I have done quite a fair amount of additional customization work with this plugin and how it works with Gravity Forms. I happen to have built this exact feature which extends GF with an Advanced tab selector to either encrypt the stored data for a field or not, along with several other features including file storage encryption on-the-fly, improvements to hiding the encryption method, and an attempt at making the database stored GF encrypted data far less useful if someone gets it.
In it’s current state, with symmetric encryption, the key is stored in the wp_options table. And the code for running the decryption routines are obviously right here…so I worked around that a bit.
If the plugin author is willing, perhaps we could open it up and co-author / co-support it? I would consider adopting it too.
I would have to spend a bit of time pulling all my changes together and getting them in shape for sharing though. ??