This challenge provides an introduction to utilizing a crib! The cipher is
designed to encrypt four bytes at a time, which are combined with a "counter".
This string is XORed with a secret key, and then output. Here, due to the setup,
we are able to utilize the knowledge that the first four characters are wctf
to immediately find the secret key! We know that the first eight bytes are
wctf\x00\x00\x00\x00 XORed with the key, so by reversing the XOR, we can find
the key and decrypt all subsequent blocks.