not an expert on tax laws, but I think they don't need to know what is inside to tax it. It is your own responsibility to report everything as accurate as you can. But if one day in the future they know, for some reason, something doesn't match with what you reported, then get ready for the consequences. 

quark

privacy

Do you close the door of your house with your key when you go out?

Thanks for the analogy. The thing is even if you have the keys for your house and you lock it, you will pay taxes. If bitcoin remains fully private, no way to tax it as we don’t know who belong to. 