I am doing a project in cryptography and while reading a tag generation algorithm for files, I got stuck with the meaning of $\Pi$. The screenshot of the algorithm is inserted below.

![Fig](https://i.stack.imgur.com/lou8d.jpg)

Here the *KeyGen* algorithm generates 1 public key and 2 secret keys and the *TagGen* algorithm takes a file $M$ (which is divided into $n$ blocks which is again divided into $s$ sectors) and the secret keys generated by the KeyGen algorithm. TagGen has to compute tags for blocks of the file $M$. I am stuck with how the data tags are computed here.