Access control: RFID card opens a stepper door, on an Arduino Uno

// An access control system: the right card opens the door.
//
//   RC522 SDA (SS) -> pin 10   SCK -> 13   MOSI -> 11   MISO -> 12   RST -> 9
//   RC522 3.3V -> 3.3V   GND -> GND
//   stepper driver IN1..IN4 -> pins 2, 3, 4, 5   + -> 5V   - -> GND
//   I2C LCD GND -> GND   VCC -> 5V   SDA -> A4   SCL -> A5
//
// The door knows one card, the first one on the reader's list. A known card
// turns the stepper a quarter turn (512 of its 2048 steps), waits, and turns
// it back; any other card is refused. Stepper.h steps the motor's four coils
// in order; the Stepper wants its pins as IN1, IN3, IN2, IN4.
//
// Click the reader and hold a card to it.

#include <SPI.h>
#include <MFRC522.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include <Stepper.h>

const int SS_PIN = 10;
const int RST_PIN = 9;
const int STEPS_PER_TURN = 2048;
const int DOOR_STEPS = 512;
const byte KNOWN[4] = {0xC3, 0x5A, 0x1E, 0x94};

MFRC522 reader(SS_PIN, RST_PIN);
LiquidCrystal_I2C lcd(0x27, 16, 2);
Stepper door(STEPS_PER_TURN, 2, 4, 3, 5);

void prompt() {
  lcd.clear();
  lcd.print("Scan your card");
}

bool known(const byte *uid, byte size) {
  if (size != 4) return false;
  for (byte i = 0; i < 4; i++) {
    if (uid[i] != KNOWN[i]) return false;
  }
  return true;
}

void setup() {
  Serial.begin(9600);
  SPI.begin();
  reader.PCD_Init();
  door.setSpeed(10);
  lcd.init();
  lcd.backlight();
  prompt();
  Serial.println("Mokxi Uno: access control. Card C3 5A 1E 94 opens the door.");
}

void loop() {
  if (!reader.PICC_IsNewCardPresent() || !reader.PICC_ReadCardSerial()) return;
  lcd.clear();
  if (known(reader.uid.uidByte, reader.uid.size)) {
    lcd.print("Access granted");
    Serial.println("access granted: opening");
    door.step(DOOR_STEPS);
    delay(2000);
    door.step(-DOOR_STEPS);
  } else {
    lcd.print("Access denied");
    Serial.println("access denied");
    delay(2000);
  }
  reader.PICC_HaltA();
  reader.PCD_StopCrypto1();
  prompt();
}