Hierarchies arise in the context of access control whenever the user population can be modeled as a set of partially ordered classes (represented as a directed graph). A user with access privileges for a class obtains access to objects stored at that class and all descendant classes in the hierarchy. The problem of key management for such hierarchies then consists of assigning a key to each class in the hierarchy so that keys for descendant classes can be obtained via efficient key derivation.We propose a solution to this problem with the following properties: (1) the space complexity of the public information is the same as that of storing the hierarchy; (2) the private information at a class consists of a single key associated with that class; (3) updates (i.e., revocations and additions) are handled locally in the hierarchy; (4) the scheme is provably secure against collusion; and (5) each node can derive the key of any of its descendant with a number of symmetric-key operations bounded by the length of the path between the nodes. Whereas many previous schemes had some of these properties, ours is the first that satisfies all of them. The security of our scheme is based on pseudorandom functions, without reliance on the Random Oracle Model.Another substantial contribution of this work is that we are able to lower the key derivation time at the expense of modestly increasing the public storage associated with the hierarchy. Insertion of additional, so-called shortcut, edges, allows to lower the key derivation to a small constant number of steps for graphs that are total orders and trees by increasing the total number of edges by a small asymptotic factor such as O(log*n) for an n-node hierarchy. For more general access hierarchies of dimension d, we use a technique that consists of adding dummy nodes and dimension reduction. The key derivation work for such graphs is then linear in d and the increase in the number of edges is by the factor O(logd − 1n) compared to the one-dimensional case.Finally, by making simple modifications to our scheme, we show how to handle extensions proposed by Crampton [2003] of the standard hierarchies to “limited depth” and reverse inheritance.

Dynamic and Efficient Key Management for Access Hierarchies

Current access control models typically assume that resources are under the strict custody of a trusted party which monitors each access request to verify if it is compliant with the specified access control policy. There are many scenarios where this approach is becoming no longer adequate. Many clear trends in Web technology are creating a need for owners of sensitive information to manage access to it by legitimate users using the services of honest but curious third parties, that is, parties trusted with providing the required service but not authorized to read the actual data content. In this scenario, the data owner encrypts the data before outsourcing and stores them at the server. Only the data owner and users with knowledge of the key will be able to decrypt the data. Possible access authorizations are to be enforced by the owner. In this article, we address the problem of enforcing selective access on outsourced data without need of involving the owner in the access control process. The solution puts forward a novel approach that combines cryptography with authorizations, thus enforcing access control via selective encryption. The article presents a formal model for access control management and illustrates how an authorization policy can be translated into an equivalent encryption policy while minimizing the amount of keys and cryptographic tokens to be managed. The article also introduces a two-layer encryption approach that allows the data owner to outsource, besides the data, the complete management of the authorization policy itself, thus providing efficiency and scalability in dealing with policy updates. We also discuss experimental results showing that our approach is able to efficiently manage complex scenarios.

/pdf/encryption-policies-for-regulating-access-to-outsourced-data-4jcchwnais.pdf

Encryption policies for regulating access to outsourced data

We show how to create a master key scheme for controlling access to a set of services. Each master key is a concise representation for a list of service keys, such that only service keys in this list can be computed easily from the master key. Our scheme is more flexible than others, permitting hierarchical organization and expansion of the set of services.

/pdf/flexible-access-control-with-master-keys-2uw540wdl3.pdf

Flexible access control with master keys

A time-bound hierarchical key assignment scheme is a method to assign time-dependent encryption keys to a set of classes in a partially ordered hierarchy, in such a way that the key of a higher class can be used to derive the keys of all classes lower down in the hierarchy, according to temporal constraints.In this paper we design and analyze time-bound hierarchical key assignment schemes which are provably-secure and efficient. We first consider the unconditionally secure setting and we show a tight lower bound on the size of the private information distributed to each class. Then, we consider the computationally secure setting and obtain several results: We first prove that a recently proposed scheme is insecure against collusion attacks. Hence, motivated by the need for provably-secure schemes, we propose two different constructions for time-bound hierarchical key assignment schemes. The first one is based on symmetric encryption schemes, whereas, the second one makes use of bilinear maps. These appear to be the first constructions of time-bound hierarchical key assignment schemes which are simultaneously practical and provably-secure.

https://link.springer.com/content/pdf/10.1007%2Fs00145-010-9094-6.pdf

Provably-secure time-bound hierarchical key assignment schemes

Database outsourcing is becoming increasingly popular introducing a new paradigm, called database-as-a-service (DAS), where an organization's database is stored at an external service provider. In such a scenario, access control is a very important issue, especially if the data owner wishes to publish her data for external use.In this paper, we first present our approach for the implementation of access control through selective encryption. The focus of the paper is then the presentation of the experimental results, which demonstrate the applicability of our proposal.

/pdf/key-management-for-multi-user-encrypted-databases-5gn6x3kzcg.pdf

Key management for multi-user encrypted databases

A cryptographic scheme for controlling access to information within a group of users organized in a hierarchy was proposed in [1]. The scheme enables a user at some level to compute from his own cryptographic key the keys of the users below him in the organization.

An Optimal Algorithm for Assigning Cryptographic Keys to Control Access in a Hierarchy

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An Optimal Algorithm for Assigning Cryptographic Keys to Control Access in a Hierarchy