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