Ridha Khedri

On a formal semantics of tabular expressions

Abstract Parnas et al. (Janicki et al., in: Brink, Kahl, Schmidt (Eds.), Relational Methods in Computer Science, Springer, Berlin, 1997; Parnas, Commun. ACM 26 (8) (1983) 572–581; Parnas and Madey, Sci. Comput. Programm. 25 (1) (1995) 41–61; Parnas et al., IEEE Trans. Software Eng. 20 (12) (1994) 948–976) advocate the use of relational model for documenting the intended behaviour of programs. In this method, tabular expressions (or tables) are used to improve readability so that formal documentation can replace conventional documentation. Parnas (CRL Report 260, Telecommunications Research Institute of Ontario (TRIO), Mcmaster University, Hamilton, Ontario, Canada, 1992) describes several classes of tables and provides their formal syntax and semantics. In this paper, an alternative, more general and more homogeneous semantics is proposed. The model covers all known types of tables used in Software Engineering.

Feature algebra

Based on experience from the hardware industry, product families have entered the software development process as well, since software developers often prefer not to build a single product but rather a family of similar products that share at least one common functionality while having well-identified variabilities. Such shared commonalities, also called features, reach from common hardware parts to software artefacts such as requirements, architectural properties, components, middleware, or code. We use idempotent semirings as the basis for a feature algebra that allows a formal treatment of the above notions as well as calculations with them. In particular models of feature algebra the elements are sets of products, i.e. product families. We extend the algebra to cover product lines, refinement, product development and product classification. Finally we briefly describe a prototype implementation of one particular model.

Integration of sequential scenarios

We give a formal relation-based definition of scenarios and we show how different scenarios can be integrated to obtain a more global view of user-system interactions. We restrict ourselves to the sequential case, meaning that we suppose that there is only one user (thus, the scenarios we wish to integrate cannot occur concurrently). Our view of scenarios is state-based, rather than event-based, like most of the other approaches, and can be grafted to the well-established specification language Z. Also, the end product of scenario integration, the specification of the functional aspects of the system, is given as a relation; this specification can be refined using independently developed methods. Our formal description is coupled with a diagram-based, transition-system like, presentation of scenarios, which is better suited to communication between clients and specifiers.

An algebra of product families

Experience from recent years has shown that it is often advantageous not to build a single product but rather a family of similar products that share at least one common functionality while having well-identified variabilities. Such product families are built from elementary features that reach from hardware parts to software artefacts such as requirements, architectural elements or patterns, components, middleware, or code. We use the well established mathematical structure of idempotent semirings as the basis for a product family algebra that allows a formal treatment of the above notions. A particular application of the algebra concerns the multi-view reconciliation problem that arises when complex systems are modelled. We use algebraic integration constraints linking features in one view to features in the same or a different view and show in several examples the suitability of this approach for a wide class of integration constraint formulations. Our approach is illustrated with a Haskell prototype implementation of one particular model of product family algebra.

Exploring Covert Channels

Covert channels pose a threat to system security for many reasons. One of the most significant security concerns surrounding the use of covert channels in computer and information systems involves confidentiality and the ability to leak confidential information from a high level security user to a low level one covertly. There are many differing views surrounding the ideas of covert channels and steganography with debates igniting over the existence of a relationship between the two concepts. This debate can be resolved with a model to provide a perception of covert channel communication to yield a better understanding of covert channels. In this paper, we propose a model to perceive covert channel communication. We use the proposed model to explore the relationship between covert channels, steganography and watermarking. The intent is to provide a better understanding of covert channel communication in an attempt to develop investigative support for confidentiality.

Requirements Analysis for Engineering Computation: A Systematic Approach for Improving Reliability

This paper argues that the reliability of engineering computation can be significantly improved by adopting software engineering methodologies for requirements analysis and specification. The argument centers around the fact that the only way to judge the reliability of a system is by comparison to a specification of the requirements. This paper also points to methods for documenting the requirements. In particular, a requirements template is proposed for specifying engineering computation software. To make the mathematical specification easily understandable by all stakeholders, the requirements documentation employs the technique of using tabular expressions. To clarify the presentation, this paper includes a case study of the documentation for a system for analyzing statically determinant beams.

Verification of Information Flow in Agent-Based Systems

Analyzing information flow is beneficial for ensuring the satisfiability of security policies during the exchange of information between the agents of a system. In the literature, models such as Bell-LaPadula model and the Chinese Wall model are proposed to capture and govern the exchange of information among agents. Also, we find several verification techniques for analyzing information flow within programs or multi-agent systems. However, these models and techniques assume the atomicity of the exchanged information, which means that the information cannot be decomposed or combined with other pieces of information. Also, the policies of their models prohibit any transfer of information from a high level agent to a low level agent. In this paper, we propose a technique that relaxes these assumptions. Indeed, the proposed technique allows classifying information into frames and articulating finer granularity policies that involve information, its elements, or its frames. Also, it allows for information manipulation through several operations such as focusing and combining information. Relaxing the atomicity of information assumption permits an analysis that takes into account the ability of an agent to link elements of information in order to evolve its knowledge.

Algebraic View Reconciliation

Embedded systems such as automotive systems are very complex to specify. Since it is difficult to capture all their requirements or their design in one single model, approaches working with several system views are adopted. The main problem there is to keep these views coherent; the issue is known as view reconciliation. This paper proposes an algebraic solution. It uses sets of integration constraints that link (families of) system features in one view to other (families of) features in the same or a different view. Both, families and constraints, are formalised using a feature algebra. Besides presenting a constraint relation and its mathematical properties, the paper shows in several examples the suitability of this approach for a wide class of integration constraint formulations.

On the Necessary Conditions for Covert Channel Existence: A State-of-the-Art Survey

Abstract With the ability to leak confidential information in a secret manner, covert channels pose a significant threat to the confidentiality of a system. Due to this threat, the identification of covert channel existence has become an important part of the evaluation of secure systems. In this paper, we present a state-of-the-art survey discussing the conditions for covert channel existence found in the literature and we point to their inadequacy. We also examine how conditions for covert channel existence are handled by information theory. We propose a set of necessary and verifiable conditions for covert channel existence in systems of communicating agents. We aim to provide an improved understanding of covert channel communication and to build a foundation for developing effective and efficient mechanisms for mitigating covert channels in systems of communicating agents at the early stages of software development

Endowing Concurrent Kleene Algebra with Communication Actions

Communication is integral to the understanding of agent interactions in concurrent systems. In this paper,we propose a mathematical framework for communication and concurrency called Communicating Concurrent Kleene Algebra (C2KA). C2KAextends concurrent Kleene algebra with the notion of communication actions. This extension captures both the influence of external stimuli on agent behaviour aswell as the communication and concurrency of communicating agents.