A data processing device for connecting between a storage device and a controlling device (100, 101), the data processing device comprising: encryption means for encrypting data which is received from the controlling device (100, 101) and is to be transmitted to the storage device; decryption means for decrypting data which is encrypted, is received from the storage device and is to be transmitted to the controlling device (100, 101); memory for storing seed information for generating key information that is used for the encryption and decryption, wherein the encryption means and decryption means are arranged respectively to encrypt and decrypt data using key information generated on the basis of the seed information and unique information from the controlling device (100, 101); receiving means for receiving a command from the controlling device (100, 101); determining means for determining whether the command received by the receiving means is executable; authentication means for executing authentication between the controlling device (100, 101) and the data processing device; and controlling means for controlling an operation mode of the data processing device so that: before the authentication by the authentication means is executed and when the data processing device is powered on, the data processing device initially operates only in an intercept mode in which any information sent by the controlling device (100, 101) is not transmitted to the storage device, so that accesses from the controlling device (100, 101) to the storage device are intercepted; and after the authentication by the authentication means is executed and the controlling device (100, 101) is authenticated as valid, the data processing device operates in a normal mode (S702, S703, S704) in which access to the storage device from the controlling device (100, 101) is permitted; wherein the determining means determines whether the received command is executable, depending on the operation mode of the data processing device, such that in the normal mode the determining means determines whether each command issued by the controlling device (100, 101) is executable at the storage device. encryption means for encrypting data which is received from the controlling device (100, 101) and is to be transmitted to the storage device; decryption means for decrypting data which is encrypted, is received from the storage device and is to be transmitted to the controlling device (100, 101); memory for storing seed information for generating key information that is used for the encryption and decryption, wherein the encryption means and decryption means are arranged respectively to encrypt and decrypt data using key information generated on the basis of the seed information and unique information from the controlling device (100, 101); receiving means for receiving a command from the controlling device (100, 101); determining means for determining whether the command received by the receiving means is executable; authentication means for executing authentication between the controlling device (100, 101) and the data processing device; and controlling means for controlling an operation mode of the data processing device so that: before the authentication by the authentication means is executed and when the data processing device is powered on, the data processing device initially operates only in an intercept mode in which any information sent by the controlling device (100, 101) is not transmitted to the storage device, so that accesses from the controlling device (100, 101) to the storage device are intercepted; and after the authentication by the authentication means is executed and the controlling device (100, 101) is authenticated as valid, the data processing device operates in a normal mode (S702, S703, S704) in which access to the storage device from the controlling device (100, 101) is permitted; wherein the determining means determines whether the received command is executable, depending on the operation mode of the data processing device, such that in the normal mode the determining means determines whether each command issued by the controlling device (100, 101) is executable at the storage device.