Passwords storage class
You don't need to declare password to just use on the next line and never again. Use the literal directly:

self.key = hashlib.sha256('password').digest()


You have some confusing names, like s and iv. Renaming these to be more explicit, like string for s would make it more readable. As would adding docstrings or (less preferably) comments to explain the context of what each function is exactly. At the very least your overall class should have a docstring for a user to know about the general practice of the class.