A coded passage in “Kryptos,” Jim Sanborn’s sculpture at CIA headquarters, has gone unsolved for decades. After he announced its auction, two researchers stumbled on the answer.

It is clear that the sprawling city of Teotihuacan near Mexico City was a major metropolis of the ancient world, but what do ...

A single leaked hint exposed the CIA’s most puzzling code. But dozens of the world’s toughest ciphers are still waiting to be ...