G to the XY mod Q, and this is how we're computing the key when the XY values-- one is a secret value, and one is the value that was sent in the previous message-- and the assumption is that this value, which is G to the XY mod Q, and that's the value that's computed as the key in the Diffie-Hellman protocol, is indistinguishable from random even to an attacker who knows G, G to the X, G to the Y, and Q-- these are the messages that would be intercepted. 