Right now Bitcoin doesn't come under any currency or it cannot be controlled by any Authority or government in any country because it has not been made legal in many countries and that's the reason it is decentralized. But if you are holding the bitcoins in the form of assets for long-term investing or if you are trading the bitcoins for short-term profits then you are getting the capital gains which indirectly means it is the source of income for you and you have to pay a certain amount of tax on the capital gains according to your normal tax slab rate in your country.
If you are from Indian Country then you can make a note of this TWO ARTICLE LINKED BELOW, where everything is clearly explained in detail.
http://www.businesstoday.in/money/tax/bitcoin-gains-tax-cryptocurrency-income-from-other-sources-capital-gains-short-term-gains-income-tax-returns/story/263434.html
https://timesofindia.indiatimes.com/business/india-business/bitcoin-sale-profit-is-taxable-capital-gains-for-investor/articleshow/58691027.cms