A treatise on the Rilke cryptogram

Abstract The ‘Rilke Cryptogram’ is a possible cipher of German, WWII origin. This study summarises the text’s history, and examines explanations for its nature. Statistical techniques are applied to an updated transcription. The text consists of 18,760 characters in 3,305 unique groups of four, arranged into 670 rows by seven columns on 33 pages of an otherwise ordinary book. The distribution of these groups is not particularly Zipfian, suggesting the text is likely not a code or monoalphabetic substitution cipher. Its alphabet is longer than English and German (46 characters). Character and N-gram entropy analyses suggest the text is less ordered than English and German, but more ordered than random text. Machine-learning modelling suggests the text is a substitution cipher, but frequency and Kasiski-Kerckhoff analyses suggest otherwise. Sukhotin’s algorithm results for the text are somewhat consistent with results for German, though N-gram distributions do not strongly resemble English or German. Statistical analysis of physical typewriter key distances suggest the text’s groupings are highly unlikely to appear at random, and are consistent with intentional ‘lazy’ typing. The text is inconsistent with machine ciphers of its era (e.g., Enigma), and other features are not entirely consistent with non-cipher explanations. Further investigation is required.