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+<title>Day 14 - Advent of Code 2016</title>
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+Oh, hello! Funny seeing you here.
+
+I appreciate your enthusiasm, but you aren't going to find much down here.
+There certainly aren't clues to any of the puzzles. The best surprises don't
+even appear in the source until you unlock them for real.
+
+Please be careful with automated requests; I'm not Google, and I can only take
+so much traffic. Please be considerate so that everyone gets to play.
+
+If you're curious about how Advent of Code works, it's running on some custom
+Perl code. Other than a few integrations (auth, analytics, ads, social media),
+I built the whole thing myself, including the design, animations, prose, and
+all of the puzzles.
+
+The puzzles probably took the longest; the easiest ones were around 45 minutes
+each, but the harder ones took 2-3 hours, and a few even longer than that. A
+lot of effort went into building this thing - I hope you're enjoying playing it
+as much as I enjoyed making it for you!
+
+If you'd like to hang out, I'm @ericwastl on Twitter.
+
+- Eric Wastl
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+-->
+<body>
+<header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2016/about">[About]</a></li><li><a href="/2016/support">[AoC++]</a></li><li><a href="/2016/events">[Events]</a></li><li><a href="/2016/settings">[Settings]</a></li><li><a href="/2016/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <span class="supporter">(AoC++)</span> <span class="star-count">28*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap">sub y{</span><a href="/2016">2016</a><span class="title-event-wrap">}</span></h1><nav><ul><li><a href="/2016">[Calendar]</a></li><li><a href="/2016/leaderboard">[Leaderboard]</a></li><li><a href="/2016/stats">[Stats]</a></li><li><a href="/2016/sponsors">[Sponsors]</a></li></ul></nav></div></header>
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+<div id="sponsor"><div class="quiet">Our <a href="/2016/sponsors">sponsors</a> help make AoC possible:</div><p><a href="http://www.novetta.com/careers/#opportunities" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);">Novetta</a> - Unleash your imagination. Innovate at Novetta.</p></div>
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+<main>
+<article class="day-desc"><h2>--- Day 14: One-Time Pad ---</h2><p>In order to communicate securely with Santa while you're on this mission, you've been using a <a href="https://en.wikipedia.org/wiki/One-time_pad">one-time pad</a> that you <a href="https://en.wikipedia.org/wiki/Security_through_obscurity">generate</a> using a <span title="This also happens to be the plot of World War II.">pre-agreed algorithm</span>. Unfortunately, you've run out of keys in your one-time pad, and so you need to generate some more.</p>
+<p>To generate keys, you first get a stream of random data by taking the <a href="https://en.wikipedia.org/wiki/MD5">MD5</a> of a pre-arranged <a href="https://en.wikipedia.org/wiki/Salt_(cryptography)">salt</a> (your puzzle input) and an increasing integer index (starting with <code>0</code>, and represented in decimal); the resulting MD5 hash should be represented as a string of <em>lowercase</em> hexadecimal digits.</p>
+<p>However, not all of these MD5 hashes are <em>keys</em>, and you need <code>64</code> new keys for your one-time pad. A hash is a key <em>only if</em>:</p>
+<ul>
+<li>It contains <em>three</em> of the same character in a row, like <code>777</code>. Only consider the first such triplet in a hash.</li>
+<li>One of the next <code>1000</code> hashes in the stream contains that same character <em>five</em> times in a row, like <code>77777</code>.</li>
+</ul>
+<p>Considering future hashes for five-of-a-kind sequences does not cause those hashes to be skipped; instead, regardless of whether the current hash is a key, always resume testing for keys starting with the very next hash.</p>
+<p>For example, if the pre-arranged salt is <code>abc</code>:</p>
+<ul>
+<li>The first index which produces a triple is <code>18</code>, because the MD5 hash of <code>abc18</code> contains <code>...cc38887a5...</code>. However, index <code>18</code> does not count as a key for your one-time pad, because none of the next thousand hashes (index <code>19</code> through index <code>1018</code>) contain <code>88888</code>.</li>
+<li>The next index which produces a triple is <code>39</code>; the hash of <code>abc39</code> contains <code>eee</code>. It is also the first key: one of the next thousand hashes (the one at index 816) contains <code>eeeee</code>.</li>
+<li>None of the next six triples are keys, but the one after that, at index <code>92</code>, is: it contains <code>999</code> and index <code>200</code> contains <code>99999</code>.</li>
+<li>Eventually, index <code>22728</code> meets all of the criteria to generate the <code>64</code>th key.</li>
+</ul>
+<p>So, using our example salt of <code>abc</code>, index <code>22728</code> produces the <code>64</code>th key.</p>
+<p>Given the actual salt in your puzzle input, <em>what index</em> produces your <code>64</code>th one-time pad key?</p>
+</article>
+<p>Your puzzle answer was <code>25427</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p>Of course, in order to make this process <a href="https://en.wikipedia.org/wiki/MD5#Security">even more secure</a>, you've also implemented <a href="https://en.wikipedia.org/wiki/Key_stretching">key stretching</a>.</p>
+<p>Key stretching forces attackers to spend more time generating hashes. Unfortunately, it forces everyone else to spend more time, too.</p>
+<p>To implement key stretching, whenever you generate a hash, before you use it, you first find the MD5 hash of that hash, then the MD5 hash of <em>that</em> hash, and so on, a total of <em><code>2016</code> additional hashings</em>. Always use lowercase hexadecimal representations of hashes.</p>
+<p>For example, to find the stretched hash for index <code>0</code> and salt <code>abc</code>:</p>
+<ul>
+<li>Find the MD5 hash of <code>abc0</code>: <code>577571be4de9dcce85a041ba0410f29f</code>.</li>
+<li>Then, find the MD5 hash of that hash: <code>eec80a0c92dc8a0777c619d9bb51e910</code>.</li>
+<li>Then, find the MD5 hash of that hash: <code>16062ce768787384c81fe17a7a60c7e3</code>.</li>
+<li>...repeat many times...</li>
+<li>Then, find the MD5 hash of that hash: <code>a107ff634856bb300138cac6568c0f24</code>.</li>
+</ul>
+<p>So, the stretched hash for index <code>0</code> in this situation is <code>a107ff...</code>. In the end, you find the original hash (one use of MD5), then find the hash-of-the-previous-hash <code>2016</code> times, for a total of <code>2017</code> uses of MD5.</p>
+<p>The rest of the process remains the same, but now the keys are entirely different. Again for salt <code>abc</code>:</p>
+<ul>
+<li>The first triple (<code>222</code>, at index <code>5</code>) has no matching <code>22222</code> in the next thousand hashes.</li>
+<li>The second triple (<code>eee</code>, at index <code>10</code>) hash a matching <code>eeeee</code> at index <code>89</code>, and so it is the first key.</li>
+<li>Eventually, index <code>22551</code> produces the <code>64</code>th key (triple <code>fff</code> with matching <code>fffff</code> at index <code>22859</code>.</li>
+</ul>
+<p>Given the actual salt in your puzzle input and using <code>2016</code> extra MD5 calls of key stretching, <em>what index</em> now produces your <code>64</code>th one-time pad key?</p>
+</article>
+<p>Your puzzle answer was <code>22045</code>.</p><p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
+<p>At this point, you should <a href="/2016">return to your advent calendar</a> and try another puzzle.</p>
+<p>Your puzzle input was <code class="puzzle-input">yjdafjpo</code>.</p>
+<p>You can also <span class="share">[Share<span class="share-content">on
+ <a href="https://twitter.com/intent/tweet?text=I%27ve+completed+%22One%2DTime+Pad%22+%2D+Day+14+%2D+Advent+of+Code+2016&url=http%3A%2F%2Fadventofcode%2Ecom%2F2016%2Fday%2F14&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
+ <a href="https://plus.google.com/share?url=http%3A%2F%2Fadventofcode%2Ecom%2F2016%2Fday%2F14" target="_blank">Google+</a>
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+></span>]</span> this puzzle.</p>
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