RSA is a strong public-key cryptosystem, which is generally used for authentication protocols. In this paper, we propose a resource efficient design for RSA based on a new architecture whose complexity in resource but memory grows little with the security level of RSA. Furthermore, we optimize the Montgomery modular multiplication algorithm in high-radix by avoiding the final comparison and implement some effective countermeasures against side channel attacks in our design. Our implementation trades off time and area requirements and improves the time?area factor when compared that of Blum's and Nedhah's.