Add solution for 4l

exam/ex.tex

@@ -236,7 +236,15 @@
       for transmission of public keys so that they can fit in commonly used
       packet sizes.
 
-    \item \TODO
+    \item One disadvantage of the scheme is that it inherently has a non-zero
+      decoding failure rate (DFR). For ephemeral keys and the lowest security
+      level, the authors advertise an error probability of $14$ out of $1.2\cdot
+      10^9$ decodes. The DFR can be lowered by choosing different parameters,
+      but the rate is arguably still too high for practical use. 
+
+      For long term keys the authors state that 95 out of 100 keys (lowest
+      security level) provide a DFR of $2^{-64}$, which is also arguably too low
+      for extended use well into the future.
 
   \end{enumerate}