So go ahead and add the user custom fields through the UX?
That’s going to be a challenge since you don’t seem familiar with Rails or Discourse plugins. It does appear that there are hooks for alternate encryption methods, but I’m not quite sure just how easy it’lll be to add another and override the default on in a plugin without some pretty good insight into how Rails and Discourse allow for such overrides.
You and probably do that with webhooks through the UX.