Another one for my Waganono collection :) Your serial number "decrypts" (xor) a ciphered message in the crackme. The crackme verifies proper decryption by using a keyed hash message authentication code. You get a French message and a song as a reward! You can see patterns in the ciphertext that clue you to the length of the key. Next, individual bytes of the key can be toggled to maximize the amount of "normal" text in trial decryptions.