TELETEXT TRANSMISSION SYSTEMS

摘要

A teletext decoder for a teletext transmission system has a logic processor which is programmed to descramble teletext information containing scrambled text and encrypted cipher messages. The logic processor operates according to a decoding method to decrypt various cipher keys and to use the decrypted cipher keys to descramble pages of teletext text. The decoding method involves a one-way function which is the same as that used for scrambling/encryption at the transmission end of the teletext system. Each byte of a teletext message is descrambled by exclusive-ORing it with a respective decode byte that is derived from the one-way function. Figure 2 illustrates the one-way function used.