Context: I'm working on a web application that would deal with user data, and I'm looking for some best practices and tips for respecting the privacy of the user as much as possible. Ideally, I'd like all data to be completely encrypted client side so that it would be almost impossible for me (or a hacker) to mishandle the data, but I know little about what technologies could help me with this goal.

What are some things I can do to ensure user data remains theirs and is never leaked to malicious actors?