  This paper presents a scheme named code hopping (CodeHop) for gaussian
wiretap channels based on nonsystematic low-density parity-check (LDPC) codes.
Different from traditional communications, in the CodeHop scheme, the
legitimate receiver (Bob) rapidly switches the parity-check matrix upon each
correctly received source message block. Given an authenticated public feedback
channel, the transmitter's (Alice) parity-check matrix can also be synchronized
with the receiver's. As a result, once an eavesdropper (Eve) erroneously
decodes a message block, she may not be able to follow the update of subsequent
parity-check matrices. Thus, the average BER of Eve will be very close to $0.5$
if the transmitted number of message blocks is large enough. Focused on the
measure of security gap defined as the difference of channel quality between
Bob and Eve, numerical results show that the CodeHop scheme outperforms other
solutions by sufficiently reducing the security gap without sacrificing the
error-correcting performance of Bob.
