I have $\Pi=(Gen,Enc,Dec)$ and let it be semantically secure public-key encryption scheme. Security parameter is $n$, then the message space of plaintext is always $\lbrace 0, 1 \rbrace^n$.
By using $\Pi$ I want to construct key exchange protocol $\Theta$. There should be 2 rounds (i.e. one for Alice and one for Bob). It must be secure against eavesdroppers (and it'd should be possible to prove it :) ).
Of course the only assumption is security of $\Pi$. For example Diffie–Hellman key exchange protocol fits to this exercise (if we assume somethink), but I don't know how to generalize it.
P.S. The key Alice and Bob establish $\in \lbrace 0, 1 \rbrace^n$.