  Physical layer key generation (PKG) is a promising means to provide
on-the-fly shared secret keys by exploiting the intrinsic randomness of the
radio channel. However, the performance of PKG is highly dependent on the
propagation environments. Due to its feature of controlling the wireless
environment, reconfigurable intelligent surface~(RIS) is appealing to be
applied in PKG. In this paper, in contrast to the existing literature, we
investigate both the constructive and destructive effects of RIS on the PKG
scheme. For the constructive aspect, we have identified static and
wave-blockage environments as two RIS-empowered-PKG applications in future
wireless systems. In particular, our experimental results in a static
environment showed that RIS can enhance the entropy of the secret key,
achieving a key generation rate (KGR) of 97.39 bit/s with a bit disagreement
rate (BDR) of 0.083. In multi-user systems where some remote users are in worse
channel conditions, the proposed RIS-assisted PKG algorithm improves the sum
secret key rate by more than 2 dB, compared to the literature. Furthermore, we
point out that RIS could be utilized by an attacker to perform new jamming and
leakage attacks and give countermeasures, respectively. Finally, we outline
future research directions for PKG systems in light of the RIS.
