Exploiting Multi-Antenna Non-Reciprocal Channels for Shared Secret Key Generation

Traditional physical-layer (PHY)-based secret key generation schemes typically assume reciprocity of forward and reverse channels in order for facilitated shared secrecy. However, frequency-selective channels for frequency division duplex links as well as calibration difference of transceiver hardwares can invalidate reciprocity assumption and can cause severe key error floors between users. In this paper, we propose two practical PHY-based secret key generation schemes for multiple-input and multiple-output wireless links over non-reciprocal physical channels. One is pilot assistant secret key (PASKey) generation and the other is eigenvalue-based secret key (EVSKey) generation. In PASKey, both legitimate users estimate the same composite channel, while EVSKey exploits the properties of eigenvalues to generate secret bits. Both are effective against secrecy key theft by eavesdroppers. Furthermore, without adopting public training signals, EVSKey manages to generate secret bits even when nearby eavesdroppers collude. Our two schemes are shown to be more efficient and can achieve lower key error ratio than the existing methods.