Cloudflare speak TLS 1.3 0-RTT with Origin Backend?


I have just started enabling Nginx with TLS 1.3 via BoringSSL on my Cloudflare origin backend servers.

But curious if

  1. Cloudflare have any plans for Edge servers to speak and communicate with origin backends via TLS 1.3 ? And
  2. If so, will we one day be able to speak TLS 1.3 0-RTT between Cloudflare Edge and origin backends ?
  3. If TLS 1.3 0-RTT between Edge and origin is planned, would it be enough for origin backends to implement Cloudflare Authenticated Origin Pulls and CF Full SLL, to protect against 0-RTT replay attacks on the origin backend servers ?



Tls 1.3 &

Hi George,

These are some great questions.

  1. We have supported TLS 1.3 to the origin since draft 22. However, there have been several revisions since then (including the subsequent RFC) and not all drafts are compatible nor do all current versions of crypto libraries support all drafts. If your origin supports draft 22, you’ll be able to use TLS 1.3 between Cloudflare and your origin today. Otherwise, now that BoringSSL has recently added support for the RFC version of TLS 1.3, it’s on our roadmap to update to this version as well.
  2. We don’t currently have 0-RTT enabled between Cloudflare and the origin for two reasons. First, to mitigate any potential for replay attacks and second, the benefits of 0-RTT are less impactful between servers communicating over a high-bandwidth connection. We may revisit this decision in the future.
  3. No, mutually-authenticated TLS connections won’t be enough to mitigate any replay attacks with 0-RTT. The server still needs to guarantee that a ClientHello message with 0-RTT data is only accepted exactly once and the client needs to limit how it retries initiating 0-RTT connections.



Hi @Max

  1. Good to know that if the origin and CF edge servers can speak the same TLS 1.3 version, that it will work :slight_smile: BoringSSL is removing all TLS 1.3 drafts for TLS 1.3 RFC only So looking forward to Cloudflare’s update to TLS 1.3 RFC version for speaking with origin servers.
  2. As yes CF edge server’s would be much closer to origins (with maybe exception of free plan CF users and Sydney/Australia origins where CF doesn’t necessarily use CF Edge from Sydney but Singapore instead ? IIRC CF Sydney edge reserved for higher plan CF plans ?). But 0-RTT would still benefit slow 3G speed connections ? edit: nevermind that question, CF to origin will never have those speeds heh
  3. On nginx origin, would limiting early data 0-RTT to just GET requests be good workaround on nginx origin side ? I guess this would be something a write up at Cloudflare in more detail for what to do to secure origins using TLS 1.3 0-RTT when used with Cloudflare when TLS 1.3 becomes more prevalent on CF customer’s origin servers as well. Of course that is provided if/when CF would enable 0-RTT for communication with origins :slight_smile:



@Max OpenSSL 1.1.1 final release is out now so here’s to seeing Cloudflare talking TLS 1.3 with backends that can speak TLS 1.3 final version :slight_smile:

I am ready for it :slight_smile:

nginx -V
nginx version: nginx/1.15.3 (110918-135811)
built by gcc 8.2.1 20180907 (GCC)
built with OpenSSL 1.1.1 11 Sep 2018
TLS SNI support enabled