In this paper, we study some computational security assump-tions involve in two cryptographic applications related to the RSA cryp-tosystem. To this end, we use exponential sums to bound the statistical distances between these distributions and the uniform distribution. We are interesting studying the k least (or most) significant bits of x e mod N , where N is a RSA modulus when x is restricted to a small part of [0, N). First of all, we provide the first rigorous evidence that the cryptographic pseudo-random generator proposed by Micali and Schnorr is based on firm foundations. This proof is missing in the original paper and do not cover the parameters chosen by the authors. Consequently, we extend the proof to get a new result closer to the parameters using a recent work of Wooley on exponential sums and we show some limitations of our technique. Finally, we look at the semantic security of the RSA padding scheme called PKCS#1 v1.5 which is still used a lot in practice. We show that parts of the ciphertexts are indistinguisable from uniform bitstrings.