An S-box inversion was an algorithm used in computer decryption.
Day 1 Edit
When Milo Pressman was brought into CTU Los Angeles to decrypt a key card, he asked Jamey Farrell whether she had tried using an S-box inversion on it. She replied that she hadn't, and he told her it was the fastest way to access a file that size. Later, after Jack Bauer had switched the card for a different one, Milo claimed he had computed the inverse but it wasn't working. Jamey suggested that it was because the sectors were not linked, which temporarily solved the problem. ("Day 1: 5:00am-6:00am", "Day 1: 6:00am-7:00am")