BL Add MS 61575 includes papers related to William Blencowe's codebreaking (f.1-36) (reported to the Earl of Sunderland, Secretary of State in 1706-1710) (All Souls College ... (Dokumen PUB); Deism in Enlightenment England: Theology, Politics, and Newtonian Public Science (Dokumen PUB)). Add MS 61491-61665 are the Sunderland Archive, acquired by the British Library from the collection of the Blenheim Palace (Alsop (1982), "New Light on Joseph Addison" (JSTOR), n.1))
The following outlines ciphertexts in this volume. Two appear to be undeciphered (see Torcy's and Villars').
F.9 (DECODE R8746) is Blencowe's letter reporting his decipherment (16 October 1707), in which he explains that the reading of "437" is his guess, while he was pretty sure about "763" being "le Roy de France." The deciphered letter in question is in f.11-12 (DECODE R8747), which is the deciphered plaintext in which "437" and "763" are written as they are with their conjectured meanings.
It is titled "Lettre d'un envoye de Prince Ragotski pres l'electeur de Baviere et qui a retourne en Hongrie pres ce Prince. Dechiffre" and endorsed "Lettre d'un Ministre de Prince Ragotski a quelqu'un pres l'Electeur de Baviere", 13 September 1707. (Both Rákóczi and the Elector of Bavaria were allied with Louis XIV in their struggle with the Habsburgs at this time in the midst of the War of the Spanish Succession.)
The period of Rákóczi's open anti-Habsburg activities (1703-1711) coincides with the time when cryptography in Hungary reached its high watermark, according to Benedek Láng (2015), Real Life Cryptology, Ciphers and Secrets in Early Modern Hungary, Amsterdam University Press, which details Hungarian ciphers (p.52). A reference book on ciphers from this period was published in 1974 by Révay Zoltán (ibid. p.25, n.48). Many materials related to Ferenc Rákóczi from the Hungarian archives are in DECODE. (One, R633, is a beautifully written sheet of cipher.)
The deciphered letter is in French, one of the preferred languages in political correspondence in Hungary (Láng p.55).
According to Láng (2015), Rákóczi's advocate at the Elector of Bavaria used the same cipher with Chamillard (French secretary for war) and French ambassador Des Alleurs in Latin and in French as well as with Rákóczi and (Pál?) Ráday in Hungarian (p.93). This may be the cipher used in the original of f.9.
F.15-22 are intercepted letters from Hermann van Petkum (Pettecum, Pettekum) from 1709. He was an envoy for Holstein-Gottorp at the Hague from 1695 (University of Nottingham (I have seen only Google snippet)) and later at London (British History Online). Actually, he worked as an intermediary between the Dutch and the French (Churchill (1933), Marlborough His Life and Times; I was reminded of this book by Karl de Leeuw (2000), Cryptology and Statecraft in the Dutch Republic (pdf), p.144).
In early 1709, the Duke of Marlborough obtained the whole correspondence between Petkum and the French foreign minister Torcy and Blencowe deciphered them. Translation of eleven letters are in Add MS 32306, 34518 (Churchill ii, p.496-497). Soon, Petkum's secret negotiation was taken over by an official channel beteen Rouillé and Heinsius, while Marlborough found out about their negotation by obtaining their correspondence. Because of the hardline attitude of the British, the negotiation at this time came to nothing (ibid. p.525-528)
The letters in Add MS 61575 are:
f.15-16 (DECODE R8748), 11 April 1709, largely deciphered in f.18 (the first half of R8750)
f.17 (R8749), 16 April 1709, largely deciphered in f.19 (the second half of R8750)
f.20 (R8751), 19 April 1709, largely deciphered in f.21-22 (R8752)
It appears the British intelligence later acquired the original cipher, which is in f.69-71 (R8766). This is a two-part code, with figures running up to 774. This is good in that letters, syllables, and words are mixed. This may be counted as one of Louis XVI's great ciphers (see another article). It is remarkable that Blencowe could solve a cipher of such complexity.
On the other hand, comparison with the original cipher shows Blencowe's deciphering was sometimes incorrect albeit correct in general sense. Blencowe considered "13" was a homophone for DE, but it is actually "avec." The deciphering part of the original cipher has at least one error. It gives "48" and "104" for "de", but "104" should be "140" according to the enciphering part as well as from my reconstruction.
F.27 (DECODE R8753) is an anonymous letter in French from 10 February 1710. A copy with partial interlinear decipherment is in f.29 (R8754).
The cipher used is a two-part code, with figures running up to at least 409. Letters, syllables, and word fragments are mixed.
F.38-41 (DECODE R8755) is an intercepted letter from the French foreign minister Torcy to French ministers sent to the peace talk at Geertruidenberg, dated 3 April 1710.
This is endorsed "M Blencow cannot decypher them." The cipher must be a great cipher like Petkum's above. My transcription is here. High frequency symbols are: 50 (12x), 42 (10x), 122 (9x), 451 (9x), 5 (8x), 107 (8x), 236 (8x), 77 (7x), 135 (7x), 175 (7x), 269 (7x), 411 (7x), 821 (7x).
F.44 (DECODE R8756) is a copy of a letter from Marshal Villars to the Abbé de Polignac, dated 1 June 1710.
The Abbé de Polignac (Wikipedia) was one of the French plenipotentiaries sent to the peace talk at Geertruidenberg. Marshal Villars was wounded in the Battle of Malplaquet in 1709 and had just come back to the front at this time.
It appears this is also left undeciphered by Blencowe. High frequency symbols are: 340 (10x), 28 (5x), 32 (4x), 91 (4x), 158 (4x), 248 (4x), 267 (4x), 371 (4x), 95 (3x), 100 (3x), 119 (3x), 459 (3x). From this, it appears Villars and Torcy used different ciphers with French plenipotentiaries.
My transcription is here.
The following continue to describe ciphers from the volume, but I'm not sure of their connection to Blencowe.
F.60 (DECODE R8761) is titled "On a ajouté a ce Chiffre les mots suivans", annotated "c.1712-14". It includes some instructions in French. This appendix to some cipher in French includes numbers 1049-1196.
F.61 (R8762) is another copy without a title and instructions, but it has additional entries: 892 (Monsieur), 369 (Bernstorf), St. Jean (Bollingbroke), Raby (Strafford).
F.72-75 (DECODE R8767) is an anonymous cipher with figures 1-996. The entries include letters, syllables, and words in alphabetical order. There is a note "Il y a quellques erreurs qui sont nulls[?] dans l'autre copie."
F.76-77 (DECODE R8768), endorsed "Chifre pour mr le marquis de Rose" and annotated "1715-1718", is a two-part code with numbers 1-578.
F.78 (DECODE R8769) is a fragmentary code list in French.
F.79 (DECODE 8770) is a cipher, with numbers 5-67 for letters (without k) and 90-184 for syllables and double consonants.
Add MS 61575 also includes materials in English.
F.51-52 (DECODE R8757) is a "Cypher with the Earle of Nottingham", annotated "c.1702". It includes numbers 34-699 for letters, syllables, and words and 700-750 for words and names (THE=601). It has a note: "all numbers above and under 34 and all numbers ending in 2, 3, and 6 are nulls" ("above" must mean physically above on the paper).
F.53-54 (R8758) is an English cipher with numbers 30-83 for letters and 200-311 for names (beginning with "200 D. of Marlborough, 201 Duchess of Marlborough, ..."). Annotated "1704-6", with a note apparently to the effect that the dating is based on "Secretary Hedges". Endorsed "Lady Marlb.[?] Cypher" (my reading of the handwriting is not sure enough to say this belonged to Sarah Churchill, Duchess of Marlborough.)
F.56 (first half of R8759) is an English cipher with numbers 20-101 for letters and 104-224 for names and words, annotated "1704-6". (F.55 has an archivist's note "Borrowed from D-23.")
F.57 (second half of R8759) is a code list in English, annotated "1705-6".
F.58-59 (R8760) is an English cipher with numbers 5-83 for letters and 100-223 for names, annotated "c.1706-8".
Endorsed "D[?].M. Cypher" (Duchess of Marlborough?).
F.62-63 (R8763) is an English cipher with numbers 1-113 for letters and names, annotated "c 1712-13".
It appears "additions and changes" are made but the handwriting is hard to read.
F.64-65 (R8764) is a code list beginning with "1 Mr Stanhope", annotated "c. 1712-13". (This is in the hand of Sarah Churchill, Duchess of Marlborough, according to the annotation to a complete key in Add MS 32256, f.31 (DECODE R9130).)
F.66-68 (R8765) is a code list beginning with "1 Mr Harley", annotated "c. 1712-13".
Three ciphertexts in transposition cipher in the volume are described in another article.
Héder, M ; Megyesi, B. The DECODE Database of Historical Ciphers and Keys: Version 2. In: Dahlke, C; Megyesi, B (eds.) Proceedings of the 5th International Conference on Historical Cryptology HistoCrypt 2022. Linkoping, Sweden : LiU E-Press (2022) pp. 111-114. , 4 p. [pdf]
Megyesi Beáta, Esslinger Bernhard, Fornés Alicia, Kopal Nils, Láng Benedek, Lasry George, Leeuw Karl de, Pettersson Eva, Wacker Arno, Waldispühl Michelle. Decryption of historical manuscripts: the DECRYPT project. CRYPTOLOGIA 44 : 6 pp. 545-559. , 15 p. (2020) [link]
Megyesi, B., Blomqvist, N., and Pettersson, E. (2019) The DECODE Database: Collection of Historical Ciphers and Keys. In Proceedings of the 2nd International Conference on Historical Cryptology. HistoCrypt 2019, June 23-25, 2019, Mons, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [pdf]