Re: Getting randomic integers

[David Given <dg@...>]

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On 17/01/10 19:27, Florian Weimer wrote:
[...]
>> /dev/urandom is a variant of /dev/random that, when the entropy pool is
>> empty, will return fake random numbers generated with a PRNG. As such it
>> is not suitable for crypto purposes.
> 
> This is not true, the data is perfectly usable for almost any purpose
> (unless there are bugs or breathtaking advancements in the open crypto
> literature).

Hey, I'm just paraphrasing the man page:

> A  read  from  the  /dev/urandom device will not block waiting for more
> entropy.  As a result, if  there  is  not  sufficient  entropy  in  the
> entropy  pool,  the  returned  values are theoretically vulnerable to a
> cryptographic attack on the algorithms used by the  driver.   Knowledge
> of how to do this is not available in the current non-classified liter‐
> ature, but it is theoretically possible that such an attack may  exist.
> If this is a concern in your application, use /dev/random instead.

I'll agree that the chance of anybody successfully exploiting
/dev/urandom's lack-of-randomness are so close to nil as not to be worth
worrying about, but when dealing with cryptography one always equates
'theoretically possible' to 'danger, Will Robinson!'.

Of course, must people *aren't* interested in cryptography, even the
ones who think they are. We use this stuff at work for our gaming
platform; all we do with it is to read four bytes from /dev/random to
seed our own general purpose PRNG. We used to have a hideous bug
perpetrated by a now ex-coworker who decided that since we *might* use
our random number stream for crypto purposes one day we should be
reading numbers directly from /dev/random. This had the entertaining
result that some games would run for five seconds and then lock up until
you touched the mouse.

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