/dev/random vs. /dev/urandom

[zeveck]

What is the difference?

Is it insecure to use /dev/urandom for deriving an IV? a key?

[RiBBiT]

/dev/random is more "random" and thus more secure. I would claim though, that for most purposes, /dev/urandom is sufficient.

Read more here:
http://en.wikipedia.org/wiki//dev/random
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[pmgas]

Hi

The difference between /dev/random and /dev/urandom is, that one of them (I don't know which one) uses a pseudorandomnumbergenerator to generate "random" values. The other one uses real random values, collected from process numbers, mouse movement ....
So, if I have the choice, I would pick the one with the real number output, but you can also use the other one, in my opinion. You don't need very much random data for a key or an IV ... Imho good PRNGs are secure enough

hth
peda

[Janne Pikkarainen]

/dev/random generates real randomness by listening mouse movements, keyboard activity, network events and all kind of stuff what's going on in your server. The biggest problem you may see with /dev/random is that especially on a busy server your system may not generate new entropy fast enough, if processes are reading /dev/random a lot. Running out of entropy blocks processes until they get the randomness they wanted. Personally I've seen pop3d of Cyrus IMAPd getting slow and/or stuck because of kernel run out of entropy. You may check anytime if there's entropy left in your system with cat /proc/sys/kernel/random/entropy_avail.

Some processors and/or motherboards may have a hardware-based random generator, which helps the situation. Also there are other ways to make generating real randomness faster: for example, there is a daemon which listens to a sound files you feed to it and generates randomness from your music.

In workstation use /dev/random does not run out very easily, though.

/dev/urandom generates random numbers as well, but they're not "real" randomness. Instead /dev/urandom just pops out numbers out of its head, being very fast in that. For most applications /dev/urandom is more than sufficient.

And what if application really wants to use /dev/random instead of /dev/urandom, and your application gets slow and buggy because of that? No worries! Then you may emerge rng-tools and run rngd (/etc/init.d/rngd start). It then uses /dev/urandom for filling entropy pool of /dev/random in case there's not HW generated randomness available.
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