I&#x27;m not a crypto master, but as I understand it, while it&#x27;s possible to find md5 collision, it&#x27;s many times more difficult in hmac, because it&#x27;s:<p><pre><code>    hash((secret+pad1) | hash((secret+pad2) | message))
</code></pre>
so you would have to find a collision of one key that matches collision of another key, so you&#x27;re back to relying on basic birthday attack rather than any specific hash weakness.<p>If you&#x27;re looking for an actual proof, it&#x27;s at <a href="http:&#x2F;&#x2F;cseweb.ucsd.edu&#x2F;~mihir&#x2F;papers&#x2F;hmac-new.html" rel="nofollow">http:&#x2F;&#x2F;cseweb.ucsd.edu&#x2F;~mihir&#x2F;papers&#x2F;hmac-new.html</a>