ENCYCLOPEDIA OF Espionage, Intelligence, and Security Volume ...

Codes and CiphersA 1968 miniature Kroger’s codebook containing a series of numbers that was used by spies to decode messages from Moscow, displayed beside an enlargedphotocopy of the text. ©HULTON-DEUTSCH COLLECTION/CORBIS.the invention of ciphering machines in the early twentiethcentury; codes and simple ciphers were the only feasiblemethods of ciphering. Yet, a cipher that is simple toimplement is proportionately simple to crack, and a crackedcipher can be disastrous. It is better to have to communicate”in the clear“—to send messages that can be easilyread by the enemy—than to suppose that one’s communicationsare secret when they are not. Mary, Queen of Scots(1542–1567) was executed for treason on the basis ofdeciphered letters that frankly discussed plans for murderingQueen Elizabeth of England; likewise, simple ciphersused by the Confederacy during the U.S. Civil Warwere easily cracked by Union cryptographers. What ismore, even more sophisticated ciphers, such as the Enigmacipher used by Nazi Germany during World War II orimplemented today on digital computers, are subject toattack. As soon as any new cipher is invented, someone,somewhere starts attacking it. The result is that ciphers,like some antibiotics, have limited lifespans, and must beregularly replaced.Historical perspective. Throughout much of the ancientworld, writing was either completely unknown or was anarcane art accessible only to priests. There was littlemotive, therefore, to develop coding or ciphering. Eventually,however, writing came to serve military, personal,and commercial as well as sacred purposes, creating aneed for secure communications. To meet this need, ciphersbased on scrambling the order of plaintext charactersor on substituting other characters for them weredeveloped. The first recorded use of ciphering was by theGreek general Lysander in the fifth century B.C. The Kamasutra,a Hindu text compiled in the A.D. fourth century frommanuscripts dating back as far as the fourth century B.C.,recommends monoalphabetic substitution ciphering—thereplacement of each letter of a plaintext message with adifferent letter of the alphabet—as one of the 64 arts to bemastered by an ideally-educated woman. By the firstcentury B.C., codes had also been developed.Cryptography fell out of use during the early MiddleAges, but Arab scholars during the heyday of medievalMuslim civilization, the Abbasid caliphate (A.D. 750–1258),revived it. Muslim writers not only ciphered, but inventedcryptanalysis, the systematic breaking of ciphers. NinthcenturyArab philosopher Abu Yusuf al-Kindi wrote theearliest known description of the cryptanalytic techniqueknown as frequency analysis, which breaks substitution226 Encyclopedia of Espionage, Intelligence, and Security