Abstract

Recently, XUE-CAO (Q.S.Xue, Z.F.Cao, Factoring based proxy signature schemes, Journal of Computational and Applied Mathematics, 195(2006) 229-241) proposed one normal proxy signature scheme and one multi-proxy signature scheme which are based on the hard problem of factoring large integer. In this paper, however, we show that XUE-CAO's two schemes are insecure against the replaced warrant attack: the original signer can prepare a new warrant and then remove the original warrant from the proxy signature to forge a new and valid normal proxy signature (or multi-proxy signature) with the new warrant. To avoid the problem, one improved normal proxy signature scheme and one improved multi-proxy signature scheme are proposed, and their security and performance are analyzed as well.

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