DATA ENCRYPTION DEVICE, DECODER AND ITS PROGRAM RECODING MEDIUM

摘要

PROBLEM TO BE SOLVED: To decode encrypted data by an access to an optional point on a time base of encrypted consecutive medium data in the output feedback OFB mode of a stream encryption. SOLUTION: A random number generator 11 generates a random number based on contents in a register 12 at an encryption side, the random number is fed back to the register to generate a succeeding random number, the random number series and input data are exclusively 0Red to obtain encrypted data, contents of the register 12 are extracted periodically and inserted to the encrypted data at a synthesizer 15 and insert identification information is inserted to the encrypted data without encrypting the information just before the insertion of the contents of the register 12 (Figure B). An extraction device 22 detects the inserted identification information from the received encrypted data at a decoder side (Figure C) and extracts the register contents, sets the contents to a register 23 to start generation of a random number by a random number generator 24.