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, wherein the TEK process block includes: a structure analysis block for analyzing an MPEG structure included in the received TEK process data and a MAC (Media Access Control) structure buried in the MPEG structure to output MAC state information data that represents a state and meaning of MAC data having the MAC structure; a decryption block for identifying encrypted part of the TEK process data by referring to the MAC state information data, decrypting the encrypted part using TEK data for cryptanalysis, and integrating a result of the decryption with unencrypted part of the TEK process data, wherein the structure analysis block includes: an MPEG header analysis block for analyzing an MPEG header of the MPEG structure included in the TEK process data to output a MAC data position signal which indicates a position of the MAC data and a MAC data head position signal which indicates a position of a leading byte of a MAC frame; a MAC header analysis block for receiving the MAC data position signal and the MAC data head position signal and determining a state information for fields included in a MAC header of the MAC structure except for an extension header and a MACMM (MAC Management Message) header, wherein the MAC header analysis block outputs extension header position information data which indicates a position of the extension header when the TEK process data includes the extension header, and the MAC header analysis block outputs MACMM header position information data which indicates a position of the MACMM header when the TEK process data includes the MACMM header; an extension header analysis block for receiving the extension header position information data and checking fields of the extension header to output extension header state information data which represents state information of the extension header; and a MACMM header analysis block for receiving the MACMM header position information data and checking fields of the MACMM header to output MACMM header state information data which represents state information of the MACMM header, wherein the MAC header analysis block receives the extension header state information data and the MACMM header state information data and generates the MAC state information data based on state information of the fields included in the MAC header except for the extension header and MACMM header, the state information of the extension header which is represented by the extension header state information data, and the state information of the MACMM header which is represented by the MACMM header state information data.