Equinux, maker of VPN Tracker, Mail Designer Pro, and other products has a fascinating look at the tech behind App Store gift and promo code cards that can be scanned and redeemed with an iPhone or Mac’s camera. The company wanted to make scannable cards of its own to hand out at events, so they began investigating how the alphanumeric codes found on the back of the cards work. It turns out there’s quite a bit to it.