Double-Eavesdroppers Cooperative Attack on iJam

The iJam scheme is a novel physical-layer approach for secret key generation, where the generation of secret bits can be very fast and essentially independent of channel variations. The basic idea is that Alice, as the legitimate user, sends an OFDM symbol twice and Bob, as the legitimate receiver, randomly jams either the original transmission sample, or the corresponding sample in the repetition. Since Bob knows the locations of jamming, it can pick the correct samples from the received signal, and restructure them to obtain a clean signal. But the eavesdropper does not know which received signal is jammed and which one is clean, so it cannot correctly decode the data. However, the robustness of jamming depends on the eavesdropper's location with respect to the sender and jammer. Although iJam takes two-way exchange to overcome the eavesdropper position sensitive issues, the security of iJam has not been resolved fully. In order to crack iJam scheme completely, we propose a double-eavesdroppers cooperative attack, which can be well employed to crack the iJam scheme. Extensive simulation results validate our claim.