Picosecond-Scale Secret Key Generation in Free Space

Abstract

Symmetric key cryptography is commonly employed in wireless networks, and is considered to be secure even in the emerging quantum computation paradigm. Typically, the establishment of a shared secret key is achieved through asymmetric cryptography, in which legitimate users leverage computationally acquired public and private key pairs to agree on a symmetric key. Unfortunately, this method requires a trusted third party, introduces computational costs to users, and incurs delays on the order of 10s to 100s of milliseconds for Internet round-trip time to connect with the third party. Moreover, asymmetric cryptography has well-known vulnerabilities to quantum computing. In this work, we propose a rapid picosecond-scale symmetric key generation method via a free-space wireless channel and without requiring a third party. In particular, we devise a technique for the legitimate users to use transmissive reconfigurable metasurfaces to generate random perturbations in their free-space channel from which they can extract reciprocal random frequency signatures and thus extract random bits. Our method also ensures key confidentiality by spatially randomizing the modulated pilot signal, such that any adversarial node in the environment obtains a completely different bit sequence compared to the legitimate users. We experimentally demonstrate our method and show that the extracted keys pass the standard randomness tests and that our method renders eavesdroppers’ observations statistically independent from those of the legitimate users, provided that they have more than a few degrees of angular separation. Due to our use of picosecond-scale pulses as pilot signals, this method yields several orders of magnitude increase in the bit generation rate compared to prior methods that leverage mobility-induced changes in channel response, while maintaining a low bit mismatch rate.