Use BCL Curve25519 for Windows 10+

[scott-xu]

Beginning in Windows 10, CNG provides support for Curve25519.
See https://learn.microsoft.com/en-us/windows/win32/seccng/cng-named-elliptic-curves

[Rob-Hague]

lgtm otherwise

[Rob-Hague]

actually, it is not working on my win10 22H2 machine (connected aborted). develop branch works fine, using curve25519-sha256

[scott-xu]

actually, it is not working on my win10 22H2 machine (connected aborted). develop branch works fine, using curve25519-sha256

Based on your description, it does not throw exception when get the named curve. May related to agreement derivation.

[Rob-Hague]

Right, see wireshark.zip. I see the bad capture (on this branch) is sending ECDH init with a key length of 65 bytes (containing 32 trailing zeroes), and the server closes the connection. The good capture (on develop) is sending with 32 byte key, and everything works fine

[Rob-Hague]

this works

diff --git a/src/Renci.SshNet/Security/KeyExchangeEC.BclImpl.cs b/src/Renci.SshNet/Security/KeyExchangeEC.BclImpl.cs
index 54757719..571614f6 100644
--- a/src/Renci.SshNet/Security/KeyExchangeEC.BclImpl.cs
+++ b/src/Renci.SshNet/Security/KeyExchangeEC.BclImpl.cs
@@ -23,17 +23,15 @@ public override byte[] GenerateClientECPoint()

                 var q = _clientECDH.PublicKey.ExportParameters().Q;

-                return EncodeECPoint(q);
+                return q.X;
             }

             public override byte[] CalculateAgreement(byte[] serverECPoint)
             {
-                var q = DecodeECPoint(serverECPoint);
-
                 var parameters = new ECParameters
                 {
                     Curve = _curve,
-                    Q = q,
+                    Q = new ECPoint { X = serverECPoint, Y = new byte[serverECPoint.Length] },
                 };

                 using var serverECDH = ECDiffieHellman.Create(parameters);

[scott-xu]

That's really strange. Not sure why integration test passes.