The idea of encoding data arises in many ways. We have already seen the way in which information is encoded in identification numbers, such as the ISBN or driver’s license numbers. Another important form of encoding that we saw is the addition of check digits for error correction, which is used both in identification numbers and in sending data.

In this chapter we shall look at another reason for encoding—secrecy. Sometimes you encode something and only tell certain selected people how to decode it. This form of secret encoding is called encryption, and the study of it is cryptography.

It has been widely used in military operations, but nowadays it is very common in commerce. For example, the use of a PIN number with your bank card is a form of encryption.

The original message, in everyday language, is often called a plaintext, and the encoded version is the corresponding ciphertext. (“Cipher” is another word for “code”.) The process of moving from ciphertext to plaintext (the opposite of encryption) is called decryption. The special information required by your ally to decrypt your message is called a key.

Suppose you send a secret message. If your enemy intercepts it, he will try to figure out what it means by working out how you did the encoding—usually, he tries to find the key. This is called cryptanalysis, or codebreaking. The study of cryptography and cryptanalysis is collectively called cryptology.

There are many mystery stories in which cryptography takes an important part.

If you want to read some of these, I recommend The Adventure of the Dancing Men,  a Sherlock Holmes story by Sir Arthur Conan Doyle, Edgar Allen Poe’s The Gold Bug, or The Nine Tailors by Dorothy L. Sayers.