A Secret Key Generation System Based on Multipath Channel Randomness: RSSI vs CSSI

We design and analyze a method to extract secret keys from the randomness inherent to wireless channels. We study a channel model for the IEEE 802.11a orthogonal frequency-division multiplexing channel. We analyze the fundamental limits of key generation for this model and describe an implementation based on error correcting codes. We study key extraction based both on channel state information (CSI) and on received signal strength indicators (RSSI). Due to the reduction in the degree-of-freedom when going from CSI to RSSI, the rate of key extraction based on CSI is far higher than that based on RSSI. This suggests that making CSI information available to higher layers would greatly assist in key generation. We prototype a key generation system based on low-density parity-check codes. We describe the implementation and performance of two systems: one based on binary LDPC codes and the other (useful at higher signal-to-noise ratios) based on four-ary LDPC codes.