The encryption/decryption device includes: a data structure analysis block for receiving encrypted data or data to be encrypted and outputting control data and also the encrypted data or the data to be encrypted as processing block input data; a data control block for determining a mode selection signal according to the control data; and a shared processing block for performing encryption or decryption for the processing block input data and outputting the result. The shared processing block is configured to have the ability to perform encryption and decryption in either of the CBC mode and the CFB mode by performing ECB processing using input key data, and performs encryption or decryption in the mode indicated by the mode selection signal.