Nice clear answer, thanks.
But just because the cops could get a warrant doesn't mean the suspect really has their evidence.
They can "compel" Professor Plum to produce the candlestick, but if he can't, he can't. And if he can, he's probably sealed his own fate.
So I'm with GP - I don't see how compelling one to provide evidence against herself is anything but compelling self-incriminating testimony. If the cops can't crack the password, their evidence that she had the password should be all they can produce at trial.