Showing papers by "Chris J. Mitchell published in 2006"

Effects of preexposure on stimulus discrimination: An investigation of the mechanisms responsible for human perceptual learning.

Abstract: The effect of preexposure on human perceptual learning was investigated in four experiments. In Experiments 1a and 1b, participants were preexposed to one pair of visual stimuli on an intermixed schedule (AX/BX) and one on a blocked schedule (CX_DX). The ability to discriminate between AX and BX and between CX and DX was then assessed by examining the extent to which key presses assigned to each member of a stimulus pair generalized to the other member (Experiment 1a) and by looking at the accuracy of same–different responses (Experiment 1b). Stimuli were more easily discriminated following intermixed than following blocked preexposure on both the generalization and same–different tasks. This suggests that two stimuli are more perceptually distinct after intermixed preexposure. Experiments 2a and 2b investigated the mechanisms responsible for perceptual learning using same–different tasks. The results support the suggestion that the enhanced discrimination observed after intermixed preexposure is due to i...

Forward blocking in human learning sometimes reflects the failure to encode a cue–outcome relationship

Abstract: In two “allergist” causal judgement experiments, participants were trained with a blocking design (A + |AB+). The procedure allowed different food cues to be paired with different fictitious allergic reactions. On test, participants were asked to rate the causal efficacy of the target cues and to recall the particular allergic reaction (outcome) that had followed each cue during training. Forward blocking was observed on the causal judgement measure and on the outcome recall measure in both Experiment 1 and Experiment 2. A backward blocking contingency was also trained in Experiment 2 (AB + |A+). Backward blocking was not observed either on the causal judgement or on the outcome recall measure. The evidence from the recall measure suggests that forward blocking in this task results from a failure to encode the B–outcome relationship during training. Associative and nonassociative mechanisms of forward blocking are discussed.

Toward a Broader View of Security Protocols

Abstract: Computer and network security researchers usually focus on the security of computers and networks. Although it might seem as if there is more than enough insecurity here to keep all of us fully occupied for the foreseeable future, this narrow view of our domain may actually be contributing to the very problems that we are trying to solve. We miss important insights from, and opportunities to make contributions to, a larger world that has been grappling with security since long before the computer was invented.

A service discovery threat model for ad hoc networks

Abstract: The dynamic yet vulnerable nature of an hoc network presents many new security and privacy challenges. Securing the process of service discovery is one of them. Novel solutions are therefore required. However, in order for appropriate security measures to be devised, all possible security threats must first be identified and thoroughly analysed. In this paper, we present a threat model for service discovery in ad hoc networks. Based on these threats, we proceed to derive the security services required to achieve secure service discovery.

Modelling e-business security using business processes

Abstract: Organisations (enterprises, businesses, government institutions, etc.) have changed their way of doing business from a traditional approach to embrace e-business processes. This change makes the perimeter security approach inappropriate for such organisations. The well-known and widely used security mechanisms, including cryptography-based tools and techniques, cannot provide a sufficient level of security without being a part of a comprehensive organisational approach/philosophy. This approach must be different from the current dominant approach, i.e. perimeter security, and must focus on different organisational components. In this paper we suggest a process security approach, and describe ongoing research with the aim of developing an e-business security model based on this new, process security, approach.

The Dancing Bear : A New Way of Composing Ciphers. Authors' reply

Abstract: This note presents a new way of composing cryptographic primitives which makes some novel combinations possible. For example, one can do threshold decryption using standard block ciphers, or using an arbitrary mix of different decryption algorithms - such as any three keys out of two AES keys, a 3DES key, an RSA key and a one-time pad. We also provide a new way to combine different types of primitive, such as encryption and signature. For example, Alice can construct a convertible signature that only Bob can verify, but which he can make world-verifiable using an AES key. We can incorporate even more exotic primitives, such as micropayments and puzzles, into compound constructs. Previously, there had been two basic ways to combine cryptographic primitives. One could either design a compound primitive, perhaps using the homomorphic properties of discrete exponentiation, or one could embed several primitives into a protocol. Neither is ideal for all applications, and both have been extremely vulnerable to design errors. We provide a third construction that also allows the designer to do new things. We show, for example, how to incorporate cyclic dominance into a cryptographic mechanism, and how it might be used in a digital election scheme. Our new construction not only complements existing ways of composing crypto primitives; it also has the virtue of simplicity.