When Is It Random Enough?

TheCamper asks: "The generation of random numbers is very important in many areas, especially encryption. Pseudo Random numbers created by software is simply not good enough. Many key generation applications ask the user to move the mouse or bang on the keyboard to add to the randomness. You can also purchase a (very expensive) hardware random number generator to make truly random numbers. Wanting the randomness of a hardware random number generator without wanting to pay for or build my own, I was wondering if crinkling cellophane (or the like) into my computer's microphone would be considered random enough for serious encryption key generation." What entropy sources would you use for the generation of strong encryption keys?

Comments

Define "strong encryption key".

[rjh]

IAAGSSTS (I Am A Grad Student Studying This Shit).

There are two different concerns going on here; the first is the strength of the key and the second is its lifetime. If you really desperately need a truly random 128-bit session key, then take out a quarter and start flipping; it takes about five minutes and you're done. But if you're in a situation where your applications will be changing keys every second, then you don't want rekeying to take five minutes.

Honestly, the best advice is to look long and hard at your reasoning for trying to roll your own generator. If you can point out precise reasons why you need truly random numbers and back your reasons up with references to the literature, then great, break out a quarter. If you can point out precise reasons why existing PRNGs are all insufficient for your task, then great, try to roll your own.

Otherwise, find a good pseudorandom number generator--and by "good", I mean "well-understood with good analysis and well-known behavior", such as the ANSI X9.17 pseudorandom number generator. Read up on its weaknesses and where it fails and how it fails. Avoid those failure modes.

Creating good PRNGs is extraordinarily hard. Trying to roll your own generator is fraught with risk; even when experienced professionals do it, they fail more often than they succeed. If you just want to learn about PRNGs and RNGs, then sure, go for it; I'm all for that. However, be very, very careful before you put your handrolled system into production code.

ob. dilbert

[syrinx]

Accounting troll: "That's our random number generator."

RNG: "nine. nine. nine. nine. nine. nine."

Dilbert: "Are you sure that's random?"

Accounting troll: "That's the problem with randomness; you can never be sure."