Quantifying the Sensitivity and Unclonability of Optical Physical Unclonable Functions

Due to their unmatched entropy, complexity, and security level, optical physical unclonable functions (PUFs) currently receive a lot of interest in the literature. Despite the large body of existing works, herein, one of their core features in detail is studied, namely, their physical unclonability. This article tackles this fundamental and yet largely unaddressed issue. In simulations and/or experiments, the sensitivity of diffraction-based optical responses is investigated with respect to various small alterations such as variation in position, size, and number of the scatterers, as well as perturbations in the spatial alignment between the PUF and the measurement apparatus. The analysis focuses on 2D optical PUFs because of their relevance in integrated applications and the need to reply to security concerns that can be raised when the physical structure of the geometry is accessible. Among the results of this study, the sensitivity analysis shows that a positional perturbation of scatterers on the order of 30 nm, that is, far below the wavelength of the probing laser light of 632 nm wavelength, is sufficient to invalidate the PUF response and thus detect forgery attempt. These results support and quantify the high adversarial efforts required to clone optical PUFs, even for 2D layouts.