Asymmetric cryptography with physical unclonable keys

Secure communication is of paramount importance in modern society. Asymmetric cryptography methods such as the widely used RSA cryptosystem allow secure exchange of information between parties who have never previously shared keys. However, the existing asymmetric cryptographic schemes rely on unproven mathematical assumptions for security. Further, the digital keys used in their implementation are susceptible to copying that might remain unnoticed. Here, we introduce a secure communication method based on Physical Unclonable Keys (PUKs), which we call PUK-Enabled Asymmetric Communication (PEAC). PEAC uses physical keys and thus overcomes the problem of unnoticed copying. As all the information about the PUK is allowed to be public, PEAC does not require the safekeeping of any digital information. Using optical PUKs realized in opaque scattering materials, we transmit messages in an error-corrected way employing off-the-shelf equipment. Information is transmitted as patterned wavefronts of few-photon wavepackets which can be successfully decrypted only with the receiver's PUK. The security of PEAC assumes technological constraints in distinguishing between different few-photon wavefronts. A heuristic argument for the security of PEAC is outlined focusing on a specific attack, namely state estimation. We demonstrate secure transmission of messages over a 2 m free-space line-of-sight quantum channel. PEAC enables new directions for physical key based cryptography.