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Recently, a number of authentication schemes have been proposed for multimedia data. The main requirement for such authentication systems is that minor modifications which do not alter the content of the data preserve the authenticity of the data, whereas modifications which do modify the content render the data not authentic. These schemes can be classified into two classes depending on the underlying model of image authentication. We look at some of the advantages and disadvantages of these schemes and their relationship with limitations of the underlying model of image authentication. In particular, we study feature-based algorithms and hash-based algorithms. The main disadvantage of feature-based algorithms is that similar images generate similar features, and therefore it is possible for a forger to generate dissimilar images with the same features. On the other hand, the class of hash-based algorithms utilizes a cryptographic digital signature scheme and inherits the security of digital signatures to thwart forgery attacks. The main disadvantage of hash-based algorithms is that the image needs to be modified in order to be made authenticatable. We propose a multimedia authentication scheme which combines some of the best features of these two classes of algorithms. The proposed scheme utilizes cryptographic digital signature schemes and the data does not need to be modified in order to be made authenticatable. We show how results in sphere packings and coverings can be useful in the design. Several applications including the authentication of images on CD-ROM and handwritten documents are discussed.