I think it is wrong to call bitcoin a "private" currency.
On the contrary, imo bitcoin is much more of a public currency that any national currency is.
It seems to me that Bitcoin is not a privatr currency, Why? For me the definition of private meaning only you or a few people can know about Bitcoin like. But if you says Public meaning broad people know about Bitcoin so it was. We can not call Bitcoin as a private currency but a public currency because many people knew about this king of currency and some people now used it.