On a Resynchronization Weakness in a Class of Combiners with Memory

In some applications for synchronous stream ciphers frequent resynchronization or resynchronization upon request may be necessary. We describe a weakness in a class of combiners with one-bit memory which makes them vulnerable in such applications requesting resynchronization. A correlation attack based on chi-square criterion, which in some aspects complements the attack studied by Daemen et. al., is presented. Stream ciphers are an important class of encryption algorithms. They encrypt individual characters of a plaintext one at a time, using encryption transformation, which varies with time. We consider a weakness of synchronous stream ciphers with one-bit memory. Our investigations justify the designers intuitive understanding that in conservative design of such combiners should not be used linear memory functions, despite their easy implementation.