Who said anything about using a valid key (or algorithm?)
Take input data.
Determine what you want as output data.
Create a 'decryption' process that will simply convert the first into the latter. If they don't have to reveal the code that does this it doesn't even need to be an algorithm, just a straight switch.
Wouldn't work on a whole disk obviously, but for a single file I can see that fooling a lot of non-technical people.
Biting the hand that feeds IT
