The Influence of Language Models on Decryption of German Historical Ciphers
Sammanfattning: This thesis assesses the influence of language models on decryption of historical German ciphers. Previous research on language identification and cleartext detection indicates that it is beneficial to use historical language models (LM) while dealing with historical ciphers as they can outperform models trained on present-day data. To date, no systematic investigation has considered the impact of choosing different LMs for the decryption of ciphers. Therefore, we conducted a series of experiments with the aim of exploring this assumption. Using historical data from the HistCorp collection and Project Gutenberg, we have created 3-gram, 4-gram and 5-gram models, as well as constructed substitution ciphers for testing of the models. The results show that in most cases language models trained on historical data perform better than the larger modern models, while the most consistent results for the tested ciphers gave the 4-gram models.
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