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Comment by hyperbovine | Hacker News Reader

$ dd if=/dev/random of=nonbullshittestfile bs=1M count=500

That would take forever, at least on my system, as I don't really have a lot of entropy available. Just for kicks I have started this when I began writing this reply. And now I have the huge amount of 200 Byte in this test file. So thats about 10 bytes per second.

But if you take /dev/urandom you can get quite a bit more. On this system it is about 7Mbyte/s.

What's the difference between /dev/random and /dev/urandom? I've only ever used urandom and it spits out data as fast as I can consume it.

/dev/random will block while waiting to collect entropy from the system. /dev/urandom will be satisfied with pseudorandom numbers. That's fine for many applications (e.g. a file filled with garbage) but not acceptable for things like cryptography.

That entropy, by the way, is derived from the keyboard and mouse devices. If you want /dev/random to go faster, move your mouse and type more ..

Can it be configured to use other sources?

Does this mean that if I've got a server with no mouse/keyboard attached, /dev/random will block forever?

Logging into my slicehost server, and running cat /dev/random | hexdump -C seems to support this, more or less - only a few lines get output unless I start typing into the terminal - then it goes marginally faster.

It will also use other interrupt timings in the creation of the entropy, most notably hard drives since they're rather random on when they reply back due to it having to rotate to the right place. Not sure how this works on an SSD. I also believe if you've got some kind of hardware RNG it will use that too.

You can buy hardware random number generators.

http://www.entropykey.co.uk/

http://www.idquantique.com/true-random-number-generator/prod...

http://www.gamesbyemail.com/dicegenerator

...among many others.

Interesting. I don't see /dev/random blocking on Mac OS X (13" MacBook Pro). I wonder what the source is?

On OS X, /dev/random and /dev/urandom are the same thing (both acting like the traditional /dev/urandom).

/dev/random uses environmental noise for entropy, and can be depleted rather quickly.

/dev/urandom supplements its entropy with a PRNG so that it never blocks..

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