Many historians and linguists are working individually and in an uncoordinated fashion on the identification and decryption of historical ciphers. This is a time-consuming process as they often work without access to automatic methods and processes that can accelerate the decipherment. At the same time, computer scientists and cryptologists are developing algorithms to decrypt various cipher types without having access to a large number of original ciphertexts. In this paper, we describe the DECRYPT project aiming at the creation of resources and tools for historical cryptology by bringing the expertise of various disciplines together for collecting data, exchanging methods for faster progress to transcribe, decrypt and contextualize historical encrypted manuscripts. We present our goals and work-in progress of a general approach for analyzing historical encrypted manuscripts using standardized methods and a new set of state-of-the-art tools. We release the data and tools as open-source hoping that all mentioned disciplines would benefit and contribute to the research infrastructure of historical cryptology.

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.

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