User Authentication Schemes for Mobile and Handheld Devices

User authentication is a difficulty for every system providing safe access to precious, private information, or personalized services. It is a continual problem, particularly with mobile and handheld devices such as Personal Digital Assistants (PDAs). User authentication is the primary line of defense for a handheld device that comes into the hands of an unauthorized individual. Password or Personal Identification Number (PIN) based authentication is the leading mechanism for verifying the identity of actual device users but this method has been shown to have considerable drawbacks. For example, users tend to pick PIN or passwords that can be easily guessed. On the other hand, if a password is hard to guess, then it is often hard to remember.