\end{itemize}
\end{slide}
-%% IFF systems
-\begin{exercise}
- %% A nice open-ended question. Need to sort out IFF details (e.g.,
- %% aircraft numbering or whatever.
- An \emph{identify friend-or-foe} (IFF) system works as follows. Each
- `friendly' aircraft knows a common secret key. When a new aircraft is
- detected, a \emph{challenge} $x$ is sent. If a correct \emph{response} is
- returned, the new aircraft is presumed friendly; otherwise it is attacked.
- Discuss the security requirements of IFF systems and construct a formal
- model. Decide which primitive is best suited to the job and relate the
- security notions. What sorts of resource constraints can be imposed on
- adversaries attacking an IFF system?
- \answer%
- No prizes for guessing that a MAC is what's wanted, this being the MAC
- section.
-\end{exercise}
-
\xcalways\subsection{Basic results}\x
\begin{slide}