OATH TOTP in Opa
OATH is a standard for two factor authentication (i.e. RSA thingy which generates a unique code every minute). The purpose of this code is to demonstrate how to implement things in Opa.
I also implemented Google's KeyUriFormat, so everything is easy with the Google Authenticator app.
Note: if you are going to use this in a real system, you might want to keep track of which tokens was last used (i.e. synchronize clocks)