972Unsolved Historical Ciphers
99 129 378 389 222 344 15 412 7 22 19 206 262
p o u r · m o n s i e u r · d e
underline = reversed syllable · 99–250 nulls

Paris → the Archduke’s camp · letter cipher and syllabic number code · March 1649 · read

The prince de Conti’s ciphered mémoires, March 1649

BnF fr. 3854 nos. 41–43, the Frondeurs’ secret instructions to their envoys with Archduke Leopold Wilhelm during the Rueil conference · two mémoires “avec chiffre”, and a third “avec chiffre et déchiffrement”

The two “undeciphered” mémoires turned out to be in print (Lasry 2023), and were read again from the leaves. The deciphered one was not finished: its first twelve lines had never been read

Daniel Bourdeau · solved

Summary. The catalogue of new targets listed Conti’s mémoires of 26 and 27 March 1649 as two ciphers left without a decipherment. It assumed the third mémoire, which has one, would be the crib.

01 The documents

BnF Français 3854 (Gallica btv1b52520094g) is a recueil of Fronde papers from the prince de Conti’s side, January to April 1649. It holds his instructions to his envoys, the marquis de Noirmoutier and Geoffroy de Laigue, the Archduke’s Spanish answers, and three pieces in cipher that Omont describes as follows:

The catalogue’s “verify first” was to confirm that the three share one cipher. They do not. Nos. 41 and 42 are written in lower-case letters mixed with clear words. No. 43 is written in numbers, with a clerk’s letters and syllables above most of them.

BnF fr. 3854 f. 113r, the head of no. 41: A Paris ce 26 Mars 1649, and the first lines of the letter cipher mixed with clear words such as Monsieur, choses, comme, Il sera
No. 41, f. 113r: “A Paris ce 26 Mars 1649”, then the letter cipher inline with clear words. Jcctg nle Monsieur de nrgmtild is “Affin que Monsieur de Noirmo[u]stier”. Bibliothèque nationale de France.

02 Nos. 41 and 42: found already read, and read again

Prior art. A web search before any work found George Lasry, “Armand de Bourbon’s Poly-Homophonic Cipher – 1649”, HistoCrypt 2023. Lasry deciphered both letters, printed their text with a translation, and described the system.

Read again here. All three pages were transcribed sign by sign from the full-resolution images. A beam search over each sign’s possible letters, scored with a French quadgram model and then a period word list, picked the readings. The text agrees with Lasry’s throughout, with three additions:

“Monsieur de Tenerande”, with whom Longueville is to treat, reads the same under the key; the person is unidentified. “Dix” or “six mil hommes” cannot be settled, because one sign serves both d and s.

Depuis la lettre que je vous escrivi hier j’ay cru vous debvoir advertir qu’on a donné aujourd’huy arrest au parlement qui ordonne à ses députez de se joindre à ceux de son Altesse le prince de Conti pour insister à demander l’éloignement du cardinal Mazarin. On ne doute plus après cela de la rupture … Ne concluez rien pourtant que vous n’ayez le dernier advis, mais si elle arrive, mettez-vous en estat d’en faciliter les suittes. No. 42, 27 March 1649

03 No. 43: a number code, and twelve lines nobody read

BnF fr. 3854 f. 117r, the first thirteen lines of no. 43: rows of numbers with clear words (quil, principalement, bonnes intentions, jay jugé apropos de vous en, que vous puissies, par les, nous sommes tousiours dans), and the first interlinear gloss po ur p r o cu at the foot
No. 43, f. 117r. Twelve lines of numbers and clear words with nothing written above them; the clerk’s interlinear decipherment starts only at the foot (po ur p r o cu, “pour procu[rer]”). Bibliothèque nationale de France.

The clerk who deciphered no. 43 wrote a letter or syllable above almost every number from f. 117r line 13 to the end of f. 120r. He wrote nothing over the first twelve lines. Those 264 groups carry the address and the purpose of the mémoire.

The key, rebuilt from his glosses. Agents transcribed number/gloss pairs page by page, and the tallies (key43.txt) show a regular design:

The opening, decoded. Of the 264 groups:

The first draft was checked against the rest of the mémoire. Nine of the context and structural values were later found glossed on ff. 117r–120r, all as predicted: 279 de, 364 la, 354 ne (underlined, en), 443 te, 445 su, 37 o, 43 h, 86 n, and 36 p. None was contradicted.

Pour Monsieur de Legue. Parce que nous ne doutons que le cardinal ne donne une mauvaise interprétation à toutes ses actions, et qu’il ne face principalement ses efforts pour mettre l’archiduc dans la défiance de nos bonnes intentions sur le sujet de la conférence que [le] parlement a accordée, j’ay jugé à propos de vous en faire instruire, afin que vous puissiés destourner les mauvais effets des artifices du cardinal, et faire comprendre à l’archiduc que ce pourparler n’a esté causé et résolu que par les retardemens que l’archiduc apporte à nous secourir, et que nous sommes tousiours dans nos mesmes desseins … No. 43, f. 117r, lines 1–12, read here for the first time

04 What no. 43 says

The last line of f. 120r is glossed à Paris le 5me mars 1649. That is the day after the Rueil conference opened between the court and the parlement. Conti writes to Laigue, who is with Archduke Leopold Wilhelm, to keep the Spanish alliance alive while the Frondeurs negotiate with Mazarin’s side.

The complete edited text of the mémoire is in conti1649/READING43.md.

Where it fits. The three mémoires frame the Frondeurs’ double game. On 5 March, Conti promises the Archduke that the conference is only a way to buy time. On 26–27 March, he tells Noirmoutier that the talks are about to break on Normandy and Mazarin, and that he should be ready. The Peace of Rueil was signed on 11 March, in between. The parlement’s arrest of 27 March shows the generals still pressing the Mazarin article after the peace.

05 What remains uncertain

06 Sources