Roberto De Prisco

Revisiting the Paxos Algorithm

This paper develops a new I/O automaton model called the Clock General Timed Automaton (Clock GTA) model. The Clock GTA is based on the General Timed Automaton (GTA) of Lynch and Vaandrager. The Clock GTA provides a systematic way of describing timing-based systems in which there is a notion of “normal” timing behavior, but that do not necessarily always exhibit this “normal” behavior. It can be used for practical time performance analysis based on the stabilization of the physical system.

Optimal colored threshold visual cryptography schemes

Visual cryptography schemes allow the encoding of a secret image into n shares which are distributed to the participants. The shares are such that only qualified subsets of participants can “visually” recover the secret image. Usually the secret image consist of black and white pixels. In colored threshold visual cryptography schemes the secret image is composed of pixels taken from a given set of c colors. The pixels expansion and the contrast of a scheme are two measures of the goodness of the scheme.In this paper, we study c-color (k,n)-threshold visual cryptography schemes and provide a characterization of contrast-optimal schemes. More specifically we prove that there exists a contrast-optimal scheme that is a member of a special set of schemes, which we call canonical schemes, and that satisfy strong symmetry properties.Then we use canonical schemes to provide a constructive proof of optimality, with respect to the pixel expansion, of c-color (n,n)-threshold visual cryptography schemes.Finally, we provide constructions of c-color (2,n)-threshold schemes whose pixels expansion improves on previously proposed schemes.

Revisiting the PAXOS algorithm

The Paxos algorithm is an efficient and highly fault-tolerant algorithm, devised by Lamport, for reaching consensus in a distributed system. Although it appears to be practical, it seems to be not widely known or understood. This thesis contains a new presentation of the Paxos algorithm, based on a formal decomposition into several interacting components. It also contains a correctness proof and a time performance and fault-tolerance analysis. The presentation is built upon a general timed automaton (GTA) model. The correctness proof uses automaton composition and invariant assertion methods. The time performance and fault-tolerance analysis is conditional on the stabilization of the underlying physical system behavior starting from some point in an execution. In order to formalize this stabilization, a special type of GTA called a Clock GTA is defined.

Time-optimal message-efficient work performance in the presence of faults

Method and apparatus for determining diameter and wall thickness of hollow microspheres or shells wherein terminal velocities of shells traveling in fluid-filled conduits of differing diameters are measured. A wall-effect factor is determined as a ratio of the terminal velocities, and shell outside diameter may then be ascertained as a predetermined empirical function of wall-effect factor. For shells of known outside diameter, wall thickness may then be ascertained as a predetermined empirical function of terminal velocity in either conduit.

A dynamic view-oriented group communication service

View-oriented group communication services are widely used for fault-tolerant distributed computing. For applications involving coherent data, it is importaut to know when a process has a primary view of the current group membership, usually defined as a view containing a majority out of a static universe of processes. For high availability in a system where processes can join and leave routinely, some researchers have suggested def?.ning primary views dynamically, depending on having enough members in common with recent views. We present a new formal automaton specification, DVS, for the safety guarantees made by a practical group communication service providing a dynamic notion of primary view. We demonstrate the value of DVS by showing both how it can be implemented and how it can be used in an application. First, we present a distributed algorithm based on a group membership algorithm of Lotem, Keidar and Dolev; our version integrates communication with the membership service, uses iuformation from the application processes saying when a view has been prepared for computation by the application, and uses a static view-oriented service internally. We prove that this algorithm implements DVS. Second, we present an application algorithm that is a variant of an algorithm of Amir, Dolev, Keidar, Melliar-Smith and Moser, modified to use DVS instead of a static service. We prove that it implements a (non-group-oriented) totally-orderedbroadcast service.

Performing tasks on synchronous restartable message-passing processors

Summary. This work considers the problem of performing t tasks in a distributed system of p fault-prone processors. This problem, called do-all herein, was introduced by Dwork, Halpern and Waarts. The solutions presented here are for the model of computation that abstracts a synchronous message-passing distributed system with processor stop-failures and restarts. We present two new algorithms based on a new aggressive coordination paradigm by which multiple coordinators may be active as the result of failures. The first algorithm is tolerant of

A Dynamic Primary Configuration Group Communication Service

f < p

On the Relation of Random Grid and Deterministic Visual Cryptography

stop-failures and does not allow restarts. Its available processor steps (work) complexity is

Do You Trust Your Phone

S = {\cal O}((t + p\log p/\log\log p) \cdot\log f)

An Evolutionary Music Composer Algorithm for Bass Harmonization

and its message complexity is