bugtraq - Re: /dev/random is probably not

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Date: Sat, 02 Jul 2005 16:08:13 +0200
From: Thomas Wana <thomas@...a.at>
To: "Charles M. Hannum" <mycroft@...bsd.org>
Cc: bugtraq@...urityfocus.com
Subject: Re: /dev/random is probably not


Charles M. Hannum wrote:
> Most implementations of /dev/random (or so-called "entropy gathering daemons") 
> rely on disk I/O timings as a primary source of randomness.  This is based on 
> a CRYPTO '94 paper[1] that analyzed randomness from air turbulence inside the 
> drive case.

At least on Linux, the entropy pool is not only filled with data
from I/O timings, but also from networking I/O, user input events,
etc etc.[1]

Which specific implementations do you mean with "most implementations"?

Tom

[1] man 4 random

> 
> I was recently introduced to Don Davis and, being the sort of person who 
> rethinks everything, I began to question the correctness of this methodology.  
> While I have found no fault with the original analysis (and have not actually 
> considered it much), I have found three major problems with the way it is 
> implemented in current systems.  I have not written exploits for these 
> problems, but I believe it is readily apparent that such exploits could be 
> written.
> 
> a) Most modern IDE drives, at least, ship with write-behind caching enabled.  
> This means that a typical write returns a successful status after the data is 
> written into the drive's buffer, before the drive even begins the process of 
> writing the data to the medium.  Therefore, if we do not overflow the buffer 
> and get stuck waiting for previous data to be flushed, the timing will not 
> include any air turbulence whatsoever, and should have nearly constant time.
> 
> b) At least one implementation uses *all* "disk" type devices -- including 
> flash devices, which we expect to have nearly constant time -- for timing.  
> This is obviously a bogus source of entropy.
> 
> c) Even if we turned off write-behind caching, and so our timings did include 
> air turbulence, consider how a typical application is written.  It waits for, 
> say, a read() to complete and then immediately does something else.  By 
> timing how long this higher-level operation (read(), or possibly even a 
> remote request via HTTP, SMTP, etc.) takes, we can apply an adjustment factor 
> and determine with a reasonable probability how long the actual disk I/O 
> took.
> 
> Using any of these strategies, it is possible for us to know the input data to 
> the RNG -- either by measurement or by stuffing -- and, therefore, quite 
> possibly determine the future output of the RNG.
> 
> 
> Have a nice holiday weekend.
> 
> 
> [1] D. Davis, R. Ihaka, P.R. Fenstermacher, "Cryptographic Randomness from Air 
> Turbulence in Disk Drives", in  Advances in Cryptology -- CRYPTO '94 
> Conference Proceedings, edited by Yvo G. Desmedt, pp.114--120. Lecture Notes 
> in Computer Science #839. Heidelberg: Springer-Verlag, 1994.
>