People who want to understand cryptography usually start by reading a handbook. They start with definitions, then they get into protocols, then they move on to algorithms, and finally they learn the history, without ever getting through the first chapter.
The reason is that there is a lot of confusion about what cryptography is. The basic idea is easy to grasp: when you send someone a message, you want it to be secure. But in practice, that’s much harder than it sounds.
Cryptography turns ordinary information into secret information. Cryptography is a branch of mathematics, but it can also be described as a technology, or even a form of art. It has been around for thousands of years (and was the dominant mode of communication in the second millennium BC), but only recently has its full importance become apparent. Cryptography is used to protect information. For example, it is used to protect bank accounts, credit card numbers, and other kinds of sensitive personal data. And because cryptography is so important to modern life, even people who don’t know much about it are worried about its security.
In theory, cryptography has nothing to do with secrecy or information security. In theory encryption is just about hiding something in plain sight: the way to hide something is not to keep it secret but to make people believe that it isn’t there.
But in practice cryptography involves a lot more than just hiding things; it often includes concealing information from one person while revealing it to another person; and sometimes the concealment and the revelation can be done at once.
The best-known example of this kind of thing is probably steganography, which means “covered writing.” The oldest examples of steganography are coins with messages written on them by famous generals in
Cryptography is just a special case of information security. The more general term is information security, and it’s what happens when you’re trying to keep people from reading your email, stealing your credit card number, or taking over your computer.
There are any number of methods you can use to do that, but the most common is encryption. A cryptosystem is a way to keep something secret.
In practice, the two things go hand in hand: if the cryptosystem lets people read encrypted messages, it will not keep them secret.
In the late 19th century, Western governments gradually realized that they couldn’t trust their own telegraph networks; in fact, no one could. So they invented cryptography to secure their messages. And that led to the development of other forms of cryptography.
Cryptography is what is used to protect bank accounts, credit cards and e-mail, for example. It’s also used for secure instant messaging, secure Voice over IP (VoIP) telephony and secure Web browsing.
It’s also used for encrypting data on hard drives and CDs, on DVDs, on flash drives and memory sticks, on routers and switches, and so on. The reason it’s used so widely is that it’s very easy to use. You just have to decide what you want to keep secret–something like a password–and then use it along with a key to encrypt the data: “Encrypt text with DES” or “Encrypt text with IDEA”, for example.
Cryptography is the science of secrecy. It is also the art of hiding messages so that only people you want to read them can. Sometimes cryptography involves tricks, like encrypting messages with a secret code, or using secret keys–but those are just examples. What really counts is that the person who wants to read your message can’t read it without your knowing it in advance.
Cryptography is one of the oldest known forms of communication. It goes back at least as far as chicken bones and cave drawings, and probably goes back a lot further than that.
Cryptography has a long history, but it is still an active field of research. For example, new mathematical tools are being developed all the time, and some of those tools will be used for encryption in the future, but we don’t know yet whether or not any particular one will be used for that purpose, or when.
Cryptography is the science of making secret communications impossible without using a key. The word comes from the Greek for “hidden writing.”
Cryptography is a subject you don’t learn in school. It’s not a secret, but it’s not something that can be taught in a book or a lecture. It’s the same kind of thing as computer programming. You have to learn by doing.