, x 231) of the n register stages (302) of the implemented NLFSR is representable by a second subset (x 24. , x 193) of a linear feedback shift register, LFSR (116), , x 193) of the n register stages (302) of the implemented NLFSR is representable by at least one register stage (x 0. A device (110) for generating a keystream (128) for ciphering or deciphering a data stream (122), the device comprising:Ī nonlinear feedback shift register, NLFSR (112), including n register stages (302) wherein the NLFSR is implemented in a Galois configuration, wherein at least one register stage (x 0. A computer program product, optionally stored on a computer-readable medium, the computer program product comprising program code portions which, when executed on a computing device cause said computing device to perform the steps of claim 1.ģ. Wherein the nonlinear output function (118) includes linear terms and wherein the linear terms included in the nonlinear output function (118) depend only on the first subset of the n register stages (302) of the implemented NLFSR.Ģ. Outputting (204) the keystream (128) from a nonlinear output function (118), wherein an input of the nonlinear output function (118) is coupled to at least two of the n register stages (302) of the implemented NLFSR (112), and Wherein the implemented NLFSR (112) is transformable to the second NLFSR (114), which is an n-bit NLFSR (114), and which has only two nontrivial feedback functions of type f n − 1 x 0, x 1, …, x n − 1 = x 0 + f L x 1, …, x n − 1 + f N x 1, …, x n − 1 and f n − 1 − k x 0, x 1, …, x n − 1 = x n − k + f N x 1 − k, …, x n − 1 − k, wherein "-" is subtraction modulo n, wherein f L is a linear function and f N is a non-linear function and , x 255) of a second NLFSR (114), wherein a number of register stages (x 217, x 255) receiving a nonlinear feedback in the second NLFSR (114) is greater than one and less than a number of register stages (x 193, x 197, x 201, x 295, x 209, x 213, x 217, x 231, x 235, x 239, x 243, x 247, x 251, x 255) receiving a nonlinear feedback (309) in the implemented NLFSR (112), and Wherein at least one register stage (x 0. Operating (202) a nonlinear feedback shift register, NLFSR (112), including n register stages (302) wherein the NLFSR is implemented in a Galois configuration, A method (200) of generating a keystream (128) for ciphering or deciphering a data stream (122), the method comprising:
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