| 摘要 |
A data communication system is provided in which a time required for a wiretapper to decrypt an encrypted text is significantly increased so that concealment is improved. In a data transmitting apparatus ( 17105 ), a multilevel encoding part ( 111 ) switches a plurality of key information to generate a multilevel code sequence in which the average values of signal levels are different, and then combines the generated multilevel code sequence with information data to generate a multilevel signal having a level corresponding to the combination of the two signal levels. A light modulating part ( 125 ) converts the multilevel signal into a modulated signal of a predetermined modulation scheme and transmits it. In a data receiving apparatus ( 17205 ), a light demodulating part ( 219 ) demodulates the received modulated signal into the multilevel signal. A multilevel decoding part ( 212 ) switches a plurality of key information to generate a multilevel code sequence in which the average values of signal levels are different, and then identifies the multilevel signal based on this generated multilevel code sequence to regenerate the information data.
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