Add support for X25519DiffieHellman from BCL on .NET 11.0+ and .NET FW

Directory.Packages.props

@@ -11,7 +11,7 @@
     <PackageVersion Include="GitHubActionsTestLogger" Version="3.0.5" />
     <PackageVersion Include="Meziantou.Analyzer" Version="3.0.125" />
     <!-- Should stay on LTS .NET releases. -->
-    <PackageVersion Include="Microsoft.Bcl.Cryptography" Version="10.0.10" />
+    <PackageVersion Include="Microsoft.Bcl.Cryptography" Version="11.0.0-preview.7.26381.103" />
     <PackageVersion Include="Microsoft.Extensions.Logging.Abstractions" Version="8.0.3" />
     <PackageVersion Include="Microsoft.Extensions.Logging.Console" Version="10.0.10" />
     <PackageVersion Include="MSTest" Version="4.3.3" />

src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl.cs

@@ -1,4 +1,4 @@
-#if NET
+#if NET && !NET11_0_OR_GREATER
 using System.Security.Cryptography;
 
 namespace Renci.SshNet.Security

src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl2.cs

@@ -0,0 +1,41 @@
+#if NETFRAMEWORK || NET11_0_OR_GREATER
+using System.Security.Cryptography;
+
+namespace Renci.SshNet.Security
+{
+    internal partial class KeyExchangeECCurve25519
+    {
+        protected sealed class BclImpl : Impl
+        {
+            private readonly X25519DiffieHellman _clientX25519DH;
+
+            public BclImpl()
+            {
+                _clientX25519DH = X25519DiffieHellman.GenerateKey();
+            }
+
+            public override byte[] GenerateClientPublicKey()
+            {
+                return _clientX25519DH.ExportPublicKey();
+            }
+
+            public override byte[] CalculateAgreement(byte[] serverPublicKey)
+            {
+                using var serverX25519DH = X25519DiffieHellman.ImportPublicKey(serverPublicKey);
+
+                return _clientX25519DH.DeriveRawSecretAgreement(serverX25519DH.ExportPublicKey());
+            }
+
+            protected override void Dispose(bool disposing)
+            {
+                base.Dispose(disposing);
+
+                if (disposing)
+                {
+                    _clientX25519DH.Dispose();
+                }
+            }
+        }
+    }
+}
+#endif

src/Renci.SshNet/Security/KeyExchangeECCurve25519.cs

@@ -8,10 +8,10 @@ namespace Renci.SshNet.Security
     internal partial class KeyExchangeECCurve25519 : KeyExchangeEC
     {
 #pragma warning disable SA1401 // Fields should be private
-#if NET
-        protected Impl _impl;
-#else
+#if NETSTANDARD
         protected BouncyCastleImpl _impl;
+#else
+        protected Impl _impl;
 #endif
 #pragma warning restore SA1401 // Fields should be private
 
@@ -38,12 +38,18 @@ protected override int HashSize
         public override void Start(Session session, KeyExchangeInitMessage message, bool sendClientInitMessage)
         {
             base.Start(session, message, sendClientInitMessage);
-#if NET
+#if NET && !NET11_0_OR_GREATER
             if (System.OperatingSystem.IsWindowsVersionAtLeast(10))
             {
                 _impl = new BclImpl();
             }
             else
+#elif NETFRAMEWORK || NET11_0_OR_GREATER
+            if (X25519DiffieHellman.IsSupported)
+            {
+                _impl = new BclImpl();
+            }
+            else
 #endif
             {
                 _impl = new BouncyCastleImpl();