Optical Cryptosystem Based on Synchronization of Hyperchaos Generated by a Delayed Feedback Tunable Laser Diode
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t Secure communications based on chaos have b investigated for some years, especially in the area radiofrequency transmissions. Signal encoding a decoding is achieved using a carrier whose amplitu fluctuates chaotically. Compared with conventional da encryption techniques in which the key is a pseudorand binary number that controls the encryption algorithm, b which is slow, chaos is used as a coding key embod directly in the structure of the carrier. At least two clas sical methods have been demonstrated for communica with chaos [1]. The first method due to Ott, Grebogi, an Yorke [1] utilizes controlling chaos. The dynamics of chaotic oscillator is made to follow prescribed orbits the attractor by using small perturbations, thus allowing message to be encoded in the chaotic wave form. A v different concept developed by Pecora and Carroll [ uses the idea of synchronized chaos for secure comm cations. In this case, a small information-bearing sign is masked by a large chaotic signal. The chaotic cod generating system is divided in two subsystems, name the master and the slave. The slave is replicated at receiver. The master subsystem is used to synchron the two slave subsystems. The message signal is ad to the chaotic signal generated by the slave subsystem the transmitter and this composite signal is transmitt to the receiver. When the two subsystems are synch nized, the message can be reproduced by subtracting chaotic part of the composite signal. So far most of t studies were implemented with electrical circuits featu ing low dimensional attractors, such as the double scr or Chua’s attractor that has a single positive Lyapun exponent [2]. The simple chaotic processes thus obtain can, however, be defeated by an eavesdropper without synchronized receiver, using unmasking signal process techniques which work well for simple chaotic system [3]. One way to solve this security problem is by usin hyperchaotic systems with multiple positive Lyapuno exponents to mask the message. A recent theoret work [4] indicates the possibility of synchronizing hyperchaotic chaos with just one transmitted sign Practical realizations remain, however, to be develop The question of optical synchronization of chaos h