MFRC522: RFID cards and key fobs on an RC522

// RFID door: two of the four cards open it, the other two do not.
//
// Tap a card beside the reader to lay it on the antenna. The sketch reads its
// UID, looks it up in the list below, and lights the green LED for a card on
// the list or the red one for a card that is not. Every UID is printed, so a
// new card is added by tapping it, copying its UID from the serial monitor
// into ALLOWED, and running again.
//
// PICC_HaltA() at the end puts the card to sleep: it stays quiet until it
// leaves the field. That is why a card left on the reader opens the door once
// and not once a loop. Tap it off and on again to read it again.

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

MFRC522 reader(10, 9); // SDA (chip select) on 10, RST on 9

const int GREEN_LED = 7;
const int RED_LED = 6;

// The cards that open the door: the white card and the blue key fob.
const byte ALLOWED[][4] = {
    {0xC3, 0x5A, 0x1E, 0x94},
    {0x7B, 0x02, 0xD4, 0x19},
};
const int ALLOWED_COUNT = sizeof(ALLOWED) / sizeof(ALLOWED[0]);

bool onTheList()
{
    if (reader.uid.size != 4) {
        return false; // the seven-byte sticker is never on it
    }
    for (int card = 0; card < ALLOWED_COUNT; card++) {
        bool same = true;
        for (int i = 0; i < 4; i++) {
            if (reader.uid.uidByte[i] != ALLOWED[card][i]) {
                same = false;
            }
        }
        if (same) {
            return true;
        }
    }
    return false;
}

void setup()
{
    Serial.begin(9600);
    pinMode(GREEN_LED, OUTPUT);
    pinMode(RED_LED, OUTPUT);
    SPI.begin();
    reader.PCD_Init();
    reader.PCD_DumpVersionToSerial();
    Serial.println("Tap a card on the reader.");
}

void loop()
{
    if (!reader.PICC_IsNewCardPresent() || !reader.PICC_ReadCardSerial()) {
        return;
    }
    Serial.print("UID:");
    for (byte i = 0; i < reader.uid.size; i++) {
        Serial.print(reader.uid.uidByte[i] < 0x10 ? " 0" : " ");
        Serial.print(reader.uid.uidByte[i], HEX);
    }
    Serial.println();

    int led = onTheList() ? GREEN_LED : RED_LED;
    Serial.println(led == GREEN_LED ? "Welcome in." : "Not this card.");
    reader.PICC_HaltA();

    digitalWrite(led, HIGH);
    delay(1500);
    digitalWrite(led, LOW);
}