flag.java

import java.io.Console;

public class Oracle {
  private static final int FLAG_LENGTH = 42;
  
  private static final byte[] CHECK = new byte[] { 
      48, 6, 122, -86, -73, -59, 78, 84, 105, -119, 
      -36, -118, 70, 17, 101, -85, 55, -38, -91, 32, 
      -18, -107, 53, 99, -74, 67, 89, 120, -41, 122, 
      -100, -70, 34, -111, 21, Byte.MIN_VALUE, 78, 27, 123, -103, 
      36, 87 };
  
  private static byte[] numbers;
  
  private static void firstPass() {
    for (byte b = 0; b < 42; b++)
      numbers[b] = (byte)(numbers[b] ^ 3 * b * b + 5 * b + 101 + b % 2); 
  }
  
  private static void secondPass() {
    byte[] arrayOfByte = new byte[42];
    for (byte b = 0; b < 42; b++)
      arrayOfByte[b] = (byte)(numbers[(b + 42 - 1) % 42] << 4 | (numbers[b] & 0xFF) >> 4); 
    numbers = arrayOfByte;
  }
  
  private static void thirdPass() {
    for (byte b = 0; b < 42; b++)
      numbers[b] = (byte)(numbers[b] + 7 * b * b + 31 * b + 127 + b % 2); 
  }
  
  private static void fail() {
    System.out.println("\nThat's not the flag. Try again.");
    System.exit(1);
  }
  
  public static void main(String[] paramArrayOfString) {
    Console console = System.console();
    //numbers = console.readLine("Enter flag: ", new Object[0]).getBytes();
    numbers = paramArrayOfString[0].getBytes();
    if (numbers.length != 42)
      fail(); 
    firstPass();
    secondPass();
    thirdPass();
    int i = 0;
    for (byte b = 0; b < 42; b++) {
      System.out.print(numbers[b]+" ");
      i |= CHECK[b] ^ numbers[b]; 
    }
    if (i != 0)
      fail(); 
    System.out.println("\nGood job. You found the flag!");
  }
}