--- a/hws/hw05.tex	Sat Sep 23 14:19:09 2017 +0100
+++ b/hws/hw05.tex	Sat Sep 23 19:32:21 2017 +0100
@@ -79,6 +79,20 @@
 achieve that the transponder $T$ authenticates itself to the car
 $C$? Does the car authenticate itself to the transponder?
 
+\item What are the main disadvantages of the following
+      protocol that establishes a mutual key between two
+      parties $A$ and $B$ with the help of a mutually trusted
+      third party $S$:
+
+\begin{center}
+\begin{tabular}{l@{\hspace{2mm}}l}
+$A \to S :$ & $A, B$\\
+$S \to A :$ & $\{K_{AB}\}_{K_{AS}}$ and $\{\{K_{AB}\}_{K_{BS}} \}_{K_{AS}}$\\
+$A \to B :$ & $\{K_{AB}\}_{K_{BS}}$\\
+$A \to B :$ & $\{m\}_{K_{AB}}$\\
+\end{tabular}
+\end{center}
+
 
 \item \POSTSCRIPT
 \end{enumerate}