Every SHA1 hash starts with an initalization vector, which is a kind of ur-hash:<p><pre><code>    67452301EFCDAB8998BADCFE10325476C3D2E1F0
</code></pre>
The function of SHA1 is to transform this ur-hash into the hash of a specific stream of data.<p>In a freestart collision, attackers start from something other than the initialization vector. Think of it as if they&#x27;re starting from the hash of some piece of data and then hashing more data into it (that&#x27;s essentially what they&#x27;d be doing).