Add secure mode

config/config.go

 	"fmt"
 	"net"
 	"strconv"
+	"strings"
 
 	"github.com/juju/errors"
 )
 	publicIPv6 net.IP, publicIPv6Port uint16,
 	statsIP net.IP, statsPort uint16,
 	secret, adtag string) (*Config, error) {
+	if strings.HasPrefix(secret, "dd") {
+		secret = secret[2:]
+	}
 	if len(secret) != 32 {
 		return nil, errors.New("Telegram demands secret of length 32")
 	}

mtproto/connection_options.go

 	ConnectionTypeUnknown ConnectionType = iota
 	ConnectionTypeAbridged
 	ConnectionTypeIntermediate
+	ConnectionTypeSecure
 )
 
 // ConnectionProtocol* define which connection protocols to use.
 var (
 	ConnectionTagAbridged     = []byte{0xef, 0xef, 0xef, 0xef}
 	ConnectionTagIntermediate = []byte{0xee, 0xee, 0xee, 0xee}
+	ConnectionTagSecure       = []byte{0xdd, 0xdd, 0xdd, 0xdd}
 )
 
 // Tag maps connection type to the corresponding handshake tag.
 		return ConnectionTagAbridged, nil
 	case ConnectionTypeIntermediate:
 		return ConnectionTagIntermediate, nil
+	case ConnectionTypeSecure:
+		return ConnectionTagSecure, nil
 	default:
 		return nil, errors.Errorf("Unknown connection type %d", t)
 	}
 	if bytes.Equal(magic, ConnectionTagAbridged) {
 		return ConnectionTypeAbridged, nil
 	}
+	if bytes.Equal(magic, ConnectionTagSecure) {
+		return ConnectionTypeSecure, nil
+	}
 
 	return ConnectionTypeUnknown, errors.New("Unknown handshake protocol")
 }