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# Re: Defeating MITM with Eric's Secure Phone

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[ To: Cryptography, cypherpunks ## Date: 10/10/97 ##
Subject: Re: Defeating MITM with Eric's Secure Phone ]

>Date: Thu, 9 Oct 1997 21:02:53 -0700
>From: Bill Frantz <[email protected]>
>Subject: Re: Defeating MITM with Eric's Secure Phone
>Cc: [email protected], [email protected]

I wrote (and Bill commented 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.''

Bill responded with some nice reasoning:
>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.

This is *almost* right.  We need to add one more thing,
though:

1.      Alice calls Mallory, thinking she's calling Bob.
She reads the first three digits to him.  He makes the
connection fall apart.  At the same time, Mallory calls Bob,
pretending to be Alice, and causes the connection to fall
apart at the same time.

2.      Alice calls Bob again, re-establishes their
connection, and talks to him.  Both seem to have had the
same thing happen, so it's believeable that it was just
noise on the line.  (Where I am, this isn't so uncommon that
it would imply an active attack.)

3.      Mallory now knows three of these words in the
dictionary.  He lies low for the next few calls before
trying the same trick again, until he learns most or all the
dictionary entries.

This implies a couple of things:  First, Alice and Bob
ought to be suspiscious of line noise that conveniently
clobbers them during reading of checksums.  Second, Alice
and Bob shouldn't keep the same 16-word dictionary forever.
If they change it once a week, this may be enough to make
the attack I describe above unworkable.

Note that this all works only when the phones/dictionaries
are used only by single users.

I also made a poorly-thought-through comment about using
6-digit hex secrets, and adding or XORing them into the
checksums.  This is vulnerable to a trivial attack, of
course.  The thing is, this doesn't need to be as complex or
cryptographically secure as a hash function or MAC, because
of the way it's used.  We can probably also expect
one-day-only keys for this application.

I can think of dozens of things that *ought* to work here,
and still be strong enough to resist the limited possible
attacks, but I can't seem to convince myself of the security
of any that are simple enough to use with a calculator.  If
I settled on one, it would be

New checksum = (((old checksum * C0) mod P)+ C1) mod 2^{64}.

where P is a random 64-bit prime, C0 is a random 64-bit
number between 0 and P-1 inclusive, and C1 is a random
64-bit number.  If P,C0, and C1 are all unknown, I *think*
that's secure.  An attacker given the high-order half of
this doesn't seem to have the information needed to guess
its low-order half on a different checksum, nor to reliably
learn the value of P.

This is essentially the IBC-Hash message authentication
code, which is provably secure if the key is used once.
I can't see a way that this could be attacked in this
application, given one-time use of the keys.  Can anyone
else?  (I may be missing something obvious again.)

>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."

I wish I could claim this, but it's been around for a while.
Didn't some of the PGPphone people come up with this idea?
(Or was it users of older, government-issued secure phones?)

>Bill Frantz       | Internal surveillance      | Periwinkle --
Consulting
>(408)356-8506     | helped make the USSR the   | 16345 Englewood
Ave.
>[email protected] | nation it is today.        | Los Gatos, CA
95032, USA

--John Kelsey, Counterpane Systems, [email protected]
PGP 2.6 fingerprint = 4FE2 F421 100F BB0A 03D1 FE06 A435 7E36

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--John Kelsey, Counterpane Systems, [email protected]
PGP 2.6 fingerprint = 4FE2 F421 100F BB0A 03D1 FE06 A435 7E36

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