ACE Working Group M. Tiloca
Internet-Draft RISE AB
Intended status: Standards Track J. Park
Expires: December 20, 2020 Universitaet Duisburg-Essen
F. Palombini
Ericsson AB
June 18, 2020
Key Management for OSCORE Groups in ACE
draft-ietf-ace-key-groupcomm-oscore-07
Abstract
This specification defines an application profile of the ACE
framework for Authentication and Authorization, to request and
provision keying material in group communication scenarios that are
based on CoAP and secured with Group Object Security for Constrained
RESTful Environments (OSCORE). This application profile delegates
the authentication and authorization of Clients that join an OSCORE
group through a Resource Server acting as Group Manager for that
group. This application profile leverages protocol-specific
transport profiles of ACE to achieve communication security, server
authentication and proof-of-possession for a key owned by the Client
and bound to an OAuth 2.0 access token.
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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This Internet-Draft will expire on December 20, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Overview of the Joining Process . . . . . . . . . . . . . 6
2.2. Overview of the Group Rekeying Process . . . . . . . . . 6
3. Joining Node to Authorization Server . . . . . . . . . . . . 7
3.1. Authorization Request . . . . . . . . . . . . . . . . . . 7
3.2. Authorization Response . . . . . . . . . . . . . . . . . 8
4. Interface at the Group Manager . . . . . . . . . . . . . . . 8
4.1. GET Handler . . . . . . . . . . . . . . . . . . . . . . . 8
5. Token POST and Group Joining . . . . . . . . . . . . . . . . 9
5.1. Token Post . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Sending the Joining Request . . . . . . . . . . . . . . . 10
5.2.1. Value of the N_S Challenge . . . . . . . . . . . . . 11
5.3. Processing the Joining Request . . . . . . . . . . . . . 12
5.4. Joining Response . . . . . . . . . . . . . . . . . . . . 13
5.5. ACE Groupcomm Policy for Group OSCORE Pairwise Mode
Support . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Public Keys of Joining Nodes . . . . . . . . . . . . . . . . 16
7. Retrieval of Updated Keying Material . . . . . . . . . . . . 18
7.1. Retrieval of Group Keying Material . . . . . . . . . . . 18
7.2. Retrieval of Group Keying Material and Sender ID . . . . 19
8. Retrieval of New Keying Material . . . . . . . . . . . . . . 19
9. Retrieval of Public Keys of Group Members . . . . . . . . . . 20
10. Update of Public Key . . . . . . . . . . . . . . . . . . . . 21
11. Retrieval of Group Policies . . . . . . . . . . . . . . . . . 21
12. Retrieval of Keying Material Version . . . . . . . . . . . . 22
13. Retrieval of Group Status . . . . . . . . . . . . . . . . . . 22
14. Request to Leave the Group . . . . . . . . . . . . . . . . . 23
15. Removal of a Group Member . . . . . . . . . . . . . . . . . . 23
16. Group Rekeying Process . . . . . . . . . . . . . . . . . . . 24
17. Security Considerations . . . . . . . . . . . . . . . . . . . 26
17.1. Management of OSCORE Groups . . . . . . . . . . . . . . 26
17.2. Size of Nonces for Signature Challenge . . . . . . . . . 27
17.3. Reusage of Nonces for Signature Challenge . . . . . . . 28
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
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18.1. ACE Groupcomm Profile Registry . . . . . . . . . . . . . 28
18.2. ACE Groupcomm Key Registry . . . . . . . . . . . . . . . 29
18.3. OSCORE Security Context Parameters Registry . . . . . . 29
18.4. Sequence Number Synchronization Method Registry . . . . 30
18.5. ACE Groupcomm Parameters Registry . . . . . . . . . . . 31
18.6. ACE Groupcomm Policy Registry . . . . . . . . . . . . . 31
18.7. TLS Exporter Label Registry . . . . . . . . . . . . . . 32
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
19.1. Normative References . . . . . . . . . . . . . . . . . . 32
19.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Profile Requirements . . . . . . . . . . . . . . . . 35
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 37
B.1. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 37
B.2. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 38
B.3. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 38
B.4. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 39
B.5. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 39
B.6. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 40
B.7. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 41
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
Object Security for Constrained RESTful Environments (OSCORE)
[RFC8613] is a method for application-layer protection of the
Constrained Application Protocol (CoAP) [RFC7252], using CBOR Object
Signing and Encryption (COSE)
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
enabling end-to-end security of CoAP payload and options.
As described in [I-D.ietf-core-oscore-groupcomm], Group OSCORE is
used to protect CoAP group communication over IP multicast
[I-D.ietf-core-groupcomm-bis]. This relies on a Group Manager, which
is responsible for managing an OSCORE group and enables the group
members to exchange CoAP messages secured with Group OSCORE. The
Group Manager can be responsible for multiple groups, coordinates the
joining process of new group members, and is entrusted with the
distribution and renewal of group keying material.
This specification is an application profile of
[I-D.ietf-ace-key-groupcomm], which itself builds on the ACE
framework for Authentication and Authorization
[I-D.ietf-ace-oauth-authz]. Message exchanges among the participants
as well as message formats and processing follow what specified in
[I-D.ietf-ace-key-groupcomm] for provisioning and renewing keying
material in group communication scenarios, where Group OSCORE is used
to protect CoAP group communication over IP multicast.
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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:
o 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 particular, this includes Client (C),
Resource Server (RS), and Authorization Server (AS).
o The terms and concepts related to the CoAP protocol described in
[RFC7252][I-D.ietf-core-groupcomm-bis]. 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".
o The terms and concept related to the message formats and
processing specified in [I-D.ietf-ace-key-groupcomm], for
provisioning and renewing keying material in group communication
scenarios.
o The terms and concepts for protection and processing of CoAP
messages through OSCORE [RFC8613] and through Group OSCORE
[I-D.ietf-core-oscore-groupcomm] in group communication scenarios.
These include the concept of Group Manager, as the entity
responsible for a set of groups where communications are secured
with Group OSCORE. In this specification, the Group Manager acts
as Resource Server.
Additionally, this document makes use of the following terminology.
o Group name is used as a synonym for group identifier in
[I-D.ietf-ace-key-groupcomm].
o Requester: member of an OSCORE group that sends request messages
to other members of the group.
o Responder: member of an OSCORE group that receives request
messages from other members of the group. A responder may reply
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back, by sending a response message to the requester which has
sent the request message.
o Monitor: member of an OSCORE group that is configured as responder
and never replies back to requesters after receiving request
messages. This corresponds to the term "silent server" used in
[I-D.ietf-core-oscore-groupcomm].
o Signature verifier: entity external to the OSCORE group and
intended to verify the countersignature of messages exchanged in
the group. An authorized signature verifier does not join the
OSCORE group as an actual member, yet it can retrieve the public
keys of the current group members from the Group Manager.
2. Protocol Overview
Group communication for CoAP over IP multicast has been enabled in
[I-D.ietf-core-groupcomm-bis] and can be secured with Group Object
Security for Constrained RESTful Environments (OSCORE) [RFC8613] 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 [I-D.ietf-ace-key-groupcomm]
and [I-D.ietf-ace-oauth-authz] to perform a number of authentication,
authorization and key distribution actions, as defined in Section 2
of [I-D.ietf-ace-key-groupcomm], for an OSCORE group.
With reference to [I-D.ietf-ace-key-groupcomm]:
o The node wishing to joining the OSCORE group, i.e. the joining
node, is the Client.
o The Group Manager is the Key Distribution Center (KDC), acting as
a Resource Server.
o The Authorization Server associated to the Group Manager is the
AS.
All communications between the involved entities MUST be secured.
In particular, communications between the Client and the Group
Manager leverage protocol-specific transport profiles of ACE to
achieve communication security, proof-of-possession and server
authentication. Note that it is expected that in the commonly
referred base-case of this specification, the transport profile to
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use is pre-configured and well-known to nodes participating in
constrained applications.
2.1. Overview of the Joining Process
A node performs the steps described in Section 4.2 of
[I-D.ietf-ace-key-groupcomm] in order to join an OSCORE group. The
format and processing of messages exchanged among the participants
are further specified in Section 3 and Section 5 of this document.
2.2. Overview of the Group Rekeying Process
If the application requires backward and forward security, the Group
Manager MUST generate new keying material and distribute it 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.
The keying material distributed through a group rekeying MUST
include:
o a new Group Identifier (Gid) for the group, used as ID Context
parameter of the OSCORE Common Security Context of that group (see
Section 2 of [I-D.ietf-core-oscore-groupcomm]). Note that the Gid
differs from the plain group name introduced in Section 1.1, which
is a plain, stable and invariant identifier, with no cryptographic
relevance and meaning.
o a new value for the Master Secret parameter of the OSCORE Common
Security Context of that group (see Section 2 of
[I-D.ietf-core-oscore-groupcomm]).
Also, the distributed keying material MAY include a new value for the
Master Salt parameter of the OSCORE Common Security Context of that
group.
Upon generating the new group keying material and before starting its
distribution, the Group Manager MUST increment the version number of
the group keying material. When rekeying a group, the Group Manager
MUST preserve the current value of the Sender ID of each member in
that group.
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The Group Manager MUST support the Group Rekeying Process described
in Section 16. 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]. Note that what is
defined in [I-D.ietf-ace-key-groupcomm] applies, and only additions
or modifications to that specification are defined here.
3.1. Authorization Request
The Authorization Request message is as defined in Section 3.1 of
[I-D.ietf-ace-key-groupcomm], with the following additions.
o If the 'scope' parameter is present:
* The group name of each OSCORE group to join under the Group
Manager is encoded as a CBOR text string (REQ1).
* Accepted values for role identifiers in the OSCORE group to
join are: "requester", "responder", and "monitor" (REQ2).
Possible combinations are: ["requester" , "responder"]. An
additional role identifier is "verifier", denoting an external
signature verifier that does not join the OSCORE group. Each
role identifier MUST be encoded as a CBOR integer (REQ2), by
using for abbreviation the values specified in Figure 1 (OPT7)
(see Appendix A).
+-----------+------------+
| Name | CBOR Value |
+-----------+------------+
| requester | TBD8 |
| responder | TBD9 |
| monitor | TBD10 |
| verifier | TBD11 |
+-----------+------------+
Figure 1: CBOR Abbreviations for Role Identifiers in the Group
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3.2. Authorization Response
The Authorization Response message is as defined in Section 3.2 of
[I-D.ietf-ace-key-groupcomm], with the following additions:
o The AS MUST include the 'expires_in' parameter. Other means for
the AS to specify the lifetime of Access Tokens are out of the
scope of this specification.
o The AS MUST include the 'scope' parameter, 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 each
scope entry MUST be present, and includes the role or CBOR array
of roles that the joining node is actually authorized to take in
the OSCORE group for that scope entry, encoded as specified in
Section 3.1 of this document.
4. Interface at the Group Manager
The Group Manager provides the interface defined in Section 4.1 of
[I-D.ietf-ace-key-groupcomm], with the following additional resource:
o /group-oscore/GROUPNAME/active: this sub-resource is fixed and
supports the GET method, whose handler is defined in Section 4.1.
4.1. GET Handler
The handler expects a GET request.
The handler verifies that the group identifier of the /group-
oscore/GROUPNAME/active path is a subset of the 'scope' stored in the
Access Token associated to the requesting client. If verification
fails, the Group Manager MUST respond with a 4.01 (Unauthorized)
error message.
If verification succeeds, the handler returns a 2.05 (Content)
message containing the CBOR simple value True if the group is
currently active, or the CBOR simple value False otherwise. The
group is considered active if it is set to allow new members to join,
and if communication within the group is expected.
The method to set the current group status, i.e. active or inactive,
is out of the scope of this specification, and is defined for the
administrator interface of the Group Manager specified in
[I-D.tiloca-ace-oscore-gm-admin].
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5. Token POST and Group Joining
The following subsections describe the interactions between the
joining node and the Group Manager, i.e. the sending of the Access
Token and the Request-Response exchange to join the OSCORE group.
The message exchange between the joining node and the KDC consists of
the messages defined in Section 3.3 and 4.2 of
[I-D.ietf-ace-key-groupcomm]. Note that what is defined in
[I-D.ietf-ace-key-groupcomm] applies, and only additions or
modifications to that specification are defined here.
A signature verifier provides the Group Manager with an Access Token,
as described in Section 5.1, just as any another joining node does.
However, unlike candidate group members, it does not join any OSCORE
group, i.e. it does not perform the joining process defined in
Section 5.2. After a successful token posting, a signature verifier
is authorized to perform only the operations specified in Section 9,
to retrieve the public keys of group members, and only for the OSCORE
groups specified in the validated Access Token. The Group Manager
MUST respond with a 4.01 (Unauthorized) error message, in case a
signature verifier attempts to access any other endpoint than /group-
oscore/GROUPNAME/pub-key at the Group Manager.
5.1. Token Post
The Token post exchange is defined in Section 3.3 of
[I-D.ietf-ace-key-groupcomm].
Additionally to what defined in [I-D.ietf-ace-key-groupcomm], the
following applies.
o The 'kdcchallenge' parameter contains a dedicated nonce N_S
generated by the Group Manager. For the N_S value, it is
RECOMMENDED to use a 8-byte long random nonce. The joining node
may use this nonce in order to prove the possession of its own
private key, upon joining the group (see Section 5.2).
The 'kdcchallenge' parameter MAY be omitted from the 2.01
(Created) response, if the 'scope' of the Access Token includes
only the role "monitor" or only the role "verifier", for each of
the specified groups.
o If the 'sign_info' parameter is present in the response, the
following applies for each element 'sign_info_entry'.
* In the 'id' element, every group name is encoded as a CBOR text
string (REQ1) (see Appendix A).
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* 'sign_alg' takes value from the "Value" column of the "COSE
Algorithms" Registry [COSE.Algorithms], if not encoding the
CBOR simple value Null.
* If not encoding the CBOR simple value Null, 'sign_parameters'
is a CBOR array including the following two elements:
+ 'sign_alg_capab', encoded as a CBOR array. Its precise
format and value is the same as the COSE capabilities entry
in the "Capabilities" column of the "COSE Algorithms"
Registry [COSE.Algorithms], for the algorithm indicated in
'sign_alg' (REQ4).
+ 'sign_key_type_capab', encoded as a CBOR array. Its precise
format and value is the same as the COSE capabilities entry
in the "Capabilities" column of the "COSE Key Types"
Registry [COSE.Key.Types], for the algorithm indicated in
'sign_alg' (REQ4).
* If not encoding the CBOR simple value Null,
'sign_key_parameters' is a CBOR array. Its precise format and
value is the same as the COSE capabilities entry in the
"Capabilities" column of the "COSE Key Types" Registry
[COSE.Key.Types], for the algorithm indicated in 'sign_alg'
(REQ5).
* If 'pub_key_enc_res' is present, it takes value 1 ("COSE_Key")
from the 'Confirmation Key' column of the "CWT Confirmation
Method" Registry defined in [RFC8747], so indicating that
public keys in the OSCORE group are encoded as COSE Keys
[I-D.ietf-cose-rfc8152bis-struct]. Future specifications may
define additional values for this parameter.
Note that, other than through the above parameters as defined in
Section 3.3 of [I-D.ietf-ace-key-groupcomm], the joining node MAY
have previously retrieved this information by other means, e.g. by
using the approach described in [I-D.tiloca-core-oscore-discovery].
Additionally, if allowed by the used transport profile of ACE, the
joining node may instead provide the Access Token to the Group
Manager by other means, e.g. during a secure session establishment
(see Section 3.3.1 of [I-D.ietf-ace-dtls-authorize]).
5.2. Sending the Joining Request
The joining node requests to join the OSCORE group, by sending a
Joining Request message to the related group-membership resource at
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the Group Manager, as per Section 4.2 of
[I-D.ietf-ace-key-groupcomm].
Additionally to what defined in [I-D.ietf-ace-key-groupcomm], the
following applies.
o The string "group-oscore" is used instead of "ace-group" (see
Section 4.1 of [I-D.ietf-ace-key-groupcomm]) as the top level path
to the group-membership resource. The url-path /group-oscore/ is
a default name of this specifications: implementations are not
required to use this name, and can define their own instead.
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 joining process (see Section 6).
Otherwise, this parameter MUST NOT be present.
o 'cnonce' contains a dedicated nonce N_C generated by the joining
node. For the N_C value, it is RECOMMENDED to use a 8-byte long
random nonce.
o The signature encoded in the 'client_cred_verify' parameter is
computed by the joining node by using the same private key and
countersignature algorithm it intends to use for signing messages
in the OSCORE group. Moreover, N_S is as defined in
Section 5.2.1.
5.2.1. Value of the N_S Challenge
The value of the N_S challenge is determined as follows.
1. If the joining node has posted the Access Token to the /authz-
info endpoint of the Group Manager as in Section 5.1, N_S takes
the same value of the most recent 'kdcchallenge' parameter
received by the joining node from the Group Manager. This can be
either the one specified in the 2.01 (Created) response to the
Token POST, or the one possibly specified in a 4.00 (Bad Request)
response to a following Joining Request (see Section 5.3).
2. If the Token posting has relied on the DTLS profile of ACE
[I-D.ietf-ace-dtls-authorize] with the Access Token as content of
the "psk_identity" field of the ClientKeyExchange message
[RFC6347], N_S is an exporter value computed as defined in
Section 7.5 of [RFC8446]. Specifically, N_S is exported from the
DTLS session between the joining node and the Group Manager,
using an empty 'context_value', 32 bytes as 'key_length', and the
exporter label "EXPORTER-ACE-Sign-Challenge-coap-group-oscore-
app" defined in Section 18.7 of this specification.
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It is up to applications to define how N_S is computed in further
alternative settings.
Section 17.3 provides security considerations on the reusage of the
N_S challenge.
5.3. Processing the Joining Request
The Group Manager processes the Joining Request as defined in
Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm]. Additionally, the
following applies.
o In case the Joining Request does not include the 'client_cred'
parameter, the joining process fails if the Group Manager either:
i) does not store a public key with an accepted format for the
joining node; or ii) stores multiple public keys with an accepted
format for the joining node.
o To compute the signature contained in 'client_cred_verify', the GM
considers: i) as signed value, N_S concatenated with N_C, where
N_S is determined as described in Section 5.2.1, while N_C is the
nonce provided in the 'cnonce' parameter of the Joining Request;
ii) the countersignature algorithm used in the OSCORE group, and
possible correponding parameters; and iii) the public key of the
joining node, either retrieved from the 'client_cred' parameter,
or already stored as acquired from previous interactions with the
joining node.
o A 4.00 Bad Request response from the Group Manager to the joining
node MUST have content format application/ace+cbor. The response
payload is a CBOR map which MUST contain the 'sign_info'
parameter, including a single element 'sign_info_entry' pertaining
the OSCORE group that the joining node tried to join with the
Joining Request.
o The Group Manager MUST return a 4.00 (Bad Request) response in
case the Joining Request includes the 'client_cred' parameter but
does not include both the 'cnonce' and 'client_cred_verify'
parameters.
o The Group Manager MUST return a 4.00 (Bad Request) response in
case it cannot retrieve a public key with an accepted format for
the joining node, either from the 'client_cred' parameter or as
already stored.
o When receiving a 4.00 Bad Request response, the joining node
SHOULD send a new Joining Request to the Group Manager, where:
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* The 'cnonce' parameter MUST include a new dedicated nonce N_C
generated by the joining node.
* The 'client_cred' parameter MUST include a public key
compatible with the encoding, countersignature algorithm and
possible associated parameters indicated by the Group Manager.
* The 'client_cred_verify' parameter MUST include a signature
computed as described in Section 5.2, by using the public key
indicated in the current 'client_cred' parameter, with the
countersignature algorithm and possible associated parameters
indicated by the Group Manager. If the error response from the
Group Manager included the 'kdcchallenge' parameter, the
joining node MUST use its content as new N_S challenge to
compute the signature.
5.4. Joining Response
If the processing of the Joining Request described in Section 5.3 is
successful, the Group Manager updates the group membership by
registering the joining node NODENAME as a new member of the OSCORE
group GROUPNAME, as described in Section 4.1.2.1 of
[I-D.ietf-ace-key-groupcomm].
If the joining node is not exclusively configured as monitor, the
Group Manager performs also the following actions.
o The Group Manager selects an available OSCORE Sender ID in the
OSCORE group, and exclusively assigns it to the joining node.
o The Group Manager stores the association between i) the public key
of the joining node; and ii) the Group Identifier (Gid), i.e. the
OSCORE ID Context, associated to the OSCORE group together with
the OSCORE Sender ID assigned to the joining node in the group.
The Group Manager MUST keep this association updated over time.
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 success Joining
Response is formatted as defined in Section 4.1.2.1 of
[I-D.ietf-ace-key-groupcomm], with the following additions:
o The 'gkty' parameter identifies a key of type
"Group_OSCORE_Security_Context object", defined in Section 18.2 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
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[I-D.ietf-core-oscore-groupcomm]. This parameter has as value 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', 'cs_params', 'cs_key_params' and
'cs_key_enc' defined in Section 18.3 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, as
described above. This parameter is not present if the node
joins the group exclusively as monitor, 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), i.e. the OSCORE ID Context of the
OSCORE group.
* The 'cs_alg' parameter MUST be present and specifies the
algorithm used to countersign messages in the group. This
parameter takes values from the "Value" column of the "COSE
Algorithms" Registry [COSE.Algorithms].
* The 'cs_params' parameter MAY be present and specifies the
parameters for the counter signature algorithm. This parameter
is a CBOR array, which includes the following two elements:
+ 'sign_alg_capab', with the same encoding as defined in
Section 5.1. The value is the same as in the Token Post
response where the 'sign_parameters' value was non-null.
+ 'sign_key_type_capab', with the same encoding as defined in
Section 5.1. The value is the same as in the Token Post
response where the 'sign_parameters' value was non-null.
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* The 'cs_key_params' parameter MAY be present and specifies the
parameters for the key used with the counter signature
algorithm. This parameter is a CBOR array, with the same non-
null encoding and value as 'sign_key_parameters' of the
Section 5.1.
* The 'cs_key_enc' parameter MAY be present and specifies the
encoding of the public keys of the group members. This
parameter is a CBOR integer, whose value is 1 ("COSE_Key")
taken from the 'Confirmation Key' column of the "CWT
Confirmation Method" Registry defined in [RFC8747], so
indicating that public keys in the OSCORE group are encoded as
COSE Keys [I-D.ietf-cose-rfc8152bis-struct]. Future
specifications may define additional values for this parameter.
If this parameter is not present, 1 ("COSE_Key") MUST be
assumed as default value.
o The 'num' parameter MUST be present.
o The 'ace-groupcomm-profile' parameter MUST be present and has
value coap_group_oscore_app (TBD1), which is defined in
Section 18.1 of this specification.
o The 'exp' parameter MUST be present.
o The 'pub_keys' parameter, if present, includes the public keys of
the group members that are relevant to the joining node. That is,
it includes: i) the public keys of the responders 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 responder or
monitor. If public keys are encoded as COSE_Keys, each of them
has as 'kid' the Sender ID that the corresponding owner has in the
group, thus used as group member identifier.
o The 'group_policies' parameter SHOULD be present, and SHOULD
include the elements "Sequence Number Synchronization Method" and
"Key Update Check Interval" defined in Section 4.1.2 of
[I-D.ietf-ace-key-groupcomm], as well as the element "Group OSCORE
Pairwise Mode Support" defined in Section 5.5 of this
specification.
Finally, the joining node uses the information received in the
Joining Response to set up the OSCORE Security Context, as described
in Section 2 of [I-D.ietf-core-oscore-groupcomm]. In addition, the
joining node maintains an association between each public key
retrieved from the 'pub_keys' parameter and the role(s) that the
corresponding group member has in the group.
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From then on, the joining node can exchange group messages secured
with Group OSCORE as described in [I-D.ietf-core-oscore-groupcomm].
When doing so:
o The joining node MUST NOT process an incoming request message, if
signed by a group member whose public key is not associated to the
role "Requester".
o The joining node MUST NOT process an incoming response message, if
signed by a group member whose public key is not associated to the
role "Responder".
If the application requires backward security, the Group Manager MUST
generate updated security parameters and group keying material, and
provide it to the current group members upon the new node's joining
(see Section 16). As a consequence, the joining node is not able to
access secure communication in the group occurred prior its joining.
5.5. ACE Groupcomm Policy for Group OSCORE Pairwise Mode Support
This specifications defines the group policy "Group OSCORE Pairwise
Mode Support", for which it registers an entry in the "ACE Groupcomm
Policy" IANA Registry defined in Section 8.8 of
[I-D.ietf-ace-key-groupcomm].
The corresponding element in the 'group_policies' parameter of the
Joining Response (see Section 5.4) encodes the CBOR simple value
True, if the OSCORE group supports the pairwise mode of Group OSCORE
[I-D.ietf-core-oscore-groupcomm], or the CBOR simple value False
otherwise (REQ14).
6. Public Keys of Joining Nodes
Source authentication of a message sent within the group and
protected with Group OSCORE is ensured by means of a digital counter
signature embedded in the message (in group mode), or by integrity-
protecting the message with pairwise keying material derived from the
asymmetric keys of sender and recipient (in pairwise mode).
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.
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In particular, one of the following four cases can occur when a new
node joins an OSCORE group.
o The joining node is going to join the group exclusively as
monitor. 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, which thus does not have to perform any
check related to the public key encoding, or to a countersignature
algorithm and possible associated parameters for that joining
node. In case that joining node still provides a public key in
the 'client_cred' parameter of the Joining Request (see
Section 5.2), the Group Manager silently ignores that parameter,
as well as related the parameters 'cnonce' and
'client_cred_verify'.
o The Group Manager already acquired the public key of the joining
node during a past joining process. In this case, the joining
node MAY choose not to provide again its own public key to the
Group Manager, in order to limit the size of the Joining Request.
The joining node MUST provide its own public key again if it has
provided the Group Manager with multiple public keys during past
joining processes, intended for different OSCORE groups. If the
joining node provides its own public key, the Group Manager
performs consistency checks as per Section 5.3 and, in case of
success, considers it as the public key associated to the joining
node in the OSCORE group.
o The joining node and the Group Manager use an asymmetric proof-of-
possession key to establish a secure communication channel. Then,
two cases can occur.
1. The proof-of-possession key is compatible with the encoding as
well as with the counter signature algorithm and possible
associated parameters used in the OSCORE group. Then, the
Group Manager considers the proof-of-possession key as the
public key associated to the joining node in the OSCORE group.
If the joining node is aware that the proof-of-possession key
is also valid for the OSCORE group, it MAY not provide it
again as its own public key to the Group Manager. The joining
node MUST provide its own public key again if it has provided
the Group Manager with multiple public keys during past
joining processes, intended for different OSCORE groups. If
the joining node provides its own public key in the
'client_cred' parameter of the Joining Request (see
Section 5.2), the Group Manager performs consistency checks as
per Section 5.3 and, in case of success, considers it as the
public key associated to the joining node in the OSCORE group.
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2. The proof-of-possession key is not compatible with the
encoding or with the counter signature algorithm and possible
associated parameters used in the OSCORE group. In this case,
the joining node MUST provide a different compatible public
key to the Group Manager in the 'client_cred' parameter of the
Joining Request (see Section 5.2). Then, the Group Manager
performs consistency checks on this latest provided public key
as per Section 5.3 and, in case of success, considers it as
the public key associated to the joining node in the OSCORE
group.
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 joining process with that Group
Manager for the first time, the joining node specifies its own
public key in the 'client_cred' parameter of the Joining Request
targeting the group-membership endpoint (see Section 5.2).
7. Retrieval of Updated Keying Material
At some point, a group member considers the OSCORE Security Context
invalid and to be renewed. This happens, for instance, after a
number of unsuccessful security processing of incoming messages from
other group members, or when the Security Context expires as
specified by the 'exp' parameter of the Joining Response.
When this happens, the group member retrieves updated security
parameters and group keying material. This can occur in the two
different ways described below.
7.1. Retrieval of Group Keying Material
If the group member wants to retrieve only the latest group keying
material, it sends a Key Distribution Request to the Group Manager.
In particular, it sends a CoAP GET request to the endpoint /group-
oscore/GROUPNAME at the Group Manager.
The Group Manager processes the Key Distribution Request according to
Section 4.1.2.2 of [I-D.ietf-ace-key-groupcomm]. The Key
Distribution Response is formatted as defined in Section 4.1.2.2 of
[I-D.ietf-ace-key-groupcomm]. In particular, the 'key' parameter is
formatted as defined in Section 5.4 of this specification, with the
difference that it does not include the 'clientId' parameter.
Upon receiving the Key Distribution Response, the group member
retrieves the updated security parameters and group keying material,
and, if they differ from the current ones, use them to set up the new
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OSCORE Security Context as described in Section 2 of
[I-D.ietf-core-oscore-groupcomm].
7.2. Retrieval of Group Keying Material and Sender ID
If the group member wants to retrieve the latest group keying
material as well as the Sender ID that it has in the OSCORE group, it
sends a Key Distribution Request to the Group Manager.
In particular, it sends a CoAP GET request to the endpoint /group-
oscore/GROUPNAME/nodes/NODENAME at the Group Manager.
The Group Manager processes the Key Distribution Request according to
Section 4.1.6.2 of [I-D.ietf-ace-key-groupcomm]. The Key
Distribution Response is formatted as defined in Section 4.1.6.2 of
[I-D.ietf-ace-key-groupcomm].
In particular, the 'key' parameter is formatted as defined in
Section 5.4 of this specification, with the difference that if the
requesting group member is configured exclusively as monitor, no
'clientId' is specified within the 'key' parameter. Note that, in
any other case, the current Sender ID of the group member is not
specified as a separate parameter, but rather specified as 'clientId'
within the 'key' parameter.
Upon receiving the Key Distribution Response, the group member
retrieves the updated security parameters, group keying material and
Sender ID, and, if they differ from the current ones, use them to set
up the new OSCORE Security Context as described in Section 2 of
[I-D.ietf-core-oscore-groupcomm].
8. Retrieval of New Keying Material
As discussed in Section 2.4.2 of [I-D.ietf-core-oscore-groupcomm], a
group member may at some point exhaust its Sender Sequence Numbers in
the group.
When this happens, the group member MUST send a Key Renewal Request
message to the Group Manager, as per Section 4.4 of
[I-D.ietf-ace-key-groupcomm]. In particular, it sends a CoAP PUT
request to the endpoint /group-oscore/GROUPNAME/nodes/NODENAME at the
Group Manager.
Upon receiving the Key Renewal Request, the Group Manager processes
it as defined in Section 4.1.6.1 of [I-D.ietf-ace-key-groupcomm], and
performs one of the following actions.
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1. If the requesting group member is configured exclusively as
monitor, the Group Manager replies with a 4.00 (Bad Request)
error response.
2. Otherwise, depending on the configured policies (OPT8), the Group
Manager takes one of the following actions.
a. The Group Manager rekeys the OSCORE group. That is, the
Group Manager generates new group keying material for that group
(see Section 16), and replies to the group member with a group
rekeying message as defined in Section 16, providing the new
group keying material. Then, the Group Manager rekeys the rest
of the OSCORE group, as discussed in Section 16.
b. The Group Manager generates a new Sender ID for that group
member and replies with a Key Renewal Response, formatted as
defined in Section 4.1.6.1 of [I-D.ietf-ace-key-groupcomm]. In
particular, the CBOR Map in the response payload includes a
single parameter 'clientId' defined in Section 18.5 of this
document, specifying the new Sender ID of the group member
encoded as a CBOR byte string.
9. Retrieval of Public Keys of Group Members
A group member or a signature verifier may need to retrieve the
public keys of (other) group members. To this end, the group member
or signature verifier sends a Public Key Request message to the Group
Manager, as per Section 4.5 of [I-D.ietf-ace-key-groupcomm]. In
particular, it sends the request to the endpoint /group-
oscore/GROUPNAME/pub-key at the Group Manager.
If the Public Key Request uses the method FETCH, the Public Key
Request is formatted as defined in Section 4.1.3.1 of
[I-D.ietf-ace-key-groupcomm]. In particular, each element of the
'get_pub_keys' parameter is a CBOR byte string, which encodes the
Sender ID of the group member for which the associated public key is
requested.
Upon receiving the Public Key Request, the Group Manager processes it
as per Section 4.1.3.1 or 4.1.3.2 of [I-D.ietf-ace-key-groupcomm],
depending on the request method being FETCH or GET, respectively.
Additionally, if the Public Key Request uses the method FETCH, the
Group Manager silently ignores identifiers included in the
'get_pub_keys' parameter of the request that are not associated to
any current group member.
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The success Public Key Response is formatted as defined in
Section 4.1.3.1 or 4.1.3.2 of [I-D.ietf-ace-key-groupcomm], depending
on the request method being FETCH or GET, respectively.
10. Update of Public Key
A group member may need to provide the Group Manager with its new
public key to use in the group from then on, hence replacing the
current one. This can be the case, for instance, if the
countersignature algorithm and possible associated parameters used in
the OSCORE group have been changed, and the current public key is not
compatible with them.
To this end, the group member sends a Public Key Update Request
message to the Group Manager, as per Section 4.6 of
[I-D.ietf-ace-key-groupcomm]. In particular, it sends a CoAP POST
request to the endpoint /group-oscore/GROUPNAME/nodes/NODENAME/pub-
key at the Group Manager.
Upon receiving the Group Leaving Request, the Group Manager processes
it as per Section 4.1.7.1 of [I-D.ietf-ace-key-groupcomm], with the
following additions.
o If the requesting group member is configured exclusively as
monitor, the Group Manager replies with a 4.00 (Bad request) error
response.
o The N_S signature challenge is computed as per point (3) in
Section 5.2.1 (REQ17).
o If the request is successfully processed, the Group Manager stores
the association between i) the new public key of the group member;
and ii) the Group Identifier (Gid), i.e. the OSCORE ID Context,
associated to the OSCORE group together with the OSCORE Sender ID
assigned to the group member in the group. The Group Manager MUST
keep this association updated over time.
11. Retrieval of Group Policies
A group member may request the current policies used in the OSCORE
group. To this end, the group member sends a Policies Request, as
per Section 4.7 of [I-D.ietf-ace-key-groupcomm]. In particular, it
sends a CoAP GET request to the endpoint /group-oscore/GROUPNAME/
policies at the Group Manager, where GROUPNAME is the name of the
OSCORE group.
Upon receiving the Policies Request, the Group Manager processes it
as per Section 4.1.4.1 of [I-D.ietf-ace-key-groupcomm]. The success
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Policies Response is formatted as defined in Section 4.1.4.1 of
[I-D.ietf-ace-key-groupcomm].
12. Retrieval of Keying Material Version
A group member may request the current version of the keying material
used in the OSCORE group. To this end, the group member sends a
Version Request, as per Section 4.8 of [I-D.ietf-ace-key-groupcomm].
In particular, it sends a CoAP GET request to the endpoint /group-
oscore/GROUPNAME/ctx-num at the Group Manager, where GROUPNAME is the
name of the OSCORE group.
Upon receiving the Version Request, the Group Manager processes it as
per Section 4.1.5.1 of [I-D.ietf-ace-key-groupcomm]. The success
Version Response is formatted as defined in Section 4.1.5.1 of
[I-D.ietf-ace-key-groupcomm].
13. Retrieval of Group Status
A group member may request the current status of the the OSCORE
group, i.e. active or inactive. To this end, the group member sends
a Group Status Request to the Group Manager.
In particular, the group member sends a CoAP GET request to the
endpoint /group-oscore/GROUPNAME/active at the Group Manager defined
in Section 4 of this specification, where GROUPNAME is the name of
the OSCORE group. The success Group Version Response is formatted as
defined in Section 4 of this specification.
Upon learning from a 2.05 (Content) response that the group is
currently inactive, the group member SHOULD stop taking part in
communications within the group, until it becomes active again.
Upon learning from a 2.05 (Content) response that the group has
become active again, the group member can resume taking part in
communications within the group.
Figure 2 gives an overview of the exchange described above.
Group Group
Member Manager
| |
|------ Group Status Request: GET ace-group/GID/active ------>|
| |
|<---------- Group Status Response: 2.05 (Content) -----------|
| |
Figure 2: Message Flow of Group Status Request-Response
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14. Request to Leave the Group
A group member may request to leave the OSCORE group. To this end,
the group member sends a Group Leaving Request, as per Section 4.9 of
[I-D.ietf-ace-key-groupcomm]. In particular, it sends a CoAP DELETE
request to the endpoint /group-oscore/GROUPNAME/nodes/NODENAME at the
Group Manager.
Upon receiving the Group Leaving Request, the Group Manager processes
it as per Section 4.1.6.3 of [I-D.ietf-ace-key-groupcomm].
15. Removal of a Group Member
Other than after a spontaneous request to the Group Manager as
described in Section 14, a node may be forcibly removed from the
OSCORE group, e.g. due to expired or revoked authorization.
If, upon joining the group (see Section 5.2), the leaving node
specified a URI in the 'control_path' parameter defined in
Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm], the Group Manager
MUST inform the leaving node of its eviction, by sending a DELETE
request targeting the URI specified in the 'control_path' parameter
(OPT9).
If the leaving node is not configured exclusively as monitor, the
Group Manager performs the following actions.
o The Group Manager frees the OSCORE Sender ID value of the leaving
node, which becomes available for possible upcoming joining nodes.
o The Group Manager cancels the association between, on one hand,
the public key of the leaving node and, on the other hand, the
Group Identifier (Gid) associated to the OSCORE group together
with the freed OSCORE Sender ID value. The Group Manager deletes
the public key of the leaving node, if that public key has no
remaining association with any pair (Gid, Sender ID).
If the application requires forward security, the Group Manager MUST
generate updated security parameters and group keying material, and
provide it to the remaining group members (see Section 16). As a
consequence, the leaving node is not 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.
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16. Group Rekeying Process
In order to rekey the OSCORE group, the Group Manager distributes a
new Group Identifier (Gid), i.e. a new OSCORE ID Context; a new
OSCORE Master Secret; and, optionally, a new OSCORE Master Salt for
that group. When doing so, the Group Manager MUST increment the
version number of the group keying material, before starting its
distribution.
Furthermore, the Group Manager MUST preserve the same unchanged
Sender IDs for all group members. This avoids affecting the
retrieval of public keys from the Group Manager as well as the
verification of message countersignatures.
The Group Manager MUST support at least the following group rekeying
scheme. Future application profiles may define alternative message
formats and distribution schemes.
As group rekeying message, the Group Manager uses the same format of
the Joining Response message in Section 5.4. In particular:
o Only the parameters 'gkty', 'key', 'num', 'ace-groupcomm-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 joining process. In particular, each rekeying
message can target the 'control_path' URI path defined in
Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm] (OPT9), if provided
by the intended recipient upon joining the group (see Section 5.2).
It is RECOMMENDED that the Group Manager gets confirmation of
successful distribution from the group members, and admits a maximum
number of individual retransmissions to non-confirming group members.
This approach requires group members to act (also) as servers, in
order to correctly handle unsolicited group rekeying messages from
the Group Manager. In particular, if a group member and the Group
Manager use OSCORE [RFC8613] to secure their pairwise communications,
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the group member MUST create a Replay Window in its own Recipient
Context upon establishing the OSCORE Security Context with the Group
Manager, e.g. by means of the OSCORE profile of ACE
[I-D.ietf-ace-oscore-profile].
Group members and the Group Manager SHOULD additionally support
alternative rekeying approaches that do not require group members to
act (also) as servers. A number of such approaches are defined in
Section 4.3 of [I-D.ietf-ace-key-groupcomm]. In particular, a group
member may subscribe for updates to the group-membership resource of
the group, at the endpoint /group-oscore/GROUPNAME/nodes/NODENAME of
the Group Manager. This can rely on CoAP Observe [RFC7641] or on a
full-fledged Pub-Sub model [I-D.ietf-core-coap-pubsub] with the Group
Manager acting as Broker.
In case the rekeying terminates and some group members have not
received the new keying material, they will not be able to correctly
process following secured messages exchanged in the group. These
group members will eventually contact the Group Manager, in order to
retrieve the current keying material and its version.
Some of these group members may be in multiple groups, each
associated to a different Group Manager. When failing to correctly
process messages secured with the new keying material, these group
members may not have sufficient information to determine which exact
Group Manager they should contact, in order to retrieve the current
keying material they are missing.
If the Gid is formatted as described in Appendix C of
[I-D.ietf-core-oscore-groupcomm], the Group Prefix can be used as a
hint to determine the right Group Manager, as long as no collisions
among Group Prefixes are experienced. Otherwise, a group member
needs to contact the Group Manager of each group, e.g. by first
requesting only the version of the current group keying material (see
Section 12) and then possibly requesting the current keying material
(see Section 7.1).
Furthermore, some of these group members can be in multiple groups,
all of which associated to the same Group Manager. In this case,
these group members may also not have sufficient information to
determine which exact group they should refer to, when contacting the
right Group Manager. Hence, they need to contact a Group Manager
multiple times, i.e. separately for each group they belong to and
associated to that Group Manager.
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17. Security Considerations
Security considerations for this profile 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
transport profile of ACE signalled by the AS, such as
[I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-oscore-profile].
The following security considerations also apply for this profile.
17.1. Management of OSCORE Groups
This profile 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 3.1 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 group 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
repository of public keys of group members, and provides them upon
request.
o Synchronization of sequence numbers (see Section 6.1 of
[I-D.ietf-core-oscore-groupcomm]). This concerns how a responder
node that has just joined an OSCORE group can synchronize with the
sequence number of requesters in the same group.
Before sending the Joining Response, the Group Manager MUST verify
that the joining node actually owns the associated private key. To
this end, the Group Manager can rely on the proof-of-possession
challenge-response defined in Section 5. Alternatively, the joining
node can use its own public key as asymmetric proof-of-possession key
to establish a secure channel with the Group Manager, e.g. as in
Section 3.2 of [I-D.ietf-ace-dtls-authorize]. However, this requires
such proof-of-possession key to be compatible with the encoding as
well as with the countersignature algorithm and possible associated
parameters used in the OSCORE group.
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A node may have joined multiple OSCORE groups under different non-
synchronized Group Managers. Therefore, it can happen that those
OSCORE groups have the same Group Identifier (Gid). It follows that,
upon receiving a Group OSCORE message addressed to one of those
groups, the node would have multiple Security Contexts matching with
the Gid in the incoming message. It is up to the application to
decide how to handle such collisions of Group Identifiers, e.g. by
trying to process the incoming message using one Security Context at
the time until the right one is found.
17.2. Size of Nonces for Signature Challenge
With reference to the Joining Request message in Section 5.2, the
proof-of-possession signature included in 'client_cred_verify' is
computed over the challenge N_C | N_S, where | denotes concatenation.
For the N_C challenge share, it is RECOMMENDED to use a 8-byte long
random nonce. Furthermore, N_C is always conveyed in the 'cnonce'
parameter of the Joining Request, which is always sent over the
secure communication channel between the joining node and the Group
Manager.
As defined in Section 5.2.1, the way the N_S value is computed
depends on the particular way the joining node provides the Group
Manager with the Access Token, as well as on following interactions
between the two.
o If the Access Token is not explicitly posted to the /authz-info
endpoint of the Group Manager, then N_S is computed as a 32-byte
long challenge share (see points 2 of Section 5.2.1).
o If the Access Token has been explicitly posted to the /authz-info
endpoint of the Group Manager, N_S takes the most recent value
specified to the client by the Group Manager in the 'kdcchallenge'
parameter (see point 1 of Section 5.2.1). This is specified
either in the 2.01 response to the Token Post (see Section 5.1),
or in a 4.00 response to a following Joining Request (see
Section 5.3). In either case, it is RECOMMENDED to use a 8-byte
long random challenge as value for N_S.
If we consider both N_C and N_S to take 8-byte long values, the
following considerations hold.
o Let us consider both N_C and N_S as taking random values, and the
Group Manager to never change the value of the N_S provided to a
Client during the lifetime of an Access Token. Then, as per the
birthday paradox, the average collision for N_S will happen after
2^32 new posted Access Tokens, while the average collision for N_C
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will happen after 2^32 new Joining Requests. This amounts to
considerably more token provisionings than the expected new
joinings of OSCORE groups under a same Group Manager, as well as
to considerably more requests to join OSCORE groups from a same
Client using a same Access Token under a same Group Manager.
o Section 7 of [I-D.ietf-ace-oscore-profile] as well Appendix B.2 of
[RFC8613] recommend the use of 8-byte random values as well.
Unlike in those cases, the values of N_C and N_S considered in
this specification are not used for as sensitive operations as the
derivation of a Security Context, with possible implications in
the security of AEAD ciphers.
17.3. Reusage of Nonces for Signature Challenge
As long as the Group Manager preserves the same N_S value currently
associated to an Access Token, i.e. the latest value provided to a
Client in a 'kdcchallenge' parameter, the Client is able to
successfully reuse the same signature challenge for multiple Joining
Requests to that Group Manager.
In particular, the client can reuse the same N_C value for every
Joining Request to the Group Manager, and combine it with the same
unchanged N_S value. This results in reusing the same signature
challenge for producing the signature to include in the
'client_cred_verify' parameter of the Joining Requests.
Unless the Group Manager maintains a list of N_C values already used
by that Client since the latest update to the N_S value associated to
the Access Token, the Group Manager can be forced to falsely believe
that the Client possesses its own private key at that point in time,
upon verifying the signature in the 'client_cred_verify' parameter.
18. 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.
18.1. ACE Groupcomm Profile Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Profile" Registry defined in Section 8.7 of
[I-D.ietf-ace-key-groupcomm].
o Name: coap_group_oscore_app
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o Description: Application 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: TBD1
o Reference: [[This specification]] (Section 5.4)
18.2. ACE Groupcomm Key Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Key" Registry defined in Section 8.6 of [I-D.ietf-ace-key-groupcomm].
o Name: Group_OSCORE_Security_Context object
o Key Type Value: TBD2
o Profile: "coap_group_oscore_app", defined in Section 18.1 of this
specification.
o Description: A Group_OSCORE_Security_Context object encoded as
described in Section 5.4 of this specification.
o Reference: [[This specification]] (Section 5.4)
18.3. OSCORE Security Context Parameters Registry
IANA is asked to register the following entries in the "OSCORE
Security Context Parameters" Registry defined in Section 9.4 of
[I-D.ietf-ace-oscore-profile].
o Name: cs_alg
o CBOR Label: TBD3
o CBOR Type: tstr / int
o Registry: COSE Algorithm Values (ECDSA, EdDSA)
o Description: OSCORE Counter Signature Algorithm Value
o Reference: [[This specification]] (Section 5.4)
o Name: cs_params
o CBOR Label: TBD4
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o CBOR Type: array
o Registry: Counter Signatures Parameters
o Description: OSCORE Counter Signature Algorithm Additional
Parameters
o Reference: [[This specification]] (Section 5.4)
o Name: cs_key_params
o CBOR Label: TBD5
o CBOR Type: array
o Registry: Counter Signatures Key Parameters
o Description: OSCORE Counter Signature Key Additional Parameters
o Reference: [[This specification]] (Section 5.4)
o Name: cs_key_enc
o CBOR Label: TBD6
o CBOR Type: integer
o Registry: ACE Public Key Encoding
o Description: Encoding of Public Keys to be used with the OSCORE
Counter Signature Algorithm
o Reference: [[This specification]] (Section 5.4)
18.4. Sequence Number Synchronization Method Registry
IANA is asked to register the following entries in the "Sequence
Number Synchronization Method" Registry defined in Section 8.9 of
[I-D.ietf-ace-key-groupcomm].
o Name: Best effort
o Value: 1
o Description: No action is taken.
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o Reference: [I-D.ietf-core-oscore-groupcomm] (Appendix E.1)
o Name: Baseline
o Value: 2
o Description: The first received request sets the baseline
reference point, and is discarded with no delivery to the
application.
o Reference: [I-D.ietf-core-oscore-groupcomm] (Appendix E.2)
o Name: Echo challenge-response
o Value: 3
o Description: Challenge response using the Echo Option for CoAP
from [I-D.ietf-core-echo-request-tag].
o Reference: [I-D.ietf-core-oscore-groupcomm] (Appendix E.3)
18.5. ACE Groupcomm Parameters Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Parameters" Registry defined in Section 8.5 of
[I-D.ietf-ace-key-groupcomm].
o Name: clientId
o CBOR Key: TBD7
o CBOR Type: Byte string
o Reference: [[This specification]] (Section 8)
18.6. ACE Groupcomm Policy Registry
IANA is asked to register the following entry in the "ACE Groupcomm
Policy" Registry defined in Section 8.8 of
[I-D.ietf-ace-key-groupcomm].
o Name: Group OSCORE Pairwise Mode Support
o CBOR Key: TBD8
o CBOR Type: Simple value
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o Description: True if the OSCORE group supports the pairwise mode
of Group OSCORE [I-D.ietf-core-oscore-groupcomm], False otherwise.
o Reference: [[This specification]] (Section 5.5)
18.7. TLS Exporter Label Registry
IANA is asked to register the following entry in the "TLS Exporter
Label" Registry defined in Section 6 of [RFC5705] and updated in
Section 12 of [RFC8447].
o Value: EXPORTER-ACE-Sign-Challenge-coap-group-oscore-app
o DTLS-OK: Y
o Recommended: N
o Reference: [[This specification]] (Section 5.2.1)
19. References
19.1. Normative References
[COSE.Algorithms]
IANA, "COSE Algorithms",
<https://www.iana.org/assignments/cose/
cose.xhtml#algorithms>.
[COSE.Key.Types]
IANA, "COSE Key Types",
<https://www.iana.org/assignments/cose/
cose.xhtml#key-type>.
[I-D.ietf-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-ietf-ace-key-groupcomm-07
(work in progress), June 2020.
[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-33
(work in progress), February 2020.
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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", draft-ietf-ace-
oscore-profile-10 (work in progress), March 2020.
[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-08 (work in progress),
April 2020.
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-09
(work in progress), June 2020.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", draft-ietf-cose-rfc8152bis-
struct-10 (work in progress), June 2020.
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
19.2. 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-10 (work in progress), May 2020.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-09 (work in
progress), September 2019.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", draft-ietf-core-echo-
request-tag-09 (work in progress), March 2020.
[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", draft-
ietf-core-groupcomm-bis-00 (work in progress), March 2020.
[I-D.tiloca-ace-oscore-gm-admin]
Tiloca, M., Hoeglund, R., Stok, P., Palombini, F., and K.
Hartke, "Admin Interface for the OSCORE Group Manager",
draft-tiloca-ace-oscore-gm-admin-01 (work in progress),
March 2020.
[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-05 (work in progress), March 2020.
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[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>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
Appendix A. Profile Requirements
This appendix lists the specifications on this application profile of
ACE, based on the requiremens defined in Appendix A of
[I-D.ietf-ace-key-groupcomm].
o REQ1 - Specify the encoding and value of the identifier of group,
for scope entries of 'scope': see Section 3.1 and Section 5.1.
o REQ2 - Specify the encoding and value of roles, for scope entries
of 'scope': see Section 3.1.
o REQ3 - if used, specify the acceptable values for 'sign_alg':
values from the "Value" column of the "COSE Algorithms" Registry
[COSE.Algorithms].
o REQ4 - If used, specify the acceptable values for
'sign_parameters': values from the COSE capabilities in the "COSE
Algorithms" Registry [COSE.Algorithms] and from the COSE
capabilities in the "COSE Key Types" Registry [COSE.Key.Types].
o REQ5 - If used, specify the acceptable values for
'sign_key_parameters': values from the COSE capabilities in the
"COSE Key Types" Registry [COSE.Key.Types].
o REQ6 - If used, specify the acceptable values for 'pub_key_enc': 1
("COSE_Key") from the 'Confirmation Key' column of the "CWT
Confirmation Method" Registry defined in [RFC8747]. Future
specifications may define additional values for this parameter.
o REQ7 - Format of the 'key' value: see Section 5.4.
o REQ8 - Acceptable values of 'gkty': Group_OSCORE_Security_Context
object (see Section 5.4).
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o REQ9: Specify the format of the identifiers of group members: see
Section 5.4 and Section 9.
o REQ10 - Specify the communication protocol that the members of the
group must use: CoAP, possibly over IP multicast.
o REQ11 - Specify the security protocols that the group members must
use to protect their communication: Group OSCORE.
o REQ12 - Specify and register the application profile identifier:
coap_group_oscore_app (see Section 18.1).
o REQ13 - Specify policies at the KDC to handle member ids that are
not included in 'get_pub_keys': see Section 9.
o REQ14 - If used, specify the format and content of
'group_policies' and its entries: see Section 5.4; the three
values defined and registered, as content of the entry "Sequence
Number Synchronization Method" (see Section 18.4); the defined and
registered encoding of the entry "Group OSCORE Pairwise Mode
Support" (see Section 18.6).
o REQ15 - Specify the format of newly-generated individual keying
material for group members, or of the information to derive it,
and corresponding CBOR label: see Section 8.
o REQ16 - Specify how the communication is secured between the
Client and KDC: by means of any transport profile of ACE
[I-D.ietf-ace-oauth-authz] between Client and Group Manager that
complies with the requirements in Appendix C of
[I-D.ietf-ace-oauth-authz].
o REQ17: Specify how the nonce N_S is generated, if the token is not
being posted (e.g. if it is used directly to validate TLS
instead): see Section 5.2.1.
o REQ18: Specify if 'mgt_key_material' used, and if yes specify its
format and content: not used in this version of the profile.
o OPT1 (Optional) - Specify the encoding of public keys, of
'client_cred', and of 'pub_keys' if COSE_Keys are not used: no.
o OPT2 (Optional) - Specify the negotiation of parameter values for
signature algorithm and signature keys, if 'sign_info' and
'pub_key_enc' are not used: possible early discovery by using the
approach based on the CoRE Resource Directory described in
[I-D.tiloca-core-oscore-discovery].
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o OPT3 (Optional) - Specify the encoding of 'pub_keys_repos' if the
default is not used: no.
o OPT4 (Optional) - Specify policies that instruct clients to retain
unsuccessfully decrypted messages and for how long, so that they
can be decrypted after getting updated keying material: no.
o OPT5 (Optional) - Specify the behavior of the handler in case of
failure to retrieve a public key for the specific node: send a
4.00 Bad Request response to a Joining Request (see Section 5.3).
o OPT6 (Optional) - Specify possible or required payload formats for
specific error cases: send a 4.00 Bad Request response to a
Joining Request (see Section 5.3).
o OPT7 (Optional) - Specify CBOR values to use for abbreviating
identifiers of roles in the group or topic (see Section 3.1).
o OPT8 (Optional) - Specify policies for the KDC to perform group
rekeying after receiving a Key Renewal Request: no.
o OPT9 (Optional) - Specify the functionalities implemented at the
'control_path' resource hosted at the Client, including message
exchange encoding and other details (see Section 4.1.2.1 of
[I-D.ietf-ace-key-groupcomm]): see Section 15 for the eviction of
a group member; see Section 16 for the group rekeying process.
o OPT10 (Optional) - Specify how the identifier of the sender's
public key is included in the group request: no.
Appendix B. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
B.1. Version -06 to -07
o Alignments with draft-ietf-core-oscore-groupcomm.
o New format of 'sign_info', using the COSE capabilities.
o New format of Joining Response parameters, using the COSE
capabilities.
o Considerations on group rekeying.
o Editorial revision.
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B.2. Version -05 to -06
o Added role of external signature verifier.
o Parameter 'rsnonce' renamed to 'kdcchallenge'.
o Parameter 'kdcchallenge' may be omitted in some cases.
o Clarified difference between group name and OSCORE Gid.
o Removed the role combination ["requester", "monitor"].
o Admit implicit scope and audience in the Authorization Request.
o New format for the 'sign_info' parameter.
o Scope not mandatory to include in the Joining Request.
o Group policy about supporting Group OSCORE in pairwise mode.
o Possible individual rekeying of a single requesting node combined
with a group rekeying.
o Security considerations on reusage of signature challenges.
o Addressing optional requirement OPT9 from draft-ietf-ace-key-
groupcomm
o Editorial improvements.
B.3. Version -04 to -05
o Nonce N_S also in error responses to the Joining Requests.
o Supporting single Access Token for multiple groups/topics.
o Supporting legal requesters/responders using the 'peer_roles'
parameter.
o Registered and used dedicated label for TLS Exporter.
o Added method for uploading a new public key to the Group Manager.
o Added resource and method for retrieving the current group status.
o Fixed inconsistency in retrieving group keying material only.
o Clarified retrieval of keying material for monitor-only members.
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o Clarification on incrementing version number when rekeying the
group.
o Clarification on what is re-distributed with the group rekeying.
o Security considerations on the size of the nonces used for the
signature challenge.
o Added CBOR values to abbreviate role identifiers in the group.
B.4. Version -03 to -04
o New abstract.
o Moved general content to draft-ietf-ace-key-groupcomm
o Terminology: node name; node resource.
o Creation and pointing at node resource.
o Updated Group Manager API (REST methods and offered services).
o Size of challenges 'cnonce' and 'rsnonce'.
o Value of 'rsnonce' for reused or non-traditionally-posted tokens.
o Removed reference to RFC 7390.
o New requirements from draft-ietf-ace-key-groupcomm
o Editorial improvements.
B.5. Version -02 to -03
o New sections, aligned with the interface of ace-key-groupcomm .
o Exchange of information on the countersignature algorithm and
related parameters, during the Token POST (Section 4.1).
o Nonce 'rsnonce' from the Group Manager to the Client
(Section 4.1).
o Client PoP signature in the Key Distribution Request upon joining
(Section 4.2).
o Local actions on the Group Manager, upon a new node's joining
(Section 4.2).
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o Local actions on the Group Manager, upon a node's leaving
(Section 12).
o IANA registration in ACE Groupcomm Parameters Registry.
o More fulfilled profile requirements (Appendix A).
B.6. Version -01 to -02
o Editorial fixes.
o Changed: "listener" to "responder"; "pure listener" to "monitor".
o Changed profile name to "coap_group_oscore_app", to reflect it is
an application profile.
o Added the 'type' parameter for all requests to a Join Resource.
o Added parameters to indicate the encoding of public keys.
o Challenge-response for proof-of-possession of signature keys
(Section 4).
o Renamed 'key_info' parameter to 'sign_info'; updated its format;
extended to include also parameters of the countersignature key
(Section 4.1).
o Code 4.00 (Bad request), in responses to joining nodes providing
an invalid public key (Section 4.3).
o Clarifications on provisioning and checking of public keys
(Sections 4 and 6).
o Extended discussion on group rekeying and possible different
approaches (Section 7).
o Extended security considerations: proof-of-possession of signature
keys; collision of OSCORE Group Identifiers (Section 8).
o Registered three entries in the IANA Registry "Sequence Number
Synchronization Method Registry" (Section 9).
o Registered one public key encoding in the "ACE Public Key
Encoding" IANA Registry (Section 9).
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B.7. 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, Carsten
Bormann, Martin Gunnarsson, Rikard Hoeglund, Daniel Migault, 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
Tiloca, et al. Expires December 20, 2020 [Page 41]
Internet-Draft Key Management for OSCORE Groups in ACE June 2020
Francesca Palombini
Ericsson AB
Torshamnsgatan 23
Kista SE-16440 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
Tiloca, et al. Expires December 20, 2020 [Page 42]
