between the Client and the RS. If any verification fails, the RS
MUST respond with a 4.01 (Unauthorized) error response.
Further recommendations, considerations and behaviors defined in
Section 4.2 of [I-D.ietf-ace-oscore-profile] hold for this document.
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A.3.3. OSCORE Setup - Client Side
Once having received the 2.01 (Created) response from the RS,
following an unprotected POST request to the /authz-info endpoint,
the Client MUST extract the nonce N2 from the 'nonce2' parameter, and
the Client identifier from the 'ace_server_recipientid' parameter in
the CBOR map of the response payload. Note that this identifier is
used by C as Sender ID in the pairwise OSCORE Security Context to be
established with the RS, and is different as well as unrelated to the
Sender ID of C in the OSCORE group.
Then, the Client performs the following actions, in order to set up
and fully derive the pairwise OSCORE Security Context for
communicating with the RS.
* The Client MUST set the ID Context of the pairwise OSCORE Security
Context as the concatenation of: i) GID, i.e., the Group
Identifier of the OSCORE group, as specified by the Client in the
'context_id' parameter of the Client-to-AS request; ii) the nonce
N1; iii) the nonce N2; and iv) CID, i.e., the value in the
contextId parameter of the OSCORE_Input_Material provided in the
'cnf' parameter of the Access Token response from the AS. The
concatenation occurs in this order: ID Context = GID | N1 | N2 |
CID, where | denotes byte string concatenation.
* The Client MUST set the updated Master Salt of the pairwise OSCORE
Security Context as the concatenation of SaltInput, MSalt, the
nonce N1, the nonce N2 and GMSalt, where: i) SaltInput is the
Sender ID that the Client has in the OSCORE group, which is known
to the Client as a member of the OSCORE group; ii) MSalt is the
(optional) Master Salt in the pairwise OSCORE Security Context
(received from the AS in the Token); and iii) GMSalt is the
(optional) Master Salt in the Group OSCORE Security Context, which
is known to the Client as a member of the OSCORE group. The
concatenation occurs in this order: Master Salt = SaltInput |
MSalt | N1 | N2 | GMSalt, where | denotes byte string
concatenation. Optional values, if not specified, are not
included in the concatenation. The five parameters SaltInput,
MSalt, N1, N2 and GMSalt are to be concatenated as encoded CBOR
byte strings. An example of Master Salt construction using CBOR
encoding is given in Figure 16.
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SaltInput, MSalt, N1, N2 and GMSalt, in CBOR diagnostic notation:
SaltInput = h'00'
MSalt = h'f9af838368e353e78888e1426bd94e6f'
N1 = h'018a278f7faab55a'
N2 = h'25a8991cd700ac01'
GMSalt = h'99'
SaltInput, MSalt, N1, N2 and GMSalt, as CBOR encoded byte strings:
SaltInput = 0x4100
MSalt = 0x50f9af838368e353e78888e1426bd94e6f
N1 = 0x48018a278f7faab55a
N2 = 0x4825a8991cd700ac01
GMSalt = 0x4199
Master Salt = 0x41 00
50 f9af838368e353e78888e1426bd94e6f
48 018a278f7faab55a
48 25a8991cd700ac01
41 99
Figure 16: Example of Master Salt construction using CBOR encoding.
* The Client MUST set the Master Secret of the pairwise OSCORE
Security Context to the concatenation of MSec and GMSec, where: i)
MSec is the value of the 'ms' parameter in the
OSCORE_Input_Material of the 'cnf' parameter, received from the AS
in Appendix A.2.2; while ii) GMSec is the Master Secret of the
Group OSCORE Security Context, which is known to the Client as a
member of the OSCORE group.
* The Client MUST set the Recipient ID as ace_client_recipientid,
sent as described in Appendix A.3.1.
* The Client MUST set the Sender ID as ace_server_recipientid,
received as described in Appendix A.3.1.
* The Client MUST set the AEAD Algorithm, ID Context, HKDF, and
OSCORE Version as indicated in the corresponding parameters
received from the AS in Appendix A.2.2, if present in the
OSCORE_Input_Material of the 'cnf' parameter. In case these
parameters are omitted, the default values SHALL be used as
described in Sections 3.2 and 5.4 of [RFC8613].
Finally, the client MUST derive the complete pairwise OSCORE Security
Context following Section 3.2.1 of [RFC8613].
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From then on, when communicating with the RS to access the resources
as specified by the authorization information, the Client MUST use
the newly established pairwise OSCORE Security Context or the Group
OSCORE Security Context of the OSCORE Group where both the Client and
the RS are members.
If any of the expected parameters is missing (e.g., any of the
mandatory parameters from the AS or the RS), or if
ace_client_recipientid equals ace_server_recipientid (and as a
consequence the Sender and Recipient Keys derived would be equal, see
Section 3.3 of [RFC8613]), then the client MUST stop the exchange,
and MUST NOT derive the pairwise OSCORE Security Context. The Client
MAY restart the exchange, to get the correct security input material.
The Client can use this pairwise OSCORE Security Context to send
requests to the RS protected with OSCORE. Besides, the Client can
use the Group OSCORE Security Context for protecting unicast requests
to the RS, or multicast requests to the OSCORE group including also
the RS. Mutual authentication as group members is only achieved
after the client has successfully verified the Group OSCORE protected
response from the RS. Similarly, mutual authentication as OSCORE
peers is only achieved after the client has successfully verified the
OSCORE protected response from the RS, using the pairwise OSCORE
Security Context.
Note that the ID Context of the pairwise OSCORE Security Context can
be assigned by the AS, communicated and set in both the RS and Client
after the exchange specified in this profile is executed.
Subsequently, the Client and RS can update their ID Context by
running a mechanism such as the one defined in Appendix B.2 of
[RFC8613] if they both support it and are configured to do so. In
that case, the ID Context in the pairwise OSCORE Security Context
will not match the "contextId" parameter of the corresponding
OSCORE_Input_Material. Running the procedure in Appendix B.2 of
[RFC8613] results in the keying material in the pairwise OSCORE
Security Contexts of the Client and RS being updated. The Client can
achieve the same result by re-posting the Access Token to the
unprotected /authz-info endpoint at the RS, as described in
Section 4.1 of [I-D.ietf-ace-oscore-profile], although without
updating the ID Context.
A.3.4. OSCORE Setup - Resource Server Side
After validation of the Access Token as defined in Appendix A.3.2 and
after sending the 2.01 (Created) response to an unprotected POST
request to the /authz-info endpoint, the RS performs the following
actions, in order to set up and fully derive the pairwise OSCORE
Security Context created to communicate with the Client.
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* The RS MUST set the ID Context of the pairwise OSCORE Security
Context as the concatenation of: i) GID, i.e., the Group
Identifier of the OSCORE group, as specified in the 'contextId'
parameter of the OSCORE_Input_Material, in the 'cnf' claim of the
Access Token received from the Client (see Appendix A.3.1); ii)
the nonce N1; iii) the nonce N2; and iv) CID which is the value in
the contextId parameter of the OSCORE_Input_Material provided in
the 'cnf' claim of the Access Token. The concatenation occurs in
this order: ID Context = GID | N1 | N2 | CID, where | denotes byte
string concatenation.
* The RS MUST set the new Master Salt of the pairwise OSCORE
Security Context as the concatenation of SaltInput, MSalt, the
nonce N1, the nonce N2 and GMSalt, where: i) SaltInput is the
Sender ID that the Client has in the OSCORE group, as specified in
the 'salt_input' claim included in the Access Token received from
the Client (see Appendix A.3.1); ii) MSalt is the (optional)
Master Salt in the pairwise OSCORE Security Context as specified
in the 'salt' parameter in the OSCORE_Input_Material of the 'cnf'
claim, included in the Access Token received from the Client; and
iii) GMSalt is the (optional) Master Salt in the Group OSCORE
Security Context, which is known to the RS as a member of the
OSCORE group. The concatenation occurs in this order: Master Salt
= SaltInput | MSalt | N1 | N2 | GMSalt, where | denotes byte
string concatenation. Optional values, if not specified, are not
included in the concatenation. The same considerations for
building the Master Salt, considering the inputs as encoded CBOR
byte strings as in Figure 16, hold also for the RS.
* The RS MUST set the Master Secret of the pairwise OSCORE Security
Context to the concatenation of MSec and GMSec, where: i) MSec is
the value of the 'ms' parameter in the OSCORE_Input_Material of
the 'cnf' claim, included in the Access Token received from the
Client (see Appendix A.3.1); while ii) GMSec is the Master Secret
of the Group OSCORE Security Context, which is known to the RS as
a member of the OSCORE group.
* The RS MUST set the Recipient ID as ace_server_recipientid, sent
as described in Appendix A.3.2.
* The RS MUST set the Sender ID as ace_client_recipientid, received
as described in Appendix A.3.2.
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* The RS MUST set the AEAD Algorithm, ID Context, HKDF, and OSCORE
Version from the corresponding parameters received from the Client
in the Access Token (see Appendix A.3.1), if present in the
OSCORE_Input_Material of the 'cnf' claim. In case these
parameters are omitted, the default values SHALL be used as
described in Sections 3.2 and 5.4 of [RFC8613].
Finally, the RS MUST derive the complete pairwise OSCORE Security
Context following Section 3.2.1 of [RFC8613].
Once having completed the derivation above, the RS MUST associate the
authorization information from the Access Token with the just
established pairwise OSCORE Security Context. Furthermore, as
defined in Section 4.2, the RS MUST associate the authorization
information from the Access Token with the Group OSCORE Security
Context.
Then, the RS uses this pairwise OSCORE Security Context to verify
requests from and send responses to the Client protected with OSCORE,
when this Security Context is used. If OSCORE verification fails,
error responses are used, as specified in Section 8 of [RFC8613].
Besides, the RS uses the Group OSCORE Security Context to verify
(multicast) requests from and send responses to the Client protected
with Group OSCORE. When processing an incoming request protected
with Group OSCORE, the RS MUST consider as valid authentication
credential of the Client only the authentication credential
associated with the stored Access Token. As defined in
Appendix A.3.6, a change of authentication credential in the group
requires the Client to upload to the RS a new Access Token, where the
'client_cred' claim specifies a COSE Key equivalent to the new
authentication credential that the Client has in the group.
If Group OSCORE verification fails, error responses are used, as
specified in Sections 8 and 9 of [I-D.ietf-core-oscore-groupcomm].
Additionally, for every incoming request, if OSCORE or Group OSCORE
verification succeeds, the verification of access rights is performed
as described in Appendix A.3.5.
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After the deletion of the Access Token related to a pairwise OSCORE
Security Context and to a Group OSCORE Security Context, due to, for
example, expiration, the RS MUST NOT use the pairwise OSCORE Security
Context. The RS MUST respond with an unprotected 4.01 (Unauthorized)
error message to received requests that correspond to a pairwise
OSCORE Security Context with a deleted Access Token. Also, if the
Client uses the Group OSCORE Security Context to send a request for
any resource intended for OSCORE group members and that requires an
active Access Token, the RS MUST respond with a 4.01 (Unauthorized)
error message protected with the Group OSCORE Security Context.
The same considerations, related to the value of the ID Context
changing, as in Appendix A.3.3 hold also for the RS.
A.3.5. Access Rights Verification
The RS MUST follow the procedures defined in Section 4.4.
Additionally, if the RS receives an OSCORE-protected request from a
Client, the RS processes it according to [RFC8613].
If the OSCORE verification succeeds, and the target resource requires
authorization, the RS retrieves the authorization information from
the Access Token associated with the pairwise OSCORE Security Context
and to the Group OSCORE Security Context. Then, the RS MUST verify
that the action requested on the resource is authorized.
The response code MUST be 4.01 (Unauthorized) if the RS has no valid
Access Token for the Client.
A.3.6. Change of Client's Authentication Credential in the Group
During its membership in the OSCORE group, the client might change
the authentication credential it uses in the group. When this
happens, the Client uploads the new authentication credential to the
Group Manager, as defined in Section 11 of
[I-D.ietf-ace-key-groupcomm-oscore].
After that, the Client may still have an Access Token previously
uploaded to the RS, which is not expired yet and still valid to the
best of the Client's knowledge. Then, 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
Appendix A.2.1 for the update of access rights, i.e., with the
'req_cnf' parameter including only a 'kid' field. In particular,
when sending the POST request to the AS, the Client indicates:
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* The current Group Identifier of the OSCORE group, as value of
the 'context_id' parameter.
* The current Sender ID it has in the OSCORE group, as value of
the 'salt_input' parameter.
* The public key of the new authentication credential it uses in
the OSCORE group, as value of the 'client_cred' parameter. In
particular, the specified public key is the COSE Key
equivalent to the new authentication credential that the
Client uses in the OSCORE group.
* 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.
* The same current or instead new set of access rights, as value
of the 'scope' parameter.
2. After receiving the response from the AS (see Appendix A.2.2),
the Client performs the same exchanges with the RS as defined in
Appendix A.3, with the following difference: the POST request to
/authz-info for uploading the new Access Token MUST be protected
with the pairwise OSCORE Security Context shared with the RS.
When receiving the new Access Token, the RS performs the same steps
defined in Appendix A.3.2. In particular, no new pairwise OSCORE
Security Context is established between the Client and the RS.
A.4. Secure Communication with the AS
The same considerations for secure communication with the AS as
defined in Section 5 hold.
A.5. Discarding the Security Context
The Client and the RS MUST follow what is defined in Section 6 of
[I-D.ietf-ace-oscore-profile] about discarding the pairwise OSCORE
Security Context.
Additionally, they MUST follow what is defined in the main mode of
the profile (see Section 6), with respect to the Group OSCORE
Security Context.
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
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SHOULD request a new Access Token and post it to the RS, in order to
establish a new pairwise OSCORE Security Context and bind it to the
Group OSCORE Security Context obtained upon re-joining the group.
A.6. CBOR Mappings
The new parameters defined in this document MUST be mapped to CBOR
types as specified in Figure 6, with the following addition, using
the given integer abbreviation for the map key.
/----------------+----------+------------\
| Parameter name | CBOR Key | Value Type |
|----------------+----------+------------|
| client_cred | TBD | map |
\----------------+----------+------------/
Figure 17: CBOR mappings for new parameters.
The new claims defined in this document MUST be mapped to CBOR types
as specified in Figure 7, with the following addition, using the
given integer abbreviation for the map key.
/--------------+----------+------------\
| Claim name | CBOR Key | Value Type |
|--------------+----------+------------|
| client_cred | TBD | map |
\--------------+----------+------------/
Figure 18: CBOR mappings for new claims.
A.7. Security Considerations
The dual mode of this profile inherits the security considerations
from the main mode (see Section 8), as well as from the security
considerations of the OSCORE profile of ACE
[I-D.ietf-ace-oscore-profile]. Also, the security considerations
about OSCORE [RFC8613] hold for the dual mode of this profile, as to
the specific use of OSCORE.
Unlike the main mode and consistently with Section 6.1 of
[I-D.ietf-ace-oauth-authz], the dual mode of this profile cannot be
used to issue an Access Token for an audience that comprises multiple
RSs. This is because the proof-of-possession key bound to an Access
Token is the OSCORE Master Secret included in the
OSCORE_Input_Material object of the 'cnf' claim, and it has to be
shared only between the Client and one RS.
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A.8. Privacy Considerations
The same privacy considerations as defined in the main mode of this
profile apply (see Section 9).
In addition, as this profile mode also uses OSCORE, the privacy
considerations from [RFC8613] apply as well, as to the specific use
of OSCORE.
Furthermore, this profile mode inherits the privacy considerations
from the OSCORE profile of ACE [I-D.ietf-ace-oscore-profile].
Appendix B. Profile Requirements
This appendix lists the specifications on this profile based on the
requirements of the ACE framework, as requested in Appendix C of
[I-D.ietf-ace-oauth-authz].
* (Optional) discovery process of how the Client finds the right AS
for an RS it wants to send a request to: Not specified.
* Communication protocol the Client and the RS must use: CoAP.
* Security protocol(s) the Client and RS must use: Group OSCORE,
i.e., exchange of secure messages by using a pre-established Group
OSCORE Security Context. The optional dual mode defined in
Appendix A additionally uses OSCORE, i.e., establishment of a
pairwise OSCORE Security Context and exchange of secure messages.
* 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 the mutual authentication is
not completed before the Client has verified an OSCORE or a Group
OSCORE response using the corresponding security context.
* Content-format of the protocol messages: "application/ace+cbor".
* Proof-of-Possession protocol(s) and how to select one; which key
types (e.g., symmetric/asymmetric) supported: Group OSCORE
algorithms; distributed and verified asymmetric keys. In the
optional dual mode defined in Appendix A: OSCORE algorithms; pre-
established symmetric keys.
* profile identifier: coap_group_oscore
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* (Optional) how the RS talks to the AS for introspection: HTTP/CoAP
(+ TLS/DTLS/OSCORE).
* How the client talks to the AS for requesting a token: HTTP/CoAP
(+ TLS/DTLS/OSCORE).
* How/if the authz-info endpoint is protected: Not protected.
* (Optional) other methods of token transport than the authz-info
endpoint: Not specified.
Acknowledgments
The authors sincerely thank Benjamin Kaduk, John Mattsson, Dave
Robin, Jim Schaad and Goeran 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
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
Ludwig Seitz
Combitech
Djäknegatan 31
SE-21135 Malmö Malmö
Sweden
Email: ludwig.seitz@combitech.com
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Francesca Palombini
Ericsson AB
Torshamnsgatan 23
SE-16440 Stockholm Kista
Sweden
Email: francesca.palombini@ericsson.com
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