http: add support for client certificates

Add support for automatically sending client TLS certificates using the
Git configuration setting 'http.sslAutoClientCert'.

This setting is currently only present in Git for Windows, and there is
only respected when the SSL backend is "schannel".

src/shared/Core/Constants.cs

@@ -158,6 +158,7 @@ public static class Http
                 public const string SslBackend = "sslBackend";
                 public const string SslVerify = "sslVerify";
                 public const string SslCaInfo = "sslCAInfo";
+                public const string SslAutoClientCert = "sslAutoClientCert";
             }
 
             public static class Remote

src/shared/Core/HttpClientFactory.cs

@@ -76,6 +76,20 @@ public HttpClient CreateClient()
                 handler = new HttpClientHandler();
             }
 
+            // Trace Git's chosen SSL/TLS backend
+            _trace.WriteLine($"Git's SSL/TLS backend is: {_settings.TlsBackend}");
+
+            // Mirror Git for Windows and only send client TLS certificates automatically if we're using
+            // the schannel backend _and_ the user has opted in to sending them.
+            if (_settings.TlsBackend == TlsBackend.Schannel &&
+                _settings.AutomaticallyUseClientCertificates)
+            {
+                _trace.WriteLine("Configured to automatically send TLS client certificates.");
+                handler.ClientCertificateOptions = ClientCertificateOption.Automatic;
+            }
+
+            // Configure server certificate verification and warn if we're bypassing validation
+
             // IsCertificateVerificationEnabled takes precedence over custom TLS cert verification
             if (!_settings.IsCertificateVerificationEnabled)
             {

src/shared/Core/Settings.cs

@@ -119,6 +119,11 @@ public interface ISettings : IDisposable
         /// </summary>
         bool IsCertificateVerificationEnabled { get; }
 
+        /// <summary>
+        /// Automatically send client TLS certificates.
+        /// </summary>
+        bool AutomaticallyUseClientCertificates { get; }
+
         /// <summary>
         /// Get the proxy setting if configured, or null otherwise.
         /// </summary>
@@ -563,6 +568,9 @@ public bool IsCertificateVerificationEnabled
             }
         }
 
+        public bool AutomaticallyUseClientCertificates =>
+            !TryGetSetting(null, KnownGitCfg.Credential.SectionName, KnownGitCfg.Http.SslAutoClientCert, out string value) && value.ToBooleanyOrDefault(false);
+
         public string CustomCertificateBundlePath =>
             TryGetPathSetting(KnownEnvars.GitSslCaInfo, KnownGitCfg.Http.SectionName, KnownGitCfg.Http.SslCaInfo, out string value) ? value : null;
 

src/shared/TestInfrastructure/Objects/TestSettings.cs

@@ -31,6 +31,8 @@ public class TestSettings : ISettings
 
         public bool IsCertificateVerificationEnabled { get; set; } = true;
 
+        public bool AutomaticallyUseClientCertificates { get; set; }
+
         public ProxyConfiguration ProxyConfiguration { get; set; }
 
         public string ParentWindowId { get; set; }