What amount range are we talking to sponsor a client side encryption / decryption?
I would see it built up as a plugin.
The core implementation could be quite simple, to have separate input fields (Encrypted input fields) so that encryption/decryption is done on the fly for each change either direction. All the data would be reflected to a bubble pseudo hidden input-field, whom would store the changed data in its encrypted form, this pseudo field would have the STANDARD bubble input field implementation for dataflow (real time etc.), just be hidden.
On any server event with changed data coming back to the user from an external change, the pseudo field would notify the encrypt-field through a decrypt function. Either through an event based implementation or as a listener (onchange).
All decryption would be done via a long encryption/decryption key that could be stored on bubble in its encrypted format.
You would only need a function to generate or decrypt that key on bubble front-end and store it /retrieve it temporarily to/from localstorage. Popup with “Enter your password to display encrypted data”.
Alot of this implementation already exists in native JS / jquery.
See here for inspirations:
http://tiddlywiki.com/ (see under encryption)
What do you think?