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\section*{Homework 5}

\HEADER

\begin{enumerate}
\item Imagine you are researching security products
      (e.g.~CCTV, alarms etc) on a helpful website. They ask
      you for your address details? Think about whether this
      can be bad for you.


\item What can attacker that controls the network do to a communication
between a client and a server?

\item Before starting a TCP connection, client and servers
perform a three-way handshake. Describe how can this three-way
handshake can be abused by an attacker? 

\item Consider the following simple mutual authentication protocol:

\begin{center}
\begin{tabular}{ll}
$A \to B$: & $N_a$\\  
$B \to A$: & $\{N_a, N_b\}_{K_{ab}}$\\
$A \to B$: & $N_b$\\
\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 What is the main problem with the following
authentication protocol where $A$ sends $B$ mutually
shared key?

\begin{center}
$A \to B: K_{AB}$
\end{center}

\item Nonces are unpredicatble random numbers used in protocols.
Consider the following protocol

\begin{center}
\begin{tabular}{ll}
$A \to B$: & $N$\\  
$B \to A$: & $\{N + 1\}_{K_{ab}}$\\
\end{tabular}
\end{center}

Write down three facts that $A$ can infer after this protocol has been
successfully completed?

\item Write down a protocol which establishes a secret key
between $A$ and $B$ using a mutually trusted third party $S$.
You can assume $A$ and $S$, respectively $B$ and $S$, share 
secret keys.

\item Consider the following protocol between a car and a
key transponder: 

\begin{enumerate}
\item $C$ generates a random number $N$
\item $C$ calculates $(F,G) = \{N\}_K$
\item $C \to T$: $N, F$
\item $T$ calculates $(F',G') = \{N\}_K$
\item $T$ checks that $F = F'$
\item $T \to C$: $N, G'$
\item $C$ checks that $G = G'$
\end{enumerate}

In Step 2 and 4 a message is split into two halves. Explain
what the purpose of this split is?  Assume the key $K$ is shared 
only between the car and the transponder.  Does the protocol 
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}
\end{document}

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