Cryptography, the science of encoding communications so that only the intended recipient can understand them, is ancient. In almost every civilization, cryptography appeared almost as soon as there was writing. For example, in 1500 B.C.E. a Mesopotamian scribe, using cuneiform signs that had different syllabic interpretations (akin to spelling "sh" as "ti," as in nation), disguised a formula for pottery glazes. According to the Greek historian Herodotus, in the fifth century B.C.E. a Greek at the Persian court used steganography, or hiding one message within another, to send a letter urging revolt against the Persians. In the fourth century B.C.E. the Spartans developed a transposition algorithm that relied on wrapping a sheet of papyrus around a wooden staff; in the same period, the Indian political classic the Arthasastra urged cryptanalysis as a means of obtaining intelligence. In the fifteenth...