One of the principal challenges of resource sharing on data communication network is its security. This paper presents a design of data encryption and decryption using RSA algorithm with a specific message block size. RSA is based on several mathematical principles in number theory. This paper proposed an implementation of a complete RSA encrypt/decrypt solution based on the study of RSA public key cryptosystem. A cryptosystem is simply an algorithm that can convert input data into something unrecognizable (encryption) and convert the unrecognizable data back to its original form (decryption), security depends on the algorithm while the internal structure of the rigor of mathematics.