From 22564bb2a332e6455afd0ea66780044ffcbf035b Mon Sep 17 00:00:00 2001
From: Kenneth Hsu <kennethhsu@gmail.com>
Date: Tue, 18 May 2021 09:17:13 -0700
Subject: Cache knowledge of ECDSA support.

---
 Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs | 54 +++++++++++++++----------
 1 file changed, 32 insertions(+), 22 deletions(-)

(limited to 'Duplicati/Library/Backend')

diff --git a/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs b/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs
index 3237d8a4e..1fca6aad7 100644
--- a/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs
+++ b/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs
@@ -59,6 +59,8 @@ namespace Duplicati.Library.Backend
 
         private SftpClient m_con;
 
+        private readonly Lazy<bool> supportsECDSA = new Lazy<bool>(SSHv2.SupportsECDSA, true);
+
         public SSHv2()
         {
         }
@@ -334,32 +336,40 @@ namespace Duplicati.Library.Backend
             m_con = con;
         }
 
+        /// <summary>
+        /// SSH.NET relies on the System.Security.Cryptography.ECDsaCng class for
+        /// ECDSA algorithms, which is not implemented in Mono (as of 6.12.0.144).
+        /// This prevents clients from connecting if one of the ECDSA algorithms is
+        /// chosen as the host key algorithm.  In this case, we will prevent the
+        /// client from advertising support for ECDSA algorithms.
+        /// </summary>
+        /// <seealso href="https://github.com/mono/mono/blob/mono-6.12.0.144/mcs/class/referencesource/System.Core/System/Security/Cryptography/ECDsaCng.cs">Mono ECDsaCng implementation.</seealso>
+        /// <returns>Whether ECDSA algorithms are supported or not.</returns>
+        private static bool SupportsECDSA()
+        {
+            try
+            {
+                ECDsaCng unused = new ECDsaCng();
+                return true;
+            }
+            catch (NotImplementedException)
+            {
+                return false;
+            }
+            catch
+            {
+                return true;
+            }
+        }
+
         private void TryConnect(SftpClient client)
         {
-            if (Utility.Utility.IsMono)
+            if (!this.supportsECDSA.Value)
             {
-                // SSH.NET relies on the System.Security.Cryptography.ECDsaCng class for
-                // ECDSA algorithms, which is not implemented in Mono (as of 6.12.0.144).
-                // This prevents clients from connecting if one of the ECDSA algorithms is
-                // chosen as the host key algorithm.  In this case, we will prevent the
-                // client from advertising support for ECDSA algorithms.
-                //
-                // See https://github.com/mono/mono/blob/mono-6.12.0.144/mcs/class/referencesource/System.Core/System/Security/Cryptography/ECDsaCng.cs
-                try
-                {
-                    ECDsaCng unused = new ECDsaCng();
-                }
-                catch (NotImplementedException)
-                {
-                    List<string> ecdsaKeys = client.ConnectionInfo.HostKeyAlgorithms.Keys.Where(x => x.StartsWith("ecdsa")).ToList();
-                    foreach (string key in ecdsaKeys)
-                    {
-                        client.ConnectionInfo.HostKeyAlgorithms.Remove(key);
-                    }
-                }
-                catch
+                List<string> ecdsaKeys = client.ConnectionInfo.HostKeyAlgorithms.Keys.Where(x => x.StartsWith("ecdsa")).ToList();
+                foreach (string key in ecdsaKeys)
                 {
-                    // Ignore other exceptions and assume that we have ECDSA support.
+                    client.ConnectionInfo.HostKeyAlgorithms.Remove(key);
                 }
             }
 
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