Cocquet to Mangot, Rome, 13 November 1616: the cipher read

Document
Paris, BnF, Clairambault 369, ff. 316-317 (Gallica btv1b9000782k, views 328-330). Coquet, secretary of the French ambassador in Rome, to Claude Mangot, secretary of state, Rome, 13 November 1616.
New
A first reading of the cipher passages. The cipher was first solved from context, with the crib "a don pedro". The key sheet was found afterwards. We found no earlier use of this sheet for this letter.
Not ours
The key. It is a period document: Paris, BnF, ms. français 18009, f. 156, "Double du chiffre baillé à monsieur le marquis de Treinel" (Gallica btv1b90638776, view 181). The marquis de Tresnel was the ambassador, and Coquet was his secretary.

The key

The cipher has several signs for each letter, numbers and underlined letters for common words, nulls, and two signs that double the letter before them. Here is each sign of the worked lines, cut from the letter and set under its letter. The names of the signs are ours.

sign deldelaA, row 2sign delxdelxaA, row 3sign ffaA, row 1asign ccbB, row 2bsign zzcC, row 3csign 55dD, row 2sign JJdD, row 1dsign LLeE, row 1sign ggeE, row 3sign pipieE, row 4sign tteE, row 2esign CCgG, row 1gsign ddddhH, row 3hsign qqiI, row 1sign ububiI, row 3isign xxlL, row 1lsign 22mM, row 1sign ffffmM, row 2msign ssnN, row 1sign xixinN, row 3nsign mumuoO, row 3sign slsloO, row 1sign ssssoO, row 2osign $$pP, row 2sign ZbZbpP, row 1psign epeprR, row 1rsign nbnbsS, row 2sign ththsS, row 3ssign gamgamtT, row 1sign obobtT, row 3sign otottT, row 2tsign HHuV, row 3sign etaetauV, row 1sign wwuV, row 2usign 66yY, row 1ysign oooonullrule sign DBLDBLdoublesrulesign TTdoublesrule sign n_n_leword list sign 8686queword list sign 8989quilword list 

In each box: the sign, our name for it, its value, and its place on the key sheet (column letter and row of the alphabet; "word list" = the list of numbers and underlined letters; "rule" = a note on the sheet). Z and S are z and s written larger. On the sheet the column V serves for u and v.

The key sheet of the marquis de Tresnel, BnF fr. 18009, f. 156
The key sheet, reduced: BnF, ms. français 18009, f. 156. The alphabet is the band at the top: 22 columns, A to Z, with two to four signs under each letter. Source: gallica.bnf.fr / BnF.
Key sheet, alphabet, columns A to M
The alphabet of the key sheet, left half (columns A to M). Row 1 is the first sign under a letter. Source: gallica.bnf.fr / BnF.
Key sheet, alphabet, columns N to Z
The alphabet of the key sheet, right half (columns N to Z). Source: gallica.bnf.fr / BnF.

Our transcription of the whole sheet (259 rows, one pass): key_fr18009_f156.tsv. The decoder with the key table: decode.py.

Worked example

The first cipher run fills three lines of f. 317r. Each box holds one sign: its image, our name for it, and its value by the key. The line under a group of boxes gives the word; the word division is ours. Spelling is that of the cipher (u and v are one letter).

Line 1

Cipher line 1 of the lower half of f. 317r
Clairambault 369, f. 317r, cipher line 1 (set in two rows here). Source: gallica.bnf.fr / BnF.
sign 8686quequesign n_n_lelesign JJdsign wwusign zzcducsign JJdsign ggedesign 22msign mumuosign ssnsign otottsign deldelasign n_n_lesign mumuosign xixinmontaleonsign deldelasign wwusign slslosign qqisign gamgamtauoitsign ffffmsign ffasign ssnsign 55dsign pipiemandesign 8686quequesign ththssign deldelasasign 22msign deldelasign qqisign LLesign ththssign otottsign ttemaieste

Line 2

Cipher line 2 of the lower half of f. 317r
Clairambault 369, f. 317r, cipher line 2 (set in two rows here). Source: gallica.bnf.fr / BnF.
sign deldelasign etaetausign mumuosign ububisign obobtauoitsign HHusign xixinunsign ththssign 66ysysign CCgsign epeprsign deldelasign ssnsign JJdgrandsign TTdoubles: dsign ttesign ththssign wwusign mumuosign 66ysign LLesign 22msign LLesign ssnsign gamgamtdesuoyementsign ddddhsign deldelasign wwusign xxlsign ototthaultsign ggesign gamgamtetsign ccbsign delxdelxasign nbnbsbassign 8989quilquilsign zzcsign epeprsign mumuosign 66ycroy

Line 3

Cipher line 3 of the lower half of f. 317r
Clairambault 369, f. 317r, cipher line 3 (set in two rows here). Source: gallica.bnf.fr / BnF.
sign ssssosign qqisign otottoitsign 8686quequesign LLesign xxlsign n_n_leellesign xixinsign ggenesign $$psign mumuosign wwusign TTdoubles: usign slslosign qqisign otottpouuoitsign ZbZbpsign deldelasign ththssign DBLDBLdoubles: ssign ttesign epeprpasser

que le duc de Montaleon auoit mande que sa maieste auoit un sy grand desuoyement hault et bas qu'il croyoit qu'elle ne pouuoit passer

that the duke of Monteleone had written that His Majesty had so great a flux upward and downward (vomiting and diarrhoea) that he believed that His Majesty could not last [two hours].

The word "croyoit" runs over the end of line 2. The cipher has "que elle"; the reading file writes "qu'elle". The signs T and DBL double the letter before them: granddesuoyement, pouuuoit, passer.

Line 5

Cipher line 5 of the lower half of f. 317r
Clairambault 369, f. 317r, line 5: the cipher run between clear French. Source: gallica.bnf.fr / BnF.
sign deldelaasign JJdsign mumuosign ssndonsign $$psign ggesign JJdsign epeprsign mumuopedrosign oooonull sign oooonull 

[cest homme est dangereux et mande force choses] a don Pedro

[this man is dangerous and writes many things] to Don Pedro

The two last signs are nulls. "a don pedro" was the crib that opened the cipher.

How sure

MeasureValue
Cipher signs189, in nine runs inside clear French
Signs identical in the two transcription passes (the second blind)179 (94.7 %); no difference changes a word
Sign values of the context solution that agree with the key sheet43 of 43
Four-letter-group score of the reading-10.47; 2,000 shuffled keys: mean -15.42, best -13.13

Outside check. The key sheet itself. The solution from context came first; all 43 sign values of that solution agree with the sheet, with no conflict. The sheet then corrected five points (the code 89 = "qu'il", the words "hault" and "quant", the sign for "il", and the doubling sign).

What is not resolved

French text with the cipher passages, and an English translation: reading.md. Folder: cocquet-1616.