Cryptography, the use of codes and ciphers, began thousands of years ago. Until recent decades, it has been the story of what might be called classical cryptography — that is, of methods of encryption that use pen and paper, or perhaps simple mechanical aids. In the early 20th century, the invention of complex mechanical and electromechanical machines, such as the Enigma rotor machine, provided more sophisticated and efficient means of encryption; and the subsequent introduction of electronics and computing has allowed elaborate schemes of still greater complexity, most of which are entirely unsuited to pen and paper. The development of cryptography has been paralleled by the development of cryptanalysis — the "breaking" of codes and ciphers. The discovery and application, early on, of frequency analysis to the reading of encrypted communications has, on occasion, altered the course of history. Thus the Zimmermann Telegram triggered the United States' entry into World War I; and Allies reading of Nazi Germany's ciphers shortened World War II, in some evaluations by as much as two years. Until the 1960s, secure cryptography was largely the preserve of governments. Two events have since brought it squarely into the public domain: the creation of a public encryption standard (DES), and the invention of public-key cryptography.
Antiquity
The earliest known deliberate transformation of a text is found in non-standard hieroglyphs carved into the wall of a tomb from the Old Kingdom of Egypt circa 1900 BC. This non-standard script was formed by replacing similar looking pictograms with each other and use of the rebus principle, where pictograms that begin with a sound come to represent that sound. These are not thought to be serious attempts at secret communications, however, but rather to have been attempts at mystery, intrigue, or even amusement for literate onlookers. Some clay tablets from Mesopotamia somewhat later are clearly meant to protect information—one dated near 1500 BC was found to encrypt a craftsman's recipe for pottery glaze, presumably commercially valuable. It was encoded by replacing cuneiform symbols with other symbols in their least common values (pronunciations). Furthermore, Hebrew scholars made use of simple monoalphabetic substitution ciphers (such as the Atbash cipher) beginning perhaps around 600 to 500 BC. The Kama Sutra, estimated to have been composed in India between 400 BC to 300 AD, lists 64 arts recommended for a better quality of life, including a reference to Mlecchita vikalpa, "the art of understanding writing in cypher, and the writing of words in a peculiar way." This was recommended for private communication between lovers. As the Kama Sutra only contains a general reference in a list and not a description, it is unclear what precisely it referred to at the time. Later commentaries on the Kama Sutra offer detailed instructions for substitution ciphers, but these were composed between the tenth and thirteenth centuries AD. Parts of the Egyptian demotic Greek Magical Papyri were written in a cypher script. The ancient Greeks are said to have known of ciphers. The scytale transposition cipher was used by the Spartan military, but it is not definitively known whether the scytale was for encryption, authentication, or avoiding bad omens in speech. Herodotus tells us of secret messages physically concealed beneath wax on wooden tablets or as a tattoo on a slave's head concealed by regrown hair, although these are not properly examples of cryptography per se as the message, once known, is directly readable; this is known as steganography. Another Greek method was developed by Polybius (now called the "Polybius Square"). Originally a method of fire signaling over long distances, it later evolved into a method of cryptography. The Greek writer Aeneas Tacticus had also devised an early form of the pinhole cipher, and a method of cryptography where vowels were replaced by dots. The Romans were also knowledgeable about cryptography. For instance, the Caesar cipher of Julius Caesar and its variations, used for clandestine communication.
Medieval cryptography
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