Wrong Design of Cipher Keys: Analysis of Historical Cipher Keys From the Hessisches Staatsarchiv Marburg Used in the Thirty Years’ War

Abstract

Nomenclator is a complex encryption system consisting of several different simpler encryption systems used together during the encryption. It is one of the main encryption systems used before the twentieth century. In some cases, there are large collections of historical ciphers preserved in archives. Those from a particular time period or geographic location are very valuable and can bring insights to the cipher design from a specific time/location. This paper provides the first detailed empirical analysis of historical cipher keys from the Thirty Years’War deposited in Hessisches Staatsarchiv Marburg. We describe a large variety of analyzed keys with a focus on those properties (poorly designed keys) that can decrease the security of the cipher. We further show that these properties alone do not imply a bad design, sometimes a combination of several properties is needed and at the same time a bad use of the encryption key.