title: Breaking SHA1 using a length extension attack
body: <p>I'm implementing this flaw of SHA1:</p>
  
  <p><a href="https://blog.whitehatsec.com/hash-length-extension-attacks/" rel="nofollow">https://blog.whitehatsec.com/hash-length-extension-attacks/</a></p>
  
  <p>I need to SHA1 a chunk of data, then using the resulting hash as the starting vector, I will SHA1 a second chunk of data.</p>
  
  <p>Like:</p>
  
  <pre><code>firstHash = SHA1(fisrtchunk)
  setSHA1StartingVector(firstHash)
  secondHash = SHA1(secondchunk)
  </code></pre>
  
  <h3>Do you any library who let you apply SHA1 using a custom starting vector?</h3>