Until the twentieth century, cryptography—the scrambling of messages to make them obscure to everybody except an intended recipient—was a relatively straightforward business, based on principles used since antiquity. The exiled Jewish scribes who wrote the Book of Jeremiah, for example, sometimes obscured the word "Babylon" by using what is now known as the Atbash cipher, in which letters at opposite ends of the Hebrew alphabet were swapped. (If applied to our alphabet, the cipher would make A into Z, B into Y, and TAKE BACK JERUSALEM into GZPV YZXP QVIFHZOVN.) Julius Caesar, too, used a simple code, known today simply as the Caesar cipher, which was formed by substituting each letter of the alphabet with the one that came three places after it. (In this system, BARBARIANS COMING would become EDUEDULDQV FRPLQJ.)
Then computers came along. Ciphers like those used by Caesar and the Jewish scribes soon became almost laughably vulnerable to what are called "brute-force attacks"—that is, continuous computer trials of all possible coding combinations. This was a serious problem for modern governments, which increasingly felt the acute need for secure and private global communications, and which therefore devoted themselves to the development of advanced new cryptographic systems. (One of the main reasons computers were invented, in fact, was the effort during the Second World War to develop efficient ways to crack enemy codes.) The result of all this has been that, as the cryptographer Bruce Schneier describes it in Applied Cryptography (1995),
The United States' National Security Agency (NSA), and their counterparts in the former Soviet Union, England, France, Israel, and elsewhere have spent billions of dollars in the very serious game of securing their own communications while trying to break everyone else's. Private individuals, with far less expertise and budget, have been powerless to protect their own privacy against these governments.
As Schneier suggests, governments basically took over cryptography in the postwar era, and in the United States, where most of the innovations were taking place, all open discussion of the field was stifled—to the extent that cryptography and writing about cryptography were classified as "munitions," which made their export illegal. The government's argument was that it needed absolute secrecy and control of cryptography in order to gain and keep the upper hand against terrorists, hackers, and other criminals.