An Improvement of ElGamal-like Cryptosystem with Periodic Bitwise Operations

Abstract

Hwang et al. (2002) proposed an ElGamal-like cryptosystem which aimed to improve effectiveness of the ElGamal cryptosystem for encrypting large messages, particularly in terms of computational complexity and the storage of ciphertext. Later, Wang et al. (2006) proved that the ElGamal-like cryptosystem is less secure than the ElGamal cryptosystem, and then proposed an improved version to increase the security. In this paper, we improve the scheme of Wang et al. (2006) by using different bitwise operations, which are assigned according to a certain periodic sequence, while encrypting different blocks of plaintext. The results show that our scheme can provide larger period length of the coefficients for encryption and more resistance to a ciphertext-only attack and a known-plaintext attack than the scheme of Wang et al. (2006).