A French diplomatic cipher used by Charles de Danzay was recently reconstructed by Sergey Ryabov. It is a common homophonic cipher, but it may have a potential interest with regard to the cryptographic practice at the time because Danzay's career as ambassador to Denmark from 1548 to 1589 (according to Daussy, he was only a resident for a first few years) was by far the longest in the sixteenth century.
Ryabov's work showed Danzay used the same cipher at least in 1574 and 1578. (His paper discusses the importance of the content about the escalation of Danish-Swedish tensions etc. but it is outside the scope herein.)
The letter from 1574 is from Danzay to Henry III, dated 14 October 1574 (BnF fr. 4736 (BnF metadata here somehow spells "D'Anzay" with an apostrophe), f.87), which was listed in my list of unsolved ciphers here. The other is from Danzay to Henry III, dated 28 February 1578 (BnF fr.2812, f.45). Ryabov succeeded in reconstructing the cipher from the decipherment in the margins and thereby deciphering the yet unsolved cipher paragraph in the former letter.
Another ciphertext in a letter from 25 November 1575 in Cinq Cents de Colbert 398 (f.63) uses the same cipher.
The reconstructed cipher is as follows.
Apart from the null symbols given here, a few plaintext words are used as nulls in the fr.4736 specimen.
(By re-deciphering the ciphertext, one can see that the decipherment in the margin is not a letter-by-letter decipherment (as is often the case): "navires de guerre" is shortened to "navires", etc. etc.
My table above is slightly different from Ryabov's. It is partly because in working with handwritten ciphertext, there is always a question of whether two symbols with the same meaning are two distinct symbols or variants in handwriting of the same symbol. (I also include handwriting variants in my tables from time to time.)
It would be interesting to see how long Danzay used the same cipher.
BnF fr.20140 includes ciphertext in four letters from Danzay in the reign of Henry II:
f.16 10 January 1557 (to Henry II, deciphered in separate sheets)
f.24 10 January 1557 (to Cardinal of Lorraine, deciphered in the margin)
f.30 27 January 1557 (to Henry II, deciphered in the margin)
f.35 27 January 1557 (to Cardinal of Lorraine, not deciphered)
The reconstructed cipher is as follows.
It can be seen most double letters are represented by the letter itself with a diacritic sign above or below.
In these letters, many nulls are used, not only at the beginning of ciphertext but also between words or even within words. Moreover, there are many different symbols for use as nulls (my reconstruction above does not cover all). While these are better compared to the specimens from the 1570s above, this does not necessarily mean that the French cryptographic practices were better in 1557. As I observed in another article, there are other letters that employ many nulls, some of which are from the 1570s.
In the 1557 specimens above, plaintext words are used as nulls but some of them (e.g., "pour", "le") are real plaintext. So, in this case, words used as nulls must have been specifically defined in the key. (But sometimes, the same word seems to be used for both null and plaintext, e.g., "il", "quand".)
It will be interesting to find and compare other ciphers used during Danzay's career.
Danzay's letters are compiled in Correspondance de Charles Dantzai, ministre de France à la Cour de Danemarck (1824) from Swedish archives and C.F.Bricka (ed.) (1901) from Danish archives, but I have not seen (images of) the original manuscript.
Searching the French archives online returns (besides those already mentioned) BnF fr.15967, Dupuy 937, Mélanges de Colbert 16 (to Danzay), but I do not find Danzay's cipher in these.
Sergey Ryabov (2025), "Secrets of the Foreign Policy of the Last Valois in Northern Europe", Quaestio Rossica, vol.13, no.4, DOI: https://doi.org/10.15826/qr.2025.4.1034 (Academia.edu)
Correspondance de Charles Dantzai, ministre de France à la Cour de Danemarck (1824), vol. ix de la collection Handlingar rôrande Skandinaviens Historia, Stockholm, chez Elmén et Granberg [I have not seen this.]
C.F.Bricka (ed.) (1901), Indberetninger fra Charles de Dançay til det franske hof om forholdene i Norden 1567-1573, I Kommission hos C.A. Reitzel, Copenhagen (HathiTrust)
Holger Frederik Rördam (1897), Résidents français près la cour de Danemark au xvie siècle, extrait du Bulletin de l'Académie royale des sciences et des lettres du Danemark for 1897, Copenhagen (HathiTrust)
Alfred Richard (1910), Un Diplomate poitevin du xvie siècle. Charles de Danzay, ambassadeur de France en Danemark, Poitiers (Internet Archive)
Hugues Daussy (2004),"Un diplomate protestant au service d'un roi catholique : Charles de Danzay, ambassadeur de France au Danemark (1515-1589)", Élites et notables de l'Ouest, xvie-xxe siècle, edited by Frédérique Pitou, Presses universitaires de Rennes, DOI: https://doi.org/10.4000/154oe (OpenEdition Books)
S. Tomokiyo (2019-2023), French ciphers during the Reigns of Charles IX and Henry III