True, but not by outright cracking the encryption.
They will get your password instead, by implanting your keyboard, putting a camera behind you on the wall, or grabbing you just after you've entered your password.
Good point. Another thing: even if they can decrypt it, they'd save that for Osama types, not ruin it over a small tax case. Otherwise bad guys would stop using it. Maybe decrypt but not use in court...
One tool is parallel construction. First they find out what you did through an illegal/classified method, then they use the benefit of knowing the answer to construct a way to figure out the same information legally.
For example, an agency illegally taps your phone and find out you'll be driving with something illegal in your car along a certain route. The agency then tells some state troopers to notice $your_car driving unsafely at $location and pull you over for a routine traffic stop and search your car.
It's unlikely they'd take even that risk on a tax case, though.
They will get your password instead, by implanting your keyboard, putting a camera behind you on the wall, or grabbing you just after you've entered your password.