Problem is that they don't need to crack, can be an unlucky collision. Chances are small but they are still there if there is no way to guarantee a unique secret. But how does SyncApp know which computer has the files beloning to my secret? If I enter my secret on another computer the original computers need to know what computer was added and how to reach that computer. And it wouldn't be less secure if it used two secrets. Compare it to having an account for each computer to log on to and from which to manage sharing folders with other accounts and a seperate secret that is also needed to access folders but isn't stored centrally. It will take decades for a super computer to guess that specific secret but it can take but a second for a random computer to accidentally generate a secret that is allready in use somewhere else. Give a thousand monkeys a thousand typewriters and there will be a day when a monkey writes Hamlet. With no guarantee that a secret is unique there will always be a change, however small, that someone will get your files.