972Unsolved Historical Ciphers

Fontainebleau and Nijmegen, 1635 · Valtellina, 1636 · digits and invented signs within clear French

Rohan papers and the Franco-Dutch letters, 1635–36

Louis XIII’s letter to Châtillon and Brézé and Châtillon’s letter to Servien (1635) have long clear passages beside the cipher; the clear text was transcribed and the ciphered passages were not deciphered. The Savoy and Canisy letters to Rohan (1636) are printed deciphered and could serve as known plaintext, but their ciphertext lies beyond the end of the Gallica reproduction of the manuscript. No earlier decipherment of the 1635 cipher passages was found.

Daniel Bourdeau · posted · updated

Method: not solved · Extent: text not obtained

Result. The clear French of BnF Français 3758, nos. 100 and 104, was transcribed. The enciphered passages were not deciphered. BnF Français 5190 contains d’Émery’s cipher letter of 25 January 1636 and the Savoy and Canisy letters of 27 May, but its supplied Gallica reproduction ends at manuscript page 80, before all three items. The latter two are printed deciphered in the 1758 Mémoires et lettres, but without their ciphertext a key cannot be reconstructed from them. No key was recovered.

01 The witnesses

BnF Français 3758 contains no. 100, Louis XIII to marshals Châtillon and Brézé, Fontainebleau, 30 June 1635 (Gallica views 126–130; manuscript pp. 114–118). No. 104 is Châtillon to Servien, Nijmegen, 3 August 1635 (views 135–140; manuscript pp. 123–128). The second item is sometimes grouped with the letters to the two marshals, but Servien is its addressee. Both letters mix ordinary handwriting with sequences of digits and invented signs.

The catalogue of BnF Français 5190 identifies nos. 171–172 as d’Émery’s cipher letter of 25 January 1636 and Rohan’s reply; nos. 214–215 are the cipher letters from the duke of Savoy and Canisy of 27 May, received 6 June. The physical volume has 208 folios. Its linked Gallica reproduction offers 90 views, the last showing manuscript p. 80. Requests for later view numbers return that same last image.

02 The printed Rohan letters

The 1758 Mémoires et lettres, volume III, prints the Savoy and Canisy letters as clear French under headings that call them “en chiffres”. Savoy asks Rohan to lodge 2,000 infantry and 200 cavalry in the Val Mesolcina to block the Germans’ passage, while threatening the Francesca post to draw enemy forces away. He says 6,000 men intended for Canisy’s army are being sent to Rohan instead. Canisy confirms the king’s and Savoy’s agreement, describes an intended approach to the Ticino, and urges Rohan to hold ready for a junction.

This printed text settles what those two letters say. It does not settle which signs in the missing manuscript stand for their words, nor whether d’Émery used the same key.

03 Louis XIII to Châtillon and Brézé

The king begins by warning that Châtillon’s last dispatch was sent “n’estant point en chiffre”: had the enemy intercepted it, they would have learned too much. He is pleased with the junction of his army and the States’ and with the two marshals’ understanding with the prince of Orange. In a clear passage he accounts for armies in Germany, Italy and the Valtellina, saying that the Valtellina force numbers “plus de douze mil hommes de pied et cinq cens chevaux”. He rejects an obligation to fund yet another attack in Flanders, though he promises an additional force under Chaulnes and Rantzau. Later he presses for exact musters, condemns officers’ fraudulent returns, asks the marshals to write more often and to use cipher for important news, and reports the imperial crossing at Philippsburg.

Several military proposals, directions of movement, and parts of the discussion of money and supply sit in cipher. The copy also contains clear fragments separated from the running text, including a paragraph about a convoy due by 20 August. These fragments cannot simply be joined into a continuous letter without checking the encrypted sections.

04 Châtillon to Servien

Châtillon reports the fall of Schenkenschans and blames the States’ delayed repairs and a garrison reduced to two weak companies. He criticises Orange’s indecision and writes that the allied armies still have 25,000 infantry and 6,000 cavalry, yet hesitated to occupy a position two leagues from Nijmegen opposite the lost fort. From there they might have blocked its reinforcement and recovered it quickly. He says the French army alone is keeping the States reassured.

The letter comments on its own cipher. Châtillon says Servien reproached him for writing too freely without one, but “vous ne m’en aviez point donné”. Brézé then gave him the cipher the two had used on their German journey, “duquel je me sers en quelques lignes de cette lettre”. That sentence shows the enciphered lines were inserted on purpose; it gives none of their values. Châtillon goes on to describe disease, poor bread, depleted infantry, scarce provisions, and the decision to cross the Rhine for safety. He closes by saying he has written “en bon François sans y employer aucun artifice” because Servien needs to know the army’s true condition.

05 What is needed to decipher the passages

The first requirement is images of Français 5190 nos. 171–172 and 214–215. The Savoy and Canisy ciphertext, paired with the printed plaintext, could test or rebuild a Rohan key; d’Émery’s text could then be tried against it. For the 1635 letters, Marie-Catherine Vignal-Souleyreau’s L’œil et la plume de Richelieu: Correspondances croisées, 1635 (2020) identifies Bibliothèque Sainte-Geneviève MS 3338 as a second copy of the Châtillon correspondence. The full edition or that manuscript may supply matching clear passages. Neither was available here. The visually similar 1637 Saint-Chamond cipher has not been verified as a key for these 1635 letters.

06 Sources

The longer clear-prose transcription and source audit are in the repository’s research report; the research notes and profile are in rohan1636/.