Resynchronization Weaknesses in Synchronous Stream Ciphers

In some applications for synchronous stream ciphers, the risk of loss of synchronization cannot be eliminated completely. In these cases frequent synchronization or resynchronization upon request may be necessary. In the paper it is shown that this can lead to significant deterioration of the cryptographic security. A powerful general attack on nonlinearly filtered linear (over ℤ₂) systems is presented. This attack is further refined to efficiently cryptanalyze a linear system with a multiplexer as output function.