you cannot expect player to input a key for the game to run. In fact it's even worse, when the user knows the key he/she can hack the resouces easier. Key HAS TO be in the exe.

In fact if the algorithm requires a key, some way other it will be used somewhere in your app, which if you look from this point, is always hackable.

It's rather more logical to use a harcoded function as key instead of a number/string.

Even after all this, files will be decrypted to ram, which defies the use of the encryption.