of research, namely property testing, machine learning theory, and Boolean analysis. ML attacks can also apply to PUFs because most of the pre and post-processing methods applied until now ignore the effect of correlations between PUF-circuit outputs. For instance, obtaining one bit by comparing two ring oscillator outputs is a method to decrease the correlation. However, this method does not remove all correlations. Therefore, the classic transforms from the signal-processing literature are applied to raw PUF-circuit outputs to decorrelate them before quantizing the outputs in the transform domain to generate bit sequences. Such decorrelation methods can help to overcome the correlation-based information leakages about the PUF outputs even if the ambient temperature and supply voltage change. Optical PUFs Optical PUFs rely on a random optical multiple-scattering medium, which serves as a token. Optical PUFs offer a promising approach to developing entity authentication schemes that are robust against many of the aforementioned attacks. However, their security against emulation attacks can be ensured only in the case of quantum readout (see below), or when the database of challenge-response pairs is somehow encrypted. Optical PUFs can be made very easily: a varnish containing glitter, a metallic paint, or a frosted finish obtained by sandblasting a surface, for example, are practically impossible to clone. Their appearance changes depending on the point of view and the lighting. Authentication of an optical PUF requires a photographic acquisition to measure the luminosity of several of its parts and the comparison of this acquisition with another previously made from the same point of view. This acquisition must be supplemented by an additional acquisition either from another point of view, or under different lighting to verify that this results in a modification of the appearance of the PUF. This can be done with a smartphone, without additional equipment, using optical means to determine the position in which the smartphone is in relation to the PUF. Theoretical investigations suggest that optical PUFs with nonlinear multiple-scattering media, may be more robust than their linear counterparts against the potential cloning of the medium. See also Hardware Trojan Quantum Readout of PUFs Random number generation Defense strategy (computing) References External links "Physical Unclonable Functions and Applications", by Srini Devadas and others, MIT Ultra-low-cost true randomness AND physical fingerprinting "Mixed-signal physically unclonable function with CMOS capacitive cells", by Kamal Kamal and Radu Muresan Cryptographic primitives Applications of randomness