Internet DRAFT - draft-gerdes-ace-a2a
draft-gerdes-ace-a2a
ACE Working Group S. Gerdes
Internet-Draft Universitaet Bremen TZI
Intended status: Informational September 11, 2015
Expires: March 14, 2016
Managing the Authorization to Authorize in the Lifecycle of a
Constrained Device
draft-gerdes-ace-a2a-01
Abstract
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.
Resource-constrained nodes benefit from delegating 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 security relationships between constrained
nodes and their Authorization Managers can be established and managed
in a RESTful way, thus providing for a flexible authorization
solution for the whole lifecycle of a constrained node.
Status of This Memo
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 14, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Authorization to Authorize . . . . . . . . . . . . . . . . . 3
4. Assigning a new Authorization Manager . . . . . . . . . . . . 4
5. Authorization Transitions in the Lifecycle of Constrained
Nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Manufacturing . . . . . . . . . . . . . . . . . . . . . . 5
5.2. Commissioning . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Decommissioning . . . . . . . . . . . . . . . . . . . . . 6
5.4. Handover . . . . . . . . . . . . . . . . . . . . . . . . 6
5.5. Maintenance . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
As shown in [I-D.ietf-ace-actors], constrained nodes can benefit from
being closely coupled to a less constrained node called Authorization
Server or Authorization Manager (AM). The AM helps its constrained
node with authentication and authorization tasks. Authorization
solutions such as the delegated CoAP Authentication and Authorization
Framework (DCAF) [I-D.gerdes-ace-dcaf-authorize] define the
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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 the security relationship between a
constrained node and its Authorization Manager can be managed in a
RESTful way, thus providing for 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].
o Readers should be familiar with the concepts and terminology
introduced in [I-D.ietf-ace-actors] and
[I-D.gerdes-ace-dcaf-authorize]
3. Authorization to Authorize
The Authorization Manager helps its constrained node to determine the
authorization of another node, e.g. if it is allowed to access an
item of interest or to provide information about such an item.
Principals can easily configure authorization information on the AM
since it has the necessary user interface. AM provides the
authorization information to the constrained node: It is authorized
to define authorizations.
The constrained node needs keying material to determine if the
authorization information was really provided by its AM. We call
this keying material K_AM. Depending on the authorization solution,
symmetric or asymmetric keys can be used. For symmetric solutions,
the constrained node and the AM share a key. For asymmetric
solutions, K_AM is AM's public key.
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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 AM. 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 node. 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 node, the AM-key resource must be
changed. In this case, the constrained node always acts as the
server, even if it is otherwise 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.
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:
o request a ticket from the former AM.
o use a preconfigured ticket.
With the help of the ticket, the constrained device can determine
that request is authorized. In DCAF, it can additionally be used to
establish a DTLS session between client and server. 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 Nodes
The lifecycle of a constrained node can be roughly divided into six
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
nodes fullfill their purpose in life, sometimes alternated with a
maintenance phase. Some nodes are sold during their lifetime and
need to be decommissioned and recommissioned in the handover phase.
At the end of the node's lifecycle, the node is decommissioned in the
decommissioning phase.
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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:
o Provisioning with AM service: K_AM is provisioned to the new node
and the manufacturer provides an Authorization Manager service.
o Provisioning only: K_AM is provisioned to the new node but the
manufacturer does not provide an Authorization Manager service.
o No provisioning: No K_AM is provisioned to the newly manufactured
node.
In the provisioning with AM service case, the manufacturer provides
an own AM service. Future principals can use the AM service to
request a ticket for their own AM or might even continue to use the
manufacturer's AM if they don't want to maintain their own. The
node's AM-URI resource is set to the URI of the manufacturer's AM.
Additionally, the manufacturer configures the K_AM keying material on
the AM and the constrained node. Depending on the used solution
shared symmetric keys or asymmetric key pairs are used. For
symmetric solutions, a shared secret must be generated and provided
to constrained node and AM. Each constrained node SHOULD be
provisioned with an individual unique key. For asymmetric solutions,
key pairs must be generated on the constrained node and the AM. The
AM's public key is stored as K_AM in the AM-key resource.
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.
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5.2. Commissioning
In the commissioning phase, the principal of the node has changed.
The new principal needs to be able to take over the control over the
node 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
node (see also Section 4).
To assign a new Authorization Manager, the procedure described in
Section 4 is used.
The constrained node MUST end all existing communications and delete
all Tickets that were issued by the former AM.
5.3. Decommissioning
If a device is discarded or sold, the principal of the node changes.
To make sure that nobody who gets hold of the device afterwards is
able to misuse it, permissions for the node must be revoked.
The constrained node must be deregistered from the AM. AM MUST NOT
issue any new tickets for the constrained node and SHOULD revoke
tickets on communication partners of this node.
Already existing tickets and session keys have to be removed from the
decommissioned node.
5.4. Handover
A change of ownership of a node often entails that the relationship
between the former AM and the constrained node must be canceled.
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.
5.5. Maintenance
During the lifecycle of a constrained node, Authorization Managers
sometimes need to be exchanged, e.g. because they are replaced by a
newer model. In this case, the former AM should issue a ticket for
the new AM before it is decommissioned. The AM-Key SHOULD be deleted
from the old AS to prevent it from issuing new tickets before the AM-
Key is changed. Old tickets issued by the AM do not need to be
revoked.
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6. Security Considerations
o What do we do if the key for changing the AM is lost?
o K_AM must be protected. The entity that has K_AM is in control of
the constrained node.
o It might be difficult to protect a preconfigured K_AM.
o If the PSK is printed onto the device, everyone who has access to
the device can use it.
o If a new AM-key is transmitted this transmission must be protected
very well.
7. IANA Considerations
None
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)", draft-
gerdes-ace-dcaf-authorize-02 (work in progress), March
2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[I-D.ietf-ace-actors]
Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
architecture for authorization in constrained
environments", draft-ietf-ace-actors-00 (work in
progress), August 2015.
Author's Address
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Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org
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