  This paper describes the security weakness of a recently proposed improved
chaotic encryption method based on the modulation of a signal generated by a
chaotic system with an appropriately chosen scalar signal. The aim of the
improvement is to avoid the breaking of chaotic encryption schemes by means of
the return map attack introduced by Perez and Cerdeira. A method of attack
based on taking the absolute value of the ciphertext is presented, that allows
for the cancellation of the modulation scalar signal and the determination of
some system parameters that play the role of system key. The proposed improved
method is shown to be compromised without any knowledge of the chaotic system
parameter values and even without knowing the transmitter structure.
