Ciphertext only Reconstruction of LFSR-based Stream Ciphers
Résumé
This report presents an operational reconstruction technique of most stream ciphers. We primarily expose it for systems where several linear feedback shift registers (LFSR) are combined by a nonlinear Boolean function. With only short ciphertexts, it is shown how to completely recover the different feedback polynomials and the combining function, when the algorithm is totally unknown. Estimates of necessary cipherlength and experimental results are detailed.