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\noindent \textbf{CS426: Computer Security\hspace*{\fill}Fall 2010}
\begin{center}
{\LARGE Homework \#3}
\end{center}

\paragraph{Due date \& time:} 1:30pm on November 5, 2010.
Hand in at the beginning of class (preferred), or email to the TA
(twykoff@purdue.edu) by the due time.

\vspace*{-0.1in}\paragraph{Late Policy:} You have three extra days in total for all
your homeworks and projects.  Any portion of a day used counts as one day; that is,
you have to use integer number of late days each time. If you emailed your homework
to the TA by 1:30pm the day after it is due, then you have used one extra day. If
you exhaust your three late days, any late homework won't be graded.


\vspace*{-0.1in}\paragraph{Additional Instructions:} (1) The
submitted homework must be typed. Using Latex is recommended, but
not required.
\\


\begin{description}

\item[Problem 1 (10 pts)]
Identify and explain the buffer-overflow vulnerabilities in the following C code
snippets, and give a corrected version of the code.

\begin{enumerate}
 \item
\begin{alltt}
    char  buf[20];
    char  prefix[] = "http://";
    strcpy(buf, prefix);
    strncat(buf, path, sizeof(buf));

\end{alltt}

 \item
\begin{alltt}
    char buf[32];
    strncpy(buf, data, strlen(data));

\end{alltt}

 \item
\begin{alltt}
    #define MAX_BUF  256
    void BadCode(char * input)
    \{
        short  len;
        char  buf[MAX_BUF];
        len = strlen(input);
        if (len < MAX_BUF)
            strcpy(buf, input);
    \}
\end{alltt}
\end{enumerate}


 \item[Problem 2 (5 pts)]
Write a C function that contains a heap-overflow vulnerability, which is caused by an
integer overflow vulnerability.
 \\

 \item[Problem 3 (5 pts) ]
Recall that a security label consists of a security level and a set
of categories.  Given the security levels \emph{public},
\emph{confidential}, and \emph{strictly confidential}, and the
categories FACULTY, STAFF, and STUDENTS. How many labels can be
constructed?  Which labels dominate the label (confidential,
{STUDENT})? More generally, how many security labels can be
constructed given $n$ security levels and $m$ categories.
 \\

 \item[Problem 4 (10 pts)]
Explain the following terminologies: Trusted Computer Base, Trusted
Path, discretionary access control, mandatory access control, covert
channels.
 \\


 \item[Problem 5 (15 pts)]
Read Part IA of the ``The Protection of Information in Computer Systems'' by Saltzer and
Schroeder.  For each of the eight principles listed there, write your understanding of
the principle, give an instance of where it has been applied or where it should be
applied.
 \\

 \item[Problem 6 (20 pts) ]
Read the following two articles
\begin{itemize}
 \item
Ken Thompson's ``Reflections on Trusting Trust''
 \item
David A. Wheeler's ``Countering Trusting Trust through Diverse Double-Compiling''
\end{itemize}

Write a brief summary which should include: (i) How the attack described in
Thompson's article work? How can it be used to compromise the security of real world
systems? What are the most effective ways to defend against the attack?  What have
you learned.
 \\

 \item[Problem 7 Clark-Wilson (20 points)]
Read the Clark-Wilson paper.

\begin{itemize}
 \item
D.D. Clark and D.R. Wilson. "A Comparison of Commercial and Military Computer
Security Policies".
\end{itemize}

Answer the following questions.

\begin{itemize}
 \item
How would you compare the Biba integrity models and the Clark-Wilson integrity model.

 \item
List the two or three most significant new insights you took away from the
Clark-Wilson paper and the most significant flaws or weaknesses of it (if any).\\
\end{itemize}

 \item[Problem 8 Common Criteria Evaluation (15 points)]

Read the Common Criteria evaluation report of the Green Hills Software INTEGRITY-178B Separation Kernel, and discuss your assessment of the security of the software based on the report.  

\end{description}

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