Opportunistic Encryption For Firefox

Firefox 37 brings more encryption to the web through opportunistic encryption of some http:// based resources. It will be released the week of March 31st.

OE provides unauthenticated encryption over TLS for data that would otherwise be carried via clear text. This creates some confidentiality in the face of passive eavesdropping, and also provides you much better integrity protection for your data than raw TCP does when dealing with random network noise. The server setup for it is trivial.

These are indeed nice bonuses for http:// - but it still isn't as nice as https://. If you can run https you should - full stop. Don't make me repeat it :) Only https protects you from active man in the middle attackers.

But if you have long tail of legacy content that you cannot yet get migrated to https, commonly due to mixed-content rules and interactions with third parties, OE provides a mechanism for an encrypted transport of http:// data. That's a strict improvement over the cleartext alternative.

Two simple steps to configure a server for OE

1. Install a TLS based h2 or spdy server on a separate port. 443 is a good choice :). You can use a self-signed certificate if you like because OE is not authenticated.
2. Add a response header Alt-Svc: h2=":443" or spdy/3.1 if you are using a spdy enabled server like nginx. 

When the browser consumes that response header it will start to verify the fact that there is a HTTP/2 service on port 443. When a session with that port is established it will start routing the requests it would normally send in cleartext to port 80 onto port 443 with encryption instead. There will be no delay in responsiveness because the new connection is fully established in the background before being used. If the alternative service (port 443) becomes unavailable or cannot be verified Firefox will automatically return to using cleartext on port 80. Clients that don't speak the right protocols just ignore the header and continue to use port 80.

This mapping is saved and used in the future. It is important to understand that while the transaction is being routed to a different port the origin of the resource hasn't changed (i.e. if the cleartext origin was http://www.example.com:80 then the origin, including the http scheme and the port 80, are unchanged even if it routed to port 443 over TLS). OE is not available with HTTP/1 servers because that protocol does not carry the scheme as part of each transaction which is a necessary ingredient for the Alt-Svc approach.

You can control some details about how long the Alt-Svc mappings last and some other details. The Internet-Draft is helpful as a reference. As the technology matures we will be tracking it;  the recent HTTP working group meeting in Dallas decided this was ready to proceed to last call status in the working group.