From an Egyptian tomb to the internet: four thousand years of secret writing
Around 1900 BC, the tomb of an Egyptian named Khnumhotep II used inscriptions that swapped ordinary symbols for unusual ones, one of the earliest known examples of encryption. The goal has barely changed since: scramble a message so that only someone holding the right key can read it.
Encryption turns readable plaintext into unreadable ciphertext. It does not stop anyone intercepting a message; it just keeps the meaning out of reach. Greeks and Romans used symbol substitution for military messages, most famously the Caesar cipher, which shifts each letter a fixed number of places along the alphabet. Around 800 AD the Arab mathematician al-Kindi showed how to crack such ciphers by counting letters: in English the commonest letter is E, so the most frequent symbol in the ciphertext probably stands for it. Polyalphabetic ciphers, described by al-Qalqashandi and by Leon Battista Alberti in 1465, fought back by changing the substitution as the message went on.
Machines followed. About 1790 Thomas Jefferson designed a wheel cipher to jumble messages of up to 36 characters, though it was never built. US Army Major Joseph Mauborgne independently created a similar device, the M-94, in 1917, and the army used it until 1942. Germany's Enigma machine in the Second World War changed its settings daily, and the Allies combined codebreaking methods with electromechanical machines to narrow down each day's possibilities.
Modern schemes come in two kinds. In symmetric encryption both sides share the same key, as with Enigma's daily setting. Public-key encryption, first described in a secret document in 1973 and then publicly by Diffie and Hellman, lets anyone encrypt with a published key while only the recipient can decrypt; their key exchange lets two parties agree a secret over an open channel. RSA, developed in 1977, builds its keys from two prime numbers, and in 1991 Phil Zimmermann released Pretty Good Privacy free with its source code.
Key length matters. The Data Encryption Standard's 56-bit key offered about 72 quadrillion possibilities, yet a purpose-built machine from the Electronic Frontier Foundation cracked it in 1999 in 22 hours and 15 minutes. Today's standards, such as AES with 256-bit keys, protect everything from online shopping to cash machines.
Source: Encryption
