--- a/hws/hw04.tex	Tue Oct 22 14:47:43 2013 +0100
+++ b/hws/hw04.tex	Mon Oct 28 11:53:07 2013 +0000
@@ -10,6 +10,66 @@
 \begin{enumerate}
 \item Explain what is meant by \emph{Kerckhoffs' principle}.
 
+\item How can a system that separates between \emph{users} and \emph{root} be of any 
+help with buffer overflow attacks?
+
+\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 Explain what are the differences between dictionary and brute forcing attacks  against  passwords.
+
+\item In the context of which information flow should be protected, explain briefly the 
+differences between the {\it read rule} of the Bell-LaPadula access
+policy and the Biba access policy. Do the same for the {\it write rule}.
+
+\item A Unix directory might look as follows:
+
+\begin{center}
+\begin{verbatim}
+$ ls -ld . * */*
+drwxr-xr-x 1 ping staff  32768 Apr  2 2010 .
+-rw----r-- 1 ping students  31359 Jul 24 2011 manual.txt
+-r--rw--w- 1 bob students   4359 Jul 24 2011 report.txt
+-rwsr--r-x 1 bob students 141359 Jun  1 2013 microedit
+dr--r-xr-x 1 bob staff  32768 Jul 23 2011 src
+-rw-r--r-- 1 bob staff  81359 Feb 28 2012 src/code.c
+-r--rw---- 1 emma students    959 Jan 23 2012 src/code.h
+\end{verbatim}
+\end{center}
+
+with group memberships assigned as follows:
+\begin{center}
+\begin{tabular}{ll}
+Members of group staff: & ping, bob, emma\\ 
+Members of group students: & emma\\
+\end{tabular}
+\end{center}
+
+The file microedit is a text editor, which allows its users to open, edit and 
+save files. Note carefully that microedit has set its setuid flag. 
+Fill in the access control matrix below that shows for each of the above five files, 
+whether ping, bob, or emma are able to obtain the right to read (R) or replace (W) its 
+contents using the editor microedit.\bigskip
+
+\begin{center}
+\begin{tabular}{r|c|c|c|c|c}
+        & manual.txt & report.txt & microedit & src/code.c & src/code.h \\\hline
+ping & & & & &\\\hline
+bob & & & & &\\\hline
+emma   & & & & &\\
+\end{tabular}
+\end{center}
+
 
 \end{enumerate}