Key Provisioning for Group Communication using ACE
draft-palombini-ace-key-groupcomm-02

Abstract

This document defines message formats and procedures for requesting and distributing group keying material using the ACE framework, to protect communications between group members.

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 April 25, 2019.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. Overview
• 3. Authorization to Join a Group
• 3.1. Authorization Request
• 3.2. Authorization Response
• 3.3. Token Post
• 4. Key Distribution
• 4.1. Key Distribution Request
• 4.2. Key Distribution Response
• 5. Removal of a Node from the Group
• 5.1. Expired Authorization
• 5.2. Request to Leave the Group
• 6. Retrieval of Updated Keying Material
• 6.1. Key Re-Distribution Request
• 6.2. Key Re-Distribution Response
• 7. Retrieval of Public Keys for Group Members
• 7.1. Public Key Request
• 7.2. Public Key Response
• 8. Security Considerations
• 9. IANA Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Acknowledgments
• Authors' Addresses

1. Introduction

This document expands the ACE framework [I-D.ietf-ace-oauth-authz] to define the format of messages used to request, distribute and renew the keying material in a group communication scenario, e.g. based on multicast [RFC7390] or on publishing-subscribing [I-D.ietf-core-coap-pubsub].

Profiles that use group communication can build on this document to specify the selection of the message parameters defined in this document to use and their values. Known applications that can benefit from this document would be, for example, profiles addressing group communication based on multicast [RFC7390] or publishing/subscribing [I-D.ietf-core-coap-pubsub] in ACE.

If the application requires backward and forward security, updated keying material is generated and distributed to the group members (rekeying), when membership changes. A key management scheme performs the actual distribution of the updated keying material to the group. In particular, the key management scheme rekeys the current group members when a new node joins the group, and the remaining group members when a node leaves the group. This document provides a message format for group rekeying that allows to fulfill these requirements. Rekeying mechanisms can be based on [RFC2093], [RFC2094] and [RFC2627].

1.1. Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119]. These words may also appear in this document in lowercase, absent their normative meanings.

Readers are expected to be familiar with the terms and concepts described in [I-D.ietf-ace-oauth-authz] and [RFC8152], such as Authorization Server (AS) and Resource Server (RS).

2. Overview

+------------+                  +-----------+
|     AS     |                  |    KDC    |
|            |        .-------->|           |
+------------+       /          +-----------+
      ^             / 
      |            /                  
      v           /                           +-----------+
+------------+   /      +------------+        |+-----------+
|   Client   |<-'       | Dispatcher |        ||+-----------+
|            |<-------->|    (RS)    |<------->||   Group   |
+------------+          +------------+         +|  members  |
                                                +-----------+

Figure 1: Key Distribution Participants

The following participants (see Figure 1) take part in the authorization and key distribution.

• Client (C): node that wants to join the group communication. It can request write and/or read rights.
• Authorization Server (AS): same as AS in the ACE Framework; it enforces access policies, and knows if a node is allowed to join the group with write and/or read rights.
• Key Distribution Center (KDC): maintains the keying material to protect group communications, and provides it to Clients authorized to join the group. During the first part of the exchange (Section 3), it takes the role of the RS in the ACE Framework. During the second part (Section 4), which is not based on the ACE Framework, it distributes the keying material. In addition, it provides the latest keying material to group members when requested. If required by the application, the KDC renews and re-distributes the keying material in the group when membership changes.
• Dispatcher: entity through which the Clients communicate with the group and which distributes messages to the group members. Examples of dispatchers are: the Broker node in a pub-sub setting; a relayer node for group communication that delivers group messages as multiple unicast messages to all group members; an implicit entity as in a multicast communication setting, where messages are transmitted to a multicast IP address and delivered on the transport channel.

This document specifies the message flows and formats for:

• Authorizing a new node to join the group (Section 3), and providing it with the group keying material to communicate with the other group members (Section 4).
• Removing of a current member from the group (Section 5).
• Retrieving keying material as a current group member (Section 6 and Section 7).
• Renewing and re-distributing the group keying material (rekeying) upon a membership change in the group (Section 4.2 and Section 5).

Figure 2 provides a high level overview of the message flow for a node joining a group communication setting.

C                              AS     KDC   Dispatcher          Group
|                              |       |        |               Member
|                              |       |        | \               |
|     Authorization Request    |       |        | | Defined       |
|----------------------------->|       |        | | in the ACE    |
|                              |       |        | | framework     |
|     Authorization Response   |       |        | |               |
|<-----------------------------|       |        | |               |
|                              |       |        | |               |
|--------- Token Post ---------------->|        | /               |
|                                      |        |                 |
|---- Key Distribution Request ------->|        |                 |
|                                      |        |                 |
|<--- Key Distribution Response ------ | --- Group Rekeying ----->|
|                                               |                 |
|<================== Protected communication ===|================>|
|                                               |                 |

Figure 2: Message Flow Upon New Node's Joining

The exchange of Authorization Request and Authorization Response between Client and AS MUST be secured, as specified by the ACE profile used between Client and KDC.

The exchange of Key Distribution Request and Key Distribution Response between Client and KDC MUST be secured, as a result of the ACE profile used between Client and KDC.

All further communications between the Client and the KDC MUST be secured, for instance with the same security mechanism used for the Key Distribution exchange.

All further communications between a Client and the other group members MUST be secured using the keying material provided in Section 4.

3. Authorization to Join a Group

This section describes in detail the format of messages exchanged by the participants when a node requests access to a group. The first part of the exchange is based on ACE [I-D.ietf-ace-oauth-authz].

As defined in [I-D.ietf-ace-oauth-authz], the Client requests from the AS an authorization to join the group through the KDC (see Section 3.1). If the request is approved and authorization is granted, the AS provides the Client with a proof-of-possession access token and parameters to securely communicate with the KDC (see Section 3.2). Communications between the Client and the AS MUST be secured, and depends on the profile of ACE used.

Figure 3 gives an overview of the exchange described above.

Client                                            AS  KDC
   |                                               |   |
   |---- Authorization Request: POST /token ------>|   |
   |                                               |   |
   |<--- Authorization Response: 2.01 (Created) ---|   |
   |                                               |   |
   |----- POST Token: POST /authz-info --------------->|
   |                                                   |

Figure 3: Message Flow of Join Authorization

3.1. Authorization Request

The Authorization Request sent from the Client to the AS is as defined in Section 5.6.1 of [I-D.ietf-ace-oauth-authz] and MUST contain the following parameters:

• ‘grant_type’, with value “client_credentials”.

Additionally, the Authorization Request MAY contain the following parameters, which, if included, MUST have the corresponding values:

• ‘scope’, with value the identifier of the specific group or topic the Client wishes to access, and optionally the role(s) the Client wishes to take. This value is a CBOR array encoded as a byte string, which contains:
  • As first element, the identifier of the specific group or topic.
  • Optionally, as second element, the role (or CBOR array of roles) the Client wishes to take in the group.

  The encoding of the group or topic identifier and of the role identifiers is application specific.

• ‘req_aud’, as defined in Section 3.1 of [I-D.ietf-ace-oauth-params], with value an identifier of the KDC.
• ‘req_cnf’, as defined in Section 3.1 of [I-D.ietf-ace-oauth-params], optionally containing the public key or the certificate of the Client, if it wishes to communicate that to the AS.

• Other additional parameters as defined in [I-D.ietf-ace-oauth-authz], if necessary.

3.2. Authorization Response

The Authorization Response sent from the AS to the Client is as defined in Section 5.6.2 of [I-D.ietf-ace-oauth-authz] and MUST contain the following parameters:

• ‘access_token’, containing the proof-of-possession access token.
• ‘cnf’ if symmetric keys are used, not present if asymmetric keys are used. This parameter is defined in Section 3.2 of [I-D.ietf-ace-oauth-params] and contains the symmetric proof-of-possession key that the Client is supposed to use with the KDC.
• ‘rs_cnf’ if asymmetric keys are used, not present if symmetric keys are used. This parameter is as defined in Section 3.2 of [I-D.ietf-ace-oauth-params] and contains information about the public key of the KDC.
• ‘exp’, contains the lifetime in seconds of the access token. This parameter MAY be omitted if the application defines how the expiration time is communicated to the Client via other means, or if it establishes a default value.

Additionally, the Authorization Response MAY contain the following parameters, which, if included, MUST have the corresponding values:

• ‘scope’, which mirrors the ‘scope’ parameter in the Authorization Request (see Section 3.1). Its value is a CBOR array encoded as a byte string, containing:
  • As first element, the identifier of the specific group or topic the Client is authorized to access.
  • Optionally, as second element, the role (or CBOR array of roles) the Client is authorized to take in the group.

  The encoding of the group or topic identifier and of the role identifiers is application specific.

• Other additional parameters as defined in [I-D.ietf-ace-oauth-authz], if necessary.

The access token MUST contain all the parameters defined above (including the same ‘scope’ as in this message, if present, or the ‘scope’ of the Authorization Request otherwise), and additionally other optional parameters the profile requires.

When receiving an Authorization Request from a Client that was previously authorized, and which still owns a valid non expired access token, the AS can simply reply with an Authorization Response including a new access token.

3.3. Token Post

The Client sends a CoAP POST request including the access token to the KDC, as specified in section 5.8.1 of [I-D.ietf-ace-oauth-authz]. If the specific ACE profile defines it, the Client MAY use a different endpoint than /authz-info at the KDC to post the access token to. After successful verification, the Client is authorized to receive the group keying material from the KDC and join the group.

Note that this step could be merged with the following message from the Client to the KDC, namely Key Distribution Request.

4. Key Distribution

This section defines how the keying material used for group communication is distributed from the KDC to the Client, when joining the group as a new member.

If not previously established, the Client and the KDC MUST first establish a pairwise secure communication channel using ACE. The exchange of Key Distribution Request-Response MUST occur over that secure channel. The Client and the KDC MAY use that same secure channel to protect further pairwise communications, that MUST be secured.

During this exchange, the Client sends a request to the AS, specifying the group it wishes to join (see Section 4.1). Then, the KDC verifies the access token and that the Client is authorized to join that group; if so, it provides the Client with the keying material to securely communicate with the member of the group (see Section 4.2).

Figure 4 gives an overview of the exchange described above.

Client                                               KDC
   |                                                  |
   |---- Key Distribution Request: POST /group-id --->|
   |                                                  |
   |<--- Key Distribution Response: 2.01 (Created) ---|
   |                                                  |

Figure 4: Message Flow of Key Distribution to a New Group Member

The same set of message can also be used for the following cases, when the Client is already a group member:

• The Client wishes to (re-)get the current keying material, for cases such as expiration, loss or suspected mismatch, due to e.g. reboot or missed group rekeying. This is further discussed in Section 6.
• The Client wishes to (re-)get the public keys of other group members, e.g. if it is aware of new nodes joining the group after itself. This is further discussed in Section 7.

Additionally, the format of the payload of the Key Distribution Response (Section 4.2) can be reused for messages sent by the KDC to distribute updated group keying material, in case of a new node joining the group or of a current member leaving the group. The key management scheme used to send such messages could rely on, e.g., multicast in case of a new node joining or unicast in case of a node leaving the group.

Note that proof-of-possession to bind the access token to the Client is performed by using the proof-of-possession key bound to the access token for establishing secure communication between the Client and the KDC.

4.1. Key Distribution Request

The Client sends a Key Distribution request to the KDC. This corresponds to a CoAP POST request to the endpoint in the KDC associated to the group to join. The endpoint in the KDC is associated to the ‘scope’ value of the Authorization Request/Response. The payload of this request is a CBOR Map which MAY contain the following fields, which, if included, MUST have the corresponding values:

• ‘scope’, with value the specific resource that the Client is authorized to access (i.e. group or topic identifier) and role(s), encoded as in Section 3.1.

• ‘get_pub_keys’, if the Client wishes to receive the public keys of the other nodes in the group from the KDC. The value is an empty CBOR Array. This parameter may be present if the KDC stores the public keys of the nodes in the group and distributes them to the Client; it is useless to have here if the set of public keys of the members of the group is known in another way, e.g. it was provided by the AS.

• ‘client_cred’, with value the public key or certificate of the Client. If the KDC is managing (collecting from/distributing to the Client) the public keys of the group members, this field contains the public key of the Client.
• ‘pub_keys_repos’, can be present if a certificate is present in the ‘client_cred’ field, with value a list of public key repositories storing the certificate of the Client.

4.2. Key Distribution Response

The KDC verifies the access token and, if verification succeeds, sends a Key Distribution success Response to the Client. This corresponds to a 2.01 Created message. The payload of this response is a CBOR Map which MUST contain the following fields:

• ‘key’, used to send the keying material to the Client, as a COSE_Key ([RFC8152]) containing the following parameters:
  • ‘kty’, as defined in [RFC8152].
  • ‘k’, as defined in [RFC8152].
  • ‘exp’ (optionally), as defined below. This parameter is RECOMMENDED to be included in the COSE_Key. If omitted, the authorization server SHOULD provide the expiration time via other means or document the default value.
  • ‘alg’ (optionally), as defined in [RFC8152].
  • ‘kid’ (optionally), as defined in [RFC8152].
  • ‘base iv’ (optionally), as defined in [RFC8152].
  • ‘clientID’ (optionally), as defined in [I-D.ietf-ace-oscore-profile].
  • ‘serverID’ (optionally), as defined in [I-D.ietf-ace-oscore-profile].
  • ‘kdf’ (optionally), as defined in [I-D.ietf-ace-oscore-profile].
  • ‘slt’ (optionally), as defined in [I-D.ietf-ace-oscore-profile].
  • ‘cs_alg’ (optionally), containing the algorithm value to countersign the message, taken from Table 5 and 6 of [RFC8152].

The parameter ‘exp’ identifies the expiration time in seconds after which the COSE_Key is not valid anymore for secure communication in the group. A summary of ‘exp’ can be found in Figure 5.

+------+-------+----------------+------------+-----------------+
| Name | Label | CBOR Type      | Value      | Description     |
|      |       |                | Registry   |                 |
+------+-------+----------------+------------+-----------------+
| exp  | TBD   | Integer or     | COSE Key   | Expiration time |
|      |       | floating-point | Common     | in seconds      |
|      |       | number         | Parameters |                 |
+------+-------+----------------+------------+-----------------+

Figure 5: COSE Key Common Header Parameter 'exp'

Optionally, the Key Distribution Response MAY contain the following parameters, which, if included, MUST have the corresponding values:

• ‘pub_keys’, may only be present if ‘get_pub_keys’ was present in the Key Distribution Request; this parameter is a COSE_KeySet (see [RFC8152]), which contains the public keys of all the members of the group.
• ‘group_policies’, with value a list of parameters indicating how the group handles specific management aspects. This includes, for instance, approaches to achieve synchronization of sequence numbers among group members. The exact format of this parameter is specific to the profile.
• ‘mgt_key_material’, with value the administrative keying material to participate in the group rekeying performed by the KDC. The exact format and content depend on the specific rekeying scheme used in the group, which may be specified in the profile.

Specific profiles need to specify how exactly the keying material is used to protect the group communication.

If the application requires backward security, the KDC SHALL generate new group keying material and securely distribute it to all the current group members, using the message format defined in this section. Application profiles may define alternative message formats.

TBD: define for verification failure

5. Removal of a Node from the Group

This section describes at a high level how a node can be removed from the group.

If the application requires forward security, the KDC SHALL generate new group keying material and securely distribute it to all the current group members but the leaving node, using the message format defined in Section 4.2. Application profiles may define alternative message formats.

5.1. Expired Authorization

If the node is not authorized anymore, the AS can directly communicate that to the KDC. Alternatively, the access token might have expired. If Token introspection is provided by the AS, the KDC can use it as per Section 5.7 of [I-D.ietf-ace-oauth-authz], in order to verify that the access token is still valid.

Either case, once aware that a node is not authorized anymore, the KDC has to remove the unauthorized node from the list of group members, if the KDC keeps track of that.

5.2. Request to Leave the Group

A node can actively request to leave the group. In this case, the Client can send a request formatted as follows to the KDC, to abandon the group.

TBD: Format of the message to leave the group

The KDC should then remove the leaving node from the list of group members, if the KDC keeps track of that.

Note that, after having left the group, a node may wish to join it again. Then, as long as the node is still authorized to join the group, i.e. it has a still valid access token, it can re-request to join the group directly to the KDC without needing to retrieve a new access token from the AS. This means that the KDC needs to keep track of nodes with valid access tokens, before deleting all information about the leaving node.

6. Retrieval of Updated Keying Material

A node stops using the group keying material upon its expiration, according to the ‘exp’ parameter specified in the retained COSE Key. Then, if it wants to continue participating in the group communication, the node has to request new updated keying material to the KDC.

The Client may perform the same request to the KDC also upon receiving messages from other group members without being able to correctly decrypt them. This may be due to a previous update of the group keying material (rekeying) triggered by the KDC, that the Client was not able to receive or decrypt.

Note that policies can be set up so that the Client sends a request to the KDC only after a given number of unsuccessfully decrypted incoming messages.

6.1. Key Re-Distribution Request

To request a re-distribution of keying material, the Client sends a shortened Key Distribution Request to the KDC (Section 4.1), formatted as follows. The payload MUST contain only the following field:

• ‘scope’, which contains only the identifier of the specific group or topic, encoded as in Section 3.1. That is, the role field is not present.

6.2. Key Re-Distribution Response

The KDC receiving a Key Re-Distribution Request MUST check that it is storing a valid access token from that client for that scope.

TODO: defines error response if it does not have it / is not valid.

The KDC replies to the Client with a Key Distribution Response containing the ‘key’ parameter, and optionally ‘group_policies’ and ‘mgt_key_material’, as specified in Section 4.2. Note that this response might simply re-provide the same keying material currently owned by the Client, if it has not been renewed.

7. Retrieval of Public Keys for Group Members

In case the KDC maintains the public keys of group members, a node in the group can contact the KDC to request public keys of either all group members or a specified subset, using the messages defined below.

Figure 6 gives an overview of the exchange described above.

Client                                         KDC
   |                                            |
   |---- Public Key Request: POST /group-id --->|
   |                                            |
   |<--- Public Key Response: 2.01 (Created) ---|
   |                                            |

Figure 6: Message Flow of Public Key Request-Response

Note that these messages can be combined with the Key Re-Distribution messages in Section 6, to request at the same time the keying material and the public keys. In this case, either a new endpoint at the KDC may be used, or additional information needs to be sent in the request payload, to distinguish these combined messages from the Public Key messages described below, since they would be identical otherwise.

7.1. Public Key Request

To request public keys, the Client sends a shortened Key Distribution Request to the KDC (Section 4.1), formatted as follows. The payload of this request MUST contain the following fields:

• ‘get_pub_keys’, which has as value a CBOR array including either:
  • no elements, i.e. an empty array, in order to request the public key of all current group members; or
  • N elements, each of which is the identifier of a group member, in order to request the public key of the specified nodes.
• ‘scope’, which contains only the identifier of the specific group or topic, encoded as in Section 3.1. That is, the role field is not present.

7.2. Public Key Response

The KDC replies to the Client with a Key Distribution Response containing only the ‘pub_keys’ parameter, as specified in Section 4.2. The payload of this response contains the following field:

• ‘pub_keys’, which contains either:
  • the public keys of all the members of the group, if the ‘get_pub_keys’ parameter of the Public Key request was an empty array; or
  • the public keys of the group members with the identifiers specified in the ‘get_pub_keys’ parameter of the Public Key request.

The KDC ignores possible identifiers included in the ‘get_pub_keys’ parameter of the Public Key request if they are not associated to any current group member.

8. Security Considerations

The KDC must renew the group keying material upon its expiration.

The KDC should renew the keying material upon group membership change, and should provide it to the current group members through the rekeying scheme used in the group.

9. IANA Considerations

The following registration is required for the COSE Key Common Parameter Registry specified in Section 16.5 of [RFC8152]:

• Name: exp
• Label: TBD
• CBOR Type: Integer or floating-point number
• Value Registry: COSE Key Common Parameters
• Description: Identifies the expiration time in seconds of the COSE Key
• Reference: [[this specification]]

10. References

10.1. Normative References

10.2. Informative References

Acknowledgments

The following individuals were helpful in shaping this document: Ben Kaduk, John Mattsson, Jim Schaad, Ludwig Seitz, Göran Selander and Peter van der Stok.

The work on this document has been partly supported by the EIT-Digital High Impact Initiative ACTIVE.

Authors' Addresses