Many people have now heard of the EFF-backed free certificate authority Let's Encrypt. Not only is it free of charge, it has also introduced a fully automated mechanism for certificate renewals, eliminating a tedious chore that has imposed upon busy sysadmins everywhere for many years.

These two benefits - elimination of cost and elimination of annual maintenance effort - imply that server operators can now deploy certificates for far more services than they would have previously.

The TLS chapter of the RTC Quick Start Guide has been updated with details about Let's Encrypt so anybody installing SIP or XMPP can use Let's Encrypt from the outset.

For example, somebody hosting basic Drupal or Wordpress sites for family, friends and small community organizations can now offer them all full HTTPS encryption, WebRTC, SIP and XMPP without having to explain annual renewal fees or worry about losing time in their evenings and weekends renewing certificates manually.

Even people who were willing to pay for a single certificate for their main web site may have snubbed their nose at the expense and ongoing effort of having certificates for their SMTP mail server, IMAP server, VPN gateway, SIP proxy, XMPP server, WebSocket and TURN servers too. Now they can all have certificates.

Early efforts at SIP were doomed without encryption

In the early days, SIP messages would be transported across the public Internet in UDP datagrams without any encryption. SIP itself wasn't originally designed for NAT and a variety of home routers were created with "NAT helper" algorithms that would detect and modify SIP packets to try and work through NAT. Sadly, in many cases these attempts to help actually clash with each other and lead to further instability. Conversely, many rogue ISPs could easily detect and punish VoIP users by blocking their calls or even cutting their DSL line. Operating SIP over TLS, usually on the HTTPS port (TCP port 443) has been an effective way to quash all of these different issues.

While the example of SIP is one of the most extreme, it helps demonstrate the benefits of making encryption universal to ensure stability and cut out the "man-in-the-middle", regardless of whether he is trying to help or hinder the end user.

Is one certificate enough?

Modern SIP, XMPP and WebRTC require additional services, TURN servers and WebSocket servers. If they are all operated on port 443 then it is necessary to use different hostnames for each of them (e.g. turn.example.org and ws.example.org. Each different hostname requires a certificate. Let's Encrypt can provide those additional certificates too, without additional cost or effort.

The future with Let's Encrypt

The initial version of the Let's Encrypt client, certbot, fully automates the workflow for people using popular web servers such as Apache and nginx. The manual or certonly modes can be used for other services but hopefully certbot will evolve to integrate with many other popular applications too.

Currently, Let's Encrypt's certbot tool issues certificates to servers running on TCP port 443 or 80. These are considered to be a privileged ports whereas any port over 1023, including the default ports used by applications such as SIP (5061), XMPP (5222, 5269) and TURN (5349), are not privileged ports. As long as certbot maintains this policy, it is generally necessary to either run a web server for the domain associated with each certificate or run the services themselves on port 443. There are other mechanisms for domain validation and various other clients supporting different subsets of them. Running the services themselves on port 443 turns out to be a good idea anyway as it ensures that RTC services can be reached through HTTP proxy servers who fail to let the HTTP CONNECT method access any other ports.

Many configuration tasks are already scripted during the installation of packages on a GNU/Linux distribution (such as Debian or Fedora) or when setting up services using cloud images (for example, in Docker or OpenStack). Due to the heavily standardized nature of Let's Encrypt and the widespread availability of the tools, many of these package installation scripts can be easily adapted to find or create Let's Encrypt certificates on the target system, ensuring every service is running with TLS protection from the minute it goes live.

If you have questions about Let's Encrypt for RTC or want to share your experiences, please come and discuss it on the Free-RTC mailing list.