—Recent theoretical and practical work has shown
that physical layer security techniques have the potential to
significantly strengthen the security of wireless networks. We
propose a practical security scheme by which two terminals
(say Alice and Bob) are able to exploit the randomness of
wireless fading channels to exchange data in an informationtheoretically secure way. To ensure that a potential eavesdropper (say Eve) is unable to decode any useful information,
Alice sends useful symbols to Bob only when the instantaneous
secrecy capacity is strictly positive. In the remaining time, a
specially designed class of LDPC codes is used for reconciliation,
thus allowing the extraction of a secret key, which can be distilled using privacy amplification. We believe this opportunistic
approach can be used effectively as a physical layer complement
to existing cryptographic protocols.