Mary Queen of Scots — The Castelnau Letters

Mary's Castelnau cipher is a sophisticated Renaissance nomenclator — the same class of system used in the Babington Plot correspondence (1586), which was broken by Thomas Phelippes. The Castelnau cipher is substantially more complex:

• Symbol alphabet: ~100 purpose-designed symbols representing letters and common syllables, not the Roman alphabet, making frequency-alphabet mapping non-trivial.
• Code groups: Named persons, places, and frequent political terms replaced by dedicated codewords (e.g., a specific symbol for "Monsieur de Castelnau," another for "Her Majesty the Queen of England").
• Nulls: Non-semantic symbols interspersed to inflate symbol frequency counts and frustrate statistical attacks.
• Special symbols: The recovered system also includes signs that modify how neighbouring symbols are read. The paper documents their functions; this is not evidence of a separate double-encryption layer.

The plaintext language is French — an additional barrier for analysts who might expect English or Latin in Mary's correspondence.

The attack method, developed by George Lasry, is a variant of the simulated annealing / hill-climbing approach used in modern historical cryptanalysis. The key components:

1. Transcription: The team converted the handwritten ciphertext symbols into machine-readable identifiers. The approximately 50,000 figure describes plaintext words, not cipher characters.
2. Null identification: Statistical analysis of symbol frequency distributions identified probable null characters (symbols appearing with anomalous frequency patterns inconsistent with natural French).
3. Codeword separation: Compound symbols and high-frequency sequences were tentatively identified as code groups (names and words), isolated, and set aside while the substitution alphabet was attacked first.
4. Computer-assisted search: Candidate substitutions were evaluated using language statistics; human analysis then resolved symbols, words and context that the initial output did not explain.
5. Manual refinement: Once the hill-climber converged on a near-correct key, Lasry, Biermann, and Tomokiyo examined readable passages, identified remaining errors, and resolved codeword assignments from context.

Computer-assisted cryptanalysis and archival interpretation complemented one another. Readable fragments helped identify the writer, after which historical context helped refine the decipherment.

The following numbers and mappings are invented teaching examples of nomenclator notation. They are not transcriptions of Mary’s manuscripts or entries from the recovered key.

Each number represents a single item in the nomenclator table. A null symbol (cryptographer's decoy) might appear as · ·98· · or · ·100· · — a high-frequency interspersed symbol that does not contribute to plaintext.

Illustrative entries only; consult the paper for the actual recovered symbols and assignments:

The full content of the deciphered letters is reported in the Cryptologia paper and its supplementary materials. Key categories of newly revealed content include:

• Captivity conditions: Mary describes her treatment in captivity, her access to correspondence and to her attendants, and her physical health — primary-source details not previously available from the cipher side of the archive.
• Political strategy: Mary outlines her diplomatic positioning — appealing to the French crown for intervention, requesting Castelnau relay messages to specific parties at the French court, and describing her view of her prospects in England.
• Undocumented intermediaries: The letters name individuals not previously known to have been involved in Mary's communications network, expanding the known cast of her diplomatic circle.
• Assessment of Walsingham: Mary's own characterisation of Sir Francis Walsingham and her awareness of surveillance — a rare first-person account of Tudor intelligence from the surveilled rather than the surveiller.

Lasry, G., Biermann, N., & Tomokiyo, S. (2023). “Deciphering Mary Stuart’s lost letters from 1578-1584.” Cryptologia, 47(2), 101–202. Published online 8 February 2023; the “2022” in the DOI is not the publication year.

Original manuscripts: Several collections at the Bibliothèque nationale de France. The article’s document inventory supplies the individual archival references; this is not a single manuscript volume.

The article documents the cipher, its recovery, letter texts and historical interpretation. No HistoCrypt 2021 presentation of this result is established here.

Cryptiana context: Satoshi Tomokiyo's wider Cryptiana work on Renaissance diplomatic ciphers provides the archival and technical backdrop for why the Castelnau nomenclator could be recognised, classified, and finally attacked computationally in the first place.

See also: George Lasry exhibit · Babington Plot Cipher · Further Reading: Cryptologia journal