The plugin stores all license keys in an encrypted fashion inside the database. During activation the plugin will create the following directory.
Inside this directory, the plugin creates two files (defuse.txt and secret.txt). The contents of these files are used to encrypt, decrypt, and hash all license keys.
Before doing anything else, please create a backup of these two files and store them somewhere safe. If anything happens, and these files are deleted or overwritten, you will permanently lose all license keys stored in your database!