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Comment by mountbob | Hacker News Reader

> "might generate easily predicted cryptographic keys under certain extremely rare circumstances"

no, he's not saying that - he's saying that there's no such thing as entropy depletion - and so urandom is secure. Which makes me ask after urandom was created - why EVER bother using /dev/random with it's blocking flaw/deficiency?

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There is no reason to use /dev/random other than cargo culting devs that believe that /dev/urandom can supposedly run out of entropy.

Use /dev/urandom. Don't use /dev/random. On sane systems (FreeBSD for example), /dev/urandom is a symlink to /dev/random and /dev/random only blocks once, upon startup to gather entropy, after that it never blocks!

I don't think it's a "cargo cult" if the official docs mention and warn about entropy pool exhaustion.

The official docs are wrong.

Then submit a patch to fix them.

Instead of telling other people what to do, maybe you should just do it yourself.

Corey Csuhta filed an issue for this four months ago, but no one has responded to it: https://bugzilla.kernel.org/show_bug.cgi?id=71211

There's no such thing as entropy depletion, but there is such a thing as an insufficiently seeded CSPRNG - which means that /dev/urandom is not secure by design: it does not protect you against that failure mode, and in fact people rely on fragile hacks implemented by distributions to try to seed /dev/urandom properly as soon as possible in the bootup sequence. These hacks could easily break if somebody does not know exactly what they're doing while touching the boot sequence.

/dev/random is also stupid, but it does protect you against that particular failure mode.

He said there's no such thing as entropy depletion for urandom after it's been seeded. But the seed still has to come from somewhere, and one possible source for that seed is /dev/random.

/dev/random actually points to the same RNG as /dev/urandom, according to http://www.2uo.de/myths-about-urandom/ (and other sources I recall reading but can't find). So you wouldn't use /dev/random to seed /dev/urandom, but you (or the kernel) might use something else to seed both.

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