\documentclass{article}+
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\usepackage{charter}+
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\usepackage{hyperref}+
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\usepackage{amssymb}+
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+
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\begin{document}+
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+
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\section*{Homework 4}+
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+
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\begin{enumerate}+
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\item Explain what is meant by \emph{Kerckhoffs' principle}.+
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+
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\item How can a system that separates between \emph{users} and \emph{root} be of any +
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help with buffer overflow attacks?+
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+
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\item Consider the following simple mutual authentication protocol:+
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+
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\begin{center}+
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\begin{tabular}{ll}+
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$A \rightarrow B$: & $N_a$\\  +
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$B \rightarrow A$: & $\{N_a, N_b\}_{K_{ab}}$\\+
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$A \rightarrow B$: & $N_b$\\+
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\end{tabular}+
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\end{center}+
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+
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Explain how an attacker $B'$ can launch an impersonation attack by +
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intercepting all messages for $B$ and make $A$ decrypt her own challenges.+
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+
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\item Explain what are the differences between dictionary and brute forcing attacks  against  passwords.+
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+
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\item In the context of which information flow should be protected, explain briefly the +
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differences between the {\it read rule} of the Bell-LaPadula access+
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policy and the Biba access policy. Do the same for the {\it write rule}.+
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+
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\item A Unix directory might look as follows:+
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+
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\begin{center}+
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\begin{verbatim}+
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$ ls -ld . * */*+
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drwxr-xr-x 1 ping staff  32768 Apr  2 2010 .+
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-rw----r-- 1 ping students  31359 Jul 24 2011 manual.txt+
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-r--rw--w- 1 bob students   4359 Jul 24 2011 report.txt+
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-rwsr--r-x 1 bob students 141359 Jun  1 2013 microedit+
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dr--r-xr-x 1 bob staff  32768 Jul 23 2011 src+
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-rw-r--r-- 1 bob staff  81359 Feb 28 2012 src/code.c+
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-r--rw---- 1 emma students    959 Jan 23 2012 src/code.h+
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\end{verbatim}+
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\end{center}+
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+
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with group memberships assigned as follows:+
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\begin{center}+
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\begin{tabular}{ll}+
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Members of group staff: & ping, bob, emma\\ +
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Members of group students: & emma\\+
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\end{tabular}+
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\end{center}+
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+
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The file microedit is a text editor, which allows its users to open, edit and +
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save files. Note carefully that microedit has set its setuid flag. +
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Fill in the access control matrix below that shows for each of the above five files, +
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whether ping, bob, or emma are able to obtain the right to read (R) or replace (W) its +
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contents using the editor microedit.\bigskip+
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+
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\begin{center}+
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\begin{tabular}{r|c|c|c|c|c}+
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        & manual.txt & report.txt & microedit & src/code.c & src/code.h \\\hline+
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ping & & & & &\\\hline+
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bob & & & & &\\\hline+
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emma   & & & & &\\+
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\end{tabular}+
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\end{center}+
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+
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+
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\end{enumerate}+
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+
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+
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\end{document}+
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+
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%%% Local Variables: +
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%%% mode: latex+
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%%% TeX-master: t+
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%%% End: +
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