Cryptanalysis and Improvement of Akleylek et al.'s cryptosystem

Akleylek et al. [S. Akleylek, L. Emmungil and U. Nuriyev, Algorithm for peer-to-peer security, journal of Appl. Comput. Math., Vol. 6(2), pp.258-264, 2007.], introduced a modified algorithm with steganographic approach for security in peer-to-peer (P2P) network. In this cryptosystem, Akleylek et al. attempt to increase the security of P2P network by connecting the ElGamal cryptosystem with knapsack problem. We show that this combination leak the security and makes the hybrid cryptosystem vulnerable to ciphertext only attack. Thus, in the network, an attacker can apply this attack and simply can recover the original message (plaintext) from any challenge ciphertext. Moreover, we show that the receiver cannot decrypt the ciphertext in polynomial time and so, the proposed cryptosystem is completely impractical. We modify this cryptosystem to increase security and efficiency.