  We propose a simple method for secure digital signal transmission by making
some modifications in the single-step parameter modulation technique proposed
earlier into overcome certain inherent deficiencies. In the modified method,
the parameter modulation is effectively regulated or controlled by the chaotic
signal obtained from the transmitting chaotic system so that it has the maximum
security. Then, the same idea is also extended to the multistep parameter
modulation technique. It is found that both the methods are secure against
ciphertext (return map) and plaintex attacks. We have illustrated these methods
by means of the Lorenz system.
