title: Length extension attack doubts
body: <p>So I've been studying this concept of length extension attacks and there are few things that I noticed during my study about it which are not very bright to me.</p>
  <p>1.Research papers are explaining how you can append some type of data to the end and make newly formed data. For example</p>
  <p>Desired New Data: count=10&amp;lat=37.351&amp;user_id=1&amp;long=-119.827&amp;<strong>waffle</strong>=eggo&amp;<strong>waffle=liege</strong></p>
  <p>(notice 2 waffles). My question is if a parser function on the server side can track duplicate attributes, could then the entire length extension attack be nonsense? Because the server would notice duplicate attributes. Is a proper parser that is made to check any duplicates a good solution versus length extension attacks? I'm aware of HMAC approach and other protections, but specifically talking just about parsers here now.</p>
  <p>2.Research says that only vulnerable data is H(key|message). They claim that H(message|key) won't work for the attacker because we would have to append a new key (which we obviously don't know). My question is why would we have to append a new key? We don't do it when we are attacking H(key|message). Why can't we rely on the fact that we will pass the verification test (we would create the correct hash) and that if the parser tries to extract the key from it, that it would take the only key in the block we send out and resume from there? Why would we have to send 2 keys? Why doesn't attack against H(message|key) work?</p>