🚀 go-pugleaf

Consider two parties X and Y who have never met and never exchanged
information. They wish to establish a confidential and authenticated
channel. Since they can't communicate directly, they need to send their
messages to Z, who (supposedly) routes them to the recipient.

X and Y can use Diffie-Hellman to establish a secret key using public
messages that Z can read. However, Z might intelligently change these
messages to create a transparent channel between X and Y. Z will be able
to read and change all messages sent over this supposedly secure channel.

Is it possible for X and Y to detect if Z has created a transparent
channel between them, for example by using the fact that both have
calculated different secret keys. I know authentication is possible
if all communication between the parties is conversational, since X
can send Y a hashed value of its next block and the key, wait for Ys reply 
and send the real block. Z cannot fake this, since it doesn't know the
original block of data. However, this type of check is impractical
for everyday use on network layers.

Any suggestions are welcome.
 
Ronald
roana@cs.utwente.nl