Включить tls 1.2 Android 4.X - Xamarin Android -

Question

Мне нужно включить TLS 1.2 на устройствах до версии 5.0.Я реализовал TLSSocketFactory:

public class TLSSocketFactory : SSLSocketFactory
{
......
}

Реализация TlSSocketFactory

и реализовал AndroidCustomClientHandler

AndroidCustomClientHandler

Мой запрос к серверу:

_client.SendAsync(aRequest, cancellationToken.Token);

aRequest является типом HttpRequestMessage.

Я получаю исключение:

Javax.Net.Ssl.SSLHandshakeException: 
java.security.cert.CertPathValidatorException: Trust anchor for ` . 
certification path not found. --->` 
Java.Security.Cert.CertificateException: 
java.security.cert.CertPathValidatorException: Trust anchor for certification path not found. ---> Java.Security.Cert.CertPathValidatorException: Trust anchor for certification path not found.
   --- End of inner exception stack trace ---
   --- End of inner exception stack trace ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at Java.Interop.JniEnvironment+InstanceMethods.CallVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x00069] in <7802aa64ad574c33adca332a3fa9706a>:0 
  at Java.Interop.JniPeerMembers+JniInstanceMethods.InvokeAbstractVoidMethod (System.String encodedMember, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x00014] in <7802aa64ad574c33adca332a3fa9706a>:0 
  at Javax.Net.Ssl.HttpsURLConnectionInvoker.Connect () [0x00000] in /Users/builder/data/lanes/5945/dffc5912/source/monodroid/external/xamarin-android/src/Mono.Android/obj/Release/android-26/mcw/Javax.Net.Ssl.HttpsURLConnection.cs:417 
  at Xamarin.Android.Net.AndroidClientHandler+<>c__DisplayClass42_0.<ConnectAsync>b__0 () [0x0005a] in /Users/builder/data/lanes/5945/dffc5912/source/monodroid/external/xamarin-android/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs:308 
  at System.Threading.Tasks.Task.InnerInvoke () [0x0000f] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Threading.Tasks.Task.Execute () [0x00010] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x0003e] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.ConfiguredTaskAwaitable+ConfiguredTaskAwaiter.GetResult () [0x00000] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at Xamarin.Android.Net.AndroidClientHandler+<DoProcessRequest>d__44.MoveNext () [0x000e4] in /Users/builder/data/lanes/5945/dffc5912/source/monodroid/external/xamarin-android/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs:357 
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x0003e] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1+ConfiguredTaskAwaiter[TResult].GetResult () [0x00000] in <f32579baafc1404fa37ba3ec1abdc0bd>:0 
  at Xamarin.Android.Net.AndroidClientHandler+<SendAsync>d__39.MoveNext () [0x001e2] in /Users/builder/data/lanes/5945/dffc5912/source/monodroid/external/xamarin-android/src/Mono.Android/Xamarin.Android.Net/AndroidClientHandler.cs:275 

Как мне решитьпроблема ??Большое спасибо.