Naya Nagy

The maximum flow problem: a real-time approach

The dynamic version of the maximum flow problem allows the graph underlying the flow network to change over time. The graph receives corrections to its structure or capacities and consequently the value of the maximum flow is modified. Several correction types are treated: edge capacity corrections and constant degree vertex additions/deletions. These corrections arrive in real time. In this paper, parallel and sequential solutions to the real-time maximum flow problem are developed on the reconfigurable multiple bus machine model and on the random access machine model, respectively. The parallel solution successfully meets the deadlines imposed in real time, while the sequential one fails to do so.The two solutions are then applied to a real-time process scheduler, an extension of Stones static two-processor allocation problem. The scheduler allows processes to be created and destroyed, the amount of communication between two processes to change with time, and so on. The parallel algorithm is always able to compute the optimal schedule, while the solution obtained sequentially is only an approximation. The sequential solution gets worse with each new deadline to be met. In fact, after a sufficient number of steps, the quality improvement provided by the parallel approach over the sequential one is superlinear in the number of processors used by the parallel model.

AUTHENTICATED QUANTUM KEY DISTRIBUTION WITHOUT CLASSICAL COMMUNICATION

The aim of quantum key distribution protocols is to establish a secret key among two parties with high security confidence. Such algorithms generally require a quantum channel and an authenticated classical channel. This paper presents a totally new perception of communication in such protocols. The quantum communication alone satisfies all needs of array communication between the two parties. Even so, the quantum communication channel does not need to be protected or authenticated whatsoever. As such, our algorithm is a purely quantum key distribution algorithm. The only certain identification of the two parties is through public keys.

Computing with uncertainty and its implications to universality

It is known that there exist computational problems that can be solved on a parallel computer, yet are impossible to be solved sequentially. Specifically, no general purpose sequential model of computation, such as the Turing machine or the random access machine, can simulate a large family of computations (e.g. solutions to certain real-time problems), each of which is capable of being carried out readily by a particular parallel computer. We extend the scope of such problems to the class of problems with uncertain time constraints. The first type of time constraints refers to uncertain time requirements on the input data, that is when and for how long are the input data available. The second type of time constraints refers to uncertain deadlines on when outputs are to be produced. Our main objective is to exhibit computational problems in which it is very difficult to find out (read ‘compute’) what to do and when to do it. Furthermore, problems with uncertain time constraints, as described here, prove once more that it is impossible to define a ‘universal computer’, that is a computer able to perform (through simulation or otherwise) all computations that are executable on other computers. Finally, one of the contributions of this paper is to promote the study of a topic, conspicuously absent to date from theoretical computer science, namely the role of physical time and physical space in computation. The focus of our work is to analyse the effect of external natural phenomena on the various components of a computational process, namely the input phase, the calculation phase (including the algorithm and the computing agents themselves) and the output phase.

Quantum security in wireless sensor networks

Security in sensor networks, though an important issue for widely available wireless networks, has been studied less extensively than other properties of these networks, such as, for example, their reliability. The few security schemes proposed so far are based on classical cryptography. In contrast, the present paper develops a new security solution, based on quantum cryptography. The scheme developed here comes with the advantages quantum cryptography has over classical cryptography, namely, effectively unbreakable keys and therefore effectively unconditionally secure messages. Our security system ensures privacy of the measured data field in the presence of an intruder who listens to messages broadcast in the field.

KEY DISTRIBUTION VERSUS KEY ENHANCEMENT IN QUANTUM CRYPTOGRAPHY

It has been said that quantum cryptography in general offers a secure solution to the problem of key enhancement. This means that two parties who already share a small secret key, can use quantum protocols to establish a new large secret key. This large secret key can be arbitrarily long and is unbreakable. Thus, to date, the main contribution of quantum cryptography has been believed to be quantum key enhancement. This paper shows that quantum cryptography can do significantly more. The quantum protocol described here distributes an unbreakable secret key to the two parties by relying on public information only. This is the first time that quantum cryptography is shown to be able to produce secret information using only public information. This contribution is also unique for cryptography in general, classical and quantum.

A QUANTUM CRYPTOGRAPHIC SOLUTION TO THE PROBLEM OF ACCESS CONTROL IN A HIERARCHY

Access control in a hierarchy refers to a selective access to a database. A large number of users work with the same database. These users are organized in a hierarchical structure and therefore have different access rights to the data. This paper offers a solution to the problem of access control in a hierarchy based on quantum cryptography. Each user has two keys: a classical key and a quantum key. Our scheme offers several security advantages over the classical schemes to date. It protects users from identity theft and prevents collusion attacks. Most importantly though, our scheme adapts to dynamic changes of the user hierarchy: users may join, leave, or change position in the hierarchy, without affecting the rest of the user structure.

Quantum Wireless Sensor Networks

Security in sensor networks, though an important issue for widely available wireless networks, has been studied less extensively than other properties of these networks, such as, for example, their reliability. The few security schemes proposed so far are based on classical cryptography. In contrast, the present paper develops a totally new security solution, based on quantum cryptography. The scheme developed here comes with the advantages quantum cryptography has over classical cryptography, namely, effectively unbreakable keys and therefore unbreakable messages. Our security system ensures privacy of the measured data field in the presence of an intruder listening to messages broadcasted in the field.

Quantum authenticated key distribution

Quantum key distribution algorithms use a quantum communication channel with quantum information and a classical communication channel for binary information. The classical channel, in all algorithms to date, was required to be authenticated. Moreover, Lomonaco

ASPECTS OF BIOMOLECULAR COMPUTING

This paper is intended as a survey of the state of the art of some branches of Biomolecular Computing. Biomolecular Computing aims to use biological hardware (biomare), rather than chips, to build a computer. We discuss the following three main research directions: DNA computing, membrane systems, and gene assembly in ciliates. DNA computing combines practical results together with theoretical algorithm design. Various search problems have been implemented using DNA strands. Membrane systems are a family of computational models inspired by the membrane structure of living cells. The process of gene assembly in ciliates has been formalized as an abstract computational model. Biomolecular Computing is a field in full development, with the promise of important results from the perspective of both Computer Science (models of computation) and Biology (understanding biological processes).

Carving Secret Messages out of Public Information

This study shows that secret information can be shared or passed from a sender to a receiver even if not encoded in a secret message. In the protocol designed in this study, no parts of the original secret information ever travel via communication channels between the source and the destination, no encoding/decoding key is ever used. The two communicating partners, Alice and Bob, are endowed with coherent qubits that can be read and set and keep their quantum values over time. Additionally, there exists a central authority that is capable of identifying Alice and Bob to share with each half of entangled qubit pairs. The central authority also performs entanglement swapping. Our protocol relies on the assumption that public information can be protected, an assumption present in all cryptographic protocols. Also any classical communication channel need not be authenticated. As each piece of secret information has a distinct public encoding, the protocol is equivalent to a one-time pad protocol.