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= Linear Feedback Shift Register =
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A LFSR is set of rules to alter a set of bits. They are useful to psudeo random
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number generators, and as key generators for stream ciphers.
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All LFSRs are cyclical in nature, and after a set amount of time will repeat
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back into themselves. The initial state of the bits in the LFSR is called the
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seed.
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The maximum period for a _n_ bit shift register is
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2^n - 1
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2021-12-15 19:45:01 +00:00
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An LFSR can be generalized as a recurrence relationship where
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- The preceding terms are not raised to a power
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- There are no added constants
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2021-12-15 19:00:01 +00:00
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A *tap* is where a bit is read and fed back into itself.
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2021-12-15 19:30:01 +00:00
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== Reverse Engineering ==
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An LFSR generates values based on a linear expression modulous 2, therefore we
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can reverse engineer the state of the LFSR based on a sequence we are given.
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This can be done using the Berlekamp-Massey algorithm.
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