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<a href="https://f3real.github.io/blockchain_ctf1011.html"
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Blockchain CTF Lvl. 10-11
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<span class="published">
<i class="fa fa-calendar padding-small"></i><time datetime="2018-12-22T10:01:00+01:00"> Sat 22 December 2018</time>
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<p>In this post, we will take a look at the last two challenges 10 and 11.</p>
<p>So let's start:</p>
<div class="toc">
<ul>
<li><a href="#lvl-10-slot-machine">Lvl 10 Slot Machine</a></li>
<li><a href="#lvl-11-rainy-day-fund">Lvl 11 Rainy Day Fund</a></li>
</ul>
</div>
<h2 id="lvl-10-slot-machine">Lvl 10 Slot Machine</h2>
<div class="highlight"><pre><span></span><code><span class="k">pragma solidity</span><span class="w"> </span><span class="err">0.4.24</span><span class="p">;</span>
<span class="kt">import</span><span class="w"> </span><span class="s2">"../CtfFramework.sol"</span><span class="p">;</span>
<span class="kt">import</span><span class="w"> </span><span class="s2">"../../node_modules/openzeppelin-solidity/contracts/math/SafeMath.sol"</span><span class="p">;</span>
<span class="k">contract</span><span class="w"> </span><span class="ni">SlotMachine</span><span class="w"> </span><span class="kt">is</span><span class="w"> </span>CtfFramework<span class="p">{</span>
<span class="w"> </span>using<span class="w"> </span>SafeMath<span class="w"> </span><span class="kt">for</span><span class="w"> </span><span class="kt">uint256</span><span class="p">;</span>
<span class="w"> </span><span class="kt">uint256</span><span class="w"> </span><span class="k">public </span><span class="nv">winner</span><span class="p">;</span>
<span class="w"> </span><span class="kt">constructor</span><span class="p">(</span><span class="kt">address</span><span class="w"> </span><span class="nv">_ctfLauncher</span><span class="p">,</span><span class="w"> </span><span class="kt">address</span><span class="w"> </span><span class="nv">_player</span><span class="p">)</span><span class="w"> </span><span class="kt">public</span><span class="w"> </span><span class="kt">payable</span>
<span class="w"> </span>CtfFramework<span class="p">(</span>_ctfLauncher<span class="p">,</span><span class="w"> </span>_player<span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span>winner<span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="m m-Decimal">5</span><span class="w"> </span>ether<span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">function</span><span class="p">()</span><span class="w"> </span><span class="kt">external</span><span class="w"> </span><span class="kt">payable</span><span class="w"> </span>ctf<span class="p">{</span>
<span class="w"> </span><span class="kt">require</span><span class="p">(</span><span class="k">msg.value</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="m m-Decimal">1</span><span class="w"> </span>szabo<span class="p">,</span><span class="w"> </span><span class="s2">"Incorrect Transaction Value"</span><span class="p">);</span>
<span class="w"> </span><span class="kt">if</span><span class="w"> </span><span class="p">(</span><span class="kt">address</span><span class="p">(</span><span class="kt">this</span><span class="p">).</span>balance<span class="w"> </span><span class="o">>=</span><span class="w"> </span>winner<span class="p">){</span>
<span class="w"> </span><span class="k">msg.sender</span><span class="p">.</span>transfer<span class="p">(</span><span class="kt">address</span><span class="p">(</span><span class="kt">this</span><span class="p">).</span>balance<span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
<p>To win we need to make a contract balance greater or equal than 5 eth. But we see that we can only send 1 <code>szabo</code> (0.000001 eth) at the time to contract. </p>
<p>There are three ways to avoid activating fallback function:</p>
<ol>
<li>miner block rewards</li>
<li>using <code>selfdestruct</code></li>
<li>send ether to contract before it is created</li>
</ol>
<p>Since we don't want to mine, we will go with approach two. Function <code>selfdestruct(address)</code> destroys contract and transfers all balance to a specified address. It is a way of cleaning contracts from the blockchain.</p>
<p>Our solution:</p>
<div class="highlight"><pre><span></span><code><span class="kt">function</span><span class="w"> </span><span class="nv">endIt</span><span class="p">()</span><span class="w"> </span><span class="kt">external</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">require</span><span class="p">(</span><span class="k">msg.sender</span><span class="w"> </span><span class="o">==</span><span class="w"> </span>owner<span class="p">);</span>
<span class="w"> </span>selfdestruct<span class="p">(</span>slotContractAddress<span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
<h2 id="lvl-11-rainy-day-fund">Lvl 11 Rainy Day Fund</h2>
<div class="highlight"><pre><span></span><code><span class="k">pragma solidity</span><span class="w"> </span><span class="err">0.4.24</span><span class="p">;</span>
<span class="kt">import</span><span class="w"> </span><span class="s2">"../CtfFramework.sol"</span><span class="p">;</span>
<span class="k">contract</span><span class="w"> </span><span class="ni">DebugAuthorizer</span><span class="p">{</span><span class="w"> </span>
<span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="k">public </span><span class="nv">debugMode</span><span class="p">;</span>
<span class="w"> </span><span class="kt">constructor</span><span class="p">()</span><span class="w"> </span><span class="kt">public</span><span class="w"> </span><span class="kt">payable</span><span class="p">{</span>
<span class="w"> </span><span class="kt">if</span><span class="p">(</span><span class="kt">address</span><span class="p">(</span><span class="kt">this</span><span class="p">).</span>balance<span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="m m-Decimal">1</span><span class="p">.</span><span class="m m-Decimal">337</span><span class="w"> </span>ether<span class="p">){</span>
<span class="w"> </span>debugMode<span class="o">=</span><span class="kt">true</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="k">contract</span><span class="w"> </span><span class="ni">RainyDayFund</span><span class="w"> </span><span class="kt">is</span><span class="w"> </span>CtfFramework<span class="p">{</span>
<span class="w"> </span><span class="kt">address</span><span class="w"> </span><span class="k">public </span><span class="nv">developer</span><span class="p">;</span>
<span class="w"> </span><span class="kt">mapping</span><span class="p">(</span><span class="kt">address</span><span class="o">=></span><span class="kt">bool</span><span class="p">)</span><span class="w"> </span><span class="kt">public</span><span class="w"> </span>fundManagerEnabled<span class="p">;</span>
<span class="w"> </span>DebugAuthorizer<span class="w"> </span><span class="kt">public</span><span class="w"> </span>debugAuthorizer<span class="p">;</span>
<span class="w"> </span><span class="kt">constructor</span><span class="p">(</span><span class="kt">address</span><span class="w"> </span><span class="nv">_ctfLauncher</span><span class="p">,</span><span class="w"> </span><span class="kt">address</span><span class="w"> </span><span class="nv">_player</span><span class="p">)</span><span class="w"> </span><span class="kt">public</span><span class="w"> </span><span class="kt">payable</span>
<span class="w"> </span>CtfFramework<span class="p">(</span>_ctfLauncher<span class="p">,</span><span class="w"> </span>_player<span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">//debugAuthorizer = (new DebugAuthorizer).value(1.337 ether)(); // Debug mode only used during development</span>
<span class="w"> </span>debugAuthorizer<span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kt">new</span><span class="w"> </span>DebugAuthorizer<span class="p">();</span>
<span class="w"> </span>developer<span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">msg.sender</span><span class="p">;</span>
<span class="w"> </span>fundManagerEnabled<span class="p">[</span><span class="k">msg.sender</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kt">true</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">modifier</span><span class="w"> </span>isManager<span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">require</span><span class="p">(</span>fundManagerEnabled<span class="p">[</span><span class="k">msg.sender</span><span class="p">]</span><span class="w"> </span><span class="o">||</span><span class="w"> </span>debugAuthorizer<span class="p">.</span>debugMode<span class="p">()</span><span class="w"> </span><span class="o">||</span><span class="w"> </span><span class="k">msg.sender</span><span class="w"> </span><span class="o">==</span><span class="w"> </span>developer<span class="p">,</span><span class="w"> </span><span class="s2">"Unauthorized: Not a Fund Manager"</span><span class="p">);</span>
<span class="w"> </span>_<span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">function</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="kt">external</span><span class="w"> </span><span class="kt">payable</span><span class="w"> </span>ctf<span class="p">{</span>
<span class="w"> </span><span class="c1">// Anyone can add to the fund </span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">function</span><span class="w"> </span><span class="nv">addFundManager</span><span class="p">(</span><span class="kt">address</span><span class="w"> </span><span class="nv">_newManager</span><span class="p">)</span><span class="w"> </span><span class="kt">external</span><span class="w"> </span>isManager<span class="w"> </span>ctf<span class="p">{</span>
<span class="w"> </span>fundManagerEnabled<span class="p">[</span>_newManager<span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kt">true</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">function</span><span class="w"> </span><span class="nv">removeFundManager</span><span class="p">(</span><span class="kt">address</span><span class="w"> </span><span class="nv">_previousManager</span><span class="p">)</span><span class="w"> </span><span class="kt">external</span><span class="w"> </span>isManager<span class="w"> </span>ctf<span class="p">{</span>
<span class="w"> </span>fundManagerEnabled<span class="p">[</span>_previousManager<span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kt">false</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="kt">function</span><span class="w"> </span><span class="nv">withdraw</span><span class="p">()</span><span class="w"> </span><span class="kt">external</span><span class="w"> </span>isManager<span class="w"> </span>ctf<span class="p">{</span>
<span class="w"> </span><span class="k">msg.sender</span><span class="p">.</span>transfer<span class="p">(</span><span class="kt">address</span><span class="p">(</span><span class="kt">this</span><span class="p">).</span>balance<span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
<p>We see that there is modifier <code>isManager</code> blocking us to withdraw funds from the contract. To bypass it we can either be contract creator or be in debug mode.</p>
<p>If we look at the contract <code>DebugAuthorizer</code> it sets <code>debugMode</code> to true only if balance is 1.337 eth during creation. This means that we need to send eth to contract before it is created.</p>
<p>Contract addresses in ethereum are deterministic and can be calculated using the following formula:
<code>keccak(RLP(sender address, nonce)))</code></p>
<p>RLP stands for Recursive Length Prefix, a type of encoding used by ethereum. A good explanation can be found on <a href="https://github.com/ethereum/wiki/wiki/RLP">wiki</a>.</p>
<p>In solidity we can calculate address as follows:</p>
<div class="highlight"><pre><span></span><code><span class="nx">function</span><span class="w"> </span><span class="nx">addressFrom</span><span class="p">(</span><span class="nx">address</span><span class="w"> </span><span class="nx">_origin</span><span class="p">,</span><span class="w"> </span><span class="nx">uint</span><span class="w"> </span><span class="nx">_nonce</span><span class="p">)</span><span class="w"> </span><span class="kd">external</span><span class="w"> </span><span class="nx">pure</span><span class="w"> </span><span class="nx">returns</span><span class="w"> </span><span class="p">(</span><span class="nx">address</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nx">address</span><span class="p">(</span><span class="nx">keccak256</span><span class="p">(</span><span class="nx">byte</span><span class="p">(</span><span class="mh">0xd6</span><span class="p">),</span><span class="w"> </span><span class="nx">byte</span><span class="p">(</span><span class="mh">0x94</span><span class="p">),</span><span class="w"> </span><span class="nx">_origin</span><span class="p">,</span><span class="w"> </span><span class="nx">byte</span><span class="p">(</span><span class="nx">_nonce</span><span class="p">)));</span>
<span class="p">}</span>
</code></pre></div>
<p>Let's explain these magic numbers.</p>
<ol>
<li>Since we are RLP encoding array <code>[address, nonce]</code>, the first number we have to provide is <code>0xd6</code> = <code>0xc0</code> (for lists of total length less than 55 bytes, as defined in the standard) + <code>0x16</code> (length of the list in our case)</li>
<li><code>0x94</code> = <code>0x80</code> (constant for strings 0-55 bytes long) + <code>0x14</code> (length of address without prefix <code>0x</code>, 20 bytes)</li>
<li>nonce has no RLP prefix since we assume here that it will be smaller than <code>0x7f</code>, otherwise we would need to add it as well as modify the first byte.</li>
</ol>
<p>Since we now know how to calculate future address lets look at how we can get <code>nonce</code>. The nonce is a number that is sequentially incremented after each transaction, starting from 0 in case of wallet addresses.</p>
<p>Contracts are a bit special, their <code>nonce</code> value starts at 1 (since EIP 161) and gets only incremented when a contract creates another contract. To find current nonce we can use <code>etherscan</code> and count number of contract creations (under <code>Internal Txns</code> tab) or we can get the same number using javascript.</p>
<div class="highlight"><pre><span></span><code><span class="nx">web3</span><span class="p">.</span><span class="nx">eth</span><span class="p">.</span><span class="nx">getTransactionCount</span><span class="p">(</span><span class="s2">"0xed0d5160c642492b3b482e006f67679f5b6223a2"</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="nx">err</span><span class="p">,</span><span class="nx">res</span><span class="p">)=>{</span><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">err</span><span class="p">,</span><span class="nx">res</span><span class="p">);})</span>
</code></pre></div>
<p>Looking at the source, we see that we need to calculate new address two times:</p>
<ol>
<li>first find the current nonce of <code>developer</code> contract and calculate the address of the new <code>RainyDayFund</code> contract that will be created.</li>
<li>since <code>RainyDayFund</code> contract will create new contract <code>DebugAuthorizer</code> in the constructor, we need to calculate the address of that contract as well (<code>nonce</code> will be 1 since it is the first contract created from <code>RainyDayFund</code>)</li>
</ol>
<p>After getting both addresses we need to send 1.337 eth to address we calculated for <code>DebugAuthorizer</code> contract and reset challenge from the dashboard to redeploy <code>RainyDayFund</code>. If we did everything correctly we can just call withdraw and win :D</p>
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