--- a/hws/hw05.tex	Tue Sep 26 12:03:24 2017 +0100
+++ b/hws/hw05.tex	Tue Sep 26 12:10:41 2017 +0100
@@ -79,6 +79,30 @@
 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 Explain briefly the purpose of the certification authority in
+  the public-private key encryption scheme.
+
+\item Explain briefly what is meant by a certification authority
+  becoming ``too big to fail'' when it has issued a large number 
+  of certificates.
+
+\item In which situations does it make sense to install
+  invalid (self-signed) certificates?
+
 
 \item \POSTSCRIPT
 \end{enumerate}