# Cryptography in the NTM (Prague)

Photography by Carlos Dorce

I hope this is my last post about the NTM in Prague after talking about the astronomical instruments and Kircher’s Organum Mathematicum! Opposite the astronomical exhibition, there is another about secrets and adults and children can play with the cryptography (if they know the Czec language!):

Ché Guevara used a number code when communicating with Fidel Castro. He transposed each letter in the text to a number […]. He then wrote those numbers one behind the other. Below that line he wrote a second line of numbers, known only to him and Castro, which was used only once. He then added both lines, number per number, and below each set of numbers he wrote (the last digit of) the sum. This give a third line of numbers. Only that row was transmitted. When Castro subtracted the second line from the third, he had the first line as the result.

The Ché method cannot be cracked because the key (the second line of numbers) is random, as long as the message, is only used once.

One of the oldest methods for hidding a message is the Ribbon Code:

The code system of winding ribbons around a shaft was already known to the Greeks of the seventh century BC. The generals of Sparta received a staff in deep secrecy, of which the other half was kept by the magistrates of the city. Messages were written on a leather belt.

The Greek called such a stick a scytale and code experts today still use that name. The scytale is an example of a ‘transposition code’, a code in which the letters of a text are mixed up according to a specified recipe. The drawback of a scytale is that the spy can already guess from the ribbons which coding technique was used. It is then only a matter of patience to find the right diameter of the shaft. Gabrielle Petit, a heroine of the Belgian resistance during World War I, passed messages on silk ribbons, which she camouflaged as part of her clothes.

Photography by Carlos Dorce

The Caesar code is also explained:

Photography by Carlos Dorce

The French cardinal Richelieu (1585-1642), a famous and powerful intrigant, fully used this technique to stay in contact with his agents and spies. They were each given a mask, to put over his letters.

In this method, you and your partner each have the same grid with holes. You write a message in the holes, and afterwards you fill in the remaining space with innocent text. This technique is known as the ‘Cardan grille’ after Girolamo Cardano, of Cardan joint fame, who invented the grille in 1550. It is very difficult to crack.

Photography by Carlos Dorce

Finally, a space for the Enigma machine:

The Enigma was the German top coding machine during World War II. The Germans thought their messages to be uncrackable. Which would indeed have been the case, if they operators hadn’t made procedural mistakes and the British hadn’t secured a code book. The mathematical genius Alan Turing designed the first automatic calculators to do the enormous amount of computations needed to check all possibilities. It is assumed Turing and his crew shortened the war with two years.

Photography by Carlos Dorce

LocationNational Technical Museum in Prague (map)