From: "Theodore Ts'o" <tytso@mit.edu>
To: David Lang <dlang@digitalinsight.com>
Cc: Bernard Normier <bernard@zeroc.com>, linux-kernel@vger.kernel.org
Subject: Re: Concurrent access to /dev/urandom
Date: Thu, 9 Dec 2004 01:36:54 -0500 [thread overview]
Message-ID: <20041209063654.GA9324@thunk.org> (raw)
In-Reply-To: <Pine.LNX.4.60.0412081905140.17193@dlang.diginsite.com>
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
next prev parent reply other threads:[~2004-12-09 6:37 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-11-27 20:45 Concurrent access to /dev/urandom Bernard Normier
2004-11-27 20:56 ` Jan Engelhardt
2004-11-27 21:15 ` Bernard Normier
2004-11-27 21:22 ` Jan Engelhardt
2004-11-28 20:58 ` Bernard Normier
2004-12-07 23:41 ` Bernard Normier
2004-12-08 1:28 ` Theodore Ts'o
2004-12-08 1:56 ` Bernard Normier
2004-12-08 19:21 ` Theodore Ts'o
2004-12-08 20:15 ` Bernard Normier
2004-12-08 21:56 ` Matt Mackall
2004-12-09 1:57 ` Theodore Ts'o
2004-12-09 2:46 ` andyliu
2004-12-09 4:55 ` Matt Mackall
2004-12-09 2:58 ` Matt Mackall
2004-12-09 21:29 ` Matt Mackall
2004-12-10 4:47 ` Matt Mackall
2004-12-10 16:35 ` Theodore Ts'o
2004-12-10 18:28 ` Matt Mackall
2004-12-10 21:28 ` Theodore Ts'o
2004-12-10 22:23 ` Matt Mackall
2004-12-11 0:22 ` Adam Heath
2004-12-11 1:10 ` Matt Mackall
2004-12-11 17:33 ` Theodore Ts'o
2004-12-11 19:58 ` Adam Heath
2004-12-11 20:40 ` Matt Mackall
2004-12-12 16:19 ` Pavel Machek
2004-12-11 0:19 ` Adam Heath
2004-12-09 3:10 ` David Lang
2004-12-09 4:52 ` Matt Mackall
2004-12-09 6:36 ` Theodore Ts'o [this message]
2004-11-29 22:47 ` Jon Masters
2004-11-29 23:14 ` Bernard Normier
2004-11-29 23:43 ` Sven-Haegar Koch
2004-11-30 2:31 ` David Schwartz
2004-11-30 4:14 ` Kyle Moffett
2004-11-30 8:23 ` Jan Engelhardt
2004-11-30 18:50 ` David Schwartz
2004-11-29 23:42 ` David Wagner
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