Although today’s ciphers are effectively unbreakable, researchers continue to develop new security systems. For example, steganography does not hide the meaning of a message, but rather hides the very existence of the message. Previous techniques include invisible ink and microdots. Today, texts can be hidden within jpeg images, so that the image appears unchanged. One of the motivations for modern steganography is the fear that cryptography might be banned by a totalitarian regime that wants to spy on its subjects. But a dictator cannot ban a technology that by definition is hard to find.

In addition to research conducted by industry and in universities, there are still cryptographers working in government labs. The biggest employer of mathematicians in the world is the American National Security Agency. So, although there is plenty of cryptography in the open that can be studied, it is still true to say that to some extent the science of secrecy continues to be a secret science.

Simon Singh is a writer and broadcaster specialising in mathematics and science. He is the author of The Code Book, a history of codes and codebreaking. His website (www.simonsingh.net) contains some cryptography sections, some encryption tools and details of an interactive CD-ROM version of The Code Book.