Re: Secure Phone: Making man in the middle audible.

[Adam Back <aba@dcs.ex.ac.uk>]

James Donald <jamesd@echeque.com> writes:
> Suppose that Bob's computer from time to time formulates a plan to
> do groups of a particular size and form, and sends Ann's computer a
> hash of that plan and the DH negotiated shared secret.
> 
> So malloc must decompress Bob's speech packets, repacketize them, 
> and recompress them,
> 
> Often he will not be able to send off a packet, until he has received 
> two of Bob's packets.
> 
> So this triples the delay, and increases the speech degradation.

Good.  Sounds as though this could work.  Much better than my stego
ramblings.

One minor point: if I was Mallet faced with the problem your solution
presents the MITM with I would compose a plan which used a whole row
of minimum sized packets (or small upward random variations).  That
would minimise the chance that I would have to wait to compose
packets.  Clearly you can fix this: you can insist on a certain
standard deviation, or whatever as part of the protocol.

Another point is that you are making the unstated assumption that you
can't stream packets.  That is, you are assuming that you must receive
the whole packet before you can start to resend decoded and
re-encoded.  Perhaps this is already the case anyway, I don't know
much about audio compression algorithms or codecs.  Eric can tell us
whether this will already be the case for his algorithm set.  If not,
I think it should be easy enough to enforce this property (immediate
solution which comes to mind: send the bits in a packet backwards!)


I'm wondering also whether you couldn't generalise the sort of
stretched interlock protocol which is going on here, with in your
solution the readers ears telling him something is wrong, to come up
with something where the firmware in Eric's fine box could detect
and just flip out of secure mode, or flash the leds, or whatever in
protest to indicate it had detected a MITM attempt.

Perhaps what you have already is enough -- if the inter-packet delay
gets longer than whatever you caculate as the highest possible time
which could normally happen you flag MITM.  If you can ensure that
transatlantic delays are lower than the delays MITM will introduce
with this protocol, you've got it covered.

Being paranoid, I still see a couple of dangers: 

- if the packets are quite small you may be able to have a neural net
setup fill in the gaps when the packets lengths arrive awkwardly for
the MITM without it sounding too strange.

- there is no guarantee of real-time, you can make something sound like
it is coming in real time when it has lag.  For example: if you insert
some throat clearing, or ponderous "errr".  Some peoples speech
patterns are that way already.

This second one causes me to suggest that you'd be safer to have the
box detect the MITM induced lag if you can, rather than rely on human
ears.

I'm going to think on this some more.

Adam
-- 
Now officially an EAR violation...
Have *you* exported RSA today? --> http://www.dcs.ex.ac.uk/~aba/rsa/

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