Interest that you pay to purchase investments is a deductible interest expense on form 4952. It is an itemized tax deduction on schedule A, and it is one of the few itemized tax deductions that was not eliminated in the tax reform act of 2017.
However, the amount of the deduction is limited to the amount of income you report from those investments. If you are holding the cryptocurrency, and have not sold it, so you have not realized the capital gains, you will have no actual interest deduction. However, you still report the interest on form 4952 so it carries forward year to year, and you can take the deduction when and if you sell the investment for a taxable capital gain.
