> That property is hardly unique to LFSRs.  Since pseudo-random number
> generators are deterministic algorithms run on deterministic machines
> with finite memory, they ALL repeat.
indeed however with a large enough internal state, the right algorithm and a
sufficiantly random initial seed value predicting the next output from the
previous outputs can be made practically impossible.

it really depends, if its just got to look random to the untrained eye then
you'll probablly get away with 8 bits of state but if it has to defeat
trained cryptographers you will need FAR more.

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