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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.

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    I've never heard of any game being "protected" like this. What games are supposed to have used it? But a DRM system like you described would be easy to crack, just copy the game after it's been unlocked. They'd need to use some actual copy protection method (eg. key disks) to prevent copying, and during the 90's these were all cracked. – Ross Ridge Jun 12 at 3:04
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    Besides, the product from the ads you linked is using a parallel-port dongle. The right to run the software is granted by this physical object that you had to plug (and having this dongle was the only requirement to run the software, which could be copied from machine to machine). It is not a scheme where you send a code to the software company and get back some sort of license file. So, either the ads are for another product, or you are mistaken on the way it worked. – dim Jun 12 at 12:01
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    Software Sentinel was the dongle in the ad, yes. I want to say that one of the companies I worked for in the late 1980s or early 1990s used it. IIRC, there was a C library that we had to link in to our software. Depending on which of my former employers it was, we charged our customers between $200 and $2000 for the software. That was in ~1990 US dollars, so that would be roughly $400 to $4000 in 2018. I bet there's an old issue of Byte that describes how it works. – shoover Jun 12 at 18:43
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    @RossRidge One accessible example of a CD with a decrypt-on-purchase system -- though not the Rainbow Technologies one the question is about -- is on the official Quake shareware release (archive.org/details/cdrom-quake-shareware). The point is not copy protection -- this was mostly used to deliver games where, as here, the regular full retail releases didn't use copy protection either. – rakslice Jun 13 at 9:30
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    @rakslice Apparently it used some sort of week symmetric encryption: doomworld.com/forum/topic/… – Ross Ridge Jun 13 at 13:29

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