# Why does the Content Scrambling System have two different length LFSRs

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?

• 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?
– fgrieu
Sep 28 '18 at 6:09