Diffie´s Wireless Phone: Heterodyning-Based Physical-Layer Encryption

We propose a physical-layer encryption scheme inspired by Diffie´s telephone system. Instead of the usual XOR-then-modulate encryption scheme, we propose using modulate-then-add, and show that the decryption operation can be implemented using the circuit operation of heterodyning. We then show that a slightly modified superheterodyne receiver performs decryption at no additional cost compared to existing receivers. Our proposed cryptosystem is significant to the research community for the following reasons: 1) Our physical-layer encryption scheme uniquely outperforms other physical-layer security protocols by guaranteeing positive conditional secrecy capacity as long as Bob´s signal-to-interference-and-noise ratio is above a threshold, even if Eve´s channel condition is the same or significantly better than Bob´s or if the channel between Alice and Bob is static, and 2) Our physical-layer encryption scheme shows that by removing a filter that the wireless circuit community has long considered to be necessary in the superheterodyne design, the modified receiver offers intriguing security features.