Abstract

. In the past few years a lot of attention has been paid to the use of special logics to analyse cryptographic protocols, foremost among these being the logic of Burrows, Abadi and Needham (the BAN logic). These logics have been successful in finding weaknesses in various examples. In this paper a limitation of the BAN logic is illustrated with two examples. These show that it is easy for the BAN logic to approve protocols that are in practice unsound. 1 Introduction In recent years there has been great interest in the design and analysis of secure protocols. Various new techniques have been developed and used to find a great variety of different attacks on such protocols. One of the most important of these techniques is the Logic of Authentication of Burrows, Abadi and Needham [2], (the `BAN logic') which transforms a protocol into a special form and then uses logical rules to analyse it. The BAN logic has been used to find new weaknesses in various cryptographic protocols. A ...

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