CSS uses two LFSRs of length 17 and 25.

If we have two periodic functions $f(t)$ and $g(t)$ and consider the function $h(t) = (f(t), g(t))$, then $h$ will have period $pq/\gcd(p, q)$ where $p$ and $q$ are the period of $f$ and $g$, respectively, and $\gcd$ is the greatest common divisor.

How does this principle explain the choice for two different LFSRs?

  • $\begingroup$ This principle can explain the period of the generator, and allow comparing that to the period of a generator with 17+25 bits. But "explain the choice for two different LFSRs".. Could it be the Question? $\endgroup$ – fgrieu Sep 28 '18 at 6:09

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