Add solution for 1i

exam/ex.tex

@@ -89,7 +89,9 @@
       EWCDM construction, however, provides such a \emph{fallback} security
       guarantee and is of high practical relevance.
 
-    \item \TODO
+    \item Poly1305 is also a message authentication code (MAC), which we
+      discussed in the lecture.
+
     \item \TODO
     \item \TODO
   \end{enumerate}