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From: daw@mozart.cs.berkeley.edu (David Wagner)
Newsgroups: isaac.lists.linux-kernel
Subject: Re: [RFC][PATCH] Make cryptoapi non-optional?
Date: Fri, 22 Aug 2003 04:28:39 +0000 (UTC)
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David Lang  wrote:
>On Mon, 18 Aug 2003, Andreas Dilger wrote:
>> It wasn't even vanishingly small...  We had a problem in our code where
>> two readers got the same 64-bit value calling get_random_bytes(), and
>> we were depending on this 64-bit value being unique.  This problem was
>> fixed by putting a spinlock around the call to get_random_bytes().
>
>if the number is truely random then there should be a (small but finite)
>chance that any two reads will return the same data. counting on a random
>number to be unique is not a good idea.

Have you ever worked out how small a number 2^-64 is?
You might be surprised.  For most practical purposes, 2^-64 is
effectively zero.

If you see a 64-bit values repeat twice in a row when querying a
cryptographic pseudorandom generator, the crypto-PRNG is almost surely
broken.