This sounds like you want to implement some kind of password manager similar to Keepass or 1Password, where users can store tons of passwords and encrypt the whole thing with one single master password, right?
Well, if the master password is then reasonably long and difficult to guess or crack, then this is a reasonable thing to do.
When I would have such a task on my list, I would start reading tons of books on cryptography and cryptanalysis, or even taking a good crypto course, simply out of fear that I might end up producing only snake oil security - something that looks good but hardly secures anything.