Signal Protocol is a modern, open source, strong encryption protocol for asynchronous messaging
systems. We use Signal Protocol as the foundation for our own private messaging app, Signal, but
also make Signal Protocol libraries available for other applications to use.
Our Signal Protocol libraries are open source, licensed GPLv3. We like the GPL for the quality control that it
provides. If someone publicly says that they’re using our software, we want to see if they’ve made
any modifications, and whether they’re using it correctly. This helps to increase transparency and
accountability in deployments of our software, which we feel are important for end-to-end encryption.