Re: Defeating MITM with Eric's Secure Phone

[Bill Frantz <frantz@netcom.com>]

At 12:08 PM -0700 10/9/97, Adam Back wrote:
>John Kelsey <kelsey@plnet.net> writes:
>> Adam Back <aba@dcs.ex.ac.uk> writes:
>> [computationally infeasible jobs for MITMs]
>> I prefer to work on the more immediately useful problem: How can I
>> secure my use of the (very nicely done) Comsec secure phones using
>> existing infrastructure?  I am concerned with the MITM voice
>> impersonation attack, since that's the easiest attack on the
>> system.
>
>We were discussing this problem before turning to talking about
>automated methods.  I think Eric Blossom suggested this earlier on:
>
>> 1.	Exchange PGP-encrypted e-mail establishing a set of
>> sixteen different words, labeled for 0..f in each direction.
>> Thus:
>>
>> 0. Dilbert 1. Alpha 2. Cable 3. Swordsman ... f. Marxist
>>
>> Now, the checksum reading is very hard to spoof.  Suppose I
>> get 0x33f. I say ``My checksum is Swordsman Swordsman
>> Marxist, or 33f.''
>
>It seems like a good solution.  An interesting question might be how
>many times can you use the same table without starting to leak values.
>Perhaps it doesn't matter that much because the MITM can't exactly use
>brute force on the problem otherwise you will know he's there.  He has
>to act non-passively to extract information.  (Presuming the protocol
>exchanges part of the information hashed for the challenge is
>encrypted with the negotiated key).
>
>> Now, the problem with this is that it's too cumbersome.
>
>What would be nice would be able to have information on one sheet of
>paper which you could continue to use for lots of communications,
>without need for calculator, or computer, or more emailed tables.

When I suggested using code words to exchange the checksum, I thought you
would have to use them in one-time-pad mode to be secure.  The following
argument makes me think you can reuse them several times, changing them at
about the same rate as you would change a symmetric crypto key.

Assume that the contents of the paper are secret between Alice and Bob.
When Alice calls Bob, she reads the word coresponding to the first digit of
the checksum.  Either Mallory is in the middle or he isn't.  If he isn't,
no problem.  The word list remains secure.

If he is in the middle, he has 15 chances in 16 of being caught on the
first exchange.  He only survives if the first digit of the Alice-Mallory
connection is the same as the first digit of the Mallory-Bob connection.
He now knows the word for one value and can continue to play 1 out of 16
times.

The probability he can survive the next word that Bob reads to Alice is
harder to calculate.  He can survive if the second digit of the Mallory-Bob
connection is the same as the second digit of the Alice-Mallory connection,
or the second digit of the Alice-Mallory connection is the same as the
first digit on that connection.  Without doing the math, Mallory's survival
probability becomes very small as the exchange continues.

If Alice and Bob catch Mallory, they talk about the weather and exchange a
new list by email.  If they don't, there is a very high probability that
the word list has not been compromised, and they can safely continue to use
it for the next call.


BTW - I really like John's idea of doing another exchange later in the
conversation.  Perhaps something like, "You know, I was dancing the Foxtrot
with my wife 9 days ago at 5AM."


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