[Python-Dev] BDFL ruling request: should we block forever waiting for high-quality random bits?

[Steven D'Aprano]

On Thu, Jun 09, 2016 at 06:21:32PM +0100, Paul Moore wrote:

> If we put the specific issue of applications that run very early in
> system startup to one side, is there a possibility of running out of
> entropy during normal system use? Even for a tiny duration?

With /dev/urandom, I believe the answer to that is no.

On most platforms other than Linux, /dev/urandom is exactly the same as 
/dev/random, and both can only block straight after the machine has 
booted up before enough entropy has been collected. Then they will run 
forever without blocking. (Or at least until you reboot.)

On Linux, /dev/random *will* block, at unpredictable times, but 
fortunately we're not using /dev/random. We're using Urandom. Apart from 
just after boot up, /dev/urandom on Linux will also run forever without 
blocking, just like the other platforms.

The critical difference is just after booting up:

- Linux /dev/urandom doesn't block, but it might return predictable, 
  poor-quality pseudo-random bytes (i.e. a potential exploit);

- Other OSes may block for potentially many minutes (i.e. a 
  potential DOS).

Two links which may help explain what's happening:

http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/

http://security.stackexchange.com/a/42955



-- 
Steve