The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

by Simon Singh

The urge to discover secrets is deeply ingrained in human nature; even the least curious mind is roused by the promise of sharing knowledge withheld from others. Some are fortunate enough to find a job which consists in the solution of mysteries, but most of us are driven to sublimate this urge by the solving of artificial puzzles devised for our entertainment. Detective stories or crossword puzzles cater for the majority; the solution of secret codes may be the pursuit of a few.

It has been said that the First World War was the chemists’ war, because mustard gas and chlorine were employed for the first time, and that the Second World War was the physicists’ war, because the atom bomb was detonated. Similarly, it has been argued that the Third World War would be the mathematicians’ war, because mathematicians will have control over the next great weapon of war—information.

The word “code” refers to a very particular type of secret communication, one that has declined in use over the centuries. In a code, a word or phrase is replaced with a word, number or symbol.

The alternative to a code is a cipher, a technique that acts at a more fundamental level, by replacing letters rather than whole words.

Secret communication achieved by hiding the existence of a message is known as steganography, derived from the Greek words steganos, meaning “covered,” and graphein, meaning “to write.”

cryptography itself can be divided into two branches, known as transposition and substitution

In transposition each letter retains its identity but changes its position, whereas in substitution each letter changes its identity but retains its position.

Each distinct cipher can be considered in terms of a general encrypting method, known as the algorithm, and a key, which specifies the exact details of a particular encryption.

“A man is crazy who writes a secret in any other way than one which will conceal it from the vulgar.”

Technically, a code is defined as substitution at the level of words or phrases, whereas a cipher is defined as substitution at the level of letters.

A nomenclator is a system of encryption that relies on a cipher alphabet, which is used to encrypt the majority of a message, and a limited list of codewords.