Equinux blog:

Apple is known for doing things with more attention to detail than most companies. So it should come as no surprise that even App Store gift cards with their promo codes have a few secret details that help make the experience more Apple-like.

So what powers the simple App Store promo codes? Secret fonts, special dimensions, and many more.

Today, we uncover these secrets.

And:

Apple’s App Store gift cards have a special trick: you can simply hold one up to your iPhone or Mac’s camera and it’ll automatically scan in the code and redeem the card for you. As developers, we thought it’d be cool to print some of our own promo code cards to give away at events, so we tried to create our own scannable cards. Turns out, there’s more to it than meets the eye…

This is some fascinating reverse engineering. My concern is that the post exposes font details that might be used to break Apple’s carefully built promo card system. If so, I’d expect a pretty rapid response by Apple.