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<h1>
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title="Permalink to The Sharif University CTF 2014">
The Sharif University CTF 2014
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<abbr class="published" title="2014-09-26T04:20:00">
Fri 26 September 2014 </abbr>
<span class="label"> Category</span>
<a href="./category/reverse-engineering.html"><i class="icon-folder-open"></i>Reverse Engineering</a>
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<a href="./tag/ctf.html"><i class="icon-tag"></i>CTF</a>
<a href="./tag/steganography.html"><i class="icon-tag"></i>Steganography</a>
<a href="./tag/jad.html"><i class="icon-tag"></i>jad</a>
<a href="./tag/outguess.html"><i class="icon-tag"></i>OutGuess</a>
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<p>It looked like a fun CTF, but I did not have the chance to play for too long. I managed to solve a few problems in the morning and the solutions are below.</p>
<h2>Avatar: Steganography</h2>
<p>The challenge starts with:</p>
<blockquote>
<p>A terrorist has changed his picture in a social network. What is the hidden message?</p>
</blockquote>
<p>And the following image:</p>
<p><img alt="" src="http://i.imgur.com/6NIu64us.jpg" /></p>
<p>For this problem, I use <a href="http://www.outguess.org/download.php">OutGuess</a>, which can be installed as:</p>
<div class="highlight"><pre><span class="nv">$ </span>tar -zxvf outguess-0.2.tar.gz
<span class="nv">$ </span><span class="nb">cd </span>outguess
<span class="nv">$ </span>./configure <span class="o">&&</span> make
</pre></div>
<p>Running it will give us the flag:</p>
<div class="highlight"><pre><span class="nv">$ </span>./outguess -r lamb.jpg pass.txt
Reading ../lamb.jpg....
Extracting usable bits: 28734 bits
Steg retrieve: seed: 94, len: 41
<span class="nv">$ </span>cat pass.txt
We should blow up the bridge at midnight
</pre></div>
<hr />
<h2>What is this: Steganography</h2>
<p>This challenge has a very short text:</p>
<blockquote>
<p>Find the flag.</p>
</blockquote>
<p>Together with two pictures:</p>
<p><img alt="" src="http://i.imgur.com/sbSwBju.jpg" />
<img alt="" src="http://i.imgur.com/mpcIKWt.jpg" /></p>
<p>After the usual inspection (tail, file, diff, compare), I applied my knowledge of a former astrophysicist to inspect what would happen if I added or subtracted the picture. I wrote the following script:</p>
<div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">from</span> <span class="nn">scipy.misc</span> <span class="kn">import</span> <span class="n">imread</span><span class="p">,</span> <span class="n">imsave</span>
<span class="k">def</span> <span class="nf">compare_images</span><span class="p">(</span><span class="n">img1</span><span class="p">,</span> <span class="n">img2</span><span class="p">):</span>
<span class="n">diff</span> <span class="o">=</span> <span class="n">img1</span> <span class="o">+</span> <span class="n">img2</span>
<span class="n">imsave</span><span class="p">(</span><span class="s">'sum.png'</span><span class="p">,</span> <span class="n">diff</span><span class="p">)</span>
<span class="n">diff</span> <span class="o">=</span> <span class="n">img1</span> <span class="o">-</span> <span class="n">img2</span>
<span class="n">imsave</span><span class="p">(</span><span class="s">'diff.png'</span><span class="p">,</span> <span class="n">diff</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="n">file1</span><span class="p">,</span> <span class="n">file2</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">:</span><span class="mi">1</span><span class="o">+</span><span class="mi">2</span><span class="p">]</span>
<span class="n">img1</span> <span class="o">=</span> <span class="n">imread</span><span class="p">(</span><span class="n">file1</span><span class="p">)</span><span class="o">.</span><span class="n">astype</span><span class="p">(</span><span class="nb">float</span><span class="p">)</span>
<span class="n">img2</span> <span class="o">=</span> <span class="n">imread</span><span class="p">(</span><span class="n">file2</span><span class="p">)</span><span class="o">.</span><span class="n">astype</span><span class="p">(</span><span class="nb">float</span><span class="p">)</span>
<span class="n">compare_images</span><span class="p">(</span><span class="n">img1</span><span class="p">,</span> <span class="n">img2</span><span class="p">)</span>
</pre></div>
<p>Running it, give us the flag!</p>
<p><img alt="" src="http://i.imgur.com/mvj8OJl.png" /></p>
<hr />
<h2>Guess the number: Reverse Engineering</h2>
<p>This problem starts with another not very informative text:</p>
<blockquote>
<p>Guess the number and find the flag.</p>
</blockquote>
<p>Then it gives us a <em>java class</em> file. It was clear that we needed to decompile it. I'm using
<a href="http://varaneckas.com/jad/">jad</a>for this task:</p>
<div class="highlight"><pre><span class="nv">$ </span>jad guess.class
</pre></div>
<p>Now, opening this file in a text editor, we can see how to generate the flag:</p>
<div class="highlight"><pre><span class="err">```</span><span class="n">java</span>
<span class="c1">// Decompiled by Jad v1.5.8e. Copyright 2001 Pavel Kouznetsov.</span>
<span class="c1">// Jad home page: http://www.geocities.com/kpdus/jad.html</span>
<span class="c1">// Decompiler options: packimports(3)</span>
<span class="c1">// Source File Name: guess.java</span>
<span class="o">(...)</span>
<span class="n">String</span> <span class="n">str_one</span> <span class="o">=</span> <span class="s">"4b64ca12ace755516c178f72d05d7061"</span><span class="o">;</span>
<span class="n">String</span> <span class="n">str_two</span> <span class="o">=</span> <span class="s">"ecd44646cfe5994ebeb35bf922e25dba"</span><span class="o">;</span>
<span class="n">String</span> <span class="n">answer</span> <span class="o">=</span> <span class="n">XOR</span><span class="o">(</span><span class="n">str_one</span><span class="o">,</span> <span class="n">str_two</span><span class="o">);</span>
<span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">((</span><span class="k">new</span> <span class="n">StringBuilder</span><span class="o">(</span><span class="s">"your flag is: "</span><span class="o">)).</span><span class="na">append</span><span class="o">(</span><span class="n">answer</span><span class="o">).</span><span class="na">toString</span><span class="o">());</span>
</pre></div>
<p>Running the modified version gives us:</p>
<div class="highlight"><pre><span class="n">$</span> <span class="n">javac</span> <span class="o">-</span><span class="n">g</span> <span class="n">guess</span><span class="o">.</span><span class="na">java</span>
<span class="n">$</span> <span class="n">java</span> <span class="n">guess</span>
<span class="n">your</span> <span class="n">flag</span> <span class="nl">is:</span> <span class="n">a7b08c546302cc1fd2a4d48bf2bf2ddb</span>
</pre></div>
<hr />
<h2>Sudoku image encryption - cryptography</h2>
<p>This challenge starts with the following text:</p>
<blockquote>
<p>Row Major Order</p>
</blockquote>
<p>And it gives us two pictures: a map and a sudoku.</p>
<p><img alt="" src="http://i.imgur.com/U9Zq4wp.png" /></p>
<p>We solve the sudoku and write the solution in a script to reorder the blocks:</p>
<div class="highlight"><pre><span class="kn">from</span> <span class="nn">PIL</span> <span class="kn">import</span> <span class="n">Image</span>
<span class="c"># solved sudoku</span>
<span class="n">sudoku</span> <span class="o">=</span> <span class="s">'''</span>
<span class="s">964127538</span>
<span class="s">712385694</span>
<span class="s">385496712</span>
<span class="s">491578263</span>
<span class="s">238614975</span>
<span class="s">576239841</span>
<span class="s">627843159</span>
<span class="s">153962487</span>
<span class="s">849751326</span>
<span class="s">'''</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">sudoku</span><span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="s">'</span><span class="se">\n</span><span class="s">'</span><span class="p">,</span> <span class="s">''</span><span class="p">)</span>
<span class="n">image</span> <span class="o">=</span> <span class="n">Image</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'image.png'</span><span class="p">)</span><span class="o">.</span><span class="n">convert</span><span class="p">(</span><span class="s">'RGB'</span><span class="p">)</span>
<span class="n">out</span> <span class="o">=</span> <span class="n">Image</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s">'RGB'</span><span class="p">,</span> <span class="n">image</span><span class="o">.</span><span class="n">size</span><span class="p">)</span>
<span class="k">for</span> <span class="n">j</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">9</span><span class="p">):</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">9</span><span class="p">):</span>
<span class="n">img_cell</span> <span class="o">=</span> <span class="n">image</span><span class="o">.</span><span class="n">crop</span><span class="p">((</span><span class="n">i</span> <span class="o">*</span> <span class="mi">50</span><span class="p">,</span> <span class="n">j</span> <span class="o">*</span> <span class="mi">50</span><span class="p">,</span> <span class="n">i</span> <span class="o">*</span> <span class="mi">50</span> <span class="o">+</span> <span class="mi">50</span><span class="p">,</span> <span class="n">j</span> <span class="o">*</span> <span class="mi">50</span> <span class="o">+</span> <span class="mi">50</span><span class="p">))</span>
<span class="n">c</span> <span class="o">=</span> <span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">s</span><span class="p">[</span><span class="n">j</span> <span class="o">*</span> <span class="mi">9</span> <span class="o">+</span> <span class="n">i</span><span class="p">])</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span> <span class="o">*</span> <span class="mi">50</span>
<span class="n">out</span><span class="o">.</span><span class="n">paste</span><span class="p">(</span><span class="n">img_cell</span><span class="p">,</span> <span class="p">(</span><span class="n">c</span><span class="p">,</span> <span class="n">j</span> <span class="o">*</span> <span class="mi">50</span><span class="p">))</span>
<span class="n">out</span><span class="o">.</span><span class="n">save</span><span class="p">(</span><span class="s">'out_image.png'</span><span class="p">)</span>
</pre></div>
<p>This gives us our flag:</p>
<p><img alt="" src="http://i.imgur.com/sA7JPrl.png" /></p>
<p><strong> Hack all the things! </strong></p>
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