972Unsolved Historical Ciphers
Tout a esté Z φ = ♯ ς Γ 7 △ □ y υ S S que je vous ay adverty par ma derniere…

? → the duc de Mercœur, Brittany · cipher of Greek letters, digits and graphic signs · 26 June [c. 1586]

“MR” to the Duke of Mercœur, 26 June [c. 1586]

Duc de Mercœur
toDuc de Mercœur

BnF français 15564 f. 151: a slip of League correspondence, clear French with six runs of cipher. The cipher was transcribed but not deciphered. Lasry’s key for the volume and two other League keys of 1586 do not fit, and a ciphertext-only solver found no more on the real text than on a shuffled copy. Lasry had already reported that the slip does not decipher with his key; no decipherment is known.

Daniel Bourdeau · posted · updated

Method: not solved · Extent: text not obtained

Summary. The catalogue listed f. 151 of fr. 15564 as a letter to Mercœur with no known decipherment and suggested testing George Lasry’s keys for the volume first. Lasry recovered the key of the Guise→Mercœur letters on ff. 27, 78, 119 and 142 in 2022. Tomokiyo reports that Lasry also looked at f. 151 and found it “does not decipher with the same key and is probably too short for cryptanalysis.” The check here confirms it; the two symbol sets have no symbol in common. f. 151 writes Greek letters, squares and triangles, digits and Latin letters. Lasry’s key uses cursive letter forms with Roman-numeral codes. The Mayenne–Forget key and the Clairambault 357 numerical cipher, both from 1586, do not match either. The six cipher runs total 217 symbols of 55 kinds. A homophonic substitution solver (simulated annealing, French language model) scores the real text no better than a shuffled copy, so the ciphertext-only attack finds nothing. The cleartext is transcribed below. It is a planning letter from someone close to Mercœur, waiting on his presence, signed with an unidentified “MR” monogram.

01 The slip

f. 151 is a narrow slip mounted on a guard at the right of Gallica view 164, foliated 151 at the top right with an older number 226 at the foot. It carries eight lines of clear French in a neat secretary hand. The cipher runs are set inside the clear sentences. It is dated “Ce xxvje juin” and signed with a monogram read as MR. The year is not written; the catalogue gives c. 1586.

The first two lines of f. 151: clear French with runs of Greek-letter and geometric signs
BnF fr. 15564 f. 151, the head of the slip: Tout a esté [cipher] que je vous ay adverty…, then the second cipher run. Image: Bibliothèque nationale de France, fr. 15564 f. 151, via Gallica btv1b9064027v view 164.

02 The cleartext

Tout a esté [C1] que je vous ay adverty par ma derniere du ..e de ce mois. Et voy bien que [C2]. Toutes choses y estans disposées et preparées ainsi que j’estime que vous aurez esté adverty [C3] assister … sans vre presence, afin que nous eussions cest heur que desirons (?) tout plus tost … Je regrette infiniment nre malheur si [C4] Je ne faillray de vous tenir adverty de [C5] … Mais nous avons depuis estimé qu’il viendra plus a propos d’attendre encores un peu … le tout ensemble [C6] mais esperons beaucoup par sa valeur. Ce xxvje juin. [monogram] MR

“Everything has been […] as I told you in my last of the … of this month … everything there being arranged and prepared … without your presence, so that we might have the good fortune we desire … I regret our misfortune extremely if […] … but we have since judged it better to wait a little longer … but we hope much from his valour.” Words marked … or (?) are uncertain readings of the hand.

03 What was tried

04 What stays open

None of the six cipher runs is deciphered, and the MR monogram is unidentified. Another letter in the same key would give the material to recover it: the monogrammist’s other letters to Mercœur in fr. 15564–15566 or in the Mercœur papers. The key itself would decipher the runs directly.

05 Sources

Transcription, solver and control: mercoeur1586/.