ACE Working Group S. Gerdes
Internet-Draft Universität Bremen TZI
Intended status: Informational March 09, 2015
Expires: September 10, 2015

Managing the Authorization to Authorize in the Lifecycle of a Constrained Device


Constrained nodes are devices which are limited in terms of processing power, memory, non-volatile storage and transmission capacity. Due to these constraints, commonly used security protocols are not easily applicable. Nevertheless, an authentication and authorization solution is needed to ensure the security of these devices.

During the lifecycle of a constrained device, responsibility for managing authorization policies for the constrained device may change several times. To ensure the security of the constrained devices, the authorization to authorize must be transferred to the new principal in a secure way.

The Delegated CoAP Authorization Framework (DCAF) specifies how resource-constrained nodes can delegate defined authentication- and authorization-related tasks to less-constrained devices called Authorization Managers, thus limiting the hardware requirements of the security solution for the constrained devices.

This document defines how DCAF can be used to manage the Authorization Manager of a constrained device and introduces a flexible authorization solution for the whole lifecycle of a constrained device.

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Table of Contents

1. Introduction

As shown in [I-D.gerdes-ace-actors], constrained devices can benefit from being closely coupled to a less constrained device, the Authorization Manager (AM). The AM helps its constrained devices with authentication and authorization tasks. The delegated CoAP Authentication and Authorization Framework (DCAF) [I-D.gerdes-ace-dcaf-authorize] defines the communication flow between client, server and their respective Authorization Managers, thus relieving constrained nodes from managing keys for numerous devices while ensuring that the constrained devices are able to enforce the authorization policies of their principals.

Since the constrained devices strongly rely on their Authorization Managers for security-related tasks, the connection between the constrained device and its respective AM needs to be especially protected. This is particularly difficult at transitions between different phases in the lifecycle of a constrained device. These transitions often comprise a change of the device ownership and therefore might often entail that the principal that controls the authorization policies changes. One way of transferring this authorization to authorize is to change which Authorization Manager is responsible for a constrained device.

This document defines how DCAF can be used to manage the Authorization Manager of a constrained device and introduces a flexible authorization solution for the whole lifecycle of a constrained device.

2. 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 RFC 2119 [RFC2119].

3. Authorization to Authorize

AM helps its constrained device to determine the authorization of another device, e.g. if it is allowed to access an item of interest or to provide information about such an item. Some security-related tasks must be conducted by the constrained device itself, such as message authentication and the enforcement of authorization policies. However, the information needed for these tasks are provided by the AM which represents the principal’s will to the constrained device. Principals can easily configure the AM since it has the necessary user interface. In particular, AM provides authorization information to the constrained device: It is authorized to define authorizations.

The constrained device shares a symmetric key with its AM. We call this key K_AM. The constrained device uses this key to determine if the authorization information was provided by the AM.

K_AM is stored in a resource which we call AM-Key, e.g. /am/key. The key belongs to a URI which is the address of the Authorization Manager. The URI is stored in resource that we call AM-URI, e.g. /am/uri.

The AM-key resource needs special protection because the entity which controls K_AM is in control of the constrained device. Therefore, the AM-key resource MUST be access-protected and SHOULD be write-only.

4. Assigning a new Authorization Manager

To assign a new AM to a constrained device, the AM-key resource must be changed. In this case, the constrained device always acts as the Server, even if it is generally used as a client. The client in this communication SHOULD be the new AM.

To change the value of a resource representation, a ticket is needed. DCAF tickets consist of two parts, the ticket face and the verifier. While the client uses the verifier as a session key, the Server can derive the session key from the ticket face and the AM-key.

To change the AM-key (/am/key) and AM-URI (/am/uri) resources, the client needs a ticket that authorizes it to use PUT on these resources. There are three possibilities for a client to get this ticket:

With the help of the ticket, client and server establish a DTLS session. The new K_AM and the URI of the new AM can then be securely transmitted to the Server.

The new K_AM MUST NOT be disclosed to others. If the authorization ticket is requested from the former AM, the client MUST NOT include the new K_AM in the Access Request Message.

If the client is not the new AM, the new K_AM MUST be transmitted to the new AM and removed from the client.

5. Authorization Transitions in the Lifecycle of Constrained Devices

The lifecycle of a constrained device consists of several phases. The device is created in the manufacturing phase. Devices are then sold to customers who introduce them to their networks during the commissioning phase. In the operation phase, constrained devices fullfill their purpose in life, sometimes alternated with a maintenance phase. Some devices are sold during their lifetime and need to be decommissioned and recommissioned in the handover phase. At the end of the device’s lifecycle, the device is decommissioned in the decommissioning phase.

Apart from the operation phase, mechanisms for changing the authorization to authorize are needed in every phase of the lifecycle.

5.1. Manufacturing

In the manufacturing phase, the manufacturer can choose one of the following options for the initial key provisioning:

In the provisioning with AM service case, the manufacturer provides an own AM service. Future principals can use the AM service if they don’t want to maintain an own AM. The manufacturer sets the AM-URI resource to the URI of the manufacturer’s AM and writes the initial K_AM into the AM-key resource. Additionally, K_AM is provided to the Authorization Manager. Each constrained device SHOULD be provisioned with an individual unique key.

In the provisioning only case, the manufacturer does not provide an AM service. The AM-key resource is set to the initial K_AM. The AM-URI resource is left empty. K_AM has to be made available to the new principal, e.g. by encoding it into a QR code and printing it onto a sheet of paper which is delivered with the device, or onto the device itself. K_AM SHOULD be kept secret.

In the no provisioning case, the AM-key resource is not initialized and MUST be unprotected. The new principal will then be able to write an AM-key into this resource without the need for an authorization ticket.

5.2. Commissioning

In the commissioning phase, the principal of the device has changed. The new principal needs to be able to take over the control over the device by defining authorization policies. To achieve this, principals will either use the Authorization Manager service of the manufacturer (if available) or need to assign a new Authorization Manager to the device (see also Section 4).

To assign a new Authorization Manager, the procedure described in Section 4 is used.

5.3. Decommissioning

If a device is discarded or sold, the principal of the device changes. To make sure that nobody who gets hold of the device afterwards is able to misuse it, permissions for the device must be revoked.

The principal removes authorizations for the constrained device from the AM. Since the AM is used to negotiate tickets for new connections with other devices, the decommissioned device will not be able to request new connections afterwards.

Already existing tickets and session keys have to be removed from the decommissioned device. In particular, for Servers the ticket faces and derived session keys need to be erased, for Clients the Verifiers must be deleted.

5.4. Handover and Maintenance

During the lifecycle of a constrained device, Authorization Managers sometimes need to be exchanged for maintenance reasons or because the device is sold. In both cases, the relationship between the former AM and the constrained device must be broken.

The exchange of the AM consists of a decomissioning as described in Section 5.3 followed by a commissioning as described in Section 5.2. Before the decommissioning, one of the mechanisms described in Section 4 for the commissioning MUST be used to create an authorization ticket for assigning the new AM.

6. Security Considerations

7. IANA Considerations


8. References

8.1. Normative References

[I-D.gerdes-ace-dcaf-authorize] Gerdes, S., Bergmann, O. and C. Bormann, "Delegated CoAP Authentication and Authorization Framework (DCAF)", Internet-Draft draft-gerdes-ace-dcaf-authorize-01, February 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2. Informative References

[I-D.gerdes-ace-actors] Gerdes, S., "Actors in the ACE Architecture", Internet-Draft draft-gerdes-ace-actors-02, October 2014.

Author's Address

Stefanie Gerdes Universität Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63906 EMail: