alt Hacker NewsYou’re throwing a lot of bits away, though. If you used four bits per number then you could pack 16 random digits into one 64-bit draw. But a rejection-sampling branch will be taken much more often.
You’re throwing a lot of bits away, though. If you used four bits per number then you could pack 16 random digits into one 64-bit draw. But a rejection-sampling branch will be taken much more often.
If you want to split a 64-bit random number into 16 4-bit random numbers, the quality of the RNG that generates the 64-bit random numbers must be extremely high. Only cryptographic RNGs may have such a high quality.
Any non-cryptographic PRNG is useful only if it is much faster than a cryptographic RNG, e.g. one using AES, which in many modern CPUs needs around 10 clock cycles to generate a 128-bit random number (less than that, even less than half of that, in some more recent CPUs).
So non-cryptographic PRNGs must generate a 64-bit number in less than 1 nanosecond to be competitive. Many older PRNGs are not this fast, so they are completely obsolete.
Such PRNGs do not offer any guarantee that if you take more than one piece of a generated number they will not be correlated. So the rule is that you can not make 2 or more random numbers from 1 random number provided by a PRNG.