arXiv subjects

Jason Crampton

Publications and source records attributed to Jason Crampton.

26 records · Page 2Linked to original sources

arXiv2014

Publicly Verifiable Outsourced Computation with a Key Distribution Centre

The combination of software-as-a-service and the increasing use of mobile devices gives rise to a considerable difference in computational power between servers and clients. Thus, there is a desire for clients to outsource the evaluation of complex functions to a server and to be able to verify that the resulting value is correct. Previous work in this area of Publicly Verifiable Outsourced Computation (PVC) requires a costly pre-processing stage. However, in many practical situations multiple clients will be interested in the same set of core functions and will make use of the same servers. Thus, the pre-processing phase may be performed many more times than is necessary. In this paper we introduce a Key Distribution Center (KDC) that handles the generation and distribution of the keys that are required to support PVC, thereby eliminating this redundancy. We define a number of new security models and functionalities that arise with the introduction of the KDC, and present a construction of such a scheme built upon Key-Policy Attribute-based Encryption.

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arXiv2014

Path Conditions and Principal Matching: A New Approach to Access Control

Traditional authorization policies are user-centric, in the sense that authorization is defined, ultimately, in terms of user identities. We believe that this user-centric approach is inappropriate for many applications, and that what should determine authorization is the relationships that exist between entities in the system. While recent research has considered the possibility of specifying authorization policies based on the relationships that exist between peers in social networks, we are not aware of the application of these ideas to general computing systems. We develop a formal access control model that makes use of ideas from relationship-based access control and a two-stage method for evaluating policies. Our policies are defined using path conditions, which are similar to regular expressions. We define semantics for path conditions, which we use to develop a rigorous method for evaluating policies. We describe the algorithm required to evaluate policies and establish its complexity. Finally, we illustrate the advantages of our model using an example and describe a preliminary implementation of our algorithm.

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arXiv2014

Iterative Plan Construction for the Workflow Satisfiability Problem

The \emph{Workflow Satisfiability Problem (WSP)} is a problem of practical interest that arises whenever tasks need to be performed by authorized users, subject to constraints defined by business rules. We are required to decide whether there exists a \emph{plan} -- an assignment of tasks to authorized users -- such that all constraints are satisfied. Several bespoke algorithms have been constructed for solving the WSP, optimised to deal with constraints (business rules) of particular types. It is natural to see the WSP as a subclass of the {\em Constraint Satisfaction Problem (CSP)} in which the variables are tasks and the domain is the set of users. What makes the WSP distinctive as a CSP is that we can assume that the number of tasks is very small compared to the number of users. This is in sharp contrast with traditional CSP models where the domain is small and the number of variables is very large. As such, it is appropriate to ask for which constraint languages the WSP is fixed-parameter tractable (FPT), parameterized by the number of tasks. We have identified a new FPT constraint language, user-independent constraint, that includes many of the constraints of interest in business processing systems. We are also able to prove that the union of FPT languages remains FPT if they satisfy a simple compatibility condition. In this paper we present our generic algorithm, in which plans are grouped into equivalence classes, each class being associated with a \emph{pattern}. We demonstrate that our generic algorithm has running time $O^*(2^{k\log k})$, where $k$ is the number of tasks, for the language of user-independent constraints. We also show that there is no algorithm of running time $O^*(2^{o(k\log k)})$ for user-independent constraints unless the Exponential Time Hypothesis fails.

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arXiv2013

On the Use of Key Assignment Schemes in Authentication Protocols

Key Assignment Schemes (KASs) have been extensively studied in the context of cryptographically-enforced access control, where derived keys are used to decrypt protected resources. In this paper, we explore the use of KASs in entity authentication protocols, where we use derived keys to encrypt challenges. This novel use of KASs permits the efficient authentication of an entity in accordance with an authentication policy by associating entities with security labels representing specific services. Cryptographic keys are associated with each security label and demonstrating knowledge of an appropriate key is used as the basis for authentication. Thus, by controlling the distribution of such keys, restrictions may be efficiently placed upon the circumstances under which an entity may be authenticated and the services to which they may gain access. In this work, we explore how both standardized protocols and novel constructions may be developed to authenticate entities as members of a group associated to a particular security label, whilst protecting the long-term secrets in the system. We also see that such constructions may allow for authentication whilst preserving anonymity, and that by including a trusted third party we can achieve the authentication of individual identities and authentication based on timestamps without the need for synchronized clocks.

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arXiv2013

Constraint Expressions and Workflow Satisfiability

A workflow specification defines a set of steps and the order in which those steps must be executed. Security requirements and business rules may impose constraints on which users are permitted to perform those steps. A workflow specification is said to be satisfiable if there exists an assignment of authorized users to workflow steps that satisfies all the constraints. An algorithm for determining whether such an assignment exists is important, both as a static analysis tool for workflow specifications, and for the construction of run-time reference monitors for workflow management systems. We develop new methods for determining workflow satisfiability based on the concept of constraint expressions, which were introduced recently by Khan and Fong. These methods are surprising versatile, enabling us to develop algorithms for, and determine the complexity of, a number of different problems related to workflow satisfiability.

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arXiv2013

On the Parameterized Complexity and Kernelization of the Workflow Satisfiability Problem

A workflow specification defines a set of steps and the order in which those steps must be executed. Security requirements may impose constraints on which groups of users are permitted to perform subsets of those steps. A workflow specification is said to be satisfiable if there exists an assignment of users to workflow steps that satisfies all the constraints. An algorithm for determining whether such an assignment exists is important, both as a static analysis tool for workflow specifications, and for the construction of run-time reference monitors for workflow management systems. Finding such an assignment is a hard problem in general, but work by Wang and Li in 2010 using the theory of parameterized complexity suggests that efficient algorithms exist under reasonable assumptions about workflow specifications. In this paper, we improve the complexity bounds for the workflow satisfiability problem. We also generalize and extend the types of constraints that may be defined in a workflow specification and prove that the satisfiability problem remains fixed-parameter tractable for such constraints. Finally, we consider preprocessing for the problem and prove that in an important special case, in polynomial time, we can reduce the given input into an equivalent one, where the number of users is at most the number of steps. We also show that no such reduction exists for two natural extensions of this case, which bounds the number of users by a polynomial in the number of steps, provided a widely-accepted complexity-theoretical assumption holds.

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arXiv2011

PTaCL: A Language for Attribute-Based Access Control in Open Systems

Many languages and algebras have been proposed in recent years for the specification of authorization policies. For some proposals, such as XACML, the main motivation is to address real-world requirements, typically by providing a complex policy language with somewhat informal evaluation methods; others try to provide a greater degree of formality (particularly with respect to policy evaluation) but support far fewer features. In short, there are very few proposals that combine a rich set of language features with a well-defined semantics, and even fewer that do this for authorization policies for attribute-based access control in open environments. In this paper, we decompose the problem of policy specification into two distinct sub-languages: the policy target language (PTL) for target specification, which determines when a policy should be evaluated; and the policy composition language (PCL) for building more complex policies from existing ones. We define syntax and semantics for two such languages and demonstrate that they can be both simple and expressive. PTaCL, the language obtained by combining the features of these two sub-languages, supports the specification of a wide range of policies. However, the power of PTaCL means that it is possible to define policies that could produce unexpected results. We provide an analysis of how PTL should be restricted and how policies written in PCL should be evaluated to minimize the likelihood of undesirable results.

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arXiv2011

Practical Constructions for the Efficient Cryptographic Enforcement of Interval-Based Access Control Policies

The enforcement of access control policies using cryptography has received considerable attention in recent years and the security of such enforcement schemes is increasingly well understood. Recent work in the area has considered the efficient enforcement of temporal and geo-spatial access control policies, and asymptotic results for the time and space complexity of efficient enforcement schemes have been obtained. However, for practical purposes, it is useful to have explicit bounds for the complexity of enforcement schemes. In this paper, we consider interval-based access control policies, of which temporal and geo-spatial access control policies are special cases. We define enforcement schemes for interval-based access control policies for which it is possible, in almost all cases, to obtain exact values for the schemes' complexity, thereby subsuming a substantial body of work in the literature. Moreover, our enforcement schemes are more practical than existing schemes, in the sense that they operate in the same way as standard cryptographic enforcement schemes, unlike other efficient schemes in the literature. The main difference between our approach and earlier work is that we develop techniques that are specific to the cryptographic enforcement of interval-based access control policies, rather than applying generic techniques that give rise to complex constructions and asymptotic bounds.

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