Re: [dm-crypt] [ANNOUNCE] cryptsetup 1.2.0-rc1 (test release candidate)

[Arno Wagner <arno@wagner.name>]

On Thu, Nov 18, 2010 at 02:01:13PM +0100, Milan Broz wrote:
> On 11/18/2010 01:40 PM, Arno Wagner wrote:
> >> I think it is not only about starved situations, thats just practical
> >> impact of using this interface.
> >> Ipsec need to set key too and cannot wait for entropy.
> > 
> > It has to. No entropy - no security. The entropy does not
> > nee to be "fresh", but it needs to be there.
> 
> Maybe I said it wrong - RNG of course must be seeded (using entropy).
> But this is in initialisation phase. It must wait forever here
> if there is no entropy.
> But once seeded, it should produce strong enough stream of data,

Ah, yes, I see we are in agreement. Once properly seesed
it _must_ perform. 

> optionally mixed with environmental noise.

That is not stricly needed, agreed, but it helps to
add more entropy if the crypto-PRNG has slight weaknesses
or slight information leakage. It is more of a safety-net
for these cases. It is essential if an installation 
including seed is copied or restored from backup.


I also see you had a piece of information I was not aware
of so far (but which is logical in hindsight), namely that
the kernel does not reseed the PRNG from its former state
after a reboot. It seems in Debian, this is done by 
/etc/init.d/urandom, i.e. by the distribution.

Thinking about this, I believe the only clean solution would 
have been to drop /dev/urandom from the kernel entirely, as
it runs into the seeding problem after each reboot and has
to do so unless the kernel was allowed to write the filesystem,
which would cause a whole lot of other issues. It would
seem that while the kernel is the place to gather entropy 
(and /dev/random does a reasonable job of that), it is not
the place for a dependable cryptographic PRNG service. 


Ok, I think I really see your point now ;-)

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
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