Most of us conduct many of our daily personal, business and government transactions electronically. We do so many things online—from staying in touch with friends to buying and selling everything, including the kitchen sink—that getting comprehensive information about most people is as easy as logging, or recording, their online activities.
And for various reasons, ISPs are already logging our activities, such as which sites we have visited and when. They are not alone. Many entities we interact with online—stores, newspapers, dating sites, and the like—keep close tabs on us as well. Thus, if we value privacy, we face the challenge of how to take advantage of everything the Internet has to offer without giving up our privacy.
An amazing discovery of modern cryptography is that virtually any task involving electronic communication can be carried out privately. Many people, including the editors of most dictionaries, mistakenly think that “cryptography” is synonymous with the study of encryption. But modern cryptography encompasses much more. It provides mathematical methods for protecting communication and computation against all kinds of malicious behavior—that is, tools for protecting our privacy and security.
Cryptography (or cryptology; from Greek κρυπτός, kryptos, “hidden, secret”; and γράφω, gráphō, “I write”, or -λογία, -logia, respectively) is the science that practices and studies how to hide information. Modern cryptography intersects the disciplines of mathematics, computer science, and engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.
The earliest forms of secret writing required little more than local pen and paper analogs, as most people could not read. More literacy, or opponent literacy, required actual cryptography. The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g., ‘hello world’ becomes ‘ehlol owrdl’ in a trivially simple rearrangement scheme), and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters (e.g., ‘fly at once’ becomes ‘gmz bu podf’ by replacing each letter with the one following it in the English alphabet).
Simple versions of either offered little confidentiality from enterprising opponents, and still don’t. An early substitution cipher was the Caesar cipher, in which each letter in the plaintext was replaced by a letter some fixed number of positions further down the alphabet. It was named after Julius Caesar who is reported to have used it, with a shift of 3, to communicate with his generals during his military campaigns, just like EXCESS-3 code in boolean algebra.
Encryption attempts to ensure secrecy in communications, such as those of spies, military leaders, and diplomats. There is record of several early Hebrew ciphers as well. Cryptography is recommended in the Kama Sutra as a way for lovers to communicate without inconvenient discovery. Steganography (i.e., hiding even the existence of a message so as to keep it confidential) was also first developed in ancient times. An early example, from Herodotus, concealed a message – a tattoo on a slave’s shaved head – under the regrown hair. More modern examples of steganography include the use of invisible ink, microdots, and digital watermarks to conceal information.