I made this plugin for myself https://encrypt-decrypt-lists.bubbleapps.io/version-test
where the project required to encrypt lists of messages in the DB to go in a repeating group. (eg these were chat messages). It’s really no different to all the other Encryption/Decryption plugins except it takes lists - which is what you need for Repeating Groups.
Otherwise I’d have to do what @jared.gibb 
suggests 
which is totally viable.
The example page shows you how you might show a list of decrypted messages in a repeating group (using server side actions, custom states). All the other caveats and warnings folks are advising in this thread hold true. At the end of the day, your secret key is in Bubble itself, where you are also storing the data.
If you really want to understand Bubble security - read @petter security book.
http://forum.bubble.io/t/the-ultimate-guide-to-bubble-security-is-out-300-pages-of-privacy-and-security-content/200580