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\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 Before starting a TCP connection, client and servers
perform a three-way handshake:

\begin{center}
\begin{tabular}{rl}
$A \rightarrow S$: & SYN\\
$S \rightarrow A$: & SYN-ACK\\
$A \rightarrow S$: & ACK\\
\end{tabular}
\end{center}

How can this protocol be abused causing trouble on the server?

\end{enumerate}
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