Summary: In this letter, we propose a symmetric key block cipher algorithm utilizing the essence of chaos. The proposed cryptosystem does not use explicitly the system parameter or initial condition of the chaotic map as a secret key. However, these parameters are generated by an external secret key of variable length (maximum 128-bits) and used in our algorithm. The encryption of each block of plaintext has been made dependent on the secret key and the cryptosystem is further made robust against any reasonable attack by using the feedback technique. Simulation results show that the proposed cryptosystem requires less time to encrypt the plaintext as compared to the existing chaotic cryptosystems and further produces the ciphertext having flat distribution of same size as the plaintext.