code: def to_string(self):
          
          data = self.ndef_str
          if self.ndef_type == _NDEF_URI_TYPE:
              data = self._encode_ndef_uri_type(data)
          elif self.ndef_type == _NDEF_TEXT_TYPE:
              data = self._encode_ndef_text_params(data)
          if len(data) > _NDEF_DATA_SIZE:
              raise YubiKeyNEO_USBHIDError("NDEF payload too long")
          # typedef struct {
          #   unsigned char len;                  // Payload length
          #   unsigned char type;                 // NDEF type specifier
          #   unsigned char data[NDEF_DATA_SIZE]; // Payload size
          #   unsigned char curAccCode[ACC_CODE_SIZE]; // Access code
          # } YKNDEF;
          #
          fmt = '< B B %ss %ss' % (_NDEF_DATA_SIZE, _ACC_CODE_SIZE)
          first = struct.pack(fmt,
                              len(data),
                              self.ndef_type,
                              data.ljust(_NDEF_DATA_SIZE, b'\0'),
                              self.access_code,
                              )
          #crc = 0xffff - yubico_util.crc16(first)
          #second = first + struct.pack('<H', crc) + self.unlock_code
          return first
docs: Return the current NDEF as a string (always 64 bytes).