Hua Liu

Defending Web Services against Denial of Service Attacks Using Client Puzzles

The interoperable and loosely-coupled web services architecture, while beneficial, can be resource-intensive, and is thus susceptible to denial of service (DoS) attacks in which an attacker can use a relatively insignificant amount of resources to exhaust the computational resources of a web service. We investigate the effectiveness of defending web services from DoS attacks using client puzzles, a cryptographic countermeasure which provides a form of gradual authentication by requiring the client to solve some computationally difficult problems before access is granted. In particular, we describe a mechanism for integrating a hash-based puzzle into existing web services frameworks and analyze the effectiveness of the countermeasure using a variety of scenarios on a network test bed. Client puzzles are an effective defence against flooding attacks. They can also mitigate certain types of semantic-based attacks, although they may not be the optimal solution.

Nonlinear Acoustoelastic Interactions of Lamb Waves with LiNbO3 Films Deposited on Sapphire Substrates

The influence of a biasing electric field on the propagation of the lowest-order antisymmetrical a0 Lamb wave modes in a bi-layered piezoelectric plate is investigated in this paper. It is found that the velocity shifts for the a0 mode due to presence of the bias field on the 10- µm LiNbO3 film structure are comparable with those observed in surface acoustic waves and Lamb waves in LiNbO3 plates. The fractional change in phase velocity of the layered piezoelectric structure is a linear function of the biasing electric field and can be used in voltage sensors.

Mathematical Models for the Hydrodynamic Characteristics of Abrasive Waterjets

Predictive models for the particle velocity in an abrasive waterjet (AWJ) are developed following a CFD (computational fluid dynamics) study. A numerical study is then carried out to assess the models. It is shown that the predictive models can adequately predict this particle characteristic in an AWJ.