RFID authentication

#include <SPI.h>
#include <MFRC522.h>

#define SS_PIN 10
#define RST_PIN 9
 
MFRC522 rfid(SS_PIN, RST_PIN); // Instance of the class

byte nuidPICC[4];
String Rfid_Serial;

void setup() { 
  Serial.begin(9600);
  SPI.begin(); 
  rfid.PCD_Init();
}
 
void loop() {

  if ( ! rfid.PICC_IsNewCardPresent())
    return;

  if ( ! rfid.PICC_ReadCardSerial())
    return;

  if (rfid.uid.uidByte[0] != nuidPICC[0] || 
      rfid.uid.uidByte[1] != nuidPICC[1] || 
      rfid.uid.uidByte[2] != nuidPICC[2] || 
      rfid.uid.uidByte[3] != nuidPICC[3] ) 
      {  for (byte i = 0; i < 4; i++) {  nuidPICC[i] = rfid.uid.uidByte[i];  }
   
      printDec(rfid.uid.uidByte, rfid.uid.size);
      Serial.println(Rfid_Serial);
  }
  rfid.PICC_HaltA();
  rfid.PCD_StopCrypto1();
}

void printDec(byte *buffer, byte bufferSize) {
  Rfid_Serial="";
  for (byte i = 0; i < bufferSize; i++) 
  {
    Rfid_Serial=Rfid_Serial + buffer[i];
  }
}