code: private static byte[] concat(byte[] a, byte[] b)
      {
          byte[] c = new byte[a.length + b.length];
          System.arraycopy(a, 0, c, 0, a.length);
          System.arraycopy(b, 0, c, a.length, b.length);
  
          return c;
      }
docs: Concatenates two byte arrays and returns the resulting byte array.
  
  @param a  first byte array
  @param b  second byte array
  @return  byte array containing first and second byte array