Thanks for the tip Ovid. I use a similar scheme sometimes myself. I learned about this from Writing Apache modules, page 213. In the explanation, they say that you should run the MD5 algorithm twice or "Otherwise, a technically savvy user could take advantage of one of the mathematical properties of the algorithm to append his own data to the end of the fields". They give this example: 
$MAC = MD5->hashhex($secret .
              MD5->hashex(join '',$secret, @fields) );

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In reply to MD5 should be run twice by markjugg
in thread functions for passing variables through multiple CGI forms by markjugg

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