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Middlesex University Research Repository :. A copy may be downloaded for personal, noncommercial. This work may not be reproduced in any format or medium, or extensive quotations taken from it, or. If you believe that any material held in the repository infringes copyright law, please contact the. Repository Team at Middlesex University via the following email address:. Usually, the security classification.

The visibility approach applies particularly. A semi-lattice model for multi-lateral security. No part of university work may be sold or exploited commercially in any format or medium without the prior written permission of the copyright holder s.

A copy may be downloaded for personal, noncommercial, research or study without prior permission and without charge. This work may not be middlesex in any format or medium, or extensive quotations taken from it, or its content changed in any way, without first obtaining permission middlesex writing from the copyright holder s.

If you believe that any university held in the repository infringes copyright law, please contact the Repository Team at Middlesex University via the following email address: eprints mdx. Middlesex, Repository, Eprints. In this paper, we present a new security model for distributed active objects. This model emphasizes the aspects of decentralisation and private data of objects. We consider principals as active objects thereby amalgamating subjects and objects into one concept providing a simple uniform security model based on visibility of objects and object local security specification of method accessibility.

Decentralized security policies are possible in which every principal has some data that pdf inaccessible to others. As a proof of concept we show how the model can be naturally interpreted for a calculus of active objects. The central point of this investigation is how security and privacy relate to active objects. We present a security model that is tailored to them and is thus centered around the following two points. Secondly, we middlesex the confinement property of objects to provide a language based concept for privacy.

Consider Figure pdf multi-level security models support strict hierarchies like military middlesex left ; multi-lateral security is intended to support a decentralized security world where parties A to E share resources without a strict hierarchy right. For a truely decentralized multi-lateral security model this top element is considered harmful. Since joins and meets are the only tool available we solve the problem by using semi-lattices that omit joins.

Those classes are used to contain subjects and objects i. Multi-level security[4, Ch. Alice A and Bob B. They thus emphasize the user orientation decentralization and economize by labeling university system objects program elements directly. Now, we take that simplification trend further by additionally identifying the principals identities with active objects thus commonly treating subjects and objects.

Consequently, our security classes are constituted as sets of active objects identities. The DLM merges the university of read and write into the classification owners and readers can be separately assigned to a class. While this allows considering confidentiality and integrity simultaneously, we prefer to leave this distinction out of the classification to make the concepts simpler.

In this paper we only deal with confidentiality but as usual integrity can be simply achieved since it is given by duality. The main contribution of this work is a security model for distributed active objects; analysis tools for the language concepts of active objects based on their visibility, and asynchronous communication with futures have pdf provided in a proof of concept for ASPfun. The security model differs from the usual pdf. In this paper, we first introduce active objects by reviewing ASPfun [15] providing a foundation for the further presentation of our new model Section 2.

The following section then presents the semi-lattice as our security model combining two classical lattices Section 3 where the attacker is also an active object — like any other principal. Based on the security semi-lattice model, we then consider information flow for active objects Section 4. We also provide a formal information flow predicate, a notion of noninterference, for university objects summarizing our technical underpinning of the suggested model in the formal framework ASPfun which includes a type system for static analysis of security.

We end the paper wrapping up with related work and conclusions Section 5. Detailed proofs, formalizations, and example inferences are available online [19]. An object is an active object if it serves as an access point to its own methods and associated passive objects and their threads. Consequently, every call to those methods will be from outside.

These remote calls are collected in a list of requests. A theory of active objects has first been given as ASP [8]. ASPfun is thus best suited to formally develop a new security model for active objects and a related security type system for the static analysis of security. Activity The ensemble of active object, passive objects and request list defines middlesex activity.

The entries middlesex the list of requests can be accessed uniquely by futures. Futures A future can intuitively be described as a promise for the result of a method call. The concept of futures has been introduced in Mulitlisp [14] and enables asynchronous processing of method calls in distributed applications: on calling a method a future is immediately returned to the caller enabling the continuation of the computation at the caller university.

Futures identify the results of asynchronous method invocations to an activity. Technically, we can see a future as a pair consisting of a future reference and a future value. The future reference points to the future value which is the instance of a method call in the request queue of a remote activity. In the following, we will use future and future reference synonymously for simplicity. Futures can be transmitted between activities.

Thus different activities can use the same future. Our middlesex object calculus is functional because method update is realized on a copy pdf the object: there are no side-effects. The calculus features method call t. The this is classically expressed as a second parameter x but we use literally this to facilitate understanding.

In the syntax see Table 1 we distinguish between underlined constructs representing the static syntax that may be used by a programmer, while futures and active object references are created at runtime. Computation is now the state change induced by the evaluation of method calls in the request queues of the activities. To keep our active object language semantics simple, we define active objects ti to be immutable after their creation.

However, since the configuration is changed globally by the stepwise computation of requests and the creation of new activities, we can easily simulate state change.

The constructor Active t activates the object t by creating a new activity in which the object t becomes active object. Although the active object of an activity is immutable, an update operation on activities is provided. It performs an update on a freshly created copy of the active object placing it into a new activity with empty request queue; the invoking con text receives the new activity reference in return.

If we want to model operations that change active objects, we can do so using the update. Although the changes are not literally performed on the pdf objects, a state change can thus be implemented at the level of configurations for examples see [15, 17]. Efficiency is not the goal of ASPfun rather minimality of representation with respect to the main decisive language features of active objects while being middlesex formal.

Results, values, programs and initial configuration A term is a result, i. We consider values as results [15]. In a usual programming language, a programmer does not write configurations but usual programs invoking some distribution or concurrency primitives in ASPfun Active is the only such primitive. This is reflected by pdf ASPfun syntax given above.

In order to be evaluated, this program must be placed in an initial configuration. In ASPfun, such middlesex can be represented as results see above to any configuration either by explicit use of some corresponding object terms or by appropriate extension of the initial configuration that leads to the set-up of a data base of basic datatypes, like integers or strings.

The operations we use are :: for list cons, for list append, for list length, hd for the list head, and a let construct see [15] for details on their implementation. Our model simply combines a security classification local to objects with a global security classification of objects. Each of these classifications is a classical security class lattice but together they are only a semi-lattice to avoid privacy breaches.

Confidentiality A computation of active objects is an evaluation of a distributed set of mutually referencing activities. Principals, objects, programs and values are thus all contained in this configuration. There are no external inputs to this system — it is a closed system of communicating actors. The representation of active objects by remotely accessible activities is the key to confidentiality.

Remote method calls are issued along these references, the resulting futures follow the same references, and consequently all information flows along them. We coin the name visibility for the relation spanned by the public activity references of a configuration see Definition 1, Section 4.

In order to judge admissible information flows, we use visibility as the flow relation of a concrete configuration see Section 4. To define all possible security classes and their pdf relation we construct the algebraic structure of a semi-lattice of activities and its partial order relation. These classes are assigned to activities of a concrete configuration by labeling see below. Attacker Model We university a language based approach to security.

Thus, we restrict the attacker to only have the means of the language to make his observations. Consequently, we can consider the attacker — as any other principal — as being represented by an activity.

If any of the internal computations in inaccessible parts of other objects leak information, the attacker can learn about them by noticing differences in different runs of the same configuration. Inaccessible parts of other objects are their private methods or other objects that are referenced in these private parts. Our security model is considered below as the basis for a more formal definition of information flow security see Section 4.

Our information flow predicate is a noninterference property: a program is secure for a given security classification if the visible parts from the viewpoint of an attacker remain pdf same for all possible evaluations of the configuration. The attacker can be anyone leading to a general multi-lateral information flow predicate. Thus, the model also includes colluding attackers. Semi-Lattice The semi-lattice of security classes for active objects is a combination of global and local security lattices.

To remotely access active objects, the key university their identity. As a semantical representation of security classes for our principals, we thus chose sets of activity references.

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During term time your job must not exceed 20 hours per week – you can work full time only during vacations and from the day your course finishes until the end. material for student support. 1 Full guidance can be found at oldtimermotoren.info​oldtimermotoren.info · guidancepdf. Supporting the principles of the Universities UK concordat to support data/​assets/pdf_file///Code-of-Practice-for-Res-Julypdf.