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ESIGN is an e-cient signature scheme that has been pro- posed in the early nineties (see (14)). Recently, an efiort was made to lay ESIGN on flrm foundations, using the methodology of provable security. A security proof (15) in the random oracle model, along the lines of (2), appeared in support for ESIGN. However, several unexpected di-cul- ties were found. Firstly, it was observed in (20), that the proof from (15) holds in a more restricted model of security than claimed. Even if it is quite easy to restore the usual security level, as suggested in (9), this shows that the methodology of security proofs is more subtle than it at flrst appears. Secondly, it was found that the proof needs the additional assumption that e is prime to '(n), thus excluding the case where e is a small power of two, a very attractive parameter choice. The di-culty here lies in the simulation of the random oracle, since it relies on the distribution of e-th powers, which is not completely understood from a mathematical point of view, at least when e is not prime to '(n). In this paper, we prove that the set of e-th power modulo an RSA modulus n, which is a product of two equal size integers p,q, is almost uniformly dis- tributed on any large enough interval. This property allows to complete the security proof of ESIGN. We actually ofier two proofs of our result: one is based on two-dimensional lattice reduction, and the the other uses Dirichlet characters. Besides yielding better bounds, the latter is one new example of the use of analytic number theory in cryptography.

Almost Uniform Density of Power Residues and the Provable Security of ESIGN