Interactive secret key generation over reciprocal fading channels

Abstract—We study a two-terminal secret-key generation problem over a two-way, approximately reciprocal, block-fading channel. The channel gains between the legitimate terminals are not revealed to any terminal, whereas the channel gains of the eaves-dropper are revealed perfectly to the eavesdropper. We study a separation based scheme that involves a training phase followed by a communication phase within each block. The training phase generates correlated estimates of the channel state sequence be-tween the two terminals, whereas the communication phase generates correlated source sequences between the two terminals. A portion of the secret-key is gen-erated from the correlated channel state sequences by creating omniscience between the legitimate terminals and the remainder of the secret-key is generated from the correlated source sequences. An upper bound on the secret-key capacity is also established by reducing the setup to a coherent channel by providing genie-aided side information to the receivers. We observe that the diffrence between the upper and lower bound decreases as 1 T in the high signal-to-noise-ratio (SNR) regime. Numerical results indicate that the proposed scheme achieves significant gains over the commonly used training-only schemes even for moderate SNR. I