Historical cryptanalysis: How to crack secret messages from the Vatican

Shortly after the invention of writing, people developed the need to hide important written information in such a way that only certain recipients could read it. Cryptography, i.e. the science and art of secret writing, has therefore developed steadily over the last four thousand years. Today there are many historical documents, some of which are several hundred years old and mysteriously encrypted, in archives all over the world – for example in the Vatican Apostolic Archive (formerly known as the Secret Archive) or in the House, Court and State Archives in Vienna. Experts assume that – even if only one percent of all archived documents are encrypted – together they would fill hundreds of meters of archive corridors; a large part of these documents has not yet been decrypted.

A lot of work for the cryptanalysts – the natural opponents of the cryptographers – trying to decrypt encrypted texts. And their work is made even more difficult: In most cases, the encrypted historical documents are not archived centrally, are not digitally accessible, are scattered across various shelves and boxes in the archives and must first be painstakingly compiled by interested researchers. Again and again historians stumble over encrypted documents, but can do little with them because decryption is not part of their practice.

The DECRYPT project

The decoding of historical texts is an interdisciplinary task that neither historians nor computer scientists can handle alone. The DECRYPT project and its predecessor called DECODE help to achieve joint success. The aim of the research cooperation is to research historical cryptology on a large scale, systematically and in an interdisciplinary manner. The project is managed from Uppsala in Sweden. A team of historians, historical and computational linguists, philologists, computer scientists and cryptologists from Germany, France, Great Britain, Israel, the Netherlands, Slovakia, Spain and Hungary are working together to bring together the encrypted past in the form of original documents. They are analyzed with the help of computer-aided methods and the results are presented to a broad public in an understandable way.