Hi Narayana,
Qdabra's Electronic Signature Template Part doesn't support encrypting the data in SharePoint because that would require decrypting it in the form which would require yet another PIN or password! :).
I believe the documentation just recommends setting up permissions on the SharePoint list to only allow the owner to see their row including the PIN. In the form itself, you can hide the PIN by changing the foreground text color of the PIN field text to white (same as background), or changing the font to a symbol or wingdif font. I believe there is also provide an asterisk font too.