An apparatus for enciphering digital information data with improved cipher strength by subjecting digital information data to enciphered process responding to random number data or pseudo-random number data produced in accordance with key data. The apparatus comprises an enciphering portion for producing the enciphered digital information data, a cipher producing portion for producing, in response to the key data, a cipher data from which random number data or pseudo-random number data are obtained to be supplied to the enciphering portion, a register for supplying the cipher producing portion with input data, and an initial value data generator for supplying the register with enciphered initial value data.