def to_string(self):
        """
        Return the current NDEF as a string (always 64 bytes).
        """
        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
Return the current NDEF as a string (always 64 bytes).