hack

// Mifare Ultracard writer
// Written by: Samuel Duclos
// Creative Commons License

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

#define RST_PIN 9
#define SS_PIN 10

char i = 0, k = 0, input[] = {',',',',',',',',',',',',',',','};
byte page = 0, buffer[4];

MFRC522 mfrc522(SS_PIN, RST_PIN);

void setup() {
  Serial.begin(9600);
  SPI.begin();
  mfrc522.PCD_Init();
  Serial.println("Ultracard writer by Samuel Duclos, under Creative Commons License");
  Serial.println("Writings to pages 2 and 3 are permanent!!");
  Serial.println("Enter page# [02-0F]:");
}

void loop() {
  while (!Serial.available());
  Serial.readBytesUntil('\n', input, 6);
  page = (byte)strtol((char *)input, NULL, 16);
  Serial.println("Enter 4 bytes separated by \"ENTER\" strokes:");

  do {
    // for(char j = 0; j < sizeof(input); j++) input[j] = 0;
    memset(input, 0, sizeof(input));
    k = Serial.available();
    while (k == Serial.available());
    Serial.readBytesUntil('\n', input, 6);
    buffer[i] = (byte)strtol((char *)input, NULL, 16);
  } while (++i < 3);

Serial.println("You may now scan the card...");

  while (1) {
    if(!mfrc522.PICC_IsNewCardPresent()) continue;
    if (!mfrc522.PICC_ReadCardSerial()) continue;
    Serial.println(
      mfrc522.GetStatusCodeName(
        mfrc522.MIFARE_Ultralight_Write(
          page, (byte *)buffer, 4)));
  }
}