Myths about /dev/urandom

There are a few things about /dev/urandom and /dev/random that are repeated again and again. Still they are false. /dev/urandom is insecure. Always use /dev/random for cryptographic purposes. Fact: /dev/urandom is the preferred source of cryptographic randomness on UNIX-like systems. /dev/urandom is a pseudo random number generator, a PRNG, while /dev/random is a “true” random number generator. Fact: Both /dev/urandom and /dev/random are using the exact same CSPRNG (a cryptographically secure pseudorandom number generator). They only differ in very few ways that have nothing to do with “true” randomness. /dev/random is unambiguously the better choice for cryptography. Even if /dev/urandom were comparably secure, there’s no reason to choose the latter. Fact: /dev/random has a very nasty problem: it blocks. But that’s good! /dev/random gives out exactly as much randomness as it has entropy in its pool. /dev/urandom will give you insecure random numbers, even though it has long run out of entropy. Fact: No. Even disregarding issues like availability and subsequent manipulation by users, the issue of entropy “running low” is a straw man. About 256 bits of entropy are enough to get computationally secure numbers for a long, long time. And the fun only starts here: how does /dev/random know how much entropy there is available to give out? Stay tuned! But cryptographers always talk about constant re-seeding. Doesn’t that contradict your last point? Fact: You got me! Kind of. It is true, the random number generator is constantly re-seeded using whatever entropy the system can lay its hands on. But that has (partly) other reasons. Look, I don’t claim that injecting entropy is bad. It’s good. I just claim that it’s bad to block when the entropy estimate is low. That’s all good and nice, but even the man page for /dev/(u)random contradicts you! Does anyone who knows about this stuff actually agree with you? Fact: No, it really doesn’t. It seems to imply that /dev/urandom is insecure for cryptographic use, unless you really understand all that cryptographic jargon. The man page does recommend the use of /dev/random in some cases (it doesn’t hurt, in my opinion, but is not strictly necessary), but it also recommends /dev/urandom as the device to use for “normal” cryptographic use. And while appeal to authority is usually nothing to be proud of, in cryptographic issues you’re generally right to be careful and try to get the opinion of a domain expert. And yes, quite a few experts share my view that /dev/urandom is the go-to solution for your random number needs in a cryptography context on UNIX-like systems. Obviously, their opinions influenced mine, not the other way around. Hard to believe, right? I must certainly be wrong! Well, read on and let me try to convince you. I tried to keep it out, but I fear there are two preliminaries to be taken care of, before we can really tackle all those points. Namely, what is randomness, or better: what kind of randomness am I talking about here? And, even more important, I’m really not being condescending. I have written this document to have a thing to point to, when this discussion comes up again. More than 140 characters. Without repeating myself again and again. Being able to hone the writing and the arguments itself, benefitting many discussions in many venues. And I’m certainly willing to hear differing opinions. I’m just saying that it won’t be enough to state that /dev/urandom is bad. You need to identify the points you’re disagreeing with and engage them. Actually, I used to believe that /dev/urandom was insecure myself, a few years ago. And it’s something you and I almost had to believe, because all those highly respected people on Usenet, in web forums and today on Twitter told us. Even the man page seems to say so. Who were we to dismiss their convincing argument about “entropy running low”? This misconception isn’t so rampant because people are stupid, it is because with a little knowledge about cryptography (namely some vague idea what entropy is) it’s very easy to be convinced of it. Intuition almost forces us there. Unfortunately, intuition is often wrong in cryptography. So it is here. What does it mean for random numbers to be “truly random”? I don’t want to dive into that issue too deep, because it quickly gets philosophical. Discussions have been known to unravel quickly, because everyone can wax about their favorite model of randomness, without paying attention to anyone else. Or even making himself understood. I believe that the “gold standard” for “true randomness” are quantum effects. Observe a photon pass through a semi-transparent mirror. Or not. Observe some radioactive material emit alpha particles. It’s the best idea we have when it comes to randomness in the world. Other people might reasonably believe that those effects aren’t truly random. Or even that there is no randomness in the world at all. Let a million flowers bloom.

Myths about /dev/urandom

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