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The goal of this thesis is to study the statistical methods allowing to decrypt messages where a brute attack will be unfeasible. 

After the description of encryption techniques used in pre-modern cryptography and in the Second World War, we will present methods for breaking these classical ciphers with a brief presentation of the cryptanalysis needed to break wartime ciphers. Then, a bayesian method introduced by Turing in a wartime paper for breaking Vigenère cipher will be developed. This work will also investigate the use of Markov Chain Monte Carlo to attack substitution cipher and transposition cipher. Finally, we will discuss how to measure the performances of some techniques presented in the previous chapters based on simulations.

cryptography --- statistics --- cryptanalysis --- code breaking --- Kasiski --- index of coincidence --- Markov chain Monte Carlo --- Turing --- substitution cipher --- transposition cipher --- Vigenère cipher --- Physique, chimie, mathématiques & sciences de la terre > Mathématiques