From: Adam Back <aba@dcs.ex.ac.uk>
Date: Tue, 7 Oct 1997 05:55:29 +0800
To: eb@comsec.com
Subject: using PGP email to authenticate Eric's Secure phone
In-Reply-To: <199710061905.MAA22945@comsec.com>
Message-ID: <199710062149.WAA00897@server.test.net>
MIME-Version: 1.0
Content-Type: text/plain




Eric Blossom <eb@comsec.com> writes:
> What the commitment prevents is a birthday attack on the verification
> code by Mallet.  Mallet has to be able to come up with a g^x' that
> when concatenated with g^y and hashed computes the same verification
> code as g^x concatenated with g^y and hashed.

Yes.  But that just means that you need commitments to prevent a MITM
brute forcing a key with the same partial hash.

I don't see that commitments on DH parameters do anything to prevent
someone who can impersonate voices, as far as I can see it is all
going to collapse back to whether the attacker can impersonate voices
and splice in without audible noise.

> >On the other hand, using persistent key public key crypto, Tim has
> >been signing his posts recently, and I have an ancient public key of
> >his stashed away which his new key is signed with.  If we were able to
> >construct a protocol to bolt on top of the reading of hashes, we could
> >have much greater protection against MITM.
> 
> Agreed.  The primary difficulty is getting the public keys into the
> unit.  And agreeing on what kind of certificate to use...  
> My preference (for patent reasons) would be to use DSA or ElGamal
> signatures.

How about touch tone keypad (phone).  Bit tedious?  Or temporarily
plug unit into a PC's modem port?

What about... a key server on a phone number.  You call key directory
services, you type in phone number, and your phone downloads
certificates and phone numbers, and uploads it's own certificate.
Also put the keyserver on the internet.

Too much complexity probably, if most of your users won't be using it
as it will add to cost.

But I do think it would be a good idea for you to include
documentation on a good secure way to use a PGP signature to exchange
use-once keys suitable for printing on a sheet of A4 which would keep
a user going for a few hundred calls.  Plus easy to follow description
of how to use.  Your suggestion in another post in this thread was a
challenge response.  Say you printed a matrix of random numbers or
words which were exchanged before hand via PGP.  And then use the
digits of the hash on the LCD screen to do a table lookup.  The
attacker won't be able to do his MITM because he won't know the table,
and so won't know what value he should read.

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

print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<>
)]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`