Thoroughly wash and dry the limes, then zest them.
Sprinkle the zest onto wax paper or parchment paper, then cover loosely with a paper towel. Let sit at room temperature until completely dry (about 1 day).
Add chili powder, cumin, oregano and salt to a mortar.
Add the dried lime zest (you should have about 1 teaspoon), then grind everything with a pestle until well combined.
Store in an airtight container at room temperature until ready to use.
Recipe notes:
*One medium-sized lime will yield a little more than 1 teaspoon of fresh lime zest. This will be a little more than ½ teaspoon when dried.