Classical optical cryptography

This paper describes a cryptographic technique based on coherent optical communication for fiber or free space networks. A pseudo-random cipher-stream is used to band-spread an optical carrier with coded data. The legitimate receiver uses the agreed upon key to modulate its local oscillator and the resulting beat signal will uncover the band-spread signal. An eavesdropper who does not have the key will find the spread signal with too low signal-to-noise ratio to perform any useful determination of the message sequence. Theoretical bounds based on Shannon´s Theory of Secrecy is used to show the strength of the encoding scheme which is expected to be superior.