A number of games from the early 1990s appear to be protected with a DRM system developed by Rainbow Technologies Ltd. The system allows content in the games to be unlocked by sending the manufacturer a code and getting one in return.
The system appears to make use of information in this patent.
In 1998 it advertised the ability to sell software over the internet (typically shipped on CD, only the unlock codes were sent via the internet).
It appears to be some kind of public key crypto. The game/app has a private key and an "install key" that is generated during installation. The user sends the install key plus money to the manufacturer, and they send back an unlock key that unlocks selected features.
I have been unable to find any more information on the system, how it works and if it was ever cracked.