PRNGs sometimes have ugly faces.
PRNG designed by NSA was easy to predict if you know a secret (discrete logarithm) almost surely known by the NSA.
You have to know a lot more than the algorithm, you have to know the seed value. This, in practice, can act as a key to make practically unbreakable ciphers. Now the diehards will say that you can't make such a claim, but in truth eventually the probability of an attacker knowing enough of the free variables (64 bit seed value, 1% or more sequential characters of plaintext being used repeatedly across different ciphertexts IN THE SAME LOCATION) becomes vanishingly small. Trying to get the key value securely to your recipient is another problem which may be outside cryptology, but is certainly an interesting historical problem for politicians and generals. Marxos