Deciphering a Short Papal Cipher from 1721

Abstract

As part of the DECRYPT project, hundreds of enciphered papal letters from the 16th, 17th, and 18th centuries were deciphered, and dozens of keys were recovered. Several ciphertexts remained unsolved, despite the use of sophisticated computerized algorithms, and were offered as public challenges in 2019. One of those is a short letter sent by the nuncio in Bruxelles to Rome, on October 9, 1721. It consists of groups of digit codes, separated by commas. After improving the algorithm, and with some manual work, the authors were able to recover most of the key and of the plaintext. In this article, they present the method they used to recover the key, and the decrypted message.