Public-Key Cryptography Question?

Consider the following protocol:

A → B : <A, {K}pk(B)>

B → A : <B, {K}pk(A)>

First, A generates a fresh key K and sends it encrypted with the public key of B. Only B will be able to decrypt this message. In this way, B learns K and B also knows that this message comes from A as indicated in the first part of the message he received. Hence, B

answers to A by sending again the key, this time encrypted with the public key of A.

Show that an attacker can learn the key K generated by an honest agent A to another honest

agent B.