I'm betting there's probably something that generates the key from a vastly smaller player input, i.e what gameobjects you interacted with, in what order, or what did you press/place somwhere. But that also means that the entropy is probably in the bruteforcable range, and once you find the function that decrypts the secrets, it should be pretty easy to find the function that generates the key, and the inputs it takes.
When handling passwords, it is standard practice to use an intentionally costly (in CPU, memory, or both) algorithm to derive the encryption key from the password. Maybe the dev can reuse this? The resulting delay could easily be masked with some animation.