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    <p>Well, each character of <code>md5</code> is a hex bit. That means it can have one of 16 possible values. So if you're only using the first 4 "hex-bits", that means you can have <code>16 * 16 * 16 * 16</code> or <code>16^4</code> or 65536 or <code>2^16</code> possibilities. </p> <p>So, that means that the total available "space" for results is only 16 bits wide. Now, according to the <a href="http://en.wikipedia.org/wiki/Birthday_attack" rel="nofollow">Birthday Attack/Problem</a>, there are the following chances for collision:</p> <ul> <li><code>50%</code> chance -> <code>300</code> entries</li> <li><code>1%</code> chance -> <code>36</code> entries</li> <li><code>0.0000001%</code> chance -> <code>2</code> entries.</li> </ul> <p>So there is quite a high chance for collisions.</p> <p>Now, you say you need a 4 character hash. Depending on the exact requirements, you can do:</p> <ul> <li>4 hex-bits for <code>16^4</code> (65,536) possible values </li> <li>4 alpha bits for <code>26^4</code> (456,976) possible values </li> <li>4 alpha numeric bits for <code>36^4</code> (1,679,616) possible values </li> <li>4 ascii printable bits for about <code>93^4</code> (74,805,201) possible values (assuming ASCII 33 -> 126)</li> <li>4 full bytes for <code>256^4</code> (4,294,967,296) possible values.</li> </ul> <p>Now, which you choose will depend on the actual use case. Does the hash need to be transmitted to a browser? How are you storing it, etc.</p> <p>I'll give an example of each (In PHP, but should be easy to translate / see what's going on):</p> <p><strong>4 Hex-Bits</strong>:</p> <pre><code>$hash = substr(md5($data), 0, 4); </code></pre> <p><strong>4 Alpha bits</strong>:</p> <pre><code>$hash = substr(base_convert(md5($data), 16, 26)0, 4); $hash = str_replace(range(0, 9), range('S', 'Z'), $hash); </code></pre> <p><strong>4 Alpha Numeric bits</strong>:</p> <pre><code>$hash = substr(base_convert(md5($data), 16, 36), 0, 4); </code></pre> <p><strong>4 Printable Assci Bits</strong>:</p> <pre><code>$hash = hash('md5', $data, true); // We want the raw bytes $out = ''; for ($i = 0; $i &lt; 4; $i++) { $out .= chr((ord($hash[$i]) % 93) + 33); } </code></pre> <p><strong>4 full bytes</strong>:</p> <pre><code>$hash = substr(hash('md5', $data, true), 0, 4); // We want the raw bytes </code></pre>
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    1. POsubstr md5 collision
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    1. COJust a quick bug fix, for your '4 Alpha bits' solution, I think the second line should be: $hash = str_replace(range(0, 9), range('Q', 'Z'), $hash);
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    2. COCome to think of it, the second one gives a-z + Q-Z as a range = 36 ^ 4 possible values. .. and the whole should read: $hash = substr(base_convert(md5($data), 16, 26),0, 4); $hash = str_replace(range(0, 9), range('q', 'z'), $hash);
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