972Unsolved Historical Ciphers
22 . 38 . 23 . 3 . 40 . 36 . 27 . 20 . 6 . 30 . 1 . 3 . 7 . 24 . 35 . 45 . 22 . 18
E X C U S A T I O N   Z U   M A C H E N
A = 24/36 · E = 22/34 · 145 = ich · 152 = Kayser · 198 = Prinz

Brussels → brother · homophonic alphabet with a small nomenclator · 18 Nov 1721

A letter between the Windischgrätz brothers, Brussels 1721

SOA Plzeň, pracoviště Klášter u Nepomuka, Rodinný archiv Windischgrätzů, inv. nr. 1433, karton 202 (DECODE R5029, no decipherment on record). A German letter in clear with thirteen enciphered passages. The key Jakub Mírka reconstructed in 2023 from the brothers’ other letters, attached to the record, was applied unchanged: 148 of the 181 cipher groups (82%) decrypt. No earlier decipherment of this letter was found.

Daniel Bourdeau · posted · updated

Method: read with known key · Extent: partial (82% of the cipher)

Summary. The catalogue listed this as a letter from Leopold Viktorin von Windischgrätz to his brother Ernst Friedrich, Brussels, 18 November 1721, eight pages, with no decipherment and two keys of the family series on DECODE. Neither of those keys (R5017, R5018, the Emperor’s) is the right one. The right one is on the record itself: a spreadsheet by Jakub Mírka (19 June 2023) reconstructing the brothers’ own cipher from their letters of late 1721, most of which carry interlinear decipherments. His code list includes codes that occur in this letter, but no decipherment of it was published, and DECODE lists none. Applied here, it decrypts every letter-spelled passage to German at once: eine Excusation zu machen, verliehret, Succession, niemahlen anzunehmen, Dienste zu. Of 181 cipher groups, 148 (82%) have a value; 33 groups, 23 distinct nomenclator codes, stay open because neither the key nor the letter fixes them. No cryptanalysis was needed. The work was transcribing the numbers and applying a published key.

01 The document and the key

Five DECODE images: the first page (“Brüssel den 18. 9bris 721”, “Hochgebohrner Graf”), two openings, and the last page (seen from both sides). The text is ordinary German and French in clear; the secrets are written as dotted numbers in the line.

Mírka’s key gives each letter two numbers: A = 24 or 36, B = 12 or 48, C = 23 or 35, E = 22 or 34, and so on to Z = 1 or 37; the first column runs 24 to 1 down a fixed order of the alphabet, the second is the first plus 12 or 36. Numbers above 48 are a small nomenclator, of which he identifies 54 Althann, 85 der/dem, 86 die, 135 hat, 145 ich, 152 Kayser, 197 Plan, 198 Prinz, with guesses for 99 [Engländer?], 103 [Emp…?], 151 [K…?], 167 [Micosch?] and others.

First page of the letter: clear German with lines of dotted numbers
First page: “dass 85. 103. 7.20.16. gar 18.32.35.9.15. 130. 209. willen 152. selbst widerrathen 146. eine 22.38.23.3.40.36.27.20.6.30. 1.3. 7.24.35.45.22.18.” = dass der [103] mir gar nicht [130 209] willen, [der] Kayser selbst widerrathen [146] eine Excusation zu machen. Image: Státní oblastní archiv v Plzni, Rodinný archiv Windischgrätzů inv. 1433, karton 202, first page (image p2), via DECODE R5029.

02 The decipherment

Cipher in capitals, codes in brackets (open ones by number), cleartext around them in lower case.

The writer, waiting for the Congress of Cambrai to open, complains that he gets no support; the Emperor himself advised an excuse; a patron [205] works under hand, perhaps with the Prince (Eugene); love for [103] is being lost daily, even by the Emperor, after the courier finally brought the Prince’s plan; the English [?] want to go home and “I shall succumb, the Prince triumph”. In clear, the Madrid courier has brought Pozobueno orders on the exchange of renunciations. The matter (Cambrai, Pozobueno, Beretti Landi) fits Ernst Friedrich, the imperial plenipotentiary at the Congress, better than Leopold Viktorin; DECODE’s direction is kept here but is worth checking against the hands.

03 What remains uncertain

04 Sources

Transcription, key table and decoder: windischgraetz1721/.