True, but purely mathematically every cipher is vulnerable to the "get lucky and guess the key right the first time" attack (OTP excepted of course).

Almost all crypto operates in the realm of absurdly close to certain.

If it's random, then the probability that the same case of output will occur forever is zero. Here the probability of hitting the terminating case occurs with p=(1 - 250/256). You aren't going to randomly roll 6/256 enough times in a row to notice within the age of the universe.

This is the standard method for selecting a uniform range from a source that is not an exact multiple. The worst-case probability of having to draw again can always be kept under 0.5. For example, I want a number in the range of (0 <= n < 129).