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In this paper, an observer-based secure communication system composed of four chaotic oscillators is proposed. Observer based synchronization is achieved between two of these oscillators and employed as a transmitter and a receiver. The other two oscillators are indirectly coupled and are employed as keystream generators. The novelty lies in the generation of the same chaotic keystream both in the transmitter and receiver side for encryption and decryption purposes. We show, in particular, that it is possible to synchronize the two keystream generators even though they are not directly coupled. So doing, an estimation of the keystream is obtained allowing decrypting the message. The performance of the proposed communication scheme is shown via simulation using the Chua and Lorenz oscillators.