Key generation and physical layer encryption for secure wireless communications

Wireless communication is vulnerable due to its broadcast nature. This thesis aims to enhance wireless security by performing physical layer encryption to randomize the transmission waveforms and securely generating the encryption keys from the randomness of the common wireless channel between two users. Orthogonal frequency-division multiplexing (OFDM) is studied as an example in this thesis as it is one of the most popular wireless techniques. In order to avoid passive and active attacks, this thesis proposes an OFDM physical layer encryption scheme employing subcarrier obfuscation and training symbol resequencing. Subcarrier obfuscation reserves several OFDM subcarriers for dummy data transmission and training symbol resequencing uses a secure and random sequence to replace the training symbol defined in the standard. The scheme has been evaluated to be robust to various attacks and has a good performance in terms of search space, key rate and complexity. Key generation is a promising alternative to public key cryptography for the establishment of cryptographic keys. This thesis presents to generate keys efficiently from the channel responses of individual OFDM subcarriers. The OFDM subcarrier's channel responses are theoretically proved to be an ideal random source for key generation. The efficiency is achieved by extracting keys from both time and frequency domain and improving cross-correlation of channel measurements. The proposed key generation system is prototyped in a statistical channel model and evaluated in terms of randomness, key generation rate and key disagreement rate. The proposed key generation system is experimentally studied by implementation in the wireless open-access research platform (WARP). The key generation principles, i.e., temporal variation, channel reciprocity and spatial decorrelation, are studied by extensive experiments carried out in the anechoic chamber, reverberation room and office with static, object moving and mobile scenarios. The cross-correlation relationship between the channel measurements is also experimentally studied.