Showing papers by "Chris J. Mitchell published in 1996"

Comments on the S/KEY user authentication scheme

Abstract: We give a critical analysis of the security properties of the S/KEY user authentication system.

Intrinsic reinforcing properties of putatively neutral stimuli in an instrumental two-lever discrimination task

Abstract: Four experiments examined the influence of a stimulus presented after one response in a two-lever choice task. In Experiment 1, food-deprived rats trained on a concurrent variable-interval extinction schedule responded more often on the extinction lever when such responding periodically produced a visual stimulus than when it did not. In Experiments 2 and 3, a similar signal-induced enhancement effect was found even when food was delivered randomly with respect to responding on both levers or when no food was presented. In Experiment 4, a response-contingent visual stimulus elevated responding to the lever on which it was presented, but an auditory cue suppressed responding. These findings indicate that visual stimuli may possess intrinsically reinforcing properties for rats.

A method for constructing decodable de Bruijn sequences

Abstract: We present two related methods of construction for de Bruijn (1946) sequences, both based on interleaving "smaller" de Bruijn sequences. Sequences obtained using these construction methods have the advantage that they can be "decoded" very efficiently, i.e., the position within the sequence of any particular "window" can be found very simply. Sequences with simple decoding algorithms are of considerable practical importance in position location applications.

Key Escrow in Mutually Mistrusting Domains

Abstract: In this paper we present a key escrow system which meets possible requirements for international key escrow, where different domains may not trust each other. In this system multiple third parties, who are trusted collectively but not individually, perform the dual role of providing users with key management services and providing authorised agencies in the relevant domains with warranted access to the users' communications. We propose two escrowed key agreement mechanisms, both designed for the case where the pair of communicating users are in different domains, in which the pair of users and all the third parties jointly generate a cryptographic key for end-to-end encryption. The fact that all entities are involved in the key generation process helps make it more difficult for deviant users to subvert the escrowed key by using a hidden ‘shadow-key’. The first mechanism makes use of a single set of key escrow agencies moderately trusted by mutually mistrusting domains. ! The second mechanism uses a transferable and verifiable secret sharing scheme to transfer key shares between two groups of key escrow agencies, where one group is in each domain.

Redundant Integer Representations and Fast Exponentiation

Abstract: In this paper two modifications to the standard square and multiply method for exponentiation are discussed. The first, using a signed-digit representation of the exponent, has been examined previously by a number of authors, and we present a new precise and simple mathematical analysis of its performance. The second, a new technique, uses a different redundant representation of the exponent, which we call a string replacement representation; the performance of this new method is analysed and compared with previously proposed methods. The techniques considered in this paper have application in the implementation of cryptographic algorithms such as RSA, where modular exponentiations of very large integers need to be calculated.

Tailoring authentication protocols to match underlying mechanisms

Abstract: Authentication protocols are constructed using certain fundamental security mechanisms. This paper discusses how the properties of the underlying mechanisms affect the design of authentication protocols. We firstly illustrate factors affecting the selection of protocols generally. These factors include the properties of the environment for authentication protocols and the resources of the authenticating entities. We then consider a number of authentication protocols which are based on mechanisms satisfying different conditions than those required for the ISO/IEC 9798 protocols, in particular the use of non-random nonces and the provision of identity privacy for the communicating parties.

Authentication Schemes, Perfect Local Randomizers, Perfect Secrecy and Secret Sharing Schemes

Abstract: In this paper we use results on authentication schemes to derive alternative proofs for results on perfect local randomnes in pseudo-random sequences, on block cipher systems which afford perfect secrecy against known plaintext attacks and on secret sharing schemes.