In 1992 Denes and Keedwell proposed an authentication scheme in which a message is divided into blocks of length t and a set of signature characters is obtained by taking the product of the elements of each block. An analysis of the security of Denes and Keedwell's scheme will be presented in this paper. It will be shown that the method used to divide the message into the blocks impacts on the security of the scheme. Also it will be shown that under certain circumstances the t-ary product is

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... unique, and so there are classes of binary operations all of which produce the same signature or authentication tag.