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We present an empirical study on historical keys in their original form from Early Modern Times (1400-1800) in Europe. We describe the internal structure of keys, and specify what was encoded and how. We present some trends of the construction of historical keys over time. Some of these trends have been sensed but never systematically documented by crypto historians, some other trends however are revealed here for the first time.

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Studying original cipher keys constructed throughout history gives important insights into encryption methods and cipher systems. We can study the type of encryption used, the code structure and their corresponding plaintext entities, be it letters, morphemes, words, or named entities. The insights can lead us to better decryption methods, and the understanding of the development of historical ciphers. In this paper, we present a tool for automatic key structure extraction that describes the symbol system and the code structure along with the encoded plaintext features and the mapping between the two. The tool is aimed at the empirical study of historical keys given transcribed keys.

We give an overview of the development of European historical cipher keys originating from early Modern times. We describe the nature and the structure of the keys with a special focus on the nomenclatures. We analyze what was encoded and how and take into account chronological and regional differences. The study is based on the analysis of over 1,600 cipher keys, collected from archives and libraries in 10 European countries. We show that historical cipher keys evolved over time and became more secure, shown by the symbol set used for encoding, the code length and the code types presented in the key, the size of the nomenclature, as well as the diversity and complexity of linguistic entities that are chosen to be encoded.