[Cryptography] Non-Authenticated Key Agreement

Bill Cox waywardgeek at gmail.com
Wed Sep 23 10:45:24 EDT 2015

On Tue, Sep 22, 2015 at 10:11 PM, Davy Durham <ddurham at davyandbeth.com>

> Okay, so I've lurked on this list for a couple of weeks and wasn't sure if
> it was a safe place to propose crypto ideas, but given the recent threads,
> and the plethora of friendly discuss, I feel that I can do this :)  And, I
> agree with Rule #1 of crypto, namely not to invent your own.  But I think
> the unstated rest of that thought is "... and then to put it into practice
> without gobs of scrutiny".  Since I'm not doing that, here goes.

I know others on this list might dump on arm-chair crypto attempts, but so
as long as non-proven crypt is not put into practice, it's all fun.
Besides, it's nice to be able to break a crypto system before breakfast :)

> Alice wishes to send Bob a piece of information d.  Alice and Bob have not
> exchanged any information previously.
> Alice makes up a random key, ka, and Bob makes up a random key, kb.
> The following sequence diagram allows Alice to send d to Bob while the d
> remains protected from eavesdropping in between.
>    Alice                                           Bob
> d = data
> ka = random bits
> d' = E(d, ka)
>                          d'
>       ---------------------------------------->
>                                               kb = random bits
>                                               d'' = E(d', kb)
>                          d''
>       <----------------------------------------
> The problem in your crypto system happens right here.  Eve simply computes
d' XOR d'', and the result is kb, revealing Bob's secret key.

Your basic intuition is right, though.  If we can find an encryption
function E(d, k) such that E(E(d, k1), k2) == E(E(d, k2), k1) where E is a
secure symmetric key encryption, then you can use it to create a public key
crypto system.

If you look at regular DH, it almost fits into this model.  Instead of user
data d, use a constant g.  Then you need E(E(g, k1), k2) == E(E(g, k2),
k1).  If E(g, k) = g^k mod p, then E(E(g, k1), k2) = (g^k1)^k2 = g^(k1*k2)
= E(E(g, k2), k1).

One hard part in public key crypto is finding candidate functions E with
this property.  It leads naturally to investigating Abelian groups.


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