It&#x27;s a collision if you can choose the IV. That&#x27;s not much practical use, it&#x27;s just an interesting demonstration that confirms research is slowly progressing towards a real collision.<p>SHA-1 uses a Merkle-Damgard construction, which means the input is split into fixed-size blocks and the output from one block becomes the IV for the next. This is because SHA-1 operates on a fixed-size, so if you have more than 1 block of data, it&#x27;s chained together like this:<p><pre><code>  char *SHA1(char *IV, char *block);

  Digest = SHA1(SHA1(SHA1(StartIV, block1), block2), block3);
</code></pre>
A freestart collision is a collision if you can choose the IV. That&#x27;s not much use, because we don&#x27;t know how to find a block (or chain of blocks) that produce the IV we want.