Hi Miguel,
I would do the encryption on the Server side in C#. https://dzone.com/articles/json-web-encryption-jwe-in-net-core
Once sent over to the client, you can maybe decrypt the json by having a .Net DLL on the client side with a function that does this. Use the DLL Importer Tool to easily get access to that .Net function.
I'm not sure if Appeon is thinking of enhancing the JSON generation with encryption possibilities. (I think I read that somewhere, but maybe it was not even a post about Powerbuilder).
saludos,
MiguelL