RSA cryptosystem is an earlier public key cryptosystem. 1978, Rivest, Shamir and Adleman of Massachusetts Institute of Technology (MIT) put forward an asymmetric (public key) cryptosystem based on number theory, which is called RSA cryptosystem, in their paper entitled "Methods of obtaining digital signature and public key cryptosystem". RSA is a block cipher system based on "prime decomposition of large integers is a difficult problem".