bool  SocketTlsHandshake(
   int           socket,               // socket
   const string  host                  // host address
   );

//+------------------------------------------------------------------+
//|                                           SocketTlsHandshake.mq5 |
//|                                  Copyright 2024, MetaQuotes Ltd. |
//|                                             https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2024, MetaQuotes Ltd."
#property link      "https://www.mql5.com
#property version     "1.00"

#define   SERVER    "smtp.gmail.com"
#define   PORT      587
//+------------------------------------------------------------------+
//| Switch manually to secure connection                             |
//+------------------------------------------------------------------+
bool TlsHandshake(int socket)
  {
//--- get server greeting
   string rsp;

   if(!RecvString(socket,rsp))
      return(false);
//--- greet server
   if(!SendString(socket,"EHLO my.domain.com\r\n"))
      return(false);
//--- get a server response with a list of supported commands
   if(!RecvString(socket,rsp))
      return(false);
//--- print greeting
   Print("SERVER: ",rsp);
//--- inform the server that we want to switch from an insecure connection to a secure one using TLS
   if(!SendString(socket,"STARTTLS\r\n"))
      return(false);
//--- get server response
   if(!RecvString(socket,rsp))
      return(false);
//--- in the example, we do not check the server response about readiness to switch to TLS ('Ready to start TLS')

//--- initiate a secure TLS (SSL) connection to the specified host using the TLS Handshake protocol
   if(SocketTlsHandshake(socket,InpTimeout))
      return(true);

   Print("SocketTlsHandshake() failed. Error ",GetLastError());
   return(false);
  }
//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart(void)
  {
//--- create a socket and get its handle
   int socket=SocketCreate();

   if(socket==INVALID_HANDLE)
     {
      Print("SocketCreate() failed. Error ",GetLastError());
      return;
     }
//--- connect to SERVER via PORT
   if(!SocketConnect(socket,SERVER,PORT,10000))
     {
      Print("SocketConnect() failed. Error ",GetLastError());
     }
   else
     {
      //--- insecure connection established
      PrintFormat("%s connection has been established to %s:%d",(PORT==443 ? "A secured" : "An unsecured"),SERVER,PORT);
      //--- switch to secure connection
      if(PORT!=443 && TlsHandshake(socket))
        {
         PrintFormat("Unsecured connection to %s:%d switched to secured",SERVER,PORT);
         //--- if connection is protected by certificate, display its data
         string   subject,issuer,serial,thumbprint;
         datetime expiration;

         if(SocketTlsCertificate(socket,subject,issuer,serial,thumbprint,expiration))
           {
            Print("TLS certificate:");
            Print("   Owner:      ",subject);
            Print("   Issuer:     ",issuer);
            Print("   Number:     ",serial);
            Print("   Print:      ",thumbprint);
            Print("   Expiration: ",expiration);
           }
        }
     }
//--- close the socket after use
   SocketClose(socket);
   /*
   result:
   An unsecured connection has been established to smtp.gmail.com:587
   SERVER: 220 smtp.gmail.com ESMTP a640c23a62f3a-a9b1f298319sm82305866b.105 - gsmtp

   SERVER: 250-smtp.gmail.com at your service, [37.193.40.122]
   250-SIZE 35882577
   250-8BITMIME
   250-STARTTLS
   250-ENHANCEDSTATUSCODES
   250-PIPELINING
   250-CHUNKING
   250 SMTPUTF8

   SERVER: 220 2.0.0 Ready to start TLS

   SocketTlsHandshake(): A secure connection to smtp.gmail.com:587 is now established
   TLS certificate:
      Owner:  /CN=smtp.gmail.com
      Issuer:  /C=US/O=Google Trust Services/CN=WR2
      Number:     1f:f4:db:2a:5a:e6:dc:52:0a:4c:05:ce:81:cc:c3:f7
      Print: d6be8af229b5329cd3d4c2789c02aa94f89b421c
      Expiration: 2024.12.30 08:25:30
   */
  }
//+------------------------------------------------------------------+
//| Send string to server                                            |
//+------------------------------------------------------------------+
bool SendString(int socket,const string str)
  {
//--- convert the string to a character array
   uchar data[];
   int   size=StringToCharArray(str,data,0,str.Length(),CP_UTF8);
//--- send data to the socket
   ResetLastError();
   if(SocketSend(socket,data,size)==size)
      return(true);
//--- data sending error
   Print("Failed to send data to server. Error ",GetLastError());
   return false;
  }
//+------------------------------------------------------------------+
//| Get a string from the server                                     |
//+------------------------------------------------------------------+
bool RecvString(int socket,string& result,uint timeout_ms=1000)
  {
//--- wait for data to appear on the socket
   ulong wait_time_end=GetMicrosecondCount()+timeout_ms*1000;

   while(!SocketIsReadable(socket))
     {
      Sleep(10);
      //--- timed out waiting for data - return NULL as response
      if(wait_time_end<GetMicrosecondCount())
        {
         Print("ERROR: No response from server");
         return(false);
        }
     }
//--- read data from socket
   uchar  data[128];
   uint   size=0;
   string resp=NULL;

   do
     {
      uchar b[1];
      int   n=SocketRead(socket,b,1,1000);

      if(n < 0)
         break;

      if(n)
        {
         data[size++]=b[0];

         if(size==data.Size())
           {
            resp += CharArrayToString(data,0,data.Size(),CP_UTF8);
            size = 0;
           }
        }
     }
   while(SocketIsReadable(socket));
//--- copy the read data into the string
   if(size)
      resp+=CharArrayToString(data,0,size,CP_UTF8);
//--- if string is empty, then error
   if(!resp.Length())
     {
      Print("ERROR: No response from server");
      return(false);
     }
//--- return the string
   result=resp;
   return(true);
  }