At this moment, it seems that we can buy the crypto currencies and store them in the various wallets or on the exchanges. The only references in these wallets are the public keys and private keys.
If our transactions occur only via such keys on the internet, with no relation to our current physical world, then privacy is preserved. An example would be purchasing one crypto currency with another crypto currency.
What if we bought a physical good or service? Say buying a house, a car or insurance. Then personal data will need to be provide to register ownership under the current physical world (eg getting title to the house or car).
Should the seller make a profit or gain in selling the house, wouldn't the government want to apply their existing tax laws to collect their tax revenue on the gain? What about their wanting to collect Sales Tax (also called VAT or GST)?
I understand that these maybe transitional issues. But they do seem to give the current authorities various desires and opportunities to try to gain "control" (whatever the extent) over the usage of cryptos.
When I see the new developments, such as smart contracts, I begin to worry that the authorities maybe waiting for the various technologies, apps, coins, tokens to mature and settle to a mature enough stage for them to try implementing control due to the inter-connection between the crypto world and the current physical world.
Perhaps my worry maybe misplaced. But I do not see much written on this aspect.
I saw Bix Wier (no matter whether you may agree with his theories) talked about The Good Guys vs The Bad Guys. He seems to have considered that a totally "complete parallel system" must be designed and implemented by The Good Guys before they will crashed the current Bad Guys system. If so, then there's still quite a way to go.
I'm pretty sure you pay income tax for any profit made from crypto currencies. The US Government is treating it like a commodity such as gold and silver. Any profits made in USD is taxed as income tax. But contact your tax professional because there may be a different tax bracket.
The State is quickly catching on. http://www.zerohedge.com/news/2017-07-25/sec-cracks-down-initial-coin-offerings-concludes-tokens-are-subject-securities-laws