For 35 years, amateur and professional cryptographers have tried to crack the code on Kryptos, a majestic sculpture that sits behind CIA headquarters in Langley, Virginia. In the 1990s, the CIA, NSA, and a Rand Corporation computer scientist independently came up with translations for three of the sculpture’s four panels of scrambled letters. But the final segment, known as K4, was encoded with knottier techniques and remains unsolved. This failure has only deepened the obsession of thousands of...