Yes. Cryptocurrencies are taxable according to this IRS memo. They are taxable as income if you sell them less than 1 year (365 days) of buying them. They are taxable as long-term capital gains (capital gains for short) if you sell them after 1 year of buying them. They are not taxable if you never sell the crypto assets that you buy. All of this applies to US citizens at home and abroad. Can you interpret this IRS memo as unofficial and unenforced, and not pay taxes for 2017? At your own risk.

https://medium.com/@david_97757/the-ten-commandments-of-crypto-taxes-8ab9c532c500