After visiting the exhibition about Leonardo da Vinci and the Russian Emperors and Empresses’ tombs in the church, I decided to finish my visit to Peter and Paul Fortress entering in the jail. All the cells of the jail have posters next to their doors with explanations about famous Russian people who were there. Of course I didn’t read all the posters but there was one which aroused my interest. The prisoners were isolated and they couldn’t talk to each other. So they developed a cryptographic code to communicate:
Tapping was the only way of communication between prisoners in single cells. They used so called “prison alphabet”. Letters in alphabetical order were accumulated in a table of six lines and five columns. Each letter was tapped in two steps, number of a line, then number of a column. Prisoners “spoke” tapping the floor, walls or legs of a bedstead.
This encrypting method is also known as Polybius square. Polybius was a Greek historian who lived in the IInd century BC and he is supposed to have invented the method.