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ACE labs puzzle
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<i class="fa fa-calendar padding-small"></i><time datetime="2021-08-21T10:01:00+02:00"> Sat 21 August 2021</time>
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<p>Let's take a look at short assembly puzzle from the ACE labs.</p>
<div class="highlight"><pre><span></span><code><span class="w"> </span><span class="nf">AND</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span>
<span class="w"> </span><span class="no">CALL</span><span class="w"> </span><span class="no">$</span><span class="w"> </span><span class="err">+</span><span class="w"> </span><span class="mi">0xA</span><span class="w"> </span>
<span class="w"> </span><span class="no">ENTER</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span>
<span class="w"> </span><span class="nf">LAHF</span><span class="w"> </span>
<span class="w"> </span><span class="no">ADD</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="mi">0xC829</span>
<span class="w"> </span><span class="nf">BSWAP</span><span class="w"> </span><span class="no">eax</span>
<span class="w"> </span><span class="nf">SHR</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="mi">0x13</span><span class="w"> </span>
</code></pre></div>
<p>Besides this snippet there is just a textbox were we can enter our answer.</p>
<p>Just looking at intructions we see that some of them are going to be skipped due to <code>CALL</code>. The <code>$</code> is NASM evaluates to the assembly position of the line containing it. In our case this means we are going to jump 10 bytes from current address.
We can compile this snippet using <code>nasm</code>.</p>
<div class="highlight"><pre><span></span><code>nasm -felf64 snippet.asm
</code></pre></div>
<p>And decompile it using <code>objdump</code> </p>
<div class="highlight"><pre><span></span><code>objdump -dw -Mintel <output>
</code></pre></div>
<p>to see hex representation of assembly. </p>
<div class="highlight"><pre><span></span><code> 0000000000000000 <.text>:
0: 83 e0 00 and eax,0x0
3: e8 05 00 00 00 call 0xd
8: c8 00 00 00 enter 0x0,0x0
c: 9f lahf
d: 05 29 c8 00 00 add eax,0xc829
12: 0f c8 bswap eax
14: c1 e8 13 shr eax,0x13
</code></pre></div>
<p>Jumping 10 bytes gets us to the <code>ADD</code> instruction. Since <code>EAX</code> is zeroed out at the start with <code>AND</code> instruction, this will just put 0xC829 in it. Bitswapped this becomes 0x29c80000 (since we have 32bit register), and once we shift right by 0x13 we get 0x539.
In decimal this is 1337, which is also the answer to the puzzle.</p>
<div class="highlight"><pre><span></span><code><span class="nb">print</span><span class="p">(</span><span class="nb">hex</span><span class="p">(</span><span class="mh">0x29c80000</span> <span class="o">>></span> <span class="mh">0x13</span><span class="p">))</span>
<span class="mh">0x539</span>
</code></pre></div>
<p>We can also just link the file we got from <code>nasm</code> and debug it.</p>
<div class="highlight"><pre><span></span><code>ld <nasm_output> -o <output_name>
gdb <output_name>
</code></pre></div>
<p>In <code>gdb</code> we can put breakpoint at first instruction using <code>starti</code> and continue stepping one instruction at a time using <code>si</code> until we reach end of the snippet. If we look at value in rax register at the end we will see same value 1337 (<code>layout reg</code> enables register view in <code>gdb</code>).</p>
<p><img alt="Debugger view" class="img-fluid centerimage" src="https://f3real.github.io/images/2021_8_21_debugger.png"></p>
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