bouncy.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Sockets;
using System.Text;
using System.Threading.Tasks;
using Org.BouncyCastle.Math;
using Org.BouncyCastle.Crypto.Tls;

namespace test_ssl_bouncy
{
    class Program
    {        
        static void Main(string[] args)
        {
            TcpClient client = new TcpClient("api.telegram.org", 443);
            NetworkStream targetStream = client.GetStream();
            targetStream.ReadTimeout = 10000;
            targetStream.WriteTimeout = 10000;

            TlsClientProtocol protocol = new TlsClientProtocol(targetStream, new Org.BouncyCastle.Security.SecureRandom());

            protocol.Connect(new MyTlsClient()); // error 71 "insufficient_security"
        }
        
        private class MyTlsClient : DefaultTlsClient
        {
            public override TlsAuthentication GetAuthentication() => new MyTlsAuthentication();
            public override int[] GetCipherSuites() => new[] { CipherSuite.TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 };
            public override void NotifyAlertRaised(byte alertLevel, byte alertDescription, string message, Exception cause)
            {
                base.NotifyAlertRaised(alertLevel, alertDescription, message, cause);
                Console.WriteLine("AlertLevel: " + alertLevel);
                Console.WriteLine("AlertDescription: " + alertDescription);
                Console.WriteLine("Message: " + message);
                Console.WriteLine("Exception: " + cause);
            }
        }

        private class MyTlsAuthentication : TlsAuthentication
        {
            public TlsCredentials GetClientCredentials(CertificateRequest certificateRequest) => null;
            public void NotifyServerCertificate(Certificate serverCertificate) { }
        }
    }
}