I have a C# application making a successful TCP TLS 1.0 connection with mutual authentication to another company's server. It is implemented using SslStream class. We are just one of many clients of this very large organisation.
This TCP link above must undergo TLS 1.2 + SHA2 upgrade. After doing all necessary steps on our side and successful testing with our local servers we are still failing all attempts to connect to the remote server. A lengthy investigation revealed that during TLS Handshake the server is sending to us a certificate request with only option for Signature Hash Algorithm = SHA1-RSA (see picture below). Our cerificate is SHA256. As a result, SslStream is not sending our certificate to the server at all and the server sees this as a Handshake Failure and closes down the connection.
The reason for this nuisance is that our counterparty uses very old (10.x) version of F5 firewall to terminate SSL. It only sends SHA1/RSA Signature Hash Algorithm in the certificate request although it supports the client’s SHA2 certificates.
While admitting the facts above, our counterparty is unable to upgrade F5 soon enough. They suggested to ignore the requested Signature Hash Algorithm and send our SHA256 certificate anyway. Apparently other clients connecting to them were able to do that somehow.
Unfortunately,.Net's SslStream does not provide that level of fine tuning for TLS handshake.
Hence the question: is it possible to ignore the server's request for SHA1RSA-based certificate at all? What options do I have? Are there SslStream alternatives that implement TLS 1.2? Is there an open-source third-party solution? Any suggestion would be helpful. Thanks in advance.
