French Ciphers during the reign of Louis XIV are outlined in another article. The present article describes a cipher in an intercepted letter of June 1676 from French ambassadors at Nimwegen to the King. At the time, France was engaged in the Dutch War (Wikipedia) but Louis XIV had sought peace after the campaign in 1674. Despite mutual distrust among the allies against the French, Nijmegen (Nimwegen) was accepted as the place for peace negotiations. But it was only at the end of 1676 that all the ambassadors arrived there (Baxter, William III, p.128-130) for negotiations for the Peace Treaty of Nijmegen (Wikipedia).
The deciphering of the letter is described in a contemporary treatise found in the archives in Brussels and printed in Devos et al. (1967) (p.72 ff.).
The cipher gathered from the treatise is as follows.
This employs Arabic figures with two accents to represent single letters (possibly monoalphabetic) and figures with a grave accent, a tilde, an overbar, an umlaut, or a circumflex to represent syllables or small words in generally alphabetical order. These figures are within the range 1-99 with one exception: 100 with an umlaut for "ter". Some three-digit figures are code numbers.
Use of diacritics is also seen in one cipher during the reign of Henry IV (see another article) but is not seen among other French ciphers during the reign of Louis XIV described in another article, of which the earliest one, by Louvois, Minister of War, dates from 1676, the same year as the letter above. Further specimens are wanted to discern whether Louvois' cipher of 1676 introduced the change in the French ciphers.
The striking weakness in this cipher is that figures with two accents are exclusively assigned to single letters. This provides a breach for codebreakers. The author of the treatise says the cipher proposed for the Secretary of State avoided this weak point by mixing ciphers for the same kind of referents among various columns (p.88-89). Among the Spanish ciphers in the 17th century transcribed in another work of Devos (see another article), use of the term "column" reminds one of the type of Spanish ciphers such as Cg.56 (1698) but the same precaution could have been attained by earlier Spanish ciphers via other schemes. It is yet to be found out whether the proposal of the author of the treatise was put into practice.
The above-mentioned treatise, titled "Traité de l'art de déchiffrer", in the Archives Générales du Royaume in Brussels is printed in:
Devos et al. (1967), L'art de deschifrer: traité de déchiffrement du XVIIe siècle de la Secrétairerie d'État et de Guerre Espagnole.
It is in French and was probably written by someone involved in Spanish deciphering activities in the Low Countries. In discussing regular/irregular assignment of syllables to code numbers, the author mentions deciphering of a letter sent to the Low Countries "two years ago" by the Secretary of State by the order of the Spanish King for experimenting in the art of deciphering (p.55).
The author of the treatise, who had the original of the above-mentioned letter of June 1676 in his possession, is not known and Devos gives a safe dating of the treatise as 1676-1714. The several references to the actual deciphering activities may help further study. P.48 mentions identifying an "X" led to deciphering of a letter from Louvois to Comte d'Estrade. According to page 53, a technique for identifying homophones was employed in deciphering in the Low Countries "four months ago" a letter from the minister of Sweden in Paris to the Count of Rybenach [Rübenach], in which "Meclenbourg" occurred frequently.
A letter used in demonstration of deciphering of a Spanish despatch is said to be a portion of a report of the "success of 14 August" and reads: "Donde los hechos hablan de por si es ocioso al discurso ponderarlos y teniendo el presente tantas circunstancias que solo la admiracion puede celebrarlos dignamente no se gastaran en su narracion mas palabras de las que bastaren a referirlas." (p.70) (This text appears to refer to no specific event. One event of this date is the Battle of Saint-Denis (Wikipedia) in 1678 but it may not be called a success.)