Add binding solution for 5d

exam/ex.tex

@@ -300,7 +300,11 @@
       $\mathcal{G}_{\mathsf{ch}}$ isomorphic to $\mathcal{G}$ as input and
       outputs $\top$ if the result matches $\mathcal{G}'$ and $\bot$ otherwise.
 
-    \item \TODO
+    \item Computational binding: Suppose $\mathsf{Comm}(\psi,\mathcal{G}_0) =
+      \mathsf{Comm}(\psi,\mathcal{G}_1)$. This means that $\psi(\mathcal{G}_0) =
+      \psi(\mathcal{G}_1)$ and the adversary has found an isomorphism which maps
+      two different graphs to the same output which corresponds to solving the
+      CGI problem.
 
     \item If $G_{ch}=\phi_{ch}(G)$ and $G'=\psi(G)$, it follows that
       $G=\phi_{ch}^{-1}(G_{ch})$ and therefore $G'=\psi(\phi_{ch}^{-1}(G_{ch}))$