We’ve published the core of Authinator3 as open source under the ISC license: gitlab.com/jowolf/authinator3.

Authinator is a web login gateway for firewalls. Authenticate over HTTPS, click once, and your current IP address is granted passage through the firewall - everyone else sees a wall. It replaces both permanently-open management ports and brittle port-knocking schemes with something anyone can operate from a phone: a login page.

Authinator has guarded our own production gateways in one form or another since 2007.