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

by Simon Singh

frequency analysis,

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.

homophonic substitution cipher. Here, each letter is replaced with a variety of substitutes, the number of potential substitutes being proportional to the frequency of the letter.

“It saved lives. Not only Allied and Russian lives but, by shortening the war, German, Italian, and Japanese lives as well. Some people alive after World War II might not have been but for these solutions. That is the debt that the world owes to the codebreakers; that is the crowning human value of their triumphs.”

Cryptography is a science of deduction and controlled experiment; hypotheses are formed, tested and often discarded. But the residue which passes the test grows until finally there comes a point when the experimenter feels solid ground beneath his feet: his hypotheses cohere, and fragments of sense emerge from their camouflage. The code “breaks.” Perhaps this is best defined as the point when the likely leads appear faster than they can be followed up. It is like the initiation of a chain reaction in atomic physics; once the critical threshold is passed, the reaction propagates itself.

Now picture the following scenario. As before, Alice wants to send an intensely personal message to Bob. Again, she puts her secret message in an iron box, padlocks it and sends it to Bob. When the box arrives, Bob adds his own padlock and sends the box back to Alice. When Alice receives the box, it is now secured by two padlocks. She removes her own padlock, leaving just Bob’s padlock to secure the box. Finally she sends the box back to Bob. And here is the crucial difference: Bob can now open the box because it is secured only with his own padlock, to which he alone has the key.

Solution to key distribution problem

“last on, first off.”

Hellman’s idea relied on a one-way function of the form Yx (mod P). Initially, Alice and Bob agree on values for Y and P.

Solving the order problem of key distribution problem

In a symmetric cipher the encryption key is the same as the decryption key, so Alice and Bob must take enormous precautions to ensure that the key does not fall into Eve’s hands. This is the root of the key distribution problem.

This system of asymmetric cryptography, known as RSA, is said to be a form of public key cryptography.