If you AES-256 decrypt it with the ST-0001 Key you get the following message.
The final challenge is intended as a small reward for the cleverness and creativity of individuals and clans during the game thus far. Please follow these instructions carefully to avoid leaking any information to rival hunters.
The first challenge rewards the individual or clan with the most keys. To submit, please message https://keybase.io/toshitreasure with all of the keys you collected over the course of the game, one key per line. The clan or individual with the most keys submitted by Friday, October 16th at midnight EST will be contacted via the Keybase address the keys were sent from to receive this prize.
The secondary challenge rewards the individuals who received unique keys during the course of the game. To submit, please message https://keybase.io/toshitreasure with any unique keys you collected, one key per line. Each clan or individual who submits a unique key will also be contacted via the Keybase address the keys were sent from to receive this prize. If you aren't sure whether a key is unique or not, or if you have a key which may have only been found by you despite potentially being discoverable by multiple hunters, do submit! Any key which is only received once will be eligible for a prize.
Thanks again to everyone who played. Losing one of our core members was devastating for our team, and we also deeply regret that we couldn't make this game into the ongoing adventure we had hoped for at the outset. We're grateful to have gotten to have some amazing moments with hunters and puzzles, and wish you all the best in future puzzles and hunts!