Add solution for 5a

exam/ex.tex

@@ -264,7 +264,19 @@
   \item \textbf{(33 points)} 
   \begin{enumerate}
 
-    \item \TODO
+    \item Let there be an adversary $\mathcal{A}$ which breaks CGI. We can then
+      construct an adversary $\mathcal{B}$ which breaks CGI2.
+
+      Suppose $\mathcal{B}$ is given a CGI2 instance
+      $(\mathcal{G}_a,\mathcal{G}_b)$ where $a\neq b$ and $\mathcal{G}_a$ and
+      $\mathcal{G}_b$ are in the set of $2^{130}$ graphs isomorphic to
+      $\mathcal{G}$. The goal of $\mathcal{B}$ is to find an isomorphism $\phi$
+      with non-negligible advantage such that $\mathcal{G}_a =
+      \phi(\mathcal{G}_b)$. $\mathcal{B}$ will give
+      $(\mathcal{G}_a,\mathcal{G}_b)$ to $\mathcal{A}$ and $\mathcal{A}$ will
+      output an isomorphism $\phi$ which satisfies $\mathcal{G}_a =
+      \phi(\mathcal{G}_b)$. $\mathcal{B}$ can then take this isomorphism and
+      apply it to its own problem to obtain the solution.
 
     \item \TODO