1. An apparatus for computing a T-function based Stream Cipher (TSC)-4 comprising two T-function units and a nonlinear filter for receiving bits output from the two T-function units and generating an 8-bit output sequence per clock, wherein each of the T-function units comprises: a first register for storing an internal state value of lower N bits; an N-bit internal state updater for updating the internal state value of the lower N bits stored in the first register; an intermediate result register for storing an intermediate result value output from the N-bit internal state updater; a second register for storing an internal state value of upper M bits; and an M-bit internal state updater for updating the internal state value of the upper M bits stored in the second register using the value stored in the intermediate result register.