Let's say I open a vertical option spread for a $100 credit. The stock prices moves how I thought it would, but quicker than I expected, so I close the position early for a $30 debit. I have a net credit of $70. Just for clarity:
+100 opened - 30 closed ---- + 70 net profit
When filing US federal taxes on this income, am I taxed on the $70 net credit or on the $100 initial credit?