The famous historian Francesco Guicciardini (1483-1540) warned against writing even in cipher any potentially incriminating ideas. One of his maxims written about 1530 advised to be "careful not to communicate by letters; for these are often intercepted, and furnish proof which cannot be controverted. And though nowadays there may be many cautious methods of writing, there have also been discovered many aids for their interpretation." (no.193 in Counsels and Reflections of Francesco Guicciardini, trans. Nanine Hill Thomson (1890) (Internet Archive), (Google)) (I came to know of this quotation by Michael Warner, The Rise and Fall of Intelligence: An International Security History, p.15; it is also quoted in French by Speziali (1955) in n.3).
The present article deals with another Guicciardini from sixty years later.
A cipher used in three letters (in Italian) of November and December 1593 from Giacomo Guicciardini, secretary of the Grand Duke of Tuscany, to Sillery (Wikipedia), a French ambassador who negotiated with the Swiss, is reconstructed in Speziali (1955). The ciphertext consists of figures, which represent letters and syllables in units of two figures except for three-figure code elements.
The continuous stream of figures in the ciphertext suggests this cipher is Italian (Florentine), rather than French. (Speziali (1955) considers it is Florentine from different reasoning (p.196-197).)
A similar numerical cipher with syllable representation is found in no.73 of the Nevers collection (another article).
Another cipher used by (probably the same) Guicciardini, secretary of the Grand Duke, found among diplomatic correspondence addressed to Henry IV in 1590, is presented in Monts de Savasse (1997). The cipher is different from the above in being more irregular and being polyphonic. (I rearranged it as below in the hope of finding some pattern, but there seems to be no regularity in the arrangement.)
There is one other polyphonic cipher (1594) related to the Grand Duke of Tuscany (no.62 of the Nevers collection), which may have been used in correspondence addressed to the duke (see another article).
According to Hammer (1999) (p.179-180), a cipher used by James (Giacomo in Italian) Guicciardini is found in Cecil MS329/3 (which I have not seen). This Guicciardini was born in London and was employed by the Earl of Essex as an agent at Florence in late 1593. He was to serve as a secret contact between England and the Grand Duke of Tuscany, who wanted England to endorse the conversion of Henry IV of France to the Catholic religion. On behalf of the duke, Guicciardini send intelligence about Spanish military intensions, papal overtures to Henry IV, etc.
Use of cipher in James Guicciardini's later correspondence (undeciphered) is recorded in Cecil Papers (British History Online). It appears only some names are ciphered. (If this Guicciardini is different from Giacomo above, the cipher may be English, rather than Florentine.)
BnF Dupuy 63 includes a letter from the Grand Duke of Tuscany, Ferdinand II, to Francesco Guicciardini, ambassador to Philip II of Spain, 23 April 1597 (f.48). Francesco Guicciardini was an ambassador to Philip II in 1593-1602 (Goudriaan, E.J. (2015) "The cultural importance of Florentine patricians", Universiteit Leiden (pdf), p.86).
The cipher can be reconstructed as follows.
The digits are written continuously without break, while the cipher symbols are formed of either one or two digits (variable-length symbols). To allow the (authentic) recipient to decipher, 0, 1, and 5 are reserved to be used singly (except when "0" is used in "20" or "30", in which cases a dot is placed under "0".)
The enciphering clerk of this specific letter breaks a line even in the middle of a two-digit symbols. For example, the last digit "2" on the first line and the first digit on the second line "4" form a symbol for "c" (24).
Pierre Speziali (1955), 'Aspects de la Cryptographie au XVIe Siecle', Bibliotheque d'Humanisme et Renaissance, Tome XVII
Jacques de Monts de Savasse (1997), "Les chiffres de la correspondance diplomatique des ambassadeurs d'Henri IV, en l'année 1590", in Pierre Albert (direction) Correspondre jadis et naguère, 120e Cogrès national des société historiques scientifiques, Aix-en-Province, 1995, p.219-228
Paul E. J. Hammer (1999), The Polarisation of Elizabethan Politics: The Political Career of Robert Devereux, 2nd Earl of Essex, 1585-1597