If you can persuade the court that you really don't have the password or decryption key they asked for, then it's not an offense not to provide it.
If you really have forgotten it but the court concludes that on the balance of probabilities you are lying about that, _then_ you are boned. Especially if there really wasn't any child porn on your computer and remembering the password would have let you prove that.
If you are in a game where you are playing/betting against other players, not just the house, and you are better than the other players by more than the house margin, then you can win in the long run. Many of the people who think they are in that situation are idiots, but it is possible.