How to store encrypted data?

I'm new in ruby on rails, and i'm developing and application that will have very sensitive data (api keys from other websites) and i need to store it encrypted in a db but without knowing them anytime. Let me explain myself: -The form ask the user for his api keys -Encrypt them -Store it in the db

The main question is, how i encrypt them in such a way i can use it later (still without knowing them)

Sorry if the question is silly, but i can't find a way to do it, and thanks