  Physical layer key generation is a promising candidate for cryptographic key
establishment between two wireless communication parties. It offers
information-theoretic security and is an attractive alternative to public-key
techniques. Here, the inherent randomness of wireless radio channels is used as
a shared entropy source to generate cryptographic key material. However,
practical implementations often suffer from static channel conditions which
exhibit a limited amount of randomness. In the past, considerable research
efforts have been made to address this fundamental limitation. However, current
solutions are not generic or require dedicated hardware extensions such as
reconfigurable antennas. In this paper, we propose a novel wireless key
generation architecture based on randomized channel responses from an
intelligent reflecting surface (IRS). Due to its passive nature, a cooperative
IRS is well-suited to provide randomness for conventional resource-constrained
radios. We conduct the first practical studies to successfully demonstrate
IRS-based physical-layer key generation with an OFDM system. In a static
environment, using a single subcarrier only, our IRS-assisted prototype system
achieves a key generation rate (KGR) of 97.39 bps with 6.5% key disagreement
rate (KDR) after quantization, while passing standard randomness tests.
