But none of these may be as secure as [mircemk]’s Arduino-based door lock that employs a smartphone’s flashlight as a pass code. At first blush, this seems horribly insecure.

This system was designed for a garage door. The bistable relay serves to disable the communication between the sensor and the Arduino in case there are more than 3 false scanner fingerprints, or ...

This proves that you have access to the secret, and can only be used once. To secure his Lego mini-figures, [Luca] built an authentication system using Google Authenticator and Arduino.