Knock Lock – Unlock a Door with a Secret Knock (2009)

http://news.ycombinator.com/item?id=924237

http://news.ycombinator.com/item?id=927299

I wonder how easy a similar set-up with a Raspberry Pi would be. The option to use the same hardware, but provide a web service to control the door as well, or other authentication methods would be cool to see.