[tor-relays] Compatibility issue with OpenSSL 1.1.1a

Hi, folks!

You should know that there is a compatibility issue between Tor and OpenSSL 1.1.1a, when TLS 1.3 is in use. Only OpenSSL 1.1.1a is affected; other OpenSSL versions are not. The effect here is that Tor relays using this version of OpenSSL will not be able to negotiate TLS 1.3 connections with one another.

This is caused by a regression in OpenSSL 1.1.1a's implementation of tls13_hkdf_expand() function. For more information, see https://trac.torproject.org/projects/tor/ticket/28616

We're looking into possible mitigations.

best wishes,