Does a TLS client needs to support one of the named groups (curves?) supported by the server for TLS handshake to succeed?
I'm running a web site using Windows Server 2016. The web site has thousands of users and provides a REST API which many users interact with from various environments. Some of them are running older software like Windows XP and what not.
To make the web site more secure, I'm placing a web application firewall (WAF) in front of it to detect various attacks (like SQL injection and similar).
Historically when changing TLS configuration on the server, I've had various issues with clients who can no longer connect, for example due to mismatch in cipher support, old TLS/SSL-versions and the certificate chain not beeing fully included in the server response.
I've used Qualys SSL labs to compare the TLS configuration in the WAF with that of the old Windows Server 2016. The only thing which differs is that the old server supports a curve named "x25519" while the WAF does not. The old server also supports secp256r1, secp384r1 which is supported by the WAF.
The WAF instead support secp256r1, secp521r1, brainpoolP512r1, brainpoolP384r1, secp384r1, brainpoolP256r1, secp256k1, sect571r1, sect571k1, sect409k1, sect409r1, sect283k1, sect283r1.
What I'm trying to understand is whether I risk breaking backwards compatibility with clients if I remove the support for x25519.