Keygen.sh developed a good API based licensing service in the cloud which they later open sourced and worked out well for them (the main dev blogged about it a

Keygen.sh developed a good API based licensing service in the cloud which they later open sourced and worked out well for them (the main dev blogged about it and explained the reasoning. I think with security products like this, open sourcing can be a good idea as it allows users to inspect the code and feel more confident that it is indeed secure. Plus you get feedback (and maybe even help) you wouldn’t otherwise get. And you can charge for services, etc even if open source (if you want).