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copied!<p>Forget about the endians: it's just a way name for a way to encode information.</p> <p>Let's follow the wikipedia <a href="http://en.wikipedia.org/wiki/MD5#Algorithm" rel="nofollow noreferrer">MD5</a> article. You start with an input message. It can be arbitrarily long: MD5 hashes for 2GB ISO files are routinely created, just like hashes for strings a dozen characters long (e.g. for passwords).</p> <p>The hash will be contained in registers <code>a</code> , <code>b</code>, <code>c</code> and <code>d</code>. These registers are initialized with special values (<code>h0-h3</code>).</p> <p>The algorithm breaks the input into 16 4-byte chunks ("sixteen 32-bit little-endian words") and applies specific logical operations (functions <code>F</code>, <code>G</code>, <code>H</code> and <code>I</code>) on parts of the input and the current state of registers <code>a</code> , <code>b</code>, <code>c</code> and <code>d</code>. It does this 64 times for each set of 16 4-byte chunks.</p> <p>When all of the chunks are processed, what remains in <code>a</code> , <code>b</code>, <code>c</code> and <code>d</code> is the final hash, the one you might get by invoking <code>md5sum testfile.txt</code>.</p> <p>Update:</p> <p>If you just want to be able to calculate a hash, implementing it yourself makes no sense because it's been done and tested for probably every significant language out there:</p> <p><a href="http://docs.python.org/library/md5.html" rel="nofollow noreferrer">Python</a>:</p> <pre><code>import md5 md5.new("Nobody inspects the spammish repetition").digest() </code></pre> <p>SQL (<a href="http://dev.mysql.com/doc/refman/5.1/en/encryption-functions.html#function_md5" rel="nofollow noreferrer">MySQL</a>):</p> <pre><code>SELECT MD5('Nobody inspects the spammish repetition') </code></pre> <p><a href="http://snippets.dzone.com/posts/show/3686" rel="nofollow noreferrer">Java</a>:</p> <pre><code>String s="Nobody inspects the spammish repetition"; MessageDigest m=MessageDigest.getInstance("MD5"); m.update(s.getBytes(),0,s.length()); System.out.println(new BigInteger(1,m.digest()).toString(16)); </code></pre> <p>etc.</p>

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