Is zakat due on the principal only, or on the principal and its profits?

If the capital reaches the nisab and a year passes over it, zakat becomes obligatory on it. The profit generated from it is also subject to zakat along with it, provided that the profit was gained through lawful means. However, if the profit was usurious (interest-based), one must repent from it and return the excess amount to its rightful owners if they are known. Otherwise, it should be spent on charitable causes. Zakat is not obligatory on such usurious profit due to the lack of complete ownership.