Optimizing the AES S-Box using SAT

Non-trivial linear straight-line programs over the Galois field of two elements occur frequently in applications such as encryption or high-performance computing. Finding the shortest linear straight-line program for a given set of linear forms is known to be MaxSNP-complete, i.e., there is no ε-approximation for the problem unless <math>P = NP</math>.This paper reiterates a non-approximative approach for finding the shortest linear straight-line program. After showing how to search for a circuit of XOR gates with the minimal number of such gates by a reduction of the associated decision problem ("Is there a program of length <math>k</math>?") to satisfiability of propositional logic, we show that using modern SAT solvers, provably optimal solutions to interesting problem instances from cryptography can be obtained. We substantiate this claim by a case study on optimizing the AES S-Box.

Polynomial force approximations and multifrequency atomic force microscopy

We present polynomial force reconstruction from experimental intermodulation atomic force microscopy (ImAFM) data. We study the tip–surface force during a slow surface approach and compare the results with amplitude-dependence force spectroscopy (ADFS). Based on polynomial force reconstruction we generate high-resolution surface-property maps of polymer blend samples. The polynomial method is described as a special example of a more general approximative force reconstruction, where the aim is to determine model parameters that best approximate the measured force spectrum. This approximative approach is not limited to spectral data, and we demonstrate how it can be adapted to a force quadrature picture.