So I checked out MIT's puzzle. Why is this called a puzzle? The current solution looks like brute force to me. Admittedly there are a lot of technicality and ingenuity to keep track of the intermediate steps.

The answer is also the encrypted text of a one-time pad. Since one-time pad z is quite long and the way of encryption is xor, the answer must be of a comparable length. Otherwise one can read off quite a lot of info from z.