If you want to enter a 'password' then you need to read a sequence of keys and buffer them until the password is complete, and then validate it. You will need some way to tell when the sequence of entering the password starts and ends. For example, you might require the user to press the right sequence of keys within a time limit. You probably also want to provide some way to cancel the last key entered, or cancel the password entry completely. In order to be usable, I think you need to give the user feedback when you have started password input, when you have accepted a keypress as part of a password, and when the password has passed or failed validation, If you don't want to use a display then a sounder or flashing LED would be alternative options.