I'm interested in writing a replacement login screen for Ubuntu that would present the user with a puzzle rather than prompt for a password. I'm looking for some advice on how to go about creating this. I'm a programmer by profession with years of experience, but am not familiar enough with Linux application programming to know how to begin this particular project. Thank you!
You could probably do this as an authentication module for PAM (Linux Pluggable Authentication Modules). PAM is configured in configuration files in /etc/pam.d. Each file in this directory defines a PAM service by specifying a set of PAM modules and how they should work together. You could write a new authentication module and replace the current authentication module in the services where you want to use the new login scheme.