  Physical-layer key generation (PKG) can generate symmetric keys between two
communication ends based on the reciprocal uplink and downlink channels. By
smartly reconfiguring the radio signal propagation, intelligent reflecting
surface (IRS) is able to improve the secret key rate of PKG. However, existing
works involving IRS-assisted PKG are concentrated in single-antenna wireless
networks. So this paper investigates the problem of PKG in the IRS-assisted
multiple-input single-output (MISO) system, which aims to maximize the secret
key rate by optimally designing the IRS passive beamforming. First, we analyze
the correlation between channel state information (CSI) of eavesdropper and
legitimate ends and derive the expression of the upper bound of secret key rate
under passive eavesdropping attack. Then, an optimal algorithm for designing
IRS reflecting coefficients based on Semi-Definite Relaxation (SDR) and Taylor
expansion is proposed to maximize the secret key rate. Numerical results show
that our optimal IRS-assisted PKG scheme can achieve much higher secret key
rate when compared with two benchmark schemes.
