Linear Feedback Shift Register (LFSR)
2n-1 non-null sequences
-1 non-null sequences if one of them has a 2n-1 period length, then all have it.
Primitive feedback functions (primitive polynomials)
All non-null sequences have a 2 n–1 period length.
Issue
If you know N consecutive bits of the output, you know the entire sequence ahead.
The output must be mixed with something else ...
Generators using many LFSR: A5/1 (GSM)
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