In wireless communication systems, the conventional secret key exchange is based on the public key cryptography, which requires complex computations to retain the secrecy level of these key bits. The proposed physical layer-based algorithms have shown promising performance to extract secret keys from the privately shared randomness relying on the reciprocal channel state between both communicated nodes. In this study, the authors propose a physical layer key exchange method which transmits the key bits by encoding them within some phase randomisation (PR) sequences that are privately indexed to a specific channel criterion. The PR sequences only randomise the data phases and thus no efficiency reduction will be incurred. In fact, by choosing a pool of randomisation sequences with certain statistical properties, they could also be used to condition the signal to meet physical layer transmission requirements such as bandwidth, envelope and so on. They quantify the potential of the proposed method by demonstrating it within the context of a multiple-input multiple-output orthogonal frequency division multiplexing system. The results reveal that, relative to existing techniques, the proposed method offers superior key error rate performance at lower computational complexity with better secrecy level.