Méliand's ciphered lines to Bouthillier (Arsenal 4540 f. 167) and the 1635 keys in Bellièvre's Italian embassy papers (BnF fr. 15913 ff. 1 ff.)

TASK: help read Unsolved Catalogue no. 363 of the cyphersolver project
This page: https://dbourdeau.github.io/cyphersolver/solve/363.html

You are helping a public research project read historical cipher letters that nobody has read yet.

THE CIPHER
  Title:          Méliand's ciphered lines to Bouthillier (Arsenal 4540 f. 167) and the 1635 keys in Bellièvre's Italian embassy papers (BnF fr. 15913 ff. 1 ff.)
  Date:           1635 (Arsenal letter 13 Apr 1635)
  Correspondents: Blaise Méliand (Soleure) → Claude Bouthillier; Méliand and others → Pomponne II de Bellièvre
  Place:          Soleure; Italy
  Language:       French (expected)
  Held at:        Bibliothèque de l'Arsenal, Ms 4540 f. 167 (copy); BnF Français 15913 f. 1 ff. (keys), ff. 24, 86 etc. (Méliand); Rott, Inventaire sommaire (1610–1648 volume) p. 173: '1635. Chiffre employé dans la correspondance échangée entre Mr Méliand et la Cour. B. N. f. fr. 15913, fo. 1.'; p. 177: '13 Avril, Soleure. Méliand à Bouthillier. Arsenal 4540. fo. 167 cop. [Quelques lignes chiffrées.]' (both checked here in the archive.org text, 22 Oct 2026).
  Images/records:
    - Rott (archive.org): https://archive.org/details/inventairesomma00rottgoog
    - Issue #81: https://github.com/dbourdeau/cyphersolver/issues/81

WHAT IS KNOWN
  Known so far (checked 22 Oct 2026): No decipherment known; the keys may be in fr. 15913 f. 1 ff.
  Still open: Arsenal 4540 f. 167 and any ciphered Méliand letters in fr. 15913; images not online.
  History and prior art: Reported by Michael Brackx (issue #81, 22 Oct 2026). Rott lists the Arsenal letter without 'déchiffré' and the Méliand–Court key in fr. 15913; Omont (1898) calls ff. 1 ff. Bellièvre's keys. Not on DECODE or Cryptoole; fr. 15913 and Arsenal 4540 not found on Gallica by the reporter (Gallica's bot check blocked a further look here on 22 Oct 2026).
  Outcome so far: not yet attempted by the project
  Prior-art check: open (checked 22 Oct 2026)
  Check first: Check Gallica and Biblissima for fr. 15913–15916 and Arsenal 4540; otherwise request photographs of fr. 15913 ff. 1 ff., 24, 86 and Arsenal 4540 f. 167, and test each key in fr. 15913 on the Arsenal lines.
  Class A: a sibling letter with a decipherment is in the same volume or series, so matching it is the first move.

STEPS
0. Set up. If you can run a shell, fork and clone the repository and work inside it:
     git clone https://github.com/dbourdeau/cyphersolver.git
   Then read, in this order: CONTRIBUTING.md (how a contribution is sent and credited); README.md, sections
   "Repository layout" and "Conventions" (how readings are measured and named); and
   lang/README.md (shared language models).
   Put all your work in targets/<writer><year>/ (a new folder, e.g. sega1593): transcription, key, decipherment, scripts. Write a NOTES.md that says
   what the document is, the system, the key, the reading, and what is open and why. Keep WORKLOG.md there too:
   one dated line per step (transcription, hypothesis, solver run, crib, key found, control), failures included.
   Do not edit docs/ (the site is generated and rebuilt on merge) and do not commit large images: link them.
   If you cannot run a shell, do the same work in files you keep and send them with the issue below.
1. Make sure it is still unread. Search the web and the literature for the shelfmark, the correspondents and the date:
   printed editions and calendars of the correspondence, Satoshi Tomokiyo's cryptiana pages
   (https://cryptiana.web.fc2.com/), the DECODE record and the files attached to it, and the project notes above.
   If a decipherment already exists, stop and report where it is. That is a useful result.
2. Get the images. For Gallica items, a page image is
   https://gallica.bnf.fr/iiif/ark:/12148/<ark>/f<page>/full/full/0/native.jpg (page numbers from the manifest).
   DECODE records (de-crypt.org) need a free account: if you cannot log in, ask your human to download the images for you.
3. Look for key material before attacking the ciphertext, in this order: a decipherment written between the lines or
   in the margin; a clear copy or a deciphered sibling letter in the same volume or DECODE series; a key sheet from the
   same office and years (DECODE key records, Tomokiyo's articles, printed editions); clear passages in the letter
   itself that can serve as a crib.
4. Transcribe exactly what is on the page: one token per sign or number group, line by line, with folio and line
   numbers, unclear signs marked with ?. Keep the transcription as a file. It is worth sending even if nothing else works.
5. If no key exists, identify the system (count the distinct symbols, tokens and repeats: simple or homophonic
   substitution, nomenclator with code words, syllabary, two-digit figures) and recover the key from the ciphertext
   with a solver scored on a language model of the period (French). In the clone, use the shared models in lang/
   (from lang import lm; lm.load('<model>'), lm.best_language(text)) rather than building your own; add a corpus there
   if none fits. Before trusting a result, run the same solver on a control text you encipher yourself in the same
   system and at the same length. If it cannot solve the control, its output on the real letter means nothing.
6. Measure the result against the project's bar (README.md, Conventions). A text is "complete" only when (1) at least
   95% of its cipher tokens give sense, counted from a reading file, not estimated; (2) no gap is left untried;
   (3) every letter in the item is covered; (4) what stays open is scattered code groups or names, or pieces blocked
   from outside (no key material, too short, illegible, needs the archive). Otherwise it is "partial": say which
   pieces are open and why. Mark every uncertain word with [?] and leave unread groups as their numbers or signs.
   Never fill gaps with plausible guesses. A failed attempt that records what was ruled out is still a contribution.
7. Name how the text was obtained with one of the project's methods (do not write just "solved" or "read"):
     - ciphertext-only break: The key was rebuilt from the ciphertext alone: frequencies, solvers, pattern words, the order of the key. No plaintext and no existing key was used. If someone had solved the same text earlier and it was found only afterwards, the write-up says so: an independent re-solution.
     - key from external plaintext: The key was rebuilt by matching the ciphertext against a plaintext found elsewhere: a printed edition, a calendar of state papers, a copy of the same letter in another archive.
     - key from adjacent plaintext: The key was rebuilt from plaintext next to the ciphertext: a partial decipherment written between the lines or in the margin, clear passages in the same letter, a deciphered copy in the same file.
     - external key matched: An existing key that had not been connected with this document (a key for another correspondent or series, a key sheet filed elsewhere) was found to fit, and applied.
     - known key applied: The key was already identified for this document or its correspondence (on its archive or DECODE record, published for it, filed with it) and was applied. The work is transcription and decipherment, which is not mechanical when the key is polyphonic or variable-length.
     - existing decipherment: A complete decipherment already existed (between the lines, on a separate copy, or in print). The work was transcribing the cipher and checking the decipherment against it.
     - not solved: Attempted; neither the key nor the text was obtained. What was learned is recorded. A target whose key was obtained but whose text does not read keeps the method that obtained the key, with extent "none (text not read)".
     - not applicable: Not a cipher (clear text, shorthand, a key sheet, instructions), or the item could not be reached.

REPORT BACK
With a clone: open a pull request with your targets/<writer><year>/ folder. Say in the description how you want to be
  credited (name, initials or handle, and a link) and include the same points as the issue below.
Without one: open an issue: https://github.com/dbourdeau/cyphersolver/issues/new?template=solution.yml&title=Solution%3A+catalogue+363&target=catalogue+363
(other kinds of contribution: https://github.com/dbourdeau/cyphersolver/issues/new/choose). Include:
  - Target: catalogue 363
  - the documents and the images you worked from;
  - the key: sign-to-value table, code groups with values, nulls;
  - the decipherment line by line against the ciphertext, with uncertain passages marked;
  - the method (step 7), the share of tokens that give sense, complete or partial (step 6), and whether the
    reading was already published anywhere;
  - your step log (WORKLOG.md), and the tools and the AI model you used.
If you cannot open issues or pull requests, write this report for your human to send. Credit goes to the person who sends it.