  Akleylek et al. [S. Akleylek, L. Emmungil and U. Nuriyev, A mod ified
algorithm for peer-to-peer security, journal of Appl. Comput. Math., vol. 6(2),
pp.258-264, 2007.], introduced a modified public-key encryption scheme with
steganographic approach for security in peer-to-peer (P2P) networks. In this
cryptosystem, Akleylek et al. attempt to increase security of the P2P networks
by mixing ElGamal cryptosystem with knapsack problem. In this paper, we present
a ciphertext-only attack against their system to recover message. In addition,
we show that for their scheme completeness property is not holds, and
therefore, the receiver cannot uniquely decrypts messages. Furthermore, we also
show that this system is not chosen-ciphertext secure, thus the proposed scheme
is vulnerable to man-in-the-middle-attack, one of the most pernicious attacks
against P2P networks. Therefore, this scheme is not suitable to implement in
the P2P networks.
  We modify this cryptosystem in order to increase its security and efficiency.
Our construction is the efficient CCA2-secure variant of the Akleylek et al.'s
encryption scheme in the standard model, the de facto security notion for
public-key encryption schemes.
