From d0961a7767de94fa36aabf1c9c679a16db73cb1b Mon Sep 17 00:00:00 2001
From: Scott Xu <scott-xu@msn.com>
Date: Tue, 25 Aug 2026 22:36:26 +0800
Subject: [PATCH] Add support for X25519DiffieHellman from BCL on .NET 11.0+
 and .NET Framework

---
 Directory.Packages.props                      |  2 +-
 .../KeyExchangeECCurve25519.BclImpl.cs        |  2 +-
 .../KeyExchangeECCurve25519.BclImpl2.cs       | 41 +++++++++++++++++++
 .../Security/KeyExchangeECCurve25519.cs       | 14 +++++--
 4 files changed, 53 insertions(+), 6 deletions(-)
 create mode 100644 src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl2.cs

diff --git a/Directory.Packages.props b/Directory.Packages.props
index 0ffc049d3..c3af9d182 100644
--- a/Directory.Packages.props
+++ b/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" />
diff --git a/src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl.cs b/src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl.cs
index de3ab55e2..83badc0e4 100644
--- a/src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl.cs
+++ b/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
diff --git a/src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl2.cs b/src/Renci.SshNet/Security/KeyExchangeECCurve25519.BclImpl2.cs
new file mode 100644
index 000000000..95e05a713
--- /dev/null
+++ b/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
diff --git a/src/Renci.SshNet/Security/KeyExchangeECCurve25519.cs b/src/Renci.SshNet/Security/KeyExchangeECCurve25519.cs
index 3ce8d1f81..873a46103 100644
--- a/src/Renci.SshNet/Security/KeyExchangeECCurve25519.cs
+++ b/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();