Cryptanalysis of MUX-LFSR based scramblers
Joan Daemen, René Govaerts, Joos P. L. Vandewalle · 1993
A recursive attack is presented that is applicable to synchronous stream ciphers consisting of a finite state machine with a linear state-transition function and a multiplexer as output function. A variant of this attack can be used to build a cheap system that bypasses on-line the scrambling of video signals as advised in [1] . 1 The Attacked System The scrambler can be described by a finite state machine model. The internal state at a time t is denoted by s t , a binary vector with n components s t 0 to s t n\\Gamma1 . The linear state transition can be expressed by the matrix equation s t+1 = As t (1) with A an invertible binary n \\Theta n matrix. The output function is a multiplexer. A multiplexer with q address inputs has 2 q data inputs. The output is equal to the data input selected by the address input bits. The output function is completely specified by the indices of the components that are the address and data inputs. This finite state machine is used as a pseudor...