Any source. But again they'll be levying tax on both exchange and withdrawal to fiat, and the exchange between crypto to crypto is hard to track.
Wow, they would be getting a lot of money that way. Not only do they get money from the exchange, they get some from us every time we cash out. Feels rather unfair considering we got no help from them when we risked our money buying something that financial institutions are warning against. And then when we make it big they get a slice of the pie....
Gov't trying to tax withdrawal even more might just be what will convince people to just use digital currencies for purchases rather than exchange them for fiat.