ACE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track J. Park
Expires: September 9, 2019 Universitaet Duisburg-Essen
F. Palombini
Ericsson AB
March 08, 2019
Key Management for OSCORE Groups in ACE
draft-ietf-ace-key-groupcomm-oscore-01
Abstract
This document describes a method to request and provision keying
material in group communication scenarios where communications are
based on CoAP and secured with Object Security for Constrained
RESTful Environments (OSCORE). The proposed method delegates the
authentication and authorization of new client nodes that join an
OSCORE group through a Group Manager server. This approach builds on
the ACE framework for Authentication and Authorization, and leverages
protocol-specific profiles of ACE to achieve communication security,
proof-of-possession and server authentication.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 9, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Relation to Other Documents . . . . . . . . . . . . . . . 5
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Overview of the Join Process . . . . . . . . . . . . . . 7
2.2. Overview of the Group Rekeying Process . . . . . . . . . 8
3. Joining Node to Authorization Server . . . . . . . . . . . . 8
3.1. Authorization Request . . . . . . . . . . . . . . . . . . 8
3.2. Authorization Response . . . . . . . . . . . . . . . . . 9
4. Joining Node to Group Manager . . . . . . . . . . . . . . . . 10
4.1. Token Post . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. 'key info' Parameter . . . . . . . . . . . . . . . . 11
4.2. Join Request . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Join Response . . . . . . . . . . . . . . . . . . . . . . 12
5. Leaving of a Group Member . . . . . . . . . . . . . . . . . . 14
6. Public Keys of Joining Nodes . . . . . . . . . . . . . . . . 15
7. Group Rekeying Process . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9.1. ACE Authorization Server Request Creation Hints Registry 17
9.2. ACE Groupcomm Key Registry . . . . . . . . . . . . . . . 17
9.3. OSCORE Security Context Parameters Registry . . . . . . . 18
9.4. ACE Groupcomm Profile Registry . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Document Updates . . . . . . . . . . . . . . . . . . 20
A.1. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Object Security for Constrained RESTful Environments (OSCORE)
[I-D.ietf-core-object-security] is a method for application-layer
protection of the Constrained Application Protocol (CoAP) [RFC7252],
using CBOR Object Signing and Encryption (COSE) [RFC8152] and
enabling end-to-end security of CoAP payload and options.
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As described in [I-D.ietf-core-oscore-groupcomm], OSCORE may be used
to protect CoAP group communication over IP multicast [RFC7390].
This relies on a Group Manager, which is responsible for managing an
OSCORE group, where members exchange CoAP messages secured with
OSCORE. The Group Manager can be responsible for multiple groups,
coordinates the join process of new group members, and is entrusted
with the distribution and renewal of group keying material.
This specification builds on the ACE framework for Authentication and
Authorization [I-D.ietf-ace-oauth-authz] and defines a method to:
o Authorize a node to join an OSCORE group, and provide it with the
group keying material to communicate with other group members.
o Provide updated keying material to group members upon request.
o Renew the group keying material and distribute it to the OSCORE
group (rekeying) upon changes in the group membership.
A client node joins an OSCORE group through a resource server acting
as Group Manager for that group. The join process relies on an
Access Token, which is bound to a proof-of-possession key and
authorizes the client to access a specific join resource at the Group
Manager.
Messages exchanged among the participants follow the formats defined
in [I-D.ietf-ace-key-groupcomm] for provisioning and renewing keying
material in group communication scenarios.
In order to achieve communication security, proof-of-possession and
server authentication, the client and the Group Manager leverage
protocol-specific profiles of ACE. These include also possible
forthcoming profiles that comply with the requirements in Appendix C
of [I-D.ietf-ace-oauth-authz].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in the ACE framework for authentication and authorization
[I-D.ietf-ace-oauth-authz]. The terminology for entities in the
considered architecture is defined in OAuth 2.0 [RFC6749]. In
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particular, this includes Client (C), Resource Server (RS), and
Authorization Server (AS).
Readers are expected to be familiar with the terms and concepts
related to the CoAP protocol described in [RFC7252][RFC7390]. 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".
Readers are expected to be familiar with the terms and concepts for
protection and processing of CoAP messages through OSCORE
[I-D.ietf-core-object-security] also in group communication scenarios
[I-D.ietf-core-oscore-groupcomm]. These include the concept of Group
Manager, as the entity responsible for a set of groups where
communications are secured with OSCORE. In this specification, the
Group Manager acts as Resource Server.
This document refers also to the following terminology.
o Joining node: a network node intending to join an OSCORE group,
where communication is based on CoAP [RFC7390] and secured with
OSCORE as described in [I-D.ietf-core-oscore-groupcomm].
o Join process: the process through which a joining node becomes a
member of an OSCORE group. The join process is enforced and
assisted by the Group Manager responsible for that group.
o Join resource: a resource hosted by the Group Manager, associated
to an OSCORE group under that Group Manager. A join resource is
identifiable with the Group Identifier (Gid) of the respective
group. A joining node accesses a join resource to start the join
process and become a member of that group.
o Join endpoint: an endpoint at the Group Manager associated to a
join resource.
o Requester: member of an OSCORE group that sends request messages
to other members of the group.
o Listener: member of an OSCORE group that receives request messages
from other members of the group. A listener may reply back, by
sending a response message to the requester which has sent the
request message.
o Pure listener: member of a group that is configured as listener
and never replies back to requesters after receiving request
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messages. This corresponds to the term "silent server" used in
[I-D.ietf-core-oscore-groupcomm].
o Group rekeying process: the process through which the Group
Manager renews the security parameters and group keying material,
and (re-)distributes them to the OSCORE group members.
1.2. Relation to Other Documents
Figure 1 overviews the main documents related to this specification.
Arrows and asterisk-arrows denote normative references and
informative refences, respectively.
+---------------------------------------+
| |
+----------------|--------------+ |
| | | |
| v v Key Management
Pub-sub ---> Key Groupcomm ---> ACE Framework <--- for OSCORE Groups
profile * [[WG]] [[WG]] [[This document]]
| * * ^ ^ | |
| * * * * | |
| * * * *************** | |
| *********** * * * | |
| * * * * +--------------+ |
ACE | * * * * | |
-----|-*--------------*--------------*-*-|--------------------|-------
CoRE | * * * * | |
v v v * * v v
CoRE CoRE OSCORE -------------> OSCORE
Pubsub Groupcomm <*** Groupcomm <************* [[WG]]
[[WG]] [[RFC7390]] [[WG]]
Figure 1: Related Documents
2. Protocol Overview
Group communication for CoAP over IP multicast has been enabled in
[RFC7390] and can be secured with Object Security for Constrained
RESTful Environments (OSCORE) [I-D.ietf-core-object-security] as
described in [I-D.ietf-core-oscore-groupcomm]. A network node joins
an OSCORE group by interacting with the responsible Group Manager.
Once registered in the group, the new node can securely exchange
messages with other group members.
This specification describes how to use the ACE framework for
authentication and authorization [I-D.ietf-ace-oauth-authz] to:
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o Enable a node to join an OSCORE group through the Group Manager
and receive the security parameters and keying material to
communicate with the other members of the gorup.
o Enable members of OSCORE groups to retrieve updated group keying
material from the Group Manager.
o Enable the Group Manager to renew the security parameters and
group keying material, and to (re-)distribute them to the members
of the OSCORE group (rekeying).
With reference to the ACE framework and the terminology defined in
OAuth 2.0 [RFC6749]:
o The Group Manager acts as Resource Server (RS), and hosts one join
resource for each OSCORE group it manages. Each join resource is
exported by a distinct join endpoint. During the join process,
the Group Manager provides joining nodes with the parameters and
keying material for taking part to secure communications in the
OSCORE group. The Group Manager also maintains the group keying
material and performs the group rekeying process to distribute
updated keying material to the group members.
o The joining node acts as Client (C), and requests to join an
OSCORE group by accessing the related join endpoint at the Group
Manager.
o The Authorization Server (AS) authorizes joining nodes to join
OSCORE groups under their respective Group Manager. Multiple
Group Managers can be associated to the same AS. The AS MAY
release Access Tokens for other purposes than joining OSCORE
groups under registered Group Managers. For example, the AS may
also release Access Tokens for accessing resources hosted by
members of OSCORE groups.
All communications between the involved entities rely on the CoAP
protocol and MUST be secured.
In particular, communications between the joining node and the Group
Manager leverage protocol-specific profiles of ACE to achieve
communication security, proof-of-possession and server
authentication. To this end, the AS must signal the specific profile
to use, consistently with requirements and assumptions defined in the
ACE framework [I-D.ietf-ace-oauth-authz].
With reference to the AS, communications between the joining node and
the AS (/token endpoint) as well as between the Group Manager and the
AS (/introspect endpoint) can be secured by different means, for
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instance using DTLS [RFC6347] or OSCORE
[I-D.ietf-core-object-security]. Further details on how the AS
secures communications (with the joining node and the Group Manager)
depend on the specifically used profile of ACE, and are out of the
scope of this specification.
2.1. Overview of the Join Process
A node performs the following steps in order to join an OSCORE group.
Messages exchanged among the participants follow the formats defined
in [I-D.ietf-ace-key-groupcomm], and are further specified in
Section 3 and Section 4 of this document. The Group Manager acts as
the Key Distribution Center (KDC) defined in
[I-D.ietf-ace-key-groupcomm].
1. The joining node requests an Access Token from the AS, in order
to access a join resource on the Group Manager and hence join the
associated OSCORE group (see Section 3). The joining node will
start or continue using a secure communication channel with the
Group Manager, according to the response from the AS.
2. The joining node transfers authentication and authorization
information to the Group Manager by posting the obtained Access
Token (see Section 4). After that, a joining node must have a
secure communication channel established with the Group Manager,
before starting to join an OSCORE group under that Group Manager
(see Section 4). Possible ways to provide a secure communication
channel are DTLS [RFC6347] and OSCORE
[I-D.ietf-core-object-security].
3. The joining node starts the join process to become a member of
the OSCORE group, by accessing the related join resource hosted
by the Group Manager (see Section 4).
4. At the end of the join process, the joining node has received
from the Group Manager the parameters and keying material to
securely communicate with the other members of the OSCORE group.
5. The joining node and the Group Manager maintain the secure
channel, to support possible future communications.
All further communications between the joining node and the Group
Manager MUST be secured, for instance with the same secure channel
mentioned in step 2.
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2.2. Overview of the Group Rekeying Process
If the application requires backward and forward security, the Group
Manager MUST generate new security parameters and group keying
material, and distribute them to the group (rekeying) upon membership
changes.
That is, the group is rekeyed when a node joins the group as a new
member, or after a current member leaves the group. By doing so, a
joining node cannot access communications in the group prior its
joining, while a leaving node cannot access communications in the
group after its leaving.
Parameters and keying material include a new Group Identifier (Gid)
for the group and a new Master Secret for the OSCORE Common Security
Context of that group (see Section 2 of
[I-D.ietf-core-oscore-groupcomm]).
The Group Manager MUST support the Group Rekeying Process described
in Section 7. Future application profiles may define alternative
message formats and distribution schemes to perform group rekeying.
3. Joining Node to Authorization Server
This section describes how the joining node interacts with the AS in
order to be authorized to join an OSCORE group under a given Group
Manager. In particular, it considers a joining node that intends to
contact that Group Manager for the first time.
The message exchange between the joining node and the AS consists of
the messages Authorization Request and Authorization Response defined
in Section 3 of [I-D.ietf-ace-key-groupcomm].
In case the specific AS associated to the Group Manager is unknown to
the joining node, the latter can rely on mechanisms like the
Unauthorized Resource Request message described in Section 5.1.1 of
[I-D.ietf-ace-oauth-authz] to discover the correct AS to contact.
3.1. Authorization Request
The joining node contacts the AS, in order to request an Access Token
for accessing the join resource hosted by the Group Manager and
associated to the OSCORE group. The Access Token request sent to the
/token endpoint follows the format of the Authorization Request
message defined in Section 3.1 of [I-D.ietf-ace-key-groupcomm]. In
particular:
o The 'scope' parameter MUST be present and MUST include:
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* in the first element, either the Group Identifier (Gid) of the
group to join under the Group Manager, or a value from which
the Group Manager can derive the Gid of the group to join. It
is up to the application to define how the Group Manager
possibly performs the derivation of the full Gid. Appendix C of
[I-D.ietf-core-oscore-groupcomm] provides an example of
structured Gid, composed of a fixed part, namely Group Prefix,
and a variable part, namely Group Epoch.
* in the second element, the role(s) that the joining node
intends to have in the group it intends to join. Possible
values are: "requester"; "listener"; and "pure listener".
Possible combinations are: ["requester" , "listener"];
["requester" , "pure listener"].
o The 'audience' parameter MUST be present and is set to the
identifier of the Group Manager.
3.2. Authorization Response
The AS is responsible for authorizing the joining node to join
specific OSCORE groups, according to join policies enforced on behalf
of the respective Group Manager.
In case of successful authorization, the AS releases an Access Token
bound to a proof-of-possession key associated to the joining node.
Then, the AS provides the joining node with the Access Token as part
of an Access Token response, which follows the format of the
Authorization Response message defined in Section 3.2 of
[I-D.ietf-ace-key-groupcomm].
The 'exp' parameter MUST be present. Other means for the AS to
specify the lifetime of Access Tokens are out of the scope of this
specification.
The AS must include the 'scope' parameter in the response when the
value included in the Access Token differs from the one specified by
the joining node in the request. In such a case, the second element
of 'scope' MUST be present and includes the role(s) that the joining
node is actually authorized to take in the group, encoded as
specified in Section 3.1 of this document.
Also, the 'profile' parameter indicates the specific profile of ACE
to use for securing communications between the joining node and the
Group Manager (see Section 5.6.4.3 of [I-D.ietf-ace-oauth-authz]).
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In particular, if symmetric keys are used, the AS generates a proof-
of-possession key, binds it to the Access Token, and provides it to
the joining node in the 'cnf' parameter of the Access Token response.
Instead, if asymmetric keys are used, the joining node provides its
own public key to the AS in the 'req_cnf' parameter of the Access
Token request. Then, the AS uses it as proof-of-possession key bound
to the Access Token, and provides the joining node with the Group
Manager's public key in the 'rs_cnf' parameter of the Access Token
response.
4. Joining Node to Group Manager
The following subsections describe the interactions between the
joining node and the Group Manager, i.e. the Access Token post and
the Request-Response exchange to join the OSCORE group.
4.1. Token Post
The joining node posts the Access Token to the /authz-info endpoint
at the Group Manager, according to the Token post defined in
Section 3.3 of [I-D.ietf-ace-key-groupcomm].
If the joining node is not aware of the countersignature algorithm
and related parameters used in the OSCORE group, it may want to get
that information from the Group Manager. In such a case, the CoAP
POST request uses the Content-Format "application/ace+cbor" defined
in Section 8.14 of [I-D.ietf-ace-oauth-authz], and includes also the
parameter 'key info' defined in Section 4.1.1 and registered in
Section 9.1, encoding the CBOR simple value Null. Alternatively, the
joining node may retrieve that information by other means, e.g. by
using the approach described in [I-D.tiloca-core-oscore-discovery].
If the Access Token is valid, the Group Manager responds to the POST
request with a 2.01 (Created) response, according to what is
specified in the signalled profile of ACE.
The payload of the 2.01 (Created) response MAY be a CBOR map
including a 'key info' parameter, which MUST be present if the POST
request included the 'key info' parameter with value Null. If
present, the 'key info' parameter of the 2.01 (Created) response is a
CBOR array formatted as follows:
o The first element is an integer or a text string, indicating the
counter signature algorithm used in the OSCORE group. This
parameter takes values from Tables 5 and 6 of [RFC8152].
o The second element indicates the parameters of the counter
signature algorithm. Its structure depends on the value of the
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first element, and is defined in the Counter Signature Parameters
Registry (see Section 9.1 of [I-D.ietf-core-oscore-groupcomm]).
This parameter MUST be omitted if there are no parameters for that
algorithm value.
The CDDL notation of the 'key info' parameter is given below.
key_info = [
sign_alg : int / tstr,
? sign_parameters : any
]
Finally, the joining node establishes a secure channel with the Group
Manager, according to what is specified in the Access Token response
and the signalled profile of ACE.
4.1.1. 'key info' Parameter
The 'key info' parameter is an OPTIONAL parameter of the AS Request
Creation Hints message defined in Section 5.1.2. of
[I-D.ietf-ace-oauth-authz]. This parameter contains information
about the key to be used in the security association between the
Client and the RS. Its format is application specific.
4.2. Join Request
Once a secure communication channel with the Group Manager has been
established, the joining node requests to join the OSCORE group, by
accessing the related join resource at the Group Manager.
In particular, the joining node sends to the Group Manager a
confirmable CoAP request, using the method POST and targeting the
join endpoint associated to that group. This join request follows
the format and processing of the Key Distribution Request message
defined in Section 4.1 of [I-D.ietf-ace-key-groupcomm]. In
particular:
o The 'get_pub_keys' parameter is present only if the joining node
wants to retrieve the public keys of the group members from the
Group Manager during the join process (see Section 6). Otherwise,
this parameter MUST NOT be present.
o The 'client_cred' parameter, if present, includes the public key
of the joining node. In case the joining node knows the
countersignature algorithm and possible associated parameters used
in the OSCORE group, the included public key MUST be in a
consistent format. This parameter MAY be omitted if: i) the
joining node is asking to access the group exclusively as pure
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listener; or ii) the Group Manager already acquired this
information, for instance during a previous join process. In any
other case, this parameter MUST be present.
4.3. Join Response
The Group Manager processes the request according to
[I-D.ietf-ace-oauth-authz] and Section 4.2 of
[I-D.ietf-ace-key-groupcomm]. If this yields a positive outcome, the
Group Manager performs the following check. In case the Join Request
included the 'client_cred' parameter, the Group Manager checks that
the public key of the joining node is consistent with the counter
signature algorithm and possible associated parameters used in the
OSCORE group.
If the public key of the joining node does not have an accepted
format, the Group Manager MUST reply to the joining node with a 2.01
(Created) response. The payload of this response is a CBOR map,
which includes a 'key info' parameter formatted as in the Token POST
response in Section 4.1. Upon receiving this response, the joining
node SHOULD send a new Join Request to the Group Manager, with the
'client_cred' parameter including its own public key in a format
consistent with the countersignature algorithm and possible
associated parameters indicated by the Group Manager.
Otherwise, the Group Manager updates the group membership by
registering the joining node as a new member of the OSCORE group.
Then, the Group Manager replies to the joining node providing the
updated security parameters and keying meterial necessary to
participate in the group communication. This join response follows
the format and processing of the Key Distribution success Response
message defined in Section 4.2 of [I-D.ietf-ace-key-groupcomm]. In
particular:
o The 'kty' parameter identifies a key of type
"Group_OSCORE_Security_Context object", defined in Section 9.2 of
this specification.
o The 'key' parameter includes what the joining node needs in order
to set up the OSCORE Security Context as per Section 2 of
[I-D.ietf-core-oscore-groupcomm]. This parameter includes a
Group_OSCORE_Security_Context object, which is defined in this
specification and extends the OSCORE_Security_Context object
encoded in CBOR as defined in Section 3.2.1 of
[I-D.ietf-ace-oscore-profile]. In particular, it contains the
additional parameters 'cs_alg' and 'cs_params' defined in
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Section 9.3 of this specification. More specifically, the 'key'
parameter is composed as follows.
* The 'ms' parameter MUST be present and includes the OSCORE
Master Secret value.
* The 'clientId' parameter, if present, has as value the OSCORE
Sender ID assigned to the joining node by the Group Manager.
This parameter is not present if the node joins the group
exclusively as pure listener, according to what specified in
the Access Token (see Section 3.2). In any other case, this
parameter MUST be present.
* The 'hkdf' parameter, if present, has as value the KDF
algorithm used in the group.
* The 'alg' parameter, if present, has as value the AEAD
algorithm used in the group.
* The 'salt' parameter, if present, has as value the OSCORE
Master Salt.
* The 'contextId' parameter MUST be present and has as value the
Group Identifier (Gid) associated to the OSCORE group.
* The 'rpl' parameter, if present, specifies the OSCORE Replay
Window Size and Type value.
* The 'cs_alg' parameter MUST be present and specifies the
algorithm used to countersign messages in the group. This
parameter takes values from Tables 5 and 6 of [RFC8152].
* The 'cs_params' parameter MAY be present and specifies the
additional parameters for the counter signature algorithm.
This parameter is encoded as specified in Section 2 of
[I-D.ietf-core-oscore-groupcomm].
o The 'profile' parameter MUST be present and has value
"coap_group_oscore", which is defined in Section 9.4 of this
specification.
o The 'exp' parameter MUST be present and specifies the expiration
time in seconds after which the OSCORE Security Context derived
from the 'key' parameter is not valid anymore.
o The 'pub_keys' parameter is present only if the 'get_pub_keys'
parameter was present in the join request. If present, this
parameter includes the public keys of the group members that are
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relevant to the joining node. That is, it includes: i) the public
keys of the non-pure listeners currently in the group, in case the
joining node is configured (also) as requester; and ii) the public
keys of the requesters currently in the group, in case the joining
node is configured (also) as listener or pure listener.
o The 'group_policies' parameter SHOULD be present and includes a
list of parameters indicating particular policies enforced in the
group. For instance, it can indicate the method to achieve
synchronization of sequence numbers among group members (see
Appendix E of [I-D.ietf-core-oscore-groupcomm]).
Finally, the joining node uses the information received in the join
response to set up the OSCORE Security Context, as described in
Section 2 of [I-D.ietf-core-oscore-groupcomm]. From then on, the
joining node can exchange group messages secured with OSCORE as
described in [I-D.ietf-core-oscore-groupcomm].
If the application requires backward security, the Group Manager
SHALL generate updated security parameters and group keying material,
and provide it to all the current group members (see Section 7).
When the OSCORE Security Context expires, as specified by the 'exp'
parameter of the join response, the node considers it invalid and to
be renewed. Then, the node retrieves updated security parameters and
keying material, by exchanging shortened Join Request and Join
Response messages with the Group Manager, according to the approach
defined in Section 6 of [I-D.ietf-ace-key-groupcomm]. Finally, the
node uses the updated security parameters and keying material to set
up the new OSCORE Security Context as described in Section 2 of
[I-D.ietf-core-oscore-groupcomm].
5. Leaving of a Group Member
A node may be removed from the OSCORE group, due to expired or
revoked authorization, or after its own request to the Group Manager.
If the application requires forward security, the Group Manager SHALL
generate updated security parameters and group keying material, and
provide it to the remaining group members (see Section 7). The
leaving node must not be able to acquire the new security parameters
and group keying material distributed after its leaving.
Same considerations in Section 5 of [I-D.ietf-ace-key-groupcomm]
apply here as well, considering the Group Manager acting as KDC. In
particular, a node requests to leave the OSCORE group as described in
Section 5.2 of [I-D.ietf-ace-key-groupcomm].
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6. Public Keys of Joining Nodes
Source authentication of OSCORE messages exchanged within the group
is ensured by means of digital counter signatures (see Sections 2 and
3 of [I-D.ietf-core-oscore-groupcomm]). Therefore, group members
must be able to retrieve each other's public key from a trusted key
repository, in order to verify source authenticity of incoming group
messages.
As also discussed in [I-D.ietf-core-oscore-groupcomm], the Group
Manager acts as trusted repository of the public keys of the group
members, and provides those public keys to group members if requested
to. Upon joining an OSCORE group, a joining node is thus expected to
provide its own public key to the Group Manager.
In particular, four cases can occur when a new node joins a group.
o The joining node is going to join the group exclusively as pure
listener. That is, it is not going to send messages to the group,
and hence to produce signatures with its own private key. In this
case, the joining node is not required to provide its own public
key to the Group Manager upon joining the group.
o The Group Manager already acquired the public key of the joining
node during a previous join process. In this case, the joining
node may not provide again its own public key to the Group
Manager, in order to limit the size of the join request.
o The joining node and the Group Manager use an asymmetric proof-of-
possession key to establish a secure communication channel. In
this case, the Group Manager stores the proof-of-possession key
conveyed in the Access Token as the public key of the joining
node.
o The joining node and the Group Manager use a symmetric proof-of-
possession key to establish a secure communication channel. In
this case, upon performing a join process with that Group Manager
for the first time, the joining node specifies its own public key
in the 'client_cred' parameter of the join request targeting the
join endpoint (see Section 4.2).
Furthermore, as described in Section 4.2, the joining node may have
explicitly requested the Group Manager to retrieve the public keys of
the current group members, i.e. through the 'get_pub_keys' parameter
in the join request. In this case, the Group Manager includes also
such public keys in the 'pub_keys' parameter of the join response
(see Section 4.3).
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Later on as a group member, the node may need to retrieve the public
keys of other group members. The node can do that by exchanging
shortened Join Request and Join Response messages with the Group
Manager, according to the approach defined in Section 7 of
[I-D.ietf-ace-key-groupcomm].
7. Group Rekeying Process
In order to rekey the OSCORE group, the Group Manager distributes a
new Group ID of the group and a new OSCORE Master Secret for that
group. To this end, the Group Manager MUST support at least the
following group rekeying scheme. Future application profiles may
define alternative message formats and distribution schemes.
The Group Manager uses the same format of the Join Response message
in Section 4.3. In particular:
o Only the parameters 'kty', 'key', 'profile' and 'exp' are present.
o The 'ms' parameter of the 'key' parameter specifies the new OSCORE
Master Secret value.
o The 'contextId' parameter of the 'key' parameter specifies the new
Group ID.
The Group Manager separately sends a group rekeying message to each
group member to be rekeyed. Each rekeying message MUST be secured
with the pairwise secure communication channel between the Group
Manager and the group member used during the join process.
8. Security Considerations
The method described in this document leverages the following
management aspects related to OSCORE groups and discussed in the
sections of [I-D.ietf-core-oscore-groupcomm] referred below.
o Management of group keying material (see Section 2.1 of
[I-D.ietf-core-oscore-groupcomm]). The Group Manager is
responsible for the renewal and re-distribution of the keying
material in the groups of its competence (rekeying). According to
the specific application requirements, this can include rekeying
the group upon changes in its membership. In particular, renewing
the keying material is required upon a new node's joining or a
current node's leaving, in case backward security and forward
security have to be preserved, respectively.
o Provisioning and retrieval of public keys (see Section 2 of
[I-D.ietf-core-oscore-groupcomm]). The Group Manager acts as key
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repository of public keys of group members, and provides them upon
request.
o Synchronization of sequence numbers (see Section 5 of
[I-D.ietf-core-oscore-groupcomm]). This concerns how a listener
node that has just joined an OSCORE group can synchronize with the
sequence number of requesters in the same group.
Before sending the join response, the Group Manager should verify
that the joining node actually owns the associated private key, for
instance by performing a proof-of-possession challenge-response,
whose details are out of the scope of this specification.
Further security considerations are inherited from
[I-D.ietf-ace-key-groupcomm], the ACE framework for Authentication
and Authorization [I-D.ietf-ace-oauth-authz], and the specific
profile of ACE signalled by the AS, such as
[I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-oscore-profile].
9. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[This
specification]]" with the RFC number of this specification and delete
this paragraph.
This document has the following actions for IANA.
9.1. ACE Authorization Server Request Creation Hints Registry
IANA is asked to register the following entry in the "ACE
Authorization Server Request Creation Hints" Registry defined in
Section 8.1 of [I-D.ietf-ace-oauth-authz].
o Name: key info
o CBOR Key: TBD (range -256 to 255)
o Value Type: any
o Reference: [[This specification]]
9.2. ACE Groupcomm Key Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Key" Registry defined in Section 9.1 of [I-D.ietf-ace-key-groupcomm].
o Name: Group_OSCORE_Security_Context object
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o Key Type Value: TBD
o Profile: "coap_group_oscore", defined in Section 9.4 of this
specification.
o Description: A Group_OSCORE_Security_Context object encoded as
described in Section 4.3 of this specification.
o Reference: [[This specification]]
9.3. OSCORE Security Context Parameters Registry
IANA is asked to register the following entries in the "OSCORE
Security Context Parameters" Registry defined in Section 9.2 of
[I-D.ietf-ace-oscore-profile].
o Name: cs_alg
o CBOR Label: TBD
o CBOR Type: tstr / int
o Registry: COSE Algorithm Values (ECDSA, EdDSA)
o Description: OSCORE Counter Signature Algorithm Value
o Reference: [[This specification]]
o Name: cs_params
o CBOR Label: TBD
o CBOR Type: bstr
o Registry: Counter Signatures Parameters
o Description: OSCORE Counter Signature Algorithm Additional
Parameters
o Reference: [[This specification]]
9.4. ACE Groupcomm Profile Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Profile" Registry defined in Section 9.2 of
[I-D.ietf-ace-key-groupcomm].
o Name: coap_group_oscore
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o Description: Profile to provision keying material for
participating in group communication protected with Group OSCORE
as per [I-D.ietf-core-oscore-groupcomm].
o CBOR Value: TBD
o Reference: [[This specification]]
10. References
10.1. Normative References
[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-ietf-ace-key-groupcomm-00
(work in progress), December 2018.
[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)", draft-ietf-ace-oauth-authz-22
(work in progress), March 2019.
[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", draft-ietf-ace-
oscore-profile-07 (work in progress), February 2019.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-16 (work in
progress), March 2019.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., and J. Park,
"Group OSCORE - Secure Group Communication for CoAP",
draft-ietf-core-oscore-groupcomm-03 (work in progress),
October 2018.
[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>.
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. 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)", draft-ietf-ace-dtls-
authorize-06 (work in progress), February 2019.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
Groups with the CoRE Resource Directory", draft-tiloca-
core-oscore-discovery-01 (work in progress), January 2019.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
Appendix A. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
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A.1. Version -00 to -01
o Changed name of 'req_aud' to 'audience' in the Authorization
Request (Section 3.1).
o Added negotiation of countersignature algorithm/parameters between
Client and Group Manager (Section 4).
o Updated format of the Key Distribution Response as a whole
(Section 4.3).
o Added parameter 'cs_params' in the 'key' parameter of the Key
Distribution Response (Section 4.3).
o New IANA registrations in the "ACE Authorization Server Request
Creation Hints" Registry, "ACE Groupcomm Key" Registry, "OSCORE
Security Context Parameters" Registry and "ACE Groupcomm Profile"
Registry (Section 9).
Acknowledgments
The authors sincerely thank Santiago Aragon, Stefan Beck, Martin
Gunnarsson, Rikard Hoeglund, Jim Schaad, Ludwig Seitz, Goeran
Selander and Peter van der Stok 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 EIT-Digital High Impact
Initiative ACTIVE.
Authors' Addresses
Marco Tiloca
RISE AB
Isafjordsgatan 22
Kista SE-164 29 Stockholm
Sweden
Email: marco.tiloca@ri.se
Jiye Park
Universitaet Duisburg-Essen
Schuetzenbahn 70
Essen 45127
Germany
Email: ji-ye.park@uni-due.de
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Francesca Palombini
Ericsson AB
Torshamnsgatan 23
Kista SE-16440 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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