Page "Cryptographically secure pseudorandom number generator" Paragraph 13

:: Example: If the CSPRNG under consideration produces output by computing bits of π in sequence, starting from some unknown point in the binary expansion, it may well satisfy the next-bit test and thus be statistically random, as π appears to be a random sequence.

( This would be guaranteed if π is a normal number, for example.

) However, this algorithm is not cryptographically secure ; an attacker who determines which bit of pi ( i. e. the state of the algorithm ) is currently in use will be able to calculate all preceding bits as well.