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Re: /dev/random is probably not
From: Kai Howells <kai.howells () icorp com au>
Date: Thu, 7 Jul 2005 10:53:06 +1000

As for the issue of getting randomness on a freshly-booted system, Mac OS X will collect entropy over time and dump some to disk to be reloaded next time the system reboots.
From the random (4) manpage:

OPERATION
The random device implements the Yarrow pseudo random number generator algorithm and maintains its entropy pool. Addditional entropy is fed to the generator regularly by the SecurityServer daemon from random jitter measurements of the kernel. SecurityServer is also responsible for peri- odically saving some entropy to disk and reloading it during startup to
     provide entropy in early system operation.

You may feed additional entropy to the generator by writing it to the random device, though this is not required in a normal operating environ-
     ment.

Now this raises some interesting issues - such as where is the entropy written to, and how much does this pool of entropy set the state of the RNG after bootup - ie, if an attacker had control of this file, could they influence the RNG in a deterministic fashion after forcing a reboot?

Kai Howells

On 06/07/2005, at 3:48 PM, Thomas wrote:

Linux cited using keyboard interrupts.  How many of those happen on
a web server in a rack, in an air conditioned computer room somewhere ?
How many happen when you open up your web browser and select your
internet banking web site from your bookmarks?


To complete the list, Linux uses:
    - block-device access
    - interrupt occurence
    - keyboard
    - mouse
    - freedback from pool extraction
    - pool extraction timing (doesn't matter)

Even w/o devices such as keyboard and mouse Linux starts
producing "a bit" entropy on an old notebook w/ just one hdd after
about 2200 events (the end-phase of a booting  SuSE Linux 9.0 system)

Fortunately the pool is initialized in two stages... not perfect but
sufficient for most systems.

Twisting and stirring the bits should scatter entropy evenly in the pool.
Afterwards hashing the pool contents, feeding back the hash value,
and "folding" the hash value should be enough to stop every useful
attack.

Nevertheless I think it's time to retire for Linux' /dev/random implementation
and use new approaches like Ferguson's Fortuna.



What the original article was getting at is that perhaps not all of
the information you think of as random information going into your
PRNG is actually random.  If that happens then even though the
output of the PRNG "looks random", it may be predictable.


Unfortunately yes. At least for Linux I am not sure how accurate
the entropy estimation really is. At least during boot it is much too
optimistic.



Darren


Thomas Biege

--
Tom <tom () electric-sheep org>
fingerprint = F055 43E5 1F3C 4F4F 9182  CD59 DBC6 111A 8516 8DBF



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