linux-kernel - Re: /dev/urandom uses uninit bytes, leaks user data

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Message-ID: <476747DC.4040309@davidnewall.com>
Date:	Tue, 18 Dec 2007 14:39:00 +1030
From:	David Newall <david@...idnewall.com>
To:	Theodore Tso <tytso@....edu>, David Newall <david@...idnewall.com>,
	Andy Lutomirski <luto@...ealbox.com>,
	John Reiser <jreiser@...Wagon.com>,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org,
	security@...nel.org
Subject: Re: /dev/urandom uses uninit bytes, leaks user data

Theodore Tso wrote:
> On Tue, Dec 18, 2007 at 01:43:28PM +1030, David Newall wrote:
>   
>> On a server, keyboard and mouse are rarely used.  As you've described it, 
>> that leaves only the disk, and during the boot process, disk accesses and 
>> timing are somewhat predictable.  Whether this is sufficient to break the 
>> RNG is (clearly) a matter of debate.
>>     
>
> In normal operaiton, entropy is accumlated on the system, extracted
> via /dev/urandom at shutdown, and then loaded back into the system
> when it boots up.

Thus, the entropy saved at shutdown can be known at boot-time.  (You can 
examine the saved entropy on disk.)


> If you have a server, the best thing you can do is use a hardware
> random number generator, if it exists.  Fortunately a number of
> hardware platforms, such as IBM blades and Thinkpads, come with TPM
> modules that include hardware RNG's.  That's ultimately the best way
> to solve these issues.

Just how random are they?  Do they turn out to be quite predictable if 
you're IBM?
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