Bugtraq: Re: ssh host key generation in Red Hat Linux

[Crispin Cowan <crispin () immunix com>]

Kent Borg wrote:

> I recently installed Red Hat Linux 9 and noticed on the first boot a
> message about generating ssh host keys.  Isn't that a dangerous thing
> to do on the first boot?  Where is the installation going to get
> enough good entropy so early in its life?
>
> Maybe the paranoid thing to do is, as part of configuring a machine,
> to regenerate those keys once user interaction (or other entropy
> source) has had time to really stir the Linux entropy pool.
SSH is likely getting it's entropy from /dev/random. The kernel will 
decide whether there is enough entropy in the /dev/random entropy pool, 
and block reads until the pool fills.
This pool, in turn, is going to have pleanty of entropy generated by 
timing jitter in disk I/O interrupts.
To experiment with this, run the command:

cat /dev/random | od -cx
 

It will dump for a while and then stop. Then type a key. Then move your 
mouse. Wait for a cron job to start up and watch what it does. Etc. etc.
Disclaimer: there is dispute in the crypto community about the hashing 
done in /dev/urandom (note the 'u') which never blocks. /dev/urandom 
just recycles the entropy pool with a PRNG, and people have variable 
faith in the quality of PRNG's.
Crispin

--
Crispin Cowan, Ph.D.           http://immunix.com/~crispin/
Chief Scientist, Immunix       http://immunix.com
           http://www.immunix.com/shop/