--- a/hws/hw05.tex	Fri Oct 10 15:49:23 2014 +0100
+++ b/hws/hw05.tex	Fri Oct 10 16:14:55 2014 +0100
@@ -8,6 +8,20 @@
 \section*{Homework 5}
 
 \begin{enumerate}
+\item Consider the following simple mutual authentication protocol:
+
+\begin{center}
+\begin{tabular}{ll}
+$A \rightarrow B$: & $N_a$\\  
+$B \rightarrow A$: & $\{N_a, N_b\}_{K_{ab}}$\\
+$A \rightarrow B$: & $N_b$\\
+\end{tabular}
+\end{center}
+
+Explain how an attacker $B'$ can launch an impersonation attack by 
+intercepting all messages for $B$ and make $A$ decrypt her own challenges.
+
+
 \item Access control is about deciding whether a principal that
 issues a request should be trusted on this request. Explain
 how such decision problems can be solved by using logic.