The challenge name itself is a hint. Cross means X. So it's XOR.
By xoring the first few bytes with the flag format you can guess the key: `VERYSECUREKEY` and decrypt the rest:
```RaziCTF{d0nt_r3pe4T_kEy_1N_X0R} You cannot be buried in obscurity , you are exposed upon a grand theater to the view of the world . If your actions are upright and benevolent , be assured they will augment your power and happiness.```