Diffie-Hellman Weakness Weakness

[norm@netcom.com (Norman Hardy)]

It has been mentioned several places that the Diffie-Hellman key exchange
algorithm is subject to the man-in-the-meddle attack. There is
a weakness in the attack that I understand. I suppose that
the attack goes as follows where I am the man in the middle:
 
I am able to install an active wire tap that allows me to substitute the
data traveling in either direction. I have a fast computer to help me.
I want to conceal my activity but learn what transpires.
 
Upon receiving signals to begin DH protocol I respond to each side separatly
"lets go". I establish a secret session key with each side. I am unable to
cause the two keys to be equal except by passing the b^x going one way and b^y
going the other. In this case I know neither x or y and can't read the traffic.
I must choose my own random numbers zx and zy and replace b^x with b^zx and
b^y with b^zy. X and Y now enter secure mode with the secret keys b^(x*zy)
between me and X and b^(zx*y) between me and Y. I can read the traffic.
If the connection is digitized voice and if X should happen to mention the
low ten bits of b^zy to Y then Y would notice the discrepency since Y knows
that he sent b^y. The jig is up. I don't know how to do voice recognition
so as to intercept the vocal quotation of b^zy and change it to a quotation
of b^y in a way that Y would not notice. I would have to simulate X's
voice.
 
Curiously there seems to be no analog of this precaution for digital DH
communicators. If there is a secret protocol for comparing b^y over the
nominally secured channel then there may as well have been a secret key
in the first place. If there is a public protocol for comparing b^y then
I can follow that protocol my self.