Implementing and tracking software licensing

I am looking at introducing a licencing scheme for my software, and trying to be sensible about how to do so. I know I will need to have ways of producing product licences locked to machines, testing the licence at set points in program execution, and keep track of all customers and the required details. I have been studying several open source licensing solutions, and each have different ways of operating. One will use an ECDSA, one uses RSA, another seems to use self signed certificates. Some implement XML license files, others use the FlexLM format. All seem to use a variety of different ways of identifying the target licensed system.

I'd prefer not to hand roll something completely from first principles, but I don't want to use just any implementation and be stuck to it. The common denominators seem to be:

• Open standard private-public key
• Unique machine identifier
• Supplier side license generator

What would you consider the minimum specification for a licensing system, what standards, file formats, libraries, trackers etc.?