Deciphering a Letter to Duke Wilhelm V of Bavaria from the Cologne War (12 June 1583)

This paper presents the decipherment of the encrypted passages of a German letter to Duke Wilhelm V of Bavaria, dated 12 June 1583 and kept in the Bayerisches Hauptstaatsarchiv in Munich (Kurbayern Äußeres Archiv 4591, ff. 274–277). The cipher had been transcribed and attacked before, without result. It turns out to be a simple substitution of the writer's Bavarian German, with Latin words mixed in. Two details made it harder than it looks. The writer used a stroke that doubles the preceding letter, and the transcription gives one label to some pairs of signs that the plaintext requires to be different. With these taken into account, one key reads about nine words in ten. The letter reports on the Cologne war shortly after the election of the Duke's brother Ernst as Archbishop of Cologne. Its most sensitive passage warns that a woman called Susanna was apparently to be brought from Freising to Liège, which "would cause a great scandal in these parts", and asks that Ernst be kept from such things "with discretion and tact". The reading given here rests on a published transcription and has not yet been checked against the original.