This paper proposes a robustness evaluation of an RSA cryptosystem against collision attacks and correlation electromagnetic analysis. Our hardware co-processor is based on the Residue Number System (RNS) in order to perform modular operations over large numbers. To increase its robustness against Side-Channel Analysis, we implemented two different countermeasures. The first one spatially permutates the elements of the RNS bases in order to blur electromagnetic emanations. The second countermeasure aims at randomizing RNS bases before each modular exponentiation. To the best knowledge of authors, this is the first paper that explores the robustness of RNS-RSA against EM analyses.