When to ask for login?

I own an online store for video games and use the Stripe API for handling payments.

This is the current order of events: Add to cart → Checkout → Enter Card Info → Charge → Download Item. To battle piracy, I recently installed a login page into the games to detect whether the user has bought them.

Because of this, I need to get game buyers to login, but due to limitations in my installation of Stripe, I can't put the login between the checkout and enter card info before checkout.

In terms of UX, which other place would be the best place to ask the user to login?