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SubjectRe: /dev/urandom uses uninit bytes, leaks user data
Theodore Tso wrote:
> On Tue, Dec 18, 2007 at 02:39:00PM +1030, David Newall wrote:
>> Thus, the entropy saved at shutdown can be known at boot-time. (You can
>> examine the saved entropy on disk.)
>>
>
> If you can examine the saved entropy on disk, you can also introduce a
> trojan horse kernel that logs all keystrokes and all generated entropy
> to the FBI carnivore server --- you can replace the gpg binary with
> one which ships copies of the session keys to the CIA --- and you can
> replace the freeswan server with one which generates emphermal keys by
> encrypting the current timestamp with a key known only by the NSA. So
> if the attacker has access to your disk between shutdown and boot up,
> you are *done*. /dev/random is the least of your worries.
>
> Really, why is it that people are so enamored about proposing these
> totally bogus scenarios? Yes, if you have direct physical access to
> your machine, you can compromise it. But there are far easier ways
> that such a vulnerability can be exploited, rather than making it easy
> to carry out an cryptoanalytic attack on /dev/random.
>
> (And yes, after using the saved state to load the entropy at
> boot-time, the saved state file is overwritten, and if you're
> paranoid, you can scrub the disk after it is read and mixed into the
> entropy pool. And yes, the saved state is *not* the entropy pool used
> during the previous boot, but entropy extracted using SHA-1 based
> CRNG.)
>
>>> 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?
>
> They use a noise diode, so they are as good as any other hardware
> random number generator. Of course, you ultimately have to trust the
> supplier of your hardware not to do something screwy, and Thinkpads
> are now made by Lenovo, which has caused some US Government types to
> get paranoid --- but that's why the best way to do things is to get
> entropy from as many places as possible, and mix it all into
> /dev/random. If any one of them is unknown to the attacker, he's stuck.
>
In another thread I believe I mentioned things an attacker can't know
(unless your system is already compromised) like fan speed, CPU
temperature, etc.

--
Bill Davidsen <davidsen@tmr.com>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot


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