two tickets for the danish public transport system, for residents and for tourists, with some desired and undesired side-effect, depending what your interests are:

officially tourists should aim for the single ticket with rfid: called "milfare ultralight". no cryptography, but also more expensive than the normal one and no discount for seniors, but after all that seems less important.

the other ticket solution seems to be for residents: provided with the "milfare classic chip", cryptography is added as money can be stored: anyhow: hacked

The first reported attack was designed by two students at the University of Amsterdam, Pieter Siekerman and Maurits van der Schee. They analyzed the single-use ticket and showed its vulnerabilites in a report. They also showed how a used single-use card could be given eternal life by resetting it to its original "unused" state. For this work they won the Joop Bautz award. | Andrew S. Tanenbaum