Options: SecurityOptions options = new SecurityOptions(SecureProtocol.Ssl3);
options.Certificate = GetCertificate();
options.Entity = ConnectionEnd.Client;
options.CommonName = "server";
options.VerificationType = CredentialVerification.Manual;
options.Verifier = new CertVerifyEventHandler(CheckServerCertificate);
options.Flags = SecurityFlags.MutualAuthentication;
options.AllowedAlgorithms = SslAlgorithms.SECURE_CIPHERS;

When the IIS does not require client certificate, all is OK, but otherwise it seems, that is something wrong with the certificate (Loaded is OK, contains private key).
I've also try this in Java and all is OK.
Could someone help?
Thanks
Paul

In options I've set
options.AllowedAlgorithms = SslAlgorithms.SECURE_CIPHERS;, but the instance of SecureSocket has property ActiveEncryption set to "RSA_RC4_128_MD5" ????

Paul