The potential weakness of the Y-00 direct encryption protocol when the
encryption box ENC in Y-00 is not chosen properly is demonstrated in a fast
correlation attack by S. Donnet et al in Phys. Lett. A 35, 6 (2006) 406-410. In
this paper, we show how this weakness can be eliminated with a proper design of
ENC. In particular, we present a Y-00 configuration that is more secure than
AES under known-plaintext attack. It is also shown that under any
ciphertext-only attack, full information-theoretic security on the Y-00 seed
key is obtained for any ENC when proper deliberate signal randomization is
employed
