Signal is licensed under the AGPLv3 and as such, Free and Open Source.
Discussion
"Signal is licensed under the AGPLv3 and as such, Free and Open Source."
Correct. Unfortunately it's also rigidly centralised. So state-level actors only need to block one set of servers and it's unusable (eg Signal is very hard to use in China due to the Great Firewall). Matrix is a better bet, both E2EE and Free and Open Source software, *and* a decentralised protocol anyone can implement.