Re: Concurrent access to /dev/urandom

[Theodore Ts'o]

On Wed, Dec 08, 2004 at 07:10:16PM -0800, David Lang wrote:
> 
> pulling data from /dev/random or /dev/urandom will not ensure that you 
> don't have duplicates.
> 
> remember the birthday paradox. it says that if you get a group of 26 
> people in a room you have about a 50% chance that two of these people have 
> the same birthday.
> 
> now that's only with numbers in the range of 1-365 if you pull 16 bit 
> numbers (1-65536) you will need a much larger group to see the problem, 
> but if you pull enough you WILL see the problem eventually.

Yes, but a UUID is 128 bits.  So the chance of a collision is 1 in
2**62.  (There are 4 version/type bits, so there are 122 bits of
randomness in a type-4, randomly generated UUID).  Assuming a
correctly working random number generator, yes, it's possible that you
could get a duplicate.  It's also possible that all of the oxygen
molecules could suddenly randomly end up in the other part of the
room, and you would suffocate.  But it's not very likely.  2**62 is a
rather large number....

						- Ted
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/