1. A device for performing bidirectional control in digital bidirectional communication, comprising: an interface block for converting a format of input downstream data to generate downward data; a CPU which receives the downward data and realizes a MAC (Media Access Control) function; and a TEK process block for receiving TEK (Traffic Encryption Key) process data obtained from the downward data, analyzing a data structure of the TEK process data, and performing decryption processing based on a result of the analysis.