A password is a secret word or string of characters used to authenticate a user's identity and allow access to a protected resource. Passwords have been used for centuries, with sentries challenging those seeking entry to supply a password. Common passwords today are used to access operating systems, phones, TVs, ATMs and more. A 2016 study found the most commonly used but least secure passwords were "123456", "password", and variations using numbers. Strong passwords contain a mix of uppercase and lowercase letters, numbers, and symbols, and are at least 15 characters long. Methods for creating strong, memorable passwords include deriving them from personal facts, patterns, or prime numbers.