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In computing, a linear-feedback shift register (LFSR) is a shift register whose input bit is a linear function of its previous state. The most commonly used linear function of single bits is exclusive-or (XOR). Thus, an LFSR is most often a shift register whose input bit is driven by the XOR of some bits of the overall shift register value.
This example uses two Galois LFRSs to produce the output pseudorandom bitstream. The Python code can be used to encrypt and decrypt a file or any bytestream ...
The Berlekamp–Massey algorithm is an algorithm that will find the shortest linear-feedback shift register (LFSR) for a given binary output sequence. The algorithm will also find the minimal polynomial of a linearly recurrent sequence in an arbitrary field .
Example of generating an 8-bit CRC. The generator is a Galois-type shift register with XOR gates placed according to powers (white numbers) of x in the generator polynomial. The message stream may be any length. After it has been shifted through the register, followed by 8 zeroes, the result in the register is the checksum.
For example, a Fibonacci LFSR with taps at 000_0011 in state 000_0010 will step forward to step 100_0001 and then 110_0000. If we bit-reverse that final state (producing 000_0011) and load it into the "mirrored" Fibonacci LFSR -- i.e., the one with taps at 100_0001 -- then stepping it forward produces state 100_0001 (the bit-reversal of 100 ...
This example will use the connection polynomial x 8 + x 4 + x 3 + x 2 + 1, and an initial register fill of 1 0 1 1 0 1 1 0. Below table lists, for each iteration of the LFSR, its intermediate output before self-shrinking, as well as the final generator output. The tap positions defined by the connection polynomial are marked with blue headings.
The MixColumns operation performed by the Rijndael cipher or Advanced Encryption Standard is, along with the ShiftRows step, its primary source of diffusion.. Each column of bytes is treated as a four-term polynomial () = + + +, each byte representing an element in the Galois field ().
Unfortunately, most programming languages make the latter much easier to write (X % r), so it is very commonly used. The generator is not sensitive to the choice of c, as long as it is relatively prime to the modulus (e.g. if m is a power of 2, then c must be odd), so the value c=1 is commonly chosen.