Using SshClient on Linux under Wine throws System.Security.Cryptography.CryptographicException

SSH.NET Version: 2026.0.0
.NET Version: 10.0.11
Wine Version: 11.0

When trying to connect a SshClient from a Windows Application running under Wine, I get the following exception:

System.PlatformNotSupportedException: The specified curve 'Curve25519' or its parameters are not valid for this platform.
---> System.Security.Cryptography.CryptographicException: Unknown error (0x80090029)
at System.Security.Cryptography.CngKey.Create(CngAlgorithm algorithm, String keyName, CngKeyCreationParameters creationParameters)
at System.Security.Cryptography.CngAlgorithmCore.GetOrGenerateKey(Nullable`1 curve)
--- End of inner exception stack trace ---
at System.Security.Cryptography.CngAlgorithmCore.GetOrGenerateKey(Nullable`1 curve)
at System.Security.Cryptography.ECDiffieHellmanCng.GenerateKey(ECCurve curve)
at Renci.SshNet.Security.KeyExchangeECCurve25519.BclImpl.GenerateClientPublicKey()
at Renci.SshNet.Security.KeyExchangeECCurve25519.StartImpl()
at Renci.SshNet.Security.KeyExchangeECCurve25519.Start(Session session, KeyExchangeInitMessage message, Boolean sendClientInitMessage)
at Renci.SshNet.Session.OnKeyExchangeInitReceived(KeyExchangeInitMessage message)
at Renci.SshNet.Messages.Transport.KeyExchangeInitMessage.Process(Session session)
at Renci.SshNet.Session.MessageListener()
--- End of stack trace from previous location ---
at Renci.SshNet.Session.WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout)
at Renci.SshNet.Session.WaitOnHandle(WaitHandle waitHandle)
at Renci.SshNet.Session.Connect()
at Renci.SshNet.BaseClient.CreateAndConnectSession()
at Renci.SshNet.BaseClient.Connect()
at <my code>

Code executed:

var sshClient = new Renci.SshNet.SshClient(hostname, port, username, password);
sshClient.Connect();

I understand this is a Wine specific issue, because the bcrypt.dll / ncrypt.dll are heavily stubbed under Wine and Windows CNG (ECDH kex algorithms) is not fully supported as of right now. So not really a SSH.NET bug.

I was able to circumvent the issue by removing ECDH key exchange algorithms before connecting via my SshClient:

foreach (var algo in sshClient.ConnectionInfo.KeyExchangeAlgorithms.Keys.ToList())
{
    if (!algo.StartsWith("diffie-hellman-group", StringComparison.Ordinal))
        sshClient.ConnectionInfo.KeyExchangeAlgorithms.Remove(algo);
}

Obviously this is not really a good solution.

As a suggestion, it would be nice to have an option to switch to the BouncyCastle implementations via e.g. some config option.

For example in KeyExchangeECCurve25519:

        public override void Start(Session session, KeyExchangeInitMessage message, bool sendClientInitMessage)
        {
            base.Start(session, message, sendClientInitMessage);
#if NET
            if (!_forceBouncyCastleImpl && System.OperatingSystem.IsWindowsVersionAtLeast(10))
            {
                _impl = new BclImpl();
            }
            else
#endif
            {
                _impl = new BouncyCastleImpl();
            }

            StartImpl();
        }

where _forceBouncyCastleImpl is just some placeholder to force using the BouncyCastle implementation that can be somehow configured.

(As a side note: This issues did not occur with older SSH.NET versions or on .NET Framework)

Start at KeyExchangeECCurve25519 and inspect how SshClient exposes ConnectionInfo.KeyExchangeAlgorithms. Review the BclImpl and BouncyCastleImpl selection shown in the issue, then determine how a caller could configure the implementation before connecting. Done means the configured BouncyCastle path can be selected for Wine without manually removing key-exchange algorithms.