This book examines the use of cryptography in both real and fictional crimes—a topic that is rarely broached. It discusses famous crimes, such as that of the Zodiac Killer, that revolve around cryptic messages and current uses of encryption that make solving cases harder and harder. It then draws parallels with the use of cryptography and secret writing in crime fiction, starting with Edgar Allan Poe and Arthur Conan Doyle, claiming that there is an implicit principle in all such writing—namely, that if the cryptogram is deciphered then the crime itself reveals its structure. The general conclusion drawn is that solving crimes is akin to solving cryptograms, as the crime fiction writers suggested. Cases of cryptographic crime, from unsolved cold cases to the Mafia crimes, are discussed and mapped against this basic theoretical assumption. The book concludes by suggesting that by studying cryptographic crimes the key to understanding crime may be revealed.