How Caesar Hid His Messages in Plain Sight
Two thousand years ago, a rider carried a letter across enemy territory, knowing that if he was caught, the letter would be read, the plan would die, and so would he. Julius Caesar had a fix for that. He assumed his letters would be caught. So he wrote them in a way that made reading them useless. The trick was almost embarrassingly simple. When Caesar had something confidential to say, he wrote each letter as the letter three places further down the alphabet. A became D. B became E. The Roman writer Suetonius, who read Caesar's private letters, tells us exactly how it worked: to read one, you substitute the fourth letter of the alphabet, D, for A, and so on. So "ATTACK AT DAWN" became "DWWDFN DW GDZQ". To an enemy who intercepted it, the message looked like gibberish in an unknown language. Most people in the ancient world could barely read their own language, let alone spot a pattern in a scrambled one. The letter could be c...